| With the development of lightweight and high-speed vehicles,the role and status of high-strength aluminum alloy welding technology are increasingly prominent,which has been widely used in high-speed trains,aerospace,ships,large aircraft and automobile manufacturing and other fields.A7204 aluminum alloy belongs to al-zn-mg high-strength aluminum alloy which can be strengthened by heat treatment.It has the advantages of good extrusion performance,excellent welding performance and high quality of post-welding welding seam.It is widely used in structural materials of high-speed trains.Laser welding has the advantages of high energy density,fast welding speed,narrow post-welding heat affected zone,etc.However,during laser deep fusion welding of A7204 aluminum alloy,due to the instability of keyhole,it is easy to cause weld joint softening and serious porosity problems.The optical fiber laser-melt variable polarity of inert gas shielded welding(VPTIG)hybrid wire filling welding,can make full use of VPTIG positive and reverse current value and duty ratio is adjustable,the greatest degree of strengthening cathode atomizing clean surface oxidation film,further reducing the welding heat input at the same time,improve the mechanical properties of weld and increase joint,sure to the prominent comparative advantage in aluminum alloy welding.In aluminum alloy wire filling welding,the composition of weld metal determines the structure and performance of the joint.Through reasonable selection of welding wire and then adjusting the structure and performance of the joint,it becomes an effective means to further improve the welding quality of aluminum alloy car body.By filling ER5087,ER5356 and ER4047 respectively,the microstructure and mechanical properties of hybrid welded joints under different filling wire conditions were compared and analyzed,and the joint softening behavior and mechanism were studied.The results show that ER5087 wire microstructure is the smallest,and the dendrite is relatively fine,followed by the ER5356 welding seam,and the microstructure of the ER4047 welding seam is relatively developed and thick.After 90 days of natural aging,there was no significant difference in the tensile properties of the three filler wire joints,and most of them were fractured in the heat-affected zone near the parent metal.There are obvious aging hardening zone and recrystallization softening zone in heat affected zone,and natural aging has obvious effect on softening in heat affected zone.The results of transmission electron microscopy(TEM)show that the decrease of precipitate and coarsening in the softening zone of recrystallization is the main reason of joint softening.The addition of three kinds of welding wires makes the overall mechanical properties of the welding seams not change much.Considering that among all kinds of micro-alloying elements of metal materials,Nb element has a significant effect.At the same time,Nb is a solid solution strengthening and inhalation hydrogen fixing element,and niobium strengthening mainly consists of fine crystal strengthening and dispersion strengthening.Therefore,niobium microalloying was considered on the basis of the original welding seam.Fiber laser-vptig hybrid filaments were used to weld the A7204 aluminum alloy with A thickness of 6 mm.The micro-alloying of the welding seam was achieved by presetting niobium foil of different thickness on the butt surface of the test plate to be welded.When the laser power was 9 kW,the welding current was 200 A,the welding speed and wire feeding speed were 6 m/min,the welded joint with good internal defection was obtained.The cross section microstructure of the weld without niobium microalloying was fine equiaxed crystal,columnar crystal and equiaxed dendrite.After adding niobium,the porosity of welded joint decreased obviously,columnar crystal and secondary dendrite disappeared in the center of welding seam,and fine equiaxed crystal and grain size were presented.Comparison of microhardness in weld zone between laser-vptig and the welded joints after microalloying of 0.025 mm and 0.01 mm thick niobium foil shows a state of gradual increase,and the softening phenomenon is obviously weakened.By comparing the tensile strength of the four groups of joints,it can be seen that the welded joints of niobium micro-alloying aluminum alloy can significantly improve the tensile strength,yield strength and elongation.After 60 days of natural aging,the tensile strength reaches 92% of the base metal,and the yield strength is about 91% ofthe base metal.It can be seen that the microstructure and mechanical properties of niobium microalloyed aluminum alloy are improved obviously after welding. |
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