U.S. patent application number 10/070527 was filed with the patent office on 2002-09-26 for method and device for atomizing molten metals.
Invention is credited to Edlinger, Alfred.
Application Number | 20020134198 10/070527 |
Document ID | / |
Family ID | 3686507 |
Filed Date | 2002-09-26 |
United States Patent
Application |
20020134198 |
Kind Code |
A1 |
Edlinger, Alfred |
September 26, 2002 |
Method and device for atomizing molten metals
Abstract
In a method for atomizing metal melts, in which the liquid metal
bath is sprayed from a tundish via an outlet opening by the aid of
a gas into a cooling chamber, or onto a surface to be coated while
compacting the comminuted particles by the aid of a propellant gas,
the liquid metal melt via an annular gap is introduced into the
outlet opening, into which a hot gas having a temperature of
between 250.degree. C. and 1300.degree. C. and a supercritical
pressure of between 2 and 30 bars is ejected through a Laval nozzle
concentrically with said opening. The hot gas is contacted with the
melt bath at a speed exceeding supersonic speed, with a radial
outwardly directed component or with a twist. The device for
carrying out the method includes a melt tundish (1) and an
immersion tube (4) immersed in the melt (2) while forming an
annular gap surrounding the outlet opening for the melt (2) and a
lance (7) for the ejection of a propellant gas, wherein the
height-adjustable lance (7) carries a Laval nozzle (9).
Inventors: |
Edlinger, Alfred;
(Bartholomaberg, AT) |
Correspondence
Address: |
Robert J Schneider
Chapman & Cutler
111 West Monroe Street
Chicago
IL
60603
US
|
Family ID: |
3686507 |
Appl. No.: |
10/070527 |
Filed: |
March 6, 2002 |
PCT Filed: |
July 6, 2001 |
PCT NO: |
PCT/AT01/00225 |
Current U.S.
Class: |
75/338 ;
266/202 |
Current CPC
Class: |
B22F 3/115 20130101;
C21B 2400/062 20180801; B22F 2998/00 20130101; C21B 2400/072
20180801; B22F 2009/0892 20130101; C21B 2400/026 20180801; B22F
9/082 20130101; C22C 1/1042 20130101; C21B 3/08 20130101; B22F
2009/0824 20130101; B22F 2009/088 20130101; B22F 2998/00
20130101 |
Class at
Publication: |
75/338 ;
266/202 |
International
Class: |
B22F 009/08 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 7, 2000 |
AT |
A1169/2000 |
Claims
1. A method for atomizing metal melts, in which the liquid metal
bath is sprayed from a tundish via an outlet opening by the aid of
a gas into a cooling chamber, or onto a surface to be coated while
compacting the comminuted particles by the aid of a propellant,
characterized in that the liquid metal melt via an annular gap is
introduced into the outlet opening, into which a hot gas having a
temperature of between 250.degree. C. and 1300.degree. C. and a
supercritical pressure of between 2 and 30 bars is ejected through
a Laval nozzle concentrically with said opening, and that the hot
gas is contacted with the melt bath at a speed exceeding supersonic
speed, with a radial outwardly directed component or with a
twist.
2. A method according to claim 1, characterized in that the hot gas
is ejected via a deflector body.
3. A method according to claim 1 or 2, characterized in that a
lance comprising the Laval nozzle for the hot gas is conducted
concentrically in a tube while forming an annular space, and that
reactive gases such as, e.g., CO, H.sub.2, O.sub.2 or H.sub.2O
vapor, and/or inert gases such as, e.g., N.sub.2 or Ar, and/or
carbides such as, e.g., WC, TiC or VC, are sucked in via said
annular space.
4. A method according to claim 3, characterized in that reactive
metal powders or additives such as, e.g., SiC, Al.sub.2O.sub.3 or
Y.sub.2O.sub.3 are charged into the gas flow sucked in.
5. A method according to any one of claims 1 to 4, characterized in
that the hot gas is heated in a heat exchanger surrounding the melt
particles ejected.
6. A method according to any one of claims 1 to 5, characterized in
that extremely fine particles of the solidifying melt, which ascend
within the cooling chamber, are sucked off below the entry of the
melt flow and discharged via a sluice.
7. A method according to any one of claims 1 to 6, characterized in
that a pressure of 1.5 to 25 bars is maintained within the
tundish.
8. A method according to any one of claims 1 to 7, characterized in
that a pressure of 1.5 to 10 bars is maintained within the cooling
chamber.
