Chapter G.7 - 7XXX series with copper

Abstract

The properties of Al–Zn–Mg–Cu alloys depend largely on the sum of the concentrations of the main alloying elements Zn+Mg+Cu: alloys with more than 9 wt.% total alloying elements exhibit the highest strength but at the expense of corrosion resistance; between 6 and 8 wt.% it is possible to find a compromise between mechanical properties and corrosion resistance; and between 5 and 6 wt.%, corrosion problems are reduced. These alloys are sensitive to several forms of corrosion: pitting corrosion, intergranular corrosion, exfoliation corrosion and stress corrosion cracking, especially in the short traverse ST direction. They are therefore unusable in aggressive environments without adequate and effective protection. By modifying the microstructure, heat treatments have a major influence on the mechanical properties and on the susceptibility to stress corrosion and exfoliation corrosion. The quench rate is a major factor influencing the corrosion resistance of copper-containing 7XXX alloys. It must be at least 150°C per second. The practice of double ageing, temper T73, was introduced to improve stress corrosion resistance and temper T76 to improve exfoliation corrosion resistance. Retrogression re-ageing (RRA) treatments combine the advantage of good resistance to stress corrosion equivalent to that of temper T73, while maintaining the mechanical characteristics of temper T6.