U.S. patent application number 12/033486 was filed with the patent office on 2009-08-20 for casting molds for use in a directional solidification process and methods of making.
This patent application is currently assigned to General Electric Company. Invention is credited to Andrew John Elliott, Michael Francis Xavier Gigliotti, Shyh-Chin Huang, Roger Petterson, Stephen Francis Rutkowski.
Application Number | 20090205799 12/033486 |
Document ID | / |
Family ID | 40954026 |
Filed Date | 2009-08-20 |
United States Patent
Application |
20090205799 |
Kind Code |
A1 |
Elliott; Andrew John ; et
al. |
August 20, 2009 |
CASTING MOLDS FOR USE IN A DIRECTIONAL SOLIDIFICATION PROCESS AND
METHODS OF MAKING
Abstract
Molds 16 for casting molten materials using a directional
solidification process and methods for forming a barrier layer
between a liquid cooling medium 20 and a molten material of a
casting process are provided. According to an embodiment, a mold 16
for casting a molten material comprises an inner surface at least
partially coated with a metal oxide slurry 32 comprising metal
oxide particles, wherein the metal oxide slurry 32 is capable of
inhibiting a liquid cooling medium 20 from contacting a molten
metal or metal alloy when the molten metal or metal alloy is
disposed within an interior of the mold 16 and the mold 16 is
disposed in the liquid cooling medium 20.
Inventors: |
Elliott; Andrew John;
(Greer, SC) ; Gigliotti; Michael Francis Xavier;
(Scotia, NY) ; Huang; Shyh-Chin; (Latham, NY)
; Petterson; Roger; (Fultonville, NY) ; Rutkowski;
Stephen Francis; (Duanesburg, NY) |
Correspondence
Address: |
CANTOR COLBURN, LLP
20 Church Street, 22nd Floor
Hartford
CT
06103
US
|
Assignee: |
General Electric Company
Schenectady
NY
|
Family ID: |
40954026 |
Appl. No.: |
12/033486 |
Filed: |
February 19, 2008 |
Current U.S.
Class: |
164/128 ;
249/114.1; 427/133 |
Current CPC
Class: |
B22C 3/00 20130101; B22D
27/045 20130101; B22C 9/04 20130101 |
Class at
Publication: |
164/128 ;
249/114.1; 427/133 |
International
Class: |
B22D 27/04 20060101
B22D027/04; B22C 9/00 20060101 B22C009/00 |
Claims
1. A mold for casting a molten material, comprising: an inner
surface at least partially coated with a metal oxide slurry
comprising metal oxide particles, wherein the metal oxide slurry is
capable of inhibiting a liquid cooling medium from contacting a
molten metal or metal alloy when the molten metal or metal alloy is
disposed within an interior of the mold and the mold is disposed in
the liquid cooling medium.
2. The mold of claim 1, wherein the metal oxide particles comprise
aluminum oxide, an alkaline earth metal oxide, a transition metal
oxide, a rare earth element oxide, or a combination comprising at
least one of the foregoing metal oxides.
3. The mold of claim 1, wherein the metal oxide particles have a
diameter of about 20 micrometers to about 500 micrometers.
4. The mold of claim 1, further comprising a primary stucco layer
or another metal oxide layer disposed upon the metal oxide slurry,
wherein the another metal oxide layer and the metal oxide slurry
have dissimilar compositions.
5. The mold of claim 5, further comprising at least one additional
metal oxide slurry and stucco layer stack disposed upon the primary
stucco layer.
6. A method for forming a barrier layer between a molten material
and a liquid cooling medium of a casting process, comprising:
disposing a metal oxide slurry comprising metal oxide particles
upon the inner surface of the mold.
7. The method of claim 6, wherein said disposing the metal oxide
slurry comprises dip or spraying coating the inner surface of the
mold.
8. The method of claim 6, wherein the metal oxide particles
comprise aluminum oxide, an alkaline earth metal oxide, a
transition metal oxide, a rare earth element oxide, or a
combination comprising at least one of the foregoing metal
oxides.
9. The method of claim 6, further comprising disposing a molten
metal or metal alloy within an interior of the mold to cause the
metal oxide slurry to react and form the barrier layer adjacent to
the molten metal or metal alloy.
10. The method of claim 6, wherein the metal oxide particles have a
diameter of about 20 micrometers to about 500 micrometers.
11. The method of claim 6, further comprising disposing a primary
stucco layer or another metal oxide layer upon the metal oxide
slurry, wherein the another metal oxide layer and the metal oxide
slurry have dissimilar compositions.
12. The method of claim 11, further comprising disposing at least
one additional metal oxide slurry and stucco layer stack upon the
primary stucco layer.
