Abstract:
A weld process cuitable for repairing precipitation-strengthened superalloys, and particularly gamma prime-strengthened nickel-based superalloys. The process entails forming a weldment in a cavity present in a surface of an article formed of a precipitation-strengthened superalloy. The cavity has a root region and a cap region between the root region and the surface of the article. A solid body formed of a superalloy composition is placed in the root region of the cavity so as to occupy a first portion but not a second portion of the root region. A first filler material formed of a solid solution-strengthened superalloy is then weld-deposited in the second portion of the root region. Subsequently, a second filler material formed of a precipitation-strengthened superalloy is weld-deposited in the cap region of the cavity.

Description:
BACKGROUND OF THE INVENTION 
     The present invention generally relates to welding processes and materials. More particularly, this invention relates to a process for welding precipitation-strengthened superalloys that are prone to cracking when welded. 
     Superalloys are used in the manufacture of components that must operate at high temperatures, such as buckets, nozzles, combustors, and transition pieces of industrial gas turbines. During the operation of such components under strenuous high temperature conditions, various types of damage or deterioration can occur, including wear and cracks. Because the cost of components formed from superalloys is relatively high, it is more desirable to repair these components than to replace them. For the same reason, new-make components that require repair due to manufacturing flaws are also preferably repaired instead of being scrapped. 
     Methods for repairing nickel-base superalloys have included gas tungsten arc welding (GTAW) techniques. GTAW is known as a high heat input process that can produce a heat-affected zone (HAZ) in the base metal surrounding the weldment. A filler is typically used in GTAW repairs, with the choice of filler material being between a ductile filler or a filler whose chemistry closely matches that of the base metal. An advantage of using a ductile filler is a reduced tendency for cracking in the weldment. On the other hand, a significant advantage of using a filler whose chemistry closely matches the base metal is the ability to more nearly maintain within the component the desired properties of the superalloy base material. 
     Directionally-solidified (DS) and single-crystal (SX) castings formed of precipitation-strengthened nickel-base superalloys have proven to be particularly difficult to weld. Though an equiaxed (EA) precipitation-strengthened nickel-based superalloy filler wire having a composition similar to that of the superalloy base material being welded would provide an optimum weld repair, the result is often solidification shrinkage, hot tears, and cracking during and after the welding processes, and strain age cracking due to gamma prime (γ′) precipitation (principally Ni 3 (Al,Ti)) during post-weld vacuum heat treatment. Cracking is particularly likely in the termination region of the weldment. Further complicating the termination of the weldment is the typical geometry of the superalloy article being welded. 
     In view of the above, improved methods are required for welding precipitation-strengthened superalloys that will yield crack-free weldments. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides a weld process suitable for repairing precipitation-strengthened superalloys, and particularly gamma prime-strengthened nickel-based superalloys. The process of this invention minimizes solidification shrinkage, the incidence of hot tears and cracking during and after the welding processes, and the incidence of strain age cracking during post-weld heat treatment. 
     The process generally entails forming a weldment in a cavity present in a surface of an article formed of a precipitation-strengthened superalloy. The cavity has a root region and a cap region between the root region and the surface of the article. A body formed of a superalloy composition is placed in the root region of the cavity to occupy a first portion but not a second portion of the root region. The superalloy composition of the body may be a precipitation-strengthened superalloy or a solid solution-strengthened superalloy, and may be more ductile than the precipitation-strengthened superalloy of the article. A first filler material formed of a solid solution-strengthened superalloy is then weld-deposited in the second portion of the root region. Subsequently, a second filler material formed of a precipitation-strengthened superalloy is weld-deposited in the cap region of the cavity. 
