applied sciences
Review Principles and Characteristics of Different EDM Processes in Machining Tool and Die Steels
Jaber E. Abu Qudeiri 1,* , Aiman Zaiout 1 , Abdel-Hamid I. Mourad 1 , Mustufa Haider Abidi 2 and Ahmed Elkaseer 3,4 1 Mechanical Engineering Department, College of Engineering, United Arab Emirate University, Al Ain 15551, UAE; [email protected] (A.Z.); [email protected] (A-H.I.M.) 2 Raytheon Chair for Systems Engineering (RCSE), Advanced Manufacturing Institute, King Saud University, Riyadh 11421, Saudi Arabia; [email protected] 3 Department of Production Engineering and Mechanical Design, Faculty of Engineering, Port Said University, Port Said 42526, Egypt; [email protected] 4 Institute for Automation and Applied Informatics, Karlsruhe Institute of Technology, 76344 Karlsruhe, Germany * Correspondence: [email protected]
Received: 29 January 2020; Accepted: 10 March 2020; Published: 19 March 2020
Abstract: Electric discharge machining (EDM) is one of the most efficient manufacturing technologies used in highly accurate processing of all electrically conductive materials irrespective of their mechanical properties. It is a non-contact thermal energy process applied to a wide range of applications, such as in the aerospace, automotive, tools, molds and dies, and surgical implements, especially for the hard-to-cut materials with simple or complex shapes and geometries. Applications to molds, tools, and dies are among the large-scale initial applications of this process. Machining these items is especially difficult as they are made of hard-to-machine materials, they have very complex shapes of high accuracy, and their surface characteristics are sensitive to machining conditions. The review of this kind with an emphasis on tool and die materials is extremely useful to relevant professions, practitioners, and researchers. This review provides an overview of the studies related to EDM with regard to selection of the process, material, and operating parameters, the effect on responses, various process variants, and new techniques adopted to enhance process performance. This paper reviews research studies on the EDM of different grades of tool steel materials. This article (i) pans out the reported literature in a modular manner with a focus on experimental and theoretical studies aimed at improving process performance, including material removal rate, surface quality, and tool wear rate, among others, (ii) examines evaluation models and techniques used to determine process conditions, and (iii) discusses the developments in EDM and outlines the trends for future research. The conclusion section of the article carves out precise highlights and gaps from each section, thus making the article easy to navigate and extremely useful to the related research community.
Keywords: EDM; tool steels; hard-to-cut; machining; process parameters; performance measures
1. Introduction In recent years, rapid developments in aerospace, medical instruments, transportation, and many other industrial sectors increased the need for new materials with favorable characteristics. In addition to unique characteristics, most modern materials need special manufacturing processes to enable them to be machined with ease [1,2]. Most of these materials are usually difficult to cut by conventional manufacturing processes [3–7]. The unique characteristics of these hard-to-cut materials increase their applications, which further drive manufacturers to explore new machining processes with reasonable cost and high precision [8,9].
Appl. Sci. 2020, 10, 2082; doi:10.3390/app10062082 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 2082 2 of 46
Tool steels and other tool materials (e.g., carbides) are such widely used hard-to-cut materials because of their high hardness and abrasion wear resistance, in addition to their ability to withstand high load and to operate in rapidly changing temperatures [6]. Tool steels have a wide range of applications, including stamping and metal-working dies, cutting tools, hammers, and machine parts [10]. Applications of these tools in the manufacturing sector is very large; thus, there exists huge machining requirements of tools, tooling, dies, and molds [11]. Before being put to use, these steels are subjected to heat treatment to meet the required properties for specific application [12,13]. In addition to iron and carbon, tool steels have in them other elements (e.g., Cr, W, V, Mo, etc.) to increase their strength, hardness, hot strength and hot hardness, and wear resistance. Although these steels can be machined by conventional methods, they come with serious concerns with regard to very poor tool life and part accuracy [14]. Electric discharge machining (EDM) is one of the most advanced manufacturing methods used to successfully machine conductive hard-to-cut materials [8,15–19]. EDM is the process of choice to machine hard-to-cut materials widely used in modern industries to facilitate accurate machining [20–25], complex shape machining, and better surface integrity. The process is utilized to machine electrically conductive materials by applying repetitive sparks between electrode and workpiece. Unlike in mechanical machining, no deforming force is required between the electrode and the workpiece, and the machining takes place without actual contact between them [23,26–28]. There are a large number of variants of the EDM process such as sinking EDM, wire EDM, micro-EDM, powder-mixed EDM, and dry EDM; all of these possess work on the same mechanism of material removal. Developments of variants make the process more versatile and suitable for relatively big and micro-scale machining areas. Several review papers related to EDM were published in recent years such as references [29–35], among others. Furthermore, some other articles presented a discussion of specific objectives; for example, Barenji et al. [36] developed a model for prediction of material removal rate (MRR) and tool wear rate (TWR) for the EDM of AISI D6 tool steel. They reported that higher values of pulse-on time resulted in higher MRR and lesser TWR. Long et al. [37] used powder-mixed EDM for machining die steels. Titanium powder was used for mixing, and surface quality was analyzed. It was revealed that the quality of surface layer was improved at optimal parameters. Shabgard et al. [38] studied the effects of the key input variables of wire EDM of ASP30 tool steel. The output responses under consideration were MRR and surface roughness. The results revealed that an increase in spraying pressure of dielectric fluid led to a higher MRR and surface roughness. EDM of AISI M42 high-speed tool steel alloy was conducted to study the effect of major input parameters on MRR. It was revealed that tool polarity was the most influential factor and, at negative polarity, maximum MRR was achieved. P20 tool steel was machined using wire EDM, and pulse-on time, pulse-off time, peak current, and spark gap voltage were varied. The output responses under study were kerf width and MRR. The best combination of parameters was reported to achieve maximum MRR [39]. Sharma and Sinha [40] applied rotary-EDM to machine AISI D2 tool steel using a copper electrode. MRR, TWR, and machining rate were studied by varying input parameters (peak current, voltage, duty cycle, and electrode rotation speed). Bahgat et al. [41] conducted experiments to study the effect of major input variables on MRR, electrode wear ratio, and surface roughness while machining H13 die steel. It was reported that higher MRR and lower electrode wear rate were achieved using a copper electrode, whereas lower surface roughness was attained with a brass electrode. Gopal et al. [42] compared the performance of unprocessed and equal channel angular pressing (ECAP)-processed copper electrode while machining AISI H13 tool steel using EDM. It was reported that the triple-ECAP-passed electrode gave better machining quality. Despite many existing review papers, to the best of authors’ knowledge, there is no study that reviewed the EDM process specifically for tool and die steels. Since tool and die steels have usage in a wide range of applications and they are difficult to cut with the conventional manufacturing processes, non-conventional processes such as EDM are becoming prevalent for their machining. Generally, one of the largest uses of the EDM process is in tool-, die-, and mold-making. All these industries mostly use various kinds of tool steels. EDM remains one of the most popular processes used for their fabrication. Appl. Sci. 2020, 10, x FOR PEER REVIEW 3 of 45
Generally, one of the largest uses of the EDM process is in tool-, die-, and mold-making. All these Appl. Sci. 2020, 10, 2082 3 of 46 industries mostly use various kinds of tool steels. EDM remains one of the most popular processes used for their fabrication. Therefore, it is important to have in-depth knowledge of this subject and, Therefore,hence, the it isaim important of this research to have in-depth paper is knowledge to provide of comprehensive this subject and, information hence, the aim and of details this research of this paperprocess. is to It provide is important comprehensive for review information articles such and as detailsthis one of to this bring process. out the It isintricacies important of for processes, review articlesparameter–response such as this one co-relations, to bring out and the intricaciescritical analys of processes,is of their parameter–response response to better utilize co-relations, them. andThis criticalwill prove analysis to be of of their great response use to tounderstand better utilize failures, them. as This well will as provein-service to be performance of great use toissues, understand of tools failures,and tooling. as well The as present in-service article performance aims to provide issues, an of overview tools and of tooling. the significant The present contributions article aims of EDM to provideto machining an overview of tool ofsteels. the significantThis paper contributionsgives the state of of EDM the art to on machining current research of tool steels. studies This conducted paper givesin all the EDM state variants of the artfor onmachining current researchdifferent studiesgrades conductedof tool materials. in all EDMThe paper variants begins for machiningwith a brief diintroductionfferent grades ofof EDM tool and materials. its variants, The paper and then begins intr withoduces a brief its working introduction principle. of EDM It further and its variants,discusses andthe thenEDM introduces process parameters its working and principle. its performance It further meas discussesures. The the main EDM text process of this parameters article provides and its a performancemeticulous review measures. on major The main areas text of EDM of this research article provides using different a meticulous grades reviewof tool onmaterials. major areas The last of EDMsection research of the using paper diff erentdraws grades conclusions, of tool materials. and trends The lastof sectionthe reviewed of the paper bodies draws of conclusions,research are andsubsequently trends of thedrawn. reviewed Figure bodies 1 shows of research the EDM are processes subsequently and their drawn. main Figure process1 shows parameters the EDM and processesoutput (performance) and their main measures. process parameters and output (performance) measures.
•Discharge voltage •Peak current •Pulse duration •Pulse interval EDM Processes •Electrode gap •Polarity •Material Removal Rate •Sinking EDM •Pulse wave form •Electrode Wear Rate •Wire EDM •Dielectic •Surface Quality •Micro-EDM •Workpiece rotation •Powder-mixed EDM •Electrode rotation Preformance •Dry EDM measures Process Parameters
Figure 1. The processes of electric discharge machining (EDM) and their process parameters and performance measures. Figure 1. The processes of electric discharge machining (EDM) and their process parameters and 2. EDMperformance Process Details measures. 2.1. Working Principle of EDM 2. EDM Process Details The EDM process was invented in the 1940s [23]. The process uses thermoelectric energy to erode workpieces2.1. Working due Principle to rapidly of EDM recurring electrical discharges (sparks) between the non-contact electrode and workpieceThe EDM [43 process–46]. The was discharge invented current in the flows 1940s from [23]. between The process the electrode uses thermoelectric and workpiece energy through to theerode small workpieces distance between due to them,rapidly called recurring a “gap”, electric whichal discharges is filled with (sparks) dielectric between fluid. Dielectricthe non-contact fluid alsoelectrode simultaneously and workpiece flushes [43–46]. the gap The continuously discharge duringcurrent theflows machining from between and carries the electrode away debris and (particleworkpiece erosion through produced the small as a distance result of between machining) them, and ca restoreslled a “gap”, the sparking which is condition filled with in dielectric the gap. Afluid. proper Dielectric gap is carefully fluid also selected simultaneously to a value flushes such that the it gap generates continuously surgecurrent during bythe breaking machining down and thecarries dielectric away film debris under (particle a set voltage.erosion produced There are as no a cuttingresult of forces machining) used between and restores the electrode the sparking and thecondition workpiece in the as theregap. A is proper no contact gap betweenis carefully them. selected This conditionto a value eliminatessuch that it vibration generates and surge problems current arisingby breaking due to down machining the dielectric forces [47 film,48]. under a set voltage. There are no cutting forces used between the electrode and the workpiece as there is no contact between them. This condition eliminates vibration and problems arising due to machining forces [47,48]. Appl. Sci. 2020, 10, 2082 4 of 46
2.2. EDM Process Parameters The EDM process parameters which drive this process are divided into two types, namely, electrical and non-electrical parameters. The major electrical parameters are discharge voltage, peak current, pulse duration and interval (pulse-on and pulse-off times), electrode gap, polarity, and pulse wave form. Important non-electrical parameters include flushing of the dielectric fluid, workpiece rotation, electrode rotation, etc. These parameters are discussed below. The discharge or machining voltage is the average voltage across the spark gap during machining. The discharge voltage directly influences the regulation of the size spark gap and overcut [49–52]. Normally, electrode and workpiece materials of high electrical conductivity use low voltage. In contrast, higher voltage is considered with materials of low conductivity. This parameter has a direct effect on the material removal rate (MRR), tool wear rate (TWR), and machining accuracy [53–57]. An increase in current increases MRR and TWR and adversely affects the accuracy. These characteristics of the parameter current and its responses opened the door for electrodes of high wear resistance that can be used in high-current conditions [47]. The pulse-on time is the time during which the discharge is applied. The amount of energy generated during pulse-on time has a direct influence on the MRR [54–60]. Accordingly, increasing the discharge energy by applying longer pulse-on time also increases the MRR [61]. The pulse-off time is the time during which there is no discharge. During this time, the debris as a result of sparking and erosion is flushed out of the gap between the workpiece and the electrode [62]. Flushing improves the ionization conditions and avoids the formation of an insulating layer; thus, proper selection of pulse-off time provides stable machining [63–65]. A shorter period of this time increases MRR as long as enough flushing of debris takes place. Otherwise, it may result in unsuitable conditions during the next pulse-on time period [66]. Furthermore, a pulse wave has many forms such as rectangular and trapezoidal waves or even a composite geometric form. It is demonstrated in the literature that, among standard forms, the trapezoidal wave form generator minimizes the electrode wear [67]. Recently, other generators were developed such as a typical one which initially facilitates the main pulse by producing a high-voltage pulse with a low current of narrower duration before the main pulse [63]. The polarity in EDM depends on many factors, including electrode and workpiece materials, current density, and pulse length [68,69]. Either the electrode or the workpiece has a positive charge polarity and the other has a negative charge polarity. Negative electrode polarity is recommended for high-precision machining when the MRR is high. In the wire EDM process, the electrode “wire” usually has a negative polarity to keep machining rate high, and, since the wire wear keeps on moving continuously during arcing, its wear rate is less. The electrode and workpiece are located at a small predetermined distance called the “discharge gap” which is controlled by the discharge gap servo [70]. A discharge gap on the order of 0.005 mm and 0.1 mm is usually maintained. Finishing or high-precision machining requires a relatively low voltage in the gap on the order of 50 V and 300 V, as too high a voltage reduces machining precision. Non-electrical parameters include flushing of the dielectric fluid, workpiece rotation, and electrode rotation. The function of the dielectric fluid is to provide insulation against premature discharging, reduce the temperature during machining, and clean away the debris from the machining area [71]. Good dielectric fluids should have characteristics such as high dielectric strength, flushing ability, fast recovery after breakdown, etc. [72]. In the case of a sinker type EDM, hydrocarbon- and silicon-based dielectric oil and kerosene are used after raising the flash point. Meanwhile, de-ionized water and oil are used in wire EDM. Moreover, some sinker EDMs also use de-ionized water in high-precision machining, such as in fine hole drilling. Many studies were conducted recently to explore oil-based synthetics to avoid harmful effects on workers and the environment [73–75]. Importantly, the dielectric type and flushing method influence MRR, TWR, and surface roughness (SR) [76–78]. The dielectric flushing conditions can be improved with workpiece and electrode rotation [79,80]. This improvement in flushing caused by electrode rotation achieves better SR and higher MRR [81–83] and minimizes the density of cracks on the surface and recast layer [76]. The effect of the EDM process parameters Appl. Sci. 2020, 10, x FOR PEER REVIEW 5 of 45
Appl. Sci. 2020 10 because of ,the, 2082stochastic nature of the discharge mechanism [84]. Several researchers performed5 of 46 studies related to EDM processes and explored the influence of process parameters on the onperformance the response measures parameters [21,54,58,85,86]. are difficult to explain because of the stochastic nature of the discharge mechanism [84]. Several researchers performed studies related to EDM processes and explored the2.3. influencePerformance of processMeasures/Parameters parameters on the performance measures [21,54,58,85,86]. Performance parameters are the factors that measure performance of the EDM process. There 2.3. Performance Measures/Parameters are numerous parameters, but the main performance measure parameters are MRR, TWR, and surfacePerformance quality. The parameters MRR is the are erosion the factors rate that of work measurepiece performance surface; it also of the defines EDM the process. measure There of arethe numerousmachining parameters,speed. A large but number the main of performance studies were measureperformed parameters with a focus are MRR,on ways TWR, and and techniques surface quality.that result The in MRR an improvement is the erosion ratein MRR of workpiece [87–91]. On surface; the other it also hand, defines TWR the is measure a performance of the machining measure speed.for the Aerosion large numberrate of the of studiestool electrode. were performed This parameter with a focushas a ondirect ways influence and techniques on the geometry that result and in anaccuracy improvement of the machined in MRR [87 cavity–91]. Ondue the to other a contin hand,ued TWR change is a performancein the electrode measure profile for theduring erosion the ratemachining of the toolprocess. electrode. Apart from This the parameter accuracy has and a geometry direct influence of the part on the being geometry machined, and it accuracy also affects of thethe machinedlife of the cavitytool. A due focus to aon continued reducing change TWR is in extremely the electrode important profile duringbecause the wear machining of the electrode process. Apartaffects fromthe electrode the accuracy profile and and geometry leads to of lower the part accuracy being machined,[85,92,93]. The it also surface affects integrity the life ofis theanother tool. Aimportant focus on response reducing which TWRis is extremely representative important of machined because surface wear of quality the electrode or conditions. affects the The electrode surface profilequality andis a leadscomposite to lower characteristic accuracy [85with,92 ,components93]. The surface such integrity as SR, extent is another of the important heat-affected response zone which(HAZ), is recast representative layer thickness, of machined and micro-crack surface quality density. or conditions. Many research The surface studies quality were isintroduced a composite to characteristicexplore utilization with components of the EDM such process as SR, extent in surf of theace heat-a treatment,ffected zoneand (HAZ),they reported recast layer the thickness, surface andeffectiveness micro-crack caused density. by the Many process research [94]. studies were introduced to explore utilization of the EDM process in surface treatment, and they reported the surface effectiveness caused by the process [94]. 2.4. Various EDM Process Variants 2.4. Various EDM Process Variants 2.4.1. Sinking EDM or Sinker Type EDM 2.4.1. Sinking EDM or Sinker Type EDM The sinker type EDM set-up is schematically shown in Figure 2. In the sinking type EDM process,The sinkertwo machining type EDM systems set-up is are schematically applied. In shown the first in Figure system,2. In a thecontrolled sinking typeelectrical EDM process,spark is tworepeated machining until the systems electrode are applied.shape is replicated In the first over system, the aworkpiece controlled su electricalrface. In sparkthe second is repeated system, until the theelectrode electrode moves shape in three-dimensional is replicated over (3D) the workpiecespace to machine surface. a complex In the second 3D shape system, on work the surface. electrode A moveshybrid inof three-dimensionalthe two cutting systems, (3D) space however, to machine is possible a complex but remains 3D shape to be on reported. work surface. The sinking A hybrid EDM of theprocess two cuttinghas a characteristic systems, however, sharp rise is possible in the temperature but remains to(i.e., be 8000 reported. to 12,000 The sinking°C) of the EDM machining process hasarea. a EDM characteristic machines sharp have rise a unit in the control temperature system (i.e.,that controls 8000 to 12,000 and monitors◦C) of the the machining machining area. variables EDM machinesand shows have the process a unit control execution system sequence. that controls A wide and range monitors of electrode the machining materials variables are used, and the showschoice theof which process also execution depends sequence. on the workpiece A wide rangematerials; of electrode copper and materials copper-doped are used, metals the choice with ofgraphite, which alsoamong depends others, on are the widely workpiece used materials; as the electrode copper andmaterials copper-doped in the sinking metals EDM with graphite,process. amongDue to others,electrode are wear, widely the usedelectrode as the is electrodeoften re-shaped materials to execute in the sinking the finishing EDM process.operations. Due Normally, to electrode this wear,process the uses electrode a hydrocarbon is often re-shaped dielectric tofluid execute because the of finishing its positive operations. effect on Normally, the SR and this electrode process wear uses arate hydrocarbon (EWR). dielectric fluid because of its positive effect on the SR and electrode wear rate (EWR).
