Edge-crack free and high mechanical properties of AA7075 sheets by using cryorolling and subsequent aging

Abstract

In this study, AA7075 sheets were processed by cryorolling and room-temperature rolling respectively, and subsequent aging. The microstructure evolution and mechanical properties of AA7075 sheets were investigated by hardness measurements, tensile tests, X-ray diffraction, scanning electron microscopy and transmission electron microscopy. Results show that with the rolling reduction ratio of 80%, the room-temperature rolled samples had massive edge cracks, while the cryorolled samples were in good shape and without any cracks. The improved ductility of materials in cryogenic environments, the reduced precipitation and the absence of shear bands in sheets during cryorolling contributed to the reduced edge cracks. Compared to solution treated samples, the yield strength and ultimate tensile strength of cryorolled samples were increased by 155% and 44%, respectively, and the elongation decreased as a loss, which is obviously better than that of room-temperature rolled samples. After the subsequent aging treatment, the strength and ductility of the rolled samples were simultaneously improved, the yield strength of cryorolled + peak-aged sample was increased by 44 MPa and the elongation was increased by 68% compared with those of the cryorolled samples. Compared with room-temperature rolled + peak-aged samples, the strength of cryorolled + peak-aged sample was higher, and the elongation to failure and uniform elongation were increased by 58% and 72%, respectively. Numerous nanosized GP zones and η' phases were precipitated after aging, and the precipitation strengthening is the main contributor to the strength enhancement of the samples after aging. The improved ductility of cryorolled + peak-aged samples is mainly due to the combined effect of dislocation annihilation, uniform deformation and fine precipitates in them.