Achieving superior grain refinement and microstructural stability for enhanced strength-ductility synergy in a fusion-welded Mg-Gd-Y-Zr alloy

Fusion zone (FZ) of Mg-RE alloy joints usually exhibits severe abnormal grain coarsening (AGC) during post-weld heat treatment (PWHT), deteriorating the comprehensive mechanical properties. To overcome this long-standing challenge, a microstructural stabilization strategy was applied by developing a novel filler wire to in-situ form thermally stable 14H-LPSO phases within FZ of a Mg-9Gd-3Y-0.5Zr alloy. These basal-oriented LPSO precipitates provide exceptional pinning at grain boundaries, suppressing AGC during PWHT without compromising strength. In addition, rapid solidification of fusion welding enables homogeneous LPSO distribution and thus eliminates the stress concentration in FZ. The resultant TIG-welded joint achieves a record ultimate tensile strength of 364.7 MPa with a joint efficiency of 97.2%, representing a 41% strength enhancement over conventional Mg-RE alloy joints. This work establishes a paradigm for stabilizing FZ through controlled phase engineering.