Fluxing of Aluminum Alloys

Fluxes should be used when melting aluminum because this alloy rapidly forms a layer of oxide (primarily alumina) on all surfaces exposed to an oxygen-containing atmosphere. In aluminum melting, and especially in the remelting of returns or other scrap, oxide formation and nonmetallic impurities are common. Impurities appear in the form of liquid and solid inclusions that persist through melt solidification into the casting. Inclusions can originate from dirty tools, sand and other molding debris, sludge (iron-chromium-nickel intermetallic compounds commonly found in die casting alloys), metalworking lubricant residues, and the oxidation of alloying elements and/or the base metal. Oxidation accelerates as temperature increases. Fine oxide particles in molten aluminum tend to remain suspended because its density is close to that of aluminum and its high specific surface area slows both flotation and settling. Moreover, oxides that separate from the melt tend to envelop substantial amounts of usable metallic aluminum.

The term fluxing, in the broadest sense, applies to a treatment technique to the melt containing such impurities and inclusions as those mentioned above. Fluxing of the melt facilitates the agglomeration and separation of such undesirable constituents from the melt.

Fluxing is temperature dependent. The temperature must be high enough to achieve good physical separation or the desired chemical reaction. At sufficiently high temperatures, the fluidity of both the metal and the fluxing agent is likely to be very high, which provides for good contact between the two and better reactivity.

Flux Composition

The specific compounds or chemical reagents used in fluxes depend on the specific purpose of the flux. Most fluxing compounds consist of inorganic salt mixtures. The various constituents of these salts or other materials in the flux serve to:

· Form low-melting high-fluidity compounds at use temperature, as is the case with sodium chloride (NaCl)-potassium chloride (KCl) mixtures
· Decompose at use temperature to generate anions, such as nitrates, carbonates, and sulfates, capable of reacting with impurity constituents in the melt. This creates impurity metal oxides or other compounds with densities different from that of the base melt and facilitates physical separation
· Act as fillers to lower the cost per pound or to provide a matrix or carrier for active ingredients or adequately cover the melt

· Absorb or agglomerate reaction products from the fluxing action

Excellent book on aluminum melt treatment is Treatment of Liquid Aluminum-Silicon Alloys by John E. Gruzleski, Bernard Closset.