1. Introduction
The usage of superalloys in an aerospace turbine engine is almost 50% of the total materials, and nickel alloy (Ni-alloy) shares account for ~40% [
1]. Nickel-based superalloys are the family of metallic alloys that the aerospace manufacturing industries have widely adopted, especially in structural components due to outstanding properties such as resistance to corrosion, fracture toughness, better yield and high-temperature strength. However, this alloy is in a category of difficult-to-machine materials is due to low thermal conductivity and greater susceptibility toward work hardening [
2]. Among Ni- alloys, Inconel 718 is still the most widely consumed alloy. Compared to turning and milling, data on the drilling process is limited [
3]. So far, conventional drilling methods have been found to be less effective and less productive; therefore, researchers have adopted various techniques to improve the machining efficiency for drilling Inconel 718. Previously, peck drilling [
4] and pilot hole-making strategies [
5] were also adopted by the researchers, but poor hole quality and low productivity, respectively, were the common problems associated with these approaches. A thorough summary of literature reviews on drilling of Inconel 718 in particular, and the machining of nickel alloys in general, is presented below for context building.
Several researchers have attempted to change the cutting environment for holes’ quality improvement while drilling Inconel 718. Nearly all researchers have employed cemented carbide twist drills while studying the variations in the cutting environment while drilling Inconel 718. Shah et al. [
6] drilled 50% more holes at a maximum Vc of 20 m/min using liquid CO
2 compared to liquid nitrogen due to less chipping and abrasion wear with the former. Reductions in surface roughness (11%), power consumption (19%), and thrust force (14%) were noticed with liquid CO
2, but deteriorating effects of carbon dioxide usage were reported in terms of environment, human health, and natural resources depletion. Thus, liquid nitrogen was recommended as the superior cryogenic coolant over carbon dioxide because of its lesser environmental damage. Eskandari et al. [
2] recommended an efficient method of cooling for drilling Inconel 718 by adding graphene nanoplatelets blended in cutting fluid at a constant Vc of 30 m/min and
f of 0.05 mm/rev. The particle size of graphene was within the range of 7 µm. By applying the cutting fluid having graphene in it caused the reduction in the value of cutting torque from 6 Nm to 2 Nm. Moreover, cutting with graphene-based cutting fluid resulted in approximately 10% improvement in surface roughness. Additionally, deformation zone depth and subsurface strain magnitude were lower in the presence of graphene-based cutting fluid than in flooded cooling. A reduction of ~33% in the cutting temperature was recorded with the former cutting scheme. At the same time, the authors did not recommend dry cutting of Inconel 718 due to Sa values escalating to 35 µm. Three modes of cutting environment was investigated by Girinon et al. [
7] (internal high-pressure cooling (via drill), external low-pressure cooling, and dry condition) on the resulting residual stresses and surface quality. Cutting speed (c) and feed rates (
f) were constant at 24 m/min and 0.10 mm/rev, respectively. Girinon recommended internal high-pressure cooling for compressive residual stresses and limited strain hardened layer. Contrary to internal high-pressure cooling, dry drilling produced severe tensile residual stresses and a thick strain-hardened layer below the machined surface. Ucak et al. [
8] analyzed the holes drilled in Inconel 718 by investigating the thrust forces, torque, rise in temperature, surface quality, and tool life under various drilling conditions at constant Vc of 15 m/min and 0.02 mm/rev
f. With a cryogenic cutting environment, a substantial reduction in cutting temperature was noticed, along with an improvement in integrity of the hole and quality of the hole. However, an increase in thrust force and a reduction in tool life were seen along with tool chipping when cutting in a cryogenic environment due to an increase in the hardness and strength of the workpiece material. Better results in terms of lower surface roughness and higher tool life were achieved when applying Flood cooling. Furthermore, TiAlN coating dramatically enhanced the tool life. Khanna et al. [
3] compared dry drilling with the cryogenic environment at a constant speed of 19 m/min and 0.02 mm/rev. The latter arrangement not only improved the tool life by about 87% but also reduced the torque by about 30%. In addition, improvement in hole quality attributes was also recorded with a decrease in circularity error (up to 51 %), cylindricity error (up to 77 %), and roughness value (up to 48 %). Furthermore, dry drilling was not recommended by the authors considering the tool life and hole integrity. Apart from drilling, oil emulsion was suggested to be the better feasible option when turning Inconel 718 as referred by Amigo et al. [
9]. In their work, CO
2 cryogenic cooling was recommended for Hayness 263 for providing a good balance between environmental and technical aspects. As per Pereira et al. [
10], cryoMQL, which is a combination of MQL and cryogenic technique, showed a 57% increase in tool life in the milling of Inconel 718 in comparison to MQL technology. However, the performance of this technique was inferior in comparison to wet machining. It is important to mention that the use of cutting fluid is mandatory in turning, drilling, and milling of Ni-based superalloys such as Inconel 718, RR 1000,Haynes 282 and Waspaloy [
11,
12,
13,
14].
