U.S. patent application number 09/924962 was filed with the patent office on 2002-12-12 for impregnated alumina-based core and method.
Invention is credited to Caccavale, Charles F., Frank, Gregory R., Haaland, Rodney S., Kaulius, Alfred P. JR., Keller, Ronald J..
Application Number | 20020185243 09/924962 |
Document ID | / |
Family ID | 27127263 |
Filed Date | 2002-12-12 |
United States Patent
Application |
20020185243 |
Kind Code |
A1 |
Frank, Gregory R. ; et
al. |
December 12, 2002 |
IMPREGNATED ALUMINA-BASED CORE AND METHOD
Abstract
An impregnated fired porous alumina-based ceramic core for use
in an investment shell mold in the casting of molten metals and
alloys wherein the core is impregnated with yttria to improve core
creep resistance at elevated casting temperatures and times.
Inventors: |
Frank, Gregory R.;
(Morristown, TN) ; Keller, Ronald J.; (Grand
Haven, MI) ; Haaland, Rodney S.; (Suffolk, VA)
; Caccavale, Charles F.; (Wharton, NJ) ; Kaulius,
Alfred P. JR.; (Muskegon, MI) |
Correspondence
Address: |
Mr. Edward J. Timmer
Walnut Woods Centre
5955 W. Main Street
Kalamazoo
MI
49009
US
|
Family ID: |
27127263 |
Appl. No.: |
09/924962 |
Filed: |
August 8, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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09924962 |
Aug 8, 2001 |
|
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09854851 |
May 14, 2001 |
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Current U.S.
Class: |
164/31 ; 164/27;
164/520 |
Current CPC
Class: |
B22C 9/12 20130101; C04B
41/87 20130101; C04B 41/4539 20130101; C04B 38/00 20130101; C04B
35/117 20130101; C04B 35/10 20130101; C04B 2111/00879 20130101;
C04B 41/009 20130101; C04B 35/624 20130101; C04B 2235/3225
20130101; C04B 41/009 20130101; C04B 2235/80 20130101; C04B 35/111
20130101; C04B 41/5045 20130101; B22C 9/10 20130101; C04B 41/009
20130101; C04B 35/6303 20130101; C04B 2235/3206 20130101; C04B
41/5045 20130101; C04B 2235/6022 20130101; C04B 41/009 20130101;
C04B 2235/3222 20130101; C04B 2235/96 20130101 |
Class at
Publication: |
164/31 ; 164/27;
164/520 |
International
Class: |
B22C 009/10 |
Claims
We claim:
1. A method of treating a fired porous alumina-based ceramic core
for use in the casting of molten metallic materials, comprising
impregnating said core with yttria to improve core creep resistance
at elevated casting temperature.
2. The method of claim 1 wherein said core is impregnated by
immersing it in colloidal yttria.
3. The method of claim 1 wherein the core is impregnated by
immersing it in a medium including a yttria precursor.
4. The method of claim 1 wherein said core is made from a mixture
of alumina particles, yttria particles and a binder.
5. The method of claim 1 wherein said core is made from a mixture
of alumina particles, yttrium bearing particles and a binder.
6. The method of claim 2 including drying said core after
impregnation.
7. An impregnated fired, porous alumina-based core that includes
yttria impregnant in pores of the core and that exhibits a
substantial increase in creep resistance at an elevated casting
temperature as compared to an unimpregnated fired, porous
alumina-based core.
8. The core of claim 7 wherein the yttria impregnant in the core
pores is present in an amount of about 1% to about 5% by weight of
the core.
9. The core of claim 7 which includes a microstructure comprising
alumina particles and a yttria-bearing constituent.
10. The core of claim 9 wherein the yttria-bearing constituent
comprises 3Y.sub.2O.sub.3:5Al.sub.2O.sub.3.
Description
[0001] This application is a continuation-in-part application of
Ser. No. 09/854,851 filed May 14, 2001.
FIELD OF THE INVENTION
[0002] The present invention relates to a fired, porous
alumina-based ceramic core impregnated with yttria to improve core
creep resistance at elevated casting temperature used for casting
metallic materials, especially reactive superalloys, and a method
of improving core creep resistance by such impregnation.
