Effect of Ti Addition in AISI 316L stainless steel using Directed Energy Deposition (DED) Division of Advanced Nuclear Engineering Pohang University of Science and Technology

Abstract

The additive manufacturing process has gained significant attention for its abil- ity to produce net-shaped parts with high mechanical properties. AISI 316L stainless steel is widely studied for additive manufacturing (AM) due to its excellent printabil- ity, mechanical properties, and corrosion resistance. It is used in various fields includ- ing nuclear reactors, biomedical applications, and the aerospace industry. Stainless steel’s high corrosion resistance is attributed to the Cr2O3 surface film. However, pit- ting corrosion can occur due to local penetration of this protective oxide film, leading to exposure to the surrounding environment. MnS inclusions play a major role in pit initiation and propagation in stainless steel produced by conventional casting. These MnS inclusions, formed by adding Mn to improve material machinability, dissolve more readily than Cr2O3, exposing the metal to aggressive electrolytes. However, in additive manufacturing, Mn for de- oxidation and machinability is not essential, and the deoxidizing element Si can be replaced by another element. In this study, Ti was added to 316L stainless steel and Mn, Si were eliminated to capture sulfur and oxygen effectively by forming Ti-S and Ti-O, thus eliminating Fe-S and reducing oxygen content. The casting process, powder manufacturing process, and DED printing process were conducted. The powder, microstructure, hardness, and corrosion properties were analyzed. The microstructure of 316L-Ti-AM specimens exhibited equiaxed grains with an average size of 5.49μm, compared to 46.49μm in 316L-AM. The fabricated powder was homogeneous and had a circular shape, with avalanche energy lower than 1.4mJ/kg, indicating excellent flowability. Additionally, other flowability measure- ments confirmed the high quality of the powder. The 316L-Ti-AM showed about 5% of the BCC phase, while 316L-AM was fully FCC. The hardness results indicated that 316L-Ti-AM had a higher hardness (226.30 Hv) compared to 316L-AM (196.66 Hv). The primary strengthening mechanisms for 316L-Ti-AM were solid solution strengthening, grain boundary strengthening, and dislocation strengthening, whereas 316L-AM was mainly influenced by Orowan strengthening. There was no significant difference in corrosion resistance between 316L-AM and 316L-Ti-AM. Despite grain size refinement and fine cell structure potentially enhancing pitting resistance, no sig- nificant impact on pitting resistance was observed.