Investigation of Quench Sensitivity and Microstructure Evolution During Isothermal Treatment in 2195 Al-Li Alloy

To investigate the quenching sensitivity of the 2195 Al-Li alloy rolled sheet and guide the design of the quenching process, the time-temperature-property (TTP) curves of this material were researched through interrupted quenching experiments. The differential scanning calorimetry (DSC) and transmission electron microscope (TEM) were used to characterize the evolution of precipitates during isothermal treatment. The results of this essay demonstrated that the nose temperature of 2195 Al-Li alloy is around 370 degrees C and the temperature range of quenching sensitivity is 340 degrees C to 400 degrees C. The microstructure observation revealed that the T-1 particles precipitate and grow rapidly at the temperature from 340 to 400 degrees C, which is due to the high nucleation rate of phase and fast solute diffusion kinetics, especially at the nose temperature. The needle-shaped theta'/theta '' and T-1 particles grow up quickly as the isothermal preservation time prolonged, leading to the decrease of the supersaturated solid solution of the matrix. This will reduce the number of the age-induced precipitate and weaken the subsequent age hardening effect. Therefore, the rate of cooling should be increased in the quenching sensitivity range (340-400 degrees C) to inhibit the precipitation of the second phase and obtain excellent mechanical properties. While in other temperature ranges, the cooling rate should be decreased appropriately to reduce residual stress. The appropriate average cooling rate is recommended to be around 13 degrees C s(-1) at the temperature from 340 to 400 degrees C.