Abstract:
A method for repairing a turbine blade ( 10 ) wherein the tip ( 16 ) of the blade is removed ( 41,43 ) and a replacement cap is attached by welding ( 49 ). The cap may consist of a plate ( 48 ) attached by welding and a squealer ( 54 ) formed by depositing weld material ( 52 ), as illustrated in FIG. 3. The plate and/or squealer may be formed from a material different from the material of the airfoil portion ( 42 ) of the blade in order to optimize the performance of the blade.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to the field of turbine blades, and more particularly to the field of the repair of the tip portion of turbine blades. 
     FIG. 1 illustrates a turbine blade  10  as is known in the prior art for use in power generating turbines, such as in the first row of blades of a gas or combustion turbine. Turbine blade  10  includes a blade root  12 , an airfoil portion  14 , and a tip portion  16 . The blade root  12  is designed to be inserted into and retained by a disc on a rotating shaft (not shown) of the turbine. Airfoil portion  14  is shaped to extract energy from combustion gases passing over the airfoil portion  14 , thereby imparting rotating mechanical energy to the turbine shaft. For modern gas turbine engines, airfoil portion  14  is designed to include one or more cooling passages formed below the surface of the airfoil for the passage of cooling air necessary to insure the integrity of the blade material in the hot combustion gas environment. Such cooling passages may be formed in a forged blade by a drilling process or may be formed directly in a cast material blade. For cast turbine blades, the cooling passages are formed by supporting a ceramic core within the volume of the mold as the material of the blade is cast. In order to support the ceramic core in its proper position during the casting process, it is necessary to extend a portion of the core to the edge of the casting, thereby creating one or more openings in the tip portion  16  of the as-cast blade. These openings must then be sealed during the fabrication of the blade in order to assure the proper flow of the cooling air within the turbine blade  10 . If the size of the opening is sufficiently small, it may be sealed by a weld plug  18  formed on the tip  16  of the blade  10 . For larger openings it may be necessary to cover the opening with a cap such as one or more plates  20  in order to seal the opening. U.S. Pat. No. 4,073,599 issued on Feb. 14, 1978, to Allen et al. describes such a blade tip closure design, and it is incorporated by reference herein. Plates  20  are mechanically restrained by the structure of the blade tip  16  and are held in position and sealed by one or more brazed joints  21 . It may be appreciated that the assembly and brazing of plates  20  can be a difficult and expensive process. Furthermore, in spite of efforts to maintain the core in its proper position during the casting process, many cast blades are rejected due to a minimum wall violation caused by unintended movement of the core resulting in an end of a cooling passage being located proximate a surface of a tip end of the airfoil of the blade. 
     Turbine blade  10  is designed to rotate within a casing (not shown). It is important for the blade tip  16  to fit precisely within the casing in order to minimize the passage of combustion gases around the blade tip  16 , since such bypass gases impart no energy to the airfoil section  14 . The blade tip  16  is provided with a squealer  22  which is a raised lip extending around the periphery of the blade tip  16 . Squealer  22  gets its name from the sound that is produced in the event of a mechanical interference between the blade tip  16  and the casing. Ideally the squealer  22  is sized to fit within the casing without rubbing but with a minimum of space there between. 
     It is known that turbine blades  10  may develop one or more cracks  24  near the tip  16  of the blade  10  due to low cycle fatigue stresses imparted on the blade tip  16  during the operation of the turbine. If a crack  24  extends beyond a critical dimension, the turbine blade  10  must be removed from service and/or repaired in order to prevent catastrophic failure of the blade and turbine. It can be appreciated that a crack  24  may be repaired by removing the material adjacent to the crack  24  to form a crack repair volume, and then filling the crack repair volume with weld metal. However, the presence of braze joint  21  utilized to secure plates  20  in position can complicate the repair process, since weld integrity is adversely affected when applied over a braze material. 
     In light of the limitations of the prior art designs, it is desirable to provide a method for repairing a cracked hollow turbine blade which overcomes the problems associated with the presence of braze material in the proximity of the cracked area. It is also desired to provide a method of manufacturing a hollow turbine blade that precludes the possibility for a repair in the area of a braze joint. Furthermore, it is desired to provide a turbine blade having improved level of performance to prevent the occurrence of cracks near the blade tip. 
     SUMMARY OF THE INVENTION 
     These and other objects of the invention are met in a method of repairing a turbine blade, the blade having a plurality of cooling passages formed therein extending to a tip of the blade, the blade further having a cap brazed over an end of a cooling passage at the tip and a squealer portion extending beyond the cap, the method including the steps of: removing the squealer portion, cap, and all braze material from the blade to form a repair surface on the tip; forming a replacement cap sized to span the plurality of ends of the cooling passages; attaching the replacement cap to the repair surface by welding to seal the cooling passage ends; forming a replacement squealer portion by welding 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a prior art turbine blade having a crack formed in the tip thereof. 
     FIG. 2 is a partial sectional view of the turbine blade of FIG.  1 . 
     FIG. 3 is a partial sectional view of a turbine blade illustrating a repair made in accordance with the present invention. 
     FIG. 4 is a schematic representation of the steps of a method for repairing a turbine blade in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 2 illustrates a partial sectional view of the prior art turbine blade  10  of FIG. 1 as seen along section  2 — 2  of FIG.  1 . Plate  20  and squealer  22  are seen in sectional view in FIG.  2 . For the embodiment shown, the walls  28  of the turbine blade  10  are integrally cast with the squealer portion  22 . A ceramic core (not shown) is in place during the casting process to form cooling passage  26 , as well as to form the internal webs  30 . The notches  32 , within which plate  20  is retained, are formed by machining slots into the internal webs  30 . Braze material  34  is utilized to hold plate  20  in position within notches  32 . Notches  32  provide a reactive force to counteract the forces imposed upon plate  20  during the operation of the turbine into which the blade  10  is installed. As described above, a crack  24  illustrated in FIG. 1 may extend to portions of blade  10  containing the brazed material  34 . 
