Abstract:
An orthogonal stitch-weave method and fiber architecture. The architecture allows near-net-shape composite preforms to be fabricated, thereby reducing costs associated with complex preform shapes and increasing desired strengths of the composite.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0001]     This invention was made with government support under Contract No. F33615-C-2893. The government may have certain rights to the invention. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates generally to composite preforms and methods for making composite preforms.  
       BACKGROUND OF THE INVENTION  
       [0003]     In a gas turbine engine, air is pressurized in a compressor and mixed with fuel in a combustor and ignited for generating hot combustion gasses which flow downstream through several turbine stages. A turbine stage includes stationary turbine vanes extending radially between inner and outer castings. The vanes are hollow with openings that mate to openings within the castings. The vanes have airfoil configurations for guiding the combustion gasses between corresponding turbine rotor blades disposed downstream of the vanes. The blades are mounted to the perimeter of a rotor disk that rotates during operation for providing power to turn the compressor or fan during operation. Since the turbine vanes are heated during operation by the hot combustion gasses, which flow thereover, cooling air bled from the compressor, is channeled inside the vanes for cooling thereof.  
         [0004]     In order to increase the efficiency and the performance of gas turbine engines so as to provide increased thrust-to-weight ratios, lower emissions and improved specific fuel consumption, engine turbines are tasked to operate at higher temperatures. As the higher temperatures reach and surpass the limits of the material comprising the components in the hot section of the engine and in particular the turbine section of the engine, new materials must be developed.  
         [0005]     As the engine operating temperatures have increased, new methods of cooling the high temperature alloys comprising the combustors and the turbine airfoils have been developed. For example, ceramic thermal barrier coatings (TBCs) have been applied to the surfaces of components in the stream of the hot effluent gases of combustion to reduce the heat transfer rate and to provide thermal protection to the underlying metal and allow the component to withstand higher temperatures. These improvements helped to reduce the peak temperatures and thermal gradients. Simultaneously, ceramic matrix composites were developed as substitutes for the high temperature alloys. The ceramic matrix composites (CMCs) in many cases provided an improved temperature and density advantage over the metals, making them the material of choice when higher operating temperatures were desired.  
         [0006]     A number of techniques have been used to manufacture turbine engine components. One such technique includes manufacturing turbine blades from CMCs. One method of manufacturing CMC components, set forth in U.S. Pat. Nos. 5,015,540; 5,330,854; and 5,336,350; incorporated herein by reference in their entirety and assigned to the assignee of the present invention, relates to the production of silicon carbide matrix composites containing fibrous material that is infiltrated with molten silicon, herein referred to as the Silcomp process. The fibers generally have diameters of about 140 micrometers or greater, which prevent intricate, complex shapes, such as turbine blade components, from being manufactured by the Silcomp process.  
         [0007]     Another technique of manufacturing CMC turbine blades is the method known as the slurry cast melt infiltration (MI) process. A technical description of such a slurry cast MI method is described in detail in U.S. Pat. No. 6,280,550 B1, which is assigned to the Assignee of the present invention and which is incorporated herein by reference. In one method of manufacturing using the slurry cast MI method, CMCs are produced by initially providing plies of balanced two-dimensional (2D) woven cloth comprising silicon carbide (SiC)-containing fibers, having two weave directions at substantially 90° angles to each other, with substantially the same number of fibers running in both directions of the weave. The use of two dimensional plies prevents intricate, complex shapes, such as turbine blade components, from being manufactured.  
         [0008]     One disadvantage of existing methods for production of CMC preforms is that the fibers that provide advantageous structural properties to a composite are not positioned and interlocked or interwoven to provide an optimal fiber architecture. Conventional methods of three-dimensional composite performing typically involve fabricating woven, or braided shapes that do not provide the desired complex preform shapes. What is needed is a method of weaving a three-dimensional composite preform that provides a tailored fiber architecture, which may be, assembled “near-net-shape”.  
       SUMMARY OF THE INVENTION  
       [0009]     Improvements in manufacturing technology and materials are the keys to increased performance and reduced costs for many articles. As an example, continuing and often interrelated improvements in processes and materials have resulted in major increases in the performance of aircraft gas turbine engines, such as the improvements of the present invention. The present invention is a novel method for manufacturing a turbine engine component made from a ceramic matrix composite (CMC) by weaving a three-dimensional composite preform that provides a tailored fiber architecture that may be assembled near-net-shape. A near-net-shape is a shape for a composite preform that, after weaving, requires little or no trimming prior to final fabrication of the composite. The present invention produces a component tailored to an intricate, complex shape having the advantage of a fine unit cell size inside the weave, wherein the resultant composite has superior mechanical properties and erosion resistance.  
