REVIEW OF WIRE ELECTRIC DISCHARGE MACHINING (WEDM) OF NIMONIC ALLOYS OF DIFFERENT GRADES

REVIEW OF WIRE ELECTRIC DISCHARGE MACHINING (WEDM) OF NIMONIC ALLOYS OF DIFFERENT GRADES Sarpreet Singh a*, Harvinder Singh b a* M.Tech Student, Department of Mechanical Engineering, Punjabi University, Patiala, India. b Assistant Professor, Department of Mechanical Engineering, Punjabi University, Patiala, India. *e-mail: singh.sarpreet9976@gmail.com ABSTRACT Wire electrical discharge (WEDM) is one of the developments of electrical discharge machining (EDM). WEDM is used for machining of hard materials which cannot be machined by conventional machining process. Nimonic alloys are used in aerospace industry for its strength at high temperature. This paper represents tutorial introduction, history and review of research work carried out in area of wire electrical discharge machining. The machining mechanism, application, observation and various parameters like pulse duration, pulse interval, peak current, servo voltage dielectric flow rate, gap voltage wire tension, wire feed rate and their effects also discussed in this paper. Keywords: Optimization, Process Parameters, Review, Wire cut EDM, Nimonic 1. Introduction Wire Electrical discharge machining (WEDM) has been developed as an alternative for machining nickel-based alloys. WEDM is considered a competitive machining technology due its contact free material removal mechanisms in which no deformation occurs. As a result, slender and fragile jobs can be machined conveniently. The resulting low process forces of WEDM are capable of creating burr-free microstructures with high aspect ratio. EDM can be successfully employed to machine electrically conductive materials. The Physical and metallurgical properties of the work material, such as strength, toughness, microstructure, etc., is no barrier to its applications. The WEDM was first introduced to industry in 1960s by SWISS firm, this machine works simply and the change of wire is limited to copper and brass only. In 1975 its popularity was rapidly increasing, it can machine the electrical conductive material of any kind. Wire-EDM (W-EDM) is becoming more popular in industry because it can be fully automated, and complex geometrical shapes can be machined in one setup. 2. WEDM WEDM is electro thermal, non-contact, force free cutting process. In the WEDM process electric spark is produce and the wire is used to cut the electric conductive materials. The difficult profiles, delicate geometries, complex parts, precision components irrespective of hardness and toughness are easily machined. To cut the material it should conduct electricity and cut by coated or uncoated wire of EDM process. The dielectric cutting fluid feed continuously to the work piece which electorates erodes the material from the work piece using melting and vaporization of material by producing spark and very high temperature is obtained during machining process The wire should moves along the defined path. The wire does not touch the work piece. To start the machining either drill a hole in the work piece or start from the edge. If the amount of stock removed from the electrode become greater than the amount of being removed from work piece the wire electrode brakes and discharge is stopped. For the taper profiles the wire from the top and bottom is inclined to make precise taper profiles or complex files. The dielectric cutting fluid cool and flush the eroded material from the work piece. For automated control a computer numeric control is used for better accuracy. There is never any mechanical contact between

the wire and work piece. These early machines were extremely slow but today, machines are equipped with automatic wire threading and can cut over 20 times faster, Carl Sommer and Steev Sommer,[1]. Fig. 1 schematic Diagram of WEDM The effectiveness of the whole process depends on number of input process parameters such as pulse on time, pulse off time, servo voltage, peak current, dielectric flow rate, wire feed, and wire tension. The important machining responses include material removal rate (MRR), surface roughness (Ra), Kerf (width of cut), wire wear ratio (WWR) and surface integrity factors. In this paper description of various process parameters and their influence respective responses have been presented. Literature is classified based on the response parameters. Different modeling and optimization methods proposed by various researchers are also discussed. Finally the recommendations and future trends in WEDM research have been outlined. 3. Nimonic alloys and its applications Application of nimonic alloys is increasing day by say due to superior properties. Nickel based alloy is used in many industries for resistance to corrosion and their retention of strength as well as other mechanical properties in extreme temperatures. Nimonic alloy used in manufacturing of aero engine components because of high specific strength. Where others material exposure to elevated temperature. The metal begins to crack, deform, corrode, fatigue etc. but nimonic alloys known for retention of mechanical properties, such as impact strength, yield strength and hardness in temperature as high as 1100 C depending upon the grade of alloys. Nimonic alloys are useful in industries where erosion or abrasion of material could damage the product or aesthetic appearance is required. The nominal chemical composition of nimonic alloys ranges from 38 to 76 wt.% nickel, up to 27 wt.% chromium and 20 wt.%, cobalt. Other elements are tungsten (W), Tantalum (Ta) and molybdenum (Mo) may be added to increase their strength and oxidation Properties. Different properties are obtained according to the different grades. 4. WEDM Process Parameters The process parameters can affects the quality of machining or cutting in wire EDM process, the main goal of wire EDM is to achieve high productivity, accuracy and surface finishing. The optimum utilization of capacity of WEDM process is proper selection of process parameters.

