Effect of Ultrasonic Impact Path Spacing on the Cutting Mechanism of Inconel 718 Alloy

A three-dimensional coupled finite element model incorporating the cutting tool, workpiece, and simulated impact pin was developed using ABAQUS/Explicit to investigate the machining behavior of Inconel 718 under conventional cutting and ultrasonic impact-assisted cutting conditions. Three ultrasonic impact trajectory spacings of 2, 3, and 4 mm were considered, where the trajectory spacing denotes the transverse distance between the centerlines of adjacent ultrasonic impact trajectories. Single-factor analyses were conducted by varying the cutting speed, depth of cut, and tool rake angle. The investigated cutting speeds ranged from 400 to 2000 mm/min, the depths of cut from 0.09 to 0.21 mm, and the tool angle from 10° to 30°. Experiments were performed using a KZUIT-20C ultrasonic system operating at 20 kHz with a 4 mm diameter impact head and a Mitsubishi MV820 CNC machine tool equipped with a titanium-alloy cutting tool. The numerical model was established based on the Johnson–Cook constitutive model and validated against experimentally measured cutting forces, with a maximum relative error of 11.23%. The results show that the ultrasonic impact trajectory spacing has a pronounced influence on the subsequent cutting-force response of Inconel 718. With increasing cutting speed, the X-direction cutting force generally increases under all investigated conditions, whereas the Y-direction force exhibits a more condition-dependent response. At a cutting speed of 2000 mm/min, the X-direction cutting forces under conventional cutting and 2, 3, and 4 mm trajectory spacings are 53.48, 56.38, 56.45, and 40.49 N, respectively. The 4 mm spacing consistently produces the lowest X- and Y-direction cutting forces over the investigated cutting-speed range. As the depth of cut increases, the X-direction cutting force generally increases, particularly at larger cutting depths, while the 4 mm condition maintains a comparatively low and smooth force response. The Y-direction cutting force shows stronger fluctuations depending on the trajectory spacing and cutting depth. Variation in tool angle produces relatively moderate changes in the X-direction force, whereas the Y-direction force exhibits a more pronounced condition-dependent response, including a localized increase under the 3 mm spacing condition. The residual stress and tool-temperature responses further demonstrate that the spatial distribution of ultrasonic impact trajectories affects the subsequent thermomechanical behavior of the machined material. Within the investigated parameter range, the 4 mm trajectory spacing generally maintains relatively low cutting-force levels and a comparatively stable thermal response. These results suggest that an appropriate separation between adjacent impact trajectories can reduce excessive interaction between neighboring impact-affected regions and thereby modify the mechanical resistance encountered during subsequent material removal. The present findings provide a numerical and experimental basis for selecting suitable ultrasonic impact trajectory spacing and machining parameters for the high-performance machining of Inconel 718.