U.S. patent number RE44,426 [Application Number 13/233,492] was granted by the patent office on 2013-08-13 for continuous casting of foamed bulk amorphous alloys.
This patent grant is currently assigned to Crucible Intellectual Property, LLC. The grantee listed for this patent is James Kang. Invention is credited to James Kang.
United States Patent |
RE44,426 |
Kang |
August 13, 2013 |
Continuous casting of foamed bulk amorphous alloys
Abstract
Methods and apparatuses for the continuous casting of solid foam
structures with varying bubble density from bulk solidifying
amorphous alloys are provided. Continuously cast solid foam
structures having bubble densities in the range of from 50 percent
up to 95% by volume are also provided.
Inventors: |
Kang; James (Laguna Niguel,
CA) |
Applicant: |
Name |
City |
State |
Country |
Type |
Kang; James |
Laguna Niguel |
CA |
US |
|
|
Assignee: |
Crucible Intellectual Property,
LLC (Rancho Santa Margarita, CA)
|
Family
ID: |
33302325 |
Appl.
No.: |
13/233,492 |
Filed: |
April 14, 2004 |
PCT
Filed: |
April 14, 2004 |
PCT No.: |
PCT/US2004/011909 |
371(c)(1),(2),(4) Date: |
April 23, 2007 |
PCT
Pub. No.: |
WO2004/091828 |
PCT
Pub. Date: |
October 28, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
10552496 |
Apr 23, 2007 |
7588071 |
Sep 15, 2009 |
|
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Current U.S.
Class: |
164/463; 164/423;
164/79 |
Current CPC
Class: |
B22D
11/0611 (20130101); B22D 25/005 (20130101); B22D
11/0622 (20130101); B22D 11/0631 (20130101); C22C
1/08 (20130101); C22C 2001/086 (20130101) |
Current International
Class: |
B22D
11/06 (20060101); B22D 27/00 (20060101) |
Field of
Search: |
;164/463,423,479-482,79 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2075551 |
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2236325 |
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2236325 |
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57109242 |
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57109242 |
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61238423 |
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JP |
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06-264200 |
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Sep 1994 |
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JP |
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06264200 |
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Sep 1994 |
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JP |
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2000-256811 |
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Sep 2000 |
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JP |
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2000256811 |
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Sep 2000 |
|
JP |
|
2000277127 |
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Oct 2000 |
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JP |
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2000277127 |
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Oct 2000 |
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JP |
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2001303218 |
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Oct 2001 |
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JP |
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2001303218 |
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Oct 2001 |
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JP |
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Other References
American Society for Metals, "Forging and Casting", Metals
Handbook, Jan. 1970, vol. 5, 8th Edition, 16 pgs. cited by examiner
.
Inoue et al., "Mg-Cu-Y Bulk Amorphous Alloys with High Tensile
Strength Produced by a High-Pressure Die Casting Method", Materials
Transactions, JIM, vol. 33, No. 10, pp. 937-945. cited by examiner
.
Inoue et al., "Bulky La-A1-TM (TM=Transition Metal) Amorphous
Alloys with High Tensile Strength Produced by a High-Pressure Die
Casting Method", Materials Transactions, JIM, vol. 34, No. 4, 1993,
pp. 351-358. cited by examiner .
Kato et al., Production of Bulk Amorphous Mg85Y10Cu5 Alloy by
Extrusion of Atomized Amorphous Powder, Materials Transactions,
JIM, vol. 35, No. 2, 1994, pp. 125-129. cited by examiner .
Kawamura et al., Full Strength Compacts by Extrusion of Glassy
Metal Powder at the Supercooled Liquid State, American Institute of
Physics, May 30, 1995, vol. 67, No. 14, pp. 2008-2010. cited by
examiner .
Polk et al., "The Effect of Oxygen Additions on the Properties of
Amorphous Transition Metal Alloys", pp. 220-230. cited by examiner
.
"Interbike Buyer Official Show Guide", advertisement, 1995, 1 page.
cited by examiner .
Primedia, Inc., "Interbike Official Show Guide Content", 1 page.
cited by examiner .
Amorphous Metal Research, "Interbike Exhibitors", 1995 Interbike
Buyer, p. 171, 1 pg. cited by examiner .
UES, Inc. Software Products Center, "ProCAST . . . not just for
castings!", Sep. 30, 1996, 1 pg. cited by examiner .
