Effect of Pre-Equal Channel Angular Pressing Homogenization on Microstructure and Mechanical Properties of As-Cast 7050 Al Alloy

An effective homogenization scheme is crucial for successful room temperature equal channel angular pressing (RT-ECAP) deformation of the 7050 aluminum (Al) alloy. This study investigated the effects of different single-stage homogenization heat treatments (ST), double-stage homogenization heat treatments (DT) and subsequent RT-ECAP deformation on as-cast 7050 Al alloy. Multiscale observations of microstructure evolution were conducted using differential scanning calorimetry, optical microscopy, x-ray diffraction and scanning electron microscopy equipped with energy-dispersive spectroscopy. Mechanical properties and crack depth variations were analyzed using universal testing machine and depth testing platform. The results show that the microstructure of as-cast 7050 Al alloy exhibits severe dendritic segregation, with non-equilibrium phase areas fraction of approximately 4.1% and coarse grain boundaries around 5.14 μm. With increase in temperatures and times of ST, dendritic segregation gradually diminishes and the non-equilibrium second phase dissolves into the matrix. After DT of 7050 Al alloy at 465 °C-24 h + 475 °C-4 h, only 0.7% of insoluble residual phases remained and the grain boundaries became fine with a width of 1.08 μm, representing reductions of approximately 82.9 and 79% compared to the as-cast, respectively. Additionally, owing to significant improvements in the microstructure, no cracks were observed in the subsequent RT-ECAP of 7050 Al alloy. The ultimate tensile strength (UTS) and yield strength (YS) of 7050 Al alloy decreased from (472 ± 4) and (457 ± 15) MPa in as-cast state to (450 ± 17) and (335 ± 26) MPa after DT at 465 °C-24 h + 475 °C-4 h. The fracture elongation of 7050 Al alloy was increased from (5.3 ± 1.1) to (7.0 ± 0.3)% in as-cast state. Subsequently RT-ECAP, the UTS and YS of 7050Al alloy were increased to (554 ± 3) and (525 ± 26) MPa, respectively, and the fracture elongation was reduced to (6.5 ± 0.3)%. After the DT of 465 °C-24 h + 475 °C-4 h, significant improvements were achieved in the microstructure, simultaneously enhancing the plasticity and the RT-ECAP workability of 7050 Al alloy with limited strength reduction, indicating this scheme as the reasonable DT scheme.