EP3429787B1 - Process to produde and repair an abradable layer of a turbine ring - Google Patents

Description

Background of the invention

This disclosure relates to a method of manufacturing a turbine ring for a turbomachine.

In many rotating machines, it is now known to provide the stator ring with abradable tracks facing the top of the rotor blades. Such tracks are made using so-called “abradable” materials which, when they come into contact with the rotating blades, wear out more easily than the latter. A minimum clearance is thus ensured between the rotor and the stator, improving the performance of the rotating machine, without running the risk of damaging the vanes in the event of friction of the latter on the stator. On the contrary, such friction erodes the abradable track, which makes it possible to automatically adjust the diameter of the stator ring as close as possible to the rotor. Thus, such abradable tracks are often installed in turbomachine compressors.

On the other hand, their use is rarer in the turbines of such turbomachines, and especially in high pressure turbines in which extreme physicochemical conditions prevail.

In fact, the burnt gases from the combustion chamber open into the high pressure turbine at very high temperature and pressure levels, which leads to the premature erosion of the conventional abradable tracks.

Therefore, in order to protect the turbine ring, it is often preferred to provide the latter with a coating of the thermal barrier type, the materials and high density of which, too high for the coating to be effectively abradable, make it possible to protect the turbine ring. ring against erosion and corrosion.

However, it will naturally be understood that in such a case the integrity of the vanes is no longer ensured in the event of contact with the stator, which necessitates providing a greater clearance between the rotor and the stator and therefore increases the leakage rate at the top of the blades and thus reduces the performance of the turbine.

Moreover, due to the point friction with the blades and the heat of the burnt gases, the coating may be damaged and less protect the stator. Ratel and. al., SURFACE AND COATINGS TECHNOLOGY, vol. 205, no. 5, (2010), pages 1183-1188 , discloses a process for forming an abradable MCrAIY (M = Co) layer by the SPS method with the addition of talc. EP1837103A1 discloses an SPS process for the simultaneous formation of several layers of an aluminum-based composite which are subsequently separated.

Purpose and summary of the invention

The present disclosure aims to remedy at least in part these drawbacks.

To this end, the present disclosure relates to a method of manufacturing at least two abradable plates for a turbomachine turbine ring, according to claim 1 attached.

By “cobalt-based” is meant a metal powder in which the cobalt has the highest percentage by mass. Likewise, the term “nickel-based” is understood to mean a metal powder in which the nickel has the highest percentage by mass. Thus, for example, a metal powder comprising 38% by mass of cobalt and 32% by mass of nickel will be designated as a cobalt-based powder, the cobalt being the chemical element whose percentage by mass is the greatest in the powder. metallic.

Metal powders based on cobalt or nickel are powders which, once sintered, exhibit good resistance at high temperature. They can thus fulfill the dual function of abradable and thermal shield. For example, mention may be made of the CoNiCrAlY superalloys. These metal powders also have the advantage of having a chemical composition similar to the chemical composition of the material forming the turbine ring, for example the AM1 or N5 superalloys.

The powder based on a flux makes it possible to reduce the sintering temperature of the mixture of powders.

The SPS sintering process, in accordance with the English acronym for “Spark Plasma Sintering”, also known under the name FAST sintering, in accordance with the English acronym for “Field Assisted Sintering Technology”, or flash sintering, is a sintering process during which , a powder is subjected simultaneously to a pulsating current of high intensity and to a uniaxial pressure in order to form a sintered material. SPS sintering is generally carried out in a controlled atmosphere and can be assisted by heat treatment.

The SPS sintering time is relatively short and SPS sintering allows a choice of starting powders which is relatively limited. In fact, SPS sintering makes it possible in particular to sinter, that is to say to densify, materials whose welding is relatively complicated to achieve, or even impossible, because these materials crack easily when they are heated. Due to the choice of SPS sintering and the short duration of this sintering, it is therefore possible to produce an abradable layer with a very wide variety of materials.

