Keywords

15.1 Introduction

EDM is used in manufacturing, automotive and aerospace industries for production of complex geometries and finishing of parts [1]. Even nowadays some nonconductive material can also be machined for example zirconia and alumina but it requires coating of conductive material over it to initiate sparking. Once the spark is initiated hydrocarbon oil degenerates into hydrogen and carbon which helps in further sparking and material gets removed. Material from workpiece is removed using thermal energy by erosive effect. It is different from other traditional machining techniques as it does not have physical contact between tool and workpiece which eradicates many problem like chatter, vibration, mechanical stresses which reduces chances of tool failure [2] and even delicate parts can be easily machined like washing machine agitators. It has wide application in production of mould, dies [3, 4], medical equipment, optical instruments, thin-walled separators in nuclear industry [5] and sports equipment etc. EDM is replacing other traditional machining methods like drilling, grinding, polishing and milling etc. [6]. Figure 15.1 shows important output and input parameters of EDM. Figure 15.2 explains classification of spark erosion machining.

Fig. 15.1
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Important output and input parameters for the EDM process [7]

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Fig. 15.2
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Classification of spark erosion machining [8]

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In EDM as shown in Fig. 15.3 replica of tool is formed on workpiece. Material from workpiece is removed using thermal energy produced by sparking between workpiece and tool filled with dielectric fluid between them. EDM is mostly used method for the production of close tolerance shapes, 3D complex shapes. Thermal energy causes the vaporization of material from workpiece and material is removed.

Fig. 15.3
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ZNC 250 electric discharge machine

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Advantages of EDM machining are:

  • Complicated shapes can easily be made by EDM machining.

  • Elements with close tolerances can be manufacture by EDM

  • Material with any hardness and toughness can be easily machined by EDM.

  • As there is no contact between workpiece and tool so material can be machined without any distortion.

  • EDM machining is more suitable for delicate material which cannot take stress of traditional machining methods.

  • Mirror surface finish can also be obtained by EDM machining by introducing powder additives in dielectric fluid.

  • Due to chip formation in traditional machining methods burs are left on workpiece where as in EDM machining no burs are left on workpiece.

Generally, tool and workpiece are connected with positive and negative polarity respectively where the tool behaves as cathode and workpiece as an anode. Dielectric fluid is an electrically neutral liquid that has some dielectric constant. As the dielectric fluid has some breakdown voltage therefore potential applied between tool and workpiece should be greater than its breakdown voltage otherwise the current will not flow between tool and workpiece. There is RC relaxation circuit in which capacitance gets charged due to the supply of DC power voltage and after that, it gets discharged. When the capacitor discharges its charge between tool and workpiece with minimum resistance. The spark occurs between tool and workpiece which has minimum resistance. There is a minimum resistance between the inter-electrode as there is less gap between these electrodes.

A very large amount of potential is supplied between tool and workpiece when the capacitor gets discharged after charging. An electric field is generated due to electrostatic force which directs electrons from tool towards the workpiece. The electron will move towards workpiece in dielectric fluid when the bond energy of electron is less. Then electron travels with very high velocity and this velocity depends on the potential which is supplied through the capacitor. The electron will move towards the workpiece and collide with the molecules. This collision between them results in the ionization of the molecules. The collision between electron and molecules generates more positive ions and electrons due to which high concentration of these ions between tool and workpiece within the minimum electrode gap takes place. The positive ion moves in direction of the tool while electrons move towards the workpiece which results in the sparking known as “Plasma Channel”.

The electron collides with the workpiece at a very high velocity. The kinetic energy of these electrons converts into thermal energy which removes the material from the workpiece. This material is removed due to vaporization and melting. The collision of positively charged ions is also responsible for material removal but as there concentration is very less so they result in very less material removal. The plasma channel collapses when the supply of potential between tool and workpiece is stopped. The pressure is generated which removes molten material from the workpiece.

