Abstract:
A method of assembling together by brazing first and second parts made of composite material, each of the first and second parts having an assembly face for brazing with the assembly face of the other part, the method including making at least one perforation in the assembly face of the first part; interposing capillary elements between the assembly faces of the first and second parts made of composite material; placing the first and second parts facing each other while inserting a peg in each perforation of the first part; placing a brazing composition in contact with a portion of the capillary elements; and applying heat treatment to liquefy the brazing composition so as to cause the molten brazing composition to spread by capillarity between the assembly faces of the composite material parts.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application of the U.S. National Stage of PCT/FR2013/051672 filed Jul. 12, 2013, which in turn claims priority to French Application No. 1256949, filed Jul. 18, 2012. The contents of both applications are incorporated herein by reference in their entirety. 
     BACKGROUND OF THE INVENTION 
     The invention relates to using brazing to assemble together thermostructural composite materials, and in particular ceramic matrix composite (CMC) materials. CMC materials are typically formed by a porous substrate, such as a porous fiber substrate, that has been densified by a ceramic matrix. The fibers of the substrate may be made of carbon or of ceramic. By way of example, the matrix may be made of a refractory ceramic such as a refractory carbide, nitride, boride, or oxide. CMC materials, such as for example C/SiC composite materials (carbon fiber reinforcement with a silicon carbide matrix), are remarkable for their mechanical properties, which make them suitable for constituting structural elements, and for their ability to conserve those properties at high temperatures. 
     When making structures out of ceramic matrix composite materials, it is common to construct them from independent elements made of CMC materials, which elements are assembled together by brazing. Nevertheless, brazing ceramic matrix composite materials together is technically difficult, and the breaking stress of the brazed joint is often insufficient relative to the forces to which the assembly might be subjected, in particular shear forces. 
     Pieces made of CMC materials present a high degree of surface roughness, which prevents the brazing composition spreading well in the zone where the brazed joint is formed, thereby creating zones of weakness in the brazed connection. 
     Also, while the brazing composition is diffusing between the assembly faces of the parts to be brazed together, gas is given off due to reactions between the brazing composition and the ceramic particles used for conveying the brazing composition by capillarity between the assembly faces. Some of that gas becomes imprisoned inside the brazed joint, thereby leading to a brazed joint that is porous, and in which there remain local braze-free areas between the two surfaces. That lack of material leads to defects in the connection between the two parts, and consequently degrades the quality of the assembly by weakening the connection that is made. 
     OBJECT AND SUMMARY OF THE INVENTION 
     An object of the invention is to provide a method that enables composite material parts to be assembled together by brazing, but without the above-specified drawbacks, and that makes it possible in particular to make brazed connections having improved mechanical strength against shear forces. 
     This object is achieved with a method of assembling together by brazing first and second parts made of composite material, each of the first and second parts having an assembly face for brazing with the assembly face of the other part, the method comprising the following steps:
         making at least one perforation in the assembly face of the first part;   interposing capillary elements between the assembly faces of the first and second parts made of composite material;   placing the first and second parts facing each other while inserting a peg in each perforation of the first part;   placing a brazing composition in contact with a portion of the capillary elements; and   applying heat treatment to liquefy the brazing composition so as to cause the molten brazing composition to spread by capillarity between the assembly faces of the composite material parts.       

     Thus, by inserting a peg in at least one perforation present in at least one of the two composite material parts, it is possible to anchor the brazed joint securely in the material of the part and to reinforce significantly the strength of the brazed connection, in particular in shear. 
     The perforation formed in the assembly face of one or both parts for assembling together may optionally open out in the face opposite from the assembly face. 
     In a first aspect of the invention, clearance is provided between the inside wall of each perforation and the outside wall of the peg inserted in the perforation. The presence of such clearance enables degassing to take place during the brazing cycle, with gas being evacuated via the end of the or each perforation that opens out outside the assembly faces. 
     Also, the presence of brazing in the perforation(s) may be checked merely by visual inspection from the face into which the perforation(s) open(s) out. Specifically, partial or complete absence of brazing in the clearance present between a peg and a perforation indicates that the brazed joint has not been formed in uniform manner over the entire brazing zone and that the connection might then include zones of weakness. 
     In a second aspect of the invention, at least one first perforation is formed in the assembly face of the first part, while a second perforation is formed in the assembly face of the second part. The first and second perforations are brought into alignment when the first and second parts are caused to face each other, a peg being inserted in the first and second perforations. 
     In a third aspect of the invention, the second composite material part includes a peg on its bonding face, the peg being inserted in the perforation of the first part when the first and second parts are placed facing each other. Under such circumstances, the peg may be machined directly on the bonding face of the second part. 
     In a fourth aspect of the invention, each peg is made out of a material similar to the composite material of the first and second parts. The term “similar” is used of a material to mean either the same material as the material of the first and second parts, or a material having a coefficient of thermal expansion that differs from the coefficient of thermal expansion of the composite material of the first and second parts by no more than 40%. 
     In a fifth aspect of the invention, a plurality of grooves are also made in the assembly face of at least one of the two composite material parts, at least some of said grooves opening out into one or more faces adjacent to the assembly face of the part. 
