Blanketing Atmosphere for Molten Aluminum-Lithium Or Pure Lithium

Europaisches Patentamt European Patent Office © Publication number: 0 268 841 Office europeen ctes brevets A1

EUROPEAN PATENT APPLICATION

® Blanketing atmosphere for molten aluminum-lithium or pure lithium.

© Blanketing of molten aluminum-lithium alloys is performed under a nontoxic and noncorrosive dichlorodifluoromethane containing gas atmosphere, which produces a thin self-passivating fluxing film on the melt surface. The blanketing atmosphere protects the melt from oxidation, burning, and lithium evaporation, improves alloy cleanliness and can be used in any furnace, transfer or casting operation. The blanketing atmosphere can be applied in the entire range of commercial or master aluminum- lithium alloys including pure lithium melts. The dichlorodifluoromethane concentration in the blanketing atmosphere can range from 0.05 to 100 vol% with the remainder being an inert gas such as argon.

00 00 CO CM

CL 111

Xerox Copy Centre 0 268 841

BLANKETING ATMOSPHERE FOR MOLTEN ALUMINUM-LITHIUM ALLOYS OR PURE LITHIUM

continuously monitoring the ingot casting rate and Field of the Invention continuously adding a measured and controlled amount of molten lithium beneath the surface of This invention relates to the production of the molten aluminum stream as it flows to the ingot aluminum-lithium alloys, and more particularly to 5 casting station. At the contact location of the lithium the protective atmospheres for the operations of and aluminum, a mixture of argon and chlorine melting, holding, alloying, stirring, degassing, mold and/or other inert and reactive fluxing gases is casting, and direct chill casting of aluminum-lithium injected through a vaned, rotating dispenser. The alloys. patent further discloses that the introduction of the 70 lithium into the aluminum must be below the surface of the aluminum in order to minimize the Background of the Invention occurrence of oxidation, fuming and hydrogen absorption. The production of aluminum-lithium alloys has Both U.S. Patent 4,248,630 and 4,556,535 become of commercial interest, due to the com- 75 counterbalance the detrimental effects of lithium bination of mechanical properties and light weight reactivity by means of minimizing time between the which these alloys exhibit. Unfortunately, molten alloying and casting, neither process deals effec- aluminum-lithium alloys are very reactive with air tively with problems of submerged injection of a which makes their production and fabrication cor- premelted lithium charge, inert blanketing, lithium respondingly difficult. 20 evaporation and melt hydrogen pick-up. Both sys- The surface of an Al-Li bath reveals chemical tems suffer from the lack of proper melt surface behavior of molten lithium rather than aluminum protection for inert gas bubbling and handling oper- thus causing the bath to: (1) burn on contact with ations. air thus forming an excessive dross layer with the Batch processes utilizing molten salt fluxes are generation of toxic fumes resulting in poor lithium 25 an alternative to the continuous systems, discussed recovery and hazardous work conditions; (2) attract above, which are expensive and inflexible in opera- hydrogen from the atmosphere, including traces of tion, particularly when operating ranges or alloy water vapor, which increases hydrogen absorption changes are required. These fluxes, which are and results in higher porosity levels and a loss of comprised primarily of lithium chloride or lithium the desired mechanical properties; and (3) become 30 fluoride, are applied to the surface of the lithium practically unskimmable thus preventing proper containing bath whereby they eliminate a part of stirring and degassing of the melt since any disrup- the problem related to the lithium reactivity and still tion of the generated dross will increase the rate at achieve a lithium recovery of approximately 80 which further quantities of dross are formed. To wt%. Unfortunately, disruptions in the bath surface overcome these enumerated difficulties, several so- 35 whether by stirring or degassing or any other lutions have been offered in the literature. movement in the bath breaks the flux layer and U.S. Patent 4,248,630 discloses a process for exposes the metal to ambient air resulting in violent adding alloying elements, including highly reactive oxidation of the lithium. Also, fluxes are highly metals such as lithium, to molten aluminum so that corrosive to the refractory linings of the furnace normally occurring oxidation reactions of such ele- 40 and related casting equipment and materials of ments with the stmosphere is minimized. Basically, construction. The fluxes are also known to deterio- the process requires that all other alloying ele- rate the metal cleanliness and contaminate the ments except lithium be added to the molten alu- environment as well as the equipment including minum and the melt be degassed and filtered. melting, mixing, holding, and alloying furnaces, Upon completion of the degassing/filtering step the 45 metal transfer troughs, casting stations, direct-chill lithium is introduced into a mixing crucible as the liners and molds. Difficulties associated with stor- final step prior to casting. The desired concentra- age and handling of the fluxes frequently cause a tion of the lithium is achieved by controlling the carry over of moisture into the aluminum-lithium relative amount of lithium and the alloyed melt. melt .and the subsequent oxidation and hydrogen Uniformity of the mixture is achieved by mechani- so pick-up. cal stirring. The mixing crucible and all other cru- Other solutions such as blanketing with a pure cibles in with lithium may be present are kept dry inert atmosphere eliminate the flux method under an argon blanket. drawbacks, however, these require tightly enclosed U.S. Patent 4,556,535 discloses a process for pots and troughs and therefore are not flexible forming aluminum-lithium alloys which comprises enough to be used in various stages of aluminum- 0 268 841