From last few years, to increase MRR, tool is given ultrasonic assisted vibration and tool vibrates, it increases flushing away of debris from workpiece which increases MRR and SR remains unaffected. Rotation of tool also increases the MRR. Some appropriate powder additives in dielectric fluid are being used to improve SR and decreases tool wear. Water as dielectric fluid is being used over other hydrocarbon dielectric fluid for safe environment as decomposition of hydrocarbon oil releases carbon monoxide and methane which is harmful for environment.

15.2 Research Background

Modification of surface for biomaterials is very important for improved surface morphology. There are various techniques available for surface modification like CVD, anodising, PVD and nitriding etc. but due to weak adhesion, bond cannot sustain under repeated loading conditions in corrosive environment and these techniques have become inefficient. Before these techniques coating is required i.e., grit blasting and shot peening which results in increase of production cost [9]. EDM has been recognized for Surface modification of biomaterials in recent years [10]. Elements machined by EDM shows good biocompatibility. Human osteoblast cell reveals better cell proliferation when machined by EDM. EDM machined surface requires no surface preparation as required by other techniques. Corrosion resistance and hardness is improved when machined by EDM which is desirable for biomedical implants. EDM machined samples are preferable substances for MG-63 cells of human osteoblast as per adhesion and growth is concerned [9, 11, 12].

Texture and nature of implant surface defines the behaviour of cells and tissue [13]. More is the surface roughness of elements more is the surface area available for interaction with the bone. Higher surface roughness of biomaterial shows better bone fixation with it and in growth of tissues is revealed with higher surface roughness [13,14,15,16]. Main aim of increasing surface roughness is to improve cellular activity. Higher surface roughness obtained by EDM results in deposition of oxide and carbide layers on material which enhances adhesion and ingrowth of bone derived cells [17]. EDM machined surfaces shows improved wear resistance [18, 19]. Porosity of EDM machined surface is increased [20] which enhances the growth of mineralized tissue into pore space which allows body fluid to be transported through interconnected pores [21]. Deposition of oxide and carbide layer after EDM machining results in increase of corrosion resistance and biocompatibility [9]. Table 15.1 explains different types of biomaterials.

Table 15.1 Characteristics of metallic biomaterials (grades ranged from 5 excellent score, to 1, poor score) [27]
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SS 316L is mostly used biomaterial as shown in Fig. 15.4 due to low cost, excellent fabrication properties, high fatigue resistance, easy availability, good corrosion resistance, high fracture toughness and good biocompatibility [22,23,24]. In stainless steel 316L, L stands for low carbon percentage i.e., 0.03%. Table 15.2 shows SS 316L’s chemical composition. Graphite was used as tool for machining as shown in Fig. 15.5. Graphite produces more carbide phases on EDM machined surfaces which increases the corrosion resistance of workpiece. Graphite as electrode gives higher surface roughness [25, 26].

Fig. 15.4
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Machined workpiece

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Table 15.2 Chemical composition of stainless steel 316L [28]
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Fig. 15.5
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Graphite electrode

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Graphite is non-metal which is commonly used in industries as electrode for EDM machining. Graphite has high melting point which makes it more stable than other electrode material. It is comparatively cheaper than other electrodes. It has low density and can be easily machined into desired shape of electrodes. Graphite produces more carbide phases on EDM machined surfaces which increases corrosion resistance of workpiece.

15.3 Research Methods

Whole work was carried out on ZNC-250 Electric Discharge Machine. Schematic diagram of experimental setup is shown in Fig. 15.6. ZNC 250 die sinking EDM was used which has 3 axis x, y and z. Range of x, y and z axis is 250 mm, 250 mm and 200 mm respectively. It has maximum capacity of operating platform of 200 mm. It has maximum processing speed of 200mm3/min. It consumes maximum power of 6 kW. The machine can hold workpiece of maximum of 200 kg and tool of 25 kg.

