Nihat Akkus

A finite-element model for the superplastic bulging deformation of Ti-alloy pipe

Abstract The superplastic bulging deformation process of Ti-alloy pipe under constant strain rate at the apex has been analyzed using a finite-element model (FEM). For the simulation of the process and the prediction of the final thickness distribution of the deformed part, MARC-MENTAT was used as a computational tool. An incremental approach based on a rigid-plastic flow formulation was employed. Special sub-routines were developed and linked to the main program for the superplastic material properties and the results of pressure calculations of constant apex-strain-rate deformation for each step. By FE modelling it is confirmed that the constant apex-strain-rate deformation process is superior to the constant pressure deformation process with respect to wall-thickness bulging. The results of calculation are compared to experimental results.

Superplasticity dome forming of machined sheets

An experimental work on the superplastic bulge forming of machined sheets is presented in this study. Unlike the previously employed incremental-iterative method, a reverse deformation model was used to estimate the initial thickness distribution of the machined sheets from which a constant final thickness can be obtained when the shape of the bulged sheet is hemisphere. The reverse deformation model was obtained by modifying previously-known models, which were based on the axisymmetric membrane and the incremental strain theory. Bulge forming experiments were conducted on machined sheets of Al alloy, A5083, at about 530 C under constant pressure control mode. The result of this simulation to estimate the final constant thickness distribution agreed well with the experiment, and confirmed that the reverse deformation model can be successfully applied in optimizing the thickness distribution of the starting sheets in order to obtain the desired final thickness distribution of the free bulged hemispherical product.

Influence of Preforming on the Final Thickness Distribution of the Superplastically Deformed Domes

This study reports the influence of preforming on the final thickness distribution of superplastically formed domes. It is very common to see about 50% thickness difference between apex and periphery of domes formed from initially uniform blanks. If the initial shape of the blank is modified properly by preforming, the stress-strain distribution of the test piece during bulge forming can be changed in order to reduce the non-uniformity of the final thickness distribution of dome. Ti-alloy sheets of thickness 0.83 mm of SP-700 containing 4.5%Al, 3%V, 2%Fe, 2%Mo were used as test material. The blank sheets were initially preformed to a torus type shape and then subjected to final hemispherical bulge forming. Both preforming and final bulge formings were conducted at 800 °C. Influences of shape parameters of preforming dies were examined and confirmed that optimized preforming would be effective to reduce the non-uniformity of the final thickness distribution of formed dome.