Browsing by Author "Biro, Elliot"

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A Study on High-Frequency Induction Welding of TRIP 690 Tubes using Mechanical Tests and Computer Simulations
(University of Waterloo, 2025-01-02) Okoroafor, Sydney; Biro, Elliot
The High-Frequency Induction Welding (HFIW) process is increasingly being adopted for manufacturing tubes used in hydroformed Advanced High Strength Steel (AHSS) components, specifically TRIP 690, in the automotive industry. This trend is driven by strict government climate legislation on automobiles that promotes the development of lightweight materials (AHSS) and efficient manufacturing techniques in the industry. Despite its advantages, the HFIW of TRIP 690 faces significant challenges, particularly the recurring issue of oxide inclusion defects. These defects are often undetectable by conventional tube mill inspection technologies and can only be identified through destructive mechanical testing. These defects also lead to poor mechanical properties and potential failures during complex loading scenarios like tube hydroforming. These oxide inclusion defects have not been explored in literature, leading to a critical knowledge gap in the HFIW of TRIP 690 that reduces the yield of high-quality TRIP 690 tubes during the HFIW process. This research aims to bridge the knowledge gaps associated with the HFIW of TRIP 690. It first investigates the influence of welding parameters and oxide inclusions on the mechanical properties of TRIP 690 tubes. Key findings indicate that the Ring Hoop Tensile (RHT) test yields reliable mechanical property data, revealing notable discrepancies when compared to traditional flat sheet data. The study also establishes that welding power and speed significantly affect Ultimate Tensile Strength (UTS), Uniform Elongation (UE), and fracture toughness. Optimal operating regions are identified through mechanical properties-process mapping, linking these properties to the mechanical properties-thermal process map for heat input and temperatures at the vee apex experienced in the vee region. Furthermore, the presence of oxide inclusions is shown to detrimentally impact mechanical performance, resulting in substantial reductions in UTS, elongation, and toughness. A preliminary mesoscale Finite Element Analysis (FEA) model demonstrates the potential to predict failure behavior in samples containing oxide inclusions through simulations. In addition, this research explores the thermal dynamics of the vee region during the HFIW process. A numerical model developed in COMSOL Multiphysics integrates the thermal modeling with experimental validation from tube mill trials, providing a comprehensive analysis of how critical parameters—such as heat flux, coil-to-weld point distance, and the thermophysical properties of TRIP 690—affect weld quality. Through this study useful tools have been developed that can aid in the optimization of HFIW of TRIP 690.
A Study on Improving the Mechanical Performance by Controlling the Halo Ring in the Q&P 980 Steel Resistance Spot Welds
(Elsevier, 2022-01-17) Ramachandran, Dileep Chandran; Figueredo, Bruna; Sherepenko, Oleksii; Jin, Woosung; Park, Yeong-Do; Biro, Elliot
This study focuses on improving the mechanical performance of third-generation Q&P steel resistance spot welds using a double-pulse welding cycle. Single and double-pulse welding schedules were implemented to assess the mechanical performance of the welds. Single-pulse welds exhibited poor cross-tension strength (CTS) values, failed around the fusion zone, and were accompanied by poor energy absorption capability. However, the double-pulse schedule showed improved CTS values by 33%, with an associated 110% increase in absorbed energy. The failure path observed from interrupted cross-tension tests showed that, in welds made using both pulsing schedules, failure proceeded along the fusion boundary and CGHAZ. In the single-pulse welds failed in brittle fashion, whereas the welds made with a double-pulse schedule exhibited a mixed (ductile and brittle) fracture morphology. The high-density microhardness mapping confirmed the presence of a localized softened zone (halo ring) adjacent to the fusion boundary in single-pulse welds. Strong elemental partitioning of Mn, Si, and C in the vicinity of the fusion boundary during long welding time was the primary cause for the halo formation. However, the halo ring was eliminated by performing a double-pulse weld schedule with 30 ms cooling time in between pulses; resulting in improved mechanical properties.
Characterization and microstructure-based modeling of spot weld failure in hot stamped steel
(University of Waterloo, 2022-02-08) Mohamadizadeh, Alireza; Biro, Elliot; Worswick, Michael
The performance of automotive safety components in a crash event depends on the mechanical properties of the sheet metal as well as the failure behavior of the spot welds that are used for sheet metal assembly. Unlike many practical and numerical methods for quantitative and qualitative failure characterization of uniform sheet metals, spot weld failure analysis methods are complicated by the complex microstructure, non-uniform properties and loading conditions around the weld and the fact that spot weld area is not accessible during failure. Therefore, current spot weld failure analysis is typically limited to post-failure observations and peak load measurement. Without having a proper way to characterize the spot weld failure, simple load-based failure criteria were used to predict the onset of failure. In these models the effect of microstructure, through-thickness damage progression, failure mode, the location of failure, post-failure energy absorption, and fracture paths are not considered. In the present work, spot weld failure for several automotive-grade press-hardening steels, Usibor®1500-AS, Ductibor®1000-AS, and Ductibor®500-AS, is investigated considering the effect of microstructure on failure and a hardness-mapping approach to implement local material properties into meso-scale models. To this end, resistance spot weld process optimization was performed using a combination of experiment and process simulation for five different material conditions including the three aforementioned hot-stamped alloys, as well as two tailored hot stamped conditions using lower quench rates for Usibor®1500-AS. Using the optimized welding settings, a transient softened zone at the fusion boundary, the halo ring, was formed in the as-hot-stamped Usibor®1500-AS spot welds. Optical and electron microscopy showed that, the halo ring is a ~100 μm wide band with a minimum hardness of 472 HV around the weld nugget which leads to partial thickness failure and pull-out through a shear-assisted fracture along the band. A novel in-situ¬ failure characterization test method using modified double half-weld (DHW) specimens coupled with Digital Image Correlation (DIC) was developed to capture the failure event on the cross-section of the welds under shear and normal loading. To demonstrate the capability of the new in-situ testing technique for different failure modes, modified RSW parameters were used to eliminate the halo ring in hot-stamped Usibor®1500-AS spot weld and to promote interfacial failure in the hot-stamped Ductibor®1000-AS spot weld. The DIC results revealed that failure is initiated at the weld notch by Corona debonding regardless of spot weld microstructure and loading condition. In the presence of the halo ring, failure occurs by strain localization and shear band formation at the halo ring. In the absence of the halo ring, fracture occurs parallel to the load under normal loading and within the softened HAZ under shear load. Using the DHW+DIC technique, interfacial failure in the as-hot-stamped Ductibor®1000-AS spot weld was found to be a ductile shear-dominant event rather than brittle fracture, as is commonly asserted for interfacial failure in the literature. A hardness/microstructure mapping approach was used to assign local material properties to a discretized spot weld geometry model which was created with a 3D meshing strategy with nominal element size of 60 μm. The constitutive models and fracture surfaces for the HAZ were calibrated based on tensile and plane-strain V-bend test results and implemented in finite element models of lap-shear and cross-tension tests. The models were able to predict partial and complete pull-out as well as interfacial failure responses, depending upon the alloy and welding process conditions. The predicted failure modes and mechanisms, location of failure, and through-thickness damage progression matched the experimental observations and the predicted failure loads were within 6.3% of measured values. From the current research, comprising detailed microstructure characterization, development of the novel in-situ failure analysis techniques, and meso-scale through-thickness modeling, a fundamental understanding of spot weld failure mechanisms and damage progression has been established, which was the key outcome of this research.