Warren M. Rohsenow, "Heat Transfer", Handbook of Engineering, 1936,
Section 12, pp. 1113-1119. cited by applicant.
|
Primary Examiner: Walker; Keith
Assistant Examiner: Yoon; Kevin E
Attorney, Agent or Firm: Pillsbury Winthrop Shaw Pittman
LLP
Claims
What is claimed is:
1. A method of manufacturing a continuous sheet of a metallic glass
foam from a bulk-solidifying amorphous alloy comprising: providing
a quantity of a bulk solidifying amorphous alloy foam precursor at
a casting temperature around the melting temperature of the alloy;
stabilizing the bulk solidifying amorphous alloy at a casting
temperature below the melting temperature [T.sub.m] of the alloy
and above the temperature at which crystallization occurs on the
shortest time scale for the alloy [T.sub.NOSE] such that the
viscosity of the bulk solidifying amorphous alloy is from about 0.1
to 10,000 poise; introducing the stabilized bulk solidifying
amorphous alloy foam precursor onto a moving casting body such that
a continuous sheet of heated bulk solidifying amorphous alloy is
formed thereon; and quenching the heated bulk solidifying amorphous
alloy foam precursor at a quenching rate sufficiently fast such
that the bulk solidifying amorphous alloy remains in a
substantially amorphous phase to form a solid amorphous continuous
foam sheet having a thickness of at least 0.1 mm.
2. The method according to claim 1, wherein the precursor is formed
by providing a molten bulk-solidifying amorphous alloy; and
introducing a plurality of gas bubbles to the molten alloy at a
temperature above the liquidus temperature of the molten alloy to
form a pre-cursor.
3. The method of claim 1, wherein the viscosity of the bulk
solidifying amorphous alloy at the "melting temperature" Tm of the
bulk solidifying amorphous alloy is from about 10 to 100 poise.
4. The method of claim 1, wherein the viscosity of the bulk
solidifying amorphous alloy at the "melting temperature" Tm of the
bulk solidifying amorphous alloy is from about 1 to 1000 poise.
5. The method of claim 1, wherein the critical cooling rate of the
bulk solidifying amorphous alloy is less than 1,000.degree.
C./sec.
6. The method of claim 1, wherein the critical cooling rate of the
bulk solidifying amorphous alloy is less than 10.degree.
C./sec.
7. The method according to claim 2, wherein the gas bubbles are
introduced to the molten alloy by stirring the molten alloy.
8. The method according to claim 2, wherein the gas bubbles are
introduced to the molten alloy by adding an gas releasing agent to
the molten alloy.
9. The method according to claim 1, wherein a volume fraction of
<30% of a plurality of bubbles having sizes between 1 .mu.m and
1 mm are introduced to the molten alloy.
10. The method according to claim 1, wherein at least 50% by volume
of the metallic glass foam has an amorphous atomic structure.
11. The method according to claim 1, further including homogenizing
the expanded bubbles by mechanically stirring the pre-cursor.
12. The method according to claim 1, wherein the step of
introducing gas bubbles to form the pre-cursor occurs at a pressure
up to 50 bar or more.
13. The method according to claim 1, wherein the bubbles of the
metallic foam have a size distribution of about 10 .mu.m.
14. The method according to claim 1, wherein the bulk solidifying
amorphous alloy is a Zr-base amorphous alloy.
15. The method of claim 1, wherein the quenching occurs on the
casting body.
16. The method of claim 1, wherein the casting body is selected
from the group consisting of a wheel, a belt, double-roll
wheels.
17. The method of claim 1, wherein the casting body is formed from
a material having a high thermal conductivity.
18. The method of claim 1, wherein the casting body is formed of a
material selected from the group consisting of copper, chromium
copper, beryllium copper, dispersion hardening alloys, and
oxygen-free copper.
19. The method of claim 1, wherein the casting body is at least one
of either highly polished or chrome-plated.
20. The method of claim 1, wherein the casting body moves at a rate
of 0.5 to 10 cm/sec.
21. The method of claim 1, the casting temperature of the alloy is
stabilized in a viscosity regime of 1 to 1,000 poise.
22. The method of claim 1, wherein the casting temperature of the
alloy is stabilized in a viscosity regime of 10 to 100 poise.
23. The method of claim 1, wherein the foam sheet has a thickness
of 0.5 to 3 mm.
