Development of a Friction Law Respecting Plastic Surface Smoothing

Abstract

Friction has an essential influence on metal forming processes and affects the mould filling strongly. Numerical simulation is widely used because they allow for a efficient product design without the time and cost intensive production of prototype moulds. The quality of the simulation results and thus their reliability is determined by the accuracy of the modelling. For this purpose the applied friction law is of great importance. Characteristic of sheet-bulk metal forming is the coexistence of moderate contact pressures like in sheet metal forming and high contact pressures like in bulk metal forming. The Coulomb friction law is suitable for the sheet metal forming process but it predicts too high friction forces for high contact pressures. On the other hand the Tresca friction law is suitable for bulk metal forming but overestimates the friction for low contact pressures. A smooth transition between the Coulomb and Tresca friction law is described by the Shaw friction law and the Wanheim-Bay friction law. An unresolved problem remains the influence of plastic surface smoothing of structured workpiece surfaces. The tribological properties of the surface are altered by the plastic deformation of the surface roughness. As a consequence the real area of contact and thus the friction are larger in unloading and reloading than in the first loading at the same surface pressure. This plays a role in forming processes with multiple stages, where the surface is smoothed by prior forming operations like for example the forming of tailored blanks. Therefore efforts have been made in the numerical modelling of elasto-plastic surface deformation with a halfspace model. This model allows for the efficient modelling of large rough surfaces because it uses only a surface mesh and not an numerically expensive volume mesh like a Finite-Element model. This halfspace model is calibrated and verified with experimental investigations. A friction law taking into account the plastic surface deformation has been developed based on the halfspace simulations. It distinguishes between first loading, where the current surface pressure is higher than all surface pressures which occurred previously, and unloading or reloading, where the friction is higher because the surface is smoothed plastically in a previous load step, where the surface pressure was higher than currently.