The Effect of Cu and Cr on Clustering and Precipitation in Al-Mg-Si Alloys

Abstract

A group of alloys based on pure ternary Al-0.4 wt.%Mg-1.0 wt.%Si are used to study the effect of Cu and Cr on clustering and precipitation in Al-Mg-Si alloys. Differential Scanning Calorimetry (DSC) is performed for samples naturally aged for different times after solution heat treatment and ice water quenching. Three clustering processes are observed in all the alloys. The fundamental clustering sequence does not change by additional elements. However, the Cu containing alloy shows less clustering and the first clustering event is hindered. Kissinger analysis reveals that the 2 nd and 3 rd processes have very similar effective activation energies. Therefore, a model incorporating only two independent reactions is used to obtain kinetic parameters. It is found that the first clustering process starts with a low effective activation energy of 50 kJ/mol and has a mechanism similar to mixed nucleation while the latter two processes are governed by a higher activation energy of 79 kJ/mol and have a mechanism similar to particle growth. During precipitation, the Cr containing alloy shows a similar precipitation heat signal in DSC as the pure ternary, thus having negligible effect on precipitation. On the other hand, the formation of " is less dominant in the Cu-containing alloy while precipitation of other phases before reaching the peak-aged condition is possible.