Modelling of Metal Powder Production Using a Wax Atomiser

Abstract

The gas atomisation process for the production of fine metal powders was modelled using a laboratory atomiser in which wax was atomised under high air pressure. A ‘confined design’ atomising nozzle was used in a vertically upward direction. The wax particles obtained covered a wide range of sizes and showed good sphericity. Particle sizing using a Malvern laser particle sizer indicated that size distribution obeyed the Rosin- Rammler law, rather than the log-normal law applicable to metal powders. Theoretical considerations indicate that this is caused by the comparatively high viscosity, low surface tension, and low density of wax, and that the same liquid break· up mechanism is likely to be applicable to wax and liquid metals. Under a given set of gas flow conditions, the median diameter of the powder produced increased in proportion to the square root of the flowrate of wax supplied to the atomiser. Operation at higher wax temperatures led to refinement of particle size as a result of more favourable physical properties (lower surface tension and viscosity). Gas pressure (in the range 1–2·2 MPa) had no significant effect on particle size when the mean diameters of powders made using the same amount of gas per unit weight of powder at different pressures were compared. These three effects follow closely, and hence modelJthe behaviour of liquid metals (aluminium). Quantitative prediction of aluminium powder size from the wax results, using the Lubanska equation, underestimates the median diameter by 60% , indicating that this correlation is unsuitable for extrapolation. However, present tests indicate that the median diameters of wax and aluminium powders obtained under a given set of gas flow conditions are approximately the same. This observation forms a better basis for predicting metal powder size from wax results. PM/0517