Happy New Year! We are pleased to provide this contribution by Dr. Daniel Schaeffler, President, Engineering Quality Solutions, Inc. and Technical Editor, AHSS Applications Guidelines.
Forty years ago, the metal forming community needed to figure out how to stamp a new exotic family of steels making inroads into automotive body construction. These grades, called High Strength Low Alloy steels, were much stronger than the commonplace mild steels, and were more formable than the high-strength options available at that time. Initially, only a few steelmakers were able to offer these new grades, but over time more companies added the equipment and know-how necessary to support their customers with these products. Automakers and their supply chain stampers needed to adapt as an increasing number of parts transitioned to HSLA steels.
Fast-forward a few decades, and metal formers are facing similar challenges. Successful forming and joining of Advanced High-Strength Steels is made easier with processes that are tuned to work with the characteristics associated with these alloys. One such technique to improve formability is to employ Active Binder Force Control.
In conventional stamping, a draw ring applies pressure around the binder in order to control the sheet metal flow into the cavity. The ring may be referred to as a binder plate, draw pad, pressure pad, or blank holder. Creating the restraining force typically is done with urethane springs, coil springs, gas springs (like air or nitrogen), or press cushion systems actuated by gas or hydraulic cylinders.
Where the traditional approach applies binder pressure uniformly throughout the press stroke, modern stamping presses can be equipped with cushions having multipoint-control systems (see Figure 1 example). The associated pressure profile can be adjusted around the panel and throughout the stroke to optimize metal flow, prevent splits and wrinkles, and minimize thinning.
Figure 1. An Example of Multi-Point Press Forming Method1
Incorporating Active Binder Control capabilities has several benefits for the press shop, panel quality, and product design, including:
- A segmented blankholder combined with individually programmable hydraulic cylinders, sometimes called a flexible binder, allows for precise control of one segment independent of the others.
- Pulsating blank holder force has been shown to reduce press tonnage requirements and increase metal flow, with the frequency and amplitude being key variables that must be adjusted based on the grade and thickness of interest.
- Pre-acceleration of the cushion reduces shock loading, which minimizes the press-damaging snap-through loads associated with reverse tonnage.
The merits of a variable blank holder force on AHSS springback were documented in a 2004 conference paper2. With the traditional constant binder force approach, springback in the form of side-wall curl was seen in parts made from either a DP590 grade or a mild steel grade used as a control. Increasing the constant binder force helped to reduce springback in the mild steel part.
In Figure 2, CBF reflects tests conducted with constant binder force. VBF-LH and VBF-MLH reflect variable binder force tests conducted with a low-high force profile sequence and a medium-low-high force profile, respectively.
Figure 2: Variable Binder Force Reduces Springback2
By employing a variable binder force, springback of both the mild steel and the DP 590 material was substantially reduced. Employing either variable binder force approach reduced the thinning from forming the DP 590 material, resulting in a more uniform strain distribution across the entire channel profile (Figure 3).
Figure 3: Uniform Strain Distribution Achieved with Variable Binder Force2
More recently, a presentation from 2018 showed CP 800 panel quality improvements associated with variable blank holder force capabilities3. Panel results from a constant binder force of 300 kN and 400 kN are shown in Figures 4 and 5, respectively. Both exhibit severe wrinkling in the flange. Applying 500 kN binder force was not feasible due to exceeding the press tonnage curve limits throughout the stroke.
Figure 4: CP800 Panel Formed with Constant Binder Force of 300 kN, and Associated Close-Up of Flange3
Figure 5: CP800 Panel Formed with Constant Binder Force of 400 kN, and Associated Close-Up of Flange3
Figure 6 shows the panel produced with a variable binder force. The chosen profile fit within the press tonnage requirements and minimized wrinkles.
Figure 6: CP800 Panel Formed with Variable Binder Force Ramping from 300 kN to 600 kN, and Associated Close-Up of Flange3
Active drawbead control is an offshoot of these techniques, allowing for the magnitude and timing of drawbead engagement to be optimized for the requirements of each part. A description of using stake beads to minimize springback is available in a previous AHSS Insights blog here – active drawbead control is one approach to actuate beads.
The initial laboratory studies relating to active binder force control go back nearly 20 years ago. In the coming years, more information will enter the public domain on how metal formers are using these concepts in production. When you look to purchase a servo press, be sure to ask your press manufacturer about programmable cushions.