         [0010]     The method of the present invention is particularly suitable for fabrication of turbine airfoil components for gas turbine engines. In particular, the method of the present invention is suitable for the fabrication of stationary turbine vanes for use in gas turbine engines. The method of the present invention is particularly suitable for composites having complex and/or intricate shapes, such as stationary turbine vanes, because the method allows fabrication of preforms of a complex and/or intricate shape that does not require trimming (i.e., near-net-shape) prior to final fabrication into the composite.  
         [0011]     The present invention provides a method of weaving a preform that includes positioning a predetermined number of warp pins on a base. The warp pins on the base are positioned to form a path substantially forming the outline of a desired preform shape. A predetermined number of dummy pins are positioned on the base, outlining the path formed by the warp pins. The dummy pin&#39;s help facilitate placement of the weft tow and stitcher tow by providing support for the tows along the path formed by the warp pins. The method further includes positioning a first predetermined number of weft tows adjacent the warp pins to form a general outline of the desired preform shape. A predetermined number of stitcher tows are positioned adjacent the warp and dummy pins. A predetermined number of stitcher tows are positioned adjacent and substantially perpendicular to the first predetermined number of weft tows. The warp pins and the dummy pins are advanced through the base to maintain a length of pin sufficient to place the weft and stitcher tows. A second predetermined number of weft tows are positioned adjacent the warp pins in the path formed by the warp pins, in a direction opposite the direction of the first predetermined number of weft tows. The second predetermined number of weft tows are laid upon the previously placed weft tows and stitcher tows. A second predetermined number of stitcher tows are positioned adjacent and substantially perpendicular to the second predetermined number of weft tows. The method is repeated in order to build a thickness of weft tows and stitcher tows. The plurality of weft tows and stitcher tows placed adjacent the warp pins and dummy pins form the general outline of the desired preform shape having a build dimension perpendicular to the weft tows and stitcher tows. When a predetermined thickness for the build dimension is reached, the warp and dummy pins are removed and the warp pins are replaced by warp tows. The resultant preform is near-net-shape having an interlocked and interwoven fiber architecture.  
         [0012]     Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]      FIG. 1  is a sectional view of a hollow turbine for a gas turbine engine vane, illustrating the geometry of the vane along a radial axis of the engine.  
         [0014]      FIG. 2  is a perspective view of the orthogonal stitch-weave pattern of the present invention.  
         [0015]      FIG. 3  is a flow chart illustrating an exemplary method in accordance with the present invention.  
         [0016]      FIG. 4  is a sectional view illustrating an initial step of creating the stitch-weave of  FIG. 2 .  
         [0017]      FIG. 5  is a sectional view illustrating the arrangement of  FIG. 4 , with a further step of creating the stitch-weave of  FIG. 2 .  
         [0018]      FIG. 6  is a sectional view illustrating the arrangement of  FIG. 5 , with a further step of creating the stitch-weave of  FIG. 2 .  
         [0019]      FIG. 7  is a sectional view illustrating the arrangement of  FIG. 6 , with a further step of creating the stitch-weave of  FIG. 2 .  
         [0020]      FIG. 8  is a variation of the stitch-weave method shown in  FIGS. 4 through 7 .  
         [0021]      FIG. 9  is a variation of the stitch-weave shown in  FIG. 8 , illustrating an exemplary pattern used to create joints in complex shapes, in accordance with the present invention.  
         [0022]      FIG. 10  is a sectional view of the hollow vane of  FIG. 1 , illustrating the stitch-weave pattern of  FIG. 2 , in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]      FIG. 1  illustrates a sectional view of vane  20 . Vane  20  is a relatively complex three dimensional shape that includes a structural stiffener  40  and cooling air channels  42 . During engine operation, cool, compressed air is bled from a compressor section of a gas turbine engine (not shown) and directed through channels  42  to cool vane  20 .  