BACKGROUND OF THE INVENTION
[0003] In casting hollow gas turbine engine blades and vanes
(airfoils) using conventional equiaxed techniques to produce
equiaxed grain castings and directional solidification (DS)
techniques to produce columnar grain or single crystal castings, a
fired ceramic core is positioned in an investment shell mold to
form internal cooling passageways in the airfoil. During service in
the gas turbine engine, cooling air is directed through the
passageways to maintain airfoil temperature within an acceptable
range.
[0004] Green (unfired) ceramic cores typically are formed to
desired core configuration by injection molding, transfer molding
or pouring of an appropriate ceramic core material that includes
one or more ceramic powders, a fugitive binder such as wax,
polyproplylene, polyolefin, prehydrolized ethyl silicate, and other
additives into a suitably shaped core die. After the green core is
removed from the die, it is subjected to firing at elevated
(superambient) temperature in one or more steps to remove the
fugitive binder and sinter and strengthen the core for use in
casting metallic material, such as a nickel or cobalt base
superalloy. As a result of removal of the binder and fugitive
filler material, if present, the fired ceramic core is porous.
[0005] The fired, porous ceramic cores used in investment casting
of hollow turbine engine superalloy airfoils typically have an
airfoil shape with a quite thin cross-section trailing edge region.
U.S. Pat. No. 4,837,187 describes an alumina-based ceramic core
formed, prior to sintering, from an admixture of alumina particles,
yttria particles and a binder followed by debinding and sintering
for use in investment casting of hollow airfoils from reactive
superalloys, such as yttrium-bearing nickel base superalloys.
Although such alumina-based cores have been used in production with
success for years, the cores can exhibit a tendency to creep (move)
at the high casting temperatures and extended time-at-temperature
involved in DS casting of columnar grain and single crystal hollow
superalloy airfoils (e.g. 1480 to 1600 degrees C. for 1/2 to 3
hours). As a result, positive core wall location and control
features typically are provided in the form of platinum pins and/or
alumina pins located between the core and ceramic shell mold wall
in which the core is disposed at selected locations to maintain
core position relative to the shell mold and counter the tendency
of the core to creep at high DS (columnar grain and single crystal)
casting temperature for extended time.
[0006] U.S. Pat. No. 5,580,837 describes an alumina-based ceramic
core formed, prior to sintering, from an admixture of alumina
particles, yttria aluminate particles and a binder followed by
debinding and sintering for use in investment casting of hollow
airfoils from reactive superalloys, such as yttrium-bearing nickel
base superalloys.
[0007] An object of the present invention is to improve the creep
resistance of such alumina-based cores as described in the above
patents at elevated casting temperature used for casting metallic
materials, such as especially reactive nickel based
superalloys.
SUMMARY OF THE INVENTION
[0008] An embodiment of the present invention provides an
impregnated fired porous alumina-based ceramic core for use in a
mold in the casting of molten metals and alloys wherein the core is
impregnated with yttria to improve core creep resistance at
elevated casting temperature.
[0009] Another embodiment of the invention involves impregnating
the fired porous alumina-based ceramic core with colloidal yttria
or other impregnating medium that will provide yttria in pores of
the core to improve core creep resistance.
[0010] Still a further embodiment of the invention provides an
impregnated fired, porous alumina-based core that includes yttria
impregnant in pores of the core and that exhibits a substantial
increase in creep resistance at elevated casting temperature and
time as compared to an unimpregnated fired, porous alumina-based
core. The yttria impregnant in the core pores preferably is present
in an amount of about 1% to about 5% by weight of the core (based
on weight gain of the dried impregnated core).
[0011] The above objects and advantages of the present invention
will become more readily apparent from the following detailed
description taken with the following drawings.
DESCRIPTION OF THE DRAWINGS
[0012] The FIGURE is a perspective view of a fired, porous ceramic
core for a gas turbine airfoil that can be made pursuant to the
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0013] The invention provides an impregnated alumina-based ceramic
core especially useful in casting of hollow gas turbine engine
blades and vanes (airfoils) using conventional equiaxed and
directional solidification (DS) techniques from reactive metallic
materials, especially reactive nickel base superalloys. For
example, the invention can be practiced to cast reactive nickel
base superalloys that include yttrium to make hollow airfoil
castings. Such reactive Y-bearing nickel base superalloys are well
known, for example, as General Electric Rene N5 nickel base
superalloy and PWA 1487 nickel base superalloy. The invention is
not limited to airfoil shaped cores as any other fired, porous
ceramic core can be made by practice of the invention.
