The laser beam melting (LBM) process is a powder-based additive manufacturing able to produce parts layer-by-layer in a hermetic chamber. Selection of the appropriate process parameters and fabrication conditions plays a fundamental role in the final properties. Investigating the interaction between the laser and the powder bed helps to understand the melt-pool behavior. In fact, even if additive manufacturing processes offer appreciable advantages, some processing parameters have to be studied to improve the properties of the products. The conventional LBM process is carried out in a chamber filled with an inert gas, at approximately atmospheric pressure. During the melting of a metal powder, gas can be caught in the liquid and may cause pores to form. This work investigates the effect of vacuum pressure in the chamber (up to 10⁻² mbar) on avoiding oxidation and improving the quality of the welding of stainless steel 316L. However, by modifying the residual pressure of the chamber, physical phenomena such as surface tension, Marangoni convection, and recoil pressure are amplified, creating more instabilities in the melt pool. This paper presents an initial analysis of the behavior of the powder under combinations of different residual pressures and laser speeds/powers. High-speed image acquisition brings to light the evolution of the spatters in vacuum compared to atmospheric pressure. Particle recovery coupled with Scanning Electron Microscopy (SEM) analyses shows the organization and morphologies of the ejections. Then, sectional views of single scan tracks are compared and related to the spattering. A considerable evolution in the results of the melting is found as one passes from atmospheric pressure to residual pressure. Hence, a powder pre-sintering solution is investigated in order to remedy the instabilities in vacuum and improve the process.

激光束熔化 (LBM) 工艺是一种基于粉末的增材制造，能够在密闭室中逐层生产零件。选择合适的工艺参数和制造条件对最终性能起着重要作用。研究激光和粉末床之间的相互作用有助于了解熔池行为。事实上，即使增材制造工艺提供了明显的优势，也必须研究一些工艺参数以改善产品的性能。传统的 LBM 工艺是在一个充满惰性气体的腔室中，在接近大气压的压力下进行的。在金属粉末的熔化过程中，气体可能会被困在液体中，并可能导致气孔形成。这项工作调查了腔室中真空压力的影响（高达 10⁻² mbar) 在避免氧化和提高不锈钢 316L 焊接质量方面的作用。然而，通过改变腔室的残余压力，表面张力、马兰戈尼对流和反冲压力等物理现象被放大，在熔池中产生更多的不稳定性。本文对粉末在不同残余压力和激光速度/功率组合下的行为进行了初步分析。与大气压相比，高速图像采集揭示了真空中飞溅的演变。粒子回收结合扫描电子显微镜 (SEM) 分析显示了喷射的组织和形态。然后，比较单个扫描轨迹的截面图并与飞溅相关。当人们从大气压变为残余压力时，发现熔化结果发生了相当大的变化。因此，研究了一种粉末预烧结解决方案，以弥补真空中的不稳定性并改进工艺。