U.S. patent application number 14/182874 was filed with the patent office on 2015-08-20 for methods for directional solidification casting.
This patent application is currently assigned to General Electric Company. The applicant listed for this patent is General Electric Company. Invention is credited to Francis Johnson, Stephen Francis Rutkowski, Wanming Zhang, Qi Zhao, Min Zou.
Application Number | 20150231696 14/182874 |
Document ID | / |
Family ID | 53797280 |
Filed Date | 2015-08-20 |
United States Patent
Application |
20150231696 |
Kind Code |
A1 |
Zhao; Qi ; et al. |
August 20, 2015 |
METHODS FOR DIRECTIONAL SOLIDIFICATION CASTING
Abstract
A method of directionally solidifying a molten alloy is
presented. The molten alloy is disposed in a shell mold that has a
thermal conductivity value greater than about 2 W/m-K. During the
direction solidification, heat is transferred from the shell mold
to a cooling region with a heat extraction rate greater than about
120 W/m.sup.2.
Inventors: |
Zhao; Qi; (Niskayuna,
NY) ; Johnson; Francis; (Clifton Park, NY) ;
Rutkowski; Stephen Francis; (Duanesburg, NY) ; Zou;
Min; (Niskayuna, NY) ; Zhang; Wanming;
(Clifton Park, NY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
General Electric Company |
Schenectady |
NY |
US |
|
|
Assignee: |
General Electric Company
Schenectady
NY
|
Family ID: |
53797280 |
Appl. No.: |
14/182874 |
Filed: |
February 18, 2014 |
Current U.S.
Class: |
164/122.1 |
Current CPC
Class: |
B22C 9/04 20130101; B22D
21/06 20130101; B22C 7/02 20130101; B22D 27/045 20130101 |
International
Class: |
B22D 27/04 20060101
B22D027/04; B22D 21/06 20060101 B22D021/06 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND
DEVELOPMENT
[0001] This invention was made with Government support under
contract number DE-EE0005573, awarded by the U.S. Department of
Energy. The Government has certain rights in the invention.
Claims
1. A method comprising directionally solidifying a molten alloy
disposed in a shell mold by transferring heat from the shell mold
to a cooling region with a heat extraction rate greater than about
120 W/m.sup.2.
2. The method of claim 1, wherein directionally solidifying the
molten alloy comprises the steps of; disposing the molten alloy
into the shell mold; and contacting the shell mold containing the
alloy with the cooling region.
3. The method of claim 2, wherein contacting the shell mold with
the cooling region comprises withdrawing the shell mold from a
heating region into the cooling region.
4. The method of claim 3, wherein withdrawing the shell mold
comprises withdrawing the shell mold with a withdrawal rate ranging
from about 50 millimeters/hour to about 400 millimeters/hour.
5. The method of claim 3, wherein withdrawing the shell mold
comprises withdrawing the shell mold with a withdrawal rate ranging
from about 200 millimeters/hour to about 300 millimeters/hour.
6. The method of claim 1, wherein directionally solidifying the
molten alloy comprises maintaining a superheat of the molten alloy
at least about 20 degrees Celsius.
7. The method of claim 1, wherein the shell mold has a thermal
conductivity value greater than about 2 W/m-K.
8. The method of claim 1, wherein the shell mold has a thermal
conductivity value in a range from about 5 W/m-K to about 15
W/m-K.
9. The method of claim 1, wherein transferring heat from the shell
mold to the cooling region comprises transferring heat with the
heat extraction rate in a range from about 150 W/m.sup.2 to about
300 W/m.sup.2.
10. The method of claim 1, wherein the alloy comprises a
nickel-based alloy, a cobalt-based alloy, or an iron-based
alloy.
11. The method of claim 1, wherein the cooling region comprises a
liquid coolant.
12. A method comprising directionally solidifying a molten alloy
disposed in a shell mold, wherein the shell mold has a thermal
conductivity value greater than about 2 W/m-K.
13. The method of claim 12, wherein the shell mold comprises a
support having a face coat disposed on an inner surface of the
support and a seal coat disposed on the outer surface of the
support.
14. The method of claim 13, wherein the support comprises a
material having a thermal conductivity greater than about 285
W/m-K.
15. The method of claim 14, wherein the material comprises silicon
carbide, aluminum nitride, diamond, graphite, or a combination of
any of the foregoing.
16. The method of claim 12, wherein directionally solidifying the
molten alloy comprises the steps of; disposing the molten alloy
into the shell mold; and contacting the shell mold containing the
molten alloy with a cooling region.
17. The method of claim 16, wherein contacting the shell mold with
the cooling region comprises withdrawing the shell mold from a
heating region into the cooling region.
18. The method of claim 17, wherein withdrawing the shell mold
comprises withdrawing the shell mold with a withdrawal rate ranging
from about 50 millimeters/hour to about 400 millimeters/hour.
19. The method of claim 17, wherein withdrawing the shell mold
comprises withdrawing the shell mold with a withdrawal rate ranging
from about 100 millimeters/hour to about 300 millimeters/hour.
20. The method of claim 16, wherein contacting the shell mold with
the cooling region comprises transferring heat from the shell mold
to the cooling region with a heat extraction rate greater than
about 120 W/m.sup.2.
21. The method of claim 16, wherein contacting the shell mold with
the cooling region comprises transferring heat from the shell mold
to the cooling region with a heat extraction rate in a range from
about 150 W/m.sup.2 to about 300 W/m.sup.2.
22. The method of claim 12, wherein directionally solidifying the
molten alloy comprises maintaining a superheat of the molten alloy
at least about 20 degrees Celsius.
23. The method of claim 12, wherein the shell mold has a thermal
conductivity value in a range from about 5 W/m-K to about 15
W/m-K.
24. The method of claim 12, wherein the alloy comprises a
nickel-based, a cobalt-based, or an iron-based alloy.
Description
BACKGROUND
[0002] The present disclosure generally relates to directional
solidification, and more particularly, to directional
solidification processes for casting large sized components at a
large scale.
[0003] Directional solidification (DS) techniques enable the
solidification of materials with grains aligned in a specific
direction. Specifically, directional and single crystal (SC)
structures are formed to improve the mechanical and metallurgical
properties of the cast materials at their operating use
temperatures. Various industrial DS/SC components of nickel-based
alloys, iron-based alloys and cobalt-based alloys are traditionally
produced by using the well-known Bridgman process. However, due to
the thick ceramic molds and the poor cooling condition used in this
process, it is difficult to maintain the required high temperature
gradient necessary for the desired solidification structure.
[0004] Demand for a more efficient DS/SC casting with a high
temperature gradient led to the development of modified techniques,
such as liquid metal cooling (LMC). Although LMC results in higher
cooling rates and finer microstructures compared to the
conventional processes, it is still not widely applied in the
industrial field due to the high equipment investments and lower
throughput. Additionally, the heat transfer through the thick shell
mold is not significantly improved by the LMC technique, this
shortcoming is a barrier to further increases in thermal gradient,
particularly in the casting of large-sized DS/SC components.
[0005] One way to improve throughput would be to increase the
production rate at which components are being cast, which is
typically limited by the rate of heat extraction, the rate of
solidification of the alloy and the rate of withdrawal in the
process. Because of this, it is a challenge to attain the correct
microstructure during directional solidification and produce large
castings with high production rate. Accordingly, there remains a
need for improvements in directional solidification processes for
large scale casting.
BRIEF DESCRIPTION
[0006] One embodiment of the invention is directed to a method for
directionally solidifying a molten alloy disposed in a shell mold
by transferring heat from the shell mold to a cooling region with a
heat extraction rate greater than about 120 W/m.sup.2.
[0007] Another embodiment of the invention relates to a method that
includes directionally solidifying a molten alloy disposed in a
shell mold, wherein the shell mold has a thermal conductivity value
greater than about 2 W/m-K.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] These and other features and aspects 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.
