EP1595620B1 - Broken mould moulding method - Google Patents

Description

The present invention relates to the manufacture of parts such as metal vanes with complex geometries according to the technique known as lost-wax foundry.

For the manufacture of turbofan bladders, such as parts of rotors or stators, or structural parts according to this technique, it is first made a model wax or other equivalent material easily removable later. If necessary we group together several models into one cluster. A ceramic mold is made around this model by dipping in a first slip to form a first layer of material in contact with its surface. The surface of this layer is sanded in order to reinforce it and facilitate the attachment of the next layer, and the assembly is dried: this constitutes the stuccage and drying operations respectively. The soaking operation is then repeated in slips of possibly different compositions, an operation always associated with the successive operations of stuccage and drying. A ceramic shell made of a plurality of layers is thus produced. The slips are composed of particles of ceramic materials, in particular a flour, such as alumina, mullite, zircon or other, with a mineral colloidal binder and adjuvants where appropriate depending on the desired rheology. These adjuvants make it possible to control and stabilize the characteristics of the different types of layers, while avoiding the effects of the different physicochemical characteristics of the raw materials constituting the slips. It may be a wetting agent, a fluidizer or a texturizer depending, for the latter, the desired thickness for the deposit.

The carapace mold is then dewaxed, which is an operation by which the material constituting the original model is removed. After elimination of the model, we obtain a ceramic mold whose cavity reproduces all the details of the model. The mold then undergoes heat treatment at high temperature or "cooking", which gives it the necessary mechanical properties.

The shell mold is thus ready for the manufacture of the metal part by casting. After checking the internal and external integrity of the shell mold, the next step is to pour a molten metal into the mold cavity and then to the solidify. In the field of lost-wax foundry, there are currently several solidification techniques, and therefore several casting techniques, depending on the nature of the alloy and the expected properties of the part resulting from the casting. It may be directed solidification with columnar structure (DS), directed solidification with monocrystalline structure (SX) or equiaxed solidification (EX) respectively. The first two families of parts concern superalloys for parts subjected to strong thermal and mechanical stresses in the turbojet engine, such as HP turbine blades.

After casting of the alloy, the shell is broken by a shaking operation, and the manufacture of the metal part is completed.

Methods of producing shell molds are described in particular in the documents published under the references US5618633 and EP0399727 , which disclose different types of slip.

During the molding step, several types of shells can be made through several processes. Each carapace must have specific properties that ensure the desired type of solidification. For example, for equiaxed solidification, several different processes can be implemented, one using an ethyl silicate binder, another using a colloidal silica binder. For directed solidification, the shells can be made from different fillers, based on silico-aluminous, silica-zircon or silica.

In order to simplify and standardize the processes used, there is a need for a so-called "unique" structure shell whose properties would enable it to be used in the various cases of solidification.

On the other hand, for reasons of compliance with environmental standards and costs, there is also a need to avoid the use of alcohol-based binders such as ethyl silicate.

For reasons of rejection costs, it is also desirable to develop a shell structure not including zircon. This material, even a weakly radioactive one, requires the establishment of waste treatment procedures that are very demanding both industrially and financially.

The invention achieves these objectives with the following method.

• trempage dans une première barbotine contenant des particules céramiques et un liant, dépôt de particules de sable sur la couche et à sécher ladite couche, de manière à former la couche de contact,
• trempage dans une deuxième barbotine contenant des particules céramiques, un liant, dépôt de particules de sable sur ladite couche et à sécher celle-ci, de manière à former la couche intermédiaire
• trempage dans au moins une troisième barbotine contenant des particules céramiques, un liant, dépôt de particules de sable sur ladite couche, à sécher celle-ci, de manière à former une couche de renfort, On répète la formation de couches de renfort jusqu'à obtenir une épaisseur de moule carapace définie.

The multi-layer ceramic shell mold manufacturing method, including at least one contact layer, an intermediate layer and a plurality of layers reinforcement from a model in wax or other similar material, consists in carrying out the following operations:

• dipping in a first slip containing ceramic particles and a binder, depositing sand particles on the layer and drying said layer, so as to form the contact layer,
• dipping in a second slip containing ceramic particles, a binder, deposition of sand particles on said layer and drying it, so as to form the intermediate layer
• dipping in at least a third slip containing ceramic particles, a binder, deposition of sand particles on said layer, to dry it, so as to form a reinforcing layer, the formation of reinforcement layers is repeated up to obtain a defined shell mold thickness.

