US2107282A - Method of remelting and refining magnesium and light metal alloys containing magnesium - Google Patents

Description

Patented Feb. 8, 1938 UNITED STATES PATENT OFFIC Adolf Beck, Bitterfeld, Germany, assignor, by

mesne assignments, to Magnesium Development Corporation, a corporation of Delaware No Drawing. Application July 10, 1936, Serial No. 90,006. 1 In Germany July 19, 1935 7 Claims.

This invention relates to a method of re-melting and refining magnesium and light metal a1- loys containing magnesium, more particularly high percentage magnesium-base alloys.

It has of late frequently been asserted that the saline melts employed for re-melting and refining magnesium and high percentage magnesium-base alloys, and containing, besides chlorides of the alkaline-earth metals, andon occasion of the alkali metals, additions of thickening or inspissating substances such as metal oxides or fluorides, melt only at temperatures at which the metal to be treated is already in a molten state, and that, in consequence, the tendency of the molten metal to relatively extensive oxidation cannot be entirely suppressed by the fiuxing agent, which was supplied in the form of powder,

in the bottom of the crucible, or strewn over the metal to be melted, since at the melting temperature of the metal said fiuxing agent was still unmolten and thus incapable of protecting the metal from oxidation by the formation of a coherent layer.

In order to obviate this drawback, it has been proposed to employ fluxes fusing below the melting point of the metal under treatment, the intention being for the fiux, by melting first, to

form a protective cover on the metal which has not yet melted, or is in course of melting. However, in the case of these fluxes, which contain no inspissating agents, or only small amounts thereof, the thickening eflect obtainable by the addition of such inspissating agents is of course absent, that is to say, owing to their high fluidity,

or low surface tension at higher temperatureeven at such as are only about IUD-150 C. above the melting point of the metals under treatment-it is impossible to obtain a clean separation of the flux from the metallic regulus after the treatment is finished; and there is therefore always a risk of traces of fiux being entrained by, or included in, the metal, on pouring, such traces afterwards giving rise to the corrosion phenomena which, for a long time, made the 45 commercial employment of magnesium. or its alloys impossible.

Nevertheless, by suitably modifying the known process of re-melting and refining light metals, with the aid of chloride fluxes containing inspis- 59 sating or thickening agents, it is possible, on the one hand, to prevent any extensive oxidation of the metal during re-melting and, on the other, to secure a clean separation of the flux from the treated metal, under all conditions.

According to the present invention the known process of melting and refining magnesium and light metal alloys containing magnesium is modified with a view to ensuring an efiicient protection, from oxidation, of the metal to be treated, during the melting stage by the presence of an easily fusible flux, whilst at the same retaining the advantages resulting from the use of inspissated fluxes, namely complete removal of the flux from the metal after completion of the refining operation. I

To this end, according to the invention, the metal under treatment is first melted down in the presence of a thinly fluid, i. e., non-inspissated, flux containing one or more chlorides and having a melting point which is not substantially higher than that of the metal, and which on occasion, may be substantially below the latter. -Substances which have been found particularly suitable for this purpose are anhydrous carnallite, or mixtures still richer in potassium chloride such as for example, 2KCLMgCl2, having a melting point of 580 C. or less, that is to say, at least about 70 C. below the melting point of pure magnesium. The desired efiect may, however, also be obtained with pure anhydrous magnesium chloride, containing additions of chlorides other than potassium chloride, tending to lower the melting point.

In carrying out the invention, it is immaterial whether, by using a somewhat larger quantity of the flux, the metal melts down into a sump of flux in the bottom of the crucible, or whether the fiux be strewn as powder, over the metal to be melted, or added by degrees in accordance with the progress of the melting operation. In any event, the readily fusible fiux will afiord extensive protection to the metal against substantial oxidation during the melting operation. As soon as a substantial sump of molten metal has formed, further portions of flux may be applied, as a mobile protective coating, to the surface of the metal, in order to protect this portion of the metal from any extensive oxidation.

When the whole of the metal under treatment is in a molten state, the' melt is energetically stirred in the usual manner, in order to enable the flux to take up any impurities contained in the metal. This operation can be carried -out, without substantial oxidation of the metal, at temperatures below about 800 C. During this cleansing action, the known inspissating or thickening agents may be added to the metal bath, in suitable amounts for inspissating the flux already present, or alternatively, suitable amounts (according to circumstances) may be added, of a flux of fundamentally the same composition as that of the known fluxes containing inspissating or thickening agents, but preferably with a larger proportion of these latter than usual, in order that, in being contacted with the mobile flux already present in the metal bath, the newly added flux may also have an inspissating eifect thereon. The known mixtures of magnesium chloride with magnesium fluoride, calcium fluoride and/or magnesium oxide, or of calcium chloride with magnesium fluoride, are specially suitable for this purpose.

After the stirring has been completed, which, in the case of small units, usually does not take any longer than about 1 minute, but may be continued for a considerable time, or be frequently repeated, if necessary, in accordance with the amount of impurities contained in the metal, the melt is covered, in known manner, with a protective layer of a flux containing a normal proportion of inspissating agents, and is then heated to considerably abovethe pouring temperature for the purpose of separating the final traces of agglomerated flux from the metallic regulus. The metal is thereupon poured, on occasion after cooling to pouring temperature, and the coherent solid covering of flux is held back by suitable means such as rakes or the like.

