Method For Melting Titanium Alloys

Abstract

This invention relates to a method of making a homogeneous ingot of a titanium base alloy by mixing particles of titanium with a master alloy in the form of fine granules, said master alloy containing 30 to 75 percent by weight of molybdenum, 25 to 70 percent by weight of at least one of the elements selected from the group consisting of chromium, titanium, zirconium, nickel and copper, but not over 30 percent chromium, not over 25 percent titanium, not over 40 percent zirconium, not over 10 percent nickel and not over 10 percent copper, compacting the mixture into the form of a compacted article and then melting said compacted article in a vacuum.

Parent Case Text

This application is a continuation-in-part of copending application Ser. No. 523,866, filed Feb. 1, 1966, now U.S. Pat. No. 3,508,910.

Description

It has long been known and appreciated that it is exceedingly difficult to produce molybdenum-containing beta-type titanium-base alloys on a commercial scale. Titanium-base alloys of the alpha-beta type, such as the alloy comprising 7 percent aluminum, 4 percent molybdenum, balance titanium, are known, and these alloys can be melted as a homogeneous composition rather readily by using a master alloy consisting of aluminum and molybdenum. In making beta-type titanium-base alloys, this approach is not available because aluminum in the preparation required to form suitable master alloys with molybdenum is harmful to the properties of such titanium-base alloys.

Various alternative procedures have been considered for effecting the inclusion of molybdenum in alloys of this type and obtaining a homogeneous composition, but all of these are considerably more tedious and complex in commercial use than the practice which is made possible by the use of master alloys in accordance with the instant invention.

The earliest known procedure for the production of relatively high-molybdenum titanium-base alloys involved the repeated vacuum-induction melting of small quantites, for example, about 50 grams, of the alloy, starting with titanium sponge or powder and powders of molybdenum and the other alloy constituents. Although a homogeneous alloy can be obtained in this manner, this method is entirely too time-consuming and costly to be useful in the production of molybdenum-containing beta-type titanium-base alloys on a commercial scale.

Another method which has been considered for use in producing such alloys involves the mixing of titanium powder with molybdenum powder and other alloy constituents to form a homogeneous powder mixture, compacting the powder mixture into a briquette, and then vacuum consumable-electrode melting such a briquette. Powdered molybdenum is required as a starting material, and the mixing step is exceedingly difficult to perform adequately on anything but a very small scale, because the molybdenum powder is more dense than the other involved powders. The finest-particle-size molybdenum powder commercially available is nevertheless sufficiently large, and the rate of diffusion of molybdenum in solid titanium is sufficiently low, that it takes an impractically long time to achieve homogeneity in even a perfectly blended powder-metallurgy compact. The long diffusion time is aggravated by the necessity to use relatively coarse titanium granules to avoid excessive oxygen pickup. Moreover, if one melts even a perfectly blended electrode, the titanium melts and the solid molybdenum particles drop from the melting electrode into the molten titanium pool. The latter is very reducing, i.e., nonoxidizing, and the molybdenum particles doubtless have chemically clean surfaces wetted by titanium. They are also high melting, and so do not immediately dissolve in the titanium; they are denser, and so drop rapidly to the bottom of the molten titanium bath--here, it is believed, there is a liquid-solid slush, and also there is often some swirl and the interface between solid and liquid is usually concave upwards, with the result that some of the chemically clean-surfaced molybdenum particles become jostled together and thus become sintered into agglomerates to form the large dense inclusions subsequently found in the ingot. A master alloy of zirconium-molybdenum, on the other hand, is lower melting and less dense than molybdenum, and so melts quickly into liquid solution into the molten titanium, and in the instant invention advantage is taken of this fact.

Yet another possibility is the mixing of titanium powder with molybdenum powder and other alloy constituent powders in the manner described above, followed by compaction to a high theoretical density (somewhat in excess of 90 percent), for example, in an argon-filled or evacuated enclosure or under other suitable protective conditions. Although such a method avoids the expense of the final melting step, the expense of the initial powder-mixing step and the use of molybdenum powder militate strongly against the commercial usefulness of such approach. The tendency of titanium powder to become contaminated with oxygen, as noted above, is a further drawback. Similarly, dense metallic compacts produced in this manner have been found too brittle to be useful and/or too inclined, when welded, to cause weld porosity to be os use in any applications involving welding.

