Casting core

Abstract

The invention relates to a casting core (102) comprising a main part (11) made of molybdenum or a molybdenum alloy, characterised in that it comprises, on the surface of the main part, at least two protuberances (12) made of a refractory material, the whole of the main part and the protuberances being covered with an anti-oxidation coating (13).

Description

Description Title of the invention: Foundry core Technical Field

[0001] This presentation concerns foundry tools for the manufacture of metal parts and more specifically the foundry cores used for the production of hollow parts.

[0002] The present disclosure further relates to a method of manufacturing such a foundry core, and to a method of casting a hollow part of metallic material using a foundry core. Prior art

[0003] The lost wax casting process is known from the literature, and allows to obtain a metal part directly to the desired dimensions thanks to the use of a wax model of the part to be obtained, by 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 by means of a foundry core having the shape of the desired recess. The foundry core is positioned in the mold, so that the molten metal introduced into the mold cannot occupy the space of the foundry core.

[0005] Subsequent removal of the core allows a recess to be obtained in the metal part 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 solid metal part to create the desired recess, thus making it possible to simply create recesses with complex geometries.

[0007] However, the foundry core is in contact with the metal being poured into the mold, and as such must withstand the temperatures involved. In addition, it is important that the core is easily removable once the metal part has been obtained, in order to create the desired porosity.

[0008] Core compositions are known, such as molybdenum or molybdenum alloys (sometimes referred to as RMC for the English acronym "Refractory Metal Core").

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

[0010] However, even with such a coating, current foundry cores are not entirely satisfactory. Indeed, it has been found that the parts necessary for positioning the core in the foundry mold are more subject to friction with the mold or during handling. Such friction can damage the coating, which then no longer provides sufficient protection for the part. As a result, the foundry core oxidizes, and the cast part does not conform to what was expected.

[0011] Conversely, wanting to avoid this friction through careful handling requires drastically slowing down the core positioning stages, which harms the industrial competitiveness of this method.

[0012] There therefore remains a need for new foundry cores that are more resistant to oxidation than those of the prior art. Statement of the invention

[0013] The present invention aims precisely to respond to the problem set out above.

[0014] To this end, it proposes, in one embodiment, a foundry core comprising a main part made of molybdenum or molybdenum alloy, characterized in that it comprises on the surface of the main part at least two protuberances composed of a refractory material, the whole of the main part and the protuberances being covered with a protective coating against oxidation.

[0015] The inventors have in fact found that such a core addresses the problems of the cores of the prior art.

[0016] On the one hand, the main part of the core remains consistent with prior art cores and therefore does not require a complete change in investment casting tools and processes.

[0017] On the other hand, the protrusions allow easy anchoring of the core in the mold and therefore precise positioning of the main part of the core for the foundry process.

[0018] Also, the protrusions made of refractory material are not subject to oxidation. Therefore, even though the protective coating on the protrusions would be degraded due to handling and when in contact with the mold, the integrity of the core is not compromised.

[0019] The inventors also found that it was particularly advantageous to deposit the protective coating over the entire core after fixing the protrusions, because this ensures the continuity of the coating and in particular prevents the fixing of the protrusions from risking degrading the coating of the main part of the core.

[0020] For all the above reasons, a core is thus obtained which has excellent oxidation resistance properties while being easily usable in the lost wax casting processes already developed.

[0021] In one embodiment, the oxidation protective coating is chosen from coatings comprising at least, from the core and towards the outside, a bonding layer and a protective layer.

[0022] Preferably, the bonding layer is chosen to have a coefficient of thermal expansion close to the substrate. For example, the bonding layer may be chosen from a layer of titanium nitrocarbide TiCN, titanium carbide TiC, nitride TiN, silicon carbide SiC, hafnium carbide HfC, or aluminum nitride AIN.

[0023] Preferably, the protective layer is a layer of alumina AI2O3.

[0024] Thus, in one embodiment, the oxidation protective coating comprises at least, from the core and towards the outside, a bonding layer and a protective layer, the bonding layer being chosen from a layer of titanium nitrocarbide TiCN, titanium carbide TiC, TiN nitride, silicon carbide SiC, hafnium carbide HfC, or aluminum nitride AIN and the protective layer being a layer of alumina AI2O3.

[0025] In such an embodiment, the thickness of the bonding layer may be between 2 and 10 μm.

[0026] In such an embodiment, the thickness of the protective layer may be between 5 and 50 μm.

