Meanwhile, the high spatial and temporal resolutions afforded by the 3rd generation high-energy synchrotron facility and the state-of-the-art beamline instruments have enabled the quantification of the pore moving trajectories at different locations in the melt pool (Supplementary Movie 1). Absorption and phase contrast in the X-ray image, generated by different features, permit an easy identification of the micro-pores, melt pool boundary, and vapor depression zone, as indicated in Fig. 1c, and a representative X-ray movie is presented in Supplementary Movie 1. A representative single-pulse X-ray image is depicted in Fig. Single-pulse (100 ps pulse width) X-ray imaging was conducted with a recording rate of 135,776 frames per second (see “Methods” section for details). In order to probe pore motion in every location in the melt pool, AlSi10Mg plate samples, with uniformly dispersed pores (diameters of 10–60 µm), were built by the LPBF as the substrates, as shown in Fig. The in-situ X-ray imaging experiment setup consists of a powder bed system (a 100 µm layer of powder on a substrate sandwiched between two glassy carbon plates), a selective laser melting system (to scan the powder bed and create a melt pool), and a high-speed X-ray imaging system (to capture the dynamics of the LPBF process) 15, 16, 17. The in-situ high-speed X-ray imaging experiment to capture the dynamics of pore motion and elimination during LPBF is schematically shown in Fig. In-situ characterization of pore dynamics during LPBF The thermocapillary force driven pore elimination mechanism revealed here could be used to design 3D printing approaches to achieve pore-free 3D printing of metals. We identify that the high thermocapillary force induced by the high temperature gradient in the laser interaction region can overcome the drag force induced by melt flow to rapidly eliminate pores from the melt pool during LPBF process. With complementary multi-physics modeling, we find that the pore moving behavior is governed by the competition of the temperature gradient induced thermocapillary force and the melt flow induced drag force. Here, we reveal the highly dynamic and complex motions of micro-pores in the melt pool during LPBF process by using the high-speed hard X-ray imaging technique, with high resolutions (100 ps temporal resolution and ~2 µm spatial resolution). Earlier works, involving the use of X-ray imaging to visualize pore motion in a laser melt pool, achieved some success 11, 13, 14, but the resolutions afforded by a lab source, or a mid-energy synchrotron facility, are not sufficient to capture some of the faster motions of those micro-pores. However, because of the small sizes and high velocity of the pores, as well as the opaque nature of metals, it has been very challenging to probe the motion of these micro-pores in-situ and in real time. Therefore, it is critical to uncover the dynamics and mechanisms of pore evolution and elimination in the melt pool during the LPBF process and identify mechanisms for eliminating pores during the printing process, in order to obtain as-printed parts with very low or zero porosity. For example, the hot isostatic pressing (HIP) cannot close the surface pores 5 and the gas pores closed by HIP can reopen and grow during subsequent heat treatment 12. It is very challenging to completely eliminate pores in the printed parts by post processing. Thus, pores have been ubiquitously observed in as-printed parts 4. Those pores in the melt pool cannot be effectively eliminated by buoyant force 9, a commonly used mechanism that eliminates pores from liquid 10, because the high drag force, that is induced by the strong melt flow in the LPBF process, traps the pores within the melt pool 11. Many mechanisms can cause pores to form in the melt pool during the printing process (e.g., pore transfer from feedstock powder 6, instability of depression zone during printing process 7, vaporization of volatile elements 8, gas precipitation 9). However, the parts printed by the LPBF normally contain many more pores than those made by conventional methods 4, which severely hinders their applications, because pore is one of the most detrimental defects that cause failure of parts 5. Laser powder bed fusion (LPBF) is a 3D printing technology (also known as additive manufacturing) that can print metal parts with complex geometries directly from digital models without the design constraints of traditional manufacturing routes, which has the potential to revolutionize biomedical, aerospace, and defense industries 1, 2, 3.