Method and apparatus for epitaxial solidification

Disclosed are a mold, apparatus and method for obtaining articles of controlled crystallographic orientation using solidification from seeds. The starter section of a directional solidification mold is adapted to both contain a seed and receive molten metal which 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 section 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.

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 crystallographic orientation. 
2. Description of the Prior Art 
lt is well known that great improvements in the performance of metal 
structures 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. Pat. No. 3,260,505 and Piearcey, U.S. 
Pat. No. 3,494,709. The principal objective of the prior art apparatus, 
methods, and articles has been to provide structures which have enhanced 
properties along the principal axis of the article, that is, the principal 
axis of the article is typically the solidification growth axis or the 
axis along which the solidification front is caused to move. 
When metals are directionally solidified, they often by nature solidify or 
grow faster in one crystallographic orientation than others. For example, 
in nickel base superalloys the &lt;001&gt; orientation is found to predominate. 
As a result, single crystal castings made by means disclosed in U.S. Pat. 
No. 3,494,709, mentioned above, will have the &lt;001&gt; orientation lying 
along the growth axis. Therefore, to produce another crystallographic 
orientation along the principal axis of solidification specialized 
techniques must be used. 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 of 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 influenced by its crystallographic orientation. For example, 
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 of 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 
location in a part is determined, the orientation will be the same in 
another region in the absence of 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 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. 
Pat. No. 1,793,672 and other publications dates from the 1920's. Delano in 
U.S. Pat. No. 2,791,813 describes structures with controlled 
crystallographic orientations in which seed crystals are used to attain 
the desired result. Barrow et al in U.S. Pat. No. 3,759,310 describes an 
apparatus and electric arc method for making single crystal articles with 
a consumable electrode in which a seed crystal at the bottom of the mold 
is used. More recently, Petrov et al in U.S. Pat. No. 3,857,436 describes 
an improved method and apparatus for manufacturing 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 describes further 
refinements. Also, Copley U.S. Pat. No. 3,598,169, discloses the casting 
of relatively flat objects using seed wedges and accomplishing radially 
outward solidification. 
With the exception of Barrow, all the aforementioned techniques anticipate 
heating the mold prior to the introduction of the molten metal. The 
practice in the prior art 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 contacts the 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 techniques to produce 
articles in quantity on an economic basis. According to one of these 
techniques, described in King et al, U.S. Pat. No. 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 
since superheating often leads to increased time and cost, undesired 
vaporization of elements, and increased degradation of the mold. To 
separately heat the seed or to include the seed with the mold when the 
mold is being heated after the methods of the older art is also 
disadvantageous, both from the mechanical and manufacturing complications 
and because the seed can become unduly oxidized or otherwise contaminated. 
Another consideration during the manufacture of articles of controlled 
primary and secondary crystallographic orientation is that after 
manufacture, the orientation of the seed must be, first, accurately 
defined by suitable inspection techniques and, second, controlled 
precisely with respect to the axes of the articles being cast. 
Accordingly, the providing of seeds for casting can represent a 
significant cost. It 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, recover for reuse is difficult. 
SUMMARY 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, a mold has an article cavity connected by a 
selector section to a starter cavity, wherein the starter cavity is 
adapted to contain a seed and receive surplus molten material which is 
caused by the mold design to flow over the seed to thereby heat and melt a 
part of it, and remove any undesirable contamination film which is 
present. The mold is best used conjunctively with a chill plate. 
Preferably the seed projects into the starter cavity to allow the surplus 
molten metal to surround it and thereby further heat it and aid subsequent 
solidification. A barrier layer, such as a ceramic coating, may be 
provided on selected portions of the seed to facilitate its removal from 
the solid metal casting for reuse. In one embodiment, 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 another embodiment of the invention, a mold has an article section 
connected to the starter section by a straight selector section. 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, which then passes 
through the selector section, whereupon it is discharged so it impinges on 
the surface of the seed. 
The invention is suitable for the production of cast articles of any alloy, 
in any desired controlled structure producible 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, overcoming 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 
introduction 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 compared to growth without 
seeding, allowing the realization of benefits from primary and secondary 
orientation control. Single crystal mold design can be simplified and 
initial solidification rates increased, thereby increasing production 
yield. 
The foregoing and other objects, features, and advantages of the present 
invention will become more apparent from the detailed description of the 
preferred embodiment and the accompanying drawings which follow.

