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.
Forming a liquid cover or blanket over the melt, these fluxes reduce oxidation and gas absorption during melting and holding. Such covers are frequently added tat the start of the melt to form a liquid layer to act as a scavenger bringing oxides to the surface for easy removal.
Wet dross consists of a mixture of oxides and small aluminum globules. Some droplets may be over 1/2" in diameter. These large size droplets may be as much as 60 to 80% of the dross volume. The 200 mesh or finer is the lesser volume; however, it represents an appreciable volume of metal and an attempt must be made to recover it. The wetting action of the flux promotes coalescence, bringing these fine droplets together and forming larger drops which are much easier to recover.
Most Asbury Fluxmaster drossing-off fluxes are not exothermic but rely on superior wetting action to effectively wet-out and separate non-metallic from useable metal, thus minimizing smoke and fumes recovering maximum metal yield from dross.
Many competitive drossing-off fluxes depend on an exothermic reaction to oxidize the fine -200 mesh free aluminum in the dross. Resulting heat is intended to remelt the +200 mesh metal back into the melt and free from dross. Unfortunately, such fluxes evolve considerable heat, smoke, and fumes not allowable under the air pollution controls prevalent in major industrial areas. Another problem is that the thermitic reaction, once begun, can consume excessive amounts of contained metal in dross ranging to 40% or higher of the dross' original metal content.
The principal function of degassing is to strip out absorbed hydrogen which, if allowed to remain in the metal, contributed to casting porosity. At the same time an effective degasser will entrain and sweep out oxides.
Chlorine, which has been used extensively s a degasser in the past has the faults of toxicity, corrosiveness and extreme air pollution problems particularly sensitive in areas like Los Angeles basin where chlorine use requires complex and expensive fume-recovery systems. In addition, chlorine reacts chemically with both aluminum and magnesium. Three pounds of chlorine can combine with one pound of magnesium or three-fourths of a pound of aluminum, metal that can never be recovered.
Nitrogen produces a wet dross high in metallics and is considered by many to be a mechanical type cleaner. It can also react with magnesium alloys to form aluminum and magnesium nitrides. This not only reduces magnesium and aluminum content of the melt but increased the amount of dross-forming, non-metallic impurity.
The most effective degassers depend on fully chlorine-saturated hydrocarbons which are hydrogen free. Asbury Fluxmaster degassers are products, in tablet form, containing precisely measured chlorinated hydrocarbons. They release a large number of more effective smaller chlorine bubbles over a sustained amount of time to give optimum cleaning and are free of the toxicity, noxious fumes and corrosiveness encountered with gaseous chlorine.
The importance of proper fluxing and degassing cannot be over-emphasized. Proper fluxing will improve finish and mechanical properties of the casting by removal on non-metallic inclusions. Fluidity is also improved.
Asbury Fluxmaster provides a full line of fluxes, degassers and grain refiners for normal situations. We also welcome the opportunity to custom tailor special products for specific problems.
Controlling grain formation is the key to creating high quality aluminum alloys. A fine, uniform grain structure heightens the mechanical properties of an alloy, improves feeding, reduces shrinkage, defects, and provides more homogeneous castings.
Tizox®, from Asbury Fluxmaster, combines two highly effective grain refiners, titanium and boron, with an active fluxing medium n convenient one-pound or four-ounce tablets. The tablets are manufactured under strict SPC standards to ensure high recovery, fast dissolution, and excellent nucleation.
Tizox is available with titanium contents ranging from 16% to 95% and boron contents from 0% to 4%.