Materials and Manufacturing Processes ISSN: 1042-6914 (Print) 1532-2475 (Online) Journal homepage: https://www.tandfonline.com/loi/lmmp20 Nonedible vegetable oil-based cutting fluids for machining processes – a review Rahul Katna, M. Suhaib & Narayan Agrawal To cite this article: Rahul Katna, M. Suhaib & Narayan Agrawal (2019): Nonedible vegetable oilbased cutting fluids for machining processes – a review, Materials and Manufacturing Processes, DOI: 10.1080/10426914.2019.1697446 To link to this article: https://doi.org/10.1080/10426914.2019.1697446 Published online: 17 Dec 2019. Submit your article to this journal View related articles View Crossmark data Full Terms & Conditions of access and use can be found at https://www.tandfonline.com/action/journalInformation?journalCode=lmmp20 MATERIALS AND MANUFACTURING PROCESSES https://doi.org/10.1080/10426914.2019.1697446 Nonedible vegetable oil-based cutting fluids for machining processes – a review Rahul Katnaa, M. Suhaiba, and Narayan Agrawalb a Department of Mechanical Engineering, Faculty of Engineering and Technology, Jamia Millia Islamia, New Delhi, India; bDepartment of Tool Engineering, Delhi Institute of Tool Engineering, New Delhi, India ABSTRACT ARTICLE HISTORY Cutting fluids are an inherent part of the modern manufacturing system. With the increase in the industrialization and development of new materials and processes, there has been a need for developing new cutting fluids with superior performance. For achieving this, cutting fluids have been produced from mineral oil and other additives. The additives used in the cutting fluids are carcinogenic in nature and are harmful to the workers and the environment. Mostly edible oils have been used as cutting fluids in machining. But it has been seen that nonedible oils have the potential to be used in the manufacturing sector to mitigate the detrimental effects of conventionally used cutting fluids without compromising on machining efficiency. The purpose of this review article is to apprise the readers with the current trend in the application of nonedible vegetable oils and their application in machining. This paper entails various aspects of cutting fluids and an up to date and exhaustive review on the latest literature on the effectiveness of nonedible oils and its modified versions, blended oils, ionic liquids, and nanoparticles as additives is done to understand the efficacy of nonedible vegetable oil-based cutting fluids. Received 16 October 2019 Accepted 14 November 2019 Introduction Cutting fluids are essential requirements in the modern manufacturing sector. Given the current rate of utilization of cutting fluids, approximately 39.4 million Mt (metric ton), the requirement would reach 43.9 million Mt in 2022[1]. During machining process metal shearing takes place resulting in chip formation and almost 99% the external mechanical energy gets converted into heat.[2–5] Heat gets generated from the primary zone, secondary zone andtertiary zone or tool-chip contact area[6,7] as shown in Fig. 1. This high temperature distributes over the cutting tool, shown in Fig. 2, and causes detrimental effects such as dimensional deformation of the workpiece, loss of hot hardness of tool, a higher rate of tool wear leading to premature failure of tool and loss of surface integrity.[10,11] Cutting fluids control the temperature rise by providing adequate cooling and lubrication between tool and workpiece.[12,13] Moreover, cutting fluids also help in increasing tool life, reducing vibrations by providing damping effect, reduction in BUE formation, producing short and manageable chips during machining.[15–18] The absence of cutting fluids causes increased power consumption, rapid tool wear, and poor surface finish.[9,19–23] Even a small quantity of lubricant as in MQL method of fluid delivery, can enhance the machining quality by reducing surface roughness and lowering cutting forces.[24–27] Vegetable oils, animal fat and lard have also been used as a lubricant because of their good thermal and lubrication characteristics.[28–33] Quality of lubricant film is vital as the metal cutting process has some contact between the mating surfaces, whilst complete detachment of metal layers renders KEYWORDS Cutting; fluids; nonedible; vegetable; oils; sustainable; disposal; manufacturing; Green; environment the machining process ineffective. Proper cutting fluid selection is important as the performance of cutting fluid depends on the type of machining operation.[34–36] Figure 3 shows a relation between cooling property and lubrication effectiveness of conventional fluids. Cutting fluids work by establishing close contact with the surfaces and getting adsorbed on the surface followed by physisorption or chemisorption.[38,39] Figure 4 shows the different possible ways of interaction. The molecules of cutting fluids break the existing bonds on the surface to get absorbed on the workpiece surface.[41] Chemisorption, being stronger than physisorption, gives intense interaction with the workpiece surface and thereby better lubricity.[42] Presence of additives such as sulfur increases the chemisorption which was proved in grinding operation done with cutting fluid with sulfur as additive.[43–45] The presence of additives has shown enhanced lubrication effect and even a slight variation in additive constituent affects the machining output.[46–49] Several experiments done with different types of lubricants have shown that the lubricity property of a cutting fluid increases due to the way the additive molecules interact on the surface of the workpiece.[50–54] The various theories of cutting fluid mechanisms are briefly described in Table 1. Since the efficacy of a cutting fluid is affected by the type of machining operation, the characteristics such as viscosity, strength of lubricant film, thermal conductivity, and specific heat capacity are important factors to be considered in a good cutting fluid selection for a particular machining operation. Operations like tapping and drilling require more lubrication action in comparison to high-speed machining operations like CONTACT Rahul Katna katnarahul@gmail.com Department of Mechanical Engineering, Faculty of Engineering and TechnologyJamia Millia Islamia, New Delhi, 110025 Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/lmmp. © 2019 Taylor & Francis 2 R. KATNA ET AL. Figure 1. Zones of heat generation[3,8,9]. Figure 2. Temperature distribution in cutting tool[14]. Figure 3. Lubrication vs. cooling property of cutting fluids[37]. turning, milling, and grinding in which more cooling action is required. High cutting speed increases the temperature during machining which affects the viscosity of the cutting fluid and as such a lubricant having a higher viscosity index (mineral oils with naphthenic base) is preferred to a lubricant having a lower viscosity index (petroleum base mineral oil). A proper cutting fluid selection program is important with regard to machining operation as well as the material of the workpiece.[36,69,70] Although cutting fluids have immensely improved productivity, they have many disadvantages.[71,72] These have negative effect on human and environment.[59– 61,73–75] Shockingly, a huge 12 mn ton waste cutting fluid is MATERIALS AND MANUFACTURING PROCESSES 3 stock for cutting fluid formulation. The driving force for nonedible oils is health safety, environmental friendliness and ease in availability of nonedible oils.[26,71–76,85–90] The use of nanoparticles and ionic liquids as additives has increased the performance in terms of lubricity by 100 percent. Owing to this, new ionic liquids are being tested with nonedible oils as base oils in order to improve their performance. Nonedible oils such as Jatropha oil, Karanja oil, etc. have been successfully tested as biodiesel and they offer a high degree of lubricity. Their use is now gaining ground as cutting fluids in the manufacturing industry. Ionic liquids appear to be one of the most promising lubricant additives for superior performance of vegetable oils than the conventionally used lubricants. Recent trends in the usage of nonedible oils and their formulations based on blends, additives and ionic liquids as cutting fluids are described in this paper. The next section comprises classification of conventional cutting fluids and various drawbacks of conventional cutting fluids. Further section describe the performance of straight non edible vegetable oils in machining and various methods of improving the performance of nonedible vegetable oils by different methods such as blending, modification, and use of additives. Conventional cutting fluids classification Figure 4. Possible ways of interaction on the surface of the workpiece[40]. disposed of in the environment annually.[76] Untreated disposal is harmful to aquatic life. Proper disposal requires neutralization methods, which add up the total cost incurred on use of cutting fluid. The rising health and environment hazard caused by cutting fluids have advocated research for finding a substitute to the conventionally used mineral oil-based cutting fluids. The recent decade has witnessed investigation on oils from vegetables as a potential lubricant for machining processes.[48,63–70,49,77–84] Edible oils have been used for a long time and recently researchers have shown interest in Nonedible oils as base The cutting fluids are usually made from petroleum products and mineral oils. Cutting fluid requirements depends on the type of application and machining condition. A low-speed operation requires more lubrication action against a highspeed machining operation which requires more cooling than lubrication.[91–93] Conventionally, cutting fluids are categorized as Straight oils, soluble oils, synthetic and semisynthetic oils, and gas-based lubricants. Straight oils Straight oils are petroleum or mineral oils without water.[94] They are also called as neat oils. These are used as-is or “straight” without adding any water or being diluted. These are formulated from mineral oils or vegetable oils by mixing them with different additives as performance enhancers and find application where more lubrication is required than Table 1. Theories of cutting fluid working mechanism. Theory name Adsorption theory Description References Cutting fluids interact via physisorption, chemisorption, and chemical reaction. The stronger intensity of interaction of [36,38,39,55] molecules on the surface of the workpiece takes place in the presence of –NH, -F, and –OH molecules due to the availability of free electrons. A stronger interaction enhances the degree of lubricity. Hence, the use of additives increases lubricity which was substantiated by many researchers also. Different chemicals react differently which was proved that in order to attain some level of lubrication as sulfur at room temperature, the higher temperature was required with phosphorus. [55–60] Rehbinder effect Surface-active agents reduce the hardness of the workpiece. Bond breaking starts from micro-cracks which are inherently present in all materials. Cutting fluid enters into these cracks and weakens the existing bonds and new bond formation takes place. However, it is a surface phenomenon and does not hold for the bulk material This has poor reproducibility. William and Tabor Model Molecules of cutting fluids are transported via a number of capillary channels. This model postulated that instead of [50,61] adsorption, capillarity is more important. Godlevski Model Based on capillarity but considers entry as liquid phase, conversion into microdroplets, and filling of the capillary as [54,62] gaseous phase. Conical capillary model Considers cutting fluid transfer due to conical capillary formation. [63] Marangoni effect Cutting fluid tends to move away from the high temperature zone. If capillary forces dominate the Marangoni number, [64–68] cutting fluid can penetrate and reach the machining zone. 4 R. KATNA ET AL. cooling. Among the other types of cutting fluids, these provide the highest cushioning effect. However, these have drawbacks like poor thermal stability and heat transfer capacity, high flammability, high cost, and low efficiency at high cutting speeds.[95,96] Additives are added to improve machining efficiency at extreme loads, to reduce foaming and mist formation. These are used in threading, broaching, drilling, gear cutting, etc.[70] Contact with water causes rancidity and makes them susceptible to microbial attack. These drawbacks limit the application of straight oils in high-speed machining operations. It was seen that mixing water with oil can offer a significant improvement in machining performance in comparison to using straight oils alone. Thus soluble oils were formulated. Soluble oils Soluble oils are emulsifiable concentrates which readily dissolve in water and provide good cooling effect and lubrication.[97] Water is the main ingredient ~95% in this type. In order to make oil soluble in water, it is mixed with additives like surfactants, emulsifiers, and dispersants. Some other additives such as corrosion inhibitor, biocides, antifoam, etc. are added. They provide excellent cooling in addition to lubrication and are used widely in high-speed machining like milling, turning, and grinding.[82,93,98] However, they are prone to emulsion breakdown and microbial attack and precipitation on the workpiece and machine parts.