Functions of Flux Components in Aluminum Melt Refining
Fluxes are solid substances used in molten aluminum treatment. To refine an aluminum melt effectively, a flux should have a melting point below the refining temperature, exhibit good wettability in the molten state, and have a lower density than the aluminum melt.
Molecular chlorides such as CCl₄, SiCl₄, and AlCl₃ can be used individually as refining agents. Ionic chlorides such as LiCl, NaCl, KCl, and MgCl₂ are suitable for preparing mixed-salt fluxes. To achieve better overall performance, most industrial fluxes are formulated as mixtures of several salts.
Sodium Chloride and Potassium Chloride
NaCl and KCl are the basic components of many mixed-salt fluxes. As chemically inert salts, they help reduce the melting point of the flux mixture. When distributed over the surface of molten aluminum, they also form a protective layer that limits oxidation and gas absorption.
A eutectic mixture containing 45% NaCl and 55% KCl has a relatively low melting point of approximately 650°C, allowing it to remain liquid at normal aluminum refining temperatures.
At these temperatures, the densities of NaCl and KCl are approximately 1.55 g/cm³ and 1.50 g/cm³, respectively, which are considerably lower than the density of molten aluminum, typically 2.3–2.4 g/cm³. The flux therefore floats on the melt surface.
Molten NaCl and KCl also have good fluidity and wetting properties, allowing the flux to spread evenly over the aluminum melt. Because these alkali-metal chlorides do not react chemically with molten aluminum, their eutectic mixture is one of the most widely used base materials in aluminum-alloy flux formulations.
Fluoride Additions
Fluorides such as NaF, Na₃AlF₆ (cryolite), and CaF₂ are often added to chloride-based mixed fluxes. Their main function is to improve refining performance by adsorbing or dissolving Al₂O₃ inclusions. They can also adjust the flux melting characteristics and enhance the overall purification effect.
CaF₂ increases the surface tension of the mixed flux, causing flux droplets containing adsorbed oxides to become more spherical. This promotes separation between the flux and the molten aluminum.
Because the flux is less dense than the aluminum melt, dispersed flux droplets rise to the surface during refining and carry the adsorbed oxide inclusions with them. The liquid flux layer then remains on the melt surface, protecting the aluminum from further oxidation and gas absorption.
Adding a small quantity of fluoride to the NaCl–KCl eutectic mixture, typically 3%–5% Na₃AlF₆ or approximately 1.5% CaF₂, significantly improves its ability to capture oxide inclusions. This enhances the refining and degassing effect.
Fluoride additions also increase the interfacial tension between the flux and molten aluminum. As a result, the two phases are less likely to mix with each other, making it easier to remove the slag before casting.
Magnesium Chloride
In No. 2 flux, which is used for refining high-magnesium aluminum alloys, MgCl₂ is the principal purification component.
Chlorides used in mixed-salt fluxes generally have relatively low surface tension and good wettability, making them suitable as covering agents. NaCl and KCl are therefore commonly used to protect the surface of molten aluminum.
