Method for producing a shell mold for manufacturing a turbomachine part, such as a turbomachine blade

Abstract

Method for producing a manufacturing mold by molding a turbomachine part, such as a turbomachine blade, the method comprising: a) producing by additive manufacturing the mold of the part by successive depositions of layers of extruded wire (10) so as to form an internal surface (13) complementary to the external surface of the turbomachine part, b) during step a) or after step a), smoothing or machining the internal surface (13) of the mold so as to obtain a surface condition corresponding to that of the turbomachine part. Abstract figure: Figure 1

Description

Title of the invention: Method for producing a shell mold for manufacturing a turbomachine part, such as a turbomachine blade Technical field

[0001] The present disclosure relates to the field of turbomachine blades, in particular blades obtained by casting a molten alloy into a shell mold. The present disclosure relates more specifically to the field of producing such a shell mold for manufacturing a turbomachine part. Prior art

[0002] Conventionally, a shell mold, or mold for manufacturing by casting, is obtained by the so-called lost wax casting technique, which consists first of all in making a model in wax, or in any other material easily removable thereafter, of the part to be made; this model can include an internal part forming a ceramic core which represents the cavities that one wishes to see appear inside the blade. The wax model is then dipped several times in slips consisting of a suspension of ceramic particles, followed by so-called stuccoing and drying operations, to make a shell mold.

[0003] The carapace mold is then dewaxed, which is an operation by which the wax or material constituting the original model is removed from the carapace. After this removal, a ceramic mold is obtained whose cavity reproduces all the shapes of the blade and which still contains the ceramic core intended to generate the internal cavities of the latter. The mold then undergoes a high-temperature heat treatment or "firing" which gives it the necessary mechanical properties.

[0004] The shell mold is then ready for the manufacture of the metal part by casting. After checking the internal and external integrity of the shell mold, the next step is to pour a molten metal, which occupies the voids between the inner wall of the shell mold and the core, then to solidify it. In the field of lost wax casting, several solidification techniques and several casting techniques are currently distinguished, depending on the nature of the alloy and the expected properties of the part resulting from the casting. This may be directional solidification with a columnar structure (DS), directional solidification with a monocrystalline structure (SX) or equiaxial solidification (EX).

[0005] After casting the alloy, the shell is removed by a shake-out operation. In another step, the ceramic core that remains is chemically removed. enclosed in the resulting blade. The resulting metal blade then undergoes finishing operations which allow the finished part to be obtained.

[0006] Examples of the production of turbine blades using the lost wax casting technique are given in the applicant's patent applications FR2875425 and FR2874186.

[0007] In this conventional process, the properties of the shell mold, in particular the thermomechanical properties, play a key role in the quality of the metal part produced. Before the liquid metal is poured, the initially cold mold is directly introduced into the melting furnace at more than 1000°C. It must therefore withstand a significant thermal shock, with a rapid temperature variation greater than 1000°C, leading to a risk of macro and micro cracks developing. Then, the shell mold is filled with the liquid metal at more than 1500°C. The significant hydrostatic pressure can lead to the rupture of the shell mold. The mold then remains at this temperature for several tens of minutes, hence a risk of the mold becoming brittle.During cooling and solidification of the metal, the shell mold must not be too rigid so as not to constrain the metal part during its removal (i.e. when its dimensions decrease after cooling). Indeed, excessive local stress generated by the mold risks causing defects such as recrystallized grains and / or cracks on the metal part. After solidification (around 1300°C) and during cooling to room temperature, the mold must be as brittle as possible to facilitate demoulding and avoid any stress on the metal part.

[0008] The main limitations of this conventional method are that it does not allow the uniformity of the thickness of the shell mold to be controlled, in particular as a function of its curvatures, and that the properties of the shell mold cannot be locally adapted in terms of, for example, composition, microstructure or even thickness, in order to guarantee the production of a part obtained by casting which corresponds to the criteria for non-scrappage.

[0009] The additive manufacturing technique of the shell mold attempts to overcome these drawbacks, and in particular makes it possible to overcome the manufacturability limits of the standard process, by locally adapting the thickness, microstructure and composition of the shell according to the thermomechanical needs of the mold. The additive process also makes it possible to manage the compressive strength of the shell mold, so as not to generate stresses in the metal part during cooling, for example by localized reduction of its thickness or by insertion of specific architectures in certain areas of the shell mold.

