## 3 Experimental Results and Discussion
### The Effect of HE Parameters on Specific Material Removal Rate and Surface Roughness  
The effects of HE voltage on specific material removal rates are shown in Figure 9. The specific material removal rate (96.5%) of Inconel 718 with HE treatment under 5 V was much higher than that of the original workpiece (69.1%), and no obvious changes in specific material removal rates were observed with increasing voltage from 5 V to 20 V.  
Figure 8: Scratching grooves of Inconel 718 ((**a**) is the initial portion, (**b**) is the middle portion, and (**c**) is the end portion) and 5 V HE samples ((**d**) is the initial portion, (**e**) is the middle portion, and (**f**) is the end portion). Figure 9: The effect of HE voltages on specific material removal rates. The effects of HE time on specific material removal rate are shown in Figure 10. With the increase in HE time, the specific material removal rate firstly increased and then decreased. It was demonstrated that when the HE time was less than 10 min, HE effectively improved the specific material removal rate from 69.1% to 96.5%. However, the specific material removal rate decreased to 72.2% when extending HE time to 20 min, and the removal rate even dropped to 56.6% when the HE time reached 40 min. To explore the mechanism of HE time on specific material removal rates, this study investigated the effects of HE time on material hardness, as increasing hardness would increase tool wear and thus reduces specific material removal rates. In this experiment, nanoindentation tests were performed for 10 cycles for each sample, and the load force was 10 mN. As shown in Figure 11, when the HE time was less than 10 min, there is little change in surface hardness. Meanwhile, the plasticity of the Inconel 718 alloy decreased rapidly by 66.1% within 10 min of HE treatment [27]. A large number of stress lines appeared in the subsurface, accompanied by a few microcracks. During the machining process, this subsurface stress is easily released, significantly reducing the energy required for deformation. As a result, compared to the original Inconel 718, the total fracture energy is reduced less. Consequently, the surface of the hydrogenated specimens shows numerous micro fractures. Therefore, the cooperation of lower hardness and plasticity facilitates the improvement in specific material removal rates. As the HE time increased from 10 min to 40 min, the surface hardness dramatically increased from 340 HV to 512 HV. According to [PERSON] et al. [28], the reason for such a change in hardness is due to the \(\gamma^{\prime}\) phase precipitates. The much higher surface hardness overcompensated for the effect from lower plasticity, ultimately leading to a decrease in specific material removal rates. To investigate the material removal mechanism, a study was conducted on blind hole machining with a depth of 0.1 mm. The roughness at the bottom surface of the blind hole was measured. As shown in Figure 12, when the HE time increased, the surface roughness of the hole bottom decreased. The minimum value of roughness was 0.177 \(\upmu\)m at 40 min HE. As shown in Figure 8, better surface quality was obtained with the HE samples. After HEUM, britteness material removal occurred on the machined surface. To provide more intuitive evidence of the surface roughness, this study used SEM to observe the surface morphology of the blind hole (Figure 13). In the presence of HE treatment, the material removal mode changed to brittle deformation, with only 2 \(\upmu\)m long fractures remaining at the bottom surface, and the fracture length was comparable to the abrasive diameter, effectively improving the surface roughness of the hole bottom (Figure 13b-e). In contrast, the material removal mode exhibited plastic deformation, with a large number of residual  
Figure 10: The effect of HE times on specific material removal rate. scratches and abrasive adhesion on the surface after processing without HE treatment (Figure 13a).