Claims:1. A braze alloy composition (16) comprising:
nickel, cobalt, or a combination of nickel and cobalt;
about 5 weight percent to about 30 weight percent germanium; and
about 15 weight percent to about 60 weight percent palladium.

2. The braze alloy composition (16) of claim 1, wherein the braze alloy composition (16) is free of silicon, boron, or both silicon and boron.

3. The braze alloy composition (16) of claim 1, wherein the braze alloy composition (16) is free of titanium, zirconium, hafnium, and vanadium.

4. The braze alloy composition (16) of claim 1, wherein nickel, cobalt, or the combination of nickel and cobalt is present in an amount of at least about 30 weight percent.

5. The braze alloy composition (16) of claim 1, wherein germanium is present in a range from about 5 weight percent to about 25 weight percent.

6. The braze alloy composition (16) of claim 1, wherein palladium is present in a range from about 20 weight percent to about 45 weight percent.

7. The braze alloy composition (16) of claim 1, further comprising aluminum in a range of from about 0.5 weight percent to about 10 weight percent.

8. The braze alloy composition (16) of claim 1, further comprising at least about 7 weight percent chromium.

9. The braze alloy composition (16) of claim 1, comprising a discontinuous phase comprising palladium-germanium.

10. The braze alloy composition (16) of claim 1, having a ductility level of at least about 2%.

11. The braze alloy composition (16) of claim 1, having a melting point in the range of about 950 degrees Celsius to about 1260 degrees Celsius.

12. A superalloy article (10) having a crack (12) that is filled with the braze alloy composition (16) of claim 1.

13. An article (20) comprising at least two metallic components (22, 24) joined together by the braze alloy composition (16) of claim 1.

14. A composite composition (14) comprising the braze alloy composition (16) in accordance with claim 1 and at least one nickel-based superalloy powder (18) having a melting temperature greater than the melting temperature of the braze alloy composition (16).

15. The composite composition (14) of claim 14, wherein a weight ratio of the braze alloy composition (16) to the nickel-based superalloy powder (18) is in the range of about 95 : 5 to about 20 : 80.

16. A superalloy article (10) having a crack (12) that is filled with the composite composition (14) of claim 14.

17. A gas turbine engine comprising at least two metallic components (22, 24) joined together by the composite composition (14) of claim 14.

18. A method for joining a metallic component (22) to another metallic component (24) by brazing, comprising:
(i) introducing a braze alloy composition (16) between the metallic components (22, 24); and

(ii) heating the braze alloy composition (16) to form a braze joint between the metallic components (22, 24);

wherein the braze alloy composition (16) comprises:
nickel, cobalt, or a combination of nickel and cobalt;
about 5 weight percent to about 30 weight percent germanium; and
about 15 weight percent to about 60 weight percent palladium.

19. The method of claim 18, wherein the heating step is carried out at a brazing temperature in a range of about 950 degrees Celsius to about 1260 degrees Celsius and in vacuum or in an atmosphere comprising argon, helium, hydrogen, or combinations thereof.

