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Understanding aluminum degassing.



To meet the demands of today's aluminum casting market, foundries have focused on improving molten metal quality by developing processes to produce clean metal. These processes focus on eliminating a variety of impurities (i.e. inclusions) that pose serious problems in the production of quality castings. At the forefront of these impurities is hydrogen, the only gas soluble in molten aluminum, because it creates porosity in castings.

This article discusses hydrogen absorption during aluminum melting and casting and assesses methods to evaluate and remove hydrogen.

Role of Solidification

The solubility solubility

Degree to which a substance dissolves in a solvent to make a solution (usually expressed as grams of solute per litre of solvent). Solubility of one fluid (liquid or gas) in another may be complete (totally miscible; e.g.
 of hydrogen in molten aluminum increases with temperature (Fig. 1). A degree of 'alloy specificity' also exists--at a given molten metal temperature, less hydrogen is soluble in 319 alloy than in 356 alloy, for example. However, both exhibit substantially reduced solubility in the solid state.

Solidifying metal must reject the hydrogen or resultant castings will suffer from porosity. Here, solidification plays a role. For a given hydrogen concentration in the liquid state, the faster the solidification rate, the fewer problems with resultant porosity. Because not all sections of the casting solidify at the same rate, ample opportunity exists for hydrogen buildup in the remaining liquid metal during solidification. If the hydrogen partial pressure in this remaining liquid exceeds a critical level, a gas molecule forms from the association of hydrogen atoms, creating porosity.

Shrinkage Porosity vs. Gas Porosity Abstract
Determining the true porosity of a gas filled formation has always been a problem. While gas is a hydrocarbon, similar to oil, the physical properties of the fluids are very different, making it very hard to correctly quantify the total amount of gas in a formation.
 

Not all porosity can be blamed on hydrogen. Figure 2 portrays a classical distinction between shrinkage porosity and gas porosity. Shrinkage porosity occurs as the metal solidifies; a 6% volume change occurs when an aluminum alloy transforms from a liquid to a solid. This change is accommodated in normal casting practice through the use of risers. Shrinkage porosity indicates a lack of feed metal reaching the place where it is needed--the last portion of the casting to solidify. Consequently, the resultant porosity follows the solidification path and the dendrite dendrite: see nervous system; synapse.  boundaries, and shrinkage porosity displays a ragged, irregular edge.

By contrast, gas porosity shows a more distinctive regular contour, often round. Casting porosity is often a combination of the two. Porosity also may be due to a reaction from molding materials and coatings or simply air entrapment entrapment, in law, the instigation of a crime in the attempt to obtain cause for a criminal prosecution. Situations in which a government operative merely provides the occasion for the commission of a criminal act (e.g.  due to inadequate venting.

Identifying Hydrogen Presence

Hydrogen enters molten aluminum easily. Major sources include absorption from the atmosphere, products of fossil fuel fossil fuel: see energy, sources of; fuel.
fossil fuel

Any of a class of materials of biologic origin occurring within the Earth's crust that can be used as a source of energy. Fossil fuels include coal, petroleum, and natural gas.
 decomposition (hydrocarbons), adherent adherent /ad·her·ent/ (-ent) sticking or holding fast, or having such qualities.  condensation on dirty tools, chemical flux materials and alloy additives. Hydrogen absorption also increases with higher ambient relative humidity relative humidity
n.
The ratio of the amount of water vapor in the air at a specific temperature to the maximum amount that the air could hold at that temperature, expressed as a percentage.
.

Assessing the level of hydrogen in the melt can be done in several ways. Hydrogen determination methods fall into two categories--real time and laboratory. In real time, the main method employed is closed-loop recirculation Noun 1. recirculation - circulation again
circulation - the spread or transmission of something (as news or money) to a wider group or area
. In this technique, a probe introduces a small amount of carrier gas into contact with the molten aluminum. Hydrogen in the melt diffuses into the carrier gas and is allowed to equilibrate e·quil·i·brate  
v. e·quil·i·brat·ed, e·quil·i·brat·ing, e·quil·i·brates

v.intr.
To be in or bring about equilibrium.

v.tr.
To maintain in or bring into equilibrium.
.

The resultant increased partial pressure is related to the concentration of hydrogen in the melt. Through computerized data analysis, results are reported in actual hydrogen content expressed as ml [H.sub.2]/100g Al.

