EP1502679A1

EP1502679A1 - Method for casting a directionally solidified or single crystal article

Info

Publication number: EP1502679A1
Application number: EP03405567A
Authority: EP
Inventors: Martin Balliel
Original assignee: Alstom Technology AG
Current assignee: General Electric Technology GmbH
Priority date: 2003-07-30
Filing date: 2003-07-30
Publication date: 2005-02-02
Anticipated expiration: 2023-07-30
Also published as: EP1502679B1

Abstract

It is disclosed a method of casting a directionally solidified (DS) or single crystal (SX) article with a casting furnace comprising a heating chamber (4), a cooling chamber (5), a separating baffle (3) between the heating and the cooling chamber (4, 5). The method comprises the preheating a shell mould (12) within a separate preheating chamber (22), loading the shell mould (12) into the heating chamber (4), where liquid metal (6a) is fed into the shell mould (12), withdrawing the shell mould (12) from the heating chamber (4) through the baffle (3) to the cooling chamber (5) thereby directionally solidifying the liquid metal (15), and finally unloading the mould (12) to a separate cool-down chamber (23).

Description

FIELD OF INVENTION

The invention relates to a method for casting a directionally solidified article according to the independent claim.

STATE OF THE ART

The invention proceeds from a process for producing a directionally solidified casting and from an apparatus for carrying out the process as is described, for example, in US-A-3,532,155. The process described serves to produce the guide vanes and rotor blades of gas turbines and makes use of a furnace which can be evacuated. This furnace has two chambers which are separated from one another by a water-cooled wall and are arranged one above the other, the upper chamber of which is designed so that it can be heated and has a pivotable melting crucible for receiving material to be cast, for example a nickel base alloy. The lower chamber, which is connected to this heating chamber by an opening in the water-cooled wall, is designed so that it can be cooled and has walls through which water flows. A driving rod which passes through the bottom of this cooling chamber and through the opening in the water-cooled wall bears a cooling plate through which water flows and which forms the base of a casting mould located in the heating chamber.
When carrying out the process, first of all the alloy which has been liquefied in the melting crucible is poured into the casting mould located in the heating chamber. A narrow zone of directionally solidified alloy is thus formed above the cooling plate forming the base of the mould. As the casting mould is moved downwards into the cooling chamber, this mould is guided through the opening provided in the water-cooled wall. A solidification front which delimits the zone of directionally solidified alloy migrates from the bottom upwards through the entire casting mould, forming a directionally solidified casting.
A further process for producing a directionally solidified casting is disclosed in US-A-3,763,926. In this process, a casting mould filled with a molten alloy is gradually and continuously immersed into a tin bath heated to approximately 260°C. This achieves a particularly rapid removal of heat from the casting mould. The directionally solidified casting formed by this process is distinguished by a microstructure which has a low level of inhomogeneities. When producing gas turbine blades of comparable design, it is possible using this process to achieve α values which are almost twice as high as when using the process according to US-A-3,532,155. However, in order to avoid unwanted gas-forming reactions, which can damage the apparatus used in carrying out this process, this process requires a particularly accurate temperature control. In addition, the wall thickness of the casting mould has to be made larger than in the process according to US-A-3,532,155.
US-A-5,168,916 discloses a foundry installation designed for the fabrication of metal parts with an oriented structure, the installation being of a type comprising a casting chamber communicating with a lock for the introduction and extraction of a mould, via a first opening sealable by a first airtight gate apparatus for casting and for cooling the mould placed in the chamber. In accordance with the invention, the installation includes, in addition, a mould preheating and degassing chamber communicating with the lock via a second opening sealable by a second airtight gate.
US-A-5,921,310 discloses a process which serves to produce a directionally solidified casting and uses an alloy located in a casting mould. The casting mould is guided from a heating chamber into a cooling chamber. The heating chamber is here at a temperature above the liquidus temperature of the alloy, and the cooling chamber is at a temperature below the solidus temperature of the alloy. The heating chamber and the cooling chamber are separated from one another by a baffle, aligned transversely to the guidance direction, having an opening for the casting mould. When carrying out the process, a solidification front is formed, beneath which the directionally solidified casting is formed. The part of the casting mould which is guided into the cooling chamber is cooled with a flow of inert gas. As a result, castings which are practically free of defects are achieved with relatively high throughput times.
Due to economical considerations, in the process according to US-A-5,921,310 generally the highest possible number of components is cast in one mould. For two or more components per mould the components with their shell mould around them shadow each other substantially from the thermal radiation from the surrounding heaters in the heating chamber, and in the cooling chamber shadow each other from the gas jets for enhanced cooling below the baffle and for outgoing radiation from the moulds into the cooling chamber. This causes an inhomogeneous heat input to and output from the components. Such shadowed mould surfaces are heated less than other adjacent mould surfaces at the same mould height when they are in the heating chamber, and they are cooled less, when they are below the baffle in the cooling chamber. In consequence this may cause the isotherms at the solidification front to bend substantially as the reductions in heating and respectively cooling are not necessarily equal. Additionally, it causes the thermal gradient at the solidification front to drop substantially due to the decreased heat flux from a shadowed and therefore relatively colder area in the heating chamber to a shadowed and therefore relatively hotter area below the baffle in the cooling chamber. This local drop in thermal gradient in conjunction with the risk of bending of the isotherms at the solidification front causes a decrease in metallurgical quality of the components, in particular a higher risk for freckle formation, increase in dendrite arm spacing, higher risk for stray grain formation and for directionally solidified (DS) polycrystals a higher risk for inclined DS grain boundaries and consequently coarsening of DS grains. Overall such a decrease in metallurgical quality becomes worse by increasing the number of components cast per mould and thereby limits the number of components per mould, when the quality drops below the acceptance limit.