Moreover, the SPS sintering being carried out under uniaxial pressure exerted by the mold on the powder layer, the shrinkage due to the sintering of the powder layer to give the abradable plate is limited to the direction of application of the pressure. No shrinkage of the powder layer is therefore observed in directions perpendicular to the direction of application of the pressure. Also, controlling the dimensions of the abradable plate is relatively simple.

At least two layers of the powder mixture are placed in the mold, the two layers being spaced apart from each other by a chemically inert insert.

It is thus possible to produce several abradable plates in a single SPS sintering step. For example, ten layers of the mixture of powders can thus be deposited, each layer being separated from the adjacent layer by a chemically inert insert. Ten abradable plates can thus be formed, each having a thickness which may vary from 1 to 5 mm, each of the abradable plates being separated from an adjacent abradable plate by a chemically inert insert.

The chemically inert insert helps to reduce or even eliminate chemical reactions between the powder mixture layers during SPS sintering.

Each powder mixture layer being separated from the adjacent layer by a chemically inert insert, the powder mixture layers do not sinter on each other and it is therefore easy to make several abradable plates which do not stick together. to the other.

The chemically inert insert can also be placed between the powder mixture layer and the mold.

The chemically inert insert makes it possible to reduce, or even eliminate, the chemical reactions between the powder mixture layer and the mold during SPS sintering and therefore reduce, or even avoid, the sticking of the abradable plate with the parts of the mold.

The chemically inert insert also makes it possible to reduce, or even avoid, the formation of a layer of carbide on the surface of the abradable plate in contact with the mold. It is sought to avoid the formation of this layer of carbide which, if it does form, must be removed from the abradable plate before its use.

The chemically inert insert can comprise boron nitride or corundum.

The term “chemically inert insert comprising boron nitride” is understood to mean an insert which comprises at least 95% by mass of boron nitride. Likewise, the term “chemically inert insert comprising corundum” is understood to mean an insert which comprises at least 95% by mass of corundum.

The chemically inert insert can take the form of a layer of boron nitride deposited using a spray on the mold. The chemically inert insert can also take the form of a plate reproducing the shape of the abradable plate. Thus, the chemically inert insert makes it possible, during the SPS sintering step, to give its shape to the abradable plate.

Boron nitride can form an outer layer of the chemically inert insert.

The chemically inert insert may be a plate of dense material covered with a layer of boron nitride deposited using a spray on the plate.

The fluxing element is silicon or boron.

The mixture of powders may comprise a percentage by mass of the fluxing element less than or equal to 5% by mass, preferably less than or equal to 3% by mass.

The mold can be made of graphite and the SPS sintering can be carried out at a temperature greater than or equal to 800 ° C, preferably greater than or equal to 900 ° C.

SPS sintering is carried out at a pressure greater than or equal to 10 MPa, preferably greater than or equal to 20 MPa, even more preferably greater than or equal to 30M Pa.

The mold can be made of tungsten carbide and the SPS sintering can be carried out at a temperature greater than or equal to 500 ° C, preferably greater than or equal to 600 ° C.

SPS sintering can be carried out at a pressure greater than or equal to 100 MPa, preferably greater than or equal to 200 MPa, even more preferably greater than or equal to 300 MPa.

The present disclosure also relates to a method of repairing a turbine ring for a turbomachine, comprising the steps as defined in claim 7 attached.

The fluxing element included in the mixture of powders used to form the abradable plate also makes it possible to facilitate the process of brazing the abradable plate on the turbine ring.

The brazing of the abradable plate on the turbine ring makes it possible to avoid the direct deposition of a new abradable coating on the ring or the ring sector. Indeed,

After brazing the abradable plate to the turbine ring, a free surface of the brazed abradable plate can be machined.

