Achieving grain refinement and microstructural stability in repair-welded high-strength Mg–Gd–Y–Zr alloy

Abnormal Grain Coarsening (AGC) usually occurs during post-weld heat treatment of repaired Mg–9Gd–3Y–0.5Zr alloy joints and severely deteriorates their mechanical properties. Here, Zn-containing filler wires (0wt%–2wt%) were designed to introduce thermally stable LPSO phases into the FZ via TIG repair welding. Zn microalloying introduces 18R-LPSO phases in the as-welded FZ, which transform into blocky and lamellar 14H-LPSO phases during solution treatment, providing effective grain boundary pinning. AGC is completely suppressed at 1wt% Zn, with the T6-treated FZ refined to 19 μm. Quantitative analysis of strengthening contributions reveals a competitive precipitation relationship between LPSO and β-series phases due to RE element consumption. Consequently, when Zn content reaches 2wt%, the reduced amount of β-series precipitates leads to a slight decrease in joint strength. The 1Zn joint achieves optimal properties: YS = 220 MPa, UTS = 309 MPa, EL = 2.6%, representing 16.6% and 225% improvements in UTS and EL over the Zn-free joint. A schematic illustration summarizing the microstructural evolution and fracture behavior was proposed. This work provides an effective strategy to enhance the mechanical performance of repaired Mg-RE alloy joints, significantly reducing production costs and expanding aerospace applications.