Effect of layered microstructure on superplastic forming property of AA8090 Al–Li alloy

Superplastic bulge forming of commercial grade AA8090 Al–Li alloy sheet was done. The sheet contained three layers of distinct microstructural features along thickness direction. In order to understand the effect of such microstructure on the forming parameters, the blanks of three different layers, viz. surface layer, middle layer and composite layer, comprising of different microstructures, were obtained from the as-received sheet. Three different forming pressures—low, intermediate and high, corresponding to initial strain rates of 4 × 10−4, 1 × 10−3 and 5 × 10−3 s−1, respectively, were employed for forming of each layer. The superplastic forming characteristics, including thickness and bulge profile, of surface layer were found to be superior to middle layer. It is interesting to find that the composite layer also exhibited better bulge profile and more uniform thickness distribution than middle layer. The difference in forming characteristics among different layers can be attributed to the presence of favorable equiaxed microstructure in the surface layer and unfavorable elongated grains in the middle layer.