Patent Abstract:
A method, including: providing a layer of powder material ( 106 ) on a substrate ( 12 ) having protruding rib material ( 26 ); and traversing an energy beam ( 100 ) across the layer of powder material to form a cladding layer ( 10 ) around and bonded to the protruding rib material, wherein the cladding layer defines a layer of an airfoil skin ( 94 ).

Full Description:
FIELD OF THE INVENTION 
     The invention relates to building up an airfoil by depositing cladding layers using an energy beam and control optics. In particular, the invention relates to encircling protruding rib material by the cladding layers. 
     BACKGROUND OF THE INVENTION 
     Blades used in the turbine section of gas turbine engines are exposed to combustion gases, high mechanical force, and foreign object impact. This, coupled with the high operating temperature, create high levels of stress in the blade. Blade tips, blade airfoil sections, and blade platforms are particularly susceptible to stress related damages, including areas of wear and cracks. Blade tips, (also known as tip caps), include blade tip shelves (an end piece of the airfoil) and blade squealers (elevated material surrounding the blade tip). The cracks may extend from the tip of the airfoil downward toward the platform, sometimes extending past the blade shelf adjacent the blade tip. 
     It is known to replace worn or cracked blade squealers with non-structural replacement material. This replacement material is considered non-structural primarily because the stresses are relatively low in this location, and as a result, consequences of damage are relatively minimal in terms of performance. Unfortunately, cracking is very often found below (toward the platform) the tip shelf, extending into the airfoil body. For example, the cracks may extend 30 mm below the blade tip. Replacement of this material (below the squealer) is more difficult and must be considered to be of a more structural requirement, wherein certain minimum mechanical properties must be attained in order to sustain the greater stresses encountered in the airfoil body. 
     For the most difficult to weld superalloys, there is no known process to replace such extensive portions of a turbine blade. Grinding out and re-welding cracks using a hot box to maximize material ductility during the process has met with limited success. Cutting off the entire distressed blade tip and welding is not possible for at least two reasons. First, the material itself does not accommodate butt welding. It would crack due to shrinkage stresses and high restraint. Second, ribs disposed within the airfoil (serving structural function and cooling air management) could not be accessed for butt welding. Consequently, there is room in the art for improved methods of building and/or repairing blade airfoils. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is explained in the following description in view of the drawings that show: 
         FIG. 1  schematically shows an exemplary embodiment of paths followed by an energy beam when forming an exemplary embodiment of a cladding layer, where the paths are superimposed on the cladding layer and the cladding layer is disposed on an exemplary embodiment of a substrate. 
         FIG. 2  is a schematic perspective view of the cladding layer of  FIG. 1  being formed on the substrate toward a beginning of the formation process. 
         FIG. 3  is a schematic sectional view along A-A of  FIG. 2  after several cladding layers have been formed. 
         FIG. 4  schematically shows exemplary embodiments of patterns followed by the energy beam when forming the cladding layer adjacent ribs, where the patterns are superimposed on the cladding layer and the cladding layer is disposed on the substrate. 
         FIGS. 5-6  schematically show exemplary embodiments of patterns followed by the energy beam while forming exemplary embodiments of a blade tip shelf. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present inventors have devised a method of building up an airfoil section of a gas turbine component having an airfoil skin structurally supported by internal ribs, such as a turbine blade. This is accomplished by forming layers of cladding on a bonding surface of a substrate using scanning optics to a melt powder placed on the bonding surface. The cladding layers are deposited around existing rib material that protrudes above the bonding surface, and the cladding layer bonds to the bonding surface and the protruding rib material. The scanning optics generate two melt pools that simultaneously travel along different paths on opposite sides of the protruding rib material to form each cladding layer. Each cladding layer forms a layer of the airfoil, includes side sections, and may include at least one rib section to create a rib where there is no protruding rib material already present. Being able to form an airfoil skin around existing rib sections enables building up of new airfoil sections and repair of existing airfoil sections in a manner not previously possible. With respect to worn airfoils, the inventors have recognized that the ribs are rarely distressed and so it may be most efficient to leave them in place for a repair using the disclosed methods. 
     In an exemplary embodiment where the substrate is a superalloy, the powder material may include a superalloy metal powder and a flux as described in U.S. patent publication number 2013/0140278 to Bruck et al. and incorporated in its entirety by reference herein. The ability to clad superalloys in this manner, together with the advanced scanning optics now available (e.g. Cambridge Technology Lightning II 4 kW, Scanlab powerSCAN 4 kW, Trumpf PFO 3D 8 kw and IPG 8 kW), and the deposition pattern disclosed herein enables buildup and repair of superalloy components that was not previously possible. 
