Study On The Narrow Gap Laser-MIG Hybrid Welding Process And Joint Microstructure And Properties Of Duplex Stainless Steel Assisted By Alternating Magnetic Field

2205 duplex stainless steel has been widely used in various fields such as chemical pipelines,building structures,and petroleum transportation due to its excellent corrosion resistance,good plasticity,and weldability.When using traditional welding methods to weld thick plates of duplex stainless steel,problems such as multiple welding passes,high heat input,low efficiency,and imbalanced dual-phase ratio of welded joints may occur.By combining narrow gap groove with laser-MIG hybrid welding,the filling of weld metal can be reduced,welding heat input can be reduced,and welding efficiency and comprehensive performance of joints can be improved.However,narrow gap grooves can lead to poor sidewall fusion,and the depth to width ratio of a single weld seam is large,making it difficult for gas in the molten pool to escape,resulting in high sensitivity to porosity and sidewall incomplete fusion defects,which limits the application of this technology.This article introduces alternating magnetic field into the narrow gap laser-MIG hybrid welding of 2205 duplex stainless steel thick plate,and studies the effect of alternating magnetic field on plasma behavior and weld formation in narrow gap groove.The mechanism of alternating magnetic field action on plasma is elucidated,and multi-layer single pass laser-MIG hybrid welding butt tests are conducted on 25 mm thick duplex stainless steel under different magnetic field parameters to explore the effect of alternating magnetic field on the microstructure transformation of the joint,The mechanical properties and corrosion resistance of laser-MIG hybrid welding joints were tested and analyzed,and the internal relationship between "alternating magnetic field microstructure properties" was established.The main conclusions are as follows:The plasma behavior under alternating magnetic field was observed and analyzed through high-speed photography.The results showed that under the action of alternating magnetic field,the arc oscillated laterally,and the oscillation amplitude increased with the increase of magnetic field intensity.When the magnetic field frequency is 17 Hz,the arc oscillation amplitude is maximum.When the magnetic field frequency reaches 50 Hz,the arc rotation speed and oscillation frequency increase.The arc swings left and right in the narrow gap groove,which can increase the heat input to the side wall and reduce the generation of incomplete fusion defects on the inner wall of the groove.The formation of each weld seam in a narrow gap under different alternating magnetic field parameters was studied.The results showed that after applying the alternating magnetic field,the transition between the inner wall of the narrow gap groove and the weld seam became smoother,the weld seam spread better,and the weld surface became smoother.In the narrow gap laser-MIG hybrid welding butt test of 25 mm 2205 duplex stainless steel,there were no incomplete fusion and porosity defects in the joint when the magnetic field intensity was 20 m T and the frequency was 50 Hz.Using OM,SEM&EDS,XRD and EBSD test methods,the effect of alternating magnetic field on the transformation of weld microstructure was studied 25.4%,15.4%,and 6.3%.Applying an alternating magnetic field will make the diffusion of alloy elements more thorough and distribute more evenly in the two phases.The difference in Mo element content in the two phases gradually decreases with the increase of magnetic field frequency.The electron backscatter diffraction experiment shows that the alternating magnetic field increases the austenite content in the weld seam and refines the grain size;After applying a 30 m T magnetic field,the large angle grain boundaries in ferrite increase,the texture strength increases,and the austenite phase shifts from Cu texture to Brass texture,Cube texture,and Rotated Cube texture.The maximum density of the orientation distribution of ferrite and austenite has been improved compared to the absence of an external magnetic field.The effect of alternating magnetic field on the mechanical properties and corrosion resistance of duplex stainless steel joints was studied through tensile,impact,hardness,and electrochemical testing methods.The results showed that the tensile specimens all fractured at the base metal,and the tensile strength of the weld seam was higher than that of the base metal;The alternating magnetic field increases the hardness of each weld bead and makes the distribution of hardness more uniform;When the magnetic field intensity is 30 m T and the magnetic field frequency is 50 Hz,the impact absorption energy reaches its maximum value of 168 J,which is 29.2% higher than that without adding a magnetic field.Through potentiodynamic scanning testing and electrochemical impedance spectroscopy testing,it was found that with the increase of magnetic field intensity,the self corrosion potential of the sample shifts forward,the pitting potential increases,and the surface polarization resistance Rp and weld capacitance arc radius of the sample gradually increase.This indicates that the addition of alternating magnetic field improves the uniformity of weld fabric and improves the corrosion resistance of the weld.