Abstract:
An assembly including a metal piece, a piece made of ceramic material, and at least one intermediate connection element assembled to each of the pieces by brazing. The intermediate connection element includes a deformable sheet presenting at least two flat zones brazed to respective ones of the pieces, the two flat zones being interconnected by a deformable zone presenting at least two free undulations oriented in alternation towards the metal piece and towards the piece made of ceramic material.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to the general field of brazing together two materials that present different thermomechanical properties. 
         [0002]    More precisely, the invention applies to brazing together a metal piece and a piece made of ceramic material, e.g. based on silicon carbide (SiC) and/or carbon. 
         [0003]    The piece made of ceramic material may be constituted by solid silicon carbide. It may also be constituted by a thermostructural composite, and in particular by a ceramic matrix composite (CMC) reinforced by silicon carbide or carbon fibers. 
         [0004]    Thermostructural composite materials are characterized by mechanical properties that make them suitable for constituting structural parts, while also conserving these mechanical properties at high temperatures. They are constituted by fiber reinforcement densified by a matrix of refractory material that fills in the pores of the fiber reinforcement, at least in part. The choice of materials for the fibers and for the ceramic is typically made amongst carbon and ceramics (i.e. materials that are neither metallic nor organic), and in particular silicon carbide (SiC). 
         [0005]    By way of example, the invention can be used for assembling together a piece made of ceramic material with a metal piece made of an alloy of titanium, aluminum, and vanadium (TA6V) or of Inconel 718 (registered trademark), an alloy based on nickel and having the composition NiCr19Fe19Nb5Mo3. 
         [0006]    The mechanical properties of pieces made of ceramic material and the fact that these properties are conserved at high temperature make them materials that are particularly suitable for making pieces that are subjected to high levels of thermomechanical stress, in particular in aviation applications (engine parts, fairing elements). When ceramic materials are reinforced by silicon carbide or carbon fibers, they constitute an alternative to metallic materials, presenting numerous advantages, in particular in terms of weight savings and of operational lifetime. 
         [0007]    Conventionally, pieces made of ceramic material and metal pieces are assembled together by a mechanical connection of the riveting or bolting type, but such a connection can sometimes be unsuitable for reasons of size, difficulty of implementation, or weight. 
         [0008]    Furthermore, known homogeneous assembly methods for use with ceramic materials and involving organic precursors of ceramics are not suitable for heterogeneous assemblies between a ceramic material and a metal. 
         [0009]    Known brazing techniques used for making homogeneous connections between two ceramic materials can be difficult to use for heterogeneously brazing a ceramic material on a metal because of the very different thermomechanical and chemical behaviors of ceramic materials and metals. 
         [0010]    A metal alloy based on titanium, aluminum, and vanadium presents a coefficient of expansion that is about two to three times greater than that of ceramic materials. 
         [0011]    More precisely, the coefficient of expansion of such an alloy at 500° C. is about 10×10 −6 K −1 ±15%, while the coefficient for a CMC is about 2.5×10 −6 K −1  to 4.0×10 −6 K −1 ±15%. 
         [0012]    Thus, for a 30 millimeter (mm) assembly, an expansion offset of 0.2 mm is observed on cooling the assembly from the solidification temperature of the brazing composition to ambient temperature. 
         [0013]    Such an offset leads to high levels of stress appearing in the two pieces, and in particular to compression forces in zones of the brazed joint adjacent to the ceramic, and traction forces in zones adjacent to the metal piece. These stresses can give rise to local deformations that might cause one of the pieces to break or lead to reduced strength of the brazed joint. 
         [0014]    Such deformations are irreversible in the metal piece. In the ceramic piece, in particular when made of a CMC, these deformations can lead to brittle type breakage. Such breakage can occur suddenly if the stress is too high. Breakage can also occur by damage building up successively under cyclical stressing. 
       OBJECT AND SUMMARY OF THE INVENTION 
       [0015]    A main object of the present invention is thus to mitigate such drawbacks by proposing an assembly comprising a metal piece, a piece made of ceramic material, and at least one intermediate connection element assembled to each of said pieces by brazing, the intermediate connection element being constituted by a deformable sheet presenting brazed flat zones and deformable zones. 
