Method for manufacturing 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: - formation of a fibrous 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 fibrous reinforcement densified by said matrix, the step of forming the fibrous preform comprising draping the fibers (1) on a surface of a mandrel (2) having protrusions (3),and the mandrel (2) and the protrusions (3) can each be made of a fusible material 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 heat treatment step for transformation. 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. More particularly, it relates to acoustic skins made of composite material included in 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 exhaust thereof, multi-perforated acoustic skins made of composite material are used.

[0003] In certain prior art methods, multi-perforated acoustic skins made of composite material are produced by drilling and machining numerous small holes through the composite skin. These processes generally involve drilling these holes manually one by one or mechanically using a mat with raised dots, which entails a very significant manufacturing time. This technique is particularly used for drilling holes with a diameter ranging from 1 millimeter to several millimeters.

[0004] Other methods for manufacturing multi-perforated acoustic skins made of composite material include a step of perforating the skin after the consolidation step of the composite material. US 6190602 and EP 3590843 are known in this regard. US 6190602 proposes depositing the composite material obtained after the consolidation step onto a substrate and inserting a flexible drilling device having pins or nails through the assembly comprising the composite material and the substrate. This step is followed by the removal of the drilling device. EP 3590843, for its part, proposes a method in which inserts are manually introduced into a layer of the composite material obtained after the consolidation step. This step is followed, as in the previous case, by the removal of some of the inserts.

[0005] Other prior art methods propose integrating the formation of perforations during the formation of the preform. This is the case in document CN211975529, which proposes draping, in different directions, a surface with several layers of fibrous reinforcements so as to form perforations, into which small studs are inserted. Furthermore, document WO2017017367 proposes a method for manufacturing an acoustic panel having a sandwich structure. In this case, the draping onto a mold involves a first layer of plies. A layer of fibrous reinforcement is created, and rounded blocks, each with a small stud made of fusible material, are placed on top of it. A second layer of plies is then deposited on these blocks. Perforations in the second layer and cells between the two layers are created during the densification stage.

[0006] However, all these processes are inefficient for various reasons. Manufacturing processes involving drilling or perforation steps require the use of particularly expensive drilling or cutting tools. Furthermore, the tools for creating such perforations generally have drill bits or pins that require frequent replacement.

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

[0008] The main purpose 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 disadvantages.

[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 in order to obtain a multi-perforated acoustic skin made of composite material comprising a fibrous reinforcement densified by said matrix,

[0012] characterized in that the fibrous preform formation step includes draping fibers over a surface of a mandrel having protrusions, and in that the mandrel and the protrusions are each made of a fusible material at a temperature lower than the heat treatment temperature of transforming the precursor into a matrix so as to eliminate said mandrel and said protrusions during the heat treatment transformation step.

[0013] Thus, the manufacturing process makes it possible to obtain multi-perforated acoustic skins made of composite material that do not require drilling or mechanical machining, nor the use of specific tooling that is time-consuming and expensive to implement. The process of the invention therefore allows for a more economical manufacture of multi-perforated acoustic skins compared to prior art manufacturing solutions. The process of the invention also makes it possible to manufacture multi-perforated acoustic skins with complex shapes, and more particularly... Acoustic skins for annular-shaped parts. The process according to the invention also makes it possible to shorten the manufacturing time of the skins because the removal of the mandrel and protrusions 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 thermal treatment of transformation of the precursor into a matrix.

[0015] According to a particular feature of the process, the preform formation step may include draping dry fibers over the mandrel surface followed by an impregnation step of the fibrous preform with the precursor material of a matrix.

[0016] According to another particular feature of the process, the preform formation step may include draping pre-impregnated fibers with the precursor material of a matrix onto the surface of the mandrel.

[0017] According to a particular feature of the process, the draping with fibers can be carried out by winding the fibers. Winding the fibers makes it possible to obtain materials with very good mechanical properties and to arrange the fibers optimally in the direction of the stresses to be borne.

