Improving microstructural and mechanical properties of dissimilar friction stir welded AZ61/AZ40 joint

This study focuses on the microstructure evolution and mechanical properties of dissimilar magnesium alloy friction stir welded AZ61/AZ40 joints achieved at different traverse speeds (50–130 mm/min) and a constant rotation speed (1200 rpm). The surfaces of the welds are relatively smooth without any obvious surface defects except for the FSW joint at a traverse speed of 50 mm/min. The nugget zone (NZ) is bowl-shaped due to the tapered probe. The Mg-based alloys were sufficiently mixed with each other in the NZ, and the interface was irregular. In addition, the NZ exhibits fine equiaxed grains due to dynamic recrystallization (DRX), and the grain size decreases with increasing traverse speed. The welded joints show a relatively discontinuous microhardness, and the lowest microhardness occurs in the thermo-mechanically affected zone (TMAZ) on the advancing side (AS). The strength increases as the traverse speed increases from 50 to 70 mm/min and then decreases as the traverse speed increases continually. An exceptionally high tensile strength of 235 MPa was achieved at a traverse speed of 70 mm/min. The combined effects of high-density dislocations and fine second phases (η-Al₈Mn₅ and β-Mg₁₇Al₁₂ phase) promote mechanical properties.