Surface quality investigation in surfactant-based EDM of Inconel 617 using deep cryogenically treated electrodes

Surface quality governs the physical, mechanical, tribological, physio-chemical, and biological properties of materials. Considering the excellent mechanical characteristics of Inconel 617 and the nature of its intended applications, electric discharge machining is generally engaged. However, there is still a need to uplift the surface quality of machined parts to improve their working life and performance. Therefore, this study revealed the potential of deep cryogenically treated (DCT) electrodes under dielectrics modified with span and tween in the said context using a full factorial experimental design. Experimental findings are deeply investigated by employing 3D surface profilometry, scanning electron, and optical microscopy. Cryogenically treated electrode(s) have demonstrated a better surface quality in comparison to the non-treated electrodes, such as DCT Cu has provided a 25.5% lower roughness value than non-treated Cu. Referring to the role of additives, there exists a reduction of 32.5% in roughness when DCT brass is used instead of typical brass using a mixture of span-20 (S-20) and kerosene as a dielectric. The surface finish given by the cryogenically treated brass is 18.99% better when compared to the average value given by all cryogenically treated electrodes considered in this study. It has also been revealed that tween-based dielectrics gave 83% better surface finish than span-based dielectrics with DCT electrodes.


Introduction
Hard to cut nickel (Ni)-based superalloys, categorically Inconel 617, got popularity and acceptance of their usage in various fields of engineering such as high range of temperature areas, turbines, boilers, and combustion cans [1]. The motive for the approval of the said alloy in different areas is linked with the outstanding properties of Ni-based alloy, i.e., greater corrosion resistance, small density, outstanding strength, and high oxidative strength [2]. Hard to manufacture Ni alloy is notable for its great strength, limited thermal conductivity, and fast strain hardening, all of which make conventional cutting methods problematic [3]. A non-traditional setup is arranged for the cutting of the Inconel 617. Non-traditional machining procedures involve electric discharge cutting (EDM), wire-EDM, electric discharge milling, and drilling [4]. EDM die sinker has been preferred over the other machining processes for the machining of the said alloy due to its remarkable surface finish, perfect geometrical accuracy, and the thin tool geometry [5].
EDM, or spark erosion process, is a prevalent and acceptable technique over traditional processes due to creating intricate shapes and structures, dies and molds with high precision regardless of taking material types such as heattreated steels, composites, and superalloys [6]. Researchers have also declared that the performance of EDM has proved to be the best in terms of machining the various kinds of materials, i.e., composites, Ni-superalloys [7]. The concept behind the operation of an EDM machine is that it removes material by vaporization and melting the work surface. The electrode provides the appropriate voltage to create polarization of the dielectric media. This creates repeating sparks in discharge gap [4,8]. The cavity produced as a result of sparking is dependent of the shape of the tool [9].
A major role is played by the dielectric fluid utilized in EDM operations. The cutting operation is carried out with the dielectric fluid; however, irrespective of the cutting operation, it executed the functions like, cooling of the workpiece and tool, flushing away the eroded material during the EDM [10,11]. Various kinds of dielectric mediums have been used in the EDM, but kerosene oil is engaged in this study to determine the effect of different electrodes on the surface of the said alloy [12,13]. Usually, kerosene has high thermal conductivity, high breakdown voltage, and good dielectric strength but low flash point and high rate of flammability [14]. To uplift the output responses in the EDM, various additives have also been engaged with the dielectric fluid. The additives used in the dielectric medium during the EDM behave as the connection between the workpiece and electrode material. Thereof, a bond link is developed at certain locations between the tool and workpiece during the pulse on-time through these additives [15]. The additives like metallic powder increase the spark concentration by extending the plasma channel due to which a uniform and regular sparking executes and this helps in the uniform erosion of material and reducing the surface roughness (SR) of the machined material [16,17].
