US3508914A - Methods of forming and purifying nickel-titanium containing alloys - Google Patents

Description

United States Patent U.S. Cl. 75-135 8 Claims ABSTRACT OF THE DISCLOSURE Methods for forming and treating titanium-nickel alloys which include preparing the alloys by induction melting the nickel, adding the titanium to the molten nickel, contacting the resulting alloy with carbon while under vacuum. The alloys are further cast in suitable graphite molds with the solidification process proceeding from the bottom toward the top of the mold.

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

This invention pertains to the formation and treatment of alloys and more particularly to the formation and treatment of alloys comprising a highly reactive metal and a relatively unreactive metal.

Many techniques and methods have been tried in an effort to find an effective means for forming alloys containing a highly reactive metal. For example, attempts to form alloys of the highly reactive metal titanium by fusion techniques were not very successful due to the high reactivity of molten titanium with the interstitial elements, oxygen, hydrogen, nitrogen and carbon. Thus, the use of well known ceramic crucibles, e.g., SiO A1 0 etc., for forming titanium alloys resulted in a disasterous contamination due to the interaction of the titanium and the oxygen of the crucible. The use of high purity dense graphite crucibles was not much more successful since carbide impurities resulted from the interaction between the titanium and carbon of the crucible. The use of techniques such as special electrical induction fields to minimize graphite contact and thus minimize carbide formation were a little more successful, but they are difiicult and costly.

As a result of the failure of fusion methods, the alloys of highly reactive metals are generally prepared by arcmelting techniques, e.g., titanium-nickel alloys have been formed by both consumable and non-consumable methods, employing a water-cooled copper crucible. These techniques, however, sufier from the following disadvantages:

(1) Composition control is difficult due to the lack of stirring capability in the overall melt.

(2) Little or no opportunity exists for alloy purification during melting.

(3) Costly multiple arc-melting operations are necessary to promote chemical homogeneity in the cast ingots.

(4) Only limited cast shapes can be produced.

Accordingly, it is an object of this invention to provide a novel method for forming alloys comprising a highly reactive metal and a relatively unreactive metal.

It is another object to provide a novel method for producing the above mentioned alloys that promotes chemical homogeneity.

It is a further object to provide a novel method for forming and casting the above mentioned alloys.

It is still another object to provide a novel method for purifying the above mentioned alloys.

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It is still a further object of this invention to provide a novel method for forming and casting nickel-titanium alloys.

It is yet another object of this invention to remove oxygen impurities from nickel-titanium alloys.

These and other objects will become more readily apparent from reading the following detailed description of the invention.

The method of this invention for forming the alloy generally comprises melting the relatively unreactive component of the alloy in a container formed from a stable material, adding to and melting in the container the reactive metal component and pouring the resulting alloy into a suitable mold for solidification. The melting operation is performed in an inert atmosphere, i.e., any atmosphere that excludes atmospheric contaminants including, for example, a vacuum; a rare gas such as argon, helium, etc., and the like, with the use of a rare gas at atmospheric pressure or higher being preferred since it prevents the leakage of air into the system.

The container is generally formed from either thoria or magnesia with magnesia being preferred due to its lower cost and less toxic nature. It is to be understood, however, that the term container formed from a stable material is not limited to containers wholly made from such materials but also includes containers made from other materials that have their inner surfaces coated with stable materials. The stable material used for forming the container generally has a purity of at least about 97%, with at least about 99% being preferred, in order to lessen the possibilities of oxygen impurities resulting from the interaction of the reactive metal component with the oxide impurities generally found in such materials.

The relatively unreactive material is melted in the container first since it has been found that the molten unreactive component has a moderating effect upon the reactive metal component which reduces metal-container interaction. Since this moderating effect is considerably reduced when the atomic ratio of reactive component to unreactive component exceeds about 2 to 1, the method of this invention is most effective on alloys that fall within this ratio.

The components are generally melted in a low frequency induction furnace since such an operation has a good mixing effect which promotes chemical homogeneity. Although low frequency induction melting techniques are preferred because they produce superior alloys at a lower cost, it is to be understood that other melting techniques may be employed so long as they are utilized in an inert atmosphere and the relatively unreactive component is melted first in a container formed from a stable material such as magnesia or thoria.

