The availability of laser sources with a power of 20 kW upwards prepared the ground for laser beam welding of up to 20 mm thick metal parts. Challenges are the prevention of gravity-driven melt drop-out and the control of the dynamics mainly due to the Marangoni flow.
Coupled numerical turbulent fluid flow, thermal and electromagnetic simulations and experimental validation with aluminum AlMg3 and stainless steel AISI 304 were done for alternating and steady magnetic fields perpendicular to the process direction. The first can prevent melt sagging in full-penetration welding by Lorentz forces in the melt induced by an AC magnet located below the weld specimen counteracting gravitational forces. The latter controls the Marangoni flow by Lorentz braking forces in the melt by the so-called Hartmann effect.
The simulations show that the drop-out of aluminum and stainless steel can be avoided for 20 mm thick full-penetration welds with moderate magnetic flux densities of 70 mT and 95 mT at oscillation frequencies of 450 Hz and 3 kHz, respectively. The experiments are in good agreement but show somewhat larger values for steel, whose weakly ferromagnetic properties are a possible reason. The investigations with steady magnetic fields reveal the possibility to mitigate the dynamics significantly beginning with around 500 mT at laser penetration depths of approximately 20 mm.