U.S. patent application number 14/550094 was filed with the patent office on 2016-05-26 for casting cores and methods for making.
The applicant listed for this patent is General Electric Company. Invention is credited to James Anthony Brewer, Sylvia Marie DeCarr, Frederic Joseph Klug, Anthony Mark Thompson.
Application Number | 20160144423 14/550094 |
Document ID | / |
Family ID | 56009285 |
Filed Date | 2016-05-26 |
United States Patent
Application |
20160144423 |
Kind Code |
A1 |
Thompson; Anthony Mark ; et
al. |
May 26, 2016 |
CASTING CORES AND METHODS FOR MAKING
Abstract
Embodiments of the present invention include methods for forming
ceramic articles, such as cores and core dies used in investment
casting. The method may provide ceramic articles having improved
strength and smoothness compared to articles manufactured by
conventional methods. One embodiment is a method that comprises
introducing a liquid comprising a silicon-bearing fluid into pores
of a porous ceramic body; and heating the porous ceramic body in an
oxygen-bearing atmosphere to form silica within the pores of the
body.
Inventors: |
Thompson; Anthony Mark;
(Niskayuna, NY) ; Brewer; James Anthony; (Scotia,
NY) ; DeCarr; Sylvia Marie; (Schenectady, NY)
; Klug; Frederic Joseph; (Schenectady, NY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
General Electric Company |
Schenectady |
NY |
US |
|
|
Family ID: |
56009285 |
Appl. No.: |
14/550094 |
Filed: |
November 21, 2014 |
Current U.S.
Class: |
164/33 |
Current CPC
Class: |
B22C 9/10 20130101; B22C
3/00 20130101 |
International
Class: |
B22C 9/10 20060101
B22C009/10; B22C 3/00 20060101 B22C003/00 |
Claims
1. A method comprising: introducing a liquid comprising a
silicon-bearing fluid into pores of a porous ceramic body; and
heating the porous ceramic body in an oxygen-bearing atmosphere to
form silica within the pores of the body.
2. The method of claim 1, wherein the fluid comprises a siloxane or
a silane.
3. The method of claim 1, wherein the liquid is neat.
4. The method of claim 1, wherein the liquid further comprises a
second fluid.
5. The method of claim 4, wherein the second fluid has a lower
viscosity than the silicon-bearing fluid.
6. The method of claim 4, further comprising volatilizing the
second fluid after the liquid has been introduced into the pores of
the ceramic body.
7. The method of claim 1, wherein reacting comprises heating the
fluid to a temperature above about 200 degrees Celsius.
8. The method of claim 1, wherein the liquid comprises a mixture of
silicone monomers.
9. The method of claim 8, further comprising curing the mixture of
silicone monomers prior to the reacting step.
10. The method of claim 9, wherein the liquid further comprises a
catalyst, and wherein curing comprises thermally curing the mixture
of silicone monomers in the presence of the catalyst.
11. The method of claim 1, wherein the liquid further comprises a
plurality of particles suspended within the silicon-bearing
fluid.
12. The method of claim 11, wherein the plurality of particles
comprises oxide particles.
13. The method of claim 1, wherein introducing comprises performing
vacuum infiltration of the liquid into the pores of the body.
14. The method of claim 1, wherein the porous ceramic body
comprises a core for investment casting.
15. The method of claim 1, wherein the ceramic body comprises
silica, zirconium silicate, or a combination of zirconium silicate
and silica.
Description
BACKGROUND
[0001] This disclosure generally relates to investment casting, and
more particularly, relates to materials for use in forming the
ceramic cores employed in investment casting.
[0002] The manufacture of gas turbine components, such as turbine
blades and nozzles, requires that the parts be manufactured with
accurate dimensions having tight tolerances. Investment casting is
a technique commonly employed for manufacturing these parts. The
dimensional control of the casting is closely related to the
dimensional control of a ceramic insert, known as the core (which
is typically used to form internal surfaces, such as cooling
passageways, within the casting), as well as the mold, also known
as the shell (which typically corresponds to the external surfaces
of the casting). In this respect, it is important to be able to
manufacture the core and shell to dimensional precision
corresponding to the dimensions of the desired metal casting, e.g.,
turbine blade, nozzle, and the like.
