Abstract:
A process and apparatus for solid freeform fabrication and repair of components on existing bodies (such as turbine blades), the innovative process and apparatus as well as the resultant product having the following advantages: a) Can build on existing 3-D surfaces. Not limited to horizontal flat surfaces, b) Usable for metals that are difficult to weld. c) Robust process that is adaptable to new damage modes. d) No shielding of the melt pool by inert gas is needed. e) Wide range of powder sizes.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to the field of forming a turbine blade auxiliary component directly onto the turbine blade, and particularly to forming a snubber directly onto the turbine blade wherein the snubber properties and structure conform to the properties and structure of the turbine blade. 
       BACKGROUND OF THE INVENTION 
       [0002]    In power generation, turbine blades are subjected to a demanding range of performance requirements including withstanding high temperatures and temperature fluctuations, high pressures, high speed rotation, vibrational loading/stress, metal fatigue, irregular part geometry, and nearly uninterrupted turbine engine usage. Coincidentally, auxiliary blade components, such as blade snubbers, are also challenged to endure these extreme operating conditions. 
         [0003]    In turbine engines snubbers are provided as an interface between pairs of blades and help to minimize the vibrational loading and stresses experienced by the blades. Snubbers provide additional stiffness to the blades which in turn impacts the blade design options—such as allowing for reduced axial blade width or optimization of the blade frequency response. Snubbers are designed and selected to function in the extremely demanding operating environment of the turbine blade. 
         [0004]    Typical methods of forming and joining machine parts presents limitations constraining the use of these techniques in forming snubbers and in integrating snubbers with turbine blades. For example, some methods require excavating portions of the turbine blade before adding on material used to construct a snubber. These excavated areas present modifications to the original blade design which range from changing the overall aerodynamics of the blade to introducing reductions in the structural integrity of the blade. 
         [0005]    Other methods of forming and joining machine parts involve subjecting the blade and snubber to high temperatures, such as via the use of a welding process, which consequentially, may alter the physical characteristics of the blade. 
         [0006]    Since, overall, snubbers are used to enhance turbine engine performance, there is a need optimize both the design of the snubber, the composition of the snubber, as well as the techniques used to attach the snubber onto the turbine blade. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The invention is explained in the following description in view of the drawings that show: 
           [0008]      FIG. 1  illustrates a turbine blade with a snubber provided near the mid-span. 
           [0009]      FIG. 2  presents an exploded view indicating the snubber positioning on the turbine blade. 
           [0010]      FIG. 3  is a side sectional view of a preform showing aspects of an embodiment of the invention. 
           [0011]      FIG. 4A  is a top view of a layer of a snubber. 
           [0012]      FIG. 4B  is a top view of a layer of a snubber. 
           [0013]      FIG. 5A  is a side view of a snubber layer under formation on a single grain blade. 
           [0014]      FIG. 5B  is a side view of a snubber layer under formation on a multiple grain blade. 
           [0015]      FIG. 6  is a side view of a snubber layer being formed on a single grain blade. 
           [0016]      FIG. 7  is a side view of a snubber layer formed on to a single grain blade. 
           [0017]      FIG. 8  is a side view of a snubber layer being formed on a multiple grain blade. 
           [0018]      FIG. 9  is a side view of a snubber layer formed on to a multiple grain blade. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]    The present inventors have developed a method of forming a secondary structure on to the surface of a cast metal component by melting successive layers of metal powder on to the cast metal component using a directed energy beam wherein the melting does not encompass a full grain depth of the cast metal component. 
         [0020]    Particularly, the present inventors have developed a method of forming a snubber directly onto a directionally solidified cast turbine blade wherein the snubber properties and structure conforms to or matches the properties and structure of the turbine blade. Consequently, the inventors have developed a turbine blade snubber with mechanical properties which are custom matched to or aligned with mechanical properties of the host turbine blade. 
         [0021]    Advantageously and particularly the turbine blade plus snubber combination developed by the inventors includes a common grain orientation at least at their union thereby maintaining the mechanical properties of the turbine blade. 