The powder's addition in the dielectric medium encountered a severe issue of agglomeration which impedes the sparking through the dielectric medium and hence, the performance of EDM has been restricted [18,19]. The formation of agglomeration during the EDM results in the reduction of sparking and hence, output responses are affected. In order to address this problem, surfactants are added in dielectric medium which lower down the surface tension of the dielectric medium and enhance the proper mixing and movement of additives in dielectric medium [20]. The surfactant word is termed as the surface active agent, which has both the hydrophilic and lipophilic nature [21]. The additives, i.e., surfactants improve the dispersion of nanopowder, boost the dielectric conductivity, by lowering down the surface tension and scattering of the nano-powder during the EDM in the dielectric medium. Hence, the surface quality improves by resolving the said issue [22,23]. Emulsifiers that fall under the category of surfactants also employed in the dielectric fluid for the better surface finish [24].
The poor surface finish is also another major concern which arises during the EDM of materials. Researchers are trying to lower down the said problem by putting some additives, i.e., surfactants, metallic powders in the dielectric medium. But some of the researchers addressed this issue by employing cryogenically treated electrodes against the base material during the EDM. The cryogenic treatment on the electrodes significantly enhanced the properties (wear, hardness, strength, toughness, and electrical properties) of electrodes that helps in the reduction of poor surface finish [25]. Ozdemir [26] performed the shallow cryogenic treatment at − 84 °C on the high Cr cast iron and low-carbon cast steel and yielded that the toughness, hardness, and wear characteristics have been improved after the said treatment. The author also concluded that the microstructure also improved by the shallow cryogenic treatment. Senthilkumar and Rajendran [27] evaluated the cryogenic treatment effect on the En 19 steel, and found that after the shallow cryogenic treatment, wear properties of the said material increased by 114%, and after the deep cryogenic treatment, 214% increment was found in the wear properties of En 19 steel.
There is evidence to suggest that cryogenic treatment of the tool (or tools) can boost EDM cycle efficiency. Grewal and Dhiman et al. [28] investigated the influence of cryogenic treatment on the copper electrode against the workpiece of EN24 steel for the measurement of SR. The authors claimed that by applying the cryogenic treatment, surface finish enhanced by 15.75% compared to the surface achieved without the cryogenic treatment. Ram et al. [29] performed the experiment using non-treated-steel, i.e., EN31 as the workpiece material, and cryogenically treated electrodes (copper, brass, tungsten and graphite) in their research to find out the output responses, i.e., MRR and SR. The authors yielded that both the output responses were improved using the cryogenically treated electrodes compared to the nontreated electrodes. M2 high-speed steel is machined during the EDM against the cryogenically treated Cu electrode to examine the SR of the said workpiece material. Srivastava and Pandey et al. [30] claimed that the output response (SR) was improved and better surface finish achieved after the EDM due to the cryogenically treated Cu electrode. AbdulKareem et al. [31] executed an experiment by engaged the squared cryogenically treated Cu electrode on the Ti-6Al-4 V alloy as workpiece material. The authors found that cryogenic treatment reduced the melting and vaporizing of the electrode which resulted in the reduction of SR of the base material.
The EDM of AISI 304 steel has been investigated by employing cryogenically treated Cu and brass electrodes. Jefferson and Hariharan et al. [32] exposed that output response increased by engaging the cryogenically treated electrodes. Bhaumik and Maity [33] used Ti-5Al-2.5Sn alloy as the base material against three different kinds of electrode, i.e., copper, brass, and zinc and EDM oil as the dielectric during the machining of the said alloy to investigate the output response SR. The authors claimed that among the three electrodes, Cu has given the lowest value of SR. Li et al. [34] performed the EDM of nickel-based alloy and concluded that SR was mainly influenced by the peak current and pulse on time. Singh and Singh [35] performed a comparison of cryogenically treated and non-treated electrodes in their research by employing the Taguchi L9 array. The authors yielded that there was an improvement of 7.99% in the SR value by using the cryogenically treated electrodes compared to the non-cryogenic treated electrodes. Sugunakar et al. [36] studied the graphite powder's addition to the kerosene dielectric along with the surfactant, i.e., Span-20 and concluded that the SR decreased by 5.49%. The improvement in the output responses (MRR and SR) was reported by the Kolli and Kumar [37] when 4.0 to 6.0 g/l surfactant was added to the dielectric medium during the machining operation of Ti-6Al-4 V.
