US3290742A - Grain refining process - Google Patents

Description

Dec; 13, 1966 v. PETRovlcH 'GRAIN REFINING PROCESS Filed Feb. 6, 1964 XCMEmWW W xmbtmwm.

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INVENTOR. Wadi if' Paf/'ow' h United States Patent O 3,290,742 GRAIN REFINING PROCESS Vladimir Petrovich, Midland, Mich., assignor to The Dow Chemical Company, Midland, Mich., a corporation of Delaware Filed Feb. 6, 1964, Ser. No. 343,067 9 Claims. (Cl. 22-200.1)

This invention relates to grainrefining and more particularly relates to a novel method of grain refining a m-agnesium-base alloy containing zinc and misch metal.

When casting magnesium alloys containing up to 1.0 percent of r-are earth metals and at least about 0.3 percent zinc, lfor example, the ASTM designated magnesiumbase alloy ZE10, which are usually cast at a temperature within the ran-ge lof from about 1300 to l350 F., columnar ygrains and sO-called stringers, as defined below, tend to form in the outer portions commencing near the chilled surface and on in toward the center of the cast ingots or billets rather than equiaxed grains which are more desirable. Such columnar structure is particularly undesirable in castings intended `for rolling in that it may cause the metal to crack severely when lrolled. However, when equiaxed grains are present the rollability of the alloy is increased t-ogether with its strength potential.

The term columnar grains as used herein refers to a course structure of parallel columns of lgrains having the long axis perpendicular to the casting surface, whereas, the term stringers refers to a band at or near the casting surface comprising shorten columnar grains but, :like the columnar grains, not equiaxed in structure. Accordingly, the -tenm equiaxed Igrain refers to a grain structure in a magnesium-.base alloy wherein the axis Iof substantially all the grains is substantially the same length 'across 'any 4'given cross isection thereof. y

Various methods have been and are presently practiced to obtain preferably a long lasting grain refinement effect in magnesiumbase alloys such as, for example, by adding metallic elements to the alloy as nucleating agents such as, for example, zirconium, iron, aluminum, and carbon. The addition of such agents, however, to `grain refine said ZE10 alloy in Vaddition to being economically undesirable, has been found to be unsatisfactory in that by adding zirconium, for example, the `cleanliness of the molten alloy deteriorates, that is, the num-ber of oxide particles and flux inclusions increases in castings. The addition of iron onV the other hand rdoes not normally result in a lasting grain refinement at high alloying and casting temperatures. In addi-tion, iron may create corrosion problems in the alloy. Furthermore, carbon treatments are not effective on alloys, such as ZE10, which do not contain aluminum. In view of the above difficulties,

therefore, it would be highly desirable in the art to have a particular simple and economical process for grain 'refining zinc and misch metalqcontaining magnesiumbase alloys, whereby a long lasting Igrain refinement effect may be obt-ained without the need of additional equipment or process steps.

The term long-lasting grain refinement effect as used herein means a grain refinement effect which persists throughout the period of settling, holding and casting withlout additional treatment in a large batch of the molten magnesium-base alloy specified herein to be grain refined in accordance with the present invention, for example, in batches containing 600 to 4000 pounds of molten metal.

An object of Ithe present invention, therefore, is to l provide a novel process for grain refining magnesiumbase alloys containing zinc and rare earth metals.

Another object of the present invention lis to provide a simple and economical method of imparting a lasting grain refinement to magnesiumabase alloys containing up 3,290,742 Patented Dec. 13, 1966 to about 1.0 percent of rare ear-th metals and at least about 0.3 percent of zinc. A further object is to provide :such a method for grain refining magnesium-base alloy ZE10 castings.

The appended two-figured lithograph, which will be discussed more `fully hereinafter, 4of a photomicrograph taken at a 25 magnification of the magnesiumsbase alloy ZZElO shows in FIGURE 1 the alloy when treated in accordance with the present invention and, as shown in FIGURE 2, when not so-treated, thus illustrating the fine equiaxed grain structure obtainable in said alloy when processed by the -method of the present invention.

In general, the above and other objects and advantages have surprisingly -been found obtainable, in accordance with the present invention, by casting or holding the magnesium-base alloy to be treated in the casting pot just prior to and during pouring at a temperature within the range of from about 1-190J F. to about 1240 F., and preferably between 1220 F'. and 1240 F. The time and other temperatures, such as those employed in melting, alloying, .and `settling the alloy prior to being cast, do not affect the grain refining, provided the aforesaid temperatures in the casting pot are observed just prior to and during pouring (casting) of the molten alloy. Higher temperatures than about 1240 F. should be avoided during casting, particularly when casting the ZE10 alloy. For example, the line carrying mol-ten metal from the cas-ting pot to the mold shouldrnot raise the temperature of the molten alloy above 1240 F.

