A mathematical model for cooling and rapid solidification of molten metal droplets

During the spray forming process, a continuous molten metal stream is atomized by impinging high speed inert gas jets. In the generated spray cone, the resulting metal droplets are rapidly cooled by the huge temperature difference to the surrounding gas phase and thereby partly solidify. After a certain flight and residence time inside the spray cone, the droplets impinge on the substrate and form the product (deposit). The material properties of this product depend on several process parameters and especially on the thermal stale of the deposited droplets at impingement. Smaller droplets cool very fast and may impinge onto the product in a completely solidified state as solid metal powder particles. Larger droplets contain a higher amount of thermal energy and impact during the state of phase change or even still completely liquid. Therefore, describing the thermal history of metal droplets during flight in the spray cone is of great importance. In this contribution, a mathematical model is introduced to describe the cooling and solidification of individual metal droplets in the spray cone during the droplet-gas interaction in flight. By introducing this model into a standard two phase flow simulation model for the spray cone description, it is possible to calculate the transient droplet temperature and solid fraction contents of individual particles depending on overall process parameters and flight path. (C) 2000 Editions scientifiques et medicales Elsevier SAS.