FIELD OF THE INVENTION
[0001] This invention relates to amorphous metal powders and in particular powders made
with compositions of known glass forming alloys.
Description of the Prior Art
[0002] Economic methods of fabricating various metallic glasses in the form of filaments,
wire, ribbon or strip in large quantities necessary for practical applications are
well established as the existing state of the art. Metallic glasses exhibit extraordinary
magnetic, mechanical and chemical properties and are thus of great interest as engineering
materials. In the form of wire, ribbon or strip, metallic glasses might have potential
applications as tire cord, reinforcing elements in composite materials, soft magnetic
cores in motors and transformers, cutlery, tape recording head and many other engineering
applications.
[0003] Numerous conventional metals and alloys such as iron and steels of various grades,
nickel, copper and aluminum are commercially produced as powders. In the majority
of the cases these powders are subsequently consolidated by powder metallurgical methods
into various commercial products having useful properties. Over the past two decades,
innovative metallurgical techniques led to fabrication of powder metallurgical parts
with properties superior to wrought and cast products of many alloys, thereby causing
vastly increased technological demand for metal powders.
[0004] Methods for obtaining metal in powder form are known. For example, relatively fine
metal powder can be made by several processes involving atomization of molten metal.
A method of making steel powder having, after compaction, a high density and superior
physical properties has, for example, been disclosed by Robert A. Huseby in United
States Patent No. 3,325,277, issued June 13, 1967. The Huseby method involves impinging
a jet of molten steel against a flat, sheet-like stream of water flowing at high velocity
to atomize the molten steel to obtain agglomerates of solid particles of high density.
[0005] U.S.P. 3,598,567 to Grant discloses atomization from a liquid metal bath, the atomized
particles or droplets being rapidly solidified, and then advantageously rapidly quenched
to low temperatures to avoid coarse particle precipitation and/or growth. As the liquid
particles are produced, they are delivered to a quenching medium, such as refrigerated
air, nitrogen, or argon and more advantageously, wet steam, water brine or even a
cold metal substrate of high heat conductivity metal, such as copper, silver, steel
and the like. The rate of cooling to achieve a fine dendrite spacing of the phases
should be at least about 100°C/sec. and where cooling on a metal substrate is employed,
may range up to about 10
6 or l08•
C/sec. With regard to the latter, the high rate of cooling is achieved by projecting the
finely divided liquid droplets of metal at high velocity against the metal substrate.
The metal powder produced in this manner has a finely refined structure, is substantially
free from segregation and is capable of being hot worked into a hard metal shape by
hot consolidating the powder mass, for instance, by hot extrusion.
[0006] U.S.P. 3,646,177 to Thompson discloses a method for producing powdered metals and
alloys that are free from oxidation by a process which involves atomizing molten metal
with a fluid jet to form discrete particles of the molten metal and directing the
stream into a reservoir of an inert cryogenic liquid to solidify the particles under
protection from oxidation during cooling.
[0007] U.S.P. 3,764,295 to Lindskog discloses a method for making steel powder wherein a
jet of atomizing fluid is directed against a stream of molten steel to atomize the
molten steel into particles consisting of a metallic core and an oxide skin, and thereafter
the particles are allowed to solidify.
[0008] U.S.P. 3,813,196 to Backstrom discloses a device for atomizing molten metals wherein
a first jet of an atomizing fluid is directed against a jet of molten metal to form
a combined stream of the molten metal and the first jet of atomizing fluid. Then a
second jet of atomizing fluid impinges the combined stream at a certain angular relationship
to the molten metal stream and as a result of the specific arrangement of the jet
embodied in particular nozzles and their orientation a fine, very uniform powder is
obtained which consists of smooth, substantially spherical particles.
[0009] Amorphous metal alloys and articles made therefrom are disclosed by Chen and Polk
in United States Patent 3,856,513. This patent discloses metal alloy compositions
which are obtained in the amorphous state and are superior to such previously known
alloys based on the same metals. These compositions are easily quenched to the amorphous
state and possess desirable physical properties. This patent discloses that powders
of such amorphous metals with particle size ranging from about 0.0004 to 0.010 inch
(0.001016 to 0.254 cm) can be made by atomizing the molten alloy to droplets of this
size, and then quenching these droplets in a liquid such as water; refrigerated brine
or liquid nitrogen.
[0010] Metallic glasses in the form of powders have useful applications. Powders of metallic
glasses have most of the unique properties of the same alloys in the glassy bulk form,
e.g. ribbon, filament or wire. Soft magnetic metallic glasses in the form of powders
can be cold pressed into magnetic cores. Also, metallic glass powders can be powder
metallurgically hot consolidated or thermoinechanically pressed into discrete structural
processed shapes and parts having useful mechanical properties. Complex composite
materials composed of both metallic glass phases and crystalline metallic or nonmetallic
phases can be designed and fabricated by powder metallurgical techniques to provide
the exceptional properties required in the highly sophisticated and demanding aerospace,
electronic and nuclear industries.
[0011] U.S.P. 2,825,108 to Pond discloses a method for making metallic filaments directly
from the melt by directing a jet of molten metal against the inner surface of a rapidly
rotating cup shaped chill body. Progressive reduction of the ejection velocity of
metal melt results in shorter and shorter filaments until the length to width ratio
of the filament approaches unity and the filament becomes a particle of flake powder.
[0012] A method for making metal flakes suitable for making metal powder for powder metallurgical
purposes is disclosed by Lundgren in German Offenlegungsschrift 2,555,131 published
August 12, 1976. The process involves impinging a jet of molten metal against a rotating
flat disc. Relatively thin, brittle and easily shattered essentially dentrite free
metal flakes are obtained with between amorphous and microcrystalline structure, from
which a metal powder can be obtained by shattering and grinding, for instance in a
ball mill.
[0013] There remains a need for methods for making amorphous (glassy) metal powder having
good properties for use in metallurgical processes.
SUMARY OF THE INVENTION
[0014] The present invention provides a method for making metallic glass powder. A jet of
molten glass- forming metal alloy is formed and then momentum is transferred from
a moving body onto said jet to atomize said jet into a stream of discrete molten metal
droplets. The flow of the atomized molten metal droplets is directed against a travelling
chill surface to impinge the atomized molten metal droplets against the travelling
chill surface to effect rapid solidification thereon.
