Abstract:
Molds  16  for casting molten materials using a directional solidification process and methods for forming a barrier layer between a liquid cooling medium  20  and a molten material of a casting process are provided. According to an embodiment, a mold  16  for casting a molten material comprises an inner surface at least partially coated with a metal oxide slurry  32  comprising metal oxide particles, wherein the metal oxide slurry  32  is capable of inhibiting a liquid cooling medium  20  from contacting a molten metal or metal alloy when the molten metal or metal alloy is disposed within an interior of the mold  16  and the mold  16  is disposed in the liquid cooling medium  20.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This disclosure relates generally to directional solidification casting and, more specifically, to molds for casting molten materials such as metals or metal alloys using a liquid cooled directional solidification process. 
         [0002]    Directional solidification casting is a method for producing gas turbine components and the like with columnar and single crystal growth structures. Generally, a desired single crystal growth structure is created at the base of a vertically disposed mold defining a part. Then, a single crystal solidification front is propagated through the structure under the influence of a moving thermal gradient. 
         [0003]    Materials that have been cast using directional solidification include steel and superalloy parts. In addition to composition, the crystal grain characteristics of a superalloy can determine superalloy properties. For example, the strength of a superalloy is determined in part by grain size. At high temperatures, deformation processes are diffusion controlled and diffusion along grain boundaries is much higher than within grains. Hence, at high temperatures, large grain structures can be stronger than fine grain structures. The failure of a superalloy can originate at grain boundaries oriented perpendicular to the direction of an applied stress. Casting a superalloy to produce an elongated columnar structure with unidirectional crystals aligned substantially parallel to the long axis of the casting can reduce grain boundaries normal to the primary stress axis. This grain boundary reduction can, in turn, almost entirely eliminate grain boundary failure modes. 
         [0004]    During directional solidification, crystals of nickel, cobalt or iron-based superalloys are characterized by a “dendritic” morphology. Dendritic refers to a form of crystal growth where forming solid extends into still molten liquid as an array of fine branched needles. Spacing between the needles in the solidification direction is called “primary dendrite arm spacing.” A temperature gradient can be impressed in front of an advancing solidification front to avoid nucleation and growth of parasitic dendritic grains. The magnitude of the required gradient is proportional to the speed of solidification. For this reason, it is desirable to control the speed of displacement of the solidification front, which can be on the order of a fraction of a centimeter to several centimeters per hour. 
         [0005]    Liquid metal cooled directional solidification processes have been developed to allow the speed of displacement of the solidification front to be carefully controlled. One such process involves passing the alloy material through a heating zone and then into a cooling zone. The heating zone can include an induction coil or resistance heater while the cooling zone can include a liquid metal bath. In another process, the liquid metal bath can be utilized both for heating and cooling to provide an improved planar solidification front for the casting of complex articles. 
       SUMMARY OF THE INVENTION 
       [0006]    Disclosed herein are molds for casting molten materials, methods for forming a barrier layer on an inner surface of such molds, and methods for casting a molten material. According to an embodiment, a mold for casting a molten material comprises an inner surface at least partially coated with a metal oxide slurry comprising metal oxide particles, wherein the metal oxide slurry is capable of inhibiting a liquid cooling medium from contacting a molten metal or metal alloy when the molten metal or metal alloy is disposed within an interior of the mold and the mold is disposed in the liquid cooling medium. 
         [0007]    In another embodiment, a method for forming a barrier layer between a molten material and a liquid cooling medium of a casting process comprises: disposing a metal oxide slurry comprising metal oxide particles upon an inner surface of the mold. 
