Atoms arrange themselves in three-dimensional patterns called lattices. Think about billiard balls in multiple layers. The balls can be one layer directly above the prior one, or they can be shifted and rest in the crevice formed by adjacent balls in the layer below. The balls are all the same material, but the gap size changes with different arrangements.
A crystal lattice shows atoms in a defined, repetitive pattern. In this simple example the atoms can arrange in a square or triangle pattern. The atom size is the same, but the gap size where small elements like carbon can go depends on the lattice arrangement.
Ferrite and Austenite: The Building Blocks of Steel
This is what happens with steel, which for most automotive grades are at least 97% iron. At temperatures below 725 °C, a maximum of only 0.02% carbon fits in the gap between the iron atoms. This orientation is called ferrite. At higher temperatures, a different atomic orientation is stable, which we call austenite. Up to 2% carbon can fit into this arrangement of atoms. For low-carbon steels under normal conditions, austenite cannot exist at room temperature – when the steel is slowly cooled, it changes from austenite to a combination of ferrite and a mixture of phases called pearlite.
Ferrite: Soft, Ductile, and Low-Strength
100% iron is very soft. Ferrite at room temperature is iron with no more than 80 parts per million carbon. That’s really close to pure iron, so when discussing ferrite, think of something soft, low-strength, and ductile.
Carbon-Manganese Steels: Structural Strength
If additional strength is needed, then more alloying elements must be used in addition to carbon. The next most cost-effective alloying element for strengthening is manganese which produces higher-strength steels called carbon-manganese steels. These grades have limited ductility, especially at higher carbon and manganese contents, so they are used in structural applications that do not need a lot of formability and are therefore also called structural steels (SS).
Microalloying: The Birth of HSLA Steels
Around 1980, steelmakers rolled out a new approach to getting higher strength levels while minimizing the loss of elongation usually seen with higher strengths. They accomplish this by strengthening the ferrite through the addition of very small quantities of titanium, niobium, and vanadium to form carbide and nitride precipitates. These microalloying additions are used in precipitation hardening of the ferrite to create High Strength Low Alloy (HSLA) steels.
Rapid Cooling Enables Martensite Formation
The steels discussed to this point are produced with relatively slow cooling. However, investments by the steel industry resulted in equipment capable of reaching rapid cooling (quenching) rates that allow for production of a very high strength phase called martensite.
Martensite wasn’t commonly found as a microstructural componentduring most of the history of automotive sheet steels due to the limited number of companies having an annealing line with appropriate quenching capabilities. This started to change around the turn of the millennium when newer annealing lines were installed with capability to achieve complex thermal cycles. This allowed for production of the first generation of Advanced High-Strength Steels (AHSS), including grades that have a microstructure of only martensite.
Dual Phase (DP) Steels: Balancing Strength and Ductility
Dual Phase (DP) steels are the most common AHSS. Ferrite and martensite are the two phases in DP steels: ferrite is super-soft and comprises the majority of the microstructure, while martensite is super-hard and takes up 10% (590DP) to 40% (980DP) of the microstructure. The more martensite, the stronger the steel. Since most of the structure is ferrite, these steels have exceptional elongation as measured in a tensile test for the strength level.
TRIP Steels: Transformation-Induced Plasticity
For as good as DP steels are in tensile ductility, TRIP steels are even better. The magic of these grades comes from retained austenite. Austenite is a very ductile phase. What makes this a special phase is that as austenite-containing steels deform, the atoms rearrange and the austenite transforms into martensite, giving the steel enhanced ductility — which researchers state as greater plasticity. Another way of saying that this enhanced ductility comes from austenite transforming to martensite is that these steels have Transformation Induced Plasticity (TRIP).
Ferrite-Bainite and Complex Phase (CP) Steels: Improved Edge Ductility
In both DP and TRIP steels, the large hardness difference between ferrite and martensite leads to crack initiation sites and results in poor cut-edge ductility during stretch flanging. For applications like stretch flanging that need improved cut-edge ductility, one option are the Ferrite-Bainite grades. Bainite is a little lower in strength than martensite but has higher elongation and toughness. Another option are Complex Phase (CP) steels, which have a microstructure of bainite and precipitation-strengthened ferrite, with martensite and retained austenite also present in lower amounts. Lacking soft ferrite, these steels have relatively high yield strength and low elongation as measured in a tensile test, but the bainite leads to exceptional cut-edge ductility as measured in a hole expansion test.
