EP3326734B1 - Method for producing a foundry ceramic core - Google Patents

Description

Technical area :

The present invention relates to a method of manufacturing a ceramic foundry core for the production of a complex cavity hollow casting part by lost-wax casting, such as a gas turbine rotor or stator, of a motor. airplane, reactor, a combustion nozzle or the like, said core being an image of the complex cavity of the hollow part to be manufactured.

Prior art:

The technique of manufacturing metal parts by precision casting with lost wax is widely used and particularly used to manufacture precision hollow metal parts, which can have very complex internal and external forms, such as for example rotors and stators of gas turbines, aircraft engines, reactors, combustion nozzles, etc., these parts being used in various fields such as energy, aeronautics, aerospace, etc. These pieces are very technical and their internal and external forms are dictated by aeraulic constraints. They are hollowed out for reasons of weight but especially to house a network of internal channels allowing the circulation of a cooling fluid. These examples are of course not limiting.

This lost wax foundry manufacturing technique is very complex to implement and requires a plurality of intermediate manufacturing steps each complex and tedious to perform, making this manufacturing process particularly long and expensive. Thus, any new part to be manufactured, or any modification or evolution that one wishes to bring to an existing part, requires a very long detrimental in the research and development phase to generate new parts and optimize aerodynamic characteristics, aeraulics, etc. said pieces.

One of the steps of this lost wax foundry fabrication process is to manufacture a ceramic core having a complex geometry and walls or partitions that can be very thin, of the order of a millimeter, since this core must be hollowed out and perforated. to be able to define the exact and precise interior volume of the hollow part to be manufactured. This ceramic core is preferably made of a technical ceramic material or any other compatible material, that is to say which has a high mechanical strength and a high hardness and which withstands very high temperatures given the temperature of melting of metals and metal alloys which are of the order of 1500 ° C. In addition, this technical ceramic material or the like must be able to dissolve chemically to release the complex interior cavity of the hollow part obtained after casting. This ceramic core is intended to be embedded in a wax blank obtained by molding and whose outer geometry defines the external volume of the hollow part to be manufactured. The wax blank is dipped in a ceramic bath to coat it with a hard ceramic shell. The ceramic shell is raised in temperature to the melting temperature of the wax allowing the removal of the wax that flows from the shell leaving inside the shell a negative volume defined between the inner wall of the carapace. the carapace and the outer wall of the inner core. The molten metal is then cast inside the ceramic shell. After cooling, the outer ceramic shell and the inner core are removed by shaking to clear the resulting hollow part. The foundry technique makes it possible to obtain quality finished parts without any subsequent finishing operation.

Conventionally, the ceramic casting cores are manufactured by molding in a multi-drawer mold. The manufacture of the mold is particularly tedious because the prints, which are the negative images of the core to be made, are very complex and make the design of the mold and its manufacture very expensive and very long. As an example, the average manufacturing time of such a mold is about a year and represents an investment of about one million euros.

Various manufacturing processes have been developed to partially overcome these disadvantages and to try to reduce the time and cost of manufacturing the molds and consequently the cost of production of the ceramic casting cores.

One of the techniques consists in providing a contact machining step of a previously molded ceramic core blank, with or without an inner recess. This machining step allows either to machine the recess as such, or to perfect the inner recess already partially produced by molding, or to deburr the blank obtained. Depending on whether the machining step is performed on a raw and flexible ceramic, that is to say before cooking, or on a cooked and hard ceramic, it can be carried out either by removal of material such as by milling or by abrasion, as the examples described in the publications FR 2 878 458 A1 , FR 2 930 188 A1 and FR 2900850 A1 . Another technique consists in providing a non-contact machining step of a previously molded ceramic core blank, this machining step being performed on a fired ceramic, by laser or ultrasound to perfect the dimensional characteristics of said core, such as the examples described in the publications US 5,465,780 A and WO 97/02914 A1 .

These contact or non-contact machining techniques do not, however, make it possible to dispense with the preliminary molding step of a ceramic core blank, imposing the constraints mentioned above. The time required and the investment of the molds are therefore not substantially reduced.

