US2472025A - Method of treatment of magnesiumbase alloys - Google Patents

Description

tented May 31, 1949 umrso sures T OFFICE METHOD OF TREATMENT OF MAGNESIUM- BASE ALLOYS John S. Peake. Bloomington, Ind., and Frank E. Robbins. Midland. Mich, assignors to The Dow Chemical C mpany, Midland, Mich, a corporation of Delaware No Drawing. Application November 5, 1947, Serial No. 784,302

4 Claims.

The invention relates to methods of treatin molten magnesium-base alloys. It more particularly concerns an improved method of treating a molten magnesium-base alloy whereby castings made therefrom exhibit a finer grain structure than that of similar castings made conventional- 1y. The invention has additional advantages as will appear as the description proceeds.

The term magnesium-base alloy is used herein and in the appended claims to mean an alloy of magnesium in which the magnesium content is at least about 85 per cent, the balance being one or more elements alloyable with magnesium. The usual alloying elements are aluminum, manganese and zinc, although small amounts of other elements such as calcium, cerium, and silicon are sometimes used. The most common of the structural magnesium-base alloys are the ternary alloys of magnesium, aluminum, and manganese, and the quaternary alloys of magnesium, aluminum, manganese, and zinc. Standard specifications of these ternary and quaternary magnesium-base alloys are found in the Dowmetal Magnesium Alloys Data Book, published October 1. 1944, by The Dow Chemical Company, Midland, Michigan.

It is known in a general way that the finer ,the grain size of a cast magnesium-base alloy article the better the tensile strength and related concomitant properties. It is a desideratum in the art of producing a magnesium casting for its structure to be of fine grain. As regards obtaining a fine grain size, it is known that the thermal history of the magnesium-base alloy in the molten state is a factor. It has been found,

for example, that by a prolonged superheating of the molten metal to at least 1472 F., i. e. 392 Fahrenheit degrees or more above the melting point (a temperature between 1600 and 1700 F. is usually employed), after the operation of relatively high temperature to which the molten i metal must be subjected to produce a given degree of grain size reduction promotes oxidation of metal and consumption of flux, if used, and the life of the melting equipment is shortened due to the more rapid deterioration which occurs at the high temperatures used. In addition, there is an increased hazard attendant upon handling this metal as it is very active chemically at the higher temperatures involved. Another disadvantage is that the time required to produce the superheating efiect is relatively long and this immobilizes melting equipment which otherwise could be used for the production of molten metal for castings. Still another disadvantage is that the superheated metal, being so much hotter than generally desirable for making castings, must be cooled to a suitable casting temperature before pouring into molds. This latter feature involves a further disadvantage in that the superheating effect rapidly disappears as the metal is cooled before casting so that the cooled superheat-treated metal must be poured into the molds soon after cooling if the grain refining effect of the superheating is to be retained. No method is now commercially available by which a body of a molten magnesium casting alloy may be maintained for long at casting temperature without grain coarsening occurring.

We have now discovered that by adding a carbide of an alkali earth metal to the conventional saline flux normally used for fluxing and protecting magnesium-base alloys in the molten state and maintaining an excess of the carbide in the flux while fiuxing the molten metal, castings made from the so-fiuxed molten metal exhibit a finer grain structure than that obtained when the metal is similarly treated with a saline fiux but in the absence of the added carbide.

