Keywords

1 Introduction

In today’s technological era, manufacturing industries are facing a major challenge from the materials that are very difficult to cut/machine such as ceramics, super alloys, and composites along with precise requirements of the design like high surface quality, complex shapes, high bending stiffness, high strength, good damping capacity, better fatigue characteristics, and machining costs. The engineering challenges posed by the rapid growth in the development of such materials owed to the emergence of new and advanced machining processes, also known as “Unconventional Machining Methods”. The word “unconventional” is used because metals like tungsten, hardened stainless steel, tantalum, and some high strength alloys, composites cannot be worked or processed upon by conventional methods. Nowadays, the nonconventional machining routes are more commonly used in the manufacturing industries in place of the traditional tools for machining. For the last few years, Electric Discharge Machine (EDM) is used to deal with the machining of such category of materials in the desired size and shape as well as for the required accuracy [1]. Electrical Discharge Machining is an example of advanced unconventional machining method (Fig. 1), wherein, material erosion takes place by the repeated action of spark between tool and workpiece in the presence of dielectric; so it becomes important to select machining parameters to achieving optimal machining performance [2]. Though, EDM possesses a major drawback of its reluctance to be used for large-scale production because of low MRR and poor surface finish, an attempt was made to enhance the surface properties of workpiece by mixing various types of powder in dielectric fluid. Favorable results were witnessed along with decrease in thickness of the recast layer as compared to conventional EDM [3]. Re-deposition of the eroded material back into machined surface came out to be one of the extreme difficulties in the way of high metal removal rate. Under high temperature and rapid cooling phenomenon, the hydroxides and melting metal debris solidify to form a recast layer [4]. Hence, a more efficient and convenient flushing system was proposed in order to improve process performance that made use of a abrasive particles mixed dielectric fluid jet [5]. MRR of EDM largely depends on the flushing conditions and to enhance the flushing conditions of EDM tube electrodes of different diameters were fabricated and then after surface quality and machining efficiency were investigated. Results showed that increase in internal diameter enhanced the surface finish and material removal rate [4]. Further, to improve the flushing conditions different shaped diameter electrodes were fabricated. The double-hole tube electrode was conformed as to have the optimal structure for relevant higher MRR [6]. The conventional EDM could be improved in terms of machining characteristics by combining various mechanisms jointly (“Hybridizing EDM”) during machining to attain preferred surface integrity and stability of process. The development of hybrid machining is intended to utilize of the combined benefits of individual process and to avoid or plunge their limitations at the same time exhibited by the individual constituents [7, 8].

Fig. 1
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Copper electrodes with variable hole diameter

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Abrasive jet machining (AJM) is one of the suitable options for machining of brittle and hard materials efficiently in surface finishing, deburring, and cleaning of surfaces, and is the focus of concern for many researchers for fabricating micro holes, slits etc. [9]. Tsai et al. [10] employed AJM to enhance the surface finish of tungsten carbide and SKD 61 steel machined by EDM. The results indicated the improvement in surface roughness from 1.3 to 0.7 mm within short time interval. Lin et al. [11] used the hybrid process of EDM and AJM under several parameters and concluded that abrasive particles used in the hybrid process not only improved the surface finish but material removal rate of SKD 61 steel as well.

The main motive behind this study is to analyze the performance of abrasive particles assisted EDM operation on machining characteristics, for instance, MRR, TWR, and SR and recast layer formation. The prominent process parameters such as current, pulse-on, pulse-off, tube electrode inner diameter, and type of dielectric were chosen to analyze their effects on performance of process. Moreover, to develop a mathematical model for machine parameters and machining characteristics, Taguchi’s design approach (DOE) was selected. Finally, process parameters were optimized to get the desired surface finish, material removal rate, and tool erosion rate.

2 Materials and Methods

2.1 Material

Two hollow electrodes of electrolytic copper having an outer diameter of 15 mm and internal diameters of 2 and 4 mm, respectively, were used for the abrasive mixed dielectric to flow through the tool, as shown in Fig. 1. Al–SiC MMC workpiece in a rectangular shape (2 mm × 2 mm) had been procured from CPS Technologies, USA (Fig. 2). The properties of the workpiece material are tabulated in Table 1 and selected process parameters in Table 2. Commercial grade EDM oil was used as the dielectric medium and abrasive particles of SiC having size of 220 mesh were used in 30 gm/L for abrasive-assisted machining.

