U.S. patent application number 13/226816 was filed with the patent office on 2011-12-29 for shell molds and processes for forming shell molds.
This patent application is currently assigned to GENERAL ELECTRIC COMPANY. Invention is credited to Michael Francis Xavier Gigliotti, Shyh-Chin Huang, Adegboyega Masud Makinde, Roger John Petterson, Stephen Francis Rutkowski, Venkat Subramaniam Venkataramani.
Application Number | 20110315338 13/226816 |
Document ID | / |
Family ID | 41110694 |
Filed Date | 2011-12-29 |
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United States Patent
Application |
20110315338 |
Kind Code |
A1 |
Gigliotti; Michael Francis Xavier ;
et al. |
December 29, 2011 |
SHELL MOLDS AND PROCESSES FOR FORMING SHELL MOLDS
Abstract
Shell molds and processes for making the shell molds that
exhibit high emissivity in the red and infrared regions. In this
manner, thermal resistance within a gap formed between solidifying
cast metal and the interior mold surface is decreased. In one
embodiment, the facecoat region is formed from a slurry composition
comprising an aluminum oxide, a green chromium oxide and a silicon
dioxide. In another embodiment, the facecoat region is formed from
a slurry composition including zirconium silicate and silica with
stucco layer of alumina is included.
Inventors: |
Gigliotti; Michael Francis
Xavier; (Glenville, NY) ; Huang; Shyh-Chin;
(Latham, NY) ; Makinde; Adegboyega Masud;
(Niskayuna, NY) ; Petterson; Roger John; (Sun City
West, AZ) ; Rutkowski; Stephen Francis; (Duanesburg,
NY) ; Venkataramani; Venkat Subramaniam; (Clifton
Park, NY) |
Assignee: |
GENERAL ELECTRIC COMPANY
Schenectady
NY
|
Family ID: |
41110694 |
Appl. No.: |
13/226816 |
Filed: |
September 7, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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12179749 |
Jul 25, 2008 |
8033320 |
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13226816 |
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Current U.S.
Class: |
164/24 ;
164/361 |
Current CPC
Class: |
B22C 9/04 20130101 |
Class at
Publication: |
164/24 ;
164/361 |
International
Class: |
B22C 9/02 20060101
B22C009/02; B22C 9/12 20060101 B22C009/12 |
Claims
1. A shell mold for casting molten material to form an article,
comprising: a facecoat disposed on an inner surface of the shell
mold that contacts the molten material during use thereof, said
facecoat having a phase comprising a high-emissivity alumina solid
solution, wherein the high emissivity alumina solid solution is
formed from a slurry comprising zirconium silicate and colloidal
silica with a stucco comprising aluminum oxide.
2. The shell mold of claim 1, wherein the zirconium silicate is in
an amount from 70 to 95% by weight, and colloidal silica is 5 to
about 30% by weight, wherein the weight percents are based on a
total solid content of the slurry composition after drying.
3. The shell mold of claim 1, wherein the aluminum oxide stucco
layer has a grain size of 200 mesh to 40 mesh.
4. The shell mold of claim 1, wherein the stucco further comprises
titanium dioxide or chromium oxide.
5. The shell mold of claim 1, wherein the slurry further comprises
a refractory material selected from a group consisting of, FeO,
Fe.sub.2O.sub.3, TiO.sub.2, TaC, TiC, SiC, HfC, ZrC, oxides
thereof, and combinations thereof.
6. A process for forming a shell mold, the process comprising:
preparing a fugitive pattern; dipping said pattern in a slurry
composition to form a facecoat layer contacts the fugitive pattern,
the slurry composition comprising an aluminum oxide, a green
chromium oxide, and a silicon dioxide; depositing a stucco layer
onto the facecoat layer; drying the shell; and firing the shell at
a temperature greater than a melting point of a metal to be
cast.
7. The process for forming a shell mold of claim 6, wherein the
aluminum oxide, the green chromium oxide, and the silicon dioxide
form, upon firing, substantially mullite and corundum
8. The process for forming a shell mold of claim 6, wherein the
aluminum oxide is in an amount of 70 to about 95 weight percent;
the chromium oxide in an amount greater than 0 to about 9 weight
percent, the silicon dioxide in an amount greater than 0 to about
27 weight percent, wherein the weight percents are based on total
solids of the slurry.
