# Experimental Study of Tool Wear in Electrochemical Discharge Machining
## 4 Discussion  
### Tool Wear  
In electrochemical discharge machining, when the voltage does not reached the critical voltage, there are not enough bubbles generated by the electrochemical reaction to form an insulating gas film on the cathode. When the critical voltage is reached, the generation rate and escape rate of the bubbles are equal, which can form an insulating gas film on the cathode and discharge to generate sparks. In general, more electrochemical energy can be provided by increasing the voltage, thus resulting in more frequent spark discharges. However, tool wear may also occur. Therefore, the cathode critical voltage of 28 V was chosen as the starting voltage in the experiment. Within the range of 28-64 V, the cathode weight loss is shown in Figure 5a. It can be seen that when the voltage is below 44 V, the cathode has almost no loss. On the one hand, the negative cathode weight loss may be caused by measurement error due to the impurities attached to the cathode when it is exposed to air. On the other hand, according to the study of [PERSON] and [PERSON] [25], in the sodium hydroxide electrolyte, granular or flocculent particles may condense on the cathode surface after the cooling of electrolyte, and this may also cause an increase in the weight of the cathode, as shown in Figure 6b,c. This phenomenon was also found at high voltage, as shown in Figure 6d. Above 44 V, the cathode weight loss increases sharply with the increase in voltage. Similarly, with the increase in frequency, duty cycle and electrolyte concentration, the cathode weight loss shows an increasing trend, but no linear relationship seems to be presented between the speed and the cathode weight loss, and the variation range of the cathode weight loss is very small. The cathode length loss is shown in Figure 5b. It can be seen that at low voltages, the cathode length almost does not change; when the voltage exceeds 58 V, the cathode length presents significant reduction. The shortening of the cathode changes the immersion depth. As can be seen from Figure 3, the tool wear mainly occurs at the end and the bulge part of the edge, and the loss is diffused from the edge tip to the root. This phenomenon is consistent with the findings of [PERSON] et al. [6]. They pointed out that a higher current density will be generated near the edge of the tool electrode, and more severe wear is expected to occur at the edge of the tool electrode. From the perspective of energy, high voltage provides more energy, and the energy transferred to the cathode in the form of thermal energy through spark discharge will also increase. When the local temperature of the cathode reaches the melting point, local melting and even the vaporization of material may occur. However, the spark discharge is denser at the end of the cathode and at the part with the edge bulge. Therefore, the tool wear is more likely to occur at these locations, as shown in Figure 6e,f. In addition, the tungsten carbide cathode is manufactured with powder metallurgy. [PERSON] and [PERSON] [16] pointed out that the surface temperature of tool electrode will rise to 2800 \({}^{\circ}\)C at the voltage of 50 V. At high temperatures, the tungsten carbide material may melt and remain in the electrolyte in the form of powder. There is also less debris remaining on the electrode surface. Therefore, at higher voltages (>56 V), there is dense concave shape loss in the spiral inner concave of the cathode. When the duty cycle increases, the spark discharge time t2 will also increase accordingly, and more energy will be transferred to the cathode. More severe tool wear has occurred in this case. The input energy decreases with the increase in frequency. According to the research by [PERSON] et al. [26], the critical voltage is proportional to frequency. At lower pulse frequencies, the critical voltage will increase sharply, and more energy is required to generate spark discharges. Therefore, in the process of increasing frequency, the energy required to generate spark discharges declines.