Fig. 15.6
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Experimental setup

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SR is a measure of texture of workpiece measured after machining was done. SR is measured by Mitutoyo SURFTEST SJ-210 as shown in Fig. 15.7. SS 316L was used with dimension 32 × 32 × 150 mm as work piece material. Graphite electrode with dimension 20 × 20 × 100 mm was used as tool.

Fig. 15.7
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Mitutoyo SURFTEST SJ-210

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Surface roughness is measured by contact between Mitutoyo SURFTEST SJ-210 and workpiece. Technique used here for measuring surface roughness is post process technique and measured by stylus profilometer which has diamond stylus tip. It is moved over machined workpiece and surface roughness is measured. This instrument uses differential inductance method for detection method. It has measuring range of 360 µm (− 200 µm to + 160 µm). Radius of tip is 5 µm. Stylus is made of diamond. Radius of skid curvature is 40 mm. Drive range of detector is 21 mm. Its least count is 0.001 µm. Bottom configuration has V-shaped trough. The surface roughness is measured by Mitutoyo SURFTEST SJ-210 and data stored in it can be recovered by SPC, Printer or by inserting memory card in it. Here memory card has been used for retrieval of data. Following steps has been performed for insertion of memory card into it and for storing data in it as shown in Fig. 15.8.

Fig. 15.8
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a To insert memory card and b to store data perform following functions in instrument

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In EDM Dielectric fluid use here is Divyol spark erosion oil—25. Machining has to be carried out in absence of oxygen in order to avoid oxidation, when oxidation occurs it leads to poor surface conductivity of workpiece and hinders the further machining of workpiece. That’s why dielectric fluid is used as medium to have oxygen free environment for machining. It should have enough sturdy dielectric resistance so that is does not break down easily and while sparking it should be thermally resistant. Most commonly used dielectric fluid are kerosine and deionised water. Tap water can not be used as dielectric fluid as it ionises easily due to presence of salts in it. Dielectric fluid flows over workpiece and tool and removes eroded material from workpiece and helps in further efficient machining of workpiece.

SR has been calculated and examined using Taguchi by Minitab 18. Various steps in Taguchi has been shown in Fig. 15.9. Orthogonal array minimizes number of trials, so it is most effective as it handles number of variables and vast data. Three different parameters i.e., Tp, Id and Vd are varied and their effect on SR has been studied and design matrix for various parameters are shown in Table 15.3. L9 orthogonal array was selected for evaluating maximum SR. Larger the better option was selected. ANOVA was used to identify most significant factor.

Fig. 15.9
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Flow chart for steps in Taguchi design

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Table 15.3 Design matrix
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15.4 Result Discussion

Three parameters Tp, Id and Vd are varied and their effect has been studied. In this study high range of parameters has been opted as the aim is to increase SR of biomaterial stainless steel 316L for biomedical application. Here Toff has been kept constant as with various study it was concluded that Toff has very less effect or variation on SR. Here Toff taken was 40 μs. As high range of Tp and Id has been opted which causes more material removal and more debris, so to provide sufficient time for flushing away of eroded material Toff was selected so.

Table 15.4 shows experimental results of machined surface of SS 316L. Larger the better option was considered in Taguchi as the aim was to increase SR of material for biomedical use.

Table 15.4 Design matrix
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Main effect of plot for means graph has been obtained by Taguchi method by software Minitab 18. Main effect plot for means reveals optimization of process parameters at which one can get optimised result for response. Here SR is response and current, pulse on time and servo voltage are factors and for optimization of results higher the better has been chosen. Results of Fig. 15.10 reveals that maximum SR is obtained at combination of parameter Id 18 A, Tp 650 μs and Vd 5 V.

Fig. 15.10
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Main effect plot for means of SR

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Figure 15.11 shows how surface roughness is varied when there is increase in parameters Id and Tp. Here as we can see in the graph when Tp increases from 50 to 650 μs for Id 10 A there is increases in SR. similarly for Id 14 A and 18 A for increase in Tp SR is increasing. This graph clearly shows that SR increases with increase in Id and Tp.

