METHOD FOR MANUFACTURING AN ABRADABLE PLATE AND REPAIRING A TURBINE SHROUD

Abstract

The invention relates to a method for manufacturing an abradable plate (32) for a turbomachine turbine shroud (24, 26), the method comprising 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, and making the abradable plate (32) by subjecting the powder mixture layer to a method of SPS sintering. The invention also provides a method of preparing a turbine shroud (24, 26) for a turbomachine.

Description

BACKGROUND OF THE INVENTION

The present disclosure relates to a method for manufacturing a turbine shroud for a turbomachine.

In numerous rotary machines, it is now known to provide the ring of the stator with abradable tracks facing the tips of the blades of the rotor. Such tracks are made using so-called “abradable” materials, which, when they come into contact with rotating blades, become worn more easily than the blades themselves. This serves to ensure minimum clearance between the rotor and the stator, thereby improving the performance of the rotary machine, without running the risk of damaging the blades in the event of them rubbing against the stator. On the contrary, such rubbing erodes the abradable track, thereby acting automatically to match the diameter of the ring of the stator as closely as possible to the rotor. Thus, such abradable tracks are often installed in turbomachine compressors.

In contrast, use of such tracks is less common in the turbines of such turbomachines, and in particular in the high pressure turbines in which physico-chemical conditions are extreme.

Specifically, the burnt gas coming from the combustion chamber flows into the high-pressure turbine at very high levels of temperature and pressure, thereby leading to premature wear of conventional abradable tracks.

Under such circumstances, in order to protect the turbine shroud, it is often preferred to provide it with a thermal barrier type coating made of materials that serve to protect the shroud against erosion and corrosion and that present density that is high, too high for the coating to be effectively abradable.

Nevertheless, under such circumstances, it can naturally be understood that the integrity of the blades is no longer ensured in the event of coming into contact with the stator, which makes it necessary to provide greater clearance between the rotor and the stator, and therefore increases the rate of leakage past the tips of the blades, thus reducing the performance of the turbine.

Furthermore, because of spots of rubbing against the blades and because of the temperature of the burnt gas, the coating can become damaged, thereby providing the stator with less protection.

OBJECT AND SUMMARY OF THE INVENTION

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

To this end, the present disclosure relates to a method for manufacturing an abradable plate for a turbomachine turbine shroud, 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; and

making the abradable plate by subjecting the powder mixture layer to a method of SPS sintering.

The term “cobalt-based” is used to mean a metal powder in which cobalt presents the greatest percentage by weight. Likewise, the term “nickel-based” is used to mean a metal powder in which nickel presents the greatest percentage by weight. Thus, by way of example, a metal powder comprising 38% by weight cobalt and 32% by weight nickel is referred to as a cobalt based powder, since cobalt is the chemical element having the greatest percentage by weight in the metal powder.

Cobalt- or nickel-based metal powders are powders that present good high-temperature strength after sintering. They can thus perform the two functions of being abradable and of providing a heat shield. For example, mention may be made of CoNiCrAlY superalloys. These metal powders also have the advantage of presenting a chemical composition that is similar to the chemical composition of the material forming the turbine shroud, e.g. AM1 or N5 superalloy.

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

The SPS sintering method (SPS standing for “spark plasma sintering”) is also known as field assisted sintering technology (FAST), or as flash sintering, and it is a method of sintering during which a powder is subjected simultaneously to high-current pulses and to uniaxial pressure in order to form a sintered material. SPS sintering is generally performed under a controlled atmosphere, and it may be assisted by heat treatment.

The duration of SPS sintering is relatively short, and SPS sintering makes it possible to select starting powders with relatively few limitations. Specifically, SPS sintering makes it possible in particular to sinter, i.e. to densify, materials that are relatively complicated to weld, or indeed impossible to weld, because they are materials that crack easily when heated. As a result of selecting SPS sintering and of the short duration of such sintering, it becomes possible to make an abradable layer out of a very wide variety of materials.

Furthermore, since SPS sintering is performed under uniaxial pressure exerted by the mold on the powder layer, the shrinkage of the powder layer that results from the sintering for producing the abradable plate is restricted to the direction in which pressure is applied. No shrinkage of the powder layer is thus to be observed in directions perpendicular to the direction in which pressure is applied. Thus, it is relatively simple to control the dimensions of the abradable plate.

