Microstructural Evolution of 6061 Alloy during Isothermal Heat Treatment

The semi-solid billet of 6061 aluminum alloy was prepared by the near-liquidus semi-continuous casting (LSC) with rosette or near-spheroide grains. The pre-deformation processing was applied before partial remelting to further improve the microstructure and properties of the semi-solid alloy. The effects of different processing parameters, such as holding temperature and holding time, on the semisolid microstructures during partial remelting have been investigated. It was found that the optimal partial remelting parameters should be 630 degrees C and 10-15 min for 6061 alloy cold rolled with 60% reduction in height of pre-deformation. The coarsening rates were anasysed by Lifshitz-Slyozov-Wagner (LSW) theory. The pre-deformed 6061 alloy exhibits lower coarsening rate constants than that of the as-cast one, and also lower than other alloys processed by different method found in previous literature. It is because the coarsening rate is associated with the initial microstructure and composition of the alloy. The secondary phases in the alloy inhibit the migration of the liquid film grain boundaries. The microstructure obtained by using the combination of near-liquidus semi-continuous casting and pre-deformation treatment is better than that without pre-deformation processing, which demonstrates that the used method is promising for fabricating high quality semi-solid alloys.