,e flow field distribution in an interelectrode gap is one of the important factors that affect the machining accuracy and surfacequality in the electrochemical machining (ECM) process for aircraft blades. In the ECM process, some process parameters, e.g.,machining clearance, processing voltage, and solution concentration, may result in electrolyte fluid field to be complex andunstable, whichmakes it very difficult to predict and control themachining accuracy of ECM.,erefore, 30 sets of experiments forcooling hole making in ECM were carried out, and furthermore, the machining accuracy and stability of cooling hole wereconcentrated. In addition, the flow channel of the geometrical model of the gap flow field was established and analyzed accordingto the electrolyte flow state simulation by CFD. ,e effects of the flow velocity mode on the machining accuracy and stability forcooling hole making were investigated and determined in detail.
1. Introduction
With the development of aviation technology, modernaircraft engines can generate higher power for the same unitsize and therefore improve fuel efficiency. Turbine bladespossess a large number of cooling channel holes (as shown inFigure 1), and the typical cooling hole diameter and aspectratio are in the range of 1–4mm and 40–200, respectively.Generally speaking, turbine blades are made of super-heat-resistant alloys, such as nickel alloy and titanium alloy.Peetermans and Lehmann [1] stressed that turbine blades aremade of single-crystal nickel-based superalloys that canwithstand the highest temperatures and loads. Liu et al. [2]pointed out that although the military and civil high bypasspressure ratio turbofan engines has adopted the existingmature materials and technologies, it still has special re-quirements in the manufacture of some key parts, whichneed to be tackled. Bilgi et al. [3] explained that it is difficultfor conventional hole making processes to drill these holes innickel-based superalloys due to their low thermal conduc-tivity, high toughness, high work hardening, and specialaspect ratios. Fortunately, the emergence of nontraditionalmachining has effectively solved the above problems. ,ey
are (i) electrical discharge machining (EDM), (ii) laser beammachining (LBM), and (iii) electrochemical machining(ECM). Research works by Liu et al. [4] showed that tra-ditional nanosecond laser drilling is a high-efficiency andlow-cost manufacturing process for hole making, but un-avoidable recasting layer or evenmicrocracks may appear onthe machined surface due to laser heat transfer. Wang [5]pointed out that the EDM can achieve high perforationspeed and machining efficiency and also can easily producethe recasting layer. Zhang et al. [6] in their work indicatedthat electrochemical drilling is an important method forproducing small holes in difficult-to-machine materials suchas titanium alloy and nickel-based alloy. A study by Jain andPandey showed that the accuracy of holes could greatly beimproved by use of bits rather than bare tools as ECM tools,and they found that the overcut difference between the topand bottom of the drilled hole is less than 5mm [7]. On theshop floor, the use of bit can be used to drilling holes. Aliet al. [8] conducted a partial design experiment with STEMto study the effects of important process parameters on holediameter and hole taper, provided the guidelines forselecting process conditions, and developed the processmodel of selecting the process parameters for a desired hole
HindawiAdvances in Materials Science and EngineeringVolume 2019, Article ID 4219323, 11 pageshttps://doi.org/10.1155/2019/4219323
quality. ,is provides a basis for the parameter selection ofthe electrochemical machining hole. Chen et al. [9] con-ducted orthogonal experiments to investigate ECM of Ti60to determine the influences of some electrochemical processparameters on the surface roughness. ,ey found that usingsuitably optimized parameters for ECM can greatly decreasethe surface roughness of a workpiece. And the optimizationparameters have been successfully applied in blisk blades.Wang et al. [10] used wedge-shaped electrodes in the STEMprocess to machine an inclined cooling hole and obtainedhigh-quality holes with large inclination angles. Li et al. [11]used vacuum extraction shaped tube electrolytic machining(VE-STEM) to manufacture cooling holes of Inconel 718. Intheir study, the effects of key process parameters, i.e., appliedvoltage, electrolyte concentration, and tool feed rate, onprocess efficiency, form accuracy, and process stability hasbeen investigated. ,e effect size was determined using anANOVA analysis. From the results of the electrochemicaldrilling process using the regression analysis, ANOVA, andTaguchi technique, Rao et al. [12] completed the optimi-zation of machining parameters and established a radialovercut model. By using this model and referring to theoptimization parameters, the quality of the holes in theactual production can be improved. Gao et al. [13] managedto eliminate the blackened layer by traditional wire elec-trochemical micromachining (WEEM) of type 304 stainlesssteel through using the double-pulsed WEMM method.Rajurkar et al. [14] pointed out that ECM is widely used inturbine blademaking because it has no tool electrode loss, noresidual stresses on the machined surface, and comparablehigh material removal rate. However, improving the ma-chining accuracy and processing stability of ECM has beenproved to be a challenging task because the physical features(e.g., electrolytic products and electrolyte flow field) in ECMarea are hard to control during the process.
