Abstract:
A composite ceramic core that, in combination with a shell mold, is suitable for use in a casting process to produce metal alloy components. The core and casting process make use of a highly leachable interior layer in combination with an exterior layer that is less reactive than the interior layer in the presence of common alloying elements. The interior layer contains at least one hollow channel that allows a point of entry for a leaching solution, as well as exit for gaseous byproducts generated during the casting process.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention generally relates to casting processes and materials. More particularly, this invention relates to cores and processes for casting reactive metal alloys, such as a steels (including stainless steels), superalloys, titanium-base alloys, etc. 
         [0002]    Metal alloy materials can be formed into components by various casting techniques, a notable example being investment casting (lost wax) processes. Investment casting typically entails dipping a wax or plastic model or pattern of the desired component into a slurry comprising a binder and a refractory particulate material to form a slurry layer on the pattern. A common material for the binder is a silica-based material, for example, colloidal silica. A stucco coating of a refractory particulate material is typically applied to the surface of the slurry layer, after which the slurry/stucco coating is dried. The preceding steps may be repeated any number of times to form a shell mold of suitable thickness around the wax pattern. The wax pattern can then be eliminated from the shell mold, such as by heating, after which the mold is fired to sinter the refractory particulate material and achieve a suitable strength. 
         [0003]    To produce hollow components, such as turbine blades and vanes having intricate air-cooling channels, one or more cores must be positioned within the shell mold to define the cooling channels and any other required internal features. Cores are typically made using a plasticized ceramic mixture that is injection molded or transfer molded in a die or mold, and then hardened by firing or baking. Typical ceramic compositions contain silica and/or alumina. One or more fired cores are then positioned within a pattern die cavity into which a wax, plastic or other suitably low-melting material is introduced to form the wax pattern. The pattern with its internal core(s) can then be used to form a shell mold as described above. Once the shell mold is completed and the pattern selectively removed to leave the shell mold and core(s), the shell mold can be filled with a molten metal, which is then allowed to solidify to form the desired component. The mold and core are then removed to leave the cast component with one or more internal passages where the core(s) formerly resided. Removal of silica-based and alumina-based cores is performed by a leaching process with an agitated caustic solution (typically aqueous solutions of NaOH or KOH) in an autoclave at high pressures (e.g., about 100 to 500 psi; about 0.7 to 3.5 MPa) and temperatures (e.g., about 200° C.), with typical treatments requiring about ten to twenty hours, depending on the size and intricacy of the core. 
         [0004]    From the above, it can be appreciated that shell molds and cores used in investment casting processes must exhibit sufficient strength and integrity to ensure that the component will have the required dimensions, including wall thicknesses resulting from the location of each core relative to the shell mold. Additional challenges are encountered when attempting to form hollow castings of reactive materials, including stainless steel alloys, as a result of their reactivity. Cores made of fine particles having a high silica content have been used in the past due to their relatively high leach rate. However, silica devitrifies at high temperatures (e.g., about 1200° C.) causing the core to gradually lose strength and distort during a casting process. Silica also reacts with alloying elements, such as Al, Ni, Cr, Y, Zr, etc., which may cause surface depletion zones, internal oxidation and other deleterious effects. For example, oxidation and loss of aluminum, nickel, chromium, yttrium and/or zirconium may cause rejection of an expensive casting. In addition, a product of the oxidation reaction is SiO, which is gaseous at pour temperature and can become trapped in a casting to form gas defects. Also, the reaction may cause a core body and the metal casting to tightly stick to each other, with the result that the core is more difficult to remove. 
         [0005]    Cores made of high-content alumina tend to be inert or at least less reactive to metal alloys. However, alumina cores tend to have a very low leach rate, implying that a much longer leach operation is needed to completely remove the core body from a solidified article. Due to the longer leach operations, the leach agent may attack the metal casting and form reaction pits. For stainless steels, the pits are often easily visible even after sand blast of a casting. 
