The welding of Zn-coated steel thin sheets is of verygreat importance for the automotive industry. Previousstudies have shown the main involved physicalprocesses. The zinc vapor coming out from theinterface of the two sheets is violently expelled insidethe keyhole and expels the melt pool, for noncontrolled conditions. When using CO2 lasers, wehave previously shown that an elongated laser spotproduces an elongated keyhole, which is efficient forsuppressing this effect. We recently adopted a similarapproach for Nd:Yag laser welding. We observed thatan elongated spot was not necessary for achievinggood weld seams. Several diagnostics were used inorder to understand these interesting results. Highspeed video camera visualizations of the top and thebottom of the keyhole during process show thedynamics of the keyhole hydrodynamic behavior. Itappears that the role of the reflected beam on the frontkeyhole wall is crucial for an efficient stabilization ofthe process. Our dynamic keyhole modeling, thatincludes ray tracing, totally confirms this interpretationand explains the results for very different experimentalconditions (effect of welding speed, laser intensity,variable sheet thickness, laser beam intensitydistribution) that will be presented.
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