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ScienceDirect Procedia CIRP 42 (2016) 51 – 56

18th CIRP Conference on Electro Physical and Chemical Machining (ISEM XVIII)

Optimizing Machining Parameters of Compound Machining of Inconel718 H. Dong, Y. Liu*, Y. Shen, X. Wang College of Mechanical and Electronic Engineering, China Un iversity of Petroleum, Qingdao 266580, China

* Corresponding author. Tel.: +86-0546-8392303; fax: +86-0546-8393620. E-mail address: [email protected]

Abstract The compound machining (CM ) compounded with electrical discharge machining (EDM ) and arc machining is a novel approach to process difficult-to-machine materials especially for the aerospace materials such as Inconel718. An exciting material removal rate (M RR) and a sm all diameter of overcut (DOC) can be received in this approach. A suitable selection of machining parameters plays a key role in CM of Inconel718. Therefore, a various machining parameters affecting M RR and DOC such as peak current, electrode rotation speed and peak voltage were analyzed. This paper highlights the combined influence of these parameters in CM . A 23 factorial with central composite design (CCD) and a total of 20 experiments were conducted in this study. The mathematical model of objective function between the machining parameters and the dependent variable was obtained by utilizing the response surface method (RSM ). Finally, the optimized machining parameters can be acquired in CM of Inconel718.

©©2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license 2016 The Authors. Published by Elsevier B.V. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of 18th CIRP Conference on Electro Physical and Chemical M achining (ISEM Peer-review under responsibility of the organizing committee of 18th CIRP Conference on Electro Physical and Chemical Machining XVIII). (ISEM XVIII) Keywords: EDM, machining parameters, response surface method, material removal rate, diameter of overcut

1. Introduction The nickel-based superalloy such as Inconel718 has temperature strength, high corrosion and oxidation resistance [1]. It can keep the stability of mechanical and chemical properties at extreme temperature. This superalloy is widely used in aerospace industry as ideal materials for aero-engine components particularly [2]. Nevertheless, it is difficult to be mach ined using the traditional processing methods such as milling, broaching or grinding due to its low thermal conductivity and extreme high strength [3]. Electrical discharge machining (EDM ) is a non-contact manufacturing approach between electrode and workpiece which can erode material depending on thermoelectric process between anode and cathode regardless of the hardness and strength of the workpiece. Therefor EDM is an alternative to traditional processing methods in machining Āhard-to-cutā materials. EDM including wire EDM and sinking EDM are widely used in aerospace industry to process nickel-based

alloys [4-6]. Nevertheless, the low efficiency of this conventional EDM limits the spread of this technology. Wang et al. [7] p roposed a compound machining (CM) which co mbined electrical d ischarge machin ing (EDM) with arc machin ing. The proposed CM utilizes EDM to break down the dielectric between electrode and workpiece, and arc mach ining to melt workpiece material. By rotation o f the electrode and flushing of the working flu id, it can remove material fro m the workpiece. It imp roves material removal rate (MMR) great ly by the using of arc processing and increases the discharge gap by the using of EDM. It overcomes the lo w efficiency of EDM and the low percentage of energy input in working area. The maximu m MRR of this CM reached 15062 mm3 /min when processing Inconel718 [8]. A suitable selection of machin ing parameters plays a key role in CM of Inconel718. Optimizat ion of machin ing parameters in EDM has investigated by many researchers. I. Puertas et al. [9] carried out a model of Ra and Rq parameters in the function of current, pulse on time and pulse off time in sinking EDM using a full factorial design. M.S. Hewidy et al.

2212-8271 © 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of 18th CIRP Conference on Electro Physical and Chemical Machining (ISEM XVIII) doi:10.1016/j.procir.2016.02.185

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[10] studied the effect of peak current, duty factor, wire tension and water pressure on volumetric metal removal rate, wear ratio, and surface roughness when machining Inconel 601 using wire EDM . They obtained the response function by utilizing response surface method (RSM ). An optimal setting of process parameters wh ich in fluences the MRR and surface roughness in power mixed EDM was acquired by H.K. Kansal [11] using RSM. After that, they conducted the confirmat ion experiments to verify the optimal conditions. In this study, a various manufacturing parameters influencing MRR and diameter of overcut (DOC) such as peak current, electrode rotation speed and peak voltage were analyzed. The mathematical model of object ive function between the machining parameters and the dependent variable was acquired by utilizing the response surface method (RSM).An optimized process conditions were obtained in CM of Inconel718.

