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Thread: Degassing confusion

  1. #11

    flux and degassing

    I found some material to clarify degasing versus fluxing.

    All gases are insoluable in Aluminum except hydrogen. Any gas pumped into the melt will "collects the hydrogen but also removes particulate matter, that is, inclusions, by flotation".

    Not sure if this was mentioned but the chlorine Anon is talking about is produced from the decomposition of the solid hexachloroethane tablets.

    Aluminum Foundry Degassing

    Degassing of aluminum alloy melts is a critical melt treatment technology necessary to provide readiness of the melt for casting into many applications (Ref 1, Ref 2, Ref 3, Ref 4, Ref 5, Ref 6, Ref 7, Ref 8). Hydrogen is the only gas that is soluble in aluminum, as monatomic hydrogen. Figure 1 exhibits the hydrogen solubility of typical aluminum-silicon foundry alloys. Clearly, a substantial amount of hydrogen can be present in the liquid state, but as the melt solidifies, there is a substantial reduction in the amount of hydrogen that can exist in the solid state. How this change takes place will determine whether porosity will form. If not removed or reduced to an acceptable level determined by prior process and quality studies, the gas dissolved in molten aluminum, if not treated, can result in detrimental properties to castings.


    ]Fig. 1 Hydrogen solubility in aluminum casting alloys. Source: Ref 2[/h]
    Figure 2 is a classic example of gas porosity developed during solidification in an aluminum alloy casting. Loss in mechanical properties, such as tensile strength, elongation, fatigue strength, and fracture toughness, are common results. In addition, interconnected porosity can result in “leakers” during pressure-tightness testing and blistering upon subsequent heat treatment as the pores grow and open up; this is especially a problem on the surface when surface finish/cosmetic properties are required.


    Fig. 2 Typical porosity configurations in aluminum-silicon casting alloys. (a) Shrinkage pore found within a casting. (b) Gas pore in an Al-8% Si alloy. (c) Microporosity (gas plus shrinkage). (d) Microporosity (gas plus shrinkage). Source: Ref 2

    However, gas can arise not only from entrapped hydrogen dissolved during the melting process and evolving as gas during the liquid-solid transition, but also gas can result from aspirated air or gas evolution from molding materials (i.e., cores, binders, etc.). Thus, it is not totally clear in Fig. 2(a) that the porosity is due to evolution of hydrogen gas. Observable porosity may also very well be caused by lack of feeding, that is, shrinkage, during solidification. In fact, Fig. 2 depicts porosity that follows very well the interdendritic structure and hence is very likely shrinkage, not gas.
    In Fig. 2(b), the rounded porosity would suggest gas evolution during final stages of solidification and hence could very well be caused by hydrogen, However, further microscopic investigation may be necessary, since this porosity may also be caused by entrapped air or lubricant decomposition. In actual practice, it is therefore best to degas the melt to an acceptable level before casting. A typical target value, below which hydrogen-caused porosity is not normally a problem in the final casting, is that the degassing process should achieve a level of 0.15 mL H2/100 g Al, the usual standard of measurement n a quantified test such as Alscan (Alcan Aluminum Ltd.). If one is instead using the reduced-pressure test as a measure of minimal porosity potential, it is not possible to accurately predict lack of hydrogen-caused porosity by this means alone. However, reaching a high percentage value in the reduced-pressure test related to the specific alloy theoretical density is a good measure, for instance, 2.62 in A356, whose theoretical density is 2.68 g/cm3.
    Hydrogen Sources

    Given the propensity of aluminum to readily absorb hydrogen, there are many ways that hydrogen can and will always be present in a melt to a certain degree. Fuel-fired furnaces (natural gas, oil) have hydrogen available from fossil fuel decomposition. Water vapor is almost always present in the atmosphere, even at high temperatures directly in contact with the melting furnace. Indeed, high humidity conditions exacerbate not only hydrogen dissolution in the melt but also can retard the ability of any melt treatment to remove hydrogen, because a degassed melt can then readily redissolve additional hydrogen. Ingot, additives, and fluxes are other common sources of hydrogen. In the case of fluxes, fluxing compounds are usually mixtures of various salts, including halides, and become very hygroscopic when exposed to air. In addition, the tools used to add and work in fluxes—rakes, spoons, and so on—will often retain flux and oxide debris after use, which will then adsorb moisture from the air as they cool down. Reuse of these tools, without intermediate cleaning, not only poses a potential safety hazard but also can reintroduce hydrogen. Scrap castings, machine turnings and borings, die cast trim press scrap, gates and risers, and sand and other nonmetallic molding material debris are additional sources of hydrogen.

    Aluminum Degassing

    There are many means available to minimize hydrogen pickup or to degas a melt. Noting the increased solubility with temperature, as depicted in Fig. 1, merely minimizing the melting or holding times and temperatures will lessen the hydrogen absorption. Reducing the temperature will result in natural outgassing, given time, which is not always available. Therefore, additional degassing measures are often needed.
    The basic principle of practical foundry degassing is that any gaseous component has the ability of collecting hydrogen gas, since all gases are insoluble in aluminum, with the exception of hydrogen, which exists as a single atom. Thus, a purge or process gas can function as in Fig. 3, depicting gas fluxing, which not only collects the hydrogen but also removes particulate matter, that is, inclusions, by flotation.


    Fig. 3 Schematic of gas fluxing in molten aluminum

    The first application of this principle was mainly through the decomposition of a solid product that reacted in molten aluminum. Historically, the degassing material commonly employed was a hexachloroethane tablet (C2Cl6). When submerged and plunged into the melt, the hexachloroethane decomposed into its respective components. The chlorine component immediately forms a metastable aluminum chloride gaseous compound, which is insoluble in the melt and served as the purge gas. With any purge gas, the monatomic hydrogen diffuses into the purge gas bubble, combines with another hydrogen atom to form molecular hydrogen, and the bubble rises to the surface, where both gaseous species are released. The fluxing components in the hexachloroethane tablet, which are often halide salts, served not only to bind the tablet, but these salt components do aid in wetting of the gas bubble in the aluminum, reducing the interfacial energy and allowing easier transmittal of the hydrogen atoms into the collector aluminum chloride gas bubble. This use of hexachloroethane tablets has given way to other degas methodologies, in most instances because of environmental concerns and more efficient technologies. However, some foundries still employ this technique, especially on small crucible furnaces.
    The second most popular degassing technique in historical use—and still used today (200—is a simple flux wand or static lance. This can be a steel pipe or, more commonly, a graphite lance. A purge gas such as nitrogen or argon is injected under low pressure (less than 207 kPa, or 30 psi) into the melt. The insoluble nitrogen or argon gas bubble then rises within the melt, and the hydrogen atoms diffuse to this process gas bubble and form a hydrogen molecule. When the bubble reaches the surface of the melt, the hydrogen is released. If the relative humidity in the atmosphere is particularly high, this will be a dynamic process, because new hydrogen may become absorbed into the melt.
    Fundamentally, the lance degassing technique is relatively inefficient, as is seen later in this section, in relation to newer technologies. The reason for this is the relatively large gas bubble that emanates from the bottom of the lance and then agglomerates and rises quickly to the surface. The rate of rise increases with bubble size, and the bubble accelerates as it climbs toward the surface. Fig. 4(a) depicts a water model showing that a static lance only influences a small volume of fluid surrounding the lance, unless it is manually moved about. While this is possible in relatively small-diameter melting and holding vessels, it is impractical in larger ones.

