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 blank is through-cut across the thickness.

Description:
BACKGROUND OF THE INVENTION 
   The invention relates to investment casting. More particularly, it relates to the investment casting of superalloy turbine engine components. 
   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 invention is described in respect to the production of particular superalloy castings, however it is understood that the invention is not so limited. 
   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. 
   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. No. 6,637,500 of Shah et al. and U.S. Pat. No. 6,929,054 of Beals et al (the disclosures of which are incorporated by reference herein as if set forth at length) disclose use of ceramic and refractory metal core combinations. 
     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. With such a configuration, the transition between the passageway  36  and the outlet slot  50  may be relatively abrupt and may create relatively thick areas  52  and  54  of the pressure and suction side walls. 
   SUMMARY OF THE INVENTION 
   One aspect of the invention 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 length and width 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. 
   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, grinding, and mechanical machining. The through-cutting may comprise forming a plurality of through-apertures and a plurality of recesses. After the through-cutting, the blank may be bent to at least partially contract the recesses. The thinning may comprise machining a downstream-tapering portion and leaving a thicker portion downstream of the downstream-tapering portion. The core may be coated. The core may be overmolded with a ceramic core or assembled to a pre-molded ceramic core. The thinning may form a mounting flange by thinning from both the first and second faces. The mounting flange may be overmolded by a ceramic core or inserted into a mating slot of a pre-molded ceramic core. 
   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. 
   Another aspect of the invention involves an investment casting core having a metallic core element and a ceramic core. The metallic core element has a flange extending from a second portion, the second portion thicker than the flange. The ceramic casting core has a slot receiving the flange and slot shoulders abutting shoulders of the second portion. A smooth continuous taper may span a junction between the metallic casting core element and the ceramic casting core. The slot may be pre-molded or formed by overmolding the metallic casting core element. 
   The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a partial streamwise sectional view of a trailing edge portion of a prior art airfoil cast within a ceramic shell. 
       FIG. 2  is a partial streamwise sectional view of a modified airfoil. 
       FIG. 3  is a view of a composite core for casting the airfoil of  FIG. 2 . 
       FIG. 4  is a streamwise sectional view of a trailing portion of the composite core of  FIG. 3 . 
       FIG. 5  is a trailing edge view of the composite core of  FIG. 3 . 
       FIG. 6  is a flowchart of a core manufacture process. 
       FIG. 7  is an end view of a core precursor. 
       FIG. 8  is an end view of the precursor of  FIG. 7  after a first local thinning from a first face. 
       FIG. 9  is an end view of the precursor of  FIG. 8  after additional thinning from the first face and an opposite second face to form a mounting flange. 
       FIG. 10  is a first face plan view of the precursor of  FIG. 9  after a through-cutting. 
       FIG. 11  is a simplified view of a core formed by bending the precursor of  FIG. 10  at a plurality of recesses. 
       FIG. 12  is a flowchart of an investment casting method. 
       FIG. 13  is a partial first face view of a first alternate core. 
       FIG. 14  is a partial first face view of a second alternate core. 
       FIG. 15  is a partial first face view of a third alternate core. 
       FIG. 16  is a view of a fourth alternate core. 
       FIG. 17  is a view of a fifth alternate core. 
       FIG. 18  is an end view of a sixth alternate core. 
       FIG. 19  is an end view of a seventh alternate core. 
       FIG. 20  is an end view of an eighth alternate core. 
       FIG. 21  is an end view of a ninth alternate core. 
       FIG. 22  is an end view of a tenth alternate core. 
   

   Like reference numbers and designations in the various drawings indicate like elements. 
   DETAILED DESCRIPTION 
     FIG. 2  shows a reengineered airfoil  60  which may be based upon the exemplary airfoil  20 . The airfoil  60  has a relatively gently transitioning junction  62  between a trailing feed passageway/cavity  64  and an outlet slot  66 . For example, a leading portion  68  of the slot  66  has a downstream-tapering thickness profile which tends to reduce the peak thickness of the pressure and suction side walls  70  and  72  (thereby reducing part mass, improving part cooling, and reducing resistance to the cooling airflow). Similar smooth transitions have been attempted with purely ceramic cores. However, such purely ceramic cores then suffer breakage problems if fine features of the outlet slot are to be cast. 
     FIG. 3  shows a portion of a core assembly  80  for casting the passageways  64  and  66  of  FIG. 2 . The core  80  includes a ceramic core element/portion  82  and a refractory metal core (RMC) element/portion  84  (also shown in broken lines in  FIG. 2 ). For purposes of illustration, remaining portions of the ceramic core element  82  are not shown. Additionally, apertures within both of the elements  82  and  84  are also not shown. 