Chandrashekarappa et al. [14] determined the SR, MRR, and EWR during the EDM process of HcHcr with three different types of electrodes. The authors concluded that graphite electrode gave the highest MRR, minimum EWR, and SR compared to copper and brass in the presence of distilled water. Sharma et al. [38] performed the machining operation on Inconel 625 to investigate the EWR, SR, and MRR in the presence of kerosene oil against the Cu electrode. The authors claimed that there was a close relationship between the predicted values and the experimental values. However, the model precision of about 97.82% for SR, 95.55% for MRR, and 90.35% for EWR was found through ANFIS. Singh et al. [39] used the mathematical models to estimate the value of SR in argon assisted EDM. The authors revealed that ANFIS model demonstrated to be a fitted model compared to the artificial neural network (ANN) and semi-empirical models for SR. Singaravel et al. [40] used two types of dielectrics, i.e., sunflower oil and kerosene oil to determine the machining responses of EDM for Inconel 800. The authors found that sunflower oil is the best option for obtaining the least value of SR and EWR; however, the biodegradable dielectric gave the highest magnitude of MRR compared to kerosene oil. Payal et al. [41] carried out a study to measure the EWR, SR, and MRR for the EDM of Inconel 825 under the process parameters and in the presence of dielectric fluid. The authors found that tool material, discharge current, and pulse on time were the input parameters for EWR, and pulse on time, discharge current, and tool material were the significant factors for SR, while discharge current and tool material were the significant factor for MRR.
The performance of surfactant-based dielectric through cryogenically treated electrodes has yet to be determined based on the literature listed above. Furthermore, the capability of surfactant(s) for EDM of Inconel 617 through cryogenically treated electrodes has not been thoroughly explored. This research investigated the SR of cryogenically treated and non-cryogenically treated electrodes in various dielectrics. The experiment used cryogenically treated and non-cryogenic treated copper, brass, and graphite electrodes with five different modified dielectrics: pure kerosene, span 20 accompanying pure kerosene (S-20-kerosene), span 80 accompanying pure kerosene (S-80-kerosene), tween 20 accompanying pure kerosene (T-20-kerosene), tween 80 accompanying pure kerosene (T-80-kerosene). Microscopical, scanning electron microscopy (SEM), 3D surfaces profilometry images have been used to validate and support the discussion. The optimal dielectric and electrode for the job are proposed to get the minimum SR for the hard to cut material.

Materials and methods
The performance of cryogenically treated and non-cryogenic treated electrodes was determined against the Ni-based superalloy by employing the five kinds of modified dielectric media during the machining. The output parameter was the SR, and its selection was used to explain the single most important process yield. Optical microscopy was executed to find out the chemical composition of the Inconel 617. Table 1 lists the most imperative characteristics of the workpiece. The measurements of the rectangular workpiece used in this instance are 6 cm × 6 cm × 0.5 cm.
Copper (Cu), brass, and graphite electrodes, each measuring 9 mm in diameter, were employed in this study. Cryogenic treatment and its effect on the performance of these electrode materials were both studied. The electrodes were placed inside the nitrogen chamber and brought into contact with liquid nitrogen maintained at − 185 °C to complete the cryogenic treatment. The cryogenic treatment was completed after the electrodes   Table 2 lists the chemical characteristics of dielectric kerosene. Based on the results of the experiments, the optimal machine settings were determined and presented in Table 3. In Table 4, we can see the primary features of the chosen surfactants. The process's sensitivity to variations in input parameters was investigated through pilot runs. During preliminary testing, the procedure was refined, and its refined parameters were used in subsequent experimentation to achieve a complete machining impression on the workpiece. Additionally, these settings lessen the possibility of tool and workpiece burns. The concentration of surfactant in the kerosene dielectric was also an important factor to establish. The final choice was made by comparing preliminary experimental attempts to the provided selection criterion. However, a preliminary framework was gleaned from the existing literature. According to the preliminary results, a surfactant concentration of 6% is better suited to the stated criteria. To ensure proper surfactant and kerosene mixing, a separate container with a motorized stirrer was constructed. Throughout the experiment, the dielectric and surfactant will be mixed uniformly owing to the stirrer mechanism. As illustrated in Fig. 1, the experiment was carried out using an EDM machine (Model: RJ230).
