other project report or dissertation on the basis of which a degree or award was conferred .... suitable filler wire rod is introduced in the weld pool. The electric ... fatigue tests showed that the FSW joint had a fatigue life fairly close to that of the.
FATIGUE BEHAVIOUR OF WELDED JOINTS A PROJECT REPORT Submitted by
AKSHANSH MISRA [Reg No: 1021310334] SARAVANAN M [Reg No: 1021310333] ANAND SINGH [Reg No: 1021310338]
Under the guidance of
Dr. A RAZAL ROSE, Ph.D. (Associate Professor, School of Mechanical Engineering)
in partial fulfillment for the award of the degree of BACHELOR OF TECHNOLOGY in
MECHANICAL ENGINEERING of
FACULTY OF ENGINEERING & TECHNOLOGY
S.R.M. Nagar, Kattankulathur, Kancheepuram District
MAY 2017
i
SRM UNIVERSITY (Under Section 3 of UGC Act, 1956)
BONAFIDE CERTIFICATE
Certified that this project report titled “FATIGUE BEHAVIOUR OF WELDED JOINTS” is the bonafide work of “AKSHANSH MISHRA [Reg No: 1021310334], SARAVANAN M [Reg No: 1021310333], ANAND SINGH [Reg No: 1021310338] who carried out the project work under my supervision. Certified further, that to the best of my knowledge the work reported herein does not form any other project report or dissertation on the basis of which a degree or award was conferred on an earlier occasion on this or any other candidate.
SIGNATURE
SIGNATURE
Dr. A. RAZAL ROSE GUIDE Associate Professor (Sr.G) Department of Mechanical Engineering
Dr. S. PRABHU, Ph.D HEAD OF THE DEPARTMENT Department of Mechanical Engineering
Signature of the Internal Examiner
Signature of the External Examiner
ii
ABSTRACT The effect of Friction Stir Welding and the Tungsten Inert Gas Welding on the fatigue behavior of AA6061 aluminium alloy has been studied. To reveal the influence of the welding parameters, different travel speeds of the welding tool have been used to provide weld seams with varying micro structural features. Crack initiation as well as crack propagation behavior under fatigue loading has been investigated with respect to the local microstructure at the crack initiation sites and along the crack path. Fatigue cracks were mostly initiated around the stir zone and the adjacent thermomechanical affected zone independent from hardness distributions in the weld seams. In some specimens, defect-like feature was observed at the crack origins, which shortened the fatigue lives. It has been found that while the effect of the tool travel speed on the fatigue lifetime seems to be little, the varying and complex local microstructure in the weld seam basically affects both the crack initiation sites and the crack propagation paths.
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ACKNOWLEDGEMENT We hereby express our sincere gratitude to the Management of SRM University, for their kind encouragement bestowed upon us to complete this project. We take this opportunity to thank Dr. Muthamizhchelvan, Director (E&T), SRM University of providing us with excellent infrastructure and facilities for the development of this project. We are greatly indebted to Dean, School of Mechanical Engineering, SRM University, Dr. Kingsly jeba Singh and Professor and Head, Department of Mechanical Engineering, SRM University, Dr. S. Prabhu, for his motivation and guidance through the course of this project work. His advice, ideas and constant support has encouraged and helped us to get through in difficult times. We express our deep gratitude to our guide Dr. A. Razal Rose, Associate Professor(Sr.G), Department of Mechanical Engineering, for his encouragement and guidance towards the development of this project, for his support, confidence as well as awe-inspiring advice throughout the project. Again we express our profound gratitude to our Project Coordinator, Dr. Mohammed Iqbal, Associate Professor (Sr.G), Department of Mechanical Engineering for his encouragement and guidance towards the development of this project. We express our sincere and heartfelt gratitude to the Faculty of the Department of Mechanical Engineering, SRM University. We would like to express our gratitude to our Parents for having supported for us in every step in life. Lastly we express our gratitude to the Almighty God, and all those who have extended their valuable help and suggestions, cooperation, whether directly or indirectly during the progress of the project and thereby aided the successful completion of the project.
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TABLE OF CONTENTS LIST OF TABLES
viii
LIST OF FIGURES
ix
LIST OF ABBREVIATIONS
xi
1 INTRODUCTION
1
1.1 Overview
1
1.2 Aim of the Work
2
2 LITERATURE SURVEY
3
3 DESIGN AND FABRICATION OF FIXTURE
7
3.1 Importance of Fixture Design
7
3.2 Design of Fixture
7
3.2.1 Objective of Fixture Design
11
3.2.2 Fabrication of Fixture
12
4 FUNDAMENTALS OF FRICTION STIR WELDING
13
4.1 Introduction of Friction Stir Welding
13
4.2 Applications of FSW
13
4.2.1 Applicability of Friction Stir Welding
16
4.3 Macroscopic Process in Friction Stir Welding
16
4.4 Tool Material
17
4.4.1 Tool Material Selection 4.5 Welding Tool Design
17 18
4.5.1 Tool Design Dimension
20
4.6 Tool Materials Properties
20
4.7 Heat Treatment Process of Welding Tool
21
v
5 FRICTION STIR WELDING OF SIMILAR METALS
24
5.1 Experimental Setup
24
5.2 Experimental Procedure
27
5.3 Process Parameters in FSW Process
28
5.4 Microstructure Characterization of welded joints
31
5.4.1 Stir Zone
31
5.4.2 Flow Arm Zone
31
5.4.3 Thermo Mechanically Affected Zone
32
5.4.4 Heat Affected Zone
32
5.5 Hardness Test
33
5.6 Tensile Test
35
6 TUNGSTEN INERT GAS WELDING
36
6.1 Introduction
36
6.2 Experimental procedure
37
6.2.1 TIG Weld Torch
37
6.2.2 Process Parameters
38
6.2.3 Source of Power
38
6.2.4 Operation of TIG Welding
39
6.3 Advantages of TIG Welding
39
6.4 Applications of TIG Welding
40
6.5 Microstructure for TIG Welded joints
40
6.6 Hardness Test
41
6.7 Tensile Test
42
7 FATIGUE BEHAVIOUR OF WELDED JOINTS
43
7.1 Introduction
43
7.2 Fatigue Failure
43
7.3 Fatigue Failure Mechanism
43
7.4 Fatigue Failure Stages
44 vi
7.5 Fatigue Test Specimen
45
7.6 Fatigue Measurement Assembly
45
7.7 Fatigue Measurement Instrument
45
7.8 Procedure
46
7.9 Observation
46
8 CONCLUSIONS
48
9 REFRENCES
49
vii
LIST OF TABLES
3.1 Components used in Fixture design
9
3.2 Chemical Composition of Carbon Steel A36
10
3.3 Chemical Composition of E24 Steel
10
3.4 Chemical Composition of E8 Steel
11
4.1 Tool Design Dimension
20
4.2 Chemical Composition of H13 Steel
20
4.3 Mechanical Properties of H13 Steel
20
5.1 Chemical Properties of AA6061 Aluminium alloy
26
5.2 Mechanical Properties of AA6061 Aluminium alloy
26
5.3 Parameters in FSW Process
29
5.4 Tensile Test Value for FSW joints
35
6.1 Hardness Value of TIG welded joints
41
6.2 Tensile Test Value for TIG welded joint s
42
7.1 Fatigue Test observation for FSW joints
46
7.2 Fatigue Test observation for TIG Welded joints
47
viii
LIST OF FIGURES 3.1 Front view of Fixture Design
8
3.2 Top view of Fixture Design
9
3.3 Channels used in Fixture
9
3.4 Backing Plate of Fixture
10
3.5 Pressure Plate in Fixture
11
3.6 Assembly of Fixture
12
4.1 Schematic diagram of Pressure and Time
14
4.2 Friction Stir Welding Process
14
4.3 Tool for Friction Stir Welding
18
4.4 FSW tool Design with Thread
19
4.5 FSW tool Design without Thread
19
4.6 Quenching of Welding Tool
21
4.7 Welding tool with Thread after quenching process
22
4.8 Welding tool without Thread after quenching process
23
5.1 CNC vertical Milling Machine
24
5.2 Fixture mounted on vertical milling machine
25
5.3 Friction Stir Welding Process
29
5.4 Friction Stir Welded AA6061 aluminum alloy
30
5.5 Heat affected zone in FSW process
31
5.6 Typical Microstructures at different region
32
5.7 Micro Vickers Hardness Tester Apparatus
33
5.8 Hardness value of FSW joints
34
6.1 TIG Welding Machine Apparatus
37
6.2 TIG Welding Torch
37
6.3 TIG Welding Power Source
38
6.4 TIG Welding-Weld Zone
40
6.5 TIG Welding-Interface zone
41
6.6 TIG Welding-Heat Affected zone
41
7.1 Fatigue Testing Specimen
45
7.2 S-N Curve for FSW joint
46
7.3 E-N Curve for FSW joint
47 ix
7.4 S-N Curve for TIG welded joint
47
7.5 E-N Curve for TIG welded joint
48
x
LIST OF ABBREVIATIONS FSW – Friction Stir Welding TIG – Tungsten Inert Gas Welding SZ – Stir Zone HZ – Heat Affected Zone TMAZ – Thermo Mechanically Affected Zone GTAW – Gas Tungsten Arc Welding Mg – Magnesium Al – Aluminium D.C – Direct Current A.C – Alternate Current
xi
CHAPTER 1 INTRODUCTION
