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The effect of machining on surface integrity of titanium alloy Ti–6% Al–4% V

The effect of machining on surface integrity of titanium alloy Ti–6% Al–4% V
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  Journal of Materials Processing Technology 166 (2005) 188–192 The effect of machining on surface integrity of titaniumalloy Ti–6% Al–4% V C.H. Che-Haron a , ∗ , A. Jawaid b a  Department of Mechanical and Materials Engineering, Universiti Kebangsaan Malaysia,43600 UKM Bangi, Selangor, Malaysia b School of Engineering, Coventry University, Coventry CV1 5FB, UK  Received 28 January 2002; received in revised form 6 July 2004; accepted 13 August 2004 Abstract This paper gives the investigation on surface integrity of rough machining of titanium alloy Ti–6% Al–4% V with uncoated carbidecutting tools. The experiments were carried out under dry cutting conditions. The cutting speeds selected in the experiment were 100, 75,60 and 45mmin − 1 . The depth of cut was kept constant at 2.0mm. The feed rates used in the experiment were 0.35 and 0.25mmrev − 1 . Twotypes of insert were used in the experiments. For a range of cutting speeds, feeds, and depths of cut, measurements of surface roughness of machined surface, microhardness and work hardening backed up with scanning electron microscope were taken. The surface of titanium alloyis easily damaged during machining operations due to their poor machinability. The machined surface experienced microstructure alterationand increment in microhardness on the top white layer ( ≤ 10  m) of the machined surface. Severe microstructure alteration was observedwhen machining with the dull tool. In addition, surface roughness values obtained were within the limit (<6  m) stipulated by ISO for roughmachining.© 2004 Elsevier B.V. All rights reserved. Keywords:  Surface integrity; Titanium alloy; Carbide tool 1. Introduction Titaniumalloysareextremelydifficulttomachinemateri-als. The machinability of titanium and its alloys is generallyconsidered to be poor owing to several inherent properties of materials.Titaniumandtitaniumalloyshavelowthermalcon-ductivityandhighchemicalreactivitywithmanycuttingtoolmaterials. Its low thermal conductivity increases the temper-atureatthecuttingedgeofthetool.Hence,onmachining,thecutting tools wear off very rapidly due to high cutting tem-perature and strong adhesion between tool and workpiecematerial. Additionally, the low modulus of elasticity of ti-tanium alloys and its high strength at elevated temperaturefurther impair its machinability.Machining of titanium alloys at higher cutting speed willcause rapid chipping at the cutting edge which leads to catas- ∗ Corresponding author. Fax: +60 3 8259 659.  E-mail address:  chase@eng.ukm.my (C.H. Che-Haron). trophic failure of the inserts [1]. A higher cutting speed also results in rapid cratering and/or plastic deformation of thecutting edge. This is due to the temperature generated whichtends to be concentrated at the cutting edge closer to the noseof the inserts. The heat affected zone is very small whencutting titanium alloys. The smaller heat affected area pro-duced is as a result of the shorter chip/tool contact length. Itis mainly for this reason that the cutting speeds are limited toabout 45mmin − 1 when using straight grade cemented car-bides (WC–Co) [2]. The rapid tool failure and chipping atthe cutting edge has resulted in poor surface finish of the ma-chined surface. It has caused not only higher surface rough-ness values but also higher microhardness values and severemicrostructure alteration.Titaniumalloysaregenerallyusedforacomponent,whichrequires the greatest reliability, and therefore the surface in-tegrity must be maintained. According to Field and Kahles[3], when machining any component it is essential to satisfysurface integrity requirements. However, during machining 0924-0136/$ – see front matter © 2004 Elsevier B.V. All rights reserved.doi:10.1016/j.jmatprotec.2004.08.012  C.H. Che-Haron, A. Jawaid / Journal of Materials Processing Technology 166 (2005) 188–192  189Table 1Nominal chemical composition of the titanium alloysWork material Chemical composition (wt.