Assessment of constitutive equations in modelling the hot deformability ofsome overaged Al-Mg-Si alloys with varying solute contents

In this study five constitutive models with varying numbers of fitting para meters are assessed for modelling the stress-strain curves of four high-pur ity Al-Mg-Si alloys. Four of the five models are composed of the general ex ponential saturation equation as proposed by Sah et al. (J.P. Sah, G.J. Ric hardson, C.M. Sellars, J. Austr, Inst. Met. 14 (1969) 292), with a strain r ate dependence which is either power law-like, exponential, power law-like with a temperature dependent strain rate sensitivity, or hyperbolic sine-li ke. The fifth model consists of a single modified hyperbolic sine equation. The compositions of the experimental alloys facilitated the examination of the effect of solute Mg and Si content on the parameters of the constituti ve equations. The alloys were plane strain compression tested in an over-ag ed condition at temperatures and at strain rates relevant to hot rolling an d hot extrusion. A good fit of the experimental curves with just one parame ter set per alloy is obtained for the four models based on the exponential saturation equation, whereas the modified hyperbolic sine law yields a less satisfactory fit. On the basis of the quality of the fit, the complexity o f the model, and the fit parameter values obtained, it is concluded that th e Sah-saturation equation combined with the hyperbolic sine law is the most favourable of the five models. It is shown that the deviations between the apparent activation energy values obtained and the literature value of the activation energy for lattice diffusion in pure aluminium can be attribute d to the temperature dependence of the solubility of Mg and Si. (C) 1999 El sevier Science S.A. All rights reserved.