EP3487649B1 - Process for manufacturing a shell mold - Google Patents

Description

The present invention relates to the manufacture of a foundry mold in a so-called “lost wax” process, for the manufacture of precision metal parts. This type of mold is also called a shell mold.

The production of “lost wax” molds is known and widely used in particular for the manufacture of precision parts having complex geometries or precision parts in very small or even single series.

To manufacture a lost wax mold, a model of the part to be manufactured is first produced in wax or in a removable material which can be melted or easily removed from the manufactured mold.

The model is successively soaked, sandblasted and / or coated with a reinforcement and dried. The quenching operation is carried out in one or more slip (s). The sandblasting operation, also called stuccoing, consists in reinforcing the deposit formed by the layer of slip deposited on the model during quenching. After each quenching and sanding and / or coating operation, the water is removed from the various layers. Then, the model is eliminated for example during a passage in an autoclave (treatment by pressure and temperature). Finally, the mold undergoes a heat treatment in order to give it the characteristics necessary for the casting of the metal.

For the fabrication of precision metal parts, a mold must be stable when casting molten metal. By stable is meant that the molten metal must not cause the mold material to react in such a way that the latter deforms.

In order for the mold to have a perfect surface condition for the production of a part, it is important that the composition of the first layer in contact with the model, commonly called the contact layer, is chemically compatible and closely matches the profile of the model. . This contact layer is the result of soaking the model in a contact slip. The contact layer must be homogeneous, stable, fluid, dense, non-reactive with the molten metal of the precision part to be manufactured and compatible with the following layers of the mold. Furthermore, the expansion coefficients of the contact layer and of the following layers constituting the mold must be compatible so as to avoid any damage caused by a difference in thermal expansion of the layers.

It is known to use different slips which comprise either alumina, or zircon, or either electro-fused silica. Each of these compounds has at least one particular drawback. For example, alumina is not compatible with certain alloys that make up the precision metal parts to be produced, electro-fused silica lacks refractoriness, and zircon, in addition to being radioactive, loses stability over time. as the temperature of the molten alloy increases.

The document GB 2 017 118 describes a method of making a shell mold.

The document US 5,927,379 discloses a method and composition for forming shells for precision casting as well as methods of casting metal articles using such shells.

The document JP H06 15404 describes a self-collapsing casting mold.

The document EP 0 479 672 describes a shell mold usable in lost wax casting.

The document EP 2 153 919 describes a shell mold and a method of making shell molds.

The object of the invention is in particular to provide a simple, effective and economical solution to these problems.

To this end, the invention proposes a method of manufacturing a shell mold with several layers, including at least one contact layer, from a model of a part to be manufactured in wax or other material. similar, the method comprising a step of dipping the model in a contact slip forming the contact layer and comprising a binder and a powder, the powder comprising a mullite-zirconia composite.

The use of a mullite-zirconia composite powder makes it possible to limit the chemical interactions between the shell mold and the metal alloy introduced by casting into the shell mold. The aforementioned composite is preferably essentially or almost exclusively formed of mullite and zirconia. Obviously, it is understood that it could include impurities in negligible quantity. These impurities can be calcium or sodium, in particular. According to one characteristic of the invention, the binder can be inorganic or organic or even be a mixture of organic and inorganic compounds.

The mullite-zirconia composite powder makes it possible in particular to produce a contact slip having good rheological stability, good chemical inertia with respect to the molten alloy and the manufacture of which is controlled.

It will be recalled that a composite is a material composed of several elementary components, the association of which confers on all the properties that none of the components taken separately possesses.

The mullite-zirconia composite powder can be obtained by chemical synthesis using a mullite precursor such as alumina and / or silica and a zirconia precursor such as zircon. The grains of the powder are then formed from an aggregate of mullite and zirconia.

Preferably, the grains of the mullite-zirconia composite powder have an average size of between 5 and 20 µm and a size distribution ranging from a submicron size up to a size of 100 µm.

