Effect of ultrasonic vibration on residual stress and microstructure in resistance spot welding of aluminum/steel dissimilar materials

The reliable joining of aluminum/steel dissimilar materials remains a critical challenge in lightweight manufacturing. This study aims to systematically investigate the influence of ultrasonic vibration assistance on the microstructure, residual stress, and mechanical properties of aluminum/steel resistance spot welded joints. A comparative analysis was conducted between conventional resistance spot welding and ultrasonic vibration-assisted resistance spot welding joints. Electron backscatter diffraction analysis revealed that ultrasonic treatment effectively relaxed the localized strain in critical zones of the weld, particularly alleviating strain concentration at the aluminum/steel interface. XRD residual stress measurements indicated that the peak macroscopic tensile residual stress in the joint was reduced by over 50% due to the release of microscale strain and optimization of the thermo-mechanical coupling field. The synergistic optimization of both microstructure and stress state promoted a transition in the fracture mode from brittle interfacial fracture to ductile button fracture, with the fracture morphology changing from cleavage river patterns to uniform dimples. Finite element simulation results further validated the modulatory effect of ultrasonic vibration on the welding thermal cycle and stress evolution. This study demonstrates that the ultrasonic vibration-assisted process provides an effective pathway for achieving high-performance spot welding of aluminum/steel dissimilar materials through optimization of the interfacial reactions, crystalline structure, and stress field.