Method for manufacturing an improved core for the lost-wax casting process.

Abstract

Method for manufacturing an improved core for a lost-wax casting process. The invention relates to a method for manufacturing a core for a lost-wax casting process comprising: - a step of obtaining the core 100 from ceramic material by an additive manufacturing process; - a step of impregnating the open porosity of the core with a reinforcing composition, the reinforcing composition comprising an aqueous solution of polyvinyl alcohol with a degree of hydrolysis between 82% and 98.4%, and at a concentration between 35 gL-1 and 165 gL-1; the impregnation step being carried out at a pressure less than or equal to 1 bar; - a draining step; and then - a drying step. Figure for the abstract: Fig. 2.

Description

Title of the invention: Method for manufacturing an improved core for the lost-wax casting process. technical field

[0001] The invention falls within the field of ceramic cores for the manufacture of cavities in metal parts obtained by lost-wax casting processes. Previous technique

[0002] Increasing the operating temperature of aeronautical turbomachinery allows for an increase in their efficiency and therefore fuel savings.

[0003] However, the increase in operating temperatures subjects the constituent materials of the turbomachines to more aggressive conditions.

[0004] This increases, for example, the cooling requirements of blades located in hot zones. To meet this need, new cooling circuits have been proposed for such blades.

[0005] These new cooling circuits are better suited to the new constraints, and offer better cooling of the turbomachine blades.

[0006] Conventionally, hollow cooling circuits in metallic material blades are obtained via lost-wax manufacturing processes.

[0007] In such processes, a wax model of the blade is made around a core the size of the cavity, or cavities as the case may be. A ceramic mold, called a shell, is formed around the wax model, and then the wax is removed. Metal is formed in the shell in place of the removed wax, and thus around the core.

[0008] The subsequent removal of the core allows the cooling circuit to be formed in the metal blade.

[0009] The increasing complexity of cooling circuits requires the use of new methods for manufacturing the core.

[0010] Indeed, although the cores of prior art processes could be obtained by ceramic injection processes, such methods do not allow the geometric resolution required for new cooling circuits to be obtained.

[0011] In addition, and regardless of the process chosen for manufacturing the ceramic core, it is usually proposed to impregnate the cores to increase their mechanical properties.

[0012] This step of the process poses additional difficulties in the prior art since some of the core impregnation resins that could be used are subject to strict regulation or are not recommended to be used.

[0013] Thus, there remains a need for improvement for the new generation ceramic cores, whose properties would be at least identical to those of the prior art cores but whose geometry could be even more finely controlled than the cores obtained by a ceramic injection process. Description of the invention

[0014] The present exposition aims to address at least one of the problems described above.

[0015] To this end, it relates, according to one of its aspects, to a method for manufacturing a core for a lost-wax casting process comprising: - a step of obtaining the core in ceramic material by an additive manufacturing process; - a step of impregnating the open porosity of the core with a reinforcing composition, the reinforcing composition comprising an aqueous solution of polyvinyl alcohol with a degree of hydrolysis between 82% and 98.4%, and with a concentration between 35 gL 1 and 165 gL 1; the impregnation step being carried out at a pressure less than or equal to 1 bar, - a draining step; then - a drying step.

[0016] It is to the credit of the inventors that they have developed a reinforcing composition which allows a ceramic material core obtained by an additive manufacturing process to exhibit mechanical strength and a surface finish compatible with application in a lost-wax casting process.

[0017] First, the step of obtaining the core from ceramic material by an additive manufacturing process ensures that the core can have dimensions perfectly adapted to the cooling circuits for new turbomachine blades. Furthermore, it is possible to obtain, by additive manufacturing, cores whose geometry would not be demoldable.

[0018] In particular, the additive manufacturing process makes it possible to achieve shapes that are impossible to achieve by prior art manufacturing processes, especially those by powder compaction.

[0019] Furthermore, in a process, the reinforcing composition gives the cores sufficient mechanical properties to withstand the wax injection step required in any lost-wax casting process. In particular, the cores are at least as good as those of the prior art in terms of mechanical properties.

