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3B2v8:06a=w ðDec 5 2003Þ:51cþmodel

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ED:VijayBNPAGN:anu SCAN:

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doi:10.1016/j.ijm

�CorrespondE-mail addr

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aguiarpr@feb.u

International Journal of Machine Tools & Manufacture ] (]]]]) ]]]–]]]

www.elsevier.com/locate/ijmactool

F

Analysis of surface integrity for minimum quantity lubricant—MQL ingrinding

Leonardo Roberto da Silvaa,�, Eduardo Carlos Bianchib, Ronaldo Yoshinobu Fusseb,Rodrigo Eduardo Cataic, Thiago Valle Franc-ab, Paulo Roberto Aguiard

aDepartment of Mechanical, CEFET, Belo Horizonte, Av. Amazonas, 5253, Jd. Nova Suıc-a, CEP: 30480-000, MG, BrazilbDepartment of Mechanical Engineering, UNESP, Av. Eng. Luiz E. C. Coube, s/n, Bauru, SP, Brazil

cUNESP, Guaratingueta, SP, BrazildDepartment of Electrical Engineering, UNESP, Bauru, SP, Brazil

Received 30 June 2005; received in revised form 28 February 2006; accepted 6 March 2006

CTED PROO

Abstract

The quality of machined components is currently of high interest, for the market demands mechanical components of increasingly high

performance, not only from the standpoint of functionality but also from that of safety. Components produced through operations

involving the removal of material display surface irregularities resulting not only from the action of the tool itself, but also from other

factors that contribute to their superficial texture. This texture can exert a decisive influence on the application and performance of the

machined component. This article analyzes the behavior of the minimum quantity lubricant (MQL) technique and compares it with the

conventional cooling method. To this end, an optimized fluid application method was devised using a specially designed nozzle, by the

authors, through which a minimum amount of oil is sprayed in a compressed air flow, thus meeting environmental requirements. This

paper, therefore, explores and discusses the concept of the MQL in the grinding process. The performance of the MQL technique in the

grinding process was evaluated based on an analysis of the surface integrity (roughness, residual stress, microstructure and

microhardness). The results presented here are expected to lead to technological and ecological gains in the grinding process using MQL.

r 2006 Elsevier Ltd. All rights reserved.

Keywords: Surface integrity; Minimum quantity lubricant (MQL); Nozzle and grinding

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UNCORRE1. Introduction

In recent years, energy consumption, air pollution andindustrial waste have been the focus of special attention onthe part of public authorities. The environment has becomeone of the most important subjects within the context ofmodern life, for its degradation directly impacts humanity.Driven by pressure from environmental agencies, politi-cians have drawn up increasingly strict legislation aimed atprotecting the environment and preserving natural energyresources. These combined factors have led the industrialsector, research centers and universities to seek alternative

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e front matter r 2006 Elsevier Ltd. All rights reserved.

achtools.2006.03.015

ing author. Tel.: +5514 3103 6119.

esses: lrsilva@feb.unesp.br (L.R. da Silva), bianchi@fe-

. Bianchi), ryfusse@feb.unesp.br (R.Y. Fusse), rcatai@-

(R.E. Catai), thiagovf@feb.unesp.br (T.V. Franc-a),

nesp.br (P.R. Aguiar).

production processes, creating technologies that minimizeor avoid the production of environmentally aggressiveresidues.Emulsion-based cooling fluids for machining are still

widely used in large quantities in industrial metal-mechan-ical processes, generating high consumption and disposalcosts and harming the environment. The growing need forenvironmentally correct production techniques and therapidly rising cost involved in the disposal of cutting fluidjustify the demand for an alternative to the grindingprocess with fluid. In the last decade, however, the goal ofresearches has been to strongly limit the use of cooling and/or lubricating fluids in metal-mechanical productionprocesses. Dry machining and minimum quantity lubricant(MQL) machining have caught the attention of researchersand technicians in the field of machining as an alternativeto traditional fluids. The drastic reduction or even the

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Nomenclature

vs grinding wheel speed (m/s)vf infeed rate (mm/min)vw workpiece speed (m/s)a depth of cut (mm)ts spark-out time (s)

Ra surface roughness (mm)Al2O3 aluminum oxideTT heat treatmentMQL minimum quantity of lubricantABNT Brazilian Society for Technical StandardsSEM scanning electron microscopy

L.R. da Silva et al. / International Journal of Machine Tools & Manufacture ] (]]]]) ]]]–]]]2

UNCORRECT

complete elimination of this fluid can undoubtedly lead tohigher temperatures in the process, causing reduced cuttingtool output, loss of dimensional and geometrical precisionof the workpieces and variations in the machine’s thermalbehavior. When abrasive tools are used, a reduction incutting fluid may make it difficult to keep the grindingwheel’s pores clean, favoring the tendency for clogging andthus strongly contributing to the aforementioned negativefactors [1–5].

