EP3414031B1 - Method for forming dust-removal holes for a turbine blade and associated ceramic core - Google Patents

Description

Field of the invention

The present invention relates to the general field of turbomachine turbine blades, and more particularly to turbine blades provided with integrated cooling circuits produced by the lost wax casting technique.

Prior art

In a manner known per se, a turbomachine comprises a combustion chamber in which air and fuel are mixed before being burned there. The gases resulting from this combustion flow downstream of the combustion chamber and then supply a high pressure turbine and a low pressure turbine. Each turbine has one or more rows of fixed blades (called distributors) alternating with one or more rows of moving blades (called moving wheels), spaced circumferentially around the turbine rotor. These turbine blades are subjected to very high temperatures of the combustion gases, which reach values well above those which these blades which are in direct contact with these gases can withstand without damage, which necessarily implies ensuring their continuous cooling. by an integrated cooling circuit which, when it is desired to ensure efficient and precise cooling without significantly increasing the air flow and without penalizing engine performance, includes multiple cavities. The hollow blades thus formed are manufactured by the so-called “lost wax” foundry process which requires the use of a model part or core whose external surface corresponds to the internal surface of the finished blade, as described in the applications FR2986982 Or FR2961552 filed in the name of the plaintiff.

The air necessary for the operation of the engine generally contains various dusts (in particular fine sand) which can accumulate in the cooling circuits of the turbine blades, causing the evacuation orifices at the outlet of the cavities to be blocked and thus threatening the air. integrity of dawn. To overcome this problem, the blades of turbine are equipped at the top of the cavity with calibrated dust removal holes, obtained by high precision machining or from connecting rods in alumina or quartz inserted in the ceramic core, and whose role is to generate these holes and guarantee the evacuation of these particles after their release (that is to say their dissolution).

However, the use of these connecting rods poses certain problems. First of all, alumina rods are very difficult to remove with basic solutions (or under the standard shaking conditions of ceramic cores) and require a significant residence time, a very high sodium or potash concentration and high temperatures and pressures. very high levels likely to be aggressive towards the alloy (stress corrosion). Likewise, quartz rods have low mechanical strength, thus penalizing their use in a lost wax casting process where the core, which has a different coefficient of thermal expansion (CTE) and is also often of a different composition, undergoes several mechanical stresses. . Next, the use of rods is not applicable in all core manufacturing processes. In the case, for example, of cores produced by additive manufacturing or cores obtained by machining in a ceramic block, the rods cannot be embedded in the core during manufacturing (unlike the injection molding process). Finally, the use of rods is not applicable to all core geometries, particularly those using thin plates whose rods must then conform to the shape.

In addition, the assembly of several ceramic cores is generally done by the foot and the head of the cores (non-functional parts of the core), the excess material resulting from casting (consequence of the assembly of the cores) must be eliminated during the machining of the bathtub which requires the use of various resurfacing (refilling) or plate brazing techniques. However, these brazed plates are not robust (they can become detached and it may therefore be necessary to thicken them locally) and the resurfacing is often not precise (uncertain depth of filling). Therefore, the drilling of the connecting rods forming the dust removal holes turns out to be particularly delicate because this calibrated drilling is carried out in the previously sealed location, therefore with a smaller diameter while still respecting the minimum dimensions to evacuate debris. Requests WO2015/195110 Or US2010/303625 illustrate such drilling of ceramic rods by EDM.

Object and summary of the invention

The present invention therefore aims to overcome the aforementioned drawbacks by proposing a geometric arrangement of the core making it possible to simply obtain dust removal holes more reliably than currently and in particular without harming the robustness of this core. Another aim is to eliminate the final operation of drilling the bathtub of the prior art to obtain these orifices.

