U.S. patent application number 13/336470 was filed with the patent office on 2013-06-27 for casting methods for making articles having a fine equiaxed grain structure.
This patent application is currently assigned to GENERAL ELECTRIC COMPANY. The applicant listed for this patent is Stephen Joseph Balsone, Andrew John Elliott, Michael Francis Xavier Gigliotti, JR., Shyh-Chin Huang, Roger John Petterson, Stephen Francis Rutkowski, Pazhayannur Ramanathan Subramanian, Akane Suzuki. Invention is credited to Stephen Joseph Balsone, Andrew John Elliott, Michael Francis Xavier Gigliotti, JR., Shyh-Chin Huang, Roger John Petterson, Stephen Francis Rutkowski, Pazhayannur Ramanathan Subramanian, Akane Suzuki.
Application Number | 20130160967 13/336470 |
Document ID | / |
Family ID | 48575778 |
Filed Date | 2013-06-27 |
United States Patent
Application |
20130160967 |
Kind Code |
A1 |
Suzuki; Akane ; et
al. |
June 27, 2013 |
CASTING METHODS FOR MAKING ARTICLES HAVING A FINE EQUIAXED GRAIN
STRUCTURE
Abstract
Methods for casting a metallic material to form a component are
described. The component can be a superalloy-containing turbine
part, for example. The general method includes the step of pouring
the metallic material, in molten form, into an investment mold; and
then rapidly immersing the entire investment mold into a bath that
contains a low-melting liquid coolant metal, so as to achieve
substantially uniform, multi-directional heat transfer out of the
molten material. The molten material that solidifies to form the
component is characterized by a fine-grained, equiaxed grain
structure. Related embodiments include the use of two ingots that
constitute the superalloy material. One ingot includes the
oxygen-reactive elements, and is prepared by a vacuum-melting
technique. The other ingot includes the remainder of the elements,
and can be prepared by a number of techniques, such as air-melting
processes.
Inventors: |
Suzuki; Akane; (Clifton
Park, NY) ; Balsone; Stephen Joseph; (Simpsonville,
SC) ; Elliott; Andrew John; (Westminster, SC)
; Gigliotti, JR.; Michael Francis Xavier; (Scotia,
NY) ; Huang; Shyh-Chin; (Latham, NY) ;
Petterson; Roger John; (Sun City West, AZ) ;
Rutkowski; Stephen Francis; (Duanesburg, NY) ;
Subramanian; Pazhayannur Ramanathan; (Niskayuna,
NY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Suzuki; Akane
Balsone; Stephen Joseph
Elliott; Andrew John
Gigliotti, JR.; Michael Francis Xavier
Huang; Shyh-Chin
Petterson; Roger John
Rutkowski; Stephen Francis
Subramanian; Pazhayannur Ramanathan |
Clifton Park
Simpsonville
Westminster
Scotia
Latham
Sun City West
Duanesburg
Niskayuna |
NY
SC
SC
NY
NY
AZ
NY
NY |
US
US
US
US
US
US
US
US |
|
|
Assignee: |
GENERAL ELECTRIC COMPANY
Schenectady
NY
|
Family ID: |
48575778 |
Appl. No.: |
13/336470 |
Filed: |
December 23, 2011 |
Current U.S.
Class: |
164/493 ;
164/121; 164/128; 164/495 |
Current CPC
Class: |
B22D 27/04 20130101 |
Class at
Publication: |
164/493 ;
164/128; 164/121; 164/495 |
International
Class: |
B22D 27/04 20060101
B22D027/04; B22D 25/02 20060101 B22D025/02; B22D 27/02 20060101
B22D027/02 |
Claims
1. A method of casting a metallic material to form a component,
comprising the following steps: (a) pouring the metallic material,
in molten form, into an investment mold; and (b) rapidly immersing
the entire investment mold into a bath comprising a low-melting
liquid coolant metal, so as to achieve substantially uniform,
multi-directional heat transfer out of the molten material, thereby
solidifying the molten material to form the component, and
providing a fine-grained, equiaxed grain structure thereto.
2. The method of claim 1, wherein the investment mold is
pre-heated, in a vacuum or an inert atmosphere.
3. The method of claim 1, wherein the investment mold comprises an
interior surface that includes a nucleating agent for enhancing the
formation of the equiaxed grain structure.
4. The method of claim 3, wherein the nucleating agent comprises at
least one cobalt-containing oxide.
5. The method of claim 1, wherein the bath has a mass that is at
least 4 times the total mass of the mold and the cast metal.
6. The method of claim 1, wherein the rate of immersion is defined
by a withdrawal rate of at least about 380 cm (150 inches) per
hour.
7. The method of claim 1, wherein the metallic material is at a
temperature of about 50.degree. C. to about 100.degree. C. above
its melting point, while being poured into the mold.
8. The method of claim 1, wherein the liquid coolant metal is at a
temperature of about 700.degree. C. to about 1400.degree. C. below
the melting point of the metallic material, while the metallic
material is poured into the mold.