[0010] It is known from document US 2019 / 0001403Al to manufacture a shell mold with core using stereolithography. This additive process consists of building a layer-by-layer part by selective polymerization of a section of a layer of a reactive system which is a dispersion of ceramic particles in a photopolymerizable resin. This process has a high dimensional resolution, but the size of the ceramic grains necessary for resistance to thermal shock and the dimensional stability of the mold will impose a layer thickness of at least the order of a millimeter, which implies an internal mold surface roughness that is too high compared to the required requirements.

[0011] Documents EP3030367B1 and US7533713B2 also disclose an additive process using binder jetting. This binder jetting process is also a layer-by-layer additive process which consists of agglomerating a desired section of a powder bed by ejecting binder droplets. It is a technology making it possible to meet the final dimensional requirement of a cluster shell mold (large shell mold). However, it has the disadvantage of poor surface finish and insufficient mechanical strength. Furthermore, the binder jetting process is by nature mono-material.

[0012] Of all the known processes, none can meet all the criteria sought for a shell mold with cores. In particular, none can combine high dimensional accuracy with the production of a part in several materials.

[0013] Furthermore, the search for increased engine performance involves, in particular, more efficient cooling of the turbine blades located immediately downstream of the combustion chamber. This requirement necessitates the formation inside these blades of more elaborate internal cavities for circulation of the cooling fluid. These cavities are obtained by producing ceramic cores having the shape of the final cavities, this or these cores being arranged in the shell mold. However, the known methods do not allow the manufacture of a multi-material part, that is to say a shell mold with its core or cores, while having the dimensional precision mentioned above.

[0014] The invention therefore aims in particular to provide a simple solution to the problems of the prior art described above by the simplified manufacture of a shell mold. Summary

[0015] To this end, the present disclosure proposes a method for producing a shell mold for manufacturing a turbomachine part, such as a turbomachine blade, the method comprising: a) production by additive manufacturing of the mold of the part by successive deposits of layers of extruded wire so as to form an internal surface complementary to the external surface of the turbomachine part, b) during step a) or after step a), smoothing or machining the internal surface of the mold so as to obtain a surface condition corresponding to that of the turbomachine part.

[0016] The method thus makes it possible to modify, during the manufacture of the mold, the internal surface condition which will be in contact with the liquid metal during casting. Indeed, during a conventional additive manufacturing process by molten wire deposition or by extrusion (commonly known by the English term "robocasting"), the mold obtained can be complex, but the surface condition limited, in particular due to the size and shape of the material wire used, the positioning precision of the deposition head relative to the manufacturing plate but also the shrinkage of the material during drying and the mechanical strength of the latter as the part is constructed. The finishing, by smoothing and / or machining, used during the method of the present disclosure makes it possible to rectify the surface condition of the part, by finishing the internal surface as the material layer is deposited, and therefore as it is constructed.This step of modifying the internal surface is carried out during the construction of the mold so that all the surfaces of the mold in contact with the liquid metal during casting are accessible to the tool at a certain point in the manufacturing process for finishing. Even the most difficult surfaces due to the complex geometry of the mold are therefore accessible. The process therefore makes it possible to manufacture a mold with an optimal surface finish, simply and quickly. This process therefore has the advantage of simplified implementation, as it does not require complex foundry steps.

[0017] Alternatively, steps a) and b) may be repeated to form successive layers of extruded wire until the inner surface of the mold is formed.

[0018] The smoothing / machining step can further be carried out by means of a milling head, preferably configured to obtain a surface roughness Ra of less than 5 μm, preferably less than 3 μm. The surface condition obtained is thus equivalent to that of the surface of the shell mold obtained by a conventional process such as lost wax casting.

[0019] Alternatively or additionally, the smoothing / machining step is carried out by means of a planing tool, preferably configured to obtain a surface roughness Ra of less than 5 pm, preferably less than 3 pm.

[0020] Further, the extruded yarn layers may comprise a first material and a second material, the first material being different from the second material.

[0021] According to an alternative, during the process, for: - step a), a first extrusion head and a second extrusion head are provided, each depositing one or more layers of extruded wire of different material, - step b) a smoothing tool is provided.