Yet another method for producing molybdenum-containing beta-type titanium-base alloys is that disclosed in U.S. Pat. No. 3,269,825, issued Aug. 30, 1966. This involves mixing powders of molybdenum and tin, rolling the molybdenum-tin mixture to extreme thinness so that it forms flakes, placing the flakes uniformly over the surface of a bed of titanium powder, building up the bed by adding additional layers of titanium powder and flakes, and then briquetting and double vacuum-consumable-electrode melting as indicated above. This procedure is very time consuming and expensive and requires considerable care and skill on the part of the persons practicing it.

Thus, it will be seen that the art of titanium metallurgy has been faced for a period of at least 10 years with a problem, the production of alloys of this type on a large scale, for which no satisfactory solution has yet been proposed.

In accordance with the instant invention, it has been discovered that by making first a master alloy of molybdenum with zirconium, alone or with one or more other elements, and comminuting that master alloy into fine granular form, and then mixing the fine granular master alloy with granular powder or sponge titanium, it becomes possible to produce, far more readily than in any manner hitherto known, beta-type titanium-base alloys containing substantial quantities of molybdenum. It has further been discovered that by replacing part, or possibly all, of the zirconium with iron, the melting point of the master alloy can be lowered still further, and the ease with which such molybdenum-containing beta-type titanium-base alloys can be produced is yet further enchanced. The introduction of iron into the master alloy has the further advantage that it makes possible the use as a starting material of a relatively inexpensive material, ferromolybdenum, in place of the relatively expensive material, powdered molybdenum, hitherto considered necessary as a source of the molybdenum in the alloys. This latter aspect is based upon work that has revealed, rather surprisingly, that although silicon, even in amounts as small as 0.1 percent by weight, tends to affect detrimentally the cold workability of the desired titanium-base alloys, nevertheless the amounts of silicon that are present as an impurity in ferromolybdenum are sufficiently small that when ferromolybdenum is used in the formation of the master alloy which is ultimately incorporated in a molybdenum-containing beta-type titanium-base alloy, the silicon present does not worsen the properties of the product titanium-base alloy substantially, at least if the silicon content of the ferromolybdenum is less than about 1 percent.

It has also been discovered that the melting point of the master alloy can be further lowered, and the properties of the alloy thus produced can be further enhanced, by the inclusion in the master alloy of a substantial proportion of chromium, up to about 30 percent. If desired, up to about 10 percent in total amount of one or more elements selected from the group consisting of manganese, hafnium, columbium, tantalum, vanadium, nickel, copper, and cobalt may be incorporated in the master alloy with a view to further lowering its melting point and/or enhancing the properties of the titanium-base alloy to be produced by its use.

In brief summary, the instant invention comprises the concept of providing, in fine granular form, a molybdenum-containing master alloy containing 20 to 40 percent zirconium, and at least about 30 percent molybdenum. In the broadest aspect of the invention, the zirconium may be replaced with iron in whole or in part on the basis of about one part for one by weight, and preferably this is done, as aforesaid, by using the commercial ferromolybdenum as a source of at least part of the molybdenum in the master alloy. Commercial ferromolybdenum contains about 55 to 75 percent of molybdenum by weight, the balance being substantially iron, so that in most circumstances only a relatively small part of the molybdenum contained in the master alloy would need to be supplied in the form of pure molybdenum. It is considered essential that zirconium be included in a master alloy intended for use in the production of titanium-base alloys, because otherwise the final titanium-base alloy has a relatively high content of iron, and its ductility suffers.

In its broadest aspects, the instant invention comprises master alloys that contain 30 to 75 percent molybdenum, 0 to 40 percent zirconium, 0 to 20 percent iron, 0 to 30 percent chromium, and 0 to 25 percent titanium, but 25 to 70 percent in total amount of one or more of the elements zirconium, iron, chromium, and titanium. Preferably, such alloys contain 20 to 40 percent of zirconium, and desirably, also at least 3 percent in total amount of an element selected from the group consisting of iron and chromium. It is understood that other alloying elements desired in the final titanium-base alloy may in many cases be included in the master alloy without detriments. For example, tin, vanadium and aluminum could be so added singly or in combination in amounts up to 25, 35 and 10 percent respectively. More specifically, one desirable range of molybdenum-zirconium-iron master alloys consists of 40 to 75 percent molybdenum, 25 to 35 percent zirconium, and 5 to 20 percent iron. Another example of a master alloy within the scope of the present invention is one consisting essentially of 45 to 70 percent molybdenum, 25 to 35 percent zirconium, 5 to 20 percent iron, 3 to 10 percent chromium, and 15 to 25 percent titanium.

