Manufacturing process for a refractory meral core

Abstract

The present invention relates to a process (20) for preparing a foundry core (18) made of refractory metal comprising: - a step E1 of manufacturing a green core (10) by shaping a mixture comprising a mixture of refractory metal particles and binder particles, the refractory metal comprising molybdenum, - a step E2 of removing the binder from the green core to obtain a brown core (14), and - a step E3 of sintering the brown core in the presence of a carbon source so as to form, on at least a part of the surface of the brown core (16), a protective layer (12) of molybdenum carbide having a thickness of between 5 µm and 500 µm.

Description

Title of the invention: Method for manufacturing a refractory metal core. FIELD OF THE INVENTION

[0001] The invention relates to the manufacture of a refractory metal core comprising molybdenum and coated with a layer of molybdenum carbide, this core being intended for use in a lost-wax casting process. The invention also relates to a method for manufacturing a complex metal alloy part using the refractory metal core. 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 poured 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 (or 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 thus subsequently 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 shell at the end of the process is a nickel- or cobalt-based alloy. Furthermore, casting cores must be easily manufacturable to obtain a part with the desired geometry 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 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] However, a disadvantage of refractory metal cores is that, during their manufacture, a parasitic layer may be formed on their surface.

[0009] In particular, during the manufacturing of the core by powder injection molding, known as Metal Injection Molding (MIM), or by additive manufacturing with binder jetting, known as Metal Binder Jetting (MBJ), a sintering step is performed. During this sintering step, a layer of molybdenum carbide, particularly Mo2C, can inevitably form on the surface of the core. This layer is more or less thick and irregular, thus imprecisely defining the core's geometry.

[0010] Furthermore, when the core is used in a lost-wax casting process, the 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 shell baking process. To prevent the formation of this unwanted layer, it is necessary to coat the core with a protective layer.

[0011] To prevent these oxidation reactions on the core surface during the lost-wax casting process, it is necessary to protect this surface before the core is used in the lost-wax casting process. However, the irregular molybdenum carbide layer that inevitably forms during core fabrication makes it difficult to form a protective layer while maintaining the desired core geometry. Description of the invention

[0012] To this end, the invention relates to a method for manufacturing a refractory metal foundry core comprising: - a step El of manufacturing a green core by shaping a mixture comprising a mixture of refractory metal particles and binder particles, the refractory metal comprising molybdenum, - a step E2 of removing the binder from the green core to obtain a brown core, and - a step E3 of sintering the brown core in the presence of a carbon source so as to form, on at least part of the surface of the brown core, a protective layer of molybdenum carbide with a thickness between 5 pm and 500 pm.

[0013] This process makes it possible to obtain a refractory metal core comprising molybdenum, at least part of whose surface is protected by the protective layer. The process allows the protective layer to be formed in a controlled manner with the desired thickness and regularity. In particular, the variation in thickness of the protective layer is at most 10%. The refractory metal core, and in particular the part of its surface covered by the protective layer, therefore does not need to undergo the finishing step usually performed to remove the undesirable carbon layer.

[0014] Furthermore, when the refractory metal core is used during firing in the presence of oxygen, such as during shell firing in the case where the refractory metal core is used in a lost-wax casting process, the protective layer prevents any oxidation of the core surface.

[0015] Thus, when the refractory metal core is used in a lost-wax casting process, the surface covered with the molybdenum carbide layer, preferably the entire surface, is protected from oxygen during shell baking.

[0016] In particular, the molybdenum carbide layer exerts an oxygen barrier function up to 1200°C for several tens of minutes.

[0017] The thickness of the molybdenum carbide layer is measured by optical microscopy or scanning electron microscopy (SEM). Step 11 - Fabrication of the green core

[0018] The manufacture of the green core can be carried out by powder injection molding, known under the anglicism "Metal Injection Moulding" (MIM), or by additive manufacturing with binder jetting, known under the anglicism "Meta Binder Jetting" (MBJ).

[0019] These methods make it possible to obtain a green core of the desired shape.

[0020] The refractory metal comprising molybdenum can be chosen from molybdenum, titanium zirconium molybdenum (TZM), and molybdenum rhenium (Mo-Re).

[0021] The binder particles comprise a polymer selected from polyethylene, polypropylene, polyoxymethylene, polyvinylpyrrolidone, and one of their mixtures.

[0022] The binder particles may further comprise a solvent selected from water, ethylene glycol, 2-butoxyethanol, and one of their mixtures.

[0023] In the precursor mixture, the volume ratio of refractory metal particles to binder particles can range from 85 / 15 to 75 / 25.

