National Institute of Technology, Rourkela , Odisha, INDIA. National Institute of Technology, Rourkela , Odisha, INDIA

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1 Optmzaton of Electro-Dscharge Machnng Responses of Super Alloy Inconel 718: Use of Satsfacton Functon Approach Combned wth Taguch Phlosophy Rahul a, Saurav Datta b, Bbhut Bhusan Bswal c, Sba Sankar Mahapatra d a,c Department of Industral Desgn Natonal Insttute of Technology, Rourkela , Odsha, INDIA b.d Department of Mechancal Engneerng Natonal Insttute of Technology, Rourkela , Odsha, INDIA Abstract Inconel 718 s a Nckel-based super alloy whch possesses hgh strength, hgh temperature resstance. Ths super alloy s manly used n the applcatons of automoble, aerospace and defence ndustres. Due to the extreme toughness and work hardenng characterstc of the alloy, the problem of machnng Incone1 718 has become a great challenge. In ths work, machnablty aspects of Inconel 718 have been expermentally nvestgated durng Electro- Dscharge Machnng (EDM) usng Copper tool electrode. Based on fve-factor-fve-level L 25 Orthogonal Array (OA), experments have been conducted by varyng the followng controllable process parameters vz. OCV (gap voltage), peak current (I P ), Pulse-on Tme (T on ), Duty Factor (τ) and Flushng Pressure (F p ). Machnng performance has been evaluated n terms of multple responses such as Electrode Wear Rato (EWR), Roughness average R a (of the EDMed surface), Surface Crack Densty (SCD) and Whte Layer Thckness (WLT). Aforesad performance features have been treated as objectve functons towards optmzaton assumng that these correspond to Lower-s- Better (LB) requrement. The goal has been to determne the best settng of controllable process parameters wthn selected expermental doman to acheve satsfactory machnng performance. Owng to the lmtaton of Taguch s optmzaton phlosophy.e. t cannot solve mult-response optmzaton problem; the concept of satsfacton functon has been attempted n ths research to obtan satsfacton values of ndvdual responses, thus, facltatng aggregaton of mult-response features nto an equvalent sngle ndex. A dstance measure has been computed next whch bascally determnes the separaton of each expermental settng (alternatve) wth respect to the deal expectaton (satsfacton). Fnally, ths dstance functon has been optmzed (mnmzed) by Taguch method. Keywords: Inconel 718; Electro-Dscharge Machnng (EDM); Orthogonal Array (OA); satsfacton functon; Taguch s optmzaton phlosophy 1. Introducton and State of Art Inconel 718 s a Nckel-based super alloy [1]. It fnds huge applcaton manly n the aerospace ndustry, partcularly, n the hot sectons of gas turbne engnes due to ts extreme hgh temperature strength and hgh corroson resstance of Inconel 718. But pecular characterstcs of Inconel 718 such as low thermal conductvty, strong work hardenng tendency, presence of abrasve carbde partcles, affnty to react wth tool materal etc. make t dffcult-to-machne. Hence, t s classfed as Dffcult-to-Cut materal [2]. It has been experenced that for machnng of hgh-strength, super-tough, less electrcally conductve materals such as the aerospace materals ncludng Inconel 718, conventonal routes or tradtonal machnng processes are not sutable [3]. In ths context, non-conventonal machnng operatons especally EDM has been found ample applcatons towards processng of such super alloys to obtan ntrcate shape of the fnshed part component wth desred level of accuracy/dmensonal precson, good surface fnsh. EDM process s carred out due to the thermoelectrcal energy between the work pece and the tool electrode. A pulse dscharge occurs n a small gap between the work pece and the electrode; the process removes unwanted materal (debrs) from the parent metal through meltng and vaporsng. The electrode and the work pece must have electrcal conductvty n order to generate the spark

