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State of Art on Minimum Quantity Lubrication in Grinding Process
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ScienceDirect Materials Today: Proceedings 5 (2018) 19638–19647
www.materialstoday.com/proceedings
ICMPC_2018
State of Art on Minimum Quantity Lubrication in Grinding Process M. Bhuyana, A. Sarmahb, K.K. Gajranic, A. Pandeyc, T.G. Thulkarc, M.R. Sankarc* a
Department of Mechanical Engineering, Assam Engineering College, Guwahati-781013, India Department of Mechanical Engineering, National Institute of Technology, Silchar-788010, India c Department of Mechanical Engineering, Indian Institute of Technology Guwahati, Guwahati-781039, India b
Abstract Most of the machining process use cutting fluids for better cooling and lubrication, which ensures good machinability at low specific energy consumption. However, the conventional cutting fluids pose serious health hazards to operators along with environmental pollution. Furthermore, the cost incurred in the purification and disposal of cutting fluids is sometimes greater than the cutting tool. Grinding process involves higher specific energies, which give rise to high heat generation in grinding region and the penetration ability of liquid grinding fluids (flood cooling) into the contact zone becomes poor. Therefore, attention towards alternative methods of cutting fluid application has bolstered owing to the operational and environmental cost pressures. Various studies have shown that a prominent substitute to flood cooling in grinding is minimum quantity lubrication (MQL) grinding or near dry grinding (NDG). In this study, the performance parameters of MQL grinding are reviewed in terms of surface quality, grinding forces, wheel wear, thermal considerations and ecological safety. Moreover, MQL can be a viable alternative to dry and flood cooling in grinding process. © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of Materials Processing and characterization. Keywords:Grinding forces; wheel wear; surface integrity; thermal model; coolant lubricant; grinding performance.
1. Introduction Grinding is one of the conventional machining cum finishing processes that help to achieve dimensional accuracy and better surface finish of a workpiece. The abrasive particles have a high negative rake angle that makes it distinctive from conventional machining processes. Heat generation in the grinding zone is higher than most other
* Corresponding author. Tel.: +361-2582684; E-mail address:[email protected] 2214-7853© 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of Materials Processing and characterization.
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machining processes because of high specific cutting energy. Effectiveness in grinding can be achieved with efficient removal of heat from the grinding zone. The most efficient, economic and environmentally friendly process to remove the unwanted heat is through MQL. MQL is widely used in turning, milling and drilling [1-2]. However, it is relatively new to grinding. This paper gives a peek into the use of MQL with grinding and its various possible improvisations. J.A. Sanchez et al. [3] presented an improvement in MQL with the introduction of CO2 to freeze the lubricating oil over the grit to penetrate into the grinding zone. The effective penetration of cutting fluid in conventional MQL technique is attained by impingement of fluid on the grinding zone by forcing it through high-pressure air jet (Fig. 1). When nanofluids forced through the jet, reaches the grinding zone to provide the desired cooling and lubricating effect. Nanoparticles, reaching the interface, act as ball bearings between the grinding wheel and the workpiece [4]. The removal of excessive heat leads to decrease in specific cutting energy and increase in surface finish.The following sections and subsections give various aspects of MQL in grinding operation as shown in below Fig 1 and Fig 2.
Fig. 1: Schematic view of lubricant penetration in the grinding zone [35]
Fig. 2: An illustration showing abrasive particle fracture acting as lubricant sources and flat wear area [6]
2. Surface integrity in minimum quantity lubrication Silva et al. [5] experimented on tempered ABNT 4340 steel. The average surface roughness values (Ra) were considerably reduced when MQL was employed, but also resulted in high residual stresses, which can be accounted by the non-uniform heating and cooling of the workpiece surface. The sub-surface alterations were less in both MQL and conventional cooling. In another study, Mao et al. [6] observed the effects of MQL grinding on hardened AISI-52100 steel. The oil-water lubrication provided good results by reducing the thickness of the affected layer and grinding temperature whereas pure oil lubrication resulted in a superior surface finish. Moreover, Tawakoli et al. [7] investigated the effect of MQL on grinding of 42CrMo4 soft steel and 100Cr6 hardened steel. It was observed that 100Cr6 steel showed better surface integrity than 42CrMo4 steel. It was also seen that MQL coupled with high MRR resulted in improved Ra compared to the dry and flood cooling. The material removal mechanism in MQL grinding is mostly shearing and fracturing unlike in conventional plastic deformation. Afterwards, Tawakoli et al. [8] demonstrated the sensitivity of the process towards the cutting fluid while using different types of Al2O3 wheels. The use of vitrified bond segmented grinding (SG) wheels produced less thermal damages and material side flows on the surface. Tawakoli et al. [10] pointed out that grindability of 100Cr6 steel increases substantially with the use of MQL oil and surface damage was reduced as compared to dry, wet and MQL-water-miscible conditions. Sadeghi et al. [11] investigated the surface grinding on AISI 4140 steel under MQL, whose surface roughness tends to have a higher value than that of other grinding modes. Grinding may be restricted to softer materials thus restricting its usability towards harder materials [12]. Afterwards, Sadeghi et al. [13] reported the grinding of Ti– 6Al–4V titanium alloy under dry, wet and MQL lubrication schemes, which produced more or less same results as Sadeghi et al [11]. In most of the study, MQL proved to be a superior alternative. The comparisons were made on the basis of surface roughness, Barkhausen noise, specific energy consumption and wheel wear. It was noted that all the above-mentioned output responses were reduced in the minimum coolant grinding (MCG) system when compared to the conventional cooling system. However, the Barkhausen noise in MCG was slightly on the higher side [14]. The reduction in specific energy consumption is attributed to effective lubrication in the grinding area as shown in Fig. 2. Li et al. [15] observed that cryogenic cooling reduced the surface roughness and increased the surface integrity to a considerable extent as compared to its oil and dry versions. A significant reduction of 30-50% was observed in the
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longitudinal Ra parameter values. The tensile residual stress of the ground surface was reduced by a notable amount because of the lower grinding zone temperature and reduced friction between ground surface and chip. Liu et al. [16], noted that in surface grinding of the Ti-6Al-4V, the surface roughness was inversely dependent upon the feed rates. It was due to the accelerated bond rupture and grit fracture with increasing feed rates. Thus, MQL can improve the surface integrity by removing the heat from the grinding zone. Wang et al. [17] argued that the grinding quality of the workpiece is dependent on the coolant-lubricant types. Experiments were conducted to study the values of surface roughness with various OW (oil filmed water droplets) fluids such as OW fluids with cooled air, ester oils and water under MQL lubrication. It was observed that OW fluids were more eco-friendly and emission-free as compared to their emulsion counterparts. OW fluids also exhibited lower surface roughness and higher grinding ratios. Minimum quantity cooling lubrication (MQCL) technique using the CBN wheel proved to be favorable in terms of surface integrity values. Residual surface compressive stresses were optimized when MQL was applied with the conventional cooling system [18]. Xiu et al. [19] studied point grinding in MQL with the help of CBN wheel. The grinding power and grinding temperature was considerably reduced in point grinding due to less contact area, weak airflow barrier effect and an increase of grinding fluid effect. In addition, it was observed that after a certain jet pressure, the influence of the grinding fluid on the surface quality was reduced. Liu et al. [20] investigated cooling-air grinding performance, which proposes that grinding quality can be improved by lowering the temperature of the ambient air. Grinding at lower feed rate gave a better grinding finish. This is because the heat generated during grinding was efficiently removed by the cool air. The surface finish, in case of grinding with cooling air, was always less than that obtained in conventional grinding. Moreover, cryogenic cooling