1. Introduction
For specific applications where conventional steels do not perform well, Inconel 718 is employed in the manufacturing and aerospace industries. The outstanding properties which make the alloy a first choice for aero-industry applications include superior endurance of mechanical properties at elevated temperatures, fatigue and corrosion resistance, higher creep strength, excellent engineering attributes, and an exceptional weight-to-thrust ratio [
1,
2]. This alloy is widely used by manufacturing industries because of its excellent thermo-mechanical functionality to make a variety of components for aircraft engines, including turbine discs, blades, combustors, and casings, as well as extrusion dies and containers, hot work tools, and dies [
3,
4].
Since the alloy is used for high-tech applications, it must be processed in several shapes as per functionality. Due to the alloy’s attractive physical properties, specific processing challenges are associated with it because of its hard-to-cut nature and poor thermal conductivity [
5]. These challenges exponentially increase the machinability cost. There are several developments in conventional machining setups to overcome the cost barrier. These developments include customized lubrication systems, novel cutting tools, and thermally aided processing. However, these developments restrict production capabilities. Recently, the demand for the alloy has increased in hot structures and components of gas turbines and aero-engines, which are required as components with complex geometrical profiles [
6]. Some examples where complex geometrical profiles are required include turbine discs with firtree profiles and fan discs, as shown in
Figure 1. Therefore, the components’ dimensional accuracy and higher surface quality are critical for the application.
In the past decade, complex geometries have been processed through traditional machining processes. These conventional machining processes include turning, milling, and drilling. As the material possesses superior mechanical properties, the processing is limited because of tool wear, localized mechanical stresses at the surface and subsurface, and machinability challenges because of high hardness [
7]. These limitations, along with poor production efficiency and performance, bound the machinability through conventional methods because of the thermo-mechanical properties of the alloy. Such problems limit productivity and push towards the consideration of non-conventional processes.
Among non-conventional processes, wire electric discharge machining (WEDM) has gained popularity as an attractive alternative to process hard-to-cut materials with desirable quality attributes. Wire electric discharge machining (WEDM) is a variant of electric discharge machining die sinking with a replacement of fixed electrodes to continuously moving electrodes [
8]. The primary function of the process is removing the material through successive electric discharges occurring at a certain frequency. The fundamental material removal mechanism is arguable to date. However, the basic understanding involves thermal conduction. This condition mechanism is administered by generating heat from spark channeling and dissipation in the tool electrode and workpiece. The discharge channels result in material melting and vaporizing, followed by flushing [
9]. Therefore, the process is categorized into subtractive technologies for producing two- and three-dimensional features: through-drilling, irregular shape manufacturing, inclined hole making, and complex geometries. As mentioned earlier, the fabrication of the shapes is complicated by a single process on hard-to-cut alloy such as Inconel 718.
However, the process is also used for surface texturing, enhancing surface hardness through the redeposition of electrode material as carbides, oxides, and surface alloying. In a nutshell, the process employs a thermo-electrical mechanism to machine the workpiece where sparking causes melting and evaporation of unwanted material. The tool electrode, which has a diameter of 0.05–0.30 mm, is a conductive wire which constantly moves by maintaining an inter-electrode gap of 10–100 μm [
1]. Since there is no physical interaction between the tool and workpiece, the process is capable of machining any electrically conductive material of different dimensions [
10]. However, the tool electrode is recommended to have higher thermal and electrical conductivity because of the involvement of high temperature while eroding the workpiece. Welling [
3] made a comparison of surface quality and geometrical accuracy between wire electric discharge machining, grinding, and broaching for an Inconel 718 firtree-shaped (as shown in
Figure 1a) jet engine component. In terms of geometric accuracy and overall surface integrity, the wire electric discharge machining process outperformed the conventional processes. The authors recommended the WEDM as a technologically advanced and attractive alternative for producing firtree profiles in Inconel 718.
Figure 1.
Application of complex profiles used in aero-industry: (
a) turbine disk firtree profiles of Inconel 718 [
6]; (
b) fan disk of heat-resistant alloy [
11].
Figure 1.
Application of complex profiles used in aero-industry: (
a) turbine disk firtree profiles of Inconel 718 [
6]; (
b) fan disk of heat-resistant alloy [
11].
