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Superplastic forming of titanium assemblies

Superplastic forming of titanium assemblies description/claims

1) Field of the Invention

The present invention relates to the forming and bonding of structural members and, more particularly, relates to the use of different grain titanium for superplastic forming and/or diffusion bonding.

2) Description of Related Art

Superplastic forming (SPF) generally refers to a process in which a material is superplastically deformed beyond its normal limits of plastic deformation. Superplastic forming can be performed with certain materials that exhibit superplastic properties within limited ranges of temperature and strain rate. For example, workpieces formed of titanium alloys are typically superplastically formed in a temperature range between about 1450° F. and 1850° F. at a strain rate up to about 3×10−4 per second.

Diffusion bonding (DB) generally refers to a process of joining members using heat and pressure to form a solid-state coalescence between the materials of the joined members. Joining by diffusion bonding occurs at a temperature below the melting point of the materials that are being joined, and the coalescence therebetween is produced with loads below those that would cause macroscopic deformation of the article.

According to one conventional process, superplastic forming is performed by providing one or more superplastically formable metal sheets in a die cavity defined between cooperable dies, heating the sheets to an elevated temperature at which the sheets exhibit superplasticity, and then using a gas to apply differential pressures to the opposite sides of the sheets in order to form the sheets. The pressure is selected to strain the material at a strain rate that is within its superplasticity range at the elevated temperature, stretch the sheet, and cause it to assume the shape of the die surface. In this way, the sheet can be formed to a complex shape defined by the dies.

Further, in some cases, superplastic forming and diffusion bonding are performed in a combined forming/bonding operation. For example, in one typical combined SPF/DB process, three metal sheets are stacked to form a pack. A stop-off material is selectively provided between the sheets to prevent portions of the adjacent surfaces of the sheets from being bonded. The pack is heated and compressed in a die cavity with sufficient gas pressure so that the adjacent portions of the sheets that are not treated with the stop-off material are joined by diffusion bonding. Thereafter, a pressurized gas is injected between the sheets to inflate the pack, and thereby superplastically form the pack to a configuration defined by the surface of the die cavity. This process is described further in U.S. Pat. No. 3,927,817 to Hamilton, et al. Such a combined SPF/DB process can be used, e.g., to produce complex honeycomb sandwich structures that are formed and diffusion bonded to define hollow internal cells. Generally, the simplicity of the superplastic forming and/or diffusion bonding processes can result in lighter and less expensive structures with fewer fasteners and higher potential geometric complexity. Applications of SPF and/or DB include the manufacturing of parts for aircraft, other aerospace structures, non-aerospace vehicles and structures, and the like.

The individual sheets of a pack for forming according to the foregoing conventional process are typically provided as a flat sheets in a stacked relationship. FIG. 1 illustrates a portion of a three-sheet pack after being diffusion bonded and superplastically formed according to the conventional process. As shown, the space S between the outer sheets (or “face sheets”) F1, F2 has been expanded by the gas and the middle sheet (or “inner sheet” or “core sheet”) C has been superplastically formed to a corrugated or otherwise non-linear shape so that the middle sheet C extends in alternating directions between the outer sheets F1, F2 and defines the cells of the pack. As the outer sheets are expanded outward, the middle sheet tends to exert a reactive force on the outer sheets, thereby causing the outer sheets to be deformed. The effect of this reactive force is shown in FIG. 1 as deformation of the outer sheet where the middle sheet is connected thereto. In particular, instead of the outer sheet defining a flat surface, the outer sheet has been deformed to form a depression M, typically referred to as “markoff,” on its surface opposite the connection to the middle sheet.

Such markoff of the outer sheets of a pack during superplastic forming can be reduced by providing a middle sheet that is significantly thinner than the outer sheets, thereby increasing the relative stiffness of the outer sheets and reducing the inward force on the outer sheets during forming. The ratio of the thickness of the middle sheet to the thickness of each outer sheet is typically no more than about 25%. Therefore, if the design requirements for a particular application require a thicker middle sheet, superplastic forming is not typically used. The production of two-sheet assemblies and assemblies having other numbers of sheets can similarly be limited by a desire to avoid markoff.

While the conventional methods for SPF/DB processing have proven effective for manufacturing a variety of structural assemblies, including assemblies formed of titanium, there exists a continued need for improved SPF/DB methods and assemblies. In particular, the method should allow the production of assemblies with a greater range of desired dimensions, including assemblies with sheets of particular dimensions.

