The advantages of superplastic forming include the ability to make part shapes not possible with conventional forming, reduced forming stresses, improved formability with essentially no springback and reduced machining costs. The mechanism of superplasticity was covered in Chapter 2 on Aluminum. In general, titanium alloys exhibit much higher superplastic elongations than aluminum alloys, and there are a much wider variety of titanium alloys that exhibit super-plasticity. In addition, for titanium, SPF can be combined with diffusion bonding
(DB) to make large one piece unitized structures. Since superplasticity depends on microstructure, the fine grain equiaxed two-phase alpha-beta alloys exhibit inherent grain stability and are therefore resistant to grain growth at elevated temperature. Although the optimum structure depends on the specific alloy, the optimum volume fraction of beta is about 20% in Ti-6-4. While internal void formation by cavitation is a concern with aluminum and some other alloys, it has not been a problem in titanium alloys; therefore, the additional complication of back pressure to suppress cavitation is not required. Superplastic titanium alloys include Ti-6-4, Ti-6242S, Ti-6-22-22S, Ti-3-2.5, Ti-8-1-1, Ti-1100, Timetal 550, and SP700.
In the single-sheet SPF process, illustrated in Fig. 4.18, a single sheet of metal is sealed around its periphery between an upper and lower die. The lower die is either machined to the desired part shape or a die inset is placed in the lower die box. The dies and sheet are heated to the SPF temperature, and argon gas pressure is used to slowly form the sheet down over the tool. The lower cavity is maintained under vacuum to prevent atmospheric contamination. After the sheet is heated to its superplastic temperature range, argon gas is injected through inlets in the upper die. This pressurizes the cavity above the metal sheet forcing it to superplastically form to the shape of the lower die. Gas pressurization is slowly applied so that the strains in the sheet are maintained in the superplastic
Fig. 4.18. Single Sheet SPF
range. Typical forming cycles for Ti-6-4 are 100-200psi at 1650-1750°F for 30min to 4h.
During the forming operation, the metal sheet is being reduced uniformly in thickness; however, where the sheet makes contact with the die it sticks and no longer thins-out. Therefore, if the sheet is formed down over a male die, it will touch the top of the die first and this area will be the thickest. The thickness tapers down along the sides of the die to its thinnest point in the bottom corners which are formed last. For the same reason, when a sheet is formed into a female cavity, the first areas that make contact are the center of the bottom and the top of the sides. These areas are the thickest. The thickness tapers down the sides to the thinnest point in the bottom corners which again form last. To reduce these variations in thickness, overlay forming can be used.
In overlay forming, the sheet that will become the final part is cut smaller than the tool periphery. A sacrificial overlay sheet is then placed on top of it and clamped to the tool periphery. As gas is injected into the upper die cavity, the overlay sheet forms down over the lower die, forming the part blank simultaneously. While overlay forming does help to minimize thickness variations, it requires a sacrificial sheet for each run that is discarded. Dies for titanium SPF are high temperature steels such as ESCO 49C (Fe-22Cr-4Ni-9Mn-5Co) lubricated with either boron nitride or yttria.
For titanium alloys, superplastic forming (SPF) can be combined with diffusion bonding (DB), a processes known as superplastic forming/diffusion bonding (SPF/DB) to form one piece unitized structure. Titanium is very amenable to DB because the thin protective oxide layer (TiO2) dissolves into the titanium above 1150° F leaving a clean surface. Several processes have been developed including two-, three-, and four-sheet processes.
In the two-sheet process, shown in Fig. 4.19, two sheets are welded around the periphery to form a closed envelope. The sheets can be welded by either resistance seam welding or laser welding; what is important is that the weld joints are vacuum tight and capable of resisting up to 200 psi gas pressure. The welded pack is placed in the die and heated to the forming temperature. Argon gas is used to form the lower sheet into the corrugated die cavity. Once the lower sheet is formed, the bladder below the lower die is inflated to push the lower skin up against the upper skin and the two are diffusion bonded together. To prevent the two sheets from sticking together prior to forming of the lower sheet, stop-off agents such as boron nitride or yttria suspended in an acrylic binder can be used, or a slight positive pressure can be used initially to keep the two sheets separate. The inner moldline of a two-sheet door is shown in Fig. 4.20, illustrating the stiffeners that were formed in the lower die during the process. This SPF/DB door replaced a built-up structure that consisted of two skins, seven formers, six intercostals, ten miscellaneous pieces, and several hundred fasteners.
The three-sheet process is shown in Fig. 4.21. The three sheets are welded around the periphery; however, in this process selected areas of the center sheet
Metal Bag Pressurized
Fig. 4.19. Two-Sheet SPF/DB
Silk Screen Stop-Off Pattern
Silk Screen Stop-Off Pattern
Fig. 4.21. Three-Sheet SPF/DB Process
Fig. 4.21. Three-Sheet SPF/DB Process are masked with a stop-off material to prevent bonding. The pack is then placed between the dies and the sheets are diffusion bonded together except at the locations that have been masked. After DB, gas pressure is applied to each side of the center sheet to expand the substructure. The areas of the center sheet that were masked stretch between the top and bottom sheets to form the stiffening ribs.
The four-sheet process,19 shown in Fig. 4.22, utilizes two sheets that form the skin (skin pack) and two sheets that form the substructure (core pack). The two core pack skins are first selectively welded together in a pattern that will form the substructure. The two skins for the skin pack are then welded to the periphery of the core pack. Although either resistance seam welding or laser welding can be used, one advantage of laser welding is that an automated laser welder can be programmed to make weld patterns and resulting core geometries other than rectangular, possibly resulting in more structurally efficient substructure designs. The total pack is then placed in the die, heated to the SPF temperature, the face sheets are expanded against the tool to form the skins, and then the core pack is expanded to form the substructure. The process is not limited to four sheets; experimental structures with five and more sheets have been designed and fabricated.20 A distinct advantage of both the three- and four-sheet processes is that the tooling is much simpler; instead of having cavities for substructure as
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