U.S. patent application number 12/970104 was filed with the patent office on 2012-06-21 for unidirectional solidification process and apparatus and single-crystal seed therefor.
This patent application is currently assigned to GENERAL ELECTRIC COMPANY. Invention is credited to Gregory Keith Bouse, Andrew John Elliott, Ganjiang Feng, Shan Liu, Kenneth Burrell Potter.
Application Number | 20120152483 12/970104 |
Document ID | / |
Family ID | 46232815 |
Filed Date | 2012-06-21 |
United States Patent
Application |
20120152483 |
Kind Code |
A1 |
Feng; Ganjiang ; et
al. |
June 21, 2012 |
UNIDIRECTIONAL SOLIDIFICATION PROCESS AND APPARATUS AND
SINGLE-CRYSTAL SEED THEREFOR
Abstract
A single-crystal seed, apparatus and process for producing a
casting having a single-crystal (SX) microstructure. The seed has a
geometry that includes a vertex capable of destabilizing an oxide
film that forms at the interface between the seed and a molten
metal during the casting process, and thereby promotes a continuous
single-crystal grain growth and reduces grain misorientation
defects that can initiate from the seed/metal interface.
Inventors: |
Feng; Ganjiang; (Greenville,
SC) ; Bouse; Gregory Keith; (Greer, SC) ;
Potter; Kenneth Burrell; (Simpsonville, SC) ;
Elliott; Andrew John; (Westminster, SC) ; Liu;
Shan; (Central, SC) |
Assignee: |
GENERAL ELECTRIC COMPANY
Schenectady
NY
|
Family ID: |
46232815 |
Appl. No.: |
12/970104 |
Filed: |
December 16, 2010 |
Current U.S.
Class: |
164/122.2 ;
164/338.1; 164/412 |
Current CPC
Class: |
C30B 29/52 20130101;
C30B 11/14 20130101; B22D 27/045 20130101 |
Class at
Publication: |
164/122.2 ;
164/412; 164/338.1 |
International
Class: |
B22D 27/04 20060101
B22D027/04; B22C 23/00 20060101 B22C023/00 |
Claims
1. A seed for use in a unidirectional casting process, the seed
comprising a body having a single-crystal microstructure and at
least a first surface region, the first surface region defining a
vertex of the body that protrudes away from the body, the vertex
being adapted to destabilize an oxide film attempting to form on
the first surface region when the first surface region is contacted
by a molten metal during a mold filling step of the casting
process, the vertex destabilizing the oxide film as a result of
surface tension of the oxide film at the vertex being sufficiently
high to cause the oxide film to collapse as the oxide film is
forming on the first surface region during the casting process.
2. The seed according to claim 1, wherein the body of the seed has
an axis that passes through the vertex and about which the first
surface region has rotational symmetry.
3. The seed according to claim 1, wherein the first surface region
of the body is a lateral surface of a conical portion of the
body.
4. The seed according to claim 3, wherein the conical portion of
the body has an axis that passes through the vertex and about which
the first surface region has a rotational symmetry, and the first
surface region is at an angle of about 20 to about 40 degrees to
the axis.
5. The seed according to claim 1, wherein the body of the seed has
an axis that passes through the vertex and a crystallographic
orientation oriented parallel to the axis.
6. The seed according to claim 1, wherein the body of the seed is
formed of a seed alloy containing at least one reactive element
chosen from the group consisting of aluminum, titanium, yttrium,
and rare-earth metals.
7. The seed according to claim 1, wherein the body of the seed is
formed of a seed alloy containing aluminum.
8. An apparatus for unidirectionally casting an alloy, the
apparatus comprising: a mold having a base and a mold cavity
adjacent thereto, the mold cavity being adapted to contain a molten
quantity of the alloy during solidification thereof to yield a
unidirectionally-solidified casting defined by the mold cavity; a
heating zone adapted to heat the mold and the molten quantity of
the alloy therein to a heating temperature above the liquidus
temperature of the alloy; a cooling zone adapted to cool the mold
and the molten quantity of the alloy therein to a cooling
temperature below the solidus temperature of the alloy to cause
unidirectional solidification of the molten quantity of the alloy
and thereby yield the unidirectionally-solidified casting; and a
single-crystal seed disposed in the base of the mold and coupled to
the mold cavity so that the molten quantity of the alloy
epitaxially solidifies based on a crystallographic orientation of
the seed, the seed comprising a body having at least a first
surface region, the first surface region defining a vertex of the
body that protrudes away from the body, the vertex being adapted to
destabilize an oxide film attempting to form on the first surface
region when the first surface region is contacted by a molten metal
during a mold filling step of the casting process, the vertex
destabilizing the oxide film as a result of surface tension of the
oxide film at the vertex being sufficiently high to cause the oxide
film to collapse as the oxide film is forming on the first surface
region during the casting process.
