Liquid Metal Embrittlement Research Articles

Avoidance of liquid metal embrittlement during resistance spot welding by heat input dependent hold time adaption

ABSTRACT Liquid metal embrittlement (LME) cracking can occur during resistance spot welding (RSW) in zinc-coated advanced high-strength steels (AHSS) for automotive production. In this study, a methodological variation of hold time is performed to investigate the process-related crack influence factors. A combination of numerical and experimental investigations confirms, that the extent of heat dissipation and re-heating of the sheet surface can be influenced and thus the degree of crack formation can be controlled in a targeted manner by the parameterisation of the hold time. The temperature and stress history of crack-free and crack-afflicted spot welds are analysed and a conclusion on the borders defining the LME active region is derived.

The effect of silicon content on liquid-metal-embrittlement susceptibility in resistance spot welding of galvanized dual-phase steel

Abstract Liquid metal embrittlement (LME) can have a significant effect on the mechanical properties of resistance spot welding (RSW) advanced high strength steels (AHSS). An understanding of the role of alloying elements such as silicon (Si) on microstructure details and RSW parameters, both of which influence LME cracking is essential for design of LME-resistant AHSS. The present study has shown that increasing Si-content from 0.7 wt.% to 1.8 wt.% increased LME susceptibility in dual phase (DP) steels. It has been demonstrated that increasing Si led to higher heat inputs and early expulsion due to the higher resistivity of the higher Si-content grade during RSW that was related to more severe LME-cracking. Furthermore, increasing Si-content had a significant effect on microstructural evolution during processing prior to RSW, including increased ferrite grain size, decarburization layer depth, and internal oxides density, all of which were related to an increased LME crack susceptibility.

Grain Boundary Complexion Transitions

Grain boundaries can undergo phase-like transitions, called complexion transitions, in which their structure, composition, and properties change discontinuously as temperature, bulk composition, and other parameters are varied. Grain boundary complexion transitions can lead to rapid changes in the macroscopic properties of polycrystalline metals and ceramics and are responsible for a variety of materials phenomena as diverse as activated sintering and liquid-metal embrittlement. The property changes caused by grain boundary complexion transitions can be beneficial or detrimental. Grain boundary complexion engineering exploits beneficial complexion transitions to improve the processing, properties, and performance of materials. Here, we review the thermodynamic fundamentals of grain boundary complexion transitions, highlight the strongest experimental and computationalevidence for these transitions, clarify a number of important misconceptions, discuss the advantages of grain boundary complexion engineering, and summarize existing research challenges.

Reducing liquid metal embrittlement cracking in resistance spot welding of Q&P980 steel

ABSTRACT The galvanized ultra-high-strength steels are prone to liquid metal embrittlement (LME) cracking during resistance spot welding (RSW), cracking is facilitated by the specific temperature and stress conditions on the steel surface. A two-dimensional electro-thermomechanical model was established to simulate the RSW of a galvanized Q&P980 steel, which was susceptible to LME. Temperature and stress histories in the welding area were obtained from simulation, they could well explain the occurrence of LME. A mild steel cover sheet was used during welding, which was placed between the Q&P980 steel and the electrode. It was found that the cover sheet modified the temperature and stress states on the surface of Q&P980 steels and made them less satisfying to LME, and consequently suppressed LME cracking.

Role of liquid Zn and α-Fe(Zn) on liquid metal embrittlement of medium Mn steel: An ex-situ microstructural analysis of galvannealed coating during high temperature tensile test

