Carbon-fiber-reinforced plastic (CFRP) has seen a significant surge in its usage across multiple industries. The global CFRP market is expected to expand from 201.20 kilotons in 2023 to 276.18 kilotons by 2028, representing a compound annual growth rate (CAGR) of 6.54% [1]. This growth can be attributed to its exceptional physical and mechanical properties, including low density (1.7–1.95 g/cm³), high elastic modulus (240–800 GPa), high tensile strength (2500–4800 MPa), as well as impressive corrosion and fatigue resistance [2–4]. In the aerospace industry, the incorporation of CFRP has resulted in substantial reductions in the weight of aircraft, such as the Boeing 787 (53 wt%) and Airbus A350 XWB (50 wt%). For instance, the Boeing 787 has achieved a 22% reduction in fuel consumption compared to the Boeing 767 (3 wt%), and the Airbus A350 XWB requires 50% less frequent structural maintenance and airframe checks than the Airbus A380 [5]. One notable characteristic of CFRP composites is their ability to be manufactured to near-net shape using various techniques, including compression moulding, autoclave moulding, and filament winding [6]. However, achieving the final product shape often involves secondary operations that demand high dimensional accuracy and quality for assembly However, achieving the final product shape often involves secondary operations that demand high dimensional accuracy and quality for assembly [6]. These assembly processes frequently involve the creation of bolt or rivet holes, which must be machined with precision according to tight target dimensions [7]. The efficiency of this machining is crucial, particularly for smaller aircraft, which may require over 100,000 holes, while larger aircraft may need well over 1,000,000 holes [8, 9]. Moreover, CFRP's anisotropic nature, structural inhomogeneity, highly abrasive fibre composition [10], and poor thermal conductivity (ranging from 0.4 to 4.6 W/mK) [11] pose challenges for conventional machining methods.
Drilling constitutes the primary operation employed for perforating holes in composite materials. Notably, the U.S. aerospace industry alone consumes an estimated 250 million twist drills annually for this purpose [12]. In conventional drilling of carbon fibre-reinforced plastic (CFRP), several challenges arise due to the material's inherent characteristics. Specifically, its anisotropic and structurally inhomogeneous nature can result in undesirable outcomes such as fibre pull-out and fibre-matrix debonding [13]. Additionally, CFRP's limited thermal conductivity exacerbates the situation by causing heat to accumulate at the cutting zone, leading to thermal-induced wear on the cutting tool [6]. This wear is further aggravated by the abrasive properties of carbon fibres [14]. Under dry drilling conditions, the localized heat can even induce thermal damage to the epoxy matrix [15]. Overcoming these challenges is made more complex by the practical limitations of applying coolant when drilling holes of considerable depth [16]. The primary defect encountered during CFRP drilling is delamination, a phenomenon occurring when the thrust force exerted by the drill exceeds the interlaminar fracture toughness of the material's layers [13]. Delamination can have far-reaching repercussions, including compromised assembly tolerances, reduced structural integrity, long-term performance degradation, and is a major contributor to part rejections during aircraft assembly, accounting for approximately 60% of such rejections [17]. Moreover, conventionally drilled CFRP workpieces exhibit a range of prevalent defects, including burrs, tears, excessive surface roughness, matrix degradation (often linked to glass transition failure), suboptimal hole accuracy, surface cavities (associated with specific fibre fraction modes), and internal cracks (involving both fibre and matrix cracking) [18, 19].
An investigation into the conventional drilling of CFRP was undertaken by Shyha et al. [20]. This study systematically examined the influence of various parameters, including drill type (stepped/conventional), surface condition (uncoated/coated), point angle (118°/140°), helix angle (24°/30°), cutting speed (3200/9600 RPM), and feed rate (0.1/0.2 mm per revolution), on both tool life and hole quality. Through the application of analysis of variance (ANOVA), the study identified drill type (contributing 37.2% PCR) and feed rate (contributing 32.6% PCR) as the most significant factors (at a 5% significance level) affecting tool life. Similarly, ANOVA analysis revealed that drill type had a PCR of 40% for the hole entry delamination factor (DF), followed by a 15% contribution from the feed rate, with both hole entry and exit delamination factors consistently below values of 1.3. Jia et al. [21] proposed an innovative intermittent-sawtooth drill structure designed to reverse the cutting direction, thereby generating support to prevent delamination at the hole exit. Experimental results comparing a conventional one-shot drill with and without the intermittent-sawtooth structure demonstrated improved productivity and hole quality when using the structure. However, this approach suffered from increased tool wear and reduced tool life [21]. Empirical findings indicated that the introduction of a support plate underneath the laminate workpiece effectively inhibited hole exit delamination [7]. Tsao et al. [22] developed an analytical model for determining the critical thrust force at the onset of delamination and concluded that the use of a support plate in conventional CFRP drilling allowed for a higher critical thrust force. Consequently, a higher feed rate could be employed without incurring delamination damage.
