US3857705A - Small grain promoting aluminum-titanium-boron mother alloy - Google Patents

Abstract

A small grain promoting aluminum-titanium boron mother alloy containing 3.5 to 7.5 wt% of titanium and 0.1 to 0.3 wt% of boron in a ratio by weight of boron/titanium of 1:20-40. The alloy is preferably prepared by admixing finely divided titanium alkali fluoride and alkali borofluoride to molten aluminum at temperatures not exceeding 900*C, the aluminum reducing the titanium and boron in such fluorides to free metal. The mother alloy can be dispersed into the mass of molten aluminum to be improved which is then ready to be cast into a desired shape.

Description

United States Patent 1191 Miyasaka et al.

[ Dec. 31, 1974 [5 SMALL GRAIN PROMOTING 2,931,722 4/1960 Urban 75/138 ALUMINUM TITANIUM BORON MOTHER 3,503,738 3/1970 Cooper 75/138 ALLOY P E R D nmary xammer can [75] Inventors ggi i r gs s i gg :b Attorney, Agent, or Firm-William J. Daniel Japan 57 [73] Assignee: Nippon Light Metal Research 1 ABSTRACT Laboratory, Ltd Tokyo, j pa A skrlnall l%rain promoting3 galumiplgm-titanhfim boron mot er a 0y contaming to wt% 0 titanium [22] Filed 1973 and 0.1 to 0.3 wt% of boron in a ratio by weight of bo- [21] App]. No.: 330,094 ron/titanium of 1:20-40. The alloy is preferably prepared by admixing finely divided titanium alkali fluoride and alkali borofluoride to molten aluminum at [52] US. Cl. 75/138, 75/68 R, 75/68 B temperatures not exceeding 900C, the aluminum [51] 1] Cl. C22C 21/00 ducing the titanium and boron in Such fluorides to free [58] Fleld Of Search 75/138, 68 R, 68 B metal. The mother alloy can be dispersed into the mass of molten aluminum to be improved which is {56] References cued then ready to be cast into a desired shape.

UNITED STATES PATENTS 2,578,098 12/1951 Southard 75/138 6 4 Draw F'gures f A 5 h 0 a 0L 0 v 3.... 7 F3 0. l-Pu 9% \N.I ml I 0 MW 4 I Q! m m i M I T m w m 8 M. A dd M W .u\ n: b V m r u n! I N p A l u 3 div J A. 1...... m [Kr f u I. flu nn an I. B r .31 t .I o c I S 5 e m p 4 1 v c a 1 m f .1 J F i U v v s M r A2: w L 1 Q. t. 2 s 5. m 4. .z. Hr a T R A w M m z B R m 1| P 3 e B r M m w r N plo a tr. F o T r A .m P M M m FIG. 28.

H 500 Macrostructure of 99.5% Al. Slab FIG. 2A.

LSM Macrostructure ,of 99.5% A1. Slab SMALL GRAIN PROMOTING ALUMINUM-TITANIUM-BORON MOTHER ALLOY FIELD OF THE INVENTION The present invention relates to an aluminumtitanium-boron mother alloy which is adapted to be added to a molten mass of aluminum or aluminum alloy for castings to promote the formation of small crystal grains in such castings.

In casting molded structures or ingots of aluminum or aluminum alloys, it has been the practice to add a small quantity of certain metal elements to the mass of molten aluminum or aluminum alloy in order to promote reduction in the size of the crystal grains of the resulting castings and thus obtain sound aluminum castings with a uniform structure. It is well known that among those elements which promote small grain formation, titanium and boron are most significant in effect, and that their effect is particularly outstanding when both are added to the molten metal at the same time.

For ease of handling, it is preferable to use a mother alloy of aluminum containing titanium and boron as the medium for adding both titanium and boron to molten aluminum and one alloy designed for this purpose is commercially available. This commercial alloy, however, has the disadvantage that its grain size reducing effect decreases with the retention time of the molten metal before casting. Therefore, in aluminum casting using a furnace of large capacity, in which a long time is required for the molten metal to be freed of gases and impurities and for all the molten metal to be poured into the molds, there can occur a significant difference in the ultimate grain size from the beginning to the end of the casting operation. As a result, while the castings obtained at the beginning of the casting operation have desirable extremely fine crystal grains and uniform structures, the castings obtained later on in the casting operation have coarse crystal grains and non-uniform structures. It is theorized that an aluminum-titanium boron alloy of this kind is effective for reducing the grain size in aluminum castings because the intermetallic compounds TiAl and TiB formed in the mother alloy act when added to aluminum on each other to greatly promote the generation of crystalline nuclei of aluminum.

GENERAL DESCRIPTION OF THE INVENTION After extensive studies, it has been found that if the TiAl crystallizing in the mother alloy has a fine grain with almost no acicular, i.e., needlelike, crystals, while the crystal grains of TiB are uniformly distributed in the mother alloy without forming aggregates, the small grain promoting effect of the alloy can be substantially prolonged or extended and that these desirable properties are critically related to a narrow range of proportions of the titanium and boron in the mother alloy wherein the ratio of boron to titanium is about 1:20 -40 by weight. It has also been found that a suitable mother alloy can be readily prepared using finely divided tita' nium alkali fluoride and alkali borofluoride as the sources of titanium and boron, these fluoride powders being added to molten aluminum of the mother alloy in proportions such that the ratio of boron to titanium contained in the alloy is about 1:20 -40 by weight, the melting temperature being as low as possible below about 900C.

The present invention provides a small grain promoting aluminum-titanium-boron mother alloy which is prepared by adding to a small amount of molten aluminum titanium alkali fluoride powder and alkali borofluoride powder in amounts such that titanium and boron are contained in the alloy in the ranges of 3.5 to 7.5 wt% and 0.1 to 0.3 wt%, respectively, and the ratio by weight of boron to titanium is about 1:2040 by weight, the melting temperature being kept below about 900C.

SPECIFIC DESCRIPTION OF THE INVENTION The criticality of the narrow range of weight ratios of boron to titanium in the mother alloy for achievement of the results of the invention both with respect to the grain size reducing effect as well as the duration of that effect is demonstrated by data obtained from a series of comparative experiments and tabulated in the following Table I. In these examples, a high-frequency induction furnace was used to prepare the various alloy samples. Aluminum of 99.8% purity was melted in the crucible at 820C, and to this molten aluminum were added titanium potassium fluoride powder in an amount giving a content of metallic titanium in the alloy of 5 wt%, and potassium borofluoride powder as needed to make the ratio by weight of boron to titanium vary from 121-1 :70. After mixing for 4-5 minutes, the resultant molten alloy was skimmed and poured into a mold. Aluminum-titanium-boron mother alloys thus obtained were added to the casting mass of molten aluminum of 99.7% purity held in a 2-ton melting furnace at 750C to give a content of titanium of 0.025 wt%. Immediately after addition, the molten aluminum was stirred and kept at the same temperature for a predetermined time as specified in Table I. The molten aluminum was then cast into cylindrical ingots 200 mm in diameter and cooled with water. The resulting ingots were inspected by a polarizing microscope to determine the diameter of the crystalline grains therein. The average diameter for 400 crystalline grains for each such inspection is specified in the table, together with the character of the crystalline structure of the alloy in each sample.

TABLE I Retention Time After Addition (Min) Ex. B/Ti 30 I20 300 No. Ratio Crys Grn Crys Grn Crys Grn in the Str Diam Str Diam Slr Diam Alloy (f (1 I No Ad C* C C* dition 2 III G A 372 do. do. 3 H5 G 208 G A 450 do. 4 I110 do. 189 do. 300 do. 5 l/l5 do. I70 do. 238 G A 6 H20 do. I76 G 205 do. 335 7 H25 do. I74 do. I80 G 225 8 H30 do. I76 do. I83 do. I98 9 H35 do. I87 do. 191 do. 203 I0 l/ do. 239 G A 30I G A 349 ll l/ do. 285 do. 305 do. 425

