U.S. patent application number 13/169207 was filed with the patent office on 2012-12-27 for production of atomized powder for glassy aluminum-based alloys.
This patent application is currently assigned to UNITED TECHNOLOGIES CORPORATION. Invention is credited to Thomas J. Watson.
Application Number | 20120325051 13/169207 |
Document ID | / |
Family ID | 45930617 |
Filed Date | 2012-12-27 |
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United States Patent
Application |
20120325051 |
Kind Code |
A1 |
Watson; Thomas J. |
December 27, 2012 |
PRODUCTION OF ATOMIZED POWDER FOR GLASSY ALUMINUM-BASED ALLOYS
Abstract
A system and method for producing atomized powder for glassy
aluminum-based alloys in an inert gas atmosphere. A melt chamber
melts the alloy and it is atomized to form powder. The powder is
deposited in at least one catch tank.
Inventors: |
Watson; Thomas J.; (South
Windsor, CT) |
Assignee: |
UNITED TECHNOLOGIES
CORPORATION
Hartford
CT
|
Family ID: |
45930617 |
Appl. No.: |
13/169207 |
Filed: |
June 27, 2011 |
Current U.S.
Class: |
75/338 ;
425/7 |
Current CPC
Class: |
B22F 9/002 20130101;
B22F 9/08 20130101; C22C 1/0416 20130101; C22C 45/08 20130101 |
Class at
Publication: |
75/338 ;
425/7 |
International
Class: |
B22F 9/08 20060101
B22F009/08 |
Claims
1. A system for producing atomized powder for glassy or partially
devitrified aluminum-based alloys, comprising: a melt chamber
having a closed top, including an inert gas inlet for providing a
positive pressure of inert gas therein; a source of inert gas
adapted to supply inert gas to the inlet; a crucible for melting
aluminum alloy therein, the crucible having an outlet for
delivering molten aluminum alloy; an atomization chamber positioned
to receive molten aluminum alloy from the crucible and produce fine
aluminum alloy powder from the molten alloy, including inert gas
inlets for maintaining a positive pressure of inert gas; inert gas
inlets on the chamber for making powder; and at least one catch
tank for receiving powder produced in the atomization chamber while
maintaining a positive pressure of inert gas.
2. The system of claim 1, wherein a first catch tank is disposed
for first receipt of the powder.
3. The system of claim 2, wherein a first cyclone catch tank is
disposed to receive powder downstream from the first catch
tank.
4. The system of claim 3, wherein a second cyclone catch tank is
disposed to receive powder downstream from the first cyclone catch
tank.
5. The system of claim 4, wherein the first catch tank, the first
cyclone tank and the second cyclone tank have isolation valves for
closing access thereto.
6. The system of claim 1, wherein the atomization chamber is
maintained at a dew point of 35.degree. F. to -110.degree. F.
(-37.2.degree. C. to -78.9.degree. C.).
7. The system of claim 1, the atomization chamber has a gas
composition of a mixture of helium and at least one inert gas
wherein the ratio of helium to inert gas ranges from 100 in.sup.3/0
to 50 in.sup.3/50 in.sup.3.
8. The system of claim 1, the crucible is adapted to heat the alloy
to an upper temperature ranging from 1600.degree. F. (871.degree.
C.) to 2200.degree. F. (1204.degree. C.).
9. The system of claim 1, wherein the aluminum alloy is a
devitrified glass-forming aluminum alloy having a nanometer-sized
grain structure and nanometer-sized intermetallic phase or
phases.
11. A method for producing atomized powder for glassy
aluminum-based alloys, comprising the steps of: providing a
positive pressure of inert gas in a melt chamber having a closed
top using an inert gas inlet; supplying inert gas to the inlet from
a source of inert gas; melting an aluminum alloy in a crucible and
delivering molten aluminum alloy out of the crucible; receiving the
molten aluminum alloy from the crucible into an atomization chamber
and forming fine aluminum alloy powder from the molten alloy, while
maintaining a positive pressure of inert gas in the atomization
chamber; and receiving powder produced in the atomization chamber
in at least one catch tank while maintaining a positive pressure of
inert gas therein.
12. The method of claim 11, wherein the powder is transferred from
the atomization chamber to a first catch tank.
13. The method of claim 12, wherein a portion of the powder is
transferred from the atomization chamber downstream from the first
catch tank to a first cyclone catch tank.