As the electrolyte flows through the gap in the ECMprocess, the metal workpiece material is dissolved and pro-duces electrolytic products constantly, which are dischargedby the circulation of the electrolyte flow. ,erefore, thedischarge velocity of the electrolytic products is determinedby the distribution of the electrolyte flow velocity and velocityfield mode, which affects the manufacturing efficiency of thecooling hole. In addition, the electrolytic products, whichwere not discharged in time and stranded in the electrolyte
flow passage, may usually result in the electrolyte flow fielddisorder and eddy appearance. ,e eddy can further reducethe update speed of the electrolyte in the ECM gap. With thecontinuous process of ECM, the workpiece material disso-lution is slower and slower in eddy area, and the machiningefficiency of ECM is reduced. In addition, short circuit burnsand serious electrolyte flowmarks can appear together, whichaffect the machining accuracy and stability seriously. Re-cently, with the rapid developments of the computationalfluid dynamics (CFD), some numerical simulations wereconcentrated on the machining accuracy and process stabilityof hole making in ECM. Bilgi et al. [15] used the STEMprocess with pulsed power supply to investigate themanufacturing process of Ni-based alloy deep holes andoptimize the process parameters. Based on the theory of ECMgap distribution, Bilgi et al. [16] established a predictionmodel for the machining gap in ECM of tube electrodes andgot more accurate results than anyone else had done before.,e proposedmodel helps in determining the side gap currentduring electrochemical drilling and would help in evaluatingthe depth-averaged radial overcut. Hu et al. [17] managed todecrease the stray corrosion in trepanning ECM throughusing forward flow of compressed air to blow into the cathodeto form a gas film on themachined surface and carried out thenumerical simulations of the distributions of compressed airand electrolytes in the machining area. In their research, thegas film layer could reduce stray corrosion and taper angleand improve the machining surface quality and accuracy ofthe workpiece. Wu et al. [18] used CFD to simulate the eddydistribution in cavity electrolysis process, obtained the eddystatus and pressure distribution in the ECM gap, and thenguided the optimization design of tool cathode. Zhu et al. [19]put forward a new dynamic lateral flow mode and verified itsrationality through simulation by using computational fluiddynamics software and experiment. However, the main goalsof the above research relative to CFD of flow field in ECM arehow to improve the design accuracy of tool cathode andoptimize the process parameters.,e CFD simulations for theelectrolyte flow velocity field mode are relatively scarce. ,estudies on the effects of the electrolyte flow velocity fieldmodeon machining efficiency in ECM are also still less.
,is paper mainly studied the influences of ECM pa-rameters (applied voltage U and electrolyte concentration ξ)on the machining accuracy, stability, and efficiency in
(a) (b) (c)
Figure 1: (a) Aircraft engine and (c) cooling holes on (b) the turbine blade’s surface.
2 Advances in Materials Science and Engineering
aircraft cooling hole making by the STEM process. ,emodes of the electrolyte flow field were determined by CFD.Moreover, the effects of the electrolyte flow field mode onEMC efficiency were analyzed based on CFD computingresults in detail.
