Abstract:
Boron and silicon free braze alloys are useful for structural repair of superalloy gas turbine engine components. The braze alloy compositions include nickel, chromium, titanium, and at least one of zirconium and hafnium. All of the above elements are metallic and form ductile bonds within and across the braze interface when compared to non-metallic bonds of boron and silicon.

Description:
[0001]    This application is a continuation-in-part of U.S. patent application No. 14/251,691, filed 14 Apr. 2014 (attorney docket number 2013P24727US), incorporated by reference herein. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates generally to the field of materials technology, and more particularly to the joining or repair of superalloy materials with a brazing process. 
       BACKGROUND OF THE INVENTION 
       [0003]    It is recognized that the repair of superalloy materials is made difficult due to their susceptibility to weld solidification cracking and strain age cracking. The term “superalloy” is used herein as it is commonly used in the art; i.e., a highly corrosion and oxidation resistant alloy that exhibits excellent mechanical strength and resistance to creep at high temperatures. (http://en.wikipedia.org/wiki/Superalloy) Superalloys typically include a high nickel or cobalt content. Examples of superalloys include alloys sold under the trademarks and brand names Hastelloy, Inconel alloys (e.g. IN 738, IN 792, IN 939), Rene alloys (e.g. Rene N5, Rene 80, Rene 142), Haynes alloys, Mar M, CM 247, CM 247 LC, C263, 718, X-750, ECY 768, 282, X45, PWA 1483 and CMSX (e.g. CMSX-4) single crystal alloys. 
         [0004]    Brazing processes are used to repair superalloy materials in some applications. While a braze joint is generally understood to be mechanically weaker than a weld joint and to have a lower acceptable operating temperature due to the relatively low melting temperature of the braze material, braze repairs may be acceptable in certain lower stress and/or lower temperature applications. 
         [0005]    Typical braze materials using boron or silicon as the melting point depressant material are of limited value with superalloy substrate materials because they create deleterious phases which reduce the ductility of the joint and repaired region. Boron and silicon free braze alloys incorporating hafnium and/or zirconium have been developed for which mechanical properties of up to 80% of the base superalloy properties are claimed. 
         [0006]    The present inventor has developed several boron and silicon free braze alloys utilizing titanium as a melting point depressant material, including those disclosed in co-pending United States patent application publication numbers US 2013/0302647 A1 (attorney docket number 2011 P24893US) and US 2014/0007988 A1 (attorney docket number 2011P25126US01). Titanium based braze alloys can provide brazed joints having mechanical strength close to (e.g. more than 80% of) the repaired substrate material properties such that the repair can be considered a structural repair and can be used in relatively high stressed regions of the component. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The invention is explained in the following description in view of the drawings that show: 
           [0008]      FIG. 1  is a side sectional view of a gas turbine vane wherein a large gap discontinuity is being repaired by brazing. 
           [0009]      FIG. 2  is a side section view of a gas turbine vane wherein a narrow gap discontinuity being repaired by brazing. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0010]    Braze alloys are disclosed herein that are particularly useful with superalloys, including Rene 80 and IN 939 superalloy material, for example when repairing a gas turbine engine blade or vane that has been found to have service-induced cracks on its platform or airfoil section. Rene 80 components present a particular challenge because they may be solution heat treated at 1,205° C., which is lower than some other alloys, such as Alloy 247 components which may be solution heat treated at 1,235° C. Alloys disclosed below may be formulated to have liquidus and solidus temperatures and melting temperature ranges that can be tailored to work cooperatively with a solution heat treatment temperature regiment for the particular superalloy material of the component to accomplish a braze and to fuse and to homogenization the braze joint during solution heat treatment. Such braze materials may be selected to have melting temperature ranges inclusive of or below the solution heat treating temperature (i.e. the peak hold temperature used during the regiment) for particular superalloy substrate materials of interest. The homogenization and solidification of the braze joint advantageously does not introduce any new elemental constituent into the superalloy substrate material that was not already present in that material. 
         [0011]    Alloys disclosed herein include nickel, chromium and titanium, as well as at least one of zirconium and hafnium, which can be stated as Ni—Cr—Ti—(Zr and/or Hf). All compositions described herein are given in weight percent. 
         [0012]    The titanium and zirconium and/or hafnium of the disclosed alloys function to reduce the melting point of the alloys. The compositions include sufficient amounts of titanium and zirconium and/or hafnium such that they exhibit a melting temperature range approximately within the range of 1,100-1,200° C. Advantageously, alloys disclosed herein exhibit a melting temperature range that includes or is below the 1,205° C. solution heat treatment temperature of Rene 80 alloy, which makes them particularly well suited for use with that alloy, as well as with alloys such as CM 247 and IN 939 which have solution heat treatment holding temperatures higher than that of Rene 80. 
         [0013]    The titanium content of the disclosed alloys functions to increase the strength of the alloys. 
         [0014]    The zirconium and/or hafnium content of the disclosed alloys functions to increase the ductility of the alloys. 
         [0015]    A Ni—Cr—Ti—Zr braze alloy may be desired when the superalloy component to be repaired or joined contains no or little hafnium. A Ni—Cr—Ti—Hf braze alloy may be desired when the superalloy component to be repaired or joined contains no or little zirconium. A Ni—Cr—Ti—Zr—Hf braze alloy may be desired when the superalloy component to be repaired or joined contains all of these elements. 
