Fluid flow and bubble evolution in the melt pool of high-power (>2500 W) laser additive manufacturing of aluminum captured by X-ray imaging

With the continuous advancement of industrial technology, the demand for large and high-performance parts in industries such as aerospace, automobile, and energy are growing. High-power additive manufacturing can achieve a deep melt pool and fast forming speed, which is particularly important for the manufacturing of mass production, large-size parts and high melting point metals. In view of the fact that the keyhole mode under high-power will induce the pore generation, the in-situ X-ray imaging technology is used in this work to observe the entire process of fluid flow, bubble generation and evolution in the high-power laser melting of aluminum alloy. By comparing and analyzing the metallurgical behaviors of the fluid in the melt pool before and after the laser is turned off, the effect of keyhole on the fluid flow is revealed, that is, the keyhole leads to two relatively independent flow loops in the melt pool. Four bubble formation mechanisms are elucidated, namely, the bubbles formed by entrainment of the shielding gas through the free surface, bubbles formed at the melt pool boundary, bubbles formed at the crack, and bubbles formed at the keyhole bottom. Four bubble evolution mechanisms are also clarified. The bubbles may escape from the free surface, merge into large bubbles, be captured by the keyhole, and be captured by the solidification interface to form pores. We also reveal the dynamic process of bubbles being captured by the solidification interface to form pores. Under the interface constraint, the large-sized bubble splits into a pore and a new small bubble. While the small-sized bubble completely forms a pore. Finally, the characteristic of the bubble in the melt pool after the laser is turned off under high-power printing is clarified, that is, a large number of small bubbles appear and finally turn into pores. This work shows the phenomena in high-power additive manufacturing, which are different from those in low-power additive manufacturing. Two independent circulation flows occur in the obtained large-sized melt pool, and the large-sized bubbles split into small-sized pores during the capture process, as well as a large number of small bubbles occurs in the melt pool after the laser is turned off. The results provide theoretical support for optimizing processes, reducing defects, and improving component performance for high-power additive manufacturing, as well as the welding process.