20. A method for repairing a superalloy article (10) of a turbine engine that includes at least one crack (12) in a surface of the superalloy article (10), comprising the following steps:
(a) preparing a composite composition (14), comprising a braze alloy composition (16) and a nickel-based superalloy powder (18) having a melting temperature greater than the melting temperature of the braze alloy composition (16);
(b) applying the composite composition (14) to the surface, over the at least one crack (12); and
(c) heating the composite composition (14), so as to cause it to flow and fill the at least one crack (12), and metallurgically bond to the surface upon cooling;
wherein the braze alloy composition (16) comprises:
nickel, cobalt, or a combination of nickel and cobalt;
about 5 weight percent to about 30 weight percent germanium; and
about 15 weight percent to about 60 weight percent palladium.
, Description:[0001] This disclosure generally relates to braze alloy compositions. In some specific embodiments, the disclosure relates to ductile braze alloy compositions useful in sealing and repair processes for various types of turbine engines.
BACKGROUND
[0002] High temperature cobalt-base and nickel-based superalloys are used in the manufacture of high temperature operating gas turbine engine components, including the nozzles, combustors, and turbine vanes and blades. During the operation of such components under strenuous high temperature conditions, various types of damage or deterioration can occur. For example, erosion and cracks tend to develop at the trailing edge of nozzles during service, due to stresses that are aggravated by frequent thermal cycling. Over time, the severe operating conditions of the nozzles can develop cracks that measure up to one millimeter wide and fifty millimeters or more in length. Because the cost of components formed from high temperature cobalt and nickel-based superalloys is relatively high, it is typically more desirable to repair these components than to replace them.
[0003] Brazing has become one of the most effective repair techniques in recent years, and many different braze materials are available. Just a few examples are set forth in U.S. Patents 6,165,290 (Rabinkin), 6,530,971 (Cohen et al), 6,520,401 (Miglietti), and 7,651,023 (Huang et al.). In addition to repairing cracks, the braze materials are often used for joining superalloy components together, and for providing critical seals in turbine engines, e.g. abradable honeycomb seals that need to be attached to carrier structures within a stator-rotor interface.
[0004] While many commercial braze alloys are effective for several applications, there continue to be serious needs for improvement in very specialized situations. In general, there are three key requirements for braze materials, whether used in a joining process or a repair process. First, they should be capable of being applied effectively to the component(s), e.g., with sufficient flow and wettability characteristics. Second, they must be capable of eventually solidifying into a joint or fill-material that exhibits strength, ductility, and oxidation resistance. Third, the materials must be cost-effective for a given application.
[0005] As technical requirements and other industrial needs increase, it is becoming more difficult to satisfy these general requirements for braze materials. For example, many brazing operations for gas turbine components continue to require braze materials with demanding flow characteristics. The materials must also melt at temperatures low enough to protect the base material or workpiece from becoming overheated or otherwise damaged. It is therefore often necessary to incorporate significant amounts of metalloid elements such as boron and silicon into the braze compositions.
[0006] However, significant levels of boron and silicon can be detrimental to the final braze product. For example, these elements tend to form brittle, hard intermetallic phases in the braze microstructure. The presence of boron may lead to the loss of ductility due to the presence of boride phases like Ni3B.
[0007] With these general concerns in mind, new braze compositions for use with nickel-based superalloys would be very welcome in the art. The compositions should have a melting point low enough for many current brazing operations (e.g., for turbine engines). They should also be free of boron, which can decrease braze integrity through unfavorable phase formation.
[0008] Moreover, the braze compositions should have flow and wettability characteristics which facilitate joint-forming or cavity-filling processes. The compositions should also be generally compatible with the component being brazed, e.g., in terms of microstructure. Furthermore, after solidifying, the braze compositions should exhibit the necessary characteristics for a given end use application, e.g., a desirable level of strength, ductility, and oxidation resistance. The braze compositions should also be cost-effective for given end use objectives.
BRIEF DESCRIPTION
[0009] One embodiment of the disclosure is directed to a braze alloy composition, including:
nickel, cobalt, or a combination thereof;

about 5 weight percent to about 30 weight percent germanium; and

about 15 weight percent to about 60 weight percent palladium.

