Note: Descriptions are shown in the official language in which they were submitted.
RD 14~363
-- 1 --
ME_HOD OF ATOMIZATION OF MELT FROM A CLOSELY
COUPLED NOZZLE~!_ APPARATUS AND PRODUCT FORMED
BACKGROUND OF_THE INVENTION
Rapid Particle Solidification
This invention relates generally to the
production of powders from a liquid melt by
atomization and solidification. More particularly it
relates to the preparation of higher temperature
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 of superalloys.
There is a well established need for an
economic means of producing powders of superalloys.
Such powders can be used in making superally articles
by powder metallurgy techniques. The present
industrial need for such powders is expanding and will
continue to expand as the damand for superally
articles expands.
Presently only about 3~ of powder produced
industrially is smaller than 10 microns and the cost
of such powder is accordingly very high.
A major cost component of fine powders,
prepared by atomization and useful in industrial
applications, is the cost of the gas used in the
RD 14~363
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atomization. At present the cost of the gas increases
as the percentage of fine powder sought in an atomized
sample is increased. Also as finer and -finer powders
are sought the quantity of gas per unit of mass of
powder produced increases. The gases consumed in
producing powder, particularly the inert yases such as
argon, are expensive.
There is at present a growing industrial
demand for finer powders. Accordingly there 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 of fine powder is influenced
by the surface tension of the melt from 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 of fine powder of less than 37
micrometers average diameter from molten metals having
high surface tensions is of the order o-f 25 weight %
to about 40 weight %.
Fine powders of less -than 37 microme-ters (or
microns) of certain metals are used in low pressure
plasma spray applications. In preparing sucll powders
by presently available industrial processes as much
as 60-75% of the 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 of finer
powder which can be produced by a process or apparatus
RD 14~63
-- 3
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 of the particle squared.
The following expression is accordingly
pertinent to this relationship:
Tp ~ 1
D
where
T is the rate of cooling of the particle
and
D is the particle diameter.
Accordingly, if the average size of the
diameter of the particles of the composition is
reduced in half then the rate of cooling is increased
by a factor of about 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 particularly those
in which the rate of cooling of the particle is
significant to the properties achieved. For example
there is a need for rapidly solidified powders of size
smaller than 37 microns and particularly for the
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 of the
RD 14863
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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
siæe range. The present invention improves the
capability for producing such powder on an industrial
scale.
If particles of 100 micron size are produced
by a first process from a given molten liquid metal
for a given application, and it is then learned how to
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
advantageous for rapid cooling and for the attendant
benefits which derive from rapid cooling of certain
molten materials. ~ovel amorphous and related
properties may be achieved in this way. The present
invention makes possible the production of powders
with such small particle size with attendant rapid
cooling.
The powder metallurgy technology presently
has a need for fine and ultrafine particles and
particles in the size range of 10 to 37 microns in
diameter. Particles having average particles ln the
~D~
RD 14863
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particle size range of 10 micron to 37 micron are
produced by this novel process o-f this invention.
'rhe 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 higher sintering 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 of smaller size particles more difficult.
ACCRETION ON PRIOR ART NOZZLES
A major problem associated with prior art
gas atomization nozzles and methods has been the
solidification of specks and globules of the atomized
high temperature alloy on the nozzle surfaces. The
resulting buildup on the nozzle has sometimes caused
the termination of the atomization 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 yases from direct
impingement at high energy onto the emerging stream of
liquid metal. In severe cases the buildup of solid
deposit at the nozzle tip has caused the buildup
deposit to break away from the nozzle. In such case
the result has sometimes been a contamination of the
powder being formed with material from the nozzle or
from the melt delivery system.
In conventional apparatus the problem of the
~z~
RD 14863
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build up of solidified high temperature material at
the gas nozzle or at the molten metal orifice is
solved by keeping the gas nozz:Le fairly remo-te from
the atomization region as explained more fully below.
The problems of a prvgressive accretion of
numerous specks and globules of solidified melt on the
atomizing nozzle is most acute for the very higll
temperature melts and particularly for the molten
metals which have high metling temperatures.
LOWER TEMPERATURE PRIOR ART ATOMIZATION
There is a great deal of difference between
the practices which may be employed with low
temperature 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 low temperature spray may
include materials which are liquid at room temperature
and those which become liquid at temperatures up to
about 300C. The atomization of materials at these
lower temperatures and particularly of materials which
are liquid at room temperature is not attended by the
occlusion of frozen metal on the spray nozzle to
anywhere near the degree which occurs when high
temperature molten metals or other high temperature
materials are employed. Accretion of lower
temperature material on an atomization nozzle does not
lead to destruction of elements of the nozzle itself.
Also at the lower temperatures there is far less
reaction and interaction between the metal being
atomized and the melt delivery tube or the materials
of other parts or the atomization nozzle. A metal
melt delivery tube can be used to atomize materials at
or below 300C but ceramic delivery systems must be
used at the higher temperatures of 1000C, 1500C
and 2000C and above.
