Abstract:
A method for manufacturing an investment casting core uses a metallic blank having a thickness between parallel first and second faces less than a width and length transverse thereto. The blank is locally thinned from at least one of the first and second faces. The local thinning forms a taper on a leading portion of the RMC. The blank is through-cut across the thickness. The blank is inserted into the leading portion into a slot in a pre-formed ceramic core.

Description:
US GOVERNMENT RIGHTS 
       [0001]    The invention was made with US Government support under contract W911W6-08-2-0001 awarded by the US Army. The US Government has certain rights in the invention. 
     
    
     BACKGROUND 
       [0002]    The disclosure relates to investment casting. More particularly, it relates to the investment casting of superalloy turbine engine components. 
         [0003]    Investment casting is a commonly used technique for forming metallic components having complex geometries, especially hollow components, and is used in the fabrication of superalloy gas turbine engine components. The disclosure is described in respect to the production of particular superalloy castings, however it is understood that the disclosure is not so limited. 
         [0004]    Gas turbine engines are widely used in aircraft propulsion, electric power generation, and ship propulsion. In gas turbine engine applications, efficiency is a prime objective. Improved gas turbine engine efficiency can be obtained by operating at higher temperatures, however current operating temperatures in the turbine section exceed the melting points of the superalloy materials used in turbine components. Consequently, it is a general practice to provide air cooling. Cooling is provided by flowing relatively cool air from the compressor section of the engine through passages in the turbine components to be cooled. Such cooling comes with an associated cost in engine efficiency. Consequently, there is a strong desire to provide enhanced specific cooling, maximizing the amount of cooling benefit obtained from a given amount of cooling air. This may be obtained by the use of fine, precisely located, cooling passageway sections. 
         [0005]    The cooling passageway sections may be cast over casting cores. Ceramic casting cores may be formed by molding a mixture of ceramic powder and binder material by injecting the mixture into hardened steel dies. After removal from the dies, the green cores are thermally post-processed to remove the binder and fired to sinter the ceramic powder together. The trend toward finer cooling features has taxed core manufacturing techniques. The fine features may be difficult to manufacture and/or, once manufactured, may prove fragile. Commonly-assigned U.S. Pat. Nos. 6,637,500 of Shah et al. and 6,929,054 of Beals et al and Pre-grant Publication 2007/261814 of Luczak (the disclosures of which are incorporated by reference herein as if set forth at length) disclose use of ceramic and refractory metal core combinations. 
         [0006]      FIG. 1  shows a trailing edge portion of a turbine airfoil  20  as cast within a shell  22 . For casting the internal passageways, the shell contains a core assembly. The exemplary core assembly includes a ceramic feed core having spanwise legs  30 ,  32 , and  34  for casting associated passageway legs. The leg  34  casts a trailing spanwise passageway  36 . The core assembly also includes metallic cores, of which cores  40 ,  42 , and  44  are shown. The exemplary metallic cores are formed of refractory metal sheet stock. The core  40  forms a pressure side outlet circuit, the core  42  forms a suction side outlet circuit, and the core  44  forms a trailing edge outlet slot  50 . The outlet slot  50  is fed from the passageway  36 . During core assembly, a leading portion of the core  44  is secured within a mating slot of the trailing leg  34  of the ceramic core. 
       SUMMARY 
       [0007]    One aspect of the disclosure involves a method for manufacturing an investment casting core from a metallic blank. The blank has a thickness between parallel first and second faces less than a width and length transverse thereto. The blank is locally thinned from at least one of the first and second faces. The blank is through-cut across the thickness. The blank is inserted into the leading portion into a slot in a pre-formed ceramic core. 
         [0008]    In various implementations, through-cutting may comprise at least one of laser cutting, liquid jet cutting, and EDM. The thinning may comprise at least one of EDM, ECM, MDP, and mechanical machining. 
         [0009]    In an investment casting method, the investment casting core may be at least partially overmolded by a pattern-forming material for forming a pattern. The pattern may be shelled. The pattern-forming material may be removed from the shelled pattern for forming a shell. Molten alloy may be introduced to the shell. The shell may be removed. The method may be used to form a gas turbine engine component. An exemplary component is an airfoil wherein the core forms trailing edge outlet passageways. 
