DIRECTIONAL SOLIDIFICATION COOLING FURNACE AND COOLING PROCESS USING SUCH A FURNACE

Abstract

A directional solidification cooling furnace for metal casting part comprises: a cylindrical internal enclosure having a vertical central axis and a mold support arranged in the internal enclosure; the internal enclosure comprising a casting zone and a cooling zone, the casting zone and the cooling zone being superposed one on the other; the casting and cooling zones being thermally insulated from each other when the mold support is arranged in the casting zone by means of a heat shield that is stationary and by means of a second heat shield that is carried by the mold support; the casting zone including at least a first heating device, and the cooling zone including a second heating device.

Claims

1. A directional solidification cooling furnace for metal casting part, the furnace comprising: a cylindrical internal enclosure having a vertical central axis; and a mold support arranged in the internal enclosure; the internal enclosure comprising: a casting zone; and a cooling zone, the casting zone and the cooling zone being superposed one on the other; the casting and cooling zones being thermally insulated from each other when the mold support is arranged in the casting zone by means of a heat shield that is stationary and by means of a second heat shield that is carried by the mold support; the casting zone including at least a first heating device, and the cooling zone including a second heating device, the first and second heating devices being configured so that the temperature of the casting zone is higher than the temperature of the cooling zone; and the cooling zone including an upper portion and a lower portion that are superposed one on the other and that are thermally insulated from each other by a third heat shield, the upper portion of the cooling zone including the second heating device.

2. A furnace according to claim 1, wherein the upper portion of the cooling zone is removable.

3. A furnace according to claim 1, wherein the second heating device comprises an induction susceptor.

4. A furnace according to claim 1, wherein the second heating device comprises an electrical resistance.

5. A furnace according to claim 1, wherein the internal enclosure has a diameter greater than or equal to 20 cm.

6. A furnace according to claim 1, wherein the casting zone has an upper portion and a lower portion that are thermally insulated from each other by a fourth heat shield, the upper portion including an upper heating device and the lower portion including a lower heating device.

7. A method of directional solidification cooling of a metal casting part using the furnace according to claim 1, the method comprising the steps of: fastening the upper portion of the cooling zone on the furnace; adjusting the casting zone to a casting temperature and the cooling zone to a cooling temperature, the temperature of the upper portion of the cooling zone being higher than or equal to 700 C.; and progressively cooling the metal casting part by moving the mold support inside the furnace from the casting zone towards the cooling zone.

8. A method according to claim 7, wherein the temperature difference between the casting zone and the liquid metal lies in the range 0 C. to 50 C., the temperature of the casting zone being lower than the temperature of the liquid metal.

9. A method according to claim 7, wherein the temperature of the upper portion of the cooling zone is greater than or equal to 700 C.

10. A method according to claim 7, wherein during cooling of the metal casting part, the cooling rate at a given point of the metal casting part is less than 0.30 C./s.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0052] The invention and its advantages may be better understood on reading the following detailed description of various embodiments of the invention given as non-limiting examples. This description refers to the accompanying sheets of figures, in which:

[0053] FIG. 1 is a side view of a shell mold including a casting cluster;

[0054] FIG. 2 is a diagrammatic section view of a cooling furnace;

[0055] FIG. 3A is a diagrammatic section view of the FIG. 2 furnace, the FIG. 1 mold being arranged in the casting zone, and FIG. 3B is a diagrammatic section view of the furnace and of the mold during directional solidification;

[0056] FIG. 4 is a graph showing how temperature varies at a point of a part for varying temperature of the removable portion; and

[0057] FIG. 5 shows the thermal stresses in a metal part, comparing the use of a conventional furnace with the use of an furnace in accordance with the present disclosure.

DETAILED DESCRIPTION OF EMBODIMENTS

[0058] An example furnace 20 of the present disclosure and an example cooling method by directional solidification for use with blades made by casting are described below with reference to FIGS. 1 to 5.

[0059] Blades are fabricated by a casting method. A first step in this casting method consists in fabricating a model of the blades and in grouping together a plurality of models so as to form a cluster enabling a mold to be fabricated, as described in the following step.

[0060] In a second step, a shell mold 1 is fabricated from the wax cluster.

[0061] The last operation of the second step consists in eliminating the wax of the cluster model from the shell mold 1. Wax is eliminated by raising the shell mold 1 to a temperature higher than the melting temperature of the wax.

[0062] In a third step, a cluster 10 of blades 12 (FIG. 1) is formed in the shell mold 1 by casting molten metal into the shell mold 1. Molten metal is cast into the shell mold 1 from the top portion of the mold, referred to as a casting bush 14. During this step, the shell mold 1 is in a casting zone A of a cooling furnace 20.

[0063] In a fourth step, the metal present in the shell mold is cooled and its solidifies in a cooling zone B of the cooling furnace 20.

[0064] Finally, in a fifth step, after the cluster 10 has been released from the shell mold 1 by a knocking-out method, each of the blades 12 is separated from the remainder of the cluster 10 and is finished by completion methods, e.g. machining methods.

