CASTING MOLD COMPOSITION WITH IMPROVED DETECTABILITY
FOR INCLUSIONS AND METHOD OF CASTING
BACKGROUND
[001] The present disclosure relates generally to casting mold compositions, and
methods for casting titanium and titanium alloys.
[002] Modern gas turbines, especially aircraft engines, must satisfy the highest
demands with respect to reliability, weight, power, economy, and operating service
life. In the development of aircraft engines, the material selection, the search for new
suitable materials, as well as the search for new production methods, among other
things, play an important role in meeting standards and satisfying the demand.
[003] The materials used for aircraft engines or other gas turbines include titanium
alloys, nickel alloys (also called super alloys) and high strength steels. Titanium
alloys are generally used for compressor parts, nickel alloys are suitable for the hot
parts of the aircraft engine, and the high strength steels are used, for example, for
compressor housings and turbine housings. The highly loaded or stressed gas turbine
components, such as, components for a compressor, for example, are forged parts.
Components for a turbine, on the other hand, are typically fabricated as investment
cast parts.
[004] Although investment casting is not a new process, the investment casting
market continues to grow as the demand for more intricate and complicated parts
increase. Because of the great demand for high quality, precision castings, there
continuously remains a need to develop new ways to make investment castings more
quickly, efficiently, cheaply and of higher quality.
[005] Conventional investment mold compounds that consist of fused silica,
cristobalite, gypsum, or the like, that are used in casting jewelry and dental prostheses
industries are not suitable for casting reactive alloys, such as titanium alloys. One
reason is because there is a reaction between mold titanium and the investment mold.
It is difficult to investment cast titanium and titanium alloys and similar reactive
metals in ceramic molds because of the titanium's high affinity for elements such as,
oxygen, nitrogen, and carbon. At elevated temperatures, titanium and its alloys can
react with the mold facecoat.
[006] The properties of the final casting are greatly deteriorated if any reaction
occurs between the molten alloy and the mold. The form of this deterioration can
include a poor surface finish due to gas bubbles, or in more serious cases, the
chemistry, microstructure, and properties of the casting are compromised. Asperities
and/or pits on the surfaces of cast alloy components can reduce aerodynamic
performance in, for example, turbine blade applications, and increase wear and
friction in rotating or reciprocating part applications. Therefore, there is a need in the
art for new, practical and useful casting mold compositions and methods for detecting
inclusions in reactive alloys, such as titanium alloys.
SUMMARY
[007] Aspects of the present disclosure provide casting mold compositions, methods of
casting, and cast articles that overcome the limitations of the state of the art. Some aspect
of the disclosure may be directed toward the fabrication of components for the aerospace
industry, for example, engine turbine blades. Further aspects may be employed in the
fabrication of a component in any industry, in particular, those components containing
titanium and/or titanium alloys.
[008] One aspect of the disclosure is a mold composition for casting a titaniumcontaining
article, comprising: a calcium aluminate cement comprising calcium
monoaluminate, calcium dialuminate, and mayenite; and an X-ray or Neutron-ray
detectable element. Another aspect of the present disclosure is a titanium-containing
article casting -mold composition, comprising: calcium aluminate; and an X-ray or
Neutron-ray detectable element. In one embodiment, the calcium aluminate cement
forms an intrinsic facecoat of less than about 100 microns when the mold composition
forms a mold. In one embodiment, the X-ray or Neutron-ray detectable elements are
mixed within the mold. In another embodiment, the X-ray or Neutron-ray detectable
elements are mixed within the mold and become part of the intrinsic facecoat. In one
embodiment, the mold composition does not have an extrinsic facecoat.
[009] In one embodiment, the mold composition further comprises oxide particles.
The oxide particles may comprise at least one of aluminum oxide particles,
magnesium oxide particles, calcium oxide particles, zirconium oxide particles, and
titanium oxide particles. Moreover, in some instances, the oxide particles comprise
hollow oxide particles, for example, the hollow oxide particles comprise hollow
aluminum oxide particles. In a specific embodiment, the oxide particles are
aluminum oxide particles.
[0010] In another embodiment, X-ray or Neutron-ray detectable element
comprises ytterbium, hafnium, gadolinium, tungsten, thorium, uranium, yttrium,
dysprosium, erbium, cerium, and compositions thereof. The X-ray or Neutron-ray
detectable element may be in the range of about 1 to about 4 weight percent in the
mold composition. The radiographically dense element may be more radiographically
dense than the oxide particles. In one embodiment, radiographically dense element is
more radiographically dense than calcium aluminate and comprises one or more of
ytterbium, hafnium, gadolinium, tungsten, thorium, uranium, yttrium, dysprosium,
erbium, cerium and compositions thereof.
[0011] One aspect of the present disclosure is a method for detecting sub
surface ceramic inclusions in a titanium or titanium alloy casting, said method
comprising: combining calcium aluminate, at least one element more radiographically
dense than the calcium aluminate, and a liquid to form a slurry; forming a mold
having the calcium aluminate and the radiographically dense element from the slurry;
introducing a titanium aluminide-containing metal to the radiographically dense
element-bearing mold; solidifying said titanium aluminide-containing metal to form
an article in the mold; removing the solidified titanium aluminide-containing metal
article from said mold; subjecting the solidified titanium aluminide-containing article
to radiographic inspection to provide a radiograph; and examining said radiograph for
the presence of the radiographically dense element on or in the article.
[0012] In one embodiment, the method further comprises removing the
radiographically dense element from the article. The removing the radiographically
dense element from the article may comprise one or more steps of machining,
grinding, polishing, or welding. The combining step may further comprise combining
oxide particles with the slurry. In one embodiment, the oxide particles comprise
hollow oxide particles, for example, hollow aluminum oxide particles.
[0013] In one embodiment, the method comprises minimizing the presence of
mold material inclusions in titanium aluminide-containing cast articles. The titaniumcontaining
cast article may comprise an engine or turbine, or a component of a
turbine. For example, the titanium-containing cast article comprises a turbine blade.
The titanium-containing cast article may be a titanium aluminide containing engine, a
titanium aluminide containing turbine, or a titanium aluminide containing turbine
blade.
[0014] One aspect of the present disclosure is a mold composition comprising:
calcium aluminate cement comprising calcium monoaluminate, calcium dialuminate,
and mayenite; and at least one element more radiographically dense than the calcium
aluminate cement. Another aspect of the present disclosure is a mold composition
comprising calcium aluminate and at least one element more radiographically dense
than the calcium aluminate. In one embodiment, the mold composition further
comprises oxide particles. In a related embodiment, the radiographically dense
element is further more radiographically dense than the oxide particles.
[0015] Another aspect of the present disclosure is a mold composition for
casting titanium-containing articles, comprising: calcium aluminate; and an X-ray or
Neutron-ray detectable element. For instance, an aspect of the present disclosure may
be uniquely suited to providing mold compositions to be used in molds for casting
titanium-containing and/or titanium alloy-containing articles or components, for
example, titanium containing turbine blades.
[0016] These and other aspects, features, and advantages of this disclosure
will become apparent from the following detailed description of the various aspects of
the disclosure taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE FIGURES
[0017] The subject matter, which is regarded as the invention, is particularly
pointed out and distinctly claimed in the claims at the conclusion of the specification.
The foregoing and other features and advantages of the present disclosure will be
readily understood from the following detailed description of aspects of the disclosure
taken in conjunction with the accompanying drawings in which:
[0018] Figure 1 is a diagram that depicts the percentage of aluminum oxide on
the x axis and temperature on the y axis, showing various calcium oxide-aluminum
oxide composition ranges for the calcium aluminate cements, and shows particular
aluminum oxide percentages and temperature ranges for the compositions according
to disclosed embodiments.
[0019] Figure 2a and 2b show one example of the mold microstructure after
high temperature firing. The backscattered scanning electron microscope images of
the cross section of the mold fired at 1000 degrees Celsius are shown, wherein Figure
2a points to the alumina particles present and Figure 2b points to the calcium
aluminate cement.
