Abstract:
A method of manufacturing an assembly ( 10 ), including: positioning a first component ( 12 ) and a second component ( 14 ) in a desired positional relationship with each other; and building-up a locking component ( 16 ) by depositing layer after layer of material onto a surface ( 24, 26 ) of the assembly until a completed locking component is formed in-situ that holds the first component and the second component in the desired positional relationship.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to mechanical joining of an assembly via a component formed in-situ via a layer-by-layer additive manufacturing process. 
       BACKGROUND OF THE INVENTION 
       [0002]    In the field of gas turbine engines various parts of a single component may have widely varying operational requirements. Certain materials may be well suited for the operating requirements of one of the various parts, while another material may be better suited for the operating requirements of another. Modular components have therefore been used to tailor the materials used to the varying operational requirements. In this manner more expensive or difficult-to-fabricate materials may be limited to those parts of the component where needed, while less expensive or easier-to-fabricate materials can be used elsewhere. Furthermore, this modular approach to manufacturing a component allows for the replacement of individual modules rather than an entire component to extend service life of the component. 
         [0003]    Joining of these components conventionally includes metallurgical joining such as welding and brazing. However, there are many high-temperature turbine materials that are very difficult to weld without cracking. This is less of a problem for braze joints, but braze joints are only as strong as the braze material. Mechanical joining offers advantages when joining dissimilar materials or materials that are difficult to weld. However, there are frequently concerns that mechanical joints may fail during service and liberate hardware into the engine. Consequently, there remains room in the art for improvement. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]    The invention is explained in the following description in view of the drawings that show: 
           [0005]      FIG. 1  is a schematic representation of an exemplary embodiment of the assembly. 
           [0006]      FIG. 2  is a schematic representation of an alternate exemplary embodiment of the assembly. 
           [0007]      FIG. 3  is a schematic representation of yet another alternate exemplary embodiment of the assembly. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0008]    The present inventor has devised an innovative approach for joining components to form an assembly. Specifically, components of the assembly are held relative to each other in a positional relationship that they are to have when they are part of the component. While the components are being held in the desired positional relationship a locking component that completes the assembly is formed in place on one of the other components of the assembly via an additive manufacturing process, where the locking component is formed layer-by-layer. An interlocking relationship within the assembly holds the assembly together, and the locking component ensures that interlocking elements of the interlocking relationship stay engaged with each other. The locking component therefore forms part of the assembly and is effective to ensure the interlocking relationship remains intact, thereby holding the assembly together. 
         [0009]      FIG. 1  shows a schematic longitudinal cross-section of an exemplary embodiment of an assembly  10  having an elongated shape similar to an airfoil. The assembly  10  includes a first component  12 , a second component  14 , and a locking component  16 . In this exemplary embodiment the assembly  10  is held together by two interlocking relationships  18  formed by a first interlocking feature  20  and a second interlocking feature  22  that engage each other. So long as the interlocking features  20 ,  22  are engaged with each other the interlocking relationship  18  is formed and this holds the assembly  10  together. Thus, the locking component  16  is configured to be part of the component  10  and to simultaneously ensure the interlocking relationship  18  remains intact. In this exemplary embodiment the first interlocking feature  20  for each interlocking relationship  18  is formed as part of the locking component  16 . However, the locking component  16  may or may not include geometry that forms part of the interlocking relationship  18 . Instead, the locking component  16  is formed to ensure the interlocking relationship  18  remain intact, regardless of which components of the assembly  10  actually include the geometric features that form the interlocking relationship  18 . 
