Parting composition

Abstract

 A parting composition contains alpha-olefin oligomer having a viscosity in the range of about 1-3 cs at 450°F. The parting composition in one aspect can be blended with a fatty ester or fatty alcohol. The parting composition is useful for the continuous casting of molten metal such as aluminum or aluminum alloy, which may contain lithium. A casting process for continuously casting molten metal, particularly containing lithium, through a mold or header includes applying a parting lubricant containing alpha-olefin oligomer. The lithium-­containing alloy may be continuously cast by direct chill with a direct chill coolant which is a moisture barrier liquid employing the disclosed parting compositions.

Description

[0001] This invention relates to parting compositions or mold lubricants for the continuous casting of molten metal, and to a process for casting molten metal in a mold, particularly for casting a lithium containing alloy.

[0002] Coninuous casting is widely used to form ingot from molten metal. The molten metal is introduced to one end of a mold which usually is cooled by heat exchange against a coolant such as water. Solidified metal is withdrawn from the other end of the mold. The soldified metal withdrawn from the mold has a solid wall surrounding a molten interior. Further cooling of the solid metal is required to solidify the entire cross section of the ingot.

[0003] Since the finished ingot will undergo working processes such as rolling or extrusion, it is critically important to avoid surface defects on the ingot. These defects, which include tears and blemishes with or without bleedouts of molten metal, interfere with the subsequent working processes for transforming the finished ingot into plate, sheet, extrusions, etc.

[0004] Surface defects in the ingot usually are controllable by adjusting casting conditions. The molten metal must be cooled at a rate sufficiently slow to avoid folds in the ingot which otherwise require deep scalping of the ingot to provide an adequate surface for subsequent working. Further, the molten metal should be cooled sufficiently fast to avoid the danger of tears or blemishes and particularly to avoid bleedouts of molten metal through the thin solidified skin of the ingot as it is withdrawn from the mold.

[0005] A parting composition or mold lubricant is applied to the mold of the continuous casting process to facilitate achieving a smooth ingot surface. In the continuous casting of aluminum or aluminum alloys, the most commonly used mold lubricant today is castor oil. However, castor oil is very viscous and difficult to apply to molds in a uniform fashion, especially in cold weather. Futher, castor oil undergoes polymerization at temperatures for the casting of aluminum. The polymerization produces a varnish-like film on the molds, which may generate tears and unsatisfactory surfaces on the finished ingot.

[0006] A parting composition or mold lubricant should meet several important requirements. It should have a viscosity at room temperature which permits easy and uniform application to the mold. The lubricant should have a viscosity at higher temperatures, e.g., at mold-ingot interface casting temperatures so that a stable lubricant film is maintained. The lubricant should have a high resistance to thermal degradation. The lubricant should resist polymerization to avoid the varnish-like deposits on the mold which produce unsatisfactory ingot surface. The lubricant should permit a high heat transfer from the molten metal to the mold. The lubricant should separate from the ingot coolant to avoid contaminating the coolant for re-circulation.

[0007] Several attempts have been made to achieve an ingot casting lubricant which satisfies all of the foregoing requirements. Smith et al., U.S. Patent No. 3,524,751, discloses a parting composition of 60%-80% by weight castor oil and 40%-20% of an alkyl ester of an acetylated hydroxy fatty acid, such as n-butyl acetyl ricinoleate.

[0008] Gardner et al., Canadian Patent No. 925,070, discloses a lubricant for horizontal continuous casting of aluminum including polybutene alone or with other ingredients, such as vegetable oil, castor oil, or rape seed oil; animal oil, such as lard oil or methyl lardate; or mineral oil, such as petroleum fractions.

[0009] According to the present invention there is provided a parting composition of the continuous casting of molten metal, containing alpha-olefin oligomer having a viscosity in the range of about 1-3 cs at 232°C. (450°F.).

[0010] Also according to the present invention there is provided a parting composition for continuously casting molten metal, comprising a lubricant substantially free from fatty esters.

[0011] Further, according to the present invention there is provided a casting process comprising casting molten metal in a mold, and applying to the mold the parting composition of the present invention.

[0012] According to the present invention there also is provided a process for direct chill casting a lithium-­containing alloy by direct chill with a chill coolant, and providing a moisture barrier liquid containing a vegetable oil or synthetic hydrocarbon having a flash point higher than about 204°C. (400°F.) to reduce water absorption in said coolant.

[0013] This invention thus also provides a process for direct chill casting a lithium-containing alloy including continuously casting the lithium-containing alloy by direct chill with a coolant and providing a moisture barrier liquid to reduce water absorption in the coolant. The moisture barrier liquid composition includes vegetable oils or synthetic hydrocarbons having flash points higher than about 204°C. (400°F.). In one aspect, the moisture barrier liquid comprises alpha-olefin oligomer.