         [0010]    Another aspect of the disclosure involves an investment casting core having a metallic core element and a ceramic core. The metallic core element has a tapered leading portion, an intermediate portion downstream of the tapered leading portion, and a trailing portion downstream of the intermediate portion and thicker than the intermediate portion. The ceramic casting core has a slot receiving the leading portion. 
         [0011]    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 
         [0012]      FIG. 1  is a partial streamwise sectional view of a trailing edge portion of a prior art airfoil cast within a ceramic shell. 
           [0013]      FIG. 2  is a partial streamwise sectional view of a modified airfoil. 
           [0014]      FIG. 2A  is an enlarged view of a portion of  FIG. 2 . 
           [0015]      FIG. 3  is a partially schematic/simplified view of a pattern including the core assembly. 
           [0016]      FIG. 4  is a partially schematic/simplified view of a blade cast in a shell formed over the pattern. 
           [0017]      FIG. 5  is an enlarged partial pressure side view of a discharge slot of the blade of  FIG. 4 . 
           [0018]      FIG. 6  is a flowchart of a core manufacture process. 
       
    
    
       [0019]    Like reference numbers and designations in the various drawings indicate like elements. 
       DETAILED DESCRIPTION 
       [0020]      FIG. 2  shows an alternative refractory metal core  60  which has a leading/upstream edge/end  62  and a trailing/downstream edge/end  64 . The exploded view of  FIG. 3  shows an inboard end  66  and an outboard end  68 . As is discussed further below, an upstream-most portion  70  extending aft from the leading edge/end  62  is configured to be received within and mate with a trailing slot  72  of a trailing leg  74  of a ceramic feedcore  76 . The RMC  60  has an intermediate portion  80  which casts the majority of the ultimate trailing edge discharge slot. In the exemplary RMC  60 , along this region  80 , the RMC pressure side/surface  82  and suction side/surface  84  are separated by an essentially constant RMC thickness T 1  ( FIG. 2A ). Downstream of the portion  80 , the exemplary RMC thickens. A relatively thick portion  86  having an essentially constant thickness shown as T 3  extends to the trailing end/edge  64 . Of this portion  86 , a smaller upstream portion  88  casts pressure side discharge openings in the airfoil. 
         [0021]      FIG. 3  (a partially schematic/simplified view of a pattern) shows the portion  80  having holes  100  for casting posts within the slot.  FIG. 3  further shows the portion  88  as having streamwise elongate tapering holes  102  which are interspersed with intact portions  104 . The intact portions  104  cast pressure side openings from the trailing edge discharge slot; whereas the holes  102  cast walls therebetween. 
         [0022]    In the exemplary core assembly, the feedcore slot  72  and RMC portion  70  both have an upstream-ward taper. The exemplary thickness T 2  of the RMC at the leading edge is less than T 1  (e.g., 30-60%). The exemplary RMC taper is essentially constant at an angle of θ 1  over a streamwise length L 1 . The exemplary taper is provided by relieving/beveling only one of the two faces  82  and  84  (the face  84  in the exemplary embodiment with a bevel facet/surface  110 ). The exemplary relief provides the taper angle θ 1 . Exemplary θ 1  are 0.1-3.0°, more narrowly 1.0-2.5°. Exemplary taper length L 1  is coincident with or slightly less than a depth D 1  of the slot. The exemplary slot has an opening  120  having a height H 1  which may be greater than T 1  and has a base  122  with a height H 2  which is greater than T 2 . A portion of the slot between respective slot walls  124  and  126  and the RMC may be filled with an adhesive or slurry  130 . The exemplary streamwise cross-section of the RMC is shown as generally arcuate with concavity along the pressure side and convexity along the suction side so as to correspond to a median of the airfoil cross-section. 
