Porous metal bodies used for attenuating aviation turbine noise

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DESCRIPTION

Metal porous body suitable for attenuating the noise of aeronautical turbines The invention relates to the manufacture of metal porous bodies.

The noise emission of a commercial aircraft, mainly due to the engines, can reach 155 dB in the immediate proximity of the aircraft at takeoff. This value above the threshold of auditory pain evaluated at 120 dB reaches even 90 dB at 400 m from the source. Therefore, it is desirable to lower this noise emission level. One way to try to solve this problem is to absorb noise at one of its emission points, that is, at the level of the engines ... Solutions have already been applied to the cold parts of the engines, but the hot parts have not they are currently the subject of no acoustic treatment. Therefore, it is desirable to develop a material that has an acoustic absorption function for the hot parts of aircraft engines. To do this, one way in which it is contemplated is to develop a turbine capable of absorbing, in part, the noise produced inside the engine.

Honeycomb structures, well known in the aeronautical world, can be adapted for acoustic absorption. These structures are then associated with perforated skins that partially close the elemental cells. The elementary cells, with a diameter greater than 1 mm, thus form resonant acoustic cavities that trap the waves that penetrate through the perforations. These structures lead to insufficient acoustic properties, since they are Helmholtz-type resonators, which cannot absorb very specific frequencies. The phenomenon used is based on the four-wave resonance. Only frequencies that have a wavelength close to four times the depth of the elementary cells and their harmonics are efficiently absorbed. EP 0036356 describes a porous metal body with channels.

Now, effective acoustic absorption at the turbine level for the noise produced by the combustion chamber and the different blades of the turbines and of the high-pressure compressors implies an effect on a wide frequency spectrum.

The object of the invention is to provide a porous structure that has improved acoustic properties relative to known structures.

The invention mainly relates according to claim 1 to a metallic porous body having two opposite main faces and suitable for attenuating the noise produced or transmitted by a gas stream, which sweeps through a first of said main faces, said body presenting pores in the form of cylindrical channels, whose axes extend substantially according to straight lines perpendicular to said first face, that open through a first of its ends in said first face and closed at its opposite end, each channel having a diameter between approximately 0.1 and 0.3 mm and being located, on at least a part of its length, at a The minimum distance from their close neighbors is between approximately 0.02 and 0.3 mm and the ratio of the length to the diameter of the channels is greater than 10 and preferably on the order of 102.

The metallic structure described here has a porosity that can exceed 70%, whose volumetric mass is compatible with aeronautical applications.

This structure behaves as an excellent absorber of noise, in particular for frequencies above 1 kHz, as shown by the application of classical analytical acoustic absorption models (propagation of an acoustic wave inside a tube by Kirchhoff in 1857). The open cells of this “micro-honeycomb” are large enough to allow the sound wave, in the frequency domain of the order of 1 kHz or more, to penetrate the structure, but small enough to provide the necessary specific surface area. to attenuate the acoustic energy by viscoacoustic dissipation in the fluid contained within the porous material. This dissipation is due to the shear of the fluid in the boundary layer that appears on the internal walls of the porous structure.

For a diameter less than 0.1 mm, the wave no longer penetrates the structure effectively. For a diameter greater than 0.3 mm, the quarter-wave resonance phenomenon becomes preponderant again.

The cylindrical channels, whose diameter is between 0.1 and 0.3 mm, favor the dissipation of the energy of the acoustic wave in the internal gas shears that occur in the boundary layers that appear on the walls of the channels.

If the diameter of the cylindrical channels is greater than 0.3 mm, the total surface of the walls is insufficient.

The absorption mechanism of this new structure is due to a viscous dissipation in the gas, whereas, by way of comparison, a classical acoustic absorption system uses the principle of the Helmholtz resonator valid exclusively for the absorption of a particular frequency and must be combined, in order to absorb a higher frequency spectrum, with non-structural porous materials.

The compilation of the state of the art tends to show that any noise absorber based on the principle of the Helmholtz resonator will necessarily be thick, since to cover the entire frequency range at

ES 2 658 684 T3 absorb, you will have to associate other different materials (honeycombs, felts, etc.) in different thicknesses with the resonant structure. However, this contribution to the thickness may imply a not insignificant overweight.