     FIG. 3 illustrates a turbine blade  40  manufactured or repaired in accordance with the present invention. The walls  42  of blade  40  correspond to the walls  28  of blade  10  of FIG.  2 . Walls  42  form a portion of the boundary of a cooling passage  44  that is formed during the casting of the blade  40 . These portions of blade  40  may be formed during the manufacturing of a new blade or may be the result of a partial repair process to a blade  10  taken out of service from a turbine. In accordance with the present invention, blade  10  of FIG. 2 may be repaired to become blade  40  of FIG. 3 by a sequence of steps illustrated schematically in FIG.  4 . The first step  41  is to remove the squealer  22 , cap plate  20 , and all brazed material  34  from blade  10 . The removal of these structures results in the formation of a repair surface  46  on the tip  16  of blade  40 . The repair surface  46  is preferably flat and will expose the ends of each of the cooling passages  44 . If the blade  10  had been rejected for a minimum wall violation, it may be possible to remove sufficient material to remove the portion of the blade  10  containing the minimum wall violation. The webs  30  of FIG. 2 are removed in step  43  to improve thermal characteristics at the tip and so that the webs  30  will not interfere with the ability to obtain a successful weld. In addition, removal of the webs  30  will expand the size of the opening of cooling passages  44  on repair surface  46 . By improving the access to cooling passage  44 , the inside surfaces  50  of the walls  42  become more accessible for nondestructive examination (NDE). It may be appreciated that the prior art blade designs utilize a web to minimize the core print opening at the tip of the blade, thereby making it easier to close the core print holes, either by attaching a plate by brazing or by welding to form a plug if the core print hole is small enough. A significant amount of effort is currently being expending in the casting industry to minimize core print hole size. However, the web material at the tip of the blade makes it more difficult to cool the tip of the blade. The current invention eliminates the problems associated with having a web for both newly manufactured and repaired blades. 
     A replacement cap, illustrated in FIG. 3 as plate  48 , is then formed in step  45  to span cooling passage  44 . Advantageously, a single plate  48  may be used to cover a plurality or all of the cooling passages  44  formed in blade  40  since the repair surface  46  is a single flat surface across the entire cross section of the blade  40 . In this manner, the multiple plate design of the prior art blade  10  illustrated in FIG. 1 is eliminated. The plate  48  may be sized to span passage  44  leaving just a small gap between the edges of plate  48  and the edges of airfoil walls  42  to facilitate the subsequent welding process discussed below. 
     The material of plate  48  is chosen to facilitate the welding of the plate to the airfoil walls  42 . In one embodiment the blade  40  is formed of a cast nickel-based super alloy such as IN-738LC, and both the plate  48  and the weld material  52  used to secure the plate onto the repair surface  46  are selected to be the same material as the blade  40 . For a typical gas turbine row  1  blade, plate  48  may be in the range of 0.060-0.100 inches in thickness. Plate  48  may be held in place by mechanical means or by a tack weld as shown in step  47  of FIG. 4 before it is welded to the repair surface  46  in step  49 . In one embodiment of the present invention, the welding process utilized in step  49  is a high temperature TIG welding process. The applicants have found that for blades cast either conventionally, directionally solidified, or as a single crystal from either IN-738, Mar M247, or CM 247LC material, a pre-heat and an in-process welding temperature of between 1,650-1,950 degrees Fahrenheit will provide acceptable results. If a tack weld is used in step  47 , the tack weld and its heat affected zone are consumed during the welding of step  49  in order to obtain the more desirable material properties associate with a high-temperate TIG welding process. 
     In the event that the original blade  10  that is repaired to form blade  40  had developed one or more cracks  24 , as illustrated in FIG. 1, the repair process may include step  51  of removing material adjacent the crack  24  to form a crack repair volume, and step  53  of filling the crack repair volume by welding. Step  55  indicates that nondestructive examination of the blade  40  may be conducted before or after the welding of the replacement cap and/or the repair of any cracks that may be present. For a newly manufactured blade, it may be appreciated that steps  41 , 43 , 51 , 53  are not necessary, but are replaced by the manufacturing of a new blade body including airfoil section  42  as shown in FIG.  3 . 
     In some applications, it may be necessary or desirable to conduct step  57  of forming a curved surface on the top surface of plate  48 . Step  59  indicates that a replacement squealer portion  54  is formed by a welding process wherein layers of weld material are deposited to form the general shape of squealers  54 . Conventional or laser welding may be utilized for step  59 . Step  61  indicates that the final shape of the blade tip  16  and squealers  54  are formed by a process such as final machine, grinding, EDM, or other material shaping process. 
     In one embodiment of the present invention, the step  59  of forming the squealer portion may be performed using a weld filler material that is different than the weld filler material utilized in step  49  of welding the replacement cap  48  onto repair surface  46 . The material selected for the root  12  and airfoil  14  sections of a turbine blade are primarily selected for their high temperature, high stress, creep properties. However, the tip  16  portion of a turbine blade  40  experiences a different set of operating perimeters than the lower portions of the blade, and failures in the tip portion  16  are usually the result of low cycle fatigue, oxidation and corrosion. Therefore, it may be desirable to select the material for plate  48  and/or replacement squealers  54  to have different properties than airfoil walls  42 . 
     The above described embodiments of the present invention are provided by way of illustration, not limitation. Accordingly, the full scope of the applicants&#39; invention is as claimed below.