Drilling of Al 2 O 3 ceramic using ultrasonic assisted electrochemical discharge machining process

Abstract

Alumina is one of the most important ceramic materials in the industry due to its advantageous properties, such as electrical resistivity and high hardness. The machining of this material encounters several difficulties, and it is usually machined using non-traditional machining processes. Among these processes, ultrasonic-assisted electrochemical discharge machining has been widely used for glass machining. However, this machining process is expected to be accompanied by acceptable results in the case of alumina. Therefore, in this study, the effects of process parameters such as ultrasonic vibration amplitude, voltage, pulse-on time, and pulse-off time on material removal rate, depth, overcut, and taper angle have been experimentally studied in the machining of alumina. The results revealed that hole depth increased up to 52% using ultrasonic vibrations with an amplitude of 34 µm.

Introduction

Aluminium oxide (Al₂O₃) is one of the most important ceramic materials in the industry, with different applications in aerospace, chemical, mechanical, electrical and medical devices. High hardness, electrical resistivity, wear resistance and corrosion resistance has made Al₂O₃ as an appropriate candidate for cutting tool material in hard machining of difficult to cut materials such as alloy steels.^1–3 Due to these elevated properties, the machinability of Al₂O₃ is low; hence particular methods must be utilized for processing this material.

Nowadays, green machining of Al₂O₃ dramatically reduces traditional machining costs; in this method, ceramics will be machined before the final production stage and then placed in the furnace for final curing. ¹ However, this technique has serious limitations from the viewpoint of dimensional accuracy.

Ultrasonic machining is another technique which has been used by several researchers for machining Al₂O₃; in this method, abrasive particles are thrown towards the workpiece by severe impacts produced by ultrasonic vibrations, and consequently, the material removal process is performed.^2–4 However, ultrasonic machining suffers from low-dimensional accuracy, and therefore cannot be used for machining of precise features such as narrow grooves.

Al₂O₃ can also be machined using laser beam machining; in this method, laser beam is focused on the workpiece surface, and due to highly concentrated heat source, the ceramic is melted or evaporated.^3,5,6 In this case, due to alumina's low heat transfer coefficient, the intense heat may lead to the generation of cracks or even cause failure. ⁷

Electrochemical discharge machining (ECDM) has been introduced as one of the best methods for machining of hard and non-conductive materials such as glasses and ceramics; this technique, which is a combination of electrical discharge machining and electrochemical machining, was first introduced by Kura Fuji for glass drilling. ⁸ In Figure 1, the cathode (tool) and the anode are connected to the negative and positive poles of the power supply, respectively, and both of them are placed in an electrolyte solution. The electrolysis process establishes a current in the electrolyte solution, forming a gas film around the tool. By increasing the voltage, sparks are generated from the tool towards the electrolyte solution. If the workpiece is placed adjacent to the tool, the sparks impact the workpiece, and machining is accomplished. This method can machine non-conductive materials such as ceramics and glasses.^9,10

Figure 1.

Schematic of electrochemical discharge machining.

In 1999, Bhattacharyya et al. investigated the effects of electrolyte concentration and voltage on the material removal rate (MRR) and overcut of Al₂O₃ in the drilling process. The results showed that increasing the voltage and electrolyte concentration would increase MRR; however, the low-dimensional accuracy of the hole was reported in this study. ¹¹

In 2000, Jain et al. investigated the effect of electrolyte temperature on MRR, overcut and depth in the drilling of alumina while the tool had a rotational motion during the machining process. The results showed that overcut would be raised by increasing the temperature of the electrolyte. ¹² These authors also conducted a study in 2002 on the effect of abrasive electrodes on MRR and drilling depth in machining of alumina; their findings showed that MRR and drilling depth were increased by using abrasive electrodes. ¹³

In 2007, Chak et al. performed the drilling process of alumina using an abrasive rotating electrode and applied pulsed voltage. The results revealed the possibility of drilling holes with higher depths. Additionally, using the pulsed voltage prevented the creation of cracks and overcuts in the machining area. ¹⁴ In another work in 2008, these authors used two different tools to drill alumina; one in the form of a tube and the other in the form of an abrasive electrode, both with rotational motions. The results indicated that higher-depth holes with higher quality would be obtained using abrasive electrodes ¹⁵ ; this improvement in machining performance was mainly due to the additional material removal caused by abrasive electrode.

