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1 Contents lists available at ScienceDiect CIRP Annals Manuactuing Technology Jounal homeage: Investigations on tuning Ti-6Al-V titanium alloy using sue-inished tool edge geomety geneated by mico-machining ocess (MMP) Anil K. Sivastava a (), Xueing Zhang b, Tim Bell c and Steve Cadigan a a TechSolve, Inc., Cincinnati, USA b Shanghai Jiao Tong Univesity, Shanghai, PRC c MicoTek Finishing, Cincinnati, USA Submitted by S. R. T. Kumaa (), Pennsylvania, USA This ae esents investigations on high seed tuning o Ti-6Al-V alloy using sue-inished cutting edge insets geneated by mico-machining ocess (MMP). In ode to bette undestand the inluence o sue-inished cutting edges and thei imact on active oce comonents, tool ace iction is analyzed. The tool-chi-wok iction coeicients ae obtained analytically using measued cutting oces unde othogonal cutting conditions and chi chaacteistics. The cutting oces and chi mohology ae edicted accuately using a two dimensional inite element model (FEM) using ABAQUS. The tuning tests conducted unde looded coolant conditions show that sue-inished cutting edge insets substantially enhance the tool lie. Keywods: Ti-6Al-V, Sue inished tool, tool ace iction. Intoduction Titanium alloys such as Ti-6Al-V have high seciic stength, high toughness, high coosion esistance, low density, bio-comatibility and ae extensively used in aeosace, biomedical, chemical and many othe industies. These alloys ae diicult to machine due to thei inheent oeties such as low themal conductivity, low modulus o elasticity and high chemical eactivity with othe mateials at elevated temeatues []. The machining o titanium (Ti-6Al-V) alloy geneates seated and cyclic chis which ae consideed to be caused by adiabatic sheaing [2-]. Liteatue suvey eveals that many exeimental and numeical investigations have been made to study the cutting eomance o this alloy with the aim o enhancing the machinability and oductivity [-8]. Some investigations have shown that machining eomance o titanium alloys is inceased by imoving cutting tool mateial/geomety and coatings [5-6]. In this ae, the imact o sue-inish eaed cutting edge o the uncoated tungsten cabide (WC/Co) insets on machining eomance is investigated o ossible imovements in machining o Ti 6Al V alloy. Also, two dimensional (2-D) inite element model (FEM) is used to edict cutting oces ecisely. In the ast, most o the numeical investigations have been made using DEFORM sotwae o two dimensional (2-D) othogonal cutting o Ti-6Al-V alloy [9-]. DEFORM is based on the modiied- Langangian igid-astic FEM, which cannot achieve the astic deomation and theeoe the esidual stesses geneated om elastic ecovey ate unloading ae not consideed duing the analysis []. Duing esent investigations, ABAQUS has been used o numeical investigations that takes cae o this asect and ovecomes DEFORM s inheent disadvantage [5-7]. To ecisely edict cutting oces and chi mohology, it is imotant to detemine the tool-chi-wok iction coeicients. Shaw [8] oosed an aoach o calculating tool-chi iction coeicient using sha tool. Howeve, cutting tools, in geneal, have a honed o chameed cutting edge and it is easonable to conside the tool-edge eect on cutting oces and tool temeatues [8]. This equies the detemination o tool-chiwok iction coeicients. In most liteatues, tool-chi-wok iction coeicients have been detemined emiically. Such as, an aveage value o iction coeicient o.7 to.9 have been ecommended in DEFORM sotwae []. Umbello [9] in his analysis set tool-wok iction coeicient as.7 while Ozel and Sima [, 2] adoted tool-wok iction coeicient o.85 o uncoated tool and.9 o coated tool. Seveal othe investigatos consideed tool-chi iction coeicients as. [, 7] and.5 [, 5, 7] based on emiical omula [8]. It is obvious that the esumed o emiical detemination o tool-chi-wok iction coeicients cannot edict the machining eomance accuately. This ae ooses a new iction model to detemine the iction coeicients o chi-tool-wok based on Shaw s othogonal cutting oce model by incooating 2-D exeimental oces and chi mohology. The tool-chi-wok iction coeicients ae obtained analytically. A FEM is