    Selecting a Process Gas. Since all gases except hydrogen are insoluble in molten aluminum, there are theoretically many choices of which gas to use.
    Nitrogen is the most commonly employed and is the least expensive. Nitrogen creates a wet dross; that is, the dross that is formed is high in metallic aluminum content. Furthermore, since the density of nitrogen is comparable to air overall, there is a tendency to pick up moisture in nitrogen lines as fittings deteriorate and humid air is aspirated into the delivery lines. For this reason, all nitrogen sources should include an in-line dryer. Nitrogen purity should be at least 99.99% for any critical casting applications.

    Argon, while significantly more expensive than nitrogen, produces a less metal-rich dross. Argon is more inert, and, being heavier, provides a protective cover over the melt during degassing, precluding not only hydrogen absorption but further oxidation as well. It is also easier to keep the argon supply drier and cleaner as hoses and fittings deteriorate. While argon is the preferred process gas for in-line degassing treatment in the production of wrought alloys, it is not extensively used in foundry casting because of the higher cost.

    Active Halogens. Historically, a small percentage of chlorine (5 to 10%) was used with an inert gas when degassing with a static lance. The chlorine enhanced the reaction between the melt and the hydrogen atoms because it lowered the interfacial tension at the purge gas bubble surface (through the formation of aluminum chloride, a metastable gaseous phase) and facilitated the diffusion of the hydrogen atom into the purge gas bubble. Several commercial mixtures of halogen gases—Freon (CCl2F2) (Dupont) being the most popular—were employed in the past. With the growing concern and difficulties with the environmental impact of chlorine use, sulfur hexafluoride (SF6) gained prominence 20 years ago as a replacement for chlorine. However, subsequent experience showed that SF6 also posed environmental problems with ozone depletion, and the gas is very corrosive, even in small amounts, to foundry metal structures.
    Fortunately, with the advent of rotor degassing and the resultant better kinetics of reaction and mixing, it has been found that chlorine does not enhance the degassing rate or efficiency when the purge/process gas bubbles are sufficiently small, as they are with rotor impeller degassing. Rather, when chlorine is used, there is better wetting of the nonmetallic inclusions during the degassing process, so that metal cleanliness is improved, and the burden on any subsequent particulate removal process is lessened, which in turn creates a better-quality cast product.

    Other Considerations for the Degassing Process. There are other issues that must be taken into account during the degassing process.
    Temperature. It is well understood that hydrogen absorption in the melt increases with increasing temperature. Conversely, hydrogen solubility and partial pressure of hydrogen gas in the melt decrease rapidly as the melt temperature is decreased. It follows that minimizing the temperature will result in lower hydrogen value.
    Given sufficient time, allowing the temperature to drop will result in natural outgassing. However, the injection of a purge gas at normal standard temperature and pressure creates an endothermic reaction, and heat will therefore be lost at an accelerated rate. A general rule-of-thumb is that degassing a metal volume of 455 kg (1000 lb) in an unheated refractory vessel for 5 to 10 min will result in a temperature drop of nearly 28 °C (50 °F). Whatever the actual magnitude of the temperature drop due to gas injection/treatment time, this reduction must be accommodated in relation to the necessary temperature that must be maintained as the treated melt is transferred into the mold or into the casting station(s). A degassing process should nevertheless be carried out at the lowest practical temperature possible.

    Grain Refining. Most aluminum melts in sand, permanent mold, and low-pressure foundry production require grain refinement to achieve the best mechanical properties in the finished casting. In most instances, grain refiner is added during the degassing process, because the increased mixing and agitation facilitates the dissolution and reaction of the grain refiner itself and provides more uniform dispersion throughout the melt or treated volume.

    Modification. Historically, the element sodium was used to provide modification of the eutectic silicon component in aluminum casting alloys—a necessary practice, together with grain refining, to produce desired mechanical properties. In current practice, sodium has given way to a strontium addition for many reasons, including safety in handling and in the environment, better consistency, and reduced fade, which is the loss of modification with time. Because both sodium and strontium are prone to oxidation (although strontium vapor pressure is substantially lower than sodium), and the degassing mixing creates more convection during treatment, degassing should take place before modification, so as not to lose the benefits of the dissolved modifier due to oxidation. If chlorine is used, even in small quantities, in the degassing process, both sodium and strontium will react chemically to form solid chloride reaction products.

    Dross Formation and Skimming. The action of the purge gas injection process by whatever means—lance, rotor, or porous plug/disperser—will result in flotation of debris and some degree of particulate inclusion content within the melt. Naturally, the degassing process creates dross. This must be thoroughly skimmed off the surface to the greatest practical extent possible to avoid transfer into the casting mold or casting station.

    Determining Hydrogen Content. To control hydrogen content, one must be able to measure the hydrogen content in the melt and the subsequent result of any hydrogen-removal treatment. Laboratory evaluations are possible using vacuum fusion (Leco technique) and subvacuum fusion on previously solidified test specimens. Obviously, laboratory measurements cannot be used to make real-time decisions but rather are useful in process development. Real-time determinations are limited to the simple reduced-pressure test (RPT) and its variants, and in situ techniques, such as Alscan, continuous hydrogen analysis by pressure evaluation, and others. The RPTs are the simplest, but these tests are a measure of molten metal cleanliness and not absolute hydrogen content per se. The presence of inclusions in the melt during an RPT evaluation will create exaggerated porosity nucleated during solidification and a resultant lower density value. The Alscan test and others of this type do generate a real-time result through the principles of recirculating gas of a known standard hydrogen content and the thermal conductivity of dilute solutions.

