Abstract:
An investment casting pattern is formed by installing a first core to a first element of a molding die to leave a first portion of the first core protruding from the first element. After the installing, the first element is assembled with a feed core and a second element of the molding die so that the first portion contacts the feed core. A material is molded at least partially over the first core and the feed core. The first portion has one or more surface area enhancements.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The invention relates to investment casting. More particularly, the invention relates to casting of film cooling holes in gas turbine engine components.  
         [0002]     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.  
         [0003]     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 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.  
         [0004]     A well developed field exists regarding the investment casting of internally-cooled turbine engine parts such as blades, vanes, seals, combustors, and other components. 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 a well known fashion. The wax may be removed such as by melting, e.g., 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.  
         [0005]     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 may then be 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 cores defining fine features may be difficult to manufacture and/or, once manufactured, may prove fragile.  
         [0006]     A variety of post-casting techniques were traditionally used to form the fine features. A most basic technique is conventional drilling. Laser drilling is another. Electrical discharge machining or electro-discharge machining (EDM) has also been applied. For example, in machining a row of cooling holes, it is known to use an EDM electrode of a comb-like shape with teeth having complementary shape to the holes to be formed. Various EDM techniques, electrodes, and hole shapes are shown in U.S. Pat. No. 3,604,884 of Olsson, U.S. Pat. No. 4,197,443 of Sidenstick, U.S. Pat. No. 4,819,325 of Cross et al., U.S. Pat. No. 4,922,076 of Cross et al., U.S. Pat. No. 5,382,133 of Moore et al., U.S. Pat. No. 5,605,639 of Banks et al., and U.S. Pat. No. 5,637,239 of Adamski et al. The hole shapes produced by such EDM techniques are limited by electrode insertion constraints.  
         [0007]     Commonly-assigned U.S. Pat. No. 6,637,500 of Shah et al. discloses exemplary use of a ceramic and refractory metal core combination. With such combinations, 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. As is the case with the use of multiple ceramic cores, 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. It may be difficult to assemble 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).  
         [0008]     Separately from the development of RMCs, various techniques for positioning the ceramic cores in the pattern molds and resulting shells have been developed. U.S. Pat. No. 5,296,308 of Caccavale et al. discloses use of small projections unitarily formed with the feed portions of the ceramic core to position a ceramic core in the die for overmolding the pattern wax. Such projections may then tend to maintain alignment of the core within the shell after shelling and dewaxing.  
         [0009]     Commonly assigned U.S. patent application Ser. No. 10/891,660, filed Jul. 14, 2004, and entitled “INVESTMENT CASTING” discloses use of comb-like RMCs to position a ceramic core. The RMC may have tapering tines flexed to bias the ceramic core toward the desired position. The disclosure of this &#39;660 application is incorporated by reference as if set forth at length.  
         [0010]     Nevertheless, there remains room for further improvement in core assembly techniques.  
       SUMMARY OF THE INVENTION  
       [0011]     One aspect of the invention involves a method for forming an investment casting pattern. A first core is installed to a first element of a molding die to leave a first portion of the first core protruding from the first element. After the installing, the first element is assembled with a feed core and a second element of the molding die so that the first portion contacts the feed core. A material is molded at least partially over the first core and the feed core. The first portion has one or more surface area enhancements.  
         [0012]     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  
       [0013]      FIG. 1  is a partially cutaway view of a turbine blade.  
         [0014]      FIG. 2  is a partial sectional view of an airfoil of the blade of  FIG. 1 , taken along line  2 - 2 .  
         [0015]      FIG. 3  is a partial sectional view of an airfoil of  FIG. 2 , taken along line  3 - 3 .  
         [0016]      FIG. 4  is a sectional view of a first discharge/outlet passageway of the airfoil of  FIG. 2 .  
         [0017]      FIG. 5  is a sectional view of a second discharge/outlet passageway of the airfoil of  FIG. 2 .  
         [0018]      FIG. 6  is a sectional view of a third discharge/outlet passageway of the airfoil of  FIG. 2 .  
         [0019]      FIG. 7  is a sectional view of a pattern-forming die.  
         [0020]      FIG. 8  is a view of a refractory metal core for use in the die of  FIG. 7 .  
         [0021]      FIG. 9  is a partial view of a refractory metal core tine for forming the passageway of  FIG. 4 .  
         [0022]      FIG. 10  is a partial view of a refractory metal core tine for forming the passageway of  FIG. 5 .  
         [0023]      FIG. 11  is a partial view of a refractory metal core tine for forming the passageway of  FIG. 6 .  
         [0024]      FIG. 12  is a sectional view of an alternate airfoil.  
     
    
       [0025]     Like reference numbers and designations in the various drawings indicate like elements.  