9. A device for carrying out the method according to any one of
claims 1 to 8, including a melt tundish (1) and an immersion tube
(4) immersed in the melt (2) while forming an annular gap
surrounding the outlet opening for the melt (2) and a lance (7) for
the ejection of a propellant gas, characterized in that the
height-adjustable lance (7) carries a Laval nozzle (9).
10. A device according to claim 9, characterized in that a
deflector body (10) is arranged in a height-adjustable manner in
the widening opening region of the Laval nozzle (9) or following
thereupon, viewed in the flow direction, the clear cross section
between the nozzle (9) and the deflector body (10) being designed
to increase in the axial direction towards the outlet end and to be
larger than the narrowest cross section of the Laval nozzle
(9).
11. A device according to claim 9 or 10, characterized in that the
lance (7) opens in the outlet opening of the tundish (1) below the
lower edge of the immersion tube (4).
12. A device according to any one of claims 9 to 11, characterized
in that the outer diameter of the lance (7) is smaller than the
clear diameter of the immersion tube (4) and the lance (7) is
sealingly guided through a lid (6) of the immersion tube (4), and
that a duct (24) for the supply of gases and/or reactive metal
powders and/or additives opens into the space of the immersion tube
(4) surrounding the lance (7).
13. A device according to any one of claims 9 to 10, characterized
in that the deflector body (10) is designed as a cone having
deflector surfaces provided on its jacket.
14. A device according to claim 13, characterized in that the
deflector surfaces extend in an S-likely curved manner and, in the
peripheral direction, terminate so as to be directed at the tangent
of the base circle of the conical body each under the same angle.
Description
[0001] The invention relates to a method for atomizing metal melts,
in which the liquid metal bath is sprayed from a tundish via an
outlet opening by the aid of a gas into a cooling chamber, or onto
a surface to be coated while compacting the comminuted particles by
the aid of a propellant gas, as well as a device for carrying out
said method.
[0002] In order to obtain dense metal coatings, it has already been
proposed to eject such metals from a melt bath by the aid of
propellant gases onto a surface to be coated or any other target
with the still molten droplets solidifying during impingement on
the surface to be coated or any other target (substrate), thus
causing the coating to be accordingly compressed or compacted. When
atomizing molten metals by the aid of propellants, an inert
propellant gas jet is usually employed at ambient temperature, the
known processes all requiring a relatively high propellant gas
consumption and, as a rule, also a relatively high propellant gas
pressure. A number of nozzle geometries have been proposed for the
atomization and compaction of such atomized metal particles. The
economy of such methods, as a rule, has, however, been
substantially determined by the propellant gas amount and
propellant gas pressure required.
[0003] The invention aims to provide a method of the initially
defined kind, by which it is feasible to atomize molten metals
efficiently and by using substantially smaller-structured devices
while substantially lowering the necessary amount of propellant
gas, whereby a substantially finer atomization is to be ensured and
the option to incorporate also other components into the atomized
metal melt is to be provided, at the same time. To solve this
object, the method according to the invention essentially consists
in that the liquid metal melt via an annular gap is introduced into
the outlet opening, into which a hot gas having a temperature of
between 250.degree. C. and 1300.degree. C. and a supercritical
pressure of between 2 and 30 bars is ejected through a Laval nozzle
concentrically with said opening, and that the hot gas is contacted
with the melt bath at a speed exceeding supersonic speed, with a
radial outwardly directed component or with a twist. By using hot
gases having temperatures of between 250.degree. C. and
1300.degree. C. and a supercritical pressure of between 2 and 30
bars as opposed to the known methods, the viscosity of the
propellant gas is substantially increased in view of known methods,
whereby shearing forces will act more efficiently and a finer
comminution of the metal melt into particularly small particles
having diameters d.sub.50 of below 10 .mu.m will be obtained. At
the same time, it is feasible to reduce the propellant gas
consumption to 1/3 to 1/5 as against the use of propellant gases at
the usually low temperatures, thus yielding substantial advantages
as regards the economy of metal pulverization processes. Another
advantage consists in that the metal melt does not freeze in the
melt runout due to smaller temperature differences. By introducing
the liquid melt via an annular gap into the outlet opening it has
become feasible to influence the inflow of liquid melt and hence
the flow rate per time unit in a simple manner by an appropriate
adjustment of said annular gap, and by introducing the propellant
gas concentrically with the outlet opening it has become feasible
to use the structural component defining the annular gap as a
second concentric tube, i.e., as a suction tube to suck in further
substances. By contacting the hot gas with the melt bath at a speed
exceeding sonic speed with a radial outwardly directed component or
with a twist, which is feasible, in particular, by effecting the
ejection under a supercritical pressure via a Laval nozzle, it is
feasible to transmit high shearing forces at reduced propellant gas
amounts, thus ensuring a particularly efficient and rapid
comminution during the impingement on the metal melt by rapidly
braking the propellant gas jet, which is more highly viscous on
account of the elevated temperatures. By the hot gas being ejected
in the interior of the melt jacket and contacted with the melt bath
with a radially outwardly directed component, the gas is forced to
pass through the melt jacket, thereby tearing the melt jacket open.
