Investigation On The Process And Microstructure Properties Of The Double Wire MIG Welded For 7N01 High Strength Aluminum Alloy

As the most suitable material for modern high-speed rail in China and the most suitable material for welded structural parts,7N01 high-strength aluminum alloy has good extrusion resistance and excellent welding performance.7N01 aluminum alloy is the main AI-Zn-Mg aluminum alloy with additive elements of Zn and Mg.The elements of Zn and Mg are dissolved in the Al matrix to form the main strengthening phase equilibrium phaseη（Mg₂Zn）,non-equilibrium phaseη（Mg₂Zn）and T phase（Al₂Mg₂Zn₃）.With the development of industry,traditional single MIG welding technology has been difficult to meet the needs of industrial production.Based on single MIG welding technology,a new welding technology such as double wire MIG has been developed.The double wire MIG welding technology has the advantages of low cost,easy operation and high deposition rate.Therefore,it has an important potential in improving production efficiency.As a new type of arc welding technology suitable for aluminum alloy welding,the double wire MIG welding has not only high efficient but also good joint quality.In this paper,the droplet transfer mode,microstructure and mechanical properties of 7N01 high strength aluminum alloy welded by double wire MIG welding were analyzed.The experimental results show that when the phase angle is 180°different from the same frequency,the interval between adjacent pulses is very uniform,and there is no phenomenon of loss and overlap.The two arcs are alternately undergoing pulse droplet transfer.The interference between the two arcs is reduced effectively.The weld is well formed when the welding process is a transition form of one pulse and one drop.It is found that the weld microstructure of single wire MIG welding and double wire MIG welding is composed of coarse dendrite and a few equiaxed crystals.There areη-Mg₂Zn strengthening phases andα（Al）+η-Mg₂Zn（+AlFeMnSi）binary and ternary eutectic structures in and between dendrites.The tensile strength of single wire welding joint is 290 MPa,the elongation after break is4.7%.The tensile strength of double wire welding joint is 289.4 MPa,the elongation after break is 4.7%.The difference between double wire welding joint and single wire is not different,and the breaking position occurs in the weld zone.After 180°positive bending and back bending test,the double wire welding joint has no damage phenomenon and shows good bending resistance.The fatigue limit of double wire MIG welded joint can reach 140 MPa.