Abstract:
A method for bonding a first part made of a composite to a second part to a predetermined dimension is disclosed. The method includes placing an insert into the first part, a first surface of the insert protruding beyond the predetermined dimension; machining the insert until the first surface is at the predetermined dimension; placing the second part on the insert; and fastening the second part via the insert. The use of an insert makes it possible to produce, with high precision, a reproducible assembly, which can still be dismantled.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to the field of composites and to their application in particular in aeronautical turbomachines. 
     In the aeronautical field in particular, a constant objective is to produce parts of great mechanical strength in an unfavorable environment for a minimal weight and size. Thus, certain parts are produced from a ceramic matrix composite (CMC), such as for example an SiC/SiC composite produced for applications of long duration at high temperatures. Such a composite consists of a multidirectional reinforcement made of silicon carbide and a matrix, also made of silicon carbide. The latter gives the composite exceptional thermomechanical fatigue resistance, in an oxidizing environment, at temperatures possibly up to 1400° C. These composites are currently used for the manufacture of aircraft engine parts, namely nozzle flaps, combustion chamber and reheat system. It is the fibers that take up the loads, the matrix providing a function of binder for the rest of the part and of protecting and isolating the fibers, which must not come into contact with the oxidizing atmosphere. 
     The difficulties of integrating a composite, in particular a CMC, in any environment are connected with its manufacturing tolerances: the geometrical envelope of this composite may vary up to ±0.55 mm in thickness. 
     When such a part is to be assembled with another part made of a CMC or a metal, the practice hitherto has been to use the technique of matching. However, with this technique it is not possible to dimension the parts separately —it is the assembly that is dimensioned. Used in development, this method cannot be economically transposed to mass production. In particular, matching eliminates interchangeability of the parts, since no unique definition exists. In addition, matching is an expensive method both in terms of production and after-sales service for spares. 
     BRIEF SUMMARY OF THE INVENTION 
     The subject of the invention is therefore a method for joining, with high precision, a first part made of a composite, especially of the CMC (ceramic matrix composite) type having itself a relatively high manufacturing tolerance, to a second part, while still allowing possible replacement of the second part, for example within the context of a repair. 
     This objective can be achieved with a method for bonding a first part made of a ceramic matrix composite to a second part, to a defined dimension, wherein an insert is fitted onto said first part, said insert protruding beyond said dimension, the insert is machined until its surface is at said defined dimension, said second part is placed on the insert and the second part is fastened via the insert. 
     Thanks to the use of a preferably metal insert, which can thus be machined with high precision, it is possible, in a very simple and inexpensive manner, to position the second part relative to the first at the desired dimension. There would be a risk of impairing the properties of the composite if the matrix of the composite were to be machined, possibly directly, over a large portion of the bearing surface between the two parts, this operation baring the fibers. 
     More precisely, a housing is drilled out in the first part in order to house the insert therein. Advantageously, an insert with a cylindrical portion and a shoulder is used, the insert is placed via its cylindrical portion in the housing as far as its shoulder, and the insert is fastened by means of a washer that is brazed to the face on the opposite side from the shoulder. This way of fixing the insert in the housing of the first part allows optimum preservation of material without any risk of corrosion. In particular, a brazing temperature above the operating temperature is chosen. Thus, as the metal insert expands more strongly than the CMC composite, it is sufficient to install and fit the washer hot. The metal, on cooling, contracts and the CMC plate is pinched between the washer and the shoulder upon cooling, or in operation, down to a temperature below the brazing temperature. 
     Depending on the application, that portion of the insert with the shoulder and/or the cylindrical portion of the insert are machined. 
     Preferably, the second part is fastened to the first part by means of a rivet, a welded stud or a bolt that passes through the insert along the axis of the cylindrical portion. 
     The subject of the invention is also an insert for implementing the method, in which a first part of defined thickness is bonded to a second part to a defined assembly dimension. The insert is one in which the cylindrical portion is of greater height than the thickness of the first part, the shoulder having a protrusion that can be machined off. Advantageously, the insert has a central bore passing through the cylindrical portion and the shoulder. More particularly, the shoulder has a clamping means for immobilizing the insert during the machining operation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described in detail with reference to the appended drawings in which: 
         FIG. 1  shows, in perspective, a flame holder arm of an afterburner device in a turbojet; 
         FIG. 2  is a sectional view along the direction II-II of the arm of  FIG. 1 ; 
         FIG. 3  is a sectional partial view of a first part to be assembled with an insert according to the invention; 
         FIG. 4  shows the two parts assembled in accordance with the invention; 
         FIG. 5  shows one embodiment of the insert, seen from above; 
         FIG. 6  shows another embodiment of the insert; and 
         FIGS. 7 and 8  show another application of the assembly method according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows, as does  FIG. 2  in cross section, a flame holder arm  10 , as used in an afterburner system of a turbojet. This arm is made of a composite as described in patent application FR 04/00651 filed by the Applicant. The composite, of the CMC type, allows a one-piece structure to be produced, here with a V-shape with two walls  14  and  15  diverging from each other from an edge  17 . The arm includes two flanges  20  and  22  for fixing to the external casing of the afterburner channel. The elongate cavity  16 , provided between the two walls and turned toward the downstream of the channel, serves for delivering an air/fuel mixture into the stream of the primary flow. A cover or screen  25 , shown in exploded view in  FIG. 1 , partially closes off the cavity  16 , while still leaving a peripheral space P through which the fuel mixture is injected into the gas stream. It is important to have a precise assembly since the gap between the walls  14  and  15  of the arm and those of the cap  25  controls the flow rate of the mixture. A problem therefore arises owing to the nature of the composite from which the components are made. It is not possible to compensate for the geometrical variations in the part by machining the composite in order to adjust the position of one part relative to the other because of the risk of fiber corrosion when the working temperature of the part is above 300° C. Moreover, an assembly using a matching technique would not permit a repair to be made with a replacement for one element, since the dimensions would be lost. 
