Study of microstructure and wear properties of laser borided Inconel 718

Abstract Effect of laser power density (LPD) on microstructure and tribological properties of laser borided Inconel 718 is studied. Boriding is carried out using laser powder bed fusion based additive manufacturing process at three LPDs of 17.0, 22.6 and 28.3 kW cm−2. Laser power density played significant role on surface roughness, homogeneity and depth of the borided layer. Higher laser power density led to lower surface roughness, improved homogeneity and higher depth of borided layer. Structural, chemical and microstructural characterization of the borided layer confirmed the formation of Nickel boride (Ni2B/Ni3B), Chromium boride (CrB/Cr2B) and Iron boride (FeB/Fe2B) along with the matrix γ (Ni-Cr-Fe) phase in borided layer. The content of γ phase in borided layer is found to be increased with increase in LPD due to dilution effect. The presence of different boride phases in the borided layer caused significant improvement in hardness (∼5 times) and wear resistance (∼8 times) than the substrate Inconel 718 for LPD of 17.0 kW cm−2. At higher LPD, due to increased content of soft γ phase, both hardness and wear resistance are reduced. Across the borided layer, microstructural morphology is changed from cellular to equiaxed dendrite with a clear columnar to equiaxed transition. The formation of different boride phases and microstructural evolution in the borided layer is discussed in view of available phase equilibria information of Ni-Cr-Fe-B system and thermo-kinetic theory. The findings of present study will be helpful in developing the Ni baser super alloys with improved tribological properties for advanced technological applications under extreme service conditions.