Table 1 The properties of Al–SiC MMC
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Fig. 2
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SEM of unmachined Al–SiC (37 vol%SiC/A356)

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2.2 DOE–Taguchi Approach

In this experimental work, two replications are performed at random order to find out the S/N ratio for more accurate output. The S/N ratio is considered as measure of performance in terms of the ratio of amount of preferred signal power to the unwanted noise which is symbolized as:

  • “larger is better”: when aim is to maximize the response

    $$S/N = - 10*\log \left( {\sum {\left( {1/Y^{2} } \right)/n} } \right)$$

  • “smaller is better”: when aim is to minimize the response

    $$\begin{aligned} & S/N = - 10*\log \left( {\sum {\left( {Y^{2} } \right)/n} } \right) \\ & {\text{Considering}}\;Y = {\text{output}}\;{\text{variable}};\;n = {\text{experiments}} \\ \end{aligned}$$

In Taguchi designed experimentation, higher values of the S/N ratio identify the eminent process parameter settings that minimize the impact of the noise factors. The DOE procedure was implemented to identify and scrutinize the prominent parameters systematically, which was then verified with the Taguchi-based method. Taguchi’s L16 orthogonal experimental design was framed to draw valid conclusions during abrasive-assisted ED machining of Al–SiC MMC and the factors assignment was done using Minitab 15.

The results of the investigations have been represented graphically for the critical discussion on MRR, TWR and SR of the electrical discharge machining process for the various investigations. The experiments were planned as per Taguchi’s L16 array in the design of experiments which helped in reducing the number of experiments (Table 3).

Table 2 Input parameters and their levels
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2.3 Experimentation

The series of experiments were carried out on EDM (OSCARMAX, S645, ZNC), wherein a specially fabricated tank with a pump, for homogeneous mixing of suspended abrasive particles was mounted. The SiC particulates (220 mesh) were mixed in EDM oil of commercial grade as dielectric media with dilution of 30 gm/L and were continuously stirred to avoid settling down. The scheme of designed setup along with a brief illustration of the layout is shown in Fig. 3. The various trails were conducted using L16 orthogonal array by varying the various input factors at their respective levels.

Fig. 3
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EDM used for experimentation and abrasive jet machining setup

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For comparative analysis, experiments were conducted with jet through the electrode hole under a pressure of 1 bar for both, nonabrasive and abrasive fluids. The polarity of the tool was positive. To reveal the particular micrographic structure, the specimen was grounded using grinding machine and electrode tips were polished using emery paper. The workpiece was designated negative and tool as positive polarity. A precision electronic balance (0.001 g precision) was employed to measure workpiece and electrode weights prior and after machining. Sixteen different experiments were performed to determine the MRR, TWR, and SR of Al–SiC MMC by using different parameters as shown in Table 3. SEMs and EDS were carried out to examine the microstructure of machined specimen.

Table 3 Experimental results
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3 Results and Discussions

3.1 Influences on MRR

Table 3 demonstrates the planned experimental design and results obtained thereafter. The various outcomes were analyzed through ANOVA by using Design Expert software to test the significance of the model adopted (Table 4). From the table, it is clear that MRR is the function of peak current, pulse-on, flushing type, and pulse-off. It has been exhibited in Fig. 4 (main effects plot) and Table 4  for material removal rate that the peak current was the most significant factor augmenting the MMR steeply apart from pulse-on duration being other factor affecting the concerned response significantly.

Table 4 Analysis of variance (ANOVA) for material removal rate
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Table 5 Analysis of variance (ANOVA) for TWR
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Fig. 4
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Main effect plot for material removal rate (MRR)

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The material removal mechanism has been elaborated in scanning electron microscope (SEM) analysis as well. The enhanced MRR can be attributed to the high spark energy that mechanically improved the erosion volume through the melt pool from the surface of workpiece. On the contrary, it is witnessed that the MRR decreased steeply when flushing was done with SiC abrasive particulates mixed dielectric medium for the enhanced pulse-off time. The reason for lower MRR (with abrasive) was due to entrapment of molten material in the machining zone and the subsequent deposition of SiC abrasive particles on the workpiece surface (Fig. 5). It is also observed that perhaps the selected SiC particulates must have demonstrated a shielding effect while abrasive jet-assisted ED machining of MMC.

