Instead of performing many friction experiments, friction and lubrication conditions can now be simulated using the TriboForm software. Fast, easy-to-use and cost effective. This is demonstrated through an comparison between friction experiments and TriboForm simulations.
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The quality of sheet metal formed parts is strongly dependent on the tribology-, friction-, and lubrication conditions that are acting in the production process. These friction conditions are dependent on the utilized sheet material, lubricant and tooling material. This combination of factors is known as the tribology system. Choosing the optimal tribology system in an early stage of the development process enables the design of high quality products at minimal time and cost investment.
To analyze the effect of the tribology system on the final part quality, there is an industrial need to quantify the corresponding frictional behavior and include an accurate description of this behavior in sheet metal forming simulations. One way to determine the frictional behavior for a specific tribology system is to execute friction tests. For this purpose, different friction test set-ups are being used in industry.
In this reference case, the TriboForm software is used to simulate the frictional behavior instead of performing many friction experiments. Using the TriboForm software, friction simulations are performed for a specific combination of sheet material, lubricant and tooling material. The simulated coefficients of friction are experimentally validated using friction experiments.
In this reference case, different sheet materials and lubricants are tested and numerically evaluated using the TriboForm software. The following tribology systems are considered:
- Uncoated mild steel
- Zinc-coated mild steel
- Aluminium 6xxx series
- 1st generation prelube (for the mild steels)
- Washing oil (for the mild steels)
- Hotmelt (for the aluminium material)
- Tool steel
As an input for the TriboForm software, the real 3D surface topographies of the sheet and tooling material are used. These surface topography measurements can be imported by the user or imported from the TriboForm Library. Moreover, the material properties of the sheet material are imported in the TriboForm software. For the lubricants, the corresponding viscosity properties are imported. Finally, tribology tests have been performed for the considered tribology systems based on which the TriboForm software is calibrated. Next, the TriboForm software is ready to simulate friction conditions for the varying tribology systems.
The video’s below show a projection of the measured tooling surface topography of the friction test and the sheet material surface topography of the zinc-coated mild steel in the TriboForm software. Using this TriboForm simulation model, the friction conditions are calculated by loading and sliding the tool surface over the sheet surface. The type and amount of lubricant applied to the sheet surface can be defined in the TriboForm software by the user.
TriboForm simulation of a tooling surface topography (top) loading and sliding of a sheet surface topography (bottom) for a low lubrication amount
TriboForm simulation of a tooling surface topography (top) loading and sliding of a sheet surface topography (bottom) for a high lubrication amount
The friction conditions are calculated in the TriboForm software for pre-defined ranges of process conditions, i.e. nominal contact pressure, plastic strain in the sheet material, relative sliding velocity and interface temperature. Below, only the friction results as a function of nominal contact pressure will be considered and experimentally validated using friction test results.
Replacing friction experiments
The figures below show the experimental and simulation results of the friction conditions as a function of contact pressure applied to the sheet material. Results are shown for the varying tribology systems. The TriboForm simulation results show a good overall agreement with the experimental friction results, both considering the magnitude and the trend.
This reference case shows that the TriboForm software enables simulating friction conditions for varying tribology systems, i.e. resulting in a generally applicable approach for friction characterization under industrial sheet metal forming process conditions. Benefits of the TriboForm software are the following:
- the TriboForm software input parameters can be efficiently collected from a database or measured with minimal effort.
- it reduces the demand for experimental friction testing as further described in the business case.
- the resulting friction prediction can be readily imported in metal forming simulations of forming processes using the TriboForm FEM Plug-In.
TriboForm enables more accurate sheet metal forming simulations including the friction behavior corresponding to the tribology system as applied in stamping production, and design best-of-class sheet metal formed parts at minimal time and cost investment.
The TriboForm software reduces the demand for friction experiments. A single TriboForm simulation provides a greater amount of information (>60x) at a reduced time- and cost investment (Return Of Investment: 6x) as demonstrated in the following business case.
Friction test procedure VDA 230-213 – Section 5.6 Friction Properties
As a reference, the test procedure for determination of friction properties according to the VDA 230-213 norm are used. The VDA 230-213 norm prescribes the execution of friction testing considering the complete tribology system. Parameters that influence the final friction properties are: tool material, sheet material, coating, surface texture, lubricant type, lubrication amount, contact normal stress, sliding velocity and temperature. In the current business case, an example tribology system is considered as indicated in the norm:
- Sheet material type: DC05 ZE 75/75 (1 type)
- Lubricant type: prelube (1 type)
- Lubrication amount: 0,6 g/m2 and 2.0 g/m2 (2 amounts, resulting in 2 tribology systems)
- Tooling material: GGG 70 (1 type)
- Total of 2 tribology systems
The test parameters according to VDA 230-213 are:
- Contact normal stress: Starting at 2 MPa, with steps of ≤2 MPa increasing until fracture (typically resulting in 6 pressure settings, maximum up to 14 MPa)
- Sliding velocity: 1 mm/s, 50 mm/s and 400 mm/s (3 settings)
- Temperature: 40˚C, 60˚C and 80˚C (3 settings)
- Total of 54 experimental settings (6 x 3 x 3)
The friction experiments should be performed in triplicates at varying settings.
This results in a total of 162 experiments to be performed to determine the friction properties.
With an average price of € 200,- per friction test, this results in experimental costs of € 32.400.
TriboForm simulation of friction properties
A single TriboForm simulation provides friction properties for an infinite number of process settings. When accounting for:
- 10 contact normal stresses (user defined range between: 0 – 100 MPa)
- 10 sliding velocities (user defined range between: 1 mm/s – 1000 mm/s)
- 10 temperatures (user defined range between: 10˚C – 100˚C)
- 10 levels of straining of the sheet material (user defined range between: 0 – 0.5)
Compared to friction experiments, a single TriboForm simulation provides an equivalent value of (10 x 10 x 10 x 10 x €200) = € 2.000.000 (!)
Finally, the tribology system and friction behavior that is analyzed in friction testing will be significantly different from the tribology system in stamping production. That is:
- The roughness of the tools in try-out or stamping production will be different and show a strong variation.
- The lubrication conditions in production will be different and show a strong variation.
- The roughness of the sheet material in production will show a stronger variation.
Note that friction experiments therefore have to be repeated in case the roughness of the tooling changes, the roughness of the sheet material changes, or the lubrication conditions changes. The TriboForm software enables the simulation and variation of these conditions, by simply executing additional TriboForm simulations. Therefore, use the TriboForm software to create your friction model and:
- Simulate varying tool roughness conditions
- Simulate varying sheet roughness conditions
- Simulate varying lubrication conditions
Import the resulting friction behavior (and variation) in sheet metal forming simulations using the TriboForm FEM Plug-In, and simulate the stamping process with the friction behavior corresponding to the actual conditions in production.
For more information, please contact Dr. Ir. J. Hol, CTO.
Courtesy of Tata Steel.