9. The process for forming a shell mold of claim 6, wherein the
stucco layer is formed of aluminum oxide.
10. The process for forming a shell mold of claim 6, further
comprising depositing secondary layers of the slurry
composition.
11. The process for forming a shell mold of claim 6, wherein the
temperature is within a range of 1200.degree. C. to about
1800.degree. C. and for a period of about 5 minutes to about 10
hours.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. patent application
Ser. No. 12/179,749, entitled "HIGH EMITTANCE SHELL MOLDS FOR
DIRECTIONAL CASTING," filed 25 Jul. 2008, which is herein
incorporated by reference.
BACKGROUND
[0002] The present disclosure generally relates to shell molds for
directional casting, and more particular, to high emittance shell
mold compositions that provide a high thermal gradient.
[0003] In the manufacture of components, such as nickel based
superalloy turbine blades and vanes for turbine engines,
directional solidification (DS) investment casting techniques have
been employed in the past to produce columnar grain and single
crystal casting microstructures having improved mechanical
properties at the high temperatures encountered in the turbine
section of the engine.
[0004] For directional solidification of superalloys, the
solid-liquid interface needs a high thermal gradient to yield good
cast microstructure. In order to provide a high thermal gradient,
heat needs to be removed from the solid casting. However, during
the casting process, the metal shrinks away from the mold after the
metal solidifies upon cooling; thus, the heat must radiate across
an air gap from the surface of the metal to the surface of the
mold, from where it can be conducted away. The shrinkage associated
with solidification and cooling is a consideration for many casting
processes as it affects the casting dimensions and the formation of
hot tear cracks as well as contributing to other defects. In
continuous casting processes, the molds are often tapered to
account for the shrinkage but generally require a fundamental
understanding of the shrinkage phenomena during the solidification
and cooling of a solidifying shell.
[0005] Conventional mold ceramics are selected for strength and
chemical inertness. For directional solidification of superalloys,
the mold material is typically selected from quartz, fused silica,
zircon, alumina, aluminosilicate, and yttria. Typically the process
for forming the molds includes dipping a wax pattern into a slurry
comprising a binder and a refractory material, so as to coat the
pattern with a layer of slurry. The binder is often a silica-based
material. Colloidal silica is very popular for this purpose, and is
widely used for investment-casting molds. Commercially available
colloidal silica grades of this type often have a silica content of
approximately 10%-50%. Oftentimes a stucco coating of dry
refractory material is then applied to the surface of the slurry
layer. The resulting stucco-containing slurry layer is allowed to
dry. Additional slurry-stucco layers are applied as appropriate, to
create a shell mold around the wax model having a suitable
thickness. After thorough drying, the wax model is eliminated from
the shell mold, and the mold is fired.
[0006] Sometimes, before the shell has cooled from this high
temperature heating, the shell is filled with molten metal.
Alternately, the mold is cooled to room temperature, and is stored
for later use. Subsequent re-heating of the mold will be controlled
so as not to cause cracking. Various methods have been used to
introduce molten metal into shells including gravity, pressure,
vacuum and centrifugal methods. When the molten metal in the
casting mold has solidified and cooled sufficiently, the casting
may be removed from the shell.
[0007] Facecoats are sometimes used to form a protective barrier
between the molten casting metal and the surface of the shell mold.
For example, U.S. Pat. No. 6,676,381 (Subramanian et al.) describes
a facecoat based on yttria or at least one rare earth metal and
other inorganic components, such as oxides, silicides, silicates,
and sulfides. The facecoat compositions are most often in the form
of slurries, which generally include a binder material along with a
refractory material such as the yttria component. When a molten
reactive casting metal is delivered into the shell mold, the
facecoat prevents the undesirable reaction between the casting
metal and the walls of the mold, i.e., the walls underneath the
facecoat. Facecoats can sometimes be used, for the same purpose, to
protect the portion of a core (within the shell mold), which would
normally come into contact with the casting metal.
[0008] The solidification rate of the molten metal in an investment
casting mold significantly affects the microstructure, strength,
and quality of the casting. If the solidification rate is too
rapid, the metal may not have enough time to feed liquid metal to
accommodate the shrinkage on solidification, resulting in porosity.