         [0022]    For directionally solidified single crystal superalloy turbine blades, the inventors have recognized the benefit of adding auxiliary components to the turbine blade while preserving the blade grain orientation achieved during casting of the turbine blade. 
         [0023]    As discussed above, while attaching auxiliary components, such as snubbers, to a turbine blade has been accomplished using a variety of techniques such as welding, using these techniques to form a snubber directly on the turbine blade usually results in a degradation of the blade&#39;s mechanical properties such as undesired reorientation of the blade&#39;s directional solidification and reduction in blade wall thickness. 
         [0024]    The present inventors have developed a process and apparatus for solid freeform fabrication and repair of components on existing bodies (such as turbine blades), the innovative process and apparatus as well as the resultant product having the following advantages: 
         [0025]    a) Can build on existing 3-D surfaces. Not limited to horizontal flat surfaces. 
         [0026]    b) Usable for metals that are difficult to weld. 
         [0027]    c) Robust process that is adaptable to new damage modes. 
         [0028]    d) No shielding of the melt pool by inert gas is needed. 
         [0029]    e) Wide range of powder sizes. 
         [0030]    An unbounded metal powder compound or a preform of metal powder and other constituents such as flux powder is created that contains metal to be added to a component (such as a blade snubber) being additively fabricated or repaired. 
         [0031]    “Unbound” means loose, as opposed to consolidated, compacted, and/or sintered together into a block or other self-supporting form. A benefit of unbound particles is that laser energy penetrates to a greater depth by reflection between the particles than with a solid preform such as is described later herein. The particles may constitute respective metal and flux particles mixed in a pre-determined volume ratio, or the particles may constitute metal particles coated with or containing flux, such as are described in United States patent application publication US 2013/0136868 dated 30 May 2013, incorporated by reference herein. 
         [0032]    The selected metal powder(s) (provided via the compound or preform) may be constrained in a distribution that defines a shape of a layer or slice of the component, such as a blade snubber or other turbine component. 
         [0033]    The metal powder compound or metal powder preform is preplaced on a component surface for repair, or a previous layer. 
         [0034]    The compound or preform is then melted by a directed energy, such as a laser beam or other form of energy. This forms a layer of metal and an over-layer of slag that shields and insulates the layer during processing. The slag is then removed, and a subsequent layer may be added. 
         [0035]    Referring to the figures,  FIG. 1  shows a turbine blade  200  with a snubber  300  positioned near its midspan. The blade  200  (or airfoil) generally includes at least a blade root  201 , a blade tip portion  202 , and a blade platform  203 . 
         [0036]      FIG. 2  presents an exploded view indicating the snubber  300  positioning on the turbine blade  200 . As shown, the turbine blade  200  includes an opening  215  formed in the surface or wall  210  of the blade  200 . The blade  200  may be formed by known methods such as investment casting and the blade  200  may be hollow, for example, to minimize blade weight or to provide a path for fluid flow such as airflow. 
         [0037]    Further the blade  200  can be formed to include a selected directionally solidified grain pattern  220  or a preferred blade grain structure  220 , as shown in  FIG. 5A  thru  FIG. 9 . The blade may be cast with a single crystal grain structure  220  as shown in  FIGS. 5A ,  6 , and  7 . The blade may be cast with a multiple crystal grain structure  220  as shown in  FIGS. 5B ,  8 , and  9 . The grain structure of the blade (including a single crystal structure or multi-crystal structure) may extend from the blade root  201  or platform  203  toward the tip portion  202  of the blade (or airfoil)  200 . 
         [0038]    As explained in more detail below, the snubber  300  may be constructed to match or correspond to the blade grain structure  220 . The snubber  300  generally begins with a snubber first layer  370  formed from a deposit of metal powder and may include other constituents, such as insulator material, flux material, or other additives such as dry ice. 