Another difference is that the thermal
RD 14863
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gradient throuyh the wall of a melt delivery tube from
the melt to the atomizing gas increases as the
temperature of the melt to be atomized lncreases. For
an atomization system of constant geometry greater gas
flow is required as the heat of the 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, of the order
of a thousand degrees centigrade or more a droplet can
solidify and adhere instantly to a lower temperature
surface. At the higher temperatures materials are
more active chemically and can form stronger bonds at
surfaces whcih they contact than molten materials at
lower temperatures.
CONVENTION ATOMIZATION
LOSS OF GAS ENERGY
To avoid having such high temperature
droplets adhere to the portion of the apparatus which
is cooled by the gas supply mechanism, prior art high
temperature atomization apparatus has supplied the gas
from a jet or jets which are relatively remote from
the surface of the stream i-tself impacted by the jets.
l~here the nozzle is remote from the
atomization region there is an appreciable reduction
in the energy of the 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 as the
gas traverses the distance from the nozzle to the melt
stream. The energy loss has been estimated to be in
excess of 90% of the initial energy for certain
designs of the molten metal atomizing equipmerlt
currently in use~ Accordingly the processes employing
gas jets rernote from contact with a stream or body of
RD 14863
-- 8
molten material to be atomized are uneconomical in
usage of gas as much gas i5 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 melt stream to
atomizing gas supply orifices are illustrated and
described in U.S. Patents 4,272,463; 3,588,951;
3,428,718; 3,646,176; 4,0~0,126; 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 ~laving
the gas jet very closeiy 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 build up
of the liquid on the nozzle. Some paint spray nozzles
for example have this type of construction~
In the book entitled "The Production of
Metal Powders by Atomization" authored by John Keith
Beddow and printed by Hayden Publishers, there is a
reference made on page 45 to various designs of
nozzles for the 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 port for the development and
delivery of a liquid metal stream. The gas is
delivered from an annular gas jet surrounding the
center port. I'he Beddow nozzles have a superficial
similarity to that illustrated in Figure 1 of this
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
2~
RD 14863
~ _
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 at the bottom. This
splashed metal freezes, more
liquid metal accretes and at
some later stage of this
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 aginst 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 of the cone of molten metal. In the practice
of certain modes of the present invention atomizing
gas is directed against all surfaces of the melt
RD 14863
-- 10 --
stream. The inverted cone of the 3,988,084 patent
resembles the inverted cone formed during conventional
remotely coupled gas atomization of a descending
liquid metal stream described above in that the gas
acts on only one side of the web of liquid metal at
the lower edge of the inverted cone. The web spreads
over the inverted 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 Depart~ent 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.
BRIEF SUMMARY OF THE INVENTION
An object of the present invention is to
produce fine metal powder directly from the li~uid
state and without necessarily employing a secondary
process such as commutating or otherwise subdividing
material formed initially in a ribbon or foil or strip
of similar solid state.
Another object is to produce powder from a
melt with a substantially higher percentage of finer
particles.
30Another 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.
3~
RD 14863
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Another object is to produce powder from
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 powder from a melt by gas atomization and to
do so ~conomically.
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 part apparent and
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
have 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 limit the distance
from the point where the gas becomes free flowing to
the point where the melt becomes free flowing.
BRIEF DESCRIPTION OE' THE FIGURES
_
The description of the invention to follow
will be better understood by reference to the
accompanying drawings in which:
Figure 1 is a vertical sectional view of one
type of gas atomization nozzle useful in the practice
of the present invention.
Figure 2 is a detail of the atomization tip
as in Figure 1 illus-trating certain dimensions A and B.
RD 1~863
- 12 -
Figure 3 is a plot oE 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.
Figure 5 is an elevational of an alternative
melt delivery tube for inclusion in the apparatus of
Fig. 1.
Figure 6 is a side elevational view of the
tube of Figure 5.
Figure 7 is a bottom plan view of the tube
of Figure 5 illustrating the slot form of orifice.
Figure 8 is a view as in Figure 7
illustrating a cross form of orifice.
Figure 9 is a vertical section of an
atomization nozzle as in Fig. 1 but slightly modified.
Figure 10 is also a slightly modified
atomization nozzle.
DESCRIPTIO~ OF A PREFERRED EMBODIMENT
. .
Conventional apparatus for producing powder
from 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 3 by the curves A, B, C and D. From
examination of these 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 of the low yield
of the smaller par-ticle powder which is formed in such
products the cost of the production of the ultrafine
RD 14863
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powder can be excessive ranging up to hundreds and
even thousands of dollars per pound.
The graphs of Fiyure 3, and illustratively
curve E of Figure 2, shows that the range of particle
sizes produced by the methods of this invention when
operated in a fine powder mode are significantly
better than the particle size range of exsiting
conventional processes. The data on which the curves
A, B, C and D of Figure 3 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 powder formed
from melts of superalloys. The data from which Curve
E was prepared was also data from the preparation of
powder from a superalloy 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.
