Experimental study on high-efficiency DC short electric arc milling of titanium alloy Ti6Al4V

Short electric arc milling (SEAM) is an efficient electrical discharge machining method, especially for the efficient removal of difficult-to-machine conductive materials with high hardness, high toughness, and wear resistance. In this study, titanium alloy Ti–6Al–4V is used as the research object to conduct machining experiments. The material removal mechanism of SEAM technology is studied using a DC power supply and different tool electrode materials (copper, graphite, Q235 steel, and titanium). The energy distribution of the discharge gap is analyzed using a data acquisition system and a high-speed camera. The arc is found to move with the spindle rotation in the process of arc discharge, and multi-point discharge occurs in the process of single-arc discharge. The voltage and current waveforms and the radius of the etched particles during the experiment are measured; the material removal rate (MRR) and relative tool wear rate (RTWR) are calculated; and the surface and cross-section micromorphology and hardness are analyzed. The experimental results show that when the electrode material is graphite, the maximum feed rate is 650 mm/min; the MRR can reach 17268 mm3/min; the ideal maximum MRR is more than 65,000 mm3/min; and the RTWR is only 1.27%. When the electrode material is Q235 steel, the minimum surface roughness is 35.04 μm, and this material has good stability under different input voltages. When the electrode material is copper, the hardness of the resolidified layer is close to that of the base material, which is beneficial for further processing. The lowest specific energy consumption is 18.26 kJ/cm3 when titanium is used as the electrode material.


Introduction
Titanium alloy Ti-6Al-4V has good mechanical properties such as high strength, good corrosion resistance, and high heat resistance. It is widely used in the manufacturing of aircraft engine compressor parts and rockets, missiles, and high-speed aircraft structural parts, as reported by Shokrani et al. [1]. However, Dandekar et al. found that due to the low thermal conductivity of titanium alloy, it is difficult to expel the heat generated, resulting in extremely high temperatures in the cutting deformation area and a small range near the cutting edge, thus significantly shortening the tool life and resulting in a low processing efficiency and high cost [2]. However, as a non-contact machining method, electrical discharge machining (EDM) effectively overcomes the difficulty in the traditional machining of titanium alloys.
Many researchers have focused on reducing the electrode loss, improving the material removal rate, and exploring the machining mechanism. Kunieda proposed electrical discharge machining in gas, which improved the machining efficiency and reduced the electrode loss significantly [3]. Li et al. proposed EDM-assisted milling to process Ti-6Al-4V alloys. The results show that compared with traditional milling, the electrode loss and cutting force are significantly reduced, and the surface quality is improved [4]. Ghoreishi et al. proposed ultrasonic-vibration-assisted electrical discharge machining to improve the machining efficiency and reduce the short circuit rate [5]. Mao.et al improved the machining efficiency of EDM drilling by changing the electrode shape and achieved a greater effective cutting depth [6]. Zhao et al. proposed blasting erosion arc machining (BEAM) using multi-hole electrodes, which significantly improved the processing efficiency [7]. Han et al. proposed replacing a pulse power supply with a DC power supply to generate moving arcs, which significantly improved the MRR of processing [8]. Although the above research on EDM technology makes the machining performance of titanium alloys better than that of traditional machining in terms of the tool loss, surface quality, and dimensional accuracy, it is insufficient for applications requiring mass removal of materials. Short electric arc machining (SEAM) technology has significant advantages in this regard.
The SEAM proposed by Zhou et al. is a type of machining method that uses instantaneous high temperatures generated by a short electric arc discharge between electrodes to remove conductive metals under the action of a certain proportion of gas-liquid mixture medium at a certain pressure. It uses low voltage and high current source to generate continuous arc in a water-air mixture medium or air to quickly remove conductive metal materials [9]. Wang et al. proposed a high-speed EDM and arc hybrid machining method with DC voltage and pulse voltage in parallel, which significantly improved the MRR under the condition that the electrode loss is similar to that of conventional EDM [10]. Zhu et al. used a highfrequency (500 Hz), high-voltage power (1000 V) parallel DC power supply and found that, the higher the pulse voltage, the higher the MRR and the lower the tool wear rate (TWR) [11]. Shen et al. used a high-frequency pulse voltage (310 V/5 kHz) and a low-frequency pulse voltage (70 V/80 Hz) parallel hybrid power supply for EDM milling, realizing improved machining stability and MRR [12]. Kou and Han proposed a moving arc high-speed EDM milling method to process titanium alloys, studied the arc elimination mechanism and process, and obtained high MRR and low electrode loss [13]. Liu et al. studied the effect of electrode polarity on DC short arc milling of titanium alloys and found that electrode negative machining can yield a higher machining efficiency [14]. Li et al. studied the influence of different electrode materials on pulse short arc milling and found that graphite provides a lower electrode loss rate, whereas red copper provides better surface quality [15]. Chen et al. studied short arc milling GH4169 with a pulse power supply and further revealed the material removal mechanism by analyzing the influence of process parameters and electrode polarity changes on machining performance [16]. In summary, SEAM has shown excellent performance in machining shaft parts made of difficult-tomachine materials. It inherits the advantages of the EDM while being cost-effective and highly efficient.
In this study, the DC SEAM method is used to analyze the influence of different electrode materials on titanium alloy Ti-6Al-4V. With the help of a high-speed camera, the formation process, discharge characteristics, and erosion mechanism of DC arc are explored, which lays a theoretical foundation for the further development of SEAM.