14. The method of claim 13, wherein an additional portion of the
powder is transferred from the atomization chamber downstream from
the first cyclone catch tank to a second cyclone catch tank, and
wherein the first catch tank, the first cyclone tank and the second
cyclone tank have isolation valves for closing access thereto.
15. The method of claim 11, wherein the atomization chamber is
maintained at a dew point of 35.degree. F. to -110.degree. F.
(-37.2.degree. C. to -78.9.degree. C.).
16.
17. The method of claim 11, the atomization chamber has a gas
composition of a mixture of helium and at least one inert gas
wherein the ratio of helium to inert gas ranges from 100 in.sup.3/0
to 50 in.sup.3/50 in.sup.3.
18. The method of claim 11, wherein the crucible is adapted to heat
the alloy to an upper temperature ranging from 1600.degree. F.
(871.degree. C.) to 2200.degree. F. (1204.degree. C.).
19. The method of claim 1, wherein the aluminum alloy is a
devitrified glass-forming aluminum alloy having a nanometer-sized
grain structure and nanometer-sized intermetallic phase or
phases.
20. A method for producing atomized powder devitrified
glass-forming aluminum alloys having a nanometer-sized grain
structure and nanometer-sized intermetallic phase or phases,
comprising the steps of: melting the alloy in a chamber having a
positive pressure of inert gas; melting an aluminum alloy in a
crucible adapted to heat the alloy to an upper temperature ranging
from 1600.degree. F. (871.degree. C.) to 2200.degree. F.
(1204.degree. C.), wherein the atomization chamber is maintained at
a dew point of 35.degree. F. to -110.degree. F. (-37.2.degree. C.
to -78.9.degree. C.); atomize the molten ally to a fine aluminum
alloy powder; and collecting the powder produced by atomization
while maintaining a positive pressure of inert gas.
Description
CROSS-REFERENCE TO RELATED APPLICATION(S)
[0001] This application is related to the following co-pending
applications that are filed on even date herewith and are assigned
to the same assignee: DIFFUSION BONDING OF GLASSY ALUMINUM-BASED
ALLOYS, Ser. No. ______, Attorney Docket No.
PA0009506U-U73.12-665KL; MASTER ALLOY PRODUCTION FOR GLASSY
ALUMINUM-BASED ALLOYS, Ser. No. ______, Attorney Docket No.
PA0009509U-U73.12-666KL; EXTRUSION OF GLASSY ALUMINUM-BASED ALLOYS,
Ser. No. ______, Attorney Docket No. PA0009510U-U73.12-667KL; and
FORGING OF GLASSY ALUMINUM-BASED ALLOYS, Ser. No. ______; Attorney
Docket No. PA0009508U-U73.12-671KL. All referenced incorporated
herein.
BACKGROUND
[0002] Aluminum alloys are important in many industries. Glassy
Al-based alloys and their devitrified derivatives are currently
being considered for applications in the aerospace industry. These
alloys involve the addition of rare earth and transition metal
elements. These alloys have high strength and, when processed
appropriately, have high ductility.
[0003] Because these alloys are processed via the powder metallurgy
approach, one of the key requirements for high ductility is control
of the uptake of hydrogen. While all Al-based alloys are sensitive
to hydrogen, alloys containing rare earth elements are particularly
susceptible to the effects of hydrogen during alloy production.
[0004] The powder for Al-based powder metallurgy alloys can be
produced using gas-atomization. When atomized powder of pure
aluminum or Al-based alloys is produced, the process normally
involves the use of an inert gas such as nitrogen that is injected
into a molten metal stream at high pressure. The gas is not
recycled because it is relatively inexpensive. However, in the case
of prior alloys, no concern has been made for oxygen and/or
hydrogen uptake because the presence of oxygen and/or hydrogen does
not affect the strength of prior alloys. From the standpoint of
ductility, the prior art involves the removal of hydrogen and
oxygen during high temperature degassing. This approach will not
work for glassy or partially devitrified nano-scale microstructures
because of the thermal instability of such microstructures while in
powder form.
[0005] Al-based alloys such as Al--Y--Ni--Co alloys are devitrified
glass-forming aluminum alloys that derive their strength from a
nanometer-sized grain structure and nanometer-sized intermetallic
phase or phases. Examples of such alloys are disclosed in co-owned
U.S. Pat. Nos. 6,974,510 and 7,413,621, the disclosures of which
are incorporated herein by reference in their entirety.