     In view of the above, the process of this invention yields a weldment in which the cap region of the weldment is formed by an equiaxed, precipitation-strengthened superalloy whose chemistry can be approximately the same as the precipitation-strengthened superalloy forming the base metal of the article, while the root region of the weldment is formed to contain at least one solid solution-strengthened superalloy whose chemistry differs from those of the precipitation-strengthened superalloys to provide different properties, most notably, greater ductility. The combination of a high-strength solid body in the root region of the weldment, a filler formed of a ductile solution-strengthened superalloy in the root region of the weldment, and a precipitation-strengthened superalloy in the cap region of the weldment is believed to enable the process of this invention to yield crack-free repairs, including the weldment termination region, as a result of reducing solidification shrinkage, hot tears, and strain age cracking that are inherent with precipitations-strengthened superalloys, such as gamma prime-strengthened nickel-based alloys. In view of this benefit, the weld process of this invention is capable of promoting full life capability to a weldment. The welding technique of this invention is particularly beneficial in regions of an article where root pass drop-through is a concern, such as where the weldment has a wide, long, and deep geometry. 
     Other objects and advantages of this invention will be better appreciated from the following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically represents a lateral cross-sectional view of a weldment formed by performing a welding process in accordance with the present invention. 
         FIG. 2  represents a plan view of a surface cavity prepared for receiving the weldment of  FIG. 1 . 
         FIGS. 3 through 7  are scanned images illustrating steps performed in the welding process of this invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  schematically represents a weldment  10  formed in a component  16  by a welding process that enables the weldment  10  to exhibit enhanced robustness. The invention will be described in reference to the component  16  being a single-crystal casting formed of a gamma prime-strengthened (principally Ni 3 (Al,Ti)) nickel-based superalloy (hereinafter, gamma-prime nickel superalloy), as is often the case for nozzles (vanes), buckets (blades), and other components located within the combustors and turbine sections of industrial and aircraft gas turbines. Particularly notable examples of gamma-prime nickel superalloys include René 125, René 80, René N5, René N4, René 108, GTD-111™, GTD-444™, IN738, IN792, MAR-M200, MAR-M247, CMSX-3, CMSX-4, PWA1480, PWA1483, and PWA1484, each of which has a relatively high gamma prime content as a result of the significant amounts of aluminum and/or titanium they contain. However, it is foreseeable that the advantages of this invention could be obtained with components formed by other methods from a variety of materials that are prone to cracking or tearing during manufacture or repair by welding. 
     The weldment  10  is represented as being formed within a cavity  12  in a surface  14  of the component  16 , and particularly through a wall section of the component  16 . The weldment  10  includes a cap  18  that is adjacent the surface  14  of the component  16  and completely covers a root  20  of the weldment  10  beneath the cap  18 . Suitable relative volumes for the cap  18  and root  20  will depend on thermal expansion stresses induced during processing. Typically, the cap  18  may constitute about 10 to about 50%, more preferably about 15 to about 30%, of the total depth of the weldment  10 . The root  20  has a central portion  22  and a filler portion  24  that surrounds the central portion  22 . The central portion  22  may contain a precipitation-strengthened or solid solution-strengthened superalloy, while the filler portion  24  is formed of a solid solution-strengthened superalloy. As such, the superalloys of the central and filler portions  22  and  24  may be the same or different. Generally, the alloys should share similar physical and mechanical properties, though a stronger alloy for the central portion  22  is believed to be preferable if the cavity  12  is relatively large. For use with a single-crystal gamma-prime nickel superalloy, the central portion  22  may be formed of the same or similar gamma-prime nickel superalloy, or a solid solution-strengthened nickel-based superalloy containing sufficient nickel, chromium, cobalt, and molybdenum to yield a desirable combination of metallurgical stability, strength, and oxidation resistance at high temperatures, the latter of which can be enhanced by additions of aluminum. The filler portion  24  also benefits from solid solution-strengthened nickel-based superalloys of the type noted for the central portion  22 . Particularly notable solid solution-strengthened nickel-based superalloys that exhibit suitable ductility for use as the central and filler portions  22  and  24  of the weldment  10  include IN600, IN617, IN625, Nimonic 263, and Haynes 230. These alloys contain very little if any gamma prime phase (and therefore are not susceptible to strain age cracking), and exhibit high ductility at temperatures sustained during the processing of this invention as well as processing and service temperature ranges typical for nozzles (vanes), buckets (blades), and other components located within the combustors and turbine sections of industrial and aircraft gas turbines. 
