materials

Article Self-Flushing in EDM Drilling of Ti6Al4V Using Rotating Shaped Electrodes

Manu Goiogana 1 and Ahmed Elkaseer 2,3,*

1 IK4-Tekniker, Advanced Manufacturing Technologies Unit, Iñaki Goenaga 5, 20600 Eibar, Spain; [email protected] 2 Karlsruhe Institute of Technology (KIT), Institute for Automation and Applied Informatics (IAI), Karlsruhe 76344, Germany 3 Port Said University, Faculty of Engineering, Department of Production Engineering and Mechanical design, Port Fuad 42526, Egypt * Correspondence: [email protected]; Tel.: +49-0721-608-25754



Received: 22 February 2019; Accepted: 22 March 2019; Published: 26 March 2019


Abstract: This article reports an experimental investigation of the efficacy of self-flushing in the Electrical Discharge Machining (EDM) process in terms of tool wear rate (TWR), hole taper angle and material removal rate (MRR). In addition to a plain cylindrical shape, electrodes of different cross sections (slotted cylindrical, sharp-cornered triangular, round-cornered triangular, sharp-cornered square, round-cornered square, sharp-cornered hexagonal and round-cornered hexagonal) were designed as a means of inducing debris egress and then fabricated in graphite. EDM drilling trials using the rotating shaped electrodes were carried out on a Ti6Al4V workpiece. The results revealed that, although a low TWR and minimum hole taper angle were achieved using a plain cylindrical electrode, the usage of rotating shaped electrodes provided self-flushing of the dielectric fluid during the EDM process, which led to an improvement in MRR compared to that achieved with a plain cylindrical electrode. Besides, in general, the electrodes with rounded corners are associated with a lower TWR, a lower hole taper angle and a higher MRR when compared to the electrodes with sharp corners. Considering these results, it was concluded that different process attributes, i.e., TWR, hole taper angle and MRR, are all greatly affected by the electrode shape, and thus, the proper selection of the electrode shape is a precondition to attain a specific response from the EDM process.

Keywords: EDM; Ti6Al4V; rotating shaped electrode; self-flushing; debris egress; MRR; tool wear; hole taper angle

1. Introduction There is a growing tendency to exploit advanced materials in a wide range of applications, e.g., aerospace, marine and power engines. This is due to their superior attributes in terms of high strength/weight ratio, wear and shock resistance, and thermal and electrical properties that enable these materials to fulfil critical requirements under severe working conditions [1,2]. The titanium-based superalloy Ti6Al4V is a good example of such a material, one of the so-called difficult-to-cut materials; however, its unique mechanical properties preclude the economical processing of this superalloy using conventional machining processes [3–5]. Electrical Discharge Machining (EDM) erodes a workpiece material by applying electric sparks regardless of the material’s mechanical properties, and it has shown a good potential in producing real, 3-D, intricate features in advanced difficult-to-cut materials [5–7]. In EDM, the mechanism eroding the workpiece material is a thermoelectric process between electrical, conductive electrodes (the tool and workpiece). This produces discrete electric discharges between the electrodes and generates a

Materials 2019, 12, 989; doi:10.3390/ma12060989 www.mdpi.com/journal/materials Materials 2019, 12, 989 2 of 20 high temperature plasma channel, where instantaneous thermal dissipation takes place. The local high temperature melts both the workpiece and tool [8]. Then, the eroded material solidifies in the form of debris (solidified eroded particles), and finally, the flushing of a dielectric fluid evacuates the debris [7]. This process occurs repeatedly, enabling a continuous machining of the workpiece. Thus, EDM is considered an enabling technology to effectively process difficult-to-cut materials such as Ti6Al4V and Inconel alloys [9,10]. Micro-EDM has also shown a high potential in producing complex features with high characteristics at the microscale [2,8] regardless of the mechanical properties of the workpiece materials, e.g., hardness or high-strength materials [2]. A range of governing parameters, e.g., voltage between the electrodes, maximum current, interelectrode gap, flushing dielectric, charge frequency and duty cycle, all influence the EDM process [2,3,11]. Nevertheless, among these EDM process parameters, flushing of the interelectrode gap is one of the most significant variables [12,13]. An efficient flushing removes the debris from the gap and, thus, enables the continuity of the eroding process [2] and helps maintain a high-precision EDM process. That is, the ejection of debris from the machining gap is important for a high performance of the EDM process, especially in the case of deep sinking/drilling operations [13–15]. Varying some EDM parameters mentioned above, like duty factor and charge frequency, can induce local flushing in the interelectrode gap, but this research work focused on the self-flushing phenomenon provoked by different electrode geometries. It is worth emphasizing that a strong flushing does not necessarily lead to optimal machining: A very high dielectric pressure could continually sweep away the plasma channel leading to a low material removal rate (MRR), and for this reason, flushing has to be implemented in a suitably controlled manner [16]. The presence of debris contamination in the interelectrode gap has a trade-off effect on the productivity of the EDM process. The presence of an appropriate amount of debris could initiate some “random” discharge sparks, which in turn could lead to a higher MRR. In contrast, in the worst-case scenario, this debris could induce short circuits with the subsequent stoppage of the process that would lower the MRR [17,18]. Therefore, one can argue that appropriate flushing leads to a high EDM performance in terms of maximum MRR, a low tool wear rate and the high-precision EDM of difficult-to-cut alloys. In view of the importance of flushing on the performance of the EDM process, this research study examines the effectiveness of self-flushing techniques (in terms of tool wear rate (TWR), hole taper angle and Material Removal Rate (MRR)) using rotating shaped electrodes. The structure of this paper is as follows. Firstly, the state-of-the-art flushing approaches are reviewed, the proposed approach reported in this paper is located within the context of the existing literature and the hypothesis examined is also given. Then, the experimental work is presented, followed by the analysis and discussion of the obtained results. Next, conclusions are drawn, and finally, possible future perspectives are outlined.

2. State-of-the-Art Flushing Approaches Different approaches have been applied to develop a flushing process and to maximise the MRR during the EDM process. A nozzle outside the machining zone can direct a jet of dielectric towards the gap between the electrodes [19]. However, this is not an effective technique for large depths of machining because the flushing pressure exerted is insufficient for an effective removal of the debris [17]. As for internal flushing, Munz et al. [12] investigated the use of flow rates between 5 L/h and 25 L/h on debris removal and process performance. They found that increasing the dielectric flow rate improved decontamination in the electrode gap, flushing away gas bubbles and debris more rapidly, which improved the process performance in terms of the feed rate. Nonetheless, it was also found that an excessive flushing turned into a negative process behavior and caused a drop in the feed rate. Flushing introduced through a multi-hole bundled electrode when machining Ti6Al4V was proposed by Gu et al. [20], and it was concluded that the proposed technique allowed for a better EDM Materials 2019, 12, 989 3 of 20 performance process compared with conventional EDM using a solid electrode. Similarly, Li et al. [21] also experimentally tested the response of EDM processes to the use of a multi-hole bunched electrode and showed how using this special tool affected flushing and the machining performance in terms of TWR and MRR. It was found that the multi-hole bunched electrode was able to withstand a greater peak current, giving a higher MRR but a greater TWR than a conventional solid electrode with single hole flushing. Nevertheless, the multi-hole inner flushing technique needs a special clamp for each external profile of the shape to be machined. For the EDM drilling of deep holes, Tanjilul et al. [22] suggested an innovative system for debris removal, combining simultaneously flushing and vacuuming. They achieved improved surface quality and better drilling times using such a system. These researchers also developed a novel computational fluid dynamics (CFD) model to simulate the performance of the proposed debris removal system. Ultrasonic vibrations have been suggested as a way to improve the flushing of the EDM process, especially for difficult-to-cut materials [23] and high aspect ratio blind holes [24]. This technique has shown promising increases in MRR [13], a higher process efficiency [25] and significant improvements in the obtainable surface quality in EDM finishing operations [26]. Although ultrasonic assisted EDM processes are widely used in industry, further investigation is still needed to help understand the ultrasonic vibration-assisted EDM process, dominant variables, process modelling and optimization, especially its application to deep micro/conventional drilling [27]. Other EDM techniques have included using planetary motion (lateral movement of the tool that is of similar geometric shape to that being machined), with a rotating tool to obtain a more homogenous distribution of the tool wear and to improve productivity [28]. Self-flushing using an electrode planetary motion was implemented to produce blind micro-holes with a high aspect ratio [29]. Nevertheless, using this technique, the removal rates were restricted [17]. Ziada and Koshy [17] suggested a novel method inspired by the properties of the Reuleaux Triangle (RT) that enabled flushing to be induced by the rotation and relative movement of the electrodes. This pioneering arrangement allowed the machining of both regular and non-regular shapes with sharp corners. The use of the RT electrodes improved flushing and helped make the MRR independent of machining depth. However, it required a complex tool and trajectory design, and further applied investigation work should be done. An alternative flushing approach is to periodically retract the tool during the machining process to allow the removal of the debris and adulterated dielectric. Masuzawa and Heuvelman proposed the self-flushing method in which the tool motion could take place in more than one axis. This motion could be controlled in a way that the electrode with these movements acts as a pump, and in this way, the gap was continually regenerated [30]. However, the main limit on this technique was that there was a loss of machining time [17]. They did not use any rotatory movement of the electrode. Another method is to use a rotating electrode to remove debris. Dwivedi and Choudhury [31] investigated both theoretically and experimentally the machining of high chromium and high carbon AISI D3 steel using rotating cylindrical electrodes. This technique resulted in a higher MRR because of the improved debris clearance and spark quality. These researchers claimed that the use of a rotating tool increased MMR by 41% and reduced surface roughness (Ra) by 12% comparing with the values obtained using stationary electrodes. Nonetheless, these tests were carried out in 5 mm thick metallic flats, and the results might vary in high aspect ratio blind holes. Pellicer et al. [32] proved that, in micro EDM, it is possible to improve the flushing by shaping the tool/electrodes such that the electrode was capable of cutting features in minimum time. Shaped electrodes, the so-called self-flushing electrodes, could effectively remove accumulated debris from the machining zone and, thus, eliminate the need for flushing during the machining process [33]. Plaza et al. [34] tested the effect of using an electrode of helical shape on electrode wear, MRR and micro-hole quality for EDM of Ti6Al4V. The goal was to improve the efficiency of the removal of debris from deep holes and, thus, to reduce the machining time. The parameters investigated were flute depth and helix angle. With a hole diameter of 800 µm, a reduction in the machining time of 37% was obtained with a Materials 2019, 12, 989 4 of 20

