Die wear is affected by, but not limited to, steel strength and surface friction, contact pressure, sliding velocity, temperature, die surface treatment and lubrication. As the material strength and hardness is increased, an increase in die wear occurs which leads to quality issues in the stamping. In addition, frequent die wear requires more frequent die maintenance or replacement, affecting turnaround times, productivity and cost.
Actions can be taken to prevent excessive wear on die materials when forming Advanced High-Strength Steels. To prevent such wear, new die materials and better coatings have been developed to maintain their hardness without compromising the toughness of the material. Hard material coatings and nitriding have also been used to improve the tribological properties of die surfaces.
Tool and Die Materials
In general, the considerably higher strength levels associated with AHSS grades exerts proportionally increased forces on the die material. AHSS might reach strength levels four to five times higher than mild steels, approaching 2000 MPa. This is based on complex metallurgy that delivers high concentrations of martensite and very high work hardening rates.
The higher forces required to form AHSS require increased attention to tool and die material specifications. Key requirements that need to be specified are the stiffness and toughness of the die substrate, as well as the surface treatments and hardness of the die coating. Life and performance of a draw die is determined by the amount of wear/galling that accumulates during forming, which then defines specific die maintenance intervals. When selecting die materials, important considerations are:
- Sheet metal being processed, characterized by strength, thickness, and surface coating.
- Die construction, machinability, radii sharpness, surface finish, and die hardness, specifically on draw beads and radii.
- Cost per part.
Thickness reduction for weight saving is one primary reason for applications of AHSS. Unfortunately, the reduced thickness of the steel increases the tendency to wrinkle. Higher blankholder loads are required to suppress these wrinkles. Any formation of wrinkles will increase the local load and accelerate the wear effects. Figure 1 shows a draw die with severe die wear due to excessive wrinkling on a DP780 part. It is not uncommon to replace these high wear areas with a more durable tool steel insert to minimize this type of excessive wear condition.
Figure 1: Draw die with significant wear due to excessive wrinkling on a DP780 part.
Tool steel inserts for forming dies must be selected according to the severity of the forming. Surface coatings are recommended for DP 350/600 and higher grades. When coatings are used, it is important that the substrate has sufficient hardness/strength to avoid plastic deformation of the tool surface, even locally. Therefore, a separate surface hardening, such as nitriding, is recommended before the coating is applied. Prior to coating application, it is important to use the die in in pre-production to provide time to adjust forming conditions and establish die performance Surface roughness must be as low as possible before coating. Ra values below 0.2 µm are recommended. Steel inserts with a TiC/TiN coating are recommended for local high-pressure conditions that cause accelerated die wear and zinc flaking.
Table 1, provided by the Auto/Steel Partnership, describes recommended construction materials for stamping dies and components, based on significant research efforts. North American Automotive Metric Standards (NAAMS) are the product of a consortium between Ford, GM, FCA and various North America automotive steel producers.
Table 1 – Recommended Materials for Specific Die Components – NAAMS Standards
Tool steels for cutting, trimming, and punching operations must be similarly selected. For these operations, tensile strength is more important than yield strength. Tool hardness between 58 and 62 HRC is recommended. Coatings may be used to reduce tool wear, but for the highest strength steels (above 1000 MPa tensile strength) coatings may fail due to local deformation of the die material substrate.
To prevent this, hardening of the substrate prior to coating is again strongly recommended. High performance tool steels, such as powder metallurgy (PM) grades, can be expensive but are justified because of their low wear rate and increased life. Ceramic tool inserts have extreme hardness for wear resistance, high heat resistance, and optimum tribological behavior, but have poor machinability and severe brittleness. High costs are offset by reduced maintenance and increased productivity. While not commonly used, the ceramic tool inserts offer a possible solution to high
interface loading and wear.
The following study is a computerized analysis of the energy required to form a cross member with a hat-profile and a bottom embossment at the end of the stroke (Figure 1).
Figure 1: Cross section of a component having a longitudinal embossment to improve stiffness locally.1
Energy requirements increase with punch displacement, as shown in Figure 2, here energy curves are shown for Mild, HSLA 250/350, and DP 350/600 steels. The three dots indicate the start of the embossment formation at a punch depth of 85 mm.
Figure 2: Computerized analysis showing the increase in energy needed to form the component with different steel grades. Forming the embossment begins at 85 mm of punch travel.1
The last increment of punch travel to 98 mm requires significantly higher energy, as shown in Figure 3. Throughout the punch travel however, the two higher strength steels appear to maintain a constant proportional increase over the Mild steel.
