Mould for monocrystalline casting

Abstract

The invention relates to the field of monocrystalline casting, and more specifically to a mold (1) for monocrystalline casting, and also to fabricating and using the mold. In particular, the mold (1) presents a mold cavity (7) comprising a first volume (7a), a second volume (7b) and a grain duct (4). The second volume (7b) is situated on the first volume (7a), being in communication therewith, and includes at least one horizontal projection relative to the first volume (7a). The grain duct (4) has a bottom end (4a) connected to the first volume (7a) and a top end (7b) adjacent to said horizontal projection of the second volume (7b). The mold (1) also has a separator member (11) interposed between the second volume (7b) of the mold cavity (7) and the top end (4b) of the grain duct (4).

Claims

1. A monocrystalline casting mold comprising: a mold cavity comprising: a first volume; a second volume situated on the first volume, in communication therewith, and having at least one horizontal projection relative to the first volume; and a grain duct with a bottom end connected to the first volume and a top end adjacent to said horizontal projection of the second volume; the mold further comprising a separator member interposed between said second volume of the mold cavity and the top end of the grain duct.

2. The mold according to claim 1, wherein the top end of the grain duct has a width that is substantially equal to said horizontal projection of the second volume.

3. The mold according to claim 1, wherein the first volume of the mold cavity corresponds to a turbomachine blade body and the second volume of the mold cavity corresponds to a turbomachine blade platform.

4. The mold according to claim 1, also presenting, under the mold cavity, a starter cavity connected to the mold cavity by a selector channel.

5. A method of fabricating the monocrystalline casting mold according to claim 1, the method comprising the following steps: making a model reproducing at least the shape of the mold cavity; coating said model in a refractory material so as to form at least the mold cavity; and emptying out at least the mold cavity wherein said separator member is inserted in said model before the coating step.

6. The method according to claim 5, wherein said model is made of a material that melts at a temperature lower than said refractory material and is emptied out from the mold cavity in the liquid state.

7. The method according to claim 5, wherein said coating step is performed by dipping the model in a slip, dusting the model with a refractory sand in order to form a shell around the model, and sintering the shell in order to consolidate it.

8. A method of using a mold for monocrystalline casting according to claim 1 the method comprising the following steps: vacuum casting a metal material in the liquid state into the mold cavity; causing the metal material to solidify in directional manner from the bottom of the mold cavity towards the top; and knocking out the mold, including the separator member interposed between the second volume of the mold cavity and the top end of the grain duct.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

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

(2) FIG. 1 is a diagrammatic view of a monocrystalline casting installation with a mold constituting an embodiment of the invention;

(3) FIG. 2 is a diagrammatic perspective view of a model for producing the FIG. 1 mold; and

(4) FIGS. 3, 4, and 5 are diagrams showing the progress of a cooling and solidification front in a casting method in the FIG. 1 installation.

DETAILED DESCRIPTION OF THE INVENTION

(5) FIG. 1 shows how progressive cooling of molten metal for obtaining directional solidification can typically be performed in a casting method.

(6) The mold 1 used in this method comprises a pouring bush 5 and a base 6. While the mold 1 is being extracted from the heater chamber 3, the base 6 is directly in contact with a soleplate 2. The mold 1 also has a molding cavity 7. It is also possible to arrange a plurality of molding cavities in a cluster in the same mold. The molding cavity 7 is connected to the pouring bush 5 by a feed channel 8 into which molten metal penetrates while it is being poured. The molding cavity 7 is also generally connected at the bottom via a baffle-shaped selector channel 9 to a smaller starter cavity 10 in the base 6. In the embodiment shown, the molding cavity 7 has a first volume 7a and a second volume 7b situated directly above the first volume 7a, and in communication therewith, while being substantially wider in a horizontal plane, so as to present at least one very significant horizontal projection relative to the first volume 7a. More specifically, in the embodiment shown, the mold 1 is for producing turbomachine blades. Consequently, the first volume 7a corresponds to a blade body and the second volume 7b corresponds to a blade platform.

(7) The molding cavity 7 also has a grain duct 4 with a bottom end 4a directly connected to the first volume 7a and a top end 4b adjacent to the horizontal projection of the second volume 7b. In the embodiment shown, said top end 4b is wider than the remainder of the grain duct 4 so as to be adjacent to said horizontal projection of the second volume 7b over the entire width L of the second volume 7b. Although adjacent, the top end 4b of the grain duct 4 and said second volume 7b are not in direct communication, since they are separated by a rod-shaped separator member 11.

(8) By way of example, the separator member 11 may be made of ceramic material. Although in the embodiment shown it is in the form of a cylindrical rod of circular cross-section, other cross-sections and other general shapes could alternatively be adopted, depending on circumstances. The dimensions and the thermal conductivity of the material of the rod may be selected so as to provide good thermal contact between the top end 4b of the grain duct 4 and the adjacent horizontal projection of the second volume 7b of the mold cavity 7.

(9) The mold 1 may be produced by the so-called lost-wax or lost-pattern method. A first step in such a method is to create a model 12, such as that shown in FIG. 2. The model 12 is for forming the mold cavity 7 and also the starter cavity 10, the selector channel 9, the pouring bush 5, and the feed channel 8, which are all hollow in the mold 1. The model is obtained using a material having a low melting temperature, such as a suitable wax or resin. When it is intended to produce a large number of parts, it is possible in particular to produce these elements by injecting the wax or resin into a permanent mold. In order to support the model 12, a support rod 20 made of refractory material, e.g. of ceramic, is incorporated in the model 12, connecting its main body 7, corresponding to the mold cavity 7 to its base (not shown), corresponding to the starter cavity 10. In order to secure the support rod 20 to the model 12, it is possible to make use of the natural adhesion of the material of the model 12, or to make use of a suitable adhesive. The separator member 11 may also be incorporated in the same manner in the model 12, between the volume of the main body 7 corresponding to the top end 4b of the grain duct 4 and the volume corresponding to the horizontal projection adjacent to the second volume 7b of the mold cavity 7.

