Effect of Rapid Cooling on Microstructure and Mechanical Properties in Friction Stir Welded Twin-Induced Plasticity Steel

High Mn twin-induced plasticity (TWIP) steel are widely used because of their outstanding mechanical properties. In the welded joint of high Mn steel, Mn vapor and segregation reduce the quality of the joints. Friction stir welding (FSW) can obtain high-quality, defect-free joints but low yield strength. This work analyzes the microstructure evolution and deformation behavior of TWIP steel FSW joints in different micro-zones based on changing cooling conditions. Compared to the case under air-cooling, a strong shear stress was identified in the stirring zone (SZ) under water-cooling conditions because water-cooling has higher dislocation density and finer grains in the SZ and heat-affected zone (HAZ). Under water-cooling, the highest yield strength of the joint reached 519 MPa, respectively, since the non-uniformity of joint grain size can promote the accumulation of geometrically necessary dislocations (GNDs). In addition, rapid cooling shortens the thermal diffusion distance of HAZ and retains more twin boundaries, which enhances the twin dynamics and leads to twin-induced hardening. The present work provides further insights into the influence of the deformation mechanism of FSW joints of TWIP steel.