Aluminium (Al) and its alloys are susceptible to a unique form of chemical corrosion known as hydrogen-induced cracking, which is caused by 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, 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).
The melt in molten aluminium processing may absorb hydrogen from the atmosphere and decompose the water that collects or condenses on tools such as the transfer ladle or crucible furnace. Should the melt absorb excessive amounts of hydrogen, it may affect the end-product porosity and contribute towards increased amounts of scrappage. Eliminating gas absorption from molten aluminium is unfeasible, so metallurgists focus on removing hydrogen from the melt before casting.
Explaining Molten Aluminium Degassing
Molten aluminium degassing 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 molten aluminium hydrogen removal compared to flux degassing.
Sialon for Molten Aluminium Degassing
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.
Rotary degassers engineered using sialon ceramics are uniquely suited to withstand the demanding thermochemical conditions of molten aluminium processing. Syalon 101 is an advanced engineering material that is non-reactive to molten aluminium over extensive periods of exposure (<1000 hours). It has found wide-ranging application in non-ferrous metal handling and forming and exhibits good potential for use in essential degassing procedures.
Sialon rotary degassers can successfully achieve equilibrium of molten aluminium in a ladle prior to forming, improving casting yields and reducing unnecessary costs associated with scrappage.
Molten Aluminium Processing with International Syalons
International Syalons is the UK’s leading authority on sialon technical ceramics for industrial applications. We have developed an innovative range of technical materials for molten aluminium handling, including Syalon 101.
If you would like any more information about using sialons, read our previous blog posts: