1. Introduction
The Inconel 718 superalloy is one of the most important alloys belonging to the nickel-based superalloy family. This superalloy is mainly used in the aerospace industry to manufacture aerospace parts and gas engine components. In gas engines, about 50% of components are manufactured from nickel-based superalloys. The material is intended for heat treatment recipients, e.g., gas turbine blades, turbine vanes, etc. [
1,
2,
3,
4]. The application of Inconel 718 in the aviation industry results from its unique properties, such as high oxidation resistance, corrosion resistance in aggressive environments, resistance to creep, thermal fatigue, very good mechanical properties at both high temperatures (up to 923 °K) and cryogenic temperatures, and its high mechanical strength under these conditions [
1,
5,
6,
7,
8].
Components of modern gas turbine engines, such as turbine blades, work in high-temperature conditions (in the range of 823.15–1373.15 °K). This causes the production of elements from advanced engineering materials including nickel-based superalloys. Among these superalloys, Inconel 718 is one of the most widely used. To increase the durability of this superalloy, a considerable number of holes are made in the structure of turbine blades (20,000–40,000) with a diameter of 0.3–5 mm and an aspect ratio of (40–600):1 (depth-to-diameter ratio) [
9,
10,
11,
12]. These holes are determined as “cooling holes” due to the ability of cooling agents to flow through them (gas or liquid), which reduces the temperature of the component material [
13,
14]. The dimensional shape accuracy and the quality of the holes’ inner surface affect the efficiency of the cooling process. Non-conventional methods are often chosen to drill the cooling holes, such as the electrical discharge process (EDM) [
15,
16,
17,
18].
The combination of the chemical composition of Inconel 718 ensures its excellent mechanical properties. The alloy elements such as Ni and Cr provide resistance to corrosion, oxidation, carburizing, and other damage mechanisms acting at high temperatures. Ni and Cr crystallize as a
γ phase (face center cubic). Additionally, Al, Ti, Nb, Co, Cu, and W are added to increase mechanical and corrosion resistance. Nb is added to form hardening precipitates
γ” (Ni
3Nb, body-centered tetragonal metastable phase). Ti and Al are added to precipitate the intermetallic
γ’ form (Ni
3 Ti, Al, simple cubic crystal). These alloy elements characterize a lower influence than
γ” particles. The two phases (
γ’ and
γ”) provide the high strength of the Inconel 718 superalloy (ultimate tensile strength of 1.1 GPa). Further, C is added to precipitate forming MC carbides (M = Ti or Nb). The C content must be low enough to enable Nb and Ti precipitation in the form of
γ’ and
γ” particles [
3,
8,
19].
The properties of Inconel 718 (such as high hardness, high toughness, very poor thermal conductivity, work hardening, and the presence of highly abrasive carbide particles) make it difficult to machine in the process of constructing aircraft components using conventional methods. The strength of this material at elevated temperatures is quite high, which affects the machining of the superalloy, requiring extreme cutting force. In effect, an enormous amount of heat is delivered to the tool tip. During the machining process, Inconel 718 has a strong tendency to weld to the tool and to form build-up on the edge of the tool. As a consequence, the tool faster wears and the workpiece plastically deforms. Due to these issues, Inconel 718 belongs to the “difficult-to-cut” materials [
1,
17,
20,
21,
22]. To improve the machinability of the Inconel 718 superalloy by using conventional machining, one should optimize the process parameters using the response surface methodology (RSM) or an analysis-of-variance (ANOVA) [
23]. For the above reasons, to machine the Inconel 718 superalloy, non-conventional methods are preferable than conventional machining. Nowadays, electrical discharge machining is among the most effective methods to machine the material [
20,
24].
When electrodischarge machining the Inconel 718 superalloy, the mechanical properties of the material do not significantly affect the process, which means the forces occurring between the tool and the workpiece surface are negligible or do not take place. The EDM process involves electrical discharges, during which a plasma channel is formed, characterized by a high temperature (about 10,000 °K and locally more than 20,000 °K even for a short pulse duration [
25]). In effect, the material is removed by melting, evaporation, and disruption under thermal stresses, which is not affected by its toughness. The heat flux on the workpiece is generally higher than on the tool electrode; therefore, the material removal rate is maximized, and wear of the electrode is minimized. The creation and movements of ions and electrons inside the plasma channel allow electricity to pass through the electrode and the workpiece. The bombardment of ions is related to the highly concentrated electricity flow and conduction of heat from the plasma to the electrodes, which heats the material on both the workpiece and tool electrode sides. During the discharge time, a bubble of vapor is created around the plasma due to the high temperature. The plasma enlarges during the discharge. When the discharge ends, the plasma collapses, and the material is then ejected. The rapid cooling during the pulse-off time leads to the formation of spherical debris, a typical shape that is obtained when a liquid is rapidly solidified [
26,
27,
28].