       DETAILED DESCRIPTION  
       [0026]      FIG. 1  shows an exemplary turbine element in the form of a blade  20 . The blade has an airfoil  22  extending from a root  24  at a platform  26  to a tip  28 . A blade attachment root  30  depends from the platform  26  and includes an exemplary pair of inlet ports  32  to a cooling passageway network  34  within the blade. The network  34  extends to a number of outlets located on the surface of the airfoil. Exemplary outlets include arrays of outlets  40  near an airfoil leading edge  42 . Additional outlets  44 ,  45 , and  46  are arrayed downstream toward the trailing edge  48 .  
         [0027]      FIG. 2  shows the airfoil  22  as including pressure and suction side surfaces  50  and  52 .  FIG. 2  further shows a leading leg  54  of the passageway network and a second leg  56 . In the exemplary airflow, the second leg  56  feeds cooling air to the leading leg  54  via connecting impingement passageways  58 . The leading leg  54  (an impingement cavity), in turn, feeds a number of discharge/outlet passageways  60 ,  62 ,  64 ,  66 ,  68 ,  70 , and  72 . In the exemplary airflow, there are spanwise groups of each of these discharge passageways. For ease of illustration, these discharge passageways are all shown in elevation although each has an at-most partial intersection with the cut/view plane. These discharge passageways extend to outlets on the airfoil surface (e.g., the outlets  40  for the discharge passageways  60 - 68 ). Each of the discharge passageways  60 - 68  includes an inlet  80  at the leading leg  54 . For enhanced cooling of the tip region  82 , the passageways  60 - 68  spiral ( FIG. 3 ), thereby increasing the length per passageway and decreasing the maximum spacing between passageways (e.g., relative to a similar number of similar cross-section straight passageways). Such spiraling is shown in U.S. Pat. No. 5,486,093.  
         [0028]     The exemplary passageways  60 - 68  have generally circular cross-sections provided with a longitudinally-varying surface enhancement.  FIG. 4  shows an exemplary enhancement in the form of circumscribing annular protrusions  100 . These protrusions  100  may function to disturb the laminar flow in the passage and increase the heat transfer between the airfoil and the cooling air.  FIG. 4  further shows a flow metering orifice  102  defined by a relatively large annular protrusion  104 . The orifice  102  may be sized to provide a desired flow through the associated outlet passageway (e.g., less than 50% of the cross-sectional area of adjacent portions of the passageway and, more narrowly, 10-30%). The exemplary orifice  102  is relatively upstream (i.e., near to the passageway leg  54 ).  FIG. 5  shows an alternate enhancement in the form of one or more spiral arrays of bumps  110  (e.g., hemispherical bumps) although other shapes may also be employed. Such bumps may provide enhanced heat transfer and turbulence generation.  FIG. 6  shows another alternate enhancement in the form of one or more spiral protrusions or ribs  120 . The spiral ribs are flow disturbers and also flow guides to produce spiral flow in the cooling air along the direction of the outlet passageway.  
         [0029]     The various cooling enhancement means may be used singularly or in combination. The ability to easily form these small diameter curved holes provides for added heat extraction from the airfoil wall through an increase in convective length of the outlet passageway.  
         [0030]     The outlet passageways are advantageously formed during casting of the blade. The outlet passageways may be formed over sacrificial casting cores.  FIG. 7  shows a die  200  for molding wax over an assembly of investment casting cores  202 ,  204 ,  206 ,  208 ,  210 ,  212 ,  214 , and  216  (additional cores not shown). The exemplary cores  202 - 214  are refractory metal cores (RMCs) whereas the exemplary core  216  is a molded ceramic feed core. The feed core  216  has portions for forming the legs of the passageway network  34 . The RMCs have tines  220  for forming the discharge passageways. The exemplary RMCs may include a refractory metal substrate and, optionally, a coating (e.g., ceramic). Exemplary RMC substrate materials include Mo, Nb, Ta, and W alone or in combination and in elemental form, alloy, intermetallic, and the like. The exemplary RMCs may be comb-like, having a back or spine  222  from which a row of the tines  220  extend. The spine may have spring biasing tabs as disclosed in the &#39;660 application. Other forms are possible.  
         [0031]     The exemplary spines  222  have first and second faces  224  and  225  and inboard and outboard ends  226  and  227 . The spines  222  have first and second lateral ends  228  and  229  ( FIG. 8 ). The tines  220  extend from roots  230  at the spine inboard end  227  to tips  232 .  
         [0032]      FIG. 9  shows a tine having annular recesses  236  for casting the protrusions  100  of  FIG. 4 . The tine further includes a deeper annular recess  238  for casting the metering protrusion  104  and leaving the associated metering orifice  102 .  FIG. 10  shows a tine having recesses  240  for forming the bumps  110  of  FIG. 5 .  FIG. 11  shows a tine having spiral recesses  242  for forming the protrusions  120  of  FIG. 6 .  