reduced the grinding zone temperature by effective cooling, leading to less wheel wear. The surface roughness of the workpiece surface was also reduced [21]. Inoue et al. [22] compared to air cooling MQL with air and dry grinding to conclude that grinding resistance is proportional to the depth of cut which in turn is proportional to Ra. Li et al. [23] analysed the MQL technique with an approach appurtenant to fluid dynamics. MQL besides reducing the hydrodynamic lift force also lowered the cost of grinding fluid, thus, authenticating superior performance of MQL supply over conventional flood cooling. Liao et al. [24] investigated grinding of Ti-6Al-4V by flooded injection of water emulsified cutting fluid, flooded injection of nano-modifier, MQL of water emulsified cutting fluid and MQL of nano-modifier. The wheel surface was observed for the above cases at the loaded region. MQL with nano-modifier showed the lowest extent of wheel loading because of the increase of surface energy (the surface area of nanoparticles is inversely proportional to their sizes), Lotus effect exhibited by nanoparticles and the high pressurized air of MQL. Meng et al. [25] did a comparative theoretical study of MQL grinding method, traditional grinding fluid supply method and dry grinding along with experimental study in terms of surface finish. It was observed that MQL gave the best surface finish compared to a traditional grinding fluid method and dry grinding as the oil mist will be easier to break in the grinding zone against the gas barrier. It reduced the friction between the sliding abrasive particle and workpiece which contributed to better surface finish. Adibi et al. [26] conducted a grinding experiment on carbonfiber reinforced SiC matrix composites in MQL condition and compared with fluid and dry grinding. They reported that the surface roughness quality for MQL grinding was 75.26% higher when compared with dry grinding, while fluid grinding resulted in a surface 70.14% poorer surface quality than dry grinding. Highest G-ratio was achieved during MQL grinding with a 115.38% increase relative to dry grinding. Wang et al. [27] reported the MQL grinding of nickel alloy GH4169 by various nanofluids such as MoS2, Al2O3, carbon nanotubes (CNTs), nanodiamond (ND), SiO2 and ZrO2 infused in pure palm oil and water-soluble liquid. Al2O3 nanofluid yielded the lowest sliding friction coefficient of 0.348, lowest surface roughness of 0.302μm, lowest specific grinding energy of 82.13 J/mm3 and also the highest G-ratio (wheel wear) of 35.94. 2. Grinding forces in minimum quantity lubrication Sadeghi et al. [11] investigated the surface grinding of AISI 4140 hardened steel using MQL. They reported that the specific energy and grinding forces in MQL were least as compared to its wet and dry counterparts due to reduction in material ductility, ideal chip formation mechanism and retained abrasive particle sharpness. Barczak et al. [12] reported better results when specific MQL conditions were considered with respect to conventional flood cooling for plane surface grinding. Friction was found to be low and reasonably good material removal rate was achieved.
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Sadeghi et al. [13] experimented on grinding of Ti-6Al-4V under MQL condition. They reported that lubricant flow and delivery pressure played an important role in minimizing grinding force. An optimum result was achieved at a lubricant flow of 60ml/h and a pressure of 0.4 MPa. There was a considerable decrease tangential and perpendicular forces in MQL as compared to flood cooling. Guo et al. [28], too, studied surface grinding of Ti-6Al4V alloy under MQL conditions and noted that grinding depth has a considerable influence on specific energy, grinding force and force ratio. In addition, MQL machining results in improved surface finish as compared to fluid and dry grinding. Surface damages like plastic deformation, side flow and redisposition were lower in the case of MQL owing to high penetration in the contact zone. Lubricant with high coolant concentration at high pressure and medium flow rate produced the best surface finish [29]. Excellent delivery of cutting fluid is achieved at a nozzle direction of 15ᵒ while grinding Ti-6Al-4V [30]. Li et al. [15] observed that cryogenic cooling is an effective means of grinding a nickel-based superalloy without incurring any environmental hazards. It was observed that the grinding forces and the specific grinding energy under cryogenic lubrication were reduced substantially mainly due to the retentivity of grit sharpness and less plowing that is attributed to the generated inert atmosphere. Similar results were previously reported by Li et al. [21]. Liu et al. [16] stated that grinding force ratio and specific energy were following an inverse relation with the feed rates. This was due to the transition of material removal mode from plowing to chip forming. It was also found that relatively lower depths were responsible for higher grinding forces This is because sliding and plowing were predominant material removal modes in that region which related to the depth in an inverse fashion. Wang et al. [17] observed that the coolant-lubricant types influenced the grinding quality of the workpiece. Different OWs are produced by combination of cooled air, ester oils and water was tested under MQL lubrication. It was observed that OW fluids produced lower values of grinding forces than their emulsion counterparts. Yang et al. [31] studied a simulation model to report the characteristics of cylindrical grinding. The adiabatic heating induced by strain hardening and high strain rate increases the grinding zone temperature as well as grinding forces. The grinding forces and the grinding temperature is greatly influenced by the depth of cut. Adibi et al. [26] reported the grinding of carbon-fiber-reinforced SiC matrix composites under MQL conditions and compared it with fluid and dry grinding conditions. Dry grinding resulted in the highest grinding forces with MQL grinding developing the lowest grinding forces. MQL grinding resulted in reduced tangential force and normal grinding force by 38.88% and 31.16% respectively as compared to dry grinding condition. Benkai et al. [33] compared MQL grinding on a high-temperature nickel alloy with different base oils such as palm oil, castor oil, soybean oil, sunflower oil, corn oil, rapeseed oil and peanut oil. Palm oil yielded the lowest grinding temperature and energy ratio coefficient and the second lowest grinding force making it the optimum choice as MQL grinding base oil. Guo et al. [34] investigated the effect of different oils such as soybean, maize, rapeseed, palm and sunflower oils when added to castor oil in a 1:1 ratio on its lubricating properties. They reported that all the mixed base oils had superior properties than castor oil. However, soybean/castor oil resulted to be the best. The specific tangential and normal grinding forces were reduced by 27.03% and 23.15%, when compared with pure castor oil. Sinha et al. [35] compared the surface grinding of Inconel 718 under dry, flood cooling and soluble oil under MQL conditions. The comparisons were done on the basis of cutting forces and surface roughness. The normal and tangential grinding forces, both, were least in MQL grinding process. However, the surface roughness of the finished surface was higher than dry and flood cooled grinding process. 3. Effects on wheel performance in minimum quantity lubrication grinding Sadeghi et al. [11] reported that wheels with coarser abrasive particles and higher porosity are the best choice for MQL in grinding process as they have less active abrasive particles and smaller wear flat area. The position of the nozzle with respect to grinding wheels plays a vital role in the performance of oil mist as argued by Tawakoli et al. [36], was found to be 10º-20º from the workpiece surface. Tawakoli et al. [10] also pointed out that MQL grinding with fine abrasive particle vitrified corundum and low porosity showed less satisfactory performance. On the other
