CASTING SLURRY FOR THE PRODUCTION OF SHELL MOLDS

Abstract

A casting slurry for producing shell molds for casting parts includes a metal alloy, the slurry includes powder particles and a binder, the binder includes colloidal yttrium oxide, and the powder particles includes calcia-stabilized zirconia.

Claims

1. A casting slurry for producing shell molds for casting parts comprising a metal alloy, the slurry comprising powder particles and a binder, characterized in that the binder comprises colloidal yttrium oxide, and in that the powder particles comprise calcia-stabilized zirconia, a mass ratio of the calcia-stabilized zirconia in the slurry being comprised between 65% and 75%.

2. The slurry as claimed in claim 1, the slurry being a contact slurry configured to come into contact with the metal of the part to be molded.

3. The slurry as claimed in claim 1, wherein the mass content of calcium oxide in the calcia-stabilized zirconia is comprised between 1% and 20%.

4. The slurry as claimed in claim 1, wherein the viscosity of the slurry is comprised between 0.1 and 2 Pa.s.

5. The casting slurry as claimed in claim 1, configured for the production of shell molds for casting parts comprising a titanium aluminide-based metal alloy.

6. Use of the casting slurry as claimed in claim 1 for the production of a shell mold.

7. A process for producing a shell mold for casting parts, the process comprising the steps of: providing a model of a part to be produced; dipping the model in a contact slurry as claimed in claim 1; sandblasting the dipped model in a contact sand comprising yttrium oxide; drying the layer obtained by the preceding steps; dipping the model in a reinforcement slurry, sandblasting the model dipped in a reinforcement sand, and drying the layer obtained, until a desired shell mold thickness is obtained; removing the part model.

8. A shell mold obtained by the process as claimed in claim 7.

9. The slurry as claimed in claim 1, wherein the mass ratio of the calcia-stabilized zirconia in the slurry is between 68% and 72%.

10. The slurry as claimed in claim 1, wherein the mass ratio of the calcia-stabilized zirconia in the slurry is equal to 70%.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0034] The invention and its advantages will be better understood upon reading the detailed description below of various embodiments of the invention given by way of non-limiting examples. This description refers to the appended pages of figures, wherein:

[0035] FIG. 1 schematically represents the steps of a process for producing a shell mold for casting processes;

[0036] FIG. 2 is a graph showing change in the viscosity of a control slurry, and of the slurry of the present disclosure, as a function of shear stress.

DESCRIPTION OF THE EMBODIMENTS

[0037] The process for producing aeronautical parts, in particular a turbine blade or a turbine blade cluster, is a casting process. The various steps of this process are described for example in the document FR3031921.

[0038] The first step of this process consists in creating a wax cluster model, also called ‘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 wax removal is done by heating the shell mold in an autoclave (or the like) at a temperature greater than the melting temperature of the wax. In a third step, the metal blade cluster is formed in the shell mold by pouring molten metal into the shell mold. In a fourth step, after the metal has cooled and solidified in the shell mold, the cluster is removed from the shell mold. Finally, in a fifth step, each of the blades is separated from the rest of the cluster and finished by finishing processes such as machining.

[0039] The invention relates in particular to the production of the shell mold in which the metal casting will be carried out, and more specifically to the contact slurry used for the production of this mold. The various steps of this process are illustrated in FIG. 1.

[0040] The first step (step S1) comprises providing a model made of wax, or other equivalent material that can be easily discharged later, of the part. In a second step, the wax model is dipped into a first slurry, the contact slurry (step S2), comprising powder particles and a binder. Sandblasting, i.e., deposition of sand particles called contact stucco, is then carried out, followed by a drying of the layer obtained (step S3). This sandblasting step reinforces the layer and facilitates the adhesion of the next layer.

[0041] The layer thus obtained is then dipped in a second slurry, called reinforcement slurry (step S4). A deposition of sand particles, called reinforcement stucco, is then carried out, followed by a drying of the layer obtained (step S5). Steps S4 and S5 are repeated N times, until a determined thickness of shell mold is obtained. Finally, when the desired thickness is reached, a dewaxing step, consisting of removing the wax model from the model, followed by heat treatment, is performed (step S6). After removal of the wax model, a ceramic shell mold whose cavity is a negative reproduction of all the details of the part to be molded is obtained. The heat treatment includes the firing of the mold obtained, the firing temperature preferably being comprised between 1000 and 1200° C.

[0042] The slurries used are composed of particles of ceramic materials, in particular alumina, mullite, zirconia or others, with a mineral colloidal binder and, if need be, adjuvants such as wetting agents or antifoam agents.

