Abstract:
A method of constructing a hollow fiber reinforced structure is disclosed. A spindle structure is provided for defining a preform shape. A resin binder or matrix is applied to a fiber material and the fiber material is placed around the spindle structure to create a hollow preform having a shape defined by the spindle structure. The preform structure is reshaped into a desired shape defined by the interior surface of a hollow mold and the exterior surface of an expandable insert structure. The reshaped preform structure is further conditioned to create a finished part. A hollow filament wound ceramic matrix composite airfoil suitable for use in a gas turbine engine may be constructed using the disclosed method.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a method of constructing a hollow fiber reinforced structure and, more specifically, relates to a method of forming a fiber reinforced ceramic matrix composite structure having a complex shape, such as an airfoil for use in a gas turbine engine. 
       BACKGROUND OF THE INVENTION 
       [0002]    Certain ceramics are attractive materials for use in structures that must be light weight yet maintain high strength at high operating temperatures. In particular, the airfoils of blades and vanes used in gas turbine engines must withstand exposure to hot combustion gasses under high pressure while also minimizing the airfoil weight to operating efficiencies. Moreover, such airfoils generally have complex geometric shapes, typically requiring a substantially convex suction side surface and a substantially concave pressure side surface. Monolithic ceramics, though readily formable into complex shapes, are brittle, and may not meet reliability requirements in such demanding applications without reinforcement. 
         [0003]    It is known that ceramic materials may be reinforced by introducing fibers into the material during the production process. The fibers add strength to the structure resulting in a less brittle part. Various methods are known to produce a fiber reinforced part. For example, fibers may be woven into a fabric, multiple plies of fabric may be stacked to a desired thickness and the layered fiber mat formed into a desired shape in a mold creating a preform. The ceramic material may be introduced to the fabric before or after lay-up by various methods including chemical vapor infiltration, directed metal oxidation or by sol-gel processes. Subsequently, the preform is sintered to form a shaped ceramic matrix composite structure, hereinafter referred to as a CMC structure. 
         [0004]    U. S. Pat. No. 6,660,115 B2 to Butler, et al. discloses a method of manufacturing a fiber reinforced CMC structure by vacuum impregnating a layered stack of fiber laminates with a slurry of ceramic sol, filler particles and a solvent. Because the laminates are impregnated in a vacuum, this method is suitable for forming structures having geometrically complex exterior shapes, such as an airfoil, by performing the vacuum impregnation process within a shaped hollow mold. 
         [0005]    It is also known to produce a sol-gel CMC structure by filament winding. One such filament winding method comprises passing each fiber through a solution of ceramic material, winding the impregnated fiber on a mandrel of the desired shape, converting the sol to a gel and heating to convert the gel to a ceramic matrix. 
         [0006]    Another method of constructing a filament wound CMC structure is called fiber placement. In this method, a moving head lays fabric on a complex geometry while simultaneously curing the product, allowing for stable placement. The fiber placement method is generally limited to materials and structures that may be sequentially cured, such as thermoplastics. 
         [0007]    There continues to be a need for an efficient method of forming CMC structures having complex geometries and, in particular, for a method of forming an airfoil or similar shaped part for use in a turbine engine. 
       SUMMARY OF THE INVENTION 
       [0008]    In accordance with one aspect of the invention, a method of constructing a hollow fiber reinforced structure is provided. The method comprises providing a spindle structure for defining a preform structure, providing a fiber structure comprising a fiber material and a matrix, and placing the fiber structure around the spindle structure to create a preform structure having an interior volume. The preform structure is located within a mold having an interior surface, and an insert structure is moved within the interior volume of the preform structure. The insert structure has an exterior surface and is expandable in at least one direction. The preform structure is reshaped into a predetermined shape defined by the interior surface of the mold and the exterior surface of the insert structure to create a reshaped preform structure, and the reshaped preform structure is then conditioned, creating a finished hollow fiber reinforced structure. 
