Abstract:
An investment casting pattern is formed by forming a metallic first core element including at least one recess. The first core element is engaged to at least a mating one of an element of the die and a second core element. The recess serves to retain the first core element relative to the mating one. The die is assembled and a sacrificial material is introduced to the die to at least partially embed the first core element. The recess may be pre-formed prior to cutting the first core element from a larger sheet of material.

Description:
BACKGROUND OF THE INVENTION 
   The invention relates to investment casting. More particularly, the invention relates to the forming of core-containing patterns for investment forming investment casting molds. 
   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. 
   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. 
   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. 
   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. 
   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. 
   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. 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). 
   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. 
   Nevertheless, there remains room for further improvement in core assembly techniques. 
   SUMMARY OF THE INVENTION 
   One aspect of the invention involves a method for forming an investment casting pattern. A metallic first core element is formed including at least one recess. The first core element is engaged to at least a mating one of an element of a die and a second core element (if present). The recess serves to retain the first core element relative to the mating one. The die is assembled. Sacrificial material (e.g., wax) is introduced to the die to at least partially embed the first core element. 
   Various implementations involve forming the first core element from sheet stock having opposite first and second faces. The at least one recess may include a first recess in the first face and a second aligned recess in the second face. The first and second recesses may be elongate channels. The engaging may involve translating a first portion of the first core into a slot in the mating one so that a projecting portion of the mating one within the slot is received into the at least one recess so as to provide a mechanical back-locking effect. The forming may involve forming a regular pattern of recesses including the at least one recess. The engaging may leave exposed a number of the recesses of the regular pattern. The regular pattern may be pre-formed in flat sheet stock. The metallic first core element may be cut and/or shaped from such sheet stock. 
   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 view of a refractory metal-based sheet for forming one or more investment casting cores. 
       FIG. 2  is a partial view of an alternate sheet. 
       FIG. 3  is a view of a core cut from the sheet of  FIG. 1  engaged to a pattern-forming die component. 
       FIG. 4  is an end view of a slot in the component of  FIG. 3  accommodating the RMC. 
       FIG. 5  is a view of an alternate die component accommodating the RMC. 
       FIG. 6  is a view of the RMC within a pattern-forming die. 
       FIG. 7  is a sectional view of an alternate RMC within an alternate pattern-forming die. 
       FIG. 8  is a view of the RMC held by an insert of the die of  FIG. 7 . 
   

   Like reference numbers and designations in the various drawings indicate like elements. 
   DETAILED DESCRIPTION 
     FIG. 1  shows a refractory metal-based sheet  20  for forming refractory metal cores for investment casting. Exemplary sheet materials include Mo, Nb, Ta, and W, alone or in combination and in elemental form, alloys, intermetallics, and the like. The exemplary sheet  20  is initially essentially flat having a thickness T between first and second surfaces  22  and  24 . Exemplary thicknesses T are 0.2–5.0 mm. The sheet has a width W between perimeter edge surfaces  26  and  28  and a length L between perimeter end surfaces  30  and  32 . Exemplary widths and lengths are much larger than T and may be from several centimeters upward. 
   According to one aspect of the invention, the sheet  20  may be pre-formed with surface features or other enhancements to serve one or more useful functions during the investment casting process. The exemplary sheet of  FIG. 1  has enhancements including a first regular array of channel recesses  34  in the surface  22 . The exemplary recesses  34  are linear at a constant spacing S. The exemplary recesses  34  have approximately semi-circular cross-sections. In the exemplary sheet, a similar array of similar recesses  36  is formed in the surface  24 . In the exemplary sheet, the recesses  34  and  36  are at the same spacing and are parallel to and in-phase with each other, although other configurations are possible. 
     FIG. 1  further shows additional enhancements in the form of an array of lines of through-apertures  38  extending between the surfaces  22  and  24 . The exemplary lines of through-apertures  38  are alternatingly interspersed with the recesses  34  and  36  at the spacing S. Within each line, the apertures have an on-center spacing S 2 . The exemplary through-apertures are formed with a circular cross-section of diameter D. Among various alternatives are arrays of blind recesses (e.g., dimples  40  ( FIG. 2 )). 
   The enhancements may be formed in an initial unenhanced sheet by a variety of means including one or more of embossing, engraving, etching, and drilling/milling (e.g., photo-etching, laser etching, chemical milling, and the like). Once so formed, individual RMCs might be cut from the larger sheet and optionally further shaped (e.g., via stamping, bending, or other forming/shaping technique). 
   The enhancements may serve one or more of several purposes. The enhancements may provide for registration and/or engagement/retention of the RMC with one or more of a pattern-forming mold, another core (e.g., a molded ceramic core), and an investment casting shell formed over a pattern. The enhancements may provide features of the ultimate casting. For example, through-apertures may provide posts for enhanced heat transfer and/or structural integrity. Blind recesses may provide enhanced heat transfer due to increased surface area, increased turbulence, and the like. 
