For aircraft production, the drilling operation is very important as almost 100,000 holes have been drilled in a single engine craft [4] that is almost 40–60% of the material removal process [5]. Therefore, drilling of Ti-6Al-4V alloys requires a profound understanding of the material's characteristics and behavior under machining conditions. Significant research is dedicated to study the essential considerations for successful drilling, including cooling strategies, drilling strategy, tool selection, and cutting parameters [6–11].
In the context of machining, dry drilling is considered an eco-friendly alternative, but it faces challenges when working with low thermal conductivity materials like titanium alloys, leading to high temperatures and tool failure [6–7]. To combat this, the traditional method of flood cooling, employing oil-based systems, has long been used to ease the machining of tough materials like titanium [12]. Cutting fluids are critical in flood cooling, reducing temperatures and friction, enhancing tool life and workpiece quality [13–15]. However, mineral-based cutting fluids pose environmental and safety concerns, driving the manufacturing community's shift towards biodegradable vegetable-based oils as a more sustainable choice [16–18]. With the detrimental impact of conventional cutting fluids on hole surface quality and environmental concerns in mind, the MQL (minimum quantity lubrication) method has emerged as an eco-efficient, cost-effective alternative. MQL employs a minimal amount of cutting fluid, typically a mixture of oil and air, which is directed precisely onto the tool's cutting edge. While MQL has been effective in reducing cutting forces by approximately 6.5% [19], it still faces limitations when dealing with hard-to-machine materials [20]. To address this, researchers have explored cryogenic strategies involving liquid nitrogen and liquid carbon dioxide, comparing their results with traditional flood coolant and MQL. This investigation has revealed a notable reduction in cutting forces, with decreases of 9% and 10%, respectively [2]. Additionally, the use of graphene oxide as a cutting fluid has demonstrated a remarkable 17.21% reduction in cutting force compared to traditional fluids [21]. In a separate study, researchers have successfully reduced burr height by employing cryogenic oil, while the use of boron oil-based MQL has shown promise in improving the surface quality of holes [22]. Furthermore, the integration of MQL with MoS4 has proven effective in enhancing surface integrity, irrespective of the tool's geometry [23]. When considering factors such as surface roughness (SR), cutting temperature, cutting force, tool wear, chip formation, surface morphology, and hardness, the research indicates that the most optimal combination is MQL with a mixture of 20 percent MoS2 and 80 percent cotton seed oil [24].
In the realm of drilling strategies, it has been observed that step drilling significantly extends tool life, offering a threefold increase compared to continuous drilling [25]. However, it's worth noting that step drilling can introduce challenges related to chip adhesion and the formation of a build-up edge (BUE), adversely affecting hole surface quality, including aspects like surface roughness, accuracy, and roundness. When dealing with the continuous drilling of titanium alloys, several critical issues surface, including burr formation, the development of a build-up edge, and the diffusion of chips. These issues are primarily attributed to the material's high chemical reactivity, which is further exacerbated by the elevated drilling temperatures [26–27].
In terms of tool selection for hard material machining, indexable drilling and solid-type twist drills are two distinctive cutting tool options commonly employed in the field. Indexable drills, with their replaceable inserts featuring multiple cutting edges, offer versatility and cost-effectiveness for a wide spectrum of materials. They are distinguished by their precision and effective chip evacuation mechanisms, attributes arising from the insert and body design. Consequently, indexable drills are well-suited for high-speed and high-feed drilling applications, surpassing solid drills in this regard [28–30]. Additionally, the cost factor favors indexable drilling as it eliminates the need for regrinding, recoating, and maintaining spare tools, rendering it a more economical choice [31]. Notably, when dealing with challenging materials like Inconel 718, known for its toughness, indexable drills have demonstrated remarkable performance at a feed rate of 0.08mm/rev, successfully mitigating issues such as surface defects and chip adhesion [32].
Non-conventional machining methods provide effective solutions to the limitations of conventional techniques. Laser drilling, utilizing a CO2-based laser, excels in micro-hole machining, ensuring high-quality results [33]. Process variables like flushing pressure, laser power, and pulse frequency influence characteristics such as taper, spatter area, and heat-affected zone. An influential parameter revealed by a study is laser power [34]. Additionally, another study emphasizes that the choice of pulse width, pulse frequency, and trepanning speed can regulate cutting temperature and hole quality [35]. However, it's crucial to note that laser drilling involves high radiation and temperature levels, leading to concerns about recast layer formation and the heat-affected zone. Therefore, operating the equipment requires special care. Electric-discharge machining (EDM) has been explored as an approach for creating holes in titanium alloys, employing combinations of graphite and CuW electrodes. The findings indicate a notable material removal rate of 22.5 × 103 mm3/min with a hole depth of 0.5 mm when using a graphite electrode, whereas CuW electrodes yielded a rate of 4 × 103 mm3/min [36]. However, a key challenge associated with EDM is the effective removal of debris, particularly in deep hole drilling applications. Furthermore, the workpiece material must possess conductivity for EDM to be viable [37]. Despite the advantages of EDM, such as precision, the high operational costs and safety concerns associated with non-conventional machining methods limit their utilization in the drilling of titanium alloys. The literature underscores the need to explore alternative approaches to machining titanium alloys that are cost-effective and less environmentally critical or hazardous compared to non-conventional methods.
An evident research gap comes to light within the domain of machining titanium alloys when considering the existing literature and the objectives of the present research. Previous investigations have indeed delved into the use of diverse cutting strategies and tool types for boring holes in titanium; however, there remains a dearth of systematic exploration concerning the performance of an indexable insert drill under various cooling methodologies and distinct feed rates.. It is imperative to bridge this research gap as it is crucial for achieving a comprehensive grasp of machining process for titanium alloys. This endeavor not only stands to boost productivity but also holds the potential to curtail operational expenses. Ultimately, the benefits of this research labor have the capacity to significantly advance the development of more effective and efficient machining techniques, especially pertinent in industries like aerospace, automotive, and medical, where the application of titanium alloys is prevalent.
Section 2 of the paper will provide an in-depth exposition of the materials and methodologies employed in this research. Subsequently, Section 3 will conduct a thorough examination of the findings and engage in a comprehensive discussion. Finally, the paper will reach its conclusion in Section 4, drawing overarching insights and offering recommendations for prospective research endeavors.