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) being configured such that the molten metal fills a cavity (121) in a main filling direction and forms a component, the shell (1) comprising at least one heat shield (13) arranged in the vicinity of the at least one mold (12) and extending substantially in the filling direction along the at least one mold (12), the at least one heat shield (13) further comprising an outer wall (131) 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" in the mold, where cooling is more rapid. To achieve this, it is important to control the temperature and solidification rate to ensure uniform crystal growth.

[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 for introducing 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 in a principal filling direction and forms a part, the shell comprising at least one heat shield arranged in the vicinity of at least one mold and extending substantially in the filling direction along at least one mold, the at least one heat shield further comprising an external wall 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 and helps to significantly reduce heat losses that cause manufacturing irregularities in parts, such as the formation of porosities, parasitic grains, 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 results in a particularly effective solution for ensuring uniform and gradual cooling of the part. The low surface roughness of the heat shield provides a "mirror" effect, helping to reflect radiation from the part and contain heat within the molds of the shell. The high emissivity, meanwhile, ensures permeability to radiation and helps prevent heat from passing through the heat shield and escaping.

[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 applicable to existing shells.

[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 outer wall of at least a thermal screen is less than 1 pm.

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

[0017] According to one improvement, the cavity of at least one mold includes a molding portion comprising an internal wall of a shape complementary to that of the at least one part, the at least one heat shield extending at least partially opposite, in a direction transverse to the filling direction, the molding portion of the at least one mold.

[0018] In this way, the heat is contained at the level of the parts of the mold which have the function of preforming the part, and the formation of "defects" on the conveying channels is avoided, which are in any case removed after adjustment.

[0019] According to one improvement, the cavity of at least one mold includes a feed portion located upstream, in a direction of molten metal flow, of the molding portion, at least one heat shield extending at least partially opposite, in a direction transverse to the direction of filling, said feed portion.

[0020] Thus, uniformity of cooling is ensured from the upstream end of the part, to avoid a polycrystalline "start".

[0021] According to one improvement, at least one heat shield has a face opposite one of at least one mold, said face extending substantially perpendicularly to a radial direction around the filling direction.

[0022] In this way, at least one thermal screen more effectively blocks heat radiation that is emitted mainly radially to the filling direction.

[0023] According to an improvement, at least one thermal screen is glued to the shell using refractory cement.

[0024] According to an improvement, at least one thermal screen includes a fastening portion, and is fixed to an anchor point of the carapace which includes a complementary fastening portion, by cooperation between the fastening portion of the at least one thermal screen and the complementary fastening portion of the carapace.

[0025] According to an improvement, the shell includes a bonding layer between the attachment portion of at least one thermal screen and the complementary attachment portion of the shell, arranged so as to cover at least the entirety of the attachment portion of at least one thermal screen, so that the at least one thermal screen is in contact with the bonding layer but is not in contact with the shell.

[0026] According to an improvement, the bonding layer comprises one of the following materials: Ta2O5, HfO2, Al2O3-3SiO2, AhO3-2SiO2.

[0027] According to one improvement, at least one heat shield is fixed to the central casting well of the shell, and is interposed between the central well and at least one of the at least one mold.

[0028] According to one improvement, at least one thermal screen extends annularly around the central well.

[0029] According to an improvement, at least one thermal screen comprises a material from: an oxide from: Sm2Hf2O?, DyTasOs + CaO, LaMnOs, MgO, ThÛ2 SiÛ2 or a non-oxide material from: MoSi2, SiC, MoSi2-SiC-AhO3, HfB2-SiC-TaSi2, HfB2-SiC-TaSi2-lr, SisN4, ZrC, SiÛ2 doped with Graphene, 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, and mixtures thereof.

[0030] Advantageously, one of the materials or a combination of them is chosen to make the thermal screen, so that it is highly emissive and resistant to temperatures above 1000°.

[0031] According to one improvement, at least one heat shield comprises silicon carbide SiC.

[0032] This disclosure also relates to a manufacturing process for a shell as described above, which includes steps that consist of: to create a model in disposable material of at least one part arranged around a central cylindrical section, dip the model made of disposable material into a ceramic slip, to form a shell around the model so as to create at least one mold around the plurality of parts and a central pouring well around the central cylindrical part, remove 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 in a filling direction, attachment of at least one thermal screen on the carapace, the at least one thermal screen comprising an outer wall 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.