9. The apparatus according to claim 8, wherein the body of the seed
has an axis that passes through the vertex and about which the
first surface region has rotational symmetry.
10. The apparatus according to claim 8, wherein the first surface
region of the body is a lateral surface of a conical portion of the
body.
11. The apparatus according to claim 10, wherein the conical
portion of the body has an axis that passes through the vertex and
about which the first surface region has a rotational symmetry, and
the first surface region is at an angle of about 20 to about 40
degrees to the axis.
12. The apparatus according to claim 8, wherein the body of the
seed is formed of a seed alloy containing at least one reactive
element chosen from the group consisting of aluminum, titanium,
yttrium, and rare-earth metals.
13. A process of unidirectionally casting an alloy, the process
comprising: providing a mold having a base and a mold cavity
adjacent thereto; placing a single-crystal seed in the base of the
mold, the seed comprising a body having a single-crystal
microstructure and at least a first surface region, the first
surface region defining a vertex of the body that protrudes away
from the body; introducing a molten quantity of the alloy into the
mold cavity; and then cooling the mold to cause unidirectional
solidification of the molten quantity of the alloy within the mold
and produce a unidirectionally-solidified casting having a columnar
crystal structure, the molten quantity of the alloy contacting the
seed so that the molten quantity epitaxially solidifies based on a
crystallographic orientation of the seed, the vertex of the body of
the seed destabilizing an oxide film forming on the first surface
region as a result of surface tension of the oxide film at the
vertex being sufficiently high to cause the oxide film to collapse
as the oxide film is forming on the first surface region.
14. The process according to claim 13, wherein the body of the seed
has an axis that passes through the vertex and about which the
first surface region has rotational symmetry.
15. The process according to claim 13, wherein the first surface
region of the body is a lateral surface of a conical portion of the
body.
16. The process according to claim 15, wherein the conical portion
of the body has an axis that passes through the vertex and about
which the first surface region has a rotational symmetry, and the
first surface region is at an angle of about 20 to about 40 degrees
to the axis.
17. The process according to claim 13, wherein the body of the seed
has an axis that passes through the vertex and a crystallographic
orientation oriented parallel to the axis.
18. The process according to claim 13, wherein the body of the seed
is formed of a seed alloy containing at least one reactive element
chosen from the group consisting of aluminum, titanium, yttrium,
and rare-earth metals.
19. The process according to claim 13, wherein the alloy is a
nickel-base, cobalt-base or iron-base superalloy that contains at
least one reactive element chosen from the group consisting of
aluminum, titanium, yttrium, and rare-earth metals.
20. The process according to claim 13, wherein the
unidirectionally-solidified casting is a component of a 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 a process and apparatus capable of reducing defects
in single-crystal (SX) castings, including but not limited to cast
components of gas turbines and other high temperature
applications.
[0002] Components of gas turbines, such as buckets (blades),
nozzles (vanes) 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 an ongoing effort to develop components, and
particularly turbine buckets, nozzles, and combustor 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 unidirectional casting techniques to have
directionally-solidified (DS) or single-crystal (SX)
microstructures, characterized by an optimized crystal orientation
along the crystal growth direction to produce columnar
polycrystalline or single-crystal articles.
[0003] As known in the art, directional casting techniques for
producing DS and SX 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 is
represented in FIGS. 1 and 2 as an apparatus 10 that employs a
Bridgman-type furnace to create a heating zone 26 surrounding a
shell mold 12, and a cooling zone 42 beneath the mold 12. The zones
26 and 42 may be referred to as "hot" and "cold" zones,
respectively, which denote their temperatures relative to the
melting temperature of the alloy being solidified. The mold 12 has
an internal cavity 14 corresponding to the desired shape of a
casting 32 (FIG. 2), represented as a turbine bucket. As such, FIG.