9. The method of claim 1, wherein the molten metallic material
being poured into the mold includes dispersed solid particles of
the metallic material; and the solid particles comprise less than
about 2% of the total weight of the metallic material.
10. The method of claim 9, wherein the solid particles within the
molten material are obtained by subjecting the molten material to
at least one solidification-melting cycle, prior to pouring the
molten material into the mold.
11. The method of claim 1, wherein multiple investment molds are
immersed in the bath to form multiple components; and the
investment molds are arranged in the bath to provide maximum,
multi-directional heat transfer out of the molten material.
12. The method of claim 11, wherein the investment molds are
arranged in a general star-shape, relative to each other, along the
longest access of each mold.
13. The method of claim 1, wherein the metallic material comprises
a superalloy based on nickel, cobalt, iron, or combinations
thereof.
14. The method of claim 13, wherein the component is a turbine
engine part.
15. The method of claim 1, wherein the metallic material for the
component comprises a group of elements generally unreactive with
oxygen; and also comprises at least one oxygen-reactive
element.
16. The method of claim 15, wherein the metallic material to be
cast is produced by preparing a first ingot by a vacuum-melting
technique, wherein the first ingot comprises all of the
oxygen-reactive elements and at least one base element selected
from nickel, cobalt, or iron; preparing a second ingot by either an
air-melting technique, an inert gas technique, or a vacuum-melting
technique, wherein the second ingot comprises all of the generally
unreactive elements; attaching the two ingots together, or placing
the two ingots together, to form a casting charge; and melting the
charge and pouring the molten material into the investment
mold.
17. The method of claim 16, wherein the vacuum-melting technique is
selected from the group consisting of vacuum induction melting;
vacuum arc re-melting; and non-consumable arc melting; and the
air-melting technique is selected from the group consisting of air
casting and argon oxygen decarburization.
18. The method of claim 16; wherein the first ingot comprises at
least one of nickel, cobalt; and iron; and at least one of
aluminum, titanium, zirconium, hafnium, and the rare earth
metals.
19. A method of casting a nickel-based superalloy to form a turbine
engine component, comprising the steps of: (i) preparing a first
ingot by a vacuum-melting technique, wherein the first ingot
comprises nickel and all elements in the superalloy that are
oxygen-reactive; and preparing a second ingot by either an
air-melting technique, an inert gas technique, or a vacuum-melting
technique, wherein the second ingot comprises the superalloy
elements that are generally non-reactive with oxygen; (ii)
attaching the two ingots together, or placing the two ingots
together, to form a casting charge; and melting the charge and
pouring the molten material into an investment mold; and (iii)
rapidly immersing the entire investment mold into a bath comprising
a low-melting liquid coolant metal, so as to achieve substantially
uniform, multi-directional heat transfer out of the molten
material, thereby solidifying the molten material to form the
component, and providing a fine-grained, equiaxed grain structure
thereto.
20. The method of claim 19, wherein the investment mold comprises
an interior surface that includes a nucleating agent for enhancing
the formation of the equiaxed grain structure.
Description
BACKGROUND
[0001] This disclosure is generally related to metallic components,
and methods for manufacturing those components. In some specific
embodiments, the disclosure is related to cast metallic articles,
often formed from nickel- or cobalt-based superalloys; and related,
specialized casting methods.
[0002] A number of metals and metal alloys are employed in
demanding applications, in terms of strength, oxidation resistance,
and/or high temperature resistance. Examples include titanium,
vanadium, molybdenum, and superalloys based on nickel, cobalt, or
iron. The superalloys are especially suitable for high-temperature
applications, such as, for example, gas turbine engine components
of aircraft engines and power generation equipment. Very often,
these components are manufactured by casting processes, such as
investment-casting. While metal casting has been practiced for
thousands of years, the techniques have become quite sophisticated
in modern times, due in part to the high level of integrity
required for cast parts such as jet engine blades.
[0003] The integrity and overall quality of the metal component is
determined in part by its crystalline structure, e.g., the grain
size and orientation of the grains in the component. The desired
grain structure is, in turn, often dependent on the projected
operating temperature of the part. As an example in the case of gas
turbine components formed from various superalloys, the turbine
blades (buckets) in the combustor of the turbine may be exposed to
temperatures as high as 900-1150.degree. C. These components
usually have a directionally solidified (DS) columnar grain
structure, or a single crystal structure, to resist
high-temperature creep failure and other degrading effects.
[0004] In contrast, engine components that are subjected to lower
operating temperatures often benefit from a very different grain
structure. For example, gas turbine wheels and discs, while having
their own set of performance requirements, often operate at
temperatures of about 650-700.degree. C. In many cases, it is very
desirable that these components have a fine equiaxed grain
structure.
[0005] Although fine equiaxed grain structures are commonly
obtained in small castings, they are relatively difficult to
produce in large, complex parts, such as the gas turbine airfoils
and structural components. The investment casting techniques
typically produce cast components having a mixture of columnar and
equiaxed grains. This is often the case for large components with
thick sections (e.g., sections more than about 10 mm thick).
Obtaining the desired fine-grain structure can be especially
difficult if the component has a complex geometry, with a wide
variation in sectional thickness.
