The fusion zone (FZ) hardness of resistance spot welds is a crucial factor affecting the Received 5 August 2023performance and durability of the welds. Failure mode transition, tendency to fail in Accepted 26 August 2023interfacial mode, interfacial failure load, and the impact of liquid metal embrittlement Available online 29 August 2023cracks on the weld strength are influenced by the FZ hardness. Therefore, accurately predicting the FZ hardness in resistance spot welds made on automotive steels is essential. A simple thermal model is used to calculate the time required for the temperature to drop from 800 C to 500 C (Dt 5 ). With the aid of continuous cooling transformation (CCT) diagrams and experimental confirmation, it is shown that in most automotive steels, the FZ exhibits an almost full martensitic microstructure. Generally, the FZ hardness tends to increase in the order of interstitial-free (IF), drawing quality specially killed (DQSK), high strength low alloy (HSLA), ferrite-martensite dual-phase steels, transformation induced plasticity (TRIP), and quench & partitioning (Q&P) steels. To predict the FZ hardness, data-driven regression-based models have been developed based on carbon content, carbon equivalent concept, and the strengthening mechanisms of the martensite. Among these models, the model based on martensite strengthening mechanisms showed the best per- formance and robustness in estimating the FZ hardness. The assessment of the strengthening mechanism of the FZ showed that most important factors determining the FZ hardness are dislocation hardening due to carbon atoms, interface hardening due to block boundaries, and solid solution hardening due to substitutional alloying elements, respectively. A simple relation is proposed to estimate the load-bearing capacity of auto- motive steel spot welds during interfacial failure based on the FZ hardness. © 2023 The Author(s). Published by Elsevier B.V. This is an open access article under the CC