[99] Also, it is found that mineral oil-based soluble oils have to be mixed with EP additives for attaining good performance at higher loads. For overcoming these problems, semi-synthetic and synthetic cutting oils were formulated. Such cutting fluids showed better thermal-oxidative properties and better chemical properties than mineral oils. with/without the combination of high pressurized air.[103] Liquefied gases at low temperatures perform an excellent function of reducing heat and keeping the temperature low. These are used in machining hardened steels and aerospace alloys and materials having low thermal conductivity. They have many limitations like high setup cost, change in surface topology at low temperatures. Gas-based coolants may also be used with oils in the form of mist. A summary of cutting fluids is given in Table 2. The additives used in each category of cutting fluid are listed in Table 3. Drawbacks of conventional cutting fluids Additives in cutting fluids enhance the performance remarkably. Though mineral oils are relatively cheaper, they have limitations like loss of performance due to oxidative instability and loss in viscosity at elevated temperature, low-temperature solidification, and susceptibility to explode in the presence of an oxidizing agent. In addition to this, the various additives used as performance enhancers pose danger to humans and the environment. Although cutting fluids have immensely affected the productivity, efficiency, and quality of machining in the manufacturing sector, yet they have many disadvantages. Most of the additives which are added are toxic to the human and environment.[106] They pose a huge risk to the workers who are in direct contact and who are in the vicinity of their application.[71,72,107,108] Cutting fluids, being toxic, affect the environment’s health and cause irreversible damage.[109–112] Also, the toxicity of cutting fluids increases over time upon storage.[113] A study showed that cutting fluids caused tumors in mice.[114] Many more studies have shown dangerous results in humans. A summary of the effects on conventional cutting fluids is shown in Fig. 5. Semi-synthetic and synthetic oils The semisynthetic category is water-soluble cutting fluids with very little oil content and a high amount of additives.[100] These have excellent protection against bacterial attack, corrosion resistance, lubricity, and less dragout (loss of oil with chips). The lower viscosity of these enables easy cleaning of a workpiece. However, they form a precipitate with hard water and foam too much. Synthetic oils do not have base oil but contain soap polymers and other additives like corrosion inhibitor, bactericide, and fungicide, defoamers, and emulsifiers.[31,101,102] They have longer sump life and are easily cleanable from workpiece due to less stickiness. However, they cause a number of health hazards like skin irritation, itching, etc. Also, they tend to leave a gummy residue when mixed with tramp oil. In spite of all the good properties, these still suffer from poor lubricity and not suitable at extreme loads where they suffer from breaking of the lubricant film. These are also expensive than other types of cutting fluids. Gas based coolants Gas-based coolant is another class of cutting fluids that utilizes gases like carbon dioxide, nitrogen, helium, and argon Health hazards to workers Thousands of workers are affected by the toxic effect of the constituents of cutting fluids.[75,115] IARC reported that the constituent (polyaromatic ring) of petroleum-based cutting fluids are carcinogenic and result in skin cancer.[116] The constituents of cutting fluids are toxic and have low biodegradability and cause a lot of environmental problems and diseases like respiratory problems, cancer, dermal conditions, and genetic disorder based on the type of constituent, exposure duration, and exposure quantity.[117] The combined exposure to metallic ions, chemicals, and constituents of cutting fluids can be far more harmful as compared to the exposure of single products individually and can produce unexpected results including synergistic toxic effect.[108,118–120] Skin protection creams used by workers have tertiary amines that react with cutting fluids to form other carcinogen nitrosamines thereby antagonizing the effect. Skin disorders like eczema, irritation occur due to cutting fluid concentration and high alkalinity of cutting fluid when the worker dips hands into the cutting fluid sump or gets in contact with cutting fluid during workpiece unloading, splashing, and during machining in the form of mist. Workers develop skin irritation, oil acne, folliculitis, MATERIALS AND MANUFACTURING PROCESSES 5 Table 2. Cutting fluid type, advantages, limitation, and applications.[91–94,99,102,104,105] Cutting fluid Straight oil Soluble oil Semi-Synthetic oil Synthetic oil Gas-based Advantages Benefits ● ● ● ● Excellent lubricity Excellent rust inhibition Useful in heavy cut machining Highest cushioning effect among other cutting fluids ● ● ● ● ● ● Good lubricity Excellent cooling Protection against corrosion by leaving a fine layer of oil on the workpiece surface ● ● ● ● ● ● Good cooling Good lubricity Good antimicrobial properties Can be used for ferrous and nonferrous materials Less dragout loss than soluble oil Easy cleaning due to less viscosity ● ● ● ● ● ● ● Excellent cooling Excellent microbial control Noninflammable High flashpoint Less foaming Good protection against rust Minimum dragout loss ● ● ● ● ● Faster cutting speeds Higher surface integrity The higher material removal rate Higher tool life No oily residue dermatitis, hair follicle blockage, inflammatory pimples due to frequent contact with the cutting fluids.[70,121–125] Some additives such as chlorine can cause chloracne and yellow cysts and constant skin irritation leading to skin defetering.[126–129] Prolonged exposure results in various types of skin cancers.[130–132] Different types of cancers were seen in workers involved in different machines. Workers involved in grinding operations are more susceptible to develop esophageal cancer while turners are susceptible to scrotal and pancreatic cancer.[133–135] Dermatitis is often seen in workers.[136,137] A study on workers exposed to cutting fluid showed DNA damage in mononuclear blood cell.[138] However, in another study, limited evidence of liver and biliary tract cancer was found associated with cutting fluids.[139] This indicates that the effect of cutting fluid depends on the type of exposure and duration of exposure. Regular inhalation of the mist of cutting fluids causes respiratory disorders in workers. A study done on 81 workers with an exposure of 2.8 years revealed mist particles of cutting fluids in respiratory tracts.[140] From the results, it is seen that higher exposure level is seen in workers who handled more workpieces and in the unenclosed machine as compared to workers operating machines with enclosures and the same number of workpieces. The mist generated during machining increases as cutting fluid is thrown off the surface of the rotating workpiece at high speed and gets atomized.[141,142] ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● Disadvantages Limitation Poor cooling Misting and smoke generation Only suitable for low-speed machining Sticking to surface Difficulty in cleaning surface Huge dragout loss Susceptible to contamination by tramp oil Dragout loss Susceptible to bacterial attack Loss due to evaporation Tendency to form precipitate Misting problem Less lubricity than soluble oils because of less base oil Foaming problem Not compatible with hard water Easy contamination with other machine fluids Can form deposits Poor lubricity due to the absence of oils Contaminated by other machine lubricants A high amount of additives Mist generation Gummy residue on interaction with machine lubricants High setup cost High coolant replacement cost Surface embrittlement Not suitable for all materials Not suitable for mass production ● ● ● ● ● Applications Tapping Drilling Broaching Honing Low-speed cutting ● ● ● ● Turning Milling Grinding High-speed machining ● ● ● ● Turning Milling Grinding High-speed machining ● ● ● ● ● ● Low speed as well as highspeed machining Tapping Drilling Turning Milling Grinding ● ● ● Machining hardened steels Aerospace alloys Superalloys machining Mist inhalation causes irritation and inflammation of respiratory tract and symptoms such as watery eyes, sore throat, nose bleeds, excessive coughing, increased phlegm, etc. are common.[143–147] In addition to the constituent of cutting fluids, microbial contamination is a causal agent for respiratory diseases.[148] Long-term exposure and delayed treatment cause lung cancer in such workers.[149] NIOSH[73] concluded that “substantial evidence existed on the finding that risk of cancer in various parts of (larynx, rectum, brain, colon, skin, and bladder) among workers exposed to MWF before the 1970s increased upon exposure to metalworking fluids.” As per a German ordinance, all the patients who got in contact with cutting fluids contracted allergic contact eczema.[74] Hazard to environment Cutting fluids are affected by selective microbial growth due to the alkalinity of the cutting fluid.[150] Stagnation creates a foul smell and increases bacterial contamination and the fluid has to be treated before disposal as contaminated fluid poses risk to the environment. Untreated disposal releases formaldehyde which is a classified carcinogen.[103,151] Aquatic plants are affected by these cutting fluids. In a research, serious damage to the photosynthesis process of plants was discovered due to cutting fluids.[152] The R. KATNA ET AL. 6 Table 3. Additives and their application. Additive type Function Emulsifier Example To stabilize oil in water emulsion of soluble oil, synthetic, and semisynthetic oil Coemulsifier Dispersant Tackifier Antioxidant Antifoam Corrosion inhibitor Viscosity index improver Fungicides and bactericides Extreme pressure additive Chelator Passivating agent Friction modifier Coupling agent Buffer Nonionic: polysorbate, fatty alcohol, ethoxylates Anionic: SLES, soap of potassium, sulfonates, Cationic: Quaternary salts of ammonia Amphoteric: Betaines (CAPB) Enhances the function of emulsifier and helps in reducing the surface Anionic surfactants work as co-surfactant for nonionic tension of oil surfactant, ethoxylates, sulfonates Prevents agglomeration of particles on the surface Sulfonate, sulfonated castor oil, sodium petroleum sulfonate Increases “Tack,” i.e. tendency to stick to the metal surface to increase Natural rubber, EVA, acrylics the effectiveness of soluble oils To prevent auto-oxidation of oils ZDDP, amines To prevent or reduce foaming tendency Silicone oil To prevent rusting due to water Sulfonates, ZDDP, amine derivatives To reduce viscosity variation with temperature Long-chain polymers To prevent bacterial contamination which may reduce the life and Phenolic compounds efficiency of cutting fluid Increase high load capacity Sulfonated compounds Combines with metallic ions and helps in maintaining the EDTA effectiveness of cutting fluid Helps in protecting the freshly generated surface from corrosive Aromatic compounds atmosphere Helps in improving lubricity and decreasing friction between surfaces Sulfonated oils, natural oils, lard Helps in binding the constituents of the cutting fluid together Butanol, cyclohexanol Helps in maintaining the pH of the solution basic Ammonium chloride and ammonium hydroxide mixture for pH around 9 SKIN DISORDER DNA MANIPULATION CARCINOGENIC TUMOR AND CANCER RESPIRATORY AILMENTS ENVIRONMENT HAZARD ADDITIONAL DISPOSAL COST Figure 5. Drawbacks of conventional cutting fluids. metalworking fluids caused severe ultrastructural damage in chloroplast and caused a reduced number of chloroplasts in the maize plant. The presence of NDELA (N-nitrosodiethanolamine) in cutting fluids induces liver tumor, breaking of DNA strand, and neoplasm in rats.[153] NDELA is carcinogenic in animals and induces liver and respiratory tract tumors in the respiratory tract and liver, and neoplasm in rats.[154–157] Costs associates with disposal Costs of cutting fluids are around 17% of total production cost as opposed to tooling which is only 4%.[88,158,159] Proper disposal requires a reduction in COD (chemical oxygen demand) level and neutral pH before releasing the cutting fluid into water bodies.[160,161] Suitable treatments for neutralization include microfiltration, ultrafiltration, adsorption, chemical coagulation, etc.[162–167] New methods include treatment with ozone which neutralizes the cutting fluid and makes it safe for disposal.[168] Recently a method “Freeze-thaw method” was developed to recover oil from spent cutting fluid without the need of any chemical agent.[169] Other methods of cutting fluid treatment involve adsorption of organic matter from cutting fluid by the use of activated carbon fibers.[170] Table 4 shows the biodegradability of different oils. Nonedible vegetable oil based cutting fluids Due to the drawbacks of the conventional cutting fluids and growing demand for finding an environmentally friendly alternative, the focus of researchers and Govt bodies these days is on developing efficient products and conserving the environment by phasing out petroleum products. Growth in demand for environment-friendly cutting fluid has opened gates for the non-edible vegetable oils as a substitute to the conventionally used cutting fluid.[105] An estimate of a 10% growth rate is predicted in the use of these biodegradable cutting fluids.