This definitely brings about an essential advantage, which resides
in the formation of a monogram powder, which formation is promoted
by the radial tearing open of the hollow-cylindrical melt jacket.
As the melt jacket is being torn open radially, it causes the
formation of a uniform ligament in the radial direction and, after
this, extremely uniform droplets. The monogram powder is
excellently suitable for use in powder-metallurgical processes.
[0004] The flow conditions of the hot gas streaming out through the
Laval nozzle may also be adjusted in a manner so as to form an
underexpanded propellant jet. This will subsequently result in
pressure bursts in the range of Mach's nodes with expansion volumes
lying between such Mach's nodes. Due to vibration interferences in
the jet, shearing stresses will be introduced into the melt
droplets, thus causing a rise in frequency with supercritical
conditions increasing and a respective reduction of the distances
of Mach's nodes in the axial direction of the propellant gas jet.
The fact that an underexpanded jet is ejected causes an immediate
expansion after the emergence from the nozzle. In a configuration
of this type, the distance to a surface to be coated may be chosen
to be extremely short such that small-structured devices will do.
Advantageously, the hot gas is ejected through a deflector body so
as to enable the effective cross section of emergence from the
Laval nozzle to be adapted to the respective requirements by a
suitable adjustment of the deflector body. The use of a deflector
body also serves to impart on the outflowing hot gas an appropriate
additional flow component directed radially outwards and/or a
twist.
[0005] Advantageously, the method according to the invention is
realized in a manner that a lance comprising the Laval nozzle for
the hot gas is conducted concentrically in a tube while forming an
annular space, and that reactive gases such as, e.g., CO, H.sub.2,
O.sub.2 or H.sub.2O vapor, and/or inert gases such as, e.g.,
N.sub.2 or Ar, and/or carbides such as, e.g., WC, TiC or VC, are
sucked in via said annular space. The tube surrounding the lance
with the Laval nozzle, by its lower edge defines the annular gap
required for the access of the liquid metal melt, while an annular
space is, at the time, formed between the lance and the tube for
the aspiration of reactive gases and/or inert gases. Such a
configuration allows for a preferred method control, by which metal
powders or additives such as, e.g., SiC, Al.sub.2O.sub.3 or
Y.sub.2O.sub.3 and/or carbides are charged into the aspirated gas
flow, thus ensuring a high degree of adjustability of the atomizing
process to different requirements by means of a particularly simple
structural configuration of the device.
[0006] The radiation heat of the metal melt ejected by the hot
propellant gas and effectively atomized during ejection may be used
to heat the hot gas, to which end it is preferably proceeded in a
manner that the hot gas is heated in a heat exchanger surrounding
the melt particles ejected.
[0007] By using a hot gas, particularly small particles are formed
as pointed out already in the beginning, thus leading to the
formation of a flow of extremely fine particles, which is directed
outwardly in a vortex-like manner, in the cooling chamber besides a
downwardly directed flow. These extremely fine particles are again
sucked into the downwardly directed flow of atomized melt, where
they partially serve to rapidly cool the atomized melt. In order to
reduce the portion of extremely fine particles which are effective
for cooling, yet partially impede the efficient comminution of the
particles, and, in particular, in order to ensure that such
extremely fine particles will not cause caking in the region of the
outlet opening or mouth of the tundish, it is advantageously
proceeded in a manner that extremely fine particles of the
solidifying melt, which ascend within the cooling chamber, are
sucked off below the entry of the melt flow and discharged via a
sluice. The option to suck in additional solid substances such as,
for instance, silicon carbide, Al.sub.2O.sub.3 or Y.sub.2O.sub.3 in
fine-powder form via the annular space, also allows for the
obtainment of metal--matrix composite materials as well as
ceramic--metal composite materials, and hence particularly
wear-resistant coatings. Unlike with complex-design discrete spray
nozzles, it is feasible by means of a single Laval nozzle and a
consecutively arranged deflector body via which merely the hot
propellant gas is ejected, to take into account all the set objects
at a substantially reduced fuel consumption, the only thing
required, in detail, being the appropriate adjustability of the
tube to adjust the annular gap as well as the appropriate selection
of the aspirated gases. Furthermore, the desired jet geometry may
be influenced, and adapted to the selected substances, in a simple
manner by an appropriate axial displaceability of the hot gas
nozzle, or of the deflector body, and/or an appropriate exchange of
the deflector body. In the main, the process control according to
the invention renders feasible the efficient atomization of any
sort of metal melts while also enabling the atomization of alloys
and, in particular, ferroalloys such as, for instance, FeV, FeCr,
FeW, FeTi or FeMo.
