GB2155049A - Method of atomization of melt from a closely coupled nozzle, apparatus and product formed - Google Patents

Abstract

In atomizing a body of molten material having elevated melting point by directing the molten body as a stream into an atomization zone and directing a stream of atomizing gas into said stream to atomize and disperse it, the orifice from which the gas is delivered is positioned closely proximate the surface of the stream to be atomized, e.g. less than 0.45 inches way. A large percentage of fine particles is thereby produced. In an embodiment gas at high pressure enters plenum chamber 20 through conduit 30, emerges from annular gas orifice 22 to impinge on the stream of molten metal descending through tube 12 and emerging from the end 16. <IMAGE>

Description

SPECIFICATION Method of atomization of meltfrom a closely coupled nozzle, apparatus and product formed BACKGROUND OFTHEINVENTION Rapid Particle Solidification This invention relates generally to the production of powders from a liquid melt by atomization and solidification. More particularly it relatesto the preparation of highertemperature materials in finely divided form by fluid atomization and to the apparatus in which such process is performed and the product obtained by the process.

For example it may be applied to the production of powders from melts ofsuperalloys.

Thereis a well established need for an economic means producing powders ofsuperalloys. Such powderscan be used in making superalloy articles by powder metallurgy techniques. The present industrial need forsuch powders is expanding and will continue to expand the demand for superalloy articles expands.

Presently only about 3% of powder produced industrially is smallerthan 10 microns and the cost of such powder is accordingly very high.

A major cost component offine powders, prepared by atomization and useful in industrial applications, is the cost of the gas used in the atomization. At present the cost ofthe gas increases as the percentage of fine powder sought in an atomized sample is increased.

Also as finer and finer powders aresoughtthe quantity of gas per unit of mass of powder produced increases. The gases consumed in producing powder, particularly the inert gases such as argon, are expensive.

There is at present a growing industrial demand for finer powders. Accordinglythere is a need to develop gas atomization techniques and apparatus which can increase the efficiency of converting molten alloy into powder, and to conserve the gas consumed in producing powder in a desired size range, particularly where the desired size range are growing smaller and smaller.

The production offine powder is influenced by the surface tension of the meltfrom which the fine powder is produced. For melts of high surface tension production of fine powder is more difficult and consumes more gas and energy. The present typical industrial yield offine powder of less than 37 micrometersaveragediameterfrom molten metals having high surface tensions is ofthe order of 25 weight % to about 40 weight %.

Fine powders of less than 37 micrometers (or microns) of certain metals are used in low pressure plasma spray applications. In preparing such powders by presently available industrial processes as much as 60-75% ofthe powder must be scrapped because it is oversize. This need to selectively remove only the finer powder and to scrap the oversize powder increases the cost of usable powder.

Fine powder also has uses in the quickly changing and growing field of rapid solidification materials.

Generally the larger percentage offiner powder which can be produced by a process or apparatus the more useful the process or apparatus is in rapid solidification technology.

It is known that the rate of solidification of a molten particle of relatively small size in a convective environment such as a flowing fluid or body of fluid material is roughly proportional to the inverse of the diameter ofthe particle squared.

The following expression is accordingly pertinentto this relationship.

o Tpos 1 Dp2 where Tp is the rate of cooling ofthe particle and Dp is the particle diameter.

Accordingly, if the average size of the diameter of the particles ofthe composition is reduced in halfthen the rate of cooling is increased by a factor ofabout four. If the average diameter is reduced in half again the overall cooling rate is increased sixteen fold.

It is desirable to produce powders of small particle size for some applications pa rticularly those in which the rate of cooling of the particle is significantto the properties achieved. For example there is a need for rapidlysolifified powders ofsizesmallerthan 37 microns and particularlyforthe production of such powders by economic means.

In addition, for certain applications it is important also to have particles which have a small spectrum of particle sizes. Accordingly, if particles of a 100 micron size are desired for certain applications a process which produces most ofthe particles in the 80-120 micron range would have a significant advantage for many applications of such particles as compared for example to a process which produces most particles in the 60 to 140 micron range. There is also a significant economic advantage in being able to produce powder having a known or predictable average particle size as well as particle size range. The present invention improves the capabilityforproduc- ing such powder on an industrial scale.

If particles of 100 micron size are produced by a first process from a given molten liquid metalfora given application, and it is then learned howto produce particles with a 50 micron average size, this second process would permit a much more rapid cooling and solidification of the particles formed from the same molten liquid metal. The present invention teaches a method by which smaller particles may be formed in higher percentage from melts, including molten liquid metal. A more rapid solidification rate of such particles is achieved by this novel process partly because the particles produced are themselves smaller on the average and also because the production is repeatable and reproducible on an industrial scale.

The achievement of small particle size is advantageousfor rapid cooling and forthe attendant benefits which derive from rapid cooling of certain molten materials. Novel amorphos and related properties may be achieved in this way. The present invention makes possiblethe production of powders with such small particle size with attendant rapid cooling.

The power metallurgy technology presently has a need forfine and ultrafine particles and particles in the size range of 1 Oto 37 microns in diameter. Particles having average particles in the particle size range of 10 micron to 37 micron are produced by this novel process ofthis invention.

The attainment of the smaller particle size may be found important in consolidation of the material by conventional powder metallurgy inasmuch as it has been observed that powder of smaller particle size can result in highersintering rate. Also it can be significant in the consolidation of the small particle size material with a material of larger particle size where such consolidation is found desirable based on higher packing density.

Present trends in powder metallurgy are creating great interest in fine metal powders, that is, in powders having diameters less than 37 microns in diameter and also in ultrafine powders specifically powders having diameters of less than 10 microns.

High surface tension in a melt material makes the formation ofsmallersize particles more difficult.

Conventional apparatus for producing powderfrom molten metals by atomization results in products depending on preparation methods and materials which have relatively broad spectra of particle sizes.

The broad spectra of particle sizes are represented in Figure 2 by the curves A, B, C and D. From examination ofthese curves it is evident that the particles range all the way from particle sizes of less than 10 micron to more than 100 microns. The percentage of particles of fine powder, i.e. less than 37 micron) produced by conventional technology is the range of about ~0 to 40%, and the percentage of ultrafine powder, i.e. less than '10 micron, produced is in the range of ~0-3%.

