This study investigated the influence of resistance spot welding (RSW) process parameters on nugget formation and joint strength in dissimilar low-carbon equivalent (LCE) and multiphase (MP) advanced high-strength steel (AHSS) sheets. A factorial experimental design was used to evaluate the effects of welding current, weld time, and cooling time for a multi-pulse weld schedule on cross-tension peak load and nugget diameter. The results indicated that welding current and weld time were the dominant factors that influenced peak load and nugget diameter, and significant interaction effects were observed. The highest peak loads and nugget diameters were obtained within a process window corresponding to welding currents between 7.5 and 10.0 kA and weld times between 130 and 200 ms. Cooling time was observed to influence the transition region between no-weld conditions and expulsion-related conditions, suggesting an effect on failure behaviour. A positive linear correlation between nugget diameter and peak load was identified, indicating that increasing nugget size improved joint strength. However, analysis within the measured nugget diameter range revealed multiple failure modes, suggesting that nugget diameter alone was insufficient for accurately characterizing weld failure behaviour. These findings established a favorable process window for dissimilar AHSS RSW and provide insight into the relationship between process parameters and mechanical performance.

Third generation Advanced High Strength Steels (3gAHSS) crucial for the future of automotive design. When Zn-coated for corrosion protection and subjected to resistance spot welding. they can be prone to liquid metal embrittlement cracking (LME) The influence of hold time variation on LME in the electrode indent above the expulsion limit was investigated in this study. Welds were conducted with hold times of 100 ms, 167 ms, 400 ms, and 1000 ms. LME severity was measured using cracking index, revealing that LME severity peaked at a hold time of 167 ms and reduced with both longer hold times, corresponding the observations in the literature, but also decreased for shorter hold time, contradicting the literature. Finite Element (FE) simulations were performed to understand this behavior, highlighting that the timing of electrode retraction significantly affects the cooling and reheating of the weld, altering the thermal stresses that cause LME. A short hold time (100 ms) prompts radial heat dissipation into the bulk, minimizing surface thermal gradients in radial direction and thus leading to reduced LME. Conversely, at 167 ms (standard AWS hold time), heat extraction by the electrodes towards the sheet surface creates steep radial temperature gradient at the electrode-sheet interface, increasing thermal stress at the surface and promoting LME. At longer hold times (400 and 1000 ms), the prolonged electrode contact cooled and supported the sheet surface longer, resulting in lower temperatures and stress levels at electrode release, leading to less severe LME compared to moderate hold times.

This study investigates the impact of boron (B) on the mechanical properties and recrystallization behaviour of cold rolled and batch-annealed titanium (Ti) microalloyed high-strength low-alloy (HSLA) steels. HSLA steels are essential in industries such as automotive and construction due to their superior mechanical properties and costeffectiveness. The addition of microalloying elements, particularly Ti, has shown significant promise in enhancing these properties through mechanisms like precipitation strengthening and controlled recrystallization. However, the role of B in this context remains underexplored. Here, three alloy compositions were systematically examined: a reference low carbon steel base alloy with no Ti or B (Ref), the base alloy with added Ti (Ref + Ti), and the base alloy with both Ti and B additions (Ref + Ti + B). The findings showed that microalloying with Ti + B can provide excellent strength improvements in the hot rolled strip, mainly due to increased hardenability that promotes the formation of finer ferrite grains with a high dislocation density. However, the recovery and recrystallization behaviour is such that it is not possible to reproduce these benefits after cold rolling and batch annealing, highlighting the complexity of optimizing microalloying strategies for cold-rolled and batch annealed products. The insights gained from this study provide a deeper understanding of the mechanisms governing the recrystallization of Ti and B microalloyed HSLA steels, paving the way for the development of advanced materials with tailored properties for critical applications.

Steel is an essential material in modern infrastructure and industry, but its production is associated with significant carbon dioxide emissions. Biocarbon utilization in electric arc furnace (EAF) steelmaking represents a promising pathway toward reducing the carbon footprint of steel production. This review draws new perspectives on the current state of biocarbon utilization in EAF steelmaking by collectively examining the literature from multiple scales of testing, from laboratory experiments to industrial trials. The scientific insights from each scale are defined and the results are collectively pooled to give a comprehensive understanding of biocarbon’s performance for EAF applications. Several recent progressions are identified along with critical limitations, such as biocarbon’s high reactivity or low density. However, solution pathways like agglomeration are established from the thorough understanding developed by this study. These insights aim to enhance the progression of biocarbon utilization in the EAF process, ultimately facilitating the development of more efficient and sustainable steelmaking. The proposed areas for future research, such as optimizing key biocarbon properties or improved injection systems, are expected to have significant impact on the next phase of biocarbon adoption.