With the advent of additive manufacturing, new techniques have emerged for making ceramic cores on 3D printing machines from digitized 3D core models, such as the examples described in publications DE102008037534 A1 and DE 102005021664 A1 . However, the materials compatible with this new technique pose problems of shrinkage after foundry because they are difficult to dissolve. In fact, they do not correspond to the ceramics currently qualified for the lost-wax foundry of hollow parts in series because their composition has been obtained empirically in traditional processes, and has not yet been reproduced in 3D printing. In addition, the cost of obtaining a ceramic core in 3D printing is about twenty times greater than that obtained by molding. This cost is totally prohibitive and out of the price that the market is willing to accept. Indeed, there is currently no ceramic core obtained by qualified 3D printing on a "serial" part, which proves the inadequacy of this solution.

The publication WO 2015/051916 A1 proposes to machine on a numerically controlled machine the ceramic core and the outer blank of lost wax disposed around said core without however specify the operating mode given the difficulties in machining said core.

Presentation of the invention

The present invention proposes a new manufacturing process for solving the problems mentioned above, to substantially shorten the manufacturing process of the ceramic cores for the lost wax foundry, and correspondingly reduce the investment cost for the purpose. to reduce the cycle and development cost of new gas turbine parts, aircraft engines, engines, combustion nozzles and any complex cavity hollow part, to provide flexibility in the management of industrial projects , to allow an evolution of the geometry of already existing pieces. By way of example only, the development time of the manufacturing method according to the invention can be divided by a coefficient of 10 and its cost by a coefficient of 40 relative to the process of classic molding. This new manufacturing process also allows the production of pre-series and the manufacture of parts on demand.

For this purpose, the invention relates to a manufacturing method of the kind indicated in the preamble, characterized in that said core is manufactured by machining a block of cooked ceramic material by mechanical removal of material, in that the operation machining tool comprises at least a first machining step for producing a first machined surface in said block of material, and a second machining step for producing a second machined surface in said block of material, substantially opposite said first machined surface, and in that, prior to said second machining step, is applied to all or part of said first machined surface a reinforcing layer in a stiffening solution to protect said block of material from breakage the second machining step and it is expected solidification of said reinforcing layer before starting the second machining step.

Thus, this manufacturing method by machining goes against a prejudice that is to say that the machining of a ceramic core by mechanical removal of material is difficult or impossible. For example, publications US 5,565,780 A and WO 2001/89738 A1 clearly indicate the impossibility of such machining by traditional techniques that implement a machining tool in contact with the core, and the size limit machining below which it is impossible to use a cutting tool . Indeed, this core to be hollowed out and perforated significantly, the vacuum may represent more than 30% of said core, and its walls or walls remaining often very thin, of the order of one millimeter, the core is particularly fragile and brittle . Also, the mechanical machining by contact can not be obtained without damaging or breaking part or all of the ceramic core due to vibrations that the cutting tool generates inside the core which cause the rupture of the weakened areas.

If the machining operation comprises several machining steps, then the application of a reinforcing layer is renewed before each new machining step on all or part of a surface of said block of material substantially opposite to said new surface to be machined.

Prior to the application of said reinforcing layer, one can clean and degrease said block of material to promote the attachment of said stiffening solution.

As a stiffening solution, it is advantageous to use a machining glue in the liquid or semi-liquid state having machinable and dissolvable properties. And one can apply said reinforcing layer in one or more applications of stiffening solution.

Depending on the surface to be reinforced on said block of material, one can apply said stiffening solution by means of a brush, or by gravity by pouring said solution on said block of material.

In order to machine said core in said block of material, a numerically controlled multi-axis machining center and diamond cutting tools are advantageously used.

To machine said core in said block of material, a block of material having at least two parallel opposite faces arranged to form two clamping faces on which the jaws of a clamping vice are applied is preferably used. a machining machine.

Prior to the machining operation of said core, it is advantageously machined in said block of material at least one reference surface for disassembling and reassembling said block of material on a machining machine with a precision of less than 0 , 05mm.

Preferably, after the machining operation of said core, it is proceeded to the removal of the reinforcing layer or layers. To carry out this withdrawal, the said core may be dipped in a solvent bath, or the said core may be subjected to a temperature corresponding to the melting temperature of the stiffening solution.

In this case, the said core is suspended from a bracket to allow the removal of the molten stiffening solution by gravity flow.

Brief description of the drawings:

The present invention and its advantages will appear better in the following description of an embodiment given by way of non-limiting example, with reference to the appended drawings, in which the Figures 1 to 4 show schematically and in front view several steps of a method of manufacturing a ceramic ceramic core according to the invention, in which the figure 1 illustrates the mounting of a ceramic block between two clamping jaws of a machining machine for machining a first face of a blank of said core, the figure 2 illustrates the application of a stiffening solution on the first machined face of the blank, the figure 3 illustrates the machining of a second face of the blank of said core, located opposite the first machined and stiffened face, and the figure 4 illustrates the removal of the stiffening solution after the machining of the second face of the blank.

