Microstructure and Mechanical Property of Refill Friction Stir Spot Welded Joint of Al-Mg Alloy

5A06 Al aluminium alloy sheets in thickness of 2.0 mm was welded by refill friction stir spot welding (FSPW). The microstructure, tensile-shear strength and cross-tension strength of the joint were tested. Results indicate that the joint can be divided in nugget zone, thermo-mechanically affected zone, thermal affected zone, pressure affected zone and the base material, respectively. The microstructure was mainly α-Al solid solution, and β-Mg2Al3 intermetallic phase distributed in the matrix. With the welding time increased, the tensile-shear strength and cross-tension strength of the FSPW joints rapid increased at first and then slowly declined. When the rotational speed was 2 000 r·min-1 and the welding time was 3.5 s, the highest tensile-shear load reached 8 194 N, and the highest cross-tension load reached 3 656 N.