[0033] According to an improvement, the process includes a step of identifying one or more cold zones likely to contain a cold spot, and of attaching at least one thermal shield in the vicinity of each identified cold zone. For example, this identification step may include a numerical simulation and / or a post-melting analysis of a test piece to determine the cold zones.

[0034] According to an improvement, the method includes a step in which a supplementary fastening portion is made from an arrangement in the model, and in which the step of fastening at least one heat shield to the shell includes the mechanical cooperation between a fastening portion of at least one heat shield and said supplementary fastening portion of the shell.

[0035] According to an improvement, the step of fixing at least one thermal screen includes gluing at least one thermal screen onto the shell using refractory cement.

[0036] According to an improvement, the step of creating a model from disposable material includes producing at least one form intended to create at least one heat shield during the molding of at least one part. In this way, the at least one heat shield is made of material along with the at least one part and is directly obtained during its molding.

[0037] In particular, the shell as described in this disclosure is of interest for the manufacture of aerodynamic parts, especially for turbine blades. Brief description of the drawings

[0038] Other features, details, and advantages will become apparent upon reading the detailed description below and analyzing the attached drawings, on which:

[0039] [Fig. 1] shows a schematic representation of the main stages in the manufacture of a shell, according to an example,

[0040] [Fig. 2] shows a schematic top view of a carapace as an example from this disclosure,

[0041] [Fig. 3] shows a schematic cross-sectional view along axis AA of Figure 2 of a carapace as an example in this disclosure,

[0042] [Fig. 4] shows a schematic perspective view of a carapace as an example from this disclosure,

[0043] [Fig. 5] shows two schematic top views, 5A and 5B, each showing an example of the arrangement and shape of a heat shield according to this disclosure,

[0044] [Fig. 6] shows a schematic cross-sectional view along the same plane as that of Figure 2, on which one can see in detail a possible embodiment of an anchor point for attaching a thermal screen according to an example in this disclosure,

[0045] [Fig. 7] shows a schematic cross-sectional view of a thermal screen separated from a portion of the shell by a bonding layer, as an example in this disclosure,

[0046] [Fig. 8] 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

[0047] 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.

[0048] 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.

[0049] 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., this refers, unless otherwise specified, 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%.

[0050] 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.

[0051] Generally, such a shell is made of a silico-aluminous material, including for example mullite, alumina, silica and mixtures thereof.

[0052] 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.

[0053] 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.

[0054] 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.

[0055] 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.

[0056] 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.

[0057] 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.

[0058] 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 at least one metal part in a single casting.

[0059] 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.

[0060] 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.

[0061] 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.

[0062] 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.

[0063] Reference is now made to Figure 2, which schematically illustrates a carapace according to an example from this disclosure.

[0064] 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.

[0065] The shell 1 further comprises at least one mold 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 substantially straight portion along axis B.

[0066] In one example, axis B extends vertically.

[0067] 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.

[0068] 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.

[0069] According to this disclosure, the shell includes at least one heat shield 13 configured to control and uniformly cool the metal contained in the internal cavity 123 of each mold 12.

[0070] According to one example, and in particular as represented in the diagram in Figure 2, the thermal screen 13 can comprise a plurality of N modules 13.1, 13.2, ... 13.N, distinct from each other, and each positioned in the vicinity of a mold 12. Alternatively, some modules can be linked to others by connecting elements 136, for example to give more structure to the whole.

[0071] For the remainder of this text, we consider a three-dimensional frame of reference, of variable orientation, comprising a first vertical direction V, a longitudinal direction L oriented towards the axis B of the central well, and a transverse direction T oriented perpendicular to the direction L, together forming a plane normal to the vertical direction V.

[0072] According to the non-limiting example of Figure 2, the modules each extend in width in a straight line along the transverse direction T, between the central well 11 and a mold 12, and have a thickness along the longitudinal direction L.

[0073] Figure 3 schematically represents the example of Figure 2 viewed along the cross-section of axis A. In this example, the central well 11 extends lengthwise along the vertical direction V, which corresponds to a potentially advantageous configuration because the metal flow is thus assisted by gravity. Furthermore, the mold 12 is arranged so that the longest dimension of the part, typically its elongation direction, is substantially parallel to the vertical direction V.

[0074] This may be the case, in particular, in the specific instance of applying this disclosure to turbine blades. Indeed, the molds used to form the blades can extend significantly vertically, as illustrated in Figure 1.

[0075] Thus, in general, the shell 1 according to this disclosure presents a general direction of filling.

[0076] According to one example, the filling direction can be defined as extending along the axis passing through both a center of a metal inlet upstream of the mold 12 and a center of a metal outlet downstream of the mold 12.

