US2620270A - Method of improving magnesium and the binary magnesium-base alloy of magnesium and manganese - Google Patents

Description

J. K. HANEY ETAL METHOD OF IwRovma MASNESIUMAND THE] BINARY MAGNESIUM-BASEALLOY 0F MAGNESIUM, AND MANGANESE Filed April 5, 1950 awe/WW Joseph K. Haney Richard K. Fade/oak Patented Dec. 2, 1952 METHOD OF IMPROVING MAGNESIUM AND THE BINARY MAGNESIUM-BASE ALLOY OF MAGNESIUM AND MANGANESE Joseph K. Haney and Richard K. Paddock, Midland, Mich., assignors to The Dow Chemical Company, Midland, Mich, a corporation of Delaware Application April 3, 1950, Serial No. 153,616

5 Claims. 1

The invention relates to methods of improving magnesium and the binary magnesium-base alloy of magnesium and manganese. It more particularly concerns an improved method of producing a finely crystalline structure in casting of these metals.

When magnesium of the usual commercial quality and the well-known binary magnesiumbase alloy of magnesium and manganese which may contain up to about 2.5 per cent of manganese, the balance being magnesium, are solidified from the molten state, as in casting an ingot or other form thereof, the crystal structure is almost invariably coarse and columnar.

The coarse crystal or grain structur of castings of these metals predisposes them to cracking and disintegrating on being mechanically Worked, as by rolling, and militates against the development of high tensile and compressive strengths and high elongation. Other magnesium-base alloys are oftentimes of coarse structure on being cast but methods are available by which grain refinement can be had at least with some of these alloys.

For example, it is common practice with magnesium-base alloys containing upwards of about 3 per cent of aluminum to grain refine them by superheating. In the superheating process, the molten alloy is held at an elevated temperature,

e. g. about 360 Fahrenheit degrees or more above the melting point, for about 10 to 30 minutes before cooling to casting temperature and pouring into the mold. On applying this method to either commercial or pure magnesium itself or to the said binary magnesium-manganese alloy, no rain refinement is obtained. Another method that has been suggested for producing a grain refining effect is to bubble nitrogen through the molten metal for a short time prior to casting. While this method produces some grain refining efiect on some of the magnesium-base alloys, such as those containing upwards of about 3 per cent of aluminum, the method does not refine the grain of magnesium or the said binary alloy of magnesium and manganese.

For refining the grain of the usual binary magnesium-base magnesium-manganese alloy, it has been proposed to add a few tenths of 1 per cent of calcium. However, this has the disadvantage that a rather larger addition of calcium is thus called for, thereby producing essentially a ternary alloy of magnesium, manganese, and calcium, and it is by no means certain that a grain refining efiect will be obtained consistently.

Smaller calcium additions have no significant nesium or the magnesium-base-manganese binary alloy can be grain refined consistently.

It is the principal object of the invention to provide a method of insuring the production of an equiaxed finely crystalline structure in a casting of magnesium and the binary alloy of magnesium and manganese, the manganese content being up to about 2.5 per cent.

A specific object is to provide a method of grain refining the usual binary magnesium-base alloy having a nominal manganese content of about 2 per cent, the balance being commercial magnesium.

Other objects and advantages will appear as the description of the invention proceeds.

The invention is based upon the discovery that by bubbling nitrogen through a melt of either vmagnesium itself or the aforesaid binary alloy containing up to about 2.5 per cent manganese while there is present in the melt from 0.0025 to 0.015 per cent of aluminum, thereafter adding to the melt between 0.01 to 0.2 per cent of calcium and promptly casting the so-treated melt, a grain refined casting is obtained. The amounts of aluminum and calcium which are thus employed to obtain grain refinement of magnesium and the said binary alloy, in conjunction with the nitrogen treatment, are insufiicient to alter the usual values of the mechanical properties of these metals in the unrefined state. The amount of nitrogen bubbled through the melt does not appear to be sharply critical, although the duration of the bubbling preferably exceeds about 5 minutes, to minutes being generally preferred. The larger the quantity of metal in general the longer the duration of the treatment with the nitrogen. For example, 50 pounds of molten metal may be bubbled with nitrogen sufficiently in about 1 0 minutes; 4000 pounds in about 15 to minutes.

