US2472757A - Mechanical method for grain refining magnesium alloys - Google Patents

Description

2 Sheets-Sheet 1 June 7, 1949. J. 5. PEAKE MECHANICAL METHOD FOR GRAIN REFINING MAGNESIUM ALLOYS Filed Feb. 16, 1945 INVENTOR. Jo/m 6T pea/(e A TTORNEYS June 7, 1949. J. 5. PEAKE 2,472,757

MECHANICAL METHOD FOR GRAIN REFINING MAGNESIUM ALLOYS Filed Feb. 16, 1945 2 Sheets-Sheet 2 I N V EN TOR. Jofin S. Pea/4e A TTORNE Y5 Patented June 7, 1949 MECHANICAL METHOD FOR GRAIN REFIN- ING MAGNESIUM ALLOYS John S. Peake, Midland, Mich., assignor to The Dow Chemical Company, Midland, Mich., a corporation of Delaware Application February 16, 1945, Serial No. 578,254

6 Claims. (Cl. 75-67) The invention relates to a method for treat ing molten magnesium-base alloys prior to casting. It more particularly concerns a method for treating the molten metal prior to casting into molds. This latter feature involves a further disadvantage in that the superheat eflect rapidly disappears as the metal is cooled before casting so that the cooled superheat-treated whereby reduced grain size and enhanced physimetal must be poured into the molds soon after cal properties are secured in the cast metal. cooling if the grain refining effect of the super- It is known that in a general way, the finer heating is to be retained. the grain size of the cast metal the better the I have now discovered that by subjecting the tensile strength and related concomitant propmetal while in the molten state to a violent agierties. It is also known that the thermal history 0 tation (as distinguished from the ordinary stirof the metal in the molten state has some effect ring used in purification under a flux) by which on the grain size of the cast metal. It has been the liquid metal undergoes intense shearing found, for example, that by superheating the causing contiguous portions of liquid to slide over molten metal to at least 1472 F., i. e. 392 Fahreneach other at a high rate, the grain size of heit degrees or more above the melting point (a castings made of the so-treated metal is greatly temperature between 1600 F. and 1700 F. is reduced, and, at least, the usual advantages of usually employed), after the operation ofpurigrain size reduction are obtained without many fication by puddling the metal under a suitable of the disadvantages of the superheating method flux before casting, the grain size of castings heretofore used to reduce grain size. In parmade therefrom is markedly smaller than that ticular, this result is obtained at a lower temof castings of non-superheated metal and the perature and in less time than by the method mechanical properties are improved. heretofore known. As a consequence, less time This procedure of superheating before casting, is required to prepare the metal for making as a method of reducing grain size and improving castings having a fine grain structure, less loss properties, has certain disadvantages which 5 of metal occurs, consumption of flux, if used, is leave much to be desired. Some of these are reduced, and the life of melting equipment is prothat the relatively high temperature to which longed and its output increased. the molten metal must be subjected to produce The invention, then, consists of the method a given degree of grain size reduction promotes hereinafter fully described and particularly oxidation of metal and consumption of flux, if pointed out in the claims, the annexed drawing used, and the life of the melting equipment is and following description setting forth a mode shortened due to the more rapid deterioration of carrying out the invention and apparatus which occurs at the higher temperatures used. the efor. In addition, there is an increased hazard at- In the drawings: tendant upon handling this metal as it is very Fig. 1 is a side elevation largely in section of active chemically at the higher temperatures apparatus for maintaining and treating metal involved. Another disadvantage is that the time in the melted state in accordance with the inrequired to produce the superheating effect is vention, relatively long and this immobilizes melting Fig. 2 is an enlarged side elevation of a portion equipment which otherwise could be used for of the apparatus of Fig. 1. the production of molten metal for castings. Fig. 3 is a plan view of the upper of the two Still another disadvantage is that the superapertured plates of the portion of the appaheated metal, being so much hotter than generratus shown enlarged in Fig. 2. ally desirable for making castings, must be cooled Fig. 4 is a plan view of the lower of the two to a suitable casting temperature before pouring apertured plates.

