Coated lost-wax casting core

Abstract

Coated Lost-Wax Casting Core The invention relates to a casting core (10) comprising a main part (100) of molybdenum or molybdenum alloy, characterized in that it comprises a coating on the external surface of its main part comprising, from the core outwards, a first layer of titanium carbonitride TiCN (101) and a second layer of alumina Al2O3 (102). Figure for the abstract: Fig. 1.

Description

Title of the invention: Coated lost-wax casting core technical field

[0001] The present exposition relates to foundry tools for the manufacture of metal parts and more specifically to foundry cores used for the production of hollow parts.

[0002] The present exposition relates more specifically to a particular coating for such cores. Previous technique

[0003] The lost-wax casting process is known from the literature and allows a metal part to be obtained directly to the desired dimensions by using a wax model of the part to be obtained, forming a mold around the model and removing the wax to obtain in the mold a cavity in the shape of the wax model, and therefore of the desired part.

[0004] When the part is hollow, that is, when it has recesses, it is possible to create these recesses using a casting core shaped like the desired recess. The casting core is positioned in the mold so that the molten metal introduced into the mold cannot occupy the space of the casting core.

[0005] The subsequent removal of the core makes it possible to obtain in the metal part a recess having the shape of the foundry core in place of the foundry core initially present.

[0006] The use of such a foundry core makes it possible to avoid subsequent machining of a massive metal part to create the desired recess, thus making it possible to easily create recesses with complex geometries.

[0007] However, the casting core is in contact with the metal being poured into the mold and must therefore withstand the temperatures involved. Furthermore, it is important that the core be easily removable once the metallic part is obtained, in order to create the desired porosity.

[0008] Core compositions are known, for example ceramic compositions, for example with silica, alumina and / or zirconia bases. Other compositions are also known based on molybdenum or molybdenum alloys (sometimes called RMC for the English acronym "Refractory Metal Core").

[0009] However, these alloys do not exhibit sufficient temperature resistance to oxidation for direct application in foundry processes and are also soluble in nickel-based superalloys. Therefore, it is generally recommended to coat them with a protective coating against oxidation.

[0010] Furthermore, the coating must be capable of protecting the core from the molten metal that will be poured into the mold to form the desired part. Conversely, the coating must prevent the constituent elements of the core or the coating from dissolving in the molten metal.

[0011] However, it is interesting to continue developing the available coatings to obtain solutions different from those currently available and even more adapted to foundry cores. Description of the invention

[0012] The present invention is specifically designed to meet this need.

[0013] For this purpose, it relates according to a first of its aspects, to a foundry core comprising a main part of molybdenum or molybdenum alloy, characterized in that it comprises a coating on the external surface of its main part comprising, from the core outwards, a first layer of titanium carbonitride TiCN and a second layer of alumina A12O3.

[0014] It is to the credit of the inventors that they determined that a coating comprising these two layers provided excellent protection of the foundry core.

[0015] More specifically, the first layer of titanium carbonitride (TiCN) has a coefficient of thermal expansion very close to that of molybdenum or molybdenum alloys. The first layer therefore helps to limit cracking that could occur during the thermal cycling experienced by casting cores.

[0016] In addition, titanium carbonitride TiCN has excellent chemical compatibility with molybdenum and molybdenum alloys, which promotes on the one hand the adhesion of the coating to the surface of the core and on the other hand the integrity of the coating during the casting stages.

[0017] The second layer of the coating has excellent high-temperature stability and excellent adhesion to the first layer.

[0018] In one embodiment, the first layer of titanium carbonitride TiCN is deposited directly in contact with the external surface of the main part of the core.

[0019] The inventors have indeed determined that it is in no way necessary to place a layer between the titanium carbonitride layer TiCN and the molybdenum or molybdenum alloy layer forming the main part.

[0020] This results in a simplified process without loss of property for the coating or its adhesion quality.

[0021] In one embodiment, the first layer of titanium carbonitride TiCN has a thickness between 1.0 pm and 30 pm.