[0065] The invention relates in particular to the cooling furnace 20 and to the method of solidification performed during the fourth step described above.

[0066] This solidification method, referred to as directional solidification is performed by means of the furnace 20 (FIG. 2).

[0067] The furnace 20 has a cylindrical wall 22 with a vertical central axis X, and a top wall 24 arranged at the top end of the cylindrical wall 22, perpendicularly to the axis X, so that the cylindrical wall 22 and the top wall 24 form an internal enclosure 26 of the furnace. The top wall includes an orifice 240 positioned substantially in the center of the wall 24.

[0068] The furnace is made up of a casting zone A and a cooling zone B that are superposed one on the other so that the casting zone A is above the cooling zone B. The casting and cooling zones A and B are thermally insulated from each other by a first heat shield 31, which may be made of a material that is not thermally conductive and that is inserted in the wall 22. For example, the first heat shield 31 may be made of compressed graphite paper or of a sandwich comprising a layer of felt compressed between two layers of graphite possessing emissivity in the range 0.4 to 0.8 as a function of temperature (e.g. as sold under the name Papeyx).

[0069] The furnace 20 also has a horizontal mold support 28 arranged inside the internal enclosure 26 and fastened on a jack 29 that serves to move the support 28 vertically upwards or downwards. The mold support 28 includes a second heat shield 32 so that when the mold 1 is positioned on the mold support 28, the mold 1 is thermally insulated from the remainder of the internal enclosure 26 that is situated under the second heat shield 32. Thus, when the mold 1 is in the casting zone A, it is thermally insulated from the cooling zone B by the first heat shield 31 and the second heat shield 32.

[0070] Furthermore, the cooling zone B itself has an upper portion B and a lower portion B, the upper and lower portions B and B being superposed one on the other so that the upper portion B is arranged above the lower portion B. The upper and lower portions B and B are thermally insulated from each other by a third heat shield 33. The upper portion B also has a heating device 60 comprising a susceptor 62 and a heating coil 64. The lower portion B constituting the bottom portion of the furnace 20 is connected to a stand 70.

[0071] The upper portion B of the cooling zone B is removable. The heating device 60 is thus adapted as a function of the parts that need to be cooled, of their dimensions, of their alloys. This also makes it possible to simplify and facilitate maintenance operations for operators.

[0072] The casting zone A also has an upper portion A and a lower portion A, the upper and lower portions A and A being superposed one on the other such that the upper portion A is arranged above the lower portion A. The upper and lower portions A and A are thermally insulated from each other by a fourth heat shield 34. The upper portion A includes a heating device 40 comprising a susceptor 42 and a heating coil 44. The susceptor 42 may be a graphite tube arranged inside the internal enclosure 26 so as to be pressed against the wall 22 of the furnace 20. The heating coil 44 may be a copper coil surrounding the outer wall 22, serving to create a magnetic field that has the effect of heating the susceptor 42. The susceptor thus also heats the internal enclosure 26 by radiation. Furthermore, the internal enclosure 26 may be evacuated, so as to preserve the graphite susceptor from any oxidation. Alternatively, the internal enclosure 26 may also be partially evacuated with an inert gas, e.g. argon, being present.

[0073] The lower portion A also has a heating device 50 comprising a susceptor 52 and a heating coil 54, the hater device 50 of the lower portion A being distinct from the heating device 40 of the upper portion A, so as to be able to heat the portions independently of each other, and thereby control the temperature gradient within the internal enclosure 29 in the casting zone A.

[0074] In the present example, the inside diameter of the cylindrical wall lies in the range 200 millimeters (mm) to 1000 mm. The casting zone extends vertically over a height of 1 meter (m). These dimensions make it possible to work with clusters of larger size, including a larger number of blades of height that may lie in the range 200 mm to 300 mm. The removable upper portion B extends vertically over a height lying in the range 150 mm to 300 mm.

[0075] There follows a description of a method of cooling metal cast blades by directional solidification using the above-described furnace.

[0076] Firstly, the upper portion B of the cooling zone is fastened to the furnace 20.

[0077] Beforehand, a casting step, as shown in FIG. 3A, consists in placing the mold 1 in the casting zone A and in positioning it on the support 28, which is itself situated in the casting zone A. The mold 1 is positioned in such a manner that the casting bush 14 faces the orifice 240 in the top wall 24 of the furnace 20. Metal in the liquid state at a temperature lying in the rang 1480 C. to 1600 C. and contained in a crucible 80 is then poured into the bush 14 via the orifice 240 until the mold 1 is almost completely filled, the casting bush 14 being filled in part only.

[0078] In parallel with this casting step, the heating devices 40 and 50 are adjusted so as to heat the mold 1 by thermal radiation so as to keep it at a temperature lying in the range 1480 C. to 1600 C. The temperature of the casting zone is thus less than or equal to the temperature of the liquid metal, the difference lying in the range 0 C. to 50 C. Thus, the temperature of the liquid metal cast into the mold 1 remains higher than the melting temperature of the metal so as to avoid unwanted solidification in the mold 1 throughout the entire casting step. Furthermore, the mold 1 is thermally insulated from the cooling zone B by the first and second shields 31 and 32.

