METHOD OF FORMING A DIFFUSION BONDED JOINT

Abstract

A method of forming a diffusion bonded joint comprises the steps of: providing a first component having a first faying surface; providing a second component having a second faying surface; applying a lamellar coating to at least one of the first faying surface and the second faying surface; and bringing the first and second faying surfaces into contact in a diffusion bonding operation to form the diffusion bonded joint.

Claims

1. A method of forming a diffusion bonded joint, the method comprising the steps of: providing a first component (51) having a first faying surface; providing a second component (52) having a second faying surface; applying a lamellar coating (53) to at least one of the first faying surface and the second faying surface, the lamellar coating (53) comprising a plurality of layers of differing composition, the layers of the lamellar coating (53) being composed of the same elements as a bulk material of one or more of the first component (51) and the second component (52), the lamellar coating (53) having substantially the same ratio of elements as the bulk material of one or more of the first component (51) and the second component (52); and bringing the first and second faying surfaces into contact in a diffusion bonding operation to form the diffusion bonded joint.

2. The method according to claim 1, wherein the lamellar coating (53) comprises adjacent layers respectively comprising elements with different atomic radii.

3. The method according to claim 2, wherein adjacent layers in the lamellar coating (53) comprise elements having a ratio of atomic radii of 1.15 or more, the ratio being defined as the ratio of larger elements to smaller elements based on calculated atomic radii, optionally 1.18 or more.

4. The method according to claim 1, wherein the lamellar coating (53) has a thickness of from 10 to 70 microns, optionally from 20 to 60 microns, further optionally from 30 to 50 microns.

5. The method according to claim 1, wherein individual layers within the lamellar coating (53) each have a thickness of from 0.10 to 1.25 microns, optionally from 0.40 to 1.10 microns, and further optionally from 0.50 to 1.00 microns.

6. The method according to claim 1, wherein individual layers within the lamellar coating (53) are formed by electron beam physical vapour deposition.

7. The method according to claim 1, further comprising applying the lamellar coating (53) in a vacuum.

8. The method according to claim 7, further comprising maintaining the first and second faying surfaces in a vacuum between the step of applying the lamellar coating (53) and the step of bringing the first and second faying surfaces into contact in a diffusion bonding operation.

9. The method according to claim 1, wherein the diffusion bonding operation is initiated by a local heat source at a part of the joint, such as a chemical or electrical fuse, or a radiant heat source.

10. The method according to claim 9, further comprising removing the part of the joint at which the local heat source was applied.

11. The method according to claim 1, wherein the diffusion bonding operation is performed under isostatic pressure.

12. The method according to claim 1, wherein the lamellar coating (53) comprises a layer of transition metal adjacent to a layer of transition metal enriched with Zirconium, adjacent to a further layer of transition metal, adjacent to a layer of transition metal enriched with Aluminium.

13. The method according to claim 1, wherein the lamellar coating (53) comprises a layer of transition metal adjacent to a layer of transition metal enriched with Tantalum, adjacent to a further layer of transition metal, adjacent to a layer transition metal enriched with Boron.

14. The method according to claim 1, wherein the lamellar coating (53) comprises a layer of transition metal adjacent to a layer of transition metal enriched with Tungsten, adjacent to a second layer of transition metal, adjacent to a layer of transition metal enriched with Titanium, adjacent to a third layer of transition metal, adjacent to a layer of transition metal enriched with Carbon.

Description

[0050] Embodiments will now be described by way of example only, with reference to the Figures, in which:

[0051] FIG. 1 is a sectional side view of a gas turbine engine;

[0052] FIG. 2 is a close up sectional side view of an upstream portion of a gas turbine engine;

[0053] FIG. 3 is a partially cut-away view of a gearbox for a gas turbine engine; and

[0054] FIG. 4 is a schematic representation of a process for creating a diffusion bond in a localised encapsulation arrangement.

[0055] FIG. 1 illustrates a gas turbine engine 10 having a principal rotational axis 9. The engine 10 comprises an air intake 12 and a propulsive fan 23 that generates two airflows: a core airflow A and a bypass airflow B. The gas turbine engine 10 comprises a core 11 that receives the core airflow A. The engine core 11 comprises, in axial flow series, a low pressure compressor 14, a high-pressure compressor 15, combustion equipment 16, a high-pressure turbine 17, a low pressure turbine 19 and a core exhaust nozzle 20. A nacelle 21 surrounds the gas turbine engine 10 and defines a bypass duct 22 and a bypass exhaust nozzle 18. The bypass airflow B flows through the bypass duct 22. The fan 23 is attached to and driven by the low pressure turbine 19 via a shaft 26 and an epicyclic gearbox 30.

