Current Status of Hard Turning in Manufacturing: Aspects of Cooling Strategy and Sustainability

Abstract

In recent years, hard turning has been found to be a well-known substitute for traditional grinding for acquiring the finish quality of hardened steel without sacrificing productivity. There are many issues that should be carefully understood and dealt with to attain efficacious performance in hard turning. This article discusses modern manufacturing challenges with a focus on analyzing the current state of the art of the hard turning process in terms of ensuring more environmentally friendly manufacturing through the use of greener cooling methods such as dry, wet/flood cooling, the minimum quantity of lubricant (MQL), high-pressure jet cooling, solid lubricant, nanofluids, ionic liquids (ILs), cryogenic cooling, and hybrid cooling. Nanofluids combined with the MQL system were found to be the superior cooling technique in comparison to dry, wet/flood, and MQL. Cryo-machining also provided superior performance by limiting the cutting temperature during hard turning. The performance of hybrid cooling (MQL + cryogenic) seems to have been superior to MQL and cryogenic coolant alone because it combined the benefits of lubrication and cooling from MQL and cryogenic systems, respectively. The addition of ILs to base fluids or nanofluids improves the thermal properties of the mixed fluid, resulting in better surface quality, lower tool wear, and longer tool life. Furthermore, the purpose of this study is to summarize the various LCA software used for analyzing the sustainability of the hard turning process. Overall, this paper can serve as a resource for researchers and manufacturers working in the field of sustainable machining.

1. Introduction

Manufacturers all over the world are constantly looking for ways to reduce manufacturing costs in order to remain sustainable and competitive in the market. Metal machining is a value-added operation in which improper raw material size and shape are transformed into accurate dimensions and high-quality products through the use of cutting tools, machine tools, and the environment. Traditional manufacturing processes, such as casting, forming, forging, and so on, are incapable of producing the desired shape, size, accuracy, and finished product, necessitating the use of machining to meet the needs of both manufacturers and users. A large number of conventional machining methods are available, namely turning, milling, planning, grinding, boring, etc. Hardened steel workpiece machining is gaining high attention due to its several applications in the automobile, press-tool, die-mold, gear, bearing, and aeronautical industries, as displayed in Figure 1 [1]. Earlier, only the grinding process was used to machine hardened steel. For two decades, the application of the turning process for machining-harden steel has grown in popularity due to several challenges (low metal removal rate, low production rate, less flexibility, low setup and cycle time, and high cost-effectiveness) in the grinding process and the development of a new generation turning tool. Hard turning was a popular term for the process of turning hardened steel (45–68 HRC). Hard turning is a machining activity in which the cylindrical surface of hardened steel (45–68 HRC) is machined using single-point cutting tools [2,3,4,5].

The hard turning process was invented by the automotive industry to machine difficult-to-cut material on the shop floor for the production of various parts, including transmission shafts, roller bearings, crankshafts, crank pins, and various automotive parts. It also finds an application in the die and mold-making industries, as its job is to give shape to raw material (hardened steel) to convert it into a die or mold. Hard turning is widely used in the aerospace industry, as it is used to manufacture an aerospace engine part that needs maximum accuracy and superior quality finished products [2,5]. The application of hard turning in various sectors was mostly made possible due to the development of cutting tool materials and novel surface coating techniques. All of these applications of hard turning suggest it turning has successfully replaced the traditional grinding process. Hard turning is often termed “finish hard turning” due to its wide application in finishing operations while machining a component.

Nowadays, the hard turning process for heat-treated steels has favorably replaced cylindrical grinding due to its enormous benefits [2,5].

➢

Lower energy consumption

➢

Higher material removal rate

➢

Lower investment in machine tool

➢

Lower machining cost per piece

➢

Multiple machining in one setup

➢

Higher flexibility to accommodate complicated contour part

➢

Suitable for interrupting machining

➢

Minimum tool inventory

➢

Environment friendly

➢

Low residual stress

➢

Improvement in surface quality

➢

Higher dimensional and shape accuracy

➢

Advantageous for Process reliability

The hard turning has benefits in surface quality improvement compared to grinding processes due to implementation of a new generation of cutting tools and advanced cooling systems in hard turning. In general, surface roughness greater than 1.6 micron was established as a rejecting limit in grinding hard to cut metals [6,7]. According to Puerto [8], the range of surface roughness obtained in the grinding process varied from 0.1 micron to 2 microns. There are several research papers reported the surface roughness value within the mentioned range 0.1 microns to 2 microns or less than 1.6 microns. Ozel et al. [9] found surface roughness (0.259 and 0.38 microns) in hard turning with a depth of cut of 0.254 mm and a feed rate of 0.05–0.08 mm/rev. In hard turning AISI D2 steel using corn oil-MQL cooling, Arsene et al. [10] discovered roughness values in the range of 0.151–0.452 microns.

Choudhury and Chinchanikar [1] also showed the qualitative benefits of hard turning over grinding using a radar plot displayed in Figure 2. Apart from benefits, some limitations were also reported by researchers, which have been listed below [2]:

➢

The cost associated with tooling is considerably greater than grinding.

➢

Chatter is produced due to high cutting pressure in the turning of long and thin products.

➢

A highly rigid machine tool is needed for a higher degree of accuracy.

➢

Quality of produced surface and dimensional accuracy deteriorated with the tool wear growth even under the limiting criterion of tool life.

➢

Residual stress and white layer formation on the finished surface retard the performance of machining.

Cutting tools are crucial for achieving sustainable hard turning. The capabilities of CBN, PCBN, and ceramic tools for turning hardened steel are widely acknowledged. However, because these tools are more expensive, researchers are looking for a possible replacement in order to reduce the turning costs. In the last 20 years, a lot of research has been done to develop new tool materials and ceramic coatings on tool substrates to achieve superior surface finishes along with enormous productivity at a low cost in hard turning. To achieve the aforementioned goal, the tool material needs to have a number of desirable qualities, including sufficient hot hardness, increased mechanical strength, high fracture toughness, increased thermal shock resistance, increased abrasion resistance, increased resistance to adhesion/diffusion, and chemical inertness to prevent chemical interactions between the tool and the work material during machining. It is typically not feasible to obtain all these superior qualities in a single cutting tool material. The advancement of tool coating technology undoubtedly facilitates the highest percentage of these superior qualities, allowing it to provide the highest levels of productivity, performance, and economy in terms of machining. Multi-layered coatings of different ceramic materials, such as titanium carbide (TiC), titanium carbide nitride (TiCN), alumina (Al₂O₃), titanium nitrate (TiN), and titanium aluminum nitride (TiAlN), were used as coating materials for different cutting inserts. These coatings can facilitate increased temperature strength and greater wear resistance [3]. For the development of various coated cutting inserts, two tool coating techniques, PVD (physical vapour deposition) and CVD (chemical vapour deposition), are now widely used [3,4].

Hard turning in dry cutting surroundings is very advantageous due to the high amount of heat generated, which causes thermal softening of the work material and makes shear deformation easier. However, tool failure accelerates due to rubbing of the flank surface caused by heat and friction, reducing tool life and surface quality. According to the researchers, various types of tool wear occurred during the machining of hardened steels in dry environments [3,4,5], and these have been classified as crater wear, flank wear, nose wear, notch wear built-up-edge (BUE), plastic deformation, chipping, and catastrophic breakage [5], as shown in Figure 3. Crater wear is caused by increased cutting speed, cutting feed, and depth of cut, which degrade surface quality and reduce chip control, whereas tool flank and nose wear are caused by increased cutting speed and feed, which degrade surface quality and dimensional accuracy of the test piece. Notch wear occurred as a result of increased cutting speed and feed, which accelerated cutting forces and burr formation. Lower wear-resistant tool material was also a factor in the formation of tool crater wear, flank wear, nose wear, and notch wear. Chipping wear encourages abrasion, diffusion, and plastic deformation. Improper rigidity of work-tool holding devices, as well as tougher tools, contribute to the chipping phenomenon. A higher sum of heat generation in the cutting region as cutting speed, feed, and depth of cut increase also accelerates cutting tool tip plastic deformation, cutting edge breakage, and surface degradation. Due to adhesion, built-up-edge (BUE) developed on the cutting edge, affecting surface quality and increasing tool wear, cutting force variations, and vibration. Catastrophic failure results in an abrupt increase in cutting forces as well as a degradation in surface quality. Furthermore, at higher speeds, the cutting temperature exceeds, resulting in increased cutting force, poor surface finish, unfavorable residual stresses, and microstructure [5]. As a result of these issues, dry, hard turning may be impractical for achieving the desired machinability. In order to improve machinability in hard turning, proper lubrication and cooling medium are required. The primary functions of cutting fluid are to cool the workpiece and insert it, to remove chips from the cutting zone, and to provide lubrication at rubbing contact surfaces to reduce friction.

Flood/wet cooling is completely unsuitable for hard machining work because it is unsustainable and adds an extra financial burden to the machinist for the disposal and upkeep of the used fluids. In fact, statistics showed that in the United States, about 155 million gallons of used coolants were released into the open environment [11]. Cutting fluids’ airborne particles negatively affect a number of chronic human health conditions, including asthma, allergic reactions, skin rashes, and dermatological issues [11]. Furthermore, cleaning cutting fluids adds time and expense and has a negative impact on the environment. As a result, excessive use of flood lubrication should be avoided or reduced. Low-viscosity, water-based cutting fluids may therefore be an appropriate technique for simplicity in cleaning. Consequently, there is a constant search for alternative solutions that reduce environmental problems.

Environmentally conscious machining, also known as “green cutting technology”, is a newer concept of the twenty-first century that arose as a result of stringent environmental regulations. It includes dry machining, mist cooling, near-dry machining, or machining with minimum cutting fluid use, vegetable-oil-based cutting fluids/lubrication, compressed air cooling, nanofluid, cryogenic cooling, and hybrid cooling. Implementation of MQL (minimum quantity lubrication) demonstrated an improvement in tool life and surface finish by minimizing heat generation in the cutting zone, thereby reducing environmental issues and costs [5,12,13,14]. In MQL, a small amount of coolant was sprayed over the turning zone, saving money on coolant and eliminating the need to dispose of used coolant. Another preferable solution for cooling in hard machining is a high-pressure cooling technique, which produces segmented chips, lower cutting forces, longer insert life, and satisfactory machined surface quality [5,12,14,15]. Spray cooling (SC) is another green cooling technology used in hard steel machining [16,17,18]. In this cooling strategy, compressed air is mixed with water in a mixing nozzle before being sprayed over the cutting region [19,20,21]. This high-pressure spray cooling not only lowers the temperature and cutting forces, but it also lowers the need for cutting fluids. It is regarded as a developing technology in cutting fluid applications for reducing friction at the chip-tool interface, controlling temperature, improving heat transfer, and improving machining performance. In recent years, the use of nanoparticles with basic coolant/lubricant has been highly praised because it helps to improve the heat transfer coefficient of basic coolants, resulting in significant reductions in cutting temperature, wear, tool life, machining forces, surface roughness, power consumption, and specific cutting forces [14,22,23]. Cryogenic cooling is another emerging technique for hard turning applications, where tool life improves due to better coolant penetration into a narrow gap at the chip-tool interface [5,12,14]. Moreover, solid lubricants and vegetable oils were also widely used as coolants in hard cutting [5,12,24,25,26]. Ionic liquids added to MQL or nanofluid MQL provide superior hard machining performance [27]. Overall, it can be observed that a number of green coolants have been successfully used in hard machining to date. Therefore, choosing the best sustainable cooling strategy is crucial for achieving better results in hard turning.

Nowadays, industries are keenly interested in using sustainable machining for manufacturing their products. Sustainability can be evaluated considering three aspects such as social, environmental, and economical. It can be evaluated qualitatively using the Pugh matrix method, and quantitatively analyzed using life cycle assessment (LCA).

The proposed work provided a detailed review of hard turning under dry conditions, nanofluid-MQL, cryogenic, and hybrid cooling. Moreover, the specifics of various ionic liquids used as cutting coolant additives in machining were summarized and discussed. Furthermore, the various sustainability assessment methods for machining applications are summarized. A list of LCA software used in machining was reported, which had not previously been presented in any review work. This paper will help you choose the best coolant strategy for machining hard materials. With the help of this research, learners can adopt the most appropriate life cycle assessment software (LCA) to quantitatively analyze the sustainability of the machining research.

2. Hard Turning under Dry Condition

Hard machining under dry conditions has been very popular for the past two decades due to the development of various new cutting tool materials that have successfully replaced cylindrical grinding. The following sections provide an overview of the performance of various hard machining cutting tools.

2.1. Performance of CBN/PCBN Cutting Tool

CBN/PCBN cutting tools have been extensively used in hard machining due to their higher hardness, higher micro-hardness, and higher capability to resist abrasion. According to Qian and Hossan [28], CBN and PCBN tools were highly applicable in the machining of heat-treated steel because of their high resistance capability against abrasive wear, high hardness, and resistance against chemical diffusion at elevated temperatures. Kishawy and Elbestawi [29] used a PCBN cutting tool to observe various types of surface defects, such as material side flow, micro-cracks, and cavities, on the finished surface of D2 steel. The various modes of wear, namely crater wear, flank wear, notch wear, and chipping of the cutting edge, were perceived in hard machining. Shalaby et al. [30] investigated the performance of hardened AISI D2 steel using PCBN, coated PCBN, and ceramic cutting tools. The turning force components were measured, and the comparative results for each tool material are shown in Figure 4. The radial component force (Fy) was the greatest, followed by the cutting (Fz) and axial (Fx) forces. The ceramic tool produced the lowest values of the three cutting force elements, followed by the PCBN, while the coated PCBN tool material produced the biggest force components. Moreover, abrasive marks were noted on the flank wear for both uncoated and coated PCBN tools because the tool’s flank edge rubbed against highly hard components (such as chromium and carbide) present in the work material. The faster cutting speeds enabled a consistent plastic flow of the work material on the tool face, resulting in continuous and tangled chips as allowed for in Figure 5. The ceramic tool produced the smoothest chip underside, shown in Figure 5, suggesting the least amount of chip adherence to the tool face. PCBN was next, and the coated PCBN had the most chip adhesion to the tool face.

Dosbaeva et al. [31] noticed that in high precision machining when high dimensional accuracy was demanded, the PCBN cutting tool can be the superior choice over the carbide tool because of its lower rate of wear progress in the initial stages of cutting tool wear under turning of AISI D2 steel. Kishawy [32] discovered that cutting temperature increased as a result of both leading speed and feed, but cutting speed had a greater impact on temperature increment than feed. In addition, the temperature rose as the tool wear rate increased during the turning of a hardened AISI D2 work-piece with a PCBN insert. Arsecularatne et al. [33] found that the tool life was significantly improved in the machining of AISI D2 steel with a PCBN cutting tool at the lowest cutting speed. Abrasion and grooving were the main forms of wear found in hard AISI D2 steel machining. Ozturk et al. [34] conducted an experiment on AISI 4140 steel with three different micro-textured tools. It was concluded that texturing inserts were not suitable for machining AISI 4140 steel in a dry condition.

