Investigation on the ageing behaviour and hardening mechanisms of 5A90 Al–Li alloy U-shaped parts formed by electric resistance heating forming-quenching process

Abstract

The electric resistance heating technology was introduced into the solution heating, forming-quenching with cold die integrated process for improving the dimensional accuracy of the 5A90 Al–Li alloy U-shaped parts. In order to evaluate the effect of current density on the forming quality of U-shaped parts, the springback and mechanical properties of formed parts were analyzed. The dimensional accuracy of formed parts increased with the current density. The U-shaped part with the minimum springback angle (0.43°) was obtained under the current density of 12.76 A/mm 2 . The increase in hardness of the typical positions of U-shaped parts was mainly due to the spherical δ′ phase with the proper matching of volume fraction and size. The hardness of the bottom fillet increased from 82.4 HV to 113.53 HV after ageing at 160 °C for 10 h. A mechanistic model referring to grain boundaries, dislocation accumulation, solid solution and precipitation was established to describe the hardening mechanisms. The δ′ phase not only contributed to the precipitation hardening mechanism but also affected the solid solution hardening by changing the concentration of solute atoms. The precipitate hardening caused by δ' phase with the perfect antiphase boundaries was the dominant mechanism, which afforded 35.6 HV with the volume fraction of 31.51% in the peak-aged sample. • The EHFQ process improves the dimensional accuracy of the 5A90 Al-Li alloy U-shaped parts. • The precipitation behavior and matching relation of δ′ phase are studied, which affect the mechanical properties of parts. • A mechanistic model is established to describe the hardening mechanisms and the contribution of each mechanism is analyzed.