Abstract:
A method of assembling a stator apparatus includes providing a first vane having an airfoil section located between a first platform section and a second platform section, positioning a first shroud ring adjacent to the first vane, welding the first platform section of the first vane to the first shroud ring relative to a first edge of the of the first platform section of the first vane, and welding the first platform section of the first vane to the first shroud ring relative to a second edge of the of the first platform section. The second edge is located opposite the first edge.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to an airfoil apparatus for gas turbine engines and a method for fabricating such an apparatus. 
   Gas turbine engines typically include a number of airfoil structures that interact with fluids that pass through the engine. Some of those airfoil structures comprise portions of non-rotating stator (or vane) structures. Stator structures are often made from forged components that are installed between a pair of shroud (or casing) rings through brazed connections. Brazing is a convenient and effective technique for joining airfoils to shroud rings to fabricate the stator structure. However, brazing can form a relatively low-strength joint that may not withstand relatively high stresses at or near the braze location. In essence, brazing can produce joints that are not as strong as the forged material of the stator structure. In order to compensate for the lower mechanical properties of brazed connections, stablugs have been added to stator structures. Stablugs are thickened portions of the stator structure that help keep braze materials away from the airfoil, which is thin and typically experiences relatively high stresses during engine operation. Stablugs can take a variety of cross-sectional shapes, including “racetrack” shapes (i.e., having linear side portions and rounded end portions that generally do not match that of the stator structure) as well as “airfoil” shapes that generally correspond to the aerodynamic contour of the airfoil. Regardless of the cross-sectional shape, the stablug must extend into the main gas flowpath of the engine adjacent to the airfoil. For example, the stablug may radially extend 0.127 cm (0.050 inch) proud into the gas flowpath for any given stator, which for low aspect ratio airfoils used in new engine designs can be over 9% of the span of the airfoil into the gas flowpath. 
   The relatively thick stablugs that extend into the gas flowpath of the engine have an undesirable impact on engine performance and efficiency (e.g., measured in terms of pressure loss), especially with stators for which it is desired to have a relatively small span. The stablugs create flow blockage at the endwalls of the stator structure in the gas flowpath. Moreover, the presence of a stablug precludes the inclusion of any stator features at that location, which is at the outer diameter or inner diameter of the airfoil where the stator is attached to the shroud rings. However, other known possibilities present cost, reliability and assembly problems. For instance, simply omitting the stablug can cause the braze joint to incur higher stresses during engine operation. Some stablugs can be partially recessed (though not entirely recessed), but recessing the stablug cannot be accomplished with high solidity stator assemblies (i.e., those with low aspect ratios and high vane counts) and does not completely eliminate the inefficiencies associated with stablug use. 
   BRIEF SUMMARY OF THE INVENTION 
   A method of assembling a stator apparatus includes providing a first vane having an airfoil section located between a first platform section and a second platform section, positioning a first shroud ring adjacent to the first vane, welding the first platform section of the first vane to the first shroud ring relative to a first edge of the of the first platform section of the first vane, and welding the first platform section of the first vane to the first shroud ring relative to a second edge of the of the first platform section. The second edge is located opposite the first edge. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a flow chart of a stator apparatus assembly method according to the present invention. 
       FIG. 2  is an exploded cross-sectional view of a stator apparatus according to the present invention prior to assembly. 
       FIGS. 3 and 4  are cross sectional views of the stator apparatus at different points during assembly. 
       FIG. 5  is a cross-sectional view of the stator apparatus when fully assembled. 
       FIG. 6  is a front view of a portion of the stator apparatus when fully assembled. 
   

   DETAILED DESCRIPTION 
     FIG. 1  is a flow chart of a stator apparatus assembly method, which is suitable for fabricating a high pressure compressor (HPC) stator nozzle or other similar flowpath structures for a gas turbine engine. The steps illustrated in  FIG. 1  are described with reference to physical structures shown in  FIGS. 2-6 . However, it should be recognized that the methods of the present invention are applicable to structures that are different from the embodiments illustrated in  FIGS. 2-6 . 
   As shown in  FIG. 1 , an initial step of the assembly method involves forming subcomponents of the apparatus (step  20 ). The subcomponents can be formed using milling, precision forging, turning and/or other known processes as desired for particular applications. The step of forming the subcomponents (step  20 ) can provide at least some desired final material properties. 
