Retrogression and re-aging treatment of Al-9.99 percent Zn-1.72 percent Cu-2.5 percent Mg-0.13 percent Zr aluminum alloy

摘要

The effect of retrogression and re-aging(RRA) heat treatment on the microstructure and mechanical properties of a low frequency electromagnetic casting alloy as Al-9.99 percent Zn-1.72 percent Cu-2.5 percent Mg-0.13 percent Zr was investigated by tensile properties test, Vickers hardness, electrical conductivity test, DSC analysis, SEM and TEM observation. The results show that RRA heat treatment can improve the stress corrosion cracking(SCC) properties with retention of the high strength of T6 level. After preaging at 100 deg C for 24 h, retrogression at 200 deg C for 7 min, and then re-aging at 100 deg C for 24 h, the alloy obtains tensile strength up to 795 MPa, yield strength up to 767 MPa, maintains 9.1 percent elongation, and electric conductivity of 35.6 percent IACS. TEM observation shows that the re-dissolution of GP zone and eta' phase in the early stage of regression leads to the decrease of hardness, then the increase in the volume fraction of eta' and eta phases leads to the increase again in the peak value, and finally the general coarsening of all particles results in a softening of the alloy. Meanwhile it is found that the conventional T6 heat treatment as the preaging and re-aging regime is not the optimum regime to the RRA treatment of the high-zinc content super-high strength aluminum alloy.