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Influence of Aluminium Content on Behaviour of Magnesium Cast. Alloys in Bentonite Sand Mould. L. A. Dobrzański. 1, a. , T. Tański. 1, b. 1Silesian University of ...

Solid State Phenomena Vols. 147-149 (2009) pp 764-769 online at http://www.scientific.net © (2009) Trans Tech Publications, Switzerland Online available since 2009/Jan/06

Influence of Aluminium Content on Behaviour of Magnesium Cast Alloys in Bentonite Sand Mould L. A. Dobrzański1, a, T. Tański1, b 1

Silesian University of Technology, Division of Materials Processing Technology and Computer Techniques in Materials Science, Institute of Engineering Materials and Biomaterials, Konarskiego St. 18a, 44-100 Gliwice, Poland a

[email protected], [email protected]

Keywords: magnesium alloys, heat treatment, structure, mechanical properties

Abstract. In this paper there is presented the structure and proprieties of the modeling cast magnesium alloys as cast state and after heat treatment, depending on the cooling medium (furnace, water, air), with different chemical composition. The improvement of the manufacturing technique and chemical composition as well as of heat treatment and cooling methods leads to the development of a material designing process for the optimal physical and mechanical properties of a new developed alloy. In the analysed alloys a structure of α solid solution and fragile phase β (Mg17Al12) occurred mainly on grain borders as well as eutectic and AlMnFe, Mg2Si phase. The investigation is carried out to testy the influence of the chemical composition and precipitation processes on the structure and mechanical properties of the magnesium cast alloys with different chemical composition in its as cast alloys and after heat treatment. Introduction The dynamic industrial development puts some higher and higher demands to the present elements and constructions. These demands belong production and research newer and newer materials for materials engineering materials with relation to predictable work conditions and arise needs [1-8]. Magnesium alloys gets a huge importance with present demands for light and reliable construction. Magnesium alloys have low density and other benefits such as: a good vibration damping and the best from among all construction materials: high dimension stability, small casting shrinkage, connection of low density and huge strength with reference to small mass, possibility to have application in machines and with ease to put recycling process, which makes possibility to logging derivative alloys a very similar quality to original material [1-12]. Magnesium alloys, which are produced, often are fill in them, especially in fewer reliable construction [3-5]. Thanks to the progresses in the filed of magnesium alloys technology currently (alloys, which are received by cooling with a hige speed – Rapid Solidification Processing, composite material on magnesium fabric – MMCs that are – Meta-Matrix Composites, casting in constant-liquid condition, rheocasting, thixomoulding, thixoforming), forming, heat treatment, technological improvement and corrosion resistance, they find wide range of use in many fields[1-3]. Generally they are applied in motor industry and machine building, but they find application in a helicopter production, planes, disc scanners, a mobile telephony, computers, bicycle elements, household and office equipment, radio engineering and an air - navigation, in chemical, power, textile and nuclear industrial. The rising tendencies of magnesium alloy production, show increased need of their application in world industry and what follows the magnesium alloys become one of the most often apply construction material our century. Therefore it is extremely important to keep a high investigation development of a light alloy issue, furthermore performing in Institute of Material Processes and Computer Technology, Institute of Engineering Materials and Biomaterials, Silesian University of Technology. The goal of this paper is to present of the investigation results of the casting magnesium alloy in its as-cast state and after heat treatment. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of the publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 157.158.18.10-06/01/09,15:08:10)

