Synergic Improvement of Plasticity and Strength of Al-Zn-Mg-Cu Alloy by Grain Refinement and Precipitates Redistribution using Cyclic Extrusion Compression

To explore the method to further improve the comprehensive performance of a high-strength Al-Zn-Mg-Cu alloy, 1-8 passes cyclic extrusion compression (CEC) experiments are carried out. By thermal-mechanical coupled numerical simulation and experiment investigation, the variation of macro field variables, size distribution, and morphological characteristics of grain structures, and precipitates are quantitatively investigated. The results show that the dynamic recrystallization occurs during CEC processing. As the number of CEC processing increases, the grain size and aspect ratio decreases monotonically, while the volume fraction of secondary phases first decreases, reaches its minimum (0.26%) at 6 passes and then increases, indicating their re-dissolution and further precipitation under severe stress field. In addition, the greatest comprehensive mechanical properties of Al-Zn-Mg-Cu alloy are achieved by 2 passes CEC with the hardness, tensile strength, yield strength, and elongation improved by 89.29%, 63.77%, 110%, 134.93%, respectively. By theoretical calculation and fractograph observations, it is found that dislocation strengthening and Hall-Petch strengthening both plays a key role in the overall strengthen, and the enhanced plasticity can be attributed to the variation of the fracture mode and the propagation path of the cracks.