| Friction Stir Welding(FSW)is a new solid-state joining technique distinguished from the common fusion welding,which was invented in the early 1990 s.The technique has simple equipments and low energy consumption.Because of welding by Solid state joining,the probability of the defects(such as shrinkage,pore,embrittlement and crack)appeared in the welding joints are lower.The welding temperature is lower than melting point of materials,the welding joints has many feature such as little deformation,small residual stress and good mechanical properties,etc..Based on the above advantage of Friction Stir Welding,it open up new ways for the duplex stainless steel as a kind of engineering structural materials,which will be widely used in manufacturing field.The super duplex stainless steel 2707 used as the experimental materials in the study were Friction Stir Welded with the welding rate of 100 mm/min,the rotation speed of mixing tools is respectively 200 rpm?300 rpm?400 rpm and 500 rpm.The microstructure,mechanical properties and corrosion resistance of the Friction Stir Welding joints of the duplex stainless steel 2707 were systematic researched under different welding parameters.The main conclusions are as following:(1)By researching the microstructure transformation of the welding joints and the effects of four different welding processes on mechanical properties and corrosion resistance,the result is obtained that optimal welding parameter is the rotation speed of 400 rpm,the overall properties of welding joints is better than base metals.(2)The macroscopic morphologies of the FSW joints of the duplex stainless steel 2707 with different rotation speed were observed.The results showed that welding joints has rough surface under the rotation speed of 200 rpm,during the rotation speed of 300 rpm and 500 rpm,it have flat and smooth surface and good formability under the rotation speed of 200 rpm.(3)When the rotation speed is 200 rpm or 300 rpm,the lower rotation speed leads to lower heat input.The peak temperature of the welding joints is 978 ?and 1018 ? respectively,which are both in the range of ?-phase precipitation,large amount of ?-phase precipitated in the welding joints.When the rotation speed is 400 rpm or 500 rpm,?-phase in the welding joints no longer exist.While two precipitation mechanism of the ?-phase were found in the FSW process.(4)The tension test results of welding joints under four different rotation speed showed that the strength of welding joints are all higher than base metals.When the rotation speed is 200 rpm or 300 rpm,the fracture of tensile samples is located in the base metal and HAZ respectively.Incomplete penetration was observed in the cross section of welding joints owing to lower heat input when the rotation speed is 200 rpm.While ?-phase precipitated in the welding joints becomes the weak location of tensile deformation process when the rotation speed of 200 rpm and 300 rpm,accordingly the fracture is located in the weld and HAZ respectively,the elongation is lower.When the rotation speed is 400 rpm or 500 rpm,no defects such as shrinkage,pore and crack was observed in the welding joints,and no ?-phase precipitated in the microstructure.The tensile fracture is located in the base metal,the elongation is near.(5)The problem of the coarse structure of welded zone and large amount of ferrite formed in the HAZ of the duplex stainless steel by melt welded was challenged resolved by FSW technique.The content of ferrite in different location of welding joints were researched with the rotation speed of 400 rpm,the results showed that the content of ferrite are varied between 50~54% in different location of welding joints,which is equivalent with base metal.The study of the recrystallized structure in the weld zone showed that ferrite and austenite in the microstructure of weld zone were both dynamic recrystallized completely when the rotation speed is between 300 rpm and 500 rpm.With the increasing of rotation speed,the size of grain gets larger.The grain size of the weld zone under three kinds of welding parameters are all smaller than base metal,while the strength and micro hardness are higher based on fine-grain strengthening theory.(6)The research on potentiodynamic polarization curve and electrochemical impedance spectrum showed that the weld joints have poor corrosion resistance under the conditions of 3.5 %NaCl solution test when the rotation speed is 200 rpm or 300 rpm.The corrosion resistance of weld joints is better than base metal totally when the rotation speed rose up to 400 rpm. |
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