Transient Titanium Alloys

Titanium and titanium alloys are extremely valuable in uses where light weight and high strength to weight ratio are important. The aircraft industry and other transportation industries in particular find such alloys highly useful. In order to fully take advantage of the strength and weight properties of titanium, it is often desirable to utilize superplastic forming techniques, e.g., to form titanium and titanium alloys in complex parts. Complex parts are parts having a shape of such complexity that they cannot be readily formed by standard casting, molding, forging, machining and welding techniques.

Superplastic forming typically is used to form sheet stock of between about 0.040 to 3/16 inch thickness. In superplastic forming, a die having the desired shape is used. A piece of stock of titanium alloy, such as a sheet of the alloy, is introduced into the die. The part is normally heated in the die. In the die, the pressure on one side of the stock is reduced and the pressure on the other side of the stock is increased, for example, to between about 200-300 psi. The difference in pressure forces the stock to flow into the die and assume the desired shape conforming to the die.

This method of forming allows complex shaped, formed parts to be manufactured which can take advantage of the high strength to weight ratios inherent in titanium alloys. However, there are limitations in this fabrication method. In general, the superplastic method is not suitable for use with unalloyed titanium and certain high temperature titanium alloys (the near alpha alloys and the like). To adequately superplastically form titanium alloys into useful parts, it is necessary for the stock to have a microstructure which contains alpha and beta phases and a fine grain size (below about 10 microns). This severely restricts the alloys which may be formed by the superplastic technique. Unalloyed titanium, for example, cannot be superplastically formed, since it has no beta phase. In addition, some titanium alloys can be formed by superplastic methods, but only at a temperature above that at which the grain structure of the alloy grows to an unacceptable size. For example, Ti-8Al-1Mo-1V can only be superplastically formed at a temperature of about 900.degree.-975.degree. C. or above. At that temperature, the grain size of the alloy rapidly grows to above 10 microns, and as a result, the formed part has an unacceptably low strength.

At present only Ti-6Al-4V alloy can be successfully formed by superplastic techniques. Even Ti-6Al-4V requires a forming temperature of 875.degree. C. or more. This temperature is sufficiently high that a high rate of die wear, and replacement expense, is experienced. In addition, the production rate is slow adding greatly to the cost of the formed parts. Due to these factors, parts formed of titanium and titanium alloys must often be fabricated by more costly conventional techniques, or with high die wear, at such slow rates, in order to retain the mechanical properties of the formed parts, that superplastic forming is not practicable. Often the reduced strength caused by the unsuitable structural properties of the formed parts requires that additional material be used to make the formed part, losing the inherent strength to weight value of titanium and titanium alloys.

Applicants have found that titanium and titanium alloys, including the high temperature (near alpha and the like titanium alloys), may be formed into alloy parts and structures of extremely complex shapes much more economically than previously possible using superplastic forming techniques. Applicants have found that titanium and titanium alloys may be formed into transient alloys which are much more suitable to superplastic forming. The transient alloys allow superplastic forming to occur at much lower temperatures and to proceed much more rapidly. Even alloys which can be successfully worked by superplastic forming can greatly benefit from applicants' invention. For example, Ti-6Al-4V can be formed by prior techniques using a forming temperature of about 875.degree. C. Using applicants' invention, parts can be produced from this alloy, as temporarily modified, at temperatures of about 800.degree. C. At this temperature the die wear and overall cost of production of parts so formed are significantly reduced. Formation of undesirable microstructures in the alloys is negligible so that the mechanical properties of the alloys and of parts formed from the alloys are not degraded.

After formation of parts from the transient alloys, the titanium and titanium alloys can be returned to their original composition, resulting in extremely light weight and strong titanium and titanium alloy parts. The superplastic parts formed may be produced at higher production rates, at low cost and from compositions which in the past would not have allowed this technique to have been used.

The invention will be more apparent from the following detailed description of the preferred embodiments and the drawings:

FIG. 1 is a comparison of a Ti-8Al-1Mo-1V conventional alloy superplastically formed in a cone die and applicants' Ti-8Al-1Mo-1V-0.2H transient alloy superplastically formed in a cone die;

FIG. 2 is a comparison of Ti-6Al-2Sn-4Zr-2Mo conventional alloy superplastically formed in a cone die and applicants' Ti-6Al-2Sn-4Zr-2Mo-0.14H transient alloy superplastically formed in a cone die;

FIG. 3 is a comparison of a Ti-6Al-4V conventional alloy superplastically formed in a cone die and applicants' Ti-6Al-4V-0.11H transient alloy superplastically formed in a cone die;

FIG. 4 is a comparison of unalloyed titanium superplastically formed in a cone die and applicants' Ti-0.22H transient alloy superplastically formed in a cone die;

FIG. 5 is a comparison of Ti-6Al-4V transient alloy formed at 860.degree. C.;

FIG. 6 is a comparison of Ti-6Al-4V transient alloy formed at 800.degree. C.;

FIG. 7 is a comparison of Ti-6Al-4V transient alloy formed at 760.degree. C.;

FIG. 8 is a comparison of Ti-6Al-2Sn-4Zr-2Mo transient alloy formed at 900.degree. C.;

FIG. 9 is a comparison of Ti-6Al-2Sn-4Zr-2Mo formed at 830.degree. C.; and

FIG. 10 is a comparison of Ti-6Al-2Sn-4Zr-2Mo transient alloy formed at 800.degree. C.

