Forming Limit Curves for Ti6Al4V Via Pneumatic Stretching

Forming limit curves (FLCs) and diagrams (FLDs) are essential tools for any sheet metal forming operation, especially in the automotive and aerospace industries. In addition to accurately describing the limits of material formability under various biaxial loading ratios, they are important for the finite element (FE) simulations of sheet-component forming operations. While the construction of FLCs is well-established for steels at ambient temperatures, their construction is challenging for lightweight alloys since the latter are typically formed at warm/high temperatures. The challenge is particularly great for titanium alloys, since forming temperatures are extremely high, exceeding the typical temperature limits for commercial mechanical stretching test setups. In this work, we shed some light on the feasibility of constructing hot formability curves for the Ti6Al4V alloy using the recently-developed “pneumatic stretching test”. The test best resembles the conditions encountered in actual superplastic forming (SPF) operations, and can be applied at very high temperatures without any frictional effects. Sheet samples are deformed at selected conditions (temperature and strain rate) by free pneumatic bulging into a set of progressive elliptical die inserts. The material in each of the formed domes is thus 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 is quantified using circle grid analysis (CGA), and the recorded planar strains are used to construct accurate friction-independent forming limit curves for the material.