Method of joining a first surface to a second surface by hot isostatic pressing without encapsulation of the surfaces, corresponding bonded component, and method of preparing such two surfaces

Abstract

The present invention relates to a method of joining a first surface to a second surface by hot isostatic pressing without encapsulation of the surfaces. The method comprises: preparing the first surface and the second surface for joining by applying a sealing material to the first surface to be joined, and pressing the first surface to the second surface to which the first surface is to be joined; placing the prepared surfaces in a hot isostatic pressing (HIP) furnace (32) without encapsulation of the surfaces; and operating the hot isostatic pressing (HIP) furnace (32) by raising pressure and temperature. A bonded component (38a, 38b) made by such a method is also provided. Also provided is a method of preparing two surfaces for joining by hot isostatic pressing without encapsulation, the method comprising: applying a sealing material to a first surface to be joined; and pressing the first surface to a second surface to which the first surface is to be joined. This provides a cost-efficient and time-efficient method for bonding complex parts.

Description

[0001]

[0002] METHOD OF JOINING A FIRST SURFACE TO A SECOND SURFACE BY HOT ISOSTATIC PRESSING WITHOUT ENCAPSULATION OF THE SURFACES, CORRESPONDING BONDED COMPONENT, AND METHOD OF PREPARING SUCH TWO SURFACES

[0003] Field of the Invention

[0004] The present invention relates to a method of preparing surfaces for joining by hot isostatic pressing (HIP) without encapsulation, a method of performing that joining, and a product thereof. In particular, the invention relates to a method of sealing an interface between two surfaces for joining in order to prevent ingress of pressurizing gas and hence to build up a compressive force at the joint interface.

[0005] Background of the Invention

[0006] Various materials, e.g. combinations of similar and dissimilar alloys, ceramics, composites and polymers can be joined by solid-state diffusion bonding. The common method of performing this is by heating the parts up to below their melting points, normally in vacuum, and applying uniaxial compressive pressure to form a bond by bringing the faying surfaces together (Figure 1). However, this requires the surfaces to be joined, as well as the end surfaces, to be parallel and straight. Diffusion bonding can also be performed using Hot Isostatic Pressing (HIP) (Figure 2). Conventionally, bonding materials using a Hot Isostatic Pressing (HIP) furnace requires “encapsulation” or “canning” of the parts to prevent any inflow of the pressurizing gas (normally argon) into the joint interface, which can hinder the formation of a bond. This encapsulation is only applicable on parts with simple shapes and profiles (normally plain, flatface or circular parts without hollows or internal cavities). Additionally, the capsule materials, which are normally bonded to the parts too, must be removed after HIP bonding, typically by machining and / or dissolving in a strong acid or alkali. This makes it a time-intensive and very expensive process.

[0007] The present invention provides a method of performing HIP-assisted diffusion bonding of parts which does not require any encapsulation. It can therefore be performed on parts with complex shapes containing hollows, multiple joints at any directions and even non-planar joint interfaces. Furthermore, as no encapsulation is required, it can significantly reduce costs and time.

[0008] Summary of the Invention

[0009] Aspects and embodiments of the present invention are set out in the appended claims.

[0010] According to a first aspect, there is provided a method of joining a first surface to a second surface by hot isostatic pressing without encapsulation of the surfaces, the method comprising: preparing the first surface and the second surface for joining by applying a sealing material to the first surface to be joined, and pressing the first surface to the second surface to which the first surface is to be joined (thereby forming prepared surfaces); placing the prepared surfaces in a hot isostatic pressing (HIP) furnace without encapsulation of the surfaces; and operating the hot isostatic pressing (HIP) furnace by raising pressure and temperature.

[0011] This provides an effective method of joining components using isostatic pressing without the need for encapsulation, which means complex shapes can be joined. It also provides a more cost-efficient and time-efficient method of joining components.

[0012] As used herein, the term ‘encapsulation’ refers to the casing, canning or enveloping configured to prevent ingress of pressurizing gas between the first surface and the second surface (i.e. the interface between the first surface and the second surface) during hot isostatic pressing (HIP).

[0013] As used herein, the term ‘sealing material’ preferably connotes any material which can cause a seal between the first surface and the second surface. This is preferably a temporary seal. In particular, the seal is (at least initially) formed by surface tension between a (liquid) sealing material and the first and second surfaces.

[0014] Typically, the first surface is a surface of a first part and the second surface is a surface of a second part.

[0015] Preferably, the sealing material has a boiling point close to or above the bonding temperature during the hot isostatic pressing. In particular, the sealing material preferably has a boiling point sufficiently high to ensure it remains liquid until bonding commences during hot isostatic pressing (in the hot isostatic pressing furnace). This can help to ensure that the seal remains until bonding commences.

[0016] Preferably, the sealing material is liquid during the pressing of the first surface to the second surface. This can help to ensure that a seal is formed by surface tension. In some implementations, the sealing material may be applied in a non-liquid state (e.g. a solid state) but is typically in a liquid state during pressing of the first surface to the second surface to form a seal.

[0017] In some implementations, the method further comprises applying the sealing material to the second surface. This can aid in ensuring good coverage of the sealing material and consequently facilitating a good seal, without any bubbles of gas remaining.

[0018] In some implementations, the method may comprise applying the sealing material to the edges of the first surface and / or second surface. This may comprise applying the sealing material only to the peripheral edges of the first surface and / or second surface. This can minimize the quantity of sealing material required, and so the quantity of sealing material which remains within the bonded component.

