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

Abstract

A method for manufacturing a metal component using lost-wax casting is provided. The component is made of, for example, nickel alloy, with a columnar or monocrystalline structure with at least one cavity of elongate shape. The method includes creating a wax model of the component with a ceramic core corresponding to the cavity, creating a shell mold around the model, placing the mold in a furnace, with the base standing on the sole of the furnace, pouring molten alloy into the shell mold, solidifying the poured metal by gradual cooling from the sole in a direction of propagation.

Claims

1. A method for manufacturing, using lost-wax casting, a metal component with a columnar or monocrystalline structure with at least one elongate-shaped cavity, comprising the steps of: producing a wax model of the component with a ceramic core corresponding to said cavity, the ceramic core comprising a first holding span at a longitudinal end and a second holding span at an opposite end; producing a shell mold around the wax model, the shell mold comprising a base, the first holding span of the ceramic core being on the same side as the base of the shell mold; eliminating the wax by dewaxing the shell mold; placing the shell mold in a furnace, the base being placed on a hearth of the furnace; pouring a molten alloy into the shell mold; solidifying the poured molten alloy by gradual cooling from the hearth in a propagation direction, wherein, during the step of producing the wax model, the second holding span comprises first surfaces that are not parallel to said propagation direction, and second surfaces that are parallel to said propagation direction; wherein, during the step of producing the wax model, the first surfaces are covered initially by a deposit of wax, and the second surfaces, which are not covered initially and previously by a deposit of wax, are directly and integrally coated by a layer of varnish, said layer of varnish having a thickness of between 3 and 5 hundredths of a millimeter; wherein the ceramic core is secured to the shell mold by an anchor between the first span of the ceramic core and an internal wall of the shell mold; wherein the second span of the ceramic core is slidably held in said internal wall of the shell mold by said layer of varnish; wherein, during and after the step of producing the shell mold, said layer of varnish prevents said internal wall of the mold from sticking to the ceramic core in said second surfaces, wherein, after the step of producing the shell mold, said second surfaces come into contact with said internal wall of the mold through said layer of varnish; wherein, during the step of dewaxing the shell mold, said layer of varnish is eliminated from said second surfaces, as well as the wax covering said first surfaces so that a free space is created between the second holding span of the ceramic core and said internal wall of the shell mold; wherein, during the progression of the solidification of the poured molten alloy, said free space left by the layer of varnish and by the wax is kept so as to prevent the second holding span of the ceramic core from coming into contact with said internal wall of the shell mold when the core expands.

2. The method according to claim 1, wherein said anchor comprises a rod passing through the first holding span and being embedded in said internal wall of the shell mold.

3. The method according to claim 2, wherein said rod is made from ceramic.

4. The method according to claim 1, for manufacturing a plurality of components, the models of said components being collected together in a cluster inside said shell mold.

5. The method according to claim 1, wherein the metal component has a columnar structure.

6. The method according to claim 1, wherein the metal component has a monocrystalline structure.

7. The method according to claim 1, wherein the metal component being a turbine engine blade, the first holding span being in an extension of an apex of a vane of the blade, the second holding span being in an extension of a root of the blade.

8. The method according to claim 1, wherein the hearth is able to move vertically between a hot region where an alloy is molten and a cold region for solidifying the alloy, the hearth itself being cooled.

9. The method according to claim 1, wherein the molten alloy includes a nickel alloy.

10. The method according to claim 1, further comprising cooling the hearth of the furnace.

11. The method according to claim 1, wherein the hearth is configured to provide directional solidification.

12. The method according to claim 1, wherein the deposit of wax has a thickness of approximately 1% of a length of the metal component.

13. The method according to claim 1, wherein after the step of dewaxing, said first surfaces of the second holding span do not come into contact with said internal wall of the shell mold.

14. The method according to claim 1, wherein after the step of dewaxing of the shell mold and eliminating of said layer of varnish, said free space comprises a first space formed by the dewaxing of said first surfaces, and a second space formed by eliminating the layer of varnish from said second surfaces of the second holding span.

15. The method according to claim 14, wherein said second space forms a sliding holding of the second holding span on said internal wall of the shell mold.