In the usual fluxing procedures for magnesium and its alloys, the fluxes are generally composed of mixtures of chlorides of alkali and alkaline earth metals and may include a small amount of a fluoride. The common chlorides, e. g. sodium chloride, potassium chloride, lithium chloride, magnesium chloride, and calcium chloride are usually used in various combinations and proportions. These fluxes are usually quite thin but, for some purposes, are inspissated by the addition of magnesium oxide or calcium fluoride, or both may be used. The most commonly used 3\ mixture generally contain magnesium chloride and either sodium chloride or potassium chloride and a small amount of calcium fluoride. Similar fluxes are used for autogenous welding of magnesium alloys. In some instances, for foundry flux, barium chloride may be present to increase the specific gravity so as to cause the flux to float the molten metal. In the conventional use of such fluxes the molten metal is covered or sprinkled .with the flux which melts forming a film or crust over the metal and a considerable quantity of flux may underlie the metal and float the metal. The molten metal is usually stirred with the flux so as to bring all themolten metal into contact with the flux, the amount of flux being from about 5 per cent to per cent of the weight of the metal, although other proportions are sometimes used. The flux has a purifying action in that non-metallic impurities such as oxide or dross are taken up by the flux. After fiuxing, as by agitating the metal and flux, additional flux may be added to ensure coverage ofthe surface of the molten metal with flux and thereby prevent excessive atmospheric attack on the molten metal.

In carrying out the invention, the carbide added tothe flux may be in granular form. It is-preferable to employ the carbide in the form of pellets or lumps, such as pieces having an average diameter in the order of /2 to of an inch. Weprefer to use commercial calcium carbide, such as that used for the manufacture of acetylene, which contains about 80 per cent by weight of 'CaCz. The carbide is added to the moltenflux preferably while in the usual melting equipment,such as the ordinary open top melting pot. The carbide usually settles to the bottom ofthe melt if not agitated. As soon as the carbide-is added, a prompt and vigorous evolution of gasusually occurs indicating carbide decomposition, the reaction subsiding generally in less than one minute. Stirring the flux with the carbide sometimes tends to promote further gas evolution. Sufiicient carbide is used so that carbide-wilrbe presentinthe flux after gas evolution hassubstantially subsided. This addition of the carbide-to the flux may be made while the flux is in the-presence of the molten magnesium alloy or before the alloy is melted under the flux or afterward.

-We-havefound that a relatively small amount of thecarbide is usually sumcient, such as about 0.1 per cent of the weight of the metalto be fluxed and asmuch-as 5 per cent, or more, may be used. Ordinarily, We preferto use an amount of commercialicalcium carbide equal to about 1 per cent.

by weight of the alloy to be fluxed, the carbide being in the form of pellets having an average diameter of about of an inch. As moisture of the atmosphere in contact with the melt of flux and metal decomposes the carbide, it is advisable from-time to timeto either add additional amounts of' carbide to the flux, or if there-be an excessof; carbide present, to occasionally-stir the fiux soastobring it'into contact with the excess of settled carbide.

:Gn being fluxed with carbide-containing flux, the molten magnesium alloy undergoes a change such that when separated from the fluxand solidified byzcooling; asain-the usual castingoperation, the solidified metal exhibits a fine grain structure, the fineness of the grain depending upon the duration: ofcontact between the molten metahand the carbide treated flux until a maximumerain. sizereduction is obtained. We have found, for example, that by merely maintaining a molten magnesium-base alloy in contact with carbide-containing molten saline flux for one or two minutes, or more, the grain size is markedly reduced. Continuance of the contact of the molten metal and carbide-containing flux does not usually significantly further reduce the grain size of the treated metal. In addition, We have found that once the grain size has been reduced, the molten, metalmay be ,left in contact with the molten carbide-containing saline flux for a long time without loss of grain size reduction efiect. This is an important advantage because the treatfidmetalneed not be heated beyond ordinary pouring temperatures and may be maintained in the molten state for long periods in readiness for pouringinto molds to give castings having a fine grainstructure. Furthermore, no added equipmentis required in the foundry to produce fine grain structure castings over that needed for conventional melting and fluxing.