         [0016]    In an embodiment, a quaternary braze alloy includes: 
         [0017]    4.5-15.0% Cr; 
         [0018]    7.0-16.5% Ti; 
         [0019]    10.0-16.0% Zr; 
         [0020]    balance Ni. 
         [0021]    One such braze alloy includes: 
         [0022]    6.5% Cr; 
         [0023]    7.5% Ti; 
         [0024]    11.0% Zr; 
         [0025]    balance Ni, 
         [0026]    which has a liquidus temperature of 1,160° C. 
         [0027]    Other alloys in this group may contain: 
         [0028]    4.5-5.5% Cr; 
         [0029]    7.0-8.0% Ti; 
         [0030]    13.0-14.0% Zr; 
         [0031]    balance Ni. 
         [0032]    A particular alloy in this group includes: 
         [0033]    4.9% Cr; 
         [0034]    7.3% Ti; 
         [0035]    13.0% Zr; 
         [0036]    balance Ni, 
         [0037]    which has a liquidus temperature of 1,180° C. This braze alloy was successfully tested to form a crack-free braze repair in a 1.2 mm wide surface opening crack in Rene 80 superalloy material. The crack was first filled with −325 mesh alloy CM 247 powder, then the braze alloy powder was applied to cover the surface of the superalloy material and heated to a solution heat treat temperature of 1,220° C. for 4 hours, wherein the braze alloy powder melted and flowed to fill the crack around the alloy CM 247 powder. 
         [0038]    In another embodiment, a quaternary braze alloy includes: 
         [0039]    4.5-5.5% Cr; 
         [0040]    7.0-8.0% Ti; 
         [0041]    18.0-19.5% Hf; 
         [0042]    balance Ni. 
         [0043]    One such braze alloy includes: 
         [0044]    4.5% Cr; 
         [0045]    8.0% Ti; 
         [0046]    18.5% Hf; 
         [0047]    balance Ni, 
         [0048]    which has a liquidus temperature of 1,160° C. 
         [0049]    In a multi-component embodiment, a braze alloy includes: 
         [0050]    3.5-5.0% Cr; 
         [0051]    7.0-9.0% Ti; 
         [0052]    9.5-12.0% Zr; 
         [0053]    18.0-19.0% Hf; 
         [0054]    balance Ni. 
         [0055]    One such braze alloy includes: 
         [0056]    4.0% Cr; 
         [0057]    8.0% Ti; 
         [0058]    10.0% Zr; p 18.5% Hf; 
         [0059]    balance Ni, 
         [0060]    which has a liquidus temperature of 1,085° C. 
         [0061]    A repair process utilizing the braze alloys described above is illustrated in  FIG. 1  where a gas turbine engine vane  10  formed of a superalloy substrate material  12  has a service-induced discontinuity  14  extending into the substrate material  12  from its surface  16 . In this embodiment, the discontinuity  14  is illustrated as a large gap crack having an opening of greater than 0.001 inches at surface  16 . After being cleaned using any known process, crack  14  is filled with an alloy-containing powder  18 , such as one containing a mixture of superalloy particles  20  and braze material particles  22 . The braze particles  22  in the alloy-containing powder  18  may constitute 5-50% by weight of the powder  18 . In other embodiments the alloy-containing powder  18  may be only superalloy particles  20 . The mesh size range of the braze material particles  22  is preferably smaller (for example −325 mesh/+2 micron) than the superalloy particles  20  (for example −120 mesh/+2 micron) in order to provide enhanced filling of the discontinuity  14 . A layer of the braze material particles  22  is disposed above the alloy-containing powder  18  to ensure a complete fill of the discontinuity during the brazing process. During a solution heat treatment of the substrate material  12 , the braze material particles  22  melt and fill the discontinuity while the superalloy particles  20  sinter together. The melting point depressant titanium, zirconium and/or hafnium contained in the braze material at a higher concentration than in the base superalloy then distributes into the surrounding superalloy material to achieve a solid homogenous joint. Advantageously, the braze material  22  contains no element that is not contained in the substrate  12  to be brazed so that no new element is introduced into the superalloy material, and the composition of the homogenized joint material is very similar to the substrate material  12 , thus providing a structural joint. 
         [0062]      FIG. 2  illustrates a different region of the substrate material  12  wherein a narrow gap discontinuity  24  (less than 0.001 inches) is repaired using only braze material particles  22  of the compositions disclosed herein. Since the width of the discontinuity  24  is limited, there is no need for filling the discontinuity with superalloy particles  20  as in  FIG. 1 , as the braze material  22  will flow into and fill the discontinuity during the solution heat treatment. 
         [0063]    In other embodiments the braze alloys disclosed herein may be formed as a foil or a wire and may be applied with any known process. Repair of a superalloy material surface containing a plurality of discontinuities may be accomplished by optionally filling the discontinuities with superalloy particles (as desired for wider openings), then conducting a heat treatment of the superalloy with a foil of one of the disclosed braze alloys disposed over the surface, causing the braze material to melt, to flow into the discontinuities and to fill around the superalloy particles, then to homogenize and to solidify as the melting temperature element diffuses into the substrate  12 . 
         [0064]    While various embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions may be made without departing from the invention herein.