U.S. patent number 3,690,368 [Application Number 05/063,738] was granted by the patent office on 1972-09-12 for casting single crystal articles.
This patent grant is currently assigned to United Aircraft Corporation. Invention is credited to Stephen M. Copley, Anthony F. Giamei, Merton F. Hornbecker, Bernard H. Kear.
United States Patent |
3,690,368 |
Copley , et al. |
September 12, 1972 |
CASTING SINGLE CRYSTAL ARTICLES
Abstract
Apparatus for the formation of single crystal articles by
directionally solidified casting techniques which substantially
eliminates the formation of heterogeneous discontinuities in the
casting.
Inventors: |
Copley; Stephen M. (Madison,
CT), Giamei; Anthony F. (Middletown, CT), Hornbecker;
Merton F. (Woodbury, CT), Kear; Bernard H. (Madison,
CT) |
Assignee: |
United Aircraft Corporation
(East Hartford, CT)
|
Family
ID: |
22051166 |
Appl.
No.: |
05/063,738 |
Filed: |
August 14, 1970 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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714722 |
Mar 20, 1968 |
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Current U.S.
Class: |
164/350; 164/34;
164/352; 164/122.2; 164/361 |
Current CPC
Class: |
C30B
29/52 (20130101); C30B 29/52 (20130101); B22D
27/045 (20130101); C30B 11/14 (20130101); C30B
11/002 (20130101); C30B 11/14 (20130101) |
Current International
Class: |
C30B
11/14 (20060101); C30B 11/00 (20060101); B22D
27/04 (20060101); B22c 009/04 (); B22c
009/22 () |
Field of
Search: |
;164/34,60,125,127,129,322,350,352,361,362 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Annear; R. Spencer
Parent Case Text
This application is a continuation-in-part of the co-pending
application, Ser. No. 714,722, filed Mar. 20, 1968 now abandoned.
Claims
We claim
1. A mold assembly for casting a plurality of single crystal
articles at one time including,
a plurality of article molds each having a crystalline growth
portion at one end thereof, said growth portion having an open
bottom end to engage a chill plate,
inner and outer mold rings defining an annular chamber in which the
article molds are positioned, with said article molds in contact
with both rings to form individual compartments in the chamber
between adjacent article molds,
a mold extension on said rings between adjacent article molds and
at the end adjacent said growth portion defining in said extension
several passages one extending downwardly from each compartment and
each having an open bottom end to engage said chill plate, each
said passage being smaller in cross sectional area than the
compartment there-above to control the amount of heat transmitted
to the chill plate,
a filling connection at the other end of each of the article molds
for filling each of said molds, and
filling extensions projecting upwardly from said inner and outer
mold rings forming passages through which the compartments in the
annular chamber may be filled.
2. A mold assembly as in claim 1 with the article molds are in
contact with said mold rings for a part of the outer surface area
of the article molds thereby to define individual casting cavities
in said annular chamber between adjacent molds.
3. A mold assembly as in claim 1 in which the article molds are
spaced apart circumferentially and are in contact with and
supported by the rings thereby dividing the annular chamber into
individual casting cavities between adjacent molds.
4. A mold assembly as in claim 1 in which the article molds are
constructed to produce a blade shape having an airfoil
configuration for part of the length and the chordwise direction of
the airfoil portion extends circumferentially of the annular
chamber.
5. A mold assembly as in claim 1 in which the article molds are
constructed to produce a blade shape having an airfoil
configuration for part of the length and the chord of the air-foil
portion extends crosswise of the annular chamber.
6. A mold assembly as in claim 5 in which the ends of the airfoil
portion of the mold are connected to the mold rings.
7. A mold assembly as in claim 4 in which the ends of the airfoil
portion of the mold are positioned in adjacent casting
cavities.
8. A mold assembly for casting directionally solidified articles,
said assembly being mounted on a chill plate for the casting
operation, said assembly including,
article forming molds each having a grain growth portion at the
bottom thereof,
inner and outer mold walls defining an annular chamber within which
said article forming molds are positioned with said molds in
engagement with said inner and outer mold walls over only a portion
of the area of the article mold walls and defining compartments in
said chamber between adjacent article molds, said inner and outer
mold walls having extensions thereon at the bottom thereof for each
compartment, each extension forming a passage therein, said growth
portions and said extensions both being open at the bottom end
thereof to engage the chill plate, and
common means for filling said article forming molds and said
compartments, the area of said passages controlling the rate of
heat loss from the material in the compartments to the chill
plate.