helix angle of 45◦. Kumar and Singh [33] fabricated a new type of electrode with inclined micro slots cut in a solid cylindrical electrode. The tool was used to drill blind holes into a Ti6Al4V workpiece, Materials 2019, 12, x FOR PEER REVIEW 4 of 20 and the effects of slot width, angle and tool speed were investigated with the goal of improving EDM theperformance. goal of improving It was EDM found per thatformance. the use ofIt was inclined found micro that slotsthe use successfully of inclined eliminated micro slots any successfully accumulated eliminateddebris, reducing any accumulated the likelihood debris, of reducing short-circuiting the likelihood and arcing. of short The-circuiting new design and was arcing. so successful The new at designeliminating was so successful debris that at the eliminating authors claimed debris that it was the self-flushing. authors claimed it was self-flushing. NastasiNastasi and and Koshy Koshy [ [3535]] have have suggested suggested improving improving rotation-induced rotation-induced debris debris removal removal by developing by developingnovel electrodes novel electrodes of suitable of shapes.suitable Flowshapes. fields Flow were fields simulated were simulated using CFD using in orderCFD toin exploreorder to the exploreability the of ability radial of and radial helical and helical slots introduced slots introduced into rotating into rotating cylindrical cylindrical tools tools to enhance to enhance MRR MRR when whendrilling drilling blind blind holes holes in 6061in 6061 aluminum aluminum alloy. alloy. The The model model suggested suggested and and the the experiment experiment verified verified that thatthe the tool tool with with a singlea single radial radial slot slot achieved achieved the the best best flushing, flushing, and and this this design design gave gave an an astounding astounding 300% 300%increase increase in MRRin MRR compared compared to a to typical a typical cylinder-shaped cylinder-shaped electrode electrode rotating rotating at the at same the speed.same speed. This was Thisachieved was achieved in holes in withholes an with aspect an aspect ratio of ratio 3. of 3. FigureFigure 1 schematically1 schematically summari summarizeszes the the state state-of-the-art-of-the-art flushing flushing techniques techniques in inthe the EDM EDM process process andand locates locates the theproposed proposed approach approach reported reported herein herein within within the the context context of ofthe the existing existing literature. literature.

FigureFigure 1. 1.AnAn overview overview of the the state-of-the-art state-of-the-art flushing flushing techniques techniques in the in Electrical the Electrical Discharge Discharge Machining Machining(EDM) process (EDM) andprocess the and positioning the positioning of this research of this research work within work thewithin context the cont of theext reported of the reported literature. literature. As it is possible to observe in Figure1, different approaches to flushing have been proposed. Nonetheless,As it is possible some to of observe them are in complex Figure or1, needdifferent special approaches equipment, to flushing and others have are been expensive proposed. in terms Nonetheless,of the electrode some designof them and are complex fabrication or need time andspecial effort. equipment Moreover,, and some others of are the expensive proposed in techniques terms of theneed electrode further investigativedesign and fabrication work in order time toand be effort. implemented Moreover and, some applied of the in proposed the Electrical techniques Discharge need further investigative work in order to be implemented and applied in the Electrical Discharge Drilling (EDD) of high aspect ratio blind holes [36]. From the reviewed literature, it is not so difficult to see that the performance of the EDM process is highly related to the debris evacuation process. Accordingly, the hypothesis followed in this research is that using rotating shaped electrodes will induce a self-flushing process. This, in turn, could improve the flushing circulation and quick

Materials 2019, 12, 989 5 of 20

Drilling (EDD) of high aspect ratio blind holes [36]. From the reviewed literature, it is not so difficult to see that the performance of the EDM process is highly related to the debris evacuation process. Accordingly, the hypothesis followed in this research is that using rotating shaped electrodes will induce a self-flushing process. This, in turn, could improve the flushing circulation and quick cleansing of debris from the sparking gap and, thus, contributes to the enhancement of the MRR of the EDD process compared to that obtained applying a plain cylindrical electrode. However, it is not expected to improve the TWR and hole taper angle simultaneously due to the smaller cross-sectional area of the shaped electrode when compared with the plain cylindrical one. In this context, the motivation for this research study was to examine the effectiveness of a self-flushing technique to enhance the machining process, combining the rotation and shaping of the electrodes. In order to induce self-flushing with rotating shaped electrodes during the EDD tests, the ease of manufacturing the electrodes has been considered when defining the cross-sectional geometry. Further details of the different electrode geometries can be found in Section 3.2. The performance of the proposed electrode shapes was examined, and their influence on TWR, hole taper angle and MRR was investigated. Due to the discharge conditions used in these tests of EDD in Ti6Al4V (shown in the next section), the results in terms of surface quality and surface integrity were not analyzed. The applied machining conditions were considered roughing conditions, and thus, it was not relevant to observe the surface quality of the workpiece. The aim of this research work was to improve the flushing using different electrode geometries and to identify the optimal conditions (electrode shape) that positively affects the EDM process responses concerning TWR, hole taper angle and MRR.

3. Experimental Setup

3.1. EDM Process The tests for this investigation were carried out in an ONA NX3 EDM machine from ONA Electroerosión S.L. (Durango, Spain) which has a positioning resolution of 1 µm. The tests were performed using POCO EDM-3 graphite electrodes (Poco Graphite, Decatur, TX, USA). More details of the geometries of the used electrodes are provided in Section 3.2. The workpiece material used in the experiments was Ti6Al4V titanium alloy. The surface to be machined was ground using a silicon carbide abrasive wheel prior to EDM machining in order to assure the same surface roughness and the same parallelism between the electrode and the workpiece in the different tests (see Figure2). An X-ray fluorescence system (model S8 Tiger from Bruker, Billerica, MA, USA) was utilized to identify the chemical compositions of the Ti6Al4V workpiece (see Table1), and the physical properties of the workpiece material are shown in Table2.

Table 1. The chemical composition of Ti6Al4V titanium alloy (weight %).

Ti Al V Fe Others 89.2 6.1 4.1 0.2 Balance

Table 2. The physical properties of Ti6Al4V titanium alloy.

Density, ρ (g/cm3) 4.43 ◦ Melting Temperature, Tm ( C) 1660 Hardness, HRc 36 Thermal Conductivity, κ (W/mK) 6.7 ◦ Specific Heat Capacity, Cp (J/kg C) 526.3 Materials 2019, 12, x FOR PEER REVIEW 5 of 20 cleansing of debris from the sparking gap and, thus, contributes to the enhancement of the MRR of the EDD process compared to that obtained applying a plain cylindrical electrode. However, it is not expected to improve the TWR and hole taper angle simultaneously due to the smaller cross-sectional area of the shaped electrode when compared with the plain cylindrical one. In this context, the motivation for this research study was to examine the effectiveness of a self-flushing technique to enhance the machining process, combining the rotation and shaping of the electrodes. In order to induce self-flushing with rotating shaped electrodes during the EDD tests, the ease of manufacturing the electrodes has been considered when defining the cross-sectional geometry. Further details of the different electrode geometries can be found in Section 3.2. The performance of the proposed electrode shapes was examined, and their influence on TWR, hole taper angle and MRR was investigated. Due to the discharge conditions used in these tests of EDD in Ti6Al4V (shown in the next section), the results in terms of surface quality and surface integrity were not analyzed. The applied machining conditions were considered roughing conditions, and thus, it was not relevant to observe the surface quality of the workpiece. The aim of this research work was to improve the flushing using different electrode geometries and to identify the optimal conditions (electrode shape) that positively affects the EDM process responses concerning TWR, hole taper angle and MRR.