TWIP Steels: Twinning-Induced Plasticity
TWIP steels containing only austenite, and as such are a high-strength, high-ductility steel. These may be written like TRIP steels, but these steels get their plasticity differently. TWIP steels deform by a mechanism known as twinning, so they are described as Twinning Induced Plasticity Steels (TWIP). Unfortunately, achieving the combination of high strength and fantastic formability requires a lot of alloying. This drives up the steelmaking complexity and cost. The alloying elements also make welding much more challenging. TWIP steels are considered second-generation advanced high-strength steels.
3rd Generation AHSS: Tailoring Microstructures for Performance
Nearly all 3rd Generation Advanced High-Strength Steels (3rd Gen AHSS or 3rd Gen) have retained austenite in the microstructure and therefore benefit from a high strength, high ductility combination through the TRIP Effect. The latest annealing lines allow for the creation of an engineered balance and distribution of ferrite, bainite, martensite, and austenite in the microstructure, providing the resultant alloy with properties that can be tailored to address the requirements and challenges of each automotive part.
The Future of Automotive Steels
Together, there are nearly 70 grades of advanced high strength steels available globally. The days of steel being simply a commodity are in the past as it relates to these highly engineered higher strength steels.
With the rise of electric vehicles, evaluating the environmental impact of each manufacturing process is essential. This article presents an EV battery enclosure welding LCA to compare the sustainability of different joining methods. As automotive manufacturers strive to reduce their carbon footprints, understanding the impact of production processes becomes crucial—especially the joining techniques used in car body assembly. Using Life Cycle Assessment, these impacts can be analyzed in terms of their reduction potential. This article focuses on the example of an all-steel EV battery case joined using several different welding methods.
Life Cycle Assessment (LCA) and Why It Matters in Automotive Welding
Life Cycle Assessment is a systematic method for evaluating the environmental impacts of a product or process throughout its life cycle—from raw material extraction to production, use, and disposal. According to DIN EN ISO 14040 standards, LCA is structured into four main components:
Goal and Scope Definition: Setting the purpose of the assessment, defining the functional unit, and establishing system boundaries.. In this article, the joining processes were in focus with associated inputs of electricity and filler materials as well as outputs of worn consumables.
Inventory Analysis: Gathering data on material and energy flows using measurements of electricity and consumables consumption as well as database values for associated material extraction and production impacts
Impact Assessment: Evaluating potential environmental impacts in different impact categories. This article shows global warming potential and acidification to compare climate change emissions and show the effect of different filler materials on acidification. Other impact categories relevant for joining processes are eutrophication, emission of photooxidants and ozone layer depletion.
Evaluation: Reviewing findings and providing recommendations regarding the environmental impact of the compared processes
Figure 1: Considered system boundaries of the LCA comprising supply of materials and energy, the joining process and disposal of consumables.
Welding Methods Compared: Processes, Assumptions, and System Boundaries
The assessed joining processes include:
Laser Remote Welding
Laser Welding with Wire
Laser Brazing
Resistance Spot Weld Bonding (RSW-Bonding) with single- and two-component adhesive
In the context of joining processes for EV battery cases, this LCA aims to compare the environmental impacts of different welding methods on a meter of weld length. The assessment uses both primary and secondary data from literature and established LCA databases. As not all data is available in-detail for welding processes, several simplifications and assumptions are required:
For example, filler wires aresimplified to pure steel or copper wire rather than taking their complex chemical composition into account. Therefore, it is likely that the actual impact of the filler material production is higher than assumed here. For electricity emissions, the German grid mix is used and to compare resistance spot welding and laser beam welding per weld length, 20 resistance spot welds per meter are assumed.
The emissions from the structural materials are not considered, as the same design is used for all welding processes. Second-order effects due to the switching a joining technology, i.e. the material savings due to reduced flange-widths for laser welding, are also not considered. The second-order material saving effects are known to be larger in comparison to the environmental impact of the joining processes and should be optimized together with the choice of joining process. Further information on these effects can be found in this study.
