Review on Performance Evaluation of Machining Characteristics using Vegetable-based Cutting Fluids – An approach towards Green Manufacturing

Lubricants play a major role in decreasing friction and wear during the machining process. Commercial metal cutting fluids are non-renewable and also produces the harmful effect to the environment as well as the operators. The preparation and disposal cost of mineral oil is an expensive one. To promote sustainable and green manufacturing eco-friendly cutting fluid is the need of an hour. Vegetable oil is preferred as an alternative tocommercial cutting fluid owing to its environmentally friendly, biodegradability, renewable, and less toxic, as well as exceptional lubricating properties. This article discusses the influence of various vegetable oil used for the material removal process and its performance. Vegetable oils significantly enhance the machining characteristics in terms of cutting force, tool wear, and surface quality


Introduction
In the machining process cutting fluids (CFs) plays a vital role in enhancing the material removal rate, and tool life, decreasing the surface finish, machining forces, and tool wear, because of its lubricant effect, cooling, and chip removal properties 1. A better surface finish was not only preferred for the visual appearance of any manufactured goods, but also better tribological characteristics. Thus, the surface finish was exceptionally essential in estimating the efficiency of machine tools which mainly depends on the cutting conditions and CFs2. In 2005, the volume of lubricants utilized in manufacturing industries was stated about 38 Mt with an anticipated rise of 1.2 % more than the succeeding decade. Almost 85 % of the mineral-based CFs were consumed around the world.The storage and disposal of mineral-based CFs are detrimentaland involve a distinct treatment by an environmental protectionagency (EPA) to eliminate the poisonous elements present inside the CF beforediscarding3. It is also calculated that the price of CFs, containingprocuring, preparation, preservation, and discardingisabout 16 % of the entire machining costs of a component. The discarding cost of CFs canbe up to 2-4 times of the procuring costs as the CFs are not recyclable and necessitateexclusive treatment preceding disposal4.In contrast, the impacts of CFs on the healthiness and the environment has been questioned in recent times. Owing to the extensive consumption of mineral-basedCFs, it caused considerable environmental pollutions throughout their life cycle5. A bullish nature in the cost for new CFs and discarding of the commercial CFs withthe mixture of new rules and regulation on the ecological and health safety, which isprojected to become even more rigorousregulation shortly, have led to inclusivesystematic research towards a green manufacturing6. Hence,the industries that paid more attention to green manufacturing via CFs have shifted from biodegradability to renewability over theyears to defend our valuableatmosphere. Green manufacturing aims to minimize harmful waste and pollution. The waste should be recyclableand non-dangerous. Further, the waste should be freely throwaway and should notrequireany additional treatment. Concurrently, several industries start to give more emphasisto green manufacturing. The several aspects of anecologicallymodifiedCFs are recorded in terms of biodegradability, harmfulness,and renewability 7. Theincreasingcall for this direction has unlocked an initiative for the progress ofvegetable-based CFs, a new biobased CF, which isecologically friendly, inexhaustible, less poisonous, extremely biodegradable, and safe forthrowing away. The recent developments in bio-based CFs by using various vegetable oils and their influence during machining was reviewed and concluded that these bio-based CFs significantly reduces the environmental issues caused by mineral-based CFs5.A detailed review on eco-friendly CFs on sustainable manufacturing was discussed by 8. The vegetable-based CFs (VBCFs) like Sunflower oil, Sesame oil, Coconut oil, Groundnut oil, Palm oil, Soybean oil, and Castor oil, etc., can be used for the machining process and enhance the machining performance in terms of surface roughness and tool life. This article will summarize the performance of VBCFs in the machining process.

