The structural integrity and safety of advanced high-strength steel (AHSS) components, particularly in the manufacturing of battery electric vehicles (BEVs), depend critically on weld quality. Conventional weld evaluation techniques are often costly and time-intensive, relying heavily on destructive testing and iterative process adjustments. To overcome these limitations, this study introduces a hybrid AI-digital twin framework (DT Core) for virtual assessment of weld quality in high-strength resistance spot welding (RSW). In this study, Simufact weld simulations are integrated with a multidisciplinary design optimization (MDO) platform (modeFRONTIER) to enable virtual optimization and scheduling of welding parameters. A combined design of experiments (DoE) and finite element method (FEM) strategy, along with trained response surface methodology (RSM) models, was applied to predict weld nugget geometry and reduce process variability. In the automotive industry, comprehensive characterization of AHSS materials provided the basis for accurate DT calibration and validation, while machine learning (ML) algorithms further enhanced predictive performance, substantially reducing reliance on physical trials. The resulting DT achieved high fidelity, with simulation results deviating less than 4

Liquid metal embrittlement (LME) poses a significant challenge in the automotive industry because it can cause circumferential surface cracks adjacent to the weldment, especially when resistance spot welding (RSW) third-generation advanced high-strength steels (3G-AHSS). Numerous studies aim to predict the onset of LME cracks during welding. Gleeble hot-tension tests are often used to understand the LME crack mechanism, including identifying the critical stress at various temperatures. An investigation developed a finite element method (FEM) to predict LME cracks; however, validation against actual welding conditions remains limited. This research assesses the critical stress from in-situ welding observations at temperatures ranging from 594 °C to 786 °C and compares these results to those from Gleeble tests. Findings indicate that the critical stress obtained from Gleeble tests directly correlates with the critical stress necessary to initiate LME crack during welding.

The mechanical properties and failure behavior of groups of resistance spot welds made from two grades of third-generation advanced high strength steels (3G-980 and 3G-1180) were investigated using two partially joined hat channel rails, which were separated under tensile-bending loading conditions. The component tests revealed 2.3

The challenge of intermetallic compound (IMC) embrittlement, resulting from thick IMC layers at the braze/ substrate interface, and the lack of a clear strategy to manipulate IMC formation, has hindered the development of reliable laser weld brazing (LWB) processes. This study addresses these challenges by introducing a novel approach to IMC manipulation during LWB of thin-gauge Zn-coated steel with Si-bronze filler on a double- flanged lap joint. By shifting IMC formation from the interface towards the interior region of the braze, the research mitigates embrittlement by developing a new IMC category, termed surrounded interface-IMCs (SIIMCs), distinct from traditional interface-IMCs (I-IMCs). The study proposes a Wire-adjusted heat input strategy to optimize brazing conditions, introducing a relative heat input equation ( HI Relative ) that correlates with various brazing defects and IMC formation. The generic scientific contribution of this work lies in identifying a critical HI Relative value of 32 J/mm for defect-free brazing, with an additional threshold of 12.44 J/mm above this level to promote a high density of SI-IMCs, occupying up to 38.2 +/- 16.9 % of the braze cross-sectional area. These SIIMCs, characterized by a shell-like Fe-Si layer and a bulky (Fe-rich)-Cu eutectic phase, enhance the mechanical performance of the brazed joints. Furthermore, this study reveals the novel role of Mn segregation in creating diffusion channels for Fe-Si IMC development, advancing the scientific understanding of IMC formation. Visualization through digital image correlation (DIC) during tensile testing showed that increasing the SI-IMC area fraction from 1.2 +/- 2.4 % to 38.2 +/- 16.9 % resulted in a 14 % increase in tensile peak load and a 350 % increase in ductility. This highlights the critical role of SI-IMCs in improving the strength and ductility of LWB joints, offering a new pathway for enhancing the performance of brazed structures.

Half-section welding has been proposed as a method to observe nugget growth throughout the resistance spot welding process. Some past studies used this technique as an in-situ monitoring approach to analyse the cracking behaviour during welding. However, there is a lack of understanding regarding the half-sectioned welding parameters, which critically impacts the liquid metal embrittlement cracking index. This study optimised half-sectioned welding parameters for liquid metal embrittlement crack analysis by developing process maps that compare the half-sectioned and full-section processes. It was found that the liquid metal embrittlement crack located at the weld shoulder can correlate to the temperature gradient in the heat-affected zone. This study proposes half-sectioned welding as a viable technique for liquid metal embrittlement analysis.

