Manufacturing process for a multi-perforated composite acoustic skin without mechanical drilling

Abstract

Method for manufacturing a multi-perforated composite acoustic skin without mechanical drilling The invention relates to a method for manufacturing a multi-perforated acoustic skin made of composite material (8) for an acoustic attenuation structure, the method comprising the following steps: - forming a fiber preform comprising a precursor material of a matrix, - carrying out a heat treatment to transform the precursor into a matrix so as to obtain a multi-perforated acoustic skin made of composite material comprising a fiber reinforcement densified by said matrix, the step of forming the fiber preform comprising draping the fibers (1) on a surface of a mandrel (2) comprising protuberances (3),and the mandrel (2) and the protrusions (3) may each be made of a material which melts at a temperature lower than the heat treatment temperature for transforming the precursor into a matrix so as to eliminate said mandrel (2) and said protrusions (3) during the transformation heat treatment step. Figure for the abstract: Fig.4,

Description

Title of the invention: Method for manufacturing a multi-perforated composite acoustic skin without mechanical drilling Technical field

[0001] The present invention relates to the general field of acoustic attenuation structures. It relates more particularly to acoustic skins made of composite material included in the acoustic attenuation structures. Prior art

[0002] In order to absorb noise over a certain range of acoustic frequencies in engines, and in particular at the level of gas turbines or their exhaust, multi-perforated acoustic skins made of composite material are used.

[0003] In some prior art methods, multi-perforated acoustic skins made of composite material are made by drilling and machining numerous small holes through the composite material skin. These methods generally involve drilling these holes manually one by one or mechanically using a mat having spikes, which involves a very long preparation time. This technique is particularly used for drilling holes with a diameter of between 1 millimeter and several millimeters.

[0004] Other methods for manufacturing multi-perforated acoustic skins made of composite material include a step of perforating the skin after the step of consolidating the composite material. In this respect, documents US6190602 and EP3590843 are known. Document US6190602 proposes depositing the composite material obtained following the consolidation step on a support and inserting a foldable drilling device having pins or nails, through the assembly comprising said composite material and the support. This step is followed by the removal of the drilling device. Document EP3590843 proposes a method in which inserts are manually introduced into a layer of the composite material obtained following the consolidation step. This step is followed, as in the previous case, by the removal of a portion of the inserts.

[0005] Other methods of the prior art propose to integrate the formation of perforations during the formation of the preform. This is the case of document CN211975529, which proposes to drape, in different directions, a surface with several layers of fibrous reinforcements so as to form perforations, into which pins are introduced. Furthermore, document WO2017017367 proposes a method for manufacturing an acoustic panel having a sandwich structure. In this case, the draping on a mold with a first layer of plies made of fibrous reinforcements is made, and on it rounded blocks each having pins made of fusible material are arranged. On these blocks, a second layer of plies is deposited. Perforations of the second layer are obtained as well as cells between the two layers during the densification step.

[0006] However, all of these methods are inefficient for various reasons. Manufacturing methods involving drilling or perforating steps require the use of particularly expensive drilling or cutting tools. In addition, the tools for forming such perforations generally have drills or pins that require frequent replacement.

[0007] The other methods have the disadvantage of requiring the implementation of numerous steps and of generating a very slow deposition of the material, which represents a significant economic cost. Statement of the invention

[0008] The main aim of the present invention is therefore to propose a solution for the manufacture of a multi-perforated acoustic skin made of composite material which does not have the aforementioned drawbacks.

[0009] To this end, the invention proposes a method for manufacturing a multi-perforated acoustic skin made of composite material for an acoustic attenuation structure, the method comprises the following steps:

[0010] - formation of a fibrous preform comprising a precursor material of a matrix,

[0011] - carrying out a heat treatment to transform the precursor into a matrix of so as to obtain a multi-perforated acoustic skin made of composite material comprising a fibrous reinforcement densified by said matrix,

[0012] characterized in that the step of forming the fiber preform comprises draping fibers on a surface of a mandrel comprising protuberances, and in that the mandrel and the protuberances are each made of a material that melts at a temperature lower than the heat treatment temperature for transforming the precursor into a matrix so as to eliminate said mandrel and said protuberances during the transformation heat treatment step.

