PWA Tschinkel Apparatus for Casting of
Directionally Solidified Articles (Liquid Metal Cooling) 1971[1973] US3763926A
Publication
Number: US3763926A
Publication
Date: 1973-10-09
Priority
Number: USD3763926A |
US1971180597A
Application
Date: 1971-09-15
Title:
APPARATUS FOR CASTING OF DIRECTIONALLY
SOLIDIFIED ARTICLES
Inventor
- w/address: Tschinkel Johann
G.,South Glastonbury,CT,US | Giamei Anthony F.,Middletown,CT,US | Kearn Bernard
H.,Madison,CT,US
Assignee/Applicant:
United Aircraft Corporation,East
Hartford,CT,US
Front Page Drawing:
Abstract:
Apparatus for casting directionally solidified articles
either columnar grained or single crystal in which the rate of solidification
is controlled by gradual immersion or submergence by a liquid coolant.
First Claim:
1. Apparatus for
solidification by columnar growth of an alloy including
* a support plate for a mold,
* a susceptor positioned above the support
plate and forming a heating chamber for the mold,
* means for filling the mold,
* a container for a cooling liquid positioned
below the susceptor,
* means for supporting the plate and mold
during heating and filling with the plate at least partially immersed in the
cooling liquid during the heating operation, and
* means for gradually surrounding the mold with
the cooling liquid from the support plate upwardly around the mold for cooling
the material poured into the mold.
Description w/Pub Language: BACKGROUND OF THE INVENTION
One concept of high
rate solidification of directionally solidified cast articles is described in
the copending application of Barrow, et al., Ser. No. 63,143 filed Aug. 12,
1970, having the same assignee as this application. The apparatus and process
of that application produced a much higher rate of directional solidification
than was previously possible by gradual withdrawal of the mold from the heating
chamber and by simultaneously exposing the mold during the solidification
process to the cooler surrounding walls of the apparatus. In this way in
addition to the downward conduction of heat through the solidified alloy to the
chill plate, a substantial heat removal was accomplished by radiation from the
walls of the mold below the liquid-solid interface. Since a fine dendritic
spacing is desirable and since the dendritic spacing decreases with increasing
rates of solidification of the alloy (the growth rate), it is desirable to
provide for more effective cooling of the mold during the solidification
process.
The dendrites formed
within the single crystal or the columnar grains in the cast article are
distinguished from the surrounding material by differences in concentration of
some constituents. Embedded carbide particles and eutectic microconstituents,
for example, tend to accumulate in the normally weaker interdendritic regions
and the strength of the alloy is decreased by such inhomogeneities. The size of
the embedded particles and pools of microconstituents is significantly reduced
by a reduction in dendritic spacing in the casting since the interdendritic
spaces are smaller with closer dendritic spacing. After the casting is
completed, it is desirable to homogenize the cast alloys by heating them at a
temperature close to the solidus temperature. Since diffusion in solids is a
slow process, this homogenization of the alloy may require hundreds of hours
where the dendritic spacing is relatively large so that normally complete
homogenization of the dendritic structure is not practical. The diffusion time
for complete homogenization at a given temperature is proportional to the
square of the distance between the dendrites so that a reduction in dendritic spacing
by a factor of 10 can reduce the annealing time by a factor of 100 thereby
bringing the required time for complete diffusion down to a few hours. In this
way the homogenization treatment would become a practical procedure. The
spacing of the dendrites is significantly reduced by more rapid solidification
of the material being cast.
STATEMENT OF THE
INVENTION
One of the principal
features of the present invention is the very rapid heat removal from the mold
in conjunction with a sharp transition between the hot and cold surroundings in
order to maintain a high thermal gradient and also a high growth rate for
making the cast article. Another feature is the use of a liquid coolant into
which the mold is immersed or in which the mold is submerged gradually for the
rapid extraction of heat from the mold, thereby obtaining the desired grain
growth within the mold. Another feature is the use of this liquid coolant to
circulate around all of the several molds in a multiple mold casting so that
the heat removal from the several molds will be the same and, accordingly, the
desired grain growth will be obtained within all of the molds. A particular
feature of the invention is the control of the dendritic growth within the
casting in such a way as to significantly reduce the distance between the
dendrites and thereby minimize the segregation of the microconstituents in the
interdendritic regions.