In automotive assembly, laser brazing (LB) is important for enabling high-precision, lightweight car body design with numerous fine details in body-in-white components [1]. To ensure the production of defect-free LB seams, there are automated nondestructive examination (NDE) techniques for real-time inspection. Interpreting real-time NDE data using artificial intelligence (AI)-powered processing systems represents a major step toward zero-defect manufacturing [2]. Using AI as a basis for quality control inspection enables replacing human decisions, allowing for quick correction of process conditions. Also, minimizing human interaction at production lines will boost efficiency in time, accuracy, and human recourse. As input/training dataset(s) are necessary to feed machine learning algorithms for AI development, the present work targeted emulating, on a laboratory scale, a broad variety of braze seam geometries and internal defects in LB joints that can occur in a manufacturing line. The main focus of the present study was to optimize the braze seam geometry and investigate conditions that manifest the braze seam imperfections, including a wavy appearance LB seam, spatter formation around the seam edge, and porosity, as important LB defects (key problem indicators). The formation of these defects will be understood with respect to variations in the process parameters, including laser power (LP), travel speed (TS), and wire feed speed (WFS).

Laser brazing (LB) offers significant advantages over other types of traditional seam joining methods: the ability to join dissimilar materials without significant melting of the substrates, reduced Zn burn-off in Zn-coated steels, and minimal heat-affected zone (HAZ) formation. However, the edge (or the tip) of laser-brazed beads can be one of the potential cracks initiations sites which results in catastrophic failure of the parts. The existing literature on LB offers no insights into the geometry and elemental segregation at the edge of the brazing bead as a result of the interaction between the laser beam and the substrate. To bridge this gap in knowledge, LB was conducted in the bead-on-plate configuration on two different types of Zn-coated hot-dip galvanized (GI) and galvannealed (GA) advanced high-strength steels (AHSS) using a Si-Bronze filler wire. LB provided an improved wettability because of the formation of a tail-like geometry at the bead edge of both coatings showing a high Zn concentration, which is opposed to what has been observed in similar studies on gas metal arc brazing (GMAB). LB had a significant effect on modifying the surface morphology of the GA-coating, which has previously not been reported in the literature. Similar Zn distribution of LB samples at the bead edge of both coatings clearly showed that the wettability of molten filler material in LB is improved relative to GMAB.

The third generation of advanced high-strength steels (3G-AHSS) has been developed to provide high strength and high ductility, which attract automakers. To protect these materials from corrosion during service, these materials are typically coated with zinc. During resistance spot welding (RSW), the zinc coating can melt, allowing it to penetrate into the grain boundaries (GBs), and lead to liquid metal embrittlement (LME) phenomena. Concerns regarding LME susceptibility have impacted the industrial application of 3G-AHSS; therefore, its mitigation has become a top focus for automakers. Several possible strategies for lowering LME severity by altering welding parameters have been proposed to mitigate LME in similar spot weld joints. However, these strategies were not tested on a dissimilar spot weld joint. Therefore, in this work, 1.4 mm gauge thickness galvanized (GI-coated) 3G-980 AHSS was joined with 0.6 mm thick Interstitial Free (IF) steel. In this work, current pulsation and ultra-short hold time were proposed to minimize LME severity. The robustness of the developed welding schedule was then tested on welds made with industrial disturbance factors such as pre-strained sheets (between 0 to 80% of 3G-980 material yield strength) and electrode misalignment (between 0° to 10° misalignment) compared to baseline parameters. In severe circumstances of disturbance factors, the resulting optimized welding schedule decreased LME cracking and showed improved resistance to LME, lowering LME severity by 41% for the extreme pre-strain condition and 27% for the extreme misalignment angle.