[0013] Thus, the manufacturing method makes it possible to obtain multi-perforated acoustic skins made of composite material which do not require drilling or mechanical machining or the use of specific tools which are long and expensive to implement. The method of the invention therefore allows for the more economical manufacture of multi-perforated acoustic skins compared to the manufacturing solutions of the prior art. The method of the invention also makes it possible to manufacture multi-perforated acoustic skins having complex shapes, and more particularly acoustic skins for annular-shaped parts. The method according to the invention also makes it possible to shorten the manufacturing time of the skins because the elimination of the mandrel and protuberances and the densification of the composite material can be carried out in the same step.

[0014] The term “fusible material” means a material capable of being eliminated under the effect of heat during the heat treatment of transformation of the precursor into a matrix.

[0015] According to a particular characteristic of the method, the step of forming the preform may comprise draping dry fibers on the surface of the mandrel followed by a step of impregnating the fiber preform with the precursor material of a matrix.

[0016] According to another particular characteristic of the method, the step of forming the preform may comprise draping fibers pre-impregnated with the precursor material of a matrix on the surface of the mandrel.

[0017] According to a particular characteristic of the method, draping with fibers can be carried out by winding fibers. Winding fibers makes it possible to obtain materials with very good mechanical properties as well as to arrange the fibers optimally in the direction of the forces to be supported.

[0018] "Fiber winding" means a method that may include a step of winding fibers onto a rotating mandrel or mold.

[0019] According to another particular characteristic of the method, the winding of fibers can be carried out with a winding angle greater than or equal to 7° relative to a central axis of the mandrel. Such a value of the winding angle makes it possible to wind the fiber precisely in the axis of the mandrel.

[0020] According to another particular characteristic of the method, the draping of the fibers can be carried out by automatic fiber placement known in English as "Automated Fiber Placement" (AFP). The automatic placement of fibers makes it possible to optimally arrange the fibers on very complex geometries and therefore to obtain parts having complex geometries with very good mechanical properties.

[0021] According to another particular characteristic of the method, the draping can be carried out with unidirectional ribbons of fibers or strips of fibers.

[0022] According to another particular characteristic of the method, the multi-perforated skin can be made of an organic matrix composite (OMC) material.

[0023] It is thus possible to obtain a multi-perforated skin which has good resistance to fatigue and corrosion while being light and economical.

[0024] According to another particular characteristic of the method, the multi-perforated skin can be made of a ceramic matrix composite (CMC) material. In this way, it is possible to obtain a multi-perforated skin which has a very high temperature resistance and with very good mechanical properties.

[0025] According to another particular characteristic of the method, the impregnation of the fibrous preform can be carried out with a solution loaded with particles of the precursor material of the matrix and the transformation heat treatment step can comprise a sintering step.

[0026] According to another particular characteristic of the method, the mandrel and the protuberances can be made of the same material.

[0027] According to another particular characteristic of the method, the mandrel may be made of a first fusible material and the protrusions may be made of a second fusible material. In such a configuration, it is possible to remove the protrusions and the mandrel at different times during the process. Such a method may be most advantageous when it is necessary to ensure optimal shaping of the perforations. In this case, the fusible material of the protrusions may be removed after the removal of the mandrel, thus allowing prolonged shaping of the protrusions. Consequently, perforations with improved diameter accuracy may be obtained.

[0028] According to another particular characteristic of the method, the second fusible material can be deformable. Thus, the protrusions can be deformed under the effect of the positioning of an envelope or a counter-mold around the mandrel. It is therefore possible to be able to optimally manage the thickness of the finished part during the transformation heat treatment step.

[0029] The invention also relates to a mandrel for implementing the method of manufacturing a multi-perforated acoustic skin made of composite material for an acoustic attenuation structure, characterized in that the mandrel comprises protuberances, and in that the mandrel and the protuberances are each made of a material that melts at a temperature lower than the temperature of the heat treatment for transforming the precursor into a matrix. Thus, it is possible to eliminate the mandrel, the protuberances and carry out the densification of the composite material during the same step. In addition, this makes it possible to avoid any step of drilling, machining and removal of the drilling tools which can be delicate and time-consuming to implement. Brief description of the drawings

[0030] Other characteristics and advantages of the present invention will emerge from the description given below, with reference to the appended drawings which illustrate exemplary embodiments thereof which are not limiting in nature.

