Title:
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Fluid flow and particle size in gas atomization for fine powders
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The production of rapidly solidified fine metal powders has
become of increasing interest in recent years as the
microstructural benefits such as refined grain size, increased
solid solubility and elimination of segregated phases has become
apparent. The cooling rate associated with solidification is the
single most important process variable affecting the
microstructure and therefore the properties of the product. Gas
atomization can produce average cooling rates of up to 10° K/sec
and is the only currently available mass production method for
fine metal powders. However the effect of process variables on
particle size are badly documented and confusing.
In order to investigate the effect of the nozzle size, geometry,
and process variables, on particle size an experimental apparatus
was constructed to carry out low temperature modelling of the
atomization process using wax. A prefilming type nozzle design
was selected for study and air was used as the atomizing gas. The
experimental apparatus permitted independent control of the gas
and liquid flowrates. Gas flow outside the nozzle was
characterized by measuring the suction created at the tip of the
nozzle, by using Schlieren photography to visualize the gas flow,
and pitot tubes to measure the Mach number.
Investigations carried out included changes in the nozzle size
and geometry, gas flow, liquid flowrate and liquid properties.
High speed photography was used to observe the process of
atomization. Detailed size analysis of the powder size produced
was carried out using a Malvern Particle Size Analyser. The
particle sizes measured were fitted to known size distributions
using a computer program, which also calculated mean diameters
and the dispersion of the particles about the mean. S.E.M. work
was also been carried out to look into the shape of the wax
particles produced to see whether they are similar to those of
metals.
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