Effects of Surface Roughening on Asperity Flattening

Abstract

A series of experiments have been conducted using aluminum strips, which manifest a strong tendency toward roughening in forming processes, to investigate the evolvement of surface roughness and its resistance to flattening. The strips are stretched to specific bulk strains and are then compressed by a smooth glass at various pressures. It is found that except for very high bulk strain, the free surface on the transverse specimen roughens more seriously and irregularly than the longitudinal one due to the fierce rotations of grains, which make the grains parallel to the straining direction and eventually mutate the topography into an isotropic one. As the roughened surface is compressed, the transverse specimen may roughen further in the case of large bulk strain. According to the finite element method (FEM) analysis using serrated surfaces, the sharper asperity undergoes more serious straining on its peak and basically such a strain increment is not affected by the degree of flattening. The ability to resist flattening, defined as the “asperity hardness,” varies with asperity angle as well as the enhanced shear strength on the asperity tip, rather than the substrate. A new quantitative description of the asperity hardness is suggested, and the new calculated contact area ratios are in good agreement with the experiments except for very high pressure where additional mechanics such as adiabatic recrystallization might happen. The concept of the additional hardening on asperity peaks can be employed to refine the prediction of asperity contact in the analysis of metal forming.