         [0009]    In accordance with another aspect of the invention, a method of constructing a hollow fiber reinforced airfoil is provided, where the airfoil had an exterior surface including a substantially convex suction side surface and a substantially concave pressure side surface. The method comprises providing a spindle structure for defining a preform structure, providing a fiber structure comprising a fiber material and a matrix, and placing the fiber structure around the spindle structure to create a preform structure having an interior volume. The preform structure is located within a mold having an interior surface defining a substantially convex interior surface for defining the substantially concave pressure surface and a substantially concave interior surface for defining the substantially convex suction side surface. An insert structure is moved within the interior volume of the preform structure. The insert has an exterior surface and is expandable in at least one direction. The preform structure is reshaped into a predetermined airfoil shape defined by the interior surface of the mold and the exterior surface of the insert structure to define a reshaped preform structure, and the reshaped preform structure is then conditioned, creating a finished fiber reinforced airfoil. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0010]    While the specification concludes with claims particularly pointing out and distinctly claiming the present invention, it is believed that the present invention will be better understood from the following description in conjunction with the accompanying Drawing Figures, in which like reference numerals identify like elements, and wherein: 
           [0011]      FIG. 1  is a diagrammatic perspective view of a spindle structure illustrating first and second spools with a partial fiber structure; 
           [0012]      FIG. 2  is a perspective view of the spindle structure of  FIG. 1  illustrating a preform structure and an insert structure partially inserted within the preform structure; 
           [0013]      FIG. 3  is a plan view illustrating the hollow preform structure within a mold and showing an insert structure in a non-expanded state within the interior volume of the preform structure; 
           [0014]      FIG. 4  is a plan view similar to  FIG. 3  showing the insert structure in an expanded state; 
           [0015]      FIG. 5  is a perspective view illustrating a reshaped preform structure within the mold; and 
           [0016]      FIG. 6  is a flow diagram illustrating the steps of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]    In the following detailed description of the preferred embodiment, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration, and not by way of limitation, a specific preferred embodiment in which the invention may be practiced. It is to be understood that other embodiments may be utilized and that changes may be made without departing from the spirit and scope of the present invention. 
         [0018]    Referring initially to  FIG. 1 , the process and apparatus of the present invention is illustrated with reference to a spindle structure  10  including a first elongated spool  14 , and a substantially smaller second elongated spool  16  located in spaced relation to the first spool  14  and extending substantially parallel to the first spool  14 . A support structure for supporting the first and second spools  14 ,  16  is generally illustrated diagrammatically and is identified by reference numeral  18 . The support structure  18  may comprise a first portion  18   a  supporting the first spool  14  for rotation about a first spool axis A 1 , and may comprise a second portion  18   b  supporting the second spool  16  for rotation about a second axis A 2 . In addition, one or both of the first and second support portions  18   a ,  18   b  may be actuated to displace the first and second spools  14 ,  16  longitudinally in a direction generally perpendicular to the axes A 1 , A 2 , such as to translate the spools  14 ,  16  toward and away from each other, as indicated by arrow  19 . 
         [0019]    In accordance with the principles of the present invention, a preform structure is formed by placement of a fiber material about the spools  14 ,  16 . As seen in  FIG. 1 , a portion of a fiber structure has been formed extending around exterior surfaces  20 ,  22  of the spools  14 ,  16 , respectively, and is illustrated by placement of a fiber structure comprising a plurality of wraps of a fiber material or tow  12  comprising one or more elongated strands of material. As is known in the art, a solution, such as a resin binder or matrix, for example a ceramic matrix slurry, may be applied to the tow  12  to prepare the tow  12  for the process of wrapping about the spools  14 ,  16 , where the solution facilitates adherence of the fibers of the tow  12  to one another during the wrapping operation, and facilitating formation of the preform structure created around the spools  14 , 16 . Alternatively, the tow  12  may comprise a pre-impregnated fiber or fibers, i.e., a prepreg that comprises a fiber that is pre-impregnated with a resin or matrix in a production process separate from the presently described process for forming the tow  12  on the spindle structure  10 . 
         [0020]    Referring to  FIG. 2 , a preform structure  24  is illustrated diagrammatically as being formed around the spools  14 ,  16  by a predetermined thickness of the fabric structure defined by the wound tow  12 . Accordingly, the shape of the preform structure  24  is substantially determined by the configuration of the spindle structure  10 , and in particular by the exterior surfaces  20 ,  22  of the first and second spools  14 ,  16 , as well as the position and orientation of the spools  14 ,  16  relative to one another. 