         [0039]    The grain orientation  320  of the snubber first layer  370  adjacent the surface  210  of the blade  200  may be matched to the blade grain structure  220 . 
         [0040]    As shown in  FIG. 2 , beyond the snubber first layer  370 , the snubber may comprise additional or subsequent layers  371 . The grain orientation of subsequent layers  371  of the snubber  300  may be created in a fashion similar to the crafting the grain orientation structure  320  of the snubber first layer  370 . Alternatively the grain orientation of the subsequent layers  371  may be formed via a unique method selected to impart the desired mechanical properties to the layer. 
         [0041]    Selectively generating a grain structure  320  of the snubber layers  370 ,  371  that matches the blade grain structure  220  is further accomplished by controlling the direction and intensity of the heat transfer and cooling of the snubber layers  370 ,  371  as they are subjected to a directed energy beam  500 , as shown in  FIGS. 5A and 5B , and subsequently cooled and solidified. 
         [0042]    For example the grain structure  320  of the snubber under formation may be selected to be parallel to the blade grain structure  220  as shown in  FIG. 5A . 
         [0043]    Control of the direction and intensity of the heat transfer and cooling experienced by the snubber layers  370 ,  371  may be accomplished in part by selectively using various system vectors  405  (as used here the system vectors encompass energy beam parameters including path, intensity, and duration, as well as programmed and computed functions and algorithms, among others). 
         [0044]    In conjunction with applying optimized system vectors  405 , laser blocking elements and material  400  (including insulation material  410  and flux material  420 ), referred to herein also as heat transfer control elements  400 , are selectively placed onto the blade surface  210  or other locations related to the applicable snubber layer  370 ,  371  to affect the formation of the subsequent snubber layer  371 . 
         [0045]    Heat transfer control material  400  having high thermal conductivity, such as graphite, induce a fine grain structure in the solidified metal powder or preform by promoting fast cooling. A laser-blocking material  400  with low thermal conductivity, such as zirconia, may be useful to induce directional solidification in the snubber by limiting a direction of heat removal to be primarily in a direction of a preferred grain orientation  320  of the snubber  300 . Thus, the grain structure of the metal in the snubber  300  can be customized and varied over the component body by selection of the surrounding heat transfer control materials  400 . 
         [0046]    Utilizing the heat flow control elements  400 , heat transfer from the directed energy beam to the cast metal component (such as the blade  200 ) and the secondary structure (such as the snubber  300  under formation) can be closely controlled to pull heat out of the solidifying metal and achieve a preferential grain growth in the secondary structure. As a result, for example, the heat transfer can be control such that the melting of the metal powder does not encompass a full grain depth of the cast metal component. 
         [0047]    Using this approach it is possible, for example, to maintain a well-defined transition from columnar (directional) grain structures to equiaxed in subsequent snubber layers  371 , thus providing a snubber having layers  370  and  371  encompassing both columnar and equiaxed grain structure features in specific areas of the same snubber. This allows for any desired combination of grain structures to be imparted to the snubber  300 . 
         [0048]    The snubber  300  may have a single crystal grain structure at the first layer  370  where the snubber is joined to the single crystal structure of the blade  200 , and thereafter subsequent snubber layers  371  may have an equiaxed grain structure, and other subsequent snubber layers  371  can have columnar grain structures. 
         [0049]      FIG. 3  is a side sectional view of a preform  322 A showing aspects of an embodiment of the invention. More particularly,  FIG. 3  shows a sectional side view of a preform  322 A (such as a preform configured as a snubber first layer  370 ) embodied as a closed container such as a bag, envelope, sleeve, or tube containing unbound particles of metal  332 ,  334  and flux  333 . 
