BACKGROUND 
     This invention relates to a method of joining first and second components, and is particularly, although not exclusively, concerned with a method of repairing a structure, such as a bladed disc of a gas turbine engine, by bonding a patch component to the damaged structure. 
     Compressors and turbines of gas turbine engines such as those used in aero engines typically include a plurality of rotor and stator vane assemblies. The rotor assemblies are designed to impart work into gases passing through the compressor and extract work from the gases passing through the turbine. The stator vane assemblies help to direct the working gases entering or exiting the rotor assemblies and thereby increase efficiency of the engine. 
     Each rotor assembly includes a disc and a plurality of blades attached to the disc so as to extend radially outwardly therefrom. Conventionally, the blades have been attached to the disc by mechanical connections such as “fir tree” type connections where a fir tree shaped blade root is received within a complementary shaped recess in the disc. This means that the blade can be readily replaced in the event that it is damaged. 
     Recent developments have resulted in integrally bladed rotor assemblies or “blisks” in which the blades are formed integrally with the disc. These have the advantage of reduced weight as compared to a standard rotor assembly and in improved aerodynamic efficiency. Such blisks are particularly applicable in the design of military aero engines. 
     In view of the nature of their applications, blisks are susceptible to damage. Blisks of gas turbine engines are high value structures. If a blisk is damaged during the manufacturing process, or during service, it is economically desirable for the damaged blisk to be repaired, rather than scrapped and replaced by an entirely new blisk. 
     Minor damage can often be rectified by conventional hand dressing and blending processes. However, more substantial damage, for example where more than 20% of an aerofoil volume is damaged or destroyed, repair can be difficult. In some circumstances, the damage may require the replacement of all or part of a blade. In order to replace the blade, it must be removed by machining it off to leave a stub, and fixing a new blade onto the stub by a suitable process. 
     It is known to remove a damaged aerofoil section and to effect a “patch” repair by joining a repair patch to the remaining part of the original aerofoil. If the joint is made by fusion welding, the resulting joint may have poor mechanical properties resulting from defects and discontinuities in the weld itself. Also, large heat affected zones are created in the repair patch and the original aerofoil part, in which the material properties may be altered. 
     Repairs have also been effected by using material deposition techniques such as Direct Laser Deposition or shaped metal deposition to regenerate the geometry of damaged components. Such processes are expensive and also result in a heat affected zone. 
     It has also been proposed, for example in U.S. Pat. No. 5,788,142, to join the metallic parts by means of an intermediate part made from a compatible powdered metal. The intermediate part is placed between the parts to be joined, and subjected to a reaction sintering and consolidation process to form the joint. Such a process requires the separate creation of the intermediate part, and the requirement for accurate alignment of both the parts to be joined and the intermediate part, which is difficult to achieve. 
     SUMMARY 
     According to the present invention there is provided a method of joining first and second components comprising:
     (a) placing a containment device in contact with the first component so that a faying surface of the first component is exposed to the interior of the containment device;   (b) introducing a sinterable powder into the containment device so that the sinterable powder accumulates on the faying surface of the first component;   (c) fluidising the sinterable powder;   (d) displacing the first and second components relatively to each other to move a faying surface of the second component and the faying surface of the first component towards each other within the containment device, thereby to compress the fluidised powder in a bonding region between the faying surfaces of the first and second components; and   (e) performing a consolidation process on the compressed powder to consolidate the powder in the bonding region to form a joint between the components.   

     The method may be conducted with at least the faying surfaces disposed in an enclosure which is evacuated or which contains an inert gas during at least steps (b) to (e). 
     The containment device may conform to the profile of at least one of the components. 
     After step (b) of the method, the accumulated powder in the containment device may be levelled to provide a powder layer of uniform thickness on the faying surface of the first component. The orientation of the first and second components may be changed after step (b) and before step (d). The orientation of the faying surface of the first component may be changed after step (b) and before step (d). For example, the faying surface of the first component may be substantially horizontal during step (b), and then rotated to a vertical orientation so that step (d) is conducted with the faying surfaces of the first and second components disposed substantially vertical. 
