chapter 1: introduction

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How to cite this thesis Surname, Initial(s). (2012) Title of the thesis or dissertation. PhD. (Chemistry)/ M.Sc. (Physics)/ M.A. (Philosophy)/M.Com. (Finance) etc. [Unpublished]: University of Johannesburg. Retrieved from: https://ujcontent.uj.ac.za/vital/access/manager/Index?site_name=Research%20Output (Accessed: Date).

LASER ADDITIVE MANUFACTURING TECHNOLOGY FOR CRACK REPAIRS IN TITANIUM ALLOY COMPONENTS by Tawanda Marazani Student Number: 201515137 Submitted in partial fulfilment of the requirements for the degree Masters in Engineering in Mechanical Engineering Faculty of Engineering and the Built Environment at the UNIVERSITY OF JOHANNESBURG Supervisor: Mr D. M. Madyira Co-supervisor: Prof. E. T. Akinlabi

January 2016

DECLARATION I, Tawanda Marazani, hereby declares that this dissertation is wholly my own work and has not been submitted anywhere else for academic credit either by myself or another person.

I understand what plagiarism implies and declare that this dissertation is my own ideas, words, phrases, arguments, graphics, figures, results, and organization except where reference is explicitly made to another’s work.

I understand further that any unethical academic behaviour, which includes plagiarism, is seen in a serious light by the University of Johannesburg and is punishable by disciplinary action.

Date:

i

January, 2016

DECLARATION| LAT for crack repairs in Titanium alloy components

ABSTRACT Laser additive technology (LAT) uses a laser beam which locally melts the target material surface. The technology has been widely used for high value and critical components mainly in the aerospace and the biomedical industries. Due to its favourable properties, titanium has become a workhorse metal, particularly the grade 5 titanium alloy (Ti-6Al-4V). Recent years have seen increased research and development studies on the application of the laser additive technology in the production of Ti-6Al-4V components. These ranged from free form fabrication, materials processing, manufacturing, maintenance and repairs.

Attempts to use the laser additive technology for the repair of cracks in Ti alloy components have been recently reported where V-grooves have been recommended. Further attempts to use narrow U-grooves for crack repairs were not successful and hence not widely adopted. There is limited published work on the use of narrow rectangular grooves for crack repairs in Ti-6Al4V.There is therefore a need to further investigate the potential repairing of U-cracks using LAT. This research work established through experimental design, mechanical and metallographic characterization, a process that was used for the laser additive repair of cracks in Ti-6Al-4V components. The preliminary repairs were made without laser re-melting. They were analysed for defects using the optical microscopy (OM) and their macrographs revealed lack of sidewall fusion, lack of interlayer fusion, lack of intralayer fusion, unmelted powder and porosity.

The matrix used for the preliminary repairs was then optimised using the observations made during the preliminary phase. It was from this preliminary phase optimization that the final experimental matrix of the research was developed. Controlled laser re-melting, reduction of the spot size diameter and lowering of the scanning speed were introduced as main process parameters of the optimized matrix. The optimized repairs were further characterized using the optical microscopy (OM) and the scanning electron microscopy (SEM). The optimized repairs were observed to have very limited defects. The energy dispersive spectroscopy (EDS) analyses revealed that the deposits were dominated by Ti, Al and V which were the main compositions of the material. The Vickers microhardness tests, microhardness-tensile strength correlations and the Charpy impact tests obtained results confirmed mechanically sound repairs with good evolving microstructural properties. ii

ABSTRACT| LAT for crack repairs in Titanium alloy components

Successful repairs of some of the samples produced were achieved through controlled re-melt runs at laser powers ranging from 1.3 kW to 2.5 kW, scanning speeds of 0.5 m/min and 2 m/min and 1 and 2 mm laser spot size diameters for 2 and 3 mm cracks respectively as the main process parameters. Considering the settings employed in this research study, the optimized settings for a successful repair was found to be at a laser power of 1.5kW, 1 mm laser spot size diameter for the 2 mm crack, 2 mm laser spot size diameter for the 3 mm crack, scanning speed of 0.5 m/min, powder feed rate of 3 rpm, and a shielding gas flow rate of 10 l/min at controlled laser re-melting applied after every two depositions of 1.4 mm height.

iii

ABSTRACT | LAT for crack repairs in Titanium alloy components

DEDICATION I dedicate this research project to my late father, Jatiwa Wellington Hwingwiri (1945-2001) who has always wished me to become a Professor someday, since my early childhood. Father, your words turned a true blessing and wherever you are, I am sure you are proud of me, for I am on the road to fulfilling your wishes.

iv

DEDICATION| LAT for crack repairs in Titanium alloy components

ACKNOWLEDGEMENTS Unto the Lord my Shepherd, I give glory for He has taken me this far. Ebenezer.

In a very special way, I would like to convey sincere gratitude and appreciation to my supervisor Mr D. M. Madyira and my Co-supervisor Prof. E. T. Akinlabi for their seamless selfless invaluable guidance, support, unmeasurable encouragement and for always being approachable throughout this Masters research study. Their guidance was key to the timeous completion of this project. I am grateful to Prof. S. Pityana who arranged the use of the CSIR National Laser Centre (NLC) laboratory for the LMD experiments and for all the technical advice during the preliminary LMD repair phase; Dr M. Tlotleng for the CSIR NLC tour and for all the technical advices; Mr B. S. Skhosane, the Laboratory Technician at the CSIR NLC for all the technical assistance throughout the LMD experiments. Special thanks go to Mr M. O. Abegunde for demonstrating the use of all the metallurgical characterization equipment. Extended to this special vote of thanks is that Mr M. O. Abegunde always made sure that the laboratory was ready for use during the time I was conducting my experiments. Of special mention is the assistance with the machining of V-notches on the Charpy impact samples that was offered by Mr W. Dott in the University of Johannesburg’s Mechanical Manufacturing laboratory. To be mentioned also is Gem Manufacturers for the wire electrical discharge machining (WEDM) of all the samples. I would also like to extend my appreciation to my fellow Masters students: Mr O. M. Ogunlana, Mr E. Nyoni, Mr E. Kula and Miss. P. Ratilal for openly sharing research ideas that contributed to the success of this work.

I would also like to thank my mother, A. Jengwa for the role she played in my Primary and Secondary education as well as her constant moral build-up, foundations without which, this could not have been reachable. I also thank my brothers and sisters and their families for the encouragement and prayers during the time of my Masters studies. Of final and most important mention, my love, wife and mother to my daughter, L. Gedi who is always there to encourage and emotionally support me, and for her many helpful suggestions during the writing of this project. I would like to express deep love to my daughter, Tisha Mutsawashe, whose understanding, hugs and cuddles were my main sources of inspiration to soldier on.

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ABSTRACT | LAT for crack repairs in Titanium alloy components

TABLE OF CONTENTS

DECLARATION ......................................................................................................................... i ABSTRACT ............................................................................................................................... ii DEDICATION .......................................................................................................................... iv ACKNOWLEDGEMENTS ....................................................................................................... v TABLE OF CONTENTS .......................................................................................................... vi LIST OF FIGURES ................................................................................................................... xi LIST OF TABLES .................................................................................................................. xvi ABBREVIATIONS .................................................................................................................xvii NOMENCLATURE ...............................................................................................................xviii GLOSSARY OF TERMS.........................................................................................................xix Chapter 1 :

INTRODUCTION ................................................................................................ 1

1.1

Background .................................................................................................................. 1

1.2

Problem Statement ....................................................................................................... 4

1.3

Research Aim ............................................................................................................... 4

1.4

Research Objectives ..................................................................................................... 4

1.5

Scope of Work ............................................................................................................. 5

1.6

Hypotheses ................................................................................................................... 5

1.7

Methodology................................................................................................................ 5

1.8

Significance of Study .................................................................................................... 6

1.9

Concluding Remarks ................................................................................................... 6

Chapter 2 :

LITERATURE REVIEW ..................................................................................... 7

2.1

Introduction .................................................................................................................. 7

2.2

Titanium and Its Alloys ............................................................................................... 7

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TABLE OF CONTENTS| LAT for crack repairs in Titanium alloy components

2.2.1

Discovery, processing and microstructure of CP-Ti ............................................ 7

2.2.2 Titanium alloys and their properties ........................................................................ 10 2.2.3 Applications of titanium alloys................................................................................ 16 2.3 Imperfections and Defects in Metals .............................................................................. 18 2.3.1

The general nature of metals .............................................................................. 18

2.3.2

Crystalline imperfections in metals .................................................................... 19

2.3.3

Defects in metals ................................................................................................ 23

2.4 Defects in Ti-6Al-4V ...................................................................................................... 30 2.4.1

Common defects in laser deposited Ti-6Al-4V components.............................. 30

2.5 Detection of Crack Defects in Metals............................................................................. 31 2.5.1

The need for crack detection in metals ............................................................... 31

2.5.2

Crack detection techniques in metals ................................................................. 32

2.6 Repair of Crack Defects in Metals ................................................................................. 35 2.6.1

Introduction to crack repair techniques in metals ............................................... 35

2.6.2

Crack repair techniques ...................................................................................... 35

2.7

Repair of Defects in Ti-6Al-4V ................................................................................. 40

2.7.1

Common repair methods for Ti-6Al-4V defects ................................................ 40

2.7.2

LMD repair of cracks in Ti-6Al-4V components ............................................... 41

2.8 Laser Metal Deposition (LMD) ...................................................................................... 45 2.8.1 Features of LMD ..................................................................................................... 45 2.8.2

Applications of LMD ......................................................................................... 47

2.8.3

Materials input and process parameters of LMD ............................................... 48

2.9

Lasers Used In Materials Processing ......................................................................... 51

2.9.1 Introduction to lasers used in materials processing ................................................. 51 2.9.2 Liquid dye, gas and solid state lasers ...................................................................... 52 2.9.3 vii

Semiconductor, fiber and eximer lasers ............................................................. 53 TABLE OF CONTENTS | LAT for crack repairs in Titanium alloy components

Wire Electrical Discharge Machining (WEDM) .................................................... 54

2.10 2.10.1

Basic principles of WEDM............................................................................... 54

2.10.2

WEDM applications and effects on Ti-6Al-4V components ............................ 55 Characterization of LMD Ti-6Al-4V Components ................................................ 55

2.11 2.11.1

Justification for materials characterization ........................................................ 55

2.11.2

Basic principles of the tensile and Charpy impact testing .............................. 56

2.11.3

Microstructural and hardness analysis of LMD Ti-6Al-4V components ......... 58

2.11.4

Mechanical characterization of LMD Ti-6Al-4V components ......................... 64 Concluding Remarks .............................................................................................. 66

2.12 Chapter 3 :

EXPERIMENTAL DESIGN AND SETUP ....................................................... 69

3.1

Introduction ................................................................................................................ 69

3.2

Aim of the Experiments ............................................................................................. 69

3.3

Description of Materials for the Experiments ............................................................ 69

3.4

Specimen Description ................................................................................................ 70

3.4.1

Tension test specimen design and preparation ................................................... 70

3.4.2

Charpy impact test specimen design and preparation ........................................ 70

3.4.3

WEDM of Ti-6Al-4V ......................................................................................... 71

3.4.4

Microhardness and Microstructure Tests Specimen preparation ....................... 74

3.5

Experimental Equipment ........................................................................................... 74

3.5.1

General equipment description ........................................................................... 74

3.6

Experimental Procedure ............................................................................................. 76

3.7

Experimental Matrix .................................................................................................. 76

3.7.1

Preliminary/trial experimental matrix ................................................................ 76

3.7.2

Optimized experimental matrix .......................................................................... 78

3.8

Sample Preparation .................................................................................................... 80

3.8.1 viii

Cutting of samples .............................................................................................. 80 TABLE OF CONTENTS | LAT for crack repairs in Titanium alloy components

3.8.2

Mounting and grinding of samples ..................................................................... 81

3.8.3 Polishing and etching of mounted samples ............................................................. 83 3.8.4 Charpy impact specimens ........................................................................................ 83 3.9

Microstructure Characterization ................................................................................ 84

3.9.1

Optical microscopy ............................................................................................. 84

3.9.2

Scanning electron microscopy (SEM) ................................................................ 85

3.9.3

Energy dispersive spectroscopy (EDS) .............................................................. 86 Mechanical Characterization .................................................................................. 86

3.10 3.10.1

Microhardness testing ......................................................................................... 86

3.10.2 Tensile testing ........................................................................................................ 88 3.10. 3 Charpy impact testing ........................................................................................... 89 Summary ................................................................................................................ 91

3.11

Chapter 4 : RESULTS AND DISCUSSION ............................................................................ 92 4.1

Introduction ................................................................................................................ 92

4.2

Characterization of Preliminary Repairs .................................................................... 92

4.2.1

Physical appearance of preliminary LMD repaired substrates ........................... 92

4.2.2

Microstructural characterization of preliminary LMD repaired samples ........... 94

4.2.3

Porosity analysis of preliminary LMD repaired samples ................................... 96

4.3

Characterization of Optimized LMD Repairs ............................................................ 98

4.3.1

Optimization of processing parameters ............................................................. 98

4.3.2

Physical appearance of optimized clads ........................................................... 100

4.3.3

Optical microscopy analysis of optimized clads .............................................. 100

4.3.4

Geometrical analysis......................................................................................... 102

4.3.5

Porosity analysis ............................................................................................... 105

4.3.6

Microstructure analysis..................................................................................... 107

4.3.7

SEM results ...................................................................................................... 114

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TABLE OF CONTENTS | LAT for crack repairs in Titanium alloy components

4.3.8

EDS results ...................................................................................................... 119

4.3.9

Microhardness testing results ........................................................................... 124

4.3.10

Tensile test results ............................................................................................ 134

4.3.11

Charpy impact test results ............................................................................. 136

4.4

Concluding Remarks ............................................................................................... 138

Chapter 5 :

CONCLUSIONS AND FUTURE WORK ....................................................... 139

5.1 Introduction .................................................................................................................. 139 5.2

Conclusions .............................................................................................................. 140

5.3

Suggested Future Work ........................................................................................... 142

REFERENCES ....................................................................................................................... 143 APPENDIX A ............................................................................................................................ 159 APPENDIX B............................................................................................................................. 162 APPENDIX C............................................................................................................................. 166 APPENDIX D ............................................................................................................................ 181

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TABLE OF CONTENTS | LAT for crack repairs in Titanium alloy components

LIST OF FIGURES Figure 1.1: (a) The titanium-built SR-71 Blackbird and (b) Customized Lower Jaw Implant [4] .................................................................................................................................................... 2 Figure 2.1: Crystal structure of (hcp) α phase and (bcc) β phase [36] ....................................... 9 Figure 2.2: Schematic diagrams on the Influence of alloying elements on phase diagrams of Titanium alloys [36] ................................................................................................................. 14 Figure 2.3: Ti-6Al-4V products: left to right, (rings, valves, connecting rods), medical implants, aero-space, marine, racing bicycles and condenser tubes for power generation [39] .............. 17 Figure 2.4: From left to right: Close-packed hexagonal unit cell structure, Face-cantered cubic unit cell structure and Body-centred cubic unit cell structure [71] .......................................... 19 Figure 2.5: Point defects [75] ................................................................................................... 21 Figure 2.6: Edge and Screw dislocation [75] ........................................................................... 21 Figure 2.7: Grain boundary defects [72] .................................................................................. 22 Figure 2.8: Volume or bulk defect: void through a metal piece [75] ....................................... 23 Figure 2.9: Centreline crack [80] .............................................................................................. 25 Figure 2.10: Crater crack [80] .................................................................................................. 26 Figure 2.11: Stages of creep cracks [80] .................................................................................. 28 Figure 2.12: Stress corrosion cracks [80] ................................................................................. 29 Figure 2.13: Hydrogen cracks [80] ........................................................................................... 29 Figure 2.14: Crack detection stages for liquid penetration inspection [92].............................. 33 Figure 2.15: Ultrasonic testing [92] .......................................................................................... 33 Figure 2.16: X-ray set-up [92] .................................................................................................. 34 Figure 2.17: X-rays falling on a work-piece with a film placed close to its rear surface [92] . 34 Figure 2.18: Magnetic flux through work-pieces, (a) a particle ridge is built up over the surface crack mouth, and (b) magnetic particles are deposited due to a subsurface crack [92] ........... 35 Figure 2.19: Stop-hole approach of arresting cracks [99] ........................................................ 38 Figure 2.20: Crack repair of aluminium using bonded composite patches [101]..................... 39 Figure 2.21: From left to right: V-groove; U-groove and U-groove with angled side walls [108] .................................................................................................................................................. 42 Figure 2.22: Left to right: LMD repaired stainless steel V-groove and the Ti-6Al-4V V-groove [108] ......................................................................................................................................... 43 xi