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hand, the porous CBN wheels with grinding oils showed a superior performance. Sensitivity to lubricant variation was found to be lowest in CBN, porous and coarse abrasive particle wheels. The latter had the least tendency for chip loading in MQL grinding. Furthermore, Tawakoli et al. [9] pointed out that least chips were loaded on the grinding wheel while using a CBM wheel with MQL due to high porosity and higher thermal conductivity which ensures the prevention of loading of chips with lower temperatures. 3. Thermal analysis of grinding in minimum quantity lubrication Ramanath et al. [37] stated that MQL can provide better results than flood injection during grinding of Ti-6Al-4V. It is because, through the use of micro-spray cutting fluid, the blanket of gas produced at the grinding zone due to the high heat generation, can be cut and hence provide the necessary lubrication. Moreover, Mao et al. [38] examined the coefficient of heat transfer of the workpiece surface by comparing the simulative and the experimental results. They reported the increase of heat transfer coefficient in nucleate-boiling zone whereas it decreased in the transition boiling. It showed insensitivity to the temperature in the non-boiling zone. A good agreement of mathematical models proposed and experimental results was observed for MQL grinding which helps in predicting the dynamics of the surface temperature of the workpiece. Hadad et al. [39] compared the measure of energy partitioning in dry, flood cooling and MQL grinding. It was seen that the energy partitioning in the case of Al2O3 wheels with flood cooling was as less as 36%. However, the combinations of resin bond CBN wheels with MQL lead to a desirable 46%. Moreover, it was also observed that the forces required to generate a temperature rise in MQL were 40-50% of the flood cooling incurring a high thermal stress. In another study, Morgan et al. [40] employed an analytical model for the thermal analysis of MQL grinding. Theoretical analysis has shown that thermal performance of MQL is same as that of wet grinding. However, it was not able to predict the presence of high-speed jet cooling effect and convection but their existence was proved by experimentations. In another study, Hadad et al. [41] compared the thermal aspects of dry, flood cooling and MQL grinding. Analytical model was proposed to analyse the process with experimentations on the conventional Al2O3 wheel and super abrasive CBN wheel. Cooling effects were studied on the trailing edge, leading edge and the grinding zone in flood and MQL conditions. It was observed that in spite of appreciable lubricating properties of MQL, the process generated some cooling effect in the trailing edge in the contact zone vicinity. This proved to be a backdrop of MQL. Moreover, Li et al. [15] noted that grinding zone temperature was reduced due to the effective cooling by the cryogenic lubrication leading to lower forces and energy consumption. Wang et al. [42] dealt with the problem of reduced grinding belt life. They used liquid nitrogen cryogenic wind jet combined MQL (LNCWJCM) technology. The experimental studies were compared to conventional cooling systems, cold air and liquid nitrogen. The work-piece used for the test was a titanium alloy. As dissipation of heat through itself is difficult in titanium, the concentration of heat leads to rise in temperature to high values. Titanium alloy being reactive at high temperature undergoes strong chemical activity which leads to abrasion wear and adhesive wear of the belt thus making it blunt. The LNWJCM technology improves the belt life without harming the environment. The high-speed jet helps in the penetration of liquid nitrogen to the very core of heat generation. Stephenson [43] theoretically and experimentally analyzed the effect of energy partitioning on the coolantlubricant type in different grinding schemes. Due to the variance of energy partitioning in the various grinding process, the grinding fluid requirement changes as it proceeds from creep feed grinding to high efficiency deep grinding (HEDG). The principle job of the grinding fluid in creep feed grinding is cooling of the grinding zone as it carries away 90% of the heat. In HEDG, the grinding fluid lubricates the grinding zone, thus maintaining abrasive life to maximum. In another study, Benkai et al. [44] compared the heat transfer performance of different nanofluids namely MoS2, CNT, polycrystalline diamond (PCD), ZrO2, SiO2 and Al2O3, with palm oil used as base oil. They reported that CNT nanofluids had the lowest grinding temperature and energy proportionality coefficient closely followed by Al2O3 nanofluids. CNT nanofluid had the highest heat transfer coefficient of 1.3*104 W/mK. Zhang et al. [45] conducted a thermal analysis in the grinding surface with different cooling approaches. MoS2, CNT, ZrO2 when mixed with the same base oil produced different grinding fluids to be used for grinding
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experiments. The lubricating effect of MoS2 was reported to be excellent. Nanofluids with CNT gave the best cooling effects with 2% volume concentration of CNT nanoparticles. 6. Nano fluid lubrication in minimum quantity lubrication A major part of the heat generated at the tool-workpiece contact region is transferred through the grinding fluids. The effectiveness of these grinding fluids depends upon the penetration to the contact region. MQL with nanofluids is a very promising approach towards attaining exceptional convection and thermal conductivity. Mao et al. [46] studied the efficiency of MQL grinding with nanofluids under various spraying parameters. The spraying direction of MQL nozzle played an important role in the application of nanofluids. Maximum grinding performance was achieved by placing the nozzle at an angle with respect to the grinding wheel. Moreover, air pressure and distance of spraying of nanofluids are important parameters to be considered in order to boost the penetration of nanofluids into the grinding zone. The grinding forces, grinding contact area temperature and surface roughness is inversely proportional to air pressure. The mist velocity decreases with increase in spraying distance. In another study, Kalita et al. [47] analyzed the process efficiency of MQL with oil-based nanolubricants in grinding of AISI 4340 steel and ductile cast iron. It was stated that MQL grinding using nanolubricants enhances the efficiency by decreasing frictional losses and energy consumption. Paraffin-based nanolubricants showed improved performance with higher nanoparticle concentration, including increase in wheel life, an increment in grinding ratio (G ratio) by 50%, a reduction of energy consumption by 53% and a lower coefficient of friction of 0.22. Silva et al [48], reported that the best performance while grinding AISI 4340 steel by a vegetable oil-based MQL can be obtained by an air flow of 26.4 m/s and a lubricant flow of 40 mL/hr. Furthermore, Kalita et al [49] conducted an experiment on the lubricating and impringing capabilities of a nanolubricant consisting of organic molecules along with nanoparticles of phosphide intercalated-MoS2 nanoparticles during the surface grinding of ductile cast iron under MQL. The nanolubricants reduced the force ratio and specific energy by 45-50%, and the abrasive wheel wear or G-ratio was reduced by 48-55% as compared with flood cooling and paraffin (base) lubrication under MQL. Mao et al. [4] stated that the quality of grinding and surface finish for hardened AISI 52100 steel improved significantly with MQL of water-based Al2O3 nanofluid when compared to dry grinding. However, wet grinding, showed superior cooling capacity and lubrication. Water-based nanofluid MQL grinding significantly decreased the grinding temperature, grinding forces and the surface roughness as compared to pure water MQL grinding. The nanoparticles served as ball bearings around the workpiece providing better lubrication than pure water MQL grinding. In another study, Zhang et al. [32] stated that on adding MoS2 nano-particles to soybean oil as base oil for jet lubrication under MQL condition, viscosity increased resulting in better lubricating property. The optimum concentration of MoS2 nano-particle in soybean base oil was found to be 6%. Wang et al. [27], further, compared six nanofluids viz. MoS2, nanodiamond (ND), CNTs, SiO2, Al2O3, and ZrO2 under MQL for grinding nickel alloy GH4169. They reported that nanoparticles with spherical molecular structure and nanofluids with higher viscosity result in better lubricating properties. The nanofluids can be ranked with respect to their lubricating properties as ZrO2 < CNTs < ND < MoS2< SiO2< Al2O3. Setti et al. [50] reported the surface grinding of Ti-6Al-4V under MQL conditions with different nanofluids namely Al2O3 and CuO nanoparticles added in different concentrations to water. MQL with Al2O3 nano-fluid significantly reduced the coefficient of friction with time, however, CuO nanofluids and soluble oil under MQL did not show any effect. Chemical reaction between Al2O3 nano-particles and titanium along with the lotus effect flushed the debris formed during grinding out of the grinding zone, thus improving the grindability of Ti-6Al-4V. Huang et al. [51] experimented on surface grinding of NAK80 mold steel under different conditions viz. dry, MQL, MQL with nanofluids and ultrasonic MQL with nanofluid. The nano-particle chosen was multi-walled CNTs. They reported considerable better results in terms of grinding temperature, grinding force, surface morphology and surface roughness, when ultrasonic assisted MQL was used. This can be credited to the reduced friction produced when nano-fluids are used, as the nano-particles interacts with the grinding wheel and surface and forms a protective layer and the superior heat transfer properties of the ultrasonic assisted nanofluids. Nanofluids with higher nanoparticle concentration in MQL grinding results in lower grinding force and temperature values [52]. Therefore, the use of nanofluids with MQL provided superior results with increased workpiece quality. A further study of mechanisms of material removal is required to optimize the performance of nanolubricants in MQL.