Figure 3: A further increase in energy is required to finish embossing.1
Setting Draw Beads
A considerable force is required to set draw beads in AHSS before part formation begins. A nitrogen die cushion may be inadequate to fully set the beads, resulting in insufficient restraining force. Binder separation may occur, resulting in a loss of control for the stamping process and excessive wrinkling of the part or addendum. Based on part geometry, the high impact load on the cushion may occur several inches from the bottom of the press stroke. Since the impact point in the stroke is both a higher velocity point and now at a de-rated press tonnage, shock loads develop and can cause damage to mechanical presses. Additional flywheel energy is dissipated by the high shock loads well above bottom dead center of the stroke. Staggering the heights of the nitrogen cylinders so they do not all engage at the same time is one way to reduce the shock load (see Figure 4).
Figure 4: Staggered nitrogen cylinders designed to reduce the initial shock load when setting draw beads by engaging at different depths in the press stroke. 2
A double-action press will set the draw beads when the outer slide approaches bottom dead center, where the full tonnage rating is available and where the slide velocity is substantially lower. This minimizes any shock loads on die and press with resultant load spikes less likely to exceed the rated press capacity.
1 G. Hartmann, “Das Spektrum Moderner Stahlfeinbleche-Festigkeiten und Auswirkungen auf die Umformung” Verschleißschutztechnik, Schopfheim, Germany (2004).
2 Courtesy of Peter J. Mooney, Managing Director, 3S Superior Stamping Solutions LLC
Header Image: Courtesy of Komatsu
Advanced High-Strength Steel (AHSS) forming challenges can lead to issues with the precision of part formation and stamping line productivity. The stamping industry is developing more advanced die designs as well as advanced manufacturing techniques to help reduce fractures and scrap associated with AHSS stamped from traditional presses. A Servo-driven Press1 is a significant option that has promising results when forming AHSS.
Characteristics of Servo Presses
A Servo Press is a press machine that uses a servomotor as the drive source. The advantage of the servomotor is that it can control both the position and speed of the output shaft compared to a constant cycle speed. In conventional mechanical presses, the press cycles at constant speed and press loads develop slowly, building power to their maximum force at bottom dead center (180-degree crank position), and then they reverse direction. In comparison, the Servo Press uses software to control press speed and position, which is much more flexible.
Servo Presses have a closed-loop feedback system to more accurately control cycle rate and loads, hence delivering a key advantage, which is the application of very high forming loads early in the stamping stroke. New forming techniques have been developed utilizing these features, and automakers are finding that with the use of the Servo Press, more complex part geometries can be achieved while maintaining dimensional precision.
Servo Presses have only been around for about 10 to 15 years, and so are considered “new technology” for the automotive industry. Recent growth in the use of Servo Presses parallels the increased use of AHSS in the structure of new automobiles. A cut-away of a Servo Press is shown below in Figure 1 to give a better understanding on the drive mechanism.
Figure 1: The Servo Press Cut-away 2
Types of Servo Presses
1) Servo motor and ball screw driven press: These Servo Presses employ ball screws to reduce the friction on the screws. Unlike traditional screw presses, this press does not require a clutch/flywheel and the slide velocity can be altered throughout forming. A notable feature of the ball screw type Servo Press is that the maximum force and slide speed are available at any slide position. This can be applied to almost all the forming methods. This capability proves to be very useful for forming with a long working stroke such as an extrusion, and for forming that requires a high-speed motion at the end of the formation.
The torque capacity of the servo motor, the load carrying capacity of the ball screw, and the reduction of the belt drive limit the maximum load. One solution to increase the power of the ball screw driven Servo Press is to increase the number of motors and driving axles (spindles). The only problem with this solution is the cost increases tremendously. Figure 2 shows a mechanism of hybrid Servo Press.
Figure 2: Operating mechanism of a hybrid Servo Press 3
2) Servomotor driven crank press: This press provides a lower-cost alternative compared to ball screw style Servo Presses. Crank presses have a high torque servomotor directly attached to the press drive shaft. A few press manufacturers developed C-Frame presses, which combine the servo driving mechanism with the conventional press structure. These presses are visually similar to mechanical presses, but the servo motor replaces the flywheel, clutch, and main motor. When the main shaft rotates at a constant speed, the stroke-time curve mirrors that of a conventional mechanical press. To increase power (for higher forming loads), crank presses are coupled with two connecting rods.