[0020] The "mechanical properties" of a core will be defined here and elsewhere in this application as the usual parameters for characterizing its behavior when subjected to mechanical stress.

[0021] The "mechanical properties" of a given core can be quantified in particular by measuring the mechanical resistance to breakage and / or measuring the mechanical resistance to bending of said core.

[0022] Finally, it is also to the inventors' credit that they have determined compositions suitable for use as reinforcing compositions. Indeed, the inventors observed that existing reinforcing compositions, particularly those based on epoxy resin, which were sufficient for prior art ceramic cores, especially those obtained by ceramic molding, did not allow ceramic cores obtained by additive manufacturing to achieve improved mechanical properties.

[0023] Without wishing to be bound by theory, the inventors are of the opinion that the known and developed reinforcing compositions for cores obtained by powder compaction do not allow for good impregnation of cores obtained by additive manufacturing. They are also of the opinion that this is due to differences in the porosity structure of the ceramic cores obtained by the two methods.

[0024] Indeed, the average diameter of the pores is smaller for ceramic cores obtained by additive manufacturing, and the properties of the reinforcing compositions must be adapted, in particular in terms of viscosity, to allow a gain in mechanical properties for these cores.

[0025] In addition, candidate strengthening compositions must be free of compounds whose handling is regulated or discouraged, for example compounds classified as carcinogenic, mutagenic or reprotoxic (also known as "CMR" by acronym) to facilitate the implementation of the new processes.

[0026] Reinforcement compositions based on an aqueous solution of polyvinyl alcohol precisely satisfy these points.

[0027] Beyond the nature of the reinforcing composition, the different stages of the process result from a succession of optimizations developed by the inventors.

[0028] In particular, the draining step was chosen to ensure that after the step of impregnating the reinforcing composition at reduced pressure, the latter does not form defects negatively impacting the surface condition of the ceramic core.

[0029] Indeed, when the draining step is omitted and the drying step is carried out directly, the reinforcing composition can solidify directly on the surface of the impregnated core, which can cause the appearance of surface defects such as roughness, bubbles, or unwanted deposits. The presence of such defects may lead to the scrapping of the core because its dimensions are crucial for obtaining an optimum cooling circuit in the final part.

[0030] Finally, the drying stage of the process is important because it allows the solidification of the entire reinforcement composition on the ceramic material core as well as in the pores of the latter, giving the entire set of mechanical properties, and in particular a resistance to breakage and their mechanical resistance to bending superior to those it would have without the reinforcement composition.

[0031] In one embodiment, the strengthening composition comprises an aqueous solution of polyvinyl alcohol with a degree of hydrolysis between 83% and 87%.

[0032] The "degree of hydrolysis" of a polyvinyl alcohol is understood in the usual and commercial sense of this term for this particular compound. In particular, the degree of hydrolysis characterizes the proportion of alcohol functional groups on the carbon chain of the polyvinyl alcohol.

[0033] As will be demonstrated by the examples provided at the end of this application, the inventors have established that a degree of hydrolysis between 83% and 87% allows the best mechanical properties to be obtained for the ceramic material core after impregnation.

[0034] In one embodiment, the reinforcing composition comprises, for more than 95% by mass, an aqueous solution of polyvinyl alcohol, or is made up of an aqueous solution of polyvinyl alcohol.

[0035] In one embodiment, the strengthening composition comprises an aqueous solution of polyvinyl alcohol at a concentration between 85 gL⁻¹ and 115 gL⁻¹

[0036] As will be demonstrated by the examples provided at the end of this application, it is for such compositions that the best mechanical properties are obtained for the ceramic material core after impregnation.

[0037] In one embodiment, the step of impregnating the reinforcing composition can be carried out at a pressure between 0.9 bar and 0.3 bar.

[0038] Indeed, and as will be illustrated by the examples, it is for these pressures that the best mechanical properties are obtained for the ceramic material core after reinforcement.

[0039] In one embodiment, the draining step is carried out for a duration of between 30 minutes and 2.0 hours.

[0040] The inventors have established that too short a time for the draining step does not allow for the proper removal of all surface defects which would impair the correct dimensions of the core.

