Effects Of Deformation And Aging Strengthening On Damage Tolerance Of 2198 Al-li Alloy

2198 aluminum-lithium alloy is widely used in the aerospace industry, owing to its high strength and excellent lightweight. The hydro-mechanical deep drawing and aging strengthening of an aluminum-lithium alloy 2198 in T3 temper were studied to realize the forming in T3 temper and the improvement of damage tolerance through aging strengthening. Uniaxial tensile test and tear test were adopted to analyze the strength and toughness. Then, high-cycle fatigue life and fatigue crack growth rate tests were applied to evaluate the fatigue properties. The microstructure of the alloy was studied by the transmission electron microscopy(TEM).Firstly, the formability and drawing of 2198-T3 was investigated. The result indicated that, the strain hardening index n was consistent at different orientations, while the plastic strain ratio r was quite different, exhibited apparent anisotropy. The plastic deformation behavior of the Al-Li alloy could be predicted accurately, as the deformation resistance curve was well described with the power function σ=σy+Kεn. Due to the quality of parts was seriously affected by the overflow pressure, blank holding gap and pre-bulging pressure during the hydro-mechanical deep drawing, numerical simulation was used to optimize the forming parameters, which were pre-bulging 2MPa, overflow pressure 25 MPa and blank holding gap 1.05 times the sheet thickness. The results of simulation were consistent with experiment ones, thus the simulation could accurately predict the deformation amount of the part.Secondly, the effects of aging temperature and time on microstructure and properties of Al-Li alloy were researched. The results showed that, the tensile strength and yield strength of the alloy increased with the rising aging temperature applied from 125℃~185℃, accompanied with the speeding of aging response. Strength and ductility changed obviously in early stages of aging which was from 2h to 20 h, while the affect was thinner in the latter part of aging in 20h~50h. Altogether, the well-matched strength and toughness could be achieved under an aging treatment of 155℃ for 20 h.Finally, the effect of pre-deformation before artificial aging process was explored based on the maximum deformation amount of 5% from the formed part. It turned out that pre-deformation significantly enhanced the novel Al-Li alloy. The pre-deformation, ranging from 1% to 5%, increased the density of dislocation in the matrix and promoted the dispersion of T1 phase which elevate the strength of the alloy. However, the increasing number of micro cracks in alloy surface and internal was generated by the increase of dislocation tangle caused by the rising of pre-deformation, consequently lead to the reduction of toughness and fatigue life, and the increment of fatigue crack growth rate. In the state of near peak aging, the increasing of pre-deformation contributed to homogenizing the distribution of T1 phase, moreover effectively avoid the formation and coarsening of PFZ, ultimately resulted in the raising of the fracture toughness. Overall, a preferable match of strength, toughness and fatigue properties could be obtained with the route of 3% pre-deformation +155℃,15 h, meanwhile, the damage tolerance of the alloy was enhanced of 2198 Al-Li alloy.