Unraveling the Role of Graphene Nanosheets in Electric Discharge Machining

Abstract

The role of graphene nanosheets shows a promising advantage over conventional machining processes. The preparation and characteristics of graphene nanosheets mixed dielectric are investigated with the help of specialized equipment. Also, the machining performance is compared at different concentrations of graphene nanosheets in the dielectric medium with conventional electrical discharge machining. Cetyl trimethyl ammonium bromide (CTAB) and sodium dodecyl sulfate (SDS) surfactants are used for suspension and machining efficiency characterization. The addition of graphene nanosheets into deionized water improves the machining performance. It gives better results at 0.2 g per 800 ml of deionized water. Material removal rate (MRR) improves by 21.27% at lower discharge energy and 114.28% at higher discharge energy. Surface roughness improves by 18% at lower discharge energy and higher spark gap. Using surfactants in the dielectric medium improves the suspension capability of graphene nanosheets in the dielectric medium. However, it reduced the machining efficiency. A slight variation or negligible variation is observed in the density and viscosity of the dielectric medium after adding graphene nanosheets into it.

1. Introduction

The dielectric medium influences the electrical discharge machining (EDM) process to a great extent. It acts as a medium for spark generation, cooling the machined surface, and washing debris from the machined surfaces. Several researchers have investigated the role of various dielectric media (deionized water, distilled water, kerosene, air, and oxygen) in EDM [1, 2].

Several investigations reported that impurities in the dielectric significantly affect EDM performance. Adding solid additives in powder into the dielectric medium created a new research area for the graphene Nanosheets Mixed Electrical Discharge Machining Process (NSMEDM) [3, 4]. Further, Kung et al. [5] moved one step ahead and used conductive powder-mixed dielectric medium to machine cobalt-bonded tungsten carbide material through EDM. The powder particles in the dielectric medium dispersed the discharge energy and enhanced the machining efficiency to a great extent. The powder concentration in the dielectric medium has a significant effect on MRR. The machining performance increases to a specific limit with the powder concentration and starts decreasing due to arcing. The grit size of powders is also one of the significant factors in powder-mixed EDM. So, Syed et al. [6] tested the electrical discharge machining (EDM) of W200 Die steel. They used both pure distilled water and a dielectric medium consisting of distilled water with suspended powder. The findings indicate that the material removal rate (MRR) increases as the grit size rises up to 27 µm but then decreases [7, 8]. Kurafuji et al. [9] were the first to employ a mixed powder dielectric in the EDM process. Numerous studies have since explored the addition of various powders to the dielectric medium, and the advantages are summarized in Table 1.

SI. No.	Ref. no.	Workpiece	Tool	Dielectric medium	Powder (size)
1.	[10]	SKH-54	Cu	EDM oil	Gr (38 ± 3 μm), Si (45 ± 3 μm), Al (45 ± 3 μm), SiC (2.36 ± 0.08 mm), Crushed, glass (2 ± 0.07 mm), MoS2 (1–3 μm)
Findings	Si and C powders are better for obtaining a fine finish. Gr and Si powders distributed the discharge in the spark gap.
2.	[11]	EN-31	Cu	Kerosene	Si (20–30 μm)
Findings	Silicon powder enhances the MRR and surface roughness.
3.	[12]	AISI 1049	Cu	EDM oil	Ti (<38 μm)
Findings	Small discharge energy and power density lead to TiC layer deposition over the machined surface. Hardness increased by threefold.
4.	[13]	AISI H13	Cu	Kerosene	MWCNT
Findings	Average MRR improved by 26.87%. Average RCL reduced by 30.89%.
5.	[14]	Ti-6Al-4 V	Cu	Kerosene	Gr (37 μm)
Findings	Improved MRR, roughness, and RLT. Stable machining.
6.	[15]	Inconel 625	Cu	Kerosene	Gr (15 μm)
Findings	Surface roughness improved—larger crater size. Surface crack density reduced. Reduction in RLT. Carbone formation due to increased pyrolysis.
7.	[16]	Ti-6Al-4 V	Cu	EDM oil	SiC
Findings	RLT increases with an increase in peak current. Hardness increases by twofold.
8.	[17]	Ti-6Al-4 V	Cu	Oil Flux ELF2	MWCNT
Findings	Better machinability is achieved at long-pulse on time and low-energy pulse. MRR is higher at long-pulse, which is associated with low current. Reduces the size of micro-surface cracks.
9.	[18]	Ti-6Al-4 V	Cu	EDM oil + Span 20	Gr (30 nm)
Findings	Surfactant reduces the agglomeration effect. Surfactants also reduce RLT and microcracks.
10.	[19]	Inconel 706	Cu	EDM oil	Al (1 μm)
Findings	109% MRR increases by changing pulse on-time from 1 μs to 6 μs and 95% when peak current changes from 3 A to 70 A, 25.9% surface roughness, and 13.7% TWR increases when peak current changes from 3A to 70A.
11.	[20]	NAK80 steel	Cu	kerosene	Si, Al, Gr, CNT
Findings	Roughness improves by 70%, and machining efficiency improves by 66% with CNT powder.