[0024] At the end of step El, a so-called "green" core, corresponding to the core which still contains binder and which does not yet have the final mechanical and geometric characteristics, is obtained.

[0025] When step E1 is carried out by powder injection molding (PIM), it includes a heating step to liquefy the binder powder. Step E2 - Binder removal

[0026] The binder removal step E2, also called the debinding step, can be carried out by thermal degradation selected from degradation, evaporation under a controlled atmosphere (possibly under vacuum), and drainage in liquid form onto a porous substrate. A brown core free of binder is then obtained.

[0027] Preferably, the debinding step is carried out under conditions that do not lead to the formation of molybdenum carbide on the core surface. Indeed, at this step it would be difficult to control the thickness of the molybdenum carbide layer.

[0028] When the debinding step E2 is performed by heat treatment, the heat treatment can be carried out during sintering. Step E2 is then performed by heating the green core from an initial temperature T2i of between 15°C and 25°C to a final temperature T2f of at least 200°C. When debinding is carried out under oxygen, the final temperature Tf is less than or equal to 400°C. When the debinding step is carried out under hydrogen or argon purging or under vacuum, the final temperature can be between 200°C and 600°C.

[0029] The rate of temperature rise between the initial temperature T2i and the final temperature T2f can be from 5°C to 30°C per minute.

[0030] At the end of the E2 debinding step, the brown core contains less than 1.0% by mass of binder, preferably less than 0.5% by mass of binder, and preferably less than 0.1% by mass of binder, relative to the total mass of the brown core. Step E3 - Brown core sintering

[0031] The brown core is then sintered in step E3.

[0032] Sintering can be carried out at a temperature below 2000°C inclusive. The duration of this sintering step is, for example, from 5 minutes to 30 minutes.

[0033] At the end of the sintering, a layer of constant thickness is obtained over the entire covered surface.

[0034] The molybdenum carbide protective layer has a maximum of 5% by volume of porosity relative to the total volume of the layer.

[0035] The E3 sintering step is preferably carried out by plasma (known by the English term Spark Plasma Sintering, SPS). The brown nucleus is then placed in a filled chamber of a powder bed that can be selected from boron carbide (B4C), silicon carbide (SiC), graphite (C), and one of their mixtures.

[0036] The enclosure is then placed under vacuum at a pressure ranging from 1 mbar to 104 mbar.

[0037] The E3 sintering step is carried out by heating the core from an initial temperature T3i between 300°C and 500°C inclusive to a final temperature T3f between 1500°C and 2000°C inclusive.

[0038] According to one possible embodiment, the E3 sintering step comprises 1 to 3 temperature steps, each step corresponding to a different temperature.

[0039] Furthermore, step E3 includes at least one temperature ramp rate of 5°C to 30°C per minute between two temperatures. The two temperatures can be the initial temperature T3i and the final temperature T3f, or the temperature of one plateau and the temperature of a subsequent plateau.

[0040] This E3 sintering step allows for the controlled formation of a molybdenum carbide layer, which can then act as a protective layer when the refractory metal core is used in a lost-wax casting process. The thickness of the protective layer ranges from 5 µm to 500 µm. The thickness is constant across the entire surface of the layer, meaning that its thickness varies by no more than 10%.

[0041] According to one possible embodiment, an additional protective layer can be deposited on the molybdenum carbide protective layer. The additional layer may comprise alumina (Al₂O₃), an alumino-forming agent such as aluminum nitride (AIN) or nickel aluminide (NiAl), which will form alumina during shell curing in air, or a nitride such as boron nitride (BN), silicon nitride (Si₃N₄), or aluminum nitride (AIN). The additional layer is preferably deposited by a method such as chemical vapor deposition (CVD), physical vapor deposition (PVD), or spray deposition. Refractory metal core

[0042] The invention also relates to a sintered refractory metal core having an outer surface, the outer surface being at least partially covered by a protective layer comprising molybdenum carbide (Mo2C), the protective layer having a thickness between 5 µm and 500 µm, the thickness being constant over the entire covered surface.

[0043] The protective layer makes it possible to protect the core against oxidation for a sufficient time so that it can be used in a lost-wax casting process for the manufacture of complex parts such as turbine blades. By Furthermore, molybdenum carbide has a hardness of approximately 1400 HV, while molybdenum has a hardness of approximately 170 HV. The protective layer, which has a coefficient of thermal expansion close to that of molybdenum, thus increases the rigidity of the refractory metal core while limiting cracking during temperature increases or decreases, thereby ensuring the integrity of the core in the casting process.