In 2016, Elhami et al. investigated the effect of adding ultrasonic vibrations on the tooltip in glass drilling. The results compromised the increase in MRR and hole depth by using ultrasonic vibrations. However, deteriorated dimensional accuracy was compensated by increasing the amplitude of vibrations as a shortcoming of this method. ¹⁶ In another research, these authors also assessed the effect of ultrasonic vibrations on the drilling speed of glass and the current intensity curve. The results showed that the spark current was reduced by ultrasonic vibrations, which caused higher precision and MRR rate. ¹⁷

In 2018, Pawariya et al. drilled the glass using a tool with both rotational motion and ultrasonic vibrations; they observed that ultrasonic vibrations remove chips from the machining area. They also disclosed the advantageous features of increasing the vibration amplitude up to a certain value; however, a further increase caused the instability of gas film. ¹⁸

In 2019, Xu et al. used abrasive electrodes and machined a groove on a workpiece made of Al₂O₃. It was inferred that the low frequency of the pulsed voltage affected the creation of a stable gas film and the subsequent enhanced MRR. However, the very low pulse voltage frequency led to low surface quality, which was compensated by increasing the tool rotation speed. ¹⁹

In 2020, Rathore et al. investigated the gas bubbles and formation of a gas film in glass drilling using ultrasonic vibrations. The results displayed that ultrasonic vibrations reduced gas film thickness, strengthening sparks, which increased depth and drilling accuracy. Through the use of ultrasonic vibrations, electrolyte could reach the machining area at higher depths and increase the drilling depth. ²⁰

The spark energy in glass drilling using ultrasonic vibrations was investigated by Singh et al. in 2021. The results revealed the formation of cavitation around the tool and an increase in spark energy. In ultrasonic-assisted electrochemical discharge machining (UAECDM), the effect of chemical machining is less than the electrochemical discharge process. The reason was associated with higher electrolyte current by the ultrasonic vibrations. ²¹

In another study in 2021, Sharma et al. studied the drilling of alumina using pulse feed. According to the results, the tool feed should be low enough to prevent collision with the workpiece. Additionally, a high voltage should be used to drill several holes simultaneously. ²²

In addition, Sharma et al. (2021) drilled alumina with a multi-tip tool. They investigated the influence of feed rate and travel distance on tool wear, tool bending, overcut, and hole depth. They found that corner tips drilled deeper than the central tip. ²³

In 2022, Singh et al. studied the spark energy in the drilling of Yttria-stabilized zirconia in ECDM at presence of a magnetic field. These authors showed that a thin and stable gas film could be created using a magnetic field, which increased sparks, sludge removal, and electrolyte current at the bottom of holes. ²⁴

By reviewing the related research in the field of machining Alumina using ECDM, some critical shortcomings of these studies were declared. These shortcomings include the lower MRR due to the high hardness of this ceramic and low hole depth resulting from the electrolyte's unreachability to the hole depth. Moreover, it was found that higher MRR, higher hole depth, and improved operation precision can be obtained by reducing gas film thickness and increasing spark energy by using ultrasonic vibrations in the electrochemical discharge process. However, previous studies used UAECDM for materials with lower hardness than alumina. Therefore, this paper aimed to drill alumina using UAECDM. Accordingly, an experimental setup capable of applying ultrasonic vibrations at the tooltip has been designed and built. Furthermore, the effects of process parameters, including voltage, the amplitude of ultrasonic vibrations, pulse-on time, and pulse-off time have been studied experimentally.

Materials and methods

Ultrasonic assisted electrochemical discharge machining setup

This research used a machining device as shown in Figure 2(a) to perform the experiments. This device is composed of an ultrasonic part, a power supply (MEGATEK MP-6005D), an ultrasonic source (KRF 1500) and a pulse power supply. As shown in Figure 2(b), the pulse power supply includes two sections: power and control. The power section stops and starts the current, while the control section, including an Arduino board (Arduino UNO) and a MOSFET (IRFP250N), controls starting and stopping sparks; consequently, a pulse voltage can be generated. The output voltage was controlled using a MOSFET consisting of three pins for the voltage inlet, voltage outlet, and connection to the Arduino.

Figure 2.

(a) Ultrasonic-assisted electrochemical discharge machining setup used in the present work. (b) Diagram of the pulse power supply.