[0019] In some implementations, the method may comprise applying the sealing material across the first surface and / or second surface. This preferably comprises applying the sealing material across the whole of the first surface and / or second surface. This preferably comprises applying the sealing material to the peripheral edges of the first surface and / or second surface and to the central portion of the first surface and / or second surface. This can help to ensure good coverage of the wetting material and can help to encourage good sealing.

[0020] Preferably, the first surface to be joined and / or the second surface to be joined comprises a metallic, ceramic or composite material. In other words, the first part (comprising the first surface) and / or the second part (comprising the second surface) may comprise a metallic, ceramic or composite material. In some implementations, the first surface to be joined and / or the second surface (and / or the first and / or second parts) to be joined may comprise a superalloy or two dissimilar superalloys. For example, a superalloy may be any high- performance metallic alloy designed to maintain strength and resist deformation at elevated temperatures. The first surface and / or the second surface (and / or the first and / or second parts) to be joined may be a single crystal. Examples of such materials include CMSX-4 (RTM), CM247, PWA 1483, Inconel 625 (RTM), Rene N4, Stainless Steel 316, and Titanium (e.g. Grades 2 & 5). In preferable implementations, the sealing material is a metal or alloy (particularly when bonding metallic, ceramic or composite materials).

[0021] In some implementations, the method may further comprise heating the first surface and / or second surface prior to applying the sealing material. This can assist with the wetting of the surface(s) by the sealing material. In some implementations, the method may comprise heating the first surface and / or second surface while and / or after applying the sealing material.

[0022] The heating of the first surface and / or second surface is preferably to a temperature above the melting point of the sealing material. This can ensure the sealing material is liquid.

[0023] In some implementations, the method may further comprise smoothing and / or polishing the first surface and / or the second surface prior to applying the sealing material. This may comprise any one or more of: grinding, polishing or lapping the surface. A smooth surface can help to facilitate better sealing and / or bonding.

[0024] In some implementations, the method may further comprise abrading the first surface and / or second surface prior to applying the sealing material. This may comprise any one or more of: grinding, polishing or lapping the surface. This may be the same or an additional step to the smoothing and / or polishing. The abrasion can disturb any oxide layer, and so help to facilitate better bonding.

[0025] In some implementations, the method may further comprise rinsing the first surface and / or second surface prior to applying the sealing material, preferably after the smoothing and / or after the abrading. This can help to remove any debris from the surface(s), and can be particularly beneficial after grinding.

[0026] The rinsing may preferably comprise rinsing with a solvent. An example of a suitable solvent is Isopropanol Alcohol (IPA).

[0027] In preferable implementations, the sealing material may be one of: gallium; indium; mercury; and solder / braze alloy.

[0028] Preferably, the first surface and the second surface have matching and / or corresponding and / or complementary profiles. This means the topography of the first surface and / or second surface is equal and opposite such that the surfaces fit together.

[0029] Preferably, the pressing of the first surface to the second surface is performed until some sealing material squeezes out from between the first surface and the second surface. This can help to ensure good sealing.

[0030] Preferably, the pressing of the first surface to the second surface is performed until sealing of the first surface and the second surface due to surface tension between the first surface and the second surface and the sealing material. (Typically surface tension between the first surface and the sealing material and between the second surface and the sealing material.)

[0031] Preferably, the method comprises initially heating the hot isostatic pressing (HIP) furnace to a temperature above the melting point of the sealing material before pressurizing the hot isostatic pressing (HIP) furnace.

[0032] The hot isostatic pressing (HIP) furnace may typically be heated to a temperature where the bonding commences, preferably below the boiling point of the sealing material.

[0033] In preferable implementations, the hot isostatic pressing (HIP) furnace is heated to a bonding temperature between 70% and 90% of the melting temperature of the material of the first surface or the second surface to be bonded, whichever has a lower melting temperature. In some instances, the first surface and the second surface may be formed of the same material.

[0034] Preferably, the method comprises heating the hot isostatic pressing (HIP) furnace to a temperature above the melting point of the sealing material before pressurizing the hot isostatic pressing (HIP) furnace.

[0035] By way of example, for superalloys: a temperature of 1 100°C to 1250°C, a pressure of 20 bar to 2000 bar (2 MPa to 200 MPa), and time of 2 to 4 hours may be used. In some instances, a pressure of 20 bar to 500 bar (2 MPa to 50 MPa) may be used. By way of a further example, for stainless steels: a temperature of 950°C to 1100°C, a pressure of 20 bar to 2000 bar (2 MPa to 200 MPa), and time of 2 to 4 hours may be used. In some instances, a pressure of 20 bar to 500 bar (2 MPa to 50 MPa) may be used. By way of a yet further example, for aluminum alloys: a temperature of 500°C to 580°C, a pressure of 10 bar to 500 bar (1 MPa to 50 MPa), and time of 1 to 2 hours may be used.

[0036] In some implementations, the method may further comprise heat treating the bonded component (comprising the first surface and second surface), typically after bonding.

[0037] According to a further aspect, there is provided a bonded component made by the method as outlined above, in particular made by any combination of the method steps as outlined above.

[0038] According to a further aspect, there is provided a method of preparing two surfaces for joining by hot isostatic pressing (HIP) without encapsulation, the method comprising: applying a sealing material to a first surface to be joined; and pressing the first surface to a second surface to which the first surface is to be joined. The method of preparing the two surfaces may comprise any of the method steps as outlined above.

[0039] The invention extends to methods and / or apparatus substantially as herein described with reference to the accompanying drawings.

