Abstract:
A turbine component repair apparatus includes: a first die having male and female halves for clamping a first section of a turbine blade with a platform and a root portion of an airfoil, the first die having a recess shaped to receive the root portion and retain a faying surface thereof in predetermined alignment; and a second die having male and female halves for clamping a repair section which defines a tip portion of the airfoil, the second die having a second recess shaped to receive the tip portion and retain a faying surface of the tip portion in predetermined alignment. The first and second dies have mating front faces configured to align their bottom surfaces in a common plane. A alignment device is removably attached to the second die to temporarily align the repair section in the absence of the male half of the second die.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of Provisional Patent Application No. 61/447,604, filed Feb. 28, 2011, currently pending. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates generally to the repair of gas turbine engine components and more particularly to methods of attaching a repair section to a portion of an existing turbine component. 
     A gas turbine engine includes a compressor that provides pressurized air to a combustor wherein the air is mixed with fuel and ignited for generating hot combustion gases. These gases flow downstream to a turbine section that extracts energy therefrom to power the compressor and provide useful work such as powering an aircraft in flight. 
     During operation, turbine components, in particular the airfoils in the compressor, are exposed to a high velocity air stream that can lead to oxidation, corrosion, physical damage, and cracking from thermal cycling. Because turbine components are complex in design, are made of relatively expensive materials, and are expensive to manufacture, it is generally desirable to repair them whenever possible. 
     One known repair method involves providing a repair section (sometimes referred to as a “SPAD”. which duplicates a portion of the airfoil. Damaged portions of the field-used airfoils are cut off and then the SPAD is welded or otherwise bonded in place. 
     Accurate and secure placement of the SPAD during the welding process is necessary to produce a satisfactory end product. The repair requires alignment of the leading and trailing edges as well as circumferential, axial and twist positions. The ability to maintain accurate orientation in a robust welding environment demands a preliminary fixture provide a stable assembly of the components in an equally robust assembly. 
     Prior art attempts to use this welding technique for 3D airfoil shapes have resulted in poorly aligned leading and trailing edges as well as misalignment in the stacking axis and twist orientation. Further attempts to correct alignment have indicated the lack of current technology to assure alignment. 
     Accordingly, there is a need for a method of repairing turbine components using a repair section or SPAD while maintaining precise alignment. 
     BRIEF SUMMARY OF THE INVENTION 
     This need is addressed by the present invention, which provides a fixture adapted to secure a field-used component and a repair section during an alignment process and also during a subsequent welding procedure. 
     According to one aspect of the invention, a turbine component repair apparatus includes: a first die having male and female halves configured to cooperatively clamp a first section of a turbine blade which includes an arcuate platform and a root portion of an airfoil extending from the platform, the first die having a first recess shaped to receive a curved surface of the root portion and configured so as to retain a first faying surface of the root portion in a first predetermined alignment relative to a first bottom surface of the first die; a second die having male and female halves configured to cooperatively clamp a repair section which defines a tip portion of the airfoil, the second die having a second recess shaped to receive a curved surface of the tip portion and configured so as to retain a second faying surface of the tip portion in a second predetermined alignment relative to a second bottom surface of the second die; the first and second die having mating front faces configured to align the first and second bottom surfaces in a common plane; and a alignment device removably attached to the front face of the second die and configured to temporarily retain the repair section in the second predetermined alignment in the absence of the male half of the second die. 
     According to another aspect of the invention, a method for repairing a metallic turbine component includes: providing an engine-run first section of a turbine blade which includes an arcuate platform, a root portion of an airfoil extending from the platform, and a first faying surface at a distal end of the root portion; placing the first section of the turbine blade in a first die having male and female halves cooperatively defining a first recess shaped to receive a curved surface of the root portion; clamping the male half of the first die to the female half so as to retain the first faying surface in a first predetermined alignment relative to a first bottom surface of the first die; providing a repair section which defines a tip portion of the airfoil and includes a second faying surface; placing the repair section in a second die having male and female halves cooperatively defining a second recess shaped to receive a curved surface of the tip portion; attaching an alignment device to a front face of the second die so as to temporarily retain the second faying surface in a second predetermined alignment relative to a second bottom surface of the second die; clamping the male half of the second die to the female half thereof so as to retain the second faying surface in the second predetermined alignment; removing the alignment device; and assembling the first and second dies to each other such that mating front faces thereof engage each other and align the first and second bottom surfaces in a common plane. 