[0044] This core is obtained in particular by the process described above.

[0045] When the refractory metal core is used in a lost-wax casting process, the surface covered with the molybdenum carbide layer, preferably the entire surface, is protected from oxygen during shell baking.

[0046] In particular, the molybdenum carbide layer exerts an oxygen barrier function up to 1200°C for several tens of minutes.

[0047] Preferably, the thickness and regularity of the molybdenum carbide layer are obtained directly during the sintering step and the layer therefore does not need to be machined to have the desired characteristics.

[0048] The protective layer is formed on at least part of the outer surface of the core, and preferably on the entire outer surface of the core.

[0049] Preferably, the protective layer is formed directly in contact with the molybdenum, which means that the refractory metal core does not have any other layer between the molybdenum and the protective layer.

[0050] The protective layer may have a coefficient of expansion between 5 x 10⁶ K⁻¹ and 9 x 10⁶ K⁻¹ inclusive. This coefficient of expansion is close to that of molybdenum, which is approximately 4.9 x 10⁶ K⁻¹, thus preventing cracking during temperature increases.

[0051] Method for manufacturing a complex part made of a metal alloy

[0052] The invention also relates to a method for manufacturing a complex part made of a metal alloy, the method comprising the use of the refractory metal casting core previously described or manufactured according to the previously described method.

[0053] In particular, the process comprises: - a step Fl of manufacturing a wax part comprising at least one core of refractory metal as previously described, the wax part being manufactured using a mold; - a step F2 of covering the wax piece with a ceramic material; - a step F3 of wax removal to obtain a ceramic shell in which the refractory metal core 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 the refractory metal core to obtain the complex metal alloy part.

[0054] 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.

[0055] This process therefore allows the manufacture of complex parts in an optimized manner since the protective layer is formed simply during the sintering step.

[0056] It is possible to use several molybdenum cores for the formation of the same complex part, each molybdenum core being able to be partially or totally covered with a protective layer of molybdenum carbide.

[0057] It is also possible to use, in addition to the molybdenum core(s), one or more ceramic cores. DESCRIPTION OF THE FIGURES

[0058] [Fig.1] represents a flowchart of the steps of a process for manufacturing a refractory metal core according to an embodiment of the invention;

[0059] [Fig. 2a] and [Fig. 2b] represent a schematic profile view of a refractory metal core manufactured according to the process of [Fig. 1]; and

[0060] [Fig. 3] represents a flowchart of the steps in a process for manufacturing a complex part using the refractory metal core of Figure 2. DETAILED DESCRIPTION OF THE INVENTION

[0061] With reference to figures 1 and 2a-b, a method for manufacturing a refractory metal core is described.

[0062] In a first step El, a green molybdenum core is manufactured by a MIM process.

[0063] For this purpose, 1 kilogram (kg) of molybdenum particles marketed by the company Plansee are mixed with 25 g of binder particles marketed by the company Exone at a temperature between 50°C and 200°C.

[0064] The resulting mixture is heated to a temperature between 100°C and 200°C until it becomes liquid, then poured into a mold to obtain the green core 10 after removal from the mold. The green core 10 is a part that can have approximately the following dimensions: a length of at most 100 mm, a width of at most 50 mm, and a thickness of at most 20 mm.

[0065] The green core 10 then undergoes a thermal debinding step E2 in an enclosure. This step is carried out by heating the green core from a temperature T2i is the ambient temperature, and T2f is a final temperature below 400°C under oxygen. The temperature rise between the initial temperature T2i and the final temperature T2f is 5°C to 30°C per minute.

[0066] A brown core 14 is then obtained and sintered in a step E3. This brown core contains less than 0.5% by mass of binder relative to the mass of molybdenum.

[0067] Sintering is carried out by plasma in the same chamber that was used for debinding. The brown core is then placed in a bed of graphite powder.

[0068] The sintering is then carried out under the following conditions: - heating from an initial temperature T3i between 300°C and 500°C to a final temperature T3f between 1500°C and 2000°C, - a temperature rise rate of between 5°C and 30°C per minute, and - a pressure of 1 mbar to 10-4 mbar.

[0069] The above conditions are maintained for a period of 5 to 30 minutes, preferably 10 to 20 minutes, allowing the sintering of the brown core 14 and the simultaneous formation of a molybdenum carbide layer 12 so as to obtain a sintered core 18.

