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

Friction stir welding (FSW) of galvanized steel offers unique advantages as it can avoid the extensive metallurgical problems associated with fusion welding. However, conventional FSW still leads to a higher temperature in the stir zone, which deteriorates the microstructure. To obtain a satisfactory weld, the welded joints of galvanized steel were created by FSW with different cooling rates employing a W-Re rotary tool with a large-diameter needle. The correlation between the microstructure evolution and mechanical behavior of joints at different cooling rates was systematically studied. The results demonstrate that the rapid cooling rate favors the refinement of ferrite and diffuse distribution of Fe3C. In addition, weakening of the texture and dispersion of dislocations were also observed after fast cooling FSW. It can be concluded that the microstructural evolution mechanism of the stirring zone (SZ) is continuous dynamic recrystallization at a slow cooling rate, while the microstructure has discontinuous dynamic recrystallization as the primary evolution mechanism at a fast cooling rate. Based on these advantages, the tensile specimens with fast cooling FSW obtained a synergistic improvement in strength-ductility, even surpassing the base material. This work provides new ideas for welding galvanized steel.