PROCESS FOR EXTRACTING A FOREIGN BODY NESTED IN A HIGH PRESSURE DISTRIBUTOR BLADE

TECHNICAL AREA

The present invention relates to a method for extracting a foreign body from a high pressure turbine engine distributor blade. It also relates to a method for detecting the presence of a foreign body in a high pressure distributor blade cavity.

The invention finds applications in the field of aeronautics and, in particular, in the field of the manufacture of high pressure distributor blades.

STATE OF THE ART

In an airplane turbojet, the high pressure distributor 100, an example of which is shown in FIG. 1, is a set of fixed blades 110 arranged between two coaxial annular platforms 120, called shrouds, and delimiting a flow path of the gas between the outlet of the combustion chamber and the inlet of the high pressure turbine. The blades 110, generally hollow, each comprise two adjacent cavities, one end of which opens out to the outside of the gas flow stream. They also have holes extending between each cavity and the vein.

In some high pressure distributors, each blade 1 10 has a sleeve embedded in each of the cavities, and pierced over its entire surface extending between the annular platforms 120, with a plurality of holes opening into the cavity. The purpose of this jacket is in particular to cool the blade in which it is embedded. In some high pressure distributors, such as the one shown in FIG. 2, it is the hollow blades which are drilled. Whether pierced or provided with a jacket, each blade cavity is closed by a closure plate 140 which regulates the cooling of said blade by obstructing the orifices of the cavity in a controlled manner.

The blades of aircraft jet engines are conventionally made from a nickel-based metal superalloy called AM1 (NiCr7Co8W5A15Ta8Ti). However, given their location, the high pressure distributor blades - also called DHP - are subjected to very high gas temperatures, in particular during the so-called "hot" operating phases of the reactor such as, for example, the take-off phase. To withstand these very high temperatures, DHP blades in AM1 are generally covered with an insulating coating forming a thermal barrier - also called BT. It is known that the thermal barrier consists of several layers each providing a distinct function. The outer layer, that is to say the most exposed to hot gases, of the thermal barrier is usually a ceramic layer. The main function of this ceramic layer is thermal insulation. The inner layer of the thermal barrier, that is to say the layer in contact with IΆM1, is a sub-layer generally in an aluminum and platinum alloy such as AIPt. The function of this sublayer is to protect the blade against oxidation and to maintain the thermal barrier on AM1.

Generally, DHP blades are manufactured according to the process shown in FIG. 3. This process firstly consists of a step 210 of producing a cast iron blade shape. At the end of this step 210, the raw blade has two cavities which are pierced, in step 220, so that they can subsequently ensure the cooling of the blade. When each cavity is fitted with a jacket, the liners are inserted into the cavities of the blade in step 230. A closure plate is then brazed above each of the cavities to close said cavities (step 240). Once the cavities of the blades are closed, a thermal barrier is deposited on the outer surface of each blade in order to protect said blade, in particular from heat. An x-ray check is then performed, in step 260, to check if the blade conforms to the specifications. This x-ray check makes it possible in particular to detect whether the cavities are empty of all foreign bodies resulting from the manufacturing operations.

Indeed, it is possible, both at the foundry stage and at the drilling, brazing or thermal barrier installation stages, that dust or other residues are deposited inside the blade, generating a defect on said blade. However, most faults result in the blade not conforming to the technical requirements in force and a non-conforming blade is discarded.

To date, it is indeed usual to discard any blade considered non-compliant following the detection of a foreign body in a cavity because it is considered that in order to remove the foreign body, it would be necessary first of all to remove the thermal barrier coating. the blade. One solution considered by aircraft jet manufacturers to rework a DHP blade is as follows:

- Strip the non-conforming blade so as to remove the thermal barrier covering said blade or at least the ceramic layer of said thermal barrier;

- Remove the closing plate;

- Remove the foreign body from the cavity;

- Solder a new closing plate on the blade in the bare state;

- Deposit a new thermal barrier; and

- Carry out a new x-ray check.

