An optimized constitutive model and microstructure characterization of a homogenized Al-Cu-Li alloy during hot deformation

Abstract

The quenching expansion tests in the temperature range 350–500 ℃ and isothermal compression tests in the range 350–500 ℃ & 0.01–10 s−1 of a homogenized Al-Cu-Li alloy were conducted to investigate the flow behavior and thermal expansion behavior. The corresponding microstructures of the tested specimens were characterized by back-scattered electron-scanning electron microscope (BSE-SEM), electron back-scattered diffraction (EBSD) and transmission electron microscope (TEM). Coarse secondary phases including Al2CuLi (T1), Al2(Cu-Ag)(Li-Mg) (Ω), Al2Cu (θ), Al2CuMg (S) dissolve with increasing temperature, which is likely to increase the coefficient of thermal expansion. The temperature rising induced by deformation heat varies from 2.29 ℃ to 38.32 ℃ with increasing strain rate and decreasing temperature. Three different constitutive models were calculated, evaluated and optimized, based on which, the two-step optimized Johnson-Cook model showed the highest precision. At the temperature ≤ 400 ℃, the existences of undeformable coarse phases (especially T1 or Ω phase) promote particle stimulated nucleation (PSN), which pronouncedly increases the volume fraction of dynamic recrystallization (DRX) and decreases the flow stress. At temperature of 450 ℃, most of undeformable coarse phases dissolved, cutting-off mechanism of S and θ phase was observed, which is no longer beneficial to PSN, and CDRX becomes the main DRX mechanism.