[0040] Brief of the Fii

[0041] One or more aspects will now be described, by way of example only and with reference to the accompanying drawings having like-reference numerals, in which:

[0042] Figure 1 shows an exemplary prior art system for performing conventional diffusion bonding using uniaxial pressure in vacuum or an inert gas;

[0043] Figure 2 shows an exemplary prior art system for performing diffusion bonding in a hot isostatic pressing (HIP) furnace after encapsulation of the parts to be bonded, the method being restricted to parts without a hollow structure or complex profile;

[0044] Figure 3 shows exemplary parts and arrangements for performing diffusion bonding by hot isostatic pressing without encapsulation;

[0045] Figure 4 shows further exemplary parts and arrangements for performing diffusion bonding by hot isostatic pressing without encapsulation;

[0046] Figure 5 shows an exemplary workflow for preparing surfaces to be joined according to the present method;

[0047] Figure 6 shows two surfaces to be joined, with sealing material applied to the edges of the faying surfaces in order to prevent inflow of pressurized gas;

[0048] Figure 7 shows a series of bonded components produced using the present method;

[0049] Figure 8 shows a bonded component from which testing rods have been cut including a tensile tested rod which withstood significant elongation without failing from the joint;

[0050] Figure 9 shows a micrograph from an edge of an insufficiently bonded component;

[0051] Figure 10 shows a micrograph of a test rod from a successfully bonded component;

[0052] Figure 11 shows a stress-displacement plot of a tensile test of a test rod from a successfully bonded part;

[0053] Figure 12 shows a further series of bonded components produced using the present method, including components formed of single crystals and / or superalloys;

[0054] Figure 13a shows a successfully bonded component formed of single crystal CMSX-4;

[0055] Figure 13b shows test rods from the successfully bonded component of single crystal CMSX- 4 of Figure 13a after tensile testing;

[0056] Figure 14 shows a stress-displacement plot of a tensile test of a test rod taken from a central position of the successfully bonded part formed of single crystal CMSX-4 of Figure 13a;

[0057] Figure 15 shows a stress-displacement plot of a tensile test of a test rod taken from an intermediate position of the successfully bonded part formed of single crystal CMSX-4 of Figure 13a;

[0058] Figure 16 shows an optical micrograph of an edge region of a successfully bonded component formed of single crystal CMSX-4;

[0059] Figure 17 shows a further optical micrograph of a region of the successfully bonded component formed of single crystal CMSX-4 of Figure 16;

[0060] Figure 18a shows a successfully bonded component formed of single crystal Rene N4 and having a complex shape and angled bond line;

[0061] Figure 18b shows the successfully bonded component formed of single crystal Rene N4 of Figure 18a from which test rods for tensile testing have been extracted, and a test rod from the successfully bonded component formed of single crystal Rene N4 of Figure 18a after tensile testing;

[0062] Figure 19 shows a stress-displacement plot of a tensile test of a test rod from the successfully bonded part formed of single crystal Rene N4 of Figures 18a and 18b;

[0063] Figure 20a shows a stress-displacement plot of a tensile test of a test rod from a successfully bonded part formed of titanium alloy Grade 2;

[0064] Figure 20b shows a test rod of the successfully bonded part formed of titanium alloy Grade 2 of Figure 20a after tensile testing; and

[0065] Figure 21 shows a micrograph of a successfully bonded component formed of single crystal PWA 1483.

[0066] Detailed Description

[0067] Conventional methods

[0068] Metals, alloys and composites can typically be joined by diffusion bonding, which is a solid- state process of joining materials by accelerating diffusion across two surfaces to be joined. This is achieved by applying pressure and raising the parts to an increased temperature, while remaining below the melting point of the materials. Figure 1 shows a conventional prior art arrangement of performing diffusion bonding 10 in which a first part 18a and a second part 18b are placed within the chamber of a vacuum (or inert gas) furnace 12. Mutually facing surfaces of the first part 18a and the second part 18b abut at an interface region 16, where the two parts 18a, 18b are to be joined. Opposite the interface region 16, the first part 18a is in contact with a first pressure plate 8a; and opposite the interface region 16, the second part 18b is in contact with a second pressure plate 8b. The furnace 12 is closed and the vacuum (or inert gas environment) applied. The first pressure plate 8a and the second pressure plate 8b apply uniaxial pressure to the first part 18a and second part 18b to be joined, such that the abutting surfaces of the first and second parts are put under pressure at the interface region 16. In addition, the temperature of the vacuum furnace 12 is raised to, and held at, a bonding temperature. Plastic deformation, including interfacial migration of the material of the abutting surfaces in the interface region 16, then occurs, to create a bond between the first part 18a and the second part 18b. Uniaxial diffusion bonding can only be used on parts which have simple shapes, planar faying surfaces, and flat top and bottom ends which come in contact with the pressure plates 8a and 8b.

[0069] Figure 2 shows an exemplary conventional prior art arrangement for performing Hot Isostatic Pressing (HIP) assisted bonding 20. In this method, the parts to be joined 28a and 28b are placed in the chamber of a pressurized furnace 22. Isostatic pressure is created in the pressurized furnace 22 due to the presence of a pressurizing gas, typically an inert gas such as argon. Conventionally, in order to prevent ingress of this gas into the joint, the parts to be joined 28a, 28b must be encapsulated (also known as “canning”). This is performed by placing the parts to be joined 28a, 28b within an envelope 24 of encapsulating material, applying a vacuum to remove any remaining air from within the envelope 24, and sealing the envelope

[0070] 24. This whole encapsulated arrangement, comprising the parts to be joined 28a, 28b and the envelope 24, is then placed within the HIP furnace 22. The temperature of the furnace 22 is raised and pressure is increased by injecting an inert gas into the furnace. A joint 26 is thereby formed between the first part 28a and second part 28b. Once removed from the furnace 22, the envelope 24 then needs to be removed. This typically comprises dissolving the envelope in an acidic or alkali solution, and as the envelope needs to be sufficiently thick to withstand the pressure of the HIP-furnace, this process can be very time-consuming and costly. Furthermore, encapsulation is not possible for more complex surface structures, such as honeycomb arrangements, as the envelope must completely surround the parts. If the parts have complex shapes or hollows, then the envelope will rupture when the hydrostatic pressure is applied, leading to ingress of gas into the joint interface and preventing bond formation.