Qin et al. [
4] employed peck drilling (seven pecks for each hole), which led to poor hole quality as measured by higher surface roughness (Ra = 2–5 µm), material smearing, cavities, grooves, and side flow. Rahim and Sasahara [
15] recommended palm oil over synthetic ester due to its effective lubrication and cooling. A reduction in surface roughness and sub-surface plastic deformation was noticed with palm oil due to its high viscosity. Karabul and Kaynak [
16] conducted dry drilling experiments on additively manufactured Inconel 718 at a Vc of 30 m/min,
f of 0.075 mm/rev. However, a high surface roughness of 2.5 µm-Ra was recorded along with an increased microhardness beneath the machined surface (extended to around 100 µm). From the above discussion, it can be noted that 30 m/min was the maximum cutting speed employed by the researchers while evaluating various cutting environments during the drilling of Inconel 718. Furthermore, it can also be commented that the drillability of Inconel 718 is not promising in a dry-cutting environment.
The second area in which researchers tried to improve the drilling efficiency of Inconel 718 is the geometric modification of the carbide twist drill bit. Pang and Wang [
1] evaluated micro-textured implanted drills at 30 m/min Vc and 0.20 mm/rev
f. The laser surface texturing technique was utilized for texturing the flank and rake surfaces. Improvements in the drilling process were noticed with lower values of tool wear, drilling temperature (up to 9%), and thrust force (up to 32 %) compared to conventional drilling. Micro-texturing enhances the air convection on the tool surface with faster heat dissipation, thus improving the aforementioned responses. Beer et al. [
17] compared the performance of standard and modified twist drills at a Vc of 35 m/min and an
f of 0.10 mm/rev. The flutes of modified carbide twist drills were ground to reduce roughness below Rz = 0.50 µm to improve chip evacuation and chip jamming problems. Additionally, a groove having a depth of 50 µm was generated at the face of the flank in parallel to the cutting edge (at a distance of 200 µm from the cutting edge) by using the laser melting method. With this modification, a ~12% improvement in the tool life was noticed as opposed to a standard drill bit. Modified twist drills generated 3–5 µm-Rz roughness, which is marginally better (10%) in comparison to the standard drill bit. The performance of modified carbide twist drills was presented in refs. [
18,
19], where the flank face was retracted via a grinding process to create a cooling flow channel (just behind the cutting edge) to improve the accessibility of the cutting fluid (flowing from the internal drill hole) towards the area of cutting edge corners. At 45 m/min Vc and 0.14 mm/rev
f, a ~3-fold tool life improvement was noticed with this modified drill compared to the standard drill bit. Additionally, the standard drill generated 28% higher roughness values than that obtained with modified drills, where the roughness was 5–7.5 µm-Rz with the latter drills [
18]. Vrabel et al. [
20] also evaluated ground carbide twist drills at Vc = 20 m/min and f = 0.035 mm/rev in the presence of an internal flood coolant supply. With this parametric combination, 24 holes were drilled. From the above discussion, it is evident that a higher Vc of up to 45 m/min can be employed with the modification in carbide twist drills, and, at the same time, improved surface quality and higher tool life could be achieved.
