Evaluation of rate-dependent forming limit for AA7075 sheets under pneumatic stretching method at elevated temperatures

The forming limit of aluminum alloy (AA) 7075 sheets depending on the deformation mode, the strain-rate, and the temperature was evaluated under the pneumatic stretching technique at elevated temperatures. In order to perform the pneumatic stretching test and to prevent the unfavorable fracture on the edge with the targeted strain-rate, the previously introduced die shape designs were utilized with the pressure-time profile. The two different kinds of strain-rate and temperature-sensitive forming limits were determined, where one is the necking-based forming limit (NFL) and the other is the fracture-based forming limit (FFL). The results show that FFL is highly deviated from NFL at 400 °C than those at 470 °C due to abrupt strain localization, which leads to high dome height at the pneumatic forming during the relatively lower elevated temperature. The microscopical analysis (optical and scanning electron) reveals that higher temperature condition, 470 °C, has early fracture due to the high diffusion of micro-voids and faster coalescence of the voids. The well-known three parameters fracture criteria e.g. Johnson-Cook (J-C), Modified Mohr-Coulomb (MMC3), Lou, and Hu fracture model were multiplicatively extended to strain-rate relationships based on the suggested general equation and independently characterized for each elevated temperature with the error analysis.