Influence of different tool polarity on short electric arc machining of GH4169

Abstract

Short electric arc machining is a recently developed high-efficiency electrical discharge machining technology. Material removal rate, tool mass wear ratio ( $\theta$ ), and workpiece surface roughness (Ra) are important indexes used to evaluate the machining performance of short electric arc machining. In order to obtain better machining effect, the nickel-based superalloy GH4169 is machined by graphite in this article. The influence of voltage, duty cycle, and flushing pressure on short electric arc machining performance is then investigated under different tool polarity conditions. Experimental results show that higher material removal rate and lower $\theta$ can be obtained by negative polarity machining, while positive polarity machining can produce better surface quality. To investigate the cause of this difference, the surface integrity of GH4169 machined by different tool polarity is studied from macro and micro perspectives.

Introduction

Nickel-based superalloys such as GH4169 display strong mechanical, thermal machining, and welding properties, as well as oxidation resistance. Due to these traits, they are widely used in fields such as aerospace, nuclear, and petroleum industries.^1–3 Due to the high strength and hardness of GH4169, tool wear is a serious issue in traditional machining.^4,5 Electrical discharge machining (EDM) is a non-traditional machining method developed to address the wear issue. ⁶ However, with the development of high-speed and ultra-high-speed milling technology, the low material removal rate (MRR) of EDM has become an obstacle to its development.^7–9 Therefore, improving the efficiency of EDM remains an important research topic in the field of EDM.

In recent years, short electric arc machining (SEAM) technology has been used to machine large high-strength, superhard, and high-toughness alloy parts such as cement rolls, coal mill rolls, and air casing. Both EDM and SEAM are non-contact machining processes, which use high temperature produced by discharge to erode materials. ¹⁰ Compared with traditional EDM, the arc column with SEAM produces higher internal temperature and more energy distribution on the workpiece. With the advantages of high material removal efficiency, stable discharge, and controllable range of recast layer and heat-affected layer, SEAM has become a vital technology for high-efficiency machining of difficult-to-machine materials. However, issues in machining characteristics such as increased tool electrode loss and relatively low surface quality of the workpiece after SEAM require further study.^11–13 The rapid development of science and technology has yielded many new algorithms which have been widely used to enhance production in various industries. For example, Shui et al. ¹⁴ used the method of minimization of mass, maximization of minimum natural frequency, and minimization of maximum deformation to optimize mechanical design characteristics and improve performance of the battery pack shell. The same algorithms can be transplanted into SEAM to optimize the machining parameters, to prevent tool electrode loss and poor surface quality; however, these improvements are based on the original machining ways, which contain certain limitations. Thus, it is necessary to combine the principle of EDM and change the machining ways to further solve these problems.

A significant polarity effect occurs during EDM, which is a non-contact-type machining method.^15,16 According to the theory of energy distribution, the energy distribution between tools and workpieces is different, so the amount of erosion between the two poles also varies. ¹⁷ In conventional EDM, when large pulse width machining is used, the large mass positive ions have enough time to accelerate, so the negative electrode removal rate is greater than the positive electrode rate, and then the workpiece should be connected with the negative electrode. When machining with small pulse width, the bombardment effect of the negative electron on the positive electrode is greater than the bombardment effect of the positive ion on the negative electrode, and then the workpiece should be connected with the positive electrode. ¹⁸ In dry EDM, larger MRR and smaller tool wear ratio (TWR) can be obtained when the tool electrode is connected with the negative electrode, but better surface quality can be obtained when the tool electrode is connected with the positive electrode. ⁸ In addition, during the process of micro-EDM, the machining accuracy of straight-through microholes can be improved by changing electrode polarity. ¹⁹ Numerous scholars have conducted relevant research on the influence of the polarity effect on EDM. However, few studies on the influence of polarity effect on SEAM. In order to obtain higher machining efficiency, lower tool loss, and better workpiece surface quality, it is necessary to study the influence of the polarity effect on SEAM.

Preparation for experiment

Experimental conditions

SEAM occurs under a certain proportion of water–gas mixture with flushing pressure. The basic experimental setup is illustrated in Figure 1. The experimental conditions are detailed as follows:

1. Experimental equipment: Computer numeral control (CNC) SEAM, CHATN digital frequency modulation pulse power supply, metallographic mosaic machine, a metallographic sample grinder, and a polishing machine.

