Cooperative deformation mechanism and its role on defect elimination in Pressure-controlled Joule-heat Forge Welding of Ti–6Al–4V/SUS316L dissimilar metals

The joining of dissimilar titanium and steel offers tremendous application potentials and a new Pressure-controlled Joule-heat Forge Welding (PJFW) method has been successfully adopted to join this hard-to-weld combination. However, the interfacial quality in PJFW is critically affected by complex multi-field coupling effects that remain insufficiently understood, thereby restricting its joint reliability. This study aims to elucidate the dominant role of interfacial dynamic deformation behavior in governing the metallurgical bonding process and defect formation during PJFW of titanium and steel. Through comprehensive experiment and simulation analyses, two distinct interfacial deformation modes, “cooperative deformation (co-deformation)” and “relative sliding”, are identified, and a quantitative parameter, Co-deformation Degree (CDD), is first proposed to evaluate the extent of co-deformation. Results reveal that the co-deformation promotes the synchronous and rapid fracture of interfacial oxide films, providing favorable conditions for extensive in-situ atomic diffusion across a quasi-static interface and significantly reducing the interfacial defect ratio, yielding a minimal defect ratio of 1.3% and a robust tensile strength of ∼500 MPa. In contrast, relative sliding repeatedly shears and tears the newly formed weld regions, drastically increasing the defect ratio to 24.5% and causing interfacial cracking. This study clarifies that promoting interfacial co-deformation while suppressing relative sliding through precise control of thermo-mechanical conditions is of great significance for optimizing the dissimilar metal welding quality.