[0010] Another embodiment relates to a composite composition including the braze alloy composition described herein, in combination with a nickel-based superalloy powder having a melting temperature greater than the melting temperature of the braze alloy composition.
[0011] Yet another embodiment contemplates a superalloy article that contains a crack filled with the braze alloy composition or the composite composition described herein, and a method for filling such a crack with the braze alloy composition or the composite composition.
[0012] Another embodiment relates to an article that includes at least two metallic components joined together by the braze alloy composition described herein.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] These and other features, aspects, and advantages of the present disclosure will become better understood when the following detailed description is read with reference to the accompanying drawings, wherein:
[0014] FIG. 1 is a photograph of one type of crack in a superalloy article.
[0015] FIG. 2 is a photograph of another type of crack in a superalloy article.
[0016] FIG. 3A is an illustration of a crack in a superalloy article being filled with a composite material, according to some embodiments of this disclosure.
[0017] FIG. 3B is an illustration of an article having two metallic components joined together using a braze alloy composition, according to some embodiments of this disclosure.
[0018] FIG. 4 is a magnified photomicrograph of a braze alloy composition, according to some embodiments of this disclosure.
[0019] FIG. 5 is a magnified photomicrograph of a braze joint fillet formed using the braze alloy composition of FIG. 4.
[0020] FIG. 6 is a magnified photomicrograph of a braze joint between two metallic components using a braze alloy composition, according to some embodiments of this disclosure.
[0021] FIG. 7 is a magnified photomicrograph of a conventional braze alloy composition.
[0022] FIG. 8 is a magnified photomicrograph of a braze joint between two metallic components using the conventional braze alloy composition of FIG. 7.
DETAILED DESCRIPTION
[0023] It should be noted that when introducing elements of various embodiments of the present invention, the articles “a,” “an,” “the,” and “said” are intended to mean that there are one or more of the elements, unless otherwise indicated. Moreover, the term “and/or” includes any and all combinations of one or more of the associated listed items. The terms “comprising,” “including,” and “having” are intended to be inclusive, and mean that there may be additional elements other than the listed elements. Approximating language, as used herein throughout the specification and claims, may be applied to modify any quantitative representation that could permissibly vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as “about” or “substantially” is not to be limited to the precise value specified. In some instances, the approximating language may correspond to the precision of an instrument for measuring the value.
[0024] Additional terminology related to braze compositions and processes may be useful in this disclosure. Typically, "brazing" uses a braze material (usually an alloy) having a lower liquidus temperature than the melting points of the components (i.e., their materials) to be joined. The braze material is brought slightly above its melting (or liquidus) temperature while protected by a suitable atmosphere (or vacuum). The braze material then flows over the components (known as wetting), and is then cooled to join the components together. As used herein, "braze alloy composition", "braze alloy", "braze material" or "brazing alloy", all refer to a composition that has the ability to wet the components to be joined, and to seal them.
[0025] A braze alloy, for a particular application, should withstand the service conditions required, and should melt at a lower temperature than the base materials; or melt at a specified temperature. As used herein, the term “liquidus temperature” generally refers to a temperature at which an alloy is transformed from a solid or semi-solid into a molten or viscous state. The liquidus temperature specifies the maximum temperature at which crystals can co-exist with the melt in thermodynamic equilibrium. Above the liquidus temperature, the alloy is homogeneous, and below the liquidus temperature, more and more crystals begin to form in the melt with time, depending on the alloy. Generally, an alloy, at its liquidus temperature, melts and forms a seal between two components to be joined. The liquidus temperature can be contrasted with a “solidus temperature”. The solidus temperature quantifies the point at which a material completely solidifies (crystallizes). The liquidus and solidus temperatures do not necessarily align or overlap. If a gap exists between the liquidus and solidus temperatures, then within that gap, the material consists of solid and liquid phases simultaneously (for example, like a “slurry”).
[0026] As used herein, the term "brazing temperature" refers to a temperature to which a brazing structure is heated to enable a braze alloy to wet the components to be joined, and to form a braze joint, seal, or filler-structure. The brazing temperature is often higher than or equal to the liquidus temperature of the braze alloy. In addition, the brazing temperature should be lower than the temperature at which the components to be joined may not remain chemically, compositionally, and mechanically stable. There may be several other factors that influence the brazing temperature selection, as those skilled in the art understand.
