Production of rapidly solidified magnesium powders by gas atomisation

Abstract

Rapidly solidified powders of magnesium were produced in a pilot plant gas atomiser. Argon gas at 1·85 MPa was used as the atomising agent in a ‘confined design’ nozzle operating vertically upwards. Oxygen was introduced into the plant at a level of 1% to cause controlled oxidation to passivate the particles. Powders were sized using dry sieving down to 32 μm and wet sieving for smaller sizes. Sauter mean diameter varied between 18·45 and 21·40 μm depending on the rate of production. The distribution of sizes was typically bimodal with the separation point of the peaks at ~34 μm. This supported the particle formation theory developed earlier which predicted that atomised metal powders consisted of two families of particles. The fine range of the particles was spherical up to ~30 μm in diameter. In the coarse range, which was virtually free of satellites, the overall round nature of the particles was preserved, but some oblong shapes were also observed. Comparison with the atomisation of aluminium shows that, despite having a lower surface tension, magnesium powders are marginally coarser than those of aluminium produced under the same conditions. This is explained in terms of the faster acceleration of magnesium droplets (by virtue of lower density) in flight during secondary breakup.

MST/915