3. Experimental Setup

3.1. EDM Process The tests for this investigation were carried out in an ONA NX3 EDM machine from ONA Electroerosión S.L. (Durango, Spain) which has a positioning resolution of 1 m. The tests were performed using POCO EDM-3 graphite electrodes (Poco Graphite, Decatur, TX, USA). More details of the geometries of the used electrodes are provided in Section 3.2. The workpiece material used in the experiments was Ti6Al4V titanium alloy. The surface to be machined was ground using a silicon carbide abrasive wheel prior to EDM machining in order to assure the same surface roughness and the same parallelism between the electrode and the workpiece in the different tests (see Figure 2). An X-ray fluorescence system (model S8 Tiger from Bruker, Billerica,Materials 2019 M,A12, ,USA 989 ) was utilized to identify the chemical compositions of the Ti6Al4V workpiece6 ( ofsee 20 Table 1), and the physical properties of the workpiece material are shown in Table 2.

Figure 2 2.. The EDM e experimentalxperimental setup setup..

During these tests, 30-mm-deep vertical blind holes with a 14 mm diameter were machined using the graphite electrodes as one of the most common electrode materials for Ti6Al4V [31]. The generic discharge conditions used for these tests are summarized in Table3 as recommended by the EDM machine manufacturer for EDM drilling with graphite electrode and Ti6Al4V titanium alloy.

Table 3. The discharge conditions.

Mean discharge Current, I (A) 32 Ionisation Voltage, U (V) 80

Pulse-On Time, ti (µs) 100

Pulse-Off Time, to (µs) 200 Tool Polarity Positive Dielectric ONA oil Rotational Speed (rpm) 40

3.2. Electrode Geometries To carry out the experimental trials, different electrode geometries were designed and fabricated in order to compare their self-flushing performance during EDM drilling operations. Electrodes with eight different cross sections were used, as shown in Table4. For inducing self-flushing, it was necessary to select noncircular cross-section profiles. Among all the different possibilities, triangular, square and hexagonal cross sections were chosen considering they were the easiest ones to design and fabricate to make them ready for the EDM tests. The profiles of the cross sections of the electrodes were selected considering the easiest non-cylindrical shapes to machine: triangular, square and hexagonal. In these cases, sharp-cornered and round-cornered cross sections were tested. Apart from that, a slotted cylindrical electrode was analyzed due to the good results obtained with slotted cylindrical electrodes by Nastasi and Koshy [35] during the machining 6061 aluminum alloy. In order to have a reference to compare with, a conventional cylindrical electrode was also examined. Materials 2019, 12, x FOR PEER REVIEW 6 of 20

Table 1. The chemical composition of Ti6Al4V titanium alloy (weight %).

Ti Al V Fe Others 89.2 6.1 4.1 0.2 Balance

Table 2. The physical properties of Ti6Al4V titanium alloy.

Density,  (g/cm3) 4.43 Melting Temperature, Tm (°C) 1660 Hardness, HRc 36 Thermal Conductivity,  (W/mK) 6.7 Specific Heat Capacity, Cp (J/kg °C) 526.3

During these tests, 30-mm-deep vertical blind holes with a 14 mm diameter were machined using the graphite electrodes as one of the most common electrode materials for Ti6Al4V [31]. The generic discharge conditions used for these tests are summarized in Table 3 as recommended by the EDM machine manufacturer for EDM drilling with graphite electrode and Ti6Al4V titanium alloy.

Table 3. The discharge conditions.

Mean discharge Current, I (A) 32 Ionisation Voltage, U (V) 80 Pulse-On Time, ti (µs) 100 Pulse-Off Time, to (µs) 200 Tool Polarity Positive Dielectric ONA oil Rotational Speed (rpm) 40

3.2. Electrode Geometries To carry out the experimental trials, different electrode geometries were designed and fabricated in order to compare their self-flushing performance during EDM drilling operations. Electrodes with eight different cross sections were used, as shown in Table 4. For inducing self-flushing, it was necessary to select noncircular cross-section profiles. Among all the different possibilities, triangular, square and hexagonal cross sections were chosen considering they were the easiest ones to design and fabricate to make them ready for the EDM tests. The profiles of the cross sections of the electrodes were selected considering the easiest non-cylindrical shapes to machine: triangular, square and hexagonal. In these cases, sharp-cornered and round-cornered cross sections were tested. Apart from that, a slotted cylindrical electrode was analyzed due to the good results obtained with slotted cylindrical electrodes by Nastasi and Koshy [35] during the machining 6061 aluminum alloy. In order to have a reference to compare with, a conventional cylindrical electrode was also examined. In all cases, the revolving external diameter of the electrode section was 14 mm. In this way, all the electrodes used in these tests had the same effective area. The value of all radii used in round- cornered electrodes was 4 mm, and in the case of slotted cylindrical electrode, the width of the slot was 2 mm and the depth was 5.5 mm. No radius was used in the case of sharp-cornered electrodes. All these electrodes were manufactured in a machining center with a simple milling operation. As the main objective was to assess the self-flushing capacity of each different cross section, no Materialsex2019ternal, 12 ,flushing 989 mechanism was used during the tests. This avoids uncertainties associated7 of 20 with external flushing and ensured only the effectiveness of self-flushing was assessed.

Table 4. The electrode designation, 3-D representation and cross section. Table 4. The electrode designation, 3-D representation and cross section.

Electrode DesignationElectrode Designation 3-D3-D Geometry and Cross Section

Cylindrical electrodeCylindrical electrode Materials 2019, 12, x FOR PEER REVIEW 7 of 20 Materials 2019, 12, x FOR PEER REVIEW 7 of 20 Materials 2019, 12, x FOR PEER REVIEW 7 of 20 Materials 2019, 12, x FOR PEER REVIEW 7 of 20 Materials 2019, 12, x FOR PEER REVIEW 7 of 20 Materials 2019, 12, x FOR PEER REVIEW 7 of 20 Materials 2019, 12, x FOR PEER REVIEW 7 of 20 Slotted cylindricalSlotted electrode cylindrical electrode Slotted cylindrical electrode Slotted cylindrical electrode Slotted cylindrical electrode Slotted cylindrical electrode Slotted cylindrical electrode Slotted cylindrical electrode

Sharp-cornered triangular electrode Sharp-corneredSharp triangular-cornered electrode triangular electrode Sharp-cornered triangular electrode Sharp-cornered triangular electrode Sharp-cornered triangular electrode Sharp-cornered triangular electrode Sharp-cornered triangular electrode

Round-cornered triangular electrode Round-cornered triangular electrode Round-corneredRound triangular-cornered electrode triangular electrode Round-cornered triangular electrode Round-cornered triangular electrode Round-cornered triangular electrode Round-cornered triangular electrode Sharp-cornered square electrode Sharp-cornered square electrode Sharp-cornered square electrode Sharp-corneredSharp square-cornered electrode square electrode Sharp-cornered square electrode Sharp-cornered square electrode Sharp-cornered square electrode Round-cornered square electrode Round-cornered square electrode Round-cornered square electrode Round-cornered square electrode Round-corneredRound square-cornered electrode square electrode Round-cornered square electrode Round-cornered square electrode Sharp-cornered hexagonal electrode Sharp-cornered hexagonal electrode Sharp-cornered hexagonal electrode Sharp-cornered hexagonal electrode Sharp-cornered hexagonal electrode Sharp-corneredSharp hexagonal-cornered electrode hexagonal electrode Sharp-cornered hexagonal electrode

