Method for manufacturing a refractory metal core

Abstract

The present invention relates to a method (20) for manufacturing a refractory metal foundry core comprising: - a step E1 of manufacturing a green core (10) by shaping a precursor mixture comprising refractory metal particles and particles of a binder, - a step E2 of forming a protective layer (12) comprising a nitride on at least a part of the surface of the green core (10), - a step E3 of removing the binder from the green core (14) to obtain a brown core, and - a step E4 of sintering the brown core to obtain a sintered core (18).

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 casting core (or core) comprising a protective layer, 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, such as turbine blades.

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

[0004] Casting cores (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 (Mo) or molybdenum (Mo) alloys, particularly the TZM type alloy (molybdenum, titanium (Ti), and zirconium (Zr)), have been used instead of ceramics to manufacture casting cores suitable for lost-wax casting. The use of refractory metals 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 or refractory metal alloy 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's curing. 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] An objective of the present invention is to avoid or limit the formation of a carbon layer on the surface of the refractory metal core during its manufacture, and in particular during the sintering stage.

[0013] Another objective of the present invention is to form a protective layer on the core in order to prevent the reaction of the refractory metal alloy with oxygen, during the manufacture of a part by lost wax casting, for example a blade, particularly during the air-baking stage.

[0014] To this end, the invention relates to a method for manufacturing a refractory metal foundry core comprising: - a step E1 of manufacturing a green core by shaping a precursor mixture comprising refractory metal particles and binder particles, - a step E2 of forming a protective layer comprising a nitride on at least part of the surface of the green core, - a step E3 of removing the binder from the green core to obtain a brown core, and - a step E4 of sintering the brown core to obtain the sintered core.

[0015] This process makes it possible to obtain a sintered core (or core) of refractory metal, at least part of whose surface is protected by the protective layer. The refractory metal core, and in particular the protective layer, therefore does not undergo the formation of a carbon layer on its surface during the sintering step, as, for example, the formation of a Mo2C layer for a molybdenum core. Consequently, no finishing step to remove this undesirable carbon layer is necessary.

[0016] 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.

[0017] The protective layer therefore protects the part of the surface of the refractory metal core that it covers from any reaction with carbon and oxygen.

[0018] Finally, the protective layer comprising a nitride makes the protected surface of the refractory metal core inert to metals such as superalloys.

[0019] Thus, when the refractory metal core is used in a lost-wax casting process, the surface covered with the nitride layer, preferably the entire surface, is protected from carbon during sintering, from oxygen during shell baking and also during the pouring of the superalloy into the shell.

[0020] In particular, the nitride layer acts as a carbon barrier up to a temperature of 1900°C under vacuum, and as an oxygen barrier up to 1200°C. Step 11 - Fabrication of the green core

[0021] 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 "Metal Binder Jetting" (MBJ).

[0022] These methods make it possible to obtain a green core of the desired shape, consisting of a mixture of refractory metal powder and binder.

[0023] The refractory metal used to form the refractory metal particles can be chosen from molybdenum, niobium, tantalum, tungsten, rhenium, hafnium, and one of their mixtures.

[0024] Preferably, the refractory metal comprises molybdenum (Mo). The refractory metal can then be chosen from molybdenum, titanium zirconium molybdenum (TZM), and molybdenum rhenium (Mo-Re).

[0025] The metal particles may have a size ranging from 5 to 50 pm. The powder preferably comprises particles of substantially spherical shape. In this description, when the particles are substantially spherical, their size is given in equivalent sphere diameter.

[0026] The binder particles comprise a polymer selected from polyethylene, polypropylene, polyoxymethylene, polyvinylpyrrolidone, and one of their mixtures.

[0027] The binder particles may further comprise a solvent selected from water, ethylene glycol, 2-butoxyethanol, and one of their mixtures.

[0028] In the precursor mixture, the volume ratio of refractory metal particles to binder particles can range from 85 / 15 to 75 / 25.

[0029] 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.

[0030] When step El is carried out by the MIM process, it includes a heating step intended to make the mixture of refractory metal powders and binder liquid. Step E2 - Formation of the protective layer

[0031] The protective layer comprises a nitride selected from boron nitride (BN), silicon nitride (Si3N4), aluminum nitride (AIN), and mixtures thereof. The protective layer may, for example, consist of a nitride selected from boron nitride (BN), silicon nitride (Si3N4), aluminum nitride (AIN), and mixtures thereof.

