Modeling of WEDM Parameters while Machining Mg-SiC Metal Matrix Composite

In this paper an attempt has been made to study the effects of the process parameters of wire cut electrical discharge machining (WEDM) on Magnesium-Silicon Carbide MMC with 5% SiC in particulate form. For the analysis, six factors, namely pulse on time, pulse off time, spark gap voltage, peak current, dielectric flushing pressure and servo feed have been taken and a Taguchi L16 orthogonal array for two levels was used. Response surface methodology was also used to develop second-order models for material removal rate (MRR) and surface roughness (SR). From the analysis of variances, it has been observed that pulse on time and pulse off time were the most significant parameters among all those observed in predicting the MRR and SR, respectively.

ANOVA; Mg-SiC; MMC; MMR; WEDM

WEDM in recent years emerged to be the most widely-used machining process with its outstanding ability to cut intricate shapes and designs without taking into account the hardness and other mechanical properties of the material (Ho et al., 2004). The principle of WEDM is illustrated in Figure 1. The mechanism of WEDM is similar to that of conventional EDM where the material is removed by erosion, due to the spark generated. However, in the case of WEDM, a reel of wire is continuously fed into the process. As the wire moves through the work piece for cutting the desired shape, the wire is fed from a supply reel to a take-up reel and a fresh wire of constant diameter is passed through the material every time so as to maintain a constant cut width throughout. One big disadvantage of WEDM is its very low material removal rate which is challenging its viability as a prominent technique to machine exotic advanced materials. Due to this situation, several types of research have been carried out to understand the effects of process parameters on the response characteristics like those of material removal rate (MRR) and surface roughness (SR) and optimize these characteristics to obtain the best performance rates, especially for MMCs. It has been found out that of the researches carried out in the field of machining MMC with EDM and WEDM, 71% of the studies are conducted on WEDM and 21% of these are in EDM (Kumar et al., 2012). Furthermore, of the studies conducted in WEDM, 22% are on optimizing the process parameters and 45% are on monitoring and controlling those process parameters (Ho et al., 2004). However, the studies, while machining magnesium MMCs for machining optimization of WEDM process parameters, have been very few. Kumar et al. (2014) investigated the relationship of the process parameters in WEDM of Mg-SiC 10% and 20% using molybdenum wire as the electrode. The response characteristics selected were MRR and SR. An L9 Taguchi orthogonal design was selected. An optimal set of process parameters were found for both MRR and SR, using signal-to-noise (S/N) ratio analysis. Furthermore, the grey relational analysis was also performed for multi-objective optimization and an optimal parameter set was revealed. Sharma et al. (2013) developed a second order model using response surface methodology (RSM) to establish the relationship between WEDM parameters, such as pulse on time, pulse off time, spark gap voltage, peak current and wire tension with MRR and SR. Shandilya et al. (2012) performed the WEDM of SiC/6061 Al MMC having 10% of SiC particles using brass wire as an electrode. Kerf width was selected as the response characteristic. A total of 29 experiments were conducted using Box Behnken Design (BBD) for four parameters and a second-order model was developed for the kerf width using RSM. The effects of the process parameters on kerf width were studied and also the surface integrity of the machine surface was examined. 

Subrahmanyam and Sarcar (2013) used the Grey-Taguchi method for the multi-objective optimization of WEDM parameters while machining hot die steel H13. MRR and SR were selected as the responses. An optimal set of parameters was revealed. Analysis of variance for the overall grey relation grade was done and the significance of the process parameters was found out. Furthemore, mathematical models were also developed for the responses. Satishkumar et al. (2011) investigated the effects of process parameters of WEDM on unreinforced Al6063 and also Al6063 reinforced with SiC 5%, 10%, 15% with a brass wire as an electrode. An L9 Taguchi orthogonal array was used and responses that were selected are MRR and SR. Linear models were developed using regressions for MRR and SR at each percentage variation of SiC. Analysis of variance was performed to study the significance of each parameter on the output responses. Puri and Bhattacharyya (2004) studied the effects of process parameters on white layer depth occurring while machining with WEDM. A rotatable Central Composite Design (CCD) was used and response surface methodology was applied to develop the mathematical models. Analysis of variance was performed to find the parametric influences of white layer depth.

