EP2540420B1

EP2540420B1 - Production of atomized powder for glassy aluminum-based alloys

Info

Publication number: EP2540420B1
Application number: EP12162575.0A
Authority: EP
Inventors: Thomas J. Watson
Original assignee: United Technologies Corp
Current assignee: RTX Corp
Priority date: 2011-06-27
Filing date: 2012-03-30
Publication date: 2015-11-04
Anticipated expiration: 2032-03-30
Also published as: US20120325051A1; EP2540420A2; EP2540420A3

Description

BACKGROUND

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.
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.
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.
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. Patents No, 6,974,510 and 7,413,621 , the disclosures of which are incorporated herein by reference in their entirety.
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.
It is necessary to find an alternative process for production of these highly reactive Al-based alloys.
Baolong Zheng et al: "Gas Atomization of Amorphous Aluminium Powder: Part II. Experimental Investigation", discloses the production of gas-atomized, Al-based amorphous powders.

SUMMARY

The present invention in at least a preferred embodiment includes a process in which rare earth containing Al-based alloys are isolated to prevent oxygen and hydrogen pickup. The process may include 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 °F to -85 °F (-37.2 °C to -65 °C), preferably as low as -110 °F (-78.9 °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 may also have the reduced oxygen and dew point noted above. The catch tanks may be cooled to prevent undesired devitrification or coarsening of fine microstructures in the powder. These catch tanks may be isolated by valves from the surrounding environment to preclude post-atomization contamination from exogenous matter as well as oxygen and hydrogen.
Additional process requirements may include control of the melt temperature with an upper limit of 1600 °F (871 °C) to 2200 °F (1204 °C), and the powder atomization rate may be 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 may be 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 may be 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 may be 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 µm).
Herein is described 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 atomization 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.
The present invention provides 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.

BRIEF DESCRIPTION OF THE DRAWINGS

Certain preferred embodiments will now be described in greater detail by way of example only and with reference to the accompanying drawings.

FIG. 1 is a diagram illustrating a production apparatus for fine aluminum alloy powder.
FIG. 2 is a block flow diagram of the process for producing fine aluminum alloy powder.

DETAILED DESCRIPTION

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. Patents No, 6,974,510 and 7,413,621 , are capable of being formed into powder using the method of this invention. Both high strength and ductility of these alloys is maintained by the method.
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 to make sure the dew point is between -35 °F (-37.2 °C) and -110 °F (-78.9 °C) and through an oxygen getter to make sure the oxygen is between 10 to 50 ppm. To preclude over-pressurization of the system 10, a pressure release valve is used. A valve system 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 enviromnent 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.
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.
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.
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 °F to -85 °F (-37.2 °C to -65 °C), preferably as low as -110 °F (-78.9 °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 an inert gas inlet. The dew point in atomization chamber 29 and catch tanks 33, 37, and 41 should also be about -35 °F to -85 °F (-37.2 °C to -65 °C), preferably as low as -110 °F (-78.9 °C). In production, the weight of metal being atomized should be between 100 pounds (45 kg) and 500 pounds (227 kg), with 300 pounds (136 kg) normally being sufficient, given the size and efficiency of the equipment.
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 to make sure the dew point is between -35 °F (-37.2 °C) and -110 °F (-78.9 °C) and through an oxygen getter to make sure the oxygen is between 10 to 50 ppm. To preclude over-pressurization of the system 10, a pressure release valve is used. A valve system 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 gauges 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.
As the powder is collected in tanks 33, 37 and 41, it is at a high temperature, such as 500 °F (260 °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.
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).
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.
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 °F to -85 °F (-37.2 °C to -65 °C), preferably as low as -110 °F (-78.9 °C). The gas pressure can vary anywhere between 50 to 1,000 psi (345 kPa to 6900 kPa), with higher pressure being desirable because the gas imparts more energy into the melt stream and results in production of finer powder.
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 in³/0 to 50 in³/50 in³. 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.
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 °F (871 °C) and 2200 °F (1204 °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 °F (10 °C) to about 400 °F (204.4 °C).
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 (45 to 2270 grams per minute), with 1 pound per minute (450 grams 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 (45 to 4500 grams) of gas per pound (450 grams) of powder.
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.
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.

Claims

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 (11) having a closed top using an inert gas inlet (15);

supplying inert gas to the inlet from a source (44) of inert gas;

melting an aluminum alloy in a crucible (21) and delivering molten aluminum alloy out of the crucible;

receiving the molten aluminum alloy from the crucible into an atomization chamber (29) 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 (33, 37, 41) while maintaining a positive pressure of inert gas therein.
The method of claim 1, wherein the powder is transferred from the atomization chamber (29) to a first catch tank (33).
The method of claim 2, wherein a portion of the powder is transferred from the atomization chamber (29) downstream from the first catch tank (33) to a first cyclone catch tank (37).
The method of claim 3, wherein an additional portion of the powder is transferred from the atomization chamber (29) downstream from the first cyclone catch tank (37) to a second cyclone catch tank (41), and wherein the first catch tank, the first cyclone tank and the second cyclone tank have isolation valves (31, 39, 43) for closing access thereto.
The method of any preceding claim, wherein the atomization chamber (29) is maintained at a dew point of 35 °F to -110 °F (-37.2 °C to -78.9 °C)
The method of any preceding claim, wherein the atomization chamber (29) 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 in³/0 to 50 in³/50 in³.
The method of any preceding claim, wherein the crucible (21) is adapted to heat the alloy to an upper temperature ranging from 1600 °F (871 °C) to 2200 °F (1204 °C).
The method of any preceding claim, wherein the aluminum alloy is a devitrified glass-forming aluminum alloy having a nanometer-sized grain structure and nanometer-sized intermetallic phase or phases.