The literature on WEDM reveals that machining complex geometries is complicated because the wire electrode deviates from the programmed track, creating overcut and undercut errors. Therefore, the manufacturing community is carrying out significant focused research to overcome the limitations of fabricating complex geometries to meet the precision requirements of the aerospace industry. For instance, Farooq et al. [
7] investigated the control of process parameters to obtain minimum geometric deviations and corner radii on Ti6Al4V. The authors machined complex curved profiles and obtained 0.250% overcut and 0.236% undercut errors in convex and concave shapes, respectively. In addition, servo voltage was considered the most controlling factor along with wire offset of 0.169–0.173 mm to attain 0.106 mm corner radii. Venkatarao and Kumar [
12] analyzed the surface quality of Inconel 718 through wire tension, pulse current, pulse on time, and pulse off time. The authors concluded that wire displacement affects profile taper, and surface quality is controlled by wire tension and pulse current. Ahmed et al. [
5] performed ultrafast drilling through electric discharge machining on Inconel 718. In the study, an additional DC power supply was integrated with the process to enhance the machining performance. The authors highlighted the significance of choosing the right tool electrode to improve the process efficiency. Dabade and Karidkar [
13] optimized electric process parameters to machine complex geometries on Inconel 718, resulting in pulse on time having a significant impact on roughness and geometric deviations. In addition, the irregular discharge energy generated by servo voltage and pulse on time reduced the dimensional accuracy of the final part. Reolon et al. [
9] investigated the process performance and surface integrity with coated and uncoated wire electrodes on Inconel 718. In the study, uncoated brass wire electrodes outperformed coated copper electrodes with a 36% increased feed rate and 80% reduced electrode consumption. The supremacy of the brass electrode was established through effective discharge energy transfer. Manoj and Narenderanath [
14] machined triangular profiles having 0°, 15°, and 30° taper angles to study inaccuracy at Hastelloy X (nickel alloy). They found that insufficient flushing attributed to the generation of wire vibrations which increased slant angle inaccuracies (taper error).
As the above literature shows, the wire electric discharge machine process depends on various factors such as workpieces, wire electrodes, dielectric attributes, process parameters, and geometric requirements. In addition, a trim-cut approach (limited to simple geometries) was applied to control the wire deflections originating from vibrations and discharge forces [
15]. Similarly, Farooq et al. compensated wire offset while machining Ti6Al4V to minimize deflection, which causes wire lag [
7]. Moreover, mathematical models have been developed to understand the discharge force and control through parametric optimization on low-radius free-form geometry [
16]. Zahoor et al. [
1] performed parametric optimization to minimize overcut and undercut through a genetic algorithm using coated electrodes on Inconel 718. Developments are underway to improve process capabilities regarding machining efficiency and better dimensional control. In this regard, different electrode materials and process optimization methodologies are opted to enhance surface integrity, reduce spark gap, and minimize dimensional errors, which are current limitations of the process. Ishfaq et al. [
17] machined AISI D2 material to reduce geometrical machining errors incurred during die repair and maintenance. Maher et al. [
18] highlighted challenges such as longer unattended operation periods, thicker workpieces, high taper angles, and complex profiles. The authors stressed effective flushing and cooling attributes to obtain higher machining performance. The cooling at the interface is important, as brass has a lesser melting temperature than Inconel 718, which introduces a cooling effect. This effect reduces the probability of wire breakage and enhances surface integrity. Zahoor et al. [
1] machined Inconel 718 and found an optimized setting of servo voltage (54.62 V), wire tension (2.6405 g), and wire feed (4.17 mm/s), pulse on time (2.99 μs), and pulse off time (22.41 μs). The authors identified the necessity of an effective flushing mechanism with discharge energy to minimize geometric errors. Naveed et al. [
19] machined curved profiles on WC-Co composite by employing wire tension (630, 730, 870 g), servo voltage (45, 50, 55 V), pulse-off-time (20, 25, 30 μs), and pulse-on-time (0.2, 0.3, 0.4 μs). The study resulted in a minimum of 6 μm radial error because the wire deflection originated from ineffective flushing.
