U.S. patent application number 10/825396 was filed with the patent office on 2005-10-20 for methods for manufacturing investment casting shells.
Invention is credited to Beals, James T., Kennard, Lea D., Mandich, Dawn D., Murray, Stephen D., Persky, Joshua E., Snyder, Jacob A., Verner, Carl R..
Application Number | 20050230078 10/825396 |
Document ID | / |
Family ID | 34940869 |
Filed Date | 2005-10-20 |
United States Patent
Application |
20050230078 |
Kind Code |
A1 |
Snyder, Jacob A. ; et
al. |
October 20, 2005 |
METHODS FOR MANUFACTURING INVESTMENT CASTING SHELLS
Abstract
An at least two step heating process is used to strengthen the
shell of an investment casting mold including a refractory metal
core. The first stage may occur under otherwise oxidizing
conditions at a low enough temperature to avoid substantial core
oxidation. The second stage may occur under essentially
non-oxidizing conditions at a higher temperature.
Inventors: |
Snyder, Jacob A.;
(Southington, CT) ; Beals, James T.; (West
Hartford, CT) ; Persky, Joshua E.; (Manchester,
CT) ; Verner, Carl R.; (Windsor, CT) ;
Kennard, Lea D.; (Manchester, CT) ; Mandich, Dawn
D.; (Monrovia, MD) ; Murray, Stephen D.;
(Marlborough, CT) |
Correspondence
Address: |
BACHMAN & LAPOINTE, P.C.
900 CHAPEL STREET
SUITE 1201
NEW HAVEN
CT
06510
US
|
Family ID: |
34940869 |
Appl. No.: |
10/825396 |
Filed: |
April 15, 2004 |
Current U.S.
Class: |
164/519 ;
164/35 |
Current CPC
Class: |
B22C 9/10 20130101; B22C
9/12 20130101; B22C 9/043 20130101 |
Class at
Publication: |
164/519 ;
164/035 |
International
Class: |
B22C 009/04 |
Claims
What is claimed is:
1. A method for forming an investment casting mold comprising:
forming a shell over a pattern comprising a hydrocarbon-based body
with a refractory metal-based core at least partially embedded in
the body; substantially removing the body from the shell;
strengthening the shell by heating in a first atmosphere of a first
composition; and further strengthening the shell by heating in a
vacuum or second atmosphere of a second composition, different than
the first composition.
2. The method of claim 1 wherein: the heating of the strengthening
is substantially at 800-1100 F; and the heating of the further
strengthening is substantially at 1400-1600 F.
3. The method of claim 1 wherein: the heating of the further
strengthening is a preheating prior to an introduction of molten
metal to the mold.
4. The method of claim 1 wherein: said first composition is more
oxidative than said second composition.
5. The method of claim 1 used to fabricate a gas turbine engine
turbine airfoil element.
6. The method of claim 1 wherein: the first composition consists in
major part of air.
7. The method of claim 6 wherein: the second composition consists
in major part of one or more inert gasses.
8. The method of claim 1 wherein: the first composition has an
oxygen partial pressure of at least fifteen kPa.
9. The method of claim 8 wherein: the second composition has an
oxygen partial pressure of no more than ten kPa.
10. The method of claim 1 further comprising: fully embedding the
refractory metal-based core in the hydrocarbon-based body.
11. The method of claim 1 wherein: the strengthening is effective
to provide the shell with a first modulus of rupture (MOR) strength
of 65-80% of a maximum MOR strength; and the further strengthening
is effective to provide the shell with a second MOR strength of at
least 85% of said maximum MOR strength.
12. The method of claim 11 wherein: after said substantially
removing, the shell has a preliminary MOR strength of no more than
50% of said maximum MOR strength.
13. A method for investment casting comprising: forming an
investment casting mold as in claim 1; introducing molten metal to
the mold; permitting the molten metal to solidify; and
destructively removing the mold.
14. The method of claim 13 wherein: a temperature of the shell does
not fall below 1200 F between the further strengthening and the
introducing.
15. A method for forming an investment casting mold comprising:
applying one or more coating layers to a sacrificial pattern having
a wax first portion and a second portion comprising a refractory
metal core; steam dewaxing of the coated pattern so as to remove a
major portion of the pattern first portion and leaving the second
portion within a shell formed by the coating layers; first heating
the shell to harden the shell and remove residues or byproducts of
the wax, the first heating being effective to provide the shell
with a first modulus of rupture (MOR) strength no more than 85% of
a maximum MOR strength; and second heating of the shell to
strengthen the shell to a second MOR strength.