Confirming the trend for environmental concernstriggered by the use of cutting fluids in machiningprocesses, as reported by several researchers and machinetool manufacturers, strong emphasis today focuses onenvironmentally correct technologies aimed at preservingthe environment and at conforming to the ISO 14000standard. On the other hand, despite persistent attempts tocompletely eliminate cutting fluids, in many cases cooling isstill essential to the economically viable service life of toolsand the surface qualities required. This is particularly truewhen strict tolerance and highly exact dimensions andshapes are required, or when the machining of criticaldifficult-to-cut materials is involved. Minimum quantitylubricant, in these cases, is an interesting alternativebecause it combines the functionality of cooling with anextremely low consumption of lubricant (usually o80ml/h). The minimization of cutting fluid has taken onincreasing relevance over the last decade [2,6–12].

The grinding process requires a considerable amount ofenergy per volume unit to remove material. During theprocess, this energy is transformed into heat, whichconcentrates in the cutting region. High temperatures cancause several types of thermal damage to the workpiece,such as superficial burning, microstructural modifications,and surface and subsurface heating of the piece, whichallows for superficial tempering and re-tempering of amaterial (in tempered steel machining) with the formationof non-softened martensite, generating undesirable residualtensile stresses and thus reducing the ultimate fatiguestrength of the machined component. Moreover, uncon-trolled thermal expansion and contraction of the pieceduring grinding contribute to errors in the dimension andshape of the final component, these phenomena leadingmainly to errors in circularity. The grinding rates utilizedtoday are limited by the maximum temperatures permis-sible in the grinding process. When these temperatures areexceeded, they may lead to deterioration of the part’squality. Thus, the sources giving rise to residual stress on

ED PROOF

the machined surface can be phase transformation, thermalstress due to irregular heating and cooling of the surfacelayer, and mechanical strain [13–17].This project aims to evaluate the performance of the

MQL technology compared with conventional cooling,applied in very small flow rates as an environmentallycorrect alternative to the cutting fluid utilized in cylindricalplunge grinding. The small amount of lubricant ispulverized in a compressed air flow, reducing the undesir-able effects involved in supplying lubrication and cooling.The evaluation of the MQL technique in the grindingprocess consisted of analyzing the surface integrity (rough-ness, residual stress, microstructure and microhardness).

2. Experimental procedure

The material used in these tests was tempered andannealed ABNT 4340 steel (0.4% C–1.8% Ni–0.8%Cr–0.23% Mo–0.68% Mn–0.23% Si), 60 HRC. Classifiedas tempering steel, it is employed in the manufacture ofpieces that require a good combination of mechanicalstrength and toughness.The tests were carried out with aluminum oxide (Al2O3)

grinding wheels (355.6� 25.4� 127mm—FE 38A60KV).The dressing operation was kept constant, using a multi-point dresser fliese-type that did not influence the outputvariables of the process.A series of preliminary tests were carried out to

determine the best lubricant and compressed air flow rate,as well as the best choice of the various types of lubricantsusing the MQL technology. Seven types of lubricants weresubjected to preliminary testing. The LB 1000 lubricantsupplied by the MQL equipment manufacturer presentedthe best performance; therefore, all the results reportedhere involve this type of lubricant.The equipment used to control the MQL was Accu-lube

provided by the manufacturer ITW Chemical ProductsLtd, which uses an oil supply pulse system and allows theair and lubricant flow rates to be adjusted separately.Fig. 1(a) shows the control unit of the MQL equipmentfixed to the grinding machine where the lubricant dosageand air flow rate adjustment are carried out. Fig. 1(b)shows details of the MQL equipment and its parts areenumerated in order to be of easy understanding aboutdescription and functioning.The nozzle employed in this work was designed by the

authors of this paper who belong to the Grinding Research

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Fig. 1. (a) MQL equipment (Accu-Lube) fixed to the grinder; (b) details of the MQL equipment. (1) Reservoir of up to 300ml; (2) valve; (3) manometer

and air filter (80–150 psi or 5.6–10.5 kgf/cm2); (4) frequency generator (clockwise increases frequency); (5) Pneumatic pump for individual adjustment; (6)

knob for lubricant flow rate adjustment; (7) metallic chassis; (8) holes for fixture or magnetic bases for fast fixture; (9) air inlet (From 80psi or 5.6 kgf/cm2);

(10) outlet for the nozzle.

