EP2945762B1 - Method for manufacturing a component using the lost wax casting method with directed cooling - Google Patents

Description

Technical area

The present invention relates to the field of metal parts, such as turbomachine blades obtained by casting metal in a shell mold and relates to a method of manufacturing these parts with directed solidification of columnar or monocrystalline type.

Prior art

The documents US-A-3,659,645 and US-B1-6,364,001 describe methods of manufacturing a part by lost wax casting.

The state of the art also includes documents GB-A-1 377 042 and SU-A1- 606 676 .

The process for manufacturing metal parts by lost wax casting comprises a succession of steps recalled below. Models of the parts to be manufactured are first produced in wax or other temporary material. Where appropriate, the models are gathered in a cluster around a central barrel also made of wax. A ceramic material shell is then formed on the models, thus assembled, by successive soaking in slips of suitable composition comprising particles of ceramic materials in suspension in a liquid, alternated with dustings of refractory sand. The wax model is then removed while consolidating the shell mold thus formed by heating. The next step consists in casting a metal alloy, in particular a nickel superalloy, in fusion in the shell mold and then in cooling the parts obtained so as to direct their solidification according to the desired crystalline structure. After solidification, the shell is removed by detaching to extract the parts. Finally, the finishing steps are carried out to remove excess material.

The cooling and solidification step is therefore controlled. The solidification of the metal alloy being the passage from the liquid phase to the solid phase, the directed solidification consists in advancing the growth of "seeds" in the bath of molten metal in a given direction, avoiding the appearance of new germs by controlling the thermal gradient and the rate of solidification. Directed solidification can be columnar or monocrystalline. Columnar directed solidification involves orienting all grain boundaries in the same direction, such that they do not contribute to the propagation of cracks. Monocrystalline directed solidification consists in completely eliminating the grain boundaries.
The directed, columnar or monocrystalline solidification is carried out in a manner known per se by placing the shell mold, open in its lower part, on a cooled hearth, then by introducing the whole into a heating equipment capable of holding the ceramic mold. at a temperature higher than the liquidus of the alloy to be molded. Once the casting has been carried out, the metal located in openings made at the bottom of the shell mold solidifies almost instantaneously on contact with the cooled sole and freezes to a limited height of the order of a centimeter on which it has an equilibrium granular structure -axis, that is to say that its solidification over this limited height takes place naturally, without any preferred direction. Above this limited height, the metal remains in the liquid state, due to the imposed external heating. The sole is moved at a controlled speed downwards so as to extract the ceramic mold from the heating device leading to a gradual cooling of the metal which continues to solidify from the lower part of the mold to its upper part.

Columnar directed solidification is achieved by maintaining an appropriate temperature gradient in magnitude and direction in the liquid-solid phase change zone, during this sole displacement operation. This makes it possible to avoid supercooling which generates new seeds ahead of the solidification front. Thus, the only seeds which allow the growth of the grains are those which preexist in the equi-axis zone solidified in contact with the cooled hearth. The columnar structure thus obtained consists of a set of narrow and elongated grains.

The monocrystalline directed solidification further comprises the interposition between the part to be molded and the cooled sole, either of a baffle or grain selector, or of a monocrystalline seed; the thermal gradient and the rate of solidification are controlled in such a way that no new seeds are created in front of the solidification front. This results in a monocrystalline molded part after cooling.

This directed solidification technique, whether columnar or monocrystalline, is commonly used to produce molded parts, and in particular turbomachine blades, when it is desirable to give the molded parts specific mechanical and physical properties. This is particularly the case when the molded parts are turbomachine blades.

In addition, in a manner known per se, during the implementation of a lost wax casting process, with or without directed solidification, weights are used in order to eliminate the porosity defects in end areas of the. parts to be manufactured. In practice, excess volumes are provided during the production of wax models, which are placed against the areas of the parts which are liable to exhibit porosity defects after solidification. When making the shell, excess volumes translate into additional volumes inside the shell, and fill with molten metal during casting, in the same way as other parts of the shell. The weights are the reserves of solidified metal which fill the additional volumes in the shell. The porosity defects, when they occur, are then displaced in the weights and are no longer located in the manufactured parts themselves. Then, once the metal has solidified and cooled, the weights are removed during a part finishing operation, for example by machining, by cutting or by grinding.

