Influence of weld size on energy input and interfacial strength during dissimilar ultrasonic welding of stainless steel to titanium

Abstract

This study explored the impact of weld area size on heat input and its subsequent effects on the microstructure, temperature, and strength at the stainless-steel–Ti interface during ultrasonic spot welding (USW) of dissimilar hard metals with a constant clamping force. It was found that a larger weld area positively affected the strength evolution in the early stages of USW by inducing local plastic flow and bond formation. However, a larger weld area led to dispersion of clamping force, leading to insufficient temperature to trigger Fe-Ti eutectoid reaction, resulting in internal fractures of α-Ti at prolonged welding. In contrast, a smaller weld area (7 ×7) concentrated energy input and accelerated temperature rise to exceed β-transus point, inducing α-to-β phase transformation through the diffusion of β-stabilizer elements from stainless steel to Ti. This phase-transformation behavior reduced the number of interfacial defects and increased the interfacial strength by 37 % (1817 N–2485 N). The findings in this work suggested the importance of elevating temperatures above the β-transus point to improve joint integrity throughout phase transformation to obtain a robust bond.