Evolution of microstructure and texture in Mg-Al-Zn alloys during electron-beam and gas tungsten arc welding
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Evolution of Microstructure and Texture in Mg-Al-Zn Alloys during Electron-Beam and Gas Tungsten Arc Welding S.H. WU, J.C. HUANG, and Y.N. WANG The evolution of microstructure and texture in the AZ-series Mg alloys subjected to electron-beam welding and gas tungsten arc welding are examined. Electron-beam welding is demonstrated to be a promising means of welding delicate Mg plates, bars, or tubes with a thickness of up to 50 mm; gas tungsten arc welding is limited to lower-end thin Mg sheets. The grains in the fusion zone (FZ) are nearly equiaxed in shape and 8 m or less in size, due to the rapid cooling rate. The as-welded FZ microhardness and tensile strength are higher than the base metals due to the smaller grain size. The weld efficiency, defined as the postweld microhardness or tensile strength at the mid-FZ region divided by that of the unwelded base metal, is around 110 to 125 pct for electron-beam welding and 97 to 110 pct for gas tungsten arc welding. There are three main texture components present in the electron-beam-welded (EBW) FZ, i.e., {1011}101 2(with TD//1120), {1121}1100 (with ND 112015 deg), and {1010}1122 (with WD 112030 deg), where TD, ND, and WD are the transverse, normal, and welding directions, respectively. The crystal growth tends to align toward the most closed-packed direction, 1120 . The texture in gas tungsten arc welded (GTAW) specimens is more diverse and complicated than the EBW counterparts, due to the limited and shallow FZ and the lower cooling rate. The cooling rates calculated by the three-dimensional (3-D) and two-dimensional (2-D) heat-transfer models are considered to be the lower and upper bounds. The cooling rate increases with decreasing Al content, increasing weld speed, and increasing distance from the weld top surface. The influences of the FZ location, welding speed, and alloy content on the resulting texture components are rationalized and discussed. I. INTRODUCTION
MAGNESIUM alloys have been used in a wide variety of structural and nonstructural applications due to their unique properties such as low density and high special strength and elastic modulus.[1,2] They are considered as advanced materials for coping with energy-conservation and environmentalpollution regulations and are used as parts in the automobile, aircraft, or aerospace industries, where lightweight metals are needed to minimize weight or reduce internal forces at high accelerations. During the past five years, applications in automobile, bicycle, and computer, communication, and consumer electronic products have become rapidly extended.[3] Among many Mg-based alloys, the AZ- (Mg-Al-Zn), AM- (Mg-AlMn), and ZK- (Mg-Zn-Zr) based alloys seem to be most popular, with the AZ91 and AZ31 alloys being the lowest priced and occupying the highest market. No matter how the alloys are processed, either via the die casting or wrought route, an appropriate bonding or joining technique is crucial for their applications. Magnesium has a high vapor pressure, low vi
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