Study of ionic liquid as effective additive for minimum quantity lubrication during titanium machining.pdf

research letter study of ionic liquid as eff ective additive for minimum quantity lubrication during titanium machining brian davis, john k. schueller, yong huang department of mechanical and aerospace engineering, university of florida, gainesville, fl 32611, usa received 1 october 2014; received in revised form 12 january 2015; accepted 12 april 2015 available online 18 april 2015 abstract the objective of this study is to investigate the eff ectiveness of ionic liquid (il), a low melting point salt, as a lubricant additive for minimum quantity lubrication (mql) implementation during titanium machining. herein 1-butyl-3-methylimidazolium hexafl uo- rophosphate (bmim-pf6 ), a prototypical il, has been used as an mql cutting fl uid additive. it is found that il-based mql can eff ectively reduce the tool wear by 60% when compared to dry cutting and 15% more than mql without bmim-pf6. it is also found that il-based mql results in the smallest cutting forces, largest saw-tooth/shear band spacing, and best surface fi nish. ? 2015 society of manufacturing engineers (sme). published by elsevier ltd. all rights reserved. keywords: machining; ionic liquid; minimum quantity lubrication 1. introduction machining innovations are often needed to improve the cutting performance during the machining of diffi cult- to-cut materials such as titanium. eff ective cooling and lubrication have been the focus of many machining innova- tions while various cutting fl uid delivery methods have been developed to better cool/lubricate the toolworkpiece interface. under most industrial settings, the cutting area is fl ooded with cutting fl uid which eff ectively cools both the tool and workpiece. however, it has been noted that fl ood cooling does not always provide adequate lubrication to the tool cutting edge even with a large fl ow rate and pres- sure at which a lubricant is applied. while fl ood cooling provides adequate cooling, minimum quantity lubrication (mql) has emerged as a disruptive environmentally con- scious machining technology for various machining pro- cesses, including turning 1,2. with a typical fl ow rate that is four orders of magnitude less than conventional fl ood cooling (550 ml/min), mql provides the advan- tageofprovidinglubricationtothetoolworkpiece interface. for improved mql performance during machining, numerous eff orts have been devoted to enhancing the per- formance of the mql fl uids, and this is often achieved by coupling the base fl uid with various additives. of them, nanoparticles have found great success in various machin- ing applications, such as turning (mos2in calcium-based grease 3 and al2o3in servocut type s oil 4), milling (sio2in ecocut ssn 332 5), and grinding (mos2in soybean oil 6, carbon nanotube in soybean oil 7, and al2o3in deionized (di) water 8,9), to name a few. cut- ting force reduction 5,6,9 and tool life improvement 3,8,9 have been frequently reported. while nanoparticles have proven their eff ectiveness as an mql additive, their application, unfortunately, is still limited by environmental concerns as well as cost. the objective of this study is to investigate the eff ective- ness of ionic liquid (il), a type of low melting point salt (100 ?c), as an additive for mql implementation during titanium machining. as salts in the liquid state, ils are /10.1016/j.mfglet.2015.04.001 2213-8463/? 2015 society of manufacturing engineers (sme). published by elsevier ltd. all rights reserved. corresponding author. tel.: +1 352 392 5520; fax: +1 352 392 7303. e-mail address: yonghufl .edu (y. huang). available online at sciencedirect manufacturing letters 5 (2015) 16 typically composed of an organic cation, the most common being imidazolium, and a weak coordinating anion such as tetrafl uoroborate (bf4 ) or hexafl uorophosphate (pf6) 10. numerous tribological tests have proven ils as excellent lubricant additives to lower both friction and wear coeffi - cients during pin/ball contact sliding tests using material pairs of steel/steel 11, aluminum/steel 1215, titanium/ steel 10,16,17, and steel/cast iron 18, to name a few. for example, a 70% reduction in the friction coeffi cient was reported during steel/aluminum sliding when using 1- ethyl-3-methylimidazolium tetrafl uoroborate(l102),a type of il, as a 1% lubricant additive in neat mineral oil 15. additionally, a reduction of 60% and 