[0009] FIG. 1 is a schematic diagram of a directional
solidification casting apparatus, according to one embodiment of
the invention;
[0010] FIG. 2 is a graphical representation of an improvement
obtained in the coercivity of an ALNICO permanent magnet formed by
using a directional solidification process according to an
embodiment of the present invention;
[0011] FIG. 3 is a graphical representation of an improvement
obtained in the remanence of an ALNICO permanent magnet formed by
using a directional solidification process according to an
embodiment of the present invention;
[0012] FIG. 4 is a graphical representation of an improvement
obtained in the energy product of an ALNICO permanent magnet formed
by using a directional solidification process according to an
embodiment of the present invention.
DETAILED DESCRIPTION
[0013] 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.
[0014] As used herein, the terms "may" and "may be" indicate a
possibility of an occurrence within a set of circumstances; a
possession of a specified property, characteristic or function;
and/or qualify another verb by expressing one or more of an
ability, capability, or possibility associated with the qualified
verb. Accordingly, usage of "may" and "may be" indicates that a
modified term is apparently appropriate, capable, or suitable for
an indicated capacity, function, or usage, while taking into
account that in some circumstances, the modified term may sometimes
not be appropriate, capable, or suitable.
[0015] In the following specification and claims, the singular
forms "a", "an" and "the" include plural referents unless the
context clearly dictates otherwise. The terms "comprising,"
"including," and "having" are intended to be inclusive, and mean
that there may be additional elements other than the listed
elements. As used herein, the term "and/or" includes any and all
combinations of one or more of the associated listed items.
[0016] The present invention relates to a method for manufacturing
directionally solidified (DS) articles using, for example a Liquid
Metal Cooling (LMC)-process. Aspects of the present invention can
be employed to produce various castings from a wide variety of
alloys including but not limited to nickel-based alloy,
cobalt-based alloy, and iron-based alloy. An alloy to be cast is
generally referred to as a casting alloy. While the advantages of
the present invention are described with reference to the permanent
magnets used in electric machines, for example ALNICO magnets, the
teachings of the present invention are generally applicable to
other components that may benefit from being unidirectionally
cast.
[0017] A directionally solidified (DS) or single crystal (SC)
casting is typically produced from a melt of a desired alloy often
prepared by known vacuum induction melting techniques. As known in
the art, heat transfer conditions during the solidification of the
casting are controlled so that a solidification front advances
unidirectionally and steadily to generate primary columnar
crystals/grains, and to avoid the nucleation and formation of
secondary grains from the melt in competition with the primary
columnar single crystal. Moreover, the ability to control the
crystal orientation of castings in the DS casting techniques proved
to be highly beneficial in the casting of magnetic alloys, which
results in improved magnetic properties along a certain direction
in the cast product. The present invention proposes improved
features and steps to enable directional solidification of large
sized castings with improved mechanical and metallurgical
properties (e.g., magnetic properties in the case of a magnetic
alloy) for high throughput applications.
[0018] FIG. 1 schematically represents a casting apparatus 10
capable of producing a directionally solidified casting by an
investment casting process. The casting apparatus 10 includes a
high temperature region 12 (heating region) and a low temperature
region 14 (cooling region). The heating region 12 usually includes
heating elements 16. The cooling zone 14 is typically located
directly beneath the heating region 12, while separated with the
heating region 12 by a baffle or heat shield 18. The cooling region
14 generally includes a tank containing a liquid coolant 20. The
coolant 20 typically includes a liquid metal bath such as, for
example, molten tin at a temperature of about 250 degrees Celsius
to about 500 degrees Celsius, or molten aluminum at a temperature
of about 700 degrees Celsius to about 900 degrees Celsius. Other
metals having low melting temperatures can potentially be used as a
liquid coolant, such as lithium, magnesium, zinc, gallium, and
indium.
[0019] The apparatus 10 further includes a shell mold 22 generally
secured to a chill plate 24, and located within the heating region
12 to be heated to a temperature at least equal to, and typically
above the liquidus temperature of a casting alloy. The shell mold
22, according to the aspects of the present invention, exhibits
high thermal conductivity.
