EP2741876B2 - Method for casting monocrystalline metal parts - Google Patents

Description

Background of the invention

The present invention relates to the field of foundry, and in particular the foundry of monocrystalline metal parts.

The traditional metal alloys are polycrystalline equiaxes: in the solid state, they form a plurality of grains of substantially identical size, typically of the order of 1 mm, but more or less random orientation. The grain boundaries are weak points in a metal part produced from such an alloy. The use of additives to reinforce these inter-grain seals, however, has the drawback of reducing the temperature of the melting point, which is particularly disadvantageous when the parts thus produced are intended to be used at high temperature.

In order to overcome this drawback, columnar polycrystalline alloys were initially proposed whose grains solidify with a determined orientation. This makes it possible, by orienting the grains in the main load direction of the metal part, to increase the resistance of these parts in a particular direction. However, even in parts subjected to highly oriented forces along a particular axis, such as turbine blades subjected to centrifugal forces, it may also be advantageous to offer increased resistance in the other axes.

With this object, since the end of 1979, new metal alloys called monocrystalline have been developed allowing the production in the foundry of pieces formed by a single grain. Typically these monocrystalline alloys are nickel alloys with a concentration of titanium and / or aluminum less than 10 mol%. Thus, after their solidification, these alloys form biphasic solids, with a first phase Y and a second phase Y '. Phase Y has a center face cubic crystal lattice, in which the atoms of nickel, aluminum and / or titanium can occupy any of the positions. On the other hand, in phase Y ', the atoms of aluminum and / or titanium form a cubic configuration, occupying the eight corners of the cube, whereas nickel atoms occupy the faces of the cube.

One of these new alloys is the "AM1" nickel alloy jointly developed by SNECMA and the ONERA laboratories, the Ecole des Mines de Paris, and IMPHY SA. The parts produced in such an alloy can achieve not only particularly high mechanical strength in all axes of effort, but also an improved thermal resistance, since it can dispense with additives intended to bind more strongly between them crystalline grains. Thus, metal parts produced from such monocrystalline alloys can be advantageously used, for example, in the hot parts of turbines.

However, even using these special alloys, it can be difficult to avoid a recrystallization phenomenon during the production of such pieces, introducing new crystalline grains, and thus new weak spots in the piece. In a traditional casting process, the molten alloy is poured into a cavity of a mold through at least one casting channel in the mold, the mold is unhooked after solidification of the alloy, to release the workpiece, and this is then subjected to a heat treatment, such as for example a quenching in which the metal is first heated, and then cooled rapidly, in order to homogenize the phases Y and Y 'in the single crystal without causing its melting .

However, the mechanical shocks to which the parts are subjected after the casting can locally destabilize the crystal lattice of the single crystal. Then, the heat treatment can trigger unwanted recrystallizations in the places thus destabilized, thereby losing the monocrystalline character of the room and introducing weak points therein. Even with great efforts, it is very difficult to avoid mechanical shocks in the handling of molds that can have a mass of several dozen pounds, especially since the shakeout of the mold involves, in itself, mechanical shocks. On the other hand, a limited reduction in the heat treatment temperature, alone, does not substantially prevent these recrystallization phenomena.

Object and summary of the invention

The present invention aims to remedy these disadvantages. For this, the invention aims to provide a foundry process that allows to largely limit the recrystallization phenomena following the heat treatment of the parts after solidification of the cast alloy in the mold.

According to one aspect of the present invention, in a casting process according to the preamble of claim 1, the casting channel comprises at least at least one transition zone adjacent to said cavity, with a rounding radius of not less than 0, 3 mm between said casting channel and said cavity in order to avoid a pronounced bend in the flow of molten alloy, bend which could give rise to a recrystallization zone of the alloy. In particular, the casting channel has, in this transition zone, an enlarged section, with respect to an upstream section, in the direction of a main axis of a section of the cavity perpendicular to the casting channel. More particularly, after casting, this transition zone could form at least one thinner metal film than the upstream casting channel, and more particularly at least one such metal film from each of two opposite sides of the casting channel. When the mold contains at least one core penetrating into said cavity and occupying a space adjacent said casting channel to form a cavity in the metal part, said transition zone may form, after casting, at least one metal veil adjacent to said core and thinner than the upstream casting channel. Each metal web adjacent the core may have an outer edge along a substantially concave line adjacent to a surface of the core. The transition zone may form at least one metal veil on each side of said core. In this case, said metal webs adjacent to the core may have outer edges joining at the ends, so as to surround the core.