2. Experimental Setup and STEM Process
Considering the characteristics of STEM, the experimentalsetup shown in Figure 2 is selected. It is composed of acontrol system based on the PC movement platform, toolcathode feed system, electrolyte supply system, and moni-toring system. ,e control system can control the multiaxisposition and motion speed of the machine tool and displaythe relevant parameters when the machine tool moves in realtime. ,e tool feed system adopts a servodrive and realizesthe closed-loop control of feed through the grating ruler withthe resolution of 1 μm. In the utilization of the electrolytesupply system, the electrolyte is ejected from the end faceoutlet of the tube electrode by using the high-pressure pumppipe and flows back to the electrolyte tank after impactingthe machining area. ,e monitoring system is used to in-spect the variation of machining current in STEM forcooling hole making.
Figure 3 is the schematic diagram of STEM for aer-oengine cooling hole making. STEM is an electrochemicalprocess to remove metal by anodic dissolution in anelectrolytic cell in which the workpiece is anode and thetool is the cathode. ,e two electrodes are immersed inthe electrolyte solution. ,e electrolyte flows out of thetube electrode inner hole, takes away the dissolvedproducts and heat, and finally completes the cooling holemaking.
Two independent process parameters were selected toconduct the parametric study, i.e., applied voltage (U) andelectrolyte concentration (ξ). All other machining pa-rameters in STEM for cooling hole making were constant.In all experiments, inlet electrolyte pressure Pinlet was0.6MPa, outlet electrolyte pressure Poutlet 0.1MPa, and toolcathode feed rate f was 0.48mm/min. Figure 3 is theschematic diagram of STEM for aeroengine cooling holemaking. ,e tool electrode is a brass tube electrode with adiameter of 0.8mm and an inner diameter of 0.3mm. Itscylindrical surface is insulated by epoxy with a thickness of50 μm in the radial direction, and 0.2mm length is leftwithout insulation at the end of the tube electrode. ,eworkpiece is made of high-temperature nickel-based alloyand Inconel 718 sheet with a thickness of 1.7mm, and itschemical composition is shown in Table 1. Since most ofthe cooling holes in the turbine blades are inclined from 15°to 60° to blades surface, all cooling holes machined inexperiments are inclined in 45° to the Inconel 718 sheetnormal direction in this study. As a result, the depths ofcooling holes are 2.4mm.
In order to verify the effects of ECM process parameterson machining accuracy and machining efficiency, the uni-lateral side gap (Δs) and machining removal rate (MRR) areused as the evaluation indexes of machining accuracy andmachining efficiency, respectively.
Δs refers to the distance between the side wall of the tubeelectrode and the inner wall of the cooling hole when theECM equilibrium state is achieved. Due to its inability tomeasure online during processing, it is generally measuredafter ECM process. ,e calculation formula is as follows:
Δs �D − d
2, (1)
where D is the diameter of the cooling holes (μm) and d isthe diameter of the tube electrode (μm).
MRR is another important parameter to evaluate theefficiency of cooling hole making. It is the weight differenceper unit time of the workpiece before and after ECM, and itscalculation formula is as follows:
MRR �M − m
ρ · T, (2)
where M is the initial weight of workpiece before ECMprocess (g), m is the weight of workpiece after ECM process(g), ρ is the density of Inconel 718 (8.24 g/mm3), and T is theECM time (min).
3. Experiment Works
3.1. Design of Experiments. In the STEM process, appliedvoltage is a key process parameter, which can not onlyestablish the electric field between the electrochemicalmachining electrodes and ensure the continuous process ofelectrochemical machining but also maintain the currentdensity during the machining process. In addition, thefunction of an electrolyte is to build the electrochemicalreaction electrode system and take away electrolyte productsand heat in time. ,erefore, its composition and concen-tration are also the vital factors in the ECM process. ,us inthis study, two independently controllable parameters se-lected to conduct the parametric study of STEM were ap-plied voltage (U) and electrolyte concentration (ξ).Electrolyte composition is NaNO3 solution. Many typicalprocess experiments on optimization of ECM parametershave been completed by researchers and scholars, whichprovide good references for the selection of experimental
Online current measurement unit
Power supply
Tool feeder
Tool holder
Vacuum pump
Workpiece holder
Motor controller
Figure 2: Experimental setup of the STEM process.
