United States Patent (19) 11) Patent Number: 4,770,697 Zurecki 45) Date of Patent: Sep. 13, 1988

54 BLANKETING ATMOSPHERE FOR 722978 10/1978 U.S.S.R. . MOLTEN ALUMNUM-LITHIUM ALLOYS 697252 11/1979 U.S.S.R...... 164/66.1 OR PURE LITHIUM OTHER PUBLICATIONS 75) Inventor: Zbigniew Zurecki, Allentown, Pa. Aluminum-Lithium Alloys II, Second International 73) Assignee: Air Products and Chemicals, Inc., Al-Li Conference, 1983, pp. 657-673. Allentown, Pa. "Phase Composition of Surface Films on Al Alloys (21) Appl. No.: 925,652 with Li..', Tarasenko et al., UDC 669.71'884:539.216.2, American Society for Metals. 22 Filed: Oct. 30, 1986 "Casting Problems Specific to Al-Li Alloys', Divecha 51 Int. Cl." ...... C22B 21/00; B22D 11/00 et al., Metallurgical Society/AIMF. 52 U.S. Cl...... 75/68 R; 420/528; Primary Examiner-Nicholas P. Godici 164/66.1; 164/68.1; 164/475 Assistant Examiner-Richard K. Seidel 58) Field of Search ...... 164/66.1, 67.1, 68.1, Attorney, Agent, or Firm-Willard Jones, II; James C. 164/259, 415, 475; 75/68 R; 420/528 Simmons; William F. Marsh 56) References Cited (57) ABSTRACT U.S. PATENT DOCUMENTS Blanketing of molten aluminum-lithium alloys is per 3,467,167 9/1969 Mahin ...... 164/56 formed under a nontoxic and noncorrosive dichlorodi 3,484,232 12/1969 Karinthi et al 75/45 fluoromethane containing gas atmosphere, which pro 3,640,702 2/1972 Karinthi et al. . 75/59 4,200,138 4/1980 Hildebrandt ...... 164/66 duces a thin self-passivating fluxing film on the melt 4,248,630 2/1981 Balmath ...... 75/135 surface. The blanketing atmosphere protects the melt 4,389,240 6/1983 Erich et al. .. 75/0.5 B from oxidation, burning, and lithium evaporation, im 4,402,741 9/1983 Pollet et al...... 75/68 R proves alloy cleanliness and can be used in any furnace, 4,522,784 6/1985 Enright et al...... 420/590 transfer or casting operation. The blanketing atmo 4,532,106 7/1985 Pickens ...... 420/528 sphere can be applied in the entire range of commercial 4,534,807 8/1985 Field et al...... 48/20.3 or master aluminum-lithium alloys including pure lith 4,556,535 12/1985 Bowman et al...... 420/580 ium melts. The dichlorodifluoromethane concentration 4,582,118 4/1986 Jacoby et al...... 164/475 in the blanketing atmosphere can range from 0.05 to 100 FOREIGN PATENT DOCUMENTS vol % with the remainder being an inert gas such as 59-19507 2/1984 Japan . argon. 2129345 5/1984 United Kingdom...... 164/475 1124540 6/1986 United Kingdom ...... 75/68 R 20 Claims, No Drawings 4,770,697 1. 2 lithium charge, inert blanketing, lithium evaporation BLANKETING ATMOSPHERE FOR MOLTEN and melt hydrogen pick-up. Both systems suffer from ALUMNUM-LITHIUM ALLOYS OR PURE the lack of proper melt surface protection for inert gas LITHIUM bubbling and handling operations. Batch processes utilizing molten salt fluxes are an FIELD OF THE INVENTION alternative to the continuous systems, discussed above, This invention relates to the production of aluminum which are expensive and inflexible in operation, particu lithium alloys, and more particularly to the protective larly when operating ranges or alloy changes are re atmospheres for the operations of melting, holding, quired. These fluxes, which are comprised primarily of alloying, stirring, degassing, mold casting, and direct 10 lithium chloride or lithium fluoride, are applied to the chill casting of aluminum-lithium alloys. surface of the lithium containing bath whereby they eliminate a part of the problem related to the lithium BACKGROUND OF THE INVENTION reactivity and still achieve a lithium recovery of ap The production of aluminum-lithium alloys has be proximately 80 wt %. Unfortunately, disruptions in the come of commercial interest, due to the combination of 15 bath surface whether by-stirring or degassing or any mechanical properties and light weight which these other movement in the bath breaks the flux layer and alloys exhibit. Unfortunately, molten aluminum-lithium exposes the metal to ambient air resulting in violent alloys are very reactive with air which makes their oxidation of the lithium. Also, fluxes are highly corro production and fabrication correspondingly difficult, sive to the refractory linings of the furnace and related The surface of an Al-Libath reveals chemical behav 20 casting equipment and materials of construction. The ior of molten lithium rather than aluminum thus causing fluxes are also known to deteriorate the metal cleanli the bath to: (1) burn on contact with air thus forming an excessive dross layer with the generation of toxic fumes ness and contaminate the environment as well as the resulting in poor lithium recovery and hazardous work equipment including melting, mixing, holding, and al conditions; (2) attract hydrogen from the atmosphere, 25 loying furnaces, metal transfer troughs, casting stations, including traces of water vapor, which increases hydro direct-chill liners and molds. Difficulties associated gen absorption and results in higher porosity levels