2 established to simulate Ti-6Al-V alloy high-seed dy cutting ocess using ABAQUS and it is validated by cutting oces and the chi mohology o dieent cutting aametes. 2. Sue-inishing o cutting tools using Mico Machining Pocess (MMP) The Mico Machining Pocess (MMP) is a unique method o sue inishing. What makes this technology tuly unique is its ability to selectively emove seciic comonents o oughness while leaving othes intact. The commecially available uncoated/coated insets have cetain amount o oughness and cutting edge chaacteistics which can be consideed as a collection o equency anges. The tem equency is used because at a mico level the eaks and valleys o the suace can be eesented as a collection o dieent wavelengths o vaying amitudes. These equencies can be chaacteized into ou dieent anges, with each successively highe ange sueimosed on the one below it. Fig.. Schematic o a cutting tool edge The lowest equency ange is the exected "Fom" o the cutting edge. Layeed on to o the Fom is the "Waviness" and layeed on to o the Waviness is the "Pimay Mico Roughness". Finally, layeed on to o the Pimay Mico Roughness is the "Seconday Mico Roughness", which esults om the oughness on the suace o the cutting tool that was imated on it duing its manuactuing ocess and is tanseed to the at being cut. Figue 2 shows cutting tool edge o a commecially available inset and sue-inished cutting edge. In ode to descibe metal mateial behaviou subjected to a lage stain, high stain ate and temeatue-deendence viscoasticity in dy cutting ocess, Ti-6Al-V constitutive model adots the ollowing Johnson-Cook (J-C) model [9] n & T T m σ = ( A + B )[ + C ln( )][ ( ) ] () & Tmelt T Wheeσ is the equivalent stess;, the equivalent astic stain; &, the astic stain ate; &, the eeence stain ate; Τ melt, the melting temeatue; Τ is tansition temeatue deined as oom temeatue 25 C; A is the initial yield stess, B is the hadening modulus, C is the stain ate deendent coeicient, n is the wokhadening exonent, m is the themal sotening coeicient, A, B, C, n and m ae mateial constants. Johnson-Cook ailue model [2] is emoyed to simulate the chi seaation behaviou and the chi cack initiation o gowth. Johnson-Cook ailue model is based on the value o the equivalent astic stain at element integation oints. Damage aamete D is deined as ( ) D = (2) Whee is the incement o the equivalent astic stain which is udated at evey analysis incement; is the equivalent stain at ailue and is exessed as: = [ d Whee + d ex( d 2 )][ + d q & ln( )][ + d & 5 T T ( T T deends on the non-dimensional equivalent astic stain ate o & / &, the atio o hydostatic essue to Mises equivalent stess ( d ~ d5 melt )] () / q, temeatue and the damage constants ). Failue is assumed to occu when the damage aamete D exceeds. Johnson-Cook model and Johnson-Cook damage constants wee obtained by Sit Hokinson Pessue Ba (SHPB) test eomed unde vaious stain ates and temeatues. The mateial aametes o Ti-6Al-V have been eoted in Table [7, 8]. Table Johnson-Cook model and its ailue model s aametes o Ti-6Al-V [7-8] A (MPa) B (MPa) n C m Fig. 2. Cutting Tool Edge (X5 Magniication). Finite Element Model (FEM) The FEM includes wok mateial constitutive model, chi seaation citeion, chi-tool-wok iction condition, mesh and bounday conditions that ae descibed below.. Mateial model o titanium alloy Ti-6Al-V Howeve, Johnson-Cook model exeimentally measued by SHPB had cetain limitation. The stain ates o the exeiment o Ti-6Al-V constitutive model eached S [8], which wee a less than the stain ates duing the actual metal cutting 5 ocesses. The tue stain ates eached ~ S [9], even highe. The Johnson-Cook constitutive model by SHPB is unable to descibe the mechanical oeties o high seed cutting. The detemination o Johnson-Cook constitutive model is the key technique o the FEM simulation o machining oeations. Modiied Johnson-Cook constitutive model is shown in Table 2, which is validated by comaing FEM esults with exeimental measuement. 2

3 Table 2 Modiied Johnson-Cook model o Ti-6Al-V A (MPa) B (MPa) n C m d d d 2 d d Mateial Poeties o Wokiece and Tool Mateials Mateial Poeties o Ti-6Al-V and tool ae shown in the ollowing Table. Table The Poeties o Ti-6Al-V and Tool [, 5] Poeties Ti-6Al-V Tool Exansion( µm m C ) 9 6 T = T +.9 ( ). 