Modeling boundary friction and starved lubrication in sheet metal forming

A new model for friction in sheet metal forming in the boundary regime was developed. The model adds an empirical correction for surface roughening to the asperity flattening model developed by Wilson and Sheu. Average coefficients of friction predicted by the model are in excellent agreement with friction measurement conducted on the sheet metal forming simulator for 1100-H14 aluminum sheet and electro-galvanized steel sheet.; The boundary friction model treats the combined influence of surface roughening (due to inhomogeneous deformation) and surface smoothing (due to asperity flattening) on the real area of contact in the presence of plastic strain. Predictions using the new model for sheet metal forming indicate that the friction coefficient tends to decrease with strain for low pressures (when roughening is of dominant importance) and tend to increase with strain for high pressures (when asperity flattening prevails). This behavior was confirmed experimentally by friction measurements for 6022-T4 aluminum sheet stretched over A2 steel pins of different diameters with a commercial lubricant. Average coefficients of friction predicted by the model were in good agreement with those measured experimentally.; A starved inlet lubrication model was incorporated into a membrane finite element model similar to that developed by Wilson, Hsu, and Huang to study the influence of starvation in axisymmetric stretch forming. Considerations of lubricant supply were used to calculate the position of the edge of the lubricant meniscus. The model utilized the new inlet film thickness calculation methodology developed by Mettes. The results of this analysis indicate that reducing the amount of lubricant on the sheet surface tends to increase friction in a similar manner to reducing forming speed.; The experimentally measured strain distributions in starved axisymmetric stretching forming experiments with limited lubricant supply have been compared with the simulated results for different forming speeds. The measured and simulated strains are in excellent quantitative agreement which validates the starved lubrication theory.