CASTING CORE

Description

TECHNICAL FIELD

The present disclosure relates to casting tools for manufacturing metal parts, and more specifically to casting cores used in the production of hollow parts.

The present disclosure also relates to a process for manufacturing such a casting core, and a process of casting a hollow part made of metal material using a casting core.

PRIOR ART

The lost-wax casting process is known in the literature, and enables a metal part to be obtained directly with the desired dimensions by using a wax model of the part to be obtained, forming a mould around the model and removing the wax to obtain a cavity in the mould in the shape of the wax model, and therefore of the desired part.

When the part is hollow, i.e. when it has recesses, it is possible to create these by means of a casting core in the shape of the desired recess. The casting core is positioned in the mould so that the molten metal introduced into the mould cannot occupy the space of the casting core.

Subsequent removal of the core produces a recess in the metal part in the shape of the casting core instead of the originally present casting core.

The use of such a casting core avoids the need for subsequent machining of a solid metal part to create the desired recess, making it easy to create recesses with complex geometries.

However, the casting core is in contact with the metal being poured into the mould, and as such must withstand the temperatures involved. In addition, it is important that the core can be easily removed once the metal part has been obtained, in order to create the desired porosity.

Various core compositions are known, such as molybdenum or molybdenum alloys (sometimes called RMC for refractory metal core).

However, these alloys do not have a sufficient temperature resistance to oxidation for direct application in foundry processes, and are also soluble in nickel-based superalloys. This is why it is generally proposed to coat them with an anti-oxidation coating.

However, even with such a coating, current casting cores are not entirely satisfactory. In fact, it has been observed that the portions required for positioning the core in the casting mould are more subject to friction with the mould or during handling. Such friction can damage the coating, which then no longer provides sufficient protection for the part. As a result, the casting core oxidises and the cast part does not conform to expectations.

Conversely, trying to avoid this friction by meticulous handling means drastically slowing down the core positioning steps, which is detrimental to the industrial competitiveness of this embodiment.

There remains therefore a need for novel foundry cores that are more resistant to oxidation than those of the prior art.

DISCLOSURE OF THE INVENTION

The present invention is specifically designed to address the problem disclosed above.

For this purpose, an embodiment proposes a casting core comprising a main portion made of molybdenum or a molybdenum alloy, characterised in that it comprises, at the surface of the main portion, at least two protuberances made of a refractory material, the whole of the main portion and the protuberances being covered with an anti-oxidation coating.

The inventors have found that such a core solved the problems of the cores of the prior art.

On the one hand, the main portion of the core remains in accordance with the cores of the prior art, and therefore does not require a complete change of lost-wax casting tools and processes.

On the other hand, the protuberances allow the core to be easily anchored in the mould and therefore enable the main portion of the core to be precisely positioned for the casting process.

The refractory material protrusions are also not subject to oxidation. As a result, even if the protective coating on the protuberances is damaged by handling and contact with the mould, the integrity of the core is not compromised.

The inventors also found that it was particularly advantageous to deposit the protective coating on the whole of the core after fixing the protuberances, because this ensures the continuity of the coating and, in particular, prevents the fixing of the protuberances from damaging the coating on the main portion of the core.

For all of the above reasons, the result is a core with excellent oxidation resistance properties that is also easy to use in the wax casting processes already developed.

In an embodiment, the anti-oxidation coating is selected from coatings comprising at least, from the core outwards, an adhesion layer and a protective layer.

Preferably, the adhesion layer is selected to have a coefficient of thermal expansion close to that of the substrate. For example, the adhesion layer can be selected from a layer of titanium carbonitride TiCN, titanium carbide TiC, titanium nitride TiN, silicon carbide SiC, hafnium carbide HfC, or aluminium nitride AlN.

Preferably, the protective layer is a layer of alumina Al₂O₃.

Thus, in an embodiment, the anti-oxidation coating comprises, from the core outwards, at least one adhesion layer and one protective layer, the adhesion layer being selected from a layer of titanium carbonitride TiCN, titanium carbide TiC, titanium nitride TiN, silicon carbide SiC, hafnium carbide HfC, or aluminium nitride AlN, and the protective layer being a layer of alumina Al₂O₃.

In an embodiment, the thickness of the adhesion layer can be between 2 and 10 μm.

In such an embodiment, the thickness of the protective layer can be between 5 and 50 μm.

