RSW of Dissimilar Steel
This article is the summary of a paper entitled, “Weld Nugget Penetration of a Four-Sheet Resistance Spot Welding Advanced High-Strength Steels”, by K. Namola, et al.N-11
Experimental Weld Nugget Penetration
The study analyzes the effect of electrode size and composition on final weld nugget size and penetration. Nugget growth patterns were analyzed and weldability issues characterized. Figure 1 shows the arrangement of the four-layer stack-ups that were tested in this study. Truncated code electrodes used were a 6-mm Class 1, 6-mm Class 3, 6-mm Class 20, 8-mm Class 1, and 10-mm Class 1. Samples were welded in the as-received condition. JAC270 is a cold rolled Mild steel with a galvanneal coating having a minimum tensile strength of 270 MPa. JSC590 and JSC980 are bare cold rolled Dual Phase steels with a minimum tensile strength of 590 MPa and 980 MPa, respectively.
Figure 1: Resistance Welding Stack and Test Electrode Combinations.N-11
Best results from the iterative trials were obtained using an 8- and 6-mm Class 1 copper electrode with the weld schedule shown in Figure 2. This weld schedule was repeated using the electrode combinations listed in Table 1. Figure 3 shows cross sections of each weld listed in Table 1.
Figure 2: Down-Selected Weld Schedule from Trials.N-11
Table 1: Nugget Penetration Using the Down-Selected Weld Schedule from Trials and Different.N-11
Figure 3: Welds Made Using the Down-Selected Schedule and Different Electrodes.N-11
Figure 4 shows cross sections of five welds made starting with new 8- and 6-mm Class 1 electrodes. As can be seen, expulsion gets progressively worse over time but penetration does not. Penetration values into the JAC 270 were determined by metallography and are shown in Table 2.
Figure 4: Welds from Repeatability Study Using the Down-Selected Weld Schedule and 8- and 6-mm Class 1 Electrodes.N-11
Table 2: Nugget Penetration into the JAC 270 During Repeatability Study.N-11
Table 3 lists the resistance measurements at the weld stack interfaces. Figure 5 shows the resistance graph of weld stack up.
Table 3: Resistance Measurements of Weld Materials and Weld Stack Interfaces.N-11
Figure 5: Resistance Graph of Weld Stack-Up.N-11
The weld force used was 2.3 kN and the current reduction values are listed in Table 4. Figure 6 and Figure 7 show the still images at each pulse. The heating pattern implies that the JAC270 is forged into the weld nugget.
Table 4: Current Reduction for High-Speed Video Welds.N-11
Figure 6: Still Images from High-Speed Video.N-11
Figure 7: Still Images from Weld Simulation of the Down-Selected Schedule using 8- and 6-mm Class 1 Electrodes.L-58
RSW of Dissimilar Steel
This article summarizes a paper entitled, “Higher than Expected Strengths from Dissimilar Configuration Advanced High-Strength Steel Spot Welds”, by E. Biro, et al.B-6
This study shows that the cross tension strength (CTS) is always higher than the strength expected from the lower strength material in the joint. Figure 1 verifies the assumption that the load bearing capacity of a heterogeneous configuration is supposed to equal the minimum strength of both homogenous assemblies. Material used in this study was a 1 mm low carbon equivalent Dual Phase 980 (DP980 LCE) steel.
Figure 1: Example of dissimilar configuration with CTS matching the “minimum rule”.B-6
The materials chosen for this study are in Table 1.
Table 1: Steel sheet samples.B-6
The material thickness combinations for all of the two sheet joints are shown in Table 2.
Table 2: Welded 2-sheet configurations.B-6
The three-sheet stackups all were made using the 1 mm DP980 LCE. These configurations were designed to understand what happens in such cases, knowing that three-sheet welding is very common in car body manufacturing. The three-sheet stackup configurations are shown in Figure 2 and are as were follows:
- a square DP980 coupon (patch) is inserted between the two classical cross-tension coupons for welding (1+patch+1 mm);
- two coupons oriented the same way welded with one coupon oriented in the transverse direction to form a cross-tension sample (1+[1+1] mm);
- same configuration as a) but the external coupon is removed by manual torsion before cross-tension testing (1+1+0 mm);
- same configuration as a), but the two coupons oriented the same way are first spot welded together strongly (with several spots) in the extremities, before the actual 3-sheet spot weld is done ([1++++1]+1 mm).
Figure 2: Three-sheet configurations based on 1mm DP980 LCE sample.B-6
The welding parameters for each configuration are listed in Table 3.
Table 3 : Welding parameters.B-6
CTS is strongly dependent on weld diameter (Figure 3).