Table 1.

Powder used in EDM and findings.

The graphene nanosheets’ selection depends on the dielectric type and workpiece material. There are two types of graphene nanosheets used in the EDM process:

Conductive: The addition of the conductive graphene nanosheets reduces the insulating strength of the dielectric medium. Therefore, the interelectrode gap/spark gap increases, giving away the easy removal of debris from the machined zone. When applying a potential difference, ample positive and negative charges form on the graphene nanosheets. Electric field intensity was higher at the nearest point between the tool and the workpiece. When the dielectric breakdown occurs at the closest point, the discharge channel is formed between the graphene nanosheets through the tool and the workpiece. The graphene nanosheets distributed the discharge over the workpiece, which widened the discharge gap. It also increases electric field intensity three times higher than conventional EDM [21]. Researchers have used many conductive graphene nanosheets in EDM, such as Al [22, 23], Gr [24, 25], Si [8, 26, 27], Cu [28], SiC [29, 30], Cr [31], B₄C [32], and W [33].

Nonconductive: Adding nonconductive nanosheets into the dielectric medium increases the electric field intensity by 1.5 times higher than conventional EDM. Therefore, the electrode gap increases by one and a half times more than in traditional EDM. Response parameters such as higher MRR and lower surface roughness were achieved compared to the conventional EDM. The EDM process has used the nonconductive powders Al₂O₃ and SiO [34, 35].

Graphene nanosheets have distinct properties from other nanosheets. It has 1500–2500 W/mK thermal conductivity at room temperature, twice that of a diamond. Electrical conductivity is 10⁴–10⁵ S/m, among the highest of any known material at room temperature, which is 13 time that of copper. The thickness of the single-layer graphene could be equal to only one atom thick, about 0.335 nm. Therefore, it is also called 2D.

2. Experimental section

The graphene nanosheet dimension emphasized an essential parameter in NSMEDM to obtain the desired results. The size of graphene nanosheets affects spark gap, surface roughness, MRR, and tool wear rate (TWR). An increase in nanosheet size increases the spark gap, whereas maximum MRR is achieved in smaller graphene nanosheets. However, TWR followed the reverse trend than MRR [34, 35]. Additive size also affects machined surface quality. The smallest nanosheet size gives a better surface finish. An Amine functionalized graphene nanosheet, called graphene nanosheets, is used in this experiment. Specification of the Amine functionalized graphene nanosheets is given in Table 2. The composition and size of graphene nanosheets are determined using a scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDX) [36] equipment, shown in Figure 1.

Thickness (Z)	5–10 nm
Dimension (X&Y)	5–10 μm
NH2 Ratio	2–5%
Purity	99%
Bulk Density	0.45 g/cm^3
Number of layers	5–10

Table 2.