According to another characteristic, the contact layer may have a thickness less than or equal to 1 mm. It is desirable to limit the thickness of the contact layer to avoid mechanically weakening the shell mold due to the presence of zirconia.

To obtain a good quality contact slip, the level of zirconia in the powder is between 5% and 90% by weight and, preferably between 10% and 50% by weight, and even more preferably between 30% and 50%.

Advantageously, the binder is colloidal silica.

To promote wetting of the contact layer on the surface of the model, the contact slip also comprises at least one wetting agent and / or at least one anti-foaming agent.

• on sable le modèle,
• on sèche le modèle sablé,
• on trempe le modèle sablé et séché dans une deuxième barbotine qui peut être de préférence dépourvue de zircone afin de lui conférer une meilleure résistance mécanique,
• on enduit le modèle trempé dans la deuxième barbotine d'un matériau de renfort,
• on sèche le modèle enduit du matériau de renfort, et
• on réalise un traitement thermique sur le modèle enduit du matériau de renfort et séché.

To produce a resistant mold allowing the manufacture of a precision part, the process comprises, following the dipping of the model in the contact slip, stages in which:

• we sand the model,
• the sandblasted model is dried,
• the sandblasted and dried model is quenched in a second slip which may preferably be devoid of zirconia in order to give it better mechanical resistance,
• the model soaked in the second slip is coated with a reinforcing material,
• the model coated with the reinforcing material is dried, and
• heat treatment is carried out on the model coated with the reinforcing material and dried.

Advantageously, the steps of soaking in the second slip, coating in the reinforcing material and drying the model coated with the reinforcing material and dried are repeated.

The succession of steps in this process and, if necessary, the repetition of certain steps make it possible to obtain a good quality mold which will withstand the manufacture of a precision part and will offer a good exterior surface condition to the part. precision manufactured.

• on introduit le liant dans récipient,
• on ajoute la poudre composite mullilte-zircone dans le mélangeur,
• on laisse le mélange de liant colloïdal minéral et de poudre composite se stabiliser.

This process, prior to dipping the model in the contact slip, comprises a phase of producing the contact slip comprising the sub-steps where:

• the binder is introduced into a container,
• the mullilt-zirconia composite powder is added to the mixer,
• the mixture of inorganic colloidal binder and composite powder is allowed to stabilize.

Advantageously, the phase of producing the contact slip also comprises a sub-step of adding the anti-foaming agent and / or the wetting agent.

In addition, the invention also relates to the use of a mold according to the method which has just been described, for the manufacture of a cast and solidified turbomachine part.

• la figure 1 est un ordinogramme montrant les étapes de réalisation d'un moule de fonderie à cire perdue selon l'invention, et
• la figure 2 est une vue schématique en coupe d'un moule de fonderie préalablement à une étape de traitement thermique.
• les figures 3 et 4 sont des images obtenues en microscopie électronique à balayage des grains de deux composites mullite-zirone différents pouvant l'un et l'autre être utilisés dans le procédé selon l'invention ;
• la figure 5 est une illustration de différents grains d'une poudre du composite mullite-zircone.

The invention will be better understood and other details, characteristics and advantages of the invention will become apparent on reading the following description given by way of non-limiting example with reference to the appended drawings in which:

• the figure 1 is a flowchart showing the production steps of a lost wax casting mold according to the invention, and
• the figure 2 is a schematic sectional view of a foundry mold prior to a heat treatment step.
• the figures 3 and 4 are images obtained by scanning electron microscopy of the grains of two different mullite-zirone composites which can both be used in the process according to the invention;
• the figure 5 is an illustration of different grains of a powder of the mullite-zirconia composite.

We have shown, on the figure 1 , a flowchart showing the steps in the manufacture of a lost wax mold 1 for the manufacture of precision parts. The name “shell mold” is also used to refer to this type of mold, however, in the remainder of the description, we will use the simplified term mold 1.

