U.S. patent application number 11/385382 was filed with the patent office on 2007-09-27 for methods and materials for attaching casting cores.
This patent application is currently assigned to United Technologies Corporation. Invention is credited to P. Brennan Reilly.
Application Number | 20070221359 11/385382 |
Document ID | / |
Family ID | 38229583 |
Filed Date | 2007-09-27 |
United States Patent
Application |
20070221359 |
Kind Code |
A1 |
Reilly; P. Brennan |
September 27, 2007 |
Methods and materials for attaching casting cores
Abstract
In a method for attaching a first casting core to a second
casting core, a first portion of the first casting core is brought
into engagement or close proximity with a second portion of the
second casting core. The first and second portions are dipped in a
ceramic slurry to form a coating around the first and second
portions. The coating is hardened to form a joint between the first
and second portions.
Inventors: |
Reilly; P. Brennan; (Boston,
MA) |
Correspondence
Address: |
BACHMAN & LAPOINTE, P.C. (P&W)
900 CHAPEL STREET
SUITE 1201
NEW HAVEN
CT
06510-2802
US
|
Assignee: |
United Technologies
Corporation
|
Family ID: |
38229583 |
Appl. No.: |
11/385382 |
Filed: |
March 21, 2006 |
Current U.S.
Class: |
164/516 ; 164/28;
164/369 |
Current CPC
Class: |
B22C 9/103 20130101 |
Class at
Publication: |
164/516 ;
164/028; 164/369 |
International
Class: |
B22C 9/04 20060101
B22C009/04; B22C 9/10 20060101 B22C009/10 |
Claims
1. A method for attaching a first casting core to a second casting
core comprising: bringing a first portion of the first casting core
into engagement with or close proximity to a second portion of the
second casting core; dipping the first portion and second portion
in a slurry to form a coating; and hardening the coating to join or
further join the first and second casting cores.
2. The method of claim 1 further comprising: bringing a third
portion of a third casting core into engagement with or close
proximity to a fourth portion of at least one of the first and
second cores and wherein said dipping, or another dipping,
comprises dipping the third arid fourth portions.
3. The method of claim 1 further comprising: attaching at least one
additional casting core to at least one of the first and second
casting cores.
4. The method of claim 1 wherein: the bringing comprises contacting
and holding in contact; and the dipping is performed after the
bringing.
5. The method of claim 1 wherein: the bringing comprises providing
a ceramic adhesive between the first portion and second portion;
and the dipping is performed after the bringing.
6. The method of claim 1 wherein: the dipping comprises multiple
dippings in different slurries; and at least one of the dippings is
performed after the bringing.
7. The method of claim 1 wherein: the bringing comprises contacting
in a fired state; and the hardening comprises drying and does not
comprise firing.
8. The method of claim 1 wherein: the bringing comprises contacting
in a green state; and the hardening comprises firing with the first
and second cores to also harden the first and second cores.
9. The method of claim 1 wherein: the coating fully encircles the
first and second portions at a junction.
10. The method of claim 1 further comprising: molding the first and
second casting cores.
11. The method of claim 1 wherein: the bringing comprises forming a
joint selected from the group consisting of: shiplap, half lap,
back-locked protuberance/recess, and tongue/groove.
12. The method of claim 1 further comprising: inserting an
insertion portion of a metallic casting core into a receiving
portion of the joined first and second casting cores.
13. The method of claim 1 used to form a turbine blade core, a
turbine vane core, a seal core or a combustor core.
14. The method of claim 1 further comprising: the hardening
comprises heating.
15. The method of claim 1 wherein the slurry comprises zircon and
aqueous colloidal silica.
16. The method of claim 1 wherein: the slurry has: a by weight
zircon content; and a by weight aqueous colloidal silica content of
20-30%, of a said zircon content.
17. The method of claim 1 wherein: the slurry comprises a
surfactant.
18. The method of claim 1 further comprising: molding the first
casting core; and forming the second casting core from a metallic
sheet.
