U.S. patent application number 13/780763 was filed with the patent office on 2014-08-28 for methods for repairing ceramic cores.
This patent application is currently assigned to GENERAL ELECTRIC COMPANY. The applicant listed for this patent is GENERAL ELECTRIC COMPANY. Invention is credited to Sylvia Marie Decarr, Martin Kin-Fei Lee, Norbert Otto Maurer, Xi Yang.
Application Number | 20140238632 13/780763 |
Document ID | / |
Family ID | 50031617 |
Filed Date | 2014-08-28 |
United States Patent
Application |
20140238632 |
Kind Code |
A1 |
Yang; Xi ; et al. |
August 28, 2014 |
METHODS FOR REPAIRING CERAMIC CORES
Abstract
A method is provided for repairing defects in a ceramic core
adapted for use in a casting process. The ceramic core is formed
from a slurry comprising at least one refractory powder material
suspended in a liquid vehicle. The method includes forming a repair
mixture comprising a liquid diluent and the at least one refractory
powder material suspended in the liquid vehicle of the slurry. The
repair mixture is applied to a region of the ceramic core that
includes at least one defect and then the ceramic core is fired to
burn off the liquid vehicle in the repair mixture and form a
ceramic composition that closes the defect.
Inventors: |
Yang; Xi; (Mason, OH)
; Lee; Martin Kin-Fei; (Schenectady, NY) ; Decarr;
Sylvia Marie; (Schenectady, NY) ; Maurer; Norbert
Otto; (Loveland, OH) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
GENERAL ELECTRIC COMPANY |
Schenectady |
NY |
US |
|
|
Assignee: |
GENERAL ELECTRIC COMPANY
Schenectady
NY
|
Family ID: |
50031617 |
Appl. No.: |
13/780763 |
Filed: |
February 28, 2013 |
Current U.S.
Class: |
164/369 ;
427/134 |
Current CPC
Class: |
B22C 1/205 20130101;
B22C 9/10 20130101 |
Class at
Publication: |
164/369 ;
427/134 |
International
Class: |
B22C 9/10 20060101
B22C009/10 |
Claims
1. A method for repairing defects in a ceramic core adapted for use
in a casting process, the ceramic core being formed from a slurry
comprising at least one refractory powder material suspended in a
liquid vehicle, the method comprising: forming a repair mixture
comprising a liquid diluent and the at least one refractory powder
material suspended in the liquid vehicle of the slurry; applying
the repair mixture to a region of the ceramic core, the region
comprising at least one defect; and firing the ceramic core to burn
off the liquid vehicle in the repair mixture and form a ceramic
composition that closes the defect.
2. The method of claim 1, wherein the at least one refractory
powder material comprises a silica refractory powder and a zircon
refractory powder.
3. The method of claim 1, wherein the liquid vehicle comprises a
siloxane binder.
4. The method of claim 1, wherein the liquid diluent is a siloxane
diluent.
5. The method of claim 1, wherein the liquid diluent is an
additional amount of the liquid vehicle.
6. The method of claim 1, wherein the repair mixture further
comprises a catalyst capable of cross-linking the liquid vehicle
and the liquid diluent.
7. The method of claim 1, wherein the repair mixture comprises, by
volume, about 30 to about 50 percent of the refractory powder
material and about 50 to about 70 percent of the liquid vehicle and
the liquid diluent combined.
8. The method of claim 7, wherein the repair mixture comprises up
to about 100 ppm of a catalyst capable of cross-linking the liquid
vehicle and the liquid diluent.
9. The method of claim 1, further comprising wetting the region
with the liquid diluent prior to applying the repair mixture.
10. The method of claim 1, wherein the ceramic core comprises a
wall defining at least one cavity therein.
11. The method of claim 10, further comprising injecting the repair
mixture into the cavity to close at least one opening in the wall
of the ceramic core prior to firing.
12. The ceramic core repaired by the method of claim 1.
13. A method for repairing defects in a silica-containing ceramic
core adapted for use in a casting process, the silica-containing
ceramic core being formed from a slurry comprising at least one
refractory powder material and a siloxane binder, the method
comprising: forming a repair mixture comprising a siloxane diluent
and the at least one refractory powder material and the siloxane
binder of the slurry; applying the repair mixture to a region of
the silica-containing ceramic core (10), the region comprising at
least one defect; and firing the silica-containing ceramic core to
burn off the siloxane binder and the siloxane diluent in the repair
mixture and form a silica-containing ceramic composition that fills
the defect.
14. The method of claim 13, wherein the repair mixture comprises a
catalyst capable of cross-linking the siloxane binder.
15. The method of claim 13, wherein the siloxane diluent has the
same composition as the siloxane binder.