16. The method of claim 15 wherein: the first heating is in an
oxidizing atmosphere; and the second heating is in vacuum or an
inert atmosphere.
17. The method of claim 15 wherein: the second heating is a
preheating prior to molten metal introduction.
18. The method of claim 15 wherein: the first MOR strength is
65-80% of said maximum MOR strength; and the second heating is
effective so that the second MOR strength is at least 85% of said
maximum MOR strength.
19. The method of claim 15 wherein: the first heating has a peak
temperature between 800 F and 1100 F; and the second heating has a
peak temperature in excess of 1500 F.
20. The method of claim 15 wherein: the first heating has a
temperature between 800 F and 1100 F for at least 2.0 hours; and
the second heating has a temperature in excess of 1500 F for at
least 1.0 hour.
21. The method of claim 15 wherein the second portion comprises:
said refractory metal core; a coating on said refractory metal
core; and a ceramic core secured to said refractory metal core
prior to the applying.
22. A method for forming an investment casting mold comprising:
applying one or more coating layers to a sacrificial pattern having
a first portion for forming a mold void and a second portion for
forming a portion of the mold; a first step for removing a major
portion of the pattern first portion and leaving the second portion
within a shell formed by the coating layers; a second step for
initial hardening of the shell effective to provide the shell with
a first modulus of rupture (MOR) strength no more than 85% of a
maximum MOR strength; and a third step for further hardening of the
shell without substantial degradation of the pattern second
portion.
23. The method of claim 22 used to fabricate a gas turbine engine
component.
24. The method of claim 22 wherein: the second step is essentially
performed under an oxygen partial pressure of at least twenty kPa.
the third step is essentially performed under an oxygen partial
pressure of no more than five kPa.
25. A method for investment casting comprising: forming an
investment casting mold as in claim 22; introducing molten metal to
the mold; permitting the molten metal to solidify; and
destructively removing the investment casting mold.
26. A system for forming an investment casting mold comprising:
means for forming a shell over a pattern, the pattern comprising a
hydrocarbon-based body with a refractory metal-based core at least
partially embedded in the body; means for substantially removing
the body from the shell; means for strengthening the shell by
heating in a first atmosphere of a first composition; and means for
further strengthening the shell by heating in a vacuum or second
atmosphere of a second composition, different than the first
composition.
Description
BACKGROUND OF THE INVENTION
[0001] (1) Field of the Invention
[0002] The invention relates to investment casting. More
particularly, the invention relates to investment casting using
molds having oxidizable cores.
[0003] (2) Description of the Related Art
[0004] Investment casting is a commonly used technique for forming
metallic components having complex geometries, especially hollow
components, and is used in the fabrication of superalloy gas
turbine engine components.
[0005] Gas turbine engines are widely used in applications
including aircraft propulsion, electric power generation, ship
propulsion, and pumps. In gas turbine engine applications,
efficiency is a prime objective.
[0006] Improved gas turbine engine efficiency can be obtained by
operating at higher temperatures, however current operating
temperatures in the turbine section exceed the melting points of
the superalloy materials used in turbine components. Consequently,
it is a general practice to provide air cooling. Cooling is
typically provided by flowing relatively cool air from the
compressor section of the engine through passages in the turbine
components to be cooled. Such cooling comes with an associated cost
in engine efficiency. Consequently, there is a strong desire to
provide enhanced specific cooling, maximizing the amount of cooling
benefit obtained from a given amount of cooling air. This may be
obtained by the use of fine, precisely located, cooling passageway
sections.
[0007] A well developed field exists regarding the investment
casting of internally-cooled turbine engine parts such as blades
and vanes. In an exemplary process, a mold is prepared having one
or more mold cavities, each having a shape generally corresponding
to the part to be cast. An exemplary process for preparing the mold
involves the use of one or more wax patterns of the part. The
patterns are formed by molding wax over ceramic cores generally
corresponding to positives of the cooling passages within the
parts. In a shelling process, a ceramic shell is formed around one
or more such patterns in well known fashion. The wax may be removed
such as by melting in an autoclave. The shell may be fired to
strengthen the shell. This leaves a mold comprising the shell
having one or more part-defining compartments which, in turn,
contain the ceramic core(s) defining the cooling passages. Molten
alloy may then be introduced to the mold to cast the part(s). Upon
cooling and solidifying of the alloy, the shell and core may be
mechanically and/or chemically removed from the molded part(s). The
part(s) can then be machined and/or treated in one or more
stages.