Because ofthe low yield ofthe smaller particle powder which is formed in such products the costofthe production of the ultrafine powder can be excessive ranging up to hundreds and even thousands of dollars per pound.

The graphs of Figure 2, and illustratively curve E of Figure 2, showsthatthe range of particle sizes produced by the methods ofthis invention when operated in a fine powder mode are significantly betterthan the particle size range of existing conventional processes. The data on which the curves A, B, C and D of Figure 2 is based is from a review article by A.

Lawly, "Atomization of Specialty Alloy Powders" which appeared in the January 1981 issue of Journal of Metals.

The data in the Journal of Metals article, and for the Curves A, B, C and D is for powderformed from melts of superalloys. The data from which Curve E was prepared was also data from the preparation of powder from a su peralloy melt so that the two sets of data are quite comparable.

It is known that there are large differences in the ease with which powder can be prepared from different families of alloys.

PARTICLE SIZE RANGES Figure 2 contains typical powder particle distribu tions for superalloy powders produced by different atomization technologies. Curve A isforArgon gas atomized powder. Curves B, C and Dare for powder produced by the rotating electrode process, rapid solidification rate process, and vacuum atomization, respectively.

The shaded area or band bordered by Curves E and F indicates the range of powder size distributions that are produced utilizing this invention when operated in the fine powder mode.

It is readily evident from the plot of the various curves of Figure 2 that the power prepared pursuantto the present process, and using the present apparatus has a range of particle sizes and cumulative particle sizes which are much smallerthan those prepared by the conventional methods particularly in the smaller size range of about60 microns and smaller.

The shaded area ofthe graph between lines E and F is an envelope displaying the region ofthe graph in which powder products may have been produced employing the methods and techniques ofthis invention to makefine powder.

From this chart it is evidentthatthe method ofthe present invention makes possible the formation of powder having between 10 and 37% of particles of 10 microns and underhand makes possible the formation of powders having between 44 and 70 cumulative percent of particles less than 37 microns.

Higheryield offine powder may be produced bythe methods and apparatus of the present invention than are produced by other gas atomization methods and devices because practice of the invention results in transfers of energy more efficiently from the atomizing gas to the liquid metal to be atomized. One way in which this improved production offines may be accomplished is by bringing the melt stream into unprecedented close proximity with the atomizing gas nozzle. This close proximity ofthe gas nozzle to the melt stream orifice is designated herein as close coupling. The advantages of the principle of close coupling has been recognized inthe literatureas discussed below, however, until now no invention has allowed the use of this principle for high temperature materials.This is due at least in part to the problem of accretion of solidified high temperature melt on the atomizing gas nozzle as well as elsewhere on the atomizing apparatus.

ACCRETION ON PRIOR ARTNOZZL ES A major problem associated with prior art gas atomization nozzles and methods has been the solidification of specks and globules ofthe atomized high temperature alloy on the nozzle surfaces. The resulting buildup on the nozzle has sometimes caused thetermination oftheatomization process. This termination has resulted from closing off of the hole through which the melt is poured or by at least partially diverting the atomizing gases from direct impingement at high energy onto the emerging stream of liquid metal. In severe cases the buildup of solid deposit atthe nozzle tip has caused the buildup deposit to break away from the nozzle.In such case the result has sometimes been a contamination ofthe powder being formed with material from the nozzle or from the melt delivery system.

In conventional apparatus the problem ofthe build up of solidified high temperature material at the gas nozzle or atthe molten metal orifice is solved by keeping the gas nozzle fairly remote from the atomization region as explained more fully below.

The problems of a progressive accretion of numerous specks and globules of solidified melt on the atomizing nozzle is mostacuteforthevery high temperature melts and particularly forthe molten metals which have high melting temperature.

LOWER TEMPERATURE PRIORARTATOMIZA TION There is a great deal of difference between the practices which may be employed with lowtempera- ture materials in forming sprays by means of impingement of streams of gas on streams of liquid and the phenomena which occurs at elevated temperatures. In general the idea of a low temperature spray may include materials which are liquid at room temperature and those which become liquid attemperatures u p to about 300"C. The atomization of materials at these lowertemperatures and particularly of materials which are liquid at room temperature is not attended bythe occlusion of frozen metal on the spray nozzle to anywhere nearthe degree which occurs when high temperature molten metals or other high temperature materials are employed.Accretion of lowertemperature material on an atomization nozzle does not lead to destruction of elements ofthe nozzle itself. Also at the lower temperatures there isfarless reaction and interaction between the metal being atomized and the melt deliverytube or the materials of other parts of the atomization nozzle. A metal melt deliverytube can be used to atomize materials at or below 300"C but ceramic delivery systems must be used at the higher temperatures of 1000 C, 1 500"C and 2000"C and above.

Another difference is that the thermal gradient through the wall of a melt deliverytubefrom the melt to the atomizing gas increases as the temperature of the meltto be atomized increases. For an atomization system of constant geometry greater gas flow is required as the heat ofthe melt is increased because of the greater quantity of heat to be removed. A greater quantity of gas per unit volume of melt atomized can cause greater tendency toward spattering and splashing of the melt in the apparatus. Where the melt is very hot, ofthe order of a thousand degrees centigrate or more a droplet can solidify and adhere instantly to a lowertemperature surface. At the higher temperatures materials are more active chemically and can form stronger bonds at surfaces which they contact than molten materials at lower temperatures.

CONVENTIONAL GAS ATOMIZATION REMOTE COUPLING While the Applicant does notwishto be bound by the accuracy ofthe representation or description which is given here it is believed that it will be helpful in bringing outthe nature and characterofthe present invention to provide a general description of atomization mechanisms as have been referred to and described in reference to the prior art and to provide a graphical representation ofthe phenomenon which occurs as prior art atomization takes place. For this purpose reference is made to Figure Awhich is a schematic representation of a prior art atomization phenomenon as it is understood to have occurred as prior art methods were employed.In thefiguretwo gas orifices 30 and 32 are shown positioned relativeto a melt stream 34 in a manner wh ich has been conventional in the prior art. Specifically the jet gas nozzles 30 and 32 are spaced a distance from the melt stream and are also angled so that they are directed toward the melt stream ata substantial distance from the nozzles. This figure is somewhat schematic and it will be understood that the nozzles 30 and 32 could in fact form a single annular nozzle surrounding the melt delivery apparatus and could be fed from a conventional gas plenum. The melt delivery apparatus 36 is also shown in a schematic form.