[0071] The present method

[0072] By way of an initial overview, the present disclosure provides a method of joining a first surface to a second surface by hot isostatic pressing without encapsulation of the surfaces, the method comprising: preparing the first surface and the second surface for joining by applying a sealing material to the first surface to be joined, and pressing the first surface to the second surface to which the first surface is to be joined; placing the prepared surfaces in a hot isostatic pressing (HIP) furnace without encapsulation of the surfaces; and operating the hot isostatic pressing (HI ) furnace by raising pressure and temperature. The first surface is a surface of a first part and the second surface is the surface of a second part.

[0073] Figure 3 shows a simple arrangement 30a for performing the present method, which facilitates exemplary HIP-assisted diffusion bonding of two parts, 38a, 38b without requiring encapsulation of those parts. The two parts 38a, 38b can simply be placed within the chamber of the HIP furnace 32, and then the temperature is raised to the bonding temperature and the pressure increased to facilitate bond formation.

[0074] Figure 4 shows a further arrangement 30b for performing the present method, which facilitates HIP-assisted diffusion bonding of five exemplary parts, 38c, 38d, 38e, 38f and 38g, which have complex shapes and different cross sections. For example, part 38c is horseshoe-shaped and the outermost ends are to be connected to part 38d. There therefore exists a hollow within the horseshoe shape; such a part could not be joined by a method requiring encapsulation as the hollow would cause the envelope to rupture when pressure is applied. Similarly, part 38e also has a complex shape. Even if encapsulation were successful, it would be very expensive and time-consuming to remove the envelope from the complex shape of the parts. In addition, the joints 36a, 36b, 36c, 36d and 38e do not need to have a planar configuration. For example, the faying surfaces of the joint 36e between parts 38f and 38g have corresponding but curved profiles. That is to say, the surfaces are not flat, but they have matching profiles such that they fit together. Furthermore, the parts do not have flat outer surfaces, so would not be suitable for joining by uniaxial diffusion bonding as these outer surfaces would not engage properly with pressure plates. Instead, using the present method, all parts 38a, 38b, 38c, 38d and 38e can be placed directly within the chamber of the HIP furnace 32 and then the temperature is raised to the bonding temperature and the pressure increased to facilitate joining of all five parts 38c- to-38g simultaneously. Also, unlike conventional uniaxial diffusion bonding, despite having different bonding areas, the bonding pressure is the same on all five joints 36a-to-36e.

[0075] Figure 5 shows an exemplary workflow for preparing the surfaces of the parts to be joined (for example 38a and 38b, and 38c-to-38g), in order to facilitate the bonding without encapsulation. In a simple implementation, the present method can start at step 108 in which a liquid sealing material with good wettability is applied to at least one of the surfaces to be joined (which can also be referred to as the ‘faying surfaces’), followed by step 110 in which the faying surfaces are pushed together to form a seal. The joints between the parts are sealed due to the surface tension between the sealing material and parts, and so as used herein, the term ‘sealing material’ refers to any material which can facilitate this seal due to surface tension. This is typically a liquid, such as a liquid metal. Thereafter, in step 112, the whole arrangement can be loaded into a HIP furnace. The HIP furnace can then be operated to cause bonding of the two faying surfaces by raising the temperature and isostatic pressure. Optional steps 102, 104 and / or 106 may be performed before applying the sealing material in step 108. These may typically depend on the parts to be joined and / or the sealing material being used, as will be explained further below.

[0076] The sealing material is liquid (typically a liquid metal) at the temperature at which it is applied to one or both of the faying surfaces. The sealing material has a low melting temperature to ensure full sealing of the faying surfaces and to prevent the pressurized gas entering the joint interfaces 36. The sealing material also has a high boiling point, above the minimum bonding temperature, so that it remains at the joints 36, in liquid state, until bond formation commences. By way of example, the sealing material may be gallium, indium or a low-melting point solder

[0077] (mercury could also be used but may be avoided due to its toxicity). In some implementations, and typically depending on the sealing material chosen, the optional step 106 of heating the faying surfaces may be performed prior to applying the sealing material. This can help to improve the wettability, and in some cases is required to transform the sealing material to the liquid state. By way of examples, the faying surfaces may be heated to 50 °C before applying liquid gallium (melting point 30 °C) or to 180 °C before applying liquid indium (melting point 157 °C). Then, in some implementations, the samples may be cooled to the room temperature after being pressed together, so that the sealing material solidifies and holds the parts together while they are being loaded in a HIP furnace.

[0078] The surfaces to be joined typically have corresponding / matching profiles, such that they fit together neatly. This can help to ensure that no air remains between the surfaces, and the joints 36 can become fully sealed. However, the surfaces do not need to be straight, unlike conventional uniaxial diffusion bonding techniques, as described above. The joint should be free from large gaps so that effective sealing can be achieved by surface tension between the faying surfaces and the liquid metal. In some implementations, the optional step 102 of grinding, polishing or lapping the faying surfaces may be employed. This (or other similar methods) can be implemented to smooth or polish the surfaces, as typically a smoother surface leads to better bonding. Gentle abrasion can also be used to disturb any metallic oxide which may have formed on the surfaces, and which could hinder sealing of the surfaces by the liquid sealing material. It has been found empirically that conventional grinding with 1200-grit or 600- grit emery paper or equivalent is adequate for both purposes. In some implementations, additionally or alternatively, step 104 may be performed, whereby the faying surfaces are rinsed prior to the application of the sealing material. The rinsing may be performed using a solvent, such as using Isopropanol Alcohol (IPA). This can remove any debris from the surfaces, and can be particularly beneficial after grinding.

