Dynamic recrystallization mechanism, grain structure evolution and mechanical behavior in friction stir welding of galvanized steel at different cooling rate

Abstract The welded joints of galvanized steel were created by friction stir welding (FSW) with different cooling rate employing a W-Re rotary tool with a large diameter needle. The microstructure evolution was studied by optical microscopy, scanning electron microscopy, and electron backscatter diffraction. The results demonstrate that the microstructural evolution mechanism of the stirring zone is discontinuous dynamic recrystallization at fast cooling rate, while the microstructure has continuous dynamic recrystallization as the primary evolution mechanism at slow cooling rate. In addition, fast cooling is more favorable to the refinement of the organization, weakening of the texture, uniform distribution of dislocations and diffusion precipitation of Fe3C. Based on this, the tensile specimens prepared at the fast cooling rate exhibited yield and fracture strengths exceeding those of the base material with less plasticity loss.