[0077] In other examples, the filling direction can be defined as the general direction of mold elongation. In the case of a turbine blade, the filling direction could be essentially the direction in which the blade extends from its root to its tip.

[0078] According to examples, the filling direction can be parallel to the vertical direction V. Indeed, depending on the case, such an orientation naturally promotes good metal flow and uniform mold filling.

[0079] In general, the heat shield as disclosed herein extends primarily along the filling direction. This means that the heat shield has at least one face facing the mold that has a principal dimension along this direction. For example, such a face facing the mold 12 may have a surface area at least twice, or even at least five times, larger than a slice or edge of the heat shield.

[0080] According to the example in Figure 3, the thermal screen 13, more particularly here a module 13.1 of the thermal screen 13, extends along the filling direction, so as to present a face opposite the mold 12 comprising at least a vertical component and a transverse component.

[0081] According to an example, the face opposite the mold of the heat shield has a surface area at least equal to 5% of the surface area corresponding to a contour of the part projected orthogonally in a radial direction to the filling direction.

[0082] 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 the heat shield 13 to extend at least partially opposite, in a direction perpendicular to the main filling direction, the molding portion 122 of the mold.

[0083] 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.

[0084] 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 least partially opposite, in a direction perpendicular to the main filling direction, said feed portion 123.

[0085] Indeed, according to one example, it may be advantageous for the thermal screen 13 to extend both opposite a part of the feed portion 123 and opposite the molding portion 122. In this way, the thermal screen 13 is placed where it is crucial to avoid the formation of cold spots which could generate a polycrystalline microstructure in the "beginning" of solidification.

[0086] According to an example illustrated in Figure 4, which shows a schematic perspective representation of a module 13.1 of a heat shield 13 and a mold 12, the module 13.1 extends circumferentially in an arc around the mold 12. More precisely, the module 13.1 not only has a transverse component and a vertical component, but also extends along a longitudinal component, radially outward to the axis B of the central well 11, so that a cross-section in a horizontal plane of the module forms a portion of an arc extending around the mold 12.

[0087] Of course, it is admitted in the context of this disclosure that the thermal shield modules 13 may be of any shape opposite and / or around the mold or a portion of the mold, so long as they extend at least in the direction of filling, generally vertical, of the mold 12.

[0088] In a specific example, the heat shield 13 can extend opposite the mold 12 and follow the general shape of a volume in which the mold 12 is contained. The heat shield can, in particular, extend over at least 50% of its surface area opposite the mold and at a substantially constant distance from an external surface of the mold 12. For example, at a distance with a variation of less than + / - 50%, or even less than + / - 20%. In this way, the heat shield 13 extends so as to "conform" to the external surface of the mold 12 to more effectively control the cooling of the metal.

[0089] According to one example, the heat shield 13 is made so that the face of the heat shield 13 which extends opposite the mold extends substantially perpendicular to a radial direction around the filling direction along which radiation is emitted due to the cooling of the molten metal in the mold 12.

[0090] In particular, figures 5A and 5B show two examples to illustrate different possibilities for arrangement and shapes of thermal screen 13.

[0091] According to the example in Figure 5A, the heat shield 13 has a single module 13.1, which extends around the entire circumference of the central well 11 at a distance from it, forming an octagonal outline. Of course, the shape of the heat shield can be related to the number of molds. In this example, the octagonal shape is relevant since there are eight distinct molds 12 distributed around the central well 11. It is possible to consider a different number of molds 12, and also different shapes of the heat shield outline 13, regardless of the number of molds.

[0092] The thermal screen 13 may have a continuous annular surface which extends around the axis B of the central well 11.

[0093] For example, a heat shield module 13 can cover several molds 12, and a mold 12 can be covered by several heat shield modules 13.

[0094] According to the example in Figure 5B, the thermal screen 13 can have several different shapes of modules. In this example, the thermal screen includes modules that extend straight across, and modules that extend in an arc around their respective mold 12.

[0095] Also, this last example illustrates, by way of non-limiting agreement, that it is possible for the thermal screen modules 13 to be linked by connecting elements 136, for example for structural reasons and for the mechanical strength of the modules.

[0096] Furthermore, in an example not shown, it is also possible for several heat shield modules 13 to extend in the vicinity of the same mold 12. In one particular example, the heat shield 13 comprises several modules arranged one above the other along the filling direction. For example, three modules arranged one above the other along the filling direction.