[0079] In some implementations, the sealing material is applied across the whole of one or both of the surfaces to be joined. This can be advantageous if there is a slightly imperfect match in the surfaces to be joined and / or the surfaces are a little rough, as it can help to ensure no air remains between the surfaces. In some other implementations, the sealing material may only be applied to the outer edges of one or both of the surfaces to be joined, which is also sufficient to achieve sealing of the surfaces (due to surface tension). This can offer the advantage of minimizing the amount of sealing material which remains in the joint after bonding has been performed. This may be preferable, for example, when joining smaller components. The sealing material may be applied to only one of the surfaces, or it may be applied to both. Applying to both of the surfaces can help to avoid any air gaps remaining, and may in particular be beneficial if there is a slightly imperfect match in the surfaces to be joined and / or the surfaces are a little rough.

[0080] Figure 6 shows an exemplary sealing of a first surface 39a of a first part 38a, formed of stainless steel 316, and a second surface 39b of a second part 38b, also formed of stainless steel 316, which are to be joined and therefore form the faying surfaces. In this illustrated implementation, the sealing material 40 (in this case, gallium) has been placed only around the outer edges of both of the surfaces. By way of example, it has been found empirically that 50 mg / cm2of gallium is sufficient to seal ajoint fully when bonding parts of Inconel (RTM) Alloy 625 (UNS designation N06625).

[0081] In step 110, the two surfaces to be joined are pushed together. This only requires a small amount of pressure, sufficient to cause a seal between the two faying surfaces 39a, 39b and the sealing material 40. This preferably acts to expel any remaining air between the two surfaces. Typically, some of the sealing material may be squeezed out, which can aid in removing any remaining air between the two surfaces. After this, the two surfaces are typically held together by surface tension due to the interaction between the liquid sealing material 40 and the two surfaces 39a and 39b.

[0082] At step 112, the parts (the first part 38a and second part 38b, joined and sealed by the sealing material 40 without encapsulation) can be loaded into the chamber of a furnace 32, which is then operated to raise the temperature to a bonding temperature and to apply isostatic pressure. As mentioned above, the sealing material prevents the ingress of pressurizing gas (typically argon) into the joint.

[0083] The sealed parts can be placed within the furnace 32 even if the sealing material 40 has solidified. (This may be the case, for example, if the faying surfaces 39a, 39b were heated to a temperature above the melting temperature of the sealing material 40 before applying the sealing material. Then the faying surfaces 39a, 39b may be cooled back down to room temperature, and if this is below the melting temperature of the sealing material, the sealing material 40 will solidify.) The furnace 32 is heated to a temperature above the melting temperature of the sealing material 40 before the chamber of the furnace is pressurized. This ensures that the sealing material 40 is liquid when pressure is applied. This can help to prevent gas leaking into the joint interfaces 36 in Figure 4, which would hinder bond formation.

[0084] This method can be used to join metals, alloys, ceramics, composites, metal-matrix composites, including joining combinations of dissimilar materials, for example metal to ceramic. The bonding mechanisms at the meeting of the faying surfaces are the same processes which facilitate bonding during hot pressing joining techniques; therefore, any materials which can be bonded by hot pressing can be joined by the presently disclosed method. Sealing materials typically used may be gallium, indium or a low-melting point solder (for example, a tin-based low melting point solder). If bonding aluminum, however, a sealing material other than gallium is recommended, for example indium or a low melting solder, as gallium penetrates and dissolves into the grain boundaries of aluminum, causing rapid disintegration. An ideal sealing material does not react with the material or materials being joined (although, in practice, it might not always be possible to avoid a reaction completely). The bonding temperature, pressure and time depend on the particular materials being joined. The bonding temperature is typically set at 70% to 90% of the melting temperature of the material(s) forming the parts to be joined (in K). By way of example, Table 1 shows exemplary values of temperature and pressure for some different metallic materials.

[0085] Table 1

[0086] Experimental Results

[0087] Figure 7 shows a series of components comprising two parts, formed of Inconel (RTM) Alloy 625, joined by the present method, and Figure 8 shows one of these components 48. The component 48 comprises parts 38a and 38b after they have been joined by diffusion bonding facilitated by the present method. From the component 48 are removed a series of rods 42, which can be used for tensile testing. The rods 42 span the joint 36, such that they are formed of a sample of the first part 38a and the second part 38b. In particular, rods 42 are removed from a series of positions within the component. As illustrated Figure 8, rods 42 are removed from a central position, B, a position near the edge of the component, C, and a position of intermediate radius, A. This can provide an indication of a quality of the bond in different regions.

[0088] Figures 9 and 10 show micrographs of samples removed from different components but both formed of Inconel (RTM) Alloy 625. In particular, Figure 9 shows a sample taken from outer position C of an exemplary sample which has not bonded effectively, to provide a comparison. The sample shows a cross-section of the line of the joint 36, indicated by the arrows. In this sample, inadequate sealing on the edges has prevented bond formation, and it is clear that there is a gap between the first surface 39a of the first part 38a and the second surface 39b of the second part 38b.

[0089] By contrast, Figure 10 shows a micrograph of Inconel (RTM) Alloy 625 which has been successfully bonded by the present bonding method using gallium as the sealing material (1250°C, 200 bar, 2 hours). The arrows show the location of the joint 36. In this case, the microstructure is homogenous and effectively seamless; there is a virtually invisible bond line and certainly no gap. This indicates that diffusion bonding has been successful.

