Apparatus for the production of magnetic powder

This invention relates to an apparatus for casting. of metal powder, especially metallic glass powder.

Amorphous metal alloys and articles made therefrom are disclosed in US-A-3,856,513. That patent teaches certain novel metal alloy compositions which are obtained in the amorphous state and are superior to previously known crystalline alloys based on the same metals. The compositions taught therein are easily quenched to the amorphous state and possess desirable physical properties. The patent discloses further that amorphous metal powders having a particle size ranging from 10 to 250 um can be made by grinding or air milling the cast ribbon.

Manufacture of magnetic articles by consolidation of permalloy and other crystalline alloy powders is known, by the DE-A-26 28 207.

By the US-A-4 142 571 an apparatus for casting ribbons of a glassy metal alloy is known, which apparatus comprises a movable chill surface, a reservoir for holding molten metal and a nozzle in communication at its top with the reservoir and having at its bottom an opening in close proximity to the chill surface said chill surface being adapted for longitudinal movement at a velocity of 100 to 2000 meters per minute. Powders cannot be cast directly by this apparatus. If the metal alloys are desired in powder form, for example to be used in a compacting method, the cast ribbons are grinded, ball milled or air milled into powders of desirable size range. To aid the pulverization process ribbon samples are subjected to an embrittlement heat treatment below the crystallization temperature of the alloy.

The EP-A-0 008 604 discloses an apparatus for the production of flake particles from a projected stream of molten material comprising: means for containing, heating, and projecting the stream of molten material at super atmospheric pressure; a heat-extracting drum having a serrated peripheral surface with the edges of each serration substantially parallel to the central axis of the drum; means for rotating the drum about its central axis; means for controlling the rate of projection of said molten material; and means for controlling the rate of rotation of the drum relative to the rate of projection of molten material during projection.

By the US-A-4149 884 an apparatus for casting polycrystalline alloys in the form of filaments or ribbons is shown. Said apparatus comprises a rotating disc having semi-circular indentations or notches in its circumferential edge. The objective of the present invention is an apparatus to cast metal powder directly into the final form having a desirable size range, especially to cast metal powder as fine particles.

The apparatus for casting of metal powder according to the invention comprising a movable chill surface (12) having a plurality of regularly spaced peaks (22) and valleys (24), a reservoir (14) for holding molten metal (16) and a nozzle (18) in communication at its top with the reservoir (14) and having at its bottom an opening (20) in close proximity to the chill surface (12), said chill surface (12) being adapted for longitudinal movement at a velocity of 100 to 2000 m/min, and the distance from the top of a peak to the bottom of a valley ranging from 0.005 cm to 0.05 cm is characterized in that said valleys are bounded on all sides by said peaks to delimit a plurality of casting cavities, that the distance between adjacent peaks ranges from 0.01 cm to 0.1 cm, and that the bottom of the nozzle is maintained at a distance between 0.02 and 1 mm from the chill surface.

The size of the powder particles thereby produced will vary, depending on the depth of the serrations and the distance therebetween. The apparatus according to the invention typically yields powder particles having a size ranging - from 0.01 to 0.1 cm.

Preferably, in the apparatus of the invention the nozzle has a slot arranged generally perpendicular to the direction of movement of said chill surface (12), the slot being defined by a pair of generally parallel lips, a first lip and a second lip numbered in the direction of movement of the chill surface, wherein said slot has a width of from 0.2 to 1 mm, measured in direction of movement of the chill surface, wherein the first lip has a width at least equal to the width of said slot, and said second lip has a width of from 1.5 to 3 times the width of said slot, and wherein the gap between the lips and the chill surface is from 0.1 to 1 times the width of the slot.

By use of such metal powders, especially molded magnetic metal alloy articles are produced by a method comprising the step of compacting ferromagnetic glass powder with static pressure at a pressing temperature in the vicinity of the glass transition temperature and below the crystallization temperatures of said alloy, and at a pressure of 69 MPa to 690 MPa. A consolidated, glassy metal alloy body is thereby formed, which is especially adapted to be post fabrication annealed at a temperature ranging from 380 to 450°C for a time period of 1 to 4 hours in the presence of a magnetic field of 0 to 800 A/m. The annealed article has improved impedance permeability and is particularly suited for use in signal and high frequency power transformers and the like.

The invention will be more fully understood and further advantages will become apparent when reference is made to the following detailed description of the preferred embodiments of the invention and the accompanying drawing, which is a schematic representation of apparatus used to cast amorphous metal powder directly from the melt, the apparatus having a serrated casting substrate according to the invention.

As shown in the drawing, the apparatus 10 has a movable chill surface 12, a reservoir 14 for holding molten metal 16 and a nozzle 18 in communication at its top with reservoir 14 and having at its bottom an opening 20 in close proximity to the chill surface 12. The chill surface 12 has a plurality of regularly spaced peaks 22 and valleys 24. Adjacent peaks are separated by a distance, d, of 0.01 cm to 0.1 cm. The distance, y (not shown), from the top of a peak to the bottom of a valley is 0.005 cm to 0.05 cm. Powder is produced directly by deposition of molten alloy on the serrated substrate (chill surface 12) which is a rotatable chill roll, an endless belt (not shown) or the like, adapted for longitudinal movement at a velocity of 100 to 2000 meters per minute. The size of the powder particles thereby produced varies directly with the magnitude of distances d and y.