Daniel J. Schaeffler, Ph.D., President, Engineering Quality Solutions, Inc.;
Chief Content Officer, 4M Partners, LLC
Danny Schaeffler is the Metallurgy and Forming Technical Editor of the next release of the AHSS Applications Guidelines available from WorldAutoSteel. Danny is co-founder of 4M Partners, LLC and founder and President of Engineering Quality Solutions (EQS). He has written for Stamping Journal and The Fabricator. Danny writes the monthly “Science of Forming” column for Metalforming Magazine. He is passionate about training the metal stamping community on sheet formability and how to make the best use out of their chosen sheet metal grades.
1 Flexible Tooling For Manufacturing 3d Panels Using Multi-Point Forming Methodology – Scientific Figure on ResearchGate.
2 Variable Binder Force for Springback Management, M. Milititsky (University of Ghent), M. Garnett, C. Du, J. Wu, L. Zhang, P.J. Belanger, J.M. Prencipe (DaimlerChrysler Corporation), and E.D. Bishop (Engineering Quality Solutions, Inc.). International Conference on Advanced High-Strength Sheet Steels for Automotive Applications Proceedings, June 6-9, 2004, Association for Iron & Steel Technology
3 Improving the Drawing Process of AHSS by Using Servo Press Technologies, David Diaz-Infante (Ohio State University), 2018 Great Designs in Steel Seminar, AISI
Article contributed by Harry Singh, Senior Product Applications Engineer, United States Steel Corporation.
Several recent studies are forecasting that; “Within the next 10 to 15 years, urban transportation will be dominated by Electric and Automated vehicles”.1 Meaning most of us will be driving Battery Electric Vehicles (BEVs) in the not-distant future. In 2011, just eight years ago, there were only three BEVs on the market with 70 to 80 miles range on a single charge. These were the first generation BEVs. Since then, the number of EVs on the market has increased, with significant improvements in range (now approaching 300 miles). BEV 2020 vehicles cover all current segments, from small cars to SUV’s and trucks (Figure 1). These vehicles will be available from most OEMs as well as several new start-up companies. The construction material for body structures of these vehicles is predominantly steel, while some of the premium vehicles ($60,000 to $100,000) are aluminium. And the prevailing OEM message seems to be “anything TESLA can do, we can do better”.
So how will this change the vehicle body structure design, choice of construction material, its implications for manufacturing and assembly, and ultimately, the impact on automotive steel?
Figure 1: Electric Vehicle Boom – Models by Style and Range Available Through 20201 CHART SUMMARY: a) Covers all current segments, b) Structures predominantly Steel, c)Some premium vehicles highlight Aluminium, d)Products from most OEMs as well as several new start-up companies.
The driver for this electrification boom is increasing affordability. The upfront cost of BEVs will become competitive on an unsubsidized basis starting in 20242. By 2030 in the U.S., almost all light duty vehicle segments will reach cost parity as battery prices continue to fall3. Forecasters, such as McKinsey, Morgan Stanley and Bloomberg, predict that about half of all new vehicle production will be electric somewhere between 2035 and 2040. However, Tesla’s CEO Elon Musk’s prediction is much more aggressive. He expects more than half of new vehicles in the U.S. will be electric within the next 10 years, roughly 10 to 15 years ahead of most other predictions.
The Main Drivers of BEV Cost Reduction
- Lithium-ion battery prices have fallen 75% since 2013, hitting $176/kWh in 2018 (Figure 2). Industry-wide prices fell due to the adoption of new cell designs and the availability of higher energy-density cathodes. Prices are expected to drop further in coming years to below $100 per kWh. Besides the reduction in cost, packaging efficiency and the cell energy density also is improving.
- Package space required by other BEV powertrain systems also is being optimized, e.g., motor, transmission, differential and power electronics. This is yielding significant weight and cost reductions, which are then directly reinvested into lower-cost structural materials, such as Advanced High-Strength Steels (AHSS) versus higher cost Aluminium, to keep the overall price of the vehicle low.
Figure 2: BEV Price Parity with Gas-powered Cars by 2024 – Main Drivers4
BEV to ICE Vehicle Structural Differences and Advantages for Steel
Figure 3: BEV to ICE Vehicle Structural Differences5
BEV packaging differences compared with ICE Vehicles are shown in Figure 3, and include:
- Narrower and compact transverse electric powertrains, leading to shorter front end, with increased occupant space for same size vehicle and larger/efficient front crash rails.