16. The method according to claim 15, wherein said sliding holding is a longitudinally guiding of the second holding span along said internal wall of the shell mold, so as to prevent the shell mold from exerting a stress on the ceramic core.

17. The method according to claim 14, wherein said first space left by the wax has a thickness of approximately 1 mm and the metal component has a length of 100 to 200 mm, and said second space left by the layer of varnish has a thickness between 3 and 5 hundredths of a millimeter.

18. The method according to claim 1, wherein the deposit of wax has a thickness of approximately 1 mm and the metal component has a length of 100 to 200 mm.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) Other characteristics and advantages will become apparent from the following description of an embodiment of the invention given by way of non-limitative example with reference to the accompanying drawings, on which

(2) FIG. 1 depicts a turbine engine blade that can be obtained according to the method of the invention;

(3) FIG. 2 depicts schematically a ceramic core for a turbine engine blade;

(4) FIG. 3 depicts the core of FIG. 2 seen in profile;

(5) FIG. 4 depicts schematically a wax model with the core of FIG. 2; p FIG. 5 depicts the shell mould seen in longitudinal section through the core;

(6) FIG. 6 depicts an example of a furnace which permits the directed solidification of molten metal in a shell mould;

(7) FIG. 7 is an enlarged view of the top end of the shell mould shown in FIG. 5.

DESCRIPTION OF AN EMBODIMENT OF THE INVENTION

(8) The present invention relates to a method for manufacturing metal components made from a nickel-based alloy for obtaining, by means of a suitable directed solidification, a columnar or monocrystalline crystalline structure.

(9) The invention relates more particularly to the manufacture of turbine engine blades like the one shown in FIG. 1; a blade 1 comprises a vane 2, a root 5 for attachment thereof to a turbine disc, and an apex 7 with a heel where applicable. Because of the operating temperatures of the turbine engine, the blades are provided with an internal cooling circuit through which a cooling fluid travels, generally air. A platform 6 between the root and the vane constitutes a portion of the radially inner wall of the gas stream. The component depicted here is a movable blade but the invention also applies to a distributor or to any other component having a core.

(10) Because of the complexity of the cooling circuit inside the component, it is advantageous to produce it by lost-wax casting with a ceramic core for forming the cavities of the cooling circuit.

(11) FIGS. 2 and 3 depict schematically a core with a simplified form, made from ceramic, used for forming the internal cavities of a turbine engine blade. The elongate-shaped core 10 comprises a branch or a plurality of branches 11 separated by spaces 12 so as, after the pouring of the metal, to form the partitions between the cavities; in the example depicted, the core comprises two branches 11 separated by a space 12. At one end, the core is extended by a span or lug 14, the function of which is to hold the core during the manufacture of the component, but which does not necessarily correspond to a part of the component, once the latter is finished. At the opposite end the core comprises a second span 16 also for holding the core during the manufacturing steps. It can be seen in FIG. 3 that the core as depicted is relatively thin compared with its length. It will be understood that, the thinner the core with respect to its length, the more sensitive it will be to buckling.

(12) This core is placed in a mould for manufacturing the wax model. The cavity of this mould is in the shape of the component to be obtained. By injecting wax into this mould, the model of the component is obtained. The spans 14 and 16 are used for holding the core in the wax mould. FIG. 4 depicts schematically this wax model 20 with the core 10 in broken lines. The model extends at a first end 24 in the extension of the vane so as to cover the span 14 and at the opposite end 26 it extends at the root. It will be noted that a portion 16A of the span 16 is not covered with wax. This portion 16A comprises surfaces parallel to the axis of the core and is coated with a varnish, the function of which is explained below.

(13) Several models are generally assembled in a cluster so as to manufacture several components simultaneously. The models are for example disposed in a drum in parallel around a vertical central cylinder and held by the ends. The bottom part is mounted on an element intended to provide the nucleation of the crystalline structure. The following step consists of forming a shell mould around the model(s). For this purpose, as is also known, the assembly is dipped in slips so as to deposit the refractory ceramic particles in successive layers. Finally, the mould is consolidated by heating and the wax eliminated by the dewaxing operation.

(14) FIG. 5 shows schematically, in longitudinal cross section, the arrangement of the invention between the core 10 and the shell 30 with regard to a single model 20.

