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survive the hostile conditions. However, care must be exercised during handling. Preheating and/or slow immersion into the molten metal are required. Impellers can become plugged with dross, sludge, rocks, and steel contaminants. Not only is pump performance reduced but shaft breakage can also occur.

Fluxing salts will generally attack graphite, silicon carbide, and oxide ceramics (alumina, silica), causing a buildup of corrosion products. At the melt surface, residual fluxes and moisture present in the air will combine with oxides to form a tenacious corrosion product that could alter pump performance and limit pump life. Consequently, a preventive maintenance schedule of periodic hot cleaning of the molten metal pump is usually prescribed by the manufacturer to remove buildups from the intake, discharge, and melt line on the pump.

References cited in this section

25. D.V. Neff, Molten Metal Pumping Systems--Current Applications and Benefits, in Light Metals, The MetallurgicalSociety, 1987, p 805

26. D.V. Neff, Molten Metal Pumping in Foundry and Foundry Ingot Applications, Trans. AFS, 1987, p 27327. P.J. Bamji and F.W. Pierson, Electromagnetic Circulation of Molten Aluminum, J. Met., 1983, p 4428. D.V. Neff, Molten Metal Pumping and Pipeline Transport in Aluminum Casthouses, in Proceedings of Energy

Workshop X, Aluminum Association, 198729. T.L. Cunard, Application of Graphite Pump to Direct Charged Aluminum Melting Furnaces, in Proceedings of

Energy Workshop X, Aluminum Association, 1987

Degassing of Aluminum Alloys

Aluminum and its alloys are very susceptible to hydrogen absorption in the molten state. Solubility is much lower in the solid state (Fig. 16). Hydrogen is the only gas that is appreciably soluble in molten aluminum. Because of the affinity of the metal for oxygen, the principal source of hydrogen absorption will be the reduction of water vapor from the atmosphere in contact with the melt:

3H2O + 2Al → 6H + Al2O3

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kheyri

Cross-Out

Fig. 16 Solubility of hydrogen in aluminum. Source: Ref 30.

The solubility of hydrogen gas increases exponentially as the temperature is raised. Therefore, it is imperative that excessive temperatures be avoided during melting to minimize gas absorption, especially if a standard degassing treatment is used based on a given initial expected gas content (Ref 6).

Alloy Sensitivity to Hydrogen Absorption

Various aluminum alloys have different sensitivities to hydrogen absorption and ensuing gas porosity if removal is not accomplished. The major alloying elements in aluminum foundry alloys are silicon, copper, and magnesium. Increasing silicon and copper contents decrease hydrogen solubility, although the effect is greater for copper (Ref 31). Magnesium, on the other hand, increases hydrogen solubility, and recent studies have shown that lithium behaves similarly (Ref 32).

Theory of Hydrogen Removal

Various thermodynamic and kinetic factors govern the removal of hydrogen from molten aluminum, and these factors have been extensively studied. Gas removal is easier or quicker at lower temperatures and higher concentrations, following the thermodynamic solubility changes with temperature. Kinetic factors, however, determine whether the hydrogen concentration will follow thermodynamic solubility and how close the concentration will come to equilibrium.

Hydrogen is removed by (Ref 33):

• Hydrogen transport in the melt to the vicinity of an inert gas bubble by a combination of convection and diffusion• Diffusive transport through a thin, stagnant layer of fluid, called a boundary layer, surrounding the bubble• The chemical adsorption onto and subsequent desorption from the bubble surface


• Diffusion of hydrogen as a gas inside the bubble of purge gas• Escape of hydrogen from the melt surface or at refractory vessel walls

Consequently, hydrogen removal is dependent on mass transfer coefficients. Two fundamental results of theoretical investigations that have been borne out by actual practice are that:

• Smaller purge gas bubbles produce more efficient degassing results• Chlorine usually improves degassing efficiency when medium or large purge gas bubbles (>6 mm, or 0.25 in.) are

present because the mass transfer coefficient is increased

A full discussion of the scientific aspects is beyond the scope of this article, but additional information on hydrogen removal is available in Ref 33, 34, 35, and 36.