The abradable plate which has just been brazed to the turbine ring may have a free surface which may not be in the extension of the free surface of the adjacent undamaged abradable coating. Also, the free surfaces of the abradable plate and of the abradable coating are machined so as to present a surface intended to face the turbine wheel which has the least amount of discontinuity possible. Indeed, if such discontinuities are present, the blade wheel could come up against these discontinuities and thus cause shocks in the turbine, which is not desirable.

Brief description of the drawings

• la figure 1 est une vue schématique en coupe longitudinale d'une turbomachine ;
• la figure 2 est une vue schématique en perspective d'un secteur d'anneau de turbine comportant une plaque abradable ;
• la figure 3 est une vue schématique en perspective d'un empilement de plaques abradables et d'inserts chimiquement inerte ;
• la figure 4 est une vue schématique en coupe d'un empilement dans le moule pour frittage SPS, selon un plan de coupe similaire au plan de coupe IV-IV de la figure 3 ;
• les figures 5A-5D sont des images réalisées au microscope électronique à balayage de la microstructure de différentes plaques abradables ;
• la figure 6 est une vue schématique d'un secteur d'anneau comprenant un revêtement abradable endommagé ;
• les figures 7A et 7B sont des vues schématiques latérales d'un anneau de turbine dont une partie du revêtement abradable a été remplacé par une plaque abradable, respectivement avant et après usinage d'une surface libre de la plaque d'abradable.

Other characteristics and advantages of the invention will emerge from the following description of embodiments of the invention, given by way of non-limiting examples, with reference to the appended figures, in which:

• the figure 1 is a schematic longitudinal sectional view of a turbomachine;
• the figure 2 is a schematic perspective view of a turbine ring sector comprising an abradable plate;
• the figure 3 is a schematic perspective view of a stack of abradable plates and chemically inert inserts;
• the figure 4 is a schematic sectional view of a stack in the SPS sintering mold, along a sectional plane similar to the sectional plane IV-IV of the figure 3 ;
• the figures 5A-5D are images produced with a scanning electron microscope of the microstructure of various abradable plates;
• the figure 6 is a schematic view of a ring sector comprising a damaged abradable coating;
• the figures 7A and 7B are schematic side views of a turbine ring, part of the abradable coating of which has been replaced by an abradable plate, respectively before and after machining of a free surface of the abradable plate.

Detailed description of the invention

The figure 1 shows, in section along a vertical plane passing through its main axis A, a bypass turbojet 10. The bypass turbojet 10 comprises, from upstream to downstream according to the circulation of the air flow, a fan 12, a compressor low pressure 14, a high pressure compressor 16, a combustion chamber 18, a high pressure turbine 20, and a low pressure turbine 22.

The high pressure turbine 20 comprises a plurality of mobile blades 20A rotating with the rotor and rectifiers 20B mounted on the stator. The stator of the turbine 20 comprises a plurality of stator rings 24 arranged vis-à-vis the mobile blades 20A of the turbine 20.

As can be seen on the figure 2 , each stator ring 24 is made in several ring sectors 26. Each ring sector 26 has an internal surface 28, an external surface 30 and an abradable plate 32 on which the mobile blades 20A of the rotor can rub.

The abradable plate 32 is brazed to the ring sector 26. The abradable plate 32 comprises a free surface 34 and a surface 36 intended to be brazed on the ring sector 26.

For example, the ring sector 26 is made from a cobalt or nickel based superalloy, such as AM1 superalloy or N5 superalloy, and the abradable plate 32 is made from a cobalt or nickel based metal powder. nickel.

In the embodiment described, the ring 24 is composed of a plurality of ring sectors 26 assembled together to form a ring 24. The ring 24 can also be made in one piece.

To manufacture an abradable plate 32, a mixture is prepared comprising a metal powder based on cobalt or nickel and a powder based on a flux. For example, the powder based on cobalt or on nickel may be a powder of the CoNiCrAIY family and the fluxing element is boron or silicon. The mixture of powders can for example comprise 2% by weight of boron.

