The Effect Of Heat Treatment Process And Cu Content On Microstructure And Properties Of The Novel Ultra-High-Strength Al-Li Alloy

The novel ultra-high-strength Al-Li alloy,as a recently exploited third-generation Al-Li alloy,is of great significance to investigate how to further optimize the heat treatment process and modify the composition of the alloy to improve strength and ductility.In this paper,the effect of T8aging treatment and Cu contents on mechanical properties（tensile properties,fatigue crack propagation rate）and microstructure of the novel ultra-high-strength Al-Li alloy was studied through tensile and fatigue crack propagation rate tests,OM,TEM,SEM,XRD,EBSD.The conclusions are as follows:（1）The effects of T8 aging temperature and time on the tensile properties of 2A55 Al-Li alloy hot-rolled plates with 10 mm thickness were studied,then the T8 aging process was optimized to establish the heat treatment regime of 2A55 ultra-high strength Al-Li alloy for industrial application.（2）The main precipitation phases of T8-aged 2A55 ultra-high-strength Al-Li alloy hot-rolled plates comprise numerous T₁ precipitates and someθ′precipitates.With the increase of pre-deformation and aging temperature,the aging kinetic is significantly accelerated and the number density of T₁ andθ′precipitates increases with the decrease of diameter.Additionally,the strengthening contribution of T₁ precipitates reduces with increasing the pre-deformation,and that ofθ’precipitates decreases in the range of 3%～6%pre-deformation,then slowly increases at the pre-deformation over 7.5%.Whereas the joint strengthening contribution of T₁andθ’precipitates decreases with higher pre-deformation.（3）With the increase of pre-deformation,the strain strengthening contribution due to dislocations introduced by pre-deformation before artificial aging gradually increases.Meanwhile,the recovery of dislocations is suppressed during artificial aging.（4）With the increase of Cu content from 3.76%to 4.19 wt.%and 4.46wt.%,the sequence of strength and fatigue crack propagation rate in T6-aged and T8-aged novel ultra-high-strength Al-Li alloy cold-rolled sheets are different.During T6 aging,the strength of UHS1（4.19 wt.%）Al-Li alloy is highest,the yield strength and tensile strength of which are 564.4MPa and 595.1 MPa,respectively,while the fatigue crack propagation rate is highest.With the increase of Cu content in T8-aged novel ultra-high-strength Al-Li alloy cold-rolled sheets,the strength of which is increased and the fatigue crack propagation rate is accelerated.The tensile strength of the T8-aged ultra-high-strength Al-Li alloy with the highest Cu content approaches 655.4 MPa with an elongation around 7%.（5）The main precipitation phases of the novel ultra-high-strength Al-Li alloy cold-rolled sheets after T6 and T8 aging are T₁ andθ′precipitates.With the increase of Cu content,the number density and diameter ofθ′precipitates increase in T6-aged alloy,whereas the number density of T₁precipitates increases first and then decreases.As the Cu content of the T8-aged alloy increases,the number density and diameter of T₁ precipitates increase.Nevertheless,the diameter of theθ′precipitates turns larger and the number density of which increases first and then decreases.