EFFECT OF WEDM PARAMETERS ON MACHINABILITY OF NIMONIC-90

Transcription

1 EFFECT OF WEDM PARAMETERS ON MACHINABILITY OF NIMONIC-90 Vinod Kumar 1, Kamal Jangra 2, Vikas Kumar 3 1 Research Scholar, Deptt. of Mechanical Engg., YMCA University of Science and Technology, Faridabad 2 Assistant Professor, Deptt. of Mechanical Engg., YMCA University of Science and Technology, Faridabad 3 Associate Professor, Deptt. of Mechanical Engg., YMCA University of Science and Technology, Faridabad 1* Vinod_kumar9400@yahoo.com Abstract Nimonic-90 is a nickel based super alloy which is specifically used in aerospace industry for jet engines, valves, blades etc owing to its high strength at high temperature, fatigue and corrosion resistance. Present work focuses on machinability of Nimonic-90 with wire electrical discharge machining (WEDM) process. Cutting speed is considered as machinability attribute in present work. Influence of WEDM parameters namely discharge current (Ip), pulse on time (Ton), pulse off time (Toff), servo voltage (SV) and wire feed rate (WF) has been investigated on cutting speed of Nimonic-90. Keywords: WEDM, Nimonic-90, Machinability, cutting speed 1. Introduction: Nickel based super alloy are widely used in high temperature and high corrosion environment application such as aerospace industry, various thermal processing, marine engineering, crude petroleum stills and chlorinated solvent. In last few years different grades of nickel super alloy have been developed such as Inconel 601, Inconel 718, Nimonic-80, Nimonic-90, and Monel etc. Machining characteristics of nickel alloys can significantly affect the working life of its components. Several attempts have been made to evaluate the machining characteristics of nickel based super alloys with conventional machining methods. Kortabarria et al., (2011) reported on residuals stress profiles on Inconel 718 developed by dry face turning. Surface integrity condition has been directly affected by the machine parts fatigue life. Different dry facing turning conditions were used for developing different residual stress profile. Then they were compared by using X- ray diffraction method, hole drilling method and Finite element method. Aspinwall et al., (2007) presented the experimental data for Nickel based super alloy when machining with profiled super abrasive grinding wheels. The tool wear of grinding wheel CBN is lower as compared to diamond (D46) grinding wheel at the high rotation speed with lower value of surface roughness. Wei, (2002) has been reported on feasibility of using milling or grinding as alternatives for the current EDM process to machine shaped hole in Inconel 718 super heat resistant alloy. The result shows that milling process of Inconel 718 can produce shaped hold with an acceptable surface roughness and geometrically accuracy efficient after optimizing the cutting condition. Soo. et al., (2011) evaluated the machinability and surface characteristics of RR1000 Nickel based super alloy in drilling and milling process. Experimental data for drilling showed that flank wear < 100 μm, when operating at 45 m/min and measured thrust forces were N. which is generally used the high pressure compressor and turbine parts. The roughness of end mill specimens achieved up to 0.8 µm when a new tool is to be used. The significant burr increased micro hardness and white layer formation when we are using worn tools. Kwong et al., (2009) concluded the influence of major flank wear of drilling tools on the work piece surface integrity and residual stress distribution for RR1000 a Nickel based super alloy. Hughes et al., (2004) had reported on the effect of cutting tools and edge geometry on tool life and surface integrity in turning of Nickel based super alloy. Surface integrity investigation on nickel based super alloy RR1000 have been published by Herbert et al., (2009) for hole making and also present a comprehensive analysis and discussions of the influence of the drills minor cutting edge to work piece surface integrity and residual stress distribution for RR1000.Imrarn et al., (2011) conducted micro drilling in Inconel 718 alloy under wet condition and analysis three different zones namely, nanostructurered surface layer, a deformed subsurface layer and an unaffected parent metal during micro drilling. The microstructure crystal misorientation, nanohardness, plastic deformation was to be studied. Wire electrical discharge machining (WEDM) process is best non-conventional machining process to machine complex geometries in high strength, high hardness materials with high precision. Several investigations have also been carried out on EDM and WEDM. Kang et al., (2003) investigated the EDM characteristics of Nickel based heat resistance alloy Hastelloy X. Pulse on time was the main factors that affect the surface integrity of the work material. Rajesha et al., (2010) reported the machining of EDM of Inconel 718 with hollow Tools. The most influential factor on MRR was discharge current and duty factor. High value of discharge current was suggested for obtaining high MRR. Krishan, (2004) reported on the performance of two graphite electrode Poco 511

