US 3041672 A
Description (OCR text may contain errors)
J. W. LYLE MAKING SPHEROIDL POWDER July -3,' .1962
med sept. 22'. 1958 mvemon JAMES WifL A-WORV@ cAs SHIELD cAs siucLD mia ' 3,041,672 BIAKINC SllillROlDAL PWDIR .lames W. Lyle, Indianapolis, Intl., nssignor to Union Cm*- bide Corporation. u corporation of New York Filed Sept. 22, 1958, Ser. No. 762,296 6 Claims. (Cl. 15S-47.3)
. roidal shaped particles are usually preferred in contrast to irregular shaped particles obtained by mechanical grinding methods.
A novel process leas been developed, according to this invention, for producing tine particles of sphcroidlal shape. 'Iltis process comprises the steps or' striking an arc between a stick electrode and a nozzle electrode, said arc being wnll-stabilizcd at least along a pcrtion of its length, introducing a consumable wire or rod into thc collimated plasma etllucnt of such are, passing a gas stream along and coextensive with such :ollimatcd plasma to shear off molten material from the molten surface of the consumable wire whereby small droplets are formed and removed from the are zone, and then solidifying and collecting the so-produccd finely-divided particlcs.
A wall-stabilized and constricted are of the tyre used in this invention is described more fully in U.S. Patent No. 2,SO6.l24 and copending application SN. 539.794, now Patent No. 2.858,4ll, dated October 28, i953.
The preferred operating condition for smallest particle size of product involves introducing the consumable wire or rod within the nozzle electrode passage at the point of maximum arc construction and maximum momento.. of the arc plasma and coeztensivc gas stream. The gas streamcmploycd should preferably be chemically inert to the molten particles so as to prevent reaction between the gas and particles in addition to shielding the hot particles from atmospheric oxidation. VIn addition, for maximum melt-oli rates of the consumable wire and highest production rates for t'ine powder, the
consumable wire is preferably an electrode carrying the bulk of the are current.
This process is especially useful for producing finely divided, spheroidal-shaped metal particles. Ilowerct', it may also be employed with metal compounds, such as the refractory metal oxides, silicides, borides, nitrides and carbides, which are ditlicult if not impossible to produce in line spheroidal form by prior art techniques.
These line particles are relatively uniform in shape and size. Their actual size appears to be dependent upon arc power and gas momentum. The sphereidal shape renders these products more free-flowing titan prior art irregular shaped particles obtained by mechanical grinding methods. 'lltey are also useful for producing sintered porous bodies having relatively uniform pore size.
This process has the additional advantage of producing fine powders of material which has such a high d:- grce of duetility that it cannot be mechanically ground. Also, it can produce powders of refractory mamials which cannot he readily melted and treated in conventional shoz-tower processes.
In the drawings: n FIG. l is a fragmentary view m vertical cross-section of apparatus for making powder according to the invention; and
FIGS. Z-S are similar Yiews of other modifications.
y United Secties Patent @ffice Patented-July 3, 1962 electrode 10 and nozzle electrode Il. Electrical power is' supplied from source 12 through leads t3 and 14. A gas stream passes down along electrode 10 and forces the arc 15 down into tite constrictcd portion 16 of the nozzle passage 17 where the arc becomes wall-stabilized. The nozzle electrode 11v is cooled below its melting point by passing cooling fluid, such a's water, from'inlet 2S through passage 29 to outlet 30.
A consumable wire I8 is preferably introduced at or near tite point of maximum nozzle constriction and maximum arc plasma momentum in order to obtain line particle size product. This process is not limited, however, to wire introduction at this point. The wire may be introduced at other points in thc nozzle or at the nozzle outlet as shown in the position of wire 19 of FIG. 7.. Vrhen the wire 18 is introduced as shovm in vFIG. l, the nozzle passage extending beyond the point of wire entry tends to improve results by focusing and controlling the position of the molten droplet stream. It also directs the gas stream and maintains improved shielding of the metal particles from atmospheric contamination. A divergent l discharge passage 20 is conveniently employed primarily to reduce the possibility of nozzle plugging caused by deposits of molten metal particles. It also prevents the formation of an undesirable shock front at the outlet which can occur with straight nozzles. A divergent anode passage is desirable in that it spreads the nozzle. electrode area and reduces current density. Iltis helps reduce nozzle erosion at high current levels. lt is preferred for the above reasons that the noule outict have an increased cross-sectional area as compared to the area of thc` nozzle at the point of ire entry. Nozzle shapes other than that shown might iso be used. 'Die molten particles 2t cntrained in the torch gas stream pass out of the apparatus and are solidi'tie and collected in a collector 22. This collector may, for e-:ampl:. contain a body of water into which thc molten particles are directed for soliditication.
