No single flux works well for all aluminum alloys. Variables which demand different fluxes include differences on oxidation rate, melting and casting temperatures, alloy end use and type of furnace, e.g., induction, reverberatory, etc.
Probably the single most important flux function is the separation of oxides and other dross materials from metallic aluminum. Because aluminum and it's alloying elements are chemically active, oxides, nitrides, carbides and sulfides are readily formed. These oxides and other impurities are carried into the metal during the melting of ingot and scrap. Turbulent transfer, excessive stirring and ladling promote further inclusion of such dross in the cast alloy. Alloys with substantial magnesium content are especially prone to high dross production. Other strong oxide contributors include moisture on charge metal and high proportion of small scrap. Improperly adjusted furnace flames can also result in an atmosphere that will rapidly oxidize molten metal.
Since oxides of aluminum have a higher density than molten aluminum, the oxides would be expected to settle to the bottom of the melt. However, the oxides will frequently contain occluded or trapped gas, thus lowering their density so that oxides remain suspended throughout the body of the melt.
The ready formulation of oxides and other dross impurities and their difficulty of removal make a function of fluxes essential to good aluminum casting. Below is a listing of different types of fluxes.
Metallurgical coke, also known as “Met” coke, is a carbon material manufactured by the “destructive distillation” of various blends of bituminous coal. Bituminous coal is a soft, medium grade coal that contains a high percentage of volatile components. Destructive distillation is performed in “coke batteries” which are banks of large enclosed kilns. Once the kilns are loaded, they are heated to approximately 1000 C in the absence of air. During the heat cycle the volatile components of the coal are released and the solid coal goes through a partial melt and subsequent re-solidification to a non-melting carbon. Volatile components include coal tar, ammonia, and literally dozens of other “products of decomposition”. Most volatile components are reclaimed or recycled.
Gilsonite is a naturally occurring high volatile, solid hydrocarbon residue. Gilsonite is found in commercial deposits only in the Uintah Basin of northeastern Utah. This unique material is similar in appearance to coal, asphalt, or coal tar pitch. However, unlike asphalt and coal tar pitch Gilsonite is considered non-hazardous.
Gilsonite can be used as an additive in ink, paint, asphalt products, foundry sand additives, and drilling mud. It also is used as a replacement for, or in addition to, coal tar pitch as a binder in refractory materials. Gilsonite is specified based on ash content and melting point. Approximately 25 grades are available which bracket melting/softening temperatures between about 240 F to over 400 F. Ash contents vary from less than 0.5% to approximately 10%.