In Situ X-ray Phase Contrast Imaging of Humping Formation During Laser Beam Welding of 1.4404 Stainless Steel

Continuous development of laser beam sources and process technologies allows higher welding speeds, partly due to the high focus ability and brilliance of fiber laser sources. However, the increas ing interest in cost-efficient production of bipolar plates for fuel cells, for example, poses a challenge due to imperfections, which occur during high-speed welding of metals. A particular defect – referred to as humping in the welding sector - manifests itself in the form of periodical accumulation of so lidified melt on the weld bead surface. The main objective of this paper is to establish a basic under standing of humping, which could allow a shift of the effect to higher welding speeds for stainless steel 316L. This is done through tests using in-situ X-ray videographic images. Factors of capillary dynamics and capillary behavior that cause humping are analyzed. Furthermore, attempts to shift the humping limit are investigated using an Adjustable Ring Mode (ARM) laser. With the flexible power control of the core to ring beam, the capillary dynamics and the occurrence of humping are closely examined. Finally, new findings with regards to the indicators for the occurrence of humping and an assessment of the use of an ARM laser to avoid humping are deduced.