FigureFigure 2.2. Schematic diagram of sinking EDM. Appl. Sci. 2020, 10, 2082 6 of 46
Appl. Sci. 2020, 10, x FOR PEER REVIEW 6 of 45 2.4.2. Wire EDM 2.4.2. Wire EDM In wire EDM, a continuously moving metallic wire of small diameter follows a well-defined path In wire EDM, a continuously moving metallic wire of small diameter follows a well-defined path to cut a workpiece. Discrete sparks between the wire and the workpiece cause erosion in the machining to cut a workpiece. Discrete sparks between the wire and the workpiece cause erosion in the area. As in sinker type EDM, the wire and workpiece do not have any contact during machining in machining area. As in sinker type EDM, the wire and workpiece do not have any contact during wire EDM. Furthermore, both of them should be immersed in dielectric fluid. The wire used in this machining in wire EDM. Furthermore, both of them should be immersed in dielectric fluid. The wire process is usually thin in diameter, as small as 0.1 mm or so; it is generally made of copper, brass, coated used in this process is usually thin in diameter, as small as 0.1 mm or so; it is generally made of steel materials, or even metals like molybdenum. A high peak current with a short duration time is copper, brass, coated steel materials, or even metals like molybdenum. A high peak current with a applied in this process. The machine control unit enables the necessary movement through a computer short duration time is applied in this process. The machine control unit enables the necessary numerical control (CNC) system to cut complicated shapes [95]; it contains a dedicated microprocessor movement through a computer numerical control (CNC) system to cut complicated shapes [95]; it to maintain the gap within a suitable range (25–50 µm). In addition, the unit controls the speed contains a dedicated microprocessor to maintain the gap within a suitable range (25–50 µm). In of the wire through the workpiece, which produces surfaces with very high accuracy. De-ionized addition, the unit controls the speed of the wire through the workpiece, which produces surfaces water is a common dielectric fluid used in this process. The wire EDM process has a wide range of with very high accuracy. De-ionized water is a common dielectric fluid used in this process. The wire applications, such as in die making, medicine, electronics, and the automotive industry [50]. Figure3 EDM process has a wide range of applications, such as in die making, medicine, electronics, and the shows a schematic diagram of wire EDM. automotive industry [50]. Figure 3 shows a schematic diagram of wire EDM.
Figure 3. Schematic diagram of wire EDM.
2.4.3. Micro-EDM Micro-EDM follows the same principle as sinking EDM and wire EDM; however, it removes material at at a a micro-scale micro-scale for for components components smaller smaller than than 100 100µm. µThism. process This process cuts workpieces cuts workpieces of micro- of micro-scale,scale, including including micro-holes, micro-holes, micro-shafts, micro-shafts, and andany any3D 3Dmicro-cavities micro-cavities [96] [96. ].The The MRR MRR will will be be in nanometers because the machined part is at the micro level. Characteristically, Characteristically, micro-EDM works on a very small pulse energy and requires voltage and current several orders lower than those used in normal EDM processes. This This proces processs can can produce produce hole hole or or shaft shaft diamet diametersers on on the the order order of of 5 5 µmµm [47]. [47]. EDM inin thethe micro-scalemicro-scale is is available available in in four four configurations, configurations, namely, namely, die die sinking sinking micro-EDM, micro-EDM, micro-wire micro- EDM,wire EDM, micro-EDM micro-EDM drilling, drilling, and micro-EDMand micro-EDM milling. milling. In micro-wire In micro-wire EDM, EDM, a wire a wire with with a diameter a diameter less thanless than 20 µ m20 isµm used is used [24]. [24].
2.4.4. Powder-Mixed EDM As the the name name implies, implies, powder powder of of suitable suitable material, material, such such as nickel as nickel or even or even ceramics, ceramics, is mixed is mixed with withdielectric dielectric fluid. fluid. The presence The presence of powder of powder makes makes the process the process mechanism mechanism totally totally different different from from the theconventional conventional EDM EDM process process [97]. [97 In]. Infact, fact, powder powder particles particles fill fill the the gap gap between between the the electrode and workpiece. When voltage is applied during machinin machining,g, the presence of particles forces the electrode and the workpiece to move away from each other at a small distance to adjust the gap area reduction filledfilled by the powder particles. The gap between electrode and workpiece may increase by 100% to µ µ 300% or from 25–50 µmm toto 50–15050–150 µmm [[98].98]. Furthermore,Furthermore, the presence of powderpowder particles between the electrode and workpiece also leads to early explosion and faster sparking, which cause a higher erosion rate.
Appl. Sci. 2020, 10, 2082 7 of 46 the electrode and workpiece also leads to early explosion and faster sparking, which cause a higher erosion rate.
2.4.5. Dry EDM Dry EDM uses high-pressured gas as the dielectric instead of liquid which is used in other EDM processes [16,99–102]. Herein, the tool electrode is in the form of a thin-walled tube through which high-pressure gas or air is supplied. The pressurized gases flow through the gap and carry the debris away from the machining area. Pressure gases also reduce the machining area temperature and reduce the harmful environmental effects. This method has several other advantages; for example, the use of gas decreases the cost of managing debris, and it enhances the machining performance and work environment with regard to worker health. The dry EDM process positively influences the MRR and reduces the electrode wear ratio [99,101,103].
3. Various Grades of Tool Steels Steels can be categorized into four groups, namely, stainless steel, tool steel, carbon steel, and alloy steel. Each of these groups has its own characteristics which make it suitable for specific applications. This paper focuses on tool steels. As the name implies, tool steels are mainly employed for making cutting and metal-working tools [12]. In order to meet the required conditions these tools encounter under service conditions, tool steels must have many properties such as the ability to withstand high load, the ability to operate in rapidly changing temperatures, high abrasive resistance, etc. Normally, the tool steels are used in hardened conditions by heat treatment, and they are subsequently tempered to meet the required properties for specific application [12]. Tool steels are high-hardness and abrasion-resistant alloy steels. In addition to iron and carbon, tool steels include many other elements to increase hardness and wear resistance and hot strength and hot hardness. Furthermore, they also possess adequate toughness which can be achieved by tempering, which is performed subsequent to hardening. Applications of tool steels include stamping and extrusion dies, cutting tools, hammers, and machine parts [10]. Properties of widely and recently used different grades of tool steels, as well as the EDM processes considered for each grade, are summarized in Table1.
Table 1. Details of various EDM processes used by researchers for different grades of tool steels.
Different Grades and Composition (Weight Corresponding Machining Properties %) Operations High-carbon and high-chromium tool steel. It AISI D2 C 1.5, Si 0.3, Mn 0.3, Mo has high resistance to wear and abrasion. D2 Die sinking EDM [85,104–110]. 1.0, Cr 12.0, Ni 0.3, V 0.8, grade is heat-treatable steel with hardness in Wire EDM [111–113] Co 1.0. the range 55–62 HRC. Its corrosion resistance Powder-mixed EDM [114–116]. depends on the percentage of chromium [117]. High-carbon, high-chromium tool steel. It has AISI D3 excellent resistance to wear and abrasion and C 2.00, Si 0.30, Mn 0.30, Die sinking EDM [118]. has good dimensional stability and high Cr 12.00 Wire EDM [119]. compressive strength. Its hardness is in the range of 58–64 HRC [120]. Similar to other grades in group D, D5 has C 1.53, Si 0.89, Mn 0.46, AISI D5 high carbon and high chromium content; it is Cr 12.00, Mo 1.00, Ni Wire EDM [121,122]. the most commonly used steel among 0.384 the group D steels [123]. In addition to high carbon and high chromium contents, D6 tool steel is alloyed with tungsten. AISI D6 Cr 12.5, C 2.05, W 1.3, Mn D6 steel has high compressive strength, high Die sinking EDM [124]. 0.8, Si 0.3 wear resistance, high surface hardness, and good hardening stability [125]. Appl. Sci. 2020, 10, 2082 8 of 46
Table 1. Cont.
Different Grades and Composition (Weight Corresponding Machining Properties %) Operations AISI H11 Cr 4.75–5.50, Mo H11 grade is one of the most commonly used Die sinking EDM [109,126]. 1.10–1.75, Si 0.80–1.20, V chromium hot-work steels. It has low carbon Dry EDM [78,127]. 0.80–1.20, C 0.32–0.45, Ni content and has good toughness and deep Powder-mixed EDM [116,128–131]. 0.3, Cu 0.25, Mn hardness due to air quenching from heat Micro-EDM [132]. 0.20–0.50, P 0.03, S 0.03 treatment [133]. O1 is oil-hardening tool steel. It has good machinability and dimensional stability in AISI O1 C 0.85–1.00, Mn 1.00–1.40, hardening. It also has a good combination of Die sinking EDM [76]. Si 0.50, Cr 0.40–0.60, Ni high surface hardness and toughness after Wire EDM [134]. 0.30, W 0.40–0.60, V 0.30, hardening and tempering. O1 grade has good Powder-mixed EDM [135,136]. Cu 0.25, P 0.0, S 0.03 resistance to wear and abrasion due to its content of tungsten and chromium [137]. OHNS: C 0.82, Si 0.18, O2 grade is oil-hardening tool steel. It has AISI O2 Mn 0.52, Cr 0.49, V 0.19, good durability, excellent wear resistance, and Powder-mixed EDM [116]. Mo 0.13, Ni 0.05; an ability to hold a good cutting edge [138]. M2 grade is molybdenum-based high speed AISI M2 C 0.78–1.05, Cr 3.75–4.50, steel (HSS). It is a medium alloyed HSS. It has Die sinking EDM [139–141]. W 5.50–6.75, Mo good machinability, well-balanced toughness, Wire EDM [142]. 4.50–5.50, V 1.75–2.20. wear resistance, and red hardiness properties [143]. SKD11 SKD 11 is high-carbon and high-chromium C 1.40–1.60, Si Max 0.40, Die sinking EDM [144–147]. alloy steel. It has high hardness and Mn Max 0.60, P Max Wire EDM [148–151]. a tempering hardening effect. It also has good 0.030, S Max 0.030, Cr Dry EDM [103,152]. resistance to wear, quenching, and less 11.0–13.0, Mo 0.80–1.20, Powder-mixed EDM deformation. Currently, it has the best wear V 0.20–0.50. [54,136,153–155]. resistance of alloy tool steel [156]. C 0.35–0.42, Si 0.80–1.20, KSD61 is hot-work steel; it has high creep, SKD61 Mn 0.25–0.50, P Max temperature fatigue resistance, and high Die sinking EDM [157]. 0.030, S Max 0.020, Cr toughness. It also has a good ability to be Powder-mixed EDM [136,158,159]. 4.80–5.50, Mo 1.00–1.50, polished and good thermal conductivity [160]. V 0.80–1.15 P20 tool steel is a chrome-moly alloy steel with P20 C 0.28–0.40, Si 0.20–0.80, a carbon content of approximately 0.35 to 0.40. Die sinking EDM [161–164]. Mn 0.60–1.00, P Max. P20 has good mirror-polish ability and less Dry EDM [165]. 0.030, S Max. 0.030, Cr texture, making finishing easier. It distributes Powder-mixed EDM [166]. 1.40–2.00, Mo 0.30–0.55. a uniform hardness level even across large blocks [167]. BÖHLER W300 is hot-work tool steel and it BÖHLER W300 C 0.36, Si 1.1, Cr 5.0, Mo has high impact strength and excellent hot Die sinking EDM [168–170]. 1.3, V 0.4 tensile properties. EN 31 is a high-carbon alloy steel. It has high EN 31 C 0.9–1.2, Si 0.1–0.3, Mn hardness with compressive strength. Die sinking EDM [23]. 0.3–0.7, Cr 1–1.6, S Max Moreover, it has high resistance against wear Powder-mixed EDM [61]. 0.025 and P Max 0.025. and abrasion. ASP 2023 is a high-alloy high-speed steel. It ASP 2023 has dimension and shape stability during heat C 1.28, Cr 4.1, Mo 5.0, W Die sinking EDM [171]. treatment. It has good toughness even for 6.4, V 3.1 Wire EDM [172]. large dimensions. ASP 2023 has high hardness and good wear resistance [173]. C45 is a medium carbon steel. It has high C45 strength and hardness. It features extreme size C 0.43–0.50, Si 0.17–0.4, Die sinking EDM [174]. accuracy, straightness, and concentricity Mn 0.50–0.8 Wire EDM [175]. combined with minimal wear in high-speed applications [176]. DC53 has exceptional toughness, wear DC 53 C 0.95, Si 1.0, Mn 0.4, Cr resistance, compressive strength, and temper Wire EDM [177,178]. 8.0, Mo 2.0, V 0.3 resistance. It also has excellent machining characteristics [179]. Appl. Sci. 2020, 10, 2082 9 of 46
Table 1. Cont.
Different Grades and Composition (Weight Corresponding Machining Properties %) Operations This grade has high abrasive resistance, DIN 1.2379 C 1.50, Si 0.30, Cr 12.0, adhesive wear resistance, and compressive Die sinking EDM [180–182]. Mo 0.80, V 0.80 strength. It also has good toughness and good dimensional stability [183]. DIN 1.2738 This grade has good toughness, wear C 0.4, Mn 1.5, Cr 1.9, Ni Die sinking EDM [184]. resistance, stability in hardness, and high 1.0, Mo 0.22 Micro-EDM [185]. hardenability. C 0.50–0.60, Si 0.10–0.40, DIN 1.2714 has good hardenability and Mn 0.65–0.95, Cr 1.0–1.2, uniform hardness over sections with big DIN 1.2714 P max. 0.03, S max. 0.03, dimensions. Furthermore, it has good strength Die sinking EDM [186]. V 0.07–0.12, Ni 1.50–1.80, and toughness in addition to its tempering Mo 0.45–0.55 resistance and dimensional stability [187]. C 2.00–2.35, Mn 0.60, Si DIN 1.2080 is high-carbon/chromium tool steel. DIN 1.2080 0.60, Cr 11.00–13.50, Ni It has very high wear resistance and Die sinking EDM [182]. 0.30, W 1.00, V 1.00, Cu compressive strength. It can be hardened with 0.25, P 0.03, S 0.03 a very slight change in size. C 0.38–0.43, Si 0.15–0.35, AISI 4340 is a heat-treatable and low-alloy AISI 4340 Mn 0.6–0.8, P 0.035, S steel containing chromium, nickel, and Die sinking EDM [188]. 0.04, Cr 0.7–0.9, Ni molybdenum. It has high toughness and 1.65–2.0, Mo 0.2–0.3 strength in the heat-treated conditions [189]. C 1.64, Cr 4.80, W 10.40, This material has the ability to maintain its S390 Co 8.00, V 4.80, Mo 2.00, strength and hardness level under extremely Wire EDM [148]. Si 0.60, Mn 0.30 high cutting temperatures. C 0.36 0, Si 0.28, Mn 1.52, It is hardened and tempered plastic mold steel. M238 HH P 0.008, S 0.001, Cr 1.88, There is reduction of hardness in the center of Die sinking EDM [74]. Mo 0.22, Ni 0.95, Al 0.021 large sizes due to the Ni-addition. This grade has very good ductility, high abrasive/adhesive wear resistance, and high Vanadis-4E C 1.4, Si 0.4, Mn 0.4, Cr compressive strength. Moreover, it has good Wire EDM [190]. 4.7, Mo 3.5, Va 3.5 dimensional stability during heat treatment, good through-hardening properties, and temper back resistance [191].
4. Research in EDM of Tool Steel In this section, the studies dealing with tool steels in EDM processes are reviewed based on the classification scheme below. Type of EDM process. The relevant studies are classified according to the EDM process used to machine the tool steel workpiece are as follows: D-S Die sinking EDM W Wire EDM µ Micro-EDM P-M Powder-mixed EDM D Dry EDM Objective function: The following objective functions are reported in the current relevant literature: Objective 1: Performance measures in EDM of tool steels Objective 2: Effect of EDM process on the surface integrity Objective 3: Development of new methods and methodologies Objective 4: Modeling and Simulation of EDM process Objective 5: Electrode material and shape in the EDM of tool steel Objective 6: Combined and hybrid processes of tool steel Objective 7: Dielectric fluid research Objective 8: Other objective functions Appl. Sci. 2020, 10, 2082 10 of 46
Using this classification scheme, Table2 chronologically lists the studies for di fferent grades of tool steel machined in different EDM process.
Table 2. Studies for different grades of tool steel machined using various EDM processes. D-S—die sinking; W—wire; µ—micro; P-M—powder-mixed; D—dry.