7. Conductivity( Wm c ) λ ( T ) = 7.9e 59.. T Young s modulus( MPa ) E( T) =.7 T+75 T 65 J / Kg. Heat caacity( C ( ) T ) C T = e 5 V Density( g / cm )..5 Passion s atio..25. Tool-chi inteace iction model Modiied Coulomb iction model is adoted to catue the chi-tool and wok-tool inteace iction attens, which contains two distinct egions: sticking egion and sliding egion. The nomal stess magnitude acting on the contact oint detemines whethe the contact aea is in sticking o sliding state. The iction oce o sliding egion ( µσ < τ ) is τ = µσ () The iction oce o sticking egion (. µσ τ ) is τ = τ (5) Whee µ is the coeicient o iction along the sliding zone, τ is the mateial shea yield stess. The commonly acceted estimation o τ is exessed as τ = σ (6) y Whee σ is the uniaxial yield low stess o the wok mateial. y Howeve, σ is estimated by emiical value [2], which is assumed 5 MPa. y. Mesh and bounday condition o othogonal cutting FEM The othogonal (2-D) machining model o Ti-6Al-V alloy geometically consists o ou ats [22]: () wok, (2) the joint laye chaacteistic o a naow band and mateial ailue candidate element, () the chi laye, and () the cutting tool as shown in Figue. The bottom suace o wok is estained in the eed diection, the let and ight ends ae estained in the cutting diection. The tool is egaded as a igid body moving let along the cutting diection. Fig.. FEM o 2D othogonal dy cutting Ti-6Al-V.5 Detemination o tool-chi-wok iction coeicients by measued cutting oces All the tests have been conducted using a Hadinge Coba CNC Tuning Cente which is inteaced with the PC and a theedimensional (-D) Kistle Dynamomete o on-line oce data acquisition. The exeimental set-u is shown in Fig.. Fig.. Exeimental set-u o Ti-6Al-V tuning test The conditions o the othogonal tuning tests ae ovided in the ollowing Table. Table Machining aametes o Ti-6Al-V othogonal tuning test Wok Mateial Tube, 5.8mm in diamete and.75 mm wall thickness Tool Holde Tye CTGPL 6 Cutting Tool (Inset TPG 2) Uncoated Cabide: (Gade K) Coated Cabide: (Gade KC5) Sue-inished Edge: (Gade K) Rake Angles and 5 Relie Angle and 6 Cutting Seeds m/min Feed Rates.76 mm/ev,.6 mm/ev and.27mm/ev Cut Length/Test.6 mm Cutting Fluid No coolant (Dy) Duing the othogonal (2-D) tuning tests, the measued oces include (i) cutting oce and (ii) thust oce, whose diection and numeical values ae shown in Figue 5 and Table 5, esectively.

4 Wokiece Rotation Feed motion Wokiece Ti-6Al-V Tool holde Fs F = (8) cos( φ + β α) Whee α is the ake angle, β is iction angle at the tool-chi inteace. F c τ h b cos( β α ) D D = sin φ cos( φ + β α ) (9) Cutting tool F t τ h b sin( β α ) D D = sin φ cos( φ + β α ) () Cutting oce F c Thust oce F t t So the coeicient o iction on the tool ace µ is exessed in Eq. (). Fig.5. Schematic o othogonal tuning Table 5 Exeimental cutting oce values in othogonal (2-D) tuning o Ti-6Al-V Feed 5 Rates Cutting Thust Cutting Thust (mm/ev) Foce (N) Foce (N) Foce (N) Foce (N) The tool-chi-wok iction coeicients ae obtained by measued cutting oces, thust oces and chi chaacteistics. Based on Mechant-tye model [8], othogonal cutting oce model is established without consideing the tool wea because the cutting length was too shot to oduce tool wea. Foces acting on the chi and tool ae analyzed in Figue 6, in which F c and F t ae coesonding to the measued cutting oce and thust oce. The F is the esult oce o the comonent F in the cutting c diection and the comonent F is F along the eed diection. The t also the esultant o iction oce F and nomal oce N on ake ace. The F is the esultant o nomal oce F acting FN and iction oce F acting on tool lank; the total oce F ae the esultant o F and F shown in Fig. 6(a). Foce acting on chi is analysed in Figue 6(b). The esultant oce F o F and F oces acting on ake ace is also N the esultant oce o nomal oce F and shea oce F sn s acting on the shea ane. F is exessed as Eq. (7). F s h b D D = τ (7) sin φ Whee h is undeomed chi thickness, b is the width o cut, D D φ is shea angle,τ is shea stess on the shea ane. Owing to the small vaiation o undeomed chi thickness and ake angle, τ is esumed to be constant and set τ based on the modiied Coulomb iction model. µ = tan β () Foce acting on tool is shown in Figue 6(a), the esolution o cutting oce and thust oce in the cutting diection and eed diection ae detemined by Eq. (2) and Eq. (). F = F + F = F + F (2) c c c c F = F + F = F + F t t t t N () Theeoe, the coeicient iction on the lank ace is calculated by Eq. (). F F F µ = = F F F c c N t t (a)foce acting on tool (b)foce acting on chi Fig.6. Mechant model with tool edge ()