[0015] The quench rate of the atomized metal on the chill surface is within the range between
about 10
4oC per second and 10
6°C per second, or higher. The particle size of the metallic glass powder as produced
is typically less than about 100 micrometers.
[0016] In accordance with one specific embodiment of the invention process a jet of molten
glass forming metal alloy is formed and impinged on a stream of an inert fluid to
atom the molten metal and to direct the flow of the atomized molten metal against
a travelling chill surface to thereby impinge the atomized molten metal against the
chill surface and to effect rapid solidification of the molten metal thereon. The
stream of inert fluid may be provided in the form of a flat sheet by forcing fluid
through a correspondingly shaped orifice under a pressure which may be greater than
about 1000 pounds per square inch (6.89 x 10
3kPa). The stream of inert fluid and the jet of metal intersect at an angle greater
than 10°.
[0017] In another embodiment a jet of molten glass forming metal alloy is deflected and
atomized by a moving solid body.
[0018] In still another embodiment, there is provided a method for making metallic glass
powder comprising the steps of forming a jet against the inner surface of a rotating
cylindrical chill body in the direction of movement of the chill body at an angle
within the range of from about 5° to 45° and preferably from about 20° to 30°. This
effects atomization of the molten alloy into a stream of droplets of molten alloy.
The droplets then impinge on the inner surface of the chill body to be rapidly quenched
to form solid particles of metallic glass powder. The powder is removed from the inner
surface of the chill body, e.g. by a mechanical scraper or a fluid stream directed
against it.
[0019] The chill surface velocity is suitably within the range from about 15 m/sec to 40
m/sec, and the jet velocity is suitably within the range of from about 5 m/sec to
15 m/sec. The jet diameter is preferably from about 0.25 to 2.5 mm.
[0020] Apparatus of the present invention for making metallic glass powder comprises a holding
vessel for holding molten glass forming metal, nozzle means which is in communication
with said holding vessel for generating a jet of the molten metal, a means for expelling
the molten metal through said nozzle means to generate a jet of the molten metal,
means for atomizing such jet of the molten metal expelled through the nozzle to atomize
said jet into a stream of discrete droplets of molten metal, and a travelling chill
surface located in the path of said stream of discrete particles of molten metal for
impingement of said droplets thereon for solidification into a glassy metal powder.
The apparatus may further comprise means for removing the glassy metal powder droplets
from the chill surface and gating means for minimizing contact between atomized molten
metal droplets and already solidified droplets on the chill surface. Providing a travelling
chill surface to the atomized particles results in a continuous fresh surface exposed
to the atomized particles for rapid quenching.
[0021] In another embodiment of the invention apparatus is provided for making metallic
glass powder comprising a holding means for holding molten metal, a nozzle in communication
with said holding means for generating a jet of molten metal, means for expelling
molten metal through said nozzle to generate a jet of molten metal, and a rotatable
cylindrical chill body providing an inner chill surface, wherein the nozzle and the
chill body are so positioned with respect to each other that a molten metal jet expelled
from the nozzle impinges against the inner surface of the chill body in the direction
of movement of the chill surface at an acute angle of from about 5° to about 45°.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022]
FIG. 1 is a sectional elevation view of an apparatus for producing glassy metallic
powders;
FIG. 2. is a sectional elevation view of a second embodiment of an apparatus for producing
glassy metallic powders;
FIG. 3 is a sectional elevation view of an apparatus for producing glassy metallic
powders by an atomization and chill process involving a rotating ceramic spinner;
FIG. 4 is a sectional elevation view of a rotating spinner for atomization of a liquid
metal jet;
FIG. 5 is a sectional elevation view of an off axis rotating circular disc for atomization
of a liquid metal jet;
FIG. 6 is a sectional elevation view of an apparatus for atomization of a liquid metal
jet employing a cam;
FIG. 7 is a sectional elevation view of an apparatus for atomization of a liquid metal
jet employing a circular saw type spinner;
FIG. 8 is a sectional elevation view of an apparatus for atomization of a liquid metal
jet employing a circular rip saw type spinner;
FIG. 9 is a photomicrograph of a sectional plane view illustrating the shape of chill
substrate cast glassy metallic powder of composition Fe40Ni40p14-B6 (atom percent) alloy; and
FIG. 10 is a sectional elevation view of an apparatus for making amorphous metal powders
in accordance with an alternate embodiment with the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0023] The present invention relates to production of metallic glass powder involving atomization
of a jet of molten glass forming metal alloy, under certain specific conditions, followed
by rapid cooling of the atomized molten metal by quenching on a moving chill surface.
The glass forming metal alloy is melted in a suitably heated crucible. Many types
of techniques for melting alloys are well known in the art. The glass forming alloys
can be melted in a vacuum or inert atmosphere in accordance with usual metal melting
practices. Induction melting or electric arc melting furnaces may be used. Desirably,
the crucibles or the linings of the inside walls of the furnace containing the melts
should be made of inert materials such as fused quartz, high purity alumina, zirconia,
magnesia, beryllia and yttria and the like. The temperature of the melt is measured
and controlled according to known standard practices. The melt is heated to a temperature
sufficiently above the freezing point of the alloy in order to allow atomization of
the molten alloy without immediate freezing during the atomization process. In general,
temperatures of about 100°C to about 400°C above the liquidus temperature are suitable.
The preferred temperature ranges from about 150°C and to 250°C above the liquidus
temperature. Furthermore, it is advantageous for the atomization process when the
viscosity of the liquid alloy is low and, in general, the viscosity decreases with
increasing temperature.
[0024] The molten alloy is expelled through a suitable nozzle to form a jet of molten metal.
[0025] The metal can be expelled through the nozzle orifice by application of pressure,
such as hydrostatic, hydraulic or gas pressure. Gas pressure, possibly in combination
with hydrostatic pressure, is preferred. The pressure acting on the liquid metal near
the nozzle is not critical so long as it results in formation of a coherent jet having
velocities within the critical range below described. Exemplary suitable pressures
range from about 15 psi to 30 psi (1.03 x 10
2 to 2.07 x 10
2 kPa). Preferably, the pressure is from about 20 psi to 25 psi (1.38 x 10
2 to 1.72 x
102 k
Pa).
