Core for casting and manufacturing processes

Abstract

The present invention relates to a foundry core assembly (1) comprising at least one refractory metal core (4), at least one ceramic core (2) and at least one attachment element (6) placed on the ceramic core (2) and / or on the refractory metal core (4), the cores (2, 4) and the at least one attachment element (6) being solid to each other, the core assembly (1) comprising a protective coating (10) covering at least a part of the refractory metal core (4) and the at least one attachment element (6), the protective coating (10) comprising at least a first protective layer.

Description

Title of the invention: Core for a casting process and manufacturing process. FIELD OF THE INVENTION

[0001] The invention relates to the manufacture of a core assembly comprising a refractory metal core, a ceramic core, and a ceramic core attachment element, the assembly being covered with a protective coating. The invention also relates to a method for manufacturing a complex metal alloy part using the core assembly. STATE OF THE ART

[0002] Lost-wax casting processes are used in particular to manufacture metal parts. Such processes are notably used in the aeronautics industry for the manufacture of gas turbine components.

[0003] A lost-wax casting process typically involves the fabrication of wax models of the metal part to be manufactured, for example by molding or additive manufacturing, the assembly of the wax models into clusters, the fabrication of the ceramic shell from the wax cluster, the removal of the wax, notably by melting it, and finally the casting of a metal alloy into the shell. The ceramic shell (or ceramic mold) must be sintered after its fabrication before the metal alloy is cast into it. The advantage of such a process is that it is possible to cast metal alloys with very high melting points, such as nickel- or cobalt-based alloys.

[0004] Casting cores can be used in these processes to manufacture complex parts. Indeed, gas turbine components, such as blades, may include recesses, cooling circuits, or thin sections. To form these complex geometries, the cores are placed in molds to create wax models and are then embedded in the shell, allowing the final part to be formed in the desired shape.

[0005] Casting cores are monolithic parts that must withstand often high temperatures, up to 1500°C, particularly when the metal alloy cast into the core is a nickel- or cobalt-based alloy. Furthermore, casting cores must be easily manufacturable to ensure that a part with the desired geometry can be obtained precisely.

[0006] Ceramic cores are widely used in these casting processes. However, this type of core does not allow for the production of highly complex parts.

[0007] Recently, refractory metals, such as molybdenum (Mo) or molybdenum (Mo) alloys, particularly the TZM type alloy (molybdenum, titanium (Ti), and zirconium (Zr)), have been used instead of ceramics to manufacture casting cores suitable for lost-wax casting. The use of refractory metal alloys makes it possible to produce cores with complex shapes, thus enabling the creation of fine and precise geometric cross-sections in the manufactured parts. Furthermore, refractory metals exhibit very good mechanical properties and can withstand very high temperatures.

[0008] It is also known to use as a core for the process an assembly comprising at least one ceramic core and at least one refractory metal core. Such an assembly allows the ceramic core(s) to be used for the simpler parts of the assembly and the refractory metal core(s) to be used for the more complex parts of the assembly.

[0009] However, a disadvantage of refractory metal alloys is that during certain stages of the lost-wax casting process, a parasitic layer may be formed on their surface.

[0010] For example, molybdenum can oxidize, forming molybdenum dioxide on its surface, or even sublime, forming molybdenum trioxide. These oxidation reactions occur above certain temperatures in air, particularly during the curing of the shell. To prevent the formation of this unwanted layer, it is necessary to coat the nucleus with a protective layer.

[0011] Furthermore, when a core assembly is used in a lost-wax casting process, the individual cores that compose it must be joined together by means of mechanical fasteners or an adhesive / fixing material. In addition, the core assembly is generally equipped with one or more fastener elements on its surface.

[0012] However, the bonding / fixing material, mechanical fasteners, and / or fastener elements may not withstand the conditions of the lost-wax casting process and may therefore be degraded during the process, as may the surface of the assembly surrounding these parts and this material. Degradation of the surface of the core assembly may alter its geometry and thus render the geometry of the part to be manufactured inaccurate. Description of the invention

[0013] An objective of the invention is to provide a set of cores whose surface and possible attachment elements do not degrade during the lost-wax casting process.

