Abstract:
In a lubricant composition suitable for use in the manufacture of aluminum alloys comprising lubricant base selected from the group consisting of solid lubricants, liquid lubricants, grease lubricants, emulsion lubricants, and dispersion lubricants, the improvement wherein the lubricant composition further comprises: an effective amount of water and surfactant or water and a compound comprising phosphates, borates, fluorides, and silicates. It is believed that mixing oil with water and surfactant or one of these compounds provides a method for uniformly distributing the surface oxide at the meniscus for casting applications, thereby reducing vertical fold formation that lead to cracks in aluminum ingots. In addition, the mixture promotes uniform heat transfer around the mold allowing the solidifying aluminum alloy to stay in contact with the mold longer and form stronger ingot shells. A process for continuous or semi-continuous casting of aluminum alloys via the use of this lubricant composition is also disclosed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation-in-part of application Ser. No. 10/974,384, filed on Oct. 24, 2004. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The invention relates to lubricant formulations for use in the casting of aluminum or aluminum alloy ingots or bodies. In particular, the invention relates to using lubricants containing water and surfactants to improve the surface quality of cast ingots or bodies, resulting in enhanced product recovery. A method for producing aluminum or aluminum alloy ingots with enhanced surface quality is also disclosed.  
       Background of the Invention  
       [0003]     The casting of alloys may be done by any number of methods known to those skilled in the art, such as direct chill casting (DC), electromagnetic casting (EMC), horizontal direct chill casting (HDC), hot top casting, continuous casting, semi-continuous casting, die casting, roll casting, and sand casting.  
         [0004]     Each of these casting methods mentioned above has a set of its own inherent problems, but with each technique, surface imperfections can be an issue. In the aluminum alloy casting art, molten metal (or melt for brevity) surface oxidation can produce various surface imperfections in cast ingots such as pits, vertical folds, oxide patches and the like, which can develop into cracks during casting or in later processing. A crack in an ingot or slab propagates during subsequent rolling, for example, leading to expensive remedial rework or scrapping of the cracked material. One mechanical means of removing surface imperfections from an aluminum alloy ingot is scalping. Scalping involves the machining off a surface layer along the rolling faces of an ingot after it has solidified. However, scalping results in lost metal.  
         [0005]     Rectangular ingot yields for high magnesium alloys, such as 7050 and other 7xxx alloys as well as 5182 and 5083 alloys are especially prone to surface defects and cracking caused by initiation at vertical folds on the surface of the ingot. In the past, beryllium has been added, usually at part per million (ppm) levels to some of these alloys to control melt surface defects, and to prevent magnesium loss due to oxidation. In addition, materials, especially those containing fluorine, such as boron trifluoride and ammonium fluoroborate, have been used to promote uniform oxide distribution and therefore reduce surface defects and cracking. However, the use of these additives can be very costly and beryllium itself may fall into disuse due to allegations regarding health, disposal, and environmental issues that it creates. Furthermore, the use of gases can create toxic and corrosive gaseous atmospheres. For these reasons, suitable replacement strategies to control the nature of oxides during casting are needed.  
         [0006]     In the casting of aluminum alloys it is also known in the art to use a mold lubricant. Satisfactory ingot surfaces can be obtained using a lubricant that is effective in keeping aluminum from sticking to the mold at high temperatures used in casting aluminum alloys. In early casting practices, greases were commonly employed as mold lubricants. However, with the advent of modern casting methods, including continuous or semi-continuous casting, free flowing oils have been used to provide continuous lubrication and have replaced the use of greases as mold lubricants.  
         [0007]     Continuous casting refers to the uninterrupted formation of a cast body or ingot. For example, the body or ingot may be cast on or between belts, as in belt casting; between blocks, as in block casting; or in a mold or die that is open at both ends, as in direct chill (DC) casting. Casting may continue indefinitely if the cast body is subsequently cut into desired lengths. Alternately, the pouring operation may be started and stopped when an ingot of desired length is obtained. The latter situation is referred to as semi-continuous casting.  
