Patent Description:
Active brazing is a method that may be used to join ceramic and metallic elements, or to join ceramic elements to each other. The alloys used to form actively brazed joints are unique from other known brazing alloys in at least some respects. Specifically, active brazing alloys based on silver, gold, or copper, for example, typically contain a certain content of "active" elements (e.g., titanium), which enables these elements, under high vacuum, to react with a ceramic surface to form a reaction layer. The reaction layer extends between the ceramic element and the molten brazing alloy to act as a connection between the ceramic element and the brazing alloy. However, active brazing alloys are generally characterized by a relatively low melting point temperature. As such, the low melting point may adversely limit the use of actively brazed joints in high-temperature environments, such as in a hot gas path in gas turbine applications. <CIT> suggests a method for manufacturing a pressure measuring cell, which has a ceramic platform and a ceramic measuring membrane, wherein the measuring membrane is joined with the platform pressure tightly by means of an active hard solder, or braze, wherein the method includes: providing the platform, the measuring membrane and the active hard solder, or braze, positioning the active hard solder, or braze, between the platform and the measuring membrane; melting the active hard solder, or braze, by irradiating the active hard solder, or braze, by means of a laser, wherein the irradiating of the active hard solder, or braze, occurs through the measuring membrane; and letting the active hard solder, or braze, solidify by cooling. Heating the solder by irradiation is performed to reduce overall heat intake and thus affect a faster cooling down of the solder such as to eliminate or at least reduce the formation of segregated phases. The teaching a based upon fast cooling of the solder. <CIT> suggests a process for bonding ceramic parts together or to metals involves coating the surface of a ceramic part with a reactive component and then soldering it to the similarly coated surface of another ceramic part or to the surface of a metal part. <CIT> is silent about the control of solder temperature over time and the kind of heating device used to heat the solder, or to perform the described method, respectively.

According to a first aspect of the invention as herein claimed, a method of processing a joint as set forth in claim <NUM> is provided. Further embodiments of the invention as herein claimed are set forth in the dependent claims.

The embodiments described herein relate to methods of processing an actively brazed joint to increase its operating temperature capabilities. The methods described herein include forming an actively brazed joint in a vacuum furnace by heating a joining metal alloy, which is between two components, to a first temperature that is higher than the liquidus temperature of the alloy. After a predetermined dwell time (typically between about <NUM> minutes and about <NUM> minutes), the brazed joint is then cooled to a second temperature that is lower than the solidus temperature of the alloy, yet higher than room temperature. The temperature within the vacuum furnace is maintained at the second temperature for a predefined duration. It is believed, without being bound by any particular theory, that maintaining the vacuum furnace at the elevated second temperature for the predefined duration facilitates initiating diffusion phenomena within a volume of the alloy. The diffusion phenomena facilitates creating regions of segregated crystallization within the volume of the alloy. The regions of segregated crystallization include increased concentrations of the individual elements contained in the alloy. For example, after this process, it was found that a layer of brazed metal, formed from the joining metal alloy, between the two components had an increased liquidus temperature as compared to the joining metal alloy in its pre-processed state. As such, the resulting actively brazed joint may be used in environments having higher maximum operating temperatures.

Unless otherwise indicated, approximating language, such as "generally," "substantially," and "about," as used herein indicates that the term so modified may apply to only an approximate degree, as would be recognized by one of ordinary skill in the art, rather than to an absolute or perfect degree. Accordingly, a value modified by a term or terms such as "about," "approximately," and "substantially" is not to be limited to the precise value specified. Additionally, unless otherwise indicated, the terms "first," "second," etc. are used herein merely as labels, and are not intended to impose ordinal, positional, or hierarchical requirements on the items to which these terms refer. Moreover, reference to, for example, a "second" item does not require or preclude the existence of, for example, a "first" or lower-numbered item or a "third" or higher-numbered item. As used herein, the term "upstream" refers to a forward or inlet end of a rotary machine, and the term "downstream" refers to a downstream or exhaust end of the rotary machine.

<FIG> is a side view illustration of an actively brazed joint <NUM>. Exemplarily shown actively brazed joint <NUM> includes a first component <NUM>, a second component <NUM>, and a volume <NUM> of a joining metal alloy. First component <NUM> and second component <NUM> may be fabricated from any material that enables actively brazed joint <NUM> to function as described herein. First component <NUM> may be fabricated from either a ceramic material or a metallic material, and second component <NUM> may be fabricated from a ceramic material. For example, the metallic material may be, but is not limited to only being, a nickel-based superalloy material. Thus, in one embodiment, first component <NUM> is a hot gas path component, such as a rotating component or a stationary component, of a gas turbine engine, and second component <NUM> is a sensor used to monitor process conditions within the gas turbine engine.