2 AF 5 and Poco EDM I in EDM of seal slots in a jet engine turbine vane. MRR in case EDM - I was higher than Poco AF 5. Ghewade and Nipanikar, (2011) has been reported the machining of Inconel 718 using WEDM with a copper electrode. The Taguchi method is used to analysis the significance effect of each parameter i.e. peak current, gap voltage, duty cycle and pulse on time an machining characteristics such Material Removal Rate,Electrode wear rate and Radial over cut and half taper angle.peak current significantly affect the Material Removal Rate and pulse on time significantly affect the Electrode wear rate. Liu et al., (2005) present a process using micro electro discharge machining combined with high frequency dither grinding to improve the surface roughness of micro hole machining of high Nickel alloy. This technique eliminate the micro cracks along with reduce surface roughness from 2.12 to 0.85 µm Rmax. Liu et al., (2006) investigated the significant machine parameters which are affecting the characteristics of micro holes in high Nickel alloy in terms of micro hole expansion, electrode depletion and material removal rate. A proper discharge current is very important to achieve optimum results. Hewidy et al.,(2005) correlated the various WEDM parameters such as peak current, duty factor, wire tension, and water pressure with the performance outputs namely metal removal rate (MRR), wear ratio and surface roughness in WEDM of Inconel 601. Aspinwall et al., (2008) presents roughing and finishing strategies of Ti- 6Al-4V and Inconel 718 after WEDM. The average recast layer thickness less than 11 µm is to be found and several trim passes showing no apparent recast. There is no significant change in work piece microhardness. Kumar et al. investigated the optimum WEDM process parameters of Incoloy 800 super alloy with multiple machining performance characteristics such as material removal rate, surface roughness and kerf by using Gray Taguchi method. Nimonic 90 is a newly developed Nickel based heat resistance super alloy with high content of Cobalt and Chromium. Processing of such type of heat resistance alloys has been an active area of research due to increasing demand of this class of material and typical problems associated with the processing. Machining of heat resistance alloys is difficult due to a combination of low thermal conductivity and high temperature strength. It is very difficult to machine Nimonic 90 by conventional machining processes. Modern machine techniques such WEDM are increasingly being used for machine such hard material. Hence, this study focused on machining of Nimonic 90 using WEDM in order to fulfill the production and quality requirement. In present work, influence of WEDM parameters namely discharge current, pulse-on time, pulse-off time, servo voltage and wire feed rate have been evaluated on machinability of Nimonic-90. Cutting speed is considered as machinability attribute. 2. Experimental Procedure The machining experiments were performed on 5 axis sprint cut (ELPUSE-40) wire EDM manufactured by Electronic M/C Tool LTD India. In present machine tool,parameters can be varied under following range; discharge current (Ip), amp; pulse on time (Ton), μs; pulse off time (Toff),10-63 μs ; servo voltage (SV), 0-90 V; dielectric flow rate (DFR), 0-12 liter per minute ; wire feed rate(wf),1-15 m/min; wire tension (WT),1-15 N. Copper coated brass wire of diameter 0.25mm was used as an electrode because of its good capability to sustain high discharge energy. Distilled water was used as a dielectric fluid with conductivity 20 S. Nimonic-90, a nickel based super-alloy having 60% Ni, 19.3% Cr, 15% Co, 3.1% Ti, 1.4% Al, was taken as a work material in the form of a rectangular sheet of 22.5 mm thickness. The density and melting point of Nimonic-90 was measured as 8.18 g/cm 3 and C respectively. Cutting speed was measured as machinability attribute for Nimonic-90, which was observed directly from monitor screen of the machine tool. Single machining variable is varied at a time to study the influence of discharge current (Ip), pulse on time (Ton), pulse off time (Toff), servo voltage (SV) and wire feed on cutting speed (CS). 3. Effect of WEDM parameters on cutting speed 3.1. Effect of discharge current The effect of discharge current on cutting speed of Nimonic 90 with wire EDM is shown in Figure 1 and 2 under the two different setting of pulse on time ( μs ) along with two different setting of pulse off time (35μs and 45μs). The other parameters were fixed such as Servo voltage 20V, Wire tension 10N, Wire feed 5 meters/min., servo feed 2080 and dielectric flow rate 10 liter per min (upper and lower nozzles). It is clear from the fig. 1 at low pulse duration (Ton =108µs), the cutting speed is increase slowly with increase of peak current. But at high pulse duration (Ton =120μs) there is sharply increase of cutting speed with increase of peak current from 40A to 80 A. Increase in the peak current leads to increase in the rate of the heat energy and hence in the rate of melting and evaporation. 512

