U.S. patent application number 11/879770 was filed with the patent office on 2012-07-12 for preformed ceramic seed well for single crystal starter seed.
This patent application is currently assigned to United Technologies Corporation. Invention is credited to Steven J. Bullied, Lea D. Kennard, John J. Marcin, Stephen D. Murray, Marc R. Sauerhoefer, Michael K. Turkington.
Application Number | 20120175075 11/879770 |
Document ID | / |
Family ID | 39855037 |
Filed Date | 2012-07-12 |
United States Patent
Application |
20120175075 |
Kind Code |
A1 |
Sauerhoefer; Marc R. ; et
al. |
July 12, 2012 |
Preformed ceramic seed well for single crystal starter seed
Abstract
A method of creating an article from investment casting includes
forming a ceramic seed well, heating the ceramic seed well to form
a hardened ceramic seed well, attaching a meltable pattern to the
hardened ceramic seed well, coating the hardened ceramic seed well
and the meltable pattern with hardenable ceramic material, heating
the hardenable ceramic material to melt the meltable pattern,
removing the meltable pattern from the hardenable ceramic material,
firing the hardenable ceramic material to form a ceramic mold
having a cavity, placing a seed within the hardened ceramic seed
well, and pouring molten metal into the hardened ceramic seed well
and the cavity. The cavity of the ceramic mold is in communication
with the hardened ceramic seed well.
Inventors: |
Sauerhoefer; Marc R.;
(Coventry, CT) ; Marcin; John J.; (Marlborough,
CT) ; Kennard; Lea D.; (Vernon, CT) ;
Turkington; Michael K.; (Manchester, CT) ; Bullied;
Steven J.; (Pomfret Center, CT) ; Murray; Stephen
D.; (Stafford Springs, CT) |
Assignee: |
United Technologies
Corporation
Hartford
CT
|
Family ID: |
39855037 |
Appl. No.: |
11/879770 |
Filed: |
July 18, 2007 |
Current U.S.
Class: |
164/23 ; 164/271;
164/6 |
Current CPC
Class: |
C30B 29/52 20130101;
B22C 9/043 20130101; C30B 11/14 20130101; C30B 11/00 20130101; B22D
27/045 20130101 |
Class at
Publication: |
164/23 ; 164/6;
164/271 |
International
Class: |
B22D 25/06 20060101
B22D025/06; B22C 1/00 20060101 B22C001/00; B22D 15/04 20060101
B22D015/04; B22C 9/02 20060101 B22C009/02 |
Claims
1. A method of casting an article, the method comprising: forming a
ceramic seed well; heating the ceramic seed well to form a hardened
ceramic seed well; attaching a meltable pattern to the hardened
ceramic seed well; coating the hardened ceramic seed well and the
meltable pattern with hardenable ceramic material; heating the
hardenable ceramic material to melt the meltable pattern; removing
the meltable pattern from the hardenable ceramic material; firing
the hardenable ceramic material to form a ceramic mold having a
cavity, wherein the cavity is in communication with the hardened
ceramic seed well; placing a seed within the hardened ceramic seed
well; and pouring molten metal into the hardened ceramic seed well
and the cavity.
2. The method of claim 1, and further comprising attaching the
meltable pattern to the hardened ceramic seed well using a
gating.
3. The method of claim 1, wherein the ceramic seed well is formed
from one of the group consisting of: extrusion processing,
injection processing, and slip cast processing.
4. The method of claim 1, wherein the meltable pattern is formed
from wax.
5. The method of claim 1, and further comprising maintaining the
hardened ceramic seed well and the ceramic mold at a temperature
higher than a temperature of the molten metal for a predetermined
amount of time.
6. The method of claim 1, wherein coating the hardened ceramic seed
well and the meltable pattern with hardenable ceramic material
comprises coating the hardened ceramic seed well and the meltable
pattern with a plurality of layers of hardenable ceramic
material.
7. The method of claim 1, wherein placing the seed within the
hardened ceramic seed well comprises utilizing a seed having
selected primary and secondary orientations.
8. The method of claim 1, wherein the hardened ceramic seed well
has a height greater than a height of the seed.
9. The method of claim 1, and further comprising positioning the
hardened ceramic seed well on a chill plate.