[0056] In use, the core airflow A is accelerated and compressed by the low pressure compressor 14 and directed into the high pressure compressor 15 where further compression takes place. The compressed air exhausted from the high pressure compressor 15 is directed into the combustion equipment 16 where it is mixed with fuel and the mixture is combusted. The resultant hot combustion products then expand through, and thereby drive, the high pressure and low pressure turbines 17, 19 before being exhausted through the nozzle 20 to provide some propulsive thrust. The high pressure turbine 17 drives the high pressure compressor 15 by a suitable interconnecting shaft 27. The fan 23 generally provides the majority of the propulsive thrust. The epicyclic gearbox 30 is a reduction gearbox.

[0057] An exemplary arrangement for a geared fan gas turbine engine 10 is shown in FIG. 2. The low pressure turbine 19 (see FIG. 1) drives the shaft 26, which is coupled to a sun wheel, or sun gear, 28 of the epicyclic gear arrangement 30. Radially outwardly of the sun gear 28 and intermeshing therewith is a plurality of planet gears 32 that are coupled together by a planet carrier 34. The planet carrier 34 constrains the planet gears 32 to precess around the sun gear 28 in synchronicity whilst enabling each planet gear 32 to rotate about its own axis. The planet carrier 34 is coupled via linkages 36 to the fan 23 in order to drive its rotation about the engine axis 9. Radially outwardly of the planet gears 32 and intermeshing therewith is an annulus or ring gear 38 that is coupled, via linkages 40, to a stationary supporting structure 24.

[0058] Note that the terms low pressure turbine and low pressure compressor as used herein may be taken to mean the lowest pressure turbine stages and lowest pressure compressor stages (i.e. not including the fan 23) respectively and/or the turbine and compressor stages that are connected together by the interconnecting shaft 26 with the lowest rotational speed in the engine (i.e. not including the gearbox output shaft that drives the fan 23). In some literature, the low pressure turbine and low pressure compressor referred to herein may alternatively be known as the intermediate pressure turbine and intermediate pressure compressor. Where such alternative nomenclature is used, the fan 23 may be referred to as a first, or lowest pressure, compression stage.

[0059] The epicyclic gearbox 30 is shown by way of example in greater detail in FIG. 3. Each of the sun gear 28, planet gears 32 and ring gear 38 comprise teeth about their periphery to intermesh with the other gears. However, for clarity only exemplary portions of the teeth are illustrated in FIG. 3. There are four planet gears 32 illustrated, although it will be apparent to the skilled reader that more or fewer planet gears 32 may be provided within the scope of the claimed invention. Practical applications of a planetary epicyclic gearbox 30 generally comprise at least three planet gears 32.

[0060] The epicyclic gearbox 30 illustrated by way of example in FIGS. 2 and 3 is of the planetary type, in that the planet carrier 34 is coupled to an output shaft via linkages 36, with the ring gear 38 fixed. However, any other suitable type of epicyclic gearbox 30 may be used. By way of further example, the epicyclic gearbox 30 may be a star arrangement, in which the planet carrier 34 is held fixed, with the ring (or annulus) gear 38 allowed to rotate. In such an arrangement the fan 23 is driven by the ring gear 38. By way of further alternative example, the gearbox 30 may be a differential gearbox in which the ring gear 38 and the planet carrier 34 are both allowed to rotate.

[0061] It will be appreciated that the arrangement shown in FIGS. 2 and 3 is by way of example only, and various alternatives are within the scope of the present disclosure. Purely by way of example, any suitable arrangement may be used for locating the gearbox 30 in the engine 10 and/or for connecting the gearbox 30 to the engine 10. By way of further example, the connections (such as the linkages 36, 40 in the FIG. 2 example) between the gearbox 30 and other parts of the engine 10 (such as the input shaft 26, the output shaft and the fixed structure 24) may have any desired degree of stiffness or flexibility. By way of further example, any suitable arrangement of the bearings between rotating and stationary parts of the engine (for example between the input and output shafts from the gearbox and the fixed structures, such as the gearbox casing) may be used, and the disclosure is not limited to the exemplary arrangement of FIG. 2. For example, where the gearbox 30 has a star arrangement (described above), the skilled person would readily understand that the arrangement of output and support linkages and bearing locations would typically be different to that shown by way of example in FIG. 2.

