Method for making an improved core for a lost wax casting process

Abstract

The invention relates to a method for manufacturing a core for a lost-wax casting process comprising: - a step of obtaining the core 100 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 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.

Description

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 turbomachinery 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 turbomachine blades.

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

[0007] In such processes, a wax model of the dawn is made around a core the size of the cavity, or cavities if applicable. 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 necessitates 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 for the geometric resolution required for new cooling circuits.

[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 in 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 recommended to be avoided.

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

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

[0015] To this end, it concerns, according to one of its aspects, a manufacturing process for 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 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.

[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 ceramic core using an additive manufacturing process ensures that the core has dimensions perfectly suited to the cooling circuits of new turbomachine blades. Furthermore, additive manufacturing makes it possible to produce cores with geometries that would otherwise be impossible to demold.

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

[0019] Furthermore, in this 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] We will define here and elsewhere in this application the "mechanical properties" of a nucleus as the usual parameters used to characterize 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 the measurement of the mechanical resistance to bending of said core.

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

[0023] Without wishing to be bound by theory, the inventors believe 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 also believe 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, particularly 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 new processes.

[0026] Reinforcing 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 series 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 performed directly, the reinforcing composition can solidify directly on the surface of the impregnated core, potentially causing surface defects such as roughness, bubbles, or unwanted deposits. The presence of such defects can lead to the core being discarded, as its dimensions are critical for achieving optimal cooling in the final part.

[0030] Finally, the drying stage of the process is important because it allows the entire reinforcing composition to solidify on the ceramic material core and in its pores, giving the entire structure mechanical properties, including resistance to breakage and mechanical resistance to bending, superior to those it would have without the reinforcing 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 the term for this particular compound. Specifically, the degree of hydrolysis characterizes the proportion of alcohol groups attached to 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] In one embodiment, the reinforcing composition according to the invention consists of an aqueous solution of polyvinyl alcohol having a concentration between 85 gL -1< and 115 gL -1<.

[0037] 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.

[0038] In one embodiment, the reinforcing composition according to the invention is an organic solution.

[0039] In one embodiment, the reinforcement composition according to the invention does not comprise refractory materials, in particular selected from zircon, mullite, bauxite, corundum, quartz, talc, mica, andalusite, kyanite and mixtures thereof.

[0040] In one embodiment, the reinforcing composition according to the invention will be removed from the core during the first heat treatment exceeding 100-200°C. It is therefore a temporary and ephemeral addition to the internal structure of the core, allowing precisely to reinforce the mechanical performance of the core during a single critical step: the wax injection.

[0041] 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.

[0042] 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.

[0043] In one embodiment, the draining step is carried out for a duration of between 30 minutes and 2.0 hours.

[0044] The inventors established that too short a time for the draining stage did not allow for the proper removal of all surface defects that would impair the correct dimensions of the core.

[0045] On the contrary, too long a draining step causes excessive 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.

[0046] Furthermore, from an industrial point of view, it is desirable to have the shortest possible steps to increase the overall efficiency of the process.

[0047] Therefore, a duration of 30 minutes to 2.0 hours for the draining stage represents an optimum here for obtaining good mechanical properties.

[0048] In one embodiment, the drying step can be carried out in the open air or in an oven at a temperature less than or equal to 200°C.

[0049] The inventors observed that air drying was sufficient to achieve the desired effect for the reinforcing composition. This resulted in a simple process, as the drying step requires no special resources.

[0050] The inventors also found that oven drying allowed better evaporation of the solvents in the strengthening composition, which accelerates the process and ensures that the active components of the strengthening composition do not risk degrading during evaporation.

[0051] 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 period of between 30 minutes and 2.0 hours.

[0052] Indeed, the inventors 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.

[0053] In one embodiment, the drying step is carried out by exposing the core to a single temperature.

[0054] In this method of embodiment, the core is not exposed to variations in the imposed temperature during the drying stage, which ensures excellent industrial reproducibility.

[0055] Indeed, a step without temperature variation can, for example, be carried out by introducing the part into an oven at a regulated temperature.

[0056] Such a step is much simpler to carry out than one that would include a heating ramp and / or a cooling ramp. This results in a simpler process to implement.

[0057] 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.

[0058] In one embodiment, the change in mass of the core during the impregnation step is between 1% and 5% of the mass of the dry core, i.e. the core before its impregnation. Brief description of the drawings

[0059] [ Fig. 1 ] There figure 1 is a schematic representation of a ceramic core obtained by additive manufacturing. Fig. 2 ] There figure 2 is a schematic representation of a blade made of metallic material obtained by a lost-wax casting process using a ceramic material core obtained in an embodiment of the invention. Description of the implementation methods

[0060] 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.

[0061] There figure 1represents very schematically a 100 core made of ceramic material obtained by additive manufacturing.

[0062] There figure 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.

[0063] 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.

[0064] 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.

[0065] As described, the 100 core made of ceramic material is obtained by an additive manufacturing process.