10. The method of claim 1, wherein attaching the meltable pattern
to the hardened ceramic seed well comprises attaching the meltable
pattern to a wax plug positioned within the hardened ceramic seed
well.
11. A seed well formed by the steps comprising: positioning a
meltable plug within a ceramic seed well preform; coating the
preform with a hardenable ceramic material; removing the plug from
within the perform; firing the preform and the hardenable ceramic
material.
12. The seed well of claim 11, wherein the preform is formed from
one of the group consisting of: extrusion processing, injection
processing, and slip cast processing.
13. The seed well of claim 11, wherein the meltable plug is formed
of wax.
14. The seed well of claim 11, wherein the preform has interior
dimensions defined by a plurality of connected walls, and wherein
the meltable plug conforms to the interior dimensions of the
preform.
15. An investment casting process comprising: attaching a first wax
pattern to a first preformed seed well; coating the first wax
pattern and the first preformed seed well with at least one layer
of ceramic material to form a first ceramic mold; heating the first
ceramic mold to melt and remove the first wax pattern; firing the
first ceramic mold to form a first hardened ceramic shell; placing
a first seed within the first preformed seed well; and pouring
molten metal into the first hardened ceramic shell and the first
preformed seed well.
16. The process of claim 15, wherein heating the ceramic mold to
form a hardened ceramic shell comprises positioning the ceramic
mold within a furnace.
17. The process of claim 15, and further comprising positioning the
ceramic mold on a chill plate.
18. The process of claim 15, wherein the preformed seed well is
foamed from one of the group consisting of: extrusion processing,
injection processing, and slip cast processing.
19. The process of claim 15, wherein attaching the wax pattern to
the preformed seed well comprises attaching the wax pattern to a
wax plug positioned within an interior area of the preformed seed
well.
20. The process of claim 15, wherein pouring molten metal into the
preformed seed well comprises pouring the molten metal over the
seed.
21. The process of claim 15, wherein placing the seed within the
preformed seed well comprises utilizing a seed having selected
primary and secondary orientations.
22. The process of claim 15, wherein the preformed seed well has a
height greater than a height of the seed.
23. The process of claim 15, wherein attaching the wax pattern to
the preformed, ceramic seed well comprises attaching a wax gating
of the wax pattern to the preformed seed well.
24. The process of claim 15, wherein the preformed seed well is
formed of a material selected from the group consisting of:
ceramic, metal, and composite material.
25. The process of claim 15, and further comprising: attaching at
least a second wax pattern to a second preformed seed well; coating
the second wax pattern and the second preformed seed well with at
least one layer of ceramic material to form a second ceramic mold;
heating the second ceramic mold to melt and remove the second wax
pattern; firing the second ceramic mold to form a second hardened
ceramic shell; placing a second seed within the second preformed
seed well; and pouring molten metal into the second hardened
ceramic shell and the second preformed seed well; wherein attaching
the second wax pattern, coating the second wax pattern and the
second preformed seed well to form a second ceramic mold, heating
the second ceramic mold to melt and remove the second wax pattern,
firing the second ceramic mold to form a second hardened ceramic
shell, placing a second seed within the second preformed seed well,
and pouring molten metal into the second hardened ceramic shell and
the second preformed seed well occur simultaneously with attaching
the first wax pattern, coating the first wax pattern and the first
preformed seed well to form a first ceramic mold, heating the first
ceramic mold to melt and remove the first wax pattern, firing the
first ceramic mold to form a first hardened ceramic shell, placing
a first seed within the first preformed seed well, and pouring
molten metal into the first hardened ceramic shell and the first
preformed seed well, respectively.
26. The process of claim 15, wherein multiple wax patterns are
attached to multiple preformed seed wells.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates generally to the field of
investment casting. In particular, the present invention relates to
shell-mold investment casting.
[0002] Investment casting is one of the primary manufacturing
processes used in the fabrication of aerospace hardware with high
technology content. These products are typically operated in
extreme environments, such as the hot section of a turbine engine,
and are exposed to high temperatures and high stresses. In the most
demanding applications, designs require the utilization of single
crystal superalloys. One of the considerations when designing
products for these applications is the orientation of the crystal
axis of the alloys. By orienting the axis in a particular
direction, the properties of the end product may be manipulated to
better fit the desired specifications. This may be accomplished in
the casting process through a seeding technique where a
directionally solidified casting is seeded with an alloy similar in
melting point as the metal to be cast but of known orientation. The
casting is grown from the metal seed by melting back a portion of
the seed and capturing the underlying crystal structure through
epitaxial growth during the casting process.
