Abstract:
A method for depositing material on a turbine airfoil having a tip wall extending past a tip cap, wherein the tip wall includes a first alloy with a single crystal microstructure. The method includes: depositing a second alloy on at least a portion of the tip wall to form a repair structure, wherein a high temperature oxidation resistance of the second alloy is greater than a high temperature oxidation resistance of the first alloy, and wherein the repair structure has a crystallographic orientation that is substantially the same as a crystallographic orientation of the tip wall.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to the repair of gas turbine engine components and more particularly to the repair of tip structures for turbine airfoils. 
     Turbine blades for gas turbine engines are commonly fabricated from hollow castings of nickel- or cobalt-based “superalloys” having a single crystal microstructure for high-temperature strength and fatigue resistance. Cast turbine blades often include a structure known as a “squealer tip”. A squealer tip is a relatively small extension, having a cross-sectional shape conforming to that of the turbine blade, either integral with or mounted on the radially outer end of the turbine blade. The utilization of squealer tips on turbine blades can effectively reduce the disadvantageous effects of rubbing between turbine blades and the shroud. 
     Turbine blades are subject to high operating temperatures in an oxidizing gas environment. In service, their tips often fail due to oxidation and thermal mechanical fatigue. When this occurs, the tips are often repaired between service intervals rather than replacing the entire blade. Known repairs of turbine blade tips involve welding at elevated temperatures with the plasma arc or gas tungsten arc (GTA) welding process, using a filler material that has high ductility so weld cracking is minimized. However, the weld repair buildup from this process is polycrystalline in nature and not single crystal. It therefore does not have the same thermal fatigue resistance as the original turbine blade. 
     BRIEF SUMMARY OF THE INVENTION 
     These and other shortcomings of the prior art are addressed by the present invention, which provides a method for repairing a tip of an airfoil which provides a repaired area having enhanced high-temperature oxidation resistance as well as high thermal fatigue resistance. 
     According to an aspect of the invention, a method is provided for depositing material on a turbine airfoil having a tip wall extending past a tip cap, wherein the tip wall includes a first alloy with a single crystal microstructure. The method includes: depositing a second alloy on at least a portion of the tip wall to form a repair structure, wherein a high temperature oxidation resistance of the second alloy is greater than a high temperature oxidation resistance of the first alloy, and wherein the repair structure has a crystallographic orientation that is substantially the same as a crystallographic orientation of the tip wall. 
     According to another aspect of the invention, a method is provided for replacing a tip wall on a turbine airfoil, wherein the turbine airfoil includes a tip wall extending past a tip cap, and wherein the turbine airfoil includes a first alloy with a single crystal microstructure. The method includes: removing the tip wall from the turbine airfoil; and depositing a second alloy on the tip cap to form a replacement tip wall, wherein a high temperature oxidation resistance of the second alloy is greater than a high temperature oxidation resistance of the first alloy, and wherein the replacement tip wall has a crystallographic orientation that is substantially the same as a crystallographic orientation of the turbine airfoil. 
     According to another aspect of the invention, a method is provided for forming a tip wall on a turbine airfoil, wherein the turbine airfoil includes a tip cap, and wherein the airfoil includes a first alloy with a single crystal microstructure, the method comprising: depositing a second alloy on the tip cap to form a tip wall, wherein a high temperature oxidation resistance of the second alloy is greater than a high temperature oxidation resistance of the first alloy, and wherein the tip wall has a crystallographic orientation that is substantially the same as a crystallographic orientation of the turbine airfoil. 
    
    
     
       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 turbine blade; 
         FIG. 2  is a schematic, side elevational view of a portion of the turbine blade of  FIG. 1  before repair; 
         FIG. 3  is a schematic, side elevational view of a portion of the turbine blade of  FIG. 3  after a cleaning and preparation step; 
         FIG. 4  is a schematic view of a laser welding apparatus; and 
         FIG. 5  is a schematic, side elevational view of a portion of the turbine blade of  FIG. 4  after a weld deposition step. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views,  FIGS. 1 and 2  illustrate an exemplary turbine blade  10 . The turbine blade  10  includes a conventional dovetail  12 , which may have any suitable form including tangs that engage complementary tangs of a dovetail slot in a rotor disk (not shown) for radially retaining the blade  10  to the disk as it rotates during operation. A blade shank  14  extends radially upwardly from the dovetail  12  and terminates in a platform  16  that projects laterally outwardly from and surrounds the shank  14 . A hollow airfoil  18  extends radially outwardly from the platform  16 . The airfoil  18  has an outer wall comprising a concave pressure side outer wall  20  and a convex suction side outer wall  22  joined together at a leading edge  24  and at a trailing edge  26 . The trailing edge  26  may incorporate trailing edge cooling passages such as the illustrated holes  28 . The airfoil  18  has a root  30  and a tip  32 . The radially outermost portion of the airfoil  18  defines a peripheral tip wall  34 , sometimes referred to as a “squealer tip”. A tip cap  36  closes off the interior of the airfoil  18  and lies recessed a small distance radially inward from the tip  32 . The airfoil  18  may take any configuration suitable for extracting energy from the hot gas stream and causing rotation of the rotor disk. The blade  10  is preferably formed as a one-piece casting of a suitable “superalloy” of a known type, such as a nickel-based superalloy (e.g., Rene 80, Rene 142, Rene N4, Rene N5) which has acceptable strength at the elevated temperatures of operation in a gas turbine engine. The blade  10  is be formed with a selected crystalline microstructure, such as single-crystal (“SX”). 