The method of this invention is generally performed, using a nickel-titanium alloy as a representative example, by first placing a dry clean magnesia or thoria crucible containing the desired weight of nickel into a low frequency induction furnace having an inert atmosphere. The nickel is melted and the amount of titanium necessary to give the desired nickel-titanium ratio is charged from a charging chute into the molten nickel. The molten titanium and nickel are intimately mixed in the crucible by the low frequency of the furnace and when alloying is complete, the molten alloy is charged into a suitable mold for solidification.

The method of this invention for removing oxygen impurities generally comprises contacting the molten alloy with carbon under vacuum. More particularly, the alloy may be purified either by induction melting the alloy under a vacuum of at least about 10' mm. in a carbon container, preferably in the form of high density graphite or by induction melting the alloy under a vacuum of at least mm. in any suitable container followed by the addition of carbon. Although the invention is not to be limited or bound by any theoretical reactions or equations, it is believed that the removal of oxygen impurities is effected, using a nickel-titanium alloy as a representative example, by the following reaction:

The purification effect of the carbon may be enhanced by adding to the molten alloy a metal that both forms an oxide having a high heat of formation and does not alloy with the metals present in the molten alloy. Thus for example, an excess of calcium or magnesium metal (based on the amount necessary to combine with the oxygen impurities present) is placed beneath the surface of the molten alloy in the graphite container, said metal and oxygen combining to form an oxide which is raked off as a slag. When the purification is completed the excess metal having a higher vapor pressure is removed by vacuum techniques. Alternatively, the purification may be promoted by bubbling hydrogen into the molten alloy in the carbon crucible, said hydrogen combining with the oxygen imuprity to form water vapor.

The alloy forming and purification techniques described herein may be performed on any alloy comprising highly reactive and relatively unreactive metal components. As representative examples of highly reactive metal components there may be mentioned Group IV metals such as hafnium, zirconium, titanium, etc., rare earth metals such as cerium, etc., and the like. As representative examples of relatively unreactive metal components there may be mentioned iron, cobalt, copper, indium, aluminum, nickel, gold, lead, and the like. The methods of this invention are particularly applicable to the 50 to 70 weight percent nickel, remainder essentially titanium alloys, especialy the stoichiometric nickel-titanium alloy (53.5 to 56.5 weight percent nickel, the remainder essentially titanium) which are described in more detail in US. Patent No. 3,174,851, granted Mar. 23, 1965, which is hereby incorporated by reference.

There are a wide variety of embodiments incorporating the teachings of this invention which may be utilized in producing a cast shape of an alloy comprising a highly reactive and relatively unreactive metal. One such embodiment comprises forming the alloy by the method of this invention, pouring the molten alloy before solidification into a carbon crucible, said alloy being kept molten under vacuum to effect purification and pouring the molten alloy into a suitable mold for solidification. Alternatively, any suitable crucible may be used followed by the addition of carbon to elfect purification.

Another embodiment comprises placing an ingot prepared by either the method of this invention or any other method into a graphite crucible, induction melting the alloy under vacuum to effect purification and pouring the molten alloy into a suitable mold for solidification. Alternatively, any suitable crucible may be employed followed by the addition of carbon to the molten alloy.

A further embodiment comprises pouring the molten alloy prepared by the method of this invention before solidification through a carbon lined funnel into a suitable mold for solidification, said operation being performed under vacuum.

Still another embodiment for providing intricate cast shapes while effecting purification comprises delivering under vacuum the alloy formed by the method of this invention in either liquid or solid form to a specially designed mold made of high purity dense dried graphite. The mold has a suitably matched induction coil around it and the induction coil is equipped with taps to regulate heating specific sections of the mold. If the charge to the mold is molten, the mold may be preheated to a ternperature in excess of the melting point of the alloy concerned. Once the mold is filled (including the filling of generous, hot top"), the induction coil is gradually closed off starting at the bottom of the mold. By solidifying the casting from the bottom of the mold to the top it is possible to produce a minimum shrinkage pipe, and it will be in the hot top section. Further, by casting in the heated mold it is possible to provide a fine cast surface, optimize casting definition in thin sections and minimize porosity within the cast section. If the charge to the mold is solid, the solid alloy is placed in a graphite hopper attached to the top of the mold, said induction coils being wound well up on the hopper. The mold and hopper is heated by induction causing the alloy concerned to melt in the hopper and run into the mold. The solidification is then performed as mentioned above placing the shrinkage pipe in the hopper.