16. The method of claim 13, wherein the at least one refractory
powder material comprises a silica refractory powder and/or a
zircon refractory powder.
17. The method of claim 13, wherein the silica-containing ceramic
core comprises a wall defining at least one cavity therein.
18. The method of claim 17, further comprising injecting the repair
mixture into the cavity to close at least one opening in the wall
of the silica-containing ceramic core prior to firing.
19. The method of claim 13, wherein the silica-containing ceramic
core is adapted for use in casting a component for installation in
a gas turbine engine.
20. The method of claim 13, wherein the repair mixture comprises,
by volume, about 30 to about 50 percent of the silica refractory
powder and the zircon refractory powder combined, about 50 to about
70 percent of the siloxane binder and the siloxane diluent
combined, and optionally up to about 100 ppm of a catalyst capable
of cross-linking the siloxane binder and the siloxane diluent.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention generally relates to casting processes
and materials. More particularly, this invention relates to cores
and processes for repairing cores formed with defects such as
cracks.
[0002] Metal alloy materials can be formed into components by
various casting techniques, a notable example being investment
casting (lost wax) processes. Investment casting typically entails
dipping a wax or plastic model or pattern of the desired component
into a slurry comprising a binder and a refractory particulate
material to form a slurry layer on the pattern. A common material
for the binder is a silica-containing material, for example,
colloidal silica. A stucco coating of a refractory particulate
material is typically applied to the surface of the slurry layer,
after which the slurry/stucco coating is dried. The preceding steps
may be repeated any number of times to form a shell mold of
suitable thickness around the wax pattern. The wax pattern can then
be eliminated from the shell mold, such as by heating, after which
the mold is fired to sinter the refractory particulate material and
achieve a suitable strength.
[0003] To produce hollow components, such as turbine blades and
vanes having intricate air-cooling channels, one or more cores must
be positioned within the shell mold to define the cooling channels
and any other required internal features. Cores are typically made
using a plasticized ceramic mixture that is injection molded or
transfer molded in a die or mold, and then hardened by firing or
baking Typical ceramic compositions contain silica and/or alumina.
For example, U.S. Pat. Nos. 7,287,573 and 7,732,526 to McNutty et
al. disclose ceramic cores formed from a slurry comprising a
ceramic powder comprising materials such as alumina, fused alumina,
fused silica, magnesia, zirconia, spinels, mullite, glass fits,
tungsten carbide, silicon carbide, boron nitride, silicon nitride,
and mixtures thereof suspended in a silicone fluid comprising
silicone monomers and/or oligomers having alkenyl and hydride
functionalities. A metal catalyst is added to the suspension to
cross-link the silicone monomers and/or oligomers yielding a rigid
core of ceramic particles in a silicone based polymeric matrix.
McNutty discloses firing the core to substantially decompose the
matrix to produce a silica char.
[0004] One or more fired cores are then positioned within a pattern
die cavity into which a wax, plastic or other suitably low-melting
material is introduced to form the wax pattern. The pattern with
its internal core(s) can then be used to form a shell mold as
described above. Once the shell mold is completed and the pattern
selectively removed to leave the shell mold and core(s), the shell
mold can be filled with a molten metal, which is then allowed to
solidify to form the desired component. The mold and core are then
removed to leave the cast component with one or more internal
passages where the core(s) formerly resided.
[0005] When forming cores as described above, defects such as
cracks and voids can occur within the core material. Cores with
defects generally cannot be used in the component casting process
as the cores may break during casting, resulting in casting
defects. Commonly, defective cores are disposed of rather than
repaired due to the difficulty in restoring the structural
integrity of the cores, especially in regards to hollow cores. An
example of a prior attempt to repair the cores is reported in U.S.
Pat. No. 4,804,562 to Ferguson et al. wherein the cores were
repaired by (a) softening a thermoplastic binder in the core; (b)
while the binder is soft, applying loose ceramic particles to the
defect, the particles having a composition similar to the overall
composition of the core; (c) allowing the binder to reharden; and
(d) heating the core to volatilize the binder and sinter the
ceramic particles to each other.
[0006] In view of the above, it can be appreciated that it would be
desirable if a method were available for repairing defective cores
that resulted in the cores having a structural integrity suitable
for use in a casting process.
BRIEF DESCRIPTION OF THE INVENTION
[0007] The present invention provides methods suitable for
repairing ceramic cores having defects, such as one or more cracks
and/or voids, to enable a core to exhibit sufficient structural
integrity suitable for use in a casting process.
[0008] According to a first aspect of the invention, a method is
provided for repairing defects in a ceramic core adapted for use in
a casting process. The ceramic core is formed from a slurry
comprising at least one refractory powder material suspended in a
liquid vehicle. The method includes forming a repair mixture
comprising a liquid diluent and the at least one refractory powder
material suspended in the liquid vehicle of the slurry. The repair
mixture is applied to a region of the ceramic core that includes at
least one defect and then the ceramic core is fired to burn off the
liquid vehicle in the repair mixture and form a ceramic composition
that closes the defect.