    
    
     
       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 engine-run turbine compressor blade, exhibiting damage from use; 
         FIG. 2  is a perspective view of the blade of  FIG. 1  after being prepared for a welding operation; 
         FIG. 3  is a side view of a repair section for use with the blade of  FIG. 1 ; 
         FIG. 4  is a rear elevational view of the repair section of  FIG. 3 ; 
         FIG. 5  is a cross-sectional view of a fixture constructed according to an aspect of the present invention; 
         FIG. 6  is an end view of a repair section die of the fixture of  FIG. 5 ; 
         FIG. 7  is a side view of the die of  FIG. 6 ; 
         FIG. 8  is a top view of the die of  FIG. 6 ; 
         FIG. 9  is an end view of a field item die of the fixture of  FIG. 5 ; 
         FIG. 10  is a side cross-sectional view of the die of  FIG. 9 ; 
         FIG. 11  is a top view of the die of  FIG. 9 ; and 
         FIG. 12  is a cross-sectional view of the fixture assembled during a welding process. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views,  FIG. 1  shows a compressor blade  10  of a gas turbine engine. It will be understood that the principles of the present invention are also applicable to other kinds of airfoils. The blade  10  includes a dovetail  12  used to mount the blade  10  to a compressor disk wheel (not shown), an arcuate platform  14 , and an airfoil  16  having a root  18 , a tip  20 , a leading edge  22 , a trailing edge  24 , a concave pressure side  26  and a convex suction side  28 . 
     Typically, such blades are made of an alloy based on at least one of the elements Ti, Fe, Ni, and Co. Nonlimiting examples of such alloys that are commercially available include Ti 6-4, Ti 6-2-4-2, A-286, C 450, IN 718, and RENE 95 alloy. 
     In operation, the blade  10  is subject to damage, especially tip and erosion damage from abrasive materials and/or foreign object impacts. The blade  10  in  FIG. 1  is shown to include both a crack “C” and a damaged area “D” where material is missing. 
       FIG. 2  shows the blade after it has  10  been prepared for a subsequent welding process by cutting, grinding, machining, or otherwise working it to remove the spanwise outer portion along a predetermined cutting plane P (seen in  FIG. 1 ). As thus prepared, the remaining portion of the blade  10  is referred to as a “field item”  30 . The preparation process exposes a faying surface  31 . The cutting plane P is selected so that pressure applied to on the field item  30  along its spanwise or stacking axis during a welding process will not end to cause misalignment. The remainder of the airfoil  16  is referred to herein as a “root portion”. 
       FIGS. 3 and 4  illustrate a repair section  32  for use to replace the removed portion of the airfoil  16 . This type of repair section may also be referred to as a “SPAD”, from the term “spare part assembly detail” or “spare part assembly drawing”. The repair section  32  mimics the spanwise outer portion of the airfoil  16  (referred to herein as a “tip portion”) and includes a leading edge  22 ′, a trailing edge  24 ′, and opposed pressure and suction sides  26 ′ and  28 ′. In the radial or spanwise direction, it extends between a tip  20 ′ and a base  18 ′. An integral sacrificial projection  34  extends from the base  18 ′. The projection  34  is generally trapezoidal in shape, with its cross-sectional area being tapered down as it extends away from the rest of the base  18 ′. The projection  34  incorporates a faying surface  36  which is planar or otherwise complementary to the faying surface  31  of the field item  30 , as is the base  18 ′. The dimensions and exact shape of the projection  34  are selected to provide for an appropriate amount of material extrusion for a specific application. A pair of small cross-section tabs  38 , for example a few thousands of a inch in length, extend from the base  18 ′ of the repair section  32 . The repair section  32  may include a sacrificial portion  40  adjacent the tip  20 ′. The inner boundary of the sacrificial portion  40  coincides with the finished tip profile, and its outer boundary is parallel to the faying surface  36 . A notch  42  is formed in the tip  20 ′. In the illustrated example the notch is V-shaped. 