In the apparatus modification shown in FIG. l the wire 1S can be in electrical contact with nozzle electrode l1 and thus become an electrode. When the wire extends into the nozzle passage the arc current can terminate at the wire instead of the nozzle. This increases wire meltoff and increases production rates of tine particles.
Another modification of apparatus suitable for carrying out the present novel process is shown in FIG. 3. In this form the consumable wire 13 is electrically insulated from nozzle electrode 1l by insulator 23. The wire 18 can then be out of the electrical circuit, or in the preferred form can be almost electrically independent of the nozzle. The main electrical connections in the preferred form of this modification are from the power supply 12 through lead 13 to the stick electrode 10 and lead 2.4 to the consumable wire 18. The nozzle electrode 11 is connected to the power supply through resistance 2S which tends to maintain the nozzle at a lower potenticn than that of the consumable wire. This apparatus can be operated at higher wire feed rates than that of FIG. l 'because higher power levels can be maintained to the wire without damage to the nozzle. This becomes important when wire feed rates as high as lbs/hr. are desired. A pilot arc is maintained between the stick electrode and the nozzle electrode in ord-:r to effect startup ofthe process and also to maintain an arc if wire feed ceases for any reason. The electrical contact from lead 28 to wire 18 is conveniently positioned externally to the tcrch in order to increase resistance heating along the wire and increase melt-off rates.
ln order to fully protect the molten particles from atmospheric. contamination prior to their soliditicatlon,
additional shielding gas can be introduced through shield process in operation.
EXAMPLE I Production of Finely-Divided Srccl Particles An apparatus of the type shown in FIG. 1 was used consisting of a. lis-inch dia. throated tungsten stick cathode and a water cooled nozzle anode having a throat section r-inch da.. and ta-inch long. The nozzle passage beyond the throat was s-inch long having a 30 divergent angle. The stick cathode was set hack J/a-inch from the throat section. An are of 150 amperes and 85 volts was maintained between the stick cathode and nozzle anode while a gas mixture of ZOO c.f.h. argon and 13.5 c.f.h. hydrogen passed along the tungsten cathode and out through the nozzle passage. A K-inch dia. steel wire (Linde No. 65 welding wire) was introduced at a rate of about 7 1bs./ hr. to the ponle passage at a point adjacent to the throat section. .e wire was in electrical contact to the nozzle anode ani thus l'ecame an effective anode as it projected into the nozzle passage. The molten particles from the wire were entrained in the gas stream and were collected in a container of water positioned about l-ft. from the torch oud-'L the resulting product particles were sul staniah? an spheroidal in shape with particle size ranging from 5 to 200 microns'in diameter. The average size based ondistribution was about l5 microns in diameter.
EXAMPLE II Prcxfuction o] Finely-Divided Steel Particle:
The same equipment was used as described in Example I above. An are of .'50 ampercs :nfl 6d volts was maintained between the stick cathode and nozzle anode while a gas mixture of 200 c.f.h. argon and '13.5' c.f.h. hydrogen passed along the tungsten cathode and out through the nozzle essage. A V10-inch dia. steel wire was introduced EXAMPLE III Production of Finely-Divided Tungsten Particles The same basic equipment described in Example I above was used. In addition a brass tube 2-in. I.D. and 4-ft. long nas attached to the torch outlet. The outlet end of this tube was immersed in water. This is shown in FIG. 5. An arc of 125 amperes and 75 volts was maintained between the stick cathode and nozzle mode utile a gas mixture of 200 c.f.h. argon and 13.5 c.f.h. hydrogen passed through the torch. A IAG-inch dia. tungsten wire was introduced at a rate of about 4.8 lbs./
tween thc stick cathode and nozzle anode while a' gas mirtturt` of 200 lc.f.h. argonj-and l3.51 c.f.li. hydrogen passed along the tungstencathode' and out through the nozzle passage. A J/m-irlch da. sapphire rod was introduced to the nozzle Vpassage at la point adjacent to the throat section. The'rod was .ten-conducting and was thus not in the electrical circuit. The molten particles lfrom the rod was entrained in the gas stream and were collected in a container of water positioned abouthI-ft.
from the torch outlet. The resulting product particles were substantially all spherical in shape with an average particle s;ze of about 6 microns. t
EXAMPLE V Production of Finely-Divided Sapphire Par-ticle:
Table I I EFFECT OF AIIC POWER ON PAlt'IICI-.E SIZE v v A verace Voltage, oils Current, Power, Particle Amps. kw. Stre- Mit-runs 310 1&6 107 61. 1H) 10. 2 ISS 00 I 0 333 Dependency of particle size on gas momentum is show-n below. The are power remained substantially constant and the win` feed rate was adjusted as required.