[0014] In the accompanying drawing:

[0015] The Figure is an elevation view, partially in section, of a schematic apparatus for the continuous casting of molten metal through direct chill.

[0016] The present invention for the continuous casting of molten metal includes a parting composition containing alpha-olefin oligomer. The parting composition contains oligomer or oligomer blend having a viscosity in the range of about 1-3 cs at 232°C. (450°F.), preferably in the range of about 2.0-2.8 cs at 232°C. (450°F).

[0017] In one aspect, a parting composition of the present invention for casting lithium-containing alloy contains less than a varnish-film forming amount of triglyceride.

[0018] Primarily, continuous casting processes for transforming molten metal into finished ingot are controlled to achieve satisfactory ingot surface. The heat transfer from the molten metal to the mold must be controlled to avoid cooling the molten metal too fast such that folds are formed in the ingot surface. On the other hand, the heat transfer cannot be so slow that tears or blemishes are formed on the ingot surface. Uncontrollable tears will lead to bleedouts which can require the scrapping of the entire ingot.

[0019] Parting lubricant applied to the mold contributes substantially to controlling the heat transfer parameter. The parting lubricant should be applied uniformly over the mold to form a substantially uniform film thickness. This uniform film thickness should be maintained at casting termperatures. High temperature viscosity of the lubricant is important to maintain this uniform film. However, viscosity typically has been considered only with respect to low temperature handling and compatibility with existing delivery systems. The casting temper­ature for transforming the molten metal into ingot is signifi­cantly higher than room temperature. For this reason, the parting composition must have a viscosity at casting temperature sufficient to maintain a uniform lubricant film on the mold and thus a uniform rate of heat transfer from the molten metal to the mold.

[0020] The parting composition of the present invention includes an alpha-olefin oligomer or oligomer blend having a viscosity in the range of about 1-3 cs or preferably 2.3-2.7 cs at 232°C (450°F.). The com­position's viscosity at 232°C. (450°F.) is determined by ASTM D-341 Chart F. It has been found that such an alpha-olefin oligomer provides a uniform film thickness of lubricant over the mold when the alpha-olefin oligomer is selected to have a viscosity in the range of about 1-3 cs or preferably 2.0-2.8 cs at 232°C. (450°F) and more preferably in the range of about 2.4-2.6 cs at 232°C.(450°F.). This parting composition has been used in the continuous casting of aluminum and aluminum alloy to achieve satisfactory ingot surface characteristics.

[0021] In another aspect, the parting composition of the present invention includes a parting composition for casting a molten lithium-containing alloy, such as aluminum-lithium. It has been found that the parting composition containing alpha-­olefin oligomer of a viscosity in the range of about 1-3 cs or preferably 2.0-2.8 cs at 232°C. (450°F.) used in the continuous casting of aluminum-­lithium achieves ingot surface characteristics entirely superior to that obtained using prior art parting compositions.

[0022] In yet another aspect the parting composition of the present invention for casting lithium-containing alloy preferably contains less than a varnish-film forming amount of fatty ester, fatty acid, or fatty alcohol. A varnish-film forming amount is intended to mean an amount which forms a detrimental varnish film on the mold, i.e., which produces tears or blemishes on the finished ingot surface requiring treatment prior to subsequent working processes, such as to fabricate plate, sheet, extrusions, etc. from the ingot. The composition of the present invention preferably contains less than about 20% by weight fatty esters including triglycerides and more preferably less than about 5%. In a most preferred embodiment, the parting composition is substantially free from triglycerides. The triglycerides to be excluded include castor oil, which is identified chemically as triglyceride of ricinoleic acid (12-hydroxy oleic acid) and mixed triglycerides of oleic, linoleic, and stearic acids.

[0023] The parting composition can be formed by blending two or more alpha-olefin oligomers having different viscosities such that an oligomer blend is achieved having the viscosity in the range of about 1-3 cs or preferably 2.0-2.8 cs at 232°C. (450°F.). For example, such a blend can be achieved by mixing 25% alpha-olefin oligomer of 8 cs viscosity at 100°C. and 75% by weight alpha-olefin oligomer having 40 cs viscosity at 100°C. The resulting oligomer blend has a viscosity of about 2.3 cs at 232°C. (450°F.), which falls in the range of about 1-3 cs or 2.0-2.8 cs at 232°C (450°F.).

[0024] In one aspect, the parting composition of the present invention requires the alpha-olefin oligomer to have a viscosity in the range of about 1-3 cs or 2.0-2.8 cs at 232°C. (450°F.) to achieve satisfactory ingot surface characteristics. Below about 1.0 cs at 232°C. (450°F.) an inordinately higher rate of lubricant flow is needed to maintain uniform film thickness. A viscosity above about 3.0 cs at 232°C (450°F.) retards heat transfer from the molten metal to the mold such that tears and blemishes form on the ingot surface.

