Various types of UV assisted EDM have been reported in the literature. These studies can be classified into three categories, such as, vibration to tool/wire, vibration to workpiece and vibration to dielectric. This section presents the summary of research studies carried out on these three kinds of UV assisted EDM.
4.1. Ultrasonic Vibration of Tool and Wire Electrode
For achieving better performance, tool horn should match the resonant frequency of the transducer (Figure 8
a). Nanu et al. [39
] carried out finite element study of horn design using ANSYS that included the UV frequency distribution at different part of the horn. This analysis helps in finding the right dimension of horn for achieving longitudinal mode vibration. Synchronization of ultrasonic generator and EDM pulse generator is imperative for best performance of UVA-EDM.
b shows a schematic representation of vibration assisted EDM as demonstrated by Kremer et al. [40
]. Their study focused on the MRR, EWR, roughness, hardness, fatigue strength and white layer generation characteristic during UV assisted EDM. The authors reported that the process performance of vibrating tool is better than the non-vibrating tool and the reason might be the better flushing condition achieved by the vibration as well as effective discharging. Although ultrasonic vibration assisted EDM shows improved MRR (Figure 9
a) and surface roughness, it also deteriorates the tool wear condition (10–15% increases) due to the porous nature of graphite tool which reacts to the cavitation mechanism of the process. Heat effected zone reported by their study is smaller than EDM and it comes with regular as well as continuous white layer. In addition, tempered and reheated layer appears to be thinner as well. However, both hardness and residual stress are not much affected by ultrasonic vibration assisted EDM (Figure 9
In the research of Hirao et al. [41
], the application of ultrasonic vibration on tool electrode resulted in an increase of machining speed even if the amplitude of vibration was 1 µm. Furthermore, the rate of abnormal discharges, which occurred because of the bridging contact between the electrode, accumulated dust and workpiece, was decreased since the tool was forcibly separated from the machined workpiece by this “pumping action.” Such tendency has a positive effect on the surface finish because of an increase in the rate of normal discharges. Comparative analysis of EDM and ultrasonic vibration assisted EDM shows not much difference in generated roughness with the change of ultrasonic vibration frequency, however, MRR seems to be improving until 66.5 kHz and then drops gradually (Figure 10
a). In addition, MRR also increases with the increase of the tool diameter as well as the ultrasonic vibration amplitude (Figure 10
b). As can be seen in Figure 10
c, UV assisted EDM with 6 µm of amplitude increases roughness of 0.2 µm compared to EDM.
Shervani-Tabar et al. [42
] investigated the dynamics of vapor bubble formation during EDM process and how it is effected by vibration application numerically by solving the boundary integral equation. During conventional EDM process, bubble generated due to electric discharge stays in somewhere middle point between the workpiece and tool with its initial volume. When ultrasonic vibration is applied to EDM, two cases are considered, first one considers discharge occurrence when tool and workpiece are in closer distance, second one considers discharge occurrence at the far distance of tool and workpiece. For second case, during bubble growth and collapse stage, tool goes away from the workpiece, whereas in third case the tool moves towards the workpiece. Their numeral investigation reported on the bubble expansion to the normal direction of tool-workpiece interface taking the shape of hourglass and finally collapses into two parts, considering only EDM process takes place. For the second case, bubble expands to its maximum value causing rapid pressure drop inside the bubble when the tool moves away due to vibration, thus resulting in a rapid vaporization and evacuation of the melted debris. This enhances the MRR. Third case, when tool again moves towards the workpiece, causes the reduction of bubble volume to the smallest one and therefore experiencing shortest lifecycle. Their experimental verification on Ti materials using forged copper electrode in die-sinking EDM shows the increase of MRR with the pulse on time, however, selection of proper process parameters as well as synchronization of vibration frequency and discharge frequency have significant effect on the MRR (Figure 11
On another study, Shervani-Tabar et al. [43
] also investigated the effect of tool and workpiece shape on the bubble dynamics after the occurrence of necking phenomena during UVA-EDM. With the creation of necking, vapor bubble existing in the spark gap splits into two parts, which causes liquid jets to be formed on the upper and lower bubble boundary. The liquid jet on the upper boundary aids in impinging on the tool surface and lower one hits on the workpiece surface. This phenomenon in turn removes the debris from the spark gap, as well as causes the erosion of both electrodes. Since the shape and the velocity of liquid jet can be influenced by the tool and workpiece shape, this study considers three cases such as: flat tool & workpiece, convex tool & workpiece and concave tool & workpiece surface. As per their numerical simulation, the jet velocity on the vapor boundary has higher magnitude and wider shape in case of concave tool & workpiece surface, therefore, increases the MRR for the concave shape surface. Shervani-Tabar et al. [44
] also investigated the bubble dynamics and hydrodynamic behaviour of liquid jet for simultaneous vibration of both the workpiece and tool numerically and reported on the bubble generation in the mid distance of spark gap due to the same frequency and amplitude of tool & workpiece. Under this condition, split parts of bubble are the mirror image of each other and they demonstrate similar dynamic behaviour. Simultaneous ultrasonic vibration causes extension of the vapor bubble lifetime before necking happens compared to the vibration assisted tool only. Also, the rate of bubble growth as well as the collapse phase is larger for simultaneous vibration causing the bubble to reach a larger volume than tool vibration. Therefore, the minimum pressure drops inside the bubble causes ejection of material and enhances the MRR. They also reported increased bubble lifetime, higher rate of growth and collapse phase with the increase of both ultrasonic vibration amplitude and frequency. As a result, it also increases the liquid jet velocity that impinges on the surface, thus increasing the MRR.
