A study of interface friction in plastic compression

Abstract An experimental study was made of friction over the interface between a cylindrical specimen and loading platen during plastic compression. The variations of normal pressure, shear stress, and friction coefficient were determined using a pair of pressure-sensitive pins installed in a platen so that at least one was oblique to the interface. Experiments were made on aluminum, primarily, with some carried out on copper and iron. Lubricant coverage included an absence of lubrication, solid films (lead foil, molybdenum disulphide, soap, teflon), and liquids (mineral oil with and without fatty-acid additions). A simple calculation of the friction hill assuming constant friction coefficient, or a lubricant film of constant shear strength, and homogeneous compression, was valid only for the restricted conditions of low coefficient (or low shear strength) and small reduction. With increasing reduction, deformation became non-homogeneous and ubricant-film breakdown began at the periphery of the specimen. It was concluded that failure started as a consequence of increasing interface area and abrasion from displacement over the platen face. For different conditions of lubrication, friction coefficient was found to vary, below a maximum of approximately 0·5, with both reduction and location in the interface. The results could be rationalized in terms of relative changes in interface shear stress and material flow pressure.