Experimental Study on Efficient Short Electric Arc Turning of Titanium Alloy

Abstract

This study investigates a novel short electric arc vertical turning method for machining titanium alloy shafts. The method was successfully applied to titanium alloy rods, and its effects on material removal rate (MRR), surface roughness, roundness, and cross-sectional morphology were analyzed at varying processing voltages. The results indicate that the MRR and surface quality improve with increased voltage, reaching a maximum of 231 mm³/min and 26 μm surface roughness at 32 V. However, surface roughness deteriorates with higher duty cycles and voltages due to unstable discharges. Roundness deviations are minimized with higher rotational speeds, which enhance uniform material removal and arc stability. Metallographic analysis revealed an increased heat-affected zone and recast layer thickness at higher voltages. This method demonstrates high machining efficiency and improved surface quality, making it suitable for titanium alloy shaft manufacturing in advanced engineering applications.

1. Introduction

Titanium alloy is a key material for aircraft engine components and structural parts. Its excellent properties such as high-temperature resistance, corrosion resistance, oxidation resistance, and so on can meet more complex use environments [1,2]. Among them, titanium alloy shaft components are widely used across biomedical, automotive, and aerospace fields due to their high strength and corrosion resistance, playing a critical role in transmitting power and supporting loads [3,4,5,6]. However, titanium alloy shafts often suffer from bending deformation and vibration during turning machining, which cannot meet processing requirements [7].

In order to solve the problems of the low stiffness, high centrifugal force, and low thermal conductivity of titanium alloys, many scholars have conducted research on improving the existing machining methods and technologies [8,9,10]. Chetan et al. studied the influences of low temperature and wet environments on turning performance. Compared with wet turning, low-temperature turning can improve machining efficiency, reduce the machining cost, extend tool life, and reduce the surface roughness at high cutting speeds [11]. Ibrahim et al. used vegetable oil as a coolant in low-temperature environments to study the influence of uncoated carbide cutting tools at different speeds on the tool surface wear. The experiment showed that low-temperature machining can achieve the sustainable machining of titanium alloys under high-speed feed [12]. In order to reduce the vibration of the workpiece during turning, Toshifumi et al. used a double-blade turning method to effectively reduce vibration in slender shafts and thin workpieces by adjusting the insert offset parameter to correspond to the turning parameters [13]. Sun Yuwen et al. established a slender workpiece model with flexible boundary constraints, verified the position-dependent stability limit considering the flexible boundary constraints through turning experiments, and improved the prediction accuracy of turning stability [14].

Electrical Discharge Machining (EDM) is a non-contact and non-macroscopic cutting force machining method that can realize the efficient machining of difficult-to-cut materials [15,16]. Many scholars have improved EDM in such aspects as optimization methods, process combinations, electrodes, and so on. Sujee et al. used the multi-response optimization method based on grey correlation analysis to achieve the maximum machining efficiency of 9.35 mm³/min and the minimum roundness deviation of 15.51 mm [17]. In order to improve machining efficiency, Sun et al. proposed a low-speed wire rod wire EDM by dividing the turning process into three stages, namely rough, semi-finished, and finished turning, and investigated the influences of rotational speed and feed rate on roundness error [18,19]. Under the optimum parameters in EDM turning, namely a titanium alloy bar 10 mm in diameter and 75 mm in length, Vikas et al. used a grey correlation analysis method to achieve a material removal rate of 12.36 mg/min and a surface roughness of 2.29 μm [20]. Jielin et al. proposed a novel EDM-turning hybrid process that utilizes EDM discharge to elevate the workpiece machining surface temperature and reduce the strength and hardness of the workpiece, making the material easier to machine. The influences of different feed rates on surface roughness were explored, and the cutting force and surface roughness values in the hybrid EDM-turning process were reduced by more than 50% compared to those achieved with low cutting parameters. [21].