Zhang et al. [45
] also reported on the increased MRR and surface roughness with the increase of voltage, current and vibration amplitude during their UV assisted EDM on ceramics with the help of steel tool (Figure 12
). This increased MRR can be explained with the effect of better removal of debris, introduction of a new dielectric into the gap, extraction of more liquid materials from the melted pool as well as less re-solidification of liquid metal, as generated by the ultrasonic vibration. Maximum value of vibration amplitude for reaching better machining performance depends on the gap distance between the tool and the workpiece.
In order to improve the MRR of insulating ceramics associated with EDM process, Praneetpongrung et al. [46
] investigated assistive electrode EDM with the aid of ultrasonic vibration and reported on the effect of tool polarity and ultrasonic vibration amplitude on MRR, EWR, roughness, conductive layer generation. For insulating ceramics like Si3
, it is important that the stray conductive layer remains on the ceramics surface to continue the EDM process, however Mohri et al. reported on the disturbance in the generation of this conductive layer due to the tool vibration [47
]. Therefore, Praneetpongrung et al. [46
] suggested that ultrasonic vibration should be applied to EDM after the transition state in order not to disturb the conductive layer generation. According to their observation, conductive layer does not adhere to the surface when positive polarity is used thus decreasing the MRR significantly, however, the negative polarity can increase the MRR. UVA-EDM with the positive polarity also offers reduced EWR due to the adherence of carbon particles on the tool surface. In addition, vibration amplitude ranging from 2.7–3.5 µm provides higher MRR with lower EWR due to the improved debris evacuation as well as increased normal discharge occurrence. On the other hand, vibration amplitude above 3.5 µm causes decrease in the MRR with the increase of EWR due to the occurrence of the abnormal discharges. Polishing followed by UVA-EDM is recommended for the enhanced surface roughness. Abduallh and Sahabgard et al. [48
] have investigated the surface integrity characteristics of tungsten carbide (WC) during UVA-EDM. Roughly 10% increment of surface roughness was observed due to the UVA compared to pure EDM. This increase in surface roughness may be because of the hot spot-core plasma as well as the imposed current, which reduces the ignition delay time and intensive micro-jets. The micro-jets aid not only in terms of improved flushing but also reduced unstable pulses (Figure 13
). They also reported on the increased tool wear due to the cavitation effect created by UV assistance. Although UVA-EDM is reported to increase the surface micro-hardness compared to EDM at higher energy condition, hardness value remains unaffected. Surface topography generated by the both processes presented in Figure 13
shows difference in terms of crater size and occurrences of arcing.
Ghoreishi et al. [49
] studied the effect of vibratory, rotary and combined vibratory-rotary tool (Figure 14
) on the MRR, TWR and surface roughness and reported on the better performance of vibratory tool in terms of MRR and TWR. Also, higher frequency vibration has more pronounced effect on the MRR especially on the finishing zone compared to the lower frequency vibration, as it can eradicate issues such as short circuiting, arcing and pulse instability into the narrow gap condition. For the finishing operation, the high frequency vibration along with the rotary motion can provide better performance compared to rotary only, pure and vibratory only EDM. On the other hand, combined vibro-rotary EDM enhances the MRR by 35% compared to vibro-EDM. However, rough EDM does not require any assistance in terms of improved flushing and can achieve maximum MRR with the higher current setting alone.