However, such a retouching process would have many drawbacks and, in particular, the fact that it would have an impact on the dimension of the blade insofar as the pickling of the part is equivalent to chemical machining of said part in AM1. Another drawback would be the risk of residue during the stripping of the thermal barrier (in particular if the AIPt underlayer is not removed), which could cause extra thicknesses during the installation of the new thermal barrier. Yet another drawback would be the risk of increased non-compliance of the blades due to the installation of the new coatings. In addition, the implementation of such a retouching process would be expensive because, the cost of a thermal barrier being high, the fact of applying this thermal barrier twice is expensive.

SUMMARY OF THE INVENTION

To respond to the problem mentioned above of discarding non-conforming blades due to a foreign body in a cavity, the applicant proposes a method of extracting the foreign body in which only the closure plate is cut and replaced by a new closing plate brazed directly on the blade coated with the thermal barrier.

According to a first aspect, the invention relates to a method for extracting at least one foreign body nestled within a blade of a high pressure distributor of a turbojet, said high pressure distributor comprising hollow and fixed blades between two annular shrouds forming a gas flow stream, each blade being coated with a thermal barrier and pierced with at least one cavity at least partially blocked by a first closure plate, brazed. This process is characterized by the fact that it comprises the following operations:

- cutting of the first closure plate,

- extraction of the foreign body out of the cavity,

- endoscopic control of an interior space of the cavity so as to check that no foreign body remains,

- positioning of a second closure plate on the cavity, and

- brazing of the second closing plate directly on the distributor with the blades coated with the thermal barrier.

This extraction process makes it possible to retouch a blade, considered non-compliant at the end of production, by removing the foreign bodies nested in said blade and causing its non-compliance, without any removal or stripping operation. the thermal barrier.

The term “foreign body” is used to refer to any dust, manufacturing residue or particle deposited or caught in a cavity of a blade of the high pressure distributor.

Advantageously, the cutting of the first wafer is carried out by grinding said wafer.

The extraction process according to the invention may include one or more of the following characteristics:

- The positioning of the second closure plate is carried out by pointing and micro balling on the high pressure distributor;

- the second closure plate is pre-sintered;

- The brazing of the second wafer is carried out in a vacuum oven, at a temperature below 1200 ° C;

- brazing comprises a thermal cycle carried out at a temperature between approximately 1160 ° C and 1200 ° C for 10 to 20 minutes then a diffusion cycle at a temperature between approximately 1150 ° C and 1200 ° C

for 2 to 4 hours;

- it comprises, after the brazing operation of the second closure plate, an x-ray check of the high pressure distributor

BRIEF DESCRIPTION OF THE FIGURES

Other advantages and characteristics of the invention will become apparent on reading the description, illustrated by the figures in which:

- Figure 1, already described, schematically shows a partial view of a high pressure distributor of the prior art;

- Figure 2, already described, schematically shows a partial view of a high pressure distributor blade of the prior art;

- Figure 3, already described, shows a functional diagram of the usual production process of high pressure distributor blades;

FIG. 4 represents a functional diagram of the method for extracting a foreign body according to the invention;

- Figure 5 shows an enlarged sectional view of the layers of materials forming the thermal barrier covering a high pressure distributor blade;

- Figures 6A and 6B show enlarged sectional views of a thermal barrier, respectively, before brazing and after re-brazing.

DETAILED DESCRIPTION OF AT LEAST ONE EMBODIMENT

An exemplary embodiment of a method of extracting a foreign body nestled in a cavity of a high pressure distributor blade is described in detail below, with reference to the accompanying drawings. This example illustrates the characteristics and advantages of the invention. It is however recalled that the invention is not limited to this example.

In the figures, identical elements are identified by identical references. For reasons of legibility of the figures, the size scales between elements represented are not respected.

FIG. 4 represents, in the form of a functional diagram, the various steps of the extraction process 300 making it possible to retouch a blade considered to be non-compliant after the inspection 260 under x-rays. This method can be applied to all parts made of AM1 covered with a thermal barrier such as, for example, a high pressure distributor blade 110 of the type shown in FIGS. 1 and 2. As explained previously, each distributor blade high pressure 110 - more simply called blade - has adjacent cavities whose walls can be pierced or inside which a jacket can be embedded. A closure plate 140 is mounted by soldering at the open end of each cavity of the blade in order to close said cavity and, optionally, to hold the jacket there.