Tang et al. [35] stated that the cutting tool temperature was improving with cutting time in turning AISI D2 work specimens with CBN inserts. Kundrák et al. [36] discovered an increase in CBN tool wear in hard turning when the cutting speed exceeds 150 m/min. When the cutting speed was varied between 120 and 150 m/min, the tool wear was almost identical. Kumar and Chauhan [37] utilized the CBN cutting insert in turning AISI H13 grade steel. The hardness of the test piece, speed, and feed all had a significant impact on machining responses such as surface quality, tool-tip temperature, and cutting force. Oh [38] investigated the relationship between cutting forces, tool abrasiveness, and the quality of the turned surface obtained when turning SKD11 steel with a CBN insert. It was proposed that a low cutting speed with a high transfer ratio was most effective for removing a large amount of metal. Bouacha et al. [39] studied the influence of cutting parameters (cutting speed Vc, feed rate f, depth of cut ap, and cutting time t) on flank wear (VB), surface roughness (Ra), feed force (Fa), tangential-cutting force (Fc), thrust force (Fp), and material removal rate (MRR) as displayed in Figure 6. The flank wear was greatly amended by Vc and t, while the roughness was principally prejudiced by t, f, and Vc. Depth of cut was the most crucial term towards affecting all cutting forces. Similarly, MRR was greatly improved by t, followed by Vc, f, and ap.

Grzesik and Żak [40] applied hard turning and ball burnishing as two sequential finishing operations while machining 41 Cr4 steel with a CBN cutting tool. Surface roughness was greatly influenced by feed during the turning process. In comparison to hard turning, the burnishing operation performed better and produced a higher quality surface. Bartarya and Choudhury [41,42] examined the significance of input machining variables on forces as well as on the roughness of the machined surface during the finish turning of an EN 31 work-specimen (60 ± 2 HRC) with an uncoated CBN insert. It was discovered that radial depth, followed by axial feed and speed, significantly influenced the roughness of the turned surface. Khamel et al. [43] examined the impact of machining input variables on tool life, cutting forces, and surface roughness in finish turning on prior heat-treated AISI 52100 work specimens with a CBN tool. Surface roughness was chiefly controlled by tool feed, followed by speed and radial depth of cutting. Rashid et al. [44] implemented a surface defect machining method to improve the quality of the surface. By applying this method, the cutting temperature and cutting forces were reduced, while a significant improvement in surface quality was noticed compared to traditional turning on prior AISI 4340 work-specimens utilizing the CBN cutting tool. Bouacha et al. [45] explained the nature of cutting forces obtained in conventional and hard turning processes with variations in cutting speed as displayed in Figure 7. The conventional turning domain was considered when job hardness was below 46 HRC. For this case, when the cutting speed was leading up to 120 m/min, all the components of forces were decreasing. Similarly, for hard turning, the cutting force components were leading with simultaneous increments in job hardness and cutting speed. In addition, when turning AISI 52100 work-specimens with the CBN tool, the surface quality decreased with increasing feed rate while increasing with increasing cutting speed.

Sahin [46] emphasized a comparison of tool life between ceramics and cubic boron nitride inserts using the Taguchi method. CBN cutting tools outperformed ceramic tools for undercutting AISI 52100 steel. Cutting speed had the greatest influence on wear at the insert surface, followed by insert hardness and rate of feed. Dureja et al. [47] characterized the wear mechanisms of inserts in turning an AISI H11 hard work-specimen with a coated (TiN) CBN cutting insert. Machining with lower speed and feed causes abrasion wear, whereas machining with high speed causes adhesive wear due to tribo-chemical action between tool and workpiece. The built-up edges were formed at a medium cutting speed. Ghani et al. [48] deliberated on the tool wear and heat partition on turning of an AISI H13 work-specimen with a CBN cutting insert and established that the wear on the tooltip was highly dependent on the intensity of the temperature evolved in cutting and the amount of heat conceded by the insert during high-speed turning. Higher temperatures reduce the bonding strength of the cutting tool, resulting in tooltip chipping or shearing-off failure. Due to the high speed, the contact area was reduced, which shifted the maximum temperature and stress zone to the cutting edge. However, greater crater wear and concentrated plastic deformation took place on the tool edge. More et al. [49] carried out the cost analysis during the turning process, which revealed the lower cost of machining with the CBN tool compared to PCBN and showed that it can be used as a substitute tool for PCBN in the turning of hardened AISI 4340 work-specimens. Morehead and Huang [50] observed segmental as well as continuous chips with saw tooth profiles in the machining of AISI 52100 steel with the CBN tool. The spacing between two adjacent saw teeth and the height of the segmental chip were reduced with tool wear growth, whereas for the continuous chip, it was increased with tool wear. Niaki et al. [51] investigated hardened AISI 4340 steel using CBN inserts with various cutting edges to find out the responses such as tool wear, cutting force, and surface roughness. It was observed that the stability of the CBN cutting was the initial factor in the determination of the tool’s life. CBN tools of grades A and B provided a 60% improvement in tool life and inferior tool wear in comparison to CBN tools grade C. Suarez et al. [52] performed a machining operation on hardened AISI D6 material with a PCBN tool under various cooling environments to determine the effects of tool wear, quality of the surface, tool life, and cutting force. According to reports, MQF + MoS₂ provides longer tool life than MQF alone. Figure 8 depicts the wear pattern of the PCBN tool in dry machining. Different-sized abrasion marks and micro-chipping on the tooltip were seen in Figure 8. At the beginning of the machining process, the tool edge radius encountered a micro-chipping structure. Further spreading in the direction of wear that was preferred caused a localized increase in machining force, resulting in large abrasive markings on the tooltip.

Rafighi et al. [53] executed experiments on the turning of AISI D2 hardened steel by implementing CBN and ceramic tools and discussed their comparative performances. CBN provided a superior surface quality over ceramic, but the cutting forces were higher on the side of the CBN tool. Nose radius and tool speed was traced to be noteworthy toward surface roughness for both tools. Kumar et al. [54] executed the turning trials on hardened AISI H13 by taking samples of different hardnesses using the CBN tool under different machining conditions. The obtained results revealed that the cutting forces and surface roughness were mostly exaggerated by the work material’s hardness. Higher work material hardness produced higher forces but displayed a superior surface finish. Bonafa et al. [55] examined the recital of MQL while turning hard AISI D6 tool steel with PCBN tools in comparison to dry conditions. It was found that, compared with elevated values of feed rate, tool wear was smaller because of the lower feed rate. When cutting speed increased, tool wear also increased when compared with dry machining. In addition, the interaction between cutting speed and feed was very important because it gave a better surface finish without compromising the products. Kumar et al. [56] performed experiments on AISI 4340 hard steel using a CBN tool (TiN-coated), noticing that increased work hardness and feed rate resulted in a larger cutting force and a higher interface temperature. This was because frictional heat production and flank surface wear increased. It was also discovered that the machining performance of CBN tools was superior in all machining situations. Tang et al. [57] examined the tooling wear performance of the PCBN tool during machining of hardened AISI D2 steel with different hardnesses under the dry conditions, and the images of wear were displayed in Figure 9. For workpiece hardness between 40 and 55 HRC, abrasive wear was seen as the main mode of tool wear on the flank surface. Similar to this, delamination and abrasive wear were discovered for jobs with a hardness of 60 HRC because of an abrupt increase in friction at tool-work piece contacts, whereas the major wear in the rake face of PCBN tools is the crater wear.

Nayak et al. [58] conducted an experiment on AISI D6 material using the CBN tool and reported that feed rate was the important parameter for surface roughness. Karthik et al. [59] investigated on hardened EN31 steel with the CBN tool and the result revealed that for different values of cutting speed and depth of cut, the surface roughness of EN31 steel augments with the raise in the feed so feed was the most leading and influencing factor for increasing of surface roughness. Gundarneeya et al. [60] applied Taguchi’s L₉ orthogonal array and ANOVA to undertake hard turning operations with a CBN tool on EN24 material. The rotational speed has mainly impacted the workpiece surface and, subsequently, the insert nose radius, while the depth of cut and feed rate have had less of an impact. However, the depth of cut influences the dimensional accuracy of components the most, followed by spindle speed and nose radius. Nikam et al. [61] evaluated tool life, tool-edge wear, cutting forces, and surface roughness in CBN insert-assisted hard machining on AISI 4140 steel. In comparison to the feeding and cutting forces, the radial force exhibits the maximum magnitude during forceful turning. The maximum tool life of 58.02 min was recorded during machining at 0.1 mm/rev feed and 200 m/min cutting speed. Balwan et al. [62] investigated the hard machining operation on EN353 steel with CBN inserts. According to ANOVA, cutting forces were significantly influenced by feed rate and depth of cut. The brief details about the used cutting tool and machining parameters for the hard turning process are shown in

Table 1. Details of used cutting tool and cutting parameters in hard turning.

Ref.	Materials with Hardness	Cutting Tool	Cutting Parameters			Responses Studied
			Vc (m/min)	f (mm/rev)	ap (mm)
[28]	AISI 52100, AISI D2, AISI H13, AISI 4340 (44, 48, 52, 58 HRC of each materials)	CBN	140, 180, 240	0.15, 0.3, 0.45	0.2	cutting force
[29]	AISI D2 steel (62 HRC)	PCBN	140, 270, 350, 500	0.05, 0.1, 0.2	0.2, 0.4, 0.6	Surface integrity Residual stress Tool wear
[30]	AISI D2 steel (52 HRC)	Coated PCBN PCBN/Mixed alumina	100	0.05	0.06	Tool wear
[31]	AISI D2 steel (52 HRC)	PCBN, CVD coated carbide insert	100	0.1	0.06	Cutting temperature
[32]	AISI D2 steel (62 HRC)	PCBN	250, 370	0.1	0.2	Cutting temperature
[33]	AISI D2 steel (62 HRC)	PCBN	70, 95, 120	0.08, 0.14, 0.20	0.5	Tool life, Material removal rate
[34]	AISI 4140 (60 HRC)	Composite coated and non coated	150	0.1	0.3	Cutting force, Cutting temperature
[37]	AISI H13 steel (45–55 HRC)	CBN	100, 125, 150	0.05, 0.1, 0.15	0.05, 0.09, 0.13	Surface roughness Cutting force
[39]	AISI 52100 (63 HRC)	CBN	130, 185, 260	0.08, 0.12, 0.16	0.2, 0.4, 0.6	Tool wear
[40]	AISI 5140 (57 ± 1 HRC)	CBN	150	0.075, 0.1, 0.125	0.15	Surface roughness, Residual stress
[41]	EN 31 (60 ± 2 HRC)	CBN	102, 132, 167, 204, 261.1	0.075, 0.113, 0.15	0.1, 0.15 0.2	Tool force, surface roughness
[42]	EN 31 (60 ± 2 HRC)	CBN	167, 204, 261	0.075, 0.113, 0.15	0.1, 0.15 0.2	Cutting force Surface roughness
[43]	AISI 52100 steel (60 HRC)	CBN	100, 140, 200	0.08, 0.12, 0.16	0.2 0.4 0.6	Cutting force Surface roughness
[44]	AISI 4340 Steel (69 HRC)	CBN	90	0.08	0.1	Cutting force Cutting temperature
[45]	AISI 52100 steel (64 HRC)	CBN	125, 176, 246	0.08, 0.12, 0.16	0.15, 0.3, 0.45	Tool wear Surface roughness
[47]	AISI H11 steel (52 HRC)	CBN	140	0.05	0.1	Flank wear Cutting temperature
[49]	AISI 4340 Steel (53 HRC)	PCBN	100, 125, 150	0.1, 0.15, 0.2	0.25	Cutting force Surface roughness
[51]	AISI 4340 Steel (62 HRC)	CBN	180	0.06	0.1	Cutting force Surface roughness Flank wear
[52]	AISI D6 steel (60 HRC)	PCBN	150, 225, 300	0.05, 0.1, 0.15	0.2	Cutting force Surface roughness Tool wear
[53]	AISI D2 steel (60 ± 1 HRC)	CBN	90	0.05	0.015	Cutting force Surface roughness
[54]	AISI H13 steel (45, 50 and 55 HRC)	CBN	75, 100, 126, 150	0.05, 0.075, 0.1, 0.125	0.05, 0.07, 0.09, 0.11	Cutting force Surface roughness
[55]	AISI D6 (59 HRC)	PCBN	160	0.05	0.05	Tool life Surface roughness
[56]	AISI 4340 steel (60 ± 2 HRC)	CBN	175	0.2	--	Cutting force Cutting temperature
[57]	AISI D2 steel (40 ± 1, 45 ± 1, 50 ± 1, 55 ± 1, and 60 ± 1 HRC)	PCBN	250	0.15	0.1	Tool wear
[58]	AISI D6 steel (54 HRC)	CBN	--	0.15	--	Tool wear Surface finish
[59]	EN31 (60 HRC)	CBN	100	0.04	0.2	Surface roughness
[60]	EN24 (45–55 HRC)	CBN	Spindle speed: 600, 400, 500	0.1, 0.15, 0.2	0.3, 0.4, 0.5	Surface roughness Dimensional accuracy
[61]	AISI 4140	CBN	150, 175, 200	0.1, 0.15, 0.2	0.25	Surface roughness Tool wear Cutting force
[62]	EN353 (62 HRC)	CBN	Speed: 700 rpm	0.24	0.8	Cutting force

.

2.2. Performance of Ceramic and Cermet Tools

In the manufacturing sector, ceramic cutting tools are used for the machining of various hard materials. High melting points, great hardness, and outstanding wear resistance are all characteristics of ceramics. According to Basak et al. [63], to get 0.8 µm of turned work surface roughness of AISI D2 alloy steel with the ceramic tool, the speed of machining should be 220 m/min, while to get a supreme rate of production, the tool-feed should be taken from 0.11 to 0.15 mm/rev. Lima et al. [64] found improved surface quality in the tuning of AISI D2 steel (58 HRC) with coated carbide, PCBN, and ceramic cutting inserts compared to cylindrical grinding. Flank wear seemed to be improved with cutting speed along with the depth of cut. Salem et al. [65] observed a saw tooth profile of chips in the orthogonal cutting of an AISI D2 work-specimen with a ceramic insert, and it occurred due to crack propagation in the quenched structure of the material. Due to the increase in cutting speed, the chips became soft and ductile; however, segmentation of the chips took place and looked similar to a saw-tooth pattern on the chips. Yuan et al. [66] examined the wear characteristics of an alumina mixed uncoated ceramic insert at diverse cutting speeds during the turning of hardened AISI D2 work-specimens and noticed tribofilm generation at the friction surface, which directly affects the tool life. Davim and Figueira [67,68] inspected the machining indicators of AISI D2 hardened alloy steel using ceramic tools. Tool wear was extremely sensitive to changes in cutting speed and time. Wear at insert surface was rapidly increased with growing speed and turning time. Kamely and Noordin [69] studied microstructure, residual stress, and the development of a white layer during the cutting of AISI D2 work specimens with a coated PVD ceramic cutting insert. The microstructure was changed due to metallurgical alteration and chemical phenomena that happened due to elevated cutting temperatures. Bensouilah et al. [70] conducted hard-turning experiments on AISI D3 hardened steel (63 HRC) using coated and uncoated ceramic tools. The tool life and surface roughness were measured with growth in machining time, and the results are shown graphically in Figure 10a,b, respectively. With a wear limit of 0.3 mm, the tool service life with the uncoated ceramic tool was 18.9 min while it was 24 min for the coated tool. The amount of flank wear significantly impacted the surface roughness. The estimated roughness of coated and untreated tools at the end of their useful lives was 1.37 and 1.46 microns, respectively.

