Materials Science Forum Vol. 1184

Abstract: In automotive sheet metal stamping, the friction coefficient for a given tribological system (sheet, lubricant, tool surface) is known to depend on contact pressure, sliding velocity and temperature. Furthermore, plastic deformation of the sheet can cause surface roughening, which will affect frictional response. Beyond these known effects, the flat strip-draw experiments using friction pads of various sizes and repeated sliding on the same sample presented in this paper indicate that the frictional response depends on the sliding distance and sliding history. Moreover, we found that third body particles generated by the wear of sheet metal in frictional processes substantially influence the level of the friction shear force and its characteristics with respect to the initial static (breakaway) peak. Our results suggest that there is a need to improve the friction models implemented in current commercially available simulation software for aluminum sheet metal stamping, to capture these substantial effects.

Abstract: Cold forging and sheet‑metal bulk forming operations typically involve severe deformation, high contact pressures, and substantial surface enlargement. As highlighted in previous studies, friction behavior under these extreme conditions is governed by temperature, contact pressure, sliding velocity, and changes in the real contact area due to surface expansion. This work presents a newly developed linear sliding tribotester designed to characterize the friction response of metal sheets subjected to sheet‑metal bulk forming conditions. The testing procedure consists of two stages. In the first stage, the sample is compressed to intentionally modify and enlarge the initial contact surface, with the degree of surface expansion controlled by the specimen geometry. In the second stage, once the surface has been altered, frictional contact is generated between the sample and a sliding table, enabling the measurement of normal and tangential forces. These force measurements are subsequently used to determine the mean coefficient of friction. The results obtained constitute the first dataset toward the development of a multi‑scale friction model for sheet‑metal bulk gear forging. This model aims to incorporate the effects of extreme contact pressures, asperity flattening, and lubricant-related hydrostatic and hydrodynamic mechanisms.

Abstract: Friction plays an important role on formability of deep drawn products. This necessitates an accurate description of friction in finite element formability analyses. It has been shown that constant coefficient of friction does not lead to precise prediction of product formability in these analyses. The multi-scale friction model developed at University of Twente takes the local contact conditions and textures of sheet metal and tools as the input at boundary and mixed lubrication regimes. To correlate the zinc coated sheet metal surface texture parameters with its formability, 60 different textures were analyzed. The multi-scale friction model is used to estimate friction for all the sheet metal surface textures. The effect of different textures on formability of the sheet metal was investigated by simulating cross-die forming using different sheet metal surface textures. The results show that different textures depict distinct formability behavior in the boundary lubrication regime (lubricant amount 0.1 gr/m²). Exploring the correlation between areal field parameters and formability of cross-die for the current dataset shows that besides surface roughness, autocorrelation length and skewness of height distributions are determining parameters.