Fig. 5 is a view of the rotor attached to the lower end of the driving shaft shown enlarged in Fig. 2.

Fig. 6 is a modification of a. portion of the apparatus of Fig. 1.

Fig. 7 is a sectional view on the line 1|' of Fig. 6.

Fig. 8 is a fragmentary view of a portion of Fig. 6.

Throughout the drawings, like numerals refer to like parts.

As shown completely assembled in Fig. 1, the apparatus comprises a melting pot I mounted by its rim 2 in a furnace setting 3 provided with a heating burner 4 and flue 5. Resting on or attached to the rim 2 is a support in the form of a cover 6 having a charging port I with a removable cover 8. Passing through the cover 6 is a valved pipe connection 9 for controlling the passage of gas into or out of the space Ill above a body ll of molten metal contained in the pot. The cover or support 6 also is provided with an opening 12 which supports the cover plate l3 on the upper side of which is mounted the agitator motor i4 and on the lower the agitator supports l5, which extend into the pot. In Figs. 1 and 2 attached to the lower ends 16 of the supports l5, there is shown the agitator baflie I! of circular form and U-shape cross section. Within the baffle are upper and lower apertured plates l8 and I9, respectively. These are attached to the baffle at diametrically opposite points by the spacing bars 2|]. As shown, these plates are provided with concentric rows of small apertures 2| which may have a diameter of from about 0.125 inch to 0.562 inch, for example. For plates having a diameter of '7 inches, I have found that three equally radially spaced circular rows of apertures, 0.75 inch apart in the row, are satisfactory, although other arrangements and spacings may be used. These details are schematically shown in Figs. 3 and 4. Between the plates l8 and I9 operates the agitator rotor 22 attached to the lower end 23 of the drive shaft 24, the upper end of which is driven by the motor l4 through the shaft coupling 25. A bearing 26 supported by arms 21 attached to the supports l5 maintains the drive shaft in alignment. The upper apertured plate I8 is provided with a central opening 28 for admitting the drive shaft leaving a substantial annular opening between the drive shaft 24 and the adjacent edge of the plate. The lower plate I9 is also provided with a central opening 29, the area of which is preferably made substantially equal to that of the annular opening 28 so as to balance the liquid thrust which is developed when molten metal passes through these openings as the rotor revolves.

Again referring to the rotor 22, more particularly as shown in Fig. 5, the upper face 30 is provided with radial blades 3| extending a short di tance, e. g. inch, above the face of the rotor.

The lower face is similarly provided with radial blades 32.

In the modification shown in Figs. 6, 7, and 8, the rotor and baflle have a modified form. As shown, the bame comprises a pair of annuli, one designated 33 and the other 34. The annuli are supported by the lower ends l6 of the supports I5 and held in spaced relation with each other by a cylindrical screen 35 attached to the outer edges of the annuli. The screen may have small meshes, about 0.02 square inch in area being preferred, although other sizes may be used, e. g. from 0.01 square inch to 0.2 square inch in area.

4 Within the centraal openings 36 of the annuli operates the rotor 31. This consists of a plate member 38 attached to the lower end 23 of the drive shaft 24 and paddle blades 39 attached to the plate member.

In accordance with the invention, the metal to be treated is melted and its temperature raised to a suitable degree, the particular temperature employed being, in general, governed by the type of protective agent used over the metal, if any, e. g. flux, gas, etc., and while at such temperature, the molten metal is subjected to an intense shearing action as by a violent agitation in which contiguous segments of the molten metal are caused to move relatively to, or slide over, each other at a high rate. The rate of relative movement may be measured in terms of the ratio of the relative rate of movement of adjacent liquid metal segments to the distance between the segments. For convenience, this ratio may be referred to as the shear gradient or velocity gradient and may be expressed as velocity per unit distance of separation or per unit of gradient. Thus, the the closer the segments the higher the velocity gradient for the same relative velocity of segments considered.

