Precipitation and Dissolution Reactions in a 6061 Aluminum Alloy

The aging processes in a 6061 commercial aluminum alloy during continuous heating at various heating rates have been studied by means of DSC measurements. Eight reactions have been detected, six exothermic and two endothermic. The activation energy for the first exothermic peak observed at about 373 K at a heating rate of 10 K min−1 is 79 kJ mol−1, almost the same as the activation energy for vacancy migration. Considering the excess vacancies exist in the as-quenched condition, this reaction can be explained in terms of vacancy cluster formation. The second and the third exothermic reactions within the large endothermic peak are thought to arise from the formation of solute atom clusters involving silicon and magnesium. The endothermic reaction is probably due to the dissolution of these atomic clusters. The activation energy for this process is close to those for the bulk diffusion of magnesium and silicon atoms in aluminum. A large and sharp exothermic peak at temperatures around 500–550 K is due to the precipitation of intermediate β″ needles. The activation energy for this process is significantly smaller than that for bulk diffusion. The reason for it is not clear at present, but may arise from the needlelike shape of the precipitates. The following two exothermic reactions which are difficult to separate clearly from the peak at 500–550 K are probably due to the precipitation of β′ needles and the stable β phase, respectively. The dissolution of stable β phase occurs at higher temperatures.