[0020] As known in the art, a shell mold is typically formed of a
support having an outer surface and an inner surface that generally
defines an internal mold cavity corresponding to the desired shape
of a resulting casting or component. The support may include a fine
particle dispersion reinforced with plurality of coarse ceramic
particulates generally called "stucco". Stucco may include a
plurality of coarse ceramic particulates arranged in one or more
layer form. A facecoat is generally disposed at the inner surface
and a sealcoat is disposed at the outer surface of the support.
[0021] According to some embodiments of the invention, a high
thermal conductivity stucco is used to build the shell mold 22. Any
high thermal conductivity ceramic material may be used in the
stucco to form a support of the shell mold 22. In one instance, the
thermal conductivity of a material used as stucco in the support is
greater than about 285 W/m-K. Non-limiting examples of high thermal
conductivity stucco materials include materials such as silicon
carbide, aluminum nitride, diamond, graphite, or any combination of
the foregoing. In certain embodiments, the support includes silicon
carbide stucco in addition to a fine particle dispersion of alumina
and silica.
[0022] The high thermal conductivity stucco increases the overall
thermal conductivity of the shell mold 22. Furthermore, a higher
percentage of high thermal conductivity stucco material in the
support may raise the thermal conductivity of the support, thereby
allowing a thinner support and thus a thinner shell mold. In one
embodiment, the stucco in the support is in a concentration greater
than about 40 volume percent of the support. In one embodiment, the
high thermal conductivity stucco is in excess of about 50 volume
percent of the support. The shell mold 22, according to the aspects
of the present invention, exhibits a high thermal conductivity. In
one embodiment, the shell mold 22 has a thermal conductivity value,
at a direction perpendicular to a surface of the mold 22, greater
than about 2 W/m-K. In some embodiments, the thermal conductivity
of the shell mold 22 is greater than about 5 W/m-K. In some
embodiments, the thermal conductivity of the shell mold 22 ranges
from about 5 W/m-K to about 15 W/m-K. In certain embodiments, the
thermal conductivity of the shell mold 22 prepared using SiC stucco
is greater than about 9 W/m-K. Various details of a high thermal
conductivity shell mold and method of manufacturing the same are
described in U.S. patent application Ser. No. 14/091,386 filed on
Nov. 27, 2013.
[0023] In some embodiments, a method is provided for directionally
solidifying a molten alloy (a casting alloy in molten form). In the
first step of the method, the shell mold 22 that is placed within
the heating region 12 is disposed with a molten alloy 26 to be
cast. The molten alloy 26 may be poured into the shell mold 22
through a feed opening (not shown) at the top of the heating region
12. At the time of charging (for example, pouring), the shell mold
22 is fully within the heating region 12, for example a casting
furnace. The shell mold 22 can be fully or partially filled with
the molten alloy 26 depending on desired shape and size of the cast
article. After filling the shell mold 22 with the molten alloy 26,
the mold 22 is brought in contact with the liquid coolant 20 in the
cooling region 14. The mold 22 is often withdrawn continuously from
the heating region 12 to the cooling region 14 through a
variable-sized opening 13 in the shield 18. The opening 13 enables
the shield 22 to fit closely around the shape of the mold 22 as it
is withdrawn from the heating region 12, through the heat shield
18, to the cooling region 14.
[0024] A direction of withdrawal 15 is indicated with an arrow. As
soon as the shell mold 22 reaches the cooling region 14,
solidification of the molten alloy 26 begins by transferring heat
from the shell mold 22 to the coolant 20. A solidified alloy 28
(also referred to as a solidified casting) is found at a lower
portion 23 of the shell mold 22, which already reached the cooling
region 14. Approximately at the border between the heating region
12 and the cooling region 14 is a solidification front 30. A
solidification front refers to a solid-liquid interface of the
casting alloy during the solidification of the alloy. The
solidification front 30 travels from the bottom of the shell mold
22 to the top in opposition to the direction of withdrawal 15.
