Effects of Cd and Sn on double-peak age-hardening behaviors of Al-Si-Cu-Mg cast alloys

The effects of trace elements Cd and Sn on precipitation process of Al-Si-Cu-Mg cast alloys were investigated in the present research. It is shown that the addition of Cd and Sn not only increases remarkably the aging peak hardness and reduces the time to reach aging peak, but also eliminates the double-aging-peak phenomenon which appears in Al-Si-Cu-Mg alloys, in Al-Si-Cu-Mg alloys the first aging peak corresponds to GP zones (especially GP Ⅱ), and the second one is caused by metastable phases. The obvious time interval of transition from GP Ⅱ to metastable phases associates with the double-aging-peak phenomenon. The results of DSC and TEM show that Cd/Sn elements suppress the formation of GP I zone, stimulate the formation of θ",θ' and θ phases, and then shorten remarkably the temperature intervals of each exothermic peak. Because the transition interval between GP Ⅱ zone and metastable phases is shortened by Cd/Sn in Al-Si-Cu-Mg cast alloys, θ'phase coexists with θ" phase in matrix of ageing peak condition, which causes effective hardening on the alloys,and at the same time, eliminates the double-aging-peak phenomenon.