WO1989000470A1

WO1989000470A1 - Double disintegration powder method

Info

Publication number: WO1989000470A1
Application number: PCT/US1988/002477
Authority: WO
Inventors: Aspi N. Patel; Robert S. Carbonara
Original assignee: Battelle Development Corporation
Priority date: 1987-07-20
Filing date: 1988-07-20
Publication date: 1989-01-26

Abstract

Method and apparatus are described for producing a fine, narrow size distribution metal powder. The method involves the two stage breakup of a liquid metal stream (4). A first stage breaks the liquid into moderate size droplets or ligaments (6) and delivers them, still in the liquid state, to a second stage. The second stage is a rapidly spinning cup (7) having a rotating water wall (8) which shears the droplets or ligaments from the first stage into very fine powder.

Description

DOUBLE DISINTEGRATION POWDER METHOD

Technical Field

There exists many ways of producing powder. But, there is a need for an efficient method for forming fine particles with a narrow size distribution from a normally solid material, such as a metal or alloy. Current techniques for making metal powder by centrifugal atomization, for example, include supplying metal to a rotating hollow element which has holes in its walls. The liquid metal is broken up and ejected by centrifugal force through the holes. The ejected metal then cools in the air and solidifies to a metal particulate having a diameter which depends on the hole size. However, the plugging of small holes prevents fine particles from being produced, and there are generally temperature limitations on such elements. Another liquid atomization technique involves the disintegration of a stream of liquid metal by a blast of pressurized fluid. This results in solidification of the droplets and the trapping of the atomizing fluid in the particulate.

Another method is shown in U.S. Patent 4,374,074. A liquid metal stream is introduced in a very controlled way to the surface of a rotating flat disk. At high speeds, the disk shears a thin layer of molten metal and expels ligaments and droplets of liquid metal.

A further method of making powder from a liquid metal stream is shown in U.S. Patents 4,394,332 and 4,405,535. The so-called rapid spinning cup (RSC) method involves introducing a stream or droplet of liquid metal to a rotating water wall to shear the droplet into finer components. But, although the RSC process is an effective method to produce rapidly-solidified powder, the production of finer metal powder requires higher rotational surface speed and/or smaller metal delivery tubes. The latter causes orifice clogging, very high pressure, low velocity (lower energy) and low production rate (production rate is a function of the stream diameter). Moreover, the supply tube size in the RSC method is fixed making it difficult to produce varying powder sizes. It would be desirable to have a high production rate process to produce fine powder with a narrow size distribution for such applications as metallic paints, inks, catalyst, and injection molding.

Summary of the Invention

It is an object of the present invention to provide a method and apparatus for producing fine metal powders.

It is also an object to produce such fine powders with a fairly narrow size distribution.

It is further an object of the invention to produce fine powder at high production rates. It is also an object to provide method and apparatus which is not constrained by metal supply orifices and which can be easily modified to produce different size powders.

In accordance with the objects, the invention is a method and apparatus for making fine powder by creating a stream of molten material, disintegrating the stream into intermediate droplets and/or fine ligaments still in the liquid state, further disintegrating the intermediate droplets and/or ligaments by flowing them to a moving wall of a centrifugally disposed quench liquid in a manner adapted to shear the intermediate droplets/ligaments and form subdivided particles and solidifying the subdivided particles in the quench liquid. The molten material preferably has a viscosity (within 25° of its equilibrium melting point) of between about 0.001-1 poise. Metals (including pure metals and alloys thereof) are the preferred materials. The first stage disintegration is preferably a gas or centrifugal atomization process. The initial breakup preferably reduces the droplets and/or ligaments to about 50-400 um.

Figures 1-4 are sectional views of three separate apparatus for producing fine powders.

Figure 5 is a graph showing the effect of RSC rotation on particle size.

Detailed Description of the Preferred Embodiments

For many applications, very fine powders having a narrow size distribution are needed. For example, injection molding requires fine powders of consistent size to produce uniform density parts. Prior methods for making powders generally attempt to fractionate a stream of molten metal in one step with a high energy fluid. To make finer powder, the diameter of the metal stream is merely reduced or more commonly, the atomizing fluid pressure is increased. But this has limitations because orifices plug when they get too small, and production rates also drop because of low metal delivery through small orifices. Higher pressures are more difficult and expensive to provide.

The present invention provides a two-step process to produce powders such that the material delivered to the second stage is already substantially reduced in size form the incoming stream. Powder is intended to mean a solid particle having an aspect ratio (length to thickness) of less than about 10 and a diameter of about 1-25 μm, preferably about 1-10 μm. Metal particles are preferred, though other normally solid materials with low viscosity in the molten state are possible.

The second stage disintegration is carried out in the inventive process by a rapid spinning cup in which a liquid quenchant is centrifugally disposed. A variety of first stage disintegration means are available.