EDM Process D-S EDM W EDM µ EDM P-M EDM D EDM [23,74,105,107,118, [54,119,121,122, 126,139,157,161, [61,116,135,154, [103,115,152, Objective 1 142,177,178,190, [132] 164,170,182,186, 158] 159,177,178] 198] 188,192–197] [91,104,105,124,141, [122,134,148,149, Objective 2 145,146,168,174, [116,131][165] 172,190,202] 182,184,199–201] [85,162,180,188,195, [132,205, Objective 3 [111,119,198] [128,135,153][152] 197,203,204] 206] [106,109,110,126, 139,140,162,163, Objective 4 [113,121,175][166][103,114] 171,193,201,207– 209] [162,182,192,196, Objective 5 [185] 210,211] Objective 6 [141,157,212][206][135][78,127,165] [61,129,130,136, Objective 7 [74,76] [115,159] 155,158] [108,112,144,147, Objective 8 169]
4.1. Performance Measures in EDM of Tool Steels Extensive literature is available relating the effect of the working parameters on the performance measures during the EDM process of tool steels. Some even introduced many techniques to improve process performance measures. Accordingly, Balasubramanian and Senthilvelan [118] studied the influence of current, pulse-on time, die electric pressure, and tool diameter on the output response in terms of MRR, TWR, and SR of EN8 and D3 steel materials machined in EDM process. The study utilized response surface methodology (RSM) to analyze parameters and applied analysis of variance (ANOVA) to identify significant process parameters. Cast copper and sintered powder metallurgy coppers were considered as tool electrodes to machine the two workpieces. The authors reported that coefficient of determinant values were above 0.90 for both materials, and the predicted value reasonably agreed with adjusted values. Furthermore, the mean value of MRR for EN8 material was higher (72.4 mm3/min) and that of TWR was lower (12.73 mm3/min) for cast electrode compared to sintered electrode. It was found that the SR value was marginally lower with sintered electrode than with cast electrode. Moreover, the mean value of MRR was higher and that of TWR was lower when machining D3 using a cast electrode compared to a sintered electrode. Furthermore, Sahu et al. [107] proposed an optimization methodology to select the best process parameters in a multi-response situation. Experiments for AISI D2 steel machining were conducted on a die sinking EDM under different combinations of process parameters. The study adopted RSM to find the influence of process parameters including discharge current, pulse-on time, duty factor, and flushing pressure on four response parameters, namely, MRR, TWR, SR, and circularity (r1/r2) of the machined component. A mathematical tool was included as a decision-making unit to obtain the relative efficiency for each experimental run, and the decision-making unit was evaluated by the LINGO system. The authors Appl. Sci. 2020, 10, 2082 11 of 46 reported the optimal setting combination as Ip (discharge current) of 7 A, Ton (pulse-on time) of 200 µs, τ (duty factor) of 90%, and Fp (flushing pressure) of 0.4 kg/cm2, and the experimental values of responses were obtained as MRR of 13.9600 mm3/min, TWR of 0.0201 mm3/min, SR = 4.9300 µm, and circularity = 0.8401. Furthermore, Haddad and Tehrani [119] studied the influence of machining parameters on the MRR in the cylindrical wire electrical discharge turning process. A statistical design of the experimental method was utilized to study AISI D3 (DIN X210Cr12) tool steel. The EDM parameters used in the study were power, voltage, pulse-off time, and spindle rotational speed; the effect of these parameters on MRR was analyzed using ANOVA. The paper also used RSM to study surface integrity. The authors reported that the only influential design factors and interaction effects for a confidence level of 95% were power, voltage, pulse-off time, and spindle rotational speed. The MRR was modeled in terms of process parameters as follows:
3 MRR mm = 8.96796 + 3.23882 power 0.68118 voltage min × − × +2.08759 pulse off time + 0.13699 spindle rotational speed × × +0.00375 voltage voltage × × 0.21999 power pulse off time − × × 0.00137 voltage spindle rotational speed. − × × Kanlayasiri and Boonmung [177,178] investigated the effect of machining parameters including pulse-on time, pulse-off time, pulse-peak current, and wire tension on the SR of wire EDMed DC53 die steel by utilizing the ANOVA technique. The authors reported that the pulse-on time and pulse-peak current were the significant variables to SR of wire EDMed DC53 die steel. Similarly, Kiyak and Cakır [164] studied the effect of EDM parameters including pulse current, pulse-on time, and pulse-off time on SR for machining 40CrMnNiMo864 tool steel (AISI P20). The authors reported that the SR was increased with higher values of pulsed current and pulse-on time parameters, and better surface finish was achieved with lower current, lower pulse-on time, and relatively higher pulse-off time. Additionally, Wu et al. [158] investigated the influence of surfactant and Al powders added to the dielectric on the SR of an SKD61 steel workpiece during the EDM process. The machining parameters for EDM optimization were also proposed. Al powders in the dielectric were agglomerated by the electrostatic force among fine Al particles, and a surfactant homogeneously separated the Al powder in the dielectric. The authors reported that the best distribution effect was found when the concentrations of the Al powder and surfactant in the dielectric were 0.1 and 0.25 g/L, respectively. An optimal SR value of 0.172 µm was achieved under the following parameters: positive polarity, discharge current of 0.3 A, pulse duration time of 1.5 µs, open circuit potential of 140 V, gap voltage of 90 V, and surfactant concentration of 0.25 g/L. Furthermore, Amorim and Weingaertner [161] studied the influence of EDM process parameters on the performance parameters for AISI P20 tool steel under finish machining. Other research studies also discussed the machining characteristics for other grades of tool steel. For example, Hasçalýk and Çayda¸s[122] studied the machining characteristics of AISI D5 tool steel in the wire EDM process, and, in another investigation [23], the effects of machining parameters on the performance parameters in EDM of En-31 tool steel were studied. The effect of process parameters on the performance parameters was also analyzed using the Taguchi method. For example, in one study [190], the effect of wire EDM process parameters was investigated for normal SR during the machining of powder metallurgical cold-worked tool steel, VANADIS 4e. The study utilized the Taguchi method to determine the optimum SR. The process parameters considered in this study included pulse-on time, pulse-off time, servo voltage, peak current, wire tension, and water pressure. The authors reported that the most important interactions impacting the SR of machined surface were between the pulse-on time and pulse-off time, pulse-on time and peak current, and pulse-off time and peak current. Furthermore, the influence of process parameters on the breathing zone concentration of respirable aerosol generated from the EDM process was analyzed using Taguchi methodology [195]. The study analyzed the constituents of aerosol generated from Appl. Sci. 2020, 10, 2082 12 of 46 the process, studied the morphology of the particulates, and suggested the control measures to reduce the risk of aerosol exposure. The authors reported that the composition of generated aerosol depends on the composition of the electrode materials and on the boiling point of its constituents. It was found that 69% of the aerosol consists of metallic particles and 12.2% consists of the hydrocarbons attached to it. The remaining portion of the aerosols consisted of carbon dust and unidentified compounds. The particle size of the aerosol was observed in the range of 25–29 nm with spherical shape. In another study [194], a combination of Taguchi and fuzzy TOPSIS (technique for order preference by simulation of ideal solution) methods was used to solve multi-response parameter optimization problems for the EDM process in a green manufacturing environment. The study developed a decision-making model to select the process parameters that achieved green EDM. A Taguchi L9 orthogonal array was utilized to analyze the sensitivity of green manufacturing attributes regarding different process parameters including peak current, pulse duration, dielectric level, and flushing pressure. Triangular fuzzy numbers were used to obtain weighing factors for the output responses, and the most suitable combinations of parameters were selected based on the TOPSIS technique. The authors reported that the optimal machining performance for green EDM was 4.5 A peak current, 261 µm pulse duration, 40 mm dielectric level, and 0.5 kg/cm2 flushing pressure. They also identified that the peak current was the most influential parameter in multi-performance characteristics. Furthermore, an investigation presented [193] the application of Taguchi’s robust design to find the optimal machining process parameters of the EDM process of precise cylindrical forms on high-speed steel. The study considered six design factors, namely, intensity supplied by the generator of the EDM machine, pulse-on time, voltage, pulse-off time, servo, and rotational speed, and it evaluated their effect on the MRR. The authors reported that the most important factors affecting the cost-effectiveness were intensity, spindle speed, servo, and pulse-on time. It was found that the actual gain of 0.023 for MRR was very close to the predicted value of 0.021. Fuzzy logic was also considered to study the influence of processes parameters on the performance parameters; for example, in one study [121], an adaptive neuro-fuzzy inference system model was developed to predict the white layer thickness and average SR achieved as a function of the process parameters. Pulse duration, open-circuit voltage, dielectric flushing pressure, and wire feed rate were taken as the model’s input features. The model combined the modeling function of fuzzy inference with the learning ability of artificial neural networks (ANN). User-friendly fuzzy–expert systems to select the EDM parameters were also developed [188]. Furthermore, Tzeng and Chen [154] applied the integration of fuzzy logic analysis with Taguchi methods to optimize the accuracy of the high-speed EDM process. Zarepour et al. [186] presented statistical analysis of electrode wear in the EDM of tool steel DIN 1.2714 used in forging dies. Moreover, in another study, Han et al. [202] utilized the finite element method to conduct thermo-analysis on material removal in the finish cut of wire EDM to study the influence of the discharge current on machining surfaces. The authors reported that the long-duration pulses could not meet the machining requirements during finish machining that had high requirements in SR, and only short-duration pulses could be used to carry out the machining. Figure4 shows the surface morphologies of long- and short-duration pulses, and Figure5 shows the SEM microphotographs of single craters under the two pulse durations. Appl. Sci. 2020,, 10,, xx FORFOR PEERPEER REVIEWREVIEW 13 of 45 short-duration pulses could be used to carry out the machining. Figure 4 shows the surface Appl.morphologies Sci. 2020, 10 ,of 2082 long- and short-duration pulses, and Figure 5 shows the SEM microphotographs13 of of 46 single craters under the two pulse durations.
Figure 4.4. Surface morphologies of long- andand short-duration pulses [[202].202].
Figure 5. SEMSEM microphotographs microphotographs of single crater craterss under different different pulsepulse durations [[202].202].
Earlier studies were alsoalso conductedconducted toto investigateinvestigate the relationship between processes and performanceperformance parameters [[54,142].54,142]. The The main main resear researchesches in optimizing process parametersparameters of EDMEDM machining areare summarizedsummarized inin TableTable3 3..
Table 3. Summary of recent researches in optimizing machining process parameters of tool steel.
Process Machining No. Authors Process Remarks parameters Performance SR was higher when using Dura graphite than when using Poco graphite. As pulse current increases, (Younis et micro-cracks increase; soft Is, EM, and CR and 1 al., 2015) EDM machining exhibited higher residual MC RS [182] stresses than medium and rough machining. Poco graphite exhibited higher residual stresses compared with Dura graphite electrode. Appl. Sci. 2020, 10, 2082 14 of 46
Table 3. Cont.
Process Machining No. Authors Process Remarks parameters Performance The waste vegetable oil-based (Valaki and Die bio-dielectric fluid can be used as an Rathod sinking Is, Vg, Ton, MRR, EWR, 2 alternate to hydrocarbon-, water-, 2015) EDM and Toff and TWR and synthetic-based dielectric fluids [74] machine for EDM. The MRR and energy efficiency were much higher with short pulse (Zhang et al. RE, D_plas, durations than with long pulse 3 2014) EDM PD and PoW and RE durations. The depth–diameter ratio [197] of the crater was higher when the workpiece was positive. The ranges of process parameters for (Sudhakara wire EDM were established as and Ton follows: pulse-on time 108–128 µs, 4 Prasanthi W EDM Toff, Vs, Ip, SR pulse-off time 47–63 µs, peak current 2014) WT, and DP 11–13 A, voltage 18–68 V, wire [190] tension 2–8 g, water pressure 8–14. The optimal parameters (I, Ton, and (Aich and Toff) to maximize the MRR were Banerjee I, Ton, and MRR and 12.0 A, 153.9865 µs, and 50.0000 µs, 5 EDM 2014) Toff SR respectively, and those to achieve [139] the best SR were 3.0 A, 200.000 µs, and 126.8332 µs, respectively. For EN-8 material, the mean MRR value was (72.4 mm3/min), it was higher for the cast electrode than for the sintered electrode. The TWR was (12.73 mm3/min); it was lower (Balasubramanian for the cast electrode than for and the sintered electrode. For die steel Ip, Ton, DP MRR, TWR, 6 Senthilvelan EDM D3, the mean value of MRR was and D_tool and SR 2014) higher for the cast electrode than for [118] the sintered electrode. The TWR was marginally lower for the cast electrode than for the sintered electrode. The mean value of SR was marginally lower for the sintered electrode than for the cast electrode. The values of discharge current (Ip), pulse-on time (Ton), duty factor(τ), and flushing pressure (Fp) that (Sahu, achieved the best quality were 7 A, Mohanty et Ip, Ton, τ, MRR, TWR, 200 µs, 90%, and 0.4 kg/m2, 7 EDM al. 2013) and Dp SR, and r1/r2 respectively. The optimal obtained [107] response parameters were MRR = 13.9600 mm3/min, TWR = 0.0201 mm3/min, Ra = 4.9300 µm, and circularity = 0.8401. Appl. Sci. 2020, 10, 2082 15 of 46
Table 3. Cont.
Process Machining No. Authors Process Remarks parameters Performance The discharge current was the main parameter effect on the MRR and the discharge duration was the main parameter effect on the TWR. There was no direct link between the grain size and the two response parameters MRR and TWR. MRR (Klocke et increases as the current increases al. 2013) MRR and and it decreases as the pulse 8 EDM I, PD, and GG [170] TWR duration and electrical conductivity of graphite grade increase. Relative TWR slightly decreases as the current increase and slightly increases as the electrical conductivity of graphite grade increase, whereas it sharply decreases as pulse duration increases. Plasma flushing efficiency increases (Shabgard as pulse current increases and it 9 et al. 2013) EDM Is and Ton PFE decreases as pulse-on time increases. [126] Recast layer thickness increases as pulse-on time increases. Best surface roughness and the minimum achievable maximum (Fan, Bai et W processing thickness were obtained 10 al. 2013) C T and SR EDM-HS upon selecting a capacitance that [198] achieved triple the charging time constant equal to pulse duration. EWR and surface roughness were significantly lower in the ultrasonic assisted cryogenically cooled copper electrode (UACEDM) process than in the conventional EDM process and MRR was approximately (Srivastava the same as for conventional EDM. and Pandey, Is, Ton, τ, and MRR, EWR, Surface integrity of the workpiece 11 EDM 2012) Vg and SR machined by UACEDM was better [141] than that machined by the conventional EDM process. In UACEDM, the density of cracks increases as the discharge current increases. Induced stress increases as pulse-on duration and crack formation increase. The rotary tool electrode improved (Teimouri the machining performance. and Baseri The magnetic field reduced 12 EDM DE, H, and w MRR and SR 2012) the inactive pulses and helped [196] the ionization. As rotational speed increases, Ra decreases. Appl. Sci. 2020, 10, 2082 16 of 46
Table 3. Cont.
Process Machining No. Authors Process Remarks parameters Performance Machining conditions allowing material transfer (of tungsten and carbon to the workpiece surface) by EDM were at a discharge current (Kumar and less than 5 A, shorter pulse-on time Ip, Ton, and 13 Batra 2012) EDM µH less than 10 µs, and longer pulse-off Toff [116] time more than 50 µs with negative polarity of the tool electrode. The most significant factor for surface modification was peak current. The optimal machining performance (Sivapira et for green EDM was with peak Ip, PD, DL, 14 al. 2011) EDM S_green current = 4.5 A, pulse duration = and DF [194] 261 µs, dielectric level = 40 mm, and flushing pressure = 0.5 kg/cm2. (Çayda¸set The developed approach greatly Wire PD, V, DP, TWL and 15 al. 2009) improved the surface roughness and EDM and S-wire Avr_SR [121] white layer thickness in wire EDM. The MRR of magnetic force-assisted EDM was almost three times as large as the value for standard EDM. Employing magnetic force-assisted EDM improved the lower relative electrode wear ratio (REWR) from 1.03% to 0.33% and reduced the SR from Ra 3.15 to 3.04 µm on average. Discharge craters were bigger and deeper, and micro-cracks were more common in standard EDM than that magnetic force-assisted EDM. In the magnetic force-assisted EDM (Lin et al. process, MRR was significantly P, Ip, PD, IH, MRR and 16 2009) EDM affected by polarity and peak current V, and Vs SR [157] and SR was significantly affected by peak current. The optimal parameters which maximized MRR were negative polarity, peak current = 5 A, auxiliary current = 1.2 A, pulse duration = 460 µs, no-load voltage = 120 V, and servo reference voltage = 10 V. The optimal parameters which achieved minimum SR were positive polarity, peak current = 20 A, auxiliary current = 0.8 A, pulse duration = 460 µs, no-load voltage = 200 V, and servo reference voltage = 10 V. Appl. Sci. 2020, 10, 2082 17 of 46
Table 3. Cont.
Process Machining No. Authors Process Remarks parameters Performance Adding 30 g/L of Span 20 to kerosene increased the MRR by 40%. Selecting proper working parameters improved MRR by 85%. SR was not deteriorated even at MRR. Adding Span 20 (30 g/L) decreases both the concentrated (Wu et al. Ip, PD, V, and discharge energy and the unstable 17 2009) EDM MRR and SR Vg discharge phenomenon. [159] The thickness of recast layer on the workpiece of kerosene was less than the thickness of pure kerosene. The surfactant increased the conductivity of kerosene and shorted the delay time, thus improved the machining efficiency. The factors most influencing the cost-effectiveness of the EDT process were intensity, spindle (Matoorian speed, servo, and pulse-on time in IN, Ton, Toff, 18 et al. 2008) EDM MRR the following combination: 6 A, V, S, and W [193] 50 µs, 20 µs, 120 V, 30 V, and 40 rpm, respectively. The actual and predicted values of MRR were 0.023 and 0.021, respectively. The copper electrode achieved higher MRR than the graphite (Haron et al. I, EM, and electrode. It was recommended to 19 2008) EDM MRR D_tool use the copper electrode for rough [192] cutting and the graphite electrode for finish cutting. The only influential design factors and interaction effects of machining (Haddad parameters on the MRR in and Tehrani Wire P, Toff, V, and 20 MRR the cylindrical wire electrical 2008) EDM w discharge turning process were [119] power, voltage, pulse-off time, and spindle rotational speed. Powder-mixed The simulation results showed that (Kansal et electric PMEDM produced smaller and I, Ton, Toff, 21 al. 2008) discharge TD shallower craters than EDM under DE, and PCH [114] machining the same set of machining (PMEDM) conditions. The main parameters of wire EDM (Kanlayasiri affecting the SR of DC53 die steel and Wire Ton, Toff, Ip, were pulse-on time and pulse-peak 22 Boonmung SR EDM and WT current. The SR increases as 2007) the pulse-on time and pulse-peak [177,178] current increase. Appl. Sci. 2020, 10, 2082 18 of 46
Table 3. Cont.
Process Machining No. Authors Process Remarks parameters Performance MRR in powder-mixed EDM was significantly affected by peak current, concentration of the silicon powder, pulse-on time, pulse-off (Kansal et time, and gain. Among all, peak Ip, Ton, Toff, al. 2007) Powder-mixed current and concentration of silicon 23 PCON, GN, MRR [115] EDM powder were the parameters most and NF influencing MRR. The optimum c parameters were peak current = 10 A, powder concentration = 4 g/L, pulse-on time = 100 µs, pulse-off time = 15 µs, and gain = 1 mm/s. The SR increases as pulsed current and pulse time increase. SR decreases as current and pulse time decrease and pulse pause time increases. For rough EDM machining, the machine power (Kiyak and Is, Ton, and should be 25% of the produced 24 Cakır 2007) EDM SR Toff power with current, pulse time, and [164] pulse pause time of 16 A, 6 µs, and 3 µs, respectively. For finish machining, the machine had 50% of produced power with current, pulse time, and pulse pause time of 8 A, 6 µs, and 3 µs, respectively. 81.5% of the high-speed EDM process variance was due to pulse time, duty cycle, and peak value of V, Pd, τ, Ip, discharge current. The best PCON, parameter combinations achieving regular Precision and precision and accuracy of (Tzeng and distance for accuracy of the high-speed EDM process were 25 Chen 2007) EDM electrode lift, the high-speed open-circuit voltage of 120 V, pulse [154] time interval EDM duration of 12 µs, duty cycle of 66%, for electrode pulse-peak current of 12 A, powder lift, and concentration of 0.5 cm3/L, regular powder size distance for electrode lift of 12 mm, time interval for electrode lift of 0.6 s, and powder size of 40 µm. Pulse-on time, current, and pre-EDM roughing as factors, along with pulse-on time/current, pulse-on (Zarepour time/pre-EDM roughing, and 26 et al. 2007) EDM Ton, I, and V TWR current/pre-EDM roughing as [186] interactions, were found to have significant effects on electrode wear of the EDM process of DIN 1.2714. Providing a selection tool enables an unskilled user to select necessary (Yilmaz et Is, PD, PI, FR, EWR, better parameters which achieve less 27 al. 2006) EDM and GC SR, and ER electrode wear, better surface [188] quality, and high erosion rate for both finish and rough machining. Appl. Sci. 2020, 10, 2082 19 of 46
Table 3. Cont.
Process Machining No. Authors Process Remarks parameters Performance The surface roughness of the workpiece in the EDM process was improved by adding surfactant and aluminum powder to the dielectric fluid. The EDM parameters which achieved optimal surface roughness (0.172 µm) were Al powder concentration of 0.1 g/L, positive polarity, peak current of (Wu et al. P, PD, V, Vg, 0.3 A, peak duration of 1.5 µs, and 28 2005) EDM PCON, and SR surfactant concentration of 0.25 g/L. [158] SCON The gap distance was increased by adding aluminum powder or surfactant to the EDM dielectric fluid. Dielectric mixed with both aluminum powder and surfactant achieved an optimally thin recast layer. The mixture also improved the SR by 60% compared to the SR under normal dielectric. MRR increases as the concentration of the silicon powder increases. SR decreases as the concentration of the silicon powder increases. Peak (Kansal et Powder-mixed Ton, τ, Ip, current and concentration of 29 al. 2005) MRR and SR EDM and PCON the silicon powder were [61] the parameters most affecting MRR and SR. MRR increases and SR decreases as the combination of peak current and concentration increase. The maximum MRR of 8 mm3/min was obtained at a discharge current of 8 A and a discharge duration of 50 µs, with positive electrode polarity and a generator under (Amorim iso-energetic mode. The minimum and average SR of 0.6 µm was obtained Is, PD, PI, V, MRR, WWR, 30 Weingaertner EDM at a discharge current of 3 A, P, and G_mod and SR 2005) discharge duration of 12.8 µs, [161] negative electrode polarity, and generator under iso-energetic mode. The volumetric relative wear for EDM with a negative electrode polarity was much higher than that with positive electrode polarity. Appl. Sci. 2020, 10, 2082 20 of 46
Table 3. Cont.
Process Machining No. Authors Process Remarks parameters Performance The thickness white layer was proportional to the magnitude of the energy impinging on that surface. The density of cracks in the white layer and SR increase as (Hasçalýk the pulse duration and open-circuit and Çayda¸s PD, V, S-wire, 31 W EDM SR and MS voltage increase. Dielectric fluid 2004) and DP pressure and wire speed did not [122] have much of an influence on SR. The surface of all workpieces was harder than the bulk material, while the heat-affected zone was softer in quenched and tempered workpieces. (Kunieda et The monotonous oscillation using 32 al. 2004) Dry EDM G and Gain MRR a piezoelectric actuator was not [103] useful in dry EDM. Among copper, copper tungsten, brass, and aluminum, copper and aluminum electrodes offered higher MRR and SR during machining of En-31 work material in EDM, where the electrodes of these two materials (Singh et al. produced low diametrical overcut. MRR, D_over, 33 2004) EDM Is and EM The copper–tungsten electrode EWR, and SR [23] offered low values of SR at high discharge currents. Copper and copper–tungsten electrodes offered low EWR. In contrast, brass resulted in the highest EWR. Among the four electrode materials, copper was the best to machine En-31 material. The parameters most affecting the average cutting speed during rough cutting were pulse-on time, pulse-off time, and pulse-peak current, and those during trim cutting were pulse-on time and constant cutting. The parameter most affecting the SR during rough (Lin et al. cutting was pulse-peak current, and 2000) those during trim cutting were [54]; Puri Ton, Toff, Ip, pulse-on time, pulse-peak voltage, and τ, Vp, S-wire, Avg_CS and 34 W EDM servo spark gap set voltage, Bhattacharyya WT, Vs, DP, G-InI dielectric flow rate, wire tool offset, 2003) and F and constant cutting speed. [142] The factors most affecting geometrical inaccuracy due to wire lag during rough cutting were pulse-on time, pulse-off time, pulse-peak current, and pulse-peak voltage, and those during trim cutting were wire tension, servo spark gap set voltage, wire tool offset, and constant cutting speed. Appl. Sci. 2020, 10, 2082 21 of 46
Table 3. Cont.