I have found that by subjecting contiguous segments of a body of the liquid metal to be treated to an intense shear in which a velocity gradient of about 30 feet per second or more per inch of metal thickness between the segments or per inch of gradient is produced, as by moving the liquid metal rapidly adjacent to a stationary surface which produces a frictional drag on the moving metal, the molten metal is sheared at a rapid rate and undergoes a change such that upon making a casting thereof the solid metal exhibits a finer grain structure in less treatment time than metal similarly melted and heated but not subjected to the aforesaid velocity gradient in the molten state. Only a few minutes is needed of the foregoing treatment to produce the grain refining eifect in the usual size pot of the molten metal.

In order to facilitate carrying out the process of the invention in the foundry, for example, I have devised an apparatus, above described, in which the method may be practiced.

In carrying out the method with the apparatus illustrated in Figs. 1 to 5, inclusive, the magnesium-containing metal to be treated is introduced into the melting pot I through the opening I which is then closed with the cover 8 as shown. The metal is melted by heating the pot with the burner 4 and the temperature suitably raised. During melting and while the metal is in the molten condition, a suitable flux 40 may be employed over the metal to protect it from oxidation or a protective gas may be admitted through pipe 9 to contact the molten metal or both gas and flux may be used. The use of a protective inert gas, e. g. argon, helium, etc., has the advantage that the effect sought in the molten metal can be obtained at about 1200 F., although for best results a preferred operating temperature is 1250 F. If flux be used instead of such a gas but in the presence of air, the operating temperature must be raised to about 1350 F. or preferably to about 1400 F. However, if carbon monoxide or dioxide be substituted for the air, the operating temperature may be as low as 1200 F., the maximum safe working temperature with carbon. dioxide and flux being about 1350 F. With inert gas alone, the operating temperature need not be so limited. In some unearth-ii:

terials may be used in place of carbon monoxide or dioxide, e. g. CCl4, C2H2, and CH4.

After the metal has been heated to the proper temperature, the liquid metal is subjected to an intense Shearing as by setting into rotation the rotor 22 of the agitator while submerged in the body of the metal, preferably maintained under a blanket of a flux or protective gas or both. As the rotor revolves, liquid metal is thereby induced to fiow through the openings 28 and 29 and through the space between the faces of the rotor and the stationary plates 18 and 19. Liquid metal also passes through the openings 2| in the plates into the same space from which it is ejected by the action of the rotor aided by the sets of radial blades 3| and 32. The fast moving metal passing adjacent to the stationary plates is subjected to frictional drag by them and thereby has its velocity reduced as compared to the portions of the metal more remote from the stationary surfaces. A velocity gradient is thus set up in liquid metal as it traverses through the various openings and through the space between the rotor faces and the adjacent stationary plates producing shear, the magnitude of which is dependent upon the speed of rotation of the rotor.

With the apparatus shown (Figs. 1-5), the proper speed for operating the rotor can be calculated readily adopting as a basis the linear velocity of a point near the periphery of the rotor relative to the stationary plates I8 and I9. This basis is predicated on the fact that liquid metal adjacent to the rotor on traversin the spaces between the rotor 22 and adjacent plates l8 and i9 acquires a velocity relative to the adjacent stationary apertured plates substantially equal to that of the blades of the rotor at the periphery while the metal adjacent to the stationary plates tends to move slowly, if at all, due to frictional drag against these plates. For example, with a rotor of 7-inch diameter rotation at 1300 R. P. M. a point near the peripheral end of a blade travels at about 55 feet per second, and, if the clearance between the blades and the adjacent apertured plates be 0.25 inch, for example, then the velocity gradient is 55:0.25, i. e. 220 feet per second per inch. Such a velocity gradient is more than ample to produce the grain refining eifect. The rotational speed of the rotor having a diameter of 7 inches and clearance of 4 inch sufficient to produce the desired effect is about 245 R. P. M.

