Effect of an Alternating Magnetic Field on the Microstructure Homogeneity and Impact Fracture Mechanism of X100 Pipeline Steel Laser-Mag Hybrid Welds

Laser-MAG hybrid welding has broad application prospects in pipeline steel welding due to the advantages of high penetration and high efficiency. Since there is very little wire and shielding gas reaching the bottom of the molten pool, the inhomogeneous microstructure appeared in the thickness direction of laser-MAG hybrid weld. Herein, the microstructure homogeneity and impact toughness of X100 pipeline steel laser-MAG hybrid welds were improved by applying an alternating magnetic field and its effect on the impact fracture mechanism was studied. The results illustrated that the alternating magnetic field affected the laser, molten pool flow, plasma and droplet excess during welding. It deviated the droplet and arc plasma from the centerline of the molten pool, improved laser penetration and suppressed Marangoni convection to stir the molten pool. Since the inclusion distribution of the weld became homogeneous, the microstructure became homogeneous in the thickness direction, and a mixed microstructure of acicular ferrite and lath martensite was obtained. The large-angle deflection of the main crack, secondary cracks in the prior propagation direction and the combined action of plastic deformation and voids improved the impact energy of homogeneous microstructure. In addition, acicular ferrite provided a basis for crack deflection and void formation due to its high effective grain boundary density. This study provides a new idea for the control of microstructure inhomogeneity in pipeline steel laser-MAG hybrid welds