    References cited in this section

    1. R. Monroe, Porosity in Castings, Trans. AFS, Vol 113, 2005, p 519–546
    2. J.E. Gruzleski and B.M. Closset, The Treatment of Liquid Aluminum-Silicon Alloys, AFS, 1990
    3. D. Apelian, Clean Metal Processing of Aluminum, Proceedings, Materials Solutions Conference '98 on Aluminum Casting Technology, ASM International, 1998
    4. G. Sigworth and T. Engh, Chemical and Kinetic Factors Related to Hydrogen Removal from Aluminum, Metall. Trans., Vol 138, 1982, p 447
    5. G. Sigworth, A Scientific Basis for Degassing Aluminum, Trans. AFS, Vol 95, 1987, p 73
    6. D.V. Neff, Understanding Aluminum Degassing, Mod. Cast., May 2002
    7. D. Apelian and M.M. Makhlouf, Ed., High Integrity Aluminum Die Casting, NADCA, April 2004
    8. M.M. Makhlouf, L. Wang, and D. Apelian, Measurement and Removal of Hydrogen in Aluminum Alloys, AFS Special Report, 1998

    Aluminum Fluxes and Fluxing Practice

    Rafael Gallo, Foseco Metallurgical David Neff, Pyrotek


    IT IS WELL KNOWN that molten aluminum casting alloys have two inherent characteristics: the tendency to absorb hydrogen gas, and the ability to readily oxidize. Hydrogen is made available at the surface of molten aluminum alloys through the reaction of the molten bath with the water vapor present in the melting environment (Ref 1, Ref 2, Ref 3, Ref 4). The reaction between water vapor and molten aluminum yields not only hydrogen gas but also the formation (within milliseconds) of an amorphous aluminum oxide (Al2O3) film on the surface bath, which acts as a protective oxidation barrier for the molten metal underneath the film. The amorphous films have been referred to as young films (Ref 5).
    The two oxidation reactions that occur when molten aluminum contacts air proceed according to the following: The hydrogen produced through the reduction of water in turn dissociates into its atomic form at the melt surface and then diffuses through the amorphous aluminum oxide film, from which it is quickly dissolved by the molten bath. The aluminum oxide films are an intrinsic part of the melting process; they protect the metal underneath the film from further oxidation. However, in actual foundry operations, the surface of the molten bath always has some movement due to one or more of the following melting practices:
    • Charging
    • Skimming
    • Cleaning
    • Degassing
    • Transferring
    • Ladling

    Any of these melting practices causes the thin aluminum oxide films to break and reoxidize, causing rapid aluminum oxide film thickening (oxide buildup).
    The constant metal movement and breaking of the aluminum oxide films cause the films to crumble, thicken, and encapsulate unoxidized molten aluminum, generating what is known as wet dross. A typical representation of this phenomenon is well observed during the filling process of transfer ladles, such as the one shown in Fig. 1. As soon as the molten metal is tapped from the melting furnace into the transfer ladle, not only does the molten stream become disturbed (from the very beginning to the end), but also a significant amount of splashing between the molten stream and the rising metal surface promotes aluminum oxidation, causing the surface of the bath to thicken with wet dross. The aluminum content of wet drosses is typically reported to be on the order of 60 to 85%, while the remaining 40 to 15% is aluminum. The amount of trapped liquid metal in the dross varies according to the melting and/or handling molten practice. The aluminum oxide is a very stable compound that cannot be reduced to aluminum under ordinary melting conditions. However, the amount of suspended liquid metal could be reduced from the 60 to 85% range to 30% by proper fluxing and drossing techniques.
    Fig. 1 Metal being tapped from a holding furnace into a transfer ladle, causing aluminum oxide films to crumble, thicken, and encapsulate unoxidized molten aluminum, generating wet dross

    Formation of dross is an intrinsic process when melting aluminum alloys. The dross is considered to be the main contributor in influencing the total metal loss during melting. Depending on the efficiency of the melting furnace, and melting practices, the amount of dross generated may be from 1 to 10% of the total metal melted.
    In addition to the formation and elimination of dross, another problem that aluminum foundries face in the molten metal bath is the nonmetallic and the metallic impurities that are suspended and floating in the bath. Impurities and aluminum oxides will remain suspended and floating in the liquid bath because they are porous and contain some gas adhering to and trapped in the pores. In this case, the oxide density is similar to that of the aluminum alloy.
    Nonmetallic and metallic impurities are introduced into the melt during the charging process, the molten metal treatment, and the handling operations. Even when melting primary ingot, nonmetallic impurities such as hydrogen and aluminum oxide are introduced. However, as would be expected, the majority of the nonmetallic and metallic impurities come from charging returns. The term returns means scrap castings, gates, trimmings, and so on. According to the type and origin of the return metal and the conditions of raw material storage, the metal can contain considerable amounts of both nonmetallic and metallic impurities.
    Hydrogen absorption, formation of dross, generation of bifilms, metallic and nonmetallic inclusions, and oxide buildup are inherent characteristics when melting and handling molten aluminum, regardless of the melting and/or holding furnace design or the energy used (gas or electricity). Therefore, it is of vital importance to understand and control such inherent characteristics, because they will greatly affect the quality of the molten metal, which in turn impacts the final casting quality with respect to porosity, shrinkage, oxides, and inclusions.
    While the existing types of inclusion material that would be present in melting and holding furnaces and/or transfer ladles will vary from foundry to foundry, their removal is essential for proper molten metal cleanliness. A number of commercially accepted melt treatment techniques are being used by aluminum foundries to remove and separate inclusions from the molten aluminum alloy prior to casting. These include various methods of fluxing, degassing, and filtration in the furnaces and the gating systems (Ref 3, Ref 6, Ref 7, Ref 8, Ref 9, Ref 10, Ref 11).
    Any of these techniques will have an impact on the melt cleanliness of the molten aluminum alloy. However, the effectiveness of evaluating their removal would rely on the melt cleanliness measurement technique being used. The strengths and weaknesses of these methods with regard to equipment requirements, sampling, sensitivity, timing, and practical means of assessing inclusion levels on the foundry floor have been fully discussed and published in the literature (Ref 12, Ref 13, Ref 14, Ref 15, Ref 16, Ref 17, Ref 18, Ref 19, Ref 20, Ref 21, Ref 22, Ref 23, Ref 24, Ref 25, Ref 26, Ref 27, Ref 28, Ref 29). Technical articles, which specifically cover fluxing as a factor for ensuring metal cleanliness, have also been published (Ref 7, Ref 30, Ref 31, Ref 32, Ref 33, Ref 34, Ref 35).
    In addition to aluminum, fluxing is also commonly done to some extent with virtually all nonferrous mill and foundry melting operations as the first step in obtaining a clean melt. Examples include:
    • Fluxing of magnesium alloys using salt fluxes or inert gas as a cover (see the article “Magnesium and Magnesium Alloy Castings” in this Volume).
    • Fluxing of copper alloys to remove gas or prevent its absorption into the melt, to reduce metal loss, and to remove specific impurities and nonmetallic inclusions (see the article “Copper and Copper Alloy Castings” in this Volume)
    • Fluxing of zinc alloys with chloride-containing fluxes that form fluid slag covers, which can be used to minimize melt loss if they are carefully skimmed from the melt before pouring (see the article “Zinc and Zinc Alloy Castings” in this Volume)