Development and Influence of Fusion Boundary Microstructure on Resistance Spot Welds in 3rd Generation Advanced High Strength Steels
(University of Waterloo, 2024-05-08) Ramachandran, Dileep Chandran; Biro, Elliot
The use of advanced high-strength steels (AHSS) is one of the design solutions for making new-generation auto bodies to build economical and environmentally friendly structures without compromising vehicle safety. Among various types of third generation AHSSs, quenched and partitioned (Q&P) steels, which are one of the material solutions due to their exceptional combination of strength and ductility compared to the conventional dual-phase (DP) steels. To produce body-in-white structures, resistance spot welding (RSW) is the predominant joining process used in the automotive industry. Hence, it is essential to investigate the resistance spot weldability of Q&P steels. Recent investigations show that during welding, partitioning of alloying elements occurs at the weld fusion boundary (FB), especially in the steels with higher carbon equivalents. A narrow region around the nugget was transformed, which has a softer microstructure than the fusion zone (FZ) and heat-affected zone (HAZ), referred to as the “halo ring”, leading to premature failures in this weakened zone. When welded (AWS recommended single pulse schedule) and tested in tensile shear and cross tension geometries, it was seen that the fracture path in Q&P RSWs occurred through the halo ring. Moreover, the fracture surfaces of such welds show an intergranular fracture, which is contrary to the previous observations. Based on these preliminary observations, the focus of the present research is to characterize the halo microstructure, find a phenomenological link between thermal history and the halo ring, and solve the discrepancies in failure behaviors by providing robust methods to eliminate halo. Modified welding schedules and post-weld heat treatments were implemented to eliminate the halo ring. The modified double-pulse schedule exhibited improved cross tensile strength (CTS) values by 33%, with an associated 110% increase in absorbed energy than the single-pulse weld. Similarly, the paint baking (PB) process was also implemented in single pulse weld, and it shows 34%, and 102% improvements in CTS and absorbed energy. Both methods improved the mechanical properties by shifting the crack path to the upper-critical heat affect zone (UCHAZ) from the halo ring. The former method modifies the grain structure at the fusion boundary, whereas the latter one redistributes the elements segregated at the grain boundaries to the grain interiors. The elemental diffusion in the halo ring has been discovered by making spot welds sandwiching low carbon (LC steel weld) and high carbon (HC steel weld) steels, respectively with Q&P steels on both sides, which is to tailor the chemical composition of the FZ. The halo formation is more prominent in welds made with LC steel rather than HC steel. It was found that the transient softened zone can be affected by differences in chemical composition between the FZ and UCHAZ. Furthermore, the mechanism of halo formation has been studied by characterizing the halo microstructure with transmission electron microscopic (TEM) analysis. TEM investigation disclosed that the microstructure within the halo ring is characterized as bainitic ferrite accompanied by needle-like cementite, specifically identified as upper bainite. Carbon diffusion towards the FZ in LC steel weld is attributed to higher activity resulting from differences in the chemical potential of carbon. This also accompanied by varying substitutional solute content towards the FZ from the UCHAZ is the reason for the halo formation. These advances in knowledge facilitated the development of strategies to mitigate halo formation through alloying adjustments (alloying with greater and lesser compositions of steels), paint baking, and welding schedule modifications.