Zn assisted liquid metal embrittlement (LME) has characteristics that differ from the general characteristics of other LME systems: the formation of Fe-Zn intermetallic compounds and the high solubility of Zn in Fe. Consequently, there is a need to investigate the influence of the Fe-Zn interaction and resulting phase composition on LME behavior. However, little research has been conducted to reveal such relationships. This paper explores the influence of high temperature phases in galvannealed coating, specifically liquid Zn and α-Fe(Zn) on the LME sensitivity of medium Mn steel using a high temperature tensile test with eight combinations of temperatures (700 °C to 900 °C) and hold times (1 s, 50 s, and 100 s). The test results are categorized as LME, microcrack and LME free cases in terms of LME behavior. The study reveals that the LME sensitivity is governed by the fraction of effective liquid Zn, which is a consequence of isothermal solidification owing to the thermodynamic stability of liquid Zn, diffusivity of Zn in Fe and the partitioning coefficient of liquid Zn. Moreover, LME is influenced by the breaking of α-Fe(Zn) layer, which directs the liquid Zn into the substrate steel. Finally, a water-tank analogy is proposed to demonstrate the effect of the fraction of liquid Zn and the breaking of α-Fe(Zn) layer on LME.

Role of Random and Coincidence Site Lattice Grain Boundaries in Liquid Metal Embrittlement of Iron (FCC)-Zn Couple

Liquid metal embrittlement (LME) has frequently been reported in various systems: Cu-Bi, Ni-Bi, Al-Ga, and Fe-Zn for more than 60 years. Although advances have been made in understanding the phenomenon, the role of grain boundary (GB) type and characteristics in LME has remained unclear. The present work shows that liquid metal penetration only occurs in random GBs, where its propagation path is a function of misorientation angle (θ) and stress component perpendicular to GB plane. In contrast, low-Σ coincidence site lattice (CSL) boundaries: Σ3 (θ = 60 deg) and Σ5 to 9 (~ θ = 40 deg) resist LME, even at the maximum stress component. Triple junctions of CSL and random GB block liquid metal penetration by modifying of the random GB misorientation angle. These findings provide insights to employ grain boundary engineering techniques to increase population of CSLs over random GBs and, hence, mitigate LME.

Liquid Metal Embrittlement of Galvanized TRIP Steels in Resistance Spot Welding

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.

The influence of alloying on Zn liquid metal embrittlement in steels

In this study we investigate Zn liquid metal embrittlement at Fe grain boundaries (GBs) by employing ab initio density functional theory (DFT) simulations. We consider three different GBs and two common alloying elements in steels (Si and Al) and evaluate how the interplay of the elements affects Zn enrichment and embrittlement of the GBs. Our simulations show that Zn has stronger tendency to enrich at both GBs and surfaces than Al and Si. By increasing the amount of Al and Si in the bulk, Zn can be reduced at both GBs and surfaces. This is quantified for a large temperature range by the newly introduced replacement potency, which shows that compared to Al, Si has a larger potency to decrease the amount of Zn at GBs and surfaces. Regarding GB embrittlement, our results demonstrate that Si a small positive, Al a small negative, and Zn a strong negative effect on GB cohesion. In combination with Zn both Al and Si are able to somewhat reduce the detrimental effect of Zn.

Effect of temperature on liquid metal embrittlement susceptibility of an Fe10Cr4Al ferritic alloy in contact with stagnant lead-bismuth eutectic

The effect of temperature on the liquid metal embrittlement (LME) susceptibility of an Fe10Cr4Al ferritic alloy exposed to stagnant oxygen-depleted lead-bismuth eutectic (LBE) was investigated using slow strain rate tensile (SSRT) tests. The results show that the total elongation to rupture of the specimens tested in LBE decreases with increasing temperature from 150 to 350 °C, while an upward trend is observed in the temperature range of 350–500 °C. Such a strong temperature dependence is called a “ductility trough”. This phenomenon is not observed in the specimens tested in Ar reference environment. Fractographic examinations reveal that LME occurs at 150 up to 450 °C and ductility fully recovers only at 500 °C. In addition, transgranular cleavage and intergranular cracking are the major fracture modes on the fracture surfaces affected by LME.