Effect of VBCFs in turning process
The application of coconut oil in addition tothe emulsion and a neat CF (immiscible with water) in the machining of AISI 304 austenitic stainless steel was studied. It was found that the performance of coconut oilwas better than other CFs. It was noticed that coconut oil had a significant effect on surface roughness and tool life followed by neat oildue to ISSN: 00333077 6359 www.psychologyandeducation.net its thermal and oxidative stability9.The performance ofCFs formulated using palm kernel oil and cottonseed oil was experimented against the commercialCF with mineral oil in turning of AISI 4340 steel.The responses like surface roughnessand cutting force were measured during the machining process through coated carbide tools. It was found that the performance of palm kernel and cottonseed oil was better than commercial CFs in terms of least surface roughness and machining force 10 During turning of Inconel-800 alloy, different nano CFs namely Aluminum oxide (Al2O3), Molybdenum disulfide (MoS2), and Graphiteunder minimum quantity lubrication (MQL) condition were applied and measured the cutting force, surface roughness, and tool wear. It was found that the performance of Graphite based nanofluid was better than other fluids. The better performance was obtained due to its good tribological and cooling properties and the structure of Graphite is more covalent, and this considerably influences its performance than other nanofluids 11. During turning of Mild steel, Copper, and Aluminium, cutting force was measured and turning was performed with groundnut oil, palm oil, kernel oil, shear butter oil, and coconut oil was used as CF. They found that palm oil and groundnut oilexhibited the better performance in terms of the least cutting force for all the three materials and concluded as the performance of groundnut was better than all other oils 12.AISI 9310 alloy steel was turned with an uncoated carbide tool using vegetable oil as minimum quantity lubrication (MQL) fluid. The responses like chip formation, surface roughness, cutting temperature, and tool wear were measured, and the outcomes were compared with dry and wet conditions. They found that cutting temperature was increased with an increase in speed and feed, chip morphology was not varied with cutting temperature and MQL aided machining performed better than dry and wet conditions in all aspects 13. The performance of nano-boric (NB) acid suspension in SAE40 and coconut oil through turning of AISI 1040 steel with cemented carbide tool was investigated. The turning process was carried out with different parameters and measured the cutting temperature, flank wear, and surface roughness. They found that due to the lubricating action of NB acid, the cutting temperature, flank wear, and surface roughness wereconsiderably decreased compared to SAE40 CF. Coconut oil with 0.5 % of NB acid suspension performed better than SAE40 CF 14. During the turning of AISI 304 steel, the influence of turning parameters and CFs on surface roughness was examined. The CFs considered were Canola based CFs with 8 % of extreme pressure (EP) additive (CCF-I), Canola based CFs with 12 % of EP additive (CCF-II), Sunflower based CFs with 8 % of EP additive (SCF-I), Sunflower based CFs with 12 % of EP additive (SCF-II), Commercial mineral-based CF (CMCF), Commercial semi-synthetic CF (CSSCF). Based on the ANOVA results, CCF-II has a greater influence on surface roughness compared to the other turning parameters and surface roughness was decreased in the succeeding order of CFs: CCF-II, SCF-I, CSSCF, SCF-II, CCF-I, & CMCF 1. An attempt was made to substitute the mineral oil andpetroleum-based emulsifier with vegetable-based alternatives (sesame oil and coconut-based emulsifier). They measured the cutting temperature, cutting force, tool wear, and surface roughness during the turning of AISI 1040 steel. They observed that formulated fluid with 10% coconutemulsifier exhibits a performance like the commercial CF 15.The various CFs were formulated with neem oil and 5%, 10%, 15%, and 20% of emulsifier. While turning EN8 steel, the performance of the formulated CF was evaluated against dry and commercial CF in terms of stability of the CF, cutting temperature, surface roughness, tool wear, and cutting force. They found that formulated CFs were biodegradable and stable. Neem oil with 5% of emulsifier reduces the temperature, tool wear,and cutting force significantly than other CFs, and neem oil with 10% of emulsifier provides the least surface roughness. The increase in %emulsifier decreases the performance due to an increase in viscosity 16. Olive oil was used as MQL based green CF in turning hardened steel and measured the surface quality and cutting temperature. The experimentally measured outcomes were compared against other coolant systems and found that Olive oil with MQL system was the best system for green manufacturing17.The performance of 3%, 7%, and 10% of emulsion and a synthetic based VBCF and mineral-based semi-synthetic CF was measured against turning of AISI 1050 steel in terms of tribological and machining characteristics. They found that both VBCFs performed better than mineral-based CFs in terms of low coefficient of friction, best wettability, considerable cooling ability, and minimum machining force18.

