Modelling of friction values depending on the point of contact as a function of contact pressure and sliding distance

Extreme process conditions, such as high contract pressures and long sliding paths, lead to locally changing friction conditions during the forging process. If the changing tribological system is not considered correctly for the process design, this can decrease the quality of the billet significantly and increase the die wear. In the numerical simulation of forging processes friction is commonly described by using the friction factor model or the combined friction model. These approaches assume that the lubrication is locally homogeneously spread and therefore use a globally constant friction value. However, these models cannot be used to reliably and accurately predict global values such as the necessary forming force simultaneously to the local quality of the billet within a single FE simulation for current industrial applications. To solve this problem, currently multiple time-consuming FE calculations with different friction values are necessary to determine all process variables with adequate accuracy.

Therefore, in the research project, a new method was developed, which allows to use location-dependent friction values in dependence of the occurring tribological loads for FE simulations. First, a novel experimental testing method was developed, to characterise the friction conditions whilst varying tribological valuables such as the sliding path, temperature and contact pressure. Through this testing method the process conditions of bulk forming processes can be recreated. From the experimental data a new friction model was derived, which takes into account the sliding velocity, temperature, contact pressure and sliding distance. The friction model was then implemented into the FE software Forge 4.0. To allow industrial user a fast and easy test application of their own friction data to the new model, a software tool was developed through which the friction input data for a simulation can be generated directly with customised parameters.

The IGF project "Location-dependent modelling of friction coefficients as a function of contact pressure and sliding distance in solid forming", IGF project no. 21648N, is funded by the German Federal Ministry for Economic Affairs and Energy via the AiF as part of the program for the promotion of joint industrial research (IGF) on the basis of a resolution of the German Bundestag. The final report is expected to be available from the Forschungsgesellschaft Stahlverformung (FSV) from the end of 2023.