[0062] Accordingly, the present disclosure extends to a gas turbine engine having any arrangement of gearbox styles (for example star or planetary), support structures, input and output shaft arrangement, and bearing locations.

[0063] Optionally, the gearbox may drive additional and/or alternative components (e.g. the intermediate pressure compressor and/or a booster compressor).

[0064] Other gas turbine engines to which the present disclosure may be applied may have alternative configurations. For example, such engines may have an alternative number of compressors and/or turbines and/or an alternative number of interconnecting shafts. By way of further example, the gas turbine engine shown in FIG. 1 has a split flow nozzle 20, 22 meaning that the flow through the bypass duct 22 has its own nozzle that is separate to and radially outside the core engine nozzle 20. However, this is not limiting, and any aspect of the present disclosure may also apply to engines in which the flow through the bypass duct 22 and the flow through the core 11 are mixed, or combined, before (or upstream of) a single nozzle, which may be referred to as a mixed flow nozzle. One or both nozzles (whether mixed or split flow) may have a fixed or variable area. Whilst the described example relates to a turbofan engine, the disclosure may apply, for example, to any type of gas turbine engine, such as an open rotor (in which the fan stage is not surrounded by a nacelle) or turboprop engine, for example. In some arrangements, the gas turbine engine 10 may not comprise a gearbox 30.

[0065] The geometry of the gas turbine engine 10, and components thereof, is defined by a conventional axis system, comprising an axial direction (which is aligned with the rotational axis 9), a radial direction (in the bottom-to-top direction in FIG. 1), and a circumferential direction (perpendicular to the page in the FIG. 1 view). The axial, radial and circumferential directions are mutually perpendicular.

[0066] As an example component, discs of the high pressure compressor 15 can be required to operate at elevated temperatures, for example perhaps 750 C. or more. Therefore, joining processes for this component, and others, have to be able to withstand such conditions. However, such components can be made of materials such as nickel based superalloys that can become sensitive to cracking if they are bulk heated to close to their melting temperature (perhaps around 1350 C. or more). This leaves a relatively small temperature window in which the jointing processes can operate.

[0067] Conventional diffusion bonding operations require high temperatures, making them unsuitable for use in these types of situation. However, the present inventor has developed a method that permits diffusion bonding at below conventional diffusion bonding temperatures.

[0068] The present diffusion bonding method involves one or both of the faying surfaces of a joint being coated in a substantially thin, lamellar structure of layers of differing composition. The method exploits the enthalpy of mixing of these layers.

[0069] An example arrangement is shown schematically in FIG. 4. First component 51 and second component 52 are to be joined together. They are shown with their faying surfaces provided with lamellar coatings 53. Each lamellar coating 53 comprises a plurality of layers, as discussed below. The faying surfaces are in a localised encapsulation arrangement 54 in this embodiment. Arrows are used to indicate that the process may occur under isostatic pressure, in order to consolidate any porosity.

[0070] By exploiting the enthalpy of mixing, the bulk component temperature, at least in the vicinity of the joint, can be elevated above ambient but below typical diffusion bonding temperatures. The skilled reader will appreciate that the specific temperatures used for conventional diffusion bonding will vary depending upon the materials being used. In any case, as the layers of dissimilar metals diffusely mix they generate heat. This local interfacial heating provides an additional pulse of heat for diffusion bonding above the background temperature of the first and second components 51, 52. As such, the volume of material above the background temperature is minimal, with minimal heat affected zones. Moreover, unlike a braze or transient liquid phase bond (TLP), there are no melting point depressants, which might otherwise diffuse along grain boundaries.

[0071] The use of the lamellar coating 53 provides a softer interface for enabling the diffusion bonding. Nonetheless, the layers of the lamellar coating 53 are preferably composed of the same elements as the bulk material of the components 51, 52 being joined (or, if dissimilar materials are being joined, the same elements as at least one of the components). Preferably, the bulk composition of the lamellar coating has substantially the same ratio of elements as the material being joined, although there may be some local variation within the lamellae.