Effect of Paint Baking on the Fusion Boundary Softening and Fracture Behavior of Q&P 980 Steel Resistance Spot Welds
(Elsevier, 2023-10-14) Ramachandran, Dileep Chandran; Betiku, Olakunle Timothy; Shojaee, Mohammad; Salandari-Rabori, Adib; Midawi, Abdelbaset R.H.; Kim, Ji-Ung; Bakhtiari, Reza; Macwan, Andrew; Biro, Elliot
During automotive assembly, vehicles undergo low-temperature heat treatment (paint baking) to harden the paint. Although paint baking occurs at a relatively low temperature, it can remarkably affect the weld's mechanical and fracture behavior. This work studies how paint baking improving the strength and fracture behavior of Q&P 980 spot welds exhibiting a halo ring; a low carbon enriched zone in the weld nugget. The mechanical behavior of the paint-baked welds reveals an increase in cross-tensile strength and absorbed energy when baked at 180 °C for 27 min. Microstructural observation showed that the martensite present in the as-welded conditions started to decompose into tempered martensite with ε-carbide in the martensitic matrix. Gleeble thermo-mechanical simulations of the upper-critical heat-affected zone (UCHAZ) were produced to understand the mechanical and fracture micro-and macro-mechanisms, before and after the paint baking process, by widening the regions of UCHAZ. The transmission electron microscopic analysis of the Gleeble simulated sample reveals the segregation of C, Mn, Al, and Cr along the prior austenitic grain boundary which will change the nature of bonding at these boundaries. Nevertheless, paint baking treatment helps to redistribute the segregated elements from the grain boundary to the grain interior and to eliminate the solidified liquation formed at the grain boundaries during welding. The transformation of martensite to decomposed martensite, elimination of solidified liquation due to the enhanced atomic mobility and growth of surrounding grains, and the redistribution of C, Mn, Al, and Cr from the grain boundary to interior regions of grains, are the main reasons for the improvement of mechanical properties and fracture behavior of the spot welds.
Effect of Paint Baking Treatment on Mechanical Properties of Resistance Spot Welded Q&P 980 Steel
(ISIJ International, 2024-05-15) Ramachandran, Dileep Chandran; Salandari-Rabori, Adib; Midawi, Adbelbaset R.H.; Macwan, Andrew; Biro, Elliot
This study investigates the impact of paint baking on the macro and micro-mechanical properties of resistance spot welds in quenched and partitioned 980 steels. It is observed that paint baking enhances both peak load and energy absorption during cross-tension tests, as indicated by load-displacement curves. Four different regions were identified from the load-displacement curves after paint baking. An intriguing observation was a quick increase in the loading rate following a prior decrease, attributed to change in crack propagation behavior rather than improved work hardening. The study further simulated the upper-critical heat-affected zone using a Gleeble thermo-mechanical simulator to evaluate flow strength and work hardening. The Kocks-Mecking strain-hardening model was employed to analyze work hardening behavior in the studied conditions.
Effect of surface condition on resistance spot welding of advanced high strength steel
(University of Waterloo, 2019-09-25) Han, Xu; Zhou, Norman; Biro, Elliot
Advanced high strength automotive sheet steel (AHSS) is used in body-in-white design to reduce vehicle weight while maintaining high crashworthiness. Surface coatings applied to AHSS to protect it from oxidation and decarburization during its processing and life cycle. Due to the characteristics of AHSS, including alloying content and thermal process requirements, a variation of final surface conditions is possible. The resistance welding process is affected by surface changes as it alters the electrical contact resistance. As a result, a change in resistance spot welding process window occurs. Without proper attention, this variation in the operation window could reduce the joint strength and results in an unpredictable failure by having an undersized nugget. In this study, two surface-related phenomena, internal oxidation, and zinc diffusion, were investigated to characterize their impact on resistance spot welding. Additionally, a heat input based electrical dynamic resistance approach was proposed to determine appropriate welding current given variations in the Zn diffusion layer resulting from heat treating during this hot stamping process for PHS steels. Promotion of internal oxidation is used in Zn galvanizing line to improve the wettability of the steel surface to the Zn pool via the enhancement of the reactive wetting. The presence of these internal oxides has shown to shift the weld lobe to higher currents, increasing the time required to generate an acceptable weld. Study of weld development showed that surface melting is responsible for this shift in the weld process window. The surface melting created a liquid contact surface between the faying surface, which reduced the electric contact resistance and heat generation at the weld faying surface. A smaller nugget was formed due to the reduction of heating. To compensate for this reduced heat generation, a higher welding current was required when RSW of internally oxidized samples. Zinc diffusion from the galvannealed coating to the steel substrate occurs when a galvannealed steel was exposed to elevated temperature during heat treatment in the press-hardening process. This formed a Fe-Zn diffusion layer. The thickness and composition of the diffusion layer were found to be dependent on heat-treatment conditions. With an increase in heat-treatment time, the electrical resistance of the steel sheet was observed to increase as well. With higher electrical resistance, less welding current was needed to weld the material. While a change in nugget size occurred when welding steels made using different heat-treatment conditions with constant welding parameters, the mechanical lap shear strength was not impacted. Martensite tempering in the heat-affected region was more severe in samples with a larger diameter weld nugget, which decreased the required stress for failure to occur, counter-acting the increase in strength gained from the larger nugget size. This work has shown that with a heat-treatment time ranging between 4 to 10 minutes, a robust resistance welding schedule can be determined to generate a mechanically sound weld. Dynamic electrical resistance has been used to monitor the weld quality. Heat input analysis was shown to reflect the weld development as it takes into account the full weld cycle. Heat input has shown to have a linear correlation with nugget size. Undersized nugget can be successfully detected and corrected by changing the welding current based on the heat input value calculated from dynamic resistance measurement.
Improving the multiscale morphological and mechanical properties of laser welded Al-Si coated 22MnB5 press-hardened steels
(University of Waterloo, 2023-03-27) Khan, Muhammad Shehryar; Zhou, Y Norman; Biro, Elliot
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 and components with tailored properties. This demand can be met by making use of tailor-welded blanks (TWBs), which allow the production of highly optimized and complex components that are simultaneously lightweight and exceptionally strong. An example of this type of component is the B-pillar which is produced by laser-welding different types of press-hardened steels (PHSs) to offer increased elongation properties where improved energy absorption is required, while also providing an increased yield strength where preservation of structural integrity under high dynamic load is required. Parts produced using TWBs offer many different advantages like weight reduction, part consolidation, and improved part performance. The laser welding of Al-Si coated PHSs to produce TWBs is known to cause the formation of a softer ferrite phase in the welded joint due to the mixing of the Al-Si coating in the molten weld pool. It has been shown that the formation of this phase is the principal reason for premature failure of these laser-welded joints. Industry is currently removing the Al-Si layer prior to welding using laser ablation but other techniques have been developed to weld through the coating by either using secondary or modified coatings. This thesis reviews the existing literature on the problems associated with the laser welding of Al-Si coated 22MnB5 steel and several recent solutions to this problem will be discussed. As part of this research, several novel solutions to solve the problem of weld softening have been considered. It has been observed that for certain given conditions, welding through a secondary or modified coating made of austenite stabilizing elements decreases the ferrite content and increases the strength of the weld without the need to remove the Al-Si coating prior to welding. The final chapter of this thesis discusses the effect of alloying Ni with 22MnB5 on the morphology, crystallography, and mechanical properties of the steel to offer insights into potential advancements in the development of new press-hardened steels. The thesis concludes with several recommendations for future work.