Illustrations of the invention and best way to achieve it:

The method of manufacturing a ceramic core 10 of ceramic material or the like according to the invention is carried out by mechanical machining of said core directly in the mass of a machinable technical ceramic block intended for precision casting, machining effected by removal of material by means of one or more cutting tools on a conventional machining machine. This machine For example, a machining center may be a numerically controlled multi-axis machining center for producing a plurality of simple to very complex shapes. Of course, any other type of mechanical machining machine may be suitable. In the embodiment described hereinafter, a five-axis milling center was used which makes it possible to machine complex shapes, which are very common in ceramic cores. There are of course machining centers specifically equipped for machining ceramics and which can improve productivity, but their cost is not always depreciable.

More particularly and with reference to the figure 1 , the manufacturing method comprises a step of mounting a ceramic block 1 between two jaws 2 of a clamping vise 3 of a machining machine (not shown) in the direction of the arrows F. The ceramic block 1 is a machinable technical ceramic blank, namely a block of fired ceramic, which presents by way of example a hardness equivalent or comparable to that of glass fiber-filled composites. This ceramic block 1 may have a parallelepipedal shape as illustrated, or any other form depending on the general shape of the core 20 to be machined, such as for example a polyhedron, a cylinder. The positioning and indexing of the ceramic block 1 on the machining machine are important to ensure the accuracy of the different machining steps regardless of the number of disassembly and reassembly of said block. Thus, when the ceramic block 1 is parallelepiped, it must have two opposite and parallel clamping faces 4 with a precision for example at most equal to 0.1 mm. The clamping height h of the two jaws 2 on the clamping faces 4 of the ceramic block 1 must be minimal but sufficient to ensure the immobilization of the ceramic block 1, and for example equal to at least 3 mm for a lower block height or equal to 30mm, and beyond this height, equal to at least 10% of the height of said block. The height H of the two jaws 2 must be large and at least equal to 70 mm to facilitate the accessibility of the machining tools to the different faces of the ceramic block 1, and in particular to its underside. The tightening of the ceramic block 1 must be controlled to apply a weak but sufficient clamping force, by example between 1 kN and 5 kN. To this end, a torque wrench will be used to tighten the two jaws 2 according to the arrows F. The values indicated above are given by way of example and have no limiting effect. Likewise, the method of mounting the ceramic block 1 on a machining machine may vary according to the shape of said block. By way of example, if it is cylindrical, a cylindrical chuck will be used and the peripheral base of said block may serve as a reference surface.

The machining of the ceramic block 1 is started by producing a reference surface 5 which will allow disassembly and reassembly of the ceramic block 1 with a precision of at most 0.05 mm. In the example illustrated, it will be possible to choose at least the lower face and one of the clamp faces 4 of the ceramic block 1 as reference surface 5 which has the advantage of remaining accessible and available until the last step of the machining process. A first machining step can then be performed on a first portion of the ceramic block 1 to produce a first machined surface 6 (see FIG. figure 2 ).

With reference to the figure 2 , this first machined surface 6 was made on the left side (in the figure) of the ceramic block 1 by releasing the corresponding angle of the block and in particular creating cavities 7. After this first machining step and before performing the next machining step, we will stiffen the machined surface 6 by applying a stiffening solution to form a reinforcing layer 8 and will wait for the solidification of this reinforcing layer 8 before starting the second machining step . Prior to this application, the ceramic block 1 must preferably be cleaned and degreased to remove dust and machining oil and thus allow the adhesion of the stiffening solution to the surface of the ceramic block 1. For this cleaning phase, an automatic washing device adapted to prevent any degradation of the ceramic can be used. The stiffening solution is then applied at least to the first machined surface 6, taking care to fill the cavities 7. This stiffening solution, which is preferably a machining glue, can be applied by any suitable means in a or several applications. The thickness of the reinforcing layer 8 obtained must be at least equal to 2 mm to obtain the expected stiffening effect. The stiffening solution can be applied when it is in the liquid state by means of a brush or by gravity by pouring it from a determined height not too high, of the order of a few centimeters, from a container containing a quantity sufficient solution. This technique for applying a stiffening solution in the liquid state is the most suitable for filling cavities 7 more than 2 mm deep. Any other method of application may of course be suitable according to the geometry of the machined surface 6 to be stiffened and according to the fluidity of the stiffening solution. The stiffening solution must be able to be cleaned in order to be removed from the ceramic block 1 after machining. If it does not have this faculty, its residues must not make impossible the use nor the functions of the obtained ceramic core. It must also retain its stiffening properties up to a temperature of at least 50 ° C, corresponding to the temperature rise experienced by the ceramic block 1 during machining even with lubrication.

