Aluminum alloy fluxes are generally composed of alkali metal, alkaline earth metal chlorides and fluorides. Its main components are KCl, NaCl, NaF, CaFm, Na3AlF6, Na2SiF6, etc. The physical and chemical properties of the flux (melting point density, viscosity, volatility, hygroscopicity and the interface effect with oxides, etc.) play a decisive role in the refining effect.
Chloride salt: Chloride salt is the most common basic component in aluminum alloy fluxes, and the mixed salt of 45%NaCl+55%KCl is the most widely used, because they have a strong ability to infiltrate solid Al2O3, inclusions and oxide film (the wetting angle with Al2o3 is more than 20 degrees) and the specific gravity of NaCl and KCl at the melting temperature is only 1.55g/cm3 and 1.50g/cm3, which is significantly less than the specific gravity of aluminum melt, so it can be spread well in aluminum melt. The oxide film on the surface of the melt is broken and adsorbed, but only the flux containing the chloride salt, the broken and adsorbed process is slow, and manual stirring must be carried out to accelerate the above process.
Chloride has low surface tension and wettability. It is suitable as a covering agent. Among them, chloride salts with molecular crystals, such as CCl4, SiCl4, AlCl3, can be used as purifiers alone, while chloride salts with ionic crystals, such as LICl, NaCl, KCl, MgCl2, are suitable for mixed salt fluxes.
Fluorine salt: Adding NaF, NaAlF6, CaF2, such as a small amount of fluorine salt to the chlorine salt mixture mainly plays a refining role, like adsorption, dissolving Al2O3. Fluorine salt can also effectively remove the oxide film on the surface of the melt and improve the degassing effect, because:
1. Fluorine salts can react with aluminum melts to produce gaseous AlF, SiF4, BF3, etc., which mechanically promote the separation of the oxide film from the aluminum melt and push the oxide membrane through and push it into the melt.
2. The current generated on the interface where the above reaction occurs also causes the oxide film to be "washed" and broken, so the presence of fluorine salts causes the destruction of the oxide film on the surface of the aluminum melt to accelerate significantly, and the hydrogen in the melt can escape more easily.
3. Fluorine salts (especially CAF2) can increase the surface tension of mixed melt salts, make the molten oxides absorbed globular, facilitate separation from the melt, reduce the loss caused by solid slag sandwich aluminum. And due to the increase of melt-melt surface tension, it accelerates the process of melt adsorption inclusion.