Improvement on microstructure and mechanical properties of Cu/Al ultrasonic spot welded joints assisted by periodic additional force

This study utilizes active control of welding clamping force through periodic additional force (PAF) to assist in fabricating ultrasonic spot-welded copper -aluminum joints. The microstructural characteristics and mechanical properties of the copper -aluminum ultrasonic spot welded with periodic additional force joints were systematically investigated. Periodic additional force was applied to the welding interface, facilitating plastic deformation and flow. This can transform the relatively smooth weld interface into a wave- and swirl -like surface with a maximum peak height of 12 mu m. Simultaneously, the mutual diffusion length of copper and aluminum at the weld interface increased from 2 mu m to 8 mu m. Additionally, the weld interface of ultrasonic spot welded with periodic additional force exhibited typical {111}(112) or {111}(110) shear textures, confirming a strong interface bonding. The effect of the periodic additional force on the thermomechanically affected zone (TMAZ) was also examined. It was discovered that lower periodic additional forces enhanced dislocation density and decreased anisotropy, leading to improved mechanical characteristics. The utilization of periodic additional force in assisting ultrasonic spot welded can significantly enhance the lap shear failure force by up to 59.98%. The paper innovatively proposes a control method of reverse periodic additional force, offering a strategy for the preparation of reliable aluminum -copper joints.