Pathways and mechanisms for layer-wise regulation during laser powder bed fusion of horizontal overhang structures

Laser powder bed fusion (LPBF) enables the freeform fabrication of complex metallic components, yet stable support-free forming of horizontal overhang closure structures remains a major challenge because of severe thermal stress accumulation, insufficient transient support, and progressive closure-region instability. Here, we present a mechanism-guided strategy for overcoming this limitation. An octagonal horizontal overhang structure with closure features was designed using AlCoCrFeNi 2.1 high-entropy alloy as a model material. By integrating in-situ visible-light monitoring, Molten-pool Time-lapse Tomography (MTT), ex-situ computed tomography (CT), and three-dimensional morphological characterization, the failure evolution of the closure region under different energy-input conditions was directly captured and systematically analyzed. The novelty of this work lies in revealing that the forming limit of horizontal overhang closure structures is governed by a dynamic competition between thermal stress accumulation and structural strength development during layer-wise deposition, rather than by geometric constraint or heat input alone. Based on this mechanism, two energy-input regulation strategies, namely an extreme-parameter strategy and a partitioned process strategy, were proposed to coordinate stress evolution and load-bearing capacity at different deposition stages. Both strategies enabled complete support-free fabrication of horizontal overhang closure structures with single-layer spans up to 20 mm. This study provides an insightful mechanistic basis and practical process-design route for LPBF overhang closure fabrication.