Surface quality of the curved overhanging structure manufactured from 316-L stainless steel by SLM

Abstract

The efficiency of fabricating an overhanging structure by selective laser melting (SLM) is an important indicator of the performance of metallic parts. This is due to the fact that defects such as warpage and adherent dross may occur during fabrication of the curved surfaces of overhanging structures. In order to investigate the optimum conditions for fabrication of the curved surfaces of the overhanging structures, experiments were carried out using 316-L stainless steel powder. Initially, the almost 100 % dense parts were fabricated. Then, a model that has a circular curved surface along the Z axis was designed. For a given fabrication depth of 25 μm, several overhanging structures were produced when the laser scanning energy input ranges from 0.15 to 0.6 J/mm. Results show that the upper surface of the almost 100 % dense cube fluctuates like ripples and that the fabrication quality of the curved surface of the overhanging structure varies greatly depending on the energy input and the obliquity angle. For a given energy input of 0.2 J/mm, the obliquity angle for fabricating a totally overhanging surface is as low as 30°. The warpage and adherent dross grow with an increase in the energy input and a decrease in the obliquity angle. Warpage may accumulate, and the accumulated warpage of many layers significantly exceeds the predetermined thickness of the layer. All the four overhanging structures fabricated using varying energy inputs have the following four zones: no dross surface, dense-sinking transition surface, totally sinking surface, and forming failure surface. In the overhanging structures, fabricated with varying laser energy parameters, the angle corresponding to each region was different. The quality of the overhanging surface can be improved by reducing the laser energy. Additionally, a better overhanging surface can be obtained by increasing the obliquity angle. The variation trend of the roughness Rz was almost the same as that of Ra, but the variation range of Rz was much larger than that of Ra. Finally, a foldable abacus with several curved-surface overhanging structures was fabricated to verify the research results. Fundamental methods for controlling and optimizing the SLM-based direct fabrication of curved surfaces of overhanging structures are proposed in this paper, from the perspectives of crafting and design.