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.

最近关于真空或低环境压力下的选择性激光熔化 (SLM) 工艺的报告表明，印刷产品的缺陷更少，表面质量更好。尽管已经通过高速成像研究了真空中 SLM 的物理过程，但控制热传递和熔体流动的潜在机制仍未得到很好的理解。在此，我们首先基于我们最近的激光焊接研究开发了可变环境压力下 SLM 的细观模型。我们模拟了 SLM 316L 不锈钢粉末在大气压和 100 Pa 环境压力下的传输现象。对于典型工艺参数（激光功率：200 W；扫描速度：2 m∙s ^-1; 粉末直径：27 μm），腔体的平均表面温度在大气压下接近 2800 K，而在 100 Pa 压力下接近 2300 K。更强劲的流体流动（平均速度：4 m∙s ^-1) 在 100 Pa 环境压力下观察到，因为蒸发引起的表面压力与环境压力之间的压差相对较大，并在较低压力下驱动流动。还表明存在影响印刷轨迹表面粗糙度的周期性波纹流（周期：14 μs）。此外，由于雷诺数小于 400 且远低于湍流的临界值，因此熔体流动显示为层流；因此，粘性耗散很重要。结果表明，在真空或较低的环境压力下，由于波纹的轨迹长度较长，波纹流更容易被粘性效应消散；因此，提高了轨道的表面质量。总结一下，