Effect of milling technique on the dispersion, sintering and hardness of WC-Co hard metals

摘要

It is well known that the microstructure and properties of a sintered body are influenced by each of the steps in its production route. Structural homogeneity and the grain size distribution are affected primarily by the mixing-milling stage. In the manufacture of hard metal, mills are used instead of mixers to produce the necessary dispersion between the hard and metallic phases. According to Moyle, the primary goal of milling in hard metal production is to reduce the agglomerates to primary particles without creating excessive amounts of fine WC particles (<0.2μm) as a result of intergranular fracture. This statement could be reformulated as follows: "the primary goal of milling is to produce a good phase dispersion and a particle size distribution suitable for the end use of the alloy". The quality of the dispersion is an important factor in any alloy because it is related to the homogeneity of the sintered structure. As to the proper particle size distribution, this depends on the use of the alloy. Hence, the need for size reduction through milling depends on the starting WC and Co powders, and fracture of the primary particles may or may not be necessary. Industrially, milling is carried out in hall or attritor mills. The former is the most common type of mill used in powder metallurgy (PIM). The latter is considered a high energy mill, capable of mixing-milling powders in short times in relation to the usual hall milling times.