C indicates a columnar structure G indicates a granular G A indicates a granular structure containing some feathery structure From Table I it is evident that the grain size reducing effectiveness of the aluminum-titanium-boron alloys prepared as described is not only pronounced over the approximate weight ratio range of 1:20-40 of boron to titanium, reaching a maximum at about 1:30, but that the duration of such effectiveness is likewise distinctly prolonged over the same range. Thus, the molten aluminum to which the mother alloy was added to a ratio within this range exhibited the size reduction effect for 300 minutes or 5 hours after addition of the alloy. On the other hand, as the ratio of boron to titanium contained in the alloy varied outside the above-mentioned range, both the grain size reducing effect per se and the duration of that effect decline so that, at the ratios smaller than l-40 or larger than l-20, the effect rapidly disappears as the holding time for the mother alloy increases beyond 30 minutes. Moreover, ingots poured from molten aluminum kept for 120 minutes or longer either lost the granular structure entirely or developed feathery crystals, making it difficult to obtain castings with uniform fine crystalline grains.

Table II shows the effect of the fusion temperature used for preparing the mother alloy on both the extent and duration of the grain size reducing effect of the resulting alloy. To prepare the alloy samples of Table II, titanium potassium fluoride powder and potassium borofluoride powder in amounts needed to give a content of 5.0 wt% of titanium and 0.l5 wt% of boron in the alloy (with a ratio by weight of boron and titanium being 1:30) were added to molten aluminum of 99.8% purity in a high-frequency induction furnace. The temperature was held in the. range according to the invention, i.e., below 900C (temperature actually observed: 800 to 850C), and at a somewhat higher level (temperature actually observed: 920 to l,l00C). The mother alloys obtained at these temperature ranges were added to the main mass of molten aluminum of 99.7% purity in the same manner as shown in Table I, and the crystal grain size in ingots cast from these batches were determined as before.

TABLE II Retention Time After Addition (Min) It is apparent from the results of Table II that the alloy (Example No. 12) according to the invention which was prepared below 900C with boron and titanium at a ratio of l-30 is superior to the alloy. (Example 13) which was prepared above 900C with boron and titanium at the same ratio as above as regards the significant properties contemplated in this invention.

The attached drawings are reproductions of microphotographs showing the crystalline microstructure of a commercial grain size reducing aluminum-titaniumboron mother alloy vs an alloy according to the invention. They also compare the crystalline macrostructure of ingots cast from molten aluminum, to which these mother alloys had been added, after holding the molten aluminum for a long time.

FIG. la and b show microscopic photographs (200)() of crystalline structure of (a) an alloy made by company M in the United States, (Ti 5.4 wt%, B 1.1 wt%, B/Ti 1/5, melting temperature unknown) and (b) an alloy according to the invention (Ti 5.2 wt%, B 0.18 wt%, B/Ti= l/30, melting temperature 830C). FIG. 2a

shows the crystalline macrostructure of an ingot ob tained from molten aluminumof 99.5% purity to which the above commercial alloy was added in the same manner as explained in connection with Table I to give a content of titanium of 0.025 wt% and which was held at 750C for 300 minutes. FIG. 2b shows crystalline macrostructure of an ingot obtained in the same manner but using the alloy specified for FIG. lb according to the invention.

In the crystalline microstructure of the commercial alloy (FIG. 1a) remarkably grown acicular crystals of TiAl are observed. On the other hand, the grains of TiB are not uniformly distributed but tend to form localized aggregates. In the aluminum ingot obtained from molten aluminum to which the commercial alloy was added and which was held for 300 minutes, a considerable quantity of feathery crystals, in addition to granular crystals, are observed, as is seen in FIG. 2a. In other words, the grain size reducing effect of the commercial alloy is not sufficient. In the structure of the alloy according to the invention (FIG. lb), however, a greater portion of the TiAl is seen as fine grains without acicular crystals and the TiB is uniformly distributed as fine crystals. In the aluminum ingot from the alloy according to the invention, a fine granular structure is observed, as appears in FIG. 2 (b), although the molten aluminum was held for as long as 300 minutes. This demonstrates the excellent grain size reducing effect of the alloy according to the invention.

