The Influence of Plastic Bulk Deformation on Surface Roughness and Frictional Behavior during Deep Drawing Processes

In Sheet Metal Forming (SMF) processes, friction does play an important role. This with respect to the increase of product quality demands and the ability of predicting these processes by for instance finite element simulations. The existing simulation models do not contain an adequate friction model. In SMF processes different contact situations can be distinguished. As a result different coefficients of friction are locally present, which influences the forming process. Experiments are performed on a testing device by which it is possible to simulate the operational conditions as present in SMF processes. This test rig is a combination of a tensile tester and a friction measuring device, by which it is possible to measure the coefficient of friction as a function of the operational conditions (velocity and contact pressure) and deformation (elastic or plastic) in a well controlled way. Friction is presented in a generalized Stribeck-curve in which the different lubrication regimes can be distinguished, i.e. Boundary Lubrication (BL) and Mixed Lubrication (ML), which are also occurring during SMF processes. In SMF processes the sheet material deforms elastically and plastically and therefore the surface roughness will change and as a consequence will influence the frictional behavior between sheet and tool. In this paper, the influence of plastic deformation on A) the surface microgeometry and B) as a consequence of that on the frictional behavior of the sheet-tool system is studied. With the aid of a 3D-surface interference microscope, the microgeometry of the deformed material has been analyzed. The result of this investigation is that the CLA-roughness due to the deformation first decreases and then increases with increasing deformation. Furthermore, friction is hardly influenced due to the change in surface roughness. No change in the shape and the level of the generalized Stribeck curve is found.