It is also known, as described in the patent FR 2724857 in the name of the applicant, a process for manufacturing monocrystalline blades, such as turbine nozzles, consisting of at least one blade between two platforms transverse to the generatrices of the blade. The method is of the type according to which the mold is supplied with molten metal at its upper part. A directed solidification is carried out, the front of which progresses vertically from bottom to top, a single crystal grain is selected by means of a selection device placed at the lower part of the mold and at the exit of which there is a single grain of predetermined orientation and direction coinciding with the vertical.

The present invention relates to the manufacture of parts having at least one cavity and the wax model of which is molded around a ceramic core. This core, during the casting of the molten metal reserves inside the part the volume corresponding to the desired cavity. For a turbomachine blade, the cavities traversed by the cooling fluid are produced in this way.

Ceramic cores for turbomachine blades comprise, according to a known method of manufacture, two bearing surfaces or retaining lugs, one at each longitudinal end. The models are prepared in such a way that an embedding or anchoring of the ceramic core is defined at the level of the area of the base of the core in the upper part of the mold. Indeed according to this technique the core and the wax model are mounted foot up and the top down. Thus after the ceramic molding operations, the ceramic shell formed blocks the core in this area. During casting, the molten metal fills the imprint released by the wax which has been previously removed. The molten metal occupies the space between the core and the shell wall. The solidification is then carried out by the pulling from top to bottom of the bottom of the furnace on which the shell is placed, the solidification progresses from the starter in which several metal grains solidify then successively in the top of the blade, the blade and the foot. By solidifying the metal creates a second anchor of the core at the end bearing in the part of the start of solidification. The core is then held at its two ends and is constrained in compression. This results in a deformation of the core by buckling. The core no longer respects its theoretical position and defects may appear on the part: metal wall thicknesses may not be respected, or the core under the effect of the stresses of the two embedments at its two ends perforates the metal wall of dawn by buckling. In both cases the part must be scrapped.

Furthermore, the positioning of the embedding at the start of solidification has the drawback of disturbing the incipient solidification front with the risk of generating parasitic grains or disorientation. In addition, in the case of a single crystal, there is a risk of faulty re-bonding of the growing edges on either side of the embedding zone.

Disclosure of the invention

The subject of the invention is therefore a method of manufacturing a part which overcomes the problems presented above.

The process, according to the invention, for manufacturing by lost wax casting of a metal part made of nickel alloy, with a columnar or monocrystalline structure with at least one elongated cavity, comprising the following steps of producing a wax model of the part with a ceramic core corresponding to said cavity, the ceramic core having a first support surface at a longitudinal end and a second support surface at the opposite end, the second surface comprising surfaces which are parallel to the direction of cooling propagation and surfaces which are not parallel to the direction of cooling propagation,
production of a shell mold around the model, the mold comprising a base and the first bearing surface of the core being on the side of the base of the shell mold,
removal of the wax by a dewaxing operation of the shell mold, placing the mold in a furnace, the base being placed on the bottom of the furnace, casting of said molten alloy in the shell mold,
Directed solidification of the cast metal by progressive cooling from the hearth in a direction of propagation, the surfaces of the second bearing surface which are not parallel to the direction of propagation of the cooling being initially covered by a deposit of wax,