The inventors have found that, with a protective coating as described above, it was possible to obtain a coating that perfectly fulfilled its function of protecting the core, and that the coating did not crack as a result of differential expansion, due to the coefficient of thermal expansion of the adhesion layer being close to that of the substrate.

In an embodiment, the coating is selected 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 AlN and a layer of alumina Al₂O₃; a layer of silicon carbide SiC and a layer of alumina Al₂O₃; or even a layer of hafnium carbide HfC and a layer of alumina Al₂O₃.

In an embodiment, the protective coating is selected 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.

Preferably, the anti-oxidation coating is a coating comprising, from the core outwards, a layer of titanium carbonitride TiCN and a layer of alumina Al₂O₃.

The proposed elements provide excellent protection for the main portion of the core against oxidation.

In an embodiment, there are precisely two protuberances.

More specifically, the inventors have found that fixing the core at two points in the mould makes it possible to ensure hypostatic fixing, i.e. leaving the core at least one degree of freedom. This is particularly advantageous because it allows thermal expansion of the core, for example when the molten metal is poured, without creating residual stresses. In fact, the differential expansion between the core and the shell mould does not create any stress during the temperature steps, because the degree of freedom given to the core allows it to slide along the shell.

In an embodiment, the protuberances are formed by a single refractory material rod passing through the main portion of the core from one side to the other.

Such a rod should be understood in the vernacular sense of the term, as an element for which one dimension is larger than the others.

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

For example, the rod can be rotationally cylindrical or cylindrical with a hexagonal or triangular surface.

Forming the protuberances in this way greatly simplifies the core manufacturing process, because the protuberances can be formed by shrinking a rod of refractory material into a cylindrical opening passing through the main portion of the core from one side to the other.

In an embodiment, the refractory rod is a hollow rod. This simplifies the coating process, particularly when it is carried out by chemical vapour deposition.

The protuberances protrude from the surface of the main portion. In an embodiment, the protuberances have a length greater than or equal to 1.0 mm, for example between 1.0 mm and 5.0 mm.

It is understood that the length of a protuberance is measured from the surface of the main portion and perpendicular to it.

In an embodiment, the refractory material of the refractory rod can be a ceramic, for example selected from alumina or zirconia.

In an embodiment, the main portion of the core has the shape of cooling circuits of a turbomachine blade.

Turbomachine blades are generally made by casting, in particular for blades in the hot section of a turbomachine, i.e. those located after the combustion chamber.

It is therefore particularly advantageous to use a casting core rather than subsequent machining in order to manufacture the cooling circuits of a turbomachine blade.

The particularly complex geometry of the cooling circuits of a turbomachine blade cannot necessarily be achieved by machining after the part has been manufactured. In addition, for single-crystal turbomachine blades, such machining is not feasible, and it is therefore preferable to use casting cores.

The casting core described enables the cooling circuits to be obtained in such a blade, without complicating the casting process.

According to another of its aspects, the invention relates to a process for manufacturing a casting core, comprising the following steps:

• • forming a main portion of the core made of molybdenum or molybdenum alloy in the desired shape;
  • disposing at least two protuberances made of refractory material at the surface of the main portion;
  • coating the main portion and the protuberances with an anti-oxidation coating.

In an embodiment, the main portion of the casting core is formed by additive manufacturing, for example by a binder jetting process. The processes can be selected from laser melting deposition (LMD), electron beam melting (EBM) and selective laser sintering (SLS), selective laser melting (SLM), powder bed melting (PBM), a multi-jet process and direct melting laser sintering (DMLS).

Additive manufacturing makes it easy to obtain shapes with complex geometries, saving on the cost of manufacturing the core.

In an embodiment, the main portion of the casting core is formed by metal injection moulding (MIM).

According to an embodiment, the disposing of the protuberances is achieved by shrinking a refractory material rod in a through-opening of the main portion.

Forming the protuberances by shrinking a rod into a through-opening in the main portion simplifies the process for disposing the protuberances compared with other methods of fixing the protuberances.

In addition, this method ensures that two protuberances are directly opposite each other, allowing more precise positioning of the casting core in the casting mould.

This method also ensures that there is no play between the protuberances and the main portion.

In an embodiment, the refractory material of the protuberances can be a ceramic, for example selected from alumina or zirconia.

In an embodiment, the core can be coated using a chemical vapour deposition (CVD) process, a physical vapour deposition (PVD) process or a liquid process such as electrodeposition.

This type of coating ensures that the coating is continuous between the main portion and the protuberances.