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7. Effects of minimum quantity lubrication on micro grinding Li et al. [53] studied the effects of the lubricating parameters and cutting conditions on the tool life and surface finish in SK3 steels. They reported that a very small quantity of cutting fluid is required for a longer tool life. Oil flow of 1.88 ml/h and air flow of 25 l/min were found to be the best combination for extracting best performance out of the system in the form of surface integrity and tool wear reduction. Lee et al. [54] discussed the effects of nanofluids in micro-grinding. In this study, ND and nano-Al2O3 particles were used to demonstrate the consequences of nanofluids on surface roughness and grinding forces. They reported that ND particles were more effective in reducing the grinding forces whereas nano- Al2O3 particles showed a superior increase in surface integrity. Volumetric concentrations and nanoparticles sizes were the governing factors of the performance of the system, with the latter having a greater impact on surface roughness. Afterwards, Lee et al. [55] used compressed chilled air to study nanofluid assisted micro grinding under MQL as grinding forces, tool wear and surface roughness varied directly with air temperature. Low depth of cut and feed were favorable for use of compressed chilled air. Electro-plated CBN grinding tool showed a greater sensitivity towards the reduction of forces as compared to the vitrified-bond tool because of its superior thermal conductivity. Surface integrity was better achieved in the case of vitrified-bond tool whereas tool wear was reduced in both the cases. Also, Lee et al. [56] investigated a micro-grinding process under MQL condition of its thermal characteristics with experimental and numerical approach. The high thermal conductivity and ball bearing effect of the ND particles restricted the subsurface temperature rise for nanofluid MQL with ND particles as compared to the compressed air lubrication and pure MQL. 8. Potential enhancements in conventional minimum quantity lubrication The process knowledge of MQL is limited and elementary and improvisation in the MQL grinding can lead to superior surface quality and a decrease in temperature of grinding zone. Nguyen et al. [57] developed a grinding wheel model efficient for coolant penetration into the grinding zone. This arrangement of cutting fluid supplied to the core of the high-temperature zone can reduce the temperature drastically as compared to the conventional cooling method. Analytical study concluded that the quantity of coolant used can be minimized using a segmented wheel and its efficiency is dependent on the wheel speed as well as on the method of coolant supply. Tsai et al. [58] studied a grinding wheel with micro-graphite particles imbued in an Al2O3 matrix to provide lubrication at the grinding zone. The newly designed graphite impregnated wheel in MQL conditions reduced the surface roughness by 25%-32%, the cutting force by 24%-36%, and cutting temperature by 8%-14% than a conventional grinding wheel. A graphite content below 0.5% produces a better surface finish, lowers grinding forces as well as temperature, lower wheel consumption and hence a prolonged life. In another study, a similar concept of the graphite impregnated wheel was proposed by Shaji et al. [59]. Dry and coolant conditions with the proposed wheel were compared. It was seen that the tangential force component and consequently the specific energy radically reduced. Surface finish was superior for harder materials than for ductile materials. Another such solid lubricant moulded grinding wheel was developed by Shaji et al. [60] with CaF2 as the lubricant. The wear of the wheel depended on the lubricant type. It was more with CaF2 than with graphite. Oliveira et al. [61] provided a method for flushing and wheel cleaning to improve MQL in CBN grinding by combining MQL with a compressed air jet. The jet nozzle was placed at an incident angle of 300o. The use of a compressed air jet positioned at an optimal angle increases the material removal rate without affecting the quality of the workpiece. Furthermore, Alberdi et al. [62] developed alternative ways to minimize the consumption of fluid in grinding. This can be achieved through nozzle design optimization and by combining MQL and low-temperature CO2 while grinding. This leads in significant improvement in wheel life with a reduction in surface finish and force ratio. The new MQL-CO2 system developed resulted in the improvement of the friction ratio. Mandal et al. [63] studied a pneumatic barrier that can suppress the formation of the air layer in the periphery of a high speed grinding wheel. The pneumatic barrier provides effective lubricant penetration at the point of application reducing the tangential and normal forces, hence decreasing the surface roughness. Moreover, Fiocchi et al. [64] noted the characteristics of the lap grinding process in regard to the surface roughness and integrity. It was observed that the smaller abrasive particle size yielded higher surface integrity for a given lap-grinding apparatus. The best surface roughness value for 800 mesh grinding wheel was 1.92 nm. As the process produced good results with a non-MQL system, enhanced performance with MQL can be expected.
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Pan et al. [65] studied the characteristics of Electrolytic In-process Dressing (ELID) with MQL. A positive potential was given to the grinding wheel and a negative potential was given to the electrode. The space between the grinding wheel and the electrode was supplied with mist, resulting in the formation of an electrolytic film during electrolytic dressing. An increase in the grinding fluid quantity increased the film thickness. This suggested the gathering of ample amount of grinding fluid in the gap leading to stable electrolytic dressing. Increase of the grinding wheel speed at a constant supply of grinding fluid resulted in the decrease of the film thickness. This is an efficient way of machining hard and brittle materials. The use of micro-emulsions greatly increases the eco-friendly properties of MQL grinding process [66]. Biachi et al [67], in another study used a vitrified CBN grinding wheel to machine tempered AISI 4340 steel. Flood cooling and MQL with water and different concentrations of oil was employed to control the heat generation. They also employed a cleaning system with compressed air to prevent wheel clogging problems and the resulting heat generation. The increase of water content in the MQL increased the cooling capacity of the lubrication system and expedited fluid atomization. This resulted in better surface roughness, lower grinding wheel wear but higher grinding power is required. The cleaning system removes the clogged chips out of the grinding wheel surface. The cutting power required is lower when cleaning system is used. In another study, Madarkar et al. [68] compared the performance of ultrasonic MQL (UMQL) with conventional MQL (CMQL) while machining Ti-6Al-4V using sunflower oil. The use of UMQL resulted in better surface roughness, decrease in the cutting forces; 38% in tangential grinding forces and 32% in normal grinding forces, with respect to dry grinding of Ti-6Al-4V, which can be credited to the fine atomization in UMQL. 9. Conclusion The paper presents various aspects of MQL application in grinding process. The grinding process efficiency and performance are studied in view of thegrinding forces, surface quality, wheel wear and thermal consideration. With huge advantages of MQL over conventional cutting fluids applications, MQL can be a viable alternative to conventional cutting fluid applications. MQL provides an extraordinary quality of lubrication which in turn can increase surface quality. Though wet grinding can provide better cooling conditions, the surface finish cannot be compared with that of the superior finish from MQL process. This compromise with cooling is the root cause for non-implementation and non-acceptance on industrial levels. But environmental friendliness, efficiency, economical and no harm to the operator are some of the advantages of MQL technology. The MQL technology is very flexible in terms that it can be optimized to achieve specific results. The knowledge of MQL in grinding is necessary and this area is expanding day by day because of its huge advantages over conventional cutting fluid applications. Acknowledgements Authors are thankful to “ELSEVIER (Licence numbers: 4280681200373, 4280800873389)” for providing the copy right permission to use figure in this manuscript.
