Abstract:
At least one feed core and at least one wall cooling core are assembled with a number of elements of a die for forming a cooled turbine engine element investment casting pattern. A sacrificial material is molded in the die. The sacrificial material is removed from the die. The removing includes extracting a first of the die elements from a compartment in a second of the die elements before disengaging the second die element from the sacrificial material. The first element includes a compartment receiving an outlet end portion of a first of the wall cooling cores in the assembly and disengages therefrom in the extraction.

Description:
U.S. GOVERNMENT RIGHTS  
       [0001]     The invention was made with U.S. Government support under contract F33615-97-C-2779 awarded by the US Air Force. The U.S. Government has certain rights in the invention. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The invention relates to investment casting. More particularly, the invention relates to investment casting of cooled 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.  
         [0004]     Gas turbine engines are widely used in aircraft propulsion, electric power generation, ship propulsion, and pumps. 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 typically provided by flowing relatively cool air, e.g., 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]     A well developed field exists regarding the investment casting of internally-cooled turbine engine parts such as blades and vanes. In an exemplary process, a mold is prepared having one or more mold cavities, each having a shape generally corresponding to the part to be cast. An exemplary process for preparing the mold involves the use of one or more wax patterns of the part. The patterns are formed by molding wax over ceramic cores generally corresponding to positives of the cooling passages within the parts. In a shelling process, a ceramic shell is formed around one or more such patterns in well known fashion. The wax may be removed such as by melting in an autoclave. The shell may be fired to harden the shell. This leaves a mold comprising the shell having one or more part-defining compartments which, in turn, contain the ceramic core(s) defining the cooling passages. Molten alloy may then be introduced to the mold to cast the part(s). Upon cooling and solidifying of the alloy, the shell and core may be mechanically and/or chemically removed from the molded part(s). The part(s) can then be machined and/or treated in one or more stages.  
         [0006]     The ceramic cores themselves may be formed by molding a mixture of ceramic powder and binder material by injecting the mixture into hardened metal 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 ceramic core manufacturing techniques. The fine features may be difficult to manufacture and/or, once manufactured, may prove fragile. Commonly-assigned co-pending U.S. Pat. No. 6,637,500 of Shah et al. discloses exemplary use of a ceramic and refractory metal core combination. Other configurations are possible. Generally, the ceramic core(s) provide the large internal features such as trunk passageways while the refractory metal core(s) provide finer features such as outlet passageways. Assembling the ceramic and refractory metal cores and maintaining their spatial relationship during wax overmolding presents numerous difficulties. A failure to maintain such relationship can produce potentially unsatisfactory part internal features. Depending upon the part geometry and associated core(s), it may be difficult to assembly fine refractory metal cores to ceramic cores. Once assembled, it may be difficult to maintain alignment. The refractory metal cores may become damaged during handling or during assembly of the overmolding die. Assuring proper die assembly and release of the injected pattern may require die complexity (e.g., a large number of separate die parts and separate pull directions to accommodate the various RMCs). U.S. patent application Ser. No. 10/867,230, by Carl Verner et al. filed Jun. 14, 2004 and entitled INVESTMENT CASTING, discloses the pre-embedding of RMCs in wax bodies shaped to help position the core assembly and facilitate die separation and pattern removal.  
       SUMMARY OF THE INVENTION  
       [0007]     One aspect of the invention involves a method for manufacturing a cooled turbine engine element investment casting pattern. At least one feed core and at least one airfoil wall cooling core are assembled with a number of elements of a die. A sacrificial material is molded in the die and is then removed from the die. The removing includes extracting a first of the die elements from a compartment in a second of the die elements before disengaging the second die element from the sacrificial material. The first element includes a compartment receiving an outlet end portion of a first of the wall cooling cores in the assembly and disengages therefrom in the extraction.  
         [0008]     In various implementations, the disengaging of the second element from the sacrificial material may include a first extraction in a first direction. The extracting of the first die element may be in a second direction off-parallel to the first direction. The first extraction may release a backlocking between the first wall cooling core and the second element. The second direction may be off-parallel to the first direction by 5-60°.  
