U.S. patent application number 12/129867 was filed with the patent office on 2009-12-03 for high thermal gradient casting with tight packing of directionally solidified casting.
Invention is credited to Laura J. Carroll, Douglas G. Konitzer, Joseph D. Rigney.
Application Number | 20090293994 12/129867 |
Document ID | / |
Family ID | 41152117 |
Filed Date | 2009-12-03 |
United States Patent
Application |
20090293994 |
Kind Code |
A1 |
Konitzer; Douglas G. ; et
al. |
December 3, 2009 |
HIGH THERMAL GRADIENT CASTING WITH TIGHT PACKING OF DIRECTIONALLY
SOLIDIFIED CASTING
Abstract
Method for forming directionally solidified articles in tightly
packed mold cavities withdrawn into liquid metal cooling bath. The
withdrawal rate and the spacing between adjacent mold cavities
cooperate to provide a high thermal gradient. The cast articles
exhibit primary dendrite arm spacing of between about 6 to about 12
mils (about 150-300 microns).
Inventors: |
Konitzer; Douglas G.; (West
Chester, OH) ; Rigney; Joseph D.; (Milford, OH)
; Carroll; Laura J.; (Ammon, ID) |
Correspondence
Address: |
GENERAL ELECTRIC COMPANY
GE AVIATION, ONE NEUMANN WAY MD H17
CINCINNATI
OH
45215
US
|
Family ID: |
41152117 |
Appl. No.: |
12/129867 |
Filed: |
May 30, 2008 |
Current U.S.
Class: |
148/426 ;
164/122.1; 164/271 |
Current CPC
Class: |
B22D 27/045 20130101;
C30B 11/003 20130101; C30B 35/002 20130101; F05C 2253/0831
20130101; C22C 19/057 20130101; C30B 11/002 20130101; F05B 2230/211
20130101; C30B 29/52 20130101 |
Class at
Publication: |
148/426 ;
164/122.1; 164/271 |
International
Class: |
C22C 19/03 20060101
C22C019/03; B22D 27/04 20060101 B22D027/04; B22C 9/00 20060101
B22C009/00 |
Claims
1. A method of making a plurality of directionally solidified
articles comprising: adding a molten superalloy metal to a
plurality of cavities in a mold in a heated zone, each cavity being
defined at least in part by an associated mold wall spaced at a
predetermined minimum spacing from an adjacent cavity, wherein each
of the cavities is shaped to form at least one cast article,
withdrawing the mold with the molten superalloy metal from the
heated zone into a liquid metal cooling tank at a predetermined
withdrawal rate; wherein the withdrawal rate and the spacing
cooperate to provide a thermal gradient sufficient to solidify the
molten metal to form a plurality of directionally solidified cast
articles each having primary dendrite arm spacing of between about
6 to about 12 mils (about 150 to about 300 microns).
2. The method according to claim 1 wherein the mold includes at
least a first cavity group located in an outer portion of the mold
and a second cavity group located in an inner portion of the
mold.
3. The method according to claim 1 wherein the minimum spacing
between adjacent cavities is between about 1/8 to about 7/8 inch
(about 3 to about 22 mm).
4. The method according to claim 1 wherein the withdrawal rate is
greater than 12 to about 50 in/hr. (greater than 30 to about 127
cm/hr).
5. The method according to claim 4 wherein the withdrawal rate is
selected from greater than about 12 to about 20 in/hr (greater than
about 30 to about 50 cm/hr), about 16 to about 30 in/hr (about 40
to about 76 cm/hr), about 20 to about 30 in/hr (about 50 to about
76 cm/hr), and about 16 to about 50 in/hr (about 40 to about 127
cm/hr).
6. The method according to claim 1 wherein adding the molten
superalloy metal includes adding a superalloy metal comprising, in
weight percent: Al 6.2, Ta 6.5, Cr 7, W 5, Mo 1.5, Re 3, Co 7.5, C
0.05, B 0.004, Hf 0.15, balance nickel and incidental
impurities.
