This document discusses different types of fluxes used in aluminum casting processes. It describes fluxes as mixtures that facilitate removing impurities from molten aluminum alloys. The main types are covering fluxes to prevent oxidation, cleaning fluxes to remove oxides, drossing fluxes to promote separating trapped aluminum from dross, and degassing fluxes containing chlorine and fluorine salts to remove hydrogen by forming gas bubbles. Each flux is designed for a specific purpose based on its chemical composition and reactions with impurities in the molten alloy.

1. Table of Contents
INTRODUCTION ..............................................................................................................................................2
FLUXING OF ALUMINUM ALLOY.......................................................................................................................2
FLUX COMPOSITION....................................................................................................................................3
CHARACTERISTIC OF FLUXES ........................................................................................................................3
TYPES OF FLUXES ........................................................................................................................................4
COVERING FLUX ......................................................................................................................................4
CLEANING FLUX.......................................................................................................................................5
DROSSING FLUX ......................................................................................................................................5
DEGASSING FLUXES .................................................................................................................................5
DEGASSING TECHNIQUES FOR ALUMINUM ..................................................................................................6
ROTARY IMPELLER DEGASSING ....................................................................................................................6
SOLID DEGASSING/ FLUX DEGASSING...........................................................................................................7
GAS PURGING .............................................................................................................................................8
REFERENCES ...................................................................................................................................................8

2. INTRODUCTION
Metal casting is to be reported prehistoric event that appear .Aluminum castings have played an
integral role in the growth of the aluminum industry since its inception in the late 19th century.
The first commercial aluminum products were castings, such as cooking utensils and decorative parts,
which exploited the novelty and utility of the new metal. Those early applications rapidly expanded to
address the requirements of a wide range of engineering specifications. Alloy development and
characterization of physical and mechanical characteristics provided the basis for new product
development through the decades that followed. Casting processes were developed to extend the
capabilities of foundries in new commercial and technical applications. The technology of molten
metal processing, solidification, and property development has been advanced to assist the foundry man
with the means of economical and reliable production of parts that consistently meet specified
requirements.
Today, aluminum alloy castings are produced in hundreds of compositions by all commercial casting
processes, including green sand, dry sand, composite mold, plaster mold, investment casting, permanent
mold, counter-gravity low-pressure casting, and pressure die casting.
The wide applicability of casting processes and process variations in the production of aluminum-base
compositions necessitates a comprehensive understanding of process characteristics and capabilities.
The selection of casting method is based on the capabilities of each process relative to the design and the
specified requirements for each part. In most cases, castings can be readily produced by more than one
technique. In these cases, economics largely based on volume of production dictate the process choice.
For other examples, specific quality or engineering requirements restrict the process choice.
FLUXING OF ALUMINUM ALLOY
The term fluxing, in the broadest sense, applies to a treatment technique to the melt containing such
impurities and inclusions as those mentioned. Fluxing of the melt facilitates the agglomeration and
separation of such undesirable constituents from the melt.
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),

3. 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.
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 compound 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.
CHARACTERISTIC OF FLUXES
The compound use for fluxing in aluminum casting should employ following characteristic.
 They should form low melting high fluidity melt at working temperature.
 They should decompose to form anions to react with impurities to separate it from melt as it has
low density.
 They should have the capability to agglomerate the impurity and inclusion so these can be
removed from melt by mechanical or physical method.

4. TYPES OF FLUXES
In industry different types of fluxes are used. These fluxes are
i. Covering flux
ii. Cleaning flux
iii. Drossing flux
iv. Degassing flux
v. Sodium modifying flux
vi. Grain refining flux
COVERING FLUX
Aluminum is chemically very active. A tough film or skin of aluminum oxide forms quickly on all
freshly exposed surfaces, especially in the molten state. Scrap or ingot additions to the melt, stirring and
agitation cause aluminum oxide to be suspended in the melt. If these oxides are included in the cast
product, they may lead to defects, so they must be removed from the melt. Most oxides are of aluminum,
but alloying elements such as magnesium, iron, copper and titanium also can form their oxides.

