近期研究表明，相比常压条件下，采用真空或低压环境的激光选区熔化（SLM）工艺可获得缺陷更少、表面质量更优的打印件。虽然通过高速摄像实验已经对SLM工艺在真空环境下的介观物理过程进行了部分研究，但该工艺下影响熔池传热和流动方面的深层机理仍不十分清楚。为此，基于最近的激光焊接模型工作，我们首次建立了变环境压力下SLM工艺的介观尺度数学模型。我们模拟了大气压和100 Pa低压环境下SLM工艺中316L粉末的输运现象。研究发现，在典型工艺参数（激光功率：200 W；扫描速度：2 ms^-1；粉末直径：27 µm）下，大气环境中熔池表面凹坑的平均温度将近2800 K，而在100 Pa环境压力时，平均温度仅为2300 K。由于在100 Pa低压条件下，蒸发引起的表面压力和环境压力之间的压差相对于大气环境下的压差更大，更易驱动高温流体流动，因此熔池表面流动速度更加剧烈（平均速度约为4 m·s^-1）。研究表明，熔池表面周期性的波浪流动（周期：14 µs）直接影响堆积体的表面粗糙度。此外，由于雷诺数小于400且远低于湍流临界值，熔融金属液流动模式为层流，因此，黏性耗散的影响至关重要。在真空或低环境压力条件下，波纹的流动轨迹更长，使得波动更容易由于黏性效应被耗散，从而改善了打印件的表面粗造度。总之，我们的数学模型阐明了变环境压力下SLM工艺实验研究中观察到的有趣输运现象的物理机制，它有望成为优化SLM工艺过程的有力工具。

Recent reports on the selective laser melting (SLM) process under a vacuum or low ambient pressure have shown fewer defects and better surface quality of the as-printed products. Although the physical process of SLM in a vacuum has been investigated by high-speed imaging, the underlying mechanisms governing the heat transfer and molten flow are still not well understood. Herein, we first developed a mesoscopic model of SLM under variable ambient pressure based on our recent laser-welding studies. We simulated the transport phenomena of SLM 316L stainless steel powders under atmospheric and 100 Pa ambient pressure. For typical process parameters (laser power: 200 W; scanning speed: 2 m∙s^-1; powder diameter: 27 μm), the average surface temperature of the cavity approached 2800 K under atmospheric pressure, while it came close to 2300 K under 100 Pa pressure. More vigorous fluid flow (average speed: 4 m∙s^-1) was observed under 100 Pa ambient pressure, because the pressure difference between the evaporation-induced surface pressure and the ambient pressure was relatively larger and drives the flow under lower pressure. It was also shown that there are periodical ripple flows (period: 14 μs) affecting the surface roughness of the as-printed track. Moreover, the molten flow was shown to be laminar because the Reynolds number is less than 400 and is far below the critical value of turbulence; thus, the viscous dissipation is significant. It was demonstrated that under a vacuum or lower ambient pressure, the ripple flow can be dissipated more easily by the viscous effect because the trajectory length of the ripple is longer; thus, the surface quality of the tracks is improved. To summarize, our model elucidates the physical mechanisms of the interesting transport phenomena that have been observed in independent experimental studies of the SLM process under variable ambient pressure, which could be a powerful tool for optimizing the SLM process in the future.