[172] Research is being done to improve and formulate bio-lubricants from nonedible oils. Nonedible oil-based cutting fluids are biodegradable environment-friendly, nontoxic and renewable, and are the most sought option as an alternative to the conventionally used cutting fluids. Environment friendliness and human safety are the essential properties required from a good cutting fluid in addition to providing good lubricity and cooling. Table 4. Biodegradability of oils.[171] Base oil Mineral oil Nonedible vegetable oil Alkylbenzene Aromatic esters PEG Polyglycerol Di-esters % biodegradation 15%–75% 75%–100% 5%–20% 0%–90% 10%–75% 5%–95% 55%–95% MATERIALS AND MANUFACTURING PROCESSES Favorable aspects of vegetable oils in machining Biodegradability and nontoxicity are the main aspects with regard to environmental friendliness. Non-edible vegetable oils are biodegradable products as they are susceptible to biochemical reaction and ultimate breakdown into carbon dioxide and hydrogen by microbes to produce biomass. Biodegradability thus ensures safe integration of biomaterial back into the carbon cycle of nature. The output of such bio-products is thus clean and contributes very little pollution and subsequently eliminates health risks to workers. Their disposal is also safe and does not impact the water bodies and the environment. Nonedible vegetable oils degrade faster than mineral oils in nature.[173] Figure 6. Structure of fatty acids in vegetable oils[189]. 7 Role of chemical structure of vegetable oils toward physiochemical properties Non-edible vegetable oils have a triglyceride structure. This structure is associated with different fatty acid chains making a complex fatty acid structure within a single molecule. Fatty acids are comprised of carbon-carbon bonds with hydrogen and other group atoms and terminate with a –COOH group (carboxylic acid). Fatty acids comprise an even number of carbon atoms consisting of 14–26 numbers of atoms. Figure 6 shows structure of come fatty acids. At some places, hydrogen atoms are replaced by a double carbon bond making the molecule monounsaturated. If this occurs at several places the molecule is called polyunsaturated. This unsaturation causes 8 R. KATNA ET AL. lowering of melting point and lowered thermal stability` of the oil than the saturated oil. This unsaturation also leads to bending and subsequent loss in the linearity of the molecule. However, it is advantageous under situations because at room temperature linear molecules are liquid or solid which limits their application as an effective lubricant.[174] Of more than 1000 fatty acids known only 20 are found in vegetable oils. Some of the fatty acids found in oils, which are of importance, are oleic acid, palmitic acid, linoleic acid, stearic acid, ricinoleic acid, linolenic acid, and lauric acid. The type of fatty acid and the degree of saturation determines the lubricity and viscosity of the oil. A higher degree of saturation makes it less susceptible to oxidation. Coconut oil has a high quantity of lauric acid and more than 90% saturation, which imparts strong resistance to oxidation. On the other hand, palm oil has the same number of saturated and unsaturated fatty acids imparting a reasonable degree of lubricity to it. The presence of alpha-tocopherol in sunflower oil prevents auto-oxidation. Ricinoleic acid is present in high quantity in castor oil and is responsible for high viscosity in castor oil. However, it is interesting to see that oxidation produces free fatty acids which have shown to improve the boundary lubrication property of these oils.[175,176] Vegetable oils work by creating boundary lubrication in which the physical adsorption layer is created. The presence of polar groups like –OH, -COOH, -COOR in vegetable oils results in strong affinity toward metal surface thereby reducing friction on account of strong layer formation. Longer chains are able to create stronger interactions. However, there are some exceptions to it like rapeseed and palm oil which comprise an equal amount of erucic acid and palmitic acid respectively.[177] Both these fatty acids have the same constituents but erucic acid has a longer carbon chain than palmitic acid. Theoretically rapeseed oil should yield small grinding force than palm oil because of the long carbon chain of palmitic acid. But, palmitic acid yielded a smaller grinding force than rapeseed oil in grinding experiments even though erucic acid in rapeseed oil has a larger carbon length.[178] Thus, in addition to the length of the carbon chain, viscosity, viscosity index, and degree of saturation/unsaturation also play a cumulative role which affects the lubricity. Fatty acid forms a soapy film on contact with a metallic surface due to chemical reaction. The molecules get distributed in a dense manner and prevent friction between metal surfaces thereby reducing wear. The carbon content influences the total adsorption energy but a minimum /adequate number of carbon atoms are necessary for the formation of a maximum density adsorption layer. The increase in the number of carbon atoms increases the strength of the adsorption film. However, it has been observed that maximum strength is obtained with the number of carbon atoms equal to 16 and beyond this the number of carbon atoms does not play any role in the strength of film.[178] Also, a higher degree of saturation imparts higher lubricity in fatty acids. A saturated fatty acid provides better lubrication than an unsaturated one. The polar nature of fatty acids generates a guided molecular level film as against the film formation in mineral oil which is aligned randomly and is thus weaker.[105,179] Table 5. Properties of vegetable oils and mineral oils.[81,187,188] Property Density Density (kg/m3) Sludging tendency Viscosity variation index Biodegradability Pour point Cold flow behavior Oxidative stability Flashpoint Vegetable oil Mineral oil Higher 890–970 Poor 100–200 80%–100% −22 to 12 degree Poor Fair High Lower 840–920 Good 100 10%–30% −15 degree Fair Fair Low Limitations in utilization of nonedible vegetable oils in machining Although saturated and unsaturated long-chains present in fatty acids impart good tribological properties but unfortunately, their performance is affected by the low oxidativestability. However, elimination of unsaturation can improve this deficiency. The flowability of the oil is influenced by the chemical structure of the oil. The chain length affects lubricity and viscosity index positively but reduces fluidity and oxidative stability and increases volatility. Branched-chain increases fluidity but reduces volatility and lubricity and unsaturation in the molecule increases oxidative instability and lubricity. Vegetable oils have higher viscosity as well as high viscosity index than mineral oils. Saturated fatty acids are superior to unsaturated fatty acids in terms of oxidative stability. The oxidation rate depends on the degree of unsaturation and also on the type of fatty acid present in the vegetable oil in as in the oxidation rate of linoleic acid is 10 times than oleic acid and that of linolenic acid is 100 times than oleic acid.[180] The unsaturation leads to better biodegradability of vegetable oils than mineral oils. However, unsaturation also raises the amount of antioxidant required to formulate a lubricant at par with mineral oil for comparable performance. The scope for improvement in unsaturated fatty acids is possible due to the double bond, which offers site for chemical derivatization. Also, for countries like India, where a large percentage of edible oils are imported, it is not recommended to use these oils for industrial application and therefore, it is recommended to use non-edible oils for manufacturing operations. Nonedible oils are a favored base stock for use in cutting fluid formulation and are a good substitute owing to their superior properties than mineral oils. These are nontoxic and totally biodegradable in nature. It is possible to cultivate these crops in arid and harsh conditions. These do not need extreme care during growth and grow well in any condition.[181] They can be grown in a variety of weather conditions without affecting production. Some commonly used non-edible oils are given in Table 10. The benefits of using nonedible oils as an alternative to mineral oils for lubrication are higher lubricity and viscosity index, fast biodegradability, and high flash point. A comparison between vegetable oil and mineral oil is shown in Table 5[67,173–175]. MATERIALS AND MANUFACTURING PROCESSES Performance of nonedible vegetable oils in machining Recently nonedible oils used in machining operations have performed better than mineral oils due to their high inherent lubricity, which forms strong intermolecular interaction on the metal surface. Also, nonedible oils have better tribological properties than mineral oils which make them a good environment-friendly alternate. Nonedible oils like Neem oil, Karanja oil, Jatropha oil, Castor oil, and cottonseed oil have been researched upon and found to be a good substitute to mineral oil in terms of performance. Nonedible oils have inherently good lubrication properties and are less volatile and are thus a good alternative for replacing mineral oil-based cutting fluids. For replacing a product either better or at least equal performance is desired. In machining operations, comparison is generally made in terms of machining forces, temperature, surface quality, and tool life and in cases where actual machining is not performed comparison is made in the form of tribological tests. Tribological tests give information about the lubricating properties of the oil. Low values of wear scar diameter and coefficient of friction indicate good lubricating properties of the test fluid. Straight nonedible vegetable oils in machining In an attempt to replace mineral oil and develop indigenous formulations for metalworking fluids, Jain and Bisht[190] used nonedible Karanja oil and rapeseed oil. Six cutting fluids were formulated by mixing various additives like emulsifiers, anticorrosion additives, and defoamers and the viscosity of these cutting fluids was matched with reference fluids by mixing with additives. The finally formulated metalworking fluids (F1L, F2L, F3L, F1M, F2M, and F3M) were compared with reference oils (R1L, R2L, R3M, and R4M). Tribological studies were done on a four-ball tester machine and the results revealed that F1L, F3L, F1M, F2M, and F3M gave the coefficient of friction 0.086, 0.089, 0.086, 0.092, and 0.089 respectively which was less than the reference oils (R1L = 0.096, R2L = 0.092, R3M = 0.098, and R4M = 0.088). However, the Figure 7. Wear comparison with different cutting fluids[177]. 9 coefficient of friction of F2L and F2M was 0.099 and 0.092 was only slightly higher than the reference oils. The wear scar diameter of the developed oils was adequate but slightly more than the values attained with the reference oils. The lowest wear scar diameter was with F3L (0.719 mm) and F2M (0.719 mm) followed by F1L (0.725 mm), F2L (0.750 mm), F1M (0.781 mm), and highest with F3M (0.812 mm). Among the reference oils, R4M gave least wear scar diameter with 0.625 mm, followed by R1L (0.666 mm), R2L (0.781 mm), and R3M (0.781mm). Nonedible vegetable oils have shown to perform better than conventional cutting fluids. Syahrullail et al[191] tested the tribological properties of Jatropha oil and compared it with an acid distillate of palm fatty acid, stamping oil, RBD Palm olein oil, and commercial hydraulic oil using a four-ball tester. At high loads, a high coefficient of friction was observed with vegetable oils with the breaking of palm olein fatty acid distillate lubricant layer at heavy load. However, Jatropha oil and palm olein oil lubricant film persisted even at high load. The low friction coefficient of the hydraulic and stamping oil was due to EP additives. Lowest wear scar diameter (3 mm) was achieved with Jatropha oil and was nearly equal to the wear scar diameter obtained with the commercial hydraulic oil (2.9 mm). Palm olein oil resulted in WSD 3.5 mm whereas high WSD was observed with palm fatty acid distillate (5 mm) due to its semisolid state at ambient room conditions and the film breaks easily at heavy loads. Least wear scar diameter was obtained with commercial stamping oil (0.6 mm). Tribological properties of Jatropha oil and palm olein oil suggest that these have the potential to be used as a lubricant in the industry. Milena et al[177] investigated the performance of Jatropha and Moringa oil in machining through tribological tests. Several metalworking fluids were compared viz Moringa oil, Jatropha oil, canola-based coolant, Jatropha-based coolant, Jatropha ester-based coolant, and mineral coolant. Cutting speed, depth of cut and feed were 1884 m/min, 2.88 mm, and 0.05 mm/tooth and 0.3 mm/tooth respectively were used in machining. Tribological tests indicated the least wear with Jatropha oil, followed by Moringa oil, mineral oil. Both 10 R. KATNA ET AL. Moringa and Jatropha showed good lubricity capacity in the tribological tests but both lost lubrication capacity due to auto-oxidation at the end of the experiment when the temperature rise was below 100°C. Jatropha oil gave the least wear scar area, followed by Moringa oil, commercial mineral oil, canola-based coolant, Jatropha-based coolant, and highest with commercial Jatropha ester-based coolant. In milling tests, Jatropha metalworking fluid showed the best performance for lubrication. The highest volume of metal removed was observed with Jatropha-based coolant at both feed rates, followed by Jatropha ester-based coolant, mineral coolant, and least material removal rate with canola based coolant. (Fig. 12) At various sliding speeds, Jatropha oil keeps a low coefficient of friction value. Figure 7 shows the wear with different cutting fluid on the Reichert test machine.