EP3894107A1

EP3894107A1 - Improved casting slurry for the production of shell molds

Info

Publication number: EP3894107A1
Application number: EP19868215.5A
Authority: EP
Inventors: Wen Zhang; Julio-Alejandro AGUILAR ORTIZ; Pierre Jean SALLOT; Juhi SHARMA
Original assignee: Safran SA
Current assignee: Safran SA
Priority date: 2018-12-11
Filing date: 2019-12-05
Publication date: 2021-10-20
Also published as: FR3089438A1; FR3089438B1; WO2020120882A1; CN113165053A; US20220048097A1; JP2022512205A

Abstract

The invention relates to a casting slurry for the production of shell molds for molding a part comprising a metal alloy, the slurry comprising powder particles and a binder, wherein the binder comprises colloidal yttria and the powder particles comprise zirconia stabilized by calcium oxide.

Description

Title of the invention: Improved foundry slip for the manufacture of shell molds

Technical area

The present presentation relates to the field of foundry, in particular the foundry processes of the lost-wax type, and more particularly the slip used in such processes, in particular for the manufacture of foundry shell molds.

Prior art

So-called lost-wax or lost-model foundry processes are

known in themselves since Antiquity. Such a method is for example described in the document FR3031921. They are particularly suitable for the production of metal parts with complex shapes. Thus, the lost model foundry is used in particular for the production of

turbomachinery or bladed wheel sectors. In the lost model foundry, the first step is normally the manufacture of a shell mold, which generally includes the production of a model of material with a comparatively low melting temperature, such as for example a wax or resin, around which is then made a shell of refractory material. After destruction of the model, most often by evacuating the material of the model from the interior of the shell mold, which gives its name to these processes, a molten metal is poured into this mold, in order to fill the cavity formed by the model. in the mold after its evacuation. Once the metal cools and solidifies, the mold can be opened or destroyed in order to recover a metal part conforming to the shape of the model.

To make the shell mold, the wax model is generally

soaked in a foundry slip, then coated with sands and dried. These operations can be repeated in order to form several layers and to obtain the desired thickness and mechanical strength for the shell mold.

However, the first layer of slip used, called contact slip, plays a key role in the quality of the molded metal parts. In Indeed, this contact slip makes it possible to form the internal surface of the shell mold, coming directly into contact with the metal of the metal part to be molded.

In the aeronautical field, the manufacture of parts such as turbine blades uses in particular these lost wax casting methods. In particular, intermetallic alloys based on titanium aluminide (TiAI), due to their low density, are frequently used for the manufacture of these blades. This type of alloy has the particularity of reacting easily with the constituents of the shell mold, the contact of this shell mold with the metal of the part can deteriorate the surface state of this part. To limit this effect, it is known to use a contact slip comprising an yttrin powder and a binder comprising colloidal yttrine. This slip nevertheless has the disadvantage of being unstable. Indeed, the contact slip having this composition tends to gel quickly, after a few hours, for example after three or four hours. This drawback limits the industrial application of this type of slip. In addition, this type of slip has a high cost.

Alternatively, some additives could have been used, but none of these additives was satisfactory to the extent that the improvement of one parameter of the slip was offset by the unacceptable degradation of another parameter.

There is therefore a need for a new type of contact slip, having increased stability over time.

Statement of the invention

The present disclosure relates to a foundry slip for the manufacture of shell molds for the molding of a part comprising an alloy

metallic, the slip comprising powder particles and a binder, the binder comprising colloidal yttrin, and the powder particles comprising zirconia stabilized by calcium oxide.

A foundry slip is a slip suitable for being used for the formation of a shell mold in which molten metal will be cast. In particular, in contrast to any suspension, such a slip comprises a binder, that is to say a compound ensuring cohesion between the powder particles and giving the shell mold its mechanical strength in raw and after sintering. The binder can be inorganic. Conventionally, the powder particles can be sand particles (also known as “flour”), in particular refractory particles, generally having a diameter between 1 micrometer and 100 micrometers.

