Mechanical Properties and Fracture Behavior of the Refill Friction Stir Spot Welding Dissimilar Aluminum Alloy Joints

Comprehensive tests were carried out to study the effect of rotation speed and plunge depth on small diameter (5 mm) 2B06Al-T42/7B04Al-T74 refill friction stir spot welding joints. The cross section microstructures, the effective connection height (ECH), and the effective connection width (ECW) were characterized. In the meantime, the thermal mechanically affected zone/stir zone and the fracture path were also described. The lap joint’s hardness, shear strength, and failure mode were analyzed by optical microscope. It was found that the effective connection area (ECA = ECH × ECW) had a good correlation with the tensile shear property. At 1200 rpm, the plunge depth benefit improved the tensile shear strength of the welded joint, while at 1500 rpm, it had the opposite effect. The optimal welding parameters of 5 mm diameter joints were 2100 rpm of rotation speed and 1.0 mm of plunge depth, under those the tensile shear force of the joint was 3196.9 N. The shear-plug type fracture path was different at different rotational speeds. The fracture path of the welded joint at high rotational speed expanded from the stirring zone at 45° finally; while, the fracture path of the welded joint at low rotational speed fractured from the thermal–mechanical affected zone finally. The fracture path at high speed was more complex than at low speed.