         [0009]     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  
       [0010]      FIG. 1  is a streamwise sectional view of a turbine airfoil element.  
         [0011]      FIG. 2  is a tip-end view of a core assembly for forming the element of  FIG. 1 .  
         [0012]      FIG. 3  is a view of a refractory metal core of the assembly of  FIG. 2 .  
         [0013]      FIG. 4  is an end view of the refractory metal core of  FIG. 3 .  
         [0014]      FIG. 5  is an inlet end view of the RMC of  FIG. 4 .  
         [0015]      FIG. 6  is an inlet end view of an alternate refractory metal core.  
         [0016]      FIG. 7  is a streamwise sectional view of a pattern-forming die. 
     
    
       [0017]     Like reference numbers and designations in the various drawings indicate like elements.  
       DETAILED DESCRIPTION  
       [0018]      FIG. 1  shows an exemplary airfoil  20  of a gas turbine engine element. An exemplary element is a blade wherein the airfoil is unitarily cast with an inboard platform and attachment root for securing the blade to a disk. Another example is a vane wherein the blade is unitarily cast with an outboard shroud and, optionally, an inboard platform. Other examples include seals, combustor panels, and the like. The exemplary airfoil  20  has a leading edge  22  and a trailing edge  24 . A generally convex suction side  26  and a generally concave pressure side  28  extend between the leading and trailing edges. In operation, an incident airflow is split into portions  500  and  502  along the suction and pressure sides (surfaces)  26  and  28 , respectively.  
         [0019]     The exemplary airfoil  20  includes an internal cooling passageway network. An exemplary network includes a plurality of spanwise extending passageway legs  30 A- 30 G from upstream to downstream. These legs carry one or more flows of cooling air (e.g., delivered through the root of a blade or the shroud of a vane). Outboard of the legs, the airfoil has suction and pressure side walls  32  and  34 . To cool the walls  32  and  34 , the passageway network includes cooling circuits  40 A- 40 E each extending from one or more of the passageway legs  30 A- 30 G to the suction or pressure sides.  
         [0020]     In the example of  FIG. 1 , there are two circuits along the suction side: an upstream circuit  40 A; and a downstream circuit  40 B. There are three circuits along the pressure side: an upstream circuit  40 C; an intermediate circuit  40 D; and a downstream circuit  40 E. Although not shown, there may be a circuit extending from the downstreammost leg  30 G to or near to the trailing edge  24 . There may also be additional circuits along a leading portion of the airfoil. Each of the circuits  40 A- 40 E has one or more inlets  42  at the associated passageway leg or legs. As is discussed in further detail below, in the exemplary airfoil, the inlets  42  of each circuit are formed as a single spanwise row of inlets. With multiple spanwise rows, however, other configurations are possible including the feeding of a given circuit from more than one of the legs. Each circuit extends to associated outlets. In the exemplary airfoil, each circuit extends to two rows of outlets  44  and  46 . As is discussed in further detail below, the exemplary outlets of each row are streamwise staggered. Between the inlets and outlets, a main portion  48  of each circuit may extend through the associated wall  32  or  34  in a convoluted fashion.  
         [0021]     In the exemplary airfoil, the circuits  40 A- 40 D are oriented as counterflow circuits (i.e., airflow through their main portions  48  is generally opposite the adjacent airflow  500  or  502 ). The exemplary circuit  40 E is positioned for parallel flow heat exchange. In the exemplary circuits, the outlets are angled slightly off-normal to the surface  26  or  28  in a direction with the associated flow  500  or  502 . For example,  FIG. 1  shows a local surface normal  504  and an axis  506  of the outlets separated by an angle θ 1 . This angle helps enhance flow through the circuit by improving entrainment of the outlet flows  508  and  510  (shown exaggerated). The angle may also help provide a film cooling effect on the surface to the extent the cool from the flows  508  and  510  air stays closer to the surface.  