[0009] According to a second aspect of the invention, a method is
provided for repairing defects in a silica-containing ceramic core
adapted for use in a casting process. The silica-containing ceramic
core is formed from a slurry comprising at least one refractory
powder and a siloxane binder. The method includes forming a repair
mixture comprising a siloxane diluent and the at least one
refractory powder material and the siloxane binder of the slurry.
The repair mixture is applied to a region of the silica-containing
ceramic core that includes at least one defect and then the
silica-containing ceramic core is fired to burn off the the
siloxane binder and the siloxane diluent in the repair mixture and
form a silica-containing ceramic composition that fills the
defect.
[0010] A technical effect of the invention is the ability to repair
defective ceramic cores that might otherwise be unsuitable for
using in the casting process. In particular, it is believed that,
by repairing a defective ceramic core with a repair mixture that is
similar in composition to the slurry used to form the ceramic core,
the repaired core will have sufficient structural integrity to
survive the casting process. In addition, it is believed that
diluting the repair mixture with a binder/diluent can promote
adhesion of the repair mixture to a ceramic core.
[0011] Other aspects and advantages of this invention will be
better appreciated from the following detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 represents a hollow ceramic core suitable for repair
by methods that are in accordance with an aspect of this
invention.
[0013] FIG. 2 represents a cross-section of a wall of the hollow
ceramic core of FIG. 1.
DETAILED DESCRIPTION OF THE INVENTION
[0014] The present invention is generally applicable to processes
for repairing cores formed with defects such as cracks. Although
the processes hereinafter will be discussed in reference to cores
comprising silica-containing materials, it is foreseeable and
therefore within the scope of the invention that the processes
could be performed on cores comprising other materials.
[0015] FIG. 1 represents a ceramic core 10 for use with a mold
assembly (not shown) suitable for investment casting a hollow
component (not shown), such as components used in gas turbine
engines, including, but not limited to, turbine blades, nozzles, or
other airfoil components, in accordance with one aspect of the
invention. The core 10 is formed from a slurry comprising at least
one refractory material suspended in a liquid vehicle. Suitable
refractory materials include, but are not limited to, silica,
zirconia, alumina, mullite, and/or the like. For example, the
slurry may comprise a silica refractory powder, a zircon refractory
powder, a siloxane binder, and a catalyst capable of cross-linking
the siloxane binder. A preferred example is a platinum group metal
(PGM) catalyst that results in cross-linking of silicone monomers
and/or oligomers of a siloxane binder, as reported in U.S. Pat.
Nos. 7,287,573 and 7,732,526 to McNutty et al., whose contents
relating to slurry compositions and methods of use are incorporated
herein. According to a preferred aspect of the invention, the
ceramic core 10 comprises at least 5 wt. % silica, and more
preferably about 25 to about 95 wt. % silica, with the balance
zircon and optionally other refractory materials. However, it is
foreseeable that the ceramic core 10 may predominately comprise
refractory materials other than silica, for example, up to 95 wt. %
zircon for specific applications. Further examples of suitable
compositions for the core 10 are described in U.S. Pat. Nos.
7,287,573 and 7,732,526 to McNutty et al. Processes capable of
forming the core 10 are well known in the art and will not be
discussed further herein.
[0016] As represented in FIG. 1, the core 10 comprises a wall 12
defining an internal cavity 14 as well as openings 16 resulting
from formation of the internal cavity 14. In order for the core 10
to be useable in an investment casting process, the openings 16 are
preferably closed and sealed. In addition, any defects in the core
10, such as one or more cracks, need to be repaired prior to use of
the core 10 in a casting process to promote the structural
integrity of the core 10 and enable the core 10 to survive the
casting process.
[0017] According to a preferred aspect of the invention, the
openings 16 and cracks in the core 10 can be repaired by applying
to the core 10 a repair mixture 20, represented in FIG. 2 as
filling a crack 18 in a wall 22 of the core 10 of FIG. 1. The
repair mixture 20 preferably comprises materials similar those used
in an original slurry from which the core 10 was formed,
specifically, the repair mixture 20 contains at least one
refractory material suspended in a liquid vehicle, and in preferred
embodiments the repair mixture 20 comprises the aforementioned
silica and zircon refractory powder materials and siloxane binder
disclosed in U.S. Pat. Nos. 7,287,573 and 7,732,526, and optionally
the catalyst disclosed in U.S. Pat. Nos. 7,287,573 and 7,732,526.
