Shell for fabricating a single-crystal component

Abstract

The invention relates to a shell (1) for fabricating at least one single-crystal component by molding using a removable material, comprising a central casting pit (11) configured for introduction of molten metal therein, the shell (1) comprising at least one mold (12) arranged around the central pit (11), each mold (12) comprising an inner cavity (121) in fluidic communication with the central pit (11), the cavity (121) being configured so that the molten metal fills the cavity (121) and forms a component, the shell (1) comprising at least one generally annular heat shield (13) arranged around the central pit (11), the at least one heat shield (13) further comprising an outer wall (131) at least partially covered with an outer coating (135) having a surface roughness Ra less than 5 µm and an emissivity E greater than 0.6 when measured at a temperature between 1400 and 1600°C.

Description

Description Title: Shell for manufacturing a single-crystal part Technical field

[0001] The scope of this disclosure is the manufacture of single-crystal metal parts, in particular aeronautical parts such as turbine blades, by a lost-wax casting process. Previous technique

[0002] Some metal parts require a specific crystalline microstructure. In certain cases, it can be advantageous for manufactured parts to have a single-crystal microstructure. "Single-crystal" means that these parts consist of only one grain and are completely free of grain boundaries. This allows the part to exhibit improved mechanical or electrical properties, such as better creep resistance.

[0003] To obtain a single-crystal part, a Bridgman-type furnace is known to be used, which allows for a controlled solidification process to "guide" the crystallization of the part. In such a process, described for example in document WO2014 / 049223, the solidification of the molten metal is controlled by a grain selector that allows the growth of a single grain in the mold and prevents the growth of unwanted grains. To achieve this, the solidification front must also be controlled and requires gradual cooling in the direction of growth, allowing the metal to crystallize in a single direction, following the structure of the single-crystal seed.

[0004] It is therefore crucial to prevent the metal from solidifying in areas other than those directly adjacent to the crystallization of the single-crystal seed. The aim is to prevent the formation of "cold spots," where cooling is more rapid, within the mold. To achieve this, it is important to control the temperature and solidification rate to ensure uniform crystal growth within the mold.

[0005] To achieve this, document WO2014049223 suggests using the shape of the shell to help control the temperature and prevent uneven cooling of the part. Specifically, it is known to incorporate additional sections of the shell that act as thermal shields to try to slow down the cooling of the mold.

[0006] However, this solution is not entirely satisfactory. Indeed, even with such additional shell sections, the inventors observed the formation of cold spots within the shell. This results in uneven solidification of the metal. Consequently, it is not possible to guarantee that the resulting part will be perfectly single-crystal.

[0007] The purpose of this disclosure is therefore to mitigate, at least in part, the drawbacks of the state of the art mentioned above. Summary

[0008] The objectives mentioned above are achieved in particular by a shell for the manufacture of at least one single-crystal part by molding using a disposable material, comprising a central pouring well configured to introduce molten metal, the shell comprising at least one mold arranged around the central well, each mold comprising an internal cavity in fluidic communication with the central well, the cavity being configured so that the molten metal fills said cavity and forms a part, the shell comprising at least one thermal screen of generally annular shape arranged around the central well, the at least one thermal screen further comprising an external wall at least partially covered with an external coating having a surface roughness Ra less than 5 pm and an emissivity E greater than 0.6 when measured at a temperature between 1400 and 1600 °C.

[0009] Thus, in a particularly effective manner, the shell according to this disclosure allows increased permeability of radiation so as to considerably reduce heat losses which can cause manufacturing irregularities in the single-crystal part, such as the formation of porosity, or "freckles".

[0010] Indeed, the inventors determined that the shell material used to form conventional thermal shields was permeable to certain types of radiation, particularly those with wavelengths between 0.5 and 5 micrometers. This explains why the state-of-the-art solution is not fully effective: these conventional thermal shields are transparent to a portion of the radiation.