.Iadd.24. A method of manufacturing a continuous sheet of a
metallic glass foam from a bulk-solidifying amorphous alloy
comprising: providing a quantity of a bulk solidifying amorphous
alloy foam precursor at a casting temperature around the melting
temperature of the alloy; stabilizing the bulk solidifying
amorphous alloy at a casting temperature below the melting
temperature (T.sub.m) of the alloy and above the temperature at
which crystallization occurs on the shortest time scale for the
alloy (T.sub.NOSE); forming the stabilized bulk solidifying
amorphous alloy foam precursor into a continuous sheet of heated
bulk solidifying amorphous alloy; and quenching the heated bulk
solidifying amorphous alloy foam precursor at a quenching rate
sufficiently fast such that the bulk solidifying amorphous alloy
remains in a substantially amorphous phase to form a solid
amorphous continuous foam sheet having a thickness of at least 0.1
mm..Iaddend.
.Iadd.25. The method of claim 24, wherein the viscosity of the
bulk-solidifying amorphous alloy at the casting temperature is from
about 0.1 to 10,000 poise..Iaddend.
Description
FIELD OF THE INVENTION
The present invention is directed to methods of continuous casting
amorphous metallic foams, and to amorphous metallic foams made from
bulk-solidifying amorphous alloys.
BACKGROUND OF THE INVENTION
Metallic foam structures (metallic solid foam or metallic cellular
solids) are known to have interesting combinations of physical
properties. Metallic foams offer high stiffness in combination with
very low specific weight, high gas permeability, and a high energy
absorption capability. As a result, these metallic foam materials
are emerging as a new engineering material. Generally, foam
structures can be classified as either open or closed porous. Open
foams are mainly used as functional materials, such as for gas
permeability membranes, while closed foams find application as
structural materials, such as energy absorbers. However, the broad
application of metallic foams has been hindered by the inability of
manufacturers to produce uniform and consistent foam structures at
low cost. Specifically, current manufacturing methods for producing
metallic foams result in an undesirably wide distribution of cell
and/or pore sizes which cannot be satisfactorily controlled. These
manufacturing limits in turn degrade the functional and structural
properties of the metallic foam materials.
The production of metallic foamed structures is generally carried
out in the liquid state above the melting temperature of the
material, though some solid-state methods have also been used. The
foaming of ordinary metals is challenging because a foam is an
inherently unstable structure. The reason for the imperfect
properties of conventional metallic foams comes from the
manufacturing process itself. For example, although a pure metal or
metal alloy can be manufactured to have a large volume fraction
(>50%) of gas bubbles, a desired bubble distribution cannot be
readily sustained for practical times while these alloys are in
their molten state. This limitation also results in difficulties in
attempts to produce continuously cast parts with different
thicknesses and dimensions.
Specifically, the time scales for the flotation of bubbles in a
foam scales with the viscosity of the material. Most conventional
alloys have a very low viscosity in the molten state. Accordingly,
the mechanical properties of these foams are degraded with the
degree of imperfection caused by the flotation and bursting of
bubbles during manufacture. In addition, the low viscosity of
commonly used liquid metals results in a short time scale for
processing, which makes the processing of metallic foam a delicate
process.
In order to remedy these shortcomings, several techniques have been
attempted. For example, to reduce the sedimentation flotation
process, Ca particles may be added to the liquid alloy. However,
the addition of Ca itself degrades the metallic nature of the base
metal as well as the resultant metallic foam. Alternatively,
foaming experiments have been performed under reduced gravity, such
as in space, to reduce the driving force for flotation, however,
the cost for manufacturing metallic foams in space is
prohibitive.
Accordingly, a need exists for improved methods of manufacturing
amorphous metallic foams.
SUMMARY OF THE INVENTION
The present invention is directed to method of continuous casting
of amorphous metallic foams in sheet or other blanks forms.
In one embodiment of the invention, the foam sheet is formed using
conventional single roll, double roll, or other chill-body
forms.
In another embodiment of the invention, the amorphous alloy foam
sheets have sheet thicknesses of from 0.1 mm to 10 mm.
In one embodiment of the invention, a bubble density less than 10%
by volume in the foam precursor is increased in the subsequent
steps to produce a solid foam material with more than 80% by volume
bubble density.
In another embodiment of the invention, the bubble density
increases by a factor of 5 or more from the initial foam precursor
into the final continuously cast solid foam material.