         [0023]    Exemplary L 1  is 0.040-0.100 inch (1-2.5 mm), more narrowly 0.050-0.075 inch (1.3 mm-9 mm). Exemplary T 1  is 0.012 inch (0.3 mm), more broadly 0.005-0.020 inch (0.13-0.5 mm) or 0.010-0.015 inch (0.25-0.38 mm). Exemplary T 2  is 0.005 inch (0.13 mm), more broadly 0.002-0.015 inch (0.05-0.38 mm) or 0.003-0.007 inch (0.08-0.18 mm) or 25-75% of T 1 , more narrowly, 40-60%. Exemplary T 3  is 0.035 inch (0.9 mm), more broadly 0.020-0.050 inch (0.5-1.3 mm) or 200-500% of T 1 , more narrowly 250-400%. Exemplary feedcore thickness at either side of the slot base  122  (shown as T 4  to the pressure side and T 5  to the suction side) may be at least 0.018 inch (0.46 mm), more narrowly 0.018-0.040 inch (0.46-1.0 mm) or 0.08-0.025 inch (0.46-0.64 mm). 
         [0024]    In an exemplary sequence  200  of manufacture ( FIG. 6 ), the RMC  84  may be machined from a strip having a thickness equal to T 3 , a greater width, and a yet greater length. In an initial stage of manufacture, gross thickness features may be machined  202  to provide the thickness T 1  of the intermediate portion and provide the bevel/taper. Specifically, the exemplary machining is from the pressure side face  82  to define the intermediate portion and from the suction side face  84  to provide the taper of the leading portion. However, the step  202  may easily be further divided. Exemplary machining may be mechanical machining or may be an abrasive grinding, electrodischarge machining (EDM), electrochemical machining (ECM), or a molecular decomposition process (MDP). 
         [0025]    Additionally, a series of through-cuts are cut  206  to define the holes/apertures  100  for forming posts  150  ( FIG. 4 ) within the outlet slot and holes/apertures  102  for forming trailing dividing walls  152  along the slot outlet  154  at the trailing edge  156 .  FIG. 4  further shows: the airfoil  160  having a leading edge  162  and a tip  164 ; the platform  170  at the inboard end of the aitrfoil; and the firtree attachment root  172  depending from the underside of the platform. The root has the inlet ports  174  to the trunks of the cooling passageway network (cast over the ceramic feedcore trunks).  FIG. 5  shows the outlet  154  as including a spanwise array of segments/portions/openings  180  along the airfoil pressure side between associated pairs of the dividing walls  152 . As is discussed above, the openings  180  are cast by the intact portions  104  of the RMC portion  88  of  FIG. 2 . A curving transition  89  ( FIG. 2 ) between the RMC portions  80  and  86 / 88  casts a curving transition  182  ( FIG. 5 ) between a main portion  184  of the slot and the openings  180 . 
         [0026]    Exemplary cutting may be via a punching/stamping operation or, alternatively, mechanical drilling, laser cutting, liquid jet cutting, and/or EDM. To provide the RMC in the desired arcuate shape corresponding to the airfoil median  500 , the RMC is bent  208  (e.g., via stamping). This bending may also form a spanwise variation (e.g., to accommodate a varying relationship in the position of the feedcore relative to the discharge slot) such as creating a net spanwise twist. An exemplary stamping is performed via one or more pressing stages in custom presses having opposing die faces contoured to mate with the RMC. The RMC may be coated  210  with a protective coating. Alternatively a coating could be applied pre-assembly. Suitable coating materials include silica, alumina, zirconia, chromia, mullite and hafnia. Preferably, the coefficient of thermal expansion (CTE) of the refractory metal and the coating are similar. Coatings may be applied by any appropriate line-of-sight or non-line-of sight technique (e.g., chemical or physical vapor deposition (CVD, PVD) methods, plasma spray methods, electrophoresis, and sol gel methods). Individual layers may typically be 0.1 to 1 mil thick. Layers of Pt, other noble metals, Cr, Si, W, and/or Al, or other non-metallic materials may be applied to the metallic core elements for oxidation protection in combination with a ceramic coating for protection from molten metal erosion and dissolution. 
         [0027]    The ceramic core may be (e.g., silica-, zircon-, or alumina-based) molded  212 . The as-molded ceramic material may include a binder. The binder may function to maintain integrity of the molded ceramic material in an unfired green state. Exemplary binders are wax-based. After the molding  212 , the preliminary core assembly may be debindered/fired  214  to harden the ceramic (e.g., by heating in an inert atmosphere or vacuum). The slot  72  may have been formed as part of the molding  212  or may be cut in the ceramic (e.g., in the green state or in the fired state). The RMC may be inserted  216  into the ceramic core to assemble and an adhesive or slurry introduced  218 . 
