Joint strength in high speed friction stir spot welded DP 980 steel

High speed friction stir spot welding was applied to 1.2 mm thick DP 980 steel sheets under different welding conditions, using PCBN tools. The range of vertical feed rates used during welding was 2.5∼102 mm per minute, while the range of spindle speeds was 2500∼6000 rpm. Extended testing was carried out for five different sets of welding conditions, until tool failure. These welding conditions resulted in vertical welding loads of 3.6∼8.2 kN and lap shear tension failure loads of 8.9∼11.1 kN. PCBN tools were shown, in the best case, to provide lap shear tension failure loads at or above 9 kN for 900 spot welds, after which tool failure caused a rapid drop in joint strength. Joint strength was shown to be strongly correlated to bond area, which was measured from weld cross sections. Failure modes of the tested joints were a function of bond area and softening that occurred in the heat-affected zone.