       FIG. 5  shows a fixture used to align and weld the repair section  32  and the field item  30  together. Its basic components are two, two-part dies, referred to as a repair section die  44  and a field item die  46 , respectively. 
       FIGS. 6-8  illustrate the repair section die  44  in more detail. It includes a male half  48  and a female half  50 . The female half  50  is generally a rectangular solid. It includes a bottom face  52 , top face  54 , back face  56 , and front face  58 . The bottom face  52  is planar and serves to provide a common datum height when assembled to the field item die  46 . The front face  58  has a generally Z-shaped profile which includes an upper vertical face  60 , a horizontal face  62 , and a lower vertical face  64 . 
     A recess  66  is formed adjacent the top face  54 , defined by side walls  68 , a curved bottom wall  70 , and an end wall  72 . A spring plunger  74  backed by a compression spring is received in a hole in the end wall  72 . The recess  66  is sized and shaped to receive the repair section  32  (shown in  FIG. 7 ) and hold it in the proper alignment. 
     A pair of spaced-apart locator slots  75  are formed in the horizontal face  62 . A compression spring  76  is received in an axially-aligned hole that communicates with the back wall of each locator slot  75 . 
     The male half  48  (seen in  FIG. 5 ) includes a body  78  which mates against the top face  54  of the female half  50 , and a block  80  which is shaped to protrude into the recess  66 . Its bottom face  82  is curved to match the repair section  32 . A spring-loaded electrical contact  84  protrudes from the bottom face  82  of the block  80  and is coupled to an electrical lead  86 . Means are provided for securing the male half  48  to the female half  50 , such as the illustrated bolts  87 . 
     The repair section die  44  is provided with a pair of locators  88 , seen in  FIGS. 6-8 . Each locator  88  includes a block-like base  90  and a vertically projecting arm  92 , and each arm  92  has a planar alignment surface  94  and a notch  96  passing through it, complementary in shape and size to the tabs  38  of the repair section  32 . The bases  90  are sized to be received in the locator slots  74  of the repair section die  44 . 
       FIGS. 9-11  illustrate the field item die  46  in more detail. It also includes a male half  98  and a female half  100 . The female half  100  is generally a rectangular solid. It includes a bottom face  102 , top face  104 , back face  106 , and front face  108 . The bottom face  102  is planar and serves to provide a common datum height when assembled to the repair section die  44 . The front face  108  has a generally Z-shaped profile which includes an upper vertical face  110 , a horizontal face  112 , and a lower vertical face  114 . 
     A recess  116  is formed adjacent the top face  104 , defined by side walls  118 , a curved bottom wall  120 , and an end wall  122 . A vertically-oriented platform recess  124  with a vertical wall  126  is disposed adjacent the back face  106  and is contiguous with the recess  116 . Compression springs  128  are received in holes in the female half  100 , communicating with the platform recess  124 , and oriented in both lateral and spanwise directions relative to the field item  30 . 
     The male half  98  (seen in  FIG. 5 ) includes a body  130  which mates against the top face  104  of the female half  100 , and a block  132  which is shaped to protrude into the recess  116 . Its bottom face  134  is curved to match the field item  30 . A spring-loaded electrical contact  136  protrudes from the bottom face  134  of the block  132  and is coupled to an electrical lead  138 . Means are provided for securing the male half  98  to the female half  100 , such as the illustrated bolts  87 . 
     One or more surfaces of the repair section die  44  and the field item die  46  are electrically insulated as needed so as to avoid current flow between the two dies and between the dies and surrounding hardware or equipment. For example, the exposed die surfaces may be coated with a nonmetallic material. 
     The fixture comprising the repair section die  44  and the field item die  46  may be used to weld a repair section  32  to a prepared field item  30  as follows. 
     As a preliminary step, the locators  88  are inserted into the locator slots  74  in the female half  50  of the repair section die  44 . They are axially compressed against the compression springs  76  and then held in place with retainers  140  that pass vertically through the bases  90  of the locators  88  and into holes in the female half  50 . The retainers  140  may be simple pins, or threaded fasteners may be used. The compression springs  76  serve to take up all axial play between the locators  88  and the female half  50 . The installed position is shown in  FIG. 7 . 