[0018] The term “fiber winding” means a process which may include a step of winding fibers onto a rotating mandrel or mold.

[0019] According to another particular feature of the process, the fiber winding can be carried out with a winding angle greater than or equal to 7° with respect to a central axis of the mandrel. Such a winding angle allows the fiber to be wound precisely along the axis of the mandrel.

[0020] According to another particular feature of the process, the fiber draping can be carried out by automated fiber placement (AFP). Automated fiber placement allows for the optimal arrangement of fibers on highly complex geometries, thus enabling the production of parts with complex geometries and very good mechanical properties.

[0021] According to another particular feature of the process, the draping can be carried out with unidirectional fiber ribbons or fiber strips.

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

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

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

[0025] According to another particular feature of the process, 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 heat treatment transformation step can include a sintering step.

[0026] According to another particular feature of the process, the mandrel and the protrusions can be made of the same material.

[0027] According to another particular feature of the process, the mandrel can be made of a first fusible material and the protrusions can 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. This process can be most advantageous when it is necessary to ensure optimal shaping of the perforations. In this case, the fusible material of the protrusions can be removed after the mandrel has been removed, thus allowing for prolonged shaping of the protrusions. Consequently, perforations with greater diameter accuracy can be obtained.

[0028] According to another particular feature of the process, the second fusible material can be deformable. Thus, the protrusions can be deformed by positioning a casing or a counter-mold around the mandrel. It is therefore possible to optimally control the thickness of the finished part during the heat treatment stage.

[0029] The invention also relates to a mandrel for implementing the method of manufacturing a multi-perforated acoustic skin made of composite material for a sound-attenuating structure, characterized in that the mandrel has protrusions, and in that the mandrel and the protrusions are each made of a material that melts at a temperature lower than the heat treatment temperature for transforming the precursor into a matrix. Thus, it is possible to eliminate the mandrel and the protrusions and to densify the composite material in a single step. Furthermore, this eliminates the need for any drilling, machining, and removal of drilling tools, which can be delicate and time-consuming. Brief description of the drawings

[0030] Other features and advantages of the present invention will become apparent from the description given below, with reference to the attached drawings which illustrate examples of embodiment without any limiting character.

[0031] [Fig-1] Fig. 1 is a flowchart of the steps in a manufacturing process for a multi-perforated acoustic skin made of composite material according to an embodiment of the invention;

[0032] [Fig.2] The [Fig.2] is a schematic perspective view of the implementation of a draping step with fibers, carried out by winding fibers according to an embodiment of the invention;

[0033] [Fig.3] The [Fig.3] is a schematic perspective view of the implementation of an impregnation step according to one embodiment of the invention;

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

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

[0036] According to an embodiment of the process of the invention described in [Fig. 1], the manufacture of a multi-perforated acoustic skin made of composite material for a sound-attenuating structure according to the invention begins with the formation of a fibrous preform (step 11) by draping the fibers 1 over a surface of a mandrel 2 having protrusions 3. The mandrel 2 constitutes a mold for the formation of the fibrous preform. The mandrel may, in particular, be rotatable about its central axis.

[0037] According to another particular feature of the process, draping can be carried out with unidirectional fiber ribbons or fiber strips.

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

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

[0040] According to a particular feature of the process, the strips can comprise a width of between 10 and 200 mm.

[0041] Unidirectional fiber ribbons or fiber strips used for draping may be made of "dry" fibers, i.e., without a matrix precursor, or of fibers pre-impregnated with a matrix precursor. In the case of dry fibers, the fibrous preform is impregnated with a matrix precursor after draping.

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

[0043] The mandrel 2 and the protrusions 3 are each made of a fusible material 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 the ability to retain these properties at high temperatures. 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 as close as possible to the fibers, and a silicon carbide phase), SiC / SiC composites (reinforcing fibers and silicon carbide matrix), and oxide / oxide composites (reinforcing fibers and alumina matrix).