Zinc coatings are generally utilized for manufacturing corrosion-resistant advanced high-strength steels (AHSS). However, in new third generation AHSS (3G-AHSS), zinc from the coating may interact with the steel substrate leading to liquid metal embrittlement (LME) cracking during resistance spot welding (RSW). A critical RSW parameter that influences the LME response of the utilized 3G-AHSS is the electrode force. This study showed that the influence of electrode force on LME depended on whether or not welds experienced expulsion. When welding with low heat input, without expulsion, LME cracking severity decreased as electrode force increased. In such cases, increased force aided with heat extraction during welding, relieving the critical stresses required by LME cracking. In contrast, when welding with high heat input, resulting in expulsion, increased force elevated LME cracking. It was shown that high force increased the sudden indentation of the electrode into the substrate (electrode collapse), leading to rapid cooling of the weld shoulder. The rapid cooling increased the thermal stresses associated with the collapse event, promoting LME. This study established that the electrode force has two distinct roles on LME. When welding below the expulsion current, high force decreased LME. On the other hand, when welding above the expulsion current, more severe LME cracking was observed at high electrode force. The results from this study show that expulsion itself (excluding its association with increased heat input) is a factor contributing to LME cracking, which highlights the importance of considering the expulsion phenomenon in designing LME resistant welding schedules.

Friction stir processing (FSP) is used to eliminate the defects and modify the microstructure of the joints made using other welding and joining processes. In transient liquid phase (TLP) bonding, the formation of deleterious phases in the bonding zone as well as diffusion affected zone (DAZ) is always challenging. In this study, FSP was applied as a post-bond treatment to modify the microstructure of TLP joints for 304 stainless steel. Microstructural analysis of the joint after FSP showed no sign of the TLP bond line previously visible in the stir zone as well as refined microstructure due to dynamic recrystallization. The eutectic phases, produced by incomplete isothermal solidification of the liquid interlayer during TLP bonding, were coarse and interconnected before FSP. After FSP, these phases became completely distributed in the stir zone, and in the thermo-mechanically affected zone, they were finer and disconnected from each other. In the thermo-mechanically affected zone, a narrow curved shape TLP bonding line was observed, and its width increased into the heat-affected zone. FSP increased the TLP joint strength by about 37% from the pre-FSP state. After FSP, the joint strength reached about 87% of that of the base metal, which is consistent with the microstructural observations showing a lack of defects and discontinuities at the TLP bond line. As a consequence, FSP can be regarded as a complementary process to improve the quality of TLP joints, maximizing their mechanical properties.

Advanced high-strength steels (AHSSs) are generally difficult to join using traditional fusion welding processes because the heat input required for steel fusion has a detrimental effect on the mechanical properties of the joint and the heat-affected zone (HAZ). An alternative non-fusion joining method called weld-brazing, has garnered serious attention in the automotive industry as it uses traditional welding heat sources to braze different types of alloys using lower melting temperature filler materials. The process offers significantly lower heat input which reduces HAZ softening while minimizing zinc burn-off and other welding defects. However, the literature available on this joining process is limited and the reported strength of these weld-brazed joints is typically very low for load-bearing applications. This study shows that weld-brazed joints can, in fact, be used for load-bearing applications by using the appropriate type of Zn-coated steel (i.e., galvanized (GI) or galvannealed (GA)) and properly controlling the joint geometry. The results showed that the type of Zn coating had a significant effect on joint strength, with the GA coating performing significantly better compared to the GI coating. A novel plasma cleaning surface treatment was used to separate the effect of root geometry from the effect of alloy composition, microstructure, and local mechanical properties of the braze at the root, showing that the root geometry and its alloying composition were the main determining factors that controlled the fracture mode and the joint strength of arc-brazed lap joints. By offering insight into the effect of two of the main factors affecting the mechanical properties of arc-brazed lap joints that have not been discussed in the existing literature, the results presented in this study are highly novel and of great relevance to the adoption of this transformative technology on a much wider scale.

Advanced high strength steels are increasingly being used in automotive structures. However, the zinc coating commonly applied for corrosion protection can add complications during the resistance spot welding (RSW) process, namely liquid metal embrittlement (LME). This study evaluates the LME susceptibility of three material grades and two coating types during RSW and provides a new approach to assess material LME susceptibility. This work provides a methodology to process hot tensile testing LME susceptibility data to calculate LME severity observed in resistance spot welding. This methodology was applied to hot tensile testing data from six steels (three grades with two different coating types) and successfully predicted the relative LME response of these materials resulting from RSW.