The electric arc furnace (EAF) is a promising approach to decarbonize the iron and steel industry. In EAF steelmaking, injecting carbon into the molten slag remains crucial for creating a foamy slag, which enhances the energy efficiency of the process and protects the furnace. Biochar (BC) has emerged as a potential alternative to traditional fossil carbon for slag foaming. However, fully replacing fossil carbon with BC poses technical challenges. In this study, the partial replacement of fossil carbon with BC is considered, in the form of a petroleum coke (petcoke) and BC blend. Interestingly, a blend of petcoke and BC matches or possibly outperforms either carbon type individually, due to a synergistic effect. Using an induction furnace to simulate EAF conditions, a synthetic slag is melted, and injection carbon is added into the slag layer. The slag foaming effectiveness of petcoke, BC, and three blend cases are studied. Thermogravimetric analysis reveals that the BC is more reactive with slag compared to petcoke, which leads to an initial high intensity of CO generation. However, the CO generation was not continuous or consistent. All experimental results are combined to propose a mechanistic description of the slag foaming behavior of BC blends.

The direct carbonation of steel slag has emerged as a promising approach for carbon dioxide (CO2) utilization and sequestration, holding potential for advancing sustainable steel production. Despite considerably high expectations for these cleaner upcycling pathways, their maturity level remains relatively low and large-scale direct carbonation of steel slag is largely untested. To facilitate steel slag carbonation on a scale necessary for a zero-carbon future economy, this article provides a comprehensive review of fundamental carbonation mechanisms and critical parameters governing the reaction process, including temperature, pressure, reaction time, liquid-to-solid ratio, and CO2 partial pressure. The study critically examines the unique interactions among these process parameters, which can either limit or enhance the process optimization. The spectrum of scientific challenges associated with this pathway, including reaction rate limitations and the carbonated product valorization, particularly as a binder or aggregate in the construction sector, are identified and addressed. These insights aim to enhance the carbonation potential of steel slag for possible cleaner upcycling implementation pathways, ultimately facilitating the development of more efficient and sustainable carbon capture utilization and sequestration (CCUS) technologies. The proposed improvements are expected to be instrumental in promoting sustainable practices, not only to foster the decarbonization of the steelmaking industry but also in aiding other hard-to-abate sectors, such as the cement and concrete industry, in achieving their own decarbonization goals.

This study investigates the effect of in-situ post-weld heat treatment (PWHT) parameters on the microstructural changes at the edge of the fusion zone (FZ) of a resistance spot weld, which is known to be susceptible to crack propagation. The in-situ PWHT parameters (cool time and post-weld time) were varied to determine the temperature regime that induces in-situ grain refinement at the edge of the FZ. Findings revealed that applying the post-weld current when the edge of the FZ is austenitic (after a short cool time) and below the melting point prompts in-situ grain refinement from columnar to a recrystallized equiaxed structure. Meanwhile, after a longer cool time, when the edge of the FZ cools below the Ms temperature, reheating the edge of the FZ above the austenite start temperature results in martensite reversion to austenite with the grain structure similar to the initial epitaxial structure after cooling. The result indicates that in-situ grain refinement can be achieved through a PWHT schedule that involves reheating the edge of the FZ to initiate austenite recrystallization. Moreover, the formation of new equiaxed grains from the austenite recrystallization process effectively retards crack propagation during cross-tension test resulting in 89 % improvement in the energy absorption capability compared to the baseline condition. Therefore, optimizing in-situ PWHT parameters to initiate austenite recrystallization at the edge of the FZ is critical to enhancing joint performance and overall crashworthiness.

Weld-brazing has emerged as a promising alternative to fusion welding for joining Zn-coated advanced high strength steels (AHSSs) due to its lower heat input, reduced Zn burn-off, and elimination of defects like porosity and blowholes. Additionally, the weld-brazing process minimizes HAZ softening, a phenomenon that can affect joint strength. While weld-brazing can potentially eliminate the liquid metal embrittlement (LME) caused by the penetration of molten Zn into grain boundaries (GBs), this study found that galvanized (GI) DP600 steels were more susceptible to intergranular penetration of Cu into the substrate during weld-brazing compared to galvannealed (GA) DP600. Although no Zn-related LME was observed during arc-brazing with Si-Bronze filler, the potential for Cu-related LME cracks to form in GI coated steels may affect the mechanical integrity of the joint.