1 represents the cavity 14 as having regions 14a, 14b and 14c that
are configured to form, respectively, an airfoil portion 34, shank
36, and dovetail 38 (FIG. 2) of the casting 32. The cavity 14 may
also contain cores (not shown) for the purpose of forming internal
structures such as cooling passages within the casting 32.
[0004] The mold 12 is shown secured to a chill plate 24 and
initially placed in the heating zone 26 (Bridgman furnace). The
heating zone 26 heats the mold 12 to a temperature above the
liquidus temperature of the alloy. The cooling zone 42 is directly
beneath the heating zone 26, and operates to cool the mold 12 and
the molten alloy 16 within by conduction, convection and/or
radiation techniques. For example, the cooling zone 42 may be a
tank containing a liquid cooling bath 46, 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 or a fluidized bed.
[0005] An insulation zone 44 defined by a baffle, heat shield or
other suitable means is between and separates the heating and
cooling zones 26 and 42. The insulation zone 44 serves as a barrier
to thermal radiation emitted by the heating zone 26, thereby
promoting a steep axial thermal gradient between the mold 12 and
the cooling bath 46. The insulation zone 44 has a variable-sized
opening 48 that, as represented in FIG. 1, enables the insulation
zone 44 to fit closely around the shape of the mold 12 as it is
withdrawn from the heating zone 26, through the insulation zone 44,
and into the liquid cooling bath 46.
[0006] Casting processes of the type represented in FIGS. 1 and 2
are typically carried out in a vacuum or an inert atmosphere. After
the mold 12 is preheated to a temperature above the liquidus
temperature of the alloy being cast, molten alloy 16 is poured into
the mold 12 and the unidirectional solidification process is
initiated by withdrawing the base of the mold 12 and chill plate 24
downwardly at a fixed withdrawal rate into the cooling zone 42,
until the mold 12 is entirely within the cooling zone 42 as
represented in FIG. 2. The insulation zone 44 is required to
maintain the high thermal gradient at the solidification front to
prevent nucleation of new grains during the directional
solidification processes. The temperature of the chill plate 24 is
preferably maintained at or near the temperature of the cooling
zone 42, such that dendritic growth begins at the lower end of the
mold 12 and the solidification front travels upward through the
mold 12.
[0007] FIGS. 1 and 2 represent a single-crystal seed 28 within a
cavity 50 at the bottom of the mold 12. The casting 32 epitaxially
grows from the seed 28, such that both the primary and secondary
crystal orientations are controlled to yield a single-crystal
casting. The seed 28 represented in FIGS. 1 and 2 has a cylindrical
shape, which is conventional for directional casting techniques,
though other shapes are known. FIGS. 1 and 2 further represent a
crystal selector 30 coupling the seed cavity 50 to the mold cavity
14, which ensures that a single crystal enters the cavity 14. A
bridge 40 connects protruding sections of the casting 32 with lower
sections of the casting 32 so that crystal nucleation at these
protruding locations can be suppressed and a unidirectional
columnar single crystal forms substantially throughout the casting
32.
[0008] Mechanical properties of DS and SX castings depend, to a
large degree, on the avoidance of grain misorientation defects, for
example, high-angle grain boundaries, equiaxed grains, and other
potential defects that may occur as a result of the directional
solidification process. The avoidance of such defects in a SX
casting depends primarily on whether the crystal orientation of the
seed 28 can be successfully extended into the casting 32. For this
purpose, the seed 28 must be properly oriented at the bottom of the
mold 12. In an ideal situation, when the molten alloy 16 is poured
into the mold 12 and makes contact with the seed 28, a portion of
the single-crystal seed 28 is re-melted. Then, as the mold 12 is
slowly withdrawn from the hot zone 26, continuous epitaxial grain
growth occurs to yield a single crystal article with an orientation
dictated by the single-crystal seed 28.
[0009] Although casting processes of the type represented in FIGS.
