Microstructure inheriting evolution and strength-plasticity collaborative improvement mechanism of multidirectional rotary forged Al7075 sheets during T6 heat treatment

Abstract

Al7075 sheets are widely used in aerospace industry and their higher strength-plasticity collaborative improvement requirement is urgent. In this study, the microstructure inheriting the evolution and mechanical properties of Al7075 sheets during multidirectional rotary forging (MRF) and T6 heat treatment are analyzed. The results show that the average grain size exhibits near-parabolic evolution with increasing MRF deformation amount. MRF20 %+T6 (20 % MRF deformation amount+T6) condition possesses the largest grain size of 72.6 μm, and its abnormal grain growth mechanism is that the medium deformation energy and high deformation heterogeneity in MRF20 % deformed grains could cause asynchronous recrystallization behavior during T6 heat treatment, and the grains with comparatively higher deformation energy get recrystallized firstly and devour adjacent grains along preferred 〈011〉 or 〈223〉 misorientation axis. MRF70 %+T6 condition possesses the finest grain size of 14.2 μm, and its fine grain inheriting mechanism is that the uniformly high deformation energy in MRF70 % deformed grains causes uniformly rapid recrystallization, and rapidly recrystallized grains effectively suppress grain boundary motion from adjacent grains. With increasing MRF deformation amount, tensile strength and elongation values both exhibit near-antiparabolic evolution. MRF70 %+T6 condition possesses the largest tensile strength (563 MPa) and elongation (17.73 %), which increases by 8.27 % and 80.55 % compared to as-annealed+T6 (MRF0 %+T6) condition (tensile strength is 520 MPa and elongation is 9.82 %), respectively. The strength-plasticity collaborative improvement is mainly because the combination of effectively inherited fine grains, refined inclusion particles, and uniformly distributed fine η’ particles after T6 heat treatment could promote smooth dislocation movement and coordinated slip behavior in most matrix grains, which contributes to the delay of stress localization and strength-plasticity collaborative improvement.