1.1 Overview The report deals with the fatigue analysis of welded joints made by the Friction Stir welding process and Tungsten Inert Gas welding process. We have generally welded similar alloys of Aluminium AA6061. Good mechanical properties and low weight make aluminium alloys commonly used engineering materials. However, aluminium has traditionally been viewed upon as difficult to weld with conventional fusion welding. Friction Stir Welding is a solid state joining process developed in 1990 and is nowadays frequently used to join aluminium alloys. The FSW process is fast and it can easily be automated. This lowers production time and manufacturing cost. It is also seen to provide superior joint integrity. While Tungsten Inert Gas Welding also known as Gas Tungsten Arc Welding is one of the popular welding process usually employed in the fabrication thin structures. The process is quite simple – The electric arc is initiated between the base metals and tungsten electrode (non consumable) and suitable filler wire rod is introduced in the weld pool. The electric DC amperage is provided to the welding process to initiate the arc and the shielding gas either argon or helium gas protects the molten metal from the contaminated. Previous studies indicate that the fatigue properties of the TIG welds differ significantly from those of conventionally welded. Due to this the level of fatigue strength and also the slope of S-N curves for laser welded joint is different compared to existing fatigue design standards for arc welding. In transportation and under varying load conditions fatigue failure is an important issue. There are many factors that make the weld critical under fatigue loading conditions. For instance, stress concentrations such as weld toe and weld root, residual stresses, unfavourable mechanical properties of the weld nugget and potential defects in the weld are major causes of weld failure in service. Weld failure leads to loss of lives and substantial costs each year all over the world. 1
Fatigue Analysis helps identify how repetitive load cycles cause structural failures. It helps you identify failures in components subjected to stresses less than yield and do not experience plastic deformation and have relatively long lives. This type of usage is commonly referred to as high-cycle fatigue. For ductile metals, high-cycle fatigue is generally considered to be greater than 100,000 cycles of operation. Calculation of high cycle fatigue requires S-N curve or stress versus number of cycles curve, in addition to linear static stress data. Whether welding together a few relatively simple parts or fabricating large, complex structures, weld fatigue is likely to be the mostcommon failure mode if the part or structure is subjected to fluctuating stresses. 1.2 Aim of the work During recent years several investigations have been made of fatigue properties of friction stir welded joints. The great majority of available data from the fatigue analysis of friction stir welded joints are concerned with uniaxial loading conditions for a simple geometry. In uniaxial loading nominal stress is normally used as reference stress and it is easy to determine. However, fatigue failure is a highly localized phenomenon in engineering components and determining the nominal stress is not always possible due to the complexity of structures and presence of stress concentrators such as notches and cracks in which many approaches based on local parameters. The aim of the project was to perform fatigue analysis of welded joints covering the following issues:
Systematic designing and fabrication of fixture for Friction Stir Welding (FSW) process.
Designing and fabrication of welding tool for carrying out FSW process.
Evaluation of parameters of FSW and Laser Welding for proper joining of plates.
Experimental fatigue analysis of welded profiles.
Influence of welding procedure on fatigue life of welded joints.
2
CHAPTER 2 LITERATURE SURVEY
M. Czechowski - Low-cycle fatigue of friction stir welded Al–Mg alloys. The author’s in this paper [1] proposes the following alloys EN-AW 5058 H321 and EN-AW 5059
H321 (Alustar) were welded by FSW (friction stir welding) method. The FSW welds showed better properties in comparison to the joints welded by the MIG method. The test of microstructure proved the proper structure of the weld which consisted of following: welded nugget, thermo-mechanically affected zone (TMAZ), heat affected zone (HAZ) and unaffected material. The surface microanalysis (EDS) showed particles containing Fe and Mn and particles containing Si and Mg in the weld nugget of 5083 alloy. These are likely to be the phases: (Mn, Fe)Al7 and Mg2Si. Particles containing Fe and Mn were visible in the weld nugget of the Alustar alloy. The lowcycle fatigue life test was carried out in the symmetric cycle: in air and in 3.5% NaCl. Fatigue life of 5083 alloy welded by FSW method, exposed in 3.5% NaCl solution was lower than that of the specimens tested in air. The Alustar alloy welded by the new FSW method demonstrated higher fatigue life in comparison to the same alloy welded by the traditional MIG method. The fatigue zone showed cleavage fracture, changing into a ductile cracking on the fracture faces of the tested specimens D.R. Ni, D.L. Chen, J. Yang, Z.Y. Ma - Low cycle fatigue properties of friction stir welded joints of a semi-solid processed AZ91D magnesium alloy. The author’s in this paper [2] proposes a semi-solid processed (thixomolded) Mg–9Al–1Zn magnesium
alloy (AZ91D) was subjected to friction stir welding (FSW), aiming at evaluating the weldability and fatigue property of the FSW joint. Microstructure analysis showed that a recystallized fine-grained microstructure was generated in the nugget zone (NZ) after FSW. The yield strength, ultimate tensile strength, and elongation of the FSW joint were obtained to be 192 MPa, 245 MPa, and 7.6%, respectively. Low-cycle fatigue tests showed that the FSW joint had a fatigue life fairly close to that of the BM, which could be well described by the Basquin and Coffin-Manson equations. Unlike the extruded magnesium alloys, the hysteresis loops of FSW joint of the 3
thixomolded AZ91D alloy were basically symmetrical, while the non-linear or pseudo elastic behaviour was still present. The FSW joint was observed to fail in the BM section rather than in the NZ. Fatigue crack initiated basically from the pores at or near the specimen surface, and crack propagation was mainly characterized by fatigue striations along with the presence of secondary cracks. G. Padmanaban, V. Balasubramanian, G. Madhusudhan Reddy - Fatigue crack growth behaviour of pulsed current gas tungsten arc, friction stir and laser beam welded AZ31B magnesium alloy joints. The author’s in this paper [3] proposes the laser beam welded joints offered better resistance against the growing fatigue cracks compared to friction stir welded and pulsed current gas tungsten arc welded AZ31B magnesium alloy joints. The formation of very fine grains in weld region, higher fusion zone hardness, uniformly distributed fine precipitates and favourable residual stress field of the weld region are the main reasons for superior fatigue performance of laser beam welded joints of AZ31B magnesium alloy. Michael Besel, Yasuko Besel, Ulises Alfaro Mercado, Toshifumi Kakiuchi, Yoshihiko Uematsu - Fatigue behaviour of friction stir welded Al–Mg–Sc alloy. The author’s in this paper [4] proposes the effect of Friction Stir Welding on the fatigue
behaviour of Al–Mg–Sc alloy has been studied. To reveal the influence of the welding parameters, different travel speeds of the welding tool have been used to provide weld seams with varying microstructural features. Crack initiation as well as crack propagation behaviour under fatigue loading has been investigated with respect to the local microstructure at the crack initiation sites and along the crack path. Fatigue cracks were mostly initiated around the stir zone and the adjacent thermo-mechanical affected zone independent from hardness distributions in the weld seams. In some specimens, defect-like feature was observed at the crack origins, which shortened the fatigue lives. It has been found that while the effect of the tool travel speed on the fatigue lifetime seems to be little, the varying and complex local microstructure in the weld seam basically affects both the crack initiation sites and the crack propagation paths.