[0003] In addition to requiring dimensional precision in the
formation of the ceramic core, the production of various turbine
components requires that the core not only be dimensionally precise
but also be sufficiently strong to maintain its shape and integrity
during the firing, wax encapsulation, shelling, and metal casting
processes. In addition, the core must be sufficiently compliant to
prevent mechanical rupture of the casting during cooling and
solidification. Further, the core materials generally must be able
to withstand temperatures commonly employed for casting of
superalloys that are used to manufacture the turbine components,
e.g., temperatures generally in excess of 1,000.degree. C. Finally,
the core must be easily removed following the metal-casting
process. The investment casting industry typically uses silica or
silica-based ceramics due to their superior leachability in the
presence of strong bases.
[0004] As explained in U.S. Pat. No. 6,494,250, ceramic cores for
investment casting typically are formed by injection molding,
transfer molding or pouring into a suitably shaped core die a
ceramic-bearing mixture that includes, among other things, ceramic
powder dispersed in a binder or carrier. After the so-called
"green" core is removed from the die, it is fired in one or more
steps to remove the carrier and strengthen the core via sintering
for use in casting operations. As a result of removal of the
carrier and other sacrificial additives, the fired ceramic core is
generally porous.
[0005] In one particular example, investment casting cores can be
made using low pressure injection molding techniques such as that
described in U.S. Pat. No. 7,287,573. The process described therein
generally includes dispersing a ceramic powder to form a slurry in
a silicone fluid, wherein the silicone fluid includes silicone
species having alkenyl and hydride functionalities. Once a stable
suspension is formed, a metallic catalyst is added and the desired
part is formed. Depending on the particular binder liquid and
metallic catalyst employed, a heating step may then be applied to
effect a catalyzed reaction among the siloxane species, thereby
curing the formed suspension into a green body. The silicone
species cross link in the mold, yielding a dispersion of ceramic
particles in a rigid silicone-based polymeric matrix. The so-formed
silicone polymeric matrix may be substantially decomposed to
produce a silica char by further heating at a higher
temperature.
[0006] In another example, additive manufacturing processes, such
as three-dimensional printing, are applied to form the dies used to
form cores (as in U.S. Pat. No. 7,413,001, for example), or
directly to form the cores themselves from ceramic-slurry "inks"
that are subsequently fired to form porous ceramic cores.
[0007] One of the challenges in ceramic core processing is that
cores formed by these various processes can be fragile and
susceptible to cracking during typical handling and/or
transportation, or during the investment casting process itself
Moreover, in many additive manufacturing processes, the surface of
the resultant ceramic core is rougher than that obtained in the
traditional injection molding process. It is often desirable to
maintain a low surface roughness in the eventual cast cooling
passages (for part life, or other considerations). Therefore, a
need remains for processes for making ceramic cores that are strong
with improved smoothness.
BRIEF DESCRIPTION
[0008] Embodiments of the present invention are provided to meet
this and other needs. One embodiment is a method that comprises
introducing a liquid comprising a silicon-bearing fluid into pores
of a porous ceramic body; and heating the porous ceramic body in an
oxygen-bearing atmosphere to form silica within the pores of the
body.
DETAILED DESCRIPTION
[0009] Approximating language, as used herein throughout the
specification and claims, may be applied to modify any quantitative
representation that could permissibly vary without resulting in a
change in the basic function to which it is related. Accordingly, a
value modified by a term or terms, such as "about", and
"substantially" is not to be limited to the precise value
specified. In some instances, the approximating language may
correspond to the precision of an instrument for measuring the
value. Here and throughout the specification and claims, range
limitations may be combined and/or interchanged; such ranges are
identified and include all the sub-ranges contained therein unless
context or language indicates otherwise.
[0010] In the following specification and the claims, the singular
forms "a", "an" and "the" include plural referents unless the
context clearly dictates otherwise. As used herein, the term "or"
is not meant to be exclusive and refers to at least one of the
referenced components being present and includes instances in which
a combination of the referenced components may be present, unless
the context clearly dictates otherwise.
[0011] As used herein, the terms "may" and "may be" indicate a
possibility of an occurrence within a set of circumstances; a
possession of a specified property, characteristic or function;
and/or qualify another verb by expressing one or more of an
ability, capability, or possibility associated with the qualified
verb. Accordingly, usage of "may" and "may be" indicates that a
modified term is apparently appropriate, capable, or suitable for
an indicated capacity, function, or usage, while taking into
account that in some circumstances, the modified term may sometimes
not be appropriate, capable, or suitable.
