Abstract
The fluid-flow stage of densification in two-phase sintering, with minimum contribution from intersolubility effects and change of particle shape, has been studied by selecting the insoluble tungsten carbide-copper system. Density determinations, photography of specimen shrinkage, and microscopic examination were carried out over a range of copper contents, with two carbide particle sizes, for sintering in a hydrogen atmosphere.
When the copper melts it flows into regions of high carbide density to form carbide/copper colonies. If these occupy a minor proportion of the compact, densification is limited and determined by the larger, “rigid” carbide part of the compact, but if the colonies predominate there is massive shrinkage on fluid flow. Overall densification subsequent to fluid flow is unaffected by the presence of the copper. The copper may, however, be redistributed within the compact as hydrogen in pores near the surface diffuses out and the pores shrink, drawing copper from central regions to form a dense skin.
As densification proceeds the carbide particles form a rigid inter-connected framework. On cooling, the copper contracts more than the solid framework so that, even if at high temperatures the compact is fully dense, shrinkage porosity results on solidification.
The structure after the fluid-flow stage is highly dependent on the initial processing. Mixing produces agglomerates of copper that, on melting, flow into the surrounding carbide matrix leaving behind large voids. Ball-milling, in contrast, yields a more uniform green structure and hence a more uniform compact after flow of the copper.
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