Log-Normal Melt Pulsation in Close-Coupled Gas Atomization
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Close-coupled gas atomization (CCGA) is the production technique of choice when fine (5 to 50 lm), highly spherical, metal powders are required. In principle, CCGA is straightforward, high-pressure gas jets impinging upon a molten metal stream are used to disrupt the stream, breaking it into a spray of fine droplets. Liquid metal is delivered down the central bore of the nozzle, wherein it wets the nozzle tip (a process termed pre-filming, which is itself dependent upon the gas flow conditions) and is stripped off the circumferential edge of the nozzle by the gas. However, the complex interaction between the high-velocity gas and the metal results in a turbulent, and often chaotic, flow with the result that the details of the process are far from being well understood. Consequently, early study into gas atomization focused on empirical correlations between median particle size and process parameters[1,2] such as gas pressure, gas flow rate, and melt flow rate. The most widely quoted of these empirical relationships is that due to Lubanska,[1] which correlates particle size with (1 + G) 1/2, where G is the gas:metal mass ratio. For many potential powder metallurgy (PM) applications, both a small median particle size and a narrow size distribution in the product are preferable. While considerable progress has been reported on improving the efficiency with which the impinging gas jets disrupt
ANDREW M. MULLIS, Professor, ROBERT F. COCHRANE, Senior Lecturer, and IAN N. MCCARTHY, Student, are with the Institute for Materials Research, University of Leeds, Leeds LS2-9JT, UK. Contact e-mail: [email protected] NICHOLAS J. ADKINS, Researcher, is with the IRC in Materials Processing, The University of Birmingham, Edgbaston, Birmingham B15-2TT, UK. Manuscript submitted October 24, 2012. Article published online May 29, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS B
the melt stream, and hence in reducing the median particle size,[3,4] most atomizer designs produce a distribution of powder sizes which span one order of magnitude or more.[4] As pointed out by Nasr et al.,[5] remelting of the out of specification product can add substantially to the cost and energy consumption of the overall process. A number of factors contribute to this spread in the particle size distribution. In particular, common foundry experience is that close-coupled gas atomizers undergo pulsation, that is, they experience variations in the amount of metal being instantaneously delivered to the atomization tip, with this pulsation frequency ranging from 5 to 50 Hz. This is commonly seen as a flickering of the luminosity of the atomization spray cone, and has been quantitatively studied by applying Fourier analysis to high-speed filming of the atomization process.[6] A significant scientific literature has emerged attributing these fluctuations to the transition between open- and closed-wake conditions.[6–8] Clearly, such pulsing of the melt causes commensurate variations in the instantaneous gas:metal ratio and thus presumably in the size of the resulting powde
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