[0027] The inventors found that with a protective coating as described above, it was possible to obtain on the one hand a coating which perfectly fulfilled its function of protecting the core, and on the other hand that the coating did not exhibit any cracks due to differential expansion due to the coefficient of thermal expansion of the bonding layer, close to that of the substrate.

[0028] In one embodiment, the coating is chosen from coatings comprising two layers and in particular those comprising: a layer of titanium carbonitride TiCN and a layer of alumina AI2O3; a layer of aluminum nitride AIN and a layer of alumina AI2O3; a layer of silicon carbide SiC and a layer of alumina AI2O3; or a layer of hafnium carbide HfC and a layer of alumina AI2O3.

[0029] In one embodiment, the protective coating is selected from coatings comprising three layers, for example comprising from the core and outwards a layer of titanium carbide TiC, a layer of titanium nitride TiN and a layer of alumina AI2O3; or a layer of titanium nitride TiN, a layer of titanium carbide TiC and a layer of alumina AI2O3.

[0030] Preferably, the oxidation protective coating is a coating comprising from the core and outwards a layer of titanium carbonitride TiCN and a layer of alumina AI2O3.

[0031] The proposed elements provide excellent protection of the main part of the core against oxidation.

[0032] In one embodiment, the protrusions are precisely two in number.

[0033] Indeed, the inventors have found that fixing the core at two points of the mold allows for hypostatic fixing, i.e. one that leaves the core at least one degree of freedom. This is particularly advantageous because it allows thermal expansion of the core, for example when pouring the molten metal, without creating residual stresses. Indeed, the differential expansion between the core and the shell mold does not create stress during the temperature steps, because the degree of freedom granted to the core allows it to slide along the shell.

[0034] In one embodiment, the protrusions are formed by a single rod of refractory material passing right through the main portion of the core.

[0035] Such a stem must be understood in the vernacular sense of this term as an element of which one dimension is larger than the others.

[0036] For example, the largest dimension of the rod is at least 5 times greater than the other dimensions of the rod.

[0037] For example, the rod can be cylindrical of revolution or cylindrical with a hexagonal or triangular surface.

[0038] Forming the protrusions in this way greatly simplifies the core manufacturing process, as they can be formed by shrink-fitting a rod of refractory material into a cylindrical opening running right through the main part of the core.

[0039] In one embodiment, the refractory rod is a hollow rod. This simplifies the coating process, particularly when it is carried out by chemical vapor deposition.

[0040] The protrusions are present protruding from the surface of the main part. In one embodiment, the protrusions have a length greater than or equal to 1.0 mm, for example between 1.0 mm and 5.0 mm.

[0041] It is understood that the length of a protrusion is measured from the surface of the main part and perpendicular to it.

[0042] In one embodiment, the refractory material of the refractory rod may be a ceramic, for example chosen from alumina or zirconia.

[0043] In one embodiment, the main part of the core has the shape of the cooling circuits of a turbomachine blade.

[0044] In fact, turbomachine blades are generally made by casting, in particular for the hot part blades of the turbomachine, i.e. those located after the combustion chamber.

[0045] It is therefore particularly advantageous to use a foundry core rather than subsequent machining to manufacture the cooling circuits of a turbomachine blade.

[0046] 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 possible, and it is therefore preferable to use foundry cores.

[0047] The foundry core described then makes it possible to obtain the cooling circuits in such a blade, without complicating the foundry process.

[0048] According to another of its aspects, the invention relates to a method of manufacturing a foundry core comprising the following steps: - forming a main part of the core from molybdenum or molybdenum alloy to the desired shape; - the arrangement of at least two protrusions made of refractory material on the surface of the main part; - coating the main part and the protrusions with a protective coating against oxidation.

[0049] In one embodiment, the formation of the main part of the foundry core is carried out by additive manufacturing, for example by a binder jetting process. The processes can be chosen from laser metal deposition (LMD), electron beam melting (EBM), selective laser sintering (SLS), selective laser melting (SLM), powder bed melting (PBM), a multi-jet process, or direct metal laser sintering (DMLS).

[0050] Additive manufacturing makes it easy to obtain shapes with complex geometries, which allows for cost savings in core manufacturing.

[0051] In one embodiment, the formation of the main part of the casting core is carried out by metal injection molding (or MIM for the English acronym "Metal Injection Molding").

[0052] In one embodiment, the arrangement of the protuberances is achieved by shrink-fitting a rod of refractory material into a through opening of the main part.

