Microstructure and mechanical properties of refill friction stir spot welded Al–Mg-Li alloy joints

1.5-mm thick Al–Mg-Li alloy plates were successfully lap welded using refill friction stir spot welding (RFSSW), and the microstructure and mechanical properties of the joints were investigated. The results show that dynamic recrystallization (DRX) has occurred in the stir zone (SZ) of the joint, forming fine equiaxed grains with an average size of 10 μm. There is a strong texture in the base material, while no obvious texture in the SZ. The void defect at the horizontal surface and the lack of mixing defect at the vertical interface are the most important factor influencing the lap shear strength and the cross tension strength, respectively. The maximum lap shear strength is 6.2 kN, and the maximum cross tension strength is 2.7 kN. In addition, there are three fracture modes that can be observed after the tensile-shear tests, i.e., parent sheet fracture, plug type fracture, and interface fracture, while only plug type fracture mode presents after the cross tension tests.