[0008] The ceramic cores themselves may be formed by molding a
mixture of ceramic powder and binder material by injecting the
mixture into hardened metal dies. After removal from the dies, the
green cores are thermally post-processed to remove the binder and
fired to sinter the ceramic powder together. The trend toward finer
cooling features has taxed core manufacturing techniques. The fine
features may be difficult to manufacture and/or, once manufactured,
may prove fragile. Commonly-assigned co-pending U.S. Pat. No.
6,637,500 of Shah et al. discloses various examples of a ceramic
and refractory metal core combination. Various refractory metals,
however, tend to oxidize at high temperatures in the vicinity of
the temperatures used to fire the shell. Thus, the shell firing may
degrade the refractory metal cores and, thereby produce potentially
unsatisfactory part internal features. Accordingly, there remains
room for further improvement in such cores and their manufacturing
techniques.
SUMMARY OF THE INVENTION
[0009] One aspect of the invention involves a method for forming an
investment casting mold. A shell is formed over a pattern
comprising a hydrocarbon-based body with a refractory metal-based
core at least partially embedded in the body. The body is then
substantially removed from the shell. The shell is strengthened by
heating in a first atmosphere of a first composition. The shell is
further strengthened by heating in a vacuum or second atmosphere of
a second composition, different than the first composition.
[0010] In various implementations, the heating of the further
strengthening step may be a preheating prior to an introduction of
molten metal to the mold. The first composition may be more
oxidative than the second composition. The method may be used to
fabricate a gas turbine engine airfoil element such as a blade or
vane. The first composition may consist, in major part (e.g., by
volume), of air. The second composition may consist, in major part,
of one or more inert gases. The first composition may have an
oxygen partial pressure of at least 15 kPa. The second composition
may have an oxygen partial pressure of no more than 10 kPa. The
strengthening may be effective to provide the shell with a first
modulus of rupture (MOR) strength of 65-80% of a maximum MOR
strength. The further strengthening may be effective to provide the
shell with a second MOR strength of at least 85% of said maximum
MOR strength. After the substantial removal of the body, the shell
may have a preliminary MOR strength of no more than 50% of said
maximum MOR strength.
[0011] Another aspect of the invention involves a method for
investment casting. Such a casting mold may be formed. Molten metal
may be introduced to the mold. The molten metal may be permitted to
solidify. The mold may be destructively removed. In various
implementations, the temperature of the shell does not fall below a
threshold (such as 1200 F) between the further strengthening and
the introduction of the molten metal.
[0012] Another aspect of the invention involves a method for
forming an investment casting mold. One or more coating layers are
applied to a sacrificial pattern having a wax first portion and a
second portion comprising refractory metal. A steam dewaxing may
remove a major portion of the pattern first portion and leave the
second portion within a shell formed by the coating layers. There
may be a first heating of the shell to harden the shell and remove
residues or byproducts of the wax. This first heating may be
effective to provide the shell with a first modulus of rupture
(MOR) strength no more than 85% of a maximum MOR strength. A second
heating of the shell may strengthen the shell to a second MOR
strength.
[0013] In various implementations, the first heating may be in an
oxidizing atmosphere and the second heating may be in vacuum or an
inert atmosphere. The second heating may be a preheating prior to
molten metal introduction. The first MOR strength may be 65-80% of
the maximum MOR strength. The second heating may be effective so
that the second MOR strength is at least 85% of the maximum MOR
strength. The first heating may have a peak temperature between 800
F and 1100 F. The second heating may have a peak temperature in
excess of 1500 F. The first heating may have a temperature between
800 F and 1100 F for at least 2.0 hours. The second heating may
have a temperature in excess of 1500 F for at least 1.0 hour. The
second portion may comprise the refractory metal core, a coating on
the refractory metal core, and a ceramic core secured to the
refractory metal core prior to the applying.
[0014] Another aspect of the invention involves a method for
forming an investment casting mold. One or more coating layers are
applied to a sacrificial pattern having a first portion for forming
a mold void and a second portion for forming a portion of the mold.
In a first step, a major portion of the pattern first portion is
removed leaving the second portion within a shell formed by the
coating layers. In a second step, the shell is initially hardened
effective to provide the shell with a first modulus of rupture
(MOR) strength no more than 85% of a maximum MOR strength. In a
third step, the shell is further hardened without substantial
degradation of the pattern second portion.
