Improving microstructural and tensile properties of AZ31B magnesium alloy joints by stationary shoulder friction stir welding

Sound AZ31B magnesium alloy welds with glossy surface appearance and little flash were achieved for the first time by lower heat input stationary shoulder friction stir welding (SSFSW) at different rotational speeds (700-1500 rpm) and a constant welding speed (50 mm/min). Results show that a bowl-shaped stir zone (SZ) with fine equiaxed grains is present, and its grain size increases as increasing the tool rotational speed. The lowest Vickers hardness appears in the thermo-mechanically affected zone (TMAZ) on the advancing side. As the rotational speed increases, the ultimate tensile strength of joints also increases, while the yield stress and elongation at fracture decrease. An exceptionally high joint efficiency of 97% has been achieved at the rotational speed of 1500 rpm. In addition, the SSFSWed joints show higher strain hardening capacity due to dynamic recrystallization in the SZs. The fracture locations of joints shift from TMAZ to SZ with the increase of rotational speed. Fracture morphologies of the SSFSWed joints are characterized as the mixture of shear and ductile fracture modes.