Surface deformation vs. bulk plastic deformation—a key for microscopic control of surfaces in metal forming

Various factors affect the formation of surface microgeometry in coining process. Major factors to be considered are the local contact pressure, global bulk plasticity, relative sliding and the resulting frictional stress. Among them bulk plasticity plays the decisive role of the development of surface microgeometry during the process of microscopic bulging and filling, even if the overall mode of deformation looks that of surface deformation. This is based on the metal forming plasticity as in the problem of the flattening surface of asperities. Surface deformation should be stressed if the normal stress with frictional stress can be activated on the very top of the forming surface microgeometry, and especially if no bulk plasticity can be utilized. This process is a local and microscopic finishing of surface layers. Relative sliding between the tool and the workpiece also enhances the forming of the surface microprojections but is prone to cause a directional unbalanced form of the microgeometry and, in an extreme case, separation by shearing off. For these reasons, relative sliding should not be over-estimated as to contribution.