Abstract
The effect of cutting speed, feed rate and tool wear on the surfaces generated when machining nickel base, Inconel 718,alloy with conventional and high pressure coolant supplies was investigated. Tool life, failure modes, surface roughness (Ra) values and component forces were recorded. Machined surfaces were examined with the SEM and optical microscopes. Microhardness analysis show evidence of hardening of the top machined surfaces. In most cases the microhardness reading tend to approach the hardness of the base material at 0.3mm below the machined surface. This is due to the austenitic structure of Inconel 718 which promote work hardening when machining as a result of the high temperature and stresses generated at the cutting interface. The hardening effect decreased with increasing coolant pressures up to 203 bar as the coolant gain access closer to the cutting interface, thus minimising the cutting interface temperature. Analysis of machined surfaces shows that severe plastic deformation occurred when machining with conventional coolant supply than with high pressure coolant supplies. Surface damage or phase transformation was not observed when machining Inconel 718 under high pressure coolant supplies. Increasing the feed rate from 0.15 mm/rev to 0.25mm/rev when machining with ceramic tools at a cutting speed of 250m/min decreased the hardness of the machined surface. An increase in feed rate from 0.25mm/rev to 0.3mm/rev increased hardness of the machined surface when machining with coated carbide tools at a speed of 30m/min due to increased shearing forces encountered and consequent strain hardening of the machined surface.