[0025] Tearing and bleedouts occur on the ingot with parting composition viscosity less than about 1.0 cs at 232°C. (450°F.). Staining and liquation occur with parting compositions having viscosity above about 3.0 cs at 232°C. (450°F.). When liquation occurs, the ingot does not cool homogeneously since the overly thick oil provides only poor heat transfer from the molten metal to the mold.

[0026] In one embodiment for the casting of molten metal such as aluminum or aluminum alloy in the absence of lithium in detrimentally reactive concentrations, the parting composition can contain another lubricant, e.g., a fatty ester such as a triglyceride lubricant, e.g., such as castor oil. The triglyceride lubricant is mixed with alpha-olefin oligomer to reduce the cost of the parting composition. The parting composition should contain at least about 0.5% by weight alpha-olefin oligomer to provide the benefit of superior casting lubricant characteristics.

[0027] The parting composition of the present invention includes a blend of alpha-olefin oligomer and a fatty ester of at least about 8 carbon atoms. Below about 8 carbon atoms, the fatty ester is unacceptable for casting molten aluminum by reason of a lower boiling point and higher vapor pressure resulting in an unacceptable lubricant characteristic.

[0028] Further, the parting composition of the present invention can include limited amounts of surface-absorbing lubricant additives having a hydrocarbon end and a polar end. Examples include triglycerides, fatty esters, fatty alcohols, fatty acids, and fatty amides. Fatty esters include organic compounds of the formula: R₁COOR₂
where R₁ = C₇-C₁₉, branched, straight, saturated, or unsaturated chained aliphatic and
R₂ = branched or straight chained C₁-C₄.
Examples include butyl stearate, methyl ricinoleate, methyl laurate and isopropyl oleate. Esters of R₁ below about C₇ are not good lubricants. Fatty esters also include synethetic and naturally occurring triglycerides, such as glycerol trioleate, castor oil, and rapeseed oil. The fatty ester such as triglyceride can be present in the parting composition in an amount of about 10%-95% by weight and more preferably about 10%-50% by weight to include, for example, a lubricant blend of about 50% by weight alpha-olefin oligomer and 50% by weight triglyceride.

[0029] The parting composition also can contain alpha-olefin oligomer and fatty alcohols of the formula: ROH
where R = branched, straight, saturated, or unsaturated C₁₀-C₂₀ alcohols.
Examples include lauryl alcohol, oleyl alcohol, iso-stearyl alcohol, and others. The parting composition can contain from about 10%-95% by weight alcohol and preferably from about 10%-50% by weight alcohol. The parting composition also can contain blends of alpha-olefin oligomer fatty ester and fatty alcohol. The fatty ester-fatty alcohol mixture (20%-80% by weight ester/20%-80% by weight alcohol) can be present in the parting composition of the present invention in an amount of about 10%-95% by weight and preferably about 10%-50% by weight.

[0030] Notwithstanding the foregoing description of the present invention of the parting composition including fatty ester, fatty acids, and fatty alcohols, the present invention in one aspect includes a parting composition having less than a varnish-forming amount of these compounds. It has been found that fatty esters including triglycerides present in a varnish-forming amount in the parting composition for the casting of lithium-containing alloys produce an unacceptable varnish on the mold. This varnish leads to tears and blemishes on the ingot, the more dramatic of which produces bleedouts. We have found that the fatty esters including triglycerides of castor oil have a tendency to form a soap in casting lithium-containing alloy.

[0031] Alpha-olefin oligomer also is known as isoparaffinic oligomer or polyalphaolefin. Alpha-olefin oligomer is a member of the class of synthetic lubricants including, cycloaliphatics, dialkylbenzene, diesters, halogenated products, phosphate esters, polyalkylene glycols, polyalphaolefins (alpha-olefin oligomers), polybutenes, polyolesters, polyphenol ethers, silicate esters, and silicate fluids. Polyalkylene glycols, phosphate esters, diesters, and polybutenes made up nearly 80% of the total synthetic industrial lubricant usage in 1980. Phosphate ester is the synthetic lubricant base stock most used in fire retardant and industrial hydraulic fluids. Alpha-olefin oligomers are formed by polymerization, or more specifically, oligomerization.

[0032] The following sequence of carefully controlled chemical reactions represents the formation of one class of alpha-olefin oligomer oils.

[0033] The oligomerization reaction can be controlled closely to yield substantially one structure of synthetic oil. The physical property of the resulting oil will be fairly constant and predictable. The preceding diagrams use decene-1 as the starting raw material. Suitable olefin oligomers are also manufactured by mixtures of C₄ - C₁₆ alpha-olefin monomers, each monomer preferably having 6-12 carbon atoms. Alpha-olefin oligomers are available commercially from Gulf Oil Company as Synfluid, i.e., under the trade name Synfluid, from Bray Oil Company as PAOL, from Mobil as Mobil SHF, from Emery Industries as Poly-x-olefin, and from Ethyl Corporation.

