Laser powder bed fusion of a support-free overhang structure using a donut-shaped laser beam

Laser powder bed fusion (LPBF) enables the rapid fabrication of complex components, yet the manufacturing of overhangs below 45° requires support structures, which limits the design freedom of LPBF. In this study, a donut beam was developed and employed for the support-free fabrication of IN718 alloy via LPBF. The effects of beam shape on the microstructure, processability, and overhang quality at 25° were systematically investigated. The results demonstrated that the donut beam offered excellent LPBF processability, enabling the fabrication of full-density IN718 parts across a broad processing window, with comparable tensile performance under both 25 °C and 650 °C conditions. In terms of microstructure, the IN718 alloy grains fabricated under the donut beam were more elongated, with an average grain ratio of 5.7, which was much higher than that of the Gaussian beam (3.7). The texture index was also considerably higher (6.2 compared to 3.2 for the Gaussian beam). The donut beam enabled the successful manufacturing of 25° overhang specimens without the addition of support structures, achieving the target height with only 0.18 mm top-surface deformation. In contrast, the Gaussian beam failed to manufacture the support-free 25° overhang specimens due to severe warpage and deformation, with some losing features by up to 8°. Numerical simulation revealed the mechanism of beam shape on dross formation and the warpage behaviour of overhang structures. This study offers positive guidance for the application of donut-shaped laser beams in the LPBF of low-angle overhang components.