In another study done by Lee et al. [50
] insulating ceramics Al2
was machined using UVA-EDM and their results suggested increased MRR when compared with the summation of MRR by UVA, EDM, however, increased MRR also enhances generated surface roughness. Increase of the discharge power or voltage, vibration amplitude and frequency aid in MRR due to the efficient discharge distribution and easier debris removal. Uhlmann et al. [51
] studied high temperature resistant materials using low frequency vibration (0–700 Hz) for high aspect ratio structure and reported on the leading effect of the vibration amplitude. Their results suggested reduced MRR when applying higher frequency along with the high amplitude due to the short circuiting phenomena. However, the high frequency and low amplitude can provide better machining efficiency due to the better flushing. Their study also reported on the 11% increment of MRR and 21% reduction of tool wear due to the UV assistance [52
Lin et al. [53
] studied the effect of SiC abrasive mixed kerosene and distilled water during UVA-EDM and reported on the enhanced MRR due to the additional removal of materials by abrasive action; however, higher concentration of SiC seems to have negative effect due to the accumulation of abrasive in the spark gap causing the unstable discharges. In general, MRR from UVA-EDM is higher than the EDM due to the larger crater size. In addition, distilled water as dielectric offers higher MRR and lower REWR (relative electrode wear ratio)) compared to kerosene. On the other hand, surface roughness generated by the kerosene appears to be better than distilled water due to the combined effect of higher cooling rate of distilled water and poor thermal conductivity of Ti. In addition, less recast layer is observed in case of distilled water due to its higher cooling rate causing the conduction of less energy into the machined area (Figure 15
Zhixin et al. [12
] proposed low cost pulse EDM along with the tool vibration in order to drill deep holes in ceramics materials and their method replaced the pulsed generator, as discharge pulse is generated by the ultrasonic resonant system. Several problems of EDM when drilling deep holes are difficulties in removal of debris, difficulties in complete material removal due to the surface tension, bonding of solid and liquid elements. However, UV assisted EDM can offer better evacuation of debris, thus, reduces metal re-solidification by creating pressure variation in the gap. Their results reported on the enhanced MRR with the increase of voltage due to the higher energy availability and better flushing. Thoe et al. [54
] also studied UVA-ED drilling operation on Ni alloy covered with nonconductive coating and reported on the enhanced MRR as well as less heat affected zone along with micro-cracks due to the stable discharge condition. In addition, increase of the current aids in an increased machining depth, however, the tool wear is also equal or double in length. Murthi et al. [55
] studied the pulse characteristic during UVA-EDM of high carbon, chromium steel and reported on the insignificant effect of UV on the ignition delay time. However, UV assisted current pulses show different shape compared to the one without UVA (Figure 16
). In general, EDM current signal has rectangular shape, whereas UV assisted EDM exhibits pre-breakdown current which might be due to the acousto-electric effect. However, due to its low magnitude density, this current does not have significant effect on machining. Nevertheless, UV-A increases the number of effective current pulses due to the reduction of the short circuiting and arcing. Lin et al. [25
] proposed a new hybrid process which combines UVA-EDM with magnetic force assistance and is applicable for large area EDM. Their results reported on the improved machining stability due to the magnetic force assisted debris evacuation (Figure 17
) as well as better flushing due to UV assistance. The MRR of this hybrid process increases along with the increase of the peak current as well as pulse duration. However, higher peak current also increases the EWR, whereas increased pulse duration decreases EWR due to the reduced density of spark energy. Also, the tool electrode polarity affects the EWR. When polarity changes from positive to negative, EWR along with MRR reduces a great deal. Their study reported on 34% more MRR and 21% reduced roughness for UVA-EDM than conventional EDM. Figure 17
b also presents the machined surface topography using hybrid process. It is obvious from these images that, increase of discharge energy aids in the enlarging and deepening of the crater size which comes with other surface defect such as micro cracks. Therefore, larger discharge energy hinders the surface integrity and to some extent compromises its application capability.
Srivastava et al. [56
] compared the performance of normal and cryogenically cooled tools with and without the assistance of UVA-EDM. Their study narrates the enhancement of MRR and EWR with the increase of current and pulse on time. Cryogenically cooled tool without UV-A exhibits lower MRR compared to the other two tools. The reason behind this can be the reduced temperature of cryogenically cooled tool, which requires longer time to get heated allowing less available heat for melting or vaporization. The MRR can be improved with the increase of pulse on time. However, after a certain rise of pulse on time, the MRR shows declining magnitude due to the frequent re-solidification of molten material. In addition, EWR for all three tools increases with the increase of pulse current. Moreover, the surface roughness for the cryogenically cooled tool with and without UV-A exhibits better finish than the normal tool due to the effective debris removal and smaller crater size. However, with the increase of pulse on time and pulse current, MRR as well as surface roughness goes higher. Iwai et al. [58
] investigated the machining of PCD(polycrystalline diamond) materials, which contains non-conductive diamond particles and is known as hard to machine material, with the UVA-EDM(ultrasonic vibration assisted) where application of ultrasonic vibration modes varies. Different ultrasonic vibration modes are presented in Figure 18
. As per their results, complex mode vibration exhibits 1.7 times better removal than without tool vibration and 3 times better than pure axial vibration. Although complex vibration mode offers higher EWR, insignificant difference in EWR is observed for without vibration and pure axial vibration cases. In addition, surface roughness associated with the complex mode is relatively less than the axial mode vibration, although the difference is not very significant.