         [0006]    Prior attempts have been made to create composite ceramic cores with improved casting properties which utilize both easily leachable silica and nearly inert alumina. Such attempts include the use of multiple joined composite cores of different materials for different parts of a casting, and chemically in-situ coating of an alumina-rich layer mix over a silica layer. As an example of the former, U.S. Pat. No. 5,498,132 proposed using finer ceramic materials to fabricate a partial core used in a trailing edge of a bucket and coarser materials to form another partial core for a leading edge cavity. The two partials are joined together with paste or a cast-in process, such as forming separate tongue and groove partials and connecting them prior to sintering. However, this process uses finer materials, often silica or a similar highly leachable material, in the trailing edge which are more reactive to metal alloys. As an example of the in-situ coating approach, dual layer cores have been created by alumina-containing gel coating, flame spraying, vapor deposition, and various other methods, each with its own well known limitations. As a particular example, U.S. Pat. No. 3,824,113 discloses a chemical in-situ method of generating a layer of up to about 25 micrometers thick on a silica-rich core body. The core is first immersed in a liquid metal bath containing one or more reactive elements, such as Al, Hf, Y, Mg, etc., and the reactions of these elements with silica yield the desired surface oxide layer. Though this process generates a seamless interface between the chemically distinct surface layer and silica-rich interior, the immersion/reaction time is critical to the thickness control of the surface layer and the time for further thickening will be exponentially increased since solid diffusion through the surface layer must occur to continue the oxidation process. Further, the silica and the surface oxide layer have coefficients of thermal expansion which differ to a degree that may be sufficient to cause spalling, cracking, and/or separation of the layers. 
         [0007]    While prior composite ceramic cores may exhibit improved properties over single material cores; there is still a desire to create a composite core which is both highly leachable and less reactive in the presence of common alloying elements. It would be desirable if an improved core and process were available that is capable of at least partly overcoming or avoiding these problems, shortcomings or disadvantages. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0008]    The present invention provides a composite ceramic core that, in combination with a shell mold, is suitable for use in a casting process to produce metal alloy components. The core and casting process make use of a highly leachable interior layer in combination with an exterior layer that is less reactive than the interior layer in the presence of common alloying elements. The interior layer contains at least one hollow channel that allows a point of entry for a leaching solution and an exit for gaseous byproducts. 
         [0009]    According to a first aspect of the invention, a core is provided for use in combination with a shell mold to cast a hollow casting from a reactive metal alloy. The core comprises a sintered particulate material interior layer with at least one hollow channel within the interior layer. Additionally, there is at least one sintered particulate material exterior layer on the surface of the interior layer that is less reactive with the reactive metal alloy than the interior layer. 
         [0010]    According to another aspect of the invention, a process is provided for creating a core for use in combination with a shell mold to cast a hollow casting of a reactive metal alloy. The process comprises the steps of forming a sintered particulate material interior layer over the surface of a preformed body. At least one sintered particulate material exterior layer is then formed on the surface of the interior layer that is less reactive with the reactive metal alloy. The preformed body is removed from the composite core to result in the core having at least one channel within the interior layer. The interior layer and exterior layer are sintered together to yield the core. 
         [0011]    According to an optional but preferred aspect of the invention; interlocking features are formed on a surface of the interior layer that retain the exterior layer. 
         [0012]    A technical effect of the invention is that the core has the ability to leach out of a casting at a relatively high rate, while reducing the likelihood of reaction with a reactive metal alloy. Another technical effect of the invention is the reduction of the likelihood that gaseous byproducts released from the interior layer will enter the casting, therefore reducing gas pore defects in locations of the component close to the core. 
         [0013]    Other aspects and advantages of this invention will be better appreciated from the following detailed description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  represents a fragmentary cross-sectional view of a mold assembly including a shell mold, a core comprising an interior layer and an exterior layer, and a wax pattern between the core and shell mold in accordance with an embodiment of the invention. 
           [0015]      FIG. 2  represents a fragmentary cross-sectional view of a reactive metal alloy that has been investment cast within the mold assembly of  FIG. 1  in accordance with an embodiment of the invention. 
