CA1142839A

CA1142839A - Method and apparatus for epitaxial solidification

Info

Publication number: CA1142839A
Application number: CA000340560A
Authority: CA
Inventors: Bruce E. Terkelsen
Original assignee: United Technologies Corp
Current assignee: RTX Corp
Priority date: 1978-12-13
Filing date: 1979-11-23
Publication date: 1983-03-15
Also published as: NL185271B; IL58882A; CH644408A5; JPS6358669B2; DK529479A; DE2949446C2; BR7908118A; SE7910137L; GB2037200B; IT7928074A0; FR2444092B1; IT1127731B; DE2949446A1; NL185271C; FR2444092A1; NL7908785A; JPS5581064A; BE880402A; DK158629B; DK158629C

Abstract

METHOD AND APPARATUS FOR EPITAXIAL SOLIDIFICATION
ABSTRACT
An apparatus and method for obtaining articles of con-trolled crystallographic orientation using epitaxial solidi-fication from seeds. The starter section of a directional solidification mold is adapted to contain a seed and receive molten metal. Molten metal is flowed over and about a seed to heat and partially melt it. A selector section of the mold has reduced cross section compared to the starter sec-tion so that only epitaxially solidified metal will be formed in the article section. A barrier layer resistive to molten metal is applied to portions of the seed to facilitate its removal and reuse.

Description

BACKGROUND OF THE INVENTION
1. Field of the Invention This invention relates to methods and apparatuses for directionally solidifying molten metals, most particularly to the production of single crystals with controlled crys-tallographic orientation.

2. Description of the Prior Art It is well known that great improvements in the per-formance of metal s~ructures can be achieved by unidirectional casting techniques which produce articles with columnar grain or single crystals. See, for example, the teachings of Ver Snyder, U.S. Patent 3,260,505 and Piearcey, U.S. Patent

3,494,709. The principal objective of the prior ar~ appara-tus, methods, and articles has been to provide structures which have enhanced properties along the principal axis o the article, that is, the principal axis of the article is typically the solidi~ication growth axis or the axis along which the solidification front is caused to move.
When metals are directionally solidified, they o~ten by nature solidify or grow faster in one crystallographic orien-tation than others. For example, in nickel base superalloys the ~001~ orientation is found to predominate. As a result, single crystal castings made by means disclosed in U.S.
Patent 3,494,709, mentioned above, will have the ~OQl~ orien-tation lying along the growth axis. Therefore, to produce another crystallographic orien~ation along the principal axis o solidification specialized techniques must be used.

g Also, the orientation of crystals with respect to the plane perpendicular to the axis of solidification is random in most directionally solidified articles unless steps are taken to achieve control. The crystallographic orientation measured along the principal axis o a casting is called the primary orientation, while the polar orientation in the plane perpendicular to the principal axis is called the secondary orientation.
The properties of a material such as a columnar grain or single crystal material are in1uenced by its crystallo-graphic orientation. For e~ample, the elastic moduli will be importantly varied in many alloys and the performance of parts under stress and strain thereby affected~ Consequently in more sophisticated applications of advanced materials, it is of increasing importance to control both the primary and secondary orientations. The crystallographic orientations o~ materials are determinable by conventional nondestructive laboratory techniques. Radiographic diffraction, e.g. by the Laue method, is most useful. Furthermore, changes in crystallographic structure can be readily ascertained by conventional grain etching. If the orientation at a loca-tion in a part is determined, the orientation will be the same in another region in the absence o~ an intervening grain boundary, and absent subtle crystal variations beyond the scope of this discussion.
A useful technique for controlling crystallographic structure in cast articles is the use of a previously made 2~3~

