FR3032969A1 - ACOUSTICAL ATTENUATION PANEL IN COMPOSITE MATERIAL, COMPRISING CAVITY NETWORKS - Google Patents

Abstract

A method of manufacturing an acoustic attenuation panel made of a ceramic matrix composite material, comprising the successive steps of: depositing in a mold (16) a lower layer of fibrous reinforcement (2) to form a lower skin, comprising fibers of ceramic material; depositing, over the lower fibrous reinforcement, a set of layers alternately comprising a temporary fugitive filling material, forming a grid (8) comprising a series of protuberances (20), and an intermediate fibrous reinforcement (10) comprising fibers made of material ceramic; depositing a top layer of fibrous reinforcement (4) to form an upper skin, comprising fibers made of ceramic material; and sintering to obtain a matrix of composite material which impregnates the fibrous reinforcements of the assembly, at a temperature permitting the elimination of the fugitive filling material.

Description

The present invention relates to the field of acoustic attenuation panels, in particular intended to equip the hot zones of gas ejection of an aircraft turbojet engine. The invention more specifically relates to a method for manufacturing an acoustic attenuation panel in a ceramic matrix composite material, as well as an aircraft turbojet engine comprising an acoustic attenuation panel according to the invention. The turbojet engines comprise aerodynamic guide surfaces for the flow of hot ejected gases, which can be subjected to high temperatures that can exceed 600 ° C., and in some cases reach 1000 ° C.

To reduce the noise emitted by the turbojet in operation, it is known to produce the aerodynamic guide surfaces by acoustic panels comprising a sandwich-type structure composed of a core material encapsulated between two skins. The central core has transverse walls forming a large number of closed cells, which may in particular have a honeycomb shape. The front skin facing the sound source, has gas passages formed by micro-perforations, opening into resonant cavities formed by the closed cells of the central core, to form Helmholtz resonators performing attenuation of acoustic emissions. emitted by the turbojet. It is known to produce skins and honeycomb core in metallic materials. However, in the aeronautical field, where mass saving is a constant concern, acoustic panels made of metallic materials add a weight which is penalizing. It is also known to provide a porous acoustic attenuation panel, comprising a central core formed by a porous foam formed from ceramic matrix composite materials, or "CMC" materials. CMC materials are particularly resistant to high temperatures, especially temperatures above 600 ° C. Such temperatures are encountered especially at the level of the hot gas ejection cone of a turbojet engine. These materials allow a saving in weight compared to the metallic materials usually used. However, the known solutions for making these panels are not ideal in terms of performance as well as simplicity of manufacture. However, there is a need for a solution making it possible to manufacture, in a simple manner, acoustic attenuation panels made of ceramic matrix composite material "CMC", comprising a porous structure adapted to specific acoustic attenuation. The present invention aimed in particular at overcoming the disadvantages of the prior art, relates for this purpose to a method of manufacturing an acoustic attenuation panel made of a ceramic matrix composite material, comprising the successive steps of: in a mold a layer or set of layers of fibrous reinforcement to form a lower skin, comprising fibers of ceramic material; depositing over the lower fibrous reinforcement a set of layers alternately comprising a temporary fugitive filling material, forming a grid comprising series of protuberances, and an intermediate fibrous reinforcement comprising fibers made of ceramic material; depositing a layer or a set of fibrous reinforcement layers to form an upper skin, comprising fibers made of ceramic material; and sintering to obtain a ceramic composite material matrix which impregnates the fibrous reinforcements, at a temperature permitting the elimination of the fugitive filling material, and allowing the matrix to be consolidated with all the fibrous reinforcements. An advantage of this manufacturing process is that by adapting the shapes of the protuberances of the grids, in particular by superimposing the different grids so that the protuberances touch each other, one obtains in a single simple and economical operation, after the disappearance of the fugitive filling material, networks of cavities each comprising a series of interconnected cavities formed by the protuberances in contact with each other. Moreover, by providing points passing through the upper fibrous reinforcement in fugitive filling material, a porosity of the upper skin 30 forming perforations connecting the internal cavities to the external medium, which ensures the acoustic attenuation, is obtained. The manufacturing method according to the invention can comprise one or more of the following characteristics, which can be combined with one another. Advantageously, the fugitive filling material has points passing through the upper fibrous reinforcement. The porosity of the upper skin opening the acoustic cavities on the external medium is thus carried out in the same sintering operation. Advantageously, the process uses for the fugitive filling material one or more materials selected from thermoplastic plastics and thermosetting plastics. These materials are economical and simple to implement. Advantageously, the fibrous reinforcements and the matrix comprise ceramics made of metal oxides. In particular, the fibrous reinforcements or matrix material may comprise at least two different ceramic materials. The local characteristics of the material are thus adapted according to the constraints. The fugitive filling material grids may comprise longitudinal and transverse rods in a plane, as well as protrusions forming inserts penetrating the adjacent fibrous reinforcing layers.