[0032] Preferably, the nitride layer comprises a nitride selected from boron nitride (BN) and silicon nitride (Si3N4). The protective layer may, for example, consist of a nitride selected from boron nitride (BN) and silicon nitride (Si3N4).

[0033] The protective layer is formed on at least part of the outer surface of the green core, and preferably on the entire outer surface of the green core.

[0034] Preferably, the protective layer is formed directly in contact with the refractory metal and the core does not have any other layer between the refractory metal and the protective layer.

[0035] The protective layer is formed by chemical vapor deposition or physical vapor deposition, preferably by chemical vapor deposition. Physical vapor deposition can be selected from Joule effect evaporation or vapor-phase electron beam evaporation, sputtering, laser ablation, molecular beam epitaxy, or electric arc deposition.

[0036] Chemical Vapor Deposition (CVD) is carried out at a temperature ranging from 700°C to 1150°C and at a pressure ranging from 50 mbar to 500 mbar.

[0037] In the case where a boron nitride (BN) layer is formed, the precursors for chemical vapor deposition are boron trichloride (BC13) and ammonia (NH3) and the carrier gas is an inert gas, such as argon.

[0038] In the case where a silicon nitride (Si3N4) layer is formed, the precursors for chemical vapor deposition are ammonia (NH3) and a silicon gas chosen from silane (SiH4), silicon tetrachloride (SiCl4) and dichlorosilane (SiCl2H2), and the carrier gas is an inert gas, such as argon.

[0039] In the case where a boron nitride (AIN) layer is formed, the precursors for chemical vapor deposition are ammonia (NH3), hydrochloric acid (HCl) and aluminium trichloride (AlCl3), and the carrier gas is an inert gas, such as argon.

[0040] Physical Vapor Deposition (PVD) is carried out at a temperature ranging from 20°C to 600°C and at a pressure ranging from 102 mbar to 105 mbar.

[0041] In the case where a boron nitride (BN) layer is formed, the spray targets for physical vapor phase deposition are a boron compound, such as boron trichloride, and the carrier gas is an inert gas, such as dinitrogen (N2).

[0042] In the case where a silicon nitride (Si3N4) layer is formed, the sputtering targets for physical vapor phase deposition are a silicon gas selected from silane (SiH4), silicon tetrachloride (SiCD) and dichlorosilane (SiCl2H2), and the carrier gas is an inert gas, such as dinitrogen.

[0043] In the case where a boron nitride (AIN) layer is formed, the sputtering targets for physical vapor phase deposition are chosen from aluminium trichloride (AlCl3), alumina (Al2O3) and boron nitride (AIN), and the carrier gas is an inert gas, such as dinitrogen.

[0044] Once the protective layer has formed, it is not necessary to treat the protective layer or to machine it. Step E3 - Binder removal step

[0045] The E3 binder removal step, also called the debinding step, is carried out on the green core and can be carried out by thermal degradation chosen from degradation, evaporation under controlled atmosphere (possibly under vacuum) and drainage in liquid state on a porous substrate.

[0046] Advantageously, the binder can be removed even in the presence of the protective layer through which it passes.

[0047] When step E3 is carried out by heat treatment, the heat treatment can be performed during sintering, for example during a first step at a temperature ranging from 300°C to 500°C. The sintering is then carried out at a second temperature step higher than the temperature of the first step.

[0048] A so-called brown core is then obtained, the brown core being devoid of binder and not yet sintered. Step E4 - Sintering of the green core

[0049] The brown core, covered at least partially with the protective layer, is then sintered in step E4.

[0050] The sintering step E4 can be carried out by conventional sintering, by microwave, by plasma (known under the Anglicism Spark Plasma Sintering, SPS), or by induction, preferably by conventional sintering or by plasma.

[0051] When the sintering is conventional, the brown core is positioned on a graphite sheet in a chamber under a non-oxidizing atmosphere, preferably at atmospheric pressure. Heating elements are used to heat the inside of the chamber to carry out the sintering at a temperature ranging from 1500°C to 2000°C for a duration ranging from several minutes to several hours.