1 and 2 are typically carried out in vacuum, a thin oxide film can
form at the interface between the molten alloy 16 and the
single-crystal seed 28 if the alloy and/or seed 28 contains
elements capable of chemically reacting with residual oxygen in the
vacuum chamber. It is understood that this oxide film is ceramic in
nature and can prohibit continuous grain growth from the seed 28,
generate misoriented grains, and cause defects in the final casting
32. The formation of an oxide film at the seed-alloy interface can
be inhibited by reducing the availability of oxygen and reactive
elements within the alloy. However, most nickel-base superalloys
used to form single-crystal castings rely on the presence of
aluminum to form Ni.sub.3Al (gamma prime) as the primary
strengthening phase for alloys used to form articles subjected to
high stresses in high temperature environments. For example, Rene
N5 (U.S. Pat. No. 6,074,602) contains about 5 to about 7 weight
percent aluminum, and CMSX-10 has a nominal aluminum content of
about 5.7 weight percent. The oxide films that form during
directional solidification of these alloys have been found to
typically be aluminum oxide (Al.sub.2O.sub.3) mixed with chromium
oxide (Cr.sub.2O.sub.3), nickel monoxide (NiO) and titanium oxide
(Ti.sub.2O.sub.3). Due to the high reactivity of aluminum with
oxygen, Al.sub.2O.sub.3 can form at partial pressures of oxygen
being as low as 10.sup.-18 torr at a pouring temperature of about
1500.degree. C., which is equivalent to a vacuum of 10.sup.-6 torr
if water is assumed to be the only residual gas. However, the
vacuum in a Bridgman system is typically not better than 10.sup.-3
torr. Furthermore, oxygen may be present as an impurity in the
molten alloy 16 and/or seed 28, can be present in the mold 12, and
can also form as a result of reactions between the mold 12 and the
molten alloy 16.
[0010] Aside from excluding aluminum from the alloy being cast,
attempts to inhibit the formation of an oxide film at the
seed-alloy interface have included excluding aluminum from the
seed, as reported in U.S. Pat. No. 6,740,176 and U.S. Published
Patent Application No. 2010/0058977. However, if aluminum is a
required constituent of the seed and/or the casting alloy, it is
very difficult to prevent the formation of an oxide film at the
seed-alloy interface.
BRIEF DESCRIPTION OF THE INVENTION
[0011] The present invention provides a process of casting an alloy
using a unidirectional casting technique to produce a casting
having a single-crystal (SX) microstructure. The invention further
provides a single-crystal seed whose geometry is able to
automatically destabilize an oxide film that attempts to form at
the interface between the seed and the molten metal during the mold
filling process, and thereby promotes a continuous single-crystal
grain growth and reduces and preferably eliminates grain
misorientation defects that would otherwise initiate from the oxide
film at the seed/metal interface.
[0012] According to a first aspect of the invention, the seed
includes a body having a single-crystal microstructure and at least
a first surface region. The first surface region defines a vertex
of the body that protrudes away from the body. The vertex is
adapted to destabilize an oxide film attempting to form on the
first surface region when the first surface region is contacted by
a molten metal during a mold filling step of the casting process.
The vertex is believed to destabilize the oxide film as a result of
surface tension of the oxide film at the vertex being sufficiently
high to cause the oxide film to collapse as the oxide film is
forming on the first surface region during the casting process.
[0013] According to other aspects of the invention, a casting
apparatus and a casting process are provided that utilize the seed
described above to cast an alloy. For example, such an apparatus
may have a mold having a base and a mold cavity adjacent thereto.
The mold cavity is adapted to contain a molten quantity of the
alloy during solidification thereof to yield a
unidirectionally-solidified casting defined by the mold cavity. A
heating zone is provided to heat the mold and the molten quantity
of the alloy therein to a heating temperature above the liquidus
temperature of the alloy. A cooling zone is provided to cool the
mold and the molten quantity of the alloy therein to a cooling
temperature below the solidus temperature of the alloy to cause
unidirectional solidification of the molten quantity of the alloy
and thereby yield the unidirectionally-solidified casting. The
single-crystal seed is disposed in the base of the mold and is
coupled to the mold cavity so that the molten quantity of the alloy
epitaxially solidifies based on a crystallographic orientation of
the seed.
[0014] According to another aspect of the invention, a process of
casting an alloy includes providing a mold having a base and a mold
cavity adjacent thereto, placing a single-crystal seed in the base
of the mold, introducing a molten quantity of the alloy into the
mold cavity, and then cooling the mold to cause unidirectional
solidification of the molten quantity of the alloy within the mold
and produce a unidirectionally-solidified casting having a columnar
crystal structure. The seed comprises a body having a
single-crystal microstructure and at least a first surface region.
The first surface region defines a vertex of the body that
protrudes away from the body. The molten quantity of the alloy
contacts the seed so that the molten quantity epitaxially
solidifies based on a crystallographic orientation of the seed. The
vertex of the body of the seed destabilizes an oxide film
attempting to form on the first surface region as a result of
surface tension of the oxide film at the vertex being sufficiently
high to cause the oxide film to collapse as the oxide film is
forming on the first surface region.