[0011] In particular, the synergy between the low roughness and high emissivity of the heat shield's outer coating results in a particularly effective solution for ensuring uniform and gradual cooling of the part. The low surface roughness of the outer coating provides a "mirror" effect, helping to reflect radiation from the part and contain heat within the molds of the outer shell. The high emissivity, meanwhile, ensures permeability to radiation and helps prevent heat from passing through the heat shield.

[0012] Therefore, the formation of cold spots is completely avoided, and uniform and progressive cooling of the part can be guaranteed in order to obtain single-crystal parts with a low scrap rate.

[0013] The proposed solution may advantageously not require modification of the shell and can therefore be applied to existing shells. Indeed, it is possible to obtain a shell according to this disclosure, starting from an existing shell geometry, by applying the external coating locally to the areas where it is desired to increase the cooling control efficiency.

[0014] The features described in the following paragraphs may optionally be implemented, independently of each other or in combination with each other:

[0015] According to an improvement, the surface roughness Ra of the external coating of at least one thermal screen is less than 1 pm

[0016] According to an improvement, the emissivity of the external coating of at least one thermal screen is greater than or equal to 0.8.

[0017] According to an improvement, the external coating has a thickness between 0.1 and 500 pm, preferably between 0.1 and 300 pm.

[0018] According to an improvement, the external coating has a porosity, preferably uniform, of less than 10%. This ensures the uniformity and density of the coating in the geometry.

[0019] According to one improvement, the coating comprises a first material, the shell, including a bonding layer between the outer coating and at least one thermal screen, the bonding layer comprising a second material different from the first material of the outer coating. In this way, the bonding layer prevents any undesirable chemical reaction that could result from contact between the material chosen for the outer coating and the material of the shell.

[0020] According to an improvement, the bonding layer includes a surface roughness Ra of less than 1.5 pm.

[0021] According to an improvement, the bonding layer comprises one material from: Ta2Û5, HfCh, Al2O3-3SiÛ2, AhO3-2SiO2, and their mixtures.

[0022] According to one improvement, at least one thermal screen came from material with the central casting well of the carapace.

[0023] According to one improvement, at least one thermal screen extends substantially perpendicular to an axis B of the central well around the central well.

[0024] According to one improvement, the heat shield has an annular shape extending radially to the axis B of the central well between a small diameter and a large diameter, a contour of the heat shield at the small diameter being continuously connected to the central well.

[0025] According to an improvement, the external coating comprises a material from: an oxide from: Siri2Hf2O7, DyTasOs + CaO, LaMnOs, MgO, ThÛ2 SiÛ2 or a non-oxide material from: MoSi2, SiC, MoSi2-SiC-AhOs, HfB2-SiC-TaSi2, HfB2-SiC-TaSi2-lr, SisN4, ZrC, Graphene-doped SiÛ2, SiC-MoSi2-TaSi, ZrB2 doped with [(SiC-WC) or (SiC-MoSi2) or (SiC-Sir^Os) or (SiC-C) or (SiC+BÆ) or (SiC-Si3N4)) and HfB2 doped with (TaSi2 or SiC)) or a metallic material from: Pt, Pd, Ir, Os, Re, Ru.

[0026] According to an improvement, the external coating comprises silicon carbide SiC.

[0027] This disclosure also relates to a manufacturing process for a shell, which includes steps that consist of: to create a model in disposable material comprising at least one part arranged around a central cylindrical part, and comprising an annular part, arranged around the central cylindrical part, dipping the model in a ceramic slip to form a shell around the model so as to create at least one mold around at least one part and a central pouring well around the cylindrical central part, as well as at least one heat shield around the annular part; removing the material that can be removed by heating so that it releases a cavity inside the shell, said cavity being intended to be filled with molten metal introduced into the central well and to form at least one part by filling at least one mold of the shell, characterized in that it comprises a step of: application of an external coating on at least one thermal screen of the shell, the external coating having a surface roughness Ra of less than 5 pm and an emissivity E greater than 0.6 when measured at a temperature between 1400 and 1600 °C.