[0020] Figure 3a and Figure 3b show one example of the mold microstructure
after high temperature firing. The backscattered scanning electron microscope images
of the cross section of the mold fired at 1000 degrees Celsius are shown, wherein
Figure 3a points to calcium aluminate cement and fine-scale alumina particles present
and Figure 3b points to an alumina particle.
[0021] Figure 4a and Figure 4b show on example of the mold microstructure
after high temperature firing. The backscattered scanning electron microscope images
of the cross section of the mold fired at 1000 degrees Celsius are shown, wherein
Figure 4a points to a large scale alumina particle and Figure 4b points to a calcium
monoaluminate particle.
[0022] Figure 5 shows one example of the mold microstructure after high
temperature firing, showing alumina and calcium monoaluminate, wherein the
calcium monoaluminate reacts with alumina to form calcium dialuminate, and
wherein the mold in one example is fired to minimize mayenite content.
[0023] Figure 6 shows one example of the mold microstructure after high
temperature firing, showing alumina and calcium monoaluminate, wherein the
calcium monoaluminate reacts with alumina to form calcium dialuminate, and
wherein the mold is fired to minimize mayenite content.
[0024] Figure 7 shows X-ray images in planar view of a cast titanium
aluminide article. Figure 7a shows an X-ray image, with arrows pointing to examples
of sub-surface inclusions and casting porosities. Figure 7b is an zoomed in view of
Figure 7a. Figure 7b shows an example of a sub-surface inclusion from the mold that
is 5.44 mm in length. Casting porosities are also indicated, with one example the
diameter of the porosity is indicated to be 0.99 mm.
[0025] Figure 8 shows a schematic of the mold with the facecoat. Figure 8a
shows the mold with the intrinsic facecoat that is approximately 100 microns thick.
The schematic shows the intrinsic facecoat with the mold cavity and calcium
aluminate mold positions also indicated. Figure 8b shows the mold with the extrinsic
facecoat that is approximately 100 microns thick. The schematic shows the extrinsic
facecoat with the mold cavity and calcium aluminate mold positions also indicated.
[0026] Figure 9 shows a flow chart, in accordance with aspects of the
disclosure, illustrating the steps of a method for detecting sub-surface ceramic
inclusions in a titanium or titanium alloy casting.
DETAILED DESCRIPTION
[0027] Embodiments of the present disclosure provide a mold and a method of
making titanium aluminide and titanium aluminide alloy castings of high structural
integrity, by providing for easy detectability of inclusions, for example, surface and/or
sub-surface inclusions, that may be present at and/or below the exterior surface of the
casting. These inclusions can be generated from the molten metal, from the mold
fabrication process, and/or from the casting process, for example, during investment
casting. In one aspect, a surface zone may form during casting as a hard, brittle
layer, known as the "alpha case" in the art, which may contain undesirable inclusions.
The thickness of this layer is usually approximately 0.03 millimeters [mm].
[0028] The manufacture of titanium based airframe components by investment
casting of titanium and its alloys in investment shell molds poses problems from the
standpoint that the castings should be cast to "near-net-shape." That is, the
components may be cast to substantially the final desired dimensions of the
component, and require little or no final treatment or machining. For example, some
castings may require only a chemical milling operation to remove any alpha case
present on the casting. However, any sub-surface ceramic inclusions located below
the alpha case in the casting are typically not removed by the chemical milling
operation. These sub-surface inclusions are not visible upon visual inspection of the
casting, even after chemical milling, and remain in the casting below the alpha case
layer. These inclusions may be formed due to the reaction between the mold facecoat
and any reactive metal in the molding medium, for example, reactive titanium
aluminide.
[0029] The present disclosure provides a new approach for casting near-netshape
titanium and titanium aluminide components, such as, turbine blades or airfoils.
Embodiments of the present disclosure provide compositions of matter for investment
casting molds and casting methods that can provide improved titanium and titanium
alloy components for example, for use in the aerospace industry. In some aspects, the
mold composition provides a mold that may contain phases that provide improved
mold strength during mold making and/or increased resistance to reaction with the
casting metal during casting. The molds according to aspects of the disclosure may
be capable of casting at high pressure, which is desirable for near-net-shape casting
methods. A mold composition, for example, containing calcium aluminate cement
and alumina particles, and preferred constituent phases, have been identified that
provide castings with improved properties.
[0030] In one aspect, the constituent phases of the mold comprise calcium
monoaluminate (CaAl20 4) . The present inventors found calcium monoaluminate
highly desirable for at least two reasons. First, it is understood by the inventors that
calcium monoaluminate is believed to promote hydraulic bond formation between the
cement particles during the initial stages of mold making, and this hydraulic bonding
is believed to provide mold strength during mold construction. Second, it is
understood by the inventors that calcium monoaluminate experiences a very low rate
of reaction with titanium and titanium aluminide based alloys. In a certain
embodiment, calcium monoaluminate is provided to the mold composition of the
present disclosure, for example, the investment molds, in the form of calcium
aluminate cement. In one aspect, the mold composition comprises a mixture of
calcium aluminate cement and alumina, that is, aluminum oxide.
[0031] In one aspect of the disclosure, the mold composition provides
minimum reaction with the alloy during casting, and the mold provides castings with
the required component properties. External properties of the casting include features
such as shape, geometry, and surface finish. Internal properties of the casting include
mechanical properties, microstructure, defects (such as pores and inclusions) below a
specified size and within allowable limits.
[0032] The mold composition of one aspect of the present disclosure provides
for low-cost casting of titanium alumnide (TiAl) turbine blades, for example, TiAl
low pressure turbine blades. The mold composition may provide the ability to cast
near-net-shape parts that require less machining and/or treatment than parts made
using conventional shell molds and gravity casting. As used herein, the expression
"near-net-shape" implies that the initial production of an article is close to the final
(net) shape of the article, reducing the need for further treatment, such as, extensive
machining and surface finishing. As used herein, the term "turbine blade" refers to
both steam turbine blades and gas turbine blades.
[0033] Accordingly, the present inventors address the challenges of producing
a mold, for example, an investment mold, that does not react significantly with
titanium and titanium aluminide alloys. In addition, according to some aspects of the
disclosure, the strength and stability of the mold allow high pressure casting
approaches, such as centrifugal casting. One of the technical advantages of aspects
of the this disclosure is that, in one aspect, the disclosure may improve the structural
integrity of net shape casting that can be generated, for example, from calcium
aluminate cement and alumina investment molds. The higher strength, for example,
higher fatigue strength, allows lighter components to be fabricated. In addition,
components having higher fatigue strength can last longer, and thus have lower lifecycle
costs.
Casting Mold Composition
[0034] Aspects of the present disclosure provide a composition of matter for
investment casting molds that can provide improved components of titanium and
titanium alloys. In one aspect of the present disclosure, calcium monoaluminate can
be provided in the form of calcium aluminate cement. Calcium aluminate cement
may be referred to as a "cement" or "binder." In certain embodiments, calcium
aluminate cement is mixed with an alumina particulates to provide a castable
investment mold mix. The calcium aluminate cement may typically be greater than
about 30% by volume in the castable mold mix. In certain embodiments, the calcium
aluminate cement is between about 30 % and about 60 % by volume in the castable
mold mix. The use of greater than 30% by volume of calcium aluminate cement in
the castable mold mix (casting mold composition) is a feature of the present
disclosure. The selection of the appropriate calcium aluminate cement chemistry and
alumina formulation are factors in the performance of the mold. In one aspect, a
sufficient amount of calcium oxide may be provided in the mold composition in order
to minimize reaction with the titanium alloy.
