Note: Descriptions are shown in the official language in which they were submitted.
~2~312~)
CASTING IN A LOW DENSITY ATMOSPHERE
BACKGROUND OF THE I~VENTION
1. Field of the Invention
The invention relates to the castiny of metal strip
directly from a melt, and more particularly to the rapid
solidification of metal directly from a melt to form
substantially continuous metal strip.
2. Description of the Prior Art
U.S. Patent No. 4,142,571 issued to M. Narasimhan
discloses a conventional apparatus and method for
rapidly quenching a stream of molten metal to form con-
tinuous metal strip. The metal can be cast in an inertatmosphere or a partial vacuum. U.S. Patent No.
3,862,658 issued to J. Bedell and U.S. Patent No.
4,202,404 issued to C. Carlson disclose flexible belts
employed to prolong contact of cast metal filament with
a quench surface.
The casting of very smooth strip has been difficult
with conventional devices because gas pockets entrapped
between the quench surface and the molten metal during
quenching form gas pocket defects. These defects, along
with other factors, cause considerable roughness on the
~uench surface side as well as the opposite, free sur-
face side of the cast strip. In some cases, the surface
defects actually e~tend through the strip, forming
perforations therein.
U.S. Patent No. 4,154,283 issued to R. Ray et al.
discloses that vacuum casting of metal strip reduces the
formation of gas pocket defects. The vacuum casting
-2- lZ~312~
system taught by Ray et al. requires specialized
chambers and pumps to produce a low pressure castiny
atmosphere. In addition, auxiliary means are required
to continuously transport the cast strip out of the
vacuum chamber. Further, in such a vacuum casting
system, the strip tends to weld excessively to the
quench surface instead of breaking away as typically
happens when casting in an ambient atmosphere.
U.S. Patent No. 4,301,855 issued to H. Suzuki et
al. discloses an apparatus for casting metal ribbon
wherein the molten metal is poured from a heated nozzle
onto the outer peripheral surface of a rotary roll. A
cover encloses the roll surface upstream of the nozzle
to provide a chamber, the atmosphere of which is
evacuated by a vacuum pump. A heater in the cover heats
the roll surface upstream from the nozzle to remove dew
droplets and gases from the roll surface. The vacuum
chamber lowers t~e density of the moving gas layer next
to the casting roll surface, thereby increasing forma-
; 20 tion of air pocket depressions in the cast ribbon. The
heater helps drive off moisture and adhered gases from
the roll surface to further decrease formation of air
pocket depressions.
The apparatus disclosed by Suzuki et al. does notpour metal onto the casting surface until that surface
has exited the vacuum chamber. ~y this procedure,
complications involved in removing a rapidly advancing
ribbon from the vacuum chamber are avoided. The ribbon
is actually cast in the open atmosphere, offsetting any
potential improvement in ribbon quality.
U.S. Patent No. 3,861,450 to Mobley, et al.
discloses a method and apparatus for making metal
filament. A disk-like, heat-extracting member rotates
to dip an edge surface thereof into a molten pool, and a
non-oxidizing gas is introduced at a critical process
region where the moving surface enters the melt. This
non-oxidizing gas can be a reducing gas, the combustion
o~ which in the atmosphere yields reducing or non-
_3_ 1 ~ ~ 3 1 2 ~oxidizing combustion products at the critical process
region. In a particular embodiment, a cover c~mposed of
carbon or graphite encloses a portion of the disk and
reacts with the oxygen adjacent the cover to produce
non-oxidizing carbon monoxide and carbon dioxide gases
which can then surround the disk portion and the entry
region of the melt.
The introduction of non-oxidizing gas, as taught by
Mobley, et al., disrupts and replaces an adherent layer
of oxidizing yas with the non-oxidizing gas. The
controlled introduction of non-oxidizing gas also
provides a barrier to prevent particulate solid
materials on the melt surface from collecting at the
critical process region where the rotating disk would
lS drag the impurities into the melt to the point of
initial filament solidification. Finally, the exclusion
of oxidizing gas and floating contaminants from the
critical region increases the stability of the filament
release point from the rotating disk by decreasing the
adhesion therebetween and promoting spontaneous release.
