**1. Introduction**

The Al-Zn-Mg-Cu series alloys have been widely used in aerospace applications [1,2], such as for wing stringers, fuselage frames, wing skins, and other critical components owing to their high strength, low density, ability to be heat-treated, etc. With the rapid development of modern industry, the high requirements of aerospace equipment tend to accelerate the invention and the application of new manufacturing technologies. Creep age forming technology is one of those advanced aluminum alloy forming technologies, which synchronizes forming and artificial aging [3]. Compared with traditional forming methods such as drawing, rolling and bending, and shot peening, the creep age forming technology has a range of advantages such as lesser material waste than milling, lower residual stress and better surface quality. Additionally, the creep age forming technology can enhance the stress corrosion resistance of the alloy and extend the service life of parts [1,4]. Therefore, it is mainly used in the manufacture of large integral panel parts [5,6] and has a broad application prospect in the field of aircraft manufacturing [7–9].

In the past, scholars tended to study two main aspects of creep age forming technology: (1) the deformation control of aluminum alloy using the creep age forming technology (Peddieson [10] and Sallah [11] studied the relationship between creep and stress relaxation in the aging process based on viscoelastic mechanics. At the same time, they also gave the calculation formulas in their studies. Ho [12] established a unified creep/stress relaxation constitutive model that includes phase precipitation, grain growth and dislocations based on creep, stress relaxation theory and aging kinetics); and (2) improvement of microstructure and macroscopic performances of aluminum alloy using creep

age forming technology. Sarioglu [13] and Brav [14] studied the fatigue crack propagation rate of 2024 aluminum alloy under different aging conditions, and found that aging treatment can improve the fatigue performance of the alloy. Lumley [15], Zhao [16] and Jin [7] reached similar conclusions. Temperature, time and stress are combined in the process of creep age forming, and are therefore regarded as the three important parameters of creep age forming technology. Yang [17] studied the effect of aging temperature on microstructure and fatigue performance. The results show that the fatigue performance of 7075 aluminum alloy increases with the increase of aging temperature; Zhan [18] tested the hardness, strength and elongation of 2124 aluminum alloy aged at 458 K under 0 MPa and 200 MPa, the results show that the strength of the alloy increases and the plasticity decreases under the stress aging condition. Zhu [19] studied the microstructure of Al-*x*Cu aluminum alloy after aging and found that the yield strength of the alloy with stress was lower than that without stress; Liu [20] studied the strength and stress corrosion fracture (SCC) behavior of 7075 aluminum alloy after aging and regression treatment; the results showed that the strength, hardness and SCC sensitivity of 7075 aluminum alloy were closely related to aging temperature and time. Liu [21] studied the influence of aging temperature and time on the microstructure and mechanical properties of 7075 aluminum alloy sheet after creep age forming; the results showed that the creep age forming performance of 7075 aluminum alloy sheet is closely related to the process parameters.

In conclusion, during the investigation of forming microstructure and mechanical properties, the effects of temperature and time have been studied more than the effects of prebending radius on microstructure and high cycle fatigue (HCF) properties. As is well known, materials and structures exposed to HCF are subject to low cyclic stress, so the plastic deformation is not obvious and is difficult to detect and prevent. Therefore, the study of HCF performance is of great significance for improving the fatigue life of materials, in particular for 7075 aluminum alloy, a typical Al-Zn-Mg-Cu aluminum alloy, which has high strength and is widely applied in the aviation and aerospace industry. This paper outlines the findings of the study of the effects of prebending radii on the hardness, tensile strength, yield strength, elongation and HCF performance of the 7075 aluminum alloy after creep age forming. The microstructure and fatigue fracture morphology were further observed by scanning electron microscopy and transmission electron microscopy. The present research work can provide reference for the improvement of creep age forming technology and engineering application of 7075 aluminum alloy.