Another feature of the instant invention is that the master alloys are provided in the form of fine granules. By this, it is meant that the particles of master alloy are substantially all of such size as will pass through a No. 3 U.S. Standard sieve and be retained upon a sieve such as a No. 80 U.S. Standard sieve, or perhaps slightly finer. The considerations in choosing a suitable size range for a given master alloy include the readiness with which the particular alloy melts and the size and chemical composition of the material with which it is to be mixed to form the desired final alloy. Master alloys that are quite readily meltable, having a low melting point in comparison with that of the material with which the master alloy is to be mixed, can often be used in the form of fairly coarse particles, e.g., with a maximum dimension of about one inch. It is generally desirable, however, to adhere to a somewhat smaller top size, such as a No. 8 U.S. Standard sieve or finer. On the other hand, it is essential, in the interest of obtaining a relatively uniform mixture of the master alloy with the other materials contained in the composition of the desired final alloy, to avoid the use of any substantial amount of particles that are so fine as to separate out or be carried away as dust. The sizes of the particles of the titanium sponge, titanium fines, or other similar material should be considered. Finer material in the master alloy can be tolerated if there is a suitable portion of the material with which it is to be mixed which is also comparatively fine. For admixture with common sponge titanium, the use of a master alloy not containing particles which will pass through a No. 80 U.S. Standard sieve is preferred.

The invention also comprises the method of using certain of the master alloys of the invention to produce homogeneous ingots of molybdenum-rich titanium-base alloys which consists in mixing said alloys in the form of fine granules with particles of titanium, preferably titanium sponge, compacting the mixture thus obtained into an object to be melted, preferably a consumable electrode, and then melting said object in the substantial absence of oxygen, nitrogen, and carbon preferably in a vacuum.

A complete understanding of the invention may be obtained from a consideration of the following specific examples, illustrating how master alloys in accordance with the present invention are made and used.

EXAMPLE I

Ten parts by weight of molybdenum chips or molybdenum rondelles are blended with 5 parts by weight of zirconium sponge, then melted in a carbon arc furnace to produce an ingot. The ingot is dumped, crushed, and screened to obtain a sized fraction which will pass through a No. 8 U.S. Standard sieve but will be retained upon a No. 30 U.S. Standard sieve. If desired, the sized material is again carbon-arc-melted to form a second ingot which is subsequently crushed and screened to obtain a material of the same consistency. Thus, it is found that the master alloy is in fine, granular form and contains about 67 percent molybdenum, about 33 percent zirconium, and only about 0.01 percent of carbon. That is how a fine, granular master alloy in accordance with the present invention is made.

Such a master alloy is used in the following manner. Fifteen parts by weight of the master alloy, prepared as mentioned above, are mixed with 4 parts by weight of tin and 81 parts by weight of sponge titanium. The mixture is blended and is then compacted into the form of briquettes, for example, about 8 inches in diameter and 10 inches high. The briquettes are then assembled to form a consumable electrode, in any desired fashion, for example, by forming a cluster composed of three strands, each strand containing 8 or more of said briquettes, a titanium rod 1 inch in diameter being used, together with a welding torch, to weld the cluster together. An adapter piece, preferably of titanium metal, is welded to the upper end of the electrode, which is then placed in a vacuum consumable-electrode melting furnace and then melted in accordance with known practices. As is known, the ingot thus produced is upended, an adapter is welded to its upper end, and the ingot is remelted, to yield an ingot of titanium-base alloy consisting essentially of 10 percent molybdenum, 5 percent zirconium, 4 percent tin, balance titanium. In this manner, a titanium-base alloy is produced which is considerably freer from dense-metal inclusions than any other titanium-base alloy of like molybdenum content produced without the use of special and costly melting practices.

EXAMPLE II

Example I was repeated, except that after the first carbon-arc melt was conducted and a sized fraction of molybdenum-zirconium alloy was obtained, there were added to 15 parts by weight of said molybdenum-zirconium alloy 5 parts by weight of titanium sponge. This mixture was thoroughly blended and carbon-arc melted a second time as in Example I, and the resulting ingot was crushed and screened. Twenty parts by weight of such master alloy were mixed with 4 parts by weight of tin and 76 parts by weight of sponge titanium, and then further treated as in Example I to yield a titanium-base alloy containing 10 weight percent molybdenum, 5 percent zirconium, 4 percent tin, balance titanium.

