OVEN AND HEATING OR PREHEATING METHOD FOR TOMOGRAPHIC ANALYSIS

Abstract

A heating or preheating furnace (40), this furnace (40) comprising an enclosure (42) and heating elements (44) for the enclosure up to a temperature exceeding 1000°C, characterized in that it further comprises a tomography device (46) comprising an X-ray emitter (48) and a receiver (50) of these rays, the enclosure (42) being disposed between the emitter (48) and the receiver (50) and comprising walls (54b) transparent to said rays. Figure for the abbreviation: Figure 3

Description

Title of the invention: OVEN AND HEATING OR PREHEATING METHOD FOR TOMOGRAPHIC ANALYSIS Technical field of the invention

[0001] The invention relates to a furnace and a heating and preheating method in particular for tomographic analysis of a ceramic mold for example for the manufacture of rotor blades of an aircraft turbomachine. Technical background

[0002] As is known, a turbomachine comprises a combustion chamber in which air and fuel are mixed before being burned. The gases from this combustion flow downstream of the combustion chamber and then feed a high-pressure turbine and a low-pressure turbine. Each turbine comprises one or more rows of fixed stator blades (called distributors) alternating with one or more rows of moving rotor blades (called runners), mounted and spaced circumferentially around the turbine rotor. These turbine blades are subjected to the very high temperatures of the combustion gases, which reach values ​​far exceeding those that these blades, in direct contact with these gases, can withstand without damage, thereby limiting their service life.

[0003] To solve this problem, it is known to equip these blades with internal cooling circuits exhibiting high levels of thermal efficiency and designed to reduce their temperature. To achieve this, the blades are hollow, and an air circulation system is established within their internal cavities. This air is generally supplied by a pressurization circuit for the internal turbine chambers, this circuit itself being supplied by one or more compressors of the turbomachine.

[0004] As described in application FR-A1-2 961 552, turbine blades are conventionally manufactured using the lost-wax casting process. The geometry of the circuits is determined, depending on its complexity, by positioning one or more ceramic cores in a first mold. The outer surface of these cores forms the inner surface of the finished blade. Wax is poured into this first mold to create an impression of the mold. The wax encloses the core(s) and is covered by a second mold, called the shell mold, which is made of ceramic and conforms to the external shape of the impression formed by the wax. The wax is then removed to obtain an assembly comprising the shell mold and one or more cores housed within it. Molten metal alloy is then poured inside This assembly, between the shell mold and the cores, is used to obtain a rough casting of the blade after solidification.

[0005] Before the casting stage, the shell mold with the cores inside is subjected to a preheating process until it reaches the pouring temperature of the liquid metal. During this preheating stage, the mold temperature can rise from 20°C to 1600°C.

[0006] Due to thermal expansion phenomena and the difference in thermal expansion coefficients between the shell mold and the cores, as well as the core fixing systems within the shell mold, it is possible for the core to shift position inside the shell mold. Such a core displacement results in the blade dimensions no longer conforming to specifications, and the blade is rejected.

[0007] However, it is necessary to properly control the interactions and movements between the shell mold and the cores in order to successfully maintain the specified dimensional tolerances.

[0008] Unfortunately, it is not possible to observe the core / shell interaction during the preheating phase. To understand the core displacements, one must rely on numerical simulation and empirical experimentation. However, there are many parameters and poorly understood conditions that reduce the accuracy of the calculations.

[0009] Beyond this situation, it is also very difficult to determine whether the displacement of the nuclei takes place during the preheating phase or during and after the pouring of the liquid metal.

[0010] The invention aims to provide a simple, efficient, and economical means of analysis to understand at least part of the origin of these drawbacks. Summary of the invention

[0011] The present invention relates to a heating or preheating oven, this oven comprising an enclosure, and heating elements for the enclosure up to a temperature above 1500°C, characterized in that it further comprises a tomography device comprising an X-ray emitter and a receiver of these rays, the enclosure being disposed between the emitter and the receiver and comprising walls transparent to said rays.