     According to the invention, the geometrical variations of the composite are compensated for by means of an insert made of metal or equivalent, which can be machined to the desired dimension. This insert fastened to the composite part provides reproducible geometric boundaries and permits one element to be easily replaced with a spare part. 
     The invention is not limited to assembling an element on a flame holder arm of an afterburner system, but to assembling all parts where one is made of a composite, especially of the CMC type, which require precise adjustment and can be repaired by part replacement. 
       FIGS. 3 and 4  show one method of assembly according to the invention. 
     The aim is to assemble a first part  31 , made of a CMC-type composite, to a second part  34 , which may also be of the CMC type or simply made of metal. If the second part is also made of a CMC, it may be advantageous to mount a second machinable insert on the second part, the contact taking place between the two machined surfaces of the two inserts. 
     As may be seen in  FIG. 3 , the part  31  has a portion of defined thickness, which constitutes the region in which it is fastened to the second part  34 . An insert  32  is housed in a hole drilled in the part  31 . The insert  32  consists of a cylindrical portion  32 A and a portion with a shoulder  32 B. The cylindrical portion is housed in the drill hole  31 ′ of the part and is long enough to emerge on the opposite side from the shoulder of the portion  32 B. A washer  33  is brazed onto the free portion of the cylindrical element  32 A. The part  31  is thus held clamped between the washer and the shoulder  32 B. The thickness E of the shouldered element  32 B of the insert is chosen so that its free face is higher than the level A. This level A is that of the desired dimension. Here this dimension is defined with respect to that face of the part  31  on the opposite side from the shoulder  32 B. The dimension could be defined with respect to another reference. 
     The procedure is as follows: 
     a drill hole  31 ′ is machined in the part  31  so as to be able to house the portion  32 A of the insert  32 . Where appropriate, that is to say when it is desired to avoid any risk of fiber corrosion in operation, the edges of the drilled region are treated so as to reconstitute a matrix for protecting the fibers of the composite. It may be seen that it would not be possible to proceed in the same way on a bearing surface, since the reconstituted matrix on a surfaced region to a precise dimension would not be of constant thickness, hence the benefit of the insert. 
     The insert  32  is placed in the drill hole and a washer  33  is brazed onto its free edge, for example by means of a braze. The materials used for the insert and the braze are for example a cobalt-based alloy such as HS 25  or L 605 . This material would be used both for a cold application (for example for the top of the arm in the example considered here) and for a hot application (for that part of the arm swept by the primary flow). 
     Brazing is preferred to welding so as to limit the stresses of thermal origin in the CMC part during assembly. 
       FIG. 4  shows the two parts bonded together according to the invention. The insert has been machined to the dimension A and the second part brought into contact with the machined face of the insert. This second part may be made of a composite like the first, or else it may be made of metal. The bonding is obtained here by riveting through the bore  32 C made through the insert  32 . The rivet  35  clamps the two free opposed faces together, one being that of the part  34  and the other being that of the cylindrical portion  32 A of the insert. 
     Thus, thanks to the solution of the invention, it is possible to assemble two parts  31  and  34  with high precision independently of the thickness tolerances of one or other of the two parts. Moreover, this assembly can be easily dismantled. It is possible to replace the part  34  with another, depending on the case, since the dimension defined by the machined insert is maintained. 
       FIG. 5  shows one embodiment of the insert. The shouldered portion  32 B in this embodiment has two flaps  32 D machined parallel to each other along the axis of the cylindrical portion  32 A. The function of this means is to install a system for clamping the insert while it is being machined to the required dimension. 
       FIG. 6  shows another embodiment, differing from that in  FIG. 5 . This has two notches  32 D′ in which a clamping tool engages during the machining operation for bringing the insert to the required dimension. 
       FIGS. 7 and 8  illustrate the implementation of the invention within the context of another application. In this case a first part  31  is to be assembled with a second part  34  and a third part  36 . The first part  31  is assembled with the second part  34  to the dimension A and the first part  31  assembled with the third part  36  to the dimension B. For this purpose, an insert  32  is used in which the cylindrical portion  32 A and the shouldered portion  32 B each have a material allowance sufficient for them to be able to be machined to the desired dimension A and B respectively. A single rivet bonds the three parts together.