Abstract:
A method of forming a component for use in a gas turbine engine includes the steps of forming an airfoil/root assembly; creating a platform assembly structure having an opening; inserting the airfoil/root assembly into the opening; and bonding the platform assembly structure to the airfoil/root assembly to form the component.

Description:
BACKGROUND 
     The present invention relates to a component, such as a blade, having an integral platform for use in a gas turbine engine and a method for forming same. 
     Turbine blades typically require integral platforms, attached to the root region, to form the inner flowpath, and to protect the rim of the disk from hot gas ingestion. The typical cantilevered platform often has to support Blade-to-blade dampers. The combined load of the overhung platform and damper results in large bending stresses at the point where the platform meets the blade root region. 
     Ceramic Matrix Composites (CMC) are desired for turbine blades due to their high temperature capability. CMC blades made from layers of cloth or unidirectional tape offer good strength in the primary radial load path. Attaching a platform onto a laminated blade is difficult, and made especially challenging due to the low interlaminar strengths of CMC&#39;s. The bending of the platform causes high internal interlaminar stresses, and is a limiter on how long the platforms can function. 
     SUMMARY 
     In accordance with the instant disclosure, there is provided a method of forming a component for use in a gas turbine engine broadly comprising the steps of: forming an airfoil/root assembly; creating a platform assembly structure having an opening; inserting said airfoil/root assembly into said opening; and bonding said platform assembly structure to said airfoil/root assembly to form said component. 
     Further, in accordance with the instant disclosure, there is provided a component for use in a gas turbine engine which broadly comprises an airfoil and a root portion formed from a ceramic matrix composite and a three dimensional platform assembly bonded to said airfoil and root portion. The purpose is to create a platform region, with the inherent complexities, which is independent of the airfoil root structure, thus isolating the large radial pull of the airfoil from the platform assembly. Additionally the extension of the platform region into the lower root portion of the blade assembly simplifies the support of the platform by using the airfoil root to clamp the platform assembly to the disk attachment. 
     Turbine blades experience large radial pull loads due to centripetal acceleration. The simplest blades consist of an airfoil connected directly to a root with no integral platform. This simplifies the design of a ceramic matrix composite blade. In ceramic matrix composite the direction of the fibers control the strength of the ceramic matrix composite. For blades in particular, a strong biasing of the fibers in the radial direction imparts high strength with minimal material. The imposed loads on a blade of this type are dominated by the radial pull and for the airfoil region this creates a large radial tension stress. Bending stress in the airfoil is typically much less. The only region of the blade with large bending stresses is in the root region where it flares outward to form the attachment feature. In this region the ILT (Interlaminar Tensile strength, which is the layer to layer bond tensile strength) of the ceramic matrix composite material is more limiting that the tensile strength. Thus minimizing ILT bending stresses through simplification, and/or reducing the mechanisms that create bending stresses and/or increasing the ILT strength are desired. 
     Adding a platform to the existing airfoil/root assembly is challenging as the centrifugal loads created by the cantilevered platform, extending outward from the airfoil, create large bending stresses in the base of the platform where it connects with the airfoil. If the ceramic matrix composite platform was integrally woven into the airfoil region, then the fibers at the airfoil to platform intersection would be exposed to both the large radial load of the airfoil and the local bending stresses of the cantilevered platform. Since ITL strength is much lower than the tensile strength, the design would be limited by the local bending stresses in the platform, and not the airfoil region, resulting in lower capability. 
     Separating the blade into and airfoil/root assembly and a platform assembly allows the ceramic matrix composite structure to be optimized for the loads imposed on those assemblies. For the airfoil/root assembly this means a structure with minimal bending loads and a relatively straight load path between the airfoil and the root region. For the platform assembly a ceramic matrix composite structure with improved ITL capability and the ability to create a platform assembly with complex features is desired. Additionally, if the platform assembly was extended to cover the attachment region of the airfoil/root assembly, the complex bending stresses in the root portion of the airfoil/root assembly could be distributed over a larger region of the root, and lower the magnitude of the ILT stresses in the root, thus increasing the capability of that region. 
     ILT strength is the strength of the bond between the layers of a ceramic matrix composite. For typical ceramic matrix composites the fiber strength is higher than the matrix strength. Thus for typical assemblies, created by stacking layers, the weaker matrix is the limiting portion, and sets the design limit to the ILT stress of the assembly. In three-dimensional woven forms, additional fibers bridge the layers, such that they increase the ILT capability. A platform assembly made from three-dimensional ceramic matrix composite would have higher ILT capability. Additionally, programmable weaving looms can create complex 3-D woven shapes that can accommodate thickness and dimensional changes through an automated process much faster than by traditional layer by layer assembly techniques. Thus it is desirable to make a ceramic matrix composite platform assembly from a 3-D woven preform for its increased ILT performance with increased complexity. 
     Bonding an airfoil/root assembly, with optimized ceramic matrix composite construction to a platform assembly with a three-dimensional ceramic matrix composite construction would create a blade assembly with the required complexity of an integral platform and the improved structural performance of due to the combined construction. Additionally the presence of the platform assembly, encasing the airfoil root region, further improves the ILT performance of the airfoil/root assembly by distributing the attachment loads imposed by the turbine disk. 