The Influence of Electrode Force on Liquid Metal Embrittlement of Third Generation Advanced High Strength Steels during Resistance Spot Welding
(University of Waterloo, 2023-05-08) Song, Shiyuan; Biro, Elliot
Zinc coatings are generally applied on the surface of advanced high-strength steels (AHSS) for corrosion resistance. However, the elevated temperature during resistance spot welding process melts zinc coating and the intact contact of liquid zinc on the underlying steel substrate with the presence of tensile stress may induce liquid metal embrittlement (LME) cracking, potentially degrade the weld strength. It is evident that the alteration of welding parameters, one of which being electrode force, affects the LME response of steels. However, the mechanisms of how electrode force influences LME cracking is not fully understood. By assessing how electrode force variation affects LME cracking severity for welds in two newly designed 3G-AHSS, with low-LME and high-LME sensitivity, this work illustrated that electrode force had a two-fold effect on LME development. When welds did not exhibit expulsion, the increase of electrode force promoted heat extraction from the joint, which in-turn reduced LME cracking severity. However, when welds experienced expulsion, the increase of electrode force, accentuated electrode collapse, thus increased rapid cooling on the weld shoulder with its associated thermal stresses, increased LME cracking severity. The welding work was performed on three heat input levels to relate the electrode force and the LME cracking severity experimentally. At each heat input level, the welding current was adjusted to compensate the influence of electrode force variation on the nugget formation and growth. Therefore, comparable nugget diameter (a measure of weld strength) was achieved regardless of various electrode force at each heat input level. The LME cracking severity of the welds made in low-LME and high-LME sensitive materials was quantified by the number of cracks, the absolute maximum crack length and the potential weld strength loss using crack index. The LME cracks were classified according to their locations on the weld where Type A cracks located at the center of the weld surface; Type B cracks located at the weld shoulder extending to the edge of the heat affected zone; Type C cracks located at the weld notch. For both low and high LME sensitive materials, Type B cracks were observed at all levels of heat input while Type C cracks were not, and Type A cracks were only observed at moderate and high heat inputs. Welds made in the low-LME sensitive material displayed an overall less susceptibility to LME cracking (crack index below 0.15). For the high-LME sensitive material, at low heat input, the LME severity decreased from a total crack index of 0.14 to 0 when the electrode force was increased from 4.4 kN to 5.4 kN. At moderate heat input, the LME severity decreased from a total crack index of 0.67 to 0.11 when the electrode force was increased from 4.4 kN to 4.9 kN, then with a further increase of electrode force to 5.4 kN, the Total crack index increased to 0.37. At high heat input, the increase of electrode force from 4.4 kN to 5.4 kN resulted in the increase of LME cracking severity on Total crack index from 0.36 to 0.77. The observation that the increase of electrode force could result in both increased and decreased LME severity, depending on situation, contrasted with the published literature where it was seen that LME cracking severity universally decreased with increasing electrode force. To understand the mechanisms resulting in different LME response seen in welds made in the two investigated materials and why the present results diverged from the literature, experimental analysis was first conducted on results from the tests on the low-LME sensitive material. From this analysis, it was seen that increasing electrode force promoted heat loss and the expulsion event increased with the increase of applied welding heat. However, there was not a linear relationship observed between the change of heat input, heat loss, expulsion condition and the LME cracking severity that related to variations in electrode force for welds made in the low-LME sensitive material. With the assistance of ANOVA analysis and linear regression modelling, it was seen that whether the weld exhibited expulsion had a significant effect on its LME behaviour. The results indicated that analysing both LME-free (20 out of 45 welds made) and LME-containing welds in a single dataset led to inaccurate prediction of LME crack length when LME cracking was modeled as a function of heat input, heat loss, and expulsion count. The majority of LME-free cracks were found at low heat input and the probability of these welds experiencing expulsion was low. In welds made in the low-LME sensitive material that contained LME, the LME cracking severity of LME-cracking welds was significantly influenced by the heat input, heat loss and the occurrence of expulsion from the statistical analysis. Conversely to the LME-free welds, the welds exhibiting LME cracking predominately experienced expulsion during welding. Welds made in the high-LME sensitive material exhibited much more LME cracking than seen in the low-LME sensitive material. From all on the welds made, there was only 5 LME-free welds out of 45 welds produced. The tested factors (i.e. heat input, heat loss and expulsion condition), which had a significant impact on LME cracking behavior on low-LME sensitive material, were then experimentally correlated with the LME cracking severity for the high-LME sensitive material. The results indicated that the influence of electrode force on LME cracking 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, the increase of electrode force aided with heat extraction during welding, relieved the critical stresses required by LME cracking. In contrast, when welding with high heat input, resulting in expulsion, the increase of electrode force elevated LME cracking. It was shown that high electrode force increased the sudden indentation of the electrode into the steel substrate (electrode collapse), leading to rapid cooling of the weld shoulder. The rapid cooling increased the thermal stresses associated with the electrode collapse event, promoting LME. 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.