Advances in Materials Science and Engineering 3
parameters in this experiment. Wang et al. [20] machinedstainless steel of 0Cr18Ni9 with vacuum extraction ofelectrolytes in NaNO3 solution. ,ey found the machiningprocess was not carried out when the tool feed rate is over15 μm/s at the applied voltage of 15V. Zhu et al. [21]proposed a new method of making multiple holes withelectrolyte-extraction supply. Based on this method, holes inthe stainless steel plate with a thickness of 2mm weremachined successfully with an applied voltage of 10V and180 g/L NaNO3 aqueous solution during the experiments.Under the processing condition of NaNO3 (electrolyteconcentration 15%, inlet pressure 0.55MPa, and appliedvoltage 20V), Wei et al. [22] obtained the wheel hub holeswith high accuracy. Based on the above references and actualECM process requirements, the full factorial design of ex-periments was conducted in this experiment, and the pa-rameter selection is shown in Table 2.
3.2. Effects of Applied Voltage on Machining Accuracy andEfficiency. As listed in Table 2, the values of the appliedvoltage used in cooling hole making are determined to be 5,7, 9, 11, and 13V, and the tool electrode feed rate is 0.48mm/min. Figures 4 and 5 describe the variation curves of theunilateral side gap Δs and MRR with the applied voltage U.
In Figure 4, the unilateral side gap Δs increases withincreasing of applied voltage at all six certain electrolyteconcentrations. For example, at the electrolyte concentrationof 15% (the other ECM process conditions are unchanged),the unilateral side gap Δs increases by more than 126.2%(from 108.5 μm to 245.5 μm) as the applied voltage increasesfrom 5V to 13V. ,e possible reasons for the unilateral sidegap Δs increase with increased applied voltage U could beattributed to the fact that as the applied voltage increases, the
current density between tube tool and cooling hole becomeshigher, which in turn leads to more workpiece materialdissolution per unit time, and then the larger unilateral sidegap.,e same variation tendency between MRR and appliedvoltage can be observed in Figure 5. MRR increases withthe increase of applied voltage when the electrolyte
Table 2: ECM experiment conditions.
Number Voltage (V) Concentration (wt.%) Feed rate(mm/min)
Element C Al Ti Cr Fe Ni Nb Mo(wt.%) 3.12 0.44 0.86 13.91 16.13 38.84 4.24 2.67
4 Advances in Materials Science and Engineering
concentration is the same. At the electrolyte concentrationof 13%, MRR increases by more than 45.4% (from0.447mm/min to 0.650mm/min) as the applied voltageincreases from 5V to 13V. Under the condition of otherECM parameters unchanged, higher applied voltage meansmore metal materials were dissolved from cooling hole perunit time.
Figure 6 shows three cooling hole samples machined bydifferent applied voltages. ,e cooling hole shown inFigure 6(a) was machined at an electrolyte concentration of5% and applied voltage of 5V. Under these conditions, theECM process stability cannot be guaranteed because theshort circuit occurred contentiously and finally formed a
“Blind Hole.” ,e reason could be attributed to the fact thatlower applied voltage and electrolyte concentration cannotprovide strong material dissolution ability. Figure 6(b)shows an acceptable cooling hole shape, but there are alsoa small amount of edge corrosion around cooling hole edgewith the occurrence of only one short circuit. In comparison,Figure 6(c) provides the best cooling hole forming withoutshort circuits. ,erefore, the increase of applied voltage cansignificantly improve process efficiency, accuracy, and sta-bility for cooling hole making.
3.3. Effects of Electrolyte Concentration on Machining Accu-racy and Efficiency. ,e electrolyte concentrations used inexperiments for cooling hole making are 5%, 7%, 9%, 11%,13%, and 15%. ,e electrode feed rate is also 0.48mm/min.Figures 7 and 8 show the variation curves of the unilateralside gap Δs and MRR with the electrolyte concentration ξ.