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ICMPC_2018
State of Art on Minimum Quantity Lubrication in Grinding Process M. Bhuyana, A. Sarmahb, K.K. Gajranic, A. Pandeyc, T.G. Thulkarc, M.R. Sankarc* a
Department of Mechanical Engineering, Assam Engineering College, Guwahati-781013, India Department of Mechanical Engineering, National Institute of Technology, Silchar-788010, India c Department of Mechanical Engineering, Indian Institute of Technology Guwahati, Guwahati-781039, India b
Abstract Most of the machining process use cutting fluids for better cooling and lubrication, which ensures good machinability at low specific energy consumption. However, the conventional cutting fluids pose serious health hazards to operators along with environmental pollution. Furthermore, the cost incurred in the purification and disposal of cutting fluids is sometimes greater than the cutting tool. Grinding process involves higher specific energies, which give rise to high heat generation in grinding region and the penetration ability of liquid grinding fluids (flood cooling) into the contact zone becomes poor. Therefore, attention towards alternative methods of cutting fluid application has bolstered owing to the operational and environmental cost pressures. Various studies have shown that a prominent substitute to flood cooling in grinding is minimum quantity lubrication (MQL) grinding or near dry grinding (NDG). In this study, the performance parameters of MQL grinding are reviewed in terms of surface quality, grinding forces, wheel wear, thermal considerations and ecological safety. Moreover, MQL can be a viable alternative to dry and flood cooling in grinding process. © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of Materials Processing and characterization. Keywords:Grinding forces; wheel wear; surface integrity; thermal model; coolant lubricant; grinding performance.
1. Introduction Grinding is one of the conventional machining cum finishing processes that help to achieve dimensional accuracy and better surface finish of a workpiece. The abrasive particles have a high negative rake angle that makes it distinctive from conventional machining processes. Heat generation in the grinding zone is higher than most other
* Corresponding author. Tel.: +361-2582684; E-mail address:[email protected] 2214-7853© 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of Materials Processing and characterization.
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machining processes because of high specific cutting energy. Effectiveness in grinding can be achieved with efficient removal of heat from the grinding zone. The most efficient, economic and environmentally friendly process to remove the unwanted heat is through MQL. MQL is widely used in turning, milling and drilling [1-2]. However, it is relatively new to grinding. This paper gives a peek into the use of MQL with grinding and its various possible improvisations. J.A. Sanchez et al. [3] presented an improvement in MQL with the introduction of CO2 to freeze the lubricating oil over the grit to penetrate into the grinding zone. The effective penetration of cutting fluid in conventional MQL technique is attained by impingement of fluid on the grinding zone by forcing it through high-pressure air jet (Fig. 1). When nanofluids forced through the jet, reaches the grinding zone to provide the desired cooling and lubricating effect. Nanoparticles, reaching the interface, act as ball bearings between the grinding wheel and the workpiece [4]. The removal of excessive heat leads to decrease in specific cutting energy and increase in surface finish.The following sections and subsections give various aspects of MQL in grinding operation as shown in below Fig 1 and Fig 2.
Fig. 1: Schematic view of lubricant penetration in the grinding zone [35]
Fig. 2: An illustration showing abrasive particle fracture acting as lubricant sources and flat wear area [6]
2. Surface integrity in minimum quantity lubrication Silva et al. [5] experimented on tempered ABNT 4340 steel. The average surface roughness values (Ra) were considerably reduced when MQL was employed, but also resulted in high residual stresses, which can be accounted by the non-uniform heating and cooling of the workpiece surface. The sub-surface alterations were less in both MQL and conventional cooling. In another study, Mao et al. [6] observed the effects of MQL grinding on hardened AISI-52100 steel. The oil-water lubrication provided good results by reducing the thickness of the affected layer and grinding temperature whereas pure oil lubrication resulted in a superior surface finish. Moreover, Tawakoli et al. [7] investigated the effect of MQL on grinding of 42CrMo4 soft steel and 100Cr6 hardened steel. It was observed that 100Cr6 steel showed better surface integrity than 42CrMo4 steel. It was also seen that MQL coupled with high MRR resulted in improved Ra compared to the dry and flood cooling. The material removal mechanism in MQL grinding is mostly shearing and fracturing unlike in conventional plastic deformation. Afterwards, Tawakoli et al. [8] demonstrated the sensitivity of the process towards the cutting fluid while using different types of Al2O3 wheels. The use of vitrified bond segmented grinding (SG) wheels produced less thermal damages and material side flows on the surface. Tawakoli et al. [10] pointed out that grindability of 100Cr6 steel increases substantially with the use of MQL oil and surface damage was reduced as compared to dry, wet and MQL-water-miscible conditions. Sadeghi et al. [11] investigated the surface grinding on AISI 4140 steel under MQL, whose surface roughness tends to have a higher value than that of other grinding modes. Grinding may be restricted to softer materials thus restricting its usability towards harder materials [12]. Afterwards, Sadeghi et al. [13] reported the grinding of Ti– 6Al–4V titanium alloy under dry, wet and MQL lubrication schemes, which produced more or less same results as Sadeghi et al [11]. In most of the study, MQL proved to be a superior alternative. The comparisons were made on the basis of surface roughness, Barkhausen noise, specific energy consumption and wheel wear. It was noted that all the above-mentioned output responses were reduced in the minimum coolant grinding (MCG) system when compared to the conventional cooling system. However, the Barkhausen noise in MCG was slightly on the higher side [14]. The reduction in specific energy consumption is attributed to effective lubrication in the grinding area as shown in Fig. 2. Li et al. [15] observed that cryogenic cooling reduced the surface roughness and increased the surface integrity to a considerable extent as compared to its oil and dry versions. A significant reduction of 30-50% was observed in the
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longitudinal Ra parameter values. The tensile residual stress of the ground surface was reduced by a notable amount because of the lower grinding zone temperature and reduced friction between ground surface and chip. Liu et al. [16], noted that in surface grinding of the Ti-6Al-4V, the surface roughness was inversely dependent upon the feed rates. It was due to the accelerated bond rupture and grit fracture with increasing feed rates. Thus, MQL can improve the surface integrity by removing the heat from the grinding zone. Wang et al. [17] argued that the grinding quality of the workpiece is dependent on the coolant-lubricant types. Experiments were conducted to study the values of surface roughness with various OW (oil filmed water droplets) fluids such as OW fluids with cooled air, ester oils and water under MQL lubrication. It was observed that OW fluids were more eco-friendly and emission-free as compared to their emulsion counterparts. OW fluids also exhibited lower surface roughness and higher grinding ratios. Minimum quantity cooling lubrication (MQCL) technique using the CBN wheel proved to be favorable in terms of surface integrity values. Residual surface compressive stresses were optimized when MQL was applied with the conventional cooling system [18]. Xiu et al. [19] studied point grinding in MQL with the help of CBN wheel. The grinding power and grinding temperature was considerably reduced in point grinding due to less contact area, weak airflow barrier effect and an increase of grinding fluid effect. In addition, it was observed that after a certain jet pressure, the influence of the grinding fluid on the surface quality was reduced. Liu et al. [20] investigated cooling-air grinding performance, which proposes that grinding quality can be improved by lowering the temperature of the ambient air. Grinding at lower feed rate gave a better grinding finish. This is because the heat generated during grinding was efficiently removed by the cool air. The surface finish, in case of grinding with cooling air, was always less than that obtained in conventional grinding. Moreover, cryogenic cooling reduced the grinding zone temperature by effective cooling, leading to less wheel wear. The surface roughness of the workpiece surface was also