Non-conventional machining of particle reinforced metal matrix composites

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Abstract

Particle reinforced metal matrix composites (PRMMCs) have proved to be extremely difficult to machine using conventional manufacturing processes due to heavy tool wear caused by the presence of the hard ceramic reinforcement. This paper presents details of an investigation into the machinability of silicon carbide particle reinforced aluminium alloy matrix composites using non-conventional machining processes such as electro discharge machining (EDM) and laser cutting. The different removal mechanisms of the different processes when machining the composite were investigated. The surface condition and sub-surface damage of the material for the different machining processes have been examined and compared. It appears that both EDM and laser are suitable processes for machining PRMMCs, laser offers significant advantages in terms of removal rate. EDM however induce less thermal damage than that was observed using the laser.

Introduction

Particle reinforced metal matrix composites (PRMMCs) represent a group of materials where the hardness, strength and resistance of the reinforcement are combined with the ductility and toughness of a matrix material. Aluminium is the most frequently used matrix material due to its low density. Because of its extreme hardness and temperature resistant properties, SiC ceramic particles are often used as a reinforcement within the Al matrix. This type of PRMMC is more frequently used in the automotive industry today, particularly in various engine components as well as brake rotors. However, the full potential of this material is hindered by the high manufacturing cost involved, mainly because of the difficulties in machining it. Machining SiC particle reinforced aluminium matrix composites using conventional machining processes such as, turning, sawing, drilling, etc. generally results in excessive tool wear due to the very abrasive nature of this material [2]. As a consequence, non-conventional machining processes, such as electro discharge machining (EDM) [7], [12], [14], laser [8], [11] and other techniques, are increasingly being applied for the machining of PRMMCs. Most of these non-conventional machining processes still need to be investigated in greater detail in order to assess their “optimal” machining conditions.

The objective of the research work reported here was an investigation of the effectiveness of non-conventional machining processes such as EDM and laser applied to machine PRMMC. The two processes are based on different removal mechanisms and lead therefore to different results, e.g. different surface integrities. The difference in removal rate, cut quality and surface integrity for the machining processes were analysed and compared to each other, to indicate the suitability of the different processes for different applications. In order to characterise the surface condition, the surface roughness, surface topography and thermally influenced sub-surface layer were analysed using both electron microscopy and white light interferometer.

Section snippets

Experimental procedure

The metal matrix composites used for the tests were an AA2618/SiC/20p and an A356/SiC/35p composite material. The first composite consists of an aluminium alloy (AA2618–2.3% Cu, 1.5 Mg, 1.2% Fe, 1.1% Ni, bal. Al) reinforced by 20% silicon carbide (SiC) ceramic particles of approximately 10–13 μm in size. This material (AA2618/SiC/20p) was produced by spray deposition followed by an extrusion process. The second composite is made from an A356 aluminium alloys (7% Si, 0.2% Cu, 0.6% Fe, 0.35% Mn,

EDM

The results obtained indicate that Al/SiC PRMMCs can be machined using EDM, despite the low electrical conductivity and the high thermal resistance of the ceramic particle. The EDM process is however slow. The effect of the machining current and the pulse on-time on the material removal rate (MRR) are shown in Fig. 1. It can be seen, the metal removal rate increases with increasing discharge current and increased pulse duration up to an optimal value and thereafter decreases for any pulse

Conclusions

As outlined above the EDM process is suitable for machining PRMMCs, the process is however slow. It is shown that the SiC ceramic particles were not melted during the machining process. This appear to suggest that the removal of the particle reinforced aluminium alloy matrix composite occurs through melting and vaporising the matrix material around the ceramic particles up to a point where the entire particle becomes detached. As a result the removal rate decreases with increased SiC particle

Acknowledgements

The authors would like to thank Materials Ireland Metal Processing Research Unit at Trinity College, Dublin for the financial and technical support.

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