Electron Beam Welding Of New Al-Li Alloy And Microstructure And Properties Of Joint

The new Al-Li alloy has excellent comprehensive properties,such as low density,high specific strength,good anti-fatigue performance and processing formability,etc.It is widely used in many high-tech fields such as aerospace,satellite,nuclear industry and so on.The Al-Li alloy is often used as welded construction in industrial production.If it is welded by using conventional fusion welding methods,there exists some problems such as weld porosity,crack and joint softening,etc,which is limited the application of Al-Li alloy to a certain extent.Electron beam welding(EBW)has the characteristics of high energy density,low heat input and narrow heat-affected zone(HAZ),it has great advantageous to weld Al-Li alloy.In this paper,the new Al-Li alloy is welded by using EBW,and the effect of welding parameters on microstructure and properties of joints is investigated,thus the welding process is optimized.Aiming at the problem of joint softening in as-welded(AW)condition,the different post-weld heat treatments(PWHT)including solution treatment and then single aging,double aging and three-step aging treatment are carried to welded joints respectively,so as to improve the microstructure and mechanical properties of joints.The effect of different PWHTs on corrosion resistance of welded joints is also evaluated.The EBW processes are conducted to Al-Li alloys.The results show that,if the welding speed is higher,the duration time of molten pool is shorter at high temperature,which is disadvantageous to the solid solution of alloying elements.At the same time,the gas is not easy to escape from molten pool,as a result,the porosity defects will be generated in weldment and causes the decrease of mechanical properties of welded joints.The microstructure of joint indicates that,in AW condition,from fusion line to weld center,it consists of fine equiaxed grain zone(EQZ),coarse grains zone,and hybrid microstructure of equiaxed grains and dendrites.The microstructure of HAZ is the same as base metal with strip-shape rolled grain.The tensile fracture of welded joint occurred to weld metal,and the fracture surface obviously presents the characteristics of ductile fracture.The different PWHTs are carried out to Al-Li alloy joints by EBW.The results show that,the mechanical properties of joints with post-weld double aging treatment are greatly improved,and the strength coefficient of joints obviously increases,which can reach 89.7% of base metal.The observation of microstructure demonstrates that,the microstructure of weldment transformed to equiaxed grain after PWHT.The EQZ is still retained after post-weld single aging treatment,while it disappeared after post-weld double aging or three-step aging treatment.The post-weld double aging treatment can promote a large number of phase T1(Al2CuLi)nucleation and growth in grain boundaries and much dispersive phase β′(Al3Zr)particles precipitated within grains.Due to the decrease of temperature in the second aging treatment,the procedure of recrystallization fully completed in weldment,thus the grain is refined in weldment,and the effect of grain refinement strengthening produced.At the same time,the increase of grain boundaries can provide more nucleation positions for phase T1,and it promotes the phase T1 to precipitate in grain boundaries,so the volume fraction of phase T1 in weld metal increases significantly.Consequently,the mechanical properties of welded joints are greatly improved.The intergranular corrosion(IGC),exfoliation corrosion(EXCO)and electrochemical corrosion measurement are carried out to welded joints respectively.The results show that the corrosion susceptibility to different weld zones of joint is different.The IGC and EXCO resistance of weldment are both good before and after PWHT.The susceptibility to IGC increases for base metal and HAZ of joint after PWHT,while the susceptibility to EXCO decreases for base metal and HAZ of joint after post-weld single aging and three-step aging treatment.The self-corrosion potential of weldment decreased after PWHT,which means that the tendency of electrochemical corrosion increases lightly.