Spatially Resolved Velocity Mapping of the Melt Plume During High-Pressure Gas Atomization of Liquid Metals
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TRODUCTION
GAS atomization of molten metal is an important industrial process used to produce highly spherical metal powders for a range of industrial uses. The applications for such powders are diverse and include use as a feedstock for additive manufacturing, the production of catalysts for chemical processing, and formulation of brazing pastes for joining materials. Additive manufacturing in particular is creating both a massively increased demand for metal powders and a drive for improved powder quality (see, e.g., the recent review of powders for ALM feedstock by Anderson et al.).[1] For each application, the metal powder must meet a defined specification with one of the principal metrics employed being the particle size distribution (PSD) of the powder. Precise control of the gas atomization process is desirable in order to constrain the PSD produced, maximizing the usable fraction of powder and thereby minimizing scrappage or recycling of powder that is outside of the required specification. However, as noted by Anderson and Terpstra,[2] the PSD of powders produced by gas atomization tends to be quite broad, typically spanning an order of magnitude or more,
T.D. BIGG and A.M. MULLIS are with the School of Chemical & Process Engineering, University of Leeds, Leeds LS2 9JT, UK. Contact E-mail: [email protected]. Manuscript submitted July 30, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS B
leading to re-melt rates in commercial production that may be as high as 65 pct. Consequently, even modest improvements in control of the PSD could result in significant cost saving. In the gas atomization process, the PSD of the powder produced is influenced by various physical processes. One of the most significant factors controlling the PSD is the way in which the jet or film of molten metal interacts with, and is broken up by, the gas stream during primary atomization. This in turn affects the way in which the molten metal droplets formed during primary atomization interact downstream with the gas during secondary atomization. Both primary and secondary break-ups are strongly influenced by gas and particle velocities and the resultant shear forces generated. Duke and Honnery[3] have studied the position and velocity of the liquid-gas interface at the point where a liquid sheet becomes unstable prior to break-up into ligaments and droplets, finding that the Reynolds number, Weber number, and the gas/liquid momentum ratio were key parameters. Zandian et al.[4] used the level sets method to investigate primary break-up of a liquid sheet by a high-pressure gas jet, demonstrating that both the Reynolds and Weber numbers* are key *The differential velocity between the gas and the melt appear as a linear factor in the Reynolds number and a quadratic factor in the Weber number.
parameters in determining the mode by which such break-up occurs. Similarly, Li and Fritsching[5]
demonstrated that the drag force between the fast moving gas and the slower moving droplets was a key parameter in determining secondary break-up. The break-up m
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