Analysis of surface integrity for minimum quantity lubricant—MQL in grinding

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  Analysis of surface integrity for minimum quantity lubricant—MQL in grinding
     U   N  C  O   R   R   E  C   T   E   D    P   R  O  O   F International Journal of Machine Tools & Manufacture  ]  ( ]]]] )  ]]]  –  ]]] Analysis of surface integrity for minimum quantity lubricant—MQL ingrinding Leonardo Roberto da Silva a,  , Eduardo Carlos Bianchi b , Ronaldo Yoshinobu Fusse b ,Rodrigo Eduardo Catai c , Thiago Valle Franc - a b , Paulo Roberto Aguiar d a Department of Mechanical, CEFET, Belo Horizonte, Av. Amazonas, 5253, Jd. Nova Suı´ c  - a, CEP: 30480-000, MG, Brazil  b Department of Mechanical Engineering, UNESP, Av. Eng. Luiz E. C. Coube, s/n, Bauru, SP, Brazil  c UNESP, Guaratingueta´ , SP, Brazil  d Department of Electrical Engineering, UNESP, Bauru, SP, Brazil  Received 30 June 2005; received in revised form 28 February 2006; accepted 6 March 2006 Abstract The quality of machined components is currently of high interest, for the market demands mechanical components of increasingly highperformance, not only from the standpoint of functionality but also from that of safety. Components produced through operationsinvolving the removal of material display surface irregularities resulting not only from the action of the tool itself, but also from otherfactors that contribute to their superficial texture. This texture can exert a decisive influence on the application and performance of themachined component. This article analyzes the behavior of the minimum quantity lubricant (MQL) technique and compares it with theconventional cooling method. To this end, an optimized fluid application method was devised using a specially designed nozzle, by theauthors, through which a minimum amount of oil is sprayed in a compressed air flow, thus meeting environmental requirements. Thispaper, therefore, explores and discusses the concept of the MQL in the grinding process. The performance of the MQL technique in thegrinding process was evaluated based on an analysis of the surface integrity (roughness, residual stress, microstructure andmicrohardness). 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 1. 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 of modern 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 alternativeproduction processes, creating technologies that minimizeor avoid the production of environmentally aggressiveresidues.Emulsion-based cooling fluids for machining are stillwidely 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 fluid justify the demand for an alternative to the grindingprocess with fluid. In the last decade, however, the goal of researches 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   ARTICLE IN PRESS 1357911131517192123252729313335373941434547495153555759616365676971737577 3B2v8 : 06a = w ð Dec52003 Þ : 51c þ model  MTM : 1921  Prod : Type:FTPpp : 1 2 7 ð col : fig : :1  5 ; 8 Þ ED:VijayBNPAGN:anu SCAN: 0890-6955/$-see front matter r 2006 Elsevier Ltd. All rights reserved.doi:10.1016/j.ijmachtools.2006.03.015  Corresponding author. Tel.: +551431036119. E-mail addresses: (L.R. da Silva), (E.C. Bianchi), (R.Y. Fusse), rcatai@- (R.E. Catai), (T.V. Franc - a), (P.R. Aguiar).     U   N  C  O   R   R   E  C   T   E   D    P   R  O  O   F 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 o 80ml/h). The minimization of cutting fluid has taken onincreasing relevance over the last decade [2,6– 12]. The grinding process requires a considerable amount of energy 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 onthe 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 theMQL 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 of pieces that require a good combination of mechanicalstrength and toughness.The tests were carried out with aluminum oxide (Al 2 O 3 )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 todetermine 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-lubeprovided 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 theauthors of this paper who belong to the Grinding Research ARTICLE IN PRESS MTM : 1921 13579111315171921232527293133353739414345474951535557596163656769717375777981838587899193959799101103105107109111113 Nomenclaturev s  grinding wheel speed (m/s) v f   infeed rate (mm/min) v w  workpiece speed (m/s) a  depth of cut ( m m) t s  spark-out time (s) R a  surface roughness ( m m)Al 2 O 3  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     U   N  C  O   