[0003] Shell-mold investment casting is a manufacturing process by
which near-net shaped articles may be fabricated while controlling
both the microstructure and crystallographic orientation of the
basic structure. The primary and secondary orientations of the
basic arrangement of atoms, as a unit cell and a larger material
lattice (stacked unit cells), are significant to the
characterization of the material and subsequently to the properties
of the cast component. These orientations are defined by the
orientation of the seed crystal. Traditionally, the shell-mold base
cavity ("seed well") is fabricated in the same manner as the shell
mold parts, by using a wax pattern. The seed well is formed using a
wax pattern to replicate seed geometry, shelling over the wax with
ceramic, and then removing the wax. After evacuation of the wax
pattern from the mold, a cavity is left in which to place a
pre-made seed with a known orientation that has already been
previously established.
[0004] Assembly of a pre-oriented seed of the parent material
within the seed well is well-established in current industry
practice as providing an initiation point of crystal growth. The
seeds are typically a cast bar with a known primary and secondary
crystal orientation that are cut and finished to a prescribed
length depending on the configuration of the mold-height and the
height capacity of the furnace. These metal seeds can take a
variety of shapes, including, but not limited to: rectangular,
cylindrical, conical, or a variety of other extruded geometric
shapes. Crystal growth proceeds along a solidification front from a
zone containing the seed's uppermost surface which has been melted
back a prescribed distance as heat is directionally extracted from
the metal seed inside the seed well in a predominant direction.
[0005] While effective, this method of forming the seed well
results in a stack up of tolerances and shrink factors, requiring a
process that must be constantly monitored. Even with precise
monitoring, there may still be variations between the dimensions of
the seed and the seed well, leading to difficulty in placing the
seed in the seed well. The mismatch between the seed and the seed
well may be caused by variability in the seed-wall pattern and
subsequent shell, in the wax seed pattern and subsequent shell, and
the casting process. The inability to accurately place and locate
the seed in the seed well can lead to an improper crystallization
start, improper orientation, improper melt-back location, and
contamination due to force-fitting. Attempts to fix these problems
have involved sanding and finishing the seed well, adjustments to
the seed, or material removal of the seed dimensions. What is
needed is an improved seed well for use in investment casting.
BRIEF SUMMARY OF THE INVENTION
[0006] A method of creating an article from investment casting
includes forming a ceramic seed well, heating the ceramic seed well
to form a hardened ceramic seed well, attaching a meltable pattern
over the hardened ceramic seed well, coating the hardened ceramic
seed well and the meltable pattern with hardenable ceramic
material, heating the hardenable ceramic material to melt the
meltable pattern, removing the meltable pattern from the hardenable
ceramic material, firing the hardenable ceramic material to form a
ceramic mold having a cavity, placing a seed within the hardened
ceramic seed well, and pouring molten metal into the hardened
ceramic seed well and the cavity. The cavity of the ceramic mold is
in communication with the hardened ceramic seed well.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] FIG. 1 is a cross-sectional view of a preformed ceramic seed
well.
[0008] FIG. 2 is a schematic view of a first step in an investment
casting process.
[0009] FIG. 3 is a schematic view of a second step in an investment
casting process using a first embodiment of a fixture.
[0010] FIG. 3A is a schematic view of the second step in the
investment casting process using a second embodiment of a
fixture.
[0011] FIG. 4 is a schematic view of a third step in the investment
casting process.
[0012] FIG. 5 is a diagram of a method of fabricating a casting
using a preformed ceramic seed well.