Mobley, et al., however, address only the problem
of oxidation at the disk surface and in the melt. The
flowing stream of non-oxidizing gas taught by Mobley, et
al. is still drawn into the molten pool by the viscous
drag of the rotating wheel and can separate the melt
from the disk edge to momentarily disturb filament
formation. The particular advantage provided by Mobley,
et al. is that the non-oxidizing gas decreases the
oxidation at the actual point of filament formation
within the melt pool. Thus, Mobley, et al. fail to
minimize the entrainment of gas that could separate and
¦ insulate the disk surface from the melt.
U.S. Patent No. 4,282,921 and U.S. Patent No.
4,262,734 issued to H. Liebermann disclose an apparatus
and method in which coaxial gas jets are employed to
reduce edge defects in rapidly quenched amorphous
strips. U.S. Patent No. 4,177,856 and U.S. Patent No.
4,144,926 issued to H. Liebermann disclose a method and
1~131Z~
-4-
apparatus in which a Reynolds number parameter is con-
trolled to reduce edge defects n rapidly quenched amor-
phous strip. Gas densities and thus Reynolds numbers,
are regulated by the use of vacuum and by employing
lower molecular wei~ht gases.
Conventional methods, however, have been unable to
adequately reduce surface defects in cast metal strip
caused by the entrapment of gas pockets. Vacuum casting
procedures have af~orded some success, but when using
vacuum casting, excessive welding of the cast strip to
the quench surface and the difficultly o removing the
cast strip from the vacuum chamber have resulted in
lower yields and increased production costs. As a
result, conventional methods have been unable to provide
a commercially acceptable process that efficiently pro-
duces smooth strip with consistent quality and uniform
cross-sectionO
5UMMARY OF THE INVENTION
The invention provides an apparatus and method for
efficiently casting smooth metal strip and substantially
preventing the formation of gas pocket defects
therein. The apparatus of the invention includes a
moving chill body having a quench surface, and includes
a no~zle means for depositing a stream of molten metal
on a quenching region of the quench surface to form the
strip. The nozzle means has a exit portion with a
nozzle orifice. A depletion means supplies a low den-
sity atmosphere at a depletion region located adjacent
to and upstream of the quenching region. A control
means substantially prevents precipitation of condensed
or solidified constituents from the low density atmo-
¦ sphere onto the depletion reyion.
In accordance with the invention there is also pro-
vided a method for casting continous metal strip. A
chill body having a quench surface is moved at a
selected speed, and a stream of molten metal is depos~
ited on a quenching region of the quench surface to form
the strip. A low density atmosphere is supplied to a
3~Zi~
-5-
depletion re~ion located adjacent to and upstream of
said quenching règion. The quench surface is heated to
a temperature that substantially prevents precipitation
of condensed or solidified constituents from the
atmosphere onto the depletion region.
The invention further provides a metal strip having
a thickness of less than about 15 micrometers in the as-
i cast state.
The method and apparatus of the invention advanta-
geously minimize the formation and entrapment of yaspockets against the quenched surface during the casting
of the strip. As a result, the invention avoids the
needs for complex vacuum casting apparatus and can be
practiced in an ambient atmosphere. The heating of the
quench surface surprisingly provides better and more
uniform cooling and quenching of the molten metal. The
low-density atmosphere and heated quench surface reduce
the formation of gas pockets operating to decrease con-
tact between the molten metal and the quench surface.
The more uniform quenching~ in turn, provides improved
physical properties in the cast strip. In particular,
the reduction of surface defects on the quenched surface
side of the strip increases the packing factor of the
material and reduces localized stress concentrations
that can cause premature fatigue failure. The
smoothness of the free surface side of the cast strip
(i.e~ the side not in contact with the quench surface of
the chill body) is also improved by the method and
apparatus of the invention. This increased smoothness
further increases the pac~ing factor of the material.
In production of amorphous metal strip, the more uniform
¦ quenching afforded by the heated quench surface and low
density atmosphere provide a more consistent and uniform
formation of the amorphous state. In manufacture of the
strip composed of magnetic material, the number and size
of strip surface discontinuities is reduced, improving
the magnetic properties of the strip.
~ urface defects due to entrapped gas pockets are
-6- 1~3~Z~
reduced, and there is much less chance for a ~as pocket
to perforate the strip. Suprisingly, very thin strips
(less than about 15 micrometers in thickness) have been
produced. These very thin strips are highly desirable
in various applications. For example, in magnetic
devices, such as inductors, reactors and hiyh frequency
electromagnetic devices, thin magnetic material
substantially reduces power losses therein. In brazing,
the use of thinner brazing foils substantially improves
the strength of the brazed joints.
