Effect of Process Parameters on Performance of EN-31 Steel using WEDM: Experimentation and Optimization

EN-31 steel is a high carbon alloy steel which possess a high grade of hardness with compressive strength and abrasion resistance, good density and less corrosion resistance. Its high strength makes EN-31 steel popular and demandable in Industrial applications. The Wire Electrical Discharge Machining (WEDM) is a variant of EDM and is commonly known as wire cutting or wire-cut EDM. Generally this machining process is used in tool and die making industries, medical and surgical industries, air craft and aerospace industries, automobile industries, and used vastly in engineering applications. A metal wire with dielectric medium travels through a path and perform machining of EN-31 steel. Material of the wire is generally preferred as Tungsten, Copper, Brass, etc. In this research, Brass has been used as wire material. The main objective of this work is to identify the optimal WEDM process parameters for high material removal rate (MRR) and grater surface finish during machining. The effect of parameters, such as Pulse On time (Ton), Pulse Off time (Toff), Table feed (T.F), Wire feed (W.F), and Servo voltage (V) have been investigated to reveal their effect on MRR and surface roughness of material. The experimental plan is based on Taguchi method using these five factors each at three levels. The major application of the material is for manufacturing of cutting tools, dies, punches etc.


INTRODUCTION
Wire Electrical Discharge Machining (WEDM) is an electro thermal production process in which a thin singlestrand metal wire in conjunction with de-ionized water (used to conduct electricity) allows the wire to cut through metal by the use of heat from electrical sparks. Due to the inherent properties of the process, WEDM can easily machine complex parts and precision components out of hard conductive materials. WEDM works by creating an electrical discharge between the wire or electrode and the workpiece. As the spark jumps across the gap, material is removed from both the workpiece and the electrode. To stop the sparking process from shorting out, a nonconductive fluid or dielectric is also applied. The waste material is removed by the dielectric, and the process continues. In WEDM, a thin single-strand metal wire, usually brass, is fed through the workpiece.
WEDM is a specialized thermal machining process capable of accurately machining parts with varying hardness or complex shapes, which have sharp edges that are very difficult to be machined by the main stream machining processes. At present, WEDM is a widespread technique used in industry for high-precision machining of all types of conductive materials such as metals, metallic alloys, graphite, or even some ceramic materials, of any hardness. Many WEDM machines have adopted the pulse generating circuit using low power for ignition and high power for machining. However, it is not suitable for finishing process since the energy generated by the highvoltage sub-circuit is too high to obtain a desired fine surface, no matter how short the pulse-on time is assigned. As newer and more exotic materials are developed, and more complex shapes are presented, conventional machining operations will continue to reach their limitations and the increased use of WEDM in manufacturing will continue to grow at an accelerated rate. The setting is easy tuned for a straight line and become more difficult for the curve or part which is involving an angle. Ninety degree angle is the most difficult section to be cut by WEDM because of the dramatically direction changing. Wire electrical discharge machining process is a highly complex, time varying & stochastic process. The process output is affected by large no of input variables. Therefore a suitable selection of input variables for the WEDM process relies heavily on the operators technology and experience because of their numerous & diverse range. WEDM is extensively used in machining of conductive materials when precision is of prime importance. Rough cutting operation in WEDM is treated as challenging one because improvement of more than one performance measures viz. Metal removal rate (MRR), surface finish & cutting width are sought to obtain precision work as shown in Fig.1 [12] used WEDM on smart NiTi60 alloys to investigate the impact of process parameters on tool wear rate, MRR & surface roughness. Sivakiran et al. [13] studied the effect of process parameters on MRR in WEDM of EN31 steel. In this paper linear regression and Taguchi's L16 orthogonal array is used. C Reddy [14] conducted experiments on WEDM based on L16 orthogonal array selecting P20 die tool steel as work material with 0.18 mm Molybdenum wire as electrode to find out higher MRR & better surface roughness. Muthu Kumar et al. [15] determined the WEDM process parameters of incoloy 800 for high material removal rate, surface roughness, kerf width based on grey-Taguchi method. Vishal Parashar et al. [16] performed experiments on stainless steel grade 304L of 10mm thickness under different cutting conditions of gap voltage, pulse on time, pulse off time, wire feed & dielectric flushing pressure for high MRR using the regression analysis and ANOVA. Rao [17] studied the effect of process parameters on MRR in WEDM, using Hot die steel (H-11) using one variable at a time approach. Kanlayasiri and Boonmung [18] investigated the effects of WEDM parameters on surface roughness of DC53, cold die steel by ANNOVA. Mahapatra & Pattanaik [19] described about parametric optimisation of WEDM process using Taguchi method, for surface roughness (Ra) & MRR. R. Ramakrushnan & Karunamoorthy [20] used Taguchi methods for L16 orthogonal arrays to measure MRR, surface roughness & wire wear ratio. BingHwa et al. [21] used WEDM for Al2O3/6061Al composites for machining performance, such as, cutting speed, width of slit & surface roughness. EI-Taweel et al. [22] investigated the effect of machining parameters of WEDM of inconel 601 using responsive surface methodology. Hewidy et al. [23] developed mathematical models correlating the various WEDM machining parameters with MRR, Wear ratio & surface roughness, using response surface methodology. Miller et al. [24] investigated the effects of spark on-time duration & spark on-time ratio on MRR & surface integrity of various advanced materials. Tosum et al. [25] investigated the effect of machining parameters on the kerf & MRR in WEDM, using Taguchi method. C.L. Lin et al. [26] described the grey relational analysis based on an orthogonal array & fuzzy based Taguchi method for optimising the responses. Marafona & Wykes [27] have investigated a new method of optimizing MRR using EDM with Copper-Tungsten electrodes, for improved MRR and a given wear ratio. Tarng et al. [28] optimized the cutting parameters using feed forward neural network through simulated annealing algorithm, for machining SUS 304 stainless steel of 10 & 15mm thickness.