         [0016]    By way of example, a brazing composition of the Ag—Mn or Ag—Cu—Ti type may be selected, thus making it possible to obtain an assembly that is strong at high temperatures of as much as 500° C. 
         [0017]    In accordance with the invention, the differential expansion between the piece made of ceramic material and the metal piece is absorbed by the deformable sheet. 
         [0018]    The assembly thus presents thermomechanical adaptation to differential expansion, while remaining as much as possible within the elastic domain. In order to be able to withstand cooling after brazing and thermal cycling in operation, it is necessary to accommodate the differential expansion between the piece made of ceramic material and the metal piece. 
         [0019]    In a preferred embodiment, the piece made of ceramic material is based on silicon carbide and/or carbon. 
         [0020]    For example, the ceramic material piece is made of solid silicon carbide. 
         [0021]    In another embodiment, the ceramic material piece comprises a ceramic matrix reinforced by silicon carbide or carbon fibers. 
         [0022]    In a preferred embodiment, the deformable sheet includes at least two deformable undulations oriented in alternation towards the metal piece and towards the piece made of ceramic material. 
         [0023]    Preferably, at least one of the deformable undulations is free, such a free undulation guaranteeing increased flexibility for the brazed connection. 
         [0024]    The height of the free undulation may advantageously be used for modifying rigidity in the fold direction. 
         [0025]    In a first variant embodiment, the deformable undulations are concentric. 
         [0026]    In an embodiment of this first variant, the intermediate connection element comprises a first flat zone that is substantially circular about an axis, a second flat zone that is substantially annular, coaxial about the first flat zone, and having an inside diameter greater than the diameter of said first zone, and undulations presenting symmetry of revolution about the above-mentioned axis. 
         [0027]    Because of this symmetry, such a connection element presents behavior that is identical regardless of the direction of the line of greatest stress. 
         [0028]    In another variant embodiment the connection element is generally in the form of a concertina-folded tape. 
         [0029]    In an advantageous embodiment of the invention, the assembly comprises a plurality of intermediate connection elements that are disposed radially around a fixed point. 
         [0030]    With such a connection element, the stresses are low in the brazed joints and rigidity can be modulated. 
         [0031]    Placing the intermediate connection elements in a radial star disposition advantageously presents concentric undulations that are interrupted radially (the interruptions being constituted by empty gaps along a plurality of radial directions), thus making it possible to reduce or eliminate tangential stresses from such concentric circles. 
         [0032]    Preferably, the intermediate connection elements are arranged in a plurality of radial directions around the fixed point. 
         [0033]    Such an embodiment enables the stiffness of the assembly between the metal piece and the piece of ceramic material to be increased. 
         [0034]    Around the fixed point as defined above, the offset due to the expansion of the metal piece increases with distance from the fixed point. 
         [0035]    In a particular embodiment of this configuration, the intermediate connection elements are of increasing flexibility on going away from the fixed point. 
         [0036]    Such an embodiment serves to compensate for the above-mentioned increase in offset. 
         [0037]    The invention also provides a turbomachine nozzle including at least one assembly as mentioned above in which the metal piece is a casing of said nozzle (or a lever), and the piece made of ceramic material is a flap of the nozzle. 
         [0038]    The invention also seeks to provide a turbomachine combustion chamber including at least one assembly as mentioned above in which the metal piece is a casing of said chamber (or a component part thereof, or a joint—i.e. a connection element—therein), and the piece made of ceramic material is a component part of said chamber. 
         [0039]    The invention also provides post-combustion equipment for a turbomachine including at least one assembly as mentioned above in which the metal piece is a post-combustion casing (or a platform of post-combustion equipment), and the piece made of ceramic material is a flame-holder arm. 
         [0040]    The invention also provides a turbomachine including at least one assembly as mentioned above. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0041]    Other characteristics and advantages of the present invention appear from the following description with reference to the accompanying drawings that show embodiments having no limiting character. In the figures: 