If the solidification rate is too slow, the casting may exhibit a
coarse microstructure. Applicants have discovered that these
drawbacks, as well as others, may be avoided or minimized by
controlling the cooling rate of the molten metal in an investment
casting mold.
[0009] Accordingly, there remains a need for molds having high heat
emittance so as to provide good cast microstructure.
BRIEF SUMMARY
[0010] Disclosed herein are high emittance mold shells and
processes for forming the high emittance mold shells. In one
embodiment, a shell mold for casting molten material to form an
article comprises a facecoat disposed on an inner surface of the
shell mold that contacts the molten material during use thereof,
said facecoat having a phase comprising a high-emissivity alumina
solid solution, wherein the high emissivity alumina solid solution
is substantially mullite and corundum.
[0011] In another embodiment, a shell mold for casting molten
material to form an article comprises a facecoat disposed on an
inner surface of the shell mold that contacts the molten material
during use thereof, said facecoat having a phase comprising a
high-emissivity alumina solid solution, wherein the high emissivity
alumina solid solution is formed from a slurry comprising zirconium
silicate and colloidal silica with a stucco comprising aluminum
oxide.
[0012] A process for forming a shell mold, the process comprises
preparing a fugitive pattern; dipping said pattern in a slurry
composition to form a facecoat layer contacts the fugitive pattern,
the slurry composition comprising an aluminum oxide, a green
chromium oxide, and a silicon dioxide; depositing a stucco layer
onto the facecoat layer; drying the shell; and firing the shell at
a temperature greater than a melting point of a metal to be
cast.
[0013] The disclosure may be understood more readily by reference
to the following detailed description of the various features of
the disclosure and the examples included therein.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] Referring now to the figures wherein the like elements are
numbered alike:
[0015] FIG. 1 is a ternary phase diagram for an aluminum oxide, a
green chromium oxide, and a silicon dioxide composition;
[0016] FIGS. 2-3 are ternary phase diagrams for an aluminum oxide,
a zirconium oxide, and a silicon dioxide composition;
[0017] FIG. 4 graphically illustrates emittance as a function of
wavelength for shell molds formed from a slurry composition of
aluminum oxide, chromium oxide and silicon dioxide;
[0018] FIG. 5 provides a micrograph illustrating grain
microstructure of a shell mold formed from a slurry composition of
aluminum oxide and silicon dioxide and further includes qualitative
elemental analysis by energy dispersive X-ray spectroscopy for
different regions of the microstructure;
[0019] FIGS. 6-7 provides micrographs at two different resolutions
illustrating grain microstructure of a shell mold formed from a
slurry composition of aluminum oxide, 3% chromium oxide and silicon
dioxide and further includes qualitative elemental analysis by
energy dispersive X-ray spectroscopy for different regions of the
microstructure;
[0020] FIGS. 8-9 provides micrographs at two different resolutions
illustrating grain microstructure of a shell mold formed from a
slurry composition of aluminum oxide, 6% chromium oxide and silicon
dioxide and further includes qualitative elemental analysis by
energy dispersive X-ray spectroscopy;
[0021] FIGS. 10-11 provides micrographs at two different
resolutions illustrating grain microstructure of a shell mold
formed from a slurry composition of aluminum oxide, 9% chromium
oxide and silicon dioxide and further includes qualitative
elemental analysis by energy dispersive X-ray spectroscopy for
different regions of the microstructure;
[0022] FIG. 12 provides a micrograph illustrating grain
microstructure of a shell mold formed from a slurry composition of
titanium dioxide, aluminum oxide, and silicon dioxide; and
[0023] FIG. 13 graphically illustrates emittance as a function of
wavelength for shell molds formed from a slurry composition of
titanium dioxide and silicon dioxide with an aluminum oxide
stucco.
