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In order to obtain reliable Cu/Al dissimilar joints, current assisted Cu/Al high-power ultrasonic welding was proposed. The effects of current on the temperature rise, plastic flow, intermetallic compound (IMC) growth, and mechanical properties of the joints in Cu/Al ultrasonic welding were investigated. The results show that the obtained hybrid welded joint is well bonded, the tensile-shear force of the hybrid welding joint is 3030 N, and the fracture mode of the hybrid welding joint is ductile-brittle composite fracture. At the same welding time of 0.2 s, with the increase of the current, the peak temperature of the Cu/Al interface increases, and the metal plastic flow and diffusion at the interface also increase. This indicates that the assisted current can significantly enhance the interfacial metallurgy; compared with the single ultrasonic welding at welding time of 0.4 s, the assisted current can meet the welding formation requirements of interface temperature and material plastic deformation, but reduce the thickness of the interface IMC layer. This is the primary reason for the enhanced Cu/Al joint by assisted current. The results provide guidance for optimizing the strength of Cu/Al current assisted ultrasonic welded joints.