U.S. patent application number 12/237562 was filed with the patent office on 2010-03-25 for unidirectionally-solidification process and castings formed thereby.
This patent application is currently assigned to General Electric Company. Invention is credited to Stephen Joseph Balsone, Andrew J. Elliott, Jon Conrad Schaeffer.
Application Number | 20100071812 12/237562 |
Document ID | / |
Family ID | 42036412 |
Filed Date | 2010-03-25 |
United States Patent
Application |
20100071812 |
Kind Code |
A1 |
Schaeffer; Jon Conrad ; et
al. |
March 25, 2010 |
UNIDIRECTIONALLY-SOLIDIFICATION PROCESS AND CASTINGS FORMED
THEREBY
Abstract
A process capable of producing large metallic castings having
lengths of one hundred centimeters or more and a unidirectional
crystal structure substantially free of freckle defects. The
process includes pouring a molten metal alloy into a preheated mold
within a heating zone, withdrawing the mold from the heating zone,
through a heat shield, and into a cooling zone to directionally
solidify the molten metal alloy, and then cooling the mold to
produce the casting and the unidirectional crystal structure
thereof. The heat shield operates as a barrier to thermal radiation
between the heating zone and the cooling zone, and the mold is
withdrawn at a rate that, in combination with the heat shield,
maintains a thermal gradient to solidify the molten metal alloy and
form primary dendrite arms having an average spacing therebetween
of about 150 to about 500 micrometers.
Inventors: |
Schaeffer; Jon Conrad;
(Simpsonville, SC) ; Balsone; Stephen Joseph;
(Simpsonville, SC) ; Elliott; Andrew J.;
(Westminster, SC) |
Correspondence
Address: |
Hartman & Hartman, P.C.
552 E. 700 N.
Valparaiso
IN
46383
US
|
Assignee: |
General Electric Company
Schenectady
NY
|
Family ID: |
42036412 |
Appl. No.: |
12/237562 |
Filed: |
September 25, 2008 |
Current U.S.
Class: |
148/404 ;
164/122.1 |
Current CPC
Class: |
C30B 11/003 20130101;
C30B 29/52 20130101; F05C 2253/083 20130101; B22D 27/045 20130101;
F05B 2230/21 20130101 |
Class at
Publication: |
148/404 ;
164/122.1 |
International
Class: |
C22C 19/03 20060101
C22C019/03; B22D 27/04 20060101 B22D027/04 |
Claims
1. A process of producing a metallic casting having a length
greater than one hundred centimeters and a unidirectional crystal
structure that is substantially free of freckle defects, the
process comprising: pouring a molten metal alloy into a cavity in a
preheated mold located within a heating zone, the cavity having the
shape of the casting; withdrawing the mold from the heating zone,
through a heat shield, and into a cooling zone to directionally
solidify the molten metal alloy, the heat shield operating as a
barrier to thermal radiation between the heating zone and the
cooling zone, the mold being withdrawn at a rate that, in
combination with the heat shield, maintains a thermal gradient of
greater than 50.degree. C./cm in the molten metal alloy to solidify
the molten metal alloy and form primary dendrite arms having an
average spacing therebetween of about 150 micrometers to about 500
micrometers; and then cooling the mold to produce the casting and
the unidirectional crystal structure thereof that is substantially
free of freckle defects having a size greater than one hundred
centimeters.
2. The process according to claim 1, wherein the casting has a mass
of at least about 18 kg.
3. The process according to claim 1, wherein the thermal gradient
is greater than 80.degree. C./cm.
4. The process according to claim 1, wherein the withdrawal rate is
greater than 1.25 mm/minute.
5. The process according to claim 1, wherein in combination the
thermal gradient and the withdrawal rate result in a cooling rate
of at least 20.degree. C./minute.
6. The process according to claim 1, wherein the average spacing
between the primary dendrite arms is about 325 micrometers up to
about 450 micrometers.