[177] Jatropha oil was tested for its tribological properties on a flat reciprocatory tribometer with stroke length 8 mm at 12 N load.[192] The experiments were done on ball on a flat reciprocatory tribometer with stroke length 8 mm at 12 N load. It was found that the coefficient of friction was high during the beginning but it reduced gradually and became almost constant near the end of the experiment. A lower value of the friction coefficient is obtained upon increasing the frequency of sliding (oscillation frequency). The mean coefficient of friction values at 2 Hz, 5 Hz, 8 Hz, and 20 Hz are 0.121 ± 0.005, 0.083 ± 0.004, 0.05 ± 0.002, and 0.043 ± 0.001 respectively. Qualitative analysis revealed that at the beginning of the motion the mean coefficient of friction is zero due to zero mean velocity. However, with an increase in the average mean velocity the mode of lubrication was found to approach mixed regime from boundary layer lubrication. As the pin starts moving again the frictional forces increase rapidly to its highest values and then become constant till the velocity of the pin becomes zero again. This corresponds Figure 8. Evolution of coefficient of friction during test[192]. to the static friction arising and changing to dynamic friction. The velocity is the highest at the mid-stroke and at this point, the frictional force is the least. At an oscillating frequency of 5 Hz, it is found that the velocity peak occurs at 172 mm/s and the maximum force is 1.98 N whereas the lowest force is 0.4 N, which could be the static friction in the beginning. At an oscillating frequency of 8 Hz, the minimum and maximum forces are 0.12 N and 1.33 N. At 20 Hz oscillating frequency, the highest velocity that occurred was 762 mm/s, the max force was 1.1 N. However, the minimum force was negative which was due to fluid inertia forces becoming larger at higher frequency than at lower frequency. Jatropha oil thus keeps the coefficient of friction low at all sliding speeds. Figure 8 shows the evolution of friction during the experiment. Klocke et al.[193] used rapeseed oil-based synthetic ester in machining austenitic stainless steel. They also used a pin on disc tribometer for finding the lubricating properties of cutting fluids. The comparison tests between the developed biolubricant and synthetically refined esters showed that the biodegradable lubricant has a high potential for utilization as a lubricant in machining. The combined effect of polar surface energy and stability at high load leads to a steady film which helps in reducing wear. Stefanescu et al.[194] e xamined the lubricity of rapeseed oil. The results show that Rapeseed oil performed superior to mineral oil in terms of reducing frictional wear during the tribological test done on four-ball tribomachine. Based on tribological tests the authors reported that rapeseed oil exhibits less coefficient of friction and this property is essential for an oil to be used in industrial application. Figure 9 shows a comparison of the result of milling tests done with nonedible oils.[177] In an attempt to find the impact of lubricant on the wear and frictional forces Agrawal et al.[195] probed the wear property of MATERIALS AND MANUFACTURING PROCESSES 11 Figure 9. Comparison of volume of material removed in milling tests done on 7050-T7451 aluminum alloy[177]. Figure 10. Wear at different load and at different speeds[195]. Table 6. Summary of tribological tests with nonedible vegetable oils. Research group and ref Jain and Bisht[190] Syahrullail et al.[191] Souza et al.[177] Ruggiero et al.[192] Klocke et al.[193] [194] Ştefănescu et al. Agrawal et al.[195] Test equipment employed Tribo test with Amsler type A135 machine Four-ball tester Material Nonedible oils used in study En8 and mild steel Karanja Rapeseed Common ball bearing Jatropha Reichert test machine Ball-on-flat reciprocatory tribometer 7050-T7451 AL Jatropha alloy Moringa AISI 52100 steel Jatropha and X210Cr12 steel Pin on disc tribometer Four-ball tester AISI 5115 and AISI M3:2 steel Plain carbon steel M2 steel Pin on disc tribometer Rapeseed Rapeseed Cottonseed Property studied Coefficient of friction Wear scar diameter Friction coefficient Wear scar diameter Main findings Wear scar diameter of formulated oils was satisfactory. The friction coefficient was less than the reference oils. At extreme pressure conditions, Jatropha oil maintained a lubricant film. Among three vegetable oils, Jatropha gave the least wear friction coefficient lied between the two commercial oils. Wear area Jatropha oil gave the lowest wear area and highest volume of Machining test material removal Coefficient of Oil showed excellent physicochemical characteristics and has friction the potential to be used as an industrial lubricant. Friction coefficient reduces decreases with rising in rubbing frequency but and remains less than 0.1 Wear scar diameter The combined effect of the polar surface energy of the oil leads to stable lubricant film and consequently less wear. Coefficient of Rapeseed oil gives less coefficient of friction at all loads than friction the mineral oil. Wear Cottonseed oil gives less wear at all loads and speeds. Drilling tests confirmed the low wear of drill with cottonseed oil than mineral oil. cottonseed oil on M2 steel with HSS. The authors compared the performance of cottonseed oil with SAE 40 oil. Experiments with steel pins made of M2 steel showed that cottonseed oil gave less wear than SAE 40 oil. This is because of better lubricity due to long, heavy and dipolar nature of cottonseed oil. Cottonseed oil gave less wear at higher loads than SAE 40 oil. At higher loads, similar wear was noted for both oils as shown in Fig.10. At a lower speed of 300 rpm, cottonseed oil gave more wear than SAE 40 oil because of its higher viscosity, which inhibits its reach into the zone. Upon increasing the speed the heat 12 R. KATNA ET AL. generation also increases which reduces its viscosity and enables effective penetration into the zone and thus less wear is seen than SAE 40 oil. Drilling test revealed better performance of cottonseed oil in reducing tool wear in comparison to SAE 40 oil. Table 6 shows summary of nonedible oils tribological tests done using nonedible vegetable oils. The structure of vegetable oils promotes good lubricity they repoto the nonedible vegetable oils. This is largely because of the chemical structure of natural oils, which promotes good lubricity and strong lubricant film formation. This is related to the amount of saturation of the molecule and fatty acid composition. However, the use of tribological tests as the sole parameter to judge the suitability of a lubricant as cutting fluids in machining is not justified as actual machining conditions differ than those during tribological tests. Actual machining tests help in determining the feasibility of oil as cutting fluid. Olawale et al.[196] used Neem oil in drilling of mild steel and reported a higher chip thickness with Neem oil in comparison to castor oil. The authors attributed higher chip thickness due to the high degree of lubricity offered by Neem oil. Neem oil performed better in tribological tests by giving minimum diameter of wear scar. Paul et al.[197] inquired into the performance of Neem oil and Karanja oil in machining mild steel on lathe with HSS cutting tool. The results show that low surface roughness was achieved with the nonedible oils in contrast to mineral oil. They observed that the temperature variation for all the cutting fluids, during machining, is the same. However, the performance of Neem oil in reducing temperature is lesser than Karanja oil. This is because Neem oil is less viscous than Karanja oil and hence the fluidity assists in better removal of heat. Katna et al.[84]performed experiments on EN8 steel using cutting fluids formulated from Neem oil. They varied the surfactant quantity from 5 to 20%and compared the performance cutting fluids with conventional cutting fluid in terms of temperature, tool wear, machining forces, and surface quality. They reported that cutting fluids formulated from Neem oil were able to outperform the conventional cutting fluid in all the measured output. Also, the longterm emulsion stability of the cutting fluids formulated from Neem oil was at par with the conventional one for an observation period of 3 months. The authors concluded that cutting fluid formulated from Neem oil is a good substitute for the conventional fluid used in the study. Figure 11 shows surface roughness achieved with different fluids formulated from Neem oil. Yakubu and Bello[198] used Neem oil in turning Aluminum manganese alloy under different machining conditions. They compared the machining performance of Neem seed oil with Figure 11. Surface roughness using different cutting fluids formulated from neem oil in turning En8 steel[84]. MATERIALS AND MANUFACTURING PROCESSES 13 Figure 12. Tool life with different cutting fluids[202]. soluble oil and dry turning at different cutting parameters. The authors reported that Neem seed oil produced 39% less surface roughness in comparison to the soluble oil. The authors also reported a 56% lower tool wear with Neem oil than soluble oil. It is thus concluded from the outcome that Neem oil is a better cutting fluid than the mineral oil. Jabba and Usman[199] used Neem oil as base oil in machining mild steel and studied tool life, surface finish, and temperature. The results obtained for temperature indicate the least temperature rise with Neem oil, succeeded by soluble oil and highest temperature with straight oil. This is due to the good thermal conductivity of Neem oil than both other oils. The average surface roughness value was least for Neem oil and highest with soluble oil. Tool wear results favored the use of Neem oil as it produces the least wear. The results show improvement in surface finish and tool life as the cutting speed was increased while reducing the cutting speed reduced the surface finish and increased the tool wear. Neem oil significantly reduces the temperature during machining than straight oil and soluble oil. In an attempt to introduce Karanja oil as a preferable cutting fluid, the authors tested Karanja oil emulsion in turning EN8 steel.[200] Karanja oil emulsions were prepared by mixing Tween 80 in Karanja oil and the concentrate thus produced was diluted to 5% in water. Machining was done at different cutting speeds (100–200 m/min), different values of feed (0.1–0.3 mm/rev) and different values of depth of cut (0.5–1.5 mm). A comparison of cutting force showed that cutting fluid developed from Karanja oil caused a reduction in thrust force by 21% than the conventional oil. Also, 15% of lower radial forces were obtained with Karanja oil emulsion than the other fluids. However, the same feed force was obtained for both fluids. The authors concluded that Karanja oil can be used as a preferable cutting fluid because it lowers the surface roughness and the machining forces. In another study, the authors[201] investigated the impact of Karanja oil on chip thickness in turning AISI 1045 steel. It was revealed that low chip thickness (11% reduction) was achieved with Karanja oil-based cutting fluid than conventional fluid. Increasing the cutting speed increases the shear plane angle thereby shortening the shear plane and the tool chip contact area which collectively reduce the chip thickness. Also, lower cutting speed yields low chip thickness, because of the increase in the contact length of the tool and chip and also a smaller shear plane. Bork et al.[202] investigated the performance of Jatropha oil-based soluble cutting oil and compared it with canola oil, synthetic ester of Jatropha and mineral oil in milling aluminum alloy 7050-T7451. Tool wear with Jatropha oil was less than 0.05mm after a machining length of 1000 m. At all the feed rates, tool life was found to be 30% higher with Jatropha oil than other oils used in the study. As to the surface roughness, least values were obtained with Jatropha oil (0.075–0.1 micron) while with canola oil the values were from 0.1 microns to 0.15 micron, for Jatropha ester the value was from 0.1 microns to 0.12 micron and for mineral oil the roughness was from 0.08 micron to 0.1 microns at different speed-feed combination. The mean radius of chips varied from 2.125 to 1.987 mm. Jatropha oil produced the highest chip radius, which confirmed the best tribological properties against all other oils used in the study. Cutting forces were highest with canola oil among other oils. It is interesting to note that Jatropha oil gave very least variation in cutting force at all feed rates owing to its inherent lubricity and sulfur content. Lower values of cutting forces indicate its superior lubrication 14 R. KATNA ET AL. Figure 13. SEM images of cutting tool under action of different cutting fluids[202]. Figure 14. Chip shapes with different cutting fluids[202]. over other oils. The results prove the effectiveness of Jatropha oil. Also, it does not contain any chlorine as present in the mineral oil which favor its use as a clean product for machining aerospace alloys. Figure 12 shows the tool life with different cutting fluids. Figure 13 shows the SEM of tools. Figure 14 shows the image of chip shapes. Vegetable oils are effective in machining even when used in small quantity. Elmunafi et al.