[0034] The parting composition of the present invention can contain an oxidation inhibitor, e.g., 2,6-di-tert-butyl paracresol.

[0035] The parting composition can contain a biocide to prevent bacteriological degradation in bulk storage.

[0036] This invention also relates to the continuous casting of molten metal in a mold.

[0037] A continuous casting process transforms molten metal into ingot for subsequent working such as by rolling or extrusion forming. The continuous casting process takes molten metal and pours it into contact with a mold which typically is water-cooled to extract heat from the molten metal through the wall of the mold. In this way, the outer part of the molten metal cools and solidifies into a shell, the shell further cooling and forming as it withdraws from the mold to form a solid, continuously formed ingot.

[0038] Metal casting processes in general have always required a lubricant for separating cast metal from a mold surface. Lard oil was commonly used as a mold lubricant for aluminum ingot casting until the mid-1950s. The lard oil was applied to molds by brushing or swabbing prior to the casting operation. Lard oil had the principal disadvantage of hardening to a highly viscous, grease-like consistency at approximately 4.4°C. (40°F.). This grease-like form interfered with continuous casting methods where free-flowing lubricant is required. Further, the grease-like lard oil would build up on molds and interfere with ingot cooling.

[0039] As continuous casting became the accepted method for forming ingot, castor oil replaced lard oil as the most commonly used mold lubricant. Castor oil is obtained from pressing seeds of the castor plant. Typically, castor oil contains a predominant amount of the triglyceride of ricinoleic acid (12-hydroxyoleic acid). The remaining portion of the castor oil comprises mixed triglycerides of oleic, linoleic, and stearic acids. Castor oil thus falls in a chemical classification known as fatty oils. These material, as a class, are practically insoluble in water and dissolve freely in organic solvents. The double bonds in hydroxyl groupings in castor oil produce many kinds of chemical reactions to form a wide variety of compounds.

[0040] Castor oil does not have the greese-like consistency of lard oil at just below room temperature. However, castor oil is very viscous and difficult to apply to molds in a uniform fashion, especially in cold weather operation. Castor oil undergoes polymerization under casting conditions and produces a varnish-like film on the mold and the ingot. This varnish-like film produces tears and unsatisfactory surface characteristics in the ingot. Further, in direct chill casting by water, castor oil does not separate from the cooling water easily to avoid contamination of the discharged water.

[0041] The disadvantages of castor oil used as a mold lubricant in continuous casting have encouraged the search for a replacement mold lubricant.

[0042] Further this invention relates to the continuous casting of a lithium-containing alloy such as aluminum-lithium alloy.

[0043] Conventionally, large ingots of high strength light metal, e.g., such as aluminum, are produced by continuous direct chill casting of molten metal using water as the direct chill coolant. A continuous ingot having a solid surface but a core which is still molten is formed in a water-cooled mold. After passing through the mold, coolant impinges directly on the hot solid ingot surface to provide direct chill cooling. The water then separates and falls from the ingot after extracting heat.

[0044] Lithium-containing alloys, such as aluminum-lithium alloys, offer substantial advantages for high technology applications such as aircraft plate, sheet, forgings, and extrusions. Light metal lithium-containing alloys are highly regarded for material properties such as low density, high strength, high modulus of elasticity, and high fracture toughness. The combination of these material properties can reduce the weight of large commercial airliners by as much as 5400 kg (six tons) or more. The resulting weight savings can reduce an aircraft's fuel consumption by 830000 liters (220,000 gallons) or more during a typical year of operation.

[0045] A process for continuously casting lithium-containing alloys into acceptable ingots of large size depends on the manner of cooling. Typically, water is used as the direct chill coolant in conventional processes. However, water coming into contact with lithium-containing alloy has been found to present a substantial risk of violent explosion.

[0046] However, a further problem has been discovered in the continuous casting of lithium-containing alloy which stands in the way of the substantial commercial development of large-scale applications such as large size ingot for aircraft plate and sheet.

[0047] It has been found that conventional parting compositions, i.e., mold lubricants, for the continuous casting of molten metal into ingot fail to provide an acceptable lubricant film between the solidifying lithium-containing alloy ingot and the mold surface.

[0048] Castor oil is the most commonly used parting composition in the continuous casting of aluminum. Castor oil is identified chemically as the triglyceride of ricinoleic acid (12-hydroxy oleic acid) which accounts for about 80%-85% by weight of commercial castor oil. The remaining portion of castor oil is composed of the mixed triglycerides of oleic, linoleic, and stearic acids. Although castor oil is used as the predominant parting composition of choice in the continuous casting of aluminum with water as the direct chill coolant, it has been found that castor oil fails to perform in casting aluminum-lithium alloy containing more than about 1.5% by weight lithium. Rather, the castor oil used as a parting composition in the continuous casting of lithium containing alloy produces substantial surface tears in ingots larger than about 15 to 30 cm(6-12 inches) in length for 2% lithium by weight and larger than only about 5 to 8 cm (2-3 inches) for 3% lithium by weight.