Fig. 15.11
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Interaction plot for SR between Id and Tp

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In Fig. 15.12 interaction plot is shown between Vd and Tp. Here we can see how graph for SR varies with these two parameters. When Vd increases from 3 to 7 V for Tp 50 μs there is linear increase in SR. For Tp 350 μs SR initially decreases then increases for increase in Vd from 3 to 7 V. For Tp 650 μs SR first increases from 3 to 5 V then it decreases from 5 to 7 V. It is concluded that increase in Tp SR. Whereas for Vd SR increases from 3 to 5 V and then SR decreases from 5 to 7 V.

Fig. 15.12
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Interaction plot for SR between Tp and Vd

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Figure 15.13 shows interaction plot between Id and Vd for SR. here for Id 14 A SR increases for increase in Vd from 3 to 7 V. For Id 14 A SR initially deceases then increases for Vd from 3 to 7 V. For Id 18 A, SR increases with increases in value of Vd from 3 to 5 V and there is further decrease in SR with increase in value of Vd from 5 to 7 V. From interaction plots shown above clearly states that SR increases as Id and Tp increases, whereas with Vd initially SR increases then decreases.

Fig. 15.13
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Interaction plot for SR between Id and Vd

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SR plot for 3D surface shown in Figs. 15.14, 15.15 and 15.16 can be seen above. It can be clearly seen that with increase of Tp and Id, SR. Surface roughness is highly influenced by Tp and then Id. Vd has very less effect on SR. Only slight change on SR can be seen due to change in Vd. From literature survey it was observed that Tp and Id highly affects SR and increases in value of these parameters increases SR. That’s why high value of these parameters was chosen as the main aim of this study was to increase SR which is beneficial for biomedical implants. Maximum surface roughness has been obtained at peak value of Tp 650 μs, peak value of Id 18 A and 5 V Vd.

Fig. 15.14
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3D surface graph for SR between Id and Tp

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Fig. 15.15
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3D surface graph for SR between Id and Vd

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Fig. 15.16
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3D surface graph for SR between Tp and Vd

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From Table 15.5 it was observed that as Tp and Id increases SR increases. Increase in Vd results in increase of SR but further increase in Vd leads to decrease of SR.

Table 15.5 Rank table for S/N ratio and control machining parameters for SR
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From Table 15.6, it was observed that Tp and Id has p value less than 0.05 which shows that these two factors are significant and affects the response i.e. SR. Whereas Vd has p value which is greater than 0.05 which shows that Vd is not significant factor and it does effect response much i.e. SR. Tp is most significant factor followed by Id. Vd has least effect on SR. As Id increases, supplied energy between tool and workpiece also increases which results in larger crater size of machined workpiece higher is the value of Id more is the SR. When Tp is increased, supply of energy duration is increased. Hence energy is supplied for longer duration more material is melted and crater size produced is large in size and results in increase of SR. Increase of SR is higher with Tp than Id. Effect of Vd on SR is not significant as change of Vd has lesser effect on SR.

Table 15.6 Rank table for S/N ratio and control machining parameters for SR
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15.5 Conclusion

Following conclusions were drawn from experimental results.

  • For SR Tp and Id are most significant factors. Out of which Tp has more effect on SR than Id.

  • With increases in Tp from 50 to 650 μs SR also increases because as Tp increases energy supplied duration increases which results in crater of larger size on workpiece and hence SR is increased.

  • Similarly as Id increases from 10 to 18 A SR increases. As Id is increased energy supplied is increased which causes higher temperature and larger crater size and hence SR is increased.

  • Vd does not have significant influence over SR, change in Vd has very less effect on SR. As Vd is increased from 3 to 5 V, there is slight increase in SR. As it further increase to 7 V, there is slight decrease in SR.

  • Maximum SR 18.244 μm is observed at set of parameters Tp 650 μs, Id 18 A and Vd 5 V.