Formation mechanism and failure behavior in synergistic double-sided probeless friction stir spot welded joints of 6061 aluminum alloy

In this study, the synergistic double-sided probeless friction stir spot welding (SDP-FSSW) was innovatively employed to weld 6061-T6 aluminum alloy with a thickness of 2 mm. The correlation between the welding process and joint forming was clarified as well as mechanical properties. The joint formation mechanism and failure behavior were revealed. The results show that effective double-sided synchronous controllable forming is achieved during the SDP-FSSW process, resulting in sound joints. Four onion rings are present in the stirring zone (SZ) of the joint, and the hook exhibits a wavy-shaped morphology at the interface. Dynamic recrystallization, disruption of oxide film, and atomic diffusion play crucial roles in promoting grain boundary migration across interfaces, grain growth, and achieving high-strength metallurgical bonding. Two types of micron-sized second phases are observed in the joint, namely AlFeSi phase and beta-Mg2Si phase. The hardness distribution of the joint cross-section is symmetrical overall, initially increasing and then decreasing with the increase of rotation speed, reaching the highest hardness value of about 95.7 HV on the upper plate of the SZ. The joint strength is attributed to both interface metallurgical bonding and mechanical interlocking formed by hook defects. The maximum value of the joint tensile-shear strength can reach 11,493 N at a rotation speed of 600 rpm, dwell time of 2 s, and plunge depth of 0.3 mm. The joint fracture modes gradually transition from mixed fracture mode to plug fracture mode with the increase of rotation speed.