reduced [21]. Inoue et al. [22] compared to air cooling MQL with air and dry grinding to conclude that grinding resistance is proportional to the depth of cut which in turn is proportional to Ra. Li et al. [23] analysed the MQL technique with an approach appurtenant to fluid dynamics. MQL besides reducing the hydrodynamic lift force also lowered the cost of grinding fluid, thus, authenticating superior performance of MQL supply over conventional flood cooling. Liao et al. [24] investigated grinding of Ti-6Al-4V by flooded injection of water emulsified cutting fluid, flooded injection of nano-modifier, MQL of water emulsified cutting fluid and MQL of nano-modifier. The wheel surface was observed for the above cases at the loaded region. MQL with nano-modifier showed the lowest extent of wheel loading because of the increase of surface energy (the surface area of nanoparticles is inversely proportional to their sizes), Lotus effect exhibited by nanoparticles and the high pressurized air of MQL. Meng et al. [25] did a comparative theoretical study of MQL grinding method, traditional grinding fluid supply method and dry grinding along with experimental study in terms of surface finish. It was observed that MQL gave the best surface finish compared to a traditional grinding fluid method and dry grinding as the oil mist will be easier to break in the grinding zone against the gas barrier. It reduced the friction between the sliding abrasive particle and workpiece which contributed to better surface finish. Adibi et al. [26] conducted a grinding experiment on carbonfiber reinforced SiC matrix composites in MQL condition and compared with fluid and dry grinding. They reported that the surface roughness quality for MQL grinding was 75.26% higher when compared with dry grinding, while fluid grinding resulted in a surface 70.14% poorer surface quality than dry grinding. Highest G-ratio was achieved during MQL grinding with a 115.38% increase relative to dry grinding. Wang et al. [27] reported the MQL grinding of nickel alloy GH4169 by various nanofluids such as MoS2, Al2O3, carbon nanotubes (CNTs), nanodiamond (ND), SiO2 and ZrO2 infused in pure palm oil and water-soluble liquid. Al2O3 nanofluid yielded the lowest sliding friction coefficient of 0.348, lowest surface roughness of 0.302μm, lowest specific grinding energy of 82.13 J/mm3 and also the highest G-ratio (wheel wear) of 35.94. 2. Grinding forces in minimum quantity lubrication Sadeghi et al. [11] investigated the surface grinding of AISI 4140 hardened steel using MQL. They reported that the specific energy and grinding forces in MQL were least as compared to its wet and dry counterparts due to reduction in material ductility, ideal chip formation mechanism and retained abrasive particle sharpness. Barczak et al. [12] reported better results when specific MQL conditions were considered with respect to conventional flood cooling for plane surface grinding. Friction was found to be low and reasonably good material removal rate was achieved.
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Sadeghi et al. [13] experimented on grinding of Ti-6Al-4V under MQL condition. They reported that lubricant flow and delivery pressure played an important role in minimizing grinding force. An optimum result was achieved at a lubricant flow of 60ml/h and a pressure of 0.4 MPa. There was a considerable decrease tangential and perpendicular forces in MQL as compared to flood cooling. Guo et al. [28], too, studied surface grinding of Ti-6Al4V alloy under MQL conditions and noted that grinding depth has a considerable influence on specific energy, grinding force and force ratio. In addition, MQL machining results in improved surface finish as compared to fluid and dry grinding. Surface damages like plastic deformation, side flow and redisposition were lower in the case of MQL owing to high penetration in the contact zone. Lubricant with high coolant concentration at high pressure and medium flow rate produced the best surface finish [29]. Excellent delivery of cutting fluid is achieved at a nozzle direction of 15ᵒ while grinding Ti-6Al-4V [30]. Li et al. [15] observed that cryogenic cooling is an effective means of grinding a nickel-based superalloy without incurring any environmental hazards. It was observed that the grinding forces and the specific grinding energy under cryogenic lubrication were reduced substantially mainly due to the retentivity of grit sharpness and less plowing that is attributed to the generated inert atmosphere. Similar results were previously reported by Li et al. [21]. Liu et al. [16] stated that grinding force ratio and specific energy were following an inverse relation with the feed rates. This was due to the transition of material removal mode from plowing to chip forming. It was also found that relatively lower depths were responsible for higher grinding forces This is because sliding and plowing were predominant material removal modes in that region which related to the depth in an inverse fashion. Wang et al. [17] observed that the coolant-lubricant types influenced the grinding quality of the workpiece. Different OWs are produced by combination of cooled air, ester oils and water was tested under MQL lubrication. It was observed that OW fluids produced lower values of grinding forces than their emulsion counterparts. Yang et al. [31] studied a simulation model to report the characteristics of cylindrical grinding. The adiabatic heating induced by strain hardening and high strain rate increases the grinding zone temperature as well as grinding forces. The grinding forces and the grinding temperature is greatly influenced by the depth of cut. Adibi et al. [26] reported the grinding of carbon-fiber-reinforced SiC matrix composites under MQL conditions and compared it with fluid and dry grinding conditions. Dry grinding resulted in the highest grinding forces with MQL grinding developing the lowest grinding forces. MQL grinding resulted in reduced tangential force and normal grinding force by 38.88% and 31.16% respectively as compared to dry grinding condition. Benkai et al. [33] compared MQL grinding on a high-temperature nickel alloy with different base oils such as palm oil, castor oil, soybean oil, sunflower oil, corn oil, rapeseed oil and peanut oil. Palm oil yielded the lowest grinding temperature and energy ratio coefficient and the second lowest grinding force making it the optimum choice as MQL grinding base oil. Guo et al. [34] investigated the effect of different oils such as soybean, maize, rapeseed, palm and sunflower oils when added to castor oil in a 1:1 ratio on its lubricating properties. They reported that all the mixed base oils had superior properties than castor oil. However, soybean/castor oil resulted to be the best. The specific tangential and normal grinding forces were reduced by 27.03% and 23.15%, when compared with pure castor oil. Sinha et al. [35] compared the surface grinding of Inconel 718 under dry, flood cooling and soluble oil under MQL conditions. The comparisons were done on the basis of cutting forces and surface roughness. The normal and tangential grinding forces, both, were least in MQL grinding process. However, the surface roughness of the finished surface was higher than dry and flood cooled grinding process. 3. Effects on wheel performance in minimum quantity lubrication grinding Sadeghi et al. [11] reported that wheels with coarser abrasive particles and higher porosity are the best choice for MQL in grinding process as they have less active abrasive particles and smaller wear flat area. The position of the nozzle with respect to grinding wheels plays a vital role in the performance of oil mist as argued by Tawakoli et al. [36], was found to be 10º-20º from the workpiece surface. Tawakoli et al. [10] also pointed out that MQL grinding with fine abrasive particle vitrified corundum and low porosity showed less satisfactory performance. On the other
