Abstract:
A method of manufacturing a component having first and second layers, the first and/or second layers including one or more depressions provided on a surface of the respective layer. The method including: arranging the first and second layers so that they face one another and with the depressions on inner facing surfaces of the layers; diffusion bonding the first and second layers together about their edges; applying a first differential pressure across each of the first and second layers to evacuate an inner space defined by the layers, thereby forming one or more depressions on an outer facing surface of the first or second layer; and applying a second differential pressure across each of the first and second layers to expand the inner space defined by the layers.

Description:
[0001]    This invention relates to a method of manufacturing a component comprising an internal structure, and particularly but not exclusively relates to applications of the method in hollow aerofoil components for turbomachines. 
       BACKGROUND 
       [0002]    It is known to manufacture hollow metallic aerofoils for example to be used as blades in a jet engine, and in particular fan blades for a turbomachine, by diffusion bonding metallic panels and inflating the volume formed between the panels, the panels forming pressure and suction surfaces of the blade. Such structures are widely used in the civil aerospace industry, for example in wide-chord fan blades, and may also be used in blinks (i.e. bladed disks), particularly in military applications. The metallic panels may include elementary metal, metal alloys and metal matrix composites and at least one of the metallic panels may be capable of superplastic extension. In one known process the surfaces of the panels to be joined are cleaned, and at least one surface of one or more of the panels is coated in preselected areas with a stop-off material to prevent diffusion bonding. The panels are arranged in a stack and the edges of the panels are welded together, except where a pipe is welded to the panels, to form an assembly. The pipe enables a vacuum, or inert gas pressure, to be applied to the interior of the assembly. The assembly is placed in an autoclave and heated so as to “bake out” the binder from the material to prevent diffusion bonding. The assembly is then evacuated, using the pipe, and the pipe is sealed. The sealed assembly is placed in a pressure vessel and is heated and pressed to diffusion bond the panels together to form an integral structure. Diffusion bonding occurs when two surfaces are pressed together under temperature, time and pressure conditions that allow atom interchange across the interface. The first pipe is removed and a second pipe is fitted to the diffusion bonded assembly at the position where the first pipe was located. The integral structure is located between appropriately shaped dies and is placed within an autoclave. The integral structure and dies are heated and pressurised fluid is supplied through the second pipe into the interior of the integral structure to cause at least one of the panels to be blow formed to produce an article matching the shape of the dies. If either of the panels exhibit superplastic characteristics it is usual for the inflation to occur at the temperature at which the panel is in its superplastic condition. 
         [0003]    The assembly may be filled or part filled by a suitable material to provide damping of the structure and therefore to reduce vibration. A suitable material may be one which possesses viscoelastic properties. Viscoelasticity is a property of a solid or liquid which when deformed exhibits both viscous and elastic behaviour through the simultaneous dissipation and storage of mechanical energy. A known method is to introduce a viscoelastic material, for example a Huntsman™ syntactic damping material or a similar product, or non-syntactic material, into the cavity by injecting or otherwise introducing the material into some or all of the cavity. This technique may be applied in a hollow assembly wherein the cavity is smooth walled with no internal structure, see GB2371095 for example. In this configuration the viscoelastic material is restrained solely by the bond between the viscoelastic material and the walls of the cavity. If this bond is not sufficient to retain the viscoelastic material during working conditions, in particular due to centrifugal loading, then, since the viscoelastic material is a parasitic mass which is unable to support its own weight, the hydrostatic load of the unrestrained material will cause the blade to fail rapidly. Accordingly, the consequences of failure of this bond are severe. 
         [0004]    It is therefore desirable to provide some form of means for retaining and restraining the viscoelastic material. An internal structure, e.g. a Warren girder type structure, may be used to provide such a restraining or retaining effect on the injected material. However, by providing a rigid internal structure the benefits of damping the aerofoil may be reduced as the aerofoil is less flexible as a result of the internal structure. This may lead to additional problems where the aerofoil prematurely fatigues or cracks as a result of the reduced flexibility. Other configurations use internal ribs, which may be attached to alternate interior walls of the aerofoil but which are not connected to one another, see for example patent application number GB0713699.7. This configuration permits damping of the assembly whilst the re-entrant features still provide a means of retaining the injected material. However, this internal configuration is complex and difficult to manufacture. 
         [0005]    Furthermore, the use of an internal structure to physically restrain the viscoelastic material inevitably adds weight to the aerofoil and thus increases the stresses on the aerofoil, in particular at the root of the aerofoil. This increases the blade off energy if the blade were to fail, which must be taken into account when designing the blade retention system. In addition, the provision of complex internal structures increases manufacturing costs and lead times. It is therefore desirable to provide an improved method of restraining a viscoelastic material within a cavity which addresses some or all of the above problems associated with the prior art methods. 
       Statements of Invention 
       [0006]    According to a first aspect of the present invention there is provided a method of manufacturing a component, e.g. by superplastic forming and diffusion bonding, wherein the component comprises first and second layers, the first and/or second layers comprising one or more features, e.g. depressions, provided on a surface of the respective layer, and wherein the method comprises: arranging the first and second layers so that they face one another and with the features on inner facing surfaces of the layers; bonding, e.g. diffusion bonding or welding, the first and second layers together about their edges; applying a first differential pressure across each of the first and second layers to evacuate an inner space defined by the layers such that the layers are deformed with the features on the inner facing surface at least partially translated to an outer facing surface of the first or second layer, thereby forming one or more features on an outer facing surface of the first or second layer; and applying a second differential pressure across each of the first and second layers to inflate the inner space defined by the layers such that the layers are deformed with the features on the outer facing surface at least partially translated back to the inner facing surface of the first or second layer. 
         [0007]    The method may further comprise applying a stop-off material in a predetermined pattern to the inner facing surfaces of the first and/or second layers. The stop-off material may prevent a diffusion bond from forming between the first and second layers across regions defined by said predetermined pattern. The stop-off material may be applied in regions outside of the one or more depressions. 
         [0008]    Both the first and second layers may comprise the one or more depressions. The depressions on the first and second layers may be arranged so that they do not overlap with the depressions on an opposing layer. 
         [0009]    A damping material may be provided in the inner space between the first and second layers. The method may further comprise injecting the damping material into the inner space between the first and second layers. 
         [0010]    The depressions may be elongate. The depressions may form one or more rows. Alternatively, the depressions may possess rotational symmetry, e.g. they may comprise one or more spots. 
         [0011]    The method may further comprise heating and pressing the first and second layers to diffusion bond the first and second layers together to form an integral structure. The method may further comprise one or more of: placing the first and second layers between appropriately shaped dies; heating the first and second layers and dies; and supplying a pressurised fluid between the first and second layers to cause at least one of the first and second layers to be superplastically formed. The method may further comprise finishing the outer facing surfaces of the first and/or second layers, e.g. by polishing, grinding or any other known finishing technique. 
         [0012]    According to a second aspect of the present invention there is provided a component manufactured according to the aforementioned method. The component may be an aerofoil structure for a turbomachine. A turbomachine may comprise the component. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    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:— 
           [0014]      FIGS. 1(   a )-( d ) show a selection of the method steps involved in manufacturing a component according to an example of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    With reference to  FIGS. 1(   a )-( d ), a component  10  according to the present invention comprises a first layer  12  and a second layer  14 , which may be arranged side by side. For example, the present invention may specifically relate to the internal core of a hollow fan blade and  FIG. 1(   a ) shows a root to tip section through two un-bonded fan blade panels. The first and second layers  12 ,  14  may be made from titanium. 
         [0016]    The first and/or second layers  12 ,  14  may comprise one or more features, e.g. depressions  22 ,  24 , on inner facing surfaces of the first and second layers  12 ,  14  respectively. By contrast, the outer facing surfaces of the first and second layers  12 ,  14  may be substantially smooth. As shown, both the first and second layers  12 ,  14  may comprise the one or more depressions  22 ,  24 . The depressions  22 ,  24  may be arranged on one of the first and second layers  12 ,  14  so that they do not overlap with the depressions  22 ,  24  on the other of the first and second layers. Furthermore, opposing depressions  22 ,  24  may be radially spaced apart. In other words, the depressions  22  on the first layer may be smaller than the spacing between the depressions  24  on the second layer and vice-versa. 