Morever, the reduction of entrapped gas pockets
markedly increases the heat conductive contact between
the molten metal and the quench surface. Thicker strips
of rapidly solidified metal can be produced. Such
thicker strip is desireable because it can be more
easily substituted for materials conventional~y used in
existing commercial applications. These thick strip
components can, suprisingly, be provided by rapid
solidification in a single quenching step in much less
time with decreased cost.
Thus, the present invention effectively minimizes
gas pocket defects on the strip surface which contacts
the quench surface, and produces strip having a smooth
surface finish and uniform physical properties. Complex
equipment and procedures associated with vacuum casting
are eliminated. The invention efficiently casts ultra
thin as well as extra thick metal strip directly from
the melt at lower cost and with higher yield. Such
ultra thin and extra thick strips are especially suited
for use in such applications as magnetic devices, and
can be substituted for conventional materials with
greater effectiveness and economy.
! BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be more fully understood and
further advantages will become apparent when reference
is made to the following detailed description of the
preferred embodiment of the invention and the accom-
panying drawings in which:
12~3~zl,
--7--
FIG . 1 shows a representative prior art apparatus
for rapidly casting metal strip;
FIG . 2 shows a schematic representation of a embo-
diment of the invention which employs an endless casting
belt;
FIG. 3 shows an embodiment of the invention which
employs a gas delivery means located coaxial with a
casting nozzle;
FIG. 4 shows an embodiment of the invention which
employs a rotatable casting wheel;
FIG. 5 shows an embodiment of the invention which
employs a flexible hugger belt to prolong contact of the
cast strip with the ~uench surface; and
FIG. 6 shows a gas velocity profile at the quench5 surface portion on which molten metal is deposited.
DES~RIPTION OF PREFERRED EM~ODIMENTS
For the purposes of the present invention and as
used in the specification and claims, a strip is a slen-
der body the transverse dimensions of which are much
smaller than its length. Thus, a strip includes wire,
ribbon, sheet and the like of regular or irregular
cross-section.
The invention is suitable for casting metal strip
composed of crystalline or amorphous metal and is parti-
cularly suited for producing metal strip which is
rapidly solidified and quenched at a rate of at least
about 104C/sec from a melt of molten metal. Such
rapidly solidified strip has improved physical proper~
ties, such as improved tensile strenyth, ductility and
magnetic properties.
FIG. 1 shows a representative prior art device for
rapidly casting continuous metal strip. Molten metal
alloy contained in crucible 2 is heated by a heating
element 3. Pressurization of the crucible with an inert
gas forces a molten stream through a nozzle 4 at the
base of the crucible and deposits the molten metal onto
a moving chill body, such as rotatable casting wheel
1. Solidified moving strip 6, after its break-away
~13~2~
--8--
point from the quench wheel is then routed onto a suit-
able winding means.
Quench surface 5 (substrate) is preferably a mate-
rial having high thermal conductivity. Suitable mate-
rials include carbon steel, stainless steel and copperbased alloys such as beryllium copper. To achieve the
quench rates of at least about 10~C per second, wheel 1
is internally cooled and rotated to provide a quench
surface that advances at a speed ranging from about 10U
_ 4000 meters per minute. Preferably, the quench sur-
face speed ranges from about 200 - 3000 meters per min-
ute. Typically, the thickness of the cast strip ranges
from 25 - 100 microns (micrometers)~
FIG. 2 shows a representative apparatus of the
invention, A moving chill body, such as endless casting
belt 7, has a chilled casting quench surface 5. Nozzle
¦ means, such as nozzle 4, deposits a stream of molten
metal onto a quenching surface 14 of quench surface 5 to
form strip 6. Nozzle 4 has an orifice 22 located at
exit portion 26. A depletion means is comprised of gas
nozzle delivery means 8, heater means 10, and gas supply
12. The depletion means supplies a gas 24 from gas
supply 12 to produce a low density atmosphere an~
directs the gas with gas nozzle 8 to a depletion region
13 located adjacent to and upstream of quenching region
14. Nozzle 8 is suitably located to direct gas 24 at
and around the depletion region 13 so that the gas 24
substantially floods the depletion region 13, providing
a low density atmosphere therewithin. A control means
104 heats the quench surface 5 to a temperature that
substantially prevents precipitation of condensed or
solidified constituents from the atmosphere onto the
depletion region 13. Valve 16 reyulates the volume and
velocity through nozzle 8. As shown in FIG. 2, gas noz-
zle 8 is located upstream of quenching region 14 and is
directed along the direction of movement of the quench
surface. Optionally, gas nozzle 8 can be located
coaxial with casting nozzle 4 as representatively shown
g ~Z~3~Z~
in FIG. 3.