metal seed which has the desired structure. If the article casting can be made to grow epitaxially from the seed, the seed structure will be reproduced.
Of course, growing objects from seeds is well known.
For instance the Bridgman method for epitaxial single crystal formation disclosed in U.S. Patent 1,793,672 and other pub-lications dates from the 1920's. Delano in U.S. Patent 2,791,813 describes structures with controlled crystallo-graphic orien~ations in which seed crystals are used to attain the desired result. Barrow et al in U.S. Patent 3,759,310 describesl an apparatus and electric arc method for making single crystal articles with a consumable elec-trode in which a seed crystal at the bottom of the mold is used. More recently, Pe~rov et al in. U.S. Patent 3,857,436 describes an improved method and apparatus for manuacturing single crystal articles. Disclosed therein are means and methods for initiating crystallization at a conical-shaped bottom chamber where abrupt super-cooling conditions are ~ created. Petrov U.~S~. Patent de6cribes further refinements.
Also,~Copley U. S. Patent 3,598,169, discloses the casting`
of relatively flat objects using seed wedges and accomplishing ;
radially ou~ward solidificatioD.
With the exception of Barrow, all the aforementioned techniques anticipate heating the mold prior to the introduc-tion of the molten metal. The practice in the prior ar~ is that the seed is in the mold during the heating. Therefore, it is also heated with the mold to a relatively high temperature although in some situations its location would indicate lesser heating. As the superheated molten metal is introduced into the mold and allowed to stabilize, it con-~acts ~he heated seed and causes it to partially melt. Of course it is necessary to melt at least part, but only a part, of the seed, and this necessitates a control over the initial and transient conditions of the seed, mold, molten metal, and other influential factors.
Much of the prior art reflects laboratory technique and is not oriented toward mass production. Now,there is a trend towards greater commercial utilization of articles having controlled crystallographic structure, such as columnar grain and single crystal gas turbine airfoils. This has impelled the development of automated casting t:echniques ~o produce articles in quantity on an economic basis. According to one of these techniques, described in King et al, U.S. Patent 3,895,672, a heated mold is clamped onto a cool chill plate just immediately prior to the introduction of molten metal into the mold. If the seed crystal is used, it is attached to the chill plate and it is, of course, correspondingly cool.
The short duration between the mating of the hot mold and the cool chill plate provide little time for the temperature of the seed to increase. The same difficulty can obtain in some of the prior art apparatus and methods. If the seed is too cold, insufficient melting will occur and epitaxy will not result. One method of overcoming this is to increasingly superheat the molten metal but to do so is disadvantageous ~z~

since superheating often leads to increased time and cost, undesired vaporization of elements, and increased degrada-tion of the mold. To separa~ely heat the seed or to include the seed wi~h the mold when the mold is being heated after the methods of the older artis also disadvantageous, both from the mechanical and manufacturing complications and be-cause the seed can become unduly oxidized or o~herwise con-taminated.
Another consideration during the manufacture of articles of controlled primary and secondary crystallographic orienta-tion is that after manufacture, the orientation of the seed must be~firs~ accurately defined by suitable inspection techniques and3second~controlled precisely with respect to the axes of the articles being cast. Accordingly, the pro-viding of seeds for casting can represent a significant cost.
I~ is, therefore, desirable that seeds be recovered from the casting process after the article is formed and reused if possible. However, when the seed is severely melted during the casting operation or surrounded by a larger quantity of solidified metal of extraneous orientation, recovery for re-use is difficult.