[0028] According to an improvement, the application of the external coating is done by dipping the shell in a slip loaded with emissive particles at high temperature.

[0029] According to another improvement, the application of the external coating is done by spraying, in particular by local spraying on the screen area in the shell.

[0030] According to an improvement, the loading rate of the loaded slip is between 50 and 75% and is stabilized using a dispersant and a binder.

[0031] According to an improvement, the particle size distribution is between 0.1 pm and 50 pm, preferably between 0.1 pm and 10 pm.

[0032] According to an improvement, the shell comprises a first outer wall part extending at a distance from at least one thermal screen and a second outer wall part comprising at least one thermal screen, the process comprising, prior to the step of applying the outer coating, a masking step in which a masking layer is applied to the first outer wall part, the masking layer being configured to prevent the outer coating from adhering to the first outer wall when the shell is dipped in the loaded slurry.

[0033] According to an improvement, the external coating is co-sintered with the shell under air at a temperature less than or equal to 1200°C after the step of removing the disposable material.

[0034] According to another improvement, the external coating is co-sintered under a controlled atmosphere, for example under argon, at a temperature less than or equal to 1200°C.

[0035] According to another improvement, the external coating is dried between 60°C and 300°C.

[0036] According to an improvement, the process includes a mechanical polishing step of the external coating, so as to obtain a surface roughness Ra of the external coating of less than 1 pm. Brief description of the drawings

[0037] Other features, details, and advantages will become apparent upon reading the detailed description below and analyzing the attached drawings, on which:

[0038] [Fig. 1] shows a schematic representation of the main stages in the manufacture of a shell, according to an example,

[0039] [Fig. 2] shows a schematic top view of a carapace as an example from this disclosure,

[0040] [Fig. 3] shows a schematic cross-sectional view along axis AA of Figure 2 of a carapace as an example in this disclosure,

[0041] [Fig. 4] shows a schematic cross-sectional view along the same plane as that of Figure 2, on which one can see in particular a masking layer according to an example of the present disclosure,

[0042] [Fig. 5] shows a schematic cross-sectional view of a coating on a portion of the carapace as an example in this disclosure,

[0043] [Fig. 6] shows a schematic cross-sectional view of a coating on a bonding layer of a portion of a carapace as an example in this disclosure,

[0044] [Fig. 7] shows a graphical representation of different temperature differences as a function of time on two potential "cold spots" measured between a shell according to the invention and a state-of-the-art shell. Description of the implementation methods

[0045] The drawings and description below contain, for the most part, elements of a definite nature. They may therefore not only serve to better explain this disclosure, but also contribute to its definition, if necessary.

[0046] In the various figures, the same reference numerals designate identical or similar elements. For the sake of brevity, only the elements that are useful for understanding the described embodiment are shown in the figures and are described in detail below.

[0047] In the description that follows, when referring to absolute positional qualifiers, such as "front," "back," "top," "bottom," "left," "right," etc., or relative positional qualifiers, such as "above," "below," "superior," "inferior," etc., or to orientational qualifiers, such as "horizontal," "vertical," etc., unless otherwise specified, this refers to the orientation of the figures. Furthermore, the term "approximately" is to be interpreted as indicating that the result obtained is as precise as the known method for measuring it, or unless otherwise stated, that the result has a margin of error of + / - 10%.

[0048] Reference is now made to Figure 1, which shows the main steps of a process for manufacturing a shell for the production of parts using a disposable material, as an example.

[0049] Generally, such a shell is made of a silico-aluminous material, including for example mullite, alumina, silica and mixtures thereof.

[0050] Typically, such a disposable material is a wax, in which case it is referred to as "lost-wax" casting. That said, it is obviously possible to use other types of disposable materials without departing from the scope of this disclosure.