[0012] The invention thus proposes to combine a high-temperature oven (above 1500°C) with a tomography device. Tomography is an imaging technique that makes it possible to reconstruct the volume of an object from a series of measurements taken from outside that object. It is therefore understandable that tomography of an object located in the oven, during the heating or preheating of that object, will make it possible to monitor the variations in the object's volume and, in particular, its de possible placements. In the case of an assembly such as described above, this will allow the detection of any relative displacements between the shell mold and the nucleus(es) located in this mold.

[0013] The invention thus makes it possible to measure, quantify and understand the phenomena leading to the displacement of the nucleus or nuclei in such an assembly.

[0014] The oven according to the invention may comprise one or more of the following features, taken individually or in combination with each other:

[0015] - the oven further comprises a tray located within the enclosure and movable for rotation around a vertical axis, this platform is intended to support an element to be heated or preheated;

[0016] — the movable tray of the oven can be connected to a movable tray or arm of the tomography device so that the device itself controls the rotation of the oven tray;

[0017] — the tomography device is capable of taking pictures of the object or of the whole being heated or preheated, these images being used for the three-dimensional reconstruction of the object or the whole at successive temperatures;

[0018] - the enclosure has a general parallelepiped shape and said walls are transparent are located on two opposite sides of the enclosure;

[0019] - the heating elements are electrical resistances;

[0020] - the heating elements extend over and along two parallel walls of the enclosure, which extends between the transparent walls; the heating elements of the oven are thus outside the field of vision of the X-ray emitter to eliminate possible interference;

[0021] - the other walls of the enclosure are thermally insulated so as, for example, to that the external environment of the enclosure does not exceed 40°C;

[0022] - the other walls of the enclosure are made of a low-insulating material density chosen from a ceramic or graphite; it may be ceramic fibres preferably rigid, ceramic preferably with very high porosity or graphite felt preferably rigid; in the present application, low density means a density less than or equal to 1 kg per cubic metre, very high porosity means a porosity greater than or equal to 70%;

[0023] - the heating elements are capable of heating the enclosure up to a temperature of 1600°C, or even 1700°C.

[0024] The present invention also relates to a method of heating or preheating by means of an oven as described above, comprising the following steps: a. positioning of an assembly comprising a ceramic shell mold and at least one ceramic core located within said mold, inside the furnace enclosure, b. heating or preheating of the assembly by means of the heating elements, and observation by tomography of said assembly during heating so as to detect relative displacements between the shell mold and said at least one core. Brief description of the figures

[0025] Other features and advantages of the invention will become apparent upon reading the detailed description that follows, for an understanding of which reference should be made to the accompanying drawings in which:

[0026] [Fig-1] The [Fig.1] is a perspective view of an aircraft turbomachine blade;

[0027] [Fig.2] Fig.2 is a schematic perspective and cross-sectional view of an assembly comprising a shell mold and at least one core, for the production of a blade of the type of that of [Fig.1];

[0028] [Fig. 3] Fig. 3 is a very schematic view of an embodiment of a preheating or heating oven according to the invention; and

[0029] [Fig.4] The [Fig.4] is another very schematic view of the furnace of the [Fig.3]. Detailed description of the invention

[0030] Figure 1 shows a hollow rotor blade 10 for an aircraft turbomachine, and in particular for a turbine of such a turbomachine. As is known, the blade 10 has a foot 12 which is intended to be received in a rotor disk (not shown) and a blade 14 which extends from this foot. The blade 14 has two walls, respectively lower surface 16 and upper surface 18, which meet at an upstream end of the blade 14 at a leading edge 20, and at an opposite downstream end of the blade at a trailing edge 22. The terms upstream and downstream refer to the flow of gases in the turbine and over the blade 14 of the blade 10.