Sources of Hydrogen in Aluminum

There are many potential sources of hydrogen in aluminum, including the furnace atmosphere, charge materials, fluxes, external components, and reactions between the molten metal and the mold.

Furnace Atmosphere. The fuel-fired furnaces sometimes used in melting can generate free hydrogen because of the incomplete combustion of fuel oil or natural gas.

Charge Materials. Ingot, scrap, and foundry returns may contain oxides, corrosion products, sand or other molding debris, and metalworking lubricants. All these contaminants are potential sources of hydrogen through the reduction of organic compounds or through the decomposition of water vapor from contained moisture.

Fluxes. Most salt fluxes used in aluminum melt treatment are hygroscopic. Damp fluxes can therefore result in hydrogen pickup in the melt from the decomposition of water.

External Components. Furnace tools such as rakes, puddlers, skimmers, and shovels can deliver hydrogen to the melt if they are not kept clean. Oxides and flux residues on such tools are particularly insidious sources of contamination because they will absorb moisture directly from the atmosphere. Furnace refractories, troughs and launders, mortars and cements, sampling ladles, hand ladles, and pouring ladles also are potential sources of hydrogen, especially if refractories are not fully cured.

Metal/Mold Reactions. If metal flow is excessively turbulent during the pouring process, air can be aspirated into the mold. If the air cannot be expelled before the start of solidification, hydrogen pickup can result. Improper gating can also cause turbulence and suctioning. Excessive moisture in green sand molds can provide a source of hydrogen as water turns to steam.

Effects of Hydrogen

When the dissolved hydrogen in molten aluminum cannot be thoroughly expelled from the casting product as it solidifies, porosity defects will result. Porosity can be very fine, widely dispersed, or localized in those areas of the casting that are the last to solidify if the entrapped gas concentration is relatively low. Hydrogen microporosity may not be particularly harmful except when pressure tightness is required. On the other hand, some gas porosity defects can be fairly large, and they appear as blowholes and cracks.

It is important to differentiate between true gas porosity and shrinkage porosity. The former is due to the presence of trapped gas, and such defects are usually rounder and shinier upon metallographic examination. The latter is due simply to inadequate feed metal during final solidification. Shrinkage porosity is usually more irregular in shape than gas porosity (Ref 37).

Porosity defects result in a loss of mechanical properties in aluminum alloys. Figure 17 shows the effect of hydrogen porosity on the mechanical properties of alloy 356.

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Fig. 17 Influence of gas content on the tensile and yield strengths of aluminum alloy 356. Source: Ref 38.

A certain amount of gas retained in aluminum during solidification can be beneficial. Permanent mold castings often contain some unremoved or unexpelled hydrogen or sometimes gas is even added. This retained gas offsets gross solidification and aids in complete mold filling and does not detract from surface characteristics or minimum mechanical properties required for most commercial castings. The more rapid chilling and solidification rate of permanent mold casting versus sand casting facilitates the acceptance of hydrogen; the faster solidification rate usually results in finer grain size and enhanced mechanical properties. The same is true for die casting, in which extremely rapid solidification under pressure greatly reduces the tendency toward hydrogen porosity.

Testing for Hydrogen The Straube-Pfeiffer test (reduced-pressure test) for the measurement of hydrogen is a qualitative method that has been commonly used. A small sample of melt is placed in a crucible in a chamber, which is then evacuated to a specified pressure and held until the metal solidifies. Bubble evolution during solidification and the appearance of the sample surface are indicators of the amount of hydrogen present. A mushroom head indicates a gassy sample, while a recessed surface would indicate a relatively gas-free sample. The solidified sample is often sectioned and the evident porosity compared with a series of standards (Fig. 18). It is important to specify the reduced pressure used for repeated tests in order to make valid comparisons. Figure 19 shows the effect of different pressures on the observed response for samples containing the same amount of gas. These standards can be correlated with analytically determined specific hydrogen results or with individual foundry and casting acceptance criteria. If such standards are used, they must be evaluated in terms of each individual alloy, casting, and foundry experience and practice.


