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SURFACE INTEGRITY OF HIGH-SPEEDFACE MILLED Ti-6Al-4V ALLOY WITH PCDTOOLSAnhai Li a , Jun Zhao a , Yongwang Dong a , Dong Wang a & XiaoxiaoChen aa Key Laboratory of High Efficiency and Clean MechanicalManufacture of MOE, School of Mechanical Engineering, ShandongUniversity , Jinan , ChinaPublished online: 22 Jul 2013.

To cite this article: Anhai Li , Jun Zhao , Yongwang Dong , Dong Wang & Xiaoxiao Chen (2013)SURFACE INTEGRITY OF HIGH-SPEED FACE MILLED Ti-6Al-4V ALLOY WITH PCD TOOLS, MachiningScience and Technology: An International Journal, 17:3, 464-482, DOI: 10.1080/10910344.2013.806180

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SURFACE INTEGRITY OF HIGH-SPEED FACE MILLEDTi-6Al-4V ALLOY WITH PCD TOOLS

Anhai Li, Jun Zhao, Yongwang Dong, Dong Wang, and Xiaoxiao ChenKey Laboratory of High Efficiency and Clean Mechanical Manufacture of MOE, School ofMechanical Engineering, Shandong University, Jinan, China

& This study is focused on the machined surface integrity of Ti-6Al-4V alloy using polycrystallinediamond (PCD) tools under wet milling condition. The surface integrity in terms of surface rough-ness, surface topography, microhardness, microstructure, and metallurgical alternations is inves-tigated. The observations and conclusions are primarily focused on the effect of cutting speed(250–2,000m=min) on the surface and subsurface of the machined Ti-6Al-4V. Experimentalresults show that machined surface integrity of Ti-6Al-4V alloy is sensitive to the variation of cut-ting speeds. Obvious machining (feed) marks can be found on the machined surfaces. Micro hard-ness examinations showed 5–20% hardening of the top machined surfaces than the bulk material.The analyses of microstructure and metallurgical alternations reveal that slight subsurface micro-structure alteration such as plastic deformation on the subsurface and no phase transformationwere observed. The evolution of crystallographic texture induced by the intense plastic deformationof the machined surface should be responsible for the modifications of the peak intensity radios inXRD patterns as well as higher peak broadening crystal structures.

Keywords high-speed milling, polycrystalline diamond (PCD) inserts, surface integrity,titanium alloy

INTRODUCTION

Titanium alloys have considerable potential of applications inaerospace industry, nuclear power, automotive, and biomedical industriesdue to their excellent strength to weight ratio and corrosion properties(Ezugwu and Wang, 1997). However, titanium alloys (specifically Ti-6Al-4V alloy) are known as difficult-to-machine materials, and machining oftitanium alloys has been an issue that needs to be improved. The low ther-mal conductivity of titanium alloys in combination with high chemical reac-tivity with tool materials, leading to the increase in cutting loads andtemperatures, as well as to extreme abrasion (Li et al., 2012a; Li et al.,

Address correspondence to Jun Zhao, School of Mechanical Engineering, Shandong University,17923 Jingshi Road, Jinan 250061, China. E-mail: zhaojun@sdu.edu.cn

Machining Science and Technology, 17:464–482Copyright # 2013 Taylor & Francis Group, LLCISSN: 1091-0344 print=1532-2483 onlineDOI: 10.1080/10910344.2013.806180

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2012b). In applications where surface finish and minimal tool changes areimportant considerations, PCD is being increasingly employed in milling ofTi-6Al-4V alloy (Corduan et al., 2003).