According to the invention, the core is made integral with the shell mold by an anchoring means between the first bearing surface of the core and the wall of the shell mold, the second bearing surface of the core being retained in the shell mold by a sliding retention means. on the wall of the shell mold,
said sliding holding means being a layer of varnish applied, before the production of the shell mold, to the surfaces of the second bearing surface which are parallel to the direction of propagation of the cooling and which are not covered with wax,
the surfaces of the second bearing surface which are parallel to the direction of propagation of the cooling, which are not covered with wax and which, after constitution of the shell mold, come into direct contact with the internal wall of the mold, being initially fully coated with the varnish layer,
the thickness of the varnish layer being between 3 and 5 hundredths of a millimeter,
said layer of varnish being removed during the dewaxing operation of the shell mold, as well as the wax covering the surfaces of the second bearing surface which are not parallel to the direction of propagation of the cooling, so that a free space is created between the second bearing surface of the core and the wall of the shell mold,
said free space created being maintained during the progression of the directed solidification, so as to prevent the second bearing surface of the core from coming into contact with the wall of the shell mold when the core expands.

The solution of the invention makes it possible to avoid the deformation of the core during the progression of the directed solidification because the core is not retained by anchoring at its two ends. It is thus not put into compression by the stresses which would result from the difference in the expansion coefficients between the mold and the core. There is also no risk of generating parasitic grains or of re-bonding defects of the main grain.

The solution of the invention also guarantees the position of the core throughout the part manufacturing phase: from the wax model to the casting and solidification of the part.

Advantageously, the anchoring means comprises a rod, more particularly of refractory ceramic, alumina for example, passing through the first bearing surface and the wall of the mold. Preferably, the ceramic rod has a small diameter of the order of a millimeter. The rod passes through the wax model and the core which have been previously drilled to a diameter slightly greater than that of the rod to prevent stresses being generated at this level.

According to the invention, the sliding holding means is formed by a space provided between the bearing surface and the wall of the mold, this space is obtained by means of a film of expansion varnish deposited on the surface of the bearing surface at the realization of the model. This is then eliminated during the dewaxing operation. of the mold. It is for example a material of the nail varnish type making it possible to obtain thicknesses of a few hundredths of a millimeter per layer. A suitable varnish for this application includes solvents, resin, nitrocellulose and plasticizers. For example, a varnish such as that “Thixotropic base” marketed under the trade name: “Peggy Sage nail varnish all formulas” can be used in the process of the present invention.

According to the invention, this film is more precisely interposed between the second bearing surface and the wall of the mold. It is applied, before the formation of the shell mold, on the surfaces of the second bearing surface which are parallel to the direction of the progress of the cooling; that is to say in the case of a bogie, parallel to the pulling direction of the bogie. Its purpose is to prevent, on the one hand, the wall of the mold from sticking to the core in this zone and, on the other hand, to create a free space, after unwinding, of small thickness allowing the longitudinal guiding of the second bearing relative to to the mold and avoiding the mold to exert a stress on the core.

In accordance with the invention, the surfaces of the second bearing surface which are not parallel to the axis of the progression of solidification, the drawing axis, are initially covered by a deposit of wax so as to leave, after dewaxing, a space between said surfaces of the second bearing surface and the wall of the mold. This space prevents, during the casting of molten metal, the contact between the wall of the shell and the second scope of the core, and avoids the stressing of the core in this zone during solidification. Typically, the thickness of this wax deposit is of the order of a millimeter for parts having a length of 100 to 200 mm, ie approximately 1% of the length of the part.

The process allows the simultaneous manufacture of several parts. The models of said parts are in this case gathered in a cluster inside a shell mold.

The method applies to the manufacture of at least one metallic part with a columnar structure, a means of germinating the crystalline structure being provided between the shell mold and the bottom of the furnace.

The method applies to the manufacture of at least one part with a monocrystalline structure, a grain selector being provided between the seed element and the shell mold.

The invention applies in particular to the manufacture of a turbine engine blade, the first range being in the extension of the top of the blade of the blade, the second range being in the extension of the root of the blade.

The method advantageously uses a furnace whose sole is movable vertically between a hot zone where the metal is molten and a cold zone for solidification of the metal, the sole itself being cooled.