Ferreira et al. [71] analyzed the comparative performance capability of the multi-radii and conventional ceramic tools on turning of H13 steel. The roughness of the machined surface was noticed to be improved with the use of multi-radii insert compared to the conventional tools. Karpuschewski et al. [72] provided the sequence-wise stages for preparation and performance of the cutting edge that were a significant element in controlling the cutting edge in the turning of heat-treated high-alloyed steel utilizing TiN coating and an uncoated ceramic tool. Bhemuni et al. [73] found the largest affecting term of flank wear was the depth of cut trailed by cutting speed and feed rate. It was also suggested to use a small magnitude of the depth of cut to avoid chatter during turning on an AISI D3 work specimen with a ceramic (Al₂O₃ and TiC) cutting insert. Pal et al. [74] emphasized the investigation of cutting forces, chip-tool-interface temperature, and roughness of the machined surface in turning on prior heat-treated AISI 4340 work-specimens with the mixed ceramic tool and noticed the increasing trend of the roughness of the machined surface with growth in feed, whereas temperature was increasing with increasing speed and job hardness. Suresh and Basavarajappa [75] investigated tool wear and surface roughness in turning hard steel AISI H13 using a PVD-applied TiCN-coated ceramic tool. Tool wear is greatly stimulated by cutting variables, while the feed is the dominant agent for surface roughness. Figure 11 shows the worn tool obtained in different cutting conditions. The flank wear faces have distinct mechanical ploughing grooves and ridges, which are indications of typical abrasive wear. However, it was discovered that at high machining conditions, abrasion was the primary mode of wear, while adhesion was observed at low machining conditions.

Elbah et al. [76] observed that the surface quality of AISI 4140 steel of hardness 60 HRC was greatly improved with inserts of wiper ceramics when compared to conventional ceramics, and the roughness value was noticed to be lower than 0.9 microns even though the tool attained the wear at the flank face limit of 0.3 mm. Mandal et al. [77] experimentally noticed that the machining at a higher speed produced a good surface finish, which is very essential for hardened steel, when subjected to the machining of an AISI 4340 work-specimen at high speed with a cutting insert (Yttria stabilized zirconia toughened alumina). Aslantas et al. [78] observed the most dominating wear type was crater as a result of chipping or insert-tip cracking in a concise span of time. Machining AISI 52100 steel with an uncoated ceramic insert generates a superior temperature compared to a coated ceramic insert, owing to the lower conductivity of the uncoated insert. The coated tool was more sensitive to the formation of BUE during machining. Meddour et al. [79] found that the nose radius and cutting feed were the utmost factors influencing the work surface roughness whereas cutting forces were highly dominant by the depth of cut. Effects of cutting feed on surface finish could be reduced by elevating the cutting speed in AISI 4140 steel turning using a mixed ceramic cutting tool. Lalwani et al. [80] discovered good quality surfaces when they machined MDN250 steel with coated ceramic cutting inserts at higher speeds, deeper cuts, and lower feed rates. Xu et al. [81] developed Ti (C, N) based (TMWNC and TMWC) cermet tools and investigated their performance in machining 32–36 HRC hard 42CrMo workpieces. The TMWC tool produced a lower cutting force in comparison to the TMWNC tool, as presented in Figure 12a. The cutting force for both tools improved with cutting speed. TMWNC’s tool produced a relatively lower cutting temperature in comparison to TMWC due to its lower thermal conductivity, as shown in Figure 12b. The tool wear images for TMWNC and TMWC tools at various cutting speeds are shown in Figure 13 and Figure 14, respectively. The wear rate of the TMWC tool was higher than that of the TMWNC tool because the TMWC tool’s adhesive wear degree was greater than that of the TMWNC tool. According to According to Khan et al. [82], job hardness was the most important determining factor for tool life. Hard turning of an AISI D2 workpiece with a wiper-ceramic insert causes notching, wear scars, and oxidation wear. Rashid et al. [83] and Agarwal et al. [84] claimed that although machining with a smaller feed rate improved surface quality, the corresponding tool wear volume was high due to the longer length of tool movement during machining. According to Godoy and Diniz [85], the dominant mechanism for ceramic tools was abrasion, whereas for the CBN insert, abrasion was dominant at the slowest speed and diffusion was dominant at the fastest speed when turning an AISI 4340 work specimen. Oliveira et al. [86] found three times the tool life of a PCBN tool compared to a ceramic tool when machining AISI 4340 steels. The major wear phenomena associated with the end of ceramic tool life were abrasion followed by diffusion, whereas an adhesive phenomenon was more sensitive for PCBN tools. The ceramic tool produced a better-quality surface than the PCBN tool. Kumar and Patel [87] mentioned that the temperature was elevated with the progress of time and tool wear, but the rate of elevation of temperature in turning on an AISI 52100 work specimen was higher when utilizing an uncoated ceramic tool compared to a coated ceramic cutting tool due to the greater thermal stability of the coated tool for a longer duration. Helical-shaped chips were reported with uncoated tool whereas longer snarled type of chips was noticed for coated tools. Panda et al. [88] confirmed the superiority of ceramic tools in the turning of hard AISI 52100 work specimens as the width of wear at the flank face and work surface roughness were achieved within the recommended values of 0.3 mm and 1.6 µm, respectively.

Kam et al. [89] performed a hard turning process on AISI 4140 steels using ceramic tools and noticed that machinability was improved due to the increases in toughness achieved by decreasing the hardness of the workpiece. In addition, the feed rate was directly correlated with the increase in vibration amplitude values. Shalaby et al. [90] investigated hardened AISI 4340 steels with two different types of alumina ceramic inserts: ZrO₂ and Al₂O₃ coated and Al₂O₃ and TiC coated. Results revealed that alumina ceramic tools with ZrO₂ added are more resistant than ceramic tools (mixed) with TiC coating. From the SEM and XPS analyses, it was established that the triboflims created at the machining zone at the time of machining affected the tool wear resistance and impacted the coefficient of friction at the tooltip interface. Arsene et al. [10] studied and examined the turning of hardened AISI D2 steel with a ceramic wiper cutting tool and found that, in terms of surface quality, cutting with a ceramic insert gives a good performance as compared to grinding. In addition, lower tool wear was observed in MQL than in dry situations, and adhesion and major chipping were also noticed. Rath et al. [91] conducted machining operations on hardened AISI D3 steel using a mixed ceramic tool (Al₂O₃ + TiCN) under dry conditions. The results showed that by selecting appropriate cutting parameters, both surface finish and machining force could be improved simultaneously.

Das et al. [92] examined the performance of hardened AISI 4340 steels with uncoated carbide and coated cermet inserts. Coated carbides showed reduced cutting force, flank wear, and surface temperature as compared to uncoated carbides, but greater feed force and radial force. Tiwari et al. [93] studied the machining responses such as surface roughness, cutting force, and chip coefficient in hard part turning of AISI 4340 steel utilizing the cermet cutting tool and observed that surface roughness increased with the radial depth of cut and axial feed. MRR improves almost linearly as the radial depth of cut increases, but slowly as feed and speed increase. The details of the workpiece, cutting parameters, and responses studied in hard turning with a ceramic tool are listed in

Table 2. Details of workpiece, cutting parameters and responses studied in hard turning using ceramic tool.

Ref.	Materials with Hardness	Cutting Parameters			Responses Studied
		V (m/min)	f (mm/rev)	d (mm)
[10]	AISI D2 steel (55 ± 1 HRC)	120, 150, 180	0.1, 0.15, 0.2	0.1	Tool wear Surface roughness
[64]	AISI D2 steel (58 HRC)	220	0.15	0.2	Cutting force Tool wear Surface roughness
[65]	AISI D2 steel (62 HRC)	50, 250	0.05, 0.2	0.2	Chip formation Cutting force
[67]	AISI D2 steel (52 HRC)	80, 150, 220	0.05, 0.1, 0.15	--	Tool wear Surface roughness
[68]	AISI D2 steel (60 HRC)	80, 115, 150	0.05, 0.1, 0.15	--	Surface roughness Flank wear
[70]	AISI D3 steel (63 HRC)	75, 105, 150, 210	0.08, 0.12, 0.16	0.1, 0.2, 0.3.0.4	Surface roughness, cutting force, Tool wear
[71]	AISI H13 (54 HRC)	80, 16, 240	0.05, 0.1, 0.25, 0.04	0.2	Surface roughness Tool wear
[72]	High alloyed steel (X123CrMoV12) (56–60 HRC)	200–300	0.05, 0.25	0.2	Cutting edge preparation
[73]	AISI D3 steel (62 HRC)	145, 155, 165	0.05, 0.075, 0.1	0.3, 0.6, 0.9	flank wear
[74]	AISI 4340 (55 HRC)	100, 120, 150, 180	0.081, 0.088, 0.113, 0.138	0.1, 0.2, 0.3, 0.4	Surface roughness, Cutting force
[76]	AISI 4140 steel (60 HRC)	80, 115, 150	0.08, 0.11, 0.14	0.1, 0.2 0.3	Flank wear, surface roughness
[77]	AISI 4340 steel (58 HRC)	420	0.24	0.5	Surface roughness
[78]	AISI 52100 steel (63 HRC)	100, 150, 200, 250	0.07, 0.11, 0.14	0.5	Tool wear
[79]	AISI 4140 steel (60 HRC)	182	0.08	0.2	Cutting force Surface roughness
[80]	MDN250 steel (50 ± 5 HRC)	93	0.04	0.2	Surface roughness
[82]	AISI D2 steel (55 & 60 HRC)	--	0.098, 0.196, 0.281	0.1, 0.15, 0.20	Tool life
[85]	AISI 4340 steel (56 HRC)	150	0.08	0.15	Tool wear Surface roughness
[87]	AISI 52100 steel (62 HRC)	189	0.12	0.5	Tool wear
[88]	AISI52100 steel (55 ± 1 HRC)	70, 110, 150, 180	0.05, 0.1, 0.15	0.2	Cutting force Surface roughness Tool wear
[89]	AISI 4140 steel (45& 52 HRC)	120, 160, 200, 240	0.05, 0.1, 0.15	0.2	Surface roughness Cutting force
[90]	AISI 4340 (52 HRC)	150, 250, 700, 1000	0.1	0.125	Tool life Surface roughness
[91]	AISI D3 steel (58–64 HRC)	280, 140, 180, 190, 245, 320	0.04, 0.05, 0.06, 0.07, 0.08	0.1, 0.3, 0.5, 0.7, 0.9	Surface roughness Cutting force Cutting temperature
[92]	AISI 4340 steel (48 HRC)	80, 100, 120	0.05, 0.1, 0.15	0.1, 0.2, 0.3	Surface temperature
[93]	AISI 4340 steel (56 HRC)	70, 140, 210	0.05, 0.1, 0.2	0.1, 0.3, 0.5	Surface roughness Material removal rate Chip reduction coefficient

.

2.3. Performance of Coated and Uncoated Carbide Tools

Nowadays, because of the availability of various advanced coated carbide tools, the use of more expensive ceramic and CBN inserts for hard machining is being reduced, which contributes to a reduction in machining cost and, as a result, product cost. In hard machining, numerous studies have been conducted to investigate the cutting ability to distinguish coated (TiN/TiCN/Al₂O₃/TiN), (TiN), (TiAlN/AlCrN), (TiAlN), (AlTiN), (TiCN/Al₂O₃/TiN), (TiCN/Al₂O₃/TiN) carbide insert and uncoated tool. The following are some of the most significant findings:

Sahoo and Sahoo [3] accomplished a comparative evaluation of the dry-machining performance of (TiN/TiCN/Al₂O₃/TiN) coated carbide with an uncoated tool. The observed growth of wear width at the flank face for the coated tool was uniform with no immature failure, whereas abrasion was noticed to be the principal wear mechanism. Sahoo and Sahoo [4] emphasized surface finish, wear of the tool flank, chip formation, and machining forces while turning the AISI 4340 rods by using uncoated and coated carbide cutting inserts. The tool life of coated tools was found to be very high in comparison to uncoated tools. Compared to the uncoated tools, better surface finish and lower machining costs were achieved during turning with a coated tool. Srithar et al. [94,95] found diminished quality of the surface with rising depth-of-cut and cutting feed, while it was improved with rising cutting speed. The forces developed seemed to improve with speed, whereas a subsequent decrement was noticed with the rise in cutting feed. Cutting feed was traced to be the greatest influencing agent for forces and surface quality in machining on an AISI D2 work-specimen with a coated carbide cutting tool. The graph depicts various surface roughness characteristics in relation to cutting speed, depth of cut, and feed rate, as shown in Figure 15. The curve for surface roughness parameters with regard to cutting speed is shown in Figure 15a. The graphs indicate that when the cutting speed increases, the surface roughness parameter decreases. The graph for surface roughness with relation to feed rate is shown in Figure 15b. The graph clearly demonstrates that when the feed rate increases, the surface roughness parameter values increase. Furthermore, when the other factors are taken into account, the experimental findings indicate that the impact of feed rate on the surface roughness parameter is greater. Figure 15c illustrates a graph of surface roughness factors as a function of cut depth. The surface roughness parameter value increases as the depth of cut rises from 0.2 to 0.6 mm, according to the graph. Silva et al. [96] presented the larger magnitudes of cutting as well as thrust forces in the turning of AISI D2 steel with an uncoated carbide cutting tool due to its greater tensile strength and hardness. The sharp increment in cutting force was noticed with the cutting feed. Sahoo [97] applied a titanium nitride-coated tool in turning of heat-treated AISI D2 steel and noticed the acceptable quality of the work surface within the accepted limit of 1.6 microns. Higher speed is attributed to the improved quality of the finish due to the absence of BUE at higher speed. Jiang et al. [98] introduced the simulation approach to validate the obtained simulated results with the results obtained during the turning of an AISI D2 work specimen with a coated (TiAlN) carbide cutting tool. Maximum shear stress between the coated layer and substrate of the tool was improved with cutting speed, while the effect of feeding was very minute. The thrust force was enhanced by the feed rate. Due to the increased deformation degree of the saw tooth chip, the thrust forces at a cutting speed of 90 m/min are greater than those at a cutting speed of 60 m/min.

