METHOD FOR MANUFACTURING A FOUNDRY CORE FOR THE PRODUCTION OF A PART BY LOST-WAX CASTING

Abstract

A method for manufacturing a foundry core (12) for producing a part by lost-wax casting, this method comprising the following steps: a) preparing a mixture of ceramic fillers and a thermoplastic binder, and shaping the core (12), b) debinding and sintering the core (12) in a sand bath (14), the core (12) being completely covered with sand (16) and undergoing a thermal firing cycle, characterized in that: during step a), metallic particles (22) are introduced into the mixture, and during step b), at least one tomographic inspection is performed through the sand (16) to detect the positions of the metallic particles (22) in the core (12). Abstract figure: Figure 1

Description

Title of the invention: MANUFACTURING METHOD OF A FOUNDRY CORE FOR THE PRODUCTION OF A PART BY LOST-WAX CASTING Technical field of the invention

[0001] The present invention relates to a method for manufacturing a foundry core for producing a part by lost-wax casting. Technical background

[0002] The production of a part by lost-wax casting relies on pouring metal into a ceramic mold that has been previously made around a wax model. This manufacturing process is used, for example, to produce parts for an aircraft turbomachine, such as turbine blades.

[0003] Due to their use in extreme temperature conditions, turbine blades are equipped with internal cooling circuits. In order to create these hollow cavities inside the part, the lost-wax casting process uses the fabrication of ceramic cores, perfectly molding these internal cavities of the blade.

[0004] Currently, the conventional process for manufacturing a core for a turbine blade is the injection of ceramic fillers, in powder form, at high or medium pressure (HP-CIM or MP-CIM) and includes three major steps: injection, debinding and sintering.

[0005] Debinding is a phase that consists of removing the thermoplastic binder mixed with the fillers before the core is formed. In the context of debinding in a granular medium (in a sand bath), two thermal debinding phases occur successively: a first debinding phase by capillary action, occurring when the binder changes to a liquid state and flows out of the core by capillary forces, and a second debinding phase consisting of degrading the remaining binder in gaseous form, occurring when the decomposition threshold of the species is reached. The temperature then increases to consolidate and densify the ceramic fillers; this is sintering. This densification is accompanied by a volumetric shrinkage that varies depending on the composition of the mixture.

[0006] In addition to allowing the removal of a major portion of the binder via liquid during debinding, the sand that constitutes the granular medium also serves to maintain the integrity of the nuclei weakened by debinding at the beginning of the sintering phase. Therefore, within the framework of a firing cycle in a granular medium, the conditions Experimental methods imply that the core must remain embedded in the sand from the beginning to the end of the thermal cycle to maintain its integrity, thus linking the debinding and sintering stages without allowing them to be separated. In fact, manipulating the cores outside the sand before the end of sintering is almost impossible; the cores are then held in place only by the pressure forces of the sand.

[0007] In the context of the continuous improvement of the lost-wax casting process and with a view to the next generations of turbine blades, with more complex core geometries allowing for more efficient cooling, the debinding-sintering cooking step is of particular interest.

[0008] Currently, the various thermal cycles used depending on the geometries are not optimized, mainly because the granular firing stage constitutes a real black box in terms of understanding the physical phenomena. Thus, in addition to being a very time-consuming step that can last several days, the thermomechanical behavior of the core is not controlled, and the post-firing state is more a matter of chance than control.

[0009] One of the main challenges is therefore to control and understand the phenomena involved between the core and its environment during cooking in order to predict its thermomechanical behavior.

[0010] To achieve this, measuring deformations and using process simulation are necessary. This allows both the modeling of the various physical phenomena and the determination of the influence of the different material / process parameters involved. Simulation thus makes it possible to understand the phenomena of thermal origin as well as the phenomena of mechanical origin potentially responsible for the rejection of the core after firing.

