Precipitation Behavior and Strengthening Mechanism of Extruded 2196 Al-Cu-Li Alloy Plate during Multi-Step Age-Hardening Treatment

In order to reveal the precipitation behavior and strengthening mechanism of extruded 2196 Al-Cu-Li alloy plate during aging treatment, one-step and two-step age-hardening treatment with different temperatures and time were conducted. According to the mechanical property tests and microstructure observation, it was found that the strengthening effect of two-step age-hardening treatment is better than that of one-step aging, and the plasticity of the alloy is also improved. This is because the Guinier–Preston zones precipitated during low-temperature pre-aging can provide nucleation particles for T1 phases in the second-stage aging process, thus promoting the uniform precipitation of fine acicular T1 phases. In addition, the size of θ' phase and grain boundary precipitates in the two-step aged alloy is small, and the width of precipitated free zone is also narrow, which promotes the improvement of mechanical properties of the plate. The optimal two-step age-hardening treatment of 2196 alloy is finally determined as 125 °C×8 h+165 °C×32 h, the yield stress reached 455 MPa, and the fracture mechanism of the corresponding alloy shows a combination of intergranular fracture and cleavage fracture.