Generally, slurries often do not adhere well to silica-containing
cores. However, investigations leading to the invention determined
that the repair mixture 20 intended to be applied to the core 10
can be rendered suitably adherent to the core 10 by diluting a
quantity of the slurry used to produce the core 10 with a suitable
liquid diluent, to promote adhesion of the repair mixture 20 to the
core 10. The diluent can be in the form of an additional amount of
the same binder that had been used in the slurry that produced the
core 10, in which case the diluent is preferably capable of being
cross-linked by the optional catalyst. Preferably, the diluent
comprises at least one siloxane (for example, those reported in
U.S. Pat. Nos. 7,287,573 and 7,732,526 to McNutty et al.) which is
believed to promote increased bonding strength. A particularly
suitable diluent includes a mixture of
tetramethyltetravinylcyclotetrasiloxane (D4Vi) and
methylhydrogenpolysiloxane in a molar ratio of about 0.5 to about
2.0.
[0018] In order to promote the structural integrity of the repairs,
the repair mixture 20 should be diluted to have not more than about
70 vol. % of solids (including the refractory powder materials) to
promote the ability of the repair mixture 20 to fully infiltrate
the crack 18, with the balance being the binder, diluent, and any
other liquid vehicle constituent. Preferably, the repair mixture 20
comprises, by volume, about 30 to about 50 vol. % refractory powder
materials, about 50 to about 70 vol. % binder and diluent combined,
and optionally up to about 100 ppm catalyst. As noted above, the
binder and diluent may be the same, for example, a siloxane, in
which case the stated amounts for the binder and diluent are simply
combined to reflect the total amount of siloxane (or other
binder/diluent) used in the repair mixture 20.
[0019] The openings 16 and cracks 18 (and/or any other defects) can
be filled through one or more applications of the repair mixture
20. In certain embodiments of the invention, a more diluted
formulation of the repair mixture 20 can be used for one or more
initial applications to the core 10 to promote the ability of the
repair mixture 20 to fully infiltrate into the cracks 18 and other
defects. It is believed that the additional amounts of diluent with
lower the viscosity of the repair mixture 20 thereby promoting
infiltration of hairline cracks. Such a more diluted repair mixture
20 may contain, by volume, about 20 to about 40 vol. % refractory
powder materials, about 60 to about 80 vol. % binder and diluent,
and optionally up to about 100 ppm catalyst.
[0020] The repair mixture 20 may be applied to any region of the
core 10, including the surface and the cavity 14, to be repaired by
any means known in the art such as, but not limited to, brushing or
injection with a manual or power actuated syringe. The surfaces of
the region may first be wetted with the diluent prior to applying
the repair mixture 20 to promote wetting of the core surfaces by
the repair mixture 20. If the core 10 is relatively small in size,
the repair mixture 20 may be used that does not contain the
catalyst. It is believed that the catalyst may change the viscosity
of the repair mixture 20 during application of the repair mixture
20, and eventually the repair mixture may become a solid prior to
completion of the repair. In addition, repair mixtures 20
containing the catalyst may have shorter life spans and therefore
can be difficult to handle and store. It is believed that if core
10 is small, the repair mixture 20 can fill and remain within a
small defect without the need for the catalyst. If the core 10 is
relatively larger in size, the repair mixture 20 preferably
contains the catalyst. It is believed that the catalyst will
promote cross-linking of the repair mixture 20 thereby improving
the durability of the repair. The repair mixture 20 may be applied
as many times as necessary to fill the crack 18 and any other
defect in the surface of the core 10. In addition, the repair
mixture 20 (without catalyst if the core 10 is small) or the
original slurry can be injected into the cavity 14 within the
hollow core 10 to close all of the openings 16 of the core 10 prior
to the core 10 being used in a casting process. Preferably, the
repair mixture 20 is applied in a manner to ensure that the
thickness of wall 12 is balanced (i.e. relatively uniform) prior to
firing. After applying the repair mixture 20, the repaired region
is allowed to dry and the surface is smoothed. If the repair
mixture 20 comprises the catalyst, the core 10 may be cured to
cross-link the binder and diluent once the repair mixture 20 has be
applied to the core 10 as desired. After the binder has
cross-linked, the core 10 is fired to burn off the binder and
diluent and sinter the refractory solids of the repair mixture 20.
The core 10 is preferably fired at a temperature of more than about
1000.degree. C. to ensure that the solids content of the repair
mixture 20 is fully sintered to bond the refractory powder
particles to each other and to the surfaces of the core 10.
[0021] While the invention has been described in terms of specific
embodiments, it is apparent that other forms could be adopted by
one skilled in the art. For example, the order and methods by which
the repair mixture 20 is applied to the core 10 could differ, and
materials and processes other than those noted could be used.
Therefore, the scope of the invention is to be limited only by the
following claims.
* * * * *