The mold 1, schematically shown in section on the figure 2 comprises a plurality of layers 2, 3, 4, 5, superimposed on each other and covering a model 6 made of wax or a similar material, that is to say a material having similar characteristics and easily removable.

The method for making the mold 1 comprises steps 100 to 700 which will now be described.

In a first step 100, the model 6, in wax, of the precision part to be manufactured is produced.

To allow the production of a perfect precision part, the model 6 is made to the exact dimensions of the precision part and includes a high quality exterior surface finish 7. Thus, only some slight roughness may be visible or detectable on the outer surface 7 of the model 6 so that the final precision part will only need a finishing pass (i.e. a machining operation) aimed at in grinding the outer surface of the precision part obtained.

Advantageously, the model 6 will have a surface condition such that a finishing pass will not be necessary and the precision part can be used directly on leaving the mold.

• limiter le risque de cassure de l'aube qui est soumise à une force centrifuge importante en utilisation, ou encore
• limiter les perturbations d'un flux d'air s'écoulant sur la surface extérieure de l'aube.

For example, the precision part to be manufactured will be a turbomachine blade which must have an exterior surface free of rough edges so as to:

• limit the risk of the blade breaking which is subjected to a significant centrifugal force in use, or else
• limit the disturbance of an air flow flowing over the outer surface of the blade.

In a second step 200, the model is dipped in a contact slip in order to form, around the model 6, a contact layer 2 which may have a thickness less than or equal to 1 mm.

The contact layer 2 has an essential role in the use of the mold 1 since it is this which will give the precision part produced its outer surface. It is thus necessary for the contact slip to be dense and resistant at the same time, and for its viscosity and its covering power to be controlled.

Viscosity and density are necessary so that during dipping, the contact slip perfectly matches the wax model 6, and more precisely the outer surface 7 of the wax model 6 without creating, between the contact slip and the surface 7 exterior of the model 6, of air bubbles which would form, on an internal surface 8 of the mold 1, a cavity conducive to the creation of roughness on the exterior surface of the precision part.

On the other hand, the resistance of the contact slip will be necessary, so that the contact layer 2 does not deform during the manufacture of the precision part.

To meet this double criterion of viscosity and resistance, the contact slip is composed of an inorganic and / or organic binder and of a powder, in this case a mullite-zirconia composite.

Preferably, the binder is an inorganic colloidal binder such as colloidal silica in an amount by mass of between 10% and 40% and, preferably, between 20% and 30%.

By way of nonlimiting examples, the inorganic binder can be sodium silicate or else ethyl silicate and the organic binder comprises water.

The powder comprises, by weight, a zirconia content of between 5% and 90% and, preferably, between 10% and 50% and even more preferably between 30% and 50%.

• liant (silice colloïdale) : 29,8% ;
• poudre composite (mullite-zircone) : 70,0% ;
• agent mouillant, anti-moussant et autres additifs : 0,2%.

According to a preferred embodiment, the mass distribution of the elements making up the contact slip is as follows:

• binder (colloidal silica): 29.8%;
• composite powder (mullite-zirconia): 70.0%;
• wetting agent, anti-foaming agent and other additives: 0.2%.

The mass distribution is given here by way of example, it being understood that a variation of the mass distribution of between 0.1% and 10% is possible.

By way of examples, the other additives which can be added can be a bactericidal agent to limit bacteria and increase the stability of the slip, or other organic binders making it possible to guarantee a uniform and resistant deposit of the contact layer 2 on model 6 in wax.

Advantageously, the contact slip also comprises a wetting agent and an anti-foaming agent.

• on introduit et mélange le liant colloïdal minéral et l'agent mouillant dans un récipient, en l'espèce un mélangeur,
• on ajoute ensuite la poudre composite mullite-zircone dans le mélangeur,
• on ajoute l'anti-moussant,
• on maintient le mélangeur en marche pendant une durée comprise entre 1 heure et 48 heures, et de préférence pendant une durée de 24 heures,
• on transfère le mélange obtenu dans un récipient permettant de réaliser le trempage du modèle, par exemple une cuve de trempée, et
• on laisse le mélange se stabiliser pendant une durée comprise entre 24 heures et 48 heures, et de préférence pendant une durée de 24 heures.