[0006] Non-uniform grain morphology and grain size can lead to
problems in the quality and performance of the cast components. In
many cases (though not all), large grain size can result in low
strength at a given operating temperature. Moreover, a columnar
grain structure--while desirable for components operating under a
specific temperature regime--can be detrimental for the
lower-temperature components referenced above. Columnar grain
morphology is characterized by continuous, intergranular
boundaries, along which cracks and "hot tears" can sometimes
develop. Also, when oriented transversely to the stress-direction
during use, the columnar grain boundaries can be weak, which can in
turn lead to premature failure of the component.
[0007] With these general considerations in mind, new methods for
casting high-performance alloys would be welcome in the art. The
techniques should be especially suitable for manufacturing
components that require a fine, equiaxed grain structure. Moreover,
the new developments should also be suitable for casting relatively
large components having complex geometries. Furthermore, the
techniques should not require substantial changes to current
casting operations that would result in significant increases in
manufacturing costs.
SUMMARY OF THE INVENTION
[0008] An embodiment of this invention is directed to a method for
casting a metallic material to form a component, comprising the
following steps:
[0009] (a) pouring the metallic material, in molten form, into an
investment mold; and
[0010] (b) rapidly immersing the entire investment mold into a bath
comprising a low-melting liquid coolant metal, so as to achieve
substantially uniform, multi-directional heat transfer out of the
molten material, thereby solidifying the molten material to form
the component, and providing a fine-grained, equiaxed grain
structure thereto.
[0011] An additional embodiment of this invention relates to a
method of casting a nickel-based superalloy to form a turbine
engine component. The method includes these steps:
[0012] (i) preparing a first ingot by a vacuum-melting technique,
wherein the first ingot comprises nickel and all elements in the
superalloy that are oxygen-reactive; and [0013] preparing a second
ingot by either an air-melting technique, an inert gas technique,
or a vacuum-melting technique, wherein the second ingot comprises
the superalloy elements that are generally non-reactive with
oxygen;
[0014] (ii) attaching the two ingots together, or placing the two
ingots together, to form a casting charge; and melting the charge
and pouring the molten material into an investment mold; and
[0015] (iii) rapidly immersing the entire investment mold into a
bath comprising a low-melting liquid coolant metal, so as to
achieve substantially uniform, multi-directional heat transfer out
of the molten material, thereby solidifying the molten material to
form the component, and providing a fine-grained, equiaxed grain
structure thereto.
DRAWINGS
[0016] These and other features, aspects, and advantages of the
present invention will become better understood when the following
detailed description is read with reference to the accompanying
drawings, in which like characters represent like parts throughout
the drawings, wherein:
[0017] FIG. 1 is a photograph of an exemplary wax mold structure,
suitable for some of the casting embodiments of the invention.
[0018] FIG. 2 is a photograph of an exemplary ceramic mold
structure, based on the wax mold structure of FIG. 1.
[0019] FIGS. 3A and 3B are cross-sectional schematics of an
exemplary casting system for embodiments of this invention, prior
to and after immersion of the associated shell mold into a coolant
bath.
[0020] FIG. 4 is an illustration of two ingots joined to each
other, each containing particular constituents of a nickel-based
superalloy.
[0021] FIG. 5 is a photomicrograph of an etched section of a
nickel-based superalloy sample, after being cast according to
embodiments of this invention.
DETAILED DESCRIPTION
[0022] In regard to this disclosure, any ranges disclosed herein
are inclusive and combinable (e.g., compositional ranges of "up to
about 25 wt %", or more specifically, "about 5 wt % to about 20 wt
%", are inclusive of the endpoints and all intermediate values of
the ranges). Moreover, the terms "first," "second," and the like,
herein do not denote any order, quantity, or importance, but rather
are used to distinguish one element from another. The terms "a" and
"an" herein do not denote a limitation of quantity, but rather
denote the presence of at least one of the referenced items.
Moreover, approximating language, as used herein throughout the
specification and claims, may be applied to modify any quantitative
representation that could permissibly vary without resulting in a
change in the basic function to which it is related. Accordingly, a
value modified by a term or terms, such as "about," is not limited
to the precise value specified. In some instances, the
approximating language may correspond to the precision of an
instrument for measuring the value.
[0023] As alluded to previously, a number of metals can be cast
according to embodiments of this invention. Examples include the
"superalloys", a term intended to embrace iron-, cobalt-, or
nickel-based alloys. The superalloys usually include one or more
additional elements to enhance their high-temperature performance.
Non-limiting examples of the additional elements include cobalt,
chromium, aluminum, tungsten, molybdenum, rhenium, ruthenium,
zirconium, carbon, titanium, tantalum, niobium, hafnium, boron,
silicon, yttrium, and the rare earth metals. (Each of the base
alloys may contain one or more of the other elements listed as base
alloys, e.g., nickel-based alloys containing cobalt and/or iron).
Other metals that can be cast according to this invention include
titanium or titanium alloys, or stainless steel alloys.
