Method for manufacturing a mold comprising cracking particles

Abstract

Method for manufacturing a mold comprising cracking particles. The invention relates to a method for manufacturing a mold for molten metal comprising at least the following steps: the formation of a mold precursor comprising at least the following substeps: dipping a wax model of the mold in a contact slip; a stucco step with a contact stucco; a drying step; thickening the formed mold precursor, comprising one or more thickening cycles, each thickening cycle comprising at least the following substeps: a dipping step in a reinforcing slip; a stucco step with a reinforcing stucco; a drying step; a finalization step of the thickened mold comprising at least the following substeps: a dewaxing step;and a heat treatment step, the process being characterized in that, during at least one thickening cycle, the reinforcing stucco comprises crack-forming particles. Figure for the abstract: None;

Description

Title of the invention: Method for manufacturing a mold comprising cracking particles technical field

[0001] The present invention relates to the field of molten metal casting processes, and more specifically to the field of molds usable for casting molten metals. Previous technique

[0002] For the manufacture of metal parts, it is common practice to pour molten metal into a mold. After cooling, the metal takes the shape of the mold in which it has been placed, thus producing an ingot or part of the desired dimensions. During such a process, the metal is introduced into the mold in liquid form, and therefore at a high temperature. During cooling, and particularly due to the different coefficients of thermal expansion between the mold and the metal, the mold exerts pressure on the metal, which can cause the appearance of crystalline defects, notably cracks or recrystallized grains that remain in the part after cooling. Such defects can impair the mechanical properties of the parts thus produced, which may be unacceptable in certain technological fields, for example, aerospace.

[0003] It has been proposed to use less robust molds to prevent them from excessively stressing the cast metals. However, it has been observed that such molds are less robust and present a risk of breakage during handling in the raw state before firing. Furthermore, there has been a risk of leakage of molten metal, particularly during pouring or at the beginning of solidification.

[0004] Thus, there remains a need for a mold for molten metal which retains sufficient mechanical properties at high temperature, while becoming brittle during cooling in order to avoid the formation of structural and / or crystalline defects in the final part during manufacturing. Description of the invention

[0005] The invention relates to a method for preparing a mold without the disadvantages of prior art molds.

[0006] To this end, a method for manufacturing a mold for molten metal is proposed, comprising at least the following steps: - the formation of a mold precursor comprising at least the following sub-steps: - dipping a wax model of the mold into a slip of contact ; - a stucco stage using a contact stucco; - a drying stage; - a thickening of the formed mold precursor, comprising one or more thickening cycles, each thickening cycle comprising at least the following sub-steps: - a soaking step in a reinforcing slip; - a stucco application stage using a reinforcing stucco, - a drying stage; - a finalization step for the thickened mold comprising at least the following sub-steps: - a dewaxing step; and - a heat treatment step,

[0007] the process being characterized in that, during at least one thickening cycle, the reinforcing stucco comprises cracking particles.

[0008] A mold obtained by the process described above has mechanical characteristics comparable to those of a prior art mold before the introduction of molten metal.

[0009] However, the cracking particles present in the mold will only cause cracks to appear in the mold during cooling, after the poured metal has solidified. The thermal behavior of the cracking particles is sufficient to induce and propagate cracks through the thickness of the mold during the thermal gradient inherent in cooling.

[0010] This cracking of the mold allows the stresses that the ceramic mold applies to the solidified metal during cooling to be relaxed.

[0011] The relaxation of the stresses that the mold applies to the metal makes it possible to considerably reduce the number of crystalline defects in the metal part.

[0012] Nevertheless, the cracking caused within the mold's thickness by the cracking particles remains highly localized and does not penetrate through. This reduces the mold's mechanical properties, thereby lowering the stresses applied to the metal, without generating an additional risk of excessive embrittlement.

[0013] Furthermore, in a mold obtained by this process, cracks are initiated and propagated only during cooling, after the metal has solidified. Thus, the mechanical properties of the mold before the pouring of molten metal are similar to those of prior art molds, and the molds thus obtained do not require any particular adaptation of the handling steps before the pouring of metal.

[0014] Cracking particles within the meaning of the invention are understood as particles that initiate and allow the propagation of cracks in a mold of the invention particularly when the temperature varies.

[0015] For example, such cracking particles may be single-crystal grains of refractory ceramic materials with anisotropic coefficients of thermal expansion, for example single-crystal grains of electrofused aluminum titanate or andalusite.