Process Machining No. Authors Process Remarks parameters Performance The recast layer becomes thicker as the pulse current and pulse-on (Guu et al. duration increase. As the peak Is, Ton, and T_RL, SR, 35 2003) EDM current is achieved, the melting of Toff and σ [105] res the material and damage of the surface and subsurface area increase. High-frequency vibration had a notable effect on the MRR. The combination of low-frequency vibration and electrode rotation did not give a satisfactory effect on MRR. The combination of ultrasonic (Ghoreishi vibration and electrode rotation led and A, w, LF, and MRR, TWR, to an increase in MRR. 36 Atkinson EDM HF and SR The combination of high-frequency 2002) vibration and electrode rotation was [135] the best for the finishing cut. In the semi-finishing cut, the vibro-rotary EDM increased MRR by 35% and 100% compared to vibratory and rotary EDM, respectively. (Kunieda The MRR and waviness could be and improved by increasing the wire 37 Furudate Dry EDM winding speed and decreasing 2001) the actual depth of cut. [152]
4.2. Effect of EDM Process on the Surface Integrity Many studies with a focus on the surface integrity of tool steel materials during machining with different EDM variants were performed. For example, in an investigation [182], the effect of electrode material on resulting residual stresses, SR, and cracks in the surface of tool steel workpieces was studied. The research compared the machining performance while using Dura graphite 11 and Poco graphite. The authors reported that the SR of DIN 1.2080 was lower with Poco graphite. Regarding the surface cracks, the results showed that the machining surface with Dura graphite 11 had more cracks and fewer micro-cracks than the machining with DIN 1.2080. Figure6a–d show the surface crack during rough machining of the two workpiece materials machined by the two electrode materials. Appl. Sci. 2020, 10, 2082 22 of 46 Appl. Sci. 2020, 10, x FOR PEER REVIEW 20 of 45
a
b
FigureFigure 6. (a) Surface6. (a) Surface cracks cracks of DIN of 1.2379DIN 1.2379 rough rough machining machin usinging using Dura Dura graphite graphite 11 electrode;11 electrode; (b )(b surface) crackssurface of DIN cracks 1.2080 of DIN rough 1.2080 machining rough machin usinging Dura usinggraphite Dura graphite 11 electrode; 11 electrode; (c) (c surface) surface cracks cracks of of DIN 1.2379DIN rough 1.2379 machining rough machining using Poco using graphite Poco graphite EDMC-3 EDMC-3 electrode; electrode; (d) surface (d) surface cracks cracks of of DIN DIN 1.2080 1.2080 rough machiningrough using machining Poco using graphite PocoEDMC-3 graphite EDMC-3 electrode electrode [182]. [182].
It was also found that the Dura graphite 11 electrode resulted in lower residual stresses compared to another electrode. Furthermore, soft EDM machining exhibited higher residual stresses as a result of a higher pulse-on duration time. Appl. Sci. 2020, 10, 2082 23 of 46
Moreover, Cao et al. [148] have comprehensively compared the surface integrity of S390 and SKD11 with multi-cutting. The authors reported that the surface roughness, white layer thickness, and surface residual stress decreased as the number of cutting passes increased. They also reported that more pinholes and slightly larger craters were produced in the finish trim cut surface of the S390 workpiece, while micro-cracks were found in the finish trim cut surface of the SKD11 workpiece. Additionally, Kumar and Batra [116] studied the surface modification of three die steel materials achieved by EDM machining with powder-mixed tungsten. Process parameters including peak current, pulse-on time, and pulse-off time were considered to study the micro-hardness of the machined surface. The study utilized the Taguchi experimental design technique to independently conduct the experiments on each work material. The authors reported that a significant amount of material was transferred from the powder suspended in the dielectric medium to the work material. For example, a range of ~0% to 3.25% tungsten in the base material was recorded in the machined surface of H13 die steel. The authors found that the favorable machining conditions for material transfer by EDM were low discharge current (less than 5 A), shorter pulse-on time (less than 10 µs), longer pulse-off time (more than 50 µs), and negative polarity of the tool electrode. The most significant factor for the phenomenon of surface modification was peak current. Nowicki et al. [174] utilized a scanning profilometer and SEM to study discharge traces and their profiles and to determine the volume of craters and flashes. SEM was used to assess the surface microstructure, whereas a scanning profilometer was utilized to determine the crater dimensions and volumes. The authors reported that the flash volumes were 50% to 90% of the crater volumes and crater diameter, and they were half of the flash diameter. Figure7 shows the types of cratersAppl. during Sci. 2020 individual, 10, x FOR PEER discharge REVIEW for different parameters. 22 of 45
Figure 7. Types of craters during individual discharge (a) for small values of peak amperage and short Figurepulse 7. Types time, (b of) for craters intermediate during individualparameter values, discharge and ( (ca)) for for high small amperage values and of peak long amperagespark discharge and short pulsetime time, [174]. (b) for intermediate parameter values, and (c) for high amperage and long spark discharge time [174]. Klink et al. [172] studied the surface integrity of ASP2023 tool steel machined by EDM. The surface finish, microstructure, micro hardness, residual stress, and element distribution were compared for CH- and water-based dielectrics. The authors reported that surface finishes of 0.1 µm and 0.2 µm were achieved for CH- and water-based dielectrics, respectively. The formation of cracks on the surface of workpieces machined by EDM was also studied in many researches. For example, in one study [146], the effect of EDM processing parameters including pulse current and pulse-on duration on the formation of surface cracks in the machined surface of SKD11 tool steel was investigated. The authors reported that increasing the pulse current or reducing the pulse-on duration suppressed the formation of surface cracks in the SKD11 machined surface, thereby improving the fatigue life. Figure 8 shows the cracks formed on the SKD11 surface machined using a pulse current of 4 A and pulse-on duration of 16 µs, while the propagation of the fatigue crack is shown in Figure 9. Appl. Sci. 2020, 10, 2082 24 of 46
Klink et al. [172] studied the surface integrity of ASP2023 tool steel machined by EDM. The surface finish, microstructure, micro hardness, residual stress, and element distribution were compared for CH- and water-based dielectrics. The authors reported that surface finishes of 0.1 µm and 0.2 µm were achieved for CH- and water-based dielectrics, respectively. The formation of cracks on the surface of workpieces machined by EDM was also studied in many researches. For example, in one study [146], the effect of EDM processing parameters including pulse current and pulse-on duration on the formation of surface cracks in the machined surface of SKD11 tool steel was investigated. The authors reported that increasing the pulse current or reducing the pulse-on duration suppressed the formation of surface cracks in the SKD11 machined surface, thereby improving the fatigue life. Figure8 shows the cracks formed on the SKD11 surface machined using a pulse current of 4 A and pulse-on duration of 16 µs, while the propagation of the fatigue crack
Appl.is shown Sci. 20202020 in,, Figure1010,, xx FORFOR9. PEERPEER REVIEWREVIEW 2323 ofof 4545
Figure 8. SEMSEM image image showing showing severe severe cracks cracks formed formed on on SKD11 SKD11 surface surface machined machined using a pulse pulse current current of 4 A and pulse-on durationduration ofof 1616 µµss [[146].146].
Figure 9. SEMSEM cross-sectional cross-sectional image showing pr propagationopagation of fatigue crack [[146].146].
Choi et al. [149] [149] studied studied the effects effects of heat trea treatmenttmenttment on on the the surface surface of of a a diedie steelsteel SKD11SKD11 machinedmachined by wire EDM. The study compared heat treatment following following fourfour manufacturingmanufacturing methods,methods, namely,namely, namely, milling and then grinding, only wire EDM, low-temperaturetemperature heatheat treatmenttreatment afterafter wirewire EDM,EDM, andand high-temperature heat treatment after wire EDM. The authors reported that milling and then grinding produced the best surface among the four methods. For all methods other than milling and thenthen grinding,grinding, aa smallsmall amountamount ofof CuCu waswas transftransferred from the brass electrode to the workpiece surfaces.surfaces. TheThe resultsresults alsoalso showedshowed thatthat thethe microstrucmicrostructureture ofof thethe surfacesurface ofof thethe firstfirst methodmethod whichwhich waswas not EDM processed was completely different from thethe otherother microstructures.microstructures. NawasNawas etet al.al. [134][134] studiedstudied thethe surfacesurface integrityintegrity generatedgenerated inin AISIAISI O1O1 totool steel using four different machining processes, namely, conventional hard turning, production grinding,inding, roughingroughing byby wirewire EDM,EDM, andand roughingroughing andand finishingfinishing byby wirewire EDM.EDM. TheThe authorsauthors reportedreported thatthat thethe plasticplastic deformationdeformation duedue toto thethe pressurepressure between tool and workpiece was the main factor thatthat generatedgenerated surfacesurface integrityintegrity inin productionproduction grinding. In hard turning, the main factor that generated surface integrity was the thermal effect. The resultsresults ofof thethe studystudy showedshowed thatthat thethe EDMEDM procesprocesss generatedgenerated thethe mostmost damageddamaged surfacesurface duedue toto thethe high tensile stress, as well as the hard and brittlettle whitewhite layerslayers generatedgenerated byby EDM.EDM. EkmekciEkmekci [184][184] studiedstudied thethe effecteffect ofof dielectricdielectric fluidfluid andand thethe tytype of electrode on white layer structure in EDM machined surfaces in terms of retainedined austeniteaustenite andand residualresidual stresses.stresses. ElectrodesElectrodes ofof coppercopper andand Appl. Sci. 2020, 10, 2082 25 of 46 milling and then grinding, only wire EDM, low-temperature heat treatment after wire EDM, and high-temperature heat treatment after wire EDM. The authors reported that milling and then grinding produced the best surface among the four methods. For all methods other than milling and then grinding, a small amount of Cu was transferred from the brass electrode to the workpiece surfaces. The results also showed that the microstructure of the surface of the first method which was not EDM processed was completely different from the other microstructures. Nawas et al. [134] studied the surface integrity generated in AISI O1 tool steel using four different machining processes, namely, conventional hard turning, production grinding, roughing by wire EDM, and roughing and finishing by wire EDM. The authors reported that the plastic deformation due to the pressure between tool and workpiece was the main factor that generated surface integrity in production grinding. In hard turning, the main factor that generated surface integrity was the thermal effect. The results of the study showed that the EDM process generated the most damaged surface due to the high tensile stress, as well as the hard and brittle white layers generated by EDM. Ekmekci [184] studied the effect of dielectric fluid and the type of electrode on white layer structure in EDM machined surfaces in terms of retained Appl. Sci. 2020, 10, x FOR PEER REVIEW 24 of 45 austenite and residual stresses. Electrodes of copper and graphite and dielectric fluids of kerosene graphiteand de-ionized and dielectric water werefluids used. of kerosene The authors and de-ionized found that water the workpiece were used. surface The authors had a high found carbon that theconcentration workpiece surface in the hydrocarbon-based had a high carbon concentrat dielectricion fluid in forthe bothhydrocarbon-based electrode materials. dielectric The fluid residual for bothstress electrode on the machined materials. surface The increasedresidual asstress the non-homogeneitieson the machined withinsurface the increased white layer as increased.the non- homogeneitiesFigure 10 shows within the cross-sectional the white layer micrographs increased. Figure of machined 10 shows surfaces. the cross-sectional A thin featureless micrographs layer was of machineddistinguished surfaces. on the A surfacethin featureless and between layer the wa overlappeds distinguished recast on layers the (Figuresurface 10anda,b). between The white the overlappedlayer remained recast featureless layers (Figure even at10a,b). piled The sections white (Figure layer remained 10c,d). Guu featureless [104] utilized even at the piled atomic sections force (Figuremicroscopy 10c,d). technique Guu [104] to analyze utilized the the surface atomic morphology, force microscopy SR, and technique micro-cracks to analyze of AISI D2the tool surface steel morphology,machined using SR, theand EDM micro-cracks process. of AISI D2 tool steel machined using the EDM process.
FigureFigure 10. 10. Cross-sectionalCross-sectional micrographs micrographs of of electr electricic discharge discharge machined machined surfaces surfaces ( (aa)) using using kerosene kerosene dielectricdielectric and and graphite graphite electrode, electrode, (b ()b using) using kerosene kerosene dielectric dielectric and and copper copper electrode, electrode, (c) using (c) using de- ionizedde-ionized water water dielectric dielectric and graphite and graphite electrode, electrode, and (d and) using (d) de-ionized using de-ionized water dielectric water dielectric and copper and electrodecopper electrode [184]. [184].
Earlier studies also evaluated the surface integrity and stresses in machined surfaces of tool steel after EDM processes. For example, in one study [199], the effect of EDM process parameters on the 3D surface topography of tool steel was investigated, while another study [168] reported the crystallographic and metallurgical properties of the white layer in the EDM of Böhler W300 ferritic steel. In another investigation [145], the residual stress in SKD11 material was measured. The effect of the machining parameters of EDM on the surface characteristics and machining damage for AISI D2 tool steel material [105] was studied. Other works studied the surface modifications of tool steel during the EDM process for AISI H13 and AISID6 [131,200]. Some studies [124] also investigated the effect of process parameters on the fatigue life of AISI D6 tool steel. Changes in the chemical composition of re-solidified layers of the electrodes and the workpieces during the EDM of T215 Cr12 die steel were also been studied [91]. Many researches focused on the resulting residual stresses in the machined surface, such as Reference [182].
4.3. Development of New Methods and Methodologies Many developments were proposed in EDM processes; for example, one study [197] proposed a new method of determining the energy distribution and plasma diameter. The authors compared the boundary of the melted material in the crater which was obtained via the metallographic method and that which was obtained from the isothermal surface of the thermal–physical model. The Appl. Sci. 2020, 10, 2082 26 of 46
Earlier studies also evaluated the surface integrity and stresses in machined surfaces of tool steel after EDM processes. For example, in one study [199], the effect of EDM process parameters on the 3D surface topography of tool steel was investigated, while another study [168] reported the crystallographic and metallurgical properties of the white layer in the EDM of Böhler W300 ferritic steel. In another investigation [145], the residual stress in SKD11 material was measured. The effect of the machining parameters of EDM on the surface characteristics and machining damage for AISI D2 tool steel material [105] was studied. Other works studied the surface modifications of tool steel during the EDM process for AISI H13 and AISID6 [131,200]. Some studies [124] also investigated the effect of process parameters on the fatigue life of AISI D6 tool steel. Changes in the chemical composition of re-solidified layers of the electrodes and the workpieces during the EDM of T215 Cr12 die steel were also been studied [91]. Many researches focused on the resulting residual stresses in the machined surface, such as Reference [182].
4.3. Development of New Methods and Methodologies Many developments were proposed in EDM processes; for example, one study [197] proposed a new method of determining the energy distribution and plasma diameter. The authors compared Appl. Sci. 2020, 10, x FOR PEER REVIEW 25 of 45 the boundary of the melted material in the crater which was obtained via the metallographic method and that which was obtained from the isothermal surface of the thermal–physical model. The boundary boundary of the melted material was calculated from the thermal–physical model using the finite of the melted material was calculated from the thermal–physical model using the finite element method. element method. This method was applied to experimentally molded steel 8407, determining the This method was applied to experimentally molded steel 8407, determining the energy distribution energy distribution and plasma diameter in different dielectrics with different polarities. Figure 11 and plasma diameter in different dielectrics with different polarities. Figure 11 shows a comparison of shows a comparison of the section view of the craters obtained using different dielectrics and the section view of the craters obtained using different dielectrics and polarities. polarities.
Figure 11. ComparisonComparison of of the the section section view view of the craters obtained using different different dielectrics and polarities [[197].197].
Fu etet al.al. [205 [205]] proposed proposed a piezoelectric a piezoelectric self-adaptive self-adaptive micro-EDM. micro-EDM. The proposed The proposed machining machining method realizedmethod self-regulationrealized self-regulation depending depending on discharge on conditions.discharge conditions. Compared withCompared conventional with conventional micro-EDM, themicro-EDM, discharge the stability discharge of the stability working of processthe workin wasg improved process was greatly, improved and the greatly, machining and the effi ciencymachining was higher,efficiency while was the higher, electrode while wear the ratio electrode was lower. wear Furthermore, ratio was Fanlower. et al. Furthermore, [198] developed Fan a multi-modeet al. [198] precisiondeveloped pulse a multi-mode power system precision based pulse on a power micro system controller based unit on and a micro double controller complex unit programmable and double logiccomplex devices. programmable The effects oflogic capacitance devices. onThe the effects achievable of capacitance processing thicknesson the achievable were analyzed processing under controllablethickness were resistance analyzed and under capacitance controllable precision resistance pulse power and capacitance mode. By properly precision selecting pulse power a capacitance mode. thatBy properly achieved selecting triple the a capacitance charging time that equal achieved to the triple pulse the duration, charging the time best equal SR to and the minimum pulse duration, single pulsethe best energy SR and were minimum obtained. single pulse energy were obtained. Maradia et al. [132] studied the capability for the implementation of die-sink EDM in meso– micro-scale machining. The study presented the process parameters that achieved high MRR and TWR with high form accuracy and precision. Additionally, Cabanes et al. [111] proposed a methodology that guarantees an early detection of instability that can be used to avoid the detrimental effects associated with both unstable machining and wire breakage. The proposed methodology established helpful procedures to understand the causes of wire breakage and instability. Tzeng [153] proposed a flexible high-speed EDM technology with geometrical transform optimization. Furthermore, Uhlmann et al. [204] studied and analyzed the machining effects developing at high peripheral speeds. The process behavior and surface topography of three different processes, namely, electrical discharge turning, electrical discharge grinding, and wire electrical discharge grinding, were compared for cold-worked steel 90MnCrV8. In the recent past, studies were conducted to assess different methods for several purposes. For example, Bleys et al. [180] developed a new wear compensation method based on real-time tool wear sensing. On the other hand, in another investigation [85], a method of optimizing for EDM performance parameters for EDM was developed. Kunieda and Furudate [152] also proposed the development of a new dry wire EDM method, and Mohri et al. [128] developed a process of finish machining on free-form surfaces using EDM.
4.4. Modeling and Simulation of EDM Process Appl. Sci. 2020, 10, 2082 27 of 46
Maradia et al. [132] studied the capability for the implementation of die-sink EDM in meso–micro-scale machining. The study presented the process parameters that achieved high MRR and TWR with high form accuracy and precision. Additionally, Cabanes et al. [111] proposed a methodology that guarantees an early detection of instability that can be used to avoid the detrimental effects associated with both unstable machining and wire breakage. The proposed methodology established helpful procedures to understand the causes of wire breakage and instability. Tzeng [153] proposed a flexible high-speed EDM technology with geometrical transform optimization. Furthermore, Uhlmann et al. [204] studied and analyzed the machining effects developing at high peripheral speeds. The process behavior and surface topography of three different processes, namely, electrical discharge turning, electrical discharge grinding, and wire electrical discharge grinding, were compared for cold-worked steel 90MnCrV8. In the recent past, studies were conducted to assess different methods for several purposes. For example, Bleys et al. [180] developed a new wear compensation method based on real-time tool wear sensing. On the other hand, in another investigation [85], a method of optimizing for EDM performance parameters for EDM was developed. Kunieda and Furudate [152] also proposed the development of a new dry wire EDM method, and Mohri et al. [128] developed a process of finish machining on free-form surfaces using EDM.