19. The method of claim 1 wherein: the first casting core comprises
a molded ceramic; and the second casting core comprises a metallic
member.
20. A method for investment casting comprising: attaching,
according to claim 1, first and second casting cores; molding a
sacrificial material partially over the first and second casting
cores; applying a shell to the sacrificial material; removing the
sacrificial material from the shell, delivering molten metal to the
shell at least partially in place of the sacrificial material;
allowing the molten metal to solidify; removing the shell and first
and second casting cores from the solidified metal.
21. A method for attaching a first casting core to a second casting
core comprising; bringing a first portion of the first casting core
into engagement with or close proximity to a second portion of the
second casting core in a first relative position; holding the first
casting core and second casting core to maintain said first
relative position; dipping the first portion and second portion in
a slurry to form a coating; and hardening the coating to join or
further join the first and second casting cores.
22. The method of claim 21 wherein: the dipping and a portion of
the hardening occur during the holding.
23. A method for attaching a first casting core to a second casting
core comprising: bringing a first portion of the first casting core
into engagement with or close proximity to a second portion of the
second casting core in a first relative position; applying a slurry
coating over the first portion and second portion by dipping; and
hardening the slurry coating to join or further join the first and
second casting cores.
24. The method of claim 23 further comprising: holding with a
fixture the first casting core and second casting core to maintain
said first relative position during at least portions of said
applying and hardening.
25. The method of claim 23 wherein: the coating filly encircles the
first and second portions at a junction.
Description
BACKGROUND OF THE INVENTION
[0001] The invention relates to investment casting. More
particularly, the invention relates to investment casting core
assemblies.
[0002] Investment casting is commonly used in the aerospace
industry. Various examples involve the casting of gas turbine
engine parts. Exemplary parts include various blades, vanes, seals,
and combustor panels. Many such parts are cast with cooling
passageways. The passageways may be formed by sacrificial casting
cores.
[0003] Exemplary cores include ceramic cores, refractory metal
cores (RMCs), and combinations thereof. In exemplary combinations,
the ceramic cores may form feed passageways whereas the RMCs may
form cooling passageways extending from the feed passageways
through walls of the associated part. It is understood that ceramic
cores may be used to form cooling passageways and metal cores may
be used to form feed passageways. Ceramic cores may be made by
molding a "green" core and then firing to harden. Refractory metal
cores may be made by casting or from sheetstock (e.g., by stamping
or cutting/forming) or by other suitable methods. The cores may be
assembled to each other and secured, for example, with a ceramic
adhesive. An exemplary ceramic adhesive is alumina-based. For
example, the adhesive may comprise alumina powder and a binder such
as colloidal silica.
[0004] The core(s) may be overmolded with a sacrificial material
(e.g., a wax) to form a pattern in a shape at least partially
corresponding to that of the part to be cast. A shell may be formed
over the pattern (e.g., a ceramic shell formed in a multi-stage
stuccoing process). The sacrificial material may be removed (e.g.,
by a steam dewaxing) leaving the core within a mold chamber formed
by the shell. The shell may be fired to harden.
[0005] Molten metal may be poured into the shell and allowed to
solidify.
[0006] After this initial casting of the part (e.g., from a nickel-
or cobalt-based superalloy), the casting shell and core(s) are
destructively removed. Exemplary shell removal is principally
mechanical. Exemplary core removal is principally chemical. For
example, the cores may be removed by chemical leaching. Exemplary
leaching involves use of an alkaline solution in an autoclave.
Exemplary leaching techniques are disclosed in U.S. Pat. Nos.
4,141,781, 6,241,000, and 6,739,380.
SUMMARY OF THE INVENTION
[0007] Accordingly, one aspect of the invention involves a method
for attaching a first casting core to a second casting core. A
first portion of the first casting core is brought into engagement
or close proximity with a second portion of the second casting
core. The first and second portions are dipped in a ceramic slurry
to form a coating around the first and second portions. The coating
is hardened to form a joint between the first and second portions.