PA~TICLE SIZE RANGES
Figure 3 contains typical powder particle
distributions for superalloy powders produced by
different atomization technologies. Curve A is for
Argon gas atomized powder. Curves B, C and D are 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 distri~utions
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 3 that the powder prepared
pursuant to the present process, and using the present
~ #~
RD 14863
- 14 -
apparatus has a range of particle sizes and cumulative
particle sizes which are much smaller than those
prepared by the conventional methods particularly in
the smaller size range of about 60 microns and smaller.
The shaded area of tlle grapih between lines E
and F is an envelope displaying the region of the
graph in Wili ch powder products may have been produced
employing the methods and techniques of this invention
to make fine powder.
From this chart it is evident that the
method of the present invention makes possible the
formation of powder having between 10 and 37~ of
particles of 10 microns and under and makes possible
the formation of powders having between 44 and 70
cumulative percent of particles less than 37 microns.
Higher yield of fine powder may be produced
by the 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 of fines may be
accomplished is by bringing the melt stream into
unprecendented close proximity with the atomizing gas
nozzle. This close proximity of the gas nozzle to the
melt stream orifice is designated herein as close
coupling. The advantages of the principle of close
coupling has been recognized in the literature as
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.
RD 14863
- 15 -
CONVENTIONAL GAS ATOMIZATION
REMOTE COUPLING
l~hile the Applicant does not wish to be
bound by the accuracy of the representation or
description which is given here it is believed that it
will be helpful in bringing out the nature and
character of the present inventon 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 of the
phenomenon which occurs as prior art atomization takes
place. For this purpose reference is made to Figure 4
which is a schematic representation of a prior art
atomization phenomenon as it is understood to have
occurred as the prior art methods were employed. In
the figure two gas orifices 30 and 32 are shown
positioned relative to a melt stream 34 in a manner
which 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 at a
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 gas jets 30
and 32 occurs. The point of confluence 38 is the
point at which two center lines or aimpoints of the
two streams of yas could meet if there were no
interference between them. They do, however, act on
RD 14863
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the melt stream as it descends and part of this action
is the formation of the inverted hollow cone
illustrate at 40 in the figure.
The next phenomena which occurs in the
conventional atomization process is the disruption of
the cone wall into ligaments or globules of melt.
This phenomena occurs in the zone shown as 42 in the
figure.
'l'he next phenomena which occurs in
conventional atomization is the breaking up or
atomization of the ligaments into droplets. This is
shown in the figure as occurring generally in the zone
below that in which the ligaments are formed. The
individual droplets or particles are represented as
formed from larger droplets or globules.
According to this schematic representation
the conventional atomization is a mul-ti-step
multi-phenomena process, the first phenomena of which
is the formation of the inverted cone; and the second
phenomena of which is the disruption of the 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 of
the droplets are formed by disruption of the ligaments
or globules.
'rhe 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.S. See,
J. Rankle and T.B. King, Met. Trans. 4 (1973)
p. 2669-2673 which describes the atomization phenomena
based on studies made with the aid of speed
photograph~.
~Vhat is distinct and novel about the process
RD 14863
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of the subject invention is tha-t the process has a
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 of
the melt as is illustrated schematically in E'igure 4
and described above.
ILLUSTRATIVE ATO~IZATION NOZZLE
Referring to Figure 1, there is illustrated
in vertical section one form of an atomization
nozzle 10 as provided pursuant to the present
invention. Numerous modifications of the forms of
atomization nozzle 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 16 from which the metal to be atomized may
emerge as a descending stream. The lower end is
provided with a lower tip 17 having tapered outer
surface 1~ in the shape of an inverted truncated
cone. The molten metal emerging from the tube 12 at
end 16 is swept by gas from an annular gas orifice
portion of the 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 beveled surface 24 and the inverted conical or
beveled surface 18 of metal supply tube 12. The
annular orifice or port 22, for exit of jets may have
surfaces formed in a beveled shape to conform
generally to the beveled surface 18 of the liner 12.
Accordingly, -the opening 22 may be defined by the
outer beveled surface 18 of liner 12, the
corresponding bevel surface 26 of the lower portion of
tlle annular gas plenum 20 and the confronting and
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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 smail 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 22 to impinge on the stream of
molten metal descending through the tube 12 and
ernerging from the end 16 of the liner 12 at tip 17.
Exit surface 24 may conveniently 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 internally threaded 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 annular top 38 having an integrally formed inner
shelf 40. An annular cone 42, which may suitably be a
ceramic, or metal, and is part of melt guide tube 12,
is supported from shelf 40 by flange 44. The shape of
outer surface 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 of tube 12 so 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
2~
RD 14863
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an outer lower surface 1~ conforming to the inner
surface 26 of annular cone 42. It also has a
mid-flange 46 which permits its vertcial location to
be precisely determined and set relative to the
S overall nozzle 10 and to conical surface 26.
An upper annular ring 4~ 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 of this
invention.
