Study On Microstructure And Toughening Of TIG Welded Joints Of Spray Formed 7055 Aluminum Alloy By Y-modified TiB₂ Particle Composite Welding Wire

Spray forming 7055 aluminum alloy is widely used in aerospace and other fields because of its light quality and high specific strength.However,its application is limited by its high sensitivity of hot cracking and low strength and plasticity of melting joint.The welding joint with traditional welding wire has high thermal crack sensitivity and mechanical property is much lower than that of base metal.Although the joint with new ceramic particle reinforced composite welding wire can effectively improve the tensile strength,its elongation is low,usually not more than 3%,which is difficult to meet the engineering application.Therefore,it becomes the key to develop new welding materials to improve joint elongation.In this study,TiB₂particles were selected as the reinforcing phase.To promote the uniform distribution of particles and improve their morphology,rare earth element Y was selected.Four kinds of modified TiB₂particle reinforced composite welding wires with different Y contents（0,0.1,0.3,0.5 wt.%）were prepared by in-situ generation method and electromagnetic stirring auxiliary means.The effects of rare earth element Y on the microstructure and mechanical properties of 7055 aluminum alloy welded by spray forming TIG welding were studied.The main results are as follows:（1）Effect of rare earth element Y on microstructure of welding materials and welds.The microstructure of the composite welding material and weld with only TiB₂ceramic particles is equiaxed,and the grain is obviously refined by adding an appropriate amount of rare earth element Y.The main structure of 0 Y welding material and weld is composed ofα-Al,Mg Zn₂,Al₂Cu,Al₂Cu Mg and TiB₂.After the addition of rare earth element Y,bright white Al₃Y hard phase distributed near TiB₂particles and grain boundaries appeared in the welding material and weld microstructure.Y can significantly improve the morphology and distribution of TiB₂particles,effectively reduce the agglomeration size of TiB₂particles,promote more uniform distribution of TiB₂particles,and at the same time,blunt the sharp edges and corners.（2）Effect of rare earth element Y on mechanical properties of welded joints.In the welded state,the tensile strength and elongation of 0 Y-7xxx welded joint reach 295 MPa and 4.92%,respectively.After the introduction of Y,the mechanical properties of the welded joint are improved,among which the tensile strength and elongation of 0.3 Y-7xxx joint are 333±5 MPa and 6.52±0.24%.Compared with 0 Y-7xxx,the fracture was increased by 12.88%and 32.52%,and all the joints with Y added showed ductile fracture.The strengthening and toughening mechanisms of welded joints are mainly fine grain strengthening,Load bearing strengthening and Zener nailing strengthening.（3）Effect of rare earth element Y on microstructure and mechanical properties of joints under heat treatment.Rare earth element Y can accelerate the precipitation behavior of weld precipitates during solution and aging and obtain more uniform and dispersedη’phase and GPⅡzone,and narrow precipitation-free zone at grain boundaries.Compared with isothermal aging,non-isothermal aging not only can obtain better strength and toughness,but also the aging time is much lower than that of isothermal aging.The tensile strength of0.3Y-7xxx joint reached 581±11 MPa under non-isothermal aging,which was higher than569±14 MPa under isothermal aging.The elongation was 5.15±0.22%.The strengthening mechanisms of heat-treated joints are fine grain strengthening,second phase strengthening,precipitation strengthening and thermal expansion coefficient strengthening.