Fig. 18 Standards for comparison with reduced-pressure test specimen used to check the amount of hydrogen gas present in aluminum alloys. Such standards must be developed for a given alloy and set of test conditions. Courtesy of Wabash Alloys.


Fig. 19 Effect of pressure on the appearance of aluminum alloy reduced-pressure test samples obtained from the same melt. (a) Pressure: 100 kPa (760 torr). (b) 12 kPa (90 torr). (c) 6 kPa (45 torr). (d) 3 kPa (22.5 torr). Courtesy of Wabash Alloys.

In the Straube-Pfeiffer test the sample behavior is influenced by general melt cleanliness as well as by hydrogen. Inclusions will nucleate hydrogen gas bubble formation during solidification; therefore, the test does not provide an absolute measurement or effect of dissolved hydrogen alone.

The Straube-Pfeiffer test is also used to determine the sample density D under specific test conditions and to correlate the result with the known full density of the alloy (Ref 39). Hydrogen gas content can then be determined from the difference in densities plus a correction for standard temperature and pressure (STP) conditions, because the gas forms in the sample during testing at a reduced pressure and at the freezing temperature of the alloy. The calculation is:

of metal + weight of gasVolume of netal + volume of gasWeightD = (Eq 5)

The weight of the gas can be neglected. Therefore:

1 Volume of netal + volume of gas of metalD Weight

= (Eq 6)

The first term on the right-hand side of Eq 5 is simply the known theoretical density D0, of the alloy in question. The measured volume of gas Vm is therefore given by:


0

1 1mV

D D= − (Eq 7)

where Vm is in milliliters per 100 g of metal.

The measured volume of gas Vm must then be corrected to the volume of gas at standard conditions VSTP using Boyle's and Charles's laws:

pressure 273 .760 273 alloy freezing temperatureSTP m

TestV V = + (Eq 8)

where test pressure is in millimeters of mercury (torr) and alloy freezing temperature is in degrees Celsius. Finally:

0

1 1STPV k

D D

= −

(Eq 9)

where K is a constant and VSTP is in milliliters per 100 g of metal.

For each alloy and set of test conditions, a different K must be calculated. For a given alloy, however, it is possible to develop a relationship between density and actual hydrogen content. Densities of samples with varying gas contents can be measured by the Archimedes principle of weighing in air and water:

a

a w

WDW W

=−

(Eq 10)

where Wa and Ww are the weights in air and water, respectively.

Nomographs can then be developed from derived results such that the hydrogen content can be determined directly from weight determination of the reduced-pressure test specimens rather than by mathematical manipulation. Figure 20 shows a nomograph developed for aluminum alloy 1100 that relates observed first bubble pressure with hydrogen content.


Fig. 20 Nomograph correlating reduced-pressure reading at appearance of first bubble with actual hydrogen content of aluminum alloy 1100. Melt temperature, 730 °C (1350 °F); gas flow rate, 6 L/min (0.2 scfm) Source: Ref 40.

Quantitative Reduced-Pressure Technique (Ref 41). The standard reduced-pressure test gives only a semiquantitative measure of hydrogen content, which is usually then visually interpreted and correlated with comparison samples exhibiting varying degrees of porosity. Inclusions nucleate porosity effects; therefore, the reduced-pressure test does result in the determination of true hydrogen content.

An instrument is available that measures hydrogen directly. A constant mass (100 g, or 0.22 lb, of metal) is placed in the chamber, and the pressure is rapidly reduced to a specific predetermined value. The sample is then allowed to solidify under vacuum in isolation from the pump.

As hydrogen gas is evolved during solidification, the gas partial pressure is measured by a calibrated Pirani gage whose output is converted continuously to a digital display of hydrogen content. The Pirani gage operates on the principle that at low pressure the thermal conductivity of any gas varies with pressure.