After the metal has been subjected to the high shear or velocity gradient by causing it to traverse the space between the rotor and the adjacent stationary apertured plates, its lateral high velocity motion is abruptly modified, as by the bafile i1, so as to prevent violent turbulence but not circulation throughout the mass of molten metal not undergoing the shearing treatment. The arresting of the lateral motion or dissipation of the kinetic energy imparted to the molten metal by the rotor 22 in producing the violent turbulence, I have found, is generally necessary in order to prevent the beneficial effect of the treatment being undone. In this connection, I

have found, that if the treated metal is thereafter maintained under conditions permitting much general motion of the batch as a whole. grain refinin effect of the high velocity gradient may be lost.

Essentially similar procedure is employed using the apparatus modified as in Figs; 6 and 7. With this form of apparatus the rotation of the rotor 31 causes a flow of metal through the meshes in the screen 35, as indicated by the arrows, and as a result both a high velocity gradient or shear is produced in each portion of the metal body which traverses the screen and the kinetic energy imparted to the metal is sufliciently dissipated to avoid loss of the grain refining effect obtained. The magnitude of the velocity gradient produced as the metal passes through the screen meshes is dependent at least upon the speed of rotation of the rotor and may be computed on the same basis as that already described for rotor 22. In this case, the linear velocity of the periphery of the rotor 31 divided by the spacing of the meshes in the screen is the velocity gradient. For example, with a 7-inch rotor rotating at 900 R. P. M.. its linear velocity is 27.5 feet per second. Then, if the distance between the screen openings, i. e. the adjacent stationary surface is inch. the velocity gradient is 2'7.5+0.125 inch or 220 feet per second per inch.

The duration of the operation of producing a high shear gradient in the liquid so that all portions of a body thereof become so treated varies with the size of the body and the eificiency of the circulation through the device producing the velocity gradient. As an example, with the apparatus illustrated in a pot about 3 feet in diameter holding about 2000 pounds of metal, the time required is about 15 minutes.

The treated metal may be allowed to remain unused as long as about 30 minutes, or more in some cases, before the grain size of the castings made therefrom increases above that obtained upon casting the metal soon after the treatment. If the treated metal is allowed to remain unused long, it is advisable to retreat it as described before casting. If desired, the treatment may be continued, as by continuing the operation of the rotor, during the withdrawal of treated metal from the pot for casting.

With the form of apparatus illustrated, the agitator together with the driving motor can be lifted bodily off the cover of the melting pot and moved to other pots for treating baths of metal therein. And, while the agitator is thus removed, the treated metal may be readily ladled into molds, if desired.

The following examples are illustrative of the practice of the method.

EXAMPLE 1 A quantity of ingots weighing six hundred pounds consisting of Dowmetal H," a magnesium-base alloy having a nominal composition of 6 per cent aluminum, 0.2 per cent manganese, and 3 per cent zinc, the balance being magnesium, was charged into a melting pot as in Fig. 1 and melted under a blanket 40 of flux (e. g. a mixture of 55 parts KCl, 34 parts MgCh, 9 parts BaCla, and 2 parts CaFz), air having access to the space ll above the molten metal II. The temperature of the molten metal was brought to 1400 F. and maintained at substantially this temperature in the ensuing treatment before which sampleswere removed and cast into test bars. These were sectioned, polished, and examined for grain size which on the average was 0.023 inch in diameter. The molten metal in the pot was then subjected to a treatment to produce a velocity gradient in the metal above 30 feet per second per inch. This treatment was effected by the use of a rotor '7 inches in diameter, operating at 1800 R. P. M. between a pair of stationary plates spaced 0.5 inch from the rotor, a baffle similar to that shown at I! being provided near the periphery of the rotor. The velocity gradient thus produced was computed to be 110 feet per second per inch of gradient, on the basis of the peripheral velocity of the rotor and the space between the faces of the rotor and the adjacent stationary plates. At one minute intervals during the treatment, samples of treated metal were removed, cast into test bars some of which were sectioned, polished, and examined for grain size, others were used for the determination of tensile strength properties. The data obtained are given below:

1 Properties measured after subjecting the test bars to conventional solution heat treatment.

EXAMPLE 2 A batch of 60 pounds of Dowmetal H, contain ing about 0.0005 per centof beryllium, was melted under a blanket of saline flux of the same composition as that in Example 1 and its temperature raised to 1250 R, an atmosphere of CO2 being maintained above the melt. An agitating device of the form shown in Fig. 1 was submerged in the melt and the rotor revolved at a speed of from about 1000 R. P. M. to 1800 R. P. M. for 45 minutes. The batch was sampled and the samples cast into test specimens which were sectioned, polished, etched and examined for grain size with the following results:

Table II Duration of Average Diam- Treatment in eter of Grains Minutes in Inches 0. 010 (before treatment) type on which the process may be practiced are set forth in the table below:

Table III Nominal Composition-Per Cent Dowmetal Alloy Al Mn Zn Mg 0. 2 1.0 Remainder. 0.2 0.5 Do. 0. 1 2. 0 D0. 0. 2 0. 6 D0. 0.1 D0. 2.0 Ca 0 l-0.2 Do.

In addition to the advantages aforementioned. the method has the further advantage of providing a way to reduce the grain size of the magnesium-base alloys containing beryllium which so far as known are not amenable to grain refinement by any conventional treatment.

I claim:

1. The method of grain refining prior to casting a molten magnesium-base alloy which comprises mechanically imparting to adjacent segments thereof a rapid relative motion and then substantially modifying their motion, said molten metal being maintained at a temperature between about 1200 F. and 1450" F., the method being performed without the introduction into the molten alloy of any carbonaceous material.

2. The method of grain refining prior to casting a molten magnesium-base alloy which comprises mechanically imparting to adjacent segments thereof a relative motion such that the relative velocity of the adjacent segments measured not over about 1 inch apart exceeds 30 feet per second and then substantially modifying their motion, said molten alloy being maintained at a temperature between about 1200 F..and 1450 F., the method being performed without the introduction into the molten alloy of any carbonaceous material.

3. The method of grain refining prior to casting a molten magnesium-base alloy which comprises mechanically imparting to adjacent segments thereof a relative motion such that the relative velocity of the adjacent segments measured not over about 1 inch apart exceeds 30 feet per second and then substantially modifying their motion, said molten alloy having a temperature of about 1350 F., the method being performed without the introduction into the molten alloy of any carbonaceous material.

4. The method of grain refining prior to casting a body of molten magnesium-base alloy which comprises maintaining a body thereof at a temperature between about 1200 F. and 1450 F., continuously withdrawing molten metal from the body and mechanically ejecting it through a fine mesh screen submerged in the body of the molten metal whereby the molten metal traversing the screen meshes undergoes a violent shearing effect. the method being performed without the introduction into the molten alloy of any carbonaceous material.

5. The method of grain refining prior to casting a body of a molten magnesium-base alloy which comprises submerging a fiuid pump therein, said pump being provided with a screen over its discharge opening, said screen being submerged in the molten metal and operating the fluid pump so as to withdraw molten metal from the body thereof and drive it through the meshes in the screen, the said molten body of alloy being maintained at a temperature between about 9 1200' F. and 1450 1"., whereby the molten metal traversing the screen meshes undergoes a violent shearing effect, the method being performed without the introduction into the molten alloy of any carbonaceous material.

6. A method of grain refining aluminum magnesium casting alloys including mechanically agitating a body of the molten alloy While at a temperature of approximately 1400 F. just prior to casting, and without the introduction thereinto of any carbonaceous material.

JOHN S. PEAKE.

REFERENCES CITED UNITED STATES PATENTS Number Name Date 1,555,956 Bakken Oct. 6, 1925 2,066,579 Schichtei Jan. 5, 1937 2,195,092 Muller et al Mar. 26, 1940 2,275,266 Meyer Mar. 3, 1942 2,380,863 Nelson et a1 July 31, 1945 OTHER REFERENCES Hultgren et al., University of California, Final Advisory Report to the War Production Board: Report No. W-147, W. P. B. Research Project NRC-550 on Control of Grain Structure and its Efiect on Quality of Magnesium Alloy Castings," April 23, 1946, 4 pp. Preface and pages 9, 19, 42, 43 and 44.

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