4
L. Boni, A. Lanciotti, C. Polese - ‘‘Size effect’’ in the fatigue behaviour of Friction Stir Welded plates. The author’s in this paper [5] proposes a Comparative fatigue tests were carried out on Friction Stir Welded specimens of a 2195-T8 aluminum– lithium alloy that differed significantly in width. The width of the larger specimens was over thirteen times greater than that of the small specimens. Fatigue results showed a clear ‘‘size effect’’, i.e. fatigue life of large specimens was about 40% of the corresponding value of small specimens. The Equivalent Initial Flaw Size methodology was adopted to correlate the two sets of results. Fatigue crack initiation life was disregarded with respect to crack propagation life, and fatigue life was evaluated only as propagation of a small pre-existing defect. Following this methodology, test results of small specimens were used to evaluate the initial equivalent flaw contained in each specimen. It was assumed that this data followed a normal distribution. The equivalent initial flaw in larger specimens was evaluated by simple geometrical considerations. A very good assessment of mean fatigue life and scatter in the fatigue results of large specimens was obtained by simulating the propagation of these defects. Calculations were carried out by taking also welding residual stresses into account, but the results demonstrated that this effect was not significant. Athanasios Toumpis, Alexander Galloway, Lars Molter, Helena Polezhayeva Systematic investigation of the fatigue performance of a friction stir welded low alloy steel. The author’s in this paper [13] proposes A comprehensive fatigue performance assessment of friction stir welded DH36 steel has been undertaken to address the relevant knowledge gap for this process on low alloy steel. A detailed set of experimental procedures specific to friction stir welding has been put forward, and the consequent study extensively examined the weld microstructure and hardness in support of the tensile and fatigue testing. The effect of varying welding parameters was also investigated. Micro structural observations have been correlated to the weldments fatigue behaviour. The typical fatigue performance of friction stir welded steel plates has been established, exhibiting fatigue lives well above the weld detail class of the International Institute of Welding even for tests at 90% of yield strength, irrespective of minor instances of surface breaking flaws which have been identified. An understanding of the manner in which these flaws impact on the fatigue performance has been established, concluding that surface breaking irregularities such
5
as these produced by the tool shoulder’s features on the weld top surface can be the dominant factor for crack initiation under fatigue loading.
6
CHAPTER 3 DESIGN AND FABRICATION OF FIXTURE
3.1 Importance of Fixture Design The design of fixture plays an important role in Friction Stir Welding process (FSW). Proper designing of fixture is the one of the major solution to the problems arising during FSW process. FSW of aluminium alloys require a careful designing of both fixture and welding tool. The fixture should be designed and fabricated in such a way that it is able to bear the high magnitude forces and high temperature during welding process. While designing the fixture certain things are kept in mind such as proper spacing given for accommodation of backing plate and the metal to be welded. Fixture is designed to support the metal plates to be joined. Fabrication of fixture is done by different machining process. The main function of fixture is to prevent the dislocation of specimen from their initial position during welding process. Another function is to maintain stability during the welding process in order to avoid any vibration effects produced during carrying out welding. 3.2 Designing of Fixture The following points should be kept in mind while designing fixture: Fixture must hold the work pieces in correct relationship during joining and it must assist and control the joining process by affording adequate support. Fixture used for hot processes must not only withstand the temperature involved, but in many cases must either accelerate or retard the flow of heat. Fixture should be designed in such a way that their heat expanded dimensions remain functional. Fixture should be designed in such a way that there is proper accommodation of both backing plate and the plate to be welded. For holding plates Clamps are required. So holes for Clamps should be screwed at suitable places. For preventing plates to get displaced from their initial position, Key is 7
required. So during designing proper Grooving is to be considered. Fixture sh should be designed in such a way that it is properly mounted on the bed of Vertical Milling Machine for carrying out welding process. Factors to be considered for fixture designing are following:
Cost of tool.
Size of the production run and rates.
Complexity of the weld.
Quality required in the weldment.
Dimensional Tolerances.
The design we made for fixture for executing our FSW project is following: Channel
Bolts Specimen plate
Pressure Plate
Backing Plate
Figure 3.1 Front view of fixture
8
Figure 3.2 Top view of fixture design Table 3.1 Components used in fixture design Components
Material Type
Quantity
1. Channel
Carbon Steel A-36
2
2. Backing Plate
EN 24 Steel
1
3. Pressure Plate
EN 8 Steel
2
4. Plates to be welded
Aluminium A 6061
2
Channel used in fabrication of fixture is made up of Carbon Steel A A-36. The schematic diagram of Channel is following:
Figure 3.3 Channel used in fixture design
9
Table 3.2 Table of Chemical Composition of Carbon Steel A36: Element
Content (%)
Carbon
0.25-0.29
Copper
0.20
Iron
98
Manganese
1.03
Phosphorous
0.040
Silicon
0.28
Sulphur
0.050
Backing plate used in fabrication of fixture is made up of EN24 Steel grade.
Figure 3.4 Backing plate of fixture Table 3.3 Table of Composition of EN24 Steel: Element
Content
Carbon
0.36
Silicon
0.10-0.35
Manganese
0.45-0.70
Sulphur
0.040 Max
Phosphorous
0.035 Max
Chromium
1.00-1.40
Nickel
1.30-1.70
10
Pressure plate used in fixture is made up of EN8 Steel grade. The main function of pressure plate is to provide normal pressure to prevent the movement of the specimens to be welded from their original position.
Figure 3.5 Pressure plate in fixture design
Table 3.4 Table Chemical Composition of E8 Steel grade: Element
Content (%)
Carbon
0.36-0.44
Silicon
0.10-0.40
Manganese
0.60-1.00
Sulphur
0.050 Max
Phosphorous
0.050 Max
3.2.1 Objective of fixture design: Following are the Fixture designing objectives: To provide proper heat control of weld zone. To hold the part in the most convenient position for welding. To provide suitable clamping to prevent distortion. To provide channels and outlets for welding atmosphere. To provide for ease of operation and maximum accessibility to the point of weld.
11
3.2.2 Fabrication of fixture: After setting of various parameters for Fixture designing, its fabrication was carried out by following operations: Grinding operation for good surface finishing: Grinding operation was carried out on backing plate. The main motive behind grinding was to provide smooth and even surface. If surface is uneven then the specimen to be welded will align at certain angle to each other which will result improper welding. Drilling operation on drilling machine: Holes were made on channels to accommodate bolts. Tool bits of different sizes were used for drilling operation as per the design specification. Two holes were drilled on each channel to accommodate bolts. Shaping operation on shaper machine: In order to create grooves on various positions with at certain distances, shaper machine was used. Milling operation on work pieces: Milling operation was carried out on work pieces to provide sharp edges and right angled edges. This operation was carried out on horizontal milling machine. The tool used during milling operation was high speed steel cutter. Fixture having good design kinematic-ally restrains the work pieces. An operational Fixture should maintain the work piece stability during the welding process. Design specifications, Factory standards, Economy, Ease of use and safety are various design criteria which must be observed during the procedure of Fixture design.