In the use of this
apparatus for making directionally solidified castings, the apparatus is
particularly adapted for making columnar grained articles as described in
VerSnyder U.S. Pat. No. 3,260, 505 or the single crystal articles of Piearcey
U.S. Pat. No. 3,494,709. These latter are a particular form of columnar grained
articles being only a single grain, rather than the plurality of parallel
grains as in VerSnyder. The apparatus is also particularly usable on the alloys
of Gell U.S. Pat. No. 3,567,526, which also describes a form of columnar
growth. Thus, the term "columnar growth" is intended to encompass all
of these several types of grain growth.
According to the
invention, the apparatus includes a heating chamber within which the mold is
positioned for raising the mold to a high temperature above the melting
temperature of the material to be cast, a container for a liquid bath below the
heating chamber and in which the mold is immersed or submerged, a device for
filling the mold, and a device for moving the mold relative to the chamber and
container for gradually immersing the filled mold into the cooling liquid and
simultaneously withdrawing it from the heating chamber. The process is carried
out by heating the mold before filling to a temperature above the melting
temperature of the material to be cast, pouring the molten material into the
mold and then gradually withdrawing the mold from the heating area and
simultaneously submerging it gradually into a liquid cooling bath thereby
establishing a steep thermal gradient in the material in the mold and causing a
vertical solidification of the material in the mold from the base of the mold
to the top at a controlled rate.
A modified form of the
invention provides for gradually reducing the heat supplied to the mold from
bottom to top and gradually filling the container with a liquid coolant. In
either form of the invention, the filled mold is gradually surrounded with a
cooling liquid from bottom to top of the mold and at the same time, the heat
supplied to the mold is gradually reduced from bottom to top by withdrawal of
the mold or by a step-by-step reduction in the heat input to the mold as the
level of the cooling bath effectively moves upwardly around the mold.
BRIEF DESCRIPTION OF
THE DRAWINGS
FIG. 1 is a vertical
sectional view through an apparatus embodying the invention.
FIG. 2 is a
fragmentary vertical sectional view of a modification.
FIG. 3 is a
fragmentary vertical view of another modification.
FIG. 4 is a transverse
sectional view through a multiple mold showing the effectiveness of liquid
cooling.
FIG. 5 is a vertical
sectional view through a modified form of apparatus.
FIG. 6 is a transverse
microstructure of a single crystal cast conventionally.
FIG. 7 is a similar
view at the same magnification of a single crystal cast by the process of the
present application.
FIG. 8 is a similar
view at the same magnification of a single crystal cast by the present process
but at a faster cooling rate.
PREFERRED EMBODIMENT
OF THE INVENTION
Referring first to
FIG. 1 which shows the apparatus of the present invention, the article to be
cast is made in a mold 2 which rests on a support or chill plate 4, the latter
being carried by a suspending shaft 6 attached as by a threaded connection 7 to
the plate. In the position of the mold shown, it is surrounded by a susceptor 8
in the form of a graphite sleeve which in turn is surrounded by induction
heating coils 10 by which the susceptor may be heated with the latter in turn
heating the mold prior to the mold filling operation. Suitable heat shields 12
are positioned at the lower end of the susceptor sleeve 8 near the periphery of
the chill plate and other heat shields 14 close the upper end of the chamber 16
formed within the susceptor 8 and within which the mold is positioned. These
heat shields 14 are in the form of a removable cover. A pouring cup 18 may be
positioned in the shield 14 at the top of the chamber.
Positioned below the
heating chamber 16 is a tank 20 which holds a liquid 22. The tank 20 may have
heating elements 24 surrounding it for raising the temperature of the bath to
the desired temperature for immersion of the mold therein and the chamber is
also preferably surrounded by cooling coils 26 adjacent the upper end of the
tank for the purpose of maintaining the desired temperature within the bath of
liquid especially as the mold is immersed therein during the solidification
process. Suitable stirring means 27 may be provided to assure a circulation of
the liquid bath when the casting process is being carried out. The tank may be
secured to the wall of the vacuum chamber not shown in which the apparatus is
positioned.