%)V Al N O H C FeTi–6% Al–4% V 4 6 0.05 0.2 0.0125 0.1 0.3 and grinding operations, the surface of titanium alloys is eas-ily damaged because of their poor machinability. As far asthe surface metallurgy of the machined component is con-cerned, the heat generated during cutting is a main sourceof damage, especially in the grinding process. Possible sur-face and subsurface alterations include: plastic deformation,microcracking, phase transformations and residual stress ef-fects. Several studies on surface integrity parameters havebeen carried out [4–8]. When machining titanium alloys in anabusivemanneranoverheatedwhitelayercanbeproducedwhich results in a layer being softer or harder than the basematerials [3].Theaimsofthisworkweretoinvestigatesurfaceintegrityeffects when machining titanium alloy Ti–6% Al–4% V. Thepaper will explain various factors and parameters involvedwhen machining titanium alloys with carbide tools. 2. Experimental procedure 2.1. Workpiece materials Theworkpiecematerialsusedinalltheexperimentswasabarofanalpha-betatitaniumalloyTi–6%Al–4%V.Thenom-inal compositions of the alloys (in wt.%) are given in Table 1[9]. The workpiece had a microstructure, which consistedof elongated alpha phase surrounded by fine, dark etching of betamatrix.TitaniumalloyTi–6%Al–4%Visawidelyusedtitanium alloy and offers high strength, depth hardenabilityand elevated temperature properties up to 400 ◦ C. The me-chanical properties of tested material are shown in Table 2[8]. 2.2. Cutting tool materials Two types of carbide inserts of ISO designation CNMG120408wereusedforthemachiningexperiments.Thecuttingtoolsusedwerestraighttungstencarbidetools.Thetoolswerethrow-awaytypeofrhombicshapewithchipbreakerandwereuncoated. Both of the inserts consisted of 94wt.% tungsten Table 2Mechanical properties of tested materialWork material Ti-64Ultimate tensile strength (MPa) 827Modulus of elasticity ( × 10 6 MPa) 11 . 3Hardness (HBS/10mm/3000kg) 250–300Table 3Properties of the cutting tools usedInsert grade (ISO) Hardness(HNV)Density(gmcc − 1 )Grainsize(  m)CNMG 120408-883-MR4 1760 14.95 1.0CNMG 120408-890-MR3 1753 14.92 0.68 carbide with 6wt.% of cobalt as binder. The properties of thecutting tools used are shown in Table 3. 2.3. Machining tests All the machining experiments were carried out on aCincinnati Milacron CNC lathe, Cinturn 10 CC, which wascontrolled by an Achramatic 850 controller. The CNC lathehas a continuously variable spindle speed. This type of latheis particularly useful when it is required to machine bars of different diameters at the same cutting speed. Throughoutexperiments, the depth of cut was kept constant at 2.0mm,and the feed rates were set at 0.25 and 0.35mmrev − 1 . Thesurface speed employed during the machining tests was 100,75, 60, and 45mmin − 1 . The machining experiments werecarried out in a dry cutting condition. The cutting conditionsused are given in Table 4. 3. Results and discussions The results and discussions are focused on the workpiecesurfaceintegrityaspectsontheroughingoperationwhenma-chining titanium alloy Ti–6% Al–4% V. The results for toollifeandtoolwearmechanismsforthesamecuttingconditionswere explained in detail in previous paper [10]. 3.1. Surface finish and surface integrity3.1.1. Surface finish Typical surface roughness values recorded when machin-ing titanium alloy Ti-64 with 883 inserts at a feed rateof 0.35mmrev − 1 under dry cutting conditions is shownin Fig. 1. Slightly higher surface roughness values wererecorded at a lower cutting speed. However, as the cut-ting speed increased, the roughness value decreased. High-est surface roughness values recorded for 883-MR4 insert Table 4Cutting conditions for the experimental work Tool tested 883-MR4 and 890-MR3Cutting speed  V   (mmin − 1 ) 100, 75, 60, 45Depth of cut (mm) 2Feed rate (mmrev − 1 ) 0.25 and 0.35Tool geometry Approach angle: 95 ◦ ; siderake angle: − 6 ◦ ; back rakeangle: − 6 ◦ ; end relief angle:6 ◦ ; side relief angle: 6 ◦  190  C.H. Che-Haron, A. Jawaid / Journal of Materials Processing Technology 166 (2005) 188–192 Fig. 1. Typical surface roughness when machining with 883 inserts at feedrate of 0.35mmrev − 1 . was 3.28  m and it was recorded at the cutting speed of 60mmin − 1 .Surfacefinishtendstobecomesmoothertowardtheendoftoollife.Thisisprobablyduetodeformationontheflank face or adherence of the workpiece material at the toolnose. Increasing the cutting speed led to higher roughnessvalues. However, the roughness values recorded were unsta-ble during the