Abstract:
A method of producing a puck for coating a turbine shroud includes: providing a mixture of a metallic powder and a binder; melting the mixture and forming the mixture into a preform having a shape conforming to the flowpath surface of the shroud; removing a majority of the binder from the preform; and heating the preform with microwave energy to remove the remainder of the binder and to sinter the metal powder together to form the puck. A turbine shroud may be repaired by bonding the puck to its flowpath surface.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This invention relates generally to sintered metallic components and more particularly to turbine shrouds coated by metal injection molding.  
         [0002]     A gas turbine engine includes one or more turbine stages having a plurality of airfoil-shaped blades attached to a rotating disk. The blades extract energy from a stream of heated combustion gases and transfer it to the disk, which is in turn connected to a shaft for powering a fan, compressor, or external load. An annular shroud encircles the tips of the turbine blades to define the outer boundary of the flowpath through each stage. The clearance between the blade tips and the shroud is minimized to prevent the leakage past the tips of the blades and maximize efficiency. The flowpath surface of the shroud is made abradable or “rub-tolerant” so that the tip of the blade can cut into it during operation. This cutting process may be permitted to occur intentionally to allow the blade to form a matched interface with the shroud, or it may simply occur through during engine operation if the provided radial clearance is exceeded.  
         [0003]     Over time the flowpath surfaces of the shrouds wear down from blade rubbing, hot gas erosion, and high-temperature corrosion. Because of the high cost of the shroud materials, they are typically repaired by restoring them to their original dimensions. One known method for this restoration is the use of thermally densified coatings (TDC). The TDC process utilizes thin “pucks”, made from compressed metallic powders, which are brazed to the shroud. These pucks have a density of approximately 70% which tends to result in distorting or “cupping” of the shroud during cooling from the braze step. In addition, the low-melt braze has a tendency to run excessively. This requires substantial hand blending work and increases the probability of having to scrap the shroud.  
         [0004]     Accordingly, there is a need for a method of coating shrouds which produces a dense flowpath surface.  
       BRIEF SUMMARY OF THE INVENTION  
       [0005]     The above-mentioned need is met by the present invention, which according to one aspect provides a method of producing a puck for coating a turbine shroud, including: providing a mixture of a metallic powder and a binder; melting the binder and forming the mixture into a preform having a preselected shape conforming to a flowpath surface of the shroud; removing a majority of the binder from the preform; and heating the preform to remove the remainder of the binder and to sinter the metal powder together to form the puck.  
         [0006]     According to another aspect of the invention, a method of repairing a turbine shroud includes: providing a turbine shroud having a flowpath surface; providing a mixture of a metallic powder and a binder; melting the binder and forming the mixture into a preform having a preselected shape conforming to a flowpath surface of the shroud; removing a majority of the binder from the preform; heating the preform to remove the remainder of the binder and to sinter the metal powder together into a puck; and bonding the preform to the flowpath surface. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]     The invention may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which:  
         [0008]      FIG. 1  is a cross-sectional view of a shroud assembly, showing a shroud segment surrounding the tip of a turbine blade, the shroud support, the shroud hanger;  
         [0009]      FIG. 2  is a perspective view of the turbine shroud of  FIG. 1 ;  
         [0010]      FIG. 3  is block diagram of a manufacturing process carried out in accordance with the present invention;  
         [0011]      FIG. 4  is a schematic side view of an injection molding apparatus;  
         [0012]      FIG. 5  is a schematic side view of a preform being removed from the mold shown in  FIG. 4 ;  
         [0013]      FIG. 6  is a schematic cross-sectional view of a preform inside a sintering chamber; and  
         [0014]      FIG. 7  is schematic perspective view of a puck being attached to a shroud segment. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]     Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views,  FIG. 1  illustrates in cross-section a portion of a high-pressure turbine (HPT) of a gas turbine engine, including a casing  10  and a shroud hanger  12  which supports a shroud segment  14 . A plurality of the arcuate shroud segments  14  are arranged circumferentially in an annular array so as to closely surround the turbine blades  16  and thereby define the outer radial flowpath boundary for the hot combustion gases flowing through the turbine stage.  