7. The method according to claim 1 wherein at least one of the mold
cavities includes an upper and lower molding region, wherein each
molding region is shaped to form a cast article.
8. The method according to claim 1 wherein the minimum spacing
between adjacent cavities is between about 1/8 to about 7/8 inch
(about 3 to about 22 mm) and the withdrawal rate is at least one of
about 12 to about 20 in/hr (about 30 to about 50 cm/hr), about 16
to about 30 in/hr (about 40 to about 76 cm/hr), and about 20 to
about 30 in/hr (about 50 to about 76 cm/hr), and about 16 to about
50 in/hr (about 40 to about 127 cm/hr).
9. The method according to claim 1 wherein the withdrawal rate and
the spacing cooperate to provide a thermal gradient sufficient to
solidify the molten metal to form the plurality of directionally
solidified cast articles each having primary dendrite arm spacing
of between about 8 to about 10 mils (about 200 to about 250
microns).
10. The method according to claim 1 wherein: adding the molten
superalloy metal includes adding a superalloy metal comprising, in
weight percent: Al 6.2, Ta 6.5, Cr 7, W 5, Mo 1.5, Re 3, Co 7.5, C
0.05, B 0.004, Hf 0.15, balance nickel and incidental impurities;
the minimum spacing between adjacent cavities is between about 1/8
inch to about 7/8 inch (about 3 mm to about 22 mm) and the
withdrawal rate is at least one of about 12 to about 20 inches/hour
(about 30 to about 50 cm/hr), about 16 to about 30 inches/hour
(about 40 to about 76 cm/hr), and about 20 to about 30 inches/hour
(about 50 to about 76 cm/hr), and about 16 to about 50 in/hr (about
40 to about 127 cm/hr); and the withdrawal rate and the spacing
cooperate to provide a thermal gradient to solidify the molten
metal to form the plurality of directionally solidified cast
articles each having primary dendrite arm spacing of between 6 to
about 10 mils (about 150 to about 250 microns).
11. A directionally solidified article prepared by the method
according to claim 1.
12. The directionally solidified article according to claim 11
comprising a blade for a gas turbine engine.
13. A mold utilized in the formation of a plurality of
directionally solidified articles prepared by the method according
to claim 1.
14. A method for reducing solidification defects in a directionally
solidified cast article comprising a high-refractory nickel base
superalloy, the method comprising: adding a molten high-refractory
nickel base superalloy metal to at least one cavity in a mold in a
heated zone, wherein the cavity is shaped to form at least one cast
article; withdrawing the mold with the molten superalloy metal from
the heated zone into a liquid metal cooling tank at a predetermined
withdrawal rate; wherein the withdrawal rate is sufficient to
provide a thermal gradient to solidify the molten metal to form at
least one directionally solidified cast article having primary
dendrite arm spacing of between about 6 to about 12 mils (about 150
to about 300 microns); and wherein the primary dendrite arm spacing
provides a reduction in solidification defects in the at least one
cast article relative to an amount of solidification defects in a
cast article comprising a comparable high-refractory nickel base
superalloy formed using a Bridgman technique.
15. A cast article comprising: a directionally solidified high
refractory nickel base superalloy and exhibiting a primary dendrite
arm spacing of from about 6 to about 12 mils.
16. The cast article according to claim 13 comprising a high
pressure gas turbine engine blade.
Description
BACKGROUND OF THE INVENTION
[0001] This invention relates generally to superalloy casting
methods, and more specifically to casting methods for producing
directionally solidified articles exhibiting fine dendrite arm
spacing.
[0002] The mechanical properties of cast superalloy articles
improve by applying directional casting techniques to produce
columnar-grained or single crystal articles.
[0003] Directional casting techniques used to manufacture such
articles start with a mold shaped to produce the desired cast
article. One such process of manufacturing directionally solidified
cast articles employs a Bridgman-type furnace and comprises the
pouring of molten metal into a mold within a heated zone. A chill
plate cools the base of the mold. Subsequent solidification of the
molten metal occurs by controlled withdrawal of the mold from the
heated zone. The mold is initially cooled through the chill plate
by conduction and then radiation as the metal solidifies upward
along the length of the mold.
