Abstract:
A flexible tool that fits over the pressure or concave side of a metallic airfoil that includes a lightweight material component for a gas turbine engine during the fabrication of the airfoil. The metallic fan airfoil includes pockets or cavities that have been machined into the airfoil in order to reduce the weight of the airfoil. The tool is a flexible body manufactured from sheets of composite material and includes an integral elastomeric seal. The tool is placed over the airfoil and forms a seal against the pressure side of the airfoil so that lightweight resin can be injected into the pockets and retained in position during curing. The flexible tool is formed by laying up thin sheets of composite material that includes fiber over a metallic master tool after partially cured elastomeric material is placed inside a marking corresponding to the airfoil perimeter. The metallic master tool also includes slot positions corresponding to preselected positions, that form integral projections which allow the flexible tool to be correctly assembled and sealed against the airfoil.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to tooling used to manufacture lightweight fan blades used in gas turbine engines. 
     BACKGROUND OF THE INVENTION 
     Gas turbines include, but are not limited to, gas turbine power generation equipment and gas turbine aircraft engines. A gas turbine includes a core engine having a high pressure compressor to compress the air flow entering the core engine, a combustor in which a mixture of fuel and the compressed air is burned to generate a propulsive gas flow, and a high pressure turbine which is rotated by the propulsive gas flow and which is connected by a larger diameter shaft to drive the high pressure compressor. A typical front fan gas turbine aircraft engine adds a low pressure turbine (located aft of the high pressure turbine) which is connected by a smaller diameter coaxial shaft to drive a front fan (located forward of the high pressure compressor) and to drive an optional low pressure compressor (located between the front fan and the high pressure compressor). The low pressure compressor sometimes is called a booster compressor or simply a booster. 
     The fan and the high and low pressure compressors and turbines have airfoils each including an airfoil portion attached to a shank portion. Rotor blades are those airfoils which are attached to a rotating gas turbine rotor disc. Stator vanes are stationary airfoils which are attached to a non-rotating gas turbine stator casing. Typically, there are alternating circumferential rows of radially-outwardly extending rotor blades and radially-inwardly extending stator vanes. When present, a first and/or last row of stator vanes (also called inlet and outlet guide vanes) may have their radially-inward ends also attached to a non-rotating gas turbine stator casing. Counter-rotating “stator” vanes are also known. 
     Conventional airfoil designs used in the compressor section at the engine typically have airfoil portions that are made entirely of metal, such as titanium, or are made entirely of a composite. A “composite” is defined to be a material having any (metal or non-metal) fiber filament embedded in any (metal or non-metal) matrix binder, but the term “composite” does not include a metal fiber embedded in a metal matrix. The term “metal” includes alloys such as titanium Alloy 6-2-4-2. An example of a composite is a material having graphite filaments embedded in an epoxy resin. 
     The all-metal blades, including costly wide-chord hollow blades, are heavier in weight which results in lower fuel performance and require sturdier blade attachments, while the lighter all-composite blades are more susceptible to damage from bird ingestion events. Known hybrid blades include a composite blade having an airfoil shape which is covered by a surface cladding (with only the blade tip and the leading and trailing edge portions of the surface cladding comprising a metal) for erosion and foreign object impacts. The fan blades typically are the largest (and therefore the heaviest) blades in a gas turbine aircraft engine, and the front fan blades are usually the first to be impacted by foreign objects such as birds. 
     Recent improvements have resulted in lighter-weight gas turbine blades, and especially a gas turbine aircraft engine fan blade that is comprised of a combination of metal and lightweight materials. These blades have been made lighter by removing metal from the pressure side of the blade. In order to retain the aerodynamic characteristics of the blade, the removed metal is replaced by the lightweight material. Restoring the aerodynamic characteristics to the blade by adding the lightweight material to replace the removed metal involves the use of specialized tooling. However, the specialized tooling that includes a special caul sheet currently used in the process of adding lightweight material to the pressure side of the fan blade in order to restore aerodynamic characteristics requires that an effective seal be formed against the blade pressure side by the caul sheet. The caul sheet currently used in the process of adding the lightweight material relies on an O-ring to form the seal with the pressure side of the blade. However, the O-ring can cause the caul sheet to stand off from the pressure side of the blade. The result is that there is a lack of good contact between the caul sheet and the pressure surface, and a step is formed in the molded surface of the lightweight material that can rise above the pressure surface up to the diameter of the O-ring. This step is undesirable, as it adversely affects the aerodynamics of the pressure side of the blade. It is time consuming to and very difficult to remove this step from the lightweight material, as the material is also very tough. What is needed is better method using improved tooling for adding lightweight material to a blade. 