In this way, during casting, this transition zone makes it possible to fill the cavity substantially simultaneously over its entire width, thus avoiding creating, during the solidification of the alloy, irregularities in the crystal structure of the single crystal. These irregularities could indeed cause, during the heat treatment step, a local recrystallization forming a weak point in the metal part.

In order to increase the production of metal parts, the mold may contain a plurality of cavities, arranged in a cluster, for molding a plurality of metal parts simultaneously.

The process according to the invention is particularly suitable for the production of certain metal parts, such as turbomachine blades. The present invention also relates to metal parts obtained by this method.

Brief description of the drawings

• la figure 1 illustre un procédé de fonderie de l'art antérieur ;
• la figure 2 illustre un procédé de fonderie suivant un mode de réalisation de la présente invention ;
• la figure 3 illustre la connexion entre un canal de coulée et une cavité de moulage d'un moule de l'art antérieur ;
• la figure 4 est une vue en perspective d'une pièce métallique produite suivant le procédé de la figure 2 ; et
• la figure 5 et une coupe transversale de la pièce métallique de la figure 4 dans le plan V-V.

The invention will be better understood and its advantages will appear better on reading the detailed description which follows, of an embodiment shown by way of non-limiting example. The description refers to the accompanying drawings in which:

• the figure 1 illustrates a foundry process of the prior art;
• the figure 2 illustrates a foundry process according to one embodiment of the present invention;
• the figure 3 illustrates the connection between a casting channel and a molding cavity of a mold of the prior art;
• the figure 4 is a perspective view of a metal part produced according to the method of figure 2 ; and
• the figure 5 and a cross-section of the metal piece of the figure 4 in the VV plane.

Detailed description of the invention

A conventional foundry process, as used for example in the production of turbomachine blades and more particularly of high pressure turbine blades, is illustrated in FIG. figure 1 . In a first step, a ceramic mold 150 is produced, typically by the lost wax process, although other conventional methods may be used alternately. This ceramic mold 150 comprises a cluster of cavities 151 connected by casting channels 152 to an orifice 153 outside the mold 150. Each cavity 151 is shaped to mold a metal part to be produced. In this case, the parts to be produced being hollow, the mold 150 also comprises cores 155 penetrating into each of the cavities 151. After this first step, in a casting step, a molten alloy 154 is poured into the orifice 153 to fill the cavities 151 through the channels of casting 152.

After solidification of the alloy, in a third step, the hammer 150 is initially shaken off, in order to release from the mold 150 the metal parts 156 united in a cluster 157. In order to eliminate the last remains of the mold 150, an additional step of water jet shaking is then carried out. In the next step S105, the individual pieces 156 are cut from the cluster 157. The cores 155 are then unchecked from each piece 156 in the next step, and the pieces 156 are finally heat-treated. This heat treatment can be, for example, quenching, in which the parts 156 are briefly heated, and then quickly cooled, to harden the alloy parts.

Among the alloys that can be used in this process, include so-called monocrystalline alloys, which allow the production of parts formed by a single crystal grain or monocrystal. However, in this process of the prior art, the heat treatment, the object of which is in fact the homogenization of the phases Y and Y 'in the single crystal, can trigger recrystallization phenomena locally weakening the parts. In order to avoid this disadvantage, in a casting process according to an embodiment of the invention illustrated on the figure 2 , the order of the operations is modified, so as to advance the heat treatment step.

So, in this process illustrated on the figure 2 the first step is also the production of a ceramic mold 250. As in the prior art, this ceramic mold 250 can also be produced by the lost wax process, or by another alternative method among those known to the person of career. In addition, as in the prior art, this ceramic mold 250 comprises a cluster of cavities 251 connected by channels 252 to an orifice 253 outside the mold 250. Each cavity 251 is also shaped to mold a metal part at produce. In addition, since the parts to be produced are also hollow, the mold 250 also comprises cores 255 penetrating into each of the cavities 251.

After the first step, and also as in the prior art, in a casting step, a molten alloy 254 is poured into the orifice 253 to fill the cavities 251 through the pouring channels 252. After solidification of the alloy in a third step, the mold 250 is also initially shaken off, in order to release the metal pieces 256 united in a cluster 257 from the mold 250. However, in this process, after this initial shakeout, gold proceeds directly to the heat treatment step. During this heat treatment, the metal parts 256, still forming a cluster 257 with still remains of the mold 250, are directly subjected to, for example, quenching, in which the parts 256 are briefly heated, and then quickly cooled.

