PROCESS FOR MANUFACTURING CERAMIC CORES FOR TURBOMACHINE BLADES

Abstract

The invention relates a process for manufacturing a ceramic foundry core having at least one thin region with a thickness “e”, in particular in of a turbomachine blade trailing edge, comprising the forming in a mold of a mixture comprising a ceramic particle filler and an organic binder, the extraction of the core from the mold, the binder removal and consolidation heat treatment of the core. The process is one in which a core is formed in said mold, said region of this core being thickened relative to the thickness “e” by an overthickness E and in which said overthickness is machined after the core has been extracted from the mold and before or after the heat treatment operation. In particular, the machining is carried out mechanically by milling, either with removal of chips on the cores before firing, or by abrasion on the fired cores.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages will become apparent on reading the following description of one method of implementing the process of the invention with reference to the appended drawings in which:

FIG. 1 is a sectional view of a cooled turbine blade;

FIG. 2 is a general view of a cooled blade core, x;

FIG. 3 is a view of a core trailing edge region having an overthickness according to the invention;

FIG. 4 is a view of a portion of the core trailing edge after the overthickness has been machined;

FIG. 5 is a graph of the variation in injection pressure as a function of the means used to obtain the desired trailing edge geometry;

FIG. 6 shows the filling of the mold as a function of the means of FIG. 5;

FIG. 7 shows the method of machining by means of a milling cutter;

FIG. 8 shows, in section on 8-8, the first tenon of FIG. 3; and

FIG. 9 shows, in section on 9-9, the first tenon of FIG. 8.

The following description corresponds to the invention applied to the formation of a foundry core for a high-pressure turbine blade in a gas turbine engine for aeronautical or terrestrial use. This presentation is not limiting.

As may be seen in FIG. 1, a turbine blade 1 comprises a pressure face PF, a suction face SF, a leading edge LE and a trailing edge TE. When this is a high-pressure turbine blade of a gas turbine engine for aeronautical use, the blade includes internal cavities, here seven cavities, namely 1A to 1G. The trailing edge has a cavity 1H extending parallel to it. It is supplied from the last cavity 1G via a plurality of mutually parallel calibrated channels 1GH for exhausting the coolant, which is air taken from the compressor.

The cavities are separated from each other by partitions, namely 1AB, 1BC, etc. When these blades are manufactured by casting a molten metal, a core must be incorporated into the shell mold, this core occupying the voids of the cavities to be formed in the blade. This core, as may be gleaned from FIG. 1, is complex.

FIG. 2 shows a core 100 obtained from a mold. It comprises a portion corresponding to the cavities of the airfoil 100A, a portion 100B corresponding to the cavities of the root of the blade and a portion 100C forming a handle for gripping the blade during manufacture. At the tip of the blade, there is also a portion 100D corresponding to what is referred to in the jargon of the field as a “squealer”.

The trailing edge of the core i.e. the portion referenced 100H resulting in the formation of the cavity 1H of FIG. 1, and the tenons 100GH resulting in the formation of the channels 1GH of FIG. 1 are shown in FIG. 3 or FIG. 4. The particular case of the first tenon 100GH1 according to the invention will be discussed later.

This core is produced by injecting molding in a mold in which the thin regions formed by the tenons 100GH must be filled. The usual technique consists in designing the mold with subparts that have a certain mobility in order to be able to extract the core after injection of the material into the mold and its solidification. As explained above, the injection into these regions is more complicated the thinner they are.

The object of the invention is to produce a core having such a complex structure without having to develop more fluid slurries or to increase the injection parameters such as the pressure or flow rate.

According to the invention, a modified mold is produced, that is to say a mold in which the core after molding has at least one thin region that is thickened.

The thickened thin region of the first tenon 100GH1 is obtained by suitably shaping the mold at this point in order to obtain such a thickened region for the first tenon 100GH1. The first tenon is the first seen from the root of the blade via which the core slurry is injected. This portion is shown in section in FIGS. 8 and 9. FIG. 8 shows the overthickness E of the tenon 100GH1 relative to the suction face 100SF of the core 100. The surfaces on the suction face side of the portions 100G and 100H lie substantially in the same plane, with the exception of this overthickness. This overthickness is determined according to the final thickness “e” that it is desired to obtain for the tenon 100GH1 and of the quality of the slurry that is injected. A channel is created with a sufficient opening for flow of the slurry during injection. In cross section, shown in FIG. 9, the outline of the overthickness E takes into account the rounded edges of the tenon. The radiusing of the rounded edges of the tenon may also be carried out by machining.

Preferably, the slurry used comprises an organic binder combined with a mineral filler. For example, the mixture is made according to the teaching of patent application EP 328 452. The core has good handling behavior and its constitution allows it to be worked by means of a milling tool by removal of chips or by abrasion.