     Next, the repair section  32  is placed into the recess  66  in the female half  50 . The notch  42  in its tip  20 ′ engages the spring plunger  74 . It is axially compressed against the spring plunger  74  and manipulated until the tabs  38  fit into the notches  96 . It is then released so that the spring plunger  74  urges it axially against the locators  88  until its axial motion stops with the tabs  38  seated in the notches  96 . The repair section  32  is thus fully aligned in a specific predetermined orientation in all three axes. 
     Referring now to  FIG. 12 , the male half  48  of the repair section die  44  is next mounted to the female half  50 . It may be secured using bolts  87 . Alternatively, clamps, or hydraulic or pneumatic clamping means may be used to secure the two halves together. The two die halves securely clamp the repair section  32 . 
     The locators  88  may then be removed, leaving the repair section  32  securely fastened in the desired alignment with the projection  34  extending axially away from the upper vertical face  60 . 
     Next, the field item  30  is placed into the recess  116  in the female half  100  of the field item die  46 . The platform  14  is received in the platform recess  124 . The compression springs urge the platform  14  against the vertical wall  126  and also preload it in a lateral direction, so that the tip  20 ′ lies flush with the upper vertical face  110 . The field item  30  is thus fully aligned in a specific predetermined orientation in all three axes. 
     Next, the male half  98  of the field item die  46  is mounted to the female half  100 . It may be secured using bolts  87 . Alternatively, clamps, or hydraulic or pneumatic clamping means may be used to secure the two halves together. The two die halves securely clamp the field item  30 . All of the clamping forces are applied through the airfoil, preventing distortion of the platform or dovetail. 
     The repair section die  44  and the field item die  46  are placed on a datum surface  142  such as a bench, table, or surface plate. They may be held in lateral alignment by fences or rails mounted to the datum surface  142 , or by rods or bars passing between the two dies (not shown). The complementary front faces  58  and  108  ensure that the dies  44  and  46  remain in the desired alignment to their respective bottom faces  52  and  102 . Means are provided for applying axial compression in the direction shown by the arrows “A”. Examples of suitable compression means include, for example, hydraulic or pneumatic cylinders. 
     Next, an electrical power supply  144  such as a welding power supply (shown schematically in  FIG. 12 ) is connected to the contacts  84  and  136 . 
     If desired, a first spacer  146  may be placed between the front faces  58  and  108  to limit their axial motion. Then, electrical current is supplied to the field item  30  and the repair section  32  through the while an axial force is applied. Electrical resistance heating causes the tabs  38  to melt and fuse to the faying surface  31  of the field item  30 , creating two spot welds which temporarily bond the repair section  32  to the field item  30 . 
     Once the spot welds are complete, the alignment of the field item  30  and the repair section  32  can be checked. If the alignment is incorrect, the two components can be cut apart at the spot welds with little to no damage. If the alignment is correct, a final weld can be made. 
     If desired, a second spacer  148  may be placed between the front faces  58  and  108  to limit their axial motion. Then, electrical current is again supplied to the field item  30  and the repair section  32  through the while an axial force is applied. Electrical resistance heating causes the projection  34  to melt and fuse to the faying surface  31  of the field item  30 . As the weld process proceeds, the projection  34  shortens in the axial direction and extrudes laterally outward. When the second weld is complete, the repair section  32  is fully bonded to the field item  30 . 
     After the welds are complete, the bonded field item  30  and repair section  32  are removed from the dies  44  and  46 . Excess material around the bond line as well as the sacrificial portion  40  may be cut, ground, and/or machined away to restore the airfoil to new-make dimensions. 
     The process and apparatus described above has several advantages over prior art repair processes. The ability to utilize the same fixture for pre-alignment and final welding provides increased quality and performance to the product by reducing the amount of individual operations and touch-time. The ability to confirm alignment of the repair section  32  and field item  30  prior to final welding in addition to control of the material flow during compression assures repeatable and quality welds. The fixture design provides repeatable insertion of the components and assures proper placement and orientation to the design intent and complementary components. Application of forces on the airfoil removes pressures and forces from the platform and dovetails. Finally, electrical components are easily serviceable and provide for the shortest path between components thus reducing the localized heating and metallurgy effects. 
     The foregoing has described a method for repairing turbine components. 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.