In order to eliminate the last remains of the mold 250, it is then possible to proceed with the water jet unsticking in the next step. Finally, the individual pieces 256 are cut out of the cluster 257, and the cores 255 are then unchecked from each piece 256, already heat-treated before the water-jet stall.

Thanks to the progress of the heat treatment step, it is possible to reduce the recrystallization phenomena during this step. However, in order to reduce this recrystallization even more completely and above all more reliably, it is also necessary to give a suitable shape to the casting channels 252. In the figure 3 , one can see the connection between a casting channel 152 and a mold cavity 151 in the mold 150 of the prior art. This connection forms very pronounced bends between the channel 152 and the cavity 151, bends which can cause the formation of recrystallization zones 160 during the heat treatment.

In the mold 250 of the method illustrated on the figure 2 , in order to avoid the formation of such recrystallization zones in every room around 252. These channels 252 comprise transition zones adjacent to the cavities 251. In this transition zone, the casting channel 252 widens progressively in the direction of a main axis X of a section S of the cavity 251 in a plane A perpendicular to the pouring channel, so that the radius of rounding between the pouring channel 252 and the cavity 251 is not less than 0.3 mm. In particular, in the illustrated embodiment, in which the mold 250 also has the core 253 adjacent to the casting channel 252, this transition zone widens on one side and the other of the core 253, as well as on the side opposite the core 253. When the cavity 251 and the channel 252 are filled with metal, the latter will thus form a web 261 on the opposite side to the core 253, and two webs 262, 263 adjacent the core 253, one on each side of the core 253, as illustrated in Figures 4 and 5 . These webs 261, 262, 263 are, perpendicular to the X axis, substantially thinner than the casting channel 252 upstream of the transition zone.

During the casting step, the presence of the transition zone thus makes it possible to distribute the flow of molten alloy substantially throughout the width of the cavity 251, thus avoiding the formation of subsequent recrystallization zones.

The monocrystalline piece 256 illustrated on the figure 4 is a turbine blade. It is illustrated in the raw state of demoulding, that is to say, with the solidified metal out of the mold release channel 252. This metal thus forms a central rod 275, sails 261, 262 and 263, and a section 276 adjacent to the blade head 265. During casting, the molten alloy flows from the blade head 265, through the blade root 266, to a casting channel 252 connected to a nozzle. another cavity 251 further downstream. The flow of molten alloy thus substantially follows the direction of the main axis Z of the blade. The web 261, which extends towards the trailing edge 267 of the blade, has an outer edge 268 with a concave upstream segment and a convex downstream segment. In cross section, this outer edge 268 has a radius of curvature R which evolves only very gradually from the central rod 275 to the enlarged section 276. The webs 262 and 263, which extend towards the leading edge 269 dawn on each side of the core 253, have respective outer edges 270,271 substantially concave along the core 253. These outer edges 270, 271 are joined by their ends above the core 253 and in front thereof, thus forming two connections 272,273, so as to surround the core 253. In cross section, these sails 262, 263 have radii of curvature R 'and R "on the surfaces adjacent to the outer edges 270, 271 in order to avoid the germination of undesirable metallurgical defects in the vicinity of the core 253. The transition surface 277 of the sails 261, 262 and 263 and the stem 275 at the enlarged section 276 is also rounded to prevent germination of such defects.