According to one embodiment, the deposited fibrous reinforcements being dry, the precursor of the ceramic matrix filters these fibrous reinforcements by means of a vector in liquid form, the method comprising a subsequent step of drying the liquid vector in temperature before the step sintering, or polymerizing a resin in the case of a preceramic resin, at a temperature below the melting point of the temporary molding material. Advantageously, each protuberance of a grid of fugitive filling material is in contact with several protuberances of the grid opposite. In particular, the distances between the axis of two adjacent protuberances of the same grid are between 100 and 4000 micrometers.

In particular, the grids can be obtained by weaving fibers, by stamping strips, by expansion or by continuous molding of a film. Advantageously, the grids comprise, in their plane parts, rods having a cross-sectional area of between 5 μm 2 and 250 000 μm 2. Advantageously, the cross-sectional area of the stems is between 20 μm 2 and 100 μm 2. Advantageously, the protuberances have a cross-sectional area of between 10,000 μm 2 and 16,000,000 μm 2, and a height of between 100 μm and 4000 μm. Advantageously, the grids comprise protuberances disposed on its two faces.

The invention also relates to an acoustic attenuation panel obtained by a method as defined above. The invention further relates to an aircraft propulsion assembly (that is to say the assembly formed by a turbojet engine equipped with a nacelle, this assembly 5 possibly including the engine pylon), the propulsion unit comprising one or several acoustic attenuation panels obtained by a method as defined above. The invention will be better understood and other features and advantages will appear more clearly on reading the following description given by way of example, with reference to the appended drawings, in which: FIG. set of layers of an acoustic panel prepared with a method according to the invention; FIG. 2 is a perspective view of a grid of this panel comprising protuberances; FIGS. 3a and 3b are views of the planar portion of grids according to variants; FIGS. 4a and 4b are side views of grids according to variants; - Figure 5 shows three grids with different protuberances; - Figure 6 is a side view of a stack of two types of grids 20 having alternately rounded and pointed protuberances; - Figure 7 is a side view of a stack of grids with rounded protuberances; and - Figure 8 is a sectional view along the section plane VIII-VIII of this stack of grids.

FIG. 1 shows the stacking of the layers to form an acoustic panel, comprising at the bottom a lower thickness consisting of at least one impervious fibrous reinforcement layer 2 deposited on a mold 16, which will form the lower skin of the panel, then a zone of intermediate thickness consisting of a set of layers forming a central core 6, and finally a zone of greater thickness consisting of fibrous reinforcement 4 which will form the upper skin. The lower and upper fibrous reinforcing layers 2 comprise fibers of ceramic material. The acoustic panel has a lower skin 2 which is acoustic wave-tight, and an upper skin 4 which is in contact with the aerodynamic flow to attenuate acoustic emissions. This upper skin 4 comprises a large number of small perforations opening on the one hand on the aerodynamic surface and on the other hand in acoustic attenuation cavities of the central core 6. The central core 6 comprises a succession of grids 8 comprising protuberances 22. Fibrous reinforcing layers 10 comprising fibers made of ceramic material can be arranged between the grids 8. From the bottom, the protuberances 22 of the first grid 8 are turned upwards, the protuberances of the grids The following are turned both up and down, and the protuberances of the last grid are facing downwards. The last grid 12 comprises above each protuberance pins 14 made of the same material, having a length sufficient to completely cross the upper layer of fibrous reinforcement 4, and protrude above. Figure 2 shows the general principle of the grids 8, having a flat portion 20 formed by intersecting longitudinal and transverse rods, comprising a large number of perforations permitting the die to pass between the two faces. The rods support rounded protuberances 22 facing upwards, evenly spaced along a grid. The assembly is made of a fugitive filling material. The rods of the plane portions 20 may in particular have sections of between 5 and 250,000 urn.sup.2. Advantageously, these sections are between 10 and 100 μm 2. Advantageously, the protuberances 22 have a cross-sectional area of between 10 000 μm 2 and 16 000 000 μm 2 and a height of between 100 μm and 4000 μm. In particular, it is possible to provide distances separating adjacent protuberances 22 on the grids 8, between 100 and 4000 micrometers. In a variant, the grids 8 can be made by various methods, in particular by weaving fibers, by stamping strips, by expansion or by continuous molding of a film. The protuberances 22 form inserts penetrating the adjacent layers of fibrous reinforcement 10. Advantageously, the fibrous reinforcements are formed by a fabric comprising loosely tightened threads, or by low density non-woven fibers, so as to obtain a rather loose set The spacing of the protuberances 22 is calculated to obtain preferentially as shown in FIG. 1, a staggered positioning of these protrusions from one grid to the other, each protuberance being supported on, by for example, four other protuberances in the grid opposite. The method of manufacturing the panel comprises the successive removal of the layers on the mold 16 which will give the overall shape of the finished panel.

