Effect of Rotational Speed on Microstructure Evolution and Mechanical Properties of Zero‐Tilt Friction Stir Welded Mg‐7.8Gd‐4.59Y‐0.92Zn‐0.46Zr Alloy

In friction stir welding (FSW) of high‐strength Mg‐RE alloys, welding‐induced thermo‐mechanical conditions strongly affect weld formation and microstructural evolution. Under a zero‐tilt configuration, altered material flow and consolidation behavior make tool rotational speed particularly important. This study investigates the effect of rotational speed (1000–1600 r/min) on defect formation, microstructural evolution, and mechanical properties in zero‐tilt FSW joints of Mg‐7.8Gd‐4.59Y‐0.92Zn‐0.46Zr alloy. At 1000 r/min, tunnel defects were observed at the weld root, mainly associated with insufficient thermo‐mechanical input and inadequate material consolidation. As rotational speed increased, the increased relative thermo‐mechanical input and shear strain rate likely promoted plasticization and material flow, and sound joints were obtained at 1200–1600 r/min. In the stir zone, coarse network‐shaped long‐period stacking ordered phases were fragmented into finer dispersed particles, and second‐phase evolution was mainly characterized by fragmentation, redistribution, and possible partial dissolution. Pronounced recrystallization, grain refinement, and basal texture weakening were also observed, with grain size decreasing from 4.91 to 1.09 μm. Although the finest grains formed at 1600 r/min, the optimum strength–ductility balance was achieved at 1500 r/min, with an ultimate tensile strength of 370 MPa, joint efficiency of 96.9%, and 145.7% higher elongation than the BM.