As can be seen in the above examples, it is essential for the production ofthe mother alloys of this invention that the ratio of boron/titanium contained in the alloy be about 1:2040 and preferably l/35 to l/25 when maximum improvement is required. Also, it is essential to form the mother alloy at fusion temperatures not exceeding about 900C.

The mother alloys are designed to be added in small quantities to the molten casting aluminum and is used in a diluted state. Therefore, the lower limit of the quantity of titanium and boron in the alloy is not an essential limitation on the usefulness of the alloy. However, an extremely low content of titanium and boron is not desirable from the standpoint of economy, since the alloy has then to be added in very large quantities to the molten aluminum to make its influence felt. According to the invention, the lower limits of content of titanium and boron are set at 3.5 wt% and 0.1 wt%, respectively. These limits are selected somewhat arbitrarily purely on the basis of what is most economic for use in the art and hence must not be given undue importance. On the other hand, too high a content of titanium and boron does impair the excellent grain size reducing effect of the present mother alloys. It is considered that such impairment is due to the fact that as the concentrations of these components in the alloy increase, the melting temperature is likely to be raised or the melting time is likely to be extended, thus forming crystals of intermetallic compounds which are less pronounced in the extent and duration of their grain size reducing effect. For this reason the upper limit for the content of titanium and boron has been set at 7.5 wt% and 0.3 wt%, respectively.

In preparing the mother alloys of the invention, a convenient way of adding the titanium alkali fluoride powder and alkali borofluoride powder to the molten aluminum held at a predetermined temperature is by wrapping appropriate quantities of these powders, to-

gether or separately, in aluminum foil or other appropriate wrapping material, as is commonly practiced in the flux treatment of molten metal. The titanium alkali fluoride powder and alkali borofluoride powder added in this manner are chemically reduced by molten aluminum and become dispersed into the mass aluminum as a metallic component of the alloy. Local formation of free titanium and boron metal in high concentrations in the molten aluminum should be avoided because they tend to produce undesirable crystal structures in the resulting alloy and decrease the duration of the grain size reducing effect. To this end, after the titanium alkali fluoride powder and alkali borofluoride powder are added, the molten casting aluminum should be slowly stirred to distribute the added powders on the free metal as uniformly therethrough as possible.

The molten metal mass to which titanium alkali fluoride powder and alkali borofluoride powder are thus admixed is allowed, if necessary, to stand for a very short time to float the residues produced as a byproduct of the reduction reaction to the surface for easy separation. After removal of these residues, the molten alloy is poured rapidly into a mold, thereby obtaining an aluminum-titanium-boron mother alloy containing substantially the predetermined content of titanium and boron.

It is preferable to complete the preparation of mother alloys of this invention within a short time after the addition of the titanium and boron. [f a longer time is taken for melting the alloy, the crystals of TiAl formed in the alloy will tend to become acicular andthe distribution of TiB uneven so that the prolonged grain size reducing effect which is a characteristic of the alloy of the invention may gradually disappear. Therefore, the melting time is desirably limited to less than 30 minutes after the addition of the powder to the molten aluminum mass.

As used above, the term alkali is intended to include potassium, sodium and any other equivalent alkaline cations having the capacity of forming fluoride complexes with titanium or boron. Also other complexes of these metals which are adapted to be reduced to release the free metal on contact with molten aluminum and give by-product residues that can be conveniently removed without deleteriously affecting the advantageous properties of the mother alloys could be substituted for those specifically identified within the scope of this invention. The size of the powdered form of these complexes is not critical and can be any size suitable for generally uniform distribution in the molten aluminum mass and for chemical reduction to free metal within the relatively short period named above. A particle range of about 20l00 mesh is satisfactory for these requirements but other sizes could be useful.