[0035] In one aspect, the mold composition, for example, the investment mold
composition, may comprise a multi-phase mixture of calcium aluminate cement and
alumina particles. The calcium aluminate cement may function as a binder, for
example, the calcium aluminate cement binder may provide the main skeletal
structure of the mold structure. The calcium aluminate cement may comprise a
continuous phase in the mold and provide strength during curing, and casting. The
mold composition may consist of calcium aluminate cement and alumina, that is,
calcium aluminate cement and alumina may comprise substantially the only
components of the mold composition, with little or no other components. In one
embodiment, the present disclosure comprises a titanium-containing article castingmold
composition comprising calcium aluminate. In another embodiment, the
casting-mold composition further comprises oxide particles, for example, hollow
oxide particles. According to aspects of the disclosure, the oxide particles may be
aluminum oxide particles, magnesium oxide particles, calcium oxide particles,
zirconium oxide particles, titanium oxide particles, and/or silica oxide particles, or
combinations thereof.
[0036] The casting-mold composition can further include aluminum oxide,
for example, in the form of hollow particles, that is, particles having a hollow core or
a substantially hollow core substantially surrounded by an oxide. These hollow
aluminum oxide particles may comprise about 99 % of aluminum oxide and have
about 0.5 millimeter [mm] or less in outside dimension, such as, width or diameter.
In certain embodiments, the hollow oxide particles may comprise hollow alumina
spheres. The hollow alumina spheres may be incorporated into the casting-mold
composition, and the hollow spheres may have a range of geometries, such as, round
particles, or irregular aggregates. In certain embodiments, the alumina may include
both round particles and hollow spheres. In one aspect, these geometries were found
to increase the fluidity of the investment mold mixture. The enhanced fluidity may
typically improve the surface finish and fidelity or accuracy of the surface features of
the final casting produced from the mold.
[0037] The aluminum oxide comprises particles ranging in outside dimension
from about 10 microns to about 10,000 microns. In certain embodiments, the
aluminum oxide comprises particles that are less than about 500 microns in outside
dimension, for example, diameter or width. The aluminum oxide may comprise from
about 0.5 % by weight to about 80 % by weight of the casting-mold composition.
Alternatively, the aluminum oxide comprises from about 40 % by weight to about 60
% by weight of the casting-mold composition.
[0038] In one embodiment, the casting-mold composition further comprises
calcium oxide. The calcium oxide may be greater than about 15% by weight and less
than about 50% by weight of the casting-mold composition. The final mold typically
may have a density of less than 2 grams/cubic centimeter and strength of greater than
500 pounds per square inch [psi]. In one embodiment, the calcium oxide is greater
than about 30%> by weight and less than about 50%> by weight of the casting-mold
composition. Alternatively, the calcium oxide is greater than about 25% by weight
and less than about 35% by weight of the casting-mold composition.
[0039] In a specific embodiment, the casting-mold composition of the present
disclosure comprises a calcium aluminate cement. The calcium aluminate cement
includes at least three phases or components comprising calcium and aluminum:
calcium monoaluminate (CaAl204), calcium dialuminate (CaAl4Oy), and mayenite
(Cai2Ali4033). The volume fraction of calcium monoaluminate may range from 0.05
to 0.95; the volume fraction of calcium dialuminate may range from 0.05 to 0.80; and
the volume fraction of mayenite may range from 0.01 to 0.30. In another example,
the volume fraction of calcium monoaluminate comprises a volume fraction of about
0.1 to about 0.8; the calcium dialuminate comprises a volume fraction of about 0.1 to
about 0.6; and the mayenite comprises a volume fraction of about 0.01 to about 0.2.
The volume fraction of calcium monoaluminate in the calcium aluminate cement may
be more than about 0.5, and the volume fraction of mayenite in the calcium aluminate
cement may be less than about 0.15. In another embodiment, the calcium aluminate
cement is more than 30% by weight of the casting-mold composition.
[0040] In one embodiment, the calcium aluminate cement has a particle size
of about 50 microns or less. A particle size of less than 50 microns is preferred for
three reasons: first, the fine particle size is believed to promote the formation of
hydraulic bonds during mold mixing and curing; second, the fine particle size is
understood to promote inter-particle sintering during firing, and this can increase the
mold strength; and third, the fine particle size is believed to improve the surface finish
of the molded article. The calcium aluminate cement may be provided as powder, and
can be used either in its intrinsic powder form, or in an agglomerated form, such as, as
spray dried agglomerates. The calcium aluminate cement can also be preblended with
fine-scale (for, example, less than 10 micron in size) alumina. The fine-scale alumina
is believed to provide an increase in strength due to sintering during high-temperature
firing. In certain instances, larger-scale alumina (that is, greater than 10 micron in
size) may also be added with or without the fine-scale alumina.
Calcium Aluminate Cement Composition
[0041] The calcium aluminate cement used in aspects of the disclosure
typically comprises three phases or components of calcium and aluminum: calcium
monoaluminate (CaAl20 4), calcium dialuminate (CaAl4Oy), and mayenite
(Cai2Ali4033). Calcium mono-aluminate is a hydraulic mineral present in calcium
alumina cement. Calcium monoaluminate ' s hydration contributes to the high early
strength of the investment mold. Mayenite is desirable in the cement because it
provides strength during the early stages of mold curing due to the fast formation of
hydraulic bonds. The mayenite is, however, typically removed during heat treatment
of the mold prior to casting.
[0042] In one aspect, the initial calcium aluminate cement formulation is
typically not at thermodynamic equilibrium after firing in the cement manufacturing
kiln. However, after mold making and high-temperature firing, the mold composition
moves towards a thermodynamically stable configuration, and this stability is
advantageous for the subsequent casting process. In one embodiment, the volume
fraction of calcium monoaluminate in the cement is greater than 0.5, and volume
fraction of mayenite is less than 0.15. The mayenite is incorporated in the mold
because it is a fast setting calcium aluminate and it is believed to provide the cement
with strength during the early stages of curing. Curing may be performed at low
temperatures, for example, temperatures between 15 degrees Celsius and 40 degrees
Celsius because the fugitive wax pattern is temperature sensitive and loses its shape
and properties on thermal exposure above about 35 degrees C. It is preferred to cure
the mold at temperatures below 30 degrees C.
[0043] The calcium aluminate cement may typically be produced by mixing
high purity alumina with high purity calcium oxide or calcium carbonate; the mixture
of compounds is typically heated to a high temperature, for example, temperatures
between 1000 and 1500 degrees C in a furnace or kiln and allowed to react.
[0044] The resulting product, known in the art as cement "clinker," that is
produced in the kiln is then crushed, ground, and sieved to produce a calcium
aluminate cement of the preferred particle size. Further, the calcium aluminate
cement is designed and processed to have a minimum quantity of impurities, such as,
minimum amounts of silica, sodium and other alkali, and iron oxide. In one aspect,
the target level for the calcium aluminate cement is that the sum of the Na20 , Si0 2,
Fe20 3, and Ti0 2 is less than about 2 weight per cent. In one embodiment, the sum of
the Na20 , Si0 2, Fe20 3, and Ti0 2 is less than about 0.05 weight per cent.
[0045] In one aspect of the disclosure, a calcium aluminate cement with bulk
alumina concentrations over 35% weight in alumina (AI2O3) and less than 65%
weight calcium oxide is provided. The maximum alumina concentration of the
cement may be about 85% (for example, about 15% CaO). In one embodiment, the
calcium aluminate cement is of high purity and contains up to 70% alumina. The
volume fraction of calcium monoaluminate may be maximized in the fired mold prior
to casting. A minimum amount of calcium oxide may be required to minimize
reaction between the casting alloy and the mold. If there is more than 50% calcium
oxide in the cement, this can lead to phases such as mayenite and tricalcium
aluminate, and these do not perform as well as the calcium monoaluminate during
casting. The preferred range for calcium oxide is less than about 50% and greater
than about 15% by weight.