The method of the invention may be applied to unalloyed magnesium, that is, either pure magnesium or magnesium of the usual commercial grades. The latter contains small amounts of impurities, the aggregate amount of which is usually less than about 0.1 per cent of the Weight of the magnesium. The invention has its principal utility as a method of controlling the fineness and form of the crystal structure of the binary alloys of magnesium and mangane se. In these binary alloys, the manganese content is usually near saturation, i. (5., it may be as high as about 2.5 per cent, although to avoid the presence of excessive amounts of primary manganese particles in the cast metal, the manganese content is usually held within the range of 0.5 to 2.2 per cent, and preferably between about 1.3 per cent and 2.0 per cent.

In carrying out the invention, the metal to be treated is melted preferably under a conventional magnesium foundry fiux or otherwise protected from atmospheric attack. These fluxes usually contain magnesium chloride and either, or both, potassium and sodium chloride as the main constituents together with a minor amount of barium chloride and calcium fluoride, as is well-known in the art. After melting the metal, a small amount of aluminum is introduced and alloyed with the melt, if not already present therein, in proper proportions, i. e. 0.0025 to 0.015 per cent by weight. Commercial magnesium and the binary magnesium-manganese alloys sometimes contain a small amount of aluminum as an impurity and the amount may be sufficient for the purpose of the invention. However, we have found that it is nevertheless desirable, even when the amount of aluminum in the metal to be treated and as melted is well within the aforesaid range of 0.0025 to 0.015 per cent, to add one or two thousandthsof a per cent more, particularly if the initial aluminum content is in the lower portion of the range, i. e. 0.0025 to about 0.009 per cent. After adjusting the aluminum concentration in the melt to within the range of 0.0025 to 0.015 per cent and preferably adding 0.001 to 0.003 per cent in addi tion to the initial proportions of aluminum when these are initially between 0.0025 and 0.009 per cent, the melt may be gently agitated to insure uniformity of distribution and alloying of the aluminum in the melt. The temperature at which to conduct the foregoing operation is not critical and may be between about 1250 and 1500 although other temperatures may be used.

.The aluminum treated melt thus obtained is then treated with nitrogen gas by bubbling it through the melt. A convenient way to carry out the nitrogen gas treatment is to insert an iron pipe into the melt so that one end is near the bottom of the melt and discharge the gas into the melt from the pipe, thereby allowing gas bubbles to rise upwardly through the melt. Agitation of the melt beyond the slight amount produced by the rising gas bubbles is unnecessary. The temperature of the melt during this operation may be between about 1300 and 1550 F., although a temperature of about 1400 F. is preferred.

Following the nitrogen gas treatment, calcium is added, the manner of addition being the same as in usual alloying practice but a much smaller amount, i. e. 0.01 to 0.2 per cent by weight, is to be present in the melt while the temperature preferablyis not over about 1400 F. We have discovered also that the grain refining effect which is had upon adding the calcium, after the nitrogen gas treatment in the presence of the requisite concentration of aluminum according to the invention, may be lost in a matter of about 1'7 to 20 minutes after the calcium addition. The treated melt, therefore, is to be cast preferably promptly, e. g. in a matter of to 15 minutes after the calcium has been incorporated.

We have also found that a melt of the said magnesium or magnesium-manganese binary alloy having the aforesaid aluminum concentration and treated with nitrogen by bubbling it through the melt may be solidified and put into storage if desired, and at a future time, on being remelted and treated with the aforesaid specified amount of calcium, will yield grain refined metal. In contrast, similar melts either not treated with nitrogen or not containing the specified amount of aluminum, though treated with nitrogen, solidified and stored, will not become grain refined on remelting and adding the specified amount of calcium. As a consequence, a stock of metal which is susceptible of grain refinement by the addition of calcium to produce a small concentration of calcium, i. e., 0.01 to 0.2 per cent in the melt, can be prepared by adjusting the aluminum concentration to the aforesaid range, bubbling nitrogen through the melt, and then casting the so-treated metal.

The invention may be further explained and exemplified by reference to the accompanying drawing setting forth a series of five photomicrographs, viz. Figs. 1 to 5, inclusive, each at a magnification of 3 diameters, of typical grain structures of cast specimens of magnesium or the magnesium-manganese binary magnesium-base alloy, showing equiaxed crystals of grain refined metal as well as progressively larger and more elongated crystals from one specimen to the next.

The photomicrograph of Fig. 1 is a typical section of fully grain refined cast magnesium or binary magnesium-base-magnesium manganese alloy. It will be observed that the crystals are equiaxed.