intermediate cutting process. As the 883-MR4toolsworeandapproachedtheendoftheirlife,theroughnessvalues recorded increased significantly, especially at cuttingspeeds of 45mmin − 1 . The dramatic increase in the surfaceroughness value at cutting speed of 45mmin − 1 was proba-bly caused by rapid tool wear at the cutting edge closer to thenose and also fracture at the nose.The surface finish generated when machining titanium al-loy Ti-64 with 890-MR3 tools under dry cutting conditionsat feed rate of 0.25mmrev − 1 is affected by the cutting speedas shown in Fig. 2. The surface roughness values recordedfor cutting speeds of 100 and 75mmin − 1 tend to increaseas the tools approached the end of their life. Highest surfaceroughness value recorded was 5.0  m and it was recordedat the end of tool life for a cutting speed of 100mmin − 1 .The surface roughness value recorded for cutting speeds of 45 and 60mmin − 1 were higher at the initial cutting processand reduced slightly as the tools approached the end of theirlife.However,furthercuttinghasshownthatthesurfacefinishgeneratedhasimprovedgraduallybeforeitincreasedslightlyas the tool failed. This is probably due to adhered materialcovering the cutting edge. Fig. 2. Typical surface roughness when machining with 890 inserts at feedrate of 0.25mmrev − 1 .Fig. 3. Microhardness value beneath the machined surface when machiningwith 883 insert. 3.1.2. Microhardness tests Work hardening of the deformed layer beneath the ma-chinedsurfaceupto0.01mmcausedhigherhardnessthantheaveragehardnessofthebasematerial.However,thehardnessofthesubsurfaceat0.02mmbelowthemachinedsurfacewasbelow the average hardness recorded for the base material.The softening effect of the material at this level was prob-ably due to over aging of titanium alloy as a result of veryhigh cutting temperature produced at the local surface. Thelow thermal conductivity of titanium alloy also caused thetemperature below the machined surface to be retained. Thehardness values at 0.07mm beneath the machined surfaceincreased drastically under all cutting conditions. Curves inFig.3suggestthathardeninghasoccurred0.07mmbelowthemachined surface. The wear on the cutting edge affects themicrostructure, the greatest surface hardening was found totake place when machining was carried out with worn tools.Further machining of the titanium alloy with the nearlyworn tools tends to increase the hardening rate of the sur-face layer. Fig. 3 shows that higher values of hardness wererecorded at the higher cutting speed for the same feed rate.Curves in Fig. 3 also suggest that minimal increment inhardness values were recorded when the feed rate was in-creased from 0.25 to 0.35mmrev − 1 at the initial cuttingstage. Significant increment in the microhardness values wasobservedwhencomparingbetweentheinitialcutandthefinalcut. When prolonged machining was carried out with higherflank wear, the hardness of the disturbed layer of the ma-chined surface increased significantly. The highest hardnessrecorded was 391HV when machining at a cutting speed of 100mmin − 1 , and feed rate of 0.35mmrev − 1 after the 883-MR4toolhasfailed.Thehighesthardnessvaluewasrecordedat 0.005mm beneath the machined surface, where the mi-crostructures were heavily deformed. The increment in thehardness value was probably due to the work-hardening ef-fect. However, when the microstructure was less disturbed,theincrementinthehardnesswassmall.Thehardnessvaluesapproached the hardness of the base material as the depthbeneath the machined surface increased. At 0.32mm be-neath the machined surface, the difference in hardness was  C.H. Che-Haron, A. Jawaid / Journal of Materials Processing Technology 166 (2005) 188–192  191Fig. 4. Microstructure of machined surface after 10s of cutting at100mmin − 1 with 883 insert. very small, which is less than 3% at the initial cutting time.However, when machining with worn tools, the hardness ap-proached the hardness of the base material only 0.42mmbeneath the machined surface. Similar behaviour was alsoobserved for 890-MR3 insert. 