Date of Award

9-1999

Degree Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical Engineering

Supervisor

Professor M. Sklad

Abstract

Particulate metal matrix composites (PMMCs) are being widely used in the aerospace and automotive industry due to their favourable properties, mainly high specific strength and wear resistance. However, machining of particulate metal matrix composites presents a great challenge to the industry as the reinforcing particles easily abrade most of the common cutting tool materials. Polycrystalline diamond (PCD) tools appear to be the most practical type of tool material for machining PMMCs. The first part of the presented research concentrates on experimentally identifying the effect of the various cutting parameters on the chip formation mechanism, which in turn influences the tool wear. Increasing the feed rate had a beneficial effect on the tool wear due to abrasion. This is attributed to the reduced contact between the tool and the abrading reinforcing particles in the chip. The experimental work also involved an evaluation of the damage introduced into the workpiece due to machining. This damage is in the form of crushed particles, cracks and voids around the particles. The surface finish of the work-piece is a reflection of the tool's wear state. The second part of the research involved building up numerical models for the cutting tool and workpiece. Computer simulation of the machining process can potentially reduce the number of experimental iterations needed to determine the optimum cutting parameters, which are the ones that produce the minimum tool wear and least workpiece damage with minimum cost. The model stress results were verified through scanning electron microscopic observations as well as transmission electron microscopic analysis. The finite element models revealed that the area beneath the machined surface experiences high tensile stresses, which cause void formation around the reinforcing particles. These voids join up to form cracks, which have serious implications on the fatigue life of the machined part. Further research into the changes introduced in the microstructure of the chip and machined work-piece is recommended, as this could help in fully understanding the stress state beneath the machined surface.

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