Patent Application
20190329317
This invention concerns the manufacture of a foundry mould in a process known as “lost-wax”, for the manufacture of precision metal parts. This type of mould is also called a shell mould.
The production of “lost wax” moulds is well known and widely used, particularly for the manufacture of precision parts with complex geometries or precision parts in very small or even unique series.
To make a lost wax mould, a model of the part to be produced is first made of wax or of a removable material that can be easily melted or removed from the manufactured mould.
The model is successively tempered, sandblasted and/or coated with reinforcing medium and dried. The quenching operation is carried out in one or more slip(s). The sandblasting operation, also called stucco, consists in reinforcing the deposit constituted by the layer of slip deposited on the model during quenching. After each quenching and sandblasting and/or coating operation, the water is removed from the different layers. Then, the model is eliminated, for example during a passage in an autoclave (pressure and temperature treatment). Lastly, the mould undergoes a heat treatment in order to give it the necessary characteristics for casting the metal.
For the manufacture of precision metal parts, a mould must be stable when casting molten metal. Stable means that the molten metal must not cause the mould material to react in such a way that it deforms.
In order for the mould to have a perfect surface finish for the production of a part, it is important that the composition of the first layer in contact with the model, commonly referred to as the contact layer, is chemically compatible and accurately 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 mould. In addition, the expansion coefficients of the contact layer and the subsequent layers constituting the mould must be compatible in order to avoid any damage caused by a difference in thermal expansion of the layers.
The use of different slip materials that include either alumina, zircon, or electrofused silica, is known in the art. Each one of these compounds has at least one particular disadvantage. For example, alumina is not compatible with certain alloys that constitute the precision metal parts to be produced, electrofused silica lacks refractoriness, and zircon, in addition to being radioactive, loses stability as the temperature of the molten alloy increases.
The invention more particularly aims at providing a simple, efficient and cost-effective solution to these problems.
To this end, the invention proposes a method of manufacturing a multi-layer shell mould, including at least one contact layer, from a model of the part to be manufactured of wax or other similar material, the method comprising a step of dipping the model into 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 limits the chemical interactions between the shell mould and the metal alloy introduced by casting into the shell mould. The above-mentioned composite is preferably mainly or almost exclusively composed of mullite and zirconia. Of course, it is understood that it may include negligible amounts of impurities. These impurities may include calcium or sodium. According to a characteristic of the invention, the binder may be inorganic or organic or a mixture of organic and inorganic compounds.
Mullite-zirconia composite powder makes it possible to produce a contact slip with good rheological stability, good chemical inertia towards the molten alloy, and which allows controlled manufacturing.
It should be remembered that a composite is a material composed of several elementary components whose combination gives to the whole properties that none of the components taken separately possess.
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 zirconia. 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 between 5 and 20 μm and a size distribution ranging from a submicron size to a size of 100 μm.
According to another characteristic, the contact layer can have a thickness of 1 mm or less. It is desirable to limit the thickness of the contact layer to avoid mechanically weakening the shell mould due to the presence of zirconia.
To obtain a good quality contact slip, the zirconia content in the powder is between 5% and 90% w/w and, preferably between 10% and 50% w/w and even more preferably between 30% and 50% w/w.
Advantageously, the binder is colloidal silica.
To promote the wetting of the contact layer on the surface of the model, the contact slip also includes at least one wetting agent and/or at least one anti-foaming agent.
To make a resistant mould for the manufacture of a precision part, the process includes, following soaking of the model in the contact slip, steps in which:
Advantageously, the steps of soaking in the second slip, coating with 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, results in a good quality mould that will resist the manufacture of a precision part and offer a good external surface finish to the manufactured precision part.
This process, prior to soaking the model in the contact slip, includes a phase of preparing the contact slip including the sub-steps wherein:
Advantageously, the contact slip manufacturing phase also includes a sub-step of adding the anti-foam and/or wetting agent.
In addition, the invention also concerns the use of a mould according to the method described above for the manufacture of a cast and solidified turbomachine part.
The invention will be better understood and other details, characteristics and advantages of the invention will become readily apparent upon reading the following description, given by way of a non limiting example with reference to the appended drawings, wherein:
Mould 1, shown in cross-section in
The process of making mould 1 includes steps 100 to 700, which will now be described.
In a first step 100, model 6 of the precision part to be manufactured is made in wax.
To ensure the production of a perfect precision part, model 6 is manufactured to the exact dimensions of the precision part and includes a high-quality external surface finish 7. Thus, only a few slight irregularities may be visible or detectable on the outer surface 7 of the model 6 so that the final precision part will only need one finishing pass (i.e. a machining operation) to grind the outer surface of the precision part thus obtained.