     SUMMARY OF THE INVENTION 
     A flexible tool is formed to fit over the pressure or concave side of a metallic airfoil that includes a lightweight material component for a gas turbine engine during fabrication of the airfoil. Typically the airfoil is a metallic fan blade. The metallic fan blade includes pockets or cavities that have been machined into the blade in order to reduce the weight of the blade. The tool is a flexible body manufactured from sheets of composite material and includes an integral elastomeric seal. 
     The flexible tool is formed by laying up thin sheets of composite material that includes fiber over a metallic master tool. As used herein, composite material is material formed from sheets of plastic resin matrix material having a fiber reinforcement, in which the fiber reinforcement may be unidirectional or bidirectional (woven). This material is sometimes referred to as prepreg. The metallic master tool has a profile that matches the profile of the pressure side of the fan blade, but includes a plurality of slots that are located at positions that correspond to locations along the perimeter of the fan blade, that is, positions just beyond the leading edge, trailing edge or tip end. As used herein, matching the profile of the pressure side of the fan blade means that the metallic tool has a surface that substantially corresponds to the contours, dimensions and curvatures as the pressure side of a corresponding metallic fan blade that is manufactured without cavities. The slot positions correspond to preselected positions, which allow the flexible caul sheet and seal to be correctly assembled to the blade. The slot depth may vary, but need only be sufficiently deep to allow the layers of composite material to be laid into them, thus forming lugs that positively locate the flexible caul sheet when it is placed on to the concave (pressure) side of the blade. 
     The elastomeric material is partially cured and is placed along the tool within an area inside the outline of the blade, which is permanently marked on the tool, such as by scribing the tool surface. Thus, the elastomeric material is placed on the tool inside of markings that correspond to the perimeter of the blade. The sheets of composite material are laid up to achieve a predetermined thickness over the elastomeric material, over the tool surface in the region outlining the blade and into the slots on the tooling surface. The predetermined thickness provides a predetermined stiffness so that the flexible tool will not deform when lightweight a material is injected under pressure into the pockets of the blade beneath the tool. The tool also includes at least one injection port corresponding to a pocket or cavity so that the lightweight material can be injected through the tool into the blade pockets. Additional ports, each corresponding to a pocket, may be added as required. A surround frame for added local stiffness is assembled from sheets of composite material and is separated from the flexible tool using a TEFLON® (polytetrafluoroethylene—PTFE) film. The surround frame extends around the perimeter of the blade outline on the tooling surface so that it overlies the partially cured elastomer and the sheets of composite material. 
     The metallic master tool with the partially cured elastomer, the laid up composite sheets and the surround frame secured thereto is then placed in an elevated temperature atmosphere under pressure to cure the composite sheets and the elastomer to form the flexible tool. After curing, the surround frame is removed from the flexible tool, which in turn is removed from the metallic master tool. The flexible tool, which now includes an integral seal extending around its perimeter formed as the partially cured elastomer cures with the composite sheet, has a profile that matches the profile of the pressure side of the blade and can now be used to facilitate the injection of lightweight material into pockets of a fan blade by positioning the flexible tool over the fan blade and securing it into position. The integral seal is concave at room temperature to facilitate assembly of the tool to the blade, but expands on heating to form an effective seal against the blade. 
     An advantage of the present invention is that the problem of standoff is eliminated. Standoff, which is caused by use of an O-ring, results in poor contact between the tool and the blade, and results in a step when a flowable, curable, lightweight material is injected into the blade pockets. 
     Another advantage of the present invention is that a plurality of identical flexible tools can be manufactured from the metallic master tool that has an indefinite life. 
     Another advantage of the present invention is that the tool is more easily located on the blade, as the integral seal is concave at room temperature and the problems associated with positioning a tool having a movable O-ring that extends away from the tool surface are eliminated. 
