Effect of Mechanical Alloying and Equal Channel Angular Extrusion Consolidation on the Structure and Properties of Tungsten Carbide-Cobalt Alloy

Tungsten carbide-cobalt (WC-Co) materials are widely used today in the area of heavy duty machining (tool bits), in cold-forming dies and in the oil field industry (drill bits). Limited ductility is exhibited by conventionally produced WC-Co parts because of the presence of a high volume fraction of coarse WC particles in a soft cobalt matrix. Recent investigations have indicated that nanostructured WC-Co parts can attain up to twice the hardness values of conventional parts with coarse microstructure. With this background in view, the authors conducted an investigation to produce nanostructured WC-Co powder using mechanical alloying in an attritor and consolidate the powder through equal channel angular extrusion (ECAE) rather than conventional powder metallurgy technique, into bulk billets. An experimental plan based on 2³ factorial experimental design was used to determine the effect of mechanical alloying variables, namely, milling time, milling speed and ball-to-powder ratio on the microhardness of billets produced using ECAE. The significance of these variables was examined with respect to the average microhardness of annealed WC-Co samples. The combination of the variables that was most beneficial to the formation of desired microstructure (nanocrystalline) and good hardness was then established. X-ray diffraction and scanning electron microscope (SEM) analysis were performed to determine the composition and grain size to give a better insight into the microstructure developed under different combination of the variables. It is hoped that the results of this investigation will help to develop the technology to produce superior quality WC-Co parts such as inserts, seats and dies required by the oil tool industry.