         [0008]     Continuous lubrication is required for fully continuous casting and offers a number of advantages for semi-continuous casting. These advantages include substantial reduction of flame and smoke, substantial reduction of dragging and tearing tendencies near the end of the cast, and allowing casting practices that produce better quality and more uniform surfaces.  
         [0009]     Despite the use of continuous lubrication during casting, a limitation of current ingot casting practice exists in the non-uniform growth of oxide at the meniscus of molten metal at the mold interface. Non-uniform oxide growth at the meniscus of the molten metal and mold interface is particularly problematic for alloying elements that rapidly oxidize in air or in air containing moisture. For example, alloys containing lithium and magnesium may oxidize rapidly and in both cases, the vapor pressure of the element is higher than that of aluminum. As a result, magnesium and lithium may diffuse to the surface of the ingot and react with oxygen or moisture in the ambient air.  
         [0010]     Distribution of the surface oxide on the ingot head and at the meniscus plays an important role in fold prevention or formation. Data from previous research shows that humid air can produce an oxide/hydroxide film that protects magnesium-containing alloys from runaway or uncontrolled magnesium oxidation at molten metal temperatures. Since the weight gain of the magnesium-containing alloy is significantly reduced in humid air as compared to dry air, the oxide layer is thinner and the oxide distribution is believed to be more uniform. Another mechanism that plays a part in the transformation of molten metal to solid metal is the heat transfer at the mold wall between the molten metal and lubricant coated mold wall.  
         [0011]     There remains a need for an effective alternative to beryllium and fluorine containing materials to prevent surface imperfections, such as vertical folds, pits, oxide patches and the like from forming during aluminum ingot casting, and to control the nature and distribution of oxides, particularly when casting aluminum that is alloyed with elements like magnesium and lithium. Such an invention would be instrumental in preventing cracks, which can form during casting or can develop in later processing. Finally, the invention preferably would have no adverse affect on alloy properties.  
         [0012]     The primary object of the present invention is to provide a lubricant composition that allows for uniform distribution of surface oxide at the meniscus formed between the molten aluminum and the mold during the continuous and semi-continuous casting of aluminum alloy ingots.  
         [0013]     Another object of the present invention is to provide a lubricant composition that promotes a uniform and controlled rate of heat transfer at the interface formed between the molten aluminum and the mold during the continuous or semi-continuous casting of aluminum alloy ingots.  
         [0014]     A still further object of this invention is to provide a casting lubricant that promotes uniform oxide distribution without requiring the use of toxic and corrosive gaseous atmospheres, and thus eliminating associated emissions and equipment corrosion.  
         [0015]     Still another object of this invention is to provide a method that promotes uniform oxide distribution on aluminum alloy ingots or castings that does not require beryllium additions to the alloy and fluorine containing atmospheres.  
         [0016]     These and other objects and advantages are met or exceeded by the instant invention, and will become more fully understood and appreciated with reference to the following description.  
       SUMMARY OF THE INVENTION  
       [0017]     In the present invention it is believed that when water and surfactant are added to casting lubricants, the improved lubricant formulation can provide a method for uniformly distributing the surface oxide at the meniscus. Uniform distribution of the oxide at the meniscus reduces vertical fold formation that can lead to cracks in the aluminum ingot. In addition, the mixture promotes uniform heat transfer around the mold. Uniform heat transfer around the mold allows the solidifying aluminum alloy to stay in contact with the mold longer and form a thicker and stronger ingot shell. Water has an extremely high heat of vaporization when compared to other liquids that can further pull heat away from the meniscus and be affecting this interaction. Uniform heat transfer will also lead to reduced vertical fold formation and associated cracking.  
