NL7908785A - Apparatus and method for the targeted solidification of molten metals. - Google Patents

Description

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Apparatus and method for the directed solidification of molten metals,

The invention relates to methods and devices for the direct solidification of molten metals, more particularly to the production of single crystals with a controlled crystallographic orientation.

It is known that great improvements in the construction of metal structures can be obtained by unidirectional casting techniques, with which objects are produced with stem crystal structure or single crystal structure.

In this connection, reference may be made, for example, to U.S. Pat. No. 3,260,505, as well as U.S. Pat. No. 3,494,709. The main object of these known devices, methods, and articles has been to provide structures that have reinforced properties along the main axis of the article, ie the main axis of the article is typically the solidification growth axis of the axis along which it clotting front moves.

When metals are solidified, they by nature tend to solidify or grow faster in one crystallographic orientation than in others. For example, in nickel base superalloys, it has been found that the (001) orientation predominates. As a result, single crystal castings made by means described in the aforementioned U.S. Pat. No. 3,494,709 will have the (001) orientation along the growth axis. Therefore, in order to produce a different crystallographic orientation along the major axis of solidification, specialized techniques must be used. Furthermore, the orientation of crystals relative to the plane perpendicular to the solidification axis is arbitrary in most directional solidified objects unless steps are taken to control this. The crystallographic orientation, measured along the major axis of a casting, is called the primary orientation, while the polar orientation in the plane perpendicular to the major axis is called the secondary orientation.

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The properties of a metal such as a stem crystal or single crystal material are affected by its crystallographic orientation. For example, the modulus of elasticity will vary significantly in many alloys, thereby affecting the action of parts under pressure and stress. Consequently, in more sophisticated applications of advanced materials, it is increasingly important to control both the primary and secondary orientations. The crystallographic orientations of materials can be determined by conventional non-destructive laboratory techniques. Radiographic diffraction, e.g. by the Laue method, is the most useful. Furthermore, changes in crystallographic structure can be easily detected by conventional crystal etching. If the orientation at a location is determined in one part, this orientation will be the same in another region in the absence of an intervening grain boundary, apart from small crystal variations outside the scope of this discussion.

A useful technique for controlling the crystallographic structure in cast articles is to use a pre-made metal seed having the desired structure. If the object casting can be done by epitaxial growth from the seed, the seed structure will be reproduced.

Obviously, the growth of objects from germs is known. In this regard, reference should be made, for example, to the Bridgman method of epitaxial single crystal formation described in U.S. Patent 1,793,672 and other publications dating from the 1920s. In U.S. Patent 2,791,813, Delano describes structures with controlled crystallographic orientations, where seed crystals are used to obtain the desired result. In US Pat. No. 3,759,310, Barrow et al. Describes an apparatus and an electric arc method for making single crystal articles with a consumable electrode using a seed crystal at the bottom of the mold. More recently, Petrov et al. In U.S. Pat. No. 3,857,436 describe an improved method and apparatus for making crystal articles. In it become; α Π 3 7 * 3

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- 3 - - Described means and methods for the application of crystallization to a conically shaped bottom chamber, where abrupt supercooling conditions are created. Further refinements are described in said US patent. Furthermore, Copley in U.S. Pat. No. 3,598,169 describes the casting of relatively flat objects using seed layers and the formation of radially outward solidification.

With the exception of Barrow, all of the aforementioned techniques anticipate heating the mold prior to the introduction of the molten metal. The practice in the known art is that the seed is in the form during heating. Consequently, it is also heated to a relatively high temperature along with the mold, although in some situations its location would indicate less heating. When the superheated molten metal is introduced into the mold and allowed to stabilize, it contacts the heated seed and partially melts it. Of course, it is necessary that at least a portion, but only a portion, of the seed melt, and this necessitates control over the initial and changing conditions of the seed, shape, molten metal, and other influencing factors.

Much of the known art concerns laboratory techniques and is not aimed at mass production. There is currently a trend towards greater commercial utilization of objects with a controlled crystallographic structure, such as stem crystal and single crystal gas turbine-3Q air blades. This has led to the development of automatic casting techniques for producing articles in large quantities on an economic basis. According to one of these techniques, described in U.S. Pat. No. 3,895,672, a heated mold is clamped to a cool casting plate immediately prior to the introduction of molten metal into the mold. If the seed crystal is used, it is attached to the casting plate and is accordingly cool. The short time between adjusting the hot mold and the 4CT cool casting plate gives little time for the temperature of 7903735> -4 - the germ to increase- The same difficulty can arise with some of the known devices and methods. If the seed is too cold, there will be insufficient melting, and no epitaxial growth.