A full factorial experimental method was used to carry out the research. Table 2 provides a summary of input parameters with levels. Each experiment was machined to a uniform depth of 0.3 mm. In total, 30 experiments were carried out according to the predetermined experimental design. The first fifteen trials were carried out with noncryogenic treated electrodes, while the last fifteen trials were carried out with cryogenically treated electrodes. Response was measured once the experiment was completed successfully. The Taylor Hobson Surface roughness meter was used to determine the specimen's SR as shown in Figs. 2. The machining profiles obtained by the non-treated and the cryogenically treated electrodes have been shown in Fig. 3. The information gathered was then evaluated using bar charts. In the findings and discussion section, 3D surfaces profilometry graphs have been added to determine surface properties. Taking into account the physical phenomena at play during Inconel 617 EDM, the obtained results are discussed in great depth. For the required response characteristics, suggestions have been made for both the best electrode material and the most effective dielectric. Table 5 pertains to the different values of SR against various electrodes, i.e., DCT, non-cryogenic treated (non-DCT), and surfactants by machining the EDM in the  kerosene oil as a dielectric. This study's primary concern is to see the effect of different DCT and non-DCT electrodes in the kerosene oil in terms of SR. A comparison of SR of electrodes with non-DCT is given in Fig. 4 to describe the change in the values with different changing surfactant nature. Some observations are given below. The cutting performance of EDM in terms of SR is evaluated with non-DCT brass, and it got the first rank in low value of SR (8.15 µm) when non-DCT brass is engaged with the pure kerosene oil, and results are similar with the Prajapati et al. [42]. The low conductivity of brass 16 × 10 6 S/m is responsible for the low value of SR. The microscopic image shown in Fig. 5b represents the shallow and very small craters that depict the high surface finish due to the non-DCT brass electrode. The machining efficiency of EDM is gauged by non-DCT Cu electrode in the pure kerosene oil as dielectric medium, and it has been given in Fig. 4 that non-DCT Cu gave the second highest value of SR (9.66 µm). The larger value of SR is due to the high value of electrical conductivity of Cu (59.6 × 10 6 S/m) among the three electrodes used in the research. The microscopic image shown in    Fig. 5a, b, and c that depict the SR value potentially related to the micrographic images. Non-DCT graphite got the first rank in high value of SR (10.15 µm), when machining performance of EDM is evaluated in pure kerosene oil as the dielectric. It has been observed that when non-DCT graphite is engaged with pure kerosene oil, it gave the burn marks on the surface of Inconel 617 due to excessive, and irregular sparking as shown in Fig. 5c.

Results and discussions
The cutting capability of non-DCT Cu electrode is measured in the kerosene oil as dielectric with S-20. It has been observed that non-DCT Cu electrode gave the smallest value of SR (7.15 µm) in the kerosene oil with S-20 and has been ranked first in the high surface finish, thereof, SR is improved by 26% compared to pure kerosene oil. The upgradation in the SR value compared to pure kerosene at same settings is due to fact that S-20 helped in to flush away the debris due to which recast layer is not supposed to build, and results in high surface finish. The microscopic image shown in Fig. 6a depicts the fined machined surface with very smaller craters compared to Fig. 5a, the reason for getting the improved surface is linked with the small HLB (8.6) of S-20 which hampers the irregular sparking. The machining performance of said Ni-alloy is determined in kerosene oil with S-20, and non-DCT graphite electrode to check the novelty of output response, i.e., SR. It has been observed from Fig. 4 that non-DCT graphite gave second highest value of SR (9.29 µm) against the Ni-based superalloy. The improvement in the value of SR is detected by 9% when S-20 is added to the pure kerosene oil. Microscopic image in Fig. 6c shows the smaller area of burn mark by the addition of S-20 in kerosene oil. This improvement in the  SR value is due to the influencing property of S-20 that it helped in