Study On The Heat Treatment Process, Microstructures And Mechanical Properties Of Cast Al-3Li-1.5Cu-0.2Zr Alloy

Al-Li-Cu alloy is regarded as a promising structural material to be used on aircrafts and spaceships due to its lower density,higher elastic modulus,higher specific strength and specific rigidity.The microstructure and properties of wrought Al-Li-Cu alloys which has relatively low Li content have been well documented,however the literature about cast Al-Li-Cu alloys are still lack of data.Due to the difference of composition and microstructures,traditional heat treatment process is also not fit for new Al-Li alloy.In this thesis,the microstructures,mechanical properties and heat treatment process of cast Al-3Li-1.5Cu-0.2Zr alloy were investigated by using optical microscopy,X-ray diffraction,scanning electron microscopy,transmission electron microscope,thermal analysis,tensile and hardness test.The experimental results show that the as-cast alloy is mainly consisted of equiaxed?-Al,Al₃Li and Al₂Cu phases.The average grain size of?-Al is about 60?m.Masses of coarse secondary phases with continuous networks morphology can be observed along the grain boundaries.After solution treatment?ST?at 540?,550?,560?,the amount of secondary phases decreases and the average size of?-Al increases to 70?m.When temperature increases to 570°C,the alloy is overburnt.Higher temperature and longer time can help to resolve intermetallic compounds but also lead to larger grain,the volume fraction of residual phases reaches 1.4%after ST at 560?for50h.XRD results indicate?-Al₂Cu almost disappear after ST,the STed alloy is mainly consisted of?-Al and??-Al₃Li phase.The elongation?EL?of alloy increases with solution temperature and reaches the maximum value of 22.1 after ST at 560?for 40h.In this condition,after ageing at 175°C for 8 h,a mechanical property with UTS=359 MPa,YS=248 MPa and EL=3.5%was obtained.Two-stage solution treatment?TSS?can further improve the mechanical properties of this alloy,especially EL.The optimum TSS is 520?×10h+560?×30h,after ageing at175°C for 8 h,a mechanical property with UTS=381 MPa,YS=245 MPa and EL=5.9%was obtained.After aging,the main hardening precipitations are?',T₁,??and core/shell Al₃?Zr,Li?particles and the evolution of PFZ also closely related to aging process.At 200?,the precipitation reaction is strong,the growth of?'is obvious and coarse T₁ and??phases are observed in samples aged for 8h.When aged at 175?,only the growth of?'is observed in samples aged for 8h,fine plate-like T1 phase and a few of??have taken place in samples aged for32h and large number of coarsening T₁ and??can be seen in peak and over aged samples.Meanwhile,the prolongation of aging time also results in the broadening of PFZ.In samples age at 150?,only coarsening?'can be seen.Optimal single stage aging process for Al-3Li-1.5Cu-0.2Zr alloy is175?×32h,a mechanical property with UTS=398 MPa,YS=303 MPa and EL=3.1%can be achieved in this condition.Two stage aging can help to precipitate more uniform?'and finer T₁ phase,which is good to the mechanical properties of Al-Li alloy.The optimal heat treatment of alloy is to solution treated at 520?×10h+560?×30h followed by aging at175?×16h+125?×16h,under this condition the UTS,YS and EL respectively are 400 MPa,298 MPa,3.9%.After optimal heat treatment,Al-3Li-1.5Cu-0.2Zr alloy possesses good thermal stability at 100??The EL still remains 3.45%when the alloy is subjected to exposure at 100?for 100h,and the hardness rises only by 19.7HV after exposure for 200h.