    References cited in this section

    1. R. Fuoco et al., Characterization of Some Types of Oxides Inclusions in Aluminum Alloy Castings, Trans. AFS, Vol 107, 1999 p 287–294
    2. P. Crepeau, Molten Aluminum Contamination: Gas, Inclusions and Dross, Aluminum Alloy Castings, AFS Fourth International Conference on Molten Aluminum Processing, 1995, p 1–13
    3. D. Apelian and S. Shivkumar, Molten Metal Filtration — Past, Present, and Future Trends, AFS Second International Conference on Molten Aluminum Processing, 1989, p 14–1 to 14–36
    4. C. E. Eckert, The Origin and Identification of Inclusions in Foundry Alloys, AFS Third International Conference on Molten Aluminum Processing, 1992, p 17–50
    5. J. Campbell, Castings, 2nd ed., Butterworth-Heinemann, Oxford, 1988, p 11–26
    6. D. Apelian, How Clean Is the Metal You Cast? The Issue of Assessment: A Status Report, AFS Third International Conference on Molten Aluminum Processing, 1992, p 1–15
    7. R. Gallo, Development, Evaluation, and Application of Granular and Powder Fluxes in Transfer Ladles, Crucible, and Reverberatory Furnaces, AFS Sixth International Conference on Molten Aluminum Processing, 2001, p 56–70
    8. L.C.B. Martins and G.K. Sigworth, Inclusion Removal by Flotation and Stirring, AFS Second al Conference on Molten Aluminum Processing, 1989, p 16.1–16.28
    9. D.V. Neff, Continuous, Re-Usable Filtration Systems for Aluminum Foundries, AFS Fourth International Conference on Molten Aluminum Processing, 1995, p 121–137
    10. J.R. Schmahl, L.S. Aubrey, and L.C.B. Martins, Recent Advancements in Molten Aluminum Filtration Technology, AFS Fourth International Conference on Molten Aluminum Processing, 1995, p 71–102
    11. L.C.B. Martins, Chemical Factors Affecting the Separation of Inclusion from Molten Aluminum, AFS Third International Conference on Molten Aluminum Processing, 1992, p 79–91
    12. P. Crepeau et al., Characterization of Oxide Sludge, Dross and Inclusions in Aluminum Melting and Holding Furnaces, AFS Third International Conference on Molten Aluminum Processing, 1992, p 51–77
    13. T.L. Mansfield and C.L. Bradshaw, Ultrasonic Inspection of Molten Aluminum, Trans. AFS, 1985, p 317–322
    14. D. Doutre et al., Aluminium Cleanliness: Methods and Applications in Process Development and Quality Control, Light Met., 1985, p 1179–1195
    15. R.I. Guthrie and J.E. Gruzleski, Quantitative Measurement of Melt Cleanliness in Aluminum Silicon Casting Alloys, AFS Fourth International Conference on Molten Aluminum Processing, 1995, p 15–28
    16. A. Simard, F.H. Samuel, et al., Capability Study of the Improved PREFIL-Footprinter to Measure Liquid Aluminum Cleanliness and Comparison with Different Techniques, AFS Sixth International Conference on Molten Aluminum Processing, 2001, p 93–112
    17. J.L. Roberge and M. Richard, Qualiflash Apparatus for Testing the Inclusion Quality of Aluminum Alloy Baths, AFS Fourth International Conference on Molten Aluminum Processing, 1995, p 29–42
    18. E.L. Rooy and E.F. Fisher, Control of Aluminum Casting Quality by Vacuum Solidification Tests, Trans. AFS, Vol 76, 1968, p 237–240
    19. D. Dispinar and J. Campbell, Critical Assessment of Reduced Pressure Test, Part 1: Porosity Phenomena, Int. J. Cast Met. Res., Vol 17 (No. 5), 2004, p 280–286
    20. D.E. Groteke, Improve Your Vacuum Test, Mod. Cast. Mag., Aug 2007, p 28–31
    21. N.D.G. Mountford, I.D. Sommerville, et al., Sound Pulses Used for On-Line Visualization of Liquid Metal Quality, Trans. AFS, 1997, p 939–946
    22. N.D.G. Mountford, I.D. Sommerville, et al., Laboratory and Industrial Validation of an Ultrasonic Sensor for Cleanliness Measurements in Liquid Metals, Proceedings of International Conference on Cast Shop Technology, TMS-AIME, 2000, p 304–310
    23. R. Gallo et al., Ultrasound for On-Line Inclusion Detection in Molten Aluminum Alloys: Technology Assessment, AFS International Conference on Structural Casting of Aluminum, 2003, p 29–42
    24. R. Gallo, Measurement of Hydrogen Content in Molten Aluminum Alloys Using Crucibles and Reverberatory Furnaces for Sand and Permanent Mold Castings, AFS Fourth International Conference on Molten Aluminum Processing, 1995, p 209–225
    25. F. Painchaud and J.P. Martin, The New Alscan Analyzer: Easy to Use, Reliable, On-Line Measurement of Hydrogen in Liquid Aluminum Alloys, AFS Second International Conference on Molten Aluminum Processing, 1989, p 20.1–20.21
    26. C. Schwandt et al., A Novel Electrochemical Hydrogen Analyzer for Use in Molten Aluminum and Its Alloys, AFS Sixth International Conference on Molten Aluminum Processing, 2001, p 149–157
    27. D.E. Groteke et al., Foundry Methods for Assessing Melt Cleanliness, AFS Fourth International Conference on Molten Aluminum Processing, 1995, p 55–69
    28. D. Dispinar and J. Campbell, “A Comparison of Methods Used to Assess Aluminum Melt Quality,” Shape Casting: TMS, The Second International Symposium, 2007
    29. D. Apelian and S. Dasgupta, Interaction of Initial Melt Cleanliness, Casting Process and Product Quality: Cleanliness Requirements Fit for a Specific Use, AFS Fifth International Conference on Molten Aluminum Processing, 1998, p 234–258
    30. K. Strauss, Applied Science in the Casting of Metals, Pergamon, 1970, p 253–256
    31. M.H. Kogan, Design and Development of Fluxing Agents for the Aluminum Foundry Alloys, AFS Second International Conference on Molten Aluminum Processing, 1989, p 25–1 to 25–12
    32. T.A. Utigard et al., The Properties and Uses of Fluxes in Molten Aluminum Processing, J. Met., Nov 1998, p 38–43
    33. S.R. Sibley Granular Fluxes for Aluminum Alloys, Environmental and Technological Advances, AFS Fourth International Conference on Molten Aluminum Processing, 1995, p 417–430
    34. D. Neff, Ensuring Quality Prior to Pouring, Mod. Cast., June 2006, p 27–29
    35. R. Gallo, Cleaner Aluminum Melts in Foundries: A Critical Review and Update Transactions, 112th AFS Casting Congress, May 17–20, 2008 (Atlanta, GA)