         [0050]    The container has walls  324 ,  326  with a sealed periphery  328 . The walls may be sheets of any type, such as fabric, film, or foil that retains the powder. The sheets may be made of a material that does not create detrimental smoke and ash, and may contribute to the flux, such as aluminum foil, or a fabric of alumina or silica fibers. The container may be quilted or subdivided by partitions  329  to retain a particle distribution that creates a desired shape of the metal layer in response to the energy beam. Such partitions  329  may also be useful for out-of-position (non-flat) material deposition applications. Some variation in thickness of the preform is tolerable, since the melt pool is self-leveling to some extent. The partitions may provide compartments of particles  332 ,  334  of different sizes and/or different materials optimized for varying requirements over the section of the component. Larger particle sizes may be provided for larger structural features, and smaller particle sizes may be used for smaller, more detailed structural features. A fabric-walled compartment may have a mesh size appropriate for retaining a respective particle size and may be varied accordingly across a preform, as appropriate, or it may be lined, such as with aluminum foil, to retain fine powdered particles. The aluminum then becomes a constituent of the alloy melt. 
         [0051]    Optionally, the periphery  328  may include a non-metallic, non-melting, laser blocking material  400 ,  330  such as graphite or zirconia, which provides an energy absorbing turn-around area for the laser scan lines outside the melt pool. This avoids excess heating of the periphery of the layer. The laser-blocking material  330  may form a solid peripheral frame to which the peripheries  328  of the walls  324 ,  326  may be attached with high-temperature cement. Such a frame provides a highly defined outer surface of the fabricated component. 
         [0052]    Optionally, particles of dry ice may be mixed with the particles  332  of metal and flux or may be contained in a peripheral or interior compartment in place of, or in addition to, the laser blocking material  330  to control heating and to supply an oxidation shield of CO 2  gas. 
         [0053]      FIGS. 4A and 4B  present a top view of a snubber  300  to be formed on a turbine blade  200  by an embodiment of the present process and apparatus. To achieve proper grain formation of the snubber  300 , such as to achieve a specified grain pattern, heat flow control elements  400  are positioned in select locations  450  about the opening  215  in the blade surface  210 . 
         [0054]    Material selected to form the snubber first layer  370  is also positioned about the opening  215  in the blade surface  210 . Insulating material  410  and flux material  420  of the heat flow control elements  400  are selected to effectuate the desired heat transfer of the constituents of the snubber first layer  370  necessary for achieving the desired grain structure of the snubber first layer  370 . The snubber segments  375  contain the metal powder composition necessary for creating unique portions of the snubber layer  370 . 
         [0055]    Subsequent snubber layers  371  are comprised of snubber segments  375  selected for creating additional subsequent snubber layers  371 . 
         [0056]    As shown, heat flow control elements  400  are provided in various locations as needed to direct the heat transfer towards the desired direction. The heat flow control elements  400  and the associated fabrication components  600  may be positioned in discrete locations around the segmented snubber layer  370  or even placed atop other heat flow control elements  400 . 
         [0057]    With the snubber layer  370  and heat flow control elements  400  positioned as desired, a system vector  405  is applied which selectively applies the directed energy beam to the snubber layer  370  and heat flow control elements  400  to generate the chosen grain pattern in the snubber under formation. 
         [0058]    The snubber layer  370  may include shaped sections of large particles as well as smaller particles and any combination therefore to form the segment. It may further contain laser-blocking borders or directed energy beam blockers  400  such as graphite for laser turn-around areas. It may also contain interior laser-blocking sections  400  to provide high definition of the interior surfaces of the component and control the grain structure. 
         [0059]    Graphite does not adhere to metal, so the laser-blocking sections  400  can be easily removed after laser processing of each layer. The laser blocking sections may be particulate or solid. Optionally, the laser-blocking sections may be allowed to accumulate layer by layer until fabrication is complete, so that each laser-blocking section is supported on the previous laser-blocking sections. Solid laser-blocking sections may have a registration feature such as protrusions on an upper surface and depressions on the lower surface thereof to register the current preform relative to the previous one. 
         [0060]    As shown in  FIG. 4B , the heat flow control elements  400  may be positioned within the opening  215  in the blade surface  210  and may be composed of a grid like structure  217  where heat flow is directionally controlled based on the properties of each grid cell  415  as well as the vector settings  405  selected for imparting directed energy to individual grid cells  415 . 
