Effect of Plastic Deformation of Bulk Material on Frictional Behavior in Dry Metal Forming

Abstract The friction behavior in dry metal forming is studied with a new tribometer. From the microscopic observation, it is found that the ratio of the flattened area starts to increase sharply when plastic deformation extends in the whole bulk. But, the nominal frictional stress increases linearly with the nominal contact pressure irrespective of the ratio of the flattened area. The surface texture of the flattened area is measured by using electron probe surface roughness analyzer. Before the start of bulk plastic deformation, the flattened area is smooth and almost the whole area seems to be real area of contact. After plastic deformation in the bulk material, the flattened area is roughened. A new friction law for dry metal forming, Coulomb's friction with constant friction coefficient μ holds up to the critical pressure and the law of constant frictional stress τ holds for the higher pressures, is proposed based on the experimental results.