2.1objectives
This experiment is conducted to determine good surface finish & high material removal rate of EN-31 steel by changing different machining parameters, such as Pulse on time (Ton), Pulse off time (Toff), Wire feed (W.F), Table feed (T.F) and Servo voltage (V).

EXPERIMENTAL WORK
The material which is taken for this experiment is EN31 steel. It is used for the manufacturing of cutting tools, dies, punches, etc. Then these values are put in the Taguchi method and are arranged automatically. By using the obtained values, machining operation is carried out. The machine can move 200mm in Z-axis, 350 mm in Y-axis, 250 mm in X-axis & 3º in U-Vaxis. This machine consists contains upper guide, lower guide, bed, wire, and wire spool. The software used in this machine is ELCAM.

Experimental Set up
The present work focuses on MRR & Surface Roughness (Ra) of EN-31 steel in WEDM. The dimension of the steel is 306*10*6 mm. The experiment is carried out in Electronica Eco Cut Machine (Fig. 2). The wire that is used as electrode is made up of Brass. The diameter of the wire is 0.25mm. The dielectric fluid is distilled water. In this machining operation, work piece acts as anode & wire as cathode. Wire is passed between the two guiders, i.e. Upper guider & Lower guider. Lower guider is also act as Nozzle. It supplies dielectric fluid during the time of cutting operation. The function of the dielectric fluid is to cool the work piece & also act as a flushing medium. Wire is supplied continuously by the help of wire spool. The limitation is that, the wire can once be used because during the time of cutting operation it losses Electrons.

Selection of Work Piece
The work piece material used in this study is EN-31 steel. It has high resisting nature against wear and can be used for components which are subjected to severe abrasion, wear or high surface loading. The chemical & mechanical composition of EN-31 steel is given in the Table 1 and 2.   If a small amount of zinc is added to Copper wire drastically reduces the conductivity. Hard brass wire typically has conductivity only 20% of the Copper wire.
4. METHODOLOGIES 4.1Taguchi Method Genichi Taguchi, a Japanese industrialist and an international consultant in the field of total quality control & assurance formulated both a philosophy and a methodology for the process of quality improvement that depends on statistical concepts, especially statistically designed experiments. The primary goals of the Taguchi methodology can be described as: i) a reduction in the variation of a product design to improve quality & lower the los imparted to the society, ii) a proper product or process implementation strategy which can further reduce the level of variation.

Signalto -Noise Ratio
Traditionally a designed experiment can be used to estimate or test the significance of certain factors on the basis of a measurable response over a set of experimental conditions. Taguchi emphasized that an addition to this, the variation of the experimental data needs to be studied. In order to facilitate this study he used the concept of signal-to-noise ratio.

Orthogonal Arrays
Orthogonal arrays are highly fractional orthogonal designs proposed by Taguchi. These designs can not only be applicable to two level factorial experiments but also investigate main effects when factors have more than two levels. Designs are also available to investigate main effects for certain mixed level experiments where the factors included do not have the same number of levels. Table 3. These values are taken according to the machine specification. Then these values are put in the Minitab software and then the machining parameters are arranged automatically. The experiments to be conducted are based on varying the process parameters which affect the machining process to obtain the required quality characteristics. Quality characteristics are the response values or output values expected out of the experiments.