LIST OF FIGURES| LAT for crack repairs in Titanium alloy components

Figure 2.23: Left (a) U-groove; Right (b) 5o U-groove top open side walls [108] .................. 43 Figure 2.24: Cracks on LMD repaired grooves without preheating [33] ................................. 44 Figure 2.25: Cracks reduced by preheating of LMD repaired grooves [33] ............................ 44 Figure 2.26: Cracks detected by SEM analysis [33] ................................................................ 44 Figure 2.27: Schematic of the Laser Material Deposition [43] ................................................ 46 Figure 2.28: LMD shielding box [42] ...................................................................................... 50 Figure 2.29: Left (a) computer integrated tensile tester [136]; Right (b) stress-strain graph [135] .................................................................................................................................................. 57 Figure 2.30: Lateral expansion [138] ....................................................................................... 58 Figure 2.31: Example of OM micrographs [12] ....................................................................... 59 Figure 2.32: SEM images [117] ............................................................................................... 60 Figure 2.33: Microstructure micrographs [145] ....................................................................... 62 Figure 2.34: SEM analysis showing (a) and (b) martensitic structure; (c) and (d) epitaxial microstructural band [145] ....................................................................................................... 62 Figure 2.35 Morphology of Sample A showing the different zones [42] ................................ 63 Figure 2.36: Microstructure of the fusion zone showing different α grains [42] ..................... 63 Figure 2.37: Morphology of the Ti-6Al-4V powder [40]......................................................... 64 Figure 2.38: Porosity analysis sample micrographs: left (higher laser power); right (lower laser power) [40] ............................................................................................................................... 64 Figure 3.1: 100 x 100 x 7 mm Ti-6Al-4V block ...................................................................... 69 Figure 3.2: 2-dimensional and 3-dimensional drawings of the Tensile Test Specimens ......... 70 Figure 3.3: 2-dimensional and 3-dimensional drawings of Charpy Impact Test specimens .... 71 Figure 3.4: WEDM cut 2-d and 3-d drawings for Tensile test Ti-6Al-4V half blocks, 9 off.. 71 Figure 3.5: 2-d and 3-d drawings for tensile test Ti-6Al-4V half blocks with a 0.5 mm WEDM induced crack, 3 off .................................................................................................................. 72 Figure 3.6: 2-d and 3-d drawings for tensile test Ti-6Al-4V half blocks with a 2 mm WEDM induced crack, 3 off .................................................................................................................. 72 Figure 3.7: 2-d and 3-d drawings for tensile test Ti-6Al-4V half blocks with a 3 mm WEDM induced crack, 3 off .................................................................................................................. 72 Figure 3.8: From left to right: 0.5, 2 and 3mm cracks WEDM cut Charpy samples x3 each . 73 Figure 3.9: (a) 0.5 mm, (b) 2 mm and (c) 3 mm cracks WEDM cut tensile samples, x3 each 73 xii

LIST OF FIGURES | LAT for crack repairs in Titanium alloy components

Figure 3.10: (a) 1 mm optimization cracks x2 and (b) microhardness and microstructure cracks x3 WEDM cut samples ............................................................................................................. 74 Figure 3.11: WEDM machine, GEM Manufacturers ............................................................... 75 Figure 3.12: Sand blasting machine ......................................................................................... 75 Figure 3.13: ND-YAG Roffin Laser with a Kuka robot .......................................................... 75 Figure 3.14: Surface weld runs height measured with the Mitutoyo vernier height gauge at CSIR NLC .......................................................................................................................................... 78 Figure 3.15: Mecatome T300 water based cut off machine ..................................................... 80 Figure 3.16: Struers Cito Press hot compression mounting machine ...................................... 81 Figure 3.17: Struers Labopol-25 grinding machine at the University of Johannesburg........... 82 Figure 3.18: Mounted, ground, polished and etched samples of control sample and optimized LMD repaired samples ............................................................................................................. 83 Figure 3.19: V-notched Charpy impact samples ...................................................................... 84 Figure 3.20: PC interfaced Olympus DP25 and Olympus SZX16 microscopes ...................... 85 Figure 3.21: Tescan 500 mm2 Scanning Electron Microscope ................................................ 86 Figure 3.22: Micro Met Scientific cc digital microhardness tester .......................................... 87 Figure 3.23: Schematic diagram for the diamond indentation design used in this work ......... 88 Figure 3.24: (a) Standard Charpy impact-V notch specimen [137], (b) Manipulated Charpy impact-V notch specimen ......................................................................................................... 89 Figure 3.25: LOS Charpy impact tester ................................................................................... 90 Figure 3.26: (a) Zero reference drag indicator position, (b) Charpy impact tester zero reference point .......................................................................................................................................... 91 Figure 4.1: Physical appearance of the preliminary LMD repaired D1, D2, D3, D4 and D5 samples ..................................................................................................................................... 93 Figure 4.2: Full optical microscopy images of preliminary LMD clads for samples D1, D2, D3, D4 and D5 at x1.6 magnification.............................................................................................. 95 Figure 4.3: Macrographs of D-samples at 5x magnification: (a) D1, (b) D2, (c) D3, (d) D4 and (e) D5 ........................................................................................................................................ 96 Figure 4.4: Porosity images for D1, D2, D3, D4 and D5 samples ........................................... 98 Figure 4.5: Maximum pore size (μm) and porosity % of samples D1, D2, D3, D4 and D5 .... 98 Figure 4.6: LMD Clads for samples A1, A2, A3, A4 and A5 ................................................ 100

xiii


Figure 4.7: Optical microscopy images of optimized A1, A2, A3, A4 and A5 clads at x 1.6 magnification .......................................................................................................................... 102 Figure 4.8: Geometrical analysis for samples A1, A3 and A4 ............................................... 104 Figure 4.9: Main geometrical properties of LMD track cross-section: Clad height H, clad width W, clad area Ac and molten area Am) [160] .......................................................................... 104 Figure 4.10: Porosity analysis images for samples A1, A2, A3, A4 and A5 ......................... 107 Figure 4.11: Microstructure of the substrate ........................................................................... 108 Figure 4.12: Microstructure of sample A1.............................................................................. 110 Figure 4.13: Microstructure of Sample A2 ............................................................................. 111 Figure 4.14: Microstructure of sample A3 .............................................................................. 112 Figure 4.15: Microstructure of sample A4 .............................................................................. 113 Figure 4.16: Microstructure of Sample A5 ............................................................................. 114 Figure 4.17: SEM analysis for Sample A1 ............................................................................. 116 Figure 4.18: SEM analysis for sample A3 .............................................................................. 117 Figure 4.19: SEM analysis for sample A4 .............................................................................. 118 Figure 4.20: SEM analysis for Ti-6Al-4V substrate and Ti-6Al-4V powder ......................... 119 Figure 4.21: Sample A1 SEM-EDS electron image ............................................................... 120 Figure 4.22: Spectrum 1 Sample A1 Chemical composition analysis .................................... 120 Figure 4.23: Spectrum 2 Sample A1 Chemical composition analysis .................................... 121 Figure 4.24: Spectrum 3 Sample A1 Chemical composition analysis .................................... 122 Figure 4.25: Spectrum 4 Sample A1 Chemical composition analysis .................................... 123 Figure 4.26: A1-Sample Across weld rows microhardness HV 0.05 plot .............................. 126 Figure 4.27: Sample A1 3d surface microhardness HV0.05 plot ........................................... 126 Figure 4.28: Sample A1 central microhardness HV 0.05 top to bottom plot ......................... 127 Figure 4.29: Sample A1 Column average microhardness HV 0.05 plot ................................ 127 Figure 4.30: Sample A3 Across weld rows microhardness HV 0.05 plot .............................. 128 Figure 4.31: Sample A3 3-d surface microhardness HV0.05 plot .......................................... 129 Figure 4.32: Sample A3 top to bottom microhardness HV 0.05 central plot ......................... 129 Figure 4.33: Sample A3 Column average microhardness HV 0.05 plot ................................ 130 Figure 4.34: Sample A4 Across weld rows microhardness HV 0.05 plot .............................. 131 Figure 4.35: Sample A4 3-d surface microhardness HV0.05 plot .......................................... 131 xiv


Figure 4.36: Sample A4 top to bottom microhardness HV 0.05 central plot ......................... 132 Figure 4.37: Sample A4 Column average microhardness HV 0.05 plot ................................ 132 Figure 4.38: Control Ti-6Al-4V parent material microhardness HV 0.05 ............................. 133 Figure 4.39: Sample average microhardness HV 0.05 ........................................................... 133 Figure 4.40: Comparison of central top to bottom hardness HV for samples A1, A3 and A4 ................................................................................................................................................ 134 Figure 4.41: Comparison of column average hardness for samples A1, A3 and A4 .............. 134 Figure 4.42: Samples Tensile strength (MPa) ........................................................................ 136 Figure 4.43: Charpy Impact samples fractured surfaces ......................................................... 137 Figure 4.44: Average V-Notch Charpy Impact energy results ............................................... 137

xv


LIST OF TABLES Table 2.1: Common alloying elements and their stabilizing effects [50] ................................. 12 Table 2.2: Mechanical properties of a 57 mm thick x 235 wide Ti-6Al-4V bar [57] .............. 13 Table 2.3: Mechanical and Thermal properties of Ti-6Al-4V and Ti-55531 [51] ................... 13 Table 2.4: Material properties of the Ti-6Al-4V alloy [56] ..................................................... 14 Table 2.5: Components of Ti-6Al-4V [63]............................................................................... 16 Table 2.6: LMD parameters for the Ti-6Al-4V V-groove; U-groove and U-groove with angled side walls [108]......................................................................................................................... 42 Table 3.1: Matrix for 0.5 mm crack size at focal length 166.67 mm ....................................... 77 Table 3.2: Matrix for 1 mm crack size at focal length 205.02 mm .......................................... 77 Table 3.3: Matrix for 2 mm crack size at focal length 178 mm ............................................... 77 Table 3.4: Matrix for 3 mm crack size at focal length 195 mm ............................................... 77 Table 3.5: Optimized Matrix for 2 mm crack size at focal length 178 mm ............................. 79 Table 3.6: Optimized Matrix for 3 mm crack size at focal length 195 mm ............................. 80 Table 3.7: Hot compression mounting setup data .................................................................... 81 Table 4.1: Geometrical properties of A1, A3 and A4 clads ................................................... 103 Table 4.2: Percentage porosity, pore count and maximum pore sizes of different samples ... 107 Table 4.3: EDS analysis results for sample A1 spectrum 1 .................................................... 120 Table 4.4: EDS analysis results for sample A1 spectrum 2 .................................................... 121 Table 4.5: EDS analysis results for sample A1 spectrum 3 .................................................... 122 Table 4.6: EDS analysis results for sample A1 spectrum 4 .................................................... 123 Table 4.7: Conversion of hardness to tensile strength ............................................................ 135

xvi

LIST OF TABLES| LAT for crack repairs in Titanium alloy components

ABBREVIATIONS Al

- Aluminium

AM

- Additive Manufacturing

ASTM

- American Standards for Testing and Materials

CP-Ti

- Commercially Pure Titanium

CSIR

- Council for Science and Industrial Research

Cu

- Copper

DST

- Department of Science and Technology

EDS

- Energy Dispersive Spectroscope

FGMs

- Functionally Graded Materials

GMAW

- Gas Metal Arc Welding

HAZ

- Heat Affected Zone

HV

- Vickers hardness

LASER

- Light Amplification by Emission of Radiation

LMD

- Laser Metal Deposition

MPa

- Mega Pascal

MMCs

- Metal Matrix Composites

mm/min

- Millimetre per minute

NDT

- Non Destructive Testing

OM

- Optical Microscpe

rpm

- Revolutions per minute

SCC

- Stress Corrosion Cracks

SEM

- Scanning Electron Microscope

SiC

- Silicon Carbide

TEM

- Transmission Electron Microscope

Ti

- Titanium

TMCs

- Titanium Metal Composites

UTS

- Ultimate Tensile Strength

WEDM

- Wire Electrical Discharge Machining

XRD

- X-Ray Diffraction

YS

- Yield Strength

xvii

ABBREVIATIONS| LAT for crack repairs in Titanium alloy components

NOMENCLATURE Symbol

Description

Units

ρ

Density

kg/m3

P

Laser Power

kW

φ

Laser spot diameter

mm

f

Powder feed rate

rpm

W

Bead width

mm

h

Bead height

mm

U

Scanning speed

m/min

ύ

Gas flow rate

l/min

xviii

NOMENCLATURE | LAT for crack repairs in Titanium alloy components

GLOSSARY OF TERMS A 

Acicular Alpha - A fine needle-like rectangular transformation product brought about through nucleation and growth.



Additive Manufacturing (AM) - refers to a process by which digital 3D design data is used to build up a component in layers by depositing material.



Ageing - this is a process during heat treatment of keeping titanium at elevated temperature for hours to allow precipitation to take place in order to gain higher strengths.



Alloy - a metal made by combining two or more metallic elements, especially to give greater strength or resistance to corrosion.



Alpha - The low temperature allotrope of titanium with a hexagonal, close-packed crystal structure.



Alpha-Beta Structure - A microstructure that contains both alpha and beta as the principal phases.



Alpha precipitation



Alpha Stabilizer - An alloying element that dissolves preferentially in the alpha phase and raises the alpha-beta transformation temperature.



Annealing - Refers to a variety of operations involving heating and slow cooling to remove stresses and alter ductility and toughness. Annealing softens the titanium making it more workable for shearing, forming and machining.

B 

Beneficiation - the transformation of a mineral (or a combination of minerals) to a higher value product (value-addition) which can either be consumed locally or exported



Beta - The high temperature allotrope of titanium with a body-centered cubic crystal structure.



Beta Isomorphous - Beta stabilizing alloying elements which are completely miscible in the beta phase.



Beta Stabilizer - An alloying element which dissolves preferentially in the beta phase and lowers the beta transformation temperature and promotes the retention of beta at room temperature.

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GLOSSARY OF TERMS | LAT for crack repairs in Titanium alloy components



Beta Transus temperature - The temperature which designates the phase boundary between the alpha-plus-beta and beta fields.



Body Centered Crystalline - a crystalline with an atom at each corner of the unit cell, and an atom in the centre of the unit cell.

C 

Charpy Impact test - also known as the Charpy V-notch test, is a standardized high strain-rate test which determines the amount of energy absorbed by a material during fracture.



Chlorination – a process where rutile ore (TiO2) reacts with chlorine gas at elevated temperatures to yield titanium tetrachloride, a colourless liquid.



Cold worked – is when a metal is shaped when it is cold



Crack – refers to a split, fracture, fissure, rupture, break, snap, cleave without a complete separation of the parts.

D 

Defect – refers to a fault, flaw or imperfection in a material which causes it unable to meet standard criteria.



Dendrites – crystals, usually formed during solidification or sublimation, which are characterized by a tree-like pattern composed of many branches; pine-tree or fir-tree crystals in the heating direction.



Design – entails the realization of a concept or idea into a configuration, drawing, model, plan or specification on which the actual output is made reference to.



Dwell time - is the time allowed for a diamond indenter to exert a load into the sample during microhardness testing, normally expressed in seconds.

E 

Elastic deformation – is a provisional object shape and size change that is selfreversing after the force is removed, enabling the object to return to its original shape and size.



Elastic modulus - The elastic modulus of an object is defined as the slope of its stress– strain curve in the elastic deformation region.



Elongation – refers to the increase in gauge length of a specimen subjected to a tension force, referenced to a gauge length of the specimen.

xx




% Elongation - the total percent increase in the gauge length of a specimen at the end of tensile test.



Equiaxed grain – refers to a polygonal crystallite, in an aggregate, whose dimensions are approximately the same in all directions.



Etchant - a corrosive chemical solution used to etch a metal to expose its grain structure.



Etching – is when etchant is applied to the prepared metal surface to reveal structural details for metallographic analysis.

F 

Face Centered Cube - is when in a crystal system, atoms are arranged at the corners and centre of each cube face of the unit cell.



Fatigue – refers to the progressive and localized structural damage or weakening of a material that is subjected to cyclic loading.



