Keywords

1 Introduction

Machining processes have been developing long along with the development of the human race, from the formation of wheel to computerized non-conventional machining. Now today’s world demands high quality, precision machining processes for obtaining the standards, economical products in minimum time to achieve the industrial goal with higher efficiency. One of the most significant emerging machining processes to achieve the required goal in the world is Electrical Discharge Machining (EDM), which is a desirable non-conventional machining process getting attention for zero force, a contact-less process which enables machining of hard materials and complex geometries, with high precision, good surface finish which is not achieved by conventional machining processes and its capability to regulate the process parameters to accomplish the requisite surface finish and dimensional accuracy. EDM has a wide-ranging application in industries such as die industries, aerospace, automotive, electronics industry, mould making, medical, micromechanics, etc.

Electrical Discharge Machining is a metal removing process, which removes metal by the means of electric spark thermal erosion with a constant electric field in a dielectric medium. The electrically conductive workpiece and tool are connected to the positive and negative terminal of the generator respectively, so they have a huge amount of free electrons to flow. When a potential difference is generated between tool and work-piece, the electric field will be developed, free electrons will be plucked in large numbers due to electrostatic force and get accelerated towards work-piece. Energized electrons will ionize the dielectric molecules generating more ions and electrons which will again undergo collision, leading to a huge increment in the concentration of ions and electrons between tool and workpiece, plasma channel will be established between tool and work-piece. Electrons will follow the least resistivity path, will move toward the work-piece in huge amount, hence spark will be generated and the work-piece will be impinged by electrons having high kinetic energy which will be converted into high-density thermal energy which results in localized melting workpiece causing material removal. This thermal erosion produces a recast layer on the machined surface with microcracks. Here due to localize non-uniform heating and rapid cooling of material generates a multi-layered Heat Affected Zone in the subsurface of the workpiece and consequently produces thermal stresses. If these stresses have magnitude more than the material’s yield stress, they will persist in the workpiece as residual stress during subsequent cooling, which contributes to fatigue crack growth, crack closure, and fracture [1].

To study the complex process of EDM, we need to study the establishment of plasma channel between workpiece and tool, thermodynamics of the spark causing thermal erosion, microstructural and metallurgical changes of material. For this study, a heat transmission thermal problem needs to be constructed to model the EDM mechanism, where the electric spark generated in EDM is the heat input, and by solving this heat transmission thermal problem for the workpiece will give us the temperature distribution in it. And to solve this numerical model finite element method is used and the impact of various process parameters on the output parameters, temperature distribution, thermal stresses and residual stresses can be investigated.

1.1 Variants of EDM

Die-sinking EDM is a variant of EDM where the final desired shape of the workpiece is complemented by the tool shape when an electric field is set up between them and thermal erosion takes place.

Powder mixed EDM is a variant of EDM process where suitable electrically conductive abrasive powder particles are added into the dielectric fluid resulting in decreases its insulation strength as these powder particles get ionized upon setting an electrical field and show accelerated zigzag motion between electrodes, leading to an increase in the inter-electrode gap, as result of which EDM performance improves and heighten surface finish is obtained in comparison to the conventional EDM.

Wire EDM is another variant of EDM where discharge occurs between metallic thin wire and workpiece, the metallic thin wire is used to cut shapes. Main cuts and trim cuts are performed on the work material using WEDM, the main cut is preferred when material removal rate (MRR) or cutting speed is a primary need and when high surface finish is more desired than the trim cut of performed at lower power setting after the main cut. [2]

2 Technique Used for the Study

  1. 2.1

    Finite element method (FEM) is a computational technique for solving differential equations developed for mathematical modelling to investigate engineering problems in the fields of heat transfer, structural analysis, fluid dynamics, and also coupled problems like thermo-structural, electromagnetic, thermochemical, etc. In FEM the domain of the problem is discretized into small elements with the help of nodes and the differential equation is solved within the boundary of the element and by combining these discrete solutions, a global solution is obtained [3]. The chances of having error in the solution are minimized by increasing the number of elements hence reducing the element size.