       FIG. 1  schematically shows an exemplary embodiment of paths followed by an energy beam when forming an exemplary embodiment of a cladding layer  10 , where the paths are superimposed on the cladding layer  10  and the cladding layer  10  is disposed on an exemplary embodiment of a substrate  12 . The cladding layer  10  may be in the shape of an airfoil and have a skin  14  having a pressure side wall  16 , a suction side wall  18 , a leading edge  20 , a trailing edge  22 , and optionally an additional rib section  24 . The additional rib section  24  represents a rib that is to be formed in addition to the already-existing ribs. The substrate  12  includes a bonding surface (not visible) under the cladding layer  10 . Rib material  26  protrudes (out of the page) above the bonding surface and each instance of the rib material  26  represents some or all of a rib  28  around which the cladding layer  10  is deposited and to which the cladding layer  10  is bonded. In the exemplary embodiment shown, the substrate  12  includes first rib material  30  and second rib material  32 . There may be any number of ribs  28 , and there may also be any number of additional rib sections  24 . 
     The first rib material  30  is not tapered toward a tip end  40 . Consequently, the first rib material  30  represents essentially a full rib, less any fillets etc associated with bonding an untapered side surface  42  of the first rib material  30  to the pressure side wall  16 . The second rib material  32  is tapered toward a tip end  40 . Consequently, the second rib material  32  represents less than a full rib. With tapered rib material, the cladding layer  10  fills in the portion of the second rib material  32  lost to the taper and bonds to a tapered side surface  44  of the second rib material  32 . Thus, whether the rib  28  includes the untapered side surface  42  or the tapered side surface  44 , it is bonded to an inner perimeter  46  of the cladding layer  10 . 
     In an exemplary embodiment without an additional rib section  24 , the pressure side wall  16  and the suction side wall  18  may be formed by an energy beam guided by scanning options to form a first path  50  and a second path  52  along which respective melt pools travel. The first path  50  may start at a first path initiation point  54  and traverse a first wall, e.g. the suction side wall  18 , until reaching a first path termination point  56 . The second path  52  may start at a second path initiation point  58  and traverse a second wall, e.g. the pressure side wall  16 , until reaching a second path termination point  60 . The first path initiation point  54  and the second path initiation point  58  may be disposed at a common initiation point  62 . The first path termination point  56  and the second path termination point  60  may be disposed at a common termination point  64 . There may be an optional runon  66  formed at any of the initiation points, such as at the common initiation point  62 . Likewise, there may be an optional runoff  68  formed at any of the termination points, such as the common termination point  64 . 
     The location of the common initiation point  62  may be selected so that a length of the first path  50  and a length of the second path  52  are the same. In such an exemplary embodiment the scanning optics may be configured to traverse the energy beam along each path at the same rate, thereby taking the same amount of time for the energy beam to traverse the first path  50  as the second path  52  (i.e. the same duration). Alternately, the first path  50  and the second path  52  may be of different lengths. In this case it may take more time to form the longer path if the energy beam traverses each path at the same rate. When the two paths are of differing length but the traversal duration is desired to be the same for each path, the scanning of the energy beam can be still adjusted so that it traverses each path in the same amount of time. For example, if the first path  50  is twenty five percent longer than the second path  52  (e.g. 125 and 100 mm respectively), then the energy beam may spend twenty five percent more time forming the first path  50  as the second path  52  (e.g. 60 and 48 seconds respectively), while traversing each path at the same traversal duration (e.g. ˜2.1 mm/sec for e.g. total process time of 108 seconds). This is made possible because the melt pool of the shorter path will remain liquefied long enough to permit the energy beam to spend more time forming the longer path, even if the power output of the energy beam is the same when forming each path. 
     When forming the cladding layer  10  a first melt pool (not shown) would follow the first path  50  and a second melt pool (not shown) would follow the second path  52 . If one of the melt pools were to be initiated and the powder material at the common initiation point melted and then solidified before the other melt pool was initiated, then the solidified material at the common initiation point  62  would be remelted by the melt pool that initiated later in time. This remelting (remelt) can be avoided by starting both melt pools at the same time, or close enough in time that only one melt pool (not shown) is formed at the common initiation point  62 . Avoiding remelt reduces the possibility for cracking and creates a stronger cladding layer. Likewise, the melt pool that traverses the first path  50  may be timed to meet with the melt pool that traverses the second path  52  such that they unite into a single melt pool at the common termination point  64 , which avoids remelt at the common termination point  64 . An optional runoff  68  may be positioned at the common termination point  64  and one or more melt pools may be extended off of the part at the runoff  68 . Forming opposite wall sections simultaneously mitigates airfoil warping, and having a continuous, uninterrupted traversal minimizes remelts, which improves the structural integrity of the cladding layer  10 . 
