Modelling of Friction in Hot Stamping
Jenny Venema, Eisso Atzema, Javad Hazrati, D.T.A. Matthews, A.H. van den Boogaard
Abstract
Hot stamping is used to produce structural automotive parts. In this process, high strength ( ̴ 1500 MPa) can be achieved with good formability and accurate geometrical tolerances due to the combination of a forming step at high temperature ( ̴ 700 °C) and the quenching step in the press. However, due to the forming at high temperatures the friction is high and the tool wear is severe. A huge number of measurements of the Coefficient of Friction (COF) have been published so far. However, contradictory findings regarding influence of different parameters such as temperature and pressure are observed and large deviations in the COF values are found. Furthermore, in finite element (FE) analyses of hot stamping processes, as yet the friction is modelled as one constant value, while it is known that the friction depends on temperature, pressure and strain. In this paper, the effect of different parameters on COF using strip-draw tests are investigated. Moreover, a multi-scale friction model for hot stamping is developed and calibrated relative to the experimental results. The friction model regards the surface characteristics of the sheet and tool, which are of major importance due to the build-up of galling. The friction model also takes temperature, pressure and strain effects into account. The friction model is finally coupled with the FE simulation of a hot stamping process.