Round-cornered hexagonal electrode Round-cornered hexagonal electrode Round-cornered hexagonal electrode Round-cornered hexagonal electrode Round-cornered hexagonal electrode Round-cornered hexagonal electrode Round-corneredRound hexagonal-cornered electrode hexagonal electrode The proposed geometry of the eight different electrodes will induce debris egress. In this paper, The proposed geometry of the eight different electrodes will induce debris egress. In this paper, the differenceThe proposed between geometry the cross of the-sectional eight different area of theelectrodes drilled willhole induce and the debris cross -egress.sectional In thisarea paper, of the the differenceThe proposed between geometry the cross of the-sectional eight different area of theelectrodes drilled willhole induceand the debris cross -egress.sectional In thisarea paper, of the theelectrode differenceThe proposed due betweento its geometry geometry the cross of is the- sectionaltermed eight different“debris area of egress theelectrodes drilled gap”. willhole Figure induce and 3athe debrisillustrates cross -egress.sectional examples In thisarea paper, of the electrodethe differenceThe proposed due betweento its geometry geometry the cross of is the- sectionaltermed eight different“debris area of egress theelectrodes drilled gap”. willhole Figure induce and 3athe debrisillustrates cross -egress.sectional examples In thisarea paper, ofof the electrodethe“debris differenceThe egress proposed due betweentogap” its geometry forgeometry thedifferent cross of is the- sectionalelectrodetermed eight different“debris areageometries, of egress theelectrodes drilled a ndgap”. 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In Figure operation. between 4, the the ratio moving of the (rotating debris egressandwhich locally showsgaps retrwith howacting) respect debris shaped tois evacuatedthe electrodewhole alongcross and- thesection the helical machinedal areas paths are hole.generated presented. In Figure between 4, the the ratio moving of the (rotating debris Asandegress the locally maingaps retr objectivewithacting) respect was shaped to to the assess electrodewhole the cross self-flushing and-section the machinedal capacity areas are hole. of presented. each In Figure different 4, the cross ratio section, of the no debris egressand locally gaps retrwithacting) respect shaped to the electrodewhole cross and-section the machinedal areas are hole. presented. In Figure 4, the ratio of the debris externalegress flushing gaps with mechanism respect wasto the used whole during cross the-section tests.al Thisareas avoids are presented. uncertainties associated with egress gaps with respect to the whole cross-sectional areas are presented. external flushing and ensured only the effectiveness of self-flushing was assessed. The proposed geometry of the eight different electrodes will induce debris egress. In this paper, the difference between the cross-sectional area of the drilled hole and the cross-sectional area of the electrode due to its geometry is termed “debris egress gap”. Figure3a illustrates examples of the “debris egress gap” for different electrode geometries, and Figure3b shows a 3-D representation of the debris egress (due to electrode shape and movements) for the sharp-cornered square electrode, which shows how debris is evacuated along the helical paths generated between the moving (rotating and

Figure 3. (a) The cross-sectional area and the debris egress gap for three different electrode Figure 3. (a) The cross-sectional area and the debris egress gap for three different electrode Figuregeometries: 3. ( afrom) The the cross left- sectionalto the right, area the and sharp the-cornered debris egresshexagonal gap electrode, for three the different round - electrodecornered geometries:Figure 3. ( afrom) The the cross left- sectionalto the right, area the and sharp the-cornered debris egresshexagonal gap electrode, for three the different round - electrodecornered geometries:Figuresquare electrode 3. ( afrom) The and the cross theleft -s sectionaltolotted the cylindricalright, area the and s electrodeharp the-cornered debris; (b) a egress3hexagonal-D representation gap electrode, for three of thethe different debrisround -egress electrodecornered for Figuresquaregeometries: electrode 3. (a from) The and the cross theleft -s sectionallottedto the cylindricalright, area the and selectrodeharp the- cornered debris; (b) a egress3hexagonal-D representation gap electrode, for three of thethe different debrisround egress- electrodecornered for squaregeometries:Figurethe sharp electrode 3.- cornered( afrom) The and the squarecross theleft s- sectionallottedto electrode the cylindricalright, area. the and selectrodeharp the-cornered debris; (b) a egress3hexagonal-D representation gap electrode, for three of thethe different debrisround egress- electrodecornered for geometries:thesquare sharp electrode-cornered from and the square theleft s tolotted electrode the cylindricalright,. the s electrodeharp-cornered; (b) a 3hexagonal-D representation electrode, of the debrisround -egresscornered for thesquaregeometries: sharp electrode-cornered from and the square theleft s lottedto electrode the cylindricalright,. the selectrodeharp-cornered; (b) a 3hexagonal-D representation electrode, of thethe debrisround egress-cornered for squarethe sharp electrode-cornered and square the slotted electrode cylindrical. electrode; (b) a 3-D representation of the debris egress for thesquare sharp electrode-cornered and square the slotted electrode cylindrical. electrode; (b) a 3-D representation of the debris egress for the sharp-cornered square electrode. the sharp-cornered square electrode.

Materials 2019, 12, x FOR PEER REVIEW 7 of 20

Slotted cylindrical electrode

Sharp-cornered triangular electrode

Round-cornered triangular electrode

Sharp-cornered square electrode

Round-cornered square electrode

Sharp-cornered hexagonal electrode

Round-cornered hexagonal electrode

The proposed geometry of the eight different electrodes will induce debris egress. In this paper, the difference between the cross-sectional area of the drilled hole and the cross-sectional area of the electrode due to its geometry is termed “debris egress gap”. Figure 3a illustrates examples of the Materials“debris 2019egress, 12, 989gap” for different electrode geometries, and Figure 3b shows a 3-D representation8 of of 20 the debris egress (due to electrode shape and movements) for the sharp-cornered square electrode, which shows how debris is evacuated along the helical paths generated between the moving (rotating locally retracting) shaped electrode and the machined hole. In Figure4, the ratio of the debris egress and locally retracting) shaped electrode and the machined hole. In Figure 4, the ratio of the debris gaps with respect to the whole cross-sectional areas are presented. egress gaps with respect to the whole cross-sectional areas are presented.

Figure 3. (a) The cross-sectional area and the debris egress gap for three different electrode geometries: fromFigure the 3. left (a to) The the right,cross the-sectional sharp-cornered area and hexagonal the debris electrode, egress the gap round-cornered for three different square electrode electrode andgeometries: the slotted from cylindrical the left electrode;to the right, (b) the a 3-D sharp representation-cornered hexagonal of the debris electrode, egress for the the round sharp-cornered-cornered Materials 2019, 12, x FOR PEER REVIEW 8 of 20 square electrode.electrode and the slotted cylindrical electrode; (b) a 3-D representation of the debris egress for the sharp-cornered square electrode.

Figure 4. TheThe percentage percentage of debris egress gaps with respect to the whole cross cross-sectional-sectional areas.areas. 3.3. Experimental Procedure 3.3. Experimental Procedure During all the tests, the different variables were monitored and recorded, such as axial position During all the tests, the different variables were monitored and recorded, such as axial position and EDM parameters. In addition, in each test, the duration of the operation, removed material and and EDM parameters. In addition, in each test, the duration of the operation, removed material and the electrode wear were measured. For that purpose, the EDM machine was connected to a computer the electrode wear were measured. For that purpose, the EDM machine was connected to a computer via an ethernet cable, and all the parameters were synchronized and recorded in the same Matlab file. via an ethernet cable, and all the parameters were synchronized and recorded in the same Matlab After concluding the tests, the collected data were analyzed using the Matlab 2015B package to detect file. After concluding the tests, the collected data were analyzed using the Matlab 2015B package to any unusual behaviour during the EDM process, to check the real process duration time (in order to detect any unusual behaviour during the EDM process, to check the real process duration time (in calculate the MRR values), to analyze the axial movement during the different stages of the machining order to calculate the MRR values), to analyze the axial movement during the different stages of the process and, in general, to analyze and evaluate the proper functioning of the process. Assessing machining process and, in general, to analyze and evaluate the proper functioning of the process. the unnecessary movements and retractions of the electrode helped to determine the quality of the Assessing the unnecessary movements and retractions of the electrode helped to determine the consistency of the process. All the tests carried out in this research work were repeated twice, and the quality of the consistency of the process. All the tests carried out in this research work were repeated average values were used to show the results and to obtain the conclusions. twice, and the average values were used to show the results and to obtain the conclusions. In order to evaluate the electrode wear, the loss in length and mass were measured. The length of the electrodes was measured before and after each EDM test to obtain the value of the longitudinal wear. In this paper, the length of the electrodes after finishing the corresponding EDM test is called the length of worn electrodes. These measurements were carried out in the EDM machine before taking out the electrodes from it, with an accuracy of micrometers. Moreover, the graphite electrodes were weighted before and after the tests. The weighting process was realized with a resolution of 0.1 mg in a Mettler Toledo scale (Model MS204S/01). It is known that graphite is a porous material, and as it was used submerged in a dielectric liquid, it was necessary to dry the electrodes thoroughly and to remove all dielectric liquid. To remove the ONA oil, each electrode was immersed in ether petroleum for 24 hours to extract the oil from the graphite’s pores. Afterwards, the electrodes were removed from the ether and placed in an oven at 100 °C until a constant weight was obtained. In this way, it was ensured that the real weight of the dried electrode was measured. The generated cavities were measured using an Aberlink Coordinate Measuring Machine (CMM, Aberlink Innovative Metrology, Gloucestershire, UK) with a TP8 touch trigger manual indexing probe head and integrated probe, with the minimum available probe diameter of 1 mm, as shown in Figure 5a. The Aberlink 3-D measurement software was utilized to conduct the measurements of the vertical 2-D profile of the cavities. These 2-D profiles were fed into the

