Advancements and challenges in overhang structure fabrication during laser powder bed fusion: a comprehensive review

Laser powder bed fusion (LPBF), a key branch of additive manufacturing (AM), enables the fabrication of complex structures such as internal channels, lattices, and topology-optimised designs. However, despite its theoretical micron-scale precision, the layer-wise process and the low thermal conductivity and instability of loose powder often lead to overhang defects, including dross and warping. Dross typically results from oversized molten pools and powder adhesion under suboptimal parameters, while warping arises from residual stresses induced by cyclic heating and cooling. Although support structures can mitigate these issues, they significantly increase post-processing costs and are often impractical for internal cavities. Therefore, achieving support-free overhang fabrication is essential for broader LPBF applications. This review examines the critical angle threshold in LPBF overhangs and the mechanisms underlying dross and warping. It is the first to comprehensively identify and categorise 23 process-related factors affecting overhang quality and summarises innovative strategies proposed in both academia and industry. Furthermore, a standardised test specimen is proposed to evaluate the mechanical properties and surface morphology of overhangs. The review also provides detailed references to guide future experimental and numerical studies, with the expectation that subsequent research will validate and extend these insights.