US3679394A - Method for casting high ti content alloys - Google Patents

Abstract

METHODS FOR FORMING AND TREATING ALLOYS CONTAINING (A) TI AND NI, (B) TI, NI AND CO, (C) TI AND CO, (D) TI, CO AND FE, AND (E) TI AND FE WHICH INCLUDE PREPARING THE ALLOYS BY INDUCTION MELTING TECHNIQUES AND PRUIFYING THE ALLOYS BY CONTACTING THE MOLTEN ALLOY WITH CARBON. THE ALLOYS ARE FURTHER CAST IN SUITABLE GRAPHITE MOLDS WITH THE SOLIDIFICATION PROCESS PROCEEDING FROM THE BOTTOM TOWARD THE TOP OF THE MOLD.

Description

.U.S. Cl. 75-110 United States Patent O 3,679,394 METHOD FOR CASTING HIGH Ti CONTENT ALLOYS William J. Buehler, Bethesda, Md., assignor to the United States of America as represented by the Secretary of the Navy No Drawing. Continuation-impart of application Ser. No. 493,940, Oct. 7, 1965, now Patent No. 3,508,914, dated Apr. 28, 1970. This application Nov. 24, 1969, Ser. No. 879,622

Int. Cl. (32% 53/00,- C22d 7/06 8 Claims ABSTRACT OF THE DISCLOSURE Methods for forming and treating alloys containing (a) Ti and Ni, (b) Ti, Ni and Co, (c) Ti and Co, (d) Ti, Co and Fe, and (e) Ti and Fe which include preparing the alloys by induction melting techniques and purifying the alloys by contacting the molten alloy with carbon. The alloys are further cast in suitable graphite molds with the solidification process proceeding from the bottom toward the top of the mold.

CROSS-REFERENCE TO RELATED APPLICATIONS This application is a continuation-in-part of applica tion Ser. No. 493,940 filed Oct. 7, 1965, now US. Pat. No. 3,508,914, issued Apr. 28, 1970.

BACKGROUND OF THE INVENTION 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 at least one relatively unreactive metal.

Many techniques and methods have been tried in an elfort 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 etc., for forming titanium alloys resulted in a disastrous 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 difficult and costly.

As a result of the failure of fusion methods, the alloys of highly reactive metals are generally prepared by aremelting 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, suffer 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.

SUMMARY OF THE INVENTION Accordingly, it is an object of this invention to provide a novel method for forming alloys comprising a highly 3,679,394 Patented July 25, 1972 reactive metal and at least one 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 a further object of this invention to provide a novel method for forming and casting alloys containing (a) Ti and Ni, (b) Ti, Ni and C0, (c) Ti and Co, (d) Ti, Co and Fe and (e) Ti and Fe.

It is a still further object of this invention to provide a method for forming and purifying metal alloys containing Ti and either (a) Ni and C0, (b) Co, (c) Co and Fe or ((1) Fe.

It is yet another object of this invention to remove oxygen impurities from high Ti content alloys.

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

DESCRIPTION OF THE PREFERRED EMBODIMENT The method of this invention for forming the alloy generally comprises melting the relatively unreactive component or components 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 or materials are melted in the container first since it has been found that the molten unreactive component or components have a moderating effect upon the reactive metal component which reduces metal-container interaction. Since this mod erating effect is considerably reduced when the atomic ratio of reactive component to unreactive component or components exceed about 2 to l, 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 or components are 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 completed, 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 10* mm. in any suitable container followed by the addition of carbon. Although the invention is not to be limited or bound by any theorotical 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 impurity to form water vapor.

The alloy forming the 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 alloys of the formulas TiNi,,Co and Tico l e wherein X is a factor from to 1 and wherein the Ti content of the alloy constitutes approximately 50 Atomic percent of the alloy and NiCo or C0,,Fe essentially constitute the remaining approximately 50 Atomic percent of the alloy, and especially to the stiochiometric nickel-titanium alloy (53.5 to 56.5 weight percent nickel, the remainder essentially titanium). The nickel-titanium alloys are described in US. Pat. 3,174,851 granted Mar. 23, 1965 and the alloys represented by the formulas TiNi Co and TiCo Fe are described in application Ser. No. 843,887, filed June 12, 1969, now U.S. Pat. No. 3,558,369, issued Jan. 26, 1971, which is a continuation-in-part of application Ser. No. 579,185, filed Sept. 9, 1966, now abandoned. The subject matter of all these applications 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 or metals. 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 effect 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 temperature 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 preformed as mentioned above placing the shrinkage pipe in the hopper.

It is to be understood that the term graphite mold is not limited to molds wholly made of graphite but also includes containers made from other materials that have their inner surfaces coated with graphite.

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 it is then poured into a mold for solidification.

The same techniques hereinabove described can be applied to alloys containing (a) Ti and Ni, (b) Ti, Ni and Co, (c) Ti and Co, ((1) Ti, Co and Fe and (e) Ti and Fe wherein the Ti content of the alloys varies from 40 to 60 atomic percent and the other component or components essentially constitute the remainder of the alloy.

What is claimed as new and desired to be secured by Letters Patent of the United States is:

1. In a method for forming and purifying a metal alloy,

selected from the group consisting of alloys comprising (a) Ti, Ni and Co,

(b) Ti and Co,

(c) Ti, Co and Fe, and

(d) Ti and Fe wherein the Ti content of the alloy varies from 40 to 60 atomic percent of the alloy, said method comprising;

(1) induction melting the metals of the alloy in an induction melting container formed froma material selected from the group consisting of magnesia and thoria, said induction melting being performed in an inert atmosphere;

(2) contacting the molten alloy with carbon, in said induction melting container said contacting being performed in an inert atmosphere, and

(3) solidifying said alloy,

the improvement comprising first induction melting the non Ti component and thereafter adding to and induction melting in the same container the titanium component of the alloy.

2. The method of claim 1 wherein said inert atmosphere is a vacuum of at least about 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.

References Cited UNITED STATES PATENTS 2,548,897 4/1951 Kroll 10 2,806,271 9/ 1957 Operhall 7549 3,188,198 6/196'5 Moore 7549 2,707,679 5/ 1 955 Lilliendahl 7 584 3,429,691 2/ 1969 McLaughlin 75--1l 2,874,953 2/1959 Rogers 7584 2,702,239 2/ 1955 Gilbert 7584 3,183,078 5/1965 Ohtake 7549 3,223,519 12/ 1965 Schippereit 75--l0 3,417,808 12/1968 Rosenberg 164--68 WINSTON A. DOUGLAS, Primary Examiner P. D. ROSENBERG, Assistant Examiner US. Cl. X.R. 7584-