The term low density atmosphere, as used in the
specification and claims hereof, means an atmosphere
having a gas density less than 1 gram per liter and
preferably, having a gas density of of less than about
0.5 grams per liter.
To obtain the desired low-density atmosphere, gas
24 is heated to at least about 800K, and more
preferably, is heated to at least about 1300K. In
general, hotter gases are preferred ~ecause they will
have lower densities and will better minimize the forma-
tion and entrapment of gas pockets between ~uench sur-
face 5 and the deposited molten metal.
Entrapped gas pockets are undesirable because they
produce ribbon surface defects that degrade the surface
smoothness. In extreme cases, the gas pockets will
cause perforations through strip ~. A very smooth
surface finish is particularly important when winding
magnetic metal strip to form magnetic cores because
surface defects reduce the packing factor of the
material. The packing factor is the volume fraction of
the actual magnetic material in the wound core (the
volume of magnetic material divided by the total core
volume) and is often expressed in percent. A smooth
surface without defects is also important in optimizing
the magnetic properties of strip 6 and in minimizing
localized stress concentrations that would otherwise
reduce the mechanical strength of the strip.
Gas pockets also insulate the deposit molten metal
from guench surface 5 and reduce the quench rate in
localized areas. The resultant, non-uniform quenchiny
¦ produces non-uniform physical properties in strip ~,
such as non-uniform strength, ductility and magnetic
properties.
For example, when casting amorphous metal strip,
gas pockets can allow undesired crystallization in
localized portions of the strip. The gas pockets and
the local crystallizations produce discontinuities which
lZ131Zt~
--10--
inhibit movement of magnetic domain walls, thereby
degrading the magnetic properties of the material.
Thus, by reducing the entrapment of gas pockets,
the invention produces high quality metal strip with
improved surface finish and improved physical
properties. For example, metal strip has been produced
with packin~ factors of at least about 80%, and up to
about 95%.
The mechanism by which gas pockets are reduced can
be more readily explained with reference to FI~. 6. The
gas boundary layer velocity profile near quench surface
5 and upstream of melt puddle 1~ is shown schematically
at 200 The maximum gas boundary layer velocity occurs
immediately adjacent to quench surface S (substrate) and
is equal to the velocity of the moving quench surface.
Thus, moving quench surface 5 ordinarily draws cool air
from the ambient atmosphere into upstream region 13 and
into quenching region 14, the region of the quench
surface upon which molten metal is æeposited. Because
of the dr~fting of relatively cool air into the
quenching region, the presence of the hot casting nozzle
and the molten metal do not sufficiently heat the local
atmosphere to significantly reduce the density thereof.
Melt puddle 18 wets the substrate surface to an
extent determined by various factors including the metal
alloy composition, the substrate composition, and the
presence of surface films. The pressure exerted by the
gas boundary layer a~ the melt-substrate interface,
however, acts to locally separate the melt from the
substrate and form entrained gas pockets which will
appear as ~lift-off n areas 44 on the ribbon underside.
¦ The stagnation pressure of the yas boundary layer
tpressure if the layer hit a rigid wall) is given by the
formula Ps= 1/2 P v2 where: P = gas density, v =
substrate velocity. Therefore, the reduction of gas
boundary layer density or substrate velocity are
important in the reduction of the size and the number of
gas pockets entrained under the molten metal puddle.
~2~31Z61
For example, removal of the gas boundary layer by
casting in vacuum can totally eliminate the lift-off
areas in the strip underside. Alternatively, a low
density gas in the boundary layer could be employed.
The selection of a low molecular weight gas (such as
helium) is one way to reduce boundary layer gas
density. However, the variety of low molecular weight
! gases which can be used in this fashion is quite
limited. A preferred manner in which to reduce the
boundary layer gas density is to use a heated gas; the
density of the gas will diminish as the inverse of the
absolute temperature. By directing the hot gas at the
upstream side of the melt puddle 18, the size and the
number of entrained gas pockets under the melt puddle
can be substantially reduced.