     Fluidisation of the powder in step (c) may be accomplished by sonic, e.g. ultrasonic, agitation of the powder, for example by means of contact or non-contact sonic transducers coupled to the first component or to the containment device. For example, fluidisation may be achieved by pulsed application of at least one power beam or electromagnetic sonic vibrator). 
     The second component may be aligned with respect to the first component during step (d) by means of a fixture that enables accurate guidance of the second component with respect to the first component. The speed of displacement of the second component with respect to the first component in step (d) may be relatively slow, for example not greater than 1 mm/s, and possibly approximately 0.5 mm/s. 
     The consolidation process of step (e) may comprise the application of heat to the second component, for example to a flat surface of the second component that is parallel to the faying surface of the second component. After the consolidation process, the joined components may be cooled at a slow cooling rate, for example a rate not greater than 10° C./min, and possibly of the order of 5° C./min. 
     After step (e), it may be necessary to perform further operations on the joined components and on the consolidated powder. For example, the second component and/or the consolidated powder may require machining to match the profile of the original aerofoil. 
     In an embodiment in accordance with the present invention, the components are metallic components and may be made from alloys typically used in the aerospace industry, such as alloys based on titanium, nickel, aluminium or steel. It is envisaged that the method of the present invention could be successfully employed with components made from alpha, alpha+beta (Ti6-4, Ti6242, Ti6246, Ti550), beta alloy (Burn Resistant Titanium—BurTi) and titanium based intermetallic alloys (gamma titanium aluminides/Ti 3 Al) and may be applicable to all refractory superalloys. 
     The sinterable powder may comprise particles of the same material as the first and second components or, where the first and second components are made from different materials, as at least one of them. 
     While one application of a method in accordance with the present invention is in the repair or regeneration of damaged aerofoils of a blisk structure, the method may be employed in the repair or original equipment manufacture of other gas turbine engine components or of components of assemblies other than gas turbine engines. 
     It is also possible for a method in accordance with the present invention to be employed in the manufacture or repair of non-metallic structures, in which the first and second components, and the powder, are made from ceramic or composite materials. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: 
         FIG. 1  is a diagrammatic perspective view of part of an integrally bladed rotor assembly or blisk; 
         FIG. 2  shows two components to be joined; 
         FIG. 3  shows the components in a different orientation; 
         FIG. 4  shows the components of  FIG. 2  supported in a fixture; and 
         FIGS. 5 to 7  show three stages in a process of joining the components. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     The blisk  10  represented in  FIG. 1  may be used in the compressor or turbine of a gas turbine aero engine. The blisk  10  has a plurality of blades  12  integrally attached to a disc  14  so as to extend radially outwardly therefrom. The blades  12  are typically of a titanium, nickel or steel alloy (a titanium 6-4 alloy commonly being used) and, during initial manufacture of the blisk, may be attached to the disc  14  by linear friction welding. 
     During the life of the blisk  10 , it is not uncommon for a single blade  12 A to become significantly damaged and to require complete or partial replacement. To replace the blade  12 A, or a damaged part of it, a new blade or patch part has to be attached. The damaged blade  12 A or part is removed so as to provide a faying surface  6  ( FIG. 5 ) to which the replacement blade or patch part is bonded. 
       FIG. 2  shows, in schematic form, a first component  2  and a second component  4 . As represented in  FIG. 2 , the components  2 ,  4  are flat plates. In a practical embodiment the first component  2  may be the hub of the blisk  10  of  FIG. 1 , and the blisk  10  may include a stub  18 . The second component  4  thus represents a patch part in the form of a replacement blade to be joined to the hub  2  as a replacement for the blade  12 A, or part of that blade, which has been broken off the hub  2 . The hub  2  has, in reality, an arcuate upper surface (with respect to the orientation shown in  FIG. 2 ). 
     Thus, as a first step of the repair method, the damaged segment of the aerofoil  12 A is removed, for example by CNC machining, to provide a faying surface  6  ( FIG. 5 ) having a surface finish of approximately 1 micron Ra. The faying surface  6  is cleaned by a suitable process, such as localised etching using a HF+HNO 3  based reagent (for titanium alloys), followed by acetone. 