[0022] In addition, the extruded wire may comprise several components chosen from at least one of: one or more ceramic powders, a dispersant, one or more binders, one or more plasticizers, one or more solvents, a lubricant. The extruded wire may further comprise particles chosen from at least one of: alumina, zirconia, zircon, silica, cristobalite, mullite, chamotte, molochite, yttrium, silicon carbide.

[0023] The present disclosure further provides a method of manufacturing a turbomachine part, such as a turbomachine blade, the method comprising casting metal into a manufacturing mold, said manufacturing mold being obtained according to the production method described above. Brief description of the drawings

[0024] Other characteristics, details and advantages will appear on reading the detailed description below, and on analyzing the attached drawings, in which: Fig.l

[0025] [Fig.l] shows a schematic diagram of an example of the production of a manufacturing mold obtained during a first phase of the mold manufacturing process. Fig. 2

[0026] [Fig.2] shows another schematic diagram of an example of the production of a manufacturing mold obtained during a second phase of the mold manufacturing process. Description of the embodiments

[0027] For performance reasons in particular, a turbomachine part, and in particular a blade, must comprise an external surface whose surface condition must be perfectly controlled, for example with a surface roughness Ra of less than 0.2. This part is obtained by casting a metal in a manufacturing mold, also called a shell mold.

[0028] The shell mold obtained by the method described below comprises an internal surface designed to directly receive the molten metal making up the part. The internal surface of the mold is therefore complementary to the external surface of the turbomachine part and must have the same surface condition characteristics as that of the part to be manufactured.

[0029] Figures 1 and 2 illustrate by way of example a method for producing the manufacturing mold. During the method, the manufacturing mold is produced by additive manufacturing of the mold of the part by successive deposition of layers of extruded wire 10 so as to form an internal surface 13 complementary to the external surface of the turbomachine part. As illustrated schematically and by way of example in Figures 1 and 2, the additive manufacturing method used may be a method using micro-extrusion. This method consists of building a part, layer by layer, by depositing a filament, or bead, extruded through an extrusion head 30, or nozzle, which moves relative to a manufacturing plate 20. The dimensional accuracy of the part obtained depends directly on the size and shape of the bead, the positioning accuracy of the extrusion head 30 relative to the manufacturing plate 20 but also on the shrinkage of the material during drying and the mechanical strength of the latter as the part is constructed.

[0030] A paste feeds the extrusion head 30 for the formation of a filament. Its rheology is therefore adapted to the extrusion process, to the maintenance of the desired shape and to good cohesion of the beads between them within a layer and between layers. The paste comprises several components which can be chosen, for example from: the ceramic powder(s), a dispersant, one or more binders, one or more plasticizers to ensure cohesion and flexibility, one or more solvents, a lubricant, possibly an agent for improving the adhesion between the deposited filaments. An inorganic binder (nanometric particles of silica, alumina, zirconia and yttrium, etc.) can be added to ensure the cohesion of the mold after debinding.

[0031] Ceramic particles may also be included in the formulation of the paste, these particles being able to be chosen for example from: alumina, zirconia, zircon, silica, cristobalite, mullite, chamotte, molochite, yttrium, silicon carbide as well as their mixtures. The particle size of the ceramic grains is adapted to the dimensions of the extrusion heads used.

[0032] According to an exemplary embodiment of the method, the layers of extruded wires 10 may comprise a first material 11 and a second material 12, the first material 11 being different from the second material 12. In this example, a first extrusion head may deposit a first layer of extruded wire comprising a first material 11 and a second extrusion head may deposit a second layer of extruded wire comprising a second material 12. The first and second materials may be deposited one after the other, thus forming a first layer of a first material 11 and a second layer of a second material 12, the second layer being distinct from the first, or the first and second materials may be deposited one after the other, forming a layer composed of two adjacent materials 11, 12 in this same layer.More generally, the process is not limited to two extrusion heads, which allows the use of as many materials as necessary. Advantageously, the possibility of depositing several materials using several controlled nozzles opens up the possibility of simultaneously building, with the same machine, the shell mold and a core representing the cavities that we want to see appear inside the blade, the mold and the core using different materials. This process, in addition to the reduction in the number of steps provided by this multi-material additive process, therefore makes it possible to reduce the number of steps required for the . manufacturing a mold / core assembly and therefore the time required for this manufacturing. In addition, the positioning of the core is directly ensured precisely in the mold during construction.

