Boundary friction measurements in sheet metal forming

A sheet metal forming simulator which stretches the sheet around a cylindrical pin was used to investigate the relationship between friction and process variables including sliding speed, interface pressure, workpiece strain and strain-rate in the boundary lubrication regime.; Measurements have been conducted with two different steel and tooling systems. The first is with smooth A2 tool steel and both uncoated and electro-galvanized steel sheet. The second is with D2 tool steel both bare and coated with different coatings (VC, NbC, CrC) with electro-galvanized sheet. In the first system, friction tends to increase with pressure and strain in contact and is independent of sliding speed, strain-rate and the kinematic conditions. Friction increases with angle of wrap or with strain in contact and also increases with contact pressure under the same contact strain for both uncoated and coated steel. Work with the second system indicates that carbide coatings tend to reduce friction and chatter in the dry tests. Friction was found to be lower with the NbC coated pin and higher with CrC compared to the other test pin materials.; Frictional behavior and material transfer of commercially pure aluminum sheet with the various monolithic and coated tool surfaces have been studied for different forming variables. Friction was found to decrease with the strain in contact which is apparently due to a decrease in contact area associated with roughening of the workpiece by plastic deformation. Mirror finished vanadium carbide coated pins yielded the lowest friction coefficient of all the test pins. No material transfer was found for any tool material with the exception of light pickup for rough vanadium coated pins in the lubricated condition.; Steady and unsteady mathematical models of a metal strip sliding and stretching over a cylindrical pin with boundary friction have been developed. The friction model treats the influence of plastic strain on the flattening of strip asperities and real area of contact. This results in friction which increases monotonically with strain during contact. The change in global friction predicted by the model is similar to that observed with bare steel and electro-galvanized steel strip with different wrap angles.