(10) In this embodiment, in order to produce the mold 1 from the non-permanent model 12, the model 12 is dipped in a slip, and is then dusted with a refractory sand. These steps of dipping and dusting can be repeated several times over until a shell has been formed of slip-impregnated sand that presents a desired thickness around the model 12.

(11) The model 12 coated in this shell can then be heated to melt and empty out the low-melting point material of the model 12 from the inside of the shell, while conserving the support rod 20 and the separator member 11. Thereafter, in a higher temperature baking step, the shell is sintered so as to consolidate the refractory sand in order to form the mold 1 in which the support rod 20 and the separator member 11 remain incorporated.

(12) The metal or metal alloy used in this casting method is poured while molten into the mold 1 via the pouring bush 5 and it fills the starter cavity 10, the selector channel 9, and the mold cavity 7 via the feed channel 8. Among metal alloys that are suitable for use in this method, there are in particular monocrystalline nickel alloys such as, in particular: AM1 and AM3 from SNECMA; and also others such as CMSX-2, CMSX-4, CMSX-6, and CMSX-10 from C-M Group; Ren 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 shows the compositions of these alloys:

(13) TABLE-US-00001 TABLE 1 Monocrystalline nickel alloy compositions in percentage by weight Alloy Cr Co Mo W Al Ti Ta Nb Re Hf C B Ni CMSX-2 8.0 5.0 0.6 8.0 5.6 1.0 6.0 Bal CMSX-4 6.5 9.6 0.6 6.4 5.6 1.0 6.5 3.0 0.1 Bal CMSX-6 10.0 5.0 3.0 4.8 4.7 6.0 0.1 Bal CMSX-10 2.0 3.0 0.4 5.0 5.7 0.2 8.0 6.0 0.03 Bal Rene N5 7.0 8.0 2.0 5.0 6.2 7.0 3.0 0.2 Bal Rene N6 4.2 12.5 1.4 6.0 5.75 7.2 5.4 0.15 0.05 0.004 Bal RR2000 10.0 15.0 3.0 5.5 4.0 Bal SRR99 8.0 5.0 10.0 5.5 2.2 12.0 Bal PWA1480 10.0 5.0 4.0 5.0 1.5 12.0 0.07 Bal PWA1484 5.0 10.0 2.0 6.0 5.6 9.0 3.0 0.1 Bal PWA1487 5.0 10.0 1.9 5.9 5.6 8.4 3.0 0.25 Bal AM1 7.0 8.0 2.0 5.0 5.0 1.8 8.0 1.0 Bal AM3 8.0 5.5 2.25 5.0 6.0 2.0 3.5 Bal

(14) While pouring, the mold 1 is maintained in a heater chamber 3 as shown in FIG. 1. Thereafter, in order to cause the molten metal to cool progressively, the mold 1 supported on a cooled and movable support 2 is extracted from the heater chamber 3 downwards along a main axis X. Since the mold 1 is cooled through its base 6 by the support 2, the molten metal begins solidifying in the starter cavity 10 and solidification propagates substantially vertically upwards in the mold 1 while it is being progressively extracted downwards from the heater chamber 3, with solidification following a front 50 as shown in FIG. 3. The choke formed by the selector channel 9, and also its baffle shape, nevertheless ensure that only one grain from among those initially seeded in the starter cavity 10 is able to continue to extend to the mold cavity 7.

(15) At the bottom end 4a of the grain duct 4, the cooling and solidification front 50 of the metal bifurcates, continuing to advance in the first volume 7a of the mold cavity 7 and also to advance in the grain duct 4, as shown in FIG. 4. Consequently, this cooling and solidification front 50 approaches substantially simultaneously the interface between the first and second volumes 7a and 7b of the mold cavity 7 and the top end 4b of the grain duct 4. Thus, because of the thermal conduction between the top end 4b of the grain duct 4 and the horizontal projection of the second volume of the mold cavity 7, the cooling and solidification front 50 can maintain in the second volume 7b a direction of advance that is substantially vertical, as shown in FIG. 5, as though the top end 4b of the grain duct 4 were actually in communication with the horizontal projection of the second volume of the mold cavity 7. This avoids any sudden change of direction in this advance in the second volume 7b, which might generate unwanted grains around the interface between the volumes 7a and 7b of the mold cavity 7.

(16) After the metal has cooled and solidified in the mold 1, the mold can be knocked out in order to release the metal part, which can then be finished by machining and/or surface treatment methods. Both knocking out the mold and performing finishing treatment on the part are made very significantly easier by the separation between the top end 4a of the grain duct 4 and the second volume 7b of the mold cavity 7, since it suffices to break a single connection between the metal part and the metal branch corresponding to the grain duct in order to separate them.

(17) Although the present invention is described with reference to a specific embodiment, it is clear that various modifications and changes may be undertaken thereon without going beyond the general ambit of the invention as defined by the claims. In addition, individual characteristics of the various embodiments mentioned may be combined in additional embodiments. Consequently, the description and the drawings should be considered in a sense that is illustrative rather than restrictive.