Finally, even due to its architecture, the material according to the invention, unlike the solutions described in the literature, is a structural element and can be dimensioned as such. In addition, thanks to the lightening generated by its porosity, its mechanical performances related to its apparent density are exceptional (structural behavior of the type of honeycomb). Also its function of absorbing noise can be considered as an additional triumph. Due to this fact, the application of this invention to aircraft engines will allow noise to be treated at its point of emission without increasing the volume.

The usual manufacturing techniques for honeycombs (welding of stamped sheets or unfolding of perforated metal sheets) are not applicable here due to the scale of the object. Reference should also be made to other techniques. One of these techniques is based on the formation from an ultra-pure nickel chemical bath. The shape and diameter of the hole will be determined by the mandrel used and the wall by the thickness of the chemical deposition.

Depending on the nature of the alloy desired to make this wall, it is possible to proceed in another way. After having returned to the electrically conductive mandrel thanks to a chemical copper deposition, it is coated with electrolytic nickel in order to give it sufficient rigidity for handling. The electrolytic deposition is then completed by a deposition of alloy powder pre-coated by a nickel-boron alloy, as described in French patent application 05.07255 of July 7, 2005 or of alloy powder dispersed in a organic binder as described in French patent application 05.07256 of July 7, 2005.

The following are optional features of the invention, complementary or replacement.

- The relationship between the length and the diameter of the channels is between approximately 90 and 110.

- The roughness of the surface of the channels is less than 0.01 mm.

- Each channel is surrounded, according to a substantially uniform angular distribution, by six other channels separated from it at a minimum distance of between approximately 0.02 and 0.3 mm.

- The axis of each of said channels forms an angle of less than 20 ° with the perpendicular to said first face at said first end.

- The body comprises nickel and / or cobalt and / or an alloy thereof, mainly a superalloy based on nickel and / or cobalt.

- Said first face is concave.

The invention also has as an object an aeronautical turbine housing comprising at least one sector constituted by a porous body as defined above, as well as a method according to claim 9 for manufacturing such a porous body, a method in which they are arranged in layers a plurality of threads each comprising a cylindrical mandrel with a diameter between approximately 0.1 and 0.3 mm of a heat destructible material, surrounded by a metal-based sheath, the sheath of each thread being in contact with the sheaths of neighboring threads in the same layer and with the sheaths of the threads of neighboring layers, and a heat treatment is carried out to eliminate the mandrels and joining the sleeves together, producing a metallic matrix.

The process according to the invention may comprise at least some of the following particularities:

- Said chuck is made of organic matter.

- Said chuck is made of carbon.

- The sleeve is formed at least in part by chemical and / or electrolytic deposition of metal on the mandrel.

- The sleeve is formed at least in part by gluing metal particles onto the mandrel and / or onto said deposition.

- Metal particles are introduced into the gaps between the threads before said heat treatment.

- Metal particles comprise a welding coating that produces a bonding of the metal particles with each other and / or with said deposition during heat treatment.

- The metallic components present are joined together during the thermal treatment by fusion of a eutectic between its constituent metals and the carbon that it prevents from the mandrel and / or from an organic binder or adhesive.

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- Before the treatment, one end of each thread is glued on a common flat support that extends perpendicular to the axes of the threads, the support is curved according to an arc of a circle, so that the axes of the threads then extend radially , and metal particles are introduced between the gaps between the threads.

- After heat treatment, said metal matrix is ​​machined to form said first concave face.

- After heat treatment, traces of carbon that remain in the channels are removed.

- Said opposite end of the channels is closed by a metal layer produced on the corresponding face of said metal matrix.

The characteristics and advantages of the invention are set out in more detail in the following description, with reference to the accompanying drawings.

Figure 1 is a partial view of the first main face of a porous body according to the invention.

Figure 2 is a partial view of the body, in section according to the line II-II of Figure 1.

Figure 3 is a sectional view of a sector of an aeronautical turbine housing according to the invention.

The invention is illustrated below by examples. All compositions are given here by weight.

