Abstract:
A method comprises casting a metallic material ( 56 ) in a mold ( 20 ) containing a core, the core having a substrate ( 40, 44 ) coated with a coating ( 42 ). A removing the metallic material from the mold and decoring leaves a casting having a layer formed by the coating. The coating comprises a ceramic having a porosity in a zone ( 50 ) near the substrate less than a porosity in a zone ( 52 ) away from the substrate.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    Benefit is claimed of U.S. Patent Application Ser. No. 61/830,288, filed Jun. 3, 2013, and entitled “Castings and Manufacture Methods”, the disclosure of which is incorporated by reference herein in its entirety as if set forth at length. 
     
    
     BACKGROUND 
       [0002]    The disclosure relates to casting of turbine engine components. More particularly, the disclosure relates to casting of superalloy components with internal cooling passageways. 
         [0003]    Gas turbine engine hot section components such as turbine blades, vanes, and air seals are often cast from superalloys (e.g., nickel-based or cobalt based). They are often cast over cores such as molded ceramic cores. Alternative cores include refractory metal cores (RMC) and RMC/ceramic core assemblies). 
         [0004]    After casting, a deshelling and decoring process leaves the internal cooling passageways where the cores had been. 
         [0005]    It may be desired to apply a thermal barrier coating (TBC) system to the casting. 
         [0006]    Coating along the internal passageways poses difficulties. 
         [0007]    U.S. Pat. Nos. 6,929,054, 7,207,373, and 7,207,374 disclose alumina protective coatings on RMCs. 
         [0008]    U.S. Pat. No. 7,802,613 discloses noble metal plating of ceramic cores (and of ceramic-coated RMCs) to improve wetting by the superalloy during casting. 
         [0009]    US Patent Application Publication 2005/0241797A1 discloses transferring an MCrAlY coating from a ceramic core to a superalloy casting. 
         [0010]    U.S. Pat. No. 7,055,574 discloses transferring a yttria-stabilized zirconia (YSZ) coating layer and an MCrAlY layer from a sand core to a cast article. 
       SUMMARY 
       [0011]    One aspect of the disclosure involves a method comprising: casting a metallic material in a mold containing a core, the core having a substrate coated with a coating. A removing the metallic material from the mold and decoring leaves a casting having a layer formed by the coating. The coating comprises a ceramic having a porosity in a zone near the substrate less than a porosity in a zone away from the substrate. 
         [0012]    A further embodiment may additionally and/or alternatively include the substrate comprising a molded first ceramic and the coating ceramic comprising a second ceramic different from the first ceramic. 
         [0013]    A further embodiment may additionally and/or alternatively include applying the second ceramic to the first ceramic by PVD. 
         [0014]    A further embodiment may additionally and/or alternatively include the first ceramic being silica-based and the second ceramic being alumina-based. 
         [0015]    A further embodiment may additionally and/or alternatively include the coating ceramic having a characteristic thickness of 1.0 to 10 mil (25 to 250 micrometers). 
         [0016]    A further embodiment may additionally and/or alternatively include the coating comprising a first layer applied by a first technique and a second layer applied by a second technique, different from the first technique. 
         [0017]    A further embodiment may additionally and/or alternatively include the first technique being a vapor deposition and the second technique not a vapor deposition. 
         [0018]    A further embodiment may additionally and/or alternatively include the second layer comprising a first sublayer and a second sublayer of differing porosities. 
         [0019]    A further embodiment may additionally and/or alternatively include the second technique being a sol-gel process. 
         [0020]    A further embodiment may additionally and/or alternatively include the coating comprising a second metallic material atop and/or intermixed with the ceramic. 
         [0021]    A further embodiment may additionally and/or alternatively include at least a majority by weight of the second metallic material diffusing into the metallic material. 
         [0022]    A further embodiment may additionally and/or alternatively include the metallic material being a nickel-based superalloy. 
         [0023]    A further embodiment may additionally and/or alternatively include the casting having an airfoil. 
         [0024]    A further embodiment may additionally and/or alternatively include applying a coating to an exterior of the casting, but not the interior. 