Short electric arc machining technology utilizes a specific proportion of a pressure gas–liquid mixture as a working medium. It employs instantaneous high temperatures generated by short arc discharges between poles to erode conductive metal materials, making it an efficient cutting method [22]. This short arc machining method exhibits good performance in milling and cement rolls, coal mill rolls, aviation magazines, and other large-scale high-strength, super-hard, and high-toughness alloys. Kou et al. used mechanical and fluid coupling arc breaking to improve the efficiency of arc machining [23]. Zhou et al. were able to significantly improve machining efficiency using a DC power supply instead of a pulse power supply, but at the same time, this strategy introduces problems such as poor surface quality and serious electrode loss [24]. Zhang et al. investigated the influences of voltage, frequency, duty cycle, flushing fluid pressure, and electrode rotational speed on the removal rate of titanium alloy material, and they found that the highest machining efficiency reached 556 mm³/min when the electrode feed rate was 30 mm/min [25]. Zhao et al. compared positive- and negative-polarity machining, and the experimental results show that negative-polarity machining has a higher machining efficiency compared with positive-polarity machining, but the surface quality is poorer and the dimensional accuracy is lower. A short electric arc machining titanium alloy milling experiment was used to successfully remove the heat-affected zone after arc machining by electrochemistry [26,27,28]. Zhao et al. analyzed MRR, relative electrode wear ratio, and specific energy consumption when processing SKD11, and they found that compared with EDM, short electric arc machining has a higher material removal rate and lower energy consumption ratio [29]. In the above research, the high-speed rotating arc breaking of the tool electrode had some problems, such as unstable discharge, chip removal difficulty, and serious electrode loss.

This paper presents a novel short electric arc efficient turning for machining titanium alloy shafts. The high-speed rotation of the workpiece enhances chip removal and arc breakage capabilities, improving processing stability and efficiency, and making it an effective approach for shaft components. Based on single-factor experiments, the influences of machining voltage, duty cycle, workpiece rotational speed, and feed rate on the performance of short electric arc turning are explored, providing a theoretical basis for optimizing process parameters.

2. Materials and Methods

2.1. Experimental Principle

A partial picture of short electric arc turning is shown in Figure 1. In the experiment, a DC power supply is used to provide discharge energy, and the rod titanium alloy workpiece is installed on the main shaft of the five-axis machining center using a torque wrench. The titanium alloy workpiece is connected to the positive electrode, and the block graphite electrode is connected to the negative electrode of the power supply. During machining, the electrode is fixed in a vise, and the workpiece is rotated at a high speed and fed to the workpiece along the Y-Axis. When the workpiece and the electrode reach the discharge gap, the working medium is pierced to form a discharge channel. In this process, the charged particles, under the action of the electric field, gain energy and form high-speed moving particles that bombard both the workpiece and the electrode (Figure 2). This interaction creates three key regions on the surface of the workpiece: the vaporization zone, the melting zone, and the heat-affected zone. The vaporization zone is where the material transitions from solid to gas due to the high temperature, while the melting zone is where the material melts, forming the core of the material removal process. The heat-affected zone is the area near the surface where high temperatures cause changes in material properties but do not result in complete melting. The scrap chips produced during machining are expelled from the workpiece by centrifugal force generated by the high-speed rotation of the workpiece, its own gravity, and the pressure of the working medium. This action helps maintain the stability of the discharge. At the same time, the workpiece is rapidly cooled by the high-speed flushing of the working medium, which aids in achieving both high machining efficiency and excellent surface quality.

