Effect of processing variables on particle size in gas atomization of rapidly solidified aluminium powders

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Abstract
The performance of a ‘confined’ type atomizing nozzle was investigated using nitrogen, argon, and helium gases at different pressures and AA 2014 metal at varying superheats and flow rates. Helium produced the finest powders with median diameters between 14·6 and 18·5 μm. Nitrogen powders showed an intermediate size while those made in argon were the coarsest (21–37 μm). Metal temperatures above 825°C led to only a small decrease in particle size which could be explained on the basis of lowered surface tension and viscosity of the melt. At lower temperatures (775°C), possible interference from premature solidification was noted. Operation at 1·56 MPa pressure offered the optimum conditions (nitrogen). There was a wastage of gas at higher pressures (2·12 MPa) and flake formation interfered at lower pressures (1·05 MPa). Powder median diameter increased in proportion to the square root of the metal flow rate and in all cases the size distribution could be satisfactorily represented by the log–normal law. The spread of the sizes, as measured by geometric standard deviation, increased at high metal flows indicating that relatively higher proportions of large particles were present in such powders. MST/671

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