Applicants' transient alloys are formed principally of titanium. By "titanium" we mean both unalloyed titanium and titanium alloys. The invention is especially valuable for unalloyed titanium and high temperature titanium alloys (such as the near alpha alloys and the like), which in some instances may not be superplastically formed without transient alloy formation. However, other titanium alloys may be used. Examples of titanium alloys from which transient alloys may be formed are Ti-8Al-1Mo-1V, Ti-5-Al-2-2.5Sn, Ti-6Al-4V, and Ti-6Al-2Sn-4Zr-2Mo. Transient alloys are prepared, for example, from the above alloys, by the addition of hydrogen into the alloy matrix. The amount of hydrogen added is not critical. One should, however, avoid hydrogen addition in such large quantities as to change the alloys to single beta phase. Hydrogen introduced into the alloy matrix at a level between about 0.01 and 0.5 weight percent of the total matrix is effective to improve forming. Normally above 0.2 percent is optimum. The transient alloys may be described by the general formula A--XH, where A is the base alloy material, including unalloyed titanium, H is hydrogen and X is a numerical value normally between about 0.01 and 0.5 percent by weight.

The transient alloys are formed from the base alloy stock, that is, from unalloyed titanium or from a titanium alloy, by heating the stock under pressure in a non-flammable gas environment for a time sufficient for hydrogen to be absorbed and dissolved into the alloy matrix. A typical composition for the non-flammable gas environment would be a gas consisting of 96 weight percent argon and four percent hydrogen gas, that is, hydrogen makes up about forty-three volume percent of the gas. The composition of the gas is not critical, but it is preferred that the hydrogen be less than about 5 weight percent, to avoid creating a flammable mixture. This hydrogen charging step, as it may be called, may be actually performed in the die itself, allowing the stock to absorb the hydrogen into the alloy matrix whie in the die chamber. Many charging cycles or conditions will yield the desired amount of hydrogen in the alloy; for example, heating in the described environment for two hours at 760.degree. C. and at 150 psi pressure has been found to produce the desired transient alloy. In general, pressures of between about 50 to 300 psi or more may be used. Temperatures of between about 600.degree.-760.degree. C. may be used. The temperature also is not critical, but it is preferred to use temperatures in the lower part of the range, since the solubility of hydrogen in titanium decreases with increasing temperature. It is preferred to keep the temperature below that at which the grain size of the alloy grows, in any event.

After the desired transient alloy has been formed, that is, after sufficient hydrogen has been dissolved in the alloy matrix, the alloy may be formed into the desired shape. This procedure follows previous practice. Typically, the pressure is released on one side of the stock and the pressure on the other side of the stock is increased, causing the stock to flow into the die and assume the desired shape of the die. After forming, the original composition of the alloy is restored by driving the hydrogen out of the alloy matrix. Typically, this is done by heaating the formed part in a vacuum, or a partial vacuum, for example, for two hours at 650.degree. C. The temperature of the restoration and other conditions are not critical. It is preferred that heating be at above about 600.degree.-650.degree. C., since at much lower temperatures the rate at which the hydrogen is released may be slow. Preferably the temperatures should be below the forming temperatures, to minimize changes in the grain structure of the alloy, or at least below the temperature at which substantial grain growth occurs.

The following illustrations demonstrate the improvement in superplastic forming which may be accomplished, using applicants' transient alloys, with titanium and titanium alloys.

As shown in FIG. 1, two duplex annealed disks of titanium alloy were superplastically formed in a special steel conicical die having a diameter of 2.15 inches and a depth of 1.8 inches. The disks were nominally 23/4 inch diameter and 0.1 inch in thickness. Both disks were of Ti-8Al-1Mo-1V initially. A transient alloy having the composition Ti-8Al-1Mo-1V-0.2H was formed from the disk shown on the right in FIG. 1. The alloy was formed in the die by charging the die with an atmosphere of 99 weight percent argon and one percent hydrogen under 150 psi pressure and at 760.degree. C. The disk was held at these conditions for two hours prior to forming.

Both disks were formed in the die at 860.degree. C. under a biaxial sample stress (.sigma.) of 18.8 MPa (2.72 Ksi) for 1.3 hours (.sigma.=P.times./2T, where P is the applied gas pressure, X is the sample radius of curvature and T is the instantaneous sample thickness.) In FIG. 1 the conventional alloy is shown on the left; it would not conform to the die under the test conditions. The transient alloy, shown on the right in FIG. 1, conformed fully to the die to make a complete complex shape. The part formed from the transient alloy was restored to the original alloy composition by heating at 650.degree. C. in a vacuum for five hours.

