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

Double-sided friction stir welding (FSW), performed with a pair of rotation tools, was studied in an attempt to increase the joining speed when applied to welding of automotive advanced high-strength steels (AHSS). The experiments were performed on butt joint of 1.6-mm-thick 1180 MPa grade steel sheets, and sound welds were obtained when the travel speed was no more than 4 m/min, fairly comparable with laser beam welding (LBW), one of the conventional automotive welding processes. It was clarified by experimental and numerical analyses that the enhancement in travel speed was due to the unique material flow interactively developed by a pair of rotation tools. Double-sided FSW was performed at 3 m/min of joining speed on butt joint of dissimilar steel sheets in a combination of 1.5 GPa and 590 MPa grades. The mechanical properties of the welded joints were evaluated by tensile, V-bending and Erichsen tests comparing with LBW, which verified that double-sided FSW exhibited sufficient mechanical properties for the press-forming to produce automotive parts. A prototype automotive part was successfully fabricated using the tailor-welded blank welded with double-sided FSW, which demonstrates its applicability to automotive production.