Effect of VBCFs in the milling process
The CFs and milling parameters were optimized using a Doptimal design with different CFs. They measured the specific energy, surface roughness, and tool life during the milling of AISI 304 steel. They found that while machining with Canola CF lest specific energy and surface roughness was attained. To enhance the tool life commercial CF was preferred. To enhance the tool life and decrease the energy consumption, all the milling parameters have to maintain at low levels 7. During milling of Aluminium 7075-T6 (Al7075-T6) alloy composite, various VBCFs (Palm oil, Soya bean, sunflower, and coconut oil) were employed, and cutting force and vibration was measured. They found that palm oil was the most suitable oil for reducing the cutting force and vibration than other VBCFs owing to its higher viscosity 19. The performance of various VBCFs like castor, groundnut, cottonseed, palm, rapeseed, and neem oil was evaluated in terms of flank wear during milling of Al7075-T6 composite. They found that castor oil performed better in terms of flank wear and vibration due to its higher viscosity. The higher viscosity of castor oil forms a layer to decrease the tool wear 20. The performance of castor oil, mustard oil, and neem oil was evaluated in terms of surface roughness, and flank wear during milling of Inconel 625. They maintained the flowrate of VBCFs as 60 ml/hr and found that castor oil exhibits the surface roughness and flank wear of 0.131 μm and 0.02077 mm respectively. The performance of castor oil was better than the other two VBCFs in terms of surface roughness and flank wear. They found reported that high viscosity castor oil developed a thin lubricating layer on the tool-workpiece ISSN: 00333077 6360 www.psychologyandeducation.net interface and decrease the flank wear 21.The performances of five vegetable oils in the drilling of AISI 316Lstainless steel were investigated and the outcomes like chip formation, cutting force, and tool life were compared against commercial mineral-based oil. They observed that the tool life can be increased up to 177% and thrust force can be decreased by 7% using vegetable oil as a lubricant22. During end milling of AISI 304 stainless steel, various CFs like sunflower and canola oil (SCF-II with 8% extreme pressure (EP), CCF-II without EP and CCF-II with 8% EP) and a commercial type of semi-synthetic CF) were employed to investigate the milling performance like milling force, surface roughness, and flank wear. They found that CCF-II with 8% EP CF exhibited better performance than other CFs 23.During milling of TC4 alloy, MQL with graphene dispersed VBCF was employed as a coolant to enhance the milling characteristics like milling temperature, milling force, surfaceroughness, and surface microhardness. The milling parameters were optimized with Taguchi -Principal component analysis -Gray relational analysis and found that graphene dispersed VBCF significantly enhance the milling characteristics 24. The performance of kapok and palm oil was evaluated in terms of cutting force, cutting sound, vibration, surface roughness, and flank wear, and the measured outcomes were compared against SAE40 oil. They developed a tool condition monitoring system based on sound and vibration signals. The milling parameters were optimized for minimum surface roughness and flank wear using Response Surface Methodology (RSM). Kapok oil significantly decreases cutting force than SAE40 and palm oil. They found that the performance of palm oil was better than the other two CFs in terms of surface roughness, flank wear, cutting sound, and vibration signals 25. The performance of VBCF along with the dispersion of Al2O3 nanoparticles (0.5, 1.0, and 1.5 vol%) was evaluated during the milling of Hastelloy C276 alloy in terms of tool life and surface roughness. They found that the addition of 1 vol% of Al2O3, tool life was significantly increased by 23% and 10% compared to 0.5 vol% and 1.5 vol% of nanoparticles due to the rise in thermal conductivity and wettability ability of CF while increasing the concentration of Al2O3 nanoparticles 26.During the face milling process, the performance of VBCFs like palm oil, and coconut oil were evaluated in terms of surface roughness, and tool wear. The measured outcomes were compared with the commercial CF. The results revealed that coconut oil exhibited better performance than other CFs due to its high thermal and oxidative stability. They found the least tool wear and worn area as 0.16 mm2, and 0.04 mm2 respectively 27. During milling of Al6061-T6 alloy, coconut oil with NB acid suspension was used as a CF. Cutting temperature and surface roughness were measured under various milling conditions as well as the flowrate of the CF. They found that cutting temperature and surface roughness decreased with an increase in the flowrate of the coconut oil. They concluded that better performance was obtained with coconut oil with 0.5 wt% of NB acid suspension 28. The performance of coconut oil with 0.5wt% and 0.9wt% of NB acid suspension was evaluated during the face milling of Inconel 625. The outcomes like surface roughness, flank wear, and spindle vibration were measured and the process parameter was optimized using Taguchi's design of experiment. 