In still another embodiment of the invention, the majority of the
bubble expansion is achieved at temperatures above Tnose and
temperatures below about Tm.
In yet another embodiment of the invention, the bubble density is
increased by a factor of 5 or more from the initial foam precursor
at temperatures above Tnose and temperatures below about Tm.
In still yet another embodiment of the invention, a bubble density
less than 10% by volume in the foam precursor is increased to more
than 80% by volume bubble density at temperatures above Tnose and
temperatures below about Tm.
In one embodiment of the invention, the melt temperature is
stabilized in a viscosity regime of 0.1 to 10,000 poise.
In another embodiment of the invention, the melt temperature is
stabilized in a viscosity regime of 1 to 1,000 poise.
In still another embodiment of the invention, the melt temperature
is stabilized in a viscosity regime of 10 to 10,000 poise.
In one embodiment of the invention, the extraction of continuous
foam sheet is preferably done at speeds of 0.1 to 50 cm/sec
In another embodiment of the invention, the extraction of
continuous foam sheet is preferably done at speeds of 0.5 to 10
cm/sec
In still another embodiment of the invention, the extraction of
continuous foam sheet is preferably done at speeds of 1 to 5
cm/sec
In one embodiment the invention is directed to continuously cast
solid foam structures having bubble densities in the range of from
50 percent up to 95% by volume.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other features and advantages of the present invention
will be better understood by reference to the following detailed
description when considered in conjunction with the accompanying
drawings wherein:
FIG. 1 is block flow diagram of an exemplary method for continuous
casting bulk solidifying amorphous alloy foams in accordance with
the current invention.
FIG. 2a is a side view in partial cross section of an exemplary
conventional apparatus for forming sheets of a molten metal
foams.
FIG. 2b is a close-up of the formation of the sheet of molten metal
foam shown in FIG. 2a.
FIG. 3 is a side view in partial cross section of an exemplary
apparatus for forming precursors of a molten bulk solidifying
amorphous alloy.
FIG. 4 is a time-temperature transformation diagram for an
exemplary continuous foam casting sequence in accordance with the
current invention.
FIG. 5 is a temperature-viscosity of an exemplary bulk solidifying
amorphous alloy in accordance with the current invention.
FIG. 6a is a graphical representation of the flotation
(sedimentation) properties of an embodiment
(Zr.sub.41Ti.sub.14Cu.sub.12Ni.sub.10Be.sub.23 (% atom.) called
VIT-1) of a suitable materials for manufacturing amorphous metallic
foams according to the current invention
FIG. 6b is a graphical representation of the flotation
(sedimentation) properties of an embodiment
(Zr.sub.41Ti.sub.14Cu.sub.12Ni.sub.10Be.sub.23 (% atom.) called
VIT-1) of a suitable materials for manufacturing amorphous metallic
foams according to the current invention as compared to pure Al
metal.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is directed to method of continuous casting
of amorphous metallic foams in sheet or other blanks forms using
bulk solidifying amorphous alloys.
For the purposes of this invention, the term amorphous means at
least 50% by volume of the alloy is in amorphous atomic structure,
and preferably at least 90% by volume of the alloy is in amorphous
atomic structure, and most preferably at least 99% by volume of the
alloy is in amorphous atomic structure.
Bulk solidifying amorphous alloys are amorphous alloys (metallic
glasses), which can be cooled at substantially lower cooling rates,
of about 500 K/sec or less, than conventional amorphous alloys and
substantially retain their amorphous atomic structure. As such,
they can be produced in thickness of 1.0 mm or more, substantially
thicker than conventional amorphous alloys, which have thicknesses
of about 0.020 mm, and which require cooling rates of 10.sup.5
K/sec or more. U.S. Pat. Nos. 5,288,344; 5,368,659; 5,618,359; and
5,735,975 (the disclosure of each of which is incorporated herein
by reference in its entirety) disclose such exemplary bulk
solidifying amorphous alloys.
One exemplary family of bulk solidifying amorphous alloys can be
described as (Zr,Ti).sub.a(Ni,Cu,Fe).sub.b(Be,Al,Si,B).sub.c, where
a is in the range of from 30 to 75, b is in the range of from 5 to
60, and c in the range of from 0 to 50 in atomic percentages.
