Abstract:
The method of repairing a metal alloy component, and the resulting repaired component. The method involves machining the component surface to remove a defect, and then placing in the resulting surface cavity a filler insert whose size and shape are predetermined so that the welding operation can be carried out to completely melt the insert while minimizing the melting of the component immediately surrounding the insert. As such, minimum mixing occurs between the materials of the insert and the component, thereby reducing the risk of cracking following the welding operation.

Description:
BACKGROUND OF INVENTION 
   1. Field of the Invention 
   The present invention generally relates to methods for weld repairing metal alloys, particularly those suitable for use in the high temperature environment of a gas turbine engine. More particularly, this invention relates to a method of performing a controlled weld repair of a defect in a component formed of such an alloy, wherein the method minimizes melting of the alloy with a filler material used to repair the defect. 
   2. Description of the Related Art 
   Hot section components of gas turbine engines, such as blades (buckets), vanes (nozzles) and combustors, are typically formed of nickel, cobalt and iron-base superalloys characterized by desirable mechanical properties at turbine operating temperatures. These components are typically used in cast form, and as a result can have point defects, e.g., ceramic inclusions, pores, etc., as well as small linear defects that require repair. Various welding techniques have been developed that are capable of repairing these defects, including tungsten inert gas (TIG) and plasma transferred arc (PTA) welding processes that must be carefully carried out to achieve acceptable welding yields and ensure that the mechanical properties of the superalloy are maintained. Use is particularly made of relatively simple manual repair methods, such as TIG with a filler material, which can be readily implemented by casting suppliers. 
   As known in the art, welding involves local melting and resolidification. To prevent cracking, an alloy being repaired by welding must be sufficiently ductile to accommodate the thermal strains that develop during welding. However, temperature resistant materials of the type used in gas turbine engines are inherently resistant to deformation, such that filler materials formed of the same alloy as the component being repaired are difficult to use at room temperature. As a result, alloys more ductile than the parent alloy are frequently used to repair superalloy components. A difficulty encountered when using a ductile filler to repair a superalloy component is that the ratio of filler to parent metal is hard to control in manual processes such as TIG. Frequently, TIG welds of superalloys and other alloys that are difficult to weld will experience cracking in the root passes of the weld due to excessive melting of the parent metal into the molten pool of filler metal. 
   In view of the above, it would be desirable if a method were available for repairing high-temperature metal alloys, by which excessive melting of the parent metal and mixing with the filler metal could be minimized. 
   SUMMARY OF INVENTION 
   The present invention provides a method of repairing a metal alloy component, such as a superalloy component of a gas turbine engine, and the resulting repaired component. The method employs a filler insert whose size and shape are predetermined so that the welding operation can be carried out to completely melt the insert while minimizing the melting of the surrounding metal alloy component. As such, minimum mixing occurs between the insert and the component, thereby reducing the risk of cracking following the welding operation. 
   The weld repair method of this invention generally comprises performing an evaluation by which the development of a weld melt pool over time in a surface of a body, e.g., a filler metal alloy, is determined so as to correlate melt pool width, depth and shape with time for a set of welding parameters. A component formed of a metal alloy (which may be the same or different than the evaluated body) and having a defect in its surface is then machined to remove the defect and create a cavity in the surface having a width, depth and shape substantially the same as a melt pool width, depth and shape correlated with a time period during the evaluation. The filler insert having approximately the same width, depth and shape as the cavity is then placed in the cavity so that the outer surface of the filler insert is juxtaposed to the cavity surface. Finally, the filler insert is heated using essentially the same set of welding parameters and for the same time period correlated during the evaluation and which served as the basis for sizing both the cavity and the insert. As a result, the filler insert is melted to form a metallurgically-bonded weld repair that fills the cavity. 
   According to a preferred aspect of the invention, the evaluation is used to determine or estimate the rate at which a melt front propagates through the filler metal alloy, or at least the location of the melt front at different time periods, for a given set of welding conditions and parameters, and this information is used when heating the filler insert so that the melt front that propagates through the insert toward its outer surface will arrive substantially simultaneously at the entire outer surface of the filler insert. Heating may then continue to melt a limited portion of the component beneath the cavity surface, such that the melted portion has a substantially uniform thickness that is intentionally limited to minimize mixing between the materials of the insert and component. As a result, a metal alloy that is relatively difficult to weld, such as a superalloy, can be repaired with an insert formed of an alloy that is more ductile and/or has a lower melting point, yet with a reduced risk of cracking during the welding operation as a result of the reduced amount of mixing in the weld. 
