Note: Descriptions are shown in the official language in which they were submitted.
~2~
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
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 yas pockets entrapped
betwe0n the quench surface and the molten metal during
quenching form gas pocket defects. These defects, along
with other factors, cause considerable rouyhness on the
quench surface side as well as the oppositet free sur-
face side of the cast strip. In some cases, the surface
defects actually extend through the strip, forming per
forations therein.
U.S. Patent No. 4,154,283 to R. Ray et al. dis-
closes that vacuum casting of metal strip reduces the
formation of ~as pocket defects. The vacuum casting
system taught by Ray et al. requires specialized
chambers and pumps to produce a low pressure casting
atomosphere. 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 evacu-
ated by a vacuum pump. A heater in the cover heats the
roll surface upstream from the nozzle to remo~e dew
droplets and gases from the roll surface. The vacuum
chamber lowers the density of the moving gas layer next
to the casting roll surface, thereby decreasiny forma-
tion of air pocket depressions in the cast ribbon. Theheater helps drive off moisture and adhered gases from
the roll surface to ~urther 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
potential improvement in ribbon quality.
U.S. Patent No. 3,861,450 to Mobley, et al. dis-
closes a method and apparatus for making metal fila-
mentO A disk-like, heat-extracting member rotates to
dip an edge surface thereof into a molten pool, and a
non-oxidiæing 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-
oxidi~ing combustion products at the critical processregion. In a particular embodiment, a cover co~posed o~
carbon or graphite encloses a portion of the disk and
reacts with the oxygen adjacent ~he 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 yas, as taught by
Moble~, et al., disrupts and replaces an adherent layer
of oxidizing gas with the non-oxidizing gas The con-
- trolled 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 drag the impurities
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 a~ the disk surface and in the melt. The
flowing stream of non-oxidizing gas tauyht by ~obley,
et al. is still drawn into the molten pool by the vis-
COU5 drag of the rotating wheel and can separate themelt 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.
~ .S. Patent No. 4,282,921 and U.S. Patent No.
4,262,734 issued to ~. Liebermann disclose an apparatus
and method in which coaxial gas jets are employed ~o
reduce edge defects in rapidly quenched amorphous
strips. U.S. Patent No~ 4,177,856 and U.~. Patent No.
4,144,926 issued to H. Liebermann disclose a method and
--4--
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 weiyht gases.
Conventional methods, howeverl have been unable to
adequately reduce surface defects in cast metal strip
¦ caused by the entrapment of gas pockets. Vacuum casting
I procedures have afforded some success, but when using
vacuum casting, excessive welding of the cast strip to
the quench surface and the difficultly oE removiny 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 e~ficiently pro-
duces smooth strip with consistent quality and uniform
cross-section.
S~MMARY O~ TH~ 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 mov-
ing chill body haviny a quench surface, and includes a
nozzle means for depositing a stream of molten metal on
a guenchiny region 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
having a temperature of at least about 800K. The gas is
supplied to a depletion region located adjcent to and
upstream of the quenching region to provide the low
density atmcsphere 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 region of the ~uench surface to form
the strip. A gas is heated to lower the density thereof
--5--
and to produce a low density atmosphere having a tem-
perature of at least about 800K, The gas is supplied to
a depletion re~ion located adjacent to and upstream of
the quenching region to provide the low density atmo-
sphere within the depletion reyion 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 yas
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
; 20 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 ~uenched
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 oE
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 manufacture of strip composed of
magnetic material~ the number and size of strip surface
discontinuities is reduced, improving the magnetic prop-
erties of the strip.
--6--
Surface defects due to entrapped gas pockets are
reduced, and there is much less chance for a gas pocket
to perforate the stripO 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 ma~netic 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
strenyth 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 so;idified metal can be produced. Such
thicker strip is desireable because it can be more
easily substituted for materials 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 time
waith decreased cost.
Thus, the present invention effectively minimizes
gas pocket defects on the strip surface which contacts
the ~uench surface, and produces strip having a smooth
surface finish and uniform physical properties. Complex
equipment and procedures associated ~ith 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 thic~ 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 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 o~ the invention and the accom-
--7--panying drawings in which:
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 sho~s 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 prolony 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 PREFERRED EMBODIMENTS
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 muchsmaller 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 ~uenched 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 strength, 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 throuyh a nozzle 4 at the
base of the crucible and deposits the molten metal onto
a moving chill body, such as rotatable casting wheel
--8--
1. Solidified moving strip 6, after its break-away
point from the quench wheel is then routed onto a suit-
able winding means.
Quench surface 5 (substratel 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
- 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 reyion 14 of quench surface 5 to
form strip 6. Nozzle 4 has an orifice 2~ located at
exit portion 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 yas with gas nozzle 8
to a depletion region 13 located adjacent to and
upstream from quenching region 14. ~ozzle 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 16 regulates the volume and velocity
through nozzle 8, As sho~n in FIG. 2, gas nozzle 8 is
located upstream of ~uenching region 14 and is directed
along the direction of movement of the quench surface.