the fast the motion of dielectric molecules, and thereof, debris moved away from the electrode material. The supremacy of EDM due to non-DCT brass electrode is also evaluated in kerosene oil with S-20, and it has been reported that non-DCT brass electrode positioned in first rank in high value of SR (10.69 µm). Addition to that, non-DCT brass gave 23% high SR value with S-20 in kerosene oil compared to pure kerosene oil. The above experimental results have direct link with the pulse on time, i.e., larger the value of pulse on time (100 µs) large will be discharge current that leads to the high melting and vaporization of the material, and hence yielded in the high value SR. The microscopic image shown in Fig. 6b depicts the high SR due deeper and larger craters compared to Fig. 5b. Figure 6a, b, and c show graphs for different Ra values taken by the Taylor Hobson SR instrument for verification, which indicate the SR level potentially related to the micrographic pictures. The machining performance of a spans representative, S-80, too was examined for EDM of the aforementioned alloy using non-DCT electrodes. The Ni-superalloy was machined with three different electrodes; the non-DCT graphite electrode had the lowest SR value (8.67 m) when S-80 was added to kerosene oil. With the smallest value of SR in S-80, burn marks are still observed on the surface of machined alloy shown in Fig. 7c, due to low HLB (hydrophilic-lipophilic balance) value (4.6) of S-80 among the other surfactants used in this research. Non-DCT brass   Fig. 7b shows the shallow and small craters that depict the high surface finish compared to S-20 in kerosene oil in Fig. 6b. The cutting performance of non-DCT Cu electrode is also evaluated in S-80, and this performance is gauged at third rank with the highest value of SR (10.69 µm) in the S-80. As per the evidence, the highest SR value given by the non-DCT Cu in S-80-kerosene dielectric is shown by the Taylor Hobson SR meter. It has been observed that there is 32% increment in the SR value when S-80 is used against the non-DCT Cu electrode compared to S-20 in the kerosene oil. The rise in the SR is linked with the high electrical conductivity of Cu which allows to pass high current. This high current then produces high discharge heat which melts and evaporates the larger particles from the base material. The microscopic image in Fig. 7a presents that there are deep and larger craters that are present when non-DCT Cu is used in S-80 compared to Fig. 6a where refined surface is presented. The cutting profile can be seen in Fig. 8a and b where SEM is performed to validate the above statement that deep craters are formed when Inconel 617 is machined against the modified dielectric S-80-kerosene.
The superiority of machining of a representative of tweens, T-20, has been examined for Inconel 617 EDM against several non-DCT electrodes. As can be seen in Fig. 4, the addition of T-20 to kerosene oil resulted in the lowest SR value (7.25 m) for a non-DCT graphite electrode,  which is equivalent to a 16.4% improvement in the machined surface over using S-80. This improved value of SR compared to S-80 is due to high HLB value (16.7) of the T-20 present in the kerosene oil. The microscopic image in Fig. 9c is very refined machined surface of Inconel 617 compared to Fig. 7c where burned marks are depicted. The cutting performance of Inconel 617 is investigated in the kerosene oil with T-20 to find out the output response, i.e., SR due to non-DCT electrodes. The non-DCT Cu electrode gives the second highest value of SR (7.35 µm), which is 31.1% improved SR value compared to S-80 against non-DCT Cu electrode. Microscopic image shown in Fig. 9a depicts the smaller craters compared to larger craters with S-80 in Fig. 7a. Similarly, cutting ability of the workpiece is also determined due to non-DCT brass electrode which gives the highest value of SR (8.07 µm) in the kerosene oil with T-20. It has been gauged that the addition of T-20 in the kerosene oil gave the 17% amended surface compared to S-80 against non-DCT brass. The microscopic image in Fig. 9b portrays the progressed machined surface compared to poor machined surface in Fig. 7b that is illustrated with S-80 in kerosene oil. For confirmation, Fig. 9a, b, and c show graphs for high Ra values taken by the Taylor Hobson SR meter, which indicate the SR value presumably associated to the micrographic pictures.
