Consolidation of tungsten-coated copper composite powder
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Powder metallurgy (P/M) methods have expanded in recent years to include net-shape fabrication of W–Cu components. Sintering of W–Cu compacts made from previous powders has led to low density and copper bleedout. This paper presents test results on the consolidation of a new powder containing particles composed of a tungsten-coated copper core. This powder effectively eliminates copper bleedout at sintering thereby making possible the net-shape manufacturing of parts. Sintered articles are characterized by microstructural homogeneity and properties comparable to those of infiltrated W–Cu composites.
I. INTRODUCTION
Infiltration of a porous tungsten skeleton with liquid copper, using a copper source external to the skeleton, is the predominant technology to make W–Cu composites. Infiltrated W–Cu composites are usually made in the form of slugs or blanks whose weight ranges from several grams to several kilograms. A number of machining operations are required to turn the infiltrated shapes into actual components. Therefore, infiltration does not allow net-shape manufacturing. Fabrication of W–Cu net shapes has become an important issue in recent years due to the increasing complexity and miniaturization (parts weighing as low as 0.5–1.0 g) of heat sinks and spreaders used for thermal management of electronic devices.1,2 Such net-shape components can be fabricated only by powder metallurgy (P/M) using powder sources that contain both the tungsten and the copper phases. Other traditional applications of W–Cu, such as electrical contacts, electrical discharge machining electrodes, spotwelding electrodes, etc., would also benefit from the versatility of P/M. The success of making P/M W–Cu composites ultimately depends on the powder source. Development of new W–Cu powder sources should be based on a thorough consideration of factors influencing the sintering of W–Cu composites. Increasing the powder fineness is a traditional way to raise its sintering activity.3 However, it may be difficult to take advantage of this activation method in manufacturing of W–Cu. Sintering of W–Cu compacts from mechanical mixtures of fine elemental powders (made, for example, by co-reduction of blends of tungsten and copper oxides) results in copper bleedout from the compact and in a low sintered density.4 Sintering activators (Fe, Co, Ni) have been suggested to promote the densification of tungsten in the solid-state J. Mater. Res., Vol. 17, No. 8, Aug 2002
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sintering regime (at temperatures below 1083 °C, the melting point of copper). It has been established5 that sintering activators cause degradation of the electrical and thermal conductivity of sintered W–Cu composites. A number of factors have been identified that complicate the sintering of fine (low micron and submicron) W–Cu powders, namely: surface oxygen, dissolved oxygen, lack of solubility, and localized densification. The high surface area of fine W–Cu powders causes retention of surface oxides mainly in the form of WO3,
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