Effect of Sleeve Plunge Depth on Interface/Mechanical Characteristics in Refill Friction Stir Spot Welded Joint

Refill friction stir spot welding was employed to produce 6061-T6 aluminum alloy joints with different sleeve plunge depths. The interface characteristics of joint-line remnant and hook are investigated by optical and scanning electron microscopy. The joint-line remnant consists of primary bonding region and secondary bonding region, and two types of hook can be identified as downward hook and upward hook. Tensile shear results demonstrate that joint-line remnant and hook make interaction effects on tensile shear properties. The optimal joint is achieved when sleeve plunge depth was 2.0 mm with the corresponding failure load of 8673.4 N. Three different types of fracture mode are exhibited in joints produced at different sleeve plunge depths, which are closely related with the morphology of interface characteristics.