9. A mold construction for producing single crystal articles
comprising:
a first ring,
a second ring positioned within and spaced from the first ring, the
first and second rings being positioned to form an annular mold
cavity therebetween;
a chill plate, the first and second rings being positioned thereon
with the cavity open at the top to receive molten metal and with
mold extensions at the bottom of the annular cavity forming a
plurality of passages terminating in heat transfer relationship
with the chill plate, and
a plurality of smaller article molds positioned within the annular
mold cavity and in contact with said rings to form individual
compartments between adjacent article molds, with one of said
passages communicating with each of said compartments, each article
mold having a crystal selecting passage at the lower end thereof
constructed to permit the entrance into each of said article molds
of a single grain of metal, and a growth zone between said passage
and the chill plate the crystal selecting passage being a
restriction to heat flow from the interior of the article molds to
the chill plate, heat from within the article molds being
transferred laterally into the metal the compartments in the
annular chamber and vertically to the chill plate through said
first mentioned passages thereby controlling the configuration of
the liquid-solid interface within the article molds and the annular
chamber with respect to the chill plate the area of the first
mentioned passages controlling the rate of heat conduction from the
compartments to the chill plate.
10. A mold construction as in claim 9 wherein,
the lower end of the crystal selecting passage on each article mold
is spaced above the chill plate, and
each article mold is in contact with and supported by said first
and second rings.
11. A mold construction as in claim 10 wherein,
the lower end of the crystal selecting passage is at least one inch
above the chill plate.
Description
This invention relates to the casting of single crystal articles
and more specifically to the elimination of stringers of equiaxed
grains or freckles at the surface of the single crystal
casting.
In casting articles in single crystal form, it is desirable to be
able to cast complex as well as simple single crystal shapes that
is to say in addition to casting rods or bars it is desirable to
produce such complex shapes as turbine blades and vanes with the
entire blade or vane including the root airfoil section and at
times a shroud all of a single crystal. The complex shaped castings
have been limited in the past by the problem of extracting heat at
the proper rate since the removal of heat from the solidifying
alloy has been accomplished primarily by the conduction of heat
from the solidifying alloy through a constriction adjacent to the
bottom of the mold. The rate of heat removal through the relatively
small cross-section of alloy in the constriction has been low and
therefore the thermal gradient and the rate of growth of the
crystal have been limited. One method for increasing the heat
extraction rate has been described in a co-pending application,
Ser. No. 714,743, filed Mar. 20, 1968 for a process for casting
single crystal shapes now U.S. Pat. No. 3,543,284. This application
has the same assignee as the present application.
One of the problems in obtaining single crystal cast defect free
articles is the occurrence of jets which result in the formation of
trails of small equiaxed grains. Such convective jets can occur
within the mushy zone during solidification. These jets lead to
erosion and breakage of the dendrite structure and thus cause the
formation of trails of equiaxed grains or "freckles". The jets are
a consequence of hydrodynamic instability within the mushy zone and
become more pronounced as the liquidus and solidus interfaces
become curved or as the mushy zone height increases (i.e. the
thermal gradient decreases). Freckle grains are a major defect
since they detrimentally effect the mechanical properties of the
casting.
SUMMARY OF THE INVENTION
One feature of this invention is an apparatus by which to cast
single crystal parts such as turbine blades and vanes without the
formation of these trails of equiaxed grains. The elimination of
these discontinuities in the surface is obtained by appropriate
control of the thermal gradient within the mold, a control of the
rate of solidification of the alloy upwardly within the mold and a
control of the wall temperature of mold surrounding the article
being cast. The latter result is obtained by surrounding the
article mold at least in part by a cavity in which a controlled
solidification can take place at a selected rate thereby to more
effectively control the temperature of the article mold as
solidification occurs.