There is a phenomena recognized in the prior art of the formation of an inverted hollow cone in the melt stream as it descends to the area where the confluence of the gas from the respective gasjets 30 and 32 occurs. The point of confluence 38 is the point at which two center lines or aimpoints ofthe two streams of gas could meet if there were no interference between them. They do, however, act on the melt stream as it descends and part ofthis action is the formation of the inverted hollow cone illustrated at 40 in the figure.

The next phenomena which occurs in the conventional atomization process is the disruption of the cone wall into ligaments orglobules of melt. This phenomena occurs in the zone shown as 42 in the figure.

The next phenomena which occurs in conventional atomization is the breaking up or atomization of the ligaments into droplets. This isshown inthefigure as occurring generally in the zone below that in which the ligaments areformed.The individual droplets or particles are represented as formed from larger droplets or globules.

According to this schematic representation the conventional atomization is a multi-step multi-phenomena process, the first phenomena ofwhich is the formation ofthe inverted cone; and the second phenomana ofwhich is the disruption ofthe cone wall into the ligaments; and the third phenomena of which is the disruption of the ligaments into droplets.

So far as the droplet formation is concerned it is seen from this description to be a secondary phenomena in the sense that a very high percentage ofthe droplets are formed by disruption of the ligaments or globules.

The most definitive work on the remotely coupled atomization of liquid metals cited in the technical literature is entitled "The Disintegration of Liquid Lead Streams by Nitrogen Jets" by J. B. See, J. Rankle and T. B. King, Met. Trans. (1973) p.2669-2673 which describestheatomization phenomena based on studies made with the aid of speed photography.

What is distinct and novel about the process of the subject invention is that the process hasa greatly reduced secondary particle formation and has a very high degree of primary direct formation of particles immediately from the melt and without the need to go through a second stage of subdivision ofthe melt as is illustrated in schematically in Figure A and described above.

CONVENTIONAL ATOMIZATION LOSS OF GAS ENERGY To avoid having such high temperature droplets adhere to the portion ofthe apparatus which is cooled bythe gas supply mechanism, prior art high temperature atomization apparatus has supplied the gas from a jet objets which are relatively remote from the surface of the stream itself impacted by the jets.

Wherethe nozzle is remote from the atomization region there is an appreciable reduction in the energy ofthe gas as it moves from the nozzle from which it is delivered to the point of impact with the liquid metal to be atomized. There are substantial diffusion and entrainment losses at the gas traverses the distance from the nozzle to the melt stream. The energy loss has been estimated to be in excess of 90% ofthe initial energyforcertain designs ofthe molten metal atomizing equipment currently in use. Accordingly the processes employing gas jets remotefrom contact with a stream or body of molten material to be atomized are uneconomical in usage of gas as much gas is needed to overcome the loss of energy which occurs in the stream of gas before the molten metal stream is contacted.

Such remote coupling of a meltstreamto atomizing gas supply orifices are illustrated and described in U.S. Patents 4,272,463,3,588,951,3,428,718, 3,646,176,4,080,126; 4,191,516 and 3,340,338 although not described in terms of remote coupling.

DISCUSSION OF THE PRIOR ART Use of metal and even plastic nozzles having the gas jet very closely proximate the liquid supply tube or orifice has been known heretofore. For example atomization of liquid at room temperature can be accomplished without serious freezing and buildup of the liquid on the nozzle. Some paint spray nozzles for example havethistype of construction.

In the book entitled: "The Production of Metal Powders byAtomization" authored by John Keith Beddow and printed by Hayden Publishers, there is a reference made on page 45 to varioius designs of nozzlesforthe production of powder metal from a molten metal stream. Such atomization involves high temperature gas atomization.

The Beddow nozzles are annular nozzles in that they have a center portforthe developmentand delivery of a liquid metal stream. The gas is delivered from an annulargasjet surrounding the center port. The Beddow nozzles have a superficial similarity to that illustrated in Figure 1 ofthis specification. The problem of buildup on annular nozzles such as those disclosed in Beddow is pointed out immediately beneath the figures on page 45 as follows: "One important problem with annular nozzles is that of 'build-up' on the metal nozzle body. This is caused by splashing of molten metal onto the inside of the nozzle, especially near the rim atthe bottom. This splashed metal freezes, more liquid metal accretes and at some later stage ofthis process the jet of air causes the hot metal build-up to ignite.In this way the operator can lose a nozzle block rather easily." Thus although such nozzle design has been known, prior art practitioners of this art have not been able to overcome the problem recited by Beddow in the gas atomization of high temperature material and particularly metals.

Other sources of information on the configuration of nozzles for use in atomization technology are found in U.S. patents. In U.S. Patent 2,997,245 a method of atomizing liquid metal employing so-called "shock waves" is described.

In U.S. Patent 3,988,084 a scheme for generating a thin stream of metal on a hollow inverted cone and intercepting the stream by an annular gas jet is described. In the scheme of patent 3,988,084 the atomization gas stream is directed against only one side of the cone of molten metal, i.e. the exterior of the cone, and no gas is directed against the other side of the cone of molten metal, i.e. the inside surface ofthe cone of molten metal. In the practice of certain modes of the present invention atomizing gas is directed against all surfaces of the melt stream. The inverted cone ofthe 3,988,084 patent resembles the inverted coneformed during conventional remotely coupled gas atomization of a descending liquid metal stream described above in that the gas acts on only one side ofthe web of liquid metal atthe lower edge of the inverted cone.The web spreads overtheinverted cone to its edge and the gas sweeps metal from the edge into a hollow converging cone.

The inventor of this application prepared a thesis entitled "The Production and Consolidation of Amorphous Metal Powder" and submitted the thesis to the Department of Mechanical Engineering at Northeastern University, Boston, Massachusetts in September, 1980. The thesis describes the use of an annular gas nozzle with a ceramic and/or graphite metal supply tube. In this thesis improvements in the production of powder having a higher proportion of finer powder from the atomization of molten metal with an annular jet of gas is reported.

BRIEFSUMMARYOF THE INVENTION An object of the present invention is to produce fine metal powderdirectlyfromthe liquid state and without necessarily employing a secondary process such as commutating or otherwise subdividing mate rialformed initially in a ribbon orfoil or strip of similar solid state.

