Abstract:
Apparatus is provided for forming aluminum alloy ingots in a sealed chamber having a source of inert gas using a crucible positioned inside the chamber for melting aluminum alloy powder. The crucible has a solid top and a source of inert gas therein. An outlet in the crucible is positioned to draw molten alloy from the crucible at a point proximate the lowest point in the crucible. A tundish adapted to control the flow of molten alloy from the crucible on a path to at least one ingot mold out of the sealed chamber

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
       [0001]    This application is related to the following co-pending applications: U.S. patent application Ser. No. 13/169,202, entitled “MASTER ALLOY PRODUCTION FOR GLASSY ALUMINUM-BASED ALLOYS” and filed on Jun. 27, 2011, and these other applications also filed on June 27, 2011 and are assigned to the same assignee: DIFFUSION BONDING OF GLASSY ALUMINUM-BASED ALLOYS, Ser. No. 13/169,194, Attorney Docket No. PA0009506U-U73.12-665; EXTRUSION OF GLASSY ALUMINUM-BASED ALLOYS, Ser. No. 13/169,204; Attorney Docket No. PA0009510U-U73.12-667KL; PRODUCTION OF ATOMIZED POWDER FOR GLASSY ALUMINUM-BASED ALLOYS, Ser. No. 13/169,207, Attorney Docket No. PA0009512U-U73.12-668KL; and FORGING OF GLASSY ALUMINUM-BASED ALLOYS, Ser. No. 13/169,210; 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]    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]    When Al-based alloys are produced in large quantities, they are often direct chill cast into molds that drop into well-like openings in the ground. For reactive materials such as Al—Li—X alloys, care must be exercised to preclude or prevent reaction of the Li with any oxidant such as air or water. For more reactive elements such as Yttrium and other rare earths, even more care is needed because exposure to water that is used to cool direct chill molds could result in fire and/or an explosion. 
         [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. The presence of hydrogen destroys the ductility of these alloys. Consequently, it is necessary to produce master alloys with hydrogen contents of 1 ppm or less. 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]    It is necessary to find an alternative process for production of these highly reactive Al-based alloys. 
       SUMMARY 
       [0007]    It has now been discovered that master alloy for devitrified glass-forming Al-based alloys can be produced in a process that avoids hydrogen pickup. The molten metal is isolated from the environment to a substantial degree. The process includes the use of a bottom-pour or side-pour crucible that is “covered” with an inert gas such as argon. The gas cover includes a physical cover on the top of the crucible into which argon or another inert gas such as nitrogen is bled into the crucible to form a positive pressure. The heavier argon forces out any air to minimize exposure of the melt to air. 
         [0008]    The metal is poured out from the side or bottom of the crucible, rather than tipping to pour out the top. It is poured into a launder or pipe that is sealed and attached to the crucible, and is also filled with an inert gas such as argon. The molten metal flows through a launder or launder/tundish combination and is deposited directly into molds, which are also filled with inert gas such as argon. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  shows one embodiment of a bottom pour furnace with a vertical feed for producing aluminum alloy ingots while avoiding hydrogen pickup. 
           [0010]      FIG. 2  shows another embodiment of a bottom pour furnace with a horizontal feed for producing aluminum alloy ingots while avoiding hydrogen pickup. 
           [0011]      FIG. 3  is a possible cross section for the horizontal feed launders of the apparatus of  FIG. 2 . 
           [0012]      FIG. 4  is a flow diagram illustrating the method of forming aluminum master alloy ingots. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]      FIG. 1  illustrates a bottom pour furnace  10  generally with a vertical feed. The entire furnace is inside an inert chamber  11 , having a solid top  17 , with gas feed  13  introducing argon or another inert gas such as nitrogen. It is more effective if the inert gas is heavier than air, as argon is, to more easily push out any air that is initially present in chamber  11 . A top-feed crucible  15  is located inside chamber  11 . Inside crucible  15  is a quantity of aluminum and various alloy elements in the form of chips, shot, rod, etc. that is to be made into master alloy ingots. 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. Nos. 6,974,510 and 7,413,621, can be formed into low oxygen and low hydrogen master alloy ingots using the method of this invention. 
