Abstract:
A method for producing ceramic articles having a complex geometry. Temporary tooling is provided having cavities corresponding in shape to the desired ceramic article. The cavities are filled with a ceramic slurry which is solidified by freezing or gelation of a polymer. The ceramic is treated to remove the original liquid portion of the slurry and the temporary tooling is removed. The ceramic is then sintered. The ceramic article thus obtained may be used to investment cast a metal article.

Description:
[0001]     Methods are disclosed for fabricating unitary multi-element ceramic casting cores for fabrication of hollow castings having multiple thin walls, complex internal passages and other complex geometries. The method involves the use of multi-part molded wax or polymer temporary tools which are joined together to form a complex temporary tool containing cavities. The cavities are filled with a ceramic slurry which is then solidified. After the ceramic slurry is solidified the temporary tooling is removed. In another embodiment, shells may be formed in conjunction with the ceramic cores.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     This invention relates to the fabrication of complex ceramic cores and combination complex unitary ceramic core shells for the production of complex castings. The method is particularly suited for the fabrication of certain components for gas turbine engines.  
         [0004]     2. Description of Related Art  
         [0005]     Hollow castings are widely used to produce gas turbine engine components. Gas turbine components are often cooled by flowing air through internal cavities. However, the use of cooling air, which is supplied from the compressor section of the engine, reduces operating efficiency. Consequently there is a desire to maximize the cooling effect of compressor cooling air to improve efficiency. Increasing cooling efficiency usually requires more complex internal passages. Gas turbine engine designers have devised many airfoil designs for improving cooling efficiency, however some of these designs have proven difficult to produce on a cost-efficient basis.  
         [0006]     In particular, designers have recently focused their attention on castings which have multiple thin walls, usually double walls. This configuration is shown, for example, in U.S. Pat. No. 5,720,431 which is incorporated herein by reference. The difficulty arises in fabricating the ceramic casting cores which define the interior of the casting.  
         [0007]     Conventional cores for single wall hollow castings, such as that shown schematically in  FIG. 1A , are commonly produced by injecting a heated ceramic powder/polymer (or wax) mixture into a split die set which contains a cavity whose contours are essentially those of the desired core. The injection molded core is cooled, the dies are opened and the core is removed. The core is then heated to remove the polymer binder and then heated at a higher temperature to sinter the ceramic powder particles to form a durable ceramic core.  
         [0008]     Split molds cannot be used to produce cores for double wall castings. The practice to date has been to fabricate these complex cores as multiple ceramic parts and then to cement or otherwise fasten these ceramic cores parts together to produce a unitary multi element core assembly. This approach has proven to be undesirable because the core parts are brittle and easily damaged, especially during handling.  
         [0009]     New types of ceramic slurries, and associated processes have recently been developed. These include gel casting which is shown for example in U.S. Pat. Nos. 5,824,250 and 4,894,194 and freeze casting which is described in U.S. Pat. Nos. 4,975,225, 5,811,171, 6,024,259, and 6,368,525.  
         [0010]     The gel casting system uses a ceramic slurry consisting of ceramic particles suspended in a carrier liquid comprised in part of a polymer precursor which polymerizes when heated. The ceramic slurry solidifies when the carrier polymerizes. The solidified article is treated to remove the polymer binder and then sintered.  
         [0011]     Freeze casting is a ceramic article preparation scheme in which a ceramic slurry, usually having an aqueous based carrier, and containing a variety of other additives, is frozen to solidify the ceramic slurry. Sublimation or vacuum dewatering is then used to remove what was originally water in the ceramic slurry. After the water is removed the article is sintered.  
       BRIEF SUMMARY OF THE INVENTION  
       [0012]     According to the invention multi-part temporary tooling is fabricated from wax or polymeric materials using injection molding. Each of the parts of the temporary tooling has a configuration which permits production using split molding dies. The multiple temporary tooling parts are assembled to form a temporary tooling assembly containing cavities which have the configuration of the desired multi-part unitary ceramic core.  
         [0013]     The cavities within the assembled temporary tooling are filled with a ceramic slurry which is preferably of a type which can be solidified by heating (e.g., a gel casting-type slurry), or by cooling (e.g., a freeze casting-type slurry).  
         [0014]     In the case of the slurry which is formulated to harden by gelation, the filled temporary tooling is heated to the appropriate temperature to cause the ceramic slurry to gel. The temporary tooling may be removed at this point by thermal process such as melting or combustion or by solvent dissolution, or by combinations of these methods. Next, the original liquid in the gel casting slurry may be removed by further heating to cause the liquid to evaporate or by (flash) freezing followed by liquid removal by sublimation or by an appropriate technique. The solidified ceramic material is then sintered.  
