Study On Plasma Characteristics And Droplet Transition Behavior Of Thick Plate Narrow-gap Laser-MIG Hybrid Welding Assisted By Alternating Magnetic Field

With the rapid development of modern industry,the thickness of steel plates used in large equipment increased a lot.Compared with traditional welding methods,narrow gap laser arc hybrid welding could greatly reduce the consumption of welding materials and energy.However,porosities and unfused side wall limited the thickness of the workpiece.In this thesis,an alternating magnetic field was applied in narrow gap laser arc hybrid welding.The electromagnetic force was used to control the arc shape,droplet transition and weld formation,so as to decrease defects and improve the mechanical properties of joints.In this thesis,a platform was built firstly to assist the test.The physical characteristics of welding plasma in narrow gap were studied by spectral diagnosis.The results showed that the plasma temperature and electron density decreased with the increase of the distance from the collecting point to the bottom of the groove.As the magnetic field intensity increased from 3m T to 30m T,the plasma temperature first increased and then decreased,reaching maximum at 12m T,the highest temperature at the bottom of the groove was 9536K,and the electron density continuously increased from 8.67×10¹⁶cm^-3 to 9.82×10¹⁶cm^-3.As the magnetic field frequency increased from 12.5Hz to 50Hz,the plasma temperature ranged from9095K to 9416K and the electron density ranged from 8.44×10¹⁶cm^-3 to 9.14×10¹⁶cm^-3 at the bottom of the groove.When the magnetic field frequency was 37.5Hz,both plasma temperature and electron density reached the maximum.The arc shape and droplet transition were filmed by high-speed camera.The results showed that the frequency and intensity of the magnetic field influenced the shape and motion of the arc obviously.When the frequency increased from 0 to 37.5Hz,the arc shape was stable,and the radius increased.When the frequency increased to 50Hz,the arc shrunk and the radius decreased significantly.As the intensity increased from 0 to 30m T,the arc deflection phenomenon became more serious.When the intensity increased to 30m T,the arc deviated seriously from the center position of the welding wire.The welding process was extremely unstable and accompanied with lots of spatters.As the intensity increased from 3m T to 30m T,the transition frequency increased first and then decreased.The transition modes were all short circuit transition.Transition frequency reached the fastest at 12m T,and the weld was well formed.With the increase of frequency from 12.5Hz to 50Hz,the transition mode changed from large droplet transition to short circuit transition and then changed to large droplet transition again.The electromagnetic stirring effect was the best at 37.5Hz,which could reduce the defects such as porosity and non-fusion.The narrow gap Laser-MIG hybrid welding test of thick plate and high strength steel was carried out by the optimized welding parameters.The results showed that dense pores and a lot of spatters appeared in the weld,the side wall was not fused,and the cover weld bead owned undercut with no magnetic field.The welding seam was uniformly distributed without defects with alternating magnetic field.Through the analysis of the microstructure and properties of the weld,the grain size decreased obviously and the nucleation rate increased with magnetic field.Compared with the condition of no alternating magnetic field,the hardness of the joints was increased.The hardness of the heat affected zone of each welding layer were in the range of 312HV?420HV,and the hardness of the weld zone were in the range of 300HV?390HV.The impact toughness of the welding seam was enhanced by adding magnetic field.The upper average impact energy increased by 36.05%,and the lower average impact energy increased by 33.91%.However,the effect of alternating magnetic field on the tensile strength of joints was not obvious.