SUMMARY OF THE INVENTION

It is aim of the present invention as written in the claims to find a method for manufacturing a series of directionally solidified articles which avoids the disadvantages of the prior art and provides a higher productivity with a substantially improved metallurgical quality of the components at the same time.
This process is substantially more economical than simply casting one component per mould in a conventional gas cooling equipment, as substantially more components are cast in a given time period and a series of components can be cast with a high degree of automation. This means a substantially increased productivity and reduced casting cost as: the casting equipment has a substantially increased production capacity in a given time period, and other casting equipment needed to cover a production volume may be reduced or eliminated and the requirement for human work, e.g. loading, unloading, operating and control, is substantially reduced on a per component basis. Additionally, this process provides a substantially improved metallurgical quality, as single components with shell mould around them are cast as separate moulds and therefore do not shadow each other from radiation from the heaters in the heating chamber and in the cooling chamber from cooling gas jets coming from the nozzles arranged below the baffle and for outgoing radiation from the moulds into the cooling chamber. To minimize the shadowing effect, if two or three articles with a longer and a shorter principal extension perpendicular to the withdrawal direction are cast, the articles within the shell mould are positioned in a row so that the shorter extensions face each other with a minimum distance of about the shorter extension and the longer extensions are aligned parallel to each other.
Further advantageous embodiments of the invention are written in the dependent claims.

SHORT DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention are illustrated in the accompanying drawings, in which

Fig. 1: shows a schematic view of a preferred embodiment of an apparatus for carrying out the method according to the invention.
Fig. 2: illustrates a schematic view of a second embodiment of an apparatus for carrying out the method according to the invention and
Fig. 3: shows the cross section along the line III - III in Fig. 2.

The drawings show only the elements important for the invention. Same elements will be numbered in the same ways in different drawings.

