EP1813365B1 - Method of manufacturing a turbomachine part comprising cooling air outlets - Google Patents

Description

The present invention relates to the cooling of turbine engine parts by air film.

To increase the performance of a gas turbine engine, it is necessary to increase the temperature of the gases at the outlet of the combustion chamber. The engine parts swept by these gases are then subjected to high thermomechanical stresses. They are protected by circulating cooling air taken from the compressor in channels arranged under the wall and by evacuating it into the gas stream through small diameter orifices arranged so as to form a film of protective gas between the wall and the flow of hot gas. The parts concerned by this treatment are essentially the distributor sectors, consisting of one or more radial blades between two platforms in ring sectors delimiting the gas stream, as well as the blades of the first turbine stages. The mechanical strength and the life of the parts are increased by this means.

The orifices are generally cylindrical holes, made in appropriate areas of the wall to be protected. In order to improve the formation of the air film along the wall, these holes are given a flared shape at its surface. These holes therefore consist of two distinct parts: a cylindrical portion calibrating the air flow and a portion shaped so as to diffuse and orient the flow of air to promote the flow in the formation zone of the cooling film . Examples of such orifices are illustrated in the patents US6183199 , EP 228338 and US 4197443 .

A known method of manufacture consists in making these holes in two stages; the flaring portion of the orifice is first machined by electroerosion, a technique also referred to as EDM for electrodischarge machining, then the bottom is pierced by means of a laser beam, for example, to produce a cylindrical channel.

According to the EDM technique, an electrode is placed at a distance from the surface to be eroded and electrical discharges are produced between it and the part. These discharges cause particles of matter and gradually erode the surface of the piece. The shape of the cavity obtained depends on the geometry of the electrode, which may be frustoconical, for example of rectangular section, or more complex with portions rounded as seen in the documents US 6,183,199 or EP228.338 . The second part, calibrated, is carried out either with the same electrode or by means of a laser beam.

The following problems are encountered with this technique.

The electrode, whatever its shape, even if it makes it possible to produce rounded wall portions inside the cavity, can not prevent sharp edges remaining. These edges are the seat of stress concentrations and present risks of crack initiation.

For mainly economic reasons, the orifices are made in series by means of electrodes cut in a plate and which are therefore arranged in a row. Such a practice does not allow an individual optimization of the geometry of the orifices according to the local profile surrounding them.

It is not possible to make this type of orifice in areas of reduced access. This is particularly the case when it comes to making holes along the blades of a two-way distributor sector in the inter-blade channel. As in this area the flared shape of the orifices is essential, it is not possible to achieve dual distributor sectors by casting in one piece. Each vane is manufactured separately and welded together to form the dispenser sector. The manufacturing cost is then higher.

The patent EP 1616642 discloses a method for making cooling orifices in the wall of a blade. In the wax model are inserted points whose shape corresponds to that of the desired orifices. This process involves interventions on the delicate mold to achieve.

These problems are solved in accordance with the invention with a method of producing cooling fluid discharge ports in the wall of a piece made by the lost-wax casting technique with formation of a pattern in a wax mold, the wall having an inner surface and an outer surface, the orifices having a first portion opening to the outer surface of the wall. This method is characterized in that it consists in forming in the wax model cavities corresponding to the first portions of said orifices of the part, by means of protuberances in the wax mold having a shape with radiated connection areas having a curved profile, complementary to that of said first portions, so that that the model has said cavities and that the output part of the foundry comprises said first portions of the preformed orifices, then machining in the casting part a second orifice portion, placing in communication the bottom of the first portion of orifice with the inner surface of the wall.

By making this orifice portion on the wax model of the part, so that it is formed by casting, one can optimize its shape easily for each emission on the profile of the vein. The heavy and expensive implementation of the electroerosion technique can be avoided and such a method is compatible with the manufacture of multi-blade foundry distributor sectors.

Most frequently, said first portion is of flared shape but the method of the invention allows any type of shape.

The connection areas between two non-coplanar surface portions of the protuberances at least in part have a curved profile so as to avoid the formation of sharp edges. They say they are radiated. The radius of curvature of the radiated surfaces is at least 0.1 mm, preferably 0.2 mm. The curvature of these surfaces is possibly progressive.

According to another characteristic, a second portion of orifice is machined in the casting piece bringing the bottom of the first portion into communication with the inner surface of the wall. The section of this second orifice portion is advantageously calibrated so as to dose the air flow. This portion is of tubular shape with circular section or other, in particular oblong, slot-shaped for example.

According to a preferred method, the machining is performed by means of a laser beam but other means can be implemented.

The invention also covers the turbomachine part obtained according to the method and having cooling air outlet orifices whose connection areas of the first portions with the outer wall of the part are radiated.

