Core-ring Laser Welding: the Influence of Ring Diameter on the Capillary Dynamics and the Formation of Pores in the Resulting Weld

Double-core fibers that deliver laser beams with adjustable amounts of power to the central core and the surrounding ring component of the fiber are of major interest for optimizing laser welding processes. However, the central focussed laser spot and the ring diameters are fixed by the fiber dimensions, the focusing optics, and the laser systems involved. This work investigates the influence of different ring beam diameters, with a constant central spot diameter, on the capillary dynamics and the formation of pores in the resulting welds in 1.4301 stainless steel (X5CrNi18-10, AISI 304). The different core-ring configurations were achieved using specially designed beam-shaping optics. The generation of pores during the welding process was examined by means of synchrotron X-ray imaging. The results show that different ring diameters have a profound effect on the melt pool geometry, the capillary shape and the location of the capillary collapse which gives rise to pore formation in the weld. In the examples presented here, larger ring diameters extended the melt pool in the direction of welding without contributing directly to the capillary evaporation. In this case the narrow, deep capillaries which are created by the core beam alone can bulge and collapse, trapping vapor and gas to create large pores. If, however, the ring beam has a small enough diameter, it can help to produce a wider capillary which does not trap gas and create pores in the same way. The mechanisms by which large bubbles in the melt can result in non-spherical pores in the weld are also explained.