Gas atomization of molten metal

An experimental investigation of the gas atomization of molten metal, a technique for producing fine metal powders used in powder metallurgy processing, has been performed. In this process, a low velocity jet of molten metal is disintegrated by a coaxial, coflowing, supersonic gas jet using a close-coupled atomization nozzle. The goal of this study was to investigate the physical processes responsible for creating fine (<20μm) particles, so as to better direct future development and optimization efforts aimed at increasing yields of fine particles and improving the economic efficiency of their production. Two close-coupled nozzles, selected from the recent literature, were evaluated on the basis of their gas-flow behavior and metal atomization performance. A simple converging and a converging-diverging nozzle were included in this study to determine the importance of using proper supersonic nozzles to achieve high yields of fine particles.; Schlieren flow visualization and pitot pressure measurements were used to study the gas flow behavior of the two nozzles at overall pressure ratios between 14–55. In addition, both nozzles were used to atomize small quantities of pure tin at gas-to-metal mass flow ratios (G:M) between 0.5–2.5 and at several pressure ratios. Flow visualization revealed that the majority of fine particles were formed during secondary breakup well away from the nozzle tip. Particle size was found to decrease with increasing supersonic jet length. The longer jets produced more complete secondary breakup due to sustained lengths of high gas dynamic pressure. Particle size analysis revealed that a 40% reduction in mean diameter was achieved with a four-fold increase in jet length. It is hypothesized that minimum particle size can be achieved if the gas jet remains supersonic for the entire distance over which secondary breakup is active. This criterion for minimum particle size was found to be proportional to G:M, suggesting a physical basis for the strong influence of this ratio on particle size. No significant difference in gas flow behavior or atomization performance was observed between the two nozzles, indicating that pressure ratio, rather than nozzle design, had the dominant influence on particle size.