Another object is to produce powderfrom a melt with a substantially higher percentage offiner particles.

Another object is to produce powder directly of more uniform particle size.

Another object is to produce powder by gas atomization more efficiently.

Another object is to provide a method and apparatus for more efficient production of powder of desired particle size by gas atomization.

Another object is to produce powderfrom higher temperature melts at low cost.

Another object is to produce useful articles of powder derived from alloys which cannot be made by conventional techniques into useful articles.

Another object is to make possible production of powder by rapid solidification techniques for use in forming novel articles of manufacture.

Another object is to produce new and distinct powderfrom a melt by gas atomization and to do so economically.

Another object is to provide a method of limiting the accretion of melt on atomizing apparatus.

Another object is to provide a method which permits long term continuous runs of atomizing apparatus.

Other objects will be in partapparentand in part pointed out in the description which follows.

In one of its broader aspects the objects can be achieved by providing an atomization apparatus having a central melt delivery tube and having a gas orifice for supply of atomizing gas surrounding said tube, and closely coupling the gas orifice to the melt delivery tube and to its orifice to limitthe distance from the pointwhere the gas becomesfree flowing to the pointwherethe melt becomesfree flowing.

BRIEF DESCRIPTION OF THE FIGURES The description ofthe invention to follow will be better understood by reference to the accompanying drawings in which: Figure lisa vertical sectional view of one type of gas atomization nozzle useful in the practice ofthe present invention.

Figure 2 is a detail ofthe atomization tip as in Figure 1 illustrating certain dimensions A and B.

Figure 3 is a plot of certain parameters relating to particle size distribution of the cumulative fraction of particles in powder samples prepared by different methods.

Figure 4 is a schematic illustration of a prior art atomization phenomena.

DESCRIPTION OFA PREFERRED EMBODIMENT ILLUSTRATIVE ATOMIZATION NOZZLE Referring to Figure 1,there is illustrated in vertical section one form of a atomization nozzle 10 as provided pursuanttothe present invention. Numerous modificationsoftheformsofatomization nozzles may also be employed in practicing this invention, all as described elsewhere in this specification.

The nozzle 10 is illustrated as having an inner ceramic liner 12 having an upper end 14 into which liquid metal to be atomized is introduced, and a lower end 16from which the metalto be atomized may emerge as a descending stream. The lower end is provided with a lowertip 17 having tapered outer surface 18 in the shape of an inverted truncated cone.

The molten metal emerging from tube 12 at end 16 is swept by gas from an annular gas orifice portion ofthe nozzle 10. The annular gas jet is made up of gas streaming from a plenum chamber 20 downwardly through an opening 22 formed between an inner bevelled surface 24 and the inverted concical or beveled surface 18 of metal supply tube 12. The annular orifice or port 22, for exit of jets of gas may have surfaces formed in a beveled shape to conform generallytothe beveled surface 18 ofthe liner 12.

Accordingly, the opening 22 may be defined by the outer beveled surface 18 of liner 12, the corresponding beveled surface 26 of the lower portion of the annular gas plenum 20 and the confronting and opposite surface 24 on plate 32 forming the lower closure of plenum 20. The lower surface 18 of liner 11 forms one side of a small land 19. The other side of land 19 is formed by the melt orifice 15 also contained in 12.

By supplying a gas at high pressure through the gas conduit 30 from a source not shown, the gas enters the annular plenum chamber 20 and emerges from the annular gas orifice 22to impinge on the stream of molten metal descending through the tube 12 and emerging from the end 16 ofthe liner 1 2 at tip 17.

Exit surface 24 may conventiently be formed on the inner edge of a plenum closure plate 32. Plate 32 may have external threads to permit it to be threaded into the lower internallythreaded edge 36 of plenum housing sidewall 34. The raising and lowering of plate 32 by turning the plate to thread its inner edge further into or out of plenum 20 has the effect of moving surface 24 relative to surface 18 and accordingly opening or closing annular orifice 22 as well as raising the orifice relative to the lower tip 17 of melt delivery tube 12.

The plenum housing 34 is made up of an annulartop 38 having an integrallyformed inner shelf40. An annular cone 42, which may suitably be a ceramic, or metal, and is partofmeltguidetube 12, is supported from shelf 40 byflange 44.The shape of outersurface 26 of cone 42 is significant in forming the inner annular surface of plenum 20 from which gas is delivered to annular orifice 22. The outer surface 26 of cone 42 may be aligned with the outer conical lower end surface 18 oftube 12so that the two surfaces form one continuous conical surface along which gas from plenum 20 passes in being discharged through annular orifice 22.

As indicated tube 12 has bottom tip 17 and an outer lower surface 18 conforming to the inner surface 26 of annularcone42. It also hasa mid-flange 46 which permits its vertical location to be precisely determined and set relative to the overall nozzle 10 and to concial surface 26.

An upper annular ring 48 has an inner depending boss 50 which presses on flange 46 to hold the tube and cone parts of the device in precise alignment.

The means for holding the nozzle assembly in the related apparatus in which molten metal is atomized is conventional and forms no part ofthis invention.

The configuration and form of gas orifice useful in practice ofthe present invention is not limited to the form illustrated in Figure 1. For certain applications a nozzle in the form of a Laval nozzle will be preferred to control expansion of gas released from the orifice 22 of 1.

Furtherthe annular jet of gas need not be formed solely by an annular orifice although such orifice is preferred. Rather the annular jet can be created by a ring of individually supplied tubular nozzles each directed toward the melt surface. The gas of such a ring can form a single annulargas jet asthegasfrom the individual nozzles converge at or nearthe melt surface.

Furtherthe angle at which gas is directed from a gas orifice toward a melt stream surface is not limited to thatshown in the figure. While some angles are prepared for certain combinations of nozzle design and melt to be atomized, it is known that atomization can be accomplished with impingement angles from a fractiona I degree to ninety degrees. Applicant has found that atomization with a nozzle as illustrated in Fig. 1 at an angle of incidence of 22 is highly effective in producing higher concentrations of fine powder than prior art methods.

Fine particles may be produced from a melt employing a nozzle as described here approximately as described with reference to Figure 3 above.