Fig. 2. Design of the nozzle (mm) used in the MQL tests.

Fig. 3. Setup showing the location of nozzle relative to the grinding wheel

and workpiece.

L.R. da Silva et al. / International Journal of Machine Tools & Manufacture ] (]]]]) ]]]–]]] 3

UNCORRGroup of Sao Paulo State University—UNESP, School ofEngineering, Bauru Campus, Sao Paulo. The design of thenozzle allowed the compressed air speed very close to thegrinding wheel speed (30m/s). This speed is required toenable the mixture (lubricant plus compressed air) topenetrate the region of contact between the tool and theworkpiece, allowing lubrication and cooling of the process.The lubricant flow rate used here was 40ml/h. A flow ratemeter and a pressure regulator equipped with a filter wereemployed to take precision measurements of the com-pressed air flow rate, which has produced the aforemen-tioned speed. The MQL system consists of a compressor, apressure regulator, a rotameter, a doser and a spray nozzle.Fig. 2 shows the nozzle developed and utilized in the testingof the MQL technique in the grinding process. The nozzlewas placed at a distance of about 35mm from the grindingwheel–workpiece interface as shown in Fig. 3.

The main input parameters [grinding wheel speed (vs),infeed rate (vf), workpiece speed (vw), depth of cut (a) and

spark-out time (ts)] were selected based on preliminarytesting. The cutting conditions selected after testingpreliminary to the definitive tests were: vs ¼ 30m=s;vf ¼ 1mm=min; vw ¼ 20m=min (average speed); a ¼

0:1mm and ts ¼ 10 s. These parameters were kept constantthroughout the tests.A synthetic emulsion in a 5% concentration was used in

the conventional cooling condition. The maximum flowrate supplied by the pump and by the machine’s originalnozzle was 8.4 l/min.

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L.R. da Silva et al. / International Journal of Machine Tools & Manufacture ] (]]]]) ]]]–]]]4

ORRECT

The surface roughness was measured by adjusting theprofilometer to a cut-off length of 0.8mm. At the end ofeach test, the average of the surface roughness values, Ra,were measured at three different points approximately 1201equidistant from each other. Scanning electron microscopy(SEM) was used to analyze possible damage caused bythermal and mechanical forces on the material’s surface.The scanning electron microscope is a highly versatiledevice which allows different types of analyses. The mainadvantages of the SEM in relation to an optical microscopeare its resolution and focal depth. The nominal values ofresidual stress were determined based on the method ofmultiple exposition (sen2C), following the SAE J784a code.In this procedure, the normal residual stress and theresidual shear stress are evaluated by adjusting (d) versus(sen2C) curves for an elliptically shaped curve, where (d) isthe interplanar distance of the analyzed crystallographicplane and (C) is the angle of the sample’s slope. Forexperimental reasons, the analyses of (2 1 1) plane of theferrite and martensite using the modulus of elasticity valuesand Poisson’s coefficient were chosen. Cobalt radiation wasemployed to determine the residual stress, with a scanningangle (2y) varying from 471 to 631, in 0.11 steps and anexposure time of 2 s. The sample’s slope angles varied from�601 to 601, with measurements taken at 101 intervals.

3. Results and discussion

The results described below refer to the best cutting,lubrication and cooling conditions found in the cylindricalplunge grinding of tempered ABNT 4340 steel for theparameters evaluated here.

3.1. Surface roughness

It is well known that the surface finish can significantlyaffect the mechanical strength of components when theyare subjected to fatigue cycles. Fig. 4 compares the meanvalues of the Ra parameter (mm) with the Al2O3 grindingwheel under conventional cooling against those obtainedwith the MQL technique. The values were obtained afterthree stages of 30 grinding cycles, in which the depth of cutfor each cycle was 100 mm. Six Ra measurements were taken

UNC

0

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0.7

Air = 30 m/s; Lub. = 40 ml/h Cooling = 8.4 l/min

Rou

ghne

ss -

Ra

�m)

Fig. 4. Roughness after 90 cycles with the Al2O3 grinding wheel

(vs ¼ 30m=s; vf ¼ 1mm=min; ts ¼ 10 s; a ¼ 100mm).

in three different positions approximately 1201 equidistantfrom each other.The analysis of the results obtained with the conven-

tional cutting fluid application system and with the MQLtechnique indicates that the application of cutting fluid byMQL technique led to a result superior to that of theconventional system due to the more efficient penetrationof the fluid into the cutting region. The MQL technique ledto lower roughness values, probably because of the moreeffective lubrication and cooling of the abrasive grains atthe work–tool interface. Efficient lubrication allows thechips to slide more easily over the tool’s surface, resultingin a better surface finish.