and with storage and handling of the fluxes frequently cause a loss of the desired mechanical properties; and (3) be a carry over of moisture into the aluminum-lithium melt come practically unskimmable thus preventing proper and the subsequent oxidation and hydrogen pick-up. stirring and degassing of the melt since any disruption Other solutions such as blanketing with a pure dry of the generated dross will increase the rate at which inert atmosphere eliminate the flux method drawbacks, further quantities of dross are formed. To overcome however, these require tightly enclosed pots and these enumerated difficulties, several solutions have troughs and therefore are not flexible enough to be used been offered in the literature. in various stages of aluminum-lithium fabrication. Fur U.S. Pat. No. 4,248,630 discloses a process for adding 35 thermore, inert atmosphere blanketing does not de alloying elements, including highly reactive metals such crease lithium evaporation from the bath, which results as lithium, to molten aluminum so that normally occur in substantial lithium losses and creates a potential ha ring oxidation reactions of such elements with the atmo zard. Inert atmosphere blanketing does not provide flux sphere is minimized. Basically, the process requires that layer cleaning properties such as preventing the hydro all other alloying elements except lithium be added to gen just removed from the bath during degassing from the molten aluminum and the melt be degassed and freely back-diffusing into the uncovered alloy, and/or filtered. Upon completion of the degassing/filtering allowing nonmetallic inclusions which have moved to step, the lithium is introduced into a mixing crucible as the bath surface during inert gas stirring to be inter the final step prior to casting. The desired concentration cepted by the flux layer. of the lithium is achieved by controlling the relative 45 amount of lithium and the alloyed melt. Uniformity of SUMMARY OF THE INVENTION the mixture is achieved by mechanical stirring. The The present invention is a protection process for use mixing crucible and all other crucibles in which lithium in melting, holding, alloying, stirring, degassing, melt may be present are kept under an argon blanket. transfer and casting processes for molten aluminum U.S. Pat. No. 4,556,535 discloses a process for form 50 lithium alloys or lithium. The process of the present ing aluminum-lithium alloys which comprises continu invention comprises blanketing the top of a molten ously monitoring the ingot casting rate and continu aluminum-lithium alloy or lithium bath with an effec ously adding a measured and controlled amount of tive amount of a nontoxic, reactive, dichlorodifluoro molten lithium beneath the surface of the molten alumi methane containing, gas atmosphere. The dichlorodiflu num stream as it flows to the ingot casting station. At 55 oromethane reacts with primarily the lithium in the melt the contact location of the lithium and aluminum, a and rapidly forms a thin fluxing layer on the surface of mixture of argon and chlorine and/or other inert and the bath. This thin layer prevents oxidation of the melt, reactive fluxing gases is injected through a vaned, rotat hydrogen absorption into the melt, and the formation of ing dispenser. The patent further discloses that the in a heavy dross layer; the thin layer is easily skimmed troduction of the lithium into the aluminum must be from the surface if necessary. The layer develops even below the surface of the aluminum in order to minimize if not all of the ambient air is evacuated from above the the occurrence of oxidation, fuming and hydrogen ab melt. sorption. Alternatively, other substitute blanketing atmo Both U.S. Pat. Nos. 4,248,630 and 4,556,535 counter spheres containing an effective amount of a halogen balance the detrimental effects of lithium reactivity by 65 compound having at least one fluorine atom and one means of minimizing time between the alloying and other halogen atom selected from the group consisting casting, however, neither process deals effectively with of chlorine, bromine and iodine, or an atmosphere com the problems of submerged injection of a premelted prising fluorine or a fluorine-containing compound and 4,770,697 3 4. one other halogen or halogen-containing compound The CCl2F2/inert gas blend is useable for the entire wherein said halogen is selected from the group consist range of aluminum-lithium alloys and aluminum-lithium ing of chlorine, bromine and iodine will work in the process of the present invention. The use of these alter master alloys up to 100% wt of lithium. The blend is native atmospheres would result in the same protective not, however, recommended for pure aluminum melts, layer. since its specific protective and fluxing properties are manifested only in presence of lithium. DETAILED DESCRIPTION OF THE PRESENT Although not being held to any particular theory as INVENTION to why the present