5 Table 6 Tool-Chi-Wok iction coeicients Vaiable tool-chi iction coeicient tool-wok iction coeicient Feed ate (mm/ev) The chi mohology o Ti-6Al-V is obseved to be sawtooth as shown in Figue 7. eak and the bottom, h min h is the distance between chi is that between chi valley and eak. The distance between C and C2 is the chi-itch, and the aveage value o which is P C. The mean distance between C and D is P. Theeoe, the cutting atio γ is exessed by the distance P C and P: γ = (5) P The measued cutting atio γ is aoximately.8 when the ake angle is and the eed ate is.27mm/ev. By combining the omulas (6) and (7), shea angle obtained is aoximately and the iction angle o tool-chi inteace is. P C cos α φ = tan sin α ( ) (6) π φ = ( β α) (7) 2 The tool-chi-wok iction coeicients ae calculated by combining the equations (7) to (). Though the same method, tool-chi-wok iction coeicients o othe cutting condition ae calculated in Table 6. Fo the same ake angle, the shea angle and tool-wok iction coeicients incease with the incease in eed ate, othewise ae tool-chi iction coeicients. Howeve, the toolwok iction coeicients ae.~6.66 times lage than that o tool-chi iction coeicients. Fo the same eed-ate, the toolchi iction coeicients incease with the incease in ake angle. The nomal stesses decease with incease in ake angle and the atio o mateial yield stength to nomal stess inceases. Table 7 Foce validation when ake angle Feed Rates (mm/ev) Measued Value Simulated value Eo (%) Cutting oce (N) Thust oce (N) Resultant oce (N) Table 8 Foce validation when ake angle 5 Feed Rates (mm/ev) Measued Value Simulated value Eo (%) Cutting oce (N) Thust oce (N) Resultant oce (N) The esults show that exeimental cutting oces have close ageement with the edicted cutting oces (eo ~ 5.7%). The eo o thust oces edicted is less than 7%, which can be uthe imoved with the accuacy o measued shea angle. The ediction eo o esultant oce is ~.6%. The ageements between the edicted and measued oces validate the FEM o othogonal (2-D) dy cutting o Ti-6Al-V alloy accuately egadless o machining conditions..2 Pedicted chi mohology and validation The edicted and measued saw-tooth chi adoting eed ate o.27 mm/ev, ake angle o, and the cutting seed o m/min ae shown in Figue 8. The aveage distance o the chi itch is.29 mm, and distance between chi eak to valley is.96 mm. The coesonding edicted values ae.56 mm and. mm, esectively. The ediction eos ae 5.2% and.% esectively. These esults veiy the FEM in tems o chi mohology.. FEM Validation Fig.7. Schematic diagam o saw-tooth mohology. Pedicted oce and validation The measued and edicted cutting oces using FEM analysis ae comaed in Tables 7 and 8 o the cutting seed o m/min and two ( o, 5 o ) ake angles. (a) simulation (b) exeiment Fig.8. Simulated chi mohology comaed with exeiment 5

6 An oblique tuning test was conducted to see the eect o sue-inishing the cutting edge on tool-lie. Beoe conducting the test, the aveage suace oughness values (R a ) o uncoated (K Gade), coated (KC5 Gade) and sue-inished cutting edge (K Gade) wee measued which wee.97,. and.8 µm, esectively, at a cut-o length o.25 mm. The esults o tool wea ae shown in Fig. 9. The esults clealy show that tool-lie substantially inceases when sue-inished cutting edge insets ae used. It seems that sue-inishing the cutting edge ovides bette contact and distibution o cutting essue and distibution o heat at wok-tool-chi contact zone, thus, enhancing the tool-lie. Fig.9. Maximum tool wea v/s machining time using uncoated, coated, and sue-inished cutting edge tool (cutting seed: m/min, eed-ate:.75 mm/ev, deth o cut:. mm, coolant: (5% vol.) Tim Sol) This conims that sue-inishing the cutting edge o the insets using MMP have a ositive imact on the tool lie and eliability o cutting tools when aied coectly. 5. Conclusions Tool-chi-wok iction coeicients ae obtained analytically using othogonal (2-D) cutting exeimental oces and chi chaacteistics. A FE model o high seed dy machining o Ti-6Al-V alloy is established using ABAQUS, which is validated in tems o edicted oces and the chi mohology. The seciic conclusions include: ) The tool-wok iction coeicients ae. to 6.66 times lage than that o tool-chi iction coeicients. The toolchi iction coeicients decease with eed-ates, and othewise ae the tool-wok iction coeicients. 2) The edicted cutting oces, thust oces and esultant oces by the FE model agee well with the exeimentally measued oces. The ediction eos ae less than 6%, 7% and % esectively. The eos ae lowe than 5.2% and.%, esectively in tems o the edicted aveage value o chi itch and between the chi valley and eak. ) Sue-inishing the cutting edge o the insets using MMP substantially enhances the tool lie duing the high seed machining o Ti-6Al-V titanium alloy. 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