Zeghni and Hashmi [99] found a higher wear resistance capability of coated substrates compared to uncoated substrates, whereas TiC introduced greater wear resistance in relation to TiN coating in the course of turning AISI D2 as well as AISI D3 grade steels. Haron et al. [100] stated that the coated tool outperformed compared to the uncoated in terms of wear during the turning of the 95 MnCrW1 steel bar. Smooth growth of wear was noticed with the coated tool, while the wear zone was concentrated on the tool-tip region. For coated tools, chipping was noticed at the end of the tool’s life. The chips’ color varied with tool wear and cutting speed.

According to Selvaraj et al. [101], the tool wear was dominated by abrasion during turning on duplex stainless steel with a coated (TiC and TiCN) carbide cutting insert at lower cutting speeds, but when turning was carried out at higher speeds, mechanisms such as diffusion, thermal softening, and notching were most prominent. Chinchanikar and Choudhury [102,103] reported that the tool wear directly influenced the radial as well as feed forces in tuning an AISI 4340 work-specimen with PVD and CVD cutting tools. Rapid wear, as well as catastrophic breakage of the tool, was more dominant for the PVD tools whereas gradual progress of wear was noticed for the CVD tool.

Gowd et al. [104] discovered a rising trend in MRR with increasing depth-of-cut, while tool wear improved with increasing speed, tool feed, and tool-radial-depth in turning AISI S2 work-specimen with carbide inserts. It was also noticed that the cutting force was improving with the rising cutting speed. Sahoo and Sahoo [105,106] studied the machinability properties such as wear in the flank, chip formation, and surface finish obtained during the turning of AISI 4340-grade steel. The tool wear in hard-turning was primarily caused by abrasion and chipping. TiN-coated carbide produces a better surface finish. Major factors for tool wear were machining speed, depth of cut, and cutting feed. The economical interpretation was performed using optimal setting data, and Rs 3.17 was found for the machining cost for the considered machining variables. Chinchanikar and Choudhury [107,108,109,110] worked on the machinability properties of AISI 4340-grade steel while machining with coated and uncoated carbide tools. The change in machining force varied proportionately with cutting depth and axial feed. In another work, the performances of TiAIN and coated carbide tools (TiCN/Al₂O₃/TiN) while turning AISI 4340-grade steel were carried out. PVD-coated tools produced less machining force than CVD-coated tools. The single-layered tool produced a lower width of wear, but for the multi-layered tools, the wear growth was increasing rapidly because of the flaking of coatings. Higher cutting forces were noticed when machining harder work pieces, while they were influenced by the cutting feed and the depth of cut. Mechanisms associated with the failure of the tool were nose wear and chipping because of abrasion and adhesion. Suresh et al. [111,112] reported the advantages of lowered processing costs, better material properties, and improved productivity in machining AISI 4340-grade steel using multi-layered carbide inserts. It was found that the cutting feed was exceedingly influenced by the particular cutting force, whereas the cutting power and wear of the cutting tool increased directly with cutting speed and axial cutting feed. In addition, greater machining speed and a lower cutting feed are attributed to the improved surface quality. Chip breaking occurred due to the higher cutting speed. Figure 16 shows the wear SEM image of a coated carbide cutting tool after machining. Wear was occurring due to high pressure and temperature. Grooves were discovered on the rake face of the tool as a result of the abrasive mechanism and material adhesion, as shown in Figure 16a. Furthermore, tool wear in the nose region was frequently deliberated due to higher pressure and thermal softening of the tool material. In Figure 16b, chipping of the cutting tool edge was shown, which shows the major tool wear at the highest values of cutting parameters. Dave et al. [113] revealed that the positive insert produced a higher rate of material removal compared to the negative insert. The rate of material removed was affected by the depth of cut when turning on EN31 and EN8 with a coated (TiN) cutting insert. Kene et al. [114] noticed that the carbide tool with PVD coating exhibited extended tool life, enhanced quality of the work surface, more dimensional precision, and a higher temperature in the turning of hard AISI 4340-grade steel without using any coolant. The cutting forces improved with the progress of wear width, whereas tool wear was affected by contact stress and interface temperature in turning. Sethi and Kumar [115] worked on the turning of EN31 work specimens using a coated carbide tool. The results suggested that the wear at the flank face increased with all turning parameters, whereas cutting speed was noticed to be a highly dominant variable, followed by depth of cut and cutting feed. Motorcu et al. [116] found the most compelling factor on interface (tool-chip) temperature was turning speed, whereas tool temperature was largely stimulated by the depth of cut. The quality of the surface was significantly influenced by turning feed when turning on AISI 4140 with a PVD coated carbide cutting insert. Lima et al. [117] stated that the turning forces improved with the hardness of the test specimen, whereas the variation in forces followed the linear trend with rising cutting feed as well as the depth of cut. The abrasion tool mechanism was more prominent towards wear, whereas sudden breakage of the tool appeared with longer machining periods under turning on AISI 4340 by means of a coated carbide tool. Das et al. [118] investigated the machining performance of hard AISI D6 material in a dry environment utilizing two types of cemented coated tools (AlTiN and AlTiSiN) and found that the scalable pulsed power plasma (SPPP)-AlTiN coated tool outperformed the other two tools in aspects of enhanced surface quality, minimized cutting force, and longer tool life. In machining, serated and saw tooth segmented type chips were generated. Due to the inadequate heat dissipation capabilities of the AlTiN tool, bigger and broader saw tool chips were produced.

Zang et al. [119] examined the cutting temperature during the machining of a hardened AISI H13 steel specimen with multi-layered coated tools. Kumar et al. [120] carried out a hard part turning on AISI D2 steel by CVD (TiN/TiCN/Al₂O₃) coated carbide and uncoated carbide insert under a dry environment and observed the responses such as tool wear, cutting temperature, surface roughness. The researcher noticed that, compared with uncoated carbide tools, coated carbide tools performed well in terms of tool life. When the temperature rises, the upper layer of Al₂O₃ acts as a thermal barrier, protecting the tool from abrasion and diffusion. The TiCN coating also provides stronger adherence to the tool, protecting it from flank wear. Allu et al. [121] evaluated surface roughness utilizing a wiper carbide insert during a hard turning operation on AISI 52100 steel in a dry cutting environment and reported the type of cutting tool was the most influential factor, followed by feed rate and nose radius. When compared to regular inserts, wiper inserts have a superior surface finish. Chavan et al. [122] used multi-layered PVD coated tungsten carbide inserts to machine harden AISI 52100 steel under various cooling conditions to determine the values of surface roughness, residual stress, microhardness, and white layer formation. In addition, compared to a dry environment, the hybrid NF-MQCL cutting environment resulted in less tool wear and less white layer formation due to the reduction of friction and temperature in the cutting zone area. Das et al. [123] accomplished a comparative study of the tool life, tool wear, cutting temperature, and surface integrity of HSN2-TiAlxN and TiCN-coated carbide tools on hardened AISI D6 steel. TiAlxN-coated carbide tools provide better machinability performance in terms of lessened flank and crater wear, higher tool life, enhanced surface integrity, and minimal cutting temperature. In addition, the TiAlxN-coated carbide tool provided considerable manufacturing cost reductions as compared to the TiCN-coated insert. Hamadi et al. [124] compared the machining performances of PVD, CVD, MTCVD, and uncoated carbide tools in a dry cutting environment using hardened AISI 4140 steel. The flank wear rate of the TiN coated tool was roughly twice that of the TICN/Al₂O₃/TiN and four times that of the TiCN/Al₂O₃, with the uncoated tool showing the most wear. The highest cutting force was found with an MTCVD (TiCN/Al₂O₃/TiN) cutting insert. In terms of surface quality, MTCVD (TiCN/Al₂O₃/TiN) cutting inserts were recognized as the leading material. The details of coated tool coatings and cutting parameters used in hard turning are shown in

Table 3. Details of coated tool coatings and cutting parameters used in hard turning.

Ref.	Materials	Cutting Tool with Coating	Cutting Parameters			Responses Studied
			V (m/min)	f (mm/rev)	d (mm)
[107]	AISI 4340 steel	CVD coated multi-layer MT-TiCN/Al2O3/TiN	100 142 150 200	0.1 0.125 0.2 0.3	0.5 0.8 1.5	Chip morphology Tool life
[108]	AISI 4340 steel	PVD-applied singlelayer TiAlN-coated tungsten-based cemented carbide inserts	100 142 200 265 300	0.1 0.15 0.2 0.25 0.3	0.5 0.1 1.5 2 2.5	Surface roughness Flank wear Tool life
[109]	AISI 4340 steel	PVD coated single-layer TiAlN carbide insert, CVD coated multi-layer MT-TiCN/Al2O3/TiN carbide inser	142 200 265 345 487	0.125	0.2	Cutting force
[110]	AISI 4340 steel	CVD with TiCN/ Al2O3/TiN coating	100 150 200	0.1 0.2 0.3	0.5 1.5 2.5	Surface roughness Tool life
[111]	AISI 4340 steel	multilayer CVD coating (TiN/TiCN/Al2O3)	140 200 260	0.1 0.18 0.26	0.6 0.8 1.0	Tool wear Surface roughness Cutting force
[112]	AISI 4340 steel	MT CVD coating (TiN/TiCN/Al2O3)	80 140 200 260	0.10 0.18 0.26	0.8 1.0 1.2	Surface roughness Cutting force
[113]	EN-8 and EN-31 steel	CVD (TiN) coated and PVD (TiN) coated	100 150	0.25 0.36	1 1.5	Material removal rate Surface roughness
[114]	EN-24 steel	Multi-layer PVD coated Ti-Al-N nano-layer carbide insert	100 150	0.15	0.25	Tool wear
[116]	AISI 4140 steel	PVD coated-TiAlN-TiN	76 114 170	0.05 0.08 0.12	0.40 0.60	Cutting temperature
[117]	AISI 4340 steel	Coated carbide insert	60 120 180	0.1 0.2 0.3 0.4	0.5 1.0 1.5 2.0	Cutting force flank wear
[118]	AISI D6 die steel	Cemented carbide (AlTiN and AlTiSiN)	40 55 90	0.04, 0.08, 0.12	0.2, 0.3, 0.4	Rough-ness, tool wear, chip morphology and cutting force
[119]	AISI H13 hard steel	Multi-layer coated tool (TiAlN/TiN and TiN/TiC/TiN)	40 60 90 120 200 250	0.2	0.25	Cutting temperature
[120]	AISI D2 steel	CVD coated carbide insert (TiN-TiCN-Al2O3) and uncoated carbide	63 108 140 182	0.04 0.08 0.12 0.16,	1.5 0.2 0.3, 0.4	Rough-ness and cutting temperature and tool wear
[121]	AISI 52100 steel	Wiper carbide insert	70 110 150	0.05 0.1, 0.15	0.1 0.2 0.3	Surface roughness,
[122]	AISI 52100 steel	PVD coated multi-layered tungsten carbide insert (TiSiN/TiAlN)	80 140 200	0.08 0.14 0.2	0.3	Surface roughness Micro hardness Residual stress White layer

.

Furthermore, in recent years, the use of customized textured cutting tools in hard turning has grown in popularity due to the potential to increase tool life without the use of coolant. Sivaiah and Bodicherla [125] used two types of surface texture tools in their experimental study, namely surface texture grooves 45° angled to the primary cutting tool edge and perforated holes tool, in addition to the nontexture conventional tool in a MQL environment. Surface textured tools (grooves 45° inclined to the principal cutting edge) considerably minimize the average surface roughness and tool flank wear. Orra and Choudhury [126] developed vertical, horizontal, and elliptical micro-textured tools and compared their hard turning performance. In comparison to a non-textured cutting tool, the micro-textured tool provided a significant decrease in the coefficient of friction, allowing for a significant reduction in cutting force. Kim et al. [127] obtained improved tribological characteristics when an EDM assisted micropatterned CBN insert was used in hard machining. The coefficients of friction, cutting force, and tool wear were considerably reduced in comparison to when a non-patterned tool was used. With reduction of cutting speed, non-patterned insert exhibited 9.7 to 11.4% gain in tool wear in contrast to patterned tool. Patel et al. [128] examined the impact of micro-textured/micro-grooved tungsten carbide (WC/Co) cutting tools on cutting forces, friction coefficients, and tool wear during dry cutting of alloy steel AISI 4340. Chip-tool contact is reduced as spacing and groove width are increased, while chip flow and adhesion into micro-grooves are increased. Song et al. [129] used self-lubricating and conventional tools to conduct dry machining trials on AISI 1045 hardened steel. When compared to the conventional tool, the self-lubricating tool embedded with graphite demonstrated excellent efficiency and stability in reducing cutting temperature and tool wear.

3. Hard Turning Performance under Different Cooling/Lubricating Conditions

Dry hard turning has certain limitations, such as higher cutting temperatures, fast tool wear, reduced surface finish, high cutting forces, high stresses, high power consumption, etc. Therefore, lots of works using different coolant/cooling techniques have been done till date to improve the hard machining machinability. This section presents a review of hard machining under different cooling and lubrication conditions. Figure 17 depicts the various cooling techniques used in hard-turning machining.

3.1. Wet/Flood Cooling

Wet or flood cooling is a machining technique in which a cooling jet is focused into the active zone to cool, lubricate, and remove chips. This approach is best suited for turning and grinding, where sparks and higher temperatures may occur due to the coolant’s water content, which is present in the emulsion [130].

Khatai et al. [131] utilized a PVD-coated tool to turn a hard component from EN31 steel with a hardness of 56 ± 1 HRC in a dry and flood cooled environment. Under flood cooling conditions, surface roughness values were found to be lower and MRR was found to be higher. According to the data, surface quality is primarily determined by feed, whereas MRR is primarily determined by depth of cut. Selvam and Sivaram [132] investigated the effect of dry, flooded, and near-dry cooling on the surface finish obtained in turning AISI 4340 steel. It was reported that MQL machining resulted in less tool wear, better surface quality, and lower cutting fluid consumption than dry and flood cooling methods. Details of the cutting tool, cutting parameters, and response studies used in the flood cooling environment are shown in

Table 4. Details of cutting tool, cutting parameters and response studies in flood cooling environment for hard turning operation.

Ref.	Test Material	Cutting Tool	Cutting Parameters			Response Studied
			Vc m/min	f mm/rev	ap mm
[132]	EN-31	PVD Multi -layer coated carbide	60 120 180 240	0.06 0.12 0.18 0.24	0.15 0.25 0.35 0.45	Surface roughness Material removal rate
[133]	AISI 4340 steel	TiN coated carbide insert	325 350 375	0.1 0.15 0.2	0.3 0.6 0.9	Surface roughness

.