  2. #12


    Aluminum Fluxes and Fluxing Practice

    Aluminum Fluxing

    Fluxing is a term commonly used in foundries, especially within the melting department work force, to refer only to the addition of chemical compounds to clean molten aluminum alloy baths in either the furnaces (melting or holding) or the transfer ladles. Fluxing is the first step in obtaining clean molten metal by preventing excessive oxide formation, removing nonmetallic inclusions from the melt, and preventing and/or removing oxide buildup on furnace walls.
    In general, fluxes may be grouped into two classes: gaseous or solids. Gaseous fluxes may be a blend of an inert and a chemically active gas that is injected into the molten bath. Solid fluxes are blends of salts, which, at the present time (2008), are the most preferred type of fluxes used in foundries, especially since gaseous mixtures with chlorine have been almost completely eliminated because of environmental concerns.
    Gas Fluxing

    Gas fluxing is the application of a gas treatment to the melt. The gas may be inert, such as argon or nitrogen, and the latter is more common because of economics. Gas mixtures may also be used, usually adding an active halogen component—chlorine or fluorine compounds—in smaller amounts, typically only 5 to 10%, to the base inert gas. The purpose of this addition is to provide a better separation of solid particulate (i.e., aluminum oxide inclusions) from the liquid metal. The principal function of gas fluxing is hydrogen removal, not dross treatment or recovery. Gas fluxing does not reduce dross but rather increases dross.

    Solid Fluxes

    Solid fluxes can be broadly categorized as passive or active fluxes. Passive fluxes protect the surface of the molten aluminum from oxidation and prevent hydrogen pickup by the melt. Active fluxes react chemically with the aluminum oxides and clean the melt more effectively.

    Foundrymen recognize the importance of furnace type; furnace design; furnace maintenance; quality grade of smelter ingot; quality of the in-house returns; condition of the skimmers, ladles, and the like; and the charging and melting practice on the starting quality of the molten metal to make a casting. Nonetheless, it seems that the foundrymen still have some concerns over the lack of general accepted information on conventional solid fluxes.
    Even though there are many different commercial fluxes on the market, the basic ingredients of such commercial fluxes are based on the very early work, conducted approximately 100 years ago, to improve the electrolytic production of aluminum by reducing the aluminum oxide using molten cryolite (Na3AlF6). Since then, both natural and synthetic cryolite and fluorides have been evaluated. Figure 2 shows the equilibrium diagram of cryolite.


    Fig. 2 Equilibrium diagram of cryolite

    The removal of aluminum oxide by halides has its foundations in basic research and development on the systems NaF-AlF3-Al2O3 and NaF-AlF3. Studies on these systems have shown that the solubility of aluminum oxide increases with the sodium fluoride content. The reaction between cryolite and aluminum oxide in the presence of a surplus of NaF takes place according to the following reactions: or In addition, halide and fluoride salts have been thoroughly researched by secondary (recycled) aluminum producers during the melting and processing of aluminum scrap and dross. The recovered aluminum results from the effectiveness with which nonmetallic and metallic impurities are removed or reduced to acceptable levels. In these operations, low-melting-point fluxes are typically used. They are basically sodium and potassium chloride with additions of fluorides and some other chlorides, such as calcium chloride and magnesium chloride.

    Salts are used in fluxes because of the following general characteristics:
    • Cost-effective
    • Easy to use
    • Combine easily with other ingredients
    • Serve as fillers for active ingredients
    • Have lower density than aluminum
    • Give ability to cover the molten surface
    • Allow a low-melting-point, high-fluidity product
    • Have capability to absorb oxides and reaction products from the fluxing action

    Solid fluxes are basically blends of sodium chloride and potassium chloride salts, with or without the addition of fluorides. In addition, small quantities of oxidizing compounds, such as carbonates, sulfates, and nitrates, are added to promote exothermic chemical reactions (Ref 7, Ref 29, Ref 30, Ref 31). Exothermic reactions are important because they prompt the coalescence of the trapped liquid aluminum particles in the dross.
    The mechanism of how salt fluxes work has been attributed to thermodynamic chemical reactions and surface tension effects between the aluminum oxide and the flux, the aluminum oxide and the molten metal, and the molten metal and the flux. This interaction between aluminum oxides, flux, and molten metal has been explained in earlier papers (Ref 11, 12, 31, 32).
    In addition, the effect of the flux on the liquid metal will depend on the chemistry of the flux used, morphology of the flux, total amount of flux added, molten metal temperature, flux contact time, rabbling (stirring) technique, and so on. Nevertheless, from the point of view of flux chemistry, it is important to understand that different combinations and proportions of ingredients will impart different flux properties, such as flux density, flux fluidity, flux wettability, and flux reactivity. These four different flux properties are responsible for the characterization of any particular flux.
    To better understand how these flux properties are accomplished, it is necessary to recognize that essentially each different ingredient provides different effects that directly influence the final property of a flux. Despite the fact that many ingredients are available, not all of them are necessary or found in each flux. Table 1 lists the characteristics of the materials used in fluxes.
    Table 1 Characteristics of materials used in fluxes

    LiCl 43.39 2.068 605 1121 1325 2417
    NaCl 58.44 2.165 801 1474 1413 2575
    KCl 74.56 1.984 770 1418 1500 2732
    CaCl2 110.99 2.150 782 1440 1600 2912
    MgCl2 95.22 2.320 714 1317 1412 2574
    AlCl3 133.34 2.440 190 374 177.8 352
    BaCl2 208.25 3.920 963 1765 1560 2840
    LiF 25.94 2.635 845 1553 1676 3049
    NaF 41.99 2.558 993 1819 1695 3083
    KF 58.10 2.480 858 1576 1505 2741
    CaF2 78.08 3.180 1423 2593 2500 4532
    MgF2 62.31 3.180 1261 2302 2239 4062
    83.98 2.882 1291(a) 2356(a)
    Na3AlF6 209.94 2.900 1010 1850
    LiNO3 68.94 2.380 264 507 600(b) 1112(b)
    NaNO3 84.99 2.261 307 585 380(b) 716(b)
    KNO3 101.11 2.109 339 642 400(b) 752(b)
    Li2SO4 109.94 2.221 859 1578 High
    Na2SO4 142.04 897 1647
    K2SO4 174.27 2.660 1069 1956 1689 3072
    CaSO4 136.14 2.610 1450 2642 High
    MgSO4 120.37 2.660 1124(b) 2055(b)
    Li2CO3 73.89 2.110 723 1333 1310 2390
    Na2CO3 105.99 2.532 851 1564 High
    K2CO3 138.21 2.420 894 1641 High
    MgCO3 84.32 2.960 350(b) 662(b)
    CaCO3 100.09 2.710 1339 2442 850 1562
    (a) Sublimes.

