The present invention relates to a new method for grain refining of the 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 crystalliza¬ tion/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 the melt is in practice intentionally added grain refining means 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 remaining rest aluminium, is nowadays the most common grain refining additive applied on 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.

US patent No. 4,793,971 discloses a method for adding alloy¬ ing/refining material having high dissolution rate by con¬ verting the alloying material into a spray of superheated alloy material and directing the spray into the molten metal at a predeterminated 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 shield¬ ing/carrier gas, supplied at a flow rate which maximizes the projection of the spray into the melt, is preferentially an ionizable gas as argon or even helium having high ionization potential which fascilitates 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 (Ti l 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 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 pro¬ vide a new method for grain refining of metals being sub¬ stantially 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 details by means of examples and referring to the drawings, Figs. 1-3, where

Fig. 1 shows schematically in a vertical cross- section an apparatus applied in a pre¬ ferred embodiment of the invention.

Fig. 2 is a photographic picture of the AISi alloy structure prior to the refining treatment, and

Fig. 3 is a photographic picture of the same alloy after the treatment,

and in conjunction with an apparatus particularly suitable to conduct the method as disclosed in US patent 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 hollow rotat¬ ing body 6 having openings 9 provided along the body's peri¬ meter and its bottom part, is connected to a source of current 3 by means of an electrode 8 situated co-axially within the shaft 7. An electrical arc 5 is generated between the electrode's lower part and the rotating parabolic melt surface provided during the rotation of the rotor body 6. A gas conduct 11 is attached to the top of the shaft 7 in order to provide the desirable atmosphere over the melt and/or to feed to the melt inert (passive) or active gases for the purpose of refining.

The movement of the rotor body 6 causes a portion of the melt inside the rotor to rotate and develop an upper surface having a paraboloid shape where centripetal forces eject the melt through the side openings of the rotor under the surface of the surrounding melt, while the melt circulates upwardly into the rotor body through the bottom opening. Thus a very efficient mixing of e.g. molten metal is achieved so that the rotor is especially suitable, besides heating, also for refining or alloying of metal melts.

During the tests with the above melt treating apparatus it has been found that by using nitrogen as a passive refining gas aluminium nitride (A1N) particles were formed and dis¬ tributed through the melt functioning as solidification nuclei, something which resulted in a fine grained structure on the cast metal.

These A1N particles are formed partly by evaporation of aluminium at temperatures over 2000°C reacting with the gaseous nitrogen and condensating inwardly on the rotating

body and/or by contacting the melt. However, most of the particles are formed in situ by reaction between the Al- elt and the atomized and ionized nitrogen generated around the electric arc. These particles having very small size are, thanks to the specially shaped rotor, distributed quickly, uniformly and efficiently through the whole volume of the melt ensuring an extremely fine and uniform distribution of appropriately sized grain refining nuclei.

More details and parameter characteristics for the new grain refining method will be apparent from the following prac¬ tical examples.

Example 1

770 kg AlSi-melt was treated batchwise by means of the above described apparatus during a period of 5 minutes. A graphite anode was applied during the trial and electric arc effect (direct current) of 50 K . Nitrogen was supplied at a ratio of 10 1/minute and the melt temperature was kept between 780-840°C. The resulting grain size on the cast metal was below 100/urn.

Figs. 2 and 3 are photographic pictures of the metal struc¬ ture before and after the nitrogen treatment of the melt, respectively. The pictures illustrate clearly the grain refining effect of the in situ provided aluminium nitrides on the AlSi-alloy.

Example 2

20 tons of AlSi-melt was passed through a treatment unit installed in a holding furnace. The same process parameters as in Example 1 were applied - arc effect of 50 KW and the nitrogen feeding rate 10 1/minute. The grain size of the cast metal was under 100 um.

As it appears from the examples, the method can be applied both batchwise and in a continuous casting process (in-line) either as a finalizing treatment step by means of an apparatus (treatment unit) readily installed in furnaces for the refining treatment or by means of a separate unit built upon the same principles - provision of nitrogen around an electric arc and an efficient transport and distribution of the AIN-particles in the melt.

This in situ formation of grain refining nuclei represents a great improvement of the refining processes known so far both with regard to efficiency and economy of grain refining of Al-alloys.

As previously mentioned there is no need for a rather ex¬ pensive master alloy rod of titanium (TiB ₂ ) as a grain re¬ fining material to be applied in the refining process. Furthermore, use of carrier gas like argon or helium in sufficient amounts/rates to enhance entry of the grain re¬ fining material is eliminated. Apart from representing addi¬ tional costs, the use of excessive gas flow rates in order to maximize the projection of superheated material spray into the melt, something which is important for the dissolu¬ tion and recovery rates of the alloying/refining material according to the disclosure of prior art, may introduce some disturbance of the process, have impact on the thermal balance and even cause a contamination of the melt by en¬ trapping of e.g. surface oxides.

In the grain refing process according to the present inven¬ tion the applied nitrogen gas rates correspond to the amount of nitrogen reqired to form grain refining nitrides. There are no secondary requirements to the gas since the extremely fine and uniform distribution of the ionized gas particles

in the melt is automatically ensured by the submerged rotat¬ ing hollow body.

It is also possible instead of direct current to apply directly alternating current. Furthermore, the graphite anode can be substituted by a plasma burner in order to provide an electric arc between inert materials, or an ac¬ tive anode can be used made of materials being dissolved in the melt and thus participating actively to the formation of nuclei. For example using a titan rod as an active anode will result in an additional provision of nuclei, preferen¬ tially Ti_ιθ ₇ , formed in situ by a reaction between the re¬ duced titanium and the melt constituents (oxides) .

Even if the above examples and description are based on the use of nitrogen and nitrides as grain refining means, it is obvious that also other materials can be used and conse¬ quently other refining means formed in the melt according to and without departing from the frame and spirit of the present invention. E.g. could carbides be formed in a similar manner as nitrides by substitution of nitrogen by a suitable carbon comprising gas.