A numerical-experimental approach to material characterization and process analysis in the blow forming process

In carrying out a superplastic forming process, the first step for the process optimization is a detailed material characterization. The achievement of a successfully formed component is strictly related to the material behaviour: commonly, several tensile tests are carried out at different temperatures and strain rates for locating the best deformation parameters, corresponding to which maximum uniform elongation of the material can be found. Additionally, characterization is needed also for estimating material parameters to numerically model the process. Numerical modelling is a necessary step when the pressure profile of a blow forming process has to be optimized. In this paper authors present a methodology for estimating material parameters on the basis of few bulge tests and for analysing the superplastic forming process. Temperature and pressure influences are analysed by means of free inflation tests. Material parameters are calculated starting from an analytical approach and refining the solution by an inverse analysis. A 3D numerical model of the process has been then created and used for evaluating the pressure profile that is able to keep the strain rate value in the sheet close to a target value. A numerical-experimental comparison has been done for testing the model capability and effectiveness of the pressure profile in a prismatic closed die blow forming test. Good agreement has been found between experimental results and numerical simulations. Thickness distribution measurements, optical microscope observations, post-forming characteristics evaluation have been performed to compare constant pressure and constant strain rate tests.