| Summary: | 博士 === 國立成功大學 === 航空太空工程學系 === 103 === Metal powders have been widely used in industries like powder metallurgy (PM), metal injection molding (MIM), cold spray (CS), and laser additive manufacturing (LAM). Among the various fabrication techniques for metal powder production, gas atomization is able to produce fine, spherical, and high purity powders with fine microstructure. Conventional gas atomization process for metal powder production is operated at high gas pressure, typically at gas pressure over 20 bar. In this research, a new gas atomizer with internal confinement has been designed for operation at low gas pressure. Atomizing gas enters the atomizer and reaches supersonic speed at the nozzle tip by flowing through an annular convergent-divergent flow channel in the atomizer. The gas is further confined in the atomizer by the internal confinement. Observation of spray patterns and investigation of the characteristics of water spray and performance of metal powder production have been carried out to understand the characteristics of the atomizer. Impingement of droplets on the inner surface of confinement is observed at the transient process owing to the pressurization of gas from ambient pressure to the setting pressure if gas enters the atomizer later than water. On the other hand, the spray immediately reaches steady state in the process where gas enters the atomizer first. From the observation of spray patterns at steady state and the simulation of gas flow behavior, it is found that the confinement of an atomizer allows the gas to have a further expansion, which means an increase in velocity. Increasing the expansion angle of confinement will increase the width of the recirculation zone and lead to a nonuniform liquid film on the boundary of the recirculation zone. The characteristics of water spray show that the pressure at the delivery tube is always negative and decreases with increasing gas pressure and confinement length. As a result, the water mass flow rate increases as gas pressure and confinement length increase. The median droplet size reduces with gas pressure, and better atomization efficiency can be achieved by increasing the confinement length. Increasing the expansion angle of the confinement will result in an increase in the pressure at the delivery tube tip and reduce atomization efficiency. In the melt experiments, copper alloy C5212 is atomized and particle sizes are reduced from 67.29 μm to 30.13 μm for the atomizer with 20 mm confinement as gas pressure increases from 4 bar to 6.5 bar. Atomizers with longer confinement can be operated at lower gas-to-melt ratio and have higher atomization efficiency. SEM photos show that the powders are all spherical in shape. In conclusion, the atomizer developed in this study performs impressively at low gas pressure and can be applied to metal powder production.
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