A characterization for the dynamic recrystallization kinetics of as-extruded 7075 aluminum alloy based on true stress–strain curves

Abstract

In a hot metal forming process, the stress–strain curves indicate the state of microstructures at the given deformation conditions, by which the prediction of DRX evolution can be performed. In order to improve the understanding of the coupling effect in dynamic recrystallization (DRX) behavior and flow behavior of as-extruded 7075 aluminum alloy, a series of isothermal upsetting experiments with height reduction of 60% were performed at the temperatures of 573K, 623K, 673K and 723K, and the strain rates of 0.01s−1, 0.1s−1, 1s−1 and 10s−1 on Gleeble 1500 thermo-mechanical simulator. By the regression analysis for Arrhenius type equation of flow behavior, the apparent activation energy of deformation was determined as Q=392.9468kJmol−1, and a dimensionless parameter controlling the stored energy was determined as Z/A=ε˙exp[(392.9468×103)/8.31T]/1.3713×1030. Based on the conventional strain hardening rate curves (dσ/dε versus σ), the characteristic points including the critical strain for DRX initiation (εc), the strain for peak stress (εp), and the strain for maximum softening rate (ε∗) were identified to express the evolution of DRX. In order to characterize the evolution of DRX volume fraction by Avrami type equation, two important parameters εc and ε∗ were described as the functions |εc|=0.058556(Z/A)0.00645 and |ɛ∗|=0.264427(Z/A)0.00702 respectively. From the Avrami type equation achieved, the evolution of DRX volume were described as following: for a fixed strain rate, the strain required for the same amount of DRX volume fraction increases with decreasing deformation temperature, in contrast, for a fixed temperature, it increases with increasing strain rate. These conclusions were verified by the microstructure observations.