The configuration and form of gas orifice
useful in practice of the present invention is not
limited to the form illustrated in Figure 1. For
certain appllcations a nozzle in the form of a Laval
nozzle will be preferred to control expansion of gas
released from the orifice 22 of Fig. 1.
Further the 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 annular gas jet
as the gas from the individual nozzles converge at or
near the melt surface.
Further the angle at which gas is directed
from a gas orifice toward a melt stream surface is not
limited to that shown 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 fractional degree to ninety degrees.
Applicant has found that atomization with a nozzle as
illustrated in Fig. 1 at an angle of incidence of 22
RD 14i363
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is highly effective in produciny higher concentrations
of fine powder than prior art methods.
Fine particles may be produced from a melt
employing a noæzle 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 solidifed droplet or particle will shown an
improvement 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. Finer powder per se
has other advantages over conventional materials.
With respect to getting higher
solidification rates one of the common observances is
a vast decrease in segregation of the constituents of
an alloy from which the particle is formed. For
example, as a result of that decrease in segregation
one can raise the incipient melting point of the
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 droplet
without segregation occurring. The net effect of that
is a homogeneous structure. By getting a homogeneous
structure the difference between the liquidus
temperature of the alloy and the solidus temperature
of the alloy is reduced and ultimately they can
approach 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
RD 14863
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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 tryping to consolidate a rapidly
solidified fine amorphous powder by the types of
techniques that have been used in the past one goes
above the transition temperature the material
crystallizes. So one can't consolidate the material
and retain the amorphous structure for most amorphous
alloys. Some amorphous alloys have been consolidated
but in the case of superalloys, 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 solidified tool
steels. Such improved properties are achieved in
articles prepared from rapidly solidified powders
produced by the nozzles of this invention.
Considering a sample of very finely divided
powder, even if the effects of cooling rate are
eliminated and just dealing in terms of particle size,
the fact that 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
pontentially than in a very large particle simply by
the definition of the material available to segreyate.
Secondly with respect to advantages of small
particle size it has been shown in the literature that
smaller metal particles tend to sinter sooner 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.
RD 14863
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Thirdly one of the problems associated with
powder metallurgy is contamination of the powder by
foreign objects. These foreign objects get mixed into
the powder and then pressed up into the part and
ultimately represent a potential failure site in the
part. If one has very fine powder the common belief
that 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 of fine
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 of the alloy segregate out as either
brittle intermetallic compounds or as 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
RD 14863
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conventional practices may have to be discarded
because they were brittle. However it is now Eound
that these alloys have useful properties if rapidly
solidified. This phenomena varies from alloy system
to alloy system, solidification rate to solidification
rate. Ultimately consolidation tec'rlniques affect
whether you can use the material or not as well.
An important feature of the present
invention is that it permits the formation of powder
from a melt with high efficiency in the utilization of
gas. The improvement 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
specification 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
NARROW RANGE 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
because the more uniform size particles will have seen
a more uniform cooling llistory. 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
RD 14863
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powder and the powder is processed through powder
metallurgy -techniques there is a limit on the
desirable properties which can be imparted to a
composition and this limit is related to the
composition and properties of the larger particles of
the 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 lower value of properties will occur.
As a general rule the smaller the particle
size and the smaller the average particle size and the
more uniform the size of smaller particle powder of an
ingredient powder used to form a solid object the more
likely that the product obtained will have certain
combinations of desirable properties in solid objects
prepared from the powder. Ideally if all particles
formed were exactly 20 microns in diameter they would
all have seen essentially the same thermal history and
the objects formed from these particles would have
properties which were 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 particle bodies. However, because of the
internal segregation of the 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
RD 14863
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powder in forming larger bodies by powder metallurgy.
The use of powder metallurgy techniques is presently
the principle route by which superior products are
achieved 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 highly desirable
combination of properties of rapidly solidified
metals. Further, the articles Eormed have a more
uniform set of properties because of the more uniform
particle size of the particles of the powder from
which the particle is formed.
One the unique features of the technology
made possible by the present invention is that it
permits a closer control of a number of the parameters
of a powder product produced by atomization as taught
in this application.
For example, it has been found possible to
alter the 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 size
from a given run is made possible regardless of the
size of particle which is selected. If, for example,
a particle size of 10 microns is selected as the
principle product size for a powder, the control of
the variables of the 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 of this
invention to produce powders which have higher
concentrations of the particles within the selected
~ L~ RD 14863
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size range.
By use of prior art processes it is possible
to produce a wide range of particle sizes in any one
lot or from 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 particle size product.
Accordingly, the present 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 of this invention is that it
not only makes possible to production of powder with a
relatively tight particle size distribution but
because of the tight distribution the particles will
have a selected microstructure. Accordingly it is
possible through use of this 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 size
for the sample being produced.
PREFERRED EMBODIMENT
ILLUSTRATIVE ATOMIZATION
An atomization zone is formed in the area of
confluence of the molten metal stream and the annular
stream of atomizing gas emerging from the annular
opening 22 at the bottom of the gas supply plenum 28.