Figure 3.6 Assembly of fixture design
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CHAPTER 4 FUNDAMENTALS OF FRICTION STIR WELDING
4.1 Introduction: Friction Stir Welding (FSW) is a solid state welding process which produces welds due to the compressive force contact of work pieces which are either rotating or moving relative to each other. The heat required to join different specimens is generated by heating due to friction at the interface. The main advantages of friction stir welding are following: It is environment friendly process. There is no generation of fumes, smoke or gases. Removal of oxides is possible after the welding process. The automation of this process is possible for mass production. The process is in solid state with narrow heat affected zone (HAZ) as material is not melted during the welding process. The weld strength is stronger than the weaker of two materials joined. 4.2 Applications The industrial application of FSW include following: Construction: Bridges, reactors for power industries, pipelines. Railway: High speed trains, container bodies, railway tankers, good wagon. Aerospace: Wings, fuselage, cryogenic fuel tanks, aviation fuel tanks, aircraft structure. Automotive: Engine and chassis cradles, wheel rims, tailored blanks, armour plate vehicles Friction stir welding (FSW) was invented at The Welding Institute (TWI) of UK in 1991 as a solid-state joining technique, and was initially applied to aluminium alloys.
13
Figure 4.1 A schematic diagram of pressure and time In friction stir welding, the pressure exceeds the yield strength of the work piece and weld cycle is fairly short. The basic concept of FSW is remarkably simple. Figure 4.2 illustrates process definitions for the tool and work piece in a butt joint configuration. A non-consumable rotating tool with a specially designed pin and shoulder is inserted into the abutting edges of sheets or plates to be joined and subsequently traversed along the joint line. The definitions of all terms are included in the taxonomy section. In this illustration, the FSW tool rotates in the counter clockwise direction and travels into the page (or left to right). The advancing side is on the right where the tool rotation direction (sense of tangential velocity) is the same as the tool travel direction and the retreating side is on the left where the tool rotation (sense of tangential velocity) is opposite the tool travel direction. Friction stir processing (FSP) is a generic adaptation of FSW, where the tool is traversed along a desired path to modify the microstructure rather than joining two pieces.
Figure 4.2 Friction stir welding process
14
Conceptually the FSP runs are similar to bead-on-plate runs. Although most of the fundamental concepts are similar, certain aspects are purposely changed in FSP to achieve the desired processing goal. For example, in friction stir welding one may want to maximize the traverse rate to increase the production rate. If one takes some of the faster examples to date such as traverse rate of 1–3 m/min for Al 5XXX alloys, at a tool rotation rate of 1,000 rpm, this would give advance per revolution as 1–3 mm/rev. The shear gradient in such runs would be very high and resultant micro structural gradient would lead to inhomogeneous grain structure. This may not matter for the joint property in terms of room temperature strength. However, if the intention is to use FSP for super plasticity, the goal would be to obtain fine grains and uniform micro structural distribution. For this, the traverse speed and tool rotation rate needs to be carefully selected to obtain fine and uniform grain size. The tool serves three primary functions, i.e., (a) heating of the work piece, (b) movement of material to produce the joint, and (c) containment of the hot metal beneath the tool shoulder. Heating is created within the work piece by friction between both the rotating tool pin and shoulder and by severe plastic deformation of the work piece. The localized heating softens material around the pin, and combined with the tool rotation and translation leads to movement of material from the front to the back of the pin thus filling the hole in the tool’s wake as the tool moves forward. The tool shoulder restricts metal flow to a level equivalent to the shoulder position, i.e., approximately to the initial work piece top surface. As a result of the tool action and influence on the work piece, when performed properly, a ‘solid state’ joint is produced, i.e., no melting. Because of various geometrical features on the tool, material movement around the pin can be complex with gradients in strain, temperature, and strain rate. Accordingly, the resulting nugget zone microstructure reflects these different thermo mechanical histories and is not homogeneous. In spite of the local micro structural in homogeneity, one of the significant benefits of this ‘solid state’ welding technique is the fully recrystallized, equi-axed, fine grain microstructure created in the nugget by the intense plastic deformation at elevated temperature. As will be seen within these chapters, the fine grain microstructure produces excellent mechanical properties, fatigue properties, enhanced formability and exceptional super plasticity.
15
4.2.1 Applicability of Friction Stir Welding: Friction stir welding is considered to be the most significant development in metal joining in decades and in addition is a “green” technology due to its energy efficiency, environmental friendliness, and versatility. As compared to the conventional welding methods, FSW consumes considerably less energy, no consumables are use such as a cover gas or flux, and no harmful emissions are created during welding, thereby making the process environmentally friendly. Further, since FSW does not involve the use of filler metal and because there is no melting, any aluminium alloy can be joined without concern for compatibility of composition or solidification cracking; issues associated with fusion welding. Also, dissimilar aluminium alloys and composites can be joined with equal ease. In contrast to traditional friction welding, a welding process limited to small axisymmetric parts that can be rotated and pushed against each other to form a joint, friction stir welding can be applied to most geometric structural shapes and to various types of joints such as butt, lap, T-butt, and fillet shapes. The most convenient joint configurations for FSW are butt and lap joints. A simple square butt joint is two plates or sheets with the same thickness are placed on a backing plate and clamped firmly to prevent the abutting joint faces from being forced apart. The backing plate is required to resist the normal forces associated with FSW and the work piece. During the initial tool plunge, the lateral forces are also fairly large and extra care is required to ensure that plates in the butt configuration do not separate. To accomplish the weld, the rotating tool is plunged into the joint line and traversed along this line while the shoulder of the tool is maintained in intimate contact with the plate surface. Tool position and penetration depth are maintained by either position control or control of the applied normal force. 4.3 Macroscopic process in Friction Stir Welding: For any manufacturing process, understanding its fundamental process mechanisms is vital for its long-term growth. Unlike fusion based joining processes, there is no perceptible melting during friction stir welding (FSW). From the operational viewpoint, a friction stir welding run can be divided into three sub-procedures or phases: (a) plunge and dwell, 16
(b) traverse and (c) retract. At the start of the plunge phase, both the tool and the work piece are at ambient temperature (except the region surrounding tool and work piece interface). When the rotating friction stir tool is gradually inserted into the work piece, the material is too cold to flow and the rubbing action creates chipping as in any machining process. The rate of insertion determines the rate of temperature rise and extent of plasticity. The process of tool insertion continues until the tool shoulder is in intimate contact with the work piece surface. At this stage, the entire tool shoulder and pin surface contribute to the frictional heating and the force starts to drop as the metallic work piece reaches critical temperature for plastic flow. For metals with higher melting point, the rotating tool is sometimes intentionally retained at this position for short durations so as to reach the desired temperature required for plastic flow. This is known as the dwell phase and is typically a fraction of the time required for plunge phase. Typically, the plunge stage is programmed for controlled plunge rate (i.e. vertical position controlled FSW) but it can be also done by controlling the force applied on the tool along its rotation axis (i.e. force controlled FSW). Of course, any combination of displacement and force controlled approach is possible. For a typical FSW run, the vertical force reaches a maximum value in this part of the run and this tends to be critical phase for the tool. 4.4 Tool Material 4.4.1 Tool Material Selection In this section we discuss the basic conceptual approach of tool material selection and then build towards the tool features and the framework for selection. Figure 4.1 schematically shows tool and labels different part of a friction stir welding tool along with illustration of the friction stir welding process. The tool has been shown again in Fig. 4.3 and illustrates the different aspects of the tool vividly.