         [0021]    For the purposes of the present description, the preform structure  24  defined by the spools  14 ,  16  comprises a preform for constructing an airfoil, such as an airfoil preform for forming a turbine vane or blade. In this configuration of the preform structure  24 , the exterior surface  20  of the first spool  14  defines an interior contour for a leading edge of the airfoil preform. Similarly, the exterior surface  22  of the second spool  16  defines an interior contour for a trailing edge of the airfoil preform. An interior volume defined by an elongated hollow space  26  extends longitudinally between the first and second spools  14 ,  16 , such that the preform structure  24  comprises a hollow structure. The hollow space  26  may form a cavity within the airfoil for conducting a cooling fluid and, in the final form of the airfoil, a structure (not shown) may be provided in the hollow space  26  to define multiple cooling passages conducting cooling fluid in a predetermined pattern for obtaining a desired cooling effect. It should be noted that the second spool  16  may be provided with a plurality of pins  28  extending rearwardly from the exterior surface  22 , as is illustrated only in  FIGS. 1 and 2 , for defining trailing edge cooling holes in the final airfoil shape for conducting a cooling fluid from the interior of the airfoil to an exterior area of the airfoil for cooling of the airfoil. That is, the pins  28  define a structure around which the tow  12  may be placed as the preform structure  24  is formed to the desired thickness. The pins  28  are optional and are disclosed solely for illustrative purposes, and it should be understood that the presence of the pins  28  is not required to achieve the object of the present invention discussed herein. 
         [0022]      FIG. 2  further illustrates movement of an insert structure  30  within the hollow space  26  defined in the preform structure  24 , and in particular illustrates movement of the insert structure  30  in an insertion direction, as depicted by arrow  32 . The insert structure  30  comprises a support member, illustrated diagrammatically at  34 , and first and second forming members  36 ,  38  mounted on the support member  34 . The support structure  34  may be provided to support the forming members  36 ,  38  for movement between a retracted position ( FIG. 3 ) and an expanded position ( FIG. 4 ) and for movement into and out of the interior area of the preform structure  24  defined by the hollow space  26 . The forming members of the insert structure  30  may initially be configured in the retracted position when the preform structure  24  is formed about the spindle structure  10 , which configuration may be provided to facilitate positioning of the insert structure  24  in association with the hollow space  26  of the preform structure  24  as it is inserted in the direction  32 . 
         [0023]    It should be noted that upon completion of placement of the fabric structure about spools  14  and  16 , the preform structure  24  is configured in a shape that conforms to the shape of the spindle structure  10 , and the ceramic matrix slurry coated or prepreg fabric structure remains in a substantially non-rigid, i.e., substantially pliable, state. For the purposes of the illustrated embodiment, it should be understood that a typical airfoil has a substantially convex suction side surface and a substantially concave pressure side surface. After wrapping on spindle structure  10 , the preform structure  24  remains non-rigid and has substantially linear sides defined by the linear distance between spools  14  and  16 , as illustrated in  FIG. 2 . A mold  40  cooperates with the insert structure  30  to reshape the preform structure  24  into a desired airfoil shape defined by the shape of interior surfaces  46 ,  48  formed in respective first and second mold halves  42 ,  44  of the mold  40  and by the outer surfaces  52 ,  54  of the forming members  36 ,  38  of insert structure  30 . This is most easily seen in  FIG. 4  where insert structure  30  is shown fully expanded to conform the preform structure  24  to the inside surfaces  46 ,  48  of the mold  40 , defining a reshaped structure  24 ′. 
         [0024]    Referring to  FIGS. 3 and 4 , substantially concurrently with the insert structure  30  moving Within the hollow space  26  of the preform structure  24 , the first mold half  42  and second mold half  44  may be actuated to move into surrounding relation around the preform structure  24 . To form the airfoil shape illustrated in the presently described embodiment, the interior surface  46  of the first mold half  42  comprises a concave surface of the final airfoil shape, and the interior surface  48  of the second mold half  44  comprises a convex surface of the final airfoil shape. The first and second mold halves  42 ,  44  may be supported by appropriate apparatus (not shown) for moving toward each other to reshape the preform structure  24 , providing surfaces  46 ,  48  substantially defining the final outer shape of an airfoil into which the exterior of the preform structure  24  may be reshaped. 
