Characterization of the superplastic behavior of a Ti6Al4V-ELI alloy bilayer sheet

The aim of the present work is to investigate the superplastic behaviour of a bilayer Ti6Al4V-ELI to numerically design a full custom prosthesis manufacturing process by means of SuperPlastic Forming (SPF); the bilayer was composed by: (i) a monolithic Titanium blank (cut by a rolled sheet); (b) a porous blank cut by a billet produced by Hot Isostatic Pressing (HIP) and subjected to a Solid State Foaming (SSF) heat treatment. Experimental bulge tests aimed to evaluate the effects of both the initial porosity level of the HIPed layer and the temperature (850 and 900 °C) of the SPF process. The bilayer sample was deformed by a constant argon gas pressure and the time evolution of the dome height was recorded during the test in order to be used as target in the IA to determine the constants of the rheological model of the porous layer, being the behavior of the monolithic layer already known. A 2D FE model of the free inflation test was integrated in an automatic optimization procedure with the aim to minimize the error between the calculated and the acquired dome height vs time curve. Finally, the obtained material constants determined were used to design the SPF process by means of the numerical simulation, identifying as a case study a zygomatic full custom prosthesis. The initial porosity level resulted to poorly affect the superplastic behavior of the bilayer at 850 °C; on the contrary, when increasing the temperature up to 900 °C, an evident reduction of the forming time was obtained. Numerical simulations showed that at 900 °C it is possible to obtain the very complex geometry of the adopted case study in about 50000 seconds.