Novel Cooling Rate Correlations in Molten Metal Gas Atomization

Abstract

The cooling rate in molten metal gas atomization is the key determining factor for the microstructure of metal powders. Mathematical expressions for cooling rates often include the melt droplet diameter and a pre-exponential factor describing the materials and gas properties. A new mathematical cooling rate correlation for rapidly solidified melt droplets is proposed based on heat flow considerations during gas atomization. The model approach takes process conditions such as gas-to-melt mass flow ratio and the initial gas temperature into account. The mathematical formulation was experimentally developed using secondary dendrite arm spacing method. For this purpose, a Cu-6wt pct Sn alloy was atomized with close-coupled (CCA) and free-fall atomization (FFA). A novel approach was made to predict the pre-exponential factor that allows the transferability to other materials. Our correlation for the cooling rate and the pre-exponential factor was validated by experimental data from the literature. The novel correlation type is valid for two different atomizing systems (FFA and CCA), suggesting that it may be applicable to entirely different gas atomization systems.