[0014] Another objective of the invention is to provide a set of cores enabling the precise manufacture of complex parts.

[0015] Another objective of the invention is to provide a set of cores on which a protective coating can be easily deposited.

[0016] The invention thus relates to a set of foundry cores comprising at least one refractory metal core, at least one ceramic core and at least one attachment element placed on the ceramic core and / or on the refractory metal core, the cores and at least one attachment element being solid to each other, the set of cores comprising a protective coating covering at least a part of the refractory metal core and at least one attachment element, the protective coating comprising at least a first protective layer.

[0017] The core assembly, thus protected by the protective coating, can be used in high-temperature processes without damaging the parts of the core assembly covered by the protective coating. For example, the core assembly can be used in a lost-wax casting process carried out at temperatures exceeding 1000°C under oxidizing conditions, while allowing the coated parts of the core assembly to retain their integrity. In particular, there is no discontinuity in the coating between the attachment element and the coated core portion. Therefore, the core assembly does not present a risk of damage to the core portion and / or the attachment element. Protective coating

[0018] The first protective layer of the protective coating may include a nitride; an oxide; a metallic material, for example including aluminium, molybdenum or nickel; a composite material; or a mixture thereof.

[0019] According to a first embodiment, the first protective layer may comprise a nitride, the nitride being able to be chosen from boron nitride (BN), silicon nitride (Si3N4), aluminium nitride (AIN), titanium carbon nitride (TiCN), and one of their mixtures.

[0020] Preferably, the first protective layer comprises a nitride selected from boron nitride (BN) and silicon nitride (Si3N4). The protective layer may, for example, consist of a nitride selected from boron nitride (BN) and silicon nitride (Si3N4).

[0021] According to another embodiment, the first protective layer comprises an oxide selected from silica (SiO2), alumina (Al2O3), zirconium oxide (ZrO2) and one of their mixtures.

[0022] According to another embodiment, the first protective layer comprises a composite material such as molybdenum disilicide (MoSi2) or the Mo(Si,Al)2 composite.

[0023] According to another possible embodiment, the first protective layer can be formed directly in contact with the surface of the parts of the cores that it covers, which means that the cores that it covers do not have any other layer between the covered parts and the first protective layer.

[0024] The protective coating may further include a second protective layer positioned directly in contact with the surface of the first protective layer, and optionally a third protective layer positioned directly in contact with the surface of the second protective layer.

[0025] The second protective layer of the protective coating may comprise a nitride, an oxide, a composite material, a metallic material, or a mixture thereof, such as those described for the first protective layer. The protective layer may be identical to or different from the first protective layer.

[0026] The third protective layer may include a nitride, an oxide, a composite material, a metallic material or a mixture thereof, such as those described for the first protective layer.

[0027] When the protective coating has three protective layers, the three layers may be made of the same material; or two of the layers may be made of the same material, the third of the layers may be made of a different material, the layer different from the other two may be the first, second or third layer; or all three layers may be made of a different material.

[0028] When it is indicated that the protective coating has several layers, this means that on at least part of the surface of the core assembly, the coating has several layers, while other parts of the assembly may have fewer protective layers or only one protective layer. Preferably, the core assembly is entirely covered by the several layers. Ceramic core

[0029] The material forming the ceramic core(s) is a non-metallic and non-organic material.

[0030] The ceramic core may include, in particular, alumina, a silica-based ceramic such as silicon dioxide, or a zirconium-based ceramic such as zirconium dioxide.

[0031] The ceramic core can conventionally be manufactured by mixing the ceramic with a binder, for example paraffin-based, and a solvent, particularly in the case The core can be manufactured by injection molding. The resulting core then undergoes a debinding step and is subsequently sintered to obtain a core with the desired mechanical properties. Alternatively, the ceramic core can be manufactured by additive manufacturing followed by a debinding step and then sintering.

