|Title of Invention||
A PROCESS AND A DEVICE FOR PRODUCING MAGNESIUM LITHIUM ALLOY
|Abstract||This invention relates to a novel process for preparing magnesium lithium alloy. Solid lithium metal is added ur.der alloying conditions to molten magnesium in the absence of conventional fluxes. The resulting alloy is free from alkali metal contamination. The alloy yield is very high. The invention also includes a device for preparinl this alloy. It consists of an encasement chamber, an alloying chamber, a casting chamber, and a means for electrically heating the alloying chamber.|
This invention relates to a process and a device for making magnesium lithium alloy.
Magnesium lithium alloys are the lightest metallic alloys whioh can be used as structural materials. Density of these alloys range from 1.35 - 1.5g/cc. They also exhibit very high specific stiffness of more than 3100 x 10 mm and is second only to beryllium. These alloys possess excellent formability due to change of hep structure of magnesium into bcc structure due to alloying with lithium. Because of these characteristics, magnesium-lithium alloys are extensively used in the manufacture of space crafts upper stages of satellite launch vehicles and the like where weight reduction is of paramount importance.
However, process of preparing these alloys by melting and casting poses formidable problems due to the extremely high reactivity of lithium. Explosive tendency of lithium often leads to fire hazards and the metal needs to be stored under protective atmosphere. Magnesium is also highly reactive in nature. The considerable difference in density between magnesium (l,74g/cc) and lithium (0.54g/cc) also poses problems related to the uniformity in composition along the length of the billets.
Several attempts have been made for preparing magnesium-lithium alloy and a brief description is given hereinbelow. In conventional flux melting technique a protective
cover of fluxes based on chlorides/fluorides of sodium, potassium and magnesium is used. In this method solid lithium was added to molten magnesium with other alloying elements if added. The alloy prepared by this method showed very low ductility as a result of sodium and potassium pick up during the alloying process.
Special flux melting technique containing fluxes of chlorides and fluorides of lithium has also been developed. Solid lithium was added to a liquid melt of magnesium and other alloying elements. Though alkali metal contamination is avoided by this method, flux becomes heavier than the alloy, leading to dross formation. Difficulty in separating the flux and metal, resulting in flux inclusion in the alloy and poor alloy yield are also observed.
In yet another known method, conventional fluxes together with special fluxes are used. Flux based on chlorides and fluorides of sodium, potassium and magnesium was adapted for magnesium melting while a flux based on chloride and fluoridr- of lithium was used for lithium melting. Molten magnesium was added to molten lithium according to this process. Though process cost IS low, difficulties like dross formation, flux inclusions and poor yield are the draw backs associated with this .nethod.
In another method soiid magnesium and solid lithium were melted in an atmosphere of argon and the pouring was done
in air. Advantages observed were minimum alkali metal contamination and improvement in alloy yield. Disadvantages were the dross inclusions during the pouring step and segregation of lithium resulting in non-uniformity in composition.
The prior art described herein above clearly indicates that none of the methods resulted in the production of premium quality cast billets free from alkali metal contamination, flux/dross inclusion and gravity segragation of lithium. Minimum loss of lithium during melting, high alloy yield after casting and recycling of casting scrap are also important considerations, especially keeping in view of the very high cost of lithium metal and limited natural resources of exploitable lithium containing ores.
The present invention has been made to overcome the above discussed difficulties faced in the process of preparing magnesium lithium alloys. The entire alloy making process including the casting step of the molten alloy is carried out in the absence of any flux. This, no doubt, eliminates flux inclusions in the billet and also significantly increases the alloy yield as flux contaminated waste metal is not at all produced during this alloy making process.
Metals for alloy making are cleaned separately. Magnesium pieces are cut and then cleaned using emery paper, acetone and 5% nitric acid. Lithium pieces stored and cut using
mineral oil as coolant. After removing the coolant oil, lithium pieces are kept in lithium container using a flow of argon preferably at the rate of 5 lit/min. Magnesium is melted in an atmosphere of argon and lithium pieces and other alloying elements are introduced into the molten magnesium. The alloy melt is preferably maintained at a temperature of 650*^0 to 700'"'C. The alloy is stirred for about 10 minutes and the molten alloy is cast into billets after evacuating the casting chamber with a vacuum pump and purging the chamber with argon.
Accordingly the present invention provides a process for making magnesium-lithium alloy which, according to this invention comprises the steps of adding solid lithium metal stored in argon to molten magnesium in an atmosphere of argon under known alloying conditions in the absence of conventional fluxes and subsequently casting the molten alloy in an atmosphere of argon.
The percentage of lithium and magnesium in the alloy is not critical and may be varied as desired.
is found to be more than 95% consistent. Maximum ineiting loss of lithium is found to be only 2% and as such
lithium recovery is more than 98%. Billets are found to be free of oxides, flux inclusions, lithium segregation and gas porosites.