99.5% in the friction coeffi cientandwearrate,respectively,was observed in titanium/steel sliding contacts using 1-octyl- 3-methylimidazolium tetrafl uoroborate (l108) when com- pared to neat mineral oil 10. unfortunately, there is still no study regarding the use of il as a cutting fl uid additive for machining applications. in this study, the eff ects of il-based mql on the titanium cutting performance in terms of tool wear, cutting force, chip morphology, and surface roughness have been exam- ined. it is expected that the resulting knowledge will help promote the further study and applications of various ils as eff ective mql additives for advanced machining operations. 2. materials and methods the inset of fig. 1 illustrates the chemical structure of1-butyl-3-methylimidazolium hexafl uorophosphate (bmim-pf6), a widely used imidazolium-based, prototypi- cal il. bmim-pf6is a viscous, colorless, hydrophobic and non-watersolubleil,makingitpossibletoformmicro-emul- sions suitable for mql applications. in this study, 1-butyl-3- methylimidazolium hexafl uorophosphate(bmim-pf6) was chosen as the prototypical il for mql machining for its wide acceptance and good lubrication performance in tribological studies 14,17,19,20. a cutting fl uid containing 0.5% (wt%) bmim-pf6(sigmaaldrich, st. louis, mo) was prepared by adding bmim-pf6drop wise in 30 min increments to di water under constant stirring. the suspen- sionwasfurtherstirredfor24 hinsideafumehoodtoensure equilibrium is reached. it is assumed that hydrolysis, if occurring, during the preparation of bmim-pf6and water suspension may only produce lowly concentrated ch3 ohf (fl uorinated methanol) 2123. furthermore, the reaction betweenbmim-pf6and waterreaches equilibrium after 24 h. as such, it is believed that it is safe to use the bmim-pf6water suspension for the proposed mql study. while bmim-pf6is used as a prototypical il additive herein, other ils should also be explored in future studies. since the objective of this exploratory study was to eval- uate the eff ectiveness of ils as lubricant additives during mql machining, di water instead of industrial mql cut- ting fl uids was chosen as the base cutting fl uid. by using di water, a typical base cutting fl uid with an excellent thermal conductivity, the possible physical and/or chemical reac- tion between the il and any undisclosed additives from industrial mql cutting fl uids may be minimized. commerciallypuregrade2titaniumroundbars (titanium metal supply, poway, ca) with a diameter of 19.1 mm(0.75inches)wereturnedusinguncoated cubicboronnitride(cbn)inserts(kennametal tpgn160308mt, latrobe, pa) on a mikron ucp 600 vario machining center (gf agie charmilles, switzerland). asshowninfig.1,themikronmillingcenterwasconfi gured for turning operation using an erickson high-speed milling chuck (kennametal, latrobe, pa) to fi x the titanium work- pieceinthespindle.thecuttinginsertwasmountedtoakis- tler 9257b dynamometer (kistler, amherst, ny) which was fi xed on the work table of the milling center. the machining test was conducted under nominal cutting conditions: cut- ting speed v = 2 m/s, feedrate f = 0.05 mm/rev, and depth of cut doc = 0.1 mm. diamond tools were not favored in stir plate stir bar il emulsion mql system workpiece cutting tool chip axial direction radial direction tangential direction n n ch3 ch3 p f ff ff f suspended il particle fig. 1. schematic of machining and mql setup. 2b. davis et al./manufacturing letters 5 (2015) 16 this study due to their brittleness and chemical affi nity with titanium. an amcol precision applicator system (amcol, hazel park, mi) was utilized to supply cutting fl uids to the cutting edge under a liquid fl ow rate of 1 ml/min and an air pressure of 7 bars. the il-based cutting fl uid container, which contained pre-mixed il cutting fl uid and a magnetic stir bar, was placed on a vmr stir plate (vmr, radnor, pa). the stir bar continuously stirred the il-based cutting fl uid emulsion before pumping the emulsion into the amcol system. tool wear was mea- sured using a stereomaster microscope (fisher scientifi c, hampton, nh). the average surface roughness of the machined workpieces was determined at fi ve evenly spaced locations along the longitudinal direction of the machined samples using a newview 7200 surface profi lometer (zygo corp, middlefi eld, ct). tool wear and chip morphology were further characterized using a jeol 6400 scanning electron microscope (jeol ltd., tokyo, japan). 