Constitutive modelling of hot deformation behaviour of metallic materials

The complexity of hot deformation behaviours of metallic materials is acknowledged in decades of study. The present work uses a new constitutive model by Zhu-Ou-Popov (ZOP model) and its modifications to predict the hot deformation behaviours of metals. The ZOP model and its modifications are introduced firstly. The basic idea of these models is centred on a set of piecewise and transition functions: the piecewise functions are used to predict flow stress at different strain ranges, whilst the transition functions enable a smooth shifting from small strain to large strain ranges. The methods for identification of four variant models are developed and given in detail. Hot compressive flow stress curves of 42CrMo at different strain rates and temperatures are used to show the validity of these models. Results show that, all of the developed models are able to predict the hot compressive behaviour of 42CrMo. Using the Arrhenius type equation and modified Zener-Holloman parameter to predict the flow stress from the yield point, the Modified model III gives the most favourable prediction accuracy with R-2 of 0.9569, whilst other models are also effective with R-2 higher than 0.91. The modifications of the ZOP model can be used to predict the yield stress and to reflect the peak stress. In addition, the applicability and advancement of the presented constitutive models are discussed. The models are considered to be effective in reflecting the occurrence and completion of dynamic recrystallization (DRX) in hot deformation of metallic materials. It is shown that the studied models are capable of predicting the peak strain and completion strain of DRX, with comparable results obtained to experimental data.