Research On Stress Corrosion Cracking Behaviour Of Friction Stir Welded 40mm-thick 7B05-T5 Aluminum Alloy

In recent years,high speed railway has been developed quickly.Taking into account the energy saving and speed raising safely,lightweight of train body and application of aluminum alloy train body have become a trend of development.5xxx(Al-Mg),6xxx(Al-Mg-Si)and 7xxx(Al-Mg-Zn)are the main applications of the aluminum alloy in high speed train body,especially 7xxx series(Al-Zn-Mg)Aluminum alloy.Because of its high strength,good extrusion performance and good welding performance,it has become one of the important materials for the welding structure of high-speed trains.While in the welding of aluminum alloy train body,the friction stir welding avoids many disadvantages of fusion welding,such as the welding deformation,porosity and hot cracking and so on.Therefore in the welding of aluminum alloy train body,the friction stir welding have the trend of replacing fusion welding.However,the microstructure and the property of the friction welded joints are not uniform along the thickness direction,which not only affects the use of the alloy structure performance and carrying capacity,but also reduces the service life of aluminum alloy components.Therefore,the researches of the stress corrosion behavior of aluminum alloy and its correlation with the microstructure have important practical significance.In this study,to the upper and central specimens from 40mm thick 7B05-T5 aluminum alloy of friction stir welding,the slow strain rate tensile test and four-point-bending test are carried out to test the stress corrosion performance.Using the methods of metallographic observation,SEM,EDS,microhardness measurement and tensile performance test to research the microstructure and properties of welded joints.And the main conclusions are coming as follows:1.The microhardness of weld cross section was high-low-higher-lower-high "W" shaped distribution feature.The hardness of the base metal was the highest in both sides.The hardness of the heat affected zone was obviously reduced,and appeared the softening zone.The hardness of the thermo mechanically affected zone and nugget zone had risen.2.The four point bending stress corrosion specimens all happened stress corrosion cracking.For the upper surface of sample,welding center on both side of 10mm to 25mm between thermo mechanically affected zone and heat affected zone corrosion observed intensive pitting corrosion characteristics.The pitting pit was arranged along the rolling direction of the base metal.Among them,the transition zone of back side of the TMAZ and HAZ had relatively serious corrosion.The deepest depth was 32.4?m.For the center layer of sample,between the weld centers of 4.5mm to 5.8mm,the corrosion of one side was obvious.Obvious corrosion furrows had been formation along the rolling direction of base metal.The corrosion depth was 22.9?m.This area was located in the junction of the after weld bead and the front weld bead.The sensitivity of the stress corrosion was higher,which was related to the microstructure and the morphology of the second phase particles;3.The results of SSRT showed that in 25? 3.5%NaCl solution,ISSRT of the upper surface of sample was 0.034,ISSRT of the center layer of the sample was 0.092.The stress corrosion sensitivity was low.In 50?3.5%NaCl solution,ISSRT of the upper surface of sample was 0.133.ISSRT of the center layer of the sample was 0.181.It showed a certain stress corrosion tendency.Compared with the surface sample,the stress corrosion sensitivity of the central layer sample was higher.4.The electrochemistry corrosion test results show that the transition zone of the TMAZ and HAZ on the upper surface weld center of sample,the electrochemical corrosion tendency was higher than that of the base metal and weld nugget zone and heat affected zone of softening zone.Compared with the forward side of the TMAZ and HAZ of the transition zone,the tendency of electrochemical corrosion was higher at the backward side of the TMAZ and HAZ of the transition zone.Compared with the upper part of the weld nugget zone,the electrochemical corrosion tendency was higher in the junction of the front weld bead and the back weld bead.5.The nanometer scale and distribution of the grain boundary precipitates are the main controlling factors for the stress corrosion susceptibility.The results of TEM analysis for the surface of the weld precipitates are indicated:The thermo mechanically affected zone was observed around 30nm continuous distribution at grain boundary precipitates phase.The dissolution and reprecipitation of the precipitates occurred in the welding zone.In the crystal and the grain boundaries,had not observed precipitate phase.The HAZ which distance from the weld center was about 30mm,had observed the precipitate phase of around 40mm was not continuous distributed in the grain boundary.The size of the grain boundary in the base mental zone was about 60nm,and the spacing was large.The fundamental reason of the higher stress corrosion tendency of the TMAZ is the tiny grain boundary precipitates and continuous distribution.