Keywords

1 Introduction

Electrical discharge machining (EDM) has become one of the most important production technologies to manufacture very accurate three-dimensional complex components on any electrically conductive material [1]. The cost of a part manufactured by EDM is essentially determined by the electrode cost, which consists of the raw material cost and the tool production [2]. The contribution of the electrode cost to the total EDM operation cost can be more than 70%. The electrode production is a major cost and time spent in the EDM process cycle, which can account for over 50% of the conventional machining costs [3]. The thermo-physical properties of the electrode, such as thermal and electrical conductivity, thermal expansion and heat needed to vaporize from room temperature, melting and boiling temperature have a considerable influence on the EDM process performance in terms of material removal rate, electrode wear and surface integrity of the workpiece [4]. When the surface temperature of the electrode is not over the boiling or melting point, electrode material removal does not occur, so materials with higher melting point are suitable as tool electrodes [5]. Electrical properties are also important, as the current is responsible for the material erosion. Higher electrical conductivity (low electrical resistivity) generally leads to lower tool wear. In addition, although the EDM process does not involve mechanical forces, the sparks produced act in a violent way, inducing stresses in a microscopic scale [6]. Thus, the structural integrity of the electrode is a significant factor determining the electrode performance. The nickel-tungsten alloy coating has good erosion resistance performance, which can effectively reduce the side wear of the tool, especially at the end and edge, maintaining the shape of the tool and improving the shape precision of the machined microhole [7, 8]. Researchers were proposed a wear-variation EDM (WV-EDM) approach to fabricate a laminated disc electrode (LDE) with stable-shape microchannels on the outer edge surface [9].

In this paper, in order to reduce the significant problem of tool wear in EDM, thin film coated copper tools were made by electroplating technique. Experiments were performed as per Taguchi based design of experiment method. The effects of the thin film coated tools on tool wear, surface roughness, and material removal rate were analyzed. It provides a new tool preparation method for EDM, and the research results have certain guiding significance for the actual production in EDM.

2 Experimental Details

Experiments were performed on titanium alloy (Ti-6Al-4V) and undergone drilling operation using EDM. Two types of electrodes were used; uncoated aluminum electrode and copper coated aluminum electrode. Selection of process parameters and its levels were selected as per literature review and machine capabilities, three input process parameters with four levels were selected for the final experimentation, as shown in Table 1. Taguchi based orthogonal array were used for experimental work with four levels of Current (I), Gap Voltage (Vg), Pulse-ON Time (TON) and L16 design of experiment were planned and conducted experiments with uncoated aluminum electrode and copper coated aluminum electrode.

Table 1. Selection of process parameters & levels
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3 Experimental Results

In this experimental work study has been conducted to investigate effect of coated and uncoated electrodes on the material removal rate (MRR), tool wear rate (TWR) and surface roughness (SR) by using Ti-6Al-4V as workpiece material, Table 2.

3.1 Performance Analysis of Material Removal Rate (MRR)

Main effect plots were plotted with help of Minitab 17 software. These graphs show the behavior of process parameters in relation to response variables. Main effect plot of MRR with uncoated & Cu coated Al electrode is shown in Fig. 1. The MRR chart is a linear graph that increases from 10 to 40 amps in both cases.

Fig. 1.
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Main effect plots for MRR with uncoated & Cu coated Al electrode

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Table 2. Results for Ti-6Al-4V using uncoated and copper coated Al electrode
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Fig. 2.
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Influence of electrode material on MRR

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High pulse time values, the drop may be explained, due to pulse-OFF values. As the duty cycle was kept constant during the experimentation, at higher values of TON corresponding TOF values are also higher due to which reduction in the MRR was observed. This phenomenon was been observed by few researchers which support our statement. The calculated MRR for two electrodes is shown in bar chart shown in Fig. 2. About 12% higher MRR is achieved for copper-coated Al electrode than uncoated Al electrode.

The higher value of electrical conductivity is the main reason for the increasing of material removal rate. The higher value of electrical conductivity can increase more energy transfer into workpiece and it causes more material removal from the workpiece. Though the electrical conductivity of Al is improved by copper coating on it, it does not reach the electrical conductivity as much as copper.

3.2 Performance Analysis of Tool Wear Rate (TWR)

The absolute values of tool wear show that there are two phenomena that regulate the wear characteristics of the tools: the erosion of the tool electrode material and the deposition of the workpiece material on the tool electrode. When high MRR conditions occur, large quantities of debris are produced and the quantity of material deposits increases on the tool electrode. In such conditions, the amount of actual material removed from the tool electrode also increases. Main effects plot for TWR is shown in Fig. 3.

Fig. 3.
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Main effect plots for MRR with uncoated & Cu coated Al electrode

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Fig. 4.
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Influence of electrode material on TWR

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Figure 4 shows the calculated electrode wear rate (EWR) for uncoated & Cu copper electrodes. The EWR is decreased by 22% after applying copper coating on Al electrode. The thermal conductivity is mainly responsible for electrode wear rate. The higher thermal conductivity of electrode material can be driven out more heat from sparking zone more quickly. This is reduced the electrode erosion. Thermal conductivity of copper-coated Al is higher compared to uncoated Al alloy. The higher thermal conductivity of copper-coated 6061Al electrode can reduce the electrode wear compared to the uncoated Al electrode.

3.3 Performance Analysis of Surface Roughness (SR)

Increase in current and pulse on time shows the increase in the surface roughness (SR) which can be determined from main effects graph, thereby increasing temperature leading to premature melting of the tool electrode. Therefore, as the current level is increased the surface roughness is increased due to increment in the spark energy. Main effects plot for SR is shown in Fig. 5.

Fig. 5.
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Main effect plots for SR with uncoated & Cu coated Al electrode

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Figure 6 shows the influence of three different electrodes on the surface roughness of workpiece. The results show that better surface is achieved using copper-coated Al electrode compared to uncoated Al electrode. The higher wear rate caused the degradation of surface of electrode. The degraded surface of electrode has generated more rough surface.

Fig. 6.
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Influence of electrode material on SR

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4 Conclusion

Copper-coated aluminium electrode was developed using electrode position process. Prior to electrode position, the surface of aluminium was prepared by anodizing process. The EDM operations were performed with newly developed electrodes. The performance of two electrodes was compared on the basis of MRR, TWR and SR. The results show that copper coating on aluminium has enhanced the performance compared to uncoated aluminium. The result shows that copper coating on aluminium alloy has significantly improved the applicability and sustainability of Al as an EDM electrode. The copper-coated aluminium can be used as EDM tool in a large scale in industry.