[0072] It will be understood that the term lamellar coating does not require that each layer within the coating 53 is necessary coextensive with the surrounding layers, nor that each layer is uniformly thick across its extent. Indeed, it will be readily understood that the technique may be extended to create more geometrically complex lamellar coatings, for example by use of masks or three-dimensional printing techniques, in which different layers may have different shapes and variable thicknesses. Sol-gel processing might be used to pre-position the interlayer material. Nonetheless, such variations will still have an essentially lamellar structure in that they will be composed, at least locally, of layers of material above and below each other.

[0073] The lamellar coating can be applied in vacuum. Preferably, the layers can be formed by electron beam physical vapour deposition. Further, the faying surfaces can optionally remain in vacuum until insertion into the diffusion bonding vessel, to prevent oxidation, condensation or contamination.

[0074] The lamellar coating can be formed of thinly deposited lamellar layers of, for example, from 0.10 to 1.25 microns, optionally from 0.40 to 1.10 microns, and further optionally from 0.50 to 1.00 microns. The different layers may comprise different elements that, together, would comprise elements of one or more of the components being joined together.

[0075] In some embodiments, there are at least four layers in the lamellar coating. Optionally there may be 30 to 100 layers, or 100 to 1000 layers.

[0076] The total thickness of the lamellar coating can be from 10 to 70 microns, optionally from 20 to 60 microns, further optionally from 30 to 50 microns.

[0077] To promote the enthalpy of mixing, the different layers can be composed of different elements having different atomic radii, e.g. transition group elements of significantly varying atomic radius. That is, the layers may each substantially consist of a single element, or perhaps a mixture of elements, with an adjacent layer substantially consisting of an element or mixture of elements with a significantly different atomic radius.

[0078] Examples of elements having smaller atomic radii include Ni, Co, Cr, Fe & Mn (which have calculated atomic radii, according to the well known Clementi values, of 149, 152, 166, 156 and 161 pm respectively).sup.1. .sup.1 See, for example, the disclosure of: E Clementi, D L Raimondi, W P Reinhardt (1963) J. Chem. Phys. 38:2686.

[0079] Examples of elements have larger atomic radii include Zr, Nb, Hf, W and Ta (which have calculated atomic radii, according to the well known Clementi values, of 206, 198, 208, 193 and 200 pm respectively).

[0080] Taking the smallest large atom (W) and the largest small atom (Co) in these examples, it can be seen that the ratios of atomic radii between the larger element in one layer to the smaller element in the adjacent layer will be 1.16 or more for layers created from the elements listed.

[0081] Aluminium (Clementi radius: 118 pm) may also be used as a relatively smaller atom. However aluminium as a pure element is limited in its maximum processing temperature, and so a layer composed of a mixture of titanium and aluminium may be used to improve processibility.

[0082] During the diffusion bonding operation, the faying surfaces are pressed together at an elevated temperature. This temperature is below conventional diffusion bonding temperatures, and could be in the range of 800 C. to 1100 C., for example. As mentioned above, the skilled reader will appreciate that the specific temperatures used for conventional diffusion bonding will vary depending upon the materials being used.

[0083] However, as the faying surfaces are pressed together, the layers of the lamellar coating(s) mix, and the latent heat of mixing is released to enable a local heating that allows the diffusion bonding to occur, without the bulk heating that might increase susceptibility to cracking.

[0084] Moreover, the pressure of the diffusion bonding operation will also act to counteract the differential strains. As such, this also contributes (in conjunction with the reduced overall exposure to elevated temperature) to reducing the opportunity for cracking in the components being joined.

[0085] In addition to the bulk heating, a local, secondary, heat source may optionally be used to initiate the diffusion bonding at a point in the joint. Such a heat source could be a chemical or electrical fuse, for example. Preferably, such a secondary heat source is used in a discardable region of the joint, i.e. a region to be machined off after the joining, to avoid any imperfections around the secondary heat source propagating into the final product.

[0086] One example of a lamellar coating that may be used for bonding two components of nickel based superalloy may including the following layers (in order): [0087] Ni [0088] Zr (i.e. much larger atoms than Ni) [0089] Ni [0090] Al (i.e. smaller atom than Ni)

[0091] In this example (and in the following examples), Ni is an example of a transition metal, and the other layers may be provided as a layer of the transition metal (i.e. Ni) enriched with noted element (e.g. the second layer may be a layer of Ni enriched with Zr). This allows for individual component elements to be provided without requiring a pure layer of a particular element.