Prior to being processed in accordance with the methods described herein, the joining metal alloy is positioned between first component <NUM> and second component <NUM>. In its pre-processed state, volume <NUM> of joining metal alloy may have a thickness T defined within a range between about <NUM> microns and about <NUM> microns, and be in the form of a braze foil. The joining metal alloy is positioned to join first component <NUM> to second component <NUM>. The joining metal alloy may include any combination of elements that enables actively brazed joint <NUM> to function as described herein. For example, in one embodiment, the joining metal alloy includes, but is not limited to only including, silver, copper, indium, and at least one active element. Exemplary active elements include, but are not limited to, titanium, hafnium, zirconium, and/or niobium. Thus, in an exemplary embodiment, the joining metal alloy includes silver defined within a range between about <NUM> percent and about <NUM> percent by weight, copper defined within a range between about <NUM> percent and about <NUM> percent by weight, indium defined within a range between about <NUM> percent and about <NUM> percent by weight, and titanium at less than about <NUM> percent by weight.

<FIG> is an enlarged cross-sectional view of a portion of actively brazed joint <NUM> shown in <FIG>, and <FIG> is an enlarged cross-sectional view of a portion of actively brazed joint <NUM> shown in <FIG>. In the exemplary embodiment, volume <NUM> of joining metal alloy includes a plurality of elements that each having a weight percentage that defines an overall composition of the joining metal alloy. By processing actively brazed joint <NUM> in accordance with the methods to be described in more detail below, a layer <NUM> of brazed metal, formed from volume <NUM> of the joining metal alloy, includes at least one region <NUM> of segregated crystallization. The weight percentage of at least one of the elements of the joining metal alloy is greater in the at least one region <NUM> than in the overall composition. It is believed, without being bound by any particular theory, that the heat treatment processing methodology described herein facilitates the segregation of phases (elements) within volume <NUM>, wherein at least one of the individual phases, or each individual phase, has a higher melting point than the joining metal alloy as a whole. As such, performing the heat treatment processing described herein on actively brazed joint <NUM> facilitates increasing the operating temperature of the brazed metal.

Referring to <FIG>, the at least one region <NUM> includes, for example, at least a first region <NUM>, a second region <NUM>, and a third region <NUM> of segregated crystallization that are each distinct from each other. In the exemplary embodiment, first region <NUM> has a greater weight percentage of a first element (e.g., silver) than within second region <NUM>, third region <NUM>, and the overall composition of the joining metal alloy. Likewise, second region <NUM> has a greater weight percentage of a second element (e.g., copper) than within first region <NUM>, third region <NUM>, and the overall composition of the joining metal alloy, and third region <NUM> has a greater weight percentage of a third element (e.g., titanium) than within first region <NUM>, second region <NUM>, and the overall composition of the joining metal alloy. In some embodiments, the first element, the second element, and the third element are the predominant element in each region <NUM>, <NUM>, and <NUM>, in that each respective element has a greater weight percentage than any other elements included in each respective region <NUM>, <NUM>, and <NUM>. The predominant element in each respective region <NUM>, <NUM>, and <NUM> may constitute less than about <NUM> percent, greater than about <NUM> percent, greater than about <NUM> percent, or greater than about <NUM> percent by weight of the composition of each region <NUM>, <NUM>, and <NUM>.

<FIG> is a flow diagram illustrating an exemplary method <NUM> of processing a joint. In the exemplary embodiment, method <NUM> includes forming <NUM> a brazed joint, such as actively brazed joint <NUM> (shown in <FIG>), in a vacuum furnace. As noted above, the joining metal alloy of the brazed joint has a solidus temperature and a liquidus temperature. During forming <NUM>, the joining metal alloy is heated in the furnace to a first temperature that is higher than the liquidus temperature to facilitate joining first component <NUM> and second component <NUM> (both shown in <FIG>) to each other. After first component <NUM> and second component <NUM> are joined together, the heat treatment processing may be performed.

