Influence of phase transformation on the micromachinability of laser powder bed fused Inconel 718

The characteristics of laser powder bed fusion (L-PBF) Inconel-718 (IN-718) differ from those of its wrought counterpart because of the rapid heating and cooling cycles involved during the process. Heat treatments are performed to enhance the characteristics of L-PBF IN-718 components before their suitability for practical applications. This investigation explores the influence of phase transformation-induced changes in the microstructure and thereby the machinability (specifically micro-turning) of L-PBF IN-718. Additionally, a comparison is made with its conventional counterparts (wrought specimens). The anisotropic characteristic of L-PBF IN-718 is only marginally affected by the standard double aging (DA) heat treatment. However, DA substantially impacts the microstructural characteristics, resulting in a notable increase in hardness of 12.2% and 20.7% for wrought and L-PBF IN-718 respectively due to precipitation of the hardening phases γ' and γ″. Experimental investigations were conducted to study the machining behaviour during micro-turning, focusing on cutting force, tool wear, surface roughness, and chip morphology. Noticeably, heat treatment led to a significant increase in the cutting force (120%), tool wear (67%), and surface roughness (33%) for the L-PBF samples, due to precipitation of γ' and γ″. Results indicate that to achieve better surface quality, it is recommended to machine the L-PBF specimens at a lower cutting speed whereas the wrought specimens at a higher cutting speed.