The pursuit of an improved machining approach to create high-quality and productive holes in CFRP has led to investigations into non-conventional cutting processes. Non-contact machining methods, including Laser Beam Machining (LBM), Pure and Abrasive Waterjet Machining (PWJM/AWJM), and Electrical Discharge Machining (EDM), have garnered attention due to their tool-less nature. Herzog et al. [23] effectively employed three high-power laser sources, including a CO2 laser, disk laser, and pulsed Nd: YAG lasers, to cut CFRP composites. However, they observed the presence of a heat-affected zone (HAZ) in the final workpiece, with tensile and bending strengths reduced to 85% of the original material. A comparative study between laser cutting and Wire Electrical Discharge Machining (WEDM) highlighted that the latter exhibited less surface damage, a smaller HAZ, and a superior surface quality [24]. Li et al. [25] explored the feasibility of employing pure and abrasive waterjets to cut plain woven CFRP laminates. This approach encountered challenges, including delamination, fibre pull-out, fibre burrs, and craters, arising from mechanical forces experienced during the jet-composite impact. Delamination factors for PWJM were not reported due to extensive damage, while those for AWJM ranged between 1.11 -1. 24 for entry and 1.07–1.08 for exit. Ming et al. [26] conducted abrasive waterjet trials on CFRP, noting a loss of dimensional accuracy near the jet exit due to reduced cutting ability stemming from kinetic energy loss. In contrast, Electrical Discharge Machining (EDM) has received limited attention in the context of CFRP machining, primarily due to the inherent challenge of achieving electrical conductivity between the electrode and the non-conductive matrix [27]. However, in more recent research, die-sink Electrical Discharge Machining (EDM) has been thoroughly examined and its feasibility in both macro and micro drilling of CFRP has been confirmed.
Gourgouletis et al. [28] conducted a study investigating the feasibility of die-sink EDM for drilling blind holes in CFRP, specifically holes measuring 4.42 mm and 8.02 mm. The findings indicated that optimal surface roughness and accuracy were achieved by employing low current, shorter pulse durations, and a positive polarity tool. The highest MRR recorded was 1.033 mm³/min and was achieved using a positive polarity tool, a pulse current of 1.5 A, and pulse-on times ranging from 500 to 750 µs. In another study by Kumaran et al. [29], the relationship between MRR, electrode wear, and hole quality in CFRP drilling was explored. This investigation considered varying pulse-off times (20, 30, and 42 µs) and current levels (46, 91, and 153 A) and introduced additional filler materials, including graphite (5 and 10 vol%) and carbon black (1 and 2 vol%). The use of these fillers effectively mitigated hole exit damage and improved electrode wear. This enhancement was attributed to the higher thermal conductivity of the matrix material, allowing for efficient dissipation of thermal energy. Additionally, the presence of fillers contributed to improved MRR, particularly when pulse-off times exceeded 20 µs, attributed to enhanced flushing efficiency. Chen et al [30] conducted an experiment to compare the disparities in CFRP EDM when employing discharge mediums of kerosene, deionized water, and graphene aqueous solution. Through a grey correlation analysis of material removal rate (MRR), electrode wear rate (EWR), and heat-affected zone (HAZ), the optimal processing parameters were identified as a current of 1.5 A, pulse-on time of 40 µs, pulse-off time of 30 µs, and a 30V gap voltage. The results revealed that, in comparison to kerosene and deionized water, the use of a graphene aqueous solution as a medium led to a significant enhancement in MRR, with improvements of 76% and 18%, respectively. Additionally, this choice of medium resulted in an 8% and 4% reduction in the HAZ. In pursuit of further refining the EDM cutting process, Pattanayak et al [31] utilized a 1 mm thick aluminum plate as a guide for drilling a CFRP plate. Remarkably, they