Effects of friction and lubrication in metal stamping processes
During the sheet metal stamping process, the sheet is always in contact with the tool. This contact is not static but dynamic because the metal plate flows over the tool surface, i.e. there is relative motion between the plate and the tool. Even though sheet and tool surfaces appear smooth without aided vision, under a microscope they reveal complex shapes.
Sheet and tool surfaces have roughness distributions that consist of a series of peaks and valleys of varying heights, depths, and spacing, as shown in Figures 1 and 2. The roughness distribution of sheet metal will vary depending on the type of material, grade and coating, while the roughness distribution of tools will vary depending on the type of material and how they are machined.
Because of these irregularities in the sheet and tool surfaces, there is resistance to relative motion. Simply put, this resistance to relative motion is called "friction", which is why lubricants are applied to metal plates to reduce their resistance and therefore reduce friction. The ratio between the friction force and the contact force of two moving objects is represented by the friction coefficient "μ", the value of which depends on the tribological system itself and the forming process, such as the temperature of the sheet, the stamping speed, the contact pressure and the strain of the sheet.
We understand where friction comes from and why we need to apply lubricant to the sheet before stamping. Now, we will focus on how the amount of lubrication affects the quality of the panel during the forming process. You can better understand the lubrication effect through the pictures below.
All panels shown in the image below are simulated in AutoForm using a friction model created with TriboForm. Note that when not using the friction model, the simulations are run with a constant friction coefficient "μ". When using the friction model, the user can change the amount of lubrication while simulating the panel; and depending on how sensitive the panel is to friction, the amount of lubrication will have different effects on the quality of the panel.
The higher the amount of lubrication, the lower the resistance to movement, i.e. the material then moves freely on the tool surface in an uncontrolled manner, creating wrinkles. Conversely, when the amount of lubrication applied to the sheet is very low, the resistance to movement is very high. This high resistance forces the metal sheet to stretch beyond the required amount, producing extensive thinning and, in some cases, extensive cracking, as shown in Figure 4.
Therefore, using the right amount of lubrication when pulling panels becomes critical, as is finding the optimal amount of lubrication required. Figure 5 shows a sheet with no wrinkles and cracks when lubricant is used correctly.
Just like any other manufacturing process, applying lubricant to the sheet can create some inconsistencies such as noise. This means that if the user decides to use 1g/m2 of lubricant on the panel, thereby producing defect-free panels, what is the probability that the robot will spray the exact amount of lubricant on the panel every time? For example, if the accuracy of the equipment is 85%, the deviation of the lubricant will be 0.85 - 1.15g/㎡, which may cause some problems if the panel is very sensitive to friction. Therefore, it is crucial to find a safe range for the amount of lubrication and ensure that the equipment sprays lubricant within the given range.
Finding the "optimal amount" of lubricant in the panel that does not create a large number of surface defects and at the same time does not show high dilution values depends on accurate simulation tools, such as using the TriboForm plug-in with AutoForm.
When considering AHSS molding tribological systems, there are three key points to consider, namely: 1) The impact of friction and tribology on springback; 2) AHSS molding produces higher temperatures, which again affects the friction behavior; 3) AHSS molding The use of different tool materials has new effects on the friction behavior in forming and simulation. The above three phenomena should be taken into account in forming simulations, which can only be achieved by using advanced friction models.
Of course, AHSS has more springback when forming, for example, automotive parts. Springback can be severely affected by the friction behavior set in the sheet metal forming simulation. This is also why you should improve the friction behavior in stamping simulations. This in turn results in better rebound predictions. Friction determines the amount of restraint in the part, and based on this, the springback behavior is affected. Furthermore, it is important to consider that when forming AHSS, higher contact pressures between the tool and the sheet are typically observed, which is why friction becomes so important, and friction causes an increase in temperature in the material, which has a negative impact on For low carbon steel, this order of magnitude will not occur. Therefore, a proper description of temperature changes and their effects on friction behavior is critical for simulating the forming of AHSS.
Additionally, AHSS forming materials require the use of tool steels that are not typically used on medium-strength steels. Instead of considering tools made of cast iron, we now have to consider the tribological effects of a harder tool made of a certain carbon and chromium content. This mold material also has an impact on tribological properties. This is why the user must take this into account along with the lubricant selection during simulation setup. A good friction model should take into account all these interrelationships when generating the friction model.
If you have an advanced friction model in your forming simulation, then you need to introduce a realistic tribology system into your sheet metal forming simulation. You will then get more accurate predictions of cracking, wrinkling, thinning and springback, all tied to the friction model you use.
