Abstract:
A method of producing a metallic component includes: providing a body made of a first alloy; providing a preform comprising a metallic powder made of a second alloy and formed in the shape of an extension of the body; and heating the preform with microwave energy to sinter the metallic powder together and to bond the preform to the body. The method may be used to make new components as well as to repair or modify existing components.

Description:
BACKGROUND OF THE INVENTION 
   This invention relates generally to high-temperature components for gas turbine engines and more particularly to turbine airfoils. 
   Thermal and mechanical loads applied to the leading and trailing edges and tips of a gas turbine engine airfoil can adversely affect the airfoil&#39;s useful life. Airfoils in gas turbine engines experience durability problems at the tip of the airfoil in the form of cracking due to thermally induced stress and material loss due to oxidation and rubbing. This can be addressed by using an alloy having increased resistance to environmental oxidation and corrosion. However, it is undesirable to upgrade the entire airfoil to a more thermal-resistant and oxidation-resistant alloy because this increases component cost and perhaps weight. 
   Materials having better high temperature properties than conventional superalloys are available. However, their increased density and cost relative to conventional superalloys discourages their use for the manufacture of complete gas turbine components, so they are typically used as coatings or as small portions of components. These highly environmentally resistant materials have proven difficult to attach to the basic airfoil alloys due to a mismatch in liquidus and solidus temperatures between the environmentally resistant alloys (higher liquidus and solidus) and the component alloys (lower liquidus and solidus). This mismatch is great enough that by the time the solidus of the environmentally resistant alloy is reached the liquidus temperature of the component alloy is far exceeded resulting in a melt away of the component. In processes to date that do join the blade alloy to the tip alloy a distinct centerline is formed in the joint. Experience predicts that this type of joint is likely to fail in either fatigue or rupture. 
   Accordingly, there is a need for a method of attaching environmentally resistant alloys to conventional superalloys. 
   BRIEF SUMMARY OF THE INVENTION 
   The above-mentioned need is met by the present invention, which according to one aspect provides a method of producing a metallic component, including: providing a body made of a first alloy; providing a preform including a metallic powder of a second alloy formed in the shape of an extension of the body; and heating the preform with microwave energy to sinter the metal powder together to bond the preform to the body. 
   According to another aspect of the invention, a method of producing an airfoil includes: providing an airfoil body made of a first alloy and having curved pressure and suction sides, a tip cap disposed between the pressure and suction sides at a radially outer end of the airfoil body and a partial height squealer tip extending radially outwards from the tip cap; providing a preform including a metallic powder of a second alloy different from the first alloy and formed in the shape of an extension of the squealer tip; positioning the preform on the squealer tip; and heating the preform with microwave energy to sinter the metal powder together in a consolidated squealer tip extension, and to bond the squealer tip extension to the body. 
   According to another aspect of the invention, a method of modifying an airfoil includes: providing an airfoil body made of a first alloy and having curved pressure and suction sides, a tip cap disposed between the pressure and suction sides at a radially outer end of the airfoil body, and a squealer tip extending radially outwards from the tip cap; removing a portion of the squealer tip so as to reduce its height in a radial direction; providing a preform comprising a metallic powder of a second alloy different from the first alloy and formed in the shape of an extension of the squealer tip; disposing the preform on the squealer tip; and heating the preform with microwave energy to sinter the metallic powder together in a consolidated squealer tip extension, and to bond the squealer tip extension to the body. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which: 
       FIG. 1  is a perspective view of an exemplary prior art turbine blade; 
       FIG. 2  is a cross-sectional view of a portion of the turbine blade of  FIG. 1 , showing a squealer tip thereof; 
       FIG. 3  is a perspective view of a preform for a squealer tip; 
       FIG. 4  is a schematic cross-sectional view of a preform inside a microwave chamber; and 
       FIG. 5  is a schematic side view of an alternative process for attaching a squealer tip preform to an airfoil. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views,  FIG. 1  depicts an exemplary turbine blade  10  for a gas turbine engine. The present invention is equally applicable to the construction of other types of metallic components, such as stationary turbine vanes, frames, combustors, and the like. The turbine blade  10  comprises an airfoil  12  having a leading edge  14 , a trailing edge  16 , a tip  18 , a platform  20 , a convex suction sidewall  22 , and a concave pressure sidewall  24 . An arcuate inner platform  26  is attached to the platform  20  of the airfoil  12 . 
   In manufacturing the airfoil  12 , the pressure and suction sidewalls  24  and  22 , a tip cap  28 , and a partial height squealer tip  30  are integrally cast as a one-piece airfoil body  32 . The airfoil body  32  is typically cast from known type of a nickel- or cobalt-based superalloy having high-temperature strength properties suitable for the intended operating conditions. Examples of known materials for constructing the airfoil body  32  include RENE 77, RENE 80, RENE 142, and RENE N4 and N5 nickel-based alloys. 
   A squealer tip extension  34  is bonded to the partial height squealer tip  30 . The squealer tip extension  34  preferably comprises an alloy which exhibits superior high-temperature oxidation resistance compared to the base alloy of the airfoil body  32 . 
   One example of a suitable material for this purpose is a rhodium-based alloy comprising from about three atomic percent to about nine atomic percent of at least one precipitation-strengthening metal selected from the group consisting of zirconium, niobium, tantalum, titanium, hafnium, and mixtures thereof; up to about four atomic percent of at least one solution-strengthening metal selected from the group consisting of molybdenum, tungsten, rhenium, and mixtures thereof; from about one atomic percent to about five atomic percent ruthenium; up to about ten atomic percent platinum; up to about ten atomic percent palladium; and the balance rhodium; the alloy further comprising a face-centered-cubic phase and an L1 2 -structured phase. 