The position of the
heating and cooling coils 24 and 26 around the tank serves to create and
strengthen convective currents in the liquid bath to circulate the liquid and
thereby maintain a more nearly constant temperature for the portion of the bath
in which the mold is being immersed. The effect of the immersion of the mold is
to heat the surrounding liquid rapidly causing an upward flow toward the
surface. The cooling coils, near the top of the liquid bath, serve to cool the
adjacent liquid and cause a downward flow along the inner surface of the tank
toward the bottom. At this point the liquid is again heated by the heating
coils and an upward flow near the middle of the tank is caused. Thus, in some
instances, the circulation of the liquid bath by the stirring device may be
omitted. It will be understood that the drawing shows the several parts merely
diagrammatically and suitable support means are provided for maintaining the
tank 20 in a predetermined relationship with the heating chamber thereabove.
The level of the liquid bath 22 is preferably such that it partially submerges
the support plate 4 therein when the mold is within the heating chamber for the
heating and pouring operation and in this way this plate serves as an effective
chill plate without the need for a circulation of coolant through the plate.
The mold is preferably
of the well known ceramic shell mold type and as shown is a multiple mold and
has two article forming portions 30 positioned on opposite sides of a central
carrying cylinder 32. The article portions 30 are shown with a cavity in the
shape of a turbine blade by way of example. The cylinder 32 is of a dimension
to fit around the vertical shaft 6 as shown. Between each of the article
portions of the mold and the central sleeve 32 are vertically filling tubes 34
communicating at their top ends with a filling ring 36, the latter at one point
being positioned directly below the pouring cup 18. Each article portion of the
mold has an upward projecting riser portion 38 terminating at a point at least
as high as the top of the filling ring 36. Below and communicating with the
article forming portion of the mold is a growth zone including a crystal
selector 40 which may be a helix defining therein a helical passage for
selecting a single crystal to grow into the article portions. The helical
passage terminates at the bottom in the main growth zone 42 in which columnar
grains are grown. The filling tubes 34 communicate with the growth zone 42 as
shown. Thus, when alloy is poured into the cup 18, it flows into the ring 36
through the tubes 34 into the growth zone and thence upwardly through the
crystal selector to fill the article portion of the mold and upwardly into the
riser. This mold arrangement is suitable for making single crystal articles.
Referring now to FIG.
2, a portion of a mold is shown which is adapted for making columnar grained
cast article instead of single crystal articles. To do this, the article mold
30' has the riser 38' at the top and a growth zone 42' at the bottom open to
the chill plate. The crystal selector of FIG. 1 is omitted and the growth zone
communicates directly with the bottom end of the article mold portion, the
dividing line being represented by the dashed line 43, FIG. 2, and it is along
this line that the growth portion of the casting would be severed from the
article itself.
Crystalline structures
of other orientations than 001 may be made by the use of a mold as shown in
FIG. 3. In this arrangement, the article mold portion 30" has the riser
38" at the top, and a growth zone 42" at the bottom. This growth zone
receives a single crystal slug 46 of the desired orientation and the base of
this slug is preferably set into a recess 48 in the support plate 4 so that
this slug will not be totally melted during heating of the mold. When the alloy
is poured, single crystal growth occurs with the dendritic orientation throughout
the arrticle the same as that of the slug 46. In FIG. 1 the crystal selector is
considered a part of the growth zone when making single crystal castings.
One particularly
suitable liquid for use in the cooling process is tin because of its low vapor
pressure and because of its low melting temperature (450° F). A suitable
temperature for the tin bath is about 500° F since clearly the lower the
temperature of the bath the faster the cooling rate will be. As above stated,
the plate 4 is partially immersed in the tin bath at the start of the casting
operation and serves as a chill plate.
The process is
desirably carried out in a vacuum or in an inert atmosphere and to this end the
apparatus is positioned within a vacuum chamber. With the apparatus in the
position of FIG. 1 and with the mold in position resting on and sutiably
secured to the support plate to prevent leakage of molten material from within
the mold, the latter is heated by energizing the induction coils 10 to raise
the temperature of the mold itself at least to the melting temperature of the
alloy and preferably to a temperature as much as about 300° F above the melting
temperature. Where the article to be cast is a turbine blade which is the shape
shown for the article portion of the mold in FIGS. 1 and 2 of the drawings, one
superalloy suitable for the purpose is Mar-M 200 although many other alloys are
equally suitable as described, for example, in the patents to VerSnyder U.S.