[0031] Inside the blade 10 there is at least one internal cavity designed to allow the circulation of a cooling airflow intended to cool the blade 10 in order to preserve its integrity. Indeed, the aforementioned gases come from the combustion chamber and are very hot.

[0032] The cooling airflow comes from an internal pressurization circuit of the turbomachine enclosures, this internal pressurization circuit being supplied with pressurized air from at least one tap made on a compressor of the turbomachine.

[0033] The internal cavity of the blade communicates with external surfaces 26 of the intrados 16 and extrados 18 walls via a plurality of cooling orifices Division 28. The presence of these channels 28 allows the external surfaces 26 of the blade 10 to be bathed in a flow of cooling air, thus limiting the action of hot gases on the external surfaces 26 of the blade 10.

[0034] Conventionally, to manufacture such a blade 10, the blade 10, equipped with its internal cavity, is first manufactured during a first molding phase using a lost-wax casting process. The orifices 28 are made by laser drilling or by electrical discharge machining (EDM) on the metallic blade 10 with the casting and solidification process.

[0035] This molding phase includes firstly a first step during which at least one ceramic core 32 is manufactured, defining at least one counterform of the internal cavity of the blade 10 to be manufactured (cf. [Fig.2]).

[0036] Next, the core 32 is placed in a metal mold whose internal walls conform to the external shape of the blade 10 to be manufactured. A wax is then injected into the metal mold, between the core 32 and the internal walls of the mold.

[0037] Once the wax has solidified, the mold is opened and a first assembly consisting of the core trapped in the solidified wax is removed.

[0038] A second mold, called the carapace mold 34, is formed on this first assembly; it is made of ceramic and conforms to an external shape of this assembly (see [Fig.2]).

[0039] The wax is removed from the shell mold 34. There is therefore only a free space 36 between the core 32 and the shell mold 34. This removal is generally carried out by melting the wax by placing the assembly in a dewaxing autoclave.

[0040] The shell mold 34 is subjected to a baking process, then it is preheated to a temperature above the melting temperature of the metal alloy used for manufacturing the blade 10.

[0041] Next, a molten metal is poured into the space 36 left free between the core 32 and the shell mold 34.

[0042] The shell mold 34 is then opened or destroyed so that only the core 32 remains trapped in the solidified metal.

[0043] The ceramic core 32 is then removed to obtain a casting blank of the hollow rotor blade 10. The removal is generally carried out by chemical dissolution in an autoclave, sometimes combined with immersion of the part in an ultrasonic bath.

[0044] The present invention provides a furnace 40, schematically illustrated in Figures 3 and 4, which can be used during one or more of the steps described above. Advantageously, it is used for preheating the assembly illustrated in [Fig. 2] comprising the core(s) 32 and the shell mold 34. For example, it is used to heat this assembly from ambient temperature to at least 1500°C, preferably at least 1600°C, and more preferably up to 1700°C. The molten metal is, for example, at 1600°C and the assembly is preheated to this temperature. The temperature allows the temperature difference between the assembly and the metal to be limited.

[0045] The furnace 40 comprises a chamber 42 and heating elements 44 for the chamber 42. The furnace 40 further comprises a tomography device 46 including an X-ray emitter 48 and a receiver 50 for these rays. The chamber 42 is located between the emitter 48 and the receiver 50 and has walls 54b that are transparent to X-rays.

[0046] In the example shown, the enclosure 42 has a general parallelepiped shape and therefore includes four lateral walls 54 forming the sides of the enclosure 42, and two walls, respectively upper 56, and lower 58.

[0047] Among the four side walls 54, there are two parallel walls 54a facing each other which are transparent to X-rays, and two other parallel and facing walls 54b which are not necessarily transparent and which may therefore be opaque to X-rays.

[0048] It is understood that the X-rays are intended to pass through the walls 54a of the enclosure 42 and are not necessarily intended to pass through the walls 54b of the enclosure 42. The heating elements 44 can be located inside the enclosure 42 and at the level of the walls 54b so as not to interfere with the transmission of the X-rays.