3) Servomotor driven linkage press: Linkage mechanisms are often used for presses to reduce the slide velocity and increase the load capacity of a given motor torque near the bottom dead center. Servomotor driven linkage presses prove to be advantageous in increasing the approaching and returning speeds by slowing down the slide speed in the working region within the stroke. They also have the advantage of maintaining high load capabilities through a considerably long working stroke.
4) Hybrid Servo Press: The knuckle joint and the linkage mechanisms are used to increase the press power with the crank shaft mechanism, and they can be combined with the ball screw mechanisms for Servo Presses.
5) Servomotor driven hydraulic press: These hydraulic presses have been developed because the high-power servomotors achieve higher slide speed compared with the conventional Servo Presses.
Advantages of Servo Presses
Servo Press technology has many advantages compared to mechanical presses when working with AHSS materials. Press manufacturers and users claim advantages in stroke, speed, energy usage, quality, tool life and uptime; these of course are dependent upon part shape and forming complexity.
1) Adjustable Stroke: The Servo Press has an adjustable stroke; the slide motion can be programmed to exert the required press load for deep draw stampings and then switch to different program routines to allow for shallow part stampings, or even blanking. This makes the Servo Press very versatile.
2) Speed: Compared to a standard mechanical press, Servo Presses manufacturers claim up to 37% increased cycle rates, which translates into better stamping plant productivity. Figure 3 shows cycle rates for comparable stroke heights for both a Servo Press and a traditional mechanical press.
Figure 3: Cycle Rates for Servo and Mechanical Presses 3
3) Energy Savings: The Servo Press has no continuously driven flywheel, cutting the costs of energy consumption. This is especially true in large capacity presses. The installed motor power is greater than that of a mechanical press whose capacity is comparable. However, throughout the stamping operation, the servo-driven motor is used only while the press is moving since the Servo Press has no continuously driven flywheel. Also contributing to nominal energy savings is the dynamic braking operation of the servo driven motor. Through this operation, the braking energy is transferred back into the power system. It is also possible to install an external energy storage feature to make up for energy peaks while reducing the nominal power drawn from the local power supply system (in cases where it is economically justified).
Figure 4 shows a comparison between a Servo Press and a traditional mechanical press with respect to energy use and storage. In a Servo Press, energy from deceleration of the slide is stored in an external device and tapped when the press motion requires more than 235 HP for each motor. The stored energy (maximum of 470 HP) is thus used during peak power requirements, enabling the facility power load to remain nearly constant, around 70 HP.
Figure 4: Energy vs. Displacement for Servo Press 3
4) Quality: Better forming stability translates into fewer part rejections.
5) Longer tool life: Decreasing the tool impact speed while simultaneously reducing the cycle time reduces impact loading, thus maximizing tool life. Increased lubrication effectiveness has been observed and using pulsating or oscillating slide motion can further extend tool life.
6) Uptime: Synchronized clutching and extended brake life allow for less frequent maintenance and better equipment uptime.
We have much more information we can share with you about these presses, more than we can fit into one blog. We encourage you to download your free copy of the AHSS Application Guidelines at worldautosteel.org and have a look at Section 3.C.6. Our next blog will provide a case study on Press Energy.
1 Dr. T. Altan, Professor at Ohio State University
2 Courtesy of AIDA America
3 Courtesy of P. Mooney archives
HEADER IMAGE SOURCE: Press line with ServoDirect Technology, courtesy of Schuler
Lubrication is an important input to almost every sheet metal forming operation. The lubricants have the following interactions with the forming process:
1. Control metal flow from the binder.
2. Redistribute strains over the punch.
3. Maximize/minimize the growth of strain gradients (deformation localization).
4. Reduce surface damage from die wear (galling and scoring).
5. Remove heat from the deformation zone.
6. Change the influence of surface coatings.
All these effects become more important as the strength of the sheet metal increases. Therefore, special attention to lubrication is required when considering AHSS.
Higher strength steels (HSLA and AHSS) have less capacity for stretch with a lower n-value compared to mild steels. Due to their strength, deformation forces to achieve the part geometry can be quite high. Additionally, these higher strength materials are often specified in thinner gages to allow for weight reduction, which makes them more prone to buckling than thicker steels. In order to maintain flatness, higher restraining forces are required [Since restraining force is a function of the coefficient of friction (C.O.F.) times the blankholder force, the restraining force falls and metal flow increases]. This combination results in higher contact pressure between the metal and the die, and higher interface temperatures. In order to counter these conditions, specialized lubricants that have both a lower coefficient of friction, and the ability to maintain chemistry and viscosity at elevated temperatures, are required.