According to previous investigations, the heat generation in the machining zone can be reduced with the employment of pulsed voltage. ²¹ In the developed setup, tool vibrates and rotates simultaneously; the vibrational movement is provided from a piezo-electric transducer. The ultrasonic vibrations provided by piezo-electric transducer is then transmitted to the tool through a horn designed to vibrate in resonance mode. Ultrasonic vibrations’ amplitude was measured by a non-contact method, using an eddy current sensor made by AEC model PU-05. In addition, the amplitude was controlled via the power rating in the power supply. Supplemental Table 1 represents the relationship between amplitude and power rating. In this device, a gravity feed system was used for moving the workpiece towards the tool. ²⁵ A plate of stainless steel with dimensions of 5 × 30 × 50 mm was used as an anode.

Design of experiments

The workpiece used in this research was alumina, with chemical compositions provided in Supplemental Table 2. The influential parameters in MRR, depth and accuracy of alumina drilling were considered as voltage, the amplitude of ultrasonic vibrations, pulse-on time, and pulse-off time. In this regard, these parameters were considered variables. Three levels for the voltage and amplitude of ultrasonic vibrations and two levels for the pulse-on time and pulse-off time were considered in Supplemental Table 3. A total of 36 experiments have been conducted based on the full factorial design of experiments methodology. The details of the parameters are provided in Supplemental Table 4.

Based on the preceding section, Al₂O₃ has a high melting temperature, so it is necessary to produce a high level of heat generation in machining zone. Therefore, this study used high voltages to create level of heat. Further, in previous investigations,^15,22 the values of 75, 85, and 95 V were commonly used. Although rotating tool speed is a critical parameter affecting gas film, ²⁶ in this paper, it was considered constant (1200 rpm), because the current work focused on the influence of ultrasonic vibration in ECDM. A new tungsten carbide drilling tool with a diameter of 1 mm was utilized for each experiment to prevent tool wear. Previous investigations revealed that using sodium hydroxide (NaOH) with a high concentration helped to create a stable gas film and enhanced machining accuracy. ²⁷ Therefore, 30% NaOH as an electrolyte was used to conduct experiments. Moreover, from the previous investigations,^14,15,23,28 it can be inferred that the apt machining time for Al₂O₃ is between 10 and 30 min, so in the current work, 10 min was selected to prevent tool wear.

Outputs measurement

The following formulas were used to calculate the MRR, overcut and aspect ratio:

$$MRR={\displaystyle \frac{W_i-W_f}{t}}$$

(1)

The MRR is calculated through equation (1), in which $W_i$ is the initial weight of the workpiece, $W_f$ is the final weight of the workpiece and t is the machining time. A scale with an accuracy of 0.1 mg was used to measure the MRR of alumina.

$$\mathrm{\Delta }D=D_h-D_t$$

(2)

The overcut is calculated from equation (2), in which $D_h$ is the hole inlet diameter and $D_t$ is the drilling diameter. The taper angle of the hole is also measured according to Supplemental Figure 1.

$$Aspectratio={\displaystyle \frac{HoleDepth}{HoleDiameter}}$$

(3)

The aspect ratio is calculated through equation (3), which is the ratio of hole depth to hole diameter.

A profilometer (LPM-D1) was used to obtain the inlet diameter ( $D_h$ ), taper angle, and hole depth. X-ray diffraction (XRD) test was conducted by utilizing an X’Pert pro MPD device made by PANalytical company to investigate the workpiece surface integrity. A solid-state PIXcel detector and a Cu k alpha tube were used in this test. Scanning electron microscopy (SEM) was utilized to photograph the hole inlet. These photos were obtained by using MIRA\\LMU device made by TESCAN company. The energy-dispersive X-ray spectroscopy (EDS) test was carried out using a DXP-X10P device made by SAMx company. Moreover, a digital oscilloscope (Good Will GDS-1072A-U) was employed to investigate the ignition process, as shown in Figure 2(a).

Results and discussion

In this section, first, the effects of voltage, the amplitude of ultrasonic vibrations, pulse-on time, and pulse-off time on the MRR, hole depth and accuracy of the hole in UAECDM of alumina were investigated. Then the surface integrity of the machined surface was studied.