The phenomena that occur during the removing process of material using the EDM process cause that the properties of the workpiece material such as thermal conductivity, density, melting point, evaporation temperature, and coefficient of thermal expansion significantly effect this process. The enthalpy of vaporization and boiling point of the tool electrode material influence the evaporating intensity of the workpiece material. The increase in these properties decreases the volume of the evaporated material of the workpiece. The tool material is affected similarly during single discharge; however, the occurrence of thermal expansion causes the stresses in the horizontal plane. This is a result of the inability of the material to expand in this plane due to the presence of a material layer with a lower temperature, which does not allow the deformation [
28].
The consequence of spark erosion is a heterogeneous spreading of the heat onto the surface workpiece and the working electrode under the pressure of the plasma channel formed during electrical discharge; therefore, the main factor influencing the erosion process is the thermal conductivity of the electrodes’ material (workpiece and tool) [
28]. In the case of Inconel 718, its thermal conductivity is low (8.9 W/(m·°K) at 298 °K [
8]), which significantly affects the process of material removal. This causes a lower amount of heat to be delivered to the workpiece material. Consequently, a considerable amount of heat penetrates the tool electrode material and occurs in the gap area (which results in insufficient flushing of the interelectrode gap). The difficulties of the flushing efficiency take place especially during the electrical discharge drilling of deep holes with a diameter of less than 1 mm [
29]. This leads to excessive tool wear and process instability. Moreover, the presence of high-temperature conditions and erosion products (eroded particles and bubbles) changes the conditions occurring in the gap area. These conditions can contribute to secondary/abnormal discharges (such as arcing and short circuits), decreasing the dimensional shape accuracy of the hole. Additionally, in [
30], it was shown that the spherical eroded particles can join into the debris chains, which are more difficult to remove from a narrow gap area.
In the case of the EDM drilling of high-aspect ratio holes (above 20:1) with a diameter of less than 1 mm, effective gap flushing is a challenge [
31]—even during the application of a high volumetric flow rate (25 L/h) [
32]. To prevent secondary discharges between debris and the hole sidewall, a satisfactory approach is to cover the sidewall of the tool electrode with a non-conductive coating. The best results are obtained for Perylene C-coated tools. This approach enables us to reach an aspect ratio of 126:1 within 1 h for micro-holes with a diameter of 0.18 mm and a depth of 10 mm. Additionally, the tool wear is two times lower when using an insulated electrode compared to using an uncoated electrode [
33]. To minimize the accumulation of debris at the hole bottom, there are several flushing methods, such as internal, external, suction-assisted flushing, flushing with different electrode movements, or vibration-supported flushing [
33,
34].
In [
35], the authors investigated the influence of three different electrode materials, such as brass, copper, and copper tungsten (CuW), on Inconel 718 machinability with the use of hybrid electrical discharge and arc machining (HEDAM). The analysis shows that for all current settings, the brass electrode allows one to achieve the highest material removal rate; however, the copper tungsten electrode provides the lowest electrode wear. The experimental research also included an investigation of the average surface roughness (
Ra) and surface characteristics of the electrodes after machining. The authors mentioned in the impact analysis of the thermal properties of Inconel 718 and the tool electrode materials on the process’s performance that brass and copper allow one to obtain less
Ra compared to using the CuW electrode.
The above analysis shows that the machinability of the nickel-based superalloys demonstrates the difficulty of machining by conventional methods; however, the thermophysical properties of these superalloys also contribute to the difficulties of hole drilling with the use of the EDM process as well (especially the drilling of deep holes with a small diameter—less than 1 mm). The EDM process is a non-contact method and is preferable for machine materials determined as “difficult–to–cut”; however, the presence of high temperatures (the heating generated from electrical discharges) during the material removal hinders the process. The main factor causing the decreased machinability of Inconel 718 is its low thermal conductivity [
35]. The differing behavior of the superalloy with increasing temperature has a significant influence on the effectivity of material removal; however, this behavior is required to achieve good hole accuracy and high EDM machining performance. Obtaining high dimensional shape accuracy of holes is a crucial issue for their application. The majority of papers concerning machining holes in a smaller range focus on the impact of the material’s thermophysical properties on the allowance removal process. The thermophysical properties of Inconel 718 in a wider range can significantly influence the appropriate selection of the machining parameters and improve the machinability of the Inconel superalloy with the use of EDM. Due to the specific behavior of this superalloy with increasing temperature, the research should be directed toward the analysis of a greater number of process parameters, including a wider range of values.