   Another suitable material for the squealer tip extension  34  is a second rhodium-based alloy comprising rhodium, platinum, and palladium, wherein the alloy comprises a microstructure that is essentially free of L1 2 -structured phase at a temperature greater than about 1000° C. More particularly, the Pd is present in an amount ranging from about 1 atomic percent to about 41 atomic percent; the Pt is present in an amount that is dependent upon the amount of palladium, such that: a) for the amount of palladium ranging from about 1 atomic percent to about 14 atomic percent, the platinum is present up to about an amount defined by the formula (40+X) atomic percent, wherein X is the amount in atomic percent of the palladium; b) for the amount of palladium ranging from about 15 atomic percent up to about 41 atomic percent, the platinum is present in an amount up to about 54 atomic percent; and the balance comprises rhodium, wherein the rhodium is present in an amount of at least 24 atomic percent. 
   Unfortunately the mismatch between the melting points of the squealer tip extension  34  and the airfoil body  32  is great enough that by the time the solidus of the environmentally resistant alloy is reached the liquidus temperature of the airfoil body alloy is far exceeded resulting in a melt away of the airfoil body  32 . 
     FIG. 3  depicts a preform  36  for a squealer tip extension for use with the present invention. The preform  36  substantially matches the peripheral shape of the partial height squealer tip  30 , and has a radial height “H”. The radial height H is selected to provide adequate protection for the airfoil body  32  from high-temperature oxidation, while minimizing the amount of material used. The radial height “H” may be a thin, foil-like dimension of about 0.127 mm (0.005 in.), or it may a more substantial thickness. In the illustrated example the radial height “H” is about 1.27 mm (0.050 in.) 
   The preform  36  may be constructed in various ways. For example, it may be prepared by a known powder metallurgy (PM) process in which a metallic powder is mixed with a lubricant and pressed into a die under high pressure. Alternatively, the preform  36  may be constructed through a metal injection molding (MIM) process in which a fine metallic powder is mixed with a plastic binder and extruded to a desired shape using plastic molding equipment. The resulting preform is chemically washed to remove a large portion of the plastic from the powder before subsequent sintering. Regardless of the process used to make the preform, the particle size of the metallic powder should be less than about 100 micrometers in diameter, to ensure compatibility with the microwave sintering process described below. 
   The process of attaching the preform  36  to the airfoil body  32  is depicted in  FIG. 4 . The preform  36  is placed on top of the partial height squealer tip  30 . If necessary, a fixture may be used to temporarily retain the preform  36  in place. The airfoil body  32  is then placed in a chamber  38  which includes means for creating a suitable atmosphere to prevent undesired oxidation of the preform  36  or other reactions during the attachment process. In the illustrated example a supply  40  of inert gas such as argon is connected to the interior of the chamber  38 . A microwave source  42  such as a known type of cavity magnetron with an output in the microwave frequency range is mounted in communication with the chamber  38 . 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. 
   The microwave source  42  is activated to irradiate the preform  48 . In the illustrated example the microwave source  42  is depicted as having a direct line-of-sight to the entire preform  36 . However, it is also possible to configure the chamber  38 , which would typically be metallic, so that the preform  36  is heated by a combination of direct and reflected microwaves. Because of the small metallic particle size in the preform  36 , the microwaves couple with the particles and heat them. The preform  36  is heated 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 melts and drives out any remaining binder. Since the microwave energy will not couple with the bulk material of the airfoil body  32 , heating of the airfoil body  32  will occur solely through conduction through the preform  36 . This will produce enough heat during the sintering process to cause the airfoil body  32  to braze itself to the preform  36  by way of capillary action. 
   The preform  36  is held at the desired temperature for a selected time period long enough to result in a consolidated squealer tip extension  34 , and to bond the squealer tip extension  34  securely to the airfoil body  32 . The heating rate (i.e. the output wattage of the microwave source) is selected depending on variables such as the mass of the preform  36 , the shape of the chamber  38  and the and the desired cycle time of the sintering process. 
   When the sintering cycle is complete, the turbine blade  10  is removed from the chamber  38  and allowed to cool. If desired, the turbine blade  10  may be subjected to additional processes such as final machining, coating, inspection, etc. in a known manner. 
     FIG. 5  illustrates an alternative method of creating a preform  36 ′. A mold or dam  43  is formed around the radially outer end of the airfoil body  32 . A core  44  may be inserted into the dam  43  to define an inner perimeter. A preform  36 ′ is then created through a metal injection molding (MIM) process using the extruding apparatus  46  shown. After the preform  36 ′ is constructed in place, it is microwave sintered as described above to consolidate it and bond it to the airfoil body  32 . If the dam  43  is constructed from a microwave-transparent material, then the preform  36 ′ may be sintered without removing the dam  43 . 
   Regardless of how the preform  36  is constructed, the airfoil body  32  and squealer tip extension  34  may be subjected to further consolidation using a known hot isostatic pressing (“HIP”) process to ensure that the squealer tip extension is substantially 100% dense. 
   It should be noted that the above-described method of attachment of a squealer tip extension is equally applicable both to new make and to repair or modification of existing components. For example, a turbine airfoil having a conventional squealer tip extension  34  may be repaired or upgraded by removing the squealer tip extension, and then attaching a new squealer tip extension as described above. 
   The foregoing has described an airfoil tip manufacture or repair process. 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.