[0097] According to this disclosure, the thermal screen 13 further comprises an external wall 131, which can be defined as a surface directly opposite at least a portion of a mold 12 intended to form a part.

[0098] According to this disclosure, this outer wall 131 has 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 surface roughness Ra value is interpreted as the average height of the peaks and troughs formed in the outer wall 131 of the heat shield 13.

[0099] Furthermore, according to this disclosure, the thermal screen 13 also exhibits 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.

[0100] The emissivity value of the outer wall 131 can typically be measured by the following method: 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 to 20 pm.

[0101] In addition, according to examples, the thermal screen 13 may have a porosity of less than 10% and / or a thickness e1 between 0.5mm and 5mm.

[0102] According to examples, the thermal screen 13 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 (composite of molybdenum disilicide, silicon carbide, and alumina), HfB2-SiC-TaSi2 (composite of hafnium diboride, silicon carbide, and tantalum disilicide), HfB2-SiC-TaSi2-lr (composite of hafnium diboride, silicon carbide, tantalum disilicide, 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).,

[0103] The thermal screen 13, or each of its modules, can be made from preforms obtained by high-temperature isostatic pressing, or by extrusion followed by sintering heat treatment, or by strip casting followed by sintering heat treatment, or by pressure casting followed by sintering heat treatment, or by additive manufacturing followed by sintering heat treatment.

[0104] In particular, sintering can allow the preform of the thermal screen 13 to have a dense homogeneous microstructure with a porosity rate, preferably uniform, of less than 10%. This ensures the uniformity and density of the thermal screen.

[0105] Depending on the case, the preforms can be adjusted to give them their final geometry, which can be relatively complex so as to extend at least partially around one or more of the shell molds. Such adjustment can be achieved through machining, milling, grinding, drilling, turning, ultrasonic machining, laser micromachining, waterjet cutting, etc., alone or in combination.

[0106] It is also possible to polish, for example by vibration, a portion of the external surface of the heat shield, to give it a surface roughness Ra of less than 5 pm, preferably 1 pm.

[0107] According to one example, it is possible that the thermal screen 13 has such a roughness value Ra only on one face opposite the mold 12 in the vicinity of which it extends.

[0108] As examples show, the thermal screen 13 can be attached to the shell in various ways. Specifically, it is possible to bond the thermal screen 13, or each module individually, to the shell using refractory cement. For example, the refractory cement could be of the silico-clay type.

[0109] According to other examples, and in particular as schematically represented in Figure 6, it is possible to provide for a purely mechanical assembly between the heat shield 13, or a module, and the shell 1. "Purely mechanical" means that no chemical adhesion contributes to the attachment. In this example, it is a gravity-based mechanical assembly, in which a fastening portion 132 of the heat shield 13 cooperates with a complementary fastening portion 112 provided at an anchor point 111 of the shell 1.

[0110] It is of course possible, within the framework of this disclosure, to employ other mechanisms involving a fastening portion 132 of the thermal screen 13 and a complementary fastening portion 112 of the shell 1.

[0111] In cases where the heat shield 13 is mounted by mechanically cooperating a fastening portion 132 of the heat shield 13 and a complementary fastening portion 112 of the shell, the heat shield 13 is then in contact with the shell, via at least one contact surface 137. In this case, it is possible that the material used for the heat shield 13 may react undesirably with the shell 1. At least for this reason, it may be advantageous to provide, between the heat shield and the shell, a bonding layer 14 made of a different material, and advantageously chemically compatible with the material of the heat shield 13.

[0112] In particular, the bonding layer 14 can be provided between the fixing portion 132 of the heat shield 13 and the complementary fixing portion 112 of the shell 1, arranged so as to cover the entirety of at least one contact surface 137 of the heat shield 13. In this way, the heat shield 13 is in contact with the bonding layer 14 but is not in contact with the shell 1. The bonding layer 14 ultimately forms an intermediate fixing layer that prevents contact between the heat shield 13 and the shell 1.

[0113] Such a bonding layer is notably represented schematically in Figure 7. According to examples, the bonding layer is advantageously composed of one of the following materials: Ta2O5 (tental oxide), HfCh (hafnium oxide), or Al2O3 (alumina) doped with silicon oxides such as 3SiO2 or 2SiO2.

[0114] As an example, the bonding layer may have a surface roughness Ra less than or equal to 1.5 pm and / or have a porosity less than 10%, and / or have a thickness e2 between 0.5 and 3mm.

[0115] Figure 8 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 than in the rest of the part.