         [0016]     The shroud segments  14 , or at least the flowpath surfaces  18  thereof, are comprised of a “rub-compliant material”, in the sense that contact with the blade tip  20  will cause wear to the flowpath surface  18  rather than the blade  16 . During engine operation, the clearance “C” between the blade  16  and flowpath surface  18  will gradually increase as the flowpath surface  18  wears away through blade rubs, mechanical erosion, oxidation, and corrosion.  
         [0017]      FIG. 2  shows one of the shroud segments  14  in more detail. The shroud segment  14  is generally arcuate with a flowpath surface  18 , an opposed interior surface  22 , a forward flange  24  defining an axially-facing leading edge  26 , an aft flange  28  defining an axially-facing trailing edge  30 , and opposed left and right sidewalls  32  and  34 . The sidewalls  32  and  34  may have seal slots  36  formed therein for receiving end seals of a known type (not shown) to prevent leakage between adjacent shroud segments  14 . The shroud segment  14  includes an outwardly-extending forward wall  38  and an outwardly-extending aft wall  40 . The forward wall  38 , aft wall  40 , sidewalls  32  and  34 , and interior surface  22  cooperate to form an open shroud plenum  42 . A forward mounting flange  44  extends from the forward wall  38 , and an aft mounting flange  46  extends from the aft wall  40 .  
         [0018]     The shroud segment  14  may be formed as a one-piece casting of a suitable superalloy, such as a nickel-based superalloy, which has acceptable strength at the elevated temperatures of operation in a gas turbine engine. At least the flowpath surface  18  of the shroud segment  14  is formed by a sacrificial or rub-compliant material. When the shroud segment  14  is excessively worn, its flowpath surface  18  may be restored to the correct dimensions by attaching a pre-formed coating member or “puck”  48 , shown in  FIG. 6 , thereto.  
         [0019]      FIG. 3  depicts the process for constructing the puck  48  according to the method of the present invention. Initially, as shown in block  50 , a metallic powder and a suitable binder are provided. The metallic powder may be a single alloy or it may be a mechanical mixture of more than one alloy. For optimum performance in the injection molding process and also for compatibility with the optional microwave heating step described below, the particle size of the metallic powder should be about 100 micrometers or less.  
         [0020]     An example of a known suitable metallic power is a combination of two nickel-based powders, where one of the powders contains a melting point depressant such as boron or silicon. Such compositions are described in U.S. Pat. No. 5,561,827 issued to Reeves et al. and assigned to the assignee of the present invention. One suitable alloy for the high-melt powder is a superalloy composition known as Rene 195, described in U.S. Pat. No. 6,565,680 issued to Jackson et al. and assigned to the assignee of the present invention. Rene 195 has a nominal composition, in weight percent, of up to about 5.1 wt % Co; about 7.2 to about 9.5 wt % Cr; about 7.4 to about 8.4 wt % Al; about 4.3 to about 5.6 wt % Ta; about 0.1 to about 0.5 wt % Si; about 0.1 to about 0.5 wt % Hf; up to about 0.05 wt % C; up to about 0.05 wt % B; about 0 to about 2.2 Re; about 2.7 to about 4.4 wt % W; and the balance Ni and typical impurities. In any case, the metallic powder must be suitable for the intended engine operating conditions and compatible with the base material of the shroud segment  14 .  
         [0021]     The binder may be any material which is chemically compatible with the metallic powder and which allows the required processing (e.g. mixing, injection, solidification, and leaching). Examples of known suitable binders include waxes and polymer resins. The binder may be provided in a powder form.  
         [0022]     The binder and the metallic powder are thoroughly mixed together, as shown in block  52 . The mixture is then heated to melt the binder and create a fluid with the metallic powder coated by the binder (block  54 ). Next, the mixture is formed into a predetermined shape at block  56 . One way of forming the mixture is to use a known injection-molding apparatus. A schematic view of an injection molding apparatus  58  including a hopper  60  and an extruder  62  with rotating screw  64  is shown in  FIG. 4 . The mixture is extruded into the cavity  66  of a mold  68 . The mold  68  may optionally be heated to avoid excessively rapid solidification of the binder which would result in a brittle preform  70 . Instead of melting the binder in a discrete batch, the mixture could be molded in a continuous manner using known injection molding equipment capable of melting the binder as it passes through the screw  64 . Once the mixture has solidified, the mold  68  is opened as shown in  FIG. 5  and the resulting uncompacted or “green” preform  70  is removed (see block  72  in  FIG. 3 ).  