[0008] According to a preferred process control, a pressure of 1.5
to 25 bars may be maintained within the tundish, while a pressure
of 1.5 to 10 bars is preferably maintained in the cooling chamber.
By observing these pressure levels, a melt saturated with pressure
gas will be obtained, the pressure gas being comprised, for
instance, of argon. The melt saturated with pressure gas
facilitates disintegration, thus enabling an altogether finer
atomization. The introduction of gas may be effected by means of
bottom tuyeres of the tundish or via an immersion lance.
[0009] The device according to the invention for carrying out said
method includes a melt tundish and an immersion tube immersed in
the melt while forming an annular gap surrounding the outlet
opening for the melt, wherein a lance is further provided for the
ejection of a propellant gas. The device according to the invention
is essentially characterized in that the height-adjustable lance
carries a Laval nozzle, wherein a deflector body is preferably
arranged in a height-adjustable manner in the widening opening
region of the Laval nozzle or following thereupon, viewed in the
flow direction, the clear cross section between the nozzle and the
deflector body being designed to increase in the axial direction
towards the outlet end and to be larger than the narrowest cross
section of the Laval nozzle. The deflector body provided in the
widening opening region of the Laval nozzle, or following
thereupon, viewed in the flow direction, may be adjusted on account
of its height adjustability with a view to minimizing the
consumption of propellant gas, wherein, in order to obtain the
desired supersonic speed, it merely has to be taken care that the
clear cross section between the inner wall of the Laval nozzle and
the deflector body is designed to be always larger than the
narrowest cross section of the Laval nozzle in the axial direction
towards the outlet end and to increase in the axial direction. The
arrangement of a deflector body is, however, not necessarily
required, and it has turned out that an efficient atomization is
feasible also without deflector body, particularly favorable
results being achieved if, as in correspondence with a preferred
further development of the device according to the invention, the
lance opens in the outlet opening of the tundish below the lower
edge of the immersion tube. To this end, the lance is arranged to
be adjustable in height.
[0010] In order to obtain an annular space suitable to suck in
additional components, the configuration advantageously is devised
such that the outer diameter of the lance is smaller than the clear
diameter of the immersion tube and the lance is sealingly guided
through a lid of the immersion tube, and that a duct for the supply
of gases and/or reactive metal powders and/or additives opens into
the space of the immersion tube surrounding the lance. An
adjustable throttle valve may be provided in the duct intended to
supply gases and/or reactive metal powders, so that the volume
between the lance and the immersion tube may optionally be
maintained under a suitable negative pressure, pulsating flows,
thus, being additionally obtainable. It is, however, also feasible
to keep the valve completely closed.
[0011] Advantageously, the deflector body is designed as a cone
having deflector surfaces provided on its jacket. A distinctive
radial component may be achieved by means of such a deflector body
if, as in correspondence with a preferred configuration, the
deflector surfaces extend in an S-likely curved manner and, in the
peripheral direction, terminate so as to be directed at the tangent
of the base circle of the conical body each under the same
angle.
[0012] In the following, the invention will be explained in more
detail by way of an exemplary embodiment of a device suitable to
carry out the method according to the invention, which is
schematically illustrated in the drawing.
[0013] In FIG. 1, a melt tundish 1 in which a metal bath 2 is kept
in the molten state is illustrated in cross section. In order to
keep this metal bath in the molten state, an inductive heating may
be provided, as is schematically indicated by coils 3.