It is important, however, to regulate pertinent
factors, such as the composition of the low-density
atmosphere, and the temperature of quench surface 5, to
substantially prevent the formation of any solid Gr
liquid matter which could precipitate onto depletion
region 13. Such precipitate, if entrained between the
melt puddle and quench surface, could produce surface
defects and degrade the strip quality.
Surprisingly, the heating of the quench surface
does not degrade the quenching of the molten metal. To
the contrary, the heating of the quench substrate and
the low density atmosphere actually improve the unifor-
mity of the quench rate by minimizing the presence of
insulating, entrapped gas pockets, and thereby improve
the quality of the cast strip.
Preferably, gas 24 is a reducing gas; i.e. it is
¦ capable of causing a chemical reduction-type reaction.
Accordingly, the gas itself is capable of undergoiny
chemical oxidation, preferably by combining with
oxygen. Suitable reducing gases include carbon monoxide
and gas mixtures thereof.
The presence of a reducing atmosphere at quench
surface 5 has distinct advantages. In particular, a
1~1312~
reducing atmosph~re minimizes the oxidation of s rip
6. In addition, the reducing at~osphere starves quench
surface 5 of oxygen and minimizes the oxidation there-
of. The reduced oxidation improves the wettability of
the quench surface and allows molten metal to be more
uniformly deposited on quench surface 5. In the case of
a copper base materials in quench surface 5, the reduced
oxidation r~nders the quench surface much more resistant
to thermally induced fatigue crack nucleation and
growth. The reducing atmosphere also depletes oxygen
from the region of nozzle 4 thereby reducing the clog-
ging of nozzle orifice 22, particularly clogging due to
oxide particulates. Optionally, additional gas nozzle
32 may be employed to provide additional reducing gas
atmospheres along selected portions of strip 6, as
representatively shown in FIG~ 2.
FIG. 4 shows an embodiment of the invention wherein
the reducing gas is capable of being ignited and burned
to form a reducing flame atmosphere. Nozzle 4 deposits
molten metal onto quench surface 5 of rotating casting
wheel l to form strip 6. The depletion means in this
embodiment is comprised of gas supply 12, yas nozzle 8
and ignition means 30. Valve 16 regulates the volume
and velocity of gas delivered through gas nozzle 8, and
a wiper brush 42 conditions quench surface 5 to help
reduce oxi~ation thereon. After gas 24 has mixed with
sufficient ox~gen, ignition means 30 ignites the gas to
produce a heated, low-density reducing atmosphere around
upstream region 13 and around quench surface region 14
where molten metal is deposited. Suitable ignition
means include spark iynition, hot filament, hot plates
! and the like. For example, in the embodiment shown in
FIG. 4, the hot casting nozzle serves as a suitable
ignition means which automatically ignites the reducing
gas upon contact therewith.
The resultant flame atmosphere forms a flame plume
28 which begins upstream of quenching region 14 and
consumes oxygen therefrom. In addition, unburned
~Z~31~
-13-
reducing gas within the plume reacts to reduce the
oxides on quench surface 5, nozzle 4 and strip 6. The
visibility of flame 28 allows easy optimization and
control of the gas flow, and plume 28 is effectively
drawn around the contour of wheel 1 by the wheel
rotation to provide an extended reducing flame
atmosphere. As a result, a hot reducing atmosphere is
located around quenchiny surface 14 and for a discrete
distant thereafter. The extended flame plume advanta-
geously provides a non-oxidizing, protective atmosphere
around strip 6 while it is cooling. Optionally, addi-
tional gas nozzles 32 and ignition means 34 can be
employed to provide additional reducing flame plumes 36
along selected portions of strip 6 to further protect
the strip from oxidation. A further advantage provided
by the hot, reducing flame plume is that the smoothness
of the free surface side of the strip (the side not in
contact with the quench surface) is significantly
improved. Experiments have shown that the mean
roughness of the rapidly solidified metal strip, as
measured by standard techniques such as pack factor, is
significantly reduced when the strip is produced in the
reducing flame plume of the invention.
Proper selection of the reducing gas and the tem-
perature to which substrate 5 i5 heated is important.
The combustion product of the burned gas should not
produce a liquid or solid phase which could precipitate
onto quench surface 5 or nozzle 4. For example,
hydrogen gas has been unsatisfactory under ordinary
conditions because the combustion product is water which
condenses onto quench surface 5. As a result, under
I conventional casting conditions, the hydrogen flame
plume does not adequately reduce the formation of gas
pockets on the quench surface side of strip 6.
