Research On Interaction Mechanism And Microstructure-properties Of Laser Arc Hybrid Welding Assisted By Alternating Magnetic Field

Laser-arc hybrid welding technology,due to the combination the advantages of laser welding and arc welding and the compensation for the shortcomings of both,had developed into a widely used welding method.However,the shape and microstructure of the laser-arc hybrid welded joint usually were inhomogeneous,which seriously affected the usability of hybrid welded joint.The alternating magnetic field was expected to improve the inhomogeneity of the laser-arc hybrid welded joint and the joint quality,because it could play the role in stirring molten pool and improving microstructure and properties of the weld.Therefore,to explore the possibility,the 6-mm-thickness 316 stainless steel was used,the interaction mechanism of alternating magnetic field assisted laser-arc hybrid welding and the influence of alternating magnetic field on microstructure and properties of laser-arc hybrid welded joint are studied.Firstly,Bead-on-plate welding was designed to monitor and analyze the dynamic behavior of arc,molten pool,welding plasma and welding current and voltage in alternating magnetic field assisted laser-arc hybrid welding by high-speed camera,spectrometer and sensor of current and voltage.It is found that the force of the charged particles in the arc was changed by alternating magnetic field,leading to more obvious compressed shape of arc.In addition,the arc exhibited wobble behavior under large magnetic induction intensity.Alternating magnetic field led to decrease of plasma temperature and density under the small magnetic induction intensity and increase of plasma temperature and density under large magnetic induction intensity.The current and voltage waveforms were more fluctuating and the period was prolonged due to addition of alternating magnetic field,the “eddy current effect” generated by the alternating magnetic field inside the molten pool caused surface of molten pool to expand outward,resulting in more severe fluctuation.Secondly,appearance,cross-sectional morphology and microstructure of arc zone and the laser zone in fusion zone of laser-arc hybrid welded joint under different alternating magnetic induction intensity after welding was studied.It is found that alternating magnetic field made the corrugated features of the weld bead more obvious,the weld bead downward,and the shape and area difference of the weld arc zone and the laser zone reduced.In addition,the Cr element of ferrite in the weld increased with the increase of the magnetic induction intensity,and the difference of Cr element content in ferrite between the arc zone and the laser zone gradually decreased.The average grain size of the weld in the arc zone decreased with the increase of the magnetic induction intensity,while the weld of the laser zone increased,leading to the decrease of grain size difference between the two zones.The proportion of large-angle grain boundary of the austenite in the weld was increased and the content and distribution of ferrite in the two zones was reduced and more uniform due to addition of alternating magnetic field.Finally,the hardness,tensile,bending,low-temperature impact and electrochemical corrosion properties of the laser-arc hybrid welded joint under different alternating magnetic induction intensity were compared.It was found that the hardness,tensile,bending,low-temperature impact and corrosion resistance of the weld of hybrid welded joint was significantly improved.The hardness of the weld in the arc zone increased with the increase of the magnetic induction intensity.The tensile strength of the welded joint reached a maximum,which was 10.3 % higher than that without magnetic field.The maximum load of front and back bend was 5.8 % and 13.0 % higher than those without magnetic field.The low-temperature impact toughness could reach to 74 J,which was 29.8 % higher than that without magnetic field.The passivation interval and the radius of capacitive reactance were significantly increased and corrosion resistance of weld was improved compared to that without alternating magnetic field.Moreover,alternating magnetic field could significantly improve the inhomogeneity of the mechanical properties of the hybrid welded joint.The longitudinal hardness distribution of the weld became more uniform with the increase of the magnetic field strength,and the difference of tensile strength and low-temperature impact energy in arc zone and the laser zone were reduced due to the addition of alternating magnetic field.The difference between the maximum front bending load and back bending load of the joint was also minimized when the magnetic field strength was 60 mT,which was only 0.04 kN.