Keywords

27.1 Introduction

Superalloys comprise one of the bright categories of alloys that possess excellent mechanical strength at elevated temperatures, superior surface stability, magnificent creep resistance, and corrosion and oxidation resistance. They have a FCC structure of austenitic type with a base of nickel, iron, cobalt, or a blend of them. They have a very high strength-to-weight ratio, are resistant to erosion, and possess metallurgic stability at elevated temperatures. Through solid solution strengthening, superalloys develop high temperature strength. The main contribution in the field of superalloys is contributed by aerospace and power industries along with the chemical industries. As they are resistant to corrosion and oxidation, therefore they have a great advantage to be used in the chemical industries. Elements like aluminum and chromium provide corrosion and oxidation resistance to superalloys as these alloys are intended to use for elevated temperature applications and this property of resistance is of great importance. They are also used in other high temperature applications like nuclear reactors and gas turbines other than their applications in aerospace industry.

Incoloy-800 is an iron base superalloy having main composition of iron–nickel–chromium. It possesses sufficient resistance to oxidation and carburization at elevated temperatures. It is of high use in petrochemical industry as it does not form embrittling sigma phase after long time exposure at 1200 °F. It is also highly resistant to stress corrosion cracking. On the other hand, in spite of its superior qualities its effective machining is still a challenge with the traditional machining methods. Incoloy-800 welds to the cutting tool if machined by these traditional methods because of its chemically reactive nature, leading to the premature tool failure. So for its productive machining, non-traditional machining methods are preferred.

In the present scenario of non-traditional machining methods, wire electrical discharge machining (WEDM) is one of the fastest extensive growing processes. Wire EDM can produce complex shapes with better surface finish and high level of accuracy and precision. Micro-machining operations and machining operations can be performed on it. It is continuously increasing popularity in aerospace, manufacturing, automobile, medicine, injection molding, and other various industries making it a hot topic of research.

Mathew [1] investigated the parameters for machining a difficult-to-machine superalloy: Inconel X-750 and Waspaloy. They analyzed the tool wear, turning forces, and surface roughness in dry turning of these superalloys. The higher amount of force is obtained for Waspaloy than Inconel X-750 for all the cutting conditions because of its higher tensile strength. Welling [2] studied the surface integrity differences between broaching and grinding compared to wire EDM on the Inconel 718 superalloy. The three-cut strategy is specially developed for the brass wire electrodes. For broaching, separate specimen with 3.2 mm thickness is selected. Xavior et al. [3] machined Inconel 718 on wire EDM and studied the effect of recast layer thickness on its mechanical characteristics. The parameters chosen are current pulse duration, peak discharge current, pulse off time, and gap voltage as it was studied that these parameters influence the recast layer thickness. Mandal et al. [4] examined the interrelationships between process parameters and performance measures of Nimonic C-263 superalloy machined on wire EDM. The process parameters chosen are pulse off time, rate flow of dielectric, servo voltage, and pulse on time. Mandal et al. [5] analyzed the wire EDM input parameters on Kerf width and surface integrity for Al 6061 alloy and found that the major factor affecting surface roughness was pulse on time followed by pulse off time and wire tension. The reasoning and explanations for surface roughness behavior have also been elaborated in several advanced machining-based studies [6,7,8]. Rajyalakshmi et al. [9] machined Inconel 825 on wire EDM and optimized its various process parameters by using Taguchi gray relation analysis. The process parameters taken are pulse off time, corner servo voltage, pulse on time, flushing pressure, servo feed rate, wire tension, wire feed, spark gap voltage. Using the gray grade value, ANOVA is formulated for identifying the significant factors. Rao et al. [10] machined Inconel 690 and applied modified flower pollination algorithm (FPA) to optimize parameters of wire EDM. The responses are compared with RSM. Goyal et al. [11] used wire EDM to optimize the surface roughness of Inconel 625. For design of experiments, Taguchi L18 (21 × 35) orthogonal array is used. Plain and cryogenic treated these two different diffused wire tool electrodes were used. Tanjilul et al. [12] studied debris removal in EDM drilling of Inconel 718 and examined the EDM flushing mechanism and debris size for efficient debris removal. Furthermore, several studies have been attempted in the domain of optimization and modeling of process variables [13,14,15,16]. Nain et al. [17] examined the effects of various input parameter responses—surface roughness and effects of recast layer using fuzzy and BP-ANN model. Rao et al. [18] optimized wire EDM process while machining Inconel 690 by applying modified (CSA) cuckoo search algorithm.