In accordance with the present invention, the article mold is
positioned within a surrounding control mold with the latter having
one or more casting cavities therein for to receive the control
portion of the hot alloy of the control of the alloy which is
poured at the same time as the alloy is poured into the article
mold. These control cavities are connected through solidification
passages to the chill plate and the article mold has a crystal
selecting passage in the lower end by which to cause the growth of
a single crystal into the article portion of the mold. Although the
mold is adapted for the production of a single cast article at one
time the construction is more adapted for the gang casting of a
plurality of articles at one time and the construction is such that
the solidified article may be readily removed from the solidified
alloy in the adjacent and partially surrounding control
cavities.
The solidification process of the present invention is carried out
by positioning the one or several article molds within the
surrounding mold and heating the entire assembly to a temperature
above the melting point of the alloy. Once the alloy is poured in
the mold assembly, directional solidification begins at the chill
plate in which the mold assembly rests and the liquid-solid
interface moves upwardly from the chill plate in both the bottom of
the article mold and in the connectors from the casting cavities to
the chill plate. The solidification of the alloy within the casting
cavities or control cavities helps to maintain the article mold at
the desired temperature during solidification and serves to control
the rate of heat removal from the article molds so that the
liquid-solid interface of the alloy within the article mold will be
retained in a substantially horizontal configuration during the
solidification of the alloy through the entire mold. This mold
construction favors higher thermal gradients and growth rates
needed for the elimination of discontinuities such as trails of
equiaxed grains in the cast surface. The rate of heat extraction by
the metal in the casting cavities is controlled to a substantial
degree by the size of the connector from these casting cavities to
the chill plate since area of solidified alloy in these passages is
a direct control of the rate of heat transfer to the chill
plate.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a horizontal sectional view through a mold assembly.
FIG. 2 is a vertical sectional view substantially along the line
2--2 of FIG. 1.
FIG. 3 is an elevational view of a portion of the mold assembly
prior to the formation of the second mold coating thereon.
FIG. 4 is a view similar to FIG. 1 of a modified form.
FIG. 5 is a vertical sectional view substantially along the line
5--5 of FIG. 4.
FIG. 6 is a fragmentary elevational view of the mold configuration
of FIG. 4.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring first to FIGS. 1-3 inclusive, the mold construction is
adapted for casting single crystal turbine blades and vanes as
described by way of example in the Piearcey U.S. Pat. No.
3,494,709. These articles are preferably cast from many of the
well-known high-temperature alloys as described in this patent or
in the VerSnyder U.S. Pat. No. 3,260,505.
The mold to be described is formed by the well-known shell molding
process in which thin mold shells are formed around disposable
patterns as for example wax patterns. The apparatus shows the mold
shells after the wax has been melted out in order that the cavities
in which the alloy is cast may be more readily distinguished. It
will be understood however that after the mold material has been
completely formed around the patterns they are melted out to leave
the cavities shown and the wax pattern did, in fact, occupy all of
the shown cavities prior to the baking of the mold to harden it and
to provide removal of the wax.
The article mold 2 has a centrally located airfoil cavity 4
extending from a root portion 6 to a shroud portion 8. Above the
shroud is the filling passage 10 and below the root 6 is the helix
12 that selects a single crystal from the columnar growth base
portion 14. The function of the growth portion 14 and helix 12 in
selecting a single crystal has been described in the co-pending
application of Copley et al., Ser. No. 806,978 filed Mar. 13, 1969
and having the same assignee as the present application.
A plurality of these article molds are positioned in a ring with
the chordwise dimension of the airfoil portion of the several
article molds extending in a generally circumferential direction as
shown in FIG. 1. The several article molds are connected together
in the circumferentially arranged relation by a disk-like filler
portion 16 for the several article molds. This portion 16 has a
chamber 18 therein communicating with all of the several article
molds.
Between adjacent article molds is a chamber 20 defined between
inner and outer mold rings 22 and 24 which are in engagement with
the surfaces of the article molds as shown in FIG. 1 leaving the
trailing edge and leading edge portions of the mold exposed to the
adjacent chambers 20. The chambers 20 are connected by passages 26
at the top communicating with the horizontal filling chamber 18 and
the same chambers communicate at the bottom by mold extension
defining a passage 28 from the chambers 20 to the chill plate 30 on
which the entire mold assemblage rests. At the top of the filler
portion 16 is a funnel-shaped pouring spout 32.