For this purpose, a ceramic core is provided used for the manufacture of a hollow turbine blade of a turbomachine using the lost wax casting technique, said blade comprising calibrated dust removal holes emanating from a top of at less one cavity and opening into a bathtub of said blade, characterized in that each of said calibrated dust removal holes is formed in a core part of a determined height sufficient to guarantee its mechanical strength, said core part comprising a through orifice of axis perpendicular to a longitudinal axis of said calibrated dust removal hole and delimiting on either side of said through orifice on the one hand a core cylinder of a determined diameter corresponding to said dust removal hole to be formed and on the other hand a remaining core volume intended to be filled after casting, so that said calibrated dust removal hole is obtained without drilling and without the use of connecting rods.

Thus, dust removal holes can be obtained directly from the foundry by injection, additive manufacturing or machining of ceramic cores without drilling or the use of connecting rods. Any possible source of differential thermal expansion is eliminated, the mechanical strength of the core is improved and correlatively the mechanical properties of the blade are thus maintained. With this core, the machining operation of the prior art having to take into account restrictive uncertainties and which can have a detrimental impact on the geometry of the plates of a multi-cavity circuit is also eliminated.

According to the envisaged embodiment, said core part can form part of a lateral column intended to create a cavity side of said blade or an inter-cavity connection zone between said at least one cavity and said bathtub.

Preferably, on a core portion corresponding to said bathtub to be created, a recessed zone is provided to allow centering of said through orifice in said connection zone, so as to guarantee better holding of said core portion during the casting.

Advantageously when the dimensioning of the blade requires it, said remaining core volume comprises at least one lateral stiffener (two stiffeners giving it a four-lobed shape) dimensioned so as to guarantee better resistance of said core part during casting.

The invention also relates to the method of forming calibrated dust removal holes in a hollow turbomachine turbine blade produced using the lost wax casting technique using a ceramic core as explained previously and any turbomachine turbine provided with a plurality of cooled blades manufactured from such a process.

Brief description of the drawings

• la figure 1 est une vue partielle d'un noyau pour aube de turbine selon l'invention,
• la figure 2 est une vue d'une partie du noyau de la figure 1 au niveau d'une plaque latérale,
• les figures 2A et 2B sont respectivement des vues après coulée et après usinage une fois la partie de noyau de la figure 2 retiré,
• la figure 3 est une vue d'une partie du noyau de la figure 1 au niveau d'une liaison avec la baignoire, et
• les figures 3A et 3B sont respectivement des vues après coulée et après usinage une fois la partie de noyau de la figure 3 retiré.

Other characteristics and advantages of the present invention will emerge from the description given below, with reference to the appended drawings which illustrate an exemplary embodiment devoid of any limiting character and in which:

• there figure 1 is a partial view of a core for a turbine blade according to the invention,
• there figure 2 is a view of part of the core of the figure 1 at the level of a side plate,
• THE Figures 2A and 2B are respectively views after casting and after machining once the core part of the figure 2 took of,
• there Figure 3 is a view of part of the core of the figure 1 at the level of a connection with the bathtub, and
• THE Figures 3A and 3B are respectively views after casting and after machining once the core part of the Figure 3 took of.

Detailed description of an embodiment

There figure 1 represents, at the level of its head assembly, a ceramic core intended for the production of a hollow turbine blade of a turbomachine. The ceramic core 10, in the example illustrated, has seven parts or columns. The first column 12, which is intended to be on the combustion gas inlet side, corresponds to a leading edge cavity which will be created after foundry, while the second column 14 corresponds to a central cavity which is adjacent. The latter receives a flow of cooling air through a pipe resulting, after foundry, from the presence of a first column base of the core. Three other columns 16, 18, 20 correspond to adjacent cavities which receive a second flow of cooling air brought by another pipe coming from the presence of a second column base of the core. Finally, the core further comprises sixth and seventh lateral columns 22, 24 corresponding to lateral cavities created after foundry and both separated from the second and third columns 14, 16 by a determined spacing necessary for the creation of 'a solid inter-cavity wall when pouring the molten metal.

The first and second columns 12 and 14 are connected to each other by a series of bridges 26, to which will correspond, after foundry, air supply orifices for cooling the leading edge cavity. Concerning the fourth column 18, other bridges 28 inclined vertically by forming thinned core regions make it possible to create stiffened blade regions. The size of the different bridges is determined to avoid their breakage when handling the core 10, which would render it unusable. The bridges are, in the example considered, distributed by being spaced substantially regularly over the height of the core, particularly at the level of the first column of the core.