Suitable stiffening solutions are, by way of example, existing machining glues such as the adhesive pastes sold under the names Araldite 2011 and Araldite 2012, the machining glue sold under the name Rigidax by the company Paramelt, or any other stiffening solution in pasty or semi-fluid form, adhesive or not, having the following particular characteristics: it must be machinable and dissoluble without causing the dissolution of the ceramic on which it is applied. The solvents which exist and which make it possible to dissolve these machining glues, adhesive pastes or any other stiffening solution may be, by way of example, a universal paint sold under the name Syntilor Chrono 10, a gelled spray cleaner marketed under the acronym 1310, a foaming cleaner sold under the name Sansil, etc. These examples are of course not limiting.

The figure 3 illustrates the ceramic block 1 remaining after the second machining step of the process which has been carried out on the right side (in the figure) of the block and during which the corresponding angle of the block has been made to create a second surface of the block. machining 9. This second machining surface 9 is substantially located opposite or at the rear of the first machining surface 6. The terms "opposite" and "rear" must not be interpreted in a restrictive sense . For example, the second machined surface may be the back of the first machined surface forming the front of the core, or the inner face of the first machined surface forming the outer face of the core. Thus, during machining, the forces and vibrations induced in the ceramic block 1 by the cutting tool or tools (not shown) are directed towards the first machined surface 6 and may cause breaks in the block However, they will have no detrimental effect on the first machined surface 6 nor on the cavities 7 since they have been protected and filled by the reinforcing layer 8.

At the end of this second machining step and before performing the next machining step of machining a third surface 10 to separate the core 20 from the remaining ceramic block 1, a stiffening solution for forming a second reinforcing layer 11 at the rear of the third surface 10 to be machined. As explained above, the remaining ceramic block 1 must be cleaned and degreased to remove dust and machining oil and thus allow the adhesion of the stiffening solution to the surface of the ceramic block 1. It is applied then the stiffening solution in the angle formed between the first machined surface 6 and the remaining portion of the ceramic block 1 opposite the third surface 10 to be machined. This second reinforcing layer 11 thus allows the maintenance of the core 20 obtained after calibration during a third machining step, namely after separation between the core 20 obtained and the remaining portion of the ceramic block 1 commonly called a heel.

The figure 4 illustrates the last step of the manufacturing method according to the invention which corresponds to the cleaning of the core 20 obtained after the third machining step which made it possible to machine the third surface 10 separating the core 20 from the ceramic block 1. In this example, the heel of the ceramic block 1 is turned by a quarter turn and held vertically by a holding flange 12. Faced with the holding flange 12, there is provided a bracket 13 arranged to support the core 20 by any suitable suspension means such as a link 14 which can pass through the openings of the core 20 to retain it when the stiffening solution has melted. The assembly is placed in a recovery tank 15 resistant at least to a temperature of the order of 200 ° C. The whole is placed in an oven, an oven or the like for at least 3h at least 120 ° C to cause the fusion of the stiffening solution 16 which will flow from the core 20 and the remaining ceramic block 1, by gravity in the bottom of the receiving tray 15. The core 20 will be positioned in such a way that the stiffening solution flows without defiling the areas of the core 20 that were not provided with it. Similarly, we will have the link 14 through the core 20 so as not to damage it. The stiffening solution recovered in the receiving pan 15 can be recycled one or more times according to its degree of impurities. Of course, any other assembly and / or technical means for removing the stiffening solution 16 of the machined ceramic core 20 may be suitable. For example, the core can be immersed in a solvent bath.

The description that has just been given of the manufacturing method according to the invention with reference to the accompanying drawings is based on an example of implementation and realization of a very simplified core and schematized in the extreme. The essence of the invention lies in the fact of regularly applying, or at each stage of the machining process, a stiffening solution on the machined and therefore weakened areas of the ceramic block 1 to prevent breakage of the ceramic.