3.2. Minimum Quantity Lubrication

Khan et al. [20] described the consequence of vegetable oil MQL on the turning performance of a low carbon alloy (AISI 9310 steel) using an uncoated carbide cutting insert. MQL applied machining exhibited a 10% decrease in mean chip-tool interface temperature, brighter and smoother chips, no built-up edge (BUE) formation, reduced wear, and improved finished surface quality when compared to dry and wet environments. Chinchankar et al. [21] explored the effects of dry, water, and coconut oil mixed cutting fluid on surface finish in turning heat-treated AISI 52100 steel using a PVD-coated carbide tool. Implementation of coconut oil at a higher cutting speed generated lower values of surface finish, whereas the roughness of the surface was primarily affected by the cutting feed. Attanasio et al. [133] found worker safety, environmental protection, and cost-effectiveness benefits in the application of MQL during the turning operation of 100 Cr6 steel with a coated carbide cutting tool. There was not much deviation in flank wear under dry and MQL surroundings, but the life of the tool was considerably improved under MQL surroundings. Elmunafi et al. [134] observed improved machining performance with MQL using castor oil as a machining fluid over dry cutting during the turning of AISI 420 stainless steel with PVD coating (TiAlN) carbide cutting inserts. However, compared to dry machining, MQL provided a longer tool life. Varadarajan et al. [135] studied the performance of MQL in hardened AISI 4340 steel using a coated (TiC, TiN, and TiCN) carbide tool in dry and wet conditions. The MQL was found to be superior in all cutting situations in terms of obtaining favorable machining forces, insert longevity, surface finishing, machining temperature, and tool chip interaction length. Chinchanikar and Choudhury [136] experimentally observed prolonged tool life when AlTiCrN coated carbide tool was used on turning of AISI 4340 steel. Cutting tool life was increased in the MQL environment as a result of lower cutting temperature generation. Tool nose wear and chipping were observed to be the leading wear patterns in both dry and MQL cutting environments. Dhar et al. [137,138] found that turning hardened AISI 1040 steel with an uncoated carbide cutting insert in a MQL environment yielded better results in terms of machining forces, tool wear, chip reduction coefficient, chip-tool interaction, machining temperatures, surface quality, and dimensional accuracy than dry cutting.

Netake and Chinchanikar [139] predicted the cutting forces and surface quality in turning heat treated AISI 52100 steel employing a PVD coated TiSiN-TiAlN carbide tool under MQL conditions. The quality of the finished surface varied linearly with cutting feed and radial depth. The force was largely impacted by cutting depth, followed by cutting feed rate. Hwang and Lee [140] noticed reduction in forces and enhancement in quality of finished work under turning of the AISI 1045 with coated carbide cutting insert under MQL compared to dry situation. Senevirathne and Punchihewa [141] varied the temperature of the aerosol in MQL assisted hard turning and found a better quality surface compared to dry and wet cooling when machining AISI P20 and AISI D2 steel with a TiCN/Al₂O₃/TiN CVD coated carbide cutting tool. Kumar et al. [142] found the favorable performance of MQL over wet assisted turning on AISI 4340 with a CBN cutting insert. Mahadi et al. [143] found the enhanced quality of surface under boric acid mixed palm kernel oil coolant compared to regular lubricants in turning AISI 431 steel with coated (TiAlN and AlCrN) cutting tool. Gürbüz and Gonülacar [144] evaluated the performance of hardened 4140 steel under dry, wet, and MQL cutting circumstances. In comparison to wet and dry machining, MQL cutting conditions were shown to significantly reduce tool wear. According to Ozbek and Sarukh [145], MQL enabled machining reduced tool wear (23%), tool vibration amplitude (45%), and cutting temperature (25%), when compared with a dry machining environment. Zbek et al. [146] investigated the machinability of Vanadis 10 steel in dry and MQL environments. When compared to dry machining, MQL revealed significant enhancements in output performances (cutting temperature, surface quality, tooling wear, life of tool, and cutting tool vibration amplitude). Cutting speed was the most effective term for tool wear (32.41%) as well as for vibration amplitude (46.22%). The details of the cutting fluid used in the MQL coolant delivery system were disclosed in

Table 5. Details about cutting fluid used in MQL delivery system in hard turning.

Ref.	Test Material	Cutting Fluid	Cutting Tool	Cutting Parameters			Response Studied
				Vc m/min	f mm/rev	ap mm
[20]	AISI 9310 steel	Vegetable oil	Uncoated carbide	223 246 348 483	0.10 0.13 0.16 0.18	1.0	Tool wear Surface roughness
[21]	AISI 52100 steel	Servo-cuts Coconut oil	PVD-coated nanolaminated carbide tool	100 125 150 175	0.1 0.15 0.2 0.25	0.1 0.2 0.3 0.4	Surface roughness
[133]	100 Cr6 steel	Easter oil	Coated carbide	300	0.1 0.15 0.2	1	Tool life
[134]	AISI 420 stainless steel	Castor oil	PVD coating (TiAlN) carbide coating	100 135 170	0.16 0.20 0.24	0.2	Tool life Surface roughness Cutting force
[135]	AISI 4340 steel	Mineral oil	Coated carbide tool	40 80 120	0.05 0.1 0.14	1.25	Surface roughness Cutting force Cutting temperature
[136]	AISI 4340 steel	Servo cuts	Coated carbide tool	100 125 150	0.088	0.3	Tool wear
[137,138]	AISI 1040	Mobil cut-102	Uncoated carbide	72 94 139 164	0.10 0.13 0.16 0.20	1.5	Cutting temperature Tool wear
[140]	AISI 1045	Air pressure	coated carbide tool	100 300	0.1 0.3	0.4 1	Surface roughness Cutting force
[141]	AISI P20 and AISI D2 steel	Pneumatic pressure	CVD coated carbide tool	150	0.5	0.5	Surface temperature
[142]	AISI 4340 steel		CBN	75 100 125 150 175	0.1 0.125 0.15 0.175 0.2	0.2	Surface roughness
[143]	AISI 431	Boric acid mixed with palm Karnel oil	Coated carbide tool	150 200	0.16 0.24	0.5 1.0	Surface roughness
[144]	AISI 4140 steel	Lubrioil Lubri Fluid-F100	Uncoated carbide tools	75 100 125	0.16 0.25 0.5	2.5	Cutting force Surface roughness Tool wear
[145]	AISI D2 steel	Vegetable-based SAMNOS ZM-22 W cutting oil	PVD coated (TiAlN-TiN) and CVD coated (TiCN-Al2O3-TiN)	60 90 120	0.09	1	Surface roughness Tool wear
[146]	Vanadis 10 steel	vegetable-based SAMNOS ZM-22 W cutting oil	Coated cemented carbide	80 100 120	0.08 0.1 0.12	1	Cutting temperature Tool flank wear

.

3.3. High Pressure Cooling

Machining using high-pressure cooling (HPC) is also gaining popularity in the metal-cutting sector as a technique for significantly enhancing the removal rate and productivity. Depending on the pressure and flow rate of the fluid jet, cooling with high pressures in turning operations is an effective way for enhancing productivity, decreasing cutting zone temperature, and improving chip control. Based on the literature, the pressure in HPC may vary from 4 MPa to 80 MPa [15]. Mia and Dhar [15] stated that the quality of the finished surface of EN24 steel was improved with the application of high-pressure jet cooling using coated carbide inserts. The surface roughness was increasing with feed while decreasing with cutting speed in both dry and HPC conditions, as displayed in Figure 18. Mia et al. [147] conducted an experiment using three different hardened steels with coated carbide tools under both dry and high-pressure coolant jets, where the depth of cut was kept constant. It was determined that, compared to dry cutting, high pressure coolant jet was successful in reducing surface roughness, tool wear, and cutting temperature. Statistical analysis revealed that for both cutting temperature and surface roughness, work piece hardness was the most significant. Kramar et al. [148] performed turning operations on hardened AISI 1045 steel utilizing high-pressure cooling and compared the performance in dry, conventional, and HPC cooling. HPC outperformed dry cooling and conventional cooling in terms of tool life, chip breakability, and reduced coolant use. Details about cutting tools and cutting parameters used in HPC are shown in

Table 6. Details of cutting tools and cutting parameters used in HPC in hard turning.

Ref.	Test Material	Cutting Tool	Cutting Parameters			Response Studied
			Vc m/min	f mm/rev	ap mm
[15]	EN24T	Coated carbide	81	0.12	-	Tool wear Surface roughness
[147]	Medium carbon steel	Coated carbide	115 161	0.12 0.14	-	Surface roughness
[148]	AISI 1045 Steel	Al2O3 coated carbide tool	98.5	0.25	2	Tool life Tool wear Cutting force

.

3.4. Solid Lubricant

Solid lubricants are substances that can reduce friction between two surfaces moving against one another without the use of a liquid medium. These lubricants can also lubricate effectively at temperatures higher than those for liquid and oil-based lubricants [149]. In recent years, many distinguished types of solid lubricants were implemented directly or via MQL in hard-turning applications. The commonly used solid lubricants in hard turning operations are as follows: Graphite, MoS₂-Molybdenum disulphide, h-BN-Hexagonal boron nitride, WS₂-Tungsten disulfide, ZnS-Zinc sulfide, CaF₂-Calcium fluoride and BaF₂-Barium fluoride. Rao and Krishna [19] utilized graphite and boric acid solid lubricants in hard turning and compared the results with dry and conventional flood/wet cooling. The particle size of 50 microns was found to be better in comparison to other sizes. Boric acid solid lubricant was providing the least surface roughness in contrast to graphite, dry cooling, and flood cooling, as illustrated in Figure 19.

Suarez et al. [52] compared the hard turning performance under dry, pure oil-MQL, and Pure oil + MoS₂ MQL environments. Both cooling solutions (MQL and MoS₂ + MQL) resulted in an increase in tool life. 150 m/min cutting speed was advised for industrial operations in order to make the process feasible and improve tool life using PCBN tools. Figure 20 illustrates that, regardless of the usage of the lubricant cooling system, a rise in cutting speed supports a steady reduction in the machining force (Fu). Furthermore, Figure 20 showed that, as compared to dry cutting, the surface quality (Ra) was typically minimized by the use of a cutting fluid by MQF, either pure oil or with the addition of MoS₂. This improvement was particularly noticeable at the lower feed rate of 0.05 mm/rev. Chinchanikar et al. [150] discovered that increasing the proportions of boric acid concentration in sunflower oil reduced cutting forces and surface roughness in hard turning. Makhesana and Patel [151] examined the effectiveness of hard turning under various conditions, including dry, conventional flood cooling, MQL, and MQL + CaF₂ solid lubricant. Additionally, the impact of particle size and concentration on surface trait and flank tool wear was examined; compared to results obtained under neat oil, flood cooling, and dry conditions, CaF₂ blended oil performed better. Larger-sized CaF₂ particles showed greater flank tool wear and surface trait under conditions of lowest cutting speed (90 m/min). In comparison to other concentrations, 20 wt% concentrations result in lesser surface quality and tooling wear as cutting speed increases. In a different study, Makhesana and Patel [152] investigated the effects of various solid lubricant concentrations when hard-turning EN 31 steel using CVD-coated carbide tools. They compared solid lubricant performance to that of dry, wet, and MQL lubrication. Temperature, surface roughness, and flank wear were all improved by using solid lubricant at a 15% concentration. Figure 21 indicated that adhesion and abrasion mechanisms accounted for the majority of tool wear. Each cutting circumstance clearly reveals abrasive marks. Solid lubricant conditions were shown to have the least tool wear in comparison to dry, wet, and MQL.

Singh and Rao [153,154] developed a new experimental setup to deliver solid lubricant powder in hard turning of bearing grade steels. It was discovered that the application of hard turning assisted by molybdenum disulphide improves machinability and surface quality significantly. Additionally, the cutting force remained essentially constant while the lubricating flow rate varied from 3 to 10 g/min. Kumar et al. [155] exhibited hard part turning experiments on AISI 4340 steel using two different solid lubricant environments (h-BN and ZnS), and the results (cutting force and chip-tool interface temperature) were in contrast to wet cooling and dry cutting results. Solid lubricant-assisted machining outperformed both dry and wet machining. The solid lubricant h-BN performed better than the ZnS lubricant. In the hard turning of AISI 4340 steel, Paul and Varadarajan [156] used a novel semi-solid assisted machining conception to manage cutting temperature, cutting force, tool wear, and surface traits. Using a special semi-solid lubricant applicator developed for this purpose, a mixture of grease and 10% graphite was applied at the cutting tool formed chip interface, work-tool interface, and back side of the chip. The results demonstrated that using grease and graphite, semi-solid lubricants, at the cutting tool-chip interface, a small amount of cutting fluid applied over the insert reduced cutting force, cutting temperature, tool wear, vibration, and enhanced surface smoothness. Similarly, in hard turning of AISI 52100 steel using a mixed ceramic tool, D.U. et al. [157] studied the effects of solid lubricants (graphite and MoS₂). Furthermore, the trials were also conducted in dry conditions, and the outcomes in solid lubricant were contrasted with those in dry settings. Dry cutting produced the roughest surface on the completed product, whereas MoS₂ solid lubricant produced the least. Additionally, the roughness decreased with increasing speed up to 125 m/min while improving after this speed. The details of the solid lubricant and cutting parameters used in hard turning are shown in

Table 7. Details of solid lubricants and cutting parameters used in hard turning.

Ref.	Types of Solid lubricants	Delivery Method	Test Material	Cutting Tool	Cutting Parameters			Response Studied
					Vc m/min	f mm/rev	ap mm
[19]	Graphite and Boric acid (50, 100, 150 and 200 µm)	Delivered into cutting zone in powder form	EN 8	Carbide insert	110	0.25	1	Cutting forces Tool temperature Surface roughness
[52]	MoS2	MQF-Minimum quantity fluid	AISI D6	PCBN	150 225 300	0.05 0.10 0.15	0.2	Tool life Resultant force Surface roughness Volume of material removed
[150]	Boric acid	Mist Solid lubricant	AISI 52100	PVD-TiSN-TiAlN coated Carbide	100 150 200	0.1 0.3	0.1 0.3	Cutting forces Surface roughness
[151]	CaF2	MQSL- minimum quantity solid lubrication	AISI 52100	PVD-TiAlN- coated carbide	90 130 170	0.2	0.5	Flank wear Surface roughness Chip-tool-interface temperature
[153,154]	MoS2 (2 µm averageparticles size)	Delivered into cutting zone in powder form	AISI 52100 steel	Mixed ceramic tool	50 75 100 125 150	0.04 0.08 0.12 0.16 0.20	0.2	Surface rough ness Cutting force
[155]	h-BN ZnS	Delivered into cutting zone in powder form	AISI 4340 steel	CBN	50 75 100 125 150	0.04 0.08 0.12 0.16 0.20	0.2	cutting force chip-tool interface temperature
[156]	Grease + 10% graphite	semi-solid lubricant applicator	AISI 4340 steel	Multicoated hard metal inserts	80 90 100	0.08 0.10 0.12	0.5	Tool vibration Surface finish Tool wear Cutting force Cutting temperature
[157]	CaF2	Minimum quantity solid lubrication	EN 31 Steel	CVD–coated carbide tool	130	0.2	0.5	Flank wear Temperature Surface roughness

Vc—Cutting speed, f—feed rate, a_p—depth of cut.