[0090] Figure 11 shows the results of a tensile test of a rod 42 from a different sample of Inconel (RTM) Alloy 625 (bonded during a different run) using gallium as the sealing material (1250°C, 200 bar, 2 hours). In particular, the stress-displacement plot of Figure 11 shows that the rod successfully withstood an elongation of approximately 71% and a reduction in cross-section of approximately 25%. The sample withstood a stress of 700 MPa (at which point the true stress was estimated to be above 1 GPa), after which the test was stopped. These results indicate that a perfect bond has been achieved, as they are comparable to results expected from a ‘monolithic’ component. As such, the bonded component has tensile properties akin to unbonded components.

[0091] Figure 12 shows a further series of components, which have been joined by diffusion bonding according to the present method. The components are formed from superalloys, including CMSX-4 (RTM), CM247, PWA 1483, Inconel 625 (RTM), Rene N4, Stainless Steel 316, and

[0092] Titanium Grade 2. Some of these components have complex shapes 40, 42, 44; for example, these include single crystal turbine blades 40 and 42, and a single crystal dovetail joint 44 formed of Rene N4. These samples have been cut apart and then re-joined by the present method in order to assess the strength of bonding in such high-performance materials and components. For example, Figures 18a and 18b indicate via a dashed line how a single crystal dovetail joint 44 formed of Rene N4 has been cut and then bonded using the present method, the dashed line also indicating the bond line 442. Such complex shapes would be impossible to join by conventional prior art diffusion bonding methods. Similarly, faying surfaces which are not planar and perpendicular to a longitudinal axis of a component would be impossible to bond by conventional prior art diffusion bonding methods. As described above, uniaxial diffusion bonding can only be used on parts which have simple shapes, planar faying surfaces, and most crucially flat top and bottom ends which make contact with the pressure plates. For conventional Hot Isostatic Pressing (HIP) assisted bonding, the parts to be joined must be encapsulated in an envelope sufficiently thick to withstand the pressure of the HIP-furnace, and this envelope must be removed after bonding. If the parts have complex shapes, especially if they have hollows, then the envelope may rupture when hydrostatic pressure is applied, which can cause gas ingress and prevent bond formation. As such, bonding parts with complex shapes such as those shown in Figure 12 by either uniaxial diffusion bonding or conventional Hot Isostatic Pressing (HIP) assisted bonding would be impossible.

[0093] Figure 13a shows a component 46 formed of single crystal superalloy CMSX-4 (RTM) and comprising a first part 46a and a second part 46b, each formed of single crystals of CMSX-4 (RTM), which were then joined by the present method at the bond line or joint 462. In particular, the first part 46a and second part 46b of the component 46 were joined using gallium as the sealing material under conditions of 2000 bar (200 MPa) hydrostatic pressure and a temperature of 1250 °C for 3 hours. Two tensile-testing rods, 464 and 466, were extracted from the component 46. One tensile-testing rod, 464, was removed from the centre of the component and one tensile-testing rod, 466, was removed from an intermediate radial position between the centre and the outer edge of the component 46.

[0094] Figure 14 shows the results of a tensile test of the rod 464 removed from the centre of the component and Figure 15 shows the results of a tensile test of the rod 466 removed from an intermediate point. Both tensile tests were performed at room temperature. In particular, Figures 14 and 15 show stress-displacement plots, both of which illustrate elastic-perfect plastic behaviour in response to loading. As such, the tensile testing of both rods 464, 466 shows comparable results to those expected from monolithic single crystals. Figure 13b shows the samples 464, 466 after tensile testing. The samples 464, 466 were observed to elongate and then fail at a location away from the bond line 462 (at the end of the samples inside the grips of tensile testing rig). The break points 4662 and 4642 of samples 466 and 464, respectively, can be seen in Figure 13b to be located close to one end of each sample, in the vicinity of where the samples 464, 466 were being held by grips, and away from the bond line 462. As such, it was not the bond line 462 which failed. Both samples failed at approximately 800 MPa, which is the expected yield stress at room temperature for a single crystal of CMSX- 4 (RTM). These results therefore indicate that single crystal behaviour, or at least near-single crystal behaviour, can be achieved after bonding using the presently disclosed method.

[0095] Figures 16 and 17 show optical micrographs of a different component 48 formed of CMSX-4 (RTM), across a bond line or joint 482. Figure 16 shows a region immediately adjacent to an edge of the component 48, while Figure 17 shows a region further away from the edge of the component 48. The regions imaged in Figures 16 and 17 overlap, and as such they comprise common features. Only a very small, barely perceptible line can be observed at the joint 482; this line is largest at the outer edge and reduces in size moving inwards from the edge. In particular, the joint 482 is virtually invisible in the region shown in Figure 17, indicating successful and effective diffusion bonding. Furthermore, there remains very little to none of the sealing material (in this case, gallium) at the joint 482. Gallium typically diffuses through the matrix of the component quickly, and the present method only requires a very small quantity to be used (sufficient to seal the faying surfaces before bonding). The result is that very little gallium remains at the joint 482. The micrographs shown in Figures 16 and 17 show the component 48 in an as-bonded state; typically a component will undergo a heat treatment after bonding. Such heat treating can further reduce any remaining sealing material at the joint 482, as the gallium diffuses throughout the matrix of the bonded component 48. This can facilitate the component becoming even more homogenized across the joint 482.