In Fig. 2, a typical partially grain refined cast specimen is shown in which the crystals are somewhat elongated and larger than those of fully refined, metal.

In Fig. 3 is shown a typical specimen of unrefined cast metal, the crystals of which are of columnar form and moderately coarse.

In Fig. 4 is shown a typical specimen of unrefined cast metal, the crystals of which are of columnar form and more coarse than those of Fig. 3.

In Fig. 5 is shown a typical specimen of unrefined cast metal, the crystals of which are of colunlnar form and more coarse than those of Fig.

These photomicrographs are to be used as standards of comparison of various grain structures exhibited by castings of either magnesium or the aforesaid magnesium-manganese binary alloy in determining when grain refinemnt has been obtained. In obtaining each cast specimen for the photomicrographs, a sample of the metal was cast in a cylindrical iron mold 1 x12". The casting was turned down to diameter and cut transversely of the long axis to obtain a cross section which was polished and then suitably etched to reveal the crystal structure. A suitable etcher is a solution of 5 grams of picric acid dissolved in enough ethyl alcohol ,tomake cc. of solution to which is added 10 cc. of distilled water and 5 cc. of glacial acetic acid. The specimen to be etched is placed face up in the solution for about 15 seconds with gentle agitation, rinsed with ethyl alcohol, and then dried in an air current.

The individual crystals whose cross section lies in the plane of the photomicrograph are manifest generally by differing one fromv the other as to color or shade of grey so that the boundaries of the adjacent crystal sections are generally manifest as sharp contrasts in shade.

Unless the metal is of refined grain, in which case all the crystals are substantially equiaxed, the crystals are generally columnar in form and extend from the outside of the specimen toward and generally perpendicularly to the central axis.

In determining whether or not grain refinement has been obtained in a melt, a test specimen is cast from the melt at the time the melt is to be cast. The test specimen is made'in the same manner as that used in preparing the standards and the appearance of the etched section (or a photomicrograph of it) at a magnification of 3 diameters, is compared with the standards by matching it with the standard it most nearly resembles. In some instances, the test specimen will have either a more refined structure than that of Fig. 1 or a coarser structure than that of Fig. 5. All others will have structures intermediate in crystal sizes between the various figures of the series. For convenience, the types of grain structures exhibited by test specimens prepared as described are given a numerical rating based upon their order in the series of standards as set forth in Table I.

TABLE I Table of grain type ratings crystals are no larger in general than those of Fig. 2 and are generally equiaxed. In other words, a structure having a grain type rating of 2 and lower is grain refined; structures whose 5 grain type rating is 3 or higher are not grain refined.

In Table II are tabulated data on four series of tests A, B, C and D on four melts illustrating 10 the effect of the aluminum content, the nitrogen 20 manganese as a typical magnesium-base binary magnesium-manganese alloy. The melts of series C and D were commercially pure mag- Appearance in structure Oftest specimen nesium. In carrying out the tests, a 50 pound rating load of the metal was melted in a steel crucible E uiaxed and finer than F1 .1 25 and maintained continuously in the molten state L i F 1 g 1 1 0 1g. Eguiaxpd and coarserthan Fi 2 during each series wh le the effect of the alu Llk F minum content, the nltrogen bubbling, and the Coarser than 4 Lik Fig 3 calcium content Were ascertained by taking 533: 30 samples as indicated in the table, casting a test (lswrsettha 8 specimen for grain type rating and analyzing Like Fig. 9 Coarser than Fig.5 for the aluminum and calcium content of the melt.

TABLE II Treatment Results-Metal Minutes analysis M 1 G f elafrzsed G e t rams o a er rain Test series 1 Kind of melt te mp metal added :5 2? trestgg type F of Na g gg Percent Percent mung Alumi- Calsampled A1 Ca num 011.1111

1 Mlg-Mn al- 1, 400 0.01 l0 0y. 2 Continued 1, 400 0. 14 3 1\ 3 Continued. 1,400 0.13 6 4 Continued. 1, 400 5 Continued. 1, 400 0. 11 0 6 Continued. 1, 400 0. 09 0 5 t i i283 8'ii on in e 9 Continued. l, 400 O. 14 5 10 Continued. l, 400 0. 01 10 B 11 Continued. l, 400 U. 14 0 12 Continued. 1, 400 0. 13 0 l3 Continued. 1, 320 0. 01 10 14 Continued. 1, 320 0.13 0 15 Continued. 1,320 0. l3 0 l6 Magnesium. l, 400 0. 01 l0 l7 Continued. 1, 100 0. 123 10 C 18 Continued. l, 400 0. ll 10 l9 Continued. 1, 400 0. 01 1O 20 Continued. l, 400 0.09 0 21 Continued. 1, 4110 0. 062 0 22 Magnesium. 1, 400 0. 01 10 23 Continued. 1,400 0. 12 7 24 Continued. 1, 400 0. 10 7 D 25 Continued 1, 400 0. 01 10 26 Continued. l, 400 0. l2 0 27 Continued. l, 5100 0. 12 l) 28 Continued. 1, 400 0. 01 10 29 Continued. 1, 10 0 0. 127 0 1 Held 1 hour.