           [0042]      FIG. 1  shows a connection element suitable for use in a first assembly in accordance with the invention; 
           [0043]      FIG. 2  is a section through an assembly of the invention using the connection piece of  FIG. 1 ; 
           [0044]      FIG. 3  shows a connection element suitable for use in a second assembly in accordance with the invention; 
           [0045]      FIG. 4  shows a connection element suitable for use in a third assembly in accordance with the invention; 
           [0046]      FIG. 5  is a fragmentary section through the connection element of  FIGS. 3 and 4 ; 
           [0047]      FIG. 6  shows an undulating surface from which intermediate connection elements of the kind shown in  FIG. 5  are obtained; 
           [0048]      FIG. 7  shows an advantageous embodiment of the invention; 
           [0049]      FIG. 8  shows a particular embodiment of a piece having openings formed therein to facilitate positioning and anchoring connection elements for assembly in accordance with the invention; and 
           [0050]      FIGS. 9A and 9B  are curves showing mechanical properties as a function of temperature for materials suitable for use in making intermediate elements, respectively elements that are ductile and elements that are reversibly deformable. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0051]      FIG. 1  shows a connection element  10  suitable for use in a first assembly in accordance with the invention. By way of example it is constituted by a plane deformable material stamped as to form a first flat zone  11  that is circular about an axis Δ, and a second flat zone  12  that is annular about the axis Δ, these two zones being interconnected by a deformable zone  13  forming substantially a truncated cone. The inside diameter of the annular zone  12  is greater than the diameter of the circular zone  11 . The substantially conical walls  13  may present inclination to a greater or lesser extent relative to the direction perpendicular to the flat zones  11  and  12 . 
         [0052]      FIG. 2  shows a section II-II (see  FIG. 1 ) through an assembly  14  of the invention made using a connection element  10  as shown in  FIG. 1 . 
         [0053]    In this assembly, a first brazed joint  15  is made between a metal piece  16  and the circular flat zone  11  of the connection element  10 . A second brazed joint  17 , preferably using the same brazing composition, is made between a piece  18  of ceramic material, e.g. a CMC, and the annular flat zone  12  of the connection element  10 . 
         [0054]    The connection element  10  shown and used in  FIGS. 1 and 2 , can be obtained from a plate of deformable material by cutting out and stamping using a substantially cylindrical stamping element. 
         [0055]    A vertical height or size for the connection element of about 2 mm is appropriate for aviation applications, however it will be understood that this size can be modified as a function of requirements specific to different applications. The proportions of the various portions of the connection element  10  may also be modified as a function the intended application. 
         [0056]    On this topic, it should be observed that modifying the geometrical parameters of the connection element  10  makes it possible to modify the magnitude of the stresses observed. 
         [0057]    The proposed size of 2 mm enables flexibility to be obtained in the tangential direction for pieces having a size of the order of about ten centimeters. Under the effect of differential expansion, for example during a thermal cycle, the conical portion  13  of the connection element  10  can absorb this expansion differential completely, or in part, without endangering the strength of the assembly. Since the material from which the connection element is made is deformable, the deformation of the cylindrical portion is of no consequence for the assembly as a whole. 
         [0058]    Nevertheless, in the embodiment shown in  FIG. 2 , high levels of stress can be observed in the brazed joints and in shear in the membrane in the connection element. 
         [0059]    The connection element  10 ′ described below with reference to  FIG. 3  serves to mitigate this drawback. 
         [0060]    The connection element  10 ′ has a deformable zone  13 ′ that is axially symmetrical about an axis Δ, this zone  13 ′ being shaped so as to comprise at least two free undulations  19  and  20  oriented alternately upwards and downwards relative to the flat zones  11  and  12  that are respectively circular and annular about the axis Δ. The structure is still advantageously made of stamped deformable material. The presence of undulations that are free, i.e. not brazed, in the deformable zone  13 ′ makes the structure more flexible. 
         [0061]      FIG. 5  shows a section V-V (see  FIG. 3 ) of the connection element  10 ′. The flat zones  11  and  12  are interconnected by the deformable zone  13 ′ that presents two free undulations  19  and  20 . 