DETAILED DESCRIPTION
[0024] Disclosed herein are casting molds that exhibit high heat
emittance in the red and infrared portions of the electromagnetic
spectrum. The facecoat of the casting mold includes emissive
compounds that advantageously increase the ability of the mold to
transfer heat to its surroundings during use thereof. In one
embodiment, the facecoat composition includes the addition of green
chromium (III) oxide to an alumina silica
(Al.sub.2O.sub.3--SiO.sub.2) mold slurry, which, as will be
described in greater detail below, yields a high emissive ceramic
mold upon firing and has exhibited an emittance greater than the
emittance of the base alumina-silica slurry without the green
chromium oxide. In this embodiment, the mold ceramic comprises
layers of Al.sub.2O.sub.3--Cr.sub.2O.sub.3--SiO.sub.2 with a stucco
of Al.sub.2O.sub.3. In another embodiment, the composition includes
the addition of zirconium oxide to an alumina-silica slurry. In
still another embodiment, the casting mold composition includes the
addition of white titanium dioxide to an alumina-silica slurry,
which yields a black, highly-emissive ceramic mold. In these
embodiments, the mold ceramic can further include the addition of
refractory oxides to the Al.sub.2O.sub.3--SiO.sub.2 slurries
including, but not limited to, Fe.sub.2O.sub.3, FeO, TiO.sub.2,
TaC, TiC, SiC, HfC, ZrC, and the like as well as oxides thereof. In
still other embodiments, the mold ceramic comprises layers of
Al.sub.2O.sub.3--ZrO.sub.2--SiO.sub.2 (doped with Cr.sub.2O.sub.3
and/or TiO.sub.2) with a stucco of Al.sub.2O.sub.3.
[0025] The general steps used to form the molds with the slurries
as generally described above include forming the desired pattern by
conventional methods. For example, a mold can be formed about a
fugitive (removable) pattern having the shape of the cast part
desired. By way of example, in making a turbine blade or vane
casting, the pattern will have the configuration of the turbine
blade or vane desired. The pattern may be made of wax, plastic, or
other removable material as noted above.
[0026] A primary mold facecoat layer for contacting the molten
metal or alloy to be cast is first formed on the pattern typically
by dipping the pattern in a ceramic slurry (coating), the
composition of which is discussed above, draining excess slurry
from the pattern, and then stuccoing the ceramic slurry while wet
with relatively coarse ceramic particulates (stucco). One or more
secondary layers may be formed on the facecoat layer by repeating
the sequence of dipping the pattern in the ceramic slurry, draining
excess slurry, and stuccoing the requisite number of times
corresponding to the number of layers desired. In one embodiment,
each slurry/stucco layer is dried prior to carrying out the next
coating and stuccoing operation. The facecoat layer and each
secondary layer, if present, include an inner region comprising the
dried ceramic slurry and outer region comprising the ceramic
stucco.
[0027] In one embodiment, the particular ceramic slurry for forming
the one or more facecoat layers includes aluminum oxide, silicate,
and green chromium oxide. In these embodiments, the ceramic stucco
can be formed of aluminum oxide (Al.sub.2O.sub.3). Both
Al.sub.2O.sub.3 and green Cr.sub.2O.sub.3 are commercially
available as dry particles, i.e., flour, in a variety of mesh
sizes. For example, the alumina can be a high-purity alumina
greater than 98% by weight Al.sub.2O.sub.3. The Al.sub.2O.sub.3
flour, when the mold is employed for the casting and directional
solidification of turbine components having a high standard of
surface finish requirements, can be acid-washed to remove
impurities, such as iron, which is detrimental to the formulation
of a suitable primary slurry. Grain sizes are considered since
surface finish of molds and mold permeability is important when an
acceptable casting is desired. A flour mixture containing a high
percentage of large grains will produce a rough inner mold wall.
This roughness is reproduced on the casting surface. Flour
containing a large percentage of "fines" can need an excessive
amount of binder and may cause mold wall "buckling". Thus, a
careful balance is made as to the mesh sizes used.
[0028] In one embodiment, the Al.sub.2O.sub.3 flour has a mesh size
of -240 mesh (less than about 60 microns) and the green
Cr.sub.2O.sub.3 flour has a mesh size -240 mesh (less than about 60
microns).
[0029] The silica is preferably in the form of colloidal silica.
Colloidal silica materials are commercially available from many
sources, such as Nalco Chemical Company and Dupont. Non-limiting
examples of such products are described by Horton in U.S. Pat. No.
4,947,927. The colloidal solution is usually diluted with
de-ionized water, to vary the silica content.
