Insight to Potential of TiB2 and CeO2 Inoculants on Microstructural Evolution in Laser Powder Bed Fusion Processed Superalloy IN718

Abstract

This investigation focuses on understanding the microstructural changes resulting from the presence of CeO2 and TiB2 inoculants in superalloy Inconel 718 (IN718) processed by laser powder bed fusion (L-PBF). It was demonstrated that additions of 1 vol pct of CeO2 or TiB2 inoculant blended with the powder feedstock effectively refined the grain structure of IN718. Interestingly, compared to the TiB2 inoculant particles that were largely unchanged in the melt pool, the CeO2 particles reacted with the melt, forming Ce(Al, Ti)O3 oxides that melted and agglomerated along the melt pool boundaries and surface during the L-PBF process. The presence of CeO2 and Ce(Al, Ti)O3 oxide particles in the melt pool contributed to decreasing the cooling rate during solidification. As a result, the as-built grain structure was noticeably finer in L-PBF IN718 containing TiB2 compared to those containing CeO2, processed under the same parameters. Detailed characterization of the as-built microstructures revealed that the TiB2 inoculant particles did modestly react with the IN718 and formed minor amounts of nano-borides formed in the vicinity of TiB2 particles. Following a solution heat treatment of 2 hours at 1150 °C, the overall fraction of borides in the microstructure increased, effectively pinning grain boundaries and preventing grain growth. These borides contributed to retaining the as-built grain structure and significantly increased the microhardness by ~ 85 HV0.5 compared to L-PBF processed IN718 without inoculant addition.