[0026] Nozzles suitable for jetting the molten alloy include, for example, those described
in U.S. Patent 2,968,062 and U.S. Patent 3,253,783, both to Probst et al. The shape
of the nozzle orifice is not critical. For convenience of fabrication, round orifices
are preferred. Such orifices may, suitably, have diameters from about 0.5 mm to 5
mm. Preferably, the diameter of the nozzle orifice is from about 0.1 mm to 1 mm. The
nozzle may be made of the same type of material as the crucible, as above discussed.
The material should be sufficiently hard to minimize erosion of the orifice during
passage of molten metal through it. The nozzle is coupled to the crucible by suitable
means. For example, it may be fitted into a machined groove in the bottom part of
the crucible and bonded by a ceramic cement. The length of the nozzle orifice is not
critical, preferably it is from about 2 mm to 30 mm.
[0027] Desirably, the jet of molten alloy expelled through the nozzle has a small diameter.
For example, the diameter of the jet may be from about 0.5 mm to 5 mm. The velocity
of the jet may be from about 2 m/sec to 10 m/sec and preferably is from about 4 m/sec
to 7 m/sec.
[0028] The velocity of the molten alloy jet is critical. When the jet speed is too low,
the jet tends to become discontinuous. When the jet has too high a speed, it becomes
very difficult to break up the jet because of the large momentum it is carrying. The
jet can be atomized by transfer of momentum from a solid or fluid material. The transfer
of momentum effects a disturbance and acceleration of the jet, causing its breaking
up into small droplets.
[0029] The jet of molten metal alloy expelled through the nozzle orifice may be atomized
by subjecting it to the action of a high pressure fluid jet. An inert fluid for the
purposes of the present invention is a fluid which does not, or for practical purposes
does not, react with the molten alloy. Fluids suitable for the purpose include inert
gases such as argon, nitrogen, hydrogen and helium; liquids such as water; liquid
metals such as thallium, tin, lead; liquid salts such as alkali halides, and the like.
Preferred atomizing fluids are argon and nitrogen. The process is desirably carried
out under a protective atmosphere, such as argon and nitrogen inside an atomization
chamber made of suitable material, such as mild or stainless steel, in order to prevent
oxidation. For this purpose and for convenience the furnace-crucible assembly is then
located inside the atomization chamber. The chamber is provided with ports for the
introduction of fluids. The whole chamber may, for example, be evacuated to 2 x 10-5
torr (2.67 x 10 N/m ) during the melting procedure of the alloy. The nozzle orifice
is kept closed by a suitable refractory stopper rod. Prior to jetting, the atomization
chamber is back filled with the gas providing the protective atmosphere, and it may
then be continuously purged at a pressure above one atmosphere, e.g., by 5 psi (34.5
kPa). The crucible is next pressurized as by filling it with argon to from about 20
to 30 psi (1.38 x 10
2 to 2.07 x 10
2 kPa), and the stopper rod is removed from the nozzle to expel the molten metal in
jet form.
[0030] Several configurations of fluid jets can be employed to effect atomization of molten
metal jets into fine droplets. The fluid jet need not be divergent or in sheet form.
For example, the jet of molten metal may be intersected by a single diverging flat
fluid stream. In that configuration the normal to the plane of the flat fluid stream
and the velocity vector of the molten metal jet desirably lies in a vertical plane
which is perpendicular to the plane of the travelling chill surface, upon which the
atomized stream of molten metal is to be impinged. The molten metal jet desirably
intersects the flat fluid stream at an angle between 5° and. 90°; and preferably between
20° and 50°.
[0031] In another suitable configuration two diverging flat streams of an inert fluid intersect
to form a V-jet with apex angle between 10° to 90° and preferably between 20° and
50°. The vertical molten metal jet is intersected by the V-jet at the apex. Four or
more flat stream jets emerging from nozzles which are equally spaced apart on a circle
can intersect to create a down cone with a well defined apex. If the number of jets
is increased, eventually an annular jet is obtained.
[0032] In all the above configurations, intersection of the molten metal jet by the fluid
jets result in the formation of a spray of molten droplets in the form of a cone.
[0033] The fluid stream can be of any suitable shape, such as a diverging flat sheet, a
V-jet or a cone. Preferred shapes of the fluid stream are V-jet and cone.
[0034] The atomized molten metal droplets are propelled toward the travelling chill surface
and the axis of the conical spray of molten metal droplets is desirably maintained
perpendicular to the surface plane of the chill surface substrate. A perpendicular
spray provides for a more concisely defined impact area on the chill surface.
[0035] The temperature of the atomizing fluid is not critical and may conveniently be from
about 25°C to 200°C. The angle between the velocity vectors of molten metal jet and
fluid can lie between about 5° and 90°. Preferably; the angle between the vectors
is from about 30° to 50°. This can be achieved by inert gas under high pressure, for
instance 100 to 1500 psi (6.89 x 10
2 to 1.03 x 10
4 k
Pa) which impinges upon the molten alloy stream and disperses the same into droplets
with their size ranging from very fine to relatively coarse. Preferred gas pressures
are in the range of about 100-400 psi (6.89 x 10
2 to 2.76 x 10
3 k
Pa), more preferably about 150-300 psi (1.03 x 10
3 to 2.07 x 10" kPa).
[0036] In addition to fluids, it is also possible to disintegrate the jet into droplets
by mechanical impact provided by a rapidly moving or rotating surface with sufficient
momentum, desirably one made of materials which are not wetted or abraded by the molten
alloys. Solid bodies providing such surface for impingement and atomization can be
shaped as flat, circular or elliptical discs, with or without teeth or varries. Exemplary
solid bodies are found in the drawing. It is necessary that the solid body provides
a surface which can impact and transfer momentum to the molten metal jet. The angle
between molten jet and surface normal of the solid body can be from about 5° to 60°.
The angle between molten jet and peripheral velocity can be from about 95° to 150°.
The solid body can be, for example, made of materials such as fused quartz, alumina,
zirconia, silicon carbide and boron nitride. Preferred materials of the solid body
are alumina and fused quartz. The materials chosen should be reasonably inert and
should not react with the molten alloy.