   From the above, it will be appreciated that various welding techniques can be employed by the method of this invention, such as a manual arc welder or an electron beam, with the same technique being employed during both the evaluation and the welding operation so that the welding parameters can be used to closely control the amount of melting that occurs during the weld repair operation. Furthermore, the evaluation of a metal alloy can be conducted so that multiple melt pool widths, depths and shapes are correlated with multiple time periods for one or more sets of welding parameters. Multiple filler inserts can then be formed to approximately have the widths, depths and shapes identified during the evaluation, allowing a particular filler insert to be selected based on the size of the defect to be repaired. As such, the repair method of this invention is highly suitable for filling cracks, porosity, flaws and other surface voids or damage that may be present in a metal alloy component, and the composition of the filler insert can be tailored to complement the composition of the component being repaired to yield a strong, crack-free weld repair. 

   
     Other objects and advantages of this invention will be better appreciated from the following detailed description. 
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  represents a step performed during an evaluation by which the development of a weld melt pool is determined over time in accordance with this invention. 
       FIG. 2  represents a point defect in a surface of a component. 
       FIGS. 3 ,  4  and  5  represent steps performed to repair the defect of  FIG. 2  by removing surface material in and around the defect to form a cavity of a predetermined size, placing a correspondingly-sized filler insert in the cavity, and then performing a welding operation that is controlled to melt the insert and a limited portion of the component immediately adjacent the insert. 
   

   DETAILED DESCRIPTION 
     FIGS. 1 through 5  represent a series of steps carried out to repair a component  10  having a surface defect  12 , as represented by FIG.  2 . The component  10  may be formed of a variety of metal alloys, including those that are relatively difficult to weld, such as nickel, cobalt and iron-based superalloys used to form cast or forged components of gas turbine engines. If the component  10  is a casting, the defect  12  will typically be a point defect, such as a ceramic inclusion, pore, etc., though the defect  12  could instead be a linear defect. 
   Representing a first step of this invention,  FIG. 1  shows a surface  24  of a metal alloy body  20  being heated with a torch  22  produced by a TIG welder  23 , such that a weld melt pool  26  has developed. As would be expected, the melt pool  26  develops over time along a weld melt front  28  that propagates radially outward and downward through the body  20  from a point nearest the torch  22 . The final size and shape of the weld melt pool  26  coincides with the farthest extent of the melt front  28  at the time the torch  22  is extinguished. For purposes of the evaluation, the propagation of this melt front  28 , and therefore the size (width and depth) and shape of the melt pool  26 , over time is recorded, as are the welding parameters used. Depending on the particular type of welding technique, such parameters may include weld current, the use of any fluxes, the position of the torch  22  relative to the surface  24 , etc., as would be appreciated by those skilled in the art. 
   The body  20  is preferably formed of the same alloy as that of the component  10  intended to be repaired though, as will become apparent from the following discussion, the body  20  can be formed of a different material as long as the weld melt front  28  will propagate through the body  20  in a manner similar to a weld melt front  38  caused to propagate through the component  10  under similar welding conditions (FIG.  4 ). As such, the term same metal alloy as used herein encompasses alloys that are sufficiently similar in terms of composition and microstructure to have similar welding properties. 
   Assuming that the body  20  has isotropic properties, the melt pool  26  and melt front  28  will generally have circular shapes at the surface  24  of the body  20 . Depending on the welding technique used, the melt pool  26  and melt front  28  may also have semispherical shapes, though a greater aspect ratio (depth vs. width) will typically be preferred for repairing many surface defects, such as the defect  12  shown in FIG.  2 . Therefore, while various welding techniques may be used to carry out the invention, electron beam or laser welding techniques will be typically preferred to repair defects that require a greater aspect ratio. For conditions in which a manual welding operation will be used, TIG and PTA welding techniques can be used. With the development of organic fluxes, the aspect ratio (depth vs. width) of a weld melt pool formed by TIG can be increased by up to 300%, making TIG a suitable candidate for many types of defects. With the TIG technique represented in  FIG. 1 , the arc is preferably initiated with the TIG machine in panel mode, and the arc current is thereafter maintained constant. 
   A suitable technique for observing the propagation of the melt front  28  and the size of the melt pool  26  is metallographic sectioning. With the evaluation represented by  FIG. 1 , a data base can be established by which the size and shape of the melt pool  26  can be recorded for any number of welding times for the welding technique and parameters used, and the sizes and shapes of the melt pools  26  and their correlated weld times cataloged. Using this same technique, the data base can be expanded to include melt pool sizes and shapes correlated with welding times for a variety of different alloys, welding techniques and parameters. 
   In  FIG. 2 , a surface region  16  of the component  10  surrounding the defect  12  has been designated. To encompass the entire defect  12 , the size and shape of the surface region  16 , as delineated by its boundary  18 , are very nearly the same as the weld melt pool  26  at the farthest extent of the weld melt front  28  in FIG.  1 . The surface region  16  is designated for removal, by which the defect  12  is eliminated from the surface  14  of the component  10  to yield a cavity  32  shown in FIG.  3 . Various techniques can be used to remove the surface region  16 , including the use of air tools equipped with carbide cutters to rough out the cavity  32 , followed by the use of a precision cutter so that the size (width and depth) and shape of the cavity  32  closely correspond to that of the surface region  16 , and are therefore very nearly the same as the weld melt pool  26  at its farthest extent in FIG.  1 . In preparation for the welding operation represented in  FIG. 3 , the surface  14  of the component  10  and the surface of the cavity  32  preferably undergo a surface treatment to remove any oxides and other surface contaminants that could interfere with the welding operation. 