RD 14863
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Accordingly, the melt guide tube 12 delivers the
liquid metal stream through the throat of the gas
nozzle to the 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 Eorm a gas nozzle having an annular gas
jet which works in cooperation with the shaped exit
end of the melt guide tube.
In other words, the provision of shaped and
configured and cooperative ends at the lower part of
the melt guide tube is one aspect of this invention as
is explained more fully herein. As will be explained
more fully below this is one of several independently
functioning phenomena which may be used in achieving
superior atomization of a variety of melts.
The close positioning of the gas orifice and
melt orifice permits the surface of the melt guide
tube to form a part of the annular gas orifice and by
doing so permits the jet of gas emerging from the gas
plenum to escape over the formed end of the melt guide
tube. This sweeping action of the gas jet on and
against the lower end of the melt guide tube has been
found to be effective in carrying off to a large
degree particles of freezing or frozen metal which
might otherwise tend to form or to 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 of the 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 of the
measures taken in the practice of this invention, the
adherence of such liquid or frozen particles is
reduced and -there is an ability of the sweeping gas to
either prevent deposit of such particles or to cause
their removal once they are deposited or accreted on
~4~
RD 14863
- 28 -
the lower end of the melt delivery tube.
In the particular configuration shown in
Figure 1 there is a continuity, conformity and
alignment between the formed lower surface of the melt
guide tube 18 and the formed surrounding surface 26 of
the gas supply plenum 20. It will be understood that
the annular gas jet can, in fact, be made up in a
number of configurations and in a number of ways.
However, the important feature which must be provided
pursuant to this aspect referred to herein as close
coupling, is an annular gas jet which is at least in
part formed by the lower formed end of the melt guide
tube and proximate to the melt surface.
The principal criteria of a material for a
melt guide tube are that the material be resistant to
the high temperature melt and that it have a high
resistance to thermal shock. Desirable
characteristics are that the material be able to be
machined or cast with a smooth surface to prevent
mechanical interlocking with accreted material; that
it be non-wetting with respect to the melt; and that
it has a low coefficient of thermal conductivity.
Boron nitride meets all of these criteria.
It has been found to be uniquely suitable as a nozzle
material in forming nozzles useful in gas atomization
of high melting metals.
A melt delivery tube such as Table 12 of
Figure 1 was made of boron nitride and was found to
work very well in this function with particular
reference to meeting the two principal criteria as set
forth above. The material was found to be resistant
to high temperature melts of metal at temperatures
of 1350C and 1750C. Further ~he Boron ~itride
delivery tube was found to be highly resistant to
thermal shock when the 1750C metal was poured
into and through the interior of the tube, while
RD 14863
-- 2g --
the atomizing yas at a temperature of
approximately -200C, flowed over the exterior
surface. It has a low coefficient of heat transfer
relative to most metals.
One source of material, a boron nitride sold
commercially by Union Carbide Corporation under the
trade designation HBR, a Union Carbide trademark, is
machinable to the configuration shown in Figure 1 and
has smooth surfaces from the machining.
Another brand, HBC( ), also available
commercially from Union Carbide Corporation is also
useful as a nozzle material although it has only about
half the fracture strength of the HBR grade.
The boron nitride used in fabricating the
melt delivery tubes was apparently wet to a limited
degree by the hot li~uid metal but the wetting was not
of such order as to prevent its use in a very
satisfactory manner as a melt delivery tube and as a
surface component exposed to the metal atomization
atmosphere.
In this last respect the boron nitride
performed exceptionally well. It resisted accretion
of high temperature molten metal better than any other
material tested.
The present invention contemplates forming
any surfaces of an atomization nozzle of boron nitride
to resist splash accretion, whether the nozzle is of
the desiyn or configuration of Figure 1 or of some
other design.
For example, forming individual gas nozzles
with boron nitride surfaces to inhibit accretion is
contemplated. Forming the number 32 of Figure 1 of
boron nitride to inhibit adhesion of molten metal
specs and the progressive accretion of such specs is
contemplated.
In general use of boron nitride on surfaces
~ D 14863
- 30 -
which are exposed -to splatter of the gas atomization
process is contemplated where the surfaces are those
which can receive accretions of frozen particles and
where such accretion interferes with -the progress of
the atornization process.
The surface 18 of Figure l is one
illustration of such surface for reasons explained
above. The gas from the annular nozzle can sweep such
parcticles which may deposit at the surface from
the surface because of the low order of adhesion
between the melt deposit and the boron nitride surface.
Surfaces of a number such as 32 can be
coated with boron nitride or inset with boron nitride
to provide the non-adherence advantages where the
other advantages of thermal sock resistance and the
like are not essential to proper functioning of the
part~
MECHAL~ISM OF CLOSE COUPLED ATOMIZATION
Auther 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 of the waves
and removes the material as droplets. By progressive
action of the high velocity gas across the surface of
the liquid body the body of liquid is disintegrated
into droplets.