The fugitive filling material is chosen to obtain its at least partial, and preferably total, elimination during the temperature sintering operation of the matrix of ceramic material which will impregnate the various fibers, in particular by oxidation of this material, by combustion, fusion or sublimation. In this case, however, the fugitive material will have to withstand the drying and / or polymerization temperature. The fugitive material may comprise one or more materials chosen from thermoplastic plastics (such as polyethylene), thermosetting plastics (for example based on epoxy), and low-melting metals (for example based on lead, tin or aluminum).

FIGS. 3a and 3b alternatively have a planar grid portion 8 formed by a film of fugitive filling material, comprising the perforations 24. In particular, the perforations 24 may be polygonal as shown in FIG. 3a, or curves as shown in FIG. 3b. The grid 8 may be made in particular by molding or thermoforming the fugitive filling material, forming at the same time the perforated planar portion 20 and the protuberances 22. FIG. 4a shows a grid 8 comprising a series of protuberances 22 facing the upper side. In particular, this type of grid is available as the first layer of the set of layers 6 forming the central core, above the lower layer of fibrous reinforcement 2 forming the lower skin. Figure 4b shows a grid 8 having a series of opposed protuberances 22, turned on both sides and aligned with each other. In particular, this type of grid is available as an intermediate layer in the set of layers 6.

FIG. 5 shows, starting from the top, a first grid 8 having on the upper side only rounded protuberances 30. It then presents a second grid 8 comprising on the upper side an alternation of protuberances of two different kinds, rounded 30 and conical 32. It finally presents a third grid 8 having on the upper side an alternation of protuberances of two kinds, rounded 30 and conical 32, each of these protuberances receiving on the lower side an opposite protuberance which is of the different kind.

FIG. 6 shows successively from the bottom a first grid 8 comprising rounded projections 30 facing upwards, a second grid having opposed conical protuberances 32 facing on both sides, a third grid having opposed rounded protuberances 30 facing each other; on both sides, and a fourth grid comprising conical protuberances 32 facing downwards. The identical spacings of the different protuberances on all the grids 8 are calculated in such a way that the conical protuberances 32 partially fit into the space between, for example, four rounded protuberances 30 of the opposite grid, and about half the height of these rounded protuberances. In this way we obtain for each protuberance several points of contact or contact surfaces with those of the opposite grid. It will be noted that, by deformation of the materials, contact surfaces are preferably obtained, giving greater passages between the cavities.

FIG. 7 shows successively from the bottom a first grid 8 comprising upwardly facing rounded protuberances 30, a second grid comprising opposite rounded protuberances rotated on both sides, and a third grid comprising rounded protuberances facing towards the outside. low.

In the same way, each rounded protuberance 30 enters the space between, for example, four rounded protuberances of the facing grid on about half the height, to obtain, in particular, four points of contact 40 shown in FIG. several contact surfaces with those of the opposite grid. After removal of the successive layers of grids 8, and 25 layers of intermediate fibrous reinforcement of ceramic material, not shown, a filtration of the precursor of the ceramic matrix 42 carried by a liquid vector, which impregnates all the fibrous reinforcements and fills all the spaces left free between the protuberances 30. In particular, a liquid compatible with the fugitive filling material is chosen so as not to mix with this material and dissolve it.

If necessary, the liquid vector is then dried to leave only the precursor of the ceramic 42, or a polymerization of a resin in the case of a preceramic resin, at a temperature below the melting temperature of the filling material. fugitive. As mentioned above, the fugitive material will be chosen to withstand the drying and / or polymerization temperature.

Alternatively, pre-impregnated fibrous reinforcements of the precursor of the ceramic matrix may be partially or completely used.

On the same panel, it is also possible to combine these different embodiments to adapt the characteristics locally. Finally, the precursor of the ceramic matrix is sintered at temperature, and optionally under pressure, to obtain an aggregation of the entire matrix which completely encompasses the ceramic fibers of the fibrous reinforcements, as well as the at least partial disappearance and total preference of the fugitive filler material which then leaves voids in place. Through the contact points 40, or possibly contact surfaces, between different protuberances 30, a passage is obtained between the various cavities left by these protuberances, which gives a network of cavities. It will be noted that the perforations of the planar portions 20 of the grids 8, formed by the spaces between the threads in the case of weaving, or by perforations on a film, allow the ceramic precursor of the matrix to pass, which constitutes a link between the two parts of the matrix being on each side, and solidarise all the layers between them.