Furthermore, those alloys can accommodate substantial amounts of
other transition metals (up to 20% atomic), including metals such
as Nb, Cr, V, Co. Accordingly, a preferable alloy family is
(Zr,Ti).sub.a(Ni,Cu).sub.b(Be).sub.c, where a is in the range of
from 40 to 75, b is in the range of from 5 to 50, and c in the
range of from 5 to 50 in atomic percentages. Still, a more
preferable composition is (Zr,Ti).sub.a(Ni,Cu).sub.b(Be).sub.c,
where a is in the range of from 45 to 65, b is in the range of from
7.5 to 35, and c in the range of from 10 to 37.5 in atomic
percentages. Another preferable alloy family is
(Zr).sub.a(Nb,Ti).sub.b(Ni,Cu).sub.c(Al).sub.d, where a is in the
range of from 45 to 65, b is in the range of from 0 to 10, c is in
the range of from 20 to 40 and d in the range of from 7.5 to 15 in
atomic percentages.
Another set of bulk-solidifying amorphous alloys are ferrous metal
(Fe, Ni, Co) based compositions, where the content of ferrous
metals is more than 50% by weight. Examples of such compositions
are disclosed in U.S. Pat. No. 6,325,868, (A. Inoue et. al., Appl.
Phys. Lett., Volume 71, p 464 (1997)), (Shen et. al., Mater.
Trans., JIM, Volume 42, p 2136 (2001)), and Japanese patent
application 2000126277 (Publ. # .2001303218 A), all of which are
incorporated herein by reference. One exemplary composition of such
alloys is Fe.sub.72Al.sub.5Ga.sub.2P.sub.11C.sub.6B.sub.4. Another
exemplary composition of such alloys is
Fe.sub.72Al.sub.7Zr.sub.10Mo.sub.5W.sub.2B.sub.15. Although, these
alloy compositions are not as processable as Zr-base alloy systems,
they can be still be processed in thicknesses around 1.0 mm or
more, sufficient enough to be utilized in the current
invention.
In general, crystalline precipitates in amorphous alloys are highly
detrimental to their properties, especially to the toughness and
strength of such materials, and as such it is generally preferred
to limit these precipitates to as small a minimum volume fraction
possible so that the alloy is substantially amorphous. However,
there are cases where ductile crystalline phases precipitate
in-situ during the processing of bulk amorphous alloys, which are
indeed beneficial to the properties of bulk amorphous alloys
especially to the toughness and ductility. The volume fraction of
such beneficial (or non-detrimental) crystalline precipitates in
the amorphous alloys can be substantial. Such bulk amorphous alloys
comprising such beneficial precipitates are also included in the
current invention. One exemplary case is disclosed in (C. C. Hays
et. al, Physical Review Letters, Vol. 84, p 2901, 2000), the
disclosure of which is incorporated herein by reference.
One exemplary method, according to the present invention, for
making foams from these bulk-solidifying amorphous alloy is shown
in FIG. 1, and comprises the following steps: 1) Providing a foam
pre-cursor above the liquidus temperature of the bulk-solidifying
amorphous alloy; 2) Stabilizing the foam precursor in a viscosity
regime of 0.1 to 10,000 poise; 3) Ejecting the foam precursor onto
the chill body of a continuous casting apparatus 4) Quenching the
precursor into an amorphous foam structure.
In the first step, a foam "pre-cursor" at temperatures above the
liquidus temperature of the alloy is created. The volume fraction
of bubbles in this precursor can be in the range of from 5% to 50%,
and the bubbles are preferably created to have a large internal
pressure by processing the pre-cursor at high pressures (up to
.about.50 bar or more).
Secondly, the precursor is stabilized at temperatures around or
below the alloy's melting temperature at viscosity regimes of 0.1
poise to 10,000 poise. This step is necessary to stabilize the
bubble distribution as well as for the continuous casting of sheet
or other blank shapes. Preferably, such stabilization is again
carried out under high pressures, up to 50 bar or more, to retain
the bubble distribution and high internal pressure in the formed
bubbles.