         [0025]    Another aspect of the disclosure involves a coated casting comprising a metallic casting having one or more internal passageways and a ceramic lining along the passageways. The ceramic lining has a porosity in a zone near the casting greater than a porosity in a zone away from the casting. 
         [0026]    A further embodiment may additionally and/or alternatively include the metallic casting at least partially filling the porosity of at least the zone near the casting. 
         [0027]    A further embodiment may additionally and/or alternatively include the metallic casting being nickel-based superalloy 
         [0028]    A further embodiment may additionally and/or alternatively include the coated casting forming a gas turbine engine component. 
         [0029]    A further embodiment may additionally and/or alternatively include the coated casting the coated casting having a thermal barrier coating on an exterior surface of differing composition from said coating. 
         [0030]    A further embodiment may additionally and/or alternatively include the casting having an airfoil. 
         [0031]    The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0032]      FIG. 1  is a sectional view of a casting mold including a shell and a coated casting core. 
           [0033]      FIG. 1A  is an enlarged view of a first portion of the core of the mold of  FIG. 1 . 
           [0034]      FIG. 1B  is an enlarged view of a second portion of the core of the mold of  FIG. 1 . 
           [0035]      FIG. 2  is an enlarged view of the first portion of the mold of  FIG. 1  after casting. 
           [0036]      FIG. 3  is a sectional view of a blade formed by the casting after deshelling/decoring and exterior coating. 
           [0037]      FIG. 3A  is an enlarged view of the first portion of the casting of  FIG. 3 . 
           [0038]      FIG. 4  is an enlarged view of a first portion of the core of the mold of  FIG. 1  with an alternate coating. 
           [0039]      FIG. 5  is an enlarged view of a first portion of the core of the mold of  FIG. 1  with an alternate coating. 
       
    
    
       [0040]    Like reference numbers and designations in the various drawings indicate like elements. 
       DETAILED DESCRIPTION 
       [0041]      FIG. 1  is a sectional view of an investment casting mold  20  comprising a shell  22  and a core  24 . The mold has an interior space  26  between a shell inner surface  28  and a core outer surface  30 . In casting, the mold interior space receives a molten alloy which solidifies to form a casting (discussed further below). The exemplary mold is for casting a turbine blade for a gas turbine engine. Other exemplary gas turbine engine components include vanes, combustor panels, and outer air seals. 
         [0042]    The exemplary core  24  comprises a substrate  40  ( FIG. 1A ) and a multi-layer coating  42 . The exemplary substrate is a ceramic substrate. An exemplary ceramic substrate is silica-based (e.g., a molded and fired silica core). Alternative substrates may be possible. One group of alternative substrates is refractory metals ( FIG. 1B ). Exemplary refractory metals for refractory metal cores (RMC) are Mo and W and such refractory metal(s) may comprise at least 50% by weight of the substrate. 
         [0043]    Core assemblies may also be relevant. One example of such assemblies is where one or more RMCs are assembled to one or more ceramic cores.  FIG. 1  shows such an assembly. In such a situation, the coating may be applied before or after core assembly and differing coatings (or lack thereof) are possible on different portions of the core or core assembly. 
         [0044]    Of the coating  42 , at least one of the layers is intended to react with the cast metal and/or survive decoring to become a portion of the ultimate cast article. 
         [0045]    A first example of the coating  42  involves an inner layer  50  ( FIG. 1A ) atop the substrate and an outer layer  52  atop the inner layer. The exemplary layers  50  and  52  are both ceramic but of differing properties. The exemplary layers  50  and  52  are intended to survive decoring and become part of the ultimate article. In a more specific example, the layers  50  and  52  are of differing porosity and/or are applied by different methods. 
         [0046]    In a yet more specific example, the layers  50  and  52  both are alumina-based. The inner layer  50  is applied to the substrate via physical vapor deposition (PVD) (e.g., electron beam physical vapor deposition (EB-PVD)), sputtering, and the like. The inner layer  50  has a relatively low porosity and high strength. The layer  52  is applied atop the inner layer  50  such as via a sol-gel process and has a higher porosity than the inner layer  50 . 