[0014] For purposes of illustration and not limitation, the FIGURE
shows a fired, porous ceramic core 10 for use in investment casting
a hollow gas turbine blade where the core has a configuration of
internal cooling passages to be formed in the blade casting. The
core 10 includes a root region 12 and an airfoil region 14. The
airfoil region 14 includes a leading edge 16 and trailing edge 18.
Openings or slots 20 of various configurations and dimensions can
be provided through the core 10 to form elongated walls, rounded
pedestals and other features in the interior of the cast blade as
well known.
[0015] In one embodiment of the invention, the ceramic core 10 is
formed by preparing a mixture of ceramic filler material and a
binder material as described in U.S. Pat. No. 4,837,187, the
teachings of which are incorporated herein by reference. For
example, as set forth in U.S. Pat. No. 4,837,187, the mixture can
comprise, prior to sintering, about 80% to 86% by weight of ceramic
filler material and about 14% to about 20% by weight of a binder
material. The ceramic filler material comprises about 66% to 95% by
weight coarse and fine Al.sub.2O.sub.3 particles, about 1% to about
20% by weight Y.sub.2O.sub.3 particles, about 1% to about 5% grain
growth inhibiting agent (e.g. only MgO), and the balance a
carbon-bearing fugitive filler material, all as described in detail
in the above '187 patent.
[0016] The binder preferably is a thermoplastic wax-based binder
system having a low melting point described in detail in the above
'187 patent. By way of example only, the binder system may be
comprised of 91% to 96% by weight of a wax system, about 1% to
about 6% by weight of an anti-segregation agent, and about 1% to
about 5% by weight of dispersing agent.
[0017] The ceramic core can be formed by conventional injection
molding, transfer molding, or pouring employed to make green
ceramic cores. For example only, in injection molding a ceramic
core shape, a fluid ceramic powder/binder mixture is heated to 80
to 125 degrees C. and injected into a steel core die having a
molding cavity having the core configuration desired. Injection
pressures in the range of 200 psi to 2000 psi are used to
pressurize the fluid ceramic/binder mixture in the molding cavity
defined by the dies. The dies may be cooled, held at room
temperature, or slightly heated depending upon the complexity of
the desired core configuration. After the ceramic/binder mixture
solidifies in the die, the die is opened, and the green, unfired
ceramic core is removed for thermal processing to remove the
fugitive binder and sinter the green ceramic core to form a fired,
porous ceramic core 10 to be used in the well known lost wax
investment casting process. Sintering achieves consolidation of the
ceramic powder particles by heating to impart strength to the core
for use in the investment casting process. Sintering of the green
ceramic core is achieved by means of heat treatment to an elevated
temperature based on the requirements of the ceramic powders
employed. Above U.S. Pat. No. 4,837,187 incorporated herein by
reference describes prebaking and sintering the alumina-based
ceramic core wherein sintering is conducted for about 48 hours
using a heating rate of about 60 degrees C. to about 120 degrees C.
per hour up to a sintering temperature in the range of about 1600
to 1700 degrees C. During sintering, the carbon-bearing fugitive
filler material is burned out of the core and leaves an
interconnected network of porosity in the sintered core.
[0018] Subsequent to sintering, the ceramic core has a
microstructure characterized by the presence of substantially
unreacted Al.sub.2O.sub.3 particles having a polycrystalline
composition consisting essentially of
3Y.sub.2O.sub.3:5Al.sub.2O.sub.3 on surfaces of the Al.sub.2O.sub.3
particles.
[0019] In another embodiment of the invention, the ceramic core 10
is formed by preparing a mixture of yttrium aluminate particles,
alumina particles, and a binder material followed by injection
molding and sintering to form a fired, porous alumina-based core as
described in U.S. Pat. No. 5,580,837, the teachings of which are
incorporated herein by reference.
[0020] The invention involves impregnating a fired, porous
alumina-based ceramic core made pursuant to U.S. Pat. Nos.
4,837,187; 5,580,837 and any other technique where the core is made
from a mixture which consists essentially of alumina particles in a
binder material, or of alumina particles and yttrium bearing
particles, such as for example only yttrium aluminate, in a binder
material. The particular core forming technique, such as injection
molding, transfer molding and pouring, and the particular thermal
processing technique form no part of the invention as conventional
core molding techniques and thermal processing techniques can be
used to make the fired, porous alumina-based ceramic core which is
treated pursuant to the invention.
