Mechanical Evaluation of Friction Stir Spot Welded Advanced High Strength Steels

Friction stir spot welding (FSSW) was used to join dual phase (uncoated DP 590, hot-dipped galvannealed (HDGA) DP 600 and DP 780) and martensitic (uncoated M190) advanced high strength steels (AHSS). The process featured a hybrid WC-Co/W-Ni-Fe bi-material FSSW tool capable of withstanding process temperatures in excess of 1000°C. Some geometric changes in the WC-Co tool face were observed due to plastic deformation. For welds produced with this tool, the lowest strength grade AHSS (DP 590) possessed the highest average lap shear strength (LSS) of 11.4 kN. Higher strength AHSS grades (DP 780 and M190) possessed lower average LSS, likely due to higher material flow stresses at elevated temperatures. Although HDGA DP 600 possesses similar mechanical properties to DP 590, its LSS was 55% lower than that of DP 590 due to the formation of a Zn-rich faying surface layer. Some welds produced in this study exceeded the maximum LSS presented in other published work for similar materials. In addition, some welds exceeded the minimum design shear strength mandated by the American Welding Society and Military Handbook for resistive spot welded steel by as much as 150% and 200%, respectively.