Molten aluminium (Al) is an extremely reactive substance. The molten metal is so reactive that it will readily decompose water in the atmosphere, or moisture on wet tools, releasing hydrogen into the melt. Even small volumes of water condensing on a crucible furnace or transfer ladle can affect the quality of final castings. Excess hydrogen can affect the distribution of porosity, and the total amount of shrinkage, ultimately increasing scrappage. Eliminating gas absorption from molten aluminium is unfeasible. So, metallurgists tend to focus on preventing hydrogen introduction into the melt and removing as much as possible prior to casting.
Why Degassing is Important
Aluminium and its alloys are susceptible to a unique form of chemical corrosion known as hydrogen-induced cracking, which is caused by the gradual diffusion of hydrogen (H) molecules through the metal’s crystal lattice. This creates a localised flaw within the alloy that can significantly impair both its tensile strength and ductility, reducing its fracture toughness, thus increasing the risk of surface fracture. While solid, the hydrogen solubility of aluminium is negligible. Molten aluminium, however, is an incredibly reactive substance that actively decomposes moisture to produce hydrogen (H).
Explaining Molten Aluminium Degasser
Molten aluminum degasser is an essential process in alloy casting. It is carried out through one of two methods: flux or rotary degassing. A rotary degasser comprises a motorized drive and a hollow, rotating shaft that directly injects an inert gas such as argon (Ar) or nitrogen (N2) into the aluminium melt. The combination of the shaft’s rotating motion and the purging of this inert gas causes a high volume of bubbles to form within the melt.
Hydrogen that has dissolved in the molten aluminium then diffuses into these bubbles and separates from the liquid phase. This provides a more efficient and clean method of hydrogen removal compared to flux degassing.
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Degassing molten aluminium can be problematic as it must be carried out in situ when the melt is maintained at temperatures exceeding 700°C (1292°F). This can induce chemical attack and thermal degradation of the rotor shaft and purge valves. Silicon nitride (Si3N4) degassing components have been widely used, but there are alternative refractory materials that are more cost-effective while offering almost identical oxidation resistance and high-temperature performance.