Material removal rate

Effect of voltage

Low MRR is one of the most critical problems in alumina machining. As shown in Figure 3, voltage is of prime importance in MRR. Increasing the machining voltage from 75 to 95 V, a 150% enhancement in the MRR was achieved. Figure 4(a) shows three spark diagrams in which the pulse-on time, pulse-off time and the amplitude of the ultrasonic vibrations were equal to constant values of 4 ms, 2 ms and 24 μm, respectively, with different voltage values. According to this figure, increasing the voltage from 75 to 85 was followed by a 27% increase in maximum spark current and a 20% increase in the number of effective sparks (in the gas film, the sparks involved in the material removal procedure are called effective sparks, while the sparks created as a result of the distance between tool and workpiece cannot involve in material removal). So, it can be concluded that increasing the voltage was associated with enhanced spark current and the subsequent increase in spark energy. ²⁹ Accordingly, the heat of the machining area was increased, and enhanced MRR was achieved. Also, with increasing the voltage, the potential difference between the tool and the electrolyte was increased and caused the creation of more effective sparks in different areas of the tool. With the further rise of the voltage up to 95 V, the spark's current remained unchanged, and the effective spark numbers were increased by only 10%. At this voltage, the maximum current of all sparks was the same, which was related to the stability of the gas film and caused to increase in the MRR.

Figure 3.

The effect of pulse-off time, pulse-on time, the amplitude of ultrasonic vibrations and voltage on MRR.

Figure 4.

Spark current diagram in (a) Different voltages. (b) Different amplitudes of ultrasonic vibrations.

Effect of pulse on/off time

Reducing pulse-off time and increasing pulse-on time were correlated with the increase of ignition time and the following increase in the number of sparks in the electrochemical discharge process. According to Figure 3, the enhanced MRR was achieved in this case. As can be seen in Figure 3, with increasing pulse-on time from 2 to 4 ms, 67% enhancement in MRR was obtained, and by decreasing the pulse-off time from 2 to 1 ms, 46% increase in MRR was accomplished.

Effect of ultrasonic vibrations amplitude

Another factor influencing the MRR is the amplitude of ultrasonic vibrations. As shown in Figure 3, with the implementation of ultrasonic vibrations, the MRR was increased by 29%, but with increasing the amplitude of ultrasonic vibrations, the MRR was reduced. Three spark diagrams with the respective pulse-on time, pulse-off time and voltage equal to 4 ms, 2 ms and 95 V and different amplitudes of ultrasonic vibrations are provided in Figure 4(b). Once gas film thickness decreases, sparks require less energy to break them, reducing spark current. As shown in Figure 4(b), the maximum spark current in UAECDM with the amplitude of 24 µm was much less than that of ECDM without ultrasonic vibration. Increasing the amplitude to 34 µm caused the maximum spark current to decline; therefore, using ultrasonic vibration reduced gas film thickness. The thickness decreased by increasing the amplitude from 24 to 34 µm. It was also proved by previous studies ¹⁷ that gas film thickness reduction occurs by adding ultrasonic vibration in the ECDM process.

Moreover, when the gas film thickness declines, sparks need less energy to break them; consequently, while using ultrasonic vibration in the ECDM process, the number of effective sparks increases, and MRR increases as a consequence. In the first diagram, no ultrasonic vibration was implemented, and low MRR was achieved. However, through ultrasonic vibrations with an amplitude of 24 µm, the effective sparks numbers and their average currents increased by 120% and 7%, respectively.

As shown in Figures 3 and 4, when the amplitude of the ultrasonic vibrations exceeded the optimum value (24 µm in this study), a reduction in MRR was observed. According to the spark diagram related to 34 µm amplitude, the current was not the same for all sparks, and an unstable gas film was formed in this case. Despite the increase in the number of effective sparks by 8%, the average sparks current decreased by 2%, followed by a 12% decrease in MRR.

Hole depth

In electrochemical discharge drilling, the penetration of electrolyte to the hole depth became impossible as the hole depth increased. Hence, no spark was created in hole depth, and sparks were only concentrated in the side walls of the tool. ¹⁷ In this regard, the advancement in hole depth would be decelerated. The effects of voltage, the amplitude of ultrasonic vibrations, pulse-on time, and pulse-off time on the hole depth are illustrated in Figure 5.

Figure 5.

The effect of pulse-off time, pulse-on time, ultrasonic vibrations amplitude and voltage on the hole depth.

