A constitutive relation of AZ80 magnesium alloy during hot deformation based on Arrhenius and Johnson–Cook model

In order to understand the constitutive behavior of as-cast AZ80 with large grain size, the uniaxial hot compression tests were carried out over a series of isothermal upsetting experiments. The maximum deformation degree was 65%. The experimental temperatures were 523 K, 573 K, 623 K and 673 K and the strain rate was 0.001 s −1 , 0.01 s −1 , 0.1 s −1 , and 1 s −1 . The stress–strain curves can be divided into three stages which are work hardening stage, softening stage, and steady-state stage at low strain rate and high temperature, while the steady-state stage cannot be observed at low forming temperature and high strain rate because of incomplete dynamic recrystallization. The Arrhenius type relation predicts the peak stress with high accuracy but cannot satisfy the strain relevant requirement. The Johnson–Cook model shows an inappropriate ability to describe the constitutive behavior in this case. Therefore, a new mathematic model (a segmented model) with high prediction accuracy based on the modified Arrhenius type relation (including strain rate) and Johnson–Cook model is proposed. The modified Arrhenius type relation is used to reflect the constitutive behavior before the peak strain and the modified Johnson–Cook model is aimed at showing the stages after peak strain.