Abstract: The formation and evolution of defects during laser powder bed fusion (LPBF) have been extensively investigated to enhance the performance of manufactured parts. However, there remains a lack of fundamental understanding regarding defect formation and elimination mechanisms during LPBF under overhang conditions. In this study, we employed a synchrotron high-speed X-ray imaging technique to track the behavior of powder spheroidization, pore formation, and escape, as well as the impact of pore growth on molten pool surface stability during overhang build using mechanically mixed Fe-Cu powder. Our findings revealed that the notable difference in the melting degree of the powder bed is the primary driving force for balling. The coalescence of spherical droplets and the continuous wetting at the melt pool boundary with the powder bed can promote melt track growth. Additionally, numerous pores emerge within the molten pool due to the “liquid phase sintering (LPS) mechanism”. To describe pore escape behavior accurately, we established and validated a pore bursting model. Furthermore, adjacent pores can interfere with each other thereby restricting pore escape and diminishing molten pool surface stability. Overall, our results elucidate defect formation mechanisms while providing guidance for mitigating spheroidization and pores in LPBF.

Key words: Synchrotron X-ray imaging, Laser powder bed fusion, Pore behavior, Molten pool surface stability