Note: Descriptions are shown in the official language in which they were submitted.
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METHODS FOR REPAIRING CERAMIC CORES
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. Patent 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 frits,
tungsten carbide,
silicon carbide, boron nitride, silicon nitride, and mixtures thereof
suspended in a
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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.
Patent 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.
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[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.
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[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.
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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.
Patent
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. Patent 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
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
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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. Patent Nos.
7,287,573 and 7,732,526, and optionally the catalyst disclosed in U.S. Patent
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. Patent 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 2Ø
[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
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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.
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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 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
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.
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