invention should work, the most The present invention is a process for protecting an 10 likely explanation is that in certain temperature ranges, alloy which comprises aluminum and lithium or pure lithium chloride passivates lithium exposed to chlorine lithium which uses a nontoxic, noncorrosive, dichloro and aluminum fluoride passivates aluminum exposed to difluoromethane containing, gas blanketing atmo fluorine, and carbon may further enhance the molten sphere, which inerts and fluxes the surfaces of melt. metal protection effect. To further the explanation, Preferably the nontoxic, noncorrosive, dichlorodifluo 15 CCl2F2 is thermally stable and inert at temperatures romethane containing, gas blanketing atmosphere is exceeding those of molten aluminum-lithium produc comprised of dichlorodifluoromethane and an inert gas, e.g. argon. tion. When exposed to the highly reactive and molten Basically, the CCl2F2/Arblanketing blend is applied lithium containing alloy surface, the CCl2F2 gas enters to the molten aluminum-lithium alloys during the melt 20 into a series of chemical reactions resulting in a complex ing, holding, alloying, stirring, degassing, melt transfer lithium chloride and lithium fluoride containing layer. and casting processes. As a result of the application the Traces of oxygen and lithium oxide, present at the melt CCl2F2 reacts with the alloy forming a passivating and surface, are combined together into a lithium carbonate self-healing viscous liquid layer which protects the product. Of these, lithium chloride and lithium carbon metal from oxidation, burning, hydrogen and/or mois 25 ate are liquid and lithium fluoride and lithium oxide are ture pick-up, hydrogen back-diffusion, and lithium loss solid at normal bath temperature. Besides, lithium chlo due to an evaporation effect. The formed liquid layer ride and lithium carbonate are characterized by a Pill can be skimmed without harm to the metal if the pro ing-Bedworth ratio of more than one, which means, cess requires a reactive gas bubbling skimming opera that their layer is compact and once formed will hinder tion for degassing and/or inclusion removal. Thus, both 30 an inerting and fluxing benefit is achieved. diffusion of reactants in either direction. Therefore, The CCl2F2/inert gas blend should be applied to the lithium chloride and lithium carbonate, as well as lith molten aluminum bath while the lithium is introduced ium bromide or iodide and unlike lithium oxide, fluoride into aluminum or at any later moment or stage of the or nitride will form a self-passivating layer. Aluminum aluminum-lithium melt processing. The gas blend (at 35 of the aluminum-lithium melt is far less reactive than mosphere) may also be contained above a pure lithium lithium and having a much larger atomic radius has a melt as weil. lower diffusivity. Yet, part of the aluminum may react CCl2F2 concentration in the blend may be varied in with the CCl2F2 and of the resultant aluminum chloride the range of 0.05 to 100 vol%; the result being the or fluoride, only the latter is protective in terms of a higher the CCl2F2 concentration the higher the rate at Pilling-Betworth ratio. It is believed that the non-pro which the resultant fluxing film is formed. The applica tective lithium fluoride and the protective aluminum tion of a 100% by volume CCl2F2 atmosphere over the fluoride will combine to form complex viscous parti melt will not cause any hazardous conditions. A cles, LisalP6. This cryolite type compound, together 0.05-5.0 volume 96 CCl2F2 concentration in the inert with lithium chloride and lithium carbonate passivate gas is preferred. The inert gas can be chosen from the 45 group consisting of Ar, He, etc. Since nitrogen is the melt surface to the point at which it is impermeable slightly reactive and nonprotective to both lithium and to the gaseous or metallic ions. The passivation process aluminum and nitrogen will cause deterioration in melt is quick and the resultant surface layer is thin and com cleanliness, in those instances where melt cleanliness is pact. Formation of the non-protective, and gaseous at not a paramount concern, nitrogen can be used as the 50 the aluminum-lithium melt temperature, aluminum inert gas. chloride is therefore not only unfavored but also kineti The dichlorodifluoromethane could be replaced by cally hindered. A further inspection of thermodynamic other reactive gases. These other reactive gases of the properties of the involved compounds shows that only blend can consist of any combination of chlorine and fluorine can replace oxygen from thin lithium oxide and fluorine bearing gases. It is believed that fluorine only 55 aluminum oxide films, which will always be present at initiates the passivating reaction and the amount of the melt surface in a foundry environment. It is con fluorine in the reactive gas need not exceed the amount of chlorine. Under a predominantly fluorine atmo cluded that fluorine atoms are necessary