Example 1

It is proposed to manufacture a porous body of pure nickel. A cylindrical wire of revolution with a diameter of 0.1 mm is used as the mandrel (the following method is applicable whatever the diameter of the wire chosen from 1 pm to 3 mm) and whatever the shape of its cross section is). It can mainly be a polyamide or polyimide thread marketed as fishing thread. A chemical nickel deposition is carried out on this wire, proceeding according to the following four stages separated by rinsing with abundant deionized water.

1. Surface preparation by degreasing and soaking.

two. Deposition by adsorption of a solid reducing agent, tin chloride SnCl2, by immersion for at least 5 min. in a saturated solution (5 g / l) of this salt.

3. Deposition on the surface to be treated of a catalyst (palladium) by reduction from an acid solution (pH = 2) to 10 g / l of PdCl2 for at least 5 min.

Four. Nickel deposition proper from a bath having the following composition:

<td>Nickel-triethylenediamine</td><td>Ni (H₂NC₂H₄NH₂) 3⁺</td><td>0.14 M</td>

<td>bland</td><td>NaOH</td><td>1 M</td>

<td>Arsenic pentoxide</td><td>As2Ü5</td><td> 6,5.10^-4 M</td>

<td>Imidazole</td><td>N2C2H4</td><td>0.3 M</td>

<td>Hydrazine hydrated</td><td>N2H4, H2O</td><td>2.06 M</td>

<td>pH</td><td></td><td> 14</td>

After immersion for one hour and thirty at 90 ° C, the wire is covered by a very pure nickel deposit with a thickness of approximately 20 µm.

This coated yarn is cut into pieces of appropriate length, on the order of 1 cm. The different pieces are then arranged parallel to each other in an alumina crucible. The pieces of a first layer rest on the flat bottom of the crucible, each one being in contact with two neighbors by diametrically opposite generatrices. Each subsequent layer is deposited on the preceding layer in a staggered fashion. The whole is finished with a weight of a few tens of grams to keep the pieces in mutual contact.

The crucible is then placed in a furnace under a vacuum better than 10 Pa and heated up to 400 ° C, at which temperature the synthetic material of the mandrel decomposes and is ingested by the pumping system.

After a period of one hour, a heating ramp is carried out at 70 ° C / min. up to 1200 ° C followed by a period of a quarter of an hour for the interdiffusion of each tube with its nearest neighbors. The whole is then refrigerated.

Following this operation, a pure nickel microporous object is obtained comprising pores in the form of cylindrical channels of revolution with a diameter D (figure 1) of approximately 100 pm. In the ideal case illustrated in the figure, each cylindrical pore 1 has six immediate neighbors 2, which are separated by a wall of

ES 2 658 684 T3 pure nickel 3 of a minimum thickness e of approximately 40 pm. The channels 2 are arranged according to a uniform angular distribution, that is, the marks 4 of their axes in the plane of figure 1 are located at the tops of a regular hexagon whose center is the mark 5 of the axis of the channel 1. In reality, the arrangement of the channels may be less regular.

Example 2

A large length of the synthetic yarn used in Example 1 is wound on a polytetrafluoroethylene (PTFE) assembly comprising six parallel cylindrical bars, the axes of which are arranged, in right projection, according to the tops of a regular hexagon. A chemical copper deposition is then carried out on this wire, which proceeds according to the following four stages separated by abundant rinsing with deionized water.

1. Surface preparation by degreasing and soaking

two. Deposition by adsorption of a solid reducing agent, tin chloride SnCL, by immersion for at least 5 min. in a saturated solution (5 g / l) of this salt.

3. Deposition on the surface to be treated of a catalyst (silver) from a neutral solution of 10 g / l of AgNÜ3 for at least 5 min.

Four. Nickel deposition proper from a bath having the following composition:

<td>Copper sulphate</td><td>CuSO4, 6H2O</td><td>0.1 M</td>

<td>Formaldehyde</td><td>FACT</td><td>0.5M</td>

<td>Double sodium and potassium tartrate</td><td>KNaC4H4O6, 4H2O</td><td>0.4 M</td>

<td>bland</td><td>NaOH</td><td>0.6 M</td>

After 30 minutes, the thread has taken on the characteristic red color of an envelope deposition.