and a depth of 2 mm were conducted on the specimen. WCe20 is widely used in arc machin ing because of its low content of radioactive element and favourable arcing properties.WCe20 with a diameter of 4mm was applied as electrode and water based dielectric fluid ( 90% of distilled water and 10 % of emulsified o il) flushed to the machin ing gap in present work. A p recision balance (Sartorius BS224S) with an accuracy of 0.1 mg was applied to measure the weight losses of the WCe20 and Inconel718. A stopwatch with an accuracy of 0.1 s was applied to record the processing time. A vernier caliper was applied to get the process width on the specimen. MRR and DOC were served as performance indicator of processing efficiency. MRR was expressed as volume of material losses in a unit interval. Moreover, DOC reflected the accuracy of processing. These two parameters can be calculated by formula 1-2 ሺ୑ ି୑ ሻ ଵ (1)  ൌ ౩ౘ ౩౗ ή ஡౩

2. Experimental details 2.1. Experimental principle Equip ment used in this experiment is a 3-axis high speed milling EDM machine. The schematic of this co mpound mach ining is shown in Fig.1. The power supply of the mach ine is co mbined EDM module with arcing machin ing module. The two power sources are isolated by diodes and connected to electrode and workpiece. The EDM module provides pulse signal (peak voltage of 220 V and maximu m current of 30 A) wh ich helps to break down the dielectric between electrode and workpiece. A DC signal (peak voltage of 70 V and maximu m current of 700 A) is supplied by arc mach ining module. The highly efficient DC power causes arc discharge which makes instant high temperature and pressure in the working gap melt materials on the workpiece. By rotation of the electrode and flushing of the working fluid, the debris fro m p rocess can be swept efficiently and the stability of machining was ensured.

୲

In wh ich M sb and M sa represent the mass of the specimen before and after processing, respectively. ρs represents the density of the specimen, here means Inconel718, and t represents the machining time. (2) DOC=Ds -De In which Ds represents the width of groove remained on the specimen after process . De represents the diameter of electrode. 2.3 Design of experiment Response surface methodology is a method of experimental design, aiming to explo re the relationships between serval independent variables and response variables. It is a sequential experimentation strategy which based on a set of designed experiments to establish a mathematic model of the interaction. Applying the model of the response, an optimu m or near optimal value can be obtained. In the general case, the RSM mathematic model can be expressed as a quadratic equation shown as formula (3) ௞

௞

௞

ܻ௨ ൌ ܾ଴ ൅ ෍ ܾ௜ ‫ ݔ‬௜ ൅ ෍ ܾ௜௜ ‫ ݔ‬ଶ௜ ൅ ෍ ܾ௜௝ ‫ݔ‬௜ ‫ݔ‬௝ ˄͵˅ ௜ୀଵ

Fig. 1.T he schematic of compound machining

2.2. Experimental setup and conditions The specimen applied in this study was Inconel718 with a size of 60 mm×10 mm×5 mm. A mach ined length of 10 mm

௜ ୀଵ

௝வ௜

Where Yu rep resent responsive variab les such as MRR and DOC and xi ( i=1, 2…k) represent independent variables affecting these processing indicators such as peak current, electrode rotation speed and peak voltage in current investigation. The terms b 0 , b i , b ii and b ij are the second order regression coefficients. Central co mposite design (CCD) is a second order design method applied broadly based on RSM. Using the smallest number of experiments, the requirements of the test can be satisfied and the equation of the response surface can be established. CCD comp rises three kind of experimental points with various characteristic, which are shown in formula 4 (4) N=mc+mr+mo Where N represents the total experimental nu mber. mc is defined as 2m that means cube points at the corners of a unit cube, representing the comprehensive test number when all the elements are selected as ±1 level. mr is defined as 2m that

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means the number of axial points or star points along the axes. mo represents the experimental central points when every element is selected as 0 level. In co mpound mach ining of Inconel718,the M RR and DOC were chosen as responsive variables and a various machin ing parameters affecting M RR and DOC such as peak current (I p ), electrode rotation speed (Re) and peak voltage (Up ) were selected as independent variables. m indicates the number of elements, thus m=3 in present work, 23 cube points, 6 axial points and a total of 20 experiments were conducted. The values of coded and actual value of each parameter applied in this work are listed in Table 1. The experimental matrix and results in the coded form are presented in Table 2. The machin ing parameters peak current, electrode rotation speed and peak voltage are signed as A,B and C respectively. T able 1. Coding of process parameters Level -1.682 -1 0 1 1.682

peak current I(A) 131.82 200 300 400 468.18

electrode rotation speed Re(r/min) 1159.1 1500 2000 2500 2840.9

peak voltage Up (V) 107.96 125 150 175 192.04

T able 2. Experimental matrix and results Experiment no. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