[0027] As used herein, the term, “braze joint fillet” refers to a casting along the outside of a braze joint that shows that the brazing material has melted and flowed along the edges or corners of a braze joint.
[0028] Moreover, as used herein for some of the embodiments, "sealing" is a function performed by a structure that joins other structures together, to reduce or prevent leakage through a joint, between the other structures. A seal structure may simply be referred to as a "seal."
[0029] As used herein, “ductility” refers to the ability of a solidified braze alloy to deform under tensile stress. Similar (though not identical) to the term “malleability”, ductility can be characterized by plasticity, i.e., the extent to which a braze structure (including solidified braze alloy) can be plastically deformed without fracture or other unacceptable degradation. For embodiments described herein, ductility is measured according to ASTM Method E 8-04, on an equiaxed casting, in as-cast condition, or after a heat treatment/anneal consisting of a solution-treatment above 800ºC and one or more aging steps between 700-1100ºC, for 0.5-10 hours.
[0030] The term, “weight percent”, as used herein, refers to a weight percent (weight %) of each referenced element in an alloy composition based on a total weight of the alloy composition, and is applicable to all incidences of the term “weight percent” as used herein throughout the specification.
[0031] Some embodiments of the disclosure are directed to a braze alloy composition that includes nickel, cobalt, or a combination thereof; germanium; and palladium. In some embodiments, the braze alloy composition is free of silicon, boron, or both silicon and boron. In some embodiments, the braze alloy composition is free of manganese. In some embodiments, the braze alloy composition is free of an active element selected from the group consisting of titanium, zirconium, hafnium, and vanadium.
[0032] The braze alloy composition is nickel-based or cobalt-based, i.e., with nickel or cobalt being the predominant element. The braze alloy composition includes nickel, cobalt, or a combination of nickel and cobalt in an amount at least 30 weight %, based on a total weight of the braze alloy composition. In some instances, nickel, cobalt, or the combination of nickel and cobalt is present in a range of from about 35 weight % to about 70 weight %, based on the total weight of the braze alloy composition. The level of nickel, cobalt, or the combination of nickel and cobalt may be in a range of from about 40 weight % to about 65 weight %, and in certain embodiments, up to about 60 weight %.
[0033] In certain embodiments, the braze alloy composition is a nickel-based braze alloy composition. Cobalt is especially desirable when a substrate (one or more components being joined or repaired) also contain cobalt. The presence of cobalt in a nickel-based braze alloy composition may enhance oxidation resistance and corrosion resistance at high temperatures, along with improving microstructural stability, as well as creep resistance. In these instances, the amount of cobalt may depend on many of the factors, as well as the amount of cobalt present in a substrate that will be attached to the braze alloy composition. Usually, the level of cobalt is in the range of about 0.5 weight % to about 40 weight %, and more specifically, about 0.5 weight % to about 30 weight %. In some instances, the preferred level of cobalt is about 1 weight % to about 10 weight %. Moreover, combinations of chromium and cobalt are sometimes preferred.
[0034] The braze alloy composition (sometimes referred to simply as the “alloy composition”) includes about 15 weight % to about 60 weight % palladium (Pd) and about 5 weight percent to about 30 weight percent germanium (Ge). Both palladium and germanium independently retain relatively high ductility for the braze joint and braze structure, while also functioning to reduce the melting point of the braze alloy composition i.e., function as a melting point depressant.
[0035] These braze alloy compositions may be characterized by the presence of a lamellar eutectic phase that provides high strength to the braze alloy composition. The lamellar eutectic phase includes an intermetallic phase that includes palladium and germanium. In one embodiment, the intermetallic phase includes Pd2Ge phase. Typically, the intermetallic phase Pd2Ge may not be desirable because it is brittle in nature. However, in the braze alloy composition as described herein, the intermetallic phase (i.e., Pd2Ge) is present discontinuously. This discontinuous phase forms the lamellar eutectic phase with nickel, cobalt, or the combination of nickel and cobalt. As used herein, the term “discontinuous phase” refers to an intermetallic phase that is not present continuously throughout the base matrix. In other words, the particles of the discontinuous phase are not continuously connected throughout the base matrix. For example, FIG. 4 shows a lamellar eutectic phase (Ni+Pd2Ge) in a braze alloy composition Ni-41.5Pd-6.6Ge-11.6Cr (in weight percent) that includes the intermetallic Pd2Ge phase present discontinuously in the nickel matrix. This intermetallic phase that is present discontinuously in the braze alloy composition may act as a strengthening phase and helps in improving the strength of the braze alloy composition.