lithium fabrication. Furthermore, inert atmosphere Basically, the CCbFa'Ar blanketing blend is ap- blanketing does not decrease lithium evaporation plied to the molten aluminium-lithium alloys during from the bath, which results in; substantial lithium the melting, holding, alloying, stirring, degassing, losses and creates a potential1 hazard. Inert at- melt transfer and casting processes. As a result of mosphere blanketing does not provide flux layer 5 the application of CCI2F2 reacts with the alloy for- cleaning properties such as preventing the hydro- ming a passivating and self-healing viscous liquid gen just removed from the bath during degassing layer which protects the metal from oxidization, from freely back-diffusing into the uncovered alloy, burning, hydrogen and/or moisture pick-up, hydro- and/or allowing nonmetallic inclusions which have gen back-diffusion, and lithium loss due to an moved to the bath surface during inert gas stirring 10 evaporation effect. The formed liquid layer can be to be intercepted by the flux layer. skimmed without harm to the metal if the process requires a reactive bubbling skimmed operation for degassing and/or inclusion removal. Thus, both an Summary of the Invention inerting and fluxing benefit is achieved. 75 The CCbFyinert gas blend should be applied to The present invention is a protection process the molten aluminium bath while the lithium is for use in melting, holding, alloying, stirring, degas- introduced into the aluminum or at any later mo- melt sing, transfer and casting processes for mol- ment or stage of the aluminum-iithium melt pro- ten aluminum-lithium alloys or lithium. The process cessing. The gas blend (atmosphere) may also be of the present invention comprises blanketing the 20 contained above a pure lithium melt as well. of top a molten aluminum-lithium alloy or lithium CCI2F2 concentration in the blend may be var- bath with an effective amount of a nontoxic, reac- ied in the range of 0.05 to 100 vol%; the result tive, dichlorodifluoromethane containing, gas atmo- being the higher the CCI2F2 concentration the high- sphere. The dichlorodifluoromethane reacts with er the rate at which the resultant fluxing film is primarily the lithium in the melt and rapidly forms a 25 formed. The application of a 100% by volume thin fluxing layer on the surface of the bath. This CCI2F2 atmosphere over the melt will not cause any thin layer prevents oxidation of the melt, hydrogen hazardous conditions. A 0.05-5.0 volume % CCI2F2 absorption into the melt, and the formation of a concentration in the inert gas is preferred. The inert heavy dross layerr the thin layer is easily skimmed gas can be chosen from the group consisting of Ar, from the surface if necessary. The layer develops 30 He, etc. Since nitrogen is slightly reactive and even if not all of the ambient air is evacuated from nonprotective to both lithium and aluminum and above the melt. nitrogen will cause deterioration in melt cleanliness, Alternatively, other substitue blanketing at- in those instances where melt cleanliness is not a mospheres containing an effective amount of a paramount concern, nitrogen can be used as the halogen compound having at least one fluorine 35 inert gas. atom and one other halogen atom selected from The dichlorodifluoromethane could be replaced the group consisting of chlorine, bromine and io- by other reactive gases. These other reactive gas- dine, or an atmosphere comprising fluorine or a es of the blend can consist of any combination of fluorine-containing compound and one other halo- chlorine and fluorine bearing gases. It is believed gen or halogen-containing compound wherein said 40 that fluorine only initiates the passivating reaction is selected halogen from the group consisting of and the amount of fluorine in the reactive gas need chlorine, bromine and iodine will work in the pro- not exceed the amount of chlorine. Under a pre- cess of the present invention. The use of these dominantly fluorine atmosphere, the metal-gas re- alternative atmospheres would result in the same action may become uncontrolled and result in protective layer. 45 burning. The chlorine of the reactive gas may be substituted by bromine or iodine. Any molecular combination of the above gas elements which may Detailed Description of the Present Invention include other elements such as carbon or sulfur, can be utilized in blanketing of the aluminum-lith- The invention present is a process for protect- 50 ium alloys or other reactive metals, however, any ing an alloy which comprises aluminium and lithium preferred embodiment should produce a nontoxic or pure lithium which uses a nontoxic, noncor- gas. Any tbxicity of the reactive gas will signifi- rosive, dichlorodifluoromethane containing, gas cantly limit the applicability of the blend in foundry blanketing atmosphere, which inerts and fluxes the operations. surfaces of melt. Preferably the nontoxic, noncor- 55 The CCl2F2/inert gas blend is useful for the rosive, dichlorodifluoromethane containing, gas entire range of aluminum-lithium alloys and blanketing atmosphere is comprised of dich- aluminum-lithium master alloys up to 100% wt of lorodifluoromethane and an inert gas, e.g. argon. lithium. The blend is not, however, recommended 0 268 841

