Direct observation of asperity deformation of specimens with random rough surfaces in upsetting and indentation processes

The trapping behavior of liquid lubricant and contact behavior of asperities at the workpiece–tool interface during upsetting and indentation are observed directly using a compression subpress which consists of a transparent die made of sapphire, a microscope with a CCD camera and a video system. The experiments are carried out without lubricant and with lubricant. Specimens used are commercially pure A1100 aluminum with a random rough surface. From these observations, the change in the fraction of real contact area is measured by an image processor. The real contact area ratios in upsetting experiments without lubricant are in good agreement with numerical ones obtained by Wanheim and Bay and Makinouchi et al. The real contact area ratios in indentation experiments without lubricant increase linearly with increasing normal stress up to 350 MPa. The real contact area ratios after upsetting and indentation experiments with lubricant reached a limiting value of about 60%.