    (b) Decomposes. Source: Ref 32

    Generally, ingredients can be classified into four major groups based on their primary influence over the mixture.
    Chlorides. Examples are aluminum chloride (AlCl3), barium chloride (BaCl2), calcium chloride (CaCl2), lithium chloride (LiCl), magnesium chloride (MgCl2), potassium chloride (KCl), and sodium chloride (NaCl).
    The melting point of these compounds, in their pure state, may range from 190 to 965 °C (375 to 1765 °F). However, they form low-temperature eutectics in combinations. In addition, it is very typical to have at least three compounds in a given flux, so that none of these compounds would be used as a single compound in a flux recipe.
    Chlorides are mainly used because of their fluidizing effects and because they are used as fillers. Fluxes based only on chloride salts should not react with molten aluminum, or at least the reaction should be negligible. In addition, these salts provide negligible effects on surface tension as compared to fluorides.

    Fluorides. Examples are simple fluorides such as aluminum fluoride (AlF3), barium fluoride (BaF2), calcium fluoride (CaF2), lithium fluoride (LiF), magnesium fluoride (MgF2), potassium fluoride (KF), sodium fluoride (NaF), and double-fluoride compounds such as sodium silicofluoride (Na2SiF6) and potassium silicofluoride (K2SiF6).
    The melting point of these compounds, in their pure state, may range from 845 to 1425 °C (1553 to 2600 °F). The high melting point of these compounds provides thickening effects on a flux.
    Fluoride salts have limited solubility for oxides. However, no fluoride salts can dissolve massive aluminum oxides. Fluoride salts act as surfactants, affecting surface tension forces between flux, liquid metal, and aluminum oxides. As the flux wets the interface between the aluminum oxide particles and the liquid metal, the adhesion force between the liquid aluminum and the oxides decreases, promoting oxide separation and metal coalescence.
    Fluorides are still the most effective compounds being used in fluxes to improve aluminum recovery from wet dross.

    Oxidizing Compounds. Examples are:
    • Nitrates such as potassium nitrate (KNO3) and sodium nitrate (NaNO3)
    • Carbonates such as calcium carbonate (CaCO3), potassium carbonate (K2CO3), and sodium carbonate (Na2CO3)
    • Sulfates such as potassium sulfate (K2SO4) and sodium sulfate (Na2SO4)

    The melting point of the nitrates ranges from 307 to 339 °C (585 to 642°F); the melting point of the carbonates ranges from 851 to 1339 °C (1564 to 2442 °F); and the melting point of the sulfates ranges from 859 to 1450 °C (1578 to 2642 °F).
    Oxidizing compounds are used to promote exothermic chemical reactions. They react with the smallest molten aluminum particles that are present in the dross, yielding aluminum oxides as well as considerable heat.
    The purpose of the exothermic reaction is twofold: 1) It allows larger pockets of entrapped aluminum to coalesce and fall back into the molten bath, and 2) it facilitates reactions between aluminum and fluorides. The exothermic reaction continues until all of the fine aluminum particles are burned.

    Solvents of Aluminum Oxides. Examples are:
    • Borax (Na2B4O7)
    • Potassium borate (K2B2O4)
    • Sodium cryolite (Na3AlF6)

    Both the Hall and Heroult patents covered the electrolysis of aluminum oxide in a bath of molten halide salts. Since then, dissolution of aluminum oxides (Al2O3) by cryolite has been documented.

    Flux Morphology

    The manufacturing of fluxes requires compliance with strict general quality requirements for the raw ingredients (commercial purity, grain size uniformity, and less than 1% moisture content) and for the manufacturing process (blending of the ingredients). Control over these variables assures consistent products and consistent performance.
    For most of the aluminum foundry history, powder fluxes have been used. Powder fluxes have been in commercial use for nearly 60 years. However, the basic formulations and compositions of the fluxes have not changed dramatically over this period. Present-day research and development in flux chemistry is still based on trial-and-error procedures.
    In the last ten years, research and development for possible new formulations and variations of traditional mixes has more closely evaluated the use of fluorides because of environmental emission concerns. It has been reported that if fluorides were removed from the formulation, it would render a powerless and inefficient flux (Ref 7, 33). At the same time, it has been reported that emissions from fluxes containing fluorides could be reduced by at least half only if the morphology of the flux changed from powder to granular. It is still prudent to mention that emissions from powder fluxes are within standard environmental limits.
    Fluoride-containing salts are being subjected to stricter environmental regulations and constraints. Therefore, fluoride-free fluxes are available. However, it is important to realize that fluoride-free fluxes will never perform as efficiently as fluxes containing fluorides. On the other hand, fluoride-free fluxes are the best option to eliminate internal smoke and odor in the plant, as well as to treat metal in electrical resistance furnaces without damaging the electrical elements.
    A granular flux may or may not have the same formulation as a powder flux. However, because of the different grain morphology, granular fluxes offer other operational and product advantages over powder fluxes.
    Granular fluxes are less polluting to the atmosphere; emissions are reduced by at least 50%. Furnace tenders appreciate the fact that granular fluxes greatly reduce smell and smoke, facilitating working closer to the furnaces during drossing operations. Granular fluxes are easier to apply and to spread over the molten surface, since they are fines and dust free. Therefore, application rates can often be reduced. Additional product benefits are the better consistency in chemistry from grain to grain and the fact that there is no segregation during flux transportation or handling.
    Figures 3 and 4 depict the difference, which is very noticeable, when applying a powder or a granular flux. Figure 3 shows that, as the furnace tender throws the powder flux, the flux falls down in a big cloud as soon as it leaves the cup. On the other hand, Fig. 4 shows that the granular flux will travel farther before falling down into the melt. That difference in traveling distance facilitates spreading and directing the granular flux inside a furnace.

    Consistency in chemistry is based on the fact that, since powder fluxes are blends of salts, each grain will represent only one of the different chemical compounds from which the flux is made. On the other hand, and because of the manufacturing process, granular fluxes have 100% grain uniformity in chemistry, since each grain will represent the same chemistry as the flux recipe. Figures 5 and 6 are used to illustrate this fact.

