US2464610A - Method for producing aluminumsilicon alloys - Google Patents

Description

Patented Mar. 15, 1949 Albert Reg-net, ,Pr-aha -Smichov, Czechoslovakia, assignor to Spole'k Pro Chemickou a ,Hutni Vyrobu,

also named UnitedJChemical and Metallurgical Works, Ltd Pi-aha, Czechoslovakia, acorporation' of Czechoslovakia I No Drawing. vApplication -.I -une 26, 1946, Serial No. 679,592. In v.GzechoslovakiaJuly 8, 1940 Section 1, Public liaw 690,-August 8,1946. Patent expires July 8, 1960 aclaims. 1

My invention relates to a methodforiproducing industrially important aluminum si'licon alloys.

It hasbeen proposed to prepare a eutectic or: a supereutectic aluminum-silicon alloy by fractional crystallization of acrude alloy containing more than 12% Si and accompanying'metallic impurities. Of these impurities, iron,'titanium and carbides have 'anespecially deleterious :eiiect on the end products preparedfromsuchalloys. However, inasmuch as the two lastimentioned impurities can be adequately eliminated by such fractional crystallization, there is no further need to consider them hereinafter.

The crude alloy is prepared from oxides ofaluminum and silicon, forinstance either by electrothermic reduction of -a mixture of kaolin and aluminum oxide with the aid of carbon, or by electrolysis of a fused bath ofbauxite, to which may be added some earth. As is well known, the separation'may beachieved in several different ways. According to one method, the crude alloy is subjected to fractionalcrystallization so that it is allowed to cool gradually from =a-completely iuse'd state to a certaintemperature which'lies in the liquidus-solidus reg-ion and which temperature corresponds "to the desired composition of the liquidus, whereupon the resulting crystals rich in silicon are'separated, either by centrifuging-or by vacuum or pressure filtration,-'fromthe' mother melt-which is concomitantly enriched in aluminum,

According to another method, theseparation is achieved by filtration, whereby thecrude alloy, previously treated thermally'soas to yield as large silicon crystals as possible, is carefuily-*heated abovethe eutectic temperature and the sieved out liquid part is separated from the solid phase, by filter-pressing for example.

The composition of theliquidus separatedin such operations is controlled by the equilibrium diagram of the system concerned, according to which Al-SiFe alloys form a ternary eutectic which melts at 577 C. and which contains,11.6% Si, and 0.80% Fe, the balance being Al. However, as accordin to this diagram the technical crude alloys contain not onlysilicon, but-also iron inasupereutectic amount, it is apparent that the liquidus-separated from thecrude alloy at a-tenlperature slightly above the eutectical will contain the smallest amounts of these metals, whereas melts isolated at highertemperatures ---wil1 .contain not only more silicon, but also more 'iron. Experiments performed with this object in mind completely confirmed the correctness of the equilibrium diagram. Thus, a liquidus separated from the-crude alloy at 579 C. showed practically an identical composition with that of the above said eutectic, whereasa separation performed at 588 C. -yielded a product containing 13.6% Si and 0.90% Fe.

Although according to some patents suchproducts, isolated from crude and impure alloys at low temperatures,-were considered to be equivalent to the .soecalled silumin, a castingalloy of. considerable technical importance and of'precisely formulated. composition, nevertheless in fact they merely complied with the specification regarding the. required content of silicon, but did not satisfy the purity requirement because they contained about -=0.85% Fe, representing an iron content almostztwice as high as the admissible maximum. Experience teaches that an iron content in excess ofi 0.5% i has a considerable harmful effect on the mechanical properties of silumin-type alloys. This; circumstance, 1 therefore, i was the principal cause why the above described production method never-became popular inindustrial practice.

"11511136 now'been discovered that the iron content is re'duced'and mechanical properties are improved if a certain amount of chromium, cobalt,

or manganese is alloyed with the crude alloy beforehand, and only thisadjusted alloy is then subjected to the separation process at a temperaturejust above the eutectical. However, it is equally: advantageous to'alloy'the Cr, 00 or Mn only into theliquidus which was first separated from .CI'LKXG'BIHOYS by known methods, without the addition of heavy metals, at a temperature close to the eutectical. This liquidus thus contained thetprescribed-amountof silicon but also had an inadmissi'bly high iron content. By the following separation of this intermediate product, at substantially the same temperature as that of the firstoperatiomthe iron content of they product is reduced to the desired extent without further changing the ratio of aluminum to silicon.