[0031] [Fig-1] [Fig.l] is a flowchart of the steps in a manufacturing process for a multi-perforated acoustic skin made of composite material in accordance with one embodiment of the invention;

[0032] [Fig.2] [Fig.2] is a schematic perspective view of the implementation of a draping step with fibers, carried out by winding fibers in accordance with one embodiment of the invention;

[0033] [Fig.3] [Fig.3] is a schematic perspective view of the implementation of an impregnation step in accordance with one embodiment of the invention;

[0034] [Fig.4] [Fig.4] is a schematic perspective view of a transformation heat treatment step in accordance with one embodiment of the invention. Description of the embodiments

[0035] The invention applies generally to the manufacture of multi-perforated composite material skins intended for use in acoustic attenuation structures present in aeronautical engines.

[0036] According to an embodiment of the method of the invention described in [Fig. 1], the manufacture of a multi-perforated acoustic skin made of composite material for an acoustic attenuation structure according to the invention begins with the formation of a fiber preform (step E1) by draping the fibers 1 on a surface of a mandrel 2 comprising protuberances 3. The mandrel 2 constitutes a mold for the formation of the fiber preform. The mandrel may in particular be movable in rotation around its central axis.

[0037] According to another particular characteristic of the method, the draping can be carried out with unidirectional ribbons of fibers or strips of fibers.

[0038] When the draping is carried out with unidirectional ribbons, several ribbons parallel to each other can be arranged simultaneously. Conversely, when the draping is carried out with strips, the strips can be arranged one by one.

[0039] According to a particular characteristic of the method, the unidirectional ribbons may have a width less than or equal to 10 mm.

[0040] According to a particular characteristic of the method, the strips can have a width of between 10 and 200 mm.

[0041] The unidirectional fiber ribbons or fiber strips used for draping may be made of "dry" fibers, i.e., fibers free of matrix precursor, or fibers pre-impregnated with a matrix precursor. In the case of dry fibers, the fiber preform is impregnated with a matrix precursor after draping.

[0042] The protuberances 3 may have a cylindrical-conical or conical or pointed shape, or even a combination of the two shapes.

[0043] The mandrel 2 as well as the protuberances 3 are each made of a material which melts at a temperature lower than the heat treatment temperature carried out during the transformation of the matrix precursor.

[0044] The mandrel 2 can be formed as in the example described here from a single part. Alternatively, it can be formed from at least two parts assembled together.

[0045] The multi-perforated skin according to the invention can be made of thermostructural composite material, that is to say a composite material having good mechanical properties and a capacity to retain these properties at high temperature. Typical thermostructural composite materials are ceramic matrix composites (CMCs). Examples of CMCs are C / SiC composites (carbon fiber reinforcement and silicon carbide matrix), C / C-SiC composites (carbon fiber reinforcement and matrix comprising a carbon phase, generally closest to the fibers, and a silicon carbide phase), SiC / SiC composites (reinforcing fibers and silicon carbide matrix) and oxide / oxide composites (reinforcing fibers and aluminous alumina matrix).

[0046] The multi-perforated skin according to the invention can also be made of organic matrix composite (OMC) material. These materials are formed of a fiber reinforcement embedded in a consolidated or hardened organic matrix. They have the advantage of having excellent mechanical properties and good corrosion resistance while being lightweight. The matrix of the OMC materials can comprise polymer resins. These resins can be present, in an unpolymerized state, in liquid and viscous form. The reinforcements can, for example, be glass fibers, carbon fibers, aramid fibers (Kevlar ®).

