Experimental research on formation mechanism of porosity in magnetic field assisted laser welding of steel

The dynamic keyhole behaviors are directly observed in laser welding of AH36 steel/glass, and Zirconia particle is used to observe the convective pattern. The weld bead formation, keyhole instability and porosity formation are analyzed. Moreover, the differences in keyhole instability and porosity formation between no magnetic field assisted and magnetic field assisted laser welding are analyzed. With the magnetic field is added, the weld width, small concave, the formation of the keyhole collapse and the convective pattern are similar in laser welding of steel, and the generated Lorentz force hinders the flow of liquid metal. Widening of bottom weld width without undercut defect can be obtained with magnetic field assisted. The lower fluctuation of the weld depth, the longer time of plasma plume above the weld pool, fewer undercuts and spatter defects contributes to the increase of the keyhole stability, and thus restrains the porosity formation. Besides, an easier keyhole open, the larger bubble escape velocity and the lower solidification rate also promote the reduction of the porosity formation.