U.S. patent application number 12/823165 was filed with the patent office on 2011-12-29 for contoured metallic casting core.
This patent application is currently assigned to UNITED TECHNOLOGIES CORPORATION. Invention is credited to Amanda J. Learned, Keith A. Santeler, Daniel A. Snyder.
Application Number | 20110315336 12/823165 |
Document ID | / |
Family ID | 44501655 |
Filed Date | 2011-12-29 |
United States Patent
Application |
20110315336 |
Kind Code |
A1 |
Snyder; Daniel A. ; et
al. |
December 29, 2011 |
Contoured Metallic Casting Core
Abstract
A method for manufacturing an investment casting core uses a
metallic blank having a thickness between parallel first and second
faces less than a width and length transverse thereto. The blank is
locally thinned from at least one of the first and second faces.
The local thinning forms a taper on a leading portion of the RMC.
The blank is through-cut across the thickness. The blank is
inserted into the leading portion into a slot in a pre-formed
ceramic core.
Inventors: |
Snyder; Daniel A.;
(Manchester, CT) ; Santeler; Keith A.; (Clifton
Heights, PA) ; Learned; Amanda J.; (Manchester,
CT) |
Assignee: |
UNITED TECHNOLOGIES
CORPORATION
Hartford
CT
|
Family ID: |
44501655 |
Appl. No.: |
12/823165 |
Filed: |
June 25, 2010 |
Current U.S.
Class: |
164/23 ; 164/271;
164/6 |
Current CPC
Class: |
B22C 9/24 20130101; B22C
9/10 20130101; B22C 9/103 20130101 |
Class at
Publication: |
164/23 ; 164/6;
164/271 |
International
Class: |
B22D 25/00 20060101
B22D025/00; B22C 9/10 20060101 B22C009/10; B22C 9/02 20060101
B22C009/02 |
Goverment Interests
US GOVERNMENT RIGHTS
[0001] The invention was made with US Government support under
contract W911W6-08-2-0001 awarded by the US Army. The US Government
has certain rights in the invention.
Claims
1. A method for manufacturing an investment casting core from a
metallic blank having a thickness between parallel first and second
faces less than a width and length transverse thereto, the method
comprising: locally thinning the blank from at least one of the
first and second faces, the local thinning forming a taper on a
leading portion of the RMC; through-cutting the blank across the
thickness; and inserting the leading portion into a slot in a
pre-formed ceramic core.
2. The method of claim 1 wherein: at least the through-cutting
comprises at least one of stamping, laser cutting, liquid jet
cutting, and EDM.
3. The method of claim 1 wherein: at least the locally thinning
comprises at least one of EDM, ECM, MDP, and mechanical
machining.
4. The method of claim 1 wherein: the through-cutting and the
locally thinning are performed separately.
5. The method of claim 1 wherein: the through-cutting comprises
forming a plurality of through-apertures.
6. The method of claim 1 wherein: the locally thinning comprises
machining a main portion and leaving a thicker portion downstream
of the main portion.
7. The method of claim 1 further comprising: coating the core.
8. The method of claim 1 wherein: the locally thinning comprises
forming the taper by thinning both of the first and second
faces.
9. The method of claim 8 wherein: the local thinning comprises
essentially uniformly removing material from a pressure side along
the taper and an intermediate portion downstream thereof and, along
the suction side, removing material only from the leading
portion.
10. The method of claim 1 wherein: the through-cutting forms
apertures within the blank.
11. A method for investment casting comprising: forming according
to claim 1 an investment casting core; molding a pattern-forming
material at least partially over the at least one investment
casting core for forming a pattern; shelling the pattern; removing
the pattern-forming material from the shelled pattern for forming a
shell; introducing molten alloy to the shell; and removing the
shell.
12. The method of claim 11 used to form a gas turbine engine
component.
13. The method of claim 11 used to form a gas turbine engine
airfoil wherein the core forms a trailing edge outlet slot.
14. An investment casting core comprising: a metallic casting core
element having: a tapered leading portion; an intermediate portion
downstream of the tapered leading portion; and a trailing portion
downstream of the intermediate portion and thicker than the
intermediate portion; and a ceramic casting core having a slot
receiving the leading portion.
15. The investment casting core of claim 14 wherein: along the
leading portion and intermediate portion, a pressure side surface
has essentially continuous concave curvature; and along a suction
side surface, the intermediate portion has essentially continuous
convex curvature and the leading portion has discontinuous
curvature so as to provide the taper.
