Microstructure and mechanical properties of friction-stir-welded high-Mg-alloyed Al–Mg alloy plates at different rotating rates

Hot-rolled high-Mg-alloyed Al–Mg alloy (Al–9.2Mg–0.8Mn–0.2Zr–0.15Ti, labeled as 5A12) plates were successfully friction stir welded at rotating rates ranging from 750 to 1500 r·min−1 at a constant welding speed of 50 mm·min−1. The joints were characterized by optical microscopy (OM), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), electron-dispersive spectroscopy (EDS), and tensile testing. All the joints are volume defect-free and exhibit fine, equiaxed dynamic recrystallization (DRX) grains with high-angle grain boundaries (HAGBs) fractions of 88.6%–93.3% in the nugget zones (NZs). The DRX grain size and the second-phase particle size in the NZs have a parabolic relation with the rotating rate. Furthermore, among the joints tested, the joint prepared at 1000 r·min−1, which has the highest ultimate tensile strength ((478 ± 3) MPa) and the largest elongation to rupture (22.5% ± 1.4%)—approximately 87.5% and 145.2% those of the base metal, respectively, exhibits the smallest grain size of 2.93 μm, as well as the smallest particle size in the NZs. These excellent mechanical properties can be ascribed to the combined effects of the fine DRX grains with high fraction of HAGBs and the fine second-phase particles with a uniform distribution.