     The blisk  10  and the patch part  4  are placed in an outer chamber that is evacuated or filled with an inert gas. The patch part  4  is supported accurately with respect to the hub  2  by means of a fixture  8 , as shown in  FIG. 4 . It will be appreciated that the arrangement is shown only diagrammatically in  FIG. 4 . The patch part  4  is spaced from the faying surface  6  of the hub  2 . An inner enclosure (not shown) such as a canister, sheath, shroud or bag encloses the patch part  4 , the fixture  8  and the region of the hub  2  containing the faying surface  6 . The inner enclosure, like the outer chamber containing the entire blisk  2 , is either evacuated or filled with an inert gas to ensure minimised oxygen content. The inert gas may, for example, be argon, and should be of high purity. The purpose of evacuating the enclosures, or filling them with inert gas, is to prevent oxidation of the various components during the following process. 
     Referring to  FIG. 5 , a containment device in the form of a conforming receptacle  16  is placed on the hub  2  to enclose the faying surface  6 . The receptacle  16  is accurately formed to have an internal contour that is complementary to the profile of the blades  12 . The receptacle  16  will thus conform to the profile of the stub  18  and will extend above the stub, conforming to the profile of the patch part  4 , which conforms to the periphery of the component joint area without flexing into the space between the patch part  4  and the faying surface  6 . 
     The receptacle  16  may be made from a polymeric material such as that available under the designation MICROSET® 101. 
     As shown in  FIG. 6 , a sinterable powder  20  is introduced into the receptacle  16  and builds up on the faying surface  6  within the receptacle  16 . The powder may be carried into the receptacle as a suspension in a flow of inert gas, or in a volatile liquid that can subsequently readily be evaporated. Alternatively, the powder may be mechanically dispensed into the receptacle  16 . Thus, any suitable means could be employed for delivering the powder to the receptacle  16 . For example, delivery could be achieved by a low velocity cold gas dynamic spray, or the powder  20  could be delivered through a feed pipe from a crucible containing sonic or sonic oscillating powder. 
     In a practical embodiment in accordance with the invention, the powder has the same nominal chemical composition as the first and second components  2 ,  4 . The powder may be manufactured by any suitable process, for example by gas atomisation. The particles of the powder may be spheroidal with a particle diameter not less than 10 microns and not more than 50 microns. The particle size, or size distribution, should be consistent throughout the powder so as to assist fluidisation of the powder. A bi-modal powder size distribution may be utilised, i.e. the powder may consist of particle size fractions of two or more different mean sizes to increase the packing density of the powder, and so minimise void sizes. Fluidisation is assisted by minimising agglomerations of the particles, the extent of which can be assessed by laser size diffraction. Particle agglomeration can be minimised if a PREP (Plasma Rotating Electrode Process) powder is used. 
     It may be desirable for the powder  20  be introduced into the receptacle  16  with the faying surface  6  in one orientation, and then for subsequent steps of the process to be conducted with the faying surface  6  in a different orientation. For example, as shown in  FIGS. 2 and 3 , the first component  2  may be arranged such that the faying surface  6  is horizontal, as shown in  FIG. 2 , during introduction of the powder  20 , and subsequently the first component  2  may be rotated such that the faying surface  6  is in a vertical orientation, as shown in  FIG. 3 , for the subsequent steps of the process. This measure can assist in ensuring adequate powder flow to all the regions of the faying surface  6 , without agglomeration. Thus, by changing the orientation of the components, gravity can be used to assist powder flow. This can be particularly desirable if the components  2  and  4  have large cross-sectional areas at the faying surfaces. In some circumstances, orientations other than horizontal as shown in  FIG. 2  and vertical as shown in  FIG. 3  may be employed. For example, it may be desirable for the subsequent steps of the process to be carried out with the faying surfaces  6 , 22  inclined at angles other than the horizontal and vertical orientations shown in  FIGS. 2 and 3 . 
       FIGS. 5 ,  6  and  7  show an open-ended receptacle  16 , open at the top end, as viewed in  FIGS. 5 to 7 . This provides an opening  28  for the admission of the powder  20 . In some variants of the process, the opening  28  may be closed, after the powder has been introduced, by a further receptacle component (not shown) to confine the powder  20  around the full extent of the joint. Prior to powder application, a rheological conformity assessment may be conducted to check on the flow characteristics of the powder and to ensure that it flows uniformly. 