The machining capability of workpiece is evaluated in the kerosene oil along with surfactant, i.e., T-80 to detect the output response, i.e., SR using non-DCT electrodes. It has been observed that non-DCT graphite electrode gives the lowest value of SR (6.33 µm) in the kerosene oil with T-80, and this value of SR in T-80 is 12.7% improved value that was obtained in the T-20. The reason for lower SR in the dielectric medium is because of low surface tension of dielectric medium due to the presence of T-80, which incorporated with dielectric medium for rapid flushing away the removed debris that minimizes the bad SR. The microscopic image in Fig. 10c shows the upgraded machined surface compared to Fig. 9c in the T-20. A 3D surface profilometry of Inconel 617 in T-80-kerosene dielectric with non-DCT graphite is shown in Fig. 11 that depicts the short heighted peaks and valleys, and the average value of Ra is small in the abovesaid case. Moreover, the SEM of the specimen at lower Ra is also attached in Fig. 12a that shows the shallow craters present on the surface of Inconel 617 which are the lower SR value. The machining performance of workpiece in the kerosene oil with the T-80 is determined with non-DCT Cu electrode, and it got the second rank in SR value (8.55 µm). The SR value is increased by 14.1% when non-DCT Cu electrode is  Fig. 10a shows the deeper and large craters compared to Fig. 9a that is captured when non-DCT Cu is engaged in the T-20 with kerosene oil. The cutting performance of non-DCT brass is evaluated in T-80 and kerosene oil, and it got the first rank in SR value (9.6 µm). It has been observed that there is an increase in the value of SR by 15% which can be seen through Fig. 10b where shallow and small craters are presented.
The complete SR result with non-DCT electrodes can be shown using the accompanying sequence. The smallest value of SR (6.33 m) was achieved using a graphite electrode that did not contain DCT in kerosene oil containing T-80. This result is 37.6 percentage points lower than the result achieved with pure kerosene oil and the same non-DCT graphite electrode. The Cu electrode also yields the lowest SR (7.15 m) when operating under the kerosene oil with S-20 dielectric. The SR is improved by 25.5% compared to when the same non-DCT Cu electrode was used with pure kerosene oil. Using kerosene oil and T-20, non-DCT graphite's SR value of 7.25 m places it third in terms of potential. When compared to the SR achieved in pure kerosene oil using a non-DCT graphite electrode, this value is an improvement of 28.6%. Figure 13 depicts the effect of cryogenic treatment of electrodes on several types of dielectric media in the EDM of claimed Ni-alloy. Figure 13 shows that when pure kerosene oil is employed as the dielectric, the cutting performance of DCT Cu is noticeably higher than that of other DCT electrodes.
The supremacy of machining of DCT Cu electrode gives the lowest value of SR (7.15 µm) in the pure kerosene oil. The reason for lower value of SR is due to cryogenic effect on the electrode, because cryogenic treatement refines the packaging of grains in the element that remvoes the uneven, and irregular sparking due to low value of SR is achieved [43]. Microscopic image in Fig. 14a shows that very small craters formed on the machined surface which portrays the refined surface of Inconel 617. The values and surfaces obtained by Taylor Hobson SR meter when DCT electrodes are machined against the pure kerosene oil as dielectric during the machining of Inconel 617 are shown in Fig. 14a, b, and c. The analysis of lowest value of SR gained in DCT Cu case with pure kerosene is shown by the SEM in Fig. 12b. The SEM explained the more deep craters gained with DCT Cu compared to the non-DCT graphite at the lowest SR value. The cutting capability of DCT graphite is also evaluated in the pure kerosene oil, and it gets the second rank in low SR value (7.6 µm). But due to the arching phenomenon in the pure kerosene oil, burn marks are produced on the surface of Ni-alloy as shown in Fig. 14c. The SR provided by DCT brass electrode in the pure kerosene oil is of maximum magnitude, i.e., 9.6 µm. It has been revealed that the surface asperities are well proven, and shallow small craters are formed on the machined surface of Inconel 617 as shown in Fig. 14b. For validation, Fig. 14a, b, and c show graphs for high Ra values taken by the Taylor Hobson SR meter, which demonstrate the SR esteem probably related to the micrographic pictures. The machining performance of DCT Cu electrode is evalualted in the kerosene oil with the S-20, and it has been found that DCT Cu gives the maximum value of SR (10.95 µm) in the kerosene with S-20 dielectric. This value of SR is 34.7% high compared to SR value in pure kerosene against the same dielectric. The reason for high SR is that Cu has high value of electrical conductivity (59.6 × 10 6 S/m), and cryogenic treatment refines the grains packing due to which high spark energy is produced that melts and vaporized the material from the workpiece, and results in the high SR that can be seen through the microscopic image in Fig. 15a that depicts the shallow craters are formed on the workpiece. To authenticate the above reason, SEM is performed at 100 µm, and it can be seen in Fig. 16 that deep and wide craters are formed on the machined surface of Inconel 617, along with that, Fig. 15a graph depicts the highest value of SR obtained by Taylor Hobson SR meter. In Fig. 17, a 3D surface profilometry of Inconel 617 in S-20-kerosene dielectric with non-DCT Cu is depicted, which displays the heighted peaks and valleys and that leads to the high value of SR. The second highest value of SR (10.0 µm) can be seen in Fig. 13 which is obtained in the case of DCT graphite in pure kerosene oil with S-20. This SR value is 24% high compared to that of in the pure kerosene oil with DCT graphite electrode, but it is important note that with increase in the SR value burn marks are reduced on the workpiece due to the addition of S-20 in the kerosene oil. S-20 helps the dielectric media to flesh away the debris due to which the chances of arcing phenomenon are minimized. The supremacy of DCT  brass electrode is also determined in the pure kerosene oil with S-20, it can be been seen from Fig. 13 that DCT brass gives the lowest value of SR (7.08 µm) in the kerosene with S-20. This value 25.5% lower than the value of SR obtained in pure kerosene oil with DCT brass electrode, and results are related with the Prajapati et al. [42].
