Influence of welding parameters on bond quality of ultrasonic welded joints for multilayer thin Cu-Al foil stacks

Abstract

Dissimilar multilayer copper (Cu) and aluminium (Al) joints find widespread applications in electrical and energy storage systems, where both electrical resistance and mechanical stability are required. The solid-state joining method, ultrasonic welding (USW), is highly promising for such material systems, as it allows joining thin layers with good conductivity while simultaneously preventing excessive intermetallic formation and porosity. However, weld quality in multilayer Cu-Al-Cu-Al assemblies is highly dependent on ultrasonic process parameters. The test material for ultrasonic welding was a Cu-Al-Cu-Al four-layer stack (0.2 mm thick). Furthermore, this study investigated the correlation between vibration amplitude, welding pressure, and welding time during joint formation. The process parameters were optimised using response surface methodology-central composite design (RSM-CCD). Lap shear strength and microstructural analyses characterised the weld joints and investigated interfacial morphology, material flow, and bonding behaviour under under-weld, good-weld, and over-weld conditions. Electron backscatter diffraction (EBSD) examined the grain structure across the weld interface. Layer-by-layer micro-hardness measurements determined the hardness variation near the weld interface. Additionally, thermal analysis and electrical resistance measurements evaluated the thermal and electrical behaviour of the welded joints. Those findings show that localised plastic deformation, mixing of interfacial materials, wave-like interface morphology, and the gradual buildup of interface micro-bonds predominantly control bonding in the Cu-Al-Cu-Al system. The results of the microstructural, thermal, and electrical analyses indicate that the current ultrasonic welding conditions can produce stable, low-resistance multilayer dissimilar metal joints. © International Institute of Welding 2026.