ADVANTAGES OF SMALL PARTICLES For many metals which are atomized a more rapidly solidified droplet or particle will show an improve ment in some properties as compared to a more slowly cooled particle. As is pointed out in the background statement the rate of rapid solidification goes up as the particle size is going down. So finer powder involves getting increased solidification rates and not just finer powder per se. Finerpowderperse has other advantage over conventional materials.

With respect two getting higher solidification rates one ofthe common observances is a vast decrease in segregation ofthe constituents of an alloy from which the particle is formed. Forexample, as a result of that decrease in segregation one can raise the incipient melting point ofthe alloy. The incipient melting point is raised essentially because the present method makes possible a homogeneous nucleation event which means essentially that the solidification will occur virtually instantaneously so that the solidified front will move rapidly through the liquid material of the dropletwithout segregation occurring. The net effect ofthat is a homogeneous structure.By getting a homogeneous structure the difference between the liquidustemperatureofthe alloy and the solidus temperature of the alloy is reduced and ultimately they can appoach one another. The benefit of that is that ultimately the incipient melting is the solidus temperature. The melting temperature of such fine particles is increased and also the potential operating temperature of the alloy has been raised. With powder prepared in this manner and pursuant to the present invention one can achieve successful consolidation with improved properties with the consolidation techniques that exist today.

If in trying to consolidate a rapidly solidified fine amorphous powderbythetypesoftechniquesthat have been used in the past one goes above the transition temperature the material crystallizes. So one can'tconsolidatethe material and retain the amporphous structure for most amorphous alloys.

Some amorphous alloys have been consolidated but in the case ofsuperalloys, which remain crystalline in the rapidly solidified form, these have been consolidated and some increase or beneficial properties have been observed in the consolidated material and especially in rapidly solidifed tool steels. Such improved properties are achieved in articles prepared from rapidly solidified powders produced by the nozzles of this invention.

Considering a sample ofveryfinely divided powder, even if the effects of cooling rate are eliminated and just dealing in terms of particle size, the factthat each particle originates from the melt and assuming that the melt is homogeneous, and allowing segregation to occur if one has a very small particle one is going to see less segregation potentially than in a very large particle simply by the definition ofthe material available to segregate.

Secondlywith respectto advantages of small particle size it has been shown in the literature that smaller metal particles tend to sintersooner at lower temperatures and in shorter times than large powder particles. There is a greater driving force for the sintering process itself. That is an economic advantage.

Thirdly one ofthe problems associated with powder metallurgy is contamination ofthe powder by foreign objects. These foreign objects get mixed into the powderandthen pressed up intothe part and ultimatelyrepresenta potential failure site in the part.

If one has very fine powder the common beliefthat one can sift the powder and eliminate these big foreign objects so that by having a finer powder one can prepare a final specimen that will have potentially smaller defects in it than are obtained if coarse powder were used.

Further considering other advantages offine powder if it were available at economic prices as processed pursuant to this invention, if one assumes 10 micron spheres versus 100 micron spheres the packing factor is the same. Accordingly it is desirable to have another set of still smaller spheres to put into those voids. But there will be voids again between the smaller spheres and the big spheres so that one would like another set of smaller spheres to fill in the smaller voids essentially. 10 micron powder can serve this need.

A relatively new area that has evolved because of rapid solidification is the development of whole new series of alloys. Because of the slower solidification rates of conventional materials the constituents ofthe alloy segregate out as either brittle intermetallic compounds orals long grain boundaries. Such materials have properties which are inferior in some aspects to rapidly solidified material.

By means of rapid solidification some of these solute materials can be kept in solution and can act as strengtheners and as a result one is now looking at new alloy compositions through rapid solidification.

These same alloys when made through conventional practices may have to be discarded because they were brittle. However it is nowfound thatthese alloys have useful properties if rapidly solidified. This phonomena varies from alloy system to alloy system, solidification rate to solidification rate. Ultimately consolidation techniquesaffectwhetheryou can use the material or not as well.

An important feature ofthe present invention is that it permits the formation of powderfrom a melt with high efficiency in the utilization of gas. The improvemenu which is obtained is quite surprising in that the finely divided powder has a higher percentage of the fine particles and it might be reasonable to assume that in order to achieve such a fine subdivision a much higher gas flow would be needed. With a much higher gas flow there would of course be a reduction in the efficiency of gas utilization. However, surprisingly I have found that by the use of the processes taught in this speicification the gas utilized actually decreases when the very fine particles are produced in the higher percentage made possible by this invention compared to conventional processes.

PARTICLE SIZE PARAMETERS NARROWRANGE OF SIZES In general there is an advantage in having powders having fine particles of relatively uniform size or with a smaller range of sizes. This is becausethemore uniform size particles will have seen a more uniform cooling history. The more uniform cooling history translates into the particles being more uniform in metallurgical properties.

Also, generally the smaller size particles are more rapidly cooled particles as set forth in the equation in the introduction to this application. Where a wide range of particle sizes is present in a powder and the powder is processed through powder metallurgy techniquesthereisa limit on the desirable properties which can be imparted to a composition and this limit is related to the composition and properties ofthe larger particles ofthe powder which goes into the composition. The larger particles will constitute a potential weak spot or spot at which lower values of incipient melting or other lowervalue of properties will occur.

Asageneral rulethesmallertheparticlesizeandthe smallerthe average particle size and the more uniform the size of smaller particle powder of an ingredient powder used to form a solid objectthe more likely that the product obtained will have certain combination of desirable properties in solid objects prepared from the powder. Ideally if all particles formed were exactly 20 microns in diametertheywould all have seen essentially the same thermal history and the objects formed from these particles would have properties which wee characteristic of the uniform size particles from which they were formed.

It would, of course, be desirable to have larger particle bodies which have been rapidly solidified at the rates which are feasible with smaller pa rticie bodies. However, because ofthe internal segregation ofthe metallurgical ingredients which occurs within a larger particle body as the larger bodies are solidified, and because there is a limit on the rate at which heat can be removed from the larger particle bodies in order to achieve such solidification, the formation of such larger particle bodies from molten metal as powder is formed by conventional atomization techniques presents a limitation on the character of powder which can be produced by conventional techniques as well as a limitation on the uses which can be made of such powder in forming iarger bodies by powder metallurgy.The use of powder metallurgy techniques is presently the principle route by which superior products are acheived using powder subjected to rapid solidification. The present invention improves both the formation of such smaller particles and the formation of larger bodies with the higher desirable combination of properties of rapidly solidified metals.