     When a melt pool approaches the untapered side surface  42  of the first rib material  26  the energy beam and/or scanning optics may change one or more operating parameters to ensure the cladding layer  10  bonds well at a junction  70  between the rib and the cladding layer  10 . For example, a traversal rate of the energy beam may be slowed or power level of the energy beam may be increased to account for additional, localized heat sinking due to the amount of material at the junction  70 . When a melt pool approaches the tapered side surface  44  the energy beam and/or scanning optics may likewise change one or more operating parameters to ensure the cladding layer  10  bonds well at a junction  70  between the rib and the cladding layer  10 . In addition, the path may be widened to ensure the cladding layer  10  reaches the tapered side surface  44 , as is discussed further below. 
     For portions of the side walls away from the junctions  70 , the power output of the energy beam may be the same for the paths made to form the cladding layer  10 . Alternately, the power output may vary. Still further, the power may be adjusted while the energy beam is traversing a path to accommodate varying heat requirements, such as the width (wall thickness) required for the airfoil. 
     In an exemplary embodiment with an additional rib section  24 , one of the paths may be varied to form the additional rib section  24 , while the other path may remain unchanged. For example, the first path  50  may remain unchanged, while the second path  52  may be changed to include the additional rib section  24 . In such an exemplary embodiment, the second path would again start from the second path initiation point  58 , which may be the common initiation point  62 , and would end at the second path termination point  60 , which may be the common termination point  64 . However, while traversing the pressure side wall  16 , the energy beam may cause the melt pool to leave the pressure side wall  16  temporarily to form the additional rib section  24 . After forming the additional rib section  24  the energy beam would cause a new melt pool to form on the pressure side wall  16  at a secondary initiation point  72  and traverse the new melt pool to the second path termination point  60 . The melt pool that forms the additional rib section  24  may be timed to arrive at a junction of the additional rib section  24  and the suction side wall  18  at the same time. This would avoid remelt at this location. It is possible that the cladding material on the pressure side wall  16  adjacent the secondary initiation point  72  and already processed by the energy beam may have solidified. Consequently, it is possible that there may be some remelt at the secondary initiation point  72  when the new melt pool is formed 
     Alternately, upon reaching the additional rib section, the energy beam  100  could be shared essentially simultaneously along three paths. In such an exemplary embodiment three melt pools could exist simultaneously. The melt pool traveling along the pressure side wall  16  could split such that one melt pool would continue along the pressure side wall  16  while another melt pool would continue along the additional rib section and meet a third melt pool at the suction side wall  18 , at which point a single melt pool would continue along the suction side wall  18 . The traversal rate of the melt pool traversing the pressure side wall  16  and the traversal rate of the melt pool traversing the suction side wall  18  could be adjusted independently so the two arrive at the common termination point  64  simultaneously. In this exemplary embodiment remelt could be avoided altogether. 
     If the cladding process generates a layer of slag on the cladding layer it may be removed as the powder material is solidified, or at the completion of the formation of the cladding layer  10 . 
     One or more cladding layers  10  may be deposited on a substrate to create or rebuild an airfoil, in which case the above process may be repeated to form as many cladding layers  10  as are necessary. 
       FIG. 2  is a schematic side view of the cladding layer  10  being formed on the substrate  12  toward a beginning of the formation process. In this exemplary embodiment the substrate  12  is an airfoil  80  having an airfoil pressure side  82 , an airfoil suction side  84 , an airfoil leading edge  86 , an airfoil trailing edge  88 , and a bonding surface  90 , which is, in this exemplary embodiment, an edge  92  of an airfoil skin  94 . An energy beam  100  emanating from an energy beam source  102  and guided by scanning optics  104  is processing powder material  106  placed on the bonding surface  90 . It can be seen that the scanning optics  104  are able to direct the energy beam  100  toward one side of the cladding layer  10  as indicated by a solid energy beam line, and then to the other side of the cladding layer, as indicated by the dotted line. The scanning optics are capable of jumping the beam from one side to the other at a jump rate of approximately 3 m/s. Consequently, two melt pools can be sustained and traversed simultaneously. During the process the powder material  106  melts, solidifies, and bonds to the bonding surface  90  to form the cladding layer  10 . 