[0024] In a typical embodiment of this invention, the metal or
metallic alloy being used to form the component is initially in the
form of powder, particulates, or ingots. The material is then
heated to a sufficient melting temperature. (In the case of
nickel-containing alloys, the alloy is usually heated to about
1350.degree. C. to about 1750.degree. C.). The molten metal may
then be poured into a mold in a casting process, to produce the
desired shape.
[0025] In most embodiments, investment casting is generally used to
make the parts. This technique is very useful for producing large
parts, and/or parts that have complex shapes, and which must be
capable of withstanding high temperatures. The investment mold is
usually made by making a pattern, using wax or another material
that can be melted away. This wax pattern is dipped in refractory
slurry, and then coated with a refractory sand which coats the
pattern and forms a shell. The shell is dried, and the process of
dipping in the slurry, sanding and drying is repeated until a
robust thickness is achieved. After these steps, the "shelled"
pattern is placed in an autoclave, and the wax is melted away. The
empty shell is then fired, resulting in a mold that can be filled
with the molten metal/alloy.
[0026] Since the shell mold is usually formed around a one-piece
pattern, (which does not have to be pulled out from the mold as in
a traditional sand casting process), very intricate parts and
undercuts can be made. The wax pattern itself is usually made by
duplication, e.g., with an injection die, or using
stereolithography. The master pattern can be fabricated by various
techniques, e.g., using a computer solid model master. Many
variations of the overall process are known to those skilled in the
art.
[0027] Preferably, the investment mold is pre-heated to about
800-1100.degree. C., to remove any residues of wax, as well as to
harden any binders that are present. The pre-heating step is
usually carried out in a vacuum, or in an inert atmosphere, but can
be carried out in air as well. Investment casting techniques are
described in many references, such as "Nickel-Based Superalloys for
Advanced Turbine Engines: Chemistry, Microstructure, and
Properties", by T. Pollock et al; and the Journal of Propulsion and
Power, Vol. 22, No. 2, March-April 2006. The metallic material can
be directed to the mold by a variety of pouring techniques. As
examples, pouring can be carried out by using gravity, pressure,
inert gas, or vacuum conditions. The preferred embodiment in some
instances is to cast in vacuum.
[0028] In some embodiments, a nucleating agent is disposed on the
interior surface of the mold. The nucleating agent--even at small
concentrations--can enhance the formation of the highly-desirable,
equiaxed grain structure. A number of compounds and materials can
serve as a nucleating agent (sometimes referred to as a "grain
refining agent"). Non-limiting examples include metals such as
cobalt, ruthenium, rhodium, iridium, and platinum; or oxides of
nickel, cobalt, or iron. Mixed oxides of silica, cobalt, and
alumina can also be used, for example. In some preferred
embodiments, the nucleating agent comprises at least one
cobalt-containing oxide, e.g., cobalt aluminate. The use of
nucleating agents for some casting applications is described, for
example, in U.S. Pat. No. 5,823,243 (T. Kelly), incorporated herein
by reference.
[0029] In a typical embodiment, the nucleating agent can be
incorporated into the interior surface of the mold in the form of a
facecoat. The facecoat can be applied in any convenient manner.
Alternatively, the nucleating agent can be applied as a "wash"
coating to the surface of the mold.
[0030] The concentration of the nucleating agent will depend on
various factors, such as the type of metal being cast; and the
identity of the particular nucleant. In some embodiments, e.g.,
using cobalt aluminate when casting a nickel-based alloy, the final
concentration of the nucleant will be in the range of about 5% to
about 10%, by weight, in a wash coating composition. However, the
concentration could be much higher, depending on the factors noted
above. Related information on the use of nucleants can also be
found in "Microstructure and Mechanical Properties of High
Temperature Creep Resisting Superalloy Rene.TM. 77 Modified
CoAl.sub.2O.sub.4", by M. Zielinswka et al; Archives of Materials
Science and Engineering; Vol. 28; Issue 10; October 2007; pp.
629-632, which is incorporated herein by reference.
[0031] The bath used to provide cooling to the filled mold (as
further described below) can be in a variety of different shapes.
The bath can be contained in a tank that may be equipped with
cooling coils. Heating elements may also be included, to adjust the
bath temperature. Moreover, a suitable stirring mechanism may be
provided to assure circulation of the liquid bath. As also
described below, the volume of the bath is calculated to
accommodate a selected number of investment molds, according to an
arrangement that maximizes a specific cooling regimen.
[0032] Low-melting liquid coolant metals for the bath are known in
the art. In many embodiments, the cooling liquid is either tin or
aluminum. Tin is especially preferred because of its low melting
temperature and low vapor pressure. In general, the liquid coolant
metal is maintained at a temperature as low as practically
possible, e.g., a temperature not much greater than the melting
point of the metal. In some typical embodiments, the liquid coolant
metal is maintained at a temperature of about 700.degree. C. to
about 1400.degree. C. below the melting point of the metallic
material being cast, while the metallic material is poured into the
mold. In the case of nickel-based superalloys that are being cast,
a suitable temperature for a tin bath is often between about
250.degree. C. and about 350.degree. C. The metallic material being
cast is usually poured into the mold (which is pre-heated), at a
material temperature of about 50.degree. C. to about 150.degree. C.
above its melting point.