     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 
     FIG. 1 depicts a cross section of a prior art caul sheet with an O-ring seal positioned over a fan blade having pockets, before being clamped in place; 
     FIG. 2 depicts a partial cross-section of a prior art caul sheet with an O-ring seal; 
     FIG. 3 is a partial cross-section of a caul sheet with an integral seal of the present invention; 
     FIG. 4 is a perspective view of a metallic tool used to manufacture a caul with an integral seal; 
     FIG. 5 is a cross section of the metallic tool of FIG. 4 with elastomeric material applied to tool surface, along lines  5 — 5 ; 
     FIG. 6 is a cross section of the metallic tool of FIG. 5 after lay-up of the sheets of the composite material over elastomeric material onto the tool surface and into the slots; 
     FIG. 7 is a cross section of the sheets of composite material arranged over the tool of FIG. 5 with separator film and surround frame in place; 
     FIG. 8 is a cross section of the cured caul sheet in place on the tool of FIG. 7 prior to cooling and after removal of the separator film and surround frame; 
     FIG. 9 is a cross section of the caul sheet at room temperature; 
     FIG. 10 is a cross section of a caul sheet assembled to a fan blade at room temperature; 
     FIG. 11 is a cross section of a caul sheet assembled to a fan blade at an elevated temperature after lightweight material has been injected into the blade pockets; 
     FIG. 12 is a cross section of a finished caul sheet at room temperature; and 
     FIG. 13 is a cross section across a lug of a finished caul sheet at an elevated temperature. 
    
    
     Whenever possible, the same reference numbers will be used throughout the Figures to refer to the same parts. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Lightweight fan blades have been made by placing a caul sheet with an O-ring over a fan blade after the pockets have been formed in the blade. FIG. 1 depicts such a caul sheet  10  positioned over such a fan blade  12  having pockets  14 . O-ring  16  fits within a slot  18  formed in the face  20  of caul sheet  10  that contacts the pressure side  22  of fan blade  12 . Injector ports  24  extend through caul sheet  10  to provide access to fan blade pockets  14 . Slot  18  is dimensioned so that it fits over a perimeter  26  on the pressure side  22  of fan blade  12  extending around pockets  14  and inboard a preselected dimension from the blade edge  28 . FIG. 2 depicts an enlarged partial cross section of FIG. 1 showing caul sheet  10  with O-ring  16 . As can be seen, O-ring  16  must be compressed into slot  18 . Because O-ring  16  is a separate piece, it has a tendency to roll when placed in contact with the pressure side  22  of blade  12 , making it difficult to position. Furthermore, O-ring  16  must be properly compressed by clamps (not shown) completely around perimeter  26  so that O-ring  18  is flat against pressure side  22  and substantially fills and extends across slot  18 . If it is not flat against the pressure side  22 , that is, if it stands off from the surface of the blade, and/or if it does not extend completely across slot  18 , then a leak path exists outside the contour of the pockets that injected material will flow into. This material creates a step at the surface of the material filling the pocket that has adverse effects on the aerodynamics of the pressure side  22  of the blade  12 . The step can be as large as the radius of O-ring  16 . This extra material cannot be removed readily without risking damage to the blade. 
     A partial cross section of the caul sheet  110  of the present invention is depicted in FIG.  3 . This caul sheet is comprised of composite material and includes an elastomeric seal  116 . The elastomeric seal  116  is manufactured so that it is integral with caul sheet  110 . At room temperature, elastomeric seal  116  has a concave contour  118  that is substantially below the contour of the surface  120  of caul sheet  110  that will contact a fan blade when assembled thereto. A smooth, convex contour  121  rises along caul sheet  110  opposite elastomeric seal  116  at room temperature. 
     Referring now to FIG. 4, a perspective view of a tool  250  used in manufacturing caul sheet  116  is depicted. Tool  250  is a replica of a solid fan blade surface without pockets  14 , similar to blade  12  depicted in FIG.  1 . Except as noted, it has the same profiles and contours of the pressure side of such a blade. Tool  250 , however, is slightly wider and slightly longer than a blade. A scribe line  252  is placed along surface  254  of tool  250 . Scribe line  252  indicates the edge of a fan blade of a preselected design. Portions of tool surface  254  extend beyond scribe line  252 . Tip portion  255  of tool  250  extends beyond the portion of scribe line  257  indicative of the tip of a fan blade, while leading edge portion  258  and trailing edge portion  259  extend beyond the leading edge portion of the scribe line  260  and trailing edge portion of scribe line  261 . Tool  250  is slightly larger than an actual fan blade of the preselected design. Tool  250  includes slots  256  at three positions in a preferred embodiment that are located in the leading edge portion  258  and the tip portion  256  of the tool outside of scribe line  252 . Depending on the blade design, tool  250  may include more slots  256  and the slots may be located at various positions, including along the trailing edge if desired. However, at least two slots  256  are required. 