The resulting composite core may be easier to manufacture than a
similarly shaped non-composite core.
[0008] In various implementations, the metallic casting core may
comprise a refractory metal-based substrate (e.g., optionally
coated). The method may be used to form a turbine blade core
assembly or a turbine vane core assembly. The slurry may be heated
to harden. The metallic casting core and ceramic casting core may
be vibrated during the introducing. The bringing may be performed
with the ceramic casting cores in a green state or in a fired
state. The slurry may comprise zircon and aqueous colloidal
silica.
[0009] 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
[0010] FIG. 1 is a flowchart of an exemplary investment casting
process.
[0011] FIG. 2 is a view of a pair of ceramic cores.
[0012] FIG. 3 is a view of the cores of FIG. 2 held assembled in a
fixture.
[0013] FIG. 4 is a view of the composite core formed from the
ceramic cores of FIG. 2 assembled with a metallic core.
[0014] FIG. 5 is a flowchart of the assembly/manufacture steps of
the composite core of FIG. 4.
[0015] FIG. 6 is a view of a ceramic core attached to a metallic
core.
[0016] Like reference numbers and designations in the various
drawings indicate like elements.
DETAILED DESCRIPTION
[0017] FIG. 1 shows an exemplary method 20 for forming an
investment casting mold. Other methods are possible, including a
variety of prior art methods and yet-developed methods. 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 or of alloys or intermetallics containing one or
more refractory metals) 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 line-of sight or non-line-of sight
technique (e.g., chemical or physical vapor deposition (CVD, PVD)
methods, plasma spray methods, electrophoresis, and sol gel
methods). Individual layers may typically be 0.1 to 1 mil thick.
Layers of Pt, other noble metals, Cr, Si, W, and/or Al, or other
non-metallic materials 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.
[0018] One or more ceramic cores may also be formed 26 (e.g., of or
containing 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. As
noted above, the assembly may include use of a ceramic slurry
discussed below. 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.
[0019] 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.
[0020] After the dewax, the shell is transferred 44 to a furnace
(e.g., containing air or other oxidizing atmosphere) in which it is
heated 46 to strengthen the shell and 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
reduce or eliminate the formation of detrimental carbides in the
metal casting. Removing carbon offers the additional advantage of
reducing the potential for clogging the vacuum pumps used in
subsequent stages of operation.
[0021] The mold may be removed from the atmospheric furnace,
allowed to cool, and inspected 48. The mold may be seeded 50, if
necessary (e.g., by locating 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. 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; and to
preheat the shell for the introduction of molten alloy to prevent
thermal shock of the shell and premature solidification of the
alloy.
[0022] 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., during or 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).
[0023] The core assembly is removed in a decoring process 68 to
leave a cast article (e.g., a metallic precursor of the ultimate
part). Inventive multi-stage decoring processes are described
below. The cast article may be machined 70, chemically and/or
thermally treated 72 and coated 74 to form the ultimate part. Some
or all of any machining or chemical or thermal treatment may be
performed before the decoring.
[0024] FIGS. 2-4 show an exemplary composite core in various stages
of manufacture. FIG. 5 is a flowchart of the assembly/manufacture
steps. FIG. 2 shows a pair of individually-molded ceramic cores 120
and 122. The exemplary cores have respective main body portions or
trunks 124 and 126 and terminal portions 128 and 130. Near
respective ends 132 and 134 of the terminal portions, the cores may
include mating features 136 and 138. Exemplary mating features are
shown as a protuberant projection 136 and complementary compartment
138. Alternative mating features include dovetail and other
back-locking features, shiplap features, halflap features, and
simple butt features. Although illustrated at terminal portions,
the mating features may be otherwise formed (e.g., one or more
pairs of mating features at intermediate locations along a core
body).