[0041] On the contrary, too long a duration of the draining step causes too much evaporation of the reinforcing composition which reduces the effectiveness of the impregnation of the part and therefore the effect on the mechanical properties of the final core.

[0042] Furthermore, from an industrial point of view, it is desirable to have the shortest possible steps to increase the overall yield of the process.

[0043] This is why a duration of 30 minutes to 2.0 hours for the draining step represents here an optimum for obtaining good mechanical properties.

[0044] In one embodiment, the drying step can be carried out in open air or in an oven at a temperature less than or equal to 200°C.

[0045] The inventors have observed, on the one hand, that air drying is sufficient to obtain the desired effect for the reinforcing composition. This results in a simple process, since the drying step then requires no special resources.

[0046] The inventors have also found that oven drying allows better evaporation of the solvents in the strengthening composition, which makes it possible to accelerate the process and to ensure that the active components of the strengthening composition do not risk degrading during the evaporation process.

[0047] In one embodiment, the drying step includes a step of introducing the core into an oven at a temperature between 70°C and 90°C for a duration between 30 minutes and 2.0 hours.

[0048] Indeed, the inventors have determined that such a temperature is an optimum allowing excellent evacuation of the water contained in the reinforcing composition, while controlling the temperature and therefore the costs associated with the drying stage.

[0049] In one embodiment, the drying step is carried out by exposing the core to a single temperature.

[0050] In such an embodiment, the core is not exposed to variations in the temperature imposed during the drying stage, which ensures excellent industrial reproducibility.

[0051] Indeed, a step without temperature variation can, for example, be carried out by introducing the part into an oven at a regulated temperature.

[0052] Such a step is much simpler to carry out than a step that would include a heating ramp and / or a cooling ramp. It therefore follows that the process is simpler to carry out.

[0053] In one embodiment, the ceramic material core comprises alumina, silica, zirconia, mullite, yttrium, silicon carbide or a mixture of two or more of these compounds.

[0054] In one embodiment, the variation in mass of the nucleus during the impregnation step is between 1% and 5% of the mass of the dry nucleus, i.e. the nucleus before its impregnation. Brief description of the drawings

[0055] [Fig-1] Fig. 1 is a schematic representation of a core made of material ceramic obtained by additive manufacturing.

[0056] [Fig.2] [Fig.2] is a schematic representation of a blade made of material metallic obtained by a lost-wax casting process using a ceramic material core obtained in an embodiment of the invention. Description of the implementation methods

[0057] The invention is now described by means of figures, which are provided for descriptive purposes to illustrate certain embodiments of the invention and which should not be interpreted as limiting the latter.

[0058] Fig. 1 represents very schematically a 100 core in ceramic material obtained by additive manufacturing.

[0059] Fig. 1 represents a core foot 111 intended to facilitate the handling of the ceramic material core during the various stages of the foundry process, and a functional portion of the core 110 intended to form the cooling circuit.

[0060] The base of the core 111 allows, for example, precise positioning of the core in a mold for a lost-wax casting process. However, the core will be machined in a final step, and its geometry is therefore not critical for the cooling circuit requirements.

[0061] In one embodiment, the core 100 may comprise alumina, silica, zirconia, mullite, yttrium, silicon carbide or a mixture of two or more of these compounds.

[0062] As described, the ceramic material core 100 is obtained by an additive manufacturing process.

[0063] Methods for creating such a 100 core are known as such and will not be described in further detail here. For example, the additive manufacturing method may be a digital light processing (DLP) method.

[0064] Such a method which includes the polymerization of a liquid photosensitive suspension contained in a vat by means of selective UV radiation.

[0065] In particular, such a method makes it possible to print cores with more complex geometries than those accessible to the ceramic powder compaction methods usually used for the manufacture of cores in ceramic material.

[0066] Once the core 100 is obtained, a reinforcing composition is applied to the surface of the core.

[0067] As indicated, the impregnation takes place at a pressure less than or equal to 1 bar.

[0068] For example, the step of impregnating the reinforcing composition can be carried out at a pressure between 0.9 and 0.3 bar.

