Optimization of machining parameters of EDM while machining Inconel 718 for form tolerance and orientation tolerance

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1 Indan Journal of Engneerng & Materals Scences Vol. 2, October 213, pp Optmzaton of machnng of EDM whle machnng Inconel 718 for form tolerance and orentaton tolerance S Dhanabalan a, K Svakumar b & C Sathya Narayanan c a Department of Mechancal Engneerng, J J College of Engneerng and Technology, Truchrapall 62 9, Inda b Department of Mechancal Engneerng, Bannar Amman Insttute of Technology, Sathyamangalam , Inda c Department of Producton Engneerng, Natonal Insttute of Technology, Truchrapall 62 15, Inda Receved 17 February 212; accepted 14 May 213 Ths paper demonstrates the effectveness of optmzng multple characterstcs of electrcal dscharge machnng (EDM) of Inconel 718 usng copper electrodes havng dfferent shapes va Taguch method-based Grey analyss. The modfed algorthm adopted here s used successfully for both detranng the optmum settngs of machnng and for combnng multple qualty characterstc nto one numercal value called Grey relatonal grade or rank. The essental lke peak current, pulse on tme and pulse off tme are chosen to conduct the experments on multple characterstcs namely materal removal rate (MRR), electrode wear rate (EWR), form tolerance namely straghtness and orentaton tolerance lke perpendcularty and angularty. Then, ANOVA s used to fnd out the parameter whch sgnfcantly affects the other. The expermental results have shown that the machnng performance on multple characterstcs n the EDM process s mproved effectvely through ths approach. Keywords: Electrcal dscharge machnng, Inconel 718, Taguch method, Form tolerance, Orentaton tolerance, Grey relatonal analyss In recent years, the Taguch method has become a powerful tool for solvng manufacturng problems lke mprovng productvty, reducng surface roughness etc. The orgnal Taguch method has been desgned to optmze a sngle performance characterstc 1,2. Handlng of multple performance characterstcs by the Taguch method requres further effectve researches. Ths s because, optmzaton of the multple performance characterstcs s concerned wth optmzaton of vector obectves. Whle optmzng the electrcal dscharge machnng (EDM) process, t s expected to have a hgher materal removal rate and a lower electrode wear rate, a good form tolerances and orentaton tolerances. Moreover, an mprovement of one performance characterstc may degrade one or more of the other performance characterstcs. Therefore, the multple performance characterstcs are much more complcated than the optmzaton of a sngle performance characterstc 3. The purpose of the ths study s to ntroduce the use of grey relatonal analyss n Taguch applcaton for multple performance characterstcs optmzaton *Correspondng author (E-mal: dhanabalan69@gmal.com) wth the usage of weghted factor 4. The orthogonal array wth the grey relatonal analyss 5-8 s used to nvestgate the multple performance characterstcs and to optmze the form tolerance and orentaton tolerance n the EDM process of machnng Inconel 718. Inconel 718 s a hgh strength temperature resstant (HSTR) nckel-based super alloy. It s extensvely used n aerospace applcatons, namely gas turbnes, rocket motors, and spacecraft, nuclear reactor, pumps and toolng 9. These components have small-szed coolng holes as they are workng n a hostle envronment 1 (.e. at hgh speed at elevated temperatures). There s also a newer verson of the alloy (718 SPF) that s used specfcally for superplastc formng. It contans substantal levels of ron, molybdenum, and nobum as well as trace amounts of ttanum and alumnum, wth a hgh level of strength and flexblty. It wll mantan good creeprupture strength at temperatures as hgh as 978 K 9. From the lterature, t s clear that Inconel 718 s a dffcult to machne materal, because of ts poor thermal propertes, hgh toughness, hgh work hardenng rate, presence of hghly abrasve carbde partcles and strong tendency to weld to the tool to