[0037] The atomized droplets are directed toward and into contact with the chill surface
provided by a moving chill body whereby they are quenched on impact and deposited
as flakes or powder. The moving chill body preferably is made of a metal of high thermal
conductivity such as copper, silver and the like. Due to rapid quenching upon impact,
the droplets solidify as glassy powders or flake. A preferred embodiment of the moving
chill body comprises a rotatably mounted copper wheel which rotates around its axis
and continuously provides a new chill surface to the impinging molten metal droplets.
Preferably a scraper is used to remove the solidified particles adhering to the moving
chill surface. To that end, the scraper may be located at that side of the rotating
wheel opposite from that at which the droplets impinge upon the rotating wheel.
[0038] Depending upon the physical spread of the stream of atomized droplets, it may occur
that liquid droplets impinge upon droplets which are already frozen and adhering to
the moving chill surface. This can be substantially minimized by providing an optional
gate which limits the geometrical area of the chill surface accessible to the stream
of droplets. The purpose of such a gate is to promote deposition of molten droplets
on the chill surface separately and isolated from each other. Another purpose of the
gate is avoiding the spreading of droplets and particles all over the apparatus and
to aid in collecting misdirected particles. This is desirable because in case a second
droplet is deposited onto an already deposited droplet, then the second droplet may
not be quenched to the glassy state. Therefore it is desirable that the velocity movement
of the chill surface and the velocity of the molten droplets and the width of the
gate be coordinated such that the deposited solidified droplets pass out of the range
allowed by the gate before an appreciable number of oncoming molten particles reaches
the substrate. Overlap of impinging particles is reduced with narrowing gate width
and, conversely, overlap increases with increasing gate width.
[0039] In any event, the length of flight of the molten droplets should be so adjusted that
they impinge on the chill surface in the molten state. This may be accomplished by
suitable choice and coordination of jet velocity, jet impingement angle and velocity
of movement of the chill surface, and optionally, gate width if a gate is employed.
The gate width depends on the angle of the impinging jet stream, on the position of
the gate with respect to the point of impingement and on the jet velocity and the
chill surface velocity, as well as on the surface tension and wetting properties of
the liquid alloy with respect to the chill surface. Furthermore, the density of droplets
in the stream, and their angular distribution, should be considered in selecting an
appropriate gate. The size of the gate depends further on the desired level of exclusion
of contamination of the glassy alloy powder by crystalline by-products. The powder
is removed from the surface of the chill body by suitable means, such as rotating
brushes or scraping means, or by blowing it off by means of a blast of air or of inert
fluid, such as nitrogen. Desirably, removal is continuously effected in the area downstream
of the area of impingement of the atomized droplets, but at a point ahead of the point
of impingement of the atomized molten droplets. Preferably, a scraper is used to remove
the solidified product adhering to the moving chill surface.
[0040] The chill cast glassy alloy powders made according to the process of the present
invention have comparatively rough and sharp edges. These particles tend to interlock
during compaction. They can be compacted in a solid body having higher green strength
but less density than a compacted body prepared from powder produced according to
the process disclosed in my co-pending commonly assigned U.S. application Ser. No.
023,411, filed March 23, 1979.
[0041] Glassy metal powders made in accordance with the invention process can be used for
powder metallurgical applications. They are also suitable for fabrication of magnetic
cores. The typical characteristics of such magnetic cores for use as stable induction
components at audio and low radio frequency ranges are permeabilities between 14 and
300 units, low core loss and stability of magnetic properties against large changes
in frequency and temperature.
[0042] The metallic glass powders of suitable size ranges can be uniformly mixed in a suitable
proportion with powders of crystalline metals and alloys such as aluminum and aluminum
base alloys, copper and copper based alloys and stainless steel. These powder mixtures
can be subsequently powder metallurgically processed, i.e. is pressed and sintered
into dense parts.
[0043] A metallic glass is an alloy product of fusion which has been cooled to rigid condition
without crystallization. Metallic glasses are characterized by having a diffuse X-ray
pattern. Such metallic glasses in general have at least some of the following properties:
high hardness and resistance to scratching, great smoothness of a glassy surface,
dimensional and shape stability, mechanical stiffness, strength and ductility and
a relatively high electrical resistance compared with related metals and alloys. Powder
comprises fine powder with particle size under 100 m, coarse powder with particle
size between 100 m and 1000 m and flake with particle size between 1000 m and 5000
m.
[0044] The terms metallic glass, or glassy metal and amorphous metal are used herein interchangeably.
[0045] Alloys suitable for use in the process of the present invention are those which upon
rapid quenching from the melt at rates in the order of at least about 10
4 to 10
6°C/sec form amorphous glassy solids. Such alloys are, for example, disclosed in U.S.P.
3,856,513, U.S.P. 3,981,722; U.S.P. 3,986,867, U.S.P. 3,989,517 as well as many others.
[0046] For example, Chen and Polk in U.S.P. 3,856,513 disclose alloys of the composition
M
aY
bZ
c, where M is one of the metals, iron, nickel, cobalt, chromium and vanadium, Y is
one of the metalloids, phosphorus, boron and carbon and Z equals aluminum, silicon,
tin, germanium, indium, antimony or beryllium with "a" equaling 60 to 90 atom percent,
"b" equaling 10 to 30 atom percent and "c" equaling 0.1 to 15 atom percent with the
proviso that the sum of a, b and c equals 100 atom percent. Preferred alloys in this
range comprises those where "a" lies in the range of 75 to 80 atom percent, "b" in
the range of 9 to 22 atom percent, "c" in the range of 1 to 3 atom percent again with
the proviso that the sum of a, b and c equals 100 atom percent. Furthermore, they
disclose alloys with the formula T.X. wherein T is a transition metal and X is one
of the elements of the groups consisting of phosphorus, boron, carbon, aluminum silicon,
tin, germanium, indium, beryllium and antimony and wherein i ranges between 70 and
87 atom percent and j ranges between 13 and 30 atom percent. However, it is pointed
out that not every alloy in this range would form a glassy metal alloy.
