Partners and International Organizations
(English)
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A, B, BG, CZ, DK, FIN, F, D, GR, H, IRL, I, LT, N, PL, P, RO, SK, SI, E, S, CH, TR, GB
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Abstract
(English)
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Among the commercial metalworking processes for the fabrication of parts, components, or semi-finished products, powder metallurgy is the one that is expanding mostly. This growth requires the development of new products and processes, like Metal Injection Moulding or Thermal Spraying. Metal powder manufacturers have therefore to increase productivity and focus on cleanliness, sphericity, size (mean diameter below 20 microns) and microstructure (nano-structured materials) as well. Commercially, metal powders are mainly obtained by atomisation of the molten metal. Gas atomised powders are highly clean and spherical. However, the mass median particle size normally exceeds 50 microns, the particle size distribution is relatively large, and gas flow rates needed to achieve fine powders can reach 1 cubic metre per kilogram of molten metal. Within the framework of this project, an atomiser is designed for the production of nano-structured, fine-sized metal powders with narrow size distribution, and with poor gas consumption. The break-up of the metallic melt into spherical droplets is carried out, under inert gas atmosphere ensuring fast cooling, at the surface of a solid resonator vibrating at ultrasonic frequency. The melt undergoes the so-called capillary-wave atomisation. Capillary-wave atomisation is known to produce liquid fogs with droplet diameter controlled by the ultrasonic parameters and with narrow size distribution. The developed technique shall therefore produce nano-structured, spherical powders with mean diameter falling into the appropriate range. Indeed, the device hereby depicted is equipped with newly developed Hammer-type transducers powered by a multifrequency generator. Compared to conventional Langevin-type transducers, these allow a better control of melt break-up. Electrolytic zinc, magnesium alloys, as well as pure silver have been successfully atomised. The atomisation mechanism is modelled. Theoretical size distributions are assessed either by a linear stability analysis of a vibrating liquid sheet, or the maximum entropy formalism is applied to the liquid fog. Experimental results are fairly in agreement with predicted particle sizes and size distributions.
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