Experimental principle
SEAM is a partial non-equilibrium thermodynamic process under the coupling effect of multiple physical fields. Its essence is to form electric arc discharge in a short distance channel by inducing a voltage breakdown between the tool electrode and workpiece. The high-density heat energy generated in the arc channel melts the parts to be machined, and the melt is eroded under the combined action of the blowing force of the high-pressure airflow and the thermal explosion force. The processing schematic of SEAM is shown in Fig. 1, mainly including CNC machine tools, power supplies, air compressors, water circulation systems, data acquisition systems, and high-speed camera systems.
As shown in Fig. 2a, as the gap between the electrodes decreases, the electric field strength between the electrodes increases sharply during operation. When the electric field strength between the two poles increases to a certain extent, electrons ionize from the cathode surface, move toward the anode, and collide with the molecules or neutral atoms in the water-gas-mixed medium, resulting in collision ionization, forming more negatively charged electrons and positively charged ions, leading to avalanche surge of the charged particles, which produces extremely high temperatures in a few microseconds [17]. Under the action of high-temperature gasification, a melting zone and a heat-affected zone are formed around the arc. The high-density heat energy generated in the arc channel melts the part to be machined, and the melt is taken away from the discharge area under the combined action of the high-pressure water-gas medium and tool electrode rotation. Figure 2b shows an image of SEAM.

Experimental conditions
The experimental system includes the following: 1. Experimental equipment: Short electric arc CNC milling machine, high power pulse power supply, metallographic microscope, metallographic sample grinding machine, and polishing machine.
2. Workpiece material: Ti-6Al-4V; its mechanical properties are listed in Table 1. 3. Tool electrode: Copper, graphite, Q235 steel, and Ti tubular electrode with an inner diameter of 6 mm and an outer diameter of 18 mm; the electrode speed is 600 r/min. Table 2 lists the thermophysical properties of the electrode and workpiece materials at room temperature.