7. The process according to claim 1, wherein the casting is
characterized by a ratio of the average spacing of the primary
dendrite arms to the length of the casting of about 0.75 to about
5.0 micrometers per centimeter.
8. The process according to claim 1, wherein the unidirectional
crystal structure has a columnar single crystal microstructure.
9. The process according to claim 1, wherein the unidirectional
crystal structure has a columnar polycrystalline
microstructure.
10. The process according to claim 1, wherein the metal alloy is
chosen from the group consisting of nickel-base superalloys and
intermetallic alloys.
11. The process according to claim 1, wherein the casting is a
component for a gas turbine.
12. The process according to claim 11, wherein the component is a
last-stage bucket of a land-based gas turbine.
13. The casting produced according to the process of claim 1.
14. The casting according to claim 13, wherein the unidirectional
crystal structure has a columnar single crystal microstructure.
15. The casting according to claim 13, wherein the unidirectional
crystal structure has a columnar polycrystalline
microstructure.
16. The casting according to claim 13, wherein the metal alloy is
chosen from the group consisting of nickel-base superalloys and
intermetallic alloys.
17. The casting according to claim 13, wherein the average spacing
between the primary dendrite arms is about 325 micrometers to about
450 micrometers.
18. The casting according to claim 13, wherein the casting is
characterized by a ratio of the average spacing of the primary
dendrite arms to the length of the casting of about 2.25 to about
3.25 micrometers per centimeter.
19. The casting according to claim 13, wherein the casting is a
component for a gas turbine with a mass of at least about 18
kg.
20. The casting according to claim 19, wherein the component is a
last-stage bucket of a land-based gas turbine.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention generally relates to materials and
processes for producing directionally-solidified castings, and
particularly to reducing defects in alloys cast as single-crystal
(SX) and directionally-solidified (DS) articles suitable for use as
components of gas turbines and other high temperature
applications.
[0002] Components of gas turbines, such as blades (buckets), vanes
(nozzles) and combustor components, are typically formed of nickel,
cobalt or iron-base superalloys characterized by desirable
mechanical properties at turbine operating temperatures. Because
the efficiency of a gas turbine is dependent on its operating
temperatures, there is a demand for components, and particularly
turbine buckets, nozzles, combustor components, and other hot gas
path components, that are capable of withstanding higher
temperatures. As the material requirements for gas turbine
components have increased, various processing methods and alloying
constituents have been used to enhance the mechanical, physical and
environmental properties of components formed from superalloys. For
example, buckets, nozzles and other components employed in
demanding applications are often cast by directional casting
techniques to have DS or SX microstructures, characterized by a
crystal orientation or growth direction in a selected direction to
produce columnar polycrystalline or single-crystal articles.
[0003] As known in the art, directional casting techniques for
producing SX and DS castings generally entail pouring a melt of the
desired alloy into an investment mold held at a temperature above
the liquidus temperature of the alloy. One such process employs a
Bridgman-type furnace to create a heated zone surrounding the mold,
and a chill plate at the base of the mold. Solidification of the
molten alloy within the mold occurs by gradually withdrawing the
mold from the heated zone and into a cooling zone, where cooling
occurs by convection and/or radiation. Solidification initiates at
the base of the mold and the solidification front progresses to the
top of the mold. Solidification is initiated and controlled within
the mold base in a manner that obtains the desired microstructure
for the casting. A high thermal gradient is required at the
solidification front to prevent nucleation of new grains during
directional solidification processes.
[0004] As known in the art, dendrites are tree-like structures that
form during the solidification of a molten metal. The spacing
between dendrite arms in a casting is influenced by the
solidification conditions of the casting, with dendrite arm spacing
varying inversely with cooling rate. As used herein, primary
dendrite arm spacing will be used to denote the average spacing
between cores of adjacent dendrites in a casting, measured by
sectioning the casting in a direction normal to the crystal growth
direction, counting the number of primary arms over the
cross-sectional area, and calculating an average spacing (typically
by assuming a square array). Secondary dendrite arm spacing can be
measured by averaging the spacing between adjacent secondary
dendrite arms observed in a section taken parallel to the crystal
growth direction. Dendrites that form during the solidification of
SX and DS castings can be distinguished from the surrounding
material by differences in concentration of certain alloy
constituents.
