The Effect of Multistage Aging on Mechanical Properties and Microstructure of Forged 7050 Aluminum Alloys

A new four-stage aging treatment, which is composed of pre-aging, retrogression, natural aging (NA) and re-aging, is designed to increase the tensile properties and the fatigue life of forged 7050 aluminum alloy in this work. As compared to traditional T74 and RRA treatments, the four-stage aging-treated sample exhibits the highest tensile properties and ductility. The tensile strength and yield strength are 627 and 608 MPa, respectively, and the elongation is 11.3%. The results of TEM and DSC show that an increased G.P zones and fine η′ phase distributed in the matrix of the alloy are subjected by this new aging processing as compared with T74 and RRA treatments. The natural aging in four-stage aging treatment accelerates the diffusion of Mg vacancies, and promotes the precipitation of G.P zones in grain interior during re-aging stage. On the basis of dislocation shearing mechanism, more dense and fine G.P zones of four-stage aging temper result in the superior tensile strength as compared to T74 and RRA temper. In addition, a combination of shearable precipitates, fine GBPs and narrow PFZs is beneficial to the slip reversibility in the plastic zone at the crack tip, thereby reducing fatigue damage accumulation and increasing the fatigue life.