[0012] Embodiments of the present invention include methods for
forming ceramic articles, such as cores and core dies used in
investment casting. The method may provide ceramic articles having
improved strength and smoothness compared to articles manufactured
by conventional methods. Fabrication of cores, for instance, having
higher strength, may allow thin or hollow parts to be fabricated at
higher yield by preventing breakage that occurs during downstream
processing. Moreover, the ability to produce smoother surfaces
ultimately may reduce surface finishing costs and improve part life
via more efficient cooling.
[0013] In one embodiment, a liquid is introduced into the pores of
a porous ceramic body. Examples of ceramic materials that may be
included in the ceramic body include oxide materials, such as
silica, alumina, and various silicate materials, among others. In
particular embodiments, the ceramic body includes silica, zirconium
silicate, or a combination of silica and zirconium silicate. Such
materials often see use in fabrication of cores for investment
casting, for instance. The introduction is done by contacting the
surface of the body with the liquid, as, for instance, by dipping
the body into a quantity of the liquid. Infiltration of the pores
by the liquid is driven at least in part by capillary action,
although in some embodiments vacuum infiltration is performed to
provide further driving force for incursion of the liquid into the
porous network. Vacuum infiltration may be followed with a step of
exposing the infiltrated body to a pressure at or exceeding ambient
pressure, to enhance the degree to which a driving force is
provided to infiltrate the liquid into the porous body.
[0014] The liquid includes a silicon-bearing fluid, such as a
siloxane or a silane, or any other silicon-bearing compound that
may serve as a precursor to a silicon-bearing ceramic material in a
process such as described herein. Non-limiting examples of suitable
silicon-bearing fluids include alkenyl siloxanes of the general
formula (I):
##STR00001##
[0015] wherein R1, R2, and R3 each independently comprise hydrogen
or a monovalent hydrocarbon, halocarbon, or halogenated hydrocarbon
radical; X a divalent hydrocarbon radical; and a is a whole number
having a value between 0 and 8, inclusive. The terms "monovalent
hydrocarbon radical" and "divalent hydrocarbon radical" as used
herein are intended to designate straight chain alkyl, branched
alkyl, aryl, aralkyl, cycloalkyl, and bicycloalkyl radicals. A
specific example of such a material is 1,3,5,7-tetravinyl
1,3,5,7-tetramethylcyclotetrasiloxane (United Chemical
Technologies, product number T2160). Further non-limiting examples
of silicon-bearing fluids include hydrosiloxanes having hydrogen
directly bonded to one or more of the silicon atoms, and therefore
containing a reactive Si--H functional group. A particular example
of such a material is a hydride-functional organosilicate resin
such as the material available from Momentive Performance Materials
under the trade name 88104EX.
[0016] The selection of any particular silicon-bearing fluid
depends in part upon the desired flow properties of the liquid. For
example, a relatively viscous liquid may prove difficult to achieve
satisfactory infiltration into the ceramic body. On the other hand,
liquids having relatively low viscosity tend to also have
relatively high vapor pressure, and if the silicon-bearing fluid
has an unduly high vapor pressure, it will tend to volatilize
during further processing before conversion to silica or other
ceramic product. In one embodiment, the liquid is neat, meaning it
consists essentially of a single species of silicon-bearing fluid.
In alternative embodiments, the liquid includes a mixture of fluid
components, wherein, for instance, in addition to the
silicon-bearing fluid there is added a second fluid, which may or
may not also include silicon. In a particular embodiment, the
second fluid has a lower viscosity than the silicon-bearing fluid,
which reduces the overall viscosity of the resultant liquid, which
may improve the ability of the liquid to infiltrate the ceramic
body. An example of such a mixture is a liquid that includes the
aforementioned 88104EX as the silicon-bearing fluid and
1,3,5,7-tetravinyl tetramethyl cyclotetrasiloxane (also known in
the art as "D4Vi" and "D4 Vinyl") as the lower viscosity second
fluid. The proportions of the mixture are selected to provide a
suitably concentrated source of silicon within the pores while also
providing a viscosity that allows acceptable penetration into the
porous network of the ceramic body.
[0017] In some embodiments, the viscosity of the liquid is in a
range up to about 1,000 centistokes, up to about 300 centistokes
where a lower viscosity is desirable, and up to about 100
centistokes in particular embodiments where even lower viscosity is
desirable (such as where the pores are fine, particularly tortuous,
and/or where deep and rapid infiltration is desired). In one
illustrative embodiment, the liquid has a viscosity in a range from
about 5 centistokes to about 30 centistokes.