         [0047]    To provide a desired porosity of the layer  52  (and, more particularly, to provide a varied or graded porosity) parameters of the sol-gel process may be controlled/varied. For example, one can vary the rate at which remaining solvents in the sol-gel material are removed to adjust the porosity and final microstructure of the layer, slowing down the rate of solvent removal will allow the sol-gel to form a more dense microstructure. 
         [0048]    The exemplary layers  50  and  52  are shown having a respective thicknesses T 1  and T 2 . Exemplary T 1  and T 2  are 0.1 to 5 mil each (2.5 to 130 micrometers) for a combined 5 to 250 micrometers (more particularly 30 to 200 micrometers). In some examples, a relatively low T 1  may be desired. For example this may involve a coating along a cooling air passageway as contrasted with a coating exposed to a gaspath. In the cooling air passageway, heat transfer through the coating is desirable (whereas it may be undesirable along the gaspath). In the cooling passageway, physical protection needs may be lower than along the gaspath (e.g., subject to less erosion). Thus the thickness T 1  in a cooling passageway may be low to provide a minimal protection (e.g. against oxidation). In such a situation, T 2  may need to be high enough to provide good attachment to the casting. Thus, exemplary T 1 &lt;T 2 . For example, exemplary T 1  is 5% to 75% of T 2 . More narrowly, T 1  is 10% to 50% of T 2 . More broadly, exemplary T 1  is 5% to 300% of T 2 . 
         [0049]    Thus, an exemplary combination involves T 1  of 0.2 mil to 2.0 mils (5 micrometers to 50 micrometers, more narrowly 10 micrometer to 40 micrometer, more broadly 3 micrometer to 100 micrometer) and T 2  of 1.0 mil to 3.0 mil (25 micrometers to 80 micrometers, more narrowly 40 micrometer to 75 micrometer, more broadly 15 micrometer to 150 micrometer). 
         [0050]    In yet more specific examples (not shown), the layer  52  has a graded porosity starting from relatively low porosity near the layer  50  and proceeding to relatively high porosity near its outer surface. An exemplary porosity variation involves: (1) essentially full density of the layer  50  (e.g., at least 95% dense, more broadly at least 90%): (2) substantially full density of the layer  52  near the layer  50  (e.g., over at least 10% local or average depth of the layer  52  (more narrowly, at least 20%)) a density of at least 95% dense, more broadly at least 90%); and (3) near the surface of the layer  52  (e.g., over at least 10% local or average depth of the layer  52  (more narrowly, at least 20%)) lower density (e.g., 15% or more porosity, more particularly, 20% or more with an exemplary 20-30%). 
         [0051]    During casting, the high porosity of the layer  52  (or the region near its outer surface) allows infiltration of casting metal  56  ( FIG. 2 ) to provide strong mechanical interlocking to resist delamination. 
         [0052]    After the cast metal has cooled, an exemplary deshelling and decoring process involves mechanically deshelling (e.g., breaking the shell) followed by chemically decoring. Exemplary decoring involves chemical leaching, such as alkaline leaching (e.g., with an aqueous solution comprising NaOH and/or KOH (exemplary concentration 25-50% molar)) and is effective to remove most if not all of the substrate while leaving most if not all of the inner layer  50 . If a refractory metal core is used, an acid leach may be used (thus a series alkaline and acid leaching may remove a core assembly). An exemplary acid leach involves a mixture of nitric, hydroflouric and hydrochloric acids. The inner layer  50  thus provides a surface  60  ( FIG. 3A ) of an internal passageway  62  in the casting and may provide thermal and/or chemical protection to the cast metal along the passageway. 
         [0053]      FIG. 3  shows a casting (e.g., of a blade having an airfoil extending from an inboard end at a platform to a tip and an attachment root (e.g., firtree) extending from an underside of the platform) which may have an exterior surface to which a conventional thermal barrier coating (TBC) system is applied (e.g., by spray and or PVD of a metallic bondcoat (e.g., MCrAlY or aluminide) and a ceramic thermal barrier coating (e.g., YSZ, GSZ, and the like). 
