Effects of axial magnetic field on Fe-Ni interface macrosegregation of 9%Ni steel welds filled with Ni-based alloy

For the 9%Ni steel joint filled with nickel-based alloy, macrosegregation and wide partial mixing zone (PMZ) at Fe-Ni interface are critical factors to determine the cryogenic toughness. The pulsed gas metal arc welding (GMAW) assisted by the alternating axial magnetic field (AMF) is proposed to improve interface morphology and composition gradient. The welding without AMF assistance and with different AMF intensity are compared. The Lorentz force on arc and droplet caused by the alternating AMF is conductive to the stable transfer of viscous nickel-based alloy. The results show that AMF is effective to suppress the macrosegregation of Fe-Ni interface resulted from the temperature difference of melting point between base metal and filler metal. The temperature and flow speed of molten pool edge is enhanced by the stirring effect of AMF. The amount of filler metal in the PMZ of base metal increases. The melting point of mixing metal in PMZ decreases. Under the action of AMF, more PMZ is stripped from base metal. The PMZ continues to mix with liquid metal after entering the molten pool, instead of solidifying inside molten pool. After the alternating 12 mT AMF was applied, the cryogenic impact absorbed energy of fusion line increased from 0.85 J/mm2 to 1.29 J/mm2.