The making of the contact slip can be carried out as follows:

• the inorganic colloidal binder and the wetting agent are introduced and mixed in a container, in this case a mixer,
• the mullite-zirconia composite powder is then added to the mixer,
• we add the anti-foaming agent,
• the mixer is kept running for a period of between 1 hour and 48 hours, and preferably for a period of 24 hours,
• the mixture obtained is transferred into a container making it possible to carry out the soaking of the model, for example a soaking tank, and
• the mixture is allowed to stabilize for a period of between 24 hours and 48 hours, and preferably for a period of 24 hours.

Following these steps, the mixture in the quenching tank is then the contact slip.

The composition of the contact slip has many advantages over the slip of the prior art, among which a better shelf life, good chemical stability, reduced manufacturing time, a non-radioactive formulation and an increased quality of the material. mold obtained.

• un temps de fabrication divisé au minimum par deux,
• une densité supérieure d'au moins 16%,
• une viscosité inférieure d'au moins 60% à la fin de la fabrication et d'environ 50% trente jours après la fin de la fabrication, et
• un meilleur recouvrement du modèle 6 en cire, notamment dans ses formes complexes, par exemple des renfoncements ou des rainures.

By way of example, compared to the slip of the prior art, the contact slip according to the invention offers:

• a manufacturing time divided at least by two,
• a higher density of at least 16%,
• a viscosity lower by at least 60% at the end of manufacture and by approximately 50% thirty days after the end of manufacture, and
• better coverage of the model 6 in wax, in particular in its complex shapes, for example recesses or grooves.

In a third step 300, the model 6 soaked in the contact slip, is sandblasted and then dried. The sandblasting is carried out in a not very aggressive with a powder which will not affect the contact layer 2 and in particular the condition of the internal surface 8 of the mold 1.

Sanding makes it possible in particular to reinforce the contact layer 2 and to facilitate the attachment of a second layer of the mold 1.

In a fourth step 400, the model 6 surrounded by the sandblasted and dried contact layer 2 is dipped in a second slip which may be of the same composition as the contact slip or of a different composition.

In a fifth step 500, the model, taken out of the second slip, is sandblasted and then dried.

At the end of step 500, a model 6 is obtained on which the contact layer 2 and a first reinforcing layer 3 are superimposed.

As shown on the flowchart of the figure 1 by the arrow in dotted lines, steps 400 and 500 can be repeated as a function of the thickness to be given to the mold 1.

In the mold example 1 shown on figure 2 , a second reinforcing layer 4 and a third reinforcing layer 5 have been superimposed on the first reinforcing layer.

However, this example of a mold 1 is in no way limiting and a greater or lesser number of reinforcing layers 3 could be provided.

In a sixth step 600, the wax model 6 is melted so that only the mold 1 remains.

Finally, in a seventh (and last) step 700, the mold 1, comprising an adequate number of reinforcing layers (here three reinforcing layers 3, 4, 5) undergoes a heat treatment, in this case baking in an oven. , in order to solidify the mold 1.

However, in general, the removal of the wax model 6 (also called the dewaxing step) is carried out before the heat treatment of the mold 1. It is also possible that the wax model 6 is removed during the step. 700 heat treatment, the temperature for consolidating the mold 1 being sufficient to melt the wax of the model 6, the steps 600 and 700 then being combined in a single step.

When the mold 1 is finished, a material, for example a metal alloy for the manufacture of the blades, can be cast in the mold 1, against the internal surface 8. After cooling, this cast material then forms the part. precision to manufacture.

To remove the precision part from the mold 1, the mold 1 can be removed mechanically (breaking the mold 1), chemically (dissolving the mold 1), or by a mechanical and chemical combination.