[0032] The resulting ceramic core can then be machined to give it its final shape.

[0033] The ceramic core can be manufactured directly in its final shape, meaning that it includes the recess(es) and / or notch(s) into which the refractory metal core(s) will be inserted. According to one possible variant, the ceramic core can be manufactured in a non-final shape and machined to create the recess(es) and / or notch(s).

[0034] Similarly, any openings allowing the insertion of one or more attachment elements can be made during the manufacture of the cores or afterwards by machining or drilling. Refractory metal core

[0035] The refractory metal core comprises a refractory metal which can be selected from molybdenum, niobium, tantalum, tungsten, rhenium, titanium, zirconium, hafnium, and one of their mixtures.

[0036] Preferably, the refractory metal comprises molybdenum (Mo). The refractory metal can then be chosen from molybdenum, titanium zirconium molybdenum (TZM), and molybdenum rhenium (Mo-Re).

[0037] The refractory metal core can, for example, be formed by machining a blank; by electron-beam melting (EBM); by selective laser melting (SLM); by wire arc additive manufacturing (WAAM); or by stereolithography (SLA). The refractory metal core can also be manufactured by metal injection molding (MIM) or by metal binder jetting (MBJ). The MIM and MBJ processes produce a so-called "green" core in which the refractory metal is mixed with a binder and which does not yet have its final mechanical and geometric characteristics.This green core must then be sintered to obtain the final core that can be used to form the set of cores. Attachment element

[0038] The attachment element can be any type of element enabling a user to manipulate the set of kernels.

[0039] The attachment element may include, for example, a rod, a ring, a hook, or any other element enabling the gripping of the core assembly.

[0040] According to one possible embodiment, the core assembly may include several attachment elements. According to this embodiment, several attachment elements may be inserted into a single core, or each attachment element may be inserted into one of the cores in the assembly. It is also possible that some cores do not include any attachment elements, while other cores include one or more.

[0041] The attachment element(s) is / are preferably inserted into the ceramic core(s).

[0042] The attachment element(s) are inserted before the protective coating is formed. Thus, the protective coating covers the interface between the attachment element and the core into which it is inserted, and the assembly does not present any unprotected area that could be damaged on or around the attachment element.

[0043] The attachment element is formed of metal or ceramic, such as silica or alumina. Set of kernels

[0044] The assembly may include several ceramic cores and / or several refractory metal cores and / or several fastening elements.

[0045] Preferably, the attachment element or attachment elements are positioned in the ceramic core(s).

[0046] When the assembly includes several attachment elements, preferably all the attachment elements and the ceramic core parts surrounding them are covered with the protective coating.

[0047] According to a preferred embodiment, the entire surface of the ceramic core or ceramic cores is covered with the protective coating.

[0048] The coating may also cover at least part of the refractory metal core. When the assembly comprises several refractory metal cores, preferably at least part of each refractory metal core is covered with the protective coating. According to a preferred embodiment, the entire surface of each of the refractory metal cores is covered with the protective coating.

[0049] According to one possible embodiment, the core assembly further comprises a fastening material for holding at least one refractory metal core and at least one ceramic core together and fixed relative to each other. When the assembly comprises several refractory metal cores and / or several ceramic cores, the fastening material holds the individual cores together and fixed relative to each other.

[0050] According to this embodiment, the fixing material comprises a compound which may be an adhesive which may be based on silica; alumina; zirconia; colloidal silica; or a mixture thereof.

[0051] According to this embodiment, the fastening material forms a joint between two cores and has an outer surface positioned between the surface of one core and the surface of the core to which it is bonded. The outer surface of the fastening material is then partially or totally covered by the protective coating. Method for manufacturing the core assembly

[0052] The set of nuclei as previously described is manufactured by a process comprising: - a step El of assembling at least one refractory metal core, at least one ceramic core, and at least one fastening element, the fastening element belonging to at least one refractory metal core or to at least one ceramic core; - a step E2 of applying the protective coating to the whole so as to cover at least part of the refractory metal core and at least one attachment element.

