EP3429784B1 - Method for manufacturing a turbine shroud for a turbomachine - 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 tip of the rotor blades. Such tracks are made using so-called "abradable" materials which, when they come into contact with the rotating blades, wear more easily than the latter. This ensures minimal clearance between the rotor and the stator, improving the performance of the rotating machine, without risking damage to the blades 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 enter the high-pressure turbine at very high temperature and pressure levels, which causes premature erosion of conventional abradable tracks.

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

However, it is naturally understood that in such a case the integrity of the blades is no longer ensured in the event of contact with the stator, which requires providing a greater clearance between the rotor and the stator and therefore increases the leakage flow at the tip of the blades and thus reduces the performance of the turbine. US 2013/177740 A1 discloses a method for the simultaneous manufacture of a porous/abradable layer on an element of a turbomachine, including a turbine ring, by the SPS method. FR 2 941 965 A1 discloses a method in which a protective layer on a turbomachine part (blade) is deposited by the SPS method in which the powder is deposited on a curved surface of the turbomachine part and an undercut is positioned on the powder before SPS sintering.

Subject and summary of the invention

This presentation aims to remedy at least part of these drawbacks.

For this purpose, the present disclosure relates to a method of manufacturing a turbine ring for a turbomachine according to claim 1.

The turbine ring is usually made in several parts, each part forming a turbine ring sector of reduced dimensions compared to the dimensions of the complete turbine ring. It is therefore simple to arrange a ring sector in a mold.

The inner surface of the turbine ring sector is the surface that faces the turbine wheel when the turbine ring is mounted in the turbine, so it is this inner surface on which the powder layer is deposited.

The SPS sintering process, in accordance with the English acronym for "Spark Plasma Sintering", also known as 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 simultaneously subjected to a high intensity pulsed current and uniaxial pressure in order to form a sintered material. SPS sintering is generally carried out in a controlled atmosphere and can be assisted by a heat treatment.

The SPS sintering time is relatively short and SPS sintering allows a relatively unrestricted choice of starting powders. Indeed, SPS sintering allows in particular to sinter, i.e. to densify, materials whose welding is relatively complicated to achieve, or even impossible, because these materials crack easily when 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.

Furthermore, since SPS sintering is performed under uniaxial pressure exerted by the lower mold and the upper mold on the powder layer, the shrinkage due to sintering of the powder layer to give the abradable layer is limited to the direction of application of the pressure. Therefore, no shrinkage of the powder layer is observed in directions perpendicular to the direction of application of the pressure. Also, the abradable layer covers the entire internal surface of the ring sector.

The turbine ring is therefore covered with an abradable layer. It is therefore possible to provide a relatively small clearance between the turbine ring and the rotor, for example the blades of a turbine wheel, and to improve the performance of the turbine, without risking damage to the blades in the event of their rubbing on the stator ring.

In addition, SPS sintering allows the formation of a diffusion layer between the abradable layer and the material forming the ring sector so that the abradable layer is firmly attached to the material forming the ring sector. The abradable layer thus formed cannot be removed from the ring sector unintentionally.

• assemblage d'une pluralité de secteurs d'anneau de turbine, la surface interne de chaque secteur d'anneau de turbine étant recouverte d'une couche d'abradable ; et
• usinage d'une surface libre de la couche d'abradable.

The method may further comprise the following steps:

• assembling a plurality of turbine ring sectors, the inner surface of each turbine ring sector being covered with an abradable layer; and
• machining of a free surface of the abradable layer.

Once several turbine ring sectors are assembled, the abradable layer of each ring sector has a free surface which may not be in line with the free surface of the adjacent ring sector. Also, the free surfaces of the different ring sectors are machined so as to present a surface intended to face the turbine wheel which 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.

The lower mold may have a shape complementary to the outer surface of the turbine ring sector.

Thus, the lower mold applies a relatively uniform pressure to the outer surface of the ring sector. However, since the lower mold has a shape complementary to the outer surface of the ring sector, the mold can accommodate variations in dimensions from one ring sector to another due to the manufacturing process of the ring sector. Indeed, the turbine sectors can for example be obtained by a casting process and the dimensions of each turbine sector can vary slightly from one turbine sector to another.

Before positioning the turbine ring sector in the lower mold and the upper mold, a layer of chemically inert material can be deposited on the lower mold and on the upper mold.

This layer of chemically inert material makes it possible to reduce chemical reactions between the powder layer and the turbine ring sector with the lower mold and the upper mold during SPS sintering. In particular, the chemically inert material makes it possible to reduce or even avoid sticking of the abradable layer and/or the ring sector with the mold parts.

The chemically inert material also makes it possible to reduce, or even avoid, the formation of a carbide layer on the free surface of the abradable layer. The aim is to avoid the formation of this carbide layer which, if it forms, must be removed from the abradable layer before use.