[0092] A second example of a lamellar coating that may be used for bonding two components of nickel based superalloy may including the following layers (in order): [0093] Ni [0094] Ta (i.e. much larger atoms than Ni) [0095] Ni [0096] B (Clementi radius: 87 pmi.e. much smaller atom than Ni)

[0097] A third example of a lamellar coating that may be used for bonding two components of nickel based superalloy may including the following layers (in order): [0098] Ni [0099] W (i.e. much larger atoms than Ni) [0100] Ni [0101] Ti (Clementi radius: 176 pm, as a substitution for Ni) [0102] Ni [0103] C (Clementi radius: 67 pm i.e. much smaller atom than Ni)

[0104] In these examples, the ratios of atomic radii between the larger element in one layer to the smaller element in the adjacent layer are all 1.18 or more, based on the Clementi radii.

[0105] The following Table 1 illustrates calculated values for enthalpy of mixing for a range of typical coating alloy compositions.

TABLE-US-00001 TABLE 1 Calculated values for enthalpy of mixing for a range of typical coating alloy compositions Elemental Composition Enthalpy of mixing (J/kg) Alloy Ni Cr Co Mo Al Ti Ta Hf Zr Nb W Mn Si Cu Fe 800 C. 900 C. 1000 C. A 26 13 42 0 5 0 0 0 0 0 14 0 0 0 0 234061 229637 215771 B 28 10 40 3 4 5 2.5 0.5 0 1 5 0 1 0 0 374597 351982 319206 C 31 14 38 0 5 0 0 0 0 0 12 0 0 0 0 228950 216212 200478 D 35.7 22.5 23.5 1.4 6.5 5.5 1.5 0 0 2 1.4 0 0 0 0 558123 546354 525727 E 39 14 24 6.5 3 3.5 6 2 0 2 0 0 0 0 0 372477 350050 308743 F 37.4 15 25.5 3 6.5 3.6 2 0 0 2 1 1 1 0 2 566505 526188 477287 G 44 14 23 0 3 0 16 0 0 0 0 0 0 0 0 289015 276509 252135 H 44.7 15 20 6 2.5 3.4 1.4 1 0 2 2 0 2 0 0 389712 370102 339463 I 37 20 16 3 4 0 3 0 0 5 2 1 1 0 8 334130 291035 262027 J 52.3 15 18.5 5 3 3.6 2 0.5 0.1 0 0 0 0 0 0 305959 287949 263252 K 52.4 15 18.5 5 3 3.6 2 0.5 0 0 0 0 0 0 0 305082 286741 262527 L 52.5 19 1 3.1 0 0 2 0 0 3 0 0.4 0.4 0.2 18.5 53344 38904 28900 M 53.7 15.3 17 4.7 3.3 4.2 1.5 0.4 0.1 0 0 0 0 0 0 346240 332247 302028 N 57.6 12 14 1.4 6 4.5 1.5 0 0 1.5 1.5 0 0 0 0 570040 557493 518908 O 58.5 8 4 2 3 1 2 0.5 0 19 2 0 0 0 0 522258 512225 483774 P 67.1 8.3 9.3 0 5.6 0 0 0 0 0 9.5 0.1 0.3 0 0 377784 366467 349550 Q 84.6 0 0 0 6.5 3.6 1.5 0 0 2.5 1.3 0 0 0 0 524163 527389 527109

[0106] This diffusion bonding technique can find wide application, not just in making joins (e.g. to join compressor blades to disks), but also in repair applications, where the processing temperature can be limited.

[0107] Moreover, the technique is not limited to nickel based superalloys, although it has been exemplified in that context. The skilled person will understand that the technique is applicable to other types of alloys too, such as for aluminium alloys High Entropy Alloys (HEAs) and Eutectic High Entropy Alloys (EHEAs).

[0108] The technique could also be used, for example, in combination with a low-rated friction welding machine, where the frictional energy can be used for initial heating before the diffusion bonding.

[0109] It will be understood that the invention is not limited to the embodiments above-described and various modifications and improvements can be made without departing from the concepts described herein. Except where mutually exclusive, any of the features may be employed separately or in combination with any other features and the disclosure extends to and includes all combinations and sub-combinations of one or more features described herein.