Liquid Metal Embrittlement Research Articles

Half-sectioned spot weld parameter optimisation for liquid metal embrittlement analysis in advanced high-strength steels

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

Additive manufacturing of multi-materials with interfacial component gradient by in-situ powder mixing and laser powder bed fusion

Defects (e.g., pores and cracks) and high stress concentration derived from the significant mutation in materials are the critical challenge for multi-material structures fabricated by laser powder bed fusion (LPBF). Introducing a transition zone between dissimilar materials with a composition gradient at the interface may be a promising approach to resolve or minimize these problems. In this work, three distinct CuSn10 copper alloy (CuA)/316 L stainless steel (SS) multi-material gradient structures (i.e., CS, SC-1, and SC-2) were fabricated by a LPBF system with in-situ powder mixing and feeding capability. A buffer layer material based on the mixture of the two base materials was introduced at the interface between CuA and SS. The effects of the building strategies and interfacial composition gradient transition paths on the interfacial bonding characteristics were investigated. The microstructure evolution and the underlying interfacial bonding mechanisms were discussed. It was found that the interfacial bonding behaviors were critically dependent on the occurrence of liquid metal embrittlement induced by Cu penetration into Fe grains, as well as the width and microstructure gradient of the diffusion zone and high stress concentration at the interface. A non-uniform melting behavior of CuA and SS powders could be induced by the distinct thermal-physical properties between the two materials, which could lead to lack-of-fusion pores at the interface. In the CS sample, printing SS on the CuA with a buffer layer material could inhibit the formation of Cu-penetration cracks and increase the width of the diffusion zone, which may enhance the interfacial metallurgical bonding. This work may enhance the essential comprehension of improving interfacial bonding strength of LPBF-processed multi-material structures.

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

Third generation (GEN3) advanced high-strength steels are increasingly used to reduce vehicle body structure weight for improved fuel economy. Zinc-induced liquid metal embrittlement (LME) has become a prevalent challenge when these steels are welded by resistance spot welding (RSW). In the literature, various numerical models have been developed to study LME during RSW of GEN3 steels. Those models tend to have a limited utility in predicting how various factors such as RSW parameters affect LME cracking. In this study, a novel LME calculation method has been developed by integrating an electro-thermo-mechanical finite element model of RSW process with an LME ratio for crack initiation established from hot tensile testing data. The onset of different types of LME cracking is tracked throughout the weld region over the entire course of RSW including the final cooling stage after electrode retraction. The model is validated using an existing set of literature data that involves different welding schedules, electrode geometries and electrode misalignments. The local temperature and stress conditions during RSW are calculated and used to understand how different welding parameters affect the severity of LME cracking. It is found that electrode misalignment exacerbates crack initiation on outer surfaces of the spot weld due to the tensile stress arisen from the off-centered squeeze force as well as the reduced surface cooling to electrodes. Initiation of cracks between the steel sheets, taking place mainly after electrode retraction, is not significantly affected by the welding schedule and the electrode tip diameter.

Critical LME elongation: a criterion measured with Gleeble correlated to LME sensitivity during spot welding

Liquid metal embrittlement may appear on multiphase high strength galvanized steels during the spot-welding process. The aim of this paper is to describe a high temperature tensile test, performed with a Gleeble machine, to evaluate the LME sensitivity of a given coated steel in conditions similar to spot welding (time cycle). The critical LME elongation, corresponding to minimum of stress derivative during force decrease, has been found as being the most relevant parameter to measure the LME phenomenon. Relevant test parameters such as temperature, strain rate, applied thermal cycle, and effect of thermocouple welding have been studied on two steels. The first, TRIP 800 steel, presents a high sensitivity to LME between 750 °C and 900 °C while the second, DP600 steel, is not sensitive. Strain rate effect reveals a competition between LME crack propagation and ductile fracture. The study also shows the need to avoid welding thermocouple on Zn-coated side. Overall, the critical LME elongation from Gleeble test is well correlated to LME cracks number and depth in spot welding.