         [0018]     Water and surfactant are added to existing lubricant bases to prepare the lubricant formulations of this invention. The lubricant formulation is mixed in a high speed mixing operation, such as blending or shearing, or any other mixing operation known by those skilled in the art to provide dispersions, emulsions, and/or true solutions. At this stage, the formulation is ready to use as a casting lubricant.  
         [0019]     In the process of casting aluminum alloy ingots, the lubricant formulation of this invention is supplied to the oil ring of a cooled continuous or semi-continuous casting mold, which subsequently lubricates the inner wall of the continuous casting mold. Molten aluminum alloy is cast into the mold. It is believed that the lubricant allows for uniform distribution of the surface oxide at the meniscus of the lubricated inner mold wall and the molten aluminum base alloy interface. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]      FIG. 1  is a flowchart for preparation of the formulation of the lubricant of the instant invention.  
         [0021]      FIG. 2  is a schematic characterization of a DC continuous casting mold used in the method of this invention.  
         [0022]      FIGS. 3   a  and  3   b  show the faces of aluminum alloy ingots cast with the use of a standard lubricant and the lubricant formulation of the present invention.  
         [0023]      FIG. 4  is a graph showing the isothermal thermogravimetric analysis of 5083 aluminum alloy in dry and wet air.  
         [0024]      FIG. 5  is a graph showing the isothermal thermogravimetric analysis of 7050 aluminum alloy in dry and wet air.  
         [0025]      FIG. 6  is a flowchart for preparation of the formulation of a lubricant with increased water content resulting from the use of a higher percentage of surfactant.  
         [0026]      FIG. 7  is a flowchart for preparation of the formulation of a lubricant with increased water content resulting from the use of a selected compound.  
         [0027]      FIG. 8  is a graph showing surfactants that increased the lubricant water content to greater than about 0.5%.  
         [0028]      FIG. 9  is a graph showing compounds or mixtures of compounds that increased the lubricant water content to greater than about 0.5%.  
         [0029]      FIG. 10  is a graph showing water solubility in formulations other than glycerol trioleate.  
         [0030]      FIG. 11  is a graph showing nonionic surfactants used to increase the lubricant water content. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0031]     The instant invention provides a casting lubricant formulation and method for using this formulation that substantially reduce vertical fold formation that can lead to cracks in an aluminum ingot. In particular, it is believed that practice of the instant invention allows for uniform distribution of the surface oxide at the meniscus of the molten aluminum alloy. In addition, practice of the instant invention leads to uniform heat transfer around a casting mold.  
         [0032]     Referring now to  FIG. 1 , a flowchart for preparation of the lubricant of this invention is presented. The invention improves on existing lubricants used in the casting of aluminum and aluminum base alloy ingots and forms, and in the general manufacture of aluminum products, using thermomechanical processes such as, but not limited to, casting, extrusion, hot and cold rolling, and forging.  
         [0033]     In a preferred embodiment, an existing aluminum alloy casting lubricant, glycerol trioleate, is used as the lubricant base. This is evidenced by box number  1  in the flow chart. Box number  2  in the flowchart evidences the amount of water and surfactant that is mixed with the lubricant base. About 0.05% to about 0.5% by weight of water could be added to the lubricant base, but about 0.1% to about 0.4% by weight of water is preferred. Similarly, less than about 0.25% by weight of surfactant could be added to the lubricant base, but about 0.05% to about 0.2% of surfactant is preferred. The types of lubricant that can be used include for example, but without limitation, glycerol trioleate, ethyl oleate, methyl oleate, butyl ricinoleate, methyl acetyl ricinoleate, butyl oleate, glycerol triacetyl ricinoleate, butyl acetyl ricinoleate, polyalphaolefins, polyisobutylenes, castor oil, peanut oil, corn oil, canola oil, cottonseed oil, olive oil, rapeseed oil, safflower oil, sesame oil, sunflower oil, soybean oil, linseed oil, coconut oil, palm kernel oil, neat&#39;s-foot oil, lard oil, tallow oil, and combinations thereof. Any type of water can be used, but soft water is preferred. For purposes of this invention, soft water is to be defined as water with a low content of polyvalent cations. It will be appreciated by those of ordinary skill in the art that polyvalent cations are ions that have more than one positive charge. Examples of polyvalent cations are calcium (Ca +2 ), magnesium (Mg +3 ), iron (Fe +2  and Fe +3 ), and aluminum (Al +3 ). The surfactant can be cationic, anionic, nonionic, or combinations thereof. The surfactant used in this invention was Kimberly Clark® Professional Pink Lotion Soap. This soap is available from the Kimberly Clark Corporation. The mixture is then subjected to high shear for about 5 minutes as represented by box number  3  in the flowchart. High shear is defined as at least 100 revolutions per minute (RPM). Shearing devices including, but not limited to, household blenders, can be used to shear the mixture. The lubricant so formulated, as represented by box number  4  in the flowchart, is applied to a casting mold in any manner that is familiar to those skilled in the art of casting aluminum alloys.  
