Improving the weldability and mechanical property of ultrasonic spot welding of Cu sheets through a surface gradient structure

The weldability of metal sheets decreases with the increase of thickness during ultrasonic spot welding. To enhance the weldability and mechanical strength of ultrasonic spot welding of original Cu (O–Cu) sheets, a surface gradient structure (SG-Cu) was designed. The optimized SG-Cu sheets exhibited a 6 μm-thick nanocrystalline layer with an average grain size of 780 nm near the surface, accompanied by a substantial reduction in friction coefficient. Microstructural characterization showed that the improved metallurgical bonding originated from discontinuous dynamic recrystallization at the welded interface, which produced randomly oriented fine equiaxed grains with active grain boundary migration. The welding interface temperature, lap shear stress and fracture work of the SG-Cu/SG-Cu joint were enhanced significantly in comparison with the O–Cu/O–Cu joint. Notably, the surface gradient structure fundamentally altered the fracture mechanisms - replacing the characteristic cleavage planes and tear ridges of O–Cu/O–Cu joints with the densely distributed micro-dimples in SG-Cu/SG-Cu joints.