[0033] It is therefore understood that the method described has the advantages over another known additive method of being simple, rapid and of allowing the construction of multi-material parts. The micro-extrusion method nevertheless leads to a surface state which corresponds to a stack of the extruded filaments, as visible as an example in the enlargement of [Fig. 1]. It can be observed that a free end 15 of a layer 10 forms an internal surface of the mold, this surface being curved and convex. A stack of several layers 10 forms at the free ends 15 of these layers a plurality of bosses. The surface state formed by these bosses poses problems of quality of the desired surface state, which does not correspond to the desired surface state on the turbomachine part to be manufactured.Indeed, each layer 10 corresponds to the deposition of a bead with a diameter of, for example, less than 1 mm, in particular between 100 and 1000 μm and for example approximately 400 μm. The internal surface of the mold obtained by stacking the layers and in particular the bosses, can therefore comprise a succession of bosses with a radius of, for example, approximately 200 μm, which is not compatible with the surface condition requirements sought for the external surface of the turbomachine part.

[0034] To overcome this drawback, the internal surface of the mold is smoothed, so as to obtain a surface condition corresponding to that of the turbomachine part. The smoothing can be carried out by a smoothing tool 40, for example a milling head or a planing tool, preferably configured to obtain a surface roughness Ra of less than 5 μm, preferably less than 3 μm. Preferably, the material residues generated by the smoothing, in particular in the case of milling, can be sucked or blown away.

[0035] The smoothing or machining of the internal surface can further be carried out during the deposition of successive layers, or after the deposition of several layers forming a part of the mold. Furthermore, the deposition of one or more layers and the smoothing of the internal surface obtained by said deposition of one or more layers can be repeated to form successive layers of extruded wire until the partial or complete formation of the internal surface of the mold. This method can thus be defined as a hybrid process, combining an additive method and a subtractive method. The step of modifying the internal surface by smoothing can be carried out during the construction of the mold so that all the surfaces of the mold in contact with the liquid metal during casting are accessible to the tool at a time during manufacturing to be retouched.

[0036] In the case of a water-based micro-extrusion paste, the construction of the manufacturing mold can be carried out under controlled hygrometry so as not to induce differential shrinkage between the different layers 10, in particular between the first layers 10 and the plate 20. After controlled drying, the assembly can be consolidated by heat treatment.

[0037] The additive micro-extrusion process also offers the advantage of being able to locally control the compressive strength of the shell mold, by locally optimizing its thickness, structure and / or composition. The stresses that may appear in the metal part when it cools below its stiffening temperature are thus limited.

Claims

Claims

1. Method for producing a shell mold for manufacturing a turbomachine part, such as a turbomachine blade, the method comprising: a) producing by additive manufacturing the mold of the part by successive depositions of layers of extruded wire (10) so as to form an internal surface (13) complementary to the external surface of the turbomachine part, b) during step a), smoothing or machining the internal surface (13) of the mold so as to obtain a surface condition corresponding to that of the turbomachine part, in which the smoothing / machining step is carried out by means of a milling head (40) and a planing tool (40), preferably configured to obtain a surface roughness Ra of less than 5 pm, preferably less than 3 pm, the method further comprising: c) repeating steps a) and b) to form successive layers of extruded wire (10) until the internal surface (13) is formed of the mold.

2. A method according to the preceding claim, wherein the layers of extruded wire (10) comprise a first material (11) and a second material (12), the first material being different from the second material.

3. Method according to claim 1 or 2, in which for: - step a), a first extrusion head and a second extrusion head are provided, each depositing one or more layers of extruded wire (10) of different material, - step b) a smoothing tool (40) is provided.

4. A method according to any one of the preceding claims wherein the extruded yarn comprises several components selected from at least one of: one or more ceramic powders, a dispersant, one or more binders, one or more plasticizers, one or more solvents and a lubricant.

5. The method of claim 4, wherein the extruded yarn further comprises particles selected from at least one of: alumina, zirconia, zircon, silica, cristobalite, mullite, chamotte, molochite, yttrium, silicon carbide.

6. A method of manufacturing a turbomachine part, such as a turbomachine blade, the method comprising casting metal into a manufacturing mold, said manufacturing mold being obtained according to the method of carrying out any one of claims 1-5.

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