Materials 2019, 12, 989 9 of 20

In order to evaluate the electrode wear, the loss in length and mass were measured. The length of the electrodes was measured before and after each EDM test to obtain the value of the longitudinal wear. In this paper, the length of the electrodes after finishing the corresponding EDM test is called the length of worn electrodes. These measurements were carried out in the EDM machine before taking out the electrodes from it, with an accuracy of micrometers. Moreover, the graphite electrodes were weighted before and after the tests. The weighting process was realized with a resolution of 0.1 mg in a Mettler Toledo scale (Model MS204S/01). It is known that graphite is a porous material, and as it was used submerged in a dielectric liquid, it was necessary to dry the electrodes thoroughly and to remove all dielectric liquid. To remove the ONA oil, each electrode was immersed in ether petroleum for 24 hours to extract the oil from the graphite’s pores. Afterwards, the electrodes were removed from the ether and placed in an oven at 100 ◦C until a constant weight was obtained. In this way, it was ensured that the real weight of the dried electrode was measured. The generated cavities were measured using an Aberlink Coordinate Measuring Machine (CMM, Aberlink Innovative Metrology, Gloucestershire, UK) with a TP8 touch trigger manual indexing probe head and integrated probe, with the minimum available probe diameter of 1 mm, as shown in Figure5a. The Aberlink 3-D measurement software was utilized to conduct the measurements of the vertical Materials 2019, 12, x FOR PEER REVIEW 9 of 20 2-D profile of the cavities. These 2-D profiles were fed into the Solidworks software to generate the Solidworks3-D geometry software of the machinedto generate entities the 3-D by geometry revolving of the the captured machined profiles entities around by revolving their central the captured axis, as profilesshown inaround Figure their5b. central axis, as shown in Figure 5b.

(a) (b) Figure 5. (a) The characterization of the machined cavities using Aberlink Coordinate Measuring FigureMachine 5. and(a) The (b) thecharacterization 3-D geometry byof theSolidworks. machined cavities using Aberlink Coordinate Measuring Machine and (b) the 3-D geometry by Solidworks. 4. Results and Discussion 4. ResultsIn this and section, Discussion the results obtained in terms of TWR, hole taper angle and MRR will be analyzed. As discussedIn this section, later, both the results hole taper obtained angle in and terms MRR of will TWR, be hole affected taper by angle the tool and wear, MRR and will thus, be analyzed the latter. Aswill dis becussed discussed later, first. both hole taper angle and MRR will be affected by the tool wear, and thus, the latter will be discussed first. 4.1. Tool Wear Rate (TWR) 4.1. ToolIn all Wear EDM Rate machining (TWR) research work, it is very important to evaluate the electrode wear because it directlyIn all EDM affects machining the final machined research work, shape it obtained is very important in the workpiece. to evaluate the electrode wear because it directThely testsaffects carried the final out machined for this paper shape are obtained not intended in the workpiece. to replicate an industrial case study. In a real industrialThe tests carried process, out several for this electrodes paper are would not beintended used in to order replicate to achieve an industrial the desired case final study. shape In ina realthe workpiece.industrial process, However, several the purpose electrodes here would was to be assess used whichin order electrode to achieve cross the section desired was final the shape most insuitable the workpiece. for self-flushing However, and, the thus, purpose would here improve was to the assess machining which results.electrode With cross this section in mind, was only the mostone electrode suitable for was self used-flushing for each and, test. thus, That would is the improve reason why the themachining electrode results. wear rateWith measured this in mind, and onlyevaluated one electrode in this paper was used may befor more each pronouncedtest. That is thanthe reason in other why papers the electrode or industrial wear processes rate measured where andmore evaluated than one in electrode this paper may may be used. be more pronounced than in other papers or industrial processes where more than one electrode may be used. The graphite electrodes were weighed before and after the tests, and the TWR was calculated as follows: Electrode's initial mass (mg) − Electrode's final mass (mg) TWR= (1) Machining time (s) As shown in Equation (1), the TWR evaluates the worn mass of the electrode per time unit. Figure 6 shows the TWR values of the different electrodes together with a picture of the initial cross section of each electrode.

Materials 2019, 12, 989 10 of 20

The graphite electrodes were weighed before and after the tests, and the TWR was calculated as follows: Electrode0s initial mass (mg) − Electrode0s final mass (mg) TWR = (1) Machining time (s) As shown in Equation (1), the TWR evaluates the worn mass of the electrode per time unit. Figure6 shows the TWR values of the different electrodes together with a picture of the initial crossMaterials section 2019, 12 of, x each FOR PEER electrode. REVIEW 10 of 20

Figure 6. The Tool Wear RateRate (TWR)(TWR) forfor differentdifferent shapedshaped electrodes.electrodes.

InIn Figure 66,, it is is not not so so difficult difficult to toobserve observe that, that, generally, generally, the theTWR TWR is lower is lower in the in case the of case shapes of shapeswith rounded with rounded corners. corners. Apart Apart from from that, that, in order in order to helpto help obtain obtain a a wider wider perspective perspective of of this this phenomenon,phenomenon, the length of worn electrodes after each EDM drilling test are collected in Table5 5 and and plottedplotted inin FigureFigure7 7. .

Table 5.5. The lengthlength ofof worn electrodeselectrodes (the(the originaloriginal lengthlength ofof allall electrodeselectrodes waswas 3030 mm).mm).

ShapeShape Length ofLength Worn ofElectrodes Worn Electrodes (mm) (mm) CylindricalCylindrical 26.15 26.15 SlottedSlotted cylindrical cylindrical 25.79 25.79 Sharp-corneredSharp-cornered triangular triangular 23.23 23.23 Round-cornered triangular 25.85 Round-cornered triangular 25.85 Sharp-cornered square 25.45 Sharp-cornered square 25.45 Round-cornered square 26.09 Round-corneredSharp-cornered square hexagonal 25.94 26.09 Sharp-corneredRound-cornered hexagonal hexagonal 26.14 25.94 Round-cornered hexagonal 26.14

Materials 2019, 12, 989 11 of 20 Materials 2019, 12, x FOR PEER REVIEW 11 of 20

Figure 7. The length of worn electrodes for different shaped electrodes. Figure 7. The length of worn electrodes for different shaped electrodes. From Table5, it can be observed that the difference in the final length of the worn electrodes is notFrom significant Table 5, except it can be in observed the case ofthat the the sharp-cornered difference in the triangle, final length especially of the sinceworn theelectrodes mean ofis thenot significant worn electrodes’ except in length the case = 25.58of the mmsharp and-cornered standard triangle deviation, especially = 0.977 since mm the and mean the of lengththe worn of sharp-corneredelectrodes’ length triangular = 25.58 mm electrode and standard is the only deviation electrode = 0.977 that mm falls and outside: the length mean of± sharptwo- standardcornered deviations.triangular electrode However, is the a more only electrode insightful that conclusion falls outside: canbe mean drawn ± two looking standard at thedeviations. percentage However, of the electrodea more insightful wear (electrode conclusion wear/original can be drawn length looking of theat the electrode percentage× 100). of the In electrode particular, wear given (electrode that the originalwear/original length length of the electrodes of the electrode was 30 × mm, 100 the). In electrode particular, wears given are found that the to vary original between length 3.85 of mm the (forelectrodes the plain was cylindrical 30 mm, electrode) the electrode and 6.77 wear mms are (for found the sharp to vary corner-triangular between 3.85 electrode), mm (for equivalentthe plain tocylindrical 12.83% and electrode) 22.57%, and respectively. 6.77 mm (for the sharp corner-triangular electrode), equivalent to 12.83% and 2As2.57 shown%, respectively. in Equation (1), the TWR was calculated from the initial and the final masses of the electrodes.As shown If there in Equation is no significant (1), the differenceTWR was incalculated the final lengthfrom the of theinitial worn and electrodes, the final masses then it canof the be assumedelectrodes. that If there the difference is no significan in thet increase difference of thein the electrode final length wear of can the be worn explained electrodes, by the then difference it can be in volumeassumed of that the the electrode difference due toin differentthe increase electrode of the shapeselectrode used wear in thecan EDM be explained trials. by the difference in volumeIt has of been the mentioned electrode due that to the different TWR is electrode lower in shapes the case used of round-cornered in the EDM trials shapes.. The unique exceptionIt has among been mentioned the non-cylindrical that the TWR electrodes is lower is the in the case case of the of round-corneredround-cornered hexagonalshapes. The electrode, unique whichexception has aamong bigger the TWR non with-cylindrical respect to electrodes that of the is sharp-cornered the case of the hexagonal round-cornered electrode. hexagonal However, theelectrode, length wh of theich wornhas a round-corneredbigger TWR with one respect is longer to thanthat of the the final sharp length-cornered of the wornhexagonal sharp-cornered electrode. hexagonalHowever, the electrode, length of as the it can worn be round seen in-cornered Table5. one is longer than the final length of the worn sharp- corneredTherefore, hexagonal in order electrode, to explain as it can why be the seen round-cornered in Table 5. hexagonal electrode results in a more pronouncedTherefore, TWR in order than to the expl sharp-corneredain why the round hexagonal-cornered electrode, hexagonal it is electrode necessary results to focus in a on more the narrownesspronounced of TWR the debris than egress the sharp gap.-cornered Because of hexagonal the cross section electrode, of the it electrodes, is necessary the debris to focus egress on gap the forcesnarrowness the electrode of the debris to induce egress self-flushing gap. Because during of the machining. cross section However, of the ifelectrodes, the aforementioned the debris egr debrisess egressgap force gaps the is not electrode big enough to induce to make self a-flushing correct flushing during machining. of the dielectric However, liquid if (observe, the aforementioned in Figure4, thedebris lowest egress values gap ofis thenot debrisbig enough egress to gap) mak possible,e a correct the wearflushing in theof lateralthe dielectric part of liquid the electrodes (observe will, in tendFigure to increase.4, the lowest This effect values will of be the more debris pronounced egress gap) if the possible debris, egress the wear gap in is divided the lateral into part six, whichof the iselectrodes the case will for the tend round-cornered to increase. This hexagonal effect will electrode. be more pronounced This is the reasonif the debris why theegress TWR gap is is bigger divided in theinto case six, ofwhich the round-cornered is the case for the hexagonal round-cornered electrode hexagonal than in the electrode. sharp-cornered This is hexagonal. the reason Awhy similar the explanationTWR is bigger can in be the inferred case forof the the round case of-cornered the slotted hexagonal cylindrical electrode electrode. than However, in the sharp in this-cornered case, the differencehexagonal. in A length similar compared explanation with the can cylindrical be inferred electrode for the is case considerable. of the slotted Thus, cylindrical the high TWR electrode. value ofHowever, the slotted in cylindrical this case, electrode the difference is attributed in length to a combination compared ofwith lateral the and cylindrical axial wear. electrode is considerable. Thus, the high TWR value of the slotted cylindrical electrode is attributed to a combination of lateral and axial wear.

