Improving the microstructure and mechanical properties of Al-Cu dissimilar joints by ultrasonic dynamic-stationary shoulder friction stir welding

Thick intermetallic compound (IMC) layers and significant residual stress exert the negative effects on Al-Cu dissimilar joints. In this study, an ultrasonic dynamic-stationary shoulder friction stir method was designed to reduce the IMC thickness and residual stress, thereby enhancing the mechanical properties of Al-Cu joints. Specifically, the tensile strengths of the Al-Cu joints at all welding parameters were improved by applying the new ultrasonic loading mode in the welding process. The maximum tensile strength reached about 247 MPa. Meanwhile, the fracture position existed at the non-interface. Compared to the dynamic-stationary shoulder friction stir welding, the ultrasonic loading allowed one to eliminate microcracks at the interface and refine the grains, enhance the dynamic recrystallization in the stir zone and reduce the IMC thickness. Meanwhile, the ultrasound weakened the deformation texture in the stir zone and decreased the residual stress of the joints. The investigation provides an efficient and stable method for realizing high-strength Al-Cu dissimilar metals welding.