[0030] In one embodiment, the slurry composition includes aluminum
oxide in an amount from 70 to about 95 percent by weight, green
chromium (III) oxide in an amount greater than 0.5 to 10% by
weight, and the silicon dioxide in an amount of greater than 0 to
about 27% wherein the amounts by weight are based on a total solid
contents of the dried slurry composition. In another embodiment,
the slurry composition includes aluminum oxide in an amount from 75
to 91 percent by weight, chromium (III) oxide in an amount from 2
to 9% by weight, and colloidal silica in an amount of about 6 to
about 16% by weight. In still another embodiment, the slurry
composition includes aluminum oxide in an amount from 79 to 90
percent by weight, chromium (III) oxide in an amount from 3 to 6%
by weight, and colloidal silica in an amount of about 7 to about
15% by weight. This mixture can be applied by dipping or brushing
the fugitive pattern with the slurry.
[0031] FIG. 1 illustrates a phase diagram of the ternary
Al.sub.2O.sub.3--Cr.sub.2O.sub.3--SiO.sub.2 composition. As shown,
the region of interest 10, wherein the ternary composition is of a
solid state (alumina solid solution phase) is at about the lower
left hand portion of the phase diagram, which indicates a higher
melting point for the composition range. In this region of interest
10, the ternary composition is in the solid state phase existing
substantially as mullite and corundum. The melting point is in
excess of 1800.degree. C.
[0032] Advantageously, the highly emissive composition can be used
to provide casting of refractory metal intermetallic composite
(RMIC) materials as well as nickel based superalloys. Examples of
applicable RMIC materials include various niobium-silicon alloys
(sometimes referred to as "niobium-silicides"). The RMIC materials
may also include a variety of other elements, such as titanium,
hafnium, aluminum, and chromium. Such materials generally have much
greater temperature capabilities than the current class of
superalloys. The melting point for a metal charge based on the RMIC
materials will of course depend on the individual constituents of
the RMIC, but is usually in the range of about 1500.degree. C. to
about 2100.degree. C.
[0033] The slurry can include additional components as may be
desired for some applications. For example, a wetting agent can be
included to ensure proper wetting of the wax pattern by the slurry.
Viscosity-control agents are also typically included. For example,
a non-ionic wetting agent is generally preferred since these are
compatible with the binder (colloidal silica) employed. Also, a
defoaming agent may be added if excessive foam is noted on the
slurry during the mixing operation. The resulting slurries are
preferably maintained at a pH high enough to maintain stability.
Various techniques can be used for this purpose, e.g., the addition
of metal hydroxides or organic hydroxides.
[0034] Optionally, a refractory metal, carbide, and/or alloyed
oxides thereof can be added or may be used in place of the chromium
(III) oxide. Suitable refractory metals, carbides, and alloyed
oxides include, without limitation, FeO, Fe.sub.2O.sub.3,
TiO.sub.2, TaC, TiC, SiC, HfC, ZrC, and the like.
[0035] The slurries described herein are prepared by standard
techniques, e.g., using conventional mixing equipment. For example,
they can be prepared by mixing the aqueous-based binder, such as
colloidal silica, with the metal or metal oxide (e.g., aluminum
oxide and green chromium oxide), and other desired additives, e.g.,
one or more compounds to maintain the pH at a desired level, as
mentioned above.
[0036] In another embodiment, the facecoat slurry composition
includes zirconium silicate (ZrSiO.sub.4) in an amount from 70 to
95% by weight, and colloidal silica 5 to about 30% by weight,
wherein the weight percents are based on a total solid content of
the slurry composition after drying. The stucco for this facecoat
slurry would include alumina with green chromium (III) oxide, or
alumina with titanium dioxide. FIGS. 2-3 provide ternary phase
diagrams of the three components. As shown in FIG. 2, zirconium
dioxide can develop in the facecoat region as a consequence of the
diffusion couple between the slurry composition and the aluminum
oxide based stucco.
[0037] In FIG. 3, the mold microstructure that is developed on heat
treatment is described. There, the various microstructures as a
function of mole percent are illustrated. With firing and
interdiffusion, the initial phases of the slurry plus stucco, e.g.,
zircon, silica, and alumina (plus chromia or titania) interdiffuse
to become a high-emissivity alumina-chromia or alumina-titania
solid solution, plus zirconium dioxide plus mullite (i.e., aluminum
silicate), and provide the mold with high emittance properties.
[0038] In a typical embodiment for making the ceramic shell molds
of this disclosure, a wax pattern having a shape and configuration
corresponding to a desired mold cavity is dipped into the slurry.