Investigation of Liquid Metal Embrittlement in Advanced High Strength Steels
(University of Waterloo, 2020-04-17) He, Liu; Biro, Elliot; Zhou, Norman
Third-generation advanced high strength steels (AHSS) are typically given a zinc coating that provides excellent resistance to corrosion. During the resistance spot welding (RSW) process, the melted zinc coating enables liquid metal embrittlement (LME) where the liquid zinc, acting as an embrittling agent, induces cracking in the weld indent, compromising weld strength. This work investigates the various factors that influence LME in AHSS and provides a viable solution to suppress LME. Hot tensile testing was first used to evalute the LME susceptiblility of the studied steels. It was discovered that the austenite content of the steels’ microstructure, Si content in the steels’ chemistry and the type of Zn coating all influence the behavior of the ductility trough of the examined steels. As the austenite content of the steel increased, the ductility loss caused by LME increased as well. Approximately 18 vol.% to 31 vol.% austenite is the minimum amount required to trigger the rise in ductility loss of all the studied steels. In addition, steels containing a low Si content are more likely to form a layer of Fe-Zn intermetallic that acts as a barrier to suppresses LME at temperatures below 670°C. It was also discovered that the GA coated steels are far less susceptible to LME than their GI coated counterparts due to it being thinner and containing 25 wt.% Fe in its coating. A mathematical model capable of estimating the crack index within the weld lobe of each material was also developed through resistance spot welding. The model showed that the weld lobe of materials where not equally affected by LME. Furthermore, it identified regions within the weld lobe where welds of sufficient size could be made while minimizing LME cracks. Using the hot tensile testing data and the results from RSW, LME susceptibility of the studied steels are ranked. QP1180GA is the most LME susceptible steel while DP980GA is the least. The ductility loss obtained via hot tensile testing shows good correlation with the intensity of LME cracks found in resistance spot welds. Finally, LME was suppressed in AHSS by placing aluminum interlayers added between the electrode and steel contact surface. Compared to welds exhibiting LME, TRIP 1100 with aluminum interlayers showed complete strength recovery while TRIP 1200 with aluminum interlayers resulted in a recovery of strength by 90%. Aluminum interlayers suppress LME of TRIP steel by formation of iron aluminides that hinder liquid zinc from coming in contact with the steel substrate, thus preventing LME.
Investigation of Liquid-Metal-Embrittlement Crack-path: Role of Grain Boundaries and Responsible Mechanism
(University of Waterloo, 2020-07-24) Razmpoosh, Mohammad Hadi; Zhou, Norman; Biro, Elliot
Liquid metal embrittlement (LME) has been reported in many structural materials, including steel, aluminum, nickel during hot working processes e.g. welding, brazing, heattreatment, leading to abrupt failure. In many of the applications, such as automotive, aerospace, nuclear industries, LME failure is considered as a serious safety concern. Over the last decades, research activities have grown considerably striving to understand the LME phenomenon. However, to date a fundamental understanding of the metallurgical and mechanical micro-events of LME has remained unclear. Moreover, the LME mechanism has been concealed behind the diverse, contradicting propositions without any robust experimental support. Hence, a comprehensive understanding of micro-events of LME calls for in-depth crack-path analysis from macroscopic, microscopic, and atomic viewpoints. The aim of this research is to explore the role of grain boundary type, and characteristics such as grain boundary misorientation angle, crystallographic plane, and grain boundary microchemistry in LME. Restrained laser beam welding was used to induce LME-cracks in various Zn-coated steels. The crack-path has been characterized to identify types and geometrical characteristics of LME-sensitive grain boundaries. It was found that LME crack-path is a function of misorientation angle and stress component perpendicular to grain boundary plane, where high-angle random (non-ordered) grain boundaries are more LME-sensitive than highly coherent low-Σ coincidence site lattice (CSL) boundaries. At higher misorientation angles, lower tensile stresses trigger grain boundary decohesion. Moreover, liquid metal selectively penetrated the grain boundaries with high-index planes due to their relatively high excess volume. The atomic-scale analysis of LME crack-path provided new insights to the inter-relation between the geometrical configuration and grain boundary chemistry. This validated the grain boundary-based LME mechanism, and revealed the micro-events leading to the embrittler-induced grain boundary decohesion. It was found that grain-boundary engineering techniques can be employed to manipulate frequency of random and CSL boundaries, which resulted in significantly improved resistance against LME.
Liquid metal embrittlement cracking in dissimilar resistance spot welding of 3rd Gen – Advanced high strength steel: mitigation methods and associated mechanisms
(University of Waterloo, 2022-12-19) Patel, Meet; Biro, Elliot
In response to the automotive industries demands for safer, lighter, and more environmentally friendly vehicles, the third generation of advanced high-strength steels (3G-AHSS) has been developed to have both high strength and high ductility. 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) cracking when combined with tensile stresses associated with the RSW process. Several possible strategies for lowering LME severity by altering welding parameters have been proposed in the literature, but these strategies have been predominantly focused on similar joints. However, as the vast majority of welds in automotive construction join materials of different thicknesses and grades, dissimilar weld performance is also of interest. Therefore, it is also of interest to understand how LME changes in dissimilar joints as compared to similar joints, as well as whether methods to reduce LME severity must take the dissimilar nature of the joint into account to improve weld quality. In this work, a 3G-AHSS galvanized (GI-coated), known to be highly susceptible to LME, with a nominal sheet thickness of 1.4 mm was welded, both to itself and to a 0.6 mm thick Interstitial Free (IF) steel. The effect of joint construction on LME cracking severity was determined by comparing the cracks resulting from welds made in both similar material and dissimilar material joints. It was found that differences in material characteristics, between the two materials, resulted in differences in temperature distribution in the similar and dissimilar joints, causing high LME severity in the dissimilar joint. An industrially viable welding schedule was developed to minimize the LME severity in dissimilar joints. The severity of the LME cracks that formed in welds made using the developed weld schedule was compared to those developed in welds made with baseline welding parameters, resulting in a 46% reduction in a severity of LME cracking. The mechanism of LME reduction was analyzed using the finite element modeling software Sysweld®. The obtained results were compared with in-situ thermography using an infrared camera. It was observed that the difference in a thermal gradient and the distance between the fusion boundary and electrode/sheet interface are responsible for LME severity. The robustness of the developed welding schedule was then tested on welds made with typical industrial disturbance factors such as pre-strained sheets (between 0 to 80% of material yield strength) and electrode misalignment (between 0° to 10° misalignment) compared to baseline parameters. When welds were made in joints affected by severe 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.