Suppression of liquid-metal-embrittlement by twin-induced grain boundary engineering approach

Grain boundary engineering (GBE) has lately been recognized as a viable approach to manipulate grain boundary characteristics to improve resistance against intergranular degradation. One of the most challenging intergranular degradation phenomena is liquid-metal-embrittlement (LME), where a reactive liquid metal penetrates along random grain boundary network. In contrast to random grain boundaries, special coincidence site lattice (CSL) boundaries has been shown to be resistant against the embrittlement. The present study investigates the feasibility of using a GBE technique to arrest LME in a Fe (FCC)-Zn couple. Two sets of low-strain-heat-treatment processing routes were used to optimize grain boundary characteristics based on: (a) Σ3 and Σ3n boundaries frequency, and (b) the material's texture and grain size evolution. The optimum characteristics resulted in significantly improved resistance against LME. The mechanism of the embrittlement arrest is discussed based on the random-grain boundary network continuity and the grain boundary triple junctions distributione.

Effect of symmetrical tilt grain boundary on the compatibility between copper and liquid lithium: Atomistic simulations

Abstract The compatibility between Cu and liquid Li directly affects the safety of fusion devices. In this work, we investigated the compatibility between Cu Ʃ3 (111), Ʃ3 (112) and Ʃ5 (310) symmetrical tilt grain boundaries (STGB) and liquid Li using molecular dynamics (MD) simulation. The Li atoms rapidly penetrate the intergranular area along Ʃ3 (112) and Ʃ5 (310) grain boundaries (GBs), resulting in the formation of liquid grooves at the junctions of GBs and solid-liquid interfaces. However, the Ʃ3 (111) GB shows its resistance to Li penetration. In addition, the interfacial alloying and dissolution of Cu atoms are accelerated by Ʃ5 (310) GB via promoting the escape of Cu atoms. Then we calculated the potential energy distribution of Cu atoms which indicates the liquid metal embrittlement (LME) effect exists between Cu bicrystal and liquid Li. And we demonstrate this embrittlement has a strong correlation with the vacancy formation energy and the segregation energy of Li atoms. Comparing with the experimental results, our simulation results explain the corrosion morphology of polycrystalline Cu in liquid Li, and point out that the LME is the root cause of Cu specimen fragmentation.

Si effect on Zn-assisted liquid metal embrittlement in Zn-coated TWIP steels: Importance of Fe-Zn alloying reaction

During resistance spot welding of Zn-coated high-strength steel sheets, Zn penetration is caused by Zn-assisted liquid metal embrittlement (Zn LME), and degrades the mechanical properties of the welded parts. When electrogalvanized twinning induced plasticity (TWIP) steels with 0–1.5 wt% Si were welded, Zn LME was exacerbated by Si addition. It was found that Si in the steel delayed the dissolution of Fe into the coating from the steel, which suspended the active formation of α-Fe(Zn). Since α-Fe(Zn) fraction in the coating was insufficient to cover the steel/coating interface in the Si added steel, it could not prevent the steel from contacting the liquid Zn during welding. As a result, liquid Zn generated higher number and deeper Zn penetration into the Si containing steel than into the Si free steel. Overall, it was revealed that Si aggravated Zn LME in TWIP steel by suppressing Fe-Zn alloying reaction, which increased the possibility of steel substrate to contact liquid Zn.

Observations of grain-boundary phase transformations in an elemental metal.

While the theory of grain boundary (GB) structure has a long history1 and the proposition that grain boundaries can undergo phase transformations was made already 50 years ago2,3, the search for these GB transitions has been in vain. The underlying assumption was that multiple stable and metastable states exist for different grain boundary orientations4,5,6. The terminology complexion was recently proposed to distinguish between those interfacial states that differ in any equilibrium thermodynamic property7. Different types of complexions and transitions between them have been characterized mostly in binary or multicomponent systems 8,9,10,11,12,13,14,15,16,17,18,19. Novel simulation schemes have provided a new perspective on the phase behavior of interfaces and showed that grain boundary transitions can occur in a multitude of material systems20,21,22,23,24. However, their direct experimental observation and transformation kinetics in an elemental metal remained elusive.Here, we show atomic scale grain boundary phase coexistence and transformations at symmetric and asymmetric [11-1] tilt grain boundaries in elemental copper (Cu). We found the coexistence of two different grain boundary structures at Σ19b grain boundaries by atomic resolution imaging. Evolutionary grain boundary structure search and clustering analysis21,25,26 finds the same structures and predicts the properties of these GB phases. Furthermore, finite temperature molecular dynamics simulations explore their coexistence and transformation kinetics. Our results demonstrate how grain boundary phases can be kinetically trapped enabling atomic scale room temperature observations.Our work paves the way for atomic scale in situ studies of grain boundary phase transformations in metallic grain boundaries. In the past, only indirect measurements have indicated the existence of interfacial transitions9,15,27,28,29. Now, their atomic scale role on the influence of abnormal grain growth, non-Arrhenius type diffusion or liquid metal embrittlement can be explored.