     In an exemplary embodiment where a flux powder is incorporated into the powder material  106  a slag  108  may form on the cladding layer  10 , which is removed before any subsequent cladding layers are deposited. In alternate exemplary embodiments the filler and flux could be preplaced in a distinct preform such as encapsulated in a sleeve that is then positioned at the process location. The filler material in the powder material may have the same chemical composition as the substrate or it may be different. 
     A dotted line defines a finished profile  110  of an unfinished portion  112  of the airfoil  80  when sufficient cladding layers  10  are deposited to complete the airfoil  80 . (Ribs are not externally visible in a finished airfoil.) The finished profile  110  may represent an airfoil  80  that is being created for the first time, or it may indicate airfoil skin  94  that was previously part of the airfoil  80  but which was removed and which must be replaced to return the airfoil to its original condition. The latter may occur, for example, when an airfoil  80  that has been in service experiences cracking at a tip  114  of the airfoil  80 . The airfoil  80  may be pulled from service and a tip end  116  of the airfoil skin  94  and the unwanted cracks therein are removed, but at least a portion of at least one of the ribs  28  remains, to permit the cladding repair operation disclosed herein. Thus, airfoil skin  94  may be removed to expose underlying rib material. If the airfoil skin  94  is removed from both the airfoil pressure side  82  and the airfoil suction side  84 , the underlying rib material may have spanned (connected to) both sides of the removed airfoil skin  94 . 
     The protruding rib material may or may not protrude all the way to the tip  114  of the airfoil  80 . For example, there may be some rib material remaining all the way to an end of the rib  28  at the tip  114  of the airfoil  80 . Alternately, some of the rib material at the tip  114  may be removed, but some protruding rib material may be left. In a non limiting example, 30 mm may be removed and cladding layers of 3 mm thickness may be formed until the 30 mm section is rebuilt. When ten layers are deposited the airfoil  80  would be returned to a finished state. An outer surface of the airfoil  80  may require finish machining. An inner surface may be accepted as is. 
       FIG. 3  is a schematic sectional view along A-A of  FIG. 2  after several cladding layers  10  have been deposited. The bonding surface  90  for a first cladding layer  130  is defined by the substrate  12  after the removal of material but before any cladding layers  10  are deposited. Once a cladding layer  10  is bonded to the substrate  12  the deposited cladding layer  10  becomes part of the substrate  12  from the perspective of the next cladding layer  10 . Consequently, the bonding surface  90  for a subsequent bonding layer is a top  132  of an immediately prior cladding layer  10 . This process repeats for each cladding layer  10   
     The junction  70  of the rib  28  and the airfoil skin  94  is oriented essentially toward a top of the page, while the tapered side surface  44  forms a taper angle  136  with the tapered side surface  44 . As a result, a taper gap  138  forms at an upper surface  140  of each layer between the junction  70  and the tapered side surface  44 . To accommodate this, the cladding layer  10  may be widened to bridge the taper gap  138  so the cladding layer  10  can bond to the tapered side surface  44 . For example, a bonding surface  90  for a second cladding layer  134  would be the top  132  of the first cladding layer  130 , which includes the edge  92  of the airfoil skin  94  plus the taper gap  138  for the first cladding layer  130 . Thus, where adjacent the rib  28 , the bonding surface  90  includes the airfoil skin  94  plus the taper gap  138  of the immediately prior cladding layer  10 . Still using the second cladding layer  134  as the example, the widening of the energy beam for each layer may take into account the increased surface area of the instant bond surface  90 , as well as the taper angle  136  within the instant bond layer, to ensure proper bonding of the cladding layer  10  to the tapered side surface  44  at both the upper surface  140  at a lower surface  142  of each cladding layer  10 . While a constant taper angle  136  is shown, the taper angle  136  may vary at one or more cladding layers  10 . The taper gap  138  may be filled in by the cladding layer  10  in a manner that creates any geometry desired, such as a stress reducing fillet, or other such feature. 
     In an embodiment the taper angle  136  may be selected to cooperate with a positioning of the energy beam source  102  and scanning optics  104  such that both sides of one rib  28  may be accessed by the energy beam  100  without having to translate the energy beam source  102 . In other words, energy beam source  102  and scanning optics  104  may be positioned such that the energy beam  100  can jump to both sides of the rib  28  through the scanning optics alone and still have line-of-site access to the areas adjacent both tapered side surfaces  44  of the rib  28 . This arrangement enables the energy beam  100  to move both melt pools past the rib  28  simultaneously and uninterrupted, while forming the proper bonds at the junctions  70 . 
