The role of friction in deformation processing

Abstract

One or more die faces are employed in most practical deformation processes. Where plastic deformation is essentially normal to the die face (Brinell hardness test) friction plays a negligible role. In most plastic flow processes, however, there is a shear component of deformation and friction plays a major role. The friction associated with a die face differs significantly from that for lightly loaded sliders. For example, Amontons' law for lightly loaded sliders does not hold for sliding on a die face. The material in the vicinity of a rigid die face does not obey the same rules of plastic flow as the material in bulk. Consequently it is generally incorrect to extend a slip line field solution, that may represent a first approximation in bulk, all the way to a die face. In some instances the deformation process for the material in bulk is grossly incompatible with that at the rigid boundary and a situation intermediate between the two extremes results. In still other instances an artificial die face is produced in situ in the deforming material and the initial geometry of the system is then drastically altered. In certain plastic flow processes it is possible to arrange hydrodynamic support at the die face. This will normally result in less die wear, but poorer finish if the viscosity of the hydrodynamic medium is such as to provide low friction. Other methods of controlling the magnitude of the friction on a die face include boundary lubrication, contaminated microcracks, a steep temperature gradient near the surface and vibration of the surface. The utility of plasticine as an analog modelling material in plastic flow studies is illustrated by some of the figures presented.