[0033] As further described below, in some detail, and depicted in
the figures, the investment mold is rapidly immersed in the bath
after the mold has been filled with the molten metal. The present
inventors discovered that rapid cooling within the bath can
dramatically influence the microstructure of the cooled casting. As
described below, a fine, equiaxed grain structure can result, which
in turn provides other important properties to the cast article.
(In some embodiments, the grains are substantially uniform as
well).
[0034] In some embodiments, the molten metallic material that is
poured into the mold includes a relatively small amount of
dispersed solid particles of the metallic material. The solid
particles can provide additional grain refinement for the final
casting. Usually, the solid particles comprise less than about 2%
of the total weight of the metallic material being cast; and in
some embodiments, less than about 1%. According to one exemplary
technique, the solid particles can be provided by subjecting the
molten casting material to at least one solidification-melting
cycle, prior to pouring the molten material into the shell mold. A
"melting-freezing" sequence can induce the formation of the
particles within the body or depth of the casting material, prior
to re-melting as the material is poured into the mold.
[0035] FIG. 1 is a depiction of an exemplary wax mold structure 10
(in simplified form), suitable for embodiments of the invention.
The structure includes a pour cup 12 formed of wax, and supported
by a number of ceramic support posts 14. The pour cup is designed
to contain the molten metal to be used in the casting. The lower
region of the pour cup 12 communicates with a number of feed tubes
or "runners" 16, also formed of wax. Each of the feed tubes 16
terminates in an interior region of a wax article mold 18.
[0036] The molds 18 have previously been formed by known techniques
to accommodate the precise shape of a part to be cast according to
the process. The molds 18 and the posts 14 can be supported on the
upper surface 20 of a base plate 22. Other general details
regarding this type of wax mold structure are provided in a number
of references, including U.S. Pat. No. 5,072,771 (Prasad); and
"Nickel-Based Superalloys for Advanced Turbine Engines: Chemistry,
Microstructure, and Properties", by T. Pollock et al; Journal of
Propulsion and Power; Vol. 22, No. 2, March-April 2006, mentioned
above. Moreover, other information regarding pour cup structures
and details for supplying the molten metal to the mold can be found
in various sources, e.g., U.S. Pat. No. 6,019,158 (Soderstrom et
al). All three of these documents are incorporated herein by
reference.
[0037] In situations where multiple molds (i.e., for multiple
components being cast simultaneously) are to be immersed in the
coolant bath, care must be taken to ensure the development of the
fine equiaxed grain structure. The present inventors discovered
that, in some embodiments, it is preferable that the investment
molds be arranged, in the wax mold structure, so as to provide
maximum, multi-directional heat transfer out of the molten
material, once the structure is filled with molten metal and
immersed in the bath. In some embodiments, the investment molds are
arranged in a general, star-shape, relative to each other, along
the longest access of each shell mold.
[0038] In other embodiments, where the shell molds might have a
relatively high aspect ratio, the longest face or surface of each
shell mold within the coolant bath is spaced from, and generally
parallel to, the longest surface of at least one other shell mold.
As one example, pairs of shell molds may have a facing surface
opposite each other, as in FIG. 1. The shell molds may generally be
situated upright, around a perimeter of the base plate of the mold
structure. In general, the surfaces of the shell molds should be
spaced in a manner to provide maximum heat transfer, as described
above.
[0039] FIG. 2 is a photograph of an exemplary ceramic mold
structure 30, based on the pattern of the wax mold structure 10,
depicted in FIG. 1. As those skilled in the art understand, the
ceramic mold is typically formed from a material such as alumina,
silica, and/or zirconia. Typically, the mold is fabricated by the
progressive build-up of the ceramic layers around a wax pattern
like that of FIG. 1. Ceramic cores can be embedded in the wax to
obtain internal structures, like cooling pathways. (In this figure,
features not specifically marked are the same as those in FIG. 1,
in ceramic form).
[0040] FIGS. 1 and 2 are generally described in terms of a single
shell mold (e.g., shell mold structure 30). However, the figures
can alternatively be described in terms of multiple shell molds,
each "fed" by a feed tube 16 (FIG. 1), and each associated with a
cast article formed in one of the specific wax molds 18. The
concept of multiple molds is expressed here in terms of a single
"mold structure", so as to simplify the overall description.
[0041] Thermal cycles are employed to remove the wax material on
which the ceramic layers were deposited, to form the ceramic
structure of FIG. 2. Thus, the pour cup, feed tubes, and article
molds (i.e., article molds 18 of FIG. 1) are all part of the
ceramic pattern at this stage. The ceramic mold structure is
usually supported on a base plate (chill plate) 32, as shown in
FIG. 2. The mold structure (referred to as an "investment mold")
can now be filled with the molten metallic material, as part of the
casting process.
[0042] The thickness of the mold in FIG. 2 can vary to some extent.