A test can be completed in about 5 min, and results with several foundry casting alloys have been shown to be accurate within 5%. However, high hydrogen readings have been obtained for wrought aluminum alloys, possibly because of the fundamental differences in oxide structure and hydrogen retention or occlusion capabilities of wrought alloy compositions.

The Telegas instrument (Ref 42) is a shop floor instrument for hydrogen determination that has been extensively used for many years (Fig. 21). Its primary application is in the wrought industry or for research purposes. The unit operates with a nitrogen carrier gas, which dissolves hydrogen from the melt. The nitrogen is bubbled through the melt for

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a sufficiently long period of time such that equilibrium is achieved with respect to hydrogen dissolved in the melt versus hydrogen dissolved in the nitrogen. The hydrogen within the nitrogen carrier gas is then measured using a Wheatstone bridge circuit and a thermal conductivity cell known as a katharometer. The instrument is precalibrated, and the result is correlated with known standards of premixed gases containing a known level of hydrogen.

Fig. 21 The Telegas unit for hydrogen determination. Courtesy of Aluminum Company of America.

Instrument readings are converted into hydrogen content by using a family of curves relating to hydrogen concentration. Correlations are available that cover the normal melt temperature range of 675 to 750 °C (1245 to 1380 °F) and hydrogen concentrations of 0.05 to 0.40 mL/100 g. Correction factors also exist for different alloy compositions because of their greater or lesser sensitivity to hydrogen. The Telegas instrument performs well when it is properly operated, and it has been found to compare very favorably with the vacuum hot extraction analytical method, with a maximum standard deviation of 0.023 mL/100 g.

Although the Telegas instrument is not difficult to operate, the ceramic probes used with this instrument are very fragile. To avoid thermal shock to the probes, they must be preheated and immersed with extreme caution.

Vibrated Vacuum Gas Test (Ref 43). The standard vacuum solidification test provides only a qualitative measure of metal quality as influenced by both hydrogen gas content and inclusions. The vibrated vacuum gas test provides several features that improve upon the standard test and sort out hydrogen effects versus inclusion-nucleated porosity. A larger-capacity pump and more sensitive vacuum gage are used to produce a better and more reproducible vacuum. A preheated outgassed refractory sample cup permits slower solidification. A controlled vibrator assists the nucleation of gas bubbles, promotes coalescence, and reduces the surface tension of the molten sample.

Results of the vibrated vacuum test have shown good differentiation between the hydrogen content effects (presence of gas bubbles) of filtered (fewer inclusions) versus unfiltered metal (Ref 43). Figure 22 shows the improvement in metal quality at any test pressure for filtered metal, correlating with lower hydrogen content as verified by hot extraction analysis. Therefore, the vibrated vacuum test can provide a more sensitive determination of metal quality as affected by hydrogen alone.


Fig. 22 Hydrogen content as a function of vibrated vacuum gas test pressure for unfiltered (a) and filtered (b) aluminum alloy. Source: Ref 43.

Methods of Degassing

Several methods are available for degassing aluminum. The simplest method is to hold the metal at a lower temperature for some period of time during which the hydrogen solubility is lower and thus provide for natural outgassing. In addition, casting conditions that permit solidification with natural expulsion of hydrogen without the attendant porosity are also desirable. However, neither of these is always possible. Deliberate degassing of the melt can be accomplished in the following ways:

• Gas purging or gas flushing• Tableted flux degassing• Mechanical mixer degassing

Gas Purging. The simplest method of degassing molten aluminum is by injecting a purging gas or gas mixture under pressure through a flux tube, pipe, lance, or wand. The tubes themselves are either ceramic-coated cast iron or steel pipe, or more often graphite.

A purge gas serves to collect hydrogen because of the lower partial pressure of the hydrogen afforded by the collector bubble versus the surrounding melt. Hydrogen diffuses into the purge gas bubble, which rises to the surface of the melt and is expelled into the atmosphere.