           [0016]      FIG. 3  represents a step in a process for fabricating a core of the type shown in  FIGS. 1 and 2 , and shows the core formed over a preformed body in accordance with a preferred aspect of this invention. 
           [0017]      FIG. 4  represents a cross-sectional view of the core of  FIG. 3  along section line A-A. 
           [0018]      FIG. 5  represents the cross-sectional view of  FIG. 4  after removal of the preformed body. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]      FIG. 1  represents a fragment of a wall section of a mold assembly  10  suitable for investment casting a hollow component in accordance with one embodiment of the invention. The invention is believed to be especially suitable for investment casting components with small, complex cavities. According to a preferred aspect of the invention, the mold assembly  10  is particularly adapted for casting reactive metals and metal alloys containing reactive elements, nonlimiting examples of which include steels (including stainless steels), superalloys, titanium-base alloys, etc., though it is foreseeable that the invention could be employed with other alloy systems. Of particular interest are metal alloys that comprise alloying elements which react at high temperatures with silica (SiO 2 ), including casting temperatures at which such alloys are molten, for example, at temperatures above 1540° C. 
         [0020]    Alloys of particular interest to the present invention contain one or more of aluminum and chromium, which are both reactive with silica at casting temperatures above 1540° C. For example, typical ranges for chromium in stainless steels are often, in weight percent, at least about 12.0% and more typically about 16.0% to about 20.0%, and aluminum may be present in stainless steels, for example, up to 2.0% weight percent. Typical ranges for chromium in nickel-base superalloys are often, in weight percent, at least about 6.0% and more typically about 10.0% to about 20.0%, and typical ranges for aluminum in nickel-base superalloys are often, in weight percent typically about 1.5% to about 3.5% for nozzles and 4.0% to 5.6% for bucket alloys. Alloys of particular interest may also contain additional elements that may be reactive at casting temperatures, nonlimiting examples of which are nickel, yttrium, and zirconium. The inclusion and amounts used of any of these elements will depend on a variety of factors, such as the base element of the alloy and the desired properties for the final alloy product, and generally all such compositions are within the scope of the invention. 
         [0021]    The mold assembly  10  of  FIG. 1  is representative of a first embodiment of the invention in which a composite ceramic core  12  comprises an interior layer  14  and at least one exterior layer  16 . The composition of the interior layer  14  is preferably selected on the basis of leachability along with other important factors, such as the ease at which it may be fabricated into complex shapes, sufficient room temperature strength to withstand pressures during injection of a wax pattern, and a sufficient elevated temperature strength to withstand the stresses due to non-uniform metal flow during casting. The exterior layer  16  is formed on the surface of the interior layer  14  to reduce the likelihood of reaction between the interior layer  14  and the alloying elements. As such, the composition and properties of the exterior layer  16  are preferably selected on the basis of minimizing any potential reactions between the interior layer  14  and a molten metal or alloy (melt) during the casting process. 
         [0022]    The mold assembly  10  is represented in  FIG. 1  as also including a shell mold  22  as the outermost member of the assembly  10 , and the core  12  is within a cavity defined by the shell mold  22 . Situated between the shell mold  22  and the core  12  is a model or pattern  20 , which may be formed of a wax, plastic or other suitable material having a suitably low melting temperature. Conventional techniques can be employed to incorporate the core  12  into the mold  22 . For example, the core  12  can be placed in a die, followed by the injection of wax around the core  12 , after which the shell mold  22  can be built up around the resulting wax-core assembly by dipping, molding, etc. Alternatively, the core  12  could be placed within the shell mold  22  after the mold  12  has been fully completed. Various other processing options are possible and within the scope of this invention. 
         [0023]    As known in the art of investment casting, the pattern  20  corresponds to the shape of a hollow component  24  to be cast from the reactive metal alloy, as represented in  FIG. 2 . The pattern  20  is removed from the shell mold  22  prior to forming the component  24 . Depending on its composition, a variety of techniques can be used to remove the pattern  20 , including such conventional techniques as flash-dewaxing, microwave heating, autoclaving, and heating in a conventional oven. 