2.2. Experimental Condition

The experiment is carried out in the self-designed short electric arc five-axis machining center, and the schematic diagram of the machining center is shown in Figure 3. It mainly consists of an external high-power power supply, air compressor, and high-pressure water pump. A pulse power supply is used during the experiment, the output voltage is up to 35 V, and the maximum current is 4000 A. The spindle provides rotation for the workpiece, and the maximum speed is 8000 r/min. The water pump supply cutting fluid is the medium, and the maximum pressure is 0.1 MPa. The experiment uses Ti₆Al₄V with a diameter of 10 mm and a length of 260 mm as the workpiece. The experiment uses 10 × 30 × 50 mm block graphite as the tool electrode. Before and after the experiment, a precision electronic scale with an accuracy of 0.01 g is used to calculate the material removal rate and electrode loss. Voltage and current data are recorded during the experiments using a DEWESOFT SIRIUSI multichannel data acquisition system with a sampling rate of up to 1 MHz. After the experiment, the surface roughness and surface morphology of the titanium alloy are measured using a VHX-6000 super-field-of-view digital microscope, and the metallographic samples are prepared by cross-cutting the workpieces using a wire-cutting machine. The samples are studied by grinding, polishing, and chemical discharge debris. The surface morphology and elemental analysis (EDS) of the micro-surfaces of the workpieces are observed using a JEOL JSEW-6460 scanning electron microscope (SEM). The cross-section of the workpiece is measured by a VHX-6000 super-field-of-view digital microscope. Finally, the roundness of the workpiece is detected by an HME02 roundness tester.

2.3. Experimental Process

2.3.1. Experimental Setup

The experimental parameters were chosen based on preliminary experiments and the current machine conditions. To ensure reliable results, each experimental setup was repeated three times. The machining polarity and flushing pressure were kept fixed throughout the experiments. The effects of machining voltage on machining efficiency, electrode wear, and surface roughness were specifically analyzed. The experimental parameters used in this study are summarized in

Table 1. Experimental processing parameters.

Parameter	Value
Polarity of tool electrodes	Positive
Power output	DC power
Depth (mm)	2
Discharge frequency (Hz)	1500
Voltage (V)	28, 29, 30 (Fixed), 31, 32
Workpiece feed speed (r/min)	1000, 1500 (Fixed), 2000, 2500
Duty cycle	20%, 30%, 40%, 50%, 60% (Fixed), 70%
Feed rate (mm/min)	1.0, 1.2, 1.4, 1.6 (Fixed), 1.8

.

2.3.2. Material Preparation

In the preliminary exploration experiment, when using negative-polarity machining, the machining energy is low, and it is easy to crash the tool, which cannot effectively remove the material. When using a DC power supply, it is not conducive to the workpiece- and electrode arc-breaking and easy-to-short-circuit phenomenon, resulting in a large diameter and depth of the surface crater and affecting the roundness of the workpiece. When the voltage is less than 20 V, the processing current is less than 50 A, and the processing is mainly grinding. When the voltage exceeds 35 V, the discharge energy is too large, intermittent short-circuits will occur, and the surface of the workpiece will show obvious ablation marks, affecting the processing quality. When the rotational speed of the workpiece exceeds 2000 r/min, the mechanical arc-breaking and chip removal effect caused by relying on the high-speed rotation of the workpiece will not be significantly improved. When the workpiece feed rate is higher than 2 mm/min, the faster the feed rate, the greater the turning depth, the greater the roundness, and the machining accuracy cannot be guaranteed. When the pressure of the flushing liquid is less than 0.1 MPa, it cannot break the arc effectively, and it is easy to short-circuit, resulting in a poor machining surface and serious electrode loss. Therefore, the machining polarity and flushing pressure are kept fixed when designing experimental parameters. The effects of machining voltage on machining efficiency, electrode loss, and surface roughness are investigated.