    References cited in this section

    1. R. Gallo, Development, Evaluation, and Application of Granular and Powder Fluxes in Transfer Ladles, Crucible, and Reverberatory Furnaces, AFS Sixth International Conference on Molten Aluminum Processing, 2001, p 56–70
    2. L.C.B. Martins, Chemical Factors Affecting the Separation of Inclusion from Molten Aluminum, AFS Third International Conference on Molten Aluminum Processing, 1992, p 79–91
    3. P. Crepeau et al., Characterization of Oxide Sludge, Dross and Inclusions in Aluminum Melting and Holding Furnaces, AFS Third International Conference on Molten Aluminum Processing, 1992, p 51–77
    4. D. Apelian and S. Dasgupta, Interaction of Initial Melt Cleanliness, Casting Process and Product Quality: Cleanliness Requirements Fit for a Specific Use, AFS Fifth International Conference on Molten Aluminum Processing, 1998, p 234–258
    5. K. Strauss, Applied Science in the Casting of Metals, Pergamon, 1970, p 253–256
    6. M.H. Kogan, Design and Development of Fluxing Agents for the Aluminum Foundry Alloys, AFS Second International Conference on Molten Aluminum Processing, 1989, p 25–1 to 25–12
    7. T.A. Utigard et al., The Properties and Uses of Fluxes in Molten Aluminum Processing, J. Met., Nov 1998, p 38–43
    8. S.R. Sibley Granular Fluxes for Aluminum Alloys, Environmental and Technological Advances, AFS Fourth International Conference on Molten Aluminum Processing, 1995, p 417–430

  3. #13
    Aluminum Fluxes and Fluxing Practice

    Categories of Fluxes

    Years of casting experience have demonstrated that proper fluxing is a key element to cost reduction and quality improvement in the liquid melt. Fluxing is the best means of obtaining clean metal by preventing excessive oxide formation, removing nonmetallic inclusions from the aluminum melt, and preventing and removing oxide buildup from furnace walls. Historically, solid fluxes have been classified into four categories, depending on their use and function at the foundry operation. These categories are:
    • Cover fluxes
    • Drossing fluxes
    • Cleaning fluxes
    • Furnace wall cleaner fluxes

    Cover, drossing, and melt-cleaning fluxes are usually manually spread, shoveled, or thrown onto the melt surface. However, drossing and melt-cleaning fluxes can also be added via rotary flux injection and/or lance flux injection. Furnace wall cleaner fluxes are typically blown (with a gunning device) onto the furnace walls at the melt line. The flux injection process introduces the powder flux in a finely dispersed stream to the bottom of the melt.
    To ensure that melt-cleaning and drossing fluxes will do their job effectively (regardless if they are either the powder or granular type), it is essential that, after manual addition, the flux be stirred into the melt to achieve as much contact as possible with the molten metal and dross layer. Based on the size of the furnace, the stirring can be accomplished manually by the furnace tender or mechanically by a forklift or mechanized vehicle. Typically, agitation and stirring of the flux is completed between 1 and 3 min in crucibles or transfer ladles and between 5 and 10 min in reverberatory furnaces with up to 41,000 kg (90,000 lb) of liquid capacity. If flux injection is used, there is no need for manual activation, since the mixing and activation would take place during the injection treatment cycle.
    It is also t to emphasize that the flux reaction efficiency depends on three interrelated factors: molten metal temperature, stirring, and activation time. Improper control of these factors will result in unreacted flux floating on top of the dross, as shown in Fig. 7.

    Cover fluxes are used, as the name implies, to cover the melt. These fluxes, which are comprised of salt compounds that form a liquid at normal aluminum melting temperatures, provide a molten barrier “blanket” on the surface of the metal to protect it against oxidation and hydrogen gas adsorption. Cover fluxes are mainly a blend of NaCl and KCl salts with small quantities of fluorides. Fluxes having these three compounds are suitable for almost all types of aluminum alloys, excluding hypereutectic aluminum alloys and aluminum-magnesium alloys with over 7% Mg.
    Over the years, a great number of different commercial cover fluxes have been developed with preferred additions and proportions of NaF, KF, Ca2F, and other compounds, such as CaCl2, for lowering the melting point of the flux, as well as for providing cleaning effects by agglomerating dirt and oxides. A typical base composition is 47.5% NaCl, 47.5% Kcl, and 5% Na3 AlF6, or cryolite. Other composition components are listed in Table 2.

    Table 2 Typical fluxing compounds employed

    AlF3 F X
    CaCl2 F
    MgCl2 F
    MnCl2 F X
    KF F
    NaF F X Na
    NaCl F
    KCl F
    CaF2 T X
    Na3AlF6 T X
    Na2SiF6 T X
    KNO3 X X
    C2Cl6 X Cl2, AlCl3
    K2CO3 X CO2
    Na2CO3 X CO2
    K2TiF6 X Ti
    KBF4 X B

    Cover fluxes, for smelting application may have slightly lower melting points than those used in foundry applications. Foundries use these types of fluxes during the melting of heavily oxidized foundry returns and machining chips, as well as when metal holding temperatures exceed the 770 to 790 °C (1420 to 1450°F) range.
    In smelting operations, cover fluxes are mixed in the rotary furnace during the melting of fine scrap, turnings, sawings, fines, and so on. Typically, cover fluxes for smelting applications have lower melting points than those used for foundry applications (e.g., 424 versus 665 °C, or 795 versus 1229 °F).

    Drossing fluxes are designed to assist in separating the entrapped aluminum from the adherent oxide skin envelope, which surrounds the aluminum in the dross layer. A high percentage of useful metallics can be lost when wet drosses are removed from the furnace; thus, many operations attempt to reduce the metallic content by in-furnace treatment of the drosses with exothermic fluxes. These contain oxidants and fluorides to generate temperature and help promote a separation of the oxides from the metallics. The exothermic reaction does consume aluminum values while generating heat and can be the source of a loss of up to 20% of the contained metallics in the dross, if the flux is applied too liberally (see the article “Dross, Melt Loss, and Fluxing of Light Alloy Melts” in this Volume for more information on drossing fluxes).
    Drossing fluxes may be based on salt blends of chlorides, simple and/or double fluorides, and oxidizing compounds. Therefore, drossing fluxes are able to react exothermically, generating heat and improving flux wettability. The wetting action of the flux promotes coalescence, which brings the fine aluminum drops tighter to form larger drops that are much easier to recover. The work of drossing fluxes is considered to be due to both the surface tension effects and the dissolution of aluminum oxides.
    There are wide commercial ranges of different flux compositions. Even though a drossing flux will always be considered as “hot or reactive flux” in furnace-tender terms, it is important to realize that the reactivity in a drossing flux is due to a combination of the oxidizers and the double-fluoride compounds. This is an important concept to understand, because even without double fluorides, the flux may look too reactive due to an excess of oxygen-bearing compounds that may be burning excessive amounts of good metallic aluminum without actually dissolving aluminum oxides in the melt.
    Some manufacturers may decrease the use of double fluorides, since they are more expensive compared to other compounds. At the same time, it is important to note that a lower-grade compound, which is cheap, may also decrease the effectiveness of the flux. The same concept will be true for any other type of flux; the quality of the chemical compounds will influence the price of the flux.
    Thus, a good drossing flux must be designed to reduce the rich metallic aluminum content of the dross. As the dross is treated with the drossing flux, it changes from a wet dross appearance (bright, shiny metallic color) to a dry dross appearance (dark, powdery). Proper flux treatment could reduce the amount of metallic aluminum content of the dross to 30%. Figure 8 shows the difference in appearance between untreated dross and dross treated with flux.