Ferrous – metals containing or consisting of iron



Formability - the ability of a metal to be plastically deformation without being damaged.



Fracture toughness - is a property which describes the ability of a material containing a crack to resist fracture, and is one of the most important properties of any material for many design criteria.



Functionally Graded Materials – (FGMs) as they are normally referred to as are innovative composite materials whose composition and microstructure vary in space following a predetermined law. The gradual change in composition and microstructure of FMGs allows for the gradient of properties and performances.

G 

Grade 5 Titanium Alloy (Ti-6Al-4V) - is the most popular Alpha-Beta Titanium alloy with 90% Ti, 6% Al, 4% V, Max 0.25% Fe and Max 0.2% O by weight.



Grain -an individual or distinct crystallite in metals that has got its own orientation or similar to the neighbouring.



Grain boundary - is the interface between two grains, or crystallites, in a polycrystalline material.



Grain growth – refers to the increase in grains size, a phenomenon usually attributed to the raise in temperature of a metal.

xxi




Grinding – is when material is removed by abrasion from the surface of a sample by use of a grinding wheel or different grades of abrasive grinding papers depending on surface requirements.

H 

Hardness is the property of a material that enables it to resist plastic deformation, usually by penetration and may also refer to resistance to scratching, abrasion or cutting.



Hardenability – the hardenability of a metal alloy is the depth up to which a material is hardened after its heat treatment process.



Hardening – is the process of increasing hardness of a metal by suitable treatment.



Hardness test - measures the resistance of a material to be penetrated or scratched by a sharp object.



Heat Affected Zone - The region of the parent metal or substrate which has not been melted, but whose mechanical properties have been altered by the heat of welding or any other thermal processes.



Heat treatment Heat Treating - Is the process of altering the properties of a metal by subjecting it to a controlled sequence of thermal cycles. Heat treatment can be performed to improve machinability, increase toughness, improve cold forming characteristics, alter hardness ad tensile strength, up and down, and to relieve residual stress as well as improve shearability.



Hexagonal close-packed – Six atoms form regular hexagon, surrounding one atom in center. Another plane is situated halfway up unit cell (c-axis), with 3 additional atoms situated at interstices of hexagonal (close-packed) planes.



Hot Isostatically pressed - (HIP) is a form of heat treatment that uses high pressure (applied by an inert gas, usually argon) to improve material properties.

 Hunter Process – a process that uses sodium as the reducing agent. I 

Ilmenite - a black mineral consisting of oxides of iron and titanium, of which it is the main ore.



Impurities - are wanted or unwanted substances inside a confined amount of material which differ from the chemical composition of the material and are either naturally occurring or added during alloying processes.

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

Interstitial impurity atoms - (much smaller than the atoms in the bulk matrix) are said to fit into the open space between the bulk atoms of the lattice structure.

K 

Kroll Process - A process for the production of titanium sponge metal where the reducing agent is magnesium.



Kroll’s Reagent - is the most popular metallographic etchant for etching titanium alloys.

L 

Laser – acronym for light amplification by simulated emission of radiation.



Laser Metal Deposition (LMD) – a process in which laser beam melts the target material surface generating a small molten pool of base material which absorbs the powder as it is delivered, creating a metallurgical bond between the substrate and the metal powder in a layer by layer fashion.

M 

Macrostructure - the structure of metal, revealed by visual examination with little or no magnification.



Magnification - ratio of the size of an image to its corresponding object. This is usually determined by linear measurement.



Martensitic - plate-like constituent having a similar appearance and mechanisms of formation to that of martensite.



Mechanical Properties - the properties of a material that reveal its elastic or inelastic behaviour when subjected to a force, indicating the elastic to plastic suitable mechanical applications.



Metal Matrix Composites - (MMC) is composite material with at least two constituent parts, one being a metal. The other material may be a different metal or another material, such as a ceramic or organic compound.



Metallography - the branch of science which relates to the constitution and structure, and their relation to the properties, of metals and alloys.



Metallurgy - The science and technology of metals and their alloys which includes their methods of extraction and use.



Metastable beta - A non-equilibrium phase that can be transformed to alpha or eutectoid products by heat or stress.

xxiii




Microstructure - the structure of a suitably prepared specimen as revealed by a microscope.



Mineral - is a naturally occurring substance, representable by a chemical formula that is usually solid and inorganic, and has a crystal structure.



Mineralogist - a person who studies minerals, which technically include all naturally occurring solid substances.



Morphology - the shape characteristics of a structure; the form and orientation of specific phase or constituent.

N 

Near-alpha alloys - contain small amount of ductile beta-phase.



Nonferrous metal - is a metal or alloy which is not ferrous, that does not contain iron in appreciable amounts.

O 

Oxidation - the addition of oxygen to a compound.



Parameter – a control variable



Phase - a physically homogeneous, mechanically separable portion of a material

P

system.

 Prior Beta Grain Size - The grain size of the beta phase prior to transformation to alpha.

 Porosity - A rounded or elongated cavity formed by gas entrapment during cooling or solidification of a weld

R 

Repair - restore damaged, faulty, or worn out material to a good and safe usable condition.



Residual stresses - the internal stress distribution locked into a material, present even after all external loading forces have been removed and are a result of the material obtaining equilibrium after it has undergone plastic deformation.



xxiv

Rutile – titanium ore which is a black beach sand containing +95% TiO2.


S  Sponge - A porous metal product of the chemical reduction of titanium tetrachloride to metal by the Kroll or Hunter process.

T 

Tensile Strength - the maximum tensile stress which a material is capable of sustaining before failure. Tensile strength is calculated from the maximum load during a tension test carried out to rupture, and the original cross-sectional area of the specimen.



Tensile Test - fundamental materials science test in which a sample is subjected to a controlled tension (slowly applied axial force) until failure. The yield strength, tensile strength, modulus of elasticity and ductility can be obtained from the test.



Titanium - the chemical element of atomic number 22, a hard silver-grey metal of the transition series, used in strong, light, corrosion-resistant alloys.



Titanium ore - The most common ore used in the titanium metals industry is rutile which is a black beach sand containing +95% TiO2. The alternate ore is ilmenite, which is only 55 to 60% TiO2 with the remainder being iron oxides.

U 

Ultimate Tensile Strength (UTS) - often shortened to Tensile Strength (TS) or ultimate strength, is the maximum stress that a material can withstand while being stretched or pulled before failing or breaking.

V 

Vickers Hardness test – is the process of indenting the test material with a diamond indenter, in the form of a right pyramid with a square base and an angle of 136 degrees between opposite faces subjected to a specific load. The full load is normally applied for 10 to 15 seconds dwell time.



Void - a pore that remains unoccupied in a composite material.



Widmanstätten Structure - A structure brought about by the formation of a new phase

W along preferred crystallographic planes of the prior phase. The Widmanstätten structure is a transformation product of the beta phase.

xxv




Wire Electrical Discharge Machining - also referred to as spark machining, spark eroding, or wire erosion, is a manufacturing process whereby a desired shape is obtained on electrically conductive material using electrical discharges (sparks).

X 

X - represents the across weld distance on the transverse cross sections of the clads in μm.

Y 

Y - represents the vertical weld distance on the transverse cross sections of the clads in μm.

xxvi


CHAPTER 1: INTRODUCTION

CHAPTER 1 : INTRODUCTION 1.1

Background

Akinlabi [1] and Van Vuuren [2] reported that titanium ore beneficiation has become one of the strategic goals of South Africa. Beneficiation entails the transformation of a mineral (or a combination of minerals) to a higher value product (value-addition) which can either be consumed locally or exported [3]. The authors added that the attractiveness of titanium and its alloys has not only caught the attention of scientists and engineers in research and development in universities and industries, but also that of the South African government and its departments. This has seen research in titanium in South Africa increasing and ranges from its mining extraction as an ore through to high-speed additive manufacturing processes, property enhancement and repair of finished products. Akinlabi [1] affirmed that these current researches and developments on titanium and its alloys are of significance to South Africa and the global economy.

According to du Preez [4], South Africa is the second largest titanium ore mineral producer in the world, after Australia. The author indicated that the dark lines which run like dark pencil marks across the sands of many South African beaches and are confused by many people as oil contamination, are in actual fact titanium mineral sands (ilmenite). It was affirmed that rather than exporting titanium as relatively cheap ore, or slag, and importing the processed titanium metal at higher prices, the Council for Scientific and Industrial Research (CSIR), in partnership with the Department of Science and Technology (DST), has launched a titanium-powder pilot plant in Pretoria. The aim is to scale up to a commercial level, the pilot plant which cuts off a number of steps in the Kroll process. The Kroll process was criticized in this report as relatively material-uneconomical and expensive with large reactors which in the end, are cut open and destroyed, contributing to the high cost of the metal. The author reported that the powdermaking technique of the plant, has been patented internationally, and is anticipated to produce titanium at significantly lower cost with the potential to transform South Africa into a global player in titanium production for both local and global benefit. In view of this, the grade 5 titanium alloy (TI-6AL-4V) which has become a workhorse metal in the aerospace and biomedical (implants) industries, should be of particular interest [5]. du Preez [4] cited the SR71 Blackbird light-weight (the fastest airplane constructed from 90% titanium) and the customized lower jaw implant, (a University of Pretoria – Central University of Technology 1

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INTRODUCTION | LAT for crack repairs in Titanium alloy components


collaboration) shown in Figure 1.1 as examples of some of the major titanium products. However, South Africa is not the only country focusing on titanium research and development. Titanium is a key metal globally. Many research works on titanium have been conducted and are still under way both locally and globally.

Figure 1.1: (a) The titanium-built SR-71 Blackbird and (b) Customized Lower Jaw Implant [4]

Titanium is used for high-value components such as biomedical implants, turbine blades and aerospace gear-landing components, mostly in complex shapes [6, 7] and has always been difficult to process using traditional or conventional methods owing to its being highly environmentally and thermally reactive and difficult to machine [8, 9, 10, 11]. This has seen increased use of Additive Manufacturing (AM) particularly Laser Metal Deposition (LMD) in the processing of titanium and its alloys for manufacturing, maintenance, repair and overhaul processes instead of parts replacement [12, 13]. The acronym LASER is Light amplification by stimulated emission of radiation. The authors cited the basic principle of LMD as a laser beam which melts the target material surface generating a small molten pool of base material which absorbs the powder as it is delivered, creating a metallurgical bond between the substrate and the metal powder in a layer by layer fashion. The authors cited LMD to have the advantage of a small and limited heat affected zone (HAZ) over its conventional counterparts. Summarised here-under are some of the studies conducted on LMD applications to Ti-alloys components.

Kelbassa et al [14], successfully repaired the damping wire worn-out grooves of a BR715 High Pressure Compressor (HPC) front drum made of titanium base alloys Ti-6Al-4V and Ti-6246, nickel base worn-out BR715 High Pressure Turbine (HPT) flange, aero engine burn out sections of HPT Nozzle Guide Vanes (NGVs) [15] and worn engine casings by LMD. LMD was used to repair defects in aerospace parts and the repaired samples were tested for microstructure analysis, Ultimate Tensile Strength (UTS), Yield Strength (YS) and % elongation to make a comparative study with the properties of the ideal substrate [11]. The test results agreed with 2

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the values of the virgin substrate, concluding the current LMD technique of aerospace parts repair as effective. LMD was also successfully applied for laser cladding of HTSH tubing, repair of Industrial Gas Turbine (IGT) knife edge seals [16] and 3-d free forming of valve ball and seat weld development [17]. LMD is also being used for: generating objects with high geometrical complexity [18] manufactured in continuous 5 axis configuration [7], repairing titanium turbine blades [12], Functionally Graded Materials (FGM) [19], build-up of reinforcement ribs on flat and large curved surfaces, deposition of ultra-thin-wall structures mainly automotive gaskets, laser hard facing [20], restoration of worn blade squealer tips, micro tools, rapid prototyping, manufacture of rocket engine components and repair of Ti compressor blade tips [21] and post-powder-deposition re-melting [22]. LMD has also found broad applications in industrial maintenance, repair and overhaul [23], fabrication of superior medical implant structures, satellite manufacturing and titanium powder metallurgy, injection moulding and die casting tooling [24]. LMD has been used for alloying, achieving good metallurgical bond and excellent mechanical properties and its use was also extended to Laser welding where due to its low heat affected zone (HAZ), weld cracks were greatly reduced [25].

Furthermore, LMD has been successfully used to create carbide-reinforced metal matrix composite (MMCs) objects [26, 27, 28, 29]. Literature shows successful mould making, regeneration and optimization of tools, substitution of component parts and the generation of metallic gradient coatings and wear-protection layers through LMD welding process with improved mechanical properties [30, 31]. Attempts have also been made to use Nd: YAG pulse laser in a free running regime for the cleaning of metal artworks but got disqualified after it was noted to cause irreversible local damage and surface melting to artworks [32]. Repair of cracks located under the tip-area of turbine blades using multi-layer cladding to replace the single crystal material was investigated [33]. It was concluded that the crack repair efforts were unsuccessful and the authors recommend further analysis of the interaction between the inductive preheating and the dendrite crack growth experienced during the repair process. Laser droplet brazing studies yielded good results of the braze joints which were tested and proven to have no evidence of brittle intermetallic phases and had good shear mechanical stability [34]. The above mentioned are few out of the many old-to-recent studies conducted on the applications of LMD. The reports reviewed on the LMD crack repairs showed that some encouraging results at both laboratory and commercial levels were obtained with further room for improvements; while some other tests failed and remain research-in-progress. 3

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There has been an increasing need for the repair of Ti and Ti- alloys components instead of new parts replacement because they are generally very complex and expensive to manufacture and may require large delivery lead time. Conventional welding processes create HAZ in the substrates close to the weld causing distortions and metallurgical property changes in the repaired substrates [11]. LASER [35] technology repair procedure is expected to produce more acceptable mechanical and metallurgical results. Of much desire is the capability of LMD to produce small or limited HAZ and small weld dilution zone which makes the weld less vulnerable to degradation of properties and yielding a better microstructure with nearly netshape results. The aim of the current work is therefore to investigate the feasibility of using LASER to repair cracks in grade 5 titanium alloy.

1.2

Problem Statement

From a repair perspective, literature shows that LMD has been limited to the repair of crackfree titanium components. However, cracks and crack-propagation are an inevitable defect in Ti-alloys in service components and any other components in engineering practice. There are very few published works on the repair of cracks through LMD and the recent repair techniques applied were unsuccessful. This research gap observed was addressed in the current work. The present research interest is thus to extend the current success on LMD repairs of components to the repair of cracks in grade 5 titanium alloy. In addition, the effects of LMD crack repair technique to be developed on the mechanical and microstructure properties of the repaired substrates will be determined; hence this study.

1.3

Research Aim

The aim of the research is to investigate the applicability of LMD technique for repair of cracks in grade 5 titanium alloy and thereafter determine the properties of the repaired substrates through mechanical testing and metallographic evaluation.

1.4

Research Objectives

The research was guided by the following objectives: 

Conduct a detailed literature study on LMD technique for repair of cracks in grade 5 titanium alloy components

4

:





Establish materials, input and process parameters for the LMD crack repair process



Design and develop the LMD experimental programme



Repair cracks on all samples using the designed and developed LMD technique



Measure the mechanical properties of the LMD repaired substrates through tensile tests, micro-hardness tests and Charpy tests



Carry out microstructure analysis on repaired substrates using metallography



Analyze the obtained results



Deduce conclusions and recommendations on the effectiveness of the proposed LMD crack repair process

1.5

Scope of Work

The project was an experimental study on crack repair in grade 5 titanium alloy through LMD. It only focused on the repair of cracks of sizes 0.5 to 5 mm on Ti-6Al-4V blocks of dimensions 100 x 100 x 7 mm, at laboratory level. Process parameters were selected based on literature and were investigated experimentally. Computer simulation is not an aspect of the current project.

1.6

Hypotheses

In this research study, laser metal deposition process was proposed to repair induced cracks in the form of rectangular grooves on Ti-6Al-4V substrate material. The samples were characterized through the evolving microstructure, hardness and Charpy impact tests. It is expected that the properties of the cracks (grooves) repaired Ti-6Al-4V substrates using the LMD process will match those of the ideal Ti-6Al-4V components in operation.