[0046] As noted above, the three phases in the calcium aluminate
cement/binder in the mold are calcium monoaluminate (CaAl20 4) , calcium
dialuminate (CaAl4Oy), and mayenite (Ca12Al140 33) . The calcium monoaluminate in
the cement/binder has three advantages over other calcium aluminate phases: 1) The
calcium monoaluminate is incorporated in the mold because it has a fast setting
response (although not as fast as mayenite) and it is believed to provide the mold with
strength during the early stages of curing. The rapid generation of mold strength
provides dimensional stability of the casting mold, and this feature improves the
dimensional consistency of the final cast component. 2) The calcium monoaluminate
is chemically very stable with regard to the titanium and titanium aluminide alloys
that are being cast. The calcium monoaluminate is preferred relative to the calcium
dialuminate, and other calcium aluminate phases with higher alumina activity; these
phases are more reactive with titanium and titanium aluminide alloys that are being
cast. 3) The calcium monoaluminate and calcium dialuminate are low expansion
phases and are understood to prevent the formation of high levels of stress in the mold
during curing, dewaxing, and subsequent casting. The thermal expansion behavior of
calcium monoaluminate is a close match with alumina.
Casting Mold Composition with Improved Detectability
[0047] There is a small difference in the X-ray density of the mold materials
(calcium aluminate cement and alumina) and titanium, and inclusions that originate
from the mold are therefore difficult to detect. In order to address this limitation,
species can be added to the ceramic investment mix to enhance X-ray detectability of
inclusions.
[0048] One aspect of the disclosure is a mold composition for casting a titaniumcontaining
article, comprising: a calcium aluminate cement comprising calcium
monoaluminate, calcium dialuminate, and mayenite; and an X-ray or Neutron-ray
detectable element. Another aspect of the present disclosure is a titanium-containing
article casting-mold composition, comprising: calcium aluminate; and an X-ray or
Neutron-ray detectable element. In one embodiment, the calcium aluminate cement
forms an intrinsic facecoat of less than about 100 microns when the mold composition
forms a mold. In one embodiment, the X-ray or Neutron-ray detectable elements are
mixed within the mold. In another embodiment, the X-ray or Neutron-ray detectable
elements are mixed within the mold and become part of the intrinsic facecoat.
[0049] There are several different methods that X-ray or Neutron-ray detectable
elements can be mixed with the mold mix. For example, the element can be added as
a liquid such as a nitrate at any stage of the mold mixing process. The element can
also be added as an oxide, as described herein. In one embodiment, the element is
combined as an oxide with alumina in a fused form, such as an erbium aluminium
garnet, or dysprosium aluminium garnet, prior to generating the mold mix. It will be
understood by someone skilled in the art of ceramic mold making that different
approaches can be employed to introduce the X-ray or Neutron-ray detectable
elements into the mold. In one embodiment, the mold composition does not have an
extrinsic facecoat.
[0050] The mold composition may further comprise oxide particles. The
oxide particles comprise particles of at least one of aluminum oxide, magnesium
oxide, calcium oxide, zirconium oxide, and titanium oxide. In a specific embodiment,
the oxide particles are aluminum oxide particles. The titanium-containing cast article
can be an engine, a turbine, or a turbine blade.
[0051] Since there is only a small difference between the X-ray density of the
mold materials (calcium aluminate cement and alumina) and the X-ray density of
titanium, inclusions that originate from the mold are difficult to detect. Here, the
inventors added certain X-ray detectable elements to their investment mix to enhance
the detectability of the sub-surface inclusions. Accordingly, one aspect of the present
disclosure is a method for detecting sub-surface ceramic inclusions in a titanium or
titanium alloy casting, comprising: combining calcium aluminate, an element more
radiographically dense than the calcium aluminate, and a liquid to form a slurry;
forming a mold having the calcium aluminate and the radiographically dense element
from the slurry; introducing a titanium aluminide-containing metal to the
radiographically dense element-bearing mold; solidifying said titanium aluminidecontaining
metal to form an article in the mold; removing the solidified titanium
aluminide-containing metal article from said mold; subjecting the solidified titanium
aluminide-containing article to radiographic inspection to provide a radiograph; and
examining said radiograph for the presence of the radiographically dense element on
or in the article. In one embodiment, the method comprises minimizing the presence
of mold material inclusions in titanium aluminide-containing cast articles.
[0052] The combining step further comprises combining oxide particles with
the slurry. A liquid, such as water, for example, deionized water, may be added to the
slurry to adjust slurry viscosity. Any viscosity measuring protocol or instrument may
be used. Typically, viscosity is adjusted to be within 8-20 seconds, preferably, 9-12
seconds, for the cement slurry mixing as determined by using the Zahn cup viscosity
measurement technique; this technique is well know to those skilled in the art. The
amount of water present in the slurry is limited so as not to diminish the green or fired
strength of the shell mold. In certain embodiments, the radiographically dense
element is more radiographically dense than the oxide particles, for example, the
radiographically dense element is more radiographically dense than calcium oxide. In
certain embodiments, the oxide particles comprise hollow oxide particles, for
example, hollow aluminum oxide particles.
[0053] One of the advantages of the present disclosure is that castings can be
produced that provide enhanced detectability of any surface and/or sub-surface
inclusions on, proximate, and/or below the surface of the casting that are typically not
detectable by visual inspection. For example, inclusions that may be located below an
alpha case layer of a titanium based casting and that are not removed by a post-cast
chemical milling operation or other surface treatments may be detected by aspects of
the disclosure. Moreover, conventional chemical milling regimes can still be used to
remove the alpha case from the casting because practicing the disclosure does not
promote further formation of alpha case on titanium based castings.
[0054] One aspect of the present disclosure provides a composition of matter
for casting molds, for example, investment casting molds, that can provide improved
X-ray or Neutron-ray inspectability for inclusions that can undesirably occur in the
casting, for example, from the casting molding. In one embodiment, this is achieved
by the addition of an element more radiographically dense than the casting mold
composition, for example, more radiographically dense than calcium aluminate. In
one aspect, the present disclosure is a mold composition for casting titaniumcontaining
articles, comprising: calcium aluminate; and an X-ray or Neutron-ray
detectable element. The titanium-containing cast article can be a titanium aluminide
engine component, a titanium aluminide turbine, or a titanium aluminide turbine
blade. In one embodiment, the X-ray or Neutron-ray detectable element that can be
used include at least one of ytterbium, hafnium, gadolinium, tungsten, thorium,
uranium, yttrium, dysprosium, erbium, cerium, and compositions thereof. These
elements are used in some instances because they are more radiographically dense
than the calcium aluminate.
[0055] One aspect of the present disclosure is a mold composition comprising:
calcium aluminate cement comprising calcium monoaluminate, calcium dialuminate,
and mayenite; and at least one element more radiographically dense than the calcium
aluminate cement. Another aspect of the present disclosure is a mold composition
comprising calcium aluminate and at least one element more radiographically dense
than the calcium aluminate. Erbium, dysprosium, and/or gadolinium-bearing calcium
aluminate cement and alumina investment mixes have the advantage of the relatively
high X-ray detectability of erbium, dysprosium, and gadolinium compared to other
elements. An additional advantage is that erbium, dysprosium, and gadolinium are
also resistant to reaction with molten titanium and titanium alloys during casting. The
erbium, dysprosium, and/or gadolinium-bearing investment mix are not radioactive
compared to Th0 2 and other radioactive bearing mold compositions and thus are
preferred in some embodiments.
[0056] The mold formulation may not form an extrinsic facecoat, such as
yttrium, when formed into a mold, but the formulation may be a homologous twophase
composition of calcium aluminate and alumina. During investment mixing,
pouring and curing, the mold forms an intrinsic facecoat of calcium aluminate in the
mold. According to an aspect of the disclosure, the intrinsic facecoat (typically less
than 100 microns thick) of calcium aluminate in the mold also contains particles of
radiographically dense elements, for example, erbium and/or dysprosium and/or
gadolinium mixed within the mold material. The erbium, dysprosium, gadolinium
bearing additions to the investment mix are used for the molds for making titanium
aluminide and titanium aluminide alloy castings because erbium, dysprosium, and
gadolinium exhibit a greater X-ray density than that of other ceramic components.
Some of the radiographically dense elements, for example, erbium, dysprosium, and
gadolinium also exhibit acceptable resistance to reaction with molten titanium
aluminide and titanium aluminide alloys during the casting operation.
