Method for manufacturing a core for a casting process

Abstract

The invention relates to a method for manufacturing a casting core, the casting core having a main part (101) made of molybdenum or molybdenum alloy comprising at least one recess and a secondary part (201), the manufacturing method comprising at least the following steps: - a step a) of arranging the secondary part in the recess of the main part, so as to form at least one protuberance at the surface of the main part; then, - a step b) of coating the entity formed of the assembly of the main part and the secondary part with a protective coating (102) providing protection against oxidation; and - a step c) of fitting an intermediate seal (301) between the main part and the secondary part of the core.

Description

Description Title of the invention: Method for manufacturing a core for a foundry process. Technical Field

[0001] This presentation concerns foundry tools for the manufacture of metal parts and more specifically a process for manufacturing a foundry core. Previous technique

[0002] 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.

[0003] When the part is hollow, meaning it has recesses, these recesses can be created using a casting core shaped like the desired recess. The casting core is positioned within the mold so that the molten metal introduced into the mold cannot occupy the space within the casting core.

[0004] The subsequent removal of the core allows a recess in the metal part to be obtained in the shape of the foundry core in place of the foundry core initially present.

[0005] The use of such a foundry core avoids the need for further machining of a massive metal part to create the desired cavity, thus making it easy to create cavity shapes with complex geometries.

[0006] 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 metal part is obtained, in order to create the desired porosity.

[0007] 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").

[0008] However, these alloys do not exhibit sufficient temperature resistance to oxidation for direct application in foundry processes and are Furthermore, they are soluble in nickel-based superalloys. Therefore, it is generally recommended to coat them with a protective oxidation coating.

[0009] 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.

[0010] However, even with such a coating, current foundry cores do not provide complete satisfaction.

[0011] Indeed, it has been observed that the parts necessary for positioning the core within the foundry mold are more prone to friction with the mold or during handling. Such friction can damage the coating, which then no longer provides sufficient protection for the part. Consequently, the foundry core oxidizes, and the cast part does not conform to expectations.

[0012] Conversely, wanting to avoid these frictions through careful handling requires drastically slowing down the core positioning steps, which harms the industrial competitiveness of this method of implementation.

[0013] There therefore remains a need for new foundry cores that are more resistant to oxidation than those of the prior art and that retain this resistance even during the handling of the cores, or during their positioning in the foundry mold. Description of the invention

[0014] The present presentation aims precisely to address this need.

[0015] To this end, the invention relates to a method for manufacturing a foundry core, the foundry core having a main part made of molybdenum or molybdenum alloy comprising at least one hollow and a secondary part, the manufacturing method comprising at least the following steps: - a step a) of arranging the secondary part in the recess of the main part, to form at least one protrusion on the surface of the main part; then, - a step b) of coating the assembly formed by the main part and the secondary part with a protective coating against oxidation; and - a step c) of providing a sealing intermediate between the main part and the secondary part of the core.

[0016] The inventors determined that the proposed process, and in particular the presence of the sealing intermediate, allowed for an excellent improvement in the protection of the cores against oxidation.

[0017] Comparatively, in prior art processes, it is generally proposed either to cover only the main part with a protective coating against oxidation, or to cover the whole formed by the assembly of the main part and the secondary part with a protective coating.

[0018] Neither of these two solutions is as satisfactory as the one proposed above.

[0019] The first solution does not allow protection of the secondary part, which then risks being damaged during the process and possibly causing a displacement of the core in the foundry mold, which compromises the part obtained in particular in that the cavity may not be positioned exactly where desired.

[0020] Furthermore, even when choosing a secondary part resistant to the temperature and oxidation conditions of a foundry process, it has been observed that the arrangement of the secondary part in the recesses of the main part after the coating of the main part could cause mechanical wear of the coating of the main part and thus locally create unprotected areas which then degrade during the pouring of metal.

[0021] The second method of the prior art allows joint protection of the primary and secondary parts of the nucleus.

[0022] However, the relative movement of the main part with respect to the secondary part during operation can cause wear on the coating.

[0023] More specifically, it is to the inventors' credit that they determined that prior art proposals did not provide sufficient protection, as the coating was mechanically stressed at the interface between the main and secondary parts, causing its deterioration.

[0024] On the contrary, the proposed process makes it possible to ensure a coating against oxidation of the core while also ensuring that the latter is not damaged solely by small relative displacements of the main part of the core with respect to the secondary part.