for pure aluminum melts, since its specific protec- sivating lithium carbonate component of the protective and fluxing properties are manifested only in tive layer. presence of lithium. Although the mechanism of CCI2F2 blanketing Although not being helt to any particular theory is speculative, aluminum-lithium and lithium melts as to why the present invention should work, the 5 are well protected by the CCI2F2 originated layer. In most likely explanation is that in certain tempera- order to demonstrate the efficacy of the present ture ranges, lithium chloride passivates lithium ex- Invention the following examples where run. posed to chlorine and aluminum fluoride passivates alluminum exposed to fluorine, and carbon may further enhance the molten metal protection effect. 10 Examples: To further the explanation, CCI2F2 is thermally sta- ble and inert at temperatures exceeding those of Example 1 molten aluminum-lithium production. When exposed to the highly reactive and molten lithium A well stirred molten aluminum-3% lithium al- containing alloy surface, the CCI2F2 gas enters into 75 loy was held under a cold transparent lid at a series of chemical reactions resulting in a com- 1300°F. The lid becomes coated with a thick me- plex lithium chloride and lithium fluoride containing tallic deposit after less than 1/2 hour if the furnace layer. Traces of oxygen and lithium oxide, present headspace were filled with argon. at the melt surface, are combined together into a A blend of 5 vol% CCI2F2 in argon gas blend lithium carbonate product. Of these, lithium chlo- 20 was then introduced into the headspace. The result ride and lithium carbonate are liquid and lithium was that a thin viscous transparent liquid layer was fluoride and lithium oxide are solid at normal bath formed on the melt surface. No deposits were temperature. Besides, lithium chloride and lithium found on the lid. carbonate are characterized by a Pilling-Bedworth Then a measured amount of ambient air, i.e. ratio of more than one, which means, that their 25 containing some water vapor, was mixed with the layer is compact and once formed will hinder diffu- CCbFa/Ar blend and introduced into the headspace sion of reactants in either direction. Therefore, lith- to simulate disturbances in the blanketing process ium chloride and lithium carbonate, as well as which may occur during casting operations in a lithium bromide or iodide and unlike lithium oxide, typical foundry environment. The result was that a fluoride or nitride will form a self-passivating layer. 30 thin viscous transparent liquid layer was found Aluminum of the aluminum-lithium melt is far less along with a powdery graphite deposit over the reactive than lithium and having a much larger molten metal surface. When the metal surface was atomic radius has a lower diffusivity. Yet, part of mechanically skimmed to remove the formed vis- the aluminum may react with the CCI2F2 and of the cous transparent layer, the freshly exposed metal resultant aluminum chloride or fluoride, only the 35 was shiny and unoxidized. The metal surface be- latter is protective in terms of a Pilling-Betworth came dull and oxidized, when concentration of air ratio. It is believed that the non-protective lithium in the blend exceeded 25 vol%. fluoride and the protective aluminum fluoride will Then the CCI2F2 concentration in the CCl2F2/Ar combine to form complex viscous particles, LJ3AIF6. blend was increased to 100% vol. The increasing This cryolite type compound, together with lithium 40 CCI2F2 concentration resulted in an increase of rate, chloride and lithium carbonate passivate the melt at which the thin transparent liquid layer was surface to the point at which it is impermeable to formed. No burning, fuming and deposits on the the gaseous or metallic ions. The passivation pro- cold lid occurred and no HF, HCI, CO, and CO2 cess is quick and the resultant surface layer is thin emissions were detected throughout the entire test- and compact. Formation of the non-protective, and 45 ing. gaseous at the aluminum-lithium melt temperature, aluminum chloride is therefore not only unfavored but also kinetically hindered. A further inspection of Example 2 thermodynamic properties of the involved com- pounds shows that only fluorine can replace oxy- 50 The CCI2F2 component of the CCyVAr blend gen from thin lithium oxide and aluminum oxide was replaced by other nontoxic reactive gas, i.e. films, which will always be present at the melt sulfur hexafluoride, which molecules contained flu- surface in a foundry environment. It is conduced orine but not chlorine atoms. This gas when tested that fluorine atoms are necessary to initiate the on pure aluminum melts produced thin elastic sur- blanketing reaction, chlorine, bromine or iodine 55 face skins. The modified blend was introduced into atoms provide material for the lithium layer pas- the aluminum-lithium furnace headspace and the sivation and carbon plays a secondary role by tests of Example 1 were repeated. The blend pro- scavenging lithium oxide and oxygen into a pas- duced a thick and lumpy unskimmable dross un- 7 0 268 841