Figure 1 shows one preferred apparatus employing gas atomization means as the first stage. A tundish 2 contains the molten material 1 at above its melting temperature. Superheat may be used to keep the material molten during the process. An orifice 3 in the tundish bottom allow a melt stream to leave the tundish. Gas jets 5 bring pressurized gas into contact with the melt stream causing the initial disintegration into intermediate droplets 6. The droplets are then blown into the liquid quenchant 8 centrifugally disposed in rotating cup 7.

The surface speed of the rotating cup may be, for example, about 5-100 m/sec in the laboratory. With appropriate equipment, speeds of 175 m/sec may be obtainable. The rapidly rotating quenchant liquid shears intermediate droplets and disintegrates and rapidly quenches them similarly to one-stage RSC. Figure 2 shows an end view of an alternative nozzle design 24 for gas atomization. The metal delivery orifice 25 is centrally located and surrounded closely by gas jets 26 directed toward the orifice 25. This nozzle on the end of a tundish is useful for atomizing the molten metal immediately after it leaves the orifice rather than downstream as shown in Figure 1.

Figure 3 shows another first stage disintegration means comprising a disk 12 having upwardly inclined walls. A flat disk could also be used but the upwardly inclined walls seem to aid in breaking the metal stream into smaller ligaments or droplets. The disk may be heated to keep the melt from losing heat and solidifying too early. It may be made of a ceramic material or of a metal with an insulating liner. According to the invention, the first stage disk 12 is generally symmetric about an axis of rotation and has a cavity with upwardly inclined walls. The walls may be straight or contoured. The cavity preferably has a bottom portion against which the molten material 14 is directed during operation as from a conduit 13. From the bottom, the molten material spreads up the inclined walls and is thrown from the rotating cup in very fine ligaments 15, which may also be broken to droplets at high speed. The ligaments/droplets then strike the liquid quenchant 11 in the rotating cup 10 and are disintegrated and cooled into solid powder 16.

On a rotating flat disk, the liquid is subject to the centrifugal force accelerating it rapidly to the edge of the disk. It is also subject to shearing forces of the rotating disk. To obtain the necessary shear for production of very fine ligaments (e.g. 10-20 μ diameter), the disk speed needs to very high.

Using a disk with an upwardly inclined wall, the speed can be substantially less than the flat disk to produce the very fine ligaments. We believe that this is because more of the liquid on the wall to reach the velocity of the cup before it is expelled. This adds additional energy for breakup of the liquid film at the cup edge.

We prefer to have straight, inclined walls but whether straight or contoured, it is desirable that the angle made by the inclined wall portion with the vertical axis of rotation be in the range of between about 5° and 65°. Product, over this range, shows significant decreases in particle size. The cup shape also is advantageous over a flat disk in allowing more variability in introducing the liquid. No particular spacing is necessary between the bottom of the disk and the point of entry of the liquid, though splashing should obviously be avoided. Moreover, an off-center pour on a flat disk can cause a wider distribution of particle sizes in the product. This condition is much less prevalent in the upward wall design because all the liquid approaches the same high tangential velocity and disintegration forces prior to breakup.

Among other factors, disk speed, disk temperature, liquid viscosity, and the interaction between liquid metal and the atomizing atmosphere affect product size. The disk is generally rotated such that the speed at the perimeter is 2-200 m/sec, though conditions and desired product requirements might dictate speeds outside this range. It is typically heated such that its temperature is at or above the liquidus temperature of the liquid material. Again, some particular requirements might dictate higher or lower temperature.

The disk and the second stage RSC may be rotated in the same direction or the reverse so long as there is a significant difference in the speeds so that the intermediate droplets/ligaments are sheared by the liquid quenchant. The disk and RSC must also be fairly close together, especially when making fine intermediate ligaments/droplets, to prevent the intermediate material from solidifying in the atmosphere before being disintegrated in the second stage. When making finer powder, it may be better not to try to reduce the size of the intermediate material, but rather to adjust the second stage disintegration. Reducing the size of the intermediate material below an optimum liquid droplet size may cause early solidification and no secondary disintegration. Figure 4 shows a schematic cross section of another preferred apparatus comprising a first stage sacrificial electrode 21. An arc is struck between the sacrificial electrode 21 and the counter electrode 27 causing melting of the sacrificial electrode. The rapid rotation flings small droplets 22 into the rotating quenchant 23 resulting in the secondary breakup.

In general, the first stage disintegration means may be any device which accepts a melt stream and reduces it to ligaments and/or droplets having a size of about 25-400 μm and which remain in liquid state until disintegrated in the second stage. Generally, the finer the intermediate material, the finer the final product and the narrower the distribution. Rotating disks, cups, cones, and drums having smooth or serrated surfaces can be used. Speeds of 100-7000 rpm are useful. Ligament size decreases with increasing speed. It should be clear that the word stream is intended to be broadly applied to any form of delivery of a quantity of molten material. For example, we intend stream to include the quantity of liquid material melted on the end of a wire by electric arc such as shown in Figure 4.