3 Figure No. 1 Figure No. 2 Increase in peak current higher over a certain limit, leads to arcing, which decrease discharge number and machining efficiency.subsequently wire to be break down because of short pulse off time.the removal time of debris particles from the gap become insufficient. But from figure no. 2, with increase of pulse off time along with pulse on time, maximum cutting speed occurs which is due to complete flushing of debris particles and available a complete deionized fluid for the next discharge. But further increase of discharge current will leads to increase of cutting speed. Maximum cutting speed of Nimonic 90 with present machine tool occurs at discharge current 200 Amp with pulse off time (Toff) 45μs Effect of pulse-on time The effect of pulse on time (Pulse duration) on cutting speed is shown in figure no. 3 for two setting of Toff =35 and Toff =40.The other parameters were kept constant under the condition of peak current 120 μs, Servo voltage 20 V, Wire feed 5 meters /min, Wire Tension 10 N, Dielectric flow rate 10 liters per min. and servo feed The cutting speed increases continuously with increase of Pulse on Time. The machining becomes unstable at high pulse duration. At high discharge energy, the amount of debris in the gap becomes too great which form an electrically conductive path between the electrode and workpiece, resulting into development of unwanted arc between them. If we increase the pulse off time with increase of pulse duration time, we can provide more time to flush away the debris. Maximum cutting speed of Nimonic 90 with present machine tool occurs at pulse on time (Ton) 118 μs with pulse off time (Toff) 40 μs Effect of pulse off time Figure no. 4 shows the effect of pulse off duration for two machining setting Ton 108µs and 120μs with fixed variables discharge current 120 A,servo voltage 20 V, dielectric flow rate 10 liter per min and servo feed 2080 and wire feed 5meters per min, Wire Tension 10. The cutting speed increases with increases in pulse off time 25 to 45μs and after that it decrease sharply decrease with increase in pulse off time. 513

4 Figure No. 3 Figure No. 4 Maximum cutting is to be found at Toff 45µs due to complete flushing of debris from the machining zones. After that further increase of pulse off time dielectric fluid produce the cooling effect on wire electrode and work material and hence decrease the cutting speed. At high pulse of duration (Ton 120µs) the cutting speed increase very fast as compared to low pulse of duration (Ton 108µs) 3.4. Effect of servo voltage The effect of servo voltage on cutting speed is shown in figure no. 5 with two different pulse durations (Ton 108 and 120) along with two different values of wire feed 5 meters per min and 6 meters per min keeping other variables fixed such as pulse off time (Toff) 35µs, servo voltage 20V, discharge current 120 A and servo feed 2080 and Wire Tension 10 N. Figure No. 5 The cutting speed decrease with increase in servo voltage from 20V to 60V for wire feed 5 m/ min and from 30V to 50 V for wire feed 6 m/min. Large servo voltage means large ionization of the dielectric fluid between 514