Brief description of the figures

• La figure 1 représente une aube de turbomachine pouvant être obtenue selon le procédé de l'invention ;
• La figure 2 représente schématiquement un noyau en céramique pour aube de turbomachine ;
• La figure 3 représente le noyau de la figure 2 vu de profil.
• La figure 4 représente schématiquement un modèle en cire avec le noyau de la figure 2 ;
• La figure 5 représente le moule carapace vu en coupe longitudinale au travers du noyau;
• La figure 6 représente un exemple de four permettant la solidification dirigée de métal coulé dans un moule carapace ;
• La figure 7 est une vue agrandie de l'extrémité haute du moule carapace montré sur la figure 5.

Other characteristics and advantages will emerge from the following description of an embodiment of the invention, given by way of non-limiting example, with reference to the appended drawings in which

• The figure 1 represents a turbomachine blade obtainable according to the method of the invention;
• The figure 2 schematically shows a ceramic core for a turbine engine blade;
• The figure 3 represents the core of the figure 2 seen in profile.
• The figure 4 schematically represents a wax model with the core of the figure 2 ;
• The figure 5 represents the shell mold seen in longitudinal section through the core;
• The figure 6 shows an example of a furnace allowing the directed solidification of metal cast in a shell mold;
• The figure 7 is an enlarged view of the upper end of the shell mold shown on the figure 5 .

Description of an embodiment of the invention

The present invention relates to a method of manufacturing metal parts made of a nickel-based alloy making it possible, through an appropriate directed solidification, to obtain a columnar or monocrystalline crystalline structure. The invention relates more particularly to the manufacture of turbomachine blades such as that shown in figure 1 ; a blade 1 comprises a blade 2, a root 5 allowing it to be attached to a turbine disk, and a top 7 with, where appropriate, a heel. Due to the operating temperatures of the turbomachine, the blades are provided with an internal cooling circuit through which a cooling fluid, generally air, passes. A platform 6 between the root and the blade constitutes a portion of the radially inner wall of the gas stream. The part shown here is a moving blade, but the invention also applies to a distributor or even to any other part having a core.
Due to the complexity of the cooling circuit inside the part, it is advantageous to produce it by lost wax casting with a ceramic core to spare the cavities of the cooling circuit.
The figures 2 and 3 schematically show a simplified ceramic core used to provide the internal cavities of a turbomachine blade. The core 10 of elongated shape comprises a branch or a plurality of branches 11 separated by spaces 12 to, after the metal has been poured, form the partitions between the cavities; in the example shown, the core comprises two branches 11 separated by a space 12. At one end, the core is extended by a bearing surface or tab 14, the function of which is to hold the core during the manufacture of the part but which does not correspond not necessarily to part of the room, once it is completed. At the opposite end, the core comprises a second bearing surface 16 for also maintaining the core during the manufacturing steps. We observe on the figure 3 that the core as shown is relatively thin compared to its length. It is understood that the finer the core is compared to its length, the more sensitive it will be to buckling.

This core is placed in a mold for the production of the wax model. The imprint of this mold is the shape of the part to be obtained. By injecting wax into this mold, the model of the part is obtained. Staves 14 and 16 are used for keeping the core in the wax mold. The figure 4 schematically represents this wax model 20 with the core 10 in dotted lines. The model extends at a first end 24 in the extension of the blade so as to cover the bearing surface 14 and at the opposite end 26, at the level of the foot. It is noted that part 16A of the scope 16 is not covered with wax. This part 16A comprises surfaces parallel to the axis of the core and is coated with a varnish, the function of which is explained below.

Several models are generally assembled in a cluster so as to manufacture several parts simultaneously. The models are for example arranged in a parallel drum around a vertical central cylinder and held by the ends. The lower part is mounted on an element intended to ensure the germination of the crystal structure. The next step is to form a shell mold around the model (s). For this purpose, as is also known, the assembly is dipped in slurries so as to deposit the refractory ceramic particles in successive layers. The mold is finally consolidated by heating and the wax removed by the dewaxing operation.

