The mechanism of sample composition variation in the selective laser melting process based on the laser-induced breakdown spectroscopy and Raman system detection

Abstract

We jointly applied laser-induced breakdown spectroscopy (LIBS) and Raman spectroscopy to microscopically detect the unavoidable, harmful residual trace metal oxides in the parts manufactured by the selective laser melting (SLM) process. The elemental and molecular compositions of 316L stainless steel (316L SS) powder and SLM-made parts were examined, and the intensity-normalized LIBS information reflected the atomic and ionic content variations during the process. The normalized LIBS intensities of Ni I 357.18 nm, Cr I 461.61 nm, and Mo I 338.58 nm lines from the SLM parts were found to be decreased by approximately 21.82%, 15.96%, and 4.11%, respectively, compared to those from the 316L SS powder. In particular, the normalized intensities of Mn II 348.29 nm and Fe II 518.68 nm lines from the SLM parts were increased by approximately 60.19% and 25.56%, respectively, compared to those from the 316L SS powder. Raman spectroscopy results indicated the presence of Fe₃O₄ (676 cm⁻¹) and α-Fe₂O₃ (500 cm⁻¹ and 1300 cm⁻¹) oxides in the SLM parts; however, these oxides were not observed in the raw 316L SS powder. These oxides may cause a decrease in the surface tension and wetting ability of the metal melt pool, and in severe cases, the metal powder in the melt pool may not fully melt and cause the spherification phenomenon.