• Mechanism of interfacial Si enrichment in hindering Fe-Zn alloying was studied. • Localized Si-enriched zone was found between the steel matrix and the Zn coating. • Si-enriched zone inhibited Zn diffusion while allowing limited Fe diffusion. • Shape of the Si-enriched region evolved with increasing annealing time. • Si-enriched regions formed dispersed particle shapes within coating middle. Retained austenite plays a significant role in third-generation advanced high-strength steels (AHSS 3. Gen.), renowned for their excellent combination of strength and ductility. Silicon (Si) is a key element in stabilizing retained austenite. However, it introduces challenges in galvannealing and welding processes in Zn-coated steels, such as inhibited Fe-Zn alloying and increased susceptibility to liquid metal embrittlement (LME). This study investigated the mechanism of Si enrichment at the Zn/steel interface and its role in suppressing Fe-Zn interdiffusion during annealing. Using advanced techniques such as high-resolution transmission electron microscopy and atomic probe tomography, and Thermo-Calc DICTRA simulations, we analyzed the diffusion behavior and microstructural evolution in Zn-coated steels with varying Si contents. Si, driven by its low solubility in liquid Zn and Fe-Zn intermetallic phases, accumulates at the interface, forming a Si-enriched region that significantly suppresses Zn diffusion while permitting limited Fe diffusion. Numerical simulations revealed that the Si-enriched layer forms via the drag effect of the Fe-Zn reaction line, progressively concentrating Si at the interface as Zn diffuses. As annealing progresses, the morphology of the Si-enriched region evolves from layered, cloud-like structures to droplets and elongated dendritic forms, driven by Zn penetration and Fe consumption. These findings provide novel insights into the role of Si enrichment in mitigating LME and optimizing the Zn-coated AHSS 3. Gen., paving the way for advancements in automotive material design.

During the laser welding of Al–Si–coated 30MnB5 steel, the incorporation of molten coating constituents into the fusion zone (FZ) promotes δ-ferrite formation, Al segregation, and heterogeneous microstructures, undermining the reliability of hot-stamped tailor-welded blanks (TWBs). To address this challenge, the thickness and geometry of the remaining Al–Si coating were precisely tailored using femtosecond laser ablation, providing a controllable pathway for regulating Al incorporation into the molten pool. TWBs with systematically varied coating geometries were fabricated and examined to establish the link between coating control, solidification dynamics, and weld performance. EBSD analysis showed that suppression of Al segregation reduced δ-ferrite content by ~30–40% and promoted a more homogeneous martensitic matrix, while KAM results confirmed higher dislocation density and improved plastic accommodation in coating-minimized conditions. Mechanical testing further demonstrated that when the Al–Si coating was controlled to an Fe 2 SiAl 7 inhibition layer, the welded joints exhibited notable improvements in hardness and strength compared to the intact-coated samples. With progressive coating removal, the hardness of FZ exceeded 550 HV and the tensile strength approached 1.7 GPa, whereas fully de-coated joints achieved performance comparable to the base metal. These findings demonstrate that femtosecond laser tailoring of the Al–Si coating thickness and remaining layer structure is a scientifically grounded and scalable strategy for improving the weldability, microstructural uniformity, and mechanical integrity of advanced high-strength steels, offering new opportunities for the reliable production of hot-stamped automotive components. • Femtosecond laser ablation was used to selectively remove Al–Si coatings in TWB joints. • Coating geometry significantly influenced weld pool flow and interface morphology. • Al dilution regulated ferrite formation and martensitic transformation in the fusion zone. • Microstructure evolution was visualized via EBSD and phase-based CCT simulations. • Enhanced strength and ductility were achieved under controlled coating removal conditions.