1.7

Methodology

Rectangular cracks/grooves of sizes 0.5 to 5 mm were induced onto grade 5 titanium alloy specimen of dimensions, 100 x 100 x 7 mm using wire electrical discharge machining (WEDM). The experimental setup was designed based on the materials, input and the process parameters and this includes, the selection of the laser machine type and size; the power, the wavelength, a computer numerical controlled tool path planning to drive the laser, laser scanning speed, powder particle sizes, shielding gas, gas flow rate, powder flow rate, beam diameter, focal length, laser spot, nozzle size and geometry, shielding mechanism, oxygen 5

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levels as well as prior and post specimen treatment. Figures obtained from the literature and experimental optimization were carefully considered to achieve the best parameters that were used for the preliminary depositions. To validate the repair process, tensile tests, microhardness tests and Charpy impact tests and microstructure analysis, were conducted on the repaired substrates and compared to those of the ideal samples. It is upon these, that findings were outlined, conclusions were drawn and the recommendations were made.

1.8

Significance of Study

The researcher has gained knowledge on LMD technique for repair of cracks in Ti-6Al-4V. Up to the time this research study was conducted, none of the consulted literature has succeeded in the repair of narrow U or rectangular shaped crack/grooves through LMD, thus necessitating the need for further investigations to find the solution to the problem. This research study has closed the gap of the noted failed attempts to successfully use the technique for the repair of cracks in Ti-6Al-4V. The laboratory test findings by the researcher are of benefit to the industry as they could be further explored and scaled up onto the repair of in service parts and products in various industries, such as aerospace parts.

1.9

Concluding Remarks

Chapter One provides the background of study, problem statement, aims and objectives, scope of work, hypotheses, methodology and significance of the research project. Chapter Two presents detailed study of the literature relevant to this work. Chapter Three presents the experimental programme required to capture the effectiveness of LMD on repair of cracks in Ti-6Al-4V and chapter Four presents the results and the discussions of the experimental investigations. Chapter Five focuses the conclusions drawn from this work and the recommendations for future work.

6

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CHAPTER 2: LITERATURE REVIEW

CHAPTER 2 : LITERATURE REVIEW 2.1

Introduction

This Chapter presents detailed literature relevant to the subject under investigation which is divided into eleven sub-sections as follows: Introduction, titanium and its alloys, defects in metals, defects in Ti-6Al-4V, detection of crack defects in metals, repair of crack defects in metals, repair of defects in Ti-6Al-4V, LMD, lasers used in materials processing, WEDM and concluding remarks.

2.2

Titanium and Its Alloys

2.2.1

Discovery, processing and microstructure of CP-Ti

Titanium was first recognized in 1791, by William Gregor, the British reverend, mineralogist, and chemist [36, 37]. The authors inferred that Gregor discovered the magnetic sand from the local river, Helford, in the Menachan Valley in Cornwall, England, and isolated ilmnite, (black sand) by removing the iron with a magnet. In describing the process, the two papers further inferred that, Gregor then treated the sand with hydrochloric acid and produced the impure oxide of a new element which he named “mechanite”, after the location. It was further reported that four years later, the Berlin chemist Martin Heinrich Klaproth independently isolated titanium oxide from a Hungarian mineral, now known as “rutile”. Klaproth is said to have named the metal “Titanium”, after the Greek mythological children of Uranos and Gaia (the Titans) who were utterly hated by their father, and were held captive in the earth’s crust, similar to the hard to extract ore.

Successful isolation of the metal by heating titanium tetrachloride (TiCl4) with sodium in a sealed bomb was reported by Leyens and Peters [36] and Duwez [37] to have been achieved by Matthew Albert Hunter from Rensselaer Polytechnic Institute in Troy, New York, at around 1910-1911. Named after the inventor, the process is still known as the, “Hunter Process.” The authors reported research in titanium processing to have continued until 1932, when Wilhelm Justin Kroll (the father of the titanium industry) produced significant quantities of titanium by combining (TiCl4) with calcium. The same findings were made by Seong et al [38]. It was 7

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LITERATURE REVIEW | LAT for crack repairs in Titanium alloy components


affirmed that during World War II, Kroll demonstrated that titanium could be extracted commercially by reducing (TiCl4) by changing the reducing agent from calcium to magnesium (Kroll process). Named after Wilhelm Justin Kroll, today the “Kroll process” was reported as the most widely used method of extracting titanium. The authors described the Kroll process as having four major steps: First, chlorination of rutile concentrate or synthetic rutile (titanium slag) to form titanium tetrachloride which is then distilled to remove metallic impurities such as iron, chromium, nickel, magnesium, and manganese. Second, magnesium reduction of titanium tetrachloride. Third, removal of the remaining magnesium and magnesium chloride by vacuum distillation where heat is applied to the sponge mass while a vacuum is maintained in the chamber, causing the residue to boil off from the sponge mass. At the end of which, the residual magnesium chloride is reported to be separated and recycled. Fourth, mechanical pushing out, of the sponge mass out of the distillation vessel, which is then sheared, and crushed. The authors postulated that Post World War II era would see titanium-based alloys being considered key materials for aircraft engines. The DuPont Company was the first to produce titanium commercially in 1948 [39].

Today, aerospace is still the prime consumer of titanium and its alloys [5, 40, 41, 42, 43, 44], but other markets such as architecture, chemical processing, biomedical, power generation, marine and offshore, sports and leisure, body jewellery and transportation have also gained increased acceptance [36, 37, 38, 39]. The authors reported titanium as not a recently discovered metal, declaring it an old metal for mineralogist, chemists and physicists, yet relatively a new engineering material. The authors attributed titanium’s utmost interest to today’s engineers, to its high corrosion resistance, low specific gravity, bio-compatibility, high specific strength, nearly perfect non-magnetic property, good creep and fatigue resistance, good high temperature properties, structurally efficient metal for critical parts, and high-performance aircraft parts just to mention a few.

Besides Russia, Australia, India, and Mexico, workable mineral deposits of titanium ore include sites in the United States, Canada, South Africa, Sierra Leone, Ukraine, Norway, and Malaysia [36, 38]. Titanium is reported to be found in abundance in the world, making it the ninth most plentiful element on the earth’s surface and the fourth most abundant structural metal in the earth’s crust exceeded only by aluminum, iron, and magnesium. Unalloyed titanium or CP-Ti 8

:



typically contains between 99%-99.5% titanium, with the balance being made up of iron and the interstitial impurity elements hydrogen, nitrogen, carbon, and oxygen [45]. The microstructure of unalloyed titanium is described as consisting of grains of alpha phase, with the possibility of small amounts of beta phase. The authors inferred that “unalloyed” grades of titanium are generally less expensive, and are easier to fabricate than alloyed, and generally stronger grades of titanium. However in contrary, the strength part is highly debatable since pure metals are reported as being of little use in engineering applications because of the demand of conflicting property requirements [46]. Alloys of titanium have a much wider use than pure metals [37].

Titanium is described as naturally occurring in oxide form in mineral sands that contain ilmenite (FeTiO3) and rutile (TiO2) ores, which contain which contains about 95 and 70% titanium, respectively [36, 38, 39]. Its existence in high impure state is said to make it difficult to process and inevitably expensive. The authors presented titanium as silvery in color, a chemical element and a transitional metal with symbol Ti, atomic number of 22 and atomic weight of 47.9. Ti exists in two allotropic modifications: a high-temperature, body-centred crystalline (BCC) lattice and a low-temperature hexagonal close-packed (HCP) lattice [47]. Figure 2.1 shows the schematic arrangement of the (hcp) and (bcc) crystal structures. Ti has a melting point of 1678℃ [48]. Classification of titanium grades is based on both mechanical properties and chemical composition, with grades 1-4 being CP-Ti [49]. It is however stated that the American Standards for Testing and Materials (AMST) recommends CP-Ti to be classified according to mechanical properties. CP-Ti undergoes an allotropic transformation from hcp (α) to bcc (β) when the temperature is raised through to 882℃ [50]. The authors reported that the α phase CP-Ti remains stable from room temperature to approximately 882°C, the point from which β phase CP-Ti remains stable to its melting point of about 1688°C.

Figure 2.1: Crystal structure of (hcp) α phase and (bcc) β phase [36]

9

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2.2.2 Titanium alloys and their properties The first titanium alloys, including today’s most popular Ti-6Al-4V, were developed in the late 1940s in the United States and today a large number of titanium alloys have paved the way for light metals to vastly expand into many industrial applications [36] . Titanium alloys are today widely used for applications requiring an excellent mechanical resistance and high strength at elevated temperatures [51]. Alloying elements dissolved in titanium are reported to enable it a wide range of physical, mechanical and metallurgical successes in many industrial applications. Pederson [50], reported alloying elements to: (i) stabilize the α phase by raising the α-β transition temperature (simple metals (SM) and many interstitial elements – Nitrogen, Oxygen and Carbon), (ii) stabilize the β phase by lowering the α-β transition temperature (transition metals (TM), noble metals and hydrogen) or (iii) act only as solid solution strengtheners with no effect to the transition temperature. Aluminium and interstitials (O, N, C), are listed as α stabilizers while chromium, niobium, copper, iron, manganese, molybdenum, tantalum and vanadium are listed as β stabilizers and tin and zirconium as neutral solutes in titanium with little effect on the transformation temperature, acting as strengtheners of the α phase [39]. The authors concluded that titanium alloys are therefore divided into three major groups, α alloys, α+β alloys and β alloys depending on the type and amount of the alloying elements which decide the phases that dominate at room temperature.

Alpha (α) alloys are stable at room temperature and contain α-stabilising elements with an hcp crystal structure forming a very high strength metal with a creep resistance that is superior to β alloys, and are preferred for high temperature applications (in the range of 316°C-593°C) but are of lesser corrosion resistance [39, 45, 52, 53]. The authors attribute α alloys’ suitablity for cryogenic applications [54] to the absence of a ductile-to-brittle transition in α alloys, a feature of β alloys. It was further reported that the close-packed structure gives α-Ti alloys a packing density of Khcp = 0.74 compared to Kbcc = 0.68 for β-Ti implying a much quicker diffusion rate of oxygen in β-Ti alloys than that involving α-Ti alloys. However, it was argued that a greater number of interstices in α-Ti alloys enable it to absorb a higher amount of oxygen in solution and deep case hardening; hence achieving a higher hardness value than α+β or β phase alloys. The papers further postulated that due to their tendency of forging defects, α-Ti alloys may not be suitable in some applications and that unlike β alloys, α alloys cannot be strengthened by heat treatment because they are single-phase. It was cited that α-Ti alloys are used most often in the annealed or recrystallized condition to eliminate the residual stresses 10

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caused by working. The authors listed aluminium as the principal alloying element in α-Ti alloys and concluded that α-Ti alloys exhibit good weldability. Typical hardness of Ti-6Al-4V in the annealed condition is Rockwell C 30-34, and about Rockwell C 35-39 in the solution and aged condition [55].

The formation of a two phase (α+β) system was attributed to the addition of controlled amounts of beta-stabilizing alloying elements which causes some beta phase to persist below the beta transus temperature, down to room temperature [39]. The α+β alloys combine both the α and β phase microstructures forming the deepest hardened layer since the α-Ti aids in preparing the oxygen to form solid solution and the β-Ti promotes the inward diffusion of oxygen to the inner layer [52, 53]. The authors reported α + β alloys as generally having good formability with properties that can be controlled through heat treatment by adjusting the morphology and distribution of retained β phase present. It was concluded that solution treatment followed by ageing at 480°C to 650 °C precipitates α Ti, resulting in a fine mixture of α and β in a matrix of retained or transformed β phase. The α + β phase is the largest group of employed Ti alloys in the aerospace industry [54].

The high percentage of beta-stabilizing elements in β-Ti alloys results in a microstructure that is metastable beta after solution annealing with possible extensive strengthening occurring by the precipitation of alpha during aging [39]. The β-Ti alloys are reported as rich in beta stabilizers and lean in alpha stabilizers in such a way that beta phase can be completely retained upon cooling to room temperature [53]. This Ti alloy class was described as offering increased fracture toughness over alpha phase with the advantage of heavy section heat treatment capability. Beta alloys containing molybdenum are hailed to offer good corrosion resistance. All β-Ti alloys contain “β-isomorphous” and are metastable and when cold worked at ambient temperature or heated at slightly elevated temperature, can partly transform to alpha phase [50]. Beta alloys have high strength, good formability and hardenability, offer lower elastic modulus and higher corrosion properties compared to other alpha and mixed alloys there by suggesting lower coefficient of elastic mismatch with the bone and favourable stress distribution and outcome [48]. Sanvik [54] outlined some of the most common alloys for each of the above as follows:

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

α-Ti alloys include Ti-8Al-1Mo-1V (Ti-811) (a near α alloy) used for compressor blades and wheels at temperatures up to 425oC, Ti-6Al-2,7Sn-4Zr-0,4Mo-0,45Si (Ti1100) (a near α alloy) which is the most creep resistant of all titanium alloys.



α + β Ti alloys in which Ti-6Al-4V (Ti-64) the first high temperature titanium alloy to be developed, and is the most widely used accounting for more than 50% of all titanium tonnage in the world, and may be used up to about 350oC. Some alloys which are more difficult to machine fall into this classification when their chemistry is adapted for high strength requirements for example – Ti-6Al-2Sn-4Zr-6Mo (Ti-6246).



Β-Ti alloys- Ti-3V-11Cr-3Al – which is finding increasing use in the aerospace frame industry due to its ability to retain high strength at lower temperatures.

The ranges and effects of some alloying elements dissolved in titanium are as in Table 2.1. Table 2.2 reports on the mechanical properties of forged and annealed Ti-6Al-4V. Table 2.3 summarises mechanical and thermal properties of Ti-6Al-4V and Ti-55531 as extracted from [51]. Table 2.4 present material properties of Ti-6Al-4V as extracted from [56].

Table 2.1: Common alloying elements and their stabilizing effects [50]

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Alloying Elements

Range (approx.) (wt. %)

Effect on Structure

Aluminium (Al)

2 to 7

α - stabilizer

Tin (Sn)

2 to 6

α - stabilizer

Vanadium (V)

2 to 20

β - stabilizer

Molybdenum (Mo)

2 to 20

β - stabilizer

Chromium (Cr)

2 to 12

β - stabilizer

Copper (Cu)

2 to 6

β - stabilizer

Zirconium (Zr)

2 to 8

α + β strengtheners

Silicon (Si)

0.2 to 1

Improves creep resistance



Table 2.2: Mechanical properties of a 57 mm thick x 235 wide Ti-6Al-4V bar [57] ~ Fatigue

Testing

0.2 % Proof

Tensile

Elastic

Elongation (%)

direction

stress (MPa)

Strength

Modulus

strength at 107

(MPa)

(GPa)

cycles ± (MPa)

Longitudinal

834

910

114

17.5

496

Long

934

934

128

17.0

427

893

978

114

12.5

565

transverse Short trannsverse

Table 2.3: Mechanical and Thermal properties of Ti-6Al-4V and Ti-55531 [51]

Thermal properties

Mechanical properties

Mechanical and Thermal properties

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Ti-6Al-4V

Ti- 55531

Density (g/cm3)

4.43

4.65

Tensile elastic modulus (GPa)

110

112

Compressive elastic modulus

--

113

Tensile strength (MPa)

931

1236

Yield Strength (MPa)

862

1174

Elongation (%)

14

6

Β transus (Tβ) (oC)

980

856

Thermal conductivity at 20oC

7.3

6.2

709

495

(GPa)

(W/mK) Specific heat 20-100oC (J/Kg/K)



Table 2.4: Material properties of the Ti-6Al-4V alloy [56]

Property

Value

Hardness (HRC)

36

Density (g/cm3)

4.42

Yield tensile strength (MPa)

870

Ultimate tensile strength (MPa)

923

Fatigue strength (MPa)

510

Modulus of elasticity (GPa)

113.8

Elongation (%)

14

Thermal Conductivity (W/ mK)

6.7

Specific Heat Capacity (J/ kgK)

560

Electrical resistivity (μΩm)

1.7

It is the influence of β-transus temperature, (882.5°C) that groups titanium alloying elements into neutral, α-stabilizers, or β-stabilizers as in Figure 2.2 [36]. The authors described the αstabilizing elements as extending the α- phase field to higher temperatures, while β–stabilizing elements shift the β - phase field to lower temperatures. Neutral elements are said to have only minor influence on the β-transus temperature.

