Achieving superplastic TWIP steel welded joint via vacuum electron beam welding

Abstract

A crack-free high-Mn TWIP steel welding joint with comparable strength and superplastic was obtained via vacuum electron beam welding (VEBW) where the microstructural evolution, mechanical behaviors and strengthening mechanism of welding joint were investigated. The results indicated that the welding joint with a typical nail-shaped welding cross-section was divided into three zones including the base metal (BM), heat-affected zone (HAZ) and fusion zone (FZ). Massive fine AlN-type precipitates with the hexagonal crystal structure were observed in welding join. The orientation relationship between the γ-austenite matrix and AlN precipitates was determined as [001]γ//[2110]AlN. In FZ, small-sized cellular grains or columnar grains with consistent growth directions were combined into large-sized columnar grains with obvious <001> preferential orientation. Compared with the base mantal, the yield strength, ultimate strength and elongation of the VEBWed specimen were slightly reduced by 6.03%, 4.7% and 13.23%, respectively. However, the impact toughness was obviously decreased from the 87.90 J–68.97 J after the VEBW process, which was mainly attributed to the brittle AlN precipitates, inhomogeneous microstructure and back stress induced by the plastic mismatch between gradient grains. The strength contributions of different regions to overall strength including of BM, HAZ and FZ in welding joint were calculated. Here, the dislocation strengthening and precipitation strengthening were the dominant strengthening mechanisms of FZ, which was attributed to the high level of density and massive fine AlN precipitates induced by micro-elements segregations during the VEBW process.