Study On The Preparation Process,Microstructure And Properties Of Aluminum Copper Lithium Alloy

Aluminum lithium alloy material has been one of the advanced lightweight aerospace structure materials due to its low density,high modulus,high specific strength and specific stiffness,and excellent combination properties.There is a significant weight saving effect for aerospace vehicle with the application of aluminum lithium alloy.The research of aluminum lithium alloy has impotant scientific significance and application value.This work,as part of the national 973 project "Study on aviation aluminum alloy with high performance" is mainly focused on the preparation process and its effect on the microstructure and performance of aluminum copper lithium alloy.The ingots with different compositions were prepared with intermediate frequency induction furnace,a resistance holding furnace and a mold with water cooling.Then thick plates and thin sheets were prepared by rolling process.Homogenization of the ingots,and solid solution treatment process and aging process of aluminum copper lithium alloy was studied.The influence of process parameters on the microstructure and mechanical properties of this alloy was studied by means of differential scanning calorimetry(DSC),optical microanalysis(OM),scanning electron microscope(SEM),energy dispersive X-ray spectroscopy(EDX)and the tests of tensile properties,hardness,electrical conductivity and so on.The experimental results show that there is a serious microsegregation in the as-cast microstrucutre with a large number of low melting point non-equilibrium eutectic phase existing on the grain boundary and in the interdendritic field.The eutectic strucutre is mainly composed of α-Al and Al2CU.The appropriate homogenization processes of 525℃×24h was obtained by studying the DSC curve,microstructure,grain size,conductivity,hardness,and etc.The influence of solid solution treatment and aging process on the microstructure and mechanical properties of the sheet with 3 mm in the thickness was studied.The results show that the proper solid solution and aging process is 510℃×1.5h+175℃×24h.The pre-deformation has a significant effect on the performance of the alloy.With the application of a 9%pre-deformation which is following the solid solution of 510℃×1.5h and before the aging treatment of 175℃×24h,the performance of the alloy is much higher than that of peak aging.The process of 510℃×1.5h+175℃×24h was used to tread thick palte.There are obvious differences of microstructure and performance along different directions of the treated thick plate.The mechanical properties along thickness direction is the worst and along rolling direction is the best.At the same time,it is found that there is a serious fluctuation of the plasticity along the thickness direction.The results show that the existence of hydrogen and oxide film is the main cause of the fluctuation.Within a certain range,the strength of the alloy increases and elongation declines slightly with the increase of the concentration of copper and lithium.The major reason could be the increase of precipitation strengthening phase which hinders the slip of dislocation and affects the mechanical properties of the alloy with the increase of the concentration of copper and lithium.