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hand, the porous CBN wheels with grinding oils showed a superior performance. Sensitivity to lubricant variation was found to be lowest in CBN, porous and coarse abrasive particle wheels. The latter had the least tendency for chip loading in MQL grinding. Furthermore, Tawakoli et al. [9] pointed out that least chips were loaded on the grinding wheel while using a CBM wheel with MQL due to high porosity and higher thermal conductivity which ensures the prevention of loading of chips with lower temperatures. 3. Thermal analysis of grinding in minimum quantity lubrication Ramanath et al. [37] stated that MQL can provide better results than flood injection during grinding of Ti-6Al-4V. It is because, through the use of micro-spray cutting fluid, the blanket of gas produced at the grinding zone due to the high heat generation, can be cut and hence provide the necessary lubrication. Moreover, Mao et al. [38] examined the coefficient of heat transfer of the workpiece surface by comparing the simulative and the experimental results. They reported the increase of heat transfer coefficient in nucleate-boiling zone whereas it decreased in the transition boiling. It showed insensitivity to the temperature in the non-boiling zone. A good agreement of mathematical models proposed and experimental results was observed for MQL grinding which helps in predicting the dynamics of the surface temperature of the workpiece. Hadad et al. [39] compared the measure of energy partitioning in dry, flood cooling and MQL grinding. It was seen that the energy partitioning in the case of Al2O3 wheels with flood cooling was as less as 36%. However, the combinations of resin bond CBN wheels with MQL lead to a desirable 46%. Moreover, it was also observed that the forces required to generate a temperature rise in MQL were 40-50% of the flood cooling incurring a high thermal stress. In another study, Morgan et al. [40] employed an analytical model for the thermal analysis of MQL grinding. Theoretical analysis has shown that thermal performance of MQL is same as that of wet grinding. However, it was not able to predict the presence of high-speed jet cooling effect and convection but their existence was proved by experimentations. In another study, Hadad et al. [41] compared the thermal aspects of dry, flood cooling and MQL grinding. Analytical model was proposed to analyse the process with experimentations on the conventional Al2O3 wheel and super abrasive CBN wheel. Cooling effects were studied on the trailing edge, leading edge and the grinding zone in flood and MQL conditions. It was observed that in spite of appreciable lubricating properties of MQL, the process generated some cooling effect in the trailing edge in the contact zone vicinity. This proved to be a backdrop of MQL. Moreover, Li et al. [15] noted that grinding zone temperature was reduced due to the effective cooling by the cryogenic lubrication leading to lower forces and energy consumption. Wang et al. [42] dealt with the problem of reduced grinding belt life. They used liquid nitrogen cryogenic wind jet combined MQL (LNCWJCM) technology. The experimental studies were compared to conventional cooling systems, cold air and liquid nitrogen. The work-piece used for the test was a titanium alloy. As dissipation of heat through itself is difficult in titanium, the concentration of heat leads to rise in temperature to high values. Titanium alloy being reactive at high temperature undergoes strong chemical activity which leads to abrasion wear and adhesive wear of the belt thus making it blunt. The LNWJCM technology improves the belt life without harming the environment. The high-speed jet helps in the penetration of liquid nitrogen to the very core of heat generation. Stephenson [43] theoretically and experimentally analyzed the effect of energy partitioning on the coolantlubricant type in different grinding schemes. Due to the variance of energy partitioning in the various grinding process, the grinding fluid requirement changes as it proceeds from creep feed grinding to high efficiency deep grinding (HEDG). The principle job of the grinding fluid in creep feed grinding is cooling of the grinding zone as it carries away 90% of the heat. In HEDG, the grinding fluid lubricates the grinding zone, thus maintaining abrasive life to maximum. In another study, Benkai et al. [44] compared the heat transfer performance of different nanofluids namely MoS2, CNT, polycrystalline diamond (PCD), ZrO2, SiO2 and Al2O3, with palm oil used as base oil. They reported that CNT nanofluids had the lowest grinding temperature and energy proportionality coefficient closely followed by Al2O3 nanofluids. CNT nanofluid had the highest heat transfer coefficient of 1.3*104 W/mK. Zhang et al. [45] conducted a thermal analysis in the grinding surface with different cooling approaches. MoS2, CNT, ZrO2 when mixed with the same base oil produced different grinding fluids to be used for grinding
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experiments. The lubricating effect of MoS2 was reported to be excellent. Nanofluids with CNT gave the best cooling effects with 2% volume concentration of CNT nanoparticles. 6. Nano fluid lubrication in minimum quantity lubrication A major part of the heat generated at the tool-workpiece contact region is transferred through the grinding fluids. The effectiveness of these grinding fluids depends upon the penetration to the contact region. MQL with nanofluids is a very promising approach towards attaining exceptional convection and thermal conductivity. Mao et al. [46] studied the efficiency of MQL grinding with nanofluids under various spraying parameters. The spraying direction of MQL nozzle played an important role in the application of nanofluids. Maximum grinding performance was achieved by placing the nozzle at an angle with respect to the grinding wheel. Moreover, air pressure and distance of spraying of nanofluids are important parameters to be considered in order to boost the penetration of nanofluids into the grinding zone. The grinding forces, grinding contact area temperature and surface roughness is inversely proportional to air pressure. The mist velocity decreases with increase in spraying distance. In another study, Kalita et al. [47] analyzed the process efficiency of MQL with oil-based nanolubricants in grinding of AISI 4340 steel and ductile cast iron. It was stated that MQL grinding using nanolubricants enhances the efficiency by decreasing frictional losses and energy consumption. Paraffin-based nanolubricants showed improved performance with higher nanoparticle concentration, including increase in wheel life, an increment in grinding ratio (G ratio) by 50%, a reduction of energy consumption by 53% and a lower coefficient of friction of 0.22. Silva et al [48], reported that the best performance while grinding AISI 4340 steel by a vegetable oil-based MQL can be obtained by an air flow of 26.4 m/s and a lubricant flow of 40 mL/hr. Furthermore, Kalita et al [49] conducted an experiment on the lubricating and impringing capabilities of a nanolubricant consisting of organic molecules along with nanoparticles of phosphide intercalated-MoS2 nanoparticles during the surface grinding of ductile cast iron under MQL. The nanolubricants reduced the force ratio and specific energy by 45-50%, and the abrasive wheel wear or G-ratio was reduced by 48-55% as compared with flood cooling and paraffin (base) lubrication under MQL. Mao et al. [4] stated that the quality of grinding and surface finish for hardened AISI 52100 steel improved significantly with MQL of water-based Al2O3 nanofluid when compared to dry grinding. However, wet grinding, showed superior cooling capacity and lubrication. Water-based nanofluid MQL grinding significantly decreased the grinding temperature, grinding forces and the surface roughness as compared to pure water MQL grinding. The nanoparticles served as ball bearings around the workpiece providing better lubrication than pure water MQL grinding. In another study, Zhang et al. [32] stated that on adding MoS2 nano-particles to soybean oil as base oil for jet lubrication under MQL condition, viscosity increased resulting in better lubricating property. The optimum concentration of MoS2 nano-particle in soybean base oil was found to be 6%. Wang et al. [27], further, compared six nanofluids viz. MoS2, nanodiamond (ND), CNTs, SiO2, Al2O3, and ZrO2 under MQL for grinding nickel alloy GH4169. They reported that nanoparticles with spherical molecular structure and nanofluids with higher viscosity result in better lubricating properties. The nanofluids can be ranked with respect to their lubricating properties as ZrO2 < CNTs < ND < MoS2< SiO2< Al2O3. Setti et al. [50] reported the surface grinding of Ti-6Al-4V under MQL conditions with different nanofluids namely Al2O3 and CuO nanoparticles added in different concentrations to water. MQL with Al2O3 nano-fluid significantly reduced the coefficient of friction with time, however, CuO nanofluids and soluble oil under MQL did not show any effect. Chemical reaction between Al2O3 nano-particles and titanium along with the lotus effect flushed the debris formed during grinding out of the grinding zone, thus improving the grindability of Ti-6Al-4V. Huang et al. [51] experimented on surface grinding of NAK80 mold steel under different conditions viz. dry, MQL, MQL with nanofluids and ultrasonic MQL with nanofluid. The nano-particle chosen was multi-walled CNTs. They reported considerable better results in terms of grinding temperature, grinding force, surface morphology and surface roughness, when ultrasonic assisted MQL was used. This can be credited to the reduced friction produced when nano-fluids are used, as the nano-particles interacts with the grinding wheel and surface and forms a protective layer and the superior heat transfer properties of the ultrasonic assisted nanofluids. Nanofluids with higher nanoparticle concentration in MQL grinding results in lower grinding force and temperature values [52]. Therefore, the use of nanofluids with MQL provided superior results with increased workpiece quality. A further study of mechanisms of material removal is required to optimize the performance of nanolubricants in MQL.
