Fracture mechanism of refill friction stir spot-welded 2024-T4 aluminum alloy

Refill friction stir spot welding (RFSSW) was applied to weld 2024-T4 aluminum alloy and effects of welding parameters on the fracture mechanisms of RFSSW tensile-shear specimens were investigated. Fracture modes are mainly determined by the bonding strengths of the lap interface and the stir zone (SZ)/thermomechanically affected zone (TMAZ) interface, which are largely influenced by heat input and sleeve plunge depth. Reasonable heat input leads to shear-plug fracture mode, which owns better mechanical properties in general. When sleeve does not penetrate into the lower plate, hook is flat and bonding ligament is continuous, shear fracture mode happens. Increased heat input improves diffusion bonding effect of the SZ/TMAZ interface but decreases hardness in SZ, which also results into shear fracture mode. Big sleeve plunge depth and downward bending hook lead to plug fracture mode. Fracture positions and fracture morphologies agree with the corresponding fracture mechanisms.