13. A method of casting a molten material, comprising: disposing
the molten material within an interior of a mold comprising an
inner surface at least partially coated with a metal oxide slurry
comprising metal oxide particles; and disposing the mold in a
liquid cooling medium to cause the molten material to solidify.
14. The method of claim 13, wherein the molten material comprises a
molten metal, a molten alloy, a molten superalloy, or a combination
comprising at least one of the foregoing materials, and wherein the
liquid cooling medium comprises a liquid metal having a melting
point less than about 700.degree. C., a liquid eutectic or near
eutectic metal alloy, or a combination comprising at least one of
the foregoing cooling mediums.
15. The method of claim 13, wherein the mold is disposed in a hot
zone at a temperature above a melting temperature of the molten
material when the mold is filled such that the metal oxide slurry
undergoes curing, and wherein the mold is moved progressively to a
cold zone comprising the liquid cooling medium at a temperature
below the melting temperature of the molten material, thereby
effecting movement of a solidification interface through the molten
material.
16. The method of claim 13, wherein the metal oxide particles
comprise aluminum oxide, an alkaline earth metal oxide, a
transition metal oxide, a rare earth element oxide, or a
combination comprising at least one of the foregoing metal
oxides.
17. The method of claim 13, wherein the mold further comprises a
primary stucco layer or another metal oxide layer disposed upon the
metal oxide slurry, wherein the another metal oxide layer and the
metal oxide slurry have dissimilar compositions.
18. The method of claim 17, wherein the mold further comprises at
least one additional metal oxide slurry and stucco layer stack
disposed upon the primary stucco layer.
19. The method of claim 13, wherein the metal oxide particles have
a diameter of about 20 micrometers to about 500 micrometers.
Description
BACKGROUND OF THE INVENTION
[0001] This disclosure relates generally to directional
solidification casting and, more specifically, to molds for casting
molten materials such as metals or metal alloys using a liquid
cooled directional solidification process.
[0002] Directional solidification casting is a method for producing
gas turbine components and the like with columnar and single
crystal growth structures. Generally, a desired single crystal
growth structure is created at the base of a vertically disposed
mold defining a part. Then, a single crystal solidification front
is propagated through the structure under the influence of a moving
thermal gradient.
[0003] Materials that have been cast using directional
solidification include steel and superalloy parts. In addition to
composition, the crystal grain characteristics of a superalloy can
determine superalloy properties. For example, the strength of a
superalloy is determined in part by grain size. At high
temperatures, deformation processes are diffusion controlled and
diffusion along grain boundaries is much higher than within grains.
Hence, at high temperatures, large grain structures can be stronger
than fine grain structures. The failure of a superalloy can
originate at grain boundaries oriented perpendicular to the
direction of an applied stress. Casting a superalloy to produce an
elongated columnar structure with unidirectional crystals aligned
substantially parallel to the long axis of the casting can reduce
grain boundaries normal to the primary stress axis. This grain
boundary reduction can, in turn, almost entirely eliminate grain
boundary failure modes.
[0004] During directional solidification, crystals of nickel,
cobalt or iron-based superalloys are characterized by a "dendritic"
morphology. Dendritic refers to a form of crystal growth where
forming solid extends into still molten liquid as an array of fine
branched needles. Spacing between the needles in the solidification
direction is called "primary dendrite arm spacing." A temperature
gradient can be impressed in front of an advancing solidification
front to avoid nucleation and growth of parasitic dendritic grains.
The magnitude of the required gradient is proportional to the speed
of solidification. For this reason, it is desirable to control the
speed of displacement of the solidification front, which can be on
the order of a fraction of a centimeter to several centimeters per
hour.
[0005] Liquid metal cooled directional solidification processes
have been developed to allow the speed of displacement of the
solidification front to be carefully controlled. One such process
involves passing the alloy material through a heating zone and then
into a cooling zone. The heating zone can include an induction coil
or resistance heater while the cooling zone can include a liquid
metal bath. In another process, the liquid metal bath can be
utilized both for heating and cooling to provide an improved planar
solidification front for the casting of complex articles.
SUMMARY OF THE INVENTION
[0006] Disclosed herein are molds for casting molten materials,
methods for forming a barrier layer on an inner surface of such
molds, and methods for casting a molten material. According to an
embodiment, a mold for casting a molten material comprises an inner
surface at least partially coated with a metal oxide slurry
comprising metal oxide particles, wherein the metal oxide slurry is
capable of inhibiting a liquid cooling medium from contacting a
molten metal or metal alloy when the molten metal or metal alloy is
disposed within an interior of the mold and the mold is disposed in
the liquid cooling medium.
[0007] In another embodiment, a method for forming a barrier layer
between a molten material and a liquid cooling medium of a casting
process comprises: disposing a metal oxide slurry comprising metal
oxide particles upon an inner surface of the mold.