    Cleaning Fluxes. Melt-cleaning fluxes are designed to remove aluminum oxides from the melt. Melt-cleaning fluxes usually have similar chlorides and oxidizing compounds as the ones used in the drossing fluxes, but in different proportions. In addition, the composition of a melt-cleaning flux will typically include only simple fluorides, compared to the simple and double fluorides that are present in drossing fluxes. Since a melt-cleaning flux is less reactive than a drossing flux, it will yield less dry dross than a drossing flux.
    The work of metal-cleaning fluxes is considered to be only due to the surface tension effects, as previously stated. It is very important not to confuse a metal-cleaning flux with a furnace wall-cleaning flux.

    Furnace Wall-Cleaning Flux. Wall-cleaning fluxes are specifically designed for the softening and removal of excessive aluminum oxide buildup that occurs on the walls of melting furnaces, especially along the melt line. This type of flux helps keep crucible and furnace walls free of oxide buildup above and below the melt line. They are usually applied with a lance or flux gun or may simply be broadcast manually onto the walls. Slightly exothermic, these fluxes, under closed-door and high-fire conditions, increase the localized temperature of the oxide buildup and thereby soften or loosen the attached oxides, permitting easier mechanical removal and subsequent skimming.
    Wall-cleaning fluxes contain the highest amounts of double-fluoride compounds, such as Na2SiF6 and Na3AlF6. The exothermic reactions that occur because of the oxidizing compounds and the double fluorides enhance more penetration of the flux into the oxide buildup, facilitating the removal of the oxide buildup at the furnace wall. Fluoride-free fluxes cannot be used effectively as a wall-cleaning flux.
    If the oxide buildup has been converted over time to the insidious form of aluminum oxide called corundum, it cannot be removed chemically. However, in its early stages, sufficient loosening of the mildly adherent product can be effected with the combination of fluxing salts and hard work.

  4. #14
    Administrator Site Admin Anon's Avatar
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    That article is an excellent reference. Do you have a link back to the source? (Also, the embedded links appear to be broken: I'd guess they were originally internal links from the way they now point to nonexistent pages on Alloy Avenue.)

    Salt = flux (helps separate metal and dross, increasing the quality of the cast part by reducing the amount of oxides included in the metal, may also improve fluidity of the metal if it's got a lot of oxides in it).

    Baking soda = nothing. Active flux additive? I don't understand what, if anything, it does to help. As far as I know it will react with the aluminum, creating more dross. It may create some heat in the process, so maybe it's useful as an additive to an active drossing flux (the idea is to heat the dross up by burning the small bits of aluminum in the presence of molten flux, allowing the larger bits to flow down into the melt). There may be better chemicals for that--I'm far from an expert on it. Nor am I sure if it's even useful in this role.

    Chlorine-releasing compounds (hexachloroethane, several hypochlorites) = active degassers (remove hydrogen by chemical action as well as diffusion, help separate oxides by mechanical flotation and stirring).

    Inert gas (nitrogen, argon, SF6) = degassers (remove hydrogen by diffusion, help separate oxides by mechanical flotation and stirring). Less effective but safer than chlorine-based degassers.
    The process of turning stumbling blocks into stepping stones can at times require the use of a large sledgehammer.

    Foundry Tutorial
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  5. #15
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    here's a link some might find interesting.


  6. #16
    Senior Member Tobho Mott's Avatar
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    Take this with a grain of salt (so to speak), and know that there could be some small errors in what I'm about to say. But I believe this is all correct at least in a general sense...

    I am just starting to experiment with fluxes myself, and to an even lesser extent with degassing. But I somehow managed to avoid getting them mixed up, so perhaps my voice of very limited experience can actually be helpful to someone here. The article pasted in above is very interesting and helpful, but for me as a newbie it was a little heavy to digest easily, and I suspect I am not the only one who is going to have to come back and read it again after a couple more months of casting and reading about casting to get the full benefit of it...

    Anon, who had some great advice for me in another thread about flux, has summed up the differences quite neatly above: flux steals metal back from oxides so that less is wasted, and degassing removes gas from the melt. As a side-effect of degassing, some unskimmed oxides may also be brought to the surface of the melt where further skimming can remove them.

    I made some salt flux by melting down some Windsor Half-Salt (half salt (NaCl) and half potassium chloride (KCl), plus a few trace ingedients that can be ignored) and pouring salt "muffin" ingots, which I then crumbled up into small pieces to be dropped on top of a melt and stirred around a minimal amount (I stop when the flux seems to have fully dissolved and spread around the top of the melt evenly, but possibly I should be trying to stir it down into the metal too? Only thing I'm sure of is too much stirring makes more dross and possibly gas problems) before skimming and pouring. On the couple of occasions I've used flux, it noticeably broke up the dross on top of the melt; less crud was left to skim off, and what I skimmed off was blackish and crumbly once it cooled instead of looking like one big solid lump of dull metal. Seems to work well; presumably you are left with a bit more metal to pour since some portion of it isn't getting skimmed off with the dross and going to waste.

    I've watched all of Myfordboy's casting videos on Youtube, and he is using washing soda as a degasser, not baking soda. In one or two of his videos he makes a point of distinguishing between the two and stating he uses one and not the other. But everything else I have read about degassing says that isn't an effective was to degas - I'm no chemist, but the scientific reasons given for that seem convincing enough to me. I've learned a lot from watching his videos though - he makes it look really easy even if he isn't degassing the way others say it should be done, and his castings look great.

    Pool shock (the bottle I have contains calcium chlorite, but I gather any chlorite pool shock should work) is one easily found degasser. I read that one good way to use it is to wrap up some in aluminum foil and then push that to the bottom of your melt and hold it there with some sort of tool with small holes drilled into it to let the gas come up as smaller bubbles rather than bigger ones once the foil melts (which I guess should happen almost immediately). Haven't built a tool to do that yet though, so I can't comment from experience or anything. I've never noticed any bubbles trapped in my castings or inside cut-off sprues etc. or anything, but I still want to try it just for the experience, in case something I cast in the future really requires it...

  7. #17
    You have to have a license for the ASM online manual. I just copied and pasted what I thought would be most interesting. Tomorrow I will fix some of the links. I thought I made that table of the types of uses for typical fluxes but I guess I didn't check after I posted it. The x's correspond to each of the types of flux purposes (and the table above that are different properties).

    I will get on that tomorrow. If you would rather I start a new thread so it could be stickied let me know.

  8. #18

  9. #19
    Administrator Site Admin Anon's Avatar
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    I can't get that link to work. I don't know if it's on my end or not...I'll try again later.

    Stickied either way: there's lots of good stuff in this thread.

    EDIT: Never mind, the link works now. (Well, it's a little bit slow, but it's not saying "Page not found" anymore.) Must have been a problem on my end.

    EDIT again: The PDF is ~50MB and 2002 pages, no wonder it was being slow. I don't have time to read it right now but it looks like an excellent reference.
    The process of turning stumbling blocks into stepping stones can at times require the use of a large sledgehammer.

    Foundry Tutorial
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  10. #20

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