In the lower mold, the chemically inert material can also be used to fill existing gaps between the lower mold and the outer surface of the turbine ring sector. Thus, the uniformity of the pressure exerted by the lower mold on the turbine ring sector and therefore on the powder layer is improved.

The chemically inert material may for example comprise boron nitride or corundum. A chemically inert material comprising boron nitride is understood to mean a material which comprises at least 95% by mass of boron nitride. Similarly, a chemically inert material comprising corundum is understood to mean a material which comprises at least 95% by mass of corundum.

The powder is a metallic powder based on cobalt or nickel.

"Cobalt-based" means a metal powder in which cobalt is the largest mass percentage. Similarly, "nickel-based" means a metal powder in which nickel is the largest mass percentage. Thus, for example, a metal powder containing 38% by mass of cobalt and 32% by mass of nickel will be referred to as a cobalt-based powder, cobalt being the chemical element in which the largest mass percentage is present in the metal powder.

Cobalt or nickel-based metal powders are powders that, once sintered, have good resistance to high temperatures. They can thus fulfill the dual function of abradable and heat shield. For example, we can cite 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 AM1 or N5 superalloys.

SPS sintering can be carried out for a period of less than or equal to 60 minutes, preferably less than or equal to 30 minutes, even more preferably less than or equal to 15 minutes.

The SPS sintering time is therefore relatively short.

The upper mold and the lower mold are 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 is 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 abradable layer may have an open porosity less than or equal to 20%, preferably less than or equal to 15%, even more preferably less than or equal to 10%.

Thanks to the SPS sintering process, it is possible, by varying the sintering parameters such as pressure, sintering temperature and/or sintering time, to vary the porosity of the abradable layer obtained. This manufacturing process for a turbine ring for a turbomachine therefore allows great flexibility.

The abradable layer may have a thickness greater than or equal to 0.5 mm, preferably greater than or equal to 4 mm and less than or equal to 15 mm, preferably less than or equal to 10 mm, even more preferably less than or equal to 5 mm.

The turbine ring may comprise a number of turbine ring sectors greater than or equal to 20, preferably greater than or equal to 30, even more preferably greater than or equal to 40.

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 couche d'abradable ;
• la figure 3 est vue en coupe d'un secteur d'anneau de turbine dans un moule pour frittage SPS, selon un plan de coupe similaire au plan de coupe III-III de la figure 2 ;
• les figures 4A et 4B sont des vues schématiques latérales de plusieurs secteurs d'anneau de turbine recouverts d'une couche d'abradable, respectivement avant et après usinage d'une surface libre de la couche d'abradable ;
• la figure 5 est une image réalisée au microscope électronique à balayage d'une interface entre un secteur d'anneau et une couche d'abradable ;
• la figure 6 représente l'évolution de la concentration de certains éléments chimiques de la couche d'abradable au secteur d'anneau ;
• les figures 7A-7D sont des images réalisées au microscope électronique à balayage de la microstructure de différentes couches d'abradable.

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

• there figure 1 is a schematic longitudinal sectional view of a turbomachine;
• there figure 2 is a schematic perspective view of a turbine ring sector having an abradable layer;
• there figure 3 is a sectional view of a turbine ring sector in a mold for SPS sintering, according to a section plane similar to section plane III-III of the figure 2 ;
• THE Figures 4A and 4B are schematic side views of several turbine ring sectors covered with an abradable layer, respectively before and after machining a free surface of the abradable layer;
• there figure 5 is a scanning electron microscope image of an interface between a ring sector and an abradable layer;
• there figure 6 represents the evolution of the concentration of certain chemical elements from the abradable layer to the ring sector;
• THE Figures 7A-7D are images made using a scanning electron microscope of the microstructure of different layers of abradable.

Detailed description of the invention

There figure 1 represents, in section along a vertical plane passing through its main axis A, a dual-flow turbojet 10. The dual-flow turbojet 10 comprises, from upstream to downstream according to the circulation of the air flow, 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 comprises a plurality of moving 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 opposite the moving blades 20A of the turbine 20.

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

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

The method of manufacturing the turbine ring 24 comprises a first step of manufacturing at least one turbine ring sector 26, for example by a foundry method.

There figure 3 shows a sectional view of the turbine ring sector 26 in a mold for SPS sintering. The mold comprises a lower mold 34 of a shape complementary to the external surface 30 of the ring sector 26.

The ring sector 26 is positioned in a lower mold 34 such that the outer surface 30 of the ring sector 26 is in contact at least partially with the lower mold 34. The lower mold 34 is therefore not in contact with the ring sector 26 over the entire outer surface 30 of the ring sector 26. The visible spaces between the ring sector 26 and the lower mold 34 make it possible to accommodate the variations in dimensions due to the manufacturing process of the different ring sectors 26. However, the shape of the lower mold 34 being complementary to the outer surface 30 of the ring sector 26, the pressure exerted by the lower mold 34 on the ring sector 26 is relatively uniform.