         [0034]     It is believed that a major benefit of the lubricant of this invention is realized in uniformly distributing surface oxides at the meniscus during DC casting of aluminum. However, it is recognized by those skilled in the art that the lubricant of this invention can be used in any thermomechanical processing of aluminum and its alloys. These processing steps include, but are not limited to casting, hot and cold rolling, forging, and extrusion.  
         [0035]     Referring now to  FIG. 2 , a cross-section of a DC casting mold  10 , which can be used to cast aluminum alloy ingots according to the instant invention, is schematically depicted. The DC casting mold  10  comprises molten metal  11  from a furnace. The molten metal is held in a trough  12 . A control pin  13  activates and deactivates the flow of molten metal  11  into a distributor bag  14 , which distributes the molten metal into the cooled mold  15 . The molten metal  11  in the cooled mold  15  may form an oxide skim  16 . The inner wall  17  of the cooled mold  15  is cooled by a liquid cooling jacket  18  that cools the mold  15  and floods the solidified ingot surface  19  with cooling liquid  20 . The liquid is preferably water, but could be any liquid suitable for cooling the ingot  22 . The liquid flows from a liquid pump (not shown) that is connected to the sides of the cooling jacket  18 . The inner wall  17  is also continuously lubricated with a formulation of the instant invention by using an oil ring  21  positioned at or near the meniscus of where the molten metal  11  in the mold  15  contacts the inner wall  17  of the cooled mold  15 . An oil ring is preferred, but other methods of continuously lubricating the mold inner wall could be used. In a preferred embodiment, the lubricant formulation comprises about 0.1% to about 0.4% by weight of water and 0.05% to about 0.2% by weight of surfactant with the remaining percentage being glycerol trioleate base. Molten metal  11  in the mold  15  solidifies into a solidified ingot  22 . The solidified ingot  22  rests on a starting block  23 . The starting block  23  rests on a starting block holder  24 . The starting block holder  24  is attached to a platen  25 . The platen can be lowered or raised by a cylinder ram  26 . As molten metal  11  in the mold  15  solidifies into a solidified ingot  22 , the cylinder ram  26  is lowered, which causes the solidified ingot  22  to also be lowered according to the directional arrows  27  superimposed onto the schematic cross section of the DC mold  10 . As the cylinder ram  26  and solidified ingot  22  are lowered, the control pin  13  is activated to allow more molten metal  11  in the trough  12  to flow into the cooled mold  15  via the distributor bag  14 , and ingots of aluminum alloy are cast, the length of such ingots being constrained only by the movement of the cylinder ram  26 . During the ingot casting operation, the solidified ingot  22  is in contact with the inner wall  17  and is continuously lubricated with the lubricant of this invention via the oil ring  21  or any other method used to continuously lubricate the mold inner wall, thusly providing a process for minimizing undesirable surface defects, such as vertical folds that were described earlier. During the practice of this invention, there is no requirement for the undesirable practice of alloying the aluminum with beryllium, nor is there any reliance on using toxic materials such as ammonium fluoroborate or boron trifluoride to prevent oxidation during casting.  
