The formability curves of the AZ31B magnesium alloy were constructed by following a novel approach that best resembles the conditions of actual Superplastic Forming (SPF) operations. Sheet samples were formed at 400 °C and a constant strain rate of 1×10−3 s−1, by free pneumatic bulging into a set of progressive elliptical die inserts. By doing so, the material in each of the formed domes was forced to undergo biaxial stretching at a distinct strain ratio, which is simply controlled by the geometry (aspect ratio) of the selected die insert. Material deformation was quantified using circle grid analysis (CGA), and the recorded planar strains were used to construct the forming limit diagram (FLD) of the material. The aforementioned was carried out with the sheet oriented either along or across the direction of major strains, in order to establish the relationship between the material’s rolling direction and the corresponding limiting strains. Great deviations between the two sets of formability curves are realised, hence a compound forming limit diagram is constructed as an improved way for characterising the material behaviour. The presented pneumatic stretching approach is shown to produce accurate friction-independent formability diagrams, with clear distinction between the safe and unsafe deformation zones, even though the developed diagrams are confined to the biaxial strain region (right side quadrant of an FLD). Moreover, the approach proves to be a viable means for providing formability maps under conditions where traditional mechanical stretching techniques (Nakajima and Marciniak tests) are limited.

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