Note: Descriptions are shown in the official language in which they were submitted.
~2~ 3
CASTING IN A THERMALLY-INDUCED, LOW DENSITY ATMO~PHERE
BACKGROUND OF THE INVENTION
- 1. Field of the Invention
The invention relates to the casting 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
ra~)idly quenching a stream of molten metal to form con-
tinuous metal strip. The metal can be cast in an inertatmosphere or a ~artial 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 flex~ible 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 yas pockets entrapped
. ~ ~
between the quench surface and the molten metal durin~
quenching form gas pocket defects. These defects, along
` 20 with other factors, cause considerable rouyhness on the
~ qu~ench surface side~as well as the opposite, free sur~
3~ faoe;~side~ of the cast strlp. In some cases, the surface
defects actually~e~xtend through the strip,~forming per-
;forat~i~o~ns the~ein.
~ ~U.S. Patent No 4~,154,283 to R. Ray et al. dis-
closes that~vacuum~castin~of metal strip reduces the
formation~of ~as~pocket~deeects. `The~vacuum castin~
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system taught by Ray et al. requires specialized
chambers and pumps to produce a low pressure casting
atomo~sphere. In addition, auxiliary means are required
to co~tinuously 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,~55 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 rolI. A
cover encloses the roll surface upstream of the nozzle
to provide a chamber, the atmosphere of which is evacu-
ated by a vacuum pump. A heater in the cover heats theroll surface upstream from the nozzle to remove dew
droplets and gases from the roll surface. The vacuum
chamber lowers the density of the moving yas layer next
to the casting roll surface, thereby decreasing forma-
tion of air pocket depressions in the cast ribbon. Theheater helps drive off moisture and adhered yases from
the roll surface to further decrease formation of air
pocket depressions.
The apparatus disclosed by Suzuki et al. does not
pour metal onto the casting surface until that surface
has exited the vacuum chamber. By this procedure, com-
plications involved in removing a rapidly advancing rib-
bon from the vacuum chamber are avoided. The ribbon is
actually cast in the open atmosphere, offsetting any
3 potential improvement in ribbon quality.
~ U.S. Patent No. 3,861,450 to Mobley, et al. dis-
closes a method and apparatus for makiny metal fila-
~ ment. 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
of which in the atmosphere yields reducing or non-
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oxidizing combustion products at the critical process
region. In a particular embodiment, a cover com~osed 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
reyion of the melt.
The introduction of non-oxidizing gas, as taught by
Mobley, et al., disrupts and replaces an adherent layer
of oxidizing gas with the non-oxidizing gas. The con-
trolled introduction of non-oxidi~ing 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 drag the im~urities
into the melt to the point of initial filament solidifi-
cation. 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
flowiny stream of non-oxidizing gas taught by Mobley,
et al. is still drawn into the molten pool by the vis-
cous 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 oxi-
dation 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 ~o.
~ ::
4,262,734 issued to HD Liebermann disclose an apparatus
and method in which coaxial gas jets are employed to
;reduce edge defects in rapidly quenched amor~hous
strips. U~S. Patent~No. 4,177,856 and U.~. Patent No.
; 4,144,926 issued to H. Liebermann disclose a method and
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apparatus in which a Reynolds number parameter is con-
trolled to reduce edge defects in rapidly quenched amor-
phous strip. Gas densities and thus Reynolds numbers,
are regulated by the use of vacuum and by employiny
lower molecular weight gases.
Conventional methods, however, have been unable to
adequately reduce surface defects in cast ~etal strip
caused by the entrapment of gas pockets. Vacuum casting
procedures have afforded some success, but when using
vacuum casting, excessive welding of the cast strip to
the quench surface and the difficultly of 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-section.
SUMMARY O~ THE I N V ENTI ON
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 mov-
ing chill body hàving a quench surface, and includes a
nozzle means for depositing a stream of molten metal on
a quenching reyion of the quench surface to form the
strip. The nozzle means has a exit portion with a noz-
zle orifice. A depletion means heats a gas to lower the
density thereof and to produce a low density atmosphere
haviny a temperature of at least about 80~K. The yas is
supplied to a depletion region located adjcent to and
upstream of the quenching region to provide the low
density atmosphere within the depletion region.
