Powder rheology and characterization of new and zinc recycled WC-6wt%Co powders

Four industrially manufactured WC-6wt%Co powders were studied in terms of bulk properties, morphology and powder rheology. Two powders, comprising 100 % fresh raw material, a 100 % zinc recycled powder and a powder comprising 60 wt% zinc recycled and 40 wt% new material, were investigated. The powders were produced from both laboratory and production mills utilizing a typical powder metallurgical process route, that produced spherically shaped, granulated powders in the 45-212 μm size range. Powder screen fractions of 45-63 μm, 90-125 μm and 150-180 μm were studied in detail using an FT4 powder rheometer as well as optical and scanning electron microscopy. Apparent density, flow rate tests and powder rheology revealed that dense granules behaved most consistently, with the smallest differences in flow rates, density and flow energy for differently sized particles, compared to hollow granules. New powder and 100 % zinc recycled powder of the 45-63 μm size range were exposed to 75 % relative humidity for 24 h and experienced higher cohesion, high flow energies and poor rheological behaviour compared to dry powders, independent of their recycling history. Shear and friction tests could not distinguish the powders clearly, while dynamic analysis, aeration, de-aeration, permeability and compressibility tests revealed that the powders' flow energy and response to air depended primarily on particle size and the presence of fines. Zinc recycled powders behaved the same way as new powders if their granule microstructure and particle size distribution was the same, with the formation of ideal dense granules resulting from slurries spray dried with a high yield stress.