Advantageously, model 6 will have such a surface finish that a finishing pass will not be necessary and the precision part can be used directly at the exit of the mould.
For example, the precision part to be manufactured will be a turbomachine blade that must have an exterior surface free of roughness in order to:
In a second step 200, the model is dipped in a contact slip to form, around model 6, a contact layer 2 which can have a thickness less than or equal to 1 mm.
Contact layer 2 has an essential role in the use of mould 1 since it will give its outer surface to the produced precision part. It is therefore necessary that the contact slip is dense and resistant at the same time, and that its viscosity and covering power are controlled.
Viscosity and density are necessary so that during soaking, 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 outer surface 7 of the model 6, air bubbles that would form, on an inner surface 8 of the mould 1, a cavity conducive to the creation of an asperity on the outer 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 dual criterion of viscosity and strength, the contact slip is composed of an inorganic and/or organic binder and a powder, in this case a mullite-zirconia composite.
Preferably, the binder is an inorganic colloidal binder such as colloidal silica in a weight percentage between 10% and 40% and, preferably, between 20% and 30%.
As examples, the inorganic binder may be sodium silicate or ethyl silicate and the organic binder includes water.
The powder contains, in weight percent, a zirconia content of between 5% and 90% and, preferably, between 10% and 50% and even more preferably between 30% and 50%.
According to a preferred embodiment, the mass distribution of the elements composing the contact slip is as follows:
The mass distribution is given here as an example, it being understood that a variation in the mass distribution between 0.1% and 10% is possible.
For example, the other additives that can be added may be a bactericidal agent to limit bacteria and increase the stability of the slip, or other organic binders to ensure a uniform and resistant deposit of the contact layer 2 on the wax model 6.
Advantageously, the contact slip also includes a wetting agent and an anti-foaming agent.
The production of the contact slip can be carried out as follows:
Following these steps, the mixture in the tempering tank is then the contact slip.
The composition of the contact slip has many advantages over the slip of the prior art, including better durability, good chemical stability, shorter manufacturing time, non-radioactive formulation and improved mould quality.
For example, compared to the slip of the prior art, the contact slip according to the invention offers:
In a third step 300, model 6, dipped in the contact slip, is sanded and then dried. Sandblasting is carried out in a gentle manner with a powder that will not affect contact layer 2 and in particular the condition of the inner surface 8 of mould 1.
Sandblasting makes it possible to reinforce contact layer 2 and facilitates the attachment of a second layer of mould 1.
In a fourth step 400, model 6 coated by the sanded and dried contact layer 2 is tempered 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, which comes out of the second slip, is sanded and then dried.
At the end of step 500, a model 6 is obtained on which the contact layer 2 and a first reinforcement layer 3 are superimposed.
As shown on the flowchart in
In the example of mould 1 shown in
However, this example of mould 1 is by no means restrictive and a higher or lower number of reinforcement layers 3 could be provided.
In a sixth step 600, the wax model 6 is melted so that only mould 1 remains.
Finally, in a seventh (and last) step 700, mould 1, comprising an adequate number of reinforcement layers (here three reinforcement layers 3, 4, 5) undergoes a heat treatment, in this case a firing in an oven, in order to solidify mould 1.
However, generally, the removal of the wax model 6 (also called the waxing step) is performed before the heat treatment of mould 1. It is also possible that the wax model 6 will be removed in heat treatment step 700, the temperature to consolidate mould 1 being sufficient to melt the wax from model 6, steps 600 and 700 then being combined in a single step.
When mould 1 is finished, a material, for example a metal alloy for the manufacture of blades, can be cast into mould 1, against the inner surface 8. After cooling, this cast material then forms the precision part to be manufactured.
To remove the precision part from mould 1, mould 1 can be removed mechanically (mould 1 breaking) or chemically (mould 1 dissolution), or by a combination of both methods.
Another advantage of choosing a mullite-zirconia composite powder for the contact slip is that contact layer 2 has a low (or no) risk of chemical reaction with a wide variety of materials that can be cast to form the precision part.
In addition, the mullite-zirconia composite ensures a good ease of use of the slip and allows the wax models 6 with complex geometries to be covered and in particular to be accommodated in grooves and other poorly accessible cavities so that all the details of the wax models 6 are reproduced on the contact layer 2.
Finally, the mullite-zirconia composite offers the advantage of not being radioactive, and can therefore be handled without specific equipment.
Reference is now made to
In the image of
| Number | Date | Country | Kind |
|---|---|---|---|
| 1657022 | Jul 2016 | FR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/FR2017/052030 | 7/21/2017 | WO | 00 |
