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The discussions relative to cold stamping are applicable to any forming operation occurring at room temperature such as roll forming, hydroforming, or conventional stamping. Similarly, hot stamping refers to any set of operations using Press Hardening Steels (or Press Quenched Steels), including those that are roll formed or fluid formed.
Automakers contemplating whether a part is cold stamped or hot formed must consider numerous factors. This blog covers some important considerations related to welding these materials for automotive applications. Most important is the discussion on Resistance Spot Welding (RSW) as it is the dominating process in automotive manufacturing.
Setting Correct Welding Parameters for Resistance Spot Welding
Specific welding parameters need to be developed for each combination of material type and thickness. In general, the Hot Press (HP) steels require more demanding process conditions. One important factor is electrode force which should be higher for the HP steel than for cold press type steel of the same thickness. The actual recommended force will depend on the strength level, and the thickness of the steel. Of course, this will affect the welding machine/welding gun force capability requirement.
Another important variable is the welding current level and even more important is the current range at which acceptable welds can be made. The current range is weldability measurement, and the best indicator of the welding process robustness in the manufacturing environment and sometime called proceed window. Note the relative range of current for different steel types. A smaller process window may require more frequent weld quality evaluation such as for weld size.
Relative Current Range (process windows) for Different Steel Types
The Effect of Coating Type on Weldability
In all cases of resistance spot welding coated steels, it is imperative to move the coating away from the weld area during and in the beginning of the weld cycle to allow a steel-to-steel weld to occur. The combination of welding current, weld time and electrode force are responsible for this coating displacement.
For all the coated steels, the ability of the coating to flow is a function of the coating type and properties, such as electrical resistivity and melting point, as well as the coating thickness.
An example of cross sectioned spot welds made on Hot Press Steel with Aluminum -Silicon coating is shown below. It shows two coating thicknesses and the displaced coating at the periphery of weld. This figure also shows the difference in current range for the different coating thickness. The thicker coating shows a smaller current range. In addition, the Al-Si coating has a much higher melting point than the zinc coatings on the cold stamped steels, making it more difficult to displace from the weld area.
Hot Press Steel with Aluminum -Silicon
Liquid Metal Embrittlement and Resistance Spot Welding
Cold-formable, coated, Advance High Strength Steels such as the 3rd Generation Advanced High Strength Steels are being widely used in automotive applications. One welding issue these materials encounter is the increased hardness in the weld area, that sometime results in brittle fracture of the weld.
Another issue is their sensitivity to Liquid Metal Embrittlement (LME) cracking. These two issues are discussed in detail on the WorldAutoSteel AHSS Guidelines website and our recently released Phase 2 Report on LME.
Resistance Spot Welding Using Current Pulsation
The most effective solution for the issues described above is using current pulsation during the welding cycle. A schematic description is shown below.
The pulsation allows much better control of the heat generation and the weld nugget development. The pulsation variables include the number of pulses (typically 2-4), the current level and time for each pulse, and the cool time between the pulses.
In summery, pulsation (and sometime current upslope) in Resistance Spot Welding proved to be beneficial for the following applications:
- Coated Cold Stamped steels
- Cold stamped Advance High Strength Steels
- Multi materials stack-ups – As described in our articles here on 3T/4T and 5T Stack-Ups
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.
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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.
Blog, RSW Modelling and Performance
Modern car bodies today are made of increasing volumes of Advanced High-Strength Steels (AHSS), the superb performance of which facilitates lightweighting concepts (see Figure 1). To join the different parts of a car body and create the crash structure, the components are usually welded to achieve a reliable connection. The most prominent welding process in automotive production is resistance spot welding. It is known for its great robustness, and easily applicable in fully automated production lines.
Figure 1: AHSS Content in Modern Car Body.W-7
There are, however, challenges to be met to guarantee a high-quality joint when the boundary conditions change, for example, when new material grades are introduced. Interaction of a liquefied zinc coating and a steel substrate can lead to small surface cracks during resistance spot welding of current AHSS, as shown in Figure 2. This so-called liquid metal embrittlement (LME) cracking is mainly governed by grain boundary penetration with zinc, and tensile stresses. The latter may be induced by various sources during the manufacturing process, especially under ‘rough’ industrial conditions. But currently, there is a lack of knowledge, regarding what is ‘rough’, and what conditions may still be tolerable.
Figure 2: Top View of LME-Afflicted Spot Weld.
The material-specific amount of tensile stresses necessary for LME enforcement can be determined by the experimental procedure ‘welding under external load’. The idea of this method, which is commonly used for comparing cracking susceptibilities of different materials to each other, is to apply increasing levels of tensile stresses to a sample during the welding process and monitor the reaction. Figure 3 shows the corresponding experimental setup.
Figure 3: Welding under external load setup.L-51
However, the known externally applied stresses are not exclusively responsible for LME, but also the welding process itself, which puts both thermally and mechanically induced stresses/strains on the sample. Here, the conventional measuring techniques fail. A numerical reproduction of the experiment grants access to the temperature, stress and strain fields present during the procedure, providing insights on the formation of LME. The electro-thermomechanical simulation model is described in detail in Modelling RSW of AHSS. It is used to simulate the welding under external load procedure (see Figure 4).