     The interior of the turbine blade  10  is mostly hollow and includes a number of internal cooling features of a known type, such as walls defining serpentine passages, ribs, turbulence promoters (“turbulators”), etc. While the turbine blade  10  is a high pressure turbine blade, the principles of the present invention are applicable to any type of turbine airfoil. 
     In operation, the turbine blade  10  is subject to a flow of high-temperature combustion gases that constitute an oxidizing environment. After a period of service, this results in defects such as fatigue cracks, examples of which are shown at “C”, and material loss from oxidation, examples of which are shown at “O” (See  FIG. 2 ). 
     The initial step in repairing such defects of the tip repair method is to strip the tip  32  of any coating materials (such as corrosion or thermal resistant coatings) that may be present. The coating material may be stripped using any suitable technique, such as grit blasting, chemical baths, and the like, or by a combination of such techniques. After stripping, the tip  32  may be cleaned, if necessary, using a process such as fluoride ion cleaning. 
     Next, any damaged portions are cut or dressed out as necessary to remove any foreign materials from the defects, and provide a void “V” in each defect location having a clean faying surface and adequate access for subsequent repair. This may be accomplished using a variety of techniques, including but not limited to, machining techniques, such as grinding and cutting. For certain applications, one or more layers may be removed from the tip wall. For other applications, one or more selected regions are removed from the tip wall. The result of this step is shown in  FIG. 3 . 
     Next, the voids V are filled using laser welding. An example of a suitable apparatus for laser welding is disclosed in U.S. Pat. No. 5,622,638 to Schell et al., assigned to the assignee of this invention, and is schematically illustrated in  FIG. 4 . The apparatus includes a laser  38 , an enclosed beam delivery conduit  40 , laser focusing optics  42 , a part positioning system  44 , a vision system  46  for part location and laser path control, an optional preheat box (not shown), and a powder feed system  48  with a powder tube  50 . The working and coordination of the individual parts of the apparatus are controlled through a computerized system controller  52 . 
     Using the apparatus shown in  FIG. 4 , molten alloy powder is deposited in the voids V in one or more passes. Alternatively, powder can be deposited and then heated to melt and fuse it to the tip wall  34 , or the filler alloy could be provided in the form of a wire. Preferably, the powder alloy composition is a material with better resistance to oxidation at high temperatures than the base alloy of the airfoil  18 . One nonlimiting example of a suitable powder composition is a nickel-based alloy having an approximate composition, in weight percentages, is as follows: 0.01-0.03 C, 7.4-7.8 Cr, 2.9-3.3 Co, 5.3-5.6 Ta, 7.6-8.0 Al, 3.7-4.0 W, 0.01-0.02 B, 0.12-0.18 Hf, 1.5-1.8 Re, 0.5-0.6 Re, balance Ni and incidental impurities. 
     The exact process parameters may vary to suit a specific application. for example, the laser beam may be operated continuously or pulsed at any frequency, and the laser duty cycle may be 0-100%. Laser power could be from about 50 W to about 1200 W. Laser wavelength may be from about 0.01 to about 100 microns. Translation speed may be about 0.01 cm/s to about 100 cm/s. Powder feed rate may be from about 0.1 g/min. to about 10 g/min. In the illustrated example, a pulsed laser beam is used, with a peak power of 200 W, pulse frequency of 5 Hz, and a 50% duty cycle. The translation speed is approximately 0.57 cm/s (0.225 in./s) 
     As shown in  FIG. 5 , the laser welding process results in a solidified weld fill “F” metallurgically bonded to the tip wall  34  at the location of each defect. With proper control of the process parameters, this process produces the same crystallographic orientation in the weld fill F (e.g. single crystal) as that of the remainder of the airfoil  18 . Once the laser welding process is finished, the weld fill F may be further formed by known processes of machining, grinding, coating, etc. to bring the tip wall  34  back to the original dimensions and condition. 
     The method described above increases repaired tip service life by increasing the oxidation resistance of the tip by compositional change. In other words the new tip material that is laser deposited has superior oxidation resistance to that of the base metal. This makes the repaired tip structure resistant to “burning away” in service. Furthermore, the laser weld repaired tip with its single crystal microstructure will provide better resistance to thermal fatigue cracking when compared to a polycrystalline weld microstructure produced with a prior art arc welding process. 
     The foregoing has described a method for repairing gas turbine engine airfoils and tip structures. 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.