Refill friction stir spot welding (refill FSSW) was used to weld 3.2-mm-thick 2060 aluminum alloy. Joint formation, defect characteristics and tensile-shear property were analyzed. Results show that keyhole can be completely eliminated under different plunge depths. However, defects such as void, unconnected welding, hook can be observed under the plunge depths of 3.4-4 mm. The size of the overall void initially increased and then decreased with the increase of the plunge depth, while the void was the smallest under the plunge depth of 3.4 mm. The unconnected defect at the lap interface gradually shrank a welding line from obvious crack. Different hook morphologies were observed under different plunge depths. The tensile-shear load of joint increased with the increase of the plunge depth and was up to the largest under the plunge depth of 4 mm. All the tensile-shear specimens fractured along the lap interface. Compared with the void, the unconnected defect had a greater influence on the tensile-shear property.

Refill friction stir spot welding (RFSSW) was used to weld the 2060 aluminum alloy with 2 mm thickness. Joint formation, defect characteristics and mechanical properties were investigated. Results show that stir zone (SZ) is clarified into dynamic recrystallization zone (DRZ) and heat extruded zone (HEZ) due to different microstructural features. The size of void near the hook tip decreases with the increase of the plunge depth. Different hook morphologies are obtained under different plunge depths. The tensile-shear load of joint with the void defect initially decreases and then increases with increasing plunge depth. The mean loads of joints under different plunge depths are in the range of 5.1-5.8 kN. The void separates the hook from lap interface, so the cracks initiating from the hook propagate along the sleeve retreating path. The hook has a larger influence on the tensile-shear load of joint than void. All the tensile specimens present a shear-plug fracture mode.