Research On Electron Beam Welding Process Optimization And Microstructure Transformation Of Al-Li Alloy

The density of 2195 aluminum-lithium alloy?Al-Li alloy?is lower than that of ordinary aluminum alloy,and its strength and plasticity are higher,so the application prospects of 2195 Al-Li alloy is excellent.Since the boiling point of the Li element is low,burning is likely to occur during welding,resulting in insufficient precipitation of the weld strengthening phase and a decrease in the mechanical properties of the joint.This paper is mainly focused on the burning loss of Li element in electron beam welding?EBW?of 2195 Al-Li alloy,which retains the strengthening phase in the weld and improves the mechanical properties of the joint.When 2195 Al-Li alloy was welded directly by EBW,there was almost no fine strengthening phase in the weld,and the grain boundary segregation was serious.The continuous brittle T₂?Al₆CuLi₃?quasicrystal phase precipitateed at the grain boundaries.The burning loss rate of Li element was as high as 36%,and the weld was obviously softened,which caused that the microhardness and tensile strength were only about 60%of the base metal.In order to reduce the burning rate of Li element,the fusion-diffusion EBW is adopted.However,although the weld penetration was reduced,the unmelted zone at the bottom of the joint failed to form an effective joint,and little?'?Al₂Cu?strengthening phase existed in the weld.After adding the nano-copper layer to the butt surface of the base metal,the hot press diffusion EBW was performed.The formation in the unmelted zone of the joint was obviously improved.Due to the low-melting eutectic reaction and the dynamic recrystallization process,the equiaxed crystal zone was formed as the shape of an inverted triangle below the fusion zone.The burning loss rate of the Li element was only 9%.As a result,the strengthening phase was effectively retained,which showed that a number of diffusely distributed T₁?Al₂CuLi?and?'?Al₂Cu?phases existing in the fusion zone.In addition,more fine T₁?Al₂CuLi?and?'?Al₂Cu?phases were formed in the equiaxed crystal zone.A large number of divergent eutectic??Al₂Cu?phases were observed at the grain boundaries.The microhardness of the fusion zone was higher than that of the EBW,and the microhardness of the equiaxed crystal zone was extremely high.The tensile strength of the joint reached70%of that of the base metal.In order to ensure the effective connection and increase the number of the strengthening phases of the weld,the EBW was carried out by adding high-strength-interlayer containing the Sc element.The microstructure of the weld was significantly optimized,all of which were extremely fine equiaxed grains.The Al₃Sc phase appeared in the weld,which is precipitated in the shape of a square inside the equiaxed grains.The Al₃Sc phase functioned as the core of the heterogeneous nucleation,which effectively refined the grains.Moreover,the Al₃Sc phase could effectively hinder the movement of the dislocations.The originally continuous T₂?Al₆CuLi₃?phase at the grain boundaries was transformed into an intermittent distribution,and its number was greatly reduced.Both T₁?Al₂CuLi?and?'?Al₂Cu?strengthening phases with extremely small dimensions appeared inside the grains.Due to the refinement effect of fine grain and the strengthening phase,the mechanical properties of the joints were significantly improved.The average microhardness of the weld was as high as 112 HV,which was 56%higher than that of the weld of EBW without the Sc element.The tensile strength of the joint was as high as 420 MPa,exceeding 80%of that of the base metal.