Basic study on the influence of liquid lubricant on contact state in cold forging processes

In cold forging processes the tribological conditions are very important for process feasibility and tool lifetime. Since boundary and mixed lubrication conditions are predominant in cold forging processes, the appearing real contact area which is influenced significantly by the topography of tool and workpiece has a major impact on the friction conditions. Against the background of current trends in cold forging like the replacement of zinc-phosphate coatings or microforming applications a better understanding of the tribological conditions is necessary. Especially the flattening behavior of single asperities under boundary lubrication conditions has to be investigated in more detail. Thus, physical models of asperities represented by pyramids with a base area of 120 x 120 μm have been flattened with a high-resolution experimental set-up which enables the in-situ observation of the developing real contact area by applying a translucent tool. Various liquid lubricants have been used. It has been revealed that the usage of certain lubricants leads to a higher resistance of the asperity against flattening which would be in favor of reduced friction. Topography analysis has been performed in order to reveal the mechanisms leading to this effect. This supporting effect of liquid lubricant has been observed during micro upsetting tests with ground topographies, too. Further investigations with different topographies are necessary in order to derive recommendations for the design of tribological systems and for a more detailed description of friction.