The cutting capability of DCT Cu is evaluated in the kerosene oil with S-80 for the EDM of Inconel 617, and it has been found that DCT Cu gives the lowest value of SR (7.95 µm) in the kerosene oil with S-80. This SR value is 27.4% improved than the value obtained in the kerosene oil with S-20 with DCT Cu electrode. The reason for improved  Fig. 18a shows the superior machined surface than the machined surface obtained in the S-20 with DCT Cu electrode. There is no significant change is observed in the SR values given by S-20 and S-80, i.e., 10 µm and 9.9 µm, respectively, when used with DCT graphite electrode. The supremacy of DCT brass is determined in terms of SR when employed against the kerosene dielectric along with the S-80, and it can be seen that DCT brass gives the first rank in the value of SR (9.95 µm). This value is 28.1% high than the value got in the S-20 with the same electrode that can be viewed by Fig. 18b where deep craters are formed which depicts the high value of SR and the graph taken by the Taylor Hobson roughness meter also formed the high peaks values that are the evidence of high SR. The machining performance of DCT electrodes using the representative of tweens, i.e., T-20 for the EDM of Inconel 617 is presented in Fig. 13. It has been found that DCT brass gives the low value of SR (7.25 µm) in the kerosene oil with T-20. The value of SR obtained in the T-20 is 27.1% less than that of the value got in the S-80 at the same electrode.
The reason for low SR value in the T-20 is due to high HLB (16.7) value of T-20 compared to other surfactants used in the study. Microscopic image in Fig. 19b shows the very refined machined surface of Inconel 617 that reflects the advanced surface compared to machined surface elaborated in Fig. 18b. The DCT Cu electrode stood second in the race of low SR for the EDM of Inconel 617 with the  1% high compared to the value got in S-80, due to the high electrical conductivity of Cu (59.6 × 10 6 S/m) as well as the cryogenic treatment that allows the passage of high amount of current, and yielded in the increase of SR value, and as results are similar with the Choudhary et al. [44], who used the same three electrodes in the study. The cutting capability of DCT graphite electrode is also gauged using T-20 in the kerosene oil for the EDM of above said alloy. It has been observed that DCT graphite stood first in SR measurement with the value of 9.65 µm that is measured in the kerosene oil with T-20. This value of SR is 2.5% lower than the value of SR measured in the kerosene oil with S-80. The microscopic image in Fig. 19c shows the more polished surface than the machined surface which is obtained by S-80 surfactant in kerosene oil.