- Lack of an exhaust system eliminates the need for the tunnel, allowing straighter/ efficient cross-members.
- No fuel tank/filler leads to more efficient rear rail load path.
- High voltage electric powertrain and large (300 litres, 500 kg) under-floor battery pack crash protection requirements result in higher safety requirements for BEV front and side structures.
- Safety. The BEV body structure load path requirements are ideal for AHSS application. The floor cross members, without the presence of the tunnel, are straight and can use very high-strength martensitic roll formed sections. Cross members can be stamped from 3rd Generation Steels offering Giga-Pascal strength and over 20% elongation. For frontal crash load management and to minimize passenger/battery compartment intrusions for increased safety, 3rd Generation steels offer the most mass/cost efficient solution. The very high strengths offered by AHSS and UHSS for the safety-critical structural members such as the rocker, rails, cross members and pillars, greatly enhance the required protection of the BEV powertrain and high energy/voltage battery systems. The battery enclosure construction greatly benefits from AHSS usage, providing protection from road-debris impacts from below the vehicle, along with fire protection into the passenger compartment. Advanced steels also enable reduced section sizes for the occupant compartment, required for improved panoramic visibility, without compromising occupant safety and comfort.
- Cost. For widespread adoption of BEVs to occur, the overall cost of the vehicle must be affordable, and its range must be above the ‘range anxiety limit’ of most drivers. Various surveys indicate this range to vary greatly from 75 miles to over 400 miles. Using steel for the vehicle structure leads to the lowest cost BEV, just as with ICE-based vehicles. The vehicle range can be increased through lightweighting and/or by increasing the size of the battery; a cost comparison of these two options is shown in Figure 4. With battery cost reduction approaching $100 per kWh, lightweighting is cost effective at approximately US$2.00 per kg saved. Lightweighting is still very important and the latest steel grades, in particular 3rd Generation steels, offer the most cost-effective lightweighting option. In comparison, if we consider lightweighting with aluminium, the cost is typically in the order of US$6.00 per kg saved. This could be cost effective if the battery cost is over $250 per kWh, which was the case a decade ago. We can see the evidence of this in OEM decisions at that time. For example, the 2011 Nissan Leaf BEV closures were aluminium; but the latest 2019 Nissan leaf BEV closures are steel.
Figure 4: BEV Range Increase – Lightweighting Cost versus Battery Cost 2020 – 2022
Battery Electric Vehicles – Boom or Bust for AHSS?
For the increased safety required for BEVs to protect the high voltage systems, the structural load paths are ideally suited for the Giga Pascal level strengths offered by AHSS and UHSS. The Battery Enclosure structure offer an additional 85 kg per vehicle opportunity, an increase of approximately 10% sheet metal over ICE vehicles. Also, using advanced steels the BEV structure can take full advantage of well-established body shop practices for manufacturing and assembly, such as stamping, roll forming and spot welding. With future increased focus on BEV affordability, safety and sustainability, steel offers the best solutions and flexibility to address these key challenges.
Harry Singh is Senior Product Application Engineer at United States Steel Corporation. He is responsible for developing technical solutions for automotive applications utilizing the U.S. Steel Advanced High-Strength Steel portfolio.
Prior to joining U.S. Steel, Harry had spent 10 years at EDAG, Inc. as Director of Lightweighting, working on vehicle design and engineering programs. Major achievements at EDAG was management the FutureSteelVehicle program for WorldAutoSteel, with full engineering, reporting and commercial responsibilities. Harry was Principal Investigator of several vehicle lightweighting studies for National Highway Traffic Safety Administration (NHTSA) to support the mid-term review and 2025 CAFE requirements.
1 Source: Bloomberg NEF
2 Report: Choosing the Electric Avenue – Unlocking Savings, Emissions Reductions, and Community Benefits of Electric Vehicles, John Farrell, 7 JUN 2017
3 Tyson, Madeline, Charlie Bloch. Breakthrough Batteries: Powering the Era of Clean Electrification. Rocky Mountain Institute, 2019.
4 2019 Sustainable Energy in America – Factbook, Bloomberg Finance L.P. 2019 and The Business Council for Sustainable Energy (Page 128)
5 Source of Images and inspiration: Don E. Malen: Mass Benchmarking Analysis of Electric Vehicles, A2Mac1, WorldAutoSteel
We once again welcome Max Biegler, Research Associate, Fraunhofer IPK, in this follow up article on Modelling Resistance Spot Welding.