[0005] Mechanical properties of DS and SX articles depend in part
on the avoidance of high-angle grain boundaries, equiaxed grains,
and defects resulting from chemical or elemental interdendritic
segregation during the directional solidification process. As an
example, depending on the particular chemistry of the superalloy,
interdendritic segregation can result in inhomogeneities such as
embedded particles and elemental microconstituents of the alloy
chemistry that accumulate in interdendritic regions and tend to
reduce the strength of the casting. The size of the embedded
particles and pools of microconstituents can be significantly
reduced by a reduction in primary dendrite arm spacing in the cast
article. Interdendritic segregation can also result in the
formation of surface freckles, which form during solidification as
chains of very small equiaxed grains. Freckles can reduce fatigue
life and act as grain initiators during the solidification process
that cause unacceptable off-axial grains. Traditional approaches
used to minimize the presence or effect of dendritic segregation
have included post-casting treatments, such as solid state
diffusion heat treatments or mechanical working. However, these
techniques are not feasible for addressing dendritic segregation in
gas turbine components and other castings that are very large or
formed with complex compositions.
[0006] The tendency for freckling has been shown to be dependent on
composition, an example being the level of tantalum and/or carbon
in an alloy. Consequently, freckling has been addressed through
careful control or modifications of superalloy compositions, as
reported in commonly-assigned U.S. Pat. Nos. 5,151,249, 6,091,141
and 6,909,988. Recently, casting process parameters such as
withdraw rate, cooling speed, and the solid-liquid interface
position have also been shown to have an effect on freckle
formation. Commonly-assigned U.S. Pat. No. 6,217,286 to Huang et
al. discloses a high-gradient casting process that reduces
freckling in castings having lengths of up to forty inches (about
one hundred centimeters). Huang et al. teach that a high thermal
gradient at the solidification front can be achieved with a baffle
placed between the mold and cooling zone, for example, a liquid
bath or impingement with an inert gas, to achieve a sufficiently
uniform primary dendrite arm spacing and reduce freckling.
[0007] Huang et al. and other prior efforts to reduce freckling and
other solidification-related defects have been limited to castings
that do not exceed lengths of forty inches (about one hundred
centimeters), due in large part to size and weight limitations
imposed by mold strength, furnace size, etc. Freckling and
size/weight complications associated with directional
solidification have essentially prevented the production and use of
single-crystal and directionally-solidified castings of sufficient
size for certain applications, including the last-stage buckets of
land-based gas turbines. An example is the last-stage buckets of
the H and FB class gas turbines used in the power-generating
industry and manufactured by the assignee of the present invention.
The lengths (about 30 inches (about 75 cm) or more), cross-sections
and weights of these buckets have rendered them very difficult to
produce as SX and DS castings, particularly with respect to
achieving microstructures that can be heat treated to obtain
desired mechanical properties. Consequently, last-stage buckets of
the H and FB class gas turbines have been limited to being produced
as equiaxed castings. However, the ability to produce these buckets
as defect-free SX and DS castings would achieve significantly
improved mechanical properties, such as creep and low-cycle fatigue
(LCF), and would therefore be of great benefit to the overall
performance and efficiency of a large gas turbine.
BRIEF DESCRIPTION OF THE INVENTION
[0008] The present invention provides a process capable of
producing metallic castings having unidirectional crystal
structures and lengths of one hundred centimeters or more, yet are
substantially free of freckle defects. The present invention also
provides metallic castings with unidirectional crystal structures
that are formed by such a process.
