Experimental and numerical investigation of a novel pinless friction stir spot welding for Al 1060 sheets

Pinless friction stir spot welding (P-FSSW) has potential applications in joining ultrathin sheets of thickness 1 mm or less. However, two issues, namely sheet tearing and weak material intermixing, hinder the widespread adoption of P-FSSW in manufacturing fields. This paper proposed the closed pinless friction stir spot welding (CP-FSSW) technique to address these issues. In this study, the CP-FSSW process was performed on Al 1060 alloys, and investigated through experimental observation and numerical modeling. The experimental results show that sound joints without sheet tearing and partially bonded interface defects were achieved. Even a small plunge depth of 0.03 mm resulted in significant deformation of the materials, with the depth of the stir zone being approximately 51 times the plunge depth. Increasing the plunge depth to 0.05 mm or 0.07 mm at a dwelling time of 2 s effectively suppresses the formation of hook defect, which was beneficial to the tensile shear load of the joints. Simulations indicate that the morphology of the hook defects at different plunge depth was highly dependent on the effective plastic strain.