Microstructure and Stress Corrosion Cracking of Spray Forming 7075 Alloy after Retrogression and Re-ageing Treatment

Abstract

A typical age-strengthening 7075 aluminum alloy with high tensile properties can be spray-formed through T6 treatment,but its stress corrosion cracking(SCC) property appears very poor after this treatment. On the other hand,the SCC property may be improved by T7 treatment but the tensile strength of the alloy then be seriously harmed. Therefore,the effects of four kinds of retrogression and re-ageing(RRA) treatments on the microstructure and mechanical properties of a spray forming 7075 alloy are investigated by using mechanical,electric and transmission electron microscope observation measures in this research. It is obtained that the tensile strength and electric conductivity of the alloy reaches up to 748 MPa and 22.8 MS/m respectively under the condition of retrogressed at 200 ℃ for 10 min,and re-aged at 120 ℃ for 24 h. During retrogression treatment,GP zone and η′ phase are dissolved in matrix,η′ phase is then re-precipitated at 200 ℃ retrogression treatment over 10 min,which leads to the slight increase of tensile strength of the alloy. Because of the precipitation of shaped-η′ phases during retrogression and the corresponding RRA treatment a lots of η′ phases re-precipitate and η phases at grain boundary may improve the SCC resistance of the alloy.