Abstract:
Tensile strength testing and high cycle fatigue testing of advanced high strength steel spot welded shear lap joints were performed for the various weld conditions. The materials used in this study were DP 980, DP 780 and TRIP 780. The microstructure and microhardness of the shear lap joints were examined in an effort to identify the effect of microstructural changes on the strength and fatigue durability of the spot weld specimens. The occurrence of interfacial failure was recorded for the differing weld processes. Several weld schedules were examined and used to produce shear lap spot weld joints, specifically varying the squeeze force and the average current. The weld force used to produce a spot weld does not have a significant effect on the fracture mode of the specimen given the average current is constant. The average current used to produce a spot weld has a significant effect on the fracture mode of the spot weld for several squeeze forces. Interfacial failure of spot welded TRIP 780 can be mitigated using a certain range of currents when welding. This appears to come as a tradeoff for sacrificing the strength of the joint. Higher values of weld strength were obtainable; however, welds that failed with higher strengths also experienced interfacial failure. A fracture mechanics approach to estimating the high cycle fatigue life of the shear lap specimen is also proposed and represents a conservative estimate of the shear lap specimen durability.
