Patent Publication Number: US-7216689-B2

Title: Investment casting

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 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. 
   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 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. 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). Accordingly, 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 first material is molded at least partially over a first core. A second material is molded at least partially over the first material. 
   In various implementations, the second material may be molded at least partially over a second core. After the first molding in a first die, the first core and first material may be assembled to the second core. The assembly may be introduced to a second die in which the second molding occurs. The first core may comprise, in major weight part, one or more refractory metals. The second core may comprise, in major weight part, one or more ceramic materials. The first molding may include positioning the first core in a first die at least in part by contacting a surface of the first die with one or more portions of the first core, said one or more portions becoming essentially flush with a surface of the first material. The first molding may include positioning the first core in a first die at least in part by positioning one or more portions of the first core in a subcompartment of a first die so that the one or more portions project from a surface of the first material after the first molding. The first molding may includes positioning the first core in a first die at least in part by placing a pre formed piece of sacrificial material between a surface of the first die a surface of the first core. 
   There may be a third molding of a third material at least partially over an alternate second core and the second molding may be at least partially over the third material. The first material and first core and the third material and alternate second core may be assembled to a third core before the second molding. The first and alternate second cores may comprise, in major part, one or more refractory metals. The third core may comprise, in major part, one or more ceramic materials. The second molding may comprises positioning the third core in a die at least in part by contacting the die with a projection unitarily formed with a remainder of the third core. The first and second materials may comprise, in major part, one or more waxes. The first and second materials may essentially be of similar composition. The first molding may be performed in a first die. The first molding may provide the first material with means for guiding insertion of the first material and first core into a second die. 
   Another aspect of the invention involves a method for forming an investment casting mold. An investment casting pattern is formed as above. One or more coating layers are applied to the pattern. The first material and the second material are substantially removed to leave the first core within a shell formed by the coating layers. In various implementations, the method may be used to fabricate a gas turbine engine airfoil element mold. 
   Another aspect of the invention involves a method for investment casting. An investment casting mold is formed as above. Molten metal is introduced to the investment casting mold. The molten metal is permitted to solidify. The investment casting mold is destructively removed. The method may be used to fabricate a gas turbine engine component. 
   Another aspect of the invention involves a component for forming an investment casting pattern. A first wax material at least partially encases a first core. The first wax material includes means for guiding insertion of the first wax material and the first core into a pattern-forming die. The first wax material may include means for maintaining a target relative position between the first core and a second core. 
   Another aspect of the invention involves a die for forming an investment casting pattern. The die includes at least one means for registering at least one core to which molding material has been pre-applied. One or more surfaces define a molding material-receiving space. A passageway is provided for introducing molding material to the molding material-receiving space. 
   In various implementations, the at least one means may further serve as means for guiding insertion of the at least one core to the die. The at least one means may include first means for registering a first such core and second means for registering a second such core. The first and second means may be formed on a single section of the die. The first and second means may be formed on respective first and second sections of the die. 
   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 core. 
       FIG. 2  is a sectional view of a die for pre-applying wax to the core of  FIG. 1 . 
       FIG. 3  is a sectional view of the die of  FIG. 2  with an alternate refractory metal core. 
       FIG. 4  is a sectional view of a core with pre-applied wax. 
       FIG. 5  is a sectional view of a die for overmolding a core assembly including cores with pre-applied wax. 
       FIG. 6  is a sectional view of an airfoil of a pattern precursor molded in the die of  FIG. 5 . 
       FIG. 7  is a sectional view of a shelled pattern from the precursor of  FIG. 6 . 
   

   Like reference numbers and designations in the various drawings indicate like elements. 