As shown on figures 3 and 4 , the mixture of powders is deposited in the form of layers in an SPS sintering mold 42. The mold 42 is for example made of graphite. The mold 42 comprises an outer mold 44 forming a chamber in which the mixture of powders is deposited. The mold 42 also has an upper piston 46 and a lower piston 48 which allow axial pressure to be applied to the powder mixture layers during the SPS sintering step.

The figure 3 shows a stack 38 comprising two abradable plates 32 between which is inserted a first chemically inert insert 40. In this example, a second chemically inert insert 40 and a third chemically inert insert 40 are also arranged on either side of the stack 38 so that each powder mixture layer is sandwiched between two chemically inert inserts 40. The chemically inert inserts 40 can for example be formed from plates of sintered boron nitride.

In the embodiment of figures 3 and 4 , each abradable plate 32 is obtained by depositing a layer of mixture of powders between two chemically inert inserts 40 and by carrying out an SPS sintering step.

We have represented on the figures 3 and 4 two stacks 38 respectively comprising two and four abradable plates 32 after SPS sintering.

Before depositing the layer of powder mixture, it is also possible to deposit a layer of boron nitride using a spray on the mold 42, in particular on the surfaces of the mold 42 which will come into contact with the mixture of powders during SPS sintering. This layer of boron nitride also forms a chemically inert insert between the mixture of powders and the mold 42.

The chemically inert inserts 40 can also be made from a material other than boron nitride. The chemically inert inserts 40 may or may not be covered with a layer of boron nitride.

The chemically inert inserts 40, in plate form or in the form of a layer, make it possible to reduce the chemical reactions between the powder mixture layer and the mold 42 during SPS sintering. The chemically inert inserts 40 make it possible in particular to reduce, or even avoid, the bonding of the powder mixture layer with the parts of the mold before SPS sintering and the bonding of the abradable plate 32 with the parts of the mold 42 after SPS sintering.

The chemically inert inserts 40 also make it possible to reduce, or even avoid, the formation of a layer of carbide on the surface of the abradable plate 32.

It is understood that the thickness of the abradable plate 32 obtained after SPS sintering depends in particular on the thickness of each powder mixture layer deposited in the mold 42 as well as on the SPS sintering parameters. The thickness of the abradable plate 32 obtained after SPS sintering can also depend on the particle size and on the morphology of the powder used. In particular, the morphology of the powder may depend on the method of manufacturing the powder. Thus, a powder produced by gas atomization or a rotating electrode will have grains of substantially spherical shape whereas a powder produced by liquid atomization will have grains of less regular shape.

The figures 5A-5D represent different microstructures of abradable plates 32, the open porosity of which is respectively about 10%, about 7%, about 3% and almost zero.

It is therefore seen that by modifying the SPS sintering parameters, such as the temperature, the pressure and the dwell time, it is possible to obtain abradable plates 32 having a different structure. For example, the figure 7A shows an abradable plate 32 obtained during an SPS sintering step at 925 ° C. for 10 minutes by applying a pressure of 20 MPa. FIG. 7D represents an abradable plate 32 obtained during an SPS sintering step at 950 ° C. for 30 minutes by applying a pressure of 40 MPa.

The figure 6 shows a top view of a ring sector 26 comprising a damaged abradable coating 50. The abradable coating 50 can be obtained by the method described above. The abradable coating 50 may also have been deposited directly on the ring sector 26 by a known method.

In the example of figure 6 , the abradable coating 50 comprises a zone 52 of damage due to the friction for example of a blade with the abradable coating 50 and a zone 54 of damage due to thermal degradation of the abradable coating 50 under the effect of hot gases. In the damaged areas 52, 54, the abradable coating 50 is damaged, i.e. its thickness is reduced compared to the original thickness of the abradable coating 50. However, in some cases in the areas damaged, the abradable coating 50 may have been completely removed and the ring 24 is then exposed.