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). In order 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 (FutureSteelVehicle ©WorldAutoSteel)
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 (©LWF)
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 my previous blog post. 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
Fraunhofer Institute for Production Systems and Design Technology IPKMax Biegler (M.Sc.) finished his studies in mechanical engineering at Technical University of Munich in 2015. He is currently working as a research associate at Fraunhofer IPK in Berlin with focus on numerical modelling of welding processes.
Take a moment and comment below on what you hope to see in this next update!
Nearly 17 years ago, WorldAutoSteel began the first compilation of the Advanced High-Strength Steels (AHSS) Application Guidelines, a volume of global best practices for forming and joining what was then a relatively new family of steels. We’ve continued to update the data and information housed in the Guidelines, recognizing the fast-paced development of AHSS products. We’re excited to inform you that we’re ramping up for another update, to be fully completed by 2021, and we’re planning for incremental content releases during the update process.
Vehicle applications of Advanced High-Strength Steels (AHSS) will continue to proliferate at automotive OEM and supplier plants as they address the challenge to achieve lighter, safer and affordable structures. And this effort now includes the additional challenges of battery protection and range improvements. The steel industry continues to meet those challenges by re-inventing steel products to be stronger, and therefore lighter, with increased forming characteristics. Consequently, we are now seeing 3rd Gen Steels becoming commercially available and automakers achieving improved safety, lightweighting and environmental performance with them.
This requires everyone in the business of making automotive structures to master the fundamentals in record time and quickly get up to speed on specific application knowledge to keep pace with process improvements that are being realized out in the field.
And that’s exactly what the AHSS Application Guidelines is meant to capture, to assist automotive designers, engineers and press shop personnel in applying these steels to vehicle manufacturing.
Our Team and Process
Forming experts Dr. Daniel Schaeffler, Engineering Quality Solutions, Inc., and George Coates, The Phoenix Group, with well-known joining expert, Menachem Kimchi, M.Sc Welding Engineering, Ohio State University, will spend the next year and a half interacting with our membership around the world to tap their expertise for new and best practices. Our membership includes steel industry subject matter experts who collaborate regularly with global manufacturers.
Additionally, we welcome best practices and lessons learned from suppliers and academicians who wish to contribute. These interactions lead to significant feedback and information, which is then captured and documented in the AHSS Application Guidelines.
In the meantime, the AHSS Application Guidelines V6.0 will remain available free for download at worldautosteel.org until the new version is complete.
A New Format Coming with this Update!
Up until now, the Guidelines have been in a PDF volume, which if you’ve downloaded it, you know what a large and daunting volume it is. This next update will bring the Guidelines into an online database that can be browsed and searched. This new online format will help you find the information for which you specifically are looking, and therefore, make it more easily available wherever in the world you may be. The best part of this online format is that we do not have to wait until the entire update is complete to begin making content available, and therefore, we’ll be able to release it in phases over the next year and a half.
The Guidelines database will be housed here at ahssinsights.org, which will get a new design to accommodate it. It will automatically integrate current and future Blog articles in searches so that, through our many expert contributors, the Guidelines will be continuously updated to reflect fresh content. Regardless of the format, the AHSS Application Guidelines will always be available free of charge.
We’ll keep you posted here on the blog and via our email blasts as information is released. If you are not subscribed to the AHSS Insights Blog, please take a moment to Subscribe in the upper right menu. You also may wish to subscribe to WorldAutoSteel’s enews to make sure you don’t miss any important information. Our firm commitment: we never pepper your inbox with unnecessary information.
We’re meeting in just a few days to prepare for the update process, and my job is to keep the whole project rolling on a smooth timetable. This will be the fourth update of AHSS Application Guidelines in which I’ve been privileged to participate. My first two experiences included many meetings with Dr. Stuart Keeler, the Guidelines technical editor for Versions 1.0 – 4.0, where he described complex metallurgy in such a way that this non-engineer could understand. Dr. Keeler’s passing last May leaves me, and others at WorldAutoSteel, with those fond memories, and the knowledge that he is smiling at us now as we continue in his footsteps.
