Relating Cooling Rates in Superheated Liquid During Solidification for Powder Characterization

When doing mathematical modeling to predict microstructural evolution during spray-forming processes, it is important to know how fast cooling occurs during solidification. Radiative, convective, and conductive heat losses are always present, and it is relatively straightforward to define cooling in the liquid phase. However, once solid begins to form, it is difficult to evaluate the mushy-zone cooling rates because both sensible and latent heat are removed simultaneously. Since mushy-zone cooling drives microstructural evolution, it is important to be able to relate liquid and solid cooling processes. A model is developed relating these two cooling rates as a function of melt thermophysical properties and includes the ability to predict how radiative transport is influenced by any phase change-induced variation in surface emissivity or specific heat. It is validated using containerless processing techniques developed for use during space testing using the ESA ISS-EML facility. The motivation of this paper is to bridge the gap between cooling rates observed in the liquid and rates which subsequently develop during mushy-zone solidification.