[0036] As observed by the present Inventors, the use of equivalent amount of silicon in place of germanium in such alloy compositions resulted in connected and continuous phases, for example brittle Pd3Si phase in the nickel matrix of a nickel-based alloy composition as shown in Figures 7 and 8. FIG. 7 shows microstructure of a braze alloy composition Ni(balance)-46.6Pd-6.2Si (in weight percent) and FIG. 8 shows a micrograph of a braze joint between two metallic components formed using the braze alloy composition of FIG. 7. These continuous brittle phases undesirably lead to a substantial loss of ductility by providing a pathway for rapid crack propagation. Therefore, as compared to silicon-containing alloy compositions (for example, that include nickel, palladium and silicon), the braze alloy composition including palladium and germanium, as described herein, provide relatively improved strength, ductility, and phase stability at high temperatures. These attributes balanced with low viscosity or “flowability” are key advantages over the use of silicon for melting point depression.
[0037] Several factors may influence the levels of palladium and germanium. They include: the specific composition of the workpiece(s) (for example, metallic components) being brazed, the desired melting point for the braze alloy composition, ductility requirements for the resulting braze joint and braze structure, oxidation resistance requirements for the braze joint, the type of brazing technique employed, and the identity of the other elements present in the braze alloy composition (which may affect the intermetallic phase and/or lamellar eutectic phase in the braze alloy composition, for example). In some embodiments, palladium is present in a range of from about 18 weight % to about 55 weight %, based on the total weight of the braze alloy composition. In some embodiments, palladium is present in a range of from about 20 weight % to about 50 weight %. Furthermore, in some specific embodiments, palladium is present in a range of from about 25 weight % to about 45 weight %.
[0038] The amount of germanium may depend on many of the factors discussed above, including formation of intermetallic phase, i.e., Pd2Ge phase. The general range for germanium may be from about 5 weight % to about 30 weight %, based on the total weight of the braze alloy composition. In specific embodiments, germanium is present in a range of from about 6.5 weight % to about 26 weight %. In some specific embodiments, germanium is present in a range of from about 12 weight % to about 22 weight %.
[0039] Levels of germanium less than about 5 weight % will not provide the melting point range often needed for these alloy compositions, e.g., about 950 degrees Celsius to about 1260 degrees Celsius. The present inventors also discovered that, for the specific braze alloy compositions described herein, high levels of germanium (higher than the upper ranges noted above) may result in undesirably low ductility levels. A high amount of germanium (for example, higher than 30 weight percent) in the braze alloy composition may result in the formation of brittle intermetallic compounds such as Ni2Ge etc., or in the formation of one or more ternary phases (for example, NiPdGe or Ni2Pd4Ge3 that are brittle in nature and detrimental for desired ductility.
[0040] In some embodiments, the braze alloy composition further includes aluminum. Aluminum can provide enhanced strength and excellent oxidation resistance, especially at high temperatures. The level of aluminum may depend on many of the factors set forth above, and may be present in an amount up to 10 weight % based on the total weight of the braze alloy composition. A high amount (>10 weight percent) of aluminum may lead to the undesirable formation of brittle intermetallic compounds such as Pd2Al (in some embodiments, as a continuous center-line compound Pd2Al that is deleterious to the braze joint ductility). In some embodiments, the level of aluminum can be as low as 0.1 weight % based on the total weight of the braze alloy composition. Usually, aluminum is present in a range of from about 0.1 weight% to 10 weight %. In some embodiments, aluminum is present in a range of from about 0.25 weight % to about 8 weight %, and for some specific embodiments, from about 0.5 weight % to about 6 weight %.
[0041] The braze alloy composition may further include chromium, which can also provide excellent oxidation and corrosion resistance, especially at high brazing temperatures. The level of chromium may depend on many of the factors set forth above, and is often present in an amount of at least about 7 weight %. In some embodiments, chromium is present in an amount at least about 12 weight %. In some embodiments, the amount of chromium can be as high as about 25 weight % or even 30 weight %, based on the total weight of the braze alloy composition. Usually, chromium is present in a range of from about 8 weight % to about 25 weight %. In some specific embodiments, chromium is present in a range of from about 10 weight % to about 20 weight %.
[0042] The braze alloy composition may also contain at least one element selected from tantalum, niobium, molybdenum, and tungsten. Each of these elements can enhance the strength of the braze joint, e.g., by strengthening the base matrix by solid solution strengthening. When used, these elements are typically present at a level (individually) of about 0.5 weight % to about 8 weight %, and in some embodiments, from about 0.5 weight % to about 5 weight %. However, their specific levels are to some degree dependent on the workpiece(s) being brazed, as well as the desired liquidus temperature for the braze alloy composition. Moreover, the required balance between strength and ductility for the braze joint is an important consideration. Iron may also sometimes be present, at the same levels as the refractory elements.