During laser welding of Al–Si coated 22MnB5 steel, the melted Al–Si coating alloys with the molten weld pool promoting α-ferrite phase formation during the heat-treatment stage of hot-stamping, which results in a fusion-zone (FZ) microstructure consisting of α-ferrite islands disbursed through a martensitic matrix. The presence of the softer ferrite phase is the main cause for premature failure of laser-welded 22MnB5 joints in the hot-stamped condition. This work showed that surface modification of the Al–Si coating using an additive manufacturing technique called electro-spark deposition (ESD) prior to laser welding prevented α-ferrite formation in the FZ post-welding and hot-stamping. This was achieved by the in-situ alloying of ferrite-suppressing carbides and austenite-stabilizing elements. These alloying agents were added to the FZ by applying different ESD-modified coatings to the material surface, which melted into the molten weld pool during laser welding, leading to the simultaneous dispersion and solid-solution strengthening of the FZ after hot-stamping, respectively. The modification of the Al–Si coating prior to welding using tungsten-carbide (WC) and Inconel 625 (In625) resulted in drastically improved mechanical properties of the welded joint in the hot-stamped condition. In fact, this study showed that by carefully modifying the as-received Al–Si coating using ESD prior to laser welding could be used as an effective method to shift failure from the FZ, where it normally occurs, to the base material (BM). This work is highly relevant to the on-going discussion in the advanced manufacturing and materials science communities regarding the production of functionally-graded components as it proposes the implementation of an advanced processing technique to achieve the production of novel materials with highly optimized properties.

Advanced high strength steels (AHSS) used in automotive structural components are protected using zinc coatings. However, the steel/zinc system creates the potential for liquid metal embrittlement (LME) during resistance spot welding (RSW). Manufacturing conditions, such as mechanical restraint, were simulated by external loading on the sample during welding. The results showed increased LME severity in all cases, but the material response to external loading was different depending on the material’s inherent susceptibility to LME. Therefore, the manufacturing conditions are more critical for certain AHSS grades. This work provides guidelines on process design for LME mitigation and awareness during manufacturing.

The mechanical performance and failure characteristics of resistance spot welds from two grades of third generation advanced high strength steels (3G-AHSS), designated 3G-980 and 3G-1180, to combined/mixed loading were investigated by conducting KS-II tests in eight different loading orientations. Due to inherent difficulty in gripping the high strength 3G-AHSS, a combination of coupon rotation, slippage, and deformation occurred especially at shear-dominated loading orientations. This study utilized a new fixture based on Arcanapparatus coupled with digital image correlation technique to track the instantaneous orientation of the KS-II coupons. This way, the actual proportion of shear and tensile components of force applied to the nugget, instead of the nominal ratio at the beginning of the tests, could be quantified experimentally, which in turn improved the calibration accuracy of a spot weld strength-based failure criterion. A novel triangulation method was proposed to minimize the influence of coupon slippage and deformation at regions away from the nugget on calculated absorbed energy. The more precise absorbed energy calculation was achieved by changing the displacement reference point from the crosshead to a local measurement between coupon halves. Energy absorption capabilities of the spot welds were calculated by separately tracking the deformation of the nugget in the shear and tensile directions. It was observed that within the tensile-dominated loading orientations (45-90) pullout failures of 3G-980 spot welds exhibit much higher post-failure energy absorption capabilities than the RSW of 3G-1180. The propagation of the formed cracks into the fusion zone was found responsible for the relatively inferior energy absorption capability of 3G-1180 joints.

Lath martensitic high-strength steels are some of the most widely used materials in structural applications, and consequently, refining and controlling the lath martensitic morphology is of significant interest to expand the versatility of the steels and their associated range of applications. It is well known that the steel’s carbon content has a pronounced effect on the lath martensitic morphology, its crystallography, and hierarchical structure, but the effect of other elements like Mn and Ni is still relatively unclear. This study provides a comprehensive investigation on the effect of Ni on the morphology, crystallography, microstructural refinement, internal transformational strain, and mechanical properties of lath martensite using advanced analytical tools at the meso-, nano-, and atomic scales to offer insights that have so far been lacking in the literature. The in situ alloying of Ni with the bulk microstructure triggered a relatively ideal martensitic transformation such that the majority of the austenite (γ) grains transformed into four martensitic packets (with each packet representing one of the four close-packed {111} planes in a given γ-grain) with a relatively homogenous distribution of the twenty-four martensitic variants that can potentially evolve in an γ-grain based on the Kurdjumov–Sachs (K–S) orientation relationship. The addition of Ni lowered the martensite start (Ms) temperature and strengthened the γ-matrix at lower temperatures, which resulted in the refinement of the packets and blocks due to an inherent need by the matrix to self-accommodate the transformational strain, whereby significantly decreasing the internal strain in the microstructure. The results of this study showed that alloying lath martensitic steels with Ni can prove to be an effective method to refine the hierarchical microstructure in a way that can be used to improve the mechanical properties of these steels.

