Insight to agglomeration and chemical reactions of CoAl2O4 inoculants in IN718 processed by selective laser melting.

• Agglomeration of CoAl 2 O 4 inoculants follows the scan direction. • Modifying the scan speeds and energy densities can inhibit the agglomeration. • CoAl 2 O 4 was reduced within the melt pool to from Co-rich particles. • The extent of melt pool remelts the Co-rich particles present in deposited layers. • Insufficient energy input may lead to a slight fraction of unreacted CoAl 2 O 4. [Display omitted] The investigation aims to clarify the influence of scan parameters on the morphology and phase constitution of CoAl 2 O 4 inoculants in Inconel 718 (IN718) superalloy processed by selective laser melting (SLM). Gas atomized IN718 powder feedstock was uniformly blended with 0.2 wt% of CoAl 2 O 4 flakes and processed with a range of laser scan parameters to understand the effect of the inoculant additions on microstructure. Similar to the reactions observed in investment cast Ni-superalloys, the CoAl 2 O 4 particles were found to chemically react with elemental Al, Cr, and Ti present in the melt pool of IN718 and become reduced into Al 2 O 3 , Cr 2 O 3 , and TiO 2 oxides after the SLM process. Analyses of single melt pool beads revealed the existence of Co-rich metallic particles distributed along the surface of the oxides. However, since SLM processing involves remelting of the previously deposited layers to minimize the occurrence of porosity, this results in the dissolution of the Co-rich metallic particles that form when the CoAl 2 O 4 particles are reduced in the melt pool. As such, the ability of the metallic Co particles to contribute to grain refinement becomes limited during SLM processing. Furthermore, the inoculant particles were found to agglomerate along the scan direction. The underlying mechanisms can be attributed to the Marangoni convection following the moving center of laser beam. It was found that decreasing energy density and increasing scan speed can effectively reduce the size and number density of the agglomerated oxide particles. Faster laser scan velocities lead to reduced convective flow within the melt pool and reduce the tendency for agglomeration. However, the reduced magnitude of energy input limited the time over which the CoAl 2 O 4 particles could react with the melt pool. This led to the formation of lack of fusion defects and incomplete chemical reaction between CoAl 2 O 4 and IN718 after the SLM process. The effectiveness of CoAl 2 O 4 inoculant particles on grain refinement in as-built SLM IN718 microstructures is strongly dependent on the melt pool physics and laser scan conditions. [ABSTRACT FROM AUTHOR]