One method of overcoming this is to increasingly overheat the molten metal, but this has drawbacks, since overheating often leads to increased time and expense, undesired evaporation of elements, and increased mold deterioration.

It is also disadvantageous to heat the seed separately or to include the seed in the mold when the mold is heated according to the methods of the known art, both because of mechanical and manufacturing complications, and because the germ can be oxidized undesirably or on other 15 contaminated.

Another consideration in the manufacture of articles with controlled primary and secondary crystallographic orientation is that after manufacture, the orientation of the seed must firstly be accurately determined by suitable inspection techniques, and secondly precisely controlled with respect to the axes of the objects to be cast. Accordingly, the provision of seeds for casting can significantly increase costs. It is therefore desirable that the seeds can be recovered from the casting process after the article has been formed and reused if possible.

However, when the seed is strongly melted during the casting operation, or surrounded by a greater amount of congealed metal of different orientation, recovery for reuse is difficult.

An object of the invention is now to provide an improved method, apparatus, and mold for the production of controlled crystallographic orientation castings using epitaxial growth from seeds having a known orientation. A further object of the invention is to ensure the protection, recovery and reuse of germs.

In accordance with the present invention, molten metal is introduced in a directional solidification form in such a manner that a portion flows over the seed to form a free flow. g 'ti? - heat and melt 5 '* part of it, and remove any unwanted contamination film that may be present. When used in conjunction with a pouring plate, the mold is configured to define a starting cavity 5 of sufficient volume to contain the seed and receive molten metal flowing over the seed.

The seed may protrude into the starting cavity to allow molten metal to flow over and heat it. A barrier layer, eg a ceramic coating, 10 may be applied to selected parts of the seed to facilitate its removal from the solidified metal casting for reuse. In one embodiment, a thermal insulation is placed on the casting plate in portions adjacent to the seed, to retard solidification of molten metal of uncontrolled orientation within the starting cavity, and to ensure that epitaxially solidified metal, starting from the seed, will be present in the object.

In one embodiment of the invention, a mold has an article section, in communication with the start section via the selector section. The start section is capable of containing the seed and providing a volume capable of absorbing a portion of the molten metal that flows around the seed to heat and melt it. The selector section is located in close proximity to the region in the start section where the seed is receptive and functions to pass only metal epitaxially solidified from the seed into the article section. In a preferred embodiment, the mold is capable of receiving molten metal 3Q through the article section, and its discharge from the selector section, through which it passes, is regulated such that the metal lands on the surface of the seed, whereby the germ is efficiently heated and melted.

The invention is suitable for the production of castings of any alloy, in any desired controlled structure, which can be produced from a seed.

A particularly useful application is the production of stem crystal or single crystal components of nickel-4Q superalloys.

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By means of the invention, a suitable melting of the seed is obtained in order to ensure the desired epitaxial growth from therein, whereby the imperfect castings which can be produced, when the seed is not melted adequately, or when the irritant layer is not completely has been removed, have been overcome. Furthermore, the invention allows the use of seed crystals, which have not been heated significantly prior to the introduction of the molten metal into the mold. In a preferred embodiment, according to the invention, the cost of germination is further reduced by ensuring that they can be easily recovered from solidified castings and then reused. The use of germs has been made more economical and therefore more applicable compared to growth without germination, allowing the realization of advantages of primary and secondary orientation control. Monocrystal molding designs can be simplified and the Initial solidification rates increased, thereby increasing the production yield.

The aforementioned and other objects, aspects and advantages of the invention will now be further elucidated with reference to the detailed description of the preferred embodiment of the invention with reference to the drawing. In the drawing: Fig. 1 shows a cross-section of a mold containing a seed mounted on a casting plate, Fig. 2 shows a cross-section of the object cavity of the mold of Fig. 1, Fig. 3 is a partly sectioned view of a germinal cavity of a mold on a casting plate, fig. 4 is a detail of the seat of the germ, fig. 5 is a partial cross-sectional view of a germ with a barrier layer around its circumference, and fig. 6 is a partial cross-sectional view of a alternative version of germ and pour plate barrier layers.

The preferred embodiment is described in 40 terms of a shape, particularly suitable for general 7 § 0? 7 9 5-7 use in the system described in the aforementioned U.S. Pat. No. 3,895,672 for the production of one-piece monocrystal nickel alloy castings, although their use is not limited thereto.