Experimental Procedure This experiment consists of 5 factors and 3 levels that are selected by Taguchi method. The factors are Ton, Toff, T.F, W.F and V shown in
There are 64 such quality characteristics. The most commonly used are: i) Larger the better, ii) Smaller the better, iii) Nominal the best, iv) Classified attribute, and v) Signed target. As the objective is to obtain the best surface finish & high MRR, it is concerned with obtaining the least value of surface roughness and large value for MRR. Hence the required quality characteristic is smaller the better, which states that the output must be as low as possible, tending to zero for surface roughness and larger the better, which states that the output must be as large as possible for high MRR.

Mechanism of MRR
Mechanism behind material removal rate of WEDM process is based on the conversion of electrical energy to thermal energy that categorise it to electro thermal process. During machining both the surfaces may have present smooth and irregularities cause minimum & maximum gap in between tool and work piece. At a given instant and at minimum point, suitable voltage is developed to produce electrostatic field for emission of electrons from the cathode and the electrons get accelerated towards anode. After achieving greater velocity, electrons collide with the dielectric molecules breaking them into negative and positive ions. As a result spark is generated with high temperature, causes melting and vaporisation of material from the work piece. MRR can be defined as follows:

Measurement of Surface Roughness
Surface roughness is the size of the surface texture. It is expressed in µm and denoted by Ra. For higher value the surface is rough and if lowers then the surface is smooth. This value is measured by a surface roughness tester. This tester is consisted with a stylus at its top. The function of stylus is to move over the machining surface, where roughness test is to be carried out. This stylus consists with a needle at its top and this needle is moved over that surface to measure the roughness.

Selection of Wire Material
International Journal of Engineering Research & Technology (IJERT) ISSN: 2278-0181 http://www.ijert.org In this chapter, we are discussing about the effect or influence of machining parameters, i.e. Ton, Toff, V, W.F, T.F. on MRR and surface roughness (Ra) with Brass wire.
In the mean time, it is to identify parameter that plays most important role during experimentation with the help of Taguchi design.  Relation between MRR & Ton is shown by the Fig.3. This figure shows that when Ton increases MRR is also increases.    7. From this plot it is clearly justified that when voltage increases MRR decreases. So I found that voltage has a big impact on MRR. There is no direct impact of Ton over Ra. It means Ra is not only depends upon Ton, but also is highly effected by some other parameters.    Roughness. The discharge energy increases with the increase of Pulse on time. Larger discharge energy causes a larger crater causing a larger surface roughness value on the work piece.  Figure 15 shows the normal probability plot, versus fits, histogram & versus order plot. This is known as Residual plots for means. This layout is necessary to cheak wheather the model meets the expectation of the analysis. The interpretation of residual plots is as follows.

Residual Plots for Means
1. Normal probability plots indicates that the data are distributed normally. It can be seen that the standardise residue lies between -0.50 & 0.50. 2. Versus fits graph indicates the variance is constant & non linear relationship exists as well as no out liers exist in the data. 3. Histogram of the data is forms an irregular shape. 4. Versus order graph shows that there are systematic effects of the data. Versus fits plot shows that all data are within the limit. Versus order plot shows that all data are arranged in a regular manner. The interaction plot of MRR is shown in Fig 17. this plot shows the interaction between the two input variables taken in this experimant. The significant interaction is shown by the Right hand top most figure. It can also be confirmed from the ANNOVA table.

International Journal of Engineering Research & Technology (IJERT)
ISSN: 2278-0181 http://www.ijert.org  Fig 19 shows that the Interaction plot for Ln St Deviation plot. Here also that two input variables are used, those are Ton & V. This figure also shows that the significant relationship between the input parameters.

Managerial Implications
Now a day WEDM is generally used in many applications, such as: automotive, aerospace, mould, tool and die making industries. Applications can also be found in the field of medical, optical, dental & jewellery parts processing. Owing to high process capability, it is widely used in manufacturing of cam wheels, special gear, bearing cage, various press tools, dies & similar intricate parts.

Limitations
Using WEDM, many other experiments can also be carried out by using some other parameters & levels. Here we have considered certain parameters such as Ton, Toff, T.F, W.F, and V etc. However, some other parameters like Tool Wear Rate, Dielectric Fluid Pressure, and Wire Tension etc can be taken into consideration in WEDM. At the same time, some other materials and Wires can also be used for machining purpose. It has a main limitation that it is difficult for machining of nonconductive materials.

Scope of Future Work
The following points may be suggested for future research work: i) Studying the effect of cutting other materials like Al and M.S., ii) Using other dielectric solution such as oil, iii) Using other wire materials such as Copper, Tungsten, & other coated wires, and iv) Studying the surface roughness in the current of WEDM using Scanning Electron Microscope.