Theifollowing example is illustrative of a mode of carrying out the invention. A quantity of a magnesium foundry flux weighting about pounds composed of approximately 34 per cent of magnesium chloride, 55 per cent of potassium chloride} percent of calcium fluoride, and about 9 per centof barium chloride was melted in a conventional open cast steel pot, the temperature being-maintainedatapproximately 1350 F. To the molten flux was added approximately 900 pounds of a: magnesium-base alloy having a nominal composition-of 10 per cent aluminum, 0.1 pen centmanganese, 0.3 per cent zinc, the balance being magnesium, and the heating continuedzuntilthe metal was melted and its temperature:raised to1about-1400 F. Approximately 5. 'poundsof commercial calcium carbide was throwninto the pot and mixed with the flux by agitating-the'flux. in the presence of the metal whereupolra prompt and vigorous evolution of gas occurred. After the cessation Of'the vigorous evolution ofi-zgas, the molten metal was sampledfrom time to' time by withdrawing a portion and forming a test-casting. This'was sectioned; polished' etched, and "the etched metal examined fongrainsize. Irr-the as-melted condition, i. e. before'add f the carbide to the flux, the metal gave test castings having' grain sizes averaging about-:-0;Q1,o inch in, diameter. Fifteen minutes after the carbide treatment, the average grain size-was;0i:0,02;:inch.

Similarwtestswere made using 40 pound melts of a, magnesium-base alloy having a nominal composition of about 9 per cent aluminum, 0.1 percent-manganese, ;2- percent zinc, the balance being magnesium. In each ofthese tests, about 10 pounds ofvfiux ofthe same composition, as:in the foregoing example, was used to which was added,,-while melted andmaintained at about 1400911, 0.5, pound .of'commercial calcium carbide. After-ithe initial vigorous evolution of gas subsided, therbatchtof treated flux and melt of magnesium. alloy was maintained at about 1400 Hand periodically-examined to determine the iainesize oftest. castings made therefrom.

The results-obtained are-tabulated in the following table. showing the average grain size of thaaast-metal; atidifierent times after the commencement of the treatmentrof the, moltenmetal with the flux-rincontact with the carbide. For comparison; theaverage grain size of the cast metalsbefore the treatment of the metal commenced, ,that is, in the as-melted state, is also ven. In; the;-tab.1 data are iven for runs of.

the same alloy composition treated in similar manner.

The data in the table show that by treating the molten metal with the molten flux containing calcium carbide, the grain size of the metal cast after such treatment progressively decreases until a few minutes have elapsed, thereafter the grain size does not materially change.

The temperature at which the beneficial effect is obtained may vary over a considerable range, the lowest efiective temperature being approximately 1350 F. It does not appear to be necessary to operate the method at temperatures above 1400 F., although higher temperatures may be used. The process may be carried out advantageously at casting temperature.

In addition to reducing the grain size, the fluxing with the carbide-containing flux has the advantage of preventing loss of metallic calcium from magnesium-base alloys containing metallic calcium as an alloying constituent. This result is in sharp contrast to that obtained with flux not containing the carbide as in the latter case, the metallic calcium content, if any, of the alloy 6 declines during conventional fluxing as by holding the metal in a molten state in contact with a conventional flux.

This application is a continuation-in-part of Our copending application, Serial No. 587,222, filed April 7, 1945, now abandoned.

We claim:

1. The method of treating an aluminum-containing magnesium-base alloy which comprises maintaining the same in the molten state at a casting temperature in contact with a saline flux therefor, the flux having about 0.1 to 5 per cent of a carbide of an alkaline earth metal in admixture therewith, separating the so-treated metal from the flux, and casting the separated metal.

2. The method according to claim 1 in which the carbide is calcium carbide.

3. The method according to claim 1 in which the flux comprises a mixture of a chloride of an alkali metal and a chloride of an alkaline earth metal and the carbide is calcium carbide.

4. The method according to claim 1 in which the flux comprises a mixture of magnesium chloride and potassium chloride and the carbide is calcium carbide.

JOHN S. PEAKE. FRANK E. ROBBINS.

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

UNITED STATES PATENTS Number Name Date 2,429,221 Davis Oct. 21, 1947 2,436,520 Mahoney et al Feb. 24, 1948 OTHER REFERENCES Battelle Memorial Institute, Columbus. Progress Report(s) on Investigation of Cast Magnesium Alloys and of the Existing Foundry Techniques and Practices--Eastwood et al., December 4, 1944, page 104; and April 1, 1945, page 223.