Figure 2.2: Schematic diagrams on the Influence of alloying elements on phase diagrams of Titanium alloys [36]

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Classes of titanium alloys that stem up from α, α + β and β include the near α, or, metastable β or stable β depending upon their room temperature microstructure [48, 51, 58]. The authors postulated that Ti-alloys form part of the light metals group, due their low density of ρ = ± 4.5g/cm³ displaying high strength at temperatures up to approximately 600°C, which is much higher than the 350°C considered as the operating temperature of a typical application such as compressor blades with a low thermal conductivity of λ = ±7W/m.K, combined with a high melting point (1650°C).

Titanium alloys are stated to have unique mechanical properties that make them eminently suited to challenging engineering applications [5]. It is reported that today titanium alloys are common, readily available engineered metals that compete directly with stainless and specialty steels, copper alloys, nickel based alloys and most composites [39]. The density of titanium alloy was reported to be about 4500 kg/m3 compared to 7800 kg/m3 for steel and 2800 kg/m3 for aluminium. The authors further stated that the tensile strength of titanium alloy is about 1000 MPa, steel 600 MPa and aluminium 450 MPa. The authors further added that titanium alloys maintain these properties up to relatively high temperatures making them good candidates for use in high temperature applications such as burners and compressor and turbine blades and discs, mainly in the aerospace industry. These amongst many others have made titanium and its alloys the much uncontested giants of many metal industries [59, 60].

Of all the titanium alloys, Ti-6Al-4V is the most specified high strength currently used in the titanium industry [58]. Ti-6Al-4V is cited as a workhorse alloy of the titanium family of alloys with a nominal composition of 6 % aluminium and 4% vanadium and the remainder being largely titanium [5, 61]. Ti-6Al-4V is reported to constitute about 60% of titanium alloy usage and is a two phase material with both alpha hcp (1678°C) and beta (bcc) (α+β) phases [36, 48]. In Ti-6Al-4V, the alloying element aluminium is the alpha phase stabilizer while vanadium stabilizes the beta phase and this alloy is considered a household name material when it comes to automotive and aerospace applications [51]. The authors attributed Ti-6Al-4V’s main attraction to its high strength-to-weight ratio when compared to steel and aluminium alloys.

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Ti-6Al-4V’s attraction is also reported to stem from its high welding, forging, machining and heat treatment capability to reach very high strength as well as its retained stability up to temperatures as high as 482oC [62]. Ti-6Al-4V is widely referred to as an aerospace alloy where it is used to manufacture mission critical components such as gears, shafts, wing sections, turbine engine, airplane body components, surgical implants and race machines [5, 39]. CRP Meccanica [63] presents the composition of Ti-6Al-4V as in Table 2.5. Ti-6Al-4V is here categorised as an α+β nonferrous Ti-alloy and made an extensive coverage on a wide range of Ti-6Al-4V properties as summarised in Table A1 and Table A2 in Appendix A. Other attractive properties of Ti alloys include: high fatigue strength and fracture toughness in air and chloride environments, low modulus of elasticity high intrinsic shock resistance, high ballistic resistance-to-density ratio, nontoxic, non-allergenic, very short radioactive half-life and excellent cryogenic properties [39]. Ti-6Al-4V is also reported to have Brinell hardness of 379, Knoop hardness of 414 and Vickers hardness of 396 [64].

Table 2.5: Components of Ti-6Al-4V [63]

Ti-6Al-4V Component

Weight %

Al

6

Fe

Maximum 0.25

O

Maximum 0.2

Ti

90

V

0

2.2.3 Applications of titanium alloys Owing to the above mentioned exceptional attributes, Ti alloys are mainly used in aerospace, marine, chemical, biomedical applications (derived from its protective oxide film) and sports equipment [39, 40, 41, 58, 61]. The refined list of its applications include, for: aerospace - civil, military and space craft, biomedical - orthopaedic implants, [65] trauma plates, dental fixtures, 16

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bone screws and surgical instruments; industrial – petrochemical, subsea, offshore, metal finishing, pulp and paper, general engineering, body jewellery, ultrasonic welding, motor racing components, bicycles, marine, power generation and sports equipment [60, 62, 66, 67].

Seong et al [38] cited the use of titanium in the aerospace for engine rotors, petrochemical industry due to its high corrosion resistance mainly for heat exchangers, tanks, process vessels and valves, and in power plants and desalination plants. The authors further reported that in US and Europe, Ti and its alloys are used in ground combat vehicles and naval applications. The authors attributed the growth in the use of Ti and its alloys in the military sector to increasing need to focus on light armaments and mobility. Expanding use of Ti and its alloys is also reported by the same authors in off-shore oil and gas production facilities, particularly for deepwater oil and gas fields; passenger cars, trucks, and heavy vehicles; geothermal facilities; architecture; medical devices and instruments; and golf clubs. The authors cited Ti-6Al-4V as the titanium alloy most commonly used in both commercially and military aircraft manufacturing for jet aircraft engines, airframes, and other components with CP-Ti mainly used in the power generation and chemical processing industries.

Titanium alloys are reported by Sandvik [54] to possess a combination of mechanical properties, thermal properties and corrosion resistance which makes them very attractive for gas turbine applications. Some of the applications of Ti-6Al-4V include: compressor blades, discs, and rings for jet engines, airframe and space capsule components, pressure vessels, rocket engine cases, helicopter rotor hubs, fasteners, shafts and subsea wellhead and riser components, ROV components, offshore and subsea oil & gas operations and critical forgings requiring high strength-to-weight ratios [55, 68]. Figure 2.3 shows some of the Ti-6Al-4V made components.

Figure 2.3: Ti-6Al-4V products: left to right, (rings, valves, connecting rods), medical implants, aero-space, marine, racing bicycles and condenser tubes for power generation [39]

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2.3 Imperfections and Defects in Metals 2.3.1

The general nature of metals

The word "metal" is derived from the Greek "metallon," which originated in a verb meaning to seek, search after, or inquire about [69]. In modern science, metals are defined as “elements that tend to lose electrons from the outer shells of their atoms, with the resulting positive ions held together by the cloud of these free electrons (electron gas) in a unique crystal structure [70, 71]. The authors described metals as an aggregation of atoms that, apart from mercury, are solid at room temperature and are held together by "metallic bonds" that result from sharing available electrons [72]. The authors termed the mechanism “metallic bonding”, giving rise to the three physical characteristics typical of solid metals namely: metals are good conductors of heat and electricity by the free movement of electrons, and they have a lustrous appearance. It was further added that the negative electron bond surrounds the positive ions that make up the crystal structure of the metal. This made the authors to arrive at three common types of lattice structure that metals belong to: close-packed hexagonal (CPH), face-centred cubic (FCC), and bodycentred cubic (BCC) [73].

In CPH, models of crystal structures can be made up of spheres stacked in close-packed layers and two arrangements are possible, one being hexagonal and the other cubic in basic structure [70, 71]. The authors postulated that in the CPH system, the spheres repeat the same position every second layer (ABABAB…) repeatedly. FCC Layers are said to be built up so that the third layer of spheres does not occupy the same position as the spheres in the first row; the structure repeats every third layer (ABCABCABC ...). It was further stated that FCC metals tend to be ductile which means they can be mechanically deformed, drawn out into wire, or hammered into sheet for example lead, aluminium, copper, silver, gold, and nickel. In the same paper, BCC metals are said to be a common type found in many metals, and is less closely packed than the FCC or CPH structures and has atoms at the corners and one atom at the centre of the cube. The atoms at the corners are said to be shared with each adjoining cube. These are illustrated in Figure 2.4. Other metal lattice structures listed by the authors include triclinic primitive; monoclinic primitive; monoclinic base centred; orthorhombic primitive; orthorhombic base centred; orthorhombic body cantered; orthorhombic face centred; tetragonal primitive; tetragonal body centred; hexagonal primitive and rhombohedral primitive. However, this section sort to only introduce the reader to the general nature of metals. It is not the objective of the current work to present all the lattices structures in detail. 18

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Figure 2.4: From left to right: Close-packed hexagonal unit cell structure, Face-cantered cubic unit cell structure and Body-centred cubic unit cell structure [71]

Metals exist as ferrous or non-ferrous metals which are further grouped into pure ferrous metals and ferrous alloys, and pure non-ferrous metals and non-ferrous alloys. Ferrous metals contain iron while non-ferrous metals do not contain iron. Pure ferrous metals include iron and its groups namely: pig iron, cast iron, white iron, grey cast iron and wrought iron. Ferrous alloys include steel and its groups namely cast steel, stainless steel and high-speed steel. Pure nonferrous metals include Copper, Aluminium, Zinc, Tin, Lead, Silver, Gold and mercury. Nonferrous alloys include brass- (Copper + Zinc), bronze – (Copper + Tin), solder – (Lead+ Tin). Most metals operate as metal alloys which with regard to mechanical characteristics, meet combined property requirements which accounts for their widespread use in structural applications [72]. The author further postulated that up to today, most every-day-use components are made from metals and metal alloys for example kitchen utensils, scissors, coins, gears, wedding rings, bolts and nuts.

2.3.2

Crystalline imperfections in metals

The above section described aspects of perfect crystals. However, in the science of space lattices, crystals are never perfect, all metals contain defects or imperfections, which are not always of adverse influence, but are sometimes exploited for very important uses in engineering [70, 72]. The discovery of crystalline defects was made in 1930, by Wagner and Schottky who through their statistical thermodynamic treatments of mixed phases showed that crystal structures are not ideal [74]. According to Wagner and Schottky, all crystalline solids will at any temperature contain vacancies and extra atoms and will as such exhibit deviations from the ideal structure. These deviations from the ideal structures are reported to be present at any temperature and occur naturally in all crystalline compounds and are called imperfections or defects. Any deviation from the ideal crystalline structure based on atomic scale was defined as a defect and this is a lattice irregularity having one or more of its dimensions on the order of an atomic diameter classified according to geometry or dimensionality of the defect. The 19

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authors thus classified crystalline imperfections into: point imperfections (zero dimensional defects), line imperfections (one dimensional defects), plane or surface imperfections (two dimensional defects) and volume imperfections (three dimensional defects).

Point defects. Point defects arise where an atom is missing or is in an irregular place in the lattice structure and self-interstitial atoms, interstitial impurity atoms, substitutional atoms, polarons excitons, colour centres, frenkel, schottkey, and vacancies are common examples [70, 72, 75]. The authors described a self-interstitial atom as an extra atom that has crowded its way into an interstitial void in the crystal structure. These are here reported to occur only in low concentrations in metals because they distort and highly stress the tightly packed lattice structure. They further described a substitutional impurity atom as an atom of a different type than the bulk atoms, which has replaced one of the bulk atoms in the lattice. Interstitial impurity atoms (much smaller than the atoms in the bulk matrix) are said to fit into the open space between the bulk atoms of the lattice structure. The authors described vacancies as empty spaces where an atom should be, but is missing saying that they are common especially at high temperatures when atoms are frequently and randomly changing their positions leaving behind empty lattice sites. It was concluded that diffusion (mass transport by atomic motion) can only occur because of vacancies. Schottkey imperfection is said to be a type of vacancy in which an atom being free from regular site, migrates through successive steps and eventually settles at the crystal surface. The combination of anion cation vacancies (in pairs) is called Schottkey imperfections and the combination of a vacancy and interstitial is called a Frankel imperfection. These defects are represented in Figure 2.5.

Line or linear defects. In line or linear defects or dislocations, groups of atoms are in irregular positions that is they deviated from perfectly periodic arrangement of atoms along a line [70, 72, 75]. The authors affirmed that the distortion is centred only along a line and therefore the imperfection can be considered as the boundary between two regions of a surface which are perfect themselves but are out of register with each other. They further stated that the line imperfection acting as boundary between the slipped and un-slipped region, lies in the slip plane and is called a dislocation. Dislocations are generated and move when a stress is applied. The strength and ductility of metals are controlled by dislocations (edge and screw) Figure 2.6 shows edge and screw dislocation.

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Figure 2.5: Point defects [75]

Figure 2.6: Edge and Screw dislocation [75]

Planar or interfacial defects. Planar or interfacial defects are interfaces between homogeneous regions of the material [72]. These are boundaries that have two dimensions and normally separate regions of the materials that have different crystal structures and/or crystallographic orientations. The author further stated that planar defects include grain boundaries, stacking faults, external surfaces, phase boundaries, and twin boundaries. The grain boundary defects appear as in Figure 2.7.

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Figure 2.7: Grain boundary defects [72]

Stacking faults arise due to imperfections in the stacking sequence of atomic planes, as an example, HCP structure ideally takes the sequential format ABCABC, and if a plane of atoms is missing from this sequence and take the disturbed format ABCAB|ABC, an intrinsic stacking fault results [70, 75]. The authors inferred that if an additional plane is inserted, say ABCA|C|BCABC, an extrinsic stacking fault results and when the sequence reverses itself about a mirror plane as in ABCABCBACBA, with C as the mirror plane, and CBA is the reverse sequence to ABC.

Volume or bulk defects. Volume or bulk defects occur on a much bigger scale than point, line and plane defects citing voids (regions where there are a large number of atoms missing from the lattice) as a common volume defect [75]. Volume defects are normally introduced during processing and fabrication steps and they include pores, cracks, foreign inclusions and other phases [72]. Acquired using a Scanning Electron Microscope (SEM) the image in Figure 2.8, is a void in a piece of metal. It is further stated that the occurrence of voids can be triggered by a number of factors. The author affirmed that when voids occur due to air bubbles becoming trapped when a material solidifies, it is commonly called porosity and when they occur due to the shrinkage of a material as it solidifies, they are called cavitation. It was further outlined that another type of bulk defect occurs when impurity atoms cluster together to form small regions

22

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of a different phase and the term ‘phase’ refers to that region of space occupied by a physically homogeneous material and are often called precipitates or inclusions.

Figure 2.8: Volume or bulk defect: void through a metal piece [75]

2.3.3

Defects in metals

Defects in metals may stem out as a result of chemical, metallurgical and physical complexities that the metal undergoes during processing in its molten state or solid state [76, 77]. They normally get carried over to fabrication and in-service of components and combined with environmental effects result in concentration of stresses at points or areas of imperfections leading to failure. Generally, the defects induced in the metal components include: unwanted metallurgical changes, holes or voids, inclusions or impurities, segregations, and solid state defects include cracks, surface defects, residual stresses and embrittlement effects. Alloying elements added to a metal introduce crystal defects and crystal imperfections have strong influence upon many properties of crystals, such as strength, electrical conductivity and hysteresis loss of ferro-magnets [72].

When molten metal is poured into an ingot mould, it cools, starts to solidify from the outer surface with the centre remaining molten, cooling at a much slower rate forming a depression at the top [77]. The authors alluded to the fact that if the source of the molten feed from the top is not maintained, this will cause the depression to be quite deep (primary pipe). In continuation, the duo stated that as the last ingot solidifies while isolated from extra molten feed, contraction cavities form at the core (secondary pipe). The same paper reported that when casting into closed moulds, sufficient molten feeding needs to be maintained to avoid formation of shrinkage cavities (normally found at the centre of complex shapes) during solidification. Hot tears are here cited as another type of shrinkage defect which occur when the mould is fully filled up prior to solidification. The authors categorised inclusions into two types, indigenous and exogenous. Indigenous inclusions are defined as small intermetallic particles (sulphides, oxides and silicates) formed by chemical reactions between various constituents of the alloy 23

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and also of the atmosphere. However, the authors stated that they do not pose much problem if well distributed throughout an ingot or casting but can pose threat to the metal if substantial portions are concentrated in one place like at the ingot centre. Exogenous inclusions are said to be large and result from accidental inclusions of foreign matter posing threat to the component integrity. The authors asserted that both indigenous and exogenous inclusions if not controlled may have deleterious effects on the metal but in some cases, they are deliberately encouraged in order to produce readily machinable metals as they provide an effective notch or chip starter. Segregation (microscopic or macroscopic in nature) is said to be a result of non-uniform rejection of elements from the solidifying metal which may lead to some regions being enriched in certain elements or phases while other regions are impoverished. These non-uniformities are reported to likely cause in-service complexities due to the local differences in element composition.