4.1. Pulse Duration Pulse duration is also called pulse on time. It is expressed in microseconds. During pulse duration discharge is produce due the voltage is applied in the gap between electrode and workpiece. More the pulse on time, higher will be the energy applied therefore more heat energy is generating during this period. During the pulse on time metal removal rate (MRR) depend upon the amount of energy applied. It is denoted by T-on[2]. 4.2. Pulse Interval Pulse interval is also called pulse off time. It is expressed in microseconds. This is time between discharges. Discharge energy not affected by pulse interval. It is pause between discharges that allow debris to solidify and flush by dielectric fluid. Wire is broken if reducing pulse off time and cutting speed also increase. It is denoted by T-off. 4.3. Peak Current Unit of peak current is amperage. It is amount of power used in discharge machining. The current increase until it reaches a present value during each pulse on time which is known as peak current, peak current governed by surface area of cut, During roughing operation high peak current is applied. It is denoted by IP. 4.4. Servo Voltage Servo Voltage is used to control the wire advance and retracts if machining voltage lowers than set servo voltage it retracts, if high then wire advances. The mean gap becomes narrow, if smaller value is set and increases the number of electric sparks, results in higher machining rates. The wire breaks if state of machining at gap may become unstable. It is denoted by SV. 4.5. Gap Voltage Gap voltage specifies the supply voltage to place on gap. Gap voltage also called open circuit voltage. Peak current is increase if gap voltage is increase, more the gap voltage more will be the electric discharge. 4.6. Dielectric Flow Rate Dielectric flow rate is rate of flow of dielectric fluid during machining. For different machining the flushing is important. Both top and bottom nozzles are used for producing flushing pressure. 4.7. Wire Feed Rate There is no wire breakage in high cutting speed, if there is low wire speed it causes to break the wire. If wire feed rate increases, cost of machining and consumption of wire increases. 4.8. Wire Tension The wire drags behind if wire tension is low and if tension is high wire stays straight. The cutting speed and accuracy increases due to increase in wire tension. The wire break, if applied tension exceeds the tensile strength of wire. The higher tension decreases the wire vibration amplitude and hence decreases the cut width so that for same discharge speed is higher. 5. Literature Based On Process Parameters 5.1. Surface Roughness Surface roughness is one of the most important responses in any machining process. So the desired surface finish is required in every machining process and to maintain the quality. The functional attributes of parts are affected by surface roughness like wear, tear, friction, heat transmission, etc. There are different roughness parameters but Ra is commonly used and all are expressed in μm. The different parameters like cutting speed,

Pulse on-time, Peak current effect the surface roughness. More the pulse on time more will be cutting speed and less will be surface roughness. Lot of research has done to reduce the surface roughness and to obtain high surface finish. V. Kumar et al. 2015[3],carried out experimental investigation to study the effect of different parameters like peak current, pulse on time, pulse off time, servo voltage, wire feed on surface roughness of nimonic 90.From the experimental results while using by varying single variable at time. Peak current, pulse on time, pulse off time provides higher noticeable effect on surface roughness. V. Kumar et al. 2015[4], carried out experimental investigation for trim cut operation in WEDM of nimonic 90. By applying statistical modeling, they found that increasing the value of pulse on time, wire feed, dielectric flow rate increase the surface roughness and increasing the servo voltage decreasing the surface roughness and dimensional shift. A. Goswami et al. 2014[5] study the multi response optimization method of nimonic 80A. Study makes use taguchi s robust design methodology. The investigation indicated that MRR and SR increasing with pulse on time and decreasing with pulse off time. Pulse on time and peak current is important factor for surface roughness. J. Pal