     In contrast to the root  20 , the cap  18  is preferably formed entirely by an equiaxed precipitation-strengthened superalloy filler  26 . If the base metal of the component  16  is formed of a single-crystal gamma-prime nickel superalloy, the cap filler  26  is more preferably a gamma-prime nickel superalloy whose chemistry is the same or similar to that of the superalloy base metal of the component  16 . For example, a suitable superalloy for the cap filler  26  may primarily differ from the superalloy base metal of the component  16  by containing grain boundary strengtheners, constituents that promote oxidation resistance, etc. At minimum, the precipitation-strengthened superalloys of the component  16  and cap  18  preferably share similar physical and mechanical properties, such as creep strength, fatigue strength, oxidation resistance, etc. Particularly notable gamma-prime nickel superalloys suitable for forming the cap  18  include René 125, René 80, René 142, René 195, René 108, GTD-111™, GTD-741™IN738, and MAR-M200. These alloys exhibit high creep strength as a result of containing large volume fractions of the gamma-prime strengthening phase, as well as generally being alloyed to exhibit a balance of strength and environmental resistance. As such, these alloys are also suitable for forming the central portion  22  if the weldment  10  in those applications where a precipitation-strengthened superalloy is preferred for the central portion  22  as discussed above. 
     The weldment  10  of  FIG. 1  is produced by a welding process that is particularly effective as a welding closure process for wide, long and deep weldment geometries where root pass drop through is critical, such as the above-noted gas turbine components.  FIGS. 2 and 3  are a schematic representation and scanned image, respectively, of a suitable geometry for the cavity  12  after being machined in preparation for the weldment  10  and welding operation. The walls  28  and  30  of the cavity  12 , which make up the perimeter of the cavity  12  as indicated by the arrows in  FIG. 2 , are preferably configured to optimize the mechanical properties of the weldment  12 . In  FIG. 2 , the lateral side walls  28  of the cavity  12  are preferably inclined at an angle of about 15 to 30 degrees from the normal axis of the cavity  12  (i.e., about 60 to 75 degrees from the surface  14  of the component  16 ), while the longitudinal end walls  30  are preferably inclined at an angle of about 40 to 55 degrees from the normal axis of the cavity  12  (i.e., about 35 to 50 degrees from the surface  14  of the component  16 ). 
     According to the present invention, the central portion  22  of the weldment  10  is formed by a solid body placed in the cavity  12  and metallurgically bonded to the walls  28  and  30  of the cavity  12  with the filler portion  24  of the weldment root  20 . The solid body is preferably centrally located within the cavity  12  and has a shape approximately congruent to the shape of the cavity  12 , thereby defining a generally uniform but limited gap that surrounds the solid body. The gap may have a width of up to about 20 mils (about 0.5 mm), more preferably about 0 to about 10 mils (about 0 to 250 mm). Tack welds, such as of the type that can be formed by a manual GTAW technique, may be used to hold the solid body in place until the filler portion  24  is deposited. The tack welds need only be of sufficient size and number to secure the solid body to the component  16  during deposition of the filler portion  24 . The materials for the filler portion  24  and cap filler  26  can then be deposited within the cavity  12 , preferably using GTAW welding processes. In particular, the filler portion  24  is deposited to fill the gap surrounding the solid body and metallurgically bond the solid body to the walls  28  and  30  of the cavity  12 , after which the weldment  10  is completed with a cap pass weld that deposits the cap filler  26  over the central and filler portions  22  and  24 . After the welding operations, the component  16  preferably undergoes a vacuum heat treatment, as conventionally practiced when welding superalloys. Though not shown as such in  FIG. 1 , the cap filler  26  can be machined so that its outer surface is substantially coplanar with the surrounding surface  14  of the component  16 . 