         [0024]      FIG. 2  is a perspective representation of the stitch-weave  48  of the present invention illustrating a plurality of fiber tows  50  arranged in a preferred architecture. As will be appreciated,  FIG. 2  depicts the pattern created by the present invention. Fiber tows  50  are made up of one or a plurality of long, relatively thin smaller fibers. As shown in  FIG. 2 , fiber tows  50  include three types including: weft tows  54 , a stitcher tows  56  and warp tows  58 . The smaller fibers making up the fiber tows  50  are any fiber suitable for the construction of preforms for ceramic matrix composites. The fibers may include, but are not limited to silicon carbide (SiC). Weft tows  54  and warp tows  58  and stitcher tows  56  are in a three dimensional orthogonal geometry. In one embodiment of the invention, the weft tows  54 , the warp tows  58  and the stitcher tows  56  are oriented at 90° angles to each other. The length of the weft tows  54  and the warp tows  58  define a first and second orthogonal directions, respectively. The stitch-weave  48  is defined by a through-thickness  60  as measured along a third orthogonal direction.  
         [0025]      FIG. 3  illustrates a preferred method in accordance with the present invention, illustrating discreet method steps. In Step  100 , warp pins  72  and dummy pins  74  are placed along a base to define a desired preform shape.  FIGS. 4 through 9  illustrate the positioning of the weft tows  54 , and stitcher tows  56  as illustrated as steps  110  and  120  in  FIG. 3 . In Step  110 , weft tows  54  are positioned, or laid, in a path adjacent to warp pins  72 , thereby outlining the desired shape (see  FIG. 4 ). In Step  120 , stitcher tows  56  are positioned, or laid, adjacent to warp pins  72  and dummy pins  74  at an angle substantially perpendicular to the positioned weft tows  54  (see  FIG. 5 ). In Step  130 , a determination is made as to whether the desired build height, as measured along the axis perpendicular to both the weft tows and the stitcher tows  56 , has been achieved. If the build height determination of Step  130  is negative, the process returns to step  110  and the steps are repeated with the positioning of the weft tows  54  and the laying of the stitcher tows  56 . If the build height determination of Step  130  is positive, the process proceeds to step  140 . In Step  140 , the warp pins  72  and dummy pins  74  are removed and warp pins  72  are replaced by warp tows  58 , thereby completing the fabrication of stitch-weave  48 .  
         [0026]      FIGS. 4 through 7  illustrate the positioning of the weft tows  54  and the stitcher tows  56  as the build height is created.  FIGS. 8 and 9  illustrate alternate embodiment for the placement of the pins and the positioning of the weft tows  54  and stitcher tows  56 . The pins shown in  FIGS. 4 through 9  comprise warp pins  72  and dummy pins  74 . Warp pins  72  are replaced by warp tows  58  when the predetermined build height is achieved. Dummy pins  74  facilitate weft tow  54  and stitcher tow  56  placement and are not replaced by the warp tows  58 .  
         [0027]      FIG. 4  illustrates the “Position Weft Tows in Desired Paths” Step  110  of  FIG. 3 , wherein warp pins  72  and dummy pins  74  are positioned and a weft tow  54  is positioned, or laid, adjacent to the warp pins  72  and dummy pins  74  in a desired path.  
         [0028]      FIG. 5  illustrates the “Lay Individual Stitcher Tows in Desired Paths” Step  120  of  FIG. 3 , wherein the stitcher tows  56  are positioned, or laid, at an angle perpendicular to the weft tows  54  and warp pins  72  and dummy pins  74 .  
         [0029]      FIG. 6  illustrates the continuation of Step  110 , immediately after Step  130 , wherein additional weft tows  54  are positioned, or laid, adjacent to the warp pins  72  in the desired path. The weft tows  54  and stitcher tows  56  build on top of each other and form a build height in a third dimension perpendicular to both the weft tows  54  and stitcher tows  56 .  
         [0030]      FIG. 7  illustrates a continuation of Step  120 , after a performance of a previous Step  120 , wherein the stitcher tows  56  are positioned, or laid, adjacent to warp pins  72  and dummy pins  74  in about a 180° orientation (i.e. the opposite direction) from the previous Step  120 . The orientations for the repeated step  120  are rotated 180° for each iteration of the method shown in  FIG. 3 . In this manner, the resultant stitch-weave  48  illustrated in  FIG. 7  is built up in the direction perpendicular to the weft tows  54  and stitcher tows  56  and is fabricated for a three dimensional shape. In order to complete the weave-stitch process of the present invention, once the predetermined build-height is achieved, the dummy pins  74  and warp pins  72  are removed and warp pins  72  are replaced with warp tows  58 . The replacement of the warp pins  72  with the warp tows  58  reinforces of the stitch-weave  48  in the direction perpendicular to both the weft tows  54  and the stitcher tows  56 .  