[0025] Furthermore, the method of the invention ensures that the integrity of the protective coating is maintained when the core is handled.

[0026] Indeed, it is during the core handling operations, particularly when placing it in a mold for the foundry, that the coating is most likely to degrade, especially due to the play that could exist between the main part and the secondary part in the absence of a sealing intermediary.

[0027] The presence of the sealing intermediary prevents this wear for cores formed by a process such as described.

[0028] In one embodiment, the secondary part of a core of the invention may be a refractory material, for example chosen from alumina or zirconia.

[0029] In one embodiment, the secondary part can be arranged in the recess of the main part by shrink fitting.

[0030] In one embodiment, the hollowing of the main part of the core can be through-hole.

[0031] This embodiment is particularly advantageous because it allows, with a single secondary part, arranged in the through opening, to have two attachment points for the core.

[0032] In one embodiment, the formation of the main part is carried out by additive manufacturing or by metal injection molding.

[0033] 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, and direct metal laser sintering (DMLS).

[0034] Additive manufacturing makes it easy to obtain shapes with complex geometries, which allows for cost savings in the manufacturing of the core.

[0035] In one embodiment, the formation of the main part of the foundry core is carried out by metal injection molding (or MIM for the English acronym "Metal Injection Molding").

[0036] In one embodiment, the secondary part forms exactly two protrusions on the surface of the main part.

[0037] Indeed, the inventors observed that fixing the core at two points on the mold ensures a hypostatic fixation, meaning that the core retains at least one degree of freedom. This is particularly advantageous because it allows for thermal expansion of the core, for example, during the pouring of molten metal, without creating residual stresses. In fact, the differential expansion between the core and the shell mold does not create stress during the temperature stages, because the degree of freedom granted to the core allows it to slide along the shell.

[0038] In one embodiment, the protrusions formed on the surface of the main part by the secondary part have a length greater than or equal to 1.0 mm, for example between 1.0 mm and 5.0 mm.

[0039] It is understood that the length of a protuberance is measured from the surface of the main part and perpendicular to it.

[0040] In one embodiment, step b) can be performed before step c).

[0041] This embodiment allows the sealing intermediate to be placed on the coating obtained at the end of step b).

[0042] Indeed, by carrying out step b) before step c), the sealing intermediate is placed on the coating which covers the space between the main part and the secondary part and thus the coating is protected at this point, while ensuring excellent adhesion of the main part to the secondary part.

[0043] The junction between the primary and secondary parts of the core is a critical point in protecting the entire core against oxidation.

[0044] Indeed, it is in this area that the coating is most stressed when the core is placed in a foundry mold.

[0045] More specifically, the secondary part ensures the function of holding the core in place in the mold and is therefore the one that will be most subject to handling operations.

[0046] If there is any play, however small, between the main part and the secondary part, the coating covering the junction between these two parts is likely to be stressed.

[0047] However, if the coating is damaged, it no longer performs its protective function, and the interface between the main and secondary parts of the core becomes a point where oxidation of the core can begin, which impairs the entire protection.

[0048] Performing step b) before step c) allows the coating to be covered with a sealing intermediate, where it is most likely to be damaged.

[0049] The sealing intermediary ensures improved retention of the main part relative to the secondary part of the core, which avoids mechanical stress on the protective coating at this location.

[0050] In one embodiment, step c) can be performed before step b).

[0051] Such an embodiment also aims to minimize the play between the main part and the secondary part.

[0052] However, this embodiment ensures that the sealing intermediate is covered by the coating and therefore the latter is itself protected against oxidation.

[0053] This embodiment offers a greater diversity of choices for the sealing intermediate.

[0054] In one embodiment, the sealing intermediate may include a ceramic adhesive, or even be made of a ceramic adhesive.

[0055] For example, ceramic adhesives can be chosen from those primarily based on silica, alumina, and zirconia. Specifically, adhesives commercially available under the Ceramabond® brand from Aremco® or ceramic adhesives from Finals Materials®.

[0056] It is to the credit of the inventors that they determined that ceramic adhesive had many advantages that made it a satisfactory sealing medium.

[0057] Firstly, ceramic adhesive can be applied at less than 400°C, which ensures that no oxidation of the core is caused during the application of the ceramic adhesive, even if it is carried out in air.