The wet coating of slurry is then at least partially dried, to form
a covering over the wax pattern. This covering serves as the first
layer of the facecoat. The pattern is then repetitively dipped into
the slurry, to build up the facecoat to a desired thickness.
[0039] In some embodiments, the facecoat comprises layers with
varying compositions or particle sizes. For example, one layer
could be formed of one silicate material such as aluminum silicate,
while an adjacent layer might be formed from zirconium silicate.
Furthermore, one or more layers may comprise fine particle size
materials, while one or more layers may comprise coarse particles,
e.g., those having an average particle size of greater than about
50 microns, and sometimes, greater than about 100 microns. The
layers (usually, about 2 to 8 for the facecoat) could continue to
alternate. The presence of the stucco layers is helpful in
providing greater strength to the mold when such an attribute is
required.
[0040] The overall thickness of the facecoat will depend on various
factors. They include the particular composition of the facecoat
material, as well as the metal being cast in the completed mold.
Usually, the facecoat has a thickness (after the mold is fired) of
about 0.05 mm to about 2 mm.
[0041] After formation of the facecoat, additional material is
deposited on the fugitive pattern, to build up the mold walls. In a
typical embodiment, the fugitive pattern is dipped in either the
same facecoat slurry, or a different slurry, or alternating
combinations of multiple slurries.
[0042] The stucco aggregate is usually in the form of coarse
particles having an average size of grain size of 200 mesh to 40
mesh. For example, the stucco material could comprise coarse
particles of yttria or yttrium monosilicate or a combination
thereof. The stucco material is an alumina-based composition. Such
materials are known in the art and described, for example, in U.S.
Pat. No. 4,247,333 (Ledder et al) and U.S. Pat. No. 6,352,101
(Ghosh et al), which are incorporated herein by reference. A
commercially available material such as fused alumina, tabular
alumina, or sintered alumina silicates, is often used, as described
in the Ledder patent, and in U.S. Pat. No. 5,143,777 (Mills).
Moreover, mixtures of alumina having two or more particle sizes
("flour sizes") can also be used.
[0043] The number of layers (i.e., secondary layers) applied over
the facecoat will of course depend on the desired thickness of the
shell mold. As a non-limiting example, about 4 to about 20 total
slurry layer/stucco layer pairs are often used for the secondary
layers. A typical shell mold, once fired, has a total wall
thickness (i.e., from the inner wall to the outer wall, and
including the facecoat) of about 0.25 cm to about 2.50 cm, and
preferably, about 0.50 cm to about 1.0 cm
[0044] The secondary layer set can be compositionally graded, so
that properties are varied across the thickness of the shell mold
wall. Other physical properties can also be adjusted by way of this
compositional grading. For example, the proportionate increase in
alumina concentration can be very beneficial when greater high
temperature-creep resistance is desired. The outermost layers of
the mold can continue to vary in terms of the alumina/chromium
oxide/silicate ratio, or could stay at a set ratio. In some
embodiments prompted by rigorous requirements for high-temperature
mold stability, the secondary layers (e.g., about 2 to about 4 of
them) farthest away from the facecoat may comprise at least about
{90%} by weight alumina, may comprise substantially all alumina.
Usually, the variation in layer composition is accomplished by the
use of multiple slurries containing the desired ingredients for a
given layer.
[0045] After the shell mold has been completed, the fugitive
material is removed by any conventional technique used in a lost
wax process. In the case when the fugitive material is a wax, for
example, flash-dewaxing can be carried out by plunging the mold
into a steam autoclave, operating at a temperature of about
100.degree. C. to about 200.degree. C. The autoclave is typically
operated under steam pressure (about 90-120 psi), for about 10-20
minutes, although these conditions can vary considerably.
[0046] In some embodiments, the mold is then pre-fired. A typical
pre-firing procedure involves heating the mold at about 800.degree.
C. to about 1150.degree. C., for about 30 minutes to about 4 hours.
The shell mold can then be fired according to conventional
techniques. The required regimen of temperature and time for the
primary firing stage will of course depend on factors such as wall
thickness, mold composition, silicate particle size, and the like.
