Study of complex chip free-surface profiles through the measurement of fractal dimension in face-milling Inconel 718

The popularity of Inconel 718 in various industries, including medical and aerospace, has risen due to its exceptional thermal and mechanical properties. However, the synergistic impact of process parameters on the very complex metallurgical properties exacerbates the stress concentration and fatigue crack initiation, leading to unprecedented tool failure, which affects the complexity of chip formation and surface integrity of the workpiece material. Various studies have evaluated the machinability of Inconel 718 by focusing on chip morphology and cutting forces. Only a few studies applied fractal theory to quantify the complexity of chip formation and machining outputs but not during interrupted CNC milling of Inconel 718 under flood cooling condition. In this work, the fractal dimension of the free surface profile of chips was measured and evaluated to investigate the synergistic impact of the cutting speed and feed rate on the surface roughness during face milling of Inconel 718. To improve the processing speed, cross-sectional images of Inconel 718 chips were captured using an optical microscope, processed via an image processing algorithm, and used to calculate the fractal dimension. The results showed that high cutting speed and feed rate reduced the complexity of chip formation and generated simpler geometry with lower fractal dimension values, while low cutting speed and feed rate resulted in more complex chips with fine features and higher fractal dimension values. The pitch distance had the highest correlation with fractal dimension value, while it was insignificant for chip compression ratio. Therefore, this research demonstrates the potential of fractal analysis to predict the complex chip morphology as a direct indicator of surface roughness and sub-optimal processing conditions during face milling of Inconel 718.