         [0014]    The alloy in crucible  15  is purged with argon or another inert gas to drive out oxygen and any other reactive gas. Hydrogen from moisture is also driven out. Crucible  15  may be any low moisture/low volatiles alumina crucible, such as those produced by St. Gobain, or a graphite crucible with a spall-free alumina coating. Typical crucibles are ceramic cylinders that are about two feet in diameter and about three feet deep. 
         [0015]    The alloy is melted in crucible  15  and exits the bottom of crucible through launder  19 , so that the flow of molten alloy is controlled by position-control door  21 . Launder may not be needed in some designs of crucible  15 . With or without launder  19 , the passage out of crucible  15  is also accomplished in an inert atmosphere via inert gas feed  23 . 
         [0016]    Tundish  25  is a funnel-shaped vessel into which the molten metal is poured. The purpose of a tundish is to allow the molten metal to reach a desired height (with a desired head pressure) so that there is a constant pour rate. It has been discovered that a slower rate precluded bubbles from forming in the melt. The height can be adjusted so there is no splashing of the metal into the molds. Flowing molten alloy  27  pours into waffle ingot molds  29  carried by conveyor belt  31 , also in an inert atmosphere. 
         [0017]    Allowing the molten alloy to drain down from the bottom of crucible  15  eliminates a major problem in prior art furnaces, in that the dross that accumulates on the top of the molten pool of alloy remains at the top and does not have to be removed until crucible  15  is cleaned prior to recharging with more alloy. Also, the dew point can be monitored, further preventing undesirable gas from contacting the sensitive elements of the alloy, thus preserving the low hydrogen/oxygen content of the master alloy. 
         [0018]    For the two embodiments as discussed herein, a hygrometer with a computer can be used for measuring the amount of moisture, and therefore hydrogen, in the gases both at the source for  13  and  23 , and within chamber  11  as a function of time. Best results are obtained when the dew point is −110° F. (−78.9° C.) or lower. A commercially available monitor such as an ALSCAN may be connected to a computer so that hydrogen readings in the melt may also be taken in real time. Similar readings in the launder can be used to monitor hydrogen there as well, which is to be as low as possible, i.e., less than 1 ppm. 
         [0019]    In an alternative embodiment, a bottom pour furnace  100  generally is shown in  FIG. 2 . A first inert chamber  111 , having a solid top  117 , is maintained in an inert state via inert gas feed  113 . Crucible  115  is filled or purged with an inert gas to drive out all reactive gasses, including hydrogen via the gas from  113 . A launder  119 , angled downward, is maintained with an inert atmosphere by a plurality of inert gas feeds  123  down stream of metal flow control door  121 . Launder  119  has a typical cross section as shown in  FIG. 3 , with a steel or other hard casing  141 , a ceramic mold or center passage  143  and the opening  145  through which the molten alloy flows. Tundish  125  controls the pour rate and pour height of molten alloy into waffle ingot molds  129  that are carried by conveyor belt  131 . Again inert gas is maintained in second inert chamber  211  by inert gas feed  213 . 
         [0020]      FIG. 4  is a flow diagram of the method of this invention. Aluminum and the required elements in the form of chips, shot, rod, etc. (Step  311 ) are selected and placed in an enclosed crucible having an inert atmosphere (Step  313 ) with a positive pressure to drive out other gasses. The input stock is melted (Step  315 ) to form a molten alloy. The molten alloy is transferred (Step  317 ) to a mold while maintaining an inert atmosphere at least until the ingot is solidified. The ingot is then removed (Step  319 ) and available for subsequent processing. 
         [0021]    Both bottom and side pouring embodiments have been found to be effective in producing satisfactory ingots. The advantage of the system of  FIG. 1  is that the system is more compact with the launder going straight down. However, if the pouring goes too fast and can&#39;t be stopped, the risk of overpouring onto the floor exists. In the system of  FIG. 2 , more space is used but there can be multiple metal flow gates to contain failure at the bottom of the furnace. 
         [0022]    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.