SUMMAR~ OF THE INVENTION
An object of the invention is to provide an improved method~ apparatus, and mold for the production of castings of controlled crystallographic orientation using epitaxial growth from seeds having a known orientation. A further object of the invention is to provide for the preservation, recovery, and reuse of seeds.
According to the invention, molten metal is introduced into a directional solidification mold in a manner which causes a portion to flow over the seed to heat and melt a part= ~S~ of it, and remove any undesirable contamination film which may be present. When used conjunctively with a chill plate, the mold is configured to define a starter cavity of sufficient volume to contain the seed and receive molten metal flowed over the seed. The seed may project into the starter cavity to allow molten metal to flow about and heat it. A barrier layer, such as a ceramic coating, may be pro-vided on selected portions of the seed to facilitate its re-moval from the solid metal casting for reuse. In one embodi-ment, thermal insulation is placed on the chill plate in portions adjacent the seed to slow solidification of molten metal of uncontrolled orientation within the starter cavity and ensure that epitaxially solidified metal originating from the seed will be present in the article.
In one embodiment of the invention, a mold has an article section connected to the starter section by a selector sec-tion. The starter section is adapted to contain the seed and to provide a volume capable of receiving a portion of the molten metal flowed about the seed to heat and melt it.
The selector section is located in close proximity to the region in the starter section where the seed is receivable and functions to only allow metal epitaxially solidified from the seed to pass into the article section. In a preferred embodiment, the mold is adapted to receive molten metal through the article section and its diseharge from selector section in which it passes is controlled to impinge on the surface of the seed and thereby effectively heat and melt the seed.
According to the invention there is provided for epitaxially casting metal, apparatus eomprised of a ehill plate, a mold in contact with the chill plate and a seed positioned on the ehill plate, the mold having in a substantially vertieal array an article cavity and a starter eavity, the starter eavity conneeted to the artiele eavity by the seleetor eavity of substantially smaller dimension than that of the starter cavity, the mold further having means for introdueing molten metal into its eavities, a portion of the seed projeet-ing into the mold starter cavity, the portion having a first surface aligned with the entrance to the selector eavity to enable epitaxial solidifieation from the first surface into the seleetor eavity, and the starter cavity having substantially greater volume than the volume of the portion of the seed pro-jeeting thereinto, providing within the starter eavity a reservoir surrounding the seed for metal which may be flowed across the seed.
Also according to the invention there is provided the method of casting metals into an article having controlled erystallographie orientation using a seed and a ehill plate which comprises: (a) placing a cold seed on a cold chill plate so that the seed projects above the surface of the chill plate with a controlled orientation thereto, (b) heating a mold and then contacting it with the chill plate in a controlled orientation thereto, so the seed is surrounded by a spaced apart cavity of the mold, without substantially heating the seed, (c) filling the mold with molten metal and thereby causing a portion of the molten metal to flow across a surface of the seed to the cavity surrounding the seed in sufficient velocity and quantity to heat and partially melt a portion of the seed and remove any contamination films thereon, and (d) epitaxially solidifying the molten metal from the seed to form an article having a crystallographic orientation determined by the seed.
The invention is suitable for the production of cast articles of any alloy, in any desired controlled structure pro-ducible from a seed. Of particular useful application is the production of columnar grain or single crystal components of nickel superalloys.
The invention achieves the appropriate melting of the seed to ensure the desired epitaxial growth therefrom, over-coming the defective castings which may be produced when the seed is not adequately melted or the contamination layer not fully removed. Further, the invention allows the use of seed crystals which are not heated substantially prior to the intro-duction of molten metal into the mold. In a preferred embodiment, it further reduces the cost of seeds by providing for their ready recovery from solidified castings and subsequent reuse. The use of seeds is made more economic and therefore more feasible com-pared to growth without seeding, allowing the realization of benefits from primary and - 8a -secondary orien~ation control. Single crystal mold design can be simplified and initial solidification rates increased, thereby increasing production yield.
The foregoing and other objects, features, and advan-tages of the present invention will become more apparent from the detailed description of the preferred embodiment and the accompanying drawings which follow.

BRIEF DESCRIPTION OF THE DRAWINGS
, Figure 1 shows a mold containing a seed mounted on a chill plate in cross section.
Figure 2 shows a transverse cross section o the article cavity of the mold in Figure 1.
Figure 3 is a partial sectional view of a seed cavity of a mold on a chill plate.
Figure 4 is a detail`of the seed seating.
Figur~e S is a partiàl sect-ional view of a seed with a barrîer layer around its peripher~
~ Fi;gure 6 is~a partial section`al view of an alternate embodiment of seed and chill plate barrier layers.