Among the alloys that can be used in this process, nickel monocrystalline alloys, such as, in particular, AM1 and AM3 from SNECMA, but also others such as CMSX-2®, CMSX-4®, CMSX- 6 ®, and CMSX-10 ® from CM Group, René® N5 and N6 from General Electric, RR2000 and SRR99 from Rolls-Royce, and PWA 1480, 1484 and 1487 from Pratt & Whitney, among others. Table 1 illustrates the compositions of these alloys: <b> Table 1: Compositions of monocrystalline nickel alloys in mass% </ b> Alloy Cr Co MB W al Ti Your Nb Re Hf VS B Or CMSX-2 8.0 5.0 0.6 8.0 5.6 1.0 6.0 - - - - - Ball CMSX-4 6.5 9.6 0.6 6.4 5.6 1.0 6.5 - 3.0 0.1 - - Ball CMSX-6 10.0 5.0 3.0 - 4.8 4.7 6.0 - - 0.1 - - Ball CMSX-10 2.0 3.0 0.4 5.0 5.7 0.2 8.0 - 6.0 0.03 - - Ball René N5 7.0 8.0 2.0 5.0 6.2 - 7.0 - 3.0 0.2 - - Ball René N6 4.2 12.5 1.4 6.0 5.75 - 7.2 - 5.4 0.15 0.05 0,004 Ball RR2000 10.0 15.0 3.0 - 5.5 4.0 - - - - - - Ball SRR99 8.0 5.0 - 10.0 5.5 2.2 12.0 - - - - - Ball PWA1480 10.0 5.0 - 4.0 5.0 1.5 12.0 - - - 0.07 - Ball PWA1484 5.0 10.0 2.0 6.0 5.6 - 9.0 - 3.0 0.1 - - Ball PWA1487 5.0 10.0 1.9 5.9 5.6 - 8.4 - 3.0 0.25 - - Ball AM1 7.0 8.0 2.0 5.0 5.0 1.8 8.0 1.0 - - - - Ball AM3 8.0 5.5 2.25 5.0 6.0 2.0 3.5 - - - - - Ball

Although the present invention has been described with reference to a specific exemplary embodiment, it is obvious that various modifications and changes can be made to these examples without departing from the general scope of the invention as defined by the claims. For example, in an alternative embodiment, the heat treatment could be performed even before the initial shakeout of the mold. In addition, individual features of the various embodiments mentioned can be combined in additional embodiments. Therefore, the description and drawings should be considered in an illustrative rather than restrictive sense.

Claims (12)

1. A foundry method of casting monocrystalline metal parts (256), the method comprising at least the steps of:

   casting a molten alloy (254) into a cavity (251) of a mold (250) through at least one casting channel (252) in the mold (250);

   subjecting the alloy to heat treatment; and

   removing the mold (250);

   characterized in that said casting channel (252) includes at least one transition zone adjacent to said cavity (251), with a rounded portion of radius not less than 0.3 mm between said casting channel (252) and said cavity (251) and presenting, relative to an upstream section, a cross-section that is enlarged in the direction of a main axis (X) of a section (S) of the cavity (251) in a plane (A) that is perpendicular to the casting channel (252).
2. A foundry method of casting monocrystalline metal parts according to claim 1, wherein, after casting, said transition zone forms at least one metal web (261, 262, 263) that is thinner than the casting channel (252) upstream.
3. A foundry method of casting monocrystalline parts (256) according to claim 2, wherein, after casting, said transition zone forms at least one metal web (261, 262, 263) on each of two opposite sides of the casting channel (252), which at least one metal web is thinner than the casting channel (252) upstream.
4. A foundry method of casting monocrystalline metal parts (256) according to claim 3, wherein said mold contains at least one core (255) penetrating into said cavity and occupying a space adjacent to said casting channel (252) so as to form a cavity in the metal part (256), and wherein, after casting, said transition zone forms at least one metal web (262, 263) adjacent to said core (255) and thinner than the casting channel (252) upstream.
5. A foundry method of casting monocrystalline metal parts (256) according to claim 4, wherein said metal web (262, 263) adjacent to the core (255) presents an outer edge (270, 271) following a substantially concave line adjacent on a surface of the core (255).
6. A foundry method of casting monocrystalline metal parts (256) according to claim 4 or claim 5, wherein, after casting, said transition zone forms at least one metal web (262, 263) adjacent to said core (255) on each of two opposite sides of the core (255).
7. A foundry method of casting monocrystalline metal parts (256) according to claim 6, wherein said metal webs (262, 263) adjacent to the core (255) present outer edges (270, 271) that join together at the ends so as to surround the core (255).
8. A foundry method according to any one of claims 1 to 7, wherein said metal part (256) is a turbine engine blade.
9. A foundry method according to any one of claims 1 to 8, wherein said mold (250) contains a plurality of cavities (251) arranged as a bunch in order to mold a plurality of metal parts (256) simultaneously.
10. A foundry method according to any one of claims 1 to 9, wherein the heat treatment is performed after the alloy has solidified in the mold (250) and before the end of mold removal.
11. A foundry method according to any one of claims 1 to 10, wherein said removal of the mold (250) comprises a first step of removal by hammering and a subsequent step of removal by water jet, said heat treatment being performed at least before the removal by water jet.
12. A monocrystalline metal part (256) produced by a foundry method according to any one of claims 1 to 11.

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