Pat. No. 3,260,505 and Piearcey U.S. Pat. No. 3, 494,709 and also in the patent
to Gell, et al. U.S. Pat. No. 3,567,526.
The alloy to be cast
is heated to a point about 300° F above the normal melting point of the alloy
so that it has a significant superheat. With the mold above the melting point
of the alloy and with the alloy itself superheated to this extent, the alloy is
poured into the mold, filling the mold at least to a point above the article
portion of the mold and preferably substantially to the level of the pouring
ring 36. Since the temperature of the support plate 4 is kept substantially at
the temperature of the liquid bath, dendritic growth immediately begins within
the growth zone 42 of the mold and as solidification continues upwardly through
the growth zone, the grain growth becomes columnar as described in the Piearcey
patent. Almost immediately after the alloy is poured and when grain growth has
begun, the support plate with the mold thereon is gradually lowered from the
heating chamber 16 so that the support plate is completely immersed and then the
mold is gradually immersed within the liquid cooling bath. As the mold moves
downward into the bath, the liquid coolant flows over the surface of the
support plate and around the various portions of the mold. Since the coolant is
in contact with all the outer surfaces of the mold, it completely surrounds the
mold and rapidly removes heat from all portions of the mold thereby increasing
the rate of solidification of the alloy in a vertical direction. The grain
selector 40 functions in much the same manner as the crystal selector in the
Piearcey patent to cause the growth of a single crystal from the main growth
portion into the article forming portion of the mold.
The mold is gradually
and continually moved downward into the liquid bath at such a rate that the
level of the cooling bath does not precede the solidus level by any substantial
amount so that the removal of heat from the mushy zone of the solidifying alloy
is vertically downward and the level of the liquid-solid interface will remain
substantially horizontal. This will assure the growth of a single crystal
within the article portion of the mold and prevent nucleation of spurious
grains along the surfaces of the mold. The high resultant thermal gradient and
the level interfaces also tend to suppress convection due to concentration
differences in the molten superalloy which could otherwise lead to a
solidification defect known as freckles.
In using a superalloy
for making turbine blades, if the blade is 4 inches in length, for example, and
the height of the growth zone 42 is preferably at least an inch, the total
height of the mold including the riser would be 8 inches. In a specific casting
operation in making a single crystal blade, this mold is heated to 2,850° F
except for the portion closely adjacent to the support plate. The alloy is
heated to 2, 850° F and is then poured into the mold which is at this time
positioned on the support plate 4 and within the heating chamber. The support
plate and the mold thereon are held in the position shown for 1 to 5 minutes
for the start of the columnar growth in the growth zone before a downward
vertical movement of the chill and mold into the liquid tin at 500° F is
started. The downward movement of the chill and mold is carried out at a
uniform rate of 120 inches per hour until the mold is immersed to a point at
least 1 inch above the top of the article portion of the mold thereby assuring
a growth of a single crystal through the entire article forming portion of the
mold.
Since the distance
that the mold must move downward to be immersed to this extent within the
liquid tin bath is 6 inches, it will be apparent that the complete operation
for adequate immersion of the mold requires only 3 minutes plus the holding
time from the time of pouring the mold for a completion of the solidification
process. The mold is then withdrawn upward and the device is preferably so
constituted that the mold is drawn up through the heating chamber to a point
thereabove, the heat shields 14 being carried upward therewith by a support
collar 44 on the shaft. With the mold and support plate completely above the
apparatus shown, removal of the mold from its position on the support plate is
done by unscrewing the plate and retracting the mold from its position around
the shaft. Any suitable mechanism not a part of this invention may be provided
for this purpose. Obviously, the suspension shaft could be moved laterally to
position the mold and plate over a suitable bench rather than over the hot
chamber 16.
The heating coils are
continuously energized and, therefore, the susceptor 8 is retained at its high
heat during the downward movement of the mold into the cooling bath so that,
above the level of the bottom of the susceptor, the mold is still kept near the
2,850° F temperature. In this way a very high thermal gradient is maintained in
the material within the mold between the level of the bottom of the susceptor
and the top of the tin bath. That is to say, the mold is surrounded by a
temperature above the melting point of the alloy throughout the entire height
of the susceptor and the lower portion of the mold is immersed in a cooling
bath at 500° F at a very short distance below the bottom end of the susceptor
thereby establishing this very high thermal gradient. The steepness of the
thermal gradient at the interface is determined to a great extent by the
spacing of the susceptor above the surface of the bath, by the temperature and
effectiveness of the bath, and by the alloy superheat.