DESCRIPTION OF THE PREFERRED EMBODIMENT
The preferred embodiment is described in terms of a mold particularly adapted to be utilized generally within the teachings of the aforementioned King et al U.S. Patent 3~895,672, for the production of one piece single crys~al nickel alloy castings~although its use is not limited to such.
A mold 20 made of a ceramic material suitable for form-ing a single crystal article is mounted on a copper chill plate 22 as shown in Figure 1. The mold is comprised of a section which defines an article cavity 24 which, as Figure 2 indicates, is configured to a gas turbine airfoil, to the production of which the present invention especially contri-butes. The mold further has a first end 26 for receiving molten metal and passing it into the article cavity and a second end 33 adapted to contact a chill plate.
A seed 28, having a predetermined crys~allographic orientation, is mounted in a recess 30 in the chill plate 22.
The seed is therefore in intimate contact with, and will be cooled by, the chill plate. Surrounding the seed i9 a starter cavity 32 defined by the second end 33, the starter section, of the mold and the chill plate 22. A selector section 3~ connects the starter cavity 32 and the article cavity 24. The selector section 34 has a substantially smaller cross sectional area than either the seed crystal cavity or article cavity.
In the pre~erred embodiment shown, the seed, starter cavity, and selector section are circular in cross section although other cross section shapes are equally functional.
The relative sizes of the respective elements is not fixed but may be put in general perspective by way of an example: When fabricating nickel superalloy articles, such as gas turbine airfoils 10 to 25 centimeters high, a seed of the superalloy with a diameter of 2 - 2.5 cm and a similar height would be preferable. The starter cavity would have a diameter of about 5 cm and the entrance to the selector sec-tion would be about 0.5 - 1.0 cm above the surface of the seed. Thus the starter cavity would have a volume of more than five times that of the seed conta;ned therein. As is pointed out below, this volume is available for receiving molten metal for heating the seed and initiating epitaxial solidification therefrom.