DETAILED DESCRIPTION
[0013] FIG. 1 shows a cross-sectional view of preformed ceramic
seed well 10 and pre-oriented seed 12 used in investment casting
process 14. Prefabrication of ceramic seed well 10 eliminates
process variations that lead to dimensional mismatches between
ceramic seed well 10 and seed 12 when wax is used to form ceramic
seed well 10. For example, variations can occur in the wax
seed-pattern, shell build, and shell cure process. Ceramic seed
well 10 is dimensionally accurate with respect to seed 12 and
produces precise geometric placement and location of seed 12. In
addition, prefabrication may further customize the thermal
conditions adjacent to seed 12 for enhanced melt back control by
controlling the thickness of ceramic seed well 10. By injecting and
firing a preformed ceramic seed well 10 out of ceramic and
assembling ceramic seed well 10 onto a mold pattern, several
tolerances and shrinkages are removed, leading to a tighter, more
consistent fit of seed 12 within seed well 10. Ceramic seed well 10
provides for a preformed geometry that allows enhanced flow across
the surface of seed 12, which may also reduce localized oxidation.
By eliminating dimensional mismatching, yields on expensive cast
parts may be increased and manufacturing costs may be reduced. The
cast articles may be subsequently used, for example, in turbine and
combustor sections of a gas turbine engine.
[0014] Ceramic seed well 10 is hollow and has inner surface 16,
first end 18, second end 20, and walls 22. Because ceramic seed
well 10 is hollow, walls 22 form an inner area 24 defined by inner
surface 16. Ceramic seed well 10 is preformed from ceramic such
that the inner dimensions of ceramic seed well 10 are substantially
the same as the dimensions of seed 12. In an exemplary embodiment,
ceramic seed well 10 may be formed of alumina or alumino-silicate
for directional solidification, columnar grained, or single crystal
(DS/SX) parts or zirconia silicate ceramic for equiaxed parts.
Although ceramic seed well 10 is discussed as being formed of
ceramic, ceramic seed well 10 may be formed of any material that
does not react with the molten metal that is subsequently poured
into ceramic seed well 10 and that has a melting point higher than
the melting point of the molten metal. For example, ceramic seed
well 10 may be formed of materials including, but not limited to:
ceramic, metallic, and composite material. In addition, although
FIG. 1 depicts ceramic seed well 10 as being rectangular in shape,
ceramic seed well 10 may take any shape without departing from the
intended scope of the present invention as long as seed 12 fits
precisely within inner area 24. In an exemplary embodiment, ceramic
seed well 10 is formed through an extrusion process.
[0015] Seed 12 is sized to fit precisely within inner area 24 of
ceramic seed well 10, although ceramic seed well 10 will have a
greater height than seed 12. Seed 12 has known primary and
secondary orientations and is typically machined from a
pre-existing single crystal of the same material or a material
having a similar melting point as the molten metal used in
investment casting process 14. Multiple seeds may also be
investment cast from a master seed.
[0016] A crucial parameter in the seeding process involves
preventing the formation of a gap between seed 12 and walls 22 of
ceramic seed well 10. If interior area 24 of ceramic seed well 10
is too small, seed 12 will not be able to be positioned within
ceramic seed well 10. If interior area 24 is too large, molten
metal will pass around seed 12 during the casting process and seed
12 may not be properly replicated. By preforming ceramic seed well
10 through an extrusion, slip cast, or injection process, shrinkage
problems during investment casting process 14 can be prevented.
[0017] FIG. 2 shows a schematic view of a first step of investment
casting process 14 using preformed ceramic seed well 10. The first
step in investment casting process 14 generally includes preformed
ceramic seed well 10, wax plug 26, wax gating 28, and wax pattern
30. To maintain the shape and dimensions of ceramic seed well 10
after manufacture, ceramic seed well 10 is fired. After ceramic
seed well 10 is in a hardened state, wax plug 26 is inserted within
ceramic seed well 10. Wax plug 26 will sufficiently conform to
interior area 24 of ceramic seed well 10 to prevent ceramic
penetration into interior area 24 of ceramic seed well 10. Although
FIG. 2 discusses a wax plug 26 being inserted within ceramic seed
well 10, ceramic seed well 10 may be blocked off by any means known
in the art to prevent ceramic from getting into interior area 24 of
ceramic seed well 10, including, but not limited to: capping off
the top of ceramic seed well 10 with a ceramic sheet.
[0018] Once wax plug 26 has plugged ceramic seed well 10, wax
gating 28 is positioned over wax plug 26. Wax gating 28 may be
attached to wax plug 26 by any means known in the art, including,
but not limited to: an adhesive such as glue or by wax welding. In
an exemplary embodiment, wax gating 28 has a helical shape.