[0047] Referring now to FIG. 1 there is shown a chamber 18 containing a crucible 20 containing
melt 22. The melt 22 comprises a metallic alloy which is heated by an induction heating
system of which the induction heating coils 24 are shown. The upper end 26 of the
crucible is tightly mounted to a pair of flanges 28 and 30 of a chamber 18. The crucible
20 is connected at its upper end 34 to a pressure chamber 36 which may be pressurized
with suitable gas coming through line 38 from a valve 40 and from another line 42
which is connected to a gas supply, e.g. a supply of argon gas. In parallel to the
gas supply line, there is a second valve 44 which is connected to a line 46 which
is in turn connected to a vacuum pump providing the possibility of evacuating the
area above the melt initially before the furnace is set in operation and before the
argon or inert gas pressure is applied.
[0048] The power for the induction heater coils 24 is supplied by supply lines 46 and 48
which are led out of the chamber 18 through seal 50. Power supplies suitable for supplying
induction power of 10 kilowatt or more are commercially available.
[0049] The liquid metal alloy is ejected from crucible 20 through nozzle 52 as a narrow
jet 54. A nozzle assembly 56 for generating a high pressure gas stream is provided,
which stream impinges upon the molten metal jet at 58 and changes the direction of
the molten metal jet to atomize it. The atomized particles are directed towards an
optional shutter 60 which has an orifice 62 suitable for selectively passing a narrow
stream of the atomized molten metal particles 64 to restrict the momentary area of
impingement-on the traveling wheel 66. The shutter assembly is held by rods 67 and
68 to flanges 28 and 30 closing the chamber 18. After passing the shutter orifice
62, the particles impinge upon a rotating disc 66. The surface speed of disc 66 in
the vicinity of impingement of the atomized molten metal is approximately 20 m/sec.
As the liquid droplets come in contact with the chill surface provided by disc 66,
they are rapidly chilled to the glassy solid state. Means for cooling disc 66 [not
shown] may optionally be provided. Disc 66 is made of high purity copper for good
thermal conductivity. The glassy solid particles are then scraped off disc 66 by means
of spring loaded scraper 71. The scraped off glassy metal powder is collected in a
collection area 70. A flange 72 provides access to the collected glassy metal powder.
Rotating disc 66 is mounted on a shaft 69 which enters chamber 18 through seal 76.
Rotating power is provided by a variable speed electrical motor 78. The motor 78 is
coupled to shaft 69 by means of flexible coupling 80. A mounting stand 82 is provided
for the electrical motor to adapt its position and to give additional stability. The
operation can be observed through a viewing port 84 which encompasses a vacuum sealed
window 86. For measuring the temperature of the melt in the crucible and, if desired,
at the point of atomization an infrared pyrometer 88 is provided which is mounted
in such position as to collect the radiation emitted from the crucible and the melt
through the viewing port 84.
[0050] Chamber 18 is further provided with a pumping port 90 connected to a vacuum system
(not shown). The whole assembly is preferably mounted on the stand 92 which insures
rigidity of the system.
[0051] FIG. 2 shows a different embodiment of the apparatus of the present invention. The
apparatus is enclosed within chamber 146. A crucible 102 is located inside susceptor
104, which can be made of any material which has stability at high temperatures with
good electrical conductivity such as tungsten, molybdenum and graphite. An induction
heating system indicated through coils 106 provides the power for melting the alloy.
Between the heating coils and the susceptor is provided an insulating layer 107 for
preventing heat losses from the susceptor.
[0052] The insulating layer 107 can be made from a number of suitable materials, for example
of fibers of high melting refractories such as zirconia. The material of the crucible
102 is a material which does not react with the melt and has sufficient stability
at the melt temperature. A stopper rod 110 reaches into the melt 122 and has at its
end a stopper 112 for closing the opening 114 in the bottom of crucible 102. Stopper
rod 110 can be made from boron nitride or other suitable material which would not
react with the melt at the high temperatures. Stopper rod 110 is introduced into crucible
102 through an opening 116 in cover 118. Upper end 120 of stopper rod 110 can be raised
and lowered to open and close opening 114 at the bottom of crucible 102. A connecting
line 121 provides for a possibly pressurized atmosphere over the melt 122 by making
a connection to a gas reservoir (not shown). The melt 122 fills part of the crucible.
At the bottom of crucible 102 there is provided nozzle 124 for ejecting a jet 126
of liquid metal. Two atomizers 128 and 130 are provided which eject pressurized gas
impinging upon the jet 126 of liquid metal at 132 to disperse the jet into a stream
134 of droplets of molten metal. The droplets are restricted with respect to their
path by a shutter 136 having an opening. Those droplets which pass through the shutter
opening then impinge upon a rotating disc 138 made from copper. Upon impact on rotating
disc 138 the liquid droplets are rapidly quenched to form particles of solid metallic
glass. The solidified particles move with the rotating copper disc 138 and are scraped
off by a spring loaded scraper 139. They fall into a collection area 142 from where
they can be removea perioaically. The copper disc 138 is mounted on a shaft 144 which
enters the chamber 146 through a seal 148 and which is driven by a motor 150. The
chamber 146 may be evacuated through a pipe 152 connected to a valve 154 and pipe
156 leading to a vacuum pump (not shown).
[0053] FIG. 3 shows an atomization and quenching system which is in many ways similar to
the system shown in FIG. 1. However, in contradistinction to FIG. 1, this system employs
a rotating spinner 160 for atomizing the molten metal jet 162 projected out of the
nozzle 164. Upon impingement onto the rotating spinner, the molten metal jet is atomized
and propelled toward shutter 168. The rotating spinner 162 comprises an oblong disc
which is preferably made from a refractory material which is not wetted by the glass
forming melts. Use of a rotating spinner permits atomization of the jet of molten
metal under vacuum.
[0054] FIG. 4 shows an enlarged view of the spinner employed in the embodiment of FIG. 3.
The spinner comprises a bar with rounded ends. The speed of the points of impingement
on the spinner are at least about four times the velocity of the jet. When the jet
of molten metal 182 impinges on the surface 184 of the spinner 186, the molten jet
is atomized and the atomized particles are propelled in the direction of rotation
of the spinner toward the chill surface [not shown].