[0025] After molding and optionally after a firing to further
harden the cores, the cores 120 and 122 may be assembled to each
other. FIG. 3 shows the cores assembled/mated 180 to each other and
held by a fixture 140 with their mating features engaged to each
other. Exemplary fixtures 140 may include clamps, robotic
actuators, and the like.
[0026] The fixture 140 may hold the assembled cores during the
application of slurry. Exemplary slurry application includes one or
more dippings 182 of the terminal portions in a slurry tank to
apply a slurry coating 150. If multiple dippings are used, they may
be of the same slurry or different slurries from different tanks to
produce a desired layering of the coating 150 (e.g., for maximized
strength of joint). The terminal portions may be slightly thinned
or recessed relative to the adjacent trunk portions so that the
build-up of slurry does not excessively increase core thickness or
provide discontinuities therein. During the coating application,
the fixture 140 may hold the engaged portions of the cores in
contact or close proximity (e.g., with a gap sufficiently sized to
allow a bonding infiltration of the slurry between the two
cores).
[0027] After coating application, the coating may be hardened. An
exemplary hardening involves drying 184 without further firing.
This may be particularly useful with pre-fired ceramic components
which are held together with wax pads. For example, an additional
ceramic core may be pre-assembled to one of the cores 120 or 122
and positioned with a wax pad. Room temperature drying of the
slurry would preserve the pad. Alternative non-firing drying might
involve heating at temperatures of up to 95.degree. C. The
hardening may, alternatively, include a firing (e.g., at
1200.degree. C. or greater) which may also harden the cores 120 and
122 (e.g., if assembled in a green state or an only partially fired
state).
[0028] The result of the hardening is to produce a joint 154 of
sufficient strength to allow further handling and processing steps
as described relative to FIG. 1. As described therein, these may
include assembly of the resulting composite ceramic core 160 to one
or more other cores such as a refractory metal core 162 (FIG. 4)
(e.g., via insertion 186 in a slot cut in the composite core or
pre-molded therein).
[0029] Exemplary slurries may be identical or similar to shell
coating slurries. Sequences may be altered relative to application
sequences for shell coating slurries to provide desired joint
strength and joint surface smoothness. For example, shell coating
slurry application sequences typically proceed from fine to coarse.
Initial fine slurry is applied to a pattern for smoothness and
subsequent coarse slurry for strength. However, it may be desired
that the last layer of slurry in the coating 150 be fine for
smoothness because the cores generally form internal features
whereas the shells generally form external features. Exemplary
slurries comprise a combination of zircon and aqueous colloidal
silica along with appropriate surfactants and other agents (e.g.,
for promoting bubble rupture).
[0030] Variations include use of the slurry coating in combination
with a ceramic adhesive in the joint. The adhesive may be
introduced during core assembly. Exemplary ceramic adhesives are
available from Cotronics Corporation of Brooklyn, N.Y., under the
trademark RESBOND. Use of ceramic adhesive may be particularly
appropriate with pre-fired ceramic cores. The ceramic adhesive
increases bond strength and may further avoid the need for a
subsequent high temperature firing.
[0031] Such adhesive may also be appropriate when the dipped
assembly includes metallic or other non-ceramic cores which may be
adversely affected by a high temperature firing. For example, FIG.
6 shows the assembly of a ceramic core 200 with a refractory metal
core 202. A portion 204 of the refractory metal core is positioned
within a slot 206 of a ceramic core and retained at least partially
by a ceramic adhesive 208. A slurry coating 210 may surround the
junction of these two cores to form a joint. The slurry 210 may be
applied as described above. If required by physical constraints or
otherwise appropriate, a portion of one or both of the cores may be
masked during slurry application (e.g., spraying, painting, or
dipping) or may be cleaned of slurry after such application. As
with the joint between ceramic cores, the overdipping of the
RMC-ceramic core joint isolates the unfired ceramic adhesive (if
any) from contact with the casting alloy to avoid adverse chemical
interaction.
[0032] 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.
* * * * *