Surprisingly, it has been found that by appropriately
adjusting the casting conditions, the water precipitate
normally produced by combustion of hydroyen gas within
depletion region 13 can be substatially avoided. For
12131; :0
-14-
example, if the quench surface 5 is maintained at a
temperature of at least about 100C., water will not
condense out of the hydrogen flame atmosphere onto the
quench surface and, therefore, will not contribute to
the formation of gas pocket defects.
The reducing gas 24 is preferably a gas that will
not only burn and consume oxygen in a strongly exother-
mic reaction, but will also produce combustion products
that will remain gaseous at quench surface temperatures
ranging from 800K to 1300K. Gases of this type comprise
practically any gas or gas mixture which when heated or
combusted produces a thermally-induced, low density
atmosphere. Preferred gases include hydrogen, carbon
monoxide, methane, propane and the like, and mixtures
thereof. Especially preferred are reducing gases that
provide an anhydrous, reducing atmosphere.
The temperature to which quench surface 5 is heated
during castin~ depends upon the composition of the
strip, the composition of the low density atmosphere
present within depletion region 13 and the composition
of the quench surface 5. Typically, the quench surface
is heated to a temperature of at least about 323K, and
preferably to a temperature of about 323K to 573K.
Quench surface temperatures of at least about 373K and,
¦ 25 most preferably of about 423K to 523K substantially
prevent precipitation of condensed or solidified
constituents from most anhydrous reducing atmospheres
onto depletion region 13.
A reducing flame atmosphere provides an efficient
means for heating the atmosphere located proximate to
melt puddle 18 to very high temperatures, in the order
of 1300 - 1400 K. Such temperatures provide very low
yas densities around the melt puddle 18. The high tem-
peratures also increase the kinetics of the reduction
reaction to further minimize the oxidation of quench
surface 5, nozzle 4 and strip 6. The presence of a hot
reducing flame at nozzle 4 also reduces thermal gra-
dients therein which might crack the nozzle.
1213iZ~
-15-
Thus, the e~bodiment of the invention employing a
reducing flame atmosphere more efficiently produces a
heated, low-density reducing atmosphere around quench
surface 5 which improves the smoothness of both sides of
the cast strip and more effectively prevents oxidation
of quench surface 5, strip 6 and casting nozzle 4.
As shown in FIG. 5, the invention may optionally
include a flexible hugger belt 38 which entrains strip 6
against quench surface 5 to prolong cooling contact
therewith. The prolonged contact improves the quenching
of strip 6 by providing a more uniform and prolonged
cooling period for the strip. Guide wheels 40 position
belt 38 in the desired hugging position along quench
surface 5, and a drive means moves belt 38 such that the
belt portion in hugging relation to quench surface 5
moves at a velocity substantially equal to the velocity
of the quench surface. Preferably, belt 38 overlaps the
marginal portions of strip 6 to directly contact and
frictionally engage quench surface 5. This frictional
engagement provides the required driving means to move
the belt.
Considerable effort has been expended to develop
devices and procedures for forming thicker strips of
rapidly solidified metal because such strip can more
easily be used as a direct substitute for materials
presently employed in existing commercial applica-
tions. Since the present invention significantly
improves the contact between the stream of molten metal
and the chilled quench surface, there is improved heat
transport away from the molten metal. The improved heat
transport, in turn, pro~ides a more uniform and more
rapid solidification of the molten metal to produce a
higher quality thick strip, i.e. strip having a
thicknes~ ranging from about 15 micrometers to as high
as about 70 micrometers and more.
Similarly, considerable effort has been expended to
form thinner strips of rapidly solidified metal. Very
thin metal strip, less than about 15 micrometers and
- lZ13~
-16-
preferably about~8 micrometers in thickness, is highly
desirable in various commercial applications. In
brazing applications, for example, the filler metals
used in brazed joint normaly have inferior mechanical
properties compared to the base metals. To optimize the
mechanical properties of a brazed assembly, the brazed
joint is made very thin. Thus, when filler material in
foil form is ~laced directly in the joint area prior to
the brazing operation, the joint strength can be
optimized by using a very thin brazing foil.
