Towards supportless laser powder bed fusion: fundamental understanding of the formation of 1^st layer overhang through parameter optimisation and in-situ high-speed thermal imaging

One of the most revolutionary aspects of Laser Powder Bed Fusion (LPBF) is to be able to lift the design constraints from conventional manufacturing. However, as a rule of thumb, any surfaces lower than 45° with respect to the horizontal plane would still require sacrificial supports in order to complete the printing. Fundamentally, it is unclear whether it is feasible to print a 1^st layer 0° overhang nor how the print parameters for the 1^st layer can be optimised. This research demonstrates that large 1^st layer 0° overhangs can be printed with a coverage above 90%. For the first time, the parameter space of laser power, scan speed and hatch spacing for the 1^st layer has been simultaneously explored efficiently. The use of the rate of change of the mushy zone is proposed for the parameter selection instead of the average melt pool temperature. Adaptive Parameter 28 (AP28: 250W_4000 mm/s_50 µm) is the best choice. Several surface morphological phenomena are discussed. The core parameter (285W_960 mm/s_110 µm) causes severe balling and its mechanism is revealed. The average melt pool temperature of the 1^st layer by the core parameter is lower than that of the bulk. The ramifications beyond the 1^st layer are also illustrated.

激光粉末床熔融（Laser Powder Bed Fusion, LPBF）最具革命性的特性之一，便是能够摆脱传统制造的设计约束。然而根据经验法则，任何与水平面夹角低于45°的表面仍需使用牺牲支撑才能完成打印。目前尚未明确，能否打印第一层的0°悬垂结构，也不清楚该第一层的打印参数应如何优化。本研究证明，覆盖率超过90%的大面积第一层0°悬垂结构可被成功打印，并首次高效同步地探索了第一层打印涉及的激光功率、扫描速度与扫描 hatch间距（hatch spacing）的参数空间。研究提出采用糊状区变化速率作为参数选择的依据，而非平均熔池温度，其中自适应参数28（Adaptive Parameter 28, AP28: 250W_4000 mm/s_50 µm）为最优参数方案。文中还讨论了多种表面形貌现象，核心参数（285W_960 mm/s_110 µm）会引发严重球化缺陷，其作用机制已被揭示，该参数对应的第一层平均熔池温度低于打印件本体的平均熔池温度。此外，本文还阐述了第一层打印参数对后续层打印的影响。