As a general, non-limiting illustration for articles being cast
from a nickel-based superalloy, and having an average thickness of
about 10 mm to about 25 mm, the average thickness of an
alumina-based mold will be in the range of about 5 mm to about 25
mm. (The overall mold size is also limited by the size of the
furnace used in the casting process).
[0043] In some embodiments, the mold may be thicker in the upper
region (i.e., the pour cup region); and in the lower region (i.e.,
the base plate region), relative to the central region, i.e., the
region encompassing the cast part itself. (This can be viewed as
the "critical cooling region"). The present inventors discovered
that this difference in thickness can be especially important for
effectively obtaining the desired microstructure, based on the
required strength and integrity of the mold, balanced against the
capacity of the mold for transferring heat out of the mold walls
during the critical cooling process.
[0044] Thus, in some embodiments, the thickness of the mold in the
cast part (central) region should be at least about 25% less than
the thickness in both the pour cup region and the base plate
region. In some preferred embodiments, the thickness of the shell
mold in the cast part region is at least about 50% less than the
thickness of the mold in the other regions. (The thickness of the
mold in the pour cup region and the base plate region need not be
identical to each other. Moreover, the thickness in the pour cup
region is sometimes graded to some degree).
[0045] FIG. 3A is a simplified depiction of an exemplary casting
system 50 for embodiments of this invention. A shell mold 52
(similar to that of FIG. 2), having at least one section shaped in
the form of a desired casting (e.g., a turbine blade), is secured
to a chill plate 54. For the purpose of depicting aspects of this
invention, the mold is depicted in an upper position in FIG. 3A.
FIG. 3B, described below, depicts the lower position, in which the
mold has been immersed in the coolant bath.
[0046] An ingot of the casting metal can be placed within a
crucible that is partially surrounded by a water-cooled induction
coil. (The ingot and crucible are conventional features not
specifically shown in the figure. Reference is made, for example,
to the Pollock article mentioned previously). Other types of
heating techniques are also possible.
[0047] The crucible that contains the ingot can be lowered into a
furnace 62 by any suitable mechanical means. One example is the
mechanical arm 64 that can be connected to a conventional drive
system. The lower end of the arm can be connected to a platform
supporting the mold and chill plate, or to any associated
structure. In this manner, the vertical motion of the investment
mold can be precisely controlled. The overall process is preferably
carried out in a vacuum or in an inert atmosphere. An ambient air
atmosphere can also be used, alone or in conjunction with the other
environment(s), as a form of cooling the mold after withdrawal from
the heating chamber.
[0048] The casting system also includes some means (not
specifically shown) for preheating the ceramic shell mold 52 to a
suitable temperature, usually above the liquidus temperature of the
casting metal or metal alloy. The system can further include a
baffle 66, situated between a lower region of the furnace 62 and an
upper region of the coolant bath 68. The baffle can assist in
obtaining a steep thermal gradient between the superheated mold and
the cooling liquid bath. The baffle may be in the form of a single
layer or multiple layers, and usually (though not always) comprises
a stiff or flexible thermal insulating material. The baffle may be
rigid, or may float. In the illustrative embodiment of FIG. 3A, an
additional floating baffle 69 is also provided. In general, the
baffle 66 can be designed to vary its fit around the shape of the
mold as the mold is withdrawn from the heating chamber, through the
baffle(s), and into the liquid coolant bath 68.
[0049] The general function of pour cup 71 was described in
reference to FIGS. 1 and 2. The pour cup receives the molten
casting metal 73 from a crucible (not shown) that is generally
situated above the cup, and communicates therewith. The particular
design and features of the pour cup are not critical to this
invention.
[0050] The coolant bath 68 contains a suitable coolant metal, as
mentioned previously, and is usually equipped with a stirrer (not
shown), or other means of agitation. The bath is often surrounded
by a liner 72 of thermal oil, that can be used to control the
temperature of the bath. The thermal oil in the liner can be
circulated through in-flow and out-flow conduits, as shown in the
Pollock article.
[0051] The size and shape of the bath can vary somewhat. Various
factors are involved, such as the size and type of castings
involved; as well as the type of furnace used; and general space
requirements. In some embodiments, the bath has a mass that is at
least 4 times (and preferably greater than 4 times) the combined
mass of the cast metal and its mold. (The figures in this
disclosure are not necessarily drawn to scale, so that various
features can be highlighted to the reader).
[0052] After the molten casting metal has been directed into shell
mold 52, the mold is rapidly immersed into coolant bath 68, as
depicted in FIG. 3B. As mentioned above, rapid immersion was found
to provide a fine, equiaxed grain structure. As used herein, a
"fine, equiaxed grain structure" refers to a population of grains
having a median aspect ratio of less than about 2.5. In some
embodiments, the cast material is characterized by an average grain
diameter of about 3 mm or less.
[0053] Rapid immersion can be characterized as a "withdrawal rate",
i.e., the withdrawal of the mold from the hot zone (furnace 62) of
the casting system. Since the investment mold moves from the
furnace to the bath with very little residence time in-between, the
withdrawal rate is effectively the "plunge rate" or "quench rate"
for moving the mold entirely into coolant bath 68. The particular
withdrawal rate will depend on various factors, such as the
identity of the casting metal, and its projected size and shape;
the liquidus temperature of the casting metal; the coolant bath
temperature and bath size; the wall thickness and overall size of
the investment mold; and the presence or absence (and type) of any
nucleating agent.
