Daimler AG: Door-outer Mercedes-Benz C-class Coupé

Daimler AG: Door-outer Mercedes-Benz C-class Coupé

In the stamping of automotive parts, friction and lubrication play a key role. To improve the quality of metal forming simulations, TriboForm is integrated to adequately describe these tribological factors and applied to the door-outer of the all new Mercedes-Benz C-Class Coupé.

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Technical case

In the stamping of automotive parts, tribology-, friction-, and lubrication conditions play a key role in achieving high quality products. In the development process of new automotive parts, it is therefore crucial to accurately account for these effects in sheet metal forming simulations. Only then, one can obtain reliable and realistic simulation results that correspond to the actual try-out and mass production conditions.

Door-outer of the Mercedes-Benz C-class CoupéDoor-outer of the Mercedes-Benz C-class Coupé

Although friction is of key importance in the production process, it is currently not considered in detail in stamping simulations. The current industrial standard is to use a constant (Coulomb) coefficient of friction. This limits the overall simulation accuracy as demonstrated in the figure below for the door-outer of the all new Mercedes-Benz C-Class Coupé. The sheet metal forming simulation results performed in AutoForm are shown at the left, using a constant coefficient of friction of µ = 0.15. The door-outer part can be safely produced based on the simulated (safe) formability. At the left side, a door-outer part taken from an initial production try out is shown. The door-outer part shows splits at two locations. Clearly, there is a large deviation between simulation results using Coulomb friction and try out production. This can be accounted to the lack of an accurate description of tribology-, friction-, and lubrication conditions in stamping simulations as demonstrated below.

Simulation and production resultsDeviation between the sheet metal forming simulation results using a constant coefficient of friction (left) and try-out in stamping production (right) for the door-outer of the all new Mercedes-Benz C-Class Coupé

In this work, the TriboForm software is used to accurately account for tribology-, friction-, and lubrication conditions in the sheet metal forming simulations. For this purpose, the TriboForm software is used in combination with AutoForm. Validation of the enhanced simulation results is performed based on door-outer parts taken from try out production. Parts have been produced at varying try out conditionsby variation of the:

  1. Drawing velocity = 49 mm/s & 15 mm/s
  2. Lubrication amount = 2,0 g/m2 & 0,5 g/m2
  3. Blank holder force = 200 kN & 300 kN

The results are presented below considering the:

  • Experimentally measured results
  • Simulation results using a constant (Coulomb) coefficient of friction of µ = 0.15 in AutoForm
  • Simulation results using the TriboForm FEM Plug-In in AutoForm

A selection of technical and business case results are described below.

Input information for the TriboForm Software

To execute a TriboForm simulation, properties of the sheet, lubricant and tooling material are inserted into the TriboForm software.  First, the type of sheet material and its surface properties are selected. The user can either choose a material from the build-in TriboForm Library or import real 3D surface measurements. For the considered door-outer part, 3D surface measurements of the sheet material have been made by confocal microscopy measurements. Next, the lubrication type and amount as applied in the metal forming process of the door-outer are entered into the TriboForm software.

Sheet and lubricant3D surface topography of the sheet material and applied lubricant

Finally, the type of tooling material and its surface conditions are entered into the TriboForm software. This can be done by using the comprehensive TriboForm Library of tooling materials, or by importing 3D surface measurements. For the considered door-outer case, 3D surface measurements of the production tooling have been made by confocal microscopy measurements and imported into the TriboForm Analyzer.

tooling3D surface topography of the stamping tools

TriboForm softwareInput information for the TriboForm Analyzer: sheet material, lubricant and tooling material

Integration of TriboForm in metal forming simulations

The TriboForm Analyzer enables the calculation and visualization of the friction conditions for the considered sheet material, lubricant and tooling material. Friction results are calculated for a wide range of process settings, i.e. nominal contact pressure, plastic strain in the sheet material, relative sliding velocity and interface temperature. Two TriboForm simulations have been performed considering a lubrication amount of 2,0 g/m2 and 0,5 g/m2.

TriboForm software approachApproach: Using the TriboForm software

Next, the friction results for the two lubrication conditions can be exported from the TriboForm Analyzer. Note that the resulting two friction files can also be used to account for the effect of stroke rate and blank holder force. This is because the friction result file accounts for the effect of contact pressure and relative sliding velocity on the friction behavior. The two friction files can be directly imported into the AutoForm software using the TriboForm FEM Plug-In. This enables advanced friction and lubrication modeling in the stamping simulation of the door-outer product.

Improved simulation accuracy for the door-outer part using the TriboForm FEM Plug-In in AutoForm.

Production try-out results

Door-outer parts are pressed in try out production for varying process conditions. For all parts, the strain distribution is measured and used for validation of the simulation results. Stamping of the parts is performed for different drawing velocities, lubrication amounts and blank holder forces according to:

  Drawing velocity Lubrication amount Blank holder force
1 49 mm/s & 15 mm/s 2,0 g/m2 200 kN
2 49 mm/s 2,0 g/m2 & 0,5 g/m2 200 kN
3 49 mm/s 2,0 g/m2 200 kN & 300 kN

 

The technical results for all three situations are summarized in the figures below. The results for varying process conditions are shown at the left and right of the figures. The resulting door-outer parts are shown at the top of each figure. The corresponding strain distributions are shown at the bottom of each figure. Also the simulated strain distribution or Forming Limit Diagram is shown at the bottom of each figure considering simulation results using a constant (Coulomb) coefficient of friction of µ = 0.15 and simulation results using the TriboForm FEM Plug-In.

