Interfacial bonding mechanism and fracture behavior in ultrasonic spot welding of copper sheets

Cu/Cu joints were fabricated using ultrasonic spot welding. Weld attributes and microstructure, welding interface temperature, effective thickness, bond density, grain size and hardness transition zone were investigated to understand the interfacial bonding mechanism, fracture behavior and mechanical properties. For the Cu/Cu joint fabricated at the welding time of 0.8 s, the gradient of the grain size along the direction from the welding line towards the base metal could be observed. The microstructure with nano-sized grains at the welding line was induced by dynamic recrystallization. Under this condition, the grain boundaries moved across the interfaces, resulting in the formation of the metallurgical bonding at the Cu/Cu interface. The mixed fracture characteristics occurred on the fracture surface.