4.4.Appl. Modeling Sci. 2020, 10 and, x FOR Simulation PEER REVIEW of EDM Process 26 of 45 Many modeling and simulation techniques were presented to find the optimal conditions for Many modeling and simulation techniques were presented to find the optimal conditions for the the EDM process. For example, Wang and Han [208] proposed a 3D model of flow field with liquid, EDM process. For example, Wang and Han [208] proposed a 3D model of flow field with liquid, gas, gas,and and solid solid phases phases for the for themachining machining gap gapin EDM. in EDM. The Themodel model analyzed analyzed the mechanisms the mechanisms of debris of debris and andbubble bubble movement movement in inthe the machining machining gap gap during during consecutive consecutive pulse pulse discharges. discharges. Experimental Experimental and and simulationsimulation model model resultsresults showedshowed thatthat thethe proposedproposed modelmodel was feasible. Much Much of of the the debris debris moves moves outsideoutside the the machiningmachining gap following following the the excluded excluded bu bubbles,bbles, representing representing the the main main path path via which via which the thedebris debris is isexcluded excluded from from the the machining machining gap. gap. Th Thee bubble bubble expansion expansion strengthened strengthened with with increased increased dischargedischarge current current and and pulse-on pulse-on time.time. FigureFigure 1212 showsshows the simulation and observation experimental experimental resultsresults of of the the bubble bubble and and debris debris movement.movement. AichAich andand Banerjee [139] [139] applied applied a a support support vector vector machine machine toto develop develop MRR MRR models models and and average average SR SR parameter paramete modelsr models using using support support vector vector regression regression with with three controlthree control parameters, parameters, namely, namely, current, current, pulse-on pulse-on time, andtime, pulse-o and pulsff time.e-off Fortime. accurate For accurate model model fitting, particlefitting, swarmparticle optimization swarm optimization was employed was employed to find the to optimal find the combination optimal combination of the internal of the parameters internal ofparameters the support of vector the support machine, vector including machine, regularization including parameter,regularization radius parameter, of the loss-insensitive radius of the hyperloss- tube,insensitive and standard hyper tube, deviation and standard of the kernel deviation function. of the kernel A high-speed function. steel A high-speed specimen steel was specimen chosen as thewas workpiece chosen as material. the workpiece material.
FigureFigure 12.12. (a()a Simulation) Simulation and and (b) observation (b) observation experimental experimental results of results the bubble of the and bubble debris andmovement debris movement[208]. [208].
Shabgard et al. [126] used the ABAQUS code finite element software to simulate the temperature distribution on the surface of an AISI H13 tool steel workpiece and the tool during a single discharge in the EDM process. Furthermore, Guo et al. [171] presented a multi-scale finite element model (FEM) for the single discharging of ASP2023 tool steel to incorporate the plasma-induced time/space-dependent Gaussian heat flux via a user subroutine. The long-standing numerical singularity of heat flux in EDM modeling was solved using the innovative functions of discharge current. The effects of discharge duration and current on the temperature profiles, crater formation, and dimensions were investigated. Zhang et al. [209] studied the characteristics of EDM plasma. A thermal–physical model and a metallographic method were introduced to study the heat flux of the plasma by comparing the boundary of the melted material in the crater. For mold steel 8407, the authors reported that the Gauss heat source was closer to the actual EDM process than the point heat source, circular heat source, and other heat source types. A novel approach for process modeling and optimization of EDM was proposed by Joshi and Pande [166]. The study integrated FEM with ANN and genetic algorithms (GA) to improve the prediction accuracy of the model with less dependency on the experimental data. FEM was also used to develop a thermo-physical model for the die sinking EDM process [163], whereas numerical analysis of the single spark operation of the EDM process was carried out considering the two- dimensional (2D) axisymmetric process continuum and an AISI P20 mold steel workpiece. Appl. Sci. 2020, 10, 2082 28 of 46
Shabgard et al. [126] used the ABAQUS code finite element software to simulate the temperature distribution on the surface of an AISI H13 tool steel workpiece and the tool during a single discharge in the EDM process. Furthermore, Guo et al. [171] presented a multi-scale finite element model (FEM) for the single discharging of ASP2023 tool steel to incorporate the plasma-induced time/space-dependent Gaussian heat flux via a user subroutine. The long-standing numerical singularity of heat flux in EDM modeling was solved using the innovative functions of discharge current. The effects of discharge duration and current on the temperature profiles, crater formation, and dimensions were investigated. Zhang et al. [209] studied the characteristics of EDM plasma. A thermal–physical model and a metallographic method were introduced to study the heat flux of the plasma by comparing the boundary of the melted material in the crater. For mold steel 8407, the authors reported that the Gauss heat source was closer to the actual EDM process than the point heat source, circular heat source, and other heat source types. A novel approach for process modeling and optimization of EDM was proposed by Joshi and Pande [166]. The study integrated FEM with ANN and genetic algorithms (GA) to improve the prediction accuracy of the model with less dependency on the experimental data. FEM was also used to develop a thermo-physical model for the die sinking EDM process [163], whereas numerical analysis of the single spark operation of the EDM process was carried out considering the two-dimensional Appl. Sci. 2020, 10, x FOR PEER REVIEW 27 of 45 (2D) axisymmetric process continuum and an AISI P20 mold steel workpiece. IzquierdoIzquierdo et et al. al. [ 106[106]] also also presented presented a a simulation simulation and and model model of of the the EDM EDM process. process. TemperatureTemperature fieldsfields within within the the workpiece workpiece generated generated by by the the superposition superposition of of multiple multipledischarges discharges were were numerically numerically calculatedcalculated using using a a finite finite di differencefference schema. schema. TheThe characteristicscharacteristics ofof thethe dischargedischarge forfor aa givengiven operation,operation, i.e.,i.e., energy energy transferred transferred onto onto the the workpiece, workpiece, diameter diameter of of the the discharge discharge channel, channel, and and material material removal removal eefficiency,fficiency, werewere estimatedestimated using inverse inverse identificat identificationion from from the the results results of the of thenumerical numerical model. model. The Theauthors authors reported reported the thevariation variation of ofvolume volume of of ma materialterial removed removed per per discharge as thethe operationoperation progressed,progressed, as as shown shown in in Figure Figure 13 13..
FigureFigure 13.13. VariationVariation ofof volumevolume ofof material material removed removed per per discharge discharge as as the the operation operation progresses progresses [ 106[106].].
BhattacharyyaBhattacharyya etet al.al. [140[140]] developeddeveloped aa comprehensivecomprehensive mathematicalmathematical modelmodel basedbased onon RSMRSM toto studystudy the the influence influence of of machining machining parameters,parameters, includingincluding peakpeak currentcurrent andand pulse-onpulse-on duration,duration, onon SR,SR, averageaverage white white layer layer thickness, thickness, and and surface surface crack crack density density of M2 of die M2 steel die machined steel machined by the EDMby the process. EDM Theprocess. author The reported author thatreported the minimum that the minimum SR was achieved SR was atachieved lower peak at lower current peak and current pulse-on and duration, pulse-on i.e.,duration, at 2 A andi.e., 20at µ2s, A while and the20 averageµs, while white the layeraverage thickness white couldlayer bethickness minimized could by keepingbe minimized the peak by keeping the peak current as low as possible (preferably in the range of 2–5 A) and by keeping pulse- on duration in the range of 163 to 510 µs. They also reported that the medium value of peak current, along with the minimum possible pulse-on time, could minimize surface crack density. Finally, it was found that the optimal combinations to achieve minimum surface roughness, white layer thickness, and surface crack density were 2 A/20 µs, 2 A/20 µs, and 9 A/20 µs, respectively. An axisymmetric 2D thermal model for powder mixed EDM was also developed by Kansal et al. [114]. To achieve different objective functions, many studies were conducted using models and simulation methods, such as one investigation [103] which reported the development of a piezoelectric servo system to improve the gap control in dry EDM. In another investigation [203], an optimization system to generate the optimal process parameters of EDM was proposed. Han et al. [175] developed a simulation method for wire EDM. Additionally, Yadav et al. [201] developed a model to represent the thermal stress of the EDM process, and Tsai et al. [109] developed a semi- empirical model for the surface finish of machined workpieces in EDM. Furthermore, Wang et al. [110] established a semi-empirical model of MRR in the EDM process. Williams and Rajurkar [113] developed a wire EDM process model to study the characteristics of machined surfaces. Cogun and Savsar [207] introduced a statistical model to study the variation of time-lag durations in the EDM process.
4.5. Electrode Material and Shape in the EDM of Tool Steel Appl. Sci. 2020, 10, 2082 29 of 46 current as low as possible (preferably in the range of 2–5 A) and by keeping pulse-on duration in the range of 163 to 510 µs. They also reported that the medium value of peak current, along with the minimum possible pulse-on time, could minimize surface crack density. Finally, it was found that the optimal combinations to achieve minimum surface roughness, white layer thickness, and surface crack density were 2 A/20 µs, 2 A/20 µs, and 9 A/20 µs, respectively. An axisymmetric 2D thermal model for powder mixed EDM was also developed by Kansal et al. [114]. To achieve different objective functions, many studies were conducted using models and simulation methods, such as one investigation [103] which reported the development of a piezoelectric servo system to improve the gap control in dry EDM. In another investigation [203], an optimization system to generate the optimal process parameters of EDM was proposed. Han et al. [175] developed a simulation method for wire EDM. Additionally, Yadav et al. [201] developed a model to represent the thermal stress of the EDM process, and Tsai et al. [109] developed a semi-empirical model for the surface finish of machined workpieces in EDM. Furthermore, Wang et al. [110] established a semi-empirical model of MRR in the EDM process. Williams and Rajurkar [113] developed a wire EDM process model to study the characteristics of machined surfaces. Cogun and Savsar [207] introduced a statistical model to study the variation of time-lag durations in the EDM process.
4.5. Electrode Material and Shape in the EDM of Tool Steel Many issues related to electrodes and the effect of electrode materials on the EDM performance were studied by many researchers. For example, Klocke et al. [170] studied the influence of a graphite grade electrode on the MRR and TWR in the sinking EDM roughing process for Böhler steel W300 (1.2343) material. The physical characteristics of graphite, including specific electric resistance, thermal conductivity, and grain size, were considered. The authors reported that there was no direct link between the performance and the grain size in term of MRR and TWR. It was found that the most important factor of the electrode regarding MRR was the electrode material electrical conductivity, while, for TWR, the most important factor was a combination of the grain size and electric resistance. Furthermore, Ekmekci and Sayar [185] studied the origin of concavity at the end-tip in the micro-EDM of blind micro-holes. The shape of tip deformation was investigated experimentally. The study considered open gap voltage, pulse energy, and tool rotation speed as the influence parameters on such tip shapes. The study implemented cross-sectional and topographical analysis on the micro-hole and micro-tool electrode to study the influence of the mentioned parameters on the final tip shape. Figure 14 shows the micro-tool electrodes and optical section views of machined micro-holes using very short pulses and open gap voltages of 60, 75, and 90 V. Finally, the study proposed an accumulation of debris phenomenon to describe the wear mechanism at the tip shapes encountered. Teimouri and Baseri [196] studied the effect of electrode rotation and external magnetic field on EDM performance. The authors reported that the machining performance was improved with a rotary electrode. The externally applied magnetic field reduced the inactive pulses, including arcing, short circuit, and open circuit, while it also helped in the ionization. It was found that the application of the magnetic field led to an improvement in surface quality, wherein SR decreased upon increasing the rotational speed. Appl. Sci. 2020, 10, x FOR PEER REVIEW 28 of 45
Many issues related to electrodes and the effect of electrode materials on the EDM performance were studied by many researchers. For example, Klocke et al. [170] studied the influence of a graphite grade electrode on the MRR and TWR in the sinking EDM roughing process for Böhler steel W300 (1.2343) material. The physical characteristics of graphite, including specific electric resistance, thermal conductivity, and grain size, were considered. The authors reported that there was no direct link between the performance and the grain size in term of MRR and TWR. It was found that the most important factor of the electrode regarding MRR was the electrode material electrical conductivity, while, for TWR, the most important factor was a combination of the grain size and electric resistance. Furthermore, Ekmekci and Sayar [185] studied the origin of concavity at the end- tip in the micro-EDM of blind micro-holes. The shape of tip deformation was investigated experimentally. The study considered open gap voltage, pulse energy, and tool rotation speed as the influence parameters on such tip shapes. The study implemented cross-sectional and topographical analysis on the micro-hole and micro-tool electrode to study the influence of the mentioned parameters on the final tip shape. Figure 14 shows the micro-tool electrodes and optical section views of machined micro-holes using very short pulses and open gap voltages of 60, 75, and 90 V. Finally, Appl.the Sci.study2020 proposed, 10, 2082 an accumulation of debris phenomenon to describe the wear mechanism30 at of the 46 tip shapes encountered.
FigureFigure 14. 14.Micro-tool Micro-tool electrodes electrodes and and optical optical section section views views of of machined machined micro-holes micro-holes using using very very short short pulsespulses and and open open gap gap voltages voltages of of ( a()a) 60 60 V, V, (b ()b 75) 75 V, V, and and (c )(c 90) 90 V V [185 [185].].
FujikiTeimouri and and Shih Baseri [78] investigated [196] studied the the influence effect of el ofectrode the electrode rotation lead, and tiltexternal angles, magnetic and dielectric field on fluidEDM flow performance. rate on MRR, The TWR,authors and reported SR in the that near-dry the machining EDM milling performance process. Thewas studyimproved developed with a arotary computational electrode. fluidThe externally dynamics applied model to magnetic predict thefield flow reduced rate ofthe dielectric inactive fluid.pulses, Haron including et al. arcing, [192] comparedshort circuit, the and influence open circuit, of copper while and it also graphite helped electrodes in the ionization. on the performance It was found of that the the EDM application process forof the XW42. magnetic The authors field led reported to an improvement that the copper in surface electrode quality, achieved wherein higher SR decreased MRR than upon the increasing graphite electrode;the rotational thus, speed. the authors recommended using the copper electrode for rough cutting and the graphite electrodeFujiki for and finish Shih cutting. [78] investigated It was found the that influence the EWR ofof thethecopper electrode electrode lead, tilt was angles, lower thanand thedielectric EWR offluid the graphiteflow rate electrode.on MRR, TWR, and SR in the near-dry EDM milling process. The study developed a computationalEarlier studies fluid dynamics related to model electrodes to predict were the also flow presented. rate of dielectric For example,fluid. Haron a et study al. [192] by Shunmugamcompared the et influence al. [211] improved of copper the and wear graphite resistance electrodes of machined on the surfaces performance using EDMof the with EDM tungsten process carbidefor XW42. power The compactauthors reported electrodes, that whereas the copper Bayramoglu electrode and achieved Duffill higher [210] studiedMRR than the the machining graphite characteristicselectrode; thus, of cylindricalthe authors tools recommended in the cutting using of 3D the shapes copper using electrode EDM. for rough cutting and the
4.6. Combined and Hybrid Processes of Tool Steel Many studies which combined two or more methods to improve the performance of the EDM process were proposed. For example, Srivastava and Pandey [141] studied the machining of M2 grade high-speed steel with EDM using an ultrasonic-assisted cryogenically cooled copper electrode (UACEDM). The study considered discharge current, pulse-on time, duty cycle, and gap voltage as process variables and MRR, TWR, and SR as performance parameters. A comparison of EDM, EDM with cryogenic cooling of the electrodes (CEDM), and UACEDM was presented. The authors reported that the UACEDM process significantly reduced TWR and SR compared to the conventional EDM process, while the MRR was close in the two processes. The surface machined by UACEDM had an abundance of cracks; the density of cracks increased with increasing discharge current. It was found that the surface integrity was better in the UACEDM process than in conventional EDM. Fujiki et al. [127] developed a control strategy for gap conditions in near-dry EDM milling by using a high-speed piezoelectric actuator. The detailed structure of a spindle system that retracts the electrode at high bandwidth was also presented. The authors reported that the proposed controller improved the MMR by about 30% compared to the conventional controller. It also improved the electrode retraction efficiency. Peng et al. [206] used micro-reversible EDM to develop a new micromachining Appl. Sci. 2020, 10, 2082 31 of 46 method for the fabrication of micro-metal structures, which combined micro-EDM deposition with the micro-EDM selective removal process. The authors reported that it was easy to achieve reversible machining from the micro-EDM deposition process to the micro-EDM selective removal process by controlling the process condition. They also fabricated micro-structures with fine surface quality and high shape precision by using the micro-reversible EDM method. Figure 15 shows an example of selective removal with micro-EDM drilling. Lin et al. [157] developed a versatile process for EDM using a magnetic force-assisted standard EDM machine. The effects of magnetic force on EDM machining characteristics were explored. Moreover, the study adopted an L18 orthogonal array based on the Taguchi method to conduct a series of experiments, and the experimental data were statistically evaluated using analysis of variance. Curodeau et al. [162] presented hybrid EDM process finish and polish machining in dielectric water with a ductile carbon–polymer composite electrode. It was found that a fine graphite grade and positive polarity current impulses improved the workpiece surface finish. Zhang et al. [213] proposed an ultrasonic vibration EDM (UEDM) in gas. The study developed a theoretical model to estimate the MRR for tool steel material. The authors reported that the UEDM improved the MRR. They found that the most important outcomes of this technique were lower pollution and TWR. Curodeau et al. [165] also proposed a hybrid EDM to automate the polishing process of tool steel cavities. The proposed method involved a dry EDM process with a thermoplastic composite electrode. Appl. Sci. 2020, 10, x FOR PEER REVIEW 30 of 45
Figure 15. 15. SelectiveSelective removal removal of ofmicro-EDM micro-EDM drilling: drilling: (a) steel (a) steelcylinder cylinder with holes; with holes;(b) 80-µm (b) micro- 80-µm micro-holehole [206]. [206].
Earlier studiesstudies alsoalso used used a a combination combination of of two two processes processes to improveto improve EDM EDM performance, performance, such such as Shu as andShu Tuand [212 Tu], [212], who studiedwho studied the eff ectthe of effect grinding of grinding and EDM and on EDM the response on the parameters,response parameters, and Ghoreishi and andGhoreishi Atkinson and [ 135Atkinson], who [135], compared who thecompared machining the characteristicsmachining characteristics in vibratory, in rotary, vibratory, and vibro-rotaryrotary, and EDM,vibro-rotary and studied EDM, theand e ffstudiedect of ultrasonic the effect vibration of ultrasonic and rotationvibration combination and rotation on combination the performance on the of theperformance EDM process. of the EDM process. In other studiesstudies [[214,215],214,215], photolithographyphotolithography technology and EDM werewere combined,combined, when processing other classes classes of of steel, steel, to to enhance enhance the the pr precisionecision in ingeometry geometry and and position, position, as well as well as the as thethroughput throughput of micro-EDM. of micro-EDM. Appl. Sci. 2020, 10, 2082 32 of 46
4.7. Research on Dielectric Fluid Many studies focused on dielectric fluids in terms of performance improvement and a reduction of the side effects on the process environment. Valaki and Rathod [74] explored the feasibility of using waste vegetable oil (WVO) as an alternative dielectric fluid. The study presented a comparison between WVO and other conventional dielectric fluids such as hydrocarbon oil and kerosene in terms of their effect on the EDM performance. The authors reported that the WVO-based bio-dielectric fluid could be used as an alternate to hydrocarbon-, water-, and synthetic-based dielectric fluids for EDM. Wu et al. [159] studied the effect of surfactant on the performance of the EDM process of SKD61 mold steel. Peak current, pulse duration, open voltage, and gap voltage were considered as process parameters. The study reported that the dispersed debris in dielectric fluid improved the effects of carbon accumulation and dreg discharge, while it reduced the unstable concentrated discharge. It was found that the particle agglomeration was reduced after surfactant molecules covered the surface of debris and carbon dregs in the kerosene solution. The authors reported that the addition of Span 20 (30 g/L) to the dielectric increased the conductivity of the dielectric and, therefore, the machining efficiency, as a result of the shorter relay time of electrical discharge. Furthermore, it was found that the MRR was improved by as much as 40%–80% with proper working parameters. Pecas and Henriques [130] studied EDM with a powder-mixed dielectric in terms of high-quality surface generation, and they compared the process performance with the conventional EDM process. It was found that the use of powder-mixed dielectric EDM reduced the SR, crater diameter, crater depth, and the white layer thickness. It was also found that the powder-mixed dielectric significantly reduced surface heterogeneity. Kansal et al. [115] studied the influence of adding silicon powder mixing into the dielectric fluid of EDM for AISI D2 die steel. The authors reported that peak current, concentration of the silicon powder, pulse-on time, pulse-off time, and gain significantly affected the MRR in powder-mixed EDM. Among all parameters, peak current and the concentration of silicon powder were the most significant parameters enhancing the MRR. Yih-Fong and Fu-Chen [155] studied the effect of additive properties on the surface of SKD-11 machined by EDM. The research experimentally studied the different thermo-physical properties of additives including aluminum, chromium, copper, and silicon carbide powders. It was found that the additive practical size in the dielectric fluid affected the surface quality of the machined surface. Furthermore, the SR improved as the particle size became smaller. However, the particle size had the opposite effect on the recast layer, whereby a smaller particle size resulted in a thicker recast layer of the EDM machined surface. The authors reported that the aluminum powder produced the best surface finish and the thinnest recast layer in the machined work, whereas the process without foreign particles and with copper powder generated the worst surface characteristics. Many other earlier studies also verified the influence of the powder-mixing on the EDM process performance for many grades of tool steels. For example, Kansal et al. [61] optimized the process parameters of powder-mixed EDM, Pecas and Henriques [129] studied the effect of silicon powder-mixed dielectric on the EDM of AISI H13 tool steel, Wong et al. [136] introduced a near-mirror-finish phenomenon in EDM using powder-mixed dielectric, and Wong et al. [76] studied the effect of flushing on the machined surface of AISI01 tool steel using EDM.
