Effect of grain recrystallization on stir zone and mechanical property behavior of TRIP 780 steel

Electron backscatter diffraction (EBSD) and transmission electron microscopy were used to determine the presence of retained austenite and displacive-type phase transformation in the stir zone of friction stir welding (FSW). Severe plastic deformations occurred in the stir zone where there was an increase in the temperature attributed to the FSW process and subsequently a grain recrystallization. Besides the recrystallization phenomena, the formation of grain evolution development in steels was resolved using EBSD. In addition, a tensile test was carried out in order to reveal the results of mechanical strength. It was found that the fracture zone occurred in the stir zone with an ultimate tensile strength of 587 MPa, a decrease of 267 MPa compared with that of the base metal. From this result, it is evident that the fracture exhibits numerous elongated dimples, distributed homogeneously, and certain locations contain cleavage fractures due to differences in the microstructure of the base metal. Microhardness profile tests of the welding regions were conducted, and the results showed that the stir zone was present with elevated hardness (near 350 HV). Characterization techniques revealed that the austenite-to-martensite transformation occurred in the stir zone, resulting in a loss of mechanical properties in the joint.