Liquid Metal Embrittlement in Resistance Spot Welding: The Thermomechanical Origin and Embrittler Transport Mechanism
(University of Waterloo, 2021-08-05) Di Giovanni, Christopher; Zhou, Norman. Y; Biro, Elliot
To increase fuel economy, demand for lighter vehicles by way of thinner parts has risen in recent years. As parts become thinner the vehicle safety cannot be comprised, leading to the development of advanced high strength steels (AHSS) which exhibit excellent strength and ductility. AHSS are often coated with zinc (Zn) to prevent corrosion, however the coating can lead to several complications, such as liquid metal embrittlement (LME). Resistance spot welding (RSW), a common automotive process, has been observed to accentuate LME cracking. During RSW, the heat buildup at the weld surface is sufficient to melt the Zn coating, leading to LME. Presently, LME cracks that are detrimental to joint integrity are undefined and thus hard to target for mitigation. Furthermore, the exact sources of stress (a prerequisite for LME) during the RSW process leading to detrimental crack formation, remains unclear. Hence, a comprehensive understanding of LME occurrence calls for an in-depth analysis of the thermomechanical aspects of the RSW process and the fundamental mechanisms leading to the onset of LME. This research aims to define, mitigate, and explain the occurrence of LME cracks during RSW. Lap shear static strength testing was used to determine the effect of LME cracks on joint performance. Detrimental cracks were defined according to their size and location. RSW process adjustments were carried out to mitigate detrimental LME cracks. It was determined that the mechanism of formation varies depending on the region the crack is located. Furthermore, it was found that the occurrence of LME in all cases was largely driven by the presence of stress. To isolate the effects of stress and temperature on the occurrence of LME, elevated temperature tensile testing was carried out using conditions that best replicate the RSW process. A co-dependence of temperature and stress for the onset of LME was observed, showing the temperature range for LME is not static as previously thought. Using interrupted testing, the onset of LME was observed in detail. Zn penetration to the AHSS grain boundaries was observed to increase with increasing applied stress until LME crack formation. A diffusion analysis revealed the fundamental LME transport mechanism to be stress-assisted grain boundary diffusion. This mechanism reconciles the observations of LME in RSW, which showed the occurrence of LME to be highly dependent on both temperature and stress.
Mechanical Properties and Failure Behavior of Resistance Spot Welded Third-Generation Advanced High Strength Steels
(University of Waterloo, 2024-09-24) Shojaee, Mohammad; Biro, Elliot; Butcher, Cliff
Acceptable crash performance and fuel efficiency are vital requirements for any modern automobile. To meet these requirements, the automotive industry is designing lighter vehicles by further adopting third-generation advanced high strength steels (3G-AHSS) within their vehicle assemblies. 3G-AHSS possess multiphase microstructures that provide a favorable combination of strength-ductility relative to existing commercial AHSS. A safe and reliable migration to 3G-AHSS within automotive body-in-white (BIW) structure demands, among other requirements, the ability to predict the onset of failure from components fabricated using common joining techniques such as resistance spot welding (RSW). A fast and reliable approach for RSW failure prediction within the automotive industry is utilizing force-based RSW failure criteria that are calibrated using critical loads/moments at the onset of RSW failure from various mechanical tests. Aside from conventional tensile shear (TS) and cross tension (CT) mechanical tests, characterizing the 3G-AHSS RSW failure strength components at various complex loading conditions can improve the calibration accuracy of experimental RSW failure loci. Some of such complex loading conditions include various ratios of shear-tension loading, characterized by KS-II tests, and tension-bending loading mode, characterized by coach peel (CP) tests. Accurate quantification of RSW mechanical performance indices, such as load-bearing capacity and energy absorption capability from single spot weld characterization technique is accompanied by unique challenges due to rotation of the joint and plastic work due to coupon deformation at regions away from the joint during mechanical testing. The influence of such unintended phenomena on extracted mechanical performance indices is commonly acknowledged but not accounted for. In this research program, the RSW process parameters were optimized for two grades of 3G-AHSS, referred to as 3G-980 and 3G-1180, via the development of a weldability lobe, and performing traditional TS and CT mechanical tests for various RSW nugget diameters while following the welding schedule recommended by AWS D8.9 standard. Thereafter, the mechanical performance of optimized and sub-optimal 3G-AHSS spot welds were characterized under various combinations of shear/tension loading ratios as well as different combinations of tension-bending loading modes. The rotation and slippage of combined loading specimens within the testing fixtures posed a challenge leading to overestimation of spot weld performance indices, such as failure load components and absorbed energy during failure. These challenges were overcome via viii quantification of rotation during mechanical tests and proposing novel post-processing methodologies that approximate local nugget displacement fields by coupling tests with stereoscopic digital image correlation (DIC) techniques. Upon attainment of critical load components and moments at various shear-tension and tension-bending loading modes, the accuracy of various force-based RSW failure criteria was evaluated independently. It was shown experimentally that while the commonly used force-based RSW failure criteria, proposed by Seeger, is fairly accurate in shear-tension loading mode, it loses accuracy by a relatively large margin in determining critical bending moments the spot welds withstand at the onset of failure. Alternative mathematical functional forms of RSW failure loci were proposed that can be readily implemented in finite element analysis for the potential improvement of 3G-AHSS RSW failure predictions. Calculations related to quantifying the energy absorption capability of the joints showed that brittle propagation of cracks into the columnar structure of fusion zone (FZ), leading to partial interfacial- partial pullout failure, significantly limits the post-failure energy absorption capability of the investigated joints in both shear-tension and tensile-bending loading conditions. The understanding of single spot weld characterization techniques were expanded to weld group (component) tests that evaluate the mechanical performance and failure characteristics as groups of spot welds separate under tensile-bending loading conditions. It was shown that the energy absorption capability of groups of spot welds is a function of the extent to which the base materials involved in the tests dissipate energy by plastically deforming throughout the tests, as well as the failure mode of the spot welds. The components made of the more ductile 3G-980 material exhibited superior energy absorption capability due to a higher degree of parent metal deformation and ductile pullout failure mode compared with the less parent metal plastic work and partial pullout failure of components from 3G-1180 material. This research program is comprised of various sections including the 3G-AHSS RSW process optimization, detailed microstructural characterizations of optimized joints, mechanical performance and failure characterization of the joints under combined shear-tensile loading using KS-II tests, tensile-bending loading using various geometries of CP test, weld-group tests, and novel post-processing techniques used for improving the accuracy of force-based RSW failure criteria, which were the key takeaways of this research.
