3.1 Broken wire problem under high energy
The current power supply method for HS-WEDM machine tools involves the use of a conductive block. The workpiece is connected to the positive pole of the pulse power supply, while the conductive block is in short-circuit contact with the wire electrode and connected to the negative pole. This configuration operates in a positive-polarity processing mode. However, during the processing of SiCp/Al, as illustrated in Fig. 3, there is a continuous and intense spark discharge phenomenon at the contact point between the wire electrode and the conductive block. The conductive block undergoes electrical erosion, quickly developing a deep groove that exceeds the diameter of the wire electrode.
When the average machining current exceeds 3 A, the sparks at the conductive block become more intense under high energy, causing a sharp increase in the depth of the etched groove and making the wire electrode more susceptible to breaking. As illustrated in the Fig. 4, compared to the conditions of low energy, the wire electrode under high energy conditions is melted at the conductive block, resulting in larger discharge craters and elongated cracks on the wire surface. Wire electrode breakage leads to replacement of the wire electrode and re-initiation of processing, which increases operational costs. Sometimes it even leads to workpiece scrapping.
3.2 Analysis of coating material on the wire electrode surface
To address the issue of wire breakage under high energy, the severe spark discharge phenomenon at the conductive block is first analyzed. The workpiece is machined using the experimental parameters shown in Table 1. The morphology of the wire electrode before and after processing is observed via scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) analysis. As illustrated in Fig. 5, randomly distributed white particles are attached to the surface of the wire electrode after processing. These particles range from 10 to 20 µm in length, with some aggregations reaching 75 µm. Table 3 presents the EDS results of the wire electrode surface before and after processing. Following processing, aluminum and silicon elements occur on the surface of the wire electrode, with the oxygen content reaching as high as 45.04% (compared with 10.64% before processing). These white particles can be identified as high-hardness, low-conductivity aluminum oxide (Al2O3) and silicon dioxide (SiO2) insulating particles.
Table 3
EDS results of wire electrode surface before and after machining (Wt%)
|
Mo
|
O
|
C
|
Al
|
Si
|
wire before machining
|
55.79
|
10.64
|
33.57
|
-
|
-
|
wire after machining
|
3.07
|
45.04
|
27.93
|
21.58
|
2.37
|
The analysis of the source of coating material on the wire electrode surface is as follows: During SiCp/Al WEDM, material transfer occurs between the positive and negative electrodes. The positive workpiece melts and gasifies under the bombardment of electrons, causing aluminum and silicon elements from the workpiece to splash onto the negative wire electrode [17]. Moreover, the water in the working fluid decomposes into oxygen at high temperatures, which rapidly reacts with the melted and gasified workpiece material to form aluminum oxide and silicon dioxide. These insulating particles are propelled onto the wire electrode by the force of discharge explosions. Owing to the random nature of sputtering and the erosion caused by positive ion bombardment on the wire electrode during positive-polarity processing, the aluminum oxide and silicon dioxide insulating particles adhering to the wire electrode are randomly distributed and vary in size. It is hypothesized that these insulating particles maintain a certain discharge gap between the wire electrode and the conductive block, contributing to the formation of sparks. The conductive block undergoes electrical erosion and experiences friction from the insulating particles, resulting in the formation of grooves.
3.3 Mechanism of spark discharge formation between the wire electrode and the conductive block
To analyze the mechanism of spark discharge formation between the wire electrode and the conductive block, an interelectrode equivalent circuit model (Fig. 7) is established based on the measurement scheme of the discharge waveform illustrated in Fig. 6. The positive pole of the pulse power supply is connected to the workpiece through the current limiting resistance R. Spark discharge occurs between B (the wire electrode) and C (the workpiece), which is equivalent to R0 (the equivalent resistance of the working fluid medium) and R1 (the equivalent resistance of the discharge channel) in parallel, and R1 and ZD1 (the voltage regulator in the working fluid) in series. No spark discharge is shown in Fig. 7(a), while in Fig. 7(b), spark discharge occurs between B (the wire electrode) and A (the conductive block), which will be analyzed further in the subsequent text. Then, the conductive block is connected to the negative pole of the power supply to form a current circuit, with the resistance of the wire electrode and the workpiece being disregarded.
Following the measurement scheme illustrated in Fig. 6, an oscilloscope is employed to capture the machining voltage and current waveforms between the conductive block and the workpiece (Fig. 8). The voltage probe records the voltage AC, which comprises the sum of voltages AB (between the conductive block and the wire electrode) and BC (between the wire electrode and the workpiece). Figure 8(a) illustrates that in the absence of sparks at the conductive block, the voltage waveform is smoother, with fewer fluctuations in the high-frequency component; the voltage is maintained at 29 V. Figure 8(b) reveals that when sparks occur at the conductive block, the waveform exhibits multiple peaks and experiences significant voltage fluctuations. For example, the wave crest M increases the voltage value from 29 V to 37 V within 2 µs. Moreover, the peak current waveform remains relatively consistent at 27 A.
