Constitutive properties and plastic instabilities in the heat-affected zones of advanced high-strength steel spot welds

Recent publications have shown that the load-bearing capability of advanced high-strength steel (AHSS) spot welds do not scale linearly with the tensile strength of the base metal. Although this unexpected degradation of welds has been linked to the tempering of the martensite phase in the heat-affected zone (HAZ), the detailed elastic–plastic stress–strain behavior in different regions of the HAZ has not been reported. In this research, the plastic flow behavior in samples subjected to simulative weld thermal cycles with peak temperatures (T_Peak) ranging from 350 to 1250 °C was evaluated. Results from this study showed that the microstructural changes in the HAZ could result in large mechanical heterogeneity in AHSS spot welds. In addition, plastic instability in the form of yield point phenomena was observed. The yield point phenomena in these simulated HAZ samples, in contrast to base metal samples, were confirmed by the observation of Lüders band and stress fluctuations before the onset of strain hardening using digital image correlation. The local plastic instability was correlated to the chemical composition of the steel, initial microstructure, and thermal cycle. The Lüders front velocity maps revealed heterogeneous nucleation and growth of multiple Lüders bands with different front speeds. Interestingly, inter-critical (Ae₁ < T_Peak < Ae₃) HAZ samples show minimum yield strength even though the maximum softening occurs in the subcritical HAZ (T_Peak < Ae₁) samples.