The following examples are illustrative of the invention but they are not intended to limit it in any manner.

EXAMPLE I A 55 weight percent nickel, remainder essentially titanium alloy may be prepared in the following manner.

2750 grams of nickel in the form of carbonyl nickel shot (99.9% pure) are placed in a magnesium oxide crucible (99.9% pure). The crucible is placed in an induction furnace which has an atmosphere of argon at a pressure of one atmosphere. The furnace is run at an induction input of approximately 3,000 cycles and the nickel in the crucible is heated to a temperature slightly in excess of 1600 C. in order to effect melting. 2,250 grams of titanium (in the form of loose sponge) are added to the molten nickel and the temperature is maintained slightly in excess of 1500 C. until the titanium and nickel are completely mixed (usually less than 5 minutes). The alloy melt is immediately poured into a suitable mold for solidification.

EXAMPLE II A nickel-titanium alloy (55 weight percent nickel, remainder essentially titanium) is purified by placing the alloy in a high density graphite crucible which is then placed into an induction furnace. A vacuum of 10* millimeters is pulled and the induction input to the furnace is approximately 3,000 cycles. The alloy is held at about 1500 C. to effect purification and is then poured into a mold for solidification.

The methods and teachings of this invention are extremely valuable since they permit the use of an induction melting technique for forming an alloy containing a highly reactive metal, retaining its advantages, i.e., low cost, chemical homogeneity, composition control, etc., while avoiding the usual disadvantages, i.e., contamination of the resulting alloy, etc. This invention will lbroaden the commercial applicability of the aforementioned alloys of US. Patent No. 3,174,851 since the formation and casting of these alloys into intricate, complex shapes is now economically feasible.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. A method for forming and purifying a metal alloy comprising, said method comprising:

(a) induction melting the nickel component in a container formed from a material selected from the group consisting of magnesia and thoria, said induction melting being performed in an inert atmosphere;

(b) adding to and melting in said container the titanium component;

(0) contacting the molten alloy with carbon, said contacting being performed under vacuum; and

(d) solidifying said alloy.

2. The method of claim 1 wherein said vacuum is at least about 10 mm,

3. The method of claim 2 wherein said contacting with carbon is performed by adding carbon to the molten alloy.

4. The method of claim 2 wherein the container is formed from magnesia.

5. The method of claim 2 wherein said contacting with carbon is performed by pouring the molten alloy into a graphite container.

6. The method of claim 2 further comprising promoting the purification by bubbling hydrogen into the molten alloy during the contacting.

7. The method of claim 2 further comprising promoting the purification by placing a metal selected from the group consisting of calcium and magnesium beneath the surface of the molten alloy during the contacting.

8. The method of claim 7 wherein the metal is calcium.

References Cited UNITED STATES PATENTS 6 2,564,498 8/1951 Nisbet 7549 2,580,273 12/1951 Bens et a1 7565 X 2,805,148 9/1957 DeLong 75-65 X 1,979,506 11/1934 Umbreit 75135 X 2,776,204 1/1957 Moore 7549 2,815,273 12/1957 Moore 7549 2,815,279 12/1957 Moore 7549 X 3,019,102 1/1962 Saarivirta 75135 X 3,116,998 1/1964 Pagonis 26634 X 3,137,566 6/1964 Thieme 7549 X 3,174,851 3/1965 Buehler et a1. 75-170 3,188,198 6/1965 Moore 7549 X OTHER REFERENCES Darmara et 211.: Vacuum Induction Melting, J our. of the Iron & Steel Inst., March 1959, pp. 266, 268 and 272.

HENRY W. TARRING II, Primary Examiner US. Cl. X.R.

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