to initiate the sphere, the metal-gas reaction may become uncon blanketing reaction, chlorine, bromine or iodine atoms trolled and result in burning. The chlorine of the reac provide material for the lithium layer passivation and tive gas may be substituted by bromine or iodine. Any carbon plays a secondary role by scavenging lithium molecular combination of the above gas elements which oxide and oxygen into a passivating lithium carbonate may include other elements such as carbon or sulfur, component of the protective layer. can be utilized in blanketing of the aluminum-lithium Although the mechanism of CCl2F2 blanketing is alloys or other reactive metals, however, any preferred 65 speculative, aluminum-lithium and lithium melts are embodiment should produce a nontoxic gas. Any toxic well protected by the CCl2F2 originated layer. In order ity of the reactive gas will significantly limit the applica to demonstrate the efficacy of the present invention the bility of the blend in foundry operations. following examples were run. 4,770,697 5 6 and back-diffusion, and lithium evaporation from the EXAMPLES melt during melting, holding, alloying, mixing or stir EXAMPLE1 ring, degassing, melt transfer, and casting operations. A well stirred molten aluminum-3% lithium alloy The process facilitates the formation of a thin and was held under a cold transparent lid at 1300 F. The lid 5 skimmable flux layer, which can actively participate in became coated with a thick metallic deposit after less the aluminum-lithium melt cleaning operations and does than hour if the furnace headspace were filled with not require application of salts, that are corrosive to the argon. fabrication equipment and contaminate molten metal, A blend of 5 vol% CCl2F2 in argon gas blend was equipment, and the environment. The nontoxic protec then introduced into the headspace. The result was that O tive and treatment atmosphere for molten aluminum a thin viscous transparent liquid layer was formed on lithium alloys which can be applied during casting or the melt surface. No deposits were found on the lid. any molten metal treatment or transfer where a gas Then a measured amount of ambient air, i.e. contain outleak is possible is safe, eliminates any fire hazards ing some water vapor, was mixed with the CCl2F2/Ar and performs even in the presence of air or water vapor blend and introduced into the headspace to simulate 15 impurities. disturbances in the blanketing process which may occur The present invention has been described with refer during casting operations in a typical foundry environ ence to several preferred embodiments thereof. How ment. The result was that a thin viscous transparent ever, these embodiments should not be considered a liquid layer was formed along with a powdery graphite limitation on the scope of the invention, which scope deposit over the molten metal surface. When the metal 20 should be ascertained by the following claims. surface was mechanically skimmed to remove the I claim: formed viscous transparent layer, the freshly exposed 1. In a process for protecting an alloy which com metal was shiny and unoxidized. The metal surface prises aluminum and lithium in a molten state by blan became dull and oxidized, when concentration of air in keting the molten alloy, the improvement comprising the blend exceeded 25 vol%. 25 carrying out the blanketing utilizing an atmosphere Then the CCF concentration in the CCl2F/Ar containing an effective amount of a halogen compound blend was increased to 100% vol. The increasing having at least one fluorine atom and one other halogen CCl2F2 concentration resulted in an increase of rate at atom selected from the group consisting of chlorine, which the thin transparent liquid layer was formed. No bromine and iodine and wherein the ratio of flourine to burning, fuming and deposits on the cold lid occurred 30 the other halogen atom in the halogen compound is less and no HF, HCl, CO, and CO2 emissions were detected than or equal to one, whereby a passivating and self throughout the entire testing. healing viscous liquid layer is formed which protects EXAMPLE 2. the molten alloy from lithium loss due to vaporization, The CCl2F2 component of the CCl2F2/Ar blend was 35 oxidation of the molten alloy, and hydrogen pick-up by replaced by other nontoxic reactive gas, i.e. sulfur hexa the molten alloy. fluoride, which molecules contained fluorine but not 2. The process of claim 1 wherein said other halogen chlorine atoms. This gas when tested on pure aluminum atom is chlorine. melts produced thin elastic surface skins. The modified 3. The process of claim 1 wherein said other halogen blend was introduced into the aluminum-lithium fur atom is bromine. nace headspace and the tests of Example 1 were re 4. In a process for protecting an alloy which con peated. The blend produced a thick and lumpy un prises aluminum and lithium in a molten state by blan skimmable dross unless the reactive gas concentration