[0043] In the context of the production of titanium aluminide (TiAl)-based aeronautical parts, the contact slurry used in step S2 comprises yttrium oxide.

[0044] The contact stucco used in step S3 may also comprise yttrium oxide. The reinforcement slurry and reinforcement stucco used in steps S4 and S5 may comprise mullite, alumina, silico-alumina, silica, zircon, zirconia or yttrium oxide, for example.

[0045] The invention relates more particularly to the contact slurry used in step S2, and in particular to the presence of colloidal yttrium oxide and calcia-stabilized zirconia (CSZ) in the powder particles therein.

[0046] In order to appreciate the influence of the presence of CSZ in a contact slurry, the inventors first studied a control slurry, denoted slurry A, intended to be used as a contact slurry for the production of a shell mold. Slurry A can have the following composition, expressed in percentages by mass: [0047] binder (colloidal yttrium oxide): 24.5%; [0048] powder particles (yttrium oxide powder): 75%; [0049] wetting agent, antifoam agent and other additives: 0.5%.

[0050] This mass distribution is given here by way of example, it being understood that a variation in mass distribution of up to 10% is possible. Slurry A does not contain CSZ.

[0051] Furthermore, the inventors have studied a slurry B which the inventors have determined exhibits similar reactivity with TiAl as slurry A, and whose powder particles comprise calcia-stabilized zirconia (CSZ), with CaO acting as a stabilizing agent. CSZ can be obtained for example by reactive sintering. The CaO content in mass percentage in the powder is comprised between 1% and 20% by weight. The slurry B thus obtained has the following mass percentages: [0052] binder (colloidal yttrium oxide): 29.8%; [0053] powder particles (CSZ): 70%, including 5% CaO; [0054] wetting agent, antifoam agent and other additives: 0.2%.

[0055] Similarly, this mass distribution is given here by way of example, it being understood that a variation of the mass distribution is possible in the ranges previously mentioned.

[0056] Slurry B also includes unavoidable impurities. Among unavoidable impurities, for example, mention may be made of silicon dioxide (SiO.sub.2), titanium dioxide (TiO.sub.2), iron oxide (Fe.sub.2O.sub.3) or alumina (Al.sub.2O.sub.3). Unavoidable impurities are defined as those elements which are not intentionally added to the composition and which are brought in with other elements.

[0057] The curves shown in FIG. 2 illustrate the influence of the composition used for the contact slurry according to the present disclosure on its stability. This figure shows the change in the dynamic viscosity η in Pa.s of the slurry, as a function of a shear applied to this slurry. These measurements are performed using a rotary rheometer with coaxial cylindrical geometries, to apply to the slurry a shear comprised between 0.1 and 100 s.sup.−1. More precisely, the dynamic viscosity η can be calculated in a non-normalized manner from the shear stress T and a shear rate Ý, according to the relationship η=τ/Ý. Curve (a) represents the viscosity of slurry A after 0.5 h, curve (b) represents the viscosity of slurry A after 2 h, curve (c) represents the viscosity of slurry A after 3.5 h, and curve (d) represents the viscosity of slurry B of the invention after 24 h. The above-mentioned times are determined from a time t0 corresponding to the end of the production of the slurry.

[0058] Curves (a) and (b) illustrating the viscosity of slurry A after 0.5 h and after 2 h are substantially coincident. For a low shear, of the order of 0.1 s.sup.−1, the viscosity of slurry A is roughly equal to 4 Pa.s after 2 h. This viscosity then increases very rapidly with time, and reaches a value greater than 25 Pa.s after 3.5 h. In other words, the slurry quickly becomes very viscous, and tends to gel.

[0059] Conversely, curve (d) illustrating the viscosity of slurry B of the invention shows that the viscosity of slurry B remains less than 1 Pa.s after 24 h, regardless of the shear applied thereto. Thus, slurry B has increased stability compared with slurry A, and remains fluid by maintaining a low viscosity even 24 h after preparation of this slurry. Furthermore, the composition of slurry B maintains a low reactivity with TiAl alloys, equivalent or even lower than that of slurry A.

[0060] Although the present invention has been described with reference to specific example embodiments, it is obvious that modifications and changes may be made to these examples without departing from the general scope of the invention as defined by the claims. In particular, individual features of the various illustrated/mentioned embodiments may be combined in additional embodiments. Consequently, the description and drawings should be considered in an illustrative rather than restrictive sense.

[0061] It is also obvious that all the features described with reference to a process are transposable, alone or in combination, to a device, and conversely, all the features described with reference to a device are transposable, alone or in combination, to a process.