Table II EFFECT 0F OAS FLOW ON PARTICLE SIZE Gas Flow A retain Pnrtlclo Slu, H yllmgcn, Argon, M tetons c.f.h. c.f.h.
It can be seen from the above tables that as the arc power and the gas momentum through the torch, as inhr. to the nozzle passage adjacent to the throat section.
The Wire was in electrical contact with the nozzle and thus beanie an effective anode as it projected into the laltc. The molten particles from the wire were en trained in the gas stream and were solidified and collected l lb 'Hier located at the end of the protective tube. n POduct particles were substantially all spheroidal l? une with an average diameter of 380 microns.
EXAP'IPL' IV Production o] Finely-Divided Sapphire Particles Eilment described in Example I above was und. #i me et N0 ampere: and 85 volts was maintained bedicated by total gas flow through the torch, are increased, the average particle size decreases.
The tine sphcroidal particles produced by the prent invention may be used as starting materials for miniature ball bearings or in powder metallurgy for making sintered porous bodies of relatively uniform pore size. This novel process has the additional advantage of producing tine powders of material, such as nickel-chromium alloy, which has such a high degree or ductility that it cannot be mechanically ground. Also, it can produce powders of refractory materials, such as tungsten carbide, which cannot be readily melted and treated in conventional shot-tower processes.
What is claimed is:
t. Method of producing finely-divided spheroidal shaped particles of metals and metal compounds by striking a wall-stabilized are between a stick electrode having an are constricting passage and a nozzle electrode, in-
troducing an consumable wire or rod composed of said y metals and metal compounds into the eollimated plasma such collima-ted ylasma through the nozzle passngeiof the formed and removed from the arc zone, and then solidi-v [ying and collecting the so-produced finely-divided particles.
2. Method of producing nely-divided spheroidaishaped particles of metals and metal compounds by striking a' wall-stabilized .virbetwcen a stick electrode and a nozzle electrode havfng an arc eonstricting passage, introducing a consumable wire or rod composed of said metals and metal compounds into the collmatcd plasma effluent of such arc, passing a gas stream coe'xtcnsive with such oollimated plasma through the nozzle passage of the nozzle electrode to shear olf molten material from the tip of the consumable wire whereby small droplets are formed and removed from the arc zone, and then solidifying and collecting the so-produced `finely-divided particles,v in which the preferred operating condition for smallest particle size involves introducing the consumable metal wire at the point of maximum are constriction and maximum momentum of the arc plasma and coextensive gas stream.
3. Method as defined by claim 2, in which, for maximum melt-off rates of the consumable wire and highest production rates for fine powder, the consumable wire is preferably an electrode carrying the bulk of the arc current.
4. Process as defined by claim 2, in which such are ellluent is conically expanded and thereby is focussed downwardly in the direction of a liquid in a container disposed thereunder for collecting such sprny'of individual spheroids.
5. Process as defined by claim 4, in which such wire is laterally fed into one side of such downwardly focussed eilluent so that such particles are sliearcd therefrom by such are ellf'ent which comprises inert gas.
6. The method of producing very small spheroidalshaped particles of metals and metal compounds by striking an arc between a trst nonconsumable electrode and a second constricting nonconsumable nozzle electrode having an expanding conical outlet, passing a gas stream' in contact with said electrodes and through said nozzle electrode to produce a collimated plasma cflluent, introducing a consumable wire or rod of said metals and metal compounds into the collimated plasma at a point of maximum are constriction and plasma momentum, whereby said vn're is melted and disintegrated into small droplets and removed from the are zone bythe plasma stream., and then solidifying and collecting the resulting spheroidal powder particles.
References Cited in the lc of this patent UNITED STATES PATENTS 1,128,175 Morf Feb. 9, 1915 1,133,508 Seboop Mar. 30, 1915 2,189,387 Wissler Feb. 6, 1940 2,269,528 Gallup Jan. 13, 1942y 2,768,279 Rava Oct. 23, 1956 2,770,708 Briggs Nov. 13, 1956 2.795.819 Lezberg et al June 18, 1957 2,806,124 Gage Sept. 1t), 1957