Friction and lubrication in metal rolling
This dissertation is concerned with the physical processes which determine friction and lubrication in metal rolling in the mixed lubrication regime, with particular attention paid to the conditions encountered when rolling aluminium foil. Two areas of relevance to the analysis of the rolling process are initially investigated. Firstly, the rheological properties of a typical aluminium foil rolling oil at high pressures and shear rates have been measured using a disc machine. The behaviour of the oil was found to be well described by the Eyring viscous model, at the shear rates and pressure likely to be found in metal rolling. Secondly, the deformation of asperities when the bulk material is deforming has been examined. The theory developed here was found to agree reasonably with experiments. The results of these investigations are used in the analysis of lubrication in metal rolling, considering the hydrodynamic buildup of oil pressure in the entry region and the crushing of the asperities both in the entry region and at the beginning of the work zone. The contact between roll and strip is divided into two regions, that under the asperities and that in the intervening valleys. Calculations for conditions appropriate to strip and foil rolling give the proportion of the two types of contact and the film thicknesses in each region. Measurements of film thicknesses with an experimental mill in a regime where roughness is unimportant were not found to agree well with an existing simple theory of lubrication. This was ascribed to uneven lubrication in the experiments. After taking this into account, the experiments in a regime where roughness was important were found to agree reasonably with the theory developed here. The effect of roughness on traction is measured in a disc machine with elastic contacts. Its behaviour is found to be determined by the bulk properties of the lubricant at the pressures and strain rates under the asperities. Theory and experiments presented in this dissertation lead to a greater understanding of the physical processes determining friction in metal rolling in the mixed lubrication regime. Film thicknesses and friction coefficients in metal rolling may now be estimated with more confidence.