Higher forming energy causes both the part and the die to increase in temperature. A study by Irmco1 measuring the temperatures on stampings produced from 350 MPa and 560 MPa steels clearly show increasing part temperatures with increasing strength levels, as shown in Table 1.
Table 1: Stamping Temperatures for High-Strength Steels.
The die temperatures are also significantly higher, and most conventional water-based or oil-based lubricants suffer viscosity reduction, with a corresponding increase in the coefficient of friction.
Ohio State University2 has performed many lubricant studies with AHSS steels; Figure 1 following shows the temperature profile on a DP900 stamping.
Figure 1: Temperature distribution for DP 900 Steel.
The highest temperatures are found on the die opening radius, and exceed 200C, reinforcing that part and die temperatures increase with increasing material strength. Without high-temperature additives, the lubricant effectiveness deteriorates, and heavy scoring and galling may result. As production speed increases (the number of parts per minute), the amount of heat generated increases, with a corresponding increase in sheet metal and die temperature.
One key to managing this heat problem when forming higher strength steels is application of a better lubricant. The chemistries of these better lubricants are less prone to viscosity changes and lubricant breakdown. Water-based lubricants disperse more heat than oil-based lubricants. Some parts may require tunnels drilled inside the tooling for circulating cooling liquids. These tunnels target hot spots (thermal gradients) that tend to localize deformation leading to failures.
Lubricants for AHSS Stampings
Some lubricant companies have developed stable, low C.O.F. lubricants; one example is the dry (barrier) lubricant. These polymer-based lubricants separate the sheet metal from the die. The dry lube C.O.F. for the same sheet metal and die combination can be 3 to 4 times lower compared to a good wet lubricant, and this performance acts to reduce the effective binder restraining force requirements. Since restraining force is a function of the coefficient of friction (C.O.F.) times the blankholder force, the restraining force increases and metal flow decreases. The net effect is a reduction in the amount of punch stretching required to form the part. Ultimately, forming strains and die wear are reduced. Fuchs Lubricants3 provides the guidelines shown in Table 2 for AHSS lubricant selection.
Table 2: Lubricant Selection Guide for AHSS.
Additives to Extreme Polymer (EP) lubricants create a protective barrier or film between the sheet metal and die. At elevated temperatures, the EP breaks down and deposits a metallic salt layer, which acts as a further temperature insulator (analogous to a surface coating on the die) and allows continued functionality of the lubricant. The complete separation of sheet metal and die by the barrier lubricant also means isolation of any differences in coating characteristics. Finally, a known and constant C.O.F. over the entire stamping greatly improves the accuracy of Computer Forming-Process Development (computerized die tryout).
1 Courtesy of Irmco (Jeff Jeffery).
2 Courtesy of the Ohio State University (Menachem Kimchi)
3 Courtesy of Fuchs Lubricants.
Friction and lubrication are important factors when trying to accurately simulate Advanced High-Strength Steel (AHSS) metal forming processes. In this blog post Dr. Johan Hol, Development Manager from TriboForm Engineering, describes frictional influences that are unique to AHSS forming and how the accuracy of predicting AHSS automotive parts forming may be increased by using an alternative friction model.
The TriboForm software is quickly becoming popular in the world of sheet metal forming and simulation. What started out as an engineering service for tool makers has now evolved into the TriboForm software that works with all major metal forming simulation solutions, including AutoForm, Pam-Stamp and LS-Dyna.
In a sheet metal stamping process, the sheet is always in contact with the tools. This contact is not static but rather dynamic because the sheet metal is flowing over the surface of the tools, i.e. there is a relative motion between the sheet and the tools. Even though the sheet and the tool surfaces look smooth from an unassisted vision; under the microscope they show a complex shape.
The sheet and the tool surfaces have a roughness profile made of a series of peaks and valleys of varying height, depth and spacing, as shown in Figures 1 and 2. The roughness profiles of the sheet metal will differ by the type, grade and the coating of the material, while that for the tools will differ by the type of the material and the way they have been machined.
Because of these irregularities in the surfaces of the sheet and the tools, there is a resistance to relative motion. In simple words, this resistance to relative motion is called ‘friction’, and this is the reason why a lubricant is applied on the sheet metal to reduce its resistance and thus friction. The ratio between the force of friction and the contact force between the two moving objects is represented by a coefficient of friction “µ” whose value will depend on the tribology system itself and the forming process, such as temperature of the sheet, ram velocity, contact pressure and the strains in the sheet.
Figure 1 (Left) : Uncoated Mild Steel Sheet of Roughness 1.5 µm.
Figure 2 (Right): Cast Iron Tool Surface of Roughness 0.4 µm.