[0043] With regard to these optional elements, the braze alloy composition is often formulated to be compatible with the composition of the superalloy of the workpiece(s), i.e., the part(s) being joined or repaired. A workpiece may be an article or a component that includes a superalloy. As an example, the workpiece may sometimes include a nickel-based superalloy, like RenèTM80 or GTDTM111, which contain relatively high levels of molybdenum. In that instance, the braze alloy composition might preferably contain molybdenum as well, e.g., about 0.1 weight % to about 4 weight %. As an example, a workpiece may be formed from an alloy like RenèTM N5, contains small amounts of tantalum (Ta). In such a case, the braze alloy composition might contain about 0.1 weight % to about 2 weight % Ta. In general, some of the braze alloy compositions used in very high-temperature applications may include at least one refractory metal, e.g., tantalum, tungsten, or molybdenum.
[0044] Other elements that are sometimes included in the braze alloy composition may be carbon and yttrium. Carbon may be beneficial in forming carbides that control grain growth; while yttrium can improve oxidation performance. Many of the factors described previously provide guidance as to the inclusion of these elements, and their relative amounts. Usually, each element is optionally present (independently) at a level in the range of from about 0.01 weight % to about 0.20 weight %.
[0045] Moreover, in some embodiments, additional phases may be present – either individually, or in combination. These additional phases may include, but not limited to, an alpha-Cr phase that forms due to additions of a high amount (> 25 weight %) of chromium and Pd2Al or PdAl that may form due to the addition of a high amount (> 5 weight %) of aluminum.
[0046] In some embodiments, the braze alloy composition is free of boron, since significant levels of boron can be detrimental to the braze joint and the braze structure. Boron promotes the formation of brittle intermetallic phases in the microstructure of the braze alloy composition. An example of such an undesirable phase is Ni3B. In some embodiments, the braze alloy composition is free of silicon. Like boron, silicon can be an effective melting point depressant. As discussed above, its presence in large amounts (for example, higher than 3 weight percent) can also result in the formation of brittle intermetallic phases. Examples of the undesirable intermetallic phases include Ni3Si, Pd3Si, Ni-Pd-Si based ternary compounds (such as Ni18Pd7Si9, Pd2NiSi), and the like. However, in some embodiments, silicon (Si) may be present as impurity in germanium, i.e., at levels less than 0.5 percent or levels that minimize the formation of the aforementioned undesirable intermetallic phases. Moreover, in many instances, the braze alloy composition is free of gallium. The melting point of gallium is near room temperature, and it is considered a liquid metal embrittlement agent that is highly corrosive to metals and alloys.
[0047] Further, the braze alloy composition is free of titanium, hafnium, zirconium, and vanadium. Each of these elements can result in a significant reduction in ductility, due to the formation of brittle intermetallic compounds with germanium, such as Ni16Ti6Ge8. If one or more of titanium, hafnium, zirconium, and vanadium were to be included because of for example, the composition of a workpiece (based on previous discussion), one or more of these active elements may be present at a level less than about 0.5 weight %, and preferably, less than about 0.25 weight %. As one illustration, a high titanium level (for example, higher than 1 weight percent) may embrittle the braze alloy composition for some end uses. In one embodiment, the braze alloy composition is free to titanium. Further, in some embodiments, the braze alloy composition is free of indium, tin, antimony, bismuth, and strontium. Moreover, in some embodiments, the braze alloy composition is free of silver, copper, and gold.
[0048] It should also be noted that in some embodiments, manganese is not desirable for inclusion in the braze alloy compositions. In some embodiments, the braze alloy composition is free of manganese. While manganese is useful as a melting point depressant in various nickel-based braze alloy compositions, its presence here is undesirable due to the formation of brittle intermetallic MnPd phase. The formation of the MnPd phase in addition to Pd2Ge phase may embrittle the alloy by increasing the total amount of brittle intermetallic phases.
[0049] Embodiments of this disclosure provide braze alloy compositions with a relatively high level of ductility, while maintaining other required braze properties, like strength and flow-properties. As mentioned previously, the presence of the intermetallic Pd2Ge phase in a discontinuous manner (compared to the continuous Pd3Si in Ni-Pd-Si alloy compositions) appears to be responsible for maintaining the ductility and providing additional strength to the braze joints and braze structures formed using the braze alloy composition. In some embodiments, the braze alloy composition is characterized by a ductility level of at least about 3%, as measured by the ASTM test described previously. In some embodiments, the ductility level is at least 5 %. This level of ductility is superior to that of various braze alloy compositions based on combinations of nickel, silicon, and boron, wherein the ductility is often less than 1%, based on the same measurement scale. With regard to flow properties, the melting point of the braze alloy composition may be in a range of from about 950 degrees Celsius to about 1260 degrees Celsius. In some specific embodiments, the melting point of the braze alloy composition is in a range of from about 1050 degrees Celsius to about 1225 degrees Celsius.