The foundry slip used in this presentation comprises a binder comprising colloidal yttrin, and powder particles comprising zirconia. Surprisingly, it was found by the inventors that the presence of CSZ (from the English “calcia stabilized zirconia”, that is to say zirconia stabilized by calcium oxide) in the particles of powder made it possible to significantly stabilize a slip comprising yttrin, and to maintain sufficient fluidity, that is to say a low viscosity. Conversely, the viscosity of a slip of the state of the art (for example a binder comprising colloidal yttrin and powder particles comprising yttrin) does not have the composition of the present description, that is to say that is, a binder comprising colloidal yttrin and powder particles comprising zirconia stabilized by calcium oxide (CSZ), tends to increase over time, causing gelation of the slip.

In the slip of the present presentation, the use of zirconia stabilized by calcium oxide makes it possible to modify the interaction between the binder and the powder particles to stabilize the slip, while retaining a low reactivity with the metals to be molded, such as for example titanium aluminide alloys (TiAI), or even a lower reactivity than a slip comprising an yttrin powder and a binder comprising colloidal ytrin. The slip thus obtained therefore has an increased service life and can be reused. The baths used can also be larger, without causing loss.

In some embodiments, the slip is a contact slip configured to come into contact with the metal of the part to be molded.

Called slip is the first slip used, which comes directly into contact with the metal of the part at the time of molding, as opposed to the following slip, called reinforcement slip and

covering the previous layers of shell mold in formation. A Contact slip is configured to follow the shape of the part and not alter it. A contact slip is often kept for longer periods of time than a reinforcing slip which is consumed more quickly, hence an increased need for stability for a contact slip. The slip according to the present disclosure is therefore particularly suitable for use as a contact slip, due to its stability over time and its non-reactivity with certain metals such as TiAI.

In certain embodiments, the mass content of calcium oxide in the zirconia stabilized by calcium oxide is between 1 and 30%, preferably between 3 and 20%, more preferably between 5 and 10 %.

In some embodiments, a mass ratio of zirconia

stabilized by calcium oxide in the slip is between 65 and 75%, preferably between 68% and 72%, more preferably equal to 70%.

In certain embodiments, a mass ratio of the binder in the slip is between 20 and 40%, preferably between 25 and 35%, more preferably equal to 29.8%.

In some embodiments, a mass ratio of additives in the slip is less than 10%, preferably between 0.1 and 5%, more preferably still between 0.5 and 2%.

In certain embodiments, the viscosity of the slip is

between 0.1 and 2 Pa.s.

More specifically, the viscosity of the slip is maintained at a value between 0.1 and 2 Pa.s for a duration at least equal to 24 hours. These values make it possible in particular to facilitate the accessibility of the slip to certain narrow areas of the model.

In some embodiments, the foundry slip is

configured for the manufacture of shell molds for the molding of a part comprising a metal alloy based on titanium aluminide.

The slip according to the present disclosure is particularly suitable for use as a contact slip, due to its stability over time. and its non-reactivity with metallic alloys based on titanium aluminide (T'AI).

The present presentation also relates to the use of a foundry slip according to any one of the preceding embodiments for the manufacture of a shell mold.

The present presentation also relates to a method of manufacturing a shell mold for molding a part, the method comprising steps of:

- supply of a model of a part to be manufactured;

- dipping the model in a contact slip according to any one of the preceding embodiments;

- sanding of the model dipped in contact sand containing yttrin;

- drying of the layer obtained by the preceding steps;

- soaking the model in a reinforcing slip, sandblasting the model soaked in reinforcing sand, and drying the layer obtained, until a desired shell mold thickness is obtained;

- removal of the part model.

In certain embodiments, the reinforcing slip comprises a binder and powder particles, the binder being chosen from: ethyl silicate, sodium silicate or colloids among which, in particular, colloidal silica, colloidal alumina, colloidal yttrin or colloidal zirconia.

In some embodiments, the powder particles

include at least one compound from alumina, mullite, zirconia, mullite-zirconia composites.

The present presentation also relates to a shell mold obtained by a process according to any one of the preceding embodiments.

The shell mold obtained by the process according to the present description makes it possible to limit the oxygen-rich reaction layer which forms on the surface of a metal part, for example a blade of an aeronautical engine, molded in this mold. shell. The reaction layer is defined here as the thickness for which the oxygen concentration is greater than at least twice the concentration measured in the base alloy. In particular, for an isothermal contact at 1600 ° C. for a period of 5 min, this reaction flask remains less than 15 μm for the part thus obtained.