After the core has been manufactured with this overthickness E on the first tenon, the next step consists in machining, in this core blank, the thickened region or regions. The machining is advantageously carried out by means of a tool as shown in FIG. 7. This is a milling cutter 200 having a cutting end 200A and a helical cutting edge or thread along its shank 200B. The milling cutter is moved perpendicular to the surface to be machined. The speed of the tool and that of its displacement are fixed. In this way the forces on the material are limited and the tool prevented from bending.

It is preferred to use a numerical control machine tool of the type having five axes of displacement, for example three axes for positioning the milling cutter in space and two axes for positioning the core. This machine can be easily programmed in order to automate the machining of the cavities, as the case may be.

FIG. 4 shows the trailing edge region of the core after it has been machined. The channels have the dimensions, in particular the thickness that they will form, apart from shrinkage, in the part upon casting the molten metal into the shell mold.

Once the machined core has been fired, it undergoes the following treatments, known per se, in the process from manufacturing foundry cores, namely binder removal, that is to say the removal of the organic binder. For this purpose, the core is heated to a sufficient temperature to degrade the organic components that it contains. The other steps consist in subsequently heating the core to the temperature for sintering the ceramic particles of which it is made. If additional consolidation is necessary, impregnation with an organic resin is carried out.

For cores machined after firing, is passed directly to the finishing and checking operations.

To demonstrate the benefit of the present solution, comparative trials were carried out with reference to FIGS. 5 and 6.

FIG. 6 a shows a phase in the filling of a mold of the prior art, indicated by the hatched lines. The thickness of the channels for forming the tenons in this example is 0.35 mm. It may be seen that the slurry is introduced via the root region of the blade and advances toward the top of the mold. The slurry is slowed down in its flow through the regions of small thickness. It cools even before having passed these regions. The slurry must therefore get past these regions. It follows that at the moment when the two propagation fronts come together, the slurry is not sufficiently fluid for a strong weld to form.

On the graph in FIG. 5, it is shown that the necessary pressure is 94 units of pressure.

FIG. 6 b shows a channel 60 on the side with the region 100H in order for the feed to be more direct. In fact, the injection pressure is lower—85 units of pressure suffice. However the weld is still not satisfactory as the slurry front remains fixed in the channels of the tenons.

FIG. 6 c shows the addition of a false tenon 70. The result is substantially the same as previously—the pressure is 85 units of pressure.

In FIG. 6d the mold has been hollowed out so as to form, on the first tenon, an overthickness according to the invention. Compared with FIG. 5, it may be seen that an injection pressure of 78 units of pressure is sufficient for the propagation front of the slurry not to be blocked in the channel. This allows the trailing edge region to be filled through the channels. It follows that no mechanical weakness affects the tenon region.

The figures essentially show the thickening of the first tenon of the core but this may be applied to all the tenons. This technique therefore makes it possible more generally to produce portions of the core that are very thin and narrow, such as the portion of the core lying close to the trailing edge and having channels for passage of the air escaping from inside the blade at the end of the cooling circuit and injected into the gas stream. However, the machining may be extended to any portion of the core for which the same freedom-of-flow problem arises.

Claims

1. A process for manufacturing a foundry core comprising at least one thin region with a thickness “e” between 0.1 and 0.5 mm in particular in a turbomachine blade trailing edge, comprising the forming in a mold of a mixture comprising a ceramic particle filler and an organic binder, the extraction from the mold, the binder removal and consolidation heat treatment of the core, wherein a core is formed in said mold, said region of this core being thickened relative to the thickness “e” by an overthickness E and wherein said overthickness is machined after the core has been extracted from the mold so as to create a channel of sufficient opening for the flow of said mixture during its injection into the mold.

2. The process as claimed in claim 1, the machining of which is carried out before the heat treatment operation.

3. The process as claimed in the preceding claim, in which the machining of the overthickness is carried out mechanically by milling with removal of chips.

4. The process as claimed in claim 1, the milling of which is carried out after the heat treatment operation.

5. The process as claimed in the preceding claim, in which the machining of the overthickness is carried out mechanically by abrasion.

6. The process as claimed in claim 5, the machining of which is carried out by means of a milling cutter by removal of material on an at least three-axis, and preferably four-axis or five-axis, milling machine.

7. The process as claimed in one of claims 1 to 6, in which the region of thickness “e” lies close to the leading edge and constitutes a tenon for forming a channel for exhausting the air for internally cooling a turbomachine blade.

8. The process as claimed in claim 7, the tenon of which is the first seen from the end in which the slurry is fed for filling the mold.

9. The process as claimed in claim 7, the machining of which includes a step of radiusing the surface of the tenon.

10. The process as claimed in one of claims 1 to 4 for the manufacture of a core comprising a plurality of said thin regions, the overthickness being applied to several thin regions.