[0050] In some embodiments, the braze alloy composition (or multiple braze alloy compositions) constitutes part of a composite composition, which also includes one or more than one nickel-based superalloy powders. The nickel-based superalloy powder has a melting temperature greater than the melting temperature of the braze alloy composition(s), and often functions as a filler material, e.g., in the case of wide-gap cracks or other sites that require filling. (In some cases, more than one brazing powder could be employed). The weight-ratio of braze alloy composition(s) to the nickel-based superalloy powder is in a range of from about 95 : 5 to about 20 : 80. In some specific embodiments, the ratio can be from about 70 : 30 to about 30 : 70.
[0051] The use of composite composition for this purpose is described, for example, in “Wide-Gap Brazing: A Practical Approach to a Difficult Process” D. Fortuna, White Paper – Wide-Gap Brazing 2002.05 (Oerlikon Metco) (2014). The composite material often exhibits an interdiffusion zone between particles of the braze alloy composition and particles of the superalloy powder, with a “metallurgical bond” formed between the particles, to provide a strong braze joint. The presence of brittle intermetallic phases in the interdiffusion zone is minimized by having the intermetallic phase in the discontinuous manner for most composite compositions according to present embodiments.
[0052] The braze alloy composition can also include a number of additives that are useful for selected purposes. The selection of any particular additive will depend on many of the factors described above, as well as the manner in which the braze alloy composition will be used, e.g., as a powder, paste, slurry, and the like. Braze slurries, for example, usually contain at least one binder and a solvent, e.g., aqueous or organic solvents. The binders are often water-based materials such as polyethylene oxide and various acrylics; or solvent-based materials.
[0053] Other additives that can be used in a braze alloy composition include dispersants, wetting agents, deflocculants, stabilizers, anti-settling agents, thickening agents, plasticizers, emollients, lubricants, surfactants, anti-foam agents, polymer gels, and curing modifiers.
[0054] It should also be understood that articles (for example, superalloy articles) that include braze joints formed using the braze alloy composition or the composite composition; and /or that can be repaired using the braze alloy composition or the composite composition constitute other embodiments of this disclosure. As one non-limiting example, turbine engine nozzles, combustors, and blades may develop cracks that can be repaired using the braze alloy composition or the composite composition described herein.
[0055] As is apparent from this disclosure, gas turbines represent desirable articles that can benefit from the use of the braze alloy compositions described herein. Gas turbine engines (e.g., turbomachinery) can be found in several different industrial environments, e.g., aviation engines, land-based turbines (heavy frame and aeroderivative), marine applications, and the like. Furthermore, in terms of other uses for the braze alloy composition, articles that include at least two metallic components joined together by a braze joint represent another embodiment of the disclosure.
[0056] Some embodiments of the present disclosure are directed to an article that includes at least two metallic components joined together by the braze alloy composition as described hereinabove. In some embodiments, the metallic components include high-temperature superalloys, for example nickel-based superalloy. The braze alloy composition includes nickel, cobalt or a combination thereof; about 5 weight percent to about 30 weight percent germanium; and about 10 weight percent to about 60 weight percent palladium. Other material details of the braze alloy compositions are described above. Some embodiments relate to a method for joining a metal component to another metal component by brazing. The method includes (i) introducing a braze alloy composition between the metal components and (ii) heating the metal components to form a braze joint (i.e., seal) between them.
[0057] The heating step for this method is typically carried out at a brazing temperature, greater than or equal to the liquidus temperature of the braze alloy composition, and less than the melting temperatures of the metal components to be joined. In some cases, the brazing temperature is in the range of from about 950 degrees Celsius to about 1260 degrees Celsius. Further, the heating step may be carried out in vacuum or in an atmosphere including argon, helium, hydrogen, or combinations thereof. Particular conditions (i.e., brazing temperature and atmosphere) may depend on a variety of factors described above.
[0058] Frequently, an appropriate brazing range is from about 1000oC to about 1250oC, while in other cases, an ideal range is from about 1100oC to about 1225oC. Moreover, in certain embodiments, the process is carried out so that the braze alloy composition, as applied, is free of aluminum, but later incorporates aluminum. As described previously, the aluminum is incorporated as part of a process that causes aluminum to diffuse from surrounding metal components (e.g., superalloys) into the braze alloy composition/braze joint, where it can increase braze joint strength in some instances.
