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A study on superplastic forming of multiple sheet with integral stiffeners

  It is well known that blow molding process is one of the common manufacturing processes in plastic and glass industry. With the invention of the coextrusion blow molding, plastic fuel tanks are now environmentally preferable to the steel tank system in the automotive industry. A similar version of this technology in manufacturing of aircraft component is known as the superplastic forming process. Superplasticity is the ability of polycrystalline material to extend plastically to large strains when deformed in certain conditions. At appropriate superplastic conditions some fine-grained ceramics and metals have exhibited tensile elongation from several hundred to several thousand percent, while most crystalline solids can elongate by, at most, three or four percent without breaking. Even though superplasticity can be obtained in some materials by phase transformation cycling, most commercially important superplastic materials exhibit “micrograin superplasticity". This kind of superplasticity involves microstructural stability of materials during deformation. A stable, fine and equiaxed grain size is essential for micrograin superplasticity. The microstructural requirements for micrograin superplasticity are 1. a fine and equiaxed grain size, usually less than 10 microns, 2. multiple phases or quasi single phase with fine precipitates to suppress grain growth and 3. high angle grain boundaries that are mobile. In order to maintain this stable microstructure during elevated temperature deformation, multiphase alloys like eutectic or eutectoid alloys with equal portions of both phases and quasisingle phase alloy with fine precipitates to inhibit grain growth during static or dynamic recrystallization have been known as adequate superplastic alloys. The strength of the secondary phase should be similar to or lower than that of the matrix, since cavities may nucleate preferentially at the interphase boundari[...]

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