In magnetic applications with high frequency elec-
tronics (over 10 kHz), power losses in magnetic devices
are proportional to the thickness (t) of the magnetic
materials. In other maynetic applications such as satu-
rable reactors, power losses are proportional to thethickness dimension of the magnetic material raised to
the second power (t2) when the material is saturated
rapidly. Thus, thin ribbon decreases the power losses
in the reactor. In addition, thin ribbon requires less
time to saturate; as a result, shorter and sharper
output pulses can be obtained from the reactor. Also,
thin ribbons decrease the induced voltage per lamination
and therefore, require less insulation between the
laminations.
In inductors for linear induction accelerators,
losses are again related to t2, and the thinner ribbon
will reduce power losses. Also, thin ribbon saturates
more easily and rapidly and can be used to produce
shorter pulse accelerators. In addition, the thinner
ribbon will require reduced insulation between the lami-
nations.
A further advantage of thin strip i3 that the strip
experiences less bending stresses when wound to a given
diameter. Excessive bending stresses will degrade the
magnetic properties through the phenomenon of magneto-
striction.
The apparatus and method of the invention are
particularly useful for forming very thin metal strip.
~Z~31;~)
-17-
Since the invention significantly reduces the size and
depth of gas pocket defects, there is less chance that
such a defect will b~ large enough to perforate the cast
strip. As a result, very thin strip can be cast because
there is less probability that a defect large enough to
perforate the strip will form. Thus, the invention can
be adapted to cast very thin metal strip, which as-cast,
is less than about 15 micrometers thick. Preferably,
0 the strip has a thickness of 12 micrometers or less.
10 More preferably, the strip thickness ranyes from 7 to 12
micrometers In addition, the thin metal strip has a
width dimension which measures at least about 1.5
millimeters, and preferably measures at l~ast about 10
mm.
-18- ~ 2 ~ 3
~ EXAMPLES
A forced-convection-cooled, plain carbon steel sub-
strate wheel used in the present investigation was 38 cm
(15 in.) in diameter, 5 cm (2 in.) wide. Initially,
nickel-base ribbons of composition Ni68cr7Fe3sl4sil8
(subscripts in atomic percent) were produced on the
steel wheel with low circumferential surface speed
(about 10 m/s or 2,000 fpm) to avoid excessive ribbon-
substrate adhesion. The substrate wheel was conditioned
continuously during the run by an idling brush wheel
inclined about 10 out of the casting direction.
~ xperiments showed that the ribbons exhibited very
little adhesion on the substrate surface. An increase
in casting pressure and an increase substrate surface
speed helped improve ribbon-substrate adhesion. All of
the ribbons cast in these initial experiments showed
significant populations of entrapped air pockets in the
underside, as is typically observed when ferrous alloy
ribbon is cast on a copper-base substrate wheel. In the
initial experiments, a dark oxidation track, which forms
on the substrate surface during ribbon casting, limits
the ribbon to substrate adhesion. A carbon monoxide
flame directed at the ribbon castin~ track upstream of
the melt puddle was then used to reduce oxidation and
promote ribbon-substrate adhesion. The combined actions
of the flame and the conditioning brush reduced the
substrate oxidation, increased adhesion and produced
ribbon having good geometric uniformity. Maynetic
properties of ferromagnetic ribbons were also
mprovedO
Experiments were conducted to determine if
substrate oxidation occurs primarily near the melt
puddle or after the point of ribbon separation from the
substrate. It was found that a reducing flame in the
immediate vicinity of the melt puddle resulted in a
ribbon casting track having substantially reduced
oxidation. The best results were obtained when the
distance between the carbon monoxide flame and the back
lZ13~L2~
--19--
of the melt puddle was less than about 2 cm (<1 inch).
Thus, experiments have shown remarkable improvement
of ribbon surface smoothness, luster, and ductility over
material cast in a conventional manner. While the
5 intrinsic wetting of a copper substrate by ferrous melts
may not be as great as the wetting of an iron-based
7 substrate, the use of a reducing flame enhances melt-
copper substrate wetting to the point where a copper
substrate is a viable material for the production of
10 high quality, defect-free strip. Such a defect-free
castiny capability allows the production of very thin
ribbon (on the order of about 7 micrometers thick).
Additionally, the improved melt-substrate contact caused
by carbon monoxide flame-assisted casting improves
15 overall quench rate and enables the production of a
given ribbon composition at a thickness greater than
usual.