3

I p (A)

Re(r/min)

Up (V)

MRR(mm /min)

DOC(mm)

1 1 1 1 -1 -1 -1 -1 1.682 -1.682 0 0 0 0 0 0 0 0 0 0

1 1 -1 -1 1 1 -1 -1 0 0 1.682 -1.682 0 0 0 0 0 0 0 0

1 -1 1 -1 1 -1 1 -1 0 0 0 0 1.682 -1.682 0 0 0 0 0 0

462.769 531.86 517.163 522.86 254.95 268.014 312.987 223.891 473.32 162.735 368.099 391.785 395.382 404.901 359.437 378.986 391.87 350.235 396.551 345.708

1.6 1.4 1.96 1.72 0.64 0.5 0.9 0.74 1.82 0.64 0.8 1.3 1.6 1.2 1.08 0.98 1.02 0.92 1.2 1

1.66×10-3 ‫ ݔ‬ଶ ‫ݔ‬ଷ-1.49×10-3 ‫ ݔ‬ଶଵ +0.02‫ ݔ‬ଶଷ (6) Where x1 , x2 , x3 represents the actual factors of peak current, electrode rotation speed and peak voltage respectively. These factors are calculated by the formula (7-9) x1=(A-300)/100 (7) x2=(B-2000)/500 (8) x3=(C-150)/ 25 (9) In order to evaluate whether the model fits the experimental data, the analysis of variance (ANOVA ) was conducted as shown in Table 3 T able 3. ANOVA for MRR source model A B C AC BC A2 C2 Residual Lack of Fit Pure Error Cor Total Std.Dev. Mean C.V.% PRESS

Sum of squares 177900 164100 719.74 15.96 2843.33 3426.02 3219.30 2918.07 7872.32 5517.28 2355.04 185800 25.61 375.68 6.82 31236.78

df 7 1 1 1 1 1 1 1 12 7 5 19 R2 Adjusted R2 Pred R2 Adeq Precision

F value 38.73 250.18 1.10 0.024 4.33 5.22 4.91 4.45

p-value Prob>F F” value of the model, the insignificant “Prob>F” value and the “Lack of fit” of 38.73 all indicate the regression model fairly fitted with the observed values. The analysis of variance above shows that this model can be used to navigate the design space.

3. Results and discussion 3.1 Analysis of MRR Using the experimental data and corresponding results listed in Table 1 and Table 2, the mathemat ic model of M RR can be obtained in this study. The equation of the fitted model for M RR can be expressed as equation 5 in terms of coded factors Yu (MRR)=376.16+109.63A-7.26B-1.08C-18.85A C-20.69BC(5) 14.87A 2 +14.16C2 It also can be translated to the equation 6 in terms of actual factors below Yu (MRR)=-377.31+3.12‫ ݔ‬ଵ+0.23‫ ݔ‬ଶ-1.27‫ ݔ‬ଷ-7.54×10-3 ‫ ݔ‬ଵ ‫ݔ‬ଷ -

Mean squares 25411 164100 719.74 15.96 2843.3 3426.0 3219.3 2918.0 656.03 788.18 471.01

(a)

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(b) Fig. 2. Contour map (a) effect of peak current and peak voltage on MRR (electrode rotation speed=2000 r/min); (b) effect of electrode rotation speed and peak voltage on MRR (peak current=300A)

In this section, contour lines with a 20 mm 3 / min interval of MRR are described in Fig. 2. The interaction of peak current (A) and peak voltage (C) on MRR is shown in Fig. 2 (a).The interaction of electrode rotation speed (B) and peak voltage (C) on MRR is shown in Fig. 2 (b). Obviously, the contour shape in Fig. 2 (b) is mo re zig zag, in other words, it is closer to the shape of ellipse more than circle, co mpared with it in Fig. 2(a). That indicates the interaction of electrode rotation speed (B) and peak voltage (C) is more significant than that of peak current (A) and peak voltage (B) on M RR, in accordance with the significant term BC described above. Fig. 3 shows the 3D response surface of effect o f electrode rotation speed and peak voltage on MRR. M RR gradually increases with the increase of electrode rotation speed when the peak voltage remains a large value. However, M RR has the decreasing tendency with the increase of electrode rotation speed at the small value of the peak voltage. The results may be explained by the fact that the machining gap increases with the peak voltage increasing, thus a high speed rotatory electrode can expel more materials and debris out of the mach ining gap when the electrode is closer to the workpiece at the small value of the peak voltage. There may be a critical

Fig. 3. Response surface of effect of electrode rotation speed and peak voltage on MRR (peak current=300 A)