         [0022]     An investment casting process is used to form the turbine element. In the investment casting process, a sacrificial material (e.g., a hydrocarbon based material such as a natural or synthetic wax) is molded over a sacrificial core assembly. The core assembly ultimately forms the passageway network. After shelling of the pattern (e.g., by a multi-stage stuccoing process) and removal of the wax (e.g., by a steam autoclave) metal is cast in the shell. Thereafter, the shell and core assembly are removed from the casting. For example, the shell may be mechanically broken away and the core assembly may be chemically leached from the casting.  
         [0023]      FIG. 2  shows an exemplary investment casting core assembly  60 . The assembly includes one or more ceramic cores, illustrated in  FIG. 2  as a single ceramic feed core  62 , and a number of refractory metal cores (RMCs)  64 A- 64 E. Exemplary RMCs are formed from molybdenum sheet stock and may have a protective coating (e.g., ceramic). Alternative RMC substrate materials include refractory metal-based alloys and intermetallics. As is discussed below, the RMCs  64 A- 64 E respectively form the circuits  40 A- 40 E in the cast part. The feed core  62  includes a proximal root  66  and a series of spanwise portions  68 A- 68 G. The spanwise portions respectively form the passageways  30 A- 30 G in the cast part.  
         [0024]     Each of the exemplary RMCs ( FIG. 3 ) includes a main body  80 . The body  80  has first and second faces  82  and  84  and may have a number of apertures  86  for forming pedestals, dividing walls, or other features in the associated circuit  40 A- 40 E. The body extends between first and second spanwise ends  88  and  90  and from an inlet end  92  to an outlet end  94 . At the inlet end, an array of tabs  96  extend from the body  80 . The tabs have proximal portions  98  bent/curved to orient the tab away from the local orientation of the body  80 . Exemplary tabs  96  have straight terminal portions  100  extending to distal ends  102 . When assembled to the feed core  60 , the distal ends  102  engage the feed core (e.g., contacting a surface of or received within a compartment of the associated spanwise portion(s)  68 A- 68 G).  
         [0025]     Similarly, at the outlet end  94 , first and second arrays of tabs  110  and  112 , respectively, extend from the body  80 . The tabs  110  and  112  have proximal portions  114  and  116 , respectively, bent/curved to orient the tab away from the local orientation of the body  80 . The exemplary tabs  110  and  112  have straight terminal portions  118  and  120 , respectively, extending to distal ends  122  and  124 . When assembled to the feed core  60 , the distal ends  122  and  124  are positioned to engage a die assembly (discussed below) for molding the pattern wax over the core assembly. In the pattern and cast part, the tabs  96  form the circuit inlets  42  and the tabs  110  and  112  form the circuit outlets  44  and  46 , respectively.  
         [0026]     As is discussed in further detail below, the terminal portions  100  of the tabs  96  have central axes  520 . The terminal portions  118  and  120  of the tabs  110  and  112  have respective central axes  522  and  524 .  FIG. 4  shows the exemplary axes  522  as parallel to each other in spanwise projection. Similarly, the exemplary axes  524  are parallel to each other in spanwise projection. In the exemplary embodiment, the axes  522  and  524  are also parallel to each other. Similarly, the exemplary axes  520  are parallel to each other. The axes may be fully parallel to each other (e.g., not merely in a spanwise projection). For example,  FIG. 5  shows the tabs  96  as parallel when viewed approximately streamwise.  FIG. 3  also shows the terminal portions  100  of the tabs  96  at an angle θ 2  to the adjacent portion of the main body  80 . The terminal portions  118  and  120  of the tabs  110  and  112  are shown at an angle θ 3  to the adjacent portion of the main body  80 . The exemplary main body  80  is curved (e.g., having appropriate streamwise convexity or concavity for the suction or pressure side, respectively, and having appropriate twist for that side). Accordingly, θ 2  and θ 3  may vary spanwise. For example, they may be well under 90° at one spanwise end, transitioning to over 90° at the other. Exemplary low values for θ 3  are less than 80°, more particularly about 30-75° or 40-70°. Exemplary larger values are the supplements (180°-x) of these.  