A layer of powder 36 is then deposited on the internal surface 28 of the ring sector 26 and the upper mold 38 is positioned on the layer of powder 36.

The SPS sintering step is then carried out, which makes it possible to obtain an abradable layer 32 made directly on the ring sector 26. For example, the upper mold 38 and the lower mold 34 can be made of graphite. They can also be made of tungsten carbide.

Before positioning the ring sector 26 in the lower mold 34, a layer of chemically inert material may be deposited in the lower mold 34 and on the upper mold 38. For example, the chemically inert material may be boron nitride applied using a spray. Boron nitride powder may also be added to fill the spaces between the ring sector 26 and the lower mold 34.

The chemically inert material can also be corundum.

The ring sector 26 coated with the abradable layer 32 is then removed from the mold.

As shown in the Figure 4A , to form a complete ring 24, several ring sectors 26 are assembled together, each ring sector 26 being covered with a layer of abradable 32. Once these turbine ring sectors 26 are assembled, the layer of abradable 32 of each ring sector has a free surface 44 which may not be in the extension of the free surface 44 of the adjacent ring sector 26. Also, the free surfaces 44 of the different ring sectors 26 are machined so as to have a machined surface 46 intended to face the turbine wheel. This machined surface 46 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.

There figure 5 is an image taken using a scanning electron microscope of an interface between a ring sector 26 and an abradable layer 32. For example, this abradable layer 32 is sintered on the ring sector 26 at 950°C, under a pressure of 40 MPa for 30 minutes.

The pressure can be applied cold, that is to say from the start of the cycle, or hot, during the sintering stage.

As can be seen from the figures 5 And 6 , the chemical composition evolves progressively, along line 40 of the figure 5 , starting from the ring sector 26 towards the abradable layer 32, defining a diffusion zone 42 at the interface between the ring sector 26 and the abradable layer 32.

THE Figures 7A-7D represent different microstructures of abradable layers 32 whose open porosity is respectively approximately 10%, approximately 7%, approximately 3% and almost zero.

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

It is understood that the thickness of the abradable layer 32 obtained after SPS sintering depends in particular on the thickness of the powder layer 36 deposited on the internal surface 28 of the ring sector 26 as well as on the SPS sintering parameters. The thickness of the abradable layer 32 obtained after SPS sintering may also depend on the particle size and morphology of the powder used. In particular, the morphology of the powder may depend on the method of manufacturing the powder. Thus, a powder manufactured by gas atomization or rotating electrode will have grains of substantially spherical shape while a powder manufactured by liquid atomization will have grains of less regular shape.

Although the present disclosure has been described with reference to a specific exemplary embodiment, it is obvious that various modifications and changes may be made to these examples without departing from the general scope of the invention as defined by the claims. Furthermore, individual features of the various embodiments recited may be combined in additional embodiments. Accordingly, the description and drawings are to be considered in an illustrative rather than restrictive sense.

Claims (5)

1. A method for manufacturing a turbine ring (24) for a turbomachine, the method comprising the following steps:

   · manufacturing at least one turbine ring sector (26) ;

   · positioning the turbine ring sector (26) in a bottom mold (34) so that an outer surface (30) of the turbine ring sector (24) is in contact at least in part with the bottom mold (34);

   · depositing a powder layer (36) on an inner surface (28) of the turbine ring sector (26), the powder being a metal powder based on cobalt or on nickel ;

   · positioning a top mold (38) on the powder layer (36); and

   · making an abradable layer (32) on the inner surface (28) by subjecting the powder layer (36) to a method of SPS sintering, the abradable layer (32) being intended to be disposed facing a turbine wheel;

   wherein the top mold (38) and the bottom mold (34) are 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 and wherein the SPS sintering is 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.
2. A method according to claim 1, further comprising the following steps:

   · assembling together a plurality of turbine ring sectors (26), the inner surface (28) of each turbine ring sector (26) being covered in an abradable layer (32); and

   · machining a free surface (44) of the abradable layer (32).
3. A method according to claim 1 or claim 2, wherein the bottom mold (34) is of shape complementary to the outer surface (30) of the turbine ring sector (26).
4. A method according to any one of claims 1 to 3, wherein before positioning the turbine ring sector (26) in the bottom mold (34) and the top mold (38), a layer of chemically inert material is deposited on the bottom mold (34) and on the top mold (38) in order to reduce the layer of abradable material and/or the ring sector sticking to portions of the mold.
5. A method according to any preceding claim, wherein the SPS sintering is performed for a duration shorter than or equal to 60 minutes, preferably shorter than or equal to 30 minutes, still more preferably shorter than or equal to 15 minutes.

Images