Microstructure and mechanical properties of dissimilar NiTi and 304 stainless steel joints produced by ultrasonic welding

Superelastic NiTi alloy and 304 stainless steel (304 SS) were joined with a Cu interlayer by ultrasonic spot welding (USW) using different welding energy inputs. The surface morphology, interfacial microstructure, mechanical properties, and fracture mechanisms of the dissimilar NiTi/304 SS USWed joints were studied. The results showed that the surface oxidation intensified with increasing ultrasonic welding energy due to mutual rubbing between tools and sheets. The weld interface microstructure exhibited voids or unbonded zones at low energy inputs, while an intimate contact was established at the joining interface when applying a higher energy input of 750 J. With increasing energy input to 750 J, the weld interface shows two interfaces due to the behavior of plastic flow of Cu interlayer. The lap-shear load of the joints first increased, achieving a maximum value of ~690 N at an energy input of 750 J, and then decreased with further increase in welding energy. Interfacial failure was observed at NiTi/Cu interface at all energy inputs, and no intermetallic compounds were found on the fracture surfaces of both the NiTi/Cu and Cu/304 SS interfaces.

Automated summary

The study found that increasing ultrasonic welding energy input intensified surface oxidation, resulting in voids or unbonded zones at low energy inputs and intimate contact at higher energy inputs. Furthermore, an increase in energy input to 750 J led to the formation of two interfaces at the weld interface, and the lap-shear load of the joints first increased to a maximum value before decreasing with further welding energy input.