According to ref. [
20], drilling is typically considered a roughing operation for the Inconel 718 alloy, and reaming is a necessary finishing operation. Sharman et al. [
5] suggested a pilot hole-making approach to drill Inconel 718. To drill 8 mm holes, firstly, 7.8 mm pilot holes were drilled, making this process less productive. They found that the surface roughness was less than 0.50 µm-Ra with the finishing operation, while it was up to 1.50 µm-Ra with the drilling operation. The sub-surface plastic deformation was also 2–3 times lower with the finishing operation compared to the drilling process. Drilling also revealed material smearing and welding of the chip to the hole walls, contrary to the finishing operation. They suggested employing finishing (either reaming or mill-boring) to meet the tight requirements of aerospace applications, as drilling of Inconel 718 is deemed insufficient.
From the above discussion, it is evident that previously published literature on the Inconel 718 drilling is focused on using cemented carbide twist drills only by using the various cutting environments or by using the geometrically modified twist drills. Additionally, dry drilling is generally not recommended for this alloy. A maximum cutting speed of 30 m/min is achieved with variations in the cutting environments, such as using liquid CO2, liquid nitrogen, and graphene-based fluids. With modified twist drills, better roughness can be achieved, and cutting speeds can be extended up to 45 m/min. Few researchers have also considered the drilling of Inconel 718 as a roughing operation, pointing out the need for some finishing operations. Therefore, the current research status for drilling Inconel 718 is that the post-drilling operations, including reaming and mill boring, are still mandatory, and drilling operation alone is not enough to meet aerospace requirements.
In general, drilling superalloys is always challenging, and researchers continuously try various techniques to improve machinability. More recently, ultrasonic peening drilling (UPD) has been proposed to increase fatigue life, induce compressive residual stresses, better surface finishes, and raise microhardness. In this methodology, ultrasonic elliptical vibrations and tool rotations are both applied clockwise [
21]. On the same analogy, ultrasonic-assisted milling has been introduced by generating impacts on the machined surface ultrasonically during end milling. Compared to conventional milling, ~16 times greater fatigue life is obtained with a better surface finish and fewer surface defects [
22].
Though the advantages of wiper inserts in turning and milling [
23,
24,
25] are well established in terms of productivity, their use in drilling is still lacking. Drill tool manufacturers have now been able to produce the wiper configuration for drilling processes. In this study, wiper inserts are evaluated along with the combination of central inserts (termed as a stepped insert) placed in a special type of tool holder, with a provision of spacious flutes for chip evacuation from the cutting zone. (Figures are shown in the next section). The expected benefits of this combination are improved surface roughness, increased productivity due to the possibility of using higher feed rates, lesser defects on the hole surface, and omitting the need for finishing operations. Hence, it is termed as one-step machining. In this study, the effect of the new wiper and central inserts on the drilling performance of Inconel 718 is evaluated in terms of tool life, tool wear, surface roughness, and surface integrity.
2. Materials and Methods
Inconel 718 was utilized as a workpiece for drilling experimentation. Initially, it was supplied in bar form, having a length of 305 mm and a diameter of 76 mm. Then, several discs measuring a diameter of 76 mm × 15 mm thickness were cut via wire electric discharge machining (EDM). The tool holder selected for the drilling operation was Coro-drill 880, manufactured by Sandvick Coromant. The Coro-drill 880 can mount two inserts: a central insert and a peripheral insert. The peripheral insert has a wiper configuration (termed as wiper insert). In wiper inserts, along with a nose radius, an additional straight edge is introduced, which keeps the cusp height of the machined surface at a lower level due to the increased contact length with the machined surface.
Figure 1a shows the schematic of the wiper insert with a nose radius and an additional straight edge. The central insert has a nose radius at the chisel edge location (in the case of a twist drill) where Vc is zero, and material removal is primarily due to extrusion with material transporting away from the center. As with twist drills, the central insert has two cutting edges to remove material by shearing (
Figure 1b). However, a central insert features steps on both cutting edges, which reduce cutting-edge involvement compared to standard twist drills, resulting in shorter chips. Both inserts are coated with TiAlN coating. Barrero et al. [
26] proposed a post-coating method of droplet elimination via drag grinding to enhance the performance of coated inserts. According to them, droplets on the coatings obstruct the sliding of the chip over the tool surface and cause abrasive wear. Removing such droplets enhanced the performance in drilling. However, the scope of this work is limited to the evaluation of novel carbide inserts in drilling; therefore, inserts were evaluated as received from the manufacturer. This aspect will be considered for future work, and the topography of coatings will be taken with an optical profilometer to analyze the droplets’ effects.