[0016] Indeed, the thermal expansion coefficients of single-crystal grains of electrofused aluminum titanate, for example, are strongly anisotropic. In particular, the thermal expansion coefficients are positive in two spatial directions and negative in the third.

[0017] Thus, the temperature change of the mold during cooling causes the electrofused aluminum titanate single-crystal grains to expand in two of the three spatial directions and contract in the other. When the grains are dispersed in the reinforcing stucco of certain layers of the mold, their anisotropic behavior causes partial detachment of the grains from the rest of the reinforcing stucco on the side where the grains contract, and the propagation of cracks in the directions where the grains expand.

[0018] This results in the appearance and propagation of cracks in the mold which reduces its mechanical resistance.

[0019] Thus, the stresses applied by the mold to the solid metal are reduced, and the crystalline defects associated with these stresses will consequently be much less present than in a prior art mold.

[0020] In one embodiment, the stucco step can be carried out by sprinkling or by fluidized bed.

[0021] In one embodiment, the contact stucco does not include cracking particles.

[0022] It has been observed that the introduction of cracking particles into the stucco of the reinforcing layers, those which give the mechanical properties to the mold, makes it possible to further reduce the stresses applied to the metal during cooling.

[0023] In one embodiment, the process comprises between 5 and 10 thickening cycles.

[0024] It has indeed been observed that 5 to 10 thickening cycles is an optimum between the duration of the mold preparation process and the final mechanical properties of the mold obtained.

[0025] Preferably, the reinforcing stucco used for the first thickening cycle does not include cracking particles.

[0026] Preferably, the reinforcing stucco used for the last thickening cycle does not include cracking particles.

[0027] One or both of the above embodiments make it possible to ensure a improved mechanical strength of the mold, particularly during the pouring of molten metal to prevent metal leakage during pouring, thus ensuring its integrity during casting.

[0028] In one embodiment, the reinforcing stucco of 3 to 5 consecutive thickening cycles may include cracking particles.

[0029] The use of cracking particles in three successive thickening cycles ensures that the mechanical properties of the mold are sufficiently degraded to allow good relaxation of the stresses applied to the solid metal during cooling.

[0030] In one embodiment, the reinforcing stucco layers which include cracking particles may comprise between 10% and 50% by mass of cracking particles.

[0031] Such a charge of cracking particles makes it possible to ensure that the mold allows the relaxation of stresses when the solidified metal has cooled, while ensuring, during the pouring of molten metal, mechanical characteristics close to those of the prior art molds.

[0032] In one embodiment, the contact stucco is identical to the reinforcing stucco in composition, possibly with a smaller particle size than the reinforcing stucco.

[0033] In one embodiment, the reinforcing stucco is identical for all thickening cycles and only the content of cracking particles varies between the thickening steps. Description of the implementation methods

[0034] As indicated, the invention relates to a method for manufacturing a mold for molten metal.

[0035] The mold manufacturing process includes a mold precursor formation step comprising at least the following substeps:

[0036] - dipping a wax model of the mold into a contact slip;

[0037] - a stucco stage using a contact stucco;

[0038] - a drying step.

[0039] The formation of a wax model of the mold provides an element around which the mold will be formed.

[0040] In particular, the wax model can have the desired shape for the final part.

[0041] Indeed, the mold will be constructed through successive thickening stages around the wax model.

[0042] In one embodiment, the wax model has the shape that the cavity of the final mold will have, as in a classic lost-wax manufacturing process.

[0043] In one embodiment, the mold can be a mold for a turbomachine part, for example a turbomachine blade.

[0044] Indeed, the crystalline state of the turbomachine parts is particularly important for obtaining the desired characteristics.

[0045] It is therefore particularly beneficial for these parts not to be constrained during the cooling of the metal, in order to avoid crystalline defects.

[0046] The technical advantages of the molds obtained by the process of the invention are therefore particularly useful in this embodiment.

[0047] The mold precursor formation step includes the formation of the first contact layer around the wax model.

[0048] This contact layer will be in direct contact with the molten metal.

[0049] In one embodiment, the contact slip can be composed of a ceramic powder, for example a powder of alumina, zircon, zirconia, mullite, electrofused silica, aluminosilicate, colloidal silica or a mixture of several of the above species.

[0050] In one embodiment, the contact stucco is selected from tabular alumina, electrofused alumina, electrofused mullite, a mullite-zirconia composite, a silico-aluminous composite, zirconia, zircon, yttrina, electrofused silica or a mixture of these compounds.