PREFERRED EMBODIMENT OF THE INVENTION

The invention of casting directionally solidified (DS) or single crystal (SX) articles such as blades or vanes or other part gas turbine engines is described in greater detail below with reference to an exemplary embodiment. In this case, Fig. 1 shows in diagrammatic representation a preferred embodiment of an apparatus for carrying out the process according to the present invention. The apparatus shown in Fig. 1 has a vacuum chamber 2 which can be evacuated by means of a vacuum system 1. The vacuum chamber 2 accommodates two chambers 4, 5 which are separated from one another by a baffle (radiation and gas flow shield) 3, which may be extended with flexible fingers or brushes 21, and are arranged one above the other, and a pivotable melting crucible 6 for receiving an alloy, for example a nickel base superalloy. The upper one 4 of the two chambers is designed so that it can be heated. The lower chamber 5, which is connected to the heating chamber 4 through an opening 7 in the baffle 3, contains a device for generating and guiding a stream of gas. This device contains a cavity with orifices or nozzles 8, which point inwardly onto a casting mould 12, as well as a system for generating gas flows 9. The gas flows emerging from the orifices or nozzles 8 are predominantly centripetally guided. A driving rod 10 passing for example through the bottom of the cooling chamber 5 bears a cooling plate 11, through which water may flow if appropriate and which forms the base of a casting shell mould 12. By means of a drive acting on the driving rod 10, this casting shell mould 12 can be guided from the heating chamber 4 through the opening 7 into the cooling chamber 5. For the reason of the present invention, the cooling chamber 5 could as well be a Liquid Metal Cooling (LMC)-Bath as known from US-B1-6,311,760 or US-A-3,763,926, furthermore the cooling chamber 5 could as well be a vacuum chamber with water-cooled walls as known from US-A-3,532,155 or a fluidized bed as known from US-A1-2002/0170698.
Above the cooling plate 11, the casting shell mould 12 has a thin-walled part 13, for example 10 mm thick, made of ceramic, which can accommodate at its bottom end towards the cooling plate 11 one or several single crystal seeds promoting the formation of single crystal articles and/or a helix initiator. By being lifted off from the cooling plate 11 or being put down on the cooling plate 11, the casting shell mould 12 can be opened or closed, respectively. At its upper end, the casting shell mould 12 is open and can be filled with molten alloy 15 from the melting crucible 6 by means of a filling device 14 inserted into the heating chamber 4. Electric heating elements 16 surrounding the casting shell mould 12 in the heating chamber 4 keep that part of the alloy which is located in the part of the casting shell mould 12 on the heating chamber 4 side above its liquidus temperature.
The cooling chamber 5 is connected to the inlet of a vacuum system 17 for removing the inflowing gas from the vacuum chamber 2 and for cooling and purifying the gas removed.
In order to produce a directionally solidified casting, first of all the casting shell mould 12 is brought into the heating chamber 4 by an upwards movement of the driving rod 10 (shown in dashed lines in the figure). Alloy which has been liquefied in the melting crucible 6 is then poured into the casting shell mould 12 by means of the filling device 14. A narrow zone of directionally solidified alloy is thus formed above the cooling plate 11 which forms the base of the mould (not shown in the figure).
As the casting shell mould 12 moves downwards into the cooling chamber 5, the ceramic part 13 of the casting shell mould 12 is successively guided through the opening 7 provided in the baffle 3. A solidification front 19 which delimits the zone of directionally solidified alloy migrates from the bottom upwards through the entire casting shell mould 12, forming a directionally solidified casting 20.
At the start of the solidification process, a high temperature gradient and a high growth rate of solid are achieved, since the material which is poured into the shell mould 12 initially strikes the cooling plate 11 directly and the heat which is to be removed from the melt is led from the solidification front through a comparatively thin layer of solidified material to the cooling plate 11. When the base of the casting shell mould 12, formed by the cooling plate 11, has penetrated a few millimetres, for example 5 to 50 mm, measured from the underside of the baffle 3, into the cooling chamber 5, inert compressed gas which does not react with the heated material, for example a noble gas, such as helium or argon, or another inert fluid is supplied from the orifices or nozzles 8. The inert gas flows emerging from the orifices or nozzles 8 impinge on the surface of the ceramic part 13 and are led away downwards along the surface. In the process, they remove heat q from the casting shell mould 12 and thus also from the already directionally solidified part of the casting shell mould content.
