Study On Laser Welding Characteristics And Compressive Buckling Behavior Of Aluminum Lithium Alloy Skin Stringer T-shaped Structure

Lightweight structure design and manufacturing technology is the core supporting technology for future civil aircraft to achieve green,efficient,energy saving,emission reduction and weight reduction.The European Airbus Cooperation has successfully realized the application of laser welded aluminum alloy skin stringer structure to replace the traditional riveted structure in the aircraft fuselage,and achieved the effect of reducing weight by 10%-20% and increasing stiffness by15%-20%.It is necessary to solve the key problems such as weld forming,joint softening,welding deformation and structural mechanical properties.In response to the urgent demand for the research of the double-sided laser simultaneous welding of Al-Li alloy skin stringer structure used by domestic large aircraft project,the research on the selection of welding wire,optimization of process parameters,weld forming,control of non fusion and porosity defects,microstructure and properties has been carried out during my master's degree.This paper makes a supplementary research on the influence of beam separation distance and spot welding process on weld forming and welding deformation,while continuing the previous process research.To proceed from reality,the influence of welding sequence and fixtures on the welding deformation on the skin stringer structure,as well as the strengthening mechanism of different post-weld heat treatment processes,is discussed in this paper.On this basis,the typical parts of skin stringer structure is manufactured by laser welding.In view of the fact that the laser welded skin stringer structure will be applied to the lower fuselage panels of the front fuselage and the middle and rear fuselage in the future,it mainly carries compression and shear loads.The buckling behavior of typical parts of skin stringer structure under compressive loads is studied in this paper by combining the experiments and finite element models.At the same time,the rationality of the finite element model is verified through electrical and optical test data.Based on the nonlinear idealized error analysis of finite element method,the influence of each error source on the compressive buckling behavior is studied.The finite element model of typical structure with equal cross-sectional area is established,which are used to make a comparation of compressive performance of the laser welded structure,the riveted structure,and the friction stir welded structure.It is found that the beam separation distance and spot welding process make a direct effect on the symmetry of cross-section of the weld and the uniformity of the internal structure through the process test research,and then the long transverse tensile strength of the joint is affected.Before the continuous welding,the use of a reasonable spot welding process can effectively control welding deformation.When the heat input of spot welding is small,the angular deformation is reduced by 64%and the flexure deformation is reduced by 89%,but the transverse tensile strength of the joint is reduced by 40MPa-60 MPa due to the inhomogeneity of the structure.In order to improve the strength of the joint,two post-weld heat treatment processes of solid solution + artificial aging and artificial aging with different aging times have been formulated.The results show that with the increase of the aging time,the solid solution + artificial aging treatment will gradually increase the strength and reduce the elongation;under the artificial aging treatment,the strength first increases and then decreases,and the elongation is less than 1%.For T3 base material,when aging at 165?×20h or 30 h,the joint strength is equivalent to that of AA2060-T8 base material,but the elongation is less than 1%;for T8 base material,under the solid solution + artificial aging 165?×20h treatment,the joint strength is almost equal to that of AA2060-T8 base material,and the elongation rate is increased to 2.7%.The strengthening mechanism of the two kinds of post-weld heat treatment on the weld structure is further explored.The Cu element in the weld under solid solution + aging heat treatment is dissolved into the matrix,and the brittle ? phase on the grain boundary disappears in a large amount,and the T1 phase and ?' phase are formed which can enhance the strength and toughness.At the same time,the spheroidization of the T phase which leads to the degree of stress concentration and the increase of the intergranular bonding strength makes an improvment of the joint strength,the strengthening mechanism of which is solid solution strengthening and second phase strengthening.AA heat-treated welds produce strength-enhancing ?'phase,but not T1 phase that increases toughness,as well the Cu content of the grain strengthening phase increases,and the original grain boundary ? phase becomes more continuous to increase the strength of the joint.The strengthening mechanism is grain boundary strengthening and second phase strengthening.On the basis of strengthening welding joint and controlling welding deformation,the compressive buckling behavior of laser welded typical parts of Al-Li alloys is studied.Compared with the actual panel structure,although the size of typical structure is smaller and the actual bearing capacity is not as good as the actual panel structure,the compressive buckling behavior reflected by the typical structure can still be used as a theoretical reference for the actual panel structure.The results show that the failure mode of the typical parts is the failure due to overall deformation and instability which make it impossible to continue the load bearing.The final failure modes include partial buckling of the skin,bending instability of the stringer,and continuous fracture of some welds.When it reaches the initial buckling load,the strain on the skin shows a strain bifurcation phenomenon,and the out-of-plane displacement of the skin shows a different number of half-wave forms.When the failure load is reached,a large out-of-plane displacement of the flange of the Stringer A appears in the negative direction of the Y axis.The relevant data is extracted from the finite element model,which is basically consistent with the experimental value and is to verify the rationality of the model.The nonlinear idealization error analysis of the above-mentioned finite element model is carried out,and the control method of the finite element model with smaller error is obtained.Boundary conditions and loading methods directly affect the eccentric loading of typical parts,which are the main factors that affect the compressive strength and failure mode of typical parts.The weld strength is improved by post-weld heat treatment,which has no effect on the compressive properties of typical parts.Eigenmode imperfections affect the buckling mode of typical parts.Geometric defects with a certain size and residual stress can increase the failure load of typical parts.As the size of the geometric defects increases or the peak value of tensile residual stress decreases,the failure load gradually increases.Finite element models with equal cross-sectional area and optimized parameters are established,which show that the failure load of the L-stringer laser welded structure was close to that of the Z-stringer friction stir welded structure and riveted structure.Compared with the welding factors,the influence of structural factors on the compressive failure load is greater.