[0059] Some embodiments present an article including a crack filled with the braze alloy composition or the composite composition as described hereinabove. In some embodiments, the article includes a superalloy article, i.e., a component formed of a superalloy material. A method for filling at least a crack in a surface of an article with the braze alloy composition or the composite composition represents another embodiment of the disclosure.
[0060] Some embodiments provide a method for repairing an article (for example, superalloy article) of a turbine engine that includes at least one crack in a surface of the article. The method includes the steps of (a) preparing a composite composition including a braze alloy composition and a nickel-based superalloy powder having a melting temperature greater than the melting temperature of the braze alloy composition; (b) applying the composite composition to the surface, over the at least one crack; and (c) heating the composite composition so as to cause it to flow and fill the at least one crack, and metallurgically bond to the surface upon cooling.
[0061] The method usually includes the following steps: (I) incorporating a braze alloy composition into the cavity, wherein the braze alloy composition is as described above; (II) heating the braze alloy composition to a brazing temperature sufficient to melt the braze alloy composition and to cause it to flow and completely fill the cavity, while not melting any surrounding material of the article; and (III) cooling the braze alloy composition so that it re-solidifies within the cavity. As also described previously, the braze alloy composition can be in the form of a composite composition with selected superalloy powders.
[0062] FIGS. 1 and 2 depict portions of nozzle sections of turbine engines, formed from nickel-based alloys. The cracks are very apparent in each figure, and represent damage to the nozzle section, requiring repair, or complete replacement of the nozzle. The method embodiments described above can be very effective for these types of crack repair.
[0063] FIG. 3A is an illustration of a relatively large crack 12 in a superalloy article 10 being filled with a composite composition 14 according to some embodiments. The superalloy article 10 may include a nickel-based superalloy. The composite composition 14 may include the braze alloy composition and a nickel-based superalloy powder, as described previously. The inset shows a magnified section of the filled crack 12 that includes melted braze alloy composition 16 along with particle/sections 18 of un-melted nickel-based superalloy powder, which is incorporated or “packed” into the crack 12.
[0064] FIG. 3B is an illustration of an article 20 that includes two metallic components 22 and 24 joined together by the braze alloy composition 16 as described hereinabove. In some embodiments, the metallic components include nickel-based superalloys.
[0065] FIGS. 4 and 5 are photomicrographs of the microstructures of a braze alloy composition and the corresponding braze joint fillet, under high-magnification. The braze alloy composition of the FIG. 4, in weight percent, is Ni-41.5Pd-6.6Ge-11.6Cr and FIG. 5 shows microstructure of the corresponding braze joint fillet formed on joining metallic components using the braze alloy composition as shown in FIG 4. FIG. 6 shows microstructure of a braze joint formed between metallic components using a braze alloy composition 40Ni-25Pd-5Al-12Ge-9Cr-9Co (in weight%). Both braze alloy compositions are within the scope of this disclosure. In each braze alloy composition (FIG. 5 and FIG. 6), the intermetallic Pd2Ge phase is discontinuous, i.e., the phase particles are disconnected from each other. The braze alloy composition of FIG. 6 further includes the intermetallic phase Pd2Al that is also present in discontinuous manner. The high ductility and high strength exhibited by these braze joint fillets compared to that of braze joint fillets formed using silicon-containing or boron-containing nickel-based braze alloys appears to have resulted from the microstructure shown in the FIG. 5 and FIG 6.
EXAMPLES
[0066] The examples that follow are merely illustrative, and should not be construed to be any sort of limitation on the scope of the claimed invention. Unless specified otherwise, all ingredients should be commercially available from such common chemical suppliers as Alpha Aesar, Inc. (Ward Hill, Massachusetts), Sigma Aldrich (St. Louis, Missouri), Spectrum Chemical Mfg. Corp. (Gardena, California), and the like.
[0067] The nickel-based braze alloy samples shown in Table 1 were prepared. For each braze alloy sample, individual elements were weighed according to the desired composition. These elements were arc-melted to provide an ingot for each composition. To ensure homogeneity of the compositions, the ingots were triple-melted. The ingots were then characterized for measuring liquidus temperatures, using a Differential Scanning Calorimeter (DSC) technique.

TABLE 1

S.No. Braze alloy composition
(weight percent) Melting temperature (oC)
1 48.7Ni-29.9Pd-21.4Ge 1071
2 44Ni-36.2Pd-19.8Ge 1081
3 43Ni- 42.5Pd- 14.5Ge 1100
4 38.7Ni-42.1Pd-19.2Ge 1050
5 40.3Ni-41.5Pd-6.6Ge-11.6Cr 1193
6 40Ni-30Pd-3Al-12Ge-15Cr 1182
7 40Ni-25Pd-5Al-12Ge-9Cr-9Co 1215

[0073] This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.