[0053] Step El includes in particular the insertion of the attachment element or attachment elements into the refractory metal core(s) and / or into the ceramic core(s).

[0054] Step El further comprises the assembly of the cores possibly using the fixing material.

[0055] The cores can be assembled before or after the insertion of the fastening element(s). Preferably, the ceramic core(s) are bonded to the refractory metal core(s), and then the fastening element(s) are positioned on the ceramic core. The fastening element(s) can, in particular, serve as a gripping element when applying the protective coating to the surface of the cores.

[0056] In step E2, which consists of applying the protective coating to the assembly, the coating is applied in such a way as to cover at least a portion of the core(s) and the attachment element(s). When the coating does not completely cover the ceramic core(s), it covers at least the portion of the ceramic surrounding the attachment element.

[0057] The protective layer can be formed by chemical vapor deposition or physical vapor deposition, preferably by chemical vapor deposition. Physical vapor deposition can be selected from direct vacuum evaporation or vapor-electron beam evaporation, sputtering, pulsed laser ablation, molecular beam epitaxy, or electric arc deposition.

[0058] Chemical Vapor Deposition (CVD) can be carried out at a temperature ranging from 700°C to 1150°C and at a pressure ranging from 50 mbar to 500 mbar.

[0059] In the case where a boron nitride (BN) layer is formed, the precursors for chemical vapor deposition are boron trichloride (BC13) and ammonia (NH3) and the carrier gas is an inert gas, such as argon.

[0060] In the case where a silicon nitride (Si3N4) layer is formed, the precursors for chemical vapor deposition are ammonia (NH3) and a silicon gas chosen from silane (SiH4), silicon tetrachloride (SiCl4) and dichlorosilane (SiCl2H2) and the carrier gas is an inert gas, such as argon.

[0061] In the case where a boron nitride (AIN) layer is formed, the precursors for chemical vapor deposition are ammonia (NH3), hydrochloric acid (HCl) and aluminium trichloride (AlCl3) and the carrier gas is an inert gas, such as argon.

[0062] In the case where an oxidized layer such as alumina (Al2O3) is formed, the precursors for chemical vapor deposition are hydrochloric acid (HCl) and aluminum trichloride (AlCl3) and the gas is an inert gas, such as argon.

[0063] Physical Vapor Deposition (PVD) is carried out at a temperature ranging from 20°C to 600°C and at a pressure ranging from 102 mbar to 105 mbar.

[0064] In the case where a boron nitride (BN) layer is formed, the sputtering targets for physical vapor phase deposition are a boron precursor, such as boron trichloride, and the carrier gas is an inert gas, such as dinitrogen (N2).

[0065] In the case where a silicon nitride (Si3N4) layer is formed, the sputtering targets for physical vapor phase deposition are a silicon gas chosen from silane (SiH4), silicon tetrachloride (SiCL) and dichlorosilane (SiCl2H2) and the carrier gas is an inert gas, such as dinitrogen.

[0066] In the case where a boron nitride (AIN) layer is formed, the spray targets for physical vapor phase deposition are aluminium trichloride (AlCl3) and the carrier gas is an inert gas, such as dinitrogen.

[0067] In the case where an oxidized layer of the alumina type is formed, the spray target for physical vapor phase deposition is alumina (Al2O3).

[0068] When the protective coating has several layers, each layer can be made as described above.

[0069] According to one possible embodiment, certain parts of the core assembly may not be covered by the protective coating. To achieve this, one or more masks may be positioned on the core assembly before the protective coating(s) are applied. Preferably, the uncovered parts are positioned on the ceramic core(s). Preferably, the entire surface of the refractory metal core(s) is covered by the protective coating.

[0070] When the protective coating has two or three protective layers, step E2 is carried out successively as many times as the coating has layers, and this is done using the same process or using a different process depending on the nature of the layer.