[0006] Owing to the reactive nature of these alloys, i.e., the
presence of rare earth elements such as Y, the presence of oxygen
can lead to fires and/or explosions. In addition, the presence of
hydrogen destroys the ductility of these alloys. When the alloy is
a glassy Al-based alloy, the high temperatures required for
degassing these materials in powder form brings about an almost
instantaneous devitrification so that the benefits of the glassy
state are lost. Also, partially devitrified derivatives of the
glassy state produce nanocrystalline microstructures that have
mechanical properties that cannot be obtained when starting out
with powder in the crystalline state.
[0007] It is necessary to find an alternative process for
production of these highly reactive Al-based alloys.
SUMMARY
[0008] The present invention includes a process in which rare earth
containing Al-based alloys are isolated to prevent oxygen and
hydrogen pickup. The process includes the gettering of oxygen and
lowering the dew point of the atmosphere above the molten metal and
throughout the rest of the system to -35.degree. F. to -85.degree.
F. (-37.2.degree. C. to -65.degree. C.), preferably as low as
-110.degree. F. (-78.9.degree. C.). When this is done, atomization
of the metal into powder is performed and thereafter caught in
catch tanks at the bottom of the atomization chamber. The catch
tanks also have the reduced oxygen and dew point noted above. The
catch tanks are cooled to prevent undesired devitrification or
coarsening of fine microstructures in the powder. These catch tanks
are isolated by valves from the surrounding environment to preclude
post-atomization contamination from exogenous matter as well as
oxygen and hydrogen.
[0009] Additional process requirements include control of the melt
temperature with an upper limit of 1600.degree. F. (871.degree. C.)
to 2200.degree. F. (1204.degree. C.) , and the powder atomization
rate is controlled to be about 0.1 to 5.0 pounds (45.4 to 2270
grams) of powder produced per minute. A gas to metal ratio is
controlled to be between about 0.1 to 10 pounds (45.4 to 4540
grams) of gas per pound of powder. Finally, the powder particle
size is controlled by controlling the nozzle size, because for any
given gas flow, a smaller nozzle will allow for a higher
gas-to-metal ratio that provides for finer powder and better
mechanical properties. The nozzle size is between 0.05 and 0.25
inches (0.0254 to 0.635 cm.), resulting in a high percentage of the
powder having a size less than -625 mesh (0.1 to 45 .mu.m).
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 is a diagram illustrating a production apparatus for
fine aluminum alloy powder.
[0011] FIG. 2 is a block flow diagram of the process for producing
fine aluminum alloy powder.
DETAILED DESCRIPTION
[0012] FIG. 1 shows production apparatus, 10, for production of
fine aluminum alloy powder. The aluminum alloy can be any alloy but
it has been discovered that the glassy devitrified alloys such as
those disclosed in co-owned U.S. Pat. No, 6,974,510 and 7,413,621,
are capable of being formed into powder using the system and method
of this invention. Both high strength and ductility of these alloys
is maintained by the system and method.
[0013] A melt chamber 11 has a closed top 13 and is filled with an
inert gas such as argon via gas inlet 15 from inert gas source 44.
Compressor 45 provides the inert gas to the melt chamber 11 and
other parts of apparatus 10, including 29, 33, 37, and 41. This gas
circulates through a dryer 46 to make sure the dew point is between
-35.degree. F. (-37.2.degree. C.) and -110.degree. F.
(-78.9.degree. C.) and through an oxygen getter 47 to make sure the
oxygen is between 10 to 50 ppm. To preclude over-pressurization of
the system 10, a pressure release valve 48 is used. A valve system
49 is used to route the gas to gas source 44. The benefit of
circulating the gas prior to melting the alloy is not only the
establishment of the correct melt conditions, but to preclude the
cost of a so-called wash heat. A wash heat is where, in this case,
pure Aluminum might be atomized to getter oxygen and hydrogen,
thereby establishing the correct environment in system 10. The
Aluminum alloys would then be atomized after this wash heat. The
problem with this approach is the possibility of
cross-contamination, creating metallurgical flaws in the
consolidated alloy. Alternatively, one could atomize the Aluminum
alloy of choice, but rare earth containing alloys are quite
expensive, and this adds cost to the overall production process,
both in materials and labor and machine time. A bottom pour
crucible 21 is located in melt chamber 11. An additional inert gas
inlet 23 provides the inert gas for atomization. A stopper rod 26
controls the opening and closing of a hole at the bottom 27 of
crucible 21.