2 392 INDIAN J. ENG. MATER. SCI., OCTOBER 213 form buld up edge 9,1. As a result, hgh tool wear has been reported durng conventonal machnng of materal. On the other hand, an alternatve method to effectvely machne ths materal, s non-tradtonal machnng processes. One of such non-conventonal machnng processes s EDM whch s wdely used to produce ntrcate shapes on any conductng metal rrespectve of ther hardness, machnablty and toughness 9,1. The EDM process, sometmes referred to as spark-eroson machnng, s a nontradtonal method of removng metal by a seres of rapdly recurrng dscrete electrcal dscharges between an electrode (the cuttng tool) and the workpece n the presence of a delectrc flud. Kuppan et al. 9 nvestgated the nfluence of EDM process n deep hole drllng of Inconel 718. The results showed that MRR s most nfluenced by peak current, duty factor and electrode rotaton, where as surface roughness s strongly nfluenced by peak current and pulse on tme. A comparatve expermental study on machnng and surface characterstcs of through and blnd holes (Ø1 mm) produced on aerospace alloys of T-6AL-4V and Inconel 718 by fast hole rotary EDM process usng tubular hallow copper and brass electrodes was conducted by Bozdana et al. 1 In practce, Bhart 1 analyzed the process wth performance measures whle machnng Inconel 718 durng de snkng EDM. Tradtonally, EDM s used to produce any component from any electrcally conductve materal but wth low MRR. In order to ncrease MRR, Ylmaz and Okka 12 used mult-channel electrodes on EDM for fast hole drllng to mprove the performances lke materal removal rate, electrode wear and surface ntegrty for the EDM whle machnng on Inconel 718 super alloy. Further, the EDM of Inconel 718 wth hallow tool was employed to ncrease the MRR and reduce EWR and surface roughness by Raesha et al. 13 Consequently, L et al. 14 proposed the nfluence of flushng on performance of EDM wth bunched electrode to further mprove the materal removal rate and to reduce electrode wear. Then, these typcal applcatons need standard desgn requrement and close tolerances n manufactured components and the data readng form tolerance and orentaton tolerance of the holes made by EDM are neglgble or non-avalable. Recently, Mustafa et al. 15 optmzed the for mcro-edm drllng of Inconel 718 superalloy on hole taper rato and hole dlaton by usng Grey relatonal analyss. From the lteratures, t s also observed that lmted works were conducted on form tolerances n electrcal dscharge machned Inconel Though many studes have been carred out on EDM process, there s no analytcal and expermental work carred out for form tolerance and orentaton tolerance namely flatness, perpendcularty and angularty. Moreover, the form tolerance and orentaton tolerance are the mportant responses n non-conventonal machnng process. Thus, ths expermental work s attempted here to evaluate the form tolerance and orentaton tolerance n EDM of Inconel 718. Taghuch technque was used to develop desgn of experments (DoE) to reduce the number of trals 3. Addtonally, the ANOVA used to found the sgnfcant parameter 16,17. In ths paper, the desgned electrodes are used to machne the features lke square and hexagonal holes and the geometrc tolerance of the above features are measured by usng CMM. The responses were optmzed by usng Grey relatonal analyss. Confrm optmzed combnaton level of machnng values of Inconel 718 was also done. Desgn of Experments (DOE) Accordng to Ln et al. 2, n electrcal dscharge machnng, removal of materal from a workpece s an electrcal spark eroson process. Common methods of evaluatng machnng performances n the EDM operaton are based on the followng performance characterstcs: materal removal rate, electrode wear rate, perpendcularty, angularty and straghtness. The above performance characterstcs are correlated wth machnng such as peak current, pulse-on tme and pulse-off tme. The