Similar to Figure 4, the unilateral side gap Δs increaseswith the increasing of electrolyte concentration at all fivecertain applied voltages. For instant, at the applied voltage of9V, the unilateral side gap Δs increased by more than 120.4%(from 81 μm to 178.5μm) as the electrolyte concentrationincreases from 5% to 13% (as shown in Figure 7). At the sametime, MRR increases from 0.425mm3/min to 0.595mm3/min,increased by 40% (as shown in Figure 8). ,is could beexplained as follows: higher electrolyte concentration meansthere exist more free moving ions in NaNO3 solution, whichcould provide stronger current carrying capacity and enhanceconductivity. In the EMC process for cooling hole making,higher electrolyte concentration leads to greater currentdensity, and the metal material dissolution rate can beaccelerated.,us, Δs andMRR increase simultaneously as theelectrolyte concentration increases.
Figure 9 shows three sample’s morphology photos ofcooling holes formed by different electrolyte concentrations(applied voltage U� 15V). Under the electrolyte concen-tration of 7%, short circuits were detected 3 times and thehole roundness were relatively poor. In addition, seriouscorrosion occurs around the cooling hole edges (as shown inFigure 9(a)). ,e hole shown in Figure 9(b) only had a shortcircuit during machining, and the roundness and edgequality were improved. By contrast, the cooling hole inFigure 9(c) machined at the electrolyte concentrationξ � 15% possesses the best shape accuracy without shortcircuits. ,erefore, the increase of electrolyte concentrationcan significantly improve the process efficiency, accuracy,and stability for cooling hole making, which is similar toapplied voltage.
4. CFD Analysis of Electrolyte Flow Field
According to fundamental theories of ECM, the electrolyteflow status can significantly impact the discharge of elec-trochemical products and further impact the machiningaccuracy and efficiency in the cooling hole making process.In addition, existence of eddies may reduce the electrolyteflow velocity and decrease the stability of ECM. Un-fortunately, the electrolyte flow status and eddy distributions
4 5 6 7 8 9 10 11 12 13 14
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f = 0.48mm/min
Voltage (V)
Uni
later
al si
de g
ap (μ
m)
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Figure 4: Variation curves of the unilateral side gap with thevoltage.
4 5 6 7 8 9 10 11 12 13 140.35
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(mm
3 /min
)
Figure 5: Variation curves of MRR with the voltage.
Advances in Materials Science and Engineering 5
in the ECM process cannot be observed and recorded di-rectly. ,erefore, a simulation study becomes a feasiblechoice in ECM.
4.1. CFDMode Building. Figure 10 is the schematic diagramof electrolyte fluid where a hole is being drilled by STEM. Inthe STEM process, the electrolyte flow field can be ap-proximately divided into two areas, bottom flow area andside flow area. ,e flow field region is composed of the toolcathode contour, workpiece contour, and interelectrode gap.In order to obtain the necessary geometric information datafor building CFD analysis CAD mode, all machined coolinghole samples were measured at five certain cross sections (asshown in Figure 11). Firstly, geometric data of Sections 1 and5 can be measured directly. Subsequently, the samples weremilled 3 times by using DMU 70 eVolution NC machinetools with the same milling depth 0.425mm, and the geo-metric data of Sections 2, 3, and 4 can be measured andcollected (as shown in Figure 12). Based on the measured 5section geometric data, the CAD modes of the cooling holewere rebuilt by using UG software, and a typical CFDanalysis mode is shown in Figure 13.
4.2. Effects of Applied Voltage and Electrolyte Concentrationon Electrolyte Flow Velocity. In this study, six cooling holesamples were selected as the objective for CFD analysis.,ree samples were machined under different appliedvoltages of 7V, 9V, and 13V with unchanged electrolyteconcentration ξ � 9% (experiment nos. 9, 15, and 27 listed inTable 2), and the other three samples were formed underdifferent electrolyte concentrations of 7%, 11%, and 15%(wt.%) with the same applied voltage of 9V (experiment nos.14, 16, and 18 listed in Table 2).
