Liquid break-up in gas atomization of fine aluminum powders
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This paper examines morphology and size distribution of aluminum powders and flash-photographs of the atomizing jet (mostly obtained during the course of the study referred to above) and relates particle size to liquid break-up in atomization. II. EXPERIMENTAL The atomization tests were carried out on the Imperial College Gas Atomizer which has been described elsewhere. [6] Aluminum alloy AA 2014 was used in all tests at 875 ~ Nitrogen, argon, and helium gases were used as the atomizing agent. The geometry and operational characteristics of the atomizing nozzles have been reported in earlier publications, tS-SJ A high-speed flash (Speediflash HS5) gun was used for photographing the jet in operation. The manufacturer's specifications were 20 J flash energy in the lamp and 12/xs effective duration to 10 pct of the peak light output. Gas flow in the atomizer had been specially designed to prevent recirculation occurring in the jet area, and this permitted a good view. 161 Powders produced in the plant were sized using a Malvern 3600 D particle sizer. The results were analyzed as described earlier tSj to obtain the Sauter mean diameter, mass median diameter, volume mean diameter, and the geometric standard deviation of the lognormal fit. For observation in the optical microscope, powder samples were mounted in bakelite or a cold setting resin and were polished to 1 p.m. For SEM studies, powder samples were mounted on a double-sided tape and were gold coated to prevent charging. III. R E S U L T S
A. Photography In confined-design nozzles, the liquid is caused to form a thin layer before it meets the gas. tSJ This filming effect was visible to the naked eye. The liquid spread radially VOLUME 20B, FEBRUARY 1989--61
into a thin film on the top of the nozzle and met the gas either at the edge or on the side of the delivery tube over a distance of up to ~ 3 mm (rundown mode), as was described earlier. 181A thin layer of metal was left in all cases at the top of the metal flow tube or on the top and the sides (rundown regime) when atomization was stopped. Photographs of the jet invariably showed that the liquid film broke up into droplets as soon as it met the high velocity gas at the tip of the nozzle. Under typical operation conditions, the largest of these droplets was ~500 /zm in diameter (Figures l(a) and (b)). In some cases, small sheets of liquid metal were observed in the jet as well as droplets. Such sheets became more frequent, especially at high metal flowrates and in poorly designed nozzles.[Tl On the other end of the range at very low flowrates (
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