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. Pat. No. 3,895,672, for the production of 
one piece single crystal nickel alloy castings, although its use is not 
limited to such. 
A mold 20 made of a ceramic material suitable for forming a single crystal 
article is mounted on a copper chill plate 22 as shown in FIG. 1. The mold 
is comprised of a section which defines an article cavity 24 which, as 
FIG. 2 indicates, is configured to a gas turbine airfoil, to the 
production of which the present invention especially contributes. 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 crystallographic 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 
is a starter cavity 32 defined by the second end 33, the starter section, 
of the mold and the chill plate 22. A selector section 35 connects the 
starter cavity 32 and the article cavity 24 at the base of which there is 
a transition section 54. The selector cavity 34 has a substantially 
smaller internal cross sectional area than either the starter cavity or 
article cavity. 
In the preferred 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 section would be about 0.5-1.0 
cm above the surface of the seed. The selector section size would be about 
3-6 mm diameter by 6-12 mm long. Thus the starter cavity would have a 
volume of more than five times that of the seed contained therein. As is 
pointed out below, this volume is available for receiving molten metal for 
heating the seed and initiating epitaxial solidification therefrom. 
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 FIG. 3, are utilized to maintain good contact between 
the mold and chill plate. Other mechanical fasteners and fixtures are 
equally suitable 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 together, the assembly is 
adapted to be placed within various apparatuses described in the prior art 
for directional solidification. 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 apparatus 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 cavity 34 
which is adapted to impinge and flow the metal across the surface 38 of 
the seed 28. The action of the molten metal on the seed surface 38 thereby 
heats it and causes it to melt and through turbulence enhances the removal 
of any deposits or films. The molten metal having passed across the 
surface of the seed is deposited in the starter cavity 32 in the reservoir 
region 33 adjacent the seed. Thus the starter cavity is configured to 
provide a portion or region which is a receiving reservoir for the extra 
molten metal used to heat the seed. Optionally, as shown in FIG. 7, a 
receiving reservoir 50 may be located apart from but continguous to the 
starter cavity 32, to which it is connected by a channel 52. This separate 
reservoir may be used independently or conjunctively with a reservoir 
within the starter cavity. 
Thus it may be seen that, first, the seed is impinged upon by molten metal 
to cleanse and heat the surface, and a reservoir is provided to receive 
the metal. Second, as an improvement, the seed is mounted so it may be 
surrounded by the molten metal to impart additional heat to it. Thus a 
distinction may be made between the molten metal resting directly between 
the seed and the selector section which is absolutely required (elsewise 
no growth can take place) and the metal used to heat the seed which can be 
said to be surplus or extra (since other means may be used to heat and 
melt the seed). 
Metal introduction to the starter cavity by a separate gate, as shown in 
U.S. Pat. No. 3,915,761, is an option. In such cases the starter cavity 
still must be configured to allow through-flow of molten metal, across the 
seed and to the region functioning as a reservoir. 
As pointed out in the background, the preferred use of the invention is in 
a method wherein the mold is separately heated prior to clamping to the 
chill plate. This process provides the greatest rapidity of production and 
also requires the maximum heating of the cold seed. But the invention will 
also be found useful in processes wherein the mold is heated after 
clamping to the chill plate, since to economize on material, the seed may 
not project much from the chill plate and therefore may need considerable 
heating beyond that which it receives by radiation from the mold. In other 
instances, the seed may not project significantly above the chill plate 
surface and thus will be unheated, regardless of the manner in which the 
mold is heated. In still further instances the inventive mold and process 
may be used without a chill plate, as where other means such as radiation 
are used to extract heat from the seed and induce epitaxial 
solidification. These have been found effective when the molten metal is 
dumped very rapidly into the article cavity, thereby producing a certain 
head of molten metal above the selector section and resultant impingement 
velocity against the seed. For other article cavity configurations and 
rates of molten metal introduction, it may be necessary to vary the 
dimensions somewhat; as for example, changing the selector section 
dimensions to restrict and prolong flow, or increasing the reservoir 
capacity. Although somewhat evident, it may be noted that the seed need 
not be of the desired crystallographic orientation throughout. Obviously, 
the portion far away from the melted back end, e.g., that embedded in the 
chill plate, can be any orientation, as its function is mechanical. Also, 
the portion that is entirely dissolved away can be any orientation. 
When the mold has been filled with metal, by withdrawal of heat through the 
chill and mold walls according to known practice, molten metal 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 the 
process. 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 from the seed crystal, it will have the same orientation as 
the seed crystal. In like fashion, the article formed in cavity 24 will 
have a similar orientation, as it takes its structure from the 
earlier-formed metal emanating from the selector section and expanding in 
dimension through the transition section 54. 
FIG. 3 illustrates in more detail the arrangement of the important elements 
of the invention in the starter section. 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 FIG. 3, the mold is oriented to the chill 
plate by means of bolts 37 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 using 
electro-optical sensors with suitable indices. Shown in the detail of FIG. 
4 are means for orienting 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 is provided 
with an integral key way 48. Other mechanical detents and locators and 
other polarizing methods will also be suitable. 
Further shown in FIGS. 3 and 4 is a ceramic shield 40 surrounding the 
circumferences of the seed 28. This is a barrier layer to prevent molten 
metal which has passed over the surface 38 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. Accordingly, after the metal in the cavity 32 has 
solidified and the entire casting is removed from the mold, the casting 
can be cut across the plane of surface 38 and the seed will thereby be 
readily detachable from the starter section casting, and with minor 
preparation can be reused. 
FIG. 5 shows an alternate embodiment of 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 molten metal during the short time it is 
exposed to it prior to solidification. It is only required that the shield 
be formed of a material which has the requisite thermal and corrosion 
resistance and is in addition of sufficient mechanical strength to not 
become loose under the action of the molten metal. Of course to achieve 
the object of the invention, the barrier layer around the seed 
circumference need not be a separate mechanical element but can be a 
coating on the seed as well. FIG. 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 
addition is a ceramic annular disc 44 which is resting on the chill 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 metal solidifying from the chill 
plate surface 36 will not have the desired crystallographic orientation of 
the seed. In particular starter cavity configurations, the presence of the 
disc 44 gives more assurance that metal having an undesired 
crystallographic orientation will not reach the selector section 34 before 
that metal epitaxially solidifying from the seed surface 38. In FIG. 6, 
the disc 44 is shown as a separate element covering the entire exposed 
chill plate in the cavity 32. However, the diametrical extent of coverage 
can be varied, for example, by decreasing the diameter of the disc 44 so 
that 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 
solidification of the article. In addition, the disc 44 may be made 
integral with the shield 40 as is shown in FIG. 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. 
FIG. 7 shows an embodiment wherein the shield for the seed and chill plate 
is integral with the mold. The disc 44 can also be configured as a coating 
on the chill plate, and the functioning of the disc can be varied by the 
thickness and thermal characteristics of 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 FIG. 
1 as an assembly, as is the common practice in the mass production of 
directionally solidified investment castings. Alternately, 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. Pat. No. 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 invention 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 
various changes and omissions in the form and detail thereof may be made 
therein without departing from the spirit and scope of the invention.