In order to avoid the environmental hazard of dielectric fluid, researchers proposed dry EDM mechanism, however, this process has limitations in terms of poor surface quality due to the inefficient flushing. Xu et al. [35
] investigated thermal stress related material removal mechanism during UVA-EDM in gas medium. As per their observation, thermal spalling which depends on the material properties is considered as the removal mechanism. This removal mechanism occurs in four steps, such as: generation of thermal stress, generation of micro-cracks, grain breaking process and particle striping. The debris generated from this process is effectively flushed out by the high-pressure gas flowing through the vibrating electrode. This volumetric material removal is not only by melting but also by cavitation erosion. They also reported on the increased MRR when pulse on time, current and voltage were increased due to effective heat transfer to the workpiece. Opposite trend was observed for the increase of pulse off time due to the inefficient heat transfer. Generation of micro-holes for micro-products also experience challenges associated with the debris evacuation and it becomes a serious issue when the generated micro-holes need to have higher aspect ratio. Therefore, the application of UV to the electrode during micro-EDM process plays a very significant role [60
]. Jahan et al. [61
] compared micro-hole fabrication with and without UV assistance and reported on the higher aspect ratio holes (16.7) for difficult to cut material tungsten carbide with UVA-micro-EDM. Their study also reported on the noticeable improvement in MRR and decreased EWR. This improvement in term of MRR, EWR and dimensional accuracy can be explained by the discharge ratio increment, reduced arcing or short-circuit phenomena, as well as the effect of flushing mechanism [62
]. Weiliang et al. [63
] conducted a similar experimental study to fabricate arrays of microelectrodes and microstructures using UVA-EDM. Their observation suggested increased MRR with the increase of voltage and increased efficiency of two-fold using 20 kHz frequency compared to pure EDM. They also reported on the cavitation effect created by UV application causing the efficient removal of debris as well as assisting in discharge effectiveness. Mahardiak et al. [64
] proposed a micro-EDM monitoring process where, based on the discharge pulse count, discharge energy can be calculated and their result shows good agreement. For the higher MRR, number of pulses increases although average pulse energy remains same. They verified their process monitoring applicability against different process conditions and shape up as well as flat head type machining. They also reported on the reduced machining time for the tool vibration and tool rotation combination compared to the one without tool vibration and rotation. Reduced machining time can be explained with the fact of reduced adherence of debris to the tool electrode due to the tool rotation and vibration, therefore avoiding the occurrence of debris attachment on the tool or workpiece surface, as shown in Figure 19
Deep hole fabrication also comes with the taperness effect. Kim et al. [65
] used resistance-capacitance (RC) type pulse generator along with UVA-EDM in order to reduce this taperness effect. Their investigation suggested to machine micro-holes as quickly as possible using higher settings of capacitances in order to avoid secondary discharge caused by the debris. The capacitance range of 1000–5000 pF shows insignificant difference between the exit and entrance side of hole. However, increasing machining time always causes overcut of the generated hole. They reported less than 1 µm diameter difference on 500 µm thick steel plate using this method, however when the thickness was increased to 1000 µm, diameter difference increased to 7 µm. Application of UVA can provide not only the reduced machining time but also effective flushing, thus aids in reducing the diameter difference. This new approach recommended use of small capacitance at first and then larger capacitance to achieve straight hole (Figure 20
Huang et al. [66
] demonstrated 60% improvement of efficiency by applying UVA while EDM of Nitinol, which is a very difficult to machine material. The amplitude of the ultrasonic vibration can be increased up to a certain value for which the machining efficiency increases, however above that, the efficiency falls again due to the collision of tool and workpiece. In addition, large amplitude affects dimensional accuracy by creating horizontal vibration of the tool. Increase of voltage aids in reduced machining time, nevertheless, provides a higher tool wear. Their study suggested to use a larger tool to increase the machining efficiency with less tool wear effect and to use higher pre-set gap to increase the flushing as well as the machining efficiency. Therefore, by manipulating the gap size, tool size and vibration amplitude, machining efficiency can be improved due to the better stirring effect. Bamberg et al. [68
] incorporated tool orbital motion instead of tool vibration while micro-EDM of Nitonol for the hole fabrication. Better surface condition as well less machining time is achieved due to the increased gap between tool and workpiece wall, which reduces the arcing or short circuit phenomena. One advantage of this technique lies in using one single standard tool for creating range of holes due to the orbital effect. Schubert et el [69
] narrated 40% process speed increase for metallic material and aspect ratio of more than 5 for ceramics materials due to the application of low frequency ultrasonic vibration to EDM. As per their study, low frequency is particularly advantageous as it does not interfere with the ultrasonic resonant system but is able to create a pressure variation in the dielectric fluid to enhance the speed of fluid.
