• 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.

• 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.

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.

This study investigates the significant factors on weld quality in the resistance spot welding of quench and partitioning advanced high-strength steel. Attention is laid emphasis on nugget formation and growth, weldability range (lobe curve), nugget size and shape (macrostructural features), coating effects, mechanical properties, and fracture behavior of resistance spot welds. As zinc-coated steel sheets are widely applied in body-in-white in automotive industry, coating's effect on weld quality is particularly significant. The research establishes correlations of nugget macrostructure, coating effects, and the following mechanical properties and fracture characteristics of the welds. Key conclusions recognize that Zn coating significantly alters the nugget formation mechanism by reducing the electrical contact resistance. Compared with uncoated samples, Zn-coated steels require a larger welding current for nugget formation because the major heat generation transforms from contact resistance to bulk resistance within the steel sheet. This reduction in contact resistance heating leads to larger nuggets for the same weld conditions. The uncoated samples show greater hardness of fusion zone and tensile-shear strength than coated samples, recording peak load-bearing capacities of 28 kN and 24 kN for medium heat input conditions, respectively. The critical welding current to transition into pullout failure mode was 9.5 kA for uncoated samples and 9.0 kA for coated samples. These findings highlight the complex interplay among coating presence, process conditions, and nugget shape attributes, all of which are crucial for predicting the mechanical performance and failure mode of resistance spot welds in Zn-coated QP980 steel.

Hydrogen embrittlement (HE) is a critical concern in advanced high-strength steels (AHSS), particularly after resistance spot welding (RSW) and laser welding (LW), where localized microstructural changes and residual stresses exacerbate susceptibility to hydrogen-induced cracking. This review comprehensively examines the hydrogen embrittlement susceptibility of RSW and LW joints by analyzing key influencing factors such as hydrogen diffusion, trapping sites, and microstructural transformations at the weld and heat-affected zones (HAZ). The study discusses various hydrogen charging methods, including acid immersion and cathodic charging, to simulate real-world hydrogen exposure conditions. To evaluate hydrogen embrittlement susceptibility, a range of mechanical and electrochemical testing techniques are reviewed. For RSW joints, methods such as slow strain rate tensile testing (SSRT), incremental load testing (ILT), and constant load testing (CLT) are explored to assess delayed fracture risks. Additionally, for LW joints, self-restraint bead-on-plate tests are examined, highlighting the role of weld pool dynamics and solidification characteristics in hydrogen trapping. Furthermore, advanced hydrogen quantification techniques, including thermal desorption spectroscopy (TDS) and gas chromatography with a thermal conductivity detector, are discussed to accurately determine hydrogen concentration and distribution in welded regions. By correlating hydrogen uptake, microstructural evolution, and embrittlement susceptibility, this review provides a systematic understanding of hydrogen embrittlement mechanisms in RSW and LW joints. The insights presented aim to support the development of optimized welding strategies and mitigation approaches, enhancing the structural reliability of AHSS for automotive and industrial applications.

In response to the increasing structural mass of electric vehicles, there is a growing need for localized reinforcement strategies that can enhance local stiffness, defined as resistance to concentrated bending deformation, while minimizing thermal distortion and maintaining full compatibility with existing arc-based manufacturing systems. This study presents a reinforcement strategy using arc-based solid wire deposition, designed to improve local stiffness-defined as the resistance to localized bending deformation-while minimizing thermal distortion. Two bead geometries were developed: an S-shaped bead for flat regions to disperse stress through curved paths, and a GUSSET bead with weaving deposition to enhance stability in corner areas. Key process parameters, including weaving amplitude and frequency, were optimized to ensure consistent bead morphology and reduced heat input. Bending tests showed that the S-shaped design(SR) improved maximum load capacity by up to 49.6% compared to conventional Gird patterns (GR). Three-dimensional surface profile measurements verified the geometric consistency and low thermal-induced distortion of the optimized reinforcement beads across different clamping conditions, thereby demonstrating the practical viability of arc-based deposition for localized structural enhancement in automotive body applications.