An oscillating (AC) magnet field was used to suppress porosity formation and to stabilize the surface of the weld pool in bead-on-plate partial penetration 4.4 kW Nd:YAG laser beam welding of AW-5754 plates in PA position. The magnet was mounted on the laser welding head. The magnet field (up to 0.4 T and 10 kHz) was oriented perpendicular the welding direction.
The analysis of the weld cross-sections and x-ray images shows a drastic reduction (up to 90%) of porosity contents in the welds. The observed effects can be explained in terms of electromagnetically (EM)
induced 'Archimedes' forces as well as the EM stirring flow in the weld pool. Moreover, usage of AC magnetic fields results in a significant reduction (up to 50%) of the surface roughness of the welds. This effect can be explained in terms of electromagnetic (EM)
contribution to the surface tension (the Garnier-Moreau effect)
The main characteristic feature of deep penetration laser beam welding is a large temperature difference between the plasma cavity (keyhole) in the centre of the weld pool and the melting/solidification front. Large temperature gradients in the weld pool result in a very intensive thermocapillary (Marangoni) convection. The weld pool surface width becomes very large and unstable. However, an externally applied oscillating magnetic field can stabilize the surface of the melt (the Garnier-Moreau effect, 1983). In the present work this technology was used to stabilize the surface of the weld pool in partial penetration 4.4 kW Nd:YAG laser beam welding of AW-5754 alloyin PA position. The AC magnet was mounted on the laser welding head. The oscillating magnet field was oriented perpendicular to the welding direction. It was found that the AC magnet field can drastically reduce the surface roughness of welds. The analysis of the x-ray images shows a rastic reduction of porosity content in the welds. This effect can be explained as a result of electromagnetic ectification of the melt.
During laser beam welding of aluminum alloys an electromagnetic field may favour pore outgassing through the top oxide layer. High frequencies cause a small penetration depth and thus exert a stabilizing effect on the weld surface. The point at which the laser beam between the two magnetic poles hits the workpiece surface is crucial to the influence of the magnetic field on the weld surface roughness. Using analyzed parameters for different laser points of application cause a change in weld surface roughness could be observed. The weld surface roughness could be reduced by 50%. The outgassing effect in terms of a reduction of pores could be observed for all parameter sets investigated.