EP2945762A1 - Method for manufacturing a component using the lost wax casting method with directed cooling - Google Patents

Abstract

The present invention relates to a method for manufacturing a metal component using the lost‑wax casting method, the component being made of nickel alloy, with a columnar or monocrystalline structure with at least one cavity of elongate shape, comprising the following steps of creating a wax model (20) of the component with a ceramic core (10) corresponding to said cavity, the ceramic core comprising a first land (14) for holding at one longitudinal end and a second land (16) for holding at the opposite end, creating a shell mould around the model, the mould comprising a base and the first land of the core being at the same end as the base, placing the mould in a furnace, with the base standing on the sole of the furnace, pouring the said molten alloy into the shell mould, directed solidification of the poured metal by gradual cooling from the sole in a direction of propagation. It is characterized in that the core (10) is secured to the shell mould by an anchoring means (40) providing anchorage between the first land (14) of the core and the wall of the mould, the second land (16) of the core being retained in the mould by a retaining means (17) that slides on the wall of the mould.

Description

 Process for manufacturing a part by lost-wax foundry and directed cooling

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 is directed to a method of manufacturing these parts with columnar or monocrystalline directed solidification.

Prior art

The process for manufacturing metal parts by lost-wax foundry comprises a succession of steps recalled below. Models of the parts to be manufactured are first elaborated in wax or in another temporary material. If necessary the models are gathered in a cluster around a central drum also in wax. A shell of ceramic material is then formed on the models, thus assembled, by successive dipping in slip of suitable composition comprising particles of ceramic materials suspended in a liquid, alternated with dusting refractory sand. The wax model is then removed while consolidating by heating the shell mold thus formed. The next step is to cast a metal alloy, especially a nickel superalloy, melt in the shell mold and then cool the parts obtained so as to direct the solidification according to the desired crystalline structure. After solidification, the shell is removed by shaking to extract the pieces. Finally, the finishing steps are carried out to eliminate excess material.

The cooling and solidification step is therefore controlled. Since the solidification of the metal alloy is the transition from the liquid phase to the solid phase, the directed solidification consists of advancing the growth of "seeds" in the molten metal bath in a given direction, avoiding the appearance of seeds. new by controlling the thermal gradient and the solidification rate. Directed solidification may be columnar or monocrystalline. Columnar directed solidification consists of orienting all grain boundaries in the same direction so that they do not contribute to the propagation of cracks. Monocrystalline direct solidification consists of completely removing the grain boundaries.

 Columnar or monocrystalline directed solidification is carried out in a manner known per se by placing the shell mold, open at its lower part, on a cooled sole, then introducing the assembly into a heating equipment capable of holding the ceramic mold. at a temperature above the liquidus of the alloy to be molded. Once poured, the metal located in openings at the bottom of the carapace mold solidifies almost instantaneously in contact with the cooled sole and freezes over a limited height of the centimeter on which it has a granular structure equi -axis, that is to say that its solidification on this limited height is carried out naturally, without privileged 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 rate downwards so as to extract the ceramic mold from the heating device leading to a progressive cooling of the metal which continues to solidify from the lower part of the mold to its upper part.

 Column directed solidification is achieved by maintaining a temperature gradient appropriate in magnitude and direction in the liquid-solid phase change zone during this sole displacement operation. This avoids supercooling generating new germs ahead of the solidification front. Thus, the only seeds that allow grain growth are those that pre-exist in the equi-axis solidified zone in contact with the cooled sole. The columnar structure thus obtained consists of a set of narrow and elongated grains.

The monocrystalline directed solidification further comprises the interposition between the molding 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 before solidification front. The result is a monocrystalline casting after cooling.

 This technique of directed solidification, whether columnar or monocrystalline, is commonly used to make molded parts, including turbomachine blades, when it is desirable to give the molded parts special 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 molding process, with or without directed solidification, weights are used, in order to eliminate the porosity defects in end zones of parts to manufacture. In practice, surplus volumes are expected when wax models are made, which are placed against the zones of the parts which are likely to have porosity defects after solidification. When making the carapace, the excess volumes result in extra volumes inside the carapace, and fill with molten metal during casting, in the same way as the other parts of the carapace. The weights are the reserves of solidified metal that fill the extra volumes in the shell. The porosity defects, when they occur, are then displaced in the flyweights and are no longer located in the parts manufactured themselves. Then, once the metal 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 F 2724857 in the name of the applicant, a method of manufacturing monocrystalline blades, such as turbine distributors, consisting of at least one blade between two transverse platforms relative to the generators of the blade. The process is of the type in which the molten metal mold is fed to its upper part. A directed solidification is carried out whose forehead progresses vertically from below upwards, a single crystal grain is selected by means of a selection device placed at the bottom of the mold and at the exit of which there is in the presence of a single grain of predetermined orientation and direction merging with the vertical. The present invention relates to the manufacture of parts having at least one cavity and whose model in wax is molded around a ceramic core. This core, during the casting of the molten metal reserves inside the room the volume corresponding to the desired cavity. For a turbomachine blade, this way the cavities traversed by the cooling fluid are produced.