I have found that the body of liquid may be
a free flowins stream of liquid melt. E'urther I have
found that a large fraction of the stream may be
disintegrated into tiny droplets directly. I have
used high speed photograph taken at about 35000 frames
per second and have observed that a plume of very fine
particles is emanated from a free flowing melt which
q3
RD 14i'363
- 31 -
is subjected to high velocity gas according -to the
close coupled atomization technique of my invention.
I have further observed that the atomization
can be carried out with gas flowing concurrent to the
flow of melt and that the atomization does not depend
on the multistep phenomena described above with
reference to Figure 4. Additionally I have found by
my hiyh speed photographic observations that no
inverted hollow cone such as illustrated in Figure 3
is formed downstream of the nozzle and that there is
no initial formation of segments or ylobules 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 at the 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 of the melt stream.
In order to induce the acceleration waves on
the liquid body surface and to induce the single step
droplet generation process of this invention as
contrasted with the multistep 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 the surface
tension of the liquid.
The instability criteria which is used is
known as the Weber instability criteria and for a
v
RD 14863
- 32 -
given numercial value of the criteria the relationship
is as follows:
We = ~ V D
where
We = Weber number
~ = gas density
V = relative velocity between the gas and
liquid
D = largest stable droplet size, and
~ = liquid surface tension
When the Weber number is in excess of
approximately 2.1 X 10 the liquid disintegrates by
the process of formation 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 formatlon from the web, or shattering of the
ligaments or globules to produce fine droplets.
The importance of the accleration wave
phenomena as used in connection with the atomization
of molten liquid by a fas is that it permits a high
energy or high intensity disruption of the body of
molten liquid into small particles. This is
particularly important where the surface tension of
the liquid of the molten body is higher. For example
in the case of the breaking up of a drop into
droplets, the atomizing of the drop is made more
difficult because of strong cohesive forces acting at
the surface of the drop acting to hold the drop into
its integral form and state. Generally if the process
RD 14863
- 33 -
is carried out effectively employing the acceleration
wave phenomena as applied by this invention once a
droplet is formed the larger liquid body the droplet
remains as such and is not recombined with other
droplets or bodies by coalascence.
The disruption of the liquid body by the gas
while the gas has a high energy content is deemed to
be responsible for the effectiveness of the 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 of the gas into the liquid body and into
impingement with the surface of the body with the hiyh
energy or high gas momentum.
Also it has been found that the angle of
impingement of the gas onto a surface of the 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
~ ~5~ RD 14863
- 34 -
gas supply plenum. The plenum, and melt guide tube
deliver a confluence of a molten metal and into the
path of an annular gas jet formed at least in part by
the lower end of the melt supply tube. In other
words, the object whicll forms the lowermost end of the
melt supply tube also forms the lowermost end of the
annular gas orifice.
Further the lower end of the melt guide tube
is preferably quite thin so that there is a very fine
edge of 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 other words, the confluence
of gas and melt occurs at the point of the wedge but
the gas is not in simple laminar flow and is effective
in disrupting the metal stream and atomizing the melt.
A still more preferred from of the lower end
of the melt delivery tube is one in which the lower
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 over the gas delivery surface toward the
melt in a direction which causes it to penetrate into
the li~uid melt exiting from the melt delivery tube.
To make the gas impact on the melt even on
both sides of the melt stream as illustrated in the
figures or in practice on all sides of the stream and
to permit symmetrical atomization from each side, or
from 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.
RD 14863
- 35 -
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.
Part of the design concept of the gas
nozzles provided pursuant to this invention is that
the surfaces which are potentially exposed to
ayglomerated material and buildup of such material are
continuously swept clean by the atomizing gas.
One of the most important controls in the
construction of the atomization nozzle as provided in
this invention is given with reference to Figure 2.
As is evident from 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 of the
gas delivery orifice. The dimension of the "A" in
some conventional nozzles is between 2 and 4 inches.
Preferably pursuant to the present invention the
dimension of "A" should be quite small and preferably
of the 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 melt to be atomized.
~ le 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 delivery tubes formed of press
boron nitride~ It is contemplated that tubes may be
formed from composite materials as for example a melt
guide tube having an inner alumina liner which is
encased in a ceramic sleeve in order to 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
RD 14863
- 36 -
atomization nozzle of Fig. 1. Two distances A and B
are shown schematically in the E'igure by two headed
arrows.
The first distance A is the shortest
distance between the bas orifice and the surface of
the melt stream which is first encountered by the 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 free flowing stream and is first
released from the containmne-t 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 of the 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 or the 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 outer surface of
the melt stream.
Distance A is preferably between 0.0 inches
and 0.250 inches, and preferably less that 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
~5~
~ D 14863
- 37 -
midline of the gas jet.
~ hile the distance A is measured alons3 the
surface of the nozzle it is not limited to the
distance on -the nozzle. This is because the actual
nozzle cons-truction is not ideal. If nozzle
construction were ideal the surface along the external
surface of Inelt delivery tube 12 would end at a
point 17 which had no radius or fla-t lower surface.
