Study of physical, chemical and microstructural characteristics of metallic powders with the parameters of water atomization process.

Abstract

In P/M industry, the metal powders are the main raw material. There are many methods available for powder production, out of which WA is one of the most attractive and important processes. The knowledge about fragmentation mechanism in WA is limited. The literature mainly focuses on the description of the influence of some fundamental variables (e.g. water pressure, melt superheating, melt-to-water mass flow rate ratio, etc.) on powder properties, but a fundamental theory is lacking. To better control WA process and the powder characteristics, it is necessary a deeper understanding of the metal particle formation mechanism. The main objective of this thesis is to study experimentally and theoretically the WA of liquid metals in order to improve our present knowledge of the process. A set of experiments was conducted with different metals and alloys. Based on the results, a new model is proposed to calculate the median particle size (D50) of atomized particles. The new model predicts the particle size for different metals considering the influence of the different operational variables, geometrical parameters and physicochemical properties of the melt. Moreover, the water sprays used for atomization have been characterised. High Speed Imaging has been applied to reveal the internal structure of the water jet and Global Sizing Velocimetry (GSV) allowed quantifying the velocity and size of the water droplets. This information was not available before in the literature for the high pressure range typically used in WA. Those data were related with the particle size and shape of the powders. In particular, new concepts to explain the particle shape of water atomised powders (e.g. the increase of the irregularity with the particle size) are proposed. Finally, the instability and breakup mechanism of a free fall liquid metal jet before reaching the atomisation zone were experimentally studied with the help of High Speed Imaging.