Microstructure And Properties Of Welded Joint Of7020Alloy For Vehicles

7020aluminium alloy is widely used in aircraft, ships, vehicles and other transportations due to its moderate strength, excellent thermal deformation properties and stress corrosion resistance and good weldability. The alloy thick plate(18mm) was welded by the method of MIG(Metal Inert Gas) weld, with the ER5183welding wire and ER5356welding wire respectively. The mechanical properties and microstructures of the7020alloy thick plate and its welded joints were investigated by optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), texture test and mechanical properties test. The main results of the study are as follows:(1) The longitudinal tensile strength was393MPa at room temperature,413MPa at-20℃and427MPa at-40℃respectively,and the elongation was15.1%,15.2%and15.4%responsibly. The transverse tensile strength was375MPa,401MPa and413MPa respectively,and the elongation was19.5%、20.1%and20.5%responsibly.(2) There existed planar anisotropy in the base metal whose texture was mainly consisted of B{110}<112>, S{124}<211>, C{112}<111>.(3) All welded joints without ribs were fractured in the weld zone. The tensile strength and elongation of the joint with ER5183were257MPa and3.8%, while the ones of the joint with ER5356were225MPa and4.2%.(4) All welded joints with ribs were fractured along the fusion zone. The tensile strength and elongation of the joint with ER5183were322MPa and4.3%, while the ones of the joint with ER5356were275MPa and4.9%.(5) The welded joints are composed of welded zone, fusion zone, heat affection zone and base metal zone. The heat affection zone could be divided into two zones, quenching zone and over-aging zone. The welded zones of two welded joints are characterized by dendritic structure. The fusion zone of the joint with ER5183is characterized by columnar grains and equiaxed crystal, while there is only columnar grains in the joint with ER5356. The fibrous organization are found in the heat affected zone and base metal zone of the two kinds of joints. Some recrystallizations occur in the quenching zone near the fusion zone, and η’ phase in over-aging zone is more coarse than ones in base metal zone.(6) For both joints, the corrosion resistance of the heat affected zone, base metal zone, welded zone increased sequentially.