Elimination of liquid metal embrittlement cracks during arc cladding of tin bronze on steel sheet

Liquid metal embrittlement cracks are generated in steel when the steel is in contact with liquid tin bronze. The elimination of liquid metal embrittlement cracks during the arc cladding of tin bronze on steel sheet was investigated. The result showed that liquid metal embrittlement cracks were generated in the bare steel sheet while no liquid metal embrittlement crack was generated in the nickel-plated steel sheet. An iron-based transition zone containing nickel was generated between steel sheet and cladding layer, and nickel had inhibitory effect on the formation of liquid metal embrittlement cracks. Strength and toughness of the steel sheet were improved with the disappearance of liquid metal embrittlement cracks.

The Role of Internal Oxides on the Liquid Metal Embrittlement Cracking During Resistance Spot Welding of the Dual Phase Steel

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.

Four Types of LME Cracks in RSW of Zn-Coated AHSS

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.

Crystallographic study of liquid-metal-embrittlement crack path

Compared to other intergranular degradation phenomena, liquid metal embrittlement (LME) is less understood. The main gap in knowledge has been the lack of fundamental information of how grain-boundary characteristics (e.g. grain boundary misorientation angle and grain boundary crystallographic plane) affects LME susceptibility. In this study, the recently developed trace analysis has been applied to the grain boundaries exhibiting LME cracking. Despite the presence of a diverse range of low- and high-index planes in the entire population of the grain boundaries, liquid metal selectively penetrated high-index random grain boundary planes such as (5 3 1), (5 1 1), and (7 3 3).

Liquid metal embrittlement sensitivity of the T91 steel in lead, in bismuth and in lead-bismuth eutectic

The liquid metal embrittlement (LME) sensitivity of the martensitic T91 steel by Pb, Bi and LBE has been studied by small punch tests at different temperatures and strain rates. The LME occurrence in the three liquid metals depends on the strain rate and on the temperature. The T91 steel loaded in Bi, Pb or LBE presents ductile behaviour at 300 °C and at 400 °C except in LBE at 300 °C at very slow strain rate for which fully brittle fracture surface was observed. It appears that the LME sensitivity of the T91 steel by LBE and bismuth is more important at 300 °C than 400 °C. No LME by lead has been observed. The most embrittling liquid metal is LBE, then bismuth while the less one is lead. Some differences in the reactive wetting of the T91 steel by the saturated oxygen liquid metal could explain the difference in LME sensitivity in the three liquid metals.

Liquid Metal Embrittlement of a Dual-Phase Al <sub>0.7</sub>CoCrFeNi High-Entropy Alloy Exposed to Oxygen-Saturated Lead-Bismuth Eutectic

This paper reports a new liquid metal embrittlement (LME) system in which a dual-phase Al0.7CoCrFeNi (equimolar fraction) high-entropy alloy (HEA) is embrittled by lead-bismuth eutectic (LBE) at 350 and 500 °C. At 350 °C, (Ni, Al)-rich BCC phase is embrittled, leading to intragrain cracking within this phase, while the predominant cracking mode changes to BCC/FCC phase boundary decohesion at 500 °C. At both temperatures, cracks are rarely seen in the (Co, Cr, Fe)-rich FCC phase, indicating that this phase is immune to LME. Furthermore, the results suggest a transition from an adsorption-dominated LME mechanism at 350 °C to a phase boundary wetting-dominated LME mechanism at 500 °C.

Liquid Metal Embrittlement in Resistance Spot Welding (Fourth Report)

Liquid Metal Embrittlement in Resistance Spot Welding (Fourth Report)