Effects of different multidirectional forging processes on the microstructure and three-dimensional mechanical properties of ultra-high strength aluminum alloys

Abstract

The 7A85 aluminum alloy was forged in multiple directions at 470 °C via four-upsetting and three-cross-stretching (4U3CS), four-upsetting and three-octagon-stretching (4U3OS), and seven-upsetting and six-cross-stretching (7U6CS), three different forging processes. Thereafter, the alloy was treated with solid solution, water quenching, and T8 two-stage aging. The microstructural evolution and three-dimensional mechanical properties of the center and edge of the specimens were detected and analyzed. Compared with 4U3CS and 4U3OS, the increased cumulative deformation of 7U6CS caused more continuous temperature rises, more severe lattice distortion, and more defect formation, such as vacancies and dislocations, which were more conducive to coarse second-phase particle dissolution, finally enhancing the driving force for aging precipitation; thus, significantly improving the yield strength (YS). Furthermore, under 7U6CS, the larger stored deformation energy provided a driving force for sufficient dynamic recrystallization. As the number of alternate loading axis changes increased, more deformation bands were interwoven and caused the expansion of the recrystallization nucleation areas, which improved the inconsistent degree of recrystallization between the center and edge. Therefore, dynamic recrystallization was the most sufficient with grain refinement especially at the edge via 7U6CS. From 4U3CS to 7U6CS, the anisotropy and uniformity of the center and edge were gradually improved. The optimal mechanical properties were obtained via 7U6CS, with average three-directional ultimate tensile strength, YS, and elongation values of 513 MPa, 464 MPa, and 10.7% at the center and 515 MPa, 473 MPa, and 11.5% at the edge, respectively.