[0055] Different spinners of suitable configurations can be used in atomizing and redirecting
the molten spray.
[0056] FIG. 5 illustrates another means for atomizing a jet of molten metal. It shows an
eccentrically mounted circular disc which rotates at a rapid speed. Optionally, the
disc may be dynamically balanced. The extremely rapid rotation of the circular disc
around an off-centered axis induces indulatory motion of the solid surface in contact
with the jet resulting in instability of the molten jet. The molten jet is thereby
atomized into small droplets of molten metal. The axis of rotation 192 of disc 194
is located about 1/2 to 1/8 of an inch (1.27 to 0.3175 cm) distant from its center
196. The surface speed of the rotating disc in the area of impingement is equal to
or greater than 60 meters per second-,
[0057] FIG. 6 shows a lobed cam shaped disc for atomization of a jet of molten metal. It
rotates around axis 202 which may be, but need not be offset from the center of the
disc. The disc is rotated to provide a circumferential surface velocity from about
1000 to about 10,000 m/sec. preferably and the jet of molten metal to be impinged
upon it preferably has a velocity in the range of 5 to 10 meters per second. Desirably,
the jet of molten metal is directed against the disc such that the stream of molten
droplets created upon impact with the spinner lies mostly outside of the enveloping
circle of the crest of the lobes.
[0058] FIG. 7 shows an additional type of spinner having saw teeth 210. The purpose of the
saw teeth 210 is to create instabilities in the molten alloy jet for producing a finely
atomized stream of molten droplets. The spinner which is desirably made of ceramic
material rotates around an axis 212, which may or may not coincide with its center.
The surface speed of such spinner can lie in the range of about 3 to 500 m/sec and
preferably in the range of about 30 to 50 m/sec.
[0059] FIG. 8 shows a different type yet of rotating spinner 222 with larger saw teeth 220.
The saw teeth of this embodiment are desirably made from a ceramic material which
is not wetted by the melt. The surface speed of the rotating spinner lies in the range
of about 3 to 500 m/sec and preferably is in the range of about 30 to 50 meters per
second.
[0060] FIG. 9 shows a photomicrograph of representative metallic glass powder which can
be obtained according to the present invention. The particles have shapes of elliptical
thin platelets, with rough edges. Such particles interlock together very well during
cold compaction, leading to preforms of superior green strength at a given pressure.
The particles have a diameter in the order of about 20 micrometers.
[0061] According to a preferred embodiment of the invention, both atomizaion of a jet of
molten glass- forming metal alloy into a stream of discrete droplets, and rapid quenching
of the droplets occur on the same chill surface provided by the inner surface of a
rapidly rotating cylindrical chill body. The glass forming metal alloy is melted in
a crucible inserted in a melting furnace. Many types of crucibles for melting alloys
are well known in the art. Particularly preferred are techniques for melting which
involve electrical arc furnaces because they are convenient and easily adaptable to
many situations found in practice. The melt is heated to a temperature sufficiently
above the freezing point of the alloy in order to allow atomization of the alloy without
immediate freezing during the atomization process. The temperature of the melt should
be within the range of from about 50° to 450°C above the liquidus line corresponding
to the melt composition, and is preferably from about 100°C to about 250
0C above the liquidus temperature. Furthermore, it is advantageous for the atomization
process when the viscosity of the liquid alloy is low and, in general, viscosity decreases
with increasing temperature.
[0062] The molten alloy is then squirted in a jet against the inner surface of the rotating
cylindrical chill body through a suitable nozzle. Desirably, the jet of molten alloy
is of small diameter. Preferably, the diameter of the jet lies in the range from about
0.25 mm to 2 mm, more preferably from about 0.25 mm to 2.5 mm. For example, a jet
diameter of about 1 mm to about 1.5 mm may be conveniently employed. The velocity
of the jet of molten metal lies suitably in the range from about 5 m/sec to about
15 m/sec and preferably from about 8 m/sec to about 12 m/sec.
[0063] The distance between the nozzle and the chill surface is desirably in the range of
between about 5 mm and.500 mm, preferably between about 100 and 150 mm. The velocity
of movement of the inner surface of the chill body is suitably within the range of
from about 15 to 40 m/sec, preferably from about 20 to 30 m/sec.
[0064] The jet of molten metal is impinged onto the chill surface provided by the inner
surface of the rotating cylindrical chill body at an angle of impingement ranging
from about 5° to 45°, preferably from 20° to 30°. The angle of impingement is defined
as the angle formed between the liquid jet and the line of tangent to the chill surface
at the point of impingement drawn in the direction opposite to the direction of rotation
of the chill surface.
[0065] When a jet of molten metal is impinged on a rapidly moving chill surface, a puddle
of molten metal is formed thereon. The normal component of the force exerted by the
liquid jet onto the puddle tends to enhance puddle stability. From a stable puddle,
a continuous ribbon may be drawn by the moving chill surface. The normal component
of the force of the liquid jet is at a maximum when the angle of impingement is 90°.
The puddle is most stable under this condition. When the angle of impingement decreases
below 90°, the horizontal component of the force exerted in the direction of movement
of the chill surface acts to destabilize the puddle. When the angle of impingement
is equal to or less than about 45°, the destabilizing force exceeds the stabilizing
force and, as a consequence, the puddle tends to disintegrate into molten droplets.
[0066] Rapid rotation of the chill body, the velocity of the jet of molten metal, and the
acute angle of impingement coact to prevent formation of an extended puddle of molten
metal and result in atomization of the metal instead. The droplets of liquid metal
resulting from atomization separate from the surface at a small angle in a stream
and impact, after traveling a short distance, again on the chill surface, whereon
they are chilled to discrete particles of glassy metal alloy.
[0067] The particle size of the metal powder so formed decreases with increasing speed of
rotation of the chill body.
[0068] The chill body is made of a metal of high thermal conductivity such as copper, silver
and the like. The rotating inner surface of the cylindrical chill body continuously
provides a new surface for the impinging metal droplets. The solidified product is
removed from the inner surface of the chill body by suitable means, such as rotating
brushes or scraping means, or by blowing it off by means of a blast of air or of inert
fluid, such as nitrogen. Desirably, removal is continuously effected in the area downstream
of the area of impingement of the atomized droplets, but at a point ahead of the point
of impingement of the metal jet. Preferably, a scraper is used to remove the solidified
product adhering to the moving chill surface.