[0015] In various implementations, the method may be used to
fabricate a gas turbine engine component. The second step may be
essentially performed under an oxygen partial pressure of at least
20 kPa. The third step may be essentially performed under an oxygen
partial pressure of no more than 5 kPa.
[0016] Another aspect of the invention involves a system for
forming an investment casting mold. Means are provided for forming
a shell over a pattern. The pattern comprises a hydrocarbon-based
body with a refractory metal-based core at least partially embedded
in the body. Means are provided for substantially removing the body
from the shell. Means are provided for strengthening the shell by
heating in a first atmosphere of a first composition. Means are
provided for further strengthening of the shell by heating in a
vacuum or a second atmosphere of a second composition, different
than the first composition.
[0017] The details of one or more embodiments of the invention are
set forth in the accompanying drawings and the description below.
Other features, objects, and advantages of the invention will be
apparent from the description and drawings, and from the
claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 is a flowchart of a first mold manufacturing process
according to principles of the invention.
[0019] FIG. 2 is a flowchart of a second mold manufacturing process
according to principles of the invention.
[0020] Like reference numbers and designations in the various
drawings indicate like elements.
DETAILED DESCRIPTION
[0021] FIG. 1 shows an exemplary method 20 for forming an
investment casting mold. One or more metallic core elements are
formed 22 (e.g., of refractory metals such as molybdenum and
niobium by stamping or otherwise cutting from sheet metal) and
coated 24. Suitable coating materials include silica, alumina,
zirconia, chromia, mullite and hafnia. Preferably, the coefficient
of thermal expansion (CTE) of the refractory metal and the coating
are similar. Coatings may be applied by any appropriate technique
(e.g., CVD, PVD, electrophoresis, and sol gel techniques).
Individual layers may typically be 0.1 to 1 mil thick. Metallic
layers of Pt, other noble metals, Cr, and Al may be applied to the
metallic core elements for oxidation protection, in combination
with a ceramic coating for protection from molten metal erosion and
dissolution.
[0022] One or more ceramic cores are also formed 26 (e.g., of
silica in a molding and firing process). One or more of the coated
metallic core elements (hereafter refractory metal cores (RMCs))
are assembled 28 to one or more of the ceramic cores. The core
assembly is then overmolded 30 with an easily sacrificed material
such as a natural or synthetic wax (e.g., via placing the assembly
in a mold and molding the wax around it). There may be multiple
such assemblies involved in a given mold.
[0023] The overmolded core assembly (or group of assemblies) forms
a casting pattern with an exterior shape largely corresponding to
the exterior shape of the part to be cast. The pattern may then be
assembled 32 to a shelling fixture (e.g., via wax welding between
end plates of the fixture). The pattern may then be shelled 34
(e.g., via one or more stages of slurry dipping, slurry spraying,
or the like). After the shell is built up, it may be dried 36. The
drying provides the shell with at least sufficient strength or
other physical integrity properties to permit subsequent
processing. For example, the shell containing the invested core
assembly may be disassembled 38 fully or partially from the
shelling fixture and then transferred 40 to a dewaxer (e.g., a
steam autoclave). In the dewaxer, a steam dewax process 42 removes
a major portion of the wax leaving the core assembly secured within
the shell. The shell and core assembly will largely form the
ultimate mold. However, the dewax process typically leaves a wax or
byproduct hydrocarbon residue on the shell interior and core
assembly.
[0024] After the dewax, the shell is transferred 44 to an
atmospheric furnace (e.g., containing air or other oxidizing
atmosphere) in which it is heated 46 to a first peak temperature
and for a first time duration effective to prestrengthen the shell.
The heating 46 may also remove any remaining wax residue (e.g., by
vaporization) and/or converting hydrocarbon residue to carbon.
Oxygen in the atmosphere reacts with the carbon to form carbon
dioxide. Removal of the carbon is advantageous to avoid the carbon
clogging the vacuum pumps used in subsequent stages of operation.