According to the invention, the method is characterized in that the ceramic particles of the slips comprise a refractory oxide or a mixture of refractory oxides without zircon, none of the layers comprising zircon so that the shell mold is not even weakly, radioactive.

Preferably the slip for the formation of the reinforcing layers is much more fluid than the second slip.

It has been found that a shell mold having this composition and this structure, at the contact layer, could be designed to be common to all types of castings according to the techniques mentioned above. It is thus possible advantageously to adjust the mechanical properties of the mold, in particular its sensitivity to thermal shocks, to satisfy the casting conditions that meet the constraints of the various solidification processes (EX, DS or SX).

Preferably and to meet the economic and environmental constraints, the binder for the various slips is a colloidal mineral solution such as colloidal silica. Similarly, to satisfy the economic constraints related to rejects, the stucco grains for the contact, intermediate and reinforcement layers are made from mullite grains and not zircon grains.

In order to control the porosity of the mold, and thus to control the sensitivity of the shell to thermal shocks, the stuccage operations are carried out with stucco grains covering a size range between 80 and 1000 microns. Furthermore, the stucco is preferably applied by dusting for the first layers, and is preferably applied by fluidized bed, for the layers from the fourth. Stucco is applied automatically, so that the movements of the robot can achieve a shell mold having a porosity after baking, between 20 and 35%. The more porous the shell is, the more it reduces its sensitivity to thermal shocks such as those produced during different types of casting.

In particular, in order to be applied to two distinct modes of solidification, the mold baking cycle comprises heating to a temperature of between 1000 and 1150 ° C, preferably between 1030 ° C and 1070 ° C.

It suffices to adapt only the contact layer to the solidification mode. Thus the first slip can be formed from mullite flours and alumina without zircon, with or without germinating.

In a particular case, for DS or SX type solidifications, the contact layer is composed mainly of mullite flour in an amount of between 40 and 80% by weight, optionally of alumina flour, a binder based on colloidal silica. , and organic adjuvants.

In the particular case of equiaxed solidification, the contact layer is composed of a mixture of alumina flours and mullite in amounts respectively of between 40 and 80% by weight and between 2 and 30% by weight, the remainder comprising a binder based on colloidal silica, a germinator, and organic adjuvants.

According to another characteristic, the second and third slip are common to any solidification process, and comprise a mixture of alumina and mullite flours in an amount of between 45 and 95% by weight, and mullite grains in an amount of between 0 and 25% by weight.

The mold structure thus defined finds, indifferently, a use
for the manufacture of a part with solidification of directed type with a columnar structure, the contact layer being formed mainly from a mullite flour,
for the manufacture of a monocrystalline structured type solidification-type part, the contact layer being formed predominantly from a mullite flour or else
for producing a piece with equiaxed type solidification, the contact layer being formed from a mixture of alumina flour and mullite.

The invention also relates to a method for manufacturing parts by casting of molten metal according to which, regardless of the type of solidification, directed with a columnar structure, directed with monocrystalline or equiaxed structure, molds having a skeleton of shells are used. common: intermediate layer and common reinforcement layer.

The invention also relates to an installation for the manufacture of parts by casting a molten metal in a shell mold comprising a mold manufacturing station and casting stations for different solidifications, said stations being fed with molds having identical reinforcement layers.

The process is described in more detail below.

The method of manufacturing shell molds for common use in all types of parts comprises a first step of manufacturing the wax model or other equivalent material known in the art. The most commonly known is wax. Depending on the type of part, the clustered models can be grouped so that several can be manufactured simultaneously. The models are shaped to the dimensions of the final pieces, to the shrinkage near the alloys.

The carapace manufacturing steps are preferably carried out by a robot whose movements are common to all types of parts, programmed to have an optimal action on the quality of the deposits made, and to overcome the geometrical aspect of the different blading.

In parallel, slips are prepared in which the models or the cluster are successively quenched to deposit a ceramic material.

We distinguish a first slip for solidification EQX.

