Laser Beam Welding And Numerical Simulation Analysis Of New Al-Li Alloy

Al-Li alloys have the characteristic of low density,high specific strength,high specific stiffness,good corrosion resistance,etc.They are regarded as one of the ideal structural materials in the aerospace field.As an advanced joining technology,laser beam welding(LBW)has the advantages of high energy density,rapid welding speed,narrow heat-affected zone(HAZ)and large depth-to-width ratio of weld.It is suitable for the welding of Al-Li alloys.Through the numerical simulation for the LBW process of Al-Li alloy,the distribution of welding temperature field and residual stress field can be obtained.The guidance can be provided for the selection of welding parameters,and the number of experiment can be greatly reduced.In this paper,the new Al-Li alloy with the thickness of 2 mm is used as the research object,and the MSC.Marc software is employed to simulate the LBW process of Al-Li alloy.The welding parameters are determined based on the results of numerical simulation.The effect of laser power,defocusing amount and welding speed on the microstructure and mechanical properties of welded joints are investigated.In order to improve the strength coefficient of welded joint,post-weld heat treatment(PWHT)is carried out for the laser beam welded joint of Al-Li alloy,and the influence of different heat treatments on the microstructural evolution and mechanical properties of welded joints are discussed.The results of numerical simulation analysis show that the laser welding parameters have great effect on the distribution of temperature field.When the laser power is too high or the welding speed is too slow,the area of weld zone will become large.When the laser power is too low or the welding speed is too rapid,incomplete penetration will be occurred to the joint.During LBW process,it has large temperature gradient in the weldment and HAZ.The simulated molten pool is well consistent with the experimental weld morphology.The simulation of stress field indicates that the residual stress is mainly concentrated in the weldment and HAZ after welding,and the residual stress is changed with the variation of welding parameters.The optimized parameters of LBW for Al-Li alloy are obtained by orthogonal experiment:laser power 3000 W,welding speed 3 m·min^-1,defocusing amount 0.The tensile strength of welded joint reaches 327.1 MPa,which is about 66%of that of base metal(BM).There are many dimples on the tensile fracture of welded joint,and it mainly presents the characteristic of ductile fracture.The measurement of microhardness for weld zone shows that the hardness of weldment is the lowest and the hardness of HAZ is relatively high.The metallographic observation shows that the microstructure of weldment presents the dendritic morphology,and there exists fine equiaxed grain zone(EQZ)near the fusion line.In as-welded(AW)condition,the quantity of strengthening phases such as?'(Al₂Cu)and?'(Al₃Li)is less in the weldment.Different solution and aging treatment procedures are carried out for the laser beam welded joints of Al-Li alloy.The grains in weldment grow up to a certain extent after heat treatment.The fine EQZ near fusion line disappears,and the dendrites in weldment are transformed into equiaxed grains.After heat treatment,a large number of strengthening phases such as??(Al₂Cu),??(Al₃Li)and T₁(Al₂Cu Li)are precipitated in weldment.The quantity of strengthening phases in weldment by double aging treatment is obviously larger than those of in the AW condition or by single aging treatment.The measurement of mechanical property shows that the hardness of weld zone is significantly increased after heat treatment.Compared with that of in the AW condition,the tensile strength and elongation of welded joint are improved after heat treatment.After double aging treatment,namely first aging treatment at high temperature and then aging treatment at low temperature,the joint has the best mechanical performance,its tensile strength reaches 446.1 MPa,which is 90%of that of BM.There are large and deep dimples on the tensile fracture of joint by double aging treatment,and the strengthening phase particles distribute at the bottom of dimples,which obviously presents the characteristic of ductile fracture.