[0096] As described above, Figure 18a shows a component in the form of a dovetail joint 44 formed of single crystal of Rene N4. The component 44 was cut along the dotted line into a first part 44a and a second part 44b. The component was deliberately cut such that the line is at an angle to the longitudinal axis of the component. These parts 44a, 44b were then joined by the presently disclosed method using gallium as the sealing material, and bonding conditions of

[0097] 1250 °C for 3 hours under 2000 bar (200 MPa) hydrostatic pressure such that a bond line or joint 442 is formed along the cut line. Figure 19 shows a stress-displacement plot of a sample extracted from this dovetail joint component 44 after bonding by the present method, and Figure 18b shows a sample 448 extracted from the dovetail joint component 44 after tensile testing. The sample withstood a stress of 1004 MPa before breaking in the grips, and therefore breaking away from the joint 442. As can be seen in Figure 18b, the break 4482 occurs at the end of the sample 448, in the vicinity of where it was held in the grips. This is away from the bond 442 line, and so failure did not occur at the bond line 442. This tensile testing behaviour is comparable to that expected of a monolithic single crystal of Rene N4 under tensile testing: such a single crystal would be expected to yield at a stress of approximately 1000 MPa at room temperature. It is worth noting that as the joint 442 is at an angle to the longitudinal (i.e. it is at a slant relative to the component 44), the component could not be joined using uniaxial diffusion bonding. As the shape is complex, it would be impossible to encapsulate the component 44 for HIP-assisted diffusion bonding and then remove the encapsulating envelope. Indeed, there would be a chance that the envelope would rupture under the hydrostatic pressure and so effective bonding would not be achieved at all. As such, the joint components, 40, 42 & 44 with internal cooling channels could not be joined by conventional diffusion methods but have been joined successfully by the method as presently disclosed.

[0098] Figure 20a shows a stress-displacement plot of a component formed of titanium Grade 2, joined according to the presently disclosed method using gallium as the sealing material and bonding conditions of 2000 bar (200 MPa) hydrostatic pressure and 1250 °C for 3 hours. The plot shows elastic behaviour followed by necking of the sample. Figure 20b shows a photograph of the sample after testing, indicating the location of the bond line 492 and the necking 494. The necking 494 occurred at a position on the sample away from the bond line 492 (i.e. the joint). Again, as failure did not occur at the bond line 492, this is indicative of successful diffusion bonding, resulting in an at least near-perfect bond.

[0099] In other experiments, titanium Grade 5 was bonded at 110 °C, 100 bar (10 MPa) for 2 hours, and such bonded samples were also observed not to break at the bond line.

[0100] Figure 21 is a micrograph showing a bonded component 50 formed of PWA 1483. The component 50 comprises a first part 50a bonded to a second part 50b via a bond line (i.e. joint) 502. The component was joined according to the present method using bonding conditions of

[0101] 2000 bar (200 MPa) hydrostatic pressure and 1250 °C for 3 hours. Each of the first part 50a and the second part 50b are formed of single crystals comprising a y matrix with y’ precipitates within that matrix. The y’ precipitates have a characteristic cuboidal shape. As shown in the micrograph of Figure 21 , the bond line 502 is approximately 1 -1 .5 pm in width and comprises y’ precipitates. Very little to no sealing material (e.g. gallium) remains at the joint 502, as can be observed at the joint 502 in Figure 21. There is a slight mismatch in the orientations of the crystal structures across the bond line 502, as indicated by the slightly differing orientations of the cuboidal y’ precipitates. It is impossible to ensure a perfect match but a near-perfect match has been observed to be sufficient to achieve behaviour similar to that of a monolithic crystal. For example, in tensile testing, failure has been observed to occur at a location away from the joint 502. As such, behaviour was observed as would be expected from a monolithic single crystal. Although the joint 502 forms a line across the component 50 with a slight mismatch in grain orientation, the joint does not act as grain boundary. As such, behaviour comparable to a monolithic single crystal is observed. Typically, in practice, heat treatment will be performed on the component 50 after bonding, which typically may promote further diffusion and homogeneity across the bond line 502.

[0102] Alternatives and Modifications

[0103] It should be understood that the present invention has been described above purely by way of example, and modifications of detail can be made within the scope of the invention.

[0104] The surface tension is different for different materials, and so the quantity of sealing material required is different for different materials.

[0105] In the case that a low melting point solder is used as the sealing material, this can be applied by applying a disk or ring of solder foil on one or both of the faying surfaces and then applying heat to transform the foil to a liquid. Then, the surfaces can be pressed together.

[0106] Alternatively to a low melting point solder, the sealing material may be a braze metal or lead. Other materials may be used as the sealing material. These can be chosen in dependence on the material or materials to be bonded, and the relevant bonding temperatures.

[0107] Each feature disclosed in the description, and (where appropriate) the claims and drawings may be provided independently or in any appropriate combination.

[0108] Reference numerals appearing in the claims are by way of illustration only and shall have no limiting effect on the scope of the claims.

[0109]

[0110] Various examples have been described above. The following numbered clauses summarise one or more aspects of some of those examples:

[0111] 1. A method of preparing two surfaces for joining by hot isostatic pressing without encapsulation, the method comprising: applying a sealing material to a first surface to be joined; and pressing the first surface to a second surface to which the first surface is to be joined.

[0112] 2. The method of clause 1 , wherein the sealing material has a boiling point sufficiently high to ensure it remains liquid until bonding commences during the hot isostatic pressing.

[0113] 3. The method of clause 1 or clause 2, wherein the sealing material is liquid during the pressing of the first surface to the second surface.

[0114] 4. The method of any preceding clause, further comprising applying the sealing material to the second surface.

[0115] 5. The method of any preceding clause, further comprising applying the sealing material to the edges of the first surface and / or second surface.