[0049] It has been found that molten metal can be continuously cast in a mold to produce ingot having preferred surface characteristics. The process of the present invention includes applying a mold lubricant containing alpha-olefin oligomer to the mold.

[0050] Alpha-olefin oligomers also are known as isoparaffinic oligomers or polyalphaolefins, and they are classified among the synthetic lubricants. Synthetic lubricants are not new. The first synthetic hydrocarbon oils were produced as early as 1877. Research concentrated on synthetic lubricants in the late 1930s and early 1940s. The second World War pointed out the inadequacies of petroleum lubricants in severe cold weather climates where mineral oil products gelled at extreme low temperatures, preventing aircraft, tanks, and other vehicles from starting. With this critical need in mind, ester lubricants were developed by German research. In 1947 the English began using ester lubricants in turboprop aircraft where mineral oil lubricants could not perform satisfactorily in high temperatures. Twelve major synthetic lubricant base stocks include cycloaliphatics, dialkylbenzene, diesters, halogenated products, phosphate esters, polyalkylene glycols, polyalphaolefins (alpha-olefin oligomers), polybutenes, polyolesters, polyphenol ethers, silicate esters, and silicate fluids. The synthetic lubricants have higher viscosity indices (VI) than mineral oil-base stock. A high VI means less change (decrease) of viscosity at higher temperatures. For this reason, the synthetic lubricants are suitable additives for crank case applications in automobiles.

[0051] It has also been found that parting composition conventionally used in the continuous casting of aluminum do not produce satisfactory results in casting lithium-containing alloys such as aluminium-lithium alloys containing lithium in an amount of more than 1.5% by weight. Lithium has been found to cleave the ester of conventional parting compositions to produce a lithium soap in a varnish-like film on the mold or header.

[0052] The lithium soap occurs according to the following equation:
R-

-O-R →R-

-O-Li.

[0053] This undesirable reaction occurs with fatty esters including triglycerides, such as castor oil and glycerol trioleate. A similar reaction also occurs with fatty acids. Fatty alcohols and polyols such as pentaerythritol form alkoxides.

[0054] The parting composition of the present invention in one aspect contains less than a varnish-film forming amount of compounds detrimentally reactive with aluminum-lithium alloy such as fatty acids, fatty alcohols, and fatty esters including triglycerides. The parting composition preferably contains less than 20% and more preferably less than 5% by weight of compounds which are detrimentally reactive with aluminum-lithium, such as fatty esters, fatty acids, and fatty alcohols. The varnish-like film which forms on the mold produces undesirable tears and bleedouts in the solidified ingot. The most preferred parting composition of the present invention includes a composition substantially free from varnish-film forming amounts of fatty esters, fatty acids, and fatty alcohols. The reaction between these varnish-forming compounds and aluminum-lithium containing more than about 1.5% by weight lithium will occur with as little as 0.1% by weight of the compounds in the parting composition. However, it does not become an insurmountable problem until the amount of varnish-forming compound exceeds a varnish-film forming amount which is detrimental to the ingot surface.

[0055] The parting composition of the present invention provides a suitable lubricant film at operating temperatures for the continuous casting of aluminium-lithium alloy. Our parting composition also provides a viscosity low enough at room temperature so that it an be pumped satisfactorily and distributed in controllable volumes to the mold.

[0056] The parting composition of the present invention has acceptable vapor pressure at casting temperatures. The parting composition provides a uniform thickness of lubricant on the mold having a high thermal and oxidative resistance. Most importantly, the parting composition of the present invention provides excellent lubrication to prevent metal sticking or transferring to the mold and to produce a smooth surface to the ingot. Such lubrication has not been found in prior art parting compositions for continuous casting processes.

[0057] The lubricating process of this invention has been used with efficient results in the continuous casting of aluminum and aluminum alloys. It has been found that the flow rate of mold lubricant can be reduced significantly.

[0058] A further embodiment of this invention provides a moisture barrier for the direct chill coolant in a direct chill continuous casting process. In casting conventional aluminum alloys, the direct chill coolant is water. However, water presents a signficant risk of violent explosion when used as a direct chill coolant for the continuous casting of aluminum-lithium alloy. The explosion risk is particularly high for aluminum alloys containing lithium in an amount above about 1.5-2% by weight. Nevertheless, aluminum-lithium alloy effectively can be continuously cast using an organic composition, e.g., such as ethylene glycol, as the direct chill coolant.

[0059] Certain preferred organic casting coolants, e.g., ethylene glycol, are hydroscopic, and moisture will accumulate in the coolant, e.g., when exposed to normal atmospheric conditions. A direct chill coolant of ethylene glycol will extract moisture from the air in amounts equal to about twice its initial volume. When the absorbed water content reaches a certain level, e.g., about 5-10% by weight or more of the coolant, e.g. of the ethylene glycol, the explosion hazard returns.