5 workpiece and wire electrode which results in high discharge energy per spark. But further increase of servo voltage will not favor in the cutting speed as the large amount of debris are unable to clear off the gap for a given pulse off time Effect of wire feed rate The effect of wire feed rate on cutting speed is represented in figure no. 6 keeping others fixed parameters such as pulse on time 108µs,pulse off time 35µs,servo voltage 20 V, discharge current 120A and servo feed 2080, Wire Tension 10 N. Figure No. 6 It is clear from figure no. 6 cutting speed increases with increases of wire feed from 3 meters per min to 5meters per min. But further increase of wire feed rate has no influence on cutting speed. It implies that with these discharge parameters, eroded debris are easily clear off from the spark gap at a wire feed rate of 5m/min. 4. Conclusions: In this work, machinability of Nimonic-90 has been evaluated on wire electrical discharge machining (WEDM) process. Cutting speed has been considered as machinability attribute in present work. Influence of WEDM parameters namely discharge current (Ip), pulse on time (Ton), pulse off time (Toff), servo voltage (SV) and wire feed rate (WF) were investigated on cutting speed of Nimonic-90. Based on the experimentation, WEDM parameters namely discharge current, pulse-on time and pulse-off time produces highly noticeable effect on cutting speed. References: Aspinwall D.K., Soo1 S.L., Curtis D.T., Mantle A.L. (2007), Profiled Superabrasive Grinding Wheels for the Machining of a Nickel Based Superalloy. Annals of the CIRP Vol. 56, pp Aspinwall D.K., Soo S.L., Berrisford A.E., Walder G. (2008), Workpiece surface roughness and integrity after WEDM of Ti 6Al 4V and Inconel 718 using minimum damage generator technology. Manufacturing Technology 57, pp Ghewade D.V., Nipanikar S.R. (2011), Experimental study of Electro Dischrge machining for Inconel material. Journal of Engineering Research and Studies,pp Herbert C.R.J., Kwong J., Kong M.C., Axinte D.A., Hardy M.C., Wither P.J. (2012), An evaluation of the evolution of workpiece surface integrity in hole making operations for a nickel-based superalloy Journal of Materials Processing Technology 212,pp Hewidy M.S., El-Taweel T.A., El-Safty M.F. (2005), Modelling the machining parameters of wire electrical discharge machining of Inconel 601 using RSM. Journal of Materials Processing Technology 169, pp Hung-Sung Liu, Biing-Hwa Yan, Fuang-Yuan Huang, Kuan-Her Qiu (2005), A study on the characterization of high nickel alloy micro-holes using micro-edm and their applications. Journal of Materials Processing Technology 169, pp Hung Sung Liu, Biing Hwa Yan, Chien Liang Chen, Fuang Yuan Huang (2006), Application of micro-edm combined with high-frequency dither grinding to micro-hole machining. International Journal of Machine Tools & Manufacture 46, pp Imran Muhammad & Mativenga Paul T. & Gholinia Ali (2011), Evaluation of surface integrity in micro drilling processfor nickel-based superalloy Int J Adv Manuf Technol 55, pp

6 Kortabarria A., Madariaga A., Fernandez E., Esnaola J.A., Arrazola P.J, (2011) A comparative study of residual stress profiles on Inconel 718 induced by dry face turning. Procedia Engineering 19, pp Kristian L. Aas (2004), Performance of two graphite electrode qualities in EDM of seal slots in a jet engine turbine vane. Journal of Materials Processing Technology 149, pp Kumar Muthu, Babu Suresh, Venkatasamy and Raajenthiren (2010), Optimization of the WEDM Parameters on Machining Incoloy800 Super alloy with Multiple Quality Characteristics. International Journal of Engineering Science and Technology Vol. 2(6), pp Kwong J.,.Axinte D.A, Withers P.J. (2009), The sensitivity of Ni-based superalloy to hole making operations: Influence of process parameters on subsurface damage and residual stress. Journal of materials processing technology 209, pp Rajesha S., Sharma A.K., and Kumar Pradeep (2011), On Electro Discharge Machining of Inconel 718 with Hollow Tool. ASM International. Sin Ho Kang, Dae Eun Kim. (2003), Investigation of EDM Characteristics of Nickel-based Heat Resistant. KSME International Journal, Vol.17, pp Soo S.L., Hood R., Aspinwall D.K., Voice W.E., Sage C. (2011), Machinability and surface integrity of RR1000 nickel based superalloy. Manufacturing Technology 60, pp Wei X. (2002), Experimental study on the machining of a shaped hole Ni based super heat resistant alloy. Journal of materials Processing Technology 129, pp