The accuracy of sheet metal forming simulations strongly depends on, amongst others, friction modelling. The industrial standard is to use the Coulomb friction model with a constant coefficient of friction. This reference case shows that friction conditions in stamping production are dependent on the applied tribology system (i.e. the applied sheet material, coating and tooling material, the lubrication and process conditions) and demonstrates how these conditions can be implemented in stamping simulations using the TriboForm software.
In this reference case of the Volvo Car Group, the TriboForm software is used for tribology-, friction- and lubrication modelling in sheet metal forming simulations. The considered application is the forming of U-bend part using varying tribology systems as commonly used in the automotive industry. The simulation results are validated using U-bend forming experiments in a mechanical press. During the forming of the U-bend products, the forming (punch) force is recorded. U-bend products have been produced using the following three tribology systems:
- DX56D+Z sheet material (0.7 mm) with a Fuchs Anticorit PLS100T lubricant (1.0 g/m2)
- DX56D+Z sheet material (0.7 mm) with a Fuchs Anticorit PLS100T lubricant (2.0 g/m2)
- AA6016 sheet material (1.0 mm) with a Fuchs Anticorit PL39SX hotmelt (1.0 g/m2)
Looking at forming process of a U-bend product, the forming (punch) force is largely determined by the friction force between the sheet material and the tooling. For the three tribology systems, a completely different punch force is recorded in the forming of the part. This demonstrates the strong effect of the tribology system on the frictional behavior. To accurately simulate the forming of the U-bend products, an accurate description of the friction conditions in the forming simulation is therefore of vital importance.
As a first step, the friction conditions per tribology system are simulated using the TriboForm Analyzer software. The resulting three friction models or files describe the frictional behavior as a function of:
- local contact pressure
- relative sliding velocity
- straining in the sheet material
- and interface temperature
As a next step, the friction files are exported from the TriboForm Analyzer and imported in to the sheet metal forming simulation software AutoForm using the TriboForm FEM Plug-In. AutoForm simulations have been performed using a constant coefficient of friction of µ = 0.15 for the zinc coated mild steel DX56D+Z material and µ =0.12 for the aluminium AA6016 material (according to the Volvo standards). In addition, AutoForm simulations have been performed including the three friction files for the varying tribology systems using the TriboForm FEM Plug-In. The results are presented below.
Animation of the time- and location dependent coefficient of friction (from 0.04 [blue] to 0.1 [red]) using the TriboForm FEM Plug-In in AutoForm.
The experimental and simulation results for the U-bend punch forces as a function of the punch stroke are presented below. A strong decrease of the punch force is observed for the DX56D+Z material by increasing the lubrication amount from 1.0 g/m2 to 2.0 g/m2. This demonstrates the strong influence of the lubrication conditions in the forming process on the frictional behavior, the resulting punch force and final part quality.
A strong improvement of the simulation accuracy is observed when using the Triboform friction files compared to a constant coefficient of friction. The agreement between experimental results and simulation results including the Triboform friction files are very good. Looking at the results for the DX56D+Z material, it is demonstrated that a constant coefficient of friction µ = 0.15 is much too high. This single value cannot represent the two situations of using a lubrication amount of 1.0 g/m2 or 2.0 g/m2 for the PLS100T lubricant.
For the AA6016 sheet material with a lubrication amount of 1 g/m2 PL39SX lubricant, a reasonable simulation prediction accuracy can be achieved using both a constant coefficient of friction and the TriboForm friction files. The best simulation accuracy is achieved using the TriboForm friction files, especially for the low blank holder force (bottom black curve). Note that the error between the experimental and simulation results using a constant coefficient of friction coefficient increases for increasing blank holder force (top black curve), which is not so much the case when using the Triboform friction files. This demonstrates that the friction conditions are dependent on the process conditions, i.e. the contact pressure, which is only accounted for in the TriboForm friction file.
Experimental and simulation results for the U-bend punch forces DX56D+Z with 1.0 g/m2 PLS100T (left), DX56D+Z with 2.0 g/m2 PLS100T (centre), AA6016 with 1.0 g/m2 PL39SX hotmelt for a high and low blank holder force (right)
It is concluded that the experimental and numerical prediction of the forming force is highly sensitive to friction. In addition to an overall improvement of the prediction accuracy of stamping simulations by using the TriboForm FEM Plug-In, the TriboForm software enables to simulate friction and the effect of varying tribology systems on the final stamped part quality. Demonstrated benefits of the TriboForm software for the Volvo Car Group are:
- the TriboForm software is based on physical models with input parameters that can be collected from a database or measured with minimal time effort and costs.
- it reduces the demand for experimental testing and enables more accurate simulations in virtual process development.
- it offers Volvo Car Group the possibility to perform sheet metal forming simulations including the friction behavior corresponding to the tribology system as applied in stamping production.
- finally, the numerical approach enables the user to perform robustness analyses including the variation of frictional behavior, and simulate sheet metal forming processes more accurately and realistically.
For more information, please contact Dr. Ir. J. Hol, CTO.
Courtesy of Volvo Cars.