Prior to the formation of spark discharge, the wire electrode and the conductive block are in a state of short-circuit contact. As illustrated in Fig. 9(a), despite the presence of Al2O3 and SiO2 insulating particles on the wire electrode surface, a substantial contact area is maintained with the conductive block, resulting in a relatively low contact resistance value, R2.
As the wire electrode slides a distance L1 to the position illustrated in Fig. 9(b), spark discharge occurs at the conductive block. Two insulating particles create a gap between the wire electrode and the conductive block. In the interelectrode equivalent circuit illustrated in Fig. 7(b), the contact resistance R2 tends toward infinity, causing a decrease in the circuit current i and a reduction in the partial voltage on the current limiting resistance R. Consequently, the voltage AC rises, and wave crests begin to appear. Under the influence of a strong electric field, the air between the wire electrode and the conductive block undergoes breakdown along the shortest path. The breakdown air exhibits good conductivity, equivalent to R3 (the resistance of the discharge channel) and ZD2 (the voltage regulator in the air) in series in Fig. 7(b). Consequently, the voltage AC increases from 29 V to 37 V, contributing to the spark discharge between the wire electrode and the conductive block, as well as between the wire electrode and the workpiece.
As the wire electrode continues to slide a distance L2 on the conductive block, the spark discharge persists until the wire electrode and the conductive block return to a state of short-circuit contact, as illustrated in Fig. 9(c), and the spark discharge ceases. The spark duration, denoted as T, represents the time during which the wire electrode and the conductive block maintain a gap, evidenced by the wave crest M lasting for 2 µs. Figure 9(c) illustrates that the time T is correlated with the size L2 of the insulating particles. Taking the measured insulation particle size of 20 µm from SEM in Fig. 5 as an example, at a wire traveling speed of 12 m/s, the continuous spark time T, calculated as \(\:T=L2/V\) is 1.6 µs. This duration aligns with the duration of the wave crest M and represents a pulse width at the microsecond level.
From the above analysis, spark discharge occurs in the air between the wire electrode and the conductive block owing to the presence of Al2O3 and SiO2 insulating particles on the wire electrode surface. This results in an increase in voltage and the appearance of wave crests. The amplitude of the wave crest represents the voltage acting on the electrodes, while the duration of the wave crest indicates the duration of the spark discharge between the wire electrode and the conductive block. Moreover, the larger the volume of Al2O3 and SiO2 insulating particles, the longer the spark discharge duration. Considering the entire pulse width, the wave crest duration, representing the action time of spark discharge at the conductive block, is variable owing to the uneven size and distribution of insulating particles adhering to the wire electrode.
3.4 Mechanism of wire electrode breakage under high energy
Experiments have revealed that increasing the average machining current intensifies sparks at the conductive block, consequently increasing the likelihood of wire electrode breakage. The mechanism behind wire electrode breakage is analyzed as follows: First, Fig. 10 illustrates the voltage and current waveforms when spark discharge occurs between the wire electrode and the conductive block at different energy levels. Under low-energy conditions of one Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) tube, the peak current is 12 A, and the voltage increases from 23 V to 32 V within 1.7 µs or 4.5 µs. In contrast, under high-energy conditions with 5 MOSFET tubes, the peak current is 34 A, and the voltage increases from 36 V to 46 V within 6.5µs. There is a notable disparity in both the amplitude and duration of the voltage wave crests under different energy levels.
Under low-energy machining conditions, the energy input is minimal, leading to a lower amplitude of voltage wave crests. Additionally, the erosion product particles generated by the molten gasification of the workpiece are small [18, 19], resulting in a limited volume of insulating particles adhering to the wire electrode. Consequently, spark discharge exhibits a short action time, characterized by a brief peak duration, typically around 1.7 µs. A longer duration, such as 4.5 µs, may occur when insulating particles accumulate at a particular location on the wire electrode. At this stage, the wire electrode and conductive block experience low-energy, short-duration spark discharge. The wire electrode can still maintain a certain level of tensile strength, which explains why it is relatively resistant to breaking during SiCp/Al machining under low-energy conditions. Conversely, during machining under high-energy conditions, the amplitude and duration of the voltage wave crest increase significantly, indicating that the wire electrode and conductive block are subjected to high-energy, long-duration spark discharge. The wire electrode remains in a high-temperature discharge state in the air for an extended period, and it melts owing to its inability to withstand the discharge energy.
Second, under high energy, the erosion depth of the conductive block rapidly increases, resulting in a rough groove surface. Additionally, the contact angle between the high-speed traveling wire electrode and the conductive block is minimal, and the wire electrode oscillations during operation further intensify the discharge formation between the conductive block and the wire electrode.
Third, the wire electrode and the conductive block remain in an unstable sliding contact state. Consequently, the wire electrode undergoes severe plastic deformation and eventually breaks owing to the combined effects of high discharge temperature, wire electrode tension, and frictional forces at the conductive block.
The utilization of the conductive block power supply method cannot meet the demand for persistent and efficient SiCp/Al WEDM under high-energy conditions.