[0054] In the case of a nickel- or cobalt-based superalloy being
cast, wherein the shell mold thickness (in the cast part region) is
in the range of about 3.5 mm (0.14 inch) to about 1 cm (0.4 inch);
and wherein a cobalt-based nucleating agent has been incorporated
into the mold surface, the withdrawal rate is usually at least
about 380 cm (150 inches) per hour, e.g., in the range of about 380
cm (150 inches) to about 510 cm (200 inches) per hour. In some
specific embodiments, the withdrawal rate is greater than about 510
cm (200 inches) per hour. The relatively high withdrawal rate
minimizes the amount of radiational cooling experienced by the
shell mold, i.e., between the furnace and coolant bath; and this
can also enhance formation of the desired grain structure. In some
embodiments directed to a conventional turbine engine component,
the entire mold is immersed in the bath within about 300 seconds
after the metallic material has been poured into the mold.
[0055] In the case of titanium-based alloys, the withdrawal rate is
usually at least about 380 cm (150 inches) per hour, and in some
preferred embodiments, at least about 510 cm (200 inches) per hour.
In the case of stainless steel alloys, the withdrawal rate is
usually at least about 380 cm (150 inches) per hour, and in some
preferred embodiments, at least about 510 cm (200 inches) per hour.
Those skilled in the casting arts will be able to determine the
most appropriate withdrawal rate for a given situation; based on
the teachings herein.
[0056] It should be emphasized that withdrawal rates for other
casting and solidification processes are, often, considerably
longer than those described above for embodiments of the present
invention. An example can be provided for an article formed of a
nickel- or cobalt-based superalloy having dimensions similar to
articles described herein, and subjected to a direct solidification
(DS) process. In that technique, the article may be withdrawn
(i.e., immersed in the coolant bath) at a rate less than about 125
cm (50 inches) per hour. The resulting microstructure of such an
article is usually very different from the fine-grained, equiaxed
grain structure described herein.
[0057] After casting is complete, conventional steps are undertaken
to release the cast article. Typically, the investment mold is
separated from the article by hammering, media blasting, vibration,
water-jetting, chemical dissolution, or some combination of these
techniques. The sprues or gates used in the molding process are cut
off, and the casting can then be subjected to other cleaning and
finishing steps, such as grinding.
[0058] The quality of the casting alloy, and the cost of preparing
and using such an alloy, can be very important factors in
manufacturing high-performance components for the present
invention. Vacuum melting techniques, such as vacuum-induction
melting and vacuum-arc remelting, can be used to produce
high-quality ingots of an alloy composition. These types of
techniques are often required when preparing alloys that include
oxygen-reactive elements, like aluminum, titanium, and zirconium.
However, in terms of equipment and operating details, the
techniques can be relatively expensive to employ--especially in the
case of very large ingots of alloy material, e.g., about 450
kilograms (1000 pounds) or more. In many embodiments for this
invention, the alloys do in fact include at least one of the
oxygen-reactive elements.
[0059] On the other hand, various air-melting techniques, such as
argon oxygen decarburization, are very attractive for preparing
alloys, e.g., wrought alloys used for plate, sheet and bar tube.
(The air-melting technique is also sometimes used as a preliminary
step to subsequent vacuum processes). The popularity of the
air-melting techniques is based in part on their lower cost, as
compared to vacuum melting. However, the air-melting techniques
cannot readily be undertaken when the desired alloy contains the
oxygen-reactive elements mentioned above.
[0060] Thus, according to embodiments of this invention in which
the metallic material contains both reactive and unreactive
elements, two separate casting compositions, e.g., ingots, are
first prepared. The first ingot comprises the elements that are
generally reactive with oxygen. As noted above, the reactive
elements usually comprise at least one of aluminum, titanium,
zirconium, hafnium, and the rare earth metals. In the case of a
superalloy, the first ingot may also comprise at least one of the
base elements, i.e., nickel, cobalt, or iron. The first ingot may
also comprise at least one of carbon, boron, silicon, and in some
cases, tantalum. The weight-ratio of base metal elements (total) to
oxidation-reactive elements (total) in the first ingot will usually
be in the range of about 90:10 to about 80:10.
[0061] The first ingot can be prepared by a vacuum-melting
technique. As alluded to previously, those techniques are known in
the art. In some preferred embodiments, the vacuum-melting
technique is either induction melting or non-consumable arc
melting.
[0062] The second ingot comprises the elements that are generally
non-reactive with oxygen. In the case of the superalloy
composition, the second ingot comprises at least one of nickel,
cobalt; and iron; and at least one of chromium, molybdenum,
tungsten, rhenium, and in some cases, tantalum. In preferred
embodiments, the composition of the second ingot is substantially
free of oxidation-reactive elements such as aluminum, zirconium,
and hafnium. In other embodiments (though not all embodiments), the
second ingot is also substantially free of at least one of carbon
or boron.