Effect of voltage

According to the discussion mentioned above in the MRR section, the spark current and, consequently, the spark energy increased by increasing the spark voltage. Also, the number of effective sparks increased. Increasing the energy and the number of effective sparks would lead to an increase in the hole depth. As shown in Figure 5, by increasing the voltage from 75 to 95 V, the hole depth was increased up to 53%.

Effect of pulse on/off time

Variations of pulse-on time and pulse-off time also affected the hole depth. Increasing the pulse-on time was associated with a higher number of effective sparks and higher depth while increasing the pulse-off time was related to a lower number of effective sparks and lower depth. As shown in Figure 5, by increasing the pulse-on time from 2 to 4 ms, the hole depth was increased by 20%, and by increasing the pulse-off time from 1 to 2 ms, the hole depth was decreased by 4%.

Effect of ultrasonic vibrations amplitude

According to Figure 5, the amplitude of ultrasonic vibrations was the other influential parameter in hole depth. As shown in this figure, ultrasonic vibrations with an amplitude of 34 µm increased the hole depth to 52% compared to the case with no ultrasonic vibrations (amplitude of 0 µm). Regarding Figure 6(a), in the electrochemical discharge process, the electrolyte could not penetrate the hole depth in higher-depth holes due to a thick gas film. Accordingly, no sparks were generated at the hole depth (tooltip), and sparks were presented only at the hole inlet.

Figure 6.

The effect of ultrasonic vibrations amplitude on the hole depth.

However, with the application of ultrasonic vibrations in the electrochemical discharge process (Figure 6(b)), the thickness of the gas film decreased, and the electrolyte was able to penetrate the hole depth. Also, reducing the thickness of gas film was followed by an increase in the number of effective sparks, and as a result, a hole with higher depth was achieved. The upward and downward motions of the tool resulted from ultrasonic vibrations, which led the electrolyte to flow into the hole depth.

Figure 7 shows three holes drilled with a voltage of 95 V and pulse-on and pule-off times of 2 ms, with different amplitudes of ultrasonic vibrations. The ultrasonic vibrations were not applied for drilling hole A, in which the minimum depth was obtained. This result was justified by the fact that the electrolyte was unable to reach the hole depth, and no effective spark was created in that area. However, in hole B, ultrasonic vibration with an amplitude of 24 µm was utilized. In this case, the electrolyte could reach the hole depth and create sparks due to the reduction of the gas film thickness, ¹⁷ which led to an increase of 76% in hole depth. By increasing the amplitude of the ultrasonic vibrations in hole C up to 34 µm, the domain of tool movement increased, and more electrolyte was pumped into the hole depth, resulting in the creation of more effective sparks and higher-depth hole. The hole depth, in this case, was increased by 100% compared to the case with no ultrasonic vibrations.

Figure 7.

The effect of amplitude on the hole depth (V: 95 V, pulse-on time and pulse-off time: 2 ms).

Dimensional accuracy

During electrochemical discharge drilling, taper-shaped holes are machined with a higher diameter at the inlet; the reason for this phenomenon was related to the higher flow of electrolyte and the higher number of effective sparks in the hole inlet compared to the hole depth. In this paper, the hole accuracy was measured using three parameters; the first parameter was overcut, which revealed the amount of machining excess at the hole inlet and obtained using equation (2), and the second parameter was the taper angle of the hole, which demonstrated the tapering of hole wall and can be obtained using the topographic images in Supplemental Figure 1. The third parameter (aspect ratio), which can be calculated through equation (3), was the ratio of the hole depth to hole diameter.

Effect of voltage

As depicted in Figure 8, the voltage was an effective parameter in the hole dimensional accuracy; as the voltage increased, the spark energy was amplified, and the MRR was also improved as a result. Regarding the creation of more sparks in the hole inlet, increasing the voltage would lead to a higher overcut. Regarding Figure 8(a), the overcut increased by 109% as the voltage increased from 75 to 95 V. As mentioned in the section ‘Hole depth’, increasing the voltage was followed by an increase in the energy and the number of sparks. Consequently, the hole depth increased, resulting in the reduction of the tapered angle.

Figure 8.

The effect of pulse-off time, pulse-on time, amplitude and voltage on (a) overcut, (b) taper angle of the hole and (c) aspect ratio.