The inert gas blown into the cooling chamber 5 can be removed from the vacuum chamber 2 by the vacuum system 17, cooled, filtered and, once it has been compressed to a few bar, fed to pipelines 18 which are operatively connected to the orifices or nozzles 8.
An exemplary embodiment of the present invention consists of a serial loading mechanism to feed a series of individual shell moulds 12 with a cooling plate 11 with only one to a maximum number of three cast components into a preheating chamber 22 which is arranged separately from the heating chamber 4. The preheating chamber 22 may be individually evacuated or at reduced pressure with inert gas to preheat the individual shell moulds 12. To transfer the shell moulds 12 into the heating chamber 4, both chambers, the preheating chamber 22 and the heating chamber 4, are connected directly with each other by temporarily opening a segment of the heater element 16 or through an opening within the heater element 16. This direct connection between the two chambers 22, 4 minimizes cooling and reheating of the moulds 12, which applies detrimental thermal strains to the in hot condition relatively weak molds 12, minimizes the number of transfer steps and associated accelerations of the preheated shell moulds 12, which may damage the in hot condition relatively weak molds 12, and minimizes heat losses. As the cooling plate 11 was already mounted to the shell mould 12 in cold condition within the preheating chamber, no mounting of preheated moulds 12 is present, which eliminates detrimental accelerations due to mounting of the moulds 12 in hot condition, and eliminates thermal shock and therefore risk of cracking and distortion of the base of the preheated moulds 12, which occurs, if the moulds 12 are mounted in hot condition to the cooling plate 11 as the hot mould base comes in direct contact with the cold cooling plate 11. As indicated by an arrow in Fig. 1, the shell mould 12 with the cooling plate 11 is then loaded sideways from the preheating chamber 22 into the heating chamber 4 of the casting furnace. The heating chamber 4 is under vacuum or reduced pressure with inert gas. In the heating chamber 4 the shell moulds 12 are filled with liquid metal 15 from the crucible 6 with filling device 14. The article is then cast by withdrawing the individual shell mould 12 into the cooling chamber 5 which is connected to the heating chamber 4 through the opening 7 and baffle 3 which may be extended with flexible fingers or brushes 21. After finishing the solidification of the article, the shell mould 12 is unloaded from the cooling chamber 5 into a separate cool-down chamber 23 which may be evacuated or filled at a given time at reduced or ambient pressure with inert gas. The gas accelerates the cool-down and may allow a shorter throughput time, if the casting of shell moulds 12 is faster than the cool-down in vacuum or at reduced pressure with inert gas. The separate cool-down chamber 23 allows a substantial increase in productivity as without the cool-down chamber 23 a subsequent shell mould has to wait for a sufficient cool-down of the preceding mould, e.g. upon venting with air to prevent oxidation of the cast article and, if the heating chamber 4 cannot be sealed airtight from the cooling chamber 5, also to prevent oxidation of the heating elements 16. Additionally, without the cool-down chamber 23, to prevent oxidation of the metal 15 or of the subsequent cast article a subsequent mould has to wait until the cooling chamber 5 is vented and re-evacuated, or until re-evacuated or washed with inert gas to remove oxygen and filled with inert gas at reduced pressure. The moulds 12 are then unloaded to a final cool-down and storage area (not shown in Fig. 1). All mentioned steps are repeated automatically with a series of individual shell moulds 12 one after the other to increase productivity of the casting furnace.
