Correlation of Local Constitutive Properties to Global Mechanical Performance of Advanced High-Strength Steel Spot Welds

Abstract

Recent results have shown that the strength of resistance spot-welded joints made from advanced high-strength steels (AHSS) do not increase linearly with their base metal strength. While the martensite tempering and subsequent softening in a narrow heat affected zone (HAZ) of these joints has been reported as a primary cause for this degradation, the quantitative effects of HAZ softening on above nonlinearity in different steels have not been explored. In this research, the role of material heterogeneity on load-displacement characteristics of dual-phase and martensitic AHSS with initial martensite volume fractions of 16, 58, and 100 pct during tension-shear (TS) and cross-tension (CT) testing was modeled with finite element method and compared with experimental measurements of global deformation and fracture behavior. Results from low-strength steels showed that the location of HAZ failure transitions from hardened to softened regions, as the nugget diameter increases from 4 to 6 and 8 mm, even with the presence of softening in the HAZ. At the same time, the results from higher strength steels showed more sensitivity to the softened region in larger nugget diameters. This result elucidates that the nonlinearity in strength of high-strength AHSS spot welds in comparison to lower strength of mild steel, is due to their intrinsic brittleness, as well as the overall geometry of the weld nugget. Our results also suggest that, while HAZ softening plays a detrimental role in DP steels, it helps to improve the ultimate load and global extension in high strength grades. Finally, several uncertainties related to finite element simulations and experiments have been highlighted. The results from this study can help in optimizing the resistance spot-welding process parameters and the design of the part made from AHSS.