         [0031]      FIG. 8  illustrates a variation of the stitch-weave  48  of  FIGS. 4-7 , wherein the dummy pins  74  and warp pins  72  are positioned in an alternate shape. Specifically, a plurality of pins  70  are positioned adjacent to one another in a portion of the preform shape. The stitch-weave  48  illustrated in  FIG. 8  is performed in the same manner as described in  FIG. 3  to fabricate a stitch-weave  48 . In this manner, stitch weave  48  will have a greater number of warp tows  58 , thereby increasing the strength of the resulting composite along the axis of the warp tows  58 . The strength of a composite preform is tailored by the ratio of warp tows  58  to stitcher tows  56 , and by other desired tow ratios.  
         [0032]      FIG. 9  illustrates a stitch weave  48  wherein a joint is created in accordance with the method of  FIG. 4 . As described herein, weft tows  54  are laid, stitcher tows  56  are laid, an additional weft tows  54  is laid, and then the same stitcher tows  56  are laid adjacent pins  70 , thereby permitting stitcher tows  56  to stitch together the resulting preform, providing a desired shape.  
         [0033]      FIG. 10  illustrates the fabrication of a preform architecture of the vane of  FIG. 2  in accordance with the method of  FIG. 3 . In  FIG. 10 , dummy pins  74  are omitted for clarity, and warp pins  72  have been replaced with warp tows  58 . As will be appreciated, many differing complex shapes can be fabricated by the teachings of the present invention. It should be noted that the positioning of weft tows  54  may include multiple weft tows  54  at differing locations within stitch-weave  48  in a single step. Thus provided, vane  20  can be constructed of a ceramic matrix composite wherein the preform is initially stitched and woven into an optimized architecture that may be tailored to provide desired properties.  
         [0034]     In another embodiment of the invention, the positioning of the stitcher tows  56  includes a step wherein the stitcher tows  56  are positioned, or laid, adjacent to warp pins  72  and dummy pins  74  at an angle substantially perpendicular to the positioned weft tows  54 , at locations along the desired preform shape in order to provide the finished ceramic matrix composite with additional transverse fiber reinforcement.  
         [0035]     In another embodiment of the invention, after the preform of near-net-shape has been woven, the preform may be processed according to any known ceramic matrix composite processing method known for ceramic cloths, including woven ceramic cloths. Examples of suitable processes for forming the final ceramic matrix composite include, but are not limited to chemical vapor infiltration, wherein a preform of woven ceramic preform is set inside a chemical vapor reactor, the preform is heated, and atoms or molecules in the form of vapor are deposited on and, preferably, within the woven ceramic preform; lanxide process, the ceramic preform is dipped into a liquid metal bath and the metal is converted to a ceramic material via oxidation; melt infiltration, wherein the woven preform is infiltrated with a carbon containing slurry, which is converted to ceramic through infiltration with molten silicon; and polymer impregnation and pyrolysis, wherein the perform is impregnated with a polymeric precursor of the matrix which is converted to ceramic through pyrolysis of the polymer at elevated temperature. Each of these process methods generally includes the step of coating the preform with a thin layer of ceramic interface (BN, BN/Si 3 N 4 ) prior to matrix processing. While the present invention has been described with respect to CMC processing, the weaving method of the present invention may also be used in preparation of any composite preform, including but not limited to, carbon-epoxy composites and carbon-carbon composites.  
         [0036]     In a preferred embodiment of the invention, after the preform of near-net-shape has been woven, the preform may be processed into a CMC by slurry cast melt infiltration. Slurry cast melt infiltration includes slurry casting a matrix, such as SiC, into the porous preform. The slurry cast preform is then subject to densification through melt infiltration with a suitable infiltrate, such as silicon.  
         [0037]     The fibers useful as tows in the present invention include, but are not limited to SiC. Alternate suitable fibers include carbon fibers; oxide fibers, such as alumina (i.e. α-Al 2 O 3 ) and alumina-silica fibers; coated fibers, such as boron nitride and carbon coated fibers; and combinations thereof.  
         [0038]     A suitable matrix for the processed preform of near-net shape CMC includes, but is not limited to silicon carbide. Alternate matrices include alumina, silica, alumina-silicate, glass, mullite, carbon, oxides of such elements as Al, Si, Hf, Y and Zr, and their mixtures and combinations thereof.  
         [0039]     While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.