[0071] The set of cores can then be used as a core in a lost-wax casting process. Method for manufacturing a complex nucleus

[0072] The invention also relates to a method for manufacturing a complex part made of a metal alloy, the method comprising the use of the set of cores previously described or manufactured according to the previously described method.

[0073] In particular, the method comprises: - a step F2 of covering a wax piece with a ceramic material; - a step F3 of wax removal to obtain a ceramic shell into which the set of cores is integrated; - a stage F4 of firing the ceramic shell to obtain the final shell; and - a step F5 of casting a metal alloy into the final shell; - a step F6 of removing the final shell and core to obtain the complex metal alloy part.

[0074] The complex part is for example an aircraft engine part such as a turbine blade having thin and complex sections as well as hollows for cooling circuits.

[0075] During step F6, the core assembly is eliminated by dissolution in a basic bath during the decoking of the formed part.

[0076] This process therefore allows the manufacturing of complex parts in an optimized manner using one or more cores of complex shape. DESCRIPTION OF THE FIGURES

[0077] [Fig.la] and [Fig.lb] represent a schematic top view of a set of kernels according to an embodiment of the invention;

[0078] [Fig. 2] represents a flowchart of a method for manufacturing a set of cores according to an embodiment of the invention; and

[0079] [Fig. 3) represents a logic diagram of a process for manufacturing a complex part using the set of cores of figures 1a and 1b according to an embodiment of the invention. DETAILED DESCRIPTION OF THE INVENTION

[0080] With reference to figures 1a, 1b and 2, a method for manufacturing a set of cores 1 according to an embodiment of the invention will be described.

[0081] The process includes the independent manufacture of a ceramic core 2, a molybdenum core 4 and an attachment element 6.

[0082] The ceramic core 2 comprises at least one element selected from silica, alumina, and zirconia, and a solvent- and paraffin-based binder. This core is manufactured by an injection molding process carried out at a temperature between 50°C and 80°C and at an injection pressure ranging from 20 to 200 bar. The core is then debound and sintered at a temperature between 1000°C and 1300°C. The ceramic core 2 is manufactured such that it includes a cavity 8 for receiving the molybdenum core 4.

[0083] The refractory metal core 4 is made of molybdenum and is manufactured by selective laser melting.

[0084] In this embodiment, the attachment element 6 is a silica part. The attachment element 6 is inserted into the ceramic core 2 by means of two orifices 9 and 9'.

[0085] The molybdenum core 4 is then inserted into the housing 8 and fixed so as to be solid to the ceramic core 2 by means of a fixing material 12 which is, in this embodiment, a silica-based adhesive.

[0086] In a step E2, once the ceramic core 2, the molybdenum core 4 and the attachment element 6 are assembled, the uncoated assembly 1 is covered with a coating 10 which, in this embodiment, covers the entire surface of the ceramic core 2, the entire surface of the molybdenum core 4 and the entire surface of the attachment element 6. The coating 10 thus also covers the interface between the attachment element 6 and the ceramic core 2 at the ports 9 and 9'.

[0087] In this embodiment, the coating 10 comprises a single layer of alumina. The layer is deposited using the chemical vapor deposition process.

[0088] The resulting set of cores 1 can then be used in a conventional lost-wax casting process according to [Fig.3] for the preparation of a complex part such as an aircraft engine blade.

[0089] With reference to [Fig. 3], the process comprises: - a step Fl of manufacturing a wax part comprising at least the set of cores 1, the wax part being manufactured using a mold; - a step F2 of covering the wax piece with a ceramic material; - a wax removal step F3 to obtain a ceramic shell into which the core 18 is integrated; - a step F4 of firing the ceramic shell to obtain the final shell; - a step F5 of pouring a metal alloy into the final shell; and - a step F6 of removing the final shell and the core set 1 to obtain the complex metal alloy part.

[0090] The protective coating 10 makes it possible, in particular, to carry out the shell baking step F4 without oxidizing the core, up to 1150°C for 1 hour during the shell baking process. The protective coating 10 also allows the core to withstand the casting of the metal alloy in step F5 to form the complex part.

