The ageing and thermal recovery behaviour of Al-Mg-Cu alloys

In this paper, the results of studies aimed at understanding the effects of both alloy composition and sheet processing on the strength, ageing and thermal recovery of Al-Mg based alloys containing additions of Cu are described. It is shown that in the solution heat treated and naturally aged condition, the effect of Mg and Cu on yield stress may be understood in terms of the expected contributions from both solid solution strengthening and any grain size strengthening. Upon ageing, at temperatures of between approximately 140 C and 200 C, the alloys show a rapid increase in strength over the first 30 minutes or so at temperature, which is followed by slightly reduced ageing kinetics. The precise ageing response has been shown to be dependent upon the levels of Mg and Cu within the alloy, and the Mg:Cu ratio, which is consistent with much previously published work on the 2xxx series (Al-Cu-(Mg)) alloys. In addition, the thermal recovery behaviour of selected alloys has been determined following tensile pre-straining to various levels (up to 15%) and the results compared with equivalent data for Cu-free alloys.