ED PROOF

3.2. Residual stress

Based on a pre-analysis of the dry condition during thetests, residual stress was also measured under the drycondition in order to compare the behavior of the residualstress under the 3 grinding conditions (conventional, MQLand dry grinding). The values were obtained after three 30-cycle stages, each cycle of 100 mm.Fig. 5 shows the values of residual stress for the samples

ground by Al2O3 wheels with MQL, conventional coolingand dry grinding. To identify how the grinding processaffected the residual stress, the residual stress was alsomeasured after turning operation followed by heat treat-ment.As indicated in the bibliographic review, grinding can

lead to microstructural transformations due to hightemperatures and displacement of the austenite in relationto the carbon, which helps diffusion. This may cause tensileor compressive stresses, depending on the material that isbeing ground and on the machining conditions.According to [11], residual stresses can be caused by

three factors: influence of thermal dilation, influence ofmicrostructural transformations in the workpiece, andmechanical influence. Thermal dilations in grinding areproportional to the temperatures generated in the process.The temperature in external layers is high, gradually

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113Fig. 5. Comparative results of residual stress at a depth of approximately

10mm below the surface after 90 cycles (vs ¼ 30m=s; vf ¼ 1mm=min; a ¼

100mm and ts ¼ 10 s).

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L.R. da Silva et al. / International Journal of Machine Tools & Manufacture ] (]]]]) ]]]–]]] 5

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decreasing in the internal layers in the direction of the core.In grinding, when the source of heat is active, the externallayers dilate more than the internal ones, leading toresidual compressive stresses on the surface. When theheat source is no longer active (cooling), the external layershould contract more, which is not permitted by the lowerlayers. The mechanical influence derives from the penetra-tion of the abrasive grain into the workpiece.

Fig. 3 shows that residual stresses were produced underboth MQL and conventional conditions. Residual com-pressive stresses are considered beneficial for the mechan-ical properties of materials, increasing their fatigue strengthand the service life of components. On the other hand,residual tensile stresses are harmful regarding mechanicalstrength, corrosion and wear. These properties areimportant when using tempered ABNT 4340 steel.

The MQL technique produced higher residual compres-sive stresses than did the conventional cooling system,which is a positive aspect. Compared with dry grinding,there was a significant increase in the residual compressivestress values under both MQL and conventional conditionstested. As mentioned previously, this residual stressbehavior was expected, once the dry condition leads tohigher temperatures in the cutting region which in turn allportion of heat nearly goes into the workpiece followed bya slow cooling, and probably altering the microstructurenear the workpiece surface.

The lubricating during the grinding process acts on adecisive way with regard to surface integrity of theworkpiece. The high friction level generated along thegrinding process is of great importance to contribute to thefinal condition of the ground workpiece. The bestperformance obtained by MQL is mainly because of thelubricity of the utilized fluid, turning out the decrease of thewheel–workpiece friction coefficient and thus preservingthe grinding wheel sharpness.

Analyzing the condition of residual stress after the heattreatment, it can be verified a significant increase in thecompressive residual stress due to the grinding conditions

UNCOR

Fig. 6. Subsurface microstructures obtained after 90 cycles (vs ¼ 30m=s; vf ¼(air ¼ 30m/s and lubri. ¼ 40ml/h).

established along the tests, providing beneficial character-istics to the mechanical properties of the workpiece tested,and in turn resulting in the improvement of the surfaceintegrity.

D PROOF

3.3. Analysis of the microstructure

Figs. 6 and 7 are micrographs of sample cross-sections,they illustrate the subsurface alterations that took place inthe samples when the Al2O3 grinding wheel was used withconventional cooling, dry condition and with the use of theMQL technique. Note that the subsurface alterationsproduced by the various lubrication and cooling conditionswere minimal, without significant differences between theconditions tested. Sandpapering and polishing the samplesmanually to ensure their planeness for the desiredmagnification was not an easy task due to the material’sgreat hardness.Analyzing the microstructures, it can be noted that the

annealed ABNT 4340 steel shows martensite structure. Theformation or not of this martensitic structure, which isruled by diffusion mechanisms of carbon, is a complexprocess dependent on the temperature, heating and coolingtime imposed by the cutting fluid. It can be noticed that themicrographs have not presented significant subsurfacealterations in all conditions tested. Probably, the amountof heat and plastic deformation that entered the workpieceduring the grinding process regarding slight conditions ofgrinding was not sufficient to produce important subsur-face alterations in the material microstructure. On theother hand, the contact time of the abrasive grains and thecooling time are very short, which make no meaningfuldifference in the subsurface. Similar results in the structureanalysis were found in [18] for plunge cylindrical grindingwhen comparing the cooling by shoe nozzle (24 l/min) andMQL technique (215ml/h). Regarding the microstructure,it has not been observed in [18] any important alterationwhen two flow rates were employed.