On the Potential Correlation between Dynamic Strain Aging and Liquid Metal Embrittlement in T91/LBE System

In the study of the liquid metal embrittlement (LME) of the T91/lead-bismuth eutectic (LBE) system, it is observed that LME occurs in a temperature interval which is similar to the temperature range where dynamic strain aging (DSA) is observed. However, the potential correlation between DSA and LME has not yet been satisfactorily investigated. This investigation for the T91/LBE system is exactly the topic of this work. For the evaluation of DSA and LME, slow strain rate tensile tests are conducted in the temperature range between 200 °C and 450 °C with strain rates of 5×10−5 s−1 and 5×10−6 s−1 in reference and a molten oxygen-depleted LBE environment. The resulting tensile properties, as well as the fracture surfaces and lateral surfaces of the failed samples, suggest a correlation between DSA and LME in the T91/LBE system. The maximum mechanical degradation of T91 is observed in the case where the effects of both DSA and LME on material properties are found to be at maximum. However, the observation of DSA was not identified as a prerequisite for LME to take place. Therefore, these results may indicate that DSA partly contributes to the ductility minimum observed in the T91/LBE system. In addition, the results of this work show that changes in the fracture surface and lateral surface are more sensitive features to claim for the potential occurrence of LME than the changes in total elongation.

Surface Oxide Removal in Preparation for Controlled Liquid Metal Embrittlement

Surface Oxide Removal in Preparation for Controlled Liquid Metal Embrittlement

Insight into grain boundaries with reduced liquid metal embrittlement susceptibility in a boron-added 3rd generation advanced high strength steel

The addition of boron to the advanced high-strength steels (AHSS) enhances grain boundary cohesion and can play a significant role in intergranular degradation phenomena such as Zn-assisted liquid metal embrittlement (LME). The objective of this study is to reveal a thorough understanding of the effect of B on LME behaviour, where the welding test results indicated substantial improvement even at relatively low B concentration. Here, we adopted the strategy of identifying LME-sensitive grain boundaries, followed by characterizing grain boundary chemistry. EBSD measurements and prior austenite grain (PAG) reconstruction provided strong evidence of intergranular LME crack propagation via PAGBs. Further transmission electron microscopy and energy dispersive X-ray spectroscopy investigations demonstrated an asymmetric Mn profile along the PAGB in B-free steel. Therefore, we propose that B addition and consequently, lower grain boundary energy appears to prevent Mn segregation. Based on our observation, we conclude that B seems to have the similar desired effect on Zn diffusion along the PAGBs and eventually mitigates Zn-assisted LME.

Effects of Substrate Microstructure and Chemical Composition on Liquid Metal Embrittlement in Galvanized 3rd Generation AHSS

Extensive efforts have been undertaken worldwide to develop new high strength steels with substantial fractions of retained austenite, for lightweight automobile manufacturing and other applications requiring improved combinations of strength and formability. These “3rd Generation” Advanced High Strength Steels (AHSS) are being implemented, and spot-welding has been found to present new challenges for these steels when Zn-based corrosion resistant coatings are involved, wherein zinc liquid metal embrittlement (LME) can occur. Some recent work is highlighted here that was designed to examine the separate effects of prior microstructure and alloy composition on LME sensitivity. LME behavior was assessed by comparing hot-ductility of steels with and without a Zn coating tested under conditions simulating spot-weld thermal cycles. Effects of prior microstructure on LME susceptibility were assessed with a single AHSS alloy composition, using annealing modifications to produce martensitic, Q&P, TBF and dual-phase substrates. The dual-phase steel exhibited less sensitivity to LME, perhaps because the Zn penetration and cracking are unable to follow (prior) austenite boundaries in this microstructure. With respect to alloy composition, carbon and manganese variations did not lead to noticeable effects on LME sensitivity, while silicon clearly leads to increased LME sensitivity. Addition of 1.3 wt. pct. aluminum to a 0.5 wt. pct silicon-containing AHSS steel further increased LME sensitivity at some test temperatures. The effects of alloying are interpreted in terms of the propensity to form an intermetallic reaction layer that consumes liquid and physically separates the substrate and liquid zinc.

