Superplastic Forming of Multipass Friction Stir Processed Aluminum-Magnesium Alloy

Multipass friction stir processing (FSP) of AA5086 Al-Mg alloy was carried out to obtain bulk fine grain material for superplastic forming. FSP produced inhomogeneous microstructure in the thickness direction. The aim of the present work was to understand superplastic forming behavior of distinct microstructural layers, i.e., nugget layer (NL) containing microstructure from nugget zone, thermo-mechanically affected/heat-affected layer (TL) containing microstructure from thermo-mechanically affected/heat-affected (TMAZ/HAZ) zone, and composite layer (CL) containing microstructure from both the above zones (nugget and TMAZ/HAZ). Superplastic forming of NL, TL, and CL blanks was carried out at constant gas pressure. Three different forming gas pressures of 0.75, 1.15, and 1.5 MPa corresponding to strain rates of 5 × 10−4 s−1, 1 × 10−3 s−1 , and 5 × 10−3 s−1, respectively, were used. Forming characteristics of CL were found to be comparable to that of NL and even better at higher forming pressures. Concomitant microstructural evolution during bulging of CL and NL plays an important role here.