[0060] The present invention provides a process for inhibiting the extraction of water from the air into the direct chill coolant. The process of the present invention includes a moisture barrier liquid which substantially covers the coolant and which is impervious to water. The moisture barrier liquid is immiscible with the coolant so that the moisture barrier liquid does not become inseparably mixed with the direct chill coolant. The moisture barrier liquid has a high flash point, i.e., higher than about 204°C. (400°F.) and preferably higher than about 260°C. (500°F.), thereby preventing fires when bleedouts pass molten metal through the ingot solid surface. The moisture barrier liquid provides a high density difference over the coolant resulting in superior gravimetric separation. The moisture barrier liquid has a low functionality such that ion-dipole, dipole-dipole, or hydrogen bonding is reduced.

[0061] The composition for providing a moisture barrier liquid in the process of the present invention can be selected from base stocks including vegetable oils such as triglycerides or triglyceride blends having flash points greater than 204°C. (400°F.) and preferably greater than 260°C. (500°F.), e.g., glycerol trioleate, castor oil, and others, and synthetic hydrocarbons such as cycloaliphatics, polyalphaolefins also known as alpha-olefin oligomers or isoparaffinic oligomers, polybutenes, and alkylated benzenes having flash points higher than 204°C. (400°F.) and preferably higher than 260°C. (500°F.).

[0062] Referring now to the Figure, a schematic apparatus is illustrated for the purpose of describing the present invention as applied to casting an aluminum alloy containing lithium. Molten metal at about 715°C. (1320°F.) is passed in line 2 through direct chill casting device 4 to interior 6 of ingot 8. Interior 6 includes a molten pool having solidus line 10 which forms initially as a solid shell 12 in a solidus temperature, e.g., on the order of about 590°C. (100°F.). Coolant at a temperature substantially below 590°C. (1100°F.) is passed in line 14 to casting device 4 which is adapted to place the coolant in thermal contact, such as including but not limited to heat transfer through a mold surface (not shown), such that molten metal 6 is continuously cast as shell 12.

[0063] Starting block 19 initially is placed directly under or inside casting device 4 to form a base 21 of ingot 8. Starting block 19 then is withdrawn to a position under the casting device (as shown) thereby permitting a continuous casting process. Shell 12 grows in thickness while ingot 8 is cooled by direct chill. Coolant at a temperature, by way of example, of about 49°C. (120°F.) is applied at 18 to the surface of shell 12 of the continuously forming ingot. Coolant liquids flow down the solid surface of the ingot as indicated by directional arrow 20, and ingot 8 is cooled by direct contact or direct chill. Coolant increases in temperature as it flows down the solid ingot surface. Warmed coolant-separates from the ingot by falling into the casting pit where it collects as a pool or reservoir 22.

[0064] A moisture barrier liquid 11 of a high flash point vegetable oil or synthetic hydrocarbon, e.g., such as, alpha-olefin oligomer having a flash point of about 274°C. (525°F.), is placed on the surface of coolant liquid in reservoir 22. The alpha-olefin oligomer is substantially immiscible and of low specific gravity relative to the coolant liquid, e.g., ethylene glycol, so that the moisture barrier will collect on the surface of coolant pool 22.

[0065] Coolant is recirculated in line 15 from reservoir 22 to join line 14.

[0066] Alpha-olefin oligomer is passed in line 17 to direct chill casting device 4 and is applied to the mold as parting composition. The alpha-olefin oligomer incorporated as parting composition lubricates the mold to reduce the friction between the mold and the thin moving ingot shell as illustrated by shell 12 in the Figure. Otherwise, the continuously forming ingot would tear on the mold surface. Such tears not only are defects on the ingot surface but also facilitate bleedouts of molten metal in direct contact with coolant. Such bleedouts are to be avoided particularly in casting lithium-containing alloys.

[0067] Moisture barrier 11 of immiscible fluid containing alpha-olefin oligomer lubricant is provided on the coolant in the reservoir, e.g., by floating the oligomer having a density less than the coolant. Barrier layer 11 acts as a substantially impermeable barrier to moisture absorption by the ethylene glycol. However, it is impractical to prevent some moisture pickup during casting and holding of the coolant in the direct chill process, and the coolant can be dried by many different drying techniques such as sparging. The moisture content of the coolant should be controlled to maintain a preferred level, such as within a predetermined range of water content in the coolant.

[0068] Aluminum-lithium alloy having a lithium content on the order of about 1.2% by weight lithium (Aluminum Association Alloy 2020) conventionally has been cast in a continuous ingot by direct chill with water, i.e., substantially 100% water. However, molten aluminum-lithium alloy containing even slightly higher amounts of lithium, such as about 1.5% to 2% or higher amounts by weight lithium, can react with a violent reaction or explosion when brought into direct contact with water or water/glycol mixtures as may occur with the bleedout during continuous direct chill casting process.

