Mechanisms controlling friction and material transfer in sliding contacts between cemented carbide and aluminum during metal forming

Cold forming of aluminum alloys is frequently associated with problems related to severe adhesion and material transfer onto the forming tools which results in high friction forces and negatively affects the surface quality of the formed parts, a phenomenon frequently named galling. In the present study, well controlled laboratory tests using a scratch testing equipment have been performed to evaluate the friction characteristics and investigate the mechanisms controlling the initial transfer of aluminum in dry sliding contact with five different cemented carbide grades. In the tests, an aluminum pin (representing the work material) with a conical tip slides against a flat, fine-polished, cemented carbide surface (representing the tool). During sliding, the mechanical contact results in plastic deformation and flattening of the work material against the tool surface, thus simulating a metal forming contact. The small scale and well-defined tribo contact in combination with post-test surface characterization using optical surface profilometry, high resolution SEM and EDS makes it possible to evaluate the influence of material transfer on the friction characteristics.The results show that sub-mu m surface irregularities in the cemented carbide surface trigger mechanical interaction with the softer aluminum surface which promotes aluminum transfer to the cemented carbide surface resulting in high friction. Common surface irregularities, promoting aluminum transfer, are sharp edges of slightly protruding carbide grains, surface steps in connection to binder phase pockets, surface steps in connection to surface pores, etc. It should be noted that even very small surface steps, < 20 nm in height, constitute efficient cutting edges able to effectively cut off the passing aluminum material and thus have a very strong impact on material transfer. In contrast, the effect of carbide composition, e.g. the presence of cubic carbides of different composition, seems to be of minor importance to reduce the adhesion and the tendency to material transfer.