Microstructural evolution and deformation behavior of friction stir welded twin-induced plasticity steel

The weldability of twin-induced plasticity (TWIP) steel with ultra-high strength via friction stir welding (FSW) technique was investigated, and microstructural evolution and deformation behavior of whole and micro-zones of FSW TWIP joint were studied for the first time. The results showed that the content of recrystallized grains in the stir zone (SZ) increased from 10.5% of basal material (BM) to 14.2%, and that of heat affected zone (HAZ) increased to 78.6%. The percentage of annealing twins decreased from 26.8% in BM to 11% in SZ, while increased to 35% in HAZ. Compared with the BM, the ultimate tensile strength and yield strength of the FSW joint increased to 1036 and 550 MPa, respectively, reaching 106.7% and 110.9% of BM, respectively. The elongation of the entire joint was 50.5%, which was lower than that of BM due to the nonuniform deformation during the tensile test. The engineering strain was mainly concentrated in BM and SZ and transferred to each other during the tensile test, while the engineering strain in HAZ was always the lowest. Finally, the tensile fracture occurred in the SZ. The order of ultimate tensile strength of micro-zones in the FSW joint was as follows: HAZ > BM ≈ SZ. The order of yield strength was as follows: HAZ > BM > SZ.