17
Figure 4.3 Tool for FSW Process The tool material needs six basic characteristics, (1) strength at ambient temperature and process temperature, (2) fatigue life at process temperature, (3) fracture toughness, (4) wear characteristics, (5) long term thermal stability, and (6) chemical stability (no or limited reaction with work piece). In the case of aluminium alloys, popular tool materials have been H13 tool steel and MP159 cobalt base super alloy. Both these tool materials would be classified as tool material A type. That is, their strength is higher than the work piece at all temperatures. The tool material can still have issues. Wear is a particular concern and weaker material can still induce wear particularly in certain condition. If the tool is made of H13 tool steel, one can find the life for the calculated peak stress at pin root by taking the S-N curve data at 500 C. The tool material selection becomes more challenging for high temperature materials, like steels, nickel base alloys and titanium alloys. 4.5 Tool Design: The tool we used during Friction Stir welding process were made up of H13 Steel and one of the tool was with thread and the other tool was without thread. The figure 4.4 represents the tool with thread and the figure 4.5 represents the tool without thread.
18
Figure 4.4 FSW tool design with thread
Figure 4.5 FSW Tool design without thread
19
4.5.1 Tool Design Dimensions: The below table shows the value of Tool Pin diameter, Tool Pin design and Shoulder diameter: Table 4.1 Tool design dimensions Tool Dimensions
Values in mm
Tool Pin diameter
6
Tool Pin length
3.9
Shoulder Diameter
18
4.6 Tool Material Properties: The below tables represents the Chemical and Mechanical properties of the H13 tool: Table 4.2 Chemical Properties of H13 Steel Components
Value (Weight %)
Carbon
0.32-0.45
Chromium
4.75-5.5
Manganese
0.2-0.5
Molybdenum
1.1-1.75
Phosphorus
0.3max
Silicon
0.8-1.12
Sulphur
0.03max
Table 4.3 Mechanical Properties of H13 Steel Components
Properties
Ultimate tensile Strength
1990Mpa
Yield Strength
1650Mpa
Elongation at break
9%
Modulus of elasticity
210Gpa
20
4.7 Heating Process of Weld Tool: After fabrication of weld tool, Quenching process is carried out. In materials science, quenching is the rapid cooling of a work piece to obtain certain material properties. A type of heat treating, quenching prevents undesired low-temperature processes, such as phase transformations, from occurring. It does this by reducing the window of time during which these undesired reactions are both thermodynamically favourable, and kinetically accessible; for instance, quenching can reduce the crystal grain size of both metallic and plastic materials, increasing their hardness.
Figure 4.6 Quenching of weld tool In metallurgy, quenching is most commonly used to harden steel by introducing martensite, in which case the steel must be rapidly cooled through its eutectoid point, the temperature at which austenite becomes unstable. In steel alloyed with metals such as nickel and manganese, the eutectoid temperature becomes much lower, but the kinetic barriers to phase transformation remain the same. This allows quenching to start at a lower temperature, making the process much easier. High speed steel also has added tungsten, which serves to raise kinetic barriers and give the illusion that the material has been cooled more rapidly than it really has. Even cooling such alloys slowly in air has most of the desired effects of quenching.
21
Although it may sound counterintuitive, the controlled cooling of a hot metal part is every bit as important as the furnace stage, for this heat treatment method controls phase changes on the downward arc of the thermal cycle. Hardening is managed in this manner, as is the deadlocking of heated metal states. Of course, the quenching bath must be carefully used, for a substandard cooling station can introduce material cracks and parts deformation. Furthermore, a superior quenching station is a highly adaptable heat treatment asset, with oil and salt baths transforming the metal part into other, more desirable grains. These are the bainite and pearlite forms, the grains that emphasize certain carbon-rich lattices while minimizing others. A well-supervised metal quenching station universally hardens a component and prepares it for tempering. Further fluid control elements, including oil and brine, enhance mastery over this rapid cooling phase, thus breathing a versatile state-altering mechanism into the process.
Figure 4.7 Weld tool with thread after quenching process
22
Figure 4.8 Weld tool without thread after quenching process
23
CHAPTER 5
FRICTION STIR WELDING OF SIMILAR METALS
5.1 Experimental Setup: The following components were parts of Experimental Setup for FSW process: 1) Vertical Milling Machine 2) Fixture 3) Backing Plate 4) Tool 5) Specimen
Figure 5.1 CNC Vertical Milling Machine
24
In Chapter 2, we have already discussed about Fixture designing. So the overall setup of fixture is mounted on the bed of Vertical Milling Machine to carry out the Friction Stir Welding Process as shown in the below diagram:
Welding tool Specimen plate
Channel
Bolt
Pressure Plate
Figure 5.2 Fixture mounted on Vertical Milling Machine In our project we used Aluminium AA6061 alloy plates and Magnesium AZ61a alloy plates.
25
Table for Chemical and Mechanical properties of Aluminium AA6061 alloy are given below Table 5.1 Chemical Properties of Aluminium AA6061 alloy Components
Amount (weight %)
Magnesium
0.8-1.2
Silicon
0.4-0.8
Iron
Max 0.7
Copper
0.15-0.40
Zinc
Max 0.25
Titanium
Max 0.15
Aluminium
Balance
Table 5.2 Mechanical properties of Aluminium AA6061 Properties
Values
Modulus of Elasticity
70-80GPa
Ultimate Tensile Strength
110-150Mpa
0.2% Proof Stress
65-150Mpa
Elongation 50mm diameter (%)
14-16Mpa
Since the friction welding process is energy efficient, the welding of Non- Ferrous metals such as aluminium and magnesium is drawing more attention. The aluminium is preferred mostly since it has more workability and it is economical. Typical applications
for
aluminium
alloy
6061
include: Aircraft
and
aerospace
components, Marine fittings, Transport, Bicycle frames, Camera lenses and Drive shafts. On the other hand application of magnesium AZ61a alloy includes: Hollow shape extrusion, Solid extrusions, Bearings caps, Bearing housings, Fitting, Rocker arm supports and Screw machine components.
26
5.2 Experimental Procedure: The flow chart for carrying out Friction Stir Welding work is given below:
Start
Strip Positioning
Tool Plunge adjustment
Tooling Traverse
YES
Visual Inspection of weld
NO Pull out/ Run off
Mechanical Testing
End
27
Firstly, the fixture was mounted on the bed of vertical milling machine. Secondly, the plates to be welded were placed on the fixture. Thirdly, the weld tool was inserted in the spindle of the Vertical Milling Machine. After that rotational speed and feed rate of varying parameter were given value in the machine. Only one number of pass was carried out during Friction Stir welding process. Quality of weld was observed. If the welding was proper then the process was carried out on same parameters. Otherwise, the parameters were changed for sound welding.
Figure 5.3 Friction stir welding process The dimensions of plates used during FSW process were of specification 150 × 100 × 4 mm. The sides and edges of the plates were properly filed in order to remove the burrs and hence smooth surface finishing was imparted on the sides which were to be welded. If proper filing is not done then there will be no proper welding and there will be some gaps after the welding process. 5.3 Process parameters in FSW process: The various parameters like Welding speed, rotational speed of weld tool and depth of plunging can be varied in order to get good joint.