[0116] 6. The method of any of clauses 1 to 4, further comprising applying the sealing material across the first surface and / or second surface.

[0117] 7. The method of any preceding clause, wherein the first surface to be joined and / or the second surface to be joined comprises a metallic, ceramic or composite material.

[0118] 8. The method of any preceding clause, wherein the sealing material is a metal or alloy.

[0119] 9. The method of any preceding clause, further comprising heating the first surface and / or second surface prior to applying the sealing material.

[0120] 10. The method of clause 9, wherein the heating is to a temperature above the melting point of the sealing material.

[0121] 11 . The method of any preceding clause, further comprising smoothing / polishing the first surface and / or the second surface prior to applying the sealing material.

[0122] 12. The method of any preceding clause, further comprising abrading the first surface and / or second surface prior to applying the sealing material.

[0123] 13. The method of any preceding clause, further comprising rinsing the first surface and / or second surface prior to applying the sealing material, preferably after the smoothing and / or after the abrading.

[0124] 14. The method of clause 13, wherein the rinsing comprises rinsing with a solvent.

[0125] 15. The method of any preceding clause, wherein the sealing material is one of: gallium; indium; mercury; and solder / braze alloy.

[0126] 16. The method of any preceding clause, wherein the first surface and the second surface have matching and / or corresponding and / or complementary profiles.

[0127] 17. The method of any preceding clause, wherein the pressing of the first surface to the second surface is performed until some sealing material squeezes out from between the first surface and the second surface.

[0128] 18. The method of any preceding clause, wherein the pressing of the first surface to the second surface is performed until sealing of the first surface and the second surface due to surface tension between the first surface and the second surface and the sealing material.

[0129] 19. A method of joining two surfaces, the method comprising: preparing two surfaces for joining by the method of any preceding clause; placing the prepared surfaces in a hot isostatic pressing (HIP) furnace; and operating the hot isostatic pressing (HIP) furnace by raising pressure and temperature.

[0130] 20. The method of clause 19, wherein the hot isostatic pressing (HIP) furnace is heated to a bonding temperature between 70% and 90% of the melting temperature of the material of the first surface and / or the second surface, whichever has a lower melting temperature.

[0131] 21. The method of any of clauses 19 or 20, further comprising heating the hot isostatic pressing (HIP) furnace to a temperature above the melting point of the sealing material before pressurizing the hot isostatic pressing (HIP) furnace.

[0132] 22. A bonded component made by the method of any of clauses 19 to 21.

Claims

CLAIMS1 . A method of joining a first surface to a second surface by hot isostatic pressing without encapsulation of the surfaces, the method comprising: preparing the first surface and the second surface for joining by applying a sealing material to the first surface to be joined, and pressing the first surface to the second surface to which the first surface is to be joined; placing the prepared surfaces in a hot isostatic pressing (HIP) furnace without encapsulation of the surfaces; and operating the hot isostatic pressing (HIP) furnace by raising pressure and temperature.

2. The method of claim 1 , wherein the sealing material has a boiling point sufficiently high to ensure it remains liquid until bonding commences during the hot isostatic pressing.

3. The method of claim 1 or claim 2, wherein the sealing material is liquid during the pressing of the first surface to the second surface.

4. The method of any preceding claim, further comprising applying the sealing material to the second surface.

5. The method of any preceding claim, further comprising applying the sealing material to the edges of the first surface and / or second surface.

6. The method of any of claims 1 to 4, further comprising applying the sealing material across the first surface and / or second surface.

7. The method of any preceding claim, wherein the first surface to be joined and / or the second surface to be joined comprises a metallic, ceramic or composite material.

8. The method of any preceding claim, wherein the first surface to be joined and / or the second surface to be joined comprises a superalloy.

9. The method of any preceding claim, wherein the sealing material is a metal or alloy.

10. The method of any preceding claim, further comprising heating the first surface and / or second surface prior to applying the sealing material.

11. The method of claim 10, wherein the heating is to a temperature above the melting point of the sealing material.

12. The method of any preceding claim, further comprising smoothing / polishing the first surface and / or the second surface prior to applying the sealing material.

13. The method of any preceding claim, further comprising abrading the first surface and / or second surface prior to applying the sealing material.

14. The method of any preceding claim, further comprising rinsing the first surface and / or second surface prior to applying the sealing material, preferably after the smoothing and / or after the abrading.

15. The method of claim 14, wherein the rinsing comprises rinsing with a solvent.

16. The method of any preceding claim, wherein the sealing material is one of: gallium; indium; mercury; and solder / braze alloy.

17. The method of any preceding claim, wherein the first surface and the second surface have matching and / or corresponding and / or complementary profiles.

18. The method of any preceding claim, wherein the pressing of the first surface to the second surface is performed until some sealing material squeezes out from between the first surface and the second surface.

19. The method of any preceding claim, wherein the pressing of the first surface to the second surface is performed until sealing of the first surface and the second surface due to surface tension between the first surface and the second surface and the sealing material.

20. The method of any preceding claim, wherein the hot isostatic pressing (HIP) furnace is heated to a bonding temperature between 70% and 90% of the melting temperature of the material of the first surface and / or the second surface, whichever has a lower melting temperature.

21. The method of any preceding claim, further comprising heating the hot isostatic pressing (HIP) furnace to a temperature above the melting point of the sealing material before pressurizing the hot isostatic pressing (HIP) furnace.

22. A bonded component made by the method of any preceding claim.

23. A method of preparing two surfaces for joining by hot isostatic pressing (HIP) without encapsulation, the method comprising: applying a sealing material to a first surface to be joined; and pressing the first surface to a second surface to which the first surface is to be joined.

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