Embodiments of the present invention provide a method of superplastically forming titanium sheets and an assembly that is formed by such a method. Titanium sheets having different granular structures are used in the method so that the different sheets are adapted to superplastically form at different temperatures. In some cases, the sheets can be formed without markoff (or without substantial markoff) occurring, even though one or more of the sheets of substantial thickness is subjected to significant forming.

According to one embodiment of the present invention, a structural assembly having a predetermined configuration is produced by superplastically forming a pack having first and second titanium sheets in a stacked configuration. The first sheet has a grain size that is at least about twice a grain size of the second sheet. For example, the first sheet can define a grain size that is greater than about 5 micron and in some cases greater than 8 micron, and the second sheet can define a grain size less than about 2 micron such as between about 0.8 and 1.2 micron. The pack is heated to at least a superplastic forming temperature of the second sheet, and the second sheet is superplastically formed to a predetermined configuration to thereby form the assembly. The second sheet can be superplastically formed at a temperature that is less than the superplastic forming temperature of the first sheet, e.g., at a temperature between about 1400° F. and 1450° F. In some cases, the second sheet is formed without superplastically forming the first sheet, e.g., with the first sheet being only nonsuperplastically formed. The second sheet can be at least 75% as thick as the first sheet. The sheets can be diffusion bonded, and the second sheet can be formed in a direction away from the first sheet. In some cases, the second sheet can also be bonded to a third sheet having a grain size that is less than the grain size of the first sheet so that the third sheet can also be superplastically formed.

According to one aspect of the invention, the first and second sheets form a first structural sub-assembly that is joined to other sub-assemblies, e.g., to form an engine exhaust heat shield. For example, a second sub-assembly can be formed by repeating the providing, heating, and superplastically forming operations used to form the first sub-assembly. The first and second sub-assemblies can be joined to opposite transverse edges of a third sub-assembly, which can also be superplastically formed. The third sub-assembly can define transversely extending channels and each of the first and second sub-assemblies can define transversely extending cells that are offset from the channels of third sub-assembly.

Embodiments of the present invention also provide a superplastically formed structural assembly. The assembly includes a first titanium sheet and a second titanium sheet that is joined to the first sheet in a stacked configuration, e.g., with the sheets joined by diffusion bonds. The second sheet is superplastically formed to a contoured configuration so that the first and second sheets define cells therebetween. Further, the first sheet has a grain size that is at least about twice a grain size of the second sheet so that the first sheet has a superplastic forming temperature that is higher than the superplastic forming temperature of the second sheet. For example, the first sheet can define a grain size of greater than about 5 micron or 8 micron and the second sheet can define a grain size less than about 2 micron such as between about 0.8 and 1.2 micron. The second sheet can be adapted to be superplastically formed at a temperature of between about 1400° F. and 1450° F. The second sheet can have a thickness that is substantial relative to the first sheet, e.g., about 75% of the thickness of the first sheet. Further, the first sheet can define a surface opposite the second sheet, and the surface can have a substantially planar configuration opposite a plurality of joints that connect the first and second sheets, i.e., without markoff or without substantial markoff of the first sheet. In some cases, the assembly can also include a third sheet that is bonded to the second sheet, the third sheet having a grain size less than the grain size of the first sheet.

The first and second sheets can define a first structural sub-assembly of an engine exhaust shield, which can also include second and third sub-assemblies. Similar to the first sub-assembly, the second sub-assembly can include first and second titanium sheets that are joined in a stacked configuration, with the second sheet superplastically formed to a contoured configuration to define cells, and with the first sheet of the second sub-assembly having a grain size that is at least about twice a grain size of the second sheet of the second sub-assembly and a correspondingly higher superplastic forming temperature. The first and second sub-assemblies can be joined to opposite transverse edges of the third sub-assembly, and each of the first and second sub-assemblies can define transversely extending cells that are longitudinally offset from transversely extending channels defined by the third sub-assembly.

Thus, the present invention provides an improved assembly and method for superplastic forming and/or diffusion bonding, in which titanium sheets having different granular structures can be used to produce the assembly. The superplastic forming can be performed at particular temperatures, such as temperatures that are below the superplastic forming temperatures of some or all of the sheets, and the formation of markoff can potentially be reduced or eliminated.

Having thus described the invention in general terms, reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein:

FIG. 1 is a perspective view illustrating a portion of a structural assembly manufactured by diffusion bonding and superplastically forming a three-sheet pack according to a conventional process;

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