As above stated, the article mold 2 with the appendages thereon is
formed by dip coating a wax or otherwise disposable pattern that is
the shape of the article together with the growth portion at the
bottom and the filling opening at the top. When the article mold is
completed the several article molds are connected to a wax pattern
that is the shape of the chamber 18. Wax is positioned between the
leading edge of one article mold and the trailing edge of the
adjacent mold in the shape of the chamber 20 and wax rods are
positioned at opposite ends which are the shape of the passages 26
and 28. With all of the wax in position, the entire assemblage is
again dip coated thereby forming the inner and outer walls 22 and
24 together with the mold 16 surrounding the chamber 18 and the
portions of the mold connecting the mold portion 16 to the
individual article molds and also the portions of the mold
extending down from the portion 16 to the top of the annular mold
portions 22 and 24. Once the entire mold is created as shown, the
assemblage is positioned in a suitable baking oven to harden the
entire mold and to melt out the wax pattern thereby leaving a mold
structure as shown in the drawings.
FIG. 3 shows an interim construction between the making of the
individual article mold 2 in this figure and the finished mold. In
the showing of FIG. 3 a wax pattern 34 has been positioned between
adjacent article molds and wax rods 36 and 38 extend from top and
bottom of the wax portion 34. At the top the rod 36 is attached to
a wax disk 40 which forms the chamber 18 in the completed mold. A
wax pouring spout not shown is also attached to the top surface of
the disk 40.
Accordingly when the mold is completed ready to have alloy poured
therein for the formation of the several cast articles the mold
assembly consists of the circumferentially spaced apart article
molds 2 positioned between inner and outer rings 22 and 24 which
define between them the control chamber 20 the latter being an
circumferentially extending chamber since the article molds are in
contact with the inner and outer mold rings and thereby break the
circumferentially defined chamber into individual compartments
which enclose the trailing edge of one of the article molds and the
leading of the adjacent article molds.
The entire mold assembly when it is ready to have alloy poured
therein is positioned within a heating device such as an induction
furnace including a cylindrical susceptor surrounded by an
induction coil and the mold is heated to a temperature above the
melting temperature of the alloy in readiness for pouring. With the
mold assembly resting on the chill plate 30, the alloy within the
mold is in contact with the chill plate through the growth portion
14 of the article mold and through the passages 28 leading to the
individual compartments of the control chamber 20. Solidification
begins at the chill plate and moves upwardly through the growth
portion and through the helix where a single crystal is selected to
grow into the article forming portion of the article mold. Columnar
grain growth extends upwardly through the passage 28 and into the
chambers 20 and it is the solidification in these chambers and the
absorption of the heat from the article mold into the solidified
alloy and the chambers 20 and the fence by conduction through the
solidified alloy and the passage 28 to the chill plate that permits
a precise control of the solidification rate and thereby the
configuration of the mushy zone that is to say the liquid-solid
interface of the solidifying alloy in the article molds. By
suitable dimensioning of the passage 28 the rate of heat removal
from the article mold may be so controlled that the liquid-solid
interface remains substantially flat and horizontal during the
entire solidification cycle. A larger overall heat conduction path
leads to a higher thermal gradient.
Referring now to FIGS. 4, 5 and 6 the article mold 52 is similar to
that described above and has the growth zone 54 at the bottom
connected by a helix 56 to the article forming portion and with an
extension 58 at the top of the article forming portion which forms
a passage for filling the article mold. Several of these article
molds are positioned in a ring with the chordwise direction of the
airfoil portion of the mold extending in a generally radially
position rather than circumferentially as in FIG. 1. Inner and
outer rings 60 and 62 define between them a substantially annular
chamber 64. The inner and outer rings communicate and are in
vertical engagement with the leading edges of the article mold by
outward and inwardly projecting ribs 66 and 68 formed on the inner
and outer mold rings respectively. As in the apparatus above
described, there is a horizontal mold portion 70 at the top of the
mold assembly and this has a horizontal chamber 72 communicating
with all of the article molds and also through vertical passages 74
in the mold communicating with the compartments of the annular
chamber 64 which are located between adjacent article molds. The
mold also has vertical passages 76 extending downwardly from the
segments of the annular chamber 64 to engage at their bottom ends
with the chill plate 78 as do the growth zones 54 of the article
mold.