Purge gas can be either inert (argon or nitrogen) or reactive (chlorine and Freon 12). Reactive gases are used in small concentrations of under 10% along with an inert gas. Reactive gases undergo an actual chemical reaction with the melt. Chlorine reacts with molten aluminum to form a gaseous AlCl3, which then serves as the purging gas. In the case of Freon, fluorine reacts to form AlF3 a solid phase.

Both chlorine and fluorine have very favorable effects on bubble surface tension and in wetting out inclusions from the melt. Therefore, gas purging that uses either of these two elements can be expected to improve degassing efficiency through more effective coalescence of hydrogen and hydrogen inclusion complexes. However, although chlorine is very effective as a purge gas, it is also very noxious, and considerable effluent may result. Therefore, it is almost always used with an inert gas.

A mixture of nitrogen, chlorine, and carbon monoxide (CO) has been successfully used for degassing (Ref 44). This trigas mixture and Freon are both reported to have lower gaseous emissions or effluent than either chlorine alone or chlorine plus inert gas mixtures (Ref 44, 45).


Results of Hydrogen Removal With Purging Gas. In general, chlorine gas or mixtures with chlorine are more beneficial than inert gases or Freon alone. Figure 23(a) shows the relative effects of three different purging gases or mixtures, while Fig. 23(b) compares the degassing achieved with argon and nitrogen.

Fig. 23 Relative efficiencies of purge gases in the degassing of aluminum alloy 356 (a) and aluminum alloy 319 (b) at constant gas flow rates. Metal density in (b) increases as gas content of the metal decreases. Source: Ref 46, 47.

Vacuum fluxing is not used on any significant commercial basis in North America, but is practiced in Europe. Recent results have shown that this technique is beneficial (Ref 48). In large (45 Mg, or 50 ton) melts, a series of alloys (99.5Al, Al0.5Mg-0.5Si, and Al-5Zn-1.1Mg-0.3Cu) was refined using argon fluxing for 30 min through porous plugs. Pressure was maintained at 7 Pa (0.001 psi). Hydrogen contents were reduced from 0.16 to 0.11 ppm, 0.24 to 0.12 ppm, and 0.17 to 0.13 ppm for the three alloys, respectively. One particular advantage of vacuum fluxing is the minimal amount of dross formed.

Hexachloroethane Degassing (Ref 49, 50). Perhaps the most common method of degassing in foundry applications is the use of hexachloroethane (C2Cl6) tablets. The tablet decomposes in the aluminum melt to form gaseous AlCl3. The rising AlCl3 gas bubbles then collect hydrogen gas and deliver the gas to the melt surface for release. The tablets may also contain salt fluxes to help wet oxide inclusions within the melt and thus enable some removal of the hydrogen gas associated with inclusions.

To be fully effective, the tablets must be kept dry and should be plunged with clean, dry tools (preferably a perforated bell-shaped plunger to permit easy gas bubble escape). The plunge should be to the full depth of the melt, and the tablets should be held in position until a gentle bubbling on the melt surface subsides. If this is not done, the lighter-density tablets will rise and float on the melt surface during decomposition, providing little or no effective degassing of the entire melt.

Tableted products are available for virtually any melt size, although their principal application is for smaller furnaces and ladles within the foundry casting industry. When properly used, C2Cl6 provides very effective degassing. However, the somewhat noxious odor of the raw tablet--as well as the sometimes strong effluent during use, particularly improper use--creates environmental difficulties that have forced many foundries to discontinue use of the tablets.

Rotary Degassing. The use of simple gases or gas mixtures in lance or flux tube degassing, although reasonably effective, is not necessarily fully efficient. The ability of this simple process to degas depends on the bubble size generated and the amount of contact area between the degassing bubbles and the melt. The bubbles are often large, and very little mixing occurs in large melts or vessels when flux tubes or lances are used.

A significant improvement was the development of rotary gas injection systems. These systems originated in the aluminum primary and mill products casting industry and are used as in-line treatment systems in auxiliary vessels between furnace and ladle or furnace and casting pit.