Figure 3: Cross-tension Strength for TRIP800 configurations.B-6
CTS for the main DP980 configurations are shown as a function of weld diameter in Figures 4 and 5.
Figure 4: Cross-tension Strength for DP980 1+1, 1+2 and 2+2 configurations.B-6
Figure 5: Cross-tension Strength for DP980 1.25+1.25, 1.25+2 and 2+2 configurations.B-6
The three-sheet configurations based on 1mm DP980 LC results are shown in Figures 6 and 7. These results again verify that dissimilar configuration performances appear above the “minimum rule” assumption described in Figure 1.
Figure 6: Cross-tension Strength for DP980 1+1, 1+1+0 and 1+patch+1 configurations.B-6
![Figure 7: Cross-tension Strength for DP980 1+1, 1+2, 1+[1+1]and [1+++1]+1 configurations.](https://ahssinsights.org/wp-content/uploads/2020/07/3114c_Fig7.jpg)
Figure 7: Cross-tension Strength for DP980 1+1, 1+2, 1+[1+1]and [1+++1]+1 configurations.B-6
The observation that CTS is greater than predicted by the “minimum rule” has been called a “positive deviation” from the expected strengths.
This work concluded that while material qualification tests are frequently based on similar welding configurations, real car body applications are quite systematically dissimilar configurations. For spot welds failing in plug mode, the strength of the assembly only depends on the weakest material strength. In case of AHSS+AHSS welded combinations, however, things turn out to be different. Similar grade but dissimilar thickness High-Strength Steel configurations have been spot welded and tested in Cross-Tension. The following main conclusions can be highlighted:
- For dissimilar thickness configurations, the cross-tensile strength is above the standard “minimum rule” assumptions, this phenomenon being called a “positive deviation”;
- Limited thermal and notch location effects can explain part of this positive deviation, but the main reason is mechanical;
- As evidenced through several analytic and numerical studies, this mechanical effect is due to the less severe local stresses at the notch in case of uneven thickness, and improves the positive deviation when the thickness ratio increases. Although widely used for material qualification and scientific purposes, similar configurations appear as the worst case in terms of cross-tension performance for high strength steels. Actual vehicle design should consider positive deviation in dissimilar configurations to maximize the potential strength of spot welds in High-Strength steels.
RSW of Dissimilar Steel
A variety of steel grades are used to manufacture vehicle body structures and closures. Welding dissimilar Advanced High-Strength Steels (AHSS) in three and four layer stack-ups requires special considerations. In this section of the Guidelines are articles summarizing papers that have investigated welding dissimilar AHSS and stack ups and discovered important factors for consideration and implementation.
RSW of Dissimilar Steel
This article is the summary of a paper entitled, “HAZ Softening of RSW of 3T Dissimilar Steel Stack-up”, Y. Lu., et al.L-15
Electromechanical Model
The study discusses the development of a 3D fully coupled thermo-electromechanical model for RSW of a three sheet (3T) stack-up of dissimilar steels. Figure 1 schematically shows the stack-up used in the study. The stack-up chosen is representative of the complex stack-ups used in BIW. Table 1 summarizes the nominal compositions of the three steels labeled in Figure 1.
Figure 1: Schematics of the 3T stack-up of 0.75-mm-thick JAC 270/1.4-mm-thick JSC 980/1.4-mm-thick JSC 590 steels.L-15
Table 1: Nominal Composition of Steels.L-15
JAC270 is a cold rolled Mild steel with a galvanneal coating having a minimum tensile strength of 270 MPa. JSC590 and JSC980 are bare cold rolled Dual Phase steels with a minimum tensile strength of 590 MPa and 980 MPa, respectively.
The electrodes used were CuZr dome-radius electrodes with a surface diameter of 6 mm. The welding parameters are listed in Table 2.
Table 2: Welding Parameters for Resistance Spot Welding of 3T Stack-Up of Steel Sheets.L-15
Figure 2 shows consistent nugget dimensions between simulation and experiment, supporting the validity of the RSW process model for 3T stack-up. The effect of welding current on nugget penetration into the thin sheet is similar to that on the nugget size. It increases rapidly at low welding current and saturates to 32% when the welding current is higher than 9 kA, as shown in Figure 2C.
Figure 2: Comparison between experimental and simulated results: A) Nugget geometry at 8 kA; B) nugget diameters; C) nugget penetration into the thin sheet as a function of welding current. In Figure 2A, the simulated nugget geometry is represented by the distribution of peak temperature (in Celsius). The two horizontal lines in Figure 2B represent the minimal nugget diameter at Interfaces A and B calculated, according to AWS D8.1M: 2007, Specification for Automotive Weld Quality Resistance Spot Welding of Steel. Due to limited number of samples available for testing, the variability in nugget dimensions at each welding current was not measuredL-15.