less the reactive gas concentration in argon ex- Claims ceeded 4- vol% and when this concentration was exceeded1 the aluminum-lithium melts burned pro- 1. In a process for protecting of an alloy which gressively increasing the metal bath temperature. comprises aluminum and lithium in a molten state Any additions of air into the blend were found to 5 by blanketing the molten alloy, the improvement facilitate the ignition and intensify burning and fum- comprising carrying out the blanketing utilizing an ing. atmosphere containing an effective amount of a halogen compound having at least one fluorine atom and one other halogen atom selected from w the group consisting of chlorine, bromine and iodine, whereby a passivating and self-healing vis- A pure lithium bath was blanketed with CCI2F2 cous liquid layer is formed. resulting in a liquid transparent layer and small 2. The process of Claim 1 wherein said other amount of a powdery graphite coating on the sur- halogen atom is chlorine. face of the melt. When the test was repeated with rs 3. The process of Claim 1 wherein said other the reactive gas of Example 2, violent burning of halogen atom is bromine. bath resulted. 4. In a process for protecting of an alloy which comprises aluminum and lithium in a molten state by blanketing the molten alloy, the improvement 20 comprising carrying out the blanketing utilizing an atmosphere containing an effective amount of dich- the tests presented in examples #1 and #2 lorofluoromethane, whereby a passivating and self- were repeated for aluminum-lithium alloys which heating viscous liquid layer is formed. contained 1.7 and 4.0wt% of lithium and for an 5. The process of Claim 4 wherein said dich- increased temperature regime of 1420°F. The test 25 lorodifluoromethane containing atmosphere is a results were the same as those previously noted. mixture of dichlorodifluoromethane and an inert As can be seen from these Examples, the gas. process of the present invention accomplishes the 6. The process of Claim 5 wherein said inert formation of protective, self-passivating and self- gas is selected from a group consisting of argon, healing thin liquid layer over the surface of molten 30 heiium or mixtures thereof. aluminum-lithium alloys, master alloys and pure 7. The process of Claim 5 wherein dich- lithium, which can protect the metal from oxidation, lorodifluoromethane comprises from 0.05 to 5.0 vol- burning, hydrogen pick-up and back-diffusion, and ume percent of said mixture. lithium evaporation from the melt during melting, 8. The process of Claim 5 wherein dich- holding, alloying, mixing or stirring, degassing, melt 35 lorodifluoromethane comprises from 0.05 to 5.0 vol- transfer, and casting operations. The process facili- ume percent of said mixture and said inert is tates the formation of a thin and skimmable flux selected from the group consisting of argon, helium layer, which can actively participate in the and mixtures thereof. aluminum-lithium melt cleaning operations and 9. The process of Claim 4 wherein said dich- does not require application of salts, that are corro- 40 lorodifluoromethane containing atmosphere is a sive to the fabrication equipment and contaminant pure dichlorodifluoromethane. molten metal, equipment, and the environment. 10. in a process for protecting of molten lithium The nontoxic protective and treatment atmosphere by blanketing the molten lithium, the improvement for molten aluminum-lithium alloys which can be comprising carrying out the blanketing utilizing an applied during casting or any molten metal treat- 45 atmosphere containing an effective amount of dich- ment or transfer where a gas outleak is possible is lorofluoromethane, whereby a passivating and self- safe, eliminates any fire hazards and performs healing viscous liquid layer is formed. even in the presence of air or water vapor impuri- 11. The process of Claim 10 wherein said ties. dichlorodifluoromethane containing atmosphere is a The present invention has been described with so mixture of dichlorofluoromethane and an inert gas. reference to several preferred embodiments there- 12. The process of Claim 11 wherein said inert of. However, these embodiments should not be gas is selected from a group consisting of argon, considered a limitation on the scope of the inven- helium or mixtures thereof. tion, which scope should be ascertained by the 13. The process of Claim 11 wherein dich- following claims. 55 lorodifluoromethane comprises from 0.05 to 5.0 volume percent of said mixture. 0 268 841 10

