Accurate determination of uniaxial flow behaviour of superplastic materials

Abstract

The design of superplastic forming technologies requires accurate description of material flow behaviour. Furthermore, as the flow curves reflect the deformation mechanisms and microstructure evolution of a material, their accurate determination is an important aspect of material science. The standard experimental method for determining superplastic flow curves is the tensile test, which encounters a significant challenge known as a gripping problem. In superplastic forming conditions, utilizing an extensometer proves difficult, leading to strain determination solely based on crosshead positions. This oversight neglects the non-uniform deformation of a specimen and the material flow occurring in the gripping region. This study presents a novel technique aimed at addressing this issue during the analysis of tensile test data, thereby establishing a reliable material model. The proposed technique was applied to construct the flow behaviour model of an aluminium alloy of the Al–Mg–Fe–Ni system at 460 °C based on the results of tensile tests in the strain rate range of $0.002-0.06s^{-1}$. The material model was developed using the hyperbolic sine equation with strain-dependent parameters, employing sequential polynomial approximation to reduce the number of utilized coefficients. This model was then used in simulations of tensile tests with various geometries to validate its accuracy.