Analysis of lap-shear force in automotive 22MnB5 boron steel sheet joints using friction stir spot welding

Friction stir spot welding has shown potential for both similar- and dissimilar-material joints, positioning it as a promising alternative to resistance spot welding. Despite the importance of similar boron steel joints, recent studies on the friction stir spot welding (FSSW) of 22MnB5 steel have focused on dissimilar joints, especially those with aluminum alloys. In this context, similar FSSW lap joints in hot-stamped 22MnB5 steel offer opportunities to investigate the weld microstructure, lap-shear force, and failure modes. This study investigates the feasibility of joining 22MnB5 steel sheets using polycrystalline cubic boron nitride tools. The test sheets underwent thermal treatment to replicate the hardness of hot-stamped steel and were welded using two tool geometries to evaluate different pin diameters. A central composite design was employed to analyze the effects of welding parameters, rotation speed (727–2073 r/min), plunge rate (0.4–2.4 mm/s), and dwell time (2.6–9.4 seconds). Higher lap-shear forces were observed with the tool having a larger pin diameter and at high rotation speeds. The rotational speed was a significant variable in the model for both geometries. At the same time, plunge rate, dwell time, and their interaction were also statistically significant for the tool with a greater pin diameter. The lap-shear force and failure mode were directly related to the extent of the stirring zone. The minimum force requirement of 10.3 kN for resistance spot welding, as specified by the American Welding Society Standard (AWS D8.1), was achieved.