[0004] Another process for making directionally solidified cast
articles is discussed in U.S. Pat. No. 3,763,926 by Tschinkel et
al. The process includes pouring molten metal into a superheated
mold positioned in a heated zone and withdrawing the mold from the
furnace into a liquid metal coolant bath. The liquid metal bath is
used to obtain high thermal gradients during the directional
solidification process.
[0005] The quality and structure of the directionally solidified
cast article still needs refinement. Certain mechanical properties
are controlled by the microstructure of the cast materials. Due to
the dendritic nature of the solidification, certain elements
segregate to the dendrite and others to the interdendritic region.
The last metal to solidify is in the interdendritic regions and
thus porosity and eutectic pools are located herein. As a result,
the properties of the cast alloy are decreased by such
inhomogeneities. The size of the porosity, carbides, and eutectic
pools is significantly reduced by a reduction in primary dendrite
arm spacing in the cast article. The primary dendrite arm spacing
is the average spacing between adjacent dendrite cores. This
spacing is measured normal to the crystal growth direction by the
average number of dendrite cores per area. Secondary dendrite arm
spacing is the average spacing between adjacent secondary dendrite
arms as observed on a section perpendicular to the growth
direction. Thus, there is a need to produce unidirectional cast
articles with minimal primary and secondary dendrite arm spacing to
achieve superior mechanical and physical properties.
[0006] Dendrite arm spacing is also directly related to the
solidification conditions during casting. Dendrite arm spacing
varies inversely with cooling rate (solidification rate times
thermal gradient). High thermal gradients are required to prevent
nucleation of new grains during directional solidification. In
known processes, the casting cavities for single crystal and
columnar-grained processes are spaced a relatively large distance
away from one another in a mold to avoid re-radiation of heat from
mold/casting to mold/casting. The spacing is used to promote
uniform thermal gradients to thereby avoid coarse dendritic
microstructure and solidification defects. Furthermore, the rate of
heat extraction from the mold/casting limits the rate at which the
mold can be withdrawn from the hot zone without forming
solidification defects.
[0007] Accordingly, it would be desirable to provide a method to
form cast parts having the desired microstructure utilizing
optimized casting packing and withdrawal rate.
BRIEF DESCRIPTION OF THE INVENTION
[0008] The above-mentioned need or needs may be met by exemplary
embodiments which provide a method of making a plurality of
directionally solidified articles. An exemplary method includes
adding a molten superalloy metal to a plurality of cavities in a
mold in a heated zone, each cavity being defined at least in part
by an associated mold wall spaced at a predetermined minimum
spacing from an adjacent cavity. Each mold cavity is shaped to form
at least one cast article. In the exemplary embodiment, the mold is
withdrawn from the heated zone into a liquid metal cooling tank at
a predetermined withdrawal rate, wherein the withdrawal rate and
the spacing cooperate to provide a thermal gradient sufficient to
solidify the molten metal to form a plurality of directionally
solidified cast articles each having primary dendrite arm spacing
of between about 6 to about 12 mils (about 150-300 microns).
[0009] An exemplary embodiment provides a method for reducing
solidification defects in a directionally solidified article. The
article may comprise a high-refractory nickel base superalloy. The
exemplary method includes adding a molten high-refractory nickel
base superalloy metal to at least one cavity in a mold in a heated
zone, wherein the cavity is shaped to form at least one cast
article and withdrawing the mold with the molten superalloy metal
from the heated zone into a liquid metal cooling tank at a
predetermined withdrawal rate. The withdrawal rate is sufficient to
provide a thermal gradient sufficient to solidify the molten metal
to form the directionally solidified cast article having primary
dendrite arm spacing of between about 6 to about 12 mils (about 150
to about 300 microns). The primary dendrite arm spacing provides a
reduction in solidification defects in at least one cast article
relative to an amount of solidification defects in a cast article
comprising a comparable high-refractory nickel base superalloy
formed using a Bridgman technique.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] The subject matter which is regarded as the invention is
particularly pointed out and distinctly claimed in the concluding
part of the specification. The invention, however, may be best
understood by reference to the following description taken in
conjunction with the accompanying drawing figures in which:
[0011] FIG. 1 is a top schematic view of a casting mold for forming
a plurality of cast articles.