.

3.5. Nanofluids

Nanofluid is an emerging coolant for hard-turning applications as it reduces tool wear, cutting force, cutting temperature, friction coefficients, and power consumption. Nanofluid synthesis has a prime role in achieving effective lubrication and cooling in machining processes. The synthesis of nano fluid is a very important aspect of getting effective lubrication and cooling results in machining. The nano fluid was synthesized using a one-step or two-step methodology. Several authors have preferred the use of a two-step methodology due to its lower budget and simple implementation [158,159]. The two-step methodology to prepare a nano fluid was displayed in Figure 22. Generally, for preparing the nano fluid mixture, two different methods were used, i.e., one step and two-steps. Most of the researchers used the two-step method because its application is simple and its synthesis cost is minimal. The performance of nano fluid was highly dependent on its concentration and its dispersion stability. The higher stability of nano fluid was more advantageous in machining applications. According to Yu and Xie’s [160] report, nano fluids had improved thermo-physical characteristics such as thermal conductivity, convective heat transfer coefficient, thermal diffusivity, and viscosity compared to base cutting fluids such as water or oil.

The addition of nanoparticles (≤100 nm) into the parent fluid provided an adequate augmentation in heat transfer thus considerably dropping the cutting temperature in the shearing area [161]. The augmentation in lubricious features of nanoparticles-mixed cutting fluid is attained by establishing a tribe-thin-film between two meshing surfaces [162], the ball-bearing effect [163], polishing impact [164], and the mending consequence [165]. Referring to Lee et al.’s [166] results, polishing and mending of nano fluids were the important approaches responsible for the enhancement in the machining procedure. Likewise, according to Peng et al. [167], adhesion among two mating surfaces and the anti-wear properties of nanoparticle-mixed coolant are enhanced by four distinct mechanisms: (a) Spherical nanoparticles roll smoothly between two meshing surfaces, reversing the slipping friction into rolling and sliding frictions. (b) Propensity to mix with friction surfaces and form a thin surface-protecting layer. (c) During the cutting process, impinged nanofluid collected on the mating surfaces and formed a thin tribo-film that compensated for the material loss; this is known as the “mending effect”. In addition, the lubricating surface roughness was reduced by friction due to the presence of nanoparticles between the contact surface, a phenomenon known as the polishing effect [168], (d) Evenly dispersed nanoparticles sustain the machining load (compressive), and reduced the stress concentration. These mechanisms are illustrated in Figure 23.

Further, many nanofluids have been utilized in hard turning to investigate the machinability of hardened steel. However, a critical analysis was made below to select the most appropriate nanofluids for machining hardened steel. Sharma et al. [25,26] presented an overview of the implementation of nano enriched coolants in machining work. Improvements in tool life and reductions in forces, temperatures, and surface roughness were reported. In another work, hybrid (Alumina + MoS₂) based nanofluids have been implemented in hard turning on AISI 304 steel with carbide insert. Al-MoS₂ mixture-based nanofluid showed better results relative to Al₂O₃-based nanofluid. Khalil et al. [169] investigated the effect of using Al₂O₃ nano lubricant while turning AISI 1050-grade steel with a coated carbide cutting tool and noticed lower tool wear as well as a very long tool life. Khajehzadeh et al. [170] experimentally studied water-based TiO₂ nanofluid using hardened AISI 4140 steel and found that lower rates of cutting tool flank wear were obtained by increasing nanoparticle concentration from 3% to 5% or reducing the nanoparticle size from 50 to 10 nm. Furthermore, it can improve the machining performance and total manufacturing cost of a component. Das et al. [171] executed a machining operation on hardened AISI 4340 steel with MQL-Al₂O₃-based nano fluid and found a relevant enhancement in tool wear relative to water-soluble materials and compressed air. Further, the variations in turning forces with flank wear were studied for all three cutting environments (compressed air, water-soluble coolant, and nanofluid). Figure 24 illustrates the impact of flank wear on turning forces. The flank wear greatly affects the intensity of forces produced in turning. The lowest range of cutting forces was found in the nanofluid machining environment in comparison to others.

In another work, Das et al. [172] utilized three different nanofluids (CuO, Al₂O₃, and Fe₂O₃) via MQL. CuO nanofluid exhibited the superior result among these nanofluids, as the least flank wear and lower surface roughness were noticed. Patole et al. [173] conducted an experiment using different parameters on the hard turning of AISI 4340 with nanofluid (multiwalled carbon nanotube) under MQL conditions. The results showed that feed rate played a vital role in the creation of lower surface roughness, followed by the depth of cut, whereas cutting speed had the least significance in producing a lower surface roughness under MQL using nanofluid. Singh et al. [174] presented the mathematical expressions to study the role of grain size on the thermal conductivity of nano solids and noticed the significant retardation of thermal conductivity with a decrease in grain size. Sharma et al. [175] found enhanced quality of the surface and improved lubricating action in machining by using higher concentrations of nano particles in nanofluid compared to traditional cutting fluid and dry conditions. In addition, minimizing trends of frictions, forces, power utilization, flank wear, and temperature were observed during machining with the nanofluid surrounding. The factors such as mode of lubrication, nano element size, nano element concentration, nozzle orientation, air pressure, and spraying distance were major terms that affected the overall machining performances. Thakur et al. [176] compared the performance of MQL and SiC-based nanofluid MQL environments in turning EN24 alloy steel and discovered that SiC-based nanofluid MQL turning outperformed MQL turning in terms of surface roughness height, cutting force, and cutting temperature.

Ibrahim et al. [177] conducted the hard turning experiments on heat-treated AISI D3 steel using MQL and ZnO nanofluid MQL (NFMQL) coolants. They used a 0.1 wt% concentration of rice bran oil-based ZnO nanofluid and compared its performance with that of MQL, dry cooling, and flood cooling. The least cutting force was obtained with nanofluid MQL. In comparison to dry and flooded machining, it was highlighted that NFMQL machining outperformed the best lubrication-cooling strategy because it led to reduced cutting force, enhanced surface morphology, and less tool wear. According to Singh et al. [178], the application of dispersed nano-sized powder in traditional coolants is attributed to lower cutting temperatures, power, specific cutting energy, tool wear, cutting forces, and friction during machining action. Further, it reduced the dimensional deviation of the test piece during machining. Duc et al. [179] evaluated the influence of coated carbide inserts in 90 CrSi steel with coated carbide tools and found that MQL with Al₂O₃ and MoS₂ nano fluids improved the performance of coated carbide inserts. In comparison to Al₂O₃ nanofluid, MoS₂ nanofluid has a considerable influence on lowering cutting force while enhancing thrust force, according to the experimental results. Al₂O₃ soybean-based nano fluid outperforms the others in terms of achieving the lowest value of surface roughness.

Krishna et al. [180] found that when the nanoboricacid particle was added to two types of base oils, the surface roughness, cutting temperature, and flank wear were all significantly reduced because of the better lubricating capability of boric acid. Among all experiments, the performance of coconut-based nanofluid seemed to be elevated compared to SAE-40-based nanofluid in hardening AISI 1040 steel with a cemented carbide cutting insert. When comparing the machinability performance of cryogenic cooling and nanofluid over hardened AISI 420, Yldrm et al. [181] found that nanofluid performed better than cryogenic cooling in the analysis of machined surfaces, while cryogenic cooling performed better in the analysis of chip morphology, tool life, tool-chip interface, and tool wear. Figure 25 shows the variations in temperature (chip-tool interface) with respect to cutting speed and feed for both nanofluid and cryogenic cooling conditions. From this comparison graph, it can be seen that cryogenic cooling offers superior cooling to nanofluid. In comparison to nanofluid, cryogenic cooling improved the tool-chip interface temperature by an average of 31.05%. This circumstance was related to liquid nitrogen’s ability to cool things down. Moreover, SEM images of the tool’s tip (Figure 26) were obtained at a speed of 100 m/min in both nanofluid and cryogenic cooling situations. In comparison to nano fluid cooling, cryogenic cooling imparted less wear, while edge fracture also occurred in both cooling conditions. Adhesion was also occurring on the face of the tool during both cooling processes. Singh et al. [182] used the MQF technique to turn AISI 304 stainless steel using an alumina-based nanofluid mixed with graphene nano platelets, which helps promote a fluid with better thermal and tribological properties. In aspects of machining force and surface roughness, the nanofluid mixed with graphene nano platelets performs well as compared to alumina nanofluid and base fluid applications. Tun et al. [183] executed turning experiments on 90 CrSi low alloy steel with MoS₂ nanofluid MQL and discovered that, in aspects of surface topography and surface roughness, MQL using MoS₂ nano additives provided an improved result in contrast to pure MQL. The white layer, burn marks, and surface deformation were also reduced as a result of improved lubricating and cooling performance.

Prasad and Srikant [184] discovered that increasing the content of nano graphite in cutting fluid improved both thermal and machining performances. Higher MQL flow rates resulted in improved machining performance when hardened AISI 1040 steel was machined with uncoated cemented carbide and HSS cutting inserts. Liew et al. [185] studied and compared the performance of CNF nanofluid and de-ionized water in machining AISI D2 steel. The findings revealed that by using CNF nanofluids instead of de-ionized water, surface characteristics and tool life were enhanced due to CNF’s superior tribological and thermal conductivity. Sharma et al. [186] performed a hard turning operation on AISI 304 steel using alumina–MWCNT hybrid nanofluid; the outcome showed that hybrid nanofluid performance was superior to cutting fluid with mixed alumina nanoparticles.

However, based on this critical review, the most common nanofluids used as metal working coolants were prepared using Al₂O₃, TiN, CuO, MOS₂, and MWCNT nano powders. The implementation of graphene, nano-sized graphite, and ZnO and Fe₂O₃ based nanofluids was very limited, especially in hard turning. The application of MgO nanofluid in hard turning was yet to be used in hard turning, although it has several favorable thermal and tribological properties. However, the cooling and lubricating performance of graphene, nano-sized graphite, ZnO, Fe₂O₃ and MgO in hard turning can be investigated in the future. The hybrid nanofluid seems to be more effective than the pure nanofluid. Hence, while making a hybrid nanofluid, the percentage volume or weight of each nano particle can be varied and investigated in hard turning. In addition, dual-nozzle-assisted nanofluid MQL with different flow rate combinations can be used further. The details of the utilized nanoparticles, base fluid, optimum concentration, and responses studied in hard turning were reported in

Table 8. Details of utilized nanoparticles, base fluid, optimum concentration and responses studied in hard turning.

Ref	Material	Nano Particle	Base Fluid	% Concentration	Optimum Concentration	Responses
[169]	AISI 1050	Al2O3	Soluble cutting oil	0.1%		Tool wear
[170]	AISI 4140	TiO2	Water based	0.5%,1.5%,3%	0.5%–3%	Cutting force, cutting temperature, flank wear
[171]	AISI 4340	Al2O3	Eco-friendly radiator coolant	2.5 gm		Cutting force, feed force, radial force
[172]	AISI 4340	Al2O3, CuO and Fe2O3	rice bran oil	0.1%		Cutting force, chip morphology, surfaces integrity
[173]	AISI 4340	MWCNT	Ethlyne glycol	2%		Tool wear, surface quality, cutting force
[176]	EN- 24	SiC (silicon carbide)	Water soluble oil	(0.5%,1.0%,1.5%)	1.5%	Surface temperature, cutting force, cutting temp
[177]	AISI D3 steel	ZnO particle	Rice bran oil	0.1%		Cutting force
[179]	90 crSi	Al2O3, MoS2	soybean oil and water-based emulsion	1% & 3%	1%	Cutting force, surface roughness
[181]	AISI 420 steel	Graphene Nano particles	Fuchs plantocut 10 SR	0.5		Cutting zone temp, surface roughness, tool life, tool wear,
[182]	AISI 304	Al2O3, Al-GnP	Servo cut oil with Deionized water	0.25%,0.75%, 1.25%	1.25%	Cutting force, surface roughness
[183]	90 CrSi low alloy steel	MoS2	Water based emulsion	1%, 2%, 3%	2%	Surface roughness, surface topology
[184]	AISI 1040	Nano graphite powder	Water soluble oil	0.15,0.3%,0.5%	0.3%	Cutting force, surface roughness, and tool wear
[185]	AISI D2 steel	CNF	Deionized water (0.1 gum Arabic)	0.1 gm		Tool life, surface roughness
[186]	AISI 304 steel	(Al2O3/Al-MWCNT)	Vagetable based emulsion	0.25%, 0.75%, 1.25%	1.25	Surface roughness and machining force

.

3.6. Cryogenic Cooling

Cryogenic technology investigates how materials behave at very low temperatures. The term “cryogenic temperature” is used in literature to describe temperatures lower than −153 °C [187,188,189,190]. Additionally, according to a report from the National Institute of Standards and Technology, a cryogenic temperature is one that is lower than −180 °C [187]. Cryogenic machining is a newly developed and environmentally friendly technique in which cryogenic coolants minimize the cutting temperature to significantly improve cutting tool performance while avoiding contamination. Cryogenic liquid has been used in cryogenic machining as a cutting fluid during the machining process. Liquid nitrogen (LN₂) and carbon dioxide (CO₂) are generally used as coolants in cryogenic machining [191]. The commonly used cryogenic temperature in machining applications was −196 °C [192] for LN₂ and −56.6 °C [193] for LCO₂. Figure 27 depicts the advantages of cryogenic machining for green manufacturing [194]. On the other hand, wet machining and MQL machining processes can cause environmental issues such as air or water pollution, requiring purifying operations. The literature review found that, due to their high productivity and good performance compared to conventional machining, cryogenic cooling machining processes contribute to the creation of additional economic, social, and environmental value. Figure 28 shows the flank wear length of a ceramic and CBN cutting insert during hard turning under various cooling conditions [194]. The CBN cutting tool with the flank jet increased the tool’s life by three times when compared to a dry ceramic cutting tool. The tool life was three times longer when compared to the ceramic cutting tool used with a rake jet for the LN₂ spray. Furthermore, the flank jet of the ceramic cutting tool extended tool life at the flank wear length.