[0058] Indeed, molybdenum or molybdenum alloys can oxidize in air at a temperature greater than or equal to 400°C.

[0059] For example, ceramic adhesive can be applied manually, for example with a brush, spatula, pipette or syringe.

[0060] Secondly, the ceramic adhesive offers excellent resistance even at temperatures above 1000°C, which allows it to hold the main part in place relative to the secondary part even at the temperatures involved during the casting of the alloy.

[0061] Finally, ceramic adhesive offers excellent chemical compatibility with the core material, namely molybdenum or molybdenum alloys, in the shape and including at least one recess.

[0062] When the sealing intermediate is a ceramic adhesive, step c) can be directly followed by a heat treatment allowing the pre-ceramicization of the ceramic adhesive.

[0063] For example, such a heat treatment may include heating to a temperature between 250°C and 350°C for a period of between 15 minutes and 4.0 hours.

[0064] Such pre-ceramicization makes it possible to obtain for the ceramic adhesive sufficient mechanical properties for a reduction of the play between the main part and the secondary part.

[0065] In particular, such pre-ceramicization allows the ceramic adhesive to fully fill any spaces that may be left free between the main part and the secondary part.

[0066] The complete ceramization of the glue will be completed when the core is exposed to higher temperatures, particularly during the firing of the shell mold under air.

[0067] When a ceramic adhesive is used as a sealing intermediate, step b) can be carried out either before or after step c).

[0068] Indeed, ceramic adhesive can be placed on the coating because it does not risk contaminating in any way the metal introduced during the subsequent casting stage, and has excellent chemical compatibility with the coating.

[0069] In one embodiment, the sealing intermediate may comprise, or even be made up of, a mixture of colloidal silica and zirconia.

[0070] Such a mixture makes it possible to have a sealing intermediate that is applicable at ambient temperature, for example at a temperature less than or equal to 30°C.

[0071] In one embodiment, step c) can be a metal brazing operation of the main part with the secondary part, in which the metal is chosen from platinum or platinum alloys.

[0072] In particular, in such an embodiment, brazing can then be carried out by vacuum laser welding

[0073] Alternatively, the metal can be chosen from aluminium or silicon, in particular to form one of the following species AIMo, AIM03 or MosSi or even MosSi or MOSÎ2.

[0074] In such an embodiment, the sealing intermediate is then made of a metal or a metallic alloy.

[0075] In such an embodiment, the sealing intermediary allows for a total elimination of the gap between the main part and the secondary part because the sealing intermediary fills this gap.

[0076] This ensures that the coating is not subjected to stresses during operation as it would be in the absence of the sealing intermediary.

[0077] However, such an intermediate deposited by brazing requires that step c) be carried out before step b).

[0078] Indeed, depositing a metallic intermediate on a protective coating could cause the metal to dissolve with the molten metal during a later casting step.

[0079] In one embodiment, step c) includes the deposition of a sealing intermediate thickness of between 10 µm and 500 µm.

[0080] In one embodiment, the sealing intermediate has a thickness between 10 µm and 500 µm.

[0081] In one embodiment, the coating of step b) can be carried out by a chemical vapor deposition (CVD) process, by a physical vapor deposition (PVD) process or by liquid means, for example by electro-deposition.

[0082] Such methods of coating implementation make it possible to ensure that the coating is continuous between the main part and the secondary part.

[0083] In one embodiment, the oxidation-protective coating is chosen from coatings comprising at least, from the core outwards, a bonding layer and a protective layer.

[0084] Preferably, the bonding layer is chosen to have a coefficient of thermal expansion close to that of the substrate. For example, the bonding layer can be chosen from a layer of titanium nitrocarbide TiCN, titanium carbide TiC, TiN nitride, silicon carbide SiC, hafnium carbide HfC, or aluminum nitride AIN.

[0085] Preferably, the protective layer is a layer of alumina Al2O3.

[0086] Thus, in one embodiment, the oxidation-protective coating comprises at least, from the core 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 aluminium nitride AIN and the protective layer being a layer of alumina Al2O3.

[0087] In such an embodiment, the thickness of the tack coat can be between 1.0 and 30 pm.

[0088] In such an embodiment, the thickness of the protective layer can be between 5.0 and 50 pm.

[0089] The inventors found that with a protective coating such as described above, it was possible to obtain on the one hand a coating that perfectly fulfilled its function of protecting the core.