The chemical composition of Inconel 718 was in line with the standard, see
Table 1.
Unlike a conventional drill, the central inserts are also provided with the chip breaker, which offers better chip breakability/chip evacuation and smooth shearing that leads to balanced drilling forces, see
Figure 2. As only one wiper insert is provided at the outer periphery along with the central insert, the question of unbalanced radial forces arises. This balance is provided by the uniform distribution of weight provided in the tool holder. Otherwise, machining with unbalanced radial forces can cause severe vibrations and affects the hole integrity. The details of the drills used are shown in
Table 2.
Makino v33i, a vertical machining center with 3 axes and a maximum RPM of 20,000 RPM and 20 HP, was utilized to drill Inconel 718. The setup for conducting the experiments when drilling is shown in
Figure 3, with a workpiece and inserts mounted on the tool holder. The cutting fluid was directed via a two-nozzle interface between the tool and workpiece with a flow rate of 20 L/min to remove excessive heat generated during the process. At first, an attempt was made to use dry drilling with the new drill. However, dry drilling resulted in excessive tool wear with catastrophic fractures of both central and peripheral inserts, even at the lowest operating parameters (see
Figure 4). This indicated the inaptness of the dry drilling operation for Inconel 718. This observation was in line with the previously reported literature [
2,
7], highlighting the significance of flood coolant. Drilling, turning, and milling of Inconel 718 is mostly conducted in the presence of cutting fluids, as surface integrity is of utmost concern during machining [
11,
27,
28]. Consequently, drilling was conducted in flooded conditions later on.
Table 3 shows the levels of input variables. Nine experiments were conducted in the flooded condition by employing a full factorial experimental design, as shown in
Table 4. Cutting speed and feed rate at three levels were used for experimentation. The Vc and
f levels were carefully selected from the literature [
4,
7,
17]. For instance, a maximum Vc of 45 m/min was reported in the studies [
18,
19] for drilling the nickel-based superalloy; however, in general, cutting speeds were within 15–30 m/min [
2,
3,
8]. Since productivity was the prime concern, a 45 m/min cutting speed was also considered in the current experimentation. The lower value of Vc at 25 m/min was to check the benefit of these inserts in terms of maximum tool life. Another objective of this work was to omit the post-drilling finishing operation (e.g., reaming) and make the drilling process a one-step operation, while keeping the
f within the range used in the previous studies [
3,
29].
Since these inserts are novel, limited inserts were supplied by the manufacturer. Even then, it was planned to replicate all experiments to ensure conformity and confidence in the results. Due to the fracture and breakage of the inserts during dry drilling, four experiments involving Test 2, Test 3, Test 4, and Test 8 were replicated. The results of these replications were within 10% of each other. Each experiment was conducted until the tool life reached the maximum flank wear criterion of 200 µm, which was set both for central and wiper inserts. If either central or wiper inserts reached the set criterion of flank wear, the test was terminated. Tool wear was measured via a coordinate measuring machine (CMM), supplied with a camera of a magnification of 70×, with a PC running the Quadra Check (QC)-5000 software. For a spindle runout of ≤5 µm, burr detection was carried out using a CMM machine with a probe at the entry and exit of the last hole of all experiments. The burr formation is quite critical in drilling as it affects dimensional accuracy, jamming, and misalignment problem, as suggested by Pena et al. [
30]. They emphasized the importance of chisel edge and chip evacuation via larger channels to avoid burr formation. They used spindle signals to detect burr formation, which was further processed via an algorithm for calculating the magnitude. The drilled holes were sliced into two halves by wire EDM to observe the surface roughness, hole surface, and sub-surface damage. Before the start of any experiment, tool images were captured to check the conformity of the edges. A portable roughness tester was used to measure the surface roughness at a cut-off length of 0.8 mm and evaluation length of 4 mm. Four random locations were selected for measuring the surface roughness of drilled holes, and their average values were reported in this study. Surface and sub-surface damage and cross-sectional images were captured via an optical microscope and a scanning electron microscope SEM (Joel 6060). Samples were prepared and etched as per standard operating practices used for Inconel 718. A summary of the research methodology used in this work is summarized in
Figure 5.