[0049] The heating elements 44 can extend on and along the two walls 54b. The heating elements 44 are, for example, electrical resistors. They are capable of heating the enclosure up to a temperature of 1500°C, or even 1600°C.

[0050] The walls 54, 56, 58 of the enclosure 42, and in particular the walls 54a, 56 and 58, are preferably thermally insulated so that, for example, the external environment of the enclosure 42 does not exceed 40°C.

[0051] The insulation of the walls is for example obtained by making them out of a low density insulating material chosen from a ceramic or a graphite.

[0052] This may be ceramic fibers, preferably rigid, ceramics, preferably with a very high porosity ratio, or a graphite felt, preferably rigid.

[0053] The oven 40 preferably includes a tray 60 located in the enclosure 42 and movable in rotation around a vertical axis A. This tray 60 is intended to support the object or assembly to be heated or preheated.

[0054] This platform 60 can be directly connected to a platform or to a movable arm of the tomography device 46 so that the device can directly control the rotational movement of the object or the assembly.

[0055] The invention also relates to a method of heating or preheating by means of the furnace 40 described above for the manufacture of a blade 10 as illustrated for example in [Fig.1].

[0056] This process comprises the following steps: a. Positioning of the assembly comprising a ceramic shell mold 34

[0057]

[0058]

[0059]

[0060]

[0061]

[0062] and at least one ceramic core 32 located in the mold 34, within the chamber 42 of the furnace 40, b. heating or preheating of the assembly using the heating elements 44, and observation by tomography of the assembly during heating so as to detect relative displacements between the shell mold 34 and the core 32. The invention makes it possible to better understand and control the displacement of the core 32 inside the shell mold 34 to improve the robustness of the manufacturing process of rotor blades with advanced cooling circuit and very high added value. Other advantages of the invention include, for example: - improvement of the dimensional quality of the blades, - validation of the numerical simulation and modeling of the core displacement, - reduction of production costs, - etc.

Claims

Demands

1. Heating or preheating furnace (40), this furnace (40) comprising: - an enclosure (42), - heating elements (44) for the enclosure up to a temperature above 1500°C, - a tomography device (46) comprising an X-ray emitter (48) and a receiver (50) for these rays, the enclosure (42) being disposed between the emitter (48) and the receiver (50) and comprising walls (54b) transparent to said rays, characterized in that the enclosure (42) has a general parallelepiped shape and comprises two transparent walls (54b) situated on two opposite sides of the enclosure (42), the heating elements (44) extending on and along two parallel walls (54b) of the enclosure (42), which extend between the transparent walls (54a).

2. Oven (40) according to claim 1, wherein it further comprises a tray (60) located in the enclosure (42) and movable in rotation about a vertical axis (A), this tray (60) being intended to support an element to be heated or preheated.

3. Oven (40) according to any one of the preceding claims, wherein the heating elements (44) are electrical resistances.

4. Oven (40) according to any one of the preceding claims, wherein the other walls (54, 56, 58) of the enclosure (42) are thermally insulated so that, for example, the external environment of the enclosure does not exceed 40°C.

5. Oven (40) according to any one of the preceding claims, wherein the other walls (54, 56, 58) of the enclosure (42) are made of a low-density insulating material selected from a ceramic or a graphite.

6. Oven (40) according to any one of the preceding claims, wherein the heating elements (44) are capable of heating the enclosure (42) up to a temperature of 1600°C, or even 1700°C.

7. A method for heating or preheating using a furnace (40) according to any one of the preceding claims, comprising the following steps: a. positioning an assembly comprising a ceramic shell mold (34) and at least one ceramic core (32) located in said mold (34), within the chamber (42) of the furnace (40), b. heating or preheating the assembly using the heating elements (44), and observation by tomography of said assembly during heating so as to detect relative displacements between the shell mold (34) and said at least one core (32).

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