However, wax gating 28 may take any variety of shapes and sizes
known in the art. Wax pattern 30 has a first end 32 connected to
wax gating 28 and a second end 34. Second end 34 of wax pattern 30
may optionally be attached to pour cup 36. Pour cup 36 may be
formed of wax or ceramic. Although wax gating 28 and wax pattern 30
are discussed as being separate pieces, wax gating 28 and wax
pattern 30 may formed as a single piece without departing from the
intended scope of the present invention.
[0019] FIG. 3 shows a schematic view of a second step of investment
casting process 14. The second step of investment casting process
14 generally includes ceramic seed well 10, wax plug 26, wax gating
28, wax pattern 30, and wax pour cup 36 which collectively will be
referred to as wax mold 38, fixture 40, ceramic material 42, and
slurry tank 44. Once wax plug 26 within ceramic seed well 10 is
properly attached to wax gating 28, wax pattern 30, and wax pour
cup 36 to form a single piece (wax mold 38), wax mold 38 is
positioned within fixture 40 for dipping into ceramic material 42
in slurry tank 44.
[0020] Fixture 40 generally includes bottom plate 46, top plate 48,
and support bars 50. Support bars 50 are mechanically attached to
bottom plate 46 and top plate 48 and connect bottom plate 46 and
top plate 48 together. Wax mold 38 is first positioned on bottom
plate 46 such that first end 18 of ceramic seed well 10 rests on
bottom plate 46. Top plate 48 is then positioned relative to wax
mold 38 such that top plate 48 abuts wax pour cup 36, or second end
34 of wax pattern 30. Bottom plate 46 and top plate 48 are secured
to support bars 50 in any suitable manner (i.e., by bolts) to
maintain wax mold 38 securely between bottom plate 46 and top plate
48. Although fixture 40 is depicted in FIG. 3 as including bottom
plate 46, top plate 48, and two support bars 50, fixture 40 may
take any form without departing from the intended scope of the
invention as long as wax mold 38 is held securely onto fixture 40.
For example, fixture 40 may optionally not include top plate 48 or
may include a different number of support bars 50. In addition,
support bars 50 of different lengths may be used to account for wax
molds of varying heights without departing from the intended scope
of the present invention.
[0021] FIG. 3A shows a second embodiment of fixture 40 having
multiple gated wax patterns 30 with attached ceramic seed wells 10
assembled on fixture 40 and attached to a single pour cup 36.
[0022] Once wax mold 38 is held securely within fixture 40, fixture
40 is lowered into slurry tank 44 to coat a first layer of ceramic
material 42 over wax mold 38. Fixture 40 with wax mold 38 is
repeatedly dipped into slurry tank 44 to form continuous layers of
ceramic material 42 over wax mold 38. Coated wax mold 38 is then
placed into a machine that applies dry ceramic onto the still wet
coat of ceramic material 42 from slurry tank 44. Typically, each
layer of ceramic material 42 is allowed to dry before dipping wax
mold 38 back into slurry tank 44. In an exemplary embodiment,
ceramic material 42 is coated onto wax mold 38 until ceramic
material 42 has a thickness of between approximately 0.25 inches
and approximately 0.5 inches. Because first end 18 of ceramic seed
well 10 and second end 34 of wax pattern 30 engage bottom plate 46
and top plate 48, respectively, first end 18 of ceramic seed well
10 and second end 34 of wax pattern 30 are not coated with ceramic
material 42. Only the sides of wax mold 38 are coated with ceramic
material 42. Although wax mold 38 is discussed as being dipped into
slurry tank 44 to coat ceramic material 42 onto wax mold 38, wax
mold 38 may be coated with ceramic material 42 by any means known
in the art without departing from the intended scope of the present
invention.