Brief description of the drawings

The invention and its advantages will be better understood on reading the

detailed description given below of various embodiments of the invention given by way of nonlimiting examples. This description refers to the pages of attached figures, on which:

[Fig. 1] FIG. 1 schematically represents the steps of a process for manufacturing a shell mold for foundry processes;

[Fig. 2] Figure 2 is a graph illustrating the evolution of the viscosity of a control slip, and of the slip of the present presentation, as a function of a shear stress.

Description of the embodiments

The process for manufacturing aeronautical parts, in particular a turbine blade or a cluster of turbine blades, is a foundry process. The various stages of this process are described for example in the document FR3031921.

A first step in this process consists in manufacturing a wax cluster model called still 'non-permanent cluster'. In a second step, the shell mold is made from the wax cluster. At the end of this operation, the wax constituting the cluster model is removed from the mold. This removal of the wax is carried out by bringing the shell mold in an autoclave oven (or other) to a temperature higher than the melting temperature of the wax. In a third step, the metal cluster of blades is formed in the shell mold by pouring molten metal into it. In a fourth step, after the metal has cooled and solidified in the shell mold, the cluster is released from the shell mold. Finally, in a fifth stage, each of the blades is separated from the rest of the cluster and finished by finishing processes, for example machining processes.

The invention relates in particular to the manufacture of the shell mold in which the casting of the metal will be carried out, and more specifically the contact slip used for the manufacture of this mold. The different steps of this process are illustrated in Figure 1.

The first step (step S1) includes the provision of a wax model, or other equivalent material that can be easily removed later, of the part. In a second step, the wax model is dipped in a first slip, the contact slip (step S2), comprising powder particles and a binder. Sandblasting, in other words a deposit of sand particles called contact stucco, is then carried out, followed by drying of the layer obtained (step S3). This sanding step makes it possible to strengthen the layer and facilitate the attachment of the next layer.

The layer thus obtained is then dipped in a second

slip, called reinforcing slip (step S4). A deposit of sand particles called reinforcing stucco is then carried out, followed by drying of the layer obtained (step S5). Steps S4 and S5 are repeated N times, until a determined shell thickness is obtained. Finally, when the desired thickness is reached, a dewaxing step, consisting of removing the wax model from the model, then heat treatment, is carried out (step S6). After elimination of the wax model, we obtain a ceramic shell mold whose cavity reproduces in negative all the details of the part to be molded. The heat treatment comprises cooking the mold obtained, the cooking temperature preferably being between 1000 and 1200 ° C.

The slip used is composed of particles of materials

ceramics, in particular alumina, mullite, zirconia or the like, with a mineral colloidal binder and additives if necessary, such as wetting agents or anti-foaming agents.

In the context of the manufacture of an aeronautical part based on titanium aluminide (TiAI), the contact slip used in step S2 comprises yttrin. The contact stucco used in step S3 may also include yttrin. The reinforcing slip and the reinforcing stucco used in steps S4 and S5 can comprise mullite, alumina, silico-aluminous, silica, zircon, zirconia or yttrine, for example.

The invention relates more particularly to the contact slip used in step S2, and in particular the presence of colloidal yttrin and of zirconia stabilized by calcium oxide (CSZ) in the powder particles which comprises that -this.

In order to assess the influence of the presence of CSZ in a contact slip, the inventors first studied a control slip, denoted slip A, intended to be used as contact slip for the manufacture of a shell mold. Slip A may have the following composition, expressed in percentages by mass:

- binder (colloidal ytrin): 24.5%;

- powder particles (yttrin powder): 75%;

- wetting agent, anti-foaming agent and other additives: 0.5%.

This mass distribution is given here by way of example, it being understood that a variation of the mass distribution of up to 10% is possible. Slip A does not have a CSZ.

Furthermore, the inventors have studied a slip B which the inventors have determined that it exhibits reactivity with TiAI similar to that of slip A, and whose powder particles comprise zirconia stabilized by calcium oxide (CSZ), CaO acting as a stabilizing agent. The CSZ can for example be obtained by reactive sintering. The level of CaO in percentage by mass present in the powder is between 1 and 20%. Slip B thus obtained has the following mass percentages:

- binder (colloidal yttrine): 29.8%;

- powder particles (CSZ): 70%, of which 5% CaO;

- wetting agent, anti-foaming agent and other additives: 0.2%.