The Mold, Casting Methods and Detecting Sub-Surface Inclusions
[0057] An investment mold is formed by formulating the investment mix of
the ceramic components, and pouring the mix into a vessel that contains a fugitive
pattern. The investment mold formed on the pattern is allowed to cure thoroughly to
form a so-called "green mold." Typically, curing of the green mold is performed for
times from 1 hour to 48 hours. Subsequently, the fugitive pattern is selectively
removed from the green mold by melting, dissolution, ignition, or other known
pattern removal technique. Typical methods for wax pattern removal include oven
dewax (less than 150 degrees C), furnace dewax (greater than 150 degrees C), steam
autoclave dewax, and microwave dewaxing.
[0058] For casting titanium alloys, and titanium aluminide and its alloys, the
green mold then is fired at a temperature above 600 degrees C, preferably 700 to 1400
degrees C, for a time period in excess of 1 hour, preferably 2 to 6 hours, to develop
mold strength for casting and to remove any undesirable residual impurities in the
mold, such as metallic species (Fe, Ni, Cr), and carbon-containing species. The
atmosphere of firing the mold is typically ambient air, although inert gas or a reducing
gas atmosphere can be used.
[0059] The firing process also removes the water from the mold and converts
the mayenite to calcium aluminate. Another purpose of the mold firing procedure is
to minimize any free silica that remains in the mold prior to casting. Other purposes
are to remove the water, increase the high temperature strength, and increase the
amount of calcium monoaluminate and calcium dialuminate.
[0060] The mold is heated from room temperature to the final firing
temperature, specifically the thermal history and the humidity profile are controlled.
The heating rate to the firing temperature, and the cooling rate after firing are
typically regulated or controlled. If the mold is heated too quickly, it can crack
internally or externally, or both; mold cracking prior to casting is highly undesirable.
In addition, if the mold is heated too quickly, the internal surface of the mold can
crack and spall off. This can lead to undesirable inclusions in the final casting, and
poor surface finish, even if there are no inclusions. Similarly, if the mold is cooled
too quickly after reaching the maximum temperature, the mold can also crack
internally or externally, or both.
[0061] The mold composition described in the present disclosure is
particularly suitable for titanium and titanium aluminide alloys. The mold
composition after firing and before casting can influence the mold properties,
particularly with regard to the constituent phases. In one embodiment, for casting
purposes, a high volume fraction of calcium monoaluminate in the mold is preferred,
for example, a volume fraction of 0.3 to 0.8. In addition, for casting purposes, it is
desirable to minimize the volume fraction of the mayenite, for example, using a
volume fraction of 0.01 to 0.2, because mayenite is water sensitive and it can provide
problems with water release and gas generation during casting. After firing, the mold
can also contain small volume fractions of aluminosilicates and calcium
aluminosilicates. The sum of the volume fraction of aluminosilicates and calcium
aluminosilicates may typically be kept to less than 5% in order to minimize reaction
of the mold with the casting.
[0062] In certain embodiments, the casting-mold composition of the present
disclosure comprises an investment casting-mold composition. The investment
casting-mold composition comprises a near-net-shape, titanium-containing metal,
investment casting mold composition. In one embodiment, the investment castingmold
composition comprises an investment casting-mold composition for casting
near-net-shape titanium aluminide articles. The near-net-shape titanium aluminide
articles comprise, for example, near-net-shape titanium aluminide turbine blades.
[0063] The selection of the correct calcium aluminate cement chemistry and
alumina formulation are factors in the performance of the mold during casting. In
terms of the calcium aluminate cement, it may be necessary to minimize the amount
of free calcium oxide in order to minimize reaction with the titanium alloy. If the
calcium oxide concentration in the cement is less than 15% by weight, the alloy reacts
with the mold because the alumina concentration is too high, and the reaction
generates undesirable oxygen concentration levels in the casting, gas bubbles, and a
poor surface finish in the cast component. If the calcium oxide concentration in the
cement is greater than 50% by weight, the mold can be sensitive to pick up of water
and carbon dioxide from the environment. As such, the calcium oxide concentration
in the investment mold may typically be kept below 50%. In one embodiment, the
calcium oxide concentration is between 15 % and 40 % by weight. Alternatively, the
calcium oxide concentration be between 25 % and 35 % by weight.
[0064] Carbon dioxide can lead to formation of calcium carbonate in the mold
during processing and prior to casting, and calcium carbonate is unstable during the
casting operation. Thus, the water and carbon dioxide in the mold can lead to poor
casting quality. If the adsorbed water level is too high, for example, greater than 0.05
weight percent, when the molten metal enters the mold during casting, the water is
released and it can react with the alloy. This leads to poor surface finish, gas bubbles
in the casting, high oxygen concentration, and poor mechanical properties. Similarly,
if the carbon dioxide level is too high, calcium carbonate can form in the mold and
when the molten metal enters the mold during casting, the calcium carbonate can
decompose generating carbon dioxide, which can react with the alloy. The resulting
calcium carbonate is less than 1 percent in weight of the mold.
[0065] Prior to casting a molten metal or alloy, the investment mold typically
is preheated to a mold casting temperature that is dependent on the particular
component geometry or alloy to be cast. For example, a typical mold preheat
temperature is 600 degrees C. Typically, the mold temperature range is 450 degrees
C to 1200 degrees C; the preferred temperature range is 450 degrees C to 750 degrees
C, and in certain cases it is 500 degrees C to 650 degrees C.
[0066] According to one aspect, the molten metal or alloy is poured into the
mold using conventional techniques which can include gravity, countergravity,
pressure, centrifugal, and other casting techniques known to those skilled in the art.
Vacuum or an inert gas atmospheres can be used. For complex shaped thin wall
geometries, techniques that use high pressure are preferred. After the solidified
titanium aluminide or alloy casting is cooled typically to less than 650 degrees, for
example, to room temperature, it is removed from the mold and finished using
conventional techniques, such as, grit blasting, water jet blasting, and polishing.
[0067] One aspect of the present disclosure is a method for detecting sub
surface ceramic inclusions in a titanium or titanium alloy casting, comprising:
combining calcium aluminate, at least one element more radiographically dense than
the calcium aluminate, and a liquid to form a slurry; forming a mold having the
calcium aluminate and the radiographically dense element from the slurry;
introducing a titanium aluminide-containing metal to the radiographically dense
element-bearing mold; solidifying said titanium aluminide-containing metal to form
an article in the mold; removing the solidified titanium aluminide-containing metal
article from said mold; subjecting the solidified titanium aluminide-containing article
to radiographic inspection to provide a radiograph; and examining said radiograph for
the presence of the radiographically dense element on or in the article. In one
embodiment, the method comprises minimizing the presence of mold material
inclusions in titanium aluminide-containing cast articles.
[0068] Between removing said fugitive pattern from the mold and preheating
the mold to a mold casting temperature, the mold is first heated to a temperature of
about 450 degrees C to about 900 degrees C, and then cooled to room temperature. In
one embodiment, the curing step is conducted at temperatures below about 30 degrees
C for between one hour to 48 hours. The removing of the fugitive pattern includes the
step of melting, dissolution, ignition, oven dewaxing, furnace dewaxing, steam
autoclave dewaxing, or microwave dewaxing. In one embodiment, after removing of
the titanium or titanium alloy from the mold, the casting may be finished with grit
blasting, water get blasting, or polishing. After the solidified casting is removed from
the mold, it is inspected by X-ray or Neutron radiography.
[0069] For the present disclosure, the solidified casting is subjected to surface
inspection and X-ray radiography after casting and finishing to detect any sub-surface
inclusion particles at any location within the casting. X-ray radiography is employed
to find inclusions that are not detectable by visual inspection of the exterior surface of
the casting. The titanium aluminide casting is subjected to X-ray radiography (film or
digital) using conventional X-ray equipment to provide an X-ray radiograph that then
is inspected or analyzed to determine if any sub-surface inclusions are present within
the titanium aluminide casting.
