Study On Welding Process And Performance Of 6N01 Aluminum Alloy Friction Stir Welding Joint For High Speed Train

With the lightweight,high-speed and high-frequency vibration of train body,it brings a greater challenges to the welding technology of aluminum alloy train body and the reliability of welded structure.The 6N01 aluminum alloy belongs to Al-Mg-Si series,which has been widely used in the field of high-speed train body manufacturing because of its medium strength,excellent formability as well as good welding performance.Friction stir welding（FSW）is one of the most potential welding technologies for the aluminum alloy.Based on the characteristics of FSW and Coulomb friction work theory,the temperature field and residual stresses field of FSW joints of 6N01 aluminum alloy（T5）were calculated by using the ANSYS finite element software.During the FEM calculation,the dual heat source models namely the body heat source and surface heat source were used to explore the evolution law of the welding process to the temperature field and the residual stress field.The method of ultrasonic residual stress detecting was used to verify the residual stresses field of the 6N01 aluminum alloy FSW joints.Based on the finite element calculation results,the flat butt welding was carried out on the thickness of 4mm 6N01 aluminum alloy by FSW.The microstructure and mechnical properties of the FSW joint were characterized by OM,microhardness tester and universal testing machine respectively and the evolution rules of welding process on microstructure and mechanical properties were explored.On this basis,the electrochemical corrosion behaviors of welded joints under different welding processes in H2SO4 solution with PH of 6.5 were analyzed by potentiodynamic polarization curve,EIS and SEM and the influence law of welding process on corrosion behavior and the corrosion mechanism of joint were clarified.The results show that:（1）The residual stress of 6N01 aluminum alloy FSW joints is mainly longitudinal residual stress,which shows obviously bimodal characteristics.When the pin-wheel speed is constant,with the increasing of welding speed,the temperature of welding process decreases,and then the degree of irregularity of the residual stress in each part of the welded joint reduce,then the value of maximum residual stress decreases.When the welding speed is constant,with the increasing of pin-wheel speed,the peak temperature increases and the degree of irregularity of the residual stress in each part of the welded joint aggravate,as well as the value of maximum residual stress increases.The effect of the pin-wheel speed has more significant influence on the temperature field and the residual stress field of 6N01 aluminum alloy FSW joint than that of the welding speed.The simulation results matched reasonably well with experiment results.（2）The grain size of the WNZ increases firstly and then decreases with the increasing of welding speed when the rotational speed is constant.Simultaneously,the grain of TMAZ and HAZ gets fine,the grain deformation of TMAZ decreases.The microhardness of WNZ decreases firstly and then increases and the microhardness of TMAZ and HAZ increases.The ultimate strength and microhardnes decrease firstly and then increases and then the elongation decreases firstly and then increases.With the increasing of welding speed,the grain size of WNZ decreases a little firstly and then increases when the rotational speed is constant.Simultaneously,the grain of TMAZ and HAZ coarsens the grain deformation of TMAZ increases.The microhardness of WNZ increases firstly and then decreases and the microhardness of TMAZ and HAZ decreases.The ultimate strength increases firstly and then decreases,and elongation increases firstly and then decreases.It can be concluded that the optimum FSW process is the rotational speed is 800 r/min and the welding speed is400 mm/min for the 6N01 aluminium alloy with the thickness of 4 mm,which the ultimate strength of the joint reaches 70%of the base metal and the strength performance meets the EU EN288-4 standard.（3）With the increasing of welding speed,the corrosion resistance of the joint increases firstly and then decreases When the rotational speed is constant.when the welding speed is constant,the corrosion resistance of the joint increases firstly and then decreases with the increasing of rotational speed.At present,when the rotational speed is 800 r/min and the welding speed is 400 mm/min is the optimum welding parameters of corrosion resistance,which are consistent with the optimum parameters of mechanical properties.The order of corrosion resistance of FSW joints of 6N01aluminum alloy is WNZ>HAZ>TMAZ.The corrosion mechanism of 6N01aluminium alloy FSW joints in H₂SO₄ with pH 6.5 mainly originates from the aggregation of excess Si at grain boundaries,microscopic galvanic cells are form and intergranular corrosion occurs,simultaneously,because the local uneven distribution of the Si element in the grain and the distribution of precipitates,there are also pits corrosion in the grain.