The said metals are'alloyed either by themselves .orin a mixture, and the added material advantageously amounts to 0.5 to 2 times the amount-of iron present in the original alloy. Under such conditions the liquidus isolated at a temperature close to the eutectic temperature contains besides 12-13.5% Si only 0.25-0.40%--Fe and .0;3-0.5% of? those metals which were added tolthezoriginal mixture in order to reduce the iron content. The, presence ofthese small amountsof heavy metals isin no way harmful to the quality of the product, on the contrary, as is even illuscomposition of silumin, such heavy metals have a very favorable effect in that they improve the metals can be added immediately to the crude alloys so that the iron is eliminated in the desired amount simultaneously with silicon, or they can be added only to the alloy of the finally desired silicon content but which still contains too much iron. This excessive iron is then eliminated in a separate process. Furthermore, the new process can be employed also for the regeneration of aluminum-silicon alloy scrap if such scrap is contaminated with an inadmissibly high content of iron which is dissolved in the alloy. Inasmuch as it is then usually desired to eliminate the iron only, but not to change the aluminumsilicon ratio, in such instances the separation is conducted at a temperature just sufiicient to keep both principal metals of the alloy in a molten state.

It is possible to introduce the heavy metals into the original crude and impure alloys by direct alloying of the pure metals or of appropriate prealloys, or by an exchange reaction between chlorides or other convenient compounds of the heavy metals and a melt of the original alloy, or finally by the simultaneous reduction of the oxides of the heavy metals in the case of electrothermic or electrolytic processes.

My new method for reducing the iron content of aluminum-silicon alloys is entirely distinct from another known method which, by the addition of Mn, Co or Cr, merely reduces the harmful effect that an excessive iron content in aluminum-silicon alloys has on castings. Alloys modified by the 01d method are suited only for pressure injected die castings, but, in view of their high Fe content they are not usable for castings in moulds or in sand, because the iron has not been eliminated but merely transformed into a structurally less harmful form. Besides, in the old process the improving effect of thesaid heavy metals can be utilized only up to a certain maximum iron content and therefore the alloys of particularly high iron content were unusable even for die-castings.

The following examples are recited here in order to illustrate the present invention:

Examples 1. A crude aluminum alloy, which was pre pared in an arc furnace and from which carbides were eliminated previously, and which contained 30.5% silicon and 1.3% iron, was gradually cooled after fusion so that the separating crystals, consisting predominantly of silicon, formed as large individual crystals as possible. As soon as the temperature dropped to 588 C., that is 11 C. above the eutectic temperature of the Al-Si-Fe system, the alloy was subjected to 4 a pressure filtration. The isolated liquid phase had a composition of 0.9% Fe, 13.6% Si, and the balance Al.

2. The crude alloy of the above said composition was subjected to a further separation process in which, however, the temperature before filtration was allowed to drop as far as 579 C., i. e. 2 0. above the eutectic temperature. The isolated liquid phase contained 0.75% Fe, 11.6% Si, balance A].

3. To this same alloy manganese was added in an amount corresponding to 1.3 times the amount of iron contained therein. By filtration at a temperature of 588 C. a fraction was obtained containing 0.48% Fe, 0.38% Mn, 13.5% Si, balance Al. 5

4. The process illustrated by Example 3 was repeated with the difference that the melt was cooled to 579 C. before filtration. The resulting liquidus contained then 0.36% Fe, 0.28% Mn. 11.6% Si, balance Al.

Although I have described certain specific em-- bodiments of my new method, I am fully aware that many variations and modifications may be made in the details of procedure above set forth without departing from the spirit of my invention. My invention, therefore, is not to be restricted except insofar as is necessitated by the prior art and by the spirit of the appended claims.

What I claim is:

1. A process for decreasing the impurity of iron in aluminum-silicon alloys to less than 0.5% said process comprising melting the impure aluminum-silicon alloy, adding thereto at least one metal selected from the group consisting of Mn, Co, and Cr,'in amounts equal to about 0.5 to 2 times the total weight of iron present in the impure alloy, and thereafter slowly cooling said alloy and separating the solidus containing the major portion of iron from the liquid phase containing less than 0.5% iron at a temperature above the fusion point of the eutectic.

2. A process for decreasing the impurity of iron in aluminum-silicon alloys to less than 0.5% said process comprising melting the impure aluminum-silicon alloy, adding thereto at least one metal selectedfrom the group consisting of Mn, Co, and Cr, in amounts equal to about 0.5 to 2 times the total weight of iron present in the impure alloy, and thereafter subjecting said alloy to fractional crystallization and separating the solidus containing the major portion of iron from the liquid phase containing less than 0.5% iron at a temperature above the fusion point of the eutectic.

ALBERT REGNER.

REFERENCES CITED The following references are of record in the file of, this patent:

UNITED STATES PATENTS OTHER REFERENCES Metal Industry (London), vol. 52 (1938), pages 631-636.