[0070] The sub-surface inclusions can originate from the investment mold
facecoat or mold facecoat as a result of erosion of the mold during mold filling,
reaction between the reactive molten metal and the mold facecoat, and/or mechanical
spallation as a result of thermal shock of the mold. When a sub-surface inclusion or
inclusions are found using the X-ray methods, the casting may be subjected to
grinding and weld repair operations to remove and replace sufficient material to
remove the objectionable inclusions; alternatively the casting may be scrapped if the
inclusion(s) is/are larger than a specified size for the required mechanical integrity of
the casting.
[0071] The solidified casting is typically subjected to surface inspection and
X-ray radiography after casting and finishing to detect any ceramic inclusion
particles, for example, sub-surface inclusion particles, at any location within the
casting. Erbium, dysprosium, and gadolinium bearing calcium aluminate cement and
alumina investment mixes are used. The erbium bearing calcium aluminate cement
and alumina investment mix can be selected from fused, calcined or sintered erbia
(erbium oxide) powder in the fused form, or other form. Fused erbia powder is
preferred as the erbia slurry component since it is more dense and resistant to
chemical reaction with a titanium aluminide or titanium aluminide alloy melt than
calcined or sintered erbia powder. A fused erbia powder can be added to the
investment mold mix during mixing, at any stage. In one embodiment, the fused erbia
powder is added with the calcium aluminate cement. A fused erbia powder
particularly useful in practicing the disclosure is available as Auercoat 4/3 from
Treibacher Auermet GmbH, A-9330 Treibach-Althofen, Austria, in the powder
particle size of -325 mesh (less than 44 microns). A calcined erbia powder useful in
practicing the disclosure is available as Auercoat 4/4 also from Treibacher Auermet
GmbH in the particle size of -325 mesh (less than 44 microns). The mesh size refers
to the U.S. Standard Screen System.
[0072] In one embodiment, the method further comprises the step of removing
the radiographically dense element from the article. This removing of the
radiographically dense element from the article can be achieved by one or more steps
of machining, grinding, polishing, or welding. Chemical milling can also be used to
remove the radiographically dense element from the article.
[0073] Since there is a risk of sub-surface inclusions becoming entrained in
the cast component and thereby reducing the strength and load-bearing capability of
the final casting, the present disclosure is directed to the detection and elimination of
these sub-surface inclusions from the castings, so as to maximize the mechanical
properties and the performance of the castings. The present disclosure provides
methods for improving the structural integrity of castings by increasing the
probability of detecting inclusions that can be generated from calcium aluminate
cement and alumina investment molds during casting of titanium aluminide.
[0074] The present disclosure also allows the detection of smaller inclusions
because of the greater X-ray contrast. The greater probability of detection of
inclusions and the greater ability to detect smaller inclusions with the modern digital
X-rays methods improve the strength and the fatigue strength of castings of titanium
alloys and titanium aluminide alloys.
[0075] The mold compositions described provide a small amount of a material
having a high Neutron absorption cross section. In one aspect, a Neutron radiograph
is prepared of the cast article. Since the titanium alloy cast article may be
substantially transparent to neutrons, the mold material will typically show up
distinctly in the resulting Neutron radiograph. In one aspect, it is believed that
Neutron exposure results in "neutron activation" of the radiographically dense
element. Neutron activation involves the interaction of the Neutron radiation with the
radiographically dense element of the casting to effect the formation of radioactive
isotopes of the radiographically dense elements of the mold composition. The
radioactive isotopes may then be detectable by conventional radioactive detecting
devices to count any radiographically dense element isotopes present in the cast
article.
[0076] A thermal Neutron beam can be obtained from a number of sources,
including a nuclear reactor, a subcritical assembly, a radioactive Neutron source, or an
accelerator. Images produced by N-ray can be recorded on a film, such as with X-ray.
This is accomplished generally by placing a part to be imaged in a Neutron beam, and
then recording the image on a film for each angle at which an image is desired. N-ray
images can also be taken in real time with modern digital detection equipment.
[0077] N-ray uses neutrons as a penetrating radiation for imaging inclusions.
All energies of neutrons, e.g., fast, epithermal, thermal and cold neutrons, can be used
for N-ray imaging. N-ray imaging is a process whereby radiation beam intensity
modulation by an object is used to identify inclusions and defects. The components
required for N-ray imaging include a source of fast neutrons, a moderator, a gamma
filter, a collimator, a conversion screen, a film image recorder or other imaging
system, a cassette, and adequate biological shielding and interlock systems.
[0078] In one aspect, the presently taught method may be used in the titanium
aluminide castings when there is an addition to the mold material that is a strong
absorber of neutrons. The Neutron absorbing additives are suitable because they have
the desired high Neutron absorption cross section. Since generally Neutron
radiographs are produced using neutrons having thermal or resonance energy levels, it
is generally preferred that the Neutron absorbing material have a high absorption
cross section for thermal neutrons. Example materials having high thermal Neutron
absorption cross sections that are compatible with the titanium aluminide mold of the
present disclosure include erbium, dysprosium, gadolinium, and mixtures thereof.
[0079] In general, the higher the Neutron absorption cross section of the
additive, the smaller the quantity required to give the desired imaging characteristics.
Generally, less than 10 weight per cent is used. For example, the X-ray or Neutronray
detectable component is in the range of about 0.5 to about 6 weight percent in the
mold composition. Good results can be obtained with erbium, dysprosium, or
gadolinium oxide in the range of about 1 to about 4 weight percent of the core
material. Gadolinium has a very high Neutron absorption cross section and produces
excellent images with small amounts in the mold. In one embodiment, solutions used
to enhance N-ray and X-ray contrast comprise nitrate, halide, sulfate, perchlorate salts
of the elements for N-ray and X-ray enhancement.
[0080] In one aspect, the selection of suitable mold additions for X-ray
contrast enhancement and detection depends upon the difference between the density
of the imaging agent and that of the titanium alloy casting. The selection of suitable
mold additions for N-ray imaging of inclusions is determined by the linear attenuation
coefficient or the thermal Neutron cross section of the imaging addition relative to
that of the cast titanium part, and throughout the whole cross section of the casting.
[0081] In one aspect of the present disclosure, the inventors selected N-ray
and X-ray contrast enhancing elements to add to the calcium aluminate investment
mold based on factors, including: the stability of the oxide of the element against the
mold metal (low reaction rate), the x-ray density in comparison with titanium, and the
N-ray moderation in comparison with titanium, and availability/cost. With these
criteria in mind, three species were identified: erbia, dysprosia, and gadolinia. Other
contrast enhancing elements such as, neodymium, samarium, europium, holmium,
ytterbium, lutetium were considered based on the above criteria, however, were not
thought to provide the same results in this application as when gadolinium, erbium
and dysprosium are used. In one embodiment, gadolinia, erbia and dysprosia are
preferred for detecting inclusions that can come from calcium aluminate molds in
casting titanium or titanium alloy.
[0082] With respect to X-ray detection of inclusions, the primary factors that
effect detectability include (1) the difference between the density of the titanium alloy
in comparison with the density of the inclusion, (2) the size, thickness, shape and
orientation of the inclusion, and (3) the thickness of the cross section of the cast
titanium alloy component. If the difference between the density of the cast material
and the inclusion is small (such as less than about 0.5 g/cc), there may not be
sufficient image contrast to detect the inclusion by X-ray. In these circumstances, Nray
is employed, provided the appropriate element is added for N-ray contrast
enhancement.
[0083] In one aspect of the present disclosure, the selection of suitable
imaging agents for X-ray detection depends upon the difference between the density
of the imaging agent and that of the metal or alloy of the casting. In one example, the
selection of suitable imaging agents for N-ray imaging of inclusions is determined by
the linear attenuation coefficient or the Neutron absorption cross section of the
material being used as an imaging agent relative to that of the metal or alloy being
cast. The difference between the linear attenuation coefficient or the Neutron
absorption cross section of the mold and that of the casting needs to be sufficient, so
that any mold inclusions can be imaged throughout the cross section of the article.
