The Role of Heterogeneous Constitutive Properties on Mechanical Behavior of Advanced High Strength Steel Spot Welds

Recent publications have shown that the load bearing capability of resistance spot welds (RSW) made from dual-phase (DP) and martensitic (MS) Advanced High Strength Steels (AHSS) does not scale linearly with the tensile strength of their base metals. Although this degradation of welds strength has been linked by researchers to a phenomenon known as heat affected zone (HAZ) softening, the individual role of HAZ softening in different grades of AHSS has not yet fully understood. This work attempts to explain the above nonlinearity in spot welds performance by using a local to global approach to understand the role of HAZ softening on the strength of DP and MS spot welded samples. Material heterogeneity in Tension-Shear (TS) and Cross-Tension (CT) samples made from six grades of AHSS is studied by using microhardness measurements on spot welds and mechanical testing and digital image correlation (DIC) on thermally simulated samples in Gleeble machine. Results showed that the HAZ softening occurs in both subcritical and intercritical HAZ and the lowest yield stress exists in the ICHAZ making this region a candidate for plastic strain concentrating and failure initiation site.Results from FE simulations showed that both the spot weld’s nugget diameter and the HAZ softening play role in the performance of TS and CT samples. Nugget diameter showed to have a dominant effect on the strength of spot welds, and it is shown that the nonlinearity between the strength of spot welds and base metal tensile strength is related to the change in failure type in sample, as the nugget diameter increases. Results also showed that the HAZ softening have a complex and different role in performance of the welds. While it reduced the peak load and extension at peak load in DP spot welds, it played an opposite role and improved the same metrics in MS spot welds. The results also suggest that the lower strength (peak load) of MS spot welds compared to DP steels, is related to the intrinsic brittle behavior of their base metal as the HAZ softening showed to increase the peak load in MS spot welds.