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

[0047] The material(s) of the mandrel 2 and the protrusions 3 are chosen taking into account the heat treatment temperature 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 heat treatment. In particular, in the case of manufacturing an acoustic skin made of 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 manufacturing an acoustic skin made of thermostructural composite material (MCM), 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 remove 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 removal of the mandrel 2 and the protrusions 3.

[0048] Depending on a particular characteristic of the process and the mandrel, the fusible material of the mandrel and / or the fusible material of the protrusions can be chosen from: plastics or metal alloys. The metal alloys may include the aluminium alloys, tin alloys, zinc alloys or a mixture thereof.

[0049] According to a particular feature of the process and the mandrel, the fusible material of the mandrel and / or the fusible material of the protrusions can be a plastic having a melting temperature between 100°C and 500°C.

[0050] According to a particular feature of the process and the mandrel, the fusible material of the mandrel can be a plastic and the fusible material of the protrusions can be a metal alloy.

[0051] The draping step can be carried out using various techniques such as: fiber winding, automated fiber placement (AFP), or manual draping, or even a combination of two or more of these techniques. Applying a predetermined tension to the fibers 1 may be necessary to ensure that they conform optimally to the surface of the mandrel 2.

[0052] Figure 2 represents a fiber winding draping operation. In the embodiment shown in Figure 2, a winding head 4 holds 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 performs this draping, the mandrel 2 can rotate about its central axis. During the draping step, the winding head 4 can move along a travel rail 5. A "travel rail" is understood to be an element used to guide the movements of the winding head 4. The fibers 1 can, for example, be grasped by the winding head 4 from a spool of fibers 6, as illustrated in Figure 2.

[0053] Draping can also be achieved by automatic fiber placement. When using this draping technique, a robot including a placement head can automatically position the fibers 1 in contact with the surface of the mandrel 2 in order to drape it (not shown).

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

[0055] Once the preform is produced, the fibrous preform is then densified in order to form a part in composite material by heat treatment of the latter 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 contain a binder. Thus, it is possible to hold the fibers and the bonded fiber layers together during the draping step. In this case, the binder used has a different composition from the precursor material of the matrix, in order 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 in which a casing or counter-mold 7 is placed around the mandrel 2, the space formed between the casing and the surface of the mandrel defining the thickness of the part. The casing may be placed in contact with the protrusions 3 of the mandrel 2. According to an alternative embodiment illustrated in [Fig. 3], the protrusions 3 and the mandrel 2 may be made of different fusible materials. The fusible material of the protrusions may be deformable, such as polypropylene or low-density polyethylene. Thus, when the inner surface of the casing 7 is placed in contact with the protrusions 3, the latter may be deformed as illustrated on the right of [Fig. 3]. This allows for optimal control of the skin thickness by compaction during the impregnation step.

[0059] In the manufacture of a multi-perforated acoustic skin made of CMO, the matrix precursor present on the pre-impregnated fibers or used to impregnate the fibrous 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, possibly 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 fibrous preform can be carried out in a manner known per se, either by liquid transfer (CVL) or by resin transfer molding (RTM). The liquid transfer 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 manufacturing a multi-perforated skin from thermostructural composite material (MCM), the matrix precursor present on the pre-impregnated fibers or used to impregnate the fibrous preform after dry fiber draping corresponds to a liquid composition containing a precursor of a ceramic material (step E2). In the case of draping with dry fibers, the fibrous texture can be immersed in a bath containing the resin and usually a solvent thereof.

[0062] Other known impregnation techniques can be used, such as passing the fibrous texture through a continuous impregnator, infusion impregnation, RTM ("Resin Transfer Moulding") impregnation, ceramic filler injection impregnation (Slurry Cast) or a silicon alloy impregnation process (MI or RMI) or a combination 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 the matrix precursor material, particularly for the manufacture of multi-perforated CMC acoustic skins. In this case, a loaded solution or slurry is injected under pressure into the preform.