• un mélange de farines d'alumine (40 - 80%) et de mullite (2 - 30%) ;
• un germinant, aluminate de cobalt (0 - 10%) ;
• un liant silice colloïdale (18 - 30%) ;
• de l'eau (0 - 5%) ;
• trois adjuvants : agents mouillant, fluidifiant et texturant ;

It includes in weight percentage:

• a mixture of flours of alumina (40-80%) and mullite (2 - 30%);
• a germinant, cobalt aluminate (0-10%);
• a colloidal silica binder (18 - 30%);
• water (0 - 5%);
• three adjuvants: wetting, thinning and texturing agents;

• un mélange de farines d'alumine (2 - 30%) et de mullite (40 - 80%) ;
• un liant silice colloïdale (18 - 30%) ;
• de l'eau (0-5%) ;
• trois adjuvants : agents mouillant, fluidifiant et texturant ;

For columnar or monocrystalline columnar solidification, the composition of the first slip in percent by weight is as follows:

• a mixture of flours of alumina (2 - 30%) and mullite (40 - 80%);
• a colloidal silica binder (18 - 30%);
• water (0-5%);
• three adjuvants: wetting, thinning and texturing agents;

• un mélange de farines d'alumine (50 - 75%) et de mullite (5 - 20%) ;
• un liant silice colloïdale (20 - 30%) ;
• de l'eau (0 - 5%) ;
• trois adjuvants : agents mouillant, fluidifiant et texturant ;

The second intermediate slip, common to all types of solidification, comprises in percentage by weight the following components:

• a mixture of flours of alumina (50-75%) and mullite (5-20%);
• a colloidal silica binder (20 - 30%);
• water (0 - 5%);
• three adjuvants: wetting, thinning and texturing agents;

• un mélange de farines d'alumine (30 - 45%) et de mullite (15 - 30%) ;
• des grains de Mullite (14 - 24%) ;
• un liant silice colloïdale (10 - 20%) ;
• de l'eau (5 - 15%) ;
• quatre adjuvants : agents mouillant, fluidifiant, texturant et de frittage ;

The third reinforcing slip, common to all types of solidification, comprises the following components in percentage by weight:

• a mixture of flours of alumina (30 - 45%) and mullite (15 - 30%);
• Mullite grains (14 - 24%);
• a colloidal silica binder (10-20%);
• water (5 - 15%);
• four adjuvants: wetting, thinning, texturizing and sintering agents;

• Le fluidifiant permet d'obtenir plus rapidement la rhéologie désirée lors de la fabrication de la couche. Il agit en tant que dispersant. Il peut appartenir à la famille des acides aminés, à la gamme des polyacrylates d'ammonium, ou à la famille des tri - acides carboxyliques à groupements alcools ;
• Le mouillant permet de faciliter le nappage de la couche lors du trempé. Il peut appartenir à la famille des alcools gras poly - alkylènes, ou alcools alkoxylates ;
• Le texturant permet d'optimiser la rhéologie de la couche afin d'obtenir des dépôts adaptés. Il peut appartenir à la famille des polymères de l'oxyde d'éthylène, des gommes de xanthane, ou des gommes de guar ;

The first 3 adjuvants respectively have the following functions:

• The fluidizer makes it possible to obtain the desired rheology more quickly during the manufacture of the layer. It acts as dispersant. It may belong to the family of amino acids, to the range of ammonium polyacrylates, or to the family of carboxylic tri-acids with alcohol groups;
• The wetting agent makes it easier to coat the layer during dipping. It may belong to the family of polyalkylene fatty alcohols, or alkoxylated alcohols;
• The texturizer makes it possible to optimize the rheology of the layer in order to obtain suitable deposits. It may belong to the family of polymers of ethylene oxide, xanthan gums, or guar gums;

For the No. 1 contact layer, once the model removed from the first slip after an immersion phase, the covered model undergoes a phase of dewatering and then topping. Then, stucco grains are applied by dusting so as not to disturb the thin layer of contact. For the stuccage operation, mullite is used, the particle size of which in this first layer is fine. It is between 80 and 250 microns. The surface condition of the final pieces depends in part.

The No. 1 layer is dried.

It is then quenched in a second slip to form a layer No. 2, said intermediate. The composition is the same whatever the method of solidification adopted.

As before, a stucco is deposited by dusting and dried. For the stuccage operation, mullite is used whose particle size is medium. It can be between 120 and 1000 microns. The state of porosity of the shells finally depends on it.

The model is then quenched in a third slip to form the layer No. 3 which is the first so-called reinforcing layer.

The identical stucco is then applied to the No. 2 layer by dusting and dried. The soaking operations are repeated in the third slip, stuccage and drying to form the layers of "reinforcement". For these reinforcement layers, the stuccage is carried out by fluidized bed.

For the last layer, a glazing operation is performed which does not include a stuccage operation.
The carapace in final can consist of 5 to 12 layers.

The soaked layers for the different layers are made in different ways and are adapted to obtain a uniform distribution of thicknesses and to avoid the formation of bubbles, especially in enclosed areas.