 The ceramic cores for the turbomachine blades comprise, according to a known method of manufacture, two bearing surfaces or lugs, one at each longitudinal end. The models are prepared in such a way that embedding or anchoring of the ceramic core is defined at the region of the foot of the core in the upper part of the mold. Indeed according to this technique the nucleus and the model in wax are mounted foot up and the top down. Thus after the ceramic molding operations, the formed ceramic shell blocks the core in this area. During casting, the molten metal fills the impression released by the wax that has been removed beforehand. The molten metal occupies the space between the core and the wall of the carapace. The solidification is then operated by drawing up and down the hearth of the furnace on which is placed the shell, 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 span in the solidification start portion. The core is then held at both ends and is forced into compression. It follows a deformation of the core by buckling. The core no longer respects its theoretical position and defects may appear on the part: thicknesses of metal wall may not be respected, or the core under the effect of the constraints of the two embeddings at both ends perforates the metal wall of dawn by buckling. In both cases the part must be scrapped.

Moreover, the positioning of the embedding at the onset of solidification has the disadvantage of disturbing the nascent solidification front with the risk of generating parasitic grains or disorientation. In addition, there is in the case of the single crystal a risk of failure to glue up the rising edges on either side of the embedding zone. Presentation of the invention

The invention therefore relates to a method of manufacturing a part that overcomes the problems presented above.

 The process, in accordance with the invention, for the production by lost-wax casting of a nickel-alloy metal part with a columnar or monocrystalline structure with at least one elongated cavity, comprising the following steps of producing a a wax model of the part with a ceramic core corresponding to said cavity, the ceramic core having a first holding surface at one longitudinal end and a second holding surface at the opposite end,

 producing a shell mold around the model, the mold comprising a base and the first range of the core being on the side of the base,

 placing the mold in an oven, the base being placed on the hearth of the furnace, pouring said molten alloy into the shell mold,

 directed solidification of the cast metal by progressive cooling from the hearth in a direction of propagation,

is characterized in that the core is secured to the shell mold by means of anchoring between the first bearing of the core and the wall of the mold, the second bearing of the core being retained in the mold by a holding means sliding on the wall of the mold.

 The solution of the invention avoids deformation of the core during the progression of the directed solidification because the core is not retained by anchoring at both ends. It is thus not put in compression by the constraints which would result from the difference of the coefficients of expansion between the mold and the core. There is also no risk of parasitic grain generation or defects in the main grain.

 The solution of the invention also ensures the position of the core during the entire phase of manufacture of the piece: from the wax model to the casting and the solidification of the piece.

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

 According to another characteristic, the sliding holding means is formed by a space formed 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. realization of the model. This is then removed during the mold dewaxing operation. This is for example a nail varnish type material to obtain thicknesses of a few hundredths of a millimeter per layer. A varnish suitable for this application includes solvents, resin, nitrocellulose and plasticizers. For example, a varnish such as "Thixotropic base" marketed under the trade name: "Peggy Sage Nail Polish All Formulas" can be used in the process of the present invention.

 This film is more precisely interposed between the second surface and the wall of the mold. It is applied, before the formation of the carapace mold, on the surfaces of the second reach which are parallel to the direction of the progression of the cooling; that is to say in the case of a mobile hearth, parallel to the direction of draft of the movable hearth. This film of varnish is preferably thin in the order of 3 to 5 hundredths of a millimeter. Its purpose is to avoid, on the one hand, that the wall of the mold comes to stick to the core in this zone and, on the other hand, to create a free space, after dewaxing, of small thickness allowing the longitudinal guidance of the second bearing relative to mold and avoiding the mold to exert stress on the core.

The surfaces of the second span which are not parallel to the axis of the progression of solidification, axis of pull, are initially covered by a wax deposit so as to provide, after dewaxing, a space between said surfaces of the second range and the wall of the mold. This space prevents, during molten metal casting, the contact between the wall of the shell and the second range of the core, and prevents stressing of the core in this area during solidification. Typically, the thickness of this wax deposit is of the order of millimeter for pieces having a length of 100 to 200 mm or about 1% of the length of the piece.