EXAMPLE III

A mixture was formed consisting of 59 parts by weight of molybdenum rondelles, 29 parts by weight of zirconium sponge and 12 parts by weight of iron pellets. The mixture was thoroughly blended, then carbon-arc-melted, screened, remelted, and again screened, as indicated in Example I. This yields a master alloy in fine, granular form, consisting essentially, by weight, of 59 percent molybdenum, 29 percent zirconium, and 12 percent iron. One hundred parts by weight of such mixture were then thoroughly blended with 24 parts by weight of tin and 466 parts by weight of titanium sponge, and then further processed as indicated in Example I, to yield a final titanium-base alloy consisting essentially of 10 percent molybdenum, 5 percent zirconium, 4 percent tin, 2 percent iron, balance titanium. The addition of iron further lowers the melting point of the master alloy and yields a final doubly consumable-electrode-melted product of improved homogeneity.

EXAMPLE IV

Example III was repeated except that ferromolybdenum was used as a source of iron, in place of iron pellets. That is, a mixture was formed consisting of 36 parts by weight of ferromolybdenum (containing 67 percent molybdenum and the balance essentially iron), 35 parts by weight of molybdenum rondelles, and 29 parts by weight of zirconium sponge. The mixture was thoroughly blended and carbon-arc-melted, screened, remelted, and again screened as indicated in Example I. One hundred parts by weight of such mixture were then thoroughly blended with 24 parts by weight of tin and 466 parts by weight of titanium sponge, and then further processed as indicated in Example I, to yield a titanium-base alloy consisting essentially of 10 percent molybdenum, 5 percent zirconium, 4 percent tin, 2 percent iron, balance titanium.

EXAMPLE V

Example IV was repeated, except that some titanium was added to the master-alloy composition immediately before the second carbon-arc-melting. That is, a mixture was formed consisting of 36 parts by weight of ferromolybdenum (containing 67 percent molybdenum and the balance essentially iron), 35 parts by weight of molybdenum rondelles, and 29 parts by weight of zirconium sponge. The mixture was thoroughly blended, then carbon-arc-melted, and screened. To this screened product there was added sufficient titanium sponge to yield a titanium content of 20 percent in the master alloy after the second carbon-arc melt. That is, 100 parts by weight of the screened product of the first carbon-arc melt were mixed with 25 parts by weight of titanium sponge, and then further processed as indicated in Example I. This yielded a screened master alloy in fine, granular form, consisting essentially, by weight of 47 percent molybdenum, 23 percent zirconium, 10 percent iron, 20 percent titanium. One hundred parts by weight of such mixture were then thoroughly blended with 19 parts by weight of tin and 351 parts by weight of titanium sponge, and then further processed as indicated in Example I, to yield a final titanium-base alloy consisting essentially of 10 percent molybdenum, 5 percent zirconium, 4 percent tin, 2 percent iron, balance titanium.

EXAMPLE VI

A mixture was formed consisting of 53 parts by weight of molybdenum rondelles, 30 parts by weight of zirconium sponge, 12 parts by weight of iron pellets, and 6 parts by weight of chromium chips. The mixture was thoroughly blended then carbon-arc-melted, screened, remelted, and again screened as indicated in Example I. One hundred and one parts by weight of such mixture were then thoroughly blended with 24 parts by weight of tin and 464 parts by weight of titanium sponge, and then further processed as indicated in Example I to yield a titanium-base alloy consisting essentially of 9 percent molybdenum, 5 percent zirconium, 4 percent tin, 2 percent iron, 1 percent chromium, balance titanium.

EXAMPLE VII

Example VI was repeated, except that ferromolybdenum was used as a source of iron in place of iron pellets. That is, a mixture was formed consisting of 36 parts by weight of ferromolybdenum (containing 67 percent molybdenum and the balance essentially iron), 29 parts by weight of molybdenum rondelles, 30 parts by weight of zirconium sponge, and 6 parts by weight of chromium chips. The mixture was thoroughly blended, then carbon-arc-melted, screened, remelted, and again screened as indicated in Example I. One hundred and one parts by weight of such mixture were then thoroughly blended with 24 parts by weight of tin and 464 parts by weight of titanium sponge, and then further processed as indicated in Example I, to yield a final titanium-base alloy consisting essentially of 9 percent molybdenum, 5 percent zirconium, 4 percent tin, 2 percent iron, 1 percent chromium, balance titanium.