keting the molten alloy, the improvement comprising in argon exceeded 4 vol% and when this concentration carrying out the blanketing utilizing an atmosphere was exceeded the aluminum-lithium melts burned pro 45 containing an effective amount of dichlorodifluoro gressively increasing the metal bath temperature. Any methane, whereby a passivating and self-healing viscous additions of air into the blend were found to facilitate liquid layer is formed which protects the molten alloy the ignition and intensify burning and fuming. from lithium loss due to vaporization, oxidation of the molten alloy, and hydrogen pick-up by the molten al EXAMPLE 3 50 loy. A pure lithium bath was blanketed with CCl2F2 re 5. The process of claim 4 wherein said dichlorodifluo sulting in a liquid transparent layer and small amount of romethane containing atmosphere is a mixture of di a powdery graphite coating on the surface of the melt. chlorodifluoromethane and an inert gas. When the test was repeated with the reactive gas of 6. The process of claim 5 wherein said inert gas is Example 2, violent burning of the bath resulted. 55 selected from a group consisting of argon, helium or mixtures thereof. EXAMPLE 4 7. The process of claim 5 wherein dichlorodifluoro The tests presented in examples #1 and #2 were methane comprises from 0.05 to 5.0 volume percent of repeated for aluminum-lithium alloys which contained said mixture. 1.7 and 4.0 wt.% of lithium and for an increased tem- 60 8. The process of claim 5 wherein dichlorodifluoro perature regime of 1420 F. The test results were the methane comprises from 0.05 to 5.0 volume percent of same as those previously noted. said mixture and said inert is selected from the group As can be seen from these Examples, the process of consisting of argon, helium and mixtures thereof. the present invention accomplishes the formation of 9. The process of claim 4 wherein said dichlorodifluo protective, self-passivating and self-healing thin liquid 65 romethane containing atmosphere is a pure dichlorodi layer over the surface of molten aluminum-lithium al fluoromethane. loys, master alloys and pure lithium, which can protect 10. In a process for protecting molten lithium by the metal from oxidation, burning, hydrogen pick-up blanketing the molten lithium, the improvement com 4,770,697 7 8 prising carrying out the blanketing utilizing an atmo ing of chlorine, bromine and iodine and wherein the sphere containing an effective amount of dichlorodiflu ratio of flourine to the other halogen in the atmosphere oromethane, whereby a passivating and self-healing is less than or equal to one, whereby a passivating and viscous liquid layer is formed. self-healing viscous liquid layer is formed which pro 11. The process of claim 10 wherein said dichlorodi fluoromethane containing atmosphere is a mixture of tects the molten alloy from lithium loss due to vaporiza dichlorodifluoromethane and an inert gas. tion, oxidation of the molten alloy, and hydrogen pick 12. The process of claim 11 wherein said inert gas is up by the molten alloy. selected from a group consisting of argon, helium or 17. The process of claim 16 wherein said other halo mixtures thereof. 10 gen atom is chlorine. 13. The process of claim 11 wherein dichlorodifluo 18. The process of claim 16 wherein said other halo romethane comprises from 0.05 to 5.0 volume percent gen atom is bromine. of said mixture. 19. A process for protecting an aluminum-lithium 14. The process of claim 11 wherein dichlorodifluo alloy during melting, casting and fabrication of wrought romethane comprises from 0.05 to 5.0 volume percent 15 shapes by enveloping the exposed surfaces with an at of said mixture and said inert is selected from the group mosphere containing an effective amount of a halogen consisting of argon, helium and mixtures thereof. compound having at least one flourine atom and one 15. The process of claim 10 wherein said dichlorodi other halogen atom selected from the group consisting fluoromethane containing atmosphere is a pure dichlo of chlorine, bromine and iodine and wherein the ratio of rodifluoromethane. 20 fluorine to the other halogen atom in the halogen com 16. In a process for protecting an alloy which com pound is less than or equal to one, whereby a passivat prises aluminum and lithium in a molten state by blan ing and self-healing viscous liquid layer is formed which keting the molten alloy, the improvement comprising protects the alloy from lithium loss due to vaporization, carrying out the blanketing utilizing an atmosphere oxidation of the alloy, and hydrogen pick-up by the containing an effective amount of fluorine or a flourine 25 alloy. containing compound and an effective amount of one 20. The process of claim 19 wherein said halogen other halogen or halogen-containing compound compound is dichlorodifluoromethane. wherein said halogen is selected from the group consist k k is k

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