Further, the rate of the
upward movement of the liquid-solid interface, the growth rate, is determined
by the rate of downward movement of the mold into the liquid bath. Since the
bath is in contact with the outer surfaces of the mold, the rate of heat
withdrawal from the mold and thus from the alloy at and below the surface of
the liquid bath by conduction will be extremely rapid. It is desirable to have
a relatively thin mold wall thereby to improve the heat transfer rate and thus
the wall thickness of the mold will be limited by the strength needed to
withstand the pressure of the material within the mold during the casting
process.
Instead of withdrawing
the mold from the heating chamber and immersing it in a liquid coolant bath,
the mold may be submerged gradually by pouring the cooling liquid into a
chamber surrounding the mold. As shown in FIG. 5, the mold 50, which is shown
as a single article mold, rests on a chill plate 52 and is surrounded by a
susceptor 54. The foot 56 of the mold is extended to overlie the entire chill
plate and to extend under the susceptor at the periphery of the chill plate.
The susceptor is held to the mold foot by cement 57 to form a liquid tight
connection at this point. The susceptor is surrounded by an induction heater 58
which consists of several axially aligned coils so that the energy supplied to
the coils may be gradually reduced from the bottom to the top of the susceptor.
A pipe 60 provides for admission of a supply of liquid coolant into the chamber
surrounding the mold. In use, the mold having been heated to the desired
temperature as above described, is filled with the superheated molten alloy and
solidification is started at the chill plate by the supply of coolant to the
passages in the chill plate. After a short period for the columnar growth to be
established in the mold at the chill plate, a cooling liquid is supplied to the
chamber and simultaneously the lowermost heating coil is turned off. The
coolant surrounds the mold and rapidly extracts heat from the mold and the
alloy therein to cause upward solidification of the alloy. The rise of coolant
in the chamber for submerging the mold is at the same rates given above for the
downward movement of the mold in FIG. 1. Except for the need for the tin to
absorb heat from the susceptor, the effect is the same in the submergence of
the mold by the poured-in coolant as in the immersion technique of FIG. 1. As
the level of the coolant rises within the chamber, successive coils are shut
down so that only the portion of the susceptor above the level of the coolant
continues to be heated.
Although the above
process has been described with respect to superalloys having a nickel or
cobalt base, it should be understood that the apparatus and process are
applicable not only to the casting of this particular type of alloy but may be
equally well adapted for the casting of other materials, for example, some of
the eutectic type of alloys among which may be the materials described in
Lemkey U.S. Pat. No. 3,552, 953 and Thompson U.S. Pat. No. 3,554,817.
Accordingly, the use of the term alloy is intended not to be limited strictly
to known superalloys of the nickel or cobalt base as in the above VerSnyder and
Piearcey patents but is more generally intended to mean any mixture of
materials subject to being directionally solidified preferably in a columnar
grained or single crystal configuration.
In the solidification
of these eutectic-type alloys, two conditions in general are highly desirable.
First, maintaining at the liquid-solid interface a large ratio of thermal
gradient to growth rate and, second, maintaining a flat and horizontal
liquid-solid interface since these eutectic-type alloys can tolerate only a
very small change from this orientation. This process offers the very high
thermal gradient desired and allows the maximum permissible growth rate which
is higher with higher thermal gradients. Since the thermal gradient is several
times as high as by other known techniques, the growth rate may also be several
times as high without affecting the ratio of thermal gradient to growth rate.
The rate of
solidification is limited by the rate of removal of heat from the alloy which
produces no excessive curvature of the solidus surface. Since the size of the
dendrites grown is a function of the rate of cooling, the shorter the time for
solidification, the closer will be the dendritic structure. In experimentation,
growth rates as high as 180 inches per hour have been realized and such rates
or higher are not unreasonable in casting, for example, blades and vanes for
gas turbine engines. The growth rate depends on the cross-sectional area of the
material in the mold and also the shape of the article since, for example, a
blade shape has a greater surface area than a circle for the same
cross-sectional area and will therefore lose heat faster.
