Atomization of Molten Metals Using the Coanda Effect

Abstract

This paper presents the results of an investigation into the fundamentals of the atomization of molten metals using the Coanda effect. Various visualization techniques were used to study the flow of the gas jet out of a Coanda nozzle, the aspiration of surrounding gas to the Coanda jet, and the atomization of molten metal. It has been shown that the Coanda gas jet is very sensitive to both nozzle surface and nozzle geometry. These design parameters control jet expansion and the efficiency of the conversion of nozzle velocity into jet momentum. The gas flowrate from a Coanda nozzle was found to be at least equivalent to that from a converging nozzle operating under frictionless and adiabatic conditions. The atomizing mechanism using the Coanda nozzle determined using high speed photography was found to follow the same sequence of events as in conventional gas atomization. However, owing to the large pressure gradient normal to the Coanda surface, the molten metal forms a semiconical sheet in the primary atomization stage. Powder samples of tin, Sn–15Pb, and cast iron atomized with a Coanda nozzle revealed a very fine microstructure which was characteristic of rapid solidification processing for the −67 μm powder. The powder size decreased with increasing nozzle pressure, and the powder was largely acicular in shape, indicating that the solidification times are faster than the spherodizing times for the alloys tested. PM/0402