         [0010]    The locking component  16  may be formed by an additive manufacturing process which can be defined as a process of joining materials to make three dimensional solid objects from a digital model, layer upon layer. This is in contrast to subtractive manufacturing methodologies which rely on the removal of material using techniques such as cutting, drilling, milling and grinding etc. One of many possible examples of an additive manufacturing process envisioned for this method is Laser Engineered Net Shaping (LENS). In this process a metal powder is injected into a molten pool created by a laser beam. The component being formed sits on a surface that may be moved under the laser&#39;s focal point, and the laser may be elevated after forming a layer in order to form another layer on top of the formed layer. The component formed is considered to be fully dense (fully sintered) and therefore fully formed when the final shape is reached. Further, the locking component may be ceramic, or as in the exemplary embodiment, it may be metal. 
         [0011]    As a result of this layer-by-layer approach, a powder metallurgy component formed via a layer-by-layer additive process has a unique microstructure when compared to components made using other powder metallurgy processes. First, a grain size in the grain structure of the component is limited to a thickness of the deposited layer in which the grain resides because a size of a pool of melted material formed by the process is limited to approximately the thickness of the layer, and the size of the weld pool limits the size of the grain. (The size of the weld pool is, in turn, controlled by the heat input from the laser and the thickness of the powder layer.) The small volume of the molten material and the fast cooling rate effectively prevent grain growth. Therefore, since each layer is essentially fully formed when deposited, any grains within the layer cannot grow to be any thicker than a thickness of the layer itself. While a subsequent layer formed on top of the first layer may melt an upper portion of the first layer in order to bond thereto, any grains present in the first layer do not grow into the second layer. 
         [0012]    Second, in the layer-by-layer approach the grains in the component would have a laminar structure as a result of the layering process. In contrast, in conventional powder metallurgy processes the individual powder particles do not melt, rather they join together via inter-diffusion when exposed to high temperatures (below the melting point) in the sintering process. The powder particles have a random orientation to each other and the interfaces between the particles become the grain boundaries. This results in a structure that is more uniformly equiaxed in conventional powder metallurgy processes. The laminar structure that results from the layer-by-layer process can lead to anisotropic properties, (where there may be differences in properties measured parallel to the build direction than properties 90 degrees to the build direction). 
         [0013]    Several advantages can be realized from this method of forming this type of assembly. For example, if the assembly  10  of  FIG. 1  is an airfoil used in a gas turbine engine, and if the first component  12  is an airfoil portion while the second component  14  is a tip coupon, (in such an exemplary embodiment the locking component  16  may take the shape of a ring or cylinder), both of these components could benefit from having different compositions. Specifically, the airfoil portion may required to have greater creep resistance in environments such as those created by hot combustion gases in a gas turbine engine, but need not necessarily be particularly abrasive. The tip coupon portion, which may encounter an abradable portion of a shroud or ring segment, may need greater abrasive properties. As often occurs in gas turbine assemblies, the best choice of material for the first component  12  may not be metallurgically compatible with the best choice of material for the second component  14 . This incompatibility makes it very difficult if not impossible to join the components via welding. Any such weld produced may be less than desirable due to cracking etc. Strength requirements may preclude the use of braze, and debris concerns may prevent the use of conventionally mechanically joined assemblies (e.g. bolting). Thus, in some instances it has not been possible to form a metallurgically ideal assembly. This method overcomes this problem by permitting the creation of an assembly that meets the varying metallurgical, cost, and reliability needs etc of the assembly without any worry associated with welding, brazing, and conventional mechanical joining. Another advantage includes the ability to replace an individual component rather than the entire assembly to extend service life. In order to facilitate disassembly of the assembly and replacement of individual components, or for any other reason deemed important, it may be desirable to form the locking component such that there is no metallurgical bond between the locking component and a surface of the component of the assembly onto which the first layer of the locking component is deposited. For example, in the exemplary embodiment of  FIG. 1 , the locking component  16  is formed on both a surface  24  of the first component  12  and a surface  26  of the second component  14 . If, on the other hand, a metallurgical bond is to be formed at an interface  28  between the surface  24  of the first component  12  and the locking component  16  and/or an interface  30  between the surface  26  of the second component  14  and the locking component  16 , then the surfaces  24 ,  26  may be appropriately cleaned to permit the metallurgical bond to form. Appropriate cleaning is known to those in the art to be similar to the cleaning necessary in welding operations to permit the formation of a proper weld. In contrast, if no metallurgical bond is to be formed at the interface  28  or the interface  30 , then the cleaning step may be eliminated. Alternately, if no bond is to be formed, an oxide layer may be allowed to form on either or both of the surface  24  of the first component  12  and the surface  26  of the second component  14 . The oxide layer may or may not be burned off during the application of the first layer of the additive manufacturing process, but in either case it is likely to prevent the formation of a metallurgical bond between the locking component  16  and any surface used as the base for the first layer. 