[0012] FIG. 2 is a cross sectional view of the casting mold taken
along the line 2-2 of FIG. 1.
[0013] FIG. 3 is a schematic representation of an alternate casting
mold.
[0014] FIG. 4 is a schematic representation showing exemplary cast
articles formed in the mold shown in FIG. 1
DETAILED DESCRIPTION OF THE INVENTION
[0015] Referring to the drawings wherein identical reference
numerals denote the same elements throughout the various views,
FIG. 1 shows a mold 10 defining a plurality of mold cavities 12.
Each mold cavity 12 is defined, at least in part, by an outer mold
wall 16 (see FIG. 2.). With reference again to FIG. 1, in an
exemplary embodiment, the mold cavities 12 are arranged in a
generally circular manner relative to a mold center 18. Further, in
an exemplary embodiment, the mold cavities 12 are arranged in at
least two groupings defined according to a relationship to the mold
center 18. For example, an exemplary mold 10 includes a first
cavity group 20 located in an outer portion 22 of the mold and a
second cavity group 30 located in an inner portion 32 of the mold.
In an exemplary embodiment, castings formed in the first cavity
group 20 exhibit microstructures substantially similar to the
microstructures of the castings formed in the second cavity group
30. In an exemplary embodiment, the molding cavities 12 are
arranged in consideration of the geometry of the cast articles
formed therein in order to maximize the number of castings that can
be made using the mold 10 while achieving the desired
microstructure. In other exemplary embodiments, the mold cavities
may be arranged in a variety of arrays such as rectangular, linear,
irregular, and the like. The spacing between adjacent cavities and
the withdrawal rate cooperate to provide the desired structure of
the cast articles, as discussed in greater detail below.
[0016] Referring to FIG. 2, in an exemplary embodiment, the mold
walls 16 of adjacent molding cavities are spaced a sufficient
distance, D, as measured from the outer surfaces 32, to allow a
cooling liquid metal to circulate and provide the necessary thermal
gradients as discussed in greater detail below. In an exemplary
embodiment, D is greater than or equal to a predetermined minimum
distance.
[0017] FIG. 3 illustrates an alternate exemplary embodiment, in
which a mold 50 includes molding cavities 52 including upper and
lower molding regions 54, 56, respectively. In the exemplary mold
50, each molding cavity 52 forms a plurality of articles (e.g., two
gas turbine engine blades).
[0018] An exemplary embodiment provides an optimum casting process
for producing directionally solidified articles, such as nozzles,
airfoils, and shrouds in a tightly packed arrangement. As used
herein, the term "directionally solidified" refers to either
columnar-grained or single crystal microstructure, as will be
understood by those having skill in the art.
[0019] The mold configuration, such as spacing between adjacent
outer mold surfaces, and withdrawal rate are interrelated. An
optimized combination of spacing and withdrawal rate enables
solidification of castings with properties similar to articles
formed by less-densely packed processes and/or by the Bridgman
method. In an exemplary embodiment, directionally solidified cast
articles may be formed in a mold arrangement having minimal spacing
D between adjacent outer mold surfaces. In an exemplary embodiment,
the spacing D may be as low as about 1/8'' (about 3 mm).
[0020] In an exemplary embodiment, the mold (e.g., mold 10, mold
50) is used in conjunction with a liquid metal coolant bath. In
order to form castings having the desired microstructure in a
densely packed mold, the liquid metal coolant must be able to
provide the necessary high temperature gradient. An exemplary
coolant is liquid tin (Sn).