It is possible to deposit at least two layers of the powder mixture in the mold, the two layers being spaced apart from each other by a chemically inert insert.

Is thus possible to make a plurality of abradable plates in a single SPS sintering step. By way of example, it is thus possible to deposit ten layers of powder mixture, each layer being separated from an adjacent layer by a chemically inert insert. It is thus possible to form ten abradable plates, each having thickness that may lie in the range 1 millimeter (mm) to 5 mm, each of the abradable plates being separated from an adjacent abradable plate by a chemically inert insert.

During SPS sintering, the chemically inert insert makes it possible to reduce chemical reactions between the layers of powder mixtures or indeed to eliminate them.

Since each layer of powder mixture is separated from the adjacent layer by a chemically inert insert, the layers of powder mixture do not sinter to one another and it is therefore easier to make a plurality of abradable plates that do not stick together.

The chemically inert insert may also be arranged between the powder mixture and the mold.

During SPS sintering, the chemically inert insert makes it possible to reduce chemical reactions between the layer of powder mixture and the mold, or even to eliminate them, and thus to reduce any sticking of the abradable plate to portions of the mold, or even to eliminate any such sticking.

The chemically inert insert also makes it possible to reduce the formation of a layer of carbide at the surface of the abradable plate that is in contact with the mold, or even to eliminate any such formation. It is desirable to avoid forming such a carbide layer since any carbide layer that is formed needs to be removed from the abradable plate before it is used.

The chemically inert insert may comprise boron nitride or corundum.

When the chemically inert insert is said to “comprise” boron nitride, that is used to mean that the insert comprises at least 95% by weight boron nitride. Likewise, when the chemically inert insert is said to “comprise” corundum, that is used to mean that the insert comprises at least 95% by weight corundum.

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

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

The chemically inert insert may be a plate of dense material covered by a layer of boron nitride deposited onto the plate by means of a spray.

The fluxing element may be silicon or boron.

The powder mixture may comprise 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.

The mold may be made of graphite, and the SPS sintering may be performed at a temperature higher than or equal to 800° C., preferably higher than or equal to 900° C.

The SPS sintering is performed at a pressure higher than or equal to 10 megapascals (MPa), preferably higher than or equal to 20 MPa, still more preferably higher than or equal to 30 MPa.

The mold may be made of tungsten carbide, and the SPS sintering may be performed at a temperature higher than or equal to 500° C., preferably higher than or equal to 600° C.

The SPS sintering may be performed at a pressure higher than or equal to 100 MPa, preferably higher than or equal to 200 MPa, still more preferably higher than or equal to 300 MPa.

The present disclosure also relates to a repair method for repairing a turbomachine turbine shroud, the method comprising the following steps:

removing a damaged abradable coating; and

brazing onto the turbine shroud an abradable plate obtained by the above-defined method.

The fluxing element included in the powder mixture used for forming the abradable plate also serves to facilitate the method of brazing the abradable plate onto the turbine shroud.

Brazing the abradable plate onto the turbine shroud makes it possible to avoid depositing a new abradable coating directly onto the shroud or onto the shroud sector.

Specifically, after the abradable plate has been brazed onto the turbine shroud, a free surface of the brazed abradable plate may be machined.

An abradable plate that has just been brazed onto the turbine shroud may present a free surface that need not necessarily extend the free surface of the adjacent undamaged abradable coating. Thus, the free surfaces of the abradable plate and of the abradable coating are machined so as to present a surface for facing the turbine wheel that presents as little discontinuity as possible. Specifically, if any such discontinuity is present, then the turbine wheel could strike against such a discontinuity, thereby leading to impacts within the turbine, which is not desirable.