[203] used MQL in machining hardened AISI 420 (47–48 HRC) at different cutting conditions. At cutting speed 100 m/min, the highest tool life obtained was 33.7 min at feed rate 0.16 m/min. However, higher feed rate caused a decrease in the tool life to 31.7 min at 0.2 mm/rev and a sharp decrease to 20.1 min at 0.24 mm/rev were observed. Machining at 135 m/min, tool life was highest at 0.16 mm.rev (8.4 min), followed by 7.4 min (0.2 mm/rev) and least tool life of 3.9 min at 0.24 mm/rev. Low tool life was achieved when machining was done at 170 m/min, the highest tool life being 6.8 min at 0.16 mm/ rev which further reduced to 2.2 min and 1.5 min at 0.2 mm/ rev 0.24 mm/rev. Nevertheless, it was established that a small quantity of castor oil was effective in increasing the tool life. During drilling of austenitic steel, Belluco et al.[188] found a better performance of vegetable oils than the mineral oils. Similarly, Nikitha and Prasad[204] used Karanja oil in MQL form in machining EN8 steel and uncoated carbide inserts. Machining was done at 110 m/min, 0.16 mm/min feed, and 0.1, 0.4, and 0.6 mm depth of cut. ANOVA was used. Experimental study on tool wear showed that MQL machining with Karanja oil provided better lubrication than flood and dry lubrication. The friction increases with an increasing speed which is due to oxidation and the wear also increases but remains less than the wear obtained with SAE 40 oil. Figure 15 shows SEM of tool used after machining.[205] To scrutinize the grinding performance of vegetable oils, the authors tried seven different oils viz corn, rapeseed, palm, castor, peanut, and soybean oil in grinding a nickel alloy.[178] Among all the other oils used in the study, Castor oil gave the least amount of thrust force (24.33 N) followed by palm oil (26.98 N), sunflower oil (30.34 N), peanut (31.10 N), soybean oil (31.54 N), rapeseed oil (31.88 N), and highest by corn oil (33.91 N). Also, least normal grinding force was achieved with Castor oil (76.83 N) followed by palm oil (87.10 N), peanut oil (91.37 N), corn oil (99.85 N), sunflower oil (101.88 N), rapeseed oil (102.53 N), and highest with soybean oil (104. 24 N). The reason for the least grinding forces with Castor oil is that it is the most viscous of all the lubricants, exhibits poor liquidity and forms a dense protection layer on the workpiece thereby exhibiting high anti friction property. Also, the long carbon chain of castor oil promotes an adsorption film which is very strong than other oils. Presence of ricinoleic acid (90.58%) promotes strength and durability to the physical film. However, in case of palm oil and rapeseed oil results obtained were opposite wherein rapeseed oil exhibited high cutting force than palm oil. Thus, only the length of carbon chain cannot be attributed to be the only factor responsible for lubrication. Figure 16 shows grinding forces with different cutting fluids.[178] Suresh et al.[206] investigated the hard turning performance of palm, castor, and groundnut oil in machining AISI D3 steel with different styles of uncoated cutting inserts. Machining done at 200 m/min with 0.09 mm/rev feed and 2 mm depth of cut with castor oil gave the least surface roughness MATERIALS AND MANUFACTURING PROCESSES 15 Figure 15. Comparison of SEM image of cutting tool with different fluids[205]. (0.7044 mm) with DNMG type insert. Also, high surface finish was observed with castor oil at all machining parameter combinations, followed by palm oil and highest with groundnut oil with different insert styles. However, it was found from ANOVA analysis that the impact of cutting fluid and insert type is negligible in comparison to cutting parameters. In order to determine the influence of lubricant on the wear and frictional forces Agrawal et al.[195] tested the influence of cottonseed oil on M2 steel. The authors compared the performance of cottonseed oil with SAE 40 oil. Experiments were carried out on pin-on-disc tribometer and pins made from M2steel. It was observed from the tribological experiments that SAE 40 was less effective in reducing wear than cottonseed oil. This is because of better lubricity due to long, heavy, and dipolar nature of cottonseed oil. Cottonseed oil gave less wear at higher loads than SAE 40 oil. At higher loads similar wear was observed for both cottonseed oil and SAE 40 oil. At lower speed of 300 rpm, cottonseed oil gave more wear than SAE 40 oil because of its higher viscosity, which inhibits its reach into the cutting zone. Upon increasing the speed, the heat generation also increases which reduces its viscosity and enables effective penetration into the zone and thus less wear is seen than SAE 40 oil. Drilling test revealed better performance of cottonseed oil in reducing tool wear in comparison to SAE 40 oil. Obi et al.[207] compared the performance of cottonseed, palm, groundnut, and shear butter oil with kerosene oil (due to gummy nature of aluminum metal). Aluminum was machined with HSS tool on lathe machine at different machining parameters. At 90 rpm, 0.5 mm/rev and 2 mm cut depth, least surface roughness was achieved with shear butter oil (32.9 micron), followed by kerosene (35 micron), cottonseed oil (35.3 micron), palm oil (38.3 micron), and highest with groundnut oil (40.9 micron). Cutting speed impacts the surface finish as it was seen that as there was an increase in the cutting speed the surface finish also increased gradually with all the cutting oils. At higher feed rate and depth of cuts, kerosene performs superior to other oils in 16 R. KATNA ET AL. improving the surface finish. Kerosene reduces temperature better than other oils due to its volatile nature. Shaikh and Siddhu[208]obtained good results in machining D2 steel with cutting fluid from nonedible vegetable oil. The experimental results indicate nearly similar surface finish and rate of material removed with cottonseed oil, mineral oil, and soybean oil with the gap among them was not more than 10%. Table 7 shows summary of machining tests done using nonedible vegetable oils Performance improvement of nonedible oils Nonedible oils have performed superior to conventionally used cutting fluids both in neat form and soluble form. They, however, are susceptible to auto oxidation and lose their properties at high loads at elevated temperature. Milena et al.[177] showed that upon oxidation the performance of nonedible oils dropped. Several other studies have shown this limitation of nonedible oils and have suggested use of additives or other processes for improving the physiochemical properties of the oils. Improving the performance of oils can be done by either blending with other oils, modification of oil or mixing nanoparticle additives. Currently, the trend is using ionic liquids in very less amount. It has been seen that ionic liquid in quantity as low as 1% has the ability to enhance the performance of nonedible oils significantly. Performance improvement by blending Agrawal and Patil[209] evaluated Aloevera oil as cutting oil in machining M2 steel and comparison was made with conventional fluid. They blended Aloevera oil with cottonseed oil in ratio 1:1. The blend thus prepared had slightly higher viscosity (40 cst) than conventional cutting fluid (38 cst). Machining was carried via MQL mode out at three levels of cutting speed feed and depth of cut using Taguchi L9 orthogonal array. At Figure 16. Grinding forces with different cutting fluids[178]. all cutting conditions, the formulated blend performed better in reducing surface roughness than the conventional cutting fluid. When machining was done at 500 rpm, 0.2 mm doc, and 0.18 mm/rev axial feed the surface roughness reduced by 16%. Formulated cutting fluid produced surface roughness 3.1 micron while the conventional cutting fluid produced 3.7 micron. Upon increasing the cut to 0.4 mm and axial feed to 0.27 mm/rev at same speed, Ra (Average roughness) value of surface increased (3.7 micron) but was still 9% less than that achieved by conventional cutting fluid (4.1 micron). On further increasing the feed and depth of cut, the surface finish reduced with the conventionally used fluid. However, the surface roughness was less than conventional cutting fluid even on increasing the machining parameters. Similarly, at lower cutting conditions the tool life was almost the same. However, on increasing the machining conditions, the tool life increased with the formulated cutting fluid in comparison to the conventional cutting fluid. In an attempt to study the characteristics of modified nonedible oils, Olawale et al.[196]tested the lubricity of oil blends prepared from Neem oil and castor oil by mixing them in different ratios. The formulated oils were pure Neem, Pure Castor, Neem90: Castor10, Neem80: Castor 20, Neem 70: Castor 30, Neem 60: Castor 40, Neem 50: Castor 50. Physiochemical investigation of Neem oil revealed high content of oleic acid (41.9%) and linolenic acid (15.9%) among other constituents. Castor oil had high ricinoleic acid (90.58%) and very less quantity of oleic acid. The highest viscosity among all the formulated oils was found for castor oil followed by 60N-40C, 50N-50C, 90N-10C, 70N, 30-C, 100N, and 80N-20C. The lubricity of all the formulated oils tested on four-ball tribometer was compared with soluble oil. Pure Neem oil gave the highest wear (0.29 mm), followed by 80N-20C (0.23mm), 70N-30C (0.18 mm), 90N-10C (0.17 mm), 100C (0.16 mm), 50N-50C (0.15 mm), and the least wear was obtained with 60N-40C (0.13 mm) blend. In actual machining tests on mild steel, chip thickness of 1.68 mm was obtained with the formulated cutting fluids whereas with soluble oil the thickness was 1.43 mm. At lower cutting speed, the chip thickness obtained with the formulated cutting fluids and soluble oil were 1.47 mm and 1.36 mm respectively. High chip thickness is associated with low coefficient of friction, which in turn is an indicator of good lubrication property. The lubricity is related to the saponification value. Higher the saponification value, higher is the boundary lubricity and lesser is the wear of the tool. Oleic acid present in seed oil is the reason for higher saponification value of Neem oil and Castor oil. Imran et al.[210] characterized the tribological properties of Jatropha oil blended with SAE 40 grade oil using Cygnus wear testing machine at a 2000 rpm and 30 N load. Blends were formulated by mixing Jatropha oil in SAE 40 oil in the ratio 10%, 20%, 30%, 40%, and 50% and the performance was compared with SAE 40 oil itself. Maximum wear of the test pin occurred for 40% blend while minimum wear occurred for 10% blend. This was due to the ability of maintaining a consistent lubricant layer of 10% blend than the 40% blend. The 30% blend, 40% blend, and 50% blend exhibited high pin wear in comparison to SAE 40 oil, 10% blend, and 20% blend. Interestingly the wear amount of 10% and 20% blend was nearly identical. All the lubricants had almost similar Surface roughness, MRR Least surface roughness achieved with castor oil. Castor oil is effective in increasing the cutting tool life. Less tool wear with cottonseed oil in comparison to SAE 40 oil Lowest surface roughness with shear butter oil followed by kerosene and then closely by cottonseed oil. Surface roughness and MRR for all oils are in close proximity with the difference not being more than 10%. Surface roughness Tool life Wear Surface roughness Castor oil gave the least grinding force. Turning Bashir and Siddhu[208] AISI D2 steel Turning Turning Drilling Turning Suresh et al. Elmunafi et al.[203] Agrawal et al.[195] Obi et al.[207] Grinding [206] Li et.al[178] High-temperature nickel-based alloy D3 steel AISI 420 steel M2 steel Aluminum Castor, palm sunflower, peanut, soybean, rapeseed, corn Castor, palm, groundnut Castor oil Cottonseed, SAE 40 oil Cottonseed, palm, groundnut, shear butter, kerosene Cottonseed, soybean, mineral oil Tool life Surface finish Machining forces Grinding force Jatropha oil Milling Bork et.al[202] Al Alloy 7050-T7451 Chip thickness Tool wear Karanja oil Karanja oil En 8 steel En8 steel Turning Turning Nizamuddin et al.[200] Nikitha and Prasad[204] Karanja oil Neem oil Al-Mg alloy En8 steel Turning Nizamuddin et al. [201] Turning Yakubu and Bello Cutting forces Neem oil emulsion performs better than conventional mineral oil in reducing machining force and temperature and increasing tool life and surface finish Neem oil produced 39% less surface finish and 55% less tool wear than conventional soluble oil. Karanja oil produced 21% less cutting force and 15% less radial force than conventional oil. Chip thickness with Karanja oil was 11% lower than conventional oil. Karanja oil via MQL reduces tool wear better than soluble Karanja oil indicating good lubricity of the straight form. Jatropha oil produced less machining forces, tool wear, and higher surface finish than others. Turning [198] Katna et al.[184] En8 steel Main findings Parameters studies Surface roughness and temperature Surface finish, temperature, tool life, machining force Surface roughness, tool wear Neem Karanja Neem oil Nonedible oils used in study Material Turning Paul and Pal[197] Mild steel Machining operation Research group Table 7. Summary of machining tests done using nonedible vegetable oils. Low surface roughness with nonedible oils than mineral oil. MATERIALS AND MANUFACTURING PROCESSES 17 coefficient of friction (0.15) except 50% blend, which had higher coefficient of friction (0.225), and this value remained almost constant throughout the experiment. However, for SAE 40 oil, the friction coefficient was high during the beginning of experiments and as machining time passed, the coefficient of friction decreased rapidly. Temperature rise during machining was the least for 10% blend (11.7 degrees), followed by 20% blend (12.8 degrees), 50% blend (13.66 degrees), 30% blend (18.65 degrees), and highest with 40% blend (25.49 degrees). This is due to the good specific heat of 10% blend than other blends. Figure 17 shows the wear of pin with different blend ratios. Suhane et al.