*3.4. Sustainable Machining Techniques*

As in the machining process, the heat produced is a major problem, and it can incur economic and technical costs either directly or indirectly [68]. Taylor [69] in the early 1900s pointed out that heat generated in the cutting zone plays a significant role in the cutting process. MWFs were used to address this, which imposes a serious challenge to the environment and the machine operators as discussed above in detail. In order to minimize the use and side effects of MWFs, several potential methods are available, including dry machining, machining with minimum quantity lubrication, machining with high-pressure jet assistance, and machining with alternative fluids such as gas, vapor, and solid lubricants.

#### 3.4.1. Dry Machining

As the name suggests, dry machining does not make use of conventional cutting fluids during the machining process. This process is only accepted by companies if it makes sure that the quality of the product is better or at least the same as when cutting fluids are used [70]. Several techniques are adopted to improve the dry machining process, such as the tool material and tool coating. In terms of the tool material, it is important to optimize the flute width, number of flutes, and margin size to have an extended tool life. Since cutting fluids are absent in this process, different methodologies are adopted to achieve the desirable finish of the workpiece, of which include the use of diamond-like carbon (DLC) coatings on the surface of tools, among others. In an investigation by Fukui et al. [71], the tribological behavior and performance of the DLC-coated tools working on the aluminum alloy workpiece were assessed. They reported that the DLC coatings on the surface of the tool resulted in improved tool life when compared with uncoated tools during the dry machining process. The comparison of the surface roughness in both cases is shown in Figure 8. It can be seen from the figure that when a DLC coating is applied, the surface roughness is lower.

**Figure 8.** The variation of the surface roughness with along the cutting length [71].

Klocke and Eisenblätter [72] carried out several investigations to implement the dry machining process in the production of cast iron, steel, aluminum, and some other materials. They reported that for the case of uncoated tools, some unwanted built-up edges were formed, and that the surface quality was also disturbed. The improvements in the dry machining process were further discussed by Sreejith and Ngoi [73]. According to their study, the dry machining process cannot match the wet machining process in many aspects, and it is only acceptable if the surface finish and other desired properties are equivalent to that obtained by the wet machining process; the authors stated that if it was to be employed, several improvements were necessary. A new system was proposed by Vereschaka et al. [74] in which the cutting tool was coated with a multi-layered, nano-scale coating, along with an ionized gas dispensing system and exciting system. They reported an improved performance in terms of the surface finish when cutting titanium alloys, steel, and nickelbased alloys. In a study by Devillez et al. [75] concerning the successful implementation of dry machining processes for Inconel®718 using a coated carbide tool, they reported that a reasonable surface finish and micro-hardness was observed, and that the values were comparable to the ones obtained in the flooded conditions. Additionally, no severe changes

were observed in the microstructure. The authors had merged different cutting techniques to reduce the cutting forces and surface roughness.

From several studies on the dry machining process, it can be inferred that although many researchers have reported successful implementations of the dry machining process for different materials such as cast iron, steel, and aluminum, etc., major technological improvements are still necessary in order to minimize the cutting forces and cutting temperatures. Furthermore, improved methods are still needed to flush out the chips that are formed during the machining process. Researchers have also reported the improved performance of coated tools such as coated carbide tools and DLC-coated tools when it comes to the surface roughness, but a high tool wear rate was still reported by many researchers.

#### 3.4.2. High Pressure Coolant Technique

Out of the several techniques to increase the machining efficiency, one of the techniques is the high-pressure coolant technique [76]. High-pressure coolant refers to the pumping of coolant at pressures exceeding 300 psi. In general, the pressures are in the range of 1000 psi. In some ultra-high-pressure coolants, the pressure reaches up to 3000 psi and therefore, solely depend on the requirements of surface being machined. There are several advantages to using of the high-pressure coolant technique, such as optimal chip control, which is accomplished by virtue of the coolant at high pressure breaking the chips into smaller pieces, preventing the chips from wrapping around the workpiece and chuck. High-pressure coolant evacuates the chips from the work area before the cutting tool gets into contact with them, therefore resulting in a better surface finish. Because of the above two benefits, the high-pressure coolant technique allows machine operators to work at increased feed rates, resulting in faster cycle times. Dahlman and Escursell [77] reported in their study that when the high-pressure coolant technique was applied, the chip control and reduction in the amount of built-up edges considerably improved during the turning process of decarburized steel. They also reported that the surface roughness was reduced as much as 80%, and that the tool wear was significantly reduced, of which the tools were prone to high temperature cracking. Ezugwu et al. [78] investigated the high-pressure coolant technique for the machining of hard metal alloys, such as Inconel 718, AISI 1045, and Tie6Ale4V steel; they also used different tool materials, such as cubic boron nitride (cBN) and TiAlN-coated carbide tools. They reported that by increasing the supply pressure of the coolant, the cooling and lubrication conditions were enhanced, along with a reduction of the cutting forces. This also resulted in the improved separation of chips and improved the surface roughness values. Kramar et al. [79] experimentally investigated different machining techniques, including the dry machining, conventional flooded machining, and the high-pressure cooling techniques for performing turning operations on piston rods which were already surface-hardened. They reported that out of all of techniques, the high-pressure cooling technique showed promising results, as the chip deformation was enhanced and the fluid consumption was reduced. However, the only shortcoming of the high-pressure coolant technique as reported by them was in its inability to reduce the depth of cut notches. The graphical interpretation of their results is shown in Figure 9. Pusavec et al. [22] conducted an experimental investigation on the cost analysis of the high-pressure cooling, conventional flood machining, and cryogenic machining techniques. They reported that the high-pressure cooling technique was 30% less costly compared to the other two techniques. Ayed et al. [80] experimentally investigated the tool deterioration and wear patterns on uncoated WC inserts, employing the conventional flooded machining and high-pressure water-jet-assisted machining. They reported promising results when compared with flooded machining with respect to the plastic deformation and flank wear of the cutting tool. They also reported a drawback of this technique, which was in its inability to reduce abrasion and adhesion wear, which led to notch wear. In a study by da Silva et al. [81], they studied the effect of the high-pressure coolant technique while machining a Ti-6Al-4V alloy with a polycrystalline diamond under high-speed conditions.

