In laser beam welding, excessive evaporation leads to bulging of the capillary tip which results in the generation of metal vapor-filled bubbles in the melt pool. These bubbles either collapse and dissolve, or solidify and remain as pores in the weld seam, thereby degrading the quality of the weld seam. We investigated the mechanism of bubble formation and collapse in detail for laser beam welding of aluminum by means of online X-ray videography. Capillary shapes were reconstructed from these high-speed videos and correlated to two kinds of processes either sensitive to pore formation or unsusceptible to pore formation. We show that the fluid dynamics in the melt pool is strongly influenced by subsequent bulging and collapsing of bubbles. Its influence on the melt flow was quantified by analyzing the trajectories of tracer particles in the melt pool. We found that the generation and collapse of bubbles is a major driver of the dynamics in the melt pool. The melt is accelerated to velocities of up to several hundreds of millimeters per second by collapsing bubbles. Similar effects were found in laser beam irradiation of transparent media, such as ice and water, which allows to resolve the generation and collapse of capillary bulging with higher temporal and spatial resolution.
Florian Fetzer, Peter Berger, Haoyue Hu, Rudolf Weber, and Thomas Graf, "Pores in laser beam welding: generation mechanism and impact on the melt flow," Proc. SPIE 10525, High-Power Laser Materials Processing: Applications, Diagnostics, and Systems VII, 105250D (Presented at SPIE LASE: February 01, 2018; Published: 15 February 2018); https://doi.org/10.1117/12.2295798.
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