Mechanisms for mitigating liquid metal embrittlement via surface decarburization in Zn-coated advanced high-strength steels

Third-generation advanced high-strength steels (AHSS 3. Gen.) have been developed to improve lightweight design and safety in automotive applications. Their superior strength–ductility balance originates from retained austenite stabilized by Si. However, Si increases susceptibility to liquid metal embrittlement (LME) in Zn-coated steels during high-temperature processes such as resistance spot welding. Decarburization is proposed as a cost-effective LME mitigation strategy, though its effectiveness remains under debate. This study demonstrates that decarburization effectively mitigates LME severity, as revealed by interrupted welding tests and microstructural analysis. Crack reclassification based on morphology shows that non-decarburized steel exhibits fewer but more severe straight-shaped (S-type) cracks (72.17 %), whereas decarburized steel exhibits a greater number of shorter, network-shaped (N-type) cracks (79.87 %). This shift in crack behavior is closely linked to decarburization-induced microstructural changes. A higher fraction of high-angle grain boundaries (HAGBs) promotes Zn dispersion across multiple GBs, reducing localized Zn accumulation. Grain growth disrupts GB connectivity and deflects Zn propagation, while internal oxide formation weakens GB cohesion. Although these factors increase crack initiation, they collectively hinder crack propagation and shift the cracking mode from concentrated and damaging to more distributed and less detrimental. Consequently, decarburization modifies the surface microstructure and redistributes liquid Zn, effectively mitigating LME severity. These findings offer new insights into LME mitigation and underscore the potential of decarburization as a practical and scalable strategy for Zn-coated AHSS 3. Gen. • Decarburization reduces severe liquid metal embrittlement crack formation. • A new classification system distinguishes severe and less severe crack types. • Decarburization promotes Zn dispersion, altering crack morphology and severity. • Microstructural analysis links grain boundary characteristics to crack behavior. • Finite element modeling reveals tensile stress role in crack formation dynamics.