Laboratory techniques Laboratory techniques are the sum of procedures used on natural sciences such as chemistry, biology, physics in order to conduct an experiment, all of them follow scientific method; while some of them involves the use of complex laboratory equipment from laboratory glassware to  are more time-consuming and use solid samples from a chill mold. The sample is machined and subjected to a vacuum sub-fusion, nitrogen carrier fusion (LECO) or vacuum fusion Vacuum fusion is an analytical chemistry technique, used for determining the oxygen, hydrogen, and sometimes nitrogen content of metals. While ineffective when used on alkali or earth metals, vacuum fusion remains a viable means when applied to almost all other metals.  analysis in the laboratory.

Actual hydrogen concentration is calculated from hydrogen partial pressure and thermal conductivity measured by these methods. Such laboratory methods are useful in process control studies but cannot be utilized as decision points in real time.

One other measure is available to foundry personnel on the shop floor-- the Reduced Pressure In thermodynamics, the reduced pressure of a fluid is defined as its actual pressure divided by its critical pressure.

 Test (RPT RPT - Unify. Report Writer Language. ) (Fig. 3). RPT can be conducted in 5 min and makes two types of determinations. The density of the sample can be determined using Archimedes principle and then compared to the theoretical density (a specific property of a given alloy composition). The sample also can be sectioned and compared with a standard chart of specimens generated from a range of test conditions/results to give a visual comparison rating.

Decisions then can be made to further treat the melt or to proceed with casting based on the quality level desired. However, the RPT result does not give a quantitative hydrogen concentration result--it is a semi-quantitative test that reveals the overall metal cleanliness of the melt sample. Inclusions present will nucleate nu·cle·ate
adj.
Nucleated.

v.
1. To form into a nucleus.

2. To serve or act as a nucleus for.

3. To provide a nucleus for.

n.
A salt of a nucleic acid.
 easier pore formation during the reduction in pressure.

Degassing degassing
(dēgas´ing),
adj related to degasification, the process by which dissolved gas is removed from water or other liquid solutions.
 Methodologies

Several methods exist to reduce hydrogen. As seen in Fig. 1, allowing the temperature to drop permits for natural outgassing--the first and simplest method to remove hydrogen.

Historically, a number of other techniques that rely on an inert, insoluble purge or collector gas have been employed. The inert gas inert gas or noble gas, any of the elements in Group 18 of the periodic table. In order of increasing atomic number they are: helium, neon, argon, krypton, xenon, and radon.  collects the soluble hydrogen atoms, allowing a hydrogen molecule to form inside the lower pressure of the collector gas bubble.

Tablet products were among the first techniques to have been widely utilized (especially in smaller melt furnaces) to degas Degas
To release and vent gases. New building materials often give off gases and odors and the air should be well circulated to remove them.

Mentioned in: Multiple Chemical Sensitivity
 the melt. The tablets usually are based on the decomposition of hexachloroethane hexachloroethane

an anthelmintic used in the treatment of fascioliasis. Poisoning manifested by narcosis, staggering and falling. Fatal cases in cattle have abomasitis and hepatic necrosis.
 ([C.sub.2][Cl.sub.6]).

In the melt, the tablet forms aluminum chloride Noun 1. aluminum chloride - a chloride used as a wood preservative or catalyst
aluminium chloride

chloride - any compound containing a chlorine atom
, an insoluble metastable met·a·sta·ble  
adj.
Of, relating to, or being an unstable and transient but relatively long-lived state of a chemical or physical system, as of a supersaturated solution or an excited atom.
 gaseous gas·e·ous
adj.
1. Of, relating to, or existing as a gas.

2. Full of or containing gas; gassy.
 phase. The aluminum chloride gas bubble serves as a collector and allows the hydrogen to be absorbed onto the bubble surface and into the bubble itself. The bubble then rises to the surface, delivering the hydrogen to the atmosphere.

Static lances, wands or flux tubes also have been used to degas the melt.

The difficulty with these methods is that the resulting gas bubble is large with a low surface area-to-volume ratio. Consequently, the gas bubbles rise rapidly to the melt surface with minimal reaction time to collect the hydrogen. Further, unless the lance is moved vigorously around the melt furnace--which is often difficult--little coverage or mixing of the entire furnace volume results, reducing the efficiency of this degassing process. Porous refractories also have been used to insert inert gas into aluminum melts, and while the bubble size is small, very little mixing of the gas bubbles and metal takes place.

Rotary Impeller Degassing

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 Noun 1. rotating shaft - a revolving rod that transmits power or motion
shaft

camshaft - has cams attached to it

crankshaft - a rotating shaft driven by (or driving) a crank
 and impeller, or rotor (Fig. 4). 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.

Figure 5 compares the relative degassing efficiency of the three techniques discussed--lance, porous plug refractory and rotating shaft/rotor.