Experimental procedure
To explore the influence of different electrode materials on the SEAM of titanium alloy Ti-6Al-4V, experiments were carried out with different input voltages as variables in this study. The detailed parameters of the experiment scheme are shown in Table 3. The parameters of the single-arc discharge experiment scheme of the different electrode materials are shown in Table 4. The energy and current dynamic change mechanism of a single arc of a graphite electrode material were studied using a high-speed camera and data acquisition system. The Ti6Al4V and tool electrodes were weighed using a precision balance having an accuracy of 0.1 mg. During the experiment, the DeweSoft SIRIUS multichannel data acquisition system with a sampling rate of 1 MHz was used to measure the gap current, gap voltage, and power output voltage. The surface roughness and three-dimensional profile of the specimens were measured using the VHX-6000 digital microscope with super-field lens. The workpieces were cut into 8 ×  8 × 6 mm3 sample blocks using a wire-cutting machine. The sample blocks were investigated by inlaying, grinding, polishing, and chemical etching. The grinding procedure was conducted to sequentially grind the machined section with P600-P3000 sandpaper. The polishing procedure involved fine polishing in the same direction using a polishing disc covered with a face cloth and a polishing agent, and the sample blocks were rinsed with water after polishing. The chemical etching procedure was performed with HF, HNO 3 , and H 2 O in the ratio of 3:6:91; the cross section of the samples was wiped for around 10 s and then cleaned with deionized water. The machined surface and cross section were visualized by cross-sectional scanning electron microscopy (SEM). The composition of the surface and cross section was examined via energy dispersive spectroscopy (EDS), and the crosssectional micro-hardness was measured using the Vickers micro-hardness test.
The calculation formulae for the MRR, RTWR, and SEC are as follows: where M wi and M wj denote the masses (g) of the workpiece before and after processing, respectively; ρ w denotes the density of the workpiece (g/cm 3 ); t is the processing time (min), M ei and M ej denote the masses (g) of the tool before and after machining (g), respectively; V refers to the output voltage (V) of the power supply during machining; I refers to the current (A) through the gap; W refers to the actual milling width; D refers to the actual milling depth; and V m is the maximum feed speed. The SEC is the specific energy consumed to remove a unit volume of material.

Results and discussion
3.1 Material removal rate and relative tool wear rate Figure 3 shows the effect of voltage variation on the MRR of different electrodes. The MRR has little difference when the voltage is low. The reason for this phenomenon is that the energy generated by the arc is limited, and the processing speed is low under the condition of low voltage. With the increase in the voltage, the MRR of the four electrode materials increases to varying degrees. The MRR of the graphite electrode has the most significant increasing trend. The MRRs of copper and graphite are similar, whereas that of titanium is the lowest. When the voltage is 30 V, the maximum feed rate of the graphite electrode can reach 650 mm/min. According to Eq. (1), the actual MRR is 17,268 mm 3 /min. This is due to the lower feed speed in the initial stage of processing, which increases the overall processing time. The removal rates of copper, Q235 steel, and titanium were 6287, 5709, and 2803 mm 3 /min, respectively, at 30 V, mainly due to the loss of electrode material.
The SEAM experiment does not involve a constant maximum feed speed, which includes the initial feed speed and the maximum feed speed, as shown in Fig. 4. In the initial feeding stage, the internal flushing fluid does not participate in the processing; only the external flushing fluid is used to wash the erosion material between the poles. If the speed is too high, the erosion material will not be discharged in time and short circuit may occur. With the movement of the electrode, the spindle feed speed can be adjusted to the maximum feed speed when the internal and external flushing fluids are functioning at the same time. Therefore, the initial feed speed will increase  Owing to the existence of the discharge gap during the SEAM process, there is a certain amount of overcut between the electrode and the workpiece. Based on a large number of experimental findings in the early stages, when graphite is used as the electrode, the width of both sides of the workpiece is set to 1 mm and the depth is set to 1 mm. Therefore, when the electrode loss is ignored, W = 20 mm and D = 5 mm in Eq.
(2); furthermore, according to Eq. (2), MRR can exceed 65,000 mm 3 /min. shows the processed images of the electrode materials under a voltage of 30 V. The RTWR of graphite electrode is the lowest, and the relative electrode loss is only 1.27% at a voltage of 30 V. This is mainly due to the higher melting point and heat capacity of graphite. The RTWRs of copper and Q235 steel are 40.44% and 47.5% respectively, and the losses are at the corners. This is because their melting and boiling points are low, and the main discharge area between the bottom and the cylindrical outer surface of the electrode is easily melted and vaporized during the processing. The RTWR of titanium is 104.29%, and the loss is mainly at the end face. This is largely due to the poor thermal conductivity of titanium, which easily leads to the melting of the electrode material at continuously high temperatures during the processing. Therefore, when titanium is used as the electrode material, an axial tool compensation is required.

