Numerical and experimental study of the enhanced melting and penetration capability of aluminum alloy gap bridging laser welding by alternating magnetic field

In order to enhance the penetration ability of aluminum alloy laser welding gap bridging, an alternating magnetic field generator was designed to assist its laser welding and a comprehensive numerical model considering assembly gap, magnetohydrodynamics (MHD) and free surface evolution was established. Comparisons were made without gap to reveal the effects of the alternating magnetic field on the thermofluidic flow and molten pool formation, as well as the effects of the gap on the distribution and magnitude of the magnetic field. It is shown that when the maximum magnetic induction strength is 39 mT and the magnetic field frequency is gradually increased from 50 Hz to 150 Hz, there is a 33.3 % enhancement of the penetration depth in the gap bridging, while the enhancement of the depth in the bead-on-plate welding is only 25.4 %. In gap bridging, the magnetic field is more stirring at the sides of the molten pool, while in the bead-on-plate welding, it is mainly at the front, back sides and at the center of the molten pool. During cooling, the alternating magnetic field at a frequency of 150 Hz boosted the flow rate significantly, by 400 % in the gap bridging compared to 116% in the bead-on-plate welding. Eventually, the weld transformed from partially penetrated (1.63 mm) to fully penetrated (2 mm) under the effect of the alternating magnetic field.