Technical tungsten powder grades prepared by hydrogen reduction are commercially available in average grain sizes from 0.1 µm (100nm) up to 100µm. The whole palette of grain sizes find their application in cemented carbides. The main portion of tungsten powder produced (80%) is directed into that production.
The starting powder for the powder metallurgical production of pure tungsten (ductile tungsten) and sintered tungsten alloys commonly covers grain sizes between 2and 6µm. Finer or coarse powder would be unsuitable. Fine powder has too high a sintering activity, not permitting the evaporation of trace impurities during the sintering process, while coarse powder leads to incomplete sintering under usual conditions.
Extremely coarse powder grained by classification (to separate any finer particles) exhibits excellent flow characteristics and is used in plasma spraying.
The purity of tungsten powder is of particular importance in PM manufacturing of tungsten metal, since during subsequent sintering further purification through evaporation is only possible to a certain extent. The demand for purity of tungsten powder has increased steadily during the last three decades. Considerable improvements in hydrometallurgy have led to concentrations fairly below 10 µg/g for most of the elements.
The reason for this enhance demand for powder purity originates in the fact that remaining impurities after sintering greatly affect the workability and properties of the final product.
By considering the impurity-to-tungsten ratio on the long path from ore concentrates to compact tungsten metal, one observes a constant increase in purity up to the stage of APT crystallization. At this stage, the maximum purity is more or less reached. Consequently, the purity of the tungsten powder depends mainly on the APT cleanliness. During APT processing to tungsten powder, the purity already decreases again. Sources of contamination are contacts with metallic tubes or boats in the respective furnaces. Slightly enhance, overall concentrations of the elements, such as Fe, Ni, Cr, and Co, are the consequence. This type of impurity occurs heterogeneously and represents small areas of locally high concentrations of foreign elements. If they are big enough, they might act as the origin of sintering defects.
Moreover, volatile elements or compounds present in the hydrogen atmosphere can be adsorbed by the tungsten powder during reduction. Typical examples are alkali metals.
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