Mastud et al. [70
] investigated reverse UVA-EDM by simulating the debris removal process with the help of Fluent. Their simulation results confirmed the short-circuiting effect due to the debris accumulation and ultrasonic vibration generated compression as well as expansion effect which increases the debris velocity. As per their results, ultrasonic vibration frequency also increases debris particle velocity due to the increased velocity of vibrating tool that transfers the momentum to debris. On the other hand, the amplitude increment increases the pressure variation of the flushing liquid, thus aids in enhancing the debris particle velocity as well. Liao et al. [71
] proposed a new inclined micro-EDM drilling with axial vibration application and reported on the enhanced drilling depth compared to horizontal set up due to the associated improved debris removal from the inclined set up (Figure 21
). Set up angle of 15° inclination along with the upward feeding of tool gives significantly improved MRR due to the gravity assistance when compared with horizontal set up. 75% increase of drilling depth is reported on Aluminium alloy in this study, whereas, aspect ratio of 26 for 100 µm hole is reported on steel.
Other than, EDM, micro-EDM; as well as wire-EDM plays a significant role for hard to cut material fabrication. Several studies reported on the improvement of process by applying vibration assisted WEDM. Guo et al. [72
] demonstrated 30% increase in the cutting efficiency as well as reduced residual stress by applying UVA to wire-EDM process. As can be seen in Figure 22
a, ultrasonic vibration can be applied both in the cutting direction as well as perpendicular to the cutting direction. Perpendicular to the cutting direction vibration increases the kerf width compared to the vibration applied in the cutting direction. Figure 22
b states the increase of cutting rate with the application of UVA. Their study also demonstrated an optimum relation between discharge energy and vibration amplitude for the higher machining rate. Spark gap is a function of discharge energy, that is, increase of energy in general increases spark gap, which can also accommodate larger vibration amplitude. However, if higher amplitude vibration is applied into the lower spark gap, it will result in a short circuit pretty often, whereas, lower amplitude with higher spark gap also does not make much impact on the process. Figure 22
c shows the relation between amplitude and current. On another study, Guo et al. [73
] also reported on the benefits of using higher frequency vibration that may cause multiple channel of discharge resulting in a better surface as well as a higher machining rate. Higher frequency also aids in reduced wire breakage due to the shifting of discharge point. Crater shape changes from rounder shape (WEDM) to elliptical shape (UVA-WEDM) due to the shifting of discharge channel. In addition, optimum distribution of uniform discharge is achieved with UVA-WEDM, thus also enhances the energy magnitudes to 15% more than the normal the WEDM. Kavtaradze et al. [74
] also suggested applying vibration to wire during EDM process to generate the superposition of vibration through numerical investigation. Lipchanskii also reported on improved process performance with the application of UVA to WEDM [75
Hoang et al. [76
] investigated on the UVA-WEDM, where the ultrasonic vibration is applied on both tool and workpiece. Their study suggested the generation of nodes and antinodes under the influence of ultrasonic vibration and their number increases with the increase of vibration frequency as seen from Figure 23
a. Ultrasonic vibration applied to the workpiece creates a larger pressure variation due to its larger area than wire and therefore, enhances the debris flushing significantly more than when vibration is applied to wire (Figure 23
b). Thus, the cutting rate for vibrating workpiece offers 1.5 times more than vibrating wire and 2.5 times larger than the pure WEDM. Although the surface finish gets better for both the tool and workpiece vibration mode, workpiece vibration provides better surface roughness compared to wire vibration due to the difference in the forced wire displacement along the node and antinode direction.