[0008] In an additional embodiment, a method for casting a molten
material comprises: disposing the molten material within an
interior of a mold comprising an inner surface at least partially
coated with a metal oxide slurry comprising metal oxide particles;
and disposing the mold in a liquid cooling medium to cause the
molten material to solidify.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] Referring now to the Figures, which are exemplary
embodiments, and wherein the like elements are numbered alike:
[0010] FIG. 1 is a schematic side view of a furnace for conducting
a directional solidification process; and
[0011] FIG. 2 is a schematic illustration of an embodiment of a
barrier layer for use upon an inner surface of a mold for casting a
molten material.
DETAILED DESCRIPTION OF THE INVENTION
[0012] FIG. 1 depicts an exemplary embodiment of a furnace 10 that
can be used to cast molten materials via directional solidification
processes. As shown, the furnace 10 can include resistance heated
conductive strips 12 (e.g., graphite strips) disposed within an
insulated furnace box 14 for preheating the box 14. The conductive
strips can be replaced by an induction coil in an alternative
embodiment. A mold positioner 18 can hold a ceramic mold 16 having
an inner surface coated with a metal oxide slurry within the
furnace box 14. An interior of the mold 16 can be filled with a
molten material that is kept in the molten state by heating the
furnace box 14 at a temperature above the melting point of the
molten material. Directional solidification can be achieved by
lowering the mold 16 containing the molten material out of the
heated furnace box 14 (i.e., the hot zone) into a liquid cooling
bath 20 (i.e., the cold zone) through an aperture 11 in the furnace
box 14. The cooling bath 20 can act as a cooling medium for the
molten material. The cooling bath 20 can be contained in a crucible
22 of metal such as a refractory metal that is heated to a
temperature below the melting point of the molten material and
above the melting point of the liquid cooling medium.
Solidification of the molten material can progress from bottom to
top within mold 16 as the mold 16 is lowered into the cooling bath
20. In particular, a solid-liquid interface can advance upward as
heat is transferred from the molten material within the mold 16 to
the liquid cooling bath 20.
[0013] As mentioned above, the inner surface of the ceramic mold
can be pre-coated with a slurry comprising metal oxide particles.
In an embodiment, the metal oxide particles can have a diameter of
about 20 micrometers (microns) to about 500 microns, more
specifically about 20 microns to about 100 microns, or even more
specifically about 20 microns to about 50 microns. Examples of
suitable metal oxide particles include but are not limited to
particles of aluminum oxide such as alumina, alkaline earth metal
oxides such as magnesium dioxide and calcium oxide, transition
metal oxides such as titania, chromia, and zirconia, rare earth
oxides such as yttria and ceria, and combinations comprising at
least one of the foregoing metal oxides. The slurry can be formed
by mixing the metal oxide particles with a liquid such as water.
The slurry can then be applied to the surface of a wax pattern,
which upon de-waxing, forms the inner surface of the mold, to form
a barrier layer adjacent to the molten material subsequently placed
within the mold. Filling the interior of the mold with the
relatively hot molten material can cause the metal oxide slurry to
cure into a solid ceramic facecoat that attaches to the surface of
the molten material upon solidification. Optionally, an additional
facecoat can be disposed on the metal oxide slurry of the mold
after the de-waxing procedure by rinsing the interior of the mold
with an additional slurry, if so desired. This additional facecoat
can serve as a protective coating that adheres to the molten
material upon solidification. It can include a dissimilar oxide
from the one used in the metal oxide slurry, such as colloidal
silica, yttria, alumina, and combinations comprising at least one
of the foregoing oxides.
[0014] If any of the liquid cooling medium, e.g., a liquid metal,
infiltrates through the main mold member during the solidification
process, the ceramic facecoat can inhibit the liquid cooling medium
from contacting the surface of the molten material within the mold.
By way of example, this infiltration of the liquid cooling medium
can occur if the mold does not seal properly or if the mold cracks
prematurely before the completion of the solidification process.
The presence of the ceramic facecoat adjacent to the surface of the
molten material can prevent or delay cross-diffusion between
components of the liquid cooling medium and the molten material and
any surface reactions between the two materials. As a result, the
composition of the molten material desirably remains substantially
the same and does not become contaminated during the solidification
process.
[0015] In another exemplary embodiment shown in FIG. 2, the inner
surface of the mold can be pre-coated with alternating layers of a
metal oxide slurry (i.e., layers 32, 36, 40, 44, and 48) and a
stucco (i.e., layers 34, 38, 42, and 46). As used herein, "stucco"
refers to a material made of an aggregate, a binder, and water that
is applied wet and hardens when it is dried. In one embodiment, the
stucco can include lime, sand, and water. The metal oxide slurry
can be applied by, e.g., dip or spray coating. The stucco can be
applied using a fluidized bed, by spraying coating, or by rain
coating. Rain coating can be performed using a device (i.e., a rain
sander) that picks up the stucco from a reservoir and distributes
it evenly (like "rain") over a large area. Suitable rain sander
devices are commercially available from Pacific Kiln &
Insulations Co., Inc.
