Tribology of flat contacts and its application in deep drawing

The difference between steel and aluminium in sheet metal forming operations, and the influence of roughness in these operations, have been studied by making deep draw and friction experiments. The friction tests have been carried out with flat contacts to simulate the conditions in the blankholder. In deep drawing the influence of roughness cannot be neglected. The punch force is affected by the roughness, and related properties such as the dimensions of the product after deep drawing and the fracture limit, are affected as well. The relative amount of this influence depends on the amount of lubricant, the type of product and the type of sheet metal, and can also vary with the state of the process. In rectangular products however the effects were less clear than in cylindrical products. For aluminium the influences of process conditions in deep drawing are stronger than for steel, caused by strong asperity flattening. When using a high speed and much lubricant at aluminium, the friction in the blankholder became so low that the blankholder force could be increased up to the limit of the press without causing fracture. Friction tests showed that for steel the friction as expressed in a Stribeck curve, barely depends on the pressure, while for aluminium the influence of pressure is strong. Both the position of the region of mixed lubrication, and the 'classic' friction coefficient at conditions of boundary lubrication are influenced by pressure. The influence of pres-sure could be linked to the phenomena encountered in the deep drawing tests success-fully. The mechanisms responsible for the generation of pressure in the lubricant at mixed lubrication have been examined. Based on these findings a model for mixed lubrication has been developed which can describe many of the observed effects including the consequence of asperity flattening.