[0009] According to a first aspect of the invention, the process
includes pouring a molten metal alloy into a cavity of a preheated
mold within a heating zone, withdrawing the mold from the heated
zone through a heat shield and into a cooling zone to directionally
solidify the molten metal alloy, and then cooling the mold to
produce the casting and the unidirectional crystal structure
thereof that is substantially free of freckle defects having a size
greater than one hundred centimeters. According to preferred
aspects of the invention, the heat shield operates as a barrier to
thermal radiation between the heated zone and the cooling zone, and
the mold is withdrawn at a rate that, in combination with the heat
shield, maintains a thermal gradient of at least 35.degree. C./cm,
for example, 50.degree. C./cm or more, to solidify the molten metal
alloy and form primary dendrite arms having an average spacing
therebetween of about 150 micrometers to about 500 micrometers. A
high cooling rate of at least about 20.degree. C./minute also
appears to be a factor in achieving the desired primary dendrite
arm spacing.
[0010] The unidirectional crystal structure of the casting can be a
columnar single crystal microstructure (SX) with a preferred single
crystal direction of <001>, though crystalline structures
having orientations other than <001> are also within the
scope of the invention, as are columnar polycrystalline
microstructures (DS). Castings that can be produced in accordance
with the invention are well suited for components of a gas turbine,
such as buckets, nozzles, and other components of gas turbines, and
may be formed of nickel-base alloys and intermetallics, for
example, a nickel aluminide (NiAl) intermetallic.
[0011] A significant advantage of this invention is that castings
produced by the process of this invention can far exceed in length,
cross-section, and/or weight what was possible with prior casting
techniques. In particular, heat-treatable, freckle-free last stage
buckets of land-based gas turbines can be produced to have
single-crystal and directionally-solidified microstructures by this
process, whereas last-stage buckets and other castings with lengths
exceeding one hundred centimeters (and correspondingly large
cross-sections and weights) were previously not possible. As such,
the reduction of the incidence of freckling is greater than was
expected for very large SX and DS castings, and the result is the
absence of freckling that would be otherwise expected in SX and DS
castings of these lengths, cross-sections and weights if produced
under conventional processing conditions.
[0012] Other objects and advantages of this invention will be
better appreciated from the following detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 is representative of a turbine bucket that can be
formed as a single-crystal casting in accordance with an embodiment
of the present invention.
[0014] FIGS. 2 and 3 represent cross-sectional views showing two
steps of a casting operation to produce a large single-crystal
turbine blade in accordance with an embodiment of this
invention.
[0015] FIG. 4 is a plot of primary dendrite arm spacing versus
casting length for castings produced in accordance with U.S. Pat.
No. 6,217,286.
[0016] FIG. 5 is a plot of primary dendrite arm spacing versus
casting length for castings produced in accordance with embodiments
of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0017] The present invention provides for the capability of
producing unidirectionally-solidified castings beyond the
capabilities and expectations of prior directional casting
technologies. The invention builds upon a discovery disclosed in
commonly-assigned U.S. Pat. No. 6,217,286 that certain
solidification process conditions, as evidenced by dendrite arm
spacing, are capable of preventing freckles in castings of up to
forty inches (about one hundred centimeters) in length. The
capability provided by the present invention was unexpected from
U.S. Pat. No. 6,217,286, as evident from the teachings of this
patent. Particularly unexpected was the ability to achieve and
maintain a sufficient thermal gradient capable of forming primary
dendrite arms with an acceptable arm spacing that would result in
the prevention of freckling. Because the thermal gradient is
dependent on the cooling rate and solidification rate
(approximately equal to the withdrawal rate) of the casting process
(a simplified statement of this relationship is that thermal
gradient is approximately equal to the cooling rate divided by the
withdrawal rate), the invention requires close control over the
cooling rate and withdrawal rate of the casting process.