[0021] The present invention strengthens such fired, porous
alumina-based cores 10 by impregnating the cores to provide yttria
impregnant a yttria film in pores of the fired, porous core. The
fired, porous alumina-based ceramic core can be impregnated with
commercially available colloidal yttria or other impregnating
medium that impregnates the core pores with yttria and dried to
substantially improve core creep resistance at elevated casting
temperature and casting time, especially those used in DS casting
of reactive nickel superalloys to make columnar grain and single
crystal castings.
[0022] For purposes of illustration and not limitation, a preferred
colloidal yttria is available as Nyacol colloidal yttria from Nano
Technologies Inc., Ashland, Mass. This commercially available
colloidal yttria includes 14% by weight yttria solids and balance
water and acetic acid.
[0023] The invention can be practiced using an aqueous or organic
solution as an impregnating medium wherein the solution includes
yttrium salts or yttrium metal-organics that are deposited in pores
of the core to comprise a yttria precursor that will form yttria in
the pores during the mold preheating stage of the casting
operation. Such a solution can comprise yttrium acetate, yttrium
nitrate, yttrium carbonate, yttrium alkoxide or other yttria
precursors.
[0024] The fired, porous alumina-based core is dried after
impregnation to remove liquid phase of the impregnant. To this end,
the impregnated core can be dried in ambient air at room
temperature or in an oven using superambient forced air at, for
example, 180 to 200 degrees F. for a time sufficient to fully dry
the core.
[0025] The impregnated fired, porous alumina-based core pursuant to
the invention includes yttria solids as a film in pores of the
core. The impregnated fired, porous core exhibits a substantial
increase in creep resistance at elevated casting temperature and
time as compared to an unimpregnated fired, porous alumina-based
core. The yttria solids in the core pores preferably are present in
an amount of about 1% to about 5% by weight of the core, the yttria
solids in the core pores being based on weight gain of the dried
impregnated core.
[0026] The following Examples are offered to illustrate but not
limit the invention. Fired, porous alumina-based cores for a first
stage gas turbine engine blade and fired, porous alumina-based core
testbars were made pursuant to the teachings of U.S. Pat. No.
4,837,187. The core testbars had dimensions of 5 inches length, 1/2
inch width and 1/4 inch thickness.
[0027] Six fired, porous cores and twelve fired, porous
alumina-based core testbars were impregnated with yttria using the
14% by weight yttria sol (colloid) commercially available as Nyacol
colloidal yttria and described above. The cores and testbars were
impregnated by immersion in the colloidal yttria at ambient
pressure for 2 minutes followed by air blow-off using 30 psi shop
compressed air to remove excess colloidal yttria from the surface
and oven drying at 180 degrees F. for 1 hour. These core testbars
are designated "A" in Table I below.
[0028] Six fired, porous cores and twelve fired, porous
alumina-based core testbars were impregnated with alumina using a
15% by weight alumina sol (colloid) made by diluting commercially
available Bluonic A colloidal alumina from Wesbond Corporation,
Wilimington, Del. For example, 3 parts of commercially available
Bluonic A colloidal alumina was diluted using one part of deionized
water to achieve the 15% by weight colloidal alumina. The cores and
testbars were impregnated by immersion in the 15% by weight
colloidal alumina at ambient pressure for 2 minutes followed by a
air blow-off using 30 psi shop compressed air to remove excess
colloidal alumina from the surface and oven drying at 180 degrees
F. for 1 hour. These core testbars are designated "B" in Table
I.
[0029] Six fired, porous cores and core testbars were impregnated
first with yttria using a 7% by weight yttria sol (colloid) made by
diluting the above commercially available 14% by weight colloidal
yttria (Nyacol colloidal yttria) with deionized water. The cores
and core testbars were impregnated by immersion in the 7% by weight
colloidal yttria at ambient pressure for 2 minutes followed by a
air blow-off using 30 psi shop compressed air to remove excess
colloidal yttria from the surface and oven drying at 180 degrees F.
for 1 hour. The cores and core testbars then were impregnated with
alumina by immersion in a 7.5% by weight colloidal alumina at
ambient pressure for 2 minutes followed by a air blow-off using 30
psi shop compressed air to remove excess colloidal alumina from the
surface and oven drying at 180 degrees F. for 1 hour. The 7.5%
colloidal alumina was made by diluting three parts of the
off-the-shelf Bluonic A colloidal alumina with 5 parts of deionized
water. These core testbars are designated "C" in Table I.
