Influence of the tool pin and shoulder on microstructure and natural aging kinetics in a friction-stir-processed 6061-T6 aluminum alloy

The influence of the stirring pin and pressing toot shoulder on the microstructural softening during friction-stir processing (FSP) and subsequent natural aging behavior was investigated for a 6061-T6 aluminum alloy. The evolution of hardness profiles in various characteristic regions of the FSP plates was investigated as a function of time from 4 to 5760 hours after the FSP through the thickness of the plates and correlated to the microstructure and residual strain profiles measured by a neutron-diffraction technique. The results show that the microstructral softening and the natural aging observed in the dynamic recrystallized zone and thermomechanically affected zone are mainly caused by the frictional heating from the tool shoulder, resulting in dissolution and reprecipitation of strengthening precipitates. On the other hand, the softening in the heat-affected zone is due to the dissolution/growth of the precipitates and is not followed by the natural aging under the current processing condition. The kinetics of the natural aging behavior is also discussed.