[0066] Methods for creating such a 100-core material are known and will not be described in further detail here. For example, additive manufacturing can be a digital light processing (DLP) method.

[0067] One such method involves the polymerization of a photosensitive liquid suspension contained in a vat using selective UV radiation.

[0068] In particular, such a method makes it possible to print cores with more complex geometries than those accessible to ceramic powder compaction methods usually used for manufacturing cores in ceramic material.

[0069] Once the core 100 is obtained, a reinforcing composition is applied to the surface of the core.

[0070] As stated, impregnation takes place at a pressure less than or equal to 1 bar.

[0071] For example, the impregnation step of the reinforcing composition can be carried out at a pressure between 0.9 and 0.3 bar.

[0072] In one embodiment, the impregnation can be carried out in two stages, a first stage of vacuuming a chamber including the ceramic core to be impregnated, and a second stage of feeding the chamber with the reinforcing composition until the reinforcing composition completely covers the part.

[0073] 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.

[0074] 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.

[0075] In addition, the minimum stress required for 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, thus hindering its propagation in the material.

[0076] 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.

[0077] 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.

[0078] Once the core impregnation step with the reinforcing composition has been completed, a draining step is carried out.

[0079] This draining step can be carried out by placing the core 100 on a vibrating support.

[0080] Alternatively, the core 100 coated with the reinforcing composition can be suspended from a support not shown so that drainage occurs by gravity.

[0081] Once drained, the 100 kernel is then subjected to a heat treatment for drying.

[0082] In one embodiment, drying can be carried out by placing the core 100 in an oven, the temperature of which is controlled.

[0083] After drying, the 100 core is ready to be used in a lost-wax casting process.

[0084] 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.

[0085] Such a dawn is notably depicted on the figure 2 .

[0086] The dotted lines in figure 2 indicate that cavity 510 is within the thickness of blade 500.

[0087] In one embodiment, the blade is an aeronautical turbomachine blade. Examples

[0088] The invention is now described by means of an example which allows several adjustable parameters of the manufacturing process to be evaluated.

[0089] For a representative comparison, the manufacturing conditions of different cores made to quantify the influence of a parameter differ from others only by that parameter.

[0090] Thus, if we wish 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.

[0091] However, for the sake of brevity, we will describe the process once here and describe all the parameters whose influence has been studied.

[0092] All the ceramic material cores in this example were obtained by a digital light processing (DLP) additive manufacturing method.

[0093] For reference, it is measured that a core not undergoing any strengthening treatment has an average flexural strength of approximately 10.0 MPa.

[0094] Such resistance to bending does not guarantee sufficient strength of the core at the time of the wax injection stage of a lost-wax process.

[0095] The preparation of a strengthening solution can be carried out in the following manner.

[0096] A commercial polyvinyl alcohol in solid form is dissolved in distilled water.

[0097] For the purposes of this example, the strengthening composition is obtained under magnetic stirring at a temperature of 80°C.

[0098] However, this is not specifically necessary and other methods for obtaining compositions with the desired concentrations and degrees of hydrolysis may be suitable.

[0099] 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-1, 100 gL-1 and 150 gL-1.

[0100] The ceramic core is then impregnated with the impregnation composition.

[0101] To achieve this, the impregnation is carried out in two stages, the first of which involves vacuum-sealing a chamber containing the ceramic core to be impregnated, and the second of which involves feeding the chamber with the reinforcing composition until the reinforcing composition completely covers the part.

[0102] Several vacuum and impregnation duration combinations 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.

[0103] Once the impregnation is complete, the kernel is left to drain.

[0104] Two draining times were tested: 1 hour and 144 hours.

[0105] Finally, after draining, drying is carried out by introducing the drained kernel into an oven.

[0106] 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).

[0107] For each sample, the flexural strength is determined for the core.

[0108] 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. [Table 2] Parameter tested Measured flexural strength Degree of hydrolysis 85%: 19.3 MPa 98%: 18.3 MPa Concentration of the strengthening composition 50 g / L: 14.5 MPa 100 g / L: 19.3 MPa 150 g / L: 13.7 MPa Value and duration of the vacuum during impregnation 0.6 bar 20 min: 18.6 MPa 0.6 bar 35min: 17.5 MPa 0.3 bar 20 min: 19.3 MPa 0.1 bar 20 min: 17.5 MPa Draining time 1 hour: 16.8 MPa 144h: 14.6 MPa Temperature of Profile 1: Profile 2: Profile 3: the oven 16.8 MPa 17.2 MPa 18.6 MPa

[0109] Table 2 illustrates on the one hand that the proposed process still makes it possible to increase the flexural strength of a ceramic core.

[0110] Furthermore, Table 2 illustrates for each parameter that preferred values ​​are indeed linked to a final increase in the core's flexural strength.

Claims

1. A process 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 g / L -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 -1 and 115 gL -1 4. A manufacturing process 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 process 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 process according to any one of claims 1 to 7, wherein the variation in mass of the core during the impregnation step is between 1% and 5% of the mass of the dry core.

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