         [0024]    After removing the pattern  20 , the melt is poured into the cavity defined by and between the shell mold  22  and core  12 .  FIG. 2  schematically represents the mold assembly  10  following the introduction and solidification of a reactive metal alloy within the shell mold cavity to form the component  24 . Aside from the materials from which they are formed as discussed below, the shell mold  22  and the core  12  can be used in substantially conventional investment casting processes, as well as other types of casting processes, and as such the casting process itself will not be discussed in any detail. 
         [0025]    According to a preferred aspect of the invention, the interior layer  14  is comprised of a silica-containing mold material, though it is foreseeable that the interior layer  14  could comprise other materials. Silica is commonly used in cores due to its high leachability. In one embodiment, the interior layer  14  predominately contains silica, which as used herein means that the interior layer  14  contains more silica by weight percent than any other individual constituent. Preferably, the interior layer  14  contains at least 70.0 wt. % silica, and more preferably about 75.0 to about 85.0 wt. % silica. Other potential constituents of the interior layer  14  include alumina (Al 2 O 3 ) in amounts of up to about 15.0 wt. %, as well as other constituents. The alumina is added to the interior layer  14  to raise the softening temperature of silica, prevent crystallization of silica into cristobalite and raise the CTE of the interior layer to be closer to that of the exterior layer. Other oxides such as MgO and Y2O3 could also be present in minor amount. 
         [0026]    As mentioned above, silica-rich compositions are believed to be desirable for use in the interior layer  14  due to their high leachability. However, at elevated temperatures silica reacts with certain elements, such as aluminum, nickel, chromium, yttrium, zirconium, etc., which may cause surface depletion effects that can negatively effect the desired properties of the component  24 . The loss of these reactive alloying elements may also cause the core  12  and the component  24  to tightly stick together, with the result that the core  12  would be more difficult to remove. A product of the oxidation reaction between reactive alloying elements and silica is silicon monoxide (SiO), which is gaseous at pour temperature and can become trapped in the component  24  and form gas defects. In addition, silica devitrifies at about 1200° C., which is much lower than the pouring temperature of steels, superalloys and titanium alloys and causes the silica to gradually lose strength and distort during the casting process. 
         [0027]    The exterior layer  16  is formed on the surface of the interior layer  14  to address the above-noted undesirable effects. In particular embodiments of the invention, the exterior layer  16  is predominately alumina (Al 2 O 3 ). Preferably, the exterior layer  16  contains at least 70.0 wt. % alumina, and more preferably about 75.0 to about 85.0 wt. % alumina. Other potential constituents of the exterior layer  16  include silica in amounts of up to about 10.0 wt. %, which improves the leachability and lowers the CTE to be closer to that of the interior layer  14 , as well as other constituents, for example, MgO and Y 2 O 3  which act as grain growth inhibitors to control the grain size of AL 2 O 3 . Although an alumina core would be difficult to remove from the component  24  by leaching, it is relatively inert to alloying elements in the cast component  24  that would likely react with silica at casting temperatures. Therefore, the presence of the exterior layer  16  on the surface of the interior layer  14  promotes the ability of the core  12  to resist reactions with alloying elements in the melt. The denser exterior layer  16  further reduces the likelihood that gaseous products released from the interior layer  14  will enter the casting, therefore reducing gas pore defects in locations close to the core  12 . The exterior layer  16  also strengthens the core  12  since alumina does not devitrify and distort during the casting process. To be an effective barrier layer, the exterior layer  16  should be no less than 20% of the local thickness, with a preferred thickness believed to be about 30 to about 40% for the purpose of protecting the interior layer  14  from the alloying elements. To minimize the negative effect that the alumina-rich exterior layer  16  would have on the leachability of the core  12 , the exterior layer  16  is preferably not greater than about 50% the local thickness. 