[0051] In one embodiment, and as described above, it is preferred not to introduce cracking particles into the contact stucco.

[0052] Indeed, contact between the cracking particles and the molten metal is not desired, because a chemical reaction could take place between these elements.

[0053] Once the mold precursor has been obtained, the process then includes thickening the mold.

[0054] The thickening of the formed mold precursor comprises one or more thickening cycles, each comprising at least the following sub-steps:

[0055] - a soaking step in a reinforcing slurry;

[0056] - a stucco stage using a reinforcing stucco,

[0057] - a drying step.

[0058] In one embodiment, the reinforcing slurry may be composed of alumina, zircon, zirconia, mullite, electrofused silica, aluminosilicate, colloidal silica or a mixture of several of the above species.

[0059] In one embodiment, the reinforcing stucco is chosen from tabular alumina, electrofused alumina, electrofused mullite, a mullite-zirconia composite, a silico-aluminous composite, zircon, electrofused silica or a mixture of these compounds.

[0060] In one embodiment, and as described above, it is possible to introduce cracking particles into the reinforcing stucco.

[0061] For example, the reinforcing stucco may comprise between 10% and 50% by mass of cracking particles.

[0062] The cracking particles can be single-crystal grains of electrofused aluminium titanate, of chemical composition Al2TiO5, as well as andalusite, of chemical composition Al2SiO5.

[0063] For example, single-crystal grains of electrofused aluminum titanate have a parallelepiped shape. These grains have an orthorhombic structure and exhibit in the three spatial directions coefficients of thermal expansion of approximately -3.106K'; 11.8.106K'; and 21.8.106K'.

[0064] These significant differences in thermal expansion coefficients along the spatial direction give the powder the desired cracking property. Furthermore, it is important that the grains be single-crystal, because polycrystalline grains would have thermal expansion coefficients in the three spatial directions that are averaged and would therefore not allow for the reproduction of the desired crack initiation and propagation effect.

[0065] During thickening, the mold forms in successive layers around the mold precursor formed in the previous step.

[0066] As described, it is not necessary for the reinforcing stucco to include cracking particles at each cycle.

[0067] In one embodiment, all intermediate layers, i.e. all layers except those formed during the first and last thickening cycle, comprise cracking particles.

[0068] For example, the process may include 5 thickening cycles, the reinforcing stucco of the second, third and fourth cycles comprising cracking particles.

[0069] As described, the location of the cracking particles makes it possible on the one hand to maintain the integrity of the mold at the time of pouring the molten metal, and on the other hand to ensure excellent stress relaxation when the already solidified metal cools in the mold.

[0070] The process of the invention further comprises a step of finalizing the thickened mold, comprising:

[0071] - a dewaxing step; and - a heat treatment step.

[0072] These steps are known as such, and allow the mold to be consolidated and the wax model to be removed.

[0073] At the end of the described process, we therefore have a mold whose mechanical properties will be degraded during the cooling of the solidified metal, in order to to reduce or even eliminate the stresses applied to the cast metal part.

[0074] In the application, the various intervals given and the expression "between ... and ..." should be understood to include the limits.

Claims

Demands

1. A method for manufacturing a mold for molten metal comprising at least the following steps: - the formation of a mold precursor comprising at least the following substeps: - dipping a wax model of the mold in a contact slip; - a stucco step with a contact stucco; - a drying step; - thickening the formed mold precursor, comprising one or more thickening cycles, each thickening cycle comprising at least the following substeps: - a dipping step in a reinforcing slip; - a stucco step with a reinforcing stucco; - a drying step; - a finalizing step of the thickened mold comprising at least the following substeps: - a dewaxing step;and - a heat treatment step, the process being characterized in that, during at least one thickening cycle, the reinforcing stucco comprises cracking particles, wherein the cracking particles are single-crystal grains of electrofused aluminum titanate or andalusite, and wherein the reinforcing stuccos comprising cracking particles comprise between 10% and 50% by mass of cracking particles.

2. A method according to claim 1, comprising between 5 and 10 thickening cycles.

3. A method according to any one of claims 1 or 2, wherein the reinforcing stucco used for the first and / or last thickening cycle does not include cracking particles.

4. A method according to any one of claims 1 to 3, wherein the Reinforcing stucco of three consecutive thickening cycles includes cracking particles.