Distinct microstructures and mechanical properties are produced in resistance spot welds (RSW) of advanced high-strength steels (AHSS). The local mechanical properties within the fusion zone (FZ) and heat-affected zones (HAZ) govern the global failure response of RSW automotive structural assemblies in crash events. In this study, a novel experimental procedure was developed to characterize the local properties of RSW sub-regions using sub-size tensile and simple shear specimens extracted from the spot weld, with local strain measurement utilizing digital image correlation (DIC) techniques. The FZ properties were obtained using a simple shear test, while the HAZ properties were characterized using a tensile test containing a central RSW. A range of AHSS grades were considered to evaluate the proposed methodology, including 3rd Gen-980, 3rd Gen-1180, and press-hardened steel (PHS1500) with respective minimum ultimate tensile strengths of 980 MPa, 1180 MPa, and 1500 MPa. Strain localization occurred wherever the hardness was lowest such as in the subcritical heat-affected zone, with limited deformation in the harder regions, such as the upper-critical HAZ and FZ. The local stress-strain responses were then compared to a hardness scaling approach to estimate the tensile properties of RSW based on the local hardness in each zone. The hardness scaling method only appears to be a viable method if the microstructures across the weld have similar microconstituents in all sub-zones across the weld.

In the automotive industry, the demand for reduced vehicle weight, improved safety and enhanced crashworthiness qualities continues to rise which introduces the need for parts with tailored properties. This demand can be met by employing tailor-welded blanks which are made of two or more types of press-hardened steels that are laser welded together and then hot-stamped to produce highly optimized parts for use in vehicle body-in-white applications. The most commonly used press-hardened steel for high strength applications is the Al-Si coated 22MnB5 grade steel that has a strength of around 1500 MPa in the hot-stamped condition. Although the Al-Si coating offers excellent barrier protection against oxidation and decarburization during hot-stamping, the Al-Si coating proves to be problematic during laser welding as it melts and mixes into the weld pool increasing the Al-content in the fusion zone, which promotes the formation of ferrite during the heat-treatment stage of hot-stamping, resulting in a dual-phase fusion zone microstructure that consists of ferrite grains embedded in a martensitic matrix. The presence of the softer ferrite phase has been shown to be the principal reason for premature failure of hot-stamped laser-welded joints. Currently, this issue is resolved by an additional, and relatively costly, manufacturing step that involves the laser ablation of the Al-Si coating prior to welding. The present work shows that this step can be eliminated from the manufacturing process by welding Al-Si coated 22MnB5 steel through a colloidal graphite coating which results in the in-situ ablation of the Al-Si coating during welding. This changes the alloying chemistry of the fusion zone which affects the resulting morphology by preventing ferrite formation which improves the mechanical properties of the weld. Failure in these welded samples was successfully shifted from the fusion zone, where it normally occurs, to the base material.

Advanced high strength steels (AHSS) used in automotive structural components are commonly protected using zinc coatings. However, the steel/zinc system creates the potential for liquid metal embrittlement (LME) cracking during welding. In this study, the effect of angular welding electrode misalignment as recommended by industry was investigated and resulted in LME cracking in the sub-critical heat affected zone (SCHAZ) which is a region where LME has not been previously observed. The largest misalignment case of 6° resulted in bending of the work piece and an angled weld nugget/heat affected zone (HAZ), causing LME cracks in the lower temperature HAZ regions. SEM and EBSD analysis characterized the LME crack area as the SCHAZ and below the Ac1 transformation temperature. Furthermore, dilatometry analysis showed the Ac1 temperature to be approximately 660 °C, meaning LME occurred below the traditional 700 °C minimum threshold. In addition to this novel observation of LME before transformation and at low temperature, it was also noted that only misalignment beyond 2° promoted LME cracking. The low angle 2° case was observed to be statistically the same using a Student’s t-test (α = 0.05) on 6 welded samples for LME crack severity compared to a standard 0° case.