         [0028]    According to a preferred aspect of the invention, the interior layer  14  and the exterior layer  16  are interlocked with each other by interlocking features  18  formed on the surface of the interior layer  14 , as represented in  FIGS. 1 through 5 . The interlocking features  18  are provided to accommodate the difference in coefficients of thermal expansion (CTE) between, for example, an interior layer  14  that is predominately silica and an exterior layer  16  that is predominately alumina. More generally, use of the interlocking features  18  is believed to be particularly desirable if the CTEs of the interior layer  14  and exterior layer  16  differ by about 50% or more. Without the interlocking features  18 , spalling and cracking might otherwise occur during casting, resulting in the failure of the core  12 . The interlocking features  18  may consist of or comprise arrays of protuberances and/or depressions, arrays of ribs, or any other structural form capable of retaining the exterior layer  16  on the interior layer  14  during the casting process. Furthermore, the interlocking features  18  may have homogeneous or heterogeneous shapes. To be effective, the interlocking features  18  preferably protrude (or are recessed) at least 20% the local thickness of exterior layer  16 . In addition, the interlocking features  18  preferably have a maximum width (in the plane of the surface of the interior layer  14 ) of about the same as the height. If in the form of ribs, trenches, or another extended feature, the interlocking features  18  may have any suitable length permitted by the size and shape of the interior layer  14 . To be effective, the interlocking features  18  should also be present in a sufficient number to retain the exterior layer  16  on the surface of the interior layer  14 . In an embodiment utilizing an array of protuberances or depressions, it is believed that the interlocking features  18  should have a density of about 1 per square centimeter of surface area of the interior layer  14 . Interlocking features  18  in the form of ribs may be relatively smooth and uniform, with a preferred width believed to be about 5% to about 10% of the local thickness of the exterior layer  16  and the width about the same as its height. 
         [0029]    According to a preferred aspect of the invention, the leachability of the core  12  is significantly improved by creating one or more channels  28  ( FIG. 5 ) within the interior of the interior layer  14 . A single channel  28  may define a single hollow space within the core  12 , or the core  12  may contain multiple channels  28  that define, for example, multiple separate hollow spaces within the core  12  or one or more series of interconnected channels within the core  12 . Each channel  28  is preferably sized and configured to increase the contact area between the interior layer  14  and a leaching solution, which is permitted to flow into the channel  28  through an opening  30  ( FIG. 5 ) to accelerate the leaching cycle. 
         [0030]    The one or more channels  28  may be created by forming the interior layer  14  over the surface of one or more preformed bodies  26 , as represented in  FIGS. 3 and 4 . The preformed bodies  26  may be formed using known techniques such as, but not limited to, casting of low-melting point tin-base alloys or machining of graphite pieces or polymer lithography. A preformed body  26  may have a relatively complicated shape, for example, multiple branches extending from a main trunk as represented by the body  26  on the lefthand side of  FIG. 3 , or a relatively uncomplicated shape as represented by the two remaining bodies  26  in  FIG. 3 . As also evident from  FIG. 3 , the core  12  and its interior layer  14  and exterior layer  16  are preferably formed so that a portion of each body  26 , for example, the trunk of each body  26 , protrudes from the core  12  with the result that, following removal of the body  26 , each channel  28  defines an opening  30  at an outermost surface of the core  12  defined by the exterior layer  16 . The bodies  26  may be made of a variety of materials, such as preformed polymers or metallic plates. A particularly suitable material for the bodies  26  is believed to be graphite, which is preferably capable of rapidly and cleanly oxidizing when sufficiently heated to yield the channels  28  in the interior of the interior layer  14 , as represented in  FIG. 5 . In such an embodiment, the thickness of the interior layer  14  between an inner surface of the interior layer  14  defined by a channel  28  and an outer surface of the interior layer  14  at the interface with the exterior layer  16 , is preferably at least 50% the local thickness to balance the requirement of adequate strength and sufficient leachability, with a preferred thickness believed to be about 60% to about 70% of the local core thickness. Gaseous products created during the casting of the component  24  are able to escape through the channels  28  and their openings  30 , reducing the likelihood of gas defects within the component  24 . 