[0051] A first step typically involves creating a model in disposable material. According to this disclosure, this model includes a cluster of part models, which might be obtained, for example, by injecting the disposable material into a reusable metal mold. The part models are intended to create molds that will later be used to produce multiple castings in a single pour.

[0052] The model also includes parts necessary for metal casting, such as a central cylindrical section for creating a pouring well, and a set of feed channels to convey the molten metal to the molds for producing the parts. These parts intended for metal casting are typically cylindrical, but it is of course possible to use other channel cross-sectional shapes, which can be constant or variable, including for the pouring well.

[0053] In general, the different parts of the model such as molds, channels and other special arrangements related to the molding process (e.g. vents) can be obtained separately and then manually assembled together to build a complete model.

[0054] The model is then dipped in a ceramic slip so that a shell forms around it. Typically, ceramic slip is a liquid suspension composed of fine ceramic particles mixed with water, configured to adhere to the surface of the model. The particles, at least in the first dipping operation, are preferably small so that they can conform to the external shape of the model as precisely as possible.

[0055] Once the slip is applied, a layer of sand (stucco) is added, shaping the material into a layer that is then left to dry, forming a solid and rigid shell around the model. This process can be repeated several times to achieve the desired thickness. Different grain sizes can be used with each repetition. Typically, slips containing larger particles are used for the outermost layers of the shell, where the precision of the resulting shape is less critical.

[0056] After the shell has completely dried, the removable model is taken off, usually by heating, leaving a hollow cavity inside the shell. This cavity is then filled with molten metal to form the multiple metal parts in a single casting.

[0057] Commonly, manufacturing processes known as "lost-wax casting" are known, in which the material to be discarded is wax. That said, it is possible to use other materials that can be discarded without departing from the scope of this disclosure.

[0058] The ceramic slip provides a refractory structure capable of withstanding the high temperatures of molten metal (on the order of temperatures exceeding 1000°C), and ensuring the precision required for the details of the final parts.

[0059] A final, unshown step involves allowing the molten metal inside the shell to cool and solidify, then removing the shell, for example by vibrating it. The metal part is then freed, the parts related to the metal casting are removed, and the part is then possibly adjusted to its functional dimensions.

[0060] These various steps are given as examples to describe a known manufacturing process using a disposable material, and to aid in understanding this document. Of course, the examples given above are in no way to be interpreted as defining the scope of this disclosure.

[0061] Reference is now made to Figure 2, which schematically illustrates a carapace according to an example from this disclosure.

[0062] The shell 1 is schematically illustrated in a top view. The shell 1 comprises, firstly, a central pouring well 11, configured for introducing molten metal. The central well 11 typically has a cylindrical shape, but it is also possible for it to have an irregular, or even variable, non-cylindrical shape.

[0063] The shell 1 further comprises a plurality of molds 12 arranged around the central well 11. The molds 12 are shown here in any shape, since this disclosure is not limited to any particular type of part. The central well 11 extends over at least a straight portion along axis B.

[0064] In one example, axis B extends vertically.

[0065] Each mold 12 comprises an outer layer, typically ceramic, defining an internal cavity 121 configured to form the part. The internal cavity of the molds typically includes a portion that has a shape complementary to that of the part, so that the molten metal, which enters the internal cavity 121 of the mold, takes the shape of the part.

[0066] To achieve this, the internal cavity 121 of each mold is in fluidic communication with the central well 11. Feeding ports 123 can be provided between each mold and the central pouring well to convey the molten metal. The feeding ports 123 can typically be hollow channels.

[0067] In one example, and particularly as shown in Figure 3, the cavity 121 of the mold 12 includes a molding portion 122 whose function is to allow the metal to conform to the shape of the part. Typically, this portion of the cavity includes an internal wall with a shape complementary to that of the part to be manufactured. In this example, it may be advantageous for at least one heat shield 13 to extend over the molding portion 122 of the mold.

