Porosity evolution and its thermodynamic mechanism of randomly packed powder-bed during selective laser melting of Inconel 718 alloy

To further investigate the porosity evolution during selective laser melting (SLM) Inconel 718 alloy, a transient mesoscale model with a randomly packed powder-bed has been proposed by finite volume method (FVM), taking consideration of the phase transition, variation of thermo-physical properties and interfacial force. The thermodynamics within molten pool and resulting porosity evolution behavior of a set of laser scanned tracks with various laser scanning speeds were studied using numerical approach. The results evidently revealed that the operating peak temperature was reduced obviously as increasing the scanning speeds. Accordingly, the high cooling rate, short lifespan and limiting depth of pool and small velocity of molten liquid flow were obtained under a high scanning speed. Scanning speed played a crucial role in determining the type of porosity in the terminally SLM-processed Inconel 718 components. At a high scanning speed of 500 mm/s, the top surface was primarily dominated by open porosity, accompanying with large-sized inter-layer porosity on the cross section, due to a limiting energy input penetrated into the powder-bed and incomplete melting of powder. By contrast, as a relatively low scanning speed of 200 mm/s was employed, the top surface appeared to be smooth free of less metallurgical porosity and no apparent inter-layer porosity on the cross section surface attributing to the escaping of porosity, indicating an well metallurgical bonding of the neighboring layer towards the building direction. Simultaneously, the physical mechanism was thoroughly discussed. The simulated distribution of porosity was found to be consistent with the experimental measurements.

为了进一步研究选择性激光熔化（SLM）Inconel 718合金过程中的孔隙率演变，通过有限体积法（FVM）提出了一种具有随机堆积粉末床的瞬态介观模型，该模型考虑了相变、热物理性质的变化和界面力。采用数值方法研究了熔池内的热力学以及一组不同激光扫描速度的激光扫描轨迹所导致的孔隙率演变行为。结果明显表明，随着扫描速度的增加，工作峰值温度明显降低。相应地，在高扫描速度下获得了高冷却速率、短熔池寿命和有限的熔池深度以及较小的熔液流动速度。扫描速度在决定最终经SLM加工的Inconel 718部件的孔隙类型方面起着至关重要的作用。在500 mm/s的高扫描速度下，由于进入粉末床的能量输入有限以及粉末未完全熔化，顶面主要由开孔孔隙主导，同时横截面上存在大尺寸的层间孔隙。相比之下，当采用200 mm/s的相对较低扫描速度时，顶面看起来光滑，冶金孔隙较少，且横截面上没有明显的层间孔隙，这是由于孔隙逸出，表明相邻层在构建方向上具有良好的冶金结合。同时，对物理机制进行了深入讨论。发现孔隙率的模拟分布与实验测量结果一致。