[0084] Gadolinium is a preferred addition to the mold for imaging using N-ray
detection of inclusions in titanium or titanium alloy castings. Gadolinium has a
very high Neutron absorption cross section. Specifically, the Neutron absorption
cross section of gadolinium is 259,000 barns, whereas the Neutron absorption cross
section of titanium is about 6.1 barns. The Neutron absorption cross section of other
elements include, dysprosium (2840 barns), erbium (659 barns), ettrium (1.3 barns),
calcium (0.4 barns), aluminum (0.2 barns). As such, the N-ray imaging capability of
calcium- and aluminum-containing inclusions, for example, is very low. (for
additional information, see the National Institute of Standards and Technology Center
for Neutron Research website). Therefore, the selection of the element is a feature of
the disclosure, and isotopes of the selected elements may be used.
[0085] In one aspect, the addition of gadolinium or dysprosium, or erbium can
substantially enhance the Neutron absorption capability with respect to titanium, and
therefore the inclusion imaging contrast capability during N-ray is substantially
enhanced. Gadolinium isotope 157 is believed to have a thermal Neutron absorption
cross section of 259,000 barns. The difference between the Neutron absorption cross
section of titanium or titanium alloys makes gadolinium particularly suitable for Nray
imaging. For metals and/or alloys other than titanium, gadolinium is also a
preferred imaging agent, primarily because of the relatively large Neutron absorption
cross of gadolinium.
[0086] One of the technical advantages of aspects of the disclosure is that it
improves the structural integrity of castings of a titanium-containing article by
allowing for improved detection of inclusions that can be generated from calcium
aluminate cement and alumina investment mixes. The disclosure also allows the
detection of smaller inclusions because of the greater X-ray contrast. The greater
probability of detection of inclusions and the greater ability to detect smaller
inclusions with the most modern digital X-rays methods improve the strength and the
fatigue strength of castings of titanium alloys and titanium aluminide alloys. The
higher strength allows lighter components, and the higher fatigue strength provides
for components with longer lives, and thus lower life-cycle costs. In one
embodiment, the component comprises a titanium aluminide turbine blade.
EXAMPLES
[0087] The disclosure, having been generally described, may be more readily
understood by reference to the following examples, which are included merely for
purposes of illustration of certain aspects and embodiments of the present disclosure,
and are not intended to limit the disclosure in any way.
The Investment Mold Composition and Formulation
[0088] A calcium aluminate cement was mixed with alumina to generate an
investment mold mix, and a range of investment mold chemistries were tested. The
investment mixture consisted of calcium aluminate cement with 70% alumina and
30% calcia, alumina particles, water, and colloidal silica.
[0089] In a first example, a typical slurry mixture for making an investment
mold consisted of 3000 grams [g] of the calcium aluminate cement, (comprising
approximately 10% by weight of mayenite, approximately 70% by weight of calcuim
monoaluminate, and approximately 20% by weight of calcium dialuminmate), 1500 g
of calcined alumina particles with a size of less than 10 microns, 2450g of high-purity
calcined alumina particles of a size range from 0.5-lmm diameter, 1650g of deionized
water, and 150g of colloidal silica.
[0090] Typical high-purity calcined alumina particles types include fused,
tabular, and levigated alumina. Typical suitable colloidal silicas include Remet LP30,
Remet SP30, Nalco 1030, Ludox. The produced mold was used for casting titanium
aluminide-containing articles such as turbine blades with a good surface finish. The
roughness (Ra) value was less than 100 microinches, and with an oxygen content of
less than 2000 parts per million [ppm]. This formulation produced a mold that was
approximately 120mm diameter and 400mm long. This formulation produced a mold
that had a density of less than 2 grams per cubic centimeter.
[0091] The mold mix was prepared by mixing the calcium aluminate cement,
water, and collodial silica in a container. A high-shear form mixing was used. If not
mixed thoroughly, the cement can gel. When the cement was in full suspension in the
mixture, the fine-scale alumina particles were added. When the fine-scale alumina
particles were fully mixed with the cement, the larger-size (for example, 0.5-1.0 mm)
alumina particles were added and mixed with the cement-alumina formulation. The
viscosity of the final mix is another factor, as it must not be too low or too high. In
addition, accelerants, and retarders can be used at selected points during the mold
making process steps. Typical individual dispersing alumina with accelerants, and
retarders include Almatis ADS-1, ADS-3, and ADW-1.
[0092] After mixing, the investment mix was poured in a controlled manner
into a vessel that contains the fugitive wax pattern. The vessel provides the external
geometry of the mold, and the fugitive pattern generates the internal geometry. The
correct pour speed is a further feature, if it is too fast air can be entrapped in the mold,
if it is too slow separation of the cement and the alumina particulate can occur.
Suitable pour speed range from about 1 to about 20 liters per minute. In one
embodiment, the pour speed is about 2 to about 6 liters per minute. In a specific
embodiment, the pour speed is about 4 liters per minute.
[0093] In a second example, a slurry mixture for making an investment mold
consisted of 3000 g of the calcium aluminate cement, (comprising approximately 10%
by weight of mayenite, approximately 70% by weight of calcium monoaluminate, and
approximately 20% by weight of calcium dialuminmate), 1500 g of calcined alumina
particles with a size of less than 10 microns, 2650g of high-purity calcined alumina
bubble of a size range from 0.5-lmm diameter, 1650g of deionized water, and 150g of
colloidal silica.
[0094] The alumina hollow particles provide a mold with a reduced density.
The weight fraction of calcium aluminate cement is 42%, and that of the alumina is
58%. This formulation produced a mold that was approximately 125mm diameter
and 400 mm long. The mold was then cured and fired at high temperature. The
produced mold was used for casting titanium aluminide-containing articles such as
turbine blades with a good surface finish. The roughness (Ra) value was less than
100, and with an oxygen content of less than 2000 ppm. This formulation produced a
mold that had a density of less than 1.8 grams per cubic centimeter.
[0095] In a third example, a slurry mixture for making an investment mold
consisted of 600 g of the calcium aluminate cement, (consisting of approximately
10% by weight of mayenite, approximately 70% by weight of calcium
monoaluminate, and approximately 20% by weight of calcium dialuminmate), 300 g
of calcined alumina particles with a size of less than 10 microns, 490g of high-purity
calcined alumina bubble of a size range from 0.5-lmm diameter, 305g of deionized
water, and 31 g of colloidal silica. This formulation produced a smaller mold for a
smaller component that was approximately 120mm diameter and 150mm long. The
mold was then cured and fired at high temperature. The produced mold was used for
casting titanium aluminide-containing articles such as turbine blades with a good
surface finish. The roughness (Ra) value was less than 100 microinches, and with an
oxygen content of less than 1600 ppm.
[0096] In a fourth example, a slurry mixture for making an investment mold
consisted of 2708 g of the calcium aluminate cement, (comprising approximately 10%
by weight of mayenite, approximately 70% by weight of calcium monoaluminate, and
approximately 20% by weight of calcium dialuminmate), 1472g of high-purity
calcined alumina bubble of a size range from 0.5-lmm diameter, 1061g of deionized
water, and 196g of colloidal silica. This formulation produced a smaller mold with a
smaller alumina content for a smaller component. The mold was then cured and fired
at high temperature. The produced mold was used for casting titanium aluminidecontaining
articles such as turbine blades.
[0097] The colloidal silica controls the rate of reaction of the calcium
aluminate phases with water, and provides mold strength during curing. This rate of
reaction of the calcium aluminate phases with water controls the working time of the
investment mold mix during mold making. This time was between about 30 seconds
and about 10 minutes. If the working time of the investment mold mix is too short,
there is insufficient time to make large molds of complex-shaped components. If the
working time of the investment mold mix is too long and the calcium aluminate
cement does not cure sufficiently quickly, separation of the fine-scale cement and the
large scale alumina can occur and this can lead to a segregated mold in which the
formulation varies and the resulting mold properties are not uniform.
[0098] The three phases in the calcium aluminate cement comprises calcium
monoaluminate (CaAl20 4), calcium dialuminate (CaAl4Oy), and mayenite
(Cai2Ali40 3 3), and the inventors made this selection to achieve several purposes.
