| 7N01 aluminum alloy has a good comprehensive performance,which is widely used in high-speed trains,subway and other rail transit vehicles.At present,traditional MIG welding is still the main welding method of aluminum alloy car body.However,it is easy to form porosity.And the grain boundary liquefied or liquefied crack is easy to occur in the fusion zone or the multi-layer welding layer of the joint part.Friction stir welding(FSW)is a new kind of solid phase connection technology,which can avoid the defects of melting welding to a certain extent.But as a result of friction stir welding joint in the process of each region has experienced different degrees of thermal-mechanical coupling effect that leads to the microstructure features change of gradient from the parent metal to stir zone.Such microstructure characteristics make welding joint show different corrosion resistance in corrosion environment.This research conduct the corrosion behavior and the microstructure characteristics of 7N01-T5 aluminum alloy FSW joint used in the high-speed train car body pillow beam.Reveal the key factors that affect corrosion behavior.It is of great significance to the development of effective measures to improve the corrosion resistance of joints.In this paper,the local corrosion behavior of 7N01-T5 aluminum alloy FSW joint was studied by means of intergranular corrosion and salt spray corrosion test.Intergranular corrosion experiment showed that the heat affected zone of joint corrosion degree was the most serious,following was forward and backward side TMAZ,the stire zone and base metal was not observed intergranular corrosion characteristic.The corrosion degree of upper and lower layers of the heat affected zone was more serious than the center layer of heat affected zone.In contrast,the corrosion degree of the upper layer,the central layer and the lower layer of TMAZ was increased in turn.Salt spray corrosion experiment showed that the HAZ of plate existed obvious erosion phenomenon,the TMAZ,SZ and BM of plate only found small amount of pitting corrosion character.The results showed that the corrosion resistance of 7N01-T5 aluminum FSW joint arrange in order from high to low,BM≈SZ>TMAZ>HAZ.In addition,the stress corrosion behavior of 7N01-T5 aluminum alloy FSW joint was studied by means of slow strain rate tension and four point bending test.Stress corrosion cracking was not occurred in four point bending stress corrosion samples.The typical characteristics of pitting corrosion were observed in HAZ center of tensile surface,which arranged along the extrusion direction of the parent metal.Especially in the area of HAZ near the sample edge,pitting corrosion transformed into corrosion furrows in the direction of the parent metal extrusion.Because the cross-section of sample did not isolate with corrosion medium in Stress corrosion experiment,which led to the formation and extension of corrosion became easier in the direction of the parent metal extrusion.In 25℃3.5%NaCl solution,the stress corrosion sensitivity of the upper and lower surface layers under slow strain rate tensile test specimens was low.ISSRT were 0.048 and 0.014 respectively.Due to parallel section of upper surface specimen only covered the sitr zone.The fracture position was located in the stir zone.Lower surface specimen covered the SZ,TMAZ and HAZ.The fracture position was located in the forward side of the heat affected zone.In 50℃ 3.5%NaCl solution,the stress corrosion sensitivity of the upper and lower surface layers under slow strain rate tensile test specimens was high.ISSRT were 0.158 and 0.144 respectively.The upper surface specimen was fractured in outside gauge.The location was the heat affected zone of the advancing side,and the lower surface of the specimen was broken in the heat affected zone of the advancing side.As we all know,in the friction stir welding process,precipitation hardening type Aluminum Alloy have different levels of strengthening phase particles dissolution,precipitation and coarsening process in the SZ,TMAZ,and HAZ.There are 30～50 nm short rod-shaped η phase on the BM grain boundary,which size is small and spacing is wide.Small n’ phase intensive distribute inside the grain.Due to the influence of welding thermal cycle in HAZ,η phase on the grain boundary start to coarse which size range from 40～100 nm.Inside the grain η’phase aggregated and grew up.Due to experience higher peak temperature in TMAZ that is closed to HAZ,η phase on the grain boundary and inside the grain η’phase dissolved.Grain boundary still remains a small amount of rod-shaped ηphase which size is 60～70 nm.The stir zone experienced severe plastic deformation and temperature rising.Strengthening phase particles on grain boundary and inside grain has obviously dissolved.The electrochemical corrosion experiment results show that SZ and BM have high corrosion potential and low corrosion current density.The HAZ has low corrosion potential and high corrosion current density.This is accorded with morphology and distribution of strengthen the particles.So,the morphology and distribution of particles inside the grain and on grain boundary are the key factors to influence the corrosion sensitivity.Moreover,the microstructure characteristics of α-Al matrix features are also main factors influencing the corrosion sensitivity.The grains in the heat affected zone far away from the TMAZ/HAZ interface still had the characteristics of fiber shape.These grain shapes are easyly to produce exfoliation corrosion.The grains of HAZ near the TMAZ/HAZ interface are equiaxed and bulky.These are prone to lead to local intergranular corrosion.The grains in the heat-affected zone are elongated along the plastic flow direction and thermally-induced by dynamic coupling.If corrosion occurs,the microcracks are easy to propagate in parallel to the elongation direction.The dynamic recrystallization of the stirring zone occurs under the strong thermo-mechanical coupling.Microstructure shows equiaxed crystal characteristics,if the occurrence of corrosion,along the crystal corrosion tendency is higher. |
PDF Full Text Request