• La figure 1 montre une aube mobile de turbine refroidie;
• La figure 2 montre une vue en coupe de la paroi au niveau d'un orifice d'évacuation d'air de refroidissement selon l'art antérieur ;
• La figure 3 montre en coupe un modèle de pièce dans son moule à cire ;
• Les figures 4 à 6 montrent les étapes de réalisation de trous évasés selon l'invention ;
• Les figures 7 et 8 montrent des vues en perspective d'un orifice évasé selon l'invention.

The invention will now be described in more detail in connection with a non-limitative embodiment illustrated in the accompanying drawings and in which:

• The figure 1 shows a turbine blade cooled turbine;
• The figure 2 shows a sectional view of the wall at a cooling air discharge orifice according to the prior art;
• The figure 3 shows in section a piece model in its wax mold;
• The Figures 4 to 6 show the steps of making flared holes according to the invention;
• The Figures 7 and 8 show perspective views of a flared orifice according to the invention.

As we see on the figure 1 , a blade 1 comprises a movable foot 3, a platform 5 and a blade 7. The blade is mounted by the foot in a suitable housing on the rim of a turbine disk. When it is of cooled type, the blade is hollow and includes cavities arranged for the circulation of cooling air. A fraction of this air is directed through the wall of the blade through calibrated orifices. Some of these orifices are simple, tubular. Other orifices 10 comprise a flared portion so as to direct the air along the wall and make it possible to form a film or film for protecting the latter. These orifices 10 with a flared portion downstream are for example arranged along the leading edge of the blade on the extrados face 10a or along a generally radial line on the underside of the blade 10b. Another example of a row of flared apertures is along the trailing edge on the intrados face at 10c.

We have shown on the figure 2 a sectional view along the plane II-II of the wall 71 of the blade through an orifice 10. There is a first flared portion 10E opening on the outer surface of the wall 71 and a tubular portion 10T. The section of this portion 10T determines the rate of cooling air through the orifice. The air jet is spread laterally in the flared portion 10E, and forms a film together with the other adjacent jets along the wall of the blade.

Due to the complexity of its geometry and the thermomechanical constraints it must resist, this type of piece is made by lost wax foundry. This technique is known below.

First of all, a model made of wax or other equivalent material is produced which comprises a foundry core which forms the internal cavities of the blading. This nucleus is itself manufactured separately and generally has a complex shape in several elementary nuclei. This core is placed in a wax mold and the wax is injected into the space between the core and the inner wall of the mold. We obtain the model incorporating the nucleus; it is the replica of the piece to be melted.

An example of a piece, here a turbine blade, is represented on the figure 3 . The wax model incorporates a core comprising a plurality of core elements 21a to 21d of ceramic material. The wax mold 30 here consists of two parts 30a and 30b each with a molding wall 30a 'and 30b' corresponding to the envelope of the piece. The mold of the example shown is simple in form but depending on the complexity of the room, it can include multiple elements.

Then, the wax model 20 is extracted from the mold 30 and is soaked in slips consisting of suspensions of ceramic particles to coat it with successive layers and to make a shell mold. After hardening the mold by cooking, the wax is removed. The piece is obtained by casting a molten metal which occupies the voids between the inner wall of the shell mold and the core. Thanks to a germ or a suitable selector and a controlled cooling, the metal solidifies according to a determined structure. Depending on the nature of the alloy and the expected properties of the part resulting from the casting, it may be directed solidification with a columnar structure, directed solidification with monocrystalline structure or equiaxial solidification respectively. The first two families of parts concern superalloys for parts subjected to high thermal and mechanical stresses in the turbojet engine, such as HP turbine blades.

According to the technique of the prior art, flared holes are formed by machining the casting piece. The hole that we see on the figure 2 is obtained by EDM machining. In particular, it can be seen that the connection area between the surface 71 ext and the flared hole 10E has an edge 10E1 that can not be avoided. A machining of this part would lead at best to the realization of a chamfer but not a rounded due in particular to the small size of this type of orifice. The machining tolerances would not allow a sufficiently precise positioning of the tool relative to the area to be machined.

According to the invention, it is proposed to make said first flared portion of the orifices directly on the wax model. Preferably the wax mold in which the wax is injected has the impression of the first portions of the orifices.

On the figure 4 , there is shown a sectional view at the inner surface 130a 'of the mold 130a and the model through a protrusion 132 of molding a first portion according to the invention. The elements of the invention corresponding to those of the prior art have the same reference increased by a hundred. The protrusion 132 has the shape of the first portion that is desired to print in the wall 120 'of the model 120 in wax. In order to satisfy the stresses of demoulding, the faces of the protuberance do not include a part forming an angle less than a demolding limit angle with respect to the demoulding direction in this zone, represented by the arrow D. When the mold is constituted of a plurality of elements with a specific insert for the protuberance or a group of protuberances, it is sufficient that the angle is defined relative to the withdrawal direction of this insert. The use of an insert has the additional advantage of facilitating the modification of the profile of the protuberances, for example in the phase of part development. Just change the insert alone to make a room with the new flared opening profile.