In this paper, we present the analysis of the results of the experiments on electrical discharge drilling of the Inconel 718 superalloy. In this study, we aimed to: investigate the machinability of Inconel 718 by using the EDM process to enable the drilling of holes with a high aspect ratio and satisfactory dimensional shape accuracy; to determine the appropriate influence of process parameters on the process’s performance; and to investigate a wider range EDM on Inconel 718—the experimental research includes the influence analysis of five machining parameters on the process’s performance. The analyzed process parameters involve parameters like open voltage, time of the impulse, current amplitude, the inlet dielectric fluid pressure, and tube electrode rotation; however, the process’s performance was analyzed in terms of drilling speed, linear tool wear, the side gap thickness, and the aspect ratio of holes.
4. Discussion
The analysis shows that the electrical discharge drilling of micro-holes with a diameter of less than 1 mm in the Inconel 718 superalloy is difficult when trying to obtain satisfactory dimensional and shape accuracy of the drilled holes and high process efficiency. Electrical discharge machining is dedicated to the machining of “difficult-to-cut” materials, which involve the chromo-nickel alloy (including Inconel 718); however, the superalloy’s properties, such as the thermal conductivity or the heat capacity, when temperatures increase during the machining process, contribute to difficulties during machining with the use of EDM. Additionally, for the electrical discharge drilling process of high-aspect ratio micro-holes, debris accumulates at the hole bottom, causing difficulties in flushing the machining area. The phenomena take place especially when the hole is deep.
In the EDD process, the amount of heat delivered to the workpiece material and into the material structure affects the removal process. The low thermal conductivity of the Inconel 718 superalloy and its increase with the increase in temperature leads to a decrease in the accuracy of drilling holes. When a lower current amplitude is applied (
I = 2.66 A) and a similarly appropriate pulse time duration (
ton = 775 µs), a significant amount of re-solidified material occurs on the edge of the hole entrance (
Figure 13b). This is the result of the heating of a greater area of material by extending the pulse time of a single pulse, and the energy delivered to the workpiece material is too small to remove material due to the lower current amplitude; however, the application of a longer pulse time,
ton = 550−999 µs, in combination with a higher current amplitude,
I = 3.99−4.65 A, affects the improvement in the hole accuracy (obtained values of
SG ≤ 100 µm) (
Figure 14b); re-solidified material is absent in this case. Usually, using the longer pulse time worsens the homogeneity of the edge of the entrance to the hole and the accuracy of holes drilled by using the EDD process. In the case of applying the Inconel 718 superalloy as a workpiece material, using longer pulse times and higher current amplitudes is favorable.
In addition, too little energy delivered to the workpiece material and an ineffective flushing of the gap area (among others by applying lower tool electrode rotation
n = 100 rpm) lead to process instability and lower process efficiency (the drilling speed about 2–3 µm/s). This is the result of short circuits and/or arc discharges during the single pulse time, which are noted in the recorded voltage and current waveforms (
Figure 7a,b and
Figure 16b).
Using deionized water as a working fluid in combination with a higher current amplitude contributes to the faster heating of deionized water in the gap area, increasing its electrical conductivity. Then, the EDD process is accompanied by electrochemical dissolution, increasing the process efficiency and dimensional accuracy of holes. The reinforced electrochemical reactions during the process are observed by the presence of cavities in the area around the top diameter when applying I ≥ 3.99 A and ton ≥ 550 µs.
The low thermal conductivity of Inconel 718 causes—when the hole is deep and difficulties in effectively flushing the gap area occur—a significant amount of heat to not penetrate the workpiece material. As such, a significant amount of heat affects the machining area and penetrates the tool electrode material. This allows debris to melt and form chains, which are more difficult to remove from the narrow gap area. Secondary discharges can also cause an excessive decrease in the dimensional and shape accuracy of the hole (causing the conicity shape of holes) and the heat penetrating the tool material increases its wear.
The electrical discharge machining process is one of the most effective methods of machining “difficult-to-cut” materials, including the chromo-nickel superalloys, according to the analyzed scientific articles, but the thermophysical properties of these superalloys mean that optimization of the process parameters is still required to obtain a satisfactory process efficiency and hole accuracy.