Further, the articles formed have a more uniform set of properties because ofthe more uniform particle size of the particles ofthe powder from which the particle is formed.

One ofthe unique features ofthe technology made possible by the present invention is that it permits a closercontrolofa number of the parameters of a powder product produced by atomization as taught in this application.

For example, it has been found possible to alterthe somewhat random distribution of particle sizes which is found in the powder products of prior art processes to permit a greater concentration of particle size of a selected value.

Secondly for a selected particle size the possibility of producing a higher yield of the sire from a given run is made possible regardless of the size of particle which is selected. If, for example, particles of 10 micron is selected as the principle product size for a powder, the control ofthe variables ofthe subject invention will make possible an emphasis in the production of the particles of that selected size.

Alternatively if particles of 50 microns or 100 microns are selected as the desired product size then the process parameters can be altered in accordance with the teaching ofthis invention to produce powders which have higher concentrations ofthe particles within the selected size range.

By use of prior art processes it is possible to produce a wide range of particle sizes in any one lot orfrom any single run. The economic advantage, however, is in being able to produce a particle size with a relatively narrow standard deviation from a selected or preselected particlesizeproduct.Accordingly,thepresent invention makes possible the production of economically more valuable powders from a given run involving the consumption of a certain amount of energy and materials.

A derivative benefit of producing powder according to the teaching ofthis invention is that it not only makes possible the production of powderwith a relativelytightparticlesizedistribution but because of the tight distribution the particles will have a selected microstructure. Accordingly it is possible through use ofthis teaching to form particles having a relatively large particle size and a tight distribution of sizes within a given sample. The larger particles because they will have undergone slower cooling will have coarser crystalline structure than those which have more rapid cooling.

Alternatively, however, by selecting those conditions which produce the finer particle size it is possible to produce a powder which is amorphous because the smaller particles are cooled more rapidly as is explained above and also because there is a very tight size distribution around the preselected sizeforthe sample being produced.

PREFERRED EMBODIMENT ILLUSTRATIVE ATOMIZA I IUN An atomization zone is formed at the area of confluence ofthe molten metal stream and the annular stream of atomizing gas emerging from the annular opening 22 at the bottom ofthe gas supply plenum 28. Accordingly, the melt guide tube 12 delivers the liquid metal stream through the throat of the gas nozzletothe atomization zone. One feature of the present invention is the provision of a gas nozzle body which cooperates with a shaped end of a melt guide tube to form a gas nozzle having an annular gas jetwhich works in cooperation with the shaped exit end ofthe meltguidetube.

In otherwords, the provision of shaped and configured and cooperative ends at the lower part of the melt guide tube is one aspect ofthis invention as is explained morefully herein. As will be explained more fully below this is one of several independently functioning phenomena which are used in achieving superior atomization of a variety of melts.

The close positioning ofthe gas orifice and melt orifice permits the surface ofthe melt guide tube to form a partoftheannulargas orifice and by doing so permits the jet of gas emerging from the gas plenum to escape over the formed end ofthe melt guide tube.

This sweeping action ofthe gas jet on and against the lower end ofthe melt guidetube has been found to be effective in carrying off to a large degree particles of freezing orfrozen metal which mightotherwisetend to form orto deposit and accrete on the lower end of the melt guide tube. The Applicant has no knowledge that such particles do not in fact accrete on the lower end ofthe tube and it is known that such adherence occurred to prior art atomization nozzles as is discussed above relative to the Beddow reference.However, because ofthis measure which are taken in the practice ofthis invention, the adherence of such liquid orfrozen particles is reduced and there is an ability of the sweeping gas to either prevent deposit of such particles orto cause their removal once they are deposited or accreted on the lower end of the melt delivery tube.

In the particularconfiguration shown in Figure 1 there is a continuity, conformity and alignment between the formed lower surface ofthe melt guide tube 18 and the formed surrounding surface 26 of the gas supply plenum 20. It will be understood thatthe annular gas jet can, in fact, be made up in a numberof configurations and in a number of ways. However, the importantfeaturewhich mustbeprovidedpursuantto this aspect referred to herein as close coupling, is an annular gasjetwhich is at least in part formed by the lowerformedendofthe meltguidetubeand proximate to the melt surface.

MECHANISM OF CLOSE COUPLEDATOMIZATION Author R. D. Ingebo in his paper on the atomization of liquids, National Aeronautics and Space Administration, Technical Paper Number 1791, has shown that a liquid body in a high velocity gas medium has waves formed at its surface and that a disruption of the liquid body occurs as high speed gas shears the liquid from the waves and from the crests ofthe waves and removes the material as droplets. By progressive action ofthe high velocity gas across the surface ofthe liquid body the body of liquid is disintegrated into droplets.

I have found that the body of liquid may be a free flowing stream of liquid melt. Further I have found that a large fraction of the stream may be disintegrated into tiny droplets directly. I have used high speed photographytaken at about 35000 frames per second and have observed that plume of very fine particles is emanated from a free flowing melt which is subjected to high velocity gas according to the close coupled atomizationtechniqueofmyinvention.

I havefurther observed thatthe atomization can be carried out with gas flowing concurrent to the flow of melt andthatthe atomization does not depend on the multistep phenomena described above with reference to Figure 4. Additionally I have found by my high speed photographic observations that no inverted hollow cone such as illustrated in Figure 3 is formed downstream ofthe nozzle and that there is no initial formation of segments or globules of melt from the web of such cone as the first step of an atomization to be followed than by further and subsequent steps as described above in reference to Figure 4.

I have further observed that the atomization occurs to a very large degree at the gas nozzle tip and may be completed atthe nozzle tip for relatively thinner melt streams.

In carrying out the process of the present invention due care must be given to the relation between the velocity of the gas and the success of the close coupling atomization ofthe melt stream.

In orderto induce the acceleration waves on the liquid body surface and to induce the single step droplet generation process ofthis invention as contrasted with the mu Itistep processes of conventional atomization, an instability criteria must be met so that the liquid body will become unstable and will break up. The instability criteria are defined in a relationship which factors in gas density, relative velocity between gas and liquid body, the largest stable droplet size and thesurfacetension of the liquid.

