Improving gas flow in gas atomization

Abstract: Gas atomization is a cost-effective processing method to produce fine, spheroidized metal powders. During the process, high velocity jets of gas are sprayed at a molten metal stream to break it into small droplets, which will solidify and form the powder particles. Gas atomized powders usually have a wide size range (1-300 μm), and the range can be up to 500 μm for free-fall gas atomization. Close-coupled gas atomization produces a higher percentage of fine particles and is more attractive to manufacturing applications (-45 μm for MIM, 20-45 μm for SLM, 45-106 μm for EBM). However, compared to the close- coupled process, free-fall type suffers less from the problem that the splashing melt can solidify on gas nozzles. It is believed that by improving the nozzle configuration and arrangement design, the yield and powder particle fineness can be improved. Gas nozzle design is one of the key factors to control gas properties and thus the powder characteristics. Visualization techniques (shadowgraphy and Schlieren imaging) and computational fluid dynamics (CFD) were used to investigate the gas flow from the free- fall atomizer. Schlieren imaging method was used to validate the CFD model, but the results did not match. Potential causes of the discrepancy that affect the CFD model include problems with discretization, input data, boundary conditions and the selection of, and parameters in the turbulence model. A qualitative parametric study was performed using this CFD model to test different designs and study the effect of gas nozzle diameter, angle, distance between melt and gas nozzles. The results showed that a smaller diameter (e.g. decreasing from1.7 mm to 1.0 mm) resulted in the recirculation intensity(maximum pressure) decreasing from  2.4 bar to  1.9  bar and  the  gas  velocity  hitting  axis  of  the  melt nozzle increasing from 301m s-1  to 426 m s-1; a smaller angle (e.g. decreasing from 30º to 10º) resulted in a decrease in both recirculation intensity (from 3.5 bar to 2.4 bar) and gas velocity hitting the axis (from 390m s-1to 301 m s-1); a smaller distance between melt and gas nozzles (e.g. reducing 30mm to 13 mm) resulted in an increase in both recirculation intensity (from 1.9 bar to 2.4 bar) and gas velocity hitting the axis (from 254m s-1to 301 m s-1).  In addition, a smaller diameter or shorter distance made the flow decay earlier. For example, thetransition point between shock units and turbulence occurred 0.301 m downstream of the melt nozzle when the gas nozzle diameter was 1.7 mm, but after 0.182 m when the diameter was reduced to 1.0 mm. It was estimated that a smaller diameter combined with larger angle and distance (e.g. ∅1.7mm, 30º, 30mm in this study) will improve gas flow in gas atomization. It was estimated that a smaller gas nozzle diameter combined with a larger angle and distance between the gas nozzles (e.g. ∅1.7mm, 30º, 30mm in this study) will improve gas flow in gas atomization. This combination brought a less intense recirculation zone and relatively high energy in the gas to break the melt in a relatively long flow region.