3. experimental results 3.1. tool wear fig. 2(a) shows the tool wear progression under three dif- ferent lubrication conditions, namely dry, mql using di water (water-based mql or mqlh2o), and mql using a water-il mixture (il-based mql or mqlh2o + il). the error bars in this study represent a one standard- deviation variation; only one tool wear progression mea- surement was done for dry cutting since the measurements are consistent with expectations. as observed, using water-based mql can reduce the tool wear by nearly half when compared to the dry cutting condition, demonstrating the advantage of mql for tool wear reduction as previously reported 1,2. when the il additive is used, the tool wear is reduced further by 60% when compared to that during dry cutting; an additional 15% reduction when compared to the water-based mql condition. the observation clearly demonstrates that using an il as a cutting fl uid additive may further improve the cutting performance of water-based mql applications, illustrating the potential of il as a promising cutting fl uid additive for mql applications. as seen from the scanning electron microscopy (sem) image of a worn tool as shown in fig. 2(b), the wear patterns are crater and fl ank wear. based on the energy dispersive spectroscopy (eds) measurements, titanium accounted for nearly 75% and 55% of the residual materials at the cutting edge under the dry and water-based mql conditions, respectively, and it is assumed that adhesion is a tool wear mechanism under these two lubrication con- ditions. for the il-based mql cutting confi guration, there is a very low deposition of titanium (15%) at the cutting edge, indicating that adhesion does not play an important role during the tool wear progression. under the gentle cut- ting conditions used, the resulting cutting temperature may not be suffi cient to initiate diff usive wear. as such, it is con- cluded that abrasion is the main wear mechanism during titanium machining under il-based mql. the tool wear reduction under il-based mql is attrib- uted to the existence of a possible protective boundary fi lm that develops due to a tribochemical reaction between the il lubricant and the tool/workpiece. the formation of such a boundary tribolayer has been attributed to the eff ec- tiveness of ils as lubricant additives during tribological testing 15,17,18,2426. it is hypothesized that the tribo- layer eff ectively mitigates the tool wear during il-based mql machining. to better understand the existence and formation of il-based protective layers on the tool surface and along the toolchip interface, future research is needed in terms of the detailed compositional characterization of the surface of worn tools. 3.2. cutting forces the three-dimensional cutting forces during the tool wear progression are shown in fig. 3. slight cutting force increases of approximately 10% are observed due to the rel- atively mild tool wear. while there is no pronounced diff er- ence among the axial thrust forces under the three lubrication conditions, the cutting forces under il-based mql are about 15% lower in the radial and tangential directions. this force reduction is attributed to the formation of a protective surface layer which reduces the frictional coeffi cient along the tool-chip interface, resulting in lower cutting forces. as observed, when there is no il additive added into the mql cutting fl uid, the diff erences flank wear (m) time (minute) 020406080100 0 20 40 60 80 100 120 140 dry mql - h2o mql - h2o + il (a) 200 m rake face flank wear (b) fig. 2. (a) tool wear progression, and (b) sem image of worn tool under il-based mql condition. b. davis et al./manufacturing letters 5 (2015) 163 of tangential cutting and radial thrust forces between dry cutting and water-based mql are negligible. 