2.3.3. Data Acquisition

In order to further reveal the machining mechanism and performance of short electric arc turning, the cross-sectional morphology, surface morphology, chemical composition, and roundness are investigated. The machining efficiency, MRR, and relative tool wear ratio (RTWR) are defined as follows:

$$MRR={\displaystyle \frac{1000(M_{wi}-M_{wj})}{{\rho }_{w}t}}({\mathrm{mm}}^3/\min )$$

(1)

$$RTWR={\displaystyle \frac{(M_{ei}-M_{ej})/{\rho }_e}{(M_{wi}-M_{wj})/{\rho }_w}}\times 100\%$$

(2)

where M_wi is the mass of the workpiece before machining (g), M_wj is the mass of the workpiece after machining (g), ρ_w is the density of the workpiece (g/cm³), and t is the machining time (s). M_ei is the mass of the electrode before machining (g), M_ej is the mass of the electrode after machining (g), and ρ_e is the density of the electrode (g/cm³).

3. Results

3.1. Gap Current and Voltage Waveform Analysis

As shown in Figure 4, the processing voltage significantly influences the discharge characteristics. At a voltage of 28 V (Figure 5a), the peak current reaches 169 A due to the low processing voltage and reduced discharge energy. The high-speed rotation of the workpiece enhances the arc-breaking capability and increases the electric field intensity, causing the plasma channel’s arc energy to be mechanically disrupted before full expansion. This interruption prolongs the current duration within the channel, limiting its growth. As the processing voltage increases, as seen in Figure 5b, the single-discharge energy rises, leading to a more intense discharge process and a peak current of 185 A, indicating more complete discharges compared to low-voltage conditions. At 30 V (Figure 5c), the peak current slightly decreases to 181 A due to an expanded discharge gap and shortened discharge duration, resulting in a relatively stable discharge state. When the voltage further increases (Figure 5d), the peak current surges to 262 A as the stronger electric field intensity allows the plasma channel to carry a higher current. However, the intense discharge effects induced by high voltage make the process unstable, often resulting in open-circuit states and significant current fluctuations.

3.2. MRR and RTWR Analysis

As shown in Figure 5, the MRR is significantly influenced by processing parameters. In Figure 5a, at a processing voltage of 28 V, the MRR is relatively low due to insufficient arc discharge energy, which leads to inadequate temperature and impact force between the electrodes. This limits the effective melting and vaporization of the workpiece material. Additionally, the low discharge energy causes physical contact and collisions between the electrode, workpiece surface, and particles in the dielectric, leading to electrode wear. When the voltage is increased to 32 V, the MRR reaches a peak of 231 mm³/min, as higher current density and single-arc energy enhance the material removal rate. However, the intense discharge also increases thermal shock on the electrode surface, accelerating thermal wear. In Figure 5b, the MRR gradually increases with a rise in the duty cycle, as the extended discharge duration enhances material removal efficiency. Nonetheless, higher-duty cycles result in elevated electrode surface temperatures, accelerating thermal wear. Figure 5c shows that MRR increases with the rotational speed of the workpiece, while the RTWR decreases. This is because high-speed rotation generates greater centrifugal force, facilitating the rapid removal of debris and heat from the machining area, preventing molten material from accumulating on the electrode and workpiece surfaces, and improving machining efficiency. However, as the rotational speed further increases, the arc’s interaction time with the workpiece surface shortens, causing a decline in MRR. Finally, as depicted in Figure 5d, the MRR initially rises and then falls with increasing feed rate. A higher feed rate increases the contact area between the workpiece and electrode, expanding the arc discharge region and improving material removal efficiency. However, when the feed rate becomes too high, the stability of arc discharges deteriorates, resulting in incomplete discharges, reduced efficiency, and even short-circuit phenomena.