Microstructural Development and Weldability Optimization in the Resistance Spot Welding of Quenched and Partitioned Steels
(University of Waterloo, 2021-04-16) do Nascimento Figueredo, Bruna; Biro, Elliot
Advanced high-strength steels (AHSS) have been widely used in the automotive industry to meet the demand for reduced fuel consumption and environmental emissions. Quenching and Partitioning (Q&P) is a heat-treatment method used to produce AHSS with higher ductility than similar strength steels, allowing for the use of thinner and lighter automotive parts due to its high formability. The mechanism behind how the microstructure of the different weld zones affects Q&P steel welds' mechanical properties is a topic that is yet to be fully understood. In this study, the resistance spot welding behavior of Q&P steels was investigated through microstructure characterization, mechanical testing, failure behavior analysis, and computational simulation. It was found that the cross tension and tensile shear test samples all failed in a partial or full button pull-out mode, with an acceptable ductility ratio, but with lower cross tension strength compared to the available studies on resistance spot welding of Q&P steels. Fractography of the testing samples showed that the crack propagated along the fusion boundary, an occurrence previously reported in the literature for Q&P980, but with no comprehensive explanation regarding the mechanism behind the behavior. This work discusses how this fracture is possibly related to the formation of a softened region at the fusion boundary, a phenomenon commonly known as halo ring. Weldability optimization was analyzed using multiple pulsing welding schedules and the study of their effects on the microstructural development of the welds and their mechanical properties. It was found that a second pulse with a higher current than the primary pulse is effective in avoiding the occurrence of halo ring and improving cross-tension strength. Multi-pulsing consistently improved overall weld strength even when normalizing the values to decouple the effect of increased weld diameter. The relationship between the occurrence of halo ring and the weldability is discussed along with alternative theories correlating the microstructure and microhardness to the observed improvements. A computational simulation was applied to investigate the mechanism behind how the described multi-pulsing weld schedule eliminates the halo. It was found through the study of the thermal cycles in the weld that a high current second pulse effectively remelts the weld nugget, avoiding the fusion boundary to remain static towards the end of the welding cycle and thus not allowing for the formation of the softened zone.
Microstructural Evolution and Formation Mechanism of the Halo Ring in Resistance Spot Welding of a 3G Advanced High Strength Steel
(Springer, 2024-09-09) Ramachandran, Dileep Chandran; Salandari-Rabori, Adib; Macwan, Andrew; Biro, Elliot
The microstructure of the halo ring has been studied in quenched and partitioned (Q&P) steel resistance spot welds. The TEM and EBSD characterizations revealed the presence of an upper bainitic microstructure in the halo ring of the three-sheet stack-up welds. Stalking faults accompanied by nano-twins were identified surrounding the cementite. Diffusion of carbon towards the molten weld pool during solidification led to the formation of bainite at the fusion boundary, triggered the localized softening.
A Statistical Approach to Quantifying Impact of Multiple Pulse Resistance Spot Welding Schedules on Liquid Metal Embrittlement Cracking
(University of Waterloo, 2019-04-26) Wintjes, Erica; Zhou, Norman; Biro, Elliot
Advanced high strength steels (AHSS) are advantageous for automotive applications due to their excellent strength and ductility. However, when coated with zinc for corrosion protection, these steels are susceptible to liquid metal embrittlement (LME) during welding. In this work, a new metric was developed to quantify LME severity and this metric was used to study the influence of multiple pulse weld schedules on LME cracking in resistance spot welds. Several conflicting reports have been released about the effect of LME on mechanical performance of resistance spot welds. In this work, a new method of LME crack quantification called a “Crack Index” was developed to link LME crack distributions in resistance spot welds to weld performance. The crack index is calculated by multiplying the lognormal median crack length by the number of cracks per weld and dividing by the sheet thickness. Because studies have established both crack size and location as vital factors affecting weld strength, both of these factors must be taken into account when characterizing LME severity. Lognormal median crack length is used as the parameter for crack size because the crack lengths measured in LME affected welds were observed to fit a lognormal distribution. Number of cracks is used to account for the probability that a crack may be found in a critical location and sheet thickness is used as a normalization factor. The crack index has a linear relationship with weld strength loss. The crack index analysis method was used to study the influence of multiple pulse welding schedules on LME severity. Pulsing was applied using two different methodologies: pulsing during the welding current to manage heat generation and a pre-pulse before the welding current to remove the zinc coating. All welds made using a double-pulse welding schedule exhibited less severe LME cracking than those made with a single pulse schedule with a similar nugget diameter. A double-pulse schedule with two equal length pulses showed the least severe LME cracking and a schedule consisting of a short pulse followed by a long pulse resulted in the most severe LME. This is due to both a difference in the amount of free zinc available for LME and the different thermal and stress profiles of the pulsing conditions. The majority of pre-pulse welding schedules caused an increase in LME cracking due to the additional heat introduced into the weld. However, a 4 kA pre-pulse (low current), applied for 3 cy (low time) was able to reduce LME cracking in TRIP1100, a LME crack susceptible alloy, by almost 30%. The 4 kA, 3 cy pre-pulse reduced the amount of free zinc for LME, without introducing too much additional heat into the weld.