Take a moment and comment below on what you hope to see in this next update!
|Kathleen (people call her Kate) started with the steel industry in 1998 when the UltraLight Steel Auto Body demonstration was making its way through technical transfers at every major automaker around the world. She has over 25 years technical writing experience and is deeply involved in the day-to-day strategy and project work at WorldAutoSteel, as well as being co-editor of the AHSS Insights blog, where she draws from a wealth of global experts to address topics and questions. Kate holds a BA in Public Relations/Communications, Wayne State University, with continued education under the tutelage of some of the finest experts in the global automotive and steel industries.
Authored by Max Biegler, Research Assistant RSW, Fraunhofer IPK, Berlin
Modelling resistance spot welding can help to understand the process and drive innovation by asking the right questions and giving new viewpoints outside of limited experimental trials. The models can calculate industrial-scale automotive assemblies and allow visualization of the highly dynamic interplay between mechanical forces, electrical currents and thermal flow during welding. Applications of such models allow efficient weldability tests necessary for new material-thickness combinations, thus well-suited for applications involving Advanced High -Strength Steels (AHSS).
Virtual resistance spot weld tests can narrow down the parameter space and reduce the amount of experiments, material consumed as well as personnel- and machine- time. They can also highlight necessary process modifications, for example the greater electrode force required by AHSS, or the impact of hold times and nugget geometry. Other applications are the evaluation of whole-part distortion to ensure good part-clearance and the investigation of stress, strain and temperature as they occur during welding. This more research-focused application is useful to study phenomena arising around the weld such as the formation of unwanted phases or cracks.
Modern Finite-Element resistance spot welding models account for electric heating, mechanical forces and heat flow into the surrounding part and the electrodes. The video shows the simulated temperature in a cross-section for two 1.5 mm DP1000 sheets:
RSW Nugget Formation from worldautosteel on Vimeo.
First, the electrodes close and then heat starts to form due to the electric current flow and agglomerates over time. The dark-red area around the sheet-sheet interface represents the molten zone, where the nugget forms after cooling.While the simulated temperature field looks plausible at first glance, the question is how to make sure that the model calculates the physically correct results. To ensure that the simulation is reliable, the user needs to understand how it works and needs to validate the simulation results against experimental tests. In this text, we will discuss which inputs and tests are needed for a basic resistance spot welding model.
At the base of the simulation stands an electro-thermomechanical resistance spot welding model. Today, there are several Finite Element software producers offering pre-made models that facilitate the input and interpretation of the data. First tests in a new software should be conducted with as many known variables as possible, i.e., a commonly used material, a weld with a lot of experimental data available etc.
As first input, a reliable material data set is required for all involved sheets. The data set must include thermal conductivity and capacity, mechanical properties like Young’s modulus, tensile strength, plastic flow behavior and the thermal expansion coefficient, as well as the electrical conductivity. As the material properties change drastically with temperature, temperature dependent data is necessary at least until 800°C. For more commonly used steels, high quality data sets are usually available in the literature or in software databases. For special materials, values for a different material of the same class can be scaled to the respective strength levels. In any case, a few tests should be conducted to make sure that the given material matches the data set. The next Figure shows an exemplary material data set for a DP1000. Most of the values were measured for a DP600 and scaled, but the changes for the thermal and electrical properties within a material class are usually small.
Figure 1: Material Data set for a DP10001
Next, meaningful boundary conditions must be chosen and validated against experiments. These include both the electrode cooling and the electrical contact resistance. To set up the thermal flow into the electrode, temperature measurements on the surface are common. In the following picture, a measurement with thermocouples during welding and the corresponding result is shown. By adjusting the thermal boundary in the model, the simulated temperatures are adjusted until a good match between simulation and experiment is visible. This calibration needs to be conducted only once when the model is established because the thermal boundary remains relatively constant for different materials and coatings.
Figure 2: Temperature measurement with thermocouples during welding and the results. The simulated temperature development is compared to the experimental curve and can be adjusted via the boundary conditions.2
The second boundary condition is the electrical contact resistance and it is strongly dependent on the coating, the surface quality and the electrode force. It needs to be determined experimentally for every new coating and for as many material thickness combinations as possible. In the measuring protocol, a reference test eliminates the bulk material resistance and allows for the determination of the contact resistances using a µOhm-capable digital multimeter.
Finally, a metallographic cross-section shows whether the nugget size and -shape matches the experiment. The graphic shows a comparison between an actual and simulated cross section with a very small deviation of 0.5 mm in the diameter. As with the temperature measurements, a small deviation is not cause for concern. The experimental measurements also exhibit scatter, and there are a couple of simplifications in the model that will reduce the accuracy but still allow for fast calculation and good evaluation of trends.