[211]tested castor oil-Karanja oil blend as a lubricant. Four different blends were prepared from Castor oil and using Karanja oil 10%, 15%, 20%, and 25%. The wear performance was measured by a four-ball tester at varying loads and speeds. The coefficient of friction modifying property of Karanja oil was commendable and Karanja as an additive in castor oil can be used in low-speed applications in industry. Lowest scar diameter was obtained with 20% blend and the highest wear scar diameter was obtained with 10% blend, followed by 15% blend and then 25% blend. Using ANOVA it was found that the blend ratio was the second most important factor after speed, which influences the wear scar diameter. Thus proper blend formulation must be taken into account. It was established that Karanja oil has good friction modifying properties as an additive in castor oil and its usage at low-speed operations is recommended. In an attempt to find an alternative to edible oil as cutting fluids Jyothi and Sharan[212] used Neem and Honge oil as a substitute to mineral oil in drilling operation of Mild steel. Neem and Honge oil were used in different proportions (1N:2H, 2N:1H, and 1N:1H). 13 mm dia and 30 mm deep holes were drilled using HSS drill bit at 800 rpm and 10 mm/rev feed. Viscosity (in N-s/ m2) of Neem oil was the highest (0.0345) among all the formulated oils, followed by Honge oil (0.0266), 1N:1H (0.01648), 1N:2H (0.0135), and 1N:1H (0.0112). During the machining process, the lowest temperature among all the formulated blends was obtained with 1N:1H blend (36.2°C). This was due to specific heat capacity, viscosity, and adhesiveness which were higher than other blends. However, a slightly lower temperature was obtained with SAE 20W40 oil than 1N:1H blend. Lowest surface roughness was achieved with 1N:1H blend among the formulated blends. Surface roughness was highest with SAE 20W40, indicating better lubrication properties of formulated oils than SAE 20W40. In a similar study, Susmitha et al.[213] formulated oil blend with Neem oil and Karanja oil by blending in ratio as 1N:2H, 2N:1H, and 1N:1H. 13 mm dia and 30 mm deep holes were drilled in mild steel using HSS drill bit at 800 rpm and 10 mm/rev feed. The performance comparison nonedible with SAE20W40 oil and measurement was done in terms of machining forces and surface finish during drilling. SAE20W40 oil produced a surface roughness of 3.5 microns, which was the highest among all the formulated blends. Least surface roughness was achieved with a 1:1 blend of Neem and Karanja oil, followed by 1N:2K, 2N:1K blend. Pure Karanja and pure Neem oil produced more surface roughness than the blends but the values were less than those achieved with SAE20W40 oil. In terms of cutting force, the least value was obtained with 1N:1K blend (169.23 N). The highest 18 R. KATNA ET AL. Figure 17. Wear with different blends[210]. cutting force was achieved with SAE20W40 oil. Other oils gave less cutting force oil than SAE oil. Habibullah et al.[214] scrutinized the tribological properties of SAE 40 oil blended with Jatropha oil in varying compositions of 1%, 2%, 3%, 4%, and 5%. Four-ball tribometer was used to compare the performance of blended oils with pure SAE 40 oil at 15 and 40 kg weight. At 15 kg load, SAE oil blended with 3%, 4%, and 5% Jatropha oils gave slightly less wear scar diameter than pure SAE 40 oil. However, at 40 kg load, 2%, 4%, and 5% blend gave less wear scar diameter than pure SAE 40 oil whereas 1% and 3% blend gave higher wear scar diameter in comparison to SAE 40 oil. Also, the addition of Jatropha oil at 3%, 4%, and 5% concentration in SAE 40 oil increases the flashpoint which enables maintenance of performance at a higher temperature. In an attempt to modify the rheology of castor oil, Guo et al.[215] mixed castor oil, and maize, soybean, sunflower, palm, rapeseed, and peanut oil in 1:1 proportion each. GH4169, a nickel-based alloy was subjected to grinding operation via the MQL method using the formulated lubricants. The lowest grinding force was obtained with Castor-Soybean blend with specific tangential force being 0.664 N and normal grinding force being 1.886 N. These values were 27.06% and 23.14% lower respectively than those obtained with pure castor oil. Palm-castor oil blend and peanut-castor oil blend reduced specific grinding force by 9% and 3.5% respectively than those obtained with castor oil alone. A significant reduction in cutting forces was obtained upon blending castor oil with other vegetable oils. Specific grinding energy was least for soybean-castor blend (60 J/mm3), followed by Maize-castor (65.21 J/mm3), rapeseed-castor (67.14 J/mm3), sunflower-castor (66.9 J/mm3), palm-castor (75.76 J/mm3), peanut-castor (78.77 J/mm3), and highest with pure castor (83.24 J/mm3). In terms of surface roughness, best surface quality is provided by the blend giving the least surface roughness. Least surface roughness was achieved with a Palm-castor blend (0.323 microns) followed by peanut-castor (0.338 microns), maize-castor (0.338 microns) soybean-castor (0.357 microns), rapeseed-castor (0.4 microns), and highest with sunflower-castor (0.473 microns). It was hence established that mixed base oils lubrication was better than lubrication effectiveness of castor oil and the soybean-castor blend performed the best. Performance improvement by modification of oils Blending of oils enhanced the performance by inducing new properties in the oils. This led the researchers to think of oil modification as a novel way of imparting new properties. Oil modification changes the molecular structure of oil thereby making them perform superior to their unmodified versions. Generally, chemical modification is done to change the oil into its TMP form via the transesterification process. Tribological properties are greatly enhanced by such modification. Methyl ester formed upon primary conversion significantly improves the friction properties of the base oil due to modification of the chemical structure.[216,217] Shashidhar and Jayaram[218] tested friction and wear property improvement of six different oils viz. raw Pongam oil (PRO), raw Jatropha oil (JRO), epoxidized versions of Pongam oil (EPRO) and Jatropha oil (EJRO), and Epoxidized versions of Jatropha Methyl ester (EJME) and Pongam methyl ester (EPME). Modification improved the oxidative stability of the oils. It was found in preliminary testing that the viscosity of the modified versions was 20% higher than the original unmodified oils and there was an improvement in the viscosity index by 30% in an epoxidized methyl ester of Pongam oil and 37% enhancement in viscosity index of epoxidized methyl ester of Jatropha oil. AISI 1040 steel pins were used as test specimens on a tribometer. For the same sliding distance, a 50% lower friction coefficient was seen for PRO in comparison to mineral oil. EPRO and EPME, however, showed increased coefficient of friction by 10% and 15% in comparison to mineral oil respectively. For JRO, EJRO, and EJME, a drop of 15%, marginal increase, and an increase of 10% in friction values were observed respectively in comparison to mineral oil. On increasing the load to 100 N, a 10% reduction in friction coefficient was observed in all formulated oils except EJRO (+40%) and EPRO (Marginally high) in comparison to mineral oil. Upon further increasing the load to 150 N, a 35% drop in the coefficient of MATERIALS AND MANUFACTURING PROCESSES 19 Figure 18. Variation of friction coefficient in modified oils[218]. friction was observed for EPRO, a 45% drop in EPME and a 60% drop in PRO. At the same load, JRP, EJRO, and AJME showed a 35% drop in friction value at a similar load in contrast to mineral oil. At the highest load of 200 N, 20%, 13%, and 30% drop is observed with PRO, EPRO, and EPME respectively. Whereas there is only a marginal reduction in friction with EJME in comparison to mineral oil, JRO and EJRO show a similar trend as PRO and EPRO. Less friction is attributed to the inherent polar nature of vegetal oils and the presence of oleic acid, which provides viscosity to the oil. However, an interesting fact is observed in wear behavior with PRO and JRO giving less wear than EJRO and EPRO. In fact, EJRO and EPRO gave high wear than the mineral oil while PRO and JRO showed low wear than mineral oil. PRO was able to reduce the wear by 65%-98% at all loads, EPRO showed 65%-70% reduction in wear at lower load while at higher loads it reduced wear by 35%-95% in comparison to mineral oil. EPME showed a 20%–90% reduction in wear at lower loads. JRO and EJRO also show a good amount of reduction in wear ranging from 50% to 90% and 70% to 908% respectively at various loads. EJME fails in reducing wear at small loads but performs well in reducing wear occurring at larger loads. The reduction in wear by using Jatropha and Pongam oil is due to the high oleic content, which helps in generating strong adsorption on the surface and hence provides good interaction among the ester chains. Figure 18 depicts the friction coefficient variation in modified oils. Owing to the hazardous problems posed by conventional mineral oil-based cutting fluids and the inferior thermooxidation stability Talib and Rahim[219] examined the Modified version of Jatropha oil in order to enhance the oxidative and thermal stability. First CJO was transformed to JME and further to TMP ester. Two oils were formulated by mixing JME and TMP respectively in two different proportions – MJO1 (3.1:1) and MJO3 (3.3:1). The performance of these oils was compared with a synthetic ester. Tribological tests showed minimum wear scar diameter with CJO (0.641 mm), followed by synthetic ester (0.693 mm), MJO1 (0.718 mm), and highest wear scar diameter with MOJ3 (0.806 mm). The coefficient of friction was least with CJO (0.059) followed by MJO1 (0.086), synthetic ester (0.097), and highest with MJO3 (0.112). Even though CJO gives the least coefficient of friction, it is not recommended to be used because of its low thermal stability. MJO1 can be used in place of synthetic ester. The actual machining process revealed the lowest cutting forces with synthetic ester because of its higher viscosity than the vegetable oils used in the study. The less viscous fluid will affect the lubricity and cause an increase in cutting forces during machining. Cutting temperatures obtained with MJO1 and MJO3 were only slightly higher than the synthetic ester. This indicated that good strength of the lubricant layer and combined viscous effect help in taking away the heat from the machining zone effectively.MJO1 performed equally well as the synthetic ester indicating that it has the potential to be used in machining. Jeevan and Jayaram[205] compared the performance of Epoxidized Jatropha (MJO) and Epoxidized Pongamia (MPO) oil in machining AA6061 with HSS on a lathe at varying cutting conditions. The performance was compared with a conventionally used mineral oil (MO). The authors used the Taguchi L27 technique and reported that MJO performed optimally followed by MPO and MO. Optimal cutting parameters for minimum surface roughness were found to be 800 rpm spindle speed, 1.5 mm doc and 0.175 mm/rev tool feed rate. For minimum cutting force, the optimal parameters were cutting speed 1600 rpm, 0.5 mm doc, and 0.1 mm/rev feed. For surface roughness, MPO performed optimally followed by MJO and MO whereas in cutting force, MJO 20 R. KATNA ET AL. performed optimally followed by MPO and MO. ANOVA analysis disclosed that cutting fluid does not impact cutting force to a large extent but has only a very minimalistic impact. However, there is a 10% contribution of cutting fluid on the surface finish than spindle speed (5.73% contribution), depth of cut (4.13% contribution), and feed (61.68% contribution). Tool wear obtained with MPO was the least at 264.9 microns, followed by MO (577.51 microns), and MJO (558.15 microns). Chemical modification of Jatropha and Pongamia oil strengthened the interaction between ester chains significantly which reduced the peeling/attrition of the tool as observed in MO. The protective layer formation in the modified oils formed at the contact surface reduces wear and friction. In another study, it was found that modified Neem oil ester used in machining EN8 steel with coated carbide caused a 40% drop in average surface roughness value and also caused a drop in the machining force than the mineral oil. Also, Modified oil has a low flash point than Neem oil but higher than conventional fluid Jeevan and Jayaram[220] explored the potential of Neem and Mahua oil in an attempt to test the feasibility of vegetable oil in drilling AISI 304L. Neem and Mahua oil have high oleic content viz 45% and 40% respectively and also high linoleic acid (15% and 14% respectively) which is a reason for their high lubricity. Cutting fluids were formulated by epoxidation of raw Neem oil and Mahua oil for enhancing their oxidation stability. 