Microstructure and deformation behavior of rapid cooling fraction stir welded Q&P1180 steel joint

In this work, Q&P1180 steels were butt welded by friction stir welding (FSW) and rapid cooling FSW (RCFSW), aiming to investigate the influence of rapid cooling on the microstructure and deformation behavior of joints. The results showed that for FSW and RCFSW joints, stir zone (SZ) composed of martensite phase, and inter-critical heat affected zone (ICHAZ) composed of ferrite/martensite dual-phase microstructure, and subcritical heat affected zone (SCHAZ) consisted of ferrite, martensite, carbide and residual austenite phases. Compared with FSW joints, the grain of HAZ was refined, the contents of residual austenite and ferrite increased, while the content of cementite decreased in RCFSW joint. The tensile strength and elongation of FSW joints were 1292 MPa and 7.5 %, respectively. Compared to FSW joints, the tensile strength of RCFSW joints slightly decreased (1237 MPa), while the elongation greatly increased to 13.5 %. During tensile deformation, the strain was mainly concentrated in HAZ of FSW joints, while the strain transferred from HAZ to basal material (BM) of RCFSW joints, and finally fractured in BM.