[0047] The material(s) of the mandrel 2 and the protrusions 3 are chosen taking into account the temperature of the heat treatment for transforming the precursor into a matrix. The melting temperature of the material(s) of the mandrel and the protrusions is lower than the temperature defined for the transformation heat treatment. In the case in particular of the manufacture of an acoustic skin made of an organic matrix composite (OMC), the fusible material of the mandrel 2 and that of the protrusions 3 each have a melting temperature less than or equal to 300°C. In the case of the manufacture of an acoustic skin made of a thermostructural composite (TMC) material, the melting temperature of each of the materials of the mandrel 2 and the protrusions 3 is between 350°C and 1000°C.Thus, it is possible to eliminate the mandrel 2 and the protrusions 3 at a temperature lower than that of the heat treatment, but also to ensure the cohesion of the fibers 1 of the preform with the matrix before the elimination of the mandrel 2 and the protrusions 3.

[0048] According to a particular characteristic of the method and of the mandrel, the fusible material of the mandrel and / or the fusible material of the protuberances can be chosen from: plastics or metal alloys. The metal alloys can include aluminum alloys, tin alloys, zinc alloys or a mixture thereof.

[0049] According to a particular characteristic of the method and of the mandrel, the fusible material of the mandrel and / or the fusible material of the protuberances may be a plastic having a melting temperature of between 100°C and 500°C.

[0050] According to a particular characteristic of the method and of the mandrel, the fusible material of the mandrel can be a plastic and the fusible material of the protuberances can be a metal alloy.

[0051] The draping step can be carried out by different techniques such as: fiber winding, automatic fiber placement known in English as "Automated Fiber Placement" (AFP) or even manual draping, or even a combination of two or more of these techniques. The application of a predetermined tension on the fibers 1 may be necessary to ensure that they optimally fit the surface of the mandrel 2.

[0052] [Fig. 2] shows a lay-up by winding fibers. In the embodiment in [Fig. 2], a winding head 4 places the fibers 1, which can be subjected to a predetermined tension, in contact with the surface of the mandrel 2. While the winding head 4 is performing this lay-up, the mandrel 2 can rotate about its central axis. During the lay-up step, the winding head 4 can move along a movement rail 5. The term "movement rail" means an element used to guide the movements of the winding head 4. The fibers 1 can, for example, be gripped by the winding head 4 from a reel of fibers 6, as illustrated in [Fig. 2].

[0053] The draping can also be carried out by automatic placement of fibers. When using this draping technique, a robot comprising a placement head can automatically arrange the fibers 1 in contact with the surface of the mandrel 2 in order to drape the latter (not shown).

[0054] The draping of fibers can be carried out according to predetermined orientations relative to a central axis of the mandrel. The winding of fibers can be carried out parallel to a central axis of the mandrel. At the end of the draping step, a fiber preform (not shown) can be obtained.

[0055] Once the preform has been produced, the fiber preform is then densified in order to form a composite material part by heat treating it in order to transform the precursor into a matrix.

[0056] In the case of draping with dry fibers on the surface of the mandrel 2, the fibers 1 may comprise a binder. Thus, it is possible to keep the fibers and the layers of fibers bonded together during the draping step. In this case, the binder used has a different composition from the precursor material of the matrix, so as to be eliminated during a step which precedes the impregnation of the preform with a matrix precursor and the heat treatment of transformation of the precursor into matrix.

[0057] The densification of the fibrous preform intended to form the fibrous reinforcement of the part to be manufactured consists of filling the porosity of the preform, in all or part of its volume, with the material constituting the matrix.

[0058] The heat treatment step of transforming the precursor into a matrix may be preceded by a step during which an envelope or a counter-mold 7 is arranged around the mandrel 2, the space formed between the envelope and the surface of the mandrel defining the thickness of the part. The envelope may be arranged in contact with the protuberances 3 of the mandrel 2. According to an alternative embodiment illustrated in [Fig. 3], the protuberances 3 and the mandrel 2 may be made of different fusible materials. The fusible material of the protuberances may be deformable, such as polypropylene or low-density polyethylene. Thus, when the internal surface of the envelope 7 is arranged in contact with the protuberances 3, the latter may be deformed as illustrated on the right of [Fig. 3]. This makes it possible to optimally manage the thickness of the skin by compaction during the impregnation step.

[0059] In the context of the manufacture of a multi-perforated acoustic skin in CMO, the matrix precursor present on the pre-impregnated fibers or used to impregnate the fiber preform after the draping of dry fibers corresponds to a liquid composition containing an organic precursor of the matrix material. The organic precursor is usually in the form of a polymer, such as a resin, optionally diluted in a solvent. Examples of resins are: polyester resins, epoxy resins and phenolic resins.