The alloy to be treated, in addition t-o containing up to 1 percent rare earth metal, such as, for example, by the addition of misch metal `(MM), and at least about 0.3 percent of zinc, should contain at least about 0.001 percent of iron. In most cases, however, such an iron content exists normally as an impurity in primary magnesium and :ordinary magnesium alloys. Iron need not, there-fore, norm-ally be added to the alloy as a specific addition.

The method of the present invention, then, for Igrain refining a magnesium-base alloy as specified hereinbefore, comprises establishing a molten mass of said alloy at a temperature within the range of from about 1190 F. to about 1240 F. and, while maintaining said temperature, casting the metal in-to the desired configuration, thereafter allowing the socast metal to solidify, thereby preparing fine grained cast articles. The present -method has the advantages of simplicity and economy in tha-t only the temperature of the molten metal need be controlled during the normal casting sequence, thus requiring no Example 1 To show how columnar grains and stringers are eliminated and fine equiaxed grains produced for a lasting grain refinement effect various zinc and rare earth metalcontaining magnesium-base alloys operable in the present novel process each having the composition indicated on Table I below were prepared in washed steel and clean graphite crucibles by melting, alloying and flux refining said alloys at 1400 F. using a crushed fused flux having a composition comprising 50 percent magnesium chloride, 25 percent potassium chloride, 20 percent barium chloride and 5 percent calcium fluoride. The melts were then held for the periods indicated in Table I, whereupon, each was sampled with a hot ladle while the melt was at a temperature within the range of from 1190 F. to 1400 F. for grain size determination and composition analysis. In the smaller melts pounds), grain size was estimated from the fractures obtained from 11/2 inch diameter by 8 inch long bars which had been cast into a permanent steel mold (P.M.). Samples using a wedge mold were also obtained from the same region of the melt for spectrographic analysis. Melts prepared for direct-chill continuous casting (commonly referred to as solidication casting) were prepared in 600 pound batches and were also sampled for grain size and composition analysis. Other than the melts used for said direct-chill castings, which were cast in diameters as indicated in Table I, the grain size of which was determined metallographically, the grain size rating of the various melts was determined by visual comparison with the naked eye of the aforesaid fracture samples and standard samples. A rating number was given qualitatively related to the grain size and type was shown below:

Equiaxed Description of Columnar Number Grain Structure Number 01 1 02 2 03 3 04 4 05 Very coarse 5 y The structure was rated with an equiaxed number only when there was a complete absence of columnar grains. If any columnar grains would appear the structure was rated with the respective columnar number.

For comparison purposes a portion of the melts prepared above as controls were processed and sampled in the same manner except that the casting temperatures employed were above 1240 F. The pertinent data and external vertical wall of the cylindrical cavity caused the metal to freeze by spraying water on the mold wall as the pouring continued and the resulting solidifying billet moved downward.

Part of the billet so formed was then cut into 2 inch slices and a representative sample slice selected for a grain sized and structure determination. The selected sample was then transversely cut into quarters and one of the quarters cut into 4 segments perpendicularly to the transverse cut. Said segments were designated 1 through 4 toward the center of the billet, and a portion of each segment was prepared for metallographic examination. A photomicrograph of the polished portions was taken and is shown as FIGURE 1 on the appended lithograph.

For comparison, the above procedure was duplicated in every respect again using the ZElO alloy except that the casting was done at a temperature above 1240 F., namely, at 1300 F. The photomicrographs obtained from the samples derived from this casting are shown as FIGURE 2 of the appended lithograph.

An examination of the grain structure-of the segment portion designated number 1 in each iigure clearly shows the absence of stringers and columnar grains in the outer portions (close to the chilled surface) of the casting of FIGURE 1 cast in accordance with the present invention as compared to the pronounced presence of same near the surface of the ZElO alloy of FIGURE 2, which was not cast in accordance with the present invention. lt should also be noted in segments 2 through 4 (which proceed to the center of the casting) that the size of the equiaxed grains in FIGURE 1 is markedly finer than that of the alloy of FIGURE 2, illustrating the exceptionally line equiaxed grains obtainable by the method of the present invention.

f' results for these controls 1s also indicated 1n Table I. 3* It 1s manlfest that var1ous modlcations can be made TYPE I Holding History C t.. Grain Structure Composition, Percent l as ing Sample Size ofMelt (Lb.) Type of Cast Temp.,

Tlme, Temp., F. Grain Width .of A1 Fe Mn MM 2 Zn Hrs. F. Rating Stringer Band 000 contr01) 0" 11a.13.o.3 1g 1,300 .04 i'wide .003 .030 .0s .13 1.30 000 wam. 11o.3 1g 1.230 .03 None .00s .021 .07 .19 1.31 000(contr01) 12 da. 13.0---. 18 1,300 1,300 .12 1V" wide .003 .035 .o3 1. 000 12d1a.Do 14 1,300 1,237 .03 Nfm@ .002 .02s .000 100 (eontro1 1%d1a. 1 10 1,350 1,300 .o .037 .072 .10 1.43 100 1%"1113 x1 17 1,350 1,220 .029 .071 .17 1. 40 100 e ntro1) 1%010 1 4 1,300 1,300 .020 .01 .19 1.34 100 11/2Il d1a. 1 0 1,300 1,220 .020 .01. .1s 1.34

l Balance essentially of magnesium. 2 MM misch metal.