;39 The starter section end 33 is placed tightly against the chill plate at its surface 36 to prevent the escape of molten metal. Means for clamping, shown as bolts in Fîgure 3, are utilized to maintain good contact between the mold and chill plate. Other mechanical fasteners and fixtures are equally sui~able so long as they are located out of the molten metal path and are adapted to holding a mold which is at a high temperature. Of course, in mass production, a criterion in the selection of clamping means is the ease and speed of engagement and release.
When the mold and chill plate are firmly clamped to-gether, the assembly is adapted to be placed within various apparatuses described in the prior art for directional soli-dification. Molten metal can be introduced and the requisite thermal gradient applied to the mold to cause directional solidification of the casting. The use of the app~ratus is as follows. Molten metal is introduced into the mold 20 through the receiving end 26, passing thereupon through the article section 24 and selector section 34 and impinging on andflowing across the surface 38 of the seed 28. The action of the molten metal on the seed surface 38 thereby heats it it and causes/to melt and through turbulence enhances the re-moval of any deposits or films. The molten metal havin~
passed across the surface of the seed is deposited in the starter cavity 32 adjacent th~ seed. Thus the starter cavi-ty functions as a receiving reservoir for the molten metal used to heat the metal. The receiving reservoir could be located apart from tlle cavity containing the seed, if desired.
Metal introduc~lon by a separate gate, as s~own in U. S. Patent ~,915,~61,is another option. In such cases the starter cavity still must be conf~gured to allow throughflow of molten metal. As the elements are conigured in the preferred embodiment of Figure 1, after passage over the seed surface, the molten metal surrounds the seed laterally and thereby further imparts heat to it.
~ hen the mold has been filled with metal, by withdrawal of heat through the chill and mold walls according to known practice, molten n~etal is caused to solidify progressively along the principal axis of the mold, that is, vertically.
Metal in the starter section will solidify first, and of course a major portion of the seed is present as a solid throughout. Inasmuch as the selector section 34 is centered above the seed 28, metal which solidifies epitaxially on the surface of the seed will desirably first reach the selector section and pass therethrough. Since the solidifying metal passing through the selector section solidified epitaxially ~rom the seed crystal, it will have the same orientation as the seed crystal. In like fashion, the article formed in ca~ity 24 will have a similar orientation, as it takes its structure from the earlier-formed material of the selector section.
Figure 3 illustrates in more detail the arrangement of the important elements of the invention in the starter sec-tion. To obtain a desired secondary orientation, it is necessary that the seed crystal be oriented in a predetermined manner with respect to the article cavity 24. This is achievable by orienting both the seed and mold in fixed relationship to the chill plate 22. As shown in Figure 3, the mold is oriented to the chill plate by means of bolts 3~ which also have the function of clamping the mold to the chill plate to prevent leakage. Of course, other orienting means can be utilized, particularly in mass production, such as polarizing of the chill plate and mold by shape at their contact points or us;ng electro-optical sensors with suitable indices. Shown in the detail of Figure 4 are means for ori-enting the seed with respect to the chill plate. Vertical or primary axis orientation is carried out by the obvious means of resting the seed on the surface of the chill plate.
The secondary orientation, or the polar orientation about the primary axis, is controlled by means of a mating slot and key. As shown, the seed crystal has a simple slot 46 across its diameter while the chill plate ls provided with an integral key way 48. Other mechanical detents and loca-tors and other polarizing methods will also be suitable.
Further shown in Figures 3 and 4 i~ a ceramic shield 40 surrounding the circumference of the seed 28.
This is a barrier layer to prevent molten metal which has passed over the surface 3~ of the seed and come to rest in the starter cavity 32 from adhering to the circumference 42 of the seed. The shield 40 will tend to inhibit melting at the seed circumference 42 and will prevent adherence of the molten metal in the cavity to the seed circumference. Ac-cordingly, after the metal in the cavity 32 has solidified and the entire casting is removed from the mold, the cast-ing can be cut across the plane of surface 33 and the seed will thereby be readily detachable from the starter section casting, and with minor preparation can be reused.
Figure 5 shows an alternate embodiment o~ the ceramic shield 40 wherein the shield is recessed into the chill plate with the seed. The shield can be constructed from a ceramic material or any other substance which is resistant to the action of the mol~en metal during the short time it is exposed to it p;~ior to solidification. It is only re-quired that the shield be formed of a material which has the req~isite thermal and corrosion resistance and is in addition of sufficient mechanical strength to not become loose under the action of the molten metal. 0 course to achieve the object of the invention, the barrier layer around the seed circurnference need not be a separate physical element but can be a coating on the seed as well. Figure 6 shows a still further embodiment of the invention in which the seed is mounted flush with a depressed region of the surface of the chill plate together with shield 40. Shown in addi~ion is a ceramic annular disc 44 which is resting on the chiPl plate surface 36 adjacent the seed. The disc 44 has the function of reducing the cooling through the chill plate, and therefore the rate of solidification of the molten metal adjacent the seed, compared to what it would be if the disc were not present. Naturally, the rnetal solidifying from the chill plate surface 36 will not have ~he desired crystallographic orientation of the seed. In particular starter cavity configurations, the presence of the disc 44 gives more assurance that metal having an unde-sired crystallographic orientation will not reach the selec-metal tor section 34 before that/epitaxially solidifying from the seed surface 38. In Figure 6, the disc 44 is shown as a separate element covering the entire e~posed chill plate in the cavity 32. However, the diametrical extent of coverage can be varied, for example, by decreasing the diameter of the disc ~4 so ~hat some of the chill plate surface at the periphery of cavity is exposed. Variation of the coverage of the chill plate would controllably vary heat extraction from the metal in cavity 32 to effect the desired solidifi-cation o the article. In addition, the disc 44 may be made integral with the shield 40 as is shown in Figure 3. As another alternate, the disc 44 can be made integral with the mold 20, in which case the inner diameter of the disc portion would be varied to control heat extraction. The disc 44 can also be configured as a coating on the chill plate, and ~he ~0 functioning of the disc can be varied by the thickness and thermal characteristics o the material of construction.
The use of the apparatus and method described herein can be adapted to the production of single parts or multiple parts. Of course, multiple pieces can be made by arranging a multiplicity of molds of the type shown in Figure 1 as an assembly, as is the common practice in the mass production of directionally solidified investment castings. Al~ernately, more than one part may be made from a single seed crystal by spreading the mold immediately above the selector section, somewhat in the manner of Petrov, U.S. Patent 3,857,436.
While the foregoing invention has been described in the preferred embodiment in terms of a single crystal casting, it is within the contemplation of the i.nvention that columnar grain castings and other epitaxially derived casting structures will be produced. The invention is usable with any castable alloy for which a suitable mold can be fabricated. It will further be understood by those skilled in the art that variolls changes and omissions in the form and detail thereof may be made therein without departing from the spirit and scope of the invention~

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. For epitaxially casting metal, apparatus comprised of a chill plate, a mold in contact with the chill plate and a seed positioned on the chill plate, the mold having in a substantially vertical array an article cavity and a starter cavity, the starter cavity connected to the article cavity by a selector cavity of substantially smaller dimension than that of the starter cavity, the mold further having means for introducing molten metal into its cavities, a portion of the seed projecting into the mold starter cavity, the portion having a first surface aligned with the entrance to the selector cavity to enable epitaxial solidification from the first surface into the selector cavity, and the starter cavity having substantially greater volume than the volume of the portion of the seed projecting thereinto, providing within the starter cavity a reservoir surrounding the seed for metal which may be flowed across the seed.