Following this operation, the wire converted into an electricity conduit is immersed in a classic electrolytic nickel deposition bath and connected to the cathode. After 20 minutes of deposition under a current density of 3 A / dm², the wire is coated with 20 pm of pure nickel.

The yarn thus coated is cut into pieces of the appropriate length. These pieces are then coated with a thickness of approximately 100 pm with a mixture of 80 parts of nickel superalloy powder sold under the name IN738 and of 20 parts of a binder composed of equal parts of an epoxy glue and of ethyl alcohol that serves as a diluent, this operation being carried out by rotating the pieces in the presence of the powder-binder mixture between a flat support surface and a flat support plate, allowing the distance between these two plates to determine the thickness of the dust deposition.

The pieces thus coated are then placed in a crucible placed in a vacuum oven as described in Example 1.

In the course of the 400 ° C interval, the mandrel material and the binder decompose and are not ingested by the pumping system. Tail decomposition involves deposition of carbon residues on the surface of each superalloy powder grain. After a period of one hour, a new heating ramp is carried out at 70 ° C / min. up to 1320 ° C followed by a period of 15 minutes for interdiffusion of each dust grain with its closest neighbors and of each tube with its closest neighbors. The whole is then refrigerated.

At the end of this operation, a microporous object made of IN738 alloy is obtained. Each pore is approximately 100 to 300 µm in diameter and is separated from neighboring pores by a superalloy wall of approximately 200 µm.

Example 3

The procedure is as in Example 2 to obtain a 20 µm coated wire of nickel cut into pieces.

On the other hand, a powder of the nickel superalloy marketed under the name Astrolloy, with a diameter of 10 pm, a weld layer based on the nickel-boron alloy of at least 1 pm thickness, is deposited on the grains, By the technique described in FR 2777215 and the powder coated in this way is mixed with 1% of methyl methacrylate marketed under the name Coatex P90, optionally diluted with water to handle the mixture. The nickel-plated wire pieces are wound in this mixture as described in Example 2 to receive an approximately 100 µm layer of coated superalloy powder.

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The pieces thus coated are then placed in a crucible placed in a vacuum oven as described in Example 1.

In the course of the 400 ° C interval, the material on the mandrel decomposes. After a period of one hour, a heating ramp is carried out at 70 ° C / min. up to 1120 ° C followed by a period of a quarter of an hour for welding each dust grain to its closest neighbors and each tube to its closest neighbors. The whole is then refrigerated.

In this way, a simple heat treatment allows both the powder grains and the tubes to be welded together. Thanks to the chemical deposition of nickel-boron alloy on the superalloy powder, the tube wall obtained after annealing is dense and homogeneous. The powder grades are welded together.

At the end of this operation, a microporous Astrolloy object is obtained. Each pore is approximately 100 to 300 µm in diameter and is separated from neighboring pores by a superalloy wall of approximately 200 µm.

Example 4

As mandrel, fiber rovings called pyrolyzed cotton are used, that is, carbon rovings obtained by carding natural cotton and pyrolysis under reduced pressure from Aragon, with a diameter of approximately 0.1 pm.

The fibers are previously nickel plated by a technique called "barreling" in a classic nickel sulfamate bath. The electrolysis is carried out for the time necessary to obtain a nickel thickness between 20 and 40 pm. The nickel-plated wicks are then cut into pieces that are mixed with dilute epoxy glue used in Example 2 in a proportion of approximately 95% mixes per 5% glue and arranged parallel to each other in a PTFE mold. After hardening of the glue, a set of high porosity is obtained. By injection with the aid of a syringe, this assembly is impregnated with the coated Astrolloy superalloy powder mixture and Coatex P90 used in Example 3. After drying in an oven at 90 ° C, the material is placed in a vertical oven under hydrogen preheated to 800 ° C. It then experiences a temperature ramp of 5 ° C per minute to a temperature of 1100 ° C. Two concomitant phenomena then occur: the nickel-boron solder that surrounds the molten Astrolloy powder grains, consequently with the powder grains being welded together, and the carbon in the wicks reacts with the hydrogen in the atmosphere furnace to form methane. After an interval of 8 hours and cooling under hydrogen to a temperature of approximately 500 ° C, in addition to a return to room temperature under argon, a porous material is obtained with pores of a diameter of approximately 0.1 mm apart. by walls, whose thickness varies between 50 and 200 pm, other smaller pores may come from the interstices between the coated fibers.