     In a sixth aspect of the invention, a plurality of orifices are also made in the assembly face of at least one of the two composite material parts, at least some of said orifices opening out into the face opposite from the assembly face of the part. 
     Thus, by the presence of grooves and/or orifices in at least one of the assembly faces of the composite material parts, additional brazed-joint anchor portions can be formed in the material of the part. In addition, the grooves and/or orifices form additional paths in the parts for degassing and for inspecting the brazed joint. 
     In a seventh aspect of the invention, the capillary elements are selected from at least one of the following elements: particles of SiC, a web of SiC, a mat of carbon, particles of carbon, and nanotubes of carbon. 
     In an eighth aspect of the invention, the composite material parts for assembling together by brazing are afterbody parts of an aeroengine. 
     In a ninth aspect of the invention, the brazing composition is a metallic composition. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other characteristics and advantages of the invention appear from the following description of particular implementations of the invention, given as nonlimiting examples, and with reference to the accompanying drawings, in which: 
         FIGS. 1A to 1E  are diagrams showing the brazing of two parts made of ceramic matrix composite material in accordance with an implementation of the invention; 
         FIG. 2  is a diagrammatic perspective view of two parts for brazing together in accordance with another implementation of the invention; 
         FIG. 3  is a diagrammatic perspective view of two parts for brazing together in accordance with another implementation of the invention; 
         FIG. 4  is a diagrammatic perspective view of two parts for brazing together in accordance with another implementation of the invention; and 
         FIG. 5  is a diagrammatic perspective view of two parts for brazing together in accordance with another implementation of the invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     The brazing assembly method of the present invention applies to any type of thermostructural ceramic matrix composite (CMC) material, i.e. to any material made up of refractory fiber reinforcement (carbon fibers or ceramic fibers) densified by a ceramic matrix that is also refractory, such as C/SiC, SiC/SiC, C/C-SiC, etc. materials. This method also applies to other types of materials that give off gas during brazing, such as C/C materials or monolithic ceramics such as SiC, SiSiC, vitreous carbon, or pyrolytic carbon. 
     In accordance with the invention, the brazed connection formed by the method of the invention is reinforced by inserting at least one peg in a perforation made in one or both of the parts for assembling together. 
     With reference to  FIGS. 1A to 1E , an implementation of a method in accordance with the invention for assembling together two CMC material parts  10  and  20  by brazing comprises the following steps. 
     As shown in  FIG. 1A , the first step consists in making a perforation in each of the parts  10  and  20 . More precisely, in the presently described example, a perforation  101  is machined in the part  10 , the perforation  101  extending from the assembly face  10   a  and opening out into the face  10   d  of the part  10  that is opposite from the assembly face  10   a,  and a perforation  201  is machined in the part  20 , the perforation  201  extending from the assembly face  20   a  of the part  20  and opening out into the face  20   d  opposite from the assembly face  20   a . In a variant implementation, one or both of the perforations  101  and  201  do not open out in the face opposite from the assembly face, as for the perforation  901  described below with reference to  FIG. 3 . Under such circumstances, the height of the peg is adapted to match the depth of the perforations. 
     Thereafter, capillary elements, in this example particles of SiC  30 , are arranged on the assembly face  10   a  of the part  10  ( FIG. 1B ). The particles of SiC are to enable the liquid brazing composition to be conveyed and to diffuse between the two parts  10  and  20  during the brazing operation. It is also possible to use other capillary elements, such as in particular: a web of SiC, a mat of carbon, particles of carbon, or nanotubes of carbon. 
     The part  20  is then positioned on the part  10  in such a manner as to cause the assembly faces  10   a  and  20   a  to face each other with the perforations  101  and  201  in alignment ( FIG. 1C ). The zone defined by the union of the facing portions of the assembly faces  10   a  and  20   a  corresponds to the brazing zone  40 , i.e. the zone where the brazed joint is to be formed. 
     Once the part  20  has been positioned on the part  10 , a shear peg  5  is inserted in the perforations  101  and  201  ( FIGS. 1C and 1D ). A brazing composition  41  in the form of a solid paste is placed on the assembly face  10   a  outside the brazing zone  40  and in contact with the particles of SiC  30  that are present outside the brazing zone  40 . The brazing composition is selected in particular as a function of its compatibility with the composite material of the parts to be assembled together, i.e. it is preferable to choose a composition that has a coefficient of thermal expansion that is close to that of the composite material and that does not react at all with the composite material or that presents controlled reactivity therewith. By way of example, use may be made of compositions based on silicon as described in the following documents: EP 806 402, U.S. Pat. No. 5,975,407, US 2008/190552 and U.S. Pat. No. 7,497,918; of alloys of silicon plus metallic silicides, of silicon plus optionally alloyed germanium, and also of metallic compositions known under the tradenames: Cusil-ABA®, Ticusil®, Incusil®, or Brasic® 
     It is also possible to use compositions corresponding to mixtures of oxides selected from the following oxides: SiO 2 , Al 2 O 3 , and MgO. 