Other additional precautions may also be recommended. These include machining the different surfaces of the ceramic block 1 from the top to the low, which preserves the rigidity of said block, and use natural diamond cutting tools or super-abrasive type PCD or CBN. The machining operations can be carried out dry or with a soluble cutting oil or other suitable lubricant. The use of a cutting oil reduces the wear of the cutting tool but requires cleaning the ceramic block 1 before each application of the stiffening solution. The cutting conditions must also be adapted to the rigidity of the ceramic block 1 and the core 20 to be machined. If it is not very hollow, of the order of approximately 30% of vacuum, it is possible to maintain high machining conditions, for example greater than 300m / min until the last machining step. If the core 20 is very hollow, for example beyond 30% vacuum, then the machining conditions should be divided by at least 2. It is still possible to complete the machining of the ceramic block 1 by traditional cutting tools with an ultrasonic pin to machine the weakest parts of the core 20, as for example the milling center Tongtai VU-5 .

Possibilities of industrial application:

It is clear from this description that the invention achieves the goals set, namely the manufacture of a ceramic core only by mechanical machining and without going through a molding step, to greatly shorten the time of completion and reduce production costs. The method of the invention thus allows to consider new developments of faster parts.

The present invention is not limited to the embodiment described but extends to any obvious modification and variant according to the present claims. In particular, the figures are only examples and are derived from the tests carried out to date to validate said method. They have no limiting effect on the scope of the invention.

Claims (15)

1. Process for producing a ceramic casting core (20) for the manufacture of a hollow part with a complex cavity by lost-wax casting, such as a rotor or stator for a gas turbine, an aircraft engine, a reactor, a combustion chamber or the like, said core (20) being an image of the complex cavity of the hollow part to be produced, characterized in that one manufactures said core (20) by machining a fired ceramic material block (1), said machining being performed by mechanical material removal by means of a cutting tool, in that the machining operation comprises at least a first machining step to realize a first machined surface (6, 7) in said material block (1), and a second machining step to realize a second machined surface (9) in said material block (1), substantially opposite to said first machined surface (6, 7), and in that, prior to said second machining step, one applies on the whole or on a part of said first machined surface (6, 7) a reinforcement layer (8, 11) made of a stiffening solution to protect said material block (1) from breaking during the second machining step and one waits for the solidification of said reinforcement layer (8, 11) before carrying out the second machining step.
2. Production process according to claim 1, characterized in that the machining operation comprises several machining steps and in that one repeats the application of a reinforcement layer (8, 11) before every new machining step on the whole or a part of a surface of said material block (1) substantially opposite to said new surface to be machined.
3. Production process according to any of claims 1 and 2, characterized in that, prior to the application of said reinforcement layer (8, 11), one cleans and degreases said material block (1) to further the adhesion of said stiffening solution.
4. Production process according to any of claims 1 and 2, characterized in that one uses as a stiffening solution a liquid or semi-liquid machining glue having machinable and dissolvable properties.
5. Production process according to claim 4, characterized in that one applies said reinforcement layer (8, 11) in one or several stiffening solution applications.
6. Production process according to claim 4, characterized in that one applies said stiffening solution with a brush on said material block (1).
7. Production process according to claim 4, characterized in that one applies said stiffening solution by gravity, pouring said solution on said material block (1).
8. Production process according to claim 1, characterized in that, to machine said core (20) in said material block (1), one uses a numerically controlled multi-axis machining center.
9. Production process according to claim 1, characterized in that, to machine said core (20) in said material block (1), one uses diamond cutting tools
10. Production process according to claim 1, characterized in that, to machine said core (20) in said material block (1), one uses a material block (1) comprising at least two parallel opposite sides arranged to form two clamping faces (4) on which the jaws (2) of a clamping vise (3) of a machining equipment are applied.
11. Production process according to claim 1, characterized in that, prior to the machining operation of said core (20), one machines in said material block (1) at least one reference surface (5) that will allow removing and putting back in place said material block (1) on a machining equipment respecting a parallelism deviation lower than 0.05mm.
12. Production process according to any of the previous claims, characterized in that, after the machining operations of said core (20), one removes the reinforcement layer(s) (8, 11).
13. Production process according to claim 12, characterized in that, to remove said reinforcement layer(s) (8, 11), one dips said core (20) in a solvent bath.
14. Production process according to claim 12, characterized in that, to remove said reinforcement layer(s) (8, 11), one subjects said core (20) to a temperature raise up to at least the melting temperature of the stiffening solution.
15. Production process according to claim 14, characterized in that one suspends said core (20) on a bracket to allow draining the molten stiffening solution by gravity flow.

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