2. Tool electrode: The tool electrode is a hollow cylindrical graphite electrode with an external diameter of 17 mm and an internal diameter of 7 mm. The rotating speed of the tool electrode is 1750 r/min.

3. Workpiece electrode: Aging solution high-temperature nickel base alloy GH4169.^20,21

4. Testing equipment: PicoScope 5204 oscilloscope, HXD-100TB microhardness tester, MJ21 forward microscope, and scanning electron microscope LE01430VP.

Figure 1.

Basic experimental setup.

Experimental principle

SEAM is a method of melting and removing materials by means of excited discharge groups between tool electrodes and workpiece electrodes under the action of a certain proportion of gas–liquid mixed dielectric with pressure. ²² The schematic diagram of SEAM is shown in Figure 2. When the pulse power supply is loaded between the tool electrode and the workpiece, a large amount of charge will be generated on the surface of the two poles. As tool electrode continues to feed, the distance between the poles decreases. Current carriers in the dielectric will collide and the collided carriers will be separated, leading to avalanche collision ionization. At this time, the dielectric is broken down and a discharge channel is rapidly formed between the two poles.

Figure 2.

Schematic diagram of SEAM.

In the discharge channel, charged particles collide with the workpiece surface at high speed, which converts kinetic energy into heat energy and transfers it to the workpiece, so that the workpiece surface is rapidly melted and gasified. In the process of heat transfer, part of the heat is absorbed by the dielectric in the gap and will be discharged from the gap together with the corrosion products of the material. In the pulse interval, the charged particles in the dielectric will combine again quickly to form neutral particles, and the dielectric properties of the medium will be restored.

In short, a complete arc discharge consists of four processes: dielectric ionization, discharge channel formation, material melting and gasification, and deionization.

Experimental process

To study the influence of the polarity effect on SEAM, the MRR, tool mass ratio wear ( $\theta$ ), and workpiece surface roughness (Ra) under different tool polarity conditions are analyzed by changing voltage, duty cycle, and flushing pressure through single-factor experiments. The experimental parameters are set as shown in Table 1. The macroscopic difference of workpiece surface after machining is analyzed, and the cause of the voltage–current difference between the two electrodes during processing is explained. Finally, the surface integrity of the workpiece after processing is analyzed from the micro-angle. The setting of experimental parameters is shown in Table 2. In this article, the calculation formulas of MRR and $\theta$ are as follows

$$\mathrm{MRR}=\frac{1000\left(M_{wi}-M_{wj}\right)}{{\rho }_w^t}\times 100\%$$

(1)

$$\theta =\frac{m_{ei}-m_{ej}}{M_{wi}-M_{wj}}\times 100\%$$

(2)

where $M_{\mathrm{wi}}$ and $M_{\mathrm{wj}}$ are the quality of the workpiece before and after machining (g); $m_{\mathrm{ei}}$ and $m_{\mathrm{ej}}$ are the quality of the tool electrodes before and after machining (g); ${\rho }_w$ is the workpiece density ( $\mathrm{g}/\mathrm{c}{\mathrm{m}}^3$ ); and t is the working time (min).

Table 1.

Parameter setting of SEAM.

Parameter	Value
Polarity of tool	Negative, positive
Voltage, U (V)	15, 20, 25, 30
Duty cycle, D (%)	30, 50, 70, 90
Flushing pressure, p (MPa)	0.2, 0.4, 0.6, 0.8
Frequency (Hz)	1000
Machining depth (mm)	2

Table 2.

Parameters of SEAM under different polarity of tool.