28
Below table represents the process parameters: Table 5.3 Parameters in FSW process
Welding
Rotational
Welding
Tool
Depth of
Run no
Speed(rpm) Speed(mm/m) Diameter(mm) Plunging(mm)
1
1600
40
6
3.75
2
1600
40
6
3.60
3
1600
35
6
3.72
4
1750
45
6
3.65
5
1800
50
6
3.70
Figure 5.4 Friction stir welded Aluminium AA6061 Alloys
High welding speed over a wide range can be used to weld this material at high tool revolution rates indicating the great potential of this technique for magnesium alloys. Equi-axed grains were observed in the stir and thermo-mechanically-affected zones. The grain size in the stir zone decreases with increasing welding speed due to lower heat input. Higher welding speeds produce slightly higher hardness in the stir zone. The yield strength increases with increasing welding speed. The tensile strength increases first with increasing welding speed but remains constant from 15 to 30 mm/s.
29
There are two tool speeds to be considered in friction-stir welding; how fast the tool rotates and how quickly it traverses along the interface. These two parameters have considerable importance and must be chosen with care to ensure a successful and efficient welding cycle. The relationship between the rotation speed, the welding speed and the heat input during welding is complex but, in general, it can be said that increasing the rotation speed or decreasing the traverse speed will result in a hotter weld. In order to produce a successful weld it is necessary that the material surrounding the tool is hot enough to enable the extensive plastic flow required and minimize the forces acting on the tool. If the material is too cold then voids or other flaws may be present in the stir zone and in extreme cases the tool may break. Excessively high heat input, on the other hand may be detrimental to the final properties of the weld. Theoretically, this could even result in defects due to the liquation of low-melting-point phases (similar to liquation cracking in fusion welds). These competing demands lead onto the concept of a "processing window": the range of processing parameters viz. tool rotation and traverse speed, which will produce a good quality weld. Within this window the resulting weld will have a sufficiently high heat input to ensure adequate material plasticity but not so high that the weld properties are excessively deteriorated. The plunge depth is defined as the depth of the lowest point of the shoulder below the surface of the welded plate and has been found to be a critical parameter for ensuring weld quality. Plunging the shoulder below the plate surface increases the pressure below the tool and helps ensure adequate forging of the material at the rear of the tool. Tilting the tool by 2–4 degrees, such that the rear of the tool is lower than the front, has been found to assist this forging process. The plunge depth needs to be correctly set, both to ensure the necessary downward pressure is achieved and to ensure that the tool fully penetrates the weld. Given the high loads required, the welding machine may deflect and so reduce the plunge depth compared to the nominal setting, which may result in flaws in the weld. On the other hand, an excessive plunge depth may result in the pin rubbing on the backing plate surface or a significant under match of the weld thickness compared to the base material. Variable load welders have been developed to automatically compensate for changes in the tool displacement while TWI has demonstrated a roller system that maintains the tool position above the weld plate. 30
5.4 Microstructure Characteristics of welded joints: The solid-state nature of the FSW process, combined with its unusual tool shape and asymmetric speed profile, results in a highly characteristic microstructure.
Figure 5.5 Heat affected zones in welding 5.4.1 Stir Zone The microstructure can be broken up into the following zones: The stir zone (also nugget, dynamically recrystallized zone) is a region of heavily deformed material that roughly corresponds to the location of the pin during welding. The grains within the stir zone are roughly equi axed and often an order of magnitude smaller than the grains in the parent material. A unique feature of the stir zone is the common occurrence of several concentric rings which has been referred to as an "onion-ring" structure. The precise origin of these rings has not been firmly established, although variations in particle number density, grain size and texture have all been suggested. 5.4.2 Flow Arm Zone The flow arm zone is on the upper surface of the weld and consists of material that is dragged by the shoulder from the retreating side of the weld, around the rear of the tool, and deposited on the advancing side.
31
5.4.3 Thermo-mechanically affected zone The thermo-mechanically affected zone (TMAZ) occurs on either side of the stir zone. In this region the strain and temperature are lower and the effect of welding on the microstructure is correspondingly smaller. Unlike the stir zone the microstructure is recognizably that of the parent material, albeit significantly deformed and rotated. Although the term TMAZ technically refers to the entire deformed region it is often used to describe any region not already covered by the terms stir zone and flow arm. 5.4.4 Heat affected Zone The heat-affected zone (HAZ) is common to all welding processes. As indicated by the name, this region is subjected to a thermal cycle but is not deformed during welding. The temperatures are lower than those in the TMAZ but may still have a significant effect if the microstructure is thermally unstable. In fact, in age-hardened aluminium alloys this region commonly exhibits the poorest mechanical properties.
Figure 5.6 Typical microstructures at different regions: (a) HAZ, (b) TMAZ, (c) BM, and (d) NZ
32
5.5 Hardness test: Hardness is a characteristic of a material, not a fundamental physical property. It is defined as the resistance to indentation, and it is determined by measuring the permanent depth of the indentation. More simply put, when using a fixed force (load) and a given indenter, the smaller the indentation, the harder the material. Indentation hardness value is obtained by measuring the depth or the area of the indentation using one of over 12 different test methods. The Vickers hardness test method, also referred to as a micro hardness test method, is mostly used for small parts, thin sections, or case depth work. The Vickers method is based on an optical measurement system. The Micro hardness test procedure, ASTM E-384, specifies a range of light loads using a diamond indenter to make an indentation which is measured and converted to a hardness value. It is very useful for testing on a wide type of materials as long as test samples are carefully prepared. A square base pyramid shaped diamond is used for testing in the Vickers scale.
Figure 5.7 Micro Vickers hardness apparatus
Typically loads are very light, ranging from a few grams to one or several kilograms, although "Macro" Vickers loads can range up to 30 kg or more. The Micro hardness methods are used to test on metals, ceramics and composites almost any type of 33
material. Since the test indentation is very small in a Vickers test, it is useful for a variety of applications: testing very thin materials lik likee foils or measuring the surface of a part, small parts or small areas, measuring individual microstructures, or measuring the depth of case hardening by sectioning a part and making a series of indentations to describe a profile of the change in hardness. The Vickers method is more commonly used. Sample preparation is usually necessary with a micro hardness test in order to provide a small enough specimen that can fit into the tester. Additionally, the sample preparation will need to make the specimen’s su surface smooth to permit a regular indentation shape and good measurement, and to ensure the sample can be held perpendicular to the indenter. Usually the prepared samples are mounted in a plastic medium to facilitate the preparation and testing. The indentations tions should be as large as possible to maximize the measurement resolution. (Error is magnified as indentation sizes decrease) The test procedure is subject to problems of operator influence on the test results.
Microhardness (HV)
82 81 80 79 78 77 76 75 74 73 72 71
-15
-10
-5
0
5
10
Distance from weld center (mm)
Figure 5.8 Hardness value of FSW joints
34
15
5.6 Tensile Test Table 5.4 Tensile Test Value of FSW Welded joints Ultimate FSW joint Area mm2
Ultimate
Gauge
Tensile
0.2%
%
Load
Length
Strength
yield
Elongation
(KN)
(mm)
(MPa)
Strength (MPa)
Specimen
58.18
17.05
50
293
170
8.8
57.87
16.88
50
290.65
165
8.3
1 Specimen 2
35
CHAPTER 6 TUNGSTEN INERT GAS WELDING
6.1 Introduction Tungsten Inert Gas welding (TIG), which is usually called Gas tungsten arc welding (GTAW), is an arc welding proce\ss that employs the heat generated by an electric arc between a non consumable tungsten electrode and the work piece. A filler rod may be fed to the arc zone. A shielding of inert gas (argon or helium) is used to avoid atmospheric contamination of the molten weld pool. It consists of Power supply; either D.C or A.C. to produce arc. A gas supply unit with pressure gauge and flow meter, a gas regulator Fitted to the gas cylinder to control the flow of gas. A TIG welding torch Special type a non-consumable tungsten electrode available with different tip shapes. A filler rod which supplies the filler metal at weld pool. Gas tungsten arc welding (GTAW) is a process in which the joining of metals is produced by heating therewith an arc between a tungsten (non consumable) electrode and the work. A shielding gas is used, normally argon done with pure tungsten or tungsten alloy rod, but multiple electrodes are sometimes used. The heated weld zone, molten metal, and tungsten electrode are shielded from the atmosphere by a covering of inert gas fed through the electrode holder. Filler metal may or may not be added. A weld is made by applying the arc so that the touching work piece and filler metal are melted and joined as the weld metal solidifies. This process is similar to other arc welding processes in that the heat is generated by an arc between a non consumable electrode and the work piece, but the equipment and electrode type distinguish it from other arc welding processes.