The instability criteria which is used is known as the Weber instability criteria and for a given numerical value of the criteria the relationship is as follows: We = pV2 D a where We = Webernumber p = gas density V = relative velocity between the gas and liquid D = largeststable droplet size, and a = liquid surface tension When the Weber number is in excess of approximately 2.1 X 103 the liquid disintegrates by the process offormation of acceleration waves on the liquid surface. The disruption process then proceeds by the high speed gas shearing the crests off these waves to form droplets.The droplets are formed directly and do not undergo cone web formation, or ligament formation from the web, or shattering ofthe ligaments or globules to produce fine droplets.

The importance ofthe acceleration wave phenomena as used in connection with the atomization of molten liquid by a gas is that it permits a high energy or high intensity disruption of the body of molten liquid into small particles. This is particularly impor tantwhere the surface tension of the liquid ofthe molten body is higher. For example in the case ofthe braking up of a drop into droplets, the atomizing ofthe drop is made more difficult because of strong cohesive forces acting at the surface ofthe drop acting to hold the drop into its integral form and state.

Generally if the process is carried out effectively employing the acceleration wave phenomena as applied by this invention once a droplet is formed from the larger liquid body the droplet remains as such and is not recombined with other droplets or bodies by coalescence.

The disruption of the liquid body by the gas while the gas has a high energy content is deemed to be responsibleforthe effectiveness ofthe present process in generating a higher percentage of smaller particles.

Surprisingly the applicant has found however that it is not necessary to use ultimate feasible speeds or energies in the atomizing gas. Rather what is necessary and advisable is to ensure that there is a delivery ofthegas into the liquid body and into impingement with the surface of the body with the high energy or high gas momentum.

Also it has been foundthattheangle ofimpinge- ment ofthe gas onto a surface ofthe liquid to be atomized is not as important as the impingement of the gas at the surface while at a high energy level.

It is further desirable to cause the gas to impinge on the molten liquid before the gas has undergone a substantial degree of lateral expansion and in fact to introduce the gas into the melt so that it can undergo at least a substantial part of its lateral expansion after the gas stream has impinged on the molten body.

MECHANISM In general one reason why the nozzle of the present invention and the method by which it is operated are so successful in achieving production of very fine and ultrafine particles of metal and other substances with relatively narrow spectrum of particle sizes is that a combination is provided to include a shaped melt supply tube working in combination with a gas supply plenum. The plenum and melt guidetube deliver a confluence of a molten metal at and into the path of an annulargasjetformed at least in part bythe lower end of the melt supply tube. In other words, the object which formsthe lowermost end ofthe meltsupply tube also forms the lowermost end of the annular gas orifice.

Furtherthe lower end ofthe melt guide tube is preferablyquitethinsothatthereisaveryfineedgeof material separating the melt from the atomizing gas at the location where the gas impacts the melt stream.

Such fine edge can preferably be achieved as the end of a wedge i.e. the lower end of the melt delivery tube and gas delivery plenum has a cross-section which is wedge shaped with the point of the wedge providing the location where the gas stream and melt stream meet. In otherwords, the confluence of gas and melt occurs atthe point ofthe wedge but the gas is not in simple laminarflow and is effective in disrupting the metal stream and atomizing the melt.

A still more preferred form of the lower end of the melt delivery tube is one in which the inner surface of the wedge is vertical and the outer surface extends out from the lower point at some acute angle to the inner surface. This configuration induces the gas to pass overthe gas delivery surface toward the melt in a direction which causes itto penetrate into the liquid melt exiting from the melt delivery tube.

To make the gas impact on the melt even on both sides ofthe melt stream as illustrated in the figures or in practice on all sides of the stream and to permit symmetrical atomization from each side, orfrom all sides, of the melt, a generally vertically descending melt stream is preferred such as would emerge from the melt delivery tube of Figure 1. However it will be understood that the same nozzle as illustrated in Figure 1 can be employed in other orientations with beneficial results and that other nozzles as provided by the subject invention can also be employed in other orientations including a vertical up orientation.

Partofthedesign conceptofthegas nozzles provided pursuantto this invention is that the surfaces which are potentially exposed to agglomerated material and buildup of such material are continuously swept clean bythe atomizing gas.

One ofthe most important controls in the construction of the atomization nozzle as provided in this invention is given with reference to Figure 2. As is evidentfrom this figure there is a dimension labelled "A" on the Figure between the tip of the melt delivery tube and the tip of the outer surface ofthe gas delivery orifice. The dimension of the "A" in some conventional nozzles is between 2 and 4 inches. Preferably pursuantto the present invention the dimension of "A" should be quite small and preferably ofthe order of 0.15 inches to 0.0 inches. The smaller the dimension "A" the more the nozzle is said to provide "close coupling" between the gas nozzle and the surface of the meltto be atomized.

The specific design of atomization nozzles which have been tried include an atomization nozzle having a graphite melt delivery tube as well as nozzles having melt deliverytu bes formed of pressed boron nitride. It is contemplated thattubes maybeformedfrom composite materials as for example a melt guide tube having an inner alumina linerwhich is encased in a ceramic sleeve in orderto isolate the ceramic from the melt and at the same time protect the alumina from the cold atomizing gas.

Figure 2 is a detail of the tip of the atomization nozzle of Fig. 1.Two distances A and B are shown schematically in the Figure bytwo headed arrows.

ThefirstdistanceA isthe shortest distance between the gas orifice and the surface ofthe melt stream which is first encountered bythe gas jet emanating from the gas orifice. In relation to the streams it is the distance from a point where the gas first becomes a freeflowingstream and isfirst releasedfrom the containment of the nozzle to the point where the molten metal first becomes a free flowing stream and is first released from the containment of the melt delivery tube.

The second distance B represents a segment of an aim line extending from the approximate middle of the orifice to the approximate middle ofthe melt stream to be atomized and in a direction along which the gas jet emitted from the orifice is aimed. It is a distance along the aim line from each portion of the annular orifice and extends from the orifice to the point where the converging aim lines intersect.

The distance B orthe length of line B is greater than that of line A partly because length B or line B extends to the midpoint of the melt stream whereas line A extends only to the outersurface of the melt stream.

Distance A is peferably between 0.0 inches and 0.250 inches, and preferably less than 0.150.