[0064] The loaded solution can, for example, be a suspension of 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 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 selected from: alumina, mullite, silica, aluminosilicates, aluminophosphates, carbides, borides, nitrides, and mixtures of such materials.

[0066] The medium or liquid phase of the solution may, for example, comprise an aqueous phase with an acidic pH (i.e., a pH less than 7) and / or an alcoholic phase containing, for example, ethanol. The slip may contain 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 also contain an organic binder such as polyvinyl alcohol (PVA), which is notably soluble in water.

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

[0068] In this case, the mandrel 2 has an outer layer in contact with the porous fibrous preform. This allows for infiltration with transverse flow through the thickness, promoting powder accumulation within the fibrous preform. The liquid slip medium is discharged through the porous mandrel 2. This porous mandrel layer can be obtained by partially sintering granules or powder of material, for example, plastic, or by bonding material granules together. The protrusions 3 can be made of a solid material—plastic or metal—and are inserted into the mandrel 2 by piercing.

[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 particles or alumina.

[0070] The process continues with a heat treatment to transform the precursor into a matrix (step E3, [Fig. 1]). It should be noted that the mandrel and protrusions are removed during step E3 ([Fig. 4]). [Fig. 4] also shows a multi-perforated skin 8 that can be obtained after the heat treatment according to step E3.

[0071] In particular, in the case of manufacturing a CMO acoustic skin, the heat treatment that transforms the precursor into a matrix, namely its polymerization, is generally carried out at a temperature between 90 °C and 380 °C. This heat treatment for transformation in the manufacture of a CMO composite material is known as "baking".

[0072] In the case of ceramic matrix formation, in particular, 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, particularly SiC or SiCN, can be polycarbosilane (PCS), polytitanocarbosilane (PTCS), or polysilazane (PSZ) type resins, while liquid carbon precursors can be resins with a relatively high coke content, such as phenolic resins. Several consecutive cycles, from impregnation to heat treatment, can be carried out to achieve the desired degree of densification.

[0073] In the case of impregnation with a solution containing refractory ceramic particles, the filled 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 within the porosity of the fibrous preform. This results in a multi-perforated acoustic skin made of composite material, for example an oxide / oxide composite material, with a fibrous reinforcement formed by the fibrous preform and exhibiting 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 making the fibrous texture with silicon carbide and / or carbon fibers and using a slurry loaded with carbide particles (e.g. SiC), boride (e.g. TiB2) or nitride (e.g. Si3N4).

[0075] The densification processes described above make it possible to produce, from the fibrous structure of the invention, primarily multi-perforated skins made of organic matrix composite (OMC) and ceramic matrix composite (CMC). Organic matrix composite (OMC) and ceramic matrix composite (CMC) materials replace metallic parts in certain sections of turbomachinery. Their use contributes to optimizing aircraft performance. 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, ...).

Claims

Demands

1. A method for manufacturing a multi-perforated acoustic skin made of composite material (8) for a sound-attenuating structure, said method comprising the following steps: - forming a fibrous 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 fibrous reinforcement densified by said matrix, characterized in that the step of forming the fibrous preform comprises draping fibers (1) onto a surface of a mandrel (2) having protrusions (3),and in that the mandrel (2) and the protrusions (3) 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 (2) and said protrusions (3) during the heat treatment transformation step.

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

3. A method according to claim 1, wherein the preform formation step 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, wherein the winding of the fibers is carried out with a winding angle greater than or equal to 7° with respect 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 placement of fibers.

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

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

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

10. A process according to claim 9 in combination with claim 2 wherein the step of impregnating the fibrous preform is carried out with a particle-loaded solution of the precursor material of the matrix and the heat treatment transformation step includes 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 of composite material for an acoustic attenuation structure according to any one of claims 1 to 11, characterized in that said mandrel (2) has protrusions (3), and in that the mandrel (2) and the protrusions (3) are each made of a material that melts at a temperature lower than the temperature of the heat treatment of transforming the precursor into a matrix.

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