EP0017723B1

EP0017723B1 - Method and apparatus for making metallic glass powder

Info

Publication number: EP0017723B1
Application number: EP80100774A
Authority: EP
Inventors: Ray Ranjan
Original assignee: Allied Corp
Current assignee: Allied Corp
Priority date: 1979-03-23
Filing date: 1980-02-15
Publication date: 1986-01-08
Also published as: CA1133671A; AU5657580A; DE3071329D1; EP0017723A1; AU530637B2

Description

This invention relates to amorphous metal powders and in particular powders made with compositions of known glass forming alloys.
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.
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.
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.
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 10⁶ or 10⁸°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.
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.
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.
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.
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.
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 thermomechanically 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.
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.
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.
There remains a need for a method for making amorphous (glassy) metal powder having good properties for use in metallurgical processes. The present invention provides a method for making metallic glass powder.
According to the invention there is provided a method for making metallic glass powder comprising the steps of forming a jet of molten, glass forming metal alloy, and impinging the jet against the inner surface of a rotating cylindrical chill body in the direction of movement of the surface at an acute angle ranging from 5° to. 45°, preferably 20 to 30°, to effect atomization of the molten alloy into a stream of droplets of molten alloy, permitting the droplets to impinge on the inner surface of the chill body to be rapidly quenched to form solid particles of metallic glass powder, and removing the metallic glass powder from the inner surface of the chill body. The metallic glass powder is removed from the inner surface of the chill body, e.g. by a mechanical scraper or a fluid stream directed against it. The chill surface velocity is suitably from 15 m/sec to 40 m/sec, and the jet velocity is suitably from 5 m/sec to 15 m/sec. The jet diameter is preferably from 0.25 to 2.5 mm.
The invention also provides apparatus for making metallic glass powder, comprising:

a) holding means for holding molten metal;
b) nozzle in communication with said holding means for generating a jet of molten metal;
c) means for expelling molten metal through said nozzle to generate a jet of molten metal; and
d) a rotatable cylindrical chill body having 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 inner chill surface at an acute angle ranging from 5° to 45°.

The present invention relates to production of metallic glass powder involving atomization of a jet of molten glass forming metal alloy followed by rapid cooling of the atomized molten metal by quenching on a moving chill surface. Both atomization 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, which is measured and controlled according to known standard procedures, should be from 50° to 450° above the liquidus line corresponding to the melt composition, and is preferably from 100°C 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, viscosity decreases with increasing temperature. The glass forming alloys can be melted in a vacuum or inert atmosphere in accordance with usual metal melting practices. 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.
.The molten alloy is expelled through a suitable nozzle to form a jet of molten metal.
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. Exemplary suitable pressures range from 15 psi to 30 psi (1.03x102 to 2.07x10² kPa). Preferably, the pressure is from 20 psi to 25 psi (1.38x10² to 1.72x10² kPa).
Nozzles suitable for jetting the molten alloy include, for example, those described in US-A-2,968,062 and US-A-3,253,783. The shape of the nozzle orifice is not critical. For convenience of fabrication, round orifices are preferred. 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 2 mm to 30 mm.
The jet of molten alloy is then impinged against the inner surface of the rotating cylindrical chill body. Desirably, the jet of molten alloy is of small diameter. Preferably, the diameter of the jet is from 0.25 mm to 8 mm, more preferably from 0.25 mm to 2.5 mm. For example, a jet diameter of 1 mm to 1.5 mm may be conveniently employed. The velocity of the jet of molten metal is suitably from 5 m/sec to 15 m/sec and preferably from 8 m/sec to 12 m/sec.
The distance between the nozzle and the chill surface is desirably between 5 mm and 500 mm, preferably between 100 and 150 mm. The velocity of movement of the inner surface of the chill body is suitably from 15 to 40 m/sec, preferably from 20 to 30 m/sec.
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 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.
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 45°, the destabilizing force exceeds the stabilizing force and, as a consequence, the puddle tends to disintegrate into molten droplets.
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 travelling a short distance, again on the chill surface, whereon they are chilled to discrete particles of glassy metal alloy.
The particle size of the metal powder so formed decreases with increasing speed of rotation of the chill body.
The chill body is made of a metal of high thermal conductivity such as copper or silver. 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 or 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.
The invention is preferably practised 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.
The chill cast glassy alloy powders made according to the method 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 EP-A-19682.
Glassy metal powders made in accordance with the invention 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.
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. pressed and sintered into dense parts.
A metallic glass is an alloy product of fusion which has been cooled to rigid condition without crystallization. Metallic glasses are characterised 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 urn, coarse powder with particle size between 100 um and 1000 µm and flake with particle size between 1000 um and 5000 um.
The terms metallic glass, or glassy metal and amorphous metal are used herein interchangeably.
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 10⁴ to 10⁶°C/sec form amorphous glassy solids. Such alloys are, for example, disclosed in US-A-3,856,513, US-A-3,981,722; US-A-3,986,867, US-A-3,989,517 and many others.
For example, Chen and Polk in US-A-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 is aluminum, silicon, tin, germanium, indium, antimony or beryllium with "a" equalling 60 to 90 atom percent, "b" equalling 10 to 30 atom percent and "c" equalling 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 are those where "a" is 75 to 80 atom percent, "b" is 9 to 22 atom percent, "c" is 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_j wherein T is a transition metal and X is one of phosphorus, boron, carbon, aluminum, silicon, tin, germanium, indium, beryllium and antimony and wherein i is between 70 and 87 atom percent and j is between 13 and 30 atom percent. However, it is pointed out that not every alloy in this range would form a glassy metal alloy.
Figure 1 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 of the order of about 20 micrometers.
Figure 2 is a sectional elevational view of an apparatus according to the invention for making amorphous metal powders.
Referring to Figure 2, 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 5° to 45° between their directions. The angle lies preferably between 20° and 30°, an angle of 25° being eminently suitable. The jet diameter is preferably from 0.25 mm to 2.5 mm.
The jet has a velocity of from 5 m/sec to 15 m/sec, and the rotation of the cylindrical chill body provides an inner surface speed of from 15 m/sec to 40 m/sec, preferably between 20 m/sec and 30 m/sec.
On impingement of the jet at an acute angle, the impinging molten metal breaks up into a stream of discrete droplets 40'. By 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.
The following Examples 1 to 3 illustrate the use of the metallic glass powder obtained by the present invention.
The following Examples 4 and 5 illustrate the present invention.