[0052] When sintering is carried out by plasma, the green core is placed in a chamber on a bed of powder which may be boron carbide (B4C), silicon carbide (SiC) or graphite (C). Sintering is preferably carried out at a temperature ranging from 1500°C to 2000°C in the chamber which is kept under vacuum at a pressure ranging from a few minutes to a few hours.

[0053] During sintering, the carbon which is necessarily present around the core, in particular in the powder bed or in the enclosure, cannot react with the refractory metal on the surface of the core when it is covered with the nitride protective layer.

[0054] During the E4 sintering step, the protective layer prevented the formation of a carbon layer on the surface of the green refractory metal core. Refractory metal core

[0055] The invention also relates to a sintered refractory metal core having an outer surface, the outer surface being at least partly covered by a protective layer comprising a nitride, the protective surface being formed before the sintering of the core.

[0056] This core is obtained in particular by the process described above.

[0057] Thus, when the refractory metal core is used in a lost-wax casting process, the surface covered with the nitride layer, preferably the entire surface, is protected from oxygen during shell baking and also during the pouring of the superalloy into the shell.

[0058] In particular, the nitride layer exerts a barrier function to carbon up to a temperature of 1900°C under vacuum and to oxygen up to 1200°C for several tens of minutes.

[0059] 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.

[0060] Preferably, the protective layer is formed directly in contact with the refractory metal, which means that the refractory metal core does not have any other layer between the refractory metal and the protective layer.

[0061] The protective layer can have a thickness between 1pm and 100 pm inclusive. The thickness of the protective layer can be controlled by selecting, for step E2, a temperature, duration, power density, and gas flow rate appropriate to the desired thickness.

[0062] The protective layer has a coefficient of expansion which is preferably close to the coefficient of expansion of the refractory metal, in particular between 2x106 K 1 and 8x108 K 1 inclusive.

[0063] Method for manufacturing a complex part made of a metal alloy

[0064] 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 core previously described or manufactured according to the previously described method, as the core.

[0065] In particular, the method 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.

[0066] 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.

[0067] The metallic alloy may in particular be a superalloy, in particular comprising a metal selected from nickel, cobalt, iron, titanium, aluminium, and one of their mixtures.

[0068] Advantageously, the core protection layer is inert with respect to superalloys, especially up to a temperature of 1900°C under vacuum.

[0069] This process therefore allows the manufacture of complex parts in an optimized manner since only one protective layer is required on the core(s), the protective layer being formed before the sintering step and allowing the following steps to be carried out without the need for an additional protective layer on the refractory metal core.

[0070] It is possible to use several refractory metal cores for the formation of the same complex part, each refractory metal core being able to be partially or totally covered with a protective layer comprising a nitride.

[0071] It is also possible to use, in addition to the refractory metal core(s), one or more ceramic cores. DESCRIPTION OF THE FIGURES

[0072] [Fig.1] represents a flowchart of the steps of a process for manufacturing a refractory metal core according to an embodiment of the invention;

[0073] [Fig.2a] and [Fig.2b] represent a schematic profile view of a refractory metal core manufactured according to the process of [Fig.1]; and

[0074] [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

[0075] With reference to figures 1 and 2a-b, a method for manufacturing a refractory metal core is described.

[0076] In a first step El, a green molybdenum core is manufactured by a MIM process.

[0077] 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.

[0078] 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.

[0079] In a second step E2, the green core 10 is coated with a boron nitride protective layer by the CVD method. The sputtering targets used are boron trichloride (BC13) and ammonia (NH3), commercially available from Bemex, and the carrier gas is argon.

[0080] The boron nitride layer is deposited over the entire surface of the green core at a temperature ranging from 700°C to 1150°C and a pressure of 50 to 500 mbar for a duration of 30 min to 8 h.

[0081] A green coated core 14 is then obtained comprising the green core 10 totally covered with a protective layer 12 of boron nitride with a thickness of 1 to 100pm.

[0082] The green core 14 is then debound and sintered during steps E3 and E4.

[0083] Sintering is carried out by plasma in a chamber on a bed of graphite powder. Prior to this, debinding is carried out at a temperature Tl ranging from 300°C to 450°C and a pressure between 1 mbar and 10⁴ mbar for a duration of 3 to 60 min, allowing all the binder contained in the core 10 to be removed to obtain a brown core 16.