Figure 4: Simulation Model of Welding Under External Load.
The videos that can be found in the link above show the corresponding temperature and plastic strain fields. As heat dissipates quickly through the water-cooled electrode, a temperature gradient towards the adjacent areas and a local temperature maximum on the surface forms. The plastic strains accumulate mainly at the electrode indentation area. The simulated strain field shows a local maximum of plastic deformation at the left edge of the electrode indentation, amplified by the externally applied stresses and the boundary conditions implied by the procedure. This area correlates with experimentally observed LME cracking sites and paths as shown in Figure 5.
The simulation shows that significant plastic strains are present during welding. When external stresses (in reality e.g. due to poor part fit-up or distorted parts) contribute to the already high load, LME cracking becomes more likely. The numerical simulation model facilitates the determination of material-specific safety limits regarding LME cracking. Parameter variations and their effects on the LME susceptibility can easily be investigated by use of the model, enabling the user to develop strict processing protocols to reduce the likelihood of LME. Finally, these experimental procedures can be adapted to other high-strength materials, to aid their application understanding and industrial set-up conditions.
Figure 5: LME Cracks in Cross Section View at Highly Strained Locations.
For more information on this topic, see the paper, co-authored by Fraunhofer and LWF Paderborn, documented in Citation F-23. You may also download the full report documenting the WorldAutoSteel LME project for which this work was conducted.
RSW Modelling and Performance
This article summarizes the findings of a paper entitled, “Prediction of Spot Weld Failure in Automotive Steels,”L-48 authored by J. H. Lim and J.W. Ha, POSCO, as presented at the 12th European LS-DYNA Conference, Koblenz, 2019.
To better predict car crashworthiness it is important to have an accurate prediction of spot weld failure. A new approach for prediction of resistance spot weld failure was proposed by POSCO researchers. This model considers the interaction of normal and bending components and calculating the stress by dividing the load by the area of plug fracture.
Background
Lee, et al.L-49 developed a model to predict spot welding failure under combined loading conditions using the following equation, based upon experimental results .
|
Equation 1 |
Where FS and FN are shear and normal failure load, respectively, and n is a shape parameter.
Later, Wung and coworkersW-38 developed a model to predict the failure mode based upon the normal load, shear load, bending and torsion as shown in Equation 2.
 |
Equation 2 |
Here, FS, FN ,Mb and Mt are normal failure load, shear failure load, failure moment and failure torsion of spot weld, respectively. α, β, γ and μ are shape parameters.
Seeger et al.S-106 proposed a model for failure criterion that describes a 3D polynomial failure surface. Spot weld failure occurs if the sum of the components of the normal, bending and shear stresses are above the surface, as shown in the Figure 1.
Figure 1: Spot weld failure model proposed by Seeger et al.S-106
The failure criterion can be expressed via Equation 3.
 |
Equation 3 |
Here, σN , σB , and τ are normal, bending and shear stress of the spot weld, respectively. And nN, nB and nc are the shape parameters. Toyota Motor CorporationL-50 has developed the stress-based failure model as shown in Equation 4.
 |
Equation 4 |
Hybrid Method to Determine Coefficients for Failure Models
This work used a unique hybrid method to determine the failure coefficients for modeling. The hybrid procedure steps are as follows:
- Failure tests are performed with respect to loading conditions.
- Finite element simulations are performed for each experiment.
- Based on the failure loads obtained in each test, the instant of onset of spot weld failure is determined. Failure loads are extracted comparing experiments with simulations.
- Post processing of those simulations gives the failure load components acting on spot welds such as normal, shear and bending loads.
These failure load components are plotted on the plane consisting of normal, shear and bending axes.
The hybrid method described above is shown in Figure 2.L-48
Figure 2: Hybrid method to obtain the failure load with respect to test conditions.L-48
New Spot Weld Failure Model
The new proposed spot weld failure model in this paper considers only plug fracture mode as a normal spot weld failure. Secondly normal and bending components considered to be dependent upon each other. Stress generated by normal and bending components is shear, and shear component generates normal stress. Lastly authors have used πdt to calculate the area of stress instead of πd2/4. The final expression is shown in the Equation 5.
 |
Equation 5 |
Here τn is the shear stress by normal load components, σS is the normal stress due to shear load component. And 
, 
, c, α and β are coefficients.
This work included verification experiments of 42 kinds of homogenous steel stack-ups and 23 heterogeneous stack-ups. The strength levels of the steels used was between 270 MPa and 1500 MPa, and thickness between 0.55 mm and 2.3 mm. These experiments were used to evaluate the model and compare the results to the Wung model.
Conclusions
Overall, this new model considers interaction between normal and bending components as they have the same loading direction and plane. The current developed model was compared with the Wung model described above and has shown better results with a desirable error, especially for asymmetric material and thickness.