     FIG. 4  shows the RMC  84  as including a leading tenon  90  received within a trailing slot or mortise  92  of the ceramic core element  82 . The exemplary tenon and slot are flat with parallel surfaces respectively facing pressure and suction sides of the airfoil. At a root of the tenon  90 , the RMC  84  expands outward with a pair of shoulders  94  and  96  engaging trailing face portions  98  and  100  of the ceramic core element  82 . These mating faces extend outward to respective suction and pressure side faces  102  and  104  of the core assembly  80 . The side faces  102  and  104  smoothly transition between the ceramic core element  82  and the RMC  84 . This junction between RMC and ceramic core falls along a tapering portion  106 . Downstream of tapering portion  106 , the RMC transitions to a straight flat portion  108  and then to a thicker portion  110  wherein the pressure side face  104  protrudes. The exemplary suction side face  102  is smooth along the tapering portion, flat portion, and thicker portion  110 . 
   In an exemplary sequence  200  of manufacture ( FIG. 6 ) The RMC  84  may be machined from a strip ( FIG. 7 ) having a thickness T, a greater width W, and a yet greater length. In an initial stage of manufacture, gross thickness features may be machined  202  to provide the smooth transition. Specifically,  FIG. 8  shows a machining from a pressure side face  120  to define the tapering region  106  and the straight region  108 . The tenon  90  ( FIG. 9 ) is then formed by machining material  204  from both the pressure side face  120  and the suction side face  122 . However, the steps  202  and  204  may easily be combined or further divided. 
   Additionally, a series of through-cuts are cut  206 . A first group of through-cuts includes recesses  140  ( FIG. 10 ) extending downstream through the tenon  90  and well into the trailing portion  110 . Others of the cuts define apertures  141 ,  142 , and  143  for forming posts  150 ,  152 , and  153  ( FIG. 2 ) within the outlet slot and apertures  144  for forming trailing dividing walls  154  along the slot outlet. To provide the RMC in the desired arcuate shape corresponding to the airfoil trailing edge, the RMC is bent  208  to partially close the recesses  140  ( FIG. 11 ). 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. 
   The RMC may be assembled in a die and the ceramic core (e.g., silica-, zircon-, or alumina-based) molded thereover. An exemplary overmolding  212  includes molding the ceramic core  82  over the tenon  90 . 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 overmolding  212 , the preliminary core assembly may be debindered/fired  214  to harden the ceramic (e.g., by heating in an inert atmosphere or vacuum). 
     FIG. 12  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. 
   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. 
   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. 
   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. 
   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). 
   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. 
     FIG. 13  shows an RMC  160  otherwise similar to the RMC  84  but wherein the apertures  141 ,  142 ,  143  and  144  are replaced by combinations of apertures  162  and wave-like slots  164 . Each of the exemplary slots  164  includes a straight leading portion  166  through the flange, a wave-like (e.g., sinusoidal) portion  168  in the RMC tapering portion and straight region, and a terminal straight portion  170  within the thicker portion. The apertures  162  are interspersed between the slots  164  in phase with the waveform. In the ultimate cast airfoil, adjacent slots  164  may form dividing walls (with passageways in between including posts cast by the apertures  162 ). 
     FIG. 14  shows an RMC  180  with similar wave-like slots  182  but lacking the apertures  162 . Accordingly, the slots may be at a closer spacing than the slots  164 .  FIG. 15  shows an RMC  190  with an array of straight slots  192  in view of the wave-like slots  182 . 
     FIG. 16  shows an RMC  300  having a spanwise variation in the angle of convergence of its tapering portion  302 . The RMC&#39;s tenon  304  and the tapering portion  302  also have as-machined spanwise curvature (e.g., as distinguished from bending at recesses). A trailing portion  306  is also thin and flat (as distinguished from the portion  110  of  FIG. 4  and, effectively a continuation of the portion  108 ). For ease of illustration, apertures are not shown. 
     FIG. 17  an RMC  320  also having spanwise curvature, but wherein the trailing portion  322  has a spanwise variation in thickness (e.g., thicker midspan and tapering toward the inboard and outboard ends). For ease of illustration, apertures are not shown. 
     FIG. 18  shows an RMC  330  otherwise similar to the RMC  84  but wherein the tapering portion  332  has arrays of dimple-like blind recesses  334  along the pressure and suction side faces. The recesses may be chemically etched, mechanically drilled, laser drilled, or the like. 
     FIG. 19  shows an RMC  340  otherwise similar to the RMC  84  but wherein the tapering portion  342  has arrays of protrusions  344  along the pressure and suction side faces. The protrusions may be formed by welding or cladding or may be left after an etching, mechanical machining, laser drilling, EDM, or the like. 
     FIG. 20  shows an RMC  350  otherwise similar to the RMC  84  but wherein the tapering portion  352  has a streamwise concavity extending  354  along the suction side face. The concavity may be formed in the initial machining. 
     FIG. 21  shows an RMC  360  otherwise similar to the RMC  84  but wherein the tapering portion  362  has a streamwise concavity extending  364  along the pressure side face. The concavity may be formed in the initial machining 
     FIG. 22  shows an RMC  370  otherwise similar to the RMC  84  but wherein the tapering portion  372  tapers along both the pressure and suction side faces. Also, the exemplary RMC  370  has a thin trailing portion  374  in place of the thick trailing portion  110 . 
   One or more embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. 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.