[0063] The second ingot can advantageously be prepared (especially
from an economic point of view) by any suitable air-melting
technique, such as air casting or argon oxygen decarburization.
However, in other embodiments, it is also possible to prepare the
second ingot by any of the suitable vacuum melting techniques
described herein. Moreover, an inert gas technique can sometimes be
used for the second ingot.
[0064] The proportion of elements within each ingot, and relative
to the other ingot, will depend in large part on the required
composition for the final casting. Other factors include the
particular melting techniques used for each ingot, as well as the
relative melting points of the two alloys. (In terms of formulating
the ingot-compositions, the melting point of each ingot should not
be so high that the melting step can be a difficult one. Moreover,
it may sometimes be desirable to "space" the melting point of one
ingot from the other, in order to enhance grain refinement in the
final, cast alloy). The weight of each ingot relative to the other
ingot will also depend on the factors noted above, along with the
intended, final metal composition. Other details regarding these
types of ingots can be found in U.S. Pat. No. 4,718,940
(McPhillips), which is incorporated herein by reference.
[0065] The two ingots can then be joined together by any convenient
technique, such as spot welding or any type of mechanical
attachment. Alternatively, they may simply be placed together or
stacked on top of each other. FIG. 4 exemplifies a nickel-based
superalloy, and provides a simple depiction of a first ingot "A",
joined to a second ingot "B". The first ingot contains nickel and
oxidation-reactive elements; while the second ingot contains nickel
and the remainder of the typical superalloy constituents. The
relative weight percentages noted in the figure are illustrative,
and can vary, as mentioned above.
[0066] In general, there is at least one economic advantage in
using the two ingots, when the bulk of the material in ingot "B" is
formed by the less expensive air-melting technique. The ingot "A",
while being prepared by the more expensive vacuum melting
technique, requires a less costly preparation because of its
relatively small size. Moreover, problems that might otherwise
arise in processing the oxidative-reactive elements are minimized
or prevented.
[0067] The two ingots can remain joined or next to each other until
they are needed to produce the desired investment-cast component.
They can then be added to a crucible within a system like that of
FIG. 1, for carrying out the casting process. The present inventors
discovered that, in addition to providing important
economic-processing advantages in some instances, the use of the
two ingots in the described technique also can result in further
enhancement in the fine-grained, equiaxed structure.
[0068] A large number of components can be cast according to
embodiments of this invention. In general, any component formed of
high-temperature/high-strength materials like the superalloys,
titanium alloys, and stainless steel alloys, can benefit from this
invention. Non-limiting examples of turbine engine components that
can be formed as described herein include turbine buckets, blades,
vanes, nozzles, combustor liners, combustor domes, and shrouds. The
inventive processes described herein are especially useful for
components having a mixture of columnar and equiaxed grains. This
is often the case for "large" components, as described above,
(e.g., those with sections having a thickness greater than about 10
mm), since obtaining the fine-grained crystal structure for those
sized components has been traditionally very difficult.
[0069] In contrast to components formed of alloys without the fine,
equiaxed grain structure, the microstructure of components formed
by embodiments of this invention can be very desirable. An
illustration can be provided in the case of turbine engine
components formed of nickel-based superalloys. In those cases in
which the alloys have the fine, equiaxed microstructure, the
components are expected to exhibit a yield strength that is at
least about 30% higher than that of a component formed of a cast
alloy having a relatively coarse-grained structure.
EXAMPLE
[0070] An ingot of a nickel-based superalloy, Rene.TM. 108, was
used in this example. The alloy had an approximate composition as
described in U.S. Pat. No. 5,897,801 (Smashey et al), incorporated
herein by reference. The ingot was melted in a crucible at
1400.degree. C., using an apparatus similar to that depicted in
FIG. 3A, and using a Ni-30Cr plug. The molten metal was directed
into an alumina-based mold that had an average mold wall thickness
(in the casting part region) of about 0.5 cm (200 mils). Prior to
receiving the metal, a facecoat that contained about 5-10% by
weight CoAl.sub.2O.sub.4 was coated onto the interior surface of
the mold. The mold had been pre-heated to 1250.degree. C.
[0071] After the molten alloy filled the mold, the mold was rapidly
immersed in a liquid tin bath maintained at a temperature of about
250.degree. C. The withdrawal rate (i.e., immersion speed) was (590
cm) (233 inches) per hour. After solidification of the cast part
(an elongate test plate), the part was removed from the mold,
cleaned, and then etched in a standard, acid-containing etching
solution.
[0072] FIG. 5 is a photomicrograph showing the microstructure of a
cross-section of the alloy, after etching. The average grain size
was about 300 microns. The grains had a median aspect ratio of less
than about 2.5. These measurements indicate that the microstructure
was considered to be fine-grained and equiaxed, according to
embodiments of this invention.
[0073] While only certain features of the invention have been
illustrated and described herein, many modifications and changes
will occur to those skilled in the art. It is, therefore, to be
understood that the appended claims are intended to cover all such
modifications and changes as fall within the true spirit of the
invention.
* * * * *