[0068] In this way, the presence of the thermal screen 13 is concentrated on the functional area of ​​the mold 12. In other words, it is possible to control the cooling in the area where molding defects are to be avoided as a priority.

[0069] Furthermore, the mold cavity may include a feed portion 123 located upstream, in a direction of molten metal flow, of the molding portion 122. This feed portion 123 is specifically configured to convey the molten metal to the internal cavity of the mold 12. In this case, the heat shield 13 may extend at the level of said feed portion 123.

[0070] Indeed, according to one example, it may be advantageous for a first thermal screen 13 to extend at the level of the feeding portion 123 and for a second thermal screen 13 to extend at the level of the molding portion 122. In this way, the evacuation of heat by radiation is prevented in an area where it is crucial to avoid the formation of cold spots, especially at the "beginning" of solidification.

[0071] According to this disclosure, the heat shield 13 further comprises an outer wall 131 covered with an external coating 135. Said coating is thus outside the heat shield 13, and therefore extends entirely outside the mold 12. The coating is also separate from the mold 12.

[0072] According to this disclosure, this external coating 135 has, at least on one external surface, a surface roughness Ra of less than 5 pm. Preferably, the surface roughness Ra is less than or equal to 1 pm. In particular, the value of the surface roughness Ra is interpreted as the average height of the peaks and troughs formed in the coating 135 of the thermal screen 13.

[0073] Furthermore, according to this disclosure, the external coating 135, and in particular at least on one external surface, has an emissivity E greater than 0.6, preferably greater than or equal to 0.8. In particular, this emissivity value is measured at a temperature between 1400 and 1600 °C. Such a temperature range corresponds substantially to that reached in a furnace used to melt the metal in the shell.

[0074] The emissivity value of the external coating 135 can typically be measured by a UV-Visible - near IR spectrometer or the Fourier transform IR spectrometer coupled to an integrating sphere whose spectral range can extend from 0.5 pm to 20 pm;

[0075] In addition, according to examples, the external coating 135 may have a porosity of less than 10% and / or a thickness e1 between 0.1 pm and 500 pm.

[0076] According to examples, coating 135 may comprise one of the following materials: Sm2Hf2O7 (samarium hafnate), DyTa3O9+CaO (dysprosium tantalum oxide and calcium oxide), LaMnO3 (lanthanum manganite), MgO (magnesium oxide), ThO2 (thorium dioxide), SiO2 (silica), or a non-oxide material from among: MoSi2 (molybdenum disilicide), SiC (silicon carbide), MoSi2-SiC-Al2O3 (molybdenum disilicide, silicon carbide, and alumina composite), HfB2-SiC-TaSi2 (hafnium diboride, silicon carbide, and tantalum disilicide composite), HfB2-SiC-TaSi2-lr (hafnium diboride, silicon carbide, tantalum disilicide composite, and iridium), Si3N4 (silicon nitride), ZrC (zirconium carbide), Graphene-doped SiO2 (graphene-doped silica), SiC-MoSi2-TaSi (silicon carbide, molybdenum disilicide and tantalum disilicide composite),ZrB2 doped with [(SiC-WC) (silicon carbide- and tungsten carbide-doped zirconium carbide) or (SiC-MoSi2) (silicon carbide and molybdenum disilicide) or (SiC-Sm2O3) (silicon carbide and samarium sesquioxide) or (SiC-C) (silicon carbide and carbon) or (SiC+B4C) (silicon carbide and boron carbide) or (SiC-Si3N4) (silicon carbide and silicon nitride)] and HfB2 doped with (TaSi2 (tantalum disilicide) or SiC (silicon carbide)) or a metallic material from among: Pt (platinum), Pd (palladium), Ir (iridium), Os (osmium), Re (rhenium), Ru (ruthenium), or mixtures thereof.

[0077] As an example, the application of the external coating 135 is carried out by dipping the shell 1 into a slurry loaded with emissive particles at high temperature. In particular, to ensure good adhesion of the loaded slurry to the surface of the shell, it may be necessary to polish the thermal screen before applying the external coating so that it achieves a surface roughness of less than 1.5 µm.