[0079] Once the casting step has finished, i.e. when the mold 1 is completely filled with liquid metal, with the exception of the layer of metal that has already solidified and that is in contact with the bottom of the mold, and after a stage of waiting prior to lowering the support, the solidification stage begins.

[0080] The support 28 is then moved downwards by the jack 29 so that the mold passes little by little from the casting zone A to the cooling zone B (FIG. 3B). The temperature in this zone is then set to a temperature of 700 C. or higher than 700 C., while being lower than the melting temperature of the metal so as to cause the metal to solidify, while the casting zone A continues to be maintained at a temperature in the range 1500 C. to 1530 C. Since the lower portion of the mold 1 is the first to penetrate into the cooling zone, the liquid metal thus begins to solidify in this lower portion of the mold. A solidification front is thus created as represented symbolically by a line 12a in FIG. 3B, which front corresponds to the interface between the liquid and solid phases of the metal. This solidification front 12a moves upwards in the reference frame of the mold 1 as the mold penetrates progressively into the cooling zone B, on the principle of directional solidification. Thus, as the support 28 continues to move downwards, the mold 1 ends up having its full height located in the bottom portion B of the cooling zone, such that all of the metal present in the mold 1 is in the solid state. The solidification stage has thus finished. The total duration of the cooling method may for example lie in the range 3600 seconds (s) to 7600 s, with the support 28 moving at a speed lying in the range 1 millimeter per second (mm/s) to 10 mm/s.

[0081] The blades 12 that are obtained are blades that are monocrystalline and hollow or solid, and made of nickel-based alloys. The term nickel-based alloy it used to designate alloys in which the weight content of nickel is in the majority. It may be understood that nickel is thus the element having the weight content in the alloy that is the greatest. These more fragile hollow or solid blades may present defects if the temperature gradients are not properly controlled during the cooling and the solidification. The above described furnace and method, and in particular the removable portion B serve to limit or even eliminate these risks by setting the temperature of this portion to a temperature that is high enough (higher than or equal to 700 C.) to minimize the temperature gradients that exist in the blades 12 in the direction of directional solidification, i.e. when the mold 1 is situated both in the casting zone A and in the cooling zone B.

[0082] FIG. 4 shows how the temperature varies at a point on the leading edge of a blade 12 for varying temperatures of the removable portion B during the solidification stage (S) and during the cooling stage (R). The dotted-line curve shows the reference situation using a copper cooler serving to maintain a cooling zone at a temperature of about 300 C., the continuous fine-line curve shows a situation using the furnace when the removable portion B is heated to 700 C., and the continuous bold-line curve shows the situation when the removable portion B is heated to 1000 C. The other curves show intermediate situations.

[0083] Although the differences between each configuration are little marked during the solidification stage, the influence of the removable portion is particularly visible during the cooling stage, starting from 700 C. For that temperature, the rate of cooling, corresponding to the slope of the curve, is 0.23 C./s such that the temperature at this point is 57 C. higher than in the reference situation. For the removable portion at a temperature of 1000 C., the rate of cooling is 0.18 C./s, such that the temperature at this point is 165 C. higher than in the reference situation. These lower rates of cooling give rise to temperature gradients that are lower, and thus to stresses that are likewise lower in the metal casting during cooling.

[0084] Furthermore, FIG. 5 shows thermal stresses in the metal of a blade by comparing the use of a conventional furnace (blades (b) on the right of FIG. 5) and an furnace of the present disclosure (blades (a) on the left in FIG. 5). The upper and lower blades show respectively the two main faces of a single blade. In FIG. 5, for the blades (b) corresponding to the conventional furnace, the zones 90 indicate zones of the blade where the stresses were the greatest. For the blades (a) corresponding to the furnace of the present disclosure, the zones 92 show zones of the blade where the stresses were the greatest. It may thus be seen that the zones 92 extend over a smaller area of the blade than do the zones 90, such that the stresses are smaller in blades cooled by the furnace 20 of the present disclosure than in a conventional furnace. More precisely, the stresses in the metal may be reduced by about 24% by means of the furnace 20 and the method of the present disclosure.

[0085] Although the present invention is described with reference to specific embodiments, it is clear that modifications and changes may be made to those embodiments without going beyond the general ambit of the invention as defined by the claims. In particular, individual characteristics of the various embodiments shown and/or mentioned may be combined in additional embodiments. Consequently, the description and the drawings should be considered as being illustrative rather than restrictive. For example, the cooling zone may have two heating devices superposed one on the other.

[0086] It is also clear that all of the characteristics described with reference to a method may be transposed, singly or in combination, to a device, and vice versa, all of the characteristics described with reference to a device may be transposed, singly or in combination, to a method.