et al. 2014[6], Study the effect of WEDM parameters like pulse on time, pulse off time, peak current, servo voltage, wire feed on surface roughness. They use statistical methodology is to analyze the result for SR. Some of the research on other materials. Lodhi et al. 2014[7] optimized of machining parameters in wire EDM if AISI D3 steel using Taguchi technique having input parameters pulse on time, pulse off time, peak current, servo voltage, wire feed. The analysis shows that the discharge current was the most influential factor on surface roughness. V. Gupta et al. 2013[8], conducted experiments on high strength low alloy steel. It was reported that MRR and Ra increase with increase pulse on time and peak current and decrease with increase in pulse off time and servo voltage and wire tension has no significant effect on MRR and Ra. 5.2. Metal Removal Rate and Cutting Speed In WEDM the material erodes from the work piece by a series of discrete sparks between the work and the tool electrode immersed in the liquid dielectric medium. These electrical discharges melt and vaporize minute amounts of the work material, which are then ejected and flushed away by the dielectric fluid. MRR directly increases with increase in pulse on time (Ton) and peak current (IP) while decreases with increase in pulse off time (Toff) and servo voltage (SV) [9]. These two factors are determine by mathematically given as MRR = cutting speed (mm/minute)* thickness of workpiece (mm) Cutting speed= horizontal distance travelled by the wire along the workpiece (mm) / time taken (minutes) S. Rao et al. 2015[10], carried out experimental research apply the ANOVA technique to analysis the MRR and SR. It was found that pulse on time and peak current are more influencing parameters.b. Jakher et al. 2015[11] study the effect of parameters on cutting speed and surface roughness. They uses taguchi method and results shows that increasing the peak current and pulse on time increases the cutting speed but it is constrained where better surface finish required. A. Goswami et al. 2014[12],carried out experimental investigation to study the effect of different parameters like peak current, pulse on time, pulse off time, servo voltage, wire feed on cutting speed and metal removal rate( MRR and CS). From experimental result CS and MRR increases with increase in pulse on time and peak current where decrease with increase in pulse off time and servo voltage. Vinod et al. 2012[13] carried out experimental investigation to different parameters on cutting speed. The experimental results shows that peak current, pulse on time and pulse off time have highly noticeable effect on cutting speed. Nithin et al.[14] used Taguchi s experimental design to obtaining the optimum machining parameters for the maximization of MRR and minimization of surface roughness separately in WEDM of brass material. They found that, the significant factors are pulse time and feed rate in both MRR and Surface finish. At higher values of feed rate and pulse duration increases the MRR and decreases the surface roughness. 5.3. Kerf (Width of cut) Kerf is one of the important performance measures in WEDM. Kerf is the measure of the amount of the material that is wasted during machining. It affects the dimensional accuracy of the finished part. Kerf of EDMed workpiece depends on gap voltage, pulse on time, pulse off time, wire feed and flushing pressure [15]. A. Goswami et al. 2015[16], investigates the effect of WEDM parameters on kerf width. They use various techniques like taguchi quality design, ANOVA and F-test affecting the kerf has been identified. The result shows that kerf width increases with decreases the pulse off time and increases with pulse on time. Jain et al. 2012[17] carried out experimental investigation to different parameters and their effect on kerf width. The experimental results shows that voltage and wire feed rate are highly significant parameters, pulse off time is less significant and pulse on time has insignificant effect on kerf width. Mahapatra and patnaik 2006[18] study