         [0017]    As depicted, the depressions  22 ,  24  may be rounded in cross-section, e.g. partially oval, elliptical or circular shaped, or they may alternatively be straight edged in cross-section, e.g. triangular, rectangular or trapezium shaped. The depressions  22 ,  24  may extend in a longitudinal direction, i.e. into the page as shown in  FIG. 1 . Alternatively, the depressions may possess rotational symmetry, e.g. they may be square or circular shaped, and there may be a plurality of depressions extending into the page as shown in  FIG. 1 . 
         [0018]    In the case of the component  10  being an aerofoil structure, the depressions  22 ,  24  on the first and second layers  12 ,  14  may correspond to ribs, which extend in a spanwise direction, i.e. from the leading edge to the trailing edge of the aerofoil structure. However, the depressions  22 ,  24  may alternatively be orientated as ridges running in a radial direction or they may be a plurality of spots, rings or dimple like shapes. 
         [0019]    The inner and outer surfaces of the first and second layers  12 ,  14  may be generated using a machining process, or any other material removal or material addition methods, including conventional methods of manufacture such as machining or forging, or non-conventional methods of manufacture such as chemical machining or metal deposition. 
         [0020]    With reference to  FIG. 1(   b ), prior to bonding the first and second layers  12 ,  14  to each other, a stop-off material  32 ,  34  may be applied in preselected areas between the first layer  12  and second layer  14 . The stop-off material  32 ,  34  prevents a diffusion bond from occurring between said layers across the preselected areas. The stop-off material  32  may be applied to the inner facing surface of the first layer  12  in regions opposing the depressions  24  in the inner facing surface of the second layer  14 . In addition or alternatively, the stop-off material  34  may be applied to the inner facing surface of the second layer  14  in regions opposing the depressions  22  in the inner facing surface of the first layer  12 . In other words, the stop-off material may be applied in regions outside of the one or more depressions  22 ,  24  (i.e. where there are no depressions in the first and second layers). The stop-off material may not be applied about edges  16 ,  18  and the remaining edges about the component&#39;s perimeter (not shown). 
         [0021]    With reference to  FIG. 1(   c ) once the stop-off material  32 ,  34  has been applied, the layers  12 ,  14  are stacked together and heat and pressure, e.g. hot isostatic pressure, are applied via dies (not shown). A diffusion bond may be formed between the respective layers, except that a diffusion bond may not be formed where the stop-off material has been applied. A diffusion bond may be formed about the edges  16 ,  18  as shown and the remaining edges (not shown) of the first and second layers  12 ,  14 . 
         [0022]    Referring still to  FIG. 1(   c ), a first differential pressure, e.g. a vacuum, may be applied between inside and outside the component  10 . The first differential pressure may evacuate an inner space defined by the first and second layers  12 ,  14 . The first and second layers  12 ,  14  may be deformed by the first differential pressure, with the depressions  22 ,  24  on the inner facing surfaces of the first and second layers at least partially being translated to an outer facing surface of the first or second layers. As a result one or more depressions  42 ,  44  on an outer facing surface of the first or second layer may be formed. In other words the material surrounding the depressions  22 ,  24  is collapsed or sucked in by the first differential pressure such that the depressions move from the inner surfaces of the first and/or second layers to the outer surfaces. As shown in  FIG. 1(   c ), a portion of the inner facing depressions  22 ,  24  may remain after the first differential pressure has been applied. 
         [0023]    With reference to  FIG. 1(   d ) once the diffusion bonds have been formed between the respective layers, a second differential pressure may be applied between inside and outside the component  10 . The second differential pressure may inflate the inner space defined by the first and second layers  12 ,  14  and the first and second layers may be inflated between hot forming dies to re-form the internal shape and form the external shape of the component  10 . Pressurised fluid (typically Argon) is supplied into the interior of the component  10  to cause at least one of the layers to be blown to produce a component matching the shape of the dies. The component  10  may also be twisted into shape during or after the inflation. 