6 mm dia and 10 mm deep holes were drilled at spindle speed 2000, 2500, and 3000 rpm and 3 × 10−3, 6 × 10−3, and 9 × 10−3 mm/rev. Cutting fluid was applied via MQL (minimum quantity lubrication) method at 2 bar pressure. Taguchi L27 was used to find optimum cutting values (viscosity, feed, and speed). Least thrust force was achieved with Mahua oil, followed by Neem oil and the highest thrust force with mineral oil. This is attributed to Mahua’s better resistance against molecular breakdown than Neem oil and mineral oil. Mahua oil performed better in providing good surface finish, followed by Neem oil and the highest surface roughness was obtained with mineral oil. This was due to better ability of epoxidized Mahua oil in maintaining a thin film of boundary lubrication than Neem oil and mineral oil. Interestingly, Neem oil was able to reduce the temperature during machining better than Mahua oil whereas the highest temperature was recorded with mineral oil. Mahua oil was able to reduce tool wear better than Neem oil and mineral oil due to improvement in its epoxidized structure. ANOVA for thrust force, surface roughness, and temperature revealed a maximum contribution of feed in all three cases, followed by the speed in all three cases and the least contribution was viscosity in all three cases. Finally, for drilling AISI 304L steel thrust, surface roughness and temperature were found to be optimum with Mahua oil. Neem and Mahua oil have high fatty content and are thus suitable for lubricant formulation. Jamshaid et al.[221]tested the friction characteristics of methyl ester prepared from cottonseed oil (COME). COME was prepared by the transesterification process of crude cottonseed oil. Tribological experiments were done on a four-ball tester as described in ASTM D4172. Five different oils that were compared were COME, DL, COME10DL90, COME20DL80, and COME50DL50 where DL is the petroleum diesel. COME10 had the highest friction coefficient at the start of the experiment, which became equal to Diesel oil as the experiment neared completion. COME had the least friction coefficient throughout the experiment followed closely by COME50. Increasing the speed of rotation from 600 to 900 rpm increased the coefficient of friction marginally but the least coefficient of friction was obtained with COME. A similar trend was observed at higher speeds of 1200 rpm and 1500 rpm. COME gave the least coefficient of friction followed by COME 50. All other blends gave a coefficient of friction higher than COME and COME 50. Thus, COME gives the highest reduction in friction coefficient and also the addition of COME in diesel oil improves the performance of diesel oil. The coefficient of friction is directly related to the scar diameter. Least scar diameter was achieved by COME, followed by COME50, COME20, COME10, and highest wear scar diameter with Diesel oil. Table 8 shows a summary of experiments done with non-edible vegetable oil blends and modified oils. Performance improvement with ionic liquids as additives Organic salts having melting points below 100°C are called ionic liquids and they comprise cations and anions. They have a wide viscosity level, superior thermal stability, high polarity, and low volatility. Ionic liquids form thin boundary layer molecular film and thus prevent contact between sliding surfaces. The ionic liquids form a densely packed tribofilm by getting adsorbed on the top layer which consequently enhances the frictional and wears properties of the lubricant. In an endeavor to improve the tribological characteristics of Jatropha oil, Sahab et al.[222] formulated different oils using modified Jatropha oil blended with ionic liquids in different weight concentrations of 1%, 5%, and 10%. [P66614] [Phosphinate] and [N1888] [NTf2] are the ionic liquids used for improving the physi0chemical properties of the base oil. Comparison is made among MJO, AIL, PIL, MJO 1% AIL, MJO 5% AIL, MJO 10% AIL, MJO 1% PIL, MJO 5% PIL, MJO 10% PIL, and synthetic ester. Jatropha oil was transformed into its methyl ester form (FAME) and then mixed with TMP in ratio 3.5:1 to produce MJO prior to doping with ionic liquids AIL and PIL. The viscosity of MJO was comparable to a synthetic ester and both these lubricants were used as a benchmark. The addition of ionic liquids increased the viscosity of MJO, which was in correspondence to the proportion of ionic liquid present in the formulated blend. Physical properties did not change much by the addition of ionic liquids in MJO, but MJO with 1% PIL showed a significant drop in viscosity at 100 degrees and also suffered a significant change in the viscosity index. Both AIL and PIL were able to resist corrosion in the copper strip corrosion test but MJO showed some tarnishing on the surface of copper. MJO+AIL 10% was successful in improving the corrosion behavior of MJO. Tribological tests showed that synthetic ester, AIL, and PIL present high value of friction coefficient. This is due to the high number of carbon atoms in the cationic and anionic constituents of the ionic liquids, which decrease the lubricant flow thereby increasing the friction. Wear scar diameter with AIL and PIL was 42% and 47% less respectively in comparison Table 8. Summary of experiments done with nonedible vegetable oil blends and modified oils. Research group Agrawal et al.[209] Material M2 steel Nonedible oils blend used in the study Aloe vera and Cottonseed (1:1) Tribotest on four-ball tester Tribotest on Cygnus wear testing Tribotest on four-ball tester Mild steel Neem and Castor (90:10, 80:20, 70:30, 60:40, 50:50), soluble Wear oil Jatropha and SAE 40 (10%, 20%, 30%, 40%, and 50%), SAE 40 Wear, Coefficient of friction Castor and Karanja (10%, 15%, 20%, and 25%) Wear Coefficient of friction Drilling Mild steel Neem and Honge (1:1, 1:2, and 2:1) Drilling Mild steel Neem and Karanja (1:1, 1:2, and 2:1) Tribotest on four-ball tester Grinding EN 31 SAE 40 and Jatropha oil (1%, 2%, 3%, 4%, and 5%) GH 4169 nickel alloy Castor oil blended with rapeseed, palm, soybean, peanut, maize, and sunflower Jeevan and Jayaram[218] Pin on disc tribometer AISI 1040 Talib and Rahim[219] Tribotest on four-ball tester Turning AISI 1045 Pongam, Jatropha, epoxidized versions of Pongam and Jatropha, Epoxidized version of Jatropha methyl ester and Pongam methyl ester, mineral oil Jatropha oil and chemically modified jatropha oil, Jamshaid et.al[221] Drilling AISI 304L Jatropha, Pongamia, Epoxidized version of Jatropha and Pongamia, mineral oil Neem, Mahua, and mineral oil-based cutting fluid Amiril et al.[222] Tribotest on four-ball tester Chrome alloy steel Cottonseed methyl ester, petroleum diesel Olawale et al.[196] Imran et al. [210] Suhane et al. [211] Jyothi et al.[212] Susmitha et al. [213] Habibullah et al.[214] Guo et al. [215] Jeevan and Jayaram [205] Aluminum Plain carbon steel AA 6061 Parameters studies Surface roughness Main findings Formulated cutting fluids have a higher viscosity than parent oils and produced less surface roughness than conventional fluid. 60:40 blend has the highest viscosity and least wear. Least wear with 10% blend, formulated oils had a low coefficient of friction than SAE 40 oil. Least wear with 20% blend Karanja oil has good friction modifying properties as additive. Surface roughness The lowest temperature obtained with 1:1 blend Temperature Lowest surface roughness with 1:1 blend. Surface roughness Least surface roughness with 1:1 blend. Cutting force The least drilling force with 1:1 blend. Wear Least wear obtained with Jatropha oil as additive increases flashpoint. Specific grinding force Least specific grinding energy for Castor-soybean Surface roughness blend and highest with unblended castor oil. Least surface roughness with Castor-Palm blend. Blended base oil lubrication is superior to unblended. Coefficient of friction Improvement in wear properties upon modification Wear of oils. Friction coefficient Wear Cutting force, surface roughness Thrust force, surface roughness, tool wear, temperature Wear, coefficient of friction Jatropha oil gives the least friction coefficient and wear scar diameter. Modified nonedible oils reduce cutting force and surface roughness more than mineral oil. Mahua oil performed best in all parameters, followed by Neem oil and the least performance of mineral oil-based cutting fluid. Modified oil had the least coefficient of friction and wear. MATERIALS AND MANUFACTURING PROCESSES Machining operation Turning 21 22 R. KATNA ET AL. Figure 19. Wear scar diameter with ionic liquids[222]. to wear scar diameter with MJO. Neat PIL provided least wear scar diameter, which was 17% less wear scar diameter with the synthetic ester. AIL presents 8% less wear than a synthetic ester. MJO 10% AIL presented 28% less wear scar diameter than neat MJO. It is observed that only a 1% PIL additive is adequate in improving the tribological properties (antifriction and antiwear) of MJO. MJO PIL 1% reduced wear scar diameter by 36% than MJO. Increasing the amount of ionic liquid in the base oil increase the degree of contact of the lubricant with the surface which enhances the contact of lubricant particles and hence the lubricity. Tapping torque tests gave the highest thrust force with MJO (70.4 N). The thrust forces reduce with more addition of AIL. However, the thrust force was raised slightly upon raising the quantity of PIL. Load carrying capacity increases with an increase in viscosity which in turn depends on the length of the carbon chain. An adequate amount of viscosity helped to lower the amount of thrust force required in the tapping torque test. MJO AIL 10% and MJO PIL 1% reduce the thrust force significantly as compared to MJO and synthetic ester. The polarity in the lubricants provides effective lubrication due to the formation of a tribological film on the surface of the metal, which protects the metal surface. AIL and PIL are potentially good additives that can enhance the performance of base oils remarkably. Figure 19 shows wear scar diameter obtained with different ionic fluids.[222] In another study, Sahab et al.[223] mixed AIL and PIL in weights of 1%, 5%, and 10% each in Jatropha oil and investigated the performance of the formulated lubricants in machining AISI 1045 steel. Uncoated tungsten carbide was used and the MQL method was employed. During machining, MJO-PIL 10% gave the highest cutting force (618 N). MJOAIL 1% gave cutting force equal to that of synthetic ester (612 N). The lowest cutting force was achieved with MJOAIL 10% with 2–4% less force than synthetic ester and MJO. MJO-PIL 1% gave 205% less cutting force in comparison to synthetic ester. Cutting forces decrease slightly upon increasing the AIL content in MJO but with PIL as an additive in MJO, the cutting force increases gradually. MJO retains high lubricating efficiency compared to synthetic ester due to its strong interaction with the metallic surface. MJO provides outstanding lubrication owing to the long carbon chain.[219,222,224] MJO-AIL 10% provided the lowest specific cutting energy, followed by MJO-PIL 1%, MJO AIL 1%, MJO, SE = MJO-AIL1% = MJO-PIL 5%, and highest with MJO-PIL 10%.This trend is due to the fact that high viscosity reduces the flow and the rate of heat transfer of the lubricant. The optimum value is needed in order to get the least specific cutting energy. In terms of reducing temperature MJO-AIL 10%, MJO-PIL1%, and MJO-PIL 5% gave less temperature than MJO and synthetic ester. MJO-AIL 1%, MJO-AIL 5%, and MJO-PIL 5% gave temperature more than MJO and synthetic ester. Ionic liquids, even in minute quantity, have shown to enhance the performance of oils significantly. Low specific cutting energy attained with their addition indicates their viability and feasibility to be used as an additive to oils used in machining operations. In yet another experiment using ionic liquids, Sani et al.[215] analyzed the machining performance of ionic liquids added in modified Jatropha oil and compared the performance with synthetic ester. It was found that MJO-AIL 10% was able to form a strong tribofilm owing to its polar nature. The boundary layer lubrication provided by MJO-AIL 10% caused 12% lower cutting force, 9% lower temperature, 7% lower surface roughness and 50% lower tool wear than a synthetic ester. However, only 1% addition of PIL to MJO was effective in reducing the cutting forces, temperature and surface roughness by 11%, 7%, and 4% respectively and also increasing the tool life by 40%. Even though 1% PIL was able to increase the performance, excessive addition of PIL deteriorated the performance and caused poor lubrication and increased surface roughness, temperature, and cutting forces. MJO-PIL 10% decreased the tool life by 10% than that by MJO. Notch wear was found predominant failure mode with abrasions MATERIALS AND MANUFACTURING PROCESSES 23 Figure 20. Cutting force with each cutting fluid[225]. and adhesion in all cases except with MJO-PIL 10%, MJOAIL10%, and MJO-PIL1% were able to prevent adhesion from both the tool and workpiece. Pannerselvam and Karthikey[226] investigated the effect of adding a small quantity of an IL–BMIMBF4) to a Metal Working Fluid (MWF) formulation applied using the MQL System in machining AISI 1040 steel. The results of surface roughness indicate superior lubricity of the oil upon the addition of ionic liquids. The addition of 1% ionic liquid causes a reduction of 40% in surface roughness in Neem oil and Neem oil emulsion. However, at 2% concentration, the change in surface roughness was only marginal. Figure 20 shows the cutting force with each cutting fluid. Shanhua et al.