Each of Examples 1 to 4 provides a porous body that has two main opposite planar faces, whose thickness is equal to the length of the pieces of wire used, of the order of 1 cm, taking into account the relationship to be respected with the diameter of the thread, and comprising cylindrical pores 1 perpendicular to these two faces and leading to them. A flat porous body can then be obtained according to the invention, the pores of which are closed at one end, covering one of the main faces with a continuous metallic layer 6 (Figure 2), for example in the form of a 0.5 mm sheet metal. thickness welded on the base body, or covering the pores with a metallic powder in suspension, by induction or projection.

A sector of an aeronautical turbine casing according to the invention can also be made by machining the base body to obtain a face with a convex arc profile and a face with a concave arc profile, then the sealing of the pores on the face is carried out. convex. In this case, the length of the pieces of wire must be greater than the thickness of the sector to be obtained, and the axes of the channels are not perpendicular to the concave face more than one half of the length of the arc, presenting an increasing inclination in relation to to the perpendicular that goes towards each of the ends of the arch.

Example 5

This time it is a question of manufacturing a sector of casing destined to an aeronautical turbine, without having to proceed with the machining required in the preceding examples. A housing with an internal diameter of approximately 1 meter is subdivided, for example, into 12 sectors.

Nickel-plated wire pieces prepared as in Example 3 and trimmed to an appropriate length are arranged vertically on a horizontal PTFE plate having a thickness of approximately 1 mm, a length and a width equal, respectively, to the length of the arc and the axial length of the sector to be made. Since the entire surface of the plate is covered by the pieces of nickel-plated wire, the end of these is glued there with a glue of the cyanoacrylate type. Once the glue has polymerized, the PTFE plate is notched, in such a way that the wire logs stretch radially outwards and present a mutual separation in the circumferential direction that grows from the plate, ensuring the nickel coating. a stiffness of the pieces. The voids formed in this way are filled with coated Astrolloy superalloy powder mix and

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Coatex P90 used in Example 3, this powder being partially substituted by hollow nickel spheres, such as spheres with a diameter of the order of 0.5 pm sold by the ATECA Company. After drying in the oven overnight at 70 ° C, the PTFE silver is removed, the set of fibers, powder and glue being mechanically solid. The whole is placed in a vacuum oven. When the pressure in the enclosure is less than about 10^-3 Pa, the assembly is brought to a temperature of 450 ° C for 1 hour in order to degas and eliminate organic products (mandrel and methyl methacrylate). The decomposition of methyl methacrylate involves a deposition of carbon residues on the surface of each grain of superalloy powder. A new heating ramp is carried out at 70 ° C / min. up to 1,320 ° C and followed by an interval of a quarter of an hour for the interdiffusion of each dust grain with its closest neighbors. The whole is then refrigerated. As in the preceding examples, the eutectic Ni-carbon has acted as a solder and has ensured the gathering of the powder grains with each other and has subsequently solidified thanks to the diffusion of the carbon in the alloy. After cooling, a porous body 10 (figure 3) is obtained in the shape of an arc of a circle crossed by a multitude of channels 11 of 0.1 pm in diameter separated from each other by walls 12 of a minimum thickness of a few hundredths of a millimeter in the vicinity of the concave face of the body and a few tenths of a millimeter in the vicinity of its convex face. The pores are then sealed by a metallic layer 13 similar to layer 6 of FIG. 2, applied on the convex face.

Sectors such as that of figure 3 can be used on the entire surface of the casing, or on only a part of it.

Although in the previous examples a wire with a circular section has been used as a mandrel due to its availability, it is also possible to use a mandrel with a non-circular section, mainly polygonal.

If necessary, an ultrasound treatment of the porous body can be carried out to eliminate traces of carbon that remain after the heat treatment on the walls of the channels and to obtain a very smooth surface.