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Experimental procedure The investigations have been carried out on test pieces of MCMgAl12Zn1, MCMgAl9Zn magnesium alloys in as-cast and after heat treatment states (Table 2). The chemical composition of the investigated materials is given in Table 1. A casting cycle of alloys has been carried out in an induction crucible furnace using a protective salt bath Flux 12 equipped with two ceramic filters at the melting temperature of 750±10ºC, suitable for the manufactured material. In order to maintain a metallurgical purity of the melting metal, a refining with a neutral gas with the industrial name of Emgesalem Flux 12 has been carried out. To improve the quality of a metal surface a protective layer Alkon M62 has been applied. The material has been cast in dies with betonite binder because of its excellent sorption properties and shaped into plates of 250x150x25. The cast alloys have been heated in an electrical vacuum furnace Classic 0816 Vak in a protective argon atmosphere. Metallographic examinations have been made on magnesium cast alloy specimens mounted in thermohardening resins. In order to disclose grain boundaries and the structure and to distinguish precisely the particular precipitaions in magnesium alloys as an etching reagent a 5% molybdenic acid has been used. The observations of the investigated cast materials have been made on the light microscope LEICA MEF4A as well as on the electron scanning microscope Opton DSM-940. The mass concentration of main elements, % Zn Mn Si Fe Mg 0,617 0,174 0,0468 0,0130 86,9507 0,77 0,21 0,037 0,011 89,7905 Table 1. Chemical composition of investigation alloy

Al 12,1 9,09

Rest 0,0985 0,0915

Conditions of solution heat treatment Temperature , °C Time of warming, h Way coolings As-cast Solution treatment 430 10 Water 430 10 Air 430 10 In furnace Aging treatment 190 15 Air Table 2. Parameters of heat treatment of investigation alloy

Sing the state of heat treatment 0 1 2 3 4

The X-ray qualitative and quantitative microanalysis and the analysis of a surface distribution of cast elements in the examined magnesium cast alloy specimens in as-cast and after heat treatment have been made on transverse microsections on the Opton DSM-940 scanning microscope with the Oxford EDS LINK ISIS dispersive radiation spectrometer at the accelerating voltage of 15 kV and on the JEOL JCXA 733 x-ray microanalizer. Observations of thin foil structure were carried out in the JEM 3010UHR firmy JEOL transmission electron microscope using an accelerating voltage of 300 kV. Tensile strength tests were made using Zwick Z100 testing machine. Discussion of experimental results The analysis of thin foils after the process of ageing has validated the fact that the structure of the magnesium cast alloy consists of the solid solution α – Mg (matrix) and an intermetallic secondary phase β – Mg17Al12 in the form of needle precipitations with different crystallographic orientations inside the matrix grains (Fig. 1). The differences of contrasts and the crossing atom bands obtained in high resolution pictures of the solid solution range α constituting the alloy matrix and the intermetallic phase β – Mg17Al12, explicitly indicate a big defect and micro deformations of lattice caused by the heat

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treatment. Moreover, the examinations of the thin magnesium cast alloy foils after the ageing process confirm the existence of a high density of crystal structure defects identified as a series of straight and parallel dislocations resembling a network (Fig. 1). a)

b)

c)

d)

Fig. 1. a) bright field image of the MCMgAl9Zn1 alloy after aging treatment with solid solution α – Mg (matrix) and an intermetallic secondary phase β – Mg17Al12 in the form of needle precipitations, b) diffraction pattern of area shown in a, c) high resolution image of the α-Mg matrix and precipitations showing dislocation and nano-scale strains, d) part of solution for diffraction pattern shown in b As a result of metallographic investigations made on the light and scanning microscopes it has been confirmed that the magnesium cast alloys MCMgAl12Zn1, MCMgAl9Zn1 in the cast state are characterized by a microstructure of the solid solution α constituting the alloy matrix as well as the β – Mg17Al12 discontinuous intermetallic phase in the forms of plates located mostly at grain boundaries. Moreover, in the vicinity of the β intermetallic phase precipitations the presence of the needle eutectics (α + β) has been revealed (Fig. 2a). In the structure of the examined magnesium cast alloys one can observe, apart from Mg17Al12 precipitations, turning grey phases, characterized by angular contour with smooth edges in the shape of hexahedrons. Out of the chemical composition examinations with the use of the EDS dispersive radiation spectrometer as well as literature data, one can conclude that it is the Mg2Si compound which, when precipitating, increases the hardness of castings. There have appeared, after the process of solutioning with cooling in water and in the air, trace quantities of the β (Mg17Al12) phase and single precipitations of a light grey phase in the structure of the alloy. There have not been noticed any locations of eutectic occurrences in the structure (Fig. 2b, 2c). After the cooling bell annealing the structure of the solid solution α with many precipitations of the secondary phase β has been revealed (locations resembling eutectics). The precipitations of the β (Mg17Al12) phase, located at grain boundaries and the light grey phase located mostly at the phase β boundary have also been observed. The structure