Technical specification of amine functionalized graphene nanosheets.

Figure 1.

SEM and EDX result of amine functionalized graphene nanosheets.

The graphene nanofluid is prepared by mixing graphene nanosheets into deionized water. First, a conical flask consisting of 800 ml of deionized water and a magnetic stirrer machine (Tarson Digital SPINNOT) stir the fluid [37]. After that, a small amount of graphene nanosheets were poured into the conical flask with an average interval of 1–2 min. After 5 hours of stirring, the nanofluid is ultrasonicated in an ultrasonic device for an hour. Figure 2 shows the process of nanofluid preparation.

Figure 2.

Graphene nanosheets mixed dielectric preparation.

3. Results and discussion

The concentration of graphene nanosheets in NSMEDM plays a vital role. Abnormal discharge occurs mainly in conventional EDM with a slight chance of short-circuiting. With the increase in nanosheet concentration in the dielectric medium, abnormal discharge reduces, and at a specific concentration, abnormal discharge is eliminated [38]. So, the dielectric gives maximum machining efficiency at a particular concentration range. On further addition of graphene nanosheets into the dielectric, the chances of short-circuiting increase by which reduction in the machine efficiency started. The excess amount of graphene nanosheets bridges the tool and the workpiece, leading to short-circuiting and abnormal discharge [39]. Figure 3 shows the effect of different graphene concentrations [no graphene (0), 0.1, 0.2, 0.4, and 0.6] in 800 ml of deionized water. It shows that at 0.2 g/800 ml, nanofluid gives better MRR. It improves results from 21.27 to 114.28% at the 0.2 g/800 ml concentration. The machined surface Ra, Rq, and Rz also improved by 18, 21, and 28%. The plot of surface roughness at different concentrations is shown in Figure 4.

Figure 3.

Effect of concentration on MRR at different parameters and concentration of graphene nanosheets.

Figure 4.

Effect of concentration on surface roughness at different parameters and concentration of graphene nanosheets.

One of the significant issues in NSMEDM is the suspension of graphene nanosheets into the dielectric medium. It was found that after a few days of suspension, graphene nanosheets settled down and were agglomerated at the bottom because of the surface tension between the graphene nanosheets and dielectric fluid. Surfactant is more likely detergents that reduce the surface tension between the solid and liquid. This allows graphene nanosheets to float for a longer time, and even after the settlement at the bottom, graphene nanosheets are kept evenly distributed at the bottom. The preliminary investigation shows that without surfactant, the suspension of graphene lasts for 2–3 hours, and the addition of surfactant increases the suspension, which can last more than a month [40].

Surfactants are widely used during the preparation of nanofluids. It increases the suspension period of the graphene nanosheets in the medium. This investigation uses SDS and CTAB with 0.2 g graphene in 800 ml deionized water. Surfactants are soluble and mix entirely into the dielectric medium, increasing the electrical conductivity. Figure 5 shows the suspension time for pure graphene and graphene with surfactant in deionized water. The electrical conductivity of the nanosheet concentration was measured, and deionized has a conductivity of 6 μs/cm, increasing up to 24 μs/cm after adding 0.2 g graphene nanosheets.

Figure 5.

Suspension of graphene nanofluid.

Further, the addition of surfactant also increases the electrical conductivity by adding 0.1 g SDS, 0.2 g SDS, 0.1 g CTAB, and 0.2 g CTAB separately, increasing electrical conductivity from 24 to 76 μs/cm, 125, 116, and 165 μs/cm, respectively. Figure 6 shows the effect of graphene nano power and surfactant on MRR. The graph shows that the highest MRR obtained at 0.2 g graphene in deionized water without any surfactant is 45 mg/min. Whereas MRR obtained with surfactants SDS at 0.1 and 0.2 g and CTAB at 0.1 and 0.2 g are 31.81 and 43 mg/min, and 28.33 and 30 mg/min, respectively. The surface roughness of Ra, Rq, and Rz is improved by adding surfactants. Table 3 shows the response values of surface roughness with varying surfactants. Figure 7 shows the effect of surfactant at different conditions on surface roughness. The dielectric medium with graphene nanosheets without surfactant gives better results than those without nanosheets and dielectric with graphene nanosheets and surfactant.

