Machining-induced surface integrity and nanocrystalline surface layers in cryogenic finishing turning of Inconel 718

In this study, the effects of cryogenic and flood cooling on the surface integrity of Inconel 718 are investigated for face turning with four selected cutting speeds of 25, 50, 75 and 100 m/min. Surface integrity of machined samples was characterized in terms of surface morphology, sub-surface microstructure, microhardness, x-ray diffraction textures, and residual stresses. While the differences between cryogenic and flood cooling were relatively limited for the majority of surface integrity metrics, a substantially increased (+80% vs flood condition) nanolayer depth was observed at the highest cutting speed of 100 m/min with cryogenic cooling. Additionally, cryogenic cooling resulted in slightly improved surface roughness and slightly increased compressive residual stress, particularly at elevated cutting speeds. Nb-rich secondary phases were detected after machining for all conditions, however, cryogenic cooling and low cutting speed led to reduced mixing of these nanocrystalized phases in the recrystallized surface layer. Based on these observations a and qualitative model for surface generation and nanocrystallization under flood and cryogenic machining conditions was proposed. Overall, the effect of cryogenic cooling on nanolayer generation was most pronounced at elevated speeds, suggesting the potential for cryogenic cooling to allow for more aggressive, yet sustainable, processing strategies with improved surface integrity.