achieved a delamination of 539.3 µm, circularity of 0.979, taper angle of -0.811354°, tool wear rate (TWR) of 0.000069g/min, and negligible burr formation. These impressive results were obtained by optimizing the processing parameters to a 4 A peak current, pulse-on and off times of 25 µs, and a flushing pressure of 0.6 MPa. Explorations into micro-scale EDM have been conducted by Kaushik et al. [32],. Response surface methodology was employed to examine the impact of voltage, capacitance, and tool rotation speed on the material removal rate (MRR), roundness error, and taper of a 250 µm diameter hole drilled through micro EDM. The study revealed that optimal MRR and hole quality were attained within the range of 150–250 nF capacitance and 1400–1600 RPM tool rotation speed, maintaining a constant voltage of 100 V. Dutta et al. [33] conducted an investigation into micro-EDM drilling of 1.4 mm through holes in twill woven CFRP laminates. Their study evaluated the influence of cutting parameters, including voltage, discharge time, and electrode rotation speed. The analysis revealed that voltage was the most significant factor affecting hole quality, contributing to 90.95% of the Percentage Contribution Rate (PCR), primarily due to its role in minimizing heat generation. Kumar et al. [34] conducted experiments to create blind holes with a diameter of 120 µm and a maximum aspect ratio of 29.17. They explored the impact of various parameters, including voltage (80, 100, and 120 V), capacitance (33, 100, and 1000 pF), speed (1000, 1500, and 2000 RPM), and tool geometries (solid, single notch, and double notch). The study identified the highest MRR when utilizing a tool speed of 1500 RPM and a single-notch electrode. Increasing the discharge energy was found to result in higher MRR and tool wear rates, along with diminished hole quality.
In addition to die-sink and micro-EDM, the viability of employing EDM as an auxiliary process has been demonstrated. Fukada et al. [35] [35] introduced an innovative cutting method that integrates conventional turning with EDM, resulting in a reduction of uncut fiber generation and a significant decrease in tool wear compared to traditional turning. Optimal results were achieved using a copper electrode under specific conditions: a 400 V gap voltage, 1 kHz discharge frequency, 7 µm discharge gap, 0.05 duty ratio, 2.0 m/min turning speed, and 0.15 mm/rev feed rate. While research on Wire Electrical Discharge Machining (WEDM) specifically for CFRP drilling remains limited, preliminary work by Abdallah et al. [36] focused on unidirectional (UD) CFRP laminates. The study assessed the effects of ignition current (3–5 A), open gap voltage (120 and 140 V), pulse-on time (0.8-1 µs), and pulse-off time (4–8 µs) on MRR and kerf widths. The highest MRR achieved was 2.41 mm³/min, with ignition current and pulse-off time identified as significant factors, contributing 48.5% and 24.3% to the PCR, respectively. No workpiece damage was reported except for rough edges and minimal adhered debris on the bottom surface. Abdallah et al. [37] further demonstrated the feasibility of WEDM for thick multidirectional CFRP plates, using high-performance wire electrodes. They achieved a maximum MRR of 15 mm³/min by optimizing parameters, primarily pulse-off time, which contributed 67.76% to the PCR. In comparison to a Topas Plus D wire, Compeed wire exhibited lower MRR (by up to ~ 40%), greater cut accuracy (~ 8%), and improved surface roughness (~ 11%). Under SEM examination, observed defects included fiber fragments, voids, excessive delamination, and adhered debris.
In light of the limited research on the application of the Wire Electrical Discharge Machining (WEDM) technique for CFRP drilling, the principal aim of this study is to explore the potential feasibility of WEDM in the drilling process. Additionally, the investigation seeks to comprehensively evaluate the effects of varying machining parameters on critical aspects, including the Material Removal Rate (MRR) and the occurrence of delamination at both the top and bottom surfaces of the CFRP material.