     Following introduction of the powder  20 , it is levelled to the top edge of the receptacle  16  by means of a levelling tool or powder placement/delivery device, linked to a vision system, or such as a roller or blade to ensure a uniform thickness of powder over the faying surface  6 . In a particular embodiment, a powder thickness of about 1150 microns to 1500 microns is desirable. 
     The patch part  4  is then offered to the hub  2 . As mentioned above, the patch part  4  is supported in the fixture  8 , which serves as an alignment frame to ensure accurate guidance of the patch part  4  towards the faying surface  6 . The corresponding faying surface  22  ( FIG. 4 ) of the patch part  4  is maintained by the fixture  8  precisely parallel to the faying surface  6  of the hub  2 . The patch part  4  is advanced towards the hub  2  with minimal pressure. The powder  20  is agitated by means of a suitable agitation device, such as piezo-electrical ultrasonic transducers  24  ( FIG. 4 ). The transducers  24  are applied to the hub  2 , at positions close to the faying surface  6 . The frequency and amplitude of the displacements created by the transducers  24  are tuned to the flow characteristics of the powder morphology and to the joint geometry size and orientation. Tuning of the transducers  24  can be achieved by, for example, an optical based closed-loop feedback process control system, including an appropriate algorithm to apply appropriate vibration parameters for the particular powder being used. 
     The transducers are disposed in repeatable, defined positions on the assembly. In  FIG. 4 , the transducers are shown mounted on the hub  2 , but they could be provided on suitable land features provided on flanks of the receptacle  16  using a non-abrasive coupling media. It is desirable for the transducers to be positioned as close as possible to the location of the joint between the components  2  and  4 . 
     Upon contact with the levelled surface of the powder  20 , the patch part  4  is advanced at a relatively slow rate, for example of the order of 0.5 mm/s, over a distance of approximately 1 mm. This causes the agitated powder to be uniaxially compressed so that it is deformed in a fluid state. The powder is compressed between the faying surface  6  of the hub  2  and the faying surface  22  of the patch part  4  to achieve a high density packed powder  20  in a bonding region between the hub  2  and the patch part  4 . The powder conforms closely to the faying surfaces  6 ,  22  without significant change in the axial alignment of the components. The compressed powder thus maintains a consistent thickness over the bonding region 
     Once compaction is complete, the receptacle  16  and the ultrasonic probes  24  are removed, and heat is applied to the patch part  4 , for example at a flat surface  26  away from the faying surface  22 , while maintaining pressure on the powder  20  between the faying surfaces  6 ,  22 . The heat may be applied by any suitable means, and heat transfer is monitored, for example by means of thermocouples or pyrometry placed at suitable locations on the patch part  4  and the hub  2 . 
     By applying the main heat to the powder  20  indirectly through the patch part  4 , it is possible to avoid over-heating the powder  20  which could result in alpha-case generation on the powder particles. Also, thermal intrusion to the surrounding blades  12  can be minimised. During this heating stage, the powder is heated to a process temperature of approximately 300° C. (for titanium) to enable plastic deformation of the powder particles under the applied pressure. This plastic deformation can be assisted by preheating the first and second components  2 ,  4  and the powder  20 , to a temperature below the process temperature. Such preheating, particularly preheating of the powder  20 , aids conformance of the powder to the faying surface  6 ,  22  by creep deformation. 
     By applying the heat to the flat surface  26 , where the patch part  4  has a relatively simple geometry, such as cuboidal, as shown in  FIG. 4 , a uniform heating rate and thermal distribution can be achieved, leading to uniform properties in the joint formed by the powder  20 . 
     Final sintering of the powder  20  is achieved in a further application of uniaxial pressure, conducted under conditions that ensure that the faying surfaces  6  and  22  remain parallel, so that a uniform thickness of the powder  20  is achieved in the bonding region. The temperature at the bonding region is raised so as to achieve full sintering of the powder both between the particles of the powder and between the powder and the faying surfaces  6 ,  22 . Typical sintering conditions are a temperature in the range 850 to 1200° C., and an applied pressure of 10 to 100 MPa (for titanium). 