A mold 20, made of ceramic material suitable for forming a single crystal object, is mounted on a copper casting plate 22, as shown in Fig. 1. The mold consists of a section defining an object cavity 24, which as shown in FIG. 2 is configured according to a gas turbine air blade for the production of which the invention particularly contributes. The mold further has a first end 26 for receiving molten metal and allowing it to pass into the article cavity, and a second end 33 capable of contacting a casting plate.

A seed 28 with an established crystallographic orientation is mounted in a recess 30 in the casting plate 22. The seed is therefore in intimate contact with, and will be cooled by, the casting plate. Surrounding the seed is a starting cavity 32, defined by the second end 33, the starting section of the mold and the pouring plate 22.

A selector section 34 connects the start cavity 32 and the object cavity 24. The selector section 34 has a substantially smaller cross section than each of the seed crystal cavity and the object cavity.

In the preferred embodiment shown, the seed, the start cavity, and the selector cavity are circular in cross-section, although other cross-sectional shapes are also functional.

The relative dimensions of the respective elements are not fixed, but can be put in general perspective by way of an example: for the manufacture of nickel superalloy articles 35 such as gas turbine air blades 10 to 25 cm high, a seed of the superalloy with a diameter of 2— 2.5 cm and corresponding height are preferred. The starter cavity should then have a diameter of about 5 cm and the entrance to the selector section should be about 0.5-1.0 cm 40 above the surface of the seed. Thus, the * »η ή * *? s s * - 8 - starter cavity a yolume of more than five times that of the germ contained therein. As explained in the following, this volume is available for the incorporation of molten metal for heating the seed and initiating epitaxial clotting from there.

The start section end 33 is placed close to the casting plate on its surface 36 to prevent molten metal from escaping. Clamping means, shown as bolts in Fig. 3, are used to maintain good contact between the mold and the casting plate. Other mechanical fasteners and fixtures are equally suitable as long as they are out of the way of the molten metal and capable of holding a high temperature mold.

Obviously, for mass production in the selection of clamping members, the criterion, ease and speed of coupling and uncoupling.

When the mold and casting plate are clamped together tightly, the assembly is suitable for placement within various devices described in the prior art directional solidification technique. Molten metal can be introduced and the required thermal gradient applied to the mold to ensure targeted solidification of the casting. The use of the device 25 is as follows: Molten metal is introduced into the mold 20 through the receiving end 26, passes through the article section 24 through the selector section 34, and falls and flows along the surface 38 of the seed 28. The action of the molten metal on the seed surface 38 causes its heating and melting, and by turbulence increases the removal of any deposits or films. The molten metal, after passing over the surface of the seed, is deposited in the starting cavity 32 adjacent to the seed. Thus, the starter cavity 35 functions as a receiving reservoir for the molten metal used for heating the metal.

The receiving reservoir may optionally be located away from the cavity containing the seed. Metal input through a separate port, as shown in U.S. Pat. No. 3,915,761, is another possibility. In such cases, the starter cavity must still be configured to allow flow of molten metal. In the configuration in the preferred embodiment of Fig. 1, the molten metal, after passing over the seed surface, surrounds the seed laterally, thereby imparting further heat to it.

When the mold is filled with metal, by drawing heat from the casting plate and the mold walls according to known practice, the molten metal is solidified progressively via the main axis of the mold, ie vertically. The metal in the starting section will solidify first, and of course a main component of the seed is present in the solid state. Insofar as the selector section 34 is centered above the seed 28, 15, metal that solidifies epitaxially on the surface of the seed will preferably first reach and pass through the selector section. Since the solidifying metal passing through the selector section solidifies epitaxially from the seed crystal, it will have the same orientation as the seed crystal. Similarly, the object formed in the cavity 24 will have a corresponding orientation as it assumes its structure from the previously formed material of the selector section.

Fig. 3 shows in more detail the implementation of the important elements of the invention in the starting section. In order to obtain a desired secondary orientation, it is necessary that the seed crystal is oriented in an established manner with respect to the object cavity 24.