Actual noæzles do have a radius or land at the point
where the external surface 18 of the melt delivery
tube meets the internal surface of the tube. As a
practical matter it is preferred to avoid having the
tip of the melt deliery tube so thin that it is
subject to cracking or breaking. The degree to which
the tip of the delivery tube can be brought to a fine
cutting edge depends on the material of which it is
constructed and the thermal and other forces to which
it is to be subjected in actual operation.
Accordingly the distance A includes the
distance along the outer tapered surface of melt
delivery tube 12 and the extension of this distance
past the end of the tube to the surface of the melt
emerging from the tube.
It will be apparent that dimension B is
directly related to the size of the orifice to which
the melt discharges as well as the angle of the cone
and the size of the gas outlet 18 thus the dimension B
is not particularly important and is automatically
fixed when the distance A is fixed for any given melt
nozzle configuration and gas outlet size. The above
dimensions for B were based on the melt outlet opening
of less than one-eighth of an inch.
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
5~
RD 14863
- 38
the mel-t stream.
It is known that the distance at which the
velocity of a free stream or jet of gas undergoes
attenuation is principa]ly a function of the jet size
or the size of the orifice from which the jet flows.
Accordingly the allowable distance at which close
coupling can be accomplished increases as the diameter
of the gas jet increases.
Economic considerations of the desired rate
of production of powder, the cost of gas, rate of gas
consumption and like factors determine the actual size
of a closely coupled gas jet to be used. However the
present invention makes feasible the economic
production of fine powder at a variety of production
rates. In fact the 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 l mm where an
apparatus as illustrated in Figure l is employed. For
a nozzle gap of l 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 of the gas,
without formation of a turbulent boundary and
attendant eddy current in the gas. E'or a nozzle as
illustrated in E'ig. l, with a gas nozzle opening of
about l mm, the distance which the converging gas can
travel over the exterior of -the melt delivery tube
RD 14863
- 39 -
without formation of a turbulent boundary layer
sufficient in thickness to result in melt accretion on
the nozzle tip has been observed to be o-E the order in
some instances of about 0.450 inches when Argon, at
plenum pressure of 4.2 MPa, has been employed as the
atomizing gas.
It will be apparent when larger flows of
molten metals are used the size of the melt outlet
orifice will have to be increased significantly as
will the gas glow orifice etc. However, it remains
important tha-t close coupling be maintained between
the gas orifice and the melt stream w~ich as above
indicated means keeping the distance travelled by the
gas stream before it initially impacts the belt so
that the gas loses little if any measurable quantities
of energy prior to impacting the melt stream i.e. the
dimension A should be maintained as a minimum.
UNSTABLE MELT STREAM
Another way in which the production of
powder from a melt may be improved pursuant to the
present invention is by atomization an agitated melt.
One way in which this may be accomplished is throuyh
the use of a gas to atomize a stream of t~le melt which
has a cross-sectional configuration resembling that of
a ribbon or strip, a star, a cross or some other
non-circular form.
It has now been recognized that one of the
most important aspects of the subject imvention is the
realization that the best powder products are produced
with very high energy interaction between the gas and
the liquid of the melt.
Also it has now been recognized that by
inducing flow patterns in the melt as it enters the
atomization zone the melt is more unstable and is more
subject to atomization than is a melt which undergoes
no internal flow undergoes laminar flow, and which
5~
R~ 14863
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enters an atomization zone with a sound regular cross
sec-tion.
Prior art practice has to a large degree
avoided the close disposition of the gas orifice to
the surface of the melt to be atomized. This practice
has grown up evidently from the difficulty which
practitioners have had with the freezing of the melt
onto the gas orifice surfaces and the occlusion of the
solidified material in the path of the gas streams as
well as in the path of the melt stream. The prior art
practice has accordingly been to provide a significant
separation between the gas jet orifice and the
location of the melt stream on which the gas from the
jet impinges. However, when a significant separation
is provided pursuant to prior art practice one result
is that the melt itself is not agitated or turbulent
by the time it drops from the nozzle and reaches the
atomization zone.
It has now been recognized that
irregularities in the flow path of the melt stream
within the melt delivery tube as well as at the exit
from the melt delivery tube can have the effect of
agitating and disturbing the flow pattern of the melt
through and from the tube in such manner as to
destabilize the melt and to assist in the atomization
process.
The agitation must occur at or near the exit
orifice from the melt delivery tube. Thus referring
to Figure 1 a melt agitation at a setback shoulder in
the mid portion of the tube would not disturb the melt
flow at the exit. However from the shoulder at the
bottom of the tube near the exit can induce
agitation. Also changes in the profile of the orifice
of the melt delivery tube exit and can assist in
agitation. Slot forms of orifice is shown in the
Figures 5, 6 and 7. In Figure 8 a double slot or
~;L~
RD 14863
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crossed slots are shown.
Effective improvements in production of fine
powder is possible through use of these orifice
configurations as described with reference -to the
apparatus of Figure 1.
