Effect of Post-weld Heat Treatment on the Dissimilar Cu/Al Joints Pro-duced by High Power Ultrasonic Spot Welding

The effect of post-weld heat treatment (PWHT) on the microstructure and mechanical properties of ultrasonic spot welded Cu/Al dissimilar joints was studied in this paper. PWHT was performed for 1 h at four different temperatures of 200°C, 300°C, 400°C and 500°C, respectively. The microstructure and mechanical properties of joints were investigated by SEM, EDS, X-ray diffractometer, Microhardness tester and Universal testing machine, respectively. Results showed that no obvious intermetallic (IMC) layer was found at the as-welded joint and joint after PWHT at 200°C. But relatively higher shear strength of 2.458 KN was obtained when the temperature of PWHT was 200°C. As the temperature of PWHT exceeded 300°C, the shear strength of joints reduced obviously, due to the formation of brittle Al–Cu IMC layer. However, when the joint was subjected to PWHT at 500°C, maximum shear strength of 3.138 KN was acquired resulted from the increase of the bonded area, though large numbers of cracks were found at the Cu/Al interface. Besides, obvious grain boundary penetration occurred on the Al base metal adjacent to the Cu/Al interface when the joint after PWHT at 500°C. The micro-hardness at the Cu/Al interface was significantly higher than that of the Cu or Al base metal. With the temperature of PWHT increasing, the micro-hardness decreased both in Cu and Al base metals, but increased significantly at the Cu/Al interface. Fracture mode of joints, both as-welded and after PWHT, were the mixed-mode of ductile and brittle fractures.