Leadebal et al. [195] conducted experiments on the hard part turning of AISI D6 tool steel with a PCBN tool under a cryogenic cooling environment. The result showed that while machining under cryogenic conditions, the surface integrity of the workpiece was changed, and it also had a lower surface roughness value in comparison with the dry condition. Kumar et al. [196] carried out machinability performance under cryogenic cooling and flood cooling during turning of hardened AISI 4340 steel by a PVD coated carbide cutting insert. Under cryogenic cooling conditions lower surface roughness value was obtained (0.4 µm) and also the life of tool up to 34 min was achieved. In cryogenic cooling conditions, 18% less cutting force and a maximum savings in power consumption of 320 w were noticed in comparison to conventional cooling conditions. Umbrello et al. [197] experimented on hardened AISI 52100 steel with a CBN tool in cryogenic and dry cutting environments. In cryo-cooling machining, the surface integrity was enhanced and the white layer thickness was reduced as compared to dry cutting conditions. According to Bicek et al. [198], the tool life greatly improved when turning was executed on hard AISI-52100 steel in cryogenic cooling. It was also suggested to select the appropriate tool geometry and edge profile to avoid tool-vibration. Tool vibration can be avoided by proper selection of cutting tool geometry and cutting-edge preparation. Cryogenic cooling had a valuable effect on surface integrity. Cetindag et al. [199] utilized both a wiper and a conventional insert during hard machining of AISI 52100 steel under various environments (MQL + CO₂ and MQL + LN₂) and observed that (MQL + CO₂) significantly reduced tool wear of both the cutting tools. A wiper ceramic insert gives a lower surface roughness value, whereas conventional inserts produce a rougher work surface. Figure 29 shows the surface roughness (Ra and Rz) values of the machined surface under different cutting conditions. The end result ensured that wiper inserts performed better than conventional inserts in all cooling conditions. Additionally, the wiper insert improves surface quality and chip breaking power when being machined at a higher speed. Among all cooling conditions, MQL + CO₂ exhibited the lowest Ra, while Rz was the least under the MQL + CO₂ cooling condition [199].

Nie et al. [200] investigated the white layer formation under dry and cryogenic cooling during the machining of AISI 52100 steel by CBN tools and found that, in cryogenic conditions, white layer formation was reduced and the hardness of the surface increased at the elevated cutting speed (300 m/min). Wu et al. [201] experimented with the hard turning of P20 mold steel and reported that tool wear was the most influencing factor for the formation of the white layer. The micro hardness of the white layer under the cryogenic condition was higher than the white layer formation under the dry cutting condition also observed white layer thickness value was higher in dry cutting comparison with the cryogenic condition. Figure 30 represents the simulation of dry and cryogenic LN₂ cooling cutting results at different flank wear conditions [201]. When the cutting tool is in new condition (unmachined), the range of the high-stress region is small, and also strain and temperature are very small, so the white layer is difficult to form. However, when tool wear increases, plastic deformation, stress regions, strain, and temperature also increase, which creates a good environment for white layer formation. At a given cutting temperature, the cutting interface creates significant stress as the workpiece material moves in relation to the cutting edge, causing the workpiece material to gradually deform plastically. Huang et al. [202] conducted a hard part turning operation on AISI 52100 steel under both cryogenic and dry cooling conditions. During machining under cryogenic conditions, chip segmentation was observed at a lower cutting speed, and it was also found that the initial hardness of the material and the cutting parameters and cooling condition were the influencing factors for changing the hardness of the material. Fernandes et al. [203] conducted a comparative experiment on hardened AISI D6 steel using a PCBN tool in dry, wet, and cryogenic cooling circumstances. The study found that, in aspects of tool life, cryogenic cooling performs better at a higher flow rate than both wet and dry cooling conditions. According to Zarate et al. [204], during the hard turning of 100 Cr6 steel by a CBN wiper-coated tool under cryogenic conditions, at a higher cutting speed of more than 300 m/min, severe tool wear was observed, which led to an increase in the machining force. In cryogenic cooling, higher tensile residual stresses were noticed on the surface, which could be caused by the faster cooling condition. The details of the cutting tool with different cryogenic coolants utilized in hard turning were reported in

Table 9. Details of cutting tool with different cryogenic coolant utilized in hard turning.

Ref.	Material	Cryogenic Coolant	Cutting Tool	Responses
[195]	AISI D6 tool steel (57 HRC)	LN2	PCBN	Tool wear tool life chip morphology
[196]	AISI 4340 (56–58HRC)	LN2	Coated carbide (Coating: TiAlN)	Surface roughness, cutting energy, cutting force and tool life
[197]	AISI 52100 steel (61 ± 1)	LN2	CBN	Higher surface quality Increased machinability,
[198]	AISI 52100 steel (183–229 HV) 850 HV	LN2	CBN	Tool life increased Thermal residual stress decreased
[199]	AISI 52100 steel	MQL + CO2 and MQL + LN	Wiper CBN tool	Reduced tool wear, lower surface roughness
[200]	AISI 52100 steel (65 HRC)	LN2	CBN tool	Reduction of temperature, increase of surface roughness
[201]	P20 Mold steel (55–57 HRC)	LN2	PCBN	White layer thickness, micro hardness
[202]	AISI 52100 steel (52, 54, 62 HRC)	LN2	PCBN	Chip morphology
[203]	AISI D6 tool steel (57 HRC)	LN2	PCBN	Nose wear, tool life
[204]	100 Cr6 steel (62 HRC)	LN2	CBN wiper coated insert	Cutting force, tool wear, residual stress, microstructure

.

3.7. Ionic Liquids

Ionic liquids (ILs) are emerging additives to enhance the tribological characteristics of lubricants due to their excellent properties, such as superior thermal stability, extraordinary oxidative stability, inferior vapour pressure, non-flammability, non-volatility, and elevated load-bearing ability [205]. ILs is a salt made up of cations and anions. Its melting point was lower than that of water (100 °C). Many researchers have used ILs as a suitable additive to base lubricants for tribological applications [206,207,208]. They have found a great reduction in tool wear and friction. Further, some researchers mix it with the base fluid of minimum quantity cooling and lubrication (MQCL) systems for different machining applications. Pandey et al. [205] utilized 1 wt% of ILs in the machining of hardened AISI D2 steel. The surface quality was enhanced considerably, but there was not much improvement in the material removal rate, and no relevant reduction in tool wear was found. It was recommended to use more wt% of ILs in the base fluid of MQL. Goindi et al. [209] stated that the addition of 1% of the weight of ILs in vegetable oil was sufficient to improve the tribology of the cutting process. The machining performance with ILs added MQL was enhanced in comparison to dry, flood, and without added ILs MQL cooling modes. In another work, Goindi et al. [210] compared the IL’s performances for rough and finish machining using different cutting parameters. For the finishing process, the cutting forces were largely influenced, while surface roughness was greatly improved for all cutting conditions. In their further research, Goindi et al. [211] implemented two distinct categories of ILs (fluorine and phosphonium bases) in machining AISI 1055 steel. The result showed that the ILs with fluorine-bases were advantageous for high-speed machining, whereas, ILs with phosphonium-bases were more beneficial when the machining speed was low. Additionally, it was recommended that the surface finish quality be improved with the higher leading viscosity of ILs. In Figure 31, surface roughness and cutting force values were shown under various cutting conditions for neat oil and for vegetable oil with IL308 (1 wt%) and IL308 (0.5 wt%) MWF [211]. According to Figure 31a, in light machine conditions, both solutions of ionic liquid IL308 with vegetable oil in a concentration of 0.5wt% and 1wt% have lower cutting forces when compared with cutting forces produced in neat vegetable oil. From Figure 31b, in heavy cutting up to 200 m/min, the 1% IL308 is most preferable to get the least force, while beyond 200 m/min speed, 0.5% ILs are suitable. Saini et al. [212] implemented two distinct ILs based on ammonium and phosphonium in turning AISI 1045 steel. A modified form of Jatropha oil was utilized as a base lubricant with three different concentrations of each ILs. Ammonium-based ILs of 10 wt% concentration and phosphonium-based ILS of 1 wt% concentration were advantageous to exhibit better performance in comparison to other concentrations. In further work, Saini et al. [213] found a 50% enhancement in the cutting life of the tool in ammonium-based ILs (10 wt%) for assisted machining in comparison to synthetic-ester fluid, while 36% was an improvement in comparison to modified jatropha oil. Davis et al. [214] included 1-butyl-3-methylimidazolium hexafluorophosphate ILs with a 0.5wt% concentration in de-ionized water as the base fluid for machining titanium alloys. Approximately 60% less flank wear was seen compared to dry, and 15% less flank wear was observed compared to MQL alone. Additionally, the distance between adjacent teeth on the chip edge was greater with ILs MQL than with MQL or dry. Figure 32 depicts the advancement of tool-wear under three distinct lubrication regimes, including dry MQL with DI water and MQL with a water–IL combination. Compared to a dry cutting environment, water-based MQL can reduce tool wear by approximately half, as shown in Figure 32a. When IL was used as an additive during machining, tool wear was decreased by 60% when compared to a dry cutting environment. In addition, a 15% decrease occurred relative to the MQL baseline condition. Figure 32b shows the tool wear obtained under the IL base MQL condition. Pham et al. [215] reported that when 1-butyl-3-methylimidazolium iodide ILs were used with MQL technology, lower surface roughness in micro-milling was observed. Furthermore, better quality with fine tool marks was seen under these ILs in comparison to dry, distilled, and other water and other ILs. Nareshbabu et al. [216] found an excess amount of reduction in surface roughness, i.e., 88% and 75%. The cutting temperature was decreased in the range of 49% and 74% in comparison to plain MQL and dry. The details of weight concentration, base fluid, and cutting parameters used in ionic liquid for hard turning are shown in

Table 10. Details about wt concentration, Base fluid, cutting parameters used in hard turning.

Ref.	ILs	Con.	Base Fluid	Cutting Parameters			Responses Studied
				V m/min	f mm/rev	d mm
[205]	Trihexyltetradecylphosphonium Chloride	1 wt%	Coconut oil	60, 120, 180	0.06, 0.12, 0.18	0.2, 0.3, 0.4	Roughness, tool wear and material removal rate
[209,210]	1-methyl 3-butylimidazoliumhexafluorophosphate 1-methyl 3-butylimidazolium tetrafluoroborate 1-methyl 3-butylimidazolium bis(trifluoromethylsulfonyl)imide	1 wt%	Vegetable oil (Canola oil)	Rough-150, Finish-200	Rough-0.3, Finish 0.1	Rough-0.8 Finish-0.3	Cutting force and surface rough-ness, Tool wear morphology
[211]	1-methyl 3-butylimidazoliumhexafluorophosphate 1-methyl 3-butylimidazolium tetrafluoroborate tributyl(nonyl) phosphoniumbis(2-ethylhexyl) phosphate	0.5 wt%; 1 wt% 3wt%	Canola oil Polyethylene Glycol (PEG) 400	150, 200, 250	0.1, 0.3	0.3, 0.8	Rough-ness and cutting force
[212,213]	methyltrioctylammoniumbis(trifluoromethylsulfonylimide) trihexyl(tetradecyl)phosphoniumbis(2,4,4-trimethylpentyl) phosphinate	1%, 5%, 10%	Modified Jatropha oil	350	0.12	--	Cutting force Specific cutting energy, Temperature, friction, chip thickness, cutting angle, Surface roughness, tool wear, tool life, material removal rate
[214]	1-butyl-3-methylimidazolium hexafluorophosphate	0.5 wt%	Deionized water	120	0.05	0.1	Tool wear, Cutting force
[215]	1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide 1-butyl-3-methylimidazolium iodide	0.02 ml	Cutting oil, Deionized water	12,000 rpm	24 m/min	0.01	Cutting force, surface roughness, surface topology, Sustain-ability
[216]	1-butyl-3-methyl imidazoliumhexaflurophosphate	0.5 wt%	Deionized water	80, 120, 160	0.05, 0.08, 0.12	0.5, 0.75, 1	Rough-ness, tempera-ture, chip thickness

.

Moreover, the safety of working with IL cooling systems for machining applications is highly essential as they have various environmental issues such as toxicity, durability, and degradation rate. According to Saini et al. [212], despite various environmental shortcomings, ILs are nevertheless regarded as ecofriendly lubricants because of their distinctive set of characteristics. Many researchers noted that by choosing the correct IL, great yields can be obtained and the quantity of waste generated during a specific reaction can be considerably reduced [217]. ILs are dominant in substituting conventional oil in realizing more effective, ecological, and sustainable processes through systematic exploitation of their unique features [218]. Literatures [205,212] reported that the hydrolytic breakdown of halogenic ILs produces highly hazardous by-products such as HF, HCl, or H₃BO₃ acids, which are said to cause environmental damage, corrosiveness, penetrativeness, and hazards. Therefore, to overcome the issues stated, halide-free ILs are now recommended for use as a cutting lubricant in metalworking processes.

3.8. Hybrid Cooling

In recent studies, hybrid cooling concepts are becoming popular in hard turning applications. Very limited research was carried so far. The use of hybrid low-pressure dry ice blasting as a lubri-cooling supply for machining, which is based on a dual-channel lubri-cooling supply, is depicted in Figure 33 [219]. The system provides a continuous supply of hybrid dry ice lubri-cooling medium. The valves controlled the pressure and flow rates perfectly. Dry ice blasting is predicated on the assumption that solid granules travel linearly owing to their velocity, penetrating the tool-chip contact with negligible gas conversion. In addition, these grain scans transfer MQL mist to the machining zone and convert it straight to gas, eliminating the need for post-cleaning [219]. Cetinda et al. [199] investigated the performance of the Cryo MQL(CO₂ + LN₂) technique in the machining of hardened AISI 52100 steel with conventional and wiper CBN inserts. Experimental results investigated that MQL + CO₂ minimized tool wear of both conventional and wiper insert but the conventional cutting tools produced rougher work surfaces under all cutting conditions.