Example 1

Magnetic composite cores from glassy metal powder

Amorphous metallic flakes with sizes ranging between 150 and 1000 um of an alloy having the composition of Fe_4oNi_4oP₁₄B₆ (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 ₁₁m. 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 by compaction under high pressures of between 200,000 and 250,000 pounds per square inch (1.38x10⁶ to 1.72x10⁶ kPa). 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 (1.72x10⁶ kPa) and annealed at 300°C for 16 hours was found to possess a permeability of 125 units.

Example 2

An amorphous metallic powder with an average particle size below about 75 11m of an alloy with the composition Mo_4oFe_4oB_2o (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 (2.76X 104 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 3

This example illustrates use of fine metallic glass powders for making high permeability magnetic cores. Metallic glass powders of composition Fe_4oNi_4oP₁₄B₆ (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.
Amorphous metallic powder of an Fe₄₀Ni₄₀P₁₄B₆ 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 (1.38x10s to 1.72x106 kPa) at room temperature. The ratios by weight of metallic glass to ceramic powder were in the range of between 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 150°C and 300°C to have improved soft magnetic properties.

Example 4

Apparatus employed is of the type and construction illustrated by Figure 2. A jet of molten alloy of the composition Fe_4oNi_4oP,₄B_s (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 cm 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 (4.14x10² to 5.52x10² 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 25 and 300 um.

Example 5

Using the apparatus employed in Example 4, a jet of a molten alloy of composition Ni₄₅Co₂₀Fe₅ Cr₁₀Mo₄B₁₆ (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°C. 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 and 1000 micrometers.

Claims

1. An apparatus for making metallic glass powder, comprising:

a) holding means (10') for holding molten metal;

b) nozzle (32') in communication with said holding means (10') for generating a jet (34') of molten metal;

c) means (24', 26', 28') for expelling molten metal through said nozzle (32') to generate a jet (34') of molten metal;

d) a rotatable cylindrical chill body (36') having an inner chill surface (38'), wherein the nozzle (32') and the chill body (36') are so positioned with respect to each other that a molten metal jet (34') expelled from the nozzle (32') impinges against the inner chill surface (38') of the chill body (36') in the direction of movement of the inner chill surface (38') at an acute angle ranging from 5° to 45°.

2. A method for making metallic glass powder, characterized by the steps of: forming a jet of molten, glass forming metal alloy, and impinging the jet against the inner surface of a rotating cylindrical chill body in the direction of movement of the surface at an acute angle ranging from 5° to 45°, to effect atomization of the molten alloy into a stream of droplets of molten alloy, permitting the droplets to impinge on the inner surface of the chill body to be rapidly quenched to form solid particles of metallic glass powder, and removing the metallic glass powder from the inner surface of the chill body.

3. A method for making metallic glass powder according to claim 2 conducted in vacuum.

4. A method according to claim 2 or 3 wherein the chill body is rotating to provide a chill surface velocity ranging from 15 m/sec to 40 m/sec and wherein the jet has a velocity ranging from 5 m/sec to 15 m/sec.

5. A method according to claim 4 wherein the chill surface velocity is from 20 to 30 m/sec and the jet has a velocity of from 8 to 12 m/sec.

6. A method according to any one of claims 2 to 5 wherein the angle of impingement is from 20 to 30°.