1. Introduction
Carbon fiber-reinforced polymer composites (CFRPCs) are gaining importance in various industrial sectors like aviation, electronics, electromagnetic interference shielding and lab-on-chip devices because of their superior mechanical properties [
1,
2,
3]. Modern industry demands the fabrication of micro-holes on special or advanced materials for microreactors and microfluidic applications. The existing machining processes whether conventional or non-conventional are practically not capable to machine CFRP composite due to their fibrous and semi-conductive nature. In the domain of conventional machining processes, the drilling and milling process were widely used for processing of CFRP though it remains less popular owing to delamination, tool wear and uncut fibers [
4,
5]. On the other hand, the Electrical Discharge Machining (EDM) has gained the interest of research fraternity for surface modification and micro-machining of CFRP [
6,
7]. Though, the application of EDM process was limited for CFRP composites due to its partial conductivity. Later on, Electrochemical Discharge Machining (ECDM) process suitably addresses all these machining constraints with respect to CFRP or other fibrous materials [
8,
9]. The pictorial view of the ECDM setup is shown in
Figure 1. The process of spark generation and machining is as follows: the regulation of electrode potential initiates the electrolysis process and generates a hydrogen layer on the surface of the tool electrode [
10]. This hydrogen layer act as an insulator between cathode and the electrolyte. As the electrode potential increases beyond a critical value, the breakdown of gas film causes spark generation at the tip and periphery of the cathode. A gap of few microns is essential between cathode and workpiece for the generation of a uniform spark at the tip of the cathode. The induced discharge causes the removal of material from the workpiece surface due to melting, vaporization, and chemical etching [
11,
12].
In recent years, researchers attempted drilling and slicing of conductive and non-conductive materials using the ECDM process. Wuthrich et al. [
13] drilled micro-holes on glass and concluded that machined surface quality becomes poor at higher machining depths. Sarkar et al. [
14] attempted micro-machining of silicon nitride ceramic and observed that material removal is dominantly affected by the applied voltage. Similar behavior has been observed by Huang et al. [
15] during generating micro holes on a stainless steel plate. Liu et al. [
16] incorporated grinding action in the ECDM process for the micromachining of metal matrix composite (MMC). Later on, Jha et al. [
17] successfully cut micro-slots on MMC using an abrasive particle-coated disc irrespective of the tool electrode in the ECDM process. Antil et al. [
18,
19] analyzed the behavior of ECDM process parameters in the machining of the fibrous polymer composite and concluded that the quality of machined hole walls becomes poor, owing to cracks and uneven heating. During the machining of carbon fiber epoxy composite, Singh et al. [
20] concluded the hat quality of drilled blind holes was poor due to uncontrolled discharge rate with a variety of process parameters. Yan et al. [
21] studied the machining of super-alloys with the incorporation of the tubular electrode in the ECDM process. From the extensive literature survey, it has been observed that there is limited information available micromachining of fibrous materials, especially CFRP materials using the ECDM process.
The superiority of machining processes depends on the optimum selection of process parameters. For assessing the excellence of machining processes, there are various techniques and methods for optimization of multi-response problems such as Grey Relational Analysis (GRA), Analytic Hierarchy Process (AHP), Response Surface Methodology (RSM), Genetic Algorithm (GA) and Technique for Order Preference by Similarity to Ideal Solution (TOPSIS) etc. Among these techniques, the TOPSIS method has gained popularity due to its simple computation and application in different fields such as mathematics, manufacturing, economics and information systems etc. [
22]. The TOPSIS method shows that the optimum solution is farthest from the worst and nearest to the positive solution. Nayak and Mahapatra [
23] successfully optimized quality characteristics in the wire EDM process by grouping of AHP and TOPSIS methods. Nguyen et al. [
24] implemented combined Taguchi-TOPSIS techniques for multi-response optimization of EDM process. Parthiban et al. [
25] optimized process parameters for Laser machining of alloys by TOPSIS method. Ladeesh and Manu [
26] used the TOPSIS method for optimization of quality characteristics in grinding assisted ECDM of glass. Kumar et al. [
27] successfully used RSM & TOPSIS method for the best selection of output quality characteristics in plasma arc machining process.
The behaviour of any machining process depends upon its input process parameters and output performance characteristics. The selection of input parameters and their level is a cumbersome job and requires a scientific and judicious approach. The input process parameters of ECDM are categorized into two categories i.e., (1) Machining Parameters, (2) Material Parameters; as represented in
Figure 2. The machining parameters i.e., applied voltage, duty cycle and polarity were related to supply of discharge energy to machining regime and inter-electrode gap deals with distance between tool electrode and auxiliary electrode of ECDM process. Moreover, the material parameters i.e., nature of electrolyte (alkaline, acidic or neutral), electrolyte concentration (affects the surface characteristics), tool electrode material (mechanical, thermal and erosion resistant properties) and workpiece (conductive or Nonconductive) plays a significant role on the output quality characteristics of the machined materials. Conversely, the output quality characteristics of ECDM are categorized into two categories i.e., (1) Machining characteristics; (2) Surface characteristics. The machining characteristics include material removal rate (MRR), surface roughness, overcut and taper. When the researches are talking about micro machining, the surface finish, overcut and taper are more prominent than MRR. The surface characteristics include micro-cracks, Heat Affected Zones (HAZs), surface damage on machined surface, recast layer formation etc. There exists an intricate connection between input and output quality characteristics of ECDM process, as shown in
Figure 2. The machining of semi-conductive and fibrous materials has various challenges in dealings with output quality characteristics. Therefore, multi-response optimization is a necessary step for better output quality characteristics of ECDM process.