The piece 101 foundry has in its wall 171 a cavity 110E corresponding to the shape of the protrusion 132 which has been applied in the wall 120 'of the wax model 120. This cavity 110E constitutes the first portion of the orifice that it is desired to dig into the wall 120 '. The formation of the cooling air discharge orifices is completed by piercing the bottom of the cavity 110E, for example by a laser beam. This piercing forms a tubular channel 110T. The section of this channel 110T is determined by the desired air flow and its shape can be advantageously circular or oblong. These two steps are illustrated by the figures 5 and 6 .

On the Figures 7 and 8 , we see an embodiment of the orifice 110 for discharging cooling air that can be obtained according to the method of the invention in a wall 171 that is to cool by air film. The different surface portions are represented with segments of generating generatrices to show the three-dimensional character.

We see the first portion 110E, of flared shape, opening on the outer surface 171ext. of the wall 171. A second portion 110T, tubular, is machined in the bottom of the first portion and opens on the inner surface 171int. of the wall 171. The cavity 110E has a bottom A, whose shape seen from above, is substantially trapezoidal. The cavity is turned downstream with respect to the flow direction of the gases. This bottom is inclined between the tubular portion 110T and the edge A1 of connection to the outer surface 171 ext. of the wall 171. The flanks L1 and L2 of the cavity are curved in the form of cylindrical sectors L1A and L2A concave, here in evolutionary profile, along their connection zone with the bottom A. The surfaces are said to be radiated. The radius of curvature of these surfaces is preferably at least 0.1 mm. and varies along the profile. The flanks L1 and L2 also comprise curved surface portions L1S and L2S, with an evolutive profile, in the direction of the surface of the wall 171ext. The flank of the cavity located transversely between the two lateral flanks L1 and L2 also comprises a convex radiated portion BS of connection with the outer surface 171ext. of the wall 171, and concave radiated portions with the flanks L1 and L2.

These radiated surface portions L1S, L2S and BS are complementary to the connecting surfaces of the protuberances 132 with the surface 130a 'of the wax mold 130a in which the model is molded. It is enough to conform correctly the protuberances to obtain a piece without sharp edge in these places.

These radiating connection portions having a radius of curvature, for example of 0.2 mm, with a minimum of 0.1 mm. They limit the thermal and mechanical stresses in these areas and reduce the occurrences of crack initiation. This improves the overall mechanical strength of the part and its life.

Another advantage compared to EDM machining is the obtaining of surfaces having a low roughness, favorable aerodynamically. For example, the roughness Ra by EDM is typically 4.5 μm. Getting a lower value is very expensive. By the foundry process it is easy to obtain a finer surface condition; Ra = 1.2 μm, for example.

Note that the line of intersection of the tubular zone 110T with the bottom of the first portion 110 E is not radiated to the extent that it is obtained by machining.

Claims (11)

1. Method for forming coolant discharge orifices (110) in a wall (171) of a part produced in accordance with the lost-wax casting process with formation of a model in a wax mould, the wall (171) comprising an inner surface (171int) and an outer surface (171ext), the orifices comprising a first portion (110E) which opens into the outer surface (171 ext) of the wall, characterised in that the method involves forming, in the wax model, cavities which correspond to the first portions (110E) of said orifices (110) in the part by means of protrusions (132) in the wax mould which are shaped with radiating connection regions and have a curved profile, this shaping being complementary to that of said first portions (110E), in such a way that the model has said cavities and the cast part comprises said first portions of the preformed orifices, then machining in the cast part a second orifice portion (110T) which connects the base of the first portion (110E) of the orifice to the inner surface (171 int) of the wall.
2. Method according to the preceding claim, wherein the cavities corresponding to said first portions (110E) of the orifices are flared in shape.
3. Method according to either claim 1 or claim 2, wherein the connection regions, at least in part, formed over the cavities corresponding to said first portions (110E) of the orifices radiate with a curved profile.
4. Method according to claim 3, wherein the connection region connecting the flanks of the cavities corresponding to said first portions (110E) of the orifices to the outer surface of the model radiates with a curved profile.
5. Method according to either claim 3 or claim 4, wherein the radius/radii of curvature of the radiating surfaces is/are at least 0.1 mm.
6. Method according to the preceding claim, wherein the radius/radii of curvature of the radiating surfaces is/are 0.2 mm.
7. Method according to any one of claims 3 to 6, wherein the radius of curvature progresses along the profile of the radiating surfaces.
8. Method according to claim 1, wherein the second portion (110T) of the orifice is machined into a tubular shape.
9. Method according to the preceding claim, wherein the machining is carried out by means of a laser beam or by EDM.
10. Turbomachine part formed in accordance with the method of any one of the preceding claims and comprising cooling air discharge orifices with wall elements, of which the regions (BS) connecting the first portions of the orifice to the outer wall (171 ext) of the part radiate with a curved profile.
11. Part according to the preceding claim, of which the radii of curvature of the radiating surfaces are at least 0.1 mm.

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