For the purpose of distinguishing a structure which is to be regarded as having been grain refined from structures which are not grain refined, the limit of coarseness is that of a struc- Referring to Table II and more particularly to the tests of series A, it will be seen that the metal as melted (row No. 1) is unrefined and remains unrefined in spite of the addition of calcium (row ture lying between Figs. 1 and 2 in which the 2) and holding molten 5 minutes thereafter (row 3). The unrefined grain type is typical of castings of melts of the alloy in which the aluminum content is less than 0.0025 per cent even though the calcium content may exceed 0.01 as with samples A2 and A3. On adding 1 gram of aluminum so as to bring the aluminum content above 0.0025 per cent and bubbling with nitrogen through the aluminum treated melt (row 4) and then adjusting the calcium content (row 5) grain refinement is obtained as shown by sample A4 taken 5 minutes after the calcium addition and sample A5 taken minutes after sample A4.

Similar results are had in the tests of series B in which as melted the alloy yielded a typical unrefined type of grain structure. The unrefined structure persisted after the addition of calcium as shown by sample B2, taken 5 minutes after the addition, showing the presence of 0.14 per cent of calcium and 0.002 per cent of aluminum (row 8). Holding minutes longer (row 9) and then adding 1 gram of aluminum and bubbling with nitrogen for 15 minutes (row 10) increased the aluminum content to 0.0044 per cent and caused a loss of calcium to below 0.01 per cent, the grain type rating thereby becoming 10. Raising the calcium content to 0.14 by adding 42 grams of calcium (row 11) resulted in the aluminum content becoming 0.0034 per cent, the calcium content 0.14 per cent and the grain showing the maximum refinement, i. e. type rating of 0 as shown by sample B5. The refinement persisted at least another 15 minutes as shown by sample B6.

The same melt thereafter yielded unrefined cast metal when 06 gram of aluminum was added and nitrogen was bubbled through the melt (row 12) As shown by sample B1, the resulting aluminum content was 0.0064 and the calcium content declined to below the minimum of 0.01 at which grain refinement would be had following the nitrogen treatment when the aluminum content is within the prescribed range. This coarse grained melt (row 13) promptly became grain refined on adding 38 grams of calcium as shown by sample B8 taken 5 minutes after the calcium addition (row 14). As shown by sample B8 both the aluminum content and the calcium content were within the prescribed ranges of the invention to achieve grain refinement on being treated with nitrogen before the calcium addition. The calcium content and the aluminum content remained within the prescribed ranges of the invention during the following 15 minutes as shown by sample B9 (row 15) and the grain type ratin remained 0.

Again referring to Table II, the C series of tests on commercially pure magnesium shows that the metal as melted exhibits maximum coarseness of structure as shown by sample Cl. This coarseness persists even with a calcium concentration of 0.123 per cent (sample C2) and 0.11 per cent (sample C3) The coarse structure still remains after adding 2.3 grams of aluminum and bubbling with nitrogen 15 minutes (row 19). Following the nitrogen treatment, the addition of 40 grams calcium promptly changes the grain type rating to 0 while the aluminum and calcium contents of the melt are 0.008 per cent and 0.09 per cent, respectively, (row 20). The grain type rating of 0 remains for at least 15 minutes after the calcium concentration adjustment as shown by sample 06. Similar results are shown in the D series of tests. Besides showin the production of grain refined metal where all three conditions of the invention are met, viz. proper aluminum concentration in the melt followed by a nitrogen bubbling and adjustment of the calcium concentration to the proper range, the data of row 28 shows that the refining effect is lost on further treat ment with nitrogen with which a loss of calcium occurs, but the refining efiect is restored upon restoring the calcium concentration to the proper range (row 29).

This application is a continuation-in-part of our copending application Serial No. 714,660, filed December 6, 1946, now abandoned.