2 [4]. Ramakrshnan and Karunamoorthy [5] developed of Artfcal Neural Network (ANN) models and performed multresponse to predct and select the best cuttng parameters of Wre Electro-Dscharge Machnng (WEDM) of Inconel 718. Experments were performed under dfferent cuttng condtons of pulse-on tme, delay tme, wre feed speed, and gnton current. The responses (namely materal removal rate and surface roughness) were optmzed concurrently usng Mult-Response Sgnal-to-Nose (MRSN) rato n addton to Taguch s parametrc desgn approach. It was dentfed that the pulse-on tme, delay tme and gnton current were the most nfluencng parameters than wre feed speed on the performance characterstcs. Newton et al. [6] conducted an expermental nvestgaton to determne the man EDM parameters responsble for recast layer formaton on Inconel 718. It was found that average recast layer thckness ncreased prmarly wth energy per spark, peak dscharge current, and current pulse duraton. The recast materal was found to possess n-plane tensle resdual stresses, as well as lower hardness and elastc modulus than the bulk materal. The peak dscharge current, current pulse duraton and energy per spark appeared to be the drvng factors n determnng average recast layer thckness. Table feed rate ndrectly nfluenced energy per spark, whch had a resultng effect on average recast layer thckness. The wre dameter and spark cycle (voltage-off tme) settngs dd not dsplay a sgnfcant effect on average recast layer thckness. Ln et al. [7] addressed exploraton of grey-taguch method for optmzaton of mcro-mllng electrcal dscharge machnng process parameters of Inconel 718 alloy to acheve multple performance characterstcs such as low electrode wear, hgh materal removal rate and low workng gap. The nfluences of peak current, pulse-on tme, pulse-off tme and spark gap on varous response features were analysed. Peak current and spark gap were found as two mportant parameters to acheve optmum results. Aggarwal et al. [8] carred out emprcal modellng of WEDM process parameters for Inconel 718 usng Response Surface Methodology (RSM). The parameters such as pulse-on tme, pulse-off tme, peak current, spark gap voltage, wre feed rate, and wre tenson etc. were selected as nput varables keepng others constant. The performance was measured n terms of cuttng rate and surface roughness. Durng ths research, the pulse-on tme appeared as the most nfluencng factor for cuttng rate and surface roughness of Inconel 718. It was observed that the cuttng rate ncreased wth the ncrease n pulse-on tme; whle, t decreased wth ncrease n spark gap voltage and pulse-off tme. For surface roughness, the pulse-on tme sgnfcantly nteracted wth peak current and spark gap voltage. It was also found that surface roughness ncreased wth the ncrease n pulse-on tme and decreased wth the ncrease n spark gap voltage. The am of the present nvestgaton s to obtan an optmal settng of process parameters ncludng OCV (gap voltage), peak current, Pulse-on Tme, Duty Factor and Flushng Pressure on performance characterstcs of EDM on Inconel 718 work materal n terms of EWR, R a, SCD and WLT. An ntegrated optmzaton route combnng satsfacton functon approach and Taguch method has been proposed heren. 2. Expermental Detals Flat Inconel 718 plates of dmenson 50505have been used as work materal. The chemcal composton (wt%) of Inconel 718 s [N-50.50, Fe-20.24, Cr-18.16, Nb-5.02, Mo-2.91, T-1.05, Al-0.62, Co-0.15, S-0.08, Mn- 0.07, Cu-0.06, C-0.05, P-0.008, Ta-0.003, B-0.003]. A pure copper rod of crcular cross secton 20 2 has been used as tool electrode (Fg.1a). The experments have been carred out on de snkng EDM setup (Make: Electronca ElektraPlusPS 50ZNC, Inda). Commercally avalable EDM ol wth specfc gravty of has been used as delectrc flud. Polarty has been kept postve (.e. workpece postve). In the present work, fve controllable process varables (parameters) have been selected based on lterature survey. The selected process parameters have been open crcut voltage (OCV) (also called gap voltage, V g ), peak current (I p ), pulse-on tme (T on ), duty factor (τ) and flushng pressure (F p ). Each process parameter has been vared at fve dscrete levels Table 1 as per confguraton of parametrc settng avalable wth the setup.