Distance B is larger than distance A and is between 0.0 and 0.6.

Also the distance B minus A is preferably less than 0.350.

Another difference between the distance A and distance B is the point in the gas jet where the distance is measured.

Distance A is measured along the surface of the nozzle whereas distance B is measured along the midline of the gas jet.

While the distance A is measured along the surface ofthe nozzle it is not limited to the distance on the nozzle. This is because the actual nozzle construction is not ideal. If nozzle construction were ideal the surface along the external surface of melt delivery tube 12 would end in a point 17 which had no radius or flat lower surface. Actual nozzles do have a radius or land atthe point where the external surface 18 ofthe meltdeliverytube meets the internal surface ofthe tube. As a practical matter it is preferred to avoid having the tip of the melt delivery tube so thin that it is subjectto cracking or breaking.The degree to which the tip ofthe deliverytube can be broughtto a fine cutting edge depends on the material of which it is constructed and the thermal and otherforces to which it is to be subjected in actual operation.

Accordinglythe distance A includes the distance along the outertapered surface of melt delivery tube 12 and the extension ofthis distance past the end of the tube to the surface ofthe melt emerging from the tube.

Close coupling may be defined as keeping the distance traversed by the gas stream between the gas orifice and the melt stream small enough so that the gas loses substantially no energy prior to impacting the melt stream.

It is known that the distance atwhich the velocity of a free stream orjet of gas undergoes attenuation is principally a function of the jet size orthe size of the orificefrom which the jetflows. Accordinglythe allowable distance at which close coupling can be accomplished increases as the diameter ofthe gas jet increases.

Economic considerations ofthe desired rate of production of powder, the cost of gas, rate of gas consumption and like factors determinetheactual size of a closely coupled gas jet be used. Howeverthe present invention makesfeasiblethe economic production of fine powder at a variety of production rates.

In factthe process is quite versatile in permitting economic production of powder at small rates as well as economic production of powder at intermediate and also at high rates by suitable adjustment of the process parameters as taught herein.

For moderate rates of gas consumption, fine powder can be produced effectively with a realistic gas size orifice of less than about 1 mm where an apparatus as illustrated in Figure 1 is employed. For a nozzle gap of 1 mm or less the close coupling separation distance for practical nozzles is less than 7.9 mm.

The distance which a jet of gas from a nozzle of a given size can travel before losing significant energy is separate and distinct from the distance a gas can travel across a solid surface, parallel to the direction of travel ofthegas, withoutformation of a turbulent boundary and attendant eddy currents in the gas. For a nozzle as illustrated in Fig. 1,with a gas nozzle opening of about 1 mm, the distance which the converging gas can travel overthe exterior ofthe melt delivery tube without formation of a turbulent boundary layer sufficient in thickness to result in melt accretion on the nozzle tip has been observed to be of the orderin some instances of about 0.450 inches when Argon, at a plenum pressure of 4.2 MPa, has been employed as the atomizing gas.

Claims (14)

1. A process for atomization of a high temperature melt which comprises forming a descending stream of said melt, forming said melt to an atomization zone through a melt delivery tube, forming a tapered end on the lower exterior of said tube, providing an annular gas orifice around the tapered lower end of said tube, supplying gas to said orifice, causing said gas to flow over and againstthe tapered end of said tube and into contact with melt emerging therefrom, and maintaining the distance from the gas orifice to the melt at less than about 0.45 inches.

2. The method of claim 1 wherein the melt is a high melting metal.

3. The method of claim 1 wherein the maintained distance is less than 0.25 inches.

4. The method of claim 1 wherein the maintained distance is about 0.1 inch.

5. A high temperature atomization nozzle comprising a meltdeliverytube having a melt delivery orifice, a gas delivery system surrounding the melt delivery tube and adapted to deliver gas from a gas delivery orifice to a melt stream emerging from said melt orifice, said gas delivery tube having a beveled external surface which extends from the tip of said tube into said gas delivery orifice, the gas delivery orifice being closely coupled to the melt delivery orifice.

6. The method of producing higher proportions of fine powderfrom molten metal which comprises providing a source of molten metal to be atomized, supplying said molten metal through a delivery vessel to an orifice to form a free flowing stream emerging from said orifice, providing a source of pressurized gas and a gas supply manifold extending around said delivery vessel, providing an annular nozzle forflow of an annular jet of gas from said nozzle, closely coupling the pointwheresaid jetstream of gas becomes free flowing and the point where said liquid metal stream emerges from said nozzle, directing said annularjet of gas against and into all surfaces of said molten metal stream as it emerges from said delivery orifice.

7. The method of claim 6 wherein the close coupling distance is 0.0 to 0.45 inches.

8. A high temperature atomization nozzle comprising a melt delivery tu be having a melt delivery orifice, a gas delivery system surrounding the melt delivery tube and adapted to deliver gas from a gas delivery orifice to a melt stream emerging from said melt orifice, said gas delivery tube having a beveled external surface which extends from the tip of said tube into said gas delivery orifice.

9. A nozzle for producing ultrafine powder in high proportion from molten high temperature metal which comprises a molten metal delivery tube of a ceramic material, a gas orificefordelivery ofajetofgastothe surface of a stream of metal from said delivery tube, the gas orifice being disposed immediately proximate the surface of a stream of molten metal from said tube.

10. An apparatusforforming a fine powderfrom a meltofa substrate with a high melting point, comprising meansforholding a bodyofthe melt, means for delivering a stream of the melt to an atomization zone, meansfordelivering an atomizing gasto said stream in said atomization zone, the orifice of said gas delivery means being closely proximate the surfaceofsaid melt in said zone.

11. The method of producing fine powder of less than 37 it in diameterfrom molten metal in high percentage which comprises introducing a stream of molten metal into an atomization zone, directing a jet of high velocity gas into said zone and against said stream of molten metal to form an acceleration wave at a surface of said stream to atomize said stream, and collecting the powderformed by the atomization.

12. The method of claim 10inwhichthedistance between the molten metal stream supplysource and the high velocity gas source is keptto a minimum so that the stream of molten metal is atomized as it is formed.

13. An article of manufacture comprising a body of high melting metal of less than full density made from consolidating powder said body having minute zones of relatively uniform properties, said zones having an average diameter of less than ten microns.

14. The article of claim 13 wherein the average diameter is less than five microns.