1. Field of the Invention
The present invention relates to processes for fabricating structural units from ordered titanium aluminide base alloys, and more particularly to methods of making metallic sandwich structures from titanium aluminide components by applying stop-off material at appropriate locations and joining sheets of the metal together through a process of diffusion bonding, and then superplastically expanding the bonded sheets to form a sandwich structure of the shape desired.
2. Discussion of the Known Prior Art
In its purest form, titanium is a relatively soft, weak and extremely ductile metal. Through additions of other elements, the base metal can be converted to an engineering material having unique characteristics, such as high strength and stiffness, resistance to corrosion, usable ductility, and low density.
Titanium is capable of existing in two or more crystalline forms. In unalloyed titanium (at up to about 785.degree. C.), the atoms arrange themselves in a hexagonal close-packed crystal array called "alpha phase". When titanium is heated above the transition temperature (beta-transus) of about 785.degree. C., the atoms rearrange into a body-centered cubic structure called "beta phase". The addition of other elements to a titanium base will generally favor one or the other of the alpha or beta phases and will respectively increase or decrease the beta-transus temperature.
Titanium-aluminum base alloys containing about 10 to 50 atomic percent Al and about 80 to 50 atomic percent Ti in addition to other alloying elements have been recognized for some time. These alloys are ordered and divided into two major groups: the alpha-2 alloys based on the intermetallic compound Ti.sub.3 Al, and the gamma alloys based on the intermetallic compound TiAl. There also exists a class of alloys containing a mixture of the alpha-2 and beta phases. These alloys, which are referred to as "titanium aluminides", exhibit excellent high-temperature strength and oxidation and creep resistance, and for these reasons have found widespread utility in aerospace applications.
However, although a multitude of processes for forming these metals have been attempted, very few have proven successful and consistent because titanium aluminide alloys are relatively brittle and difficult to process and/or fabricate at room or near-room temperatures. Indeed, a major competitor of applicant's assignee recently gave up its pursuit of a solution to this pervasive problem. This leading corporate entity apparently abandoned its research targeted at solving this long-felt problem when, after many years of fruitless effort, it consistently encountered cracking of the titanium aluminide materials during bonding and forming operations.
Some of the techniques which have been attempted for the fabrication of titanium aluminide materials include forging, extrusion, rolling, drawing, casting and powder metallurgy. Recently, superplastic forming (SPF), with or without concurrent diffusion bonding (DB), has achieved a certain prominence. This process has made it possible to form titanium aluminides in a simple manner, with significant reduction in parts such as fasteners, thereby permitting the fabrication of airframe and engine structures with significant cost and weight reduction.
For many years, it has been known that certain metals are "superplastic", i.e., have the capability of developing unusually high tensile elongations with reduced tendency toward necking. This property is exhibited by only a few metals and alloys and only within a limited temperature and strain rate range. Titanium, titanium alloys and, most recently, titanium aluminides, have been observed to exhibit superplastic characteristics equal to or greater than those of any other metals. With suitable titanium alloys, it is possible to attain an overall increase in surface area of over 300%, and recent tests have shown these high elongations to be present in titanium aluminides as well.
The advantages of superplastic forming are numerous. Very complex shapes and deep drawn parts can be readily formed. Low deformation stresses are required to form the metal at the superplastic temperature range, thereby permitting forming of parts under low pressures which minimize tool deformation and wear, allows the use of inexpensive tooling materials, and eliminates creep in the tool. Single male or female tools can be used; no spring-back occurs; no Bauschinger effect develops; multiple parts of different geometry can be made during a single operation; very small radii can be formed; and no problem with compression buckles or galling are encountered.
However, when carrying out the process of superplastic forming using titanium aluminides and similar reactive metals, it is necessary to heat and form the materials in a controlled environment to ensure cleanliness of the titanium which is particularly sensitive to oxygen, nitrogen, and water vapor content in the air at elevated temperatures. Unless the titanium aluminide is protected, it becomes embrittled and its integrity is destroyed.
Diffusion bonding refers to the metallurgical joining of surfaces of similar or dissimilar metals by applying heat and pressure for a time duration so as to cause co-mingling of atoms at the joint interface. Diffusion bonding is accomplished entirely in the solid-state at or above one-half the base metal melting point (absolute). Actual times, temperatures, and pressures will vary from metal to metal. The adjoining surfaces require preparatory cleaning, as for example by hand sanding and wiping or pickling. Surfaces must be brought within atomic distances by application of pressure. Adequate pressure must also be provided to cause some plastic flow to fill normal void areas. If pressures are too low, small voids will remain at the joint interface and the joint strength will be less than the maximum obtainable. The application of pressure also breaks up the surface oxides and surface asperites so as to present clean surfaces for bonding. The elevated temperatures used for diffusion bonding serve to accelerate diffusion of atoms at the joint interfaces as well as to provide a metal softening which aids in surface deformation thereby allowing more intimate contact for atom bonding and movement across the joint interface. The elevated temperature and application of pressure also results in diffusion of the surface contaminants into the base metal during bonding which allows metal atom-to-atom bonding and thereby strengthens the bond. Sufficient time must be allowed to ensure the strengthening of the bond by diffusion of atoms across the joint interface. A protective atmosphere for bonding is required when titanium, titanium alloys and titanium aluminides and other similar reactive metals are to be bonded.
The present invention also addresses problems associated with forming sandwich structures. A sandwich structure normally comprises a core between face sheets. Typically, fabrication of sandwich structures has taken the approach of first rolling metal foil or ribbon, then forming and joining the foil sheet into a desired cellular core configuration, and then attaching the core configuration to face sheets by brazing or spot welding. Problems encountered with the prior art methodology include the high cost of core fabrication due to excess material usage and the great difficulty of forming, excess time consumption, and cost of fabrication of the sandwich shape. Additionally, a separate operation is required to join a close out or attachment to the sandwich structure. Fabrication of an unusual shape for the sandwich structure, such as a taper, is nearly impossible. This is particularly true for the titanium aluminide material where foil is extremely difficult and expensive to produce, joining by welding or brazing has not been firmly established and the overall set-up cost and fabrication expenses have often proven prohibitive.