[0014] The powder produced from molten metal that exits the hole in
crucible bottom 27 enters an atomization chamber 29, which is a
conical hopper for catching the exiting powder. Chamber 29 includes
an isolation valve 31 that controls access of powder from chamber
29 to catch tank 33. Catch tank 33 also has a valve 31a that
separates the catch tank from chamber 29 when connected to the
chamber, and from the surrounding air when not connected.
[0015] Also part of melt chamber 11 is an outlet 35 that receives
powder from the top of atomization chamber 29. Powder is
transferred to a first cyclone catch tank 37, to which access is
controlled by isolation valve 39. First cyclone catch tank 37 also
has an isolation valve 39a that separates the catch tank 37 from
chamber 29 when connected to the chamber, and from the surrounding
air when not connected. Tank 37 catches finer powder than that in
atomization chamber 29, thus improving yield. Outlet 35 further
transfers powder to a second cyclone catch tank 41, again
controlled by isolation valve 43. Second cyclone catch tank 41
catches the finest powder produced in atomization chamber 29. Catch
tank 41 also has an isolation valve 43a that separates the catch
tank from chamber 29 when connected, and from the surrounding air
when not connected. When all catch tanks are full of powder, this
valve system allows them to be removed both during and after runs,
and a new tank added where the new tank has an internal environment
conditioned to be acceptable to that in the atomization system
10.
[0016] Closed top 13 of melt chamber 11 insures that inert gas
exerting a positive pressure will prevent moist or humid air from
entering crucible 21. It is desirable to have the dew point in melt
chamber 11, and thus in crucible 21, be as low as possible. The dew
point can range from -35.degree. F. to -85.degree. F.
(-37.2.degree. C. to -65.degree. C.), preferably as low as
-110.degree. F. (-78.9.degree. C.). In addition to dry, inert gas
in the crucible area, it is desirable to circulate dry gas in
atomization chamber 29 and catch tanks 33, 37, and 41 via inert gas
inlet 15a. The dew point in atomization chamber 29 and catch tanks
33, 37, and 41 should also be about -35.degree. F. to -85.degree.
F. (-37.2.degree. C. to -65.degree. C.), preferably as low as
-110.degree. F. (-78.9.degree. C.). In production, the weight of
metal being atomized should be between 100 pounds and 500 pounds,
with 300 pounds normally being sufficient, given the size and
efficiency of the equipment.
[0017] Once the dew point of the system has been lowered, the metal
in crucible 21 is melted and atomization is begun. Gas from gas
source 17 is pressurized by a high pressure compressor 19 and this
gas atomizes the molten metal stream via inlets 23. To minimize
cost, particularly where helium is involved, this gas is recycled
back into 17. The recycled gas is passed through a dryer 46 to make
sure the dew point is between -35.degree. F. (-37.2.degree. C.) and
-110.degree. F. (-78.9.degree. C.) and through an oxygen getter 47
to make sure the oxygen is between 10 to 50 ppm. To preclude
over-pressurization of the system 10, a pressure release valve 48
is used. A valve system 49 is used to route the gas to gas source
17. Powder formed is captured in catch tank 33 and in first cyclone
catch tank 37 and second cyclone catch tank 41. Depending on the
system, more or fewer cyclone catch tanks may be used as needed.
Pressure gages may be connected to tanks 33, 37 and 41 so their
respective valves can be closed to determine that the tanks are
capable of holding gas from the atomization process at a pressure
greater than ambient. Closing the valve and measuring the pressure
will determine if there is leakage of gas from the tanks. Leakage
will result in contamination of the powder. At times during
operation of the system, these tanks can be pressurized by an inert
gas such as dry argon, nitrogen, or a helium containing mixture of
gases.
[0018] As the powder is collected in tanks 33, 37 and 41, it is at
a high temperature, such as 500.degree. F. (260.degree. C.) and
cooling jackets may be used, such as by placement in intimate
contact with the tanks and using water or colder coolants such as
dry ice.