The Role of Microstructural Modifications in Improving the Mechanical Properties of Resistance Spot Welded Automotive Steels
(University of Waterloo, 2024-09-18) Betiku, Olakunle; Biro, Elliot
The advent of advanced high-strength steel (AHSS) in the automotive industry has evolved in recent years to satisfy the global demands for lightweight and safer vehicles. These AHSSs offer an attractive combination of strength and ductility, making them ideal for use in vehicle body-in-white structures. However, their weldability and joint performance in-service are crucial to remain competitive for selection in the automotive industry. Joining AHSS is mostly achieved by resistance spot welding (RSW), and it is envisaged to continue for the foreseeable future. Despite its advantages, the rapid cooling rates during the RSW process result in the formation of martensite in the weld fusion zone, which is known to be hard and brittle, thereby resulting in low energy absorption that is undesirable in case of a crash event. This thesis explores various metallurgical pathways that can be employed during in-situ RSW process to modify the joint microstructure and enhance the energy absorption capability of the weld. For each technique, an understanding of how different in-situ post-weld heat treatment (PWHT) parameters induce microstructural changes was investigated in this work. Furthermore, this research elucidates the microstructural evolution occurring during the non-equilibrium in-situ PWHT process and correlates these changes with the resulting mechanical properties. Grain refinement was found to be the most effective approach to improve the energy absorption capability of the weld compared to tempering, strain hardening, and paint baking processes. The refined prior austenite grain (PAG) structure was accompanied by a refinement of the substructure with high-angle grain boundary that poses more resistance to crack propagation thereby resulting in 89% improvement in energy absorption capability to failure compared to the baseline welds. It was found that the grain refinement is achieved after applying a PWHT current pulse when the edge of the FZ is solidified and in the austenitic region, rather than when the region has transformed martensite – the latter being preferred for tempering. The recrystallization schedule that induces the grain refinement was also achieved at a relatively shorter process time compared to the other PWHT techniques, which is an important criterion for industrial applicability. Additionally, the PWHT schedules adopted in this research altered the weld failure mode, causing crack propagation to deviate at the edge of the FZ during cross-tension tests. For the welds with grain refinement, the improved energy absorption capability was majorly attributed to the new equiaxed prior austenite grain structure and the change in crystallographic texture from the cleavage (001) plane in the baseline welds to the (101) plane that supports plastic deformation ahead of the crack tip, thereby retarding the crack propagation. These changes led to ductile failure, in contrast to the brittle failure observed in baseline schedules where cracks propagated into the fully martensitic FZ along the columnar structure. The findings of this research provide a unique perspective on the metallurgical transformations during in-situ RSW PWHT, offering valuable insights to the scientific community. Furthermore, these results inform the automotive industry of the optimal PWHT technique that can be employed in their manufacturing lines, enhancing the performance and safety of AHSS joints in vehicles. Keywords: Resistance spot welding (RSW) Advanced high strength steels (AHSSs) Post weld heat-treatment (PWHT) Microstructure Mechanical properties.
The Viability of using a Gleeble for Physical Simulation of High Frequency Induction Welded TRIP 690 AHSS
(University of Waterloo, 2024-11-18) Al Hussain, Syed Faique; Biro, Elliot
High Frequency Induction Welding (HFIW) is the predominant process for high volume production of small diameter tubes and pipes for hydroformed automotive and oil and gas applications. This process is well-established due to its high throughput and continuous nature which makes it ideal for industrial use. However, the HFIW process is also complicated, involving several physical phenomena occurring simultaneously such as mechanical deformation during the squeeze-out, phase transformations, large temperature gradients, high heating rates, and electromagnetic induction. These phenomena are difficult to decouple from one another, leading to gaps in the present understanding regarding how each individual phenomenon affects the formation of certain weld defects, such as oxide inclusions trapped within the bond line of the weld joint. With advances in automotive design, new high-Al TRIP steels are being used for automotive hydroforming applications, due to their capability to be used in high strength/light-weight designs. However, HFIW of these materials, such as TRIP 690, is susceptible to the formation of entrapped oxides containing aluminum (Al), manganese (Mn), and silicon (Si) within the bond line, reducing the operation window compared to other steels. In welds containing oxide inclusions, strength and ductility of the weld joint will be significantly decreased. During production, it is difficult to determine the formation of these oxides due to the dynamic and continuous nature of the HFIW process. Conducting mill trials for experimentation is not practical due to economic constraints as there are high operational costs to run a tube mill and trials result in high material usage. Thus, there is a need to be able to physically simulate the HFIW process at a laboratory scale to understand the effect of each of the individual process parameters on the formation of bond line oxide inclusions and weld quality. This study physically simulates the HFIW process in thin sheets of TRIP 690 AHSS using a Gleeble 3500 thermomechanical simulator. The results of this work demonstrated that the Gleeble could reproduce the microstructure across the bond line and heat affected zone of the HFIW produced welds. Mechanical characterization of the welds revealed a similar hardness distribution across both the Gleeble and HFIW welds. Notably, samples containing bond line oxide inclusions such as those found in HFIW welds were also recreated, and the effects of these inclusions on the tensile properties and fracture mechanism were determined. Through this study, the ideal conditions for producing oxide-free welds to ensure superior weld mechanical properties were determined.