Chill roll casting of continuous filament

Improvement in apparatus for making metal filament by depositing a stream of molten metal onto the inner surface of an annular chill roll as it is being rotated includes provision of a rotably mounted annular chill roll having an inner surface substantially parallel to its axis of rotation, and means for stripping the metal filament from the inner surface of the chill roll and for guiding the stripped metal filament away from the chill roll.

BACKGROUND OF THE INVENTION 
This invention relates to a method and apparatus for making continuous 
metal filaments, particularly amorphous metal filaments, by depositing a 
molten stream of metal onto the inner surface of an annular chill roll 
while it is being rotated around its axis to form a solid filament on that 
surface, and withdrawing the filament. 
For purposes of the present invention, a filament is a slender body whose 
transverse dimensions are much less than its length. In that context, 
filaments may be bodies such as ribbons, sheets or wires, of regular or 
irregular cross section. 
It is already known to make metal filaments by directing a jet of molten 
metal against a moving chilled quenching surface whereon it is solidified. 
One of these known methods involves chill roll casting wherein a free jet 
of molten metal is impinged upon the exterior surface of a rotating drum, 
whereon it is solidified to form a filament, which is then flung away from 
the drum by centrifugal action. Chill roll casting techniques employing 
the exterior surface of a rotating drum or cylinder have, for example, 
been described by Strange and Pim in U.S. Pat. No. 905,758. Filaments 
formed on the exterior surface of a rotating drum may be recovered 
therefrom by using nipping means, as disclosed in U.S. Pat. No. 3,856,074 
to Kavesh. The procedure described by Strange et al. may be readily 
employed to form filaments of many of the polycrystalline metals which 
possess sharp melting points, that is to say, which have solid-liquid 
transition range of less than about 5.degree. C. However, amorphous or 
glassy metals often have a transition range in the order of about 
400.degree. C. or more, through which the viscosity of the metal gradually 
increases until the critical glass transition temperature is reached, and 
it is necessary for the filament to be quenched below its glass transition 
temperature before departure from the quench roll. This is often difficult 
to achieve by the procedure of Strange et al. because centrifugal force 
tends prematurely to fling the filament away from the drum surface. 
Pond and Madden in Trans. Met. Soc. AIME, 245 (1969), pages 2475-6, 
describe a method for making metal filaments by directing a jet of molten 
metal against the inner surface of a rotating cylinder. The location of 
the jet is moved along the length of the cylinder, thereby producing a 
spiraling specimen of filament on the inner cylinder wall. Radial 
acceleration imparted to the liquid stream by rotation of the cylinder 
induces good thermal contact and spreads the stream into a flat filament 
prior to complete solidification. No provisions are made for stripping the 
filament from the drum as it is being produced, so that continuous 
production of filament is not possible, and the length of the filament so 
formed is inherently limited. 
U.S. Pat. Nos. 3,881,542 and 3,939,900, both to Polk et al., are 
respectively directed to method and apparatus for making continuous length 
shaped metal filaments by casting a stream of molten metal within a groove 
formed in the inner periphery of a rotating cylindrical chill roll. U.S. 
Pat. No. 3,881,540 to Kavesh describes a process and apparatus for making 
continuous untwisted length of metal filament by casting a molten stream 
of metal onto the inner surface of an annular chill roll which inner 
surface is inclined at an angle of 2.degree. to 30.degree. to the axis of 
rotation of the chill roll, exerting pressure on the quenched molten 
stream in contact with the inclined inner surface of the chill roll after 
solidification, and collecting the filament thus formed. 
SUMMARY OF THE INVENTION 
The present invention provides an improvement in apparatus for making metal 
filament including an annular chill roll rotably mounted around its axis 
and means for depositing a stream of molten metal onto the inner surface 
of the chill roll as it is being rotated, which improvement comprises, in 
combination, provision of (a) a rotably mounted annular chill roll having 
an inner surface substantially parallel to its axis of rotation, and (b) 
means for stripping the metal filament from the inner surface of the 
rotating chill roll beyond the point of solidification of the stream of 
molten metal deposited thereon and for guiding the stripped metal filament 
away from the inner surface of the chill roll. 