Subsequently, the viscous foam precursor is introduced onto the
chill body of a continuous casting apparatus. Schematic diagrams of
an exemplary continuous casting apparatus are provided in FIGS. 2a
and 2b. As shown in these diagrams, the continuous casting
apparatus 1 has a chill body 3 which moves relative to a injection
orifice 5, through which the melt 7 is introduced to form a
solidified sheet 9. In this specification, the apparatus is
described with reference to the section of a casting wheel 3 which
is located at the wheel's periphery and serves as a quench
substrate as used in the prior art. It will be appreciated that the
principles of the invention are also applicable, as well, to other
conventional quench substrate configurations such as a belt,
double-roll wheels, wheels having shape and structure different
from those of a wheel, or to casting wheel configurations in which
the section that serves as a quench substrate is located on the
face of the wheel or another portion of the wheel other than the
wheel's periphery. In addition, it should be understood that the
invention is also directed to apparatuses that quench the molten
alloy by other mechanisms, such as by providing a flow of coolant
fluid through axial conduits lying near the quench substrate. To
provide a steady state flow of melt through the orifice, there are
some complex relations that need to be satisfied between the
applied pressure (or gravitational pull-down), the orifice slit
size, the surface tension of the melt, the viscosity of the melt,
and the pull-out speed of the solidification front.
As shown, in the detailed view in FIG. 1b, the chill body wheel 7
travels in a clockwise direction in close proximity to a slotted
nozzle 3 defined by a left side lip 13 and a right side lip 15. As
the metal flows onto the chill body 7 it solidifies forming a
solidification front 17. Above the solidification front 17 a body
of molten metal 19 is maintained. The left side lip 13 supports the
molten metal essentially by a pumping action which results from the
constant removal of the solidified sheet 9. The rate of flow of the
molten metal is primarily controlled by the viscous flow between
the right side lip 15 and solidified sheet 9.
Once the melt is introduced onto the chill body of the continuous
casting apparatus, the viscous melt containing the high pressure
bubbles is quenched into a solid foam material. During the
quenching process, a relatively solid skin can form on the surface
of the material having contact with the chill body, whereas the
body of the viscous portion of the melt can continue to expand to
increase the volume fraction until it completely freezes. The
formed solid foam material can then be extracted form the chill
body at speeds ranging from 0.1 cm/sec to 50 cm/sec.
As discussed above, in order to prepare the pre-cursor, a gas has
to be introduced into the liquid bulk-solidifying amorphous alloy.
Any suitable method of introducing bubbles in the liquid
bulk-solidifying amorphous alloy sample may be utilized in the
current invention. In one exemplary embodiment, gas releasing
agents, such as B.sub.2O.sub.3 can be used which are mixed with the
metal alloy. During the processing, the B.sub.2O.sub.3 releases
H.sub.2O.sub.3 at elevate temperatures, which in turn forms gas
bubbles in the size range of between .about.20 .mu.m up to .about.2
mm. With bubbles within this size range no observable gradient
takes place in a typical bulk solidifying amorphous alloy
alloy.
Another method to introduce bubbles into a liquid bulk-solidifying
amorphous alloy to obtain a pre-cursor foam is by mechanical
treating. In such an embodiment, the stability of a liquid surface
can be described by comparing the inertial force to the capillary
force, according to the ratio:
.rho..times..sigma. ##EQU00001## where W is the Weber number, .rho.
is the density of the liquid, v the velocity of the moving
interface, L a typical length for bubble size, and .sigma. the
liquid's surface energy. For W<1 the liquid surface becomes
unstable and gives rise to mechanically create bubbles in the
liquid. This equation makes it possible to calculate the size of
bubbles that can be created for a given inertial force and surface
energy. For example, an object with a velocity of 10 m/s moving in
a liquid with a density of 6.7 g/cm.sup.3 and a viscosity of 1 Pas
is able to break-up bubbles with a size down to 1 .mu.m. In one
exemplary embodiment that uses a Vitreloy 106 (Zr--Nb--Ni--Cu--Al
Alloy) pre-cursor made in accordance with this mechanical method, a
bubble size distribution between 0.020 mm and 1 mm can be readily
obtained with a volume fraction of around 10%.
A schematic of an apparatus capable of creating a pre-cursor
according to this method is shown in FIG. 3. In this embodiment, a
heated crucible 20 holds the liquid alloy sample 22 and a spinning
whisk 24 is used to breakup existing bubbles 26 and create new
bubbles 28 by breaking up the surface 30 of the liquid. A bubbler
32, consisting in this embodiment of a tube through which gas may
be passed is used to create the initial bubbles. Initial bubbles
can also be created through the surface by a drag of the liquid
created by the spinning whisk.
