Ultrasonic-driven multiphysics synergistic effects enable defect-suppressed, high-performance connections of metal foils in resistance spot welding

Reliable joining of multilayer current collectors in lithium-ion batteries is a key manufacturing challenge that is critical to improving performance and safety. Existing welding technologies struggle with joining multilayer structures and newly developed materials. In this work, an ultrasonic-assisted resistance spot welding (UA-RSW) process was introduced, and corresponding experiments were conducted on typical current collector materials, specifically multilayer copper and aluminum foils, to illustrate the multiphysics synergistic enhancement effects induced by ultrasonic vibration through the lower electrode. The introduction of high-frequency longitudinal ultrasonic vibration can disrupt surface oxide films, which reduces dynamic resistance and optimizes current distribution. Consequently, more concentrated and uniform Joule heat is generated at the faying interface. Microstructural characterization shows that ultrasonic vibration can promote nugget flow, oxide fragmentation and dispersion, dendrite fragmentation, and dynamic recrystallization. These phenomena are mainly associated with ultrasonic-induced high-frequency mechanical agitation and enhanced interfacial flow. These coupling effects can refine the microstructure and promote the formation of a continuous and uniform Cu/Ni interdiffusion layer. The evolution of grain size is non-monotonic, and the refinement behavior is region-dependent and parameter-dependent. Under the best-performing parameters within the study range, a continuous and uniform diffusion layer is formed. Mechanical property tests show that the tensile-shear forces of copper foil and aluminum foil joints increased by factors of 3.5 and 1.5, respectively. The apparent stack resistance of copper foil and aluminum foil were 131.5 μΩ and 450 μΩ, respectively. Both joints of multilayer copper and aluminum foils show a transition from brittle fracture to ductile fracture. The revealed multiphysics synergistic enhancement mechanism explains how ultrasonic vibration and resistive heating jointly overcome the challenges of joining multilayer lithium-battery current collectors to tabs. This work provides a reliable joining route for the mass production of high-safety, high-energy-density batteries.