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T.B. Sercombe Department of Mining, Minerals and Materials Engineering, The University of Queensland, Qld 4072, Australia, and Interdisciplinary Research Center (IRC) in Advanced Materials, The University of Birmingham, Edgbaston, B15 2TT, United Kingdom

S.H. Huo Department of Mining, Minerals and Materials Engineering, The University of Queensland, Qld 4072, Australia

P. Calvert Department of Materials Science and Engineering, University of Arizona, Tucson, Arizona 85721

G.B. Schaffer Department of Mining, Minerals and Materials Engineering, The University of Queensland, Qld 4072, Australia (Received 2 November 2000; accepted 28 June 2001)

A series of metal-matrix composites were formed by extrusion freeform fabrication of a sinterable aluminum alloy in combination with silicon carbide particles and whiskers, carbon fibers, alumina particles, and hollow flyash cenospheres. Silicon carbide particles were most successful in that the composites retained high density with up to 20 vol% of reinforcement and the strength approximately doubles over the strength of the metal matrix alone. Comparison with simple models suggests that this unexpectedly high degree of reinforcement can be attributed to the concentration of small silicon carbide particles around the larger metal powder. This fabrication method also allows composites to be formed with hollow spheres that cannot be formed by other powder or melt methods.

I. INTRODUCTION

Metal-matrix composites, such as aluminum reinforced with silicon carbide, have long been formed by squeeze casting and solid-state extrusion. Composites can also be shaped by tool-less manufacturing techniques such as extrusion freeform fabrication (EFF). This is one of a family of rapid prototyping methods, closely related to fused deposition modeling and to multiphase jet solidification.1 For metal parts, a stepper-motor-driven syringe moves over a substrate, writing a thin layer (about 0.2 mm thick) of slurry under computer control. The slurry, of metal powders, polymer, and solvent dries rapidly to a hard layer. Successive layers are then added to build up a solid part of metal with polymer binder. In this way, metal parts can be made directly from a computeraided design solid model without prior production of a mold. Parts have been produced by EFF in ceramics,2,3 as well as thermoplastics4 and epoxy resin5 with and without fiber reinforcement. It has also recently been J. Mater. Res., Vol. 16, No. 9, Sep 2001

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shown that it is possible to use the EFF technique to align short fibers if the rheological properties of the slurry are optimized.6 To produce metal parts by the EFF technique, it is necessary that sintering to high density can be effected without compaction. This had not been possible with aluminum and its alloys until recently due to the thermodynamic stability of the aluminum sesquioxide. At 600 °C, an oxygen partial pressure of