Materials 2019, 12, 989 12 of 20 Materials 2019, 12, x FOR PEER REVIEW 12 of 20

As for the relationship betweenbetween thethe TWRTWR andand thethe debris debris egress egress gap gap of of the the electrode electrode geometry, geometry, it isit isinteresting interesting to to compare compare the the results results presented presented in in Figure Figure6 6with with that that of of Figure Figure4. 4. Comparing Comparing the the results results presented in both figures, figures, it can be concluded that there is no linear relationship between the TWR and the debris egress gap of the electrode geometry geometry.. Nevertheless, if the aforementioned exceptions (round-cornered(round-cornered hexagonal, cylindrical with with slot slot and cylindrical electrodes) electrodes) are are excluded, it it can be observed that the TWR increases as the debris egress gap gets larger. The larger the debris egress gap, th thee smaller the the area area of of the the cross cross section of of the the electrode electrode is is,, making the electrode weakerweaker and,and,thus, thus, more more susceptible susceptible to to wear. wear. A A bigger bigger debris debris egress egress gap gap results results in inathinner a thinner and and weaker weaker electrode electrode when when the the machining machining parameters parameters areare thethe samesame for all the electr electrodes.odes. Hence, in the case of the electrodes with a bigger debris egress gap, the discharge energy is more concentrated as the contact area between the electrode and the workpiece is smaller. This results in a locally more aggressive erosion processprocess andand aa higherhigher TWR.TWR. For a better understanding of the results, the resultant TWR vs. debris egress gap for different electrodes are plotted in Figure8 8..

Figure 8 8.. The r relationshipelationship between TWR and the debris egress gap.gap.

In particular, with the reduction in the debris egress gap, TWR tends to decrease. Thus, Thus, since since the the debris egress gap in the case of the cross sections of rounded corners are smaller than those related to the sharp cornered electrodes, it is possible to und understanderstand why, why, in general, the TWR in the case of roundround-cornered-cornered electrodes is lower than in the case of thethe sharp-corneredsharp-cornered ones.ones. Finally, thethe casecase ofof thethe cylindricalcylindrical electrodeelectrode is is different different from from the the other other cases, cases, as as no no self-flushing self-flushing is isachieved achieved with with this this geometry. geometry. This This is is the the reason reason why why the the laterallateral wearwear isis notnot pronounced in this this case. case. These results confirmed confirmed the initial hypothesis that that shaped shaped electrodes electrodes could could exhibit exhibit a a higher higher TWR. TWR. Nonetheless, the TWR value of the cylindrical electrode is not the lowest value among all the different different electrodes, as it was expected expected at the the beginning. beginning. The The round round-cornered-cornered square square electrode electrode presents a lower lower TWR value. Further research work will be necessary in order to clarify this result. To sum up, it can be concluded that, generally, generally, TWR is lower if if the the debris debris egress egress gap gap is is smaller, smaller, provided that the debris egress gap is not too small or narrow for self self-pumping.-pumping. Otherwise, the TWR will be increased due to the lateral electrode wear. Moreover,Moreover, in general, round-corneredround-cornered electrodes offer lowerlower TWRTWR valuesvalues than than sharp-cornered sharp-cornered electrodes. electrodes. Finally, Finally, the the cylindrical cylindrical electrode electrode shows shows a low a lowTWR TWR but notbut thenot lowest.the lowest.

4.2. Hole Taper Angle

Materials 2019, 12, 989 13 of 20

4.2. Hole Taper Angle Materials 2019, 12, x FOR PEER REVIEW 13 of 20 The hole taper is a very common dimensional defect that occurs in cutting and drilling operations. It is characterizedThe hole taper by is obtaining a very common a wider diameter dimensional at the defect top than that that occurs at the in bottomcutting in and the drilling case of operations.drilling and It getting is characterized a larger width by obtaining at the entry a wider than diameter that at the at exit the intop cutting. than that at the bottom in the case ofThe drilling hole taper and usuallygetting a is larger an undesirable width at the characteristic, entry than andthat itat is the typically exit in evaluatedcutting. using the hole taperThe angle. hole In taper this work,usually the is inneran undesirabl diametere wascharacteristic, measured and at a it depth is typically of 20 mm, evaluated as it was using neither the holepossible taper nor angle. reliable In this to determine work, the theinner diameter diameter at was the bottommeasured of the at a hole. depth A of schematic 20 mm, as definition it was neither of the possiblehole taper nor angle reliable is shown to determine in Figure the9. diameter at the bottom of the hole. A schematic definition of the hole taper angle is shown in Figure 9.

Figure 9 9.. The g geometricaleometrical definition definition of the hole taper angle angle..

The main reason for the appear appearanceance of the mentioned mentioned hole taper during the EDM EDM drilling drilling process process is the wear wear of of the the electrode. electrode. Many Many different different researchers researchers have have investigated investigated minimizing electrode wear andand,, thus, the hole taper angle. During the EDM drilling of difficult-to-cutdifficult-to-cut materials, especiallyespecially in micromachining, preventing electrode wear meansmeans enhancing productivityproductivity [[3377].]. Figure 10 shows the angle of the hole taper and the actual depth of the machined holes in millimetersmillimeters.. The values measured measured in the the holes holes machined machined with with the the eight eight different different ele electrodectrode geometries varied from 1.040 to 8.110. There was a significant significant variation of the hole taper angle depending on the electrode geometry. It is not so difficult to see that, for all drilled holes, no flat bottoms were obtained but that bottom surfaces are uneven with defects (large cone) in the center. These defects are mainly attributed to an increased current density at the bottom corner of the electrode, which results in the removal of more material at the edges than the removal from the central portion of the electrode, which in turn produces a large cone in the center of the bottom surface of the electrodes, as seen in Figure 10[38]. Comparing the results presented in Figure 10 with the percentages of debris egress gap of different electrodes represented in Figure4, it can be concluded that the larger the debris egress gap was, the larger the hole taper angle would be. The electrode is more likely to wear, associated with a larger hole taper angle, if its cross-sectional area is smaller (resulting in a larger debris egress gap). This statement can be clearly observed in Figure 11.

Materials 2019, 12, x FOR PEER REVIEW 13 of 20

The hole taper is a very common dimensional defect that occurs in cutting and drilling operations. It is characterized by obtaining a wider diameter at the top than that at the bottom in the case of drilling and getting a larger width at the entry than that at the exit in cutting. The hole taper usually is an undesirable characteristic, and it is typically evaluated using the hole taper angle. In this work, the inner diameter was measured at a depth of 20 mm, as it was neither possible nor reliable to determine the diameter at the bottom of the hole. A schematic definition of the hole taper angle is shown in Figure 9.

Figure 9. The geometrical definition of the hole taper angle.