the relationship between the MRR, SF and kerf by means of nonlinear regression analysis. The optimum search of machining parameter values for maximizing MRR and SF and minimizing kerf was formulated as a multiobjective, multivariable, non-linear optimization problem. 5.4. Wire Wear Ratio Wire wear is the ratio of weight loss of wire after machining to the initial wire weight. As WEDM is a thermoelectrical process in which material is eroded by a series of sparks between the work piece and the wire electrode, along with the workpiece material some particles from wire also will erode, this phenomenon is called wire wear and this should be kept to a minimum. Wire failure occurs in wire-edm process as a result of severity in wire wear rate, which is a function of discharge current and discharge time [19]. Goswami and kumar 2014[20], experimented to study the effect of peak current, pulse on time, pulse off time, servo voltage, wire feed and wire tension used the taguchi technique for WWR. From optimization results, it was found that all of the input parameters investigated in the study have been found to be statistically significant for WWR. Ramakrishnan and Karunamoorthy 2006[21], identified that the pulse on time and ignition current intensity have influenced more than the other parameters considered in their study on WWR. Ranganath et al. 2003[19], Wire failure occurs in wire-edm process as a result of severity in wire wear rate, which is a function of discharge current and discharge time. 5.5. Surface Integrity During machining the electrical spark is produced with high amount of heat that cause the melting or evaporation of workpiece material. Some of the material is flushed away by dielectric circulation and some part was resolidifies to the work piece. Four zones were identified considering micro hardness and micrographs in all specimens after machining. These four zones are detected by using SEM scanning electron microscope that produced a scanned image by focusing a beam of electron. Li et al. 2014[22] investigated the effect of machining parameters on the response variables such as material removal efficiency and surface integrity of IN 718 in wire EDM. From experimental results it was observed that high material efficiency can be achieved in wire EDM due to the lower thermal conductivity of the super alloy compared to steel. Liu, Li and Guo 2014[23].This study explores the process capability of wire EDM in machining Nitinol SE508 by one MC followed by four TC. From experimental results it was observed that microcracks would not propagate into the heat affected zone (HAZ) below the white layer. Amitesh goswami et al. 2015[12] studied investigation of surface integrity, MRR and wire wear ratio for WEDM of nimonic 80A and conclude that the recast layer thickness tends to increase with increased current pulse duration. Li et al. 2013[24] investigated the evolution of surface integrity from high to low discharge energy level. It was found that surface topography of machined surface showed dominant coral reel microstructure at high discharge energy level, while random micro voids are dominant at low discharge energy level. Reister et al. 2009[25] conducted experimental investigation to determine the main wire EDM parameter which contribute to recast layer formation in Inconel 718. From research work, it was found that the peak discharge current, current pulse duration, and energy per spark appeared to be the driving factor in average recast layer thickness. The wire diameter and spark cycle setting did not display a significant effect on average recast layer thickness. 6. Conclusion Wire-cut electrical discharge machining is one of the most emerging non conventional manufacturing processes for machining hard to machine materials and intricate shapes which are not possible with conventional machining methods. This is more efficient and economical for machining hard to machine materials. The effect of various parameters and setting of various parameters at their optimal levels is very much required for manufacturers. From the literature, the parameters and their effects observed are given as under 1. Higher the pulse-on time, higher will be the energy applied there by generating more amount of heat energy during this period. Material removal rate and wire wear rate increase with increase in pulse on time where as surface finish will decrease. 2. Reducing pulse off time can increase cutting speed, by allowing more productive discharges per unit time. However, reducing Off time, can overload the wire, causing wire breaks and instability of the cut by not allowing enough time to evacuate the debris before the next discharge. 3. Servo voltage acts as the reference voltage to control the wire advances and retracts. At higher value of SV the gap between work piece and wire becomes wider and it decreases the no of sparks, stabilizes electric

