Effects of tool die surface topography on friction and galling in sheet metal forming

Abstract

The evolvement of product requirements in the automotive industry, e.g. reduced weight, means that the use of advanced high strength steels (HSS, EHSS,UHSS) in automotive applications is continuously increasing. The use of high strength steels increase the tool wear in production and calls for functional tool surfaces that are durable in these severe tribological conditions. In this study the influence of tool surface topography on friction and galling has been investigated. The Design of Experiment was used to analyse the frictional response in a Bending Under Tension test and the resistance of galling in a Strip Reduction test. Two levels of tool roughness (Sa=0.1 and 0.2 μm), surface anisotropy (longitudinal and transverse), amount of prelube (rust protection oil 0.1 and 1 g/m2) and two tool materials (1 and 2) were studied. Complementary tests were performed also with Sa-values of 0.3 and 0.4 μm when sliding perpendicular to the surface lay. The results did show an interaction between the surfaces lay and roughness. The combination of high roughness and sliding along the surface lay generated high friction and wear due to deformation forces from the ploughing tool asperities. However, high roughness and sliding perpendicular to the surface lay generated low frictional response and wear and indicated hydrodynamic and/or hydrostatic lubricating phenomena despite the fact that only prelube was used. The importance of the tool surface can be stated by equal or higher factorial effects than the tool materials, although the difference in high and low roughness was just 0.1 μm (Sa). The surface parameter Δq, i.e. the root-mean-square of the slopes calculated in the sliding direction, did show the highest correlation to the response variables.