Machining of Al/SiC particulate metal matrix composites - Part II: Workpiece surface integrity

This paper deals with the surface integrity of machined Al/20%SiC particulate metal-matrix composites (PMMC). Dry high-speed turning tests, at different cutting speeds, feed rates and depths of cut, were conducted in order to investigate the effect of the various cutting parameters on the surface quality and the extent of the sub-surface damage due to machining. The cutting tests were carried out using polycrystalline diamond tools (PCD). Scanning electron microscopic images of the machined surfaces indicate the presence of grooves and holes (due to the pull-out of SiC particles). Surface roughness measurements show that the surface roughness improves with an increase in the feed rate and the cutting speed, but slightly deteriorates with an increase in the depth of cut. This was attributed to the reduction in the flank wear of the tool with an increase in feed rate, which was discussed in detail in a companion paper published earlier (El-Gallab and Sklad, J. Mater. Proc. Tech., in press). Microscopic examination of the cross-sections of the chips indicate the localization of the matrix deformation along shear bands, where the reinforcing SIC particles align themselves. Microhardness depth profiles indicate that the sub-surface damage is confined to the top 60-100 mu m. Transmission electron microscopic images of the sub-surface layers show that dislocations pile up close to the machined surface, where the matrix grains are smaller. Combining the results presented in this paper and those in the companion paper which dealt with tool wear, it is found that machining this type of composites is most economical and safe at a speed of 894 m min(-1), a depth of cut of 1.5 mm and feed rates as high as 0.45 mm rev(-1), when the surface roughness, R-max, did not exceed 2.5 mu m. These cutting parameters are much higher than those currently employed in the industry. (C) 1998 Elsevier Science S.A. All rights reserved.