Fig. 4. Response surface of effect of electrode rotation speed and peak current on MRR (peak voltage=150 V)

value of peak voltage above which the increase of electrode rotation speed has a reverse effect on the debris expelling because of the instability of the process. The effect of peak voltage and peak current on MRR is described in Fig. 4. M RR significantly increases with the increase of peak current because mo re energy is transferred to the machined zone. However, M RR decreases with the electrode rotation speed increasing at the peak current of 150V. 3.2 Analysis of DOC The equation of the mathematic model for DOC can be expressed as equation 10 in terms of coded factors Yu (MRR)=1.03+0.43A-0.15B+0.10C+0.058A 2 +0.12C2 (10) And the final equation in terms of actual factors is shown in equation 11 Yu (MRR)=-4.50+8.09‫ݔ‬ଵ -2.96h10-4 ‫ ݔ‬ଶ-0.05‫ ݔ‬ଷ+ 5.83h10-6 ‫ ݔ‬ଶଵ +1.89h10-4 ‫ ݔ‬ଶଷ (11) T able 4. ANOVA for DOC source model A B C A2 C2 Residual Lack of Fit Pure Error Cor Total Std.Dev. Mean C.V.% PRESS

Sum of squares 3.22 2.54 0.30 0.15 0.050 0.20 0.13 0.082 0.047 3.35 0.096 1.15 8.33 0.39

df 5 1 1 1 1 1 14 9 5 19 R2 Adjusted R2 Pred R2 Adeq-Pre

Mean squares 0.64 2.54 0.3 0.15 0.050 0.20 9.189h10-3 9.078h10-3 9.387h10-3

F value 70.05 275.95 32.55 15.90 5.39 22.22

p-value Prob>F F” value and the “Lack of fit” of 70.05 all indicate the regression model fairly fitted with the observed values. The analysis of variance above shows that this model can be used to navigate the design space of DOC.

rotation speed has a reverse effect on DOC above crit ical peak voltage. 4. Optimal Results In order to get the optimal co mb ination of the various mach ining parameters such as peak current, electrode rotation speed and peak voltage, the mathematic model of M RR and DOC was analy zed in present work. The independent variable value of peak current, electrode rotation speed and peak voltage was limited fro m 131 A to 468 A, 1159 r/ min to 2840 r/ min and 107 V to 192 V respectively. Finally, A relatively high MRR of 450.118 mm3 / min comb ined with a desired DOC of 0.89 mm were obtained in CM o f Inconel718 corresponding to the machining parameters ’ value of 301.67 A, 2840 r/min and 113.07 V respectively. 5. Conclusions

Fig. 5. Response surface of effect of peak current and peak voltage on DOC (electrode rotation speed=2000 r/min)

Fig. 6. Response surface of effect of peak current and electrode rotation speed on DOC (peak voltage=150 V)

A mathematical model of objective function between the mach ining parameters and the dependent variable such as MRR and DOC was proposed by utilizing the response surface methodology (RSM ) in CM of Inconel718. Th is responsive model is time saving as well as efficient in determining the co mbined influence of peak current, electrode rotation speed and peak voltage on MRR and DOC. (1) Peak current has a significant effect on M RR, wh ilst DOC main ly influenced by peak current, electrode rotation speed and peak voltage on basis of ANOVA. (2) MRR and DOC significantly increases with the increase of peak current because of more energy transferred to the mach ining zone. There may be a crit ical value of peak voltage, above which electrode rotation speed has a reverse effect on MRR. DOC decreases firstly and then has a rising tendency with the increase of peak voltage regardless of peak current and it decreases with the increase of electrode rotation speed. (3) The optimal co mbination of the various machin ing parameters is as follows: peak current of 301.67 A, electrode rotation speed of 2840 r/ min and peak voltage of 113.07 V. Moreover, a relat ively high M RR of 450.118 mm 3 / min combined with a desired DOC of 0.89 were obtained in CM of Inconel718. Acknowledgements

As shown in Fig. 5 and Fig. 6, DOC significantly increases with the increase of peak current because of more energy transferred to the machined zone. Peak current has a considerable influence on DOC and it is served as significant term in ANOVA. DOC decreases firstly and then has a rising tendency with the increase of peak voltage regardless of peak current in Fig. 5. A possible reason for these findings is discharge gap, which has the same vary ing pattern with peak voltage. And, there may be a crit ical d ischarge gap, above which the d ischarge become irregular and expands to other place, below which more material is molten, and at which the CM has the best accuracy. Fig. 6 shows the effect of peak current and electrode rotation speed on DOC, which has the decreasing tendency with the increase of electrode rotation speed. The results may be explained by the fact that electrode

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