Liquid Metal Embrittlement: Results of a 3-Year Study

Liquid Metal Embrittlement: Results of a 3-Year Study


Results of a Three-Year LME Study

WorldAutoSteel releases today the results of a three-year study on Liquid Metal Embrittlement (LME), a type of cracking that is reported to occur in the welding of Advanced High-Strength Steels (AHSS).The study results add important knowledge and data to understanding the mechanisms behind LME and thereby finding methods to control and establish parameters for preventing its occurrence. As well, the study investigated possible consequences of residual LME on part performance, as well as non-destructive methods for detecting and characterizing LME cracking, both in the laboratory and on the manufacturing line (Figure 1).

Figure 1: LME Study Scope

Figure 1: LME Study Scope

The study encompassed three different research fields, with an expert institute engaged for each:

A portfolio containing 13 anonymized AHSS grades, including dual phase (DP), martensitic (MS) and retained austenite (RA) with an ultimate tensile strength (UTS) of 800 MPa and higher, was used to set up a testing matrix, which enabled the replication of the most relevant and critical material thickness combinations (MTC). All considered MTCs show a sufficient weldability under use of standard parameters according to SEP1220-2. Additional MTCs included the joining of various strengths and thicknesses of mild steels to select AHSS in the portfolio. Figure 2 provides the welding parameters used throughout the study.

Figure 2: Study Welding Parameters

Figure 2: Study Welding Parameters

In parallel, a 3D electro-thermomechanical simulation model was set up to study LME. The model is based on temperature-dependent material data for dual phase AHSS as well as electrical and thermal contact resistance measurements and calculates local heating due to current flow as well as mechanical stresses and strains. It proved particularly useful in providing additional means to mathematically study the dynamics observed in the experimental tests. This model development was documented in two previous AHSS Insights blogs (see AHSS Insights Related Articles below).


Understanding LME

The study began by analyzing different influence factors (Figure 3) which resembled typical process deviations that might occur during car body production. The impact of the influences was analyzed by the degree of cracking observed for each factor. A select number of welding set-ups from these investigations were rebuilt digitally in the simulation model to replicate the process and study its dynamics mathematically. This further enabled the clarification of important cause-effect relationships.

Figure 3: Overview of All Applied Influence Factors (those outlined in yellow resulted in most frequent cracking.)

Figure 3: Overview of All Applied Influence Factors (those outlined in yellow resulted in most frequent cracking.)

Generally, the most frequent cracking was observed for sharp electrode geometries, increased weld times and application of external loads during welding. All three factors were closely analyzed by combining the experimental approach with the numerical approach using the simulation model.

Destructive Testing – LME Effects on Mechanical Joint Strength

A destructive testing program also was conducted for an evaluation of LME impact on mechanical joint strength and load bearing capacity in multiple conditions, including quasi-static loading, cyclic loading, crash tests and corrosion. In summary of all load cases, it can be concluded that LME cracks, which might be caused by typical process deviations (e.g. bad part fit up, worn electrodes) have a low intensity impact and do not affect the mechanical strength of the spot weld. And as previously mentioned, the study analyses showed that a complete avoidance of LME during resistance spot welding is possible by the application of measures for reducing the critical conditions from local strains and exposure to liquid zinc.


Controlling LME

In welding under external load experiments, the locations of the experimental crack occurrence showed close correlation with the strains and remaining plastic deformations computed by the simulation model. It was observed that the cracks form at the location of the highest plastic strains, and material-specific threshold values for critical strains were derived. The threshold values then were used to judge the crack formation at elongated weld times.

At the same time, the simulation model pointed out a significant difference in liquid zinc diffusion during elongated weld times. Therefore, it is concluded that liquid zinc exposure time is a second highly relevant factor for LME formation.

The results for the remaining influence factors depended on the investigated MTCs and were generally less significant. In more susceptible MTCs (AHSS welded with thick Mild steel), no significant cracking occurred when welded using standard process parameters. Light cracking was observed for most of the investigated influences, such as low electrode cooling rate, worn electrode caps, electrode positioning deviations or for gap afflicted spot welds. More intense cracking (higher penetration depth cracking) was only observed when welding under extremely high external loads (0.8 Re) or, even more, as a consequence of highly increased weld times.

For the non-susceptible MTCs, even extreme situations and weld set-ups (such as the described elongated weld times) did not result in significant LME cracks within the investigated AHSS grades.

