Addressing the abnormal grain coarsening during post-weld heat treatment of TIG repair welded joint of sand-cast Mg-Y-RE-Zr alloy

Tungsten inert gas (TIG) repair welding is one of the key methods to reduce the rejection rate of sand-cast Mg-YRE-Zr alloys containing casting defects, and welding heat treatment can further improve the joint efficiency of the repaired joints. In this study, the microstructural evolution and mechanical properties of the repaired joints of sand-cast Mg-4Y-2Nd-1Gd-0.5Zr alloy during different pre- and post-weld heat treatments have been investigated. The results indicated that abnormal grain coarsening (AGC) was formed in the fusion zone (FZ) during the post-weld solution treatment of the repaired joint. As the solution temperature increased from 480 degrees C to 520 degrees C, the average grain size of FZ of the T6-treated joint increased from 35 mu m to 657 mu m. AGC was first generated in the lower-middle part of FZ where the residual welding strain was the highest. Moreover, fine grains with high-misorientation grain boundaries in FZ were also found to facilitate the grain boundary migration and the development of AGC. The detailed formation mechanism of AGC, as well as a physical model revealing the failure procedure of the repaired joints, were proposed. The repaired joint with neither residual eutectics nor AGC was successfully prepared by a combined solution treatment of 520 degrees C x 8 h before welding and 520 degrees C x 2 h after welding, and the T6-treated joint exhibited excellent comprehensive properties with the tensile strength of 314 MPa and elongation of 4.4%.