[0069] The invention is preferably practiced in a vacuum chamber. A vacuum chamber minimizes
the heat losses by diffusion and convection during the flight of the jet and the droplets.
Furthermore, vacuum operation prevents oxidation of the molten alloy.
[0070] Referring to FIG. 10, a fused quartz crucible 10' serves as a reservoir for molten
alloy 12'. A heating means for the alloy is schematically indicated by induction coils
14', which serve to provide the energy to keep the alloy in the molten state. The
crucible 10' is kept in position by supporting means 16'. Crucible 10' is provided
with a cover 8' having a tubular connection 24' for pressurizing the metal by means
of a suitable inert gas. Valves 26' and 28' are provided to control the gas flow to
the tubular connection 24'. At the bottom of crucible 10' is a nozzle 32' for generating
a jet of molten metal 34'. Cylindrical chill body 36' rotates around its axis 37'
in the direction of the arrow with its inner surface 38' closely spaced relative to
the nozzle 32'. The vector of the velocity of the molten metal jet and of the rotating
inner cylinder surface at the impact point of the jet have an acute angle of from
about 5° to 45° between their directions. The angle lies preferably between about
20° and 30°, an angle of 25° being eminently suitable. The jet diameter is preferably
from about 0.25 mm to 2.5 mm.
[0071] The jet has a velocity of from about 5/m sec to 15 m/sec, and the rotation of the
cylindrical chill body provides an inner surface speed of from about 15 m/sec to 40
m/sec, preferably between about 20 m/sec and about 30 m/sec.
[0072] On impingement of the jet at an acute angle, the impinging molten metal breaks up
into a stream of discrete droplets 40'. 3y varying the speed of the inner surface
of the chill body, the velocity of the jet and the angle of impingement of the jet
with that surface, the size of the molten droplets, hence the size of the quenched
product particles, can be varied from fine powder to flakes. Lower chill surface speeds
result in larger particle size and, conversely, higher chill surface speeds result
in smaller particles. When the chill body rotates too fast, the product particles
tend to be small fibers.
[0073] The examples set forth below further illustrate the present invention and set forth
the best mode presently contemplated for its practice.
Example 1
[0074] This example illustrates preparation of metallic glass powder and flakes by chill
substrate casting of atomized molten metal alloy droplets-. The apparatus employed
was generally as indicated in Fig. 1. A glass forming alloy Ni
45Co
20Cr
10Fe
5Mo
4B
16 (atomic percent) was melted in a quartz crucible by means of an induction heater.
The molten alloy was expelled vertically through an orifice in the crucible as a molten
jet of 0.05 inches (0.127 cm) diameter. The molten jet was impinged by a horizontal
jet of atomizing fluid (nitrogen) under high pressure ranging from about 400 to 600
psi (2.76 x 10
3 to 4.14 x 10
3 k
Pa) and thereby atomized into small droplets and propelled towards a rotating copper
disc. The distance between the orifice and point of impact for atomization was 0.5
cm. The distance between the point of impact and the chill surface provided by the
rotating copper disc was 12 cm. The surface velocity of the chill surface was 200
cm/sec. The shutter was located 3 cm away from the point of atomization and had an
opening width of 1 cm and length of 2 cm. The resulting molten droplets in the form
of a spray were allowed to pass through the shutter opening and impacted against the
flat surface of the rotating copper disc. The chill cast particles were scraped off
the surface as they were being deposited. The surface velocity of the copper disc
in the approximate area of droplet deposition varied between about 2000 to 2500 feet
per second (609.6 to 762 meters/sec). The shutter opening, the surface speed of the
copper disc, the velocity of the jet of molten metal and the pressure of the atomizing
fluid were adjusted to minimize overlap of the droplets upon contact with the surface
of the copper disc. Quenched metallic glass particles of irregular shape as shown
in FIG. 9 were thus obtained. Their particle size ranged from about 25 micrometers
to 400u m. A small portion of the particles with particle size under 25µ m was found
to be fully crystalline. Apparently, the smaller molten droplets tended to solidify
before hitting the chill surface and hence were not quenched into the glassy state.
The larger size particles were found to be fully amorphous as determined by X-ray
analysis. By increasing the melt temperature and hence the temperature of the molten
droplets, and by decreasing the flight path, i.e. the distance between the point of
atomization and the point of impingement on the substrate, a significantly higher
yield of metallic glass particles with size smaller than 25p m could be obtained in
a typical run.
Example 2
[0075] A process of chill substrate casting of molten metal droplets is presented in this
example. Apparatus employed was generally as illustrated in FIG. 1. A verticle jet
of molten metal having the composition
Fe40Ni40B20 (atomic percent) was impinged upon and atomized into small droplets by a horizontal
high pressure (about 400 to 600 psi) (2.76 x 10
3 to 4.14 x 10
3 kPa) argon gas jet and the resultant molten droplets in the form of a spray were
impact quenched against the flat surface of a rotating copper disc. The chill cast
particles were scraped off this surface as they were being deposited. The particles
were irregular in shape due to the splatting onto the substrate. The particle size
ranged mostly between 25 µm to 400 µm, and the particles -in this size range were
found to be fully glassy. A small portion of about 5 weight percent of particles with
size smaller than 25 µm was obtained in a typical run and the alloy powders of particle
size less than 25 µm were found to be mostly crystalline. The smaller molten droplets
apparently solidified at a relatively slow rate before hitting the quench substrate
and hence remained crystalline. By decreasing the length of the flight path of the
droplets, a higher yield of metallic glass particles with a size smaller than 25 µm
can be achieved.