As above stated, the
thermal gradient is controlled by several parameters such as the amount of
superheat in the molten alloy at the time of pour, the temperature of the
liquid bath and the spacing between the bottom of the heating chamber and the
surface of the liquid bath. The thermal gradient may be quite steep and
gradients as high as 500. degree. F per inch have been obtained. Thermal
gradients as high as 1, 000° F per inch are feasible with the present invention.
The growth rate, the
rate at which the solidification front moves upward, is controlled essentially
by the rate at which heat can be removed from the mold. With the thin mold
wall, the heat removal is a function of the cross-sectional area of the alloy compared
to the surface area, the rate at which the mold is immersed in the bath and the
ability of the bath to accept the heat removal without a significant increase
in temperature. This last parameter is thus affected by the volume of the bath,
the specific heat of the material of the bath, the circulation of the bath to
keep the liquid close to the mold in motion and the external cooling means for
temperature maintenance. The growth rate is thus substantially independent of
the thermal gradient and either may be adjusted independently for optimum
results.
The effects of the
high solidification rate and the high thermal gradient of this invention is
emphasized in FIGS. 6, 7, and 8. FIG. 6 is a transverse microstructure of a
single grain at 100 magnification of Mar-M 200 alloy cast by directional
solidification techniques as in the VerSnyder patent. This shows the large
dendrites with comparable large dendrite spacing, the white areas being
eutectic microconstituents forming areas of inhomogeneity that decrease the
strength of the alloys. FIG. 7 shows a similar microstructure of the same alloy
cast by the present techniques at 25 inches per hour immersion rate with
obviously a much finer dendrite structure and closer interdendrite spacing and
smaller embedded carbide particles and eutectic microconstituents. The alloy is
thus inherently stronger and more resistant to fatigue. The smaller dendritic
structure and spacing also extends throughout the cast article and thus
provides much more uniform mechanical properties such as fatigue strength,
stress rupture and yield strength in all areas of the casting. This minimizes
the scatter of mechanical properties characteristic of more conventionally cast
articles.
FIG. 8 is a transverse
microstructure also at 100 magnification of the same alloy as in FIGS. 6 and 7
but solidified by the present techniques with a 180 inches per hour immersion
rate. When solidified at this rate, the dendritic structure and spacing is very
much smaller than at the slower immersion rate of FIG. 7, and the carbide
particles and eutectic microconstituents are also much smaller by reason of the
closer dendrite spacing. As in the casting from which the showing of FIG. 7 was
made, this microstructure prevails throughout the casting, thus assuring
uniform mechanical properties throughout the cast article.
The present apparatus
and technique permit the reproduction of the desired microstructure and the
desired mechanical properties in successive castings made so that many castings
may be made, as, for example, a complete set of turbine blades or vanes for a
gas turbine, all of which will have the same properties.
The pools of eutectic
microconstituents shown in these microstructures may be minimized or eliminated
by heating the alloy close to the solidus temperature to diffuse the materials.
If the dendrite spacing is large as in FIG. 6, the cast articles must be held
at this temperature for a long time since the diffusion time is proportional to
the square of the distance between the dendrites. The structure of FIG. 7 can
be homogenized by only a few hours of heating making such a treatment
practical. The structure of FIG. 8 would require a significantly shorter time
than FIG. 7 because of the smaller dendritic spacing.
This invention has utility
in making a plurality of castings at one time in a multiple mold since the
cooling bath will circulate within the areas between individual mold elements
and allow the individual elements of the mold to cool uniformly throughout
their cross section as well as along their length. FIG. 4 shows one of the
advantages of the liquid bath cooling in casting a plurality of directly
solidified articles at one time. This figure shows the multiple mold within a
susceptor 8. Where the mold is a multiple mold having a large number of article
mold portions 62 arranged in an outer ring and other article mold portions 64
in an inner ring as shown and all interconnected for filling simultaneously,
the entire mold is filled at one time. With such a mold structure, the outer
surfaces of the outer mold portions will lose heat rapidly by radiation to the
surrounding cooler chamber wall but the inner surfaces of the outer ring of
mold portions will be unable to lose heat at the same rate since the adjacent
surfaces are the equally hot inner mld portions 64. Thus, the growth of the
dendrite structure is irregular and the liquid-solid interface is not kept
relatively horizontal but becomes tipped. This results in slower cooling and a
slower than desirable rate of solidification. The inner ring of mold portions
48 are cooled even less rapidly since only small areas are exposed to any cool
surface to which heat may radiate and thus the major cooling must be by
conduction through the solidified alloy to the chill plate. The liquid-solid
interface of these mold portions is thus also distorted from the optimum
substantially horizontal configuration with resulting unsymmetrical and
nonuniform dendrite growth.
