The Non-Isothermal Double Ageing Behaviour of 7055 Aluminum Alloy

Thick plates of 7055 aluminum alloy are widely used as structural components, especially in the aerospace industry, due to their high strength, low density, excellent hot workability, and high stress-corrosion resistance, which are dependent on the type of thermal treatment the alloy is subjected to. Because of the heating and cooling stages in such components, non-isothermal ageing has attracted a lot of research interests. Replacing isothermal ageing with non-isothermal ageing is needed for higher efficiency and practicability. Herein, a novel isothermal-ageing technique based on double ageing is developed. Hardness test, electrical conductivity test, room-temperature tensile test, exfoliation corrosion test, DSC, and TEM analyses were employed to study the influence of non-isothermal double ageing on microstructure and properties of the 7055 aluminum alloy. The results showed that in the heating stage of the second ageing treatment, inner grains of the microstructure evolved from a three-phase coexistence state containing the GP zone, eta' phase, and alpha-Al to that containing eta' phase, eta phase, and alpha-Al. On the other hand, in the continuous cooling stage of the second ageing, GP zone and eta' phase re-precipitated, resulting in improved hardness. The eta phase on the grain boundary became coarse and discontinuously distributed, which resulted in a progressive improvement of the electrical conductivity. The heating rate and highest ageing temperature (T-p) of the second ageing stage determined the final properties. With a standard electrical conductivity of 22 MS/m, 1 degrees C/min heating rate corresponds to the T-p of 215 degrees C, while T-p of 225 degrees C is needed when heating by 3 degrees C/min. After pre-aged by 105 degrees C, 24 h and non-isothermal ageing including heating and cooling stages, the strength and exfoliation corrosion resistance of approximately 610 MPa and EB level were achieved, respectively. The alloy showed a better comprehensive performance than the T6 and T73 state ones. Additionally, the non-isothermal ageing removing the heat preservation stage realized the short process preparation.