Specific energy consumption
Through a statistical analysis of the electrical parameters obtained from the data acquisition system, the influence of voltage on the SEC of different electrode materials is shown in Fig. 6. The SEC of the different electrode materials gradually decreases with the increase in the voltage. When the voltage is 15 V, the SEC is highest for graphite, 756.2 kJ/cm 3 . This is mainly due to the lower conductivity of graphite compared with those of the other three materials, which leads to greater energy consumption. With the increase in the voltage, the increased material removal leads to a significant reduction in the energy consumption. When the voltage is 30 V, the energy consumptions of copper and Q235 steel are 107.16 kJ/cm 3 and 84.85 kJ/cm 3 respectively. The main reason is that the material removal is reduced due to excessive electrode loss. The energy consumption of titanium is the lowest and changes little with increasing voltage; it is only 18.26 kJ/cm 3 at 30 V. This phenomenon is mainly due to the low thermal conductivity of titanium, which easily leads to an increase in the temperature of the processing area and helps remove material. Compared with the other electrode materials, the voltage has little effect on the energy consumption of titanium electrode.

Surface roughness
The influence of voltage on the SR of the workpiece processed with different materials is shown in Fig. 7. In this study, the SR was expressed using the parameter Sa (arithmetic mean height of the surface). The Sa values of graphite and titanium electrode materials increase significantly with the increase in the voltage, whereas the increase is minor in the case of copper and Q235 steel electrode materials. When the voltage is 15 V, the Sa values of graphite, titanium, copper, and Q235 steel electrodes are 21.58, 30.59, 38.75, and 32.71 μm, respectively. This is because when the voltage is low, the arc energy is limited, and the processing speed is low. The melted material in the processing area can be discharged in time with the rotation of the electrode and the erosion of the working medium; hence, Sa is low. With the increase in the voltage, due to the difference in the conductivity and other thermophysical properties of the different electrode materials, the volume of the melted material in the processing area will increase to varying degrees with the increase in the arc energy, which causes the surface roughness of the workpiece to change to varying degrees.

