Factors affecting evolutions of contact area formation and bonding strength in pure Cu/Cu joints produced by ultrasonic spot welding

In this study, the effect of bonding surface conditions on the strength evolution of Cu/Cu joints during ultrasonic welding was investigated. Lap shear tests and microstructure characterisation revealed that contact area formation could be affected by bonding surface conditions and clamping force. That is, the fraction of contact area quickly evolved with smoother bonding surfaces, leading to a rapid strength increase to base metal fracture (1100 N of lap shear load after 1.0 s of welding). Comparatively, rough surface combinations exhibited a slow strength evolution owing to a lower fraction of contact areas. To overcome this situation, a higher clamping force was employed to facilitate contact area formation (1100 N of lap shear load after 0.5 s of welding).