Microstructure and Mechanical Properties of Friction Stir Welded DP1180 Steel Plates

The effect of rotation speeds from 100 to 600 rpm on the microstructure and mechanical properties of friction stir welded (FSW) DP1180 steel joints was studied. The microstructure in the different weld zones were examined using optical microscopy (OM), scanning electron microscopy (SEM), electron backscattering diffraction (EBSD), and transmission electron microscope (TEM). It was found that when welded at 100 rpm, there was an obvious boundary formed in the middle of the stir zone (SZ), which divided the SZ into two parts, namely, SZ-Ⅰ and SZ-Ⅱ. Significantly refined microstructures with an average grain size of 0.41 μm and 0.28 μm were observed in SZ-Ⅰ and SZ-Ⅱ, respectively. The SZs were mainly composed of martensite when the rotation speed increased over 200 rpm. The thermo-mechanically affected zone (TMAZ) of all joints was composed of martensite and ferrite. Microhardness tests showed that the minimum microhardness of heat affected zone (HAZ) of all joints was between 280–300 HV. It was revealed that as the rotation speed increased, the tensile strength of the joint decreased. The tensile strength of the 100 rpm joint was the highest, reaching 1094 MPa. The fracture position of the 100 rpm joint was in SZ while the fracture positions of the other joints were in HAZ.