In Figure 14, the unilateral side gap Δs and the bottomgap Δb increase with the increasing of applied voltage at theelectrolyte concentration of 9%. ,e electrolyte flow velocityin the tube electrode inner hole also increases with theincrease in applied voltage (the velocity field changes fromgreen to yellow and then to orange). In the bottom gap area,all the maximum electrolyte flow velocities are approxi-mately 30m/s under different applied voltages of 7V, 9V,and 13V. However, the unilateral gap side Δs and thebottom gap formed at the applied voltage of 13V are ob-viously larger than those of 7V and 9V, which means moreelectrochemical products could be carried out in ECM areain unit time and consequently increase the MRR.
4 5 6 7 8 9 10 11 12 13 14 15 16
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Electrolyte concentration (wt.%)
Uni
later
al si
de g
ap (μ
m)
11V13V
5V7V9V
Figure 7: Variation curves of the unilateral side gap Δs withelectrolyte concentration.
4 6 8 10 12 14 160.35
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11V13V
5V7V9V
Electrolyte concentration (wt.%)
MRR
(mm
3 /min
)
Figure 8: Variation curves of MRR with electrolyte concentration.
Similar to Figure 14, in the case of other ECM processparameters unchanged, the electrolyte flow velocity in thetube electrode inner hole increases with the electrolyteconcentration increase (from 24.85m/s to 28.35m/s), whichmeans that the electrolyte pump needs to provide moreelectrolyte solution in unit time. In addition, size of theunilateral side gap Δs and the bottom gap Δb also becomeslarger with increase in electrolyte concentration. In thebottom gap area, the high-velocity flow region of an elec-trolyte (red color region) is enlarged with electrolyte con-centration ξ variation from 7% to 15%. ,e same variationtendency may be observed in the side gap (Figures 15(a)–15(c)). A larger machining gap, more electrolyte supply, andhigher flow velocity of electrolyte make it easier to dischargeelectrochemical products and further improve MRR.
4.3. Effects of Applied Voltage and Electrolyte Concentrationon Electrolyte Flow Eddy. In this study, the influence ofapplied voltage and electrolyte concentration on electrolyteflow eddy distribution was analyzed by simulating thecooling hole samples selected (see Section 4.2).
Figures 16(a)–16(c) shows us the distribution status ofelectrolyte flow eddies with applied voltage variation atunchanged electrolyte concentration ξ � 9%. It is obviousthat the eddies are mainly locate in the bottom gap area.With the increase in applied voltage, the eddy current areabecomes larger, but the turbulent kinetic energy exhibitsdecreased tendency. For instance, at the electrolyte con-centration of 9%, the turbulent kinetic energy decreases bymore than 34.49% (from 79.32 J/kg to 51.96 J/kg) as theapplied voltage increases from 7V to 13V. Higher turbulentkinetic energy means more serious eddy current, whichcould result in metal dissolved products staying in thebottom gap area, consequently increasing short circuitprobability. According to experiment records, at the appliedvoltage of 13V and electrolyte concentration of 9%, thecooling holes were machined successfully without the oc-currence of short circuits. In comparison, four times and onetime short circuits were detected at the applied voltage of 7Vand 9V, respectively. ,erefore, ECM process stability maybe improved in the case of lower eddy current energy. Inaddition, there are some lower intensity eddies locating inthe side gap area. Because the outer wall of the tube electrode
Figure 14: Electrolyte flow velocity distribution under different applied voltages. (a) U� 7V, ξ � 9% (wt.%). (b) U� 9V, ξ � 9% (wt.%).(c) U� 13V, ξ � 9% (wt.%).
8 Advances in Materials Science and Engineering
was insulated, there is no electric field in the side gap zone.,erefore, metal dissolution does not occur in this spe-cialized zone. ,e harmful effects of low-intensity eddies onECM process stability in the side gap area may be ignorednearly.
