| Titanium alloy is a common manufacturing material for aerospace shell structures due to its high specific strength and low density.In order to enhance the mechanical properties of the welded joint and prevent the damage of the internal structure by high welding temperature,the backing plate is added inside the shell butt-weld structure to form a "T" shaped welded structure.With the continuous research and development of titanium alloy metal,titanium alloy aerospace shell structure is also facing the upgrading of metal materials and welding methods.In this paper,the macroscopic defects,microstructure,and mechanical properties of the "T" shaped welded joint which formed by two titanium alloy materials and three welding methods are studied.Combined with finite element simulation of the stress variation characteristics during the welding process,the shell structural materials and welding schemes are directionally optimized to achieve high-quality and low-cost effective structural connection.The electron beam welding method is used to weld the titanium alloy with the backing plate,and four types of welded joints with different weld morphologies are obtained by adjusting the electron beam welding process parameters.From the macro defect perspective,as the heat input energy of the electron beam increases,the no-penetration defects where between the edge position of the backing plate and the butt plate gradually decrease,and the data indicated that the degree of fusion from 44.77% increase to 88.63%.This data indicates that the lower electron beam welding speeds can effectively reduce the no-penetration depth between the backing plate and the butt plate.From the perspective of microstructure,it is found that under the action of welding linear energy,theαphase and βphase in the base material gradually transformed into interlaced mesh basket martensite phase.At the same time,the microhardness and shear force of each microzone of different specimens show a gradient characteristic of weld metal(WM)> heat affected zone(HAZ)> base material(BM).It is proves that the influence of electron beam welding process parameters on the microstructure change and mechanical properties of the welded joint is not obvious.To further analyze the impact of no-penetration defects on the welded shell structure generated by different electron beam welding speeds,the finite element simulation method is used to characterize the shell structure welding residual stresses.Simulation results show that the degree of fusion between the backing plate and the butt plate is the main factor affecting the residual stress within the shell structure,and the higher degree of fusion will reduce the structural stress and improve the service life.The finite element simulation of the welding process is conducted for the electron beam welding parameters at the welding speed is 8mm/s.The calculation results showed that as the electron beam heat source moved and constrained by the surrounding metal,the metal gradually changed from the compressive stress to the tensile stress after welding.The center of the weld metal exhibited the highest tensile stress,with the peak longitudinal and transverse residual stress of the weld being 782 MPa and515.37 MPa,respectively.The simulation results are consistent with the distribution trend of the measured results.The tungsten inert gas welding(TIG welding)and TIG+brazing hybrid welding methods are used to weld titanium alloy sheets with backing plates,respectively.The macro defects,microstructure,and mechanical properties of the "T" shaped welded joints are studied.The welding joint formed during TIG welding has a shallow fusion depth and the fusion degree is0%,which does not meet the welding requirements of the shell structure.TIG+brazing hybrid welding uses silver based brazing material as the intermediate layer between the backing plate and the butt plate,with good weld formation and the fusion degree of up to 97%.The weld joints cross-section form two distinct distributed weld morphologies,which are butt WM and braze WM.During the welding process,part of the titanium alloy plate and brazing material are melt under the influence of TIG heat source,the fusion welding process is realized between titanium alloy plate and titanium alloy plate,while the mutual diffusion reaction occurs at the titanium alloy plate-brazing area(BA)interface.The microstructure evolution of the welded joints are analyzed using optical microscopy and scanning electron microscopy(SEM),and the elemental changes in each microzone of the welded joint are characterized by X-ray diffraction(XRD)and energy dispersive spectroscopy(EDS).The results show that the microstructure in the BA is mainly composed of intermetallic compounds,and there are obvious long stripes of β-phase in the butt WM.The microhardness of welded joints presents a trend of BA > BM > HAZ > butt WM,with an average microhardness value of 429 HV in the BA.The intermetallic compounds generated in the BA significantly improve the mechanical properties,and their shear strength increases by60% compared to the BM.Combining three types welding method of titanium alloy with backing plates,the microstructure and mechanical properties of the welded joints are investigated respectively.Based on the study of electron beam welded joints of titanium alloys with backing plates,it is shown that smaller no-penetration defects have a positive effect on the structure of butt-welded shell structure.When the TIG+ brazing method is used for TC17 welding,the brazing zone fused well and there are minimal no-penetration defects between the backing plate and the butt plates.Although the increase of silver-based brazing material generates an interfacial layer between the backing plate and the butt plates,it has effectively improved the degree of fusion,reduced welding defects in the welded joint cross-section,and increased the shear strength of the joint to 2641 N.This welding method effectively and cost-effectively realizes the high-strength connection of titanium alloy with the backing plate. |
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