The principle of the rotary injector system (Fig. 24) is that gas is injected into the column or shaft of a rotating member and is released through fine openings at the base or rotor. When rotating at speeds of 300 to 500 rpm, the disperser shears the incipient gas bubble released, producing a wide dispersion of very fine bubbles for degassing. The high surface-area-to-volume ratio of degassing bubbles provides greatly increased contact area and therefore increased reaction kinetics, resulting in more efficient degassing. The rotating member also disperses the bubble array more uniformly within the reaction vessel to provide more thorough mixing of metal with the purge gas. Figure 25 shows a typical rotary degassing system used in primary and mill products casting operations. Such systems are sized according to metal volume and flow rate conditions, and either single gases such as argon, nitrogen, and chlorine or mixtures of these are used in the gas mixing and control system. The level of hydrogen removal depends on gas flow rate and initial hydrogen content as well as metal flow rate, but most commercial systems can achieve final hydrogen contents well below 0.15 mL/100 g (Fig. 26).

Fig. 24 Schematic of an in-line rotary degassing unit.


Fig. 25 Schematic of spinning nozzle inert flotation (SNIF) rotary degassing system for in-line aluminum refining. Source: Linde Division of Union Carbide Corporation.


Fig. 26 Hydrogen removal results with the SNIF rotary degasser shown in Fig. 25. Similar results can be obtained with comparable commercially available equipment. Source: Ref 51.

Degassing performance is determined by several interrelated factors, including initial hydrogen content, metal flow rate, vessel size (which determines residence time in conjunction with flow rate), purge gas flow rate, mixing capability, and specific alloy (thermodynamic factors and mass transfer coefficients). In general, all such systems have achieved satisfactory degassing results (Ref 51, 52, 53, 54, 55, 56).

Rotary degasser systems have recently been downsized for foundry applications with good results (Ref 47, 57). The degassing system is inserted directly into a crucible or ladle of metal, and the metal is degassed in a batch process before pouring.

There are optimum combinations of purge gas volume, ratio of impeller diameter to crucible diameter, and shaft revolutions per minute for best efficiency of degassing without excessive turbulence. An impeller/crucible diameter ratio of 20 to 25%, a 350 to 450 rpm shaft speed, and a gas flow rate of 18 to 20 standard cubic feet per hour (SCFH) have been found to be appropriate for one unit. Variations in impeller design, however, may alter these parameters. Gas removal results achieved with rotary degassing compare very favorably with those achieved by lance treatment. This is illustrated in Fig. 27 for a 180 kg (400 lb) heat of Alloy A357.


Fig. 27 Comparison of efficiency of rotary degassing and lance degassing of aluminum alloy A357. The rotary system used a 150 mm (6 in.) rotor at 375 rpm. Melt weight: 180 kg (400 lb). Source: Ref 57.

Other Mechanical Degassing Systems. The circulation gas injection pump (see the section "Demagging of Aluminum" in this article) can also be used for degassing. The high speed (600 to 1100 rpm) of the impeller shears the gas bubbles exiting the injection tube and creates a very fine and widely dispersed bubble pattern. In large reverberatory furnaces, the melt can be degassed through the injection and pumping action without the need for additional furnace fluxing with wands. In this manner, degassing efficiency and energy efficiency are both improved because furnace doors need not be opened, as in flux tube or lance fluxing. Large (45 Mg, or 50 ton) melts have been successfully degassed to levels of 0.15 mL/100 g and below with the gas injection pump.

Another device being used for degassing in the primary industry is a swirling tank reactor. The metal flows in a swirling pattern counter to gas flow. The gas is injected through a series of nozzles, as shown schematically in Fig. 28. Typical results obtained with this process are shown in Fig. 29.


Fig. 28 Schematic of the metal in-line treatment (MINT) degassing system. Source: Selee Corporation.


Fig. 29 Efficiency of the MINT III degassing system shown in Fig. 28 using a gas mixture of Ar-0.5/2Cl. Source: Ref 58.