[0018] The present invention reduces the tendency for freckling
while achieving mechanical properties for which SX and DS castings
are desired, particularly high temperature strength (including
creep resistance) and fatigue properties for use in such
applications as buckets (blades), nozzles (vanes), and other large
components in the hot gas flow path of a gas turbine. Of particular
interest are the very large castings required for last-stage
turbine buckets used in land-based gas turbines, whose lengths
(exceeding one hundred centimeters), cross-sections, and/or weights
have prevented their manufacture as SX and DS castings. As an
example, FIG. 1 represents a third-stage bucket 10 for a land-based
gas turbine, such as gas turbines used in the power-generating
industry. The bucket 10 has an airfoil 12 and shank 14, with a
dovetail 16 formed on the shank 14 for anchoring the bucket 10 to a
turbine disk (not shown). Depending on the particular application,
the length of the bucket 10, from tip shroud 18 to dovetail 16, may
be about 30 inches (about 75 cm) or more, including lengths of
about 50 inches (about 125 cm) or more. Furthermore, the weight
(mass) of the bucket 10 often exceeds 40 pounds (about 18 kg) and
may exceed 50 pounds (about 23 kg), with weights (mass) of even 100
pounds (about 45 kg) or more also being possible. While the
following discussion will focus on gas turbine buckets, the
invention also applies to a variety of other components, including
components within the combustion section of a gas turbine.
[0019] Buckets of the type represented in FIG. 1 are conventionally
produced as equiaxed castings of precipitation-hardened nickel-base
superalloys, such as IN738, Rene 77, or Udimet.RTM. 500. As is in
the case of first and second-stage turbine buckets, the performance
of the bucket 10 could be significantly improved if it could be
unidirectionally cast to have a columnar single crystal (SX) or
columnar polycrystalline (DS) microstructure. The advantages of
this invention will be described with reference to the bucket 10 of
FIG. 1, though the teachings of this invention are generally
applicable to other large components that could benefit from being
unidirectionally cast.
[0020] The bucket 10 would also benefit from being cast from more
advanced high-temperature materials. Of particular interest are
superalloys specifically formulated for casting as SX and DS
castings, and intermetallics such as nickel aluminide (NiAl)
intermetallic materials. Particular nonlimiting examples of
superalloys that could be used include Rene N5 (nominal
composition, by weight, about 7.5% Co, 7.0% Cr, 6.5% Ta, 6.2% Al,
5.0% W, 3.0% Re, 1.5% Mo, 0.15% Hf, 0.05% C, 0.004% B, 0.01% Y, the
balance nickel), Rene N4 (nominal composition, by weight, about
9.75% Cr, 7.5% Co, 4.2% Al, 3.5% Ti, 1.5% Mo, 6.0% W, 4.8% Ta, 0.5%
Nb, 0.15% Hf, 0.05% C, 0.004% B, the balance nickel), GTD111
(nominal composition, by weight, about 14.0% Cr, 9.5% Co, 3.0% Al,
4.9% Ti, 1.5% Mo, 3.8% W, 2.8% Ta, 0.010% C, the balance nickel),
and GTD444 (nominal composition, by weight, about 9.75% Cr, 7.5 Co,
3.5% Ti, 4.2% Al, 6% W, 1.5% Mo, 4.8% Ta, 0.08% C, 0.009% Zr,
0.009% B, the balance nickel).
[0021] As known in the art, freckles form in part as a result of
molten metal convection in the casting mold which disrupts the
unidirectional solidification process, producing irregularities
seen on SX and DS casting surfaces as little chains of equiaxed
crystals. Furthermore, freckles can act as grain initiators during
the solidification process that cause unacceptable off-axial
grains, and may reduce fatigue life of the casting. According to
one aspect of the invention, external and internal freckling can be
inhibited and even eliminated in SX and DS castings exceeding one
hundred centimeters in length (with correspondingly large
cross-sections and weights) by achieving greater control of the
primary dendrite arm spacing at these casting lengths, more
particularly achieving finer dendrite arm spacing at these casting
sizes to reduce buoyancy impact, which in turn can be attained by
improving the thermal separation between the heating and cooling
zones of a unidirectional casting process to achieve an even
greater thermal gradient (for example, 80.degree. C./cm and higher)
at the solidification front of the casting. In particular, U.S.
