| The aluminum/titanium dissimilar metal composite structure can fully exert the excellent performance of dissimilar materials,and has wide applications in the fields of aviation,aerospace,microelectronics and the like.However,due to differences in physical,chemical and metallurgical properties of dissimilar metals,the difficulty of welding is increased.Vacuum electron beam welding has high energy density,small heat-affected zone,and accurate heat input.It is one of the ideal welding methods for dissimilar metals.Therefore,this paper focuses on electron beam lap joint and butt joint test of dissimilar thickness aluminum/titanium dissimilar metal sheets,and analyzes the welding process and mechanism.For the lap joint,the 3 mm thick 7075 aluminum alloy and the 1 mm thick TC4 titanium alloy are used as the base material,and the aluminum plate is placed on the upper and the titanium plate under the lap joint.The best welding parameters are obtained by analyzing the causes of defects such as pores,cracks,shrinkage cavities,and unfusion during the welding process.Observing the microstructure of the joint,it is found that the microstructure of the aluminum alloy weld is mainly composed of columnar crystal and equiaxed crystal structure,and a small amount of fine crystal structure is formed.The a phase and ?' phase of needle-like or lamellar phase are precipitated in the weld of titanium alloy.The analysis interface found that as the heat input increased,the interdiffusion distance of the interface at the interface increased,and the interface thickness increased.When the heat input amount is small,a jagged intermetallic compound is formed at the interface.After increasing the heat input amount,the intermetallic compound grows into a cell shape by zigzag.As the heat input amount continues to increase,the intermetallic compound finally grows into a rod-like structure.Its main components are TiAl2,TiAl3 and TiAl.The mechanism study shows that the pre-intermetallic formation rate is mainly affected by its own reaction rate,and it is mainly affected by the diffusion rate of aluminum and titanium in the later stage.For the butt joint test of 3 mm thick 7075 aluminum alloy and 2 mm thick TA1 titanium alloy dissimilar metal,the conventional fusion beam welding head is not ideal.Electron beam scanning welding with added scanning parameters results in an ideal fusion welded joint.Studies have shown that electron beam scanning welding has a modification effect on the surface of the weld,and the fish scales on the surface of the weld are finer and smoother.Moreover,the weld bead is improved to increase the width of the aluminum alloy weld seam,refine the aluminum alloy weld bead,and reduce the width of the heat affected zone of the titanium alloy.Conventional electron beam welding heat source is more concentrated,the element diffusion distance is far,the island structure appears in the upper part of the joint,a small amount of lamellar intermetallic compound is formed in the middle part,and intermittent intermetallic compound appears in the lower part.The electron beam scanning welding heat source is more dispersed,the element diffusion distance is closer,and the energy distribution of the upper and lower interfaces of the welded joint is relatively uniform,which prevents the formation of the upper island structure and promotes the formation of a lamellar and uniform thickness intermetallic compound such as TiAl.According to the tensile strength test results,it is found that the conventional electron beam welded joint has three fracture modes:weld fracture,mixed fracture and interface fracture.The welded joint of electron beam scan has two fractures at the weld and fracture at the interface.In the fracture mode,the average tensile strength of the conventional electron beam welded joint is 189.20 MPa,which can reach 36%of the aluminum alloy base material,and the average tensile strength of the electron beam scanning welded joint is 259.25 MPa,which can reach the aluminum alloy base material 49%. |
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