Investigation of forging formability, microstructures and mechanical properties of pre-hardening Al-Zn-Mg-Cu alloy

A new forging process termed pre-hardening forming (PHF) process is proposed. The billets are subjected to pre-hardening treatment, then preheated, and finally forged without subsequent heat treatment. The material flow behavior and formability are investigated by uniaxial compression. The mechanical properties of the Al-Zn-Mg-Cu alloy during the PHF process are determined. Microstructure evolutions are revealed by electron backscattered diffraction and transmission electron microscopy, while the strengthening mechanism is quantified by X-ray diffraction and small-angle X-ray scattering. It is found that the pre-hardening specimens can be compressed to a very high strain level (∼70 %), indicating a good ductility similar to that of the traditional process. The higher SF and the lower texture index contribute to the plastic deformation of the Al-Zn-Mg-Cu alloy. The forming mechanism of the PHF process is the combined effect of planar slip and grain boundary slip. Although no heat treatment is carried out after forming, the strength of the compressed specimens by the PHF process reaches σ/σ 0.2  = 554 MPa/492 MPa, which exceeds the traditional forging process (σ/σ 0.2  = 542 MPa/483 MPa). Numerous thermostable GPII zones exist in the pre-hardening alloys, and the forming procedure transforms the GPII zones into dispersed η′ phases, which can contribute to the strength. The strengthening mechanism of the PHF process can be attributed to the dual effects of deformation-induced dynamic precipitation and classical discontinuous precipitation. Based on these results, aircraft components are successfully trial-produced via the PHF process. The trial production of aircraft components proves that the PHF process can not only greatly improve the mechanical properties and forming efficiency, but also significantly reduce the manufacturing energy consumption of the product, which is well-suited for industrial production, application, and promotion.