Nayak and Mahapatra [
20] investigated the effect of wire tension and speed, discharge current, pulse duration, taper angle, and part thickness on cryogenically treated Inconel 718. A minimum angular error was obtained through an optimized setting of wire tension (12 N), wire-speed (120 mm/s), discharge current (14 A), pulse duration (32 μs), and taper angle (5°). Similarly, Sharma et al. [
21] improved the machining productivity and surface quality of Inconel 706 and compared several electrode materials. The study reported that zinc-coated electrodes enhanced cutting speed but deteriorated the surface integrity. However, the hard-brass electrode improved surface quality and outperformed in terms of lower residual stresses. In addition, the hard-brass electrode with effective control over discharge energy resulted in the potential to enhance the material removal rate. The supremacy of brass wire is established as a first choice to achieve higher surface quality. Klocke et al. [
22] carried out parametric optimization with WEDM of fir tree slots in Inconel 718 and attained a 25% reduction in the recast layer and 40% productivity improvement. The literature mentioned above has established that different developments have been made to enhance geometrical accuracy and improve the surface integrity of complex profiles. In these approaches, the effect of flushing attributes on profile dimensional requirements is not studied compared to investigations on electrical parametric optimizations and the impact of electrode materials. To enhance machining performance, the aforementioned issues must be monitored and controlled. Furthermore, to strike a balance between machining efficiency, process stability, and geometric accuracy of the machined parts, flushing characteristics such as flushing pressure, flushing nozzle diameter, and flushing nozzle workpiece distance should be carefully controlled. The performance of WEDM during electro-sparks erosion is greatly influenced by controlled flushing, which involves precise flushing characteristics. Therefore, parametric evaluation of flushing pressure, flushing nozzle diameter, and flushing nozzle to workpiece distance might thereby improve the machinability in WEDM [
23]. Optimized dielectric flushing influences the quality of the machined surface as well as processing speed. A high flushing pressure reduces machining speed, while the flushing nozzle to workpiece distance and flushing nozzle diameter increase wire vibrations and consequently wire lag, reducing the dimensional accuracy of the machined workpiece. Therefore, to increase machining efficiency, the wire lag should be decreased. According to Chakraborty et al. [
24], an optimum combination of flushing features decreases the wire lag. It is well known that raising the flushing pressure and wire tension improves the component’s machinability and geometric accuracy. Electrical characteristics such as fine finish power supply, pulse width, servo voltage, and pulse current also play an essential role in improving machining performance, while enhanced debris removal is critical for stable machining. Based on the limited research that has been reported, Ehsan et al. [
25] evaluated flushing attributes to enhance the material removal rate and surface integrity. The improved flushing action while machining M42 steel resulted in a 21.99 mm
3/min material removal rate, 1.90 µm surface roughness, and 0.354 mm kerf width. Okada et al. [
26] investigated nozzle jet flushing during the machining of SKD 11 and presented a reduced degree of wire deflection and breakage, while the debris residence time and flow fields were numerically evaluated against the distribution of hydrodynamic stress in the kerf. Roy and Sanna Yellappa [
27] studied the effect of dielectric flushing conditions such as flushing pressure (0.5 to 1.5 kg/cm
2) along with electrical machining parameters while processing (relatively simple geometry) TiNiCu alloy. The study showed that flushing pressure greatly influences surface morphology and flow direction significantly controls the removal of re-solidified debris and the amount of molten metal. Bergs et al. [
28] analyzed workpiece to nozzle distance and flushing pressure as process variables to enhance the machinability of 16MnCr5 alloy. A multi-pass strategy was employed in the study to machine straight profiles for improved surface roughness and dimensional accuracy. An effective balance and more in-depth investigation were recommended in the study to correlate the flushing attributes to the quality matrix for useability in machining fir tree profiles on nickel alloys. Fujimoto et al. partially explored the essence of the flushing mechanism [
29]. The authors employed jet flushing and stressed decreasing debris stagnation to rule out process instability, wire breakage, and shape inaccuracy.
It is evident from the literature survey that most studies focused on improving surface integrity, machining efficiency, and dimensional inaccuracies. However, the influence of flushing characteristics is not comprehensively studied, which could further be integrated into other developments, such as improved electrode coatings and better parametric conditions. In addition, the local industry uses WEDM systems with limited flexibility on flushing mechanisms. In this regard, the potentiality of wire electric discharge machining is explored to enhance surface integrity and reduce dimensional inaccuracies through a novel flushing mechanism. The experimental work focuses on maintaining dimensional precision on complex geometries in hard-to-cut Inconel 718. The complexity of the profile is derived from different features used in aero-engine applications. In this regard, complex shapes are assessed based on surface, subsurface damages, and dimensional capability (spark gap, angular error, and cylindricity error). The process is optimized by incorporating one electric variable servo voltage (as indicated in the literature to be the most significant in controlling erosion science) and three flushing variables of the system. The influences are quantified through analysis of variance, analyzed through micrographs, and scanning electron microscopy. In addition, it is paramount to highlight that the mono-objective optimization and multi-objective optimization of the complex profile processing are not carried out in the literature so far for the flushing attributes. An optimized parametric solution is developed to utilize the full potential of wire electric discharge machining coupled with a flushing mechanism by offering a tradeoff addressing the requirements of the aero-manufacturing industry.