14. The process of Claim 11 wherein dichlorodifiuoromethane comprises from 0.05 to 5.0 volume percent of said mixture and said inert is selected from the group consisting of argon, helium and mixtures thereof. 5 15. The process of Claim 10 wherein said dichlorodifluoromethane containing atmosphere is a pure dichlorodifluoromethane. 16. In a process for protecting of an alloy which comprises aluminum and lithium in a molten 10 state by blanketing the molten alloy, the improvement comprising carrying out the blanketing utilizing an atmosphere comprising an effect amount of fluorine or a fluorine-containing compound and one other halogen or halogen-containing compound 75 wherein said halogen is selected from the group consisting of chlorine, bromine and iodine, whereby a passivating and self-heating viscous liquid layer is formed. 17. The process of Claim 16 wherein said other 20 halogen atom is chlorine. 18. The process of Claim 16 wherein said other halogen atom is bromine. 19. A process for protecting an aluminum-lithium alloy during melting, casting and fabrication of 25 wrought shapes by enveloping the exposed surfaces with an atmosphere containing an effective amount of a halogen compound having at least one fluorine atom and one other halogen atom selected from the group consisting of chlorine, bromine and 30 iodine. 20. The process of Claim 19 wherein said halogen compound is dichlorodifiuoromethane.

55 European Patent Number J EUROPEAN SEARCH REPORT Application Office

EP 87 11 5574

DOCUMENTS CONSIDERED TO BE RELEVANT Category Citation of document with indication, where appropriate, Relevant CLASSIFICATION OF THE of relevant passages to claim APPLICATION ant. CI.4) US-A-3 854 934 (DORE et al.) 1-6,9, * C 22 C 1/02 Claims 1,4,6,7,8 * & FR-A-2 233 406 16,19, 20 C 22 B 9/05

DE-B-2 818 495 (DIPL.-ING. HANS HORST 1,2,4-6 SCHMELZ- UND GIESSTECHNIK) * Claims 1,2; column 4, lines 45-59 * & BE-A-875 860

WELDING JOURNAL, vol. 64, no. 5, May 1,2,4-8 1985, pages 21-27, Miami, US; A.C. BICKNELL et al . : "GMA welding of aluminum with argon/freon shielding gas mixtures" * Whole document *

TECHNICAL FIELDS SEARCHED (Int. C1.4)

C 22 C C 22 B

The present search report has been drawn up for all claims Place of search Date of completion of the search Examiner THE HAGUE 19-02-1988 LIPPENS M.H.

CATEGORY OF CITED DOCUMENTS T : theory or principle underlying the invention E : earlier patent document, but published on, or X : particularly relevant if taken alone after the filing date Y : particularly relevant if combined with another D : document cited in the application document of the same category L : document cited for other reasons a A : technological background O : non-written disclosure & : member of the same patent family, corresponding G P : intermediate document document sua.