Low Temperature Hot Press Forming of a Zinc Coated Third Generation Advanced High Strength Steel

A 7 wt.% Mn steel was designed and was cast and processed to 1.5 mm sheets. The sheets were continuous annealed and coated with a Zn alloy by hot dip galvanizing before subjecting them to hot press forming cycles. The final microstructure was characterized by ultrafine ferrite grains and a high fraction of retained austenite. Excellent combinations of in-service strength-ductility-bendability were achieved for hot forming in the temperature range of 530-675 °C. The use of the low temperature hot forming minimized the liquid metal embrittlement induced cracking with Zn coating during hot forming and spot welding. Various application properties such as oxidation resistance, corrosion resistance, and springback were found at optimum levels for hot forming at 675 °C.

Negative effect of bismuth in lead on liquid metal embrittlement of a ferritic steel

Liquid metal embrittlement (LME) of a Fe-10Cr-4Al ferritic steel was studied at 375 °C in liquid Pb-Bi alloys. Slow strain rate testing (SSRT) in low oxygen conditions was used to evaluate the ductility as a function of Bi concentration. It was found that susceptibility to LME increased strongly with the Bi concentration. The steel showed a reduction in its total strain to failure, which started at 3–5 wt.% Bi. The alloying elements (Fe, Cr, and Al) have a higher solubility in Bi than pure lead (Pb), so they are expected to dissolve more readily when Bi is added to the Pb. This is believed to be part of the explanation for the observed increase of LME. Lead with up to 3 wt.% Bi induced no LME in the studied corrosion resistance FeCrAl alloy.

Liquid Metal Embrittlement Cracking in Uncoated Transformation-Induced Plasticity Steel during Consecutive Resistance Spot Welding

In the automotive production line, a single pair of electrodes is employed to produce hundreds of consecutive welds before undergoing dressing or replacement. In consecutive resistance spot welding (RSW) involving Zn-coated steels, the electrodes undergo metallurgical degradation, characterized by Cu-Zn alloying, which impacts the susceptibility to liquid metal embrittlement (LME) cracking. In the present investigation, the possibility of LME crack formation in uncoated TRIP steel joints during consecutive RSW (involving 400 welds in galvannealed and uncoated TRIP steels) was investigated. The results have shown that different Cu-Zn phases were formed on the electrode surface because of its contamination with Zn from the galvannealed coating. Therefore, during the welding of the uncoated TRIP steel, the heat generated at the electrode/sheet interface would result in the melting of the Cu-Zn phases, thereby exposing the uncoated steel surface to molten Zn and Cu, leading to LME cracking. The cracks exhibited a maximum length of approximately 30 µm at Location A (weld center) and 50 µm at Location B (shoulder of the weld). The occurrence and characteristics of the cracks differed depending on the location as the number of welds increased due to the variation in Zn content. Type A cracks did not form when the number of welds was less than 280. Several cracks with a total length of approximately 30 μm were suddenly formed between 280 and 400 welds. On the other hand, type B cracks began to appear after 40 welds. However, the number and size of these exhibited inconsistency as the number of welds increased. Overall, the results have shown that small LME cracks can form even in uncoated steels during consecutive welding of Zn-coated and uncoated steel joints.