As shown in FIG. 1, by providing points 14 of fugitive filling material on the last grid 8 at the top of all the layers of the central core 6, and passing through the upper layer of fibrous reinforcement 4, the networks of cavities Acoustics then open on the upper face of the panel by the small perforations of the upper skin 4 left by these points.

In general, the different fibers of the panel can be long or short. For the fibers and the matrix it is possible in particular to use metal oxides. The shapes of the cavity networks and their spacings as well as the density of the fibrous reinforcements, are adapted to meet in particular at each point of the panel, laterally and in thickness, to the needs of local acoustic attenuation and to the desired mechanical strength. . In particular, the type of grid and the size of the protuberances can be varied, depending on the thickness of the layers and the positioning in the panel.

Claims (17)

REVENDICATIONS1. A process for manufacturing an acoustic attenuation panel made of a ceramic matrix composite material, characterized in that it comprises the successive steps of: depositing in a mold (16) a lower layer of fibrous reinforcement (2) to form a lower skin comprising fibers made of ceramic material; depositing on top of the lower fibrous reinforcement a set of layers alternately comprising a temporary fugitive filling material, forming a grid (8) having series of protuberances (20), and an intermediate fibrous reinforcement (10) comprising fibers ceramic material; depositing a top layer of fibrous reinforcement (4) to form an upper skin, comprising fibers made of ceramic material; and sintering to obtain a matrix of composite material (42) which impregnates the fibrous reinforcements of the assembly, at a temperature permitting the elimination of the fugitive filling material, and allowing the consolidation of the matrix with the together fibrous reinforcements. 2. The manufacturing method according to claim 1, characterized in that the fugitive filling material comprises spikes (14) passing through the upper fibrous reinforcement (4). 3. Manufacturing process according to any one of the preceding claims, characterized in that it uses for the fugitive filling material one or more materials selected from thermoplastic plastics and thermosetting plastics. 4. Manufacturing process according to any one of the preceding claims, characterized in that the fibrous reinforcements (2, 4, 10) and the matrix 30 comprise ceramics metal oxides. 5. Manufacturing process according to any one of the preceding claims, characterized in that the fibrous reinforcements or the material forming the matrix, comprise at least two different ceramic materials. 35 6. Manufacturing process according to any one of the preceding claims, characterized in that the grids (8) fugitive filling material 3032969 comprise in a plane (20) longitudinal and transverse rods, and protuberances (22) forming inserts penetrating the adjacent fibrous reinforcement layers (10). 5 7. Manufacturing method according to any one of the preceding claims, characterized in that the fibrous reinforcements (2, 4) deposited being dry, the precursor of the matrix filters these fibrous reinforcements by means of a vector in liquid form, the method comprising a subsequent step of temperature drying the liquid vector prior to the sintering step, or polymerizing a resin in the case of a preceramic resin, at a temperature below the melting point of the temporary molding material. 8. Manufacturing process according to any one of the preceding claims, characterized in that the fibrous reinforcements (2, 4, 6, 8) deposited are pre-impregnated with the precursor of the ceramic matrix. 9. Manufacturing method according to any one of the preceding claims, characterized in that each protuberance (22) of a grid (8) of fugitive filling material, is in contact with several protuberances of the grid opposite . 10. The manufacturing method according to any one of the preceding claims, characterized in that the distances separating the axis of two adjacent protuberances (22) of a grid (8) are between 100 and 4000 micrometers. 25 11. Manufacturing process according to any one of the preceding claims, characterized in that the grids (8) are obtained by weaving fibers, by stamping strips, by expansion or by continuous molding of a film. 30 12. The manufacturing method according to claim 11, characterized in that the grids (8) comprise in their planar portions (20) rods having a cross sectional area of between 5 gm2 and 250 000 pm2. 13. The manufacturing method according to claim 12, characterized in that the sectional area of the rods is between 20 pm2 and 100 pm2. 3032969 11 14. The manufacturing method according to any one of the preceding claims, characterized in that the protuberances have a cross-sectional area of between 10 000 pm2 and 16 000 000 gm2, and a height of between 100 gm and 4000 gm. 5 15. The manufacturing method according to any one of the preceding claims, characterized in that the grids (8) comprise protuberances (22) disposed on their two faces. 10 16. Sound attenuation panel obtained by a method according to one of the preceding claims. 17. Aircraft propulsion unit comprising at least one acoustic attenuation panel according to the preceding claim.