[0015] A technical effect of the invention is the ability to
promote the mechanical properties of a casting, and particularly
single-crystal castings, that depend primarily on the avoidance of
potential defects that can occur during a unidirectional
solidification process due to the formation of an oxide film at the
interface between the molten metal and a single-crystal seed used
to initiate the epitaxial growth required to produce a
directionally solidified casting. In particular, a preferred aspect
of the invention is that the seed has a geometry capable of
destabilizing the oxide film to the extent that the film tends to
collapse and does not interfere with the epitaxial grain growth
from the seed during the casting process. Consequently, the seed is
able to reduce grain misorientation defects that tend to initiate
from the seed/metal interface and therefore can improve the yield
of single-crystal castings produced by the process.
[0016] Other aspects and advantages of this invention will be
better appreciated from the following detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIGS. 1 and 2 represent sectional views showing two steps of
a unidirectional casting (solidification) process to produce a
single-crystal turbine blade.
[0018] FIG. 3 schematically represents a single-crystal seed
conventionally used in unidirectional casting processes in
accordance with the prior art.
[0019] FIG. 4 schematically represents a single-crystal seed
suitable for use in a unidirectional casting process in accordance
with an embodiment of this invention.
[0020] FIGS. 5 and 6 schematically represent single-crystal seeds
evaluated during investigations leading to the present
invention.
[0021] FIGS. 7 through 10 are microphotographs showing in
cross-section four castings produced using the seeds of FIGS. 3
through 6, respectively.
DETAILED DESCRIPTION OF THE INVENTION
[0022] The present invention can be employed to produce various
castings from a wide variety of alloys, including but not limited
to nickel-base, cobalt-base and iron-base superalloy. Certain
capabilities of the invention are particularly well suited for
producing castings having a columnar single-crystal microstructure
(SX). In some cases, a preferred single-crystal direction is
<001>, though crystalline structures having orientations
other than <001> are also within the scope of the invention.
The capabilities of the invention are also particularly well suited
for producing castings from alloys that contain levels of reactive
elements above incidental or trace amounts that may otherwise be
present. Most notably, an alloy may contain aluminum at a level of
0.5 weight percent or more, which renders the alloy reactive to
oxygen in the casting environment, including the surrounding
atmosphere as well as any oxygen that might be available in the
alloy being cast and the mold and cores used to cast the alloy.
Other reactive elements of potential concern include titanium,
yttrium and rare-earth elements. In addition to aluminum, these
elements are commonly found in alloys used to produce cast articles
suitable for such applications as the hot gas flow path components
of a gas turbine, including but not limited to buckets and nozzles
of land-based gas turbines, blades and vanes of aircraft gas
turbines, as well as shrouds found in both types of gas turbines.
While the advantages of this invention will be described with
reference to SX components of a gas turbine, the teachings of this
invention can be applicable to other components that may benefit
from being unidirectionally cast.
[0023] A SX casting can be produced with the present invention from
a melt of the desired alloy, for example, prepared by known vacuum
induction melting techniques. The melt is then cast in a mold, in
particular an investment mold such as the shell mold 12 used with
the apparatus 10 represented in FIGS. 1 and 2. As such, the
previous discussion of the apparatus 10 can also be applied to the
discussion of the present invention though, as discussed below,
with at least one notable exception being the single-crystal seed
28 represented in FIGS. 1 and 2. The present invention proposes
modifications to the seed 28 that are capable of promoting the
metallurgical and mechanical properties of the casting 32 beyond
what can ordinarily be achieved with conventional unidirectional
casting techniques. The invention does not necessarily restrict or
otherwise modify other aspects of the apparatus 10. For example,
the mold 12 may be formed of conventional mold materials such as
alumina or silica, and cores may be positioned within the mold
cavity 14 to form internal passages/features in the casting.
Furthermore, liquid metal can be introduced into the mold cavity 14
through a gating system (not shown), and a riser (not shown) may be
used to feed the solidification shrinkage of the casting. As such,
the following discussion will refer to the apparatus 10 described
in reference to FIGS. 1 and 2, and aspects of the casting process
and apparatus 10 not discussed in any detail below can be, in terms
of structure, function, materials, etc., essentially as was
described in reference to FIGS. 1 and 2.
