Microstructure and properties during retrogression and re-aging treatment of a new type Al-Zn-Mg-Cu alloy profile

The effect of retrogression and re-aging(RRA) treatment on the microstructure and mechanical properties of Al-Zn-Mg-Cu alloy profile was studied by tensile,hardness,electrical conductivity test and transmission electron microscopy.The results show that suitable RRA treatment is pre-aging at 120 ℃ for 24 h,retrogression at 180 ℃ for 45 min,and then re-aging at 120 ℃ for 24 h.In this condition,the tensile strength,yield strength,elongation and electrical conductivity of the alloy profile is 613.5 MPa,599 MPa,11.1% and 39.2%IACS,respectively.Compared to T6 state,the yield strength and electrical conductivity are increased obviously with similar tensile strength and elongation.The stress corrosion cracking(SCC) resistance of alloy is improved significantly.After RRA treatment,the major precipitates in intracrystalline of the alloy are fine η′ and η phases.Discontinuous grain boundary precipitates and narrow precipitate-free zones(PFZ) along the grain boundary are observed clearly.