Analysis of single-arc discharge
To explore the single-arc discharge energy difference and current dynamic change mechanism of the different electrode materials, the data are statistically analyzed using a data acquisition system and a high-speed camera. The main purpose Tool speed (r/min) 600 is to explore the discharge mechanism of DC SEAM by comparing the discharge waveform and arc shape at the same time.
The specific procedures are as follows: 1. Clamp the tool electrode and the workpiece respectively. 2. Adjust the position of the water vapor working medium. 3. Adjust the lens position of the high-speed camera with the filter and connect it to Computer 1.
4. Connect the multi-channel data acquisition system to Computer 2 to measure the output voltage and current of the power supply through the Hall sensor.
5. Connect the positive pole of the DC power supply to the workpiece and the negative pole to the electrode; set the voltage to 30 V with a maximum current of 2000 A before powering on. 6. Manually control the spindle feed at a very low speed and immediately retract the spindle after observing the discharge; at the same time, record the discharge process with a high-speed camera.
The experimental results of the single-arc discharge at 30 V are shown in Fig. 8. Figure 8a shows the machining traces formed by single-arc discharge of the different electrode materials on the Ti-6Al-4V surface. The graphite electrode has the highest discharge energy, with the pit diameter reaching 23.99 mm, followed by copper and Q235 steel, with pit diameters of 23.28 mm and 23.03 mm, respectively. The discharge energy is lowest for the titanium electrode, and the pit diameter is 21.89 mm. Figure 8b shows the waveform corresponding to the single-arc discharge of the graphite electrode. The duration of a single-arc discharge is 123 ms, and the current waveform is unstable. Parts (c), (d), and (e) in Fig. 8b are respectively the initial stage of the discharge channel establishment, the stage of current dynamic change, and the stage of current step change. Figure 8c, d, and e show the arc dynamic change diagrams corresponding to the parts (c), (d), and (e) in Fig. 8b, respectively. As the two poles come closer, the electric field strength increases. Figure 8 c shows that at 26.845 s, the voltage between the two poles breaks down the working medium, and an arc appears with a weak arc light. The current is very low at the initial stage of the establishment of the discharge channel, but then increases rapidly and reaches 800 A in approximately 10 ms. From the position shown in Fig. 8c, we find that the position of the arc is shifted relative to the initial position with the rotation of the electrode, and it plays the role of breaking the arc under the high-speed rotation, which effectively avoids the electric discharge of the workpiece surface. This has important significance in the energy control of the arc. Figure 8d shows the arc state when the current rises steadily again under  Fig. 8b, the current begins to increase at the same time, which indicates that there are multiple discharge points in a single arc during the machining process. This will not end until all the discharge channels are extinguished at the same time, at which point the single-arc machining is complete. Figure 8e shows the corresponding arc state when the current changes step. From the corresponding current waveform shown in Fig. 8b, the current decreases from the original trend and increases by 400 A instantaneously at 26.913 s. Figure 8e shows that the arc light is getting stronger, and even a flame is produced. This may be due to the short circuit at the discharge point between the two poles, which causes the instantaneous current to increase. However, as the discharge point is eroded, the arc again tends to stabilize. Figure 9a shows the voltage and current waveforms of the copper electrode SEAM. The peak current (I P ) reaches 1109 A, and the corresponding single-arc discharge time is 59.23 ms. The discharge frequency is high, but the current is mostly low. This phenomenon is mainly due to the low heat capacity and melting point of copper and its high thermal conductivity. During the processing, secondary discharge or multiple discharges are induced due to the continuously melted particles on the electrode surface between the two poles. Figure 9b shows the voltage and current waveforms of graphite electrode SEAM, where the peak current (I P ) is 945 A, the corresponding single-arc discharge time is 48.79 ms, and the discharge frequency is the highest, which is almost continuous. This is mainly because the graphite electrode has the highest machining speed and low electrode loss, and the discharge channel  Figure 9c shows the voltage and current waveforms of the Q235 steel electrode SEAM; the peak current (I P ) is 1503

Analysis of voltage and current wave
A, and the corresponding single-arc discharge time is 11.98 ms. The discharge frequency is low, and the single-discharge time is less, which is mainly due to the low melting point and thermal conductivity of Q235 steel. The high temperature of singledischarge energy leads to a large amount of material erosion. Figure 9d shows the voltage and current waveforms of the titanium electrode SEAM. The peak current (I P ) reaches 1181 A, and the corresponding single-arc discharge time is 8.87 ms. The discharge frequency is the lowest, and the single discharge time is the smallest. This is due to the low thermal conductivity of the electrode and the workpiece; the high temperature of the machining area makes the electrode and the workpiece melt simultaneously, and the discharge gap easily undergoes a short-circuit instantaneously, resulting in a significant leakage of the single-discharge energy.