The present invention further provides a process for making metal filament 
which comprises depositing a stream of molten metal onto the inner surface 
of a rotating annular chill roll having an inner surface substantially 
parallel to its axis of rotation, stripping the metal filament formed by 
solidification of the stream of molten metal in contact with the inner 
surface of the rotating chill roll at a point beyond which solidification 
occurred, and guiding the stripped metal filament away from the inner 
surface of the chill roll. 
The method and apparatus of the present invention advantageously permit 
rapid cooling of the molten metal stream deposited on the rotating chill 
roll to and below the critical glass transition temperature as is 
necessary for the production of amorphous metal filaments, since they 
permit intimate contact of the molten metal with the quench surface over 
increased periods of time. The method and apparatus of the present 
invention are eminently suited for making filaments of polycrystalline 
metals, of alloys forming amorphous metals, and of nonductile or brittle 
alloys which are not readily formable into filaments using conventional 
processes.

DETAILED DESCRIPTION OF THE INVENTION AND OF THE PREFERRED EMBODIMENTS 
With reference to FIG. 1, apparatus employed comprises a chill roll 1 
rotably mounted around its longitudinal axis, crucible 2 equipped with 
induction heating coils for holding molten metal for spraying onto the 
inner surface of chill roll 1 through nozzle 3, and air knife 5 for 
stripping the metal filament 4 from the inner surface of the chill roll, 
and for guiding it away from the chill roll and to a suitable collection 
device (not shown). In operation, a stream of molten metal is ejected 
through nozzle 3 onto the inner surface of rotating annular chill roll 1 
whereon it forms a molten filament 4 which is solidified through intimate 
contact with the relatively cooler chill roll. The resulting solid 
filament 4 is carried by the rotation of the chill roll into the vicinity 
of air knife 5. A continuous blast of air directed by air knife 5 against 
the inner surface of the chill roll strips the solid filament 4 from the 
inner surface of the chill roll and guides the stripped filament 4 away 
from the inner surface of the chill roll and towards a suitable collection 
device. 
Operation of the embodiment of the present invention illustrated by FIG. 2 
is identical to that illustrated by FIG. 1, except that it employs blade 6 
in sliding contact with the inner surface of chill roll 1 for stripping 
the solidified metal filament from the inner surface of the chill roll, 
and employs guide 7 for guiding the stripped metal filament away from the 
chill roll and toward a suitable collection device. 
A particular advantage of employing an annular chill roll having an inner 
surface substantially parallel to its axis of rotation, as compared to use 
of chill rolls wherein the inner surface if inclined at an angle to the 
axis of rotation, is that it permits making of straight filament of 
uniform cross section. Chill rolls having an inclined cross section have a 
tendency to make filament which is thicker on one side than on the other, 
and which further is cambered, hence has a built-in twist. A particular 
advantage of forming filaments by depositing a stream of molten metal 
against the inner surface of a rotating chill roll, as compared to 
deposition of such metal against the outer surface of such chill roll, is 
that the former provides more intimate and prolonged contact of the metal 
filament with the chill roll. 
The molten material which is to be formed into a filament is heated, 
preferably in an inert atmosphere or under subatmospheric pressure to 
temperature approximately 50.degree. to 100.degree. C. above its melting 
point or higher. Ejection of the molten metal through a nozzle for 
deposition onto the chill roll may be effected by the pressure of the 
static head of the molten metal in the crucible, or by pressurizing 
crucible 2 to pressure in the order of, say, 1 to 20 p.s.i.g. or until a 
stream of molten metal is ejected through nozzle 3. Nozzle 3 may be 
provided with a tapered orifice, if desired, since tapering of the orifice 
along its length tends to enhance stability of the molten metal jet 
ejected therethrough. 
The velocity at which the stream of molten metal is ejected and the 
rotational velocity of the chill roll are interrelated. The surface speed 
of the inner surface of the annular chill roll is desirably in the range 
of from about 1 to 20, preferably from about 1 to 5, more preferably yet 
from about 1 to 2.5 times the speed of the jet of molten metal. One of the 
considerations involving the velocity of the jet is the jet stability, 
that is the velocity must be sufficient to insure a continuous uniform jet 
stream. It must not be so great as to cause breakup of the stream at the 
point of impact with the cylinder. The jet stability varies with alloy 
composition. However, in general it has been found that ejection velocity 
within the range of about 180 to 260 centimeters per second will result in 
satisfactory stable jet. It is possible to adjust the rotational speed of 
the chill roll, within the above-stated limits, to obtain filament having 
desired dimensions. 