[0053] Forming the protrusions by shrink-fitting a rod into a through opening in the main part simplifies the methods of arranging the protrusions compared to other methods of attaching the protrusions.

[0054] Additionally, this method ensures that two protrusions are directly opposite each other, allowing for more accurate positioning of the casting core in the casting mold.

[0055] Also, this method ensures the absence of play between the protrusions and the main part.

[0056] In one embodiment, the refractory material of the protrusions may be a ceramic, for example chosen from alumina or zirconia.

[0057] In one embodiment, the coating of the core can be carried out by a chemical vapor deposition (CVD) process, by physical vapor deposition (PVD) or by liquid means, for example by electro-deposition.

[0058] Such embodiments of the coating ensure that the coating is continuous between the main part and the protrusions.

[0059] This continuity ensures optimal protection of the main part of the core against oxidation, and consequently excellent resistance of the foundry core to oxidation during the pouring of molten metal.

[0060] According to another of its aspects, the invention relates to a method of manufacturing a hollow part made of metallic material by casting comprising at least the following steps: - the arrangement of a foundry core described above in a foundry mold, the foundry core being arranged in the mold so that the protrusions are in contact with the mold; - casting a molten metallic material into the mold cavity of the mold comprising the core; and - unsticking the mold and removing the core.

[0061] This manufacturing process makes it possible to obtain hollow parts made of metallic material in a simplified manner and with a lower scrap rate than the processes of the prior art.

[0062] In one embodiment, the metallic material may be a nickel or cobalt superalloy, optionally single-crystal.

[0063] In one embodiment, the hollow metal part is a turbomachine blade, for example a turbomachine hot section blade comprising cooling channels. Brief description of the drawings

[0064] [Fig. 1] Figure 1 schematically represents a foundry core in a first embodiment.

[0065] [Fig. 2] Figure 2 schematically represents a foundry core in another embodiment.

[0066] [Fig. 3] Figure 3 schematically represents a foundry core according to an embodiment of the invention placed in a mold for lost wax casting.

[0067] [Fig. 4] Figure 4 represents a flowchart schematically illustrating a method of preparing a foundry core in one embodiment.

[0068] [Fig. 5] Figure 5 represents a flowchart schematically illustrating a method of manufacturing a hollow part made of metallic material in one embodiment. Description of the embodiments

[0069] The invention is now described by means of figures, present for descriptive purposes to illustrate certain embodiments of the invention and which should not be interpreted as limiting the latter.

[0070] Figure 1 schematically represents a foundry core 101 in one embodiment.

[0071] The foundry core comprises a main part 11, two protrusions 12 on the surface of the main part 11, the whole being coated with an oxidation protective coating 13.

[0072] The protrusions 12 may be fixed by embedding, by gluing or by any other method allowing the core to be fixed in the mold, it being understood that this fixing must withstand the step of pouring the molten metal.

[0073] In the embodiment of Figure 1, the two protrusions 12 are distinct from each other in that they are formed distinctly.

[0074] Figure 2 shows an alternative embodiment which differs from Figure 1 in that the two protrusions are formed by a single rod of refractory material 15.

[0075] This embodiment makes it possible to obtain two protuberances 12 in a simplified manner. Furthermore, since this requires the upstream creation of a through opening in the main part 11 of the core 102, this embodiment ensures precise and simplified positioning of the protuberances 12 on the main part 11, and therefore better reproducibility.

[0076] In the application, the protrusions 12 made of refractory material are made of a material which is chemically inert with respect to the molten metal and resistant to the temperatures involved during the casting of the liquid metal.

[0077] For example, the refractory material may be a ceramic material, for example zirconia or alumina.

[0078] In the embodiment shown in Figures 1 and 2, the oxidation protective coating 13 is continuous, that is to say that the entire external surface of the main part 11 and the protrusions 12 is covered.

[0079] This embodiment makes it possible to ensure excellent resistance to oxidation of the main part 11 and in particular near the protrusions 12.

[0080] Indeed, it is to the credit of the inventors to have thus resolved two problems presented by the cores of the prior art not comprising protuberances composed of a refractory material.

[0081] On the one hand, the fixing of the protuberances, necessary for the correct positioning of the core 101, 102 in the mold, risks degrading the coating of the main part 11 in the area located near the protuberances 12.

[0082] On the other hand, the placement of the core 101, 102 in the mold can cause localized destruction of the protective coating 13 present on the protrusions 12. For example because of the handling tools, or because of contact with the mold.