[0069] The process of the present invention avoids such a violent reaction and maintains a moisture barrier over the coolant in the reservoir of the casting pit. The process of the present invention thereby holds the moisture or water content in the organic coolant below a predetermined level to prevent explosive reaction when direct chill casting lithium-containing alloys having more than about 1.5% by weight lithium.

[0070] The process of the present invention in one embodiment applies the moisture barrier liquid, e.g., alpha-olefin oligomer as the parting composition or mold lubricant.

[0071] The moisture barrier and parting composition of the present invention can be formed from a blend of two or more alpha-olefin oligomers. Preferably, the alpha-olefin oligomer or oligomer blend has a viscosity in the range of about 1-3 cs at 232°C. (450°F.). The composition's viscosity at 232°C. (450°F.) is determined by the method published in ASTM D445. Below about 1 centistroke, the oligomer or oligomer blend does not provide adequate lubrication without substantial increases in the rate of flow. Above about 3 centistokes, the oligomer or oligomer blend retards heat transfer from the molten metal to the mold.

[0072] The moisture barrier and parting composition combination of the present invention contains less than a varnish-film forming amount of fatty ester, fatty acid, or fatty alcohol. Fatty esters such as triglycerides and including castor oil will form varnish-like films when in contact with the lithium-containing alloy. Preferably, the moisture barrier and parting composition should contain less than about 20% by weight triglycerides. More preferably, the moisture barrier and parting composition contains less than about 5% by weight triglycerides. In the most preferred embodiment, the moisture barrier and parting composition of the present invention is substantially free from triglycerides.

[0073] The parting composition of the present invention is further described through the following actual Examples.

Example 1

[0074] Aluminum alloys containing more than about 5% by weight magnesium are harsh on prior art continuous casting parting lubricants. Such magnesium-containing aluminum alloys produce tears when casting with conventional parting lubricants. Aluminum alloys containing 8, 10, 12, and 14% by weight magnesium were continuously cast through a DC continuous casting process. DC continuous casting process refers to a process of vertical direct chill continuous casting in an open mold. A parting composition of Lubracin A1 was applied to the mold. Lubracin A1 is a trade name of Caschem Company for a mixture of 25% n-butyl acetyl ricinoleate and 75% castor oil. The finished ingot of alumimum-magnesium alloy cast using Lubracin A1 as the parting composition had significant tears on the ingot surface.

[0075] Substantially identical continuous casting processes were performed on the same aluminum-magnesium alloy composition except that the parting compositions contained alpha-olefin oligomer having a viscosity of 1-3 cs and of 2.0-2.8 cs at 232°C. (450°F.) The use of the parting compositions containing the alpha-olefin oligomer produced finished ingots having a smooth, tear-free surface.

Example 2

[0076] Aluminum-lithium alloy containing about 2% lithium by weight was continuously cast through a DC process at a rate of about 3 to 10 cm (3 inches to 4 inches) per minute with lubricant flowing at about 1 milliliter per minute. Various parting compositions were used as the mold lubricant.

[0077] A parting composition of castor oil applied to the mold produced significant tearing in the ingot.

[0078] A parting composition of glycerol trioleate, similar to castor oil but not containing hydroxyl group in the molecule, showed improvement but produced significant tears on the ingot and varnish on the mold. The varnish material was found to be a metallic soap.

[0079] A parting composition of glycerol trioleate and phosphite showed no improvement over glycerol trioleate. The combination produced tears on the ingot and varnish on the mold.

[0080] A parting composition of pentaerythritol ester looked better than the glycerol trioleate but produced tears on the ingot and varnish on the mold.

[0081] All of the four above-identified lubricants in this Example 2 were esters. The varnishes all contained metallic soap and hydroxyl functional groups which showed up on the mold.

[0082] A parting composition of polybutene produced no varnish on the mold but produced small tears on the surface of the ingot. Polybutene having an increased viscosity was tried but produced no improvement. Adding a film strength additive of fatty alcohol also did not improve the performance of the polybutene as parting lubricant. An infrared analysis of the polybutene showed an oxidation product and hydroxyl functional group. The hydroxyl group may have come from condensed coolant vapor or moisture in the air.

[0083] Parting compositions of alpha-olefin oligomers having a viscosity in the range of about 1-3 cs and of about 2.0-2.8 cs at 232°C. (450°F.) produced finished ingots having essentially no tears on the surface. The alpha-olefin oligomer permitted a reduction in the amount of lubricant applied to the mold by about 60% over the use of castor oil as the parting composition.

[0084] The parting composition of the present invention for the continuous casting of lithium-containing alloys is further described by reference to the following Example.