Porous Plug Degassing. Another method that can be used to degas by creating fine bubbles is the use of a porous disperser on the end of a flux tube or a fixed porous plug (Fig. 30). Porous plugs are graphite or ceramic materials with a very fine, interconnected porosity through which gas can be conveyed. These plugs can be installed into ladle bottoms, furnaces, or auxiliary treatment vessels. The fine porosity of the plug material allows a fine bubble size to be generated (comparable to that of rotary degassers). However, degassing efficiency is strongly related to bubble residence time, gas flow rate, depth of the melt, size of the vessel, and the ability to keep the pores in the plug open for full gas emission. Porous plug degassing can be more economical than rotary degassing systems, in which the rotors and shafts, usually graphite, require periodic replacement as they wear and oxidize.


Fig. 30 Porous plug degassing unit (a) showing fine bubbles produced in water. (b) Plug installation in vessel bottom.

Figure 31 compares the degassing obtained by using identical gas flows for porous plug, lance, and rotary degassing systems. The porous plug is more effective than a lance, and satisfactory degassing results can be obtained in many applications by controlling vessel size, gas flow rate, and residence time.

Fig. 31 Comparison of lance, porous plug, and rotary degassing efficiencies for identical gas flows. Source: Ref 36.


A combination in-line degassing/filtration system using porous plugs has recently been introduced (Ref 59). The degassing chamber is designed to accommodate all the above-mentioned variables. This system (Fig. 32) has produced final hydrogen contents of 0.06 mL/100 g at low metal flow rates (40 kg/min, or 90 lb/min) and 0.14 mL/100 g at flow rates near 330 kg/min (725 lb/min).

Fig. 32 Schematic of proprietary combination in-line degassing/filtration system. Source: Ref 59.

References cited in this section

6. Aluminum Casting Technology, American Foundrymen's Society, 198630. P.D. Hess, Measuring Hydrogen in Aluminum Alloys, J. Met., Oct 1973, p 4631. W.R. Opie and N.J. Grant, Hydrogen Solubility in Aluminum and Some Aluminum Alloys, Trans. AIME, Vol 1988,

1950, p 123732. R.Y. Lin and M. Hoch, "Solubility of Hydrogen in Molten Aluminum Alloys," Paper presented at the Annual Meeting

of AIME, New Orleans, LA, 198633. G.K. Sigworth, A Scientific Basis for Degassing Aluminum, Trans. AFS, 1987, p 7334. T.A. Engh and G.K. Sigworth, Molten Aluminum Purification, in Light Metals, The Metallurgical Society, 1982, p

983 35. G.K. Sigworth and T.A. Engh, Chemical and Kinetic Factors Related to Hydrogen Removal From Aluminum, Metall.

Trans., Vol 138, 1982, p 447 36. T.A. Engh and T. Pedersen, Removal of Hydrogen From Molten Aluminum by Gas Purging, in Light Metals, The

Metallurgical Society, 1984, p 1329 37. J.L. Jorstad, An Overview of the Need for Melt Cleanliness Control, in Proceedings of the International Molten

Aluminum Conference, American Foundrymen's Society, 1986, p 285 38. R.K. Owens, H.W. Antes, and R.E. Edelman, Effects of Nitrogen and Vacuum Degassing on Properties of a Cast

Aluminum-Silicon Magnesium Alloy (Type 356), Trans. AFS, 1957, p 424 39. H. Rosenthal and S. Lipson, Measurement of Gas in Molten Aluminum, Trans. AFS, 1955, p 30140. D.J. Neil and A.C. Burr, Initial Bubble Test for Determination of Hydrogen Content in Molten Aluminum, Trans.

AFS, 1961, p 27241. D.A. Hilton, Measurement of Hydrogen in Molten Aluminum Alloys Using a Quantitative Reduced Pressure

Technique, in Proceedings of the International Molten Aluminum Processing Conference, American Foundrymen'sSociety, 1986, p 381

42. D.A. Granger, Telegas for Determining Hydrogen in the Foundry Industry, in Proceedings of the International MoltenAluminum Processing Conference, American Foundrymen's Society, 1986, p 417


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