To repair the ring sector 26 of which the abradable coating 50 is damaged, the abradable coating 50 is removed, for example by machining, and an abradable plate 32 is brazed, for example at 1205 ° C and under vacuum, on the internal surface. 28 of ring sector 26.

As shown on figure 7A , the ring sector 26 comprising a brazed abradable plate 32 is then mounted to form the ring 24. FIG. figure 7A a ring sector 26 comprising a brazed abradable plate 32 disposed between two ring sectors 26 comprising an abradable coating 50. Once these turbine ring sectors 26 have been assembled, the abradable plate 32 has a free surface 34 which cannot be not be in the extension of the free surfaces 56 of the abradable coatings 50 of the adjacent ring sectors 26. Also, the free surfaces 34, 56 of the different ring sectors 26 are machined so as to present a machined surface 58 intended to face the turbine wheel. As shown on figure 7B , this machined surface 58 has the least possible discontinuity. Indeed, if such discontinuities are present, the blade wheel could come up against these discontinuities and thus cause shocks in the turbine, which is not desirable.

We have represented on the figures 7A and 7B a single ring sector 26 on which an abradable plate 32 has been brazed. Of course, several ring sectors 26 can be repaired, or even all of the ring sectors 26. The repaired ring sectors 26 may or may not be adjacent.

When the ring 24 is not divided or divisible into sectors, part of the abradable coating 50 can be removed from the ring corresponding to an abradable plate 32 and the abradable plate 32 can be soldered to the internal surface 28 of the ring 24 One can also remove the damaged part of the abradable coating 50 and cut or assemble several abradable plates 32 to cover the internal surface 28 of the ring thus exposed.

The inner surface 28 of the ring and the vanes are again effectively protected by an abradable coating 50 and an abradable plate 32 brazed to the ring. The ring 24 is therefore repaired.

Although the present disclosure has been described with reference to a specific exemplary embodiment, it is obvious that various modifications and changes can be made to these examples without departing from the general scope of the invention as defined by the claims.

Claims (8)

1. A method for manufacturing at least two abradable plates (32) for a turbomachine turbine shroud (24, 26), the method comprising the following steps:

   - preparing a mixture comprising a cobalt- or nickel-based metal powder and a powder based on a fluxing element;

   - depositing a layer of the powder mixture in a mold (42); and

   - making the abradable plate (32) by subjecting the powder mixture layer to a method of SPS sintering; and

   wherein at least two layers of the powder mixture are deposited in the mold (42), the two layers being spaced apart from each other by a chemically inert insert (40), so that the layers of powder mixture do not sinter to one another and are forming abradable plates that do not stick together,
   wherein the fluxing element is silicon or boron.
2. A method according to claim 1, wherein the chemically inert insert (40) comprises boron nitride or corundum.
3. A method according to claim 2, wherein boron nitride forms an outer layer of the chemically inert insert (40).
4. A method according to any preceding claim, wherein the powder mixture comprises a percentage by weight of the fluxing element that is less than or equal to 5% by weight, preferably less than or equal to 3% by weight.
5. A method according to any one of claims 1 to 4, wherein the mold (42) is made of graphite, and wherein the SPS sintering is performed at a temperature higher than or equal to 800°C, preferably higher than or equal to 900°C.
6. A method according to any one of claims 1 to 4, wherein the mold (42) is made of tungsten carbide, and wherein the SPS sintering is performed at a temperature higher than or equal to 500°C, preferably higher than or equal to 600°C.
7. A repairing method for repairing a turbine shroud (24) for a turbomachine, the method comprising the following steps:

   - removing a damaged abradable coating (50); and

   - brazing onto the turbine shroud (24, 26) an abradable plate (32) obtained in accordance with any preceding claim.
8. A repairing method according to claim 7, wherein after the abradable plate (32) has been brazed onto the turbine shroud (24, 26), a free surface (34) of the brazed abradable plate (32) is machined.

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