[0024] As noted above, the present invention is primarily directed
to the use of a single-crystal seed that differs from the
cylindrical-shaped seed 28 represented in FIGS. 1 and 2. The
conventional seed 28 is schematically represented in isolation in
FIG. 3. The seed 28 has a planar circular-shaped upper surface
region 50 and a cylindrical lower surface region 52. As evident
from FIGS. 1 and 2, the molten alloy 16 poured into the mold cavity
14 comes in contact with the upper surface region 52 of the seed
28, causing the seed 28 to melt at this surface region 52 and
initiate epitaxial growth that is consistent with the orientation
of the single-crystal seed 28 as the mold 12 is slowly withdrawn
from the hot zone 26 of the apparatus 10. In this manner, the seed
28 controls both the primary and secondary crystal orientations of
the casting 32 characteristic of a single-crystal casting.
[0025] FIG. 4 schematically represents a single-crystal seed 58 in
accordance with an embodiment of the present invention. The seed 58
can be seen to be in the form of a body having an upper surface
region 60 with a protruding conical shape and, similar to the seed
28 of FIG. 3, a lower surface region 62 having a cylindrical shape.
The body of the seed 58 is represented as being unitary, though it
is foreseeable that lower portions of the seed 58 could differ in
shape and composition from the remainder of the seed 58, and
particularly its conical-shaped upper surface region 60. As a
result of its conical shape, the upper surface region 60 defines a
vertex (apex) 64 of the body that extends or protrudes away from
the remainder of the body, including the cylindrical surface region
62 of the body. When placed in the seed cavity 50, the vertex 64
faces the mold cavity 12 such that the molten alloy 16 placed in
the cavity 14 first comes into contact with the upper surface
region 60, causing initial melting of the seed 58 to occur at the
upper surface region 60 and initiate epitaxial growth that results
in the single-crystal casting 32. Notably, whereas the epitaxial
growth direction is normal to the flat upper surface region 50 of
the conventional seed 28 of FIG. 3, the epitaxial growth direction
is not normal to any part of the conical-shaped upper surface
region 60 of the seed 58 of FIG. 5.
[0026] According to a preferred aspect of the invention, the vertex
64 of the upper surface region 60 is capable of destabilizing an
oxide film that attempts or begins to form on the interface defined
by and between the seed 58 and the molten alloy 16. Due to a very
large surface tension believed to be present at the vertex 64 of
the seed 58, any oxide film that begins to form on the surface
region 60 tends to collapse at the vertex 64, with the result that
any oxide film that has formed on the remainder of the surface
region 60 will collapse under surface tension. In contrast, an
oxide film is able to remain stable as it forms on the flat upper
surface region 50 of the conventional seed 28 of FIG. 3.
[0027] Due to its conical shape, the upper surface region 60 of the
seed 58 is a surface of revolution formed by rotating a segment of
a first line around a second line that intersects the first line.
In geometric terms, the upper surface region 60 can be described as
a lateral surface of the conical portion of the seed 58. The upper
surface region 60 is represented in FIG. 4 as a right circular
cone, in that an axis 66 that passes through the vertex 64 (and
therefore about which the upper surface region 60 has rotational
symmetry) also passes through the center of the base 68 of the cone
at a right angle, and the base 68 is a circle. However, it is
foreseeable that the upper surface region 60 could have other
conical shapes, such as an oblique cone in which the axis 66 does
not pass perpendicularly through the center of the base 68.
Furthermore, the base 68 is not required to be circular, but may
have any shape, including rectilinear. The upper surface region 60
is preferably disposed at an angle of about 20 to about 40 degrees
from the axis 66, though lesser and greater angles are foreseeable.
In addition, the height of the conical shaped defined by the upper
surface region 60 (as defined by the distance between the vertex 64
and the base 68) can vary depending on the size of the seed 58 and
the particular application in which the seed 58 is to be used,
though a suitable height is believed to be in a range of about 0.5
to about 1.5 centimeters.
[0028] Preferred crystallographic orientations for the seed 58 will
depend on the particular application, though for producing
single-crystal castings it may be preferred that the <001>
crystal axis of the seed 58 is oriented parallel to the axis 66.
Similarly, preferred materials for the seed 58 will depend on the
particular application, including the particular alloy being cast.