Example 11 To illustrate application of the grain-refining method of the 'present invention with respect specifically to directchill casting, a well known casting technique, of the magnesium-based alloy ZElO which has the nominal composition of 1.3 percent zinc, 0.2 percent misch metal (MM), and 0.002 percent iron, the balance being essentially magnesium, about 600 pounds of the alloy was melted under a protective cover of the ilux hereinbefore used and dened, whereupon it was continuously cast using a direct-chill mold to produce a 12 inch diameter billet 120 inches in length. The direct-chilled mold comprised an open-topped cylindrical cavity 12 inches in diameter having a hydraulically downwardly moving bottom part. The melt of ZElO alloy while at a ternperature Within the range of from 1220c F. to 1240 F. was continuously poured into said cavity, whereupon, after a solidified head of metal had formed against the mold bottom, said bottom was concurrently and continuously moved downward as the melt was being poured into the mold. Water pipes located annularly around the 3 D.C. direct chilled.

4 P M. permanent mold.

in the process of the present invention without departing from the spirit or scope thereof and it is understood that I limit myself only as deiined by the appended claims.

I claim:

1. A method of grain refining a magnesium-base alloy containing rare earth metals and zinc which comprises, lestablishing a molten mass of the alloy to be grain relined, casting said molten mass from a temperature within the range of from about 1190" F. to about 1240 F., thereby imparting to the cast alloy a iine equiaxed grain structure, and cooling to solidify it.

2. The method of claim 1 wherein the casting temperature is Within the range of from about 1220 F. to about 1240 F.

3. The method of claim 1 wherein the magnesium-base alloy to be grain refined contains at least about 0.001 percent by weight of iron.

4. The method of `claim 1 wherein the magnesiumbase alloy to be grain refined contains by weight up to about 1 percent of rare earth metals and at least about 0.3 percent of zinc.

5. The method of claim 1 wherein the magnesiumbase alloy to be grain refined is the ASTM designated ZE10 alloy.

6. A method of grain rening a magnesium-base alloy containing up to about 1.0 percent of misch metal and at least about 0.3 percent of zinc and at least about 0.001 percent of iron which comprises, establishing a molten mass of the alloy to be grain rened, casting said molten mass from a temperature within the range of from about 1190 F. to about 1240 F., thereby imparting to the cast alloy a structure consisting of ne equiaxed grains,x and solidifying the so-cast alloy by allowing it to cool.

7. The method of claim 6 wherein the casting temperature is within the range of from about 1220 F. to 1240 F.

8. A method of imparting a lasting ne equiaxed grain structure to a direct-chilled magnesium-base alloy casting of ZE10 which comprises, establishing a molten mass of said ZE10 alloy, continuously casting and solidifying the molten alloy into a directly chilled mold from a temperature Within the range of from about 1190 F. to about 1240 F., thereby to continuously produce a solidied casting of said ZE10 alloy having a lasting ne equiaxed grain structure throughout.

References Cited by the Examiner FOREIGN PATENTS 359,425 10/1931 Great Britain.

OTHER REFERENCES Magnesium: 1st Ed., American Magnesium Corporation, 1923, TN 799 M2 A6, page 115.

Magnesium: American Society for Metals, 1946, TN 799 M2 M18, page 73.

Journal of Metals: The Properties of Sand Cast Magnesiurn-Rare Earth Alloys, 1949, pp. 968-971.

I. SPENCER OVERHOLSER, Primary Examiner.

R. S. ANNEAR, Assistant Examiner.

Claims (1)

1. A METHOD OF GRAIN REFINING A MAGNESIUM-BASE ALLOY CONTAINING RARE EARTH METALS AND ZINC WHICH COMPRISES ESTABLISHING A MOLTEN MASS OF THE ALLOY TO BE GRAIN REFINED, CASTING SAID MOLTEN MASS FROM A TEMPERATURE WITHIN THE RANGE OF FROM ABOUT 1190*F. TO ABOUT 124*F., THEREBY IMPARTING TO THE CAST ALLOY A FINE EQUIAXED GRAIN STRUCTURE, AND COOLING TO SOLIDIFY IT.

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