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

 The method is applicable to the manufacture of at least one metal part with a columnar structure, a means of germination of the crystalline structure being formed between the mold and the hearth of the furnace.

 The method applies to the manufacture of at least one piece with a monocrystalline structure, a grain selector being formed between the germination element and the mold.

 The invention applies in particular to the manufacture of a turbomachine blade, the first scope being in line with the top of the blade of the blade, the second scope being in the extension of the root of the blade.

 The method advantageously uses an oven whose sole is vertically movable between a hot zone where the metal is melted is a cold zone of solidification of the metal, the sole being itself cooled.

Brief description of the figures

 Other characteristics and advantages will become apparent 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:

 FIG. 1 represents a turbomachine blade that can be obtained according to the method of the invention;

 Figure 2 schematically shows a ceramic core for turbomachine blade;

 Figure 3 shows the core of Figure 2 seen in profile.

 Figure 4 schematically shows a wax model with the core of Figure 2;

Figure 5 shows the carapace mold seen in longitudinal section through the core; Figure 6 shows an example of a furnace for directed solidification of cast metal in a shell mold;

 Figure 7 is an enlarged view of the upper end of the shell mold shown in Figure 5.

Description of an embodiment of the invention

The present invention relates to a method of manufacturing metal parts made of nickel-based alloy, which, by suitable directional solidification, makes it possible 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 foot 5 allowing its attachment to a turbine disk, and a vertex 7 with a bead if appropriate. Due to the operating temperatures of the turbomachine, the blades are provided with an internal cooling circuit traversed by a cooling fluid, generally air. A platform 6 between the foot and the blade constitutes a portion of the radially inner wall of the gas stream. The piece shown here is a moving blade but the invention also applies to a dispenser or to any other piece having a core.

 Due to the complexity of the cooling circuit inside the room, it is advantageous to make it by lost-wax casting with a ceramic core to protect the cavities of the cooling circuit.

Figures 2 and 3 show schematically a simplified shaped core, ceramic, used to spare the internal cavities of a turbomachine blade. The elongated core 10 comprises a branch or a plurality of branches 1 1 separated by spaces 12 for, after the casting of the metal, forming the partitions between the cavities; in the example shown, the core comprises two branches 1 1 separated by a space 12. At one end, the core is extended by a span or lug 14 whose function is to maintain the core during the manufacture of the part but which does not does not necessarily correspond to a part of the room, once it is completed. At the opposite end the core includes a second span 16 for also maintaining the core during the manufacturing steps. It is observed in Figure 3 that the core as shown is relatively thin in relation to its length. It is understood that the more the core is fine compared to its more sensitive length it will be buckling.

 This core is placed in a mold for the manufacture of the wax model. The impression of this mold is in the shape of the part to be obtained. By injecting wax into this mold, we obtain the model of the piece. Spans 14 and 16 serve to hold the core in the wax mold. Figure 4 shows schematically this model 20 in wax with the core 10 in dashed 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. Note that a portion 16A of the scope 16 is not covered with wax. This portion 16A comprises surfaces parallel to the axis of the core and is coated with a varnish whose function is explained below.

 Many models are usually clustered together to make multiple parts simultaneously. The models are for example arranged in a drum parallel to a vertical central cylinder and held by the ends. The lower part is mounted on an element intended to ensure the germination of the crystalline structure. The next step is to form a shell mold around the model or models. For this purpose, as is also known, the assembly is soaked in slips so as to deposit in successive layers the refractory ceramic particles. The mold is finally consolidated by heating and the wax removed by the dewaxing operation.

 FIG. 5 shows diagrammatically, in longitudinal section, the arrangement of the invention between the core 10 and the shell 30 at the level of a single model 20.

The first 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 being embedded therein. The rod 40 has been put in place before the shell mold is made, after the model has been drilled at the level of the bearing surface 14. The hole is of diameter slightly greater than that of the stem so that it is not created. constraints between the rod and the bearing and that the rod ensures correct positioning of the core in the model.

 The second bearing 16, opposite to the first, is initially coated with a layer of varnish 17 on the portion 16A of the core which is not covered with wax and which after constitution of the shell mold comes into direct contact with the inner wall of the mold. After dewaxing the mold, as seen in FIG. 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 varnish layer. This space 17 is thin, 3 to 5 hundredths of a millimeter. It forms a sliding support 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 the solidification are initially covered by a deposit of wax 18. This deposit of wax leaves after dewaxing a free space, likewise reference 18, which prevents the scope 16 of the core from coming into contact with the wall of the shell when the core expands, thereby avoiding the stressing of the core. Typically, the thickness of this wax deposit is of the order of one millimeter for pieces having a length of 100 to 200 mm or about 1% of the length of the piece.