3.3. chip morphology and surface roughness titanium chips were also examined to evaluate the dif- ferences due to the il-based mql condition. when machining titanium and its alloys, segmented or saw- toothed chips are commonly observed 27,28 mainly due to titaniums low thermal conductivity. in addition to being characterized based on its continuity, the chip mor- phology has also been studied based on the saw-tooth/ shear band spacing, which is a good indication of the vari- ation of material dynamic behavior during il-based mql machining. the saw-tooth spacing was fi rst measured by averaging ten spacing measurements along the chip fl ow direction, and the shear band spacing can be found by mul- tiplying the saw-tooth spacing by sinh, where h is the crack initiation angle 29. for simplicity, the crack initiation angle is assumed constant, so the saw-tooth spacing is assumed to represent the shear band spacing in this study. as shown in fig. 4, the il-based mql condition leads to the largest saw-tooth/shear band spacing, followed by the water-based mql condition, while the dry cutting condi- tion has the smallest spacing. it has been reported that the saw-tooth/shear band spac- ing increases with the feedrate 28,29 which means that the mql condition results in an equivalent eff ect of large fee- drate during chip formation. furthermore, the shear band spacing is proportional to _ c? 3 430, where _ c is the strain rate inside the primary shear zone. given that the il-based mql condition leads to the largest saw-tooth/shear band spacing, it is assumed that the strain and strain rate are reduced under il-based mql lubrication when compared to those under both dry and water-based mql. it is further found that the samples used for dry cutting have the largest surface roughness (311 nm), which is con- sistent with the surface roughness reported in 31. while there is no pronounced diff erence in the surface fi nish between the water- and il-based mql samples (250 vs. 234 nm), the il-based mql condition results in a 25% improvement when compared to the dry cutting condition. this observation demonstrates that a better surface fi nish can be achieved under mql conditions as reported before 2,32 and this il additive may further improve the quality of machined surfaces. ideally speaking, the surface fi nish in turning is a function of feedrate and tool geometry. for a given feedrate and tool geometry, the process should yield a consistent surface fi nish which is no longer valid once mql conditions are introduced. 4. conclusions and future work in this study, bmim-pf6-based mql has proven its eff ectiveness in improving the cutting performance during titanium machining as compared to dry cutting and mql without bmim-pf6. in particular, il-based mql can eff ectively reduce the tool wear by 60% when compared to dry cutting, 15% more than water-based mql, that is, mql without bmim-pf6. it is also found that il-based mql results in the smallest cutting forces, largest saw- radial forces tangential forces axial forces fig. 3. cutting force during tool wear progression. 0 2 4 6 8 10 12 14 saw-tooth spacing (m) drymql (h2o)mql (h2o + il) fig. 4. saw-tooth spacing of titanium chips under diff erent lubrication conditions (the inset scale bar is 100 lm). 4b. davis et al./manufacturing letters 5 (2015) 16 tooth/shear band spacing, and best surface fi nish when compared to both dry cutting and water-based mql. the pilot study provides some preliminary results to inspire future research on il-based mql machining. the fi ndings herein have demonstrated that ils may be an eff ective additive for improved mql performance. it is hypothesized that the cutting performance improvement during titanium machining is attributed to the formation of a protective boundary layer as a result of a tribochemi- cal reaction between the il and tool/workpiece. to further understand the underlying physics and promote the adop- tion of ils, some future work has been proposed as fol- lows.first,x-rayphotoelectronspectroscopy(xps) should be implemented to examine residual components on the tool surface to verify the existence of a possible pro- tective boundary tribolayer formed during il-based mql machining and further characterize it. second, newly devel- oped ils such as those containing a bis(trifl uoromethylsul- fonyl)imide (ntf2) anion and other ils which may be miscible with industrial cutting fl uids may be utilized to further enhance the il-based mql cutting performance while using industrial mql cutting fl uids for the machin- ing of diff erent materials. it is reported that ils containing ntf2 contribute to a substantially lower friction coeffi cient in the boundary lubrication (bl) regime due to their low viscositypressure coeffi cient and ability to form a protec- tive boundary fi lm, illustrating their potential for machin- ing operations. finally, animal and human risks from acute and chronic exposure of ils should be investigated for large-scale industrial applications. acknowledgements the study is partially supported by the u.s. national science foundation (cmmi-1404926). the authors would li