     FIG. 3  shows the placement of a filler insert  30  in the cavity  32  formed in the surface  14  of the component  10 . As shown, the insert  30  is slightly undersized relative to the cavity  32 . For example, the insert  30  may be sized to provide a diametrical clearance between the outer surface  34  of the insert  30  and the cavity  32  of about one to five percent of the diameter of the insert  30 , so as to facilitate placement of the insert  30  in the cavity  32 . According to the invention, suitable materials for the insert  30  include alloys that exhibit mechanical and thermal properties that are comparable to the material of the component  10 , e.g., a nickel-base alloy if the component  10  is formed of a nickel-base superalloy. In this sense, the insert  30  can be viewed as being formed of the same metal alloy as the component  10 , in that a weld melt front  38  ( FIG. 4 ) will propagate through the insert  30  similarly to the weld melt front  28  that propagated through the body  20  during the evaluation, as long as similar welding conditions are used. In a preferred embodiment, the insert  30  is modified to be more ductile and have a lower melting temperature than the alloy of the component  10 . As known in the art, suitable alloying constituents for this purpose include boron and silicon. 
     FIG. 4  represents the process of welding the insert  30  to the component  10  by heating the insert  30  with a torch  42  operating at essentially the same parameters as those used to perform the initial evaluation represented in FIG.  1 . To ensure consistent placement of the torch  42  relative to the component surface  14  under widely variable conditions, the TIG welder  43  is shown as being supported on a rigid support  44 . As with the body  20  of  FIG. 1 , a weld melt pool  36  has developed in the insert  30  as the result of the outward propagation of the weld melt front  38  through the insert  30  from a point nearest the torch  42 . Because of the data acquired during the evaluation of the body  20 , the size and shape of the weld melt front  38  at any given time can be accurately estimated on the basis of the time that has elapsed since the start of the welding operation. Furthermore, because the size of the insert  30  is known, the time required for the melt front  38  to reach the outer surface  34  of the insert  30  can also be accurately predicted. In addition, because the size and shape of the insert  30  and the placement of the torch  42  coincide with the size and shape of the melt pool  26  and torch placement of  FIG. 1 , the welding operation can be performed such that the melt front  38  arrives at the entire outer surface  34  of the insert  30  nearly simultaneously. 
   In accordance with a preferred aspect of the invention represented in  FIG. 5 , the melt front  38  is allowed to propagate to a substantially uniform depth into the surface of the cavity  32 , such that the melt pool  36  not only consumes the insert  30 , but also advances into a limited portion  40  of the component  10  beneath the cavity surface, such that a metallurgical bond between the insert  30  and component  10 . For this purpose, the torch  42  is permitted to operate for a very limited time beyond the time period required to form the melt pool  26  in  FIG. 1 , so that minimal melting of the component  10  occurs and therefore minimal intermixing occurs between the materials of the component  10  and insert  30 . By minimizing intermixing, the likelihood of cracking during cooldown from welding and subsequent strain age cracking is significantly reduced. 
   Following the welding operation, the component  10  is allowed to cool in accordance with known practices to further reduce the risk of weld-induced cracking. In accordance with conventional practice, the component  10  may undergo a post-weld heat treatment to temper any heat affected zone (HAZ) that may have developed in the component  10  adjacent the insert  30 , which is now in the form of a weldment that includes the portion  40  of the component  10  that was melted during welding. Finally, the surface  14  of the component  10  can be further conditioned as necessary using any suitable technique to remove any excess filler material and any surface contaminants left by the welding operation. 
   In view of the above, it can be appreciated that the repair method of this invention is conducive to developing a large catalog of inserts for the repair of a variety of alloys and defects of different sizes by individually evaluating the alloys so that multiple melt pool widths, depths and shapes are correlated with multiple weld times, and optionally for a variety of welding techniques and parameters. Based on this data, filler inserts can then be formed to have approximately the same widths, depths and shapes identified and correlated with the welding times, such that a particular filler insert can be selected from an assortment of inserts based on the alloy to be repaired, the size of the defect in the alloy, and the welding technique that will be used. Because the weld repair method of this invention minimizes mixing of the insert with the parent alloy of the component, the adverse effects of mixing are reduced, potentially allowing the inserts to be formed from a number of filler materials, including alloys that would otherwise be relatively incompatible with the alloy being repaired. 
   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.