3 3 Table 1: Machnng control parameters: Doman of varaton Parameters Unt Notaton Levels of varaton OCV/ Gap voltage (V g ) [V] A Peak current (I P ) [A] B Pulse-on-Tme (T on ) [µs] C Duty Factor (τ) [%] D Flushng Pressure (F p ) [bar] E The desgn of experment has been selected based on 5-level-5-factor L 25 Orthogonal Array (OA) shown n Table 2. Experments have been conducted as per 25 factoral settngs. Both workpece and tool have been mmersed n delectrc flud. The machnng duraton has been kept constant (10 mnutes) for each expermental run. The responses studed heren have been Electrode Wear Rato (EWR), Roughness average (R a ) and Surface Crack Densty (SCD) (on the machned surface) and Whte Layer Thckness (WLT) developed over the EDMed surface. The snapshot of EDMed workpece has been shown n (Fg. 1b). Table: 2 Desgn of experment (L 25 OA) Sl. No L 25 OA (Factors are n coded form) Sl. No L 25 OA (Contnued) (Factors are n coded form) A B C D E A B C D E The defntons of varous response measures along wth ther computatonal formulae have been provded below. 1. Electrode Wear Rato (EWR): EWR can be defned as the rato of wear weght of electrode to the wear weght of workpece expressed n percentage [9]. Ths can be computed usng the followng Eq. (1). ( We Wef ) EWR % 100% (1) ( W W ) w wf HereWe andwef are ntal and fnal weght of the tool electrode, respectvely; and, Ww and W wf are ntal and fnal weght of the workpece, respectvely. 2. Surface Roughness: Arthmetc average roughness, or R a, s the arthmetc average of the heghts of surface rregulartes (peak heghts and valleys) wth respect to the mean lne, measured wthn the samplng length. The measurement of surface roughness (R a value) of the EDMed surface has been made wth portable stylus type proflometer, Talysurf (Model: Taylor Hobson, Surtronc 3+), wth cut-off length (L c ) of 0.8 mm, sample length (L n )

4 4 of 4 mm, and flter CR ISO. Fg. 1 (a) Copper tool electrode Fg. 1(b) EDMed workpece Inconel Surface Crack Densty (SCD): To measure the surface crack densty, the top surface morphology of the EDMed surface has been studed usng Scannng Electron Mcroscopy (SEM) (Model, Joel JSM-6480LV, Japan) at magnfcaton of 500. For a partcular sample, SEM mages have been captured n three dfferent locatons and correspondng surface crack denstes have been collected. The average of these three has been consdered for that partcular specmen. For a partcular sample area, the total crack length has been measured usng PDF-X Change Vewer Software. The total crack length dvded by the specmen area provdes the measure of SCD. 4. Whte Layer Thckness (WLT): The whte layer (also called recast layer) s the result of the re-soldfcaton of the melted materal whch do not get completely flushed off from the EDMed surface by the delectrc flud. However, the whte layer formed by EDM process results an ncrease of surface roughness value; t makes the surface hard and brttle, and decreases the fatgue strength due to the presence of mcro-cracks and mcro-vods [10]. The mage of whte layer has then been vewed under SEM (Model: Joel JSM-6480LV, Japan) wth a magnfcaton of The thckness of the whte layer has been measured by ImageJ Software at fve dfferent locatons across each crosssectoned specmen and an average value has been consdered for further analyss. 3. Methodology 3.1 Satsfacton Functon The concept of the satsfacton functons as a powerful tool to aggregate smultaneously several qualty characterstcs [11]. The satsfacton functons are not necessarly lnear and symmetrc as n the case of fuzzy membershp functons. The general shape of the satsfacton functon where assocated wth devaton x ; xd the ndfference threshold o S x s the satsfacton functon x s the dssatsfacton threshold; and x s the veto threshold [12]. The DM s fully satsfed when the devaton x s wthn the nterval,. range of ndfference, x, 0 d value of 1. Outsde ths nterval, where x x d x, 0 d x Therefore, wthn the the devaton x s not penalsed and the DM s satsfacton level wll be at ts maxmal, o the DM s satsfacton functons s decreasng monotonously. Besdes, any soluton leadng to a devaton that exceeds the veto threshold x v wll be rejected. Brefly, satsfacton functon converts the devatons n the nterval of x d, x v to a satsfacton value n the nterval of 0,1. n ths work; satsfacton functon approach has been used to convert machnng performance parameters vz. EWR, R a, SCD and WLT to satsfacton values between 0 and 1, regardless of ther physcal unts. When no nformaton about the preference of the DM s avalable, the satsfacton functon s assumed to be lnear [13-17]. Usng the values obtaned from the satsfacton functons for each machnng characterstcs, a dstance functon has v