In the embodiment shown, the nozzle means has a slot arranged generally perpendicular to the direction of movement of the chill surface. The slot is defined by a pair of parallel lips, a first lip and a second lip numbered in the direction of movement of the chill surface. The slot of nozzle 18 has a width of from 0.2 to 1 millimeter, measured in the direction of movement of the chill surface. The first flip has a width at least equal to the width of the slot, and the second lip has a width of from 1.5 to 3 times the width of the slot. The gap between the lips and the chill surface is from 0.1 to 1 times the width of the slot. The preparation of a glassy alloy can be achieved by following the basic teaching set forth in US-A-3 856 513.

Powder adapted for consolidation can comprise fine powder (having particle size under 105 micrometers), coarse powder (having particle size between 105 micrometers and 300 micrometers) and flake (having particle size greater than 300 micrometers). Consolidation can be obtained by pressing glassy metal alloy powder near its glass transition and below the crystallization temperature.

In case low permeabilities (i.e., less than 25) are desired, a particle diameter of less than 105 micrometers is used. For high permeabilities (greater than 100), larger particle diameters of 300 micrometers or more are employed.

For consolidation, powders can be put in evacuated cans and then be formed to strips or isostatically pressed to discs, rings or any other desirable shape such as transformer and inductor cores, motor stators and rotor parts, and the like. Furthermore, powders can be warm pressed below the crystallization temperature and in the region of glass transition temperature into any desirable shapes of the transformer/inductor cores or motor rotor/stator segments. Consolidation is believed to result from mechanical interlocking and short-range diffusion bonding between the powder or flake particles occurring in the vicinity of the glass transition temperature. At temperatures too far below the glass transition temperature (Tg) the particles are relatively hard and are not readily deformed by shear and compressive forces exerted thereon during consolidation. Temperatures too far above Tg enhance the risk of incipient crystallization of the amorphous particles during consolidation. Generally, it has been found that interpartical bonding is best achieved during consolidation at pressing temperatures within 50°C of Tg.

The powders can also be mixed with a suitable organic binder, for instance, paraffin, polysulfone, polyimide, phenolic formaldehyde resins, and then cold pressed to suitable forms. The amount of binder can be up to 30 weight percent and is preferably less than 10 weight percent and more preferably between 0.5 and 3 weight percent for high permeability cores. Such formed alloy can have a density of at least 60 percent of the theoretical maximum. The pressed object can be cured at a relatively low temperature below the curing temperature of the binder to give more strength and then ground to final dimensions. The preferred product of this process comprises shapes suitable as magnetic components. The curing process can be performed with simultaneous application of a magnetic field.

A metallic glass is an alloy product of fusion which has been cooled to a rigid condition without crystallization.

Such metallic glasses generally 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, ductility, high electrical resistance compared with related metals and alloys thereof, and a diffuse. X-ray diffraction pattern.

The term "alloy" is used herein in the conventional sense as denoting a solid mixture of two or more metals (Condensed Chemical Dictionary, Ninth Edition, Van Norstrand Reinhold Co., New York, 1977). These alloys additionally contain admixed at least one non-metallic element. The terms "glassy metal alloy", "metallic glass", "amorphous metal alloy" and "vitreous metal alloy" are all considered equivalent as employed herein.

Examples of alloys suitable for the use in the apparatus of the present invention include the composition <chemistry id="chem0001" num="0001"><img file="EP0092091B2_D0001.tif" /></chemistry>Preferred ferromagnetic alloys according to the present invention are based on one member of the group consisting of iron, cobalt and nickel. The iron based alloys have the general composition <chemistry id="chem0002" num="0002"><img file="EP0092091B2_D0002.tif" /></chemistry>the cobalt based alloys have the general composition <chemistry id="chem0003" num="0003"><img file="EP0092091B2_D0003.tif" /></chemistry><chemistry id="chem0004" num="0004"><img file="EP0092091B2_D0004.tif" /></chemistry>and the nickel based alloys have the general composition <chemistry id="chem0005" num="0005"><img file="EP0092091B2_D0005.tif" /></chemistry><chemistry id="chem0006" num="0006"><img file="EP0092091B2_D0006.tif" /></chemistry>

An especially preferred alloy has the composition 79 atomic percent iron, 16 atomic percent boron and 5 atomic percent silicon.

Amorphous metallic powders can be compacted to fabricate parts suitable for a variety of applications such as electromagnetic cores, pole pieces and the tike. The glassy metal compacts have either high or low permeability. The resulting cores can be used as transformer cores, motor stators or rotors and in other alternating current applications. Amorphous alloys that are preferred for such applications include Fe7,B_l3Si₄, Fe₇₉B₁₆Si_s and Fe_8lB_l3Si₃._SC₂.