     Referring now to FIG. 5 which is a cross section of FIG. 4 along lines  5 — 5 , a continuous strip of partially cured elastomeric material  266  of preselected crosssection is laid on surface  254  of tool  250  inside of the perimeter of scribe line  252 . After elastomeric material  266  is placed onto tool  250 , a plurality of sheets of composite material  270  are placed over tool  250  in contact with surface  254  and over elastomeric material  266  as shown in FIG.  6 . The elastomeric material is preferably cut precisely to fit over the scribe line  252  with protrusions to fit into each of slots  256 . However, the sheets of composite when placed on surface  254  may extend beyond scribe line  252  and may be trimmed to the required size in a subsequent operation. Sheets of composite material  270  are laid into slots  256 , one of which is shown in FIGS. 5-8. The composite sheets  270  in slots  256 , when cured, form integral projections extending substantially perpendicular to the finished flexible tool or lugs that provide a positive location mechanism for caul sheet  110  when it is fitted to a blade. Tool  250  may be made of any material that can withstand elevated temperatures as will be explained. In a preferred embodiment, the material of choice is a metal capable of withstanding elevated temperatures, and the preferred metals are aluminum and its alloys. 
     Partially cured elastomeric material  266 , when fully cured, must be capable of being reheated a plurality of times without affecting the properties of the elastomeric material, and when cured, must be capable of forming a seal against the blade surface at elevated temperature and must be compatible with composite material. The composite material  270  used to form the caul body must be capable of being layed up on a tool used to form the caul body and be cured to form a composite body, and must be compatible with elastomeric material  266 . 
     The cured composite body in the form of a caul sheet must be flexible enough to accommodate the permissible variations in geometry from blade-to-blade when clamped in position, yet must be sufficiently stiff that when lightweight filler material is injected into fan blade pockets  14 , there is no deformation in the surface of the caul sheet. For elastomeric material  266  to be compatible with composite material  270 , there must be bonding as the materials are cured. While any elastomeric material  266  that has these properties may be used, urethane rubbers and fluorosilicone rubbers are acceptable. In the preferred embodiments, fluorocarbon rubbers such as Viton® available from E. I. Du Pont de Nemours are used. Composite material sheets that are acceptable include a matrix of epoxy reinforced by fibers. The sheets may include unidirectional fibers or woven fibers. The fibers that can be used include silicon carbide fiber, kevlar fiber, alumina or sapphire fiber, and graphite fiber. In a preferred embodiment, the sheets of composite material  270  are a carbon fiber/epoxy. 
     The sheets of composite material  270  are laid up to a thickness of between about 0.090-0.150 inches (0.228-0.381 cm). Each sheet or ply of composite material has a thickness of about 0.005-0.025 inches (0.013-0.064 cm). In a preferred embodiment, the thickness of the caul sheet is about ⅛ inch, about 0.120-0.130 inches (0.305-0.330 cm). As previously noted, the sheets may include woven fiber or unidirectional fiber. When sheets including unidirectional fiber are used, the sheets are placed on the tool in a quasi-tropic arrangement, that is, after the first sheet is applied, referred to as the reference sheet, at 0°, a second sheet is placed so that the fibers have an orientation of +45° to the fibers in the reference sheet, the next sheet is placed so that the fibers have an orientation of 90° to the fibers in the reference sheet, the next sheet is placed so that the fibers have an orientation of −45° to the fibers in the reference sheet, and the final sheet is placed so that the fibers are parallel to the fibers of the reference sheet. The pattern of alternating orientations is continued until the desired thickness is achieved. This quasi-tropic arrangement provides greater strength to the cured composite. While the sheet orientation pattern of 0°, 45°, 90°, (−45°) has been described, other orientation patterns may be used as desired. 
     After the composite sheets are laid up, as previously discussed they can be trimmed to an appropriate size. In the regions in which injector ports are located, small sheets (not shown) of composite material are cut to size and added to form the stiffening ribs and pads for the injector line adaptor at the injection ports. These additional sheets may be added in any other areas where additional strength or stability is required. 
     Referring back to FIG. 4, the edges of the sheets of composite material are cut so that they extend into slots  256  to form the positive location lugs for the caul sheet on the blade. The layup of composite sheets is then covered with a thin film of separator material  272  as shown in FIG.  7 . The separator material may be any material that the composite matrix will not adhere to during the curing process and serves as a barrier between the composite materials and the curing bags used during the curing process, as will be explained. TEFLON® film (polytetrafluoroethylene—PTFE) has been found to be an excellent separator material. 