We claim:

1. The method of treating magnesium and the binary magnesium-base magnesium-manganese alloy containing up to about 2.5 per cent of manganese so as to insure a grain refined structure being obtained on casting a melt thereof which comprises bubbling nitrogen through a melt of the metal while its aluminum content is from 0.0025 to 0.015 per cent, adding calcium to the so-treated melt in amount sufficient to produce a calcium concentration in the melt of from 0.01 to 0.2 per cent, and casting the calcium treated melt within 15 minutes after the addition of the calcium.

2. The method of producing a grain refined cast structure of a magnesium-base binary magnesium-manganese alloy containing up to about 2.5 per cent of manganese which comprises bubbling nitrogen through a melt of the metal while the melt is maintained at a temperature between about 1350 and 1550 F. and contains from 0.0025 to 0.015 per cent of aluminum, adding calcium to the nitrogen treated melt in amount sufficient to produce a calcium concentration in the melt of 0.01 to 0.2 per cent, and solidifying the so-treated melt within 15 minutes after the calcium addition.

3. The method of producing a grain refined cast structure of a magnesium-base alloy containing up to 2.5 per cent of manganese, the balance being magnesium which comprises heating the alloy to a tempertature between about 1350 and 1550 F., adding aluminum to the resulting melt in amount sufiicient to produce an aluminum concentration in the melt of 0.0025 to 0.015, bubbling nitrogen through the melt for from about 5 to 20 minutes, adding calcium to the melt in amount sufiicient to produce a calcium concentration in the melt of 0.01 to 0.2 per cent, and solidifying the calcium treated melt within 15 minutes after the calcium addition.

4. The method of producing a grain refined cast structure of a magnesium-base alloy containing up to 2.5 per cent of manganese and 0.0025 to 0.009 per cent of aluminum, the balance being magnesium, which comprises heating the alloy to a temperature between about 1350 and 1550 F., adding aluminum to the resulting metal in amount suificient to increase the aluminum content above that already present by an additional 0.001 to 0.006 per cent, bubbling nitrogen through the resulting melt for from 5 to 20 minutes, adding calcium to the aluminum treated melt in amount sufficient to produce a calcium concentration in the melt of 0.01 to 0.2 per cent, and solidifying the calcium treated melt within 15 minutes after the calcium addition.

5. The method of producing a grain refined cast structure of a magnesium-base alloy containing up to 2.5 per cent of manganese, the balance being magnesium which comprises heating the alloy to a temperature between about 1350 and 1550 F., adding aluminum to the resulting melt JOSEPH K. HANEY. RICHARD K. PADDOCK.

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

UNITED STATES PATENTS Number Name Date 1,720,436 Pistor July 9, 1929 1,956,971 Bars-tow May 1, 1934 2,267,862 Hanawalt Dec. 30, 1941 2,380,836 Hanawalt July 31, 1945 2,436,520 Mahoney et a1 Feb. 24, 1948 2,448,993 Mahoney et a1 Sept. 7, 1948 OTHER REFERENCES Battelle Memorial Inst. Report W-133, pp. 52, 54, 65, 66, 85, 88 and 89. July 26, 1944.

The Technology of Magnesium and Its Alloys 15 by Beck, pp. 134 and 135. 1940.

Metallography of Magnesium and its Alloys by Bulian et 2.1., published 1944 by F. A. Hughes 8a Co., Ltd., London, England, pages 33, 53 and 54.

Claims (1)

1. THE METHOD OF TREATING MAGNESIUM AND THE BINARY MAGNESIUM-BASE MAGNESIUM-MANGANESE ALLOY CONTAINING UP TO ABOUT 2.5 PER CENT OF MANGANESE SO AS TO INSURE A GRAIN REFINED STRUCTURE BEING OBTAINED ON CASTING A MELT THEREOF WHICH COMPRISES BUBBLING NITROGEN THROUGH A MELT OF THE METAL WHILE ITS ALUMINUM CONTENT IS FROM 0.0025 TO 0.015 PER CENT, ADDING CALCIUM TO THE SO-TREATED MELT IN AMOUNT SUFFICIENT TO PRODUCE A CALCIUM CONCENTRATION IN THE MELT OF FROM 0.01 TO 0.2 PER CENT, AND CASTING THE CALCIUM TREATED MELT WITHIN 15 MINUTES AFTER THE ADDITION OF THE CALCIUM.

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