The mold assemblage is made in the same manner as above described
with respect to FIGS. 1, 2 and 3. A wax pattern for example is
formed in the configuration of the article desired with a helical
projection at the bottom and a configuration corresponding to that
of the chamber in the growth zone portion of the article mold. The
wax pattern has at the top an extension that serves to form the
passage through the upward extension 58 on the article mold and
from there upwardly to communicate with a horizontal wax disk the
shape of and serving to form the chamber 72 when the second step of
making the mold is performed. When the wax pattern for the article
mold is completed it is dipped into appropriate mold forming
slurries by the usual technique of making the shell mold thereby
forming the article mold 52 with the extensions at top and bottom
as shown. This article mold is then assembled together with the was
pattern to form the chamber 72, wax patterns forming passages 74
and 76 and a ring of wax that forms the annular chamber 64 and
encloses the article molds from top to bottom of the portion of the
article mold that becomes the finished cast article that is being
cast for use. Suitable grooves are formed in this wax pattern to
communicate with the leading and trailing edges of the article mold
thereby permitting the mold forming material to contact the first
shell when the assemblage is dipped to form the secondary mold
coating.
FIG. 6 shows the assemblage of the article molds together with the
surrounding and associated wax patterns by which the secondary mold
is produced. As shown in this figure, the pattern assembly includes
the flat disk 80 having the wax rod 82 extending downwardly into
the article mold and the parallel wax rod 84 extending downwardly
to communicate with the heavy wax ring 86 that surrounds and embeds
the individual article mold for the effective portion of the
article mold. The assemblage also has the downwardly extending wax
rods 88 from the wax ring 86 down to a level coincident with the
bottoms of the growth portions 54. Grooves 90 are formed in the wax
and extend inwardly from the outer and inner vertical surfaces of
the wax to expose the leading and trailing edges of the airfoil
portion of the article mold 52. It is these grooves that permit the
formation of the connecting ribs 66 and 68.
The assemblage of FIG. 6 is then coated with mold material as by
the usual shell molding process thereby forming the secondary mold
which surrounds the wax pattern and assemblage of FIG. 6. This
secondary mold having been completed the device is placed in a
furnace where the mold is baked to harden the material thereof to
melt out the wax pattern leaving the empty casting spaces as shown
in FIGS. 4 and 5.
Once the mold is hardened ready for use, it is positioned within a
suitable heating means preferably an induction furnace and
preferably all enclosed within a vacuum and the mold is heated to a
temperature above the melting point of the alloy. The molten alloy
is then poured to fill the spaces within the mold and
solidification begins where the alloy contacts the chill plate in
the same manner as above described with respect to FIGS. 1, 2 and
3.
With respect to both modifications of mold apparatus described,
once the alloy is entirely solidified and cooled so that the mold
with the alloy therein can be handled the outer or secondary mold
is removed to as great an extent as possible and the molded parts
of the casting are pulled apart. Because of the particular
construction of the mold apparatus in both cases such that the
segments of the control ring of alloy, that is to say the alloy
that is formed within the segments of the annular chamber 20 of
FIG. 1 or the segments of the annular chamber 64, are separable one
from another, the cast segments may be readily withdrawn from one
another thereby disengaging the cast control alloy from the article
mold and the cast article within the mold. Subsequently, with the
removal of the helix of cast alloy at the bottom of the article
mold and with the removal of the cast article from the portion of
solidified alloy extending upwardly from the top of the article
portion the latter is in readiness for inspection and for machining
for use.
With the use of the alloy in the annular chambers 20 or 64 and the
effective control of the solidification rate as well as the
temperature of the article mold during solidification of the alloy
it is possible to obtain acceptable cast articles all of a single
crystal and free of the equiaxed grains that tend to form on the
surface when appropriate temperature controls are not provided.
Although the article produced by a casting technique of this type
appears to be a relatively small portion of the total amount of
alloy used in making the casting it will be readily understood that
the remainder of the cast alloy, the so-called revert, may be added
to the alloy for the subsequent casting operations and is not in
any sense wasted.
* * * * *