The main reason for the appearance of the mentioned hole taper during the EDM drilling process is the wear of the electrode. Many different researchers have investigated minimizing electrode wear and, thus, the hole taper angle. During the EDM drilling of difficult-to-cut materials, especially in micromachining, preventing electrode wear means enhancing productivity [37]. Figure 10 shows the angle of the hole taper and the actual depth of the machined holes in millimeters. The values measured in the holes machined with the eight different electrode geometries Materialsvaried from2019, 121.040, 989 to 8.110. There was a significant variation of the hole taper angle depending on14 of the 20 electrode geometry.

Materials 2019, 12, x FOR PEER REVIEW 14 of 20

Figure 10. The hole taper angle values in relation with the different electrode cross sections (the dimensions are in mm).

It is not so difficult to see that, for all drilled holes, no flat bottoms were obtained but that bottom surfaces are uneven with defects (large cone) in the center. These defects are mainly attributed to an increased current density at the bottom corner of the electrode, which results in the removal of more material at the edges than the removal from the central portion of the electrode, which in turn produces a large cone in the center of the bottom surface of the electrodes, as seen in Figure 10 [38]. Comparing the results presented in Figure 10 with the percentages of debris egress gap of different electrodes represented in Figure 4, it can be concluded that the larger the debris egress gap was, the larger the hole taper angle would be. The electrode is more likely to wear, associated with a largerFigure hole 10.taperThe angle, hole taperif its cross angle-section values inal relationarea is smaller with the (resulting different electrode in a larger cross debris sections egress (the gap).

This dimensionsstatement can are inbe mm). clearly observed in Figure 11.

Figure 1 11.1. The r relationshipelationship between the holehole taper angle and the debris egress gap.gap.

In short, the best result with regards toto thethe holehole tapertaper angleangle waswas obtainedobtained with the cylindricalcylindrical electrode, which confirmedconfirmed thethe initial hypothesis about the hole taper angle. The cylindric cylindricalal electrode was followed by the slotted cylindrical and the round round-cornered-cornered hexagonal electrodes, as they present the lowest values of thethe holehole tapertaper angle.angle. In contrast, the sharp-corneredsharp-cornered triangular electrode shows the worst results. This This is due to the weaknessweakness of the electrode geometry and,and, thus, its tendencytendency to wear axially.axially Sharp-cornered. Sharp-cornered square square and round-corneredand round-cornered triangular triangular electrodes electrodes also produced also produced relatively poorrelatively results. poor results.

4.3. Material Removal Rate (MRR) The materialmaterial removal raterate (g/min)(g/min) is is used used in in th thisis research research as a criterion criterion of machinability to assess the performance of of the the EDM EDM process process for for different different electrode electrode geometries. geometries. In Inparticular, particular, the the volumes volumes of the 3-D geometry of the machined cavities (holes) were quantified using SolidWorks, enabling the determination of the removed mass of each hole by utilizing the density of Ti6Al4V. Please note that, although the procedure followed to quantify the removed material is an indirect method, it prevents the workpiece from being taken off for frequent assessment after each EDM trial, sustaining unchanged workpiece setting conditions and minimizing the associated uncertainty errors. Moreover, these indirect quantifications are based on direct volume measurements of the machined cavities as explained in Section 3.3 and shown in Figure 5a. To determine the MRR for each EDM machining trial, the amount of removed material (in grams) was divided by the corresponding machining time and recorded using the data acquisition system previously explained in Section 3.3. Therefore, the MRR was calculated as follows:

Materials 2019, 12, 989 15 of 20 of the 3-D geometry of the machined cavities (holes) were quantified using SolidWorks, enabling the determination of the removed mass of each hole by utilizing the density of Ti6Al4V. Please note that, although the procedure followed to quantify the removed material is an indirect method, it prevents the workpiece from being taken off for frequent assessment after each EDM trial, sustaining unchanged workpiece setting conditions and minimizing the associated uncertainty errors. Moreover, these indirect quantifications are based on direct volume measurements of the machined cavities as explained in Section 3.3 and shown in Figure5a. To determine the MRR for each EDM machining trial, the amount of removed material (in grams) was divided by the corresponding machining time and recorded using the data acquisition system previouslyMaterials 2019 explained, 12, x FOR PEER in Section REVIEW 3.3 . Therefore, the MRR was calculated as follows: 15 of 20

Volume of machined hole mm3 × Workpiece density [g/mm 3] MRR = Volume of machined hole [mm3] × Workpiece density [g/mm3] (2) MRR= Machining time [min] (2) Machining time [min] Figure 1212 shows the resultant MRR for thethe eighteight differentdifferent electrodeelectrode geometries.geometries. In these EDM trials, as as previously previously explained, explained, a shaped a shaped electrode electrode simultaneously simultaneously rotates rotatesand retracts, and which retracts promotes, which self-pumpingpromotes self- ofpumping the dielectric of the fluiddielectric [26,31 fluid]. This [26,31]. evacuates This debrisevacuates along debris the helicalalong the paths helical generated paths betweengenerated the between moving the shaped moving electrode shaped andelectrode the machined and the machined hole (see hole Figure (see3b) Figure and, thus,3b) and induces, thus, self-flushinginduces self-flushing of the debris of the through debris the through gap at the the gap top at surface. the top This surface. assumption This assumption is clearly confirmed is clearly whenconfirmed looking when at thelooking results, at the as theresults, minimum as the minimum MRR value MRR (0.0983 value m/min) (0.0983 was m/min) obtained was obtained for the solid for cylindricalthe solid cylindrical electrode, electrode, with no debris with no egress debris gap. egress The resultsgap. The show results that show the slotted that the cylindrical slotted cylindrical electrode gaveelectrode a better gave result a better (MRR result = 0.1151 (MRR g/min),= 0.1151 whichg/min), can which be attributed can be attributed to the debris to the egress debris induced egress throughinduced thethrough tailor-made the tailor slot-made [32]. slot [32].

Figure 1 12.2. The Material Removal Rate ( (MRR)MRR) for different shaped electrodes.electrodes.

It is noticeablenoticeable that, apart from the triangulartriangular shape, the cross sectionssections with rounded corners produced higher MRRs than those obtained with sharp-corners.sharp-corners. This can be ascribed to the larger cross-sectionalcross-sectional areas areas of of the the square square and and hexagonal hexagonal round-cornered round-cornered cross cross sections, sections, associated associated with larger with machininglarger machining contact contact areas than areas those than with those sharp-corners. with sharp- Thecorners. exception The exception was the triangular was the trian electrode,gular whereelectrode, the sharp-cornered where the sharp electrode-cornered exhibited electrode a higher exhibited MRR a than higher the round-cornered. MRR than the round However,-cornered. in the However, in the case of the sharp-cornered triangular electrodes, the depth of the machined hole was less (22.80 mm) than the other machined holes, which had depths between 24.61 mm and 25.33 mm (see Table 5). It is worth stating that the MRR decreases with the depth of machining since debris evacuation becomes increasingly difficult the deeper the hole. This means the high MRR in the case of the sharp-cornered triangular electrode was due to the large amount of material removed in a short time at a relatively short depth, as this particular trial was associated with a high TWR and a large hole taper angle (see Figures 6 and 10). Hence, it is possible to conclude that it is not fair to compare the MRR value obtained with the sharp-cornered triangular electrode with the MRR values obtained in the other tests because the latter were obtained for deeper holes. When comparing the performance of different electrode shapes, the maximum MRR value (0.1257 g/min) was achieved with the round-cornered hexagonal electrode. This can be explained by the large contact area between the electrode and the processed material that increased the rate of erosion of the material. In addition, the high value of MRR obtained in the case of a round-cornered

Materials 2019, 12, 989 16 of 20 case of the sharp-cornered triangular electrodes, the depth of the machined hole was less (22.80 mm) than the other machined holes, which had depths between 24.61 mm and 25.33 mm (see Table5). It is worth stating that the MRR decreases with the depth of machining since debris evacuation becomes increasingly difficult the deeper the hole. This means the high MRR in the case of the sharp-cornered triangular electrode was due to the large amount of material removed in a short time at a relatively short depth, as this particular trial was associated with a high TWR and a large hole taper angle (see Figures6 and 10). Hence, it is possible to conclude that it is not fair to compare the MRR value obtained with the sharp-cornered triangular electrode with the MRR values obtained in the other tests because the latter were obtained for deeper holes. When comparing the performance of different electrode shapes, the maximum MRR value (0.1257 g/min) was achieved with the round-cornered hexagonal electrode. This can be explained by the large contact area between the electrode and the processed material that increased the rate of erosion of Materials 2019, 12, x FOR PEER REVIEW 16 of 20 the material. In addition, the high value of MRR obtained in the case of a round-cornered hexagonal electrodehexagonal can electrode be attributed can be attributed to the self-pumping to the self-pumping of the dielectric of the dielectric fluid, fluid in particular,, in particular because, because this self-pumpingthis self-pumping action action is uniformly is uniformly distributed distributed into sixinto different six different regions regions along along the circumference the circumference of the drilledof the drilled hole. This hole. homogenous This homogenous flushing flushing (debris (debris egress) egress) leads to leads a more to consistenta more consistent machining machining process and,process eventually, and, eventually a higher, a MRR. higher MRR. InIn FigureFigure 1313,, itit can be observed that, in general, the bigger the contact area between the electrode and the processedprocessed material is and, thus, the smaller the debris egress gap is, the higher the MRR obtained is duringduring thesethese EDMEDM trials.trials. The observed relationshiprelationship between the achievableachievable MRR andand debris egress gap has two exemptions to its general tendency, namelynamely the results obtainedobtained in case of thethe sharp-corneredsharp-cornered triangulartriangular electrode electrode and and the the cylindrical cylindrical electrode, electrode, and and the the possible possible reasons reasons for thisfor werethis were formerly formerly discussed. discussed.

