Evolution of microstructure and texture in Mg-Al-Zn alloys during electron-beam and gas tungsten arc welding

Abstract

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 ϳ8 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.,, and, 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,. 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.

Resistance-Spot-Welded AZ31 Magnesium Alloys: Part I. Dependence of Fusion Zone Microstructures on Second-Phase Particles

“…The differences in thermal, structural, and electrical properties also lead to differences in solidification microstructure of alloys. [1,2] It has been found that the strength of Mg alloy weldments is much lower than that of the base material and also that the weldability of Mg alloys joined by conventional fusion welding is inferior to that of steel. [1,2] In fusion welds, columnar and equiaxed grains are often the predominant macrostructural constituents.…”

Texture characterisation of hexagonal metals: Magnesium AZ91 alloy, welded by laser processing

“…This process leads to solidification in nonequilibrium conditions. It has been reported that rapidly solidified Mg alloys exhibit a change of the microstructure and especially a grain refinement [4], a formation or a modification of the texture [5] and the presence of residual stresses [6].…”

Grain Size Dependence of Yield Strength in Randomly Textured Mg-Al-Zn Alloy

“…The detailed descriptions of the experiments can be found elsewhere. 19) Furthermore, the asEBWed samples was subjected to annealing (denoted as the annealed sample) at 350 C for various time durations, resulting in various grain sizes in order to examine the effect of grain size on YS.…”

“…Fortunately, it is found that the fine-grained microstructure with weak textures can be produced by electron beam welding (EBW) in the fusion zone (FZ) of EBW magnesium alloys, owing to the high solidification cooling rate inside the fusion zone. 19) In the present paper, the tensile specimens are sampled from the FZ of the EBW AZ31 samples in order to establish the grain size dependence of YS in the randomly textured AZ31 alloy. The results are used as a base to roughly evaluate the influence of texture on grain boundary strengthening in the textured magnesium alloys, which were subjected to various thermo-mechanical processes.…”