Pat. No. 6,217,286 utilized a thermal gradient of up to about
80.degree. C./cm to obtain primary dendrite arm spacings of about
150 micrometers to less than 800 micrometers, preferably about 150
micrometers to about 650 micrometers, and more preferably about 150
micrometers to about 350 micrometers for castings of up to one
hundred centimeters. These spacings roughly correspond to arm
spacing-casting length ratios of up to about 8.0
micrometers/centimeter for a 100-cm casting. In comparison, the
present invention achieves freckle-free SX castings by narrowly
limiting the primary dendrite arm spacing to a range of about 150
micrometers to about 500 micrometers, more preferably about 250 to
about 450 micrometers, and most preferably about 325 to 450
micrometers (about 13 to about 18 mils) for castings exceeding one
hundred centimeters, corresponding to a maximum arm spacing-casting
length ratio of not more than 5 micrometers/centimeter and
preferably not more than 4.5 micrometers/centimeter for a 100-cm
casting. For comparison, FIG. 4 plots the preferred and most
preferred ranges from the U.S. Pat. No. 6,217,286 patent as lines
50 and 48, respectively, over casting lengths of four to forty
inches (about ten to about one hundred centimeters), while FIG. 5
plots the ranges 52 and 54 for, respectively, the preferred and
most preferred primary dendrite arm spacings of this invention for
castings with lengths that exceed forty inches (one hundred
centimeters) For the sake of comparison, the line 50 for the
preferred range of U.S. Pat. No. 6,217,286 is plotted beyond
casting lengths of forty inches, though there was no expectation or
suggestion in U.S. Pat. No. 6,217,286 that castings longer than
forty inches could be cast under these conditions without producing
unacceptably high levels of freckles. Furthermore, ceramic molds
available to Huang et al. limited the size of castings that could
have been considered by Huang et al.
[0022] In view of the above, an important aspect and unexpected
result of the present invention is that large castings with lengths
exceeding one hundred centimeters (with correspondingly large
cross-sections and weights) can be manufactured having a fine
dendrite arm spacing of not more than 500 micrometers, for example,
about 150 to about 500 micrometers, to avoid freckling. More
particularly, the primary dendrite arm spacing is most preferably
between 325 and 450 micrometers. The targeted spacing may be
correlated to the length of the casting, corresponding to a
spacing/length ratio of about 0.75 to about 5.0, or more narrowly
about 1.625 to about 4.5 micrometers per centimeter, and more
preferably about 2.25 to about 3.25 micrometers per centimeter.
Other potential casting defects may also be minimized with this
invention, including high angle boundaries that tend to form at
protruded sections of castings, and grains that form streaks
(slivers) in the microstructure.
[0023] FIGS. 2 and 3 represent a shell mold 20 of a type suitable
for producing a single-crystal casting of this invention. As known
in the art, the mold 20 is preferably formed of a material such as
alumina or silica, and has an internal cavity 22 corresponding to
the desired shape of a casting 32, represented as a turbine bucket.
As such, the cavity 22 is configured to produce the casting 32 with
an airfoil portion 34, shank 36, and dovetail 38, and may contain
cores (not shown) for the purpose of forming cooling passages
within the casting 32. The mold 20 is shown secured to a chill
plate 24 and placed in a heating zone 26 (for example, a Bridgman
furnace) to heat the mold 20 to a temperature equal to or above the
melting temperature of the alloy, and more particularly above the
liquidus temperature of the alloy. A cooling zone 42 is represented
as being located directly beneath the heating zone 26, and a baffle
or heat shield 44 is represented as being between and separating
the heating and cooling zones 26 and 42. The cooling zone 42 may be
a tank containing a liquid cooling bath 43, such as a molten metal,
or a radiation cooling tank that may be evacuated or contain a gas
at ambient or cooled temperature. The cooling zone 42 may also
employ gas impingement cooling (for example, see U.S. Pat. No.