Generally, the predominant constituent of the casting alloy will
also be the predominant constituent of the seed, for example, the
seed 58 will have a nickel-base alloy composition when casting a
nickel-base alloy. Notably, the effectiveness of the vertex 64 to
destabilize the formation of an oxide film allows for the seed 58
to be formed of an alloy that contains one or more reactive
elements, such as aluminum, titanium, yttrium, rare-earth metals,
and other potentially reactive elements that would otherwise be of
concern to form an oxide film.
[0029] As with the apparatus 10 and process described in reference
to FIGS. 1 and 2, casting processes performed with the seed 58 of
FIG. 4 are preferably carried out in a vacuum or an inert
atmosphere. The mold 12 is preheated prior to introducing the melt
of the desired alloy, preferably to a temperature equal to or above
the melting temperature of the alloy, and more particularly above
the liquidus temperature of the alloy, after which unidirectional
solidification is initiated by withdrawing the chill plate 24 and
the base of the mold 12 downwardly at a fixed rate through the
insulation zone 44 where solidification is initiated, and then into
the cooling zone 42 where solidification is completed. The cooling
zone 42 may contain a liquid metal cooling bath 46, or a vacuum or
ambient or cooled air for radiation cooling. Depending on
particular conditions, a single unidirectional columnar crystal
(SX) forms substantially throughout the casting 32. For example,
the seed 58 can be oriented with the seed cavity 50 so that
epitaxial growth occurs with the <100> orientation. From the
above, it should be appreciated that the overall sequence of the
unidirectional solidification process performed with the seed 58
can be similar to unidirectional solidification processes performed
with other traditional Bridgman furnaces.
[0030] In investigations leading to the present invention, a melt
of an aluminum alloy containing about 5 weight percent copper was
prepared, along with single-crystal seeds configured according to
the conventional cylindrical seed 28 of FIG. 3, the seed 58 of FIG.
4, and two additional seeds whose geometries are schematically
represented in FIGS. 5 and 6. Each seed was formed of essentially
the same Al--Cu alloy as the melt. Each of the seeds represented in
FIGS. 5 and 6 has an outer cylindrical shape and an inward conical
recess defined in its upper surface, and is therefore essentially
the inverse of the outward conical protrusion of the seed 58
represented in FIG. 4. The seed of FIG. 6 differed from that of
FIG. 5 by including a small amount of silica (SiO.sub.2) powder in
its conical recess. Each of the four seeds had a total height of
about 2.0 centimeters from top to bottom, and the cylindrical
surface region of each seed had a diameter of about 0.6 centimeter.
The height of the conical shape of the upper surface region 60 of
the seed 58 was about 0.5 centimeter, and the upper surface region
60 was disposed at an angle of about 30 degrees to the axis 66 of
the seed 58. The depth of the conical shape of each recessed
surface region of the seeds shown in FIGS. 5 and 6 was about 0.5
centimeter, and the recessed surface regions were disposed at an
angle of about 30 degrees to the axes of the seeds.
[0031] All four seeds were employed in the same or otherwise
identical molding apparatus, and roughly the same amounts of the
Al--Cu alloy were unidirectionally solidified using essentially
identical processes, including the same growth velocity and
temperature gradient. Sections of castings produced with the seeds
of FIGS. 3 through 6 are shown in FIGS. 7 through 10, respectively.
In FIG. 7, corresponding to the conventional cylindrical-shaped
seed 28 of FIG. 3, an oxide film can be clearly seen at the
interface between the cast Al--Cu alloy and the remainder of the
seed 28 (following partial melting of its upper surface region 50).
An oxide film can be similarly seen at the same interface for the
Al--Cu alloy castings produced with the seeds of FIGS. 5 and 6. In
contrast, no oxide film is evident at the interface (or elsewhere)
for the Al--Cu alloy casting produced with the seed 58 of FIG. 4.
From these results, it was concluded that a seed having an upper
surface region that defines a vertex is capable of preventing the
formation of an oxide film through some mechanism by which the
oxide film breaks and/or collapses as it attempts to form. As such,
the seed and its vertex have the ability to reduce grain
misorientation defects that can initiate from the seed/metal
interface.
[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 seed 58, the apparatus 10, and castings formed therewith could
differ from those shown, and the seed 58 could be used in a casting
process that differs from what was described above in reference to
the apparatus 10. Therefore, the scope of the invention is to be
limited only by the following claims.
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