Development of Friction Stir Welding (FSW) of High Strength Steel Sheet

Abstract

In the friction stir welding of advanced high strength steel (AHSS) sheets with tensile strength grades between 590 and 1180 N/mm 2 , 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/mm 2 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 Ac 1 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 Ac 1 , 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/mm 2 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.

Friction stir welding of steels

“…Matsushita et al 56,57 butt welded 1.6 mm-thick 780 -1180 N/mm 2 high-tensile steel sheets and used the results to evaluate the range of appropriate conditions. Using a tool of ultra-hard alloy (WC) at a rotation speed of 200 rpm to reduce the heat input, they were able to obtain a joint with no defects and only slight HAZ softening.…”

Development of double-sided friction stir welding of advanced high strength steel sheets

“…In shipbuilding, the welding speed of FSW can be technically competitive to the conventional fusion welding methods such as submerged arc welding [14]. However, welding speed for automotive AHSS is reportedly no more than 400 mm/min [15], which cannot be competitive to the conventional fusion welding for automotive such as laser beam welding.…”

“…It was reported that double-sided FSW was capable of the high-speed welding for an aluminum alloy [16,17]. However, the former researches for FSW of steels were only based on the single-sided FSW [6][7][8][9][10][11][12][13][14][15] but not on double-sided FSW.…”

Evaluation of Tensile Shear Strength for Lap Joint of Dissimilar Steels

“…The friction stir welding method was developed for the lap welding of high strength steel sheets [6]. For the lap joint by the friction stir welding, the authors reported [7] that the rotation angle around the weld bead reduced by the reduction of plate thickness.…”