[0090] 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 Al2O3; a layer of aluminium nitride AIN and a layer of alumina Al2O3; a layer of silicon carbide SiC and a layer of alumina Al2O3; or a layer of hafnium carbide HfC and a layer of alumina Al2O3.

[0091] In one embodiment, the protective coating is chosen from coatings comprising three layers, for example comprising from the core outwards a layer of titanium carbide TiC, a layer of titanium nitride TiN and a layer of alumina Al2O3; or a layer of titanium nitride TiN, a layer of titanium carbide TiC and a layer of alumina Al2O3.

[0092] In one embodiment, the protective coating may comprise silicon nitride Si3N4.

[0093] Preferably, the protective oxidation coating is a coating comprising, from the core outwards, a layer of titanium carbonitride TiCN and a layer of alumina Al2O3.

[0094] To obtain the coatings described above, chemical vapor deposition (CVD), physical vapor deposition (PVD) or spraying methods may be used.

[0095] The proposed elements provide excellent protection for the main part of the core against oxidation.

[0096] In one embodiment, the main part of the core has the shape of a cooling circuit for a turbomachine blade, or a portion of a cooling circuit for a turbomachine blade.

[0097] Indeed, turbomachine blades are generally cast, especially for the hot section blades, i.e., those located after the combustion chamber. It is therefore particularly advantageous to use a cast core rather than subsequent machining to manufacture the cooling circuits for a turbomachine blade.

[0098] Indeed, the particularly complex geometry of the cooling circuits of a turbomachine blade is not necessarily achievable through 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.

[0099] The described foundry core then makes it possible to obtain the cooling circuits in such a blade, without complicating the foundry process.

[0100] According to another aspect, the invention also relates to a method for manufacturing a hollow part made of metallic material by casting, comprising at least the following steps: - the arrangement of a foundry core as described above in a foundry mold, the foundry core being arranged in the mold so that the secondary part of the core is in contact with the mold; - the pouring of molten metallic material into the mold cavity comprising the nucleus; and - removing the mold and the core.

[0101] The mold unmolding and core removal steps can be carried out by methods known as such. Brief description of the drawings

[0102] [Fig. 1] Figure 1 schematically represents a nucleus obtained by a process according to an embodiment of the invention.

[0103] [Fig. 2] Figure 2 schematically represents a kernel identical to that of Figure 1, in a different view.

[0104] [Fig. 3] Figure 3 schematically represents a cross-sectional view of a core obtained by a process according to an embodiment of the invention.

[0105] [Fig. 4] Figure 4 schematically represents a cross-sectional view of a core produced according to an embodiment of the invention different from that of Figure 3. Description of the implementation methods

[0106] 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.

[0107] Figure 1 represents a nucleus obtained by a process according to an embodiment.

[0108] Such a core comprises a main part 101, a secondary part 201 and a sealing intermediate 301 disposed between the main part 101 and the secondary part 201.

[0109] The expression "between the main part 101 and the secondary part 201" means that the sealing intermediate 301 is disposed on both parts 101 and 201.

[0110] To be perfectly exhaustive, and as will be described in more detail in connection with Figure 3, it will be understood that a sealing intermediate is "between the main part 101 and the secondary part 201", even if it is placed on the coating provided that it is placed on the coating of both parts.

[0111] Indeed, in certain embodiments such an arrangement of the sealing intermediary allows it to give the assembly the expected properties.

[0112] To determine whether a coating is a coating of the main part or of the secondary part, one can look at whether it is arranged perpendicular to a surface of the main part or not.

[0113] 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.

[0114] Figure 1 does not show a coating for the purpose of simplifying the figure.

[0115] For the same reasons, Figure 1 is intended to be extremely schematic with regard to the shape of the main part 101 of a kernel.

[0116] In one embodiment, the main part 101 of the core may have the form of a cooling circuit for a turbomachine blade, or a portion of a cooling circuit for a turbomachine blade.

[0117] Also, in the embodiment shown in Figure 1, the main part 101 of the core has a single through recess, in which the secondary part 201 is arranged.

[0118] This embodiment is not limiting. However, it is preferred because it allows for the simple formation of two protrusions from a single secondary part 201, which allow for the fixing of the core in a lost-wax casting mold.

[0119] Frame II in Figure 1 allows visualization of the view shown in Figure 2.