Claims

Demands

1. A foundry core assembly (1) comprising at least one refractory metal core (4), at least one ceramic core (2) and at least one attachment element (6) placed on the ceramic core (2) and / or on the refractory metal core (4), the cores (2, 4) and the at least one attachment element (6) being bonded together, the core assembly (1) comprising a protective coating (10) covering at least a portion of the refractory metal core (4) and the at least one attachment element (6), the protective coating (10) comprising at least a first protective layer.

2. A core assembly (1) according to claim 1, wherein the first a protective layer comprises a nitride, an oxide, a metallic material, a composite material, or a mixture thereof.

3. A set of cores (1) according to claim 2, wherein the nitride is selected from boron nitride (BN), silicon nitride (Si3N4), aluminium nitride (AIN), titanium carbon nitride (TiCN), and a mixture thereof.

4. A set of cores (1) according to claim 2, wherein the oxide is selected from silica (SiO2), alumina (Al2O3), zirconium oxide (ZrO2), and a mixture thereof.

5. A core assembly (1) according to claim 2, wherein the composite material is selected from molybdenum disilicide (MoSi2) or Mo(Si,Al)2 composite.

6. A core assembly (1) according to any one of the preceding claims, wherein the refractory metal core (4) comprises a refractory metal selected from molybdenum, niobium, tantalum, tungsten, rhenium, titanium, zirconium, hafnium, and mixtures thereof.

7. A core assembly (1) according to any one of the preceding claims, wherein the ceramic core (2) comprises alumina, a silica-based ceramic such as silicon oxide, or a zirconium-based ceramic such as zirconium dioxide.

8. A core assembly (1) according to any one of the preceding claims, wherein the protective coating (10) covers at least a portion of the ceramic core (2).

9. A core assembly (1) according to any one of the preceding claims, wherein the attachment element (6) is formed of metal or ceramic.

10. A set of cores (1) according to any one of the preceding claims, further comprising a fixing material for keeping together and fixed to each other at least one refractory metal core (4) and at least one ceramic core (2).

11. A core assembly (1) according to claim 8, wherein the fixing material comprises an adhesive based on silica, alumina, zirconia, colloidal silica, or a mixture thereof.

12. Assembly of cores (1) according to claim 8 or 9, wherein the fixing material has an outer surface, the outer surface being at least partially covered by the protective coating (10).

13. A core assembly (1) according to any one of the preceding claims, wherein the protective coating (10) comprises two or three protective layers.

14. A method for manufacturing (20) an assembly of cores (1) according to any one of claims 1 to 13, the method comprising: - a step E1 of assembling at least one refractory metal core (4), at least one ceramic core (2), and at least one attachment element (6), the attachment element (6) belonging to at least one refractory metal core (4) or to at least one ceramic core (2); - a step E2 of applying the protective coating (10) to the assembly (1) so as to cover at least a part of the refractory metal core (4) and at least one attachment element (6).

15. Method (20) according to claim 13, wherein the first protective layer is formed by chemical vapor deposition or physical vapor deposition, the physical vapor deposition being selectable from direct vacuum or vapor-electron beam evaporation, sputtering, pulsed laser ablation, molecular beam epitaxy or electric arc deposition.

16. A method for manufacturing (30) a complex part made of a metal alloy, the method comprising the use of the core assembly (1) according to any one of claims 1 to 13 or manufactured according to the process according to any one of claims 14 and 15.

17. A manufacturing method (30) according to claim 16 comprising: - a step F1 of manufacturing a wax part comprising at least one set of cores (1), the wax part being manufactured by means of a mold; - a step F2 of coating the wax part with a ceramic material; - a step F3 of removing the wax to obtain a ceramic shell in which the core set (1) is integrated; - a step F4 of firing the ceramic shell to obtain the final shell; - a step F5 of pouring a metal alloy into the final shell; and - a step F6 of removing the final shell and the core set to obtain the complex metal alloy part.

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