Microstructure,mechanical Properties And Corrosion Resistance Of Friction Stir Welding Joints Of 6005A Aluminum Alloy

The heat treatment strengthened 6005A aluminum alloy with high strength,good extrusion performance and excellent corrosion resistance is the main material for high-speed train-body manufacturing in China.As the welding structure occupies a very core position in the production and manufacturing of high-speed trains,the welding technology of aluminum alloy is closely interrelated with the service quality,service life and safe operation of rail vehicles.Friction stir welding(FSW)can avoid a series of metallurgical problems existing in traditional welding methods,and is considered to be a potential welding technology.In this paper,the macrostructure and microstructure of 6005A alloy FSW joints with various welding parameters are characterized,and the influence mechanism of microstructure on fatigue property and corrosion behavior of joints is studied emphatically.Finally,the range of welding parameters with good comprehensive performance is obtained In addition,6005A/7N01 dissimilar aluminum alloy FSW joints are carried out to explore the effect of materials placement order on the properties.The main contents and results are as follows:The effect of welding parameters on the microstructure of 6005A aluminum alloy FSW joint is studied.The grain structure of base materials(BM)is equiaxed,and the acicular ?"phases are arranged in the grains in quantity.The welding nugget zone(WNZ)is recrystallized,and the significant decreases of grain size results in the ratio of large angle grain boundary increases.In addition,all the ?" phases dissolve back to the Al matrix during FSW process,and then form solute clusters or GP zone during natural aging.The grains of the thermo-mechanical affected zone(TMAZ)are not completely recrystallized,which have been elongated under the traction of the tool,and there are a lot of dislocations in the grains.The grain morphology of heat affected zone(HAZ)is consistent with that of BM.With the increase of welding heat input,the quantity of ? 'phase and Q' phase decreases and the size increases.The effect of welding parameters on the conventional mechanical properties of 6005A aluminum alloy FSW joint is studied.The microhardness distribution of FSW joint approximates is W-shaped.The minimum hardness is located at the HAZ,which attribute with the dissolution of ?" phase and the coarsening of ?' and Q' phases.If the heat input is too large,HAZ of FSW joint will be softened further,and if the heat input is too small,the welding defects such as hole and non-welding will be formed.Therefore,the optimized welding parameters range is:the rotational speed is 1500 rpm with the welding speed 500?900 mm/min;the welding speed is 700 mm/min with the rotational speed 1500?2400 mm/min.The fatigue behavior of FSW joint of 6005A aluminum alloy is studied.The results show that the welding parameters have a significant effect on the fatigue properties of welded joints.The fatigue strength of the joint is negatively correlated with the welding heat input.When the welding speed is 700 mm/min and the rotational speed is 2400 rpm,the fatigue strength of FSW joint is the largest.The fatigue crack propagation rate of WNZ and HAZ increases with the decrease of welding heat input.WNZ have better fatigue crack propagation resistance than HAZ.Residual stress plays a key role in promoting fatigue crack propagation.The residual stress near HAZ is much larger than that of WNZ,which leads to a higher crack propagation rate.The intergranular corrosion(IGC),exfoliation corrosion(EXCO)and stress corrosion cracking(SCC)behavior of FSW joint of 6005A aluminum alloy are investigated.The results show that the corrosion resistance of the joints decreases with the decrease of welding heat input.HAZ has the highest corrosion sensitivity.With the decrease of welding heat input,the corrosion mode of WNZ develops from pitting corrosion to pitting corrosion+localized IGC,and the corrosion mode of HAZ develops from localized IGC to comprehensive uniform corrosion.It has been found that the corrosion potential of HAZ is lower than that of WNZ,indicating that the thermodynamic possibility of electrochemical corrosion of HAZ was higher.With the increase of welding speed or the decrease of stirring speed,the corrosion potential and electrochemical impedance of WNZ and HAZ decreased gradually.The high ratio of large angle grain boundary in WNZ results in the increases the interface energy of grain boundary and makes the grain boundary more vulnerable to acid medium erosion.The corrosion sensitivity can be effectively reduced by reducing the amount and increasing the spacing of Mg2Si phases of HAZ grain boundary.The microstructure and properties of 6005A/7N01 dissimilar aluminum alloy FSW joint are studied.When the 6005A alloy is located on the advancing side(AS),the mixing of the two alloys in the WNZ is more sufficient.The tensile fracture of all dissimilar alloy FSW joints occurred on the side of 6005A alloy.When 7N01 alloy is located on the AS,the plastic deformation of 7N01 alloy in WNZ will produce more heat,which will further soften HAZ and reduce the tensile strength of joint slightly.When 7N01 alloy is on the AS,the fatigue property of FSW joint decreases significantly.Due to the poor mixing degree of the two alloys in WNZ,the fatigue crack propagates along the interface of the two alloys,which seriously reduces the fatigue properties.The large potential difference between 6005A and 7N01 alloy leads to galvanic corrosion of WNZ of FSW joint.7N01 alloy is corroded seriously as anode,while 6005A alloy has no obvious corrosion due to cathodic protection mechanism.