According to the invention, a blade considered to be non-compliant on leaving production can be retouched and the foreign body (s) extracted. In order to extract any foreign body detected inside a blade cavity during the x-ray inspection carried out at the end of the blade manufacturing process, the extraction process 300 of the invention proposes to carry out a first operation 310 of cutting the original closing plate 140, also called the first plate. This cutting of the closure plate 140 consists in removing by means of a grinding wheel all the material forming the closure plate 140. The fact of grinding the closure plate makes it possible to remove only the material forming the closure plate, without removing nor degrade the thermal barrier covering the blade.

When the closure plate 140 has been completely ground, the foreign body (s) present in the cavity can be removed by means of a suitable tool such as a pin, hook, pliers, etc., during an extraction operation 320 The extraction process 300 can be continued with an endoscope inspection operation 330. This endoscopic inspection 330 makes it possible, by inserting an endoscope inside the cavity, to examine the interior space of the cavity and in particular its interior space. interior walls to verify that all foreign bodies have been extracted. It also makes it possible to check whether the grinding operation 310 of the first closure plate has not generated new dust or other foreign matter.

After this endoscopic inspection 330, a new closure plate 140, also called a second closure plate, is positioned in front of the opening of the cavity, approximately at the location where the first closure plate was located. This positioning of the new closure plate 140 is achieved by pointing and micro-balling said plate on the blade 110 of the high pressure distributor. The operation of pointing and micro balling of the closure plate consists in prefixing the plate on the high pressure distributor blade by means of welding points made, for example, with a solid electrode for pointing and a hollow electrode for the micro balling.

Once the new closure plate is correctly positioned, said plate can be brazed to the high pressure distributor during a brazing operation 350. During this brazing operation 350, the new closure plate is brazed directly to the coated distributor blade. of the thermal barrier. The new 140 closure plate is a classic, pre-sintered closure plate, identical to the original closure plate. Brazing is carried out under vacuum, in a suitable oven, at a brazing temperature not exceeding 1180 ° C +/- 10 ° C. The brazing conditions of the new closure plate 140 are conventional soldering conditions, in which the new closure plate has a frit (c ' is to say a powder agglomerated by compression and simultaneous heating below the melting temperature), and is subjected, with the high pressure distributor, to a thermal cycle and a diffusion cycle bringing the whole to a treatment temperature which is below its melting temperature but which is sufficient for the frit to melt and conglomerate the wafer with the high pressure distributor. The thermal cycle can include, for example, cooking under vacuum for 15 minutes at 1180 ° C. The diffusion cycle, initiated following the thermal cycle, may include, for example, cooking under vacuum for 3 hours at 1175 ° C. to a thermal cycle and a diffusion cycle bringing the assembly to a treatment temperature which is below its melting temperature but which is sufficient for the frit to melt and conglomerate the wafer with the high pressure distributor. The thermal cycle can include, for example, cooking under vacuum for 15 minutes at 1180 ° C. The diffusion cycle, initiated following the thermal cycle, may include, for example, cooking under vacuum for 3 hours at 1175 ° C. to a thermal cycle and a diffusion cycle bringing the assembly to a treatment temperature which is below its melting temperature but which is sufficient for the frit to melt and conglomerate the wafer with the high pressure distributor. The thermal cycle can include, for example, cooking under vacuum for 15 minutes at 1180 ° C. The diffusion cycle, initiated following the thermal cycle, may include, for example, cooking under vacuum for 3 hours at 1175 ° C.

Unlike the brazing operation of the first closure plate, during brazing 350 of the new closure plate, the thermal barrier is present on the high pressure distributor 100. An example of an enlarged sectional view of a portion of blade in AM1 coated with this thermal barrier is shown in FIG. 5. This example shows a layer 1 10c of AM1 forming the blade 1 10, covered with a sublayer 1 10b in AIPt and a layer of ceramic 110a forming the thermal barrier.