Examination of the formed part revealed that the basic microstructural composition of the original alloy was maintained and that no degrading changes had occurred in the structure and mechanical properties of the alloy.

As shown in FIG. 2, two duplex annealed disks of titanium alloy were superplastically formed in a special steel conical die having a diameter of 2.15 inches and a depth of 1.8 inches. The disks were nominally 23/4 inch diameter and 0.1 inch in thickness. Both disks were of Ti-6Al-2Sn-4Zr-2Mo initially. A transient alloy having the composition Ti-6Al-2Sn-4Zr-2Mo-0.14H was formed from the disk shown on the right in FIG. 2. The alloy was formed in the die by charging the die with an atmosphere of 99 weight percent argon and one percent hydrogen under 150 psi pressure and at 760.degree. C. The disk was held at these conditions for two hours prior to forming.

Both disks were formed in the die at 830.degree. C. under a sample stress (.sigma.) of 18.8 MPa (2.72 Ksi) for two hours (.sigma.=P.times./2T). In FIG. 2 the conventional alloy is shown on the left; it would not conform to the die under the test conditions. The transient alloy, shown on the right in FIG. 2, conformed fully to the die to make a complete complex shape. The part formed from the transient alloy was restored to the original alloy composition by heating at 650.degree. C. in a vacuum for five hours.

Examination of the formed part revealed that the basic microstructural composition of the original alloy was maintained and that no degrading changes had occurred in the structure and mechanical properties of the alloy.

As shown in FIG. 3, two duplex annealed disks of titanium alloy were superplastically formed in a special steel conical die having a diameter of 2.15 inches and a depth of 1.8 inches. The disks were nominally 23/4 inch diameter and 0.1 inch in thickness. Both disks were of Ti-6Al-4V initially. A transient alloy having the composition Ti-6Al-4V-0.11H was formed from the disk shown on the right in FIG. 3. The alloy was formed in the die by charging the die with an atmosphere of 99 weight percent argon and one percent hydrogen under 150 psi pressure and at 760.degree. C. The disk was held at these conditions for two hours prior to forming.

Both disks were formed in the die at 900.degree. C. under a sample stress (.sigma.) of 18.8 MPa (2.72 Ksi) for two hours (.sigma.=P.times./2T). In FIG. 3 the conventional alloy is shown on the left; it would not conform to the die under the test conditions. The transient alloy, shown on the right in FIG. 3, conformed fully to the die to make a complete complex shape. The part formed from the transient alloy was restored to the original alloy composition by heating at 650.degree. C. in a vacuum for five hours.

Examination of the formed part revealed that the basic microstructural composition of the original alloy was maintained and that no degrading changes had occurred in the structure and mechanical properties of the alloy.

As shown in FIG. 4, two disks of unalloyed titanium were superplastically formed in a special steel conical die having a diameter of 2.15 inches and a depth of 1.8 inches. The disks were nominally 23/4 inch diameter and 0.1 inch in thickness. Both disks were of unalloyed titanium initially. A transient alloy having the composition Ti-0.22H was formed from the disk shown on the right in FIG. 4. The alloy was formed in the die by charging the die with an atmosphere of 99 weight percent argon and one percent hydrogen under 150 psi pressure and at 760.degree. C. The disk was held at these conditions for two hours prior to forming.

Both disks were formed in the die at 700.degree. C. under a sample stress (.sigma.) of 9.4 MPa (1.36 Ksi) for 40 minutes (.sigma.=P.times./2T). In FIG. 4 the conventional alloy is shown on the left; it would not conform to the die under the test conditions. The transient alloy, shown on the right in FIG. 4, conformed fully to the die to make a complete complex shape. The part formed from the transient alloy was restored to the original alloy composition by heating at 650.degree. C. in a vacuum for five hours.

Examination of the formed part revealed that the basic microstructural composition of the original alloy was maintained and that no degrading changes had occurred in the structure and mechanical properties of the alloy.

FIGS. 5-10 report the results of superplastic forming tests of various transient titanium alloys under differing conditions of temperature and sample strain .epsilon., where ##EQU1## L=depth of cone die. Q.sub.1 =depth of forming metal cone when contact is first made between forming metal and the die surface.

Q.sub.2 =instantaneous depth of forming metal cone.

Applicants' transient alloys allow the rapid and economical formation of complex shapes from titanium and titanium alloys by superplastic forming. The formed parts are light and strong; the structural integrity of the alloys is retained in forming. Applicants' technique is valuable and useful for all titanium alloys and has additional value when used with unalloyed titanium and high temperature titanium alloys (the near alpha alloys and the like), since it allows formation of complex shapes from these alloys by superplastic forming, which could not otherwise be formed by this method, or which, if formed would have a weakened structure.

It will be apparent to those skilled in the art that many variations and departures from the specific alloys described herein may be made without departing from the spirit and essential characteristics of the invention. The specific embodiments are to be considered in all their aspects and are for purposes of illustration and are not restrictive of the scope of the invention. The scope of the invention herein is to be determined by the claims which are appended hereto and their equivalents.