[0023] FIG. 4 shows a schematic view of a third step in investment
casting process 14 and generally includes chill plate 52, furnace
54 having hot chamber 56 and cold chamber 58 separated by thermal
baffle 60, and molten metal supply 62. After ceramic material 42
has been adequately coated onto wax mold 38 (shown in FIG. 3),
bottom and top plates 46 and 48 of fixture 40 (shown in FIG. 3) are
unbolted from bars 50 to release wax mold 38 coated with ceramic
material 42. Wax mold 38 coated with ceramic material 42 is then
heated to melt the wax forming wax plug 26, wax gating 28, wax
pattern 30, and wax pour cup 36. The wax is then poured out of
ceramic material 42. The wax may be removed by any means known in
the art. Although wax plug 26, wax gating 28, wax pattern 30, and
wax pour cup 36 are discussed as being formed of wax, wax plug 26,
wax gating 28, wax pattern 30, and wax pour cup 36 may be formed of
any meltable material.
[0024] Once the wax is removed, ceramic material 42 is then fired
to harden ceramic material 42 and form ceramic mold 64. Ceramic
mold 64 thus takes the shape of ceramic seed well 10, wax gating
28, wax pattern 30, and wax pour cup 36. Ceramic mold 64 has a
first opening 66 at a first end 68 of ceramic mold 64 and a second
opening 70 at a second end 72 of ceramic mold 64. First opening 66
of ceramic mold 64 overlaps the opening at first end 18 of ceramic
seed well 10. In addition, cavity 74 is left within ceramic mold
64. Ceramic mold 64 is then positioned in hot chamber 56 of furnace
54 and heated to a temperature higher than the melting point of the
molten metal used to form the casting. In an exemplary embodiment,
hot chamber 56 is at a temperature of between approximately 2600
degrees Fahrenheit (.degree. F.) and approximately 3000.degree. F.
Chill plate 52 is positioned within furnace 54 and is used to
extract heat from ceramic mold 64. Prior to positioning ceramic
mold 64 on chill plate 52, seed 12 is positioned within first end
18 of ceramic seed well 10. Seed 12 and ceramic mold 64 are then
positioned upon chill plate 52 such that seed 12 is positioned on
chill plate 52 with side walls 22 of ceramic seed well 10 around
seed 12. Ceramic seed well 10 must also have a height greater than
seed 12 such that a small amount of seed 12 can melt-back upon
contact with the molten metal. Before casting, ceramic mold 64 is
raised into hot chamber 56 of furnace 54 to where seed 12 is
positioned at bottom of hot chamber 56. Molten metal is poured
through second opening 70 of ceramic mold 64 from molten metal
supply 62. When the molten metal is poured into ceramic mold 64,
solidification does not initially occur within cavity 74 of ceramic
mold 64 or at walls 22 of ceramic seed well 10 because ceramic mold
64 is at a higher temperature than the molten metal. Solidification
only occurs beginning at the melt-back of top surface 76 of seed
12, once ceramic mold 64 is lowered from hot chamber 56 down into
cold chamber 58 while heat is extracted from ceramic mold 64 into
chill plate 52.
[0025] Seed 12 is used to produce a single crystal casting. Because
seed 12 is machined from a pre-existing single crystal of a
material having a similar melting point as the molten metal, when
the molten metal is introduced into ceramic mold 64, the molten
metal reaches seed 12 and melts a small portion of top surface 76
of seed 12. The liquid atoms created when seed 12 melts use the
underlying structure of the solid atoms of seed 12 and continue to
grow in the same orientation as the "seeded" orientation through
epitaxial crystal growth. As solidification proceeds, crystal
growth continues directionally upward in a direction opposite to
the heat extraction (chill plate 52). To aide this directional
alignment of upward growing crystal, ceramic mold 64 is withdrawn
vertically from hot chamber 56 and placed into cold chamber 58,
which is typically positioned below and thermally separated from
hot chamber 56 to allow solidification to progress at a uniform
rate while aligning the crystals in a directional manner. In an
exemplary embodiment, cold chamber 58 is at ambient temperature. In
addition, although cold chamber 58 is depicted in FIG. 4 as being
an integral part of furnace 54, furnaces that promote directional
crystal growth may also be used without departing from the intended
scope of the present invention.