[0018] In embodiments in which the liquid includes both a
silicon-bearing fluid and a second, lower viscosity fluid, the
method optionally further includes volatilizing the second fluid
after the liquid has been introduced into the pores of the ceramic
body. This may be achieved, for example, by heating the infiltrated
body to a suitable temperature at which the second fluid
volatilizes at an acceptable rate while leaving the silicon-bearing
fluid to remain in the pores of the body. This heating step may be
done separately from the reacting step, or it may be coincidental
with the reacting step, wherein during the heat-up to a reacting
temperature, the second fluid volatilizes as the temperature
increases. In some embodiments, the volatilizing step includes
heating the infiltrated ceramic body to a first temperature that is
above ambient but below the reacting step temperature, and holding
at this first temperature for a desired time (typically a length of
time sufficient to volatilize substantially all of the second
fluid).
[0019] Depending on the selection of first and second fluids, in
some embodiments of the present invention the liquid comprises a
mixture of silicone monomers. This mixture may be processed as
noted above, volatilizing the component of the mixture having the
lower vapor pressure, without ever performing a step to cure the
mixture, that is, cause the components of the mixture to react, to
form a cross-linked silicone polymer. However, in alternative
embodiments, the method further comprises curing the mixture of
silicone monomers. Various methods for curing silicone monomers are
widely known in the art, and the selection of which method to use
in the presently described embodiments will of course depend in
part on the selection of monomers in the liquid. Curing the
monomers involves performing a crosslinking reaction, typically
involving one of three basic reaction types: peroxide cure, which
is a free-radical reaction catalyzed by a peroxide and activated by
heat; condensation cure, which is catalyzed by a tin-bearing or
titanium-bearing compound, and which can be activated by heat or,
in some systems, by moisture (i.e., the so-called Room Temperature
Vulcanized, or RTV systems); and addition cure, catalyzed by a
complex of a platinum-group metal (often platinum or rhodium) and
activated by exposure to ultraviolet light, or by heat. As
thermally cured systems are generally convenient for use in the
method described herein, in specific embodiments the liquid further
comprises a catalyst suitable for supporting one of the thermally
activated reaction types noted above, and thus the curing step
includes thermally curing the mixture of silicone monomers in the
presence of the catalyst.
[0020] In one general, non-limiting example involving an
addition-cure reaction, the silicon-bearing fluid includes an
alkenyl siloxane of formula (I), above, and a hydrosiloxane
containing a reactive Si-H functional group. Specific examples of
suitable alkenyl siloxanes and hydrosiloxanes for use in such
embodiments of the present invention are described in, for
instance, U.S. Pat. No. 7,287,573. A metallic catalyst compound is
added to the liquid, and cross-linking of the silicone monomers may
be accomplished by utilizing a metal catalyzed reaction of the
silicone alkenyl groups and the silicon bonded hydrogen groups. The
metal catalyst, such as a platinum-group metal catalyst, can be
selected from such catalysts that are conventional and well known
in the art. Suitable metallic catalysts include, but are not
intended to be limited to, the Pt divinylsiloxane complexes as
described by Karstedt in U.S. Pat. No. 3,715,334 and U.S. Pat. No.
3,775,452; Pt-octyl alcohol reaction products as taught by
Lamoreaux in U.S. Pat. No. 3,220,972; the Pt-vinylcyclosiloxane
compounds taught by Modic in U.S. Pat. No. 3,516,946; and Ashby's
Pt-olefin complexes found in U.S. Pat. Nos. 4,288,345 and
4,421,903.
[0021] A typical thermal cure involves heating the body to a
temperature greater than about room temperature, often up to about
120.degree. C., such as a temperature in a range from about
50.degree. C. to about 100.degree. C. The time necessary to cure
the reactants is dependent on the particular monomers, solvent,
temperature, and metallic catalyst compound. Generally, the ceramic
body containing the silicon-bearing fluid is preferably heated at
an elevated temperature (i.e., greater than room temperature) for
at least about five minutes, such as a period of about 5 to about
30 minutes in order to polymerize and crosslink the monomers.
[0022] In some embodiments, the liquid is a slurry in which a
plurality of particles is suspended within the silicone-bearing
fluid. The particles are typically a ceramic material, such as an
oxide. Particular examples of suitable oxides include, but are not
limited to, yttria, zirconia, fumed silica, alumina, and magnesia.