4.8. Other Bodies of Research on Tool Steel in EDM Several other studies focused on many issues related to the EDM of tool steels. For example, Sanchez et al. [112] studied the angular error in wire EDM taper-cutting. Sanchez et al. [108] presented a methodology for the calculation of gap variation between two consecutive stages that can be applied both in roughing and in finishing regimes. Moreover, Descoeudres et al. [169] introduced a systematic investigation of the effects of plasma parameters on optical emission spectra. Furthermore, Yu et al. [147] used wavelet transform to process the waveform of the EDM process. Finally, Kao and Tarng [144] described an ANN approach for on-line monitoring of the EDM process. Appl. Sci. 2020, 10, 2082 33 of 46
Appl. Sci. 2020, 10, x FOR PEER REVIEW 32 of 45 5. Discussion 5. Discussion After reviewing the research work related to tool steel machining using the EDM process, it can be After reviewing the research work related to tool steel machining using the EDM process, it can found that the majority of studies investigated the effect of the operating parameters on the performance be found that the majority of studies investigated the effect of the operating parameters on the parameters of MRR, EWR, and surface quality. Other bodies of research were conducted to investigate, performance parameters of MRR, EWR, and surface quality. Other bodies of research were conducted solve, or study other issues, such as the electrode shape and its movement, the effect of the EDM to investigate, solve, or study other issues, such as the electrode shape and its movement, the effect process on the tool steel properties and machined surface, as well as combined and hybrid processes, of the EDM process on the tool steel properties and machined surface, as well as combined and hybrid and the effect of various dielectric fluid used in the process, among others. Researchers paid more processes, and the effect of various dielectric fluid used in the process, among others. Researchers attention to the sinking EDM and micro-EDM processes to obtain optimal and near-optimal operating paid more attention to the sinking EDM and micro-EDM processes to obtain optimal and near- parameters, which may be attributed to the popularity of these two processes. Figure 16 shows optimal operating parameters, which may be attributed to the popularity of these two processes. the percentages of the EDM processes utilized for studying tool steel machining. Figure 16 shows the percentages of the EDM processes utilized for studying tool steel machining.
Figure 16. Percentage of research related to EDM processes.
6. Conclusions This review on the state-of-the-art studies of thethe EDM processes of tool steel led to the following conclusions:
• According to the general agreement of the results,results, the main factors influencinginfluencing the MRR of • didifferentfferent tooltool steelsteel gradesgrades inin EDMEDM areare thethe dischargedischarge currentcurrent andand thethe pulse-onpulse-on time.time. The gas pressurepressure andand electrode electrode rotation rotation speed speed also havealso ahave significant a significant influence oninfluence the MRR. on Furthermore, the MRR. theFurthermore, MRR can be the improved MRR can by be using improved an electrode by using material an electrode with high material electrical with conductivity. high electrical Using powder-mixedconductivity. Using EDM powder-mixed significantly aff EDMects the significantly MRR. affects the MRR. • According toto majormajor observations observations by by the the researchers, researchers, low low SR SR is achievedis achieved at lower at lower peak peak current current and • pulse-onand pulse-on duration. duration. Furthermore, Furthermore, the medium the medium value ofvalue peak of current, peak current, along with along minimum with minimum possible pulse-onpossible pulse-on time, can time, minimize can minimize surface crack surface density. crack The density. review The revealed review thatrevealed the SR that is increasedthe SR is withincreased higher with values higher of pulsed values current of pulsed and current pulse-on and time, pulse-on whereas time, better whereas surface better finish surface is achieved finish withis achieved lower current,with lower lower current, pulse-on lower time, pulse-on and relativelytime, and higherrelatively pulse-o higherff time.pulse-off Long-duration time. Long- pulsesduration cannot pulses meet cannot the machining meet the requirementsmachining requ duringirements finish during machining finish with machining high requirements with high inrequirements SR. Furthermore, in SR. applyingFurthermore, a magnetic applying field a magn leadsetic to an field improvement leads to an inimprovement surface quality. in surface Tablequality.4 shows the general e ffect of major operating parameters on key performance measures. • Table 4 shows the general effect of major operating parameters on key performance measures. Appl. Sci. 2020, 10, x FOR PEER REVIEW 33 of 45
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Researchersma requiresattention.terials; This is however, furtherneed Researchersanother to attention. thearea current need which Researchers to trend marequiresterials; is to needfurther inhowever,vestigate to attention. the the current potential Researchers trend of EDMis needto in forvestigate to machining the potentialnon- of EDM for machining non- conductive or semi-conductive materials,conductive such or semi-conductiveas ceramics. materials, such as ceramics. • Powder-Mix EDM: Powders• of Powder-Mixdifferent material EDM:s arePowders mixed of with different dielectrics material to improves are mixed the with dielectrics to improve the machining process. This is anothermachin areaing which process. requires This further is another attention. area which Researchers requires need further to attention. Researchers need to Appl. Sci. 2020, 10, 2082 35 of 46
Extending to a Wide Range of Workpiece Materials: EDM is primarily used for conductive • materials; however, the current trend is to investigate the potential of EDM for machining non-conductive or semi-conductive materials, such as ceramics. Powder-Mix EDM: Powders of different materials are mixed with dielectrics to improve • Appl. theSci. 2020 machining, 10, x FOR process.PEER REVIEW This is another area which requires further attention. Researchers34 of 45 need to pay more attention to the machining of different tool steel grades in different EDM typespay more under attention dielectric to fluidsthe machining with diff erentof different material tool powders. steel grades There inis different a lack of EDM studies types covering under thisdielectric point. fluids with different material powders. There is a lack of studies covering this point. • UseUse ofof DiDifferentfferent Electrodes:Electrodes: InvestigatorsInvestigators cancan examineexamine thethe performanceperformance ofof thethe EDMEDM processprocess byby • usingusing variousvarious electrodeelectrode materials,materials, shapes,shapes, sizes,sizes, andand geometries.geometries. TheThe useuse ofof tubulartubular electrodeselectrodes isis inin thethe initialinitial stages,stages, andand itit requiresrequires further further attention attention to to deliver deliver promising promising results. results. • HybridHybrid oror AssistedAssisted EDM:EDM: TheThe EDMEDM processprocess hybridizedhybridized withwith somesome otherother processesprocesses providesprovides • betterbetter results.results. MagneticMagnetic force-assistedforce-assisted EDM,EDM, laserlaser withwith EDM,EDM, etc.etc. areare becomingbecoming commonlycommonly usedused methodsmethods toto overcomeovercome processprocess limitations.limitations. TheThe greatgreat improvementimprovement inin thethe performanceperformance revealedrevealed inin thethe reviewedreviewed researchresearch waswas relatedrelated toto EDMEDM withwith ultrasonicultrasonic action.action. ResearchResearch trendstrends maymay bebe directeddirected towardtoward thethe combinationcombination ofof thesethese twotwo processes.processes. • ElectrodeElectrode Cooling Cooling Methods: Methods: The The electrode electrode cooling cooling mechanism mechanism represents represents another another field of research.field of • Theresearch. cryogenic The coolingcryogenic of electrodescooling of providedelectrodes positive provided results positive in terms results of ain reduction terms of ina reduction TWR. in TWR. Electrical Discharge Turning (EDT) and Dry EDM: EDT is a very new concept and it requires more •• Electrical Discharge Turning (EDT) and Dry EDM: EDT is a very new concept and it requires research. Dry EDM is also gaining interest in the research community. more research. Dry EDM is also gaining interest in the research community. Miniaturization: More efforts are needed to extend the limits of miniaturization in micro-EDM. A •• Miniaturization: More efforts are needed to extend the limits of miniaturization in micro-EDM. smaller level of electric discharge energy is needed to overcome this limitation. Furthermore, new A smaller level of electric discharge energy is needed to overcome this limitation. Furthermore, techniques to avoid the distortion of micro-workpieces are necessary in future research. new techniques to avoid the distortion of micro-workpieces are necessary in future research. Figure 17 shows a pictographic depiction of the future research directions. •• Figure 17 shows a pictographic depiction of the future research directions.
Optimize process variables Non- conductive Miniaturization materials machining
EDM Assisted machining Powder-mix Future (such as EDM Research ultrasonic, Directions magnetic etc)
Electrode ED Turning & materials and Dry EDM shape
Different dielectric fluids
FigureFigure 17.17. Future researchresearch areasareas inin thethe EDMEDM field.field.
• The research directions can be categorized into four broad classes, which can be further allocated into sub-groups, as shown in Figure 18. Appl. Sci. 2020, 10, 2082 36 of 46
The research directions can be categorized into four broad classes, which can be further allocated • Appl. intoSci. 2020 sub-groups,, 10, x FOR PEER as shown REVIEW in Figure 18. 35 of 45
FigureFigure 18.18. ClassificationClassification ofof researchresearch directions.directions.
Author Contributions: Conceptualization, J.E.A.Q.; methodology, J.E.A.Q. and M.H.A.; software, J.E.A.Q.; Authorvalidation, Contributions: J.E.A.Q., A.Z.Conceptualization, and M.H.A.; formal J.E.A.Q.; analysis, methodology, J.E.A.Q.; investigat J.E.A.Q.ion, and J.E.A.Q., M.H.A.; A. software, E. and A-H.I.M.; J.E.A.Q.; validation,resources, J.E.A.Q.,J.E.A.Q.; A.Z. data and curation, M.H.A.; formalJ.E.A.Q., analysis, A.E. J.E.A.Q.;and A.Z.; investigation, writing—original J.E.A.Q., draft A.E. and preparation, A-H.I.M.; resources,J.E.A.Q.; J.E.A.Q.;writing—review data curation, and editing, J.E.A.Q., J.E.A.Q., A.E. and A.E. A.Z.; and writing—original A-H.I.M.; visualization, draft preparation, J.E.A.Q. and J.E.A.Q.; A.E.; supervision, writing—review J.E.A.Q.; and editing, J.E.A.Q., A.E. and A-H.I.M.; visualization, J.E.A.Q. and A.E.; supervision, J.E.A.Q.; project administration, J.E.A.Q.;project administration, funding acquisition, J.E.A.Q.; J.E.A.Q. funding All authors acquisition, have J.E.A.Q. read and agreed to the published version of the manuscript. Funding:Funding: ThisThis researchresearch waswas fundedfunded byby thethe ResearchResearch AAffairsffairs OOfficeffice at at UAE UAE University, University, grant grant number number 31N396. 31N396. ConflictsConflicts ofof Interest:Interest:The The authorsauthors declaredeclare nono conflictsconflicts of of interest. interest.
Abbreviations
Ton Pulse-on time HF High-frequency vibration Toff Pulse-off time EM Electrode material Machining condition “rough, medium, I Current MC or soft” Is Spark current GG Graphite grade Ip Peak current DE Discharge energy WT Wire tension IN Intensity DP Dielectric flushing pressure PCH Phase change FR Flushing rate GN Gain NF Nozzle flushing PCON Concentration of powder DL Dielectric level GC Gap control PD Pulse duration CR Cracks PI Pulse interval RS Residual stresses P Polarity RE Removal efficiency PoW Polarity of workpiece D_plas Plasma diameter V No-load voltage H Magnetic field intensity Vs Servo voltage r1/r2 Circularity of machined component Vp Peak voltage PFE Plasma flushing efficiency Vg Gap voltage T Achievable processing thickness G Gap distance µH Micro-hardness of the machined surface IH High-voltage auxiliary current S_green Sensitivity of green manufacturing Appl. Sci. 2020, 10, 2082 37 of 46
D_tool Tool diameter T_WL White layer thickness τ Duty factor (cycle) T_RL Recast layer thickness C Capacitance Avg_SR Average surface roughness Ratio of energy lost due to heat Temperature distribution for RE TD conduction within the workpiece powder-mixed EDM w Rotational speed ER Erosion rate S Servo speed VWR Volumetric relative wear P Power MS Metallurgical structure S-wire Wire speed D_over Diametrical overcut F Cutting speed Avg_CS Average cutting speed A Amplitudes of ultrasonic G-InI Geometrical inaccuracy f Low-frequency vibration G_mod Generator mode LF Low-frequency vibration σres Residual stress
References
1. Bilal, A.; Jahan, M.P.; Talamona, D.; Perveen, A. Electro-discharge machining of ceramics: A review. Micromachines 2019, 10, 10. [CrossRef] 2. Qudeiri, J.E.A.; Mourad, A.-H.I.; Ziout, A.; Abidi, M.H.; Elkaseer, A. Electric discharge machining of titanium and its alloys. Int. J. Adv. Manuf. Technol. 2018, 96, 1319–1339. [CrossRef] 3. Pérez, J.; Llorente, J.I.; Sanchez, J.A. Advanced cutting conditions for the milling of aeronautical alloys. J. Mater. Process. Technol. 2000, 100, 1–11. 4. Komanduri, R.; Hou, Z.-B. On thermoplastic shear instability in the machining of a titanium alloy (Ti-6Al-4V). Met. Mater. Trans. A 2002, 33, 2995. [CrossRef] 5. Cascón, I.; Sarasua, J.A.; Elkaseer, A. Tailored Chip Breaker Development for Polycrystalline Diamond Inserts: FEM-Based Design and Validation. Appl. Sci. 2019, 9, 4117. [CrossRef] 6. Elkaseer, A.; Abdelaziz, A.; Saber, M.; Nassef, A. FEM-Based Study of Precision Hard Turning of Stainless Steel 316L. Materials 2019, 12, 2522. [CrossRef] 7. Benes, J. Cutting Difficult-to-Machine Materials. Available online: https://www.americanmachinist. com/archive/cutting-tool-digest/article/21892040/cutting-difficulttomachine-materials (accessed on 27 February 2020). 8. Abu Qudeiri, J.E.; Saleh, A.; Ziout, A.; Mourad, A.-H.I.; Abidi, M.H.; Elkaseer, A. Advanced electric discharge machining of stainless steels: Assessment of the state of the art, gaps and future prospect. Materials 2019, 12, 907. [CrossRef] 9. Elkaseer, A.; Lambarri, J.; Ander Sarasua, J.; Cascón, I. On the development of a chip breaker in a metal-matrix polycrystalline diamond insert: Finite element based design with ns-laser ablation and machining verification. J. Micro Nano-Manuf. 2017, 5.[CrossRef] 10. Huyett, G.L. Engineering Handbook, Technical Information; Huyett: Minneapolis, KS, USA, 2000. 11. Elkaseer, A.M.; Dimov, S.S.; Popov, K.B.; Minev, R.M. Tool Wear in Micro-Endmilling: Material Microstructure Effects, Modeling, and Experimental Validation. J. Micro Nano-Manuf. 2014, 2.[CrossRef] 12. Lo, K.H.; Shek, C.H.; Lai, J.K.L. Recent developments in stainless steels. MSR Mater. Sci. Eng. R 2009, 65, 39–104. [CrossRef] 13. Elkaseer, A.M.; Popov, K.B.; Dimov, S.S.; Minev, R. Material microstructure effect-based investigation of tool wear in micro-endmilling of multi-phase materials. In Proceedings of the 7th International Conference on Multi-Material Micro Manufacturing, Bourg en Bresse and Oyonnax, France, 17–19 November 2010; pp. 188–191. 14. Elsanabary,S.; Elkaseer, A.; Abd-Rabbo, S.; AbdElsalam, M.; Abdou, S. Modelling and experimental validation of surface roughness in precision turning of dual-phase materials considering process uncertainties. Int. J. Interact. Des. Manuf. 2019, 13, 59–74. [CrossRef] 15. Lee, S.H.; Li, X.P. Study of the effect of machining parameters on the machining characteristics in electrical discharge machining of tungsten carbide. J. Mater. Process. Technol. 2001, 115, 344–358. [CrossRef] 16. Yu, Z.; Jun, T.; Masanori, K. Dry electrical discharge machining of cemented carbide. J. Mater. Process. Technol. 2004, 149, 353–357. [CrossRef] Appl. Sci. 2020, 10, 2082 38 of 46
