Spherical dome formation by transformation of superplasticity of titanium alloys and titanium matrix composites

Titanium alloys and titanium matrix composites are useful materials in aerospace applications due to their high strength and stiffness, good corrosion resistance and low density. The gas pressure bulging of metal sheets has become an important forming method. As the bulging process progresses, significant thinning in the sheet material becomes obvious. This paper presents a simple analytical procedure for obtaining the dome height with respect to the forming time useful to the process designer for the selection of initial blank thickness as well as non-uniform thinning in the dome after forming. By thermally cycling through their transformation temperature range, coarse-grained, polymorphic materials can be deformed superplastically, owing to the emergence of transformation mismatch plasticity (or transformation superplasticity) as a deformation mechanism. This mechanism was examined under biaxial stress conditions during thermal cycling of titanium alloys with and without discontinuous reinforcements. For the transformation superplasticity, the strain-rate sensitivity index is considered as unity. The radius of curvature, thickness and height of the dome with respect to the forming time are obtained. The analytical results were found to be reasonably in good agreement with the test results. (c) 2007 Elsevier B.V. All rights reserved.