The electric arc furnace (EAF) has the potential to significantly contribute to the decarbonization of the iron and steel industry. However, during EAF steelmaking, carbon still needs to be injected into the molten slag to initiate slag foaming, which is beneficial to the energy efficiency and protection of the furnace. To move away from fossil carbon, biocarbon has gained attention as an injection carbon agent. In this study, two biochar candidates were added to the molten slag layer of an induction furnace for steel melting, to simulate EAF steelmaking conditions. The resultant slag foaming height was measured, and a ranking in comparison to two fossil carbon candidates was developed. The results indicate that the injection biochar sample, in the form of a bio-briquette, has a considerable degree of slag foaming capacity. More work is ongoing to develop a standardized testing methodology of ranking various injection biochar candidates for their suitability and qualification for use on a larger 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.

Liquid metal embrittlement (LME) has emerged as a major concern when developing high-strength automotive steels. However, information regarding the impact of initial microstructure on LME severity is limited in the Fe/ Zn couple. Specifically, there is no consensus as to which ferritic and austenitic microstructures are more sus-ceptible to LME cracking. The present study aims to examine the LME cracking behavior of fully ferritic and austenitic microstructures under the same thermomechanical conditions. It was shown that the ferritic micro-structure has a higher sensitivity to LME crack initiation, whereas the austenitic specimen displayed a much longer average crack length, which indicates higher crack propagation rate than the ferritic specimen. It has been determined that in-situ austenite to ferrite transformations during Zn diffusion, as well as grain boundary segregation of alloying elements such as Cr and Ti, contribute to the LME propagation rate.

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.

Advanced high-strength steels protected by zinc coatings have contributed to a reduction in CO2 emissions in the automotive industry. However, the liquid metal embrittlement (LME) of the Fe/Zn couple induced by simultaneously acting stresses and high temperatures during resistance spot welding could be the cause of unexpected failure. We investigated the possible risk of LME in spot-welded martensitic steel with Zn jet vapor-deposited coating and its influence on weld strength. The weld nugget cross-sections were analyzed (optical microscopy, SEM-EDS), and their tensile shear strengths were compared with their uncoated counterparts. LME cracks were observed in all samples meeting the process window (6, 6.5, 7 kA) located at the edge of the sheet/electrode indentation area. The frequency and length of cracks increased with current, and the occurrence of Zn within cracks indicated the LME mechanism. The shear tests showed the Zn-coated sample underwent a decrease in tensile shear strength that was most evident at a welding current of 7 kA (13.2%). However, LME was excluded as a cause of lower strength. The decrease was attributed to the smaller nugget diameter and the thin slit of Zn coating remaining in the weld notch.

Metal additive manufacturing processes can produce geometrically complex and lightweight components. While conventionally manufactured components are frequently assembled to form larger parts, additive manufacturing can be used to print an entire part without needing any assembly. However, additive manufacturing processes are frequently limited in the size of the part they can produce, and it is often more economically favourable to conventionally manufacture larger or simpler geometries, such as large sheets. In this study, we demonstrate the use of a resistance joining process to facilitate the assembly of additive manufactured components. Projections are designed into additive manufactured parts to allow for joining with a conventional metal sheet. Joint performance is evaluated as a function of design choices, including the type of infill, part thickness, and proximity to adjacent joints, as well as the resistance joining process parameters. High strength joints capable of withstanding an applied torque of up to 80 Nm were obtained and functional parts were assembled to a conventionally manufactured sheet as a demonstration of the process. Incorporating projections for resistance joining into the design stage of additive manufactured parts has the potential to facilitate the use of additive manufactured components in larger assemblies and broaden the adoption of additive manufacturing in industry.

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.