         [0027]      FIG. 6  shows an alternate group of tabs  140  connected by a terminal bridging portion  142  (e.g., distinguished from the free tips of other tabs). This construction may provide greater handling robustness.  
         [0028]     The parallelism of the outlet tabs (or of groups of the outlet tabs) may facilitate pattern manufacture.  FIG. 7  shows a pattern-forming die assembly  200 . The assembly  200  includes two or more die main elements  202  and  204 . The assembly  200  also includes a number of die inserts  210 A- 210 E, each carried by an associated one of the die main elements  202  or  204 . The die assembly defines an internal surface  220  forming a compartment for containing the core assembly  60  and molding the pattern wax  222  over the core assembly  60 .  
         [0029]     For ease of reference, the die main elements  202  and  204  may be respectively identified as upper and lower die elements, although no absolute orientation is required. In general, such die elements are installed to each other by a linear insertion in a direction  540  and, after molding, are separated by extraction in an opposite direction  541 . With two such main elements, this extraction is known as a single pull. However, some pattern configurations do not permit single pull molding because the shape of the molded wax may create a backlocking effect. In such a situation, there may be an additional main element.  FIG. 7  shows, in broken line, such an additional element  224  and its associated pull direction  542 .  
         [0030]     Use of the RMCs presents additional backlocking considerations. Specifically, the tabs, if not oriented parallel to the pull of the associated die main element, may cause backlocking. To decouple tab orientation from the associated die main element pull direction, the assembly  200  utilizes the inserts  210 A- 210 E. Each of the inserts  210 A- 210 E is received in an associated compartment  230 A- 230 E in the associated die main element  202  or  204 . Each insert  210 A- 210 E includes an end surface  232  which ultimately forms a part of the surface  220 . Extending inward from the surface  232  are rows of compartments  234  and  236 . The compartments  234  and  236  are positioned to receive the terminal portions of the associated outlet tabs  110  and  112 .  
         [0031]     It can be seen in  FIG. 7  that with the inserts  210 A- 210 E in place, the RMCs backlock the upper die half  202  against extraction in the direction  541 . A similar result would occur in the absence of the inserts (i.e., if the inserts were unitarily formed with their associated die halves). One alternative to prevent such backlocking would be to orient the terminal portions  118  and  120  parallel to the direction of extraction  541 . However, this orientation could either reduce flexibility in selecting the outlet orientation or impose manufacturing difficulties.  
         [0032]     Accordingly, in an exemplary method of manufacture, the RMCs may be preassembled to the feedcore. The RMCs may be positioned relative to the feedcore such as by wax pads (not shown) between the RMC main bodies and the feedcore. The RMCs may be secured to the feedcore such as by melted wax drops or a ceramic adhesive along the contact region between the RMC inlet end terminal portions  100  and the feedcore. The die main elements are initially assembled around the core assembly  60  with the inserts  210 A- 210 E fully or slightly retracted. The inserts  210 A and  210 E are, then, inserted in respective directions  550 A- 550 E. During the insertion, the terminal portions  118  and  120  of each RMC are received by the associated compartments  234  and  236  of the associated insert  210 A- 210 E. After introduction of the wax  222 , the inserts  210 A- 210 E may be fully or partially retracted in a direction  551 A- 551 E, opposite the associated direction  550 A- 550 E. The retraction may be simultaneous or staged. In one exemplary staged retraction, the inserts in one of the die halves (e.g.,  210 A and  210 B in the upper die half  202 ) are first retracted while the other inserts  210 C- 210 E remain in place. The upper die half  202  may then be disengaged from the lower die half  204  and pattern by extraction in the direction  541 . During this extraction, the backlocking of the inserts  210 C- 210 E to their associated RMCs helps maintain the pattern engaged to the lower die half. Thereafter, the inserts  210 C- 210 E may be retracted to permit removal of the pattern from the lower die half (e.g., by lifting the pattern in the direction  541 ).  
         [0033]     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, details of the particular parts being manufactured may influence details of any particular implementation. Also, if implemented by modifying existing equipment, details of the existing equipment may influence details of any particular implementation. Accordingly, other embodiments are within the scope of the following claims.