Combinations of different particulate materials are also suitable.
Addition of these fine particles to the fluid may allow for
enhanced deposition of solid oxide material within the pores,
providing enhanced strengthening and/or smoothing of the surface of
the ceramic body. Selection of the concentration and size of
particles depends in part upon the size of the pores in the porous
body and the desired flow characteristics of the resulting slurry.
Typically the selection is made to balance the desire for efficient
strengthening or smoothing with the desire for efficient
infiltration of the liquid into the pore network of the body.
[0023] After the liquid has been introduced into the pores of the
porous ceramic body, with or without the curing step described
above, the porous ceramic body, now with a silicon-bearing material
disposed within its pores, is heated in an oxygen-bearing
atmosphere, thereby causing the silicon-bearing material to react
with the oxygen to form silica within the pores of the body. This
heating step typically involves increasing the temperature of the
ceramic body to a temperature sufficient to drive the conversion of
the material resident within the pores to silica in a desired
length of time. For instance, in some embodiments the ceramic body
is heated to a temperature greater than about 200 degrees Celsius,
and then held at this temperature for sufficient time to allow
substantially all of the convertible silicon-bearing material
resident within the porous network of the body to convert into
silica. "Convertible" silicon-bearing material in this regard means
silicon-bearing material that is present in a form amenable to
reaction with oxygen under the processing conditions; typically
this will include siloxane, silane, silicone, and like materials
that are prone to reaction with oxygen under elevated, oxidizing
conditions, but will exclude materials such as silica, silicon
carbide, and other materials having a higher stability at high
temperatures. In particular embodiments, the heating step includes
heating the body to a temperature greater than about 400 degrees
Celsius; a higher temperature generally provides faster reaction
times.
[0024] Those skilled in the art will recognize that the rate at
which the body's temperature is increased during the heating step
may be considered carefully, particularly where the silicon-bearing
material in the pores of the ceramic body is still in liquid form
or is otherwise prone to outgassing during heating. If vapor is
produced so rapidly as to unduly build up pressure within the
pores, damage may be done to the ceramic body. Thus in some
embodiments, heating is performed gradually, either through a
sufficiently slow but continuous increase, or by heating the body
through one or more intermediate temperatures, holding the body at
each intermediate temperature for intervals of time to allow for
controlled generation of vapor. The design of such "heating
profiles" is done taking into account, among other things, the
nature of the material deposited in the pores of the body, the
extent of the porous network, and the desired rate of processing.
In some embodiments, after a first heating stage to a temperature
used to form the silica within the pores, a second, higher
temperature heat treatment is performed to sinter the material in
the pores, thereby further strengthening the material and enhancing
its ability to reinforce the ceramic body. The sintering step is
typically performed at temperatures above about 900 degrees
Celsius, with 1000 degrees Celsius as a non-limiting example.
[0025] As noted above, in some embodiments of the method described
herein, the porous ceramic body includes, or is in its entirety, a
core of the type used for investment casting. Cores formed in
accordance with U.S. Pat. No. 7,287,573 referenced above, for
example, may undergo a first, lower temperature firing step, and
then a later sintering step at significantly higher temperature.
The introduction of the liquid, followed by other steps in
accordance with embodiments described herein, may be desirably
performed after the first firing step but before the sintering
step. Thus when the sintering step is performed in these
embodiments, both the ceramic body and the material resident within
the pores may be sintered simultaneously.
EXAMPLES
[0026] The following examples are presented to further illustrate
non-limiting embodiments of the present invention.
[0027] A porous ceramic test bar was vacuum infiltrated with a
liquid comprising about 45 volume % low viscosity liquid silicone
resin (Momentive 88104) and about 55 volume %
octamethylcyclotetrasiloxane (D4). The infiltrated bar was dried
overnight in a drying oven at 80 degrees Celsius, and then fired in
air at 940 degrees Celsius to convert the silicone to silica. The
treated bar and an untreated control bar were subjected to modulus
of rupture testing at ambient temperature. The treated bar showed a
49% increase in modulus of rupture compared to the untreated
control bar.
[0028] While only certain features of the invention have been
illustrated and described herein, many modifications and changes
will occur to those skilled in the art. It is, therefore, to be
understood that the appended claims are intended to cover all such
modifications and changes as fall within the true spirit of the
invention.
* * * * *