         [0025]    With the preform structure  24  surrounded by the mold  40  and the insert structure  30  located within the hollow space  26 , one or both of the forming members  36 ,  38  may be actuated, such as by an actuator illustrated diagrammatically at  49 , to pivot about a pivot location  50  located adjacent the second spool  16 . The first spool  14  includes an interior surface  56  configured to accommodate movement of the end(s) of one or both of the forming members  36 ,  38 . 
         [0026]    The preform structure  24  is engaged by the outer surfaces  52 ,  54  of the respective forming members  36 ,  38 , as the forming members  36 ,  38  move outwardly in the direction  58 , to reshape the preform structure  24  to the configuration of the reshaped structure  24 ′, as illustrated in  FIGS. 4 and 5 . The reshaped structure  24 ′ may be compressed with a predetermined pressure between the forming members  36 ,  38  and the mold halves  42 ,  44  to conform to the shape substantially defined by the interior surfaces  46 ,  48  of the mold halves  42 ,  44 . In addition, the first and second spools  14 ,  16  cooperate with the interior surfaces of the mold  40  to define the shape of leading and trailing edges of the airfoil defined by the reshaped structure  24 ′. Accordingly, the exterior surfaces  20 ,  22  of first and second spools  14 ,  16  are contoured to cooperate with the insert structure  30  such that substantially all of interior surface of the preform structure  24  is supported during the reshaping process. 
         [0027]    It should be understood that the present invention is not limited to a particular actuation mechanism for the forming members  36 ,  38  and that any actuator, including one or more actuators associated with the support structure  34  of the insert structure  30  may be provided. In addition, although the insert structure  30  is expandable in a single direction in the illustrated embodiment, other appropriate structures, including inflatable bladders or equivalent mechanisms may be provided that may be expanded in one or more directions and that perform the function of pressing the preform structure  24  outwardly into engagement with the interior surfaces  46 ,  48  of the mold  40 . 
         [0028]    It should be noted that in order to reshape preform structure  24  into an airfoil shape, it may be necessary for either or both of first and second spools  14  and  16  to rotate about axes A 1  and A 2  ( FIG. 1 ), respectively, before or during outward movement of the forming members  36 ,  38 , such as by actuation of the first and second portions  18   a ,  18   b  of the support structure  18 . Further, it may be necessary to actuate the first and second portions  18   a ,  18   b  to translate the spools  14 ,  16  in the direction  19  during or in combination with rotation of the spools  14 ,  16 . The rotation and translation of the spools  14 ,  16  may be provided to accommodate differences between the length of the sides of the preform structure  24  extending between the spools  14 ,  16  and the length of the surfaces  46 ,  48  of the mold halves  42 ,  44  as the reshaped structure  24 ′ is formed. Thus, one or both of spools  14  and  16  may be rotated about axes A 1  and A 2  as mold halves  38  and  40  are closed about spindle structure  10  with preform structure thereon. Additionally, one or both of spools  14 ,  16  may be translated toward the other spool in direction  19  to allow the preform structure  24  to conform to the interior shape of the mold halves  42  and  44 . 
         [0029]    The preform structure  24  may comprise a ceramic matrix composite material (CMC), where the ceramic matrix composite material may be any fiber reinforced ceramic matrix material or other appropriate material. The fibers and matrix material surrounding the fibers, i.e., the tow  12 , may be oxide ceramics or non-oxide ceramics, or any combination thereof. The ceramic matrix fibers may combine a matrix material with a reinforcing phase of a different composition, such as, but not limited to, mulite/silica, or of the same composition, such as, but not limited to, alumina/alumina or silicon carbide/silicon carbide. The ceramic matrix fibers may also be reinforced with plies of adjacent layers being directionally oriented to obtain the desired strength. In at least one embodiment, the preform structure may be formed from A-N720, which is available from COI Ceramics, San Diego, Calif., with mulite-alumina reinforcing fibers in an alumina matrix. The mulite-alumina reinforcing fibers may comprise materials such as are commercially available, for example, from 3M Company under the trade designations NEXTEL 720 or NEXTEL 610. 