     Other details of the ceramic matrix composite blade with integral platform using a complex weave perform are set forth in the following detailed description and the accompanying drawings wherein like reference numerals depict like elements. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic representation of a component which can be used in a gas turbine engine; 
         FIG. 2  is a sectional view of the blade of  FIG. 1 ; and 
         FIG. 3  is a flow chart showing the assembly method for forming the blade of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to the drawings, there is shown a ceramic matrix composite blade  10  for use in a gas turbine engine (not shown). The blade  10  may be a turbine blade used in the hot section of the engine. 
     The blade  10  has an airfoil portion  12  and a root portion  14 . The airfoil portion  12  and the root portion  14  may be an integral structure formed from a plurality of plies  18  of a ceramic matrix composite material as shown in  FIG. 2 . 
     As can be seen from  FIG. 1 , the blade  10  also has a platform  20  and one or more optional buttresses  22  formed from a platform assembly structure  24 . The platform assembly structure  24  is formed from a ceramic matrix composite material. The platform assembly  24  process begins with a fibrous pre-form which is in turn infiltrated with ceramic matrix to form a rigid ceramic matrix composite. The fibrous pre-form consists of a combination of three dimensional woven structures and/or portions made from chopped fibers and/or two dimensional woven cloth  26 . Two dimensional woven cloth typically has fiber/tow bundles interwoven such that a large flat sheet is created with a thickness of the sheet being approximately twice the thickness of the fiber/tow bundles. Three-dimensional woven preforms consists of fiber/tow bundles that are woven in such a manner as to have additional fiber/tow bundles such that the thickness can be increased, and complex shapes created where local thick sections can be added and still retain connectivity to the thin sections with continuous fiber/tow connectivity. As will be discussed hereinafter, the platform assembly structure  24  is bonded to the airfoil portion  12  and root assembly  14  so as to form an integral structure. 
     Referring now to  FIG. 3 , the method of forming the blade  10  includes the step  100  of forming the airfoil/root assembly by laying up plies  18  of a ceramic matrix composite material in a mold and infiltrating the plies with a matrix material. The plies  18  may be formed from a uni-directional tape and/or a fabric or woven material such that a strong primary structure is created that can transmit the radial pull of the blade airfoil  12  into the root attachment region  14 . A fabric may be made from fibers called tows. Individual tows are woven together to create the fabric. Unidirectional-Tape can be made from a collection of individual fibers or a collection of tows, bonded together to form a continuous sheet of uniform thickness. The Unidirectional tape can be cut, like fabric and stacked together with plies. After the plies  18  have been laid up, they may be joined together to form the airfoil/root assembly using low temperature polymerization, high temperature polymerization and/or pyrolosis techniques, or bonding with a Silicon interfacial layer. 
     As shown in step  102 , the platform assembly structure  24  is formed separately from the airfoil/root assembly. The platform assembly structure  24  may be formed from a ceramic matrix composite. For example, the structure  24  may be formed using a plurality of three dimensional or chopped fibers which have been infiltrated by a matrix material. Here again, bonding may be accomplished using low temperature polymerization, high temperature polymerization, and/or pyrolosis, or bonding with a Silicon interfacial layer. The structure  24  may be formed in a mold. Further, the structure  24  is formed to have a central opening  30  which extends from the top to the bottom of the structure  24 . In other words, the structure  24  has a hollow core. The structure  24  may be fabricated with one or more chord-wise spaced apart buttresses  22 . If desired, the buttresses  22  may be omitted. 
     The fibers used to form the platform assembly structure  24  may include fibers such as silicon carbide, aluminum oxide, silicone nitride, carbon, and combinations thereof. 
     The matrix used to form the platform assembly structure and/or the airfoil/root assembly may include magnesium aluminum silicate, magnesium barium aluminum silicate, lithium aluminum silicate, barium strontium aluminum silicate, bariums aluminum silicate, silicon carbide, silicon nitride, aluminum oxide, silicon aluminum oxynitride, aluminum nitride, zirconium oxide, zirconium nitride, and/or hafnium oxide. 
     In step  104 , the airfoil/root assembly is inserted into the opening  30  in the structure  24  so that the outer edge  32  of the root portion is abutted by an inner edge  34  of the structure  24 . 
     In step  106 , the platform assembly structure  24  is bonded to the airfoil/root assembly to form the blade  10 . The bonding step may be carried out by introducing the matrix material and heating to densify the ceramic matrix composite material and bond the airfoil/root assembly to the platform assembly. The platform assembly  24  may be formed so that a portion of the platform assembly  24  may extend radially inward and cover a root region of the airfoil root assembly. Alternatively the bonding step may be carried out by introducing a bonding agent such as silicon, which after bonding creates a interfacial layer between the airfoil/root assembly and the platform assembly. Silicon, deposited in a layer on the blade/attachment assembly and/or the platform assembly, would then disperse into the resulting assembly when heated and constrained appropriately, forming a continuous bond between the airfoil/attachment assembly and the platform assembly. 
     In step  108 , any protruding portion, such as fibers, may be ground off. 
     As shown in  FIG. 2 , the platform root section  40  is integrated onto the blade root section  14  such that the contact between the final blade  10  and a disk  42  occurs on the exterior surface  44  created by the three dimensional woven platform root section. 
     While the present disclosure has been described in the context of forming a turbine blade, the method could also apply to the manufacture of other components for use in a gas turbine engine. 
     There has been described herein a ceramic matrix composite blade with integral platform using complex weave perform. While the ceramic matrix composite blade has been described in the context of specific embodiments thereof, other unforeseen alternatives, modifications, and variations may become apparent to those skilled in the art having read the foregoing description. Accordingly, it is intended to embrace those alternatives, modifications and variations which fall within the broad scope of the appended claims.