[0016] The size of the stucco particles near the bottom the stacked
structure shown in FIG. 2 are smaller those near the middle and top
of the stacked structure. For example, the stucco particles near
the bottom of the stacked structure can have a diameter of about
150 microns to about 180 microns (a mesh size of about 80 to about
100), whereas those near the middle or top of the stacked structure
can have a diameter of about 250 microns to about 355 microns (a
mesh size of about 42 to about 60). Suitable sizes of the slurry
particles in the slurry layers are described above. The thickness
of each slurry layer can be about 50% to about 75% of the diameter
of the stucco particles to ensure the stucco particles are embedded
in the slurry layer but do not penetrate the layer. For example,
stucco particles having a diameter of 180 microns can need about 90
to about 135 microns of slurry thickness to lock the particles in
place.
[0017] Examples of materials that can be cast as described above
include but are not limited to metals, metal alloys, superalloys,
and combinations comprising at least one of the foregoing
materials. As used herein, the term "superalloy" refers to a nickel
(Ni), cobalt (Co), or iron (Fe) based heat resistant alloy that has
superior strength and oxidation resistance at high temperatures.
Superalloys can also include a metal such as chromium (Cr) to
impart surface stability and one or more minor constituent such as
molybdenum (Mo), tungsten (W), niobium (Nb), titanium (Ti), and/or
aluminum (Al) for strengthening purposes. The physical properties
of superalloys make them particularly useful for the manufacture of
gas turbine components.
[0018] The liquid cooling medium desirably includes a chemically
inert material having a melting point significantly below that of
the molten material, a relatively high thermal conductivity, and a
relatively low vapor pressure. Examples of suitable materials for
use in the liquid cooling medium include but are not limited to
non-flammable, non-toxic liquid metals having a melting point less
than about 700.degree. C., eutectic or near eutectic metal alloys,
and combinations comprising at least one of the foregoing cooling
mediums. Non-limiting examples of such liquid metals include
aluminum, tin, gallium, and indium.
[0019] A eutectic mixture is a combination of metals in a
proportion that is characterized by the lowest melting point of any
mixture of the same metals. The eutectic point is the lowest
temperature at which a eutectic mixture can exist in liquid phase.
The eutectic point is the lowest melting point of an alloy in
solution of two or more metals that is obtainable by varying the
proportions of the components. Eutectic alloys have definite and
minimum melting points in contrast to other combinations of the
same metals. Non-limiting examples of eutectic or near eutectic
metal alloys include binary eutectics of aluminum (Al) with copper
(Cu), germanium (Ge), magnesium (Mg), or silicon (Si) and ternary
eutectics of aluminum with copper and germanium, copper and
magnesium, copper and silicon, or magnesium and silicon. These
aluminum-based eutectic alloys, as well other eutectic alloys in
the tin, gallium and indium systems, can be used as cooling
mediums, if lower temperature or reactivity with metal is
desired.
[0020] The liquid cooling medium can be prepared as an ingot
outside of the directional solidification furnace by melting and
casting the alloy constituents into ingots. Alternatively, the
liquid cooling medium can be prepared in situ by melting
constituents within the crucible 22.
[0021] As used herein, the terms "a" and "an" do not denote a
limitation of quantity, but rather denote the presence of at least
one of the referenced items. Moreover, the endpoints of all ranges
directed to the same component or property are inclusive of the
endpoint and independently combinable (e.g., "about 5 wt % to about
20 wt %," is inclusive of the endpoints and all intermediate values
of the ranges of "about 5 wt % to about 20 wt %,"). Reference
throughout the specification to "one embodiment", "another
embodiment", "an embodiment", and so forth means that a particular
element (e.g., feature, structure, and/or characteristic) described
in connection with the embodiment is included in at least one
embodiment described herein, and may or may not be present in other
embodiments. In addition, it is to be understood that the described
elements may be combined in any suitable manner in the various
embodiments. It is also to be understood that the disclosure is not
limited by any theories described therein. Unless defined
otherwise, technical and scientific terms used herein have the same
meaning as is commonly understood by one of skill in the art to
which this invention belongs.
[0022] While the invention has been described with reference to
exemplary embodiments, it will be understood that various changes
may be made and equivalents may be substituted for elements thereof
without departing from the scope of the invention. In addition,
many modifications may be made to adapt a particular situation or
material to the teachings of the invention without departing from
the essential scope thereof. Therefore, it is intended that the
invention not be limited to the particular embodiment disclosed as
the best mode contemplated for carrying out this invention, but
that the invention will include all embodiments falling within the
scope of the appended claims.
* * * * *