| With the development of aviation technology in our country,large size,lightweight and high performance are becoming more and more important in the new aviation design concept.Integrated manufacturing technology of large titanium alloy structural parts has become a key technology in the aviation field.At present,in the field of aviation,the integral manufacturing technology is generally used in the frame,beam,landing gear and other parts of aircraft structural parts.The commonly used methods are welding+numerical control processing,integral forging+numerical control processing and material adding manufacturing+numerical control processing.Welding is more and more used in large aircraft structures because of its simple operation process and short cycle.The field of parts manufacturing.For thick plate titanium alloy,the gradient distribution of temperature during welding process results in the gradient inhomogeneity of the microstructures and mechanical properties of welded joints.This inhomogeneity has an important influence on the fracture failure of welded joints.Therefore,it is of great significance to study the manufacturing technology of TC18 titanium alloy thick plate by electron beam welding.In this paper,the relationship between the morphology,microstructure and mechanical properties of electron beam welded joint of 15mm TC18 titanium alloy thick plate was studied by means of optical microscopy(OM),scanning electron microscopy(SEM)and transmission electron microscopy(TEM),and the effect of welding process on the structure and properties of weld was analyzed.The inhomogeneity of the structure and properties along the thickness of the plate was discussed,and the mechanism of the inhomogeneity of the structure and properties was revealed.The welding process was simulated by the finite element method,and the parts where the welding stress might lead to the failure were predicted,which laid a theoretical foundation for the subsequent better guidance of the control of the structure and properties of the welded joint.The morphology,microstructure analysis and formation mechanism of TC18titanium alloy thick plate welded joint by electron beam welding show that under the conditions of acceleration voltage 150k V,welding speed 30mm/s,focusing current2085m A,electron beam 23m A,the thick plate of TC18 titanium alloy welded by electron beam is well formed,uniform and smooth,and the upper width is narrow under the condition of"nail shape".In the welding process,due to the large temperature gradient along the plate thickness direction,the microstructure and properties are uneven from top to bottom,the upper heat input is more,the holding time is long,the grain is fully grown,and its size is relatively large.On the contrary,the grain size is relatively small.The weld zone is transformed from the original isometric primary?and secondary?to?phase at first,and then with the departure of heat source,the temperature decreases,the?phase changes to fine needle or flake secondary?phase,and at the same time,there are some residual?phase.With the increase of acceleration voltage,the penetration depth and melting width of weld zone increase obviously,and the grain size of joint increases,and the width of heat affected zone widens.On the contrary,with the increase of welding speed,the upper melting width,middle melting width,lower melting width and penetration depth of the welded joint decrease obviously,and the grain size of the weld fusion zone decreases gradually.The hardness,tensile properties and hardening behavior of TC18 titanium alloy thick plate welded joint by electron beam welding show that the microhardness of TC18 titanium alloy thick plate electron beam welded joint is symmetrical.From the base metal to the weld zone,the microhardness decreases gradually.Compared with the base metal,the weld zone softens obviously,and the width of the softened zone decreases gradually along the direction of plate thickness.In addition,compared with the base metal,the strength of the welded joint is lower and the hardening ability is enhanced,and the strength and hardening ability of the welded joint are obviously enhanced from top to bottom along the direction of plate thickness,and the strength of the lower slice is about 10%higher than that of the upper part.With the increase of strain rate,it is from the top along the direction of plate thickness.The strength of the lower joint is gradually enhanced and the hardening ability is gradually reduced.With the increase of welding speed,the strength and hardening ability of the joint are gradually enhanced.With the increase of electron beam current,the yield strength and tensile strength of the joint increase at first and then decrease,but the extensibility is the opposite.With the increase of focusing current,the yield strength,the tensile strength and the extensibility of the joint show an upward trend.The high cycle fatigue properties of TC18 titanium alloy thick plate welded joints by electron beam welding are systematically studied.The mechanism of fatigue fracture is analyzed.The effects of welding speed and accelerating voltage on the fatigue properties of the joints are discussed.The results showed that along the thickness direction,the fatigue limits of the upper,middle and lower layers of TC18welded joints are 387(+0.95 MPa),426(+0.9 MPa)and 435(+1.67 MPa),respectively.Gradually,the fatigue fracture of the three-layer slices consisted of obvious fatigue crack initiation zone,propagation zone and instantaneous fracture zone,and the crack originated from the surface of the specimen.Under the conditions of strain rate of 10-2s-1,electron beam current of 23 m A and focusing current of 2085m A,the acceleration voltage of 150 k V is fixed,the welding speed is increased,and the fatigue property of the joint is gradually enhanced.Fixed welding speed of30mm/s,increased acceleration voltage,the fatigue performance of the joint decreased slightly.Based on the finite element analysis platform,the finite element model of TC18titanium alloy thick plate electron beam welding is established.Through the analysis of the temperature field and stress field evolution of the welded joint,the possible fracture position of the joint is predicted.The research results show that the side temperature field of the workpiece is a typical nail shape with upper width and lower narrow,which is in good agreement with the actual weld shape,which proves that the welding temperature field model can accurately reproduce the actual welding process.The stress occurs in and around the weld,the tensile stress is near the weld,and the compressive stress is far from the center of the weld.The maximum principal stress is distributed in and near the weld,which indicates that these areas are weak areas of welding parts and easy to produce.Cracks and other means of failure. |
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