[0025] Those familiar with the directional solidification
understand the advantage of a high thermal gradient across the
solidification front (solid-liquid interface) to yield good cast
microstructure. In order to provide a high thermal gradient, heat
needs to be removed quickly from the molten alloy. A thermal
gradient at the solidification front may be given by equation
(1).
G L .apprxeq. CH eff . submerse d cast Q heat - .rho. L [ h f + C p
. L ( T melt - T L ) ] v k L equation ( 1 ) ##EQU00001##
Where
[0026] G.sub.L is liquid thermal gradient at solidification front,
[0027] H.sub.eff, submerse shell effective submersion in liquid
coolant, [0028] d.sub.cast casting cross section equivalent
diameter, [0029] C constant determined by liquid metal (molten
alloy) properties and mixing conditions, [0030] Q.sub.heat heat
extraction rate normal to shell surface, [0031] .rho..sub.L molten
alloy density, [0032] h.sub.f casting heat of fusion, [0033]
c.sub.p,L molten alloy heat capacity, [0034] T.sub.melt molten
alloy temperature, [0035] T.sub.L liquidus temperature, [0036]
k.sub.L molten alloy thermal conductivity, [0037] v withdrawal
rate.
[0038] Referring to FIG. 1, the temperature of the heating region
12 is typically maintained higher than a liquidus temperature of
the casting alloy. A temperature difference between the temperature
of a molten alloy (T.sub.melt) in a casting furnace and the
liquidus temperature (T.sub.L) of the alloy is typically known as
"superheat" of the alloy. A sufficient amount of superheat is
typically required to achieve a highest possible thermal gradient
(G.sub.L) from the solid-liquid interface at the liquidus
temperature. On the other hand, as evident from equation (1), a low
superheat (T.sub.melt-T.sub.L) helps to enhance the thermal
gradient at the solid-liquid interface. Some embodiments of the
present invention include maintaining a superheat of the molten
alloy 26 as low as possible while ensuring full development of
thermal gradient. That is, the superheat may be maintained at a
value equal to or slightly higher than a minimum superheat required
for developing sufficient thermal gradient. A minimum value of the
superheat may depend on various process parameters including the
cooling profile, the withdrawal rate, and thickness of the baffle.
In some embodiments, the superheat of the molten alloy 26 is
maintained at least about 20 degrees Celsius, and more particularly
at least about 30 degrees Celsius. In some embodiments, the
superheat of the alloy 26 may be maintained in a range from about
30 degrees Celsius to about 50 degrees Celsius.
[0039] As mentioned previously, the baffle or heat shield 18 is
usually placed between a bottom of the heating region 12 and the
top of the cooling region 14 for shielding the cooling region 14
from the high temperature radiating from the heating region 12. The
baffle typically includes one or more openings therein to permit
passage of one or more molds there through as the molds are
withdrawn from the heating region 12 into the liquid coolant bath
20. In some embodiments, the baffle 18 is a floating baffle that
floats on the surface of the coolant 20 as shown in FIG. 1.
Materials used for constructing the floating baffle 18 should be
thermally insulating, chemically stable with respect to the liquid
coolant 20, and have a density less than that of the liquid coolant
to allow the baffle material to float on the coolant. Examples of
ceramic materials suitable for use with liquid aluminum or tin
include alumina and zirconia.
[0040] Various methods can be used for lowering the density of
these materials or other chemically compatible materials. For
example, hollow ceramic beads or bubbles may be formed of the
desired material. Use of a ceramic bead baffle may help to promote
a steep and steady thermal gradient during directional
solidification casting normal to the solidification front 30, and
to enable high withdrawal rates. The thickness of the ceramic bead
insulation can be adjusted to maintain the position of the
solid-liquid interface 30 above the level of the liquid coolant 20.
In some instances, the thickness of the ceramic bead baffle may
range from about 10 millimeters to 20 millimeters.