This process allows the production of powders which could not easily be done before. For example, tungsten is a very high melting point metal which could not be easily melted and atomized by conventional techniques. With the present process, electric arc spraying can be used as a first stage and the intermediate droplets may be further disintegrated in the RSC. The electric arc spraying can also be used to produce alloyed powder if two arc sprays, one tungsten and one copper, for example, are used as the first stage. This approach provides the opportunity to produce rapidly solidified refractory metal powders. Moreover, materials which are hard to contain, such as titanium, are also candidates for electric arc or plasma spraying as a first stage.

Example 1 - First Stage Rotating Disk

Apparatus such as shown in Figure 2 was used to produce powder according to the invention. The steel disk had upwardly inclined walls at about 65°

(from the vertical) and an outer diameter of 10 cm.

An outside cup 12.5 cm in diameter surrounded the disk.

Water was introduced into the cup to act as the quenchant. The inner disk and outer cup were rotated at different speeds and directions. A quantity of tin was melted and delivered to the first stage rotating disk. Results are shown in Table 1.

The results are also graphed in Figure 5 where the effects of outside cup and inside disk directions are shown. With the inner disk turning counterclockwise (ccw), the outer cup was rotated at 2000 rpm clockwise (cw) and 2000 rpm ccw.

Results of a trial with a stationary outer cup are also shown in Figure 5 demonstrating that particle size may be reduced more efficiently by the two stage process whether the outer cup is rotated in the same or opposite direction as the disk.

Example 2 - First Stage Arc Spray

An electric arc spray was utilized as stage one disintegration with 304 stainless steel. A 304 ss wire was melted by electric arc and several gas jets were used to atomize the resulting liquid into droplets. The intermediate droplets were immediately blown into the rotating water wall in a rapidly spinning cup. Table 2 shows the resulting powder size as a function of cup speed and melt superheat (temperature above the melting point). Increasing cup speed generally decreased the particle size.

Example 3 - First Stage Arc Spray

As evidence that the intermediate material stays in the liquid state and is further disintegrated by the second stage, a 316 stainless steel wire was melted by electric arc and sprayed by pressurized gas into stationery water (shown in Table 3 as 0 cup speed). Thereafter, the outside cup was rotated at different speeds to further reduce the first stage droplets. Table 3 shows that the intermediate material is further disintegrated by the outer cup. Melt superheat and gas pressure were varied in order to see their effects on one stage and two stage disintegration. Clearly, the gas pressure and to some extent the melt superheat have some effect on particle size in the first stage. But their effect on two stage disintegration is diluted considerably by the large effect of outer cup speed on resulting particle size.

Example 4 - First Stage Gas Atomization

A cobalt base superalloy having a composition (in weight percent) of 55 Co-23.5 Cr-10 Ni-7 W-3.5 Ta-0.20 Ti-0.7 C-0.3 Hf-0.3 Zr was melted (200°C superheat) in a pressurized crucible in proximity to a rapidly spinning cup having a centrifugally-disposed oil quenchant therein. A liquid metal stream was forced out of a 1.65 mm orifice in the bottom of the crucible and disintegrated with two argon gas jets focused on the stream. The intermediate material was directed by the jets to the rotating quenchant. The first stage gas atomization alone produced a median particle size of 265 microns (stationary RSC). At a RSC surface speed of 120 m/sec the intermediate droplets were reduced to a median particle size of 7 microns.

Claims

We Claim

1. A method for producing a fine powder from a molten mass of normally-solid material comprising the steps of (a) creating a stream of the molten material,

(b) disintegrating the stream of molten material into intermediate droplets and/or ligaments still in the liquid state,

(c) disintegrating the intermediate droplets and/or ligaments by contacting a moving wall of a centrifugally-disposed quench liquid in a manner adapted to shear the intermediate droplets and/or ligaments and form subdivided particles, and

(d) solidifying the subdivided particles to a fine powder in the quench liquid.

2. The method of Claim 1 wherein the stream is disintegrated by a rotating disk.

3. The method of Claim 2 wherein the disk has upwardly inclined walls.

4. The method of Claim 2 wherein the disk and the quench liquid are moving in opposite directions.

5. The method of Claim 2 wherein the disk and the quench liquid are moving in the same direction.

6. The method of Claim 1 wherein the stream is disintegrated by gas atomization.

7. The method of Claim 1 wherein the stream is produced by melting the tip of a solid rod and the stream is disintegrated by rapid rotation of the rod.

8. The method of Claim 1 wherein the stream is disintegrated to intermediate droplets and/or ligaments having a diameter of about 50-400 μm.

9. Apparatus for producing fine powders from a stream of a normally-solid material comprising first stage disintegration means for reducing the stream into intermediate droplets and/or ligaments, and a centrifugally-disposed quench liquid, and means for contacting the intermediate droplets and/or ligaments in the liquid state with the centrifugally-disposed quench liquid to further reduce the intermediate droplets and/or ligaments.

10. The apparatus of Claim 8 wherein the first stage disintegration means comprises a rotating disk.

11. The apparatus of Claim 9 wherein the disk has inclined walls.

12. The apparatus of Claim 8 wherein the first stage disintegration means comprises a pressurized fluid.