This invention also includes a device i.e. a special set-up for producing magnesium lithium alloy and casting the same into billets. This device consists of a lithium cleaning and encasement chamber, a lithium container, a melting and alloying chamber provided with a controlled stirring mechanism, an electric furnace for melting magnesium and lithium, a casting chamber and argon supply mechanism.
The melting and alloying chamber may consist of a melting crucible and a cover assembly, housing, a stirrer, a plunger, a stopper and a sheathed thermocouple and down sprue for allowing the flow of the alloy from the alloying chamber to the casting chamber. Means for charging the alloy constituent metals vis lithium and magnesium, into the alloy making chamber is also provided. A movable baffle arrangement may be used for charging lithium into the alloying chamber. This avoids the need for fixing a lithium charge compartment to the alloying chamber to avoid safety hazards. The baffle mechanism also assists in adding other alloying elements, if need be. This baffle is provided on the cover assembly which has a window like opening. This window may also be located on one side of the melting crucible and may be connected to the lithium container.
The casting chamber consists of a base plate, a mould box and a cover plate which may be connected to the down sprue. The casting chamber may be mounted on an adjustable trolley for adjusting the height for mating with the down sprue. Casting cavity may be formed out of graphite pieces or split type cast iron dies. Alloying chamber, lithium chamber and the casting chamber are provided with inlet and outlet for flushing them with argon gas. Means for electrically heating the alloying chamber as well as temperature regulating arrangement for controlling the temperatures are also provided. A argon is the preferred inert gas.
This device will now be described with reference to Figure I of the accompanying drawings.
Reference letters LC stands for the lithium container. CR is the crucible or the alloying chamber in which alloying takes place. SR and P represent the stopper arm and the plunger which are fixed on the cover plate assembly. The alloying chamber or crucible is provided with electrical heating means represented by letter F. The base of the alloying chamber is provided with an opening which is closed during melting and alloying stage with the graphite stopper head SG connected to the stopper rod assembly. The letters DS stand for the down sprue. The casting
chamber is indicated by CC and encloses the casting mould M. The mould M is provided with an air insulated sleeve connectable to the down sprue for allowing the molten alloy to flow into the mould. The casting chamber is mounted on an adjustable telescopic platform AP and is mounted on movable trolley TR. Reference letters I, I' and 0 represent the inlets and the outlet for argon gas which is used to flush the device prior to and during the alloying process.
Though only a specific embodiment has been described and illustrated herein, obvious alterations known to persons skilled in the art are also within the scope and ambit of the appended claims.
1, A process for making magnesium lithium alloy comprising
the step of adding solid lithium metal to molten magnesium in an
atmosphere of fergon under known alloying conditions in the
absence of conventional fluxes and subsequently casting the
molten alloy in an atmosphere of argon.
2. The process as claimed in claim 1, wherein lithium and magneisum are cut and cleaned prior to alloying.
3, The process as claimed in claim 2, wherein lithium is
cut using mineral oil as coolant and cleaned.
4, The process as claimed in claim 3, wherein lithium is cleaned
with xylene and placed in the lithium container in an atmosphere
5. The process as claimed in claim 2, wherein magnesium is cleaned with acetone and 5% nitric acid prior to melting.
6. The process as claimed in any of claims 1-5, wherein alloying is carried out at a temperature of 650^ to 700*^0 under mechanised stirring and argon atmosphere.
7. The process as claimed in any of the preceding claims wherein the molten alloy is cast after evacuation of the casting cavity and in an atmosphere of argon.
8. A device for producing magnesium lithium alloy
comprising an encasement chamber (EC), an alloying chamber(CR) provided with stirrer means, a casting chamber (CC), means (F) for electrically heating said alloying chamber (F> and means (I, I' and 0) for flushing the device with an inert gas such as argon.
9, The device as claimed in claim 8, wherein said alloying chamber consists of a crucible, a cover assembly consisting of a stirrer, a plunger, a stopper means and a thermocouple with sheath.
10. The device as claimed in claims 8 and 9, wherein the base of said alloying chamber is provided with a down sprue closable by means of a stopper provided on the lower end of said stopper rod.
11. The device as claimed in claims 9 and 10, wherein said cover assembly is provided with a window which is connectable to said encasing chamber.
12- The device as claimed in claim 11, wherein a baffle
tray is provided at said window to supply lithium and other
alloying elements from said encasement chamber to the alloying
13. The device as claimed in claim 8, wherein said casting
chamber is mounted on a movable trolley with an adjustable
14. A process for making magnesium lithium alloy substantially as herein described.
15. A device for making magnesium lithium alloy substantially as herein described with particular reference to figure I of the accompanying drawings.
|Indian Patent Application Number||1202/MAS/1999|
|PG Journal Number||50/2007|
|Date of Filing||20-Dec-1999|
|Name of Patentee||M/S. INDIAN SPACE RESEARCH ORGANISATION|
|Applicant Address||ANTARIKSH BHAVAN, NEW BEL ROAD BANGALORE - 560094,|
|PCT International Classification Number||C 22C 1/00|
|PCT International Application Number||N/A|
|PCT International Filing date|