This process provides a substantially improved metallurgical quality of the components, as single components with shell mould 12 around them are cast as separate moulds 12 and therefore do not shadow each other from radiation from the heating elements 16 in the heating chamber 4, from cooling gas jets below the baffle 3 in the cooling chamber 5, and for outgoing radiation from the mould 12 into the cooling chamber 5. Additionally, the process according to the present invention is substantially more economical than simply casting one component per mould 12 in a conventional gas cooling equipment, as substantially more components are cast in a given time period and a series of components can be cast with a high degree of automation. This means a substantially increased productivity and reduced casting cost as:
1. the casting equipment has a substantially increased production capacity in a given time period, and other casting equipment needed to cover a production volume may be reduced or eliminated,
2. the requirement for human work, e.g. loading, unloading, operating and control, is substantially reduced on a per component basis.
Using shell moulds 12 for casting two or even three components at one time increases the economy of the process by reducing the number of shell moulds 12 required for a given amount of components, while the metallurgical quality of the components would be still acceptable. Provided the cast articles have generally two principal extensions perpendicular to the withdrawal direction, a longer extension in one direction and shorter extension in a second direction, then the cast articles within the shell mould 12 are positioned in a row so that the shorter extensions face each other with a minimum distance of about the shorter extension and the longer extensions are aligned parallel to each other. Again, this minimizes the negative shadowing effect.
In a second embodiment of the invention as seen in Fig. 2 and 3 the heating chamber 4 and baffle 3 are in the shape of a slot 24, e.g. with a small width sufficient for one shell mould 12, with a length to accommodate several moulds 12 and with a similar height as in the conventional gas cooling process. Several shell moulds 12 are withdrawn concurrently from the heating chamber 4 into the cooling chamber 5 in a motion combining the vertical direction and the direction sideways along the length of the slot 24. As indicated in Fig. 3, which shows the cross section along the line III - III in Fig 2, the baffle 3, which may be extended with flexible fingers or brushes (as indicated within Fig. 1, not shown in Fig. 2 and 3), and the positioning of nozzles 8 is arranged along the sides of the slot 24. For automatically withdrawing the shell moulds 12 with the cooling plates 11 a conveyor belt can be used (not shown in Fig. 2). Thereby, the several moulds 12 with the cooling plates 11 can be mounted directly on the conveyor belt, which couples the withdrawal speed for the several moulds 12, or can be mounted via driving rods 10, which serve to vary as a function of time the withdrawal speed given by the speed and inclination of the conveyor belt. The vertical movement range of such driving rods 10 is selected to allow a desired withdrawal speed variation, e.g. half the height of a shell mold 12.
This embodiment of the invention provides a further increase of productivity as even more moulds 12 are cast in a given time period, while still providing an improved metallurgical quality, as the components with shell mould 12 around them do not shadow each other. Again, if two or three article are cast within one shell mould 12, the shadowing effect can be avoided by aligning the articles in a row so that the shorter extensions face each other with a minimum distance of about the shorter extension and the longer extensions are parallel to each other. This advantage more than compensates the relatively small decrease in metallurgical quality caused by the positioning of the nozzles 8 and a baffle 3 only from the sides of the slot 24, and not all around a single mould 12. Depending on the complexity and metallurgical quality acceptance limit of a given component, the operator has to judge whether to use this or another embodiment of the invention. For further increasing the productivity of the casting furnace a plurality of slots 24 can be operated at the same time. The plurality of slots 24 can be loaded from the same preheating chamber 22 and unloaded to the same cool-down chamber 23.
While our invention has been described by an example, it is apparent that other forms could be adopted by one skilled in the art. Accordingly, the scope of our invention is to be limited only by the attached claims.