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1131mm=min and a ¼ 100mm) 4.000� . (a) Conventional cooling, (b) MQL

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Fig. 7. Subsurface microstructures obtained after 90 cycles (vs ¼ 30m=s; vf ¼ 1mm=min and a ¼ 100mm) 4.000� . (a) Dry condition, (b) heat treatment.

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Fig. 8. Variation of microhardness as a function of depth in the

subsurface after 90 cycles (vs ¼ 30m=s; vf ¼ 1mm=min; a ¼ 100mm).

L.R. da Silva et al. / International Journal of Machine Tools & Manufacture ] (]]]]) ]]]–]]]6

CORRECT

The microstructure under dry condition shows thinnermartensitic structure in the subsurface, which is probablydue to the more effective tempering because of highertemperature and slow cooling.

All the energy spent in grinding is converted into heat inthe grinding zone, which leads to higher temperatures andpossible thermal damages in the workpiece. Severalthermal damages may occur in grinding of annealed steel,including tempering and re-hardening due to formation offragile martensite if the grinding temperature is highenough to cause the reaustenitization. These damages canbe avoided if the grinding process is carried out in such away that the maximum temperature in the grinding zone iskept below the minimum value for reaustenitization,normally near the eutectoid temperature of the carbonsteel and many alloy steels [19].

It can be noticed when analyzing the microstructures forgrinding with conventional cooling, the lack of meaningfulcircumstantial evidence of subsurface alterations in themicrostructure when the MQL technique was employed.The metallurgical alterations clearly seen for all conditionstested (MQL, conventional, dry grinding and heat treat-ment) were essentially restricted to the changes in themicrostructure. Again, the MQL technique efficiency wasalso confirmed in the microstructure analysis, reinforcingthe thesis of significant effects on cooling and lubrication,providing a positive aspect of the surface integrity of theworkpiece.

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UN3.4. Measurement of microhardness

Measurements of the microhardness were taken on thecross-sectioned samples. Fig. 8 represents the variation inmicrohardness as a function of the depth below the groundsurface using the Al2O3 grinding wheel under conventionalcooling, dry grinding and MQL. To identify the influenceof the grinding process on the microhardness, measure-ments of a sample were taken directly from the heattreatment, i.e., without grinding, as shown in the Fig. 6.

The results obtained from the microhardness measure-ments for the two lubricating and cooling conditions didnot indicate significant subsurface modifications, confirm-ing the microstructural analyses. In general, grindingoperation of steels takes place after their heat treatment.Depending on the temperature of the cutting process,annealing of the workpiece may occur during grinding,causing softening close to the finished surface. Loss ofsuperficial hardness is a complex phenomenon relating toannealing of the martensitic structure and to carbondiffusion, and is dependent on the temperature and timeinvolved in the cutting process.The author in [11] stated that, in practice, it is interesting

to combine the annealing behavior with a thermal analysisin order to predict the drop in hardness of the piece.Experimental results demonstrate that high temperaturesand long periods of exposure of the workpiece to suchtemperatures, at low velocities or with longer contactlengths of the workpiece, lead to greater losses in hardness.

4. Conclusions

An analysis of the experimental data of this study led tothe following conclusions regarding the cylindrical plungegrinding of tempered ABNT 4340 steel:

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L.R. da Silva et al. / International Journal of Machine Tools & Manufacture ] (]]]]) ]]]–]]] 7

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The MQL technique can be applied efficiently in thegrinding process, providing environmentally correct andtechnologically relevant gains.

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The Ra values were substantially reduced with the use ofthe MQL technique, probably due to excellent proper-ties of lubricity.

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No significant clogging of the grinding wheel pores wasfound with the MQL technique. �

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Residual compressive stresses were obtained under allthe conditions tested. The MQL technique led to thehighest residual stress value.

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� The use of MQL did not negatively affect the surfaceintegrity.

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No significant subsurface alterations in the microstruc-ture were detected under conventional cooling and withMQL.

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Acknowledgments

The authors are indebted to FAPESP (Brazil) for itsfinancial support of this research, and to Microquımica(Brazil) for its donation of the lubricant used in thepreliminary tests.

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[6] U. Heisel, D. Lutz, R. Wassmer, U. Walter, The minimum quantity

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