Now we know where the friction comes from and why we need to apply lubricant on the sheet before stamping. Let’s talk more about how the amount of lubrication affects the quality of a panel during forming. The following pictures give a better understanding of the effect of lubrication.
All the panels shown in the picture below are simulated in AutoForm using a friction model created by TriboForm. Please note that when a friction model is not used, the simulations are run using a constant value of friction coefficient “µ”. Using the friction model, the user can change the amount of lubrication when simulating the panels; and depending on how friction sensitive the panel is, the amount of lubrication will have different effect on the quality of the panel.
The fenders shown in Figures 3, 4 and 5 have been simulated using the same exact simulation set up, except the lubrication amount has changed and hence the part quality is different. The fender shown in Figure 3 is experiencing heavy wrinkling at the corners because of the high lubrication applied on the sheet before drawing.
The higher the amount of lubrication, the lower the resistance to motion, i.e. material then flows freely over the tool surfaces in an uncontrolled fashion, producing wrinkles. Conversely, when the amount of lubrication applied on the sheet is very low, the resistance to motion is very high. This high resistance compels the sheet metal to stretch more than required, producing a high amount of thinning, and in some cases, huge amount of splits, as shown in Figure 4.
Figure 3 (Left): Too High Lubrication.
Figure 4 (Right): Too Low Lubrication.
Therefore, it becomes critical to use the proper amount of lubrication while drawing the panel, and it becomes equally important to find out the optimum amount of lubrication needed. Figure 5 shows the fender free of wrinkles and splits with the proper use of lubrication.
Figure 5: Optimum Lubrication.
Just like any other manufacturing process, applying lubricant on the sheet will have some inconsistencies, i.e. noise. Meaning, if the user decides on using a lubrication amount of 1 g⁄m2 on the sheet to produce a defect free panel, what are the chances that the robots will spray the exact amount of lubricants on the sheet every time? If for instance, the accuracy of the equipment is 85%, then the deviation of the lubricant will be in the range of 0.85 to 1.15 g⁄m2 , and if the panel is very sensitive to friction, then it might show some problems. Therefore, it is critical to find a safe range for the amount of lubrication and making sure that the equipment sprays the lubricant in the given range.
Finding a ‘sweet spot’ for the lubrication amount where the panel does not produce a lot of surface defects and at the same time does not show a high thinning value depends on having accurate simulation tools, such as using the TriboForm Plug-In with AutoForm.
In considering the tribology system for the forming of AHSS there are three main points to consider, namely 1) the effect of friction and tribology on springback 2) forming AHSS generates higher temperatures, which again affects frictional behavior and 3) in forming of AHSS different tool materials are used which bring new effects upon the frictional behavior in forming and simulation. All three phenomena should be accounted for in the forming simulation, which can only be achieved by using advanced friction models.
Naturally AHSS has more springback when forming, e.g., automotive parts. Springback is heavily influenced by the frictional behavior which is set in sheet metal forming simulations. This is exactly why you should have an improved description of your frictional behavior in stamping simulations. This in turn leads to better springback prediction. The friction determines the amount of restraining in the part, and based upon this, the springback behavior is influenced. In addition, it is important to consider that in forming AHSS, usually higher contact pressures between the tools and sheet are observed, which is why friction becomes so important. This leads to temperature build up in the material, not seen in that order of magnitude for mild steels. A proper description of the temperature evolution, and the effect on the frictional behavior, is therefore important for simulating the forming of AHSS.
Furthermore, forming AHSS materials demands the use of tool steels, which are not normally used for medium strength steels. Instead of using tools made from cast iron now we must consider the tribological effects of tools made from a controlled amount of carbon mixed with chromium to achieve harder tools. Such tooling materials have an influence on the tribological properties as well. This is why during simulation set up the user has to account for this along with lubricant selection. A good friction model should account for all these dependencies in generating the friction models.
If you have an advanced friction model in your forming simulation, you’ll introduce a realistic tribology system in your sheet metal forming simulation. Subsequently, you’ll achieve a more accurate prediction of splitting, wrinkling, thinning and springback, which are all linked to the friction model you are using.
Dr. Johan Hol, Development Manager, TriboForm
Dr. Ir. Johan Hol obtained his PHD in the field of tribology in sheet metal forming. In 2013 he co-founded the company TriboForm Engineering, a software company providing consultancy services and software solutions in the field of virtual design and tribological modelling.
After joining the AutoForm Group in 2016, Dr. Hol became the Development Manager of TriboForm and is responsible for the development of TriboForm’s software products and technical support in the global market.