A novel in-situ temperature measurement method for liquid metal embrittlement cracking analysis

In this study, the influence of the microstructural evolution of galvannealed (GA) coating and the thermomechanical characteristics of liquid metal embrittlement (LME) cracking during resistance spot welding (RSW) of steels were investigated. The analysis of LME cracks, coating microstructure and thermomechanical characteristics at various weld regions revealed a synergistic effect of liquid Zn, α-Fe(Zn), and tensile stress on LME cracking during RSW. A continuous α-Fe(Zn) layer inhibits liquid Zn from contacting the steel substrate; hence, LME was suppressed. However, a long-duration, high-magnitude tensile stress can fragment the α-Fe(Zn) layer and enable LME cracking. Conversely, α-Fe(Zn) particles allow the easy infiltration of liquid into the steel and make the crack on tensile stress development.

Liquid metal embrittlement (LME) in Zn-coated steels is a serious issue in automotive design. The risk of rising LME surface cracks in resistance spot welding (RSW) of Zn-coated high strength steels has triggered significant research activities across the globe. This paper presents a state-of-the-art review of the various phenomena and issues related to LME during RSW. Various aspects of LME surface cracks have been described in this review, focusing on the macro- and microscopic features of LME, spot weld cracks, the sensitivity of the LME cracks towards surface locations, welding conditions, and susceptibility to high strength and galvanized steels. We also focus on the effects of various processing factors, such as temperature, stress, microstructure, and the nature of the galvanized layer, related to studies with actual spot welds LME cracks. Finally, we summarize the possible mechanisms of embrittlement and the remedies for minimizing LME cracks, with suitable guidelines to suppress surface cracks during RSW.

Zinc-coated advanced high-strength steels are known to be susceptible to liquid metal embrittlement (LME) cracking during resistance spot welding (RSW). Despite numerous reports with regard to LME during RSW, a systematic approach has not been proposed for the classification of cracks based on the cracking mechanism. The objective of this study was to characterize the LME cracks at various RSW locations, and thereby propose a classification method to identify the mechanism of the LME cracks at each location. The experimental results revealed the LME cracks were concentrated at certain weld locations and exhibited different features in terms of length, number, and orientation, owing to the synergetic effect of temperature, stress, microstructure, time of exposure to liquid zinc, and time of exposure to tensile stress at the corresponding lo-cations. Thus, the LME cracks were classified into four categories, namely type A, type B, type C, and type D, based on the formation location. The effect of time of exposure to liquid zinc and tensile stress on LME cracking revealed the time dependency of LME in RSW. The nature of contact be-tween the electrode and the sheet, and the heat input during welding, were found to be the main reasons for the difference in the thermal, mechanical, and metallurgical characteristics of various crack locations, which caused the formation of various LME crack types.

During resistance spot welding of zinc-coated advanced high-strength steels (AHSSs) for automotive production, liquid metal embrittlement (LME) cracking may occur in the event of a combination of various unfavorable influences. In this study, the interactions of different welding current levels and weld times on the tendency for LME cracking in third-generation AHSSs were investigated. LME manifested itself as high-penetration cracks around the circumference of the spot welds for welding currents closely below the expulsion limit. At the same time, the observed tendency for LME cracking showed no direct correlation with the overall heat input of the investigated welding processes. To identify a reliable indicator of the tendency for LME cracking, the local strain rate at the origin of the observed cracks was analyzed over the course of the welding process via finite element simulation. While the local strain rate showed a good correlation with the process-specific LME cracking tendency, it was difficult to interpret due to its discontinuous course. Therefore, based on the experimental measurement of electrode displacement during welding, electrode indentation velocity was proposed as a descriptive indicator for quantifying cracking tendency.

Liquid metal embrittlement in Zn-coated steel resistance spot welding: Critical electrode-contact and nugget growth for stress development and cracking

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.

Numerical modeling of liquid metal embrittlement initiation during resistance spot welding of third generation advanced high strength steel – Effect of welding schedule and electrode alignment