Despite numerous studies making an effort to attain a thorough understanding of the liquid-metal-embrittlement (LME) phenomenon, the metallurgical facet of this catastrophic event remains unclear in iron/zinc (Fe/Zn) systems. While it has been frequently reported that the presence of austenite is an essential prerequisite for LME formation, the present study showed that fully ferritic structure is prone to LME phenomenon and has a high susceptibility to LME-cracking which makes it a novel observation adding to a pool of knowledge regarding LME occurrence. The elemental distribution analysis near the LME crack-tip indicated that liquid Zn was not present which confirmed solid-state grain boundary diffusion was a plausible description of LME-cracking. The occurrence of grain dropout as well as a Zn-containing crack in grain boundary without any branches with other cracks showed that grain boundary sensitization has assisted LME-cracking.

Liquid metal embrittlement (LME) has been reported in many structural materials including steel, aluminum, brass, and nickel during hot-working processes e.g. welding, hot-deformation, brazing, heat-treatment, or in-service time. In many of the applications, such as automotive, aerospace, nuclear industries, LME is considered as a serious safety concern. Over the last decades, research activities have grown considerably striving to understand the abnormal LME phenomenon. However, a fundamental understanding of LME has not yet been unraveled due to the diverse, contradicting proposed mechanisms in different processing routes and materials. In the present overview, first various proposed mechanisms are integrated into a systematic manner. Revisiting the proposed mechanisms based on the most-recent experimental discoveries reveals that the stress-assisted grain boundary diffusion mechanism is a viable LME mechanism. The connection between sub-atomic (electronic) structure modification and the embrittlement at grain boundaries provides insights into the grain boundary decohesion during LME. In a bottom-up approach, then this overview selects Zn-induced embrittlement during assembly welding as an example case of LME. Explanations are provided to show why LME cracks are both seen to degrade and be innocuous to material’s performance. Finally, this review outlines recommended directions for future research steps to overcome LME-cracking risk, both from material and process aspects.

To improve automotive fuel economy, automobile manufacturers are minimizing the weight of the body-in-white. To do this, they are adopting new 3rd generation advanced high strength steels that have excellent strength and ductility. However, these steels are also prone to liquid metal embrittlement (LME) cracking; intergranular cracks caused by molten zinc, from the galvanized coating, penetrating the steel substrate during the resistance spot welding (RSW) process. These cracks are not acceptable to automobile manufacturers as it is unknown how LME cracks affect joint strength during weld service. To decrease LME cracking, extensive research into understanding its governing metallurgy, optimizing welding parameters, and comparing the LME sensitivity of multiple grades has been done. Most of this work was done using hot-tension testing or RSW testing. However, as there is no standard methodology for these tests, producing results that were difficult to compare. This review examined test methodologies for hot-tension and RSW testing LME severity. It was determined that the usefulness of LME testing could be improved if test methods reflected the temperature and stress-state of the welding process, facilitated comparisons between tests, and quantified results were reported. Recommendations are provided to improve hot-tension and RSW tests to meet these goals.

Grain boundaries in polycrystalline materials are a critical feature for structural applications but also create the risk of intergranular embrittlement phenomena, such as liquid metal embrittlement (LME). In LME, an aggressive liquid metal attacks the grain boundary network of a solid metal leading to fracture. Thus far, experimental work and fundamental theories on LME in the Fe/Zn system have provided contradicting results on the embrittler transport mechanism to the grain boundaries. In this study, the LME transport mechanism was determined to be stress-assisted diffusion by both experimental and numerical methods. For the first time, LME formation was incrementally observed which revealed the LME transport mechanism and the role of stress in LME onset. Stress created an additional diffusion flux that enhanced the embrittler penetration. This study provides new insight into the significance of thermomechanical state for the onset of LME and provides evidence for LME as a diffusion-based phenomenon.

The automotive industry is turning to advanced high strength steels (AHSS) to reduce vehicle weight and increase fuel efficiency. However, the zinc coating on AHSS can cause liquid metal embrittlement (LME) cracking during resistance spot welding. To understand the problem, the severity of the cracking must be measured. Typically, this is done from the weld cross-section. Currently, there is no standard procedure to determine which plane through the weld must be examined to gauge cracking severity, leading to a variety of practices for choosing a cutting plane. This work compares the magnitude and variability of LME severity measured from the plane of exhibiting the most severe surface cracking to arbitrarily chosen planes. The plane exhibiting the most severe cracks had more and longer cracks on the cross-section than the arbitrarily chosen plane, resulting in a higher crack severity measurement. This higher absolute measurement increased the relative accuracy of the examination, allowing for fewer welds to be examined to precisely determine the effect of LME mitigation methods on cracking severity, how welding parameters affect LME cracking severity and the predicted LME affected strength of a particular weld.