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This blog is a short summary of a published comprehensive research work titled: “Peculiar Roles of Nickel Diffusion in Intermetallic Compound Formation at the Dissimilar Metal Interface of Magnesium to Steel Spot Welds” Authored by Luke Walker, Carolin Fink, Colleen Hilla, Ying Lu, and Wei Zhang; Department of Materials Science and Engineering, The Ohio State University
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There is an increased need to join magnesium alloys to high-strength steels to create multi-material lightweight body structures for fuel-efficient vehicles. Lightweight vehicle structures are essential for not only improving the fuel economy of internal combustion engine automobiles but also increasing the driving range of electric vehicles by offsetting the weight of power systems like batteries.
To create these structures, lightweight metals, such as magnesium (Mg) alloys, have been incorporated into vehicle designs where they are joined to high strength steels. It is desirable to produce a metallurgical bond between Mg alloys and steels using welding. However, many dissimilar metal joints form intermetallic compounds (IMCs) that are detrimental to joint ductility and strength. Ultrasonic interlayered resistance spot welding (Ulti-RSW) is a newly developed process that has been used to create strong dissimilar joints between aluminum alloys and high-strength steels. It is a two-step process where the light metal (e.g., Al or Mg alloy) is first welded to an interlayer (or insert) material by ultrasonic spot welding (USW). Ultrasonic vibration removes surface oxides and other contaminates, producing metal-to-metal contact and, consequently, a metallurgical bond between the dissimilar metals. In the second step, the insert side of the light metal is welded to steel by the standard resistance spot welding (RSW) process.
Cross-section View Schematics of Ulti-RSW Process Development
For resistance spot welding of interlayered Mg to steel, the initial schedule attempted was a simple single pulse weld schedule that was based on what was used in our previous study for Ulti-RSW of aluminum alloy to steel . However, this single pulse weld schedule was unable to create a weld between the steel sheet and the insert when joining to Mg. Two alternative schedules were then attempted; both were aimed at increasing the heat generation at the steel-insert interface. The first alternative schedule utilized two current pulses with Pulse 1, high current displacing surface coating and oxides and Pulse 2 growing the nugget. The other pulsation schedule had two equal current pulses in terms of current and welding time.
Lap shear tensile testing was used to evaluate the joint strength using the stack-up schematically, shown below. Note the images of Mg and steel sides of a weld produced by Ulti-RSW.
Lap Shear Tensile Test Geometry and the Resultant Weld Nuggets
An example of a welded sample showed a distinct feature of the weld that is comprised of two nuggets separated by the insert: the steel nugget formed from the melting of steel and insert and the Mg nugget brazed onto the unmelted insert. This feature is the same as that of the Al-steel weld produced by Ulti-RSW in our previous work. Although the steel nugget has a smaller diameter than the Mg nugget, it is stronger than the latter, so the failure occurred on the Mg sheet side.
Joint strength depends on several factors, including base metal strength, sheet thickness, and nugget size, making it difficult to compare how strong a weld truly is from one process to another. To better compare the dissimilar joints created by different processes, joint efficiency, a “normalized” quantity was calculated for various processes used for dissimilar joining of Mg alloys to steels in the literature, and those results, along with the efficiencies of Ulti-RSW with inserts, are shown together below. Most of the literature studies also used AZ31 as the magnesium base metal. The ones with high joint efficiency (about 53%) in the literature are resistance element welding (REW) and friction stir spot welding (FSSW). In our study, Ulti-RSW with SS316 insert was able to reach an excellent joint efficiency of 71.3%, almost 20% higher than other processes.
Process Evaluation and Comparison
Thanks are given to Menachem Kimchi, Associate Professor-Practice, Dept of Materials Science, Ohio State University, and Technical Editor – Joining, AHSS Application Guidelines, for this article.
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Urbanization and waning interest in vehicle ownership point to new transport opportunities in megacities around the world. Mobility as a Service (MaaS) – characterized by autonomous, ride-sharing-friendly EVs – can be the comfortable, economical, sustainable transport solution of choice thanks to the benefits that today’s steel offers.
The WorldAutoSteel organization is working on the Steel E-Motive program, which delivers autonomous ride-sharing vehicle concepts enabled by Advanced High-Strength Steel (AHSS) products and technologies.
The Body structure design for this vehicle is shown in Figure 1. It also indicates the specific joint configuration of 5 layers AHSS sheet stack-up as shown in Table 1. Resistance spot welding parameters were developed to allow this joint to be made by a single weld. (The previous solution for this welded joint is to create one spot weld with the bottom 3 sheets indicated in the table and a second weld to join the top 2 sheets, combining the two-layer groups to 5T stack-up.)
NOTE: Click this link to read a previous AHSS Insights blog that summarizes development work and recommendations for resistance spot welding 3T and 4T AHSS stack-ups: https://bit.ly/42Alib8
Table 1. Provided materials organized in stack-up formation showing part number, name, grade, gauge in mm, and coating type. Total thickness = 6.8 mm
The same approach of utilizing multiple current pulses with short cool time in between the pulses was shown to be most effective in this case of 5T stack-up. It is important to note that in some cases, the application of a secondary force was shown to be beneficial, however, it was not used in this example.
To establish initial welding parameters simulations were conducted using the Simufact software by Hexagon. As shown in Figure 2, the final setup included a set of welding electrodes that clamped the 5-layer AHSS stack-up. Several simulations were created with a designated set of welding parameters of current, time, number of pulses, and electrode force.
Figure 2. Example of simulation and experimental results showing acceptable 5T resistance spot weld (Meets AWS Automotive specifications)
Thanks is given to Menachem Kimchi, Associate Professor-Practice, Dept of Materials Science, Ohio State University and Technical Editor – Joining, AHSS Application Guidelines, for this article.
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
![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)
