Physical and Mathematical Modeling of Water Atomization for Metal Powder Production

Water atomization (WA) of a molten metal is a well-established and cost-effective industrial approach to producing low-reactive metal powders. However, WA powders are characterized by a wide spread of particle size unsuitable for certain powder metallurgy applications. To address this limitation, this thesis examines some of the poorly understood fluid mechanics phenomena involved in WA, to develop fundamental knowledge that describes the relationships between the design and operating conditions of a water atomizer, and the particle size distribution (PSD) of the resulting powder. In the first part of the thesis, a flat fan water spray is studied by experiment and computational fluid dynamics (CFD) simulation. The disintegration of a liquid sheet into a spray is examined, an optical technique to measure a multi-range droplet size distribution (DSD) in a developing spray is presented, and an enhanced model to quantify the momentum flux of a spray where it impinges a melt stream is proposed. The model suggests that the momentum flux of a spray, and thus the atomization capacity, increases with spray pressure and decreases with the travel distance of the spray from the nozzle to the impingement point and with the spray spreading angle. In the second part of the thesis, the above findings are examined via production of a low melting point alloy (Bi-42% Sn) powder using a laboratory-scale water atomizer that was designed and built for this work. The effects of various design and operating parameters, including water pressure, melt superheat, apex angle, and spray configuration on the mass median size (d50) and standard deviation (SD) of the powder PSD are investigated, and correlations proposed. These correlations can be used to control the size features of water atomized powders produced at industrial scale, by engineering various design and operating parameters, and thus expanding the market for water atomized powders.Ph.D