[0069] In one embodiment, the impregnation can be carried out in two stages, a first of vacuuming a chamber comprising the ceramic core to be impregnated, and a second of feeding the chamber with the reinforcing composition until the reinforcing composition completely covers the part.

[0070] Such an application of the reinforcing composition should allow excellent impregnation of the ceramic material core and in particular of the open porosity of the latter by the reinforcing composition.

[0071] Without wishing to be bound by theory, the inventors are of the opinion that the improvement of the mechanical properties of the core by the impregnation of the reinforcing composition is at least partly due to the filling of the open porosities of the ceramic material core by the polymeric composition which increases the minimum stress allowing the appearance of a crack.

[0072] In addition, the minimum stress allowing a crack to propagate is also increased, because the ductile nature of the polymer present in the reinforcing composition and which covers the ceramic core allows the two walls of the crack to be bonded together, which hinders its propagation in the material.

[0073] In one embodiment, the amount of impregnated reinforcing composition can be characterized by the mass variation of the impregnated core relative to the core before impregnation.

[0074] In one embodiment, the variation in mass of the nucleus during the impregnation step is between 1% and 5% of the mass of the dry nucleus, i.e. the nucleus before its impregnation.

[0075] Once the core impregnation step with the reinforcing composition has been carried out, a draining step is carried out.

[0076] This draining step can be carried out by placing the core 100 on a vibrating support.

[0077] Alternatively, the core 100 coated with the reinforcing composition can be suspended from a support not shown so that drainage occurs by gravity.

[0078] Once drained, the core 100 is then subjected to a heat treatment for drying.

[0079] In one embodiment, drying can be carried out by placing the core 100 in an oven, the temperature of which is controlled.

[0080] After drying, the core 100 is ready to be used in a lost-wax casting process.

[0081] Such a process is known as such and ultimately allows the obtaining of a blade in metallic material 500 of which an internal cavity 510 has the shape of the core in ceramic material 100, or more precisely of its portion 110, obtained by the obtaining process described above.

[0082] Such a dawn is notably represented in [Fig.2].

[0083] The dotted lines in [Fig.2] indicate that the cavity 510 is within the thickness of the blade 500.

[0084] In one embodiment, the blade is an aeronautical turbomachine blade. Examples

[0085] The invention is now described by means of an example which allows several adjustable parameters of the manufacturing process to be evaluated.

[0086] For a representative comparison, the manufacturing conditions of different cores produced to quantify the influence of a parameter differ from others only by that parameter.

[0087] Thus, if it is desired to quantify the influence of a parameter (for example the degree of hydrolysis of polyvinyl alcohol) all the samples obtained with various degrees of hydrolysis will be subjected to the same pressure as during the impregnation of the reinforcing composition, to the same draining time, to the same drying temperature.

[0088] However, for the sake of brevity, we will describe here once the process and we will describe all the parameters whose influence has been studied.

[0089] All the ceramic material cores in this example were obtained by a digital light processing (DLP) additive manufacturing method.

[0090] By reference, it is measured that a core not undergoing any strengthening treatment has an average flexural strength of about 10.0 MPa.

[0091] Such resistance to bending does not guarantee sufficient strength of the core at the time of the wax injection step of a lost-wax process.

[0092] The preparation of a strengthening solution can be carried out in the following manner.

[0093] A commercial polyvinyl alcohol in solid form is dissolved in distilled water.

[0094] For the purposes of this example, the reinforcing composition is obtained under magnetic stirring at a temperature of 80°C.

[0095] However, this is not specifically necessary and other methods for obtaining compositions with the desired concentrations and degrees of hydrolysis may be suitable.

[0096] In the example, several strengthening compositions are prepared, with a degree of hydrolysis of polyvinyl alcohol of either 85% or 98%, and the concentration of the solution can be 50 gL*, 100 gL and 150 gL*.

[0097] The ceramic core is then impregnated by the impregnation composition.

[0098] For this, the impregnation is carried out in two stages, a first of vacuuming a chamber including the ceramic core to be impregnated, and a second of feeding the chamber with the reinforcing composition until the reinforcing composition completely covers the part.

