Effect of heat treatment on microstructure and mechanical properties of extruded Al-Zn-Mg-Cu alloy

The microstructure evolution and mechanical properties of Al-12Zn-2. 4 Mg-1. 1Cu-0. 3 Zr-0. 15Ni-0. 12Mn( mass fraction,%)alloy under three different heat treatments were investigated by means of scanning microscope,hardness test,tensile test,and impact test.The results show that the microstructure of T6-treated alloy consists of α-Al matrix, η’ and η precipitation phases. The average microhardness and tensile strength of T6-treated alloy reach 210HV and 597 MPa respectively,which are superior to that of T4-treated and T5-treatd alloy. The elongation of T6-treated alloy is slightly lower than that of T4-treated alloy because η’ and η precipitation phases have a certain mobility relative to the matrix which reduces the plasticity of the alloy. The impact fracture mechanism of T4-treated and T5-treatd alloy is brittle fracture. The impact property of T6-treated alloy is obviously improved and the fracture mechanism is ductile-brittle mixed fracture. T6 heat treatment should be adopted to the extruded Al-Zn-Mg-Cu alloy.