This burning off of the carbon may be generally coincident with
oxidation of the shell associated with the advantageous
prestrengthening of the shell. An exemplary prestrengthening
provides the shell with a fraction of its ultimate (e.g., the
maximum fully-fired) modulus of rupture (MOR) strength (e.g.,
50-90%, more narrowly 60-85% or 65-80%). For typical shell
materials, industry practice generally associates firing at a
temperature of at least 1500 F for a duration of at least one hour
as essentially fully firing the shell to achieve essentially
maximum MOR strength. In common practice the shell is maintained at
least generally isothermal for at least this period. This may
represent an increase from well below 50% of ultimate MOR strength
in the relatively green state immediately post-dewax. The
pre-harden temperature is, advantageously, sufficiently low, in
view of the oxidizing nature of the atmosphere in the atmospheric
furnace to avoid substantial oxidation of the metallic core
element(s). Despite the presence of the protective coating,
oxidation is still a substantial potential problem due to the
presence of microcracks and porosity in the coating. Oxidation can
produce coating delamination or other damage and surface
irregularities on the metallic core. Coating damage may allow
vaporization of the metallic core elements at the high subsequent
casting temperatures and/or reactions between the casting alloy and
the metallic core elements. Surface irregularities caused by the
oxidation may, in turn form imperfections in the associated
interior surfaces of the cast part--a particular problem where fine
features are being formed. The exemplary peak preharden temperature
is less than 1150 F (e.g., 800-1100 F) for a preharden time of 2-4
hours. An exemplary preharden temperature and time is about 1000 F
for about 3.5 hours.
[0025] After the prehardening, the mold may be removed from the
atmospheric furnace, allowed to cool, and inspected 48. The mold
may be seeded 50 by placing a metallic seed in the mold to
establish the ultimate crystal structure of a directionally
solidified (DS) casting or a single-crystal (SX) casting.
Nevertheless the present teachings may be applied to other DS and
SX casting techniques (e.g., wherein the shell geometry defines a
grain selector) or to casting of other microstructures.
Alternatively, the mold may have The mold may be transferred 52 to
a casting furnace (e.g., placed atop a chill plate in the furnace).
The casting furnace may be pumped down to vacuum 54 or charged with
a non-oxidizing atmosphere (e.g., inert gas) to prevent oxidation
of the casting alloy. The casting furnace is heated 56 to preheat
the mold. This preheating serves two purposes: to further harden
and strengthen the shell (e.g., by at least 5% more of ultimate MOR
strength); and to preheat the shell for the introduction of molten
alloy to prevent thermal shock and premature solidification of the
alloy. Accordingly, the preheat temperature and duration are
advantageously sufficient to substantially further harden the shell
above its prehardened condition. This may involve sintering of the
ceramic particles within the shell. Advantageous MOR is in excess
of 85%, and more particularly, in excess of 90 or 95% of ultimate
MOR. This may be achieved with a preheat temperature of at least
1200 F, more particularly, at least 1400 F with an exemplary
preheat temperature of about 1600 F. Exemplary preheat times are
approximately one hour (e.g., 0.25-4.0 hours, more narrowly,
0.75-2.0 hours).
[0026] After preheating and while still under vacuum conditions,
the molten alloy is poured 58 into the mold and the mold is allowed
to cool to solidify 60 the alloy (e.g., after withdrawal from the
furnace hot zone). After solidification, the vacuum may be broken
62 and the chilled mold removed 64 from the casting furnace. The
shell may be removed in a deshelling process 66 (e.g., mechanical
breaking of the shell) and the core assembly removed in a decoring
process 68 (e.g., a chemical process) to leave a cast article
(e.g., a metallic precursor of the ultimate part). The cast article
may be machined 70, chemically and/or thermally treated 72 and
coated 74 to form the ultimate part.
[0027] FIG. 2 shows an alternate version 100 of the exemplary
process wherein like steps are shown with like numerals. The
alternate process, however, separates the firing from the
preheating. Thus, after the inspection 48, the prehardened mold is
transferred 102 to a nonatmospheric furnace which may be separate
from the casting furnace in which casting subsequently occurs.
After transfer, the nonatmospheric furnace may be pumped down 104
to vacuum (and/or charged with an inert atmosphere such as a noble
gas or mixture thereof). After the pump down, the mold may be fired
106 at a temperature and duration similar to the preheat 56. After
firing, the vacuum may be broken 108 (or inert atmosphere otherwise
vented) and the mold removed 110. After the removal, there may be a
subsequent inspection 112, temporary storage, additional
processing, and the like. Thereafter, the mold may be seeded 114
and transferred 116 to the casting furnace. A pump down 118 may be
similar to the pump down 54. A preheat 120 may be similar to the
preheat 56 or more abrupt as the firing function will, at least
largely, already have taken place.
[0028] One or more embodiments of the present invention have been
described. Nevertheless, it will be understood that various
modifications may be made without departing from the spirit and
scope of the invention. For example, the principles may be
implemented as modifications of existing or yet-developed processes
in which cases those processes would influence or dictate
parameters of the implementation. Accordingly, other embodiments
are within the scope of the following claims.
* * * * *