Development of friction stir welding of high strength steel sheet

For friction stir welding (FSW) of advanced high strength steel (AHSS) sheets with tensile strength grades between 590 and 1180 N mm(-2), the appropriate welding condition range and the influence of welding conditions on microstructures and mechanical properties of the welds were investigated. The appropriate welding conditions to avoid defects such as the incomplete consolidation at the bottom of the weld were obtained for the steel sheets up to 1180 N mm(-2) grade. The higher tool rotation speed evidently resulted in the larger volume fraction of martensite and higher hardness in the stir zone (SZ), attributed to an increase in the peak temperature of its thermal cycle. The tensile strength of the weld joint was as high as that of the base metal for the steels up to 980 N mm(-2) grade, but slightly lower than that of the base metal for the steel of 1180 N mm(-2) grade due to the heat affected zone (HAZ) softening.