[0022] In one embodiment, the second layer of alumina A12O3 is disposed directly in contact with the titanium carbonitride layer TiCN.

[0023] In an alternative embodiment, an aluminum nitride layer AIN is disposed directly in contact with the titanium carbonitride layer TiCN, and then this aluminum nitride layer AIN is directly covered with the alumina layer A12O3.

[0024] Although optional, the intermediate layer of aluminium nitride AIN has a coefficient of thermal expansion intermediate between that of titanium carbonitride TiCN and that of alumina A12O3, which promotes good adhesion of the coating.

[0025] In particular, the presence of an intermediate layer of aluminum nitride AIN prevents the propagation of deep cracks, which is particularly advantageous when the nuclei have particularly complex shapes.

[0026] In addition to ensuring a more gradual transition in the coefficients of thermal expansion, the presence of aluminium nitride AIN makes it possible to ensure protection against oxidation, even in the case where the external alumina layer A12O3 is cracked during a casting step.

[0027] Indeed, the presence of the aluminum nitride layer AIN then allows a reaction with oxygen which can pass through the alumina layer to form with the aluminum nitride an alumina layer A12O3 impermeable to oxygen.

[0028] The integrity of the molybdenum or molybdenum alloy core is thus preserved even more.

[0029] In one embodiment, the thickness of the A12O3 alumina layer is between 5.0 pm and 50 pm.

[0030] In an embodiment where it is present, the thickness of the aluminum nitride layer AIN can be between 1.0 pm and 40 pm.

[0031] In one embodiment, the main part of the core has the shape of the cooling circuits of a turbomachine blade.

[0032] Indeed, turbomachine blades are generally made by casting, in particular for hot part turbomachine blades, i.e. those located after the combustion chamber.

[0033] It is then particularly advantageous to use a cast core rather than subsequent machining to manufacture the cooling circuits of a turbomachine blade.

[0034] Indeed, the particularly complex geometry of the cooling circuits of a turbomachine blade is not necessarily achievable by machining after the part has been manufactured. Furthermore, for single-crystal turbomachine blades, Such machining is not feasible, and it is therefore preferable to use cast cores.

[0035] The described foundry core then makes it possible to obtain the cooling circuits in such a blade, without complicating the foundry process.

[0036] According to another aspect of it, the invention also relates to a method for manufacturing a core as described above.

[0037] Indeed, the coating of the core can be obtained entirely by a chemical vapor deposition (CVD) process.

[0038] Thus, according to one embodiment, the invention relates to a method for manufacturing a core as described above, comprising at least the following steps: - a step of depositing a layer of titanium carbonitride (TiCN) by a chemical vapor deposition process onto the main part of a molybdenum or molybdenum alloy casting core; then - a step of depositing a layer of alumina A12O3 by a chemical vapor deposition process.

[0039] In one embodiment, the process allows excellent coating of a core ensuring good protection against oxidation for the whole.

[0040] In particular, the process is perfectly adapted from a foundry core which has the shape of the cooling circuits of a turbomachine blade.

[0041] Indeed, the particularly complex and narrow geometry of these cores greatly benefits from a coating made by a chemical vapor deposition process which makes it possible to cover the entire core even in areas where the geometry would be difficult to cover by other deposition methods.

[0042] In one embodiment, the step of depositing the titanium carbonitride layer TiCN by a chemical vapor deposition process is carried out at a temperature between 700°C and 1150°C, at a deposition pressure between 200 mbar and 600 mbar, using TiCl4 as a precursor.

[0043] In one embodiment, the step of depositing the A12O3 alumina layer by a chemical vapor deposition process is carried out at a temperature between 700°C and 1150°C, at a deposition pressure between 20 mbar and 500 mbar, using A1C13 and HCl as precursors.

[0044] In one embodiment, the process may further comprise, between the step of depositing the titanium carbonitride layer TiCN and the step of depositing the alumina layer Al2O3, a step of depositing an aluminium nitride layer AIN by a chemical vapor deposition process.

