Satellite formation mechanism in gas atomised powders

Abstract

An Al–1Li alloy along with tin (under argon and helium) and pure copper powders (under helium) were produced in a pilot plant gas atomiser. The pressure of the atomising gases used was 1·85 MPa, except that of argon gas in Sn powder production (1·56 MPa). The atomising agents were used in a confined design nozzle operating vertically upwards. The morphology, size, size distribution, and surface features of the powder particles used in the present study (i.e. Al, Al–Li, Cu, Mg, and Sn powders) were examined by scanning electron microscopy and by dry sieving in order to size the powders. It was observed that for satellite formation in gas atomised powders there is a need for both coarse and fine particles. Under turbulent atomisation conditions these two families of particles of different size and mass must interact with each other. The probability of this interaction increases not only when the amount of fine particles is high, arising from efficient secondary breakup depending on the type of atomising gas used and its pressure, but also when the shape of the coarse particles is irregular and the surface texture is rough. If this probability of interaction results in welding of finer particles to larger ones during in flight collisions then this increases the true satellite formation process. When the susceptibility of the material being atomised to oxidation is high, the oxygen potential of the atomisation environment and the atomising gas pressure are effective in controlling the shape irregularity of the coarse powders produced. The inhomogeneity of the surface oxide film thicknesses influences the surface roughness.