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Vacuum Inert Gas Atomization (VIGA) is a method of producing metal particles by letting high-speed inert gas coming from an annular nozzle impinge onto the strand of molten metal issuing from a central nozzle. The typical diameter of these nozzles is in the order of a few millimetres. Depending on the distance between the exits of the gas and melt nozzles they are called either free fall or closed coupled atomizers. In the latter case, the idea is that the melt flows along the tip of the nozzle to annular nozzle due to the effect of a recirculating gas region at the tip of the nozzle. This more or less pre-filming. The present set-up inverts the flow topology. The gas flows in a Laval nozzle, i.e. in the central part of the set-up and the melt feed is through an annular slit at or shortly after the smallest cross-section of the Laval nozzle. This is indeed pre-filming. Due to the expansion to sonic and supersonic conditions the gas has normally a temperature which is much lower than that of the melt. In the case of Argon expansion from room temperature to sonic condition the temperature is then -50°C. This is the more or less state in which argon is usually used for inert gas atomization of steel alloys. It is therefore not surprizing that a superheat of the melt is needed to avoid freezing of the melt in its nozzle. In the present case the gas is preheated so that its temperature is at least as high as the temperature of the melt when they first come into contact in the Laval nozzle. The working principle of such type of atomization has now been demonstrated using tin as a metal, since steel poses problems due to the much higher melting temperature. Although not many tests have been performed in the new set-up, the resulting mass median diameter of the particles is 10 µm. This value is much lower than that obtained in most VIGA atomizers. A discussion of the draw-backs of existing VIGA set-ups, the new experimental set-up, its conditions, the results using particle size analyser as well as REM images and the possible areas of application will be presented.