     The sintering process causes further deformation of the particles of the powder, and diffusion bonding between adjacent particles and between the particles and the hub  2  and the patch part  4 . A small quantity of the powder  20  may be extruded from the bonding region leaving a residue along the edges of the joint that can be removed in a subsequent process. 
     After the sintering process, the components are allowed to cool relatively slowly, for example at a rate of approximately 5° C./minute, to minimise residual stresses in the region of the joint, and to avoid any requirement for subsequent heat treatment. 
     In many applications of the method, the patch part  4  will be somewhat oversized compared to the eventual geometry of the finished blade  12 . Consequently, a final machining operation will be required to achieve the final aerofoil profile and, as mentioned above, to remove any powder residues extruded from the bonding region. 
     The process described above can be conducted manually or can be automated, at least partially. 
     For illustrative purposes,  FIG. 7  shows the patch part  4  surrounded by the powder  20 . It is envisaged that, in a practical process, the patch part  4  would be a relatively close fit in the receptacle  16 , so that the powder  20  will be constrained within the bonding region between the faying surface  6  and  22 . 
     The axis of oscillation created by the ultrasonic transducers  24  may be oriented in any direction to achieve optimum agitation of the powder  20 . In one embodiment, the oscillation may take place along an axis X ( FIG. 4 ) that is parallel to the faying surfaces  6 ,  22 , and aligned with the chordal direction of the patch part  4 . 
     The process described above provides a relatively simple and low-cost solution for repairing aerofoils with minimal distortion while maintaining excellent material properties. The resulting joints between the components  2 ,  4  may be as strong as the material of the components  2 ,  4  themselves. Although the invention has been described with reference to the repair of a blisk, the process can be applied to individual aerofoils that are connected to rotor discs or hubs by conventional mechanical means. 
     Use of a pure powder  20  without the use of a binder such as an organic binder results in joints of superior mechanical properties, since there is a reduced risk of contamination at the interfaces between the particles. 
     In the process described, the components  2  and  4 , and the powder  20  have the same composition as each other. However, the process can be adapted to join together components of dissimilar materials, while maintaining control of the mechanical properties of the joint. 
     The process is particularly suitable for repairing components made from gamma titanium aluminides and other intermetallic compositions, which are currently difficult to repair by conventional welding processes. 
     The process thus provides a means of achieving high integrity repairs of expensive components, so minimising the overall lifecycle cost of the component. In some circumstances, the process can be employed with the blisk or other damaged component in situ, without requiring its removal from its operating environment. 
     It will be appreciated that various modifications to the process may be made while not departing from the scope of the present invention. The regions of the components  2 ,  4  around the joint may be encapsulated, with either vitreous or braze based encapsulation, so that the repair method may include a hot isostatic pressing process on the powder  20  to assist in the removal of porosity from the resulting joint. It may be desirable in some circumstances to retain the receptacle  16  in position during the application of heat and pressure to the joint, and consequently it may be desirable for the receptacle  16  to be made from a material that can withstand the process conditions. For example, a ceramic sheath, instead of the polymer receptacle  16  described, may be employed. The sheath, if compatible with the substrate materials at temperature, may be retained throughout bonding to provide additional constraint if required. 
     In the process described above, the faying surfaces  6 ,  22  have generally flat profiles. It is possible to adapt the profiles of the faying surfaces  6 ,  22  to suit the requirements of the joint, and to facilitate powder compaction and filling. In the process as described, the patch part  4  is displaced uniaxially towards the hub  2  and applies uniaxial pressure to the powder  20 . In some circumstances, it may be desirable for there to be displacement between the components  2  and  4  along multiple axes either sequentially or concurrently. The relative displacement could comprise linear oscillation or circular or patterned movements. 
     It will be appreciated that the agitation of the powder could be achieved by any kind of driver instead of the piezo-electric ultrasonic transducers  24  described above. For example, electromagnetic acoustic transducers (EMATS) could be employed.
First and second components, which may be metallic components, are joined together in a process including introducing a sinterable powder between the components, the powder being retained within a receptacle, displacing the second component towards the first component to compress the powder, and subsequently applying heat and pressure to the powder to form a sintered bond.