[0078] In particular, it can be advantageous to mask off areas not to be coated prior to dipping the shell into the filled slip. Indeed, the shell may comprise a first outer wall portion 111 extending at a distance from the thermal screen 13 and a second outer wall portion 112 comprising the thermal screen 13. The process includes, prior to the step of applying the outer coating 135, a masking step in which a masking layer 113 is applied to the first outer wall portion 111. The masking layer is configured to prevent the outer coating 135 from adhering to the first outer wall 111 during the dipping of the shell 1 into the filled slip.For example, the masking layer 113 may include a material which has little or no adhesion to the filled slurry, so that during dipping the filled slurry only adheres to the second part of the outer wall 112 which includes the thermal screen 13.

[0079] According to one example, and in particular as shown in Figure 6, the shell 1 may include a bonding layer 14 between the outer coating 135 and the thermal screen 13. Such a bonding layer 14 may in particular help to prevent any undesirable chemical reaction between the coating material 135 and the thermal screen material 13 on which it is applied.

[0080] For this purpose, if the outer coating 135 comprises a first material, the bonding layer 14 formed between the outer coating 135 and the thermal screen 13 may comprise a second material different from the first material. In this way, the first material of the outer coating 135 can be chosen for its emissive properties, independently of its chemical compatibility with the material of the thermal screen 13, and the material of the bonding layer 14 can be chosen for its compatibility and chemical stability with both the material of the coating 135 and the material of the thermal screen 13.

[0081] Such a bonding layer 14 may have a thickness e2 between 1pm and 1mm, and / or a porosity of less than 20%.

[0082] According to one example, the bonding layer 14 has a surface roughness Ra of less than 1.5pm.

[0083] According to examples, the bonding layer 14 is advantageously composed of one of the following materials: Ta2O5 (tantalum oxide), HfO2 (hafnium oxide), or Al2O3 (alumina) doped with silicon oxides such as 3SiO2 or 2SiO2.

[0084] Figure 7 shows two curves that demonstrate the effectiveness of the shell according to this disclosure. Specifically, the graph shows two curves, C1 and C2, each representing measurement points taken at two different potential cold spots. Potential cold spots are defined as areas of the mold where, above a certain probability threshold, faster cooling is likely to occur than in the rest of the part.

[0085] Each curve represents temperature difference measurements over time between a conventional low-emissivity thermal screen (less than 0.4) and a thermal screen with a high-emissivity coating (greater than 0.6, specifically 0.8 in this case), as disclosed herein. Curve C1 shows a difference of more than 120°C, and curve C2 shows a difference of approximately 60°C.

[0086] These results demonstrate that shell 1, as disclosed herein, maintains a higher temperature at potential cold spots than known thermal shields. This ensures the absence of cold spots during the cooling of the single-crystal part.

Claims

Demands

1. Shell (1) for manufacturing at least one single-crystal part by molding using a disposable material, comprising a central pouring well (11) configured for introducing molten metal, the shell (1) comprising at least one mold (12) arranged around the central well (11), each mold (12) comprising an internal cavity (121) in fluidic communication with the central well (11), the cavity (121) being configured so that the molten metal fills said cavity (121) and forms a part, the shell (1) comprising at least one generally annular heat shield (13) arranged around the central well (11), the at least one heat shield (13) further comprising an outer wall (131) at least partially covered with an external coating (135) having a surface roughness Ra of less than 5 pm and an emissivity E greater than 0.6 when it is measured at a temperature between 1400 and 1600 °C.

2. Shell (1) according to the preceding claim, wherein the surface roughness Ra of the outer coating (135) of at least one thermal screen (13) is less than 1 pm.

3. Shell (1) according to any one of the preceding claims, wherein the emissivity of the outer coating (135) of at least one thermal screen (13) is greater than or equal to 0.

8.

