Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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1~)4()894
The present invention relates to the manufacture of a
metallic powder from a liquid metal.
It relates especially to a method of manufacture of a
metallic powder by projection, on a jet of liquid metal, of a gas -
under pressure delivered by at least one injector in the form of
a jet which causes the metal to burst into fine droplets, the mol-
ecules of gas being projectèd by the inj~ctor at a speed which, in
the axis of this injector, is initially and in a so-called "shock-
wave zone" substantially con~tant and equal to the local speed of
sound,and decreases beyond this zone.
It is known that it is possible to obtain metallic powder
by such a process, known as an "atomization process" by introduc-
ing into the upper portion of large vertical receptacles known as
"reactors", a jet of liquid metal obtained from a melting fur.nace
through the intermediary of a casting.ladle, and subjecting this
liquid jet at the inlet of the reactor to a powerful atomization
by jets of gas, preferably chemically neutral, which cause the
jet of metal to burst into fine droplets which become cooled as -. .
they p~oduce powder, which is collected at the base of the:reactor.
The practical application of this known process results
however in a large number of disadvantages, and especially:
- ~n upward movement of the metallic droplets towards
the atomization injectors, which can cause the obstruc-
tion of these injectors
- An excessive bursting of the spray of droplets, some
of which strike against the walls of the receptacle
before.so~idification;
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~040894
- A defective form of some of the particles of the pow-
der obtained (in the form of needles, flakes, etc)
which are troublesome in certain cases and especially
in sieving.
This method of atomization by jets of gas furthermore
results most frequently in excessively-high consumption of gas.
The essential object of the present invention is to pro-
vide a method of manufacture of metallic powder which avoids the
above-mentioned drawbacks, while at the same time reducing the
consumption of the atomization gases.
Studies carried out by various authors, for example
by Abramovitch in his work "Theory of turbulent jets" (MIT Press,
1963), have shown the existence of a so-called "shock-waves zone",
the length of which is a function essentially of the nature of the
ga~, of the injection pressure and of the injector diameter. These -
studies have established that in this shock-wave zone, the speed
of the gas is constantly equal, in the axis of the injector, to
the local speed of sound, and that it progressively decreases ~~
beyond the said zone. - -
By "local speed of sound", there is understood in this
case the speed of sound under local conditions of temperature and
pressure.
The studies and tests carried out by the authors of the
present invention have led to the conclusion that the various dis-
advantages referred to above were intimately associated wi~h the
existence of a shock-wave zone, and in particular with the posi-
tion of the point of impact of the jet of gas on the liquid metal
with respect to this zone.
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1~4~)894
The above-mentioned object is achieved by a method
of manufacture of a metallic powder by projection on to a jet
of liquid metal of a gas under pressure delivered from at least
one injector in the form of a jet which causes the metal to
burst into fine droplets, the molecules of gas being projected
from said injector with a speed which, in the axis of said
injector, is initially and in a so-called shoc~-wave zone,
substantially constant and equal to the local speed of sound,
decreasing beyond said zone, in which the distance between the
outlet extremity of said injector and the jet of metal is
regulated in such manner that, at the moment of impact with
the metal, the molecules of gas have a speed V between 50%
and 100% of the local speed of sound.
For this reason, the point of impact of the gaseous
molecules is therefore outside the shock-wave zone, which
eliminates the undesirable effect of these shock-waves.
The range of speed determined by experience, makes it
possible to obtain satisfactory results with regard to the
behaviour of the droplets of liquid metal and to the form of
the solid particles.
In accordance with a further characteristic feature of
the method according to the invention, the above-mentioned dis-
tance is regulated in such manner that the speed V is in the
vicinity of 90% of the local speed of sound.
The point of impact of the molecules of gas is thus
situated in this case slightly beyond the limit of the shock-wave
zone. The fact of choosing a speed close to the speed of sound
has the advantage of involving a minimum consumption of gas,
since a maximum speed of the molecules permits a minimum flow- -
rate of gas.
By way of a new industrial product, the invention
is also directed to the metallic powder obtained by the
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1040894
above-mentioned method, this powder being characterized by the
fact that, on the contrary to the powders obtained by known
method, it is practically free from flakes and needles.
The method according to the invention will be illustrated
below by examples given purely by way of explanation and without
any restrictive character.
In accordance with one form of embodiment, a powder is
manufactured from liquid steel by utilizing an injector of 2.5mm.
in diameter, supplied with argon under a under a pressure of 20
bars. me above-mentionned studies by Abramovitch make it pos-
sible to conclude that for such an injector and for such a pres-
sure of argon, the length of the shock-wave zone is 87.5mm. The ``
distance between the outlet extremity of the nozzle and the jet of
liquid metal was then chosen at the value of lOOm~., which corres-
ponds to a speed of the gaseous molecules of 238 m/sec., that is
to say to a speed slightly less than the local speed of sound in `
argon, which is about 265 m/sec. ` -`
Under these conditions, no substantial rise of the met-
allic droplets toward the injectors was observed, and the spread ;
of the spray of droplets was sufficiently moderate to avoid any
projection of the droplets on the walls of the receptacle before
their solidification. Furthermore, The powder obtained was free `
or practically free from particles in the form of plates or nee- `
dles, as has been shown by microscopic examination.
It has further been found that for the conditions defin-
ed above for the speed of the gaseous molecules during the impact
with the liquid metal, that is to say when V is less than the loc-
al speed of sound, there exists an optimum value of the ratio ~
;: :
between the mass flow-rate of the gas (A) and the mass flow-rate
of liquid metal(M), which makes it possible to obtain in particu-
lar a ~inimum consumption of atomization gas.
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.. . , ~ . . . . .. .. ... .
104~894
This optimim value of the ratio A is obtained by re~-
pecting the following relation:
(1) log dm = K log A ` ~ , in which: -
da M
dm is the mean ponderal diameter of the particles of powder ob-
tained;
da is the diameter of the jet of liquid metal to ~e atomized;
We is the We~er number, that is to say:
We = ~ =, in which:
~ is the density of the liquid metal
V is the qpeed of each jet of gas upon its impact with
the liquid metal;
6 is the superficial energy of the liquid metal.
~ is a coefficient which is slightly variable in depen-
dence of the nature of the atomization gas in particular, always
comprised between -0.5 and -0.7 and is preferably chosen in the
vicinity of -0.57.
According to another form of embodiment of the inven-
tion, ~aking account of this optimum value, there was obtained
from liquid steel a powder formed of spherical particles having
a mean granular size in the vicinity of 140 microns, with values
of the ratio A of the mass flow-rate of gas to the mass flow-rate
of metal of 0,2 for atomization by means of nitrogen, and 0~3
for atomization by means of argon, utilizing an atomization devi-
ce provided with injectors, the characteristics of which (diame-
ter, distance from an injector to the point of impact, etc.) are
calculated from the above relation (1), these injectors being
4 in number.
,, , , ,, . , ,. , . j . . . , -, .
104~894
The invention is not in any way limited to the produc-
tion of steel powders, nor to the use of nitrogen or argon, but
is also applicable under the same conditions to other metals or
metallic alloys, and also to other gases, although the neutral
gases are preferred by reason of their chemical inactivity.
In addition, the orientation of the injectors with respect to
the direction of flow of the metal may be in any desired direc-
tion, as may also be their number and their distribution.
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