Figure 2 All-steel EV battery case with zones marked for laser beam and resistance spot welding
LCA Results: Emissions Impact by Welding Type in EV Battery Enclosures
The results of the LCA are shown in Figure 3 for the two impact categories Global Warming Potential (i.e. CO2 equivalentemissions) and Acidification Potential (i.e. emission of SO2 equivalent). Main driving factors of emissions are electrical energy, compressed air as well as filler material in the form of steel or copper wire (for laser wire and laser brazing respectively) and adhesive for RSW bonding.
Using the German electrical grid mix, RSW bonding shows the lowest GWP impact. As it does not use any filler material, laser remote welding has the largest potential for CO2 emission reduction, if the electricity generation incorporates more renewable energy .
When analyzing the laser processes, a high idle energy consumption of the laser systems is determined. This is due to the electricity demand of the laser source, cooling and control systems. The consumption only rises slightly, when the lasers are operating. This leads to the conclusion that the overall energy efficiency of laser welding systems can be significantly improved by optimizing “beam-on times” in relation to “idle times”.
In terms of the acidification potential, laser brazing stands out with a far larger impact compared to the other processes, because of the emissions associated with mining and extraction of its copper-based filler material. The wear of copper electrode caps also contributes to this impact category.
Figure 3: Environmental impact of the different welding processes in Global Warming Potential (left) and Acidification (right) per meter of weld length.
What This LCA Reveals About Sustainable Welding for EV Manufacturing
Life Cycle Assessment provides invaluable insights into the environmental impacts of joining processes in the automotive industry. By understanding the implications of material choices and energy consumption, manufacturers can make informed decisions that promote sustainability. Both the effect of electricity consumption and filler materials on the environmental impact of automotive joining processes is discussed in this article. Joining processes are one of the major drivers of an OEM’s emissions with ample potential for optimization through LCA analysis.
Brunner-Schwer, J. Lemke, M. Biegler, T. Schmolke, S. Spohr, G. Meschut, L. Eckstein, M. Rethmeier; A life cycle assessment of joining processes in the automotive industry, illustrated by the example of an EV battery case; Laser in Manufacturing Conference, Munich, 2023
What Are Press Hardening Steel Grades and How Are They Made?
Press hardening is a special hot forming process, where the part is quenched in a forming die to receive its high hardness. It has been used in the automotive industry for over 40 years now.
The most common press hardening steel (PHS) is 22MnB5, a low carbon steel with Manganese-Boron alloying. Since it achieves a typical tensile strength of 1500 MPa after heat treatment, this material is mostly named as PHS1500 or CR1500T-MB (Cold Rolled, 1500 MPa typical Tensile strength, Manganese-Boron alloyed).B-14, V-19
The Direct Press Hardening Steel Process
The direct press hardening process involves heating the blanks over 900°C (1650°F) in an industrial furnace. The blanks are then removed from the furnace and quickly transferred to a forming die. The formed parts are not removed immediately. Instead, they are kept under force in a water-cooled tool set for quenching. With a 22MnB5 steel, the quenched part typically reaches 1500 MPa tensile strength. B-14
Coatings and Early Process Enhancements
Over the years, the first improvement on the 22MnB5 material was the application of an aluminum-silicon coating around early 2000’s. The addition of the coating did not affect its strength or elongation but improved the process as it eliminated scale formation during forming and quenching. AlSi coating however limited the process to the aforementioned direct process.B-14
Zinc Coatings and the Indirect Process
Some OEMs, especially in Europe, wanted to use Zn-based coatings for corrosion protection. The typical 22MnB5 is available with hot-dip galvanized (GI) and galvannealed (GA) coatings. Liquid metal embrittlement (LME) with Zn-based coatings is avoided with an indirect press hardening process. Forming is done at ambient temperatures, with the part subsequently heated and quenched in a press tool. These materials and techniques have been available since 2008.P-5
Figure 1 summarizes the most common 22MnB5 grades and coatings before and after the press hardening process.