Polarity of tool	Voltage, U (V)	Duty cycle, D (%)	Flushing pressure, p (MPa)
Negative, positive	25	50	0.4

Experimental results and analysis

Comparison of the influence of voltage

The influence of voltage on SEAM under different polarity conditions is shown in Figure 3. In negative SEAM, the MRR increases rapidly with the increase in voltage. When the voltage is 30 V, the maximum MRR is 261.4 mm³/min. This is because the increase in voltage is conducive to the breakdown of dielectric, which increases the average arc discharge time, that is, the arc discharge energy increases, so the MRR increases. However, $\theta$ decreases with the increase in voltage. When the voltage is 30 V, $\theta$ reaches a minimum of about 2.98%. This is attributed to the voltage increase, which enhances the chip removal effect between electrodes, reducing the short-circuit rate so the working state between electrodes is improved, and tool electrode loss is reduced. The Ra increases first, and then decreases with the increase in voltage in negative SEAM. When the voltage is 25 V, the maximum Ra is 30.86 μm. This is due to the poor effect of chip removal between electrodes in the range of 15–25 V, thus the discharge pits will become larger with an increase in the average discharge energy of the arc. When the voltage increases gradually to 30 V, the effect of removing debris between electrodes is the best. The particles can be removed well, which prevents the accumulation and makes the discharge point more uniform. Therefore, Ra first increases and then decreases.

Figure 3.

Influence of voltage under different tool polarity on SEAM performance (D = 50%, p = 0.4 MPa).

In positive SEAM, the trends of MRR, $\theta$ , and Ra are similar to those in negative SEAM, but the trends of negative SEAM are more significant. However, when voltage reaches 30 V, the maximum MRR is 132.6 mm³/min, which is about 51% of negative SEAM. In such conditions, the minimum $\theta$ is 7.35%, which is approximately 2.5 times that of the negative SEAM. This is because graphite has high melting point and low linear coefficient of thermal expansion, which can resist thermal corrosion. ¹⁷ In SEAM, the graphite electrode at the negative electrode distributes less energy than the graphite electrode at the positive electrode, so $\theta$ in negative SEAM is smaller than that in positive SEAM. When the voltage is 25 V, Ra reaches the maximum 13.35 μm in positive SEAM, which is about 0.43 times that of negative SEAM.

According to the findings, it is concluded that tool-negative SEAM can obtain higher MRR and lower $\theta$ under the same machining conditions, while tool-positive SEAM can produce smaller Ra.

Comparison of the influence of duty cycle

The influence of duty cycle on the machining performance of SEAM under different tool polarity is shown in Figure 4. The results show that MRR, $\theta$ , and Ra increase with the increase in duty cycle in both negative SEAM and positive SEAM, but the change degree of negative SEAM is more significant. This occurs because the pulse width increases with the increase in duty cycle, while pulse interval decreases, single arc discharge energy increases, energy distribution between the two electrodes increases, and the chip removal effect between the electrodes becomes worse, causing the discharge crater to deepen and become unevenly distributed. In negative polarity processing, when duty cycle is in the range of 30%–90%, the maximum MRR is 522.8 mm³/min, the minimum $\theta$ is 2.02%, and the minimum Ra is 18.5 μm. In positive polarity processing, the maximum MRR is 253.9 mm³/min, the minimum $\theta$ is 6.89%, and the minimum Ra is approximately 11.6 μm.

Figure 4.

Influence of duty cycle under different tool polarity on SEAM performance (U = 25 V, p = 0.4 MPa).

Comparison of the influence of flushing pressure

As seen in Figure 5, if the tool electrode is either negative polarity machining or positive polarity machining, the influence of flushing pressure on SEAM performance is approximately the same, that is, MRR and Ra decrease with the increase in flushing pressure. When the flushing pressure is greater than 0.4 MPa, the decreasing speed of MRR and Ra tends to be gentle gradually. This is attributed to the increase in the arc-breaking effect caused by the increase in flushing pressure, which in turn decreases the probability of discharge. Although the average discharge time and the arc discharge energy both decrease, the increase in flushing pressure is conducive to the rapid discharge of etched particles, and the stability of the machining process is improved. The distribution of discharge pits is more uniform and the surface quality of the workpiece is improved, so the decreasing trend of MRR and Ra gradually slows down. In addition, $\theta$ decreases gradually with the increase in flushing pressure. This is because flushing pressure enhances the effect of chip removal between electrodes, reducing the probability of secondary discharge, so the effect of arc breaking is more obvious. The lowering of discharge probability decreases the amount of tool removal, so $\theta$ decreases gradually.

Figure 5.

Influence of flushing pressure under different tool polarity on SEAM performance (U = 25 V, D = 50%).

