Surface Roughening and Contact Behavior in Forming of Aluminum Sheet

Abstract

The evolution of the surface topography and tool-workpiece contact behavior of an aluminum sheet during plastic deformation has been investigated by a series of indentation tests for various loads, strain rates, and bulk strains. A simple formulation for estimating the roughness of the indented surface and a model of asperity flattening have been proposed. Dynamic balance between the flattening and roughening processes, such as changes of asperity angle and spacing, is considered. Theoretical predictions of the variation of real area of contact with strain show good agreement with experiments. It is found that the very different frictional behavior of aluminum compared with steel sheets is due to the serious change of roughness, including asperity angle and spacing. When the angle is small, the increases in asperity angle and spacing have strong effect on increasing the contact area ratio. However, when the asperity angle is large, increased asperity angle has a significant contribution to the resistance to flattening. The asperity flattening velocity and the increasing rate of contact area ratio arise with the prestrain; however, they are not affected by the strain rate significantly. From FFT analysis, low frequency components (below 10 cycle/mm) due to the rotation of single grains or grain groups are observed on free surface during plastic straining. While in the contact area, in all cases, the low frequency components generated by plastic deformation are compressed.