Response Surface Approach for Optimization of Sinker Electric Discharge Machine Process Parameters on AISI 4140 Alloy Steel

Erden⁴ proposed that material removal mechanism relating to three phases of sparking,namely breakdown, discharge and erosion. Also, it was found that reversing the polarity of sparking alters the material removal phenomenon with an appreciable amount of electrode material depositing on the workpiece surface.⁵ Gadalla and Tsai⁶ investigated the material removal of WC–Co composite. They attributed the material removal to the melting and evaporation of disintegrated Co followed by the dislodging of WC gains, which have a lower electrical conductivity. However, thermal spalling contributes to the material removal mechanism during the sparking of composite ceramics. This is because the physical and mechanical properties promote abrupt temperature gradients from normal melting and evaporation.⁷ The working principle of EDM is shown in Fig.1. This technique has been developed in the late 1940s.⁸ The electrode moves toward the workpiece reducing the spark gap so that the applied voltage is high enough to ionize the dielectric fluid.⁹ Short duration discharges are generated in a liquid dielectric gap, which separates electrode and workpiece. 

The material is removed from tool and workpiece with the erosive effect of the electrical discharges.¹⁰ The dielectric fluid serves the purpose to concentrate the discharge energy into a channel of very small cross sectional areas. It also cools the two electrodes, and flushes away the products of machining from the gap. The electrical resistance of the dielectric influences the discharge energy and the time of spark initiation.¹¹ Early discharge occurs due to low resistance.If resistance is more, the capacitor will obtain higher charge value before initial discharge. A servo system is present to compares the gap voltage with a reference value and to ensure that the electrode moves at a proper rate to maintain the correct spark gap, and also to retract the electrode if short circuiting occurs. When the measured average gap voltage is higher than that of the servo reference voltage, preset by the operator, the feed speed increases.On the contrary,the feed rate decreases or the electrode is retracted when the average gap voltage is lower than the reference voltage, which is the case for smaller gap widths resulting in a smaller ignition delay. Thus, short circuits occured by eroded particles and humps of discharge a crater are avoided.

It has been observed that there is very little or no work represented on AISI 4140 & no model is available to represent MRR for AISI 4140 with combination of work piece and electrodes And hence the study is focused on AISI 4140 machining cavilty with various input parameters like current, pulse on time, gap voltage, duty cycle to obtain better MRR.These have been done using Central composite design technique of design of experiments.Central composite design technique is design of experiments (DoE) utilized because the experimental design and analyze of the results can be done with less effort and expenses. Since the method enormously reduces the number of experiments, quality loss of results must be taken into account.¹²

Figure 1: Working principle of EDM[10]

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Experimental

Test Piece And Electrode Material

The test piece material was AISI 4140 with chemical composition given in Table 1. AISI 4140  is preferred in defence and health care due to its specific properties essential in manufacturing. Components of mediums & large cross section, requiring high tensile strength & toughness for automatic engineering and gear & engine construction such as crane shafts steering knocking connecting rods are also made from this material.

Table 1: Chemical Composition(%)

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Characteristics

A 6% allowance should always be made for removal of surface defects during machining Machinability good Ideal for 60 ton tensile applications up to 100mm max. Sizes above limited ruling section will be heat treated to a hardness only, with no guarantee on mechanical properties Weldable.

Typical Applications

Axles, Connecting Rods, Drive Shafts, Crankshafts, High Tensile Bolts,  Studs And propeller Shaft Joints.

Experimental

The experiments were performed on AISI 4140 test pieces (100 mm x 100 mm x 21mm) using Electronica ENC series EDM machine shown in fig below: 

Figure :2

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Table 2: Machining conditions 

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 Measurement Procedure

The test piece was weighed before and after drilling operation using a digital precision scale. Material Removal Rate (MRR) for each experiment was calculated by the following formula:

MRR (mg/min) = initial weight – final weight / machining time                … (1)

Mathematical Modelling

Design Of Experiments

Design of Experiments (DOE) is a method to obtain useful information about a process by conducting only minimum number of experiments.¹³ Each controllable variable (I_p, T_on,V_gap,, T_{duty  cycle},Flushing pressure) can be set  on EDM machine at five consecutive levels from 1 to 5, and hence the design consisting of 30 experiments based on Central Composite Design (CCD) was generated at these levels using Minitab® statistical software.Other factors given in Table 2 were kept constant. Table 3 shows the design matrix with experimental plan. MRR predicted within error range of ± 16% (except experiment no. 29) and ± 19%, respectively.

The selected three parameters have different influences on the machining performance. The MRR is calculated 3 by dividing the volume of material removed by the actual machining time and expressed in mm³/min. The coefficients of the response surface equation were determined by using Minitab15 software.

Statistical Analysis

A five factors, five evel Central Composite design was use to determine the optimal levels of machining parameters of sinker EDM machine for AISI 4140 alloy steel. The central composite design (CCD) with a quadratic model was employed.¹⁴Five independent variables namely current, voltage gap,duty cycle,flushing pressure are chosen. Each independent variable had 5 levels which were – 1, 0 and +1. A total 30 different combinations including 8 axial runs and 6 center point runs.The value of alpha here is (-2,2) as per CCD chart for five factorial experiment.The experiments were run in random order according to a CCD configuration for five factors. The coded values above in table 3 were obtained as per the following relationships in table 4 :

A second-order polynomial equation was proposed to express the MRR as a function of independent variables.Response surface model is hereby employed as a hybrid mathematical and statistical method in which a response(MRR) of interest is affected by several variables and the objective is to optimize this response.¹³ In this study, a second-order polynomial was selected to develop empirical equations to represent responses MRR in terms of controllable variables (I_p, T_on,T_{duty cycle},V_gap):

Table 3: Experimental plan in accordance to CCD

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Table 4: Relationship between coded values and actual values of variables 

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Experimental Results & Scope

Effect of Voltage on MRR

In EDM operations, the voltage is one of the important parameters. To study the effect of voltage,the spark gap and capacitance were kept constant. With analysis, it is observed that, with increase in voltage, the MRR also increases. This occurs because of the energy discharge from the electrode increases with increase in voltage. With increase in discharge energy, higher temperatures are generated between the electrodes. This results higher in MRR.

The capacitance is also an influencing parameter in EDM. To  study the effect in capacitance, other two parameters such as voltage and spark gap are kept constant.This is another scope for further study and analysis in EDM research.

Conclusion

A Central composite design was used to conduct the experiments. The effect of important process parameters in EDM: voltage,current,duty cycle and flushing pressure on MRR has been studied. The following are some of the salient conclusions that could be drawn based on the studies:

Increase in voltage results in higher MRR and similarly higher capacitance results in higher MRR.

In case of spark gap, with increase in spark gap the MRR decreases.

The results obtained would be a good technical database for the defence/automotive industries.

References

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