         [0008]    In an additional embodiment, a method for casting a molten material comprises: disposing the molten material within an interior of a mold comprising an inner surface at least partially coated with a metal oxide slurry comprising metal oxide particles; and disposing the mold in a liquid cooling medium to cause the molten material to solidify. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    Referring now to the Figures, which are exemplary embodiments, and wherein the like elements are numbered alike: 
           [0010]      FIG. 1  is a schematic side view of a furnace for conducting a directional solidification process; and 
           [0011]      FIG. 2  is a schematic illustration of an embodiment of a barrier layer for use upon an inner surface of a mold for casting a molten material. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0012]      FIG. 1  depicts an exemplary embodiment of a furnace  10  that can be used to cast molten materials via directional solidification processes. As shown, the furnace  10  can include resistance heated conductive strips  12  (e.g., graphite strips) disposed within an insulated furnace box  14  for preheating the box  14 . The conductive strips can be replaced by an induction coil in an alternative embodiment. A mold positioner  18  can hold a ceramic mold  16  having an inner surface coated with a metal oxide slurry within the furnace box  14 . An interior of the mold  16  can be filled with a molten material that is kept in the molten state by heating the furnace box  14  at a temperature above the melting point of the molten material. Directional solidification can be achieved by lowering the mold  16  containing the molten material out of the heated furnace box  14  (i.e., the hot zone) into a liquid cooling bath  20  (i.e., the cold zone) through an aperture  11  in the furnace box  14 . The cooling bath  20  can act as a cooling medium for the molten material. The cooling bath  20  can be contained in a crucible  22  of metal such as a refractory metal that is heated to a temperature below the melting point of the molten material and above the melting point of the liquid cooling medium. Solidification of the molten material can progress from bottom to top within mold  16  as the mold  16  is lowered into the cooling bath  20 . In particular, a solid-liquid interface can advance upward as heat is transferred from the molten material within the mold  16  to the liquid cooling bath  20 . 
         [0013]    As mentioned above, the inner surface of the ceramic mold can be pre-coated with a slurry comprising metal oxide particles. In an embodiment, the metal oxide particles can have a diameter of about 20 micrometers (microns) to about 500 microns, more specifically about 20 microns to about 100 microns, or even more specifically about 20 microns to about 50 microns. Examples of suitable metal oxide particles include but are not limited to particles of aluminum oxide such as alumina, alkaline earth metal oxides such as magnesium dioxide and calcium oxide, transition metal oxides such as titania, chromia, and zirconia, rare earth oxides such as yttria and ceria, and combinations comprising at least one of the foregoing metal oxides. The slurry can be formed by mixing the metal oxide particles with a liquid such as water. The slurry can then be applied to the surface of a wax pattern, which upon de-waxing, forms the inner surface of the mold, to form a barrier layer adjacent to the molten material subsequently placed within the mold. Filling the interior of the mold with the relatively hot molten material can cause the metal oxide slurry to cure into a solid ceramic facecoat that attaches to the surface of the molten material upon solidification. Optionally, an additional facecoat can be disposed on the metal oxide slurry of the mold after the de-waxing procedure by rinsing the interior of the mold with an additional slurry, if so desired. This additional facecoat can serve as a protective coating that adheres to the molten material upon solidification. It can include a dissimilar oxide from the one used in the metal oxide slurry, such as colloidal silica, yttria, alumina, and combinations comprising at least one of the foregoing oxides. 
         [0014]    If any of the liquid cooling medium, e.g., a liquid metal, infiltrates through the main mold member during the solidification process, the ceramic facecoat can inhibit the liquid cooling medium from contacting the surface of the molten material within the mold. By way of example, this infiltration of the liquid cooling medium can occur if the mold does not seal properly or if the mold cracks prematurely before the completion of the solidification process. The presence of the ceramic facecoat adjacent to the surface of the molten material can prevent or delay cross-diffusion between components of the liquid cooling medium and the molten material and any surface reactions between the two materials. As a result, the composition of the molten material desirably remains substantially the same and does not become contaminated during the solidification process. 
         [0015]    In another exemplary embodiment shown in  FIG. 2 , the inner surface of the mold can be pre-coated with alternating layers of a metal oxide slurry (i.e., layers  32 ,  36 ,  40 ,  44 , and  48 ) and a stucco (i.e., layers  34 ,  38 ,  42 , and  46 ). As used herein, “stucco” refers to a material made of an aggregate, a binder, and water that is applied wet and hardens when it is dried. In one embodiment, the stucco can include lime, sand, and water. The metal oxide slurry can be applied by, e.g., dip or spray coating. The stucco can be applied using a fluidized bed, by spraying coating, or by rain coating. Rain coating can be performed using a device (i.e., a rain sander) that picks up the stucco from a reservoir and distributes it evenly (like “rain”) over a large area. Suitable rain sander devices are commercially available from Pacific Kiln &amp; Insulations Co., Inc. 