R   R   E  C   T   E   D    P   R  O  O   F Group of Sao Paulo State University—UNESP, School of Engineering, 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 ( v s ),infeed rate ( v f  ), workpiece speed ( v w ), depth of cut ( a ) andspark-out time ( t s )] were selected based on preliminarytesting. The cutting conditions selected after testingpreliminary to the definitive tests were:  v s  ¼ 30m = s; v f   ¼ 1mm = min;  v w  ¼ 20m = min (average speed);  a ¼ 0 : 1mm and  t s  ¼ 10s. These parameters were kept constantthroughout the tests.A synthetic emulsion in a 5% concentration was used inthe conventional cooling condition. The maximum flowrate supplied by the pump and by the machine’s srcinalnozzle was 8.4l/min. ARTICLE IN PRESS MTM : 1921 13579111315171921232527293133353739414345474951535557596163656769717375777981838587899193959799101103105107109111113 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) manometerand air filter (80–150psi or 5.6–10.5kgf/cm 2 ); (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 5.6kgf/cm 2 );(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 wheeland workpiece. L.R. da Silva et al. / International Journal of Machine Tools & Manufacture  ]  ( ]]]] )  ]]]  –  ]]]  3     U   N  C  O   R   R   E  C   T   E   D    P   R  O  O   F The surface roughness was measured by adjusting theprofilometer to a cut-off length of 0.8mm. At the end of each test, the average of the surface roughness values,  R a ,were measured at three different points approximately 120 1 equidistant 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 of residual stress were determined based on the method of multiple exposition (sen 2 C ), following the SAE J784a code.In this procedure, the normal residual stress and theresidual shear stress are evaluated by adjusting ( d  ) versus(sen 2 C ) 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 (211) 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 (2 y ) varying from 47 1  to 63 1 , in 0.1 1  steps and anexposure time of 2s. The sample’s slope angles varied from  60 1  to 60 1 , with measurements taken at 10 1  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  R a  parameter ( m m) with the Al 2 O 3  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 m m. Six  R a  measurements were takenin three different positions approximately 120 1  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. 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 m m.Fig. 5 shows the values of residual stress for the samplesground by Al 2 O 3  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 canlead 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 bythree factors: influence of thermal dilation, influence of microstructural 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 ARTICLE IN PRESS MTM : 1921 13579111315171921232527293133353739414345474951535557596163656769717375777981838587899193959799101103105107109111113 = 30 m/s; Lub. = 40 ml/hCooling = 8.4 l/min    R  o  u  g   h  n  e  s  s  -   R  a         m   ) Fig. 4. Roughness after 90 cycles with the Al 2 O 3  grinding wheel( v s  ¼ 30m = s;  v f   ¼ 1mm = min;  t s  ¼ 10s;  a ¼ 100 m m).Fig. 5. Comparative results of residual stress at a depth of approximately10 m m below the surface after 90 cycles ( v s  ¼ 30m = s;  v f   ¼ 1mm = min;  a ¼ 100 m m and  t s  ¼ 10s). L.R. da Silva et al. / International Journal of Machine Tools & Manufacture  ]  ( ]]]] )  ]]]  –  ]]] 4     U   N  C  O   R   R   E  C   T   E   D    P   R  O  O   F 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 conditionsestablished 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. 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 Al 2 O 3  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 theannealed 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 of grinding 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 (24l/min) andMQL technique (215ml/h). Regarding the microstructure,it has not been observed in [18] any important alterationwhen two flow rates were employed. ARTICLE IN PRESS MTM : 1921 13579111315171921232527293133353739414345474951535557596163656769717375777981838587899193959799101103105107109111113 Fig. 6. Subsurface microstructures obtained after 90 cycles ( v s  ¼ 30m = s;  v f   ¼ 1mm = min and  a ¼ 100 m m) 4.000  . (a) Conventional cooling, (b) MQL(air ¼ 30m/s and lubri. ¼ 40ml/h). L.R. da Silva et al. / International Journal of Machine Tools & Manufacture  ]  ( ]]]] )  ]]]  –  ]]]  5
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