[0045] For example, the step of depositing the aluminum nitride layer AIN by a chemical vapor deposition process is carried out at a temperature between 700°C and 1150°C, at a deposition pressure between 50 mbar and 500 mbar, using HCl, AlCl3 and NH3 as precursors.

[0046] In one embodiment, the coating process does not include any other deposition steps than the step of depositing a layer of titanium carbonitride TiCN by a chemical vapor deposition process, the step of depositing a layer of alumina A12O3 by a chemical vapor deposition process and optionally the step of depositing a layer of aluminium nitride AIN by a chemical vapor deposition process.

[0047] In one embodiment, the different deposition steps can be carried out in the same deposition enclosure. Brief description of the drawings

[0048] [Fig-1] Figure [Fig.1] schematically represents a nucleus in a mode of realization of the invention.

[0049] [Fig.2] Fig.2 schematically represents a kernel in another mode of realization of the invention than that of [Fig.1]. Description of the implementation methods

[0050] The invention is now described by means of figures, which are provided for descriptive purposes to illustrate certain embodiments of the invention and which should not be interpreted as limiting the latter.

[0051] Fig. 1 represents schematically a foundry core 10.

[0052] The actual complexity of the geometry of a kernel is not represented because this would be detrimental to understanding the figure.

[0053] In one embodiment, the foundry core 10 may include a main part 100 of molybdenum or molybdenum alloy.

[0054] Here and throughout the application, "main part" means the useful part of the core, i.e. the part of the core having the shape of the cavity to be obtained in the final part.

[0055] In one embodiment, the main part is understood to mean the entire molybdenum core 10.

[0056] In other embodiments, the core 10 may include, in addition to the main part 100, non-useful parts, allowing, for example, the core 10 to be attached in a foundry mold, which may not be covered by the coating.

[0057] Alternatively, and even in the case where the core includes non-useful parts, the choice may be made to cover the entire protective coating.

[0058] Fig. 1 represents an embodiment in which the main part 100 of the core 10 is directly coated with a layer of titanium carbonitride TiCN 101.

[0059] For example, the thickness ei of the titanium carbonitride layer TiCN 101 is between 1.0 pm and 30 pm.

[0060] For the purposes of this application, "thickness" has its usual meaning and characterizes the smallest dimension of extension of a layer. For example, in [Fig. 1], the thickness of a layer is measured in the direction perpendicular to the surface of the underlying layer.

[0061] For the purposes of this application a layer shall be said to be "of a compound", if it comprises by mass more than 95%, or even more than 99% or more than 99.9% of said compound.

[0062] Fig. 1 further represents an alumina layer A12O3 102, disposed directly in contact with the titanium carbonitride layer TiCN 101.

[0063] The thickness e2 of the A12O3 102 alumina layer is between 5.0 pm and 50 pm.

[0064] Preferably, the core coating does not include any other layers than the titanium carbonitride layer TiCN 101 and the alumina layer A12O3 102.

[0065] In one embodiment, the coating can be obtained by a chemical vapor deposition process.

[0066] In particular, this deposition process makes it possible to obtain layers with very high purities.

[0067] This deposition can be carried out in a furnace known as such.

[0068] Figure [Fig. 2] illustrates a foundry core 10 in another embodiment.

[0069] In the embodiment shown in [Fig.2], the core further comprises, between the titanium carbonitride layer TiCN 101 and the alumina layer A12O3 102, an aluminum nitride layer AIN 103.

[0070] As described, this layer allows for a more gradual evolution of the coefficients of thermal expansion between the main part 100 of the core and the coating layers 101, 102, 103.

[0071] This ensures even better resistance to delamination and cracking when the core is heated, for example when the latter will be placed in contact with molten metal or during a baking step of the shell mold.

[0072] In one embodiment, the thickness e3 of the aluminum nitride layer AIN can be between 1.0 pm and 40 pm.

