Effect of scandium on ageing kinetics in cast Al-Zn-Mg alloys

The effect of Sc addition in cast Al-Zn-Mg alloys with precipitation strengthening was studied different ageing conditions. Alloy (1-4) systems, the ageing at 140 °C for 16 h shows the moderate hardness value as compared to the solution treated samples. Further, the ageing at 120 °C the high hardness (360 HV) values for the Alloy-1system and the same was estimated (320HV) for the Alloy-1 for the 180 °C for 16 h. DSC curve for the all alloy system ageing at 140 °C for 16 h more endothermic peaks for the Alloy (3-4) system due to the L12-Al3Sc precipitate as compared to the Alloy (1-2) system exothermic peaks. Moreover, the TEM analysis confirmed the formation the L12-Al3Sc precipitate particles for the solution treated and aged sample. The high strength has been achieved through scandium addition in cast Al-Zn-Mg alloys through FSP (friction stir processing) by double passes. The scandium additive can retain the recrystallization behavior of Al-Zn-Mg alloys and to the increase of recrystallization temperature due to the pinning effect of fine dispersion L12-Al3Sc precipitate. It confirmed from the optical microstructures and tensile test for the T4 and T6 condition sample. The mechanical properties have been revealed through FSP and post ageing treatment at 140 °C for 16 h. The most important results due the formation of GP zones, ή precipitates, L12-Al3Sc precipitate and to improve the mechanical properties.