         [0033]     In the exemplary RMC of  FIG. 8 , a tine-to-tine pitch L 1 , may be defined as the on-center spacing/separation of adjacent tines (e.g., at their roots). The pitch may be constant or varied as may be the length and cross-sectional shape and dimensions of the tines. For example, these parameters may be varied to provide a desired cooling distribution. The array of tines has an overall length L 2 . Each spine has an overall length L 3 . These parameters may be chosen to permit a desired tooth/hole distribution in view of economy factors (e.g., it may be more economical in labor savings to have one RMC with many tines rather than a number of RMCs each with a lesser number of tines).  
         [0034]     In the exemplary RMC, proximal portions of the tines at an angle θ 1  ( FIG. 8 ) relative to an adjacent surface normal of the RMC. θ 1 , L 3 , the tine orientation, and the tine spiral characteristics need not be the same for each tine.  
         [0035]     Exemplary overall tine lengths are 0.5-13 mm, more narrowly 3.0-7.0 mm, depending essentially upon the wall thickness of the part and the overall tine angle relative to the part outer surface. Exemplary cross-sectional areas of the passageways are 0.03-0.8 mm 2 . Exemplary maximum transverse dimensions of the tines are 0.2-1.0 mm.  
         [0036]      FIG. 7  shows the RMCs positioned with their spines  222  in compartments  256  formed in the main elements  260  and  262  of the die or formed in one or more inserts or slides  264 . The tines extend so that their tips  232  contact the feed core  216 . The tines may be slightly resiliently flexed during the die assembly process to help position the feed core either during wax molding or later stages. In an exemplary implementation, the elements  260  and  262  are, respectively, pressure side and suction side elements. The compartments  256  may be shaped and dimensioned to precisely orient and position the associated spines  222 . The exemplary die elements may be formed of metal or a composite (e.g., epoxy-based). The die elements are shown assembled. The die elements may have passageways for the introduction of wax to a molding chamber surrounding the core assembly.  
         [0037]     The exemplary slide  264  is positioned in a compartment in the suction side die element  262 . The slide  264  may be retracted to release a backlocking effect between the associated core  206  and the main element  262 , allowing release of the wax pattern. The die elements may be separable by pulling in respective directions  510  and  512  after the slide  264  has been retracted in a direction  514  The directions  510 ,  512 , and  514  may correspond to an inclination of the spine(s) of the associated RMC(s). In die assembly, the spines are placed into the compartments  256  before the die elements are closed. When closed the die forms a cavity into which wax is injected to form the positive which represents the airfoil to be cast. Once the wax is solidified the die elements are separated to extract the wax pattern. The tines remain embedded in the wax. To prevent damage to the wax pattern the spine compartments  256  may be parallel to the pull plane or direction of the associated die element.  
         [0038]      FIG. 12  shows an airfoil  300  wherein the discharge/outlet passageways  302  have an upstream portion  304  of generally constant cross-section (subject to the surface area enhancements). The passageways  302  have downstream portions  306  whose cross-sections are downstream divergent. These downstream portions  306  may also have the surface area enhancements or may not. These downstream portions act as diffusers.  
         [0039]     The RMCs may be formed by any of a variety of manufacturing techniques, for example, those used to form EDM comb electrodes. For example, the substrate may be formed by milling from a refractory metal ingot or stamping and bending a refractory metal sheet, or by build up using multiple sheets. Other cutting and machining techniques include laser cutting, water jet cutting, electrochemical machining and electrical discharge machining. The tine surface enhancements may also be formed by a variety of techniques. Exemplary techniques include laser etching, grit blasting, electrical discharge machining, and photomasked chemical milling. For ease and precision, these enhancements may be formed during an intermediate stage. For example, the basic comb-like form of the RMC may be stamped. then the enhancements added to the tines, and then the tines curled to the desired spiral form.  
         [0040]     The substrate may then be coated (e.g., with a full ceramic coating or a coating limited to areas that will ultimately contact molten metal). The exemplary RMC&#39;s are intended to be illustrative of one possible general configuration. Other configurations, including simpler and more complex configurations are possible. A core precursor could be manufactured having a spine and tines and individual cores separated from the precursor, with the individual cores each having one or more of the tines. Individual cores with one to a few tines could be useful, for example, where only isolated holes or small groups thereof are desired or where it is desired that the holes be of varying shape/size, staggered out of line, of varying spacing, and the like.  
         [0041]     The foregoing teachings may be implemented in the manufacturing of pre-existing patterns (core combinations and wax shapes) or to produce novel patterns not yet designed.  
         [0042]     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 components being manufactured will influence or dictate details of any particular implementation. Thus, other core combinations may be used, including small and/or finely-featured ceramic or other cores in place of the RMCs. Dies having more than two parts may be used. Accordingly, other embodiments are within the scope of the following claims.