Figure 1: 22MnB5 before and after the hot stamping and quenching cycle. The incoming material is similar to HSLA 380 or DP600 and can be cold formed if needed. After hot stamping, typical tensile strength is around 1500 MPa (re-created after: B-18, O-8, U-9).
Measuring Performance: VDA Standards
In 2010, German Association of the Automotive Industry (VDA) developed a new bending test (238-100) to evaluate energy absorbing capacity of PHS and PQS grades.V-4 This test gave a “bending angle” measurement, which replaced – to some extent – the use of “total elongation” value for energy absorbing calculations.
PHS1800 and Beyond
In 2011, a Japanese steel maker developed the first 1800 MPa (typical) tensile strength material. The material was AlSi coated with a modified, higher carbon, 30MnB5 chemistry and Nb alloying.H-33 One Japanese OEM applied the material in their bumper beams. The higher strength allowed using 1.4 mm thick PHS1800 material, instead of 1.6 mm PHS1500.M-28
Tailored Solutions for Specific Applications
A German OEM designed a B-pillar with a PHS1500 upper section, laser welded to a lower section formed from HSLA 340LA (340 MPa yield strength) steel for improved energy absorption.S-13 It was later found that typical HSLA steels not designed for hot stamping process may show significant variation in mechanical properties depending on the cooling rate.D-22
The Rise of Press Quenched Steels (PQS)
Steel companies subsequently developed “Press Quenched Steels” (PQS) which are also HSLA but have been specifically modified to achieve consistent material properties at varying cooling rates.H-69 PQS grades are not hardenable, even after hot stamping and quenching cycle.
In 2015, a steel company in Europe developed 20MnB8 with GI coating. Chemistry with slightly lower carbon and higher manganese allowed forming to be done at lower temperatures. The company developed a new process route where the heated blank is first pre-cooled to around 500°C (930°F) and then formed and quenched – solving any LME concerns. The grade’s mechanical properties are nearly identical to 22MnB5 after quenching.K-21 Thus, it may be called PHS1500, but to differentiate the material, they are typically named CR1500T-MB-PS (PS stands for Pre-cooled Stamping).V-9
Expanding Options: Composite Steels
In 2016, two different composite steels were developed for hot stamping. These are 3-layers, hot rolled cladded grades with PQS on the outer skin and PHS1500 in the core. These were 1200 and 1400 MPa tensile strength level grades, with significantly improved bendability. There is only a commercial name for this material. To avoid using those names, the grades may be referred to as PHS1200 Sandwich and PHS1400 Sandwich.L-68
Improving Formability and Weldability
Around 2016, steel makers started developing another PHS grade which has 1000-1200 MPa tensile strength after quenching. The grade had almost similar elongation with PHS1500 (almost 5%), but higher bendability (75° vs. 50°). These grades also have lower metallurgical notch effects when spot welded. The material may be named CR1100T-MB.V-9
Multi-Step and Air-Hardening Innovations
In 2019, a Japanese steel company developed “air-hardening” 22MnSiB9-5 alloy with GA coating. After hot forming and quenching, the material had mechanical properties almost equivalent to 22MnB5. Thus, this material can also be named as PHS1500. Since the material is air-hardenable, meaning that it hardens even at very low cooling rates, it can be hot formed in a multi-station servo-mechanical-transfer press [16]. The technique is then named as “multi-step hot forming”, with the grade referred to as CR1500T-MB-MS (the last MS stands for Multi-Step).V-9
Ultra High Strength and the VDA Naming System
Since 2020, steel companies rolled out 1900 or 2000 MPa (typical) tensile strength materials. These grades are now commonly referred to as CR1900T-MB. These grades are already available uncoated, AlSi coated or GA coated.V-9
In 2021, VDA published a new standard (239-500), which standardizes the naming, chemistry and mechanical properties of PHS and PQS grades. All the grades shown in Figure 2 (excluding the sandwich) are named based on this VDA standard.V-9
Figure 2: Stress-strain curves of commercially available PHS and PQS grades after quenching. (re-created after: B-18,Y-12, R-14).
