Electron-beam welding behavior in Mg-Al-based alloys
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THE light weight, high specific strength, and recycleable characteristics of Mg-based alloys have recently attracted great attention in academic research and industry applications. Among many Mg-based alloys, the AZ- (Mg-Al-Zn) and ZK (Mg-Zn-Zr) based alloys seem to be the most popular ones. Applications in automobile, bicycle, and computer, communication, and consumer electronic products have been widely and rapidly extended.[1] No matter how the alloys are processed, either via die casting or a wrought route, such as hot extrusion or press forming, an appropriate bonding technique is needed. Low-energy arc welding of Mg alloys has been studied in numerous cases (e.g., References 2 through 8), but systemic characterizations of high-energy laser-beam welding (LBW) or electron-beam welding (EBW) on commercial AZ- or ZK-based alloys are still very limited.[9–12] Mg has a high vapor pressure, low viscosity, and low surface tension, as compared with Fe-based alloys.[13] Typically, the fusion zone of Mg alloys will be less stable, with spattering effects. A high vapor pressure will result in Mg loss during welding.[13] Mg oxides and nitrides formed during welding will cause detrimental consequences. The addition of solute elements will also affect welding performance, and it was found that a satisfactory weld can be made with Al ⬍ 10 pct and Zn ⬍ 4 pct in the AZ-based Mg alloys.[14] Figure 1 shows the binary-phase diagram[15] for the two major elements of Mg and Al in the most popular AZ-based alloys. For Mg-rich alloys, the  (Mg17Al12) phase will precipitate when the Al solute content exceeds the solubility. The typical joint efficiency for Mg alloys subjected to tungsten inert-gas arc welding could be as high as 70 to 90 S.F. SU and H.K. LIN, Graduate Students, J.C. HUANG, Professor and Chairman, and N.J. HO, Professor, are with the Institute of Materials Science and Engineering, National Sun Yat-Sen University, Taiwan 804, Republic of China. Contact e-mail: [email protected] Manuscript submitted September 25, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
pct.[2,4] The fusion line in Mg alloys with high solute contents such as AZ91 or AM60 was not distinct. Instead, a partially melted zone was present.[3] Either LBW or EBW of the AZbased alloys will result in higher joint efficiency, to even 100 pct.[8] Thicker plates, 8-mm thick, could be welded by CO2 LBW,[10] and there were nearly equiaxed grains inside the fusion zone.[10] Due to the high thermal conductivity and fast cooling rate, the heat-affected zone was very narrow, within ⬃30 to 50 m.[8] Most studies reported in the literature on high-energy welding of Mg-based alloys were centered on LBW, and only basic microstructure and mechanical properties were presented. It is expected that EBW should produce a deeper fusion zone over 10 mm through the keyhole mechanism. A much deeper weld depth would be useful in industry for joining extruded thick plates or bars. In addition to the metallographic grain-structure characterization, the texture dis
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