We have represented on the figure 5 , in longitudinal section, schematically the arrangement of the invention between the core 10 and the shell 30 at the level of a single model 20.

The first bearing surface 14 is held in the mold 30 by a refractory ceramic rod 40, which passes through it and extends into the wall of the mold 30 while being embedded therein. The rod 40 was put in place before the production of the shell mold, after the model has been drilled at the level of the bearing 14. The hole is slightly larger in diameter than that of the rod so that it is not created. of constraints between the rod and the seat and that the rod ensures correct positioning of the core in the model.

The second bearing surface 16, opposite the first, is initially coated with a layer of varnish 17 on the part 16A of the core which is not covered with wax and which, after constitution of the shell mold, comes into direct contact with the internal wall of the core. mold. After dewaxing the mold, as seen on the figure 5 , the layer having disappeared leaves a free space between the bearing surface 16 of the core and the wall of the shell mold. Reference 17 designates this free space left by the layer varnished. This space 17 is thin, 3 to 5 hundredths of a millimeter. It forms a sliding retaining means of the second bearing 16 on the wall of the shell 30.

Furthermore, the surfaces - here the horizontal surface 16B - which are not parallel to the axis of the progression of solidification are initially covered by a deposit of wax 18. This deposit of wax leaves a free space after dewaxing, likewise reference 18, which prevents the bearing surface 16 of the core from coming into contact with the wall of the shell when the core expands, thus avoiding stressing the core. Typically, the thickness of this wax deposit is of the order of a millimeter for parts having a length of 100 to 200 mm, ie approximately 1% of the length of the part.

By not being constrained, the core does not run the risk of buckling and the initial wall thicknesses of the part between the wall of the mold and the core are preserved.

The figure 5 shows, in section along the part, the shell mold 30 and the core 10 inside the mold with the branches 11, the bearing surfaces 14 and 16. The core is cut along the line VV of the figure 4 . The volume 30 ′ corresponds to the wax of the model or, after solidification of the shell, to the space between the wall of the mold and the core to be filled with the metal. The rod 40 passes through the first bearing 14; it is long enough to be anchored in the walls of the shell mold 30. In this way, the core 10 is positioned inside the shell mold 30.

After dewaxing and consolidation, the mold is placed on the bottom of a furnace equipped for directed solidification. Such a furnace 100 is shown in figure 6 . It shows an enclosure 101 provided with heating elements 102. An orifice 103 for supplying molten metal communicates with a crucible 104 which contains the charge of molten metal and which, by tilting, fills the shell mold 30 placed on the hearth. 105 from the oven. The sole is movable vertically, see the arrow, and is cooled by the circulation of water in a circuit 106 internal to its plate. The mold rests by its base on the cooled sole. The lower part of the mold is open to the sole by means of a germination member.

The manufacturing method, as explained in the preamble to the application, includes casting molten metal from crucible 104 directly in the mold 30 which is maintained at a temperature sufficient to keep the molten metal, by the heating means 102 of the enclosure 101 and where it fills the voids 30 'between the core 10 and the wall of the mold 30. As the base of the mold is in thermal contact with the hearth by the seed element, the metal solidifies forming a crystalline structure that propagates from bottom to top. The sole 105 is continuously cooled and is gradually lowered out of the heated enclosure. In the case of a monocrystalline structure, a grain selector is interposed between the germination and the solidification as is known per se.

The large temperature differences create stresses between the different areas of the mold with the metal. By the arrangement of the invention and the rod 40, the core is held by anchoring the first bearing 14 in the single lower zone for initiating solidification. As seen on the figure 7 the core is free to expand differentially in the direction of its length with respect to the shell 30 because at the opposite end of the first bearing surface, the second bearing 16 is guided along the wall of the mold thanks to the free space 17 left by the layer of varnish, removed during dewaxing of the mold.