 By not being constrained the core is not likely to flare up and the initial wall thicknesses of the part between the mold wall and the core are retained.

 FIG. 5 shows, in section along the part, the shell mold 30 and the core 10 inside the mold with the branches 1 1, the bearing surfaces 14 and 16. The section of the core is made according to the line VV of FIG. FIG. 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 by 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 floor of a furnace equipped for directed solidification. Such an oven 100 is shown in Figure 6. It shows an enclosure 101 provided with heating elements 102. An orifice 103 The molten metal supply unit communicates with a crucible 104 which contains the molten metal charge and which, when tilted, fills the shell mold 30 disposed on the hearth 105 of the furnace. The sole is movable vertically, see the arrow, and is cooled by the circulation of water in a circuit 106 internal to its tray. The mold rests with its base on the cooled sole. The lower part of the mold is opened on the sole by means of a germination member.

 The method of manufacture, as explained in the preamble of the application, comprises pouring the molten metal from crucible 104 directly into the mold 30 which is maintained at a temperature sufficient to retain the molten metal, by means of heating 102 of the chamber 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 germination element, the metal solidifies forming a crystalline structure that spreads from below upwards. Sole 105 is cooled continuously and gradually descended from the heated enclosure. In the case of a monocrystalline structure a grain selector is interposed between germination and solidification as is known per se.

 The large temperature differences create stresses between the different zones 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 only lower zone of initialization of the solidification. As seen in Figure 7 the core is free to expand differentially in the direction of its length relative to the carapace 30 because at the opposite end of the first scope, the second bearing 16 is guided along the wall of the mold thanks to the free space 17 left by the layer of varnish, eliminated during the dewaxing of the mold.

In addition, the surfaces of the second span 16 - here the horizontal surface 16B - which are not parallel to the axis of the progression of the solidification, thanks to the free space 18 formed by the wax deposit do not come into contact with the wall of the carapace. This prevents the stressing of the core. Typically, the thickness of this space corresponding to the wax deposit is of the order of one millimeter for pieces having a length of 100 to 200 mm or about 1% of the length of the piece. By not being forced the core does not risk of flaming 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 that are directed to the finishing shop are extracted.

Claims

claims 1. Process for manufacturing by lost-wax casting of a nickel-alloy metal part, with a columnar or monocrystalline structure, with at least one elongate cavity, comprising the following steps for producing a model (20) made of wax the part having a ceramic core (10) corresponding to said cavity, the ceramic core having a first holding surface (14) at one longitudinal end and a second holding surface (16) at the opposite end,  producing a shell mold (30) around the model, the mold comprising a base and the first bearing (14) of the core being on the side of the base of the mold,  placing the mold in an oven (100), the base being placed on the hearth (105) of the furnace, pouring said molten alloy into the shell mold, directed solidification of the cast metal by progressive cooling from the hearth in a direction of spread,  characterized in that the core (10) is secured to the shell mold by anchoring means (40) between the first bearing surface (14) of the core and the wall of the mold (30), the second bearing surface (16) of the core being retained in the mold by a holding means (17) sliding on the wall of the mold.  2. Method according to claim 1 wherein the anchoring means (40) comprises a rod passing through the first surface (14) and being embedded in the wall of the mold.  3. Method according to claim 2, the rod is ceramic. 4. Method according to one of the preceding claims, the sliding holding means (17) comprises the free space left by a film of varnish interposed between a portion (16A) of the second bearing (16) of the core parallel to the direction the progress of the solidification and the wall of the mold. 5. Method according to one of the preceding claims wherein a space is provided between the surfaces (16B) of the scope (16) of the core which are not parallel to the direction of the progression of the solidification.  6. Method according to one of the preceding claims for the manufacture of a plurality of parts, the models of said parts being collected in a cluster inside a carapace mold.  7. Method according to one of the preceding claims for the manufacture of at least one metal piece with columnar structure, a germinating element of the crystalline structure being formed between the mold and the sole (105) of the furnace.  8. Process according to the preceding claim for the manufacture of at least one piece with a monocrystalline structure comprising a grain selector between the seed element and the mold.  9. Method according to one of the preceding claims the part being a turbine engine blade, the first scope being in line with the top of the blade of the blade, the second scope being in the extension of the base of the blade.  10. Method according to one of the preceding claims whose sole is vertically movable between a hot zone where the metal is molten is a cold zone of solidification of the metal, the sole being itself cooled.