Current proportional distribution
To study the energy distribution of the different electrode materials in the SEAM process of titanium alloy Ti-6Al-   Figure 11a shows the 3D surface topography of the workpiece machined using the copper electrode when the voltage is 30 V. Sa and Sz are 52.05 μm and 619.75 μm, respectively, and the surface of the workpiece is relatively flat. This is due to the small energy of the single discharge, and the fact that most of the discharges are small current discharges, and the surface pits formed in the machining process are small. Figure 11b shows the 3D surface topography of the workpiece processed by the graphite electrode. Sa and Sz are 112.8 μm and 737.87 μm, respectively, and the surface quality of the workpiece is the worst. Due to the high processing speed, the rotation of the electrode, and the erosion of the working medium, the moving track of the workpiece after the arc erosion can be clearly seen. Figure 11c shows the 3D surface topography of the workpiece processed by the Q235 steel electrode. Sa and Sz are 35.04 μm and 359.61 μm, respectively. The surface quality of the workpiece is the highest, and there are evident traces of working medium scouring on the surface of the workpiece. Figure 9c shows that the peak current during the SEAM of the Q235 steel electrode is larger, which leads to a higher arc energy density. Most of the single discharge can effectively erode the matrix material and discharge it through the working medium. Figure 11d shows the 3D surface topography of the workpiece processed by titanium electrode. Sa and Sz are 70.23 μm and 690.40 μm respectively, and the surface quality of the workpiece is poor. This is due to the low thermal conductivity and good elongation of the titanium electrode during the processing, and the material easily accumulates on the surface of the workpiece and solidifies again after melting at high temperatures. Figure 12a, b, c, and d show the erosion particles of the different electrode materials in the electric discharge gap of SEAM. Figure 12e shows the quantitative statistical analysis of the percentage data of different radii, realized through the image analysis software Engauge digitizer. Figure 12a, b, c, and d show that most of the particles produced by EDM of the four electrode materials are spherical, spherical shell, or ellipsoidal, which indicate that the surface tension plays a major role in the phase transition of the particles from liquid to solid. The same was concluded by Xu et al. [18]. The statistical data in Fig. 12e show that the proportion distribution of particle radius is normal, as reported by Murti and Philip [19]. The particle radius produced by SEAM with the graphite electrode is the largest, with an average value of 574.7 μm, which is 30 times that produced by EDM. This is mainly due to the high energy density in the short arc milling process. The average radius of the particles produced by the SEAM of copper, Q235 steel, and titanium electrodes are 503.1, 402.5, and 279.9 μm, respectively. This result does not correspond to the energy density relationship of the different electrode materials, which indicates that particle formation is due to multiple factors.

Section profile dimensions
To study the influence of electrode loss on the machining dimension accuracy, the machining section is divided into an inlet section and an exit section based on the feed direction of the electrode, and the width is measured at the middle position of the machining depth. Figure 13a, b, c, and d show the cross-sectional dimension diagrams of copper, graphite, Q235 steel, and titanium electrode when the voltage is 30 V. The absolute differences in the two cross-sectional profile dimensions are 2603.7, 202.7, 2169.6, and 283.9 μm, respectively. The above data show that the cross-sectional dimension accuracy of graphite and titanium electrode materials is better, whereas the dimension deformations of copper and Q235 steel electrode are larger due to electrode loss in the processing. This is mainly due to the SEAM process. The graphite electrode loss is very small. Although the titanium electrode loss is large, it is mainly from the bottom end surfaces. Appropriate electrode compensation can maintain a good tool size; hence, the machining process of the graphite and titanium electrode maintains good tool shape integrity, resulting in higher dimensional accuracy. The main loss of the copper and titanium electrode is the corner loss. Through processing, the width and depth of the electrode EDM area gradually become smaller, resulting in poor dimensional accuracy.

