A Research On Application Of Microalloyed Al-Mg Welding Wire In Welding Of 7075 Aluminum Alloy

7×××series aluminum alloy is one heat treatable alloy,which is widely used in aerospace,marine and automotive structural parts.However,welded joints are difficult to keep high strength and excellent corrosion resistance after welding with traditional commercial welding wire.The post weld heat treatment process is a feasible way to strengthen welded joints,but hard to achieve in engineering applications.In general,the composition is an important factor to determine the structure and micrograph of the material.The rare earth element Sc is the most effective element for refining the microstructure of the aluminum alloy.In addition,the addition of trace amounts of Zr can not only reduce the effective addition amount of Sc to save cost,but also further improve the comprehensive performance of aluminum alloy.In this paper,some rare earth elements,Sc and Er etc.were added into aluminum alloy welding wire,in order to improve the mechanical properties of welded joints.To examine the welding wires,TIG welding of 7075-T651 aluminum alloy sheet was performed.The welded joints were researched by metallographic microscope,universal electronic stretching machine,Vickers hardness tester,X-ray diffraction?XRD?,field emission scanning electron microscopy?SEM?and transmission electron microscopy?TEM?analysis technology.The main conclusions are listed as following.?1?For the Al-Mg welding wires with different content of Zn,the grain in the welded joint were mainly equiaxed crystal,and the grain size was refined slightly.The addition of 1.0wt.%Zn is the best,but the effect is not remarkable.?2?With the addition of Sc,Zr and Er elements,the mechanical properties of welded joints were slightly improved,the addition of 0.3wt.%Sc and 0.15wt.%Zr is the best proportion.The ultimate tensile strength and welded joint coefficient reached to 334 MPa and 0.60.?3?After 150 days of natural aging,the mechanical properties of the welded joints were greatly improved.The ultimate tensile strength of the welded joints with the welding wire containing 0.3wt.%Sc and 0.15.wt%Zr had a value of 409 MPa.The welded joint coefficient was 0.74,which is better than welded joints with traditional commercial welding wire.?4?After post weld heat treatment?470°C×1h+water quenching+120°C×12h+air cooling?,the mechanical properties of the welded joint were further improved.The ultimate tensile strength of the welded joints with welding wire containing 0.3wt.%Sc and 0.15wt.%Zr increased to 520 MPa,and welded joint coefficient became 0.94.A large number of elements were transfered to the welded metal zone,and the precipitation of MgZn₂ phase and secondary Al₃?Sc,Zr?particles strengthened the welded metal zone.The fracture positions of the tensile samples changed from the welded metal zone to the fusion zone.