In accordance with the invention, the dust removal holes in the turbine blades necessary for the evacuation of dust (in particular fine sand) which can accumulate in the cooling circuits, are obtained by a geometric arrangement of a part of core, directly raw from the foundry, without drilling and without the use of connecting rods, whether they are the holes present at the level of the side cavities of the core or those ensuring the connection with the bathtub. If the core thus formed stands out from the existing cores, the manufacturing process lost wax of the dawn once this core has been produced is classic and consists first of all in forming an injection mold in which the core is placed before injecting the wax. The wax model thus created is then dipped in slips made of ceramic suspension to make a casting mold (also called a shell mold). Finally, the wax is removed and the shell mold is fired into which the molten metal can then be poured. Final machining operations (however simplified compared to those of the prior art) described further below will then make it possible to obtain the finished blade.

According to the invention and as illustrated in figure 2 , it is planned to arrange locally, at the level of the core part 22A, the geometry of the side columns 22, 24 (parts of the ceramic core generating the side cavities) so as to form on the one hand a core cylinder 30 of a determined diameter (calibrated in the order of 0.5mm to 0.8mm) corresponding to the dust removal hole to be made, and in addition of the smallest possible height to guarantee the mechanical strength of the plate and on the other hand a core volume 33 corresponding to the remaining space of the core and intended to be filled after casting. This geometry can be obtained conventionally by integrating a bridge type disruptor in the mold of the plate (at a through hole with a longitudinal axis delimiting in a direction perpendicular to this axis the cylinder 30 and the remaining volume 33) for the case of ceramic injection or without additional constraints for the case of additive manufacturing or by machining of cores.

When the dimensioning of the turbine blade requires it and taking into account the fragile appearance of the ceramic, it is necessary to guarantee the mechanical strength of the core by taking care not to mechanically weaken the plates obtained after casting, for example by stiffening by adding one or more stiffeners thus preventing the plates from breaking in these locations. It should be noted that such lateral stiffeners (illustrated by reference 39 on the Figure 2B ) have a very low impact on the injection (the overall section does not vary too significantly, the part lost with drilling being compensated with the addition of the stiffener).

Likewise, the ceramic cores made by injection having to be demolded, it is obviously necessary to ensure that these cores have sufficient relief in relation to a demolding axis. technical. Indeed, if this demoulding axis is not correctly oriented, the plate can be seriously weakened.
there Figure 2A illustrates the upper part of the blade (bathtub) obtained at the end of the casting (as foundry) with the two cavities 32, 34 corresponding to the two lateral columns and the excess material which surrounds them due to the assembly of these columns. On the Figure 2B , we find the same bathtub after machining this excess material and we see that, with the invention, two holes 36A, 38A; 36B, 38B are formed at each cavity (instead of just one in the prior art). One of them 36A, 36B having the dimension of the core cylinder 30 will ensure the dust removal function, the other hole 38A, 38B which has no particular function and has the dimension of the remaining core volume 33, is intended to be resealed. Thus, with the invention, the operation of filling/drilling the dust removal hole, the most delicate and presenting the least robustness in the realization of the prior art, is eliminated. The problem existing in the prior art of the uncertain depth of the filling no longer hinders the successful construction of the bathtub because it no longer has to be pierced.

The connection with the bathtub is illustrated in Figure 3 . As for the side plates, to obtain a dust removal hole, a local geometric arrangement of the connection is provided, by forming on either side of the through orifice 41 on the one hand a core cylinder 40 of a determined diameter corresponding to the diameter of the dust removal hole to be made and on the other hand the remaining volume of core 43 intended to be resealed after casting. The core cylinder also has the lowest possible height to guarantee good hold of the core and to avoid the formation of cracks. As before, the through orifice can be formed by the use of a bridge type disruptor integrated into the foundry mold. However, the space available between the cavities and the bathtub being very limited and the inter-cavity connection being thin (therefore with a small section), it is also planned to arrange on a portion of the core at the level of the bathtub create a recessed area so as to provide more space. In addition, the through orifice 41 intended to receive the disruptor therefore finds itself centered on the inter-cavity connection, better robustness is also obtained during casting.