4. Shell (1) according to any one of the preceding claims, wherein the outer coating (135) has a thickness between 0.1 and 500 pm.

5. Shell (1) according to any one of the preceding claims, wherein the outer coating (135) has a porosity of less than 10%.

6. Shell (1) according to any one of the preceding claims, wherein the coating comprises a first material, the shell (1) comprising a bonding layer (14) between the outer coating (135) and at least one thermal screen (13), the bonding layer (14) comprising a second material different from the first material of the outer coating (135).

7. Shell (1) according to the preceding claim, wherein the bonding layer (14) comprises a surface roughness Ra of less than 1.5 pm.

8. Shell (1) according to any one of claims 6 or 7, wherein the bonding layer (14) comprises a material from among: Ta2Û5, HfÛ2, AhO3-3SiO2, AhO3-2SiO2, and mixtures thereof.

9. Carapace (1) according to any one of the preceding claims, wherein at least one heat shield (13) is made of material with the central casting well (11) of the carapace (1).

10. Shell (1) according to any one of the preceding claims, wherein at least one thermal screen (13) extends substantially perpendicular to an axis B of the central well (11) around the central well (11).

11. Shell (1) according to any one of the preceding claims, wherein the outer coating (135) comprises a material from: an oxide from: Sm2Hf2O / , DyTasOs + CaO, LaMnOs,MgO, ThU2 SiU2 and mixtures thereof, or a non-oxide material from: MoSi2, SiC, MoSi2-SiC-AhO3, HfB2-SiC-TaSi2, HfB2-SiC-TaSi2-lr, SisN4, ZrC, Graphene-doped SiU2, SiC-MoSi2-TaSi, ZrB2 doped with SiC-WC or SiC-MoSi2 or SiC-Sm2O3 or SiC-C or SiC+BÆ or SiC-Si3N4 and HfB2 doped with TaSi2 OR SiC and mixtures thereof, or a metallic material from: Pt, Pd, Ir, Os, Re, Ru.

12. Shell (1) according to any one of the preceding claims, wherein the outer coating (13) comprises silicon carbide SiC.

13. A method for manufacturing a shell (1) comprising steps which consist of: producing a model of disposable material comprising at least one part arranged around a cylindrical central portion, and comprising an annular portion, arranged around the cylindrical central portion, dip the model in a ceramic slip, to form a shell around the model so as to create at least one mold (12) around at least one part and a central pouring well (11) around the central cylindrical part, as well as at least one heat shield (13) around the annular part, removing the material that can be removed by heating so that it releases a cavity inside the shell, said cavity being intended to be filled with molten metal introduced into the central well (11) and to form at least one part by filling at least one mold (12) of the shell, characterized in that it comprises a step of: application of an external coating (135) on at least one thermal screen (13) of the shell, the external coating (135) having a surface roughness Ra less than 5 pm and an emissivity E greater than 0.6 when measured at a temperature between 1400 and 1600 °C.

14. A manufacturing method according to the preceding claim, wherein the application of the external coating (135) is done by dipping the shell (1) in a slip loaded with emissive particles at high temperature.

15. A manufacturing process according to the preceding claim, wherein the loading rate of the loaded slip is between 50 and 75% and is stabilized using a dispersant and a binder.

16. A manufacturing method according to any one of claims 13 to 15, wherein the shell (1) comprises a first outer wall portion (111) extending at a distance from at least one thermal screen (13) and a second outer wall portion (112) comprising at least one thermal screen (13), the method comprising, prior to the step of applying the outer coating (135), a masking step in which a masking layer (113) is applied to the first outer wall portion (113), the masking layer being configured to prevent the outer coating (135) from adhering to the first outer wall (111) when the shell (1) is dipped in the loaded slurry.

17. A manufacturing method according to any one of claims 13 to 16, comprising a mechanical polishing step of the external coating (135), so as to obtain a surface roughness Ra of the external coating (135) of less than 1 pm.

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