By Vjaceslav Avilov, André Schneider, Marco Lammers, Andrey Gumenyuk, Michael Rethmeier

A well-known problem of partial penetration laser beam welding is keyhole-tip instability representing the main source of porosity – gas bubbles leave the keyhole near its tip. The second important problem of keyhole mode laser welding is very intensive thermocapillary (Marangoni) convection in the upper part of the weld pool. The surface tension cannot completely suppress oscillations of the weld pool surface and the re-solidified weld surface becomes very rough.

In the present work an oscillating (AC) magnetic field was used to suppress porosity formation and to stabilize the weld pool surface in bead-on-plate partial penetration up to 4.4 kW Nd:YAG laser beam welding of  AW-5754  plates in PA position. Two magnet poles (cross-section 25 x 25 mm2) were located left and right aside the weld pool. The magnetic field (up to 0.4 T(rms) and 10 kHz) was oriented perpendicularly to the welding direction. The AC power supply does not exceed 2 kW. The AC magnet weighs only 5 kg and allows easy assembly on the laser welding head.

Analyses of the weld cross-sections and x-ray images showed a drastic reduction (up to 90%) of weld porosity. 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 weld surface roughness. This effect can be explained in terms of electromagnetic contribution to the surface tension.

Figure 1.  Radiography of two partial penetration welds made without and with EM weld pool control (laser power 4 kW, welding speed 2 m min-1, shielding gas industrial grade argon, 20 l min-1, gap between the magnet poles 15 mm). The “EM acceleration” gEM is ca. times larger than the standard acceleration due to gravity g0 = 9.81m/s-2.

Figure 1. Radiography of two partial penetration welds made without and with EM weld pool control (laser power 4 kW, welding speed 2 m min-1, shielding gas industrial grade argon, 20 l min-1, gap between the magnet poles 15 mm). The “EM acceleration” gEM is ca. times larger than the standard acceleration due to gravity g0 = 9.81m/s-2.