It should be noted that there is a minimum bubble size that can be
created using these precursor-forming methods. From energy
considerations it can be derived that the minimum bubble size, is
given by: Rmin=2 Sigma/P (2) where sigma is the (surface tension)
(as in the above Weber equation), and P is the ambient pressure
during bubble creation. It should be noted the bubble size in the
foam precursor are preferably as small as possible in order to
obtain a better controlled expansion in the subsequent steps.
According to the above formula, a high ambient pressure (up to 50
bars or more) is desired during bubble formation in order to create
bubbles in smaller diameters.
As discussed, after the formation of the foam precursor, the melt
temperature is stabilized in a viscosity regime of 0.1 poise to
10,000 poise. Since the viscosity increases with decreasing
temperature, ejecting the molten amorphous alloy is preferably
carried out below Tm for processes using increased viscosity.
However, it should be noted that viscosity stabilization should be
done at temperatures above Tnose as shown in the TTT diagram
provided in FIG. 4.
Even though there is no liquid/solid crystallization transformation
for a bulk solidifying amorphous metal, a "melting temperature" Tm
(or liquidus temperature) may be defined as the temperature of the
thermodynamic melting temperature of the corresponding crystalline
phases (or the liquidus temperature of the corresponding
crystalline phases). Around the melting temperature, the viscosity
of the bulk solidifying amorphous metal generally lays in the range
0.1 poise to 10,000 poise, which is to be contrasted with the
behavior of other types of amorphous metals that have viscosities
around Tm of under 0.01 poise. In addition, higher values of
viscosity can be obtained using bulk solidifying amorphous alloys
by undercooling the material below the melting temperature Tm,
where ordinary amorphous alloys will tend to crystallize rather
rapidly. FIG. 5 shows a viscosity-temperature graph of an exemplary
bulk solidifying amorphous alloy, from the VIT-001 series of
Zr--Ti--Ni--Cu--Be family.
The specific viscosity value at which the melt is stabilized
depends on a variety of factors. One important factor is the volume
fraction and the respective bubble distribution in the precursor
foam melt. A higher viscosity is employed for a higher volume
fraction of bubbles in the precursor. Secondly, the selected
viscosity value is also dependent on the dimensions of the nozzle
through which the foam precursor melt is introduced onto the chill
body. Third, the allowable viscosity also depends on the speed the
solidified solid foam material is extracted, i.e. the relative
speed of the chill body to the nozzle. For a larger thickness of
the initial melt precursor, a higher viscosity is desired in order
to sustain a stable melt puddle over the chill body. Specifically,
the rate of flow of the molten metal is primarily controlled by the
viscous flow between the lips of the nozzle and solid strip being
formed on the chill body. For the case of a bulk solidifying
amorphous metal, it is possible to reliably continue to process a
continuous casting of a foam material even at very low wheel
rotation speeds. However, in lower viscosity melts low speed
rotation of the chill body wheel will cause the material to run and
spill over the wheel. For example, low viscosity amorphous
materials must be run over high speed chill bodies leading to a
thickness restriction for the cast sheet of a few 0.02 mm, in
contrast bulk solidifying amorphous alloys may be formed in
thicknesses up to 10 mm. Accordingly, for larger thickness
foam-strip castings, a higher viscosity is preferred and
accordingly, as higher undercooling below Tm is employed.
It should be noted that the bubble distribution and volume fraction
can be adjusted during the solidification of foam precursor into a
solid foam material. This is due to the fact that that there is no
clear liquid/solid transformation for a bulk solidifying amorphous
metal during the formation of the amorphous solid. For bulk
solidifying amorphous alloys, the molten alloy simply becomes more
and more viscous with increasing undercooling as it approaches the
solid state around the glass transition temperature. Accordingly,
the temperature of the solidification front can be around glass
transition temperature, where the alloy will practically act as a
solid for the purposes of pulling out the quenched amorphous strip
product. This unique property of bulk solidifying amorphous alloys
can be utilized to grow the bubble sizes in a controllable manner.
In other words, the foam precursor can be expanded to form higher
bubble volume fraction during its solidification into a solid foam
material. This has also the allows for the formation of solid foam
materials with a higher volume fraction of bubble distribution than
is possible using conventional metals that require processing above
the liquidus temperature.
