Deformation of OFHC copper during cutting

Abstract

Metal cutting induces larger strains and high strain rates within the primary shear zone (PSZ). The study is aimed at understanding the nature of high strain rate deformation of a highly ductile and workhardening material, namely, OFHC copper. Experimental measurements of the strain rate distribution over the primary shear zone (PSZ) have been carried out using digital image correlation (DIC) of ultra high-speed photographic images. A large depth of cut of 300um has been used to increase the width of the shear zone and resolve the deformation clearly. The experiments were carried on OFHC copper specimens with different hardness values and initial grain size. The PSZ is found to be narrower near the tool tip and wider near the free surface. The width of the PSZ for 0.1m/sec and 1m/sec increases from the tool tip to the free surface but for 3.3ms/sec it increases over the first half of its length near the cutting edge and then remains constant over the second half to the free surface. Correspondingly, the strain rate is higher near the tool tip and lower near the free surface. The PSZ for the material with smaller grain size is comprised of multiple shear bands, which are planar regions of material along which the strain is higher than in adjacent regions. The material with larger grain size typically shows a single shear band in the PSZ. Shear bands are observed to initiate near the entrance of the PSZ and remain active till they exit the PSZ. In the case of multiple shear bands, the strain rate over a band increases as it moves towards the middle of the PSZ and then decreases as it moves towards the exit of the PSZ. The mean spacing between bands near the free surface is about 25μm and 50μm for the materials with initial grain size of 30μm and 100μm respectively, which corresponds to the wavelength of the ripples along the back surface of the chip. The bands likely correspond to the strain inhomogeneity usually observed at the level of the grain size, as evidenced by the fact that the free surface of chips is always observed to be rippled.