[227] explored the tribological characteristics ionic liquids added in castor oil. The ionic liquid was mixed in wt% 0.6, 1, and 3. From the experimental results, it was found that even a small quantity of IL has the capability of delaying the occurrence of the highest friction coefficient in the wear test and also improves the wear property of castor oil. Lowest wear scar diameter was found with 1% ionic liquid. From the results, it appears that the physical mode of adsorption is more dominant than the chemical mode of adsorption with castor oil mixed with ionic liquid. Jiang et al.[228] investigated the tribological characteristics of rapeseed oil mixed with crown-type phosphate ionic liquids. Among other ionic liquids used conventionally, this type of ionic liquids shows good miscibility in rapeseed oil. The experimental outcomes show improved the tribological performance of rapeseed oil consequent to the addition of ionic liquid. It was seen that at 1% ionic liquid concentration produces the lowest friction coefficient and wear. The exceptional tribological properties are due to strong lubrication film formed between the mating surfaces. Li and Ren[229] investigated the effect of ammoniumbased ionic liquid and dioxazaborate-based ionic liquid in rapeseed oil. From the experimental results, it was found that both these ionic liquids have outstanding synergistic effects, which help in enhancing the antiwear and performance at extreme pressure of the base oil and even get enhanced if the load is increased. This synergism is due to the collaboration of tribo reaction products generated by the interaction of these two ionic liquids. Table 9 summarizes the ionic liquids used in nonedible vegetable oils. Performance improvement by use of nanoparticles as additives Talib and Rehman[182] analyzed the performance of hBN particles in 0.01%, 0.1%, and 0.5% in modified Jatropha oil (MJO and compared with MJO and CJO (crude Jatropha oil) and synthetic ester. MJO was prepared by converting it to JME and then mixing it with TMP. 12 samples were formulated viz MJO1, MJO1a, MJO1b, MJOIc, MJO3, MJO3a, MJO3b, MJO3c, MJO5, MJO5a, MJO5b, and MJO5c, where 1, 2, and 3 represent JME: TMP ratio as 3.1:1, 3.3:1, and 3.5:1 respectively with additive amount a = 0.05%, b = 0.1%, and c = 0.5% respectively. Physiochemical tests on all the samples revealed the low acid value of a modified version of Jatropha oil than its crude form (acid value 27.81 mg sodium hydroxide per gram oil). MJO5 recorded the least acid value of 0.38, followed by MJO5b (03.6), MJO5a (0.38) = MJO5c (0.38), MJO3 (0.39), MJO3a (0.4) = MJO3c (0.4), MJO3b (0.41), MJO1 (0.42), MJO1b (0.43), and MJO1b (0.45) = MJO1c (0.45). The acid value was of the synthetic ester was higher than the formulated oils (0.56). The lower acid value is vital as it is an indicator of the good quality of the oil as it is able to resist corrosion and resists the gumming and Sludging tendency. High acid value causes hydrolysis of oil on storage for the longest period of time and is undesirable. An interesting outcome is that there was no 24 Table 9. Summary of nonedible vegetable oils with ionic liquid additive. Sani et al.[223] Machining operation Material Nonedible oils and ionic liquid used in the study Cutting force, temperature Machining force, temperature, surface finish, tool life Surface roughness Lowest cutting force and temperature with ammonium-based ionic liquid. A low quantity of ionic liquids can improve the performance of oils significantly. Ammonium based IL (10%) performed best by reducing tool wear by 50%, cutting forces by 12%, temperature by 9% and surface finish improved by 7% than a synthetic ester. IL additive helped in reducing surface roughness significantly. Rapeseed oil and ammonium-based ionic liquid Coefficient of friction and alkylphenylborate and wear Ionic liquids have excellent synergistic effects in reducing friction and enhance the antiwear performances significantly. Panner et al.[226] Turning Medium carbon steel Jatropha synthetic ester, Jatropha with ammonium, and phosphonium-based IL Shanhua et al.[227] Turning AISI 1040 Jiang et al.[228] Tribotest on four-ball Plain carbon steel tester SRV oscillating AISI 52100 friction and wear tester Tribotest on four-ball GCr15 bearing steel tester Li et al. Talib and Rahim[224] Main findings P66614 gave a high coefficient of friction but less wear than Jatropha oil. P66614 additive gave the least thrust force in tapping torque test. Talib and Rahim[224] [229] Parameters studies Wear, thrust force Tribotest on four-ball AISI 52100 tester and tapping torque test Turning AISI 1045 Jatropha oil, Jatropha oil with phosphonium based IL (1%, 5%, and 10%) and ammoniumbased IL(1%, 5%, and 10%) Jatropha oil, Jatropha oil with phosphonium based IL (1%, 5%, and 10%) and ammoniumbased IL(1%, 5%, and 10%) Neem oil and 1-butyl-3-methylimidazolium tetrafluoroborate Castor oil and crown type phosphate ionic liquid Coefficient of friction Least wear with 1% ionic liquid additive. (0.6%, 1%, 2%, and 3%) and wear Rapeseed oil with Crown-type ionic liquids Friction coefficient of Improved tribological performance in comparison to base oil at varying and wear loads. Table 10. Summary of non-edible oils with nanoparticle additives. Research group Talib and Rahim[182] Singh et al.[183] Wagh and Mhaske[184] Machining operation Tapping torque test Pin on disc tribometer Tribotest on four-ball tester Material 1215 steel En 31 STEEL Dong et al.[185] Milling Carbon chromium steel Ti–6Al–4V Mahipal et al.[186] Tribotest on four-ball tester Chromium alloy steel Nonedible oils and nanoparticle used in the study Modified Jatropha oil and hBN nanoparticle (0.05%, 0.1%, and 0.5%) Pongamia and TiO2 nanoparticle (0.1%, 0.2%, and 0.3%) Karanja oil and hBN, TiO2, and ZDDP (0.5%, 1%, and 1.5%) Parameters studies Friction, viscosity index, wear Coefficient of friction, wear Coefficient of friction, wear Cottonseed oil and SiO2, Al2O3, MoS2, CNT, SiC, Temperature, force, graphite nanoparticles (1.5%) surface roughness Karanja with ZDDP (1%, 1.5%, and 2%) Friction coefficient, wear Main findings Improvement in viscosity index and a decrease in tapping torque. Tapping torque and wear increase on further addition of nanoparticle. Nanoparticle increased the friction coefficient and wear at 0.2% Addition of additives significantly improves the coefficient of friction Least wear obtained with TiO2+ ZDDP additive (1%). Least milling force and surface roughness with Al2O3 and least temperature with SiO2 Low friction and wear with 2% ZDDP additive R. KATNA ET AL. Research Group MATERIALS AND MANUFACTURING PROCESSES noteworthy difference obtained in the acid values upon the addition of hBN particles in the formulated metalworking fluids. Modification of the oils caused a decrease in the viscosity and also modified the viscosity index. Often denoted as VI, the viscosity index indicates variation in viscosity with temperature, a higher number indicating less change in viscosity at higher temperatures. The experimental results revealed that MJO5c had the highest viscosity index was of 228, followed by MJO5b (217), MJO5a (211), and MJO5 (196). However, other formulations had a viscosity index less than crude Jatropha oil. MJO3a, MJO3b, and MJO3c had viscosity index 146, 153, and 160 respectively, which was higher than the synthetic ester (137) but MJO3 had a lower viscosity index (137) than synthetic ester. MJO1, MJO1a, MJO1b, and MJO1c had viscosity index 118, 123, 126, and 129 respectively which was the lowest. It is observed that the viscosity index gets improved upon the addition of hBN particles because the distance between the particles decreases as the number of particles increases which results in improved physicochemical properties. Tapping torque test results indicated an increase in tapping torque efficiency from 2% to 18% with all the formulated oils in comparison to crude Jatropha oil. Except for MJO1c, which caused a drop in efficiency by 3%, all oils caused an increase in tapping torque efficiency. It was interesting to see an increase in the tapping torque with the addition of hBN particles. Excessive hBN particles act as abrasive particles between the sliding surfaces and generate poor lubrication due to an increase in friction and wear. Minute additions of hBN in 0.05% in MJO significantly enhance the tapping torque efficiency. Overall, MJOs with hBN additives show better performance than MJO and MJO5a showed the best performance in comparison to all other formulated samples and can be a substitute for the synthetic ester. Singh et al.[183] used TiO2 nanoparticles in Pongamia oil and evaluated their impact on friction reduction and wear. Experimental results show that an additive concentration of 0.1% least friction and consequently wear was seen. However, increasing the concentration to 0.2 percent raised the friction and also the wear amount. Also, 0.1% concentration in the base oil produced a smoother surface in comparison to base oil used without any additive particle. Wagh and Mhaske[184] evaluated the performance of HBN, TiO2, and ZDDP (zinc-dialkyldithiophosphate) as additives in Karanja oil. The tribological test shows that Karanja oil with 1% (TiO2 + ZDDP) has comparatively better wear scar diameter (WSD = 131.33 micrometer) than the commercial SAE 20W40 oil. Karanja oil with 1% (TiO2 + ZDDP) has a comparatively better coefficient of friction (µ = 0.04177), kinematic viscosity is comparatively better. Also, the flashpoint and pour point are better. Dong et al.[185] doped cottonseed oil with six different types of nanoparticles (SiO2, Al2 O3, MoS2, CNT, SiC, and graphite) at 1.5% concentration each. Milling tests revealed that the least average surface roughness value was achieved with aluminum oxide nanoparticles (0.59 μm), followed by SiO2 (0.61 μm), MoS2 (0.75 μm), CNT (0.87 μm), graphite (1.07 μm), and highest with SiC (1.29 μm). However, in temperature, it was found that CNTs could not resist temperature rise in spite of having a high value of thermal conductivity and the average temperature rise was 294.1°C and was much higher than that attained with other nanoparticles which were in the range of 54.9–87.2°C. this may be due to excess 25 nanoparticles coming together and flocking which reduced the effective heat flow. Mahipal et al.[186] experimentally found that Karanja oil without any additives produced less wear than mineral oil. The addition of ZDDP nanoparticle in the Karanja oil further reduced the wear. At 1% ZDDP concentration the wear obtained was 457.23µm while at 1.5% concentration the wear reduced to 428.56 µm and further reduced to 405.35 µm at 2% ZDDP. Further addition of ZDDP beyond 2% increases the wear as was seen with 2.5% (433.43µm) and at 3% concentration (492.87µm). Table 10 summarizes nanoparticle additives used with non-edible vegetable oils. Conclusion Natural oils have been used for centuries. However, the last many decades saw a rise in the application of petroleum products in the industry. More than half of the lubricants enter the environment upon disposal and have a negative impact on human and plant life. Vegetable oils have become a primary choice for environment-sensitive operations due to their environmental friendliness. The only limitation is the auto-oxidation and thermal instability. However, this should not deter their application as proper selection of vegetable oils has shown that these perform exceptionally well. Moreover, their properties can be improved by modification, blending, and addition of nanoparticles and, ionic liquids. One of the biggest challenges is the development of a biodegradable cutting fluid as a substitute to conventionally used cutting fluids with the current attention to the environment. The need for eco-friendly products has been felt in many organizations and research has been done. Nonedible oils are a good alternative for developing cutting fluids. There is however a lack of research in this field on testing with different oils on ferrous as well as nonferrous materials in different machining operations. Actual machining tests are required for proper feasibility identification of these oils. The development of modified versions of these oils is a highly lucrative option as it enhances the properties considerably. Studies show that the use of nonedible vegetable oils brings a combination of environment-friendliness and high performance like improved tool life and reduced energy consumption. Also, nonedible oils perform superior to the conventionally used cutting fluids in terms of friction reduction due to better boundary layer lubrication characteristics. It was apparent from the lack of research on the use of nanoparticles and ionic liquids and there is a dire need of a focused research on different oils as the industrial application of such biodegradable products will rise in the coming years. As such, many research avenues are suggested: (1) Formulation of soluble cutting fluids with different surfactants and additives as there is a lack of research on such fluids (2) Addition of nanoparticles and studying their suitability for continuous operations and loss in effectiveness study over time. However, nanoparticles being a costly affair, there is a need to look for a comparatively cheaper alternative. (3) Use of different cooling technique to enhance the effectiveness of the conventional techniques 26 R. KATNA ET AL. (4) Using MQL needs further optimization to determine optimum conditions like nozzle position with respect to the cutting tool, rate of coolant flow, the distance of nozzle from tool, and pressure range. 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