Mohammadi et al. [77
] conducted UV assisted WED-turning (Figure 24
) and investigated the effect of power, pulse off time, vibration, workpiece rotation on MRR using ANOVA method. Their study reported on significant improvement in the MRR due to the better flushing, the erosion mechanism involved with the UV application and reduced wire breakage. This improvement is valid for both roughing and finishing condition. Increase of workpiece rotation results in a higher MRR due to the cavitation effect as well as improved flushing. On the other hand, decrease of pulse off time, causes short circuiting effect due to the improper debris flushing from the interelectrode gap. Both higher MRR and lower surface roughness can be obtained by using low power input with high rotation along with high pulse off time, not to mention the UV assistance. Wang et al. [79
] presented a novel method of machining TiNi-01 by combining ultrasonic vibration (UV), magnetic field (MF) and WEDM. UVA and MF was applied together and separately to WEDM-LS process to alleviate debris evacuation from the gap. Uniform distribution of the discharging point induces reduced wire electrode break. Optical profilometer images exhibit positive impact of the hybrid method on the surface roughness (Figure 25
Wen-Jeng Hsue1 [80
] investigated ultrasonic assisted EDM (UVA-EDM), rotary ultrasonic EDM (RU-EDM), rotary EDM (R-EDM) and their effects on the tool wear and process stability where pure EDM process is found to have higher process stability index compared to other (Figure 26
a). With UVA, 49% increment of the MRR was reported compared to the EDM process, however, tool wear is affected negatively with the application of UVA (Figure 26
b,c). On the other hand, Rotary ultrasonic EDM experiences relatively low process stability compared to other processes due to the combined effect of ultrasonic vibration and rotation. In addition, tool rotation appears to be negatively impacting the MRR compared to the non-rotation condition (Figure 26
). Muttamara et al. [81
] performed investigation for improving the surface quality by using powder (TiN) mix UVA-EDM. Their study reported on the uniform TiCN layer generation with this process. It was reported that 6 A current with 50 µs pulse on time and 21 A current with 50 µs pulse on time can reduce crack generation on the surface. Frequency of ultrasonic vibration appears to have an insignificant effect on the surface roughness generation. Table 2
provides a summary of the research conducted in UV application of tool.
4.2. Ultrasonic Vibration on Workpiece
Unne et al. [83
] studied on Inconel alloy using low frequency UV assisted micro-WEDM and optimized the input parameters with the help of Box-Behnken design. As per their optimization result, capacitance had the most leading effect on MRR and kerf width; and their study also reported on 66.20% enhancement of MRR with the low frequency UV assisted micro-WEDM. Kerf dimension is influenced by the gap voltage, capacitance and ultrasonic vibration frequency. Low frequency vibration enhances the process performance due to the better flushing as well as less adhesion between the tool/workpiece surface. They conducted a similar study using low frequency UV assisted micro-ED milling and reported on the enhanced MRR with the reduced frontal tool wear. The reason for this enhancement might be because of the following fact: low frequency vibration results in the oscillation of both the dielectric and debris, thus increasing the new dielectric supply to the interelectrode gap. SEM image of the machined channel stated the presence of a greater number of globules when the discharge energy increases. Also, the application of ultrasonic vibration for same level of discharge energy increases this globules due to the rapid cooling of melted materials with the aid of enhanced dielectric circulation compared to the non-vibrating one [84
] (Figure 27
Prihandana et al. [85
] also conducted an experimentation on UVA-EDM where ultrasonic vibration is applied to the Steel workpiece and their results suggested an improved MRR, reduced tool wear as well as reduced surface roughness. During the workpiece vibration, downward movement of the workpiece is introducing a new dielectric, whereas the upward movement aids in the debris removal. Since the workpiece area is larger than the tool, debris removal effectiveness is enhanced with the movement of workpiece much more. They also investigated on the application of low frequency (300, 400, 600 Hz) and amplitude of vibration (0.75, 1 µm). Their investigation reported on the higher MRR for 1 µm amplitude with 400 Hz than 600 Hz frequency due to the decreased machining stability which resulted in a longer machining time. On the other hand, the tool wear rate was found to be lowest at 300 Hz with 0.75 µm amplitude and highest at 600 Hz. Moreover, the surface roughness can be reduced using a low amplitude ultrasonic vibration compared to the normal EDM, however, the high amplitude tends to increase the roughness. Wamia et al. [86
] conducted a similar experimentation on AlSiC metal matrix composite using low frequency ultrasonic vibration (900 Hz) for various dielectrics such as distilled water and oil. Their experiment reported on the higher MRR for distilled water compared to oil, whereas the surface roughness showed better result for oil (Figure 28
). In addition, the TWR also increases with the application of ultrasonic vibration when using water as a dielectric. Moreover, dimensional accuracy is improved irrespective of any dielectric. As per Figure 29
, MRR and TWR both increase with the increasing amplitude up to a certain magnitude, after that it shows declining values of parameter due to the increased frequency of short circuiting.