This can be achieved by orienting both the seed and the mold in a fixed relationship to the pouring plate 22. As shown in Fig. 3, the mold is oriented with respect to the pouring plate by means of bolts 37, which are also the function to clamp the mold to the casting plate to prevent leakage. Of course, other orienting means can be used, especially in mass production, such as polarization of the casting plate and mold by their shape at their contact points, or by using electro-optical sensors with appropriate directions. As shown in the 40 details of Fig. 4, there are means for orienting the seed relative to the casting plate. The vertical or primary axis orientation is performed by the obvious means of resting the seed on the surface of the casting plate. The secondary orientation, or the polar orientation around the primary axis, is controlled by the center of an adapted lock-and-key system. As shown, the seed crystal has a simple slit 46 over its diameter, which forms the lock, while the casting plate is equipped with an integral keyway 48, Other mechanical gears and locators and other polarization methods can also be used.

Also shown in FIGS. 3 and 4 is a ceramic shield 40 surrounding the periphery of seed 28. This is a barrier layer to prevent molten metal, which has passed over the surface 38 of the seed and has settled into the start cavity 32, from adhering to the periphery 42 of the seed. The shield 48 prevents melting at the periphery 42 of the seed and prevents adhesion of the molten metal in the cavity at the periphery of the seed.

Accordingly, after the metal has solidified in the cavity 32 and the entire casting is removed from the mold, the casting can be cut across the face of the surface 38, the seed being easily detachable from the starter section casting and with little preparation again can be used.

Fig. 5 shows an alternative embodiment of the ceramic shield 40, this shield extending into a recess in the casting plate together with the seed. The shield 30 can be made of a ceramic material or any other substance that can withstand the action of the molten metal for the short time that it is exposed prior to solidification.

It is only required that the shield be formed of a material that has the required heat and corrosion resistance, and in addition has sufficient mechanical strength not to be detached under the action of the molten metal. Of course, in order to achieve the object of the invention, the barrier layer around the circumference of the seed need not be a separate physical element 7908735-11, but may also consist of a coating applied to the seed. 6 shows yet another embodiment of the invention, wherein the seed is mounted in line with a depressed area of the surface of the casting plate together with shield 40. In addition, a ceramic annular disk 44 resting on the glazing plate surface 36 adjacent is shown. germinating. The disk 44 has the function of reducing cooling by the casting plate, and thereby the solidification rate of the molten metal adjacent to the seed compared to what it would have been if the disk were not present. Of course, the metal solidification from the slit plate surface 36 will not have the desired crystallographic orientation of the seed. In special cavity configurations, the presence of the disk 44 provides greater assurance that metal with an undesired crystallographic orientation will not reach selector set 34 before metal that solidifies epitaxially from the seed surface 38. In Fig. 6, the disk 44 is shown as a separate element covering the entire exposed casting plate in the cavity 32. However, the diameter excess of coating can be varied, eg by reducing the diameter of the disk 44, so that a portion of the glacier plate surface at the periphery of the cavity is exposed. Variation of the coating of the casting plate can controlably alter the heat extraction of the metal in the cavity 32 to effect a desired solidification of the article. In addition, the disk 44 may be manufactured integrally with the screen 40, as shown in Fig. 3. Alternatively, the disk 44 may be manufactured integrally with the mold 20, in which case the inner diameter of the disk portion may be varied for controlling the heat extraction. The disk 44 may also be configured as a coating on the casting plate, 35 and the functioning of the disk may be varied by the thickness and thermal characteristics of the construction material.

The use of the device and the method described here can be adapted to the production of single-40 parts or multiple parts. Obviously, 7806735 f- - 12 - multiple pieces can be made by arranging a plurality of shapes of the type shown in Fig. 1 / as an assembly, as is common in mass production of directional solidified cladding castings, Alternative 5 may be more than one part are made of a single seed crystal by spreading the shape immediately above the selector portion, in such a manner as described in U.S. Pat. No. 3,857,436.

Although the invention has been described above with reference to a preferred embodiment in terms of a single crystal casting, it is within the scope of the invention that stem crystal castings and other epitaxially developed casting structures can also be produced.

The invention is useful with any castable alloy where a suitable shape can be manufactured. It will further be apparent to those skilled in the art that various changes, modifications and omissions in shape and detail are possible without departing from the scope of the invention.