[0021] The optimized spacing between the mold walls (e.g., mold
walls 16) of adjacent mold cavities allows circulation of the
liquid metal coolant between the molding cavities in order to
extract heat. The circulating flow of the liquid tin bath provides
sufficiently high thermal gradients during cooling to produce
primary dendrite arm spacing as fine as 6 mils (about 200 microns)
when withdrawal rates equal to or greater than 12 in/hr are used.
In an exemplary embodiment, the withdrawal rate with such tight
packing of the mold cavities may be approximately 12 to 20 in/hr
(about 30-50 cm/hr).
[0022] Additionally, withdrawal rates of approximately 20 to 30
in/hr (about 50 to 76 cm/hr)may be utilized with less densely
packed molds, i.e., wider spaced outer mold surfaces. In other
exemplary embodiments, the withdrawal rate may be as high as 50
in/hr (about 127 cm/hr).
[0023] For some applications, such as high pressure gas turbine
engine blades, the optimal dendrite arm spacing may be
approximately 6 mils (about 150 microns). Practically speaking, the
primary dendrite arm spacing may be slightly larger than the
optimum value, or about 8 mils (203 micron) in thicker sections
such as a blade root.
[0024] In an exemplary embodiment, a liquid metal cooling process
is utilized to provide directionally solidified articles such as
nozzles, shrouds, and airfoils, which are formed in a densely
packed alumina/silica mold. The articles cast by the exemplary
methods exhibit desired microstructures to provide mechanical
properties that are similar, or superior, to mechanical properties
exhibited by articles cast using prior methods. It is believed that
the fine primary dendrite arm spacing promotes the reduction or
elimination of solidification defects. The exemplary methods
disclosed herein may be further useful for casting higher
refractory nickel-base superalloys. For example, superalloy
compositions including increased amounts of rhenium or tungsten,
which may provide undesirable solidification defects when cast in
prior processes, may be utilized in the exemplary embodiments
disclosed herein.
[0025] FIG. 4 illustrates a plurality of gas turbine engine blades
60 arranged as cast in an exemplary mold. Of course, other
arrangements are contemplated within the scope of the invention.
For example, the molding cavities may be provided so that during
casting, the cast articles are similarly oriented. Other
arrangements and orientations may be utilized. The desired casting
arrangement and orientation may be dependent on the type of article
cast, the casting material, the shape of the cast article, and the
like. Articles, such as blades 60, formed according to the
exemplary processes disclosed herein may exhibit a directionally
solidified microstructure with PDAS as fine as about 6 mils (about
150 microns).
EXAMPLE 1
[0026] Two mold configurations, parallel plate molds and bar molds,
were used to determine the relationship necessary for process
optimization of tightly packed directional solidified articles by
the liquid metal cooling process.
[0027] Three molds each with three parallel plates of dimensions 2
inches by 5 inches by 0.5 inches were cast in a DS withdrawal
furnace equipped with a liquid metal tin bath for high gradient
solidification. The three molds each had three plates spaced 1.5
inches, 1.0 inches, and 0.8 inches, respectively. The outer mold
surfaces were spaced 7/8 inches, 3/8 inches, and 1/8 inch apart
between adjacent outer mold surfaces.
[0028] The nickel base superalloy utilized in the exemplary molding
process has a nominal composition, in weight percent, of: Al 6.2,
Ta 6.5, Cr 7, W 5, Mo 1.5, Re 3, Co 7.5, C 0.05, B 0.004, Hf 0.15,
balance nickel and incidental impurities. This particular
composition is known as Rene N5 and is suitable for use in
directional solidification processes.
[0029] The parallel plate molds/castings were withdrawn from the
hot zone at 16 in/hr in a withdrawal furnace using the liquid metal
cooling capability. The cast plates were single crystals, free from
solidification defects and had a fine dendritic microstructure with
primary dendrite arm spacings (PDAS) as low as 8 mils.
[0030] The dendritic microstructure of the plates disposed at the
center of the mold were compared to the microstructure of the
plates disposed toward the outer regions of the mold. The dendritic
microstructure of the central cast plates were substantially
similar to the microstructure of the outer cast plates. See Table
1.