         [0028]      FIG. 6  shows an exemplary method  220  for investment casting using the core assembly. Other methods are possible, including a variety of prior art methods and yet-developed methods. The fired core assembly is then overmolded  230  with an easily sacrificed material such as a natural or synthetic wax (e.g., via placing the assembly in a mold and molding the wax around it). There may be multiple such assemblies involved in a given mold. 
         [0029]    The overmolded core assembly (or group of assemblies) forms a casting pattern with an exterior shape largely corresponding to the exterior shape of the part to be cast. The pattern may then be assembled  232  to a shelling fixture (e.g., via wax welding between end plates of the fixture). The pattern may then be shelled  234  (e.g., via one or more stages of slurry dipping, slurry spraying, or the like). After the shell is built up, it may be dried  236 . The drying provides the shell with at least sufficient strength or other physical integrity properties to permit subsequent processing. For example, the shell containing the invested core assembly may be disassembled  238  fully or partially from the shelling fixture and then transferred  240  to a dewaxer (e.g., a steam autoclave). In the dewaxer, a steam dewax process  242  removes a major portion of the wax leaving the core assembly secured within the shell. The shell and core assembly will largely form the ultimate mold. However, the dewax process typically leaves a wax or byproduct hydrocarbon residue on the shell interior and core assembly. 
         [0030]    After the dewax, the shell is transferred  244  to a furnace (e.g., containing air or other oxidizing atmosphere) in which it is heated  246  to strengthen the shell and remove any remaining wax residue (e.g., by vaporization) and/or converting hydrocarbon residue to carbon. Oxygen in the atmosphere reacts with the carbon to form carbon dioxide. Removal of the carbon is advantageous to reduce or eliminate the formation of detrimental carbides in the metal casting. Removing carbon offers the additional advantage of reducing the potential for clogging the vacuum pumps used in subsequent stages of operation. 
         [0031]    The mold may be removed from the atmospheric furnace, allowed to cool, and inspected  248 . The mold may be seeded  250  by placing a metallic seed in the mold to establish the ultimate crystal structure of a directionally solidified (DS) casting or a single-crystal (SX) casting. Nevertheless the present teachings may be applied to other DS and SX casting techniques (e.g., wherein the shell geometry defines a grain selector) or to casting of other microstructures. The mold may be transferred  252  to a casting furnace (e.g., placed atop a chill plate in the furnace). The casting furnace may be pumped down to vacuum  254  or charged with a non-oxidizing atmosphere (e.g., inert gas) to prevent oxidation of the casting alloy. The casting furnace is heated  256  to preheat the mold. This preheating serves two purposes: to further harden and strengthen the shell; and to preheat the shell for the introduction of molten alloy to prevent thermal shock and premature solidification of the alloy. 
         [0032]    After preheating and while still under vacuum conditions, the molten alloy is poured  258  into the mold and the mold is allowed to cool to solidify  260  the alloy (e.g., after withdrawal from the furnace hot zone). After solidification, the vacuum may be broken  262  and the chilled mold removed 264 from the casting furnace. The shell may be removed in a deshelling process  266  (e.g., mechanical breaking of the shell). 
         [0033]    The core assembly is removed in a decoring process  268  to leave a cast article (e.g., a metallic precursor of the ultimate part). The cast article may be machined  270 , chemically and/or thermally treated  272  and coated  274  to form the ultimate part. Some or all of any machining or chemical or thermal treatment may be performed before the decoring. 
         [0034]    One or more embodiments have been described. Nevertheless, it will be understood that various modifications may be made. For example, the principles may be implemented using modifications of various existing or yet-developed processes, apparatus, or resulting cast article structures (e.g., in a reengineering of a baseline cast article to modify cooling passageway configuration). In any such implementation, details of the baseline process, apparatus, or article may influence details of the particular implementation. Accordingly, other embodiments are within the scope of the following claims.