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of this alloy is similar to the structure of the as-cast alloy (Fig. 2d). The applied ageing process after the solution heat treatment with cooling in the air has caused the release of the β phase at grain boundaries as well as in the form of pseudo eutectic locations. There have been revealed, in the structure of the material, the parallel twinned crystals extending along the whole grain. a)

b)

50 µm

50 µm

c)

d)

50 µm

50 µm

Fig. 2. Microstructure alloy MCMgAl9Zn1: a) without heat treatment, b) after cooling in the water, c) after cooling in the air, d) after cooling with the furnace The chemical analysis of the surface element decomposition and the quantitative micro analysis made on the transverse microsections of the magnesium alloys using the EDS system have also confirmed the evident concentrations of magnesium, silicon, aluminium, manganese and iron what suggests the occurrence of precipitations containing Mg and Si with angular contours in the alloy structure as well as phases with high Mn and Al concentrations that are irregular with a non plain surface, often occurring in the forms of blocks or needles. A prevailing participation of magnesium and aluminium and a slight concentration of Zn has been ascertained in the alloy matrix as well as in the location of eutectics and big precipitations that arouse at phase boundaries identified as Mg17Al12 (Fig. 3). The results of the static tensile strength test make it possible to determine and compare the mechanical and plastic properties of the alloy without the heat treatment – As-cast and after the heat treatment (Table 3). It was found out in the strength tests that subjecting the alloy to heat treatment improves significantly its mechanical properties. Heat treatment contributes to improvement of mechanical properties, yield point and hardness with the slight reduction of the elongation. Aging treatment test pieces demonstrate the highest strength. Alloys after treatment 3 are distinguished by a significant increase of the R0.2 yield point, and alloys subjected to the treatments 1 and 2 increases slightly compared to the as-cast alloy. Elongation, however, nearly twice for the case of 1 and 2 in comparison with the as-cast state.

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b)

a) Mg

Al

Zn

Mn

20 µm

10 µm

Si

Fe

Fig. 3. The area analysis of chemical elements alloys with 12 % Al, after cooling in the air Yield point Tensile Percentage Rp0,2, strength, elongation [MPa] Rm, [MPa] A, [%] 0 129,4 170,8 2,9 1 136,9 248,6 5,6 MCMgAl12Zn1 2 141,5 251,0 4,1 3 153,7 276,1 1,0 4 149,6 294,8 1,5 0 121,2 182,4 5,3 1 126,1 241,6 12,1 MCMgAl9Zn1 2 132,1 247,7 10,1 3 158,9 266,6 2,9 4 140,4 275,3 3,8 Table 3. Mechanical properties analysis magnesium alloys

Investigation alloys

Sing the state of heat treatment

Hardness HRF 75,4 74,8 75,5 85,1 94,6 65,7 63 60,7 71,2 75,1

Summary As a result of the examinations of the thin foils made on the transmission electron microscope, that the structure of the magnesium cast alloy MCMgAl9Zn1 after the annealing constitutes a solid solution α – Mg with visible dislocation ranges. The analysis of the thin foils after the ageing process has confirmed that the structure of the magnesium cast alloy consists of the solid solution α – Mg (matrix) of the secondary phase β – Mg17Al12 evenly located in the structure. The structure creates agglomerates in the form of needle precipitations, partially coherent with the matrix placed mostly at the grain boundaries. Furthermore, the examinations of the thin foils of magnesium cast alloys after ageing confirm appearance of a high density of defects of the crystal structure in the material (Fig. 1). The results of the analysis of the EDS chemical composition confirm the presence of the main alloy additions Mg, Al, Mn, Zn and also Fe and Si included in the magnesium cast alloys in as-cast