proper selecton of machnng can result n a hgher value of materal rate, lower value of electrode wear, lower value of perpendcularty, lower value of straghtness and lower value of angularty. A total of three namely peak current, pulse on tme and pulse off tme were chosen for the controllng factor, and each parameter was desgned to have three levels, namely small, medum, and large whch are correspondngly denoted by 1, 2 and 3, as shown n Table 1. Runnng experment The chemcal composton of Inconel 718 used n ths work was analyzed by usng Bruker SI turbo alloyng Analyzer. The hardness of the workpece, Inconel 718, was found by usng the Hardness Tester

3 DHANABALAN et al.: ELECTRICAL DISCHARGE MACHINING OF INCONEL HT-7 Optel. The experments were conducted by usng de snkng SPARKONIX electrc dscharge machne havng 15 A current as maxmum ratng. The workpece, Inconel 718, n the form of dsc was connected wth postve termnal and square and hexagon profle copper electrodes were connected wth negatve termnal of the DC power supply. Kerosene was used as delectrc flud wth pressure of.2 kg/cm², and sde flushng technque was used to conduct all the experments. The weght of the electrode and workpece before machnng and after machnng were measured by usng SHIMADZU BL seres electronc balance wth an accuracy of.1 g for accuracy of every tral run. The materal removal rate (MRR) was calculated usng the formula gven as: wear weght of workpece MRR( g / mn) (1) tme of machnng The electrode wear rate (EWR) was calculated usng the formula gven as: wear weght of electrode EWR( g / mn) (2) tme of machnng The form tolerance and orentaton tolerance namely straghtness, perpendcularty and angularty were measured by usng co-ordnate measurng machne (CMM) TESA mcro-hte 3D. And then, grey based orthogonal array was used for fndng mult-obectve optmzaton. Chemcal composton The chemcal composton of Inconel 718 used n ths work s gven n Table 2. The Inconel 718 s a precptaton-hardenable nckel-chromum alloy contanng substantal levels of ron, molybdenum, and nobum whch can trace the amounts of ttanum and alumnum wth a hgh level of strength and flexblty. It possesses hgh corrosve resstance and Table 1 Machnng and ther levels Parameter Unt Level 1 Level 2 Level 3 A Peak current A B Pulse on tme µs C Pulse off tme µs hgh temperature resstance. It s sutable for the use at cryogenc temperature and a hgh temperature n the order of 978 K. The hardeness of Inconel 718 (HRB 8.13) was measured by usng hardeness tester HT-7. Though, the hardness of Inconel 718 seems to be less, t has problems n machnablty as explaned earler. Results and Dscusson The 18 expermental runs were conducted n duplcate, and the average values of MRR, EWR, perpendcularty and straghtness for square electrode, angularty and straghtness for hexagonal electrode along wth the desgn of experments (DoE) are gven n Tables 3 and 4. Mult response optmzaton usng orthogonal array wth Grey relatonal analyss A group of responses often characterze the performance of a manufactured product. These responses are generally measured by a dfferent measurement scales. The mult-response optmzaton characterstcs are complex. In ths secton, the use of orthogonal array wth Grey relatonal analyss optmzaton methodology for mult-response optmzaton s dscussed. The optmzaton of the process parameter has been explaned n the followng steps: (a) Normalzng the expermental results of MRR, EWR, perpendcularty (for square electrode), angularty (for hexagon electrode) and straghtness (for both square and hexagon electrode) of all the trals as shown n Tables 3 and 4. (b) Performng the Grey relatonal generaton and calculatng the Grey relatonal coeffcent as shown n Table 3 (for square electrode) and 4 (for hexagon electrode). (c) Calculatng the Grey relatonal grade by averagng the Grey relatonal coeffcent wth multplcaton of the weghted factor as shown n Tables 3 and 4. (d) Performng statstcal analyss of varance (ANOVA) for the nput wth the Grey relatonal grade and verfy sgnfcant parameter n affectng the process. Table 2 Chemcal composton of work materal Sn Mo Nb Zn N Co Fe Cr Sb T Al