(15) The first span 14 is held in the mould 30 by a refractory ceramic rod 40 which passes through it and extends into the wall of the mould 30, being embedded therein. The rod 40 has been fitted before the shell mould was produced, after the model was pierced at the span 14. The piercing has a diameter slightly greater than that of the rod so that stresses are not created between the rod and the span and so that the rod provides correct positioning of the core in the model.

(16) The second span 16, opposite to the first, is initially coated with a layer of varnish 17 on the part 16A of the core that is not covered with wax and which, after formation of the shell mould, comes into direct contact with the internal wall of the mould. After dewaxing of the mould, as can be seen in FIG. 5, the layer that has disappeared leaves a free space between the span 16 of the core and the wall of the shell mould. The reference 17 designates this free space left by the layer of varnish. This space 17 is thin, i.e. 3 to 5 hundredths of a millimetre. It forms a means for the sliding holding of the second span 16 on the wall of the shell 30.

(17) Moreover, the surfaceshere the horizontal surface 16Bthat are not parallel to the axis of the progression of the solidification are covered initially by a deposit of wax 18. This deposit of wax leaves a free space after dewaxing, with the same reference 18, which prevents the span 16 of the core coming into contact with the wall of the shell when the core expands. It thus prevents the stressing of the core. Typically, the thickness of this deposit of wax is approximately one millimetre for components having a length of 100 to 200 mm, that is to say approximately 1% of the length of the component.

(18) By not being stressed the core is not liable to be buckled and the initial wall thicknesses of the component between the wall of the mould and the core are preserved.

(19) FIG. 5 shows, in cross section along the component, the shell mould 30 and the core 10 inside the mould with the branches 11 and the spans 14 and 16. The cross section of the core is taken along the line VV in 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 mould and the core to be filled by the metal. The rod 40 passes through the first span 14; it is sufficiently long to be anchored in the walls of the shell mould 30. In this way, the core 10 is positioned inside the shell mould 30.

(20) After dewaxing and consolidation, the mould is placed on the hearth of a furnace equipped for directed solidification. Such a furnace 100 is shown in FIG. 6. A chamber 101 can be seen therein, provided with heating elements 102. An orifice 103 supplying molten metal communicates with a crucible 104 that contains the molten metal load and which, by tilting, fills the shell mould 30 disposed on the hearth 105 of the furnace. The hearth is able to move vertically, see the arrow, and is cooled by the circulation of water in a circuit 106 inside its plate. The mould is supported by its base on the cooled hearth. The bottom part of the mould is open onto the hearth through a nucleation member.

(21) The manufacturing method as explained in the preamble of the application comprises the pouring of molten metal from the crucible 104 directly into the mould 30, which is maintained at a sufficient temperature to keep the metal melted, by the means 102 for heating the chamber 101, and where it fills the voids 30 between the core 10 and the wall of the mould 30. As the base of the mould is in thermal contact with the hearth through the nucleation element, the metal solidifies, forming a crystalline structure that propagates upwards. The hearth 105 is cooled continuously and is lowered gradually out of the heated chamber. In the case of a monocrystalline structure, a grain selector is interposed between the nucleation and the solidification, as is known per se.

(22) The high temperature differences create stresses between the various regions of the mould with the metal. Through the arrangement of the invention and the rod 40, the core is held by anchoring the first span 14 solely in the lower solidification initialisation region. As can be seen in FIG. 7 the core is free to expand differentially in the direction of its length with respect to the shell 30 since, at the opposite end of the first span, the second span 16 is guided along the wall of the mould by means of the free space 17 left by the layer of varnish, eliminated during the dewaxing of the mould.

(23) In addition, the surfaces of the second span 16here the horizontal surface 16Bthat are not parallel to the axis of progression of the solidification, by virtue of the free space 18 formed by the depositing of wax, do not come into contact with the wall of the shell. In this way the stressing of the core is avoided. Typically, the thickness of this space corresponding to the depositing of wax is approximately one millimetre for components having a length of 100 to 200 mm, that is to say approximately 1% of the length of the component. By not being stressed the core is not liable to be buckled and the initial wall thicknesses of the component between the wall of the mould and the core are preserved.

(24) Once the metal has cooled, the mould is broken and the components are extracted and sent to the finishing workshop.