[0026] The epitaxial orientation is maintained throughout the
casting by controlling the temperature parameters of furnace 54 and
the rate by which ceramic mold 64 is withdrawn from hot chamber 56
and positioned in cold chamber 58 of furnace 54. In addition, the
helical shape formed by wax gating 28 (shown in FIG. 2) aides in
producing a single crystal casting of the desired orientation by
blocking the growth of any secondary grains that may have
spontaneously nucleated as the solidification front moved up and
away from the seed in a directional manner through the helix. The
dominant crystal continues to branch out to fill the entire cavity
74, producing a directionally solidified, single crystal with
directionality in the growth direction. Thus, the resultant seeded
single crystal casting has two directions of atomic orientation. In
addition, wax gating 28 (shown in FIG. 3) attached to ceramic seed
well 10 serves as a grain selector which aides in ensuring single
crystal growth by excluding all but one columnar grain as the
casting is solidified in a directional manner through the helix if
seed 12 fails to create epitaxial growth. The grain selector (wax
gating 28) may or may not be used depending on the success of seed
12 in creating epitaxial growth. In addition, the shape of the
grain selector may be any variety of shapes and sizes that are
known in the art.
[0027] As previously mentioned, it is important that there is no
gap between seed 12 and walls 22 of ceramic seed well 10. Thus, by
preforming ceramic seed well 10 through an extrusion, injection
slip cast, or similar process and firing ceramic seed well 10 prior
to coating ceramic seed well 10 with ceramic material 42, the
shrinkage problems that occur when wax mold 38 is heated to remove
wax mold 38 and is then fired to form ceramic mold 64, and when
ceramic mold 64 is maintained within hot chamber 56 and cold
chamber 58, are alleviated.
[0028] FIG. 5 shows a diagram of a method 100 of fabricating a
casting using preformed ceramic seed well 10. Ceramic seed well 10
is first formed from ceramic and fired to maintain its dimensions,
Box 102. In an exemplary embodiment, ceramic seed well 10 is formed
through an extrusion process. Wax plug 26 is then positioned within
ceramic seed well 10 to help facilitate attachment of wax gating 28
and wax pattern 30 to ceramic seed well 10, Box 104. In an
exemplary embodiment, wax gating 28 is positioned between ceramic
seed well 10 and wax pattern 30 and is attached to wax plug 26
within ceramic seed well 10 with an adhesive. As shown in Box 106,
wax mold 38 is then coated with ceramic material 42 from slurry
tank 44 and coated with dry ceramic. This step may be repeated as
many times as desired, depending on the desired thickness of the
resulting mold 64. In an exemplary embodiment ceramic material 42
is coated onto wax mold 38 to a thickness of between approximately
0.25 inches and approximately 0.5 inches. Ceramic material 42 is
then removed from fixture 40 and heated to remove the wax forming
wax plug 26, wax gating 28, and wax pattern 30 and fired to harden
ceramic material 42 to form ceramic mold 64, Box 108. After the wax
has been removed from ceramic mold 64 and ceramic material 42 has
been fired, seed 12 is positioned within ceramic seed well 10, Box
110. In an exemplary embodiment, seed 12 has selected primary and
secondary orientations and is sized to fit precisely within inner
area 24 of ceramic seed well 10. Ceramic mold 64 and seed 12 are
positioned on chill plate 52 and molten metal is poured over seed
12 within ceramic seed well 10 and cavity 74 of ceramic mold 64,
Box 112. Ceramic mold 64 must initially be maintained at a
temperature higher than the melting point of the molten metal such
that the molten metal contacting the surfaces of ceramic mold 64
does not begin to solidify. As heat is extracted through chill
plate 52, a casting is formed within mold cavity 74 from seed 12,
forming a single crystal casting.
[0029] The preformed ceramic seed well replaces the wax or plastic
seed well typically used in investment casting processes. The
preformed ceramic seed well is formed through an extrusion process
and is dimensioned to match the dimensions of a seed having
particular primary and secondary orientations. By preforming the
ceramic seed well, various process variations are eliminated and
thermal conditions adjacent to the seed may be customized to
enhance the melt back control of the seed. In addition, the ceramic
seed well is fired prior to being subjected to numerous heating
steps in the investment casting process, removing several
tolerances and shrinks. In addition, flow across the seed surface
is increased, reducing localized oxidation. These advantages result
in a tighter, more consistent fit of the seed within the seed well
and increase the yield on expensive cast parts.
[0030] Although the present invention has been described with
reference to preferred embodiments, workers skilled in the art will
recognize that changes may be made in form and detail without
departing from the spirit and scope of the invention.
* * * * *