Crack defects in metals. Historically, the Tsar Bell illustrates that cracking has long been a problem in metals when during a fire in 1737, a massive portion of it weighing around 11 tonnes cracked while it was in the casting pit [78]. However, during those ancient days, cracking was not so controllable. All failures are intolerable, but failure by fracture (cracking) is especially intolerable since it violates the key safety attribute of ensuring a visible collapse warning by means of prior distortion [72]. The authors cited that in 1970, the American liberty ship and an oil tanker experienced sudden fracture. According to the authors, cracks are common defects in both brittle and ductile metals: brittle fracture occurring at high constraint details with little or no warning of impending fracture while ductile fracture shows some elasticity before permanent failure. It was further stated that cracks do not occur on their own but require propagation energy from varied forms of external influences that range from thermal, mechanical, chemical and metallurgical effects and various metal processing and working methods. The authors also affirmed that the cracks normally occur from the early stages of metal processing, and are passed onto fabrication, and in-service stages and may grow through levels of threats that prompt failure. The Nalco company [79] reported that cracking may occur indirectly by fatigue or corrosion fatigue mechanisms due to the application of cyclic tensile stresses at sites of various weld defects. The company also reported of cracks forming at welds or within heat affected zones during or soon after the welding process.

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Cooling cracks. Cooling cracks may occur on the surface of bars after rolling operations due to stresses developed by uneven weld cooling [79, 80]. Nalco [81] affirmed cold cracks to run in the axial direction, without surface oxidation like seams and are also known as delayed cracks and may be surface or subsurface hydrogen induced or appear in the HAZ of welds during cooling or after hours or days after welding. The authors alluded to the fact that sources of hydrogen which leads to this type of cracks may include moisture in the electrode shielding, the shielding gas or base metal surface, or contamination of the base metal with hydrocarbon (oil or grease). The authors recommended that cold cracking can be controlled by reducing the amounts of moisture or other hydrogen-bearing contaminants in the electrode, shielding gas, and on component surfaces and preheating or post-heating.

Hot cracks. Hot cracks include several types of cracks that occur at elevated temperatures in the weld metal or HAZ [79, 81]. They occur immediately upon solidification and in general, hot cracks are usually associated with steels having high sulphur content. The HashemiteUniversity [80] and Rollett [82] listed the common types of hot cracks to include: 

Solidification Cracks: which occur near the solidification temperature of the weld metal and are caused by the presence of low melting point constituents (such as iron sulphides) that segregate during solicitation then the shrinkage of the solidified material will cause cracks to open up.



Centreline Cracks: These are longitudinal cracks along the centreline of the weld bead. They occur as a result of the low melting point impurities that move to the centre of the weld pool as the solidification progresses from the weld toe to the centre. Then shrinkage stresses of the solidified material cause cracking along the centreline. They occur when the welding speed is too high. The defect is as shown in Figure 2.9

Figure 2.9: Centreline crack [80]

25

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

Crater Cracks: These occur in the crater formed at the termination of the weld pass. Crater cracks are mostly star shaped and they are caused by three dimensional shrinkage stresses. The occurrence of crater cracks increases when welding is terminated suddenly. The defect is as shown in Figure 2.10.



Figure 2.10: Crater crack [80]

Liquidation Cracks (hot tearing): This type occurs in the HAZ when the temperature in that region reaches to the melting temperature of low melting point constituents causing them to liquidate and segregate at grain boundaries. As the weld cools down, shrinkage stresses causes the formation of small micro-scale cracks which later might link up due to applied stresses to form a continuous surface or subsurface cracks.

Hashemite-University [80] further looked into cracks that arise as a result of secondary processing discontinuities that originate during grinding, machining, heat treating, plating and related finishing operations and described them as follows: 

Grinding cracks: these develop at locations where there is a localized heating of the base metal and are usually shallow and at right angle to the grinding direction. Such cracks might be caused by the use of glazed wheels, inadequate coolant, excessive feed or grinding depth.



Pickling cracks: are hydrogen induced cracks caused by the diffusion of the hydrogen generated at the surface into the base metal. Such cracks mostly occur in materials having high residual stresses such as hardened or cold worked metals.



Heat treatment (Quenching) cracks: mostly occur during quenching especially when harsh media is used for quenching (such as cold water), oil quenching is less harsh. During quenching the material at the surface cools immediately upon contacting the liquid while the material inside take relatively longer time. This difference in cooling rate causes residual stresses in the component and could also result in cracks at the surface if the residual tensile stress is higher than the strength of the material.

26

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

Machining tears: result from the use of machining tools having dull or chipped cutting edges. Such discontinuities serve as stress raisers and can lead to premature failure of a component especially when it is subjected to fatigue loading.



Plating cracks: are surface discontinuities that can develop due to the penetration of hydrogen or hot plating material into the base metal.

Service discontinuity cracks were further presented as those that originate or develop while the component is in service [80, 83]. The authors classified the types of cracks emanating from service discontinuities as follows: 

Fatigue Cracks: When a component is subjected to fatigue stress (cyclically applied stress), fatigue cracks can develop and grow and that will eventually lead to failure even if the magnitude of the stress is smaller than the ultimate strength of the material. Fatigue cracks normally originate at the surface but in some cases can also initiate below surface. Fatigue cracks initiate at location with high stresses such as discontinuities (hole, notch, scratch, sharp corner, porosity, crack, inclusions) and can also initiate at surfaces having rough surface finish or due to the presence of tensile residual stresses.

The authors further asserted that according to Linear-Elastic Fracture Mechanics (LEFM), fatigue failure develops in three stages: 

Stage 1: development of one or more micro cracks due to the cyclic local plastic deformation at a location having high stress concentration.



Stage 2: the cracks progress from micro cracks to larger cracks (macro cracks) and keep growing making a smooth plateau-like fracture surfaces which usually have beach marks that result from variation in cyclic loading. The geometry and orientation of the beach marks can help in determining the location where the crack originated and the progress of crack growth. The direction of the crack during this stage is perpendicular to the direction of the maximum principal stress.



Stage 3: Occurs during the final stress cycle where the remaining material cannot support the load, thus resulting in a sudden fracture. The presence of the crack can (and should) be detected during the crack growth stage (stage 2) before the component suddenly fails.

Creep Cracks. The Hashemite University [80] alluded to the fact that when a metal is at a temperature greater than 0.4 to 0.5 of its absolute melting temperature and is subjected to a high enough value of stress (lower than the yield strength at room temperature but it is actually higher 27

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than the yield strength at the elevated temperature), it will keep deforming continuously until it finally fractures. Such type of deformation is here defined as creep and it is caused by the continuous initiation and healing of slipping dislocation inside the grains of the material. The author outlined that according to the rate of progress of the deformation, three stages of creep deformation can be distinguished as follows: 

Initial stage (or primary creep): the strain rate is relatively high but slows with increasing time due to work hardening.



Second stage (or steady-state creep) (secondary stage): the strain rate reaches a minimum and becomes steady due to the balance between work hardening and annealing (thermal softening).



Third stage (or tertiary creep): the strain rate exponentially increases with stress because of necking phenomena and finally the component ruptures. Creep cracks usually develop at the end of the second stage (the beginning of third stage) and they eventually lead to failure. However, when a component reaches to the third stage, its useful life is over and thus creep should be detected (by monitoring the deformation) during the second stage which takes the longest time period of the three stages. For steels, adding some alloying elements such as molybdenum and tungsten can enhance creep resistance. The creep crack stages are as shown in Figure 2.11.

Figure 2.11: Stages of creep cracks [80]

Stress Corrosion Cracks (SCC). SCCs are small sharp and usually branched cracks (as in Figure 2.12), that result from the combined effect of a static tensile stress and a corrosive environment [80, 84]. The authors postulated that the stress can either be resulting from an applied load or a residual stress. SCCs are here said to lead to a sudden failure of ductile materials without any previous plastic deformation. The authors stated that the cracks usually initiate at the surface 28

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due to the presence of pre-existing discontinuity or due to corrosive attack on the surface and once the cracks initiate at the surface, corrosive material enters the cracks and attacks the material inside forming corrosion products. The authors went further to state that the formation of the corrosion products (which have a larger volume than original metal) inside the tight cracks causes a wedging action which increase the stress at the crack tip and causes the crack to grow. The resistance to corrosion is said to be improved by plating the surface of a component by appropriate material which does not react with the environment.

Figure 2.12: Stress corrosion cracks [80]

Hydrogen Cracks. Hydrogen cracking, also known as hydrogen embrittlement results from the presence of hydrogen medium and usually occurs in conjunction with the presence of applied tensile stress or residual stress [80, 85]. The authors postulated that hydrogen can be in the metal due to previous processes such as electroplating, pickling, and welding in moist atmosphere or the melting process itself. The authors further stated that hydrogen can also come from the presence of hydrogen sulphides, water, methane or ammonia in the work environment of a component. The same papers reported that hydrogen can diffuse in the metal and initiate very small cracks at subsurface sites (usually at the grain boundaries) subjected to high values of stress. Figure 2.13 shows a schematic diagram of hydrogen cracks.

Figure 2.13: Hydrogen cracks [80]

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2.4 Defects in Ti-6Al-4V 2.4.1

Common defects in laser deposited Ti-6Al-4V components

Ti-6Al-4V has poor machinability [5, 11, 56, 86]. The authors affirmed that this is due to its high tensile strength which is maintained at elevated temperatures in combination with a low Young’s modulus, high chemical reactivity and low thermal conductivity. High strength and chemical reactivity are said to destroy the cutting tool’s upper layer causing cratering and premature tool failure and low thermal conductivity prohibits heat generated during machining to dissipate quickly from the tool edge. The authors alluded to the fact that these create high tool tip temperatures and excessive tool deformation and wear which affect the life of the cutting tool adversely. It was reported that Ti alloys are difficult to machine economically using traditional machining techniques like turning, drilling, reaming, tapping and grinding. In addition to high tool wear, the difficulties in machining Ti-6Al-4V also leads to long production times causing high cycle times which generates an increase in manufacturing costs [47]. The authors add that generally, tool wear is characterized by a loss of the ideal tool geometry influencing the process conditions. They further stated that the changed process conditions have an impact on the achievable component surfaces by means of the macroscopically visible roughness as well as microscopic defects of the surface layer. These modified conditions are here said to lead to a change of the component’s strength in highly stressed components.

Ti-6Al-4V is highly susceptible to oxidation at high temperature, requiring processing environment of oxygen ≤ 10 ppm above which, environmental conditions make it brittle [40, 42, 43, 61, 87]. Despite all its other good properties, titanium and its alloys have poor wear properties because they are reactive to contacting surfaces [41]. Ti-6Al-4V’s chemical reactivity makes it to have a tendency to weld to the cutting tool during machining leading to chipping and premature tool failure [51]. Despite its high use in the biomedical industry, Ti6Al-4V shows poor osteo-conductivity [88]. The authors further stated that many Ti-6Al-4V implants which are left to serve long are known to cause long term health problems such as Alzheimer’s, neuropathy and Osteomalacia. They added that when left long in service, Ti-6Al4V will release aluminium (Al) and vanadium (V) ions into the human body fluid system thereby becoming cyto-toxic. Van Rensburg et al [47] added that the machining of Ti-6Al-4V is generally time consuming and consumable intensive since in manufacturing production, titanium alloys are classified as hard to cut materials. 30

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Most laser deposited Ti-6Al-4V components are susceptible to porosity defects. Williams et al [89] discovered small spherical gas pores concentrated in the infill hatched region of Ti-6Al4V and attributed this to the lower energy density and less focused beam used in the infill strategy which allowed less opportunity for gas bubbles to escape the melt pool during solidification and also due to lack of powder fusion. Rarer irregular shaped pores were mostly located in the contour region and have been attributed to a lack of fusion between powder particles as a result of partial melting. High beam speed has also been found to cause this problem. Fatigue cracks are here reported to initiate at the at near-surface gas pores. The authors also found out that porosity in AM can be related to gas contamination in the powder feedstock.

Heat treatments and cooling introduce remnant phases and/or vacancy-type defects in Ti-6Al4V in sizes and concentrations that can be investigated by positron annihilation [90]. The authors stated that deformation by hot and cold-rolling introduces fresh defects (mainly dislocations) which normally agglomerate with existing defects. They further stated that such defects seem to be generated from physical displacement of atoms, a conclusion derived from the observation of shift in the characteristic peak positions in the coincidence Doppler broadening (CDBS) spectra of the samples. Due to poor surface wear resistance, Ti-6Al-4V components are prone to wear [1, 14]. It was cited turbine blades, HPC Front drum grooves, HPT flanges as some of the titanium alloy components that have been exposed to wear. Presence of undercut, deformation, cutter pull out and cutter mark defects were reported in Ti6Al-4V components used for aircraft parts [11]. Ti-6Al-4V is resistant to general corrosion but may be quickly attacked by environments that cause breakdown of the protective oxide layer including hydrofluoric (HF), hydrochloric (HCl), sulphuric and phosphoric acids [68]. It is reported that Ti-6Al-4V resists attack by pure hydrocarbons, and most chlorinated and fluorinated hydrocarbons provided that water has not caused formation of small amounts of HF and HCl acids. Ti-6Al-4V has poor abrasion resistance and fretting damage [91].

2.5 Detection of Crack Defects in Metals 2.5.1 The need for crack detection in metals Fatigue and fracture failures are a primary threat to the integrity, safety, and performance of all highly stressed mechanical structures and machineries as they can lead to serious injury or loss 31

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of life, severe environmental damage, and substantial economic loss and interference with the availability of products and services [72, 78, 83, 85, 92]. The authors postulated that design engineers should understand potential cracking mechanisms and risk factors to minimise the likelihood of in-service failure by arresting cracks (repair) before they approach the critical length for catastrophic failure. It is therefore desirable to examine the structural elements for both macro and micro cracks. The authors stated that cracks of macroscopic dimensions can be observed by the un-aided eye while those of micro dimensions must be investigated using some type of microscope.

2.5.2 Crack detection techniques in metals Irrespective of the source of the crack, components need to be examined for the presence of crack defects and non-destructive techniques (NDT) are some of the methods employed to detect potentially dangerous cracks in components. NDT is as any form of testing or inspecting materials in order to verify the structural integrity of the part without compromising the mechanical or chemical properties of the material [93, 94]. The authors further stated that NDT does not permanently alter the component being inspected. It was also stated that NDT is used in almost every field of engineering to effectively detect defects or flaws and can be applied to any types of materials including metals, ceramics, coatings and polymers of different plastics and composites. According to the authors, some of the commonly used NDT methods are: examination through human senses, liquid penetration method, ultrasonic testing, radiographic imaging, magnetic particle inspection and eddy current testing. Human senses such as sight, smell and hearing can, with experience be used to detect crack defects in components but should not be used as the sole means of detection [85].

For improved accuracy, several optical aids are used to facilitate the detection of cracks through visual inspection [92]. These include magnifying lenses, optical microscopes and endoscopes which can achieve higher magnifications and are widely used in detecting cracks in the internal surfaces of boilers, pipes, reactors, and heat exchangers. Liquid penetration inspection is stated to detect cracks which are exposed to a surface and is quite reliable and capable of identifying cracks of very small width, even cracks of just a few microns (μm). A colourful penetrant is made to enter into the cavity of a crack through the capillary action, allowed enough dwell time and excess wiped off the surface [94]. A developer (good solvent for the penetrant liquid) is 32

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reported to be laid on the surface and another liquid is applied onto the surface resulting in a part of the penetrant coming out of the crack cavity diffusing into the developer's layer on the surface thus revealing the crack as schematically elaborated in Figure 2.14 (a) to (d).

(a)

(c)

(b)

(d)

Figure 2.14: Crack detection stages for liquid penetration inspection [92]

Kumar [92] further outlined that ultrasonic testing can be used for detecting fully embedded cracks as well as surface cracks by making use of sound waves, normally, longitudinal or primary waves (particles move in the energy propagation direction), and shear or secondary waves (particles move normal to the direction of energy propagation) The author stated that if a defect exists in the work-piece, the pulse-bunch is reflected from the defect with a shorter flight time as in Figure 2.15 [92].

Figure 2.15: Ultrasonic testing [92]

It was further indicated that radiographic imaging works in a similar way to that of a camera with a flash light which we use in daily life except that the recording film is separate from the radiographic source and is placed behind the work-piece. The author further inferred that, in radiographic imaging, electromagnetic waves (X-rays or γ-rays) of very short wavelength are transmitted through a work-piece. If the material of the work-piece is not uniform, (say have a crack), the transmission of electromagnetic waves will be absorbed differently and there will 33

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be contrast on the recording film. Figure 2.16 shows the x-ray set-up and the X-rays spread on the examined components as in Figure 2.17.