Figure 17 shows us the distribution status of electrolyteflow eddies with electrolyte concentration variation at un-changed applied voltage. In Figures 17(a)–17(c), two keypoints needs special attention. On one hand, with theelectrolyte concentration increase, the turbulence kineticenergy decreases gradually. At the applied voltage of 9V, theturbulent kinetic energy of eddy decreases by more than29.88% (from 64.79 J/kg to 45.43 J/kg) as the electrolyte
concentration increases from 7% to 15%. On the other hand,the eddies show an exciting trend to leave the bottom gapand then enter the side gap region in the case of increase inelectrolyte concentration. Compared with high-intensityeddies, low-intensity eddies cannot cause serious accumu-lation of electrochemical products. ,e eddies entering theside gap region would be favorable to dissolve the metalmaterials at the bottom of cooling holes smoothly. In ad-dition, the outer wall of the tube electrode was insulated, andthe eddies may have no harmful effects on the shape ac-curacy of the machined cooling hole. Consequently, theportability of short circuit occurrence will reduce. For in-stance, two times short circuits were detected at the
Velocitycontour 1
3.107e + 001
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(c)
Figure 15: Electrolyte flow velocity distribution under different electrolyte concentrations. (a) U� 9V ξ � 7% (wt.%). (b) U� 9V ξ � 11%(wt.%). (c) U� 9V ξ � 15% (wt.%).
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Turbulencekinetic energy
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Figure 16: Electrolyte flow eddy distribution under different applied voltages. (a) U� 7V, ξ � 9% (wt.%). (b) U� 9V, ξ � 9% (wt.%).(c) U� 13V, ξ � 9% (wt.%).
Advances in Materials Science and Engineering 9
electrolyte concentration of 7% and no short circuit wasdetected at the electrolyte concentration of 11% and 15%.
5. Conclusions
In this paper, the influence of applied voltage and electrolyteconcentration on process efficiency, form accuracy, andstability of cooling hole making in STEM were investigatedby means of full factorial design of experiments. Moreover,the electrolyte flow velocity field and eddy distribution statuswere evaluated and determined by CFD simulation analysis,which possess guiding significance for the optimization ofprocess parameters of ECM. Key findings are as follows:
(i) ,e unilateral side gap Δs and MRR increase ob-viously with the increase of applied voltage andelectrolyte concentration. ,e shape accuracy ofcooling holes including hole roundness and edgequality was improved with the increasing of appliedvoltage and electrolyte concentration.
(ii) Higher process stability may be obtained in the caseof higher applied voltage and electrolyte concen-tration because the relatively large side gap Δs andthe bottom gap Δb could provide stronger dischargecapacity of electrolytic products and reduce shortcircuit portability.
(iii) With the increase of applied voltage and electrolyteconcentration, the flow velocity of electrolyte so-lution becomes faster, but the turbulence kineticenergy of eddy decreased. In addition, the eddydistribution shows the tendency to leave the bottomgap and enter the side gap with the increase inelectrolyte concentration, which will be beneficial to
improve ECM process stability for cooling holemaking.
Data Availability
,e prior studies and datasets are cited at relevant placeswithin the text as references [15–17].
Conflicts of Interest
,e authors declare that they have no conflicts of interest.
Acknowledgments
,is project was supported by the Natural Science Foun-dation of China (No. 51775321).
References
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6.479e + 001
5.837e + 001
5.196e + 001
4.554e + 001
3.912e + 001
3.271e + 001
2.629e + 001
1.987e + 001
1.346e + 001
7.040e + 000
6.237e – 001[J·kg−1]
Turbulencekinetic energy
contour 1
(a)
4.927e + 001
4.441e + 001
3.956e + 001
3.470e + 001
2.984e + 001
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2.013e + 001
1.527e + 001
1.041e + 001
5.554e + 000
6.965e – 001[J·kg−1]
Turbulencekinetic energy
contour 1
(b)
[J·kg−1]
Turbulencekinetic energy
contour 14.543e + 001
4.096e + 001
3.649e + 001
3.202e + 001
2.755e + 001
2.309e + 001
1.862e + 001
1.415e + 001
9.678e + 000
5.209e + 000
7.403e – 001
(c)
Figure 17: Electrolyte flow eddy distribution under different electrolyte concentrations. (a)U� 9V, ξ � 7%. (b)U� 9V, ξ � 11%. (c)U� 9V,ξ � 15%.
10 Advances in Materials Science and Engineering
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