Machining induced surface integrity can be generally described by itstopological, mechanical, metallurgical and chemical states of a machinedsurface (from external and internal points of view) (Jawahir et al., 2011;Ulutan and Ozel, 2011). Surface integrity has a profound effect on thepotential performance linked with fatigue, corrosion, wear and strengthof the machined parts (Novovic et al., 2004). Many factors, such as cuttingtool material, tool geometry and edge preparation, cutting conditions(cutting speed, feed rate and depth of cut, tool wear, and even workpieceinclination angle), cooling and lubrication conditions, govern the qualityand integrity of the machined surface. Surface integrity produced by cuttingtools with various tool material, edge radius, side cutting edge angle, andedge land is remarkably similar (Hughes et al., 2004; Hughes et al.,2006). Operating parameters, in particular the level of tool wear, have amajor influence on the resulting surface integrity (Hughes et al., 2006).Severe microstructure alteration such as plastic flow, tearing, deformationand white layer on top of the machined surface, can be observed whenmachining with a worn tool (Che-Haron and Jawaid, 2005). Rao et al.(2011) found that good surface integrity in terms of favorable residual stressand surface finish can be achieved under appropriate machining conditionsused with limited tool wear. Increasing feed rate will increase the compress-ive residual stresses imparted to the machined surface (Daymi et al., 2011a).

Daymi et al. (2011b) investigated the machined surface roughness,residual stress, microhardness and the microstructural observations at fourdifferent workpiece inclination angles. Ginting and Nouari (2009) alsofound that cutting conditions and tool flank wear affect significantly thesurface integrity. Thermal softening in the ageing process can cause thesoft sub-surface at the fist 50 mm, while the cyclic internal work hardeningresulted in the hard sub-surface down to 200mm under the machined sur-face. Sun and Guo (2009) investigated the surface integrity of Ti-6Al-4V endmilled using solid mill cutters, and thought that b phase becomes muchsmaller and severely deformed in the near surface with the cutting speed,but phase transformation was absent for the milling conditions. Moreover,the milled surface microhardness is about 70–90% higher than the bulkmaterial in the subsurface. Appling suitable cooling and lubrication con-ditions can relieve the work hardening of the machined surface due tolower heat generated and the consequent tempering action when machin-ing with high-pressure coolant supplies (Ezugwu et al., 2007).

Many researchers have studied the cutting performance of PCD toolswhen machining titanium alloy Ti-6Al-4V. Ezugwu et al. (2007) investigatedthe surface integrity of finished turned with PCD tools using conventional

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and high pressure coolant supplies. Nurul Amin et al. (2007) evaluated theeffectiveness of uncoatedWC-Co and PCD inserts in face milling of titaniumalloy Ti-6Al-4V and compared their applicable cutting speed ranges, metalremoval per tool life value and tool wear rates, tool wear morphology, sur-face finish, chip segmentation and chatter phenomena.

Oosthuizen et al. (2011) performed milling experiments on Ti-6Al-4Valloy and investigated the performance of PCD tools in terms of tool life,cutting forces, and surface roughness. They found that the PCD toolyielded longer tool life than a coated carbide tool at cutting speeds above100m=min, and the combined effect of high temperature coupled withabrasion accelerates the degradation of the tools. However, there are rela-tively few researches relating to surface integrity in the field of high-speedmilling of Ti-6Al-4V alloy with PCD tools.

The present work aims to evaluate the influence of cutting speed onworkpiece surface integrity. Surface integrity of the milled surfaces wassystematically characterized by surface roughness, surface topography,microhardness, microstructure and metallurgical alterations. The coherentcharacterizations provide a physical basis for the understanding of machinedcomponent performance such as fatigue life and stress corrosion in service.

EXPERIMENTAL CONDITIONS

Workpiece Materials

The workpiece material used in the machining tests was Ti-6Al-4V alloy.Ti-6Al-4V alloy is a two-phase (aþ b) titanium alloy, composed of equiaxedTi a grains (hcp) surrounded by Ti b grains (fcc), as can be seen in the typi-cal microstructure shown in Figure 1 (dark grains correspond to Ti a andlight grains to b phase). And a rectangular block of this material with a

FIGURE 1 Typical microstructure of Ti-6Al-4V alloy.

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dimension of 100� 35� 35mm was selected. The chemical compositionand thermo-mechanical properties of the material are given in Table 1and Table 2, respectively.

Cutting Tool

The geometry of the milling cutter is shown in Figure 2. The cuttingtool used in the experiment was a 125-mm-diameter end mill (catalog num-ber: R220.53-0125-09-8C, made by SECO Inc.) with a 45� major cuttingedge angle, a 10� rake angle, a 20� axial rake angle, and a �5� radial rakeangle. To avoid the influence of the tool tip run-out on the machined sