The essential role of initial powder bed state in selective laser melting of 316 L stainless steel

Abstract

Understanding the essential role of initial powder bed states (e.g., structure and temperature) in the printing process is crucial to obtain high-quality components in powder-bed-based additive manufacturing. In this article, a 3D high-fidelity model which considered the Marangoni effects and recoil pressure was established based on the computational fluid dynamics (CFD) approach to simulate the printing process of 316 L stainless steel through selective laser melting (SLM). The influences of powder bed structure and temperature on the macro- and microscopic characteristics of the molten layer during printing were comprehensively investigated. On this basis, corresponding mechanisms were analyzed. Results show that the developed numerical model can vividly reproduce the SLM process of 316 L stainless steel. With the increase of powder bed thickness from 32.02 μm (Hspreading=2 D90 and Vspreading=0.2 m/s) to 40.65 μm (Hspreading=2 D90 and Vspreading=0.01 m/s), the height of the molten layer will increase from 16.37 to 32.86 μm, the depth will decrease from 21.69 μm to 10.52 μm, and the width will decrease from 100.67 to 94.94 μm. Meanwhile, this will make the fluctuation of the depth (CV) increase from 0.31 to 0.85. When the initial temperature increases from 300 K to 773.15 K, the height, depth and width of the molten layer increase by 3.88 μm, 12.68 μm and 17.37 μm, respectively. In this case, the CV is reduced from 0.34 to 0.23, implying the improvement of the molten layer quality. Under the current laser parameters, the defects such as warping, pores, faults, and balling induced by the insufficient melting in the thick powder bed can to a certain extent be reduced or even eliminated using the high initial temperature. The melting pool collapse will occur in a thin powder bed, the resultant entrained bubble defect in the molten layer can also be properly avoided by the high initial temperature. However, a special groove structure will appear due to excessive energy input. During multi-track melting, the rough surface, irregular molten layer and the pores between the molten tracks are likely to occur due to the insufficient energy input, these problems can be properly solved by employing a high initial temperature.