         [0016]    The size of the stucco particles near the bottom the stacked structure shown in  FIG. 2  are smaller those near the middle and top of the stacked structure. For example, the stucco particles near the bottom of the stacked structure can have a diameter of about 150 microns to about 180 microns (a mesh size of about 80 to about 100), whereas those near the middle or top of the stacked structure can have a diameter of about 250 microns to about 355 microns (a mesh size of about 42 to about 60). Suitable sizes of the slurry particles in the slurry layers are described above. The thickness of each slurry layer can be about 50% to about 75% of the diameter of the stucco particles to ensure the stucco particles are embedded in the slurry layer but do not penetrate the layer. For example, stucco particles having a diameter of 180 microns can need about 90 to about 135 microns of slurry thickness to lock the particles in place. 
         [0017]    Examples of materials that can be cast as described above include but are not limited to metals, metal alloys, superalloys, and combinations comprising at least one of the foregoing materials. As used herein, the term “superalloy” refers to a nickel (Ni), cobalt (Co), or iron (Fe) based heat resistant alloy that has superior strength and oxidation resistance at high temperatures. Superalloys can also include a metal such as chromium (Cr) to impart surface stability and one or more minor constituent such as molybdenum (Mo), tungsten (W), niobium (Nb), titanium (Ti), and/or aluminum (Al) for strengthening purposes. The physical properties of superalloys make them particularly useful for the manufacture of gas turbine components. 
         [0018]    The liquid cooling medium desirably includes a chemically inert material having a melting point significantly below that of the molten material, a relatively high thermal conductivity, and a relatively low vapor pressure. Examples of suitable materials for use in the liquid cooling medium include but are not limited to non-flammable, non-toxic liquid metals having a melting point less than about 700° C., eutectic or near eutectic metal alloys, and combinations comprising at least one of the foregoing cooling mediums. Non-limiting examples of such liquid metals include aluminum, tin, gallium, and indium. 
         [0019]    A eutectic mixture is a combination of metals in a proportion that is characterized by the lowest melting point of any mixture of the same metals. The eutectic point is the lowest temperature at which a eutectic mixture can exist in liquid phase. The eutectic point is the lowest melting point of an alloy in solution of two or more metals that is obtainable by varying the proportions of the components. Eutectic alloys have definite and minimum melting points in contrast to other combinations of the same metals. Non-limiting examples of eutectic or near eutectic metal alloys include binary eutectics of aluminum (Al) with copper (Cu), germanium (Ge), magnesium (Mg), or silicon (Si) and ternary eutectics of aluminum with copper and germanium, copper and magnesium, copper and silicon, or magnesium and silicon. These aluminum-based eutectic alloys, as well other eutectic alloys in the tin, gallium and indium systems, can be used as cooling mediums, if lower temperature or reactivity with metal is desired. 
         [0020]    The liquid cooling medium can be prepared as an ingot outside of the directional solidification furnace by melting and casting the alloy constituents into ingots. Alternatively, the liquid cooling medium can be prepared in situ by melting constituents within the crucible  22 . 
         [0021]    As used herein, the terms “a” and “an” do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced items. Moreover, the endpoints of all ranges directed to the same component or property are inclusive of the endpoint and independently combinable (e.g., “about 5 wt % to about 20 wt %,” is inclusive of the endpoints and all intermediate values of the ranges of “about 5 wt % to about 20 wt %,”). Reference throughout the specification to “one embodiment”, “another embodiment”, “an embodiment”, and so forth means that a particular element (e.g., feature, structure, and/or characteristic) described in connection with the embodiment is included in at least one embodiment described herein, and may or may not be present in other embodiments. In addition, it is to be understood that the described elements may be combined in any suitable manner in the various embodiments. It is also to be understood that the disclosure is not limited by any theories described therein. Unless defined otherwise, technical and scientific terms used herein have the same meaning as is commonly understood by one of skill in the art to which this invention belongs. 
         [0022]    While the invention has been described with reference to exemplary embodiments, it will be understood that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.