Analysis of surface micromorphology
The surface micromorphologies of the workpiece processed with different electrode materials are shown in Fig. 14a-d. Evidently, there are different degrees of microcracks, spherical droplets, and resolidified layers on the surface of the four types of electrode materials. Microcracks are formed when the corrosion products that cannot be discharged in time cover the surface of the workpiece under the rapid cooling of the working medium, and the concentrated stress due to the rapid change in the temperature exceeds the yield limit stress of the material. Spherical droplets fall on the recast layer due to the surface tension and gravity of the droplets in the molten pool. A resolidified layer is formed on the surface of the workpiece because the melted metal between the two poles is not discharged in time. Figure 14e and f show the EDS spectra of the droplets and resolidified layers formed on the surface of the workpiece processed using four types of electrode materials. The main elements in the droplets and recast layers are C, O, and Ti, and the corresponding metal elements of the electrode materials. This is mainly because at high temperatures, C, O, and Ti tend to undergo chemical reactions to  Figure 15a, b, c, and d show the cross-sectional morphologies of Ti-6Al-4V workpieces processed by copper, graphite, Q235 steel, and titanium electrodes, respectively, and Fig.  15e-h show the EDS spectra of the resolidified layer, heataffected layer, and base material of the corresponding workpiece material. As shown in Fig. 15a, the thickness of the heataffected layer corresponding to copper is 44.5 μm, which is mainly due to the good thermal conductivity of copper, and most of the heat is dispersed in the discharge gap and electrode part. The maximum thickness of the heat-affected layer is for the workpiece material processed by the graphite electrode. As shown in Fig. 15b, the maximum thickness reaches 162.6 μm, which is due to the high discharge frequency and high energy density during processing. As shown in Fig. 15c, the thickness of the heat-affected layer corresponding to Q235 steel is in the range of 55.2-80.4 μm, which is greater than that in the case of copper. This is due to the low heat conductivity of Q235 and slow cooling of the working fluid. As shown in Fig. 15d, the thickness of the heat-affected layer corresponding to the titanium electrode is 38.6 μm; however, the resolidified layer is thicker and has many cracks. This is because the electrode and workpiece materials have extremely poor thermal conductivity; hence, a large amount of etchants easily accumulates on the surface of the workpiece. Most of the heat is distributed on the resolidified layer and cools rapidly with the working medium, resulting in a large number of microcracks. Figure 15e, f, g, and h show that the main elements of the electrode material and working medium increase to different degrees in the resolidified layer and heat-affected layer of each workpiece. This is mainly because in the SEAM process of titanium alloy Ti-6Al-4V, the high temperature makes the electrode and workpiece to melt and vaporize in the working medium, and a complex chemical reaction occurs, which leads to the migration and penetration of the main elements in the electrode material and working medium to the resolidified layer or heat-affected layer. The amount of migration or penetration depth is determined by the loss of the electrode itself and the chemical reaction between the electric discharge gaps. Because the main elements of the titanium electrode are consistent with the workpiece material, it is conducive to the subsequent processing and treatment of the workpiece.

Section microhardness
The microhardness test of the workpiece section after SEAM processing with different electrode materials is shown in Fig.  16. In addition to the copper electrode, the microhardness of the other three electrode materials (graphite, Q235 steel, and titanium) increases significantly near the machined surface, and the maximum hardness values are 670, 887, and 717 HV, respectively. This position is mainly concentrated in the recast layer and part of the heat-affected layer, which is mainly due to the surface of the resolidified layer containing a large number of hard and brittle Ti oxides and carbides, as well as other metal elements. The maximum hardness of the workpiece section processed by the copper electrode is 348 HV, which is because the resolidified layer on the surface of the workpiece contains a large amount of copper, and its hardness is low, which is not significantly different from that of Ti-6Al-4V; hence, the hardness of this section hardly changes.

Conclusion
In this work, we mainly studied the material erosion mechanism of different electrode materials in the DC SEAM of titanium alloy Ti-6Al-4V and explored the influence of voltage on the processing with different electrode materials in terms of the MRR, RTWR, SEC, and SR. A single-arc discharge experiment was conducted to analyze the arc motion characteristics and the cause of the dynamic change in the current. The influences of the process characteristics in the DC SEAM of titanium alloy Ti-6Al-4V were comprehensively analyzed from many aspects. The main conclusions are as follows: (1) The titanium alloy Ti-6Al-4V has the characteristics of high MRR and high energy density. When the voltage was 30 V, the maximum machining speed when using the graphite electrode was 650 mm/min; the MRR could reach 17,268 mm 3 /min; the ideal maximum MRR was more than 65,000 mm 3 /min; and the RTWR was only 1.27%. Fig. 16 Microhardness of workpiece section prepared using SEAM with different electrodes. a Copper. b Graphite. c Q235 steel. d Ti (2) In the process of DC SEAM, the arc moves with the rotation of the electrode. In the process of single-arc discharge, multi-point discharges occurred simultaneously, which affected the trend in the current. An instantaneous short circuit between the electrodes led to a stepped change in the current.
(3) The heat-affected layer of the workpiece processed by the graphite electrode had the highest thickness (108.2-162.6 μm); the thicknesses of the heat-affected layer of the workpiece processed by the copper and Q235 steel electrodes were 44.5 μm and 80.4 μm, respectively, and the thickness of the heat-affected layer of the workpiece processed by the titanium electrode was the lowest (38.6 μm).
(4) In addition to the copper electrode, the hardness values of the other three electrode materials (graphite, Q235 steel, and titanium) increased significantly near the machined surface.