This continuity ensures optimum protection of the main portion of the core against oxidation, and consequently excellent resistance of the casting core to oxidation during the pouring of molten metal.

According to another of its aspects, the invention relates to a process for manufacturing a hollow metal part by casting, comprising at least the following steps:

• • disposing a casting core described above in a casting mould, the casting core being disposed in the mould such that the protuberances are in contact with the mould;
  • pouring a molten metal material into the moulding cavity of the mould comprising the core; and
  • shakeout of the mould and removal of the core.

This manufacturing process makes it possible to obtain hollow metal parts in a simplified way and with a lower reject rate than the processes of the prior art.

In an embodiment, the metal material may be a nickel or cobalt superalloy, possibly monocrystalline.

In an embodiment, the hollow metal part is a turbomachine blade, for example a turbomachine hot section blade comprising cooling channels.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic representation of a casting core in a first embodiment.

FIG. 2 shows a schematic representation of a casting core in another embodiment.

FIG. 3 shows a schematic representation of a casting core according to an embodiment of the invention disposed in a lost-wax casting mould.

FIG. 4 shows a flow chart illustrating a process for preparing a casting core in an embodiment.

FIG. 5 shows a flow chart illustrating a process for manufacturing a hollow part from a metal material in an embodiment.

DESCRIPTION OF THE EMBODIMENTS

The invention is now described by means of figures, having the descriptive aim of illustrating certain embodiments of the invention and which must not be interpreted as limiting the latter.

FIG. 1 shows a schematic representation of a casting core 101 in an embodiment.

The casting core comprises a main portion 11, two protuberances 12 at the surface of the main portion 11, the whole being coated with an anti-oxidation coating 13.

The protuberances 12 can be fixed by embedding, bonding or any other method for ensuring the fixing of the core in the mould, it being understood that this fixing must withstand the molten metal pouring step.

In the embodiment shown in FIG. 1, the two protuberances 12 are distinct from each other, in that they are formed separately.

FIG. 2 shows an alternative embodiment which differs from FIG. 1 in that the two protuberances are formed by a single rod of refractory material 15.

This embodiment makes it possible to obtain two protuberances 12 in a simplified manner. In addition, since this requires a through-opening to be made in the main portion 11 of the core 102, this embodiment ensures precise and simplified positioning of the protuberances 12 on the main portion 11, and therefore better reproducibility.

In the application, the refractory material protuberances 12 are made of a material that is chemically inert to the molten metal and resistant to the temperatures involved in the pouring of the liquid metal.

For example, the refractory material may be a ceramic material, such as zirconia or alumina.

In the embodiment shown in FIGS. 1 and 2, the anti-oxidation coating 13 is continuous, i.e. the entire external surface of the main portion 11 and the protuberances 12 is covered.

This embodiment ensures excellent resistance to oxidation of the main portion 11, particularly close to the protuberances 12.

Indeed, it is to the inventors' credit that they have thus solved two problems presented by the cores of the prior art which do not comprise protuberances made of a refractory material.

On the one hand, the fixing of the protuberances, necessary for the correct positioning of the core 101, 102 in the mould, risks damaging the coating of the main portion 11 in the area close to the protuberances 12.

On the other hand, placing the core 101, 102 in the mould may cause localised destruction of the protective coating 13 present on the protuberances 12. This may be caused by handling tools, for example, or by contact with the mould.

A core 101, 102 according to the invention does not suffer from either of these two problems, nor does it require other methods for positioning the core 101, 102 in the mould.

The coating 13 covers both the main portion 11 and the protuberances 12. Fixing the protuberances cannot therefore damage the coating 13, which is deposited afterwards.

In addition, when the core 101, 102 is fitted, it is the protuberances 12 that come into contact with the mould. Since these are made of a refractory material, even if the coating 13 were damaged at the protuberances 12, this would have no effect on the good resistance to oxidation of the main portion 11 of the core 101, 102.

FIG. 3 schematically shows a core 102 disposed in a mould 16 for the preparation of a hollow metal part.

The mould 16 is obtained in a known manner, using a lost-wax moulding process, so that the mould cavity 20 of the mould, defined by its internal surface S_int, has the shape of the desired part.

The core 102 can be positioned, for example, by disposing the protuberances 12 in portions of the mould provided for this purpose, in this case the recesses 22.

In an embodiment, the moulding cavity 20 may comprise a useful portion 18 and a non-useful portion 24, the desired part being obtained in the useful portion 18.

