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Superplastic forming (SPF) is a near net-shape forming process which offers many advantages over conventional forming operations including low forming pressure due to low flow stress, low die cost, greater design flexibility, and the ability to shape hard metals and form complex shapes. However, low production rate due to slow forming process and limited predictive capabilities due to lack of accurate constitutive models for superplastic deformation, are the main obstacles to the widespread use of SPF. Recent advancements in finite element tools have helped in the analysis of complex superplastic forming operations. These tools can be utilized successfully in order to develop optimized superplastic forming techniques. In this work, an optimum variable strain rate scheme developed using a combined micromacro stability criterion is integrated with ABAQUS for the optimization of superplastic forming process. Finite element simulations of superplastic forming of Ti-6Al-4V sheet into a hemisphere and a box are carried out using two different forming approaches. The first approach is based on a constant strain rate scheme. The second one is based on the optimum variable strain rate scheme. It is shown that the forming time can be significantly reduced without compromising the uniformity of thickness distribution when using the proposed optimum approach. Further analysis is carried out to study the effects of strain rate, microstructural evolution and friction on the formed product. Finally the constitutive equations and stability criterion mentioned above are used to analyze the forming of dental implant superstructure, a modern industrial application of superplastic forming.

Approved for public release, distribution is unlimited / This study evaluated dome test samples of a superplastic AA5083 aluminum alloy deformed at nominally constant strain rates under biaxial strain conditions. Dome test samples resulted from gas-pressure forming of sheet material; for this study, samples were deformed at strain rates corresponding either to grain boundary sliding or dislocation creep control of deformation. Orientation Imaging Microscopy was utilized to determine texture development, grain size and grain-to-grain misorientation angle distributions for locations located along a line of latitude of the dome samples. The goal was to identify the location of the transition from grain boundary sliding to dislocation creep. Grain boundary sliding, which dominates at lower strain rates, can be recognized by a randomized texture and a higher concentration of high disorientation angles. Dislocation creep, which dominates at higher strain rates, is characterized by fiber texture formation and development of a peak at lower angles in the grain-to-grain misorientation angle distribution. / Ensign, United States Navy