         [0061]    As shown in  FIG. 5A , the blade presents a grain pattern  220  achieved during casting of the blade  200 . The snubber first layer  370  is induced to replicate the blade grain pattern  220 , in part, through the controlled transfer of heat flowing into, out of and through the snubber first layer  370  thereby effectuating the systematic heat flow control provided by the heat flow control elements  400 . Specifically, the heat flow control elements  400  are configured to help impart the dendrite grain structure of the blade  200  to at least the first layer  370  of the snubber  300 . 
         [0062]      FIG. 5A  is a side view of an embodiment designed to create a first layer  370  of the snubber of  FIG. 4 . The side view shows a layer of metal powder forming the snubber first layer  370  positioned on the surface  210  of the turbine blade  200 . As shown, the flux  420  can optionally be deposited to help form a wetted junction  380  between the blade surface  210  and the snubber first layer  370  at a metallurgical joint  385  between the blade  200  and the snubber  300 . As the metal powder of the snubber first layer  370  melts, a meniscus  386  may be formed at the wetted junction where the melted metal powder of the snubber first layer  370  converges with the blade surface  210 . 
         [0063]    Importantly, the wetted junction  380  provides for optimized heat transfer between the blade surface  210  and the snubber first layer  370  without the need for excavating a portion of the blade surface  210  as is traditionally done. As shown in  FIG. 5A  the metal powder of the snubber first layer  370  is being melted to form a snubber layer having the single grain pattern of the underlying turbine blade  200 . Note—any of the embodiments disclosed herein may include the wetted junction  380 . 
         [0064]    As shown in  FIG. 5A , with precise placement of the heat flow control elements  400  as well as the uniquely configured snubber layer  370 , only a fraction  206  of a full grain depth  219  of the blade  200  may be subjected to the melting of the metal powder and the heat of the directed energy beam. This allows the remaining grain structure to remain undisturbed. Thus, melting of the metal powder does not encompass a full grain depth  219  of the cast metal component  200 . In some embodiments where the turbine blade wall is only one grain thick, the melting proceeds to less than that full grain depth. In some embodiments where the turbine blade wall is multiple grains in thickness, the melting proceeds to less than all of the multiple grains. Advantageously, at least some of the as-cast grain structure of the wall remains un-melted during the deposition of the snubber. 
         [0065]    A grain structure  320  of the snubber under formation may be generated which is parallel to the blade grain structure  220  without disturbing the blade grain structure  220  as-cast. At least the first layer  370  of the metal powder deposited onto the blade  200  can have directionally solidified grains  320  oriented parallel to the grains  220  of the blade wall  210  as shown in  FIG. 5A . 
         [0066]    Additionally, where the snubber  300  is formed of successive layers  371 , melting of the successive layers  371  does not melt and reform a full thickness  219  of the underlying blade wall  210 . 
         [0067]    This precise control of the preferential direction of heat transfer allows the snubber  300  to be formed on the wall  210  of the blade  200  while limiting the metallurgical joint  385  to extending less than a full thickness of the blade wall  210 , which preserves the as-cast grain structure  220  of the blade wall  210  under the metallurgical joint  385 . 
         [0068]    The system vector settings  405 , the heat flow control elements  400 , and the snubber layer properties cooperate to provide for the formation of a snubber  300  which has physical characteristics and mechanical properties, such as grain structure, related to the properties of the host turbine blade. 
         [0069]      FIG. 5B  is a side view of an embodiment designed to create a layer  370  of the snubber of  FIG. 4  on to a turbine blade having a multiple grain structure. The side view shows a layer of metal powder forming the snubber first layer  370  positioned on the surface  210  of the turbine blade  200 . The as-cast grain structure  220  of the blade wall is a directionally solidified grain structure having a thickness of a plurality of grains  223  extending from the blade root  201  to the blade tip  202  and the metallurgical joint  385  does not extend into all of the plurality of grains  223 . 
