Formation of Refined Grain Size Less Than 5 nm and Nano-sized Undulations in the Bonding Interface Region of an Ultrasonic Spot Welded Cu/Ni Joint

The microstructure in the bonding interface region of an ultrasonic spot welded Cu/Ni joint was investigated. At the copper side near the bonding interface, all the refined grains with random orientations and high-angle grain boundaries are < 5 nm in size. Nano-sized undulations occur at the Cu/Ni interface. The formation of nano-sized grains and undulation characteristics is ascribed to the dynamic recrystallization as well as shear instability in ultrasonic spot welding.