7,017,646 to Balliel et al.) or a fluidized bed (for example, see
U.S. Pat. No. 6,443,213). Particularly suitable liquids for the
cooling bath 43 include molten tin at a temperature of about 235 to
about 350.degree. C. and molten aluminum at a temperature of up to
about 700.degree. C., with molten tin believed to be especially
suitable because of its low melting temperature and low vapor
pressure.
[0024] The heat shield 44 is situated to be in close contact with
the lower end of the heating zone 26 and the cooling zone 42, and
in the case of a cooling bath 43 may float on its surface. The
purpose of the heat shield 44 is to insulate the cooling zone 42
from the heating zone 26, and particularly form a barrier to
thermal radiation emitted by the heating zone 26, thereby promoting
a steep thermal gradient between the mold 20 and the cooling bath
43. The heat shield 44 may be a single layer or multiple layers of
rigid and/or flexible thermal barrier materials, such as flowing
graphite raft, a refractory felt material, or a high melting point
metal. The heat shield 44 is configured to have a variable-sized
opening 45 that, as represented in FIG. 2, enables the heat shield
44 to fit closely around the shape of the mold 20 as it is
withdrawn from the heating zone 26, through the heat shield 44, and
into the liquid cooling bath 43.
[0025] The casting process is preferably carried out in a vacuum or
an inert atmosphere, with the mold 20 preheated to a temperature
above the alloy's liquidus temperature, as a nonlimiting example,
about 1370.degree. C. to about 1600.degree. C. The molten alloy is
poured into the preheated mold 20, after which unidirectional
solidification is initiated by withdrawing the base of the mold 20
and chill plate 24 downwardly at a fixed withdrawal rate into the
cooling zone 42, until the mold 20 is entirely within the cooling
zone 42 as represented in FIG. 3. The temperature of the chill
plate 24 is preferably maintained at or near the temperature of the
cooling zone 44, such that dendritic growth begins at the lower end
of the mold 20 and the solidification front travels upward through
the mold 20. The casting 32 grows epitaxially (for example, with
the <100> orientation) based on the crystalline structure and
orientation of a small block of single-crystal seed material 28 at
the base of the mold 20, from which a single crystal forms from a
crystal selector 30, for example, a pigtail sorting structure. The
columnar single crystal becomes larger in the enlarged section of
the cavity 22. A bridge 40 connects protruding sections of the
casting 32 with lower sections of the casting 32 so that a
unidirectional columnar single crystal forms substantially
throughout the casting 32. The casting 32 is deemed to be a
substantially columnar single crystal if it does not have high
angle grain boundaries, for example, greater than about twenty
degrees.
[0026] Uniform primary dendrite arm spacings are achieved by the
strong unidirectional thermal gradients imposed on the casting 32
as a result of the heat shield 44 and the cooling zone 42.
According to a preferred aspect of the invention, the thermal
gradient at the solidification front of the casting 32 is greater
than 35.degree. C./cm, preferably greater than 50.degree. C./cm,
and more preferably greater than 80.degree. C./cm. Thermal
gradients of less than 50.degree. C./cm and particularly less than
30.degree. C./cm are believed to be unacceptable for attaining the
primary dendrite arm spacing in large castings of primary interest
to this invention. Based on their mathematical relationship, the
high thermal gradients of this invention also require high cooling
rates relative to the withdrawal rate used, the latter of which can
be up to at least twenty inches/hour (about 8.5 mm/minute).