Description
BACKGROUND
[0002] The disclosure relates to investment casting. More
particularly, it relates to the investment casting of superalloy
turbine engine components.
[0003] 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. The disclosure is described in respect
to the production of particular superalloy castings, however it is
understood that the disclosure is not so limited.
[0004] Gas turbine engines are widely used in aircraft propulsion,
electric power generation, and ship propulsion. In gas turbine
engine applications, efficiency is a prime objective. 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 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.
[0005] The cooling passageway sections may be cast over casting
cores. Ceramic casting cores may be formed by molding a mixture of
ceramic powder and binder material by injecting the mixture into
hardened steel 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 U.S. Pat. Nos. 6,637,500 of Shah
et al. and 6,929,054 of Beals et al and Pre-grant Publication
2007/261814 of Luczak (the disclosures of which are incorporated by
reference herein as if set forth at length) disclose use of ceramic
and refractory metal core combinations.
[0006] FIG. 1 shows a trailing edge portion of a turbine airfoil 20
as cast within a shell 22. For casting the internal passageways,
the shell contains a core assembly. The exemplary core assembly
includes a ceramic feed core having spanwise legs 30, 32, and 34
for casting associated passageway legs. The leg 34 casts a trailing
spanwise passageway 36. The core assembly also includes metallic
cores, of which cores 40, 42, and 44 are shown. The exemplary
metallic cores are formed of refractory metal sheet stock. The core
40 forms a pressure side outlet circuit, the core 42 forms a
suction side outlet circuit, and the core 44 forms a trailing edge
outlet slot 50. The outlet slot 50 is fed from the passageway 36.
During core assembly, a leading portion of the core 44 is secured
within a mating slot of the trailing leg 34 of the ceramic
core.
SUMMARY
[0007] One aspect of the disclosure involves a method for
manufacturing an investment casting core from a metallic blank. The
blank has a thickness between parallel first and second faces less
than a width and length transverse thereto. The blank is locally
thinned from at least one of the first and second faces. The blank
is through-cut across the thickness. The blank is inserted into the
leading portion into a slot in a pre-formed ceramic core.
[0008] In various implementations, through-cutting may comprise at
least one of laser cutting, liquid jet cutting, and EDM. The
thinning may comprise at least one of EDM, ECM, MDP, and mechanical
machining.
[0009] In an investment casting method, the investment casting core
may be at least partially overmolded by a pattern-forming material
for forming a pattern. The pattern may be shelled. The
pattern-forming material may be removed from the shelled pattern
for forming a shell. Molten alloy may be introduced to the shell.
The shell may be removed. The method may be used to form a gas
turbine engine component. An exemplary component is an airfoil
wherein the core forms trailing edge outlet passageways.
[0010] Another aspect of the disclosure involves an investment
casting core having a metallic core element and a ceramic core. The
metallic core element has a tapered leading portion, an
intermediate portion downstream of the tapered leading portion, and
a trailing portion downstream of the intermediate portion and
thicker than the intermediate portion. The ceramic casting core has
a slot receiving the leading portion.
[0011] The details of one or more embodiments are set forth in the
accompanying drawings and the description below. Other features,
objects, and advantages will be apparent from the description and
drawings, and from the claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 is a partial streamwise sectional view of a trailing
edge portion of a prior art airfoil cast within a ceramic
shell.
[0013] FIG. 2 is a partial streamwise sectional view of a modified
airfoil.
[0014] FIG. 2A is an enlarged view of a portion of FIG. 2.
[0015] FIG. 3 is a partially schematic/simplified view of a pattern
including the core assembly.
[0016] FIG. 4 is a partially schematic/simplified view of a blade
cast in a shell formed over the pattern.
[0017] FIG. 5 is an enlarged partial pressure side view of a
discharge slot of the blade of FIG. 4.
[0018] FIG. 6 is a flowchart of a core manufacture process.
[0019] Like reference numbers and designations in the various
drawings indicate like elements.