[0011] Thus, one of the main challenges is understanding the thermomechanical phenomena involved during the debinding-sintering firing cycle in order to adapt this cycle to different future core geometries and optimize the cycle time. However, modifying the thermal cycle can only be validated if there is no regression in the post-firing core quality; that is, it must be possible to ensure that a modification of the cycle will not degrade the material integrity or negatively impact the dimensions. Beyond the thermal aspects, which can be monitored relatively easily using thermocouples and instrumented sand baths, it is therefore necessary to be able to accurately monitor the mechanical properties and deformations of the core during firing, i.e., during the debinding and sintering phases.

[0012] In the scientific literature, closed granular cooking is rarely discussed. Furthermore, the debinding and sintering processes are often treated separately in cases in the study of thermomechanical behavior, so that the influence of debinding on deformations is often neglected in favor of that of sintering.

[0013] However, no method is applicable to the debinding-sintering stage of the foundry core. Indeed, the fact that the cores are immersed in sand for the entire duration of the firing makes it impossible to observe their state during debinding or sintering. For example, the use of a TMA-type analyzer is not possible because it is impossible to faithfully reproduce the experimental conditions with a probe in sand.

[0014] The present invention offers a simple, effective and economical solution to this problem. Summary of the invention

[0015] The invention relates to a method for manufacturing a foundry core for producing a part by lost-wax casting, this method comprising the following steps:

[0016] a) preparation of a mixture of ceramic fillers and a thermoplastic binder, and shaping of the core,

[0017] b) debinding and sintering of the core in a sand bath, the core being entirely covered with sand and undergoing a thermal firing cycle,

[0018] characterized in that:

[0019] in step a), metallic particles are introduced into the mixture, and

[0020] during step b), at least one tomographic inspection is carried out through the sand, even the tray, to detect the positions of metallic particles in the nucleus.

[0021] The invention thus proposes to monitor the core during its debinding and sintering. This monitoring is performed with high sensitivity using tomography. This monitoring can, in particular, allow for the experimental validation of numerical models / simulations of core deformations during its debinding and sintering, as will be described in more detail below.

[0022] The invention makes it possible to measure with unprecedented sensitivity and at unprecedented manufacturing stages the geometric deformation of a ceramic foundry core.

[0023] The method according to the invention may comprise one or more of the following features or steps, taken individually or in combination with each other:

[0024] - the metallic particles are balls;

[0025] - the metallic particles have an average size or a diameter smaller than or equal to 2mm, preferably less than or equal to 1mm, and for example on the order of 50qm;

[0026] - the metallic particles are made of nickel-based alloy;

[0027] - a first check by tomography is carried out through the sand, or even the tank, before the beginning of said cycle, then a second check by tomography is carried out through the sand, or even the tank, during said cycle;

[0028] - a third check by tomography is carried out through the sand, or even the tank, after said cycle;

[0029] - at least one tomographic image is taken during the or each check;

[0030] - the tomographic images taken during the checks are digitally analyzed in order to to determine the displacement vectors of the metallic particles.

[0031] - the shaping of the core in step a) is carried out using a press.

[0032] The present invention also relates to a foundry core for the production of a part by lost-wax casting, this core being obtained by a process as described above and comprising sintered ceramic fillers and metallic particles.

[0033] The present invention further relates to a lost-wax casting process for producing an aircraft turbomachine part, such as a blade, using a casting core as described above. Brief description of the figures

[0034] Other features and advantages will become apparent from the following description of a non-limiting embodiment of the invention with reference to the accompanying drawings in which:

[0035] [Fig-1] [Fig.1] schematically represents an installation for the implementation of a method according to the invention,

[0036] [Fig.2] [Fig.2] is an example of a tomographic image obtained by tomography of a core shaped with metal beads inside a sand bath,

[0037] [Fig. 3a-3b] Figures 3a and 3b are tomographic images taken successively during the same cooking cycle, and

[0038] [Fig.4] [Fig.4] represents the result of a numerical analysis of the two images in Figures 3a and 3b and shows displacement vectors of metallic particles. Detailed description of the invention

[0039] Fig. 1 schematically represents an installation 10 for implementing a process according to the invention for manufacturing one or more foundry core(s) 12 for producing a part by lost-wax casting.