0.5wt% suspension of NB acid in coconut oil significantly decreased the surface roughness, flank wear, and vibration by 3.92%, 6.28%, and 4.85% respectively. 0.9wt% of NB acid suspension decreased the performance due to its accumulation of particles 29. The performance of palm-castor oil (1:0.5 to 1:3) mixture in MQL condition was evaluated in terms of cutting force, surface roughness, and tool wear during the end milling of Inconel 690 alloy. TOPSIS framework was used to select the castor-palm oil mixture ratio and found as 1:2. They found that this selected mixture considerably decrease the surface roughness, cutting force, and tool wear by 16.146%, 7.971%, and 3.155% respectively 30. The summary of VBCFs and their performance in terms of the turning and milling process is shown in Table 1.

Discussion
Various researchers reported that the VBCFs withthe MQL method has better performance than the standard lubrication method (flood condition). The reasons for these results are discussed below: • The fatty acid and triglyceride are the major ingredient of vegetable oil. Owing to the presence of fatty acid and triglyceride in vegetable oils, various polar groups like -COOH and -COOR are formulated. • Figure 1illustrates the polar group of vegetable oil. It was found that extraordinary lubrication characteristics of vegetable oil are due to the presence of polar groups 5.  Around 100°C temperature, polar groups present in VBCFs respond with the workpiece and forms a metallic soap file ascribed to enhance the lubricity of the oil 34. The fatty acid chain inVBCF is made of many nonpolar methyl groups. Thus, a dispersion force called as London dispersion force is acting between the molecules as depicted in Figure 3. As aresult, the immediate dipole of the molecule makesthe neighboring molecule produce dipoles. Hence, two molecules are attracted to each other. As the number of carbon molecules is raised, subsequently dispersion force is also enhanced 35. In general, vegetable oilsdelivers high viscosity at elevated temperature and hold for a long time, which helps enhanced lubrication characteristics during the material removal process 34. Lubricant with nanoparticles is a new engineering material that has nanoparticles dispersed in a base fluid which could be an efficientapproach to decrease the wear and friction during the machining process. Nanoparticles have shown an immense potential to be an ideal and efficient alternate way to flood lubrication owing to its environmental concerns. During machining of GH4169,the lubrication ability of various VBCFs was analyzed and the following ranking was given: Maize <Rapeseed <Soybean <Sunflower<Peanut <Palm <Castor oil. This indicates maize oil had the better lubrication ability followed by rapeseed oil 35. The lubrication ability of castor oil was very less due to its very high viscosity. Though castor oil is not preferred for the machining process, palm oil can be preferred as an alternative to castor oil owing to its flowability and good polarity36.

Conclusion and Future scope
Cutting fluids (CFs) play a significant role in the material removal process and tribological applications. Owing to the cost, environmental concerns, and disposal issues, mineralbased CFs are not suitable for green and sustainable manufacturing. As an alternative to mineral-based CFs and focus on green manufacturing, VBCFs have good tribological and lubrication properties. VBCFs significantly enhance the machining characteristics in terms enhance the surface finish and tool life and decrease the cutting force and cutting energy. Further tribological properties can be enhanced by adding the additives (micro and nanoparticles) and viscosity modifiers like Ethylene-vinyl acetate copolymer (EVA) and ethyl cellulose (EC) to enhance the lubricant properties. The mixture of palm and castor and other oils can be an alternative to enhance the lubricant properties. Further research can be initiated in the direction of preparing nanofluids to enhance the machining performance and decrease the processing and disposal cost of CFs