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.