Figure 2.16: X-ray set-up [92]

Figure 2.17: X-rays falling on a work-piece with a film placed close to its rear surface [92]

Kumar [92] further reported magnetic particle as being capable of inspecting both surface and subsurface cracks in a work-piece made of ferromagnetic materials such as most steels, nickel and cobalt. According to the author, the technique is simple to use, inexpensive, quick, sensitive, and reliable, but however limited to ferromagnetic materials only. It was postulated that a strong magnetic flux is generated within a work-piece and a defect disturbs the lines of force and creates leakage of magnetic flux out of the surface of the work-piece. The same paper

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concluded that when minute magnetic particles are sprayed, they get deposited on the surface at points which are close to the crack as in Figure 2.18, thus the crack can be observed easily.

Figure 2.18: Magnetic flux through work-pieces, (a) a particle ridge is built up over the surface crack mouth, and (b) magnetic particles are deposited due to a subsurface crack [92]

2.6 Repair of Crack Defects in Metals 2.6.1 Introduction to crack repair techniques in metals When fracture critical cracks are detected in members and connections in service, several methods are employed to stop them from further growth or propagation. Fracture was defined as rupture in tension or rapid extension of a crack, leading to gross deformation, loss of function or serviceability, or complete separation of the component [85]. A number of various methods for the repair of cracks in metals are in use today. The choice is highly a function of many factors which include the nature of crack, crack position, crack orientation, crack size, crack accessibility, component application, expected repair precision, availability of tools and many others. Crack repair techniques in metals include: hammer peening, grinding (disc and burr grinding), gas tungsten arc (GTA) re-melting, stop-hole drilling, vee-and-weld, bolted plates, bonded composite patches, pulsed electron beam irradiation, and pre-stressed carbon fibre reinforced polymers (CFRP) patches [95, 96, 97]. Some of these will be discussed in detail in the next section.

2.6.2

Crack repair techniques

Hammer peening. Shallow surface cracks of up to 3 mm deep can be repaired by hammer peening [85]. The authors asserted that when hammer peening is applied to welds that have already been in service, the remaining fatigue life is at least as long as that provided by original detail when new, removing the damaging effect of prior fatigue loading cycles. It was reported 35

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that these treatments result in fatigue resistance that is at least one fatigue category greater than the original detail.

Grinding. Grinding can be used to totally remove portions of a detail containing small cracks, particularly cracks at the edges of flanges or other plates [85]. The authors further stated that the gouge created by grinding should be tapered with a 2.5:1 slope and finish grinding should be performed parallel to the applied cyclic stresses (that is, sparks from the grinding operation should fly in the direction of the primary stress, thus causing grinding scratches to be parallel to the primary stress). It was also further affirmed that small micro-cracks form at the weld toe as the weld pool cools to ambient temperature and contracts and under cyclic loading and these micro-cracks begin to propagate and become small fatigue cracks. Grinding them off has been found to be effective at shaping the weld, at the same time killing crack paths thereby enhancing fatigue strength by reducing the associated stress concentration factor. The authors reported that welds reshaped with a burr grinder have been found to have a 50% larger allowable fatigue design stress range over their untreated counterparts.

Murray and Clare [95], studied the repair of EDM induced surface cracks by pulsed electron beam irradiation. The authors used low energy irradiation and few shots to investigate the physical mechanism of crack repair and the obtained results indicated trends in crack reduction and ultimately their elimination. Under the lowest EB cathode voltage of 15 kV and with 5 and 10 shots, there was evidence of partial crack resealing. According to the authors, the circular nature of the seal suggested flow across the crack when molten then contraction upon rapid quenching, solidifying a few joints where surface tension of the melt resisted the force of contraction. Cracks induced by the EDM process were entirely eliminated from the surface, and up to 4.5μm depth of the new re-melted with an increased voltage of 25 kV and 35 kV after 20 shots at 25 kV and 35 kV. The study showed that pulsed electron beam irradiation has potential for the improvement of fatigue life and corrosive attack due to surface crack elimination. However, the process seem to be limited to cracks of micron dimensions.

Emdad and Al-Mahaidi [96] studied the effect of pre-stressed carbon fibre reinforced polymers (CFRP) patches on crack growth of centre-notched steel plates. The authors found out that multi-layers of CFRP increased the fatigue life of the plate up to 30% and showed an increase in the cracked life of between 6.5 times and 10 times. Similar findings were made by Srilakshmi 36

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et al [98]. Increases in pre-stressed patches lowered the crack growth rates increasing the crack life of the specimen. The pre-stressing technique proved to be effective and the authors recommended that the correct use of pre-stressing in general can lead to increased fatigue life of cracked members by applying a compressive force to the crack edges. The authors further elaborated that the compressive force will impose artificial crack closure on the flawed structure arresting its further growth. The authors postulated that the method currently cannot be used for double-sided repairs due to limited access and the use of the pre-stress force on one side can cause significant bending moments to the member leading to unforeseen failure as also cautioned by Ahn and Basu [97].

Ayatollahi et al [99] numerically studied fatigue life extension by crack repair in 6061-T651 aluminium alloy using stop-hole technique (Figure 2.19). The authors affirmed that drilling a hole at the crack tip turns the crack into a notch and diminishes the crack tip stress singularity thereby improving the fatigue life of the structure. Larger stop-hole diameters are here reported to have resulted in longer fatigue lives. The numerical results revealed that the presence of stop holes significantly decreased the stress concentration around the crack tip. A comparison between the reported experimental results and the obtained computational results showed that the fatigue life extension caused by the stop-hole method can be well predicted by the numerical model developed in the study. The authors recommended that the same approach can be developed to estimate the fatigue life improvement of other metallic alloys and also other cracked specimens.

In addition to stop-holes, other crack arresting methods include tensile triangles and branching of crack direction [100]. Hole drilling was described as perhaps the most widely used repair method for fatigue cracks [85]. However, the authors recommended sufficient hole diameter for successful arresting of the crack and suggested a rule of thumb that larger holes are better as long as strength and stiffness of the structure or connection is not compromised. The authors asserted that sufficient hole diameters typically in the range of 50.8 to 101.6 mm have proven successful through field experience, but depending on application a 25.4 mm hole may also be sufficient. They also added that larger holes are not always practical and many owners are uncomfortable placing such a large hole in a member. The authors concluded that flame-cut

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holes should never be used to remove crack tips as they introduce a gouged surface condition that may initiate new fatigue cracks.

Figure 2.19: Stop-hole approach of arresting cracks [99]

Bonded composite repair technology (BCRT), (Figure 2.20) was used in the analysis and repair of crack growth emanating from v-notch under stepped variable fatigue loading [101]. In the investigation, the authors studied the effect of the cyclic variable amplitude loading (CVAL) on the fatigue life of Al 7075-T6 made samples. The results indicated that the improving of the fatigue life of cracked specimens repaired with bonded composite patches is not very significant for increasing fatigue blocks. However, the authors found out that the fatigue life of repaired specimens increased considerably in the case of decreasing blocks of loading. The authors further postulated that in addition to the increase of strength due to the patch, the plastic zone formed around the crack tip due to overload, retarded the crack growth. They said that this was because the patch acted as a stress-carrying component, which in turn, attenuated the effect of plastic zone generated by the overload. Through microscopic observation the authors observed that the shear failure at the border on patched side was less compared to unrepaired specimen, an indication that the patch prevented the crack growth irrespective of the loading condition. The repair of cracked components by an adhesively bonded composite patch has gained acceptance in aerospace structures [102]. The authors reported that the repair technique reduces the stress field near the crack by bridging the stresses between the cracked plate and the composite patch which leads to retardation or complete stoppage of the crack growth.

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Figure 2.20: Crack repair of aluminium using bonded composite patches [101]

Bonded composite patch crack repair technique is also cited to have been used for the repair of cracked aluminium plate [103]. The authors postulated that the traditional repair method for cracked aircraft structure is normally fastening metal reinforcement using bolt and rivet. They reported this method to worsen the stress concentration problem due to drilling of additional fastener hole and stress concentration along the neighbourhood of repair from metal reinforcement. Ouinas [104] studied the performance of the bonded composite semi-circular patch to reduce the stress concentration and to repair the crack at the notch on aluminium plate. In the study, the authors deduced that boron epoxy patch is more important compared to a graphite epoxy patch and that using two patches reduces the values of the stress intensity factor by half, compared to the result obtained with a simple patch. The technique was found to effective when optimally used.

Vee-and-weld. Vee-and-weld is a method of weld repair for long, through-thickness cracks. Material is removed along the crack length, through three-quarter the thickness of the section that is cracked in the shape of a V [85]. The V-shaped groove is then filled with weld metal. The authors recommended air arc gouging, and grinding as the preferred methods of material removal. They however discouraged the use of a disc grinder to remove the cracked material as the crack can become blurred (or smeared) as more material is removed. The authors said that this increases the possibility of hiding or masking the crack path and leaving an embedded flaw within the repair weld. Air-arc gouging, is here recommended as it opens up the crack making the crack path easy to identify and follow. It is further stated that vee-and-weld repairs are most effective when used to repair a cracked weld detail, not cracks in unwelded base metal. The reliability of this repair method depends highly on how skilled the welder is if it is not automated. The authors concluded that repairs can be made using the shielded metal arc welding (SMAW), flux-cored arc welding (FCAW), or gas metal arc welding (GMAW) process depending on availability and site conditions. 39

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Doublers or Splice plates. Doublers or Splice plates can be used to repair through-thickness cracks [85]. The authors affirmed that this technique adds material to either increase a crosssection or provide continuity at a cracked cross-section and in turn reduce stress ranges. The authors further reported that to ensure the weld repair will have adequate fatigue resistance, doubler plates can be added after the repair is made to decrease the stress range that contributed to the original cracking thus protecting the repair. However, alignment problems have been reported to affect this technique. According to Branza et al [105] a buttering technique can also be used to further reduce weld-repair cracking and in some cases prevent it completely. The phenomenon is said to be related to alloying elements of the buttering alloy, (titanium and aluminium) and lowering these two elements, as well as controlling the amount of trace elements such as sulphur, can reduce hot cracking.

2.7

Repair of Defects in Ti-6Al-4V

2.7.1

Common repair methods for Ti-6Al-4V defects

Overally, increasing the energy density or focus of the beam was found to correlate strongly to a reduction in the level of gas porosity as sufficient energy melts the powder layers [89]. The authors inferred that for all AM platforms there is a requirement to develop a better understanding of the relationships between the process parameters and part geometry, and the size, density, and spatial distribution of pores found within a component. They added that such information is essential in developing strategies for reducing the defect content in made components. In Selective Laser Melting, which uses an inert gas atmosphere, some of these pores originate from shielding gas becoming trapped during densification of the powder. The authors recommended that high vacuum processes can reduce this effect arguing that all powder based techniques are still liable to porosity if there is contamination of the powder.

The use of laser metal deposition in the reconditioning of the worn parts was reported [14]. The technique is here rated to recondition the wear defects to near original conditions of the substrates. Undercut, deformation, cutter pull out and cutter mark defects noted in Ti-6Al-4V components used for aircraft parts were reported to be repaired by LMD. The oxidation problem of Ti-6Al-4V has been countered through argon shielding and high vacuum processes [40, 42, 43, 61, 87]. The authors recommended keeping oxygen levels below 10 ppm. Surface wear 40

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defects in Ti-6Al-4V have been reported to be improved by TiC laser coating to produce surfaces with high wear resistance [1, 44, 61]. The difficulty in machining Ti-6Al-4V and other super alloys has made many metal industries to invest heavily in Research & Development to understand how to better machine titanium. The use of specially designed tools, for example carbide and ceramics tools and coolants have been developed and through these, high standard surface finishes can be obtained from machined Ti-6Al-4V.

The following factors are reported to contribute to efficient machining of Ti-6Al-4V parts: low cutting speeds, high feed rate, large amounts of non-chlorinated cutting fluid, sharp tools and rigid tooling [68, 106]. It is further reported that Ti-6Al-4V is easily welded in the annealed condition but however, precautions must be taken to prevent oxygen, nitrogen, and hydrogen contamination. Ti-6Al-4V can be welded by the conventional techniques used with austenitic stainless steel but the HAZ must at all cost be protected by either argon, helium or a mixture because of the material's reactivity [107]. It was recommended that liquid argon is often used because of its purity and its dew point must be below -60°C. Emphasis was also made that gas flow must be carefully controlled to avoid turbulent flow and that there are glove boxes designed for the welding of Ti-6Al-4V. It was further recommended that welding be processed in a protective atmosphere and the manipulation is made through gloves sealed to the box and annealing should be performed after welding in order to have the same mechanical properties in the HAZ and the bulk. It was also suggested that Ti-6Al-4V should be avoided for applications involving low stress abrasion from hard particles but where use is inevitable, proposed surface modification by injection of carbide particles by laser cladding was suggested to improve wear properties [91]. It was also in the same paper suggested that wherever Ti-6Al4V may be subject to fretting damage, a suitable hard-facing counter-face deposit would be Stellite 6B. Most Ti-6Al-4V defects have been reported to arise from input and process parameters of the deposition process. The reviewed literature recommend broader understanding of input and process parameters to play a very important role in reducing defects in the deposited components. 2.7.2

LMD repair of cracks in Ti-6Al-4V components

Graf et al [108] investigated the use of LMD for the repair of cracks in stainless steel and titanium alloys. The authors reported that cracks can be removed by milling and then be reconditioned with new material deposition. Key to the approach of the authors was the use of 41

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different groove (crack) shapes, namely the V-groove, U-groove and the U-groove with top open angle side walls as shown in Figure 2.21. The designs of the grooves were made with the assumption that intolerable defects extend not deeper than 10 mm from the surface, therefore they chose a groove depth of 10 mm.

Figure 2.21: From left to right: V-groove; U-groove and U-groove with angled side walls [108]

The experimental study was performed using a TRUMPF TruDisk 2.0 kW Yb:Yag laser with a 3-jet powder nozzle positioned with a 5-axis machine, helium 5.0 carrier gas and argon 5.0 shielding gas, at a local inert gas atmosphere with less than 50 ppm oxygen. Powder grain size ranging between 45-125 μm were used. For Ti-6Al-4V, the parameters used were as reproduced in Table 2.8. The parameters were alternated among the three groove shapes. For both stainless steel and Ti-6Al-4V, the V-grooves and the U-groove with open top angle side walls enabled better powder delivery accessibility. The repairs made for the V-grooves were subjected to Xray testing and the V-grooves showed good side-wall fusion although lack of side wall and bottom base fusion could be detected on the images as indicated by the black rings in Figure 2.22. Also noted in the V-groove was unmelted powder.

Table 2.6: LMD parameters for the Ti-6Al-4V V-groove; U-groove and U-groove with angled side walls [108]

Welding parameters

(a)

(b)

Welding velocity (m/min)

0.5

1

Laser power (kW)

2

1

Laser spot diameter on surface

2.2

1

9.4

3.8

(mm) Powder mass flow (g/min)

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Figure 2.22: Left to right: LMD repaired stainless steel V-groove and the Ti-6Al-4V V-groove [108]

The cross-section of the U-shaped groove (Figure 2. 23 (a)) showed lack of side wall fusion defects because of inaccessibility challenges at the side-wall of the groove. The authors reported that the laser beam could not be adjusted perpendicular to the side-walls, which resulted in irregular side walls material deposition. It was further reported that part of the powder jet was impeded by the upper lip of the groove due to inaccessibility, causing irregularities in the powder stream close to the groove side-walls. The paper asserted that for the U-groove side wall regions, the safe and uniform deposition of powder was hindered and recommended wider grooves or inclined side-walls as necessary for the successful use of the technique as in Figure 2.23 (b) even pores were evident. It was then concluded that the U-groove could not be successfully repaired using LMD.

Figure 2.23: Left (a) U-groove; Right (b) 5o U-groove top open side walls [108]

Rottwinkel et al [33] made similar investigations. In their studies of “Challenges for singlecrystal (SX) crack cladding”, they used V-grooves. The results from the studies showed cross 43

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sections with cracks as shown in Figure 2. 24. Cracks are here reported to be more prevalent in the deposits without preheating. Efforts to completely eliminate the cracks by preheating were fruitless as indicated in Figure 2. 25. The repaired samples were SEM analysed and cracks were detected as shown in Figure 2.26. The authors conclude the work done in their paper as unsuccessful as it could not completely eliminate cracks in the deposits. However, they did not totally dismiss their joint efforts a total failure as the results pointed out hints that arose from the LMD key parameters that could in future studies be further optimised to achieve improved results. The study on the LMD repair of cracks in Ti-6Al-4V has been rarely investigated.

