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The use of the oscillating magnetic field as a backing support for the welding of thick components is already known. The influence of the magnetic field and the induced Lorentz forces in the melt on the melt pool geometry and the fluid flows is not yet fully investigated. The study deals with the determination of the influence of an externally applied oscillating magnetic field on the melt pool dynamics in high power laser hybrid welding process. An AC magnet was positioned under the specimen to create an electromagnetic force directed upwards to oppose dropping. To visualise the flow characteristics of the melt, several experiments were carried out using a technique specifically designed for this purpose with mild steel made of S355J2 with a wall thickness of 20 mm and a quartz glass in a butt configuration. A high-speed camera was used to monitor the geometry of the melt pool through the glass. The influence of the magnetic field orientation to the welding direction and the oscillation frequency on the molten pool was investigated for the case of the metal-glass configuration and for laser hybrid welding. The high-speed recordings were analysed with the Optical Flow Algorithm to characterise the flow within the melt pool. The element distribution over the whole seam thickness was evaluated by X-ray fluorescence (XRF). The high-speed analysis showed that in the melt pool two vortices are formed, one in the upper part and the other in the lower part. In the region where the two vortices come together, a narrow region (necking region) forms in the melt pool. The evaluation of the high-speed recordings shows that the depth of the region where the two vortices meet is strongly influenced by the oscillation frequency. Additionally, the oscillation frequency demonstrated a greater influence on the melt pool dynamics and the mixing of the elements of the filler wire.
One of the challenges of the high-power hybrid laser welding of thick steels is the sensitivity of the process of the process to manufacturing tolerances. This usually leads to a time-consuming preparation of the welding edges, such as milling. The study deals with the influence of the edge quality of milled and plasma-cut steel made of S355J2 with a wall thickness of 20 mm on the laser hybrid welded seam quality. Furthermore, the gap bridgeability and the tolerances towards edge misalignment was investigated. An AC magnet was used as backing support to prevent sagging and positioned under the workpiece, to generate an upwards directed electromagnetic pressure. The profiles of the edges and the gap on the top and root side were measured using a digital camera. Single-pass laser hybrid welds of plasma-cut edges could be welded using a laser beam power of just 13.7 kW. A gap bridgeability up to 2 mm and misalignment of edges up to 2 mm could be achieved successful. Additionally, the independence of the cutting side and the welding side was shown, so that samples were welded to the opposite side to their cutting. For evaluation of internal defects or irregularities, X-ray images were carried out. Charpy impact strength tests were performed to determine the toughness of the welds.