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7. Effects of minimum quantity lubrication on micro grinding Li et al. [53] studied the effects of the lubricating parameters and cutting conditions on the tool life and surface finish in SK3 steels. They reported that a very small quantity of cutting fluid is required for a longer tool life. Oil flow of 1.88 ml/h and air flow of 25 l/min were found to be the best combination for extracting best performance out of the system in the form of surface integrity and tool wear reduction. Lee et al. [54] discussed the effects of nanofluids in micro-grinding. In this study, ND and nano-Al2O3 particles were used to demonstrate the consequences of nanofluids on surface roughness and grinding forces. They reported that ND particles were more effective in reducing the grinding forces whereas nano- Al2O3 particles showed a superior increase in surface integrity. Volumetric concentrations and nanoparticles sizes were the governing factors of the performance of the system, with the latter having a greater impact on surface roughness. Afterwards, Lee et al. [55] used compressed chilled air to study nanofluid assisted micro grinding under MQL as grinding forces, tool wear and surface roughness varied directly with air temperature. Low depth of cut and feed were favorable for use of compressed chilled air. Electro-plated CBN grinding tool showed a greater sensitivity towards the reduction of forces as compared to the vitrified-bond tool because of its superior thermal conductivity. Surface integrity was better achieved in the case of vitrified-bond tool whereas tool wear was reduced in both the cases. Also, Lee et al. [56] investigated a micro-grinding process under MQL condition of its thermal characteristics with experimental and numerical approach. The high thermal conductivity and ball bearing effect of the ND particles restricted the subsurface temperature rise for nanofluid MQL with ND particles as compared to the compressed air lubrication and pure MQL. 8. Potential enhancements in conventional minimum quantity lubrication The process knowledge of MQL is limited and elementary and improvisation in the MQL grinding can lead to superior surface quality and a decrease in temperature of grinding zone. Nguyen et al. [57] developed a grinding wheel model efficient for coolant penetration into the grinding zone. This arrangement of cutting fluid supplied to the core of the high-temperature zone can reduce the temperature drastically as compared to the conventional cooling method. Analytical study concluded that the quantity of coolant used can be minimized using a segmented wheel and its efficiency is dependent on the wheel speed as well as on the method of coolant supply. Tsai et al. [58] studied a grinding wheel with micro-graphite particles imbued in an Al2O3 matrix to provide lubrication at the grinding zone. The newly designed graphite impregnated wheel in MQL conditions reduced the surface roughness by 25%-32%, the cutting force by 24%-36%, and cutting temperature by 8%-14% than a conventional grinding wheel. A graphite content below 0.5% produces a better surface finish, lowers grinding forces as well as temperature, lower wheel consumption and hence a prolonged life. In another study, a similar concept of the graphite impregnated wheel was proposed by Shaji et al. [59]. Dry and coolant conditions with the proposed wheel were compared. It was seen that the tangential force component and consequently the specific energy radically reduced. Surface finish was superior for harder materials than for ductile materials. Another such solid lubricant moulded grinding wheel was developed by Shaji et al. [60] with CaF2 as the lubricant. The wear of the wheel depended on the lubricant type. It was more with CaF2 than with graphite. Oliveira et al. [61] provided a method for flushing and wheel cleaning to improve MQL in CBN grinding by combining MQL with a compressed air jet. The jet nozzle was placed at an incident angle of 300o. The use of a compressed air jet positioned at an optimal angle increases the material removal rate without affecting the quality of the workpiece. Furthermore, Alberdi et al. [62] developed alternative ways to minimize the consumption of fluid in grinding. This can be achieved through nozzle design optimization and by combining MQL and low-temperature CO2 while grinding. This leads in significant improvement in wheel life with a reduction in surface finish and force ratio. The new MQL-CO2 system developed resulted in the improvement of the friction ratio. Mandal et al. [63] studied a pneumatic barrier that can suppress the formation of the air layer in the periphery of a high speed grinding wheel. The pneumatic barrier provides effective lubricant penetration at the point of application reducing the tangential and normal forces, hence decreasing the surface roughness. Moreover, Fiocchi et al. [64] noted the characteristics of the lap grinding process in regard to the surface roughness and integrity. It was observed that the smaller abrasive particle size yielded higher surface integrity for a given lap-grinding apparatus. The best surface roughness value for 800 mesh grinding wheel was 1.92 nm. As the process produced good results with a non-MQL system, enhanced performance with MQL can be expected.
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Pan et al. [65] studied the characteristics of Electrolytic In-process Dressing (ELID) with MQL. A positive potential was given to the grinding wheel and a negative potential was given to the electrode. The space between the grinding wheel and the electrode was supplied with mist, resulting in the formation of an electrolytic film during electrolytic dressing. An increase in the grinding fluid quantity increased the film thickness. This suggested the gathering of ample amount of grinding fluid in the gap leading to stable electrolytic dressing. Increase of the grinding wheel speed at a constant supply of grinding fluid resulted in the decrease of the film thickness. This is an efficient way of machining hard and brittle materials. The use of micro-emulsions greatly increases the eco-friendly properties of MQL grinding process [66]. Biachi et al [67], in another study used a vitrified CBN grinding wheel to machine tempered AISI 4340 steel. Flood cooling and MQL with water and different concentrations of oil was employed to control the heat generation. They also employed a cleaning system with compressed air to prevent wheel clogging problems and the resulting heat generation. The increase of water content in the MQL increased the cooling capacity of the lubrication system and expedited fluid atomization. This resulted in better surface roughness, lower grinding wheel wear but higher grinding power is required. The cleaning system removes the clogged chips out of the grinding wheel surface. The cutting power required is lower when cleaning system is used. In another study, Madarkar et al. [68] compared the performance of ultrasonic MQL (UMQL) with conventional MQL (CMQL) while machining Ti-6Al-4V using sunflower oil. The use of UMQL resulted in better surface roughness, decrease in the cutting forces; 38% in tangential grinding forces and 32% in normal grinding forces, with respect to dry grinding of Ti-6Al-4V, which can be credited to the fine atomization in UMQL. 9. Conclusion The paper presents various aspects of MQL application in grinding process. The grinding process efficiency and performance are studied in view of thegrinding forces, surface quality, wheel wear and thermal consideration. With huge advantages of MQL over conventional cutting fluids applications, MQL can be a viable alternative to conventional cutting fluid applications. MQL provides an extraordinary quality of lubrication which in turn can increase surface quality. Though wet grinding can provide better cooling conditions, the surface finish cannot be compared with that of the superior finish from MQL process. This compromise with cooling is the root cause for non-implementation and non-acceptance on industrial levels. But environmental friendliness, efficiency, economical and no harm to the operator are some of the advantages of MQL technology. The MQL technology is very flexible in terms that it can be optimized to achieve specific results. The knowledge of MQL in grinding is necessary and this area is expanding day by day because of its huge advantages over conventional cutting fluid applications. Acknowledgements Authors are thankful to “ELSEVIER (Licence numbers: 4280681200373, 4280800873389)” for providing the copy right permission to use figure in this manuscript.
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