First, the phases must dissolve or partially dissolve and form a suspension that can
support all the aggregate phases in the subsequent investment mold making slurry.
Second, the phases must promote setting or curing of the mold after pouring. Third,
the phases must provide strength to the mold during and after casting. Fourth, the
phases must exhibit minimum reaction with the titanium alloys that is cast in the
mold. Fifth, the mold must have a suitable thermal expansion match with the titanium
alloy casting in order to minimize the thermal stress on the part that is generated
during post-solidification cooling.
[0099] The X-ray image (Figure 7) shows a cast titanium aluminide blade that
contains a sub-surface inclusion from calcium aluminate mold. This is a very large
inclusion (5.44 mm) and can be resolved with digital enhancing techniques. Smaller
low density inclusions from calcium aluminate mold are more difficult to resolve. In
one aspect of the disclosure, the inventors used mold additions that increase the X-ray
density of the mold in order to improve the inclusion detection capability.
[00100] It is to be understood that the above description is intended to be
illustrative, and not restrictive. For example, the above-described embodiments
(and/or aspects thereof) may be used in combination with each other. In addition,
many modifications may be made to adapt a particular situation or material to the
teachings of the various embodiments without departing from their scope. While the
dimensions and types of materials described herein are intended to define the
parameters of the various embodiments, they are by no means limiting and are merely
exemplary. Many other embodiments will be apparent to those of skill in the art upon
reviewing the above description. The scope of the various embodiments should,
therefore, be determined with reference to the appended claims, along with the full
scope of equivalents to which such claims are entitled. In the appended claims, the
terms "including" and "in which" are used as the plain-English equivalents of the
respective terms "comprising" and "wherein." Moreover, in the following claims, the
terms "first," "second," and "third," etc. are used merely as labels, and are not
intended to impose numerical requirements on their objects. Further, the limitations
of the following claims are not written in means-plus-function format and are not
intended to be interpreted based on 35 U.S.C. § 112, sixth paragraph, unless and until
such claim limitations expressly use the phrase "means for" followed by a statement
of function void of further structure. It is to be understood that not necessarily all
such objects or advantages described above may be achieved in accordance with any
particular embodiment. Thus, for example, those skilled in the art will recognize that
the systems and techniques described herein may be embodied or carried out in a
manner that achieves or optimizes one advantage or group of advantages as taught
herein without necessarily achieving other objects or advantages as may be taught or
suggested herein.
[00101] While the disclosure has been described in detail in connection with
only a limited number of embodiments, it should be readily understood that the
disclosure is not limited to such disclosed embodiments. Rather, the disclosure can be
modified to incorporate any number of variations, alterations, substitutions or
equivalent arrangements not heretofore described, but which are commensurate with
the spirit and scope of the invention. Additionally, while various embodiments of the
invention have been described, it is to be understood that aspects of the invention may
include only some of the described embodiments. Accordingly, the invention is not to
be seen as limited by the foregoing description, but is only limited by the scope of the
appended claims. All publications, patents, and patent applications mentioned herein
are hereby incorporated by reference in their entirety as if each individual publication
or patent was specifically and individually indicated to be incorporated by reference.
In case of conflict, the present application, including any definitions herein, will
control.
[00102] This written description uses examples to disclose the invention,
including the best mode, and also to enable any person skilled in the art to practice the
invention, including making and using any devices or systems and performing any
incorporated methods. The patentable scope of the invention is defined by the claims,
and may include other examples that occur to those skilled in the art. Such other
examples are intended to be within the scope of the claims if they have structural
elements that do not differ from the literal language of the claims, or if they include
equivalent structural elements with insubstantial differences from the literal language
of the claims.
CLAIMS
A mold composition for casting a titanium-containing article, comprising:
a calcium aluminate cement comprising calcium monoaluminate, calcium
dialuminate, and mayenite; and
an X-ray or Neutron-ray detectable element.
The mold composition as recited in claim 1, wherein the calcium aluminate
cement forms an intrinsic facecoat of less than about 100 microns when the
mold composition forms a mold.
The mold composition as recited in claim 2, wherein the X-ray or Neutron-ray
detectable elements are mixed within the mold.
The mold composition as recited in claim 1, wherein the mold composition
does not have an extrinsic facecoat.
The mold composition as recited in claim 1, further comprising oxide
particles.
The mold composition as recited in claim 5, wherein said oxide particles
comprise at least one of aluminum oxide particles, magnesium oxide particles,
calcium oxide particles, zirconium oxide particles, and titanium oxide
particles.
7. The mold composition as recited in claim 5, wherein said oxide particles are
aluminum oxide particles.
8. The composition as recited in claim 1, wherein said X-ray or Neutron-ray
detectable element comprises ytterbium, hafnium, gadolinium, tungsten,
thorium, uranium, yttrium, dysprosium, erbium, cerium, and compositions
thereof.
9. The composition as recited in claim 1, wherein said X-ray or Neutron-ray
detectable element is in the range of about 1to about 4 weight percent in the
mold composition.
10. A method for detecting sub-surface ceramic inclusions in a titanium or
titanium alloy casting, said method comprising:
combining calcium aluminate, at least one element more
radiographically dense than the calcium aluminate, and a liquid to form a
slurry;
forming a mold having the calcium aluminate and the radiographically
dense element from the slurry;
introducing a titanium aluminide-containing metal to the
radiographically dense element-bearing mold;
solidifying said titanium aluminide-containing metal to form an article
in the mold;
removing the solidified titanium aluminide-containing metal article
from said mold;
subjecting the solidified titanium aluminide-containing article to
radiographic inspection to provide a radiograph; and
examining said radiograph for the presence of the radiographically
dense element on or in the article.
The method as recited in claim 10, further comprising removing the
radiographically dense element from the article.
The method as recited in claim 11, wherein removing the radiographically
dense element from the article comprises one or more of machining, grinding,
polishing, or welding.
The method as recited in claim 10, wherein the combining further comprises
combining oxide particles with the slurry.
The method as recited in claim 13, wherein said oxide particles comprise at
least one of aluminum oxide particles, magnesium oxide particles, calcium
oxide particles, zirconium oxide particles, and titanium oxide particles.
The method as recited in claim 13, wherein said oxide particles are aluminum
oxide particles.
The method as recited in claim 13, wherein the element is more
radiographically dense than the oxide particles.
17. The method as recited in claim 13, wherein the oxide particles comprise
hollow oxide particles.
18. The method as recited in claim 17, wherein the hollow oxide particles
comprise hollow aluminum oxide particles.
19. The method as recited in claim 10, wherein the element more radiographically
dense than the calcium aluminate comprises ytterbium, hafnium, gadolinium,
tungsten, thorium, uranium, yttrium, dysprosium, erbium, cerium and
compositions thereof.
20. The method as recited in claim 10, wherein the titanium-containing cast article
comprises a turbine blade.
2 1. The method as recited in claim 10, wherein the titanium-containing cast article
comprises a titanium aluminide turbine blade.
22. The method as recited in claim 10, further comprising minimizing the
presence of mold material inclusions in titanium aluminide-containing cast
articles.
23. A mold composition comprising:
calcium aluminate cement comprising calcium monoaluminate, calcium
dialuminate, and mayenite; and
at least one element more radiographically dense than the calcium aluminate
cement.
24. The mold composition as recited in claim 23, wherein the mold composition
further comprises oxide particles.
25. The mold composition as recited in claim 24, wherein the radiographically
dense element is further more radiographically dense than the oxide particles.
26. The mold composition as recited in claim 24, wherein said oxide particles
comprise at least one of aluminum oxide particles, magnesium oxide
particles, calcium oxide particles, zirconium oxide particles, and titanium
oxide particles.
27. The mold composition as recited in claim 24, wherein the oxide particles
comprise hollow oxide particles.
28. The mold composition as recited in claim 27, wherein the hollow oxide
particles comprise hollow aluminum oxide particles.