     The taper angle  136  may be selected to create an angle of incidence  144  between the tapered side surface  44  and the energy beam  100 . This is effective to impart more heat to the tapered side surface  44  which, in turn, improves a bond between the tapered side surface  44  and the cladding layer  10 . The angle of incidence  144  may be the same for both sides of the rib, or it may be different, depending on the local requirements. 
     While  FIGS. 2 and 3  shown the energy beam  100  processing adjacent a tapered side surface, it may still be possible to fuse the cladding layer  10  to the untapered side surface  42  when the angle of incidence  144  is zero. (I.e. when there is no direct impingement of the energy beam  100  on the untapered side surface  42 ). In this case the local plasma and available superheat contained in the molten pool may be sufficient to achieve such lateral melting and fusion. Consequently, the cladding layer  10  may be bonded to all instances of adjacent rib side surfaces. 
       FIG. 4  schematically shows an exemplary embodiment of patterns followed by the energy beam  100  when forming the cladding layer  10 . In this view the patterns are superimposed on the cladding layer  10 , which rests on the substrate  12 . In this exemplary embodiment the energy beam is guided in a circular pattern  150 . A thickness  152  of the airfoil skin  94 , and hence the cladding layer  10  may be 3.0 mm. A diameter  154  of the circular pattern  150  may be 3.5-4.0 mm and adjacent circular patterns  150  may overlap by approximately 1 mm as the energy beam traverses the second path  52 . The energy beam may have, for example, a 1 mm diameter. In this exemplary embodiment the first rib material  30  is not tapered. Consequently, the circular pattern  150  need not increase in diameter when adjacent the first rib material  30  to ensure the cladding layer  10  bonds to the untapered side surface  42 . In contrast, the second rib material  32  is tapered. The scanning optics  104  may adjust from the circular pattern  150  to a more oval pattern  156  when the pattern is adjacent the second rib material  32  in order to ensure the cladding layer  10  bonds to the tapered side surface  44 . In a non limiting exemplary embodiment there may be a first oval pattern  156  with long sides  158  separated from each other by 2 mm adjacent an overlapping second oval pattern  160  with long sides  158  likewise separated from each other by 2 mm. The result is a near uniform coverage of the bonding surface  90 , which includes the edge  92  of the airfoil skin  94  and the taper gap  138 . 
     To form the additional rib section  24 , the pattern  150  may be moved from the pressure side wall  16  to the suction side wall  18  (or the opposite direction, depending on the path chosen). Alternately, when the energy beam reaches the additional rib section the same widening of the energy beam that occurs in  FIG. 4  may also occur, but where the pattern widens such that the long sides  158  span the pressure side wall  16  and the suction side wall  18  to form the entire additional rib section  24 . This may require significant power, for example, 8-10 kW, but may expedite production where possible. Here again the exemplary embodiment is not meant to be limiting. The exact patterning may be tailored in ways known to those of ordinary skill in the art. For example, the energy beam could travel in a straight line back and forth between the pressure side wall  16  and the suction side wall  18 , advancing one beam diameter after each pass. 
       FIG. 5  schematically shows an exemplary embodiment of a pattern followed by the energy beam while forming an exemplary embodiment of a tip cap  170  of the airfoil  80 , which may be necessary to complete the airfoil  80 . An interior of the airfoil  80  may be filled with a ceramic material (e.g. zirconia, silica, alumina, titania, graphite, dry ice etc) in powder or solid form and the ceramic material may be positioned to surround an exterior of the airfoil  80 . The powder material  106  is positioned on the ceramic material that fills the airfoil  80 . In an exemplary embodiment the energy beam traverses a circular pattern  150  back and forth between the airfoil pressure side  82  and the airfoil suction side  84 . Once the tip cap  170  is complete the ceramic material may be removed, leaving a completed airfoil  80 . This exemplary embodiment is not meant to be limiting. The exact patterning may be tailored in ways known to those of ordinary skill in the art. 
     In a variation shown in  FIG. 6 , the energy beam may form the tip cap  170  in a different manner. Instead of forming distinct lateral deposits, the energy beam may be widened so that the melt pool travels from the airfoil leading edge  86  to the airfoil trailing edge  88 . This may require significant power, for example, 8-10 kW, but may expedite production where possible. This exemplary embodiment is not meant to be limiting and other patterns may be used, such as a pattern similar to the overlapping, wide oval patterns that span from the airfoil pressure side  82  to the airfoil suction side  84  disclosed above. 
     From the foregoing it can be seen that the inventors have devised an innovative method for building up an airfoil in a manner not previously possible. Consequently, this represents an improvement in the art. 
     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.

Technology Classification (CPC): 8