         [0062]    It should be observed that in  FIGS. 3 and 5 , the undulations  19  and  20  are shown as being substantially flat, however they could equally well have other shapes, for example they could be sinusoidal. 
         [0063]    Considerations of size, bulk, and proportion similar to those mentioned for the connection element shown in  FIG. 1  apply likewise to the element of  FIG. 3 . 
         [0064]    It should be observed that changes in the thickness of the stamped sheet, radii of curvature r 1 , r 2 , r 3 , r 4 , r 5 , and r 6  of the undulations, dimensions l 1 , l 2  of the undulations, heights h 1 , h 2 , and h 3  characterizing the undulations  19  and  20 , angles alp 1 , alp 2 , and alp 3  between the undulations and the perpendicular to the flat zones, and sizes l 0 , l 3  of said flat zones are all to be envisaged and constitute as many parameters for refining the stresses and the stiffness properties of the element. 
         [0065]    In the embodiments of  FIGS. 1 and 3 , the rigidity of the resulting assembly in shear presents the advantage of being isotropic in the assembly plane. 
         [0066]    The shear stresses observed at the brazed joints in an assembly using a connection element having the characteristics shown in  FIG. 3  are smaller than those observed in an assembly using a connection element as shown in  FIG. 1 . 
         [0067]      FIG. 4  shows a connection element  10 ″ that can be used in another assembly in accordance with the invention. In this embodiment, the connection element  10 ″ is generally in the form of a concertina-folded tape. In the example shown in  FIG. 4 , the section of the tape is the same as that shown in  FIG. 5 . Such a connection element could also be a stamped structure or could advantageously be obtained by folding, or by extrusion in a straight line along the direction Y. If extruded, and as shown in  FIG. 6 , an undulating surface  22  is obtained with corrugations that extend along the direction Y. 
         [0068]    The connection elements  10 ″ are then cut once every L millimeters from said undulating surface  22 . 
         [0069]    It is also possible to envisage machining a piece made of metal such as Inconel 718, e.g. by wire machining. 
         [0070]    The intermediate element  10 ″ in this embodiment has a preferred direction for deformation constituted by the direction X (tangential rigidity) and a rigid direction constituted by the direction Y (transverse rigidity). 
         [0071]    The stresses observed in the brazed joints in this embodiment are low. The resulting assembly then presents lower rigidity than that observed in the other embodiments based on an axially symmetrical connection element  10  or  10 ′, as shown in  FIGS. 1 and 3 . 
         [0072]    The geometrical parameters of the tape can be modified in order to obtain the lowest maximum stress in the brazed joints and in the structure itself for the lowest tangential rigidity (i.e. in the direction X), and the highest normal rigidity (in the direction Z). 
         [0073]    Thus, it is possible to modify the thickness e of the tape, the radii of curvature r 1 , r 2 , r 3 , r 4 , r 5 , and r 6  of the undulations, the dimensions l 1 , l 2  of the undulations, the heights h 1 , h 2 , and h 3  of the undulations, the angles alp 1 , alp 2 , and alp 3  made by the undulations relative to the perpendicular to the flat zones, and the dimensions l 0 , l 3  of the flat zones. 
         [0074]    The three heights h 1 , h 2 , h 3  characterize the two free undulations  19  and  20  of the deformable zone  13 ′. These first, second, and third heights h 1 , h 2 , h 3  correspond, as shown in  FIG. 5 , to the heights of the portions of the tape that are substantially rectilinear in profile extending respectively between the first flat zone  11  and the bottom of the undulation  19 , between the bottom of the undulation  19  and the top of the undulation  20 , and between the top of the undulation  20  and the second flat zone  12 . 
         [0075]    The person skilled in the art will understand that the height h 2  is selected to be less than or equal to the height h 1 . 
         [0076]    Nevertheless, it is advantageous also to take account of the minimum radii of curvature for the free undulations. The value of h 2  is then advantageously selected to be greater than or equal to ⅓ of h 1 . 
         [0077]    In the example described herein, the values that have been selected and that are proposed are limited to a few tenths of a millimeter since it is required that the connection between the assembled pieces should not exceed 2 mm. That said, the greater h 1  and h 2 , the more the deformations are spread and the lower the stresses. 
         [0078]    The table below shows an example of a set of geometrical parameters suitable for obtaining a good compromise between maximum stress and rigidity with an alloy tape made of Inconel 718™. 
         [0000]    
       