[0040] During the lost-wax casting process, the dimensions of the core 12 are one of the predominant, if not the most important, characteristics and must be precisely controlled during the various stages of the process. Therefore, each core undergoes a measurement step using non-destructive testing at the end of each stage of the casting process to determine if the dimensions still correspond. as expected. This is the case at the end of the debinding-sintering process, which, in addition to the expected shrinkage induced by sintering, is a step that can introduce deformations as well as defects such as cracks or blisters, more or less frequently depending on the core geometry, the thermal cycle used, and the material parameters. In fact, this step is currently responsible for the rejection of a very significant proportion of the manufactured cores.

[0041] Process simulation is the ideal tool for understanding the phenomena at play during firing and for determining the reasons for the appearance of defects and deformations. It allows, using one or more numerical models, for multiple iterations by varying the material / process parameters in order to evaluate their influence on the final dimensions. This makes it possible to modify the process according to expectations without having to carry out each experimental test, except for those necessary to validate the predictability of the chosen models.

[0042] In order to control the variability of these steps and with the aim of verifying the results of the modeling and simulations of the manufacturing steps of the ceramic cores, it is therefore necessary to have a control method which allows the geometric evolution of the cores to be measured at each of the manufacturing steps, with not just a general deformation, but with precise knowledge of the deformation of all the zones.

[0043] The process according to the invention essentially comprises two steps, a first step of preparation and shaping of the core 12, and a second step of debinding and sintering of the core.

[0044] The first step consists of preparing a mixture of ceramic fillers and a thermoplastic binder, and shaping the core 12. This first step is well known to those skilled in the art and will not be described in detail here.

[0045] Ceramic fillers are for example made of silica.

[0046] The thermoplastic binder is for example based on polymer wax.

[0047] The shaping of the core can be carried out using a press.

[0048] The second debinding and sintering step is also well known to man. profession. It is described for example in document FR-A1-3 033 509.

[0049] Figure 1 shows a sand container, i.e., a container 14 filled with sand 16 which forms a granular material. The core(s) 12 is / are intended to be immersed in the sand 16. Figure 1 shows the case where several cores 12 are immersed in the sand 12.

[0050] The core debinding and sintering step is carried out by subjecting the core to a thermal baking cycle. This cycle is performed through the sand 16, and also through the tank, meaning that the sand tank 14 with the cores 12 is subjected to this cycle.

[0051] Taking experimental measurements directly on the cores 12 is not possible throughout the entire cycle without risking skewing the results. The invention allows for monitoring the deformation of the cores during the firing cycle without needing to remove them from the sand bath and therefore without risking unintentionally inducing deformations outside the process.

[0052] According to the invention, at least one tomographic check is carried out through the sand 14 to detect any displacements or deformations of the core during the firing cycle.

[0053] For this purpose, metallic particles are introduced into the aforementioned mixture and will be monitored by tomography to analyze their possible movements.

[0054] Tomography is an imaging technique that allows the volume of an object to be reconstructed from a series of images taken from outside that object. Tomography is performed using a tomograph 18.

[0055] Tomography control is carried out through the sand 16 to detect the positions of the metallic particles in the core 12 and thus to be able to determine if parts of the core 23 undergo displacements or deformations during the firing cycle.

[0056] The metallic particles are preferably balls or ball-shaped (spherical).

[0057] The metallic particles may have an average size or diameter less than or equal to 2 mm, preferably less than or equal to 1 mm, and for example on the order of 50 µm. Determining the particle size is an important factor because they must be small enough to achieve a homogeneous mixture with the fillers and binder during the preparation of the core 12, and large enough to be detected by the tomograph 18.

[0058] The metallic particles are for example in nickel-based alloy, which prevents the mixture from being polluted during the casting of the alloy.