5 been explored to combne these values nto a composte number. Ths functon s smlar to Eucldean norm. The parametrc settng whch corresponds to the mnmum dstance d from the deal pont (deal satsfacton values for each of the performance characterstcs) s the most sutable one. 1 2 n 2 d T 1 S (2) 1 Assume that there exst a total n number of performance characterstcs; S s the satsfacton value of th response. When the value of EWR, R a, SCD and WLT (ndvdually) s at the mnmum of the set, t provdes the best satsfacton. Fg. 2 shows the satsfacton functon (Lower-s-Better; LB) that has been used heren. 3.2 Taguch Method Optmzaton of process responses (and thereby evaluaton of optmal settng of process parameters) s the key step n the Taguch method to acheve hgh product qualty as well as satsfactory process performance wthout ncreasng cost [18]. However, orgnally Taguch method was desgned to optmze sngle performance characterstcs. Accordng to Taguch method, the Sgnal-to-Nose (S/N rato) s the rato where sgnal represents the mean (desred value) of the response and nose represents the devaton. The total dstance measure (d T ) (obtaned n Eq. 2) has been treated as sngle objectve functon and fnally optmzed (mnmzed) by Taguch method. Snce d T s desred to be the mnmum, therefore, Lower-s-Better (LB) characterstc s used for S/N rato calculaton (Eq. 3). T S log y (3) N ( LB) T 1 Here y represents response characterstc value on th expermental run (corresponds to a partcular parameters settng). AlsoT ndcates the total number of trals. 4. Data Analyss: Results and Dscussons Expermental data as obtaned through experments as per desgn shown n Table 2 have been utlzed to compute satsfacton values of ndvdual responses. Satsfacton value has been carred out to elmnate crtera conflct (however, there has been no crtera conflct n the present case) and dmensonal (unt) effect. It s just a procedure for data normalzaton. For computng satsfacton values of ndvdual responses vz. EWR, R a, SCD and WLT (all correspond to LB type because least value s the most desred one), the followng formula has been used (Eq. 4). x ( j) x ( j) * max S ( j) (4) xmax ( j) xmn ( j) 1,2,...,25; j1,2..., 4 * Here ( j) s the normalzed data (satsfacton value) for th S x j s the expermental data for x 1,2,...,25 j data seres ; th th expermental run aganst j response; expermental run 1,2,...,25 j aganst x mn x max s the maxmum value of the data seres, and j 1,2,3,4. 5 th j response; s the mnmum value of the As the present research ams at optmzng mult-response features n relaton to EDM of Inconel 718 by applyng satsfacton functon and dstance measure based Taguch approach; computaton of satsfacton values for all of the responses (LB type) has been carred out as per Fg. 2. The satsfacton values aganst ndvdual responses for all expermental runs have been computed usng (Eq. 4) (Refer to Table 3). The total dstance measure (d T ) for each of the expermental runs has been computed next (usng Eq. 2) and tabulated n Table 3. The total dstance measure (d T ) has been treated as sngle objectve functon and fnally optmzed (mnmzed) by Taguch method. Table 4 exhbts mean response (S/N rato of d T ) values for dfferent factoral settngs; the same has been plotted n (Fg. 3a) n order to predct the optmal settng. The optmal settng appears as (A 4 B 2 C 1 D 5 E 5 ).e. V g =80V, I P =5A, T on =100µs, τ =85% and F P =0.6 bar. The predcted S/N rato of d T as obtaned through Taguch analyss at the settng (A 4 B 2 C 1 D 5 E 5 ) appears as ( db); whch seems to be the hghest as compared to the entres of S/N ratos for all expermental settngs (Refer to Table 3). Ths nfers that predcton result s satsfactory. Predcted optmal settng has also been verfed through confrmatory test.

