A novel Pressure-controlled Joule-heat Forge Welding (PJFW) method, developed in Osaka University, has been adopted to weld a carbon steel, in which uniform and low temperatures could be successfully achieved throughout the weld interface. However, the effect of applied pressure, which is considered the most influential factor, on the thermo-mechanical behaviours and macro-/microstructural evolution at the interface during PJFW of carbon steel has not been studied in depth, leading to a poor understanding of the fundamental interface joining mechanism. In the present study, PJFW was performed on a high-carbon steel with varying pressure conditions where the behaviours in thermal, mechanical, and metallurgical were carefully investigated via experimental and numerical approaches. The results show that applied pressure uniquely determined the peak temperature according to temperature-dependent strength variation of base metal. High-carbon-steel rods were thus well joined by PJFW at uniform temperatures lower than A1 point, which effectively prevented the brittle martensitization, while also avoided the uneven temperature issue in rotary friction welding. Appropriate thermo-mechanical condition not only provided high enough pressure to sufficiently fragment oxides, but also high enough temperature to facilitate grain boundary migration to eliminate micro-defects. Simulations confirmed that increased interfacial strain helped further disperse oxides, produce more metal fresh surfaces and promote their atomic-scale adhesion, thereby suppressing the formation of unbonded regions and voids. The clarified interface joining mechanism regarding defect closure correlated with mechanical-induced oxide fragmentation and thermal-driven grain boundary migration would provide an inspiring perspective to the community of solid-state pressure welding.