[0073] In one embodiment, the coating of the core 10 may not include any other layer than the titanium carbonitride layer TiCN 101, the aluminium nitride layer AIN 103 and the alumina layer A12O3 102.

[0074] In one embodiment, the outer layer of the coated core is the A12O3 102 alumina layer.

[0075] Indeed, this layer has all the characteristics enabling the core to exhibit the desired properties in terms of resistance to oxidation in all stages of casting, for the protection of the underlying main part 100.

[0076] In one embodiment, the core 10 can be used in a conventional lost-wax casting process comprising, after the core manufacturing step, the following steps: - the fabrication of a wax model of the part to be obtained around the foundry core; - the formation of a shell mold around the wax model, for example by dipping the wax model in a ceramic slip; - the removal of the wax, by heat treatment; - the pouring of molten metal into the shell mold in the space left vacant by the removed wax; - the solidification of molten metal by cooling; - the removal of the shell mold, for example by a mechanical step; - the removal of the core to form a cavity in the metal part.

[0077] The steps described above are characteristic of a lost-wax casting process and a core 10 as described above is perfectly suited to such a process.

Claims

Demands

1. Foundry core (10) comprising a main part (100) of molybdenum or molybdenum alloy, characterized in that it comprises a coating on the external surface of its main part comprising, from the core outwards, a first layer of titanium carbonitride TiCN (101) and a second layer of alumina Al2O3(102).

2. Foundry core (10) according to claim 1, wherein the first layer of titanium carbonitride TiCN (101) is deposited directly in contact with the external surface of the main part of the core (100).

3. Casting core (10) according to claim 1 or 2, wherein the first layer of titanium carbonitride TiCN (101) has a thickness between 1.0 pm and 30 pm.

4. Foundry core (10) according to any one of claims 1 to 3, wherein the second layer of alumina A12O3 (102) is disposed directly in contact with the titanium carbonitride layer TiCN (101).

5. Casting core (10) according to any one of claims 1 to 3, wherein an aluminium nitride layer AIN (103) is disposed directly in contact with the titanium carbonitride layer TiCN (101), and then this aluminium nitride layer AIN (103) is directly covered with the alumina layer A12O3 (102).

6. Casting core (10) according to claim 5, wherein the thickness of the aluminum nitride layer AIN can be between 1.0 pm and 40 pm.

7. Foundry core (10) according to any one of claims 1 to 6, wherein the thickness of the alumina layer A12O3 (102) is between 5.0 pm and 50 pm.

8. A method for manufacturing a foundry core (10) according to any one of claims 1 to 7 comprising at least the following steps: - a step of depositing a layer of titanium carbonitride TiCN by a chemical vapor deposition process on the main part of a foundry core made of molybdenum or molybdenum alloy; then - a step of depositing a layer of alumina A12O3 by a chemical vapor deposition process.

9. A method for manufacturing a foundry core according to claim 8, related to any one of claims 1 to 3 or 5 to 7, further comprising between the step of deposition of the titanium carbonitride layer TiCN (101) and the step of deposition of the alumina layer Al2O3(102) a step of deposition of an aluminium nitride layer AIN (103) by a chemical vapor deposition process.

10. A process according to claim 9, wherein the step of depositing the aluminum nitride layer AIN by a chemical vapor deposition process is carried out at a temperature between 700°C and 1150°C, at a deposition pressure between 50 mbar and 500 mbar, using TiCl4 as a precursor.

11. A process according to any one of claims 8 to 10, wherein the step of depositing the A12O3 alumina layer by a chemical vapor deposition process is carried out at a temperature between 700°C and 1150°C, at a deposition pressure between 20 mbar and 500 mbar, using A1C13 and HCl as precursors.

12. A process according to any one of claims 8 to 11, wherein the step of depositing the titanium carbonitride layer TiCN by a chemical vapor deposition process is carried out at a temperature between 700°C and 1150°C, at a deposition pressure between 200 mbar and 600 mbar, using HCl, AlCl3 and nH3 as precursors.

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