The time/temperature regimen for firing should be one which is
sufficient to convert substantially all free silica remaining in
the mold to one or more of the metal silicates described
previously, such as yttrium silicate. Typically, firing is carried
out at a temperature in the range of about 1200.degree. C. to about
1800.degree. C., and in other embodiment's, about 1400.degree. C.
to about 1700.degree. C. The firing time can vary significantly,
but is usually in the range of about 5 minutes to about 10 hours,
and more often, about 1 hour to about 6 hours. In preferred
embodiments, less than about 1% by weight free silica remains after
this heat treatment, in either crystalline or non-crystalline
(glass) form.
[0047] Advantageously, the casting molds as described above provide
an improved thermal gradient during directional solidification
casting processes, thereby improving casting quality. The spectral
emittance of the mold surface is increased in the gap between the
solid metal layer and the interior mold surface so as to lower
thermal resistance.
[0048] The following examples are presented for illustrative
purposes only, and are not intended to limit the scope of the
invention.
Example 1
[0049] In this example, molds were prepared from an alumina-silica
slurry containing varying amounts of green chromium oxide. The
slurries were first formed by mixing alumina powder, chromia
powder, and colloidal silica. A shell was formed by dipping a
fugitive pattern into the slurry and then sieving dry alumina
grains onto the freshly dipped pattern. The steps of dipping the
pattern into a refractory slurry and then sieving onto the freshly
dipped pattern dry refractory grains may be repeated until the
desired thickness of the shell is obtained. Each coat of slurry and
grains were air-dried before subsequent coats are applied. The
shell is then heated to a temperature of about 1000.degree. C. for
a period of time effective to stabilize the shell and then further
heated to a temperature of 1650.degree. C. for two hours to form
the mold.
[0050] FIG. 4 graphically illustrates emittance (%) over a
wavelength range for slurries with different amounts of chromia. As
shown, molds that included Cr.sub.2O.sub.3 exhibited an increase in
emittance. For molds containing 6% and 9% Cr.sub.2O.sub.3, the
emittance from about 0.4 microns to about 4 microns wavelength was
approximately 3 times greater than the control that did not contain
any Cr.sub.2O.sub.3.
[0051] FIGS. 5-11 provide scanning electron micrographs including
X-ray diffraction spectra corresponding to different regions within
the microstructure. For the various compositions containing
different amount of chromium oxide, micrographs at 1,500 and 5,000
times was examined.
Example 2
[0052] In this example, a mold was prepared from a titanium
dioxide-silica slurry (TiO.sub.2--SiO.sub.2) with an alumina
stucco. The slurry was prepared by mixing titanium dioxide into
colloidal silica. A shell was formed by dipping a fugitive pattern
into the slurry and then sieving dry alumina grains onto the
freshly dipped pattern. The steps of dipping the pattern into a
refractory slurry and then sieving onto the freshly dipped pattern
dry refractory grains may be repeated until the desired thickness
of the shell is obtained. Each coat of slurry and grains were
air-dried before subsequent coats are applied. The shell is then
heated to a temperature of about 1000.degree. C. for one hour to
stabilize the shell and then further heated to a temperature of
1600.degree. C. for one hour in a vacuum to form the mold.
[0053] FIG. 12 pictorially illustrates cross sectional views of the
mold showing the mold facecoat and as a secondary layer. Referring
back to the ternary phase diagram of FIG. 2, zirconium silicate
(ZrSiO.sub.4) formed in the facecoat region as a consequence of the
diffusion couple between the slurry composition and the
Al.sub.2O.sub.3 stucco during heat treatment. The secondary
facecoat is formed of alumina-zirconium oxide-silica.
[0054] FIG. 13 graphically illustrates emittance (%) over a
wavelength range for this example 2 mold containing titanium
dioxide and for the control mold of Example 1 that contained only
alumina and silica. For the mold containing titanium dioxide, the
emittance from about 0.4 microns to about 4 microns wavelength was
up to about 6 times greater than the control mold.
[0055] This written description uses examples to disclose the
invention, including the best mode, and also to enable any person
skilled in the art to make and use the invention. The patentable
scope of the invention is defined by the claims, and may include
other examples that occur to those skilled in the art. Such other
examples are intended to be within the scope of the claims if they
have structural elements that do not differ from the literal
language of the claims, or if they include equivalent structural
elements with insubstantial differences from the literal languages
of the claims.
* * * * *