2. The apparatus of claim 1 wherein the mold has as the means for introducing molten metal, a pouring cone connected to the article cavity and, wherein the selector cavity has a cylindrical shape oriented to impinge molten metal on the first surface of the seed contained within the starter cavity, to enable melting and removal of surface films therefrom.

3. The apparatus of claim 1, wherein the mold has an additional reservoir cavity connected to the starter cavity by a channel, to further receive molten metal flowed across the seed.

4. The apparatus of claim 1 further comprising a mold having a starter cavity with an open end, the open end in contact with and closed by the chill plate; means for holding the seed in close heat transfer relationship and predetermined orientation to the chill plate; and means for holding the mold fixedly to the chill plate, to contain molten metal and control the orientation of the mold with respect to the chill plate.

5. For epitaxially casting metal, apparatus comprised of a chill plate, a mold in contact with the chill plate and a seed positioned on the chill plate, the mold having in a sub-stantially vertical array an article cavity and a starter cavity, the starter cavity connected to the article cavity by a selector cavity of substantially smaller dimension than that of the starter cavity, the mold further having means for introducing molten metal into its cavities; characterized in that a portion of the seed is positioned within the mold starter cavity, the portion having a first surface aligned with the entrance to the selector cavity to enable epitaxial solidification from the first surface into the selectory cavity; and the starter cavity having substantially greater volume than the volume of the portion of the seed contained therein; to provide within the starter cavity a reservoir for metal which may be flowed across the seed, the seed projecting into the starter cavity to enable molten metal to surround the seed at a second surface in addition to the first surface to provide additional heating to the seed, the seed having a barrier layer at the second surface, to prevent molten metal from adhering to the seed and to facilitate its removal from the solidified casting.

6. The apparatus of claim 5 further comprising means for thermally insulating a portion of the chill plate adjacent the seed to lessen heat loss from surplus metal placed in the starter cavity.

7. The apparatus of claim 6 further comprising a mold having a starter cavity with an open end, the open end in contact with and closed by the chill plate; means for holding the seed in predetermined orientation to the chill plate;
and means for holding the mold fixedly to the chill plate, to contain molten metal and control the orientation of the mold with respect to the chill plate.

8. The apparatus of claim 5 wherein the seed is cylindrical and the barrier layer is on the circumferential surface thereof.

9. The method of casting metals into an article having controlled crystallographic orientation using a seed and a chill plate which comprises:
(a) placing a cold seed on a cold chill plate so that the seed projects above the surface of the chill plate with a controlled orientation thereto;
(b) heating a mold and then contacting it with the chill plate in a controlled orientation thereto, so the seed is surrounded by a spaced apart cavity of the mold, without substantially heat-ing the seed;
(c) filling the mold with molten metal and thereby causing a portion of the molten metal to flow across a surface of the seed to the cavity surrounding the seed in sufficient velocity and quantity to heat and partially melt a portion of the seed and remove any contamination films thereon; and (d) epitaxially solidifying the molten metal from the seed to form an article having a crstallo-graphic orientation determined by the seed.

10. The method of claim 9 further comprising: providing part of the projecting portion of the seed with a thin barrier layer resistant to the molten metal to selectively prevent adherence of molten metal to the seed, while still allowing the metal flowing into the cavity to surround and thereby heat the seed, and to facilitate removal of the seed from the casting after solidification.

11. The method of claims 9 or 10 further comprising providing an insulating layer on a portion of the chill plate surrounding the projecting seed.

12. The method of claim 9 which further comprises:
forming a mold having a first end adapted to receive molten metal, an article cavity connected to the first end, a second opposing end defining a starter cavity and adapted to contact a chill plate, and a selector cavity, the selector cavity located between the starter cavity and article cavity;
and contacting the second end of the mold with the chill plate to surround the seed with the spaced apart cavity.