Hardening programs are optimized for each type of layer, to overcome the geometric aspect of the different types of parts, and are therefore common to all references.

The interlayer drying range is optimized for each type of layer, in order to overcome the geometrical appearance of the different types of parts. The range is therefore common. The range makes it possible, for each type of layer, to dry molds with geometries as different as moving blades, distributors, or structural parts.

Final drying is carried out after the formation of the last layer, common to all types of parts.

The mussel baking cycle is the same for all types of solidification, and thus also eliminates the type of part. It includes a temperature rise phase, a bearing at the baking temperature and a cooling phase. The cooking cycle is chosen to optimize the mechanical properties of the shells so as to allow cold handling without risk of breakage, and so as to minimize the sensitivity to thermal shocks that can be generated during the various stages of casting.

It can be seen that a single cooking can be achieved instead of the two types of cooking that were previously performed to prepare the EQX, DS and SX shells for different casting modes.

Claims (17)

1. Method for producing a ceramic shell mould having a plurality of layers, including at least one contact layer, one intermediate layer and a plurality of reinforcement layers, from a model of the part to be produced, the model being made of wax or another similar material, consisting in carrying out the following successive operations:

   dipping in a first slip containing ceramic particles and a binder, depositing sand particles on said layer and drying it so as to form the contact layer,

   dipping in a second slip containing ceramic particles and a binder, depositing sand particles on said intermediate layer and drying it so as to form the intermediate layer,

   dipping in at least a third slip containing ceramic particles and a binder, depositing sand particles on said layer and drying it so as to form a reinforcement layer, the formation of reinforcement layers being repeated until a defined thickness of the shell mould is obtained,

   characterised by the fact that the ceramic particles of the slips comprise a refractory oxide or

   a mixture of refractory oxides without zircon, none of the layers comprising zircon, in such a way that the shell mould is not even slightly radioactive.
2. Method according to claim 1, wherein the refractory oxide is mullite or alumina.
3. Method according to either claim 1 or claim 2, wherein the binders for the different slips are based on mineral colloidal solutions, in particular colloidal silica.
4. Method according to claim 1, 2 or 3, wherein said sand particles consist of refractory oxide grains without zircon, in particular mullite grains.
5. Method according to claim 4, wherein the grains have a grain size of between 80 and 1000 microns.
6. Method according to either claim 4 or claim 5, wherein for certain layers, preferably the first three layers, the sand particles are applied by dusting.
7. Method according to claims 4, 5 and 6, wherein for certain layers, preferably from the fourth layer onwards, the sand particles are applied by means of a fluidised bed.
8. Method according to any one of claims 4 to 7, wherein the sand particles are applied in such a way that the shell has a porosity of between 20 and 35 % after firing.
9. Method according to claim 1, wherein drying between two successive layers is carried out according to the same route sheet, regardless of the type of part and the shape thereof.
10. Method according to claim 1, wherein the dipping is carried out by a robot which is programmed in such a way that the movements of said robot are the same regardless of the shape of the part.
11. Method according to any one of the preceding claims, wherein the sand particles are deposited on the mould automatically by a robot, in such a way that the movements of said robot are the same regardless of the shape of the part.
12. Method according to any one of the preceding claims, wherein the firing cycle of the final shell mould is the same regardless of the type of part and comprises heating to a temperature between 1000 and 1150 °C, preferably between 1030 and 1070 °C.
13. Method according to any one of the preceding claims, wherein the first slip has an alumina and mullite composition which differs depending on whether the part is produced by a directional solidification method or an equiaxed solidification method.
14. Method according to any one of the preceding claims, wherein the second and third slips comprise a mixture of alumina filler and mullite filler and mullite grains, and are common to all directional solidification and equiaxed solidification methods.
15. Method according to claim 13, wherein the first slip for directional solidification comprises predominantly mullite filler, in an amount between 40 and 80 % by weight, optionally alumina filler, a binder based on colloidal silica and organic additives.
16. Method according to claim 13, wherein the first slip for equiaxed solidification comprises a mixture of alumina filler and mullite filler, in amounts between 40 and 80 % by weight and between 2 and 30 % by weight respectively, a binder based on colloidal silica, a nucleating agent and organic additives.
17. Method according to claim 14, wherein the second and third slips are common to all solidification methods and comprise a mixture of alumina filler and mullite filler in an amount between 45 and 95 % by weight and mullite grains in an amount between 0 and 25 % by weight.