discharge and the rate of machining slows down. Whereas at smaller value of SV, the mean gap becomes narrow which leads to an increase in number of electric sparks, speed up the machining rate and unstable discharge results in wire breakage. 4. Peak current is the amount of power used in discharge machining and is measured in unit of amperage. The current increases until it reaches a preset value during each pulse on time, which is known as peak current. Peak current is governed by surface area of cut. Higher peak current is applied during roughing operation and details with large surface area. MRR directly increases with increased peak current. 5. Gap voltage is also called open circuit voltage and specifies the supply voltage to be placed on the gap, greater this value, the electric discharge becomes greater. If the gap voltage increases, the peak current will also increases, which leads to higher MRR. 6. Dielectric flow rate is the rate at which the dielectric fluid is circulated. Flushing is important for efficient machining. 7. As the wire feed rate increases, the consumption of wire and cost of machining will increase. Low wire speed will cause wire breakage in high cutting speed. 8. If the wire tension is high enough the wire stays straight otherwise wire drags behind. Within considerable range, an increase in wire tension significantly increases the cutting speed and accuracy. The higher tension decreases the wire vibration amplitude and hence decreases the cut width so that the speed is higher for the same discharge energy. However, if the applied tension exceeds the tensile strength of the wire, it leads to wire breakage. From the literature it has been observed that most of the researchers concentrated on very few number of process parameters at a time to model and optimize various responses, which may not yield accurate optimal values for the process, as the process includes number of process parameters. Further, most of the researchers have given the importance to individual response modeling and its optimization. There is a lot of scope for effective multi objective optimization. Most of the researchers experimented with tool steels, tungsten carbide, titanium alloys and aluminum metal matrices, very less work is reported on nickel base alloys (super alloys). Hence there is a scope in research on nickel based alloys using WEDM. 7. References [1] Carl Sommer, Steeve Sommer, The complete EDM Handbook, www.reliableedm.com. [2] Kansal HK, Singh S, Kumar P. Parametric optimization of powder mixed electrical discharge machining by response surface methodology. Journal of Material Processing Technology. 2005, 169(3), pp. 427-436. [3] Vinod kumar, Kamal Kumar, Vikas Kumar Study of process parameters on Surface Roughness of niminic90 in WEDM.SSRG International Journal of Mechanical Engineering, 2015, 2343-8360,pp.26-30. [4] Vinod kumar, Kamal Kumar and Kumar Jangra An experimental investigation and statistical modeling for trim cutting operation in WEDM of nimonic-90 International Journal of Industrial Engineering Computations, 2015, 2.006, pp.351-364. [5] Amitesh Goswami, Jatinder Kumar Optimization in wire-cut EDM of nimonic-80a using Taguchi s approach and utility concept Engineering Science and Technology, an international journal, 2014, vol17,pp.236-246. [6] Jasvinder Pal, Nishant Parameters optimization of wire EDM for surface roughness using taguchi technique, International Journal of Technical Research 2014, vol.3,pp.23-28. [7] Brajesh Kumar Lodhi, Sanjay Agarwal, Optimization of machining parameters in WEDM of AISI D3 Steel using Taguchi Technique, Procedia CIRP 14, 2014,pp. 194 199. [8] Neeraj Sharma, Rajesh Khanna, Rahuldev Gupta, Multi Quality Characteristics of WEDM Process Parameters with RSM, Procedia Engineering, 64, 2013, pp 710 719. [9] Singh, H., Garg, H. Effect of process parameters on material removal rate in WEDM Journal of Achievements in Materials and Manufacturing Engineering, 32, 2009, pp. 70-74. [10] Sreenivasa Rao M, Venkaiah N. Parametric optimization in machining of nimonic-273 alloy using RSM and particle swarm optimization Procedia Materials Science 10, 2015, pp.70-79.

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