They reported that by increasing the fluid pressure, the tool life increased, and the adhesion was considerably reduced, specifically at 20.3 MPa.

**Figure 9.** The effect of coolant pressure on (**a**) the feed force, and (**b**) the radial force [79].

3.4.3. Minimum Quantity Lubrication (MQL)

As has been discussed, efforts are being made by many researchers to achieve manufacturing goals that are eco-friendly in nature due to the polices regulated by governments for preventing pollution globally, with the long-term aspects of the environment in mind [82]. There are many examples of such countries, including the USA, EU, China, and Malaysia, in particular which is clearly shown by Figure 10 that the number of publications are increasing gradually on yearly basis. [83]. As machining is one of the main processes in manufacturing sectors, it is therefore considered to have a significant process and important role to play in the context of green metalworking and sustainability, as it has a direct impact over the cost, life, and performance quality of so many components [84–86]. Therefore, minimum quantity lubrication (MQL) is one of the highlighted techniques that is playing a key role in sustainable machining in the last two decades, and the research needs to work more on this process to make manufacturing environmentally friendly as per the demand of industrial sectors.

**Figure 10.** Research articles published about the advancements of MQL from 2014 to 2019 [83].

There are many researchers that are in agreement with the argument that MQL has the potential to replace the conventional methods of flooding which are used for machining processes i.e., grinding, milling, drilling, and turning [87–90]. Najiha et al. [91] reported that the MQL technique is one of the practical ways for a green manufacturing process, as it is one of the most cost-efficient techniques and also guarantees both sustainability and worker health. This claim has also been supported by many other scientists and researchers who believe that the minimum quantity of cutting fluid should be consumed in this way [88,92,93]. All of these studies depict that the MQL technique has importance in the emerging efficient and eco-friendly manufacturing techniques of the modern era. It can be seen in Figure 11 that around 7% to 17% of the cost of the manufacturing process

constitutes cutting fluid, and if replaced by minimum quantity lubrication, it will save a substantial amount of budget. Hence, a substantial amount could be saved by switching the conventional methods with the MQL technique in industries to reduce budget costs.

**Figure 11.** The quantitative distribution of manufacturing costs in industries [94].

Khan and Dhar [95] studied the benefits of using vegetable-based oil in manufacturing instead of cutting fluids, as they are very good pressure absorbents, have the ability to accelerate the material removal rate (MRR), and provide very minimum loss due to vaporization, misting, among other reasons. Moreover, many other researches have supported these advantages of MQL, especially in studies focusing on milling, drilling, and turning [19,96–98]. Dixit et al. [99] observed that synthetic oils are also very effective for machining and have similar properties to vegetable oil-based MWFs, having high boiling temperatures, low viscosities and better flash points. Moreover, some studies have revealed that synthetic oil machining is far better than both vegetable- and mineral-based oils [100].

Commercially, the MQL technique comprises five major parts, which are the cutting fluid tank, air compressor, flow control system, tubes, and spray nozzle [90]. It generally uses an atomizing method and a minimum amount of spraying, composed of an oil mixture and pressurized air sent at a flow rate below 1000 mL/h, and it directly sprays the mixture into cutting zone as has been described in many studies [101–103]. This consumes 10,000 times less cutting fluid volume as compared to the flooding technique. Furthermore, the MQL system is categorized into internal and external applications, as shown in Figure 12.

**Figure 12.** Minimum quantity lubrication delivery system categorizations [95].

All of these review studies indicate that the use of both vegetable oil-based MWFs and synthetic esters are safe to use for machining in place of conventional techniques and cutting fluids, as they are non-toxic and sustainable; MQL is a more feasible choice for machining applications due to being risk-free for the health of workers and environment.