Methods for avoidance of LME also were investigated. Changing the electrode tip geometry to larger working plane diameters and elongating the hold time proved to eliminate LME cracks. In the experiments, a change of electrode tip geometry from a 5.5 mm to an 8.0 mm (Figure 4) enabled LME-free welds even when doubling the weld times above 600 ms. Using a flat-headed cap (with small edge radii or beveled), even the most extreme welding schedules (weld times greater than 1000 ms) did not produce cracks. The in-depth analysis revealed that larger electrode tip geometries clearly reduce the local plastic deformation around the indentation. This plastic strain reduction is particularly important, as longer weld times contribute to a higher liquid zinc exposure interval, leading to a higher potential for LME cracks.

Figure 4: Electrode Geometries Used in Study Experiments

Figure 4: Electrode Geometries Used in Study Experiments

It was also seen that as more energy flows into a spot weld, it becomes more critical to parameterize an appropriate hold time. Depending on the scenario, the selection of the correct hold time alone can make the difference between cracked and crack-free welds. Insufficient hold times allow liquid zinc to remain on the steel surface and increased thermal stresses that form after the lift-off of the electrode caps. Elongated hold times reduce surface temperatures, minimizing surface stresses and thus LME potential.

Non-Destructive Testing: Laboratory and Production Capabilities

A third element of the study, and an aid in the control of LME, is the detection and characterization of LME cracks in resistance spot welds, either in laboratory or in production conditions. This work was done by the Institute of Soudure in close cooperation with LWF, IPK and WorldAutoSteel members’ and other manufacturing facilities. Ten different non-destructive techniques and systems were investigated. These techniques can be complementary, with various levels of costs, with some solutions more technically mature than others. Several techniques proved to be successful in crack detection. In order to aid the production source, techniques must not only detect but also characterize cracks to determine intensity and the effect on joint strength. Further work is required to achieve production-level characterization.

The study report provides detailed technical information concerning the experimental findings and performances of each technique/system and the possible application cost of each. Table 1 shows a summary of results:

Table 1: Summary of NDT: LME Detection and Characterization Methods

Table 1: Summary of NDT: LME Detection and Characterization Methods


Preventing LME

Suitable measures should always be adapted to the specific use case. Generally, the most effective measures for LME prevention or mitigation are:

  • Avoidance of excessive heat input (e.g. excess welding time, current).
  • Avoidance of sharp edges on spot welding electrodes; instead use electrodes with larger working plane diameter, while not increasing nugget-size.
  • Employing extended hold times to allow for sufficient heat dissipation and lower surface temperatures.
  • Avoidance of improper welding equipment (e.g. misalignments of the welding gun, highly worn electrodes, insufficient electrode cooling)

In conclusion, a key finding of this study is that LME cracks only occurred in the study experiments when there were deviations from proper welding parameters and set-up. Ensuring these preventive measures are diligently adhered to will greatly reduce or eliminate LME from the manufacturing line. For an in-depth review of the study and its findings, you can download a copy of the full report at



LME Study Authors

LME Study Authors

The LME study authors were supported by a committed team of WorldAutoSteel member companies’ Joining experts, who provided valuable guidance and feedback.



Journal Publications:



Auto/Steel Partnership LME Testing and Procedures


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Resistance Spot Welding

Resistance Spot Welding

The Resistance Spot Welding (RSW) process is often used as a model to explain the fundamental concepts behind most resistance welding processes. Figure 1 shows a standard Resistance Spot Welding arrangement in which two copper electrodes apply force and pass current through the sheets being welded. If the sheets are steel, the resistance to the flow of current of the sheets will be much higher than the copper electrodes, so the steel will get hot while the electrodes remain relatively cool. But another important characteristic exists that is critical to most resistance welding processes – the contact resistance between the parts (or sheets) being welded. As indicated on the figure, the highest resistance to the flow of current is where the sheets meet (“Resistance” 4). This fact allows a weld nugget to begin forming and grow exactly where it is needed – between the sheets.

The figure depicts the flow of current from one electrode to the other as an electrical circuit that contains seven “resistors”. Resistors 1 and 7 represent the bulk resistance of the copper electrodes, Resistors 2 and 6 represent the contact resistance between the electrodes and the sheets, Resistors 3 and 5 represent the bulk resistance of the sheets, and Resistor 4, as mentioned, represents the contact resistance between the sheets. Another fact that works strongly in the favor of resistance welding of steel is that as the steel is heated, its resistivity relative to copper increases even more. So, the initial contact resistance effectively heats the surrounding area which is in turn heated more rapidly because its resistance is higher since it is hotter. As a result, heating and weld nugget formation can occur quite rapidly. A typical weld time for RSW of steel is approximately 1/5 of a second. The current required in resistance welding is much higher than arc welding, and it is in the range of 8-15 kA.

