| The mechanics of the metalforming tool/workpiece interface under conditions of boundary lubrication are studied in detail in this dissertation. First, simple models for two elementary surface interactions are formulated: flattening of workpiece asperities against a smooth tool, and plowing of tool asperities across a smooth workpiece. An upper-bounds plasticity analysis is used to quantify the resistance of the workpiece surface to each interaction. The resistance is found to depend on the bulk strain rate of the workpiece and on the ratio of true contact area between the surfaces to the nominal contact area. Combinations of elementary interactions are studied in the context of a strip drawing analysis. A model for a stochastically rough surface is formulated, and the elementary surface interaction models are extended to this surface. The relationship between true contact area and measurable statistical properties (root mean square, skew and kurtosis) of a workpiece roughness profile flattened against a smoother tool surface is derived. The models for asperity plowing and flattening are included in a slab analysis of the flat rolling process, and computer programs are written to calculate interface friction, rolling force, torque and slip, and final strip roughness. The rolling models are tested by reducing series of aluminum strips on a laboratory rolling mill. All four combinations of rough and smooth strips and rolls are used, with and without oleic acid as a boundary lubricant. Rolling force, torque, slip, and final strip roughness are all measured. The results compare favorably with model predictions. |