Another advantage offered by the choice of a mullite-zirconia composite powder for the contact slip is that the contact layer 2 presents a low (if any) risk of chemical reaction with a wide variety of materials which can be cast to form the contact. precision part.

In addition, the mullite-zirconia composite ensures good ease of implementation of the slip and makes it possible to cover the 6 models in wax with complex geometries and in particular to be housed in the grooves and other inaccessible cavities so that all the details 6 wax models are reproduced on the contact layer 2.

Finally, the mullite-zirconia composite offers the advantage of not being radioactive, and therefore easy to handle without specific equipment.

We now refer to figures 3 and 4 which represent two images obtained by scanning electron microscopy of the grains of two different mullite-zirone composites which can both be used in the process according to the invention. The mullite-zirconia composite can be obtained by fusion synthesis ( figure 3 ) or by synthesis by reactive sintering in the solid state ( figure 4 ) followed in either case by solidification by cooling. The blocks of mullite-zirconia composite obtained then undergo micronization or ultra-fine grinding.

On the image of the figure 3 , one can distinguish several grains 9 of the mullite-zirconia composite powder, the mullite being indicated by the reference 10 and the zirconia by the reference 11. On the image of the figure 4 , we do not distinguish mullite from zirconia within a grain 9 due to greater homogeneity of the distribution of mullite and zirconia within a grain of the mullite-zirconia composite powder.

The figure 5 is a schematic illustration of several grains of a mullite-zirconia composite powder showing the diversity of grain shapes. Preferably, the grains of the mullite-zirconia composite powder have an average size of between 5 and 20 µm and a size distribution ranging from a submicron size up to a size of 100 µm.

Claims (13)

1. A method for manufacturing a shell mould (1) with several layers (2, 3, 4, 5) including at least one contact layer (2), from a model (6) of wax or other similar material of a part to be manufactured, the method comprising a step of dipping the model (6) into a contact slip forming the contact layer (2) and comprising a binder and a powder, characterized in that the powder comprises a mullite-zirconia composite.
2. A process according to claim 1, wherein the zirconia content in the powder is between 5% and 90% by weight.
3. Process according to the previous claim, in which the zirconia content in the powder is between 10% and 50% by weight.
4. A process according to claim 2 or 3, wherein the zirconia content in the powder is between 30% and 50% by weight.
5. A process according to any one of the foregoing claims, wherein the particles of the mullite-zirconia composite powder have an average size between 5 and 20 µm.
6. A process according to any one of the foregoing claims, wherein the contact layer has a thickness less than or equal to 1 mm.
7. A process according to any one of the foregoing claims, wherein the binder is colloidal silica.
8. A process according to any of the above claims, wherein the contact slip also comprises at least one wetting agent and/or at least one antifoaming agent.
9. A process according to any of the foregoing claims, which comprises, following soaking of the model (6) in the contact slip, the steps wherein:

   - the model (6) is sandblasted,

   - the sandblasted model (6) is dried,

   - the sandblasted and dried model (6) is dipped in a second slip, preferably without zirconia,

   - the model (6) dipped in the second slip is coated with a reinforcing material,

   - the model (6) coated with the reinforcing material is dried, and

   - the model (6) coated with the reinforcing material and dried is subjected to heat treatment.
10. A process according to the previous claim, in which the steps of soaking in the second slip, coating in the reinforcing material and drying the model (6) coated with the reinforcing material and dried are repeated.
11. A process according to any of the foregoing claims, which, prior to soaking the model (6) in the contact slip, comprises a phase of making the contact slip comprising the steps of:

    - introducing the mineral colloidal binder into a container, in this case a mixer,

    - adding the mullite-zirconia composite powder to the mixer,

    - allowing the mixture of mineral colloidal binder and powder to stabilize.
12. A process according to the previous claim inasmuch as it depends on claim 7, wherein the phase of making the contact slip also includes a step of adding the antifoaming and/or wetting agent.
13. Use of a mould (1) obtained by the process of any of the above claims for the manufacture of a cast and solidified turbomachine part.

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