[0083] A core 101, 102 according to the invention does not suffer from either of these two problems, nor does it require finding other methods of positioning the core 101, 102 in the mold.

[0084] In fact, the coating 13 covers both the main part 11 and the protrusions 12. The fixing of the protrusions cannot then damage the coating 13 which is deposited afterwards.

[0085] Furthermore, when the core 101, 102 is placed, it is the protrusions 12 which are in contact with the mold. Since the latter are composed of a refractory material, even if the coating 13 were damaged at the level of the protrusions 12, this would not have an impact on the good resistance to oxidation of the main part 11 of the core 101, 102.

[0086] Figure 3 schematically represents a core 102 placed in a mold 16 for the preparation of a hollow metal part.

[0087] The mold 16 is obtained in a known manner, according to a lost wax casting process, and so that the molding cavity 20 of the mold, defined by its internal surface S in t, has the shape of the desired part.

[0088] The positioning of the core 102 can be done, for example, by arranging the protrusions 12 in parts of the mold provided for this purpose, here the recesses 22.

[0089] In one embodiment, the molding cavity 20 may comprise a useful portion 18, and a non-useful portion 24, the desired part being obtained in the useful portion 18.

[0090] For example, the protrusions 12 and the recesses 22 are located in the non-useful part 24. The presence of a non-useful zone 24 makes it possible to facilitate the demolding steps and / or to carry out checks.

[0091] In this way, the final geometry of the useful part 18 of the molding cavity 20 of the mold 16 is in no way constrained by the presence of the protuberances 12 of the core, and it is thus possible to obtain a hollow part made of metallic material with the desired dimensions and shapes.

[0092] Figure 4 very schematically represents a method of manufacturing a foundry core 101, 102 as described above.

[0093] During a first step S11, the main part 11 of the core 101, 102 is formed into the desired shape.

[0094] Any method can be used for this step, particularly additive manufacturing or metal injection molding.

[0095] Both methods allow you to obtain a main part with perfectly defined geometry.

[0096] The specific processes for implementing these methods are known, and will not be described here.

[0097] In embodiments, step S11 may also comprise a particular step of creating a through opening in the main part 11, intended to accommodate the refractory material rod 15.

[0098] During a second step S12, at least two protrusions 12 are arranged on the surface of the main part 11.

[0099] It is understood that, even when the protrusions 12 are created via the insertion of a rod of refractory material 15 passing right through the main part 11, the protrusions 12 must be understood as the parts of the rod of refractory material 15 which protrude from the main part 11, and they are therefore well located on the surface of the main part 11.

[0100] For example, the second step S12 can be carried out by gluing or embedding protrusions 12 on the surface of the main part 11 created previously.

[0101] In a particular embodiment, step S12 corresponds to the insertion of a rod of refractory material 15 into a cylindrical opening provided for this purpose in the main part 11 of the core 102. The refractory material 15 may be a refractory oxide or a refractory ceramic.

[0102] It is preferred that the refractory material rod 15 be shrunk into the cylindrical opening of the main part 11.

[0103] Shrink fitting is understood here according to its usual definition in mechanics of materials, and consists of the assembly of two parts using a tight fit.

[0104] Indeed, when it is fixed by hooping, the refractory material rod 15 does not have any play with the main part 11 of the core. The inventors have found that it is preferable for the refractory material rod 15 not to have any play with the main part 11 of the core 102, because this avoids the relative movement of the refractory material rod 15 with respect to the main part 11 of the core. By such a movement the refractory material rod 15 could rub on the coating 13 of the main part 11 near the protuberances 12 and harm its integrity. In addition, the positioning of the core 102 in the mold 16 is thus more precise.

[0105] The method for preparing the foundry core 102 further comprises a step S13 of coating the core 102, formed from the main part 11 and the protrusions 12, with an oxidation-protective coating 13.

[0106] For example, this step can be carried out by a chemical vapor deposition (CVD) process or by a physical vapor deposition (PVD) process, or even by liquid means, for example by electro-deposition.

[0107] In one embodiment, the coating may comprise a layer of alumina AI2O3 and a layer of titanium carbonitride TiCN, both deposited by a chemical vapor deposition process.

[0108] Chemical vapor deposition ensures that the coating 13 covers the entire core 102, regardless of its geometry.

[0109] In an embodiment where the refractory rod 15 is hollow, step S13 may be performed by hanging the core 102 in a chemical vapor deposition furnace using a wire passing through the refractory rod 15.

