Microstructure And Properties Of 7N01 Aluminum FSW Joint After MIG Welding

At present,the friction stir welding(hereinafter referred to as FSW)process has become one of the main welding methods for aluminum alloy car bodies.However,there are no more mature repairs in the process of welding defects and post-production or service damage method.In order to meet the repairable requirements of FSW welded joints,we studied the agitation zone(shoulder affected zone and root)of 14 mm thick 7N01-T5 aluminum alloy FSW joints by using melt inert gas arc welding(hereinafter referred to as MIG)Repaired and repaired.The tensile,fatigue,corrosion and microstructure characteristics of repaired joints were studied systematically to explore the feasibility of MIG welding repair technology for FSW joint defects.In order to establish a mature and reliable FSW weld Welding repair technology to provide experimental basis.The test results of microhardness surface distribution of MIG welding joints show that the weld zone of MIG welding is a typical as-cast structure with low hardness between 60-75 HV;the hardness of the zone affected by the mixing zone and heat engine is between 100-115 HV,which is about 5-10 HV higher than that before welding,which is related to the aging strengthening in the zone caused by repairing welding.The width of softening zone at about 30 mm from the center of the stirring zone obviously increases,and its hardness value is about 85 HV,Which is generally lower than about 5 HV before welding,which is related to the intergranular η phase and intragranular η’ phase coarsening caused by repairing welding and the increase of non-precipitation band width in grain boundary.The test results of the mechanical properties of MIG welded joints show that the average tensile strength of the welded joint after welding is 306.5 MPa,the average yield strength is 212.7 MPa,the average elongation is 8.97%and the joint efficiency is 88%,It is slightly lower and the fracture location changes from the heat affected zone of the pre-weld FSW joint to the MIG weld zone,which is consistent with the lowest hardness zone determined by the hardness distribution test.The fatigue strength of the MIG weld repair specimen is 53.97 MPa,The fatigue strength is significantly lower than that of the repaired welding,and most of the fatigue crack sources are located in the root defect area of the MIG weld repairing weld zone.The results of salt spray corrosion test of MIG welding joint showed that only a small amount of pitting corrosion was observed in MIG weld zone,stirring zone,heat affected zone and base metal zone after repaired welding,which is consistent with the observation and analysis of the characteristics of the second phase particles The result is the same.There is obvious erosion in the heat affected zone about 12 mm away from the weld center due to salt spray corrosion test.The range of serious intergranular corrosion increased 6-8 mm compared with that before welding,and the grain boundary and The results of TEM observation show that the continuous precipitation of η phase on the grain boundary is the fundamental reason for the intergranular corrosion in this region.Four-point bending stress corrosion test was carried out in the 3.5%NaCl solution on the specimens taken from the upper surface of the MIG weld joint.The experimental results showed that no stress corrosion cracking occurred in the specimens and only pitting corrosion was observed in the MIG weld zone.MIG quenching zone on both sides of the seam area due to the composition and organization of the weld zone occurs macroscopic galvanic corrosion resulting in a serious degree of corrosion.A large area of pitting corrosion was observed in the MIG obsolete zone in the agitated zone.TEM observation of the retrograded MIG obsolete zone showed that the η phase precipitated continuously on the grain boundary was the cause of the serious corrosion in this zone.The influence of heat engine The characteristics of the corrosion trenches,typical of the extrusion direction along the profiles,were observed in the zone and the heat-affected zone.This phenomenon was related to the grain-boundary properties of the α-Al matrix and the continuous precipitation of the η phase in the zone along the grain boundary.Pour pit along the extrusion direction of the parent material.In summary,14 mm thick 7N01-T5 FSW aluminum alloy FSW joints in the upper and lower MIG welding,corrosion resistance of welded joints in all zones from high to low order is as follows:Base metal area ≈ MIG weld zone≈Forward MIG aging zone(stirring zone)>Forward MIG quenching zone(stirring zone)≈Rearward MIG quenching zone(stirring zone)>Rearward MIG aging zone(stirring zone)>Heat affected zone>Heat affected zone.