EFFECT OF HEAT TREATMENT ON MICROSTRUCTURES AND MECHANICAL PROPERTIES OF SS316L BY MICRO SELECTIVE LASER MELTING

Recently, micro selective laser melting (mu SLM) system equipped with finer laser beam has been developed to improve additive manufacturing resolution. The microstructures and properties of mu SLM produced metals and alloys could be different from those by conventional-size SLM, which warrants further investigation. Moreover, the widely used material, SS316L stainless steel, demonstrates unique cellular structures and excellent combination of strength and ductility after SLM. How the microstructures evolve after heat treatment and affect the mechanical properties remain to be clarified for mu SLMed SS316L. In this study, the effect of heat treatment on the microstructures and mechanical properties of mu SLMed SS316L was studied. Two heat treatment methods, namely HT650 degrees C-2h and HT950 degrees C-2h, were employed. It is found that the heat treatment has no effect on phase formation, and a preferred grain growth orientation with (110) plane along building direction and a single austenite phase was detected in all samples. Cellular structures were observed in as-printed samples and found to grow up a little bit after HT650 degrees C-2h, but disappear after HT950 degrees C-2h. Besides, more carbides are detected after HT650 degrees C-2h, while they are partially dissolved after HT950 degrees C-2h. For mechanical properties, the as-printed sample shows the best combination of strength and ductility, thanks to the strengthening effects from cellular structures and dislocations. The inferior mechanical properties after heat treatment is attributed to reduction of dislocations and disappearance of cellular structures. In addition, the presence of carbides can significantly reduce the ductility.