     FIG. 3  shows an RMC  50  cut from the sheet  20  of  FIG. 1 . The RMC  50  has side surfaces  51  and  52  from the surfaces  22  and  24 . The RMC  50  has a lateral perimeter. A portion of the perimeter can be an intact portion of the perimeter of the sheet  20 . The RMC  50  is mounted in an element of a wax molding die (e.g., a die insert  60  described in further detail below). The insert  60  has a slot formed in a first surface  61 . The slot has a base  62  and first and second sides  64  and  66 . Along the sides, elongate ribs  68  and  70  extend into the slot. The ribs  68  and  70  are complementary to an associated pair of the recesses  34  and  36  permitting the RMC  50  to be slid into the slot so as to provide a dovetail-like engagement.  FIG. 5  shows an alternate insert  70  having a slot with a base  72  and first and second sides  74  and  76 . The slot may have features (e.g., projections  78  for contacting and positioning the received portion of the RMC  50 ). Around the projections  78 , a space between the slot and the RMC may be filled via a ceramic adhesive or other accommodating material  80  to secure the RMC to the insert.  FIG. 5  further shows a cutaway ceramic core  82  receiving a second portion of the RMC  50 . The second core  82  may be cast over the RMC  50 . Alternatively, the RMC  50  may be positioned in a pre-formed slot in the ceramic core  82  and secured thereto via ceramic adhesive  84  or other securing material. 
     FIG. 6  shows a pattern-forming die assembly  100  including mating upper and lower halves  102  and  104 . The insert  60  carrying the RMC  50  is shown accommodated in a compartment  106  of the upper die half  102 . Combined internal surfaces  108  and  110  of the upper and lower die halves along with the underside  101  of the insert form a chamber for molding the pattern wax. The sacrificial pattern wax may be introduced through one or more ports  114  in the die halves or insert  60 . The wax embeds the previously protruding portion of the RMC and any similarly exposed ceramic or other core within the die. After removal of the resultant pattern from the die, a ceramic shelling process (e.g., a slurry stuccoing process) may embed the RMC portion previously received in the slot. After dewaxing, molten metal may be introduced to the shell. After metal hardening, the RMC and any other cores may be removed from the casting (e.g., via chemical leaching). 
   Especially for smaller-scale manufacturing applications, use of the pre-enhanced RMC sheet material  20  may have substantial cost benefits in providing the aforementioned utility. 
   The dovetail RMC-to-die attachment function identified above may be reproduced in other situations. For example, rather than having a regular array of the recess pairs  34  and  36 , the sheet  20  might be provided with only a single recess pair adjacent the edge  26  or even a single recess on one side  22  or  24  in the absence of an aligned recess on the other side. The enhancements across the remainder of the sheet (if any) may be otherwise formed (e.g., arrays of the apertures and/or dimples). Individual RMCs may be cut relative to the edge  26  so that the single recess or recess pair may be used to provide the dovetail interaction with the die. In yet another example, such recesses may be post-formed. 
     FIG. 7  shows an alternate pattern-forming die  200  having upper and lower halves  202  and  204 . A die insert  206  holds an RMC  208  with a protruding portion thereof extending within a die cavity  210  for receiving the pattern wax. The insert  206  may be received in an associated compartment of one or both of the die halves or otherwise mated thereto. The exemplary RMC  208  has a single aligned pair of recesses  212  and  214  in first and second side surfaces  216  and  218  adjacent a first edge  220 . Assembly of the RMC  208  to the insert  206  may be as described above. In the exemplary embodiment, along the protruding portion of the RMC  208 , the surfaces  216  and  218  are generally arcuate with the former convex and the latter concave to fall between suction and pressure sides of an airfoil to be formed on the pattern by respective die surfaces  222  and  224 . The exemplary RMC  208  has a second (leading) edge  230  distally of the insert  206 . In the exemplary embodiment, a thickness of the RMC  208  between the surfaces  216  and  218  varies with position between the edges  230  and  220 . For example, as does the airfoil, the thickness may relatively quickly increase in the downstream direction and then relatively slowly decrease so that a thickest point is in a leading half of the RMC. The RMC  208  may be fabricated by a variety of processes. A particular overall non-constant thickness (i.e., ignoring holes, recesses, and the like) may be directly prepared (e.g., by forging, extruding, or the like) or may be indirectly prepared from a constant thickness sheet (e.g., by rolling, stamping, chemical milling or etching, photo etching, electrochemical machining, electrical discharge machining, water jet machining, and the like).  FIG. 8  shows the RMC  208  as having overlapping regular arrays of through-apertures  240  and dimples  242  (in each surface) for respectively forming posts and pedestals in a slot in the ultimate cast part. The arrays may advantageously be positioned and arranged so that the individual interspersed apertures and dimples do not overlap, although other configurations are possible. In an exemplary manufacture sequence the apertures and dimples are formed along with the recesses  212  and  214  when the thickness profile is also formed in an RMC precursor. Several such RMCs may then be cut from the precursor. 
     FIG. 7  further shows several additional exemplary sacrificial cores including metallic cores that may be similarly formed to the cores described above or may be otherwise formed. A pair of RMCs  250  have first portions held in slots in the lower die half  204  and second portions contacting and optionally supporting the second surface  218  of the RMC  208 . Another RMC  260  has a first portion captured in a slot in a molded ceramic core  262  and secured thereto by a ceramic adhesive  264 . A pair of second portions of the RMC  260  are captured in the die upper half  202 . The ceramic core  262  may be held relative to the die at an end of the ceramic core or by molded-in-place bumps or by other means. 
   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 part to be cast may influence details of any particular implementation. Furthermore, the principles may be implemented in modifying an a variety of existing or yet-developed manufacturing processes for a variety of parts. The details of such processes and parts may influence the details of any implementation. Accordingly, other embodiments are within the scope of the following claims.