[0060] In the case of draping with dry fibers, the impregnation of the fiber preform can be carried out in a manner known per se using the liquid process (CVL) or by impregnation by RTM ("Resin Transfer Molding"). The liquid process consists of impregnating the preform with a liquid composition containing a precursor of the matrix material. The precursor is usually in the form of a polymer, such as a high-performance epoxy resin, possibly diluted in a solvent.

[0061] In the case of the manufacture of a multi-perforated skin made of thermostructural composite material (CMC), the matrix precursor present on the pre-impregnated fibers or used to impregnate the fiber preform after the lay-up of dry fibers corresponds to a liquid composition containing a precursor of a ceramic material (step E2). In the case of lay-up with dry fibers, the fiber texture can be immersed in a bath containing the resin and usually a solvent thereof.

[0062] Other known impregnation techniques may be used, such as passing the fibrous texture through a continuous impregnator, impregnation by infusion, impregnation by RTM ("Resin Transfer Molding"), impregnation by injection of ceramic charge (Slurry Cast) or by a process of impregnation of a silicon alloy (MI or RMI) or by following a sequence of one or more of these processes.

[0063] In certain embodiments, the impregnation step can be carried out with a solution loaded with particles of precursor material of the matrix, in particular for the manufacture of multi-perforated acoustic skins in CMC. In this case, a loaded solution or slip is injected under pressure into the preform.

[0064] The charged solution may for example be a suspension of an alumina powder in an aqueous solution.

[0065] More generally, the loaded solution may be a suspension comprising refractory ceramic particles having an average particle size of between 0.1 μm and 10 μm. The volume content of refractory ceramic particles in the slip may, before injection, be between 15% and 40%. The refractory ceramic particles may comprise a material chosen from: alumina, mullite, silica, aluminosilicates, aluminophosphates, carbides, borides, nitrides and mixtures of such materials.

[0066] The liquid medium or phase of the solution may, for example, comprise an aqueous phase having an acidic pH (i.e. a pH lower than 7) and / or an alcoholic phase comprising, for example, ethanol. The slip may comprise an acidifier such as nitric acid and the pH of the liquid medium may, for example, be between 1.5 and 4.5. The slip may, in addition, comprise an organic binder such as polyvinyl alcohol (PVA) which is in particular soluble in water.

[0067] The liquid medium or phase can be drained out of the preform allowing deposition by sedimentation of the ceramic particles in the preform.

[0068] In this case, the mandrel 2 comprises an external layer, in contact with the porous fibrous preform. Thus, infiltration can be carried out with flow transverse to the thickness, promoting the accumulation of powder within the fibrous preform. The liquid slip media is evacuated through the porous mandrel 2. This porous mandrel layer can be obtained by partial sintering of granules or powder of material, for example plastic, or by bonding granules of materials together. The protuberances 3 can be made of a solid material - plastic or metal and implanted in the mandrel 2 by stitching.

[0069] Once the injection and drainage steps have been carried out, a fibrous preform loaded with refractory ceramic particles is obtained, for example refractory ceramic oxide or alumina particles.

[0070] The process continues with a heat treatment to transform the precursor into a matrix (step E3, [Fig.l]). It should be noted that the mandrel and the protrusions are eliminated during step E3 ([Fig.4]). [Fig.4] also shows a multi-perforated skin 8 which can be obtained after the heat treatment according to step E3.

[0071] In the case in particular of the manufacture of an acoustic skin in CMO, the heat treatment for transforming the precursor into a matrix, namely its polymerization, is generally carried out at a temperature between 90°C and 380°C. The heat treatment for transformation in the context of the manufacture of a composite material in CMO is known as “baking”.

[0072] In the case in particular of the formation of a ceramic matrix, the heat treatment consists of pyrolyzing the precursor to transform the matrix into a carbon or ceramic matrix depending on the precursor used and the pyrolysis conditions. For example, liquid ceramic precursors, in particular SiC or SiCN, may be resins of the polycarbosilane (PCS) or polytitanocarbosilane (PTCS) or polysilazane (PSZ) type, while liquid carbon precursors may be resins with a relatively high coke content, such as phenolic resins. Several consecutive cycles, from impregnation to heat treatment, may be carried out to achieve the desired degree of densification.