   DETAILED DESCRIPTION 
     FIG. 1  shows an exemplary refractory metal core (RMC)  20  which may be formed by stamping and bending a refractory metal sheet and then coating the stamped/bent sheet with a full ceramic coating. The exemplary RMC  20  is intended to be illustrative of one possible general configuration. Other configurations, including simpler and more complex configurations are possible. The exemplary RMC  20  has first and second principal side surfaces or faces  22  and  24  formed from faces of the original sheetstock. After the exemplary stamping/bending process, the RMC extends between first and second ends  26  and  28  and has first and second lateral edges  30  and  32  therebetween. First and second bends  34  and  36  divide first and second end sections  38  and  40  from a central body section  42 . In the exemplary implementation, the end sections and central body sections are generally flat with the end sections at an approximate right angle to the body section. 
   The exemplary stamping process removes material to define a series of voids  44  separating a series of fine features  46 . The fine features  46  will form internal passageways in the ultimate cast part. In the exemplary embodiment, the fine features  46  are formed as an array of narrow strips extending along the entirety of the body section  42  and adjacent portions of the end sections  38  and  40 . Such strips may form a series of narrow parallel passageways through the wall of a cast airfoil. Intact distal portions  50  and  52  of the end sections  38  and  40  connect the strips to maintain their relative alignment. Additionally, the strips may be connected at one or more intervening locations by connecting portions (not shown) for further structural integrity or to enhance fluid (e.g., cooling air) flow through the ultimate passageways. In an exemplary casting process, the RMC is positioned with portion  50  embedded in a slot or other mating feature of a ceramic core and portion  52  protruding entirely out of the wax of the investment casting pattern. The portion  52  may thus be embedded in a shell formed over the pattern. When the wax is removed and metal cast in the shell, and the ceramic core(s) and refractory metal core(s) are removed, the strips  46  will form passageways through a wall of the casting from an internal passageway previously defined by the ceramic core to an exterior surface previously defined by the shell. 
     FIG. 2  shows the core  20  positioned within a wax pre-molding die  60  having first and second halves  62  and  64 . The exemplary die halves are formed of metal or of a composite (e.g., epoxy-based). The exemplary die halves are shown assembled, meeting along a parting junction  500 . Initially, with the die halves separate, the RMC  20  may be pre-positioned relative to one of the halves. For example, the portion  50  may be positioned in a slot  66  in the first half  62 . If the RMC is sufficiently rigid, this interaction alone may hold the RMC in a desired alignment. Alternatively, the RMC may be further supported directly by the die half  62  or by one or more wax pads  70  pre-positioned in the die half  62  or pre-secured to the RMC. In the exemplary implementation, a pad  70  holds the body section  42  in a predetermined alignment and spacing from adjacent surface portions of the die halves. The assembled dies define a void  72  for injection (through die passageways  74 ) with wax to pre-mold over the RMC. The second die half has a surface  80  along the parting junction  500  at least partially shaped to correspond to the shape of a ceramic core to which the RMC  20  is to be assembled. Locally, this surface is spaced apart from the body  20  by the desired spacing between the ceramic core and RMC body. The first die half  62  has a surface  82  forming an exterior lateral perimeter of the void. The first die half  62  further includes a surface  84  in which the slot  66  is located and which is positioned relative to the body  20  so that the wax therebetween (e.g., the pad  70  or other injected wax) corresponds to the desired wall shape and thickness of the part. The surface  82  has a depth beyond the surface  84  and is joined thereto by an interior lateral perimeter surface  86 . The surfaces  82  and  86  are angled to permit release of the overmolded wax from the first die half  62  after such wax is injected into the void and solidified.  FIG. 2  further shows a pull or joining/parting axis  502 . It is along this axis that the die halves are translated together and apart respectively before and after the injection of wax. In the exemplary embodiment, the RMC with the pre-molded wax may be extracted from the first die half  62  along this same axis. In alternative embodiments, this extraction may be off-parallel to the pull axis  502 . The angling of the surfaces  82  and  86  relative to this extraction direction are chosen to prevent backlocking of the injected part. As is discussed in further detail below, the angling of the surface  82  is advantageous to facilitate a second wax application stage. 