         [0015]     In the alternative embodiment of the invention, a ceramic slurry is provided which is formulated to be solidified by freezing. After the ceramic slurry contained in the temporary tooling is solidified by freezing, the temporary tooling may be removed by chemical dissolution or other suitable method. The original liquid in the frozen slurry may be removed by sublimation. If the original temporary tooling was not removed by chemical means, it may then be removed by thermal means. The ceramic material is then sintered.  
         [0016]     At the end of either of the major embodiment processes, the result is a ceramic article containing cavities which accurately reflects the original configuration of the wax or polymer temporary tooling. This core (or core/shell system) can then be used as a core in a lost wax casting process. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]      FIG. 1A  shows a section through a conventional single wall hollow airfoil;  
         [0018]      FIG. 1B  shows a section through a core used to produce the airfoil shown in  FIG. 1A ;  
         [0019]      FIG. 1C  shows a section through a core shown in  FIG. 1B  along with a surrounding shell mold;  
         [0020]      FIG. 2A  shows a cross-section through an airfoil of the type disclosed in U.S. Pat. No. 5,720,431;  
         [0021]      FIG. 2B  shows a cross-section through a core used to fabricate the airfoil shown in  FIG. 2A ;  
         [0022]      FIG. 2C  shows a section through a core as shown in  FIG. 2B  along with a surrounding integral shell mold;  
         [0023]      FIG. 3  shows a cross-section through the tooling used to produce the core whose cross-section shown in  FIG. 2B ;  
         [0024]      FIGS. 4A, 4B ,  4 C,  4 D, and  4 E show some attachment schemes which can be used to join the temporary tooling components together to form the temporary tooling shown in  FIG. 3 . 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0025]     This invention relates to the production of hollow articles having complex internal configurations and is particularly suited for fabricating cooled airfoils for use in the turbine section of gas turbine engines. Other turbine engine components such as combustor components may also be fabricated using the present invention.  
         [0026]      FIG. 1A  shows a cross section of a conventional single wall hollow airfoil in schematic form. The airfoil  10  has a leading edge  12 , a trailing edge  14 , a pressure surface  16 , and a suction surface  18 . The airfoil  10  is hollow, and has an inner surface  20 , which defines a cavity  26  and an outer surface  22 . Surfaces  22  and  24  define a wall  24 . The airfoil  10  will usually be made of a nickel or cobalt superalloy material.  FIG. 1A  shows an as-cast airfoil. Prior to being placed into service the as-cast airfoil will usually be drilled to provide cooling holes (not shown) through the wall  24  to permit a pressurized fluid to flow from the cavity  26 , through the wall  24  and then over the exterior surface  22  to protect the airfoil from excessive temperatures.  FIG. 1A  depicts a simple single wall hollow airfoil, a more advanced hollow airfoil is shown and described in U.S. Pat. No. 5,599,166 which is incorporated herein by reference.  
         [0027]     A hollow airfoil such as that shown in  FIG. 1A  will usually be formed by an investment casting process and a ceramic core will be provided to create the cavity  26 .  FIG. 1B  shows a cross sectional view of a core that would be used to produce a hollow cast airfoil such as that shown in  FIG. 1A .  FIG. 1B  shows a ceramic core  30  having an outer surface  32  which corresponds to the surface  20  shown in  FIG. 1A . A ceramic core having the shape shown in  FIG. 1B  can be fabricated by injection molding a ceramic/polymer paste into a split die assembly which separates along line X-X.  
         [0028]      FIG. 1B  shows a core  30  which might be used to form the cavity  26  of the airfoil  10  shown in  FIG. 1A .  FIG. 1C  shows how the core  30  shown in  FIG. 1B  would be used in combination with an exterior mold, shown here as shell mold  34  to provide a mold core cavity  36  which can be filled with metal to produce the hollow airfoil  10  shown in  FIG. 1A .  
         [0029]      FIG. 1C  shows the core  30 , previously described in  FIG. 1B  along with an external shell mold  34 . Together, core  30  and shell mold  34  define a generally annular space or cavity  36 . Cavity  36  has a size and configuration which are similar to the size and configuration of the airfoil  10  shown in  FIG. 1A . Airfoil  10  may be produced by pouring molten metal into cavity  36 , allowing the metal to solidify, and then removing the core  30  and the shell mold  34 .  