36
Fig 6.1 TIG Welding Machine
6.2 Experimental Procedure 6.2.1 TIG Welding Torch TIG involves the use of specially made electrode holder known as TIG Welding Torch. A tungsten electrode is inserted in the torch. A passage around the electrode is provided for the flow of inert gas to the weld zone. If the current is less than 200 amperes, air cooled torch is used; and for current more than 200 amperes, water cooled torch is used. To produce a good quality weld, tip shape tungsten electrode is chosen according to the type of power supply and the thickness of the metal to be weld.
Fig. 6.2 TIG Torch 37
6.2.2 Process Parameters for TIG Welding Power Source: D.C. (DCSP) Current Range: 160 Ampere Voltage Range: 40V Temperature Type: 2700℃ Electrode Type: Non-consumable electrode Electrode Diameter: 0.35mm Electrode Tips: Conical 6.2.3 Source of Power for Tungsten Inert Gas Welding All three types of current supplies (i.e. A.S., DCSP, and DCRP) can be used with TIG welding depending upon the metal to be welded. The choice of which consider the following: DCRP (Direct Current Reverse Polarity): For thin sheets of aluminium and magnesium alloys. DCSP (Direct Current Straight Polarity): For high melting point alloys such as alloy steels, stainless steels, heat-resisting alloys, copper alloys, nickel alloys and titanium. AC (Alternating Current): For normal sheets of aluminium and magnesium.
Fig 6.3 TIG Welding Power Source
38
6.2.4 Operation of TIG Welding Manual gas tungsten arc welding is a relatively difficult welding method, due to the coordination required by the welder. Similar to torch welding, GTAW normally requires two hands, since most applications require that the welder manually feed a filler metal into the weld area with one hand while manipulating the welding torch in the other. Maintaining a short arc length, while preventing contact between the electrode and the work piece, is also important. GTAW weld area GTAW fillet weld to strike the welding arc, a high frequency generator (similar to a Tesla coil) provides an electric spark. This spark is a conductive path for the welding current through the shielding gas and allows the arc to be initiated while the electrode and the work piece are separated, typically about 1.5–3 mm(0.06–0.12 in) apart. Once the arc is struck, the welder moves the torch in a small circle to create a welding pool, the size of which depends on the size of the electrode and the amount of current. While maintaining a constant separation between the electrode and the work piece, the operator then moves the torch back slightly and tilts it backward about 10–15 degrees from vertical. Filler metal is added manually to the front end of the weld pool as it is needed. Welders often develop a technique of rapidly alternating between moving the torch forward (to advance the weld pool) and adding filler metal. The filler rod is withdrawn from the weld pool each time the electrode advances, but it is always kept inside the gas shield to prevent oxidation of its surface and contamination of the weld. Filler rods composed of metals with a low melting temperature, such as aluminium, require that the operator maintain some distance from the arc while staying inside the gas shield. If held too close to the arc, the filler rod can melt before it makes contact with the weld puddle. As the weld nears completion, the arc current is often gradually reduced to allow the weld crater to solidify and prevent the formation of crater cracks at the end of the weld. 6.3 Advantages of TIG Welding Almost all the types of metals and alloys can be weld by this process with suitable selection of power supply i.e., AC, DCSP, or DCRP. Smooth, clean and sound weld is obtained as required in food processing equipment’s. The joints produced by this process are stronger, more ductile and corrosive resistant than produced by other process, because inert gas pushes the air out from the molten metal pool and prevents 39
the oxidation. The arc is transparent due to shielding of inert gas. This enables the welder to clearly observe the work and electrode in the weld puddle. Both ferrous and non-ferrous ous metals can be welded easily. In some cases, dissimilar metals can also be welded easily. 6.4 Applications of TIG Welding Almost all metals and alloys having various thickness and types of joint can be welded. It find its greatest application in the welding of alloy steels, stainless steels, heat-resistance resistance alloys, refractory metals, aluminium um and alloys, magnesium and alloys, titanium alloys, copper and nickel alloys, and steel coated with low melting point alloys. The process is recommended for welding very thin sheets, as this as 0.125 mm (0.005 inch). The process is capable of making smooth, clean and sound weld in aluminium um without the use of corrosive fluxes and it finds its application in Food-processing equipment’s. This process is extensively used in the fabrication of missiles, air-crafts, crafts, rockets and submarines. This process is used for welding commercially pure titanium.
6.5 Microstructure of TIG Welded Aluminium Joint
Figure 6.4 TIG Welding – Weld Zone
40
Figure 6.5 TIG Welding – Interface Zone
Figure 6.6 TIG Welding – Heat Affected Zone 6.6 Hardness Test Table 6.1 Har Hardness Value (HV) of TIG Welded joint Current (A)
Weld Zone
Fusion Zone
Heat Affected Z Zone
160
71.06
76.06
41
80.23
6.7 Tensile Test Table 6.2 Tensile Test Value of TIG Welded joints Ultimate TIG joint
Area
Ultimate
Gauge
Tensile
0.2%
%
mm2
Load
Length
Strength
yield
Elongation
(KN)
(mm)
(MPa)
Strength (MPa)
Specimen
49.6
12.9
50
260
156
5.08
51.1
12.8
50
250
158
5.12
1 Specimen 2
42
CHAPTER 7 FATIGUE BEHAVIOUR OF WELDED JOINTS
7.1 Introduction Fatigue is a phenomenon associated with variable loading or more precisely to cyclic stressing or straining of a material. Just as we human beings get fatigue when a specific task is repeatedly performed, in a similar manner metallic components subjected to variable loading get fatigue, which leads to their premature failure under specific conditions. Fatigue loading is primarily the type of loading which causes cyclic variations in the applied stress or strain on a component. Thus any variable loading is basically a fatigue loading. 7.2 Fatigue Failure: Often machine members subjected to such repeated or cyclic stressing are found to have failed even when the actual maximum stresses were below the ultimate strength of the material, and quite frequently at stress values even below the yield strength. The most distinguishing characteristics is that the failure had occurred only after the stresses have been repeated a very large number of times. Hence the failure is called fatigue failure. ASTM Definition of fatigue is the process of progressive localized permanent structural changes occurring in a material subjected to conditions that produce fluctuating stresses at some point or points and that may culminate in cracks or complete fracture after a sufficient number of fluctuations. Let us first make an attempt to understand the basic mechanism of fatigue failure. 7.3 Fatigue Failure Mechanism: A fatigue failure begins with a small crack; the initial crack may be so minute and cannot be detected. The crack usually develops at a point of localized stress concentration like discontinuity in the material, such as a change in cross section, a keyway or a hole. Once a crack is initiated, the stress concentration effect become greater and the crack propagates. Consequently the stressed area decreases in size, the stress increase in magnitude and the crack propagates more rapidly. Until finally, the 43
remaining area is unable to sustain the load and the component fails suddenly. Thus fatigue loading results in sudden, unwarned failure 7.4 Fatigue failure stages: Three stages are involved in fatigue failure namely -Crack initiation -Crack propagation -Fracture The macro mechanism of fatigue failure is briefly presented now. Crack initiation: Areas of localized stress concentrations such as fillets, notches, key ways, bolt holes and even scratches or tool marks are potential zones for crack initiation. Crack also generally originates from a geometrical discontinuity or metallurgical stress raiser like sites of inclusions. As a result of the local stress concentrations at these locations, the induced stress goes above the yield strength (in normal ductile materials) and cyclic plastic straining results due to cyclic variations in the stresses. On a macro scale the average value of the induced stress might still be below the yield strength of the material. During plastic straining slip occurs and (dislocation movements) results in gliding of planes one over the other. During the cyclic stressing, slip saturation results which makes further plastic deformation difficult. As a consequence, intrusion and extrusion occurs creating a notch like discontinuity in the material. Crack propagation: This further increases the stress levels and the process continues, propagating the cracks across the grains or along the grain boundaries, slowly increasing the crack size. As the size of the crack increases the cross sectional area resisting the applied stress decreases and reaches a thresh hold level at which it is insufficient to resist the applied stress. Final fracture: As the area becomes too insufficient to resist the induced stresses any further a sudden fracture results in the component.