On the effect of drawing velocity

The door-outer parts which are pressed in the try out production at a drawing velocity of 49 mm/s show a critical strain distribution but no fracture. Decreasing the drawing velocity to 15 mm/s results in fractures at two locations in the door outer parts (!). Reducing the drawing velocity will increase the friction in the forming process which explains the fractured parts. This clearly demonstrates that the quality of the door outer parts is strongly dependent on the friction and lubrication conditions that are acting in the actual production process.

Effect of drawing velocityEffect of drawing velocity on part quality: try out production results and simulation results

When using a constant coefficient of friction in stamping simulations, one cannot account for the effect of drawing velocity on the frictional behavior and thus on product quality. Note that therefore the simulation results using a constant coefficient of friction (in red) are similar for 49 mm/s and 15 mm/s, whereas there is a strong effect on part quality visible in the try out production results. Using the TriboForm FEM Plug-In, the effect of drawing velocity on the frictional behavior and thus on product quality can be accurately accounted for. For a drawing velocity of 49 mm/s, the simulated strain results using the TriboForm TriboForm FEM Plug-In show a good agreement with the experimental results. Decreasing the drawing velocity results in an increasingly critical strain distribution which corresponds with try out production. This clearly demonstrate the improved prediction accuracy of stamping simulations using the TriboForm software.

On the effect of lubrication amount

It is well known that the final quality of sheet metal formed parts like the door outer is strongly dependent on the applied lubrication amount used in production.  The door-outer parts which are pressed in the try out production using a lubrication amount of 2,0 g/m2 show a critical strain distribution but no fracture. Decreasing the lubrication amount to 0,5 g/m2 results in fractures at two locations in the door outer parts (!). Reducing the lubrication amount will increase the friction in the forming process which explains the fractured parts. This clearly demonstrates that the quality of the door outer parts is strongly dependent on the lubrication conditions in the actual production process.

Effect of lubrication amountEffect of lubrication amount on part quality: try out production results and simulation results

Using a constant coefficient of friction in stamping simulations, one cannot account for the effect of lubrication amount on product quality. Note that therefore the simulation results using a constant coefficient of friction (in red) are similar for 2,0 g/m2 and 0,5 g/m2, whereas there is a strong effect on part quality visible in the try out production results. Using the TriboForm FEM Plug-In, the effect of lubrication amount on the frictional behavior and thus on product quality can be accurately accounted for using varying friction files in the stamping simulations. For a lubrication amount of 2,0 g/m2, the simulated strain results using the TriboForm FEM Plug-In show a good agreement with the experimental results. Decreasing the lubrication amount results in an increasingly critical strain distribution which corresponds with try out production. This clearly demonstrate the improved prediction accuracy of stamping simulations using the TriboForm software.

On the effect of blank holder force

Finally, the results of door-outer parts pressed in the try out production using a blank holder force of 200 and 300 kN are shown. It should be noted that these parts are produced using lock-beads, so the effect of blank holder force on the draw-in in minimal. For both settings, the door outer parts are critical but no fracture is observed. Increasing the blankholder force with 100 kN shows a less critical strain distribution for the door outer part.  The same trend in the strain distribution is predicted when using the the TriboForm FEM Plug-In in the stamping simulations. The agreement between try out production results and simulation results using the the TriboForm FEM Plug-In is very good.

Effect of blank holder forceEffect of blank holder force on part quality: try out production results and simulation results

Conclusions

These results demonstrate that the quality of the door outer parts is strongly dependent on the friction and lubrication conditions that are acting in the actual production process. Changing the process settings (drawing velocity and blank holder force) and the lubrication conditions will change the tribology and friction conditions and thus the quality of the door-outer parts. A final summary of the results is provided in the figure below. In the development process of new automotive parts, it is therefore crucial to accurately account for these effects in sheet metal forming simulations using the TriboForm software. Only then, reliable and realistic simulation results can be obtained that correspond to the actual try-out and mass production conditions at Daimler AG.

summarySummary of the door-inner results: try-out production versus stamping simulations

Business case

Business case door-outerThe technical case results demonstrate that the TriboForm software enables more reliable and realistic stamping simulations with respect to critical quality factors like formability (splits, wrinkles), draw-in and springback. This enables Daimler AG to simulate the actual try-out and mass production conditions in the development process of new automotive parts more accurately, resulting in time and cost savings.

Looking at the percentual cost distribution in stamping production of a door-outer part, around 31%* of the costs can be assigned to tooling try-out. Looking at the cost breakdown of tooling try-out, different phases can be distinguished (start-up and testing, try-out loops and press-shop try-out) with the relatedd try out costs. Using TriboForm, try out conditions can now be realistically simulated in the virtual development process which saves up to one phase in try out corresponding to a savings in development time of up to 2 months*.  This results in:

  • Reduction of the total try out time: – 22%*
  • Reduction of try out costs: € 137.000* (for a single part, for a single car model)

With multiple critical parts per car model, and an increasing number of car models being developed, the TriboForm software delivers even higher savings. The technical case demonstrates that the TriboForm software empowers its users to quickly understand, simulate and solve tribology-related problems in the metal forming industry. The business case demonstrates that TriboForm enables to shrink time-lines and reduce development costs of new vehicles and results in a high Return On Investment (ROI).

[* ULSAB Cost model stamped parts, Steel association, 2013 ]

More information

For more information, please contact Dr. Ir. J. Hol, CTO.

Courtesy of Daimler AG.