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Method for producing a beta-processed alpha-beta titanium-alloy article

Method for producing a beta-processed alpha-beta titanium-alloy article description/claims

This application is a continuation of U.S. application Ser. No. 10/878,105, filed Jun. 28, 2004 which is hereby incorporated by reference in its entirety.

This invention relates to the production of alpha-beta titanium-alloy articles that are beta processed, and more particularly to improving the isotropy of the mechanical properties of the article.

Beta-processed alpha-beta titanium alloys are used to manufacture aerospace hardware such as components of gas turbine engines. These alloys have excellent mechanical properties relative to their weight, at both room temperature and moderate elevated temperatures as high as about 1200° F. The alloys are used to make parts such as fan and compressor disks, blisks, blades, shafts, and engine mounts.

An alpha-beta titanium alloy is an alloy having more titanium than any other element, and which forms predominantly two phases, alpha phase and beta phase, upon heat treatment. In titanium alloys, alpha (α) phase is a hexagonal close packed (HCP) phase thermodynamically stable at lower temperatures, beta (β) phase is a body centered cubic (BCC) phase thermodynamically stable at higher temperatures above a temperature termed the “beta transus” temperature that is a characteristic of the alloy composition, and a mixture of alpha and beta phases is thermodynamically stable at intermediate temperatures. Processing to control the relative amounts and the morphologies of these phases is used to advantage in achieving the desired properties of interest in the alloys.

One approach to preparing articles is to cast the alpha-beta titanium alloy as an ingot, to thereafter thermomechanically work the workpiece from the as-cast ingot form to approximately the final shape and size of the desired article, and to thereafter final machine the article. In beta processing, the workpiece is mechanically worked, typically by forging, at a temperature above the beta-transus temperature, and subsequently heat treated at lower temperatures to reach the desired microstructure. Beta processing is particularly useful for manufacturing large articles, because the strength of the workpiece is reduced above the beta transus temperature, and large workpieces may be mechanically worked more easily in the available metalworking equipment.

In some beta-processed alpha-beta titanium alloys, the ductility of the final article is highly anisotropic and thence strongly dependent upon the angle of the principal loading direction relative to the orientation of the prior beta grain flow that occurs during the beta-phase processing. For example, the tensile ductility measured parallel to the prior beta grain flow direction may be 2-4 times larger than the ductility measured at 45 degrees to the prior beta grain flow direction. This variability in ductility may render the material unsuitable for applications where the article is mechanically loaded in different directions in different portions of the article.

There is a need for an approach to achieving desirable mechanical properties of the beta-processed alpha-beta titanium alloys but also avoiding the anisotropy in ductility and possibly other properties that is associated with some of the beta-processed alpha-beta titanium alloys. The present invention fulfills this need, and further provides related advantages.

The present approach provides a new production procedure for beta-processing alpha-beta titanium alloys. The approach produces good mechanical properties in the final articles, while also reducing the anisotropy in ductility that is a drawback of prior processing. The technique is practiced with existing production equipment.

A method for producing a titanium-alloy article comprises the steps of providing a workpiece of an alpha-beta titanium alloy having a beta-transus temperature, and thereafter mechanically working the workpiece at a mechanical-working temperature above the beta-transus temperature. Examples of alpha-beta titanium alloys that may be processed by the present approach include alloys having a nominal composition in weight percent of Ti-6Al-2Sn-4Zr-2Mo, sometimes known as Ti-6242; Ti-6Al-2Sn-4Zr-6Mo, sometimes known as Ti-6246; Ti-6Al-2Sn-2Zr-2Mo-2Cr-0.25Si, sometimes known as Ti-6-22-22S; and Ti-5Al-4Mo-4Cr-2Sn-2Zr, sometimes known as Ti-17. The workpiece may be a precursor of a component of a gas turbine engine. A mechanical working technique of particular interest is forging.

The workpiece is thereafter solution heat treated at a solution-heat-treatment temperature of from about 175° F. to about 25° F. below the beta-transus temperature, and quenched from the solution-heat-treatment temperature. In one processing embodiment, the workpiece is solution heat treated at the solution-heat-treatment temperature of from about 175° F. to about 125° F. below the beta-transus temperature. In another processing embodiment, the workpiece is solution heat treated at the solution-heat-treatment temperature of from about 100° F. to about 25° F. below the beta-transus temperature. The method includes thereafter, overage heat treating the workpiece at an overage-heat-treatment temperature of from about 400° F. to about 275° F. below the beta-transus temperature, and cooling the workpiece from the overage-heat-treatment temperature.

After the heat treating is complete, the workpiece may be further processed, as by machining, or it may be placed into service.

In a related approach, a method for producing a titanium-alloy article comprises the steps of providing a workpiece of an alpha-beta titanium alloy having a beta-transus temperature, and thereafter mechanically working the workpiece at a mechanical-working temperature above the beta-transus temperature. The method further includes solution heat treating the workpiece at a solution-heat-treatment temperature of from about 1450° F. to about 1600° F., quenching the workpiece from the solution-heat-treatment temperature, and thereafter overage heat treating the workpiece at an overage-heat-treatment temperature of from about 1225° F. to about 1350° F., and cooling the workpiece from the overage-heat-treatment temperature. In subranges of interest, the solution-heat-treatment temperature may be from about 1450° F. to about 1500° F., or from about 1525° F. to about 1600° F. Compatible features described elsewhere may be used in relation to this embodiment of the invention as well.

In a particularly preferred embodiment, a method for producing a titanium-alloy article comprises the steps of providing a workpiece of an alpha-beta titanium alloy having a beta-transus temperature and having a nominal composition in weight percent of Ti-5Al-4Mo-4Cr-2Sn-2Zr, wherein the workpiece is a precursor of a component of a gas turbine engine. The workpiece is thereafter mechanically worked at a mechanical-working temperature above the beta-transus temperature. The method further includes thereafter solution heat treating the workpiece at a solution-heat-treatment temperature of from about 1450° F. to about 1600° F., and quenching the workpiece from the solution-heat-treatment temperature, and thereafter overage heat treating the workpiece at an overage-heat-treatment temperature of from about 1225° F. to about 1350° F., and cooling the workpiece from the overage-heat-treatment temperature.

In a related approach, a method for producing a titanium-alloy article comprises the steps of providing a workpiece of an alpha-beta titanium alloy having a beta-transus temperature, thereafter mechanically working the workpiece at a mechanical-working temperature above the beta-transus temperature, thereafter solution heat treating the workpiece at a solution-heat-treatment temperature below the beta-transus temperature, and quenching the workpiece from the solution-heat-treatment temperature; and thereafter precipitation heat treating the workpiece at a temperature of from about 1100° F. to about 1225° F. The workpiece is utilized by machining the workpiece or using the workpiece in service. The workpiece is thereafter overage heat treated at an overage-heat-treatment temperature of from about 400° F. to about 275° F. below the beta-transus temperature, and cooled from the overage-heat-treatment temperature. Optionally, after the step of utilizing and before the step of averaging, the workpiece is second solution heat treated at a second solution-heat-treatment temperature of from about 175° F. to about 25° F. below the beta-transus temperature, and quenched from the second solution-heat-treatment temperature. Any contamination resulting from these heat treatments may be removed with a macro-etch or by machining. These post-processing or post-service heat treatments restore the properties of the article.

The present approach produces acceptable mechanical properties of the beta-processed alpha-beta titanium alloys, while reducing the anisotropy of ductility in the final article. The processing may be performed using existing apparatus, and does not require a change in the beta processing. Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.

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