The inner surface of chill roll 1 providing the actual quench surface can 
be any metal having relatively high thermal conductivity. This requirement 
is particularly applicable if it is desired to make amorphous or 
metastable filaments. Preferred materials of construction include 
beryllium copper, oxygen-free copper, and stainless steel. Since, unlike 
in previously known methods for making filaments wherein the molten metal 
is deposited on the exterior surface of a rotating chill roll, intimacy of 
the contact between the filament and the chill roll surface in our method 
does not depend upon surface adhesion but instead is provided by 
centrifugal force, the inner surface of the chill roll employed in the 
process of our invention can be highly polished and, if desired, by 
provided with a highly uniform surface such as by chrome plating to obtain 
metal filament having unusually smooth surface characteristics. Such 
smooth casting surfaces could not be employed on chill rolls whereon 
filament formation is effected on the exterior surface for lack of 
sufficient adhesion between the molten metal and the surface, resulting in 
splattering and failure to obtain desired filament. To provide protection 
against erosion, corrosion or thermal fatigue, the inner surface of the 
chill roll may be coated with a suitable resistant or high melting 
coating, for example a ceramic coating or a coating of corrosion resistant 
high melting metal, which may be applied by known procedures. 
The filament may be stripped from the chill roll as soon as it is 
sufficiently solidified, but in any event at a location represented by an 
angle of rotation of the chill roll from the point of impingement of the 
jet of molten metal of less than 360.degree., generally less than about 
320.degree., desirably less than about 270.degree.. 
Means for stripping the metal filament from the inner surface of the 
rotating chill roll include jets of fluids, such as air, inert gases or 
substantially inert liquids, including low melting metals above their 
melting point, directed against the inner surface of the chill roll, 
preferably at an angle so as to lift the metal filament from the surface 
whereon it is formed. Such means further include mechanical lifting 
devices, for example scraper blades constructed of suitable material such 
as ceramics, metals, plastics or graphite which are held in intimate 
sliding contact with the inner surface of the chill roll; also brushes, 
rotating or stationary, magnetic devices, if the filament has magnetic 
properties or aspirators which lift the filaments from the surface by 
means of suction. 
Means for guiding the stripped metal film away from the chill roll and, 
optionally, towards a collection device include suitably shaped guides, 
rollers, or blasts of fluids such as air, gases or liquids. 
Detailed design of apparatus of the present invention is within the 
capability of any competent worker skilled in the art. 
The process of the present invention may be carried out in air, under 
partial or high vacuum, or in any desired atmosphere such as inert 
atmosphere as may be provided by an inert gas such as nitrogen, argon, 
helium, and the like. This can be simply accomplished by enclosing the 
apparatus in a suitable housing and evacuating it or replacing the air in 
the housing with a desired inert gas. When the process of the present 
invention is conducted under vacuum, formation of bubbles in the molten 
metal with concomitant pinhole formation in the filament is reduced. For 
that reason it is preferred to conduct the process under vacuum, say under 
absolute pressure of less than about 10 cm. Hg., preferably less than 
about 5 cm. Hg., more preferably at pressure of less than about 1000 
micron. 
In short run operation it will not ordinarily be necessary to provide 
cooling for the chill roll if it is of relatively large mass so that it 
can act as a heat sink and absorb considerable amount of heat. However, 
for longer runs, especially under vacuum, cooling of the chill roll is 
desirably provided. This may be conveniently accomplished by circulating 
water or other cooling media therethrough, expecially if the process is 
conducted under vacuum, by blowing cool air or other gases over the chill 
roll, or as by evaporative cooling as by externally contacting the chill 
roll with water or any other liquid medium which through evaporation 
provides cooling. Alternatively, a stream of a liquified gas such as 
nitrogen or carbon dioxide may be directed against the chill roll for 
cooling purposes. To enhance cooling, it may be desirable to provide 
cooling fins around the outer perimeter of the chill roll as illustrated 
in FIG. 5. If the metal filament is stripped from the chill roll by means 
of a jet of a fluid, that fluid may provide concurrent cooling of the 
chill roll. Cooling of the chill roll, if operated under vacuum, may 
desirably be provided by means of a jet of low melting metal which is also 
used for stripping the filament from the chill roll. 