The results for nugget formation during RSW of the 3T stack-up are show in Figures 3-5. Figure 2 shows that, at the start of welding, the contact pressure at interface A (thin/thick) has a higher peak and drops more quickly along the radial direction than that at interface B (thick/thick). Due to the more localized contact area (Figure 3), a high current density can be observed at interface A, as shown in Figure 4A. Additionally, due to the high current density at interface A, localized heating is generated at this interface, as shown in Figure 5A.
Figure 3: Calculated contact pressure distribution at interfaces A (thin/thick) and B (thick/thick) at a welding time of 5 ms, current of 8 kA, and electrode force of 3.4 L-15
Figure 4: Calculated current density distribution at interfaces A (thin/thick) and B (thick/thick) at welding time of A — 5 ms; B — 200 and 300 ms.L-15
Figure 5: Temperature distribution during resistance spot welding of 3T stack-up at welding times of A) 5 ms; B) 102 ms; C) 300 ms. Welding current is 8 kA and electrode force is 3.4 kN. Calculated temperature is given in Celsius.L-15
As welding time increases, the contact area is expanded, resulting in a decrease of current density. The heat generation rate is shifted from interfaces to the bulk and the peak temperature occurs near the geometrical center of the stack-up.
Figure 6 illustrates that the predicted value corresponds well with the experimental data indicating a sound fitting to the isothermal tempering experimental data.
Figure 6: Comparison of the measured hardness with JMAK calculation showing the goodness of fit of the JSC 980 tempering kinetics parameters.L-15
Figure 7 shows the predicted hardness map of RSW 3T stack-up as well as the predicted and measured hardness profiles for JSC 980.
Figure 7: A) Predicted hardness map of resistance spot welded 3T stack-up; B) predicted and measured hardness profiles along the line marked in (A) for JSC 980.L-15
RSW Modelling and Performance
The advantages of numerical simulations for resistance welding are obvious for saving time and reducing costs in product developments and process optimizations. Today’s modeling techniques can predict temperature, microstructure, stress, and hardness distribution in the weld and Heat Affected Zone (HAZ) after welding. Commercial modeling software is available which considers material type, various current modes, machine characteristics, electrode geometry, etc. An example of process simulation results for spot welding of 0.8-mm DC 06 low-carbon steel to 1.2-mm DP 600 steel is shown in Figure 1. Obviously, this technique can apply to dissimilar thicknesses, material types, and geometries. Application of adhesives is also being used with these simulations. This simulation techniques are found to be very beneficial to predict vehicle crashworthiness as it can dramatically reduce the cost of crash evaluations.
You will find several articles in this section describing RSW modelling studies and procedures.
Figure 1: Simulation results with microstructures and hardness distribution for spot welding of 0.8-mm DC06 low-carbon steel to 1.2-mm DP 600 steel.Z-1
Blog, RSW Joint Performance Testing
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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
The study encompassed three different research fields, with an expert institute engaged for each:
- Laboratory of Materials and Joining Technology (LWF) Paderborn University, Paderborn, Germany, for experimental research,
- The Institute de Soudure, Yutz, France, for investigations regarding non-destructive testing, and
- Fraunhofer Institute for Production Systems and Design Technology (IPK), Berlin, Germany, for the development of a simulation model to accurately replicate welding conditions resulting in LME.
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
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.)
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
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
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 worldautosteel.org.
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.
Related Articles: More on this study in previous AHSS Insights blogs:
Journal Publications:
- Julian Frei, Max Biegler, Michael Rethmeier, Christoph Böhne & Gerson Meschut (2019) Investigation of liquid metal embrittlement of dual phase steel joints by electro-thermomechanical spot-welding simulation, Science and Technology of Welding and Joining, 24:7, 624-633, DOI: 10.1080/13621718.2019.1582203
- Christoph Böhne, Gerson Meschut, Max Biegler, Julian Frei & Michael Rethmeier (2020) Prevention of liquid metal embrittlement cracks in resistance spot welds by adaption of electrode geometry, Science and Technology of Welding and Joining, 25:4, 303-310, DOI: 10.1080/13621718.2019.1693731
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Auto/Steel Partnership LME Testing and Procedures
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![Figure 7: Cross-tension Strength for DP980 1+1, 1+2, 1+[1+1]and [1+++1]+1 configurations.](http://ahssinsights.org/wp-content/uploads/2020/07/3114c_Fig7.jpg)