<div class="section abstract"><div class="htmlview paragraph">The application of high-strength steel sheets to car bodies is expanding to improve the crashworthiness and achieve weight reduction [<span class="xref">1</span>, <span class="xref">2</span>]. Conversely, in recent years, the occurrence of liquid metal embrittlement (LME) cracks has been discussed in resistance spot welding using a Zn-based coated high-strength steel [<span class="xref">3</span>-<span class="xref">5</span>]. This study examined the factors causing LME cracks and identified the locations of LME cracks found in resistance spot welds using a Zn-coated high-strength steel sheet. Furthermore, through an analytical approach using a scanning electron microscopy (SEM) and transmission electron microscopy (TEM), for a joint with an LME crack, it was found that (1) grain boundary fracture occurred at LME crack portion and its fracture surface was covered with Zn, (2) Zn penetrated into prior-austenite grain boundaries near the LME crack, and (3) Zn concentration decreased toward the tip of the Zn-penetrated site. From these results, it was possible to see that the concentration of Zn in the prior-austenite grain boundary in the spot weld increased during welding, the concentration exceeded the threshold, and the grain boundary melted. Additionally, the influence of the gap (also known as clearance) between the lower sheet and electrode before spot welding, as an assumed industrial noise, on the LME crack was investigated. The maximum length of an ‘inner crack’ at the outer-edge of the corona-bond was found at a certain gap. In the case of a high current, an ‘outer crack’ also occurred at the indentation center, in addition to the inner crack. Finite element (FE) analysis revealed that high tensile stress occurred at the site where the LME crack was expected, at a higher temperature over the melting point of Zn after the release of the electrode, under conditions of longer LME cracks. Utilizing FE analysis led to the prediction of the occurrence of LME cracks and a deeper understanding of LME phenomenon.</div></div>

GEN3 steels are a new family of automotive sheet steels developed and commercialized in the last three years, specifically for body-in-white applications. The high ductility in GEN3 steels is typically achieved through the transformation-induced plasticity (TRIP) effect by the addition of silicon or aluminum. When these steels are formed into parts, the TRIP effect of austenite to martensite transformation provides enhanced ductility. Typically, 10 to 12 micrometers of zinc coating (known as galvanized coating) is applied to automotive steel sheets for corrosion protection. Liquid metal embrittlement (LME) cracking can occur during resistance spot welding (RSW) of galvanized steels. LME cracking occurs when molten zinc penetrates prior austenite grain boundaries of the steel substrate. The precise role of silicon in the LME cracking behavior in TRIP and GEN3 steels is unknown. Therefore, a study was undertaken to examine the role of silicon in LME cracking behavior of GEN3 steels. The purpose was also to examine if the presence of retained austenite is required for LME cracking to occur. In this study, laboratory heats were prepared using three silicon levels. Samples cut from galvanized panels were welded using a resistance spot welding machine, and weld areas were examined metallographically for the presence of LME cracks. Gleeble® simulations were done to study the LME behavior of the three steels prepared. Base materials were examined with a scanning electron microscope using the electron back-scattered diffraction (EBSD) method to examine the nature of grain boundaries found. The effect of retained austenite in LME cracking was studied using the Gleeble®. Both RSW and Gleeble® results showed silicon promotes LME cracking in steels, predominantly in the weld heat-affected zones(HAZs). More low-energy, low-coincidence site lattice (CSL) boundaries were found as the silicon content of the steel was decreased. These boundaries do not host cracks. Higher silicon appeared to shrink the safe temperature range over which LME cracks could be avoided, thus indicating heat in-put control to limit cracks has limited windows as the silicon in steel goes up. It was shown that the presence of retained austenite in steel is not a prerequisite for LME cracking to occur.

Zn-induced liquid metal embrittlement (LME) cracks tend to be formed in hot-dip galvanizing (GI) medium-Mn advanced high-strength steel (AHSS) in the process of resistance spot welding (RSW). In the present study, the LME susceptibility of the medium-Mn AHSS with GI coating was systematically evaluated in accordance with the criterion of “Auto/Steel Partnership (A/SP)”. The samples were welded at 16 groups of distinct welding current levels from 7.0 kA to 14.5 kA in 0.5 kA increments. Some severe LME cracks were evident in the cross sections of the RSW joints even below expulsion, and more pronounced and serious LME cracks appeared at or above expulsion. It was discovered that the tendency for LME crack formation tended to worsen as the welding current increased. The maximum LME crack lengths of Type A, B, C, and D among all groups were approximately 1,285.0 μm, 980.5 μm, 1,397.0 μm, and 131.9 μm, respectively. Results revealed that the medium-Mn AHSS with GI coating displayed high LME susceptibility, which consequently failed to satisfy the necessary criterion for RSW application in the automotive industry. Moreover, the elevated Mn content was deemed as a critical factor leading to the high LME susceptibility of medium-Mn AHSS with GI coating.

One of the most effective ways to improve reactive wetting of dual phase (DP) steels during galvanizing process is annealing under relatively high oxygen partial pressure. This annealing process can lead to internal oxide formation rather than external ones. Since these oxides are mostly observed in grain boundaries (GBs), their presence could influence GB-related phenomena such as liquid metal embrittlement (LME) cracking during further manufacturing processes such as resistance spot welding (RSW). The present work has shown that internal oxides located at GBs assisted LME crack formation during RSW of DP steel. Two types of LME cracks were observed in the shoulder and center of the weld. It has been shown that at the initial stages of welding time, LME cracks initiate in the shoulder of the nugget by the Zn diffusion into oxide-bearing GBs. As welding time continues, higher temperatures and tensile stresses are applied to the steel sheets which allow center cracks to be formed more easily due to the presence of oxides which degrade the GBs.

