Machinability study of particle reinforced aluminium metal matrix composites

Abstract

In this thesis, machinability of particle-reinforced aluminium metal matrix composites, Comral-85 and DURALCANTM, has been studied. Continuous turning of round composite bars, using polycrystalline diamond (PCD) inserts has been selected as the test method. The test conditions included cutting speeds varying from 75 to 700m/min and feed rates from 0.1 to 0.4 mm/rev with constant depth of cut of 0.5 mm. The main wear mechanism of machining these Al MMC materials is abrasion by the reinforcing particles and the primary type of tool wear is flank wear. Linear regression techniques has been used to derive Taylor equations to describe the tool performance. The results show that the time required to reach the tool wear limit decreases with increased speed and feed rate. However, the volume of material removed before reaching the wear limit actually increases with the higher feed rate. This apparent anomaly has been reconciled in a modified Taylor equation. As for surface finish, the feed rate is found to be a more dominant factor than cutting speed. The higher the feed rate is, the worse the surface finish becomes. The surface finish is found to improve with tool wear at early stage because of the increase of tool nose radius; after that it starts deteriorating as a consequence of excessive tool wear. The change of feed rate is also more influential on the variation of machining forces than that of cutting speed. Using the same regression techniques, the general machining force-tool wear equations are derived. The results show that the equation derived from the feed force is better suited to monitor tool wear than that derived from the cutting force. The general relationship between tool wear and power consumption has also been established. The chip forming mechanism while machining DURALCANTM MMC has also been studied by using an explosive charged "quick-stop" device. The primary chip forming mechanism involves the initiation of cracks due to the high shear stress, followed by the decohesion of particles and matrix material within the chip due to the stress concentration on the edge of the particles. The crack propagation is enhanced through the microvoid coalescence within matrix material. The fracture and the sliding of material then follow to form semi-continuous "saw-toothed" chips.