3.9. Summary: Advantages and Limitations of Various Cooling Stratigies

Flood cooling successfully reduced temperature generation during hard turning, but its use is not advised due to high coolant consumption, hazards to the environment and the operator’s health, and difficult and expensive disposal of used coolant. By using a small volume of coolants, minimum quantity lubrication (MQL) effectively eliminates the need to dispose of used coolant, as with flood cooling. MQL offers a method for delivering various coolants, such as mineral oil, nanofluid, vegetable oil, and ionic fluid, during machining. The MQL approach enables a significant improvement in tool life and surface finish, as well as a significant decrease in cutting temperature and force. It belongs to the green cooling category as well. The operating conditions, nozzle placements, and tribological characteristics of the base coolant all have a significant impact on MQL performance. In actual use, MQL causes air pollution and fog that are harmful to user health. High-pressure cooling is an emerging technology in cutting fluid applications to minimize friction at the chip-tool interface, control the temperature, enhance the transfer of heat, and improve the machining performance. It significantly lowers cutting forces and temperatures. Due to the high pressure of the coolant supply, which ultimately improves the machining performance, it is very suitable to break the chips. The main drawbacks of this technology are found to be its excessive fluid consumption and relatively higher sound generation on machining shop floors. Another technology used in hard turning is solid lubricant. It is more effective than fluid lubricants at high loads and higher cutting speeds. It generates a higher coefficient of friction and wears faster than liquid-lubricated cooling, which makes its application limited. Nanofluid MQL cooling is another advanced cooling approach successfully utilized in hard turning. It helps to enhance the heat transfer coefficient of basic coolants, and as a result, cutting temperature, wear, machining force, and power consumption were significantly reduced. Nanofluids pose significant challenges to users in terms of synthesis and stability maintenance. Additionally, using nano fluids as a cutting coolant has a number of negative effects on one’s health in addition to being expensive. Given the current state of the economy and the environment, it would be challenging to use them as a sustainable coolant. Another cooling method used in hard turning is cryogenic cooling. It offers more effective cooling than other coolants because the coolant penetrates the cutting zone more thoroughly. It helps in providing additional economic, social, and environmental values due to their superior performance in machining. Extra or additional control setup, high capital, and maintenance costs restrict their application in general-purpose machining. Ionic liquid (ILs) is a new emerging cooling technology used in hard turning. The addition of a small volume of Ils sufficiently improved the tribological behavior of base coolant, resulting in better tool life, a lower cutting force, and better machinability. The choice of environmentally friendly ILs is essential because the coolant is chemical-based. For machining applications, halide-free ILs are advised. Furthermore, toxicity testing is always advised before use in actual machining operations. Compared to cryogenics or MQL alone, the hybrid cooling concept is gaining popularity. In many machining applications, it effectively serves as a lubricant and a cooling agent. One of the best cooling solutions currently available for metallic surfaces that are challenging to cut is hybrid (cryogenic + MQL) cooling.

4. Sustainability Assessment

In recent scenarios, manufacturing organizations are keenly interested to adopt sustainable manufacturing processes for developing the product. In a broad sense, the term “sustainability” can be defined as the system’s ability to facilitate and endure a healthy life. Manufacturing plays an important role to affects human being life by providing good jobs, improving living standards, and economic growth. However, nowadays, sustainable manufacturing is noteworthy to minimize the use of energy and natural resources. Sustainable manufacturing also aims to maximize profit and ensure societal and environmental safety throughout the product life cycle [220]. According to Davim et al. [221], sustainability involves the assessment of the economic, social, and environmental structure of a manufacturing process. Pradhan et al. [222] also state that sustainable manufacturing is an effective way to encourage the adaption of new methodologies that are economically favorable and healthier for the environment and society. Moreover, it is also suitable for lowering operational and industrial wastes, providing a hygienic environment to operators, and controlling environmental hazards. Machining is one of the significant manufacturing processes. However, in recent years, sustainable machining has grown in importance from the perspective of environmental safety. In the course of machining activities, many pollutants have been formed that need to be carefully handled so that the environment can be protected. Further, sustainable machining enables saving cutting energy, cutting fluid, and machining costs. It also helped with the recycling of coolants, cutting tools, and chips.

Nowadays, sustainable hard machining has gained significant traction in the field of machining. The sustainability of hard machining can be achieved by the implementation of appropriate cutting tool geometry, tool coatings, tool materials, cutting fluids, and a cutting fluid delivery system. Many researchers have implemented different tool coating materials in hard machining. Moganapriya et al. [223] implemented three distinct TiAlN, AlTiN, and TiAlSiN coated tools in dry machining AISI 304 stainless steel. The tool coating TiAlSiN and cutting variable values (speed—180 m/min, tool feed—0.1 mm/rev, and cutting depth—1.5 mm) are suitable to achieve sustainable results. Ahmad et al. [224] have reviewed the performance evaluation of three green cooling technologies (dry, MQL, and cryogenic) in machining 4340 steel. The cryogenic cooling provided the best performance over dry and MQL. Hybrid cooling (cryogenic-MQL) was suggested for implementation in further studies. Gajrani et al. [225] implemented self-lubricious textured cutting tools in hard turning. Further, they applied a MoS₂ coating to the textured tool and compared the performance with the uncoated textured tool. A mechanically micro-textured (perpendicular to the principal cutting edge) MoS₂ coated tool was traced to be the best tool for machining hard steel under dry conditions. According to Fernandes et al. [203], without suffering financial losses, liquid nitrogen can be viewed as a sustainable substitute for the use of mineral-based emulsions for tuning hard steels.

For sustainable machining, the hard turning process must be cost-conscious. The cost of the cutting tools significantly affects the sustainability of the machining process. According to Padhan et al. [222], the ceramic tool permitted lower cutting costs (USD 0.5) per part in hard turning AISI D3 steel in nanofluid-MQL conditions. Kim et al. [194] studied the cost analysis of hard turning AISI 52100 steel using ceramic and CBN tools. The ceramic cutting tool cost USD 15 and could use four tips, while the CBN cutting tool (two tips) cost USD 70, which confirmed that the CBN tool was 4.7 times more expansive than the ceramic cutting tool. Using the ceramic cutting tool, the number of parts that could be produced under wet, dry, cryogenic jet (rake direction), and cryogenic jet (flank direction) was 23, 11, 34, and 68, respectively. Similarly, using CBN tool, the number of parts in dry cutting was 23 while in cryogenic jet (flankdirection was 79) under same cutting parameters. Therefore, the worst performance in terms of tool cost was found with the CBN tool in a dry scenario. Dash et al. [226] applied a multi-layered (TiCN-Al₂O₃-TiN) carbide tool to hard turn AISI D3 steel in a nanofluid-MQL environment. The cost of machining each part was estimated to be around USD 0.6.

Further, many researchers reported the use of sustainability assessment tools in hard machining. Padhan et al. [222], Ross et al. [227], Das et al. [228], and Panda et al. [229] have implemented the Pugh matrix method coupled with the Kiviat radar diagram to explore sustainable hard machining. They have concluded that the Pugh matrix method coupled with the Kiviat radar diagram method is suitable to assess the sustainability of the hard machining process. By using this sustainability concept, Ross et al. [227] developed the Kiviat radar diagram (Figure 34) for sustainability and estimated the sustainability score. Figure 35 was used to find the most sustainable cooling strategies among flood, MQL, CO₂, and hybrid (CO₂ + MQL) for machining Ni-based alloys. Padhan et al. [222] stated that the nanofluid-MQL cooling concept in hard turning applications was techno-economical and eco-friendly because it provided enhanced surface quality, a low level of noise, favorable power consumption, better safety, minimal environmental impact, fantastic part cleaning, minimal cooling, and low coolant disposal costs. Bhat et al. [230] utilized the relative sustainability of the process coupled with a decision-making approach to find the sustainability of the machining process. Mia et al. [231] have also implemented the Pugh matrix tool to find sustainable machining conditions for hard turning. They have implemented dry, MQL, flood, solid-lubricant + compressed air in machining AISI 1060 hard steel. Among these cooling strategies, the MQL was found to be more sustainable due to its ability to achieve enhanced productivity without harming environmental regulations or operator health.

Further, some authors have implemented the life cycle assessment (LCA) tool for analyzing sustainability [232,233,234,235,236,237,238]. LCA is a holistic technique that covers all aspects of environmental exchanges such as emissions, energy, resources, and industrial wastes arising in the entire product development cycle [234]. According to Zanuto et al. [238] LCA is a noteworthy technique for expressing the environmental consequence allied with a product in its whole life cycle (from the stage of procurement of the raw material to production followed by disposal of waste). The LCA technique followed the ISO 14040 standard for principles and framework while following ISO 14044 for requirements and guidelines. The framework of LCA (referring to ISO 14040) can be shown pictorially in Figure 36. Moreover, the list of LCA software used in various machining works is disclosed in

Table 11. Details of LCA software used in diverse machining applications.

Ref.	LCA Software	Cooling Method	Responses Studied	Recommendations
Fernando et al. [232]	Analysis–SimaPro 8.5 software coupled with Ecoinvent database 3.4 Assessment–ReCipe 2016 midpoint (H) V1.02	Dry wet	Energy consumption Coolant consumption Surface roughness Material removal rate	Wet cooling system has higher sustainability in comparison to dry.
Mia et al. [233]	Analysis–SimaPro 8.0 software coupled with European databases Assessment–Impact 2002+ and EPS 2000	Dry Cryogenic N2 (single and dual jet)	Cutting force Chip tool interface temperature, Surface roughness Specific cutting energy Material removal rate	Dual Jets cryogenic N2 cooling system has higher sustainability in comparison to dry and single nozzle cryogenic cooling.
Campitelli et al. [234]	Analysis–Open LCA 1.4.2 software with green delta Assessment–CML 2001 method from Ecoinvent v3.1	Flood cooling MQL	Toxicity Emission	MQL was more sustainable than flood cooling.
Silva et al. [235]	Analysis–GaBi professional version-6 software Impact assessment–CML 2001	dry	Abiotic depletion prospective Acidification prospective, Freshwater aquatic toxicity prospective Human toxicity prospective Global warming prospective	Machining parameters have least influence on the results.
Gupta et al. [236]	Analysis–Simapro 8.3 software Impact assessment–EPS 2000 and ReCiPe Endpoint v1.12	minimum quantity cutting fluids (MQCF), Ranque-Hilsch Vortex Tube (RHVT) assisted MQCF	Cutting force, Power consumption, Specific cutting energy, Chips morphology, Material removal rate, Surface quality	RHVT assisted MQCF shown better sustainability over MQCF
Shi et al. [237]	Impact assessment–CML 2001 method	Flood cooling	Material consumption, Energy consumption, Machine tool parameters, Waste emissions	This method was suitable to identify distinguish environmental emissions in turning process.
Zanuto et al. [238]	Analysis–GaBi 6 coupled with SimaPro Impact assessment–CML 2001-april 13	dry	Energy consumption	TiN coated tool attributed a low impact in comparison to uncoated tool in machining

.

5. Conclusions

This paper reviews significant published experimental investigations on turning hard steel under dry and variable cooling conditions. In addition, numerous methodologies for analyzing sustainability in hard metal machining were given and critically analyzed. The following are the primary conclusions:

• Dry hard turning using a CBN/PCBN tool outperformed conventional cutting tools. Dry hard turning was not acceptable for any tool material under higher cutting parameter settings.
• Flood/wet cooling significantly reduced the temperature at the cutting zone; however, disposal of wasted cutting fluid is difficult since it pollutes the environment.
• The MQL technique enhanced cutting performance by reducing friction between tool-work surfaces.
• The performance of the MQL system was greatly affected by cutting fluid characteristics and MQL operating parameters (nozzle position, number of nozzles, nozzle diameter, nozzle to cutting zone distance, flow rate, and air pressure).
• Dual/Triple nozzles MQL applications in hardened steel machining were rarely investigated. It needs to be investigated in detail by varying the nozzle positions toward the cutting zone.
• The use of dual/triple nozzle MQL in hardened steel machining has received little attention. It must be thoroughly explored by shifting the nozzle placements toward the cutting zone.
• Vegetable oil as a cutting fluid for MQL systems is gaining popularity among researchers because of its favorable thermal, tribological, and environmental features, which make it suited for hard turning applications.
• Nanoparticles have been developed as an efficient and environmentally friendly additive for MQL cutting fluids to improve their tribological and thermal properties, resulting in increased machinability of hard steels.
• In hard turning, the most widely utilized nano materials as additives for MQL cutting fluids include Al₂O₃, TiO₂, SiO₂, CuO, ZnO, MoS₂, Fe₂O₃, MWCNT, graphene, and graphite. The performance of MgO-based nanofluids through MQL for hardened steel machining has not yet been researched, whereas the performance of ZrO2 nanofluid through MQL in hardened steel machining has rarely been investigated.
• In several machining applications, the addition of ionic liquids (ILs) to the base coolant of MQL demonstrated greater friction and wear reduction than traditional coolant alone. Additionally, ionic liquids have a tremendous potential to reduce the strength of machining forces, hence consuming less energy. However, ILs can be a good choice to achieve sustainable hard machining.
• In the future, it may be possible to increase the machinability of hardened steel by blending ionic liquids with nanofluids.
• The use of halogen-free ionic liquids should be encouraged because they are non-toxic to the environment and do not promote corrosion.
• Cryogenic coolants have the ability to rapidly remove heat from the machining zone, allowing users to perform machining tasks at a higher cutting speed than MQL and nanofluid MQL. Since cryogenic coolants were unable to effectively minimize frictions between tool-workpiece and tool-chip, hybrid cooling technology (Cryogeinc + MQL) with varied nozzle positions can be studied in depth for hardened steel machining.
• Hybrid cooling is another emerging cooling concept for metal machining applications. It can be a future perspective for machining hardened steel.
• In the machining area, sustainable hard machining has gained a lot of interest. Hard machining can be sustained by using the right cutting tool shape, tool coatings, tool materials, cutting fluids, and cutting fluid delivery systems.
• In recent research, the Pugh matrix method was used in conjunction with the Kiviat radar diagram to measure sustainability in machining. The precision of this method was determined by allocating the appropriate weight to each criterion.
• Life cycle assessment (LCA) is a new tool for estimating the sustainability of the hard turning process. LCA was employed in only a few studies; hence, it is recommended for future research in hard turning.

6. Challenges and Future Research Directions

• One of the most difficult challenges in hard turning is controlling the cutting temperature in order to improve productivity, surface integrity, and tool life. Additionally, one of the biggest difficulties for researchers and machine operators in recent years has been achieving sustainable hard turning.
• It is clear from the review that the introduction of new tools, cooling methods, and sustainable measurement techniques plays a significant role in propelling machining research to new heights for the benefit of industrial applications.
• There is always the possibility of developing newer tools for hard metal machining using nano coating technologies, which would result in longer tool life, cheaper costs, and improved productivity.
• The use of nano fluids as a cutting coolant causes a number of health issues as well as expensive costs. It would be difficult to use them as a sustainable coolant in light of the economic and ecological outlook. Therefore, in these circumstances, the MQL/hybrid MQL should be a strong candidate.
• The effective use of subcritical pressure of CO₂ gas as a lubricant reported several benefits despite of greenhouse gas requires further scope of work in difficult to machine materials.
• Future studies could successfully use green ionic liquids as additives for vegetable oil or other MQL lubricants to enhance the thermal and tribological properties of the base lubricant.
• Comparing similar cutting conditions and various cooling methods can be covered in upcoming review work.
• Future research is advised to guarantee the sustained application of novel cooling techniques in order to attract the global manufacturing sector to clean manufacturing. Future research should examine not only efficiency but also social, environmental, and economic sustainability in hard turning. More research is needed, particularly to investigate social sustainability using quantitative metrics.