Although the influence of weld process variables on LME cracking was known to be significant, limited studies have been conducted on the effect of process variables with systematic approaches in the equivalent nugget growth behavior and heat input. This study aimed to identify the effect of weld process variables on LME sensitivity with the equivalent nugget diameter and underlying mechanism with induced tensile stress for cracking. Among the welds with equivalent nugget diameters in the combination of different welding current and time, higher LME sensitivity was observed with the high welding current and short welding time combination than that with the low welding current and long welding time combination for the equivalent nugget diameter. Because a high current and short time combination resulted in faster weld nugget growth than the low welding current and long welding time combination, it rapidly increased the surface temperature along with the cooling from the electrode. These combined effects induced a higher thermal gradient and thermally induced tensile stress on the weld surface, satisfying the conditions of the LME cracking. The simulation results also confirmed that the critical weld cycle time of the LME cracking (<i>t<sub>c</sub></i>), which is the cross point between the nugget growth diameter and a contact diameter of the electrode, could be different with the combination of the weld process variables with the equivalent weld nugget size. Therefore, <i>tc</i> can be applied for the sensitivity index of LME cracking of the resistance spot weldment considering complex weld variables.

Advanced high-strength steels (AHSSs) with Zn coatings are commonly joined by the resistance spot welding (RSW) technique. However, Zn coatings could possibly cause the formation of liquid metal embrittlement (LME) cracks during the RSW process. The role of a Zn coating in the tensile-shear fatigue properties of a welding joint has not been systematically explored. In this study, the fatigue properties of tensile-shear RSW joints for bare and Zn-coated advanced high-strength steel (AHSS) specimens were comparatively studied. In particular, more severe LME cracks were triggered by employing a tilted welding electrode because much more stress was caused in the joint. LME cracks had clearly occurred in the Zn-coated steel RSW joints, as observed via optical microscopy. On the contrary, no LME cracks could be found in the RSW joints prepared with the bare steel sheets. The fatigue test results showed that the tensile-shear fatigue properties remained nearly unchanged, regardless of whether bare or Zn-coated steel was used for the RSW joints. Furthermore, Zn mapping adjacent to the crack initiation source was obtained by an electron probe micro-analyzer (EPMA), and it showed no segregation of the Zn element. Thus, the failure of the RSW joints with the Zn coating had not initiated from the LME cracks. It was concluded that the fatigue cracks were initiated by the stress concentration in the notch position between the two bonded steel sheets.

The present work studied the influence of the geometric design of the electrode on Zn-assisted liquid metal embrittlement (LME) cracking during resistance spot welding (RSW). LME cracking of the galvannealed transformation-induced plasticity (TRIP) steel welds, induced by two types of electrodes, a radius type with different radius of curvature (R), and a dome type with variable tip diameter (d), was studied both experimentally and by simulation. The current density decreased and the contact area at the electrode/sheet (E/S) interface increased with the increasing R, resulting in low temperatures and thermal stress, which subsequently led to decreased LME tendency. On the contrary, the current density decreased but the initial contact area at the E/S interface remained unchanged with increasing d, causing only a minor reduction in the temperature and hence less influence on LME cracking. These results suggested that R is the most critical design parameter of the electrode that controls LME cracking. Moreover, the radius type electrode displayed lower LME sensitivity as compared with the dome type electrode. This is attributed to the fact that the radius type electrode provides the benefits of increase in both R and d.

Abstract. Liquid Metal Embrittlement (LME) cracking is a well-documented issue encountered during resistance spot welding (RSW) of zinc-coated advanced high-strength steels (AHSS) in automotive manufacturing. Given that existing research has predominantly focused on laboratory-scale samples and lacks investigation into the load-bearing capacity of joints under crash conditions, this study aims to fill these gaps by analyzing third-generation zinc-coated AHSS. S-Rail components were produced through stamping to replicate real-world manufacturing conditions and geometries of automotive parts. To account for the disturbances typically encountered in production, samples with LME cracks were intentionally fabricated. Subsequently, a modified three-point bending test, assisted by numerical simulations, was developed to effectively apply loads to the weld spots of the S-Rail components. Results from crash tests demonstrated that observed light crack severity does not significantly compromise the joint's load-bearing capacity or lead to earlier joint failure.