Microstructure evolution and mechanical property strengthening mechanisms of Cu/Cu NPs/Cu joint fabricated by ultrasonic spot welding

Ultrasonic spot welding, as a high strain rate plastic deformation process, utilizes the high frequency sliding between the metal sheets to generate a solid-state joining. For the ultrasonic spot welded Cu/Cu joint, even with the good metallurgical bonding at the welding interface, it couldn't obtain high T-peel strength and maintain quite high T-peel load bearing capacity before failure. In order to enhance the T-peel strength and bearing capacity, Cu nanopaticles (NPs) interlayer was screen-printed on the Cu sheet surface that was to be welded. It was beneficial to enhance the temperature at the welding interface and the degree of dynamic recrystallization, obtain the nano-scaled equiaxed grains with random orientations at the welding interface, and change the microstructure evolution pattern along the weld thickness. These results are key factors to the improvement of the joint mechanical strength. In comparison with the Cu/Cu joint, the T-peel strength of the Cu/Cu NPs/Cu joint (471.7 N) was increased by 69.4%, the displacement (0.97 mm) corresponding to the T-peel strength was enhanced by 42.7%, the obviously increased fracture energy was achieved, and quite high T-peel load bearing capacity could be maintained after exceeded the T-peel strength. The mixed failure modes of brittle cleavage and quasi-cleavage as well as shallow dimples fracture occurred in the Cu/Cu joint. However, the failure mode for the Cu/Cu NPs/Cu joint was turn into the micro-void coalescence.