[0099] Several vacuum and duration pairs for impregnation were tested: - a vacuum of 0.6 bar and a duration of 20 min; - a vacuum of 0.6 bar and a duration of 35 min, but applied in several stages: a first vacuum of 20 min then 3 series of 5 minutes of vacuum with return to atmospheric pressure between each vacuum stage; - a vacuum of 0.3 bar and a duration of 20 min; and - a vacuum of 0.1 bar and a duration of 20 min.

[0100] Once the impregnation has been carried out, the core is left to drain.

[0101] Two draining times were tested: 1 hour and 144 hours.

[0102] Finally, after draining, drying is carried out by introducing the drained kernel into an oven.

[0103] Several temperature profiles for drying were tested: - profile 1: a ramp of 2°C / min from 20°C to 80°C followed by a one-hour plateau at 80°C, a ramp of 2°C / min from 80°C to 120°C followed by a thirty-minute plateau at 120°C and a ramp of -2°C / min from 120°C to 20°C; - Profile 2: a ramp of 2°C / min from 20°C to 80°C followed by a one-hour plateau at 80°C and a ramp of -2°C / min from 80°C to 20°C; and - profile 3: a one-hour plateau at 80°C (insertion into an already hot oven and uncontrolled cooling to room temperature).

[0104] For each sample, the flexural strength is determined for the core.

[0105] Table 1 below groups the results to be compared with a flexural strength of 10.0 MPa for a ceramic material core without reinforcing composition.

[0106] [Tables2] Parameter tested: Measured flexural strength; Degree of hydrolysis: 85% | 98% 19.3 MPa 18.3 MPa Concentration of the reinforcing composition: 50 g / L: 14.5 MPa 100 g / L: 19.3 MPa 150 g / L: 13.7 MPa Value and duration of pressure during impregnation: 0.6 bar 20 min: 18.6 MPa 0.6 bar 35 min: 17.5 MPa 0.3 bar 20 min: 19.3 MPa 0.1 bar 20 min: 17.5 MPa Draining time (1h): 16.8 MPa 144 h: 14.6 MPa Oven temperature: profile 1: 16.8 MPa profile 2: 17.2 MPa profile 3: 18.6 MPa

[0107] Table 2 illustrates on the one hand that the proposed process still makes it possible to increase the flexural strength of a ceramic core.

[0108] In addition, Table 2 illustrates for each parameter that preferential values ​​are indeed linked to a final increase in the flexural strength of the core.

Claims

Demands

1. A method for manufacturing a core (100) for a lost-wax casting process comprising: - a step of obtaining the core from ceramic material by an additive manufacturing process; - a step of impregnating the open porosity of the core with a reinforcing composition, the reinforcing composition comprising an aqueous solution of polyvinyl alcohol with a degree of hydrolysis between 82% and 98.4%, and at a concentration between 35 gL 1 and 165 gL 1; the impregnation step being carried out at a pressure less than or equal to 1 bar, - a draining step; then - a drying step.

2. A manufacturing process according to claim 1, wherein the reinforcing composition comprises an aqueous solution of polyvinyl alcohol with a degree of hydrolysis between 83% and 87%.

3. A manufacturing process according to claim 1 or 2, wherein the reinforcing composition comprises an aqueous solution of polyvinyl alcohol at a concentration between 85 g / L and 115 g / L

4. A manufacturing method according to any one of claims 1 to 3, wherein the draining step is carried out for a period of between 30 minutes and 2.0 hours.

5. A manufacturing method according to any one of claims 1 to 4, wherein the drying step includes a step of introducing the core into an oven at a temperature between 70°C and 90°C for a period of between 30 minutes and 2.0 hours.

6. A manufacturing method according to any one of claims 1 to 5, wherein the core (100) is not exposed to temperature variations during the drying step.

7. A manufacturing method according to any one of claims 1 to 6, wherein the ceramic material core (100) comprises alumina, silica, zirconia, mullite, yttrium, silicon carbide or a mixture of two or more of these compounds.

8. A manufacturing method according to any one of claims 1 to 7, wherein the variation in mass of the core during the step The impregnation is between 1% and 5% of the mass of the dry core.

Images