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;

* 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.

Adding Flux Injection

Another option for the rotary impeller concept is the introduction of flux injection (Fig. 6). Combining both flux injection and rotor dispersion creates a one plus one equals two-plus benefit.

Successful flux injection requires a properly constituted flux with a morphology that is granular and flows, and that melts only when it finally enters the melt. A carrier gas delivers the flux, and it is largely the carrier gas itself (usually nitrogen) that does the degassing. The rotary degasser provides the kinetic mixing between the flux, carrier gas and the metal to create full-vessel reaction.

The flux itself serves two key functions. Properly constituted, the flux can assist not only with hydrogen reduction, but also with partial removal of inclusions from the melt by virtue of the flotation action of the collector gas that is the flux carrier. Proper flux chemistry affects de-wetting for easier separation of solid inclusions from the melt through surface energy effects. Flux application by submerged injection also has the following advantages:

* a more controlled flux consumption/utilization

* less spillage and waste;

* better environmental compliance.

With or without flux injection, any degassing process creates dross. This dross is metal-rich, containing up to 85% aluminum that can be recovered. An appropriate flux composition can treat this dross in its original place, not only reducing the dross volume but also substantially decreasing the metallic content of the dross from 85% to 30% or less.

With any degassing process chosen, the specific purge gas used also affects the resulting dross.

Choosing a Purge Gas

There are discernible differences in the kind of purge or collector gas utilized in a degassing process.

Nitrogen--Nitrogen gas is the most commonly employed and is the least expensive. Nitrogen also creates a 'wet' dross, one that is rich in metallic aluminum.

Argon--Argon, while significantly more expensive than nitrogen, produces a less metal-rich dross. Argon argon (är`gŏn) [Gr.,=inert], gaseous chemical element; symbol Ar; at. no. 18; at. wt. 39.948; m.p. −189.2°C;; b.p. −185.7°C;; density 1.784 grams per liter at STP; valence 0.  is more inert, and being heavier, it provides a protective cover over the melt during degassing, precluding further oxidation and hydrogen absorption. It also is easier to keep the argon supply drier and cleaner as fittings and hoses deteriorate.

Active halogens--Active halogens See Chlorine  also have been used to assist the inert (nitrogen, argon) gas in achieving greater degassing efficiency. In the past, these have included freon ([CCl.sub.2][F.sub.2]), chlorine and sulfur hexafluoride Noun 1. sulfur hexafluoride - a colorless gas that is soluble in alcohol and ether; a powerful greenhouse gas widely used in the electrical utility industry
sulphur hexafluoride

fluoride - a salt of hydrofluoric acid
 ([SF.sub.6]). However, with a small purge gas bubble--as produced by an efficiently operated rotary degasser--the effect of an active halogen halogen (hăl`əjĕn) [Gr.,=salt-bearing], any of the chemically active elements found in Group 17 of the periodic table; the name applies especially to fluorine (symbol F), chlorine (Cl), bromine (Br), and iodine (I).  on hydrogen reduction may be minimal.

While selecting a gas to use is important, it is not the primary factor in degassing success. In addition to obvious environmental concerns with halogen gases, the specific degassing process (lance, rotor, etc.) affects the degassing performance more than which gas is used.

For a free copy of this article circle No. 344 on the Reader Action Card.

For More In formation

"Measurement and Removal of Hydrogen in Aluminum Alloys," MM. Makhlouf, L. Wang and D. Apelian, AFS A distributed file system for large, widely dispersed Unix and Windows networks from Transarc Corporation, now part of IBM. It is noted for its ease of administration and expandability and stems from Carnegie-Mellon's Andrew File System.

AFS - Andrew File System
 Special Report (1998).

"The Treatment of Liquid Aluminum-Silicon Alloys," J.E. Gruzleski and B.M. Closset (1990). Available from the AFS Library at 800/537-4237.

"RPT Gauges Aluminum Porosity," W. Rasmussen and C. E. Eckart, MODERN CASTING, March 1992.

About the Author

David Neff holds a Ph.D. in Metaullurgy from Case Western Reserve Univ. and has worked in the nonferrous metals field for 32 yr. He has been at Metaullics Systems for 18 yr and is currently the Manager, Molten Metal Treatment.
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Evgeni Bobilov (Member): Understanding aluminum degassing. 12/4/2008 6:27 AM
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Comment:Understanding aluminum degassing.(Statistical Data Included)
Author:Neff, David V.
Publication:Modern Casting
Article Type:Statistical Data Included
Geographic Code:1USA
Date:May 1, 2002
Words:1819
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