     The following step consists in performing the brazing cycle by raising the temperature so as to make the brazing composition  41  liquid, which brazing composition is then sucked by capillarity by the particles of SiC  30  and spread over the entire brazing zone  40  present between the two parts  10  and  20 . 
     As shown very diagrammatically in  FIG. 1E , a brazed joint  42  is thus obtained in contact with the assembly faces  10   a  and  20   a  of the two parts  10  and  20 . 
     During the brazing cycle, the brazing composition and the peg react together to form a solid assembly, thus forming an anchor point for the brazed joint in the material of the parts, thereby considerably reinforcing the shear strength of the brazed connection. 
     Also, clearance J is preferably left between the peg  5  and the perforations  101  and  201  ( FIG. 1D ), e.g. by making the perforations  101  and  201  with a diameter of 5.2 millimeters (mm) and the peg  5  with a diameter of 4.7 mm. Thus, because of the presence of the clearance J between the peg and the perforations, the gas produced during the brazing cycle is discharged by the perforations  101  and  201  via their portions that open out respectively into the faces  10   b  and  20   b  of the parts  10  and  20 . This thus limits the formation of pockets of gas in the brazed joint. 
     The shear peg may be machined out of a material similar to the composite material of the parts for assembling together. The shear peg may also be made out of a material presenting a coefficient of thermal expansion that is sufficiently close to that of the material of the parts for assembling together, i.e. a material having a coefficient of thermal expansion that differs from that of the material of the parts by no more than 40%. 
     In a variant implementation of the invention shown in  FIG. 2 , a shear peg  501  is machined directly on the assembly face  50   a  of a first composite material part  50  that is to be assembled by brazing with a second part  60 , which second part includes for this purpose a perforation  601  forming a housing for the peg  501 . The parts  50  and  60  are then brazed together under the same conditions as those described above. 
     In another variant implementation of the invention as shown in  FIG. 3 , the shear peg  802  is inserted into a perforation  801  machined in a first part  80  that is to be assembled by brazing with a second part  90 , which second part includes a non-through cavity  901  into which the peg  802  also penetrates. The parts  80  and  90  are then brazed together under the same conditions as those described above. 
     A plurality of shear pegs may be used for reinforcing a single brazed connection. Under such circumstances, they are inserted in distinct perforations formed in one or both parts, as described above. 
     Furthermore, one or both of the parts for assembling together by brazing may include cavities in their bonding faces, with at least some of the cavities opening out in portions or faces of the parts that are situated outside the bonding faces. 
       FIG. 4  shows a variant embodiment of the parts shown in  FIG. 1A . In  FIG. 4 , parts  110  and  210  that are to be assembled together by brazing in accordance with the method of the invention include respectively, like the parts  10  and  20  of  FIG. 1A , perforations  111  and  211  forming a housing for a shear peg  212  inserted therein before brazing. The part  110  also has grooves  112  in its assembly face  110   a,  the grooves opening out in the side faces  110   b  and  110   c  of the part  110 . Likewise, the part  210  also has grooves  212  in its assembly face  210   a,  the grooves opening out in the side faces  210   b  and  210   c  of the part  210 . The grooves  112  and  212  make it possible to form additional anchor portions for the brazed joint in the material of the parts, together with additional paths for degassing and/or visually inspecting the brazed joint. Partial or complete absence of brazing in one or more grooves indicates that the brazed joint has not been formed in uniform manner over the entire brazing zone and that the connection might then include zones of weakness. Depending on the conditions of use of the assembled parts, it is possible to define a number and/or an extent of zones of weakness that can be accepted, below which the brazed joint does not need to be re-worked. 
       FIG. 5  shows a variant embodiment of the parts shown in  FIG. 2 . In  FIG. 5 , a first part  120  includes a peg  121  on its assembly face  120   a,  while a second part  220  for assembling by brazing with the part  120  in accordance with the method of the invention includes a perforation  221  that is to form a housing for the peg  121 . The part  120  also has orifices  122  in its assembly face  120   a,  the orifices opening out in the side face  120   d  opposite from the assembly face  120   a  of the part  120 . Likewise, the part  220  also has orifices  222  in its assembly face  220   a,  the orifices opening out in the side face  220   d  opposite from the assembly face  220   a  of the part  220 . The orifices  122  and  222  make it possible to form additional anchor portions for the brazed joint in the material of the parts, together with additional paths for degassing and/or visually inspecting the brazed joint. Partial or complete absence of brazing in one or more orifices indicates that the brazed joint has not been formed in uniform manner over the entire brazing zone and that the connection might then include zones of weakness. 
     The number, the orientation, the shape, and the dimensions of the grooves or the orifices in each assembly face of the parts made of composite material may be varied. Furthermore, with grooves, the perforations opening out into the face opposite the bonding face of the part may also be machined in the bottoms of the grooves so as to add additional paths for degassing and/or inspecting the brazed joint. 
     The grooves are preferably oriented in such a manner as to form anchor portions in the brazed joint to oppose the main forces to which the assembly is to be subjected. For example, the grooves may extend in a direction substantially perpendicular to the direction of shear forces. Also, two series of grooves, each extending in different directions, may be formed in the bonding faces of the parts.