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and after the heat treatment. The chemical analysis of the surface element decomposition and the quantitative micro analysis made on the transverse microsections have also confirmed the evident concentrations of magnesium, silicon, aluminium, manganese and iron what suggests the occurrence of precipitations containing Mg and Si with angular contours, as well as phases with high Mn and Al concentrations that are irregular, with a non plain surface, often occurring in the forms of blocks or needles (Fig. 3). The different heat treatment kinds employed contributed to the improvement of mechanical properties of the alloy with the slight reduction of its plastic properties (Table 3). Acknowledgments This scientific work is fragmentary financed within the framework of scientific financial resources in the period 2007-2008 as a research and development project R15 0702 headed by Prof. L. A. Dobrzański. References [1] K.U. Kainer: Magnesium – Alloys and Technology, Wiley-VH, Weinheim, Germany, 2003. [2] H. Friedrich, S. Schumann, Research for a ”New age of magnesium in the automotive.industry”, Journal of Materials Processing Technology, Vol. 117 (2001), 276-281. [3] A. Fajkiel, P. Dudek, G. Sęk-Sas, Foundry engineering XXI c. Directions of metallurgy development and Ligot alloys casting, Publishers Institute of Foundry engineering, Cracow, 2002. [4] X. Ming-Xu, Z. Hong-Xing, Y. Sen, L. Jian-Guo, Recrystallization of preformed AZ91D magnesium alloys in the semisolid state, Materials and Design, Vol. 26 (2005), 343-349. [5] C. Yan, L. Ye, Y. W. Mai, Effect of constraint on tensile behavior of an AZ91 magnesium alloy, Materials Letters, Vol. 58 (2004), 3219-3221. [6] L. Čížek, M. Greger, L. Pawlica, L.A. Dobrzański, T. Tański: Study of selected properties of magnesium alloy AZ91 after heat treatment and forming, Journal of Materials Processing Technology, Vol. 157-158 (2004), 466-471. [7] L.A. Dobrzański, T. Tański, L. Čížek: Heat treatment impact on the structure of die-cast magnesium alloys, Journal of Achievements in Materials and Manufacturing Engineering, 20 (2007), 431-434 [8] T. Tański, L.A. Dobrzański, L. Čížek: Influence of heat treatment on structure and properties of the cast magnesium alloys, Journal of Advanced Materials Research, Vol. 15-17 (2007), 491-496. [9] R.M. Wang, A. Eliezer, E. Gutman, Microstructures and dislocations in the stressed AZ91D magnesium alloys, Materials Science and Engineering, Vol. A344 (2002), 279-287. [10] K. Iwanaga, H. Tashiro, H. Okamoto, K. Shimizu, Improvement of formability from room temperature to warm temperature in AZ31 magnesium alloy, Journal of Materials Processing Technology, Vol. 155–156 (2004), 1313-1316. [11] X. Ming-Xu, Z. Hong-Xing, Y. Sen, L. Jian-Guo, Recrystallization of preformed AZ91D magnesium alloys in the semisolid state, Materials and Design, Vol. 26 (2005), 343-349. [12] A. Kiełbus, T. Rzychoń, R. Cibis, Microstructure of AM50 die casting magnesium alloy, Journal of Achievements in Materials and Manufacturing Engineering, Vol. 18 (2006), 135-138.

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Mechatronic Systems and Materials III doi:10.4028/3-908454-04-2 Influence of Aluminium Content on Behaviour of Magnesium Cast Alloys in Bentonite Sand Mould doi:10.4028/3-908454-04-2.764

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