4.1 Effect of pulse voltage amplitude on micro-hole machining
The machined micro-holes by PA-STEM under different pulse voltage amplitude are shown in Fig. 9 and Fig. 10, it can be found that as the voltage amplitude increases, the machined micro-hole size, including the inlet diameter, the inlet rounded corner, and the machining depth, decrease continuously, indicating that the machining gap of PA-STEM is reduced and the machining accuracy is improved. At the same time, there is almost no stray corrosion around the entrance of the micro-hole. The main reason for this result is that the higher the voltage, the denser the gas film and plasma film, and the better the sidewall insulation effect of the tube electrode. In addition, the violently generated gas forms a gas-liquid two-phase flow in the machining gap, which reduces the conductivity of the electrolyte, thereby improving the overall machining accuracy. This result corresponds to the physical machining model of PA-STEM shown in Fig. 8.
4.2 Effect of tube electrode feeding rate on micro-hole machining
The tube electrode feeding rate has an important effect on the machining efficiency of PA-STEM. The machined micro-hole results with different electrode feeding rates are shown in Fig. 11 and Fig. 12. It can be found that as the feeding rate increases, the resulting diameter and depth of the micro-hole decrease continuously. At Vfeeding = 1 µm/s, the micro-hole diameter and depth are 941 µm and 1260 µm, respectively. However, at Vfeeding = 8 µm/s the diameter and depth decreased to 737 µm and 1086 µm, respectively. When the tube electrode feeding rate is slow, under the same feeding depth, the total machining time is longer, and the electrochemical reaction time is longer, so the machined micro-hole size is larger. On the contrary, when the tube electrode feeding rate is faster, the electrochemical reaction time is shortened, and the machining efficiency and accuracy are improved simultaneously. However, an excessive feeding rate is easy to cause abnormal spark discharge. As shown in Fig. 13(a), when Vfeeding = 8 µm/s, obvious thermal damage caused by spark discharge can be found at the bottom edge of the micro-hole. The principle of abnormal discharge is shown in Fig. 13(b). When the feeding rate is too fast (the feeding rate is greater than the dissolution rate of the workpiece material), the inter-electrode gap will continue to shrink. When the electric field strength of the inter-electrode gap is greater than the spark discharge threshold (about 107 V/m), high energy spark discharge will be triggered, resulting in tube electrode wear and micro-hole bottom thermal damage.
In order to further analyze the influence of feeding rate on discharge energy, the voltage-current waveforms at Vfeeding = 1 µm/s and Vfeeding = 8 µm/s were analyzed, as shown in Fig. 14. When Vfeeding = 1 µm/s (Fig. 14(a)), the gas film formed stably and the electric field strength was sufficient to induce discharge plasma. Among them, the dynamic formation of the gas film is the cause of the current waveform fluctuation. In addition, the plasma current is generally lower than 2 A, which will not lead to the power supply overpowering, so the voltage waveform is basically maintained as a square wave shape. However, at Vfeeding = 8 µm/s (Fig. 14(b)), due to the breakdown of the entire inter-electrode gap, a high energy spark discharge is formed, and the instantaneous current reaches more than 8 A. At this time, because the constant power output power supply is used, the voltage drops instantaneously while the current increases. In general, by studying the feeding rate of the tube electrode, it is concluded that Vfeeding = 4 µm/s is more suitable for high-quality micro-hole machining.
4.3 Effect of pulse frequency on micro-hole machining
The pulse frequency affects the energy of a single pulse, has an important effect on the plasma discharge behavior, and is also a key factor affecting the stability of PA-STEM processing. The resulting micro-holes under different pulse frequencies are shown in Fig. 15 and Fig. 16. It can be found that the higher the pulse frequency, the smaller the micro-hole size and the higher the machining accuracy. At f = 1 kHz, the micro-hole diameter reached 1619 µm, however, at f = 100 kHz, the micro-hole diameter was reduced by 53.06% to only 760 µm. On the other hand, although the pulse frequency has a relatively small effect on the depth of the micro-hole, it has a significant effect on the surface quality of the micro-hole. As shown in Fig. 17(a), when f = 1 kHz, there is obvious thermal damage caused by spark discharge at the bottom edge of the micro-hole, which is similar to the machining result obtained from the excessive feeding rate of the tube electrode above. At this time, the instantaneous current reaches about 8 A, and the voltage waveform drops during spark discharge. On the contrary, when f = 100 kHz, the pulse energy becomes discrete and uniform, the discharge current is lower than 1 A, and the voltage waveform is maintained as a square wave, and the bottom surface of the machined micro-hole is very smooth. Through the study of pulse frequency, it can be concluded that high pulse frequency is more suitable for high-quality micro-hole machining.
4.4 Effect of electrolyte flow rate on micro-hole machining
The electrolyte flow rate affects the electrolyte replenishment in the machining area and has an important effect on the inter-electrode electrolyte conductivity, as well as the flushing out of bubbles and electrolytic products. The micro-hole machining results under different electrolyte flow rates are shown in Fig. 18 and Fig. 19. It can be found that the micro-hole size is smaller when the electrolyte flow rate is low, and with the increase of electrolyte flow rate, the micro-hole diameter and depth increase. This result shows that the plasma stability is better at a low electrolyte flow rate. In which, when the electrolyte flow rate is low (Vflow = 1.8 ml/min), the rapidly generated gas leads to an increase in the volume fraction of the gas in the electrolyte, and a decrease in the conductivity of the local electrolyte, so the machining accuracy is improved. On the contrary, when the electrolyte flow rate is high (Vflow = 5.4 ml/min), the stability of plasma is affected by the violent flow field disturbance, which makes the precision and roundness of the micro-hole worse. Through the study of electrolyte flow rate, it is concluded that low electrolyte flow rate is more suitable for high-quality micro-hole machining.