## 1 Introduction  
Inconel 718 is highly valued in the fields of aerospace engines and gas turbines due to its excellent high-temperature strength, good thermal stability, and excellent fatigue resistance [1; 2]. However, it has severe grain boundary embrittlement at room and elevated temperatures, and the precipitation process may produce phase boundaries (PBs) of characteristic structures due to atomic diffusion [3]. Small-hole components of Inconel 718, with seemingly simple structures, play a crucial role and have been widely used in fields such as aerospace engineering, medical devices, and energy and chemical industries [4; 5; 6]. To satisfy the precision machining requirements of small holes, small tool CNC machining is generally employed to improve the dimensional and shape accuracy of small-hole components. Machining processes for small holes typically utilize tools with diameters of less than 1 mm. Due to these extremely small sizes, one of the primary reasons for tool failure is breakage, which often results from excessive thrust force or torque [7]. Inconel 718 is widely recognized as one of the most difficult materials to machine, attributed to its remarkable tensile strength and poor thermal conductivity [8]. For decades, the machining process has been a significant area of research [9; 10; 11; 12; 13]. To enhance the machining efficiency of small holes in Inconel 718, [PERSON] et al. [14] introduced a novel water-oil (W/O) nanoemulsion dielectric for high-speed grinding of this alloy. The research focused on evaluating the material removal rate (MRR) and the relative electrode wear rate. Findings revealed that the application of this innovative coolant during high-speed grinding increased the material removal rate, decreased the relative electrode wear rate, and improved surface quality. [PERSON] et al. [9] investigated methods to lower production costsby eliminating cutting fluids, conducting experiments under dry machining conditions with both coated and uncoated carbide tools. The coated carbide tool minimized welding and the formation of unstable built-up edges while also decreasing abrasive wear. [PERSON] et al. [15] compared electrochemical machining (ECM) with EDM. The surface roughness of the rectangular groove was improved from Ra 3.82 \(\upmu\)m to Ra 0.86 \(\upmu\)m after ECM. The material removal rate of ECM was determined using an L9 orthogonal array based on the Taguchi technique [16]. The optimal parameters identified to maximize the MRR were a current of four amps, a voltage of 18 V, and an electrolyte concentration of 60 g/L. However, the disadvantage of EDM is that the EDM discharge leads to rapid wear of the tool electrodes. Therefore, [PERSON] et al. [17] present a hybrid process of grinding and electrochemical removal for the precision machining of small holes. The surface finish after machining was improved to Ra 1.25 \(\upmu\)m. Therefore, tool wear is effectively suppressed, and surface quality is improved. Therefore, in the conventional small hole machining process, low efficiency, low material removal rate, and rapid wheel wear are often observed. Unlike other machining processes, ultrasonic machining (UM) typically produces workpieces with excellent surface finish and high dimensional accuracy, even when dealing with difficult-to-machine materials. This is achieved because the material removal process involves the use of extremely fine and hard abrasive particles that are capable of efficiently cutting through tough and brittle materials [18]. [PERSON] et al. [19] utilized UM to machine micropores in Zr-Cu-Ti metallic glass and found that UM does not affect the amorphous morphology of metallic glass. The quality of the small hole was observed to be very good in the higher feed rate and with the use of abrasive slurry with abrasives of a higher grit number. However, Inconel 718 has a high tearing modulus; its material removal in UM can only rely on shear, material fracture, and material displacement rather than through plastic deformation removal.