~-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 re~ion of th~e quench surface to form
the~ strip. ~A gas lS heated to lower the density thereof
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perature of at least about 800K. The yas is supplied to
a depletion reyion located adjacent to and upstream of
the quenching region to provide the low density atmo-
sphere within the depletion region and thereby substan-
tially prevent formation of gas pockets in the strip.
The invention further provides a metal strip having
a thickness of less than about 15 micrometers in the as-
cast state.
The method and apparatus of the invention advanta-
geously minimize the formation and entrapment of gas
pockets 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 heated gas
within the depletion region surprisingly provides better
and more uniform cooling and quenching of the molten
metal. The hot gas provides a low density atmos~here
that inhibits the formation of gas pockets operating to
decrease contact between the molten metal and the quench
surface. The more uniform quenching, in turn, provides
improved physical properties in the cast strip. In par-
ticular the reduction of surface defects on the quenched
surface side of the strip increases the packing factor
of the material and reduces localized stress concentra-
tions 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 packing factor of the material.
In production of amorphous metal strip, the more uniform
quenching afforded by the low density atmosphere pro-
vides a more consistent and uniform formation of the
amorphous state. In manuEac~ture of strip composed of
magnetic material~,~the number and~size of strip surface
discontinuities is~reduced, improving the magnetic prop-
erties of the strip.
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Surface defects due to entrapped yas pockets are
reduced, and there is much less chance for a yas pocket
to perforate the strip. Surprisingly, very thin strips
(less than about 15 microns in thickness) have been pro-
duced. These very thin strips are highly desirable invarious applications~ For example, in magnetic devices,
such as inductors, reactors and high frequency electro-
magnetic 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
Moreover, the reduction of entrapped gas pockets
markedly increases the heat condutive contact between
the molten metal and the quench surface. Thicker strips
of rapidly solidified metal can be produced. ~uch
thicker strip is desireable because it can be more
easily substituted for ~aterials conventionally used in
existing commercial applications. These thick strip
components can, surprisingly, be rovided by rapid solid-
ification in a single quenching step in much less timewaith 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
; 25 surface finish and uniform physical properties. Complex
equipment and procedures associated with vacuum casting
are eliminated. The inventlon 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 fr conventional materials with greater
~ effectiveness and economy.
- BRIEF DESCRIPTIO~ 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 lnvention and the accom-
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panying drawings in which:
FIG~ l shows a representative prior art apparatus
-Eor 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 hugyer belt to prolong contact of the
cast strip with the quench surface;
FIG. 6 shows a gas velocity profile at the quench
surface portion on which molten metal is deposited;
DESCRIPTION OF PREF~RRED EMBODIMENTS
For the purposes of the present invention and asused 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 reyular or irregular
cross-section.
; The invention i5 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 ~uenched at a rate of at least
about 104C/sec from a melt of molten metal. Such
~ rapidly solidified strip has improved physical proper-
: 30
ties, such as improved tensile strenyth, ductility and
: magnetic properties.
FIG. l 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
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1~ Solidi~ied moving stri~ 6, after its break-away
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 copper
based alloys such as beryllium copper. To achieve the
quench rates of at least about 104C per second, wheel 1
is internally cooled and rotated to provide a quench
surface that advances at a speed ranging from about 100
- 400~ 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, de~osits a stream of molten
metal onto a quenching region 14 of quench surface 5 to
form strip 6. Nozzle 4 has an orifice 22 located at
exit ~ortion 26. A depletion means, including gas noz-
zle delivery means 8, heater means 10, and gas supply
12, heat a gas 24 from gas supply 12 to produce a low
density atmosphere and directs the gas with gas nozzle 8
to a depletion region 13 located adjacent to and
upstream from 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.~ Valve I6 regulates the volume and velocity
through nozzle 8. As shown in FIG. 2, gas nozzle 8 is
located upstream of quenching region 14 and is directed
along the direction of movement of the quench surface.