5. CLEANING FLUX
Cleaning fluxes are necessary to remove oxides from the melt, while cover fluxes act as a barrier for the
surface of the melt against oxide formation. The same flux can generally be used for drossing flux.
Fluoride compounds in the flux increase its effectiveness and allow it to be used at lower temperatures.
However, fluorides can release harmful fumes, and as a result, for environmental reasons some
foundries prefer to use low-fluoride fluxes. Sodium-free fluxes are used in hypereutectic alloys (>12%
silicon content), since sodium can interfere with phosphorus grain refining. The fluxes available for
different temperature ranges differ primarily in their melting points.
Cleaning fluxes are designed to remove non-metallic from the melt by trapping the oxides particles as
they float out. Cleaning fluxes remove oxides suspended in the melt. Similarly to the drossing fluxes a
cleaning flux is composed of mixture of chlorides, fluorides and an oxidizing agent. These fluxes wet
the oxides for their easy removal. Cleaning fluxes generate less heat therefore their aluminum separation
effect is lower. However they possess better ability to absorb oxides inclusions from the melt.
DROSSING FLUX
Drossing fluxes promote separation of molten aluminum entrapped in the dross (sometimes up to 80%).
Besides chlorides and fluorides drossing fluxes contain oxidizing component (KNO3) reacting
exothermically with aluminum when heated. Heat generated by drossing flux improves wettability and
fluidity of the entrapped aluminum, drops of which coalescence and flow down to the melt. The dross
treated by the drossing flux is powdery and dry. It is easily removed from the furnace. Drossing fluxes
helps to reduce losses of aluminum, which makes it very economically effective particularly in
remelting aluminum scrap (chips, turnings etc.).
The salt wet and dissolve thin oxide films according to reaction
DEGASSING FLUXES
Fluxes composed of chlorine and fluorine containing salts are used for degassing molten aluminum
alloys. Degassing fluxes are commonly shaped in form of tablets. Degassing operation starts when a flux
tablet is plunged by a clean preheated perforated bell to the furnace bottom. The flux components react
with aluminum forming gaseous compounds (aluminum chloride, aluminum fluoride). The gas is
bubbling and rising through the melt. Partial pressure of hydrogen in the formed bubbles is very low
therefore it diffuses from the molten aluminum into the bubbles. The bubbles escape from the melt and
the gas is then removed by the exhausting system. The process continues until bubbling ceases.

6. Degassing flux may also be introduced by an injection method. In this case the inert gas serves as carrier
for granulated flux. Besides the degassing effect the degassing treatment allows to remove non-metallic
inclusions suspended in the melt (cleaning effect).
DEGASSING TECHNIQUES FOR ALUMINUM
A degassing treatment is commonly used in producing aluminum alloy castings. Sigworth found that
small purging bubbles are effective in removing gas, due to the large surface area for a given volume of
purging gas, and the slowed movement in the melt. However, most research work has assessed the effect
of different degassing techniques based only upon the relationship between the hydrogen content and the
degassing time. Reasonably, reducing the hydrogen content will certainly improve the mechanical
properties of the resultant aluminum alloys. The effect of the degassing technique on the quality of the
aluminum alloys includes not only controlling the hydrogen content but also the resultant quality of the
melt and its cleanliness.
During the melting of aluminum alloys, the inclusion of particles suspended in the melt can be
effectively reduced by floatation and/or sedimentation. Oxide inclusions are mostly heavier than the
base melt and so tend to fall to the bottom of the crucible. In addition, filtration can be used to remove
inclusions, significantly improving the density of the poured castings.
ROTARY IMPELLER DEGASSING
Rotary degassing works on the principle of increasing the surface area of an insert gas exposed to the
metal. The larger surface area increases the rate of transfer from metal to the inert gas. The smaller the
bubble size for a given volume of gas, the greater is the surface area.
Rotary impeller degassing, a technique borrowed from the chemical process industry that improves
mixing capability, was introduced into aluminum foundries in the mid-80s. In this technique, purge gas
is introduced to the melt through a rotating shaft and impeller, or rotor. This provides increased kinetic
mixing of the melt with the purge gas. In addition, the action of the rotor creates bubble shear, giving
rise to a broader swarm of smaller bubbles over a wider area, which increases surface-area-to-volume
ratio. These finer bubbles have a longer residence time in the metal, allowing for a higher capability of
collecting the hydrogen atoms present.
For the operating foundry person, several important variables must be considered in developing a
suitable degassing process with a rotor. The parameters that must be integrated include:
 Initial hydrogen level versus desired final hydrogen level (as determined by evaluation)
 available time for melt treatment