BRIEF DESCRIPTION OF THE DRAWINGS

Other characteristics and advantages of the invention appear from the following description of implementations of the invention, given as nonlimiting examples, and with reference to the accompanying figures, in which:

FIG. 1 is a diagrammatic longitudinal section view of a turbomachine;

FIG. 2 is a diagrammatic perspective view of a turbine shroud sector including an abradable plate;

FIG. 3 is a diagrammatic perspective view of a stack of abradable plates and of chemically inert inserts;

FIG. 4 is a diagrammatic section view of a stack in the mold for SPS sintering, on a section plane similar to the section plane IV-IV of FIG. 3;

FIGS. 5A-5D are scanning electron microscope images showing the microstructure of the various abradable plates;

FIG. 6 is a diagrammatic view of a shroud sector including a damaged abradable coating; and

FIGS. 7A and 7B are diagrammatic side views of a turbine shroud in which a portion of the abradable coating has been replaced by an abradable plate, shown respectively before and after machining a free surface of the abradable plate.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a bypass jet engine 10 seen in section on a vertical plane containing its main axis A. From upstream to downstream in the flow direction of the air stream, the bypass jet engine 10 comprises a fan 12, a low-pressure compressor 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 has a plurality of blades 20A that rotate with the rotor, and vanes 20B that are mounted on the stator. The stator of the turbine 20 has a plurality of stator shrouds 24 arranged facing the blades 20A of the turbine 20.

As can be seen in FIG. 2, each stator shroud 24 is made up of a plurality of shroud sectors 26. Each shroud sector 26 has an inner surface 28, an outer surface 30, and an abradable plate 32 against which the blades 20A of the rotor come into rubbing contact.

The abradable plate 32 is brazed onto the shroud sector 26. The abradable plate 32 has a free surface 34 and a surface 36 that is to be brazed onto the shroud sector 26.

By way of example, the shroud sector 26 is made of a cobalt- or nickel-based superalloy, such as the AM1 superalloy or the N5 superalloy, and the abradable plate 32 is obtained from a metal powder based on cobalt or on nickel.

In the described implementation, the shroud 24 is made up of a plurality of shroud sectors 26 that are assembled to one another in order to form a shroud 24. The shroud 24 could equally well be made as a single piece.

In order to fabricate an abradable plate 32, a mixture is prepared comprising a cobalt- or nickel-based metal powder and a powder based on a fluxing element. By way of example, the cobalt- or nickel-based powder may be a powder of the CoNiCrAlY family, and the fluxing element may be boron or silicon. By way of example, the powder mixture may comprise 2% by weight of boron.

As shown in FIGS. 3 and 4, the powder mixture is deposited in the form of layers in an SPS sintering mold 42. By way of example, the mold 42 is made of graphite. The mold 42 comprises an outer mold 44 forming a chamber in which the powder mixture is deposited. The mold 42 also has a top piston 46 and a bottom piston 48 that serve to apply axial pressure on the layers of powder mixture during the SPS sintering step.

FIG. 3 shows a stack 38 comprising two abradable plates 32 with a first chemically inert insert 40 inserted between them. In this example, a second chemically inert insert 40 and a third chemically inert insert 41 are also arranged on either side of the stack 38 such that each layer of powder mixture is sandwiched between two chemically inert inserts 40. By way of example, the chemically inert inserts 40 may be made from plates of sintered boron nitride.

In the implementation of FIGS. 3 and 4, each abradable plate 32 is obtained by depositing a layer of powder mixture between two chemically inert inserts 40 and by performing an SPS sintering step.

FIGS. 3 and 4 show two stacks 38 after SPS sintering, the stacks respectively comprising two and four abradable plates 32.

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

The chemically inert inserts 40 may also be made out of a material other than boron nitride. The chemically inert inserts 40 may optionally be covered in a layer of boron nitride.

The chemically inert inserts 40, whether in the form of plates or in the form of layers, serve to reduce 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 to avoid, any sticking of the powder mixture layer to portions of the mold before SPS sintering, and also any sticking of the abradable plate 32 to portions of the mold 42 after SPS sintering.

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

It can be understood that the thickness of the abradable plate 32 obtained after SPS sintering depends in particular on the thickness of each layer of powder mixture deposited in the mold 42, and also on the parameters of SPS sintering. The thickness of the abradable plate 32 obtained after SPS sintering may also depend on the grain size and on the morphology of the powder used. In particular, the morphology of the powder may depend on the method for manufacturing the powder. Thus, a powder fabricated by gaseous atomization or by a rotating electrode has grains of substantially spherical shape, while a powder fabricated by liquid atomization has grains of shape that is less regular.

FIGS. 5A-5D show various microstructures of abradable plates 32 presenting respective apparent porosities of about 10%, about 7%, about 3%, and practically zero.