The UniSteel Concept: One Alloy, Many Properties
In late 2021, researchers from China came up with a concept of using one chemistry (a modified 22MnB5) combined with different thermal processes to tailor the production of differing mechanical properties. Thus, it became possible to make a whole car from the same alloy, named as “UniSteel”.The different properties and their use areas are shown in Figure 3. The research was published in Science magazine.L-68
Figure 3: UniSteel concept: (a) material usage in a car body, (2) mechanical properties after heat treatments (re-created after R-14)
The Future: BQP and SIBORA Development
In 2025, a German consortium developed a new grade 37SiB6 and a new process route called Bainitizing, Quenching and Partitioning (BQP). Similar to the Chinese UniSteel concept, the new SIBORA (Silicon Boron with Retained Austenite) material can have various strength and elongation levels. Both the process and resulting mechanical properties are given in Figure 4. Different strength levels can be achieved by changing the bainitizing temperature between 360 and 460°C (680 and 860°F).O-15
Figure 4: (a) the BQP process (shown here is 360°C bainitizing temperature), (b) the mechanical properties after PHS or BQP processes (re-created after O-15).
We encourage you to visit this steel grades page to learn more about these grades available for Press Hardening, and head to this PHS and PQS Overview page for our PHS Primer. Thank you to Eren Billur for providing this information.
Thanks go to Eren Billur, Ph.D. for his contribution of this article to the AHSS Insights blog. Eren Billur is the Technical Manager of Billur Makine and Billur Metal Form, based in Ankara, Turkey, specializing in advanced sheet metal forming technologies. He holds a Ph.D. in Mechanical Engineering from The Ohio State University and has extensive experience in material characterization, sheet metal forming processes, and finite element simulations. Eren has contributed significantly to the understanding and application of hot stamping and advanced high-strength steels (AHSS) in the automotive industry. He is a regular columnist for MetalForming Magazine’s “Cutting Edge” column and has authored numerous scientific papers and book chapters, including contributions to the WorldAutoSteel AHSS Applications Guidelines. Passionate about advancing manufacturing knowledge, Eren provides engineering consulting, training, and simulation services worldwide, helping manufacturers optimize forming processes and successfully implement new-generation AHSS materials.
Liquid Metal Embrittlement (LME) during Resistance Spot Welding (RSW) can cause cracks when welding advanced high strength steels. Recent advances in steel metallurgy, resistance spot welding processing and accompanying simulation tools have substantially improved the way that LME can be handled in industrial practice. This article gives a brief overview of easy measures to implement when LME might potentially occur during production.
Introduction
During resistance spot welding of zinc-coated advanced high strength steels (AHSS) liquid metal embrittlement -related cracking may be observed. Since LME is often associated with a reduction of steel’s mechanical properties, it is desired to control its occurrence during production. An exemplary LME crack, forced with increased weld heat and deliberate electrode misalignment, is shown below.
Figure 1: A typical LME crack created under laboratory conditions by deliberately increasing the welding time and introducing 5° electrode tilt
Over the past several years, LME has been a a focus in welding research. It is now well-understood to the degree that it can be predicted and avoided with easy measures. Below is an overview of four key steps to address the potential of LME during automotive production.
Obtain the latest steel grades from your steel supplier
Over the past decade, steel producers have released AHSS with improved chemical compositions, helping to significantly reduce the occurrence of LME iIt is beneficial to talk with steel suppliers and ask about their latest AHSS grades, as these are likely far less sensitive to LME than previously tested grades. A recent study commissioned by WorldAutoSteel demonstrated that all five chosen material stack-ups from current production data did not show any LME even under exacerbated conditions. Only by choosing an especially difficult material stack-up could LME be forced to appear at all to conduct the study.
Read up on the current state of research for LME
WorldAutoSteel has published two studies on liquid metal embrittlement: One focused on lab conditions and the second on real-life stamped components. These studies provide an overview of all aspects of LME and how to manage and avoid LME issues.
Establish in-house testing protocols to gauge the sensitivity of your material stack-ups
To investigate LME in-house, it’s critical to establish a testing protocol that forces the cracks to appear and allows for comparison of different steels, stack-ups and welding parameters. as there There is currently no industry-wide agreed-upon testing standard.