[0041] As understood by those skilled in the art and per equation
(1), a higher heat extraction rate from a molten alloy enables
faster solidification with a higher thermal gradient at the
solidification front. A heat extraction rate is a rate at which
heat is removed from a molten alloy to be solidified when the alloy
comes in contact to a cooling region. Heat extraction from a
solidifying alloy (i.e. molten alloy) in directional castings is
generally limited by the heat transfer in the process, withdrawal
rates, etc. According to the aspects of the invention, it was
observed that the directional solidification of the molten alloy 26
disposed in the shell mold 22 occurred with a high heat extraction
rate that is greater than about 120 W/m.sup.2. Using shell mold 22
having high thermal conductivity (as discussed previously) greatly
enhances the heat transfer across the shell mold 22 to the coolant
20, and increases the heat extraction rate as well as the
solidification rate of the alloy. In some instances, the heat
extraction rate during the solidification of the alloy 26 may be
greater than about 150 W/m.sup.2. In some instances, the heat
extraction rate may range from about 150 W/m.sup.2 to about 300
W/m.sup.2.
[0042] It has been further observed that a high extraction rate may
further improve the magnetic properties of a magnetic alloy when
cast using direction solidification process as described herein.
Effects of high extraction rate on magnetic properties of ALNICO
alloy are described below with respect to an example.
[0043] An increased withdrawal rate of the shell mold 22 from the
heating region 12 to the cooling region 14 adversely affects the
thermal gradient (equation 1), however it is desirable to be high
to improve the production rate. By achieving a high heat extraction
rate, the withdrawal rate of the shell mold may be increased to a
value that maintains a high enough thermal gradient to achieve
desired microstructure and material properties and improves the
production rate of castings. It has been observed that the
withdrawal rate of the shell mold 22 during directional
solidification casting can be increased greater than about 200
millimeters/hour. The withdrawal rate of the shell mold 22 can be
increased to as high as 500 millimeters/hour, though in some
instances, the withdrawal rate of the shell mold 22 may be
maintained between about 50 millimeters/hour and 400
millimeters/hour to attain desirably high thermal gradient while
maintaining high production rate for large scale castings. In
certain embodiments, the withdrawal rate of the shell mold 22 may
be maintained between about 100 millimeters/hour to about 300
millimeters/hour.
[0044] The present invention provides advantages over conventional
directional solidification processes by enabling castings of large
size articles with high throughput. Typically, small furnaces offer
the advantage of high thermal gradient, allowing better foundry
performance (microstructure, low porosity, absence of freckles,
etc.), while in large size furnaces, for example industrial
furnaces, the thermal gradient prevailing at the solidification
front is low as well as difficult to measure. It has been a
challenge to cast a large size article or component by directional
solidification with high production rate to increase the
throughput. Aspects of the present invention enable the use of a
directional solidification process (for example LMC) to achieve and
maintain high thermal gradients normal to the advancing
solidification front, utilizing several features including high
thermal conductivity shell mold and low superheat as described
above, and thus allowing the casting of large parts as permitted by
the overall dimensions of the casting unit with high
throughput.
Example
[0045] The following example is presented for illustrative purposes
only, and is not intended to limit the scope of the invention.
[0046] A shell mold was prepared by using a high emissivity
facecoat having about 89.48 weight percent alumina
(Al.sub.2O.sub.3), 2.24 weight percent chromium oxide
(Cr.sub.2O.sub.3), and 1.78 weight percent titanium oxide
(TiO.sub.2) (all <50 microns) in a 6.5 weight percent colloidal
silica (SiO.sub.2) binder. A support part was formed by dipping a
fugitive pattern into alumina slurry reinforced with high volume
fraction of silicon carbide (SiC) stucco. Each coat of slurry and
stucco were air-dried before subsequent coats are applied. The
steps of dipping the pattern and drying were repeated until the
desired thickness of about 5 millimeters to about 6 millimeters
(mm) of the shell mold was obtained. Finally a thin layer of dark
colored high emissivity SiC sealcoat was applied to the outer
surface of the mold. After air drying, and dewaxing, the shell mold
was then heated to a temperature of about 1000 degrees Celsius. The
shell mold was subjected to an additional firing from about 1480
degrees Celsius to 1550 degrees Celsius in hydrogen for one
hour.