4 394 INDIAN J. ENG. MATER. SCI., OCTOBER 213 (e) Selectng the optmal levels of process. (f) Conductng confrmaton experment and verfyng the optmal process settng. Normalzaton of the expermental results and computng the Grey relatonal analyss for the expermental results A lnear normalzaton of the expermental results for the responses, vz., MRR, EWR, perpendcularty (for square electrode), angularty (for hexagon electrode) and straghtness (for both square and hexagon electrodes) s performed n the range between and 1, whch s called as the Grey relatonal generaton. Normalzaton s carred out to avod the effect of adoptng dfferent unts and to reduce the varablty. The normalzed results X can be expressed as y mn( y, 1, 2,3... n) X (3) max( y, 1, 2... n) mn( y, 1, 2... n) (To be used for larger the better) max( y, 1,2,3... n) y X max( y, 1,2... n) mn( y 1,2... n) (be used for smaller the better), (4) Table 3 Expermental values, normalzed values, grey coeffcent and Grey relatonal grade values for square electrodes Actual responses Normalzed responses Grey co-effcent values GRADE Sl. Peak Ton Toff MRR EWR Per. ty St. ness MRR EWR Per. ty St. ness MRR EWR Per. ty St. ness No current µs µs g/mn g/mn degrees mm g/mn g/mn degrees mm g/mn g/mn degrees mm Table 4 Expermental values, normalzed values, grey co-effcent and Grey relatonal Grade values for Hexagonal electrodes Actual responses Normalzed responses Grey co-effcent values GRADE Sl. Peak Ton Toff MRR EWR Per. ty St. ness MRR EWR Per. ty St. ness MRR EWR Per. ty St. ness No current µs µs g/mn g/mn degrees mm g/mn g/mn degrees mm g/mn g/mn degrees mm

5 DHANABALAN et al.: ELECTRICAL DISCHARGE MACHINING OF INCONEL Where y s the th expermental results n the th experment. Computng the Grey relatonal coeffcent for the normalzed values The Grey relatonal coeffcents are calculated to express the relatonshp between the deal (best 1) and the actual expermental results. The grey relatonal coeffcent δ can be expressed as mn mn x x + ξ max max x x δ x x + ξ max max x x (5) Where x s the deal normalzed results for the th performance characterstcs and ξ s the dstngushng coeffcent whch s defned n the range ξ 1. The weghted grey relatonal coeffcent s a weghtng adustment of the grey relatonal coeffcent and defnes as δ ( K) δ *( K) ω ω, n ω 1 (6) 1 In ths work, the weghtng factor ω assgned are.5,.2,.2 and.1 (5% MRR, 2% EWR, 2% for perpendcularty and angularty and 1% for straghtness) for metal removal rate, electrode wear rate, perpendcularty (square electrode), straghtness (square and hexagonal electrode) and angularty (for hexagon electrode) respectvely. Weghtng factor s assgned based on the performance characterstcs of ths study or applcaton. Computng the Grey relatonal grades The Grey relatonal grade correspondng to each performance characterstc s to be computed and the overall evaluaton of the mult response characterstc s based on the Grey relatonal grade, whch s gven by: 1 m α δ (7) m 1 Table 5 Response table for the Grey relatonal grade for square profle electrode Level 1 Level 2 Level 3 max-mn Peak current Pulse on tme Pulse off tme Where α s the Grey relatonal grade for the th experment and m s the number of performance characterstcs. The Grey relatonal grades are gven n Tables 3 and 4. The hgher Grey relatonal grade represents that the expermental result s closer to the deally normalzed value. The hgher grey relatonal grade mples the better qualty; therefore, on the bass of Grey relatonal grade, the factor effects are estmated and presented n Tables 3 and 4. Determnaton of the optmal factor and ts level combnaton The mean of the Grey relatonal grade for each level of the machnng can be calculated by averagng the Grey relatonal grade for current for experment number 1-6 for level 1, for