Still, there is a good selection of well-documented procedures to choose from. The easiest procedure is to increase the welding time until cracks start to appear – keep in mind that you need to remove the zinc coating before you can observe any cracks on the surface.
Other methods are based on so-called “Gleeble testing” or on deliberately introducing imperfections like tilted electrodes or large gaps into the welds. As you establish a testing procedure in your lab, you can use it to evaluate LME occurrence in the stack-ups that you want to implement into body-in-whites.
Think about implementing LME mitigation strategies in your most difficult welds
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)
These measures can be implemented in the planning stage and in an ongoing production environment to increase the LME-free process windows.
In Conclusion
While Liquid Metal Embrittlement may present a challenge when welding AHSS, it’s no longer an unpredictable threat. Thanks to advancements in steel development, welding techniques, and testing methods, manufacturers have the tools they need to reliably mitigate LME during production.
Staying informed, working closely with steel suppliers, implementing smart testing protocols, and applying targeted welding strategies can help automakers maintain both strength and quality in AHSS joints. With this proactive approach, LME doesn’t have to stand in the way of innovation in automotive manufacturing.
Tensile testing is one of the most basic formability characterization methods available. Results from tensile testing are a key input into metal forming simulations, but if the right information isn’t included, the simulation will not accurately reflect material behavior.
Metal forming simulation is particularly beneficial on the value-added parts made from advanced high strength steels, since accurate simulations allow for optimal processing with minimal recuts … at least when the right information is used as inputs.
Tensile Testing
During tensile testing, a standard sample shape called a dogbone is pulled in tension. Load and displacement are recorded, and which are then converted to a stress-strain curve. Strength is defined as load divided by cross-sectional area. Exactly when the cross-sectional area is measured during the test influences the results.
Before starting the pull, it’s easiest to measure the width and thickness of the test sample.
Engineering Stress-Strain Curve
At any load, the engineering stress is the load divided by this initial cross-sectional area. Engineering stress reaches a maximum at the Tensile Strength, which occurs at an engineering strain equal to Uniform Elongation. After that point, engineering stress decreases with increasing strain, progressing until the sample fractures.
However, metals get stronger with deformation through a process known as strain hardening or work hardening. As a tensile test progresses, additional load must be applied to achieve further deformation, even after the “ultimate” tensile strength is reached. Understanding true stress and true strain helps to address the need for additional load after the peak strength is reached.
During the tensile test, the width and thickness shrink as the length of the test sample increases. Although these dimensional changes are not considered when determining the engineering stress, they are of primary importance when determining true stress. At any load, the true stress is the load divided by the cross-sectional area at that instant.
True Stress-Strain Curve
The true stress – true strain curve gives an accurate view of the stress-strain relationship, one where the stress is not dropping after exceeding the tensile strength stress level.
True stress is determined by dividing the tensile load by the instantaneous area.
True stress-strain curves obtained from tensile bars are valid only through uniform elongation due to the effects of necking and the associated strain state on the calculations. Inaccuracies are introduced if the true stress-true strain curve is extrapolated beyond uniform strain, and as such a different test is needed. Biaxial bulge testing has been used to determine stress-strain curves beyond uniform elongation. Optical measuring systems based on the principles of Digital Image Correlation (DIC) are used to measure strains. The method by which this test is performed is covered in ISO 16808.
Stress-strain curves and associated parameters historically were based on engineering units, since starting dimensions are easily measured and incorporated into the calculations. These are the values you see on certified metal properties, also called metal cert sheets that you get with your steel shipments.
True stress and true strain provide a much better representation of how the material behaves as it is being deformed, which explains its use in computer forming and crash simulations.
It’s much more challenging to get accurate dimensional measurements once the test has started unless there are multiple loops of the operator stopping the test, remeasuring, then restarting the pull. This is not a practical approach.
Fortunately, there are equations that relate engineering units to true units. Conventional stress-strain curves generated in engineering units can be converted to true units for inclusion in simulation software packages.
As the industry moves to more value-added stampings, metal forming simulation is done on nearly every part. The value-added nature of parts made from advanced high strength steels requires best practices be used throughout – otherwise the results from simulation drift further away from matching reality, leading to longer development times and costly recuts.
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 HighStrength 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:
PHS steels
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.