EXAMPLE VIII

Example VII was repeated, except that titanium was introduced to the master alloy immediately before the second carbon-arc melt. That is, a mixture was formed consisting of 36 parts by weight of ferromolybdenum (containing 67 percent molybdenum and the balance essentially iron), 29 parts by weight of molybdenum rondelles, 30 parts by weight of zirconium sponge, and 6 parts by weight of chromium chips. The mixture was thoroughly blended then carbon-arc-melted and screened. To this screened product there was added sufficient titanium sponge to yield a titanium content of 20 percent in the master alloy after the second carbon-arc-melt. That is, 101 parts by weight of the screened product of the first carbon-arc-melt were mixed with 25 parts by weight of titanium sponge, and then further processed as indicate in Example I. This yielded a screened master alloy in fine, granular form having substantially the following composition: 42 percent molybdenum, 24 percent zirconium, 9 percent iron, 5 percent chromium, 20 percent titanium. One hundred twenty-six parts by weight of such mixture were then thoroughly blended with 24 parts by weight of tin and 439 parts by weight of titanium sponge, and then further processed as indicated in Example I, to yield a final titanium-base alloy consisting essentially of 9 percent molybdenum, 5 percent zirconium, 4 percent tin, 2 percent iron, 1 percent chromium, balance titanium.

EXAMPLE IX.

Example VIII was repeated, except that ferrochromium (consisting of 70 percent chromium, balance essentially iron) was used as a source of chromium in place of chromium chips, and molybdenum rondelles were used as a source of part of the molybdenum. That is, a mixture was formed consisting of 41 parts by weight of molybdenum rondelles, 33 parts by weight of ferromolybdenum, 35 parts by weight of zirconium sponge, and 10 parts by weight of ferrochromium. The mixture was thoroughly blended and then carbon-arc-melted, screened, and remelted, and again screened as indicated in Example I. One hundred nineteen parts by weight of such mixture were then thoroughly blended with 28 parts by weight of tin and 553 parts by weight of titanium sponge and further processed as indicated in Example I, to yield a final titanium-base alloy consisting essentially of 9 percent molybdenum, 5 percent zirconium, 4 percent tin, 2 percent iron, 1 percent chromium, balance titanium.

EXAMPLE X

Example IX was repeated, except that titanium was introduced to the master alloy immediately before the second carbon-arc-melt. That is, a mixture was formed consisting of 41 parts by weight of molybdenum rondelles, 33 parts by weight of ferromolybdenum, 35 parts by weight of zirconium sponge, and 10 parts by weight of Ferrochromium. The mixture was thoroughly blended then carbon-arc-melted and screened. To this screened product there was added sufficient titanium sponge to yield a titanium content of 20 percent in the master alloy after the second carbon-arc-melt. That is, 119 parts by weight of the screened product of the first carbon-arc-melt were mixed with 30 parts by weight of titanium sponge, and then further processed as indicated in Example I. This yielded a screened master alloy in fine, granular form having substantially the following composition: 42 percent molybdenum, 35 percent zirconium, 9 percent iron, 5 percent chromium, and 20 percent titanium. One hundred forty-nine parts by weight of such mixture were then thoroughly blended with 28 parts by weight of tin and 523 parts by weight of titanium sponge, and then further processed as indicated in Example I, to yield a final titanium-base alloy consisting essentially of 9 percent molybdenum, 5 percent zirconium, 4 percent tin, 2 percent iron, 1 percent chromium, balance titanium.

EXAMPLE XI

A mixture was formed consisting of 60 parts by weight of molybdenum chips, 30 parts by weight of zirconium sponge and 10 parts by weight of tin cuttings. The mixture was thoroughly blended, then tungsten-arc-melted, crushed, screened, remelted and again crushed and screened as in Example I. One hundred parts by weight of such mixture were then thoroughly blended with 400 parts by weight of titanium sponge and then further processed as in Example I, to yield a final titanium-base alloy consisting essentially of 12 percent molybdenum, 6 percent zirconium, 2 percent tin, balance titanium.

Another way in which the master alloy according to the invention may be used is to place together in a furnace in a vacuum or a protective (oxygen- and nitrogen-free) atmosphere particles of master alloy and particles of titanium, and then arc-melt, using a nonconsumable electrode, for example, of carbon or tungsten.

While we have shown and described certain embodiments of our invention, we intend to cover as well any change or modification therein which may be made without departing from the spirit and scope of the invention.

Claims

We claim:

1. A method of making a homogeneous ingot of a molybdenum-containing titanium base alloy, said method comprising the steps of mixing particles of titanium with a master alloy in the form of fine granules, said master alloy consisting essentially of 30 to 75 percent by weight of molybdenum, 25 to 70 percent by weight of at least one of the elements selected from the group consisting of chromium, titanium, zirconium, nickel and copper, but not over 30 percent chromium, not over 25 percent titanium, not over 40 percent zirconium, not over 10 percent nickel and not over 10 percent copper, compacting the mixture into the form of a compacted article and then melting said compacted article in a vacuum.

2. The method of claim 1 wherein said master alloy contains 20 to 40 percent zirconium, 30 to 75 percent molybdenum and 25 percent max. titanium.