experment number 7-12 for level 2 and for experment number for level 3. Smlarly, t s calculated for the respectve levels for pulse on tme and pulse off tme and s summarzed n Tables 5 and 6. The larger value of the Grey relatonal grade, the better s the mult response characterstcs. Performng analyss of varance (ANOVA) Furthermore, a statstcal analyss of varance (ANOVA) s performed to determne whch sgnfcantly affect the performance characterstcs. Wth the grey relatonal analyss and statstcal analyss of varance, optmal combnatons of the process are predcted. The parameter symbols typcally used n ANOVA are descrbed as: () Source The source ncludes the controllng factors A, B, C... and the error factor, e, and the sum of all observaton, T (A-Current, B-pulse on tme, and C-pulse off tme). () SS (sum of squares) SSA, SSB, SSC Denote the sum of squares of A, B, C; SSe denotes the error sum of squares; SST denotes the total varaton. Thus, the equaton for SST can be wrtten as: SST ( total var aton) CF CF( Correctonfactor) m 2 η (1) 1 m ( η ) 1 m 2 (11) Table 6 Response table for the grey relatonal grade for hexagon profle electrode Level 1 Level 2 Level 3 max-mn Peak current Pulse on tme Pulse off tme

6 396 INDIAN J. ENG. MATER. SCI., OCTOBER 213 m the total number of experments η Grey relatonal grade of ndvdual experments SSe SST SSA SSB SSC (12) () DoF (degree of freedom) DoF denotes the number of ndependent varables. In the ANOVA table, the degree of freedom for each factor s the number of ts levels -1. The total degree of freedom s the number of total measurement values -1. The error of the degree of freedom s the total degree of freedom mnus the sum of the degree of freedom of each factor. (v) P (Percentage of the contrbuton to the total varaton) SS ' P 1% ( A, B, C, e, T ) (13) SST Results of the ANOVA ndcate that peak current s the most sgnfcant than other n terms of affectng the multple responses, form tolerance and orentaton tolerance for both square and hexagon profle of the electrodes. Ths s accomplshed by separatng the total varablty of the Grey relatonal grade, whch s measured by the sum of squared devaton from the total mean of the Grey relatonal grade, nto contrbutons by each of the process and the error. The F-test s used to determne the sgnfcance. The change of the process has a sgnfcant effect on the performance characterstcs when the F-value s large. The result of the ANOVA (Tables 7 and 8) shows that peak current and pulse on tme are the sgnfcant machnng that affectng the multple performance characterstcs. Confrmaton tests The estmated Grey relatonal grade η opt usng the optmal level of the desgn can be calculated as η η + ( η η) opt (14) 1 where the total mean of the Grey relatonal grade, η s the mean of the Grey relatonal grade at the optmal level and o s the number of machnng that sgnfcant desgn that affect the multple performance characterstcs. Based on the Eq. (14), the estmated Grey relatonal grade usng the optmal machnng can be found out even for the settng whch s not avalable n the orthogonal array. Tables 9 and 1 gve a comparson of the multple process responses for ntal and optmal EDM for square and hexagon copper electrodes used for machned Inconel 718 work pece. As noted from Table 9 (Inconel 718) materal removal rate s accelerated from.116 g/mn to.132 g/mn, electrode wear s greatly reduced from.25 g/mn to.16 g/mn, the square angle s greatly reduced from 89.98º to 89.67º and the flatness s ncreased from.4 mm to.3 mm, respectvely. As noted from Table 1 (Inconel 718) materal removal rate s accelerated from.14 g/mn to.132 g/mn, electrode wear s greatly reduced from.19 g/mn to.1 g/mn, the angularty ncreased from º to 12.11º and the flatness s ncreased from.39 mm to.33 mm, respectvely. It s clearly shown that the electrode wear rate, materal removal rate, form tolerances and orentaton tolerances are greatly mproved through both approaches. Table 7 Results of ANOVA for multple performance characterstcs Inconel 718 for square profle electrode Symbol Machnng parameter Degrees of freedom Sum of squares MS F A Peak current B Pulse on tme C Pulse off tme Error Total Table 8 Results of ANOVA for multple performance characterstcs Inconel 718 for hexagonal profle electrode Symbol Machnng parameter Degrees of freedom Sum of squares MS F A Peak current B Pulse on tme C Pulse off tme Error Total

