In the friction stir welding of advanced high strength steel (AHSS) sheets with tensile strength grades between 590 and 1180 N/mm2, the proper welding condition range and the influence of the thermal history, varied by the welding conditions, on the microstructures and mechanical properties of the welds were investigated. It was verified that the proper welding conditions were present for the steel sheets up to 1180 N/mm2 grade, regarding the fact that an increase in the revolution pitch, ie, a decrease in heat input, tended to reduce the stir zone (SZ) and thus cause the incomplete consolidation at the bottom of the weld. In the regions of SZ and the heat-affected zone (HAZ) heated above Ac1 temperature, the higher tool rotation speed presumably resulted in the higher peak temperature, evidently increasing the fraction of newly transformed martensite and hardness. In the region of HAZ below Ac1, the original martensite in the base metal was tempered and the hardness was decreased, which was rather apparent with the steels with higher strength. The tensile strength of the weld joint was as high as that of the base metal for the steels up to 980 N/mm2 grade, while, for further higher grades, lower than that of the base metal because of the HAZ softening, which was effectively minimized by properly controlling the welding conditions.