         [0070]    As shown in  FIG. 5B , with precise placement of the heat flow control elements  400  as well as the uniquely configured snubber layer  370 , only a fraction  207  of the plurality of grains  223  is subjected to the directed energy beam. This allows the remaining grain structures  223  to remain undisturbed or un-encompassed by the melting of the metal powder. Further, this configuration allows the transfer of the dendrite formation of the blade&#39;s crystalline structure to the snubber under formation with minimal re-orientation of only a portion of the blade&#39;s crystalline structure  220 . 
         [0071]    Initially, a snubber layer first layer  370  (formed of a selected composition of metal powder or a preform composition) having the desired features is positioned around the blade opening  215  along with associated fabrication components  600  (such as support structures, spacers, or voids). The snubber first layer  370  and associated fabrication components  600  are subjected to the energy beam to melt the metal powder forming the snubber first layer  370  and to facilitate heat transfer in the desired direction. This induces the desired grain structure  320  in at least the snubber layer first layer  370  which appropriately corresponds to the grain structure  220  present in the turbine blade  200 . 
         [0072]    As shown in  FIG. 6 , in forming a snubber to correspond to a turbine blade  300  having a single grain thickness  219 , the metal powder of the snubber first layer  370  is placed on the surface of the turbine blade  210 . Optionally, flux may be applied to the metal powder and the blade surface to help control heat transfer. 
         [0073]    As shown in  FIG. 7 , consistent with the system vector settings  405  and the configuration of the heat transfer control elements  400 , the melted powder of the snubber first layer  370  duplicates the grain orientation  220  of the blade and a meniscus  386  may be formed at the metallurgical joint  385  or junction where the melted metal powder of the snubber first layer  370  converges with the blade surface  210 . 
         [0074]    As shown in  FIG. 8 , in forming a snubber to correspond to a turbine blade  300  having a multiple grain  223  thickness, the metal powder of the snubber first layer  370  is placed on the surface of the turbine blade  210 . Optionally, flux may be applied to the metal powder and the blade surface to help control heat transfer. 
         [0075]    As shown in  FIG. 9 , consistent with the system vector settings  405  and the configuration of the heat transfer control elements  400 , the melted powder of the snubber first layer  370  duplicates the grain orientation  220  of the blade at the grains closest to the snubber first layer  370 . As shown, the melting powder may encompass a portion or fraction  207  of the blade grains  223  while leaving the remaining blade grains  223  undisturbed. The system vectors  405  and heat transfer control elements  400  can be configured so that the fraction  207  of the blade grains influenced or encompassed by the melting metal powder reproduces the blade grain pattern of adjacent blade grains  220 / 223  with varying fidelity as desired without encompassing all the grains of the blade. 
         [0076]    For example, in  FIG. 9  the metal powder of the first layer  370  is shown subsumed within the first two blade grains nearest the snubber first layer to form a grain pattern which closely matches the grain pattern of the undisturbed blade grains  223  although not exactly. The unified grain structure formed between the first two blade grains and the snubber first layer  370  may be configured to impart selected mechanical properties as desired. 
         [0077]    Further, as shown in  FIG. 5B , the blade may have equiaxed grains and at least a portion or fraction  207  of the blade grains  223  are reoriented to conform to a grain pattern in common with the melted metal powder of the snubber first layer  370 . 
         [0078]    Additionally a meniscus  386  may be formed at the metallurgical joint  385  or junction where the melted metal powder of the snubber first layer  370  converges with the blade surface  210 . 
         [0079]    As shown in  FIG. 5A  thru  FIG. 9 , through the control of which blade grains are influenced or encompassed by the melting metal powder of the snubber layer  370 , the metallurgical joint  385  can be selected to be less than a full thickness of the blade wall  210 , which preserves the as-cast grain structure  220  the blade wall  210  under the metallurgical joint  385 . 
         [0080]    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.