         [0036]     To test the lubricant formulation, a lubricant was formulated according to the teachings of this invention as described in the following example.  
       EXAMPLE  
       [0037]     7200 grams of glycerol trioleate, 8 grams of water, and 4 grams of Kimberly Clark® Professional Pink Lotion Soap were combined and sheared, via use of a household blender, at high speed (1000 RPM) for five minutes. The lubricant formulation was used in the casting of Aluminum Alloy 5083 and 7050. Casting position  1 , which was used as the control, utilized only glycerol trioleate as the lubricant. As can be seen in  FIG. 3   a , the resultant aluminum alloy ingot was covered with vertical folds. Casting position  2  alternated between using glycerol trioleate and the lubricant formulation of the present invention. As represented in  FIG. 3   b , the resultant aluminum alloy ingot had only a few light vertical folds when the lubricant formulation of the present invention was used.  
         [0038]     The distribution of the surface oxide on the ingot head and at the meniscus plays an important role in fold prevention or formation.  FIGS. 4 and 5  represent the isothermal thermogravimetric analysis of 5083 and 7050 alloys in dry air  100  and wet air  200 , respectively. For purposes of this invention, dry air is air with a dew point of 59° F. or less and wet air with a dew point between 60° F. and 100° F. Data from previous research shows that wet air  200  can produce an oxide/hydroxide film that protects the high magnesium alloy from runaway or uncontrolled magnesium oxidation at molten metal temperatures. Since the weight gain of the magnesium-containing alloys is significantly reduced as compared to dry air  100 , the surface oxide is thinner and is believed to be more uniformly distributed. This change in oxide distribution would play a significant role in vertical fold suppression. Introducing the oxygen, in this case in the form of water mixed with oil and surfactant, provides the method for changing the metal oxide distribution at the meniscus. The critical technical part is to form a homogeneous distribution of water in the oil so water would be limited, but available uniformly over the surface of the casting mold and at the meniscus immediately before solidification. The water may be uniformly dispersed in the oil as a dispersion, emulsion, a true solution, or a combination thereof. For purposes of this application, the term dispersion is defined as the distribution of a substance, as fine particles or droplets, evenly throughout a medium, the term emulsion is defined as distributing a substance throughout a medium via use of an emulsifier, such as a surfactant, to help link the substance and the medium together, and the term true solution is defined as a homogeneous mixture formed by mixing a solid, liquid, or gaseous substance with a liquid or sometimes a gas or solid. Uniformly distributing the water in this manner reduces vertical folds and the possibility of associated cracking by also controlling the heat transfer between the molten metal and the lubricant interface on the inner wall of the mold, thereby allowing the solidified shell to stay in contact with the mold longer and form a thicker and stronger shell.  
         [0039]     A key for lubricant formulations is to have no undissolved or precipitated solid phases that can plug the small orifices delivering the lubricant to the surface of the mold. With this limitation, all lubricants formulated within this invention are effectively single phase mixtures of components, representing thermodynamically stable solutions or blends, or stable dispersions or emulsions that are defined, for the purposes of this invention, as not forming separate phases after 30 months of storage.  
         [0040]     By increasing the solubility of water in the lubricant, the tendency to have undissolved or precipitated water phase is reduced. It is generally believed that the water content of casting lubricants should be limited because of the concern for explosions. This concern is alleviated if the water can not be trapped under the aluminum. For this reason, the lubricant is added above the meniscus so the lubricant drips into the meniscus and is not trapped under the molten aluminum. In order to increase the amount of soluble water in the lubricant, surfactants other than Kimberly Clark® Professional Pink Lotion Soap have been used. As shown in  FIG. 8 , there were five surfactants that were able to increase the water content of the lubricant to greater than about 0.5 wt %.  FIG. 6  shows a flowchart for the preparation of this lubricant. As evidenced by box number  2 , the percentage of water in the lubricant is between about 0.1% and about 3.5% by weight. The percentage of surfactant is less than about 20% by weight and preferably between about 0.05% to about 10% by weight.  