What is claimed is:

1. An aluminum-titanium-boron mother alloy for addition to molten casting aluminum to promote the for' mation in the solid castings of a uniform small grain crystalline structure, said mother alloy consisting essentially of aluminum containing from about 3.5 up to about 7.5% by weight titanium and from about 0.1 up toabout 0.3% by weight boron in a weight ratio of titanium to boron of 20-4011 and being substantially free of acicular crystals.

2. The mother alloy of claim 1 wherein said weight ratio is about 2535:l.

3. The mother alloy of claim 1 wherein said weight ratio is about 30:1.

4. A method of preparing an aluminum-titaniumboron mother alloy for addition to molten casting aluminum to promote the formation of a uniform small grain crystalline structure in the solid castings, which comprises the steps of adding to a molten mass of aluminum maintained at a temperature of less than about 900C finely divided particles of an alkali titanium fluoride complex and of an alkali boron fluoride complex in amounts sufficient to give a content of titanium and boron of from about 35 up to 7.5% by weight and from about 0.1 up to 0.3% by weight, respectively, in a weight ratio of about 20-40:l, said complexes being chemically reduced by said aluminum to free metal, mixing said aluminum mass to generally uniformly distribute said titanium and boron through said aluminum, and recovering the resultant mother alloy.

5. The method of claim 4 wherein residual by products of said chemical reduction are removed from the surface of said mass which is then solidified.

6. The method of claim 4 wherein said molten mother alloy is maintained in said molten condition not longer than about 30 minutes before being solidified.

9 UNITED STATES PATENT OFFICE CERTIFICATE CF CORRECTION Patent No. 3,857,705 Dated December 31, 1974 Invent Yoshiteru MIYASAKA et a1 It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In the Heading of the Patent, add the following:

- Foreign Application Priority Data Feb. 14, 1972 Japan 14831/47 Ewigned and gealcd this second Day 0f September 1975 [SEAL] Arresr:

RUTH C. MASON C. MARSHALL DANN Arresring Officer (mnmissiom-r uj'larcnrx and Trudcmurkx FORM PO-OSO (10-69) USCOMM-DC 60376-P69

Claims (6)

1. AN ALUMINUM-TITANIUM-BORON MOTHER ALLOY FOR ADDITION TO MOLTEN CASTING ALUMINUM TO PROMOTE THE FORMATION IN THE SOLID CASTINGS OF A UNIFORM SMALL GRAIN CRYSTALLINE STRUCTURE, SAID MOTHER ALLOY CONSISTING ESSENTIALLY OF ALUMINUM CONTAINING FROM ABOUT 3.5 UP TO ABOUT 7.5% BY WEIGHT TITANIUM AND FROM ABOUT 0.1 UP TO ABOUT 0.3% BY WEIGHT BORON IN A WEIGHT RATIO OF TITANIUM TO BORON OF 20-40*1 AND BEING SUBSTANTIALLY FREE OF ACICULAR CRYSTALS.

2. The mother alloy of claim 1 wherein said weight ratio is about 25-35:1.

3. The mother alloy of claim 1 wherein said weight ratio is about 30:1.

4. A method of preparing an aluminum-titanium-boron mother alloy for addition to molten casting aluminum to promote the formation of a uniform small grain crystalline structure in the solid castings, which comprises the steps of adding to a molten mass of aluminum maintained at a temperature of less than about 900*C finely divided particles of an alkali titanium fluoride complex and of an alkali boron fluoride complex in amounts sufficient to give a content of titanium and boron of from about 3.5 up to 7.5% by weight and from about 0.1 up to 0.3% by weight, respectively, in a weight ratio of about 20-40:1, said complexes being chemically reduced by said aluminum to free metal, mixing said aluminum mass to generally uniformly distribute said titanium and boron through said aluminum, and recovering the resultant mother alloy.

5. The method of claim 4 wherein residual by-products of said chemical reduction are removed from the surface Of said mass which is then solidified.

6. The method of claim 4 wherein said molten mother alloy is maintained in said molten condition not longer than about 30 minutes before being solidified.

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