   As an alternative to use of the pad  70 , or in addition thereto, the RMC may include one or more support projections  88  and  89  ( FIG. 3 ). These may be tab-like projections tangs with distal portions bent away from adjacent material of the RMC or may take other forms. After wax molding, the tips of the projections may be essentially flush to the surface of the molded wax (i.e., not projecting/protruding and not subflush). After ultimate casting, the projections may leave small holes either to the part exterior surface or interior surface, depending upon their location in view of the particular die orientation. Many configurations are possible. In the orientation of  FIG. 3 , the one or more depending projections  88  help support the RMC. One or more at least partially oppositely directed upwardly extending projections  89  may serve to further retain the RMC (e.g., against movement due to die vibration or die orientation changes). 
     FIG. 4  shows the pre-molded RMC  90  including the RMC  20  and the pre-molding wax body  92  alter release from the die  60 . The pre-molding wax has a first surface  94  generally formed by the surface  80  of the second die  64  and from which the end portion  52  protrudes. Opposite the surface  94 , the wax body  92  has a central surface  96  associated with the surface  84  of the first die  62  and from which the first end portion  50  protrudes. The surface  96  is surrounded by a wall portion  98  protrading therebeyond and having an inner perimeter surface  100  molded byte surface  86  of the first die  62  and an outer perimeter surface  102  molded byte surface  82  of the first die  62 . 
     FIG. 5  shows three pre-molded cores  90 A,  90 B, and  90 C secured to a ceramic core  110  within a pattern die  112  in which the second wax application stage occurs. The second stage may be a main stage in which the additional wax molded over the ceramic core and pre-molded cores constitutes a majority of the total wax of the ultimate pattern. Alternatively, the additional wax may at least be of greater amount (e.g., volume) than the wax of any of the individual pre-molds. Yet alternatively, and largely influenced by the arrangement of the cores, the additional wax may be a lesser amount. 
   The exemplary ceramic core  110  is shown configured to form an airfoil element (e.g., a blade or vane of a gas turbine engine turbine section) and has leading, intermediate, and trailing sections  114 A,  114 B, and  114 C for forming corresponding main passageways and connected by a series of webs  116  for core structural integrity. In the exemplary embodiment, the first pre-molded core  90 A is mounted to a pressure side surface of the intermediate core section  114 B; the second pre-molded core  90 B is mounted to a suction side surface thereof; and the third pre-molded core  90 C is mounted to a suction side surface of the trailing core section  114 C. The distal portions  50  of the pre-molded RMCs  90 A,  90 B, and  90 C are accommodated within slots  118 ,  119 , and  120  in the associated surface of the associated ceramic core sections. These distal portions  50  may be secured in place via ceramic adhesive in the slots. Additionally, or alternatively, the surfaces  94  of the first and second pre-molded RMCs may be wax welded or otherwise adhered to the adjacent ceramic core surface. Various additional RMCs (not shown) may be secured to the ceramic core in a similar fashion or otherwise. The core assembly may then be placed in one of the die halves (e.g., a first half  122 ), with the protruding portions of the wall  98  of the second and third pre-molded cores  90 B and  90 C and their second distal portions  52  accommodated within compartments  124  and  125 . Interaction of the surfaces  102  of such pre-molded cores with the surfaces  126  and  127  of the compartments may help guide insertion of the core assembly into the die half  122  and locate and register the core assembly once inserted. Insertion may be along an axis  506 . Alternatively or additionally, the core assembly may be registered by direct contact between the ceramic core and the die half (e.g., at ends (not shown) of the ceramic core which ends ultimately protrude from the pattern and do not form internal features of the cast part). Similarly, the ceramic core may have additional positioning or retention features such as projections  128  unitarily or otherwise integrally formed with the feed portions of the ceramic core. Possible such projections are shown in U.S. Pat. No. 5,296,308 of Caccavale et al. 