Example 3
[0076] An alloy having the composition Co
45Fe
17Ni
13Cr
5MO
3B
17 (atomic percent) was squirted through an orifice of a quartz crucible to produce
a jet of molten metal of 0.08 inches (0.2032 cm) diameter. This jet was impinged onto
a flat and elliptically shaped quartz spinner similar to the oblong spinner shown
in FIG. 4 and rotating at high velocity was situated in the path of the jet of molten
metal. The quartz spinner rotating at high velocity atomized the molten jet into small
droplets. The surface speed of the spinner at the point of atomization was maintained
between 30 and 50 m/sec. These droplets passed through a shutter of adjustable opening
size and were impact quenched against a rotating chill substrate. The particles deposited
on the rotating chill substrate were scraped off by a scraper. The resulting quenched
particles having irregular shape and size in the range between 25 µm to 400 µm were
found to be fully amorphous.
Example 4
Magnetic composite cores from glassy metal powder
[0077] Amorphous metallic flakes with sizes ranging between about 150 and 1000 µm of an
alloy having the composition of Fe
40Ni
40P
14E
6 (atomic percent) were prepared by quenching an atomized stream of molten particles
on a chill surface. The resulting flakes were subsequently embrittled by annealing
below the glass transition temperature for a time of 1 hour and a temperature of 200°C
and then the flakes were subjected to dry ball milling under an atmosphere of high
purity argon atmosphere for 16 hours. There was thus obtained a powder of fine amorphous
particles of irregular shape with an average size of about 25 µm. This powder was
uniformly blended with 2 percent submicrometer size magnesium oxide particles, and
the mixture was pressed into ring shaped cores of an outer diameter of 1 inch (2.54
cm) and an inner diameter of 2 mm hy compaction under high pressures of between 200,000
and 250,000 pounds per square inch (1.38 x 10
6 to 1.72 x 10
6 k
Pa). The magnesium oxide was added to provide uniformly distributed air gaps in the
core to increase its resistivity. The compressed cores were annealed at 300°C for
2 to 16 hours. Typically a core pressed at 250,000 pounds per square inch (1.72 x
10
6 k
Pa) and annealed at 300°C for 16 hours was found to possess a permeability of 125 units.
Example 5
[0078] An amorphous metallic powder with an average particle size below about 75 µm of an
alloy with the composition Mo
40Fe
40B
20 (atomic percent) uniformly composition Mo
40Fe
40B
20 (atomic percent) was uniformly mixed in various proportions with aluminum powder
also having average particle size below about 75 µm. The resulting mixtures were hot
pressed under vacuum into cylindrical compacts, applying 4000 pounds per square inch
(2.76 x 10
4 kPa) pressure at 500°C for 1/2 hour. Since the amorphous metallic powder particles
have crystallization temperatures higher than 800°C, they therefore did not crystallize
during the hot pressing operation. The incorporation of amorphous metal alloy particles
into the aluminum matrix increased substantially the hardness of the resulting powder
metallurgical compact. Typically, an aluminum compact obtained as described above
containing only about 10 weight percent of amorphous metallic particles will have
a hardness of about 150 kilograms per square millimeter, which is much higher than
the hardness of about 20 kilograms per square millimeter typically found in annealed
pure aluminum.
Example 6
[0079] This example illustrates use of fine metallic glass powders for making high permeability
magnetic cores. Metallic glass powders of composition
Fe40Ni40P14B6 (atomic percent) are suitable for fabricating high permeability magnetic cores. The
typical characteristics of such magnetic cores for use as stable induction components
at audio and low radio frequency ranges are permeabilities between 14 and 300 units,
low core loss and stability of magnetic properties against large changes in frequency
and temperature.
[0080] Amorphous metallic powder of an Fe
40Ni
40p
14E
6 alloy with particle size less than 30 µm were blended with submicrometer ceramic
particles and pressed into ring-shaped cores using high pressures between 200,000
and 250,000
psi (1.38 x 10
6 to 1.72 x 10
6 kPa) at room temperature. The ratios by weight of metallic glass to ceramic powder
were in the range of between about 0.01 and 0.02. The ceramic powder was magnesium
oxide. Other suitable ceramic powders include aluminum oxide and yttrium oxide. The
purpose of addition of the fine ceramic particles is to provide uniformly distributed
air gaps in the core to increase its electrical resistivity. The pressed amorphous
metallic cores were subsequently annealed at temperatures below glass transition temperatures
at temperatures between about 150°C and 300°C to have improved soft magnetic properties.
Example 7
[0081] Apparatus employed is of the type and construction illustrated by the drawing. A
jet of molten alloy of the composition Fe
40Ni
40P
14B
6 (atomic percent) was formed by forcing the metal at a temperature of about 1200°C
through the nozzle. The jet of molten metal was directed against the inner surface
of the rotating cylinder at a speed of about 25 m/sec. The cylinder was constructed
of copper and had an inner diameter of 40.64 cm. It was rotated at 1175 RPM. The jet
impinged on the copper cylinder at an angle of about 25° with respect to the inner
surface of the cylinder at the point in impingement. The jet had a diameter of about
0.75 mm and was ejected from the nozzle at a velocity of about 15 m/sec. Upon impingement,
the molten alloy jet broke into a stream of small droplets which bounced off the inner
cylinder surface. The direction of motion of these droplets was forwarded in the same
direction as that of the inner cylinder surface. These molten droplets passed through
a gate with a rectangular opening and then again impacted on the surface to be quenched
into solid particles. The gate was placed about 2 om away from the point of impingement.
The gate provided an opening having a vertical width of 1 cm and a horizontal length
of 5 cm. The quenched particles were blown off the surface by a jet of nitrogen at
a pressure of 60 to 80 pounds per square inch (4.14 x 10
2 to 5.52 x 10
2 kPa) in the direction towards a collection point. The resulting quenched particles
were found to be fully glassy by X-ray diffraction analysis. About 90% of the particles
had a particle size ranging between about 25 and 300 µm.
Example 8
[0082] Using the apparatus employed in Example 1, a jet of a molten alloy of composition
Ni
45Co
20Fe
5 Cr
10Mo
4B
16 (atomic percent), having a diameter of about 1.27 mm and having a temperature of
about 1300°C was impinged against the inner surface of the rotating copper cylinder.
The angle of impingement of the jet with respect to the chill surface was about 20°.
The jet velocity was about 10 m/sec. The speed of the inner surface of the cylinder
was maintained at around 15 m/sec. Using this technique, fully glassy powder was prepared.
The particle size of the powder ranged between about 100 to 1000 micrometers.