         [0031]    As is generally conventional in the fabrication of cores for casting processes, the interior layer  14  and the exterior layer  16  of the core  12  are formed from powder materials containing particles of the ceramic compositions desired for the interior layer  14  and exterior layer  16 . 
         [0032]    In addition to composition, other aspects of the powders are believed to be important or at least preferred in order to optimize the properties and leachability of the core  12 . For example, the particle size of the powder for the exterior layer  16  is preferably finer than the particle size of the powder for the interior layer  14  to promote the strength of the exterior layer  16  and to improve the surface finish of the casting. The particle size for the powder of the interior layer  14  is preferably at least 125 μm, with a preferred powder having an average particle size of 120 mesh. The particle size for the powder of the exterior layer  16  preferably does not exceed about 90 μm, with a preferred powder having a particle size of 170 mesh. 
         [0033]    During the fabrication of the interior layer  14 , the powder is combined with a binder system, such as a wax, polyvinyl acetate (PVA), or a like polymer, to form a slurry. As known in the art, additional additives, such as defoaming agents, pH adjusters, etc., can also be incorporated into the slurry. The slurry can be prepared by standard techniques using conventional mixing equipment, and then undergo processing to form the interior layer  14 , such as by pressing, injection molding, transfer molding, or another suitable technique. Preferred binders should provide adequate green strength to the core  14  after drying, and burn off cleanly prior to or during firing (sintering). A preferred process for forming the exterior layer  16  on the interior layer  14  is believed to be a cast-in process using an appropriate slurry containing the powders for the exterior layer  16  and a suitable binder system. Following the application of the exterior layer  16 , the core  12  is dried and fired in accordance with well-known practices, with the result that the powder materials used to form the interior layer  14  and exterior layer  16  are sintered. Depending on their composition(s), the preformed bodies  26  may be removed from the core  12  during sintering. In embodiments in which the bodies  26  are formed of a graphite material, each body  26  is preferably removed during a thermal cycle performed after sintering, for example, at a temperature of about 600° C. or above, which preferably causes each body  26  to rapidly and cleanly oxidize to yield the channel  28  within the interior layer  14 . 
         [0034]    The shell mold  22  may be made of any conventional ceramic mold material suitable to cast the desired component  24  and may be created using conventional techniques that are well known in the art. After the wax pattern  20  and the shell mold  22  are formed around the core  12  and the pattern  20  is subsequently removed as described above, a melt of the desired alloy is poured into the resulting cavity defined by and between the shell mold  22  and core  12 . The molten alloy is preferably introduced into the cavity while the shell mold  22  and the core  12  are at an elevated temperature, as conventionally performed when investment casting. Following the casting operation and removal of the shell mold  22 , conventional techniques may be used to remove the core  12  from the component  24 . In the embodiment described above, wherein the interior layer  14  is predominately silica and the exterior layer  16  is predominately alumina, removal of the core  12  may generally be accomplished by known leaching techniques, for example, with the use of a caustic solution (typically aqueous solutions of NaOH or KOH) in an autoclave at high pressures (e.g., about 100 to 500 psi; about 0.7 to 3.5 MPa) and temperatures (e.g., about 200° C.). Because a leaching solution is able to enter the interior of the core  12  through the channels  28  and their openings  30 , the interior layer  14  will leach out relatively easily starting at its interior surfaces defined by the channels  28 , leaving a hollow shell defined by the residual exterior layer  16 . The exterior layer  16  has an increased surface area exposed to the leaching solution in comparison to the interior layer  14 , thereby promoting its leachability beyond what would be possible if the entire core  12  were formed entirely of the ceramic material used to form the exterior layer  16 . The removal of any remaining exterior layer  16  may be further accelerated by agitation. 
         [0035]    While the invention has been described in terms of certain embodiments, it is apparent that other forms could be adopted by one skilled in the art. Therefore, the scope of the invention is to be limited only by the following claims.