Formation of ultrasonic spot welded Cu/Cu NPs/Cu joints and the mechanical properties and electrical resistance

To enhance the mechanical properties and reduce the electrical resistance of ultrasonic spot welded (USWed) Cu/Cu joints, Cu nanoparticles (NPs) interlayer was added between the two Cu sheets. The results showed that, as the welding time increased, the cavities in the Cu NPs interlayer and at the interface of the Cu NPs interlayer/Cu sheet gradually disappeared. For the Cu/Cu NPs/Cu joint at 0.8 s welding time, unbonded areas at the welding interface could be eliminated. Lower indentation depth occurred on the top specimen, resulting in the higher effective thickness of the joint. Comparing with the corresponding properties of the Cu/Cu joints, the maximum lap shear tensile load of the Cu/Cu NPs/Cu joint was enhanced by 19.4%, and the minimum electrical resistance of the Cu/Cu NPs/Cu joint was reduced by 5.9%. The grain of the Cu NPs interlayer didn't grow up to the microscale, having an average size of 88.4 nm, and plenty of mechanical twins were formed. Metallurgical bonding could be achieved in the solid state, and no amorphous structure was generated. This nano-scaled grained and metallurgical interfacial bonding was conductive to the bonding strength of the weld interface. The maximum strain intensity of the Cu/Cu NPs/Cu joint with 0.8 s welding time at the typical deformation step of the maximum load was 69.3% higher than that of the Cu/Cu joint with the same welding time. The first peak region in the weld zone was the weakest position of the mechanical properties for the Cu/Cu NPs/Cu joint.