Phoenix Scientific Industries Ltd

Close-coupled gas atomisation

At around the same time Nick Grant, a professor at MIT, had developed the concept of ‘close-coupled’ gas atomisation. Until then, metal powder had been produced by generating a free-falling stream of molten metal from a furnace and aiming jets of water or gas in the general direction of that stream. If inert gas was used and the melt was ‘clean‘ in terms of gas and refractory inclusions, this was a perfectly good way of making spherical metal powder suitable for HIP. This process was, however, inherently inefficient in terms of usage of the kinetic energy of the atomising fluid and produced only coarse (mainly above 50 μm) powder, while the industry wanted finer powder for a range of new applications. In close-coupled gas atomisation, the atomising gas is aimed directly at the end of the ceramic tube from which the melt emerges.

This enables the energy of the gas stream to be absorbed by the melt stream much more readily, greatly increasing fine powder yields. In some cases, the powder distribution can contain more than half of its weight less than 15 μm in diameter; and this was what the newly emerging Metal Injection Moulding industry required. If the technology was to flourish, however, costs needed to be reduced.  These requirements defined the type of atomiser that PSI decided to build. Clean melts were required as this cleanliness would translate into clean powder and therefore vacuum processing of melt was required. When not operating under vacuum the whole process needed to operate under inert gas or nitrogen and take place in a highly polished stainless steel process plant more typical of the pharmaceutical industry than that found in the fume-filled caverns of the foundry industry. To produce the fine powder required by the MIM industry, close-coupled atomising nozzles had to be developed that were both efficient and reliable.