[0059] A first tomographic inspection can be carried out through the sand before the start of the firing cycle, then a second tomographic inspection is carried out through the sand during the cycle. A third tomographic inspection can also be carried out through the sand after the cycle.

[0060] At least one tomographic image is preferably taken during the inspection(s). The image(s) may be in three dimensions (3D).

[0061] Figure 2 shows an example of a photograph 20. In this photograph 20, the core 12 is seen in cross-section. This core has several parts that appear gray in the middle of the sand 16, which appears black. The magnetic particles 22 appear very bright and clear, therefore rather white. They can be easily seen, in particular, on the edges of the core 12.

[0062] The advantage of the invention lies in the fact that normally, once the firing cycle has begun, access to the cores 12 is only possible once the thermal cycle is complete, thus limiting the understanding of the thermomechanical phenomena to a state before and after firing. Here, the sand tray 14 can be removed from the oven and the cores 12 can be scanned by the tomograph 18 without having to be removed from the tray 14. For example, the same sand tray 14 described above is removed at the end of the thermal cycle and scanned directly by the tomograph 18.

[0063] These tomographic images taken during the checks are advantageously analyzed digitally in order to determine the displacement vectors of the metallic particles.

[0064] The photographs 20 can thus be compared to the photographs 20 taken before firing. This would make it possible to observe the deformations due to the displacement of the metallic particles 22 at any point during the thermal cycle, such as between the debinding and sintering phases. This would allow for differentiation between the respective influences of the debinding phase and the sintering phase, influences which are currently inseparable.

[0065] Figures 3a and 3b show a core 12 with photographs 20 taken at two different times during the firing cycle. Figure 4 represents the results of a numerical analysis of the photographs 20 in Figures 3a and 3b, which are in the form of displacement vectors VI, V2, Vn of the metallic particles 22. These displacement vectors VI, V2, Vn can then serve as validation data for modeling / simulating the thermomechanical behavior of the core 12 because the deformation of the core 12 as a function of temperature is known at any given time.

Claims

Demands

1. A method for manufacturing a foundry core (12) for producing a part by lost-wax casting, this method comprising the following steps: a) preparation of a mixture of ceramic fillers and a thermoplastic binder, and shaping of the core (12), b) debinding and sintering of the core (12) in a sand bath (14), the core (12) being completely covered with sand (16) and undergoing a thermal firing cycle, characterized in that: during step a), metallic particles (22) are introduced into the mixture, and during step b), at least one tomographic inspection is carried out through the sand (16) to detect the positions of the metallic particles (22) in the core (12).

2. A method according to claim 1, wherein the metallic particles (22) are balls.

3. A method according to claim 1 or 2, wherein the metal particles (22) have an average size or diameter less than or equal to 2mm, preferably less than or equal to 1mm, and for example of the order of 50qm.

4. A method according to any one of the preceding claims, wherein the metal particles (22) are nickel-based alloy.

5. A method according to any one of the preceding claims, wherein a first tomographic inspection is carried out through the sand (16), before the start of said cycle, and then a second tomographic inspection is carried out through the sand (16), during said cycle.

6. Method according to the preceding claim, wherein a third tomographic check is carried out through the sand (16), after said cycle.

7. A method according to any one of the preceding claims, wherein at least one tomographic image (20) is taken during the or each inspection.

8. Method according to the preceding claim, wherein the tomographic images (20) taken during the checks are analyzed digitally in order to determine vectors (VI, V2, Vn) of displacement of the metallic particles (22).

9. A method according to any one of the preceding claims, wherein the shaping of the core (12) in step a) is carried out using a press.

10. Foundry core (12) for the production of a part by lost-wax casting, this core (12) being obtained by a process according to one of the preceding claims and comprising sintered ceramic fillers and metallic particles (22).

11. Lost-wax casting method for producing an aircraft turbomachine part, such as a blade, using a casting core according to the preceding claim.

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