The present invention,
by exposing the entire periphery of each mold portion to the same cooling
liquid at substantially the same temperature, makes possible the desired rapid
and uniform heat removal by conduction through the mold to the cooling liquid
thereby to assure a substantially uniform growth rate in all the mold portions and
with the liquid-solid interface remaining substantially horizontal in all of
the mold portions and at substantially the same level during the entire
solidification process.
Claims: We claim:
1. Apparatus for
solidification by columnar growth of an alloy including
* a support plate for a mold,
* a susceptor positioned above the support
plate and forming a heating chamber for the mold,
* means for filling the mold,
* a container for a cooling liquid positioned
below the susceptor,
* means for supporting the plate and mold
during heating and filling with the plate at least partially immersed in the
cooling liquid during the heating operation, and
* means for gradually surrounding the mold with
the cooling liquid from the support plate upwardly around the mold for cooling
the material poured into the mold.
2. Apparatus as in
claim 1 in which the cooling liquid is molten tin.
3. Apparatus as in
claim 1 including
* liquid in the container to a level to
partially immerse the support plate during heating of the mold, and
* means for moving the mold and support plate
gradually into the liquid in the container after the mold is filled.
4. Apparatus for
directional solidification including
* a support plate for supporting a mold,
* a container for a liquid for cooling the
mold,
* a heating chamber directly above said
container and having means associated therewith for heating the mold, the
support plate being located at the bottom of said chamber,
* liquid in the container to a height to
partially immerse the support plate during heating of the mold in the chamber,
and to be close to the bottom of the heating chamber, and
* means for providing vertical movement between
the support plate and the chamber and container for moving the mold out of the
chamber and into the container thereby immersing the mold in a cooling liquid
therein.
5. Apparatus as in
claim 4 including
* means for controlling the temperature of the
liquid within the container.
6. Apparatus as in
claim 5 in which the container is surrounded by heating means and also cooling
means.
7. Apparatus as in
claim 5 in which the means for controlling the temperature includes a cooling
coil surrounding the container adjacent to portions of the surface of the pool
of liquid.
8. Apparatus for
directional solidification including
* a support plate for a mold,
* a container below and in a position to
surround the support plate,
* a liquid bath in said container,
* a heating chamber above said container and
having means associated therewith for heating the mold, the level of the
surface of the liquid bath being close to the bottom of the chamber and being
such that the support plate is partially immersed in the liquid while the mold
is being heated within the chamber, and
* means for moving the support plate vertically
at a controlled rate for moving the mold out of the chamber and into the
container after solidification begins on the support plate thereby immersing
the mold in the liquid.
9. Apparatus as in
claim 8 in which the surface of the liquid bath is closely below the bottom of
the heating chamber.
10. Apparatus as in
claim 8 in which the support plate is supported from above the chamber.
11. Apparatus as in
claim 8 including
* cooling means associated with the container
and located adjacent to the level of the surface of the liquid bath for cooling
the bath, and
* other means also associated with the
container and located adjacent to the bottom thereof for heating the bath.
12. Apparatus for
directional solidification of alloys and metals including
* a container for a cooling liquid,
* a susceptor positioned above the container
and forming a heating chamber for a mold,
* means for supporting the mold within the
susceptor, the bottom of the susceptor and the surface of the cooling liquid
being so closely positioned that the bottom of the mold is cooled by the
cooling liquid while the mold is in heating position within the susceptor,
* means for filling the mold while it is in
heating position, and
* means for moving the mold gradually into the
liquid in the container for controlled solidification of material in the mold.
13. Apparatus as in
claim 12 in which means are provided for controlling the temperature of the
liquid in the container.