3.3. Workpiece Surface Morphology Analysis

Figure 6 illustrates the effects of different duty cycles, processing voltages, workpiece rotational speeds, and feed rates on the surface quality of the workpiece. As shown in Figure 7a, at a processing voltage of 28 V, the surface roughness is 8.53 μm. This is attributed to the relatively low discharge intensity at lower voltages, which helps stabilize the discharge channel and reduces arc jumping and irregular melting during the discharge process, thereby improving the surface morphology. Figure 7b shows that at a duty cycle of 20%, the surface roughness is 4.12 μm due to the mild surface polishing effect of the graphite electrode under low-duty-cycle conditions. However, when the duty cycle increases to 70%, the surface roughness rises significantly to 23.82 μm. This is because the higher-duty cycle introduces current fluctuations, leading to surface erosion and a subsequent deterioration in surface roughness. In Figure 7c, the surface roughness of the workpiece initially decreases and then increases with rotational speed. At a speed of 2000 r/min, the surface roughness reaches 11.59 μm. The increase in speed enhances mechanical arc-breaking capability, shortens arc duration and single-discharge cycles, and generates greater centrifugal force to aid in chip removal, thereby improving surface quality. However, as the rotational speed increases further, the arc becomes less stable, resulting in uneven heat distribution and poorer surface quality. Figure 7d demonstrates that surface roughness decreases with increasing feed rate. At a feed rate of 1.0 mm/min, the surface roughness is 8.56 μm, as the low feed rate maintains a stable discharge gap, reducing short circuits and unstable discharges, thereby enhancing surface morphology. However, excessively high feed rates narrow the discharge gap, increase the frequency of short circuits, and cause surface erosion, leading to deteriorated surface quality.

3.4. Roundness and Circular Runout Analysis

As shown in Figure 8a, with an increase in processing voltage, the discharge energy intensifies, resulting in noticeable discharge craters on the workpiece surface. The more vigorous discharge reactions lead to deeper and larger craters, causing the maximum roundness deviation of the workpiece to reach 196 μm. In Figure 8b, at a duty cycle of 20%, the workpiece achieves the smallest roundness deviation of 81 μm and the lowest runout of 91 μm. This is because the low-duty cycle causes electrode wear, forming a smoother electrode surface that reduces localized high current density and associated erosion effects, thereby minimizing roundness errors. At a duty cycle of 70%, the roundness deviation increases significantly to 252 μm, as the extended discharge duration causes current and voltage fluctuations, open-circuit conditions, and uneven material removal, leading to greater roundness and runout errors. Figure 8c shows a negative correlation between workpiece rotational speed and roundness or runout. Higher rotational speeds promote uniform material removal, enhance arc-breaking, and improve debris evacuation. At 2000 r/min, the minimum roundness deviation of 131 μm is achieved, as the rapid rotation shortens arc-breaking and deionization processes, improving the uniformity of material removal during discharge. Finally, in Figure 8d, the minimum roundness deviation of 81 μm is observed at a feed rate of 1.8 mm/min. This is because an appropriate feed rate ensures uniform discharges, reducing roundness errors and improving machining precision.

3.5. Micro-Morphology Analysis

As shown in Figure 9a, at a processing voltage of 32 V, deep erosion craters and irregular surface features are observed on the workpiece. This is because the higher discharge intensity leads to localized melting and vaporization of the material surface. Under the influence of high-pressure flushing, deep erosion craters are formed, resulting in large irregular pits in the microstructure. In Figure 9b, at a duty cycle of 20%, the surface exhibits fewer spherical particles and appears relatively smooth. The reduced arc duration and lower energy input at low-duty cycles mitigate the high-temperature thermal effects, minimizing the rapid expansion and contraction of the material surface, reducing the formation of microcracks, and enhancing micro-surface quality. Figure 9c shows that at a rotational speed of 1000 r/min, the number of spherical particles on the surface increases, leading to a decline in micro-surface quality. The longer contact time between the arc and the workpiece at lower speeds causes localized heat concentration, resulting in uneven melting and cooling processes. Additionally, the smaller centrifugal force at low speeds makes it difficult to expel molten material, causing it to accumulate as spherical particles. In Figure 9d, at a feed rate of 1.8 mm/min, the surface has fewer spherical particles and displays more microcracks, appearing smoother overall. This is because higher feed rates induce rapid heating and cooling, leading to thermal fatigue and the formation of microcracks. Moreover, the increased feed rate enhances friction between the workpiece and the electrode, removing some of the spherical particles through abrasion.

