A numerical investigation of gas flow effects on high-pressure gas atomization due to melt tip geometry variation
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INTRODUCTION
High-pressure gas atomization (HPGA) can be an efficient method for producing high yields of ultrafine metal and alloy powders.[1,2] Currently, many commercial metal and alloy powders are being produced using HPGA, including high-alloy tool steels, nickel-based superalloys, and aluminum alloys.[3–9] Earlier studies[9–13] have shown evidence that both the system’s gas operating pressure and the melt feed tube tip geometry effectively control the gas dynamics within the atomization zone of the HPGA process. The design of the HPGA melt tip has been proven to be an essential feature of the process efficiency. Some melt tip geometric parameters can be altered, such as the melt tip extension length, s, and melt tip taper angle, a, shown in Figure 1. It is expected that the gas flow characteristics and melt aspiration will change with different melt tip designs. Operation of an HPGA system within the aspiration regime results in a stable atomization condition and has been associated with a fine powder production.[3–9] Under this favorable condition, the subambient melt tip base pressure permits the liquid metal to accelerate into contact with the gas stream. As was illustrated in the previous publications,[10–14] the liquid metal exiting the pour tube is seen to flow from the melt tip base center area to the tip edge along the melt tip base. Therefore, the pressure gradient from the melt tip base center to the melt tip base edge plays an essential role in driving this radial movement. J. MI, formerly Research Assistant, Department of Mechanical Engineering, Clemson University, is Research Associate, Metals and Ceramics Division, Ames Laboratory, Ames, IA 50011. R.S. FIGLIOLA, Professor and Chair, is with the Department of Mechanical Engineering, Clemson University, Clemson, SC 29634-0921. I.E. ANDERSON, Senior Metallurgist, is with the Metals and Ceramics Division, Ames Laboratory, Ames, IA 50011. Manuscript submitted April 30, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS B
Anderson and Rath[3] reported that efforts to use a fully retracted melt tip design at high operating pressures resulted in unstable atomization with subsequent melt flow freezeup. Extending the melt tip into the flow was found to stabilize the melt flow, and adding taper to the tip improved powder refinement. Meanwhile, the extended tip was found to develop a subambient pressure over its base for a wide range of operating pressures.[11] In an experimental study by Anderson and Figliola,[10] three different melt tip taper angles—63, 45, and 0 deg—were used to measure pressure at a single point on the melt tip base. Schlieren imaging documented very different flow fields. For a wide range of operating pressures, the a 5 63 deg tip failed to provide an aspirating effect, whereas the a 5 45 deg tip and the extended straight a 5 0 deg tip did. These results suggested that taper angle can control the aspiration capability of a melt feed tip. Indeed, commercial operators often point out that a straight tip extension will stabilize the atom
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