44
7.5 Fatigue Test Specimen:
Fig. 7.1 Fatigue Test Specimen
7.6 Fatigue Measurement Assembly: a) DC motor 220 volt 4.5 amps PMDC motor with extended shaft at both ends b) Flange attached to the motor with lever fixing holes at 5 mm, 7.5 mm, 10 mm, 15 mm from the centre of the flange. c) Position sensor d) Test sample fixing arrangement e) Proximity sensor NPN type 3mm sensing gap f) Anti-vibration vibration mountings g) Strain gauge h) 2pin, 3pin and 4pin connectors 7.7 Fatigue Measurement Instrument: a) Data Acquisition board with 4 line LCD display, Key board, PC interface software, USB cable b) PICOSCOPE 2002 DAQ system. Input from Strain amplifier board Output via another USB cable to PC 45
45
c) Strain amplifier board d) Proximity sense pulse control board e) Position sense amplifier board f) Power ON/OFF switch g) 2pin, 3pin, 4pin connectors 7.8 Procedure a) Mount the test specimen given. b) Paste the strain sensor properly over the test specimen. c) Plug in the power cord to a single phase mains supply. d) Switch on the power to the equipment 7.8 Observations Table 7.1 Fatigue Test observation for FSW joint Motor Speed
Movement
No. of cycle
Stress (MPa)
(HZ)
(rpm)
(mm)
12.49
749
0.215
2609
136
12.87
773
0.217
3479
132
13.23
798
0.217
4063
127
13.81
829
0.223
4751
123
14.01
851
0.265
5628
119
Stress Amplitude (MPa)
Frequency
140 135 130 125 120 115 0
1000
2000
3000
4000
Number of Cycle
Figure 7.2 S-N Curve for FSW joints
46
5000
6000
0.3 Movement (mm)
0.25 0.2 0.15 0.1 0.05 0
-1000
0
1000
2000
3000
4000
5000
6000
Number of cycles (N)
Figure 7.3 E-N Curve for FSW joint Table 7.2 Fatigue Test observation for TIG welded joint Motor Speed
Movement
No. of cycle
Stress (MPa)
(HZ)
(rpm)
(mm)
10.75
645
0.425
3364
125
11.11
666
0.427
3915
122
12.56
719
0.432
4582
116
13.72
785
0.444
5145
113
15.34
920
0.478
5936
109
Stress Amplitude (MPa)
Frequency
126 124 122 120 118 116 114 112 110 108 106 0
1000
2000
3000
4000
5000
Number of cycle
Figure 7.4 S-N Curve for TIG welded joints
47
6000
7000
0.49
Movement (mm)
0.48 0.47 0.46 0.45 0.44 0.43 0.42 0.41 -1000
0
1000
2000
3000
4000
5000
6000
Number of cycles (N)
Figure 7.5 E-N Curve for TIG welded joint
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7000
CHAPTER 8 CONCLUSION
The formation of fine equiaxed grains and uniformly distributed very fine strengthening precipitates in the weld region is the reason for superior tensile property of FSW joints compared to TIG joints. Tensile test shows that FSW joints have higher strength and higher ductility compared to TIG joints. Heat Affected Zone is narrower than TIG welding process. FSW requires less pre operation than TIG welding process. From Industrial perspectives, FSW is very competitive because it saves energy due to less heat input. FSW prevents joints from fusion related defects. FSW has better strength than TIG welding process. Aluminium AA6061-T6 alloy welded by FSW method shows greater fatigue life in comparison to same alloy welded by TIG welding method. Fatigue failure was generally occurred in the zone among flow arm, weld nugget and Thermo Mechanically Affected Zone (TMAZ). Fatigue limits of all joint types were close to each other.
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REFRENCES [1] M. Czechowski Low-cycle fatigue of friction stir welded Al–Mg alloys. [2] D.R. Ni, D.L. Chen, J. Yang, Z.Y. Ma Low cycle fatigue properties of friction stir welded joints of a semi-solid processed AZ91D magnesium alloy. [3] G. Padmanaban, V. Balasubramanian, G. Madhusudhan Reddy Fatigue crack growth behaviour of pulsed current gas tungsten arc, friction stir and laser beam welded AZ31B magnesium alloy joints [4] Michael Besel, Yasuko Besel, Ulises Alfaro Mercado, Toshifumi Kakiuchi, Yoshihiko Uematsu Fatigue behavior of friction stir welded Al–Mg–Sc alloy [5] L. Boni, A. Lanciotti, C. Polese ‘‘Size effect’’ in the fatigue behavior of Friction Stir Welded plates [6] V.X. Tran, J. Pan, T. Pan Fatigue behavior of spot friction welds in lap-shear and cross-tension specimens of dissimilar aluminum sheets [7] D.R. Ni, D.L. Chen, B.L. Xiao, D. Wang, Z.Y. Ma Residual stresses and high cycle fatigue properties of friction stir welded SiCp/AA2009 composites [8] H.M. Rao, J.B. Jordon, B. Ghaffari, X. Su, A.K. Khosrovaneh, M.E. Barkey, W. Yuan, M. Guo Fatigue and fracture of friction stir linear welded dissimilar aluminumto-magnesium alloys [9] Z. Barsoum, M. Khurshid, I. Barsoum Fatigue strength evaluation of friction stir welded aluminium joints using the nominal and notch stress concepts [10] Soran Hassanifard, Masoud Mohammadpour, Hossein Ahmadi Rashid A novel method for improving fatigue life of friction stir spot welded joints using localized plasticity [11] Beytullah Gungor, Erdinc Kaluc, Emel Taban, Aydin Sik Mechanical, fatigue and microstructural properties of friction stir welded 5083-H111 and 6082-T651 aluminum alloys
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[12] Yong Zhao, Zhengping Lu, Keng Yan, Linzhao Huang Microstructural characterizations and mechanical properties in underwater friction stir welding of aluminum and magnesium dissimilar alloys [13] Athanasios Toumpis, Alexander Galloway, Lars Molter, Helena Polezhayeva Systematic investigation of the fatigue performance of a friction stir welded low alloy steel [14] Banglong Fu, Guoliang Qin, Fei Li, Xiangmeng Meng, Jianzhong Zhang, Chuansong Wu Friction stir welding process of dissimilar metals of 6061T6aluminum alloy to AZ31B magnesium alloy [15] H. Wohlfahrt, Th. Nitschke-Pagel, W. Zinn Optimization of the fatigue behaviour of welded joints by means of shot peening - a comparison of results on steel and aluminium joints [16] G. M. Xie, Z. Y. Ma2 and L. Geng1 Effects of Friction Stir Welding Parameters on Microstructures and Mechanical Properties of Brass Joints
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