Friction modelling in sheet metal forming simulations: application and validation on an U-Bend product
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. However, it is known that the true friction conditions are dependent on the tribology system, i.e. the applied sheet material, coating and tooling material, the lubrication and process conditions. In this paper, an approach is presented for friction modelling in sheet metal forming simulations. The approach is applied to the forming of U-bend test parts for varying tribology systems as commonly used in the automotive indus-try. The friction conditions per system are described using a physically-based friction model included in the Triboform Software. Friction in the U-bend simulations are described as a function of local contact pressure, relative sliding velocity and straining in the sheet material. The simulation results are validated using U-bend forming experiments. It is concluded that the numerical prediction of the punch force magnitude is highly sensitive to friction. In addition to an overall improvement of the prediction accuracy of stamping simulations by including friction modelling, this approach enables to model friction and the effect of varying tribology systems on the final stamped part quality.
Experimental analysis and modeling of friction in sheet metal forming considering the influence of drawbeads
In sheet metal forming of automotive body parts, high quality standards are achieved by designing the material flow in the blankholder area. To reduce time and cost of the tool development, virtual methods are commonly used. Therefore, an accurate friction modeling is necessary to predict the material flow in the blankholder area correctly. The blankholder of industrial forming tools usually consists of a drawbead and an area with surface pressure. In this study, the influence of the drawbead on the friction is analyzed experimentally and considered in the simulation. The friction coefficient is determined before and after the drawbead pass in dependence of the normal pressure. A flattening of the surface peaks due to surface pressure and a roughening due to plastic strain are found as the main influencing factors on the friction after drawbead pass. In the simulation the friction is modeled based on the experimental results employing an enhanced method considering the influence of the drawbead. These modeling approaches are compared with the friction model of the commercial software TriboForm and the experimental results. For this purpose, the predictive accuracy of the restraining force of a drawbead with different pressure distributions on the blankholder is evaluated using a strip tensile test. This evaluation is further carried out for a different strain state at a cup drawing test. By considering the influence of the drawbead on the friction coefficient a significant increase in the predictive accuracy of the simulation can be achieved.
Submicron polymer structures with X-ray lithography and hot embossing
This article describes the process chain for replication of submicron structures with varying aspect ratios (AR) up to 6 in polymethylmethacrylate (PMMA) by hot embossing to show the capability of the entire LIGA process to fabricate structures with these dimensions. Therefore a 4.7 μm thick layer of MicroChem 950k PMMA A11 resist was spin-coated on a 2.3 μm Ti/TiOx membrane. It was patterned with X-ray lithography at the electron storage ring ANKA (2.5 GeV and λc ≈ 0.4 nm) at a dose of 4 kJ/cm3 using a Si3N4 membrane mask with 2 μm thick gold-absorbers. The samples were developed in GG/BDG and resulted in AR of 6–14. Subsequent nickel plating at 52°C resulted in a 200 μm thick nickel tool of 100 mm diameter, which was used to replicate slit-nozzles and columns in PMMA. Closely packed submicron cavities with AR 6 in the nickel shim were filled to 60% during hot embossing.