Example 3

[0085] Molten aluminum-lithium alloy at about 715°C. (1320°F.) was fed to a vertical continuous direct chill casting process. The molten metal was formed into an ingot through heat transfer from the molten metal to a mold. A parting composition was applied to the casting surface of the mold to reduce the friction between the moving ingot shell and the mold.

[0086] The process used ethylene glycol as the direct chill coolant. The aluminum alloy cast into ingot contained 2% by weight lithium. The casting rate was 7 to 10 cm (3 to 4 inches) per minute, and the lubricant flowing rate was 1 milliliter per minute.

[0087] The results of various parting compositions are shown in Table I. It was found that castor oil caused casting failure. Substantial tears fomed in the ingot surface.

[0088] Glycerol trioleate is chemically similar to castor oil but does not contain an hydroxyl group in the molecule. Although showing improvement over castor oil, glycerol trioleate produced substantial tears on the ingot and formed significant varnish on the mold. An analysis of the varnish material found metallic soap formation in the appearance of an hydroxyl functional group.

[0089] Phosphite added to the glycerol trioleate showed no improvement over glycerol trioleate. Tears were produced on the ingot and varnish found on the mold.

[0090] A more stable ester of pentaerythritol appeared to lubricate better than glycerol trioleate, but produced tears on the ingot and varnish on the mold.

[0091] A straight carbon hydrogen compound without any functional group, was tried. Polybutene produced no varnish on the mold but produced small tears on the surface of the ingot. Polybutene having an increased viscosity [about 1 cs at 232°C. (450°F.)] showed no improvement and also produced small tears on the mold. Polybutene having an added film strength additive of fatty alcohol produced no improvement over polybutene.

[0092] Alpha-olefin oligomer produced no varnish on the mold and no tears on the ingot surface. Alpha-olefin oligomer mold lubricant produced an aluminum-lithium alloy ingot containing 3% lithium by weight having no tears on the surface of the ingot. The parting composition of alpha-olefin oligomer also permitted a reduction in the amount of lubricant flow to the mold by 60% over castor oil lubricant.

Claims

1. A parting composition for the continuous casting of molten metal characterized by containing alpha-olefin oligomer having a viscosity in the range of about 1-3 cs at 232°C. (450°F.), and desirably in the range of about 2.0-2.8 cs at 232°C. (450°F.).

2. A composition as set forth in Claim 1, especially for casting a lithium-containing alloy, characterized in that said composition contains either less than a varnish-film forming amount of fatty ester, less than about 20% by weight fatty ester, or less than about 5% by weight triglyceride.

3. A parting composition according to claim 1, characterized by comprising an oxidation inhibitor such as 2,6-di-tert-butyl paracresol, and/or comprising a biocide.

4. A parting composition for continuously casting molten metal especially a lithium-containing alloy, characterized by comprising a lubricant substantially free from fatty esters, especially substantially free from triglyceride.

5. A composition as set forth in claim 4, characterized by containing alpha-olefin oligomer having a viscosity in the range of about 1-3 cs 232°C. (450°F.), preferably 2.0-2.8 cs at 232°C. (450°F.), and optionally containing a blend of two or more alpha-olefin oligomers.

6. A composition according to claim 1, characterized by containing fatty ester and at least about 0.5% by weight alpha-olefin oligomer, and desirably said fatty ester which may comprise a triglyceride is present in the range of either about 10%-95% or 10-50% by weight, and the composition optionally comprises an oxidation inhibitor.

7. A casting process, especially for continuous casting and direct chill casting, comprising casting molten metal in a mold, characterized by applying to the mold a parting composition as claimed in any one of the preceding claims.

8. A process according to claim 7, which comprises casting a lithium-containing alloy, especially for the continuous casting of aluminum-lithium, characterized by applying to a mold a parting composition which contains less than a varnish-film forming amount of triglyceride.

9. A process as set forth in claim 8, characterized in that said alloy is in aluminum alloy which contains lithium in an amount of either at least 2%, at least 2.5%, or at least 3% by weight.

10. A process for direct chill casting a lithium-­containing alloy, especially a lithium-containing alloy which comprises aluminum containing at least about 1.5% by weight lithium, comprising:
continuously casting the lithium-containing alloy by direct chill with a direct chill coolant; and characterized by
providing a moisture barrier liquid containing a vegetable oil or synthetic hydrocarbon having a flash point higher than about 204°C. (400°F.) to reduce water absorption in said coolant, with said moisture barrier liquid desirably being applied as a parting composition on a mold in the direct chill casting step, and optionally said coolant comprising a glycol.

11. A process as set forth in claim 10, characterized in that said moisture barrier liquid contains less than a varnish-film forming amount of fatty ester, fatty acid, or fatty alcohol, and desirably said liquid contains either less than about 20% by weight triglycerides, less than about 5% by weight triglycerides, or is substantially free from triglycerides.

12. A process as set forth in claim 10, characterized in that said moisture barrier liquid comprises a composition as claimed in claim 2.

Drawing