         [0023]     The preform  70  comprises metal particles suspended in the solidified binder. The preform  70  is not suitable for use as a finished component, but merely has sufficient mechanical strength to undergo further processing. At block  74  of  FIG. 3 , the preform  48  is leached to remove the majority of the binder. This may be done by submerging or washing the preform  48  with a suitable solvent which dissolves the binder but does not attack the metallic powder.  
         [0024]     Next, at block  76 , the preform  70  is sintered. As shown in  FIG. 6 , the preform  70  is placed in a chamber  78  which includes means for creating a suitable atmosphere to prevent undesired oxidation of the preform  70  or other reactions during the sintering process. In the illustrated example a supply  80  of inert gas such as argon is connected to the interior of the chamber  78 . The sintering could also be performed under a vacuum. The preform  70  is heated in a known manner, for example with a resistance heater  82 , to a temperature below the liquidus temperature of the metallic powder and high enough to cause the metallic powder particles to fuse together and consolidate. The high temperature also evaporates and drives out any remaining binder. The preform  70  is held at the desired temperature for a selected time period long enough to result in a consolidated puck  48  (see  FIG. 7 ).  
         [0025]     Alternatively, the preform  70  may be microwave sintered. To accomplish this, an optional microwave source  86  such as a known type of cavity magnetron with an output in the microwave frequency range would be mounted in communication with the chamber  78 . The microwave spectrum covers a range of about 1 GHz to 300 GHz. Within this spectrum, an output frequency of about 2.4 GHz is known to couple with and heat metallic particles without passing through solid metals.  
         [0026]     When the sintering cycle is complete, the puck  48  is removed from the chamber  78  and allowed to cool. When required, the puck  48  may be subjected to further consolidation using a known hot isostatic pressing (“HIP”) process to result in a substantially 100% dense component, as noted in block  88  of  FIG. 3 . The finished puck  48  has a mildly curved shape that conforms to that of the flowpath surface  18  and in the illustrated example has a thickness of up to about 2.5 mm (0.1 in.)  
         [0027]     The puck  48  may be used for repairing or upgrading turbine shroud segments  14  as follows. First, the shroud segment  14  is cleaned and degreased. The shroud segment  14  is then ground or grit-blasted to remove any tightly adhering oxides. Next the shroud segment  14  is acid stripped and fluoride-ion cleaned in a known manner. The puck  48  is then joined to the flowpath surface  18  preferably by an adhesive. Adhesives such as Borden&#39;s SAF-T have been found suitable. Other joining procedures, such as spot welding of the puck  48 , could also be used.  
         [0028]     The puck  48  and shroud segment  14  are heated to a selected brazing temperature under a vacuum or other suitable environment and held there for a selected time, in accordance with prior art methods. During brazing, the puck  48  bonds to the flowpath surface  18 . The shroud segment  14  with the puck  48  brazed thereto is nearly of the correct size to achieve the desired close tolerances between the extended length of the turbine blade  16  and the flowpath surface  18 . Typically, however, some final machining is required so that the flow path of the shroud has the correct dimensions. Because of the increased density of the puck  48  relative to the prior art, it will create much less of a cupping effect on the shroud segment  14 , and will have much less braze material runoff. This will avoid subsequent rework and the use of expensive braze “stop-off” materials.  
         [0029]     The foregoing has described a method of manufacturing a coating member and repairing a shroud using a coating member. While specific embodiments of the present invention have been described, it will be apparent to those skilled in the art that various modifications thereto can be made without departing from the spirit and scope of the invention. Accordingly, the foregoing description of the preferred embodiment of the invention and the best mode for practicing the invention are provided for the purpose of illustration only and not for the purpose of limitation, the invention being defined by the claims.