Influence of a retrogression and reaging (RRA)-treatment on the mechanical and microstructural characteristics of the aluminium alloy AlZn4,5Mg1

Abstract

The investigated material is the copper-free Al–Zn–Mg-alloy EN AW-7020 (AlZn4,5Mg1) in the artificial aged T6 temper. In this temper the 7xxx series alloys show a high strength level, but also a high stress corrosion sensitivity. Therefore, the retrogression and reaging (RRA)-treatment has been developed to combine stress corrosion resistance with high strength. The aim of this study is the description of the course of the mechanical behavior influenced by a RRA treatment. The changes of the mechanical properties are represented by changes in the microstructure as recorded by thermal analysis. Therefore typical mechanical properties and thermal analysis curves are determined according to temperature and time in each of the retrogression annealing and of the RRA-treatment. The temperature for retrogression annealing was between 220 and 360 °C. Reaging was performed on material at 130 °C for 16 h and 160 °C for 5 h after a storage of 7 days at room temperature. The results of the thermal analysis are generally described on the curve of the T6 tempered material and transferred to the changes by the retrogression temperature and the reaging parameter. The results show a main influence of the precipitation structure and mechanical properties after the retrogression and reaging treatment by the temperature of the retrogression annealing. For all investigated parameters, an overaging of the material after the RRA-treatment is recognizable.