     FIG. 2  is an exploded cross-sectional view of a stator apparatus prior to assembly, showing an outer shroud (or casing) ring  22 , a vane structure  24 , and an inner shroud (or casing) ring  26 . The vane structure  24  and the two shroud rings  22  and  26  are shown in  FIG. 2  in their preliminary configurations after forming (step  20 ), although final shapes of those subcomponents can differ from the preliminary configurations. For example, possible final shapes of the subcomponents are indicated in phantom in  FIG. 2  for reference. 
   As shown in  FIG. 2 , the outer shroud ring  22  includes a radially inner surface  28  (or inner diameter surface) that defines a forward edge  30  and an aft edge  32 . A forward groove  34  and an aft groove  36  are formed along the radially inner surface  28  in a generally circumferential orientation. A forward weld backstrike surface  38  and an aft weld backstrike surface  40  are formed along the forward and aft grooves  34  and  36 , respectively, and each weld backstrike surface  38  and  40  extends radially inward from adjacent portions of the inner surface  28 . A raised portion  41  extends between the weld backstrike surfaces  38  and  40  along the inner surface  28 . The functions of the weld backstrike surfaces  38  and  40  and the grooves  34  and  36  are explained in greater detail below. 
   The vane structure  24  includes an airfoil  42 , an outer platform  44  and an inner platform  46 . The outer and inner platforms  44  and  46  are generally annular structures that define portions of a gas flowpath for an engine. The airfoil  42  extends between the outer and inner platforms  44  and  46 . The outer platform  44  defines an outer surface  48  and the inner platform  46  defines a preliminary inner surface  50 . A circumferential groove  52  is formed in the outer surface  48  of the outer platform  44 . In a preferred embodiment, the vane structure  24  is formed unitarily, although it is possible in alternative embodiments for portions of the vane structure  24  to be non-unitary and attached by suitable means. 
   The inner shroud ring  26  defines a preliminary radially outer surface  54  (or outer diameter surface) and an inward facing region  56 . 
   Turning again to the flow chart of  FIG. 1 , the next step is to fixture all vanes  24  to the outer shroud ring  22  (step  58 ). Typically a plurality of circumferentially adjacent vane structures  24  are used to form an annular stator assembly for a gas turbine engine. At step  58 , all of the vane structures  24  are positioned adjacent to each other within the outer shroud ring  22  and secured in place using appropriate fixtures. 
     FIG. 3  is a cross sectional view of the vane structure  24  fixtured to the outer shroud ring  22  by an exemplary fixture  60 . Once fixtured, the vane structure  24  and the outer shroud ring  22  are welded together. Welding can be performed using conventional electron beam (EB) welding techniques. A first weld is formed at the aft edge  32  of the outer shroud ring  22  (step  64 ), and then a second weld is formed at the forward edge  30  of the outer shroud ring  22  (step  66 ) after the components are repositioned on the fixture  60 . The first weld extends along the interface between the inner surface  28  of the outer shroud ring  22  and the outer surface  48  of the outer platform  44  of the vane structure  24 , and extends from the aft edge  32  to the groove  36 . Likewise, the second weld extends along the interface between the inner surface  28  of the outer shroud ring  22  and the outer surface  48  of the outer platform  44  of the vane structure  24 , and extends from the forward edge  30  to the groove  34 . 
   The EB weld beam penetrates the areas of the first and second welds to the cavity formed by the grooves  34 , 36  and  52 , which are configured such that first and second welds do not bridge the grooves  34  and  36 . The weld backstrike surfaces  38  and  40  and the raised portion  41  of the outer shroud ring  22  are configured to help separate the first and second welds, and to limit deeper progress of the EB weld beam. This arrangement helps prevent the EB weld beam from stopping within the joint regions where the first and second welds are formed, which would be undesirable for weld integrity. The first and second welds can be formed by fixing the EB weld beam and rotating the components being welded such that welding is performed in a circumferential manner relative to substantially the entire inner diameter of the outer shroud ring  22 . 
   Next, wax is applied to the flowpath, that is, wax is applied between the airfoils  42  of adjacent vane structures  24  welded to the outer shroud ring  22  (step  68 ). Tape can be applied at flowpath joints between adjacent vane structures  24  prior to applying the wax, in order to help contain the wax. The wax gives the airfoils  42  additional rigidity during subsequent assembly processes. 
   Once wax has been applied at step  68 , the inner platform  46  of the vane structure  24  is machined to a final dimension (step  70 ). 
