Titanium and its alloys have a low machinability index and require relatively lower cutting speeds (i.e., <90 m/min) due to their low thermal conductivity, low Young’s modulus, strong chemical reactivity, and high hardness and dynamic shear strength at elevated temperatures (>500 °C) [
1,
2,
3]. However, pushing cutting conditions boundaries in machining titanium is required for faster manufacturing [
4]. Owing to low thermal conductivity such as 7.3 W/m⋅K for annealed Ti-6Al-4V, cutting titanium generates a large amount of heat close to the machining zone. The high working hardening affinity of titanium alloys can also promote high cutting forces and temperatures, leading to tool notching and excessive tool wear [
5]. Without a cutting fluid, titanium alloys are more prone to forming oxides in atmospheric environments, which can also negatively affect their mechanical properties, causing embrittlement and reduced alloy fatigue strength [
6,
7,
8]. Mineral oil-based, semi-synthetic and synthetic cutting fluids are traditionally employed due to their chemical stability. Often these fluids are blended with some additives and colloidal suspensions (e.g., nanoparticles, graphene and/or graphene oxide) to improve their lubrication and cooling efficiency [
9,
10,
11]. However, conventional cutting fluids are susceptible to microbial contamination due to the high content of toxic substances such as hydrocarbons, chemical agents (e.g., biocides), and Extreme Pressure (EP) additives which have adverse effects on the environment and human health (e.g., lung disorders, dermatitis, and cancers) [
12,
13,
14,
15,
16]. Vegetable Oils (VOs)-based cutting fluids are superior alternatives due to their high biodegradability [
17] and the distinctive chemical structure of the vegetable oil’s molecules, which may be heavy, long, and dipolar in nature. Uniformity and super-density are other unique properties of vegetable oil molecules, which afford a thick, tenacious and vigorous film layer that offers VOs a superior ability to absorb contact pressure [
18,
19,
20,
21,
22]. VOs base stocks have high thermal conductivities of up to 0.172 W/m⋅K and a low coefficient of friction (e.g., 0.03 for soybean oil) compared to 0.125 W/m⋅K and 0.07, respectively, for mineral oils [
23,
24,
25]. An adequate understanding of cutting fluid application methods in machining operations may significantly enhance heat dissipation and thus improve the surface quality of machined parts. Several cooling supply methods were introduced to control the temperature in the cutting zone to improve productivity and increase the overall performance of machining processes. Six main cooling strategies are used in machining operations [
26,
27]. Conventional flood/wet cooling is a common supply method used in machine shop floors. This method provides a steady-state stream of fluid to the machining zone with flow rates ranging from 10 L/min for single point tools to 225 L/min for multiple tool cutters [
28]. Flood cooling was evaluated against minimum quantity lubrication (MQL) and dry cutting while turning hardness steel (EN-31). A reduction (about 57.14%) in the chip–tool interface temperature was obtained when flood cooling was used [
29]. However, high cutting fluid consumption and low fluid penetration, particularly at higher cutting speeds, are the main drawbacks [
30]. High pressure cooling (HPC) was also introduced where cutting fluids are applied at high pressure up to 200 bars through customised nozzles to provide a powerful jet of fluid into the machining zone. The cooling performance of HPC was compared with flood cooling during cutting Inconel 718 using the SiAlON cutting tool. It was found that HPC was immensely helpful to extend tool life and improve chip breaking [
31]. However, the high cost of pumping systems and equipment for micro-particle filtering (i.e., <20 µm) are the main disadvantages [
30,
32]. Minimum quantity lubrication (MQL), compressed air, oil mist and cryogenic cooling were also considered as alternative cooling supply techniques to reduce the amount of cutting fluid delivered. MQL was found a useful lubricating method to enhance surface quality and tool life, while cryogenic cooling was found very effective in reducing tool wear and lengthening tool life. However, high costs associated with these cutting fluid supply systems are the principal limitation [
33,
34,
35,
36,
37].