         [0030]     A more complex airfoil is shown in  FIG. 2A , this airfoil is generally similar to that shown in U.S. Pat. No. 5,720,43. Airfoil  40  has a leading edge  42 , a trailing edge  44 , a pressure surface  46  and a suction surface  48 . Airfoil  40  has an outer wall  50  and an inner wall  52  which are generally parallel and relatively uniformly spaced apart. Outer wall  50  is connected to inner wall  52  by multiple spacers  54 . Outer wall  50 , inner wall  52 , and spacers  54  cooperate to form a stiff structure. Outer wall  50 , inner wall  52 , and spacers  54  also cooperate to form a plurality of channels  58  which are connected to central supply cavity  56 . Central supply cavity  56  is in fluid connection with each channel  58  by means of multiple apertures  60 . Enhanced cooling is provided by flowing pressurized cooling fluid into supply cavity  56 , and then through cooling holes  60 . Air flowing through cooling holes  60  impinges on the inner surface  62  of the outer wall  50  and cools wall  50 . The cooling air then flows through multiple holes (not shown), which are drilled in the outer wall  50  to provide film cooling of the outer surface  64  of outer wall  50 . In addition, the double wall construction provides strength and stiffness to the airfoil.  
         [0031]     The fabrication of an airfoil such as that shown in  FIG. 2A  by casting requires a complex core to form the interior features of the airfoil. Such a complex core is illustrated in  FIG. 2B . Core  70  includes inner ceramic element  72  whose outer surface  74  corresponds generally to the inner surface of the supply cavity  56  in  FIG. 2A . Ceramic element  70  is connected to multiple elements  76  which correspond to supply channels  58  by elements  78  which correspond to holes  60  in  FIG. 2A .  
         [0032]      FIG. 2C  shows the core assembly  70  of  FIG. 2B  surrounded by a ceramic mold  80 , the combination of core  70  and mold  80  produce a complex cavity arrangement  81 . Cavity  81  corresponds in shape to the airfoil of  FIG. 2A .  
         [0033]     It will be appreciated that the complex ceramic core shown in  FIG. 2B  cannot be fabricated by injection molding into a split die—no die parting line can be drawn which will permit separation of the dies without damaging the injection molded component.  
         [0034]     The present invention provides a process to produce ceramic cores which in cross-section are multi-part cores, such as that shown in  FIG. 2B , through which a single parting line cannot be drawn.  
         [0035]     The invention utilizes what will be termed temporary tooling. Temporary tooling in this application will be fabricated from a wax or polymeric material such as polyethylene, polypropylene and other thermoplastics including without limitation, acetyl, nylon, polyamide, polycarbonate, polystyrene, polyester, and blends thereof. These materials are selected so that they can be easily removed. The temporary tooling is fabricated in multiple elements, each of which can be produced by injection molding into a split die. The multiple elements are then joined together and used as a mold to form the ceramic core.  
         [0036]     An advantage of the invention is that the elements which are joined are made of a polymeric material and are therefor not brittle. The polymeric elements can be manipulated and joined with little likelihood of damage. This is in contrast to prior methods in which brittle ceramic elements are assembled to form the core. In the prior method, damage to the brittle ceramic elements is quite common.  
         [0037]      FIG. 2C  shows how the complex ceramic core  70  of  FIG. 2B  can be used in combination with a surrounding ceramic mold  80  to define a complex cavity  82  whose shape corresponds to the airfoil shown in  FIG. 2A . Mold  80  may be formed by solidifying a ceramic slurry, or may be formed using conventional shell molding techniques. Mold  80  and core  70  may be formed separately or in combination.  
         [0038]      FIG. 3  illustrates an exemplary arrangement which uses multi element temporary tooling to form a core for a multi wall airfoil. The temporary tooling is made in nine elements  90 ,  91 ,  92 ,  93 ,  94 ,  95 ,  96 ,  97 ,  98  and  99 . Each of elements  90 - 99  can be formed by injection molding into a split die. The temporary airfoil tooling elements have features which permit the sections to fit together in an accurate fashion. Examples of these features are shown in  FIGS. 4A, 4B ,  4 C,  4 D, and  4 E.  
         [0039]      FIGS. 4A-4E  illustrate mechanical interlocking features which may be used to join temporary tooling elements. In  FIGS. 4A-4C , the interlocking features include a protrusion or male feature on one tooling element that fits into a mating recess or female feature in the adjoining tooling element. In  FIGS. 4D and 4E , the tooling elements are joined by an independent connecting element. The connecting elements shown in  FIGS. 4D and 4E  have male features that are received within female features disposed within the tooling elements. In alternative embodiments, the male and female features may each be disposed in the other of the tooling element and connecting element, respectively.  