[0073] In the case of impregnation with a solution loaded with refractory ceramic particles, the loaded preform is subjected to a sintering heat treatment, for example in air at a temperature between 1000°C and 1200°C in order to sinter the refractory ceramic particles and thus form a refractory ceramic matrix in the porosity of the fibrous preform. A multi-perforated acoustic skin is then obtained made of composite material, for example made of Oxide / Oxide composite material, provided with a fibrous reinforcement formed by the fibrous preform and having a high matrix volume ratio with a homogeneous distribution of the refractory ceramic matrix throughout the fibrous reinforcement.

[0074] A part made of CMC composite material other than Oxide / Oxide can be obtained in the same way by producing the fibrous texture with silicon carbide and / or carbon fibers and using a slip loaded with carbide particles (for example SiC), boride (for example TiB2) or nitride (for example Si3N4).

[0075] The densification processes described above make it possible to produce, from the fibrous structure of the invention, mainly multi-perforated skins made of organic matrix composite (OMC) and ceramic matrix composite (CMC) materials. Organic matrix composite (OMC) and ceramic matrix composite (CMC) materials replace parts made of metallic material in certain parts of turbomachines. Their use contributes to optimizing the performance of aircraft. in particular by improving the efficiency of the turbomachine and reducing the overall mass of the turbomachine, significantly reducing harmful emissions to the environment (CO, CO2, NOx, etc.).

Claims

Claims

1. Method for manufacturing a multi-perforated acoustic skin made of composite material (8) for an acoustic attenuation structure, said method comprising the following steps: - forming a fiber preform comprising a precursor material of a matrix, - carrying out a heat treatment to transform the precursor into a matrix so as to obtain a multi-perforated acoustic skin made of composite material comprising a fiber reinforcement densified by said matrix, characterized in that the step of forming the fiber preform comprises draping fibers (1) on a surface of a mandrel (2) comprising protuberances (3),and in that the mandrel (2) and the protuberances (3) are each made of a material which melts at a temperature lower than the heat treatment temperature for transforming the precursor into a matrix so as to eliminate said mandrel (2) and said protuberances (3) during the transformation heat treatment step.,

2. A method according to claim 1, wherein the step of forming the preform comprises draping dry fibers (1) onto the surface of the mandrel (2) followed by a step of impregnating the fiber preform with the precursor material of a matrix.

3. A method according to claim 1, wherein the step of forming the preform comprises draping pre-impregnated fibers (1) with the precursor material of a matrix onto the surface of the mandrel (2).

4. A method according to any one of claims 1 to 3, wherein the draping with fibers (1) is carried out by winding fibers.

5. Method according to claim 4, in which the winding of the fibers is carried out with a winding angle greater than or equal to 7° relative to a central axis of the mandrel (2).

6. A method according to any one of claims 1 to 3, wherein the draping of the fibers (1) is carried out by automatic fiber placement.

7. A method according to any one of claims 1 to 6, wherein the draping is carried out with unidirectional ribbons of fibers or strips of fibers.

8. Method according to any one of claims 1 to 7, in which the multi-perforated skin (8) is made of an organic matrix composite material.

9. Method according to any one of claims 1 to 7, in which the multi-perforated skin (8) is made of a ceramic matrix composite material.

10. Method according to claim 9 in combination with claim 2 in which the step of impregnating the fibrous preform is carried out with a solution loaded with particles of the precursor material of the matrix and the transformation heat treatment step comprises a sintering step.

11. A method according to any one of claims 1 to 10, wherein the mandrel (2) is made of a first fusible material and the protrusions (3) are made of a second fusible material, said second fusible material being deformable.

12. Mandrel (2) for implementing the method of manufacturing a multi-perforated acoustic skin made of composite material for an acoustic attenuation structure according to any one of claims 1 to 11, characterized in that said mandrel (2) comprises protuberances (3), and in that the mandrel (2) and the protuberances (3) are each made of a material which melts at a temperature lower than the temperature of the heat treatment for transforming the precursor into a matrix.

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