Jiang et al. [87
] developed a new vibration platform to avoid energy loss associated with the ultrasonic vibration as well as heat effect and conducted micro-hole fabrication experiments on steel workpiece. Their results reported on five-folds increase of machining efficiency with reduced wear of tool and the reason might be the improved debris evacuation. They also reported on significantly reduced perforation time with this new voice coil motor-based platform. Shabgard et al. [88
] also experimented on AISI (American Iron and Steel Institute) tool steel workpiece using variation of pulse on time with two current settings. Their comparative analysis shows, threefold increase of MRR for smaller pulse on time with low current value due to the better debris removal as a result of pressure variation in dielectric caused by ultrasonic vibration. Also, UVA reduces the number of short circuiting, therefore, increases normal discharge frequency. However, surface roughness presents a higher value compared to pure EDM and its value increases with the increase of pulse on time that enhances craters size. Shabgard et al. [89
] also did a similar investigation on FW4 welded metal. Their study reported on a higher MRR for smaller pulse on time for ultrasonic vibration assisted EDM, however, pure EDM condition shows increasing MRR trends with increasing the pulse on time. This enhancement for UVA-EDM is due to the better debris removal by upward movement of workpiece and due to better flushing by the downward movement of workpiece. Also, TWR shows higher value for vibration assisted EDM due to the cavitation effect. This TWR value is comparatively lower when the pulse on time is low compared to pure EDM. In addition, due to improved flushing of debris, UVA-EDM presents a higher roughness of surface due to the bigger crater size. Zhang et al. [30
] proposed UVA-EDM in gas medium where high pressure gas exited from the hollow tool electrode and UV was applied to the workpiece (Figure 30
). Their study observed the dependency of MRR on several parameters, such as voltage, discharge current, pulse on time, vibration amplitude (Figure 30
b,c). However, surface roughness is mostly influenced by current and pulse on time, whereas the voltage is found to have minimum effect on it. Moreover, they also observed UVA-EDM in gas improved the MRR almost twofold compared to EDM in gas without vibration, however, pure EDM in kerosene still offers higher MRR than EDM in gas medium with ultrasonic vibration.
Teimuri et al. [90
] studied dry UVA-EDM process with the assistance of rotating magnetic field that was generated by rotating magnetic disc attached to the tool holder containing four magnets (Figure 31
). Rotating magnetic field aids in debris removal; thus, offers improved process stability in terms of higher MRR as well as better surface. However, it has negative effect on hole overcut due to the acoustic cavitation and tool wear caused by reduced ignition delay. As per their results, brass electrode offers higher MRR compared to the copper electrode due to its higher electrical conductivity. However, it also experiences more EW and roughness due to its low melting temperature. Their proposed regression models for MRR, EWR and surface roughness show a good agreement with the experimental results. Qinjing et al. [91
] also proposed another new method combining EDM and UVA mechanical machining of polycrystalline diamond workpiece using a bronze bonded diamond grinding wheel. Ultrasonic vibration amplitude applied during the process is found to have a greater effect than the frequency and it aids in an improved debris removal. They also reported on the increase of MRR with the increase of pulse on time. For given pulse on and off time, an increasing current also increases the processing speed. Increase of pulse on time also enhances the MRR up to a certain critical point due to the increased single pulse energy, then MRR falls again due to the transference of heat to the electrode and workpiece rather than the erosion purpose. Che et al. [92
] proposed horizontal UVA-EDM and reported on the three fold increment of MRR as well as 20% enhancement of process accuracy due to the effective heat transfer compared to the traditional EDM. The process setup is presented in Figure 32
. They also reported on the better surface roughness due to the improved debris evacuation caused by the horizontal vibration.
Fabrication of non-circular micro-structures always has difficulties in terms of debris evacuation due to narrow discharge gap, therefore, machining efficiency can be significantly low. Tong et al. [20
] applied UVA-EDM while fabricating non-circular microstructures on steel. Their results reported on the improvement of efficiency and discharge stability due to the improved fluidity of dielectric caused by the alternating pressure wave created by ultrasonic vibration. They also achieved 18 times higher machining efficiency as well as an increased dimensional accuracy of 10.5 μm, compared to the pure EDM condition by applying 6 kHz ultrasonic vibration with 3 μm amplitude. Figure 33
presents fabricated micro gears using UVA-EDM.