Moreover, as can be realized from Figure 8(b), the taper angle decreased by 10% through a voltage rises from 75 to 95 V. The higher increase of the hole depth over the overcut value, can be achieved by increasing the voltage; hence, the taper angle decreases with increasing voltage, Figure 8(c). By increasing voltage, the ratio of hole depth to hole diameter increased, and the aspect ratio increased consequently. As shown in Figure 8(c), by increasing voltage from 75 V to 95 V, the aspect ratio increased by 24%.

Effect of pulse on/off time

Pulse-on time and pulse-off time were the other influential parameters. Controlling the duration of ignition and the number of sparks would be possible by altering these parameters. Changing the ignition time would significantly affect the overcut value. As shown in Figure 8(a), reducing the pulse-on time from 4 to 2 ms or increasing the pulse-off time from 1 to 2 ms resulted in the reduction of the overcut by 39% on average. However, as shown in Figure 8(b), the taper angle of the hole was not affected by varying pulse-on time or pulse-off time. Increasing the ignition time caused the overcut and the hole depth were increased almost equally, and conversely, as the ignition time decreased, the hole depth and overcut decreased almost equally. Accordingly, no change in the hole taper angle was inspected. As depicted in Figure 8(c), pulse-on time and pulse-off time do not have any impact on the aspect ratio since by changing pulse-on time and pulse-off time, the hole depth and the hole diameters changed almost to the same extent.

Effect of ultrasonic vibrations amplitude

The amplitude of ultrasonic vibrations was another critical parameter. Three holes drilled with a voltage of 85 V, pulse-on time of 2 ms and pulse-off time of 1 ms, with different amplitudes of the ultrasonic vibrations, are represented in Figure 9. No ultrasonic vibration was applied in the case of hole A, and due to the presence of a thick gas film, the electrolyte could not penetrate deeply. Consequently, all the sparks were generated at the hole inlet, and a tapered-shaped was created. Nevertheless, in the case of hole B, through ultrasonic vibrations with an amplitude of 24 µm, the thickness of the gas film was reduced, and the electrolyte was able to penetrate deeper, and the subsequent increase in the hole depth and decrease in taper angle resulted. However, a reduction in the gas film thickness was associated with the increase in the number of effective sparks and the consequent increase in the overcut due to the higher number of sparks in the hole inlet than the hole depth. Applying ultrasonic vibrations with an amplitude of 34 µm in the case of hole C would result in an increase in hole depth, and a decrease in taper angle due to the greater motion range of the tool and the subsequent reachability of the electrolyte to a greater depth. As proved in previous studies, ¹⁷ the gas film thickness decreases by increasing ultrasonic vibration amplitude. The reduction of the gas film thickness caused its instability and affected the energy and the number of effective sparks. For this reason, the overcut in this hole was greatly reduced compared to the previous case.

Figure 9.

The effect of amplitude on hole dimensional accuracy (V: 85 V, pulse-on time and pulse-off time: 2 and 1 ms). (a) 0 µm, (b) 24 µm and (c) 34 µm.

As shown in Figure 8, the taper angle decreased due to the enlarged hole depth with the increasing amplitude of ultrasonic vibrations. Using ultrasonic vibrations with an amplitude of 34 µm reduced the taper angle by 19%. Applying ultrasonic vibrations would increase the overcut regarding the decreased thickness of the gas film, and the resultant increased number of effective sparks would be achieved. However, with a further increase in the amplitude of ultrasonic vibrations, the instability of the gas film would affect the energy and the number of effective sparks, resulting in the reduction of overcutting. With ultrasonic vibration with an amplitude of 34 µm, the overcut was reduced by 18%.

As represented in Figure 8(c), applying ultrasonic vibration aspect ratio increased since gas film thickness decreased; hence, electrolytes would easily flow into the hole depth. Ultrasonic vibration has more impact on the hole depth than the hole diameter. Therefore, by applying ultrasonic vibration with an amplitude of 34 µm, the aspect ratio increased by 62%. The scientific interpretation of this phenomenon is that tool movement created by ultrasonic vibration is in the direction of hole depth instead of hole diameter. According to Figure 8(c), the highest voltage and amplitude should be applied to reach the maximum aspect ratio. The maximum aspect ratio (0.6) was achieved by selecting 95 V, 34 µm, 4 ms and 2 ms for voltage, amplitude, pulse-on time and pulse-off time, respectively.