~ptiona;lly, gas nozzle 8 can be located coaxial with
cast~ing nozzle 4 as representatively shown in FIG. 3.
The term low density atmosphere, as used in the
specification and claims hersof, means an atmosphere
~ having a gas density less than l gram per liter and
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preferably, haviny a gas density of of less than about
0~5 grams per liter.
To obtain the desired low density atmosphere, ~as
24 is heated to at least about 80~K, and more prefer-
ably, is heated to at least about 1300K In general,hotter gases are preferred because they will have lower
densities and will better minimize the formation and
entrapment of gas pockets between quench surface 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 6. 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 Eactor is the volume fraction of
the actual ~aynetic ~aterial 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 optimiziny
the magnetic properties of strip 6 and in minimizing
localized stress concentrations that would otherwise
-~ reduce the fatigue resistance of the strip.
Gas pockets also insulate the deposit molten metal
from quench surface 5 and reduce the quench rate in
localized areas. The resultant, non-uniform quenching
produces non-uniEorm physical properties in strip 6,
such as non-uniform stren~th, ductility and maynetic
3~ 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
inhibit mobility of magnetic domain walls, thereb~
degrading the magnetic properties of the material.
Thus, by reducing the entrapment of yas pockets,
the~invention produces high quality metal strip with
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improved surface finish and improved physical proper-
ties. For example, metal strip has been produced with
packing 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 FIG. 6. The
gas bounaa~y layer velocity profile near ~uench surface
5 and upstream of melt puddle 18 is shown schematically
at 20. The maximum gas boundary layer velocity occurs
immediately adjacent to quench surface 5 (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 depletion region 13 and
into quenching region 14, the region of the quench sur-
face upon which molten metal is deposited. Because of
the drafting of relatively cool air into the ~uenching
region, the presence of the hot casting nozzle and the
molten metal do not sufficiently heat the local atmo-
sphere 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 at the melt-substrate interfacej how-
ever, acts to locally separate the melt from the sub-
strate and form entrained gas pockets which will appear
as "lift-off" areas 44 on the ribbon underside. The
stagnation pressure of the gas boundary layer (pressure
if the layer hit a rigid wall) is given by the formula
Ps= 1/2 pv2 where: P = gas density, v = substrate velo-
` city~ Therefore, the reduction of gas boundary layer
density or substrate velocity are important in the
r~eduction of the size and the number of gas pockets
entrained under the molten metal puddle. For example,
removal of the gas boundary layer by casting in vacuumcan totally eliminate the lift-off ar~eas in the strip
underside. Alternatlvely, a low density gas in the
boundary layer could be employed. The selection oE a
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low molecular weight gas (such as helium) is one way to
reduce boundary layer yas 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 temperatureO 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 hot, low-density
atmosphere, and the parameters of quench surface 5, to
substantially prevent the formation of any solid or
liquid matter which could precipitate onto quench sur-
face 5. Such precipitate, if entrained between the melt
puddle and quench surface, could produce surface defects
and degrade the strip quality.
~urprisingly, the heating of the gas atmosphere
located proximate to quenching region 14 to decrease the
density thereof does not degrade the quenching of the
molten metal. To the contrary, the heating actually
~ improves the uniformity of the quench rate by minimizing
-~ the presence of insulating, entrapped gas pockets, and
~ thereby improves the quality of the cast strip.
; Gases including nitrogen, helium, neon, argon
krypton, xenon and mixtures thereof, have been found
suitable for use in the present invention, provided such
gases are heated to a temperature of at least about
800K, and preferably 800~1300K, to reduce the density
thereof. Fig. 4 shows an ~embodiment of the invention in
which the aforesaid gases are supplied at low density by
a~depletion means. 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 heater
means 10. Valve 16 reyulates the volume and velocity of
gas delivered through gas nozzle 8, and a wiper brush 42
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conditions quench surface 5 to help reduce oxidation
thereon. Heater means 10 heats the gas to produce a
heated, low-density atmosphere around depletion region
13 and around quenching region 14 where molten metal is
deposited. As a result, a hot, low density atmosphere
is located around quenching region 14 and for a distance
on either side thereof. Optionallly, additional gas
nozzels 32 and heater means 33 can be employed, together
with gas supply 121 to provide additional atmospheres 36
along selected portions of strip 6 to further protect
the strip from oxidation.