Hao et al. [93
] proposed to investigate high frequency UVA-EDM in order to fabricate 3D mould cavity and reported on the improved process stability as well as discharge ratios due to the increase of favourable discharge gap. Maximum 6.5 times increase of machining efficiency was reported for 5 kHz frequency and 2.7 µm amplitude, for which the MRR achieved was 1.4 × 105 µm3
. The machining depth increases with the increase of amplitude and frequency, where frequency is the most dominant parameter. In addition, slower scanning speed also enhances the dwelling time in the favourable discharge range, therefore, aids in improving the discharge ratio as well as stable machining. Fu et al. [94
] developed a new piezoelectric self-adaptive micro-EDM process. Its working principle is based on inverse piezoelectric effect which is advantageous in terms of evacuation of debris, lowering short circuiting phenomena and accommodating self-elimination of short circuit. Their study reported on the better process stability as well as reduced machining time with this new technology. Due to reduced short circuiting, EWR is also found to be reduced. Chern et al. [95
] reported an increased MRR with the increasing voltage and a significant enhancement in the MRR compared to only EDM condition for different tool electrode materials. Chern at al considers four different cases, such as no rotation and vibration, only rotation no vibration, only vibration no rotation, both vibration and rotation and conducted micro-EDM on alloy steel (Figure 34
). Their results suggested that without both ultrasonic vibration and tool rotation, surface generated is of very poor quality (Figure 34
a), whereas, tool rotation even without ultrasonic vibration improves the surface condition comparatively (Figure 34
b). Best surface condition can be seen when both tool rotation and ultrasonic vibration are applied as a result of efficient coolant circulation due to the combined effect of vibration and tool rotation (Figure 34
Jahan et al. [23
] derived a numerical model for understanding the low frequency vibration applied to the workpiece during UVA-EDM of tungsten carbide. Their study reported on the performance improvement particularly at lower discharge energy due to the continuous change of discharge gap, thus allowing the fresh liquid to enter and evacuate the debris frequently. Stable machining process is reported due to the increased discharge ratio and reduced short circuiting. In addition, reduced short circuiting phenomenon also helps in obtaining a better rim surface as well as dimensional accuracy. They also reported on micro-hole of 17 aspect ratio with 60 µm diameter on WC with this approach (Figure 35
Garn et al. [22
] studied micro-EDM with the UV assistance to the workpiece and observed an initial delay which is a function of tool diameter, hole geometry and spark energy while machining of deep micro holes. The reason behind this initial delay is found to be arcing which results in the retraction of the tool from the workpiece which leads to the open circuit process. They also reported on the increase of short circuit numbers and decrease of their duration with the application of UVA (Figure 36
). Therefore, applied UVA can enhance the machining performance by reducing the machining time.
Gao et al. [96
] reported micro-EDM process of stainless steel while ultrasonic vibration was applied to the workpiece and achieved eight folds increase of efficiency for 0.5 mm thick workpiece compared to the traditional EDM. Workpiece vibration also enhances the aspect ratio of hole generated on tungsten workpiece. They also observed the increase of MRR with the increasing voltage. In addition, MRR increases with the application of UV is more pronounced for copper than stainless steel (Figure 37
Prihandana et al. [97
] also investigated powder mixed dielectric with and without ultrasonic vibration applied to the workpiece during micro-EDM. Effect of conductive graphite powder in dielectric without vibration increases the spark gap and enhances the debris flushing from interelectrode gap, therefore, improved machining with a smoother surface is possible to generate. However, there is an optimum magnitude of powder concentration which is 10 g/L above which, the machining stability is negatively affected due to the short circuiting caused by the excessive powder deposition on the surface. Now with the application of both vibration and powder (20 g/L), the machining time is reduced significantly. The reason behind the improved machining time might be the efficient discharge frequency due to the powder and better debris flushing due to the ultrasonic vibration. Lowest surface roughness can be achieved for 15 g/L powder concentration, where the craters generated are much well defined and circular shape than those generated during pure EDM. Sundaram et al. [98
] investigated UV assisted micro-EDM process using Taguchi design of experiment for A2 tool steel in order to optimize the process for higher MRR with the lowest EWR. Their results reported on a higher MRR when used 60% peak power with 3300 pF capacitance. The machining time is found to have a leading effect on tool wear, therefore increased tool wear with the increase of machining time is observed. Kumar et al. [99
] also investigated on EDM of AISI 1045 steel workpiece using low frequency ultrasonic vibration and observed better debris removal due to the ultrasonic vibration thus improving the surface roughness. By analysing Signal to Noise (S/N) ratios, the cutting speed is found to be influenced by pulse off time mostly and least influenced by voltage. However, for surface roughness, wire tension is the leading factor and frequency is the least significant factor. Table 3
provides an overview of EDM researches using UV assisted workpiece.