Contact pressure-dependent friction characterization by using a single sheet metal compression test

Abstract

Tribology has a major impact on process stability, product quality and tool life in metal forming processes. Today, the complex system of friction is simplified by the Coulomb friction coefficient or friction factor model for analytical and numerical process analysis. These friction values are determined in laboratory tests and assumed to be constant. Due to smoothing of the surface, displacement of lubricant from the active joint and locally different relative velocities, the friction state changes while forming. To achieve a high accuracy of finite element analysis, it is essential to consider these changing boundary conditions. Identifying strain-dependent friction coefficient or friction factor models by conventional characterization tests, such as the pin extrusion test or ring compression test, require a high number of experiments. Besides that, these tests are not suitable for low plastic strains due to their poor sensitivity. The sheet metal compression test, analyzed in this contribution, enables the description of the strain and contact pressure dependent progression of the friction values in one simple experimental setup. To determine the friction coefficients, the experimentally measured force-displacement curves will be assigned to friction coefficients in a numerically determined nomogram. The calibration of the nomogram is done analytically for the friction-free condition by using the flow curve determined in the uniaxial tensile test. The sheet metal compression test reacts sensitive to changes of the tribological system due to the high ratio of surface to specimen volume and the direct measuring of the press force as an evaluation quantity. The characterization of friction value progression is done for the materials DP600, DC06, Cu-OFE and CuZn30 with different tribological conditions. The results confirm that the friction values and thus the friction condition is changing during the forming process. Additionally, the validity limit of the friction coefficient model can be easily identified by analysing the friction coefficient progression.