Figure 13.13. The relationshiprelationship between the MRR and thethe debrisdebris egressegress gap.gap.

To sum sum up, up, one one can can say say that that the the initial initial hypothesis hypothesis for for the the MRR MRR was was proved. proved. Especially, the cylindricalcylindrical electrodeelectrode obtainedobtained thethe lowestlowest MRRMRR value when compared with the performanceperformance of shaped electrodes.electrodes. Nevertheless, it is worth emphasizing that, in general, a higher MRR was achieved when round-corneredround-cornered shapedshaped electrodeselectrodes werewere utilized.utilized. To provideprovide aa moremore comprehensivecomprehensive overviewoverview ofof thethe electrode’selectrode’s performance,performance, thethe ratioratio ofof thethe MRR toto thethe TWRTWR forfor eacheach electrodeelectrode isis presentedpresented inin FigureFigure 1414..

Materials 2019, 12, 989 17 of 20 Materials 2019, 12, x FOR PEER REVIEW 17 of 20

Figure 14.14. The ratioratio of the MRR to the TWR.TWR.

From Figure 1414,, itit cancan bebe determined determined thatthat thethe round-corneredround-cornered squaresquare electrode providesprovides the more preferable preferable performance, performance, balancing balancing the MRR the MRR and TWR. and Moreover, TWR. Moreover the sharp-cornered, the sharp hexagonal-cornered electrodehexagonal shows electrode a better shows performance a better performance when compared when withcompared the remaining with the electrodes.remaining electrodes. Finally,Finally, itit isis worthworth emphasizingemphasizing thatthat thethe electionelection ofof thethe electrodeelectrode shapeshape should depend on the processprocess criterion that the user would like to to optimize,optimize, which does not necessarily havehave to be an equilibrium between thethe differentdifferent attributes.attributes. 5. Conclusions 5. Conclusions This paper has presented an experimental study of self-flushing in the EDM drilling of Ti6Al4V This paper has presented an experimental study of self-flushing in the EDM drilling of Ti6Al4V using rotating shaped electrodes. In particular, EDM tests to drill blind holes were carried out using using rotating shaped electrodes. In particular, EDM tests to drill blind holes were carried out using rotating graphite electrodes with eight different cross sections, where TWR, hole taper angle and MRR rotating graphite electrodes with eight different cross sections, where TWR, hole taper angle and were used as quality marks. MRR were used as quality marks. The main conclusions drawn are as follows: The main conclusions drawn are as follows: • The investigation revealed revealed a a high high correlation correlation between between the the electrode electrode geometry geometry and and feasibility feasibility of ofself self-flushing-flushing when when other other EDM EDM parameters, parameters, such such as rotational as rotational speed, speed, workpiece workpiece material material and andelectrode electrode material, material, were werekept keptconstant. constant. In particular, In particular, the results the results show showa greater a greater capacity capacity of the ofself the-flushing self-flushing technique technique of the shaped of the electrodes shaped electrodes when compared when compared with the simple with the cylindrical simple cylindricalelectrode. electrode. • InIn general,general, thethe resultsresults demonstrateddemonstrated thatthat round-cornerround-corner shaped electrodes are associatedassociated with a lowerlower TWRTWR andand holehole tapertaper angleangle andand a higherhigher MRRMRR whenwhen compared to electrodeselectrodes with sharp corners.corners. In In particular, particular, the highest and lowest lowest TWR TWR values values (0.0932 (0.0932 mg/s mg/s and and 0.0578 0.0578 mg/s, mg/s, respectively)respectively) werewere foundfound inin thethe casecase ofof thethe slottedslotted cylindricalcylindrical electrodeelectrode andand thethe round-corneredround-cornered squaresquare electrode,electrode, respectively. respectively. Although, Although, the the electrode electrode wear wear varied varied between betwe 3.85en 3.85 mm mm and 7.2and mm, 7.2 themm, percentage the percentage of the electrode of the electrode wear was wear found was quite found significant quite significant and ranged and between ranged 12.83% between and 22.57%.12.83% and 22.57%. • Moreover, thethe largestlargest holehole tapertaper angle was produced for the sharp sharp-cornered-cornered triangular electrode ◦ ◦ (8.114(8.114°)),, and the smallest value (1.040(1.040°) was achieved with the cylindrical electrode. Howeve However,r, ◦ lowlow hole hole taper taper angles angles (1.292 (1.292°) were) were also also obtained obtained with the with slotted the slotted cylindrical cylindrical and round-cornered and round- cornered hexagonal electrodes. Furthermore, the investigation has shown that the round-

Materials 2019, 12, 989 18 of 20

hexagonal electrodes. Furthermore, the investigation has shown that the round-cornered hexagonal electrode exhibited the maximum MRR (0.1257 g/min), while the cylindrical electrode gave the minimum MRR (0.0983 g/min). • The findings of this research study show diverse trends for the effect of electrodes shapes on the EDM process aspects, namely TWR, hole taper angle and MRR. For a high MRR, the round-cornered hexagonal electrode is the first choice, though a cylindrical electrode has to be used if a low hole taper is to be attained, while a low TWR can be attained using a round-cornered square electrode. • The results of this research confirmed the initial hypothesis that, in general, using shaped electrodes gave better results in terms of MRR but not the case of TWR and hole taper angle when compared with the performance of the plain cylindrical electrode. Moreover, the more robust the geometry of the electrode was, the lower the value of the TWR and hole taper angle could be achieved. Nevertheless, if the debris egress gap is too narrow, the TWR will increase due to the difference in the volume of the electrode that depends on the electrode shape used.

In future work, these possible research lines are detected:

• In future work, a further investigation of self-flushing EDM processes should be conducted utilizing a wider range of electrodes with different cross sections, different radii of the rounded-corners and other polygonal electrodes, such as pentagonal or rectangular. Moreover, the self-flushing phenomenon should be evaluated in different electrode materials, for instance in copper, copper tungsten or brass. Furthermore, combinations of different tool cross sections and other flushing approaches (e.g., ultrasonic-assisted technique) should be considered. Moreover, the effect of different speeds of rotation can also be examined. In addition, the value of the debris egress gap and its distribution over the cross section could be optimized using fluid-dynamics simulation. • It is important to emphasize that the highest MRR and the lowest tool wear were not achieved using a single electrode, and thus, further research work could be conducted along these lines to improve the machining efficiency in EDM machining of blind holes. This also could entail examining new EDM strategies (the number of electrodes and different electrode shapes) to machine deep holes with a high MRR, a low TWR and a high dimensional accuracy. • Finally, an important research work should be conducted with regards to the influence of the usage of the rotating shaped electrodes and the self-flushing techniques on the surface quality and integrity of the workpiece.

Author Contributions: Both coauthors contributed equally to the work presented in this research paper. Conceptualization, M.G. and A.E.; methodology, A.E. and M.G.; software, M.G. and A.E.; validation, A.E. and M.G.; formal analysis, M.G. and A.E.; investigation, A.E. and M.G.; data curation, M.G. and A.E; writing—original draft preparation, A.E. and M.G.; writing—review and editing, A.E. and M.G.; visualization, M.G. and A.E.; funding acquisition M.G. and A.E. Funding: The authors thank the Ministry for Economic Development and Competitiveness and the Department of Industry, Innovation, Trade and Tourism of the Basque Government. The authors also acknowledge support by Deutsche Forschungsgemeinschaft and the open access publishing fund of Karlsruhe Institute of Technology. Acknowledgments: The authors gratefully acknowledge the technical help provided by Eng. Ahmed Elsayed from the Faculty of Engineering, Helwan University, Egypt, Mr. Ion Bengoetxea, Mr. Josu Leunda, Dr. Iban Quintana and Dr. Alberto Villar from IK4-Tekniker, Spain, Dr. Samuel Bigot from Cardiff University, UK and Mr. Ramy Abdallah from University of Birmingham, UK. Conflicts of Interest: The authors declare no conflict of interest.

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