Mechanical properties and microstructure evolution of ultra-high strength Al-Zn-Mg-Cu alloy processed by room temperature ECAP with post aging

Ultra-high strength Al-Zn-Mg-Cu alloy extrusion bars were processed by equal channel angular pressing (ECAP) at room temperature (RT). Post aging treatment was used to determine the optimum time of aging. ECAPed sample was divided into two types: near the upper channel (P1) and lower channel (P2). Micro-hardness and tensile tests showed that ECAPed sample reached peak aging state at 8 h, which greatly shortened the time to T6 state. The mechanical properties of ECAPed samples were also better than traditional T6 samples. Scanning electron microscopy (SEM) and electron probe microanalysis (EPMA) were used to study the phase transformation from extrusion bars to ECAPed samples. AlZnMgCu phase and Al2CuMg phase containing Zn can be found in ECAPed sample. Meanwhile, the coarsening phase Al7Cu2Fe decreased significantly. Optical microscopy (OM) and transmission electron microscopy (TEM) were used to analyze the microstructure evolution such as shear bands and grain size during ECAP process. In P1, the shear bands with width ranging from 90 to 300 µm can be found. These shear bands were divided into many thinner shear bands and became intragranular deformation bands gradually in P2. Compared with P2, the average grain size in P1 decreased greatly and the precipitate η′ phase became finer and denser. Grain boundary strengthening and precipitation strengthening in P1 were much better than P2. The diameter of the channel should not exceed 20 mm when it used to press ultra-high strength Al-Zn-Mg-Cu alloy and the 3/4 parts near the top channel of product should be used to avoid unrefined grains.