         [0054]    Some material variations involve using an oxynitride as a ceramic coating layer in place of alumina for one or both of the layers  50  and  52 . For example, silicon oxynitride (Si 2 N 2 O) has good thermal stability up to 1600° C. and would be expected to have chemical compatibility with the standard silica core materials. Additionally, these materials are commonly doped with aluminum to form SiAlON compounds with exceptional chemical inertness and corrosion resistance. These compounds can be created by reactive PVD techniques such as cathodic arc and magnetron sputtering to form useful thin films. 
         [0055]    Some variations on the dual ceramic layer or graded ceramic layer involve metal as a separate layer atop the ceramic and/or intermixed with the ceramic. The metal may improve wetting of the ceramic by the casting alloy and may fully or partially diffuse into the casting alloy (e.g., at least a majority of the metal  200  diffusing into the alloy, more particularly, at least 90% or at least 95%).  FIG. 4  shows metal  200  forming a body having a surface layer/portion  202  atop the ceramic  52  and a portion  204  intermixed to fill pores in the ceramic  52 . The layer  202  has a thickness shown as T 3 . Exemplary T 3  is less than the combined ceramic layer thickness (T 2 +T 2 ), more particularly less than each of the ceramic layers. Thus exemplary T 3  is up to 1 mil (25 micrometer), more particularly up to 10 micrometer (e.g. 0.05 micrometer to 0.5 micrometer). 
         [0056]    One example of such use of metal involves molybdenum. Exemplary molybdenum is commercially pure molybdenum. A broader range includes alloys or mixtures of at least 50% molybdenum or at least 90% by weight. Alternative metals may be used. Exemplary metals include Mo, W, Ta, Pt, Pd, and their mixtures and alloys, optionally with other components of less than plurality weight. Exemplary application techniques are deposition techniques (e.g., vapor or spray). Exemplary vapor deposition is chemical vapor deposition (CVD). Alternative techniques include plating (e.g., electroless). 
         [0057]      FIG. 5  shows a further alternative variation wherein the layer  52  is further divided into sublayers  52 - 1  and  52 - 2 , having respective thicknesses T 2-1  and T 2-2 . Both these sublayers may be broadly deposited via similar technique (e.g., sol-gel) while this may differ from the technique used to apply the layer  50 . The sublayer  52 - 1  is relatively less porous than the layer  52 - 2 . This may essentially confine metal infiltration to the sublayer  52 - 2 . Each sublayer may represent at least 15% of the thickness T 2  above, more particularly, at least 30%. In such an example, the layer  52 - 2  may serve to allow mechanical bonding between the cast alloy and the under-lying layer  52 - 2 . 
         [0058]    The exemplary mold is an investment casting mold including a shell. An exemplary shell is formed by placing the core(s) in a die to overmold the core with a sacrificial pattern-forming material (e.g., wax) to form a pattern from which portions of the core(s) protrude. The pattern is then shelled with a ceramic stucco so that the exposed core portions become embedded in the shell. In one or more steps, the shell is hardened and the wax removed to leave the interior space  26 . 
         [0059]    Alternative molds include non-shell sacrificial mold members instead of the shell. Yet further alternative molds include reusable dies used in die casting. 
         [0060]    The use of “first”, “second”, and the like in the following claims is for differentiation within the claim only and does not necessarily indicate relative or absolute importance or temporal order. Similarly, the identification in a claim of one element as “first” (or the like) does not preclude such “first” element from identifying an element that is referred to as “second” (or the like) in another claim or in the description. 
         [0061]    Where a measure is given in English units followed by a parenthetical containing SI or other units, the parenthetical&#39;s units are a conversion and should not imply a degree of precision not found in the English units. 
         [0062]    One or more embodiments have been described. Nevertheless, it will be understood that various modifications may be made. For example, when applied to an existing baseline configuration, details of such baseline may influence details of particular implementations. Accordingly, other embodiments are within the scope of the following claims.