Fig. 5
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Scanning electron microscope (SEM) analysis of workpiece

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3.2 Influences on TWR

Various machining parameter like current, hole diameter of tool, and pulse-on/off duration do not have considerable effect on TWR, as shown in Table 5, except the type of flushing which is the most significant factor affecting the wear of tool while ED machining of MMC. It is clear from the graph (Fig. 6) that SiC particulates mixed flushing dielectric resulted in the lowest tool erosion. This is due to the reason that the SiC particulates used as the abrasive particles is mixed in the dielectric re-deposited on the surface of the copper tool, thereby reducing the tool wear rate.

Fig. 6
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Main effects plot for means for TWR

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3.3 Influences on SR

The surface roughness is significantly influenced by flushing type, wherein flushing was done with pure dielectric and with abrasive mixed dielectric. The relations of surface roughness (SR) with current, hole diameter, pulse-on, flushing type, and pulse-off are shown in Fig. 7. The machining with SiC mixed dielectric resulted in the higher surface finish whereas comparatively, a rougher surface is achieved when machining is executed in pure dielectric (Table 6).

Fig. 7
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Main effects plot for surface roughness

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Table 6 Analysis of variance (ANOVA) for surface roughness
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From the scanning electron microscope (SEM) analysis (Fig. 8a), it can be observed that large and deep holes or pits are visible on the surface prominently during machining in pure dielectric oil. Surface topology is witnessed to be quite rough which resulted in higher surface roughness. On the obverse, when SiC-assisted dielectric is used for the machining process (Fig. 8b), the surface is smooth and comparatively lesser deep or large pits were formed which indicates low value of surface roughness.

Fig. 8
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SEM analysis of workpiece a without abrasives b with abrasives

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3.4 Surface Characteristics

In order to study the results of the experiments in an elaborative manner in terms of constituent elements, the EDS (Energy-dispersive X-ray spectroscopy) was performed on both types of samples machined with the conventional EDM as well as with the abrasive-assisted EDM. Figure 9 exhibits the EDS of the sample in trial 7 ED machined with the conventional EDM, i.e., flushing with the pure dielectric oil.

Fig. 9
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EDS spectrum of trial 7

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The EDS spectrum shown in Fig. 9 shows the traces of copper (electrode material) deposited on the machined surface of workpiece. The EDS of the sample in trail 13 machined with the abrasive-assisted hybrid EDM was also performed (Fig. 10).

Fig. 10
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EDS spectrum of trial 13

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The formation of oxides on the surface of the machined workpiece can be witnessed in the EDS spectrum shown in Fig. 10 that eventually led to the increase of surface hardness of the workpiece.

3.5 Recast Layer Formation

The material eroded in conventional EDM by thermo-physical behavior often results in certain damaging impacts in the form of re-deposited layer (recast layer). It has been evident through SEM images that AJEDM in SiC particulates mixed dielectric fluid has resulted in the reduction of recast layer thickness. On the other hand, nonhomogeneous metallurgical phases were witnessed in the thermally influenced region in terms of micro-cracks within the hard and brittle layer. Mostly, these micro-cracks are developed perpendicular to the machined surface and layers underneath remain unaffected by these surface alterations. The thickest recast layer has been recorded (52.428 µm) in trial 6 at high spark energy and higher pause time without abrasive jet-assisted EDM. It can be quantified to the fact that, at higher peak current, continuous sparking takes place at lowest pulse-on and high-end pulse-off duration settings causing an enormous quantity of molten debris to re-solidifies and result in a thicker recast layer at the end (Fig. 11).

Fig. 11
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SEM depicting recast layer formation for a trial 6 and b trial 8

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However, in AJEDM, almost 62% decrease in thickness of recast layer (19.763 µm) has been witnessed in trial 8 in the same set of operating parameters. The recast layer is minimized due to effectual flushing of molten debris from the electrode-workpiece gap resulting in uniformity of the recast layer.

4 Conclusions

The study was carried out on EDM of MMC with and without abrasive-assisted jet flushing conditions. The following observations are enlisted.

  1. 1.

    MRR is significantly affected by the machining parameters such as peak current, pulse-on/off as well as type of flushing conditions. It was observed that MRR enhanced significantly with the increase in spark energy at lower pulse-off time. The flushing by dielectric mixed with SiC particulates leads to sharp reduction in MRR.

  2. 2.

    The surface finish improved steeply in SiC-assisted dielectric flushing conditions.

  3. 3.

    The tool wear rate drastically reduced with SiC-assisted flushing of dielectric fluid.

  4. 4.

    The selected parameters for instance, current, hole diameter, pulse-on/off time shows insignificant effect on the TWR and SR.

  5. 5.

    Thinner recast layer was observed with AJEDM as compared to conventional EDM process.