       FIGS. 3 through 7  are scanned images of a surface cavity in a directionally-solidified gas turbine component that was cast from the GTD-111™ superalloy and underwent the weld process of this invention. The casting had a nominal composition of, in weight percent, Ni-14Cr-9.5Co-3Al-5Ti-1.6Mo-3.8W-2.8Ta-0.01C. The cavity seen in  FIG. 3  was machined in a region of the casting having a wall thickness of about 0.20 inch (about 5 mm) mm. The cavity extended completely through the casting wall and had nominal dimensions of about 0.5×2.5 inches (about 13×64 mm) at the surface of the casting. The side walls and end walls of the cavity were inclined at angles of about 25 and 45 degrees, respectively, from the normal axis of the cavity. 
       FIG. 4  is a scanned image showing the appearance of the cavity of  FIG. 3  following placement of a plate  32  within the cavity prior to the welding operation (the term “plate” as used herein is a matter of convenience, and is not intended to suggest a particular shape). The plate  32  can be seen as centrally located within the cavity and to have a shape approximately congruent to the shape of the cavity, thereby defining a generally uniform gap  34  that surrounds the plate  32 . The plate  32  is shown as being tack-welded in place using a manual GTAW technique in preparation for the welding operation. As evident from the above discussion, the plate  32  will form the central portion  22  of the weldment root  20  ( FIG. 1 ). For the investigation, the plate  32  was formed of GTD-111™, having the same nominal composition as that of the casting. A gamma-prime alloy was selected for the plate  32  on the basis of the relatively large size of the cavity being repaired, and the GTD-111™ alloy was particularly chosen for its excellent creep strength and environmental resistance. The plate  32  was cast and machined to provide a close fit with the cavity. 
       FIG. 5  is a scanned image showing the result of performing a root pass weld operation to fill the gap  34  surrounding the plate  32  with a solid solution-strengthened superalloy filler. A GTAW welding process was used to deposit the root filler which, in the specimen shown in  FIG. 5 , was formed of IN617 having a nominal composition of, in weight percent, Ni-22Cr-12Co-1Al-0.3Ti-9Mo. As a result of the welding operation, the surface of the plate  32  and the walls of the cavity were partially melted to form strong metallurgical bonds with the root filler. 
       FIG. 6  is a scanned image showing the appearance of a weldment produced by completing a cap pass over the root pass weld of  FIG. 5 . The cap filler was deposited using a GTAW welding process performed at a temperature above 1500° F. (above 815° C.). The cap filler was also formed of GTD-111™ with the same nominal composition as that for the casting and plate  32 .  FIG. 7  is an opposing view of the weldment of  FIG. 6 , and shows the root pass weld as it appears following a post-weld vacuum heat treatment that was conducted at about 1975° F. (about 1080° C.) for about two hours. At the completion of the heat treatment, no solidification shrinkage, hot tears, or strain age cracking was observed during fluorescent penetrant inspection (FPI) and metallographic examination of the specimen. 
     The results of the above investigation evidenced that relatively wide, long, and deep cavities in a gamma prime superalloy can be repaired with a robust weldment formed by a plate having essentially the same properties as the base metal, a ductile filler securing the plate within the root of the cavity, and a capping filler having the same chemistry as that of the base metal. As such, the weld procedure was concluded to be suited for producing robust weldments in the manufacturing and repairing of a variety of precipitation-strengthened components, notable examples of which are single-crystal nozzle and bucket castings for industrial gas turbines and aircraft gas turbine engines, whose thin wall sections increase the likelihood of root pass drop through. While the plate  32  used in the investigation was formed of a gamma prime superalloy, robust weldments can be produced using a plate  32  (or other suitable solid body) formed of a solid solution-strengthened superalloy and/or a superalloy significantly more ductile than the base metal being repaired, particularly when repairing cracks and other small cavities smaller than that repaired in the investigation. 
     While the invention has been described in terms of a preferred embodiment, it is apparent that other forms could be adopted by one skilled in the art. Therefore, the scope of the invention is to be limited only by the following claims.