6 In addton to that SEM mages of the EDMed Inconel 718 surfaces have been analysed too. The machned surface of Inconel 718 has been found contanng an uneven fusng structure, globules of debrs, crater marks, surface cracks, pockmarks and deposton of melted materal etc. as shown n (Fg. 3b). Representatve snaps revealng exstence of surface crack and whte layer over the EDMed surface have been shown n Fg 4a and Fg. 4b, respectvely. 6 Fg. 2. Degree of satsfacton chart for a characterstc where the mnmum value provdes the best satsfacton (Lower-s-Better; LB type) 5. Concluson The contrbutons of above mentoned work have been summarzed below. The proposed satsfacton functon and dstance based approach n conjugaton wth Taguch s phlosophy has been proposed here to determne an approprate combnaton of EDM parameters (gap voltage, peak current, pulse-on-tme, duty factor and flashng pressure) to mprove machnng performances n terms of EWR, R a, SCD and WLT, smultaneously. In vew of the lmtaton of tradtonal Taguch method to solve mult-response optmzaton problems, the proposed satsfacton functon and dstance based approach seems helpful n aggregatng multple response features nto an equvalent sngle ndex (.e. d T n the present case) whch can been optmzed fnally by Taguch method. The optmal process parameters settng appears as (A 4 B 2 C 1 D 5 E 5 ).e. OCV=80V, I P =5A, T on =100µs, τ =85% and F P =0.6 bar Mean response table (mean S/N rato of d T ) ndcates that open crcut voltage (OCV) s the most sgnfcant parameter to nfluence machnng performances. SEM mages of EDMed surfaces of Inconel 718 have also been examned as well. It has been concluded that by proper tunng of process parameters surface defects, rregulartes, probablty of formaton of cracks, whte layer etc. can be substantally reduced. Table 3: Computed satsfacton values, total dstance measure and correspondng S/N rato Sl. No. Satsfacton values of ndvdual responses Total dstance Correspondng S (1) S (2) S (3) S (4) measure (d T ) [Lower-s- Better] S/N rato [db] Predcted S/N rato [db]

7 References Table 4: Mean response (S/N rato of d T) table: Predcton of optmal settng by optmzng d T Level Mean response values at dfferent factoral levels A B C D E Delta Rank 1 # # The most sgnfcant process parameter [1] M. Rahman, W.K.H Seah, T.T. Teo, J. Mater. Process Technol. 63(1) (1997) [2] D.G. Thakur, B. Ramamoorthy, L. Vjayaraghavan, Mater. & Des. 30(5) (2009) [3] S. Ahmad, M.A. Lajs, In IOP Conference Seres: Materals Scence and Engneerng. 50 (2013) pp [4] N.M. Abbas, D.G. Solomon, M.F. Bahar, Int. J. Mach. Tool. & Manuf. 47(7) (2007) [5] R. Ramakrshnan, L. Karunamoorthy, J. Mater. Process Technol. 207(1-3) (2008) [6] TR. Newton, SN. Melkote, TR. Watkns, RM. Trejo, L. Rester, Mat. Sc. & Eng. A (2009) [7] M.Y. Ln, C.C. Tsao, C.Y. Hsu, A.H. Chou, P.C. Huang, Y.C. Ln, Trans. of Nonferr. Metal. Soc. of Chna. 23(3) (2013) [8] V. Aggarwal, S.S. Khangura, R.K, Garg, Int. J. Adv. Manuf. Technol. 79(1) (2015) [9] K.Y. Kung, J.T. Horng, K.T. Chang, Int. J. Adv. Manuf. Technol. 40(1-2) (2009) [10] B. Ekmekc, Meta. & Mater. Trans. B. 40(1) (2009) [11] J.M. Martel, B. Aoun, J. of Oper. Res. Soc. 41(12) (1990) [12] M.S. Cherf, H. Chabchoub, B. Aoun, Eur. J. Oper. Res. 186(3) (2008) [13] N.P. Suh, The prncples of desgn, Oxford: Oxford Unversty Press [14] L.H. Chen, Comp. & Oper. Res. 24(10) (1997) [15] L. Chen, Engg. Opt. 33(5) (2001) [16] O. Fat, F. Cosset, A. Bessaudou, Chemomet. Intel. Lab. Sys. 90(1) (2008) [17] A. Kentl, A.K. Kar, Int. J. of Prod. Res. 49(19) (2011) [18] U. Eşme, Arab. J. Sc. & Eng. 34(2B) (2009)

8 8 Fg. 3 (a). Predcton of optmal settng (A 4B 2C 1D 5E 5) by optmzng d T Fg. 3 (b). Characterstcs of EDMed surface of Inconel 718 obtaned at (V g=60v, I P=3A, T on=200µs, τ =75%, F P=0.5 bar) Fg.4 (a). SEM mage revealng exstence of surface cracks on EDMed Inconel 718 (V g=70v, I P=11A, T on=200µs, τ =80%, F P=0.2 bar) Fg. 4(b). SEM mage revealng exstence of whte layer on EDMed Inconel 718 (V g=70v, I P=11A, T on=200µs, τ =80%, F P=0.2 bar)