Influence of alloying elements on various properties of face-centered-cubic iron

Austenitic stainless steel 316 L, a typical face-centered-cubic (FCC) iron alloy, demonstrates great potential as structural materials in nuclear plant. However, liquid metal (Pb and Bi) embrittlement (LME) constitutes the major bottlenect in its wide application. In this work, first-principles calculations are conducted to reveal the influences of Cr, Mo, Ni, Pb, and Bi on magnetic property, surface energy, stacking fault energy, ductility, as well as electronic structure of FCC Fe under the magnetic ground state of antiferromagnetic double layer (AFMD). It is discovered that the addition of Ni could considerably increase the ductility of FCC Fe, while the effects of Cr and Mo on the increase of ductility of FCC Fe seem much smaller. On the contrary, both Pb and Bi significantly reduce the ductility of FCC Fe. Such a large decrease is mainly attributed to the more reduction of surface energy than that of unstable stacking fault energy as well as a more directional and weaker chemical bonding. The derived results could provide insight into the influence of alloying elements on the various properties of FCC Fe.

The Influence of Precooling on Liquid Metal Embrittlement and Microstructure Properties of Galvanized 22MnB5 Tubes

To address the issue of liquid metal embrittlement (LME) susceptibility in galvanized 22MnB5 steel during the hot stamping process, the material's performance is affected. This study proposes a method combining precooling and tube hydroforging to produce galvanized tubular hot stamping parts. The primary workflow comprises four distinct stages: the heating phase, thermal retention phase, precooling phase, and the tube hydroforging phase. Notably, the precooling stage employs two distinct approaches: air precooling and boiling water precooling. The composition and morphology of the zinc coating and the microstructure and mechanical properties of the 22MnB5 steel are investigated using two precooling methods at different hydroforging temperatures. The results show that when the initial hydroforging temperature is below 800 °C, the precooling combined with the tube hydroforging process eliminates LME. With increasing precooling time, the oxidation reaction forms ZnO and Fe2O3 on the coating. By comparing the composition, morphology, and mechanical properties of the coatings at hydroforging temperatures of 800 °C and 500 °C, it is concluded that the boiling water precooling aligns more effectively with the requirements of industrial production, with the optimal forming temperature being within the range of 750–800 °C.

The Identification of a New Liquid Metal Embrittlement (LME) Type in Resistance Spot Welding of Advanced High−Strength Steels on Reduced Flange Widths

Liquid metal embrittlement (LME) cracking is a phenomenon observed during resistance spot welding (RSW) of zinc−coated advanced high−strength steels (AHSS) in automotive manufacturing. In this study, severe cracks are observed at the edge of the sheet under reduced flange widths. These cracks, traversing the AHSS sheet, culminate at the edge with a width of approximately 1.2 mm. Through combined numerical and experimental investigations, and material testing, these cracks are identified and validated as a new type of LME crack. The mechanism behind this crack formation is attributed to unique geometric conditions that, when compared to center welding, amplify radial material flow by ninefold to 0.87 mm. The resultant tangential tensile stresses approximate 760 MPa, which exceed the yield strength of the examined advanced high−strength steel (AHSS) under heightened temperature conditions, and when combined with liquid zinc, promote the formation of this new type of LME crack.

Investigating the mechanism of zinc-induced liquid metal embrittlement crack initiation in austenitic microstructure

Investigating the mechanism of zinc-induced liquid metal embrittlement crack initiation in austenitic microstructure

Specificity and Embrittling Mechanism of Liquid Metal Embrittlement: First-Principles Calculations

Specificity and Embrittling Mechanism of Liquid Metal Embrittlement: First-Principles Calculations

Temperature-dependent liquid metal embrittlement of Al0.7CoCrFeNi high-entropy alloys induced by equiatomic GaInSnZn melts

The equiatomic GaInSnZn (GISZ) high-entropy alloys (HEAs) are considered to have the potential as coolants to engage the thermal management owing to their high thermal conductivity. However, the incompatibility between them and other structural materials hinders these practical applications. On the way to seeking suitable structural metals with liquid metal embrittlement (LME) resistance in GISZ environment, the Al0.7CoCrFeNi alloys are selected and their temperature-dependent LME sensitivity is investigated through high temperature and slow strain-rate tensile tests at 100, 300 and 500 ℃. The sharp decrease in tensile ductility and serious LME phenomenon are explicitly inspected at all temperatures. The transformation of the LME mechanism, which shifts from adsorption-induced reduction in cohesion of the interior or boundaries of grain colonies to grain/phase boundary wetting with temperature increasing, is corroborated by the fractographic analysis and crack propagation path. This study not only provides guidance for the LME of this new system, but also encourages a promising landscape for searching for protective measures of Al0.7CoCrFeNi alloys and new materials adapted to GISZ, thereby laying a solid foundation for promoting liquid metal cooling applied to the actual industry.