[0014] A tube 4 is immersed in the metal bath, defining an annular
gap between the bottom of the tundish 1 and the lower edge of the
tube. This tube 4 is adjustable in the height direction in the
sense of double arrow 5 so as to allow the amount of metal bath
flowing off the tundish 1 per time unit to be regulated in a simple
manner.
[0015] The tube 4 is closed by a lid 6 in which a lance 7 is
sealingly conducted in the sense of double arrow 8 so as to be
adjustable in height. On its outlet end for hot gas, the lance 7
comprises a Laval nozzle 9. By virtue of this configuration as a
Laval nozzle, sonic speed will exactly adjust in the narrowest
cross section of the Laval nozzle 9 if hot gas is supplied under
supercritical conditions, supersonic speed being reached in the
consecutive widening cross section on account of the rapid
expansion occurring. In this widening region is arranged a
deflector body 10 which is also adjustable in the axial direction
in the sense of double arrow 12 via an appropriate rod assembly 11.
Suitable adjustment of the deflector body may, thus, influence the
jet shape, whereby it merely has to be safeguarded that the
respectively effective cross section widens accordingly in the
axial direction following the narrowest point of the Laval nozzle 9
so as to ensure the attainment of supersonic speed caused by the
rapid expansion.
[0016] The propellant gas jet emerging from the lance 7 then
reaches a consecutively arranged cooling chamber 13, in which a
target 14 may, for instance, be provided. The propellant gas jet
collides with the outflowing metal bath at supersonic speed and an
appropriate viscosity on account of its high temperature so as to
effect rapid and efficient comminution, which may be applied to the
target 14 as a coating. In the absence of such a target 14, the
appropriately comminuted metal powder may be drawn off the cooling
chamber 13 via a sluice 15 provided on its lower end. The radiation
heat of the solidifying metal droplets may be exploited in a heat
exchanger 16 surrounding the cooling chamber, to which cold gas is
fed through a duct 17 and from which hot gas is drawn off through
duct 18. If the thus obtained temperature is sufficient for the
desired purposes, this hot gas may be directly fed to the lance 7
via duct 18. Further heating may be obtained by the aid of
conventional recuperative heat exchangers not illustrated in the
drawing.
[0017] In the interior of the cooling chamber 13, a further annular
duct 19 is to be seen, via which extremely fine particles may be
sucked off. These extremely fine particles may be supplied to a
screening means 21 through a duct 20 and discharged as an extremely
fine powder through a sluice 22. The amount of extremely fine
powder discharged, thus, will no longer get into the downwardly
oriented flow and have no influence on the solidification behavior
of the droplets comminuted by the propellant gas jet.
[0018] The lance 7 is guided at a distance from the inner wall of
the tube 4, leaving free an annular space 23. Additional material
may be sucked into this annular space via a duct 24, said
additional material comprising, above all, reactive gases like CO,
H.sub.2, N.sub.2, O.sub.2 or, if a partial oxidation of the metal
particles is sought, also H.sub.2O vapor. The amount aspirated in
each case may be determined by the aid of an adjustable throttle
valve 25. A number of powdery materials capable of flowing along
with a gas stream may also be sucked into this duct from a
reservoir 26 as doping agents. As dispersible solids, metal
powders, SiC, Al.sub.2O.sub.3 or even Y.sub.2O.sub.3 may, above
all, be aspirated and introduced via duct 24 into the annular space
23, from which they are aspirated via the hot gas stream and
rapidly brought into intensive contact with the metal melt.
[0019] FIG. 2 depicts a modified configuration of the propellant
gas lance, in which the lance 7 opens in the outlet opening of the
tundish 1 below the lower edge of the immersion tube 4. The lance
comprises a Laval nozzle 9, whereby the arrangement of a deflector
body may be obviated. Attempts have shown that the atomization
results are the better the deeper the propellant gas nozzle is
inserted into the melt runout.
[0020] Inert gases such as, for instance, nitrogen, argon and
helium may, be envisaged as propellant gases in the first place,
yet also reactive gases like CO, H.sub.2, optionally blended with
water vapor, may be used depending on the set object, if an
oxidative atomization is sought.
[0021] Metal melts may comprise Al, Cu, Fe, Ni, Co, Ti, Mg melts or
melts of rare earth metals or alloys thereof and, in particular,
Co-based superalloys. The powders obtained are particularly
suitable for applications in powder metallurgy, for instance for
hot isostatic pressing, but also as a charging material for MIM
processes (metal injection molding).
* * * * *