Macroscopic analysis of GH4169 surface

To more intuitively observe the effect of changing tool polarity on the surface change of GH4169 in SEAM, the following experiments are carried out, and the parameters are set as shown in Table 2. Compared with Figure 6(a) and (b), it can be seen that in negative SEAM, the surface roughness of the GH4169 is higher, and the surface brightness is darker than that of positive SEAM. The measured surface roughness, Ra, is about 30.86 μm, which is approximately 2.3 times that of positive SEAM. As shown in Figure 6(c), regardless of whether the tool is negative or positive, the end-face profile of GH4169 after SEAM is almost U-shaped. This is due to the sharp discharge effect and skin effect in SEAM, which alter the geometric shape of the tool. In addition, the tool edge loss and axial loss of positive SEAM are greater than that of negative SEAM, so that the actual machining depth of positive SEAM is less than that of negative SEAM. This also indicates that the total volume of the material removed by positive SEAM is less than that of negative SEAM. In conclusion, positive SEAM can obtain better surface quality, while negative SEAM will cause less tool wear.

Figure 6.

Surface and end-face profile of GH4169 under different tool polarity: (a) negative, (b) positive, and (c) end-face profile of GH4169 after SEAM.

Analysis of interelectrode voltage–current waveform

To investigate the influence of interelectrode voltage–current waveform, a PicoScope 5204 oscilloscope is used to collect voltage–current signals in the SEAM process. The parameters are set as shown in Table 2, and the GH4169 is machined by changing tool polarity. The variation of voltage–current waveform between electrodes is shown in Figure 7, in which channel U represents the voltage waveform, and channel i is the current waveform.

Figure 7.

Voltage–current waveform between electrodes in different tool polarity machining: (a) negative and (b) positive.

As illustrated in Figure 7, there are significant differences in the voltage–current waveforms between the electrodes of different tool polarity in the machining process. In continuous discharge machining, compared with negative SEAM, the discharge frequency of positive SEAM is lower. The arc plasma is affected by flushing pressure, and the effect of arc breaking is more obvious. In addition, the peak current of negative SEAM is obviously higher than that of positive SEAM, so the arc plasma energy of negative SEAM is also larger. This finding presents strong proof of the high efficiency of negative SEAM.

GH4169 surface integrity analysis

To further study the influence of the polarity effect on SEAM, the surface integrity of GH4169 after SEAM is analyzed from a micro perspective in the following section. The experimental parameter settings are shown in Table 2.

Scanning electron microscope analysis

The microstructure of the GH4169 surface after SEAM is presented in Figure 8. As seen in Figure 8(a), the surface discharge pits of GH4169 are large and deep, with prominent edges and rough surfaces. In Figure 8(b), the surface of GH4169 is relatively smooth, no obvious pits are found, and there is little residual material to be cooled again, indicating that positive SEAM can achieve better surface quality. To further understand the variation of different tool polarity machining, lager multiple scanning electron microscope (SEM) is used to observe the surface of GH4169 after SEAM. Compared with Figure 8(c) and (d), it can be seen that there are micro-cracks on the surface of the GH4169 after SEAM. The micro-cracks on the surface after negative SEAM are wide and long, while the micro-cracks on the surface of GH4169 after positive SEAM are narrow and short. These cracks are most likely the result of the heat generated by the arc discharge and the cooling effect of the flushing fluid. This finding also reflects that heat generated by arc discharge and the rapid cooling effect of flushing fluid in negative SEAM have a more significant effect on local stress concentration.

Figure 8.

Microstructure of GH4169 surface after SEAM: (a, c) negative; (b, d) positive.

Energy dispersive spectroscopy analysis

Material erosion in SEAM is a very complex process, which is a mixture of electric field force, magnetic force, thermal force, and hydrodynamic force. Numerous physical and chemical reactions occur during this process. For this study, the chemical composition of GH4169 surface after SEAM is analyzed by energy dispersive spectroscopy (EDS). As illustrated in Figure 9, when tool polarity varies, the chemical composition of the GH4169 surface is different. The results show that the C element on the surface of GH4169 increases after both positive and negative polarity machining, which is due to the adhesion of C element from the graphite electrode to the GH4169 surface during SEAM. The contents of Cr, Fe, Ni, Nb, and Mo elements in positive SEAM are higher than those in negative SEAM, indicating that GH4169 is closer to the matrix material after positive SEAM. This result also indicates that GH4169 still maintains good machinability after positive SEAM. In addition, the surface O element of GH4169 machined by negative SEAM is higher than that machined by positive SEAM, demonstrating that the surface of GH4169 machined by negative SEAM is easier to be oxidized than that machined by positive SEAM in air.