Figure 3: Comparison of experimental and virtual cross-sections.2
After validation, consider conducting weldability investigations with the model. Try creating virtual force / current maps and the resulting nugget diameter to generate first guesses for experimental trials. We can also gain a feeling how the quality of each weld is affected by changes in coatings or by heated electrodes when we vary the boundary conditions for contact resistance and electrode cooling. The investigation of large spot-welded assemblies is possible for part fit-up and secondary effects such as shunting. Finally, the in-depth data on temperature flow and mechanical stresses is available for research-oriented investigations, cracking and joint strength impacts.
Fraunhofer Institute for Production Systems and Design Technology IPKMax Biegler (M.Sc.) finished his studies in mechanical engineering at Technical University of Munich in 2015. He is currently working as a research associate at Fraunhofer IPK in Berlin with focus on numerical modelling of welding processes.
1 C. Schwenk, FE-Simulation des Schweißverzugs laserstrahlgeschweißter dünner Bleche – Sensitivitätsanalyse durch Variation der Werkstoffkennwerte. Berlin: BAM-Dissertationsreihe, 2007.
2 J. Frei, M. Biegler, M. Rethmeier, C. Böhne, and G. Meschut, “Investigation of liquid metal embrittlement of dual phase steel joints by electro-thermomechanical spot-welding simulation,” Science and Technology of Welding and Joining, vol. 90, pp. 1–10, 2019.
Compensate or Countermeasure? Taking Sidewall Curling Seriously
In this blog post Akshay Wankhede, Application Engineer at AutoForm, describes the causes for the springback phenomenon known as the “sidewall curl”, how its occurrence on stamped panels can be identified early through simulation of full production process, and how it can be counter-measured well ahead of die tryout. Read this post to understand the challenges of the sidewall curl effect for Advanced High-Strength Steels (AHSS).
Revisiting Sidewall Curl
The issue of the sidewall curl is a common phenomenon in sheet metal stamping which is observed on the side wall of a panel, typically following the drawing operation. This distortion results in dimensional variation, creating challenges in assembly operations, affecting productivity and subsequent part quality. Naturally, it is critical that you identify the sidewall curl ahead of time to eliminate acute problems down the road. The easiest way to recognize when a sidewall curl is occurring would be to look for any progressive angle change on the sidewall or flange area as shown in Figure 1 below.
Figure 1: Obvious Occurrence Of A Side Wall Curl – Panel B
Sidewall curl issues arise as soon as the panel comes out of the tool, where it is “free” to springback. It occurs in those areas where the strain distribution is not uniform.
To better understand the sidewall curl issue, it is important to dig deeper into the basics of the cause-effect relationship of all springback phenomenon.
In general, the elastic return effect on a metal is due to the residual stress that remains after the forming forces are removed (tool opening) or after the draw panel has been trimmed.
In the sheet metal stamping process there are three main types of stresses that can be applied to a generic section of the panel: Membrane Stress, Pure Bending Stress, and Superposed Bending Stress.
1. Membrane Stress: occurs when a specimen is uniformly stretched along its thickness beyond the elastic limit (yield point) and then allowed to relax; stresses and elastic strains are both fully released. The residual elastic strains left in the material cause the specimen to springback.
Consider the sketch in Figure 2 where the stress applied to the specimen by Force F generates a uniform stress along the thickness s0. When the force is removed the specimen springs back by the elastic strain el. Quantitatively, the elastic return is directly proportional to the applied stress and inversely proportional to the Young’s modulus E.
Figure 2: Typical Membrane Stress Applied During Tensile Test
2. Pure Bending Stress: commonly occurs when the sheet metal is bent over a radius (the radius of the wipe post during flanging for instance). In this case there is a large difference of stress and in strains between the outer and the inner layer of the sheet.
Let’s assume we have a flange-up operation as shown in Figure 3. If we take a look at the stress and relative strain of the layers along the thickness of the sheet, we can see that there is a neutral portion in the cross-section of the specimen that remains under “pure” elastic strain while the external and internal portions are in the plastic domain of the stress-strain curve; the inner layers are compressed (negative strain) while the outer layers have positive strain.
