INFLUENCE OF LIQUID-METAL PROPERTIES ON PARTICLE-SIZE OF INERT-GAS ATOMIZED POWDERS

Fine tin powders were produced in a pilot plant gas atomiser. Nitrogen gas at 1.56 MPa pressure was used as the atomising agent in a 'confin ed design' nozzle which operated vertically upwards. A range of metal flowrates from 0.864 to 1.425 kg min(-1) was studied at a melt tempera ture of 450 degrees C. Powders were sized using dry sieving down to 45 mu m and wet sieving for smaller sizes. The Sauter mean diameter of t he powders varied from 9.01 to 10.28 mu m, depending on the rate of pr oduction. The size distribution was bimodal (albeit not very well defi ned) with the peak separation at similar to 44 mu m. In the fine size range, particles were spherical, while those in the coarse range were more elongated or irregular in shape and free of satellites. Compariso n of the tin powders with copper powders from another study, AA 2014 a luminium alloy powders, and magnesium and zinc powders from previous w ork showed that the differences in mean diameter and standard deviatio n are small among these common metals at a given volumetric production rate. This confirms the overriding importance of liquid metal volume flowrate under fixed gas flow conditions in gas atomisation, while the actual physical properties of the liquid play a secondary role. Altho ugh surface tension is secondary to volume flowrate in importance for controlling particle size, the study has shown that a liquid metal wit h lower surface tension and viscosity than AA 2014 alloy, together wit h a higher density, yields finer particles.