Deformation microstructure and thermomechanical processing maps of homogenized AA2070 aluminum alloy

Abstract

The hot deformation behavior of homogenized AA2070 Al–Li–Cu alloy was studied using isothermal compression in a temperature range of 300 °C–500 °C and a strain rate range of 0.001 s-1 to 1 s-1. Deformation microstructure was characterized using electron backscatter diffraction (EBSD), transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) including weak beam dark field STEM imaging to correlate the microstructure evolution with hot deformation mechanism. TEM/STEM results show that a high density of fine T1 (Al2LiCu) phase forms in the microstructure through dynamic precipitation at compression temperatures lower than 400 °C, especially at low strain rates. Constitutive analysis shows that back stress due to dynamic precipitation of T1 phase decreases with increasing compression temperature and increasing strain rate. EBSD characterization indicates that dynamic recovery is the main softening mechanism at temperatures below 400 °C. At 400 °C and above, intermetallic phases including T1, Al20Cu2Mn3 and Al3Cu2 in the microstructure gradually dissolve, and dynamic recrystallization becomes the dominant softening mechanism, resulting in texture weakening and grain refinement. Three dynamic recrystallization mechanisms including discontinuous dynamic recrystallization (DDRX) at large angle grain boundaries, particle stimulated nucleation (PSN) and continuous dynamic recrystallization (CDRX) were observed. Based on hot work efficiency, deformation instability and microstructure evaluation, processing maps were constructed using both conventional method and Garofalo sinh equation, providing optimum processing conditions (temperature >400 °C and strain rate <0.1 s-1) for industrial forging, extrusion and rolling processes.