Single fluid atomization through the application of impulses to a melt

Impulse Atomization (IA) is a single fluid atomization technique that is capable of producing droplets of controlled size having a relatively tight distribution and a predictable cooling rate. The process has been successfully employed to produce a wide range of metal droplets including Pb-Sn alloys, aluminum alloys, copper alloys, low carbon steel and tool steel. Atomization characteristics determined from load cell measurements, video imaging and particle size analysis are discussed as a function of process characteristics. It is shown that atomization occurs by Raleigh instability and that only primary atomization of the stream is in effect. The rate of cooling of a moving molten droplet has been modeled and experimentally validated using this atomization technique. The droplet Nusselt number ranges nearly from 2 to 10, indicating that thermal conduction from the droplet to the gas is an important mechanism by which the droplet loses heat. Comparison of droplet microstructure of IA and gas atomized powders reveals that for the same size powder of the identical alloy, IA generates a finer microstructure or solidifies with a higher cooling rate. This is attributed to two-way thermal coupling (between gas and melt spray) in gas atomization being greater than in IA. These atomization and heat flow characteristics clearly demonstrate a number of unique features of this technique as well as its flexibility to meet different processing requirements for production and research. (C) 2002 Elsevier Science B.V. All rights reserved.