   The die upper half  130  may then be mated with the lower half  122 , with the first pre-molded core  90 A being accommodated within a compartment  132  in similar fashion to the accommodation of the second and third pre-molded cores  90 B and  90 C. Mating of the die halves (and their ultimate separation) may also be along the axis  506  or may be along an axis at an angle thereto. In the assembled view of  FIG. 5  it can be seen how the angling of the perimeter surfaces of the pre-molded RMCs may facilitate joining and parting of the die halves  122  and  130  without destroying the pre-molded RMCs. The angling is sufficient to prevent backlocking when the die halves are separated and when the pattern is extracted. In the illustrated embodiment, it can be seen how the end portions  52  can extend at an angle to the axis  506 . This is permitted because the walls  98  or other surrounding pre-molding structure preclude the need for the die halves to closely accommodate the portions  52 . If the die halves closely accommodated the portions  52 , the portions  52  would have to be oriented parallel to the axis  506  to permit assembly/disassembly of the die halves and/or installation or removal of the pattern. In alternative embodiments, one or more of the pre-molded cores may be assembled first to an associated mold half and then to the ceramic core as the ceramic core is put in place or as the die halves are joined. In yet alternative embodiments, the compartment for a pre-molded RMC may span two die halves. 
   After injection of the additional (main) wax into the space  140  surrounding the core assembly (through injection passageways  141  in the die halves) and solidification of such wax, the die halves are parted and the molded core assembly removed. Removal may be via an extraction along the axis  506  or potentially along an alternate axis at an angle thereto.  FIG. 6  shows the molded core assembly after removal, with tip portions  142  of the walls  98  protruding from pressure and suction side surfaces  144  and  146  of the pattern airfoil contour. These protruding portions may be cut off or otherwise removed leaving a smooth pattern surface contour from which the RMC second distal portions  52  protrude. By forming the walls  98  as structure surrounding the distal portion  52  but with protruding portions spaced apart therefrom and leaving a surrounding volume (e.g., as opposed to embedding the end  52  in a plateau) only a relatively small amount of material needs to be removed and can be removed easily without producing unacceptable irregularities in the surface contour of the resulting pattern. The wall also helps keep the distal portion clean for good subsequent adhesion to the shell. As more material is required to be removed, it becomes more difficult to remove such material while preserving a desired contour. After such removal, the pattern may be assembled to a shelling fixture (e.g., via wax welding between upper and lower end plates of the fixture) and a multilayer coating  150  ( FIG. 7 ) applied for forming a shell. After the coating dries, a dewax process (e.g., in a steam autoclave) may remove the wax from the pattern (e.g., both the pre-molding wax and the main molding wax) leaving the RMCs and ceramic core within the shell. This core and shell assembly may be fired to harden the shell. Molten metal may then be introduced to the shell to fill the spaces between the core assembly and the shell. After solidification, the shell may be destructively removed (e.g., broken away via an impact apparatus) and the core assembly destructively removed (e.g., via a chemical immersion apparatus) from the cast metal to form a part precursor. Thereafter, the precursor may be subject to machining, treatment (e.g., thermal, mechanical, or chemical), and coating (e.g., ceramic heat resistant coating) to form the ultimate component. 
   The foregoing teachings may be implemented in the manufacturing of pre-existing patterns (core combinations and wax shapes) or in to produce yet novel patterns. Whereas an existing single-stage molding process, may be relatively complex (e.g., having a large number of separate die parts and separate pull directions to accommodate the various RMCs), the main stage of a revised process may be simplified (e.g., having fewer die parts and fewer single pulls, with as few as two and one, respectively). This may simplify engineering and/or manufacturing. 
   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 at either the pre-molding or the second molding stage. However, one potential advantage of the invention is in limiting the required die complexity for forming a given pattern. Accordingly, other embodiments are within the scope of the following claims.