         [0040]      FIG. 4A  shows how tooling elements  100  and  101  maybe joined along surfaces  102  and  103 . Tooling element  100  has an under undercut groove  104  defined by surface  105  that extends below surface  102 . Protrusion  106  extends outward from surface  103  of tooling element  101 . Protrusion  106  fits into groove  104 . Protrusion  106  is split so that, when it is forced into groove  104 , undercut groove  104  will retain protrusion  106 , thereby joining tooling elements  100  and  101  across surfaces  102  and  103 .  
         [0041]      FIG. 4B  shows a similar arrangement to that shown in  FIG. 4A , wherein tooling elements  110  and  111  are joined across surfaces  112  and  113 . Protrusion  116  is forced into an interlocking relationship with recess  114 , which is defined by surface  115 . Recess  114  may be undercut and projection  116  may be split, as shown in  FIG. 4A , or projection  116  may be solid and may be force fit into recess  114 .  
         [0042]      FIG. 4C  is similar to  FIGS. 4A and 4B  in that a protrusion  126  fits into a recess  124  to hold temporary tooling elements  121  and  122  together along surfaces  122  and  123 . Projection  126 , which is shaped like a partial sphere, extends from surface  123  of element  121 . Projection  126  is sized and shaped to fit into undercut recess  124 , which is defined by surface  125 , in surface  122  of element  120 .  
         [0043]      FIGS. 4D and 4E  illustrate the use of independent connectors to hold tooling elements together. In  FIG. 4D , tooling elements  130  and  131  are held together by shaped link  132 , which fits into passages  133  and  134  which extend into articles  130  and  131  respectively.  FIG. 4E  shows the use of dog bone shaped link  148  to join articles  140  and  141 . The shaped link  148  includes a pair of protrusions  149  and  151 , connected to one another by member  153 . Protrusions  149  and  151  of link  148  fit into recesses  145  and  147 , and member  153  fits into recesses  144  and  146  in articles  140  and  141 . Bonding aids such as heat, adhesives, ultrasonic welding, and combinations thereof may be used alone, or in conjunction with mechanical interlocking arrangements such as those discussed above.  
         [0044]     The fit between the protrusion and the recess can be an interference fit; e.g., mating features that snap together, or mating features that collectively form a slight press fit, etc. Appropriate bonding agents can be used in combination with, or in place of, the interference fit. Bonds between the mating features may also be enhanced by solvent softening and/or heating, alone or in conjunction with other attachment methods.  
         [0045]     The attachment schemes shown in  FIGS. 4A, 4B , and  4 C have been described as using undercut recesses. The undercut aspect of the recess is optional especially if bonding aids such as glue, heat or ultrasonic welding are employed.  
         [0046]     The attachment schemes shown and described above are exemplary and are not limiting.  
         [0047]     The temporary tooling is usually removed after the ceramic slurry has been solidified, and either before or after the suspension carrier is removed. The temporary tooling may be removed by any means which does not adversely affect the integrity of the solidified ceramic material. In general, two techniques will be used, thermal removal and removal by solvent extraction. Thermal removal is performed by heating the temporary tooling to a temperature at which it either melts, and can be flowed out, simply evaporates, or decomposes and/or reacts with a gaseous environment to form easily removed gaseous products. Thermal removal by decomposition may be accomplished in an oxidizing atmosphere. Solvent extraction consists of dissolving the temporary tooling in an appropriate solvent. Combinations of thermal and solvent extraction processes may also be utilized. Indirect means to heat the temporary tooling, as in microwave or radio frequency waves, may also be used.  
         [0048]     A ceramic slurry consists of fine ceramic particles, having a particle size less than about 200 microns, suspended in a liquid carrier. The carrier will generally be an aqueous based liquid and will usually contain various additives, such as ceramic sols and wetting agents, depending on the ceramic particle materials used and upon the intended subsequent processing of the ceramic slurry.  
         [0049]     After solidification and removal of the carrier material, the ceramic material will be relatively soft and porous. The soft porous ceramic material may be machined. For most applications the soft porous ceramic will be sintered to reduce porosity and increase strength and hardness. Sintering is accomplished by heating the ceramic material to a temperature at which the particles interact and further bond. The temperature and time conditions required for sintering will be determined by the ceramic composition and the particle size.  
         [0050]     Referring back to  FIG. 2A , it will be appreciated that outer wall  50  contains and supports the entire structure  40  during the solidification of the slurry within the various interior cavities. The slurry may expand or contract during solidification, expansion is particularly likely when the slurry is solidified by freezing. In some situations, the outer wall  50  may be strong enough to resist the stresses resulting from slurry solidification, but it may be desirable to provide a supporting structure exterior to wall  50 .  
         [0051]     Although this invention has been shown and described with respect to the detailed embodiments thereof, it will be understood by those skilled in the art that various changes in form and detail thereof may be made without departing from the spirit and the scope of the invention.