Specialized metallurgical processes – compositions for use therei – Processes – Electrothermic processes
Patent
1990-12-19
1992-11-03
Rosenberg, Peter D.
Specialized metallurgical processes, compositions for use therei
Processes
Electrothermic processes
75 1034, 75 1043, C22B 400
Patent
active
051605333
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a new method for grain refining of a metal structure, and more particularly grain refining of light metals such as aluminium and aluminium alloys.
It is well known that in order to start the crystallization/solidification process during solidification of molten metals it is necessary, in addition to undercooling (chilling) of the melt, to provide ready crystals or nuclei in the melt. Normally, in a commercial melt there is always present a certain amount of undissolved particles/impurities such as oxides, carbides, etc. representing stabile nuclei. However, the problem is that the number of such nuclei is not sufficient, and anyway there is no possibility of achieving a reproduction control in order to provide a fine and uniform structure. Reduction of the grain size is an efficient way of improving the material characteristics, e.g. increase the strength, and therefore in practice grain refining means are intentionally added to the melt in order to increase the number of stabile nuclei, and as a result of this to achieve a fine grained structure. Titanium and especially titanium boride, usually added as a master alloy in the form of a rod or wire comprising from 4-6% Ti, 1% boron and the remainder aluminium, is nowadays the most common grain refining additive applied to aluminium and Al-alloys. Addition of titanium boride as a grain refining means functions well for most Al-alloys, but the drawback is first of all relatively high costs connected to this method, presently amounting to approximately US$6. per ton of treated aluminium, and furthermore the fact that borides are generally undesirable in aluminium.
U.S. Pat. No. 4,793,971 discloses a method for adding alloying/refining material having a high dissolution rate by converting the alloying material into a spray of superheated alloy material and directing the spray into the molten metal at a predetermined depth below the metal surface. The alloying material in the form of an elongated element (rod) having a free end is continuously fed into a spark cup through its upper inlet, and an electrical arc is maintained between the submerged metal surface and the alloying element in the spark cup. Thus provided superheated spray of the alloy material is directed by a continuous supply of shielding gas onto the submerged metal surface, where the material dissolves and disperses in the melt. The shielding/carrier gas, supplied at a flow rate which maximizes the projection of the spray into the melt, is preferentially an ionizable gas such as argon or even helium having a high ionization potential which facilitates transferring more heat into the metal.
In a particular embodiment of the invention where a metallic titanium rod is applied as a grain refining additive to molten aluminium, formation of titanium aluminide (TiAl.sub.3) in situ as grain refiner nuclei is achieved by the reaction between the vaporized titanium and the molten aluminium. This represents an improvement in terms of efficiency and economy compared to the former simple introduction of a titanium rod into molten aluminium. However, expensive titanium material is still to be used, and furthermore the disclosed method and apparatus does not seem to be able to assure a uniform distribution of titanium/titanium aluminide nuclei through the whole volume of the molten aluminium metal.
It is therefore an object of the present invention to provide a new method for grain refining of metals being substantially cheaper than the methods known so far. This is achieved by provision of a method according to the accompanying patent claims.
The invention will now be described in more detail by means of examples and referring to the drawing, where
FIG. 1 shows schematically in a vertical cross-section an apparatus applied in a preferred embodiment of the invention, method as disclosed in U.S. Pat. No. 4,568,385.
The apparatus schematically illustrated in a vertical cross-section in FIG. 1, comprises a rotor 1, submerged in molten metal 2 and comprising a hollow shaft 7 and a ho
REFERENCES:
patent: 2555507 (1951-06-01), Pratt
patent: 3390979 (1968-07-01), Greene
Myrbostad Erling
Venas Karl
Norsk Hydro a.s.
Rosenberg Peter D.
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