Pre-hole friction stir spot welding of dual-phase steels and comparison with resistance spot welding, conventional and pinless friction stir spot welding

This study investigates pre-hole FSSW to join DP590 and DP780 steels, commonly employed in the automotive sector. The primary objective is the elimination of the hook defect, which affects FSSW joints made with conventional truncated conical tools. Additionally, it investigates the influence of the alloy position in the sheet stack on joint strength and makes a comparative analysis with conventional and pinless FSSW and RSW. Microstructure and mechanical strength of joints were examined. Tensile tests were conducted and compared with AWS D8.1M standard. Cost analysis was performed considering the indirect cost of the welding machine, operator labor cost, direct cost of the welding tool/electrode, electricity cost, and cost of the pre-hole. All FSSW joints met the strength requirements of AWS D8.1M (over 11 kN). Steel position affected dissimilar joint strength. Considering the best results, the conventional FSSW joints had a strength of 18 kN when DP780 was the lower sheet, while pinless FSSW joints 25 kN and pre-hole FSSW joints 26.5 kN when DP780 was the upper sheet. Pinless and pre-hole FSSW improved strength by eliminating the keyhole defect. RSW remains the most economical joining technology to realize spot welds (0.02 € per spot weld), but pinless FSSW offers joints with higher strength and cost benefits (0.18 € per spot weld).