Intrinsic dependence of welding quality and recrystallization on the surface-contacted micro-asperity scale during ultrasonic welding of Cu–Cu joints

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

“…Longer welding time facilitated an increase in bonding fraction (comparison between Figures S3b and S3c). However, the above results and findings from other studies [22,23] suggest that higher roughness could interfere with contact area formation, leading to a slower bonding strength evolution. Based on the above, it is found that a rough surface combination retards the contact area formation and plastic flow, leading to a slower evolution of bonding strength.…”

“…Regarding the welding parameters affecting the formation of bonding areas between Cu sheets, it has been reported that rougher surface combinations yielded better joint strength because rough peaks allowed a higher degree of plastic flow at the contact points [21]. On the other hand, a recent study reports that a smoother surface combination yields a higher strength than a rougher one, which can be attributed to the difference in contact fraction between these two cases [22]. Also, it has been reported that surface roughness significantly affects the development of a steel/Ni interface, a challenging metal combination, during USW [23].…”