         
               
               
               
               
               
               
               
               
             
           
               
                   
               
               
                 e 
                 h1 
                 h2 
                 h3 
                 alp1-alp3 
                 l1-l3 
                 r1-r6 
                 l0 
               
               
                   
               
             
             
               
                 0.4 mm 
                 0.3 mm 
                 0.1 mm 
                 0.3 mm 
                 5° 
                 0.5 mm 
                 0.75 mm 
                 1.0 mm 
               
               
                   
               
             
          
         
       
     
         [0079]    To increase rigidity in shear parallel to the assembly plane, a plurality of intermediate connection elements  10 ′ are disposed radially around a fixed point  23 , as shown in  FIG. 7 . 
         [0080]    In this embodiment, the deformable sheets are directed towards the fixed point  23 . By placing the connection elements  10 ″ in concentric manner, zero relative displacement is forced between the assembled pieces for the central point  23  of the concentric arrangement. This stiffens the assembly in the assembly plane. The intermediate elements  10 ″ can be placed in such a manner that their own deformation directions point towards the fixed point  23 . 
         [0081]    This disposition mitigates the lack of rigidity in the tangential direction. Relative deformation during cooling of the two pieces to be assembled together is then oriented towards the center of the assembly, which is the fixed point  23 , so there is no relative movement between the two pieces that are to be assembled. The deformation due to differential expansion is then absorbed concentrically. Overall rigidity is obtained because of the greater rigidity of the structures in the direction perpendicular to the preferred deformation direction. 
         [0082]    In addition, given that the relative displacement between the pieces for assembly is zero at the fixed point and that it increases out to the periphery, it is advantageous to make use of structures of increasingly flexibility on going away from the fixed point. This serves to improve tangential rigidity in the region of the fixed point. 
         [0083]      FIG. 8  shows a particular embodiment of an assembly piece  30  having openings or cavities  31  formed therein for the purpose of positioning intermediate connection elements  10 ″ in a star configuration corresponding to that shown in  FIG. 7 . This arrangement enables the intermediate connection elements  10 ″ to be anchored in the piece  30  which is preferably the metal piece, or possibly the piece made of ceramic material. 
         [0084]    Implementing the invention requires the intermediate elements to be made of a material that is selected to remain within its elastic domain. It is also important to fabricate the intermediate connection elements out of a material that is as strong as possible in order to have greater latitude in mechanical weakening, i.e. improved flexibility. 
         [0085]      FIGS. 9A and 9B  show properties that need to be taken into account when selecting the material for the intermediate element. In  FIG. 9A , curve IMD representing the mechanical properties of an intermediate connection element that is solid and ductile is given for comparison with the curve HX for the metal piece that is to be assembled. A loss in mechanical properties PP (elastic limit, rupture strength) can be observed. 
         [0086]    The curve MP represents the potentially ideal mechanical properties of an intermediate connection element for brazing at a temperature T S  corresponding to the solidification temperature of the brazing compound. 
         [0087]    Below the solidification temperature T S , the intermediate element should ideally have mechanical properties that are weaker than those of the metal piece for assembly (HX) so as to enable it to act as a ductile material. It also needs to have mechanical strength properties that are at least sufficient at an operating temperature T F , where the operating temperature T F  is empirically about ⅔ of the above-mentioned solidification temperature T S , in degrees Celsius, which can be written as follows: 
         [0000]    
       
         
           
             
               T 
               S 
             
             = 
             
               
                 3 
                 2 
               
                
               
                 T 
                 F 
               
             
           
         
       
     
         [0000]    where T S  and T F  are expressed in degrees Celsius. 
         [0088]    However, the person skilled in the art knows that these requirements, represented by the curve MP cannot be achieved with an intermediate element that is solid and ductile. To mitigate the loss of property PP observed with such a material, it is necessary for the assembly to be overdimensioned. 
         [0089]      FIG. 9B  shows a curve ISC for the mechanical properties of a material for a deformable intermediate connection element that is advantageously suitable for use in the invention. This curve corresponds to a highly refractory material (e.g. alloys based on iron, nickel, chromium, aluminum, titanium) having mechanical properties that degrade little at high operating temperature. 
         [0090]    Instead of observing a loss of property PP as in  FIG. 9A , there can be seen a structural weakening margin MAS. This margin is controllable and used for adjusting the flexibility of the structure at will. 
         [0091]    The deformable intermediate connection elements may also be in the form of a one-dimensional (1D) undulating sheet, a stamped sheet, or indeed a sheet with corrugations crossing in two dimensions (2D). 
         [0092]    By way of example, materials for making these structures can be selected from the following list: 1D undulating sheet of alloy based on FeCrAlY; 2D corrugated sheet of Haynes 230 (stamped sheet); crossed 2D corrugated sheets of Nimonic 75 (one-dimensional corrugated tape inserted and brazed in the corrugation recesses of a one-dimensional corrugated sheet, embodiment not shown). 
         [0093]    Such structures can be used in particular for assembling parts made of ceramic material based on SiC with metal parts based on Inconel 718 or TA6V alloy, this list not being limiting.