[0116] Each curve represents temperature difference measurements over time between a conventional low-emissivity heat shield (approximately 0.4 here) and a high-emissivity (greater than 0.6, specifically 0.8 here) shell heat shield as described in this disclosure. Curve C1 shows a difference of approximately 50°C, and curve C2 shows a difference of over 60°C.

[0117] These results demonstrate that the shell described in this disclosure 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 such that the molten metal fills said cavity (121) in a principal filling direction and forms a part, the shell (1) comprising at least one heat shield (13) arranged in the vicinity of at least one mold (12) and extending substantially in the filling direction along at least one mold (12), the at least one heat shield (13) further comprising an outer wall (131) 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.

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

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

8.

4. Shell (1) according to any one of the preceding claims, wherein the cavity (121) due to at least one mold (12) comprises a molding portion (122) comprising an internal wall of a shape complementary to that of the at least one part, the at least one heat shield (13) extending at least partially opposite, in a direction transverse to the filling direction, the molding portion (122) of the at least one mold.

5. Shell (1) according to the preceding claim, wherein the cavity due to at least one mold comprises a feed portion (123) located upstream, in a direction of flow of the molten metal, of the molding portion (122), at least one heat shield (13) extending at least partially opposite, in a direction transverse to the direction of filling, said feed portion (123).

6. Shell (1) according to any one of the preceding claims, wherein at least one heat shield (13) has a face opposite one of at least one mold (12), said face extending substantially perpendicularly in a radial direction around the filling direction.

7. Carapace (1) according to any one of the preceding claims, wherein at least one heat shield (13) is bonded to the carapace using a refractory cement.

8. Shell (1) according to any one of the preceding claims, wherein at least one heat shield comprises a fastening portion (132) and is fixed to an anchor point (111) of the shell which comprises a complementary fastening portion (112) by cooperation between the fastening portion (132) of at least one heat shield (13) and the complementary fastening portion (112) of the shell (1).

9. Shell (1) according to the preceding claim, wherein it comprises a bonding layer (14) between the fastening portion (132) of at least one heat shield (13) and the complementary fastening portion (112) of the shell (1), arranged to cover at least the entirety of the fastening portion of at least one heat shield (13), such that at least one heat shield (13) is in contact with the bonding layer (14) but is not in contact with the carapace (1).

10. Shell (1) according to the preceding claim, wherein the bonding layer (14) comprises a material from among: Ta2Û5, HfCh, AhO3-3SiO2, AhO3-2SiO2.

11. Shell (1) according to any one of the preceding claims, wherein at least one heat shield (13) is fixed to the central casting well (11) of the shell (1), and is interposed between the central well (11) and at least one of the at least one mold (12).

12. Carapace (1) according to the preceding claim, wherein at least one thermal screen (13) extends annularly around the central well (11).

13. Shell (1) according to any one of the preceding claims, wherein at least one thermal screen (13) comprises a material selected from: an oxide from: Sm2Hf2O / , DyTasOs + CaO, LaMnOs, MgO, ThO2-SiO2 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-Si^Os 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, and their mixtures.

14. Shell (1) according to any one of the preceding claims, wherein at least one heat shield (13) comprises silicon carbide SiC.

15. A method for manufacturing a shell (1) according to any one of the preceding claims, comprising steps consisting of: to create a model in disposable material consisting of at least one part arranged around a central cylindrical section, dip the model made of disposable material into a ceramic slip, to form a shell around the model so as to create at least one mold around the plurality of parts and a central pouring well around the central cylindrical part, remove 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 in a filling direction, fixing at least one thermal screen on the shell, the at least one thermal screen comprising an outer wall 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.

16. Manufacturing method according to the preceding claim, comprising a step of identifying one or more cold zones capable of containing a cold spot, and fixing at least one thermal screen in the vicinity of each identified cold zone.

17. A manufacturing method according to any one of claims 15 and 16, wherein it comprises a step in which a supplementary fastening portion is made from an arrangement in the pattern, and in which the step of fastening at least one heat shield to the shell comprises the mechanical cooperation between a fastening portion of at least one heat shield and said supplementary fastening portion of the shell.

18. A manufacturing method according to any one of claims 15 to 17, wherein the step of fixing at least one heat shield includes bonding at least one heat shield to the shell using a refractory cement.

19. A manufacturing method according to any one of claims 15 to 18, wherein the step of making a model in disposable material includes making at least one form intended to make at least one heat shield during the molding of at least one part.

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