Microstructure and Tensile Strength of AA6082 T-joints by Corner Stationary Shoulder Friction Stir Welding: Effect of Tool Rotation Speed

Tool rotation speed is one of the most important processing parameters that govern the performance of the friction stir welds. In this study, a novel variant of friction stir welding for T-joints, which is termed as corner stationary shoulder friction stir welding (CSSFSW), is investigated regarding the effect of tool rotation speed on the microstructure and tensile strength of the T-joints. T-joints with 5 mm thickness for AA 6082-T6 plates were fabricated via a two-pass welding process by using different tool rotation speeds. The microstructure and tensile strength of the defect-free and defective T-joints were analyzed. It is found that void defects appear on the advancing side when the tool rotation speed was 1200 rpm; defect-free T-joints are obtained in the range of tool rotation speeds from 1500 to 2000 rpm. Generally, high tool rotation speed is favorable for eliminating weld defects. In the case of CSSFSW, the lower limit of the tool rotation speed for defect-free welds is higher than that in the conventional FSW, because the heat input is reduced as the stationary shoulder contributes little to the heat generation. The tensile strength of the defect-free joint made at the tool rotation speed of 1500 rpm reached a maximum in both skin and stiffener direction, while are measured to be 113 MPa in L direction and 170 MPa in T direction. The fracture morphologies of the T-joints welded at different tool rotation speeds in both skin and stiffener directions when subjected to tensile loads are documented. A conceptual model is proposed in order to discuss the weld formation in the CSSFSW configuration.