The fluxes employed in carrying out the invention may consist merely of mixturesuof the ingredients. Sometimes, however, it is preferable to pre-melt the ingredients down together and to crush the mass after solidification, because this method ensures more intimate mingling of the metal or alloy.

components. If desired, salts of a metal which is already present in the alloy, may be incorporated with the flux, in known manner, in order to prevent the said metal being washed out during the treatment with the flux. The fluxes may moreover contain additions which become reduced by the metallic melt and enter into the Example A cast-iron crucible is first strewn with a thin fluxing layer consisting of anhydrous carnallite, and is then charged with about 30 kgs.of a high percentage magnesium alloy, in the form of ingots, and heated. Directly incipient melting is observable, further quantities of the same flux are strewn in small portions over the still solid lumps of metal, and, after a metal sump has formed, the surface of this also is covered with a little anhydrous carnallite. The total amount of flux used, up to the end of the melting process, is 200 grammes. T

When the temperature reaches 720 C. the metal is thoroughly stirred for about a minute with a stirring rod, and, at the same time, a. total amount of 250 grammes of a fiux, consisting of:

4 Per cent Anhydrous MgCh 70 MgO 10 Cal: 20

Per cent MgCla. MgO 10 CaFz 10 which soon sinters in the form of a coherent,

viscous layer. Under this 1: er, the contents of the crucible are heated at about 850 C. for a short time, then cooled to the pouring temperature of 760 C. and poured in known manner.

I claim: I

1. A method of re-melting and refining magnesium and light metal alloys containing magnesium and more particularly high percentage magnesium-base alloys, which comprises melting the metal in the presence of a thinly fluid saline flux having a melting point not substantially exceeding the melting temperature of the metal, then increasing the temperature of the metal, then stirring the metal at such elevated temperature with the said flux while adding a thickening agent in a quantity sufiicient to cause inspissation of the flux already present, so as to cause said flux to combine with said inspissating agent and with the non-metallic impurities contained in the metal, and finally pouring said metal.

2. A method of re-melting and refining magnesium and light metal alloys containing magnesium and more particularly high percentage magnesium-base alloys, which comprises melting the metal in the presence of a thinly fluid saline flux having a melting point not substantially exceeding the melting temperature of the metal, then increasing the temperature of the metal, then stirring the metal at such elevated temperature with the said flux while progressively adding portions of a thickening agent in an aggregate quantity suflicient to cause inspissation of the flux already present, so as to cause said flux to combine with said inspissating agent and with the non-metallic impurities contained in the metal, and finally pouring said metal.

3. A method of re-melting and refining magnesium and light metal alloys containing magnesium and more particularly high percentage magnesium-base alloys, which comprises melting the metal in the presence of a thinly fluid fiux' containing a substantial proportion of magnesium chloride and having a melting point not substantially exceeding the melting temperature of the metal, then increasing the temperature of the metal, then stirring the metal at such elevated temperature with the said flux while adding at least one thickening agent selected from the group of metal oxides and metal fluorides in a quantity sufficient to cause inspissation of the flux already present, so as to cause said flux to combine with said inspissating agent and with the non-metallic impurities contained in the metal, and finally pouring said metal.

4. A method of re-melting and refining magnesium and light metal alloys containing magnesium and more particularly high percentage magnesium-base alloys, which comprises melting the metal in the presence of a thinly fluid fiux containing a substantial proportion of magnesium chloride and having a melting point not substantially exceeding the melting temperature of the metal, then increasing the temperature of magnesium-base alloys, which comprises melting the metal in the presence of a thinly fluid flux containing a substantial proportion of magnesium chloride and having a melting point not substantially exceeding the melting temperature of the metal, then increasing the temperature of the metal to at least about 720 ,C., then stirring the metal at such elevated temperature with the said flux while adding at least one thickening agent selected from the group of metal oxides and metal fluorides in a quantity suflicient to cause inspissation of the flux already present, so as to cause said flux to combine with said inspissating agent and with the non-metallic impurities contained in the metal, and finally pouring said metal.

6. A method of re-melting and refining magnesium and light metal alloys containing magnesium and more particularly high percentage magnesium-base alloys, which comprises melting the metal in thepresence of a thinly fluid flux containing a substantial proportion of magnesium chloride and having a melting point not substantially exceeding the melting temperature of the metal, then increasing the temperature of the metal, then stirring the metal at such elevated temperature with the said flux while adding an inspissating agent comprising magnesium oxide in a quantity sumcient to cause inspissation of the flux already present, so as to cause said flux to combine with said inspissating agent and with the non-metallic impurities contained in the metal, and finally pouring said metal.

7. A method of re-melting and refining magnesium and light metal alloys containing magnesium and more particularly high percentage magnesium-base alloys, which comprises melting the metal in the presence of a thinly fluid saline flux having a melting point not substantially exceeding the melting temperature of the metal, then increasing the temperature of the metal, then stirring the metal at such elevated temperature with the said flux while adding a thickening agent in a quantity sufficient to cause inspissation of the flux already present, so as to cause said flux to combine with said inspissating agent and with the non-metallic impurities contained in the metal, establishing a protective layer of inspissated saline flux on the surface of the molten metal, heating the metal to a temperature above about 800 C., and finally allowing the metal to cool to pouring temperature. ADOLF BECK.