Figure 9.

Surface analysis of GH4169 after different tool polarity machining: (a) negative, (b) positive, and (c) surface chemical components of GH4169.

Heat-affected layer analysis

The metallographic structure of nickel-based superalloy GH4169 consists of $\gamma \prime$ , $\gamma ″$ , $\delta$ , and NbC phases. The Ni₃Nb( $\gamma ″$ ) is the main strengthening phase, which is disk-shaped dispersed coherent precipitation in the matrix, and $\gamma \prime$ (Ni₃(Al, Ti)) is the auxiliary strengthening phase, which is spherical dispersion precipitation. ²³ In SEAM, a thick heat-affected layer is formed on the surface of GH4169 because the heat cannot be removed by the working medium in time. As can be seen from Figure 10, the thickness of heat-affected layer in tool-negative polarity machining is larger than that in positive polarity machining. This is due to the difference in the energy obtained on the surface of the workpiece, which proves that the arc energy of negative polarity machining is higher than that of positive polarity machining. This result illustrates that in SEAM, the workpiece machined by positive SEAM has superior machinability.

Figure 10.

Heat-affected layer thickness after SEAM.

The heat-affected layer is enlarged to obtain the grain variation diagram as shown in Figure 11. It can be observed that the grains gradually coarsen from the workpiece surface to the matrix direction, and long strips of structure appear near the matrix, which prevents the further growth of the matrix structure. Grain boundaries are interspersed with granular crystals, which play a strengthening role. Near the surface of the workpiece, because of the high temperature, the strengthening phase will be rapidly melted, so the grain growth process is almost not hindered by the strengthening phase. With the increase in the distance from the workpiece surface, the heat absorbed by the structure decreases gradually, and the strengthening phase cannot be completely melted. The melted particles are doped in the crystal, which hinders the growth of the grain.

Figure 11.

Grain variation in heat-affected layer.

Microhardness analysis

The GH4169 metallographic specimens were loaded on the HXD-1000TB video digital Vickers hardness tester with a force of 100 g lasting for 15 s. The hardness data were measured as shown in Figure 12, which shows that the hardness decreases gradually with the increase in distance from the surface, and eventually tends to be flat. This is because the structure absorbs more energy closer to the surface, and the grain can be fully refined, thus increasing hardness. In negative SEAM, when the distance from the surface of the GH4169 is more than 120 μm, the hardness decrease trend is gentle, while in positive SEAM, it tends to be gentle beyond 40 μm. This is due to the heat-affected layers of negative polarity and positive polarity machining, which are 118 and 36 μm, respectively. Beyond the heat-affected layer, the matrix structure is at a greater distance from the GH4169 surface, thus the structure fails to absorb more heat and the grain is not refined, so the hardness is low.

Figure 12.

Microhardness change of GH4169 surface.

Conclusion

This study analyzed the influence of voltage, duty cycle, and flushing pressure on SEAM under different tool polarity. The influence of the polarity effect on SEAM was then determined from macro and micro perspectives. The following conclusions were obtained:

1. Under the same conditions, higher MRR and lower $\theta$ can be obtained by negative polarity machining, while better surface quality can be produced by positive polarity machining.

2. After SEAM, the end-face profile of GH4169 is approximately U-shaped, and the actual machining depth of negative SEAM is greater than that of positive SEAM.

3. In continuous discharge machining, the arc-breaking effect of positive SEAM is more obvious than that of negative SEAM.

4. After negative SEAM, the surface discharge pits of GH4169 are large and deep, and the surface is rough. However, the surface of GH4169 is smooth after positive SEAM, and no obvious pits are found.

5. Compared with negative SEAM, GH4169 after positive SEAM is found to maintain relatively good machinability.

6. The thickness of the GH4169 heat-affected layer after negative SEAM is larger than that of positive SEAM.

7. After SEAM, the microhardness of the GH4169 surface decreases first and then tends to be flat with the increase in distance from the GH4169 surface.