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

FIG. 6 is a plan view of a shroud sector 26 including a damaged abradable coating 50. The abradable coating 50 may have been obtained by the method described above. The abradable coating 50 could also have been deposited directly on the shroud sector 26 by using a known method.

In the example of FIG. 6, the abradable coating 50 includes a zone 52 of damage due to rubbing, e.g. between a blade and the abradable coating 50, and a zone 54 of damage due to thermal degradation of the abradable coating 50 under the effect of hot gas. In the damaged zones 52, 54, the abradable coating 50 is damaged, i.e. its thickness has been reduced compared with the original thickness of the abradable coating 50. Nevertheless, in certain circumstances, in the damaged zones, the abradable coating 50 may have been removed completely, so that the shroud 24 is then exposed.

In order to repair the shroud sector 26 having the damaged abradable coating 50, the abradable coating 50 is removed, e.g. by machining, and then an abradable plate 32 is brazed, e.g. at 1205° C. in a vacuum, onto the inner surface 28 of the shroud sector 26.

As shown in FIG. 7A, the shroud sector 26 including a brazed abradable plate 32 is then assembled so as to form the shroud 24. FIG. 7A shows a shroud sector 26 having a brazed abradable plate 32 that is arranged between two shroud sectors 26, each having an abradable coating 50. Once the turbine shroud sectors 26 have been assembled together, the abradable plate 32 presents a free surface 34 that need not necessarily extend the free surfaces 56 of the abradable coatings 50 of the adjacent shroud sectors 26. Thus, the free surfaces 34, 56 of the various shroud sectors 26 are machined so as to present a machined surface 58 that is to face the turbine wheel. As shown in FIG. 7B, the machined surface 58 presents as little discontinuity as possible. Specifically, if any such discontinuity is present, then the turbine wheel could strike against such a discontinuity, thereby leading to impacts within the turbine, which is not desirable.

FIGS. 7A and 7B show a single shroud sector 26 having an abradable plate 32 brazed thereon. Naturally, a plurality of shroud sectors 26 could be repaired, or indeed all of the shroud sectors 26. The repaired shroud sectors 26 may be adjacent or otherwise.

When the shroud 24 is not divided or divisible into sectors, it is possible to remove a portion of the abradable coating 50 of the shroud that corresponds to an abradable plate 32 and then to braze the abradable plate 32 onto the inner surface 28 of the shroud 24. It is also possible to remove the damaged portion of the abradable coating 50 and to cut down an abradable plate 32 or to assemble together a plurality of abradable plates 32 in order to cover the inner surface 28 of the shroud that has been laid bare in this way.

The inner surface 28 of the shroud and the blades are once more protected effectively by means of an abradable coating 50 and an abradable plate 32 brazed onto the shroud. The shroud 24 is thus repaired.

Although the present disclosure is described with reference to a specific implementation, it is clear that various modifications and changes may be undertaken on those implementations without going beyond the general ambit of the invention as defined by the claims. Also, individual characteristics of the various implementations mentioned above may be combined in additional implementations. Consequently, the description and the drawings should be considered in a sense that is illustrative rather than restrictive.

Claims

1. A method for manufacturing an abradable plate for a turbomachine turbine shroud, 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; and

making the abradable plate 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, the two layers being spaced apart from each other by a chemically inert insert.

2. A method according to claim 1, wherein the chemically inert insert comprises boron nitride or corundum.

3. A method according to claim 2, wherein boron nitride forms an outer layer of the chemically inert insert.

4. A method according to claim 1, wherein the fluxing element is silicon or boron.

5. A method according to claim 1, wherein the powder mixture comprises a percentage by weight of the fluxing element that is less than or equal to 5% by weight.

6. A method according to claim 1, wherein the mold is made of graphite, and wherein the SPS sintering is performed at a temperature higher than or equal to 800° C.

7. A method according to claim 1, wherein the mold is made of tungsten carbide, and wherein the SPS sintering is performed at a temperature higher than or equal to 500° C.

8. A repairing method for repairing a turbine shroud for a turbomachine, the method comprising the following steps:

removing a damaged abradable coating; and

brazing onto the turbine shroud an abradable plate obtained in accordance with claim 1.

9. A repairing method according to claim 8, wherein after the abradable plate has been brazed onto the turbine shroud, a free surface of the brazed abradable plate is machined.