Surface integrity

In the electrochemical discharge method, some defects may occur on the workpiece's surface due to the machining zones’ high temperature. In previous investigations, nano-indentation ²¹ and 2D optical images ²² were used to test the workpiece material. However, in this paper, the workpiece was made of alumina and prepared through the powder metallurgy method. Accordingly, the device probe may be placed between the powder grains in the nano-hardness test, and an error may occur in the test result. Furthermore, 2D optical image method is not an exact method. For this reason, in this paper, the workpiece was examined using XRD and EDS methods before and after machining.

The XRD test of the workpiece before machining is depicted in Figure 10(a); according to this figure, Al₂O₃, which was characterized by prominent peaks, and magnesium aluminium oxide (Al₂Mg₁O₄), which was characterized by small peaks, were seen in the workpiece. Additionally, the XRD test of the workpiece after machining is provided in Figure 10(b); in this figure, Al₂O₃ and Al₂Mg₁O₄ were detected. Similar to the previous figure, the prominent peak belonged to Al₂O₃. The comparison of these figures is provided in Figure 10(c); according to this figure, no difference was observed. As a result, it can be inferred that the material of the workpiece was not changed due to machining.

Figure 10.

XRD test of workpiece surface (a) Before machining. (b) After machining. (c) Comparison of two diagrams.

Figure 11 shows the SEM image of a hole drilled with a voltage of 95 V and a pulse-on time of 4 ms. Although a high temperature in the machining zone was generated, no cracks in the hole inlet and the edge of it can be observed. In addition, thermal spalling did not occur at the edge of the hole where the most sparks were created.

Figure 11.

SEM image of (a) the hole inlet, (b) the edge of the hole.

The EDS test of two points in the adjacency of the hole is expressed in Figure 12. The hole was machined with a voltage of 95 V, a pulse-on time of 4 ms, and a great amount of heat was generated during machining. According to Figure 12(a), for a farther point from the hole where no machining occurred, the amount of aluminium (Al) and oxygen (O) were higher than the other elements. Moreover, Figure 12(b) displays the results of the EDS analysis for a point at the hole inlet with the highest heat resulting from the highest number of sparks, as described in the previous sections. At this point, regarding the deposition of oxygen (O) and sodium (NA) of the electrolyte (NaOH) on the workpiece at high temperatures, the weight percentage of the oxygen was enhanced. However, similar to the previous experiment, the oxygen and aluminium contents were still higher than the other elements.

Figure 12.

EDS test (a) Outside machining environment. (b) At the hole inlet.

Conclusion

This study investigates the effects of ultrasonic vibration amplitude, voltage, pulse-on time and pulse-off time on the MRR, hole depth and hole accuracy in UAECDM of alumina. The following results can be obtained in the selected range for process parameters:

Raising voltage can raise up to a 150% increase in MRR.

By increasing pulse-on time or declining pulse-off time, MRR was raised to 66%.

Applying ultrasonic vibrations in the optimal range achieved a stable thin gas film.

Using ultrasonic vibrations with an amplitude of 24 µm led to a 120% increase in the effective sparks number.

Applying ultrasonic vibration led to a 52% increase in the hole depth.

By adding ultrasonic vibration to the process, overcut and taper angle decreased by 18% and 19%, respectively.

The aspect ratio was raised to 62% by using ultrasonic vibration.

Despite the aforementioned desirable achievements in the machining of Al₂O₃, there are still significant challenges regarding tool wear and drilling deep holes.

Machined Al₂O₃ can be used as insulation and insoluble substances in electronics and chemical industries.

Tool wear in the ECDM process is high, especially in long-time machining or the machining of deep holes. Since tool wear is a significant problem in ECDM, more future research is required to establish the field further.

Supplemental Material

sj-docx-1-pie-10.1177_09544089221141339 - Supplemental material for Drilling of Al₂O₃ ceramic using ultrasonic assisted electrochemical discharge machining process

Supplemental material, sj-docx-1-pie-10.1177_09544089221141339 for Drilling of Al₂O₃ ceramic using ultrasonic assisted electrochemical discharge machining process by Mohammad Bagheri, Daniyal Sayadi, Ardeshir Hemasian Etefagh, Mohsen Khajehzadeh and Mohammad Reza Razfar in Proceedings of the Institution of Mechanical Engineers, Part E: Journal of Process Mechanical Engineering