[0110] The specific parameters of a chemical or physical vapor deposition allowing the deposition of a protective coating 13 are known to those skilled in the art. The same applies to an electro-deposition process.

[0111] Figure 5 describes a method of manufacturing a hollow part of metallic material by a lost wax casting process using a casting core as described above.

[0112] Such a method comprises a step S21 of arranging a foundry core 101, 102 in a foundry mold 16, so that the protrusions 12 of the core are in contact with the mold 16.

[0113] Preferably, and as is frequently achieved in conventional investment casting processes, the casting mold 16 may be obtained via a ceramic shell formed around a wax model of the part, for example, by dipping the wax model into a slip followed by heat treatment.

[0114] In one embodiment, the protrusions of the core are arranged in recesses 22 of the mold 16.

[0115] The method comprises a step S22 of casting a molten metallic material into the mold comprising the core 101, 102.

[0116] During this step, the molten metal fills the molding cavity 20 of the mold 16, or at least its useful part 18.

[0117] The molding cavity 20 has the shape of the part to be obtained, and the molten metal therefore takes the desired shape for the part.

[0118] The presence of the foundry core 101, 102 in the molding cavity 20 prevents the molten metal from accessing the space it occupies and the part made of metallic material is thus formed around the core.

[0119] In a subsequent step S23, the mold is detasseled and the core removed, using methods known as such to ultimately obtain a hollow part made of metallic material.

[0120] In one embodiment, once the single-crystal alloy has been cast and cooled, some of the non-useful areas are cut out, exposing the core.

[0121] The latter is then exposed to one or more chemical baths and / or one or more heat treatments in order to eliminate the core and the protuberances in refractory material.

[0122] For example, the mold can be mechanically destroyed.

[0123] For example, the foundry core can be dissolved using an acidic or basic chemical solution. Alternatively or additionally, the foundry core can be dissolved by an oxidation treatment, possibly carried out at temperature.

[0124] In one embodiment, the method for manufacturing a hollow part made of metallic material may comprise, after step S23, a machining step, for example to remove a portion of the metal which would have flowed into the non-useful zone 24 of the mold 16, and to keep only the part with the desired dimensions.

Claims

Claims

1. Foundry core (101, 102) comprising a main part (11) made of molybdenum or molybdenum alloy, characterized in that it comprises on the surface of the main part at least two protuberances (12) made of a refractory material, the whole of the main part and the protuberances being covered with an oxidation protective coating (13), the refractory material of the protuberances being a ceramic.

2. Foundry core (101, 102) according to claim 1, wherein the oxidation protective coating (13) comprises at least, from the core and outwards, a bonding layer and a protective layer, the bonding layer being selected from a layer of titanium nitrocarbide TiCN, titanium carbide TiC, TiN nitride, silicon carbide SiC, hafnium carbide HfC, or aluminum nitride AIN and the protective layer being a layer of alumina AI2O3.

3. A foundry core (102) according to claim 1 or 2, wherein the protrusions (12) are formed by a single rod of refractory material (15) passing right through the main part (11) of the core.

4. A foundry core (101, 102) according to claim 3, wherein the refractory material of the refractory rod (15) is selected from alumina or zirconia.

5. A foundry core (101, 102) according to any one of claims 1 to 4, wherein the main part (11) of the core has the shape of the cooling circuits of a turbomachine blade.

6. A method of manufacturing a foundry core (101, 102) comprising the following steps: - forming a main part (11) of the core in molybdenum or molybdenum alloy to the desired shape; - the arrangement of at least two protuberances (12) made of refractory material on the surface of the main part, the refractory material of the protuberances being a ceramic; - coating the main part and the protrusions with a protective coating against oxidation (13).

7. Manufacturing method according to claim 6, in which the arrangement of the protuberances (12) is carried out by shrinking a rod of refractory material (15) into a through opening of the main part (11).

8. A manufacturing method according to claim 6 or 7, wherein the coating of the core (101, 102) is carried out by a chemical vapor deposition process or a physical vapor deposition.

9. A manufacturing method according to any one of claims 6 to 8, wherein the formation of the main part (11) is carried out by additive manufacturing or by metal injection molding.

10. Method of manufacturing a hollow part made of metallic material by casting comprising at least the following steps: - arranging a foundry core (101, 102) according to one of claims 1 to 5 in a foundry mold (16), the foundry core being arranged in the mold so that the protrusions (12) are in contact with the mold; - pouring a molten metallic material into the molding cavity (20) of the mold comprising the core; and - unsticking the mold and removing the core.

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