The Composition and Role of Aluminium Alloy Flux

In the normal composition in the Aluminium alloy flux, the commonly recipe including fluorides and chlorides, rare earth compounds should be used as preferred additives, including rare earth halides, rare earth carbonates or other rare earth compounds. In addition, K2CO3, Na2CO3, CaCO3, NaF, KNO3, CaCl2, CaSO4 and graphite can be used as auxiliary additives as required.


The role of each component:

(1) Fluoride: It plays a refining role, it can absorb and dissolve Al2O3, and it also can properly control the melting point of mixed salts to increase the purification effect. As a surfactant, alkali metal fluoride salt can reduce the surface tension between the flux and the melt, and between the flux and the oxide inclusions. The limited solubility of alkali metal salts in oxidized inclusions ensures that the flux can penetrate the oxide film. Common fluoride salts such as Na3AlF6, CaF2 and Na2SiF6 can be added to 20% (mass fraction). CaF2 can also increase the surface tension of the mixed salt, spheroidize the molten salt of the adsorbed oxide, and easily separate from the melt.

(2) Chloride: The surface tension of the chloride in the flux is small, and it has a strong wetting effect on Al2O3, so the flux can also be used as a covering flux. Ionic chloride salts, such as KCl, MgCl2 are suitable as mixed salt fluxes. MgCl2 has good wettability with inclusions. The main purifying effect in high magnesium alloys is MgCl2. MgCl2 can reduce the melting point, the amount of flux, and the moisture absorption, but too much MgCl2 will reduce the effect of KF and NaF.
Alkali metal chloride is beneficial for the separation of oxidized inclusions and melts, and for the separation of metals and slags, but the chloride is highly hygroscopic. When ZnCl2 and C2Cl6 react with aluminum melt, AlCl3 harmful gas is generated, which will pollute the working environment and should be avoided as much as possible.

(3) Rare earth compounds: play a role in metamorphism, degassing and impurity removal. Rare earth compounds can reduce the surface tension of the molten aluminum in flux and help the flux to fully contact the gas and inclusions, thereby improving the effect of trapping gas and inclusions. The main choice is rare earth halide or rare earth carbonate. Among the rare-earth halides, fluoride is the most stable, hardly soluble in water, and can be placed in the air; while chlorides and other moisture-absorbing, are soluble in water. Rare earth halides can replace rare earth elements in aluminum. Rare earth carbonic acid compounds are also difficult to dissolve in water, can chemically react with aluminum, generate rare earth elements and CO2 to modify and purify. The rare earth can also interact with hydrogen to form stable rare earth hydrides REH2, REH3, which play a role in hydrogen fixation.

(4) K2CO3 (Na2CO3, CaCO3, NaF, KNO3): plays a role of heat insulation.

(5) Sulfate: It plays a role of heat insulation.

(6) Graphite: Insulate and remove impurities. However, at 750 ℃, the oxidation free energy of aluminum is much smaller than that of carbon, and aluminum preferentially oxidizes over graphite. Therefore, the amount of graphite added in the flux and the method of addition should be appropriate.



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