  2. 2.2

    X-Ray Diffraction (XRD) method is governed by the Bragg’s law, when a specific wavelength (\(\lambda\)) beam of X-ray falls on the workpiece’s surface, the scattered radiation undergoes interference. Bragg’s law is stated by the below equation:

    $$n\lambda = 2d\sin \theta$$

    where,

    n:

    Order of diffraction

    θ:

    Diffraction angle

    d:

    Distance between crystallographic planes

    Firstly, the strain generating the shift in crystallographic planes is evaluated assuming linear elastic distortion of the crystal lattice and then the value of stress is evaluated using Hooke’s law [4]

  3. 2.3

    Taguchi Method is a statistical methodology to enhance the quality of the product to be manufactured, by the optimization of process parameters. The main intention is to improve the quality of a product and design robust systems that are more reliable under uncontrollable conditions [5].

  4. 2.4

    Scanning electron microscopy is a technique used for the visualization and investigation of surfaces, where a fine electron beam is scanned through the fracture surface to produce a picture and the rate at which the image is scanned tells the issues. The pictures can later be advanced employing frame integration and averaging [6]. An electrically conductive workpiece is required for this method, at least the surface should be conductive, and, the workpiece should be electrically grounded to avoid the accumulation of an electrostatic charge at the surface.

  5. 2.5

    Focussed Ion Beam (FIB) milling—Digital Image Correlation (DIC). FIB is used to mill standard geometries and the relaxation causing the surface displacements is captured by the scanning electron microscope (SEM), than images are created using DIC software, these are then used with finite element modelling for calculation of the residual stresses in the workpiece [7].

3 Previous Works

Mohanty et al. [8] uses Field Emission SEM for two cases WS2 combined with de-ionized water and MoS2 powders mixed in deionized water and discovered that WS2 coated samples have a heighten density of surface crack and residual stresses develop on the recast surface than that of MoS2 powder coated samples. Yue et al. [9] performed experimental analysis using SEM, the influencing effect of thermal stress on material removal during EDM of the Cf_SiC composite and the stress distribution on the Cf_SiC composite's discharge surface was investigated. Thermal stress was generated that exceeded the tensile strengths. Srinivasa Rao et al. [10] investigated the impact of wire EDM parameters upon the residual stresses in aluminium 2014 T6 alloy with a L8 orthogonal array Taguchi method confirms that increasing the pulse on time and peak current, increases surface roughness and cutting speed. The spark gap voltage, pulse on time, peak current, and cutting speed all had a substantial impact on residual stresses.

Some researchers have conducted an experimental analysis employing the X-Ray Diffraction method and also performed computation analysis using FEM for the validation of results. Aghdeab et al. [11] using the X-ray diffraction method and FEM the residual stress arises during EDM was determined, and discovered that the residual stress boosted with raising the current, pulse on time, and pulse off time. Sundriyal et al. [12] investigated the powder mixed near-dry (PMND-EDM) process, and it was discovered that raising the concentration of metallic powder in the dielectric was assisting in broadening the plasma channel owing to an enhanced conduction all through machining and increasing the inter-electrode gap. Das and Joshi [4] used FEM model, calculated the wire safety index. The stresses arised on the wire were found to be greater than the molybdenum’s yield stress. Tlili and Ghanem [13] developed a thermomechanical model based on hydrodynamic Gruneisen-type conduct for the hydrostatic component of the stress, paired with Johnson–Cook plasticity model which reports for strain-rate-dependent stress in the scope of a shockwave condition and observed traction type residual stresses on the surface which is balanced by sublayer’s compressive stresses. Saxena [14] estimate the thermal stresses and discovered that the initial compressive stresses encircling the formed crater, after heating change its nature to tensile, and as the crater reaches greater depth, the stresses transits into compressive residual stresses which bring the system into equilibrium. Zhanga et al. [15] Examined the thermal deformation of a thin-walled sample and concluded that the force at the bottom must equate the residual stress at the top of the thin-walled sheet. Where Salvati and Korsunsky [2] assesses micro- to nano-scale residual stress, used FIB-DIC method for modeling, which was validated by FEM analysis, and discovered that peak current and pulse on time, are critical for the intensity of induced residual stresses and crack emergence.