In view of above discussions, the present experimental study has been attempted to investigate the effect of several input machining variables on the considered performance characteristic in wire-cut electrical discharge machining of Incoloy-800 superalloy. The analysis of variance test and the optimization of the selected machining response have also been performed to reveal out the impact and the behavior of the considered input factors on the machining responses statistically. Table 27.1 shows the literature review.

Table 27.1 Literature review
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27.2 Materials and Methods

The experimental work has been carried out on wire EDM (ELECTRONICA ELEKTRA MAXICUT 734) as shown in Fig. 27.1. In this study, Incoloy-800 superalloy plate of 200 × 150 mm2 area and 12 mm thickness has been used as the workpiece whose composition, properties, and workpiece figure are shown in Tables 27.2, 27.3 and Fig. 27.2, respectively. Soft brass wire of 0.25 mm diameter has been used as the tool electrode. Deionized water has been used as dielectric fluid which is flushed continuously through the gap along the wire and the sparking area to remove the debris produced during erosion. Figure 27.3 shows the dielectric flushing unit. The machine is programmed by using a NC code. Square cuts of 8 mm side have been fabricated from the workpiece as shown in Fig. 27.4.

Fig. 27.1
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Wire EDM machine setup for the experimentation

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Table 27.2 Composition of Incoloy-800
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Table 27.3 Properties of Incoloy-800
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Fig. 27.2
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Incoloy-800 workpiece for the experimentation

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Fig. 27.3
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Dielectric flushing unit

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Fig. 27.4
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Machined workpiece sample

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Design of experiments has been widely employed in the previous research articles by considering several working factors and responses [27,28,29,30,31]. According to the Taguchi method based on robust design, a L9 orthogonal array is employed for the experimentation. Based on pilot experiments carried, pulse on time, pulse off time, and peak current have been selected as the input process parameters and their levels are mentioned in Table 27.4. The various machining inputs have been kept constant as: servo voltage at 60 V, wire feed rate at 6 m/min, and the wire tension at 700 N.

Table 27.4 Controllable process parameters and their levels
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27.3 Results and Discussion

The experiments have been conducted as per the Taguchi L9 orthogonal array. The experiments have been replicated twice to distribute the error uniformly. The design matrix along with the studied response values has been detailed in Table 27.5.

Table 27.5 Design matrix and the response values (MRR)
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After attaining the MRR values, the analysis of variance test has been performed as shown in Table 27.6 and Fig. 27.5. shows variation of material removal rate under several trial runs. From the ANOVA results of MRR, it can be observed that Ton, Toff, and Ip are influencing the MRR. As the Ton increases, the MRR also gets increased with it but after attaining a highest value it slightly decreases with further increase in Ton. When the pulse on time was 1 µs, the MRR has been recorded minimum and highest at 2 µs with the very slight decrease at 3 µs. MRR gets decreased with the increase in Toff continuously at all levels. With the increase in Ip from 1 to 2 A, the MRR somewhat gets increased but it shows a drastic increase when Ip increases from 2 to 3 A. Figure 27.6 shows the main effects plot for MRR (S/N) mean data.

Table 27.6 Analysis of variance (ANOVA) for S/N ratio
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Fig. 27.5
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Variation of material removal rate under several trial runs

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Fig. 27.6
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Main effects plot for MRR (S/N)

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The peak current factor has been revealed as the most dominating parameter for the MRR followed by pulse on time, pulse off time, and wire feed rate. The optimized parametric setting for the studied machining response (MRR) has been observed as: pulse on time −2 µs, pulse off time −4 µs, and peak current −3 A, i.e., A2B1C3. At the optimized setting, the confirmatory experiment value for MRR has been revealed as 1.5439 mm/min, which is improved by 2.57% than the previously attained best value of MRR.

27.4 Conclusion

The following conclusions can be drawn from the present experimental study. These are as below:

  1. 1.

    Peak current and pulse on time have been recorded as the most influencing parameters for the MRR. An extreme increase in MRR has been observed when the peak current is increased from 2 to 3 A. It also shows an increase with the increase in pulse on time (from 1 to 2 A), but it slightly gets decreased with further increase in the pulse on time (from 2 to 3 A).

  2. 2.

    The optimized parametric setting for the studied machining response (MRR) has been observed as A2B1C3 (pulse on time −2 µs, pulse off time −4 µs, and peak current −3 A).

  3. 3.

    At the optimized setting, the confirmatory experiment value for MRR has been revealed as 1.5439 mm/min, which is improved by 2.57% than the previously attained best value of MRR.