[0019] FIG. 2 is a block flow diagram of the method of this
invention. The aluminum alloy is selected (Step 211) and placed in
a melt chamber (Step 213) where inert gas, such as argon or
nitrogen, is added to provide a positive inert gas pressure (Step
215). The alloy is then melted to form a molten alloy (Step 217)
either in the atomization crucible, or in an adjacent melt crucible
where it is transferred to the atomization crucible, while
maintaining the positive inert gas pressure (Step 219). The molten
alloy is atomized to produce fine powder (Step 221) in the inert
atmosphere. The powder is then captured in the chamber catch tank
(Step 223) and, optionally as noted above, to one or more cyclone
tanks (Step 225).
[0020] Atomization of the powder is done in a manner to provide for
high retention of the glassy state in materials having a high
solute content or to provide for maintaining the nanocrystalline
microstructure of materials having a lower solute content. The
variables that are controlled during atomization include the
atomization gas, melt temperature, powder passivation, and
atomization rate.
[0021] The atomization gas variables include oxygen content, dew
point, gas pressure and gas composition. The oxygen content should
be between 10 to 50 ppm. The dew point, as noted above, should be
about -35.degree. F. to -85.degree. F. (-37.2.degree. C. to
-65.degree. C.), preferably as low as -110.degree. F.
(-78.9.degree. C.). The gas pressure can vary anywhere between 50
to 1,000 psi, with higher pressure being desirable because the gas
imparts more energy into the melt stream and results in production
of finer powder.
[0022] For a given metal flow, higher gas pressure results in a
higher flow rate, which yields a higher gas-to-metal ratio and
results in faster cooling to yield more glassy powder or finer
nanocrystalline microstructures. The gas content by volume can be a
helium (He)/inert gas (IG) combination with a He/IG ratio of 100
in.sup.3/0 to 50 in.sup.3/50 in.sup.3. More helium is better
because helium conducts heat away from the molten powder more
efficiently than other gasses. It is noted that helium is more
expensive than other gasses; hence, the need for a recirculation
system.
[0023] The melt temperature is established through the use of
Differential Scanning Calorimity (DSC). A DSC trace can define the
temperature whereby the highest melting point phase goes into
liquid solution. Depending on the alloy composition, this upper
bound temperature can vary between 1600.degree. F. (871.degree. C.)
and 2200.degree. F. (1204.degree. C.). This temperature range keeps
all phases in liquid solution and prevents nozzle clogging. To
insure prevention of nozzle clogging, an additional increment of
temperature, known as superheat, can be added to guarantee that the
temperature does not drop below the critical level. Superheat can
range from about 50.degree. F. (10.degree. C.) to about 400.degree.
F. (204.4.degree. C.).
[0024] The atomization rate is defined as the number of pounds of
powder produced per minute. For a given gas flow rate, the lower
the powder produced per minute, the higher will be the gas-to-metal
ratio, and the higher cooling rate and thus better mechanical
properties. For good mechanical properties, powder production rate
can vary between 0.1 to 5.0 pounds per minute, with 1 pound per
minute being preferable. If the atomization rate is viewed in terms
of the gas-to-metal ratio, the optimum rate for good mechanical
properties has been found to be 0.1 to 10 pounds of gas per pound
of powder.
[0025] The powder is also subjected to powder passivation, whereby
a thin aluminum oxide shell is placed on top of the aluminum powder
particles. This is done to prevent rapid oxidation of aluminum
powder, which can be highly explosive. Powder passivation is
accomplished by adding very dry oxygen during an atomization run,
or, alternatively, by adding very dry oxygen after the powder has
cooled down to room temperature. In the former case, the amount of
oxygen added ranges from 10 to 400 ppm to produce an oxide layer of
as much as 15 to 25 nm. If oxygen is added at room temperature, the
amount can range from 10 to 1000 ppm, resulting in a thinner oxide
layer of 3 to 5 nm.
[0026] While the invention has been described with reference to an
exemplary embodiment(s), it will be understood by those skilled in
the art that various changes may be made and equivalents may be
substituted for elements thereof without departing from the scope
of the invention. In addition, many modifications may be made to
adapt a particular situation or material to the teachings of the
invention without departing from the essential scope thereof.
Therefore, it is intended that the invention not be limited to the
particular embodiment(s) disclosed, but that the invention will
include all embodiments falling within the scope of the appended
claims.
* * * * *