17. Hewidy, M.S.; El-Taweel, T.A.; El-Safty, M.F. Modelling the machining parameters of wire electrical discharge machining of Inconel 601 using RSM. J. Mater. Process. Technol. 2005, 169, 328–336. [CrossRef] 18. Chiang, K.-T.; Chang, F.-P. Optimization of the WEDM process of particle-reinforced material with multiple performance characteristics using grey relational analysis. J. Mater. Process. Technol. 2006, 180, 96–101. [CrossRef] 19. Surleraux, A.; Pernot, J.-P.; Elkaseer, A.; Bigot, S. Iterative surface warping to shape craters in micro-EDM simulation. Eng. Comput. 2016, 32, 517–531. [CrossRef] 20. Lim, H.S.; Wong, Y.S.; Rahman, M.; Lee, M.K.E. A study on the machining of high-aspect ratio micro-structures using micro-EDM. J. Mater. Process. Technol. 2003, 140, 318–325. [CrossRef] 21. Tzeng, Y.; Chen, F. A simple approach for robust design of high-speed electrical-discharge machining technology. Int. J. Mach. Tools Manuf. 2003, 43, 217–227. [CrossRef] 22. Ho, K.H.; Newman, S.T.; Rahimifard, S.; Allen, R.D. State of the art in wire electrical discharge machining (WEDM). Int. J. Mach. Tools Manuf. 2004, 44, 1247–1259. [CrossRef] 23. Singh, S.; Maheshwari, S.; Pandey, P.C. Some investigations into the electric discharge machining of hardened tool steel using different electrode materials. J. Mater. Process. Technol. 2004, 149, 272–277. [CrossRef] 24. Abbas, N.M.; Solomon, D.G.; Bahari, M.F. A review on current research trends in electrical discharge machining (EDM). Int. J. Mach. Tools Manuf. 2007, 47, 1214–1228. [CrossRef] 25. Hsieh, M.-F.; Tung, C.-J.; Yao, W.-S.; Wu, M.-C.; Liao, Y.-S. Servo design of a vertical axis drive using dual linear motors for high speed electric discharge machining. Int. J. Mach. Tools Manuf. 2007, 47, 546–554. [CrossRef] 26. McCown, B.H.; McCabe, D.E.; Russell, D.R.; Robison, D.J.; Barton, K.A.; Raffa, K.F. Stable transformation of Populus and incorporation of pest resistance by electric discharge particle acceleration. Plant Cell Rep. 1991, 9, 590–594. [CrossRef][PubMed] 27. Yan, B.H.; Huang, F.Y.; Chow, H.M.; Tsai, J.Y. Micro-hole machining of carbide by electric discharge machining. J. Mater. Process. Technol. 1999, 87, 139–145. [CrossRef] 28. Kansal, H.K.; Singh, S.; Kumar, P. Technology and research developments in powder mixed electric discharge machining (PMEDM). J. Mater. Process. Technol. 2007, 184, 32–41. [CrossRef] 29. Gaikwad, M.U.; Krishnamoorthy, A.; Jatti, V.S. Estimation of Surface Integrity Parameters in Electrical Discharge Machining (EDM) Process—A Review. In Techno-Societal 2018; Springer: Cham, Switzerland, 2020; pp. 601–614. 30. Marelli, D.; Singh, S.K.; Nagari, S.; Subbiah, R. Optimisation of machining parameters of wire-cut EDM on super alloy materials–A review. Mater. Today Proc. 2020, in press. [CrossRef] 31. Bhatia, K.; Singla, A.; Sharma, A.; Sengar, S.S.; Selokar, A. A Review on Different Dielectric Fluids and
Machining of Si3N4 and Al2O3 Composites via EDM. In Advances in Industrial and Production Engineering; Springer: Singapore, 2019; pp. 585–596. 32. Boral, S.; Sidhu, S.S.; Chatterjee, P.; Chakraborty, S.; Gugaliya, A. Multi-response Optimization of Micro-EDM Processes: A State-of-the-Art Review. In Micro-Electrical Discharge Machining Processes; Springer: Singapore, 2019; pp. 293–310. 33. Czelusniak, T.; Higa, C.F.; Torres, R.D.; Laurindo, C.A.H.; de Paiva Júnior, J.M.F.; Lohrengel, A.; Amorim, F.L. Materials used for sinking EDM electrodes: A review. J. Braz. Soc. Mech. Sci. Eng. 2019, 41, 14. [CrossRef] 34. Maity, K.P.; Choubey, M. A review on vibration-assisted EDM, micro-EDM and WEDM. Surf. Rev. Lett. 2019, 26, 1830008. [CrossRef] 35. Pant, P.; Bharti, P.S. Electrical Discharge Machining (EDM) of nickel-based nimonic alloys: A review. Mater. Today Proc. 2019, in press. [CrossRef] 36. Barenji, R.V.; Pourasl, H.H.; Khojastehnezhad, V.M. Electrical discharge machining of the AISI D6 tool steel: Prediction and modeling of the material removal rate and tool wear ratio. Precis. Eng. 2016, 45, 435–444. [CrossRef] 37. Long, B.T.; Phan, N.H.; Cuong, N.; Toan, N.D. Surface quality analysis of die steels in powder-mixed electrical discharge machining using titan powder in fine machining. Adv. Mech. Eng. 2016, 8, 1–13. [CrossRef] 38. Shabgard, M.R.; Gholipoor, A.; Hatami, M. Wire electrical discharge machining of ASP30 tool steel. J. Mech. Sci. Technol. 2016, 30, 3799–3804. [CrossRef] 39. Priyadarshini, M.; Biswas, C.K.; Behera, A. Wire Electro Discharge Machining of P20 Tool Steel. Mater. Today Proc. 2018, 5, 25519–25523. [CrossRef] Appl. Sci. 2020, 10, 2082 39 of 46
40. Sharma, A.; Sinha, A.K. Rotary electric discharge machining of AISI D2 tool steel: Present and future scope. Mater. Today Proc. 2018, 5, 18562–18567. [CrossRef] 41. Bahgat, M.M.; Shash, A.Y.; Abd-Rabou, M.; El-Mahallawi, I.S. Influence of process parameters in electrical discharge machining on H13 die steel. Heliyon 2019, 5, e01813. [CrossRef] 42. Gopal, R.; Thangadurai, K.R.; Thirunavukkarasu, K. Behavior of ECAP processed copper electrodes in electrical discharge machining of AISI H13 steel. Mater. Today Proc. 2020, 21, 295–298. [CrossRef] 43. Dauw, D.F.; Snoeys, R.; Dekeyser, W. Advanced pulse discriminating system for EDM process analysis and control. CIRP Ann. 1983, 32, 541–549. [CrossRef] 44. Gao, C.; Liu, Z. A study of ultrasonically aided micro-electrical-discharge machining by the application of workpiece vibration. J. Mater. Process. Technol. 2003, 139, 226–228. [CrossRef] 45. Marafona, J.; Chousal, J.A.G. A finite element model of EDM based on the Joule effect. Int. J. Mach. Tools Manuf. 2006, 46, 595–602. [CrossRef] 46. Gostimirovic, M.; Kovac, P.; Skoric, B.; Sekulic, M. Effect of Electrical Pulse Parameters on the Machining Performance in EDM. IJEMS 2011, 18, 411–415. 47. Ho, K.H.; Newman, S.T. State of the art electrical discharge machining (EDM). Int. J. Mach. Tools Manuf. 2003, 43, 1287–1300. [CrossRef] 48. Cui, J.; Chu, Z. Composite motion design procedure for vibration assisted small-hole EDM using one voice coil motor. Shock. Vib. 2016, 4179296. [CrossRef] 49. Ikonomou, M.G.; Blades, A.T.; Kebarle, P. Electrospray mass spectrometry of methanol and water solutions suppression of electric discharge with SF 6 gas. J. Am. Soc. Mass Spectrom. 1991, 2, 497–505. [CrossRef] 50. Anpilov, A.M.; Barkhudarov, E.M.; Bark, Y.B.; Zadiraka, Y.V.; Christofi, M.; Kozlov, Y.N.; Kossyi, I.A.; Kop’ev, V.A.; Silakov, V.P.; Taktakishvili, M.I. Electric discharge in water as a source of UV radiation, ozone and hydrogen peroxide. J. Phys. D Appl. Phys. 2001, 34, 993. [CrossRef] 51. Grigoryev, E.G.; Olevsky, E.A. Thermal processes during high-voltage electric discharge consolidation of powder materials. Scr. Mater. 2012, 66, 662–665. [CrossRef] 52. Surleraux, A.; Lepert, R.; Pernot, J.; Kerfriden, P.; Bigot, S. Machine Learning-Based Reverse Modeling Approach for Rapid Tool Shape Optimization in Die-Sinking Micro Electro Discharge Machining. ASME. J. Comput. Inf. Sci. Eng. 2020, 20.[CrossRef] 53. Liu, Y.H.; Guo, Y.F.; Liu, J.C. Electric discharge milling of polycrystalline diamond. Proc. Inst. Mech. Eng. Part B J. Eng. Manuf. 1997, 211, 643–647. [CrossRef] 54. Lin, J.L.; Wang, K.S.; Yan, B.H.; Tarng, Y.S. Optimization of the electrical discharge machining process based on the Taguchi method with fuzzy logics. J. Mater. Process. Technol. 2000, 102, 48–55. [CrossRef] 55. Kuppan, P.; Rajadurai, A.; Narayanan, S. Influence of EDM process parameters in deep hole drilling of Inconel 718. Int. J. Adv. Manuf. Technol. 2008, 38, 74–84. [CrossRef] 56. Lajis, M.A.; Radzi, H.; Amin, A. The implementation of Taguchi method on EDM process of tungsten carbide. Eur. J. Sci. Res. 2009, 26, 609–617. 57. Singh, H.; Garg, R. Effects of process parameters on material removal rate in WEDM. J. Achiev. Mater. Manuf. Eng. 2009, 32, 70–74. 58. Lin, C.L.; Lin, J.L.; Ko, T.C. Optimisation of the EDM process based on the orthogonal array with fuzzy logic and grey relational analysis method. Int. J. Adv. Manuf. Technol. 2002, 19, 271–277. [CrossRef] 59. Bhattacharyya, B.; Munda, J. Experimental investigation on the influence of electrochemical machining parameters on machining rate and accuracy in micromachining domain. Int. J. Mach. Tools Manuf. 2003, 43, 1301–1310. [CrossRef] 60. Ramakrishnan, R.; Karunamoorthy,L. Modeling and multi-response optimization of Inconel 718 on machining of CNC WEDM process. J. Mater. Process. Technol. 2008, 207, 343–349. [CrossRef] 61. Kansal, H.K.; Singh, S.; Kumar, P. Parametric optimization of powder mixed electrical discharge machining by response surface methodology. J. Mater. Process. Technol. 2005, 169, 427–436. [CrossRef] 62. Liu, Y.; Chang, H.; Zhang, W.; Ma, F.; Sha, Z.; Zhang, S. A Simulation Study of Debris Removal Process in Ultrasonic Vibration Assisted Electrical Discharge Machining (EDM) of Deep Holes. Micromachines 2018, 9, 378. [CrossRef] 63. Kumar, S.; Singh, R.; Singh, T.P.; Sethi, B.L. Surface modification by electrical discharge machining: A review. J. Mater. Process. Technol. 2009, 209, 3675–3687. [CrossRef] Appl. Sci. 2020, 10, 2082 40 of 46
64. Jahan, M.P.; Rahman, M.; Wong, Y.S. A review on the conventional and micro-electrodischarge machining of tungsten carbide. Int. J. Mach. Tools Manuf. 2011, 51, 837–858. [CrossRef] 65. Rajurkar, K.P.; Wang, W.M. Improvement of EDM Performance with Advanced Monitoring and Control Systems. J. Mech. Des. 1997, 119, 770. [CrossRef] 66. Fuller, J.E. Electrical discharge machining. ASM Handb. 1989, 16, 557–564. 67. De Bruyn, H.E. Slope Control: A great improvement in spark erosion. Ann. CIRP 1967, 16, 183–191. 68. Mohan, B.; Rajadurai, A.; Satyanarayana, K.G. Effect of SiC and rotation of electrode on electric discharge machining of Al–SiC composite. J. Mater. Process. Technol. 2002, 124, 297–304. [CrossRef] 69. Mohan, B.; Rajadurai, A.; Satyanarayana, K.G. Electric discharge machining of Al–SiC metal matrix composites using rotary tube electrode. J. Mater. Process. Technol. 2004, 153, 978–985. [CrossRef] 70. Crookall, J.R.; Heuvelman, C.J. Electro-discharge machining—the state of the art. Ann. CIRP 1971, 20, 113–120. 71. Goiogana, M.; Elkaseer, A. Self-Flushing in EDM Drilling of Ti6Al4V Using Rotating Shaped Electrodes. Materials 2019, 12, 989. [CrossRef] 72. Almacinha, J.A.; Lopes, A.M.; Rosa, P.; Marafona, J.D. How hydrogen dielectric strength forces the work voltage in the electric discharge machining. Micromachines 2018, 9, 240. [CrossRef] 73. Tsai, Y.Y.; Tseng, C.H.; Chang, C.K. Development of a combined machining method using electrorheological fluids for EDM. J. Mater. Process. Technol. 2008, 201, 565–569. [CrossRef] 74. Valaki, J.B.; Rathod, P.P. Assessment of operational feasibility of waste vegetable oil based bio-dielectric fluid for sustainable electric discharge machining (EDM). Int. J. Adv. Manuf. Technol. 2016, 87, 1509–1518. [CrossRef] 75. Shen, Y.; Liu, Y.; Zhang, Y.; Dong, H.; Sun, W.; Wang, X.; Zheng, C.; Ji, R. High-speed dry electrical discharge machining. Int. J. Mach. Tools Manuf. 2015, 93, 19–25. [CrossRef] 76. Wong, Y.S.; Lim, L.C.; Lee, L.C. Effects of flushing on electro-discharge machined surfaces. J. Mater. Process. Technol. 1995, 48, 299–305. [CrossRef] 77. Lonardo, P.M.; Bruzzone, A.A. Effect of flushing and electrode material on die sinking EDM. CIRP Annals. 1999, 48, 123–126. [CrossRef] 78. Fujiki, M.; Ni, J.; Shih, A.J. Investigation of the effects of electrode orientation and fluid flow rate in near-dry EDM milling. Int. J. Mach. Tools Manuf. 2009, 49, 749–758. [CrossRef] 79. Soni, J.S.; Chakraverti, G. Machining characteristics of titanium with rotary electro-discharge machining. Wear 1994, 171, 51–58. [CrossRef] 80. Guu, Y.H.; Hocheng, H. Effects of workpiece rotation on machinability during electrical-discharge machining. Mater. Manuf. Process. 2001, 16, 91–101. [CrossRef] 81. Kagaya, K.; Oishi,¯ Y.; Yada, K. Micro-electrodischarge machining using water as a working fluid—I: Micro-hole drilling. Precis. Eng. 1986, 8, 157–162. [CrossRef] 82. Sato, T.; Mizutani, T.; Yonemochi, K.; Kawata, K. The development of an electrodischarge machine for micro-hole boring. Precis. Eng. 1986, 8, 163–168. [CrossRef] 83. Yan, B.H.; Wang, C.C.; Liu, W.D.; Huang, F.Y. Machining characteristics of Al2O3/6061Al composite using rotary EDM with a disklike electrode. Int. J. Adv. Manuf. Technol. 2000, 16, 322–333. [CrossRef] 84. Pandit, S.M.; Mueller, T.M. Verification of on-line computer control of EDM by data dependent systems. J. Eng. Ind. 1987, 109, 117–121. [CrossRef] 85. Marafona, J.; Wykes, C. A new method of optimising material removal rate using EDM with copper–tungsten electrodes. Int. J. Mach. Tools Manuf. 2000, 40, 153–164. [CrossRef] 86. Su, J.C.; Kao, J.Y.; Tarng, Y.S. Optimisation of the electrical discharge machining process using a GA-based neural network. Int. J. Adv. Manuf. Technol. 2004, 24, 81–90. [CrossRef] 87. Erden, A. Effect of materials on the mechanism of electric discharge machining (EDM). J. Eng. Mater. Technol. 1983, 105, 132–138. [CrossRef] 88. Kaneko, T.; Tsuchiya, M. Three-dimensionally controlled EDM Using a cylindrical electrode (5th report). J. Jpn. Soc. Electr. Mach. Eng. 1985, 18, 1–16. 89. Saito, K.; Kishinami, T.; Konno, H.; Sato, M.; Takeyama, H. Development of numerical contouring control electric discharge machining (NCC-EDM). CIRP Ann. 1986, 35, 117–120. [CrossRef] 90. Kruth, J.-P.; Stevens, L.; Froyen, L.; Lauwers, B. Study of the white layer of a surface machined by die-sinking electro-discharge machining. CIRP Ann. 1995, 44, 169–172. [CrossRef] Appl. Sci. 2020, 10, 2082 41 of 46
91. Soni, J.S.; Chakraverti, G. Experimental investigation on migration of material during EDM of die steel (T215 Cr12). J. Mater. Process. Technol. 1996, 56, 439–451. [CrossRef] 92. Staelens, F.; Kruth, J.-P. A computer integrated machining strategy for planetary EDM. CIRP Ann. 1989, 38, 187–190. [CrossRef] 93. Mohri, N.; Suzuki, M.; Furuya, M.; Saito, N.; Kobayashi, A. Electrode wear process in electrical discharge machinings. CIRP Ann. 1995, 44, 165–168. [CrossRef] 94. Schumacher, B.M. EDM technology for precision workpieces with excellent surface quality. Proc. ISEM-7 1983, 124–135. 95. Sun, B.; Sato, M.; Clements, J.S. Use of a pulsed high-voltage discharge for removal of organic compounds in aqueous solution. J. Phys. D Appl. Phys. 1999, 32.[CrossRef] 96. Bigot, S.; Pernot, J.-P.; Surleraux, A.; Elkaseer, A. Micro-EDM numerical simulation and experimental validation. In Proceedings of the 10th International Conference on Multi-Material Micro Manufacture, San Sebastian, Spain, 8–10 October 2013; pp. 55–58. 97. Zhao, W.S.; Meng, Q.G.; Wang, Z.L. The application of research on powder mixed EDM in rough machining. J. Mater. Process. Technol. 2002, 129, 30–33. [CrossRef] 98. Furutania, K.; Saneto, A.; Takezawa, H.; Mohri, N.; Miyake, H. Accretion of titanium carbide by electrical discharge machining with powder suspended in working fluid. Precis. Eng. 2001, 25, 138–144. [CrossRef] 99. Kunleda, M.; Miyoshi, Y.; Takaya, T.; Nakajima, N.; ZhanBo, Y.; Yoshida, M. High speed 3D milling by dry EDM. CIRP Ann. 2003, 52, 147–150. [CrossRef] 100. YU, Z.; TAKAHASHI, J.; NAKAJIMA, N.; SANO, S.; KARATO, K.; KUNIEDA, M. Feasibility of 3-D surface machining by dry EDM. Int. J. Electr. Mach. 2005, 10, 15–20. 101. Joshi, S.; Govindan, P.; Malshe, A.; Rajurkar, K. Experimental characterization of dry EDM performed in a pulsating magnetic field. CIRP Ann. 2011, 60, 239–242. [CrossRef] 102. Kunieda, M. Study on process reaction force generated by discharge in EDM. Proc. MMSS 2000, 313, 313–324. 103. Kunieda, M.; Takaya, T.; Nakano, S. Improvement of dry EDM characteristics using piezoelectric actuator. CIRP Ann. 2004, 53, 183–186. [CrossRef] 104. Guu, Y.H. AFM surface imaging of AISI D2 tool steel machined by the EDM process. Appl. Surf. Sci. 2005, 242, 245–250. [CrossRef] 105. Guu, Y.H.; Hocheng, H.; Chou, C.Y.; Deng, C.S. Effect of electrical discharge machining on surface characteristics and machining damage of AISI D2 tool steel. Mater. Sci. Eng. A 2003, 358, 37–43. [CrossRef] 106. Izquierdo, B.; Sanchez, J.A.; Plaza, S.; Pombo, I.; Ortega, N. A numerical model of the EDM process considering the effect of multiple discharges. Int. J. Mach. Tools Manuf. 2009, 49, 220–229. [CrossRef] 107. Sahu, J.; Mohanty, C.P.; Mahapatra, S.S. A DEA approach for optimization of multiple responses in electrical discharge machining of AISI D2 steel. Procedia Eng. 2013, 51, 585–591. [CrossRef] 108. Sanchez, J.A.; de Lacalle, L.N.L.; Lamikiz, A.; Bravo, U. Study on gap variation in multi-stage planetary EDM. Int. J. Mach. Tools Manuf. 2006, 46, 1598–1603. [CrossRef] 109. Tsai, K.-M.; Wang, P.-J. Semi-empirical model of surface finish on electrical discharge machining. Int. J. Mach. Tools Manuf. 2001, 41, 1455–1477. [CrossRef] 110. Wang, P.-J.; Tsai, K.-M. Semi-empirical model on work removal and tool wear in electrical discharge machining. J. Mater. Process. Technol. 2001, 114, 1–17. [CrossRef] 111. Cabanes, I.; Portillo, E.; Marcos, M.; Sánchez, J.A. An industrial application for on-line detection of instability and wire breakage in wire EDM. J. Mater. Process. Technol. 2008, 195, 101–109. [CrossRef] 112. Sanchez, J.A.; Plaza, S.; Ortega, N.; Marcos, M.; Albizuri, J. Experimental and numerical study of angular error in wire-EDM taper-cutting. Int. J. Mach. Tools Manuf. 2008, 48, 1420–1428. [CrossRef] 113. Williams, R.E.; Rajurkar, K.P. Study of wire electrical discharge machined surface characteristics. J. Mater. Process. Technol. 1991, 28, 127–138. [CrossRef] 114. Kansal, H.K.; Singh, S.; Kumar, P. Numerical simulation of powder mixed electric discharge machining (PMEDM) using finite element method. Math. Comput. Model. 2008, 47, 1217–1237. [CrossRef] 115. Kansal, H.K.; Singh, S.; Kumar, P. Effect of silicon powder mixed EDM on machining rate of AISI D2 die steel. J. Manuf. Process. 2007, 9, 13–22. [CrossRef] 116. Kumar, S.; Batra, U. Surface modification of die steel materials by EDM method using tungsten powder-mixed dielectric. J. Manuf. Process. 2012, 14, 35–40. [CrossRef] Appl. Sci. 2020, 10, 2082 42 of 46