3.1.3. Metallurgical alterations Figs. 4 and 5 show the microstructures of the machinedsurface produced when machining with 883-MR4 tools un-der dry cutting conditions. It was found that when machiningunder dry conditions, a thin layer of disturbed or plasticallydeformed layer was formed immediately underneath the ma-chined surface. Fig. 4 shows the smooth surface with lessdisturbed layer of the machined surface observed at the earlystage (10s) of cutting for the cutting speed of 100mmin − 1 and feed rate of 0.25mmrev − 1 .Prolongedmachiningwithnearlyworntoolsproducedse-vere plastic deformation and a thicker disturbed layer on themachinedsurfaceasshowninFig.5.Themicrostructurewasobserved after machining at a cutting speed of 100mmin − 1 , Fig. 5. Microstructure of machined surface after 2min of cutting at100mmin − 1 with 883 insert.Fig. 6. Microstructure of machined surface after 10s of cutting at45mmin − 1 with 890 insert. and feed rate of 0.25mmrev − 1 for 2min, where the toolreached the end of its life.The microstructures of the machined surface producedwhen machining with 890-MR3 tools under dry cutting con-ditions are shown in Figs. 6 and 7. It was found that whenmachining under dry conditions, a very thin layer of dis-turbedorplasticallydeformedlayerwasformedimmediatelyunderneath the machined surface. Fig. 6 shows the smoothsurface with less disturbed layer of plastic flow after 10s of cutting for a cutting speed of 45mmin − 1 and feed rate of 0.35mmrev − 1 . Machining with nearly worn or worn toolsled to the generation of irregular surfaces, which consists of tearing and plastically deformed surfaces.Prolonged machining with nearly worn tools also pro-duced severe plastic deformation and thicker disturbed layeron the machined surface as shown in Fig. 7. It was ob- served that after machining for 12min at a cutting speed of 45mmin − 1 and feed rate of 0.35mmrev − 1 the tool reachedits rejection criterion. Fig. 7. Microstructure of machined surface after 12min of cutting at45mmin − 1 with 890 insert.  192  C.H. Che-Haron, A. Jawaid / Journal of Materials Processing Technology 166 (2005) 188–192 4. Conclusions The following conclusions are based on the results forturning tests with straight grade cemented carbide tools ontitanium alloy Ti-64 (Ti–6% Al–4% V):1. Straight grade cemented carbides are suitable for use inmachiningtitaniumalloy64.Thewearresistanceandcut-ting edge strength of insert CNMG 120408-883 are supe-rior to insert CNMG 120408-890 (finer grain size).2. Severe tearing and plastic deformation of the machinedsurface were observed when machining titanium alloy Ti-64,especiallyafterprolongedmachiningunderdrycuttingconditions. At the initial stages of cutting the plastic flowofmicrostructurewasnotdetected.However,attheendof cutting (when the tool failed) severe plastic flow, tearingand deformation of the microstructure was detected. Thiscausedtheformationofawhitelayerofhardenedmaterialontopofthemachinedsurface,thethicknessofwhichwasless than 0.01mm.3. The top layer of the machined surface experience work hardening process, hence the hardness is higher than theaverage hardness of the workpiece materials. However,the material beneath the top layer is softer as a result of over-aging of the materials. References [1] Z. Wang, PhD Thesis, South Bank University, UK, 1997.[2] R. Komanduri, W.R. Reed Jr., Evaluation of carbide grades and anew cutting geometry for machining titanium alloys, Wear 92 (1983)113–123.[3] M. Field, J. Kahles, Review of surface integrity of machined com-ponents, Ann. CIRP 20 (1971) 153–163.[4] G. Byrne, J. Barry, P. Young, Surface integrity of AlSi9 ma-chined with PCD cutting tools, Ann. CIRP 46 (1997) 489–492.[5] A.M. Arao, M. Wise, D. Aspinwall, Tool Life and Workpiece SurfaceIntegrity Evalutions When Machining Hardened AISI H13 and AISIE52100 Steels with Conventional Ceramic and PCBN Tool Materials,SME Technical Paper No. MR95-159, 1998.[6] W. Field, W. Koster, Surface Integrity in Conventional Machining– Chip Removal Processes, Technical Paper No. EM68, ASTME,1968.[7] D. Watson, M. Murphy, The effect of machining on surface integrity,Manuf. Eng. (1979) 199–204.[8] C.H. Che-Haron, Tool life and surface integrity in turning titaniumalloy, J. Mater. Process. Technol. 118 (2001) 231–237.[9] M.J. Donachie Jr., Titanium and Titanium Alloys; Source Book,American Society for Metals, Metals Park, OH, 1982, pp.3–19.[10] A. Jawaid, C.H. Che-Haron, Tool wear in machining of titaniumalloy Ti–6% Al–4% V, in: Proceedings of the Advances in Materi-als and Processing Technologies (AMPT’97), Portugal, July 22–27,1997, pp. 562–568.
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