Hot Cracking Susceptibility in the TIG Joint of AZ31 Mg-Alloy Plates Produced by the TRC Process with and without Intensive Melt Shearing

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1 Materials Science Forum Vol. 765 (2013) pp (2013) Trans Tech Publications, Switzerland doi: / Hot Cracking Susceptibility in the TIG Joint of AZ31 Mg-Alloy Plates Produced by the TRC Process with and without Intensive Melt Shearing Xiaohui Xue 1,2,a, Yun Wang 1,b, Ian Stone 1,c and Zhongyun Fan 1,d 1 The EPSRC Centre - LiME, BCAST, Brunel University, Uxbridge, Middlesex, UB8 3PH, UK 2 School of Materials Science and Engineering, Shanghai Jiaotong University, Shanghai, China, a xhxue@sjtu.edu.cn, b yun.wang@brunel.ac.uk, c ian.stone@brunel.ac.uk, d zhongyun.fan@brunel.ac.uk Keywords: Magnesium welding, Hot cracking susceptibility, Intensive melt shearing, Grain refinement Abstract. AZ31 Mg-alloy plates produced by both conventional twin roll casting (TRC) and the melt-conditioned TRC (MC-TRC) processes were used to compare the hot cracking susceptibility in the joints of one bead-on-plate TIG welds. The plates cast with melt shearing were employed as the welding wire. The results showed that the MC-TRC plate has higher liquefied cracking resistance in the heat affected zone (HAZ) than that of the TRC plate. The improved liquefied cracking resistance of the MC-TRC plate can be attributed to the well dispersed and uniformly distributed eutectic regions in the MC-TRC microstructure. Introduction Magnesium alloys have a hexagonal close packed structure, therefore, they are inherently more difficult to deform plastically than cubic alloys. The properties of magnesium alloys are hard to improve through thermo-mechanical processing, and this makes grain refinement important for magnesium alloys [1]. Twin roll casting (TRC) has been used to produce plates of steel, aluminium alloys and magnesium alloys [2,3]. It was recently shown that melt treatment by intensive melt shearing prior to solidification processing can provide significant grain refinement in both Al and Mg alloys [4,5]. Liquation cracking in the heat affected zone (HAZ) is one of the major welding defects, which weakens the joint of the welded structures. Huang et al. [6] reported that the AZ31 magnesium alloy has a lower hot cracking susceptibility than that of AZ61 magnesium alloy, because the higher the Al content, the more Mg 17 Al 12 phase (a low melting point phase) produced along the grain boundaries. The Mg 17 Al 12 intermetallic phase will be liquefied at the elevated temperature under the welding thermal cycle. Kimura et al. [7] investigated the effect of grain size on the cracking susceptibility of TIG spot welds of Al-Mg alloys, and their results showed that a finer grain size would lead to a lower susceptibility of cracking. In this paper, we assess the hot cracking susceptibility in the TIG welds of AZ31 Mg-alloy plates produced by the twin roll casting process with and without intensive melt shearing. Experimental Procedure The materials used in this study were AZ31 magnesium alloy plates produced by both conventional twin roll casting (TRC) and melt conditioned twin roll casting (MC-TRC). The chemical composition of the AZ31 alloy was Mg-3.15Al-0.99Zn-0.43Mn (composition are in wt.% in this paper). The dimensions of the AZ31 plates were 200 mm 15 mm 2.3 mm for the MC- TRC plate and 200 mm 15 mm 1.8 mm for the TRC plate. In the TRC process, the AZ31 alloy was melted in a steel crucible at 700 C under a protective atmosphere of N 2 containing 0.4 vol.% SF 6. The pouring temperature was 638 C and the roll speed was 6 m/min. In the MC-TRC process, 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 TTP, (ID: , Brunel University, London, United Kingdom-03/06/13,12:28:40)

2 Materials Science Forum Vol the melt was conditioned by intensive shearing using a twin screw mechanism and was then twin roll cast under the same condition as described previously. A detailed description of the intensive melt shearing and the MC-TRC processes can be found elsewhere [2,8]. The welding wire was cut and machined from the AZ31 plate produced by the MC-TRC process. Manual TIG welding was carried out with TIG AC/DC 160 equipment produced by Butters AMT Welding Ltd. to obtain one bead-on-plate welds. The parameters for the welding experiments were as follows: voltage V, current A, travel speed mm/min, and argon flow rate 10 l/min. Samples for microstructural characterisation were sectioned, ground, polished and etched following standard metallographic procedures. Results and Discussion The optical micrographs in Fig. 1 show the typical grain structure of the AZ31 alloy plate produced by the TRC and MC-TRC processes. The TRC plate shows a very coarse dendritic grain structure with an average grain size of 600 µm (Fig. 1a). Detailed examination revealed that the TRC plate contains severe centreline segregation, which is a central region of the plate containing excessive eutectic and other casting defects. In contrast, the MC-TRC plate has a fine and equiaxed grain structure (60 µm) throughout the entire thickness, and more importantly, it is free from centreline segregation (Fig. 1b). Fig. 1. Microstructures of thin AZ31 alloy plates produced by (a) the conventional TRC process and (b) the melt conditioned twin roll casting (MC-TRC) process. The grain refinement effect of intensive melt shearing has been found in many other Al- and Mg-alloys by Fan and his co-workers [4,5]. It has been confirmed that intensive melt shearing is capable of dispersing the oxide films in the alloy melts, and increases the number of active nucleating particles, resulting in a much finer grain size after solidification. Once the particles are dispersed by shearing, they will not agglomerate again during solidification [4,5]. Fig. 2 shows the general view of the one bead-on-plate TIG joints. It is seen that, although the filler metal, which was obtained from the MC-TRC plate, was re-melted and solidified during the TIG arc welding, the microstructure in the weld in both cases is fine, uniform and free from large welding defects. Further examination of the weld showed that its microstructure (Fig. 3) was even finer than that of the MC-TRC plate (see Fig. 1b).