[0031] The six bar mold configuration in which the castings were
spaced at greater than 1 inch apart produced castings with PDAS of
approximately 10 mils at a slightly faster withdrawal rate of 20
in/hr. The data suggests uniform cooling of all the plates and the
ability of the liquid tin to remove the heat uniformly from tightly
packed castings. The PDAS from the castings in the 20 in/hr run was
similar to the tightly spaced cast plates indicating the ability of
the liquid tin to uniformly cool tightly packed castings.
EXAMPLE 2
[0032] A second mold configuration was a six bar single crystal
mold in which the bars were spaced slightly more than one inch
apart (between adjacent cast surfaces). The mold was withdrawn from
the hot zone at different rates up to 50 in/hr. In this particular
example, it was found that, for cast articles of similar size to an
airfoil, rates over 35 in/hr were not optimal in terms of both
dendritic microstructure and casting defects. The 30 in/hr
withdrawal rate resulted in defect-free single crystal castings
with microstructures between 5 and 6 mils. For withdrawal rate and
microstructure comparisons, the experimental bars are believed to
be sufficiently similar to the desired airfoil structures.
[0033] Low temperature low cycle fatigue data has shown
improvements in cycle life of over an order of magnitude. The
improvement in the low temperature low cycle fatigue life is
believed to be related to the primary dendrite arm spacing. For
example improvements are found in PDAS of 10 mils as compared to 14
mils, even greater improvements with PDAS of 8 mils, and more with
PDAS of 6 mils. It is believed the prior Bridgman process is not
capable of producing these fine dendrite arm spacings in
solidification defect-free castings.
[0034] Thus, cast parts having the desired microstructure can be
formed in closely packed molding cavities by withdrawal in a liquid
coolant at faster withdrawal rates than previously known. Further,
molds containing more tightly packed directionally solidified
casting than previously realized can be used for the solidification
of single crystal or columnar-grained articles with either a
similar microstructure or a finer dendritic microstructure than
previous realized with typical Bridgman castings. Such "close
packing" may occur in a variety of mold cavity arrays including
circular, rectangular, and the like. Further, the molding cavities
are not confined to a regular or symmetric array. However, the
spacing between cavities should enable a sufficient thermal
gradient at a selected withdrawal rate to form cast articles with
the desired microstructure. Further, it is desired that each cast
article exhibit a desired microstructure regardless of cavity
location. Thus, it is desired that cast articles formed in a first
cavity group, such as an outer region of a mold be substantially
similar to articles formed in a second cavity group, such as in an
inner region of the mold. Further, this tighter packing of castings
could be realized in three dimensions, i.e., castings can be
vertically stacked as well as tightly packed around the mold. For
example, castings could be packed in a circular manner in several
layers around the center of the mold and then additionally packed a
plurality of layers vertically.
[0035] Based on the results of the Examples provided above, it is
envisioned that other nickel base superalloys may be utilized in
the exemplary processes disclosed herein to achieve the fine
primary dendrite arm spacing and thereby reduce solidification
defects in directionally solidified cast articles. The reduction in
solidification defects may be particularly useful for high
refractory nickel base compositions.
[0036] This written description uses examples to disclose the
invention, including the best mode, and also to enable any person
skilled in the art to make and use the invention. 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 languages
of the claims.
TABLE-US-00001 TABLE 1 PDAS Standard Dev. Mold Spacing Plate
Location (mils) (mils) 1/8 inch Outer Plate Top 9.5 0.2 spacing
Outer Plate Bottom 8.4 0.2 Central Top 10.5 0.6 Plate Central
Bottom 8.4 0.9 Plate 3/8 inch Outer Plate Top 8.5 0.4 spacing Outer
Plate Bottom 8.3 0.6 Central Top 9.1 0.5 Plate Central Bottom 11.3
0.2 Plate 7/8 inch Outer Plate Top 9.0 0.6 spacing Outer Plate
Bottom 10.4 1.2 Central Top 9.1 0.5 Plate Central Bottom 11.3 0.2
Plate PDAS = primary dendrite arm spacing
* * * * *