In addition, the surfaces of the second bearing 16 - here the horizontal surface 16B - which are not parallel to the axis of the progression of solidification, thanks to the free space 18 provided by the wax deposit, do not come into play. contact with the shell wall. This prevents stress on the core. Typically, the thickness of this space corresponding to the wax deposit is of the order of a millimeter for parts having a length of 100 to 200 mm, ie approximately 1% of the length of the part. By not being constrained, the core does not run the risk of buckling and the initial wall thicknesses of the part between the wall of the mold and the core are preserved.

Once the metal has cooled, the mold is broken and the parts are extracted which are sent to the finishing workshop.

Claims (8)

1. Method for manufacturing, using the lost-wax casting method, a metal component made from nickel alloy, with a columnar or monocrystalline structure with at least one elongate-shaped cavity, comprising the following steps of:

   producing a wax model (20) of the component with a ceramic core (10) corresponding to said cavity, the ceramic core (10) comprising a first holding span (14) at a longitudinal end and a second holding span (16) at the opposite end, the second span (16) comprising surfaces (16A) that are parallel to the direction of propagation of the cooling and surfaces (16B) that are not parallel to the direction of propagation of the cooling,

   producing a shell mould (30) around the model (20), the shell mould (30) comprising a base and the first span (14) of the core being on the same side as the base of the shell mould (30),

   eliminating the wax by dewaxing the shell mould (30),

   placing the shell mould (30) in a furnace (100), the base being placed on the hearth (105) of the furnace (100), pouring a molten alloy into the shell mould (30),

   carrying out a directed solidification of the poured metal by gradual cooling from the hearth (105) in a propagation direction, the surfaces (16B) of the second span (16) that are not parallel to the direction of propagation of the cooling being covered initially by a deposit of wax (18),

   characterized in that the core (10) is secured to the shell mould (30) by an anchor (40) between the first span (14) of the core and the wall of the shell mould (30), the second span (16) of the core (10) being held in the shell mould (30) by a holding means (17) sliding over the wall of the shell mould (30), said sliding holding means (17) comprising a layer of varnish (17) applied, before the production of the shell mould (30), to the surfaces (16A) of the second span (16) that are parallel to the direction of propagation of the cooling and that are not covered with wax, the surfaces (16A) of the second span (16) that are parallel to the direction of propagation of the cooling and that are not covered with wax and that, after formation of the shell mould (30), comes into direct contact with the internal wall of the mould (30), being initially fully covered with said layer of varnish (17), the layer of varnish (17) having a thickness of between 3 and 5 hundredths of a millimetre, said layer of varnish (17) being eliminated during the operation of dewaxing the shell mould (30), as well as the wax (18) covering the surfaces (16B) of the second span (16) that are not parallel to the direction of propagation of the cooling, so that a free space (17, 18) is created between the second span (16) of the core (10) and the wall of the shell mould (30),

   said free space (17, 18) created being maintained during the progression of the directed solidification, so as to prevent the second span (16) of the core (10) from coming into contact with the wall of the shell mould (30) when the core (10) expands.
2. Method according to claim 1, wherein the anchoring means (40) comprises a rod passing through the first span (14) and being embedded in the wall of the mould.
3. Method according to claim 2, wherein the rod is made from ceramic.
4. Method according to one of the preceding claims, for manufacturing a plurality of components, the models of said components being collected together in a cluster inside a shell mould (30).
5. Method according to one of the preceding claims, for manufacturing at least one metal component with a columnar structure, an element for nucleation of the crystalline structure being provided between the shell mould (30) and the hearth (105) of the furnace (100).
6. Method according to the preceding claim, for manufacturing at least one component with a monocrystalline structure, comprising a grain selector between the nucleation element and the shell mould (30).
7. Method according to one of the preceding claims, the component being a turbine engine blade, the first span being in the extension of the apex of the vane of the blade, the second span being in the extension of the root of the blade.
8. Method according to one of the preceding claims, wherein the hearth is able to move vertically between a hot region where the metal is molten and a cold region for solidification of the metal, the hearth itself being cooled.

Images