Research On Microstructure And Properties Of High Energy Beam Welded Al-Li Alloy Joint

Aluminum-lithium alloys have the advantages of low density,high specific strength,good corrosion resistance and superplastic formability,etc,and they are regarded as a potential structural material in aerospace industry.High-energy density beam welding,including vacuum electron beam welding?EBW?and laser beam welding?LBW?,has the characteristics of energy concentration,narrow heat-affected zone?HAZ?and small welding deformation,and it is advantageous to weld Al-Li alloys.However,there are some problems such as weld porosity,hot cracks and softening of weld zone during the fusion welding of Al-Li alloys.Consequently,the industry application of Al-Li alloys is limited to a certain extent.In this paper,EBW and LBW are carried out for the 2195 Al-Li alloy respectively.The microstructural evolution of welded joint is investigated under different welding conditions,and thus the welding parameters are optimized.Microstructure and mechanical properties of the two welded joints under as-welded?AW?condition are contrasted.Aiming at the softening of weld zone in AW condition,after solution treatment,single and double aging treatments are carried out for the welded joints respectively.The effect of post-weld heat treatment?PWHT?on microstructure and mechanical properties of the welded joints are investigated,so as to increase the strength cofficient of welded joint.The main research work and results are summarized as follows.?1?Microstructural evolution of the welded joints by EBW and LBW respectively under different parameters is studied.Combing with the tests of joint mechanical properties,the welding parameters are optimized.The results show that there exists a narrow fine equiaxed grain zone?EQZ?at the side of fusion zone?FZ?adjacent to fusion line in EBW joint.The FZ is mainly equiaxed grains,and some dendrites also exist.In AW condition,the phase constituent of weldment for EBW joint is mainly??Al?,and there are small amount of strengthening phases such as?,?'and?'.Among them,?and?'are the main strengthening phases,which are beneficial to the improvment of joint strength.Due to the underaging of weld metal in AW condition,the precipitation of T₁ phase is fewer in weldment.The microstructure of LBW joint is basically same to that of EBW joint,but the grain coarsening in the upper part of weldment does not occur.However,the slight grain coarsening exists in EBW joint caused by the subsequent modification welding.The optimized welding parameters are as follows.For EBW:electron beam current 13 mA,welding speed 600 mm/min,circular electron beam scanning?500 Hz,2%?;For LBW:laser power 4600 W,welding speed 5000 mm/min,defocusing amount 0 mm.?2?Mechanical properties and tensile fracture behavior of the EBW and LBW joints in AW condition are analyzed respectively.The results show that obvious softening phenomenon occurs to both joints in AW condition.The ultimate tensile strength of the EBW joint is 368.4 MPa,which is 68%of that of base metal?BM?.Mechanical properties of the LBW joint are slightly lower than that of EBW joint,and the ultimate tensile strength is 352 MPa,which is 64.6%of that of BM.The elongation of both joints is basically the same,which is a little lower than that of BM.Both joints fail in the mixed mode of ductile-brittle fracture during stretching,namely,they present the hybrid mode of microporous aggregation fracture and quasi-cleavage fracture.?3?After solution treratment,single and double aging treatments are carried out for the good joints obtained by EBW and LBW respectively.Microstructural analysis shows that the strengthening phases in both weldments are roughly identical.In AW condition,there are mainly?and?'phases.After single aging treatments?HT1,HT2?,strengthening phase such as?'and T₁ precipitates in weldment,and the size and quantity of strengthening phases increase with the elevation of aging temperature.Compared with that the single ones?HT1,HT2?,the strengthening effect is more obvious during double aging treatment?HT3?.The grains in weldment are refined to a certain extent due to the occurrence of recrystallization during double aging treatment process.The mechanical properties of welded joints in different conditions are tested.The tensile strength of EBW joint is increased to 492.5MPa after double aging treatment?HT3?,which reaches to 90.4%of that of BM.The tensile strength of LBW joint is slightly lower than that of EBW joint.It is increased to 485.2 MPa after double aging treatment?HT3?,and the strength coefficient of joint is 89.03%.In addition,PWHT also has great effect on the microhardness distribution of weld zone.The hardness of FZ is obviously increased after heat treatment,but the hardness of BM zone is decreased.The hardness curves are changed from the"V"shape in AW condition to the"hat"shape after PWHT.SEM images of joint tensile fracture in different conditions demonstrate that,the morphology of fracture surface presents the obvious characteristics of dimple fracture after single aging treatment?HT1?.During stretching,the welded joints fail in the mixed mode of ductile-brittle fracture after the heat treatments?HT2,HT3?,and there exist both dimples and small cleavage facets on the fracture surfaces.