There Figure 3A illustrates the upper part of the blade (bath) obtained at the end of the casting (as foundry) with the extension 42 resulting from the removal of the core from space d. On the Figure 3B , we find the same bathtub after machining this extension and we see that, with the invention, two holes 44, 46 are formed at the level of the bathtub. Note the presence of two lateral stiffeners 48A, 48B, giving a quadrilobed shape to the second hole (corresponding to a section of volume 43) and sized so as to guarantee the robustness of the core. In the case of ceramic injection, this also makes it possible to increase the section and guarantee better filling and in the cases of additive manufacturing and core machining, the stiffeners stiffen the connection and prevent deformation of the nuclei.

Of course, as stated previously, in the case of ceramic injection, the core must remain demouldable and therefore the added connection must also be, as well as the hole made in it.

With the invention, it is thus proposed a means of combining the functions of maintaining the core and developing dust removal holes (function usually provided by rods) adapted to any type of core manufacturing process and to any type of geometry of this nucleus.

Claims (9)

1. A ceramic core (10) used for fabricating a hollow turbine blade for a turbine engine using the lost wax casting technique, said blade including calibrated dust-removal holes emanating from a top of at least one cavity and opening out into a bathtub of said blade, the core being characterized in that each of said calibrated dust-removal holes is formed in a core portion (22A) of height that is determined to be sufficient to guarantee mechanical strength, said core portion including a through orifice (31, 41) of axis perpendicular to a longitudinal axis of said calibrated dust-removal hole and defining on either side of said through orifice firstly a core cylinder (30, 40) having a determined diameter corresponding to said dust-removal hole that is to be formed, and secondly a remaining core volume (33, 43) that is to be plugged after casting, such that said calibrated dust-removal hole is obtained without drilling and without using connection rods.
2. A ceramic core according to claim 1, characterized in that said core portion forms a portion (22A) of a side column (22, 24) that is to create a side cavity of said blade.
3. A ceramic core according to claim 1, characterized in that said core portion forms an inter-cavity connection zone between said at least one cavity and said bathtub.
4. A ceramic core according to claim 3, characterized in that it includes, on a core portion corresponding to said bathtub that is to be created, a setback zone for enabling said through orifice to be centered in said connection zone, so as to guarantee better strength for said core portion during casting.
5. A ceramic core according to claim 2 or claim 3, characterized in that said remaining core volume includes at least one lateral stiffener (39; 48A, 48B) of dimensions suitable for guaranteeing better strength for said core portion during casting.
6. A ceramic core according to claim 5, characterized in that said remaining core volume includes two lateral stiffeners (48A, 48B) facing each other and giving it a four-lobed shape.
7. The use of a ceramic core according to any one of claims 1 to 6, for fabricating a hollow turbine blade of a turbine engine using the lost wax casting technique.
8. A method of forming calibrated dust-removal holes in a hollow turbine blade for a turbine engine and made by the lost wax casting technique using a ceramic core of outside surface that is to form the inside surface of the finished blade, the method being characterized in that it comprises a step of forming a through orifice (31, 41) in a core portion (22A) of height that is determined to be sufficient to guarantee mechanical strength and in which each of said calibrated dust-removal holes is to be formed, the through orifice (31, 41) being of axis perpendicular to a longitudinal axis of said calibrated dust-removal hole and defining on either side of said through orifice firstly a core cylinder (30, 40) having a determined diameter corresponding to said dust-removal hole that is to be formed, and secondly a remaining core volume (33, 43) that is to be plugged after casting, such that said calibrated dust-removal hole is obtained without drilling and without using connection rods.
9. A method according to claim 8, characterized in that said through orifice is formed by using a bridge type disturber.

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