Precipitation behavior of spray-formed aluminum alloy 7055 during high temperature aging

The precipitation behavior of spray-formed aluminum alloy 7055 during high temperature aging at 360–440 °C was studied by using scanning electron microscopy and transmission electron microscopy, together with hardness and electrical conductivity measurements. The nucleation, growth and dissolution mechanism of η phase were analyzed based on precipitation thermodynamics. The results show that the main precipitates of the alloy during high temperature aging is η phase. At the initial rapid heating-up stage, the nucleation is dominant to influence the size and number density of η phase; during the subsequent isothermal aging, the growth rate has a dominant effect on η phase. The existence of η phase is related to its critical size at a given temperature. For the spray-formed aluminum alloy 7055, a most suitable high temperature aging treatment is 400 °C for 2 min. In this condition, the alloy forms more η phases in finer sizes, and its precipitation free zone around grain boundaries becomes narrower. Such microstructure is benefited from the balance of nucleation and growth of η phase, which in turn improves the hardness and conductivity of this alloy.