At the first introduction of the foam melt precursor onto the chill
body, a solid skin will form due to the rapid cooling of the
surface of the material. The skin thickness will be typically in
the range of a few micrometers to tens of micrometers depending on
the initial thickness of melt injection and the bubble volume
fraction. This can be beneficially utilized to form foam panels
with solid outer skins. For example, by utilizing a double-roll or
similar apparatus, a foam panel with solid skins can be formed
continuously. During such a process the inner core of the melt body
will still be in a viscous liquid regime. By employing a higher
pressure during the formation of precursor the internal pressure in
the bubbles can be made higher than the ambient pressure of the
quenching environment. Accordingly, the core of the viscous melt
will expand outwards making a foam panel (or foam sandwich) having
a thickness larger than the initial melt thickness introduced onto
the chill-body. Here, a lower viscosity in the earlier viscosity
stabilization step is preferable for a larger expansion of the
core. Since the solidification is progressive, rather than abrupt
in the case of bulk-solidifying amorphous alloys, choosing a lower
viscosity will provide a larger window for expansion of the core,
allowing for the formation of a solid foam material with a higher
volume fraction of bubbles.
As discussed above, after the charge of the amorphous alloy is
injected onto the surface of chill body, the material is cooled to
temperatures below glass transition temperature at a rate such that
the amorphous alloy retains the amorphous state upon cooling.
Preferably the cooling rate is less than 1000.degree. C. per
second, .Iadd.for example, the cooling rate can be less than
10.degree. C. per second, .Iaddend.but sufficiently high to retain
the amorphous state in the bulk solidifying amorphous alloy to
remain amorphous upon cooling. The lowest cooling rate that will
achieve the desired amorphous structure in the article is chosen
and achieved using the design of the chill body and the cooling
channels. It should be understood that although a cooling rate
range is discussed above, the actual value of the cooling rate
cannot here be specified as a fixed numerical value because the
value varies for different metal compositions, materials, and the
shape and thickness of the strip being formed. However, the value
can be determined for each case using conventional heat flow
calculations.
Although the general process discussed above is useful for a wide
variety of bulk-solidifying amorphous alloys, it should be
understood that the precise processing conditions required for any
particular bulk-solidifying amorphous alloy will differ. For
example, as discussed above, a foam consisting of a liquid metal
and gas bubbles is an unstable structure, flotation of the lighter
gas bubbles due to gravitational force takes place, leading to a
gradient of the bubbles in size and volume. The flotation velocity
of a gas bubble in any liquid metal material can be calculated
according to the Stoke's law: V.sub.sed=2
a.sup.2(.rho..sub.l-.rho..sub.g)g/9.eta. (3) where g is the
gravitational acceleration, a is the bubble radius, and
.rho..sub.l, .rho..sub.g, are the densities of the liquid and gas,
respectively.
An exemplary flotation velocity calculation made according to
Equation 1 for VIT-1 is shown in FIGS. 6a and 6b. As shown in FIG.
6a, using experimental viscosity data (as shown in FIG. 5) and a
liquid VIT-1 density of .rho.=6.0.times.10.sup.3 kg/m.sup.3, the
flotation velocities of bubbles in a VIT-1 alloy melt as a function
of bubble radius is calculated for liquid VIT-1 at 950 K ( -- ),
and 1100 K ( - - - ). FIG. 6b shows the flotation for a 1 mm gas
bubble in liquid VIT-1 ( -- ) and liquid Al ( - - - ) as a function
of T/T.sub.1.
Using such graphs, acceptable processing conditions, such as time
and temperature can be determined. For example, if the duration of
a typical manufacturing process is taken to be 60 s and an
acceptable flotation distance of .about.5 mm, processing times and
temperatures resulting in a flotation velocity smaller than
10.sup.-4 m/s would be acceptable. Therefore, in this case an
unacceptable bubble gradient can be avoided if the maximum bubble
size is less than 630 .mu.m if the VIT-1 melt is processed above
its liquidus temperature of about 950 K.
As described, the present invention allows for the continuous
casting of solid foam structures with varying bubble densities. In
one embodiment of the invention, the continuously cast solid foam
structures have a bubble density in the range of from 50 percent up
to 95% by volume. The invention further allows the use of lesser
bubble density in molten state above Tm, and increases the bubble
density (by volume) by expansion during continuous casting.
Although specific embodiments are disclosed herein, it is expected
that persons skilled in the art can and will design alternative
continuous foam sheet casting apparatuses and methods to produce
continuous amorphous alloy foam sheets that are within the scope of
the following claims either literally or under the Doctrine of
Equivalents.
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