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.激光粉末床熔融（Laser Powder Bed Fusion, LPBF）作为增材制造（Additive Manufacturing, AM）的关键分支，可制备内部流道、点阵结构以及拓扑优化设计等复杂构件。然而，尽管其理论上具备微米级加工精度，但逐层加工工艺以及松散粉末较低的导热性与不稳定性，往往会引发悬垂缺陷（overhang defects），具体包括粘屑（dross）与翘曲（warping）。粘屑通常由工艺参数欠优时熔池尺寸过大及粉末粘附所导致，而翘曲则源于往复冷热循环诱导产生的残余应力。尽管支撑结构可缓解上述缺陷，但会显著提升后处理成本，且对于内部空腔结构往往难以实施。因此，实现无支撑悬垂结构制备对于LPBF的更广泛应用至关重要。本综述聚焦LPBF悬垂结构的临界角度阈值，以及粘屑与翘曲的形成机理，首次全面梳理并归类了23项影响悬垂结构质量的工艺相关因素，并总结了学术界与工业界提出的创新解决方案。此外，本综述提出了一种标准化测试试样，用于评估悬垂结构的力学性能与表面形貌。同时，本综述提供了详尽的参考文献以指导未来的实验与数值模拟研究，并期望后续研究能够验证并拓展上述研究成果。