     After the separator material  272  is placed over the layup, a surround frame  274  is placed over the film. The surround frame extends around the periphery of tool  250 . The purpose of the surround frame is to secure the partially cured elastomeric material in position as the sheets of composite material  270  and the elastomeric material  266  are fully cured together, and to add stiffness. 
     After the sheets of composite material  270  are trimmed to size, the assembly of FIG. 7 is then placed in an autoclave and cured. The assembly is bagged using a nylon bag (not shown) placed over the TEFLON® (polytetrafluoroethylene—PTFE) film  272 . Both the nylon bag and TEFLON® (polytetrafluoroethylene—PTFE) extend around the slots  256  to the side of the tool opposite surface  264  to secure the layup in place. The sheets of composite material  270  and the elastomeric material  266  are cured in an autoclave at suitable temperatures and pressures to form caul sheet  110 . For VITON® and carbon fiber reinforced epoxy, used in the preferred embodiment of the present invention, the material is cured in an autoclave at a temperature of about 350° F. (177° C.) at a pressure of about 50 psi for about 2 hours. Different material combinations may require different temperatures, pressures and/or times to cure properly. The curing process not only causes crosslinking of the matrix material of the composite and the partially cured elastomeric material  266 , but also cross-linking occurs between the matrix material and elastomeric material  266 , forming a strong bond between the elastomeric material  266  which, on curing, forms seal  116  and the matrix material so that the seal is an integral part of the caul sheet  110 . FIG. 8 depicts the cured caul sheet  110  assembled to tool  250  while still hot. As cooling occurs, elastomeric material  266  contracts, causing a gap between a portion of the elastomeric material and tooling  250 . Two different cross-sections of cured caul sheet  110  are depicted in FIGS. 12 and 13 to illustrate the effect of elevated temperatures on the integral seal  116 . FIG. 12 illustrates a first cross section of caul sheet  110  at room temperature. At this temperature, the cross-linking between the matrix material of the composite  270  and the elastomeric material  266  restrains the elastomeric material, and concave contour  118  results as the seal contracts. Also shown is a node  280  that extends completely around the surface of caul sheet  110  formed opposite seal  116 . The amount that the elastomeric material  266  projects above surface  254  of tool  250  during fabrication of caul sheet  110  determines the contour and height of node  280 , with more material and a higher projection resulting in a node with a more severe contour and a higher height. FIG. 13 illustrates a second cross section of caul sheet  110  at an elevated temperature. At this temperature, about 300-350° F. (149°-177° C.), the cross-linking between the matrix material of the composite  270  and the elastomeric material  266  still restrains the elastomeric material, but convex contour  282  results as the seal expands. The cross sections of FIGS. 12 and 13 also illustrate one of lugs  290  formed by curing of the composite sheets that were placed slots  256 . Each lug  290  fits over an edge of fan blade  12  along the blade perimeter, and the lugs are somewhat flexible to account for small manufacturing variations among the blades of a particular design. 
     An enlarged view of caul sheet  110  and seal  116  is shown in partial cross-section in FIG. 9 at room temperature. On cooling the material of integral elastomeric seal contracts, but as a result of the strong bonding with the matrix material, it is restrained from contracting along its sides, so that concave contour forms. 
     In use, caul sheet  110  is assembled to fan blade  12  having pockets  14 , as shown in FIG. 10 by placing each of lugs  290  around the edge of the blade along a perimeter. This placement basically aligns blade  12  with the caul sheet  110 . Prior to filling pockets  14  with lightweight material  302  in the form of a lightweight, flowable, curable liquid, the assembly is heated to a temperature in the range of 220-250° F. (104-121° C.) to expand seal  116  against pressure side  22  of blade, as shown in FIG.  11 . 
     To facilitate and assure fit-up during addition and curing of lightweight material  302 , after caul sheet  110  is aligned on blade  12 , a plurality of clamping blocks  296  are assembled onto caul sheet  110  to facilitate the use of clamps  298  to secure caul sheet  110  to blade  12  while lightweight material  302 , shown in FIG. 11, is injected into pockets  14  after elastomeric seal  116  has been expanded into contact against pressure side  22  of the blade. In the preferred embodiment, each of clamping blocks  298  have a contoured surface  304  corresponding to the contour of node  280 . Clamping blocks, however, are not required to completely cover node  280  around the perimeter of the airfoil, but are present in a plurality of discrete locations around the assembly. Although the caul sheet  110  may be heated before assembling to blade  12 , in a preferred embodiment the caul sheet is assembled and clamped to the blade, and the clamped assembly is heated. This assures the proper expansion of seal  116  against pressure side  22 . 
     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.