         [0030]    Subsequent to the step of reshaping the preform structure  24  into a desired shape, i.e., the reshaped structure  24 ′, the reshaped structure  24 ′ may be subjected to a conditioning process including one or more steps to form a substantially rigid final structure. In particular, immediately after the reshaping process, the reshaped preform  24 ′ comprises a non-rigid or compliant structure and further processing is applied to rigidify the structure  24 ′ and/or to add additional material to the final structure. For example, the reshaped structure  24 ′ may initially be subjected to a drying process at a moderate temperature of approximately 150 degrees C. to 300 degrees C. while it is still within the mold  40  to get the reshaped structure  24 ′ to a green state. The drying step may be performed with or without the insert structure  30  in place. However, in the event that cooling holes are defined in the trailing edge of the preform structure  24 , the pins  28  should remain in place at least through the drying step. In a subsequent step, the green state reshaped structure  24 ′ is removed from the mold  40  and is fired in a kiln at a high temperature, such as approximately 1250 degrees C. to 1350 degrees C. to sinter the part to a high bisque state. The resulting sintered part may then be machined as necessary or other operations may be performed to finish the part. 
         [0031]    In addition to the above-noted steps, a coating or insulating layer may be provided to the sintered part to provide a hybrid ceramic structure, such as to further strengthen the part and/or to increase the temperature capability of the part. In particular, the sintered part may be provided with a coating that is cast onto appropriate places on the part within a mold. The part with the coating may then be subjected to a drying process and a firing process to produce a hybrid part at a final fired state. A description of a high temperature coating for providing an insulating hybrid oxide layer is disclosed in U.S. Pat. No. 6,733,907, which patent is incorporated herein by reference. 
         [0032]    Though the illustrated and preferred embodiment shows how the steps of the present invention may be used to create a fiber reinforced ceramic matrix composite airfoil shape, it will be apparent to those skilled in the art that other embodiments of the invention having differently shaped spools, insert structures and molds may be used to create objects having different shapes. It is further anticipated that the present invention may be used to create structures from materials other than ceramics. 
         [0033]      FIG. 6  is a flow diagram illustrating the steps of the present invention. In step  100  a fiber structure is provided and may comprise a fiber material having a matrix applied, such as a ceramic matrix slurry applied to a fiber material. As noted previously, the fiber structure may comprise a pre-impregnated material. 
         [0034]    In step  110 , a preform structure  24  is created by placing the fiber structure around a spindle structure  10 , such as by winding the fiber tow  12  around the spindle structure  10 . At the conclusion of step  110 , the completed preform structure  24  is defined by a shape established by the spindle structure  10  but remains non-rigid until further processing. 
         [0035]    In step  120 , the insert structure  30  is defined as being located within the interior volume of the hollow preform structure  24 . Although the illustrated embodiment shows the insert structure  30  being moved into the preform structure  24  after the process of placement of the material on the spindle structure  10 , it is anticipated that the insert structure  30  may either be located within the spindle structure  10  during the placement step  110  or moved therein subsequently. 
         [0036]    In step  130  the preform structure  24  is reshaped into a desired shape as defined by the spindle structure  10 , the insert structure  30  and the mold  40 , as previously described. Reshaping preform structure  24  with the mold halves  42 ,  44  and the insert structure  30  allows construction of structures having concave exterior surfaces by providing one or more convex interior surfaces on mold halves  42  or  44 . A compressive pressure may be applied to compress the preform structure  24  against the interior surfaces  46  and  48  of mold halves  42  and  44 , respectively, to complete the reshaping process and/or aid the drying process. 
         [0037]    After the reshaping step  130 , the preform structure  24  may be subjected to one or more conditioning steps  135 . The conditioning steps  135  may comprise a drying step  140 , a step  150  comprising removal of the insert structure  30  and/or the mold  40  and a step  160  of sintering the reshaped preform structure, such as by a firing process to form a final hardened part. In addition, as noted above further processing may be provided to the part, including machining and/or formation of a hybrid part through application of additional material. 
         [0038]    While particular embodiments of the present invention have been illustrated and described, it would be obvious to those skilled in the art that various other changes and modifications can be made without departing from the spirit and scope of the invention. It is therefore intended to cover in the appended claims all such changes and modifications that are within the scope of this invention.