Figure 6.

MRR response at (0) no graphene, (0.2PG) 0.2 g nanographene, 0.2 g graphene with 0.1 and 0.2 g SDS and CTAB in a dielectric, respectively.

	Input parameters	0; No graphene	0.2 g; graphene	0.1 g SDS	0.2 g SDS	0.1 g CTAB	0.2 g CTAB
Ra	8 μs, 2 A, 50 V	1.99	1.61	2.41	1.92	1.69	1.94
	8 μs, 4 A, 30 V	1.79	1.96	2.11	2.02	1.79	2.08
	8 μs, 6 A, 10 V	2.43	2.43	2.3	2.11	2.41	2.44
Rq	8 μs, 2 A, 50 V	2.55	2.01	2.99	2.45	2.12	2.38
	8 μs, 4 A, 30 V	2.24	2.27	2.58	2.58	2.21	2.62
	8 μs, 6 A, 10 V	3.08	3.08	2.87	2.67	3.04	3.21
Rz	8 μs, 2 A, 50 V	13.42	9.61	13.4	11.53	9.93	12.00
	8 μs, 4 A, 30 V	9.9	11.47	11.84	12.02	10.60	12.94
	8 μs, 6 A, 10 V	15	15	13.82	12.52	15.06	14.11

Table 3.

Surface roughness response values for SDS and CTAB surfactant.

Figure 7.

Roughness response values at no graphene, 0.1 and 0.2 g SDS and CTAB in graphene nanosheets mixed dielectric.

The characterization is based on the physical properties of the graphene nanosheets mixed with dielectric fluid, such as density, conductivity, and viscosity.

Density: A hydrometer (500 ml) with a measuring cylinder to measure the dielectric medium’s density with and without nanosheets. There is no difference in the density as a result obtained is 1000 kg/m³,which is identical in both the cases with and without nanosheets.

Electrical conductivity: The electrical conductivity of the dielectric medium with and without nanosheets is measured using a Hanna conductivity meter (Model HI 991300) and found that deionized without nanosheets has conductivity 6 μs/cm, increasing up to 24 μs/cm after adding 0.2 g graphene nanosheets into the deionized.

Viscosity test: Various ways to measure the viscosity of a fluid. Malvern Instruments made a viscometry (Bohlin Gemini 2) in the UK to measure the dielectric medium’s viscosity. Figure 8 shows the dielectric medium’s viscosity without nanosheets at room temperature (25°C). It is noticed that the viscosity of the dielectric medium increases with the addition of graphene nanosheets. The viscosity difference is high at lower shear stress and lowers at higher shear stress. The dielectric medium’s viscosity without nanosheets and graphene nanosheets at shear stress 0.007 Pa are 0.01086 and 0.02721 Pa.s, respectively.

Figure 8.

Viscosity, shear rate, and shear stress plot for the dielectric medium with and without nanosheets.

4. Conclusion

This experiment discusses the preparation and characteristics of graphene nanosheets mixed dielectric. Also, the machining performance is compared at different concentrations of graphene nanosheets in the dielectric medium with conventional EDM. CTAB and SDS surfactants are used for suspension and machining efficiency characterization.

• The addition of graphene nanosheets into deionized water improves the machining performance. It gives better results at 0.2 g per 800 ml of deionized water.

• MRR improves by 21.27% at lower discharge energy and 114.28% at higher discharge energy.

• Surface roughness improves by 18% at lower discharge energy and higher spark gap.

• Using surfactants in the dielectric medium improves the suspension capability of graphene nanosheets in the dielectric medium. However, it reduced the machining efficiency.

• A slight variation or negligible variation is observed in the density and viscosity of the dielectric medium after adding graphene nanosheets into it.