Microstructure and mechanical properties evaluation of friction stir welded boron steel

The microstructure and mechanical properties of friction stir welded boron steel in butt joint configuration are experimentally studied. Two different friction stir welding (FSW) parameter combinations are used to successfully fabricate butt joints. Microstructural analysis exhibites that the stir zone (SZ) primarily consists of fine lath martensite, while the thermo-mechanically affected zone (TMAZ) comprises bainitic ferrite and granular bainite with a small amount of martensite. The presence of granular bainite in TMAZ suggests that alloying composition affects the phase transformation. The formation of recrystallized structures with lath martensites and high dislocation density in the SZ significantly enhance the hardness of the joints compared to that of the base metal. The results of the present study suggest that FSW can be used as a method for local hardening of structural components made of boron steels, without complicated heating and rapid cooling of a conventional hot stamping process.