As shown in EIG. 5, the invention may optionally
include a flexible hugger belt 38 which entrains strip 6
against quench surface 5 to ~rolon~ coolin~ contact
therewith. The prolonged contact improves the quenchin~
of strip 6 by providing a more uniform and prolonged
cooling period for the strip. Guide wheels 40 position
belt 3~ in the desired hu~ying position along ~uench
surface 5, and a drive means moves belt 38 such that the
belt portion in hugying relation to quench surface 5
moves at a velocity substantially e~ual 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 ~. 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 siynificantly
improves the contact between the stream of molten metal
and the chilled quench surface, there is improved heat ~ 35 transport away from the molten metal. The improved heat
transport, in turn, provides a more uniform and more
rapid solidification of the molten metal to producè a
higher quality thick strip, i.e. strip haviny a thick-
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ness ranging from about 15 micrometers to as great 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
preferably about 8 micrometers in thickness, is highly
desirable in various commercial applications. In
brazing applications, for example, the filler metals
used in bra~ed 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 placed 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 maynetic
materials. In other magnetic 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 out~
put pulses can be obtained from the reactor. Also, thin
ribbons decrease the induced voltaye per lamination and
therefore, require less insulation between the lamina-
tions.
In inductors for linear induction accelerators,
losses are again related to t2, and the thinner ribbonwill 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 is that the strip
experiences less bending stresses when wound to a ~iven
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diameter. Excessive bendin~ stresses ~ill degrade the
maynetic properties through the phenomenon of magneto-
striction.
The apparatus and method of the invention are par-
ticularly useful for forming very thin metal strip.
Since the invention significantly reduces the size and
depth of gas pocket defects, there is :Less chance that
such a defect will be large enough to perforate the cast
strip. As a result, very thin strip can be cast because
thers 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, ~hich as-cast,
is less than about 15 micrometers thick. Preferably,
the cast strip has a thickness of 12 micrometers or
less. More preferably, the cast strip thickness ranges
~rom 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 least
about 10 mm,
EXAMPLES
A forced-convection-cooled, plain carbon steel sub-
strate wheel is 38 cm (15 in.) in diameter, 5 cm (2 in.)
wide. Initially, nickel-base ribbons of composition
Ni68C~7Fe3B14Si8 (subscripts in atomic percent) are
produced on the steel wheel with low circumferential
surface speed ~about 10 m/s or 2,000 fpm) to avoid
excessive ribbon-~ubstrate adhesion. The substrate
wheel is conditioned continuously during the run by an
idling brush wheel inclined about 10 out of the casting
direction.
The ribbons exhibit very little adhesion on the
substrate surfa e. An increase in casting pressure and
an increase substrate surface speed help improve ribbon-
substrate adhesion. All of the ribbons cast show
significan~ populations of entrapped air pockets in the
underside. A dark oxidation track, which forms on the
substrate surface during ribbon castin~, limits the
ribbon to substrate adhesion. A hot gas stream,
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directed at the ribbon casting track upstream of the
melt puddle, reduces oxidation and promotes ribbon-
substrate adhesion. The combined actions of the hot gas
stream and the conditioning brush reduce the substrate
oxidation, increase adhesion and produce ribbon having
good geometric uniformity.
Thus, experiments show a remarkable improve~ent of
ribbon surface smoothness, luster, and ductility over
material cast in a conventional manner. Such a defect-
free casting capability allows the production of very
thin ribbon (on the order of about 7 micrometers thick
Additionally, the improved melt-substrate contact caused
by casting in a hot ~as stream improves overall ~uench
rate and enables the production of a yiven ribbon
composition at a thickness greater than usual.
Havinc~ thus described the invention in rather full
detail, it will be understood that such detail need not
be strictly adhered to heat that various changes and
modifications may suggest themselves to one skilled in
the art, all falling within the scope of the present
invention, as defined by the subjoined claims.
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