117. Anonymous D2 Steel Properties. Available online: https://www.steelexpress.co.uk/toolsteel/D2-Steel- properties.html (accessed on 27 February 2020). 118. Balasubramanian, P.; Senthilvelan, T. Optimization of machining parameters in EDM process using cast and sintered copper electrodes. Procedia Mater. Sci. 2014, 6, 1292–1302. [CrossRef] 119. Haddad, M.J.; Tehrani, A.F. Material removal rate (MRR) study in the cylindrical wire electrical discharge turning (CWEDT) process. J. Mater. Process. Technol. 2008, 199, 369–378. [CrossRef] 120. Anonymous D3 Tool Steel Properties. Available online: https://www.steelexpress.co.uk/toolsteel/D3-Steel- properties.html (accessed on 27 February 2020). 121. Çayda¸s,U.; Hasçalık, A.; Ekici, S. An adaptive neuro-fuzzy inference system (ANFIS) model for wire-EDM. Expert Syst. Appl. 2009, 36, 6135–6139. [CrossRef] 122. Hasçalýk, A.; Çayda¸s,U. Experimental study of wire electrical discharge machining of AISI D5 tool steel. J. Mater. Process. Technol. 2004, 148, 362–367. [CrossRef] 123. AZoM. D5 Tool Steel-High-Carbon, High-Chromium, Cold-Work Steels; 2012; Available online: https://www. azom.com/article.aspx?ArticleID=6217 (accessed on 27 February 2020). 124. Zeid, O.A.A. On the effect of electrodischarge machining parameters on the fatigue life of AISI D6 tool steel. In Current Advances in Mechanical Design and Production VI, Pergamon; Elsevier: Amsterdam, The Netherlands, 1995; pp. 81–89. 125. AZoM. Grade D6 Tool Steel (UNS T30406); 2013; Available online: https://www.azom.com/article.aspx? ArticleID=9139 (accessed on 27 February 2020). 126. Shabgard, M.; Ahmadi, R.; Seyedzavvar, M.; Oliaei, S.N.B. Mathematical and numerical modeling of the effect of input-parameters on the flushing efficiency of plasma channel in EDM process. Int. J. Mach. Tools Manuf. 2013, 65, 79–87. [CrossRef] 127. Fujiki, M.; Kim, G.-Y.; Ni, J.; Shih, A.J. Gap control for near-dry EDM milling with lead angle. Int. J. Mach. Tools Manuf. 2011, 51, 77–83. [CrossRef] 128. Mohri, N.; Saito, N.; Higashi, M.; Kinoshita, N. A new process of finish machining on free surface by EDM methods. CIRP Ann. 1991, 40, 207–210. [CrossRef] 129. Pecas, P.; Henriques, E. Influence of silicon powder-mixed dielectric on conventional electrical discharge machining. Int. J. Mach. Tools Manuf. 2003, 43, 1465–1471. [CrossRef] 130. Pecas, P.; Henriques, E. Electrical discharge machining using simple and powder-mixed dielectric: The effect of the electrode area in the surface roughness and topography. J. Mater. Process. Technol. 2008, 200, 250–258. [CrossRef] 131. Simao, J.; Lee, H.G.; Aspinwall, D.K.; Dewes, R.C.; Aspinwall, E.M. Workpiece surface modification using electrical discharge machining. Int. J. Mach. Tools Manuf. 2003, 43, 121–128. [CrossRef] 132. Maradia, U.; Boccadoro, M.; Stirnimann, J.; Beltrami, I.; Kuster, F.; Wegener, K. Die-sink EDM in meso-micro machining. Procedia CIRP 2012, 1, 166–171. [CrossRef] 133. AZoM. H11 Tool Steel-Chromium Hot-Work Steels (UNS T20811); 2012; Available online: https://www.azom. com/article.aspx?ArticleID=6209 (accessed on 27 February 2020). 134. Navas, V.G.; Ferreres, I.; Marañón, J.A.; Garcia-Rosales, C.; Sevillano, J.G. Electro-discharge machining (EDM) versus hard turning and grinding—Comparison of residual stresses and surface integrity generated in AISI O1 tool steel. J. Mater. Process. Technol. 2008, 195, 186–194. [CrossRef] 135. Ghoreishi, M.; Atkinson, J. A comparative experimental study of machining characteristics in vibratory, rotary and vibro-rotary electro-discharge machining. J. Mater. Process. Technol. 2002, 120, 374–384. [CrossRef] 136. Wong, Y.S.; Lim, L.C.; Rahuman, I.; Tee, W.M. Near-mirror-finish phenomenon in EDM using powder-mixed dielectric. J. Mater. Process. Technol. 1998, 79, 30–40. [CrossRef] 137. Anonymous O1 Tool Steel. Available online: http://www.astmsteel.com/product/astm-o1-tool-steel (accessed on 27 February 2020). 138. Anonymous O2 Tool Steel. Available online: http://www.westyorkssteel.com/tool-steel/o2 (accessed on 27 February 2020). 139. Aich, U.; Banerjee, S. Modeling of EDM responses by support vector machine regression with parameters selected by particle swarm optimization. Appl. Math. Model. 2014, 38, 2800–2818. [CrossRef] 140. Bhattacharyya, B.; Gangopadhyay, S.; Sarkar, B.R. Modelling and analysis of EDMed job surface integrity. J. Mater. Process. Technol. 2007, 189, 169–177. [CrossRef] Appl. Sci. 2020, 10, 2082 43 of 46
141. Srivastava, V.; Pandey, P.M. Effect of process parameters on the performance of EDM process with ultrasonic assisted cryogenically cooled electrode. J. Manuf. Process. 2012, 14, 393–402. [CrossRef] 142. Puri, A.B.; Bhattacharyya, B. An analysis and optimisation of the geometrical inaccuracy due to wire lag phenomenon in WEDM. Int. J. Mach. tools Manuf. 2003, 43, 151–159. [CrossRef] 143. Anonymous M2 Tool Steel. Available online: http://www.astmsteel.com/product/m2-tool-steel-1-3343-hs-6- 5-2c-skh51/ (accessed on 27 February 2020). 144. Kao, J.Y.; Tarng, Y.S. A neutral-network approach for the on-line monitoring of the electrical discharge machining process. J. Mater. Process. Technol. 1997, 69, 112–119. [CrossRef] 145. Lee, H.-T.; Rehbach, W.P.; Hsu, F.-C.; Tai, T.-Y.; Hsu, E. The study of EDM hole-drilling method for measuring residual stress in SKD11 tool steel. J. Mater. Process. Technol. 2004, 149, 88–93. [CrossRef] 146. Tai, T.Y.; Lu, S.J. Improving the fatigue life of electro-discharge-machined SDK11 tool steel via the suppression of surface cracks. Int. J. Fatigue 2009, 31, 433–438. [CrossRef] 147. Yu, S.F.; Lee, B.Y.; Lin, W.S. Waveform monitoring of electric discharge machining by wavelet transform. Int. J. Adv. Manuf. Technol. 2001, 17, 339–343. [CrossRef] 148. Cao, C.; Zhang, X.; Zha, X.; Dong, C. Surface integrity of tool steels multi-cut by wire electrical discharge machining. Procedia Eng. 2014, 81, 1945–1951. [CrossRef] 149. Choi, K.-K.; Nam, W.-J.; Lee, Y.-S. Effects of heat treatment on the surface of a die steel STD11 machined by W-EDM. J. Mater. Process. Technol. 2008, 201, 580–584. [CrossRef] 150. Liao, Y.S.; Woo, J.C. The effects of machining settings on the behavior of pulse trains in the WEDM process. J. Mater. Process. Technol. 1997, 71, 433–439. [CrossRef] 151. Yan, M.-T.; Lai, Y.-P. Surface quality improvement of wire-EDM using a fine-finish power supply. Int. J. Mach. Tools Manuf. 2007, 47, 1686–1694. [CrossRef] 152. Kunieda, M.; Furudate, C. High precision finish cutting by dry WEDM. CIRP Ann. 2001, 50, 121–124. [CrossRef] 153. Tzeng, Y. Development of a flexible high-speed EDM technology with geometrical transform optimization. J. Mater. Process. Technol. 2008, 203, 355–364. [CrossRef] 154. Tzeng, Y.; Chen, F. Multi-objective optimisation of high-speed electrical discharge machining process using a Taguchi fuzzy-based approach. Mater. Des. 2007, 28, 1159–1168. [CrossRef] 155. Yih-Fong, T.; Fu-Chen, C. Investigation into some surface characteristics of electrical discharge machined SKD-11 using powder-suspension dielectric oil. J. Mater. Process. Technol. 2005, 170, 385–391. [CrossRef] 156. Anonymous the Relationship between Steel Plate Depth and Printing. Available online: https://www. finecause.com/support.php?act=view&no=96 (accessed on 27 February 2020). 157. Lin, Y.-C.; Chen, Y.-F.; Wang, D.-A.; Lee, H.-S. Optimization of machining parameters in magnetic force assisted EDM based on Taguchi method. J. Mater. Process. Technol. 2009, 209, 3374–3383. [CrossRef] 158. Wu, K.L.; Yan, B.H.; Huang, F.Y.; Chen, S.C. Improvement of surface finish on SKD steel using electro-discharge machining with aluminum and surfactant added dielectric. Int. J. Mach. Tools Manuf. 2005, 45, 1195–1201. [CrossRef] 159. Wu, K.L.; Yan, B.H.; Lee, J.-W.; Ding, C.G. Study on the characteristics of electrical discharge machining using dielectric with surfactant. J. Mater. Process. Technol. 2009, 209, 3783–3789. [CrossRef] 160. Anonymous Tool Steel (4Cr5MoSiV1/H13/SKD61). Available online: http://www.made-in-china.com/ showroom/challengemiracle/product-detailjbvnMZXVPiks/China-Tool-Steel-4Cr5MoSiV1-H13-SKD61- .html (accessed on 27 February 2020). 161. Amorim, F.L.; Weingaertner, W.L. The influence of generator actuation mode and process parameters on the performance of finish EDM of a tool steel. J. Mater. Process. Technol. 2005, 166, 411–416. [CrossRef] 162. Curodeau, A.; Marceau, L.F.; Richard, M.; Lessard, J. New EDM polishing and texturing process with conductive polymer electrodes. J. Mater. Process. Technol. 2005, 159, 17–26. [CrossRef] 163. Joshi, S.N.; Pande, S.S. Thermo-physical modeling of die-sinking EDM process. J. Manuf. Process. 2010, 12, 45–56. [CrossRef] 164. Kiyak, M.; Çakır, O. Examination of machining parameters on surface roughness in EDM of tool steel. J. Mater. Process. Technol. 2007, 191, 141–144. [CrossRef] 165. Curodeau, A.; Richard, M.; Frohn-Villeneuve, L. Molds surface finishing with new EDM process in air with thermoplastic composite electrodes. J. Mater. Process. Technol. 2004, 149, 278–283. [CrossRef] Appl. Sci. 2020, 10, 2082 44 of 46
166. Joshi, S.N.; Pande, S.S. Intelligent process modeling and optimization of die-sinking electric discharge machining. Appl. Soft Comput. 2011, 11, 2743–2755. [CrossRef] 167. Anonymous P20 Tool Steel. Available online: http://www.jaiganeshsteel.com/chemical-compositions/p20- tool-steel-chemical-composition (accessed on 27 February 2020). 168. Cusanelli, G.; Hessler-Wyser, A.; Bobard, F.; Demellayer, R.; Perez, R.; Flükiger, R. Microstructure at submicron scale of the white layer produced by EDM technique. J. Mater. Process. Technol. 2004, 149, 289–295. [CrossRef] 169. Descoeudres, A.; Hollenstein, C.; Demellayer, R.; Wälder, G. Optical emission spectroscopy of electrical discharge machining plasma. J. Phys. D Appl. Phys. 2004, 37, 875. [CrossRef] 170. Klocke, F.; Schwade, M.; Klink, A.; Veselovac, D. Analysis of material removal rate and electrode wear in sinking EDM roughing strategies using different graphite grades. Procedia CIRP 2013, 6, 163–167. [CrossRef] 171. Guo, Y.B.; Klink, A.; Klocke, F. Multiscale modeling of sinking-EDM with Gaussian heat flux via user subroutine. Procedia CIRP 2013, 6, 438–443. [CrossRef] 172. Klink, A.; Guo, Y.B.; Klocke, F. Surface integrity evolution of powder metallurgical tool steel by main cut and finishing trim cuts in wire-EDM. Procedia Eng. 2011, 19, 178–183. [CrossRef] 173. Anonymous. ASP2023, Taylor; Available online: http://www.taylorspecialsteels.co.uk/pdfdownload/asp2023. pdf (accessed on 27 February 2020). 174. Nowicki, B.; Dmowska, A.; Podolak-Lejtas, A. A new method of investigating crater and flash made by individual discharge using scanning profilometers. Wear 2011, 270, 121–126. [CrossRef] 175. Han, F.; Kunieda, M.; Sendai, T.; Imai, Y. High precision simulation of WEDM using parametric programming. CIRP Ann. 2002, 51, 165–168. [CrossRef] 176. Ravne, M. SIQUAL 0503 Steel (Steel C45). Available online: https://steelselector.sij.si/steels/C45.html (accessed on 27 February 2020). 177. Kanlayasiri, K.; Boonmung, S. An investigation on effects of wire-EDM machining parameters on surface roughness of newly developed DC53 die steel. J. Mater. Process. Technol. 2007, 187, 26–29. [CrossRef] 178. Kanlayasiri, K.; Boonmung, S. Effects of wire-EDM machining variables on surface roughness of newly developed DC 53 die steel: Design of experiments and regression model. J. Mater. Process. Technol. 2007, 192, 459–464. [CrossRef] 179. Alro. DC35 Medium Chrome Tool Steel; Available online: https://www.alro.com/datapdf/metals/dc53_complete. pdf (accessed on 27 February 2020). 180. Bleys, P.; Kruth, J.-P.; Lauwers, B. Sensing and compensation of tool wear in milling EDM. J. Mater. Process. Technol. 2004, 149, 139–146. [CrossRef] 181. Salman, Ö.; Kayacan, M.C. Evolutionary programming method for modeling the EDM parameters for roughness. J. Mater. Process. Technol. 2008, 200, 347–355. [CrossRef] 182. Younis, M.A.; Abbas, M.S.; Gouda, M.A.; Mahmoud, F.H.; Allah, S.A.A. Effect of electrode material on electrical discharge machining of tool steel surface. Ain Shams Eng. J. 2015, 6, 977–986. [CrossRef] 183. Doerrenberg. 1.2379, CPPU; Available online: https://www.doerrenberg.de/uploads/tx_c1x1downloads/1. 2379_en_01.pdf (accessed on 27 February 2020). 184. Ekmekci, B. Residual stresses and white layer in electric discharge machining (EDM). Appl. Surf. Sci. 2007, 253, 9234–9240. [CrossRef] 185. Ekmekci, B.; Sayar, A. Debris and consequences in micro electric discharge machining of micro-holes. Int. J. Mach. Tools Manuf. 2013, 65, 58–67. [CrossRef] 186. Zarepour, H.; Tehrani, A.F.; Karimi, D.; Amini, S. Statistical analysis on electrode wear in EDM of tool steel DIN 1.2714 used in forging dies. J. Mater. Process. Technol. 2007, 187, 711–714. [CrossRef] 187. Ravne. 1.2714, UTOPEX2; Available online: http://www.usbcosteels.com/pdffile/UTOPEX2.pdf (accessed on 27 February 2020). 188. Yilmaz, O.; Eyercioglu, O.; Gindy, N.N.Z. A user-friendly fuzzy-based system for the selection of electro discharge machining process parameters. J. Mater. Process. Technol. 2006, 172, 363–371. [CrossRef] 189. AZoM. AISI 4340 Alloy Steel; Available online: https://www.azom.com/article.aspx?ArticleID=6772 (accessed on 27 February 2020). 190. Sudhakara, D.; Prasanthi, G. Application of Taguchi Method for Determining Optimum Surface Roughness in Wire Electric Discharge Machining of P/M Cold Worked Tool Steel (Vanadis-4E). Procedia Eng. 2014, 97, 1565–1576. [CrossRef] Appl. Sci. 2020, 10, 2082 45 of 46
191. Uddeholm. VANADIS4 EXTRA SUPERCLEAN, UDDEHOLM; Available online: https://www.uddeholm. com/uae/en/products/uddeholm-vanadis-4-extra-superclean/ (accessed on 27 February 2020). 192. Haron, C.H.C.; Ghani, J.A.; Burhanuddin, Y.; Seong, Y.K.; Swee, C.Y. Copper and graphite electrodes performance in electrical-discharge machining of XW42 tool steel. J. Mater. Process. Technol. 2008, 201, 570–573. [CrossRef] 193. Matoorian, P.; Sulaiman, S.; Ahmad, M. An experimental study for optimization of electrical discharge turning (EDT) process. J. Mater. Process. Technol. 2008, 204, 350–356. [CrossRef] 194. Sivapirakasam, S.P.; Mathew, J.; Surianarayanan, M. Multi-attribute decision making for green electrical discharge machining. Expert Syst. Appl. 2011, 38, 8370–8374. [CrossRef] 195. Sivapirakasam, S.P.; Mathew, J.; Surianarayanan, M. Constituent analysis of aerosol generated from die sinking electrical discharge machining process. Process Saf. Environ. Prot. 2011, 89, 141–150. [CrossRef] 196. Teimouri, R.; Baseri, H. Effects of magnetic field and rotary tool on EDM performance. J. Manuf. Process. 2012, 14, 316–322. [CrossRef] 197. Zhang, Y.; Liu, Y.; Shen, Y.; Li, Z.; Ji, R.; Cai, B. A novel method of determining energy distribution and plasma diameter of EDM. Int. J. Heat Mass Transf. 2014, 75, 425–432. [CrossRef] 198. Fan, Y.; Bai, J.; Li, C.; Xu, W. Research on precision pulse power technology of WEDM. Procedia CIRP 2013, 6, 267–273. [CrossRef] 199. Ramasawmy, H.; Blunt, L. Effect of EDM process parameters on 3D surface topography. J. Mater. Process. Technol. 2004, 148, 155–164. [CrossRef] 200. Simão, J.; Aspinwall, D.; El-Menshawy, F.; Meadows, K. Surface alloying using PM composite electrode materials when electrical discharge texturing hardened AISI D2. J. Mater. Process. Technol. 2002, 127, 211–216. [CrossRef] 201. Yadav, V.; Jain, V.K.; Dixit, P.M. Thermal stresses due to electrical discharge machining. Int. J. Mach. Tools Manuf. 2002, 42, 877–888. [CrossRef] 202. Han, F.; Jiang, J.; Yu, D. Influence of discharge current on machined surfaces by thermo-analysis in finish cut of WEDM. Int. J. Mach. Tools Manuf. 2007, 47, 1187–1196. [CrossRef] 203. Fenggou, C.; Dayong, Y. The study of high efficiency and intelligent optimization system in EDM sinking process. J. Mater. Process. Technol. 2004, 149, 83–87. [CrossRef] 204. Uhlmann, E.; Piltz, S.; Jerzembeck, S. Micro-machining of cylindrical parts by electrical discharge grinding. J. Mater. Process. Technol. 2005, 160, 15–23. [CrossRef] 205. Fu, X.Z.; Zhang, Y.; Zhang, Q.H.; Zhang, J.H. Research on piezoelectric self-adaptive micro-EDM. Procedia CIRP 2013, 6, 303–308. [CrossRef] 206. Peng, Z.; Wang, Z.; Dong, Y.; Chen, H. Development of a reversible machining method for fabrication of microstructures by using micro-EDM. J. Mater. Process. Technol. 2010, 210, 129–136. [CrossRef] 207. Cogun, C.; Savsar, M. Statistical modelling of properties of discharge pulses in electric discharge machining. Int. J. Mach. Tools Manuf. 1990, 30, 467–474. [CrossRef] 208. Wang, J.; Han, F. Simulation model of debris and bubble movement in consecutive-pulse discharge of electrical discharge machining. Int. J. Mach. Tools Manuf. 2014, 77, 56–65. [CrossRef] 209. Zhang, Y.; Liu, Y.; Shen, Y.; Li, Z.; Ji, R.; Wang, F. A new method of investigation the characteristic of the heat flux of EDM plasma. Procedia CIRP 2013, 6, 450–455. [CrossRef] 210. Bayramoglu, M.; Duffill, A.W. Systematic investigation on the use of cylindrical tools for the production of 3D complex shapes on CNC EDM machines. Int. J. Mach. Tools Manuf. 1994, 34, 327–339. [CrossRef] 211. Shunmugam, M.S.; Philip, P.K.; Gangadhar, A. Improvement of wear resistance by EDM with tungsten carbide P/M electrode. Wear 1994, 171, 1–5. [CrossRef] 212. Shu, K.M.; Tu, G.C. Study of electrical discharge grinding using metal matrix composite electrodes. Int. J. Mach. Tools Manuf. 2003, 43, 845–854. [CrossRef] 213. Zhang, Q.H.; Zhang, J.H.; Ren, S.F.; Deng, J.X.; Ai, X. Study on technology of ultrasonic vibration aided electrical discharge machining in gas. J. Mater. Process. Technol. 2004, 149, 640–644. [CrossRef] Appl. Sci. 2020, 10, 2082 46 of 46
214. Li, T.; Bai, Q.; Gianchandani, Y.B. High precision batch mode micro-electro-discharge machining of metal alloys using DRIE silicon as a cutting tool. J. Micromech. Microeng. 2013, 23, 95026. [CrossRef] 215. Takahata, K.; Gianchandani, Y.B. Batch mode micro-electro-discharge machining. J. Microelectromech. Syst. 2002, 11, 102–110. [CrossRef]
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