Defeating strength-plasticity trade-off of Al–Zn–Mg–Cu alloy by spray forming and double step extrusion

A well strength-plasticity balance is necessary for industry application of high Zn 7xxx series alloys. Herein, we proposed a combination process of spray forming rapid solidification plus extrusion processes (i.e., single forward extrusion (FE alloy), double-steps: forward extrusion + backward extrusion (BE alloy)) to refine the grains and second phase particles, thereby achieving the well strength-plasticity balance of Al-10Zn-2.3Mg-1.2Cu-0.15Zr-0.05Fe (wt. %) alloy. The microstructure evolution of the spray formed ingot during extrusion and heat treatment processes has been systematically studied by various advanced characterization techniques. The results show the fine equiaxed grains with average size of 56±17μm, and the fine primary phase distributed uniformly in the Al matrix. Compared to the FE alloy, the BE alloy shows more uniform distribution of second phase particles after double-step extrusion, increasing the frequency of small particles (smaller than 0.1μm) from 45% to 70%. Double-step extrusion also increases the dislocation density after solid solution treatment, resulting in slight larger precipitate size of BE alloy. Strengthening contribution calculation indicated that the relative higher dislocation density of the BE alloy results the higher dislocation strengthening, while the slight larger precipitates result a lower precipitate strengthening about 52.5MPa than FE alloy. At last, the combination of spray forming and double-steps extrusion provide the well strength-plasticity balance with the yield strength (∼695.9±1.2 MPa), ultimate tensile strength (∼718.7±0.6 MPa) and superior elongation (∼13.77±1.2%) of BE alloy after T6 aging treatment.