[0084] The temperature in the enclosure is then brought to a second temperature T2 between 1500°C and 2000°C and a pressure between 1 mbar and 10⁴ mbar while exerting mechanical pressure on the core by means of a piston for a period of 3 to 60 min.

[0085] A final sintered core 18 formed from the core 16 covered with the boron nitride layer is then obtained.

[0086] The final core 18 obtained can then be used in a conventional lost-wax casting process according to [Fig.3] for the preparation of a complex part such as an aircraft engine blade.

[0087] With reference to [Fig. 3], the process comprises: - a step Fl of manufacturing a wax part comprising at least one 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.

[0088] The protective layer 12 makes it possible, in particular, to carry out the shell-curing step F4 without the formation of an unwanted layer. Specifically, during the shell-curing step F4, the nitride layer 12 acts as a carbon barrier up to a temperature of 1900°C under vacuum and as an oxygen barrier up to 1200°C for several tens of minutes. The protective layer 12 also allows the core to withstand the casting of the metal alloy in step F5 to form the complex part.

Claims

Demands

1. A process (20) for manufacturing a refractory metal foundry core comprising: - a step E1 of manufacturing a green core (10) by shaping a precursor mixture comprising refractory metal particles and particles of a binder, - a step E2 of forming a protective layer (12) comprising a nitride on at least a part of the surface of the green core (10), - a step E3 of removing the binder from the green core (14) to obtain a brown core, and - a step E4 of sintering the brown core to obtain a sintered core (18).

2. A method according to claim 1, wherein the nitride is selected from boron nitride (BN), silicon nitride (Si3N4), aluminium nitride (AIN), and a mixture thereof.

3. A method (20) according to claim 1 or 2, wherein the protective layer (12) is formed by chemical vapor deposition or physical vapor deposition, the physical vapor deposition being selectable from Joule or electron vapor deposition, sputtering, laser ablation, molecular beam epitaxy or electric arc deposition.

4. A method (20) according to any one of the preceding claims, wherein the green core (10) is manufactured using a method selected from powder injection molding and additive manufacturing with binder jetting.

5. A device (20) according to any one of the preceding claims, wherein the binder comprises a polymer selected from polyethylene, polypropylene, polyoxymethylene, polyvinylpyrrolidone, and mixtures thereof.

6. A method (20) according to any one of the preceding claims, wherein the sintering step is carried out by conventional sintering, by microwave, by plasma or by induction.

7. A method (20) according to any one of the preceding claims, wherein the refractory metal is selected from molybdenum, the niobium, tantalum, tungsten, rhenium, hafnium, and one of their mixtures.

8. A method (20) according to any one of claims 1 to 6, wherein the refractory metal is selected from molybdenum, titanium zirconium molybdenum (TZM), and molybdenum rhenium (Mo-Re).

9. A method (20) according to any one of the preceding claims, wherein the binder removal step E3 is carried out by thermal degradation selected from degradation, evaporation under controlled atmosphere and drainage in liquid state on a porous substrate.

10. Sintered refractory metal core (18) having an outer surface, the outer surface being at least partly covered by a protective layer (12) comprising a nitride, the protective surface (12) being formed before the sintering of the core.

11. Core (18) according to claim 10, in which the protective layer (12) is formed directly in contact with the refractory metal.

12. Core (18) according to claim 10 or 11, wherein the protective layer (12) has a thickness between 1 pm and 100 pm inclusive.

13. Core (18) according to any one of claims 10 to 12, wherein the protective layer (12) has a coefficient of expansion between 2x106 K 1 and 8x108 K 1 inclusive.

14. Method (20) of manufacturing a complex part of a metal alloy, the method comprising the use of the refractory metal core (18) according to any one of claims 10 to 13 or manufactured according to the method (20) according to any one of claims 1 to 9 for the manufacture of the complex part.

15. A manufacturing method according to claim 14 comprising: - a step Fl of manufacturing a wax part comprising at least one refractory metal core, the wax part being manufactured by means of a mold; - a step F2 of covering the wax part with a ceramic material; - a step F3 of removing the wax to obtain a ceramic shell in which the refractory metal 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 the refractory metal core (18) to obtain the complex metal alloy part.

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