         [0024]    Referring still to  FIG. 1(   d ), the second pressure differential may deform the first and second layers  12 ,  14  such that the material which had translated into the inner facing depressions  22 ,  24  is translated back out to the outer facing surfaces of the first and second layers. In other words, the depressions  42 ,  44  on the outer facing surfaces may be at least partially translated back to the inner facing surface of the first or second layer. The depressions  22 ,  24  on the inner facing surfaces of the first and second layers may therefore be substantially reformed. Accordingly, the high pressure fluid acting on the inner facing surfaces of the first and second layers  12 ,  14  may be sufficient to cause the depressions  42 ,  44  to be blown out. The first and second layers  12 ,  14  may be deformed such that the outer facing surfaces of the first and second layers  12 ,  14  are substantially smooth with the depressions  22 ,  24  having been transferred to the inner facing surfaces of the first and second layers. In addition, the high pressure fluid also creates a gap  36  between the first and second layers  12 ,  14  as the component  10  is expanded into the dies. 
         [0025]    In summary, the present invention comprises collapsing and re-inflating the first and second layers of the component in order to create the desired geometry. 
         [0026]    Once the component  10  is inflated by a high pressure fluid, the gap  36  may be filled with a damping material (not shown). The damping material may be a polymer, for example, a viscoelastic material, e.g. a Huntsman™ syntactic damping material, or a similar product, or a non-syntactic damping material. The internal cavity shape is defined in such a way that it prevents the internal media moving or viscously creeping inside the blade under extreme centrifugal load. 
         [0027]    Once fully formed, the external geometry of the component  10  may be finished as necessary, e.g. by polishing, grinding or any other finishing process. 
         [0028]    The present invention allows the component, e.g. aerofoil structure, to flex in all directions and transmit shear forces via the depressions to the internal damping material. By contrast, previously-proposed arrangements, which comprise a connector between the two layers, may not flex evenly or enable the inclusion of damping media. Furthermore, the present invention may prevent the layers of the component, e.g. aerofoil structure, from separating or delaminating. The present invention aids in preventing delamination of the damping media from the internal structure during impact or normal running stresses by having the filler mechanically linked to the internal depressions in each panel, whilst also optimising the weight of the design. 
         [0029]    Advantageously, with the present invention the internal surface geometry may be clearly defined prior to assembly of the component, whilst ensuring the post hot forming inflation cycle generates a smooth external component shape. The post hot forming machining cycles are also minimised as a result. 
         [0030]    The present invention also has an advantage over symmetrical core designs, as the flat surface opposite each depression allows for low cost silk screen printing to be used to apply stop off materials, in place of complex 3D inkjet style printing. 
         [0031]    In addition, because the inner facing surfaces of the first and second layers are initially generated directly, a high fidelity of shape is achievable in the internal surface. By contrast, shape losses occur for features which are not initially generated and are only transferred through the layers. 
         [0032]    Other advantages include a reduction in manufacturing cost due to reduced die wear associated with feature transfer, since prominent external features increase hot forming die wear. 
         [0033]    The present invention has been shown for use in an entire cavity, however it may be used in a particular area or region of a large cavity, for example acting as a patch. The component  10  may be a blade for a turbomachine, for example a compressor fan blade. The first layer  12  and second layer  14  may form the suction and pressure surfaces of a blade respectively or vice-versa. The component  10  may be orientated so that the depressions  22 ,  24  are disposed in a radial direction of the turbomachine blade. Alternatively, the component  10  may be orientated so that the depressions  22 ,  24  are disposed in a circumferential direction. In either case, the assembly described above may be repeated in the chord-wise direction and/or span-wise direction. 
         [0034]    The present invention may be used in any component where the retention of a damping media or other material is required to be held in place. This may include rotating or static components. The present invention may also be applied to similar structures, which may comprise additional layers, e.g. membranes. The present invention may also comprise the inclusion of a material addition process, forming even more complicated geometries. 
         [0035]    The present invention may be adapted for use in most hollow structure applications, in particular the internal geometry of fan blades for all types of engine (civil, military, power generation and other related applications). Examples of possible structures include: internal enclosed or vented cavities within hollow civil or military fan blades; internal enclosed or vented cavities within hollow civil or military blisks; and vibration damped static components. The present invention may be used in any field where a hollow structure is supported internally, for example to damp vibrations and/or withstand impacts. By way of example, such structures may include fan blades, containment rings, damped rotating structures, hollow structures and outlet guide vanes.