7 DHANABALAN et al.: ELECTRICAL DISCHARGE MACHINING OF INCONEL Table 9 Results of ntal and optmal electrc dscharge machnng performance square electrode Intal machnng Optmal machnng Predcton Expermental Levels A3B3C2 A3B3C2 A3B2C1 MRR, g/mn EWR, g/mn Perpendcularty, degrees Straghtness, mm Grey relatonal grade.6546* *Improvement of grey relatonal grade.1795 Table 1 Results of ntal and optmal electrc dscharge machnng performance hexagonal electrode Intal machnng Optmal machnng Predcton Expermental Levels A3B3C2 A3B3C2 A3B2C3 MRR, g/mn EWR, g/mn Angularty, degrees Straghtness, mm Grey relatonal grade.6361* *Improvement of Grey relatonal grade.625 Conclusons The paper presented the use of the Grey relatonal grade analyss based on the orthogonal array for the optmzaton of the electrcal dscharge machnng process wth the multple performance characterstcs. Grey relatonal coeffcents analyze the relatonal degree of the multple responses (electrode wear rate, materal removal rate, form tolerances and orentaton tolerances). As a result, these approaches can greatly mprove the process responses such as the electrode wear rate, materal removal rate, form tolerances and orentaton tolerances n the electrcal dscharge machnng process durng machnng of Inconel 718 by usng hexagonal and square profle copper electrodes. Confrmaton test results proved that the determned optmum combnaton of electrcal dscharge machnng satsfy the real requrement of electrcal dscharge machnng process whle machnng of Inconel 718. It s clearly shown that the multple performance characterstcs n EDM of Inconel 718 materal are greatly mproved. So the best parameter for machnng Inconel 718 by usng square electrodes s A3B2C1 and hexagonal electrodes are A3B2C3. The optmal EDM for multple performance characterstcs whle machnng Inconel 718 by usng square electrodes are peak current 12 A, pulse on tme 6 µs and pulse off tme 1 µs and hexagonal electrodes are peak current 12 A, pulse on tme 4 µs and pulse off tme 4 µs. Wth these optmum, the straghtness of square hole and hexagon hole are obtaned as.3 mm,.33 mm respectvely. For the same optmum, the angularty n square and hexagon hole s optmzed as 89.67º, 12.11º respectvely whch are n acceptable range. References 1 Narender Sngh P, Raghukandan K & Pa B C, J Mater Process Technol, (24) Ln J L & Ln C L, Int J Mach Tools Manuf, 42 (22) Jong Hyuk Jung & Won Tae Kwon, J Mech Sc Technol, 24 (5) (21) Lung Kwang Pan, Che Chung Wang, Shen Long We & Ha Feng Sher, J Mater Process Technol, 182 (27) Tosun N & Ozler L, Int J Adv Manuf Technol, 23 (24) Nhat Tosun, Int J Adv Manuf Technol, 28 (26) Tarng Y S, Juang S C & Chang C H, J Mater Process Technol, 128 (22) Huang M F & Ln T R, J Mater Sc Technol, 2 (24). 9 Kuppan P, Raadura A & Narayanan S, Int J Adv Manuf Technol, (27) DOI 1.17/s y 1 Bozdana AT, Ylmaz O, Okka M A & Flz I H, A comparatve expermental study on fast hole EDM of Inconel 718 and T-6Al-4V, 5 th Int Conf and Exhbton on Desgn and Producton on Machnes and Des / Molds, Turkey, June Bhart Pushpendra S, Maheshwar S & Sharma C, Int J Eng Sc Technol, 2(11) (21) Ylmaz Oguzhan & Okka M Al, Int J Manuf Technol, 51 (21) Raesha S, Sharma A K & Kumar Pradeep, J Mater Eng Perform, DOI: 1.17/s L Le, Ku Ln, Xuecheng & Zhao Wansheng, Int J Manuf Technol, 58 (212) Mustafa Ay, Ulas Caydas & Ahmet Hascalc, Int J Adv Manuf Technol, (212) DOI 1.17/s Kao J Y, Tsao C C, Wang S S & Hsu C Y, Int J Adv Manuf Technol, 47 (21) Pradhan B B, Masanta M, Sarkar B R & Bhattacharyya B, Int J Adv Manuf Technol, 41 (29)