Effects of Minute-Scale Preaging Time on Formability of AA2195-T34 Alloy

The influence of minute-scale preaging (MSP) at 200 °C on the microstructures and formability of 2195-T34 Al-Li alloy in the creep-aging (CA) process was systematically investigated. The elongation of the alloy before CA first increased and then decreased with MSP time (2 to 15 min), reaching a maximum of 17.86% at 5 min; the yield ratio exhibited the opposite trend, attaining a minimum of 65.55% at 5 min. When the true strain was greater than 1.4%, the strain hardening rate of the samples after MSP treatment for less than 8 min was similar to and higher than that of the samples preaged for 15 min. Compared with the non-preaged specimens, the ultimate creep deformation of samples preaged for less than 8 min was improved, reaching a maximum improvement of 29.5% at 5 min. Simultaneously, the peak aging was retarded without reducing the final peak strength (T6 level), thereby broadening the formability window. HAADF-STEM observations revealed that MSP for 5 min markedly reduces the density of G.P. zones, which delays T1 precipitation and reduces the resistance to dislocation motion. When the preaging time was less than 8 min, the number of movable dislocations increased. However, exceeding 8 min led to obvious precipitation of the T1 phase, reducing the plasticity of AA2195-T34 sheets.