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Claims (14)

1. Apparatus for solidifying molten metal into an object of controlled crystallographic orientation, characterized by a pouring plate for cooling molten metal 5 during direct solidification, a seed of known crystallographic orientation for introducing a desired epitaxial solidification into the object means for holding the seed in a fixed orientation relative to the mold in the vicinity of the casting plate so that the heat transfer from the seed to the casting plate can be achieved, a mold with an article section and a starting section shaped so that molten metal can flow around the seed, the starting section being in contact with the casting plate and defining together with the casting plate a volume capable of containing the seed and receiving a portion of the molten metal, which flows around the seed, and means for holding the mold in a fixed relationship to the casting plate, in order to orient ie the shape in relation to the germ. 2. Device according to claim 1, characterized in that the shape has a selector section connecting the object section and the start section to ensure that only epitaxially solidified material forms in the object section. 3. Device according to claim 1, characterized in that it further comprises means for dropping molten metal on the surface of the seed in order to increase its heating and surface contamination films which may interfere with the epitaxial clotting, to delete. 4. Device as claimed in claim 1, characterized in that it further comprises a seed projecting into the starting cavity such that molten metal flows, i - '- <- * IV ~ J' - '- 14 - in the starting cavity, the germ surrounds a different surface from that from which the epitaxial coagulation takes place in the object, in order to give additional heating to the germ » 5. Device according to claim 2, characterized in that it further comprises means for preventing molten metal from settling. adheres to the seed at predetermined locations to facilitate removal of the seed from the solidified casting. 6. Device according to claims 1 and 2, characterized in that it further comprises means for thermally insulating a part of the casting plate adjacent to the seed in order to reduce the heat loss of excess metal placed in the starting cavity. 7. Device according to claim 1, characterized in that the seed is a single crystal » 8. A method of casting metal into an article of controlled crystallographic orientation with the device of claims 1 to 7, using epitaxial directional solidification from a seed, which is initially substantially cooler than the melting point of the metal to be cast, with characterized in that the molten metal is dropped and flowed over the surface of the seed in sufficient amount to heat and melt a portion of the seed, and to remove surface contamination which may interfere with the epitaxial clotting from the germ. 9. A method according to claim 8, characterized by placing a seed on a casting plate in a controlled orientation relative thereto, applying a heated mold to the casting plate in a fixed orientation relative thereto to contain the seed and melted therein metal, filling the mold with molten metal under 7308735 - using an agent which ensures that a portion of the molten metal flows over the surface of the seed to a reservoir in sufficient quantity to contain a portion of the seed and partially melting, and removing any contamination films thereon, and solidifying the molten metal epitaxially to form an object from the seed with a crystallographic orientation, which is determined by the germ. 10. A method according to claim 9, characterized in that it further comprises placing a barrier layer, resistant to the molten metal, around a portion of the seed to prevent adhesion of molten metal to the seed without thereby enveloping the germ by eliminating molten metal, which would occur in the absence of the barrier layer, to facilitate removal of the germ from the casting after solidification. 11. A method according to claim 8, characterized by forming a mold having a first end capable of receiving molten metal, a second end capable of determining a starting cavity in contact with a casting plate, capable of an object cavity to contain molten metal and the molds, and a selector section, located between the second end and the object cavity, heating the mold, applying to a cool casting plate a seed, which is considerably smaller than the starting cavity, contacting the second end of the mold with the casting plate in such a way that both the seed is contained in the starting cavity and a space is provided in the cavity capable of receiving and containing molten metal, filling the mold by pouring molten metal into the first end of the mold so that this molten metal goes to the second end and flows to a reservoir thereby passing over it. surface of the seed such that a portion of the seed is heated and melted, cooling the mold so that the molten metal solidifies epitaxially from the second end into the object cavity to form an object having a crystallographic orientation, it is determined by the germ. 12. Mold for epitaxial casting of metal from a seed to an object of controlled crystallographic orientation, the mold being attached to a casting plate, and the molten metal being solidified with h. characterized in that this shape consists of: an object cavity for molding molten metal into an object, a selector cavity, connected to the object cavity to control the crystallographic orientation of metal, which grows by solidification in the object cavity from a starting cavity, 20 a starting cavity, connected to the selector cavity, and having a cross-sectional area considerably larger than the selector cavity, and a volume sufficient to contain a seed and molten metal, introduced in the form for heating the seed, and means for introduction of molten metal in the mold so that it flows over the seed to the starting cavity, where there is a volume for containing this molten metal. 13. Mold according to claim 11, characterized in that the selector cavity is a substantially straight tunnel, aligned between the start cavity and the object cavity, the tunnel axis being substantially parallel to the main growth axis of the mold form object. Mold according to claim 12, characterized in that the mold starting cavity is formed such that the heat loss to a casting plate in areas adjacent to the seed is minimized. 7908785