Influence of Superheat on Particle Shape and Size of Gas Atomised Copper Powders

Abstract

Fine powders of copper were produced in a pilot plant gas atomiser. Nitrogen gas at 1·85 MPa was used as the atomising agent in a ‘con1ined design’ nozzle which operated vertically upwards. A range of superheat in the liquid metal from 115 to 270°C was examined. The morphology, surface features, and inner structure of the powder particles were examined under SEM. The atomising jet was photographed during operation to record metal build-up at the tip of the nozzle. Dry and wet sieving and quantitative particle size measurement by SEM were used to size the powders. The powders became finer as the metal superheat was increased. Cylindrical and spheroidal particles as large as 600 μm with rough surfaces and no satellites were observed at low superheats, whereas dumb-bell shaped particles were seen during moderate superheat runs. High superheats yielded fine, spherical powder particles (with satellites) which exhibited a bimodal size distribution not observed at low superheats. This indicated that the secondary break-up mechanism was incomplete at low superheat atomisation due to premature solidification which caused an increase in the volume mean diameter and in the dispersion of powder sizes. Additionally, the influence of metal flowrate (from 1·86 to 3·87 kg min⁻¹) was also studied at the highest superheat used in the tests (270°C). This showed that both the median diameter and the geometric standard deviation increased as metal flowrate was increased under otherwise constant atomising conditions. PM/0536