[0120] Figure 2 illustrates the same elements as those in Figure 1, namely the main part of the core 101, the secondary part 201 and the sealing intermediate 301.

[0121] As with Figure 1, Figure 2 does not depict the coating for reasons of simplicity.

[0122] Figure 3 illustrates a nucleus obtained by a process of the invention in one embodiment.

[0123] Figure 3 shows coating 102.

[0124] In the embodiment shown in Figure 3, step c) is carried out after step b).

[0125] Indeed, the sealing intermediate 301 is placed on the coating 102.

[0126] Figure 3 is not shown to scale and the dimensions are not representative but only allow for understanding.

[0127] Figure 3 illustrates that the sealing intermediate 301 can be positioned "between the main part 101 and the secondary part 201" although it is not directly in contact with either of the two parts.

[0128] Indeed, in Figure 3, the sealing intermediate 301 is arranged on the coating 102 above the main part 101 and the secondary part 201.

[0129] The intermediate sealing element 301 will be considered to be "above" a primary or secondary part if it is crossed by the normal to a surface of one of these parts.

[0130] In Figure 3, the sealing intermediate 301 is positioned where the coating 201 is under the most stress.

[0131] Indeed, although the main part 101 and secondary part 201 of the core are represented directly in contact with each other, there may be some play between these two parts in reality.

[0132] Because of the existence of this play, the coating is stressed at the interface between the main part 101 and the secondary part 201, where the sealing intermediate 301 is located.

[0133] The presence of the sealing intermediate 301 ensures better fixing of the main part 101 relative to the secondary part 201, which ensures better resistance at this particular location of the coating.

[0134] Figure 4 illustrates a nucleus obtained by a process in an alternative embodiment to that shown in Figure 3.

[0135] In the embodiment shown in Figure 4, step c) of arranging a sealing intermediate 301 is carried out before step b) of coating the main 101 and secondary 201 parts of the core with a protective coating 102.

[0136] In this embodiment, the sealing intermediate 301 is then disposed directly in contact with the main part 101 and directly in contact with the secondary part 201.

[0137] This embodiment allows for excellent holding of parts 101, 201 to each other, and thus a minimization of the play which can cause during handling and / or installation of the core a degradation of the protective coating.

[0138] In the embodiment shown, the coating 102 is then placed on each of the main parts 101 and secondary parts 201 but also on the sealing intermediate 301.

Claims

Demands

1. A method for manufacturing a foundry core, the foundry core having a principal part (101) of molybdenum or molybdenum alloy comprising at least one recess and a secondary part (201), the manufacturing method comprising at least the following steps: - a step a) of arranging the secondary part in the recess of the main part, to form at least one protrusion on the surface of the main part; then, - a step b) of coating the assembly formed by the main part and the secondary part with a protective coating (102) against oxidation; and - a step c) of disposing of a sealing intermediate (301) between the main part and the secondary part of the core, the step of coating the assembly being carried out before or after the step of disposing of a sealing intermediate, the sealing intermediate (301) being a ceramic adhesive, chosen from adhesives mainly based on silica, alumina and zirconia, or the step of disposing of a sealing intermediate being followed by the step of coating the assembly, the disposing step being carried out by a metallic brazing operation of the main part with the secondary part, of a metal chosen from platinum, platinum alloys, silicon or aluminium, characterized in that the secondary part (201) forms on the surface of the main part (101) exactly two protrusions.

2. A method for manufacturing a foundry core according to claim 1, wherein step c) is directly followed by a heat treatment step comprising heating to a temperature between 250°C and 350°C for a period of between 15 minutes and 4.0 hours, where the sealing intermediate is an adhesive.

3. Method of manufacturing a foundry core according to any one of claims 1 to 2, wherein the protective coating (301) against oxidation comprises at least, from the core outwards, a tack coat and a protective coat.

4. Method of manufacturing a foundry core according to any one of claims 1 to 3, wherein the thickness of the sealing intermediate (301) is between 10 pm and 500 pm.

5. A method for manufacturing a hollow part made of metallic material by casting, comprising at least the following steps: - the arrangement of a foundry core according to any one of claims 1 to 3 in a foundry mold, the foundry core being arranged in the mold so that the secondary part of the core is in contact with the mold; - the pouring of molten metallic material into the mold cavity comprising the core; and - removing the mold and the core.

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