If the process of the present invention is conducted other than in a 
vacuum, then the width of the inner surface of the chill roll, while not 
critical, is desirable relatively narrow with respect to the width of the 
filament to be produced, to permit relatively unimpeded lateral escape of 
the atmospheric gas at or near the point of inpingement of the molten 
metal jet onto the chill roll surface, thereby minimizing buffeting of the 
jet near the chill roll surface. Under such conditions, the width of the 
inner surface of the chill roll is desirably not more than about 2 to 3 
times, preferably not more than about 11/2 times the width of the filament 
being formed. A relatively narrow inner surface of the chill roll also 
facilitates withdrawal of the filament from the chill roll. 
The shape of the jet of molten metal being directed against the inner 
surface of the chill roll is not particularly critical, provided it is 
sufficiently stable to form a continuous filament. The jet may be of 
substantially circular cross section or may be elongated to provide a 
wider filament. Alternatively, several jets may be directed against the 
chill roll in relatively close proximity so that the metal, which upon 
contacting the chill roll will spread, will form a relatively wide 
filament. 
The following examples illustrate the present invention and set forth the 
best mode presently contemplated for its practice. 
EXAMPLE I 
Apparatus employed is similar to that depicted in FIG. 1. The chill roll 
employed has inner and outer diameters of 15 and 19 inches, respectively, 
and it is 1.5 inches wide. It is rotated at a speed of 470 rpm. An air 
knife frovides a jet of air at an angle of about 90.degree. against the 
surface of the chill roll at a distance of about 270.degree. from the 
point of impingement of the metal jet onto the chill roll. A crucible 
equipped with a nozzle having 0.18 inch diameter is charged with a molten 
alloy composed of Fe.sub.25 Ni.sub.25 Cr.sub.10 B.sub.20 Co.sub.20. The 
alloy has a melting point of about 900.degree. C. The alloy in the 
crucible is pressurized to pressure of 5 p.s.i.g. by means of an argon 
blanket and is ejected through the nozzle at temperature of about 
940.degree. C. at an angle of 90.degree. with the inner surface of the 
chill roll. The ejection velocity of the metal and linear rotational 
velocity of the chill roll surface are approximately 224 and 875 
cemtimeters per second, respectively. The stream is quenched and 
transported to the air knife, and is there stripped from the inner surface 
of the chill roll and blown out of the chill roll towards a collection 
device. Upon examination using X-ray diffraction, the filament is found to 
be amorphous in structure. The filament is about 0.0025 inch thick and has 
a width of 0.026 inch. 
EXAMPLE II 
Apparatus employed is similar to that depicted in FIG. 1 except that a 
motor-driven rotating brush is substituted for the air knife and that the 
apparatus is housed in a vacuum chamber. The chill roll employed has inner 
and outer diameters of 15 and 19 inches respectively and it is 1.5 inches 
wide. It is rotated at a speed of 1300 rpm. The rotating brush is set at 
an angle of 90.degree. measured against the side of the chill roll to 
effect sideways ejection of the metal filament away from the chill roll. A 
crucible equipped with a nozzle having 0.028 inch diameter is charged with 
a molten alloy composed of Fe.sub.5 Ni.sub.45 Cr.sub.10 B.sub.16 Co.sub.20 
Mo.sub.4. The alloy has a melting point of about 1180.degree. C. The alloy 
in the crucible is pressurized to pressure of about 7 p.s.i.g. by means of 
argon blanket and is ejected through the nozzle at temperature of about 
1280.degree. C. at an angle of 90.degree. with the inner surface of the 
chill roll. The ejection velocity of the metal and linear rotational 
velocity of the chill roll surface are approximately 206 and 2419 
centimeters per second, respectively. The filament removed from the inner 
surface of the chill roll by the rotating brush is of uniform quality and 
free of pinholes. Upon examination using X-ray diffraction, the filament 
is found to be amorphous in structure. The filament is about 0.0015 inch 
thick and has a width of about 0.035 inch. 
Since various changes and modifications may be made in the invention 
without departing from the spirit and essential characteristics thereof, 
it is intended that all matter contained in the above description shall be 
interpreted illustrative only, the invention being limited only by the 
scope of the appended claims.