In WEDM, the most significant response characteristics are the material removal rate (MRR) and surface roughness (Ra). MRR stands out to be an index for productivity and surface roughness for quality. The desired requirement is that MRR should be high and Ra should be low. However, both the response characteristics are conflicting in nature and therefore an efficient method is needed to be adopted so as to optimize the process parameters to achieve the optimum machining performance. In this paper, an attempt is made to study the effects of each parameter of WEDM on MRR and SR using the Taguchi Grey Relational Analysis and to find out an optimal parameter setting. Furthermore, three of the most significant parameters will be subsequently selected from the preliminary analysis and further experiments will be conducted to develop second order models using response surface methodology.

2.    EXPERIMENTAL SET-UP

The principle of WEDM is illustrated in Figure 1. Figure 1 shows a schematic arrangement of WEDM process and surface roughness measurement of Mg-SiC.

Figure 1 Schematic diagram of WEDM process

2.1.   Material and Tool Selection

The experiment was performed in a CNC wire EDM, shown in Figure 2, using Mg-SiC (5%) as work piece material and zinc coated brass wire of 0.25 mm diameter as an electrode. The work piece material used for machining is shown in Figure 3. The dielectric fluid was de-ionized water. The MRR is expressed as the ratio of the difference in weights of the material before and after machining in relation to the machining time. The surface roughness was measured using a portable surface roughness tester. The values for surface roughness were taken at three locations for a particular machined surface after every experiment and their mean value was also taken.

2.2.   Design of Experiment

During the WEDM machining experiment, a few parameters were kept constant to analyse the performance characteristics in more detail (Saurav & Mahapatra, 2010). Table 1 gives the list of the constant parameters along with their values.

Figure 2 CNC wire cut electrical discharge machining

Figure 3 Work piece material Mg-SiC (5%)

Table 1 Parameters held constant during the experiments

In the present work, three of the most significant parameters while machining Mg-SiC in wire EDM are pulse on time (T_ON), pulse off time (T_OFF) and gap voltage (SGV), which were selected as input parameters having three levels as given in Table 2.

Table 2 Process parameters and their levels

A Box Behnken Design (BBD) of the experiment having 15 datasets from RSM was employed. The BBD experimental design matrix and responses are given in Table 3.

Table 3 Experimental results for BBD with three parameters and three levels

Taguchi Grey relational analysis is inefficient in studying the effects due to the higher order polynomials. To understand the effects in the responses due to a curvature, higher order models need to be developed. Response surface methodology (RSM) is a collection of mathematical and statistical techniques that are used for modeling and optimizing the process parameters of complex physical processes and their relationship to the output responses of the process (Myers et al., 2009). With the help of RSM, a second-order model can be developed and the quadratic and interaction effects of the process parameters can be studied. The output response can be modeled according to the second order model as shown in Equation 1 below:

                                (1)

Where y is the output response, x_i, x_j, x_k are the input process parameters,

In this study, an experimental investigation on wire cut electrical discharge machining (WEDM) was performed using magnesium-silicon carbide (5%) MMC as the work piece material and zinc-coated brass wire of 0.25 mm diameter was used as the electrode. The experiments were designed using a Box Benhken RSM Design. The following conclusions were made during the course of the work: (1) Mathematical models for both MRR and SR were developed and the models were found to be quite adequate; (2) The analysis of variance for the process parameters showed that pulse on time was the most significant parameter among all of the parameters in predicting the MRR. Furthermore, the quadratic effects were more significant than the linear effects. The quadratic effects of pulse on time, pulse off time and gap voltage were found out to be statistically more significant. Also, the interaction effect between pulse off time and gap voltage was found out to be statistically significant; (3) For SR, pulse off time was found out to be statistically significant. The quadratic effect of gap voltage was found to have a significant effect on SR.

The authors acknowledge the financial support provided by North Eastern Regional Institute of Science and Technology (NERIST), Itanagar, India for B.Tech Project and Mr. Kajal Saha, Project Manager, Tool Room and Training Centre (TRTC), Guwahati, India for his support in letting us conduct our experiments in his premises.

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