   Material is removed at the preliminary inner surface  50  of the inner platform  46  to form a finished inner surface  50 ′. Then the finished inner surface  50 ′ of the inner platform  46  is nickel flashed (i.e., nickel plated) to prepare it for brazing (step  72 ). 
   At this point, a honeycomb seal  74  can optionally be attached to the inward facing region  56  of the inner shroud ring  26  (step  76 ) (e.g., using a nickel braze). Next, the inner shroud ring  26  is machined to remove material from the preliminary outer surface  54  and define a finished outer surface  54 ′ (step  78 ). Then the finished outer surface  54 ′ of the inner shroud ring  26  is nickel flashed (i.e., nickel plated) to prepare it for brazing (step  80 ). Following nickel plating (step  80 ), the wax (and any tape) is removed from the flowpath (step  82 ), which can be accomplished by heating the wax to melt it away. The optional honeycomb seal  74  can be masked while performing nickel plating and/or brazing processes. 
   Next, the inner shroud ring  26  is slid into position inside the vane structures (step  84 ). Step  84  may require heating radially outer components (e.g., the vane structures) and/or cooling radially inner components (e.g., the inner shroud ring) in order to slide the inner shroud ring  26  into position while providing a close fit and easing assembly. Gaps between the inner shroud ring  26  and the vane structures  24  can be verified prior to brazing to assure proper fit. In order to facilitate later assembly steps, a braze foil can be tack welded to the finished outer surface  54 ′ of the inner shroud ring  26  when the ring  26  is slid into position inside the vane structures  24 . 
     FIG. 4  is a cross sectional view of the stator apparatus fixtured to the fixture  60  following assembly step  84 . The next assembly step is to braze (e.g., using a gold-nickel braze material) the inner shroud ring  26  to the vane structure  24  in order to secure those components to each other (step  86 ). The brazing process can be conducted in a conventional manner along substantially the entire interface between the finished outer surface  54 ′ of the inner shroud ring  26  and the finished inner surface  50 ′ of the inner platform  46  of the vane structure  24 . Brazing is performed circumferentially to secure all of the vane structures  24  to the inner shroud ring  26 . Brazing can be performed with the forward edge  30  facing downward in the fixture  60 . 
   After the inner shroud ring  26  has been secured to the vane structure  24  (step  86 ), sealant is applied to any circumferential gaps between adjacent vane structures  24  (step  87 ). The sealant helps to reduce undesired leakage and flow recirculation, and generally forms a non-structural bond between the adjacent vane structures  24  The sealant can be a ceramic cement, a metallic braze, a high-temperature epoxy sealant, or other suitable material. 
   Next, the outer and inner shroud rings  22  and  26  are machined to desired finished dimensions (step  88 ). At this step, the optional honeycomb seal  74  can also be ground to finished dimensions. Any further finishing steps can also be performed at this step, as desired for particular applications. For example, a timing notch can be formed in the inner shroud ring  26  through milling or electric discharge machining (EDM). Moreover, the shroud rings  22  and  26  can be optimally segmented as desired. 
     FIGS. 5 and 6  illustrate a fully assembled stator apparatus  90 . 
     FIG. 5  is a cross-sectional view and  FIG. 6  is a front view of a portion of the stator apparatus  90 . As shown in  FIG. 6 , a plurality of vane structures  24  (e.g., 100 or more vane structures) are arranged adjacent to each other in an annular configuration relative to an engine centerline CL between the outer shroud ring  22  and the inner shroud ring  26 . A sealant  92  is illustrated between adjacent vane structures  24  at both outer and inner platform  44  and  46  locations. 
   It should be recognized that the apparatus and method of the present invention provide a number of advantages. For example, a stator apparatus according to the present invention avoid the need for a stablug. Stablugs have been determined to cause a 1% loss in pressure over a non-stablug design according to the present invention for some applications, although total pressure loss will vary with the span of the airfoils of the stator apparatus and will generally be greater with relatively small span dimensions. Furthermore, the present invention provides a relatively easy and reliable assembly method that does not degrade forged material properties in the airfoils during welding or brazing processes. Subcomponents can be forged, and the assembly method can preserve forged properties. 
   Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention. The particular shape and configuration of the stator assembly can vary as desired for particular applications, for instance, the present invention applies to cantilevered stators secured only at either an outer or inner shroud ring. Moreover, the particular assembly steps involved and the order in which those steps are performed can also vary as desired for particular applications. For instance, welding can be used at the outer shroud ring and brazing at the inner shroud ring or vice-versa.