Additionally, nozzle position plays a vital role in machining operations and considerably affects tool life and cutting performance. Lopez et al. [
38] studied the influence of nozzle positions of 45° and 135° in relation to feed direction when milling Al5083 using the MQL cooling supply method. The results showed that the nozzle position at 135° achieved lower tool flank wear of 0.098 and 0.095 mm at 0.04 and 0.06 ml/min cooling rates, respectively. The 45° nozzle orientation showed inferior flank wear values of 0.14 and 0.12 mm at 0.04 and 0.06 ml/min, respectively, after a 158 m cutting length. It was found that nozzle placement is crucial in order to obtain the optimum effect of MQL cooling. Similarly, the 45° nozzle angle assisted in increasing tool life 9.25% compared with 90° nozzle angle when end-milling aged Inconel 718 under MQL+CO
2 cooling using laval coolant nozzles [
39]. A recent study [
40] has concluded that a 12.5° orientation angle increased tool life by 50% with average flank wear below 0.3 mm compared to that of 45° when high-speed milling of H13 steel using the MQL supply system. Supplying cutting fluid with and against feed direction has also been investigated [
41] when micro-milling Ti-6Al-4V using MQL and jet application cooling. Tool wear values were found to be about 1.6% less with feed direction in MQL compared to 6.15% with jet cooling. Liu et al. [
42] found that an MQL spraying nozzle position of 135° and a spraying distance of 25 mm helped to reduce cutting temperature by 2.5 and 20 °C compared to 90° nozzle angle and 45 mm spraying distance, respectively. Another recent work by [
43] has investigated the effect of a new sensor-based cutting fluid supply system on machined surface quality using minimum quantity fluid MQF cooling. The new system was evaluated against a conventional flood, high quantity flood cooling (HQF at 36 L/hr) and dry cutting conditions. The trials were performed on AISI 1045 steel utilising chemical vapour deposition (CVD) coated cutting tools and semi-synthetic cutting fluid (MicroSol 585 XT). Tests were carried out at a cutting speed of 225 m/min, a feed rate of 0.1 mm/rev and depth of cut of 1.2 mm. The results showed that lower surface roughness (Ra) value was obtained using MQF mode (0.488 µm when targeting the fluid on the tool flank and rake face) while traditional flood, HQF, and dry cutting conditions produced Ra values of 0.637, 0.689 and 0.720 µm, respectively. To date, most attention has been paid to the less fluid consumption supply systems, particularly MQL in trying to reduce cutting fluid quantity in the machining operations. Regardless of the cost of this supply system and installation, this method was considered as a lubricating method rather than cooling. This inferior cooling capacity confines the potency of MQL, especially in machining of refractory materials such as titanium and nickel alloys where the heat dissipation is paramount [
35]. Additionally, the issue of random estimation of the fluid flow rate during the cutting process under all previously mentioned supply methods opened a new avenue for developing a cost-effective and efficient (i.e., high cooling ability with minimum fluid waste) supply system. The developed system (a controlled cutting fluid impinging supply system, Cut-list) was designed to provide an accurate quantity of cutting fluid based on accurate heat generation calculation via well-targeted and controlled bespoke coherent nozzles.
The novelty of the proposed supply system relies on synchronisation between the calculated generated heat in machining zone with the exact required cutting fluid quantity to reduce its consumption and at the same time improving the machinability of titanium alloys. Additionally, the developed system can be integrated into existing machine tools, making it immensely attractive in less fluid consumption machining applications without the need to purchase a new machine tool.
Thus, the aim of the present work is to evaluate the performance of Cut-list with bench marking against a conventional flood supply system. Both systems were tested at similar machining conditions and key process indicators assessed include cutting force, workpiece temperature, tool flank wear, burr formation and surface finish. Both systems were employed during shoulder milling of Ti-6Al-4V using a vegetable oil-based cutting fluid.