In the same way, this mass distribution is given here by way of

example, it being understood that a variation of the mass distribution is possible in the above-mentioned ranges. Slip B also includes unavoidable impurities. Among the unavoidable impurities, mention may, for example, be made of silicon dioxide (Si02), titanium dioxide (Ti02), iron oxide (Fe203) or alumina (AI203). Elements which are not intentionally added to the composition and which are provided with other elements are defined as inevitable impurities.

The curves illustrated in Figure 2 illustrate the influence of the composition used for the contact slip according to this presentation on the stability thereof. This figure shows the evolution of the dynamic viscosity h in Pa.s of the slip, as a function of a shear applied to this slip. These measurements are carried out using a rotary rheometer having coaxial cylindrical geometries, making it possible to apply to the slip a shear of between 0.1 and 100 s ¹ . More precisely, the dynamic viscosity h can be calculated in a non-normalized way from the shear stress t and a shear rate Ÿ, according to the relation h = t / Ϋ. Curve (a) represents the viscosity of slip A after 0.5 h, curve (b) represents the viscosity of slip A after 2 h, curve (c) represents the viscosity of slip A after 3.5 h, and curve (d) represents the viscosity of the slip B of the invention after 24 h. The durations mentioned above are determined from an instant t0 corresponding to the end of the manufacture of the slip.

Curves (a) and (b) illustrating the viscosity of slip A after 0.5 h and after 2 h are substantially combined. For low shear, of the order of 0.1 s ¹ , the viscosity of slip A is approximately equal to 4 Pa.s after 2 h. This viscosity then increases very rapidly over time, and reaches a value greater than 25 Pa.s after 3.5 h. In other words, the slip quickly becomes very viscous, and tends to gel.

Conversely, curve (d) illustrating the viscosity of slip B of

the invention shows that the viscosity of slip B remains below 1 Pa.s after 24 h, whatever the shear applied to it. Slip B therefore has increased stability compared to slurry A, and remains fluid while retaining a low viscosity even 24 hours after the preparation of this slurry. Furthermore, the composition of slip B makes it possible to maintain a low reactivity with TiAI alloys, equivalent to or even less than that of slip A.

Although the present invention has been described with reference to

specific embodiments, it is obvious that modifications and changes can be made to these examples without departing from the general scope of the invention as defined by the claims. In particular, individual characteristics of the different embodiments

illustrated / mentioned can be combined in additional embodiments. Therefore, the description and the drawings should be considered in an illustrative rather than restrictive sense.

It is also obvious that all the characteristics described in

reference to a process can be transposed, alone or in combination, to a device, and conversely, all the characteristics described with reference to a device can be transposed, alone or in combination, to a process.

Claims

[Claim 1] Foundry slip for the manufacture of shell molds for molding a part comprising a metal alloy, the slip comprising powder particles and a binder, characterized in that the binder comprises colloidal yttrine, and in that that the powder particles comprise zirconia stabilized by calcium oxide, a mass ratio of zirconia stabilized by calcium oxide in the slip being between 65 and 75%, preferably between 68% and 72 %, preferably still equal to 70%.

[Claim 2] The slip according to claim 1, the slip being a

Contact slip configured to come into contact with the metal of the part to be molded.

[Claim 3] Barbotine according to claim 1 or 2, wherein the mass content of calcium oxide in the zirconia stabilized by calcium oxide is between 1 and 20%.

[Claim 4] A slip according to any one of claims 1 to 3, in which the viscosity of the slip is between 0.1 and 2 Pa.s.

[Claim 5] Foundry slip according to any one of

Claims 1 to 4, configured for the production of shell molds for molding a part comprising a metal alloy based on titanium aluminide.

[Claim 6] Use of a foundry slip according to any one of the preceding claims for the manufacture of a shell mold.

[Claim 7] Method for manufacturing a shell mold for molding a part, the method comprising steps of:

- Supply of a model of a part to be manufactured;

- dipping the model in a contact slip according to any one of claims 1 to 5;

- sanding of the model dipped in contact sand containing yttrin; - drying of the layer obtained by the preceding steps;

- removal of the part model.

[Claim 8] Shell mold obtained by a process according to claim 7.