Resistances associated with steel resistance spot welds.

Figure 1: Resistances associated with steel Resistance Spot Welding.

Most welding processes produce welds that provide strong visible evidence of weld quality, so visual examination is often an important approach to verifying the quality of the weld. However, with most resistance welding processes visible examination is not possible due to the “blind” weld location between the sheets or parts being welded. As a result, maintaining weld quality with processes such as Resistance Spot Welding is highly dependent on what is known as a lobe curve (Figure 2), which is basically a process window for Resistance Spot Welding. The lobe curve represents ranges of weld current and time that will produce a spot weld nugget size that has acceptable mechanical properties for the intended application. So, weld quality monitoring with resistance welding processes relies highly on the ability to monitor parameters such as current and time. During production, if a weld is made with parameters that fall outside of the lobe curve, the weld is considered unacceptable. Ultrasonic testing is also often used in the automotive industry for Nondestructive Testing (NDT) of the blind location of Resistance Spot Welding.

Resistance welding lobe curve.

Figure 2: Resistance welding lobe curve.


RSW Electrode Geometry

Electrode geometry is a very important consideration with RSW. Figure 3 shows three common shapes, but there are many more options available including unique custom designs for specific applications. The basic electrode geometry is usually selected to improve the electrical-thermal-mechanical performance of an electrode. This is generally a geometry in which the cross-sectional area increases rapidly with distance from the workpiece, thereby providing a good heat sink. The choice of shape may also include considerations such as accessibility to the part and how much surface marking of the part is acceptable. The diameter of the electrode contact area is also a consideration. Too small an area will produce undersized welds with insufficient strength, while too large an area will lead to unstable and inconsistent weld growth characteristics.

Typical RSW electrode geometries

Figure 3: Typical RSW electrode geometries (left), example geometries used by automotive manufacturers (right).


Electrodes must be able to conduct current to the part, mechanically constrain the part, and conduct heat from the part. They must be able to sustain high loads at elevated temperatures, while maintaining adequate thermal and electrical conductivity. The choice of electrode alloy for a given application is often dictated by the need to minimize electrode wear. When electrodes wear, they typically begin to “mushroom”, or grow larger in diameter. Electrode wear is accelerated when there is an alloying reaction between the electrode and the part, a common problem when welding Aluminum (Al) and coated steels. As the electrode diameter increases, the current density decreases, resulting in a decrease in the size of the weld. Since the strength of a spot-welded joint is directly related to the size of the weld nugget, electrode wear can be a big problem.

A range of Copper (Cu)-based or refractory-based electrode materials are used depending on the application. The Resistance Welding Manufacturers Association (RWMA) sorts electrode materials into three groups: A, B, and C. Group A contains the most common Cu-based alloys (see Table 1), Group B contains refractory metals and refractory metal composites, and Group C contains specialty materials such as dispersion-strengthened copper. Within the groups, they are further categorized by a class number. The general rule of thumb is as the class number goes up, the electrode strength goes up but the electrical conductivity goes down. When electrical conductivity goes down, the electrode heats more easily, resulting in premature electrode wear. The choice of electrode material involves many factors, but generally higher strength electrodes will be selected when higher strength materials are being welded. It is also important that the electrical and thermal conductivities of the electrode are much higher than those of the material being welded.

Table 1: Minimum mechanical and physical properties of Cu-based alloys for RWMA electrodes.

Table 1: Minimum mechanical and physical properties of Cu-based alloys for RWMA electrodes.A-18

The two most commonly used electrodes are the Class 1 and 2 electrodes of Group A. Class 1 electrodes [99% copper, 1% cadmium; 60 ksi UTS (forged); conductivity 92% International Annealed Copper Standard (IACS)] offer the highest electrical and thermal conductivity, and are typically used for spot welding Al alloys, magnesium alloys, brass, and bronze. Class 2 [99.2% copper, 0.8% chromium; 62 ksi UTS (forged), 82% IACS] electrodes are general-purpose electrodes for production spot and seam welding of most materials. IACS refers to a copper standard to which the electrodes are compared. Pure Cu has an IACS number of 100%.A-11, P-6, O-1