DETAILED DESCRIPTION
[0020] FIG. 2 shows an alternative refractory metal core 60 which
has a leading/upstream edge/end 62 and a trailing/downstream
edge/end 64. The exploded view of FIG. 3 shows an inboard end 66
and an outboard end 68. As is discussed further below, an
upstream-most portion 70 extending aft from the leading edge/end 62
is configured to be received within and mate with a trailing slot
72 of a trailing leg 74 of a ceramic feedcore 76. The RMC 60 has an
intermediate portion 80 which casts the majority of the ultimate
trailing edge discharge slot. In the exemplary RMC 60, along this
region 80, the RMC pressure side/surface 82 and suction
side/surface 84 are separated by an essentially constant RMC
thickness T.sub.1 (FIG. 2A). Downstream of the portion 80, the
exemplary RMC thickens. A relatively thick portion 86 having an
essentially constant thickness shown as T.sub.3 extends to the
trailing end/edge 64. Of this portion 86, a smaller upstream
portion 88 casts pressure side discharge openings in the
airfoil.
[0021] FIG. 3 (a partially schematic/simplified view of a pattern)
shows the portion 80 having holes 100 for casting posts within the
slot. FIG. 3 further shows the portion 88 as having streamwise
elongate tapering holes 102 which are interspersed with intact
portions 104. The intact portions 104 cast pressure side openings
from the trailing edge discharge slot; whereas the holes 102 cast
walls therebetween.
[0022] In the exemplary core assembly, the feedcore slot 72 and RMC
portion 70 both have an upstream-ward taper. The exemplary
thickness T.sub.2 of the RMC at the leading edge is less than
T.sub.1 (e.g., 30-60%). The exemplary RMC taper is essentially
constant at an angle of .theta..sub.1 over a streamwise length
L.sub.1. The exemplary taper is provided by relieving/beveling only
one of the two faces 82 and 84 (the face 84 in the exemplary
embodiment with a bevel facet/surface 110). The exemplary relief
provides the taper angle .theta..sub.1. Exemplary .theta..sub.1 are
0.1-3.0.degree., more narrowly 1.0-2.5.degree.. Exemplary taper
length L.sub.1 is coincident with or slightly less than a depth
D.sub.1 of the slot. The exemplary slot has an opening 120 having a
height H.sub.1 which may be greater than T.sub.1 and has a base 122
with a height H.sub.2 which is greater than T.sub.2. A portion of
the slot between respective slot walls 124 and 126 and the RMC may
be filled with an adhesive or slurry 130. The exemplary streamwise
cross-section of the RMC is shown as generally arcuate with
concavity along the pressure side and convexity along the suction
side so as to correspond to a median of the airfoil
cross-section.
[0023] Exemplary L.sub.1 is 0.040-0.100 inch (1-2.5 mm), more
narrowly 0.050-0.075 inch (1.3 mm-9 mm). Exemplary T.sub.1 is 0.012
inch (0.3 mm), more broadly 0.005-0.020 inch (0.13-0.5 mm) or
0.010-0.015 inch (0.25-0.38 mm). Exemplary T.sub.2 is 0.005 inch
(0.13 mm), more broadly 0.002-0.015 inch (0.05-0.38 mm) or
0.003-0.007 inch (0.08-0.18 mm) or 25-75% of T.sub.1, more
narrowly, 40-60%. Exemplary T.sub.3 is 0.035 inch (0.9 mm), more
broadly 0.020-0.050 inch (0.5-1.3 mm) or 200-500% of T.sub.1, more
narrowly 250-400%. Exemplary feedcore thickness at either side of
the slot base 122 (shown as T.sub.4 to the pressure side and
T.sub.5 to the suction side) may be at least 0.018 inch (0.46 mm),
more narrowly 0.018-0.040 inch (0.46-1.0 mm) or 0.08-0.025 inch
(0.46-0.64 mm).
[0024] In an exemplary sequence 200 of manufacture (FIG. 6), the
RMC 84 may be machined from a strip having a thickness equal to
T.sub.3, a greater width, and a yet greater length. In an initial
stage of manufacture, gross thickness features may be machined 202
to provide the thickness T.sub.1 of the intermediate portion and
provide the bevel/taper. Specifically, the exemplary machining is
from the pressure side face 82 to define the intermediate portion
and from the suction side face 84 to provide the taper of the
leading portion. However, the step 202 may easily be further
divided. Exemplary machining may be mechanical machining or may be
an abrasive grinding, electrodischarge machining (EDM),
electrochemical machining (ECM), or a molecular decomposition
process (MDP).
[0025] Additionally, a series of through-cuts are cut 206 to define
the holes/apertures 100 for forming posts 150 (FIG. 4) within the
outlet slot and holes/apertures 102 for forming trailing dividing
walls 152 along the slot outlet 154 at the trailing edge 156. FIG.
