Fundamental characterisation of hemisphere free bulging using superplastic 8090 Al–Li sheets

Abstract

Hemisphere free bulging of a superplastic 8090 Al–Li sheet was carried out, with particular emphasis given to the superplastic behaviour over the low strain regime ɛ=0−0·7. Various pressurising cycles, including constant pressure, constant strain rate, and multiple strain rate bulging, were performed to characterise the superplastic behaviour in terms of strain rate variation, thickness distribution, and evaluation of the strain rate sensitivity (m) of the sheet during biaxial bulging. For constant strain rate bulging, a modified Ghosh and Hamilton (GH) model and a model developed by the present authors (HL) have been used to simulate the necessary pressure profiles. Two different constitutive equations, extracted from uniaxial tensile tests, were used in the study. The modified GH model applied σ=Kεm as the constitutive equation, i.e. considering only the m value in the simulation, while the H L model incorporated the strain hardening exponent (n) additionally into the constitutive equation for the simulation: i.e. the equation σ=εɛnεm was used. The HL model can also be applied to simulate the multiple stage strain rate forming route. Based on the analyses, the material superplasticity characteristics obtained from uniaxial tensile tests were seen to be reliably applicable to equibiaxial sheet bulging. The equibiaxial straining condition was not obeyed exactly except at the pole. With the consideration of initial work hardening contribution towards the constitutive equation, the simulated results seemed to agree better with the experimental data. With further straining, the rate sensitivity increased and work hardening decreased, thereby a multiple strain rate bulging path, including a high initial rate followed by several lower strain rates, would be able to fully utilise the higher values of m and n during each straining stage. The thickness uniformity as well as the total forming time could thus be improved and shortened.MST/3107