[0027] Those skilled in the art will appreciate that a DS casting
can be produced in a similar manner, though with modifications to
the mold 20, such a growth zone at the base of the mold 20 that is
open to the chill plate 24, and omission of the seed material 28
and/or crystal selector 30.
[0028] In experiments leading to the present invention, last-stage
buckets similar to the representation of FIG. 1 were cast. The
castings were about 30 to about 50 inches (about 750 to about 1250
millimeters) in length. The compositions of the buckets were the
nickel-based superalloys Rene N4 and GTD444, both of which are
specifically formulated for SX and DS castings. Single-crystal
castings were then prepared in accordance with commercial practices
for the alloys, generally in accordance with the casting process
described above. The casting molds were about ten inches (about 25
cm) longer than their respective castings, and filled to contain up
to about 400 lbs. (about 180 kg) of molten alloy. The casting
furnace temperature was about 2750.degree. F. (about 1510.degree.
C.). Cooling was by a liquid bath of molten tin maintained at a
temperature of about 240.degree. C., and the thermal gradient in
the castings during cooling was about 85.degree. C./cm. A
conventional withdrawal rate of about three inches/hour (about 1.25
mm/minute) was used to produce a casting as a baseline comparison,
while other castings were produced using higher experimental rates
of about six to twelve inches/hour (about 2.5 to about 5
mm/minute). Based on these values, the cooling rates were about
10.degree. C./minute when using the conventional withdrawal rate
(three inches/hour), and about 20 to about 40.degree. C./minute
using the higher withdrawal rates (six and twelve inches/hour).
[0029] Following the casting operation, primary dendrite arm
spacings in the castings were measured by metallography, and
evidence of freckling was examined by macro-etching the casting
surfaces, followed by metallographic examination. From the
examinations, it was observed that the grain structure broke down
in those casting produced at the conventional withdrawal rate
(corresponding to a cooling rate of about 10.degree. C./minute).
Furthermore, the casting had many freckles at its thicker sections
and at sections where the cross-section changed, such as the tip
shroud. Dendrite spacing was about 25 to about 30 mils (about 635
to about 760 micrometers), which is within the broad range accepted
in Huang et al. Nonetheless, the casting was unable to be heat
treated due to excessive segregation in its microstructure,
resulting in incipient melting.
[0030] In contrast, the experimental castings produced at the
higher withdrawal rates (corresponding to cooling rates of about 20
to about 40.degree. C./minute) did not contain any freckles.
Furthermore, the grains were straight, evidencing that grain growth
was not influenced by other heat extraction directions. Dendrite
spacing was about 16 to about 21 mils (about 400 to about 530
micrometers), corresponding to a minimum dendrite spacing to
casting length ratio of about 0.32 .mu.m/cm. Finally, and
significantly, their microstructures allowed the castings to be
heat treated to obtain desired mechanical properties for the
buckets.
[0031] Based on these results, it was concluded that SX and DS
castings formed of superalloys formulated for SX and DS and cast to
sizes produced in the experiment (lengths of about 760 mm (about 30
inches) and longer, weights exceeding 40 pounds (about 18 kg), and
correspondingly large cross-sections benefit from relatively high
thermal gradients (preferably greater than 80.degree. C./cm and
more preferably about 85.degree. C./cm or more). The benefit of
high thermal gradients appears to also depend on the use of
relatively high cooling rates (greater than 10.degree. C./minute,
such as about 20.degree. C./minute or more) relative to withdrawal
rate. The process may also employ relatively high withdrawal rates,
for example, greater than 1.25 mm/minute, such as about 2.5 to
about 5 mm/minute, though lower and higher withdrawal rates are
also within the scope of this invention.
[0032] While the invention has been described in terms of specific
embodiments, it is apparent that other forms could be adopted by
one skilled in the art. For example, the physical configuration of
the castings could differ from that shown, and materials and
processes other than those noted could be used. Therefore, the
scope of the invention is to be limited only by the following
claims.
* * * * *