NUMBERING

1: Vacuum system
2: Vacuum chamber
3: Baffle (radiation and gas flow shield)
4: Heating chamber
5: Cooling chamber
6: Melting crucible
7: Opening
8: Nozzles
9: Inert gas flows
10: Driving rod
11: Cooling plate
12: Casting shell mould
13: Ceramic part
14: Filling device
15: Molten alloy
16: Heating element
17: Vacuum system
18: Pipelines
19: Solidification front
20: Casting
21: Flexible fingers or brushes
22: Preheating chamber
23: Cool-down chamber
24: Slot

Claims

A method of casting a directionally solidified (DS) or single crystal (SX) article with a casting furnace comprising a heating chamber (4) with at least one heating element (16), a cooling chamber (5), a separating baffle (3) between the heating and the cooling chamber (4, 5), the method comprising the steps of

(a) preheating a shell mould (12) with a cooling plate (11) for casting only one to three cast articles within a separate preheating chamber (22),

(b) connecting directly the preheating chamber (22) with the heating chamber (4) by temporarily opening a segment of the heater element (16) for passage of the shell mould (12), or through an opening within the heater element (16),

(c) loading sideways the preheated shell mould (12) with the cooling plate (11) into the heating chamber (4) of the casting furnace,

(d) feeding the shell mould (12) with liquid metal (15),

(e) withdrawing the shell mould (12) from the heating chamber (4) through the baffle (3) to the cooling chamber (5) thereby directionally solidifying the liquid metal (15) forming the cast article, and

(f) unloading the shell mould (12) to a separate cool-down chamber (23).
The method of casting a directionally solidified (DS) or single crystal (SX) article according to claim 1, further comprising the step of repeating the steps (a) to (f) automatically with a series of individual shell moulds (12) one after the other.
The method of casting a directionally solidified (DS) or single crystal (SX) article according to claim 1 or 2, further comprising the step of using a shell mould (12) for two or three cast articles, the cast articles having two principal extensions perpendicular to the withdrawal direction, a longer extension in one direction and shorter extension in a second direction, and positioning the cast article within the shell mould (12) in a row so that the shorter extensions face each other with a minimum distance of about the shorter extension and the longer extensions are aligned parallel to each other.
The method of casting a directionally solidified (DS) or single crystal (SX) article according to any of the claims 1 to 3, further comprising the step of withdrawing several moulds (12) concurrently one after the other from the heating chamber (4) which is shaped as a slot (24) in a motion combining the vertical direction and the direction sideways along the length of the slot (24) into the cooling chamber (5).
The method of casting a directionally solidified (DS) or single crystal (SX) article according to claim 4, further comprising the step of withdrawing the several moulds (12) with the cooling plates (11) by a conveyor belt, the several moulds (12) with the cooling plates (11) connected directly or via driving rods (10) to the conveyor belt.
The method of casting a directionally solidified (DS) or single crystal (SX) article according to claim 4 or 5, further comprising the step of operating a plurality of slots (24) at the same time.
The method of casting a directionally solidified (DS) or single crystal (SX) article according to claim 6, further comprising the step of loading the preheated shell moulds (12) to the plurality of slots (24) from the same preheating chamber (22).
The method of casting a directionally solidified (DS) or single crystal (SX) article according to any of the claims 6 to 7, further comprising the step of unloading the shell moulds (12) from the plurality of slots (24) to the same cool-down chamber (23).
The method of casting a directionally solidified (DS) or single crystal (SX) article according to any of the claims 1 to 8, feeding during the withdrawal of the shell moulds (12) an inert gas towards the shell mould (12) through gas nozzles (8) arranged below the separating baffle (3).
The method of casting a directionally solidified (DS) or single crystal (SX) article according to any of the claims 1 to 9, withdrawing the shell moulds (12) from the heating chamber (4) through the baffle (3) to a vacuum cooling chamber (5).
The method of casting a directionally solidified (DS) or single crystal (SX) article according to any of the claims 1 to 9, withdrawing the shell moulds (12) from the heating chamber (4) through the baffle (3) to a cooling chamber (5) which is filled with liquid metal or is a fluidized bed.
The method of casting a directionally solidified (DS) or single crystal (SX) article according to any of the claims 1 to 11, further comprising the step of preheating the shell mould (12) within the preheating chamber (22), which is evacuated or filled at a reduced pressure with inert gas.
The method of casting a directionally solidified (DS) or single crystal (SX) article according to any of the claims 1 to 12, further comprising the step of unloading the shell mould (12) to a cool-down chamber (24) which is evacuated or filled with inert gas or air at reduced or ambient pressure.
The method of casting a directionally solidified (DS) or single crystal (SX) article according to any of the claims 1 to 13, further comprising the step of producing turbine components such as vanes or blades of a gas turbine.