The cutting performance of DCT electrodes is also measured with the surfactant, i.e., T-80 for the output responses of EDM. The DCT Cu electrode gives the lowest value of SR (7.7 µm) in the kerosene oil with use of surfactant, i.e., T-80. It has been observed that the abovementioned value of SR is 11% lower than the value achieved in the T-20. Figure 20a shows the deep craters on the machined surface which is captured by using the T-20 in the kerosene oil, but Fig. 19a illustrates the shallow craters present on the machined surface that indicates the machined surface is improved from deep craters to shallow craters. The DCT graphite electrode got the second rank in SR measurement with the value of (8.0 µm) in the kerosene oil with T-80. The value achieved in the T-80 is 17% low than value obtained in the T-20 at the same electrode. Figure 20c illustrates the more refined machined surface due to DCT graphite electrode compared to other microscopic images of workpiece that are machined with the same type of electrode. The cutting capability of DCT brass in kerosene oil with T-80 gives the highest value of SR (9.6 µm), which is 24.5% high than the value accomplished in case of T-20 with the same DCT brass electrode. Microscopic image in Fig. 20b gives the brief picture that SR is increased in T-80 from 7.25 to 9.6 µm in T-20 modified dielectric medium. As an evidance, Fig. 20 shows the different graphs of Ra obtained in T-80-kerosene dielectric when Inconel 617 is machined against the DCT electrodes.
A comparison of lowest and highest value of SR is presented by 3D surface profilometry as shown in Fig. 21a and b. Figure 21a shows the 3D surface profilometry with small peaks and valleys and the average value of these peaks and valleys leads to the minimal value of SR. Similarly, the 3D surface profilometry with maximum value of SR in DCT electrodes is eleborated in Fig. 21b that depicts the more In a nutshell, different cryogenic treated electrodes have been investigated under various types of dielectric media for the EDM of Inconel 617. The minimum SR of the same magnitude, i.e., 7.08 µm, is achieved under dielectric media, i.e., pure kerosene along with S-20, as shown in Table 4. This SR magnitude is 32.5% lower than the highest SR magnitude noticed under the non-DCT dielectric medium. The second-lowest value of SR, i.e., 7.15 µm, is obtained when Inconel 617 is machined using a DCT Cu electrode with the kerosene oil. The second-lowest value of SR, i.e., 7.25 µm, is obtained when Inconel 617 is machined using a DCT brass electrode with the T-20 in the kerosene oil. The SR observed, in this case, is 31.6% lower than the highest value achieved in the non-DCT scenario. A comparison of maximum and minimum values of SR given by cryogenically and noncryogenic treated electrodes in different modified dielectrics is shown in Fig. 22.

Conclusions
The capability of EDM of hard-to-cut Inconel 617 is thoroughly investigated under various modified dielectric media and different kinds of electrodes, i.e., non-cryogenic treated electrodes and cryogenic treated electrodes. SR is examined under all possible combinations of input parameters, and results are explained in detail with the possible shreds of evidence and physics of the process. The following findings are drawn after evaluating the results of the experiments performed on Inconel 617 with various electrodes.
1. The machining proficiency of cryogenically treated electrodes averagely gave better surface finish by 11.75% compared to the non-cryogenic treated electrodes with the modified dielectrics media. 2. Among the cryogenically treated electrodes, brass performed outstandingly and gave the lowest value of SR (7.08 µm) in the modified dielectric of kerosene, i.e., S-20. The surface finish given by the cryogenically treated brass is 18.99% better/enhanced compared to the average value given by the overall cryogenically treated electrodes. 3. The machining ability of cryogenically treated electrodes in spans and tweens based modified dielectrics has also been investigated, and found that tween-based modified dielectrics gave 83% better surface finish or less surface roughness compared to the span-based modified dielectrics with cryogenically treated electrodes. 4. In case of non-cryogenically treated electrodes, T-20-kerosene modified dielectric has provided lower values of SR on overall basis as compared to the other modified dielectrics used in this study. The SR values achieved in the T-20-kerosene modified dielectric are 23.9% lower for non-cryogenic treated Cu, 0.98% for non-cryogenic treated brass, and 28.6% for non-cryogenic treated graphite than the values gained in the pure kerosene oil during the EDM of Ni-based superalloy. 5. The lowest value of SR (6.33 µm) has been achieved by engaging non-cryogenic treated graphite electrode with the T-80-kerosene modified dielectric. The said value of SR is 40.8% better than the highest value of SR achieved in case of S-20-kerosene oil with non-cryogenic treated brass electrode. 6. The performance of span-based modified dielectrics has been observed inferior by 16.3% compared to the machining ability of tween-based modified dielectrics. The primary reason for the better performance of tweens is that they both have high flash points which facilitate the impeding of high flammability. 7. The proficiency of machining of non-cryogenic-treated graphite, Cu, and brass gave the lowest SR in the order