| In recent years,China’s rail transit industry has made world-renowned achievements.At present and in the future,intelligent,green and lightweight structure will become the new main developing direction of rail transit industry.Among them,structural lightweight is highly valued because it can significantly reduce the cost of vehicle operation and maintenance.New lightweight composite materials such as carbon fiber have been successfully applied in the rail vehicle industry.Metal composite plate is expected to become one of the important candidate materials for the lightweighting of key rail transportation because of its excellent performance of multi-layer single metal.In this thesis,the forming characteristics,mechanical properties and joining mechanism of Ti/Al bimetallic laminated composite plates in railway vehicles were studied by combining experimental research and finite element simulation.The main contents include:The TA1/Al3003 bimetallic laminated composite plate was welded by MIG welding,and the macroscopic and microscopic forming characteristics of the welded joint interface were observed.The results show that: when the welding speed is 0.010 m/s,the macroscopic forming of the welded joint is better;Microscopically,α-Al dendrites and grain boundary Al+Si eutectic structures are formed in the weld area of the joint.The intergranular intermetallic compounds are mainly Ti Al3,and the grain size of the weld gradually increases along the thickness direction.The overall hardness of the joint is "M" shape;When ER4047 welding wire is used to fill,the quantity of brittle precipitated phase in the weld joint is less,and the strength of the joint is better;The grain growth of the aluminum alloy fusion zone is coarse columnar crystals,and the fusion zone presents a wide top and bottom narrow morphology;When ER4043 welding wire is used to fill,the groove form is type III,and the welding speed is 0.017 m/s,the thickness of the fusion zone of the base aluminum alloy is small,and the microhardness test results show that : the hardness of the aluminum alloy fusion zone is between the aluminum alloy base metal and the weld;Part of the α-phase on the upper surface of the titanium alloy is transformed into the β-phase,when the welding speed is 0.005m/s,the solid phase transformation zone of titanium alloy is large.The tensile results of welded joint show that the elongation of welded joint decreases slightly and the strength difference is small.By observing the microstructure morphology of titanium-aluminum welded joint,it is found that the joint has two bonding mechanisms: fusion welding and brazing.The concrete performance is: after the melting of the base aluminum alloy and the welding wire to form a fusion welding interface,and the layer of titanium alloy to form a brazing interface;Due to the uneven distribution of welding heat input,the upper surface of titanium alloy and the inclined surface at the groove react with the molten welding wire.There exist both fusion welding interface and brazing interface,and a complex interfacial reaction layer is formed in the ti-Al transition zone.When the welding speed is 0.005 m /s,the filler wire is ER4043 and type Ⅲgroove,the interface thickness of the ti-Al reaction layer is large,and the strength of the weld joint is low and the mechanical properties are poor.The microstructure observation and element distribution analysis of the interface reaction layer in the ti/Al transition zone were carried out by energy dispersive spectrometer.The results show that: the morphology of ti-Al transition interface is smooth near the titanium side and serrated or rod-shaped near the weld side.Ti and Al elements near the interface have obvious gradient changes and element diffusion.Intermetallic compounds such as Ti Al3,Ti Al,Ti3 Al and Ti5Si3 are formed in the transition interface,and the "uphill diffusion" phenomenon of Si element appears in the interface.The three-dimensional finite element model of TA1/Al3003 composite plate was established,and the welding residual stress was simulated by finite element software.The temperature field results of the welded structure model at different welding speeds were calculated and compared with the actual welded joint morphology to verify the reliability of the calculated results.At the same time,the welding residual stress and welding deformation distribution law of the composite plate were obtained,and the results showed that the welding residual stress did not exceed the material yield strength;The residual stress distribution after welding of different welding speeds and homogeneous metal plates was compared,and the results showed that: when the welding speed is 0.005 m/s,the welding residual stress is larger,and the distribution of residual stress in ti/Al composite plate is more complicated than that in homogeneous metal plate.The three-dimensional finite element model of the levitation bogie of the maglev train is established.The material of the crossbeam and longitudinal beam of the levitation bogie is titanium/aluminum composite plate,and the static strength and modal analysis of the frame are carried out according to the load under specific working conditions.The results show that:under the six abnormal load conditions,the maximum stress position of the levitation bogie is the titanium metal layer of the beam near the air spring mounting seat,and the maximum stress value is 145.9 MPa,which is far less than the allowable stress of the material and meets the static strength requirements;The levitation bogie has rich vibration pattern,good overall stiffness,and the train has a strong ability to adapt to track unevenness.Thus,the application possibility of titanium/aluminum composite plate in railway vehicles is verified. |
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