In the method of the invention, the high pressure distributor blade, which has been previously brazed with the original closure plate, is brazed a

second time during the brazing operation 350. During this second brazing operation 350, the blade 110 is coated with its thermal barrier. Although those skilled in the art would think that the thermal cycle generated to solder the new closure plate has a damaging effect on the thermal barrier of the high pressure distributor, in particular by diffusion of the underlayer, the applicant was surprised to find that 'it is not so. Indeed, on the one hand, the blade in AM1 is adapted to withstand a second thermal cycle without risk of modification of its structure. On the other hand, the ceramic layer (Zr02 / Y203) of the thermal barrier is suitable for undergoing thermal cycling at about 1180 ° C since its melting temperature exceeds 1200 ° C. Finally, as shown in Figures 6A and 6B, the sub-layer of the thermal barrier does not migrate inside the blade in AM1. Indeed, even if, under the brazing conditions stated above, the sublayer 110b changes slightly by diffusion, this has no impact on the conformity of the blade 110 of the high pressure distributor. An example of the AM1 structure with a thermal barrier before the second thermal cycle is shown in FIG. 6A. An example of the same AM1 structure with thermal barrier after the second thermal cycle (that is to say after re-brazing) is shown in FIG. 6B. A comparison of the two FIGS. 6A and 6B shows that, even if the sublayer 1 10b has slightly migrated into the part in AM1 1 10c and in the ceramic layer 1 10a,

The applicant has carried out other re-brazing tests with modulated temperatures and firing times - for example a thermal cycle for 10 to 20 minutes at temperatures between approximately 1160 ° C and 1200 ° C and a cycle of diffusion for 2 to 4 hours at temperatures between 1150 ° C and 1200 ° C - and he observed that the sublayer 110b changes little and that the microstructure is not modified neither by depletion of aluminum nor by transformation into gamma prime. The precipitate in AM1 remains between 0.3 and 0.7 microns. The thickness c1 + c2 of the underlayer remains between 40 pm and

75pm and the c2 / (c1 + c2) proportion between 50% and 80%. Likewise, the proportion of platinum (Pt) remains between 23% and 40% and the proportion of aluminum (Al) between 17% and 25%, these thickness and proportions being in accordance with the technical requirements DMP32-021.

The operation 350 of brazing the new closure plate 140 therefore makes it possible to conglomerate said new plate on the high pressure distributor blade without generating any damaging effect on the structure of the high pressure distributor. The reworked high pressure distributor will therefore perform identical to that of a standard high pressure distributor.

The extraction process 300, which has just been described, can be supplemented by an operation 360 of checking under x-rays of the cavity of the blade of the high pressure distributor. This inspection operation 360 is carried out at the end of the extraction process 300 to verify whether all the foreign bodies have indeed been removed from the cavities of the high pressure distributor blade.

Although described through a number of examples, variants and embodiments, the method of extracting foreign bodies according to the invention comprises various variants, modifications and improvements which will be evident to those skilled in the art, being understood that

these variants, modifications and improvements form part of the scope of the invention.

CLAIMS

1. A method of extracting at least one foreign body nestled within a blade (1 10) of a high pressure distributor (100) of a turbojet, said high pressure distributor comprising blades (1 10) hollow and fixed between two annular shrouds forming a gas flow stream, each blade being coated with a thermal barrier and pierced with at least one cavity at least partially blocked by a first closure plate (140), brazed, characterized in that that it comprises the following operations:

- cutout (310) of the first closure plate,

- extraction (320) of the foreign body out of the cavity,

- endoscopic control (330) of an interior space of the cavity so as to verify that no foreign body remains,

- positioning (340) of a second closure plate on the cavity, and

- brazing (350) of the second closing plate directly on the distributor with the blades coated with the thermal barrier.

2. Extraction process according to claim 1, characterized in that the cutting

(310) of the first closure plate is produced by grinding said plate.

3. Extraction method according to claim 1 or 2, characterized in that the positioning (340) of the second closure plate is achieved by pointing on the high pressure distributor.

4. Extraction method according to any one of claims 1 to 3, characterized in that the second closure plate (140) is pre-sintered.

5. Extraction process according to any one of claims 1 to 4, characterized in that the brazing (350) of the second closure plate is carried out in a vacuum oven, at a temperature below 1200 ° C.

6. The extraction process according to claim 5, characterized in that the brazing (350) of the second closure plate (140) comprises a thermal cycle carried out at a temperature between approximately 1160 ° C and 1200 ° C for 10 at 20 minutes then a diffusion cycle at a temperature between approximately 1150 ° C and 1200 ° C for 2 to 4 hours.

7. The extraction method according to any one of claims 1 to 6, characterized in that it comprises, after the brazing operation of the second closure plate, a control operation (360) under x-rays of the high pressure distributor.