Figure 2.24: Cracks on LMD repaired grooves without preheating [33]

Figure 2.25: Cracks reduced by preheating of LMD repaired grooves [33]

Figure 2.26: Cracks detected by SEM analysis [33]

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2.8 Laser Metal Deposition (LMD) 2.8.1 Features of LMD LMD’s capability to achieve small and limited HAZ makes the welds less vulnerable to degradation of properties and yields better microstructure and a smaller dilution zone [11, 14]. Most difficulties in processing Ti and its alloys which include moulding, alloying, machining and welding and many others are being solved through LMD [42]. The authors applauded LMD as a successful alternative manufacturing technique that is increasingly offsetting most of these difficulties through research and development (R&D). Laser is an acronym for light amplification by stimulated emission of radiation [65, 109, 110].

Laser technology has increasingly become global subject in many R&D forums on materials processing, manufacturing and repair [10, 14]. Laser was defined as essentially a coherent, convergent and monochromatic beam of electromagnetic radiation propagating in a straight line with negligible divergence and occur in a wide range of wavelength, energy/power and beammodes/configurations and has wide range of applications [109, 111]. However, in the current paper, focus is confined only to the use of laser in material processing, manufacturing and repair, of particular mention LMD. LMD which is also called laser engineering net shape (LENS), direct metal deposition (DMD) or laser solid forming (LSF), is a process that has a unique capability to build up complex three dimensional features in an additive way on existing components [12].

The growth in LMD and other laser technologies stems from the increasing global competition to deliver products with new design and maximum possible features in a short time frame, to meet increasing market demands [9, 112, 113]. LMD is an additive manufacturing (AM) technology that belongs to the energy deposition class as was recently grouped by the F42 committee on AM standards [41, 43]. LMD is also known as a solid freeform fabrication that can be used to manufacture near-net-shape solid components directly from the Computer Aided Design (CAD) file in a single-step waste-free process by adding layers of material [40, 42, 114, 115]. The authors asserted that AM allows one step fabrication of complex three-dimensional structures and of multi-articulated system, which could not be produced with conventional

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fabrication processes, without requiring assembly of its structural members and joints after fabrication.

LMD process, as an AM technology offers lots of advantages in material deposition and surface modification of components, repair of existing worn out parts with good metallurgical bonding, as well as building new components from scratch, most especially complex components both macro and micro [116, 117]. Its low heat input leads to low distortion and low thermal damage in the base material [108]. LMD is a good candidate for the processing of functionally graded materials parts [42, 43]. Traditional manufacturing processes are energy intensive and they generate scrap when producing complex parts [41].

In LMD, parts are built in a layer-by-layer fashion by rastering the laser and powder source across the substrate [44]. During the LMD process, powder is fed into the melt pool created by laser on the substrate, which upon solidification forms the contour defined by the CAD data information of the component [43]. The LMD process is achieved by feeding powder, which is assisted by shielding gas, into the melt pool that is generated by sharply focused collimated laser beam on the substrate [42]. The authors stated that the interaction between the substrate and the laser create the melt pool. The powder is delivered through an integrated powder delivery system and the heat generated by the melt pool as well as the laser beam causes the powder to melt and bond with the substrate as it solidifies [40]. The authors asserted that the process is repeated layer upon layer until the building process of the component is completed. The schematic diagram for the experimental setup for LMD shown in Figure 2. 27.

Figure 2.27: Schematic of the Laser Material Deposition [43]

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The LMD process has a variety of parameters that affect the quality and properties of the deposited material. Gas flow rates, laser power, powder feed-rate, powder grain size, traverse rate or scanning speed, are a few of the most important factors that affect the physical, metallurgical and mechanical properties of the deposits [40, 42, 43, 44].

2.8.2

Applications of LMD

The major strength of the LMD rapid manufacturing technique is its ability to manufacture freeform shapes, to directly create different surface coatings on a crack-free part, and to produce parts from graded porous to fully dense solid structures [65]. The author reported that LMD is gaining popularity in the fields of aerospace and biomedical manufacture due to its flexibility and cost effectiveness. In addition, LMD has many potential applications, including production of functional prototypes, short-run component fabrication, worn out component repair, material removal, joining, cutting, layered manufacturing, powder metallurgy, enhancing surface wear properties of Ti-6Al-4V by TiC (surface engineering), laser coating to produce surface with high resistance to wear and corrosion and fabrication of FGM or parts through its capability to handle more than one material simultaneously [1, 43, 44, 61].

LMD can also reduce buy-to-fly ratio for aerospace parts and the lead time for production, two factors which impact cost, as well as reducing the weight of the aircraft through elimination of parts assembly [41, 42, 43, 44]. The authors affirmed that Ti and its alloys happen to be a workhorse metal in the aerospace industry, particularly Ti-6Al-4V where due to its favourable strength to weight ratio, good thermal properties and corrosion resistance [39, 61], is used for the manufacture of aeroplane wing sections, landing gear components, body parts, and engine turbines. It is for these reasons that a lot of literature revealed recent increase in the R&D in titanium and its alloys as well as titanium laser-additive-manufacturing processes of crack-free forms [44].

In the study of Mechanical Properties of Laser-Deposited Ti-6Al-4V, LMD was used for titanium-component manufacturing and repair [44]. Due to its bio-compatibility, titanium and its alloys fit well in the biomedical applications where through LMD process, Ti medical implants can well be manufactured [40, 61]. LMD has been used in various areas namely, 47

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deposition of commercial alloy powders, deposition of carbides [1] and intermetallics, particulate reinforced metal matrix composite (PR-MMC) coatings, and advanced alloy development by laser deposition as well as many other uses in modern metal and manufacturing industry [65]. Shukla et al [41] applied LMD for Titanium Matrix Composites (TMCs) in particular, characterization of Laser Deposited Ti64/TiC (titanium alloy grade 5/titanium carbide) composite.

2.8.3

Materials input and process parameters of LMD

Mahamood et al [40] studied the effects of laser power and scanning speed on the degree and size of porosity and their study revealed that, lower scanning speed, results in lower degree porosity of smaller pore sizes. Also, their study revealed that the higher the laser power, the lower the degree of porosity. However, the pore sizes were found to increase significantly as the laser power was increased which the authors reported to be caused by gas entrapment in the melt pool. Their findings showed that various degrees of porosity can be attained by controlling the laser power and scanning speed depending on application requirements. In the study, the authors used the following materials input and process settings: Kuka robot carrying a 4 kW neodymium-yttrium-aluminium-garnet (Nd-YAG) laser and coaxial powder nozzle, 2 mm laser beam diameter, 195 mm focal distance, argon shielding mechanism of oxygen ≤ 10 parts per million (ppm), 1.44 g/min powder flow rate, 2 l/min gas flow rate, laser power of 400 W and 800 W, scanning speed varied between 0.005 m/s and 0.2 m/s and gas atomized Ti-6Al-4V powder particle size ranging between 150 -200 μm. The study was conducted on a 5 mm thick 72 x 72 mm hot rolled Ti-6Al-4V plate. Prior to the deposition process, the substrate was sandblasted, washed and degreased using acetone in order to aid the absorption of the laser beam.

Kobryn and Semiatin [44] sought to establish the effects of parameters and input materials on deposit structure, texture, magnitude and anisotropy of mechanical properties and any lack-offusion porosity for the layered manufacturing process of bulk Ti-6Al-4V deposits. Their study instead had limited work to establish the relationship between process parameters and the structure of deposits, since most of them are stated to have been achieved through trial-anderror approaches. However, the authors made an interesting finding that Hot-Isostaticallypressing (HIP) Ti-6Al-4V appears an effective pre-treatment process condition in healing lack48

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of-fusion porosity. In their tests, the stress-relieved material contained significant lack-offusion porosity while the HIP’ed material contained none. In the study, the authors used the following materials input and process settings for the experimental design, setup and control: gas-atomized pre-alloyed Ti-6Al-4V powder (composition properties closely matching those of Ti-6Al-4V plate) - particle sizes ranging between (45 and 150 μm), 13.23-mm-thick hot-rolled Ti-6Al-4V plate with an equiaxed-alpha microstructure. Prior to deposition, the plate was lapped on both sides to a surface finish of 12 to 14μm and degreased using acetone and alcohol.

In the study of Gas Flow Rate and Powder Flow Rate Effect on Properties of Laser Metal Deposited Ti-6Al-4V, Pityana et al [43], varied powder flow rate and the gas flow rate parameters to study their effect on the physical (height, width and weight of the deposit), metallurgical (microstructure) and mechanical (microhardness) properties of the deposits. Their findings were as follows: as the gas flow rate is increased, the track width, the track height and the deposit weight is reduced due to higher disturbance in powder flow path created by high gas flow rate. The track width, the track height and the deposit weight was found to increase as the powder flow rate was increased. The average Microhardness decreased with an increase in the gas flow rate and increased as the powder flow rate was increased. In the study, the authors used the following materials input and process settings for the experimental design, setup and control: 4.4 kW Nd-YAG Rofin Sinar fiber laser, the deposition process was controlled by a Kuka robot which carried the laser and the powder delivery nozzles in its end effector, constant laser power and scanning speed of 1.8 kW and 0.005 m/s respectively, 2.88 g/min – 5.76 g/min range of powder flow rate, 2 l/min - 4 l/min range of gas flow rate, 2 mm laser sport size, 195 mm focal length, argon gas shielding mechanism of oxygen ≤ 10 ppm, conducted on hot rolled 99.6% pure Ti-6Al-4V substrate 72 x 72 x 5 mm thick plate and Ti-6Al-4V powder of the same purity and of particle size range between 150 and 200 μm. The substrate was sandblasted and degreased with acetone before deposition process to improve laser power absorption and to improve metallurgical bonding of the melted powder and the substrate.

Mahamood et al [42] studied the effect of laser power on the resulting microstructure and microhardness of laser metal deposited Ti-6Al-4V powder on Ti-6Al-4V substrate. Based on the results of the preliminary experiment, a laser power of 800 W, scanning speed of 0.005 m/sec, powder flow rate of 1.44 g/min and a gas flow rate of 4 l/min produced a fully dense, 49

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pore free with good metallurgical bonded deposit. It was found that as the laser power increases, the microstructure ranges between fine martensite to thick martensite. The authors reported that the average microhardness also increases with an increase in the laser power. In the study, the authors used the following materials input and process settings for the experimental design, setup and control: 4.4 kW fiber delivered Nd-YAG laser with coaxial powder nozzles, Kuka robot to carry both the laser and powder nozzles and to control the deposition process, complete plastic shielding mechanism material attached to the end effector of the robot which covered the shielding block completely during the deposition process as in Figure 2.28, 99.6% pure Ti6Al-4V substrate of dimension 72 x 72 x 5 mm thick plate and pure Ti-6Al-4V powder of same purity and 150 to 200 μm particle size range, 0.8 kW to 3kW laser power range, scanning speed, powder feed rate and the gas flow rate kept constant at 0.005 m/sec, 1.44 g/min and 4 l/min respectively The substrate was sand blasted and cleaned with acetone prior to the deposition process.

Figure 2.28: LMD shielding box [42]

The study by Shukla et al [41] investigated the effect of laser power on the wear resistant property of laser deposited Ti64/TiC composite at low and high scanning speeds. The microstructure, micro-hardness, phases, and wear properties were extensively studied. The authors found out that the bulk of the unmelted carbide (UMC) observed at a lower laser power of 0.4 kW and 0.8 kW did not lead to an increase in the hardness because they could not provide the required reinforcing properties as a result of many loose or not well bounded UMC. The authors described this as being detrimental to the property of the parent material as the loose powder causes scratches and hence lower the microhardness when compared to the micro50

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hardness of the substrate. The authors found the quantity of UMC to reduce with the increase in laser power. They also observed that the micro-hardness increases as the laser power increases until the laser power reaches 2.0 kW and started decreasing as the laser power is increased beyond 2 kW.The authors attributed this behaviour to the fact that as the laser power is increased beyond 2 kW, more TiC powder is melted resulting in less UMC which causes less reinforcement of the UMC of the TiC powder particles in the composite and reducing the wear resistance performance of the TiC/Ti64 composite. In conclusion, the authors inferred that there is a limit to which the laser power should be increased in order to retain the required UMC to improve the microhardness of the composite. It was further observed in their research that the wear resistance was not improved at lower laser power because of large quantity of unmelted TiC particles which are easily removed during the sliding process. Furthermore, it was concluded that the optimal laser power requirement for better wear resistance performance is 2kW for the processing parameters considered in the study.

In the study, the authors used the following materials input and process settings for the experimental design, setup and control: Kuka robot to carry a 4.4 kW Nd-YAG fiber laser (Rofin Sinar) attached with coaxial nozzles, Hot rolled Ti64, 72 x 72 x 5 mm thick plates of 99.6% purity substrate in fixed position, Ti64 powder 99.6% purity and particle size range between 150 – 200 μm, TiC powder of particle size of 60 μm, glove box argon gas shielding mechanism, Oxygen level ≤10 ppm. The substrate was sandblasted and cleaned with acetone prior to deposition. Laser power ranged between 0.4 kW to 3.2kW, scanning speed, powder flow rate (total for two hoppers at equal proportions of each 1.44 g/min) and gas flow rate were all kept constant at 0.005 m/s, 2.88 g/min and 2 l/min respectively. The spot size was maintained at 2 mm at a focal distance of 195 mm above the substrate. Ahsan [65], postulated that the use of different types of the commercially available powders can give rise to different properties in deposits such as geometric accuracy, surface roughness, microstructural and mechanical properties, and defects of intralayer porosity.

2.9

Lasers Used In Materials Processing

2.9.1 Introduction to lasers used in materials processing Up to date, there are tens of thousands of laser types that are developed for various uses. Lasers are light amplifiers and the main difference between their different types is the medium used to 51

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generate the stimulate emission, that is the physical state of the active material [65, 110, 118]. The authors classified lasers as follows: gas lasers, excimer lasers, solid-state lasers, semiconductor lasers, liquid dye lasers, and fibre lasers and each class contains different lasers. However the current work focusses on the few most common that are of importance to materials processing like welding, cutting, cladding, deposition, melting, surface alloying, heat treating, machining and drilling. They are briefly described here-under.

2.9.2 Liquid dye, gas and solid state lasers Liquid dye lasers: high power direct diode lasers (HPDDLs). The HPDDL is built from a series of diode laser bars, which are a single, monolithic semiconductor substrate on which several emitters are fabricated [119]. The author reported a single bar to have a total power output of around 100 W which when multiple stacked close together, produce multiple kilo watts of laser power. It was further stated that specialized optics convert raw output into a far-field format useful for most applications in a free space delivery manner, enabling remote (up to 30 meters) delivery of the laser source from the processing area. HPDDLs are here described as very physically compact and lightweight with a wall plug (electrical conversion) efficiency, which is many times higher than for any other laser types, translating directly into lower operating cost for the system, since less electricity is required to produce a given amount of output power. The author further added that, a closed loop cooling system can be connected to the diode stack affording a typical operating lifetime of tens of thousands of hours.

Gas lasers: Carbon dioxide (CO2) lasers. CO2 lasers are the most common of the molecular gas lasers and the first generation in high powered industrial lasers [65, 110, 119]. They are reported to amplify light through molecular vibration rather than electronic translations as in other lasers. The authors reported the wall plug efficiency (optical energy out/total electrical energy into the system) of CO2 lasers to be between 10 and 12%. However, regardless of the low efficiency, the authors alluded to the fact that CO2 lasers have a good beam quality and focusability leading them being widely applied in many industrial applications such as welding, cutting, cladding, processing of glass and ceramics, soldering, hardening, brazing, cladding, melting and marking of metals, plastics and other organic materials. They further reported that the wave length of CO2 lasers varies between 9.4 𝜇m and 10.6𝜇m which is quite large and because of this, the light cannot be transferred by optical fibre and is typically achieved by 52

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mirrors, which has implications for applications. The cost of these lasers is reported to be relatively low and their power output can be more than 45 kW. They cited small size to make these lasers easy to integrate into robotic applications or even in desktop equipment hence their use in many research platforms. They further stated that coupling CO2 lasers with the right beam conditioning optics, makes their slab discharge configuration to produce a high quality Gaussian beam with a low M2 (