[0070] An additional protective layer is then deposited by CVD on the molybdenum carbide layer. The additional layer may comprise alumina (Al₂O₃), an alumino-forming agent such as aluminum nitride (AIN) or nickel aluminide (NiAl), which will form alumina during shell curing in air, or a nitride such as boron nitride (BN), silicon nitride (Si₃N₄), or aluminum nitride (AIN). Preferably, the additional layer may comprise alumina (Al₂O₃).

[0071] The sintered core 18 obtained covered with an additional layer can then be used as a core in a conventional lost-wax casting process according to [Fig.3] for the preparation of a complex part such as an aircraft engine blade.

[0072] With reference to [Fig. 3], the process comprises: - a step Fl of manufacturing a wax part comprising at least a core 18, 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 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 18 to obtain the complex metal alloy part.

[0073] The molybdenum carbide layer 12 makes it possible in particular to carry out the F4 shell baking step without the formation of a parasitic layer of molybdenum oxide, up to 1200°C for several tens of minutes during the shell baking.

Claims

Demands

1. A process (20) for preparing a foundry core (18) of refractory metal comprising: - a step E1 of manufacturing a green core (10) by shaping a mixture comprising a mixture of refractory metal particles and binder particles, the refractory metal comprising molybdenum, - a step E2 of removing the binder from the green core to obtain a brown core (14), and - a step E3 of sintering the brown core in the presence of a carbon source so as to form, on at least a part of the surface of the brown core (16), a protective layer (12) of molybdenum carbide having a thickness of between 5 pm and 500 pm.

2. Method according to claim 1, wherein step E3 is carried out at a temperature below 2000° inclusive.

3. A method according to claim 1 or 2, wherein step E3 is carried out for a duration of 5 minutes to 30 minutes.

4. A method according to any one of the preceding claims, wherein the protective layer (12) has a maximum of 5% by volume of porosity relative to the total volume of the layer.

5. A method according to any one of the preceding claims, wherein the sintering step E3 is carried out by heating the brown core (14) between an initial temperature T3i of between 300°C and 500°C inclusive and a final temperature T3f of between 1500°C and 2000°C inclusive.

6. A method according to any one of the preceding claims, wherein the sintering step E3 comprises 1 to 3 temperature steps, each step corresponding to a different temperature.

7. A method according to any one of the preceding claims, wherein step E3 includes at least a temperature rise rate of 5°C to 30°C per minute.

8. A method according to any one of the preceding claims, wherein the carbon source is in the powder bed, the carbon source being selected from boron carbide (B4C), silicon carbide (SiC), graphite (C), and a mixture thereof.

9. A method according to any one of the preceding claims, wherein step E2 is carried out by heating the green core (10) between an initial temperature T2i between 15°C and 25°C and a final temperature T2f of at least 200°C.

10. A method according to claim 9, wherein the temperature rise between the initial temperature T2i and the final temperature T2f is 5°C to 30°C per minute.

11. A method according to any one of the preceding claims, wherein the refractory metal is selected from molybdenum, titanium zirconium molybdenum (TZM), and molybdenum rhenium (Mo-Re).

12. A method according to any one of the preceding claims, wherein the binder comprises a polymer selected from polyethylene, polypropylene, polyoxymethylene, polyvinylpyrrolidone, and mixtures thereof.

13. A method according to any one of the preceding claims, wherein the volume ratio of refractory metal particles to binder particles can range from 85 / 15 to 75 / 15.

14. Sintered refractory metal foundry core (18) having an outer surface, the outer surface being at least partially covered by a protective layer (12) comprising molybdenum carbide (Mo2C), the protective layer (12) having a thickness between 5 pm and 500 pm, the thickness being constant over the entire covered surface.

15. Core according to claim 14, wherein the protective layer (12) has a maximum of 5% by volume of porosity relative to the total volume of the layer.

16. Core according to claim 14 or 15, wherein the protective layer has a coefficient of expansion between 5x106 K 1 and 9x106 K 1 inclusive.

17. Method (20) of manufacturing a complex part of a metal alloy, the method comprising the use of the casting core (18) of refractory metal according to any one of claims 14 to 16 or manufactured according to the method according to any one of claims 1 to 13.

18. A method (20) of manufacturing according to claim 17 comprising: - a step Fl of manufacturing a wax part comprising at least one refractory metal core (18), the wax part being manufactured by means of a mold; - a step F2 of covering the wax piece with a ceramic material; - a step F3 of wax removal to obtain a ceramic shell in which the refractory metal core is integrated (18); - 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 the refractory metal core (18) to obtain the complex metal alloy part.

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