Figure 3: Elastic-Plastic Portion In Case Of Pure Bending Load
The layers in the elastic portion try to go back to their original length but they are unable to completely recover original shape due to the plastically deformed portion, so the final position of the sheet is the one generated by the equilibrium of these two moments. The elastic relief moment is what causes the flange to springback by a certain angle or distance, as noted in Figure 3.
3. Superposed Bending Tension: occurs when the material is stretched and bent over a radius at the same time. Because of the stretching, the strains on the inner and the outer layers are not very different and if stretched enough, they have the same direction (all strained). Therefore, when the elastic stresses are released, the residual stresses are reduced and are more uniform. In such cases, springback is more stable and produces a lower dimensional deviation (See Figure 4).
Figure 4: Superposed Bending Tension
These conditions show that springback is caused from the strain deviation between the layers of the panel. The arising sidewall curl is a consequence of strain variation not only through the material thickness, but simultaneously along the length of the sidewall. A sidewall curl issue can also be treated as a type of springback deformation resulting from successive bending and unbending when the sheet metal is drawn over a die-radius or through a drawbead. Materials such AHSS (because of their high tensile strength) and aluminum alloys (because of their low Young’s Modulus) usually show more springback and sidewall curl problems.
Why Take Sidewall Curl Issue So Seriously?
Springback is challenging to avoid, but there are techniques that can minimize its negative effects. Other contributions in this AHSS Insights blog (see references at the end of the blog) describe using darts and beads to lock in the residual stresses and produce dimensionally accurate panels. In addition, by incorporating effective tool morphing strategies, springback can be compensated to purposefully produce dimensionally accurate panels within close tolerances. Unfortunately, sidewall curl is very difficult to fix with morphing compensation, which makes achieving a dimensionally accurate panel almost impossible to achieve. Therefore, it becomes extremely important to eliminate the sidewall curl before morphing the tools to compensate for springback during engineering prior to finishing the tools.
It is possible to examine the springback and sidewall curl risk by using simulation software tools such as AutoForm. Springback is a consequence of differences between the stresses in the layers of a sheet metal, with greater differences leading to increased sidewall curl. Evaluate bending moments in the proper orientation: If the critical radii are perpendicular to the rolling direction, then you must consider transverse properties.
Design and process changes that address springback and sidewall curl can be proven out during simulation, making it a cost-effective and efficient approach which should be incorporated into the part development process.
The Influence of Different Material and Sheet Thicknesses
Different grades do not behave the same way under all forming conditions. Higher strength of the incoming steel means higher forming tonnage, which leads to greater differences between the top and the bottom layers of the panel. This in turn leads to greater springback and in some cases greater sidewall curl. Increased thickness typically reduces both springback and curl.
The same crossmember panel shown in the pictures above has been used to investigate the effect of sidewall curl for different materials and different thicknesses. Five different materials of 1.6 mm thickness ranging from low carbon steel to high strength steel including aluminum have been used to study this effect, shown in Figure 5.
DP600 was also tested for a thicker gauge of 2.0 mm and 2.5 mm to study the effect of thickness on sidewall curl. A 5XXX series aluminum alloy is included showing the increased springback associated with the significantly lower Young’s Modulus (Elastic Modulus) characteristic of automotive aluminum alloys.
Figure 5: Springback Comparison For Different Material
Springback cannot always be completely compensated; issues such as the side wall curl, oil canning, large springback magnitudes, and the lack of robustness (repeatability) need to be addressed through improvements to process and/or product. It is important to become aware as early as possible of the potential for any such issues. The earlier this awareness is achieved, the higher the chances to look for appropriate countermeasures.
Waiting to address springback and sidewall curl in tryout is a poor strategy. A thorough study of the springback phenomenon and associated conditions by simulating the full process allows for greater understanding of potential springback related challenges. The sooner potential issues are discovered, the greater likelihood that appropriate countermeasures can be successfully deployed.
To learn more about springback management, you may want to have a look at the following links:
AHSS Insights Blogs: AHSS and Springback and Managing Springback
AutoForm Blog: AutoForm’s Springback Compensation Best Practices
AutoForm Engineering USA
Mr. Wankhede is actively engaged with AutoForm clients, utilizing AutoForm simulation software to develop stamping dies to specific requirements, supporting customer questions and resolving issues in the areas of die construction, hot forming, hydroforming and springback. Mr. Wankhede holds a Bachelor’s degree from Nagpur University in Mechanical Engineering and a Master’s Degree from the University of Missouri-Columbia in Mechanical Engineering.