For example, the protuberances 12 and the recesses 22 are located in the non-useful portion 24. The presence of a non-useful area 24 makes it easier to demould and/or carry out checks.

In this way, the final geometry of the useful portion 18 of the moulding cavity 20 of the mould 16 is not constrained in any way by the presence of the core protuberances 12, and a hollow part made of metal material with the desired dimensions and shapes can be obtained.

FIG. 4 is a very schematic representation of a process for manufacturing a casting core 101, 102 as described above.

In a first step S11, the main portion 11 of the core 101, 102 is formed in the desired shape.

Any method can be used for this step, in particular additive manufacturing or metal injection moulding.

These two methods produce a main section with a perfectly defined geometry.

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

In some embodiments, step S11 may also comprise a specific step of creating a through-opening in the main portion 11, intended to receive the refractory material rod 15.

In a second step S12, at least two protuberances 12 are disposed at the surface of the main portion 11.

It is understood that, even when the protuberances 12 are created via the insertion of a refractory material rod 15 passing through the main portion 11, the protuberances 12 are to be understood as the portions of the refractory material rod 15 which protrude from the main portion 11, and they are therefore located at the surface of the main portion 11.

For example, the second step S12 can be carried out by bonding or embedding protuberances 12 at the surface of the previously-created main portion 11.

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

It is preferred that the refractory material rod 15 is shrunk into the cylindrical opening of the main portion 11.

Shrinking is used here in its usual definition in the mechanics of materials, and consists of the assembly of two parts by means of a press fit.

In fact, when it is fixed by shrinking, the refractory material rod 15 exhibits no play with respect to the main portion 11 of the core. The inventors have found that it is preferable for the refractory material rod 15 to have no play with respect to the main portion 11 of the core 102, as this avoids relative movement of the refractory material rod 15 with respect to the main portion 11 of the core. Such movement could cause the refractory material rod 15 to rub against the coating 13 of the main portion 11 near the protuberances 12 and damage its integrity. The core 102 is also positioned more accurately in the mould 16.

The process of preparing the casting core 102 also includes a step S13 of coating the core 102, formed by the main portion 11 and the protuberances 12, with an anti-oxidation coating 13.

For example, this step can be achieved by a chemical vapour deposition (CVD) process, a physical vapour deposition (PVD) process or even a liquid process such as electrodeposition, for example.

In an embodiment, the coating may comprise a layer of alumina Al₂O₃ and a layer of titanium carbonitride TiCN, both deposited by a chemical vapour deposition process.

Chemical vapour deposition ensures that the coating 13 covers the entire core 102, whatever its geometry.

In an embodiment where the refractory material rod 15 is hollow, step S13 can be carried out by hanging the core 102 in a chemical vapour deposition furnace by means of a thread passing through the refractory material rod 15.

The specific parameters of a chemical or physical vapour deposition process for depositing a protective coating 13 are known to a person skilled in the art. The same applies to an electrodeposition process.

FIG. 5 describes a process for manufacturing a hollow part from a metal material by a lost-wax casting process using a casting core as described above.

Such a process comprises a step S21 of placing a casting core 101, 102 in a casting mould 16, so that the protuberances 12 of the core are in contact with the mould 16.

Preferably, and as is frequently done in conventional lost-wax casting processes, the casting mould 16 can be obtained via a ceramic shell formed around a wax model of the part, for example by dipping the wax model in a slip followed by heat treatment.

In an embodiment, the core protuberances are disposed in recesses 22 of the mould 16.

The process comprises a step S22 of pouring a molten metal material into the mould comprising the core 101, 102.

During this stage, the molten metal fills the moulding cavity 20 of the mould 16, or at least its useful portion 18.

The moulding cavity 20 has the shape of the part to be obtained, so the molten metal takes on the desired shape of the part.

The presence of the casting core 101, 102 in the mould cavity 20 prevents the molten metal from gaining access to the space it occupies and the metal part is thus formed around the core.

In a subsequent step S23, the mould is shaken out and the core removed, using methods known per se to enable a hollow metal part to be ultimately obtained.

In an embodiment, once the monocrystalline alloy has been cast and cooled, some of the non-useful areas are cut away, exposing the core.

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 made of refractory material.

For example, the mould can be destroyed mechanically.

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

In an embodiment, the process for manufacturing a hollow part from a metal material may include, after step S23, a machining step, for example to remove some of the metal that has flowed into the non-useful area 24 of the mould 16, and to retain only the part with the desired dimensions.