The aluminum casting fluxes are solid substances (usually a mixture of chloride and fluoride) used in aluminum smelter and foundries. The purpose is to reduce the oxidation of the melt, reduce the penetration of hydrogen in the atmosphere, absorb the non-metallic inclusions suspended in the melt, keep the furnace/ladle clear of the walls of the accumulated oxides, and reduce the content of aluminum remaining in the scum. To remove the dissolved hydrogen in the melt, refine the aluminum grains during the solidification process, modify the silicon inclusions in the silicon-containing alloy, and oxidize the excess magnesium. Different aluminum casting fluxes have different functions.
Aluminum Casting Fluxes
A refining flux composed of salts containing chlorine and fluorine is used to degas molten aluminum alloys. The components of the refining agent react with aluminum to form gaseous compounds (aluminum chloride, aluminum fluoride). The gas bubbles and rises in the melt. The partial pressure of hydrogen in the formed bubbles is very low, so it diffuses from the molten aluminum into the bubbles. The bubbles escape from the melt and then pass through the exhaust system to remove the gas. This process continues until the bubbling stops. Degassed flux can also be introduced by injection. In this case, the inert gas acts as a carrier for the granular flux. In addition to the degassing effect, the degassing treatment can also remove non-metallic inclusions suspended in the melt.
The deslagging flux promotes the separation of molten aluminum entrained in the slag. In addition to chlorides and fluorides, fluxes also contain oxidizing components, which react exothermically with aluminum when heated. The heat generated by the flux improves the wettability and fluidity of the trapped aluminum, and the aluminum droplets coalesce and flow down to the melt. The slag treated by the slag remover is powdery and dry. It is easy to remove from the furnace. Flux helps reduce aluminum loss, which makes it very economically effective, especially in remelting aluminum swarf.
Drossing Flux for Aluminum
The melting point of the covering flux is lower than that of aluminum. The covering flux is applied on the surface of the molten aluminum to melt and form a continuous layer on the surface of the molten aluminum to protect the liquid metal from oxidation and absorption of atmospheric hydrogen. The covering flux is composed of a mixture of NaCl and KCl, and may also contain some additives of CaCl2, CaF2 or KF. Fluoride further reduces the melting point of the flux and improves its cleaning ability. The disadvantage of fluorine-containing flux is that the flux releases harmful fumes at working temperature. Sodium-free protective layer flux is used to melt hypereutectic aluminum-silicon alloy. The hypereutectic alloy is modified by phosphorus, and the addition of phosphorus leads to the refinement of the primary silicon crystals. Sodium will reduce the purification effect of phosphorus in such alloys.
The cleaning flux removes oxides suspended in the melt. Similar to flux, the cleaner consists of a mixture of chloride, fluoride, and oxidant. The cleaner generates less heat, so its aluminum separation effect is lower. But they have a better ability to absorb oxide inclusions in the melt.