[0047] The shell mold thermal conductivity, measured using a
Synthetic Thermal Time-Of-Flight Imaging (STTOF) method, was about
9.3 W/m-K, which is a 540% improvement in comparison with the
conventional alumina shell molds for the DS metal as shown in Table
1. The prepared SiC shell molds of about 6 mm in thickness were
applied to make directionally solidified Fe-based alloy castings
using liquid metal cooling at withdrawal rates ranging from about
100 millimeters/hour to 300 millimeters/hour.
TABLE-US-00001 TABLE 1 Shell mold Shell mold Thermal conductivity
specimen wall thickness K (W/m-K) SiC-01 0.155 9.35 SiC-02 0.149
9.12 SiC-03 0.17 9.46 Mean 9.31 Standard 0.17 Alumina-1 0.35 1.94
Alumina-2 0.34 1.94 Almina-3 0.35 1.84 Mean 1.91 Standard 0.06
[0048] A shell mold made by using the above example was used for
LMC directional solidification processes for improved crystalline
anisotropy (reduced crystalline disorientation) and refined as-cast
structure of ALNICO permanent magnets.
[0049] When liquid metal cooling is applied for directional
solidification, heat extraction from solidifying casting is limited
by shell thermal conductivity and shell thickness. A typical heat
extraction rate for an alumina shell mold for LMC directional
solidification was found to be about 112 W/m.sup.2. Using the thin,
and high thermal conductivity shell mold of the present example for
LMC directional solidification process, the heat extraction rate of
ALNICO processing was increased to about 240 W/m.sup.2. The
increased thermal gradient of the SiC shell mold provided fine
microstructure of ALNICO magnets leading to high product yield and
better product.
[0050] A high heat extraction of the shell mold helped in faster
withdrawal of the structure thereby increasing the casting rate.
Therefore, a DS casting of ALNICO magnet using the shell mold of
the present example helped in faster ALNICO manufacturing
production, including enabling a time and energy efficient
post-cast heat treatment of the ALNICO alloy.
[0051] Furthermore, the high heat extraction rate in ALNICO alloy
casting having about 7-12 weight percent Al, about 13-26 weight
percent Ni, about 5-40 weight percent Co, up to about 6 weight
percent Cu, up to about 12 weight percent Ti, with the balance Fe,
was shown to have a significant effect on the demagnetizing
properties of the ALNICO cast alloy as can be seen from the
following example.
[0052] The effects of heat extraction rate on inherent coercivity
(Hci), remanence (Br), and energy product (BH)max are depicted in
FIG. 2, FIG. 3, and FIG. 4 respectively showing an experimental
casting by using liquid metal cooling directional solidification
according to the aspects of the present invention, as compared to a
comparative casting by using conventional Bridgman directional
solidification process.
[0053] The inherent coercivity of the experimental ALNICO casting
made by LMC process is compared with that of the comparative
casting made by Bridgman (BRG) process in FIG. 2 to demonstrate the
effects of heat extraction rate on coercivity. Heat extraction rate
in LMC ranges between 150 W/m.sup.2 to 250 W/m.sup.2, depending on
the mold materials and shell thickness. Heat extraction rate in BRG
process is about 60 W/m.sup.2 to 80 W/m.sup.2, depending on shell
surface emissivity in vacuum. It was observed that with the
increase in heat extraction rate, the inherent coercivity also
increases.
[0054] Further, it was observed from FIG. 3 that with the increase
in heat extraction rate, the inherent remanence increases, and FIG.
4 shows the increase in energy product with the increase in heat
extraction rate. Hence it can be seen that as the thermal gradient
increases the remanence and coercivity of the ALNICO magnet
increases, thereby increasing the energy product.
[0055] 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.
* * * * *