4 further shows: the airfoil 160 having a leading edge 162 and a
tip 164; the platform 170 at the inboard end of the aitrfoil; and
the firtree attachment root 172 depending from the underside of the
platform. The root has the inlet ports 174 to the trunks of the
cooling passageway network (cast over the ceramic feedcore trunks).
FIG. 5 shows the outlet 154 as including a spanwise array of
segments/portions/openings 180 along the airfoil pressure side
between associated pairs of the dividing walls 152. As is discussed
above, the openings 180 are cast by the intact portions 104 of the
RMC portion 88 of FIG. 2. A curving transition 89 (FIG. 2) between
the RMC portions 80 and 86/88 casts a curving transition 182 (FIG.
5) between a main portion 184 of the slot and the openings 180.
[0026] Exemplary cutting may be via a punching/stamping operation
or, alternatively, mechanical drilling, laser cutting, liquid jet
cutting, and/or EDM. To provide the RMC in the desired arcuate
shape corresponding to the airfoil median 500, the RMC is bent 208
(e.g., via stamping). This bending may also form a spanwise
variation (e.g., to accommodate a varying relationship in the
position of the feedcore relative to the discharge slot) such as
creating a net spanwise twist. An exemplary stamping is performed
via one or more pressing stages in custom presses having opposing
die faces contoured to mate with the RMC. The RMC may be coated 210
with a protective coating. Alternatively a coating could be applied
pre-assembly. 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.
[0027] The ceramic core may be (e.g., silica-, zircon-, or
alumina-based) molded 212. The as-molded ceramic material may
include a binder. The binder may function to maintain integrity of
the molded ceramic material in an unfired green state. Exemplary
binders are wax-based. After the molding 212, the preliminary core
assembly may be debindered/fired 214 to harden the ceramic (e.g.,
by heating in an inert atmosphere or vacuum). The slot 72 may have
been formed as part of the molding 212 or may be cut in the ceramic
(e.g., in the green state or in the fired state). The RMC may be
inserted 216 into the ceramic core to assemble and an adhesive or
slurry introduced 218.
[0028] FIG. 6 shows an exemplary method 220 for investment casting
using the core assembly. Other methods are possible, including a
variety of prior art methods and yet-developed methods. The fired
core assembly is then overmolded 230 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.
[0029] 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 232 to a shelling fixture (e.g., via wax welding between
end plates of the fixture). The pattern may then be shelled 234
(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 236. 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 238 fully or partially from the
shelling fixture and then transferred 240 to a dewaxer (e.g., a
steam autoclave). In the dewaxer, a steam dewax process 242 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.
[0030] After the dewax, the shell is transferred 244 to a furnace
(e.g., containing air or other oxidizing atmosphere) in which it is
heated 246 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.
[0031] The mold may be removed from the atmospheric furnace,
allowed to cool, and inspected 248. The mold may be seeded 250 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. The mold may be transferred 252 to a casting
furnace (e.g., placed atop a chill plate in the furnace). The
casting furnace may be pumped down to vacuum 254 or charged with a
non-oxidizing atmosphere (e.g., inert gas) to prevent oxidation of
the casting alloy. The casting furnace is heated 256 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 and premature
solidification of the alloy.
[0032] After preheating and while still under vacuum conditions,
the molten alloy is poured 258 into the mold and the mold is
allowed to cool to solidify 260 the alloy (e.g., after withdrawal
from the furnace hot zone). After solidification, the vacuum may be
broken 262 and the chilled mold removed 264 from the casting
furnace. The shell may be removed in a deshelling process 266
(e.g., mechanical breaking of the shell).
[0033] The core assembly is removed in a decoring process 268 to
leave a cast article (e.g., a metallic precursor of the ultimate
part). The cast article may be machined 270, chemically and/or
thermally treated 272 and coated 274 to form the ultimate part.
Some or all of any machining or chemical or thermal treatment may
be performed before the decoring.
[0034] One or more embodiments have been described. Nevertheless,
it will be understood that various modifications may be made. For
example, the principles may be implemented using modifications of
various existing or yet-developed processes, apparatus, or
resulting cast article structures (e.g., in a reengineering of a
baseline cast article to modify cooling passageway configuration).
In any such implementation, details of the baseline process,
apparatus, or article may influence details of the particular
implementation. Accordingly, other embodiments are within the scope
of the following claims.
* * * * *