Objective The interaction between laser and powder particles has a significant influence on the characteristics of the powders during the selective laser melting (SLM) process. However, the understanding of the transformation and evolution mechanism of the recycled powder in SLM is limited. The physical and chemical properties of powder change significantly with the increase in powder recycling times in the SLM process. Nevertheless, research on the variation of powder properties with the increase in recycling times is inadequate. Some researchers have studied the recycling of 316L stainless steel (SS), Ti6Al4V, AlSi10Mg, and CoCr alloy powders, but no consistent regulation has been obtained owing to the differences in recycling methods, forming equipment, forming processes, and the number of uses. The 316L SS powder is a widely used material owing to excellent corrosion resistance and good weldability. Therefore, studying the variation regulation of 316L SS powder characteristics with the increase in SLM recycling times is necessary to understand the variation and oxidation mechanism of the recycled powder, which helps to elucidate the reason for the unstable quality of SLM parts. The findings of this study can help understand the evolution of powder characteristics during the 316L SS powder recycling process better, judge recycling characteristics, and develop recycling standards. The study method is also applicable to other powder bed melting additive manufacturing technologies.
Methods The raw material selected was 20 kg 316L SS powder. In the early SLM experiment and small-batch processing, argon (Ar) with a purity of 99.99% was continuously filled to keep the oxygen (O2) content in the forming chamber below 5×10 -4(volume fraction) to prevent material oxidation during the forming process. After each test or production, the SS powder was strictly screened and then dried to remove water vapor. Some dried SS powder was taken as the sample for detection and analysis, and the physical and chemical characteristics were analyzed. The characteristics of the SS powder, such as particle size distribution, bulk density, tap density, fluidity, morphology, composition, O2 content, and surface microstructure, were comprehensively studied. No new powder was added to the sample powder during recycling. The particle size, physical properties, surface morphology, microstructure, and element content of SLM recycled powder were analyzed using a laser particle size analyzer, physical property tester, scanning electron microscope, and energy dispersive spectrometer.
Results and Discussions The proportion of fine and coarse SS powders, respectively, decreases and increases with increase in SLM recycling times, indicating that the particle size distribution of the SLM recycled powder was coarser and more concentrated. Owing to the formation of irregular coarse particles inducing more voids and bridging between the stacked powders, the bulk and tap densities of the recycled powders showed decreasing trends, and fluidity improved. The recycled powder had more irregular, sintered, broken, rod-shaped, and spherical particles than the virgin powder, which is owing to laser-powder-molten pool interaction. Large size powder particles with black circular spots on the surface were found in the powder that was recycled many times. EDS analysis showed that the circular oxidation spots were rich in silicon (Si) and manganese (Mn). The formation of circular spots on the surface of large particles was closely related to O affinity, high-temperature diffusion rate, and O2 partial pressure. The surface microstructure of the powder gradually changed from coarse dendrites to cellular crystals owing to the thermal radiation of the laser. In this study, the irregular particles in the recycled powders were divided into two categories: laser-induced melt ejection particles and gas entrainment-induced irregular particles. The different formation mechanisms of these two types of irregular particles were discussed in detail, respectively. The weak magnetic particles of 316L austenitic SS powders produced by phase transformation in SLM were also revealed.
Conclusions The D90 and D10 of 316L SS powder increased by 14.5% and 26.5%, respectively, after 30 recycling times in SLM. The particle size distribution of the powder coarsens with the increase in recycling times, with more rod-shaped, broken, and irregular particles in the recycled powder. The oxidation spots formed on the surface of spherical particles with larger diameters were rich in Si and Mn. The formation mechanism of oxidation spots was mainly determined by element type, O affinity, high-temperature diffusion rate, and oxidation potential. Dendrite content in the surface microstructure of the recycled powders significantly reduced. The irregular particles in the recycled powders were divided into two categories: laser-induced melt ejection particles and gas entrainment- induced irregular particles. The formation mechanisms of the two irregular particle categories were related to the molten pool instability and Ar vortex, respectively. The recycled powders also exhibited weak magnetism owing to recrystallization.