BORON NII'RIDE
It is within the scope of the present
invention to form another aspect of an annular orifice
made up entirely of the inert ceramic materials such
as boron nitride. At the very least the inert ceramic
such as boron nitride should be either provided or
deposited at least at the surface where tlle accretion
of the solidified metal is most likely to occur. The
portion at which it is deemed most likely to occur is
the portion closest to the emergence of the molten
metal. This is at and on the beveled surface 18 at
the lower end 16 of the melt guide tube.
The boron nitride also only function to
limit gas potential of the nozzle 10 since the
porosity of boron nitride is very low particularly in
relation to other normally used ceramic material.
REDIRECTED SURFACE FLOW
Essentially for gas flowing parallel to a
flat surface, the gas has a velocity component
principally in one direction. After finite travel
along the surface it is possible for the gas stream to
lift off the surface and at the intersection between
the surface and the flowing gas one will get the
formation of eddy currents. These eddy currents are
almost circular flows of gas. In the region where
such eddy currents exist the gas flow at the solid
surface due to the eddy current can be or is in the
reverse direction of the main stream of gas flow.
The eddy currents are more prevalent in the
gas flow sequence with turbulent flow than with
laminar flow. As the static pressure of the gas
RD 14863
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increases the tendency for these eddy currents to form
or for flow separation to occur is decreased. ~t
higher pressure there is a lower tendency toward flow
separation. With respect to the outside surface of
the melt delivery tube, what hapyens is that as the
gas moves along, it is redirected by the changing
contour and changing flow direction of the end
surface. This change of direction causes a
compression zone in the gas at the con-toured melt tube
end surface and causes a local buildup in static
pressure. The pressure pushes the gas down against
the surface. That is the reason for the redirectionA
If redirection of the surface is into the gas flow it
acts to eliminate flow separation. If the surface
redirection is away from gas flow it will create flow
separation. It will increase flow separation or
create it if it hasn't already occurred.
CONCAVITY OF MELT DELIVER TIP EXTERNAL SUXFACE
One way in w~ich the stagnant layer can be
swept from the exterior surface of the tube is by
altering surface configuration so that the gas ~ust
change direction as it contacts and sweeps across the
tube surface. For example, if the angle of the wedge
and the angle of the tube surface relative to that of
the surface of the metal emitted from the tube is in
the order of 15 there is a noticeable tendency for a
buildup of solid deposits on the tube surface whereas
under the same conditions of melt flow and gas flow
and configuration of the surfaces and tubes there is
noticeably less or no buildup of solid particles on a
surface which is formed at an angle 22 to the
direction of emergence of the melt. In other words if
the wedge is 15 or less a buildup of particles does
occur on the surface of the tube. Under the same set
of conditions using an angle of 22 there is
essentially no buildup of particles on the exterior
Q
RD 1~863
- 43 -
surface of the tube.
Turning now to a consideration of the
Figures 5 and 6, it will be recognized that the nozzle
structures illustrated, closely resemble that of
Figure 1 in all respects except one as described
below. Accordingly, like numbers are employed in
describing parts of the nozzle structures of Figures 5
and 6 as are employed in describing parts of Figure 1
above. r~rhe parts also have essentially the same
functions as are described above in relation to
Figure i.
The important difference concerns the
external surfaces 18 of the melt delivery tube and the
internal surface 26 of the plenum.
Surprisingly, it has been found that
relatively small differences in the angle at which
these surfaces are formed relative to the tube axis
(or melt axis) can cause relatively large differences
in nozzle performance.
In Figure 1, the beveled surfaces 18 and 26
are formed with a common angle to the melt tube axis.
The angle 22 as is evident from the Figure. This is,
accordingly, a simple uniform angle at which gas flows
along the surface to impact with the surface of the
emerging melt.
In Figure ~, the angle of surface 26 to the
tube axis is 22 but the angle of the surface 18 to
the tube axis is lower and is 15. Accordingly, gas
passing along and over this combination of surfaces is
redirected in its movement as it leaves surface 26 and
starts to move over surface 18. The pressure at the
surface 18 is increased slightly as the moving gas
makes this transition. The formation of a turbulent
wave and resultant swirling eddy currents are reduced
and the surface 18 is rendered less subject to deposit
of accreted metal or frozen melt. It has been found,
RD 14863
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in actual use of such a nozzle with this concave
combination of nozzle surfaces, that less accretion
does occur on the surface 18 and that, following a
run, the surfaces of 18 were quite clean.
By contrast, the surfaces of the tube
surface 18 of a nozzle, as depicted in Figure 10,
became quite fouled and discolored in actual use and
significantly yreater accretion occurred on the
surface 18 than occurred on the same surface of the
nozzle of Figure 1 or Figure ~. As is illustrated in
Figure 10, the surface 26 of the plenum line as 15 to
the tube axis. The external surface 18 of melt
delivery tube is at an angle of 22 to the tube axis.
The concave external surface of the melt
delivery tube and related surface of the space 42
redirect the gas f~ow to effectively limit and
restrict accretion on the surface 18 of the nozzle.