~ptionally~ gas nozzle 8 can be located coaxial with
casting nozzle ~ as representatively shown 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
- 9 -
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 8~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 factor is the volume fraction of
the actual magnetic materi~l in the wound core (the
volume of magnetic material divided by the total core
volume) and is often e~pressed in percent. A smoot
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 fatigue resistance o~ the strip.
Gas pockets also insulate the deposit molten metal
from quench surface S and reduce the quench rate in
localized areas. The resultant, non-uniform quenching
produces non-uniform physical properties in strip 6,
such as non-uniform stren~th, ductility and magnetic
properties-
For example, when castin~ 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, therebydegrading the magnetic properties of the material.
Thus, by reducing the entrapment of yas pockets,
the invention produces high quality metal strip with
-10-
improved surface finish and improved physical proper-
ties. For example~ metal strip has been produced with
packing factors of at least about ~0%, 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 boundary layer velocity profile near quench 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 moviny quench surface.
Thus, moving quench surface 5 ordinarily draws cool air
from the ambient atmosphere into depletion reyion 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
reyion, 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
yas boundary layer at the melt-substrate interface, 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
p5= 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
reduction of the size and the number of gas pockets
entrained under the molten metal puddle. For example,
removal o~ 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 yas density. However, the variety
of low molecular weight gases which can be used in this
Eashion 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
l 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 hot, low-density
atmosphere, and the parameters of quench surface 5, to
substantially prevent the formation of any soli~ 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.
Surprisingly, the heating of the yas 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 uni~ormity of the quench rate by minimizing
the presence of insulating, entrapped gas pockets, and
thereby improves the quality of the cast stripO
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 800-1300~, 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 ~ 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, gas nozzle 8 and heater
means lO. Valve 16 reyulates the volume and velocity of
gas delivered through gas nozzle 8, and a wiper brush 42
8~
-12
conditions quench surface 5 tc help reduce oxidation
thereon. Heater means 10 heats the gas to pro~uce a
heated, low-density atmosphere around depletion region
13 and around quenching reyion 1~ 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 FIG. 5, the invention may optionally
include a flexible hugger belt 38 which entrains strip 6
against quench surface 5 to prolon~ cooling contact
therewith. The prolonged contact improves the quenchiny
of strip 6 by providing a more uniform and prolonged
cooling period for the strip. Guide wheels 40 position
belt 3~ in the desired hugging position along quench
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 equal to the velocity
of the quench surface. Preferably, belt 3~ overlaps the
marginal portions of strip 6 to directly contact and
frictionally engage quench surface 5. This frictional
¦ 25 engagement provides the required driving means to move
the belt.
Considerabl~ effort has been expended to develop
devices and procedures for forming thiclser strips of
rapidly solidified metal because such strip can more
easily be used as a direct substitute ~or 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
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 produce a
higher quality thick strip, i.e. strip having a thick-
-13-
ness ran~ing from about 15 micrometers to as great as
about 70 micrometers and more.
Similarly, considerable e~fort has been expended to
form thinner strips of rapidly solidified metal. Very
5 thin metal strip, less than about 15 micrometers and
preferably about 8 micrometers in thickness, is highly
t desirable in various commercial applications. In
brazing applications, for example, the filler metals
used in brazed joint normaly have in~erior mechanical
10 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
15 optimized by using a very thin brazing foil.
In magnetic applications with high frequency elec-
tronics (over 10 kHz)l power losses in magnetic devices
are proportional to the thickness (t) of the ma~netic
materials. In other magnetic applications such as satu-
20 rable reactors, power losses are proportional to the
thickness dimension o~ 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
25 time to saturate; as a result, shorter and sharper out-
put pulses can be obtained from the reactor. Also, thin
ribbons decrease the induced volta~e 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 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 re~uire reduced insulation between the lami-
nations.
A further advantage of thin strip is that the strip
experiences less bending stresses when wound to a given
diameter. E~cessive bending stresses will degrade the
magnetic properties through the phenomenon of magneto-
S t r i C tion.
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
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,
the cast stri~ has a thickness of 12 micrometers or
less. More preferably, the cast strip thickness ranges
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 least
about lU mm.
EX~MPLE5
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
Ni6~Cv7Fe3~1~Si8 (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-substrate adhesion. The substrate
wheel is conditioned continuously during the run by an
idling brush wheel inclined about 104 out of the casting
direction.
The ribbons exhibit very little adhesion on the
substrate surface. An increase in casting pressure and
an increase substrate surface speed help improve ribbon-
substrate adhesion. All of the ribbons cast show
significant populations of entrapped air pockets in the
underside. A dark oxidation track, which forms on the
substrate surface during ribbon casting, limits the
ribbon to substrate adhesion. A hot gas stream,
-15-
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 improvement of
ribbon sur~ace smoothness, luster, and ductili~y over
¦ material cast in a conventional manner. ~uch 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 yas stream improves overall quench
rate and enables the production of a given ribbon
composition at a thickness greater than usual.
Having 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 falliny within the sco~e of the present
invention, as defined by the subjoined claimsO