         [0014]    Gas turbine engine assemblies often experience thermal growth mismatch within the assembly. This may occur when, for example, dissimilar materials wish to respond differently to a thermal change but are forced to respond identically, such as when dissimilar materials are welded together. Using the method disclosed herein, an assembly  10  can be fabricated that reduces or eliminates this problem by strategic positioning of the components. For example, in  FIG. 1  the first component  12  and the second component  14  are disposed end-to-end. This allows for each component to respond to thermal changes independently of the other. In addition a gap  32  may be built-in to the assembly  10  such that when the assembly  10  is at an operating temperature any thermal growth of the components toward each other will be accommodated by the gap  32 , thereby eliminating any stresses that may otherwise result. 
         [0015]      FIG. 2  shows a schematic longitudinal cross-section of another exemplary embodiment of the assembly  10 . Here again the first component  12  and the second component  14  are held in place by the locking component  16 . However, in this exemplary embodiment the second component  14  is assembled onto the first component  12  and the locking component  16  is then formed. The locking component  16  interlocks only with the first component  12 , while the second component is held in place simply by the presence of the first component  12  and the locking component  16 . In this exemplary embodiment the assembly  10  may be a gas turbine engine vane where the first component  12  is an airfoil component and the second component  14  is a shroud joined to the airfoil component via the locking component  16 . In such an assembly  10  hot gases are usually present on a hot gas side  40  of the second component  14 . By locating the locking component  16  on a relatively cool side  42  of the second component  14 , the material selected to be used in the locking component  16  need not withstand the operating environment created by the hot gases. Consequently, since materials that can withstand that operating environment are often more expensive and more difficult to fabricate, this configuration saves on material costs and assembly costs. 
         [0016]    In yet another exemplary embodiment that is a variation of  FIG. 2 ,  FIG. 3  depicts a longitudinal cross-section of the assembly  10 , having a third component  44 . The third component  44  may be modular in construction, for example a split ring, and the third component  44  and the first component have the first interlocking feature  20  and the second interlocking feature  22  respectively and hence form the interlocking relationship  18 . The locking component  16  holds the third component  44  in place, and hence the locking component  16  effectively ensures the interlocking relationship  18  of the first component  12  and the third component  44  is maintained. As shown an interface  46  between the locking component  16  and the third component  44  is angled with respect to a longitudinal axis  48  of the assembly  10 . This configuration is advantageous if there is no metallurgical bond at the interface  46 , since the geometry of the assembly  10  will hold the locking component  16  in place. Alternately, should there be no angle desired between the interface  46  and the longitudinal axis  48  of the assembly  10 , a metallurgical bond may be allowed to form at the interface such that the locking component  16  is metallurgically bonded to the third component  44 . This metallurgical bond would hold the locking component  16  in place and thereby hold the interlocking relationship  18  in place. Further, in this configuration the locking component  16  is isolated from the first component  12  and the second component  14 , which may prevent compatibility problems were they not isolated from each other. 
         [0017]    From the foregoing it is evident that the present inventor has created a unique way of fabricating assemblies and that this unique method can produce assemblies that solve known problems in the art. However, the solution uses technologies that can be readily applied, and so the implementation can be fast and inexpensive. Consequently, this represents an improvement in the art. 
         [0018]    While various embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions may be made without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.