3.6. Cross-Sectional Analysis

Figure 10 shows the metallographic diagram of the workpiece cross-sectional after machining with different voltages. In order to study the effect of processing voltage on the workpiece heat-affected zone and re-solidified layer, keep the workpiece speed of 1000 r/min, duty cycle of 60%, workpiece feed rate of 1.6 mm/min, and frequency of 1000 Hz. as shown in Figure 10a; when the processing voltage is 28 V, the thickness of the heat-affected zone is the smallest, at only 34 μm, and there is no obvious re-solidified layer. This is due to the low energy of the arc, which prevents it from etching the material in time. Instead, part of the process involves grinding, which removes the re-solidified layer generated by arc processing in a timely manner. As a result, no re-solidified layer remains on the surface. As shown in Figure 10b, when the machining voltage is 29 V, the thickness of the heat-affected zone is 43 μm. This is due to the fact that the heat-affected zone will be affected by the discharge energy, and the thickness of the heat-affected zone increases with the increase in the machining voltage. As shown in Figure 10c, when the processing voltage is 30 V, the thickness of the heat-affected zone is 52 μm, and the thickness of the re-solidified layer is 37 μm. At this time, it is completely detached from the grinding process, there is an obvious re-solidified layer, and the heat-affected zone is further deepened. This is due to the fact that when the discharge energy increases to arc erosion being dominant, part of the molten workpiece material and electrode material is not excluded in a timely way from the processing area; this leads to there being part of the molten material reattached to the surface of the workpiece, which is the formation of the re-solidified layer. As shown in Figure 10d, when the processing voltage is 31 V, the heat-affected zone and re-solidified layer are further deepened, the thickness of the heat-affected zone is maximized to 67 μm, and the thickness of the re-solidified layer is maximized to 39 μm.

4. Conclusions

This paper proposes a novel short electric arc vertical turning method for machining titanium alloy shafts. The machining principle of short electric arc turning was successfully investigated and applied to titanium alloy round rods with a diameter of 10 mm and a length of 260 mm. The gap voltage, current waveform, material removal rate (MRR), surface roughness, roundness deviation, electrode surface morphology, workpiece surface morphology, and cross-sectional characteristics were analyzed at different voltage settings. The main conclusions are as follows:

• The short electric arc vertical turning method proves to be an effective approach for machining titanium alloy shafts. At a machining voltage of 32 V, the maximum MRR reaches 231 mm³/min, with a surface roughness of 26 μm. This approach ensures high machining efficiency while improving surface quality. When the arc energy is low, a brief grinding process occurs, which enhances surface quality by reducing roughness. At a machining voltage of 28 V, the surface roughness improves to 8.53 μm, demonstrating the potential for fine surface finishes.
• Surface roughness increases with higher machining voltage and duty cycles. Lower voltage and duty cycle values reduce energy input, which helps improve surface quality. However, at higher duty cycles and voltages, unstable discharges lead to deterioration in surface roughness. This highlights the importance of selecting optimal parameters to balance energy input and surface finish.
• Both roundness and roundness deviation increase with higher duty cycles and machining voltage. However, these parameters significantly decrease with increased workpiece rotational speed. High-speed rotation aids in uniform material removal and improves arc stability, ultimately enhancing machining accuracy.
• The workpiece surface exhibits microcracks, spheroidized particles, and erosion pits. When the feed rate is excessively high, the rapid cooling of the workpiece leads to the formation of numerous microcracks. Metallographic analysis reveals that the heat-affected zone and re-cast layer thickness increase with higher machining voltage and duty cycles. Rational parameter control and the high-speed rotation of the workpiece can reduce thermal damage, preserving the material’s performance.
• Further investigations are needed to explore the applicability of this method to other materials and machining conditions. Additionally, optimizing the machining parameters to minimize thermal damage while maintaining high machining efficiency will be an important avenue for future research.