3 758 Light Metals Technology 2013 Fig. 2. General microstructures of the TIG welds between AZ31 alloy plates produced by (a) the TRC, and (b) the MC-TRC processes. Fig. 3. Microstructure of the TIG weld between two MC-TRC plates. During the TIG welding process, the filler wire obtained from the MC-TRC plate is re-melted and solidified under a high cooling rate provided by the fast heat extraction from the base materials. In the weld pool, the dispersed MgO particles in the MC-TRC plate are expected to remain dispersed in the melt and to be active for heterogeneous nucleation during subsequent solidification. Both the large number of dispersed MgO particles and the high cooling rate ensure an enhanced heterogeneous nucleation during solidification, resulting in a very fine weld microstructure (Fig. 3).

4 Materials Science Forum Vol Fig. 4. Optical micrographs showing the detailed microstructures of the HAZ in the welds (a) between TRC plates and (b) between MC-TRC plates. The microstructures of the heat affected zone (HAZ) for TIG welds between plates produced by both TRC and MC-TRC processes are shown in Fig. 4. The HAZ in the TRC plate maintained its coarse dendritic microstructure, but with large liquation cracks (dark regions in the plate) running between the dendritic grains (Fig. 4a). However, the microstructure of the HAZ in the MC-TRC plate after TIG welding is fine and uniform, and some isolated liquation cracks with an almost equiaxed morphology can be observed (Fig. 4b). Fig. 5 shows the detailed microstructure of the liquation cracked in the HAZ of the TRC plate (from Fig. 4a). EDX analysis has confirmed that the material in the liquation crack corresponds to the composition of α-mg/mg 17 Al 12 eutectic, which has a very fine structure due to the high cooling rate after TIG welding. Fig. 5. Liquation cracks at the grain boundaries in the HAZ of the TRC plate. Hot cracking is the main welding defect of magnesium alloys, and the present results suggest that grain refinement can significantly improve the hot cracking resistance. Huang and Kou [9] reported that the liquation cracking requires the presence of both tensile stress and the susceptible microstructure (i.e. eutectic phase). In the welded joint, the tensile stress is generated from the solidification shrinkage and thermal contraction. As a result of the intensive melt shearing, the weld metal as well as the heat affected zone (HAZ) has a fine grain structure, and the eutectic regions are therefore finely distributed throughout the entire microstructure. Each eutectic region will be small and isolated, resulting in a reduced tensile stress during solidification of such eutectic liquid. Consequently, the hot cracking tendency will be significantly reduced.

5 760 Light Metals Technology 2013 Summary Intensive melt shearing prior to solidification processing results in significant grain refinement in Mg-alloys. This idea has been applied to produce welding wires through the melt conditioned twin roll casting (MC-TRC) process. Such welding wires were used to TIG-weld AZ31 plates produced by both the MC-TRC and TRC processes. It has been confirmed that the grain refined MC-TRC plates have significantly improved hot cracking resistance compared with that of the TRC plates. This improvement can be attributed to well dispersed and uniformly distributed eutectic regions in the grain refined microstructure produced by the MC-TRC process. Acknowledgements The authors wish to thank the EPSRC for financial support under the grant for the EPSRC Centre - LiME. XHX would like to thank the China Scholarship Council for financial support. References [1] I.J. Polmear, Light alloys, from traditional alloys to nanocrystals. 4 th edition. Butterworth- Heinemann, 2006, p [2] I. Bayandorian, Z. Fan, I.C. Stone, Y. Huang and G.M. Scamans, Advances in twin roll casting of magnesium alloys, in: Z. Fan, I.C. Stone (Eds.), Solidification Science and Technology, Proc. John Hunt Int. Symp., Brunel University, Uxbridge, pp [3] C.R. Killmore, H. Creely, A. Phillips, H. Kaul, P. Campbell, M. Schueren, J.G. Williams, W. Blejde, Development of ultra-thin cast plate products by the CASTRIP process, Mater. Forum 32 (2008) [4] H.T. Li, Y. Wang, Z. Fan, Mechanisms of enhanced heterogeneous nucleation during solidification in binary Al-Mg alloys, Acta Mater. 60 (2012) [5] Z. Fan, Y. Wang, M. Xia, S. Arumuganathar, Enhanced heterogeneous nucleation in AZ91D alloy by intensive melt shearing, Acta Mater. 60 (2009) [6] C.J. Huang, C.M. Cheng, C.P. Chou, F.H. Chen, Hot cracking in AZ31 and AZ61 magnesium alloy, J. Mater. Sci. Technol. 27 (2011) [7] R. Kimura, H. Hatayama, K. Shinozaki, I. Murashima, J. Asada, M. Yoshida, Effect of grain refiner and grain size on the susceptibility of Al-Mg die casting alloy to cracking during solidification, J. Mater. Proc. Technol. 209 (2009) [8] H.T. Li, Y. Wang, M. Xia, Y. Zuo, Z. Fan, Harnessing oxides in liquid metals and alloys, in: Z. Fan, I.C. Stone (Eds.), Solidification Science and Technology, Proc. John Hunt Int. Symp., Brunel University, Uxbridge, 2011, pp [9] Huang, S. Kou, Liquation cracking in full-penetration Al-Cu welds, Weld. J. 83 (2004) 50-s- 58-s