Bhattacharya et al. [16] applied FEM to simulate generated residual stresses during WEDM cutting and the volumetric strain change caused by phase transformation with the temperature introducing a user-defined subroutine. Thermal expansion is considered in the FEM analysis to account for metallurgical phase changes. Kumara and Jilte [17], discovered that the equivalent stress appears in the tool's and workpiece’s core spark zone. The current density has a significant impact on the response output parameters. Mohapatra et al. [18] simulated for the wire using ANSYS Fluent and estimated the temperature and equivalent stress. The temperature of the wire was observed to vary up to 5500 K. At the centre of the wire length, equivalent stress was observed with a maximum effective stress of 397 MPa. Assarzadeh and Ghoreishi [19] performed an electrothermal simulation and discovered that progressive increases in discharge current create steadily deeper crater voids, implying the creation of more re-solidified materials atop the crater. Liu and Guo [20] accounted for massive random discharges to examine the residual stress arises in die-sinking EDM and investigated characteristics of sub surface’s local and average residual stress. Singh and Kumar [21] calculated the radial, tangential and axial stresses in the electrode in the static structure module and observed that the magnitude of stress hikes with hike in current values. Pradhan [1], Biswas and Pradhan [22], Pradhan [23], Pradhan and Biswas [24] investigated the correlation between parameters and the highest temperature acquired at the end of the heating cycle and the residual stress arising, by FEA modelling, and it was discovered that compressive thermal stresses formed underneath the crater and tensile stresses existed farther from the axis of symmetry and observed that by intensifying the pulse energy the thermal stresses get affected to a greater depth. Kansal et al. [25] build the FEM model for powder mixed EDM and calculated the distribution of temperature and MRR from the temperature profiles and observed that along the radius and depth of the workpiece many different process parameters affect the temperature distributions. Allen and Chen [26] simulated for micro-EDM, analysed the residual stress and investigated the effects of vital EDM parameters on tool wear ratio and the dimension of the formed crater. Yadava et al. [27] presented for EDDG, a model has been created using FEM to assess thermal stresses. The developed algorithm first determines the temperature, and then using this temperature field, estimates the thermal stress field. Computations were performed in-plane strain conditions for various grinding wheel down feeds. Das et al. [28] simulated with DEFORM to study the transient temperature and crater portrayal, the heat-affected zone and residual stresses in the workpiece.

Only that research is highlighted in Table 1, which used a suitable optimization technique such as the Finite Element Method (FEM), X-Ray diffraction method, FIB-DIC, SEM, and others for the EDM process and its variants such as WEDM, micro-EDM, powder-mixed EDM, powder-mixed near dry EDM, etc.

Table 1 Chronological ordered list of research work published related to simulation of Electric Discharge Machining from 2003 onwards
Full size table

Along with the time, researchers are considering various aspects of the simulation, such as using a gaseous dielectric medium, evaluating different tool electrodes, analysing surface wettability, and micro-hardness, developing a customised mixing chamber for creating a heterogeneous dielectric mixture, and imposing the model with an infinite-element boundary to dodge shockwave reflection on the free surface, to ensures convergence towards the equilibrium condition. For the wire-EDM procedure, the wire safety index is analysed, microcracks on the wire surface are being analysed, and the authors are attempting to describe the shape and dimension of the spark.

4 Conclusion

In this review paper, a remark on various research works carried out in the field of EDM is presented.

The observed points are follows:

  • It has been observed that thermal modelling of EDM mechanism followed by simulation is the key to investigate and predict the output parameters, temperature distribution and stresses, and these results have been validated by conducting experimental work.

  • Experimental analysis carried out by researchers used X-ray diffraction method, Taguchi method, Scanning electron microscopy method, Focussed Ion Beam (FIB) milling—Digital Image Correlation (DIC) method.

  • The experimental research shows that with increment in current pulse off time and pulse on time, increment in residual stress was observed.

  • With the increment in current and voltage level, there is an increment in the maximum temperature and the maximum residual stresses.

  • The depth at which the peak value of residual stresses will attain, increases proportionally with the pulse energy.

  • The residual stresses comes very close or exceeds to the value of yield strength of the material at vicinity of spark location.