[0030] The impregnated fired, porous core testbars were tested for
modulus of rupture (MOR) at 1520 degrees C. using a four point
bending load pursuant to ASTM standard 674-77. They were also
tested for creep resistance at 1566 degrees C. using flexure creep
test and thermal expansion using a Anter model 1161 dilatometer
available from Anter Corp., Pittsburgh, Pa. Unimpregnated fired,
porous core testbars (designated "None") were included for
comparison.
[0031] Table I sets forth the MOR results (MOR values set forth in
psi-pounds per square inch).
1TABLE I Number of Average MOR Testbar Testbars (psi) Standard
Deviation None 12 1800 224 A 12 2245 254 B 12 1984 180 C 12 1972
203
[0032] It is apparent that impregnation of the fired, porous core
testbars with yttria using the 14% by weight colloidal yttria (see
testbars designated "A") substantially improved the MOR as compared
to the MOR of the unimpregnated "NONE" core testbars. The fired,
porous core testbars impregnated with alumina (see testbars
designated "B") as described above and with yttria followed by
alumina (see testbars designated "C") as described above
substantially improved the MOR as compared to the MOR of the
unimpregnated "NONE" core testbars. However, the core testbars
impregnated with 14% by weight colloidal yttria showed the largest
increase in strength.
[0033] Table II sets forth the creep results for the impregnated
core testbars. Creep values are in inches and creep rate is in
inches/hour.
2TABLE II Creep after Testbar Creep at 2850 F 1 Hour Hold Creep
Rate at 2850 F None 0.115 0.430 0.315 A 0.161 0.231 0.071 B 0.237
0.354 0.117 C 0.186 0.281 0.094
[0034] The first column in Table II shows the average cumulative
deflection of the testbars after being ramped to 1566 degrees C. at
300 degrees C./hour. The testbars were weighted so that the bottom
span was loaded to 150 psi tensile stress at the initiation of the
test. The second column is the cumulative deflection after the
temperature ramp and a one hour hold at 1566 degrees C., which
simulates a typical DS casting temperature for columnar grain and
single crystal castings to which the core is exposed for extended
time. The third column is the rate of deflection during the one
hour hold period.
[0035] It is apparent that impregnation of the fired, porous core
testbars with yttria using the 14% by weight colloidal yttria (see
testbars designated "A") substantially improved the creep rate as
compared to the creep rate of the unimpregnated "NONE" core
testbars. For example, the creep rate was reduced about 78% for the
testbars designed "A" as compared to the "None" (unimpregnated)
testbars.
[0036] The fired, porous core testbars impregnated with alumina
(see testbars designated "B") as described above and with yttria
followed by alumina (see testbars designated "C") as described
above exhibited substantially reduced creep rates as compared to
the unimpregnated "NONE" core testbars, although the reductions in
creep rate were not as high as the reductions achieved by the core
testbars designated "A". The alumina impregnated cores exhibited
high cumulative creep (column 2 of Table 2) outside the scope of
the invention.
[0037] The thermal expansion characteristics below 1500 degrees C.
of core testbars designated "A"."B", and "C" and the unimpregnated
"None" core testbars were comparable to one another. Thus,
impregnation of the fired, porous alumina-based core with colloidal
yttria and or alumina as described above did not affect thermal
expansion characteristics.
[0038] Casting trials using 1st stage turbine blade cores pursuant
to the invention were conducted with standard single crystal
techniques to make forty two cored single crystal nickel base
superalloy blades. The casting trials resulted in a 50% reduction
in the number of scrap castings caused by core movement during
casting.
[0039] The invention is advantageous to increase creep resistance
of fired, porous alumna-based cores at elevated casting
temperatures and times. Increased core creep resistance may reduce
use of platinum or alumina pins in preparation of the core for
casting. Moreover, increased core creep resistance translates into
better control of the casting wall thickness such that thinner
casting walls may be specified for gas turbine airfoils.
[0040] Although the invention has been described with respect to
certain embodiments thereof, those skilled in the art will
appreciate that the invention is not limited to these embodiments
and changes, modifications, and the like can be made therein within
the scope of the invention as set forth in the appended claims.
* * * * *