Droplet solidification and gas-droplet thermal coupling in the atomization of a Cu-6Sn alloy

The accurate prediction of solidification cooling rates of droplets in gas atomization is critical to the production of quality powders and spray deposits. Despite many sophisticated on-line measuring instruments and numerous mathematical models, an experimentally verified analysis of droplet cooling in gas atomization is still lacking. Impulse atomization offers a unique single-fluid atomization technique with controlled gas-droplet heat transfer. This technique was, therefore, used to generate a reliable cell-spacing vs cooling-rate relationship for Cu-6Sn. Subsequently, the cell spacing was measured for gas-atomized (GA) particles that were collected with quench probes in known parts of the melt spray. Using the calibration work done on impulse atomization and the sampling positions in gas atomization, the effect of particle size, spray parameters, and location in the spray are analyzed. This analysis incorporates other on-line diagnostics in gas atomization to relate atomized droplets to GA processing conditions. A comparison between impulse atomization and gas atomization is also discussed. This work clearly demonstrates that the droplet cooling rate is more strongly affected by the gas temperature than by the droplet-gas relative velocity. In addition, the thermal Stokes number is a useful parameter to assess the amount of two-way thermal coupling in a given spray. Hence, sprays with low thermal Stokes numbers having droplets of the same size will experience lower. cooling rates and have correspondingly higher gas temperatures.