For example, method <NUM> also includes cooling <NUM> the brazed joint to a second temperature that is lower than the solidus temperature of the joining metal alloy, and maintaining <NUM> the second temperature within the vacuum furnace for a predefined duration. It is believed that maintaining <NUM> at the second temperature, within the vacuum furnace and of joining metal alloy, facilitates initiating the segregation of the phases within the joining metal alloy. The second temperature may be higher than about <NUM> percent, higher than about <NUM> percent, or higher than about <NUM> percent of the solidus temperature of the joining metal alloy in a pre-processed state. The second temperature is maintained <NUM> for a predefined duration of between about <NUM> minutes and <NUM> minutes. The vacuum furnace and the brazed joint may then be cooled and heated again to define a heating and cooling cycle. In one embodiment, the method <NUM> also includes performing <NUM> a plurality of heating and cooling cycles on the brazed joint.

In the exemplary embodiment, each heating and cooling cycle includes heating the brazed joint to a third temperature that is lower than the solidus temperature, and cooling the brazed joint from the third temperature. The third temperature may be equal to, higher than, or lower than the second temperature. The heating step in the cycle includes heating the brazed joint in the vacuum furnace set to a temperature, such as the third temperature, that is lower than the solidus temperature for a duration defined within a range between about <NUM> minutes and about <NUM> minutes. The cooling step in the cycle includes cooling the brazed joint, after the heating duration has elapsed, to an ambient temperature within a duration that is less than about <NUM> minutes, or less than about <NUM> minutes. For example, cooling of the brazed joint may be accelerated by directing a stream of compressed air towards the brazed joint. The heating step of the next heating and cooling cycle may then be initiated, by the setting the vacuum furnace to a temperature, within one minute of the brazed joint being cooled to the ambient temperature. As such, the thermal cycling is believed to accelerate and enhance the segregation of phases within the joining metal alloy. In an alternative embodiment, the cooling step in the cycle includes cooling the brazed joint, after the heating duration as elapsed, to an ambient temperature within a duration of approximately <NUM> hours.

As used herein, "ambient temperature" refers to a temperature range having a lower limit of about <NUM> degrees Celsius, and an upper limit of about <NUM> degrees Celsius.

Any number of heating and cooling cycles may be performed <NUM> on the brazed joint that enables method <NUM> to function as described herein. For example, the heat treatment processing of the brazed joint may include the performance of a predetermined number of heating and cooling cycles, such as at least <NUM> heating and cooling cycles, at least <NUM> heating and cooling cycles, or at least <NUM> heating and cooling cycles.

After completion of the predetermined number of heating and cooling cycles, the brazed joint may be subjected to a non-destructive inspection procedure and/or installed within an assembly, such as within the hot gas path of a gas turbine engine. The non-destructive inspection is used to verify the coverage, and thus the strength of, the bond between first component <NUM> and second component <NUM>. An example non-destructive inspection technique includes, but is not limited to, an ultrasonic inspection. In some embodiments, the temperature within the hot gas path of a gas turbine engine may be up to a predetermined temperature, such as about <NUM> degrees Celsius. In the exemplary embodiment, the brazed joint processed in accordance with method <NUM> is configured to operate within environments having a temperature greater than the predetermined temperature. For example, the brazed joint processed in accordance with method <NUM> has a liquidus temperature greater than the predetermined temperature.

The embodiments described herein relate to a method of processing a joint. The method and its more particular embodiments described herein facilitate increasing the liquid temperature of a joining metal alloy used in the actively brazed joint. For one instance, the processing steps of the method facilitate the formation of regions of segregated crystallization with a volume of the joining metal alloy, each region is formed from an increased concentration of individual elements of the alloy. The individual elements may have a greater liquidus temperature than the alloy as a whole. As such, the operating temperature of the actively brazed joint is facilitated to be enhanced.

The above description is meant to be exemplary only, and one skilled in the art will recognize that changes may be made to the embodiments described without departing from the scope of the invention as herein claimed is set forth in the appended claims.

Claim 1:
A method (<NUM>) of processing a joint (<NUM>), said method comprising:
forming (<NUM>) an actively brazed joint in a vacuum furnace from at least two components (<NUM>, <NUM>) coupled together by a volume (<NUM>) of a joining metal alloy having a solidus temperature and a liquidus temperature, wherein the joining metal alloy is heated to a first temperature that is higher than the liquidus temperature in the vacuum furnace; and
cooling (<NUM>) the actively brazed joint to a second temperature that is lower than the solidus temperature;
and maintaining (<NUM>) the second temperature within the vacuum furnace for a predefined duration to form at least one region (<NUM>) of segregated crystallization within the volume of the joining metal alloy, wherein the at least one region of segregated crystallization is configured to increase the liquidus temperature of a layer (<NUM>) of brazed metal, formed from the joining metal alloy, between the at least two components.