Microstructure and mechanical properties of friction stir welded joint of TRIP steel

Friction stir welding (FSW), as a new type of solid-solid welding method, was utilized to join transformation-induced plasticity (TRIP) steel. This method can avoid the solidification structural defects caused by fusion welding technology and improve the welding quality. In this study, the microstructural transformation and deformation behavior of TRIP steel during FSW were evaluated by optical microscopy, electron backscatter diffraction, and room-temperature tensile test. The results show that after FSW, martensite transformation occurred in the stir zone (SZ) and thermomechanically affected zone (TMAZ), and the retained austenite content decreased. The grain refinement and ratio of low-angle boundaries (LABs) increased. In heat-affected zone (HAZ), the retained austenite was thermally spheroidized and decomposed, the grains were coarsened, and the ratio of LABs decreased. No obvious preferred grain orientation was observed in each region of joint, and the texture strength decreased by phase transformation and grain rotation. After the tensile deformation of FSW joint, the retained austenite disappeared, the grains were refined, and the proportion of LABs increased in the base metal (BM) and HAZ. The content of retained austenite in SZ remained unchanged, but decreased in TMAZ. The grains were refined, and the ratio of LABs did not change significantly in SZ and TMAZ. FSW increased the microhardness of SZ and TMAZ by 218 HV and 93 HV, respectively, while the microhardness of HAZ decreased by 4 HV. The yield strength, tensile strength, and elongation of joint were 563 MPa, 942 MPa, and 12.8 %, respectively, reaching 96%, 104%, and 59%of BM, respectively. The tensile fracture exhibited ductile fracture characteristics. In addition, the joint exhibited strain nonuniformity during the tensile deformation, and the order of deformation degree was as follows: HAZ> TMAZ> SZ. Therefore, HAZ became a weak zone of joint mechanical properties. Continuous dynamic recrystallization and discontinuous dynamic recrystallization were the main mechanisms of grain refinement, and the martensite transformation conformed to the Kurdjumov-Sachs orientation relationship.