Duplex Stainless Steel Narrow-gap Laser-arc Hybrid Welding Technology Based On Alternating Magnetic Field

2205 duplex stainless steel thick plates have excellent strength,ductility and good corrosion resistance.And they have been widely used in many fields such as pressure vessel,ship,oil gas pipeline,nuclear power industry.There exist many difficulties in welding 2205duplex stainless steel thick plate by traditional arc welding method such as large welding heat input,low production efficiency,large residual stress after welding and deterioration of joint performance.The weld and HAZ of duplex stainless steel are subjected to the complex thermal cycle of rapid heating and cooling during the welding process.Narrow-gap laser-arc hybrid welding is a high-precision welding technology comprising of two kinds of heat sources.The narrow-gap groove significantly reduces the filling amount of weld and the welding heat input is low.Moreover,the stress and deformation of the joint after welding are also small.In recent years,it has aroused great interests to scholars both at home and abroad.However,the narrow-gap groove may lead to bad side wall fusion,the depth-width ratio of single weld is larger,and the gas in molten pool is difficult to overflow.Therefore,the high sensitivity of gas hole and side wall unfused defects limit the application of this technique.In this thesis,alternating magnetic field was introduced into narrow-gap laser arc hybrid welding of 2205 duplex stainless steel thick plate.The effects of alternating magnetic field on plasma morphology,physical properties and droplet transfer in a narrow-gap groove were studied.The mechanism of the effect of alternating magnetic field on plasma dynamic behavior and weld formation in narrow-gap groove was explained by means of SEM,XRD,EBSD,TEM and other micro-analysis methods.The microstructure characteristics of weld and HAZ in alternating magnetic field were studied.The mechanical and corrosion properties of the joint were tested and analyzed,and the relationship between alternating magnetic field,plasma,weld formation,welding defects,microstructure characteristics and corrosion resistance was established.The active control of the external alternating magnetic field on the welding quality of narrow-gap laser-arc hybrid welding is realized.The main conclusions are as follows:1.Under the action of an applied alternating magnetic field,the radial and axial plasma of the arc rotates around the end of the welding wire under the action of rotating Lorentz force with alternating directions.The radius of rotation increases with the increase of the magnetic field strength,and the arc swing amplitude increases accordingly.When the applied alternating magnetic field intensity is 40 m T and the frequency is 15 Hz,the electron temperature of the plasma near the edge of the narrow-gap groove is 8450 K,and the electron density inside the groove is 15.1×10¹⁶cm^-3.Compared with the non-magnetic field,the electron temperature and the electron density of the groove edge are increased by 23.9%and20.8%,respectively.And the melting of the base metal on both sides of the narrow-gap groove is enhanced,which is beneficial to restrain the non-fusion defects of the side wall.The droplet transfer mode changed from short circuit transfer to droplet transfer.With the increase of magnetic field intensity,droplet transfer period shortened and droplet transfer frequency increased,the frequency of droplet transfer is increased by 66.7%compared with that without alternating magnetic field.The welding process of narrow-gap laser-arc hybrid welding is optimized based on response surface method,and the butt joint of 25mm duplex stainless steel was welded with the optimized process parameters.The results show that the welded joint assisted by external magnetic field has good appearance,and is free from unfusion and porosity defects.When the magnetic field frequency is 30 Hz the average impact work of the weld is 97.7 J,which is 28.2%higher than that of the joint without alternating magnetic field.2.The results show that the two-phase uniformity Y（Y=γ%/δ%）of the welded joint is 0.850 under the applied alternating magnetic field,which is significantly higher than that without the applied alternating magnetic field.At the same time,the Mo content in the ferrite phase decreased from 6.16%to 5.87%due to the electromagnetic stirring effect of the alternating magnetic field on the molten pool.The results of EBSD and TEM show that the effect of alternating magnetic field on the preferred orientation of austenite grains and the transformation of austenite grains from Goss texture to Copper texture,the average grain size of the two phases decreases and the density of twins and dislocations increases with the alternating magnetic field.3.The results of macroscopical electrochemical tests show that the I_corr of the welded joints with alternating magnetic field is 2.64×10^-8 A·cm^-2,and the corrosion sensitivity of the welded joints with alternating magnetic field is lower than that of the welded joints without magnetic field.At the same time,both the pitting potential and the polarization resistance Rp increased,which shows that it has better corrosion resistance.The results of micro electrochemistry show that there is no cathodic current in the welded joint with magnetic field,the peak value of anodic current has little fluctuation,and the whole surface of welded joint is less corroded.The average impedance of the joint immersed in corrosive medium for 4h is60.7%,which is higher than that of the joint without magnetic field.Both the macro and micro-electrochemical tests show that applying alternating magnetic field to narrow-gap laser-arc welding of duplex stainless steel can improve the corrosion resistance of the joint.The stress corrosion susceptibility coefficient of joints with alternating magnetic field in simulated seawater solution is lower than that without magnetic field,and the tendency of stress corrosion is smaller.