         [0041]     In addition, compounds such as phosphates, borates, fluorides, and silicates have been added to increase the performance of the lubricant. Other compounds could be used, but these compounds, or mixtures containing them, were selected based on their ability to form single phase mixtures or stable dispersions or emulsions in the lubricant and their ability to react with the aluminum, or to generate a deposit on it, thus providing a surface layer at the meniscus. The surface layer acts as a barrier to control friction and minimize sticking of the aluminum to the mold and in this way, provides a second means of improving aluminum-die contact conditions. Since many forms of compounds are polar, inorganic salts and related compounds, such as salts of alkali and alkaline earth metals, their solubility in glycerol trioleate is limited. However, as shown in  FIG. 9 , 0.8% of water saturated with 6% boric acid was solubilized in glycerol trioleate with the aid of surfactants such as hexylene glycol, which enhances the stability of the mixture of water and the dissolved boric acid in the glycerol trioleate. This provides the combined benefits of the water and the inorganic friction-controlling material. Alternatively, these elements in organometallic species, such as trioctyl phosphate, can be dissolved directly in glycerol trioleate, and with the aid of hexylene glycol, can be present along with higher levels (0.8%) of dissolved water as well.  FIG. 7  shows a flowchart for the preparation of a lubricant with increased water content when mixed with a compound as described above. As evidenced by box number  2 , the percentage of water in the lubricant is between about 0.1% and about 0.8% by weight. The percentage of surfactant is less than about 20% by weight and preferably between about 0.05% to about 10% by weight.  
         [0042]      FIG. 10  shows water solubility in additional formulations. In Formulation A, which incorporates 2% boric acid with glycerol trioleate, 4.0% hexylene glycol, and 4.0% methanol, 0.8% water can be stabilized. However, in the presence of an additional 10% hexylene glycol, the water content in the formulation is increased to 2.0%. In addition, castor oil can also hold a larger amount of water, around 1.0%, than glycerol trioleate, which may be increased through the use of surfactants herein.  
         [0043]      FIG. 11  shows that when certain selected nonionic surfactants were used, instead of the surfactants in  FIG. 8 , even higher levels of water were soluble in glycerol trioleate. The highest level of soluble water obtained was 3.5 wt % using 5% Tergitol 15-S-7, a product that is well suited to stabilizing water in oil formulations due to its ability to balance suitably both its affinity for water and its affinity for lipids, such as oils, waxes, fats, and other related and derived compounds. The percentage of surfactants in  FIGS. 8 and 11  is less than about 20% by weight and preferably between about 0.05% to about 10 % by weight.  
         [0044]     It should be recognized that the dissolved water content of the lubricant base can vary with composition, manufacturing procedures, and handling and storage practices. The instant invention provides a means to increase the water content above the normal limit at a given temperature and to generate a known final water content based on analyzing the base oil initially for water content or treating it to achieve its known water content limit prior to treating with surfactant or other compounds and additional water.  
         [0045]     It should also be recognized that the metal oxide distribution at the meniscus can be changed by introduction of oxygen, in whole or in part, via the surfactants, especially the oxygen-rich non-ionic surfactants, such as hexylene glycol and the Tergitol 15-S products.  
         [0046]     It will be readily appreciated by those skilled in the art that modifications may be made to the invention without departing from the concepts disclosed in the forgoing description. Such modifications are to be considered as included within the following claims unless the claims, by their language, expressly state otherwise. Accordingly, the particular embodiments described in detail herein are illustrative only and are not limiting to the scope of the invention which is to be given the full breadth of the appended claims and any and all equivalents thereof.