Friction phenomena in hydrostatic extrusion of magnesium

When magnesium is hydrostatically extruded an inconsistent and sometimes bad surface quality is encountered. In hydrostatic extrusion the billet is surrounded by a lubricant, usually castor oil. The required pressure to deform the material is applied onto this lubricant and not directly to the billet. This means that there is oil in the contact between billet and die, where the material is deformed. In this work the process is divided into three zones: the inlet, the work and the outlet zone. The surface quality of the hydrostatic extrusion products is determined in the work zone, where the actual deformation takes place. Therefore the main focus in this work is on the friction phenomena in the work zone area. A lubrication model is developed based on the well-known Reynolds equation. To be able to calculate a film thickness using this model in the work zone a boundary condition is required. For this, also a lubrication model in the inlet zone is developed. The calculated film thickness in the work zone is compared to the roughness of the deforming material. The conclusion is that the acting lubrication regime is boundary lubrication. This means that the contact stresses are carried totally by the contacting asperities and not by the lubricant. Furthermore a contact model for the work zone is developed to be able to calculate the fraction of real contact area in the contact between billet and die. Calculations show that this ratio of area in contact is very high in the entire work zone. This is very likely the cause for the inconsistent surface quality. This could be changed by various measures, such as the use of a lubricant of which the viscosity is more strongly dependent on the pressure.