Giant segregation transition as origin of liquid metal embrittlement in the Fe-Zn system

A giant Zn segregation transition is revealed using CALPHAD-integrated density-based modeling of segregation into Fe grain boundaries (GBs). The results show that above a threshold of only a few atomic percent Zn in the alloy, a substantial amount of up to 60 at.% Zn can segregate to the GB. We found that the amount of segregation abruptly increases with decreasing temperature, while the Zn content in the alloy required for triggering the segregation transition decreases. Direct evidence of the Zn segregation transition is obtained using high-resolution scanning transmission electron microscopy. Base on the model, we trace the origin of the segregation transition back to the low cohesive energy of Zn and a miscibility gap in Fe-Zn GB, arising from the magnetic ordering effect, which is confirmed by ab-initio calculations. We also show that the massive Zn segregation resulting from the segregation transition greatly assists with liquid wetting and reduces the work of separation along the GB. The current predictions suggest that control over Zn segregation, by both alloy design and optimizing the galvanization and welding processes, may offer preventive strategies against liquid metal embrittlement.

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

Liquid metal embrittlement (LME) crack is a severe defect which can affect the joint integrity of Zn-coated advanced high-strength steel resistance spot welds (RSW). The tensile stress that causes LME cracking is a consequence of thermal contact at the electrode-sheet interface during welding. Currently, there is no systematic study to identify the critical stages of LME occurrence during welding with understanding the tensile stress development. This research aims to address this gap by combining in-situ observation, ex-situ verification, and finite element modeling to identify the critical stages in LME cracking. The study reveals that the large LME cracks form only after a critical welding time (tcrit) of 250 ms, when the growth of the weld nugget exceeds that of the electrode-sheet contact and electrode tip-face diameter. As tcrit was attained, the LME responsible thermal gradient increased from 2000 °C/mm to 4000 °C/mm, consequently tensile stress increased from 200 MPa to 300 MPa, resulting in large LME crack. A mechanistic model explaining the effect of nugget growth rate and electrode-contact growth on thermal gradient, stress development and LME cracking was then proposed and verified through cross-comparison of crack observations. This research provides valuable insights into the critical stages of LME cracking during RSW of Zn-coated steel and suggests enhancing the growth rate of electrode-contact as a solution to mitigate this issue without altering the nugget growth rate.

Determination of the Existence of Liquid Metal Embrittlement Cracks in Resistance Spot-Welded Advanced High-Strength Steel

Abstract The characteristics of liquid metal embrittlement (LME) cracks combined with the location, amount, and length of the resistance spot-welded joint of galvanized advanced high-strength steel were comprehensively investigated in this study. To better illustrate the features of LME cracks, a multilayer grinding and polishing technique was employed to more comprehensively reflect the existence of LME cracks compared with that of a single-layer technique. Based on the crack observation results of the cross section of the center weld, the longest type-I and type-II cracks were approximately 322.2 and 106.7 μm, respectively. On the contrary, the longest type-I and type-II cracks were approximately 1,062.9 and 194.7 μm, respectively, for the multilayer grinding and polishing technique. The existence of liquid zinc for more time and the resultant tensile stress at the center of the weld can trigger crack propagation, eventually resulting in type-I cracks that are longer than type-II cracks. Similarly, severe LME cracks mainly exist in the central position of the welded joint, implying that temperature and stress can have a crucial effect on the formation and propagation of LME cracks.