7.  vessel size/volume
 The relationship between rotor configuration and rpm, gas volume, surface effects (vortexing,
splash, etc.) and the time necessary and available to achieve desired degassing results.
The interplay among these variables must be determined on a case-by-case basis by the individual
foundry to achieve the optimum combination of process and equipment parameters. In general, the
optimum result achieves the necessary specifications in as short a time as possible, at the lowest cost and
without excessive turbulence.
SOLID DEGASSING/ FLUX DEGASSING
Fluxes composed of chlorine and fluorine containing salts are used for degassing molten aluminum
alloys. Degassing fluxes are commonly shaped in form of tablets. Degassing operation starts when a flux
tablet is plunged by a clean preheated perforated bell to the furnace bottom. The flux components react
with aluminum forming gaseous compounds (aluminum chloride, aluminum fluoride). The gas is
bubbling and rising through the melt. Partial pressure of hydrogen in the formed bubbles is very low
therefore it diffuses from the molten aluminum into the bubbles. The bubbles escape from the melt and
the gas is then removed by the exhausting system. The process continues until bubbling ceases.
Degassing flux may also be introduced by an injection method. In this case the inert gas serves as carrier

8. for granulated flux. Besides the degassing effect the degassing treatment allows to remove non-metallic
inclusions suspended in the melt (cleaning effect).
GAS PURGING
Gas purging is based on the difference in the partial pressures of hydrogen dissolved in the melt and
within the bubbles of the purging gas. The purging gas, usually nitrogen or argon, is introduced into the
melt by lances, nozzles, porous plugs or high—speed rotors. A bubble formed e.g. at a pore of a porous
plug has a hydrogen partial pressure of nearly zero. Hydrogen atoms dissolved in molten aluminum are
transported to the bubble by convection and via diffusion through the melt-gas boundary layer. There the
dissolved hydrogen atoms combine to gaseous hydrogen by chemical reaction. The ascending bubble
becomes larger because the metallostatic pressure decreases and hydrogen is taken up; theoretically until
the thermochemical equilibrium is reached. Normally the retention time of the gas bubbles in the melt is
in the area of seconds so that the equilibrium is mostly not reached.
Therefore the decrease of the hydrogen concentration in the melt c depends on
 the retention time t of the bubble in the melt,
 the mass-transfer coefficient j3,
 the melt volume V
 The mass-transfer area A (most important).
The mass-transfer area A is the total surface of the bubbles in the melt during gas purging. Consequently
the formation of as many and small bubbles as possible in units is essential. Furthermore the depth of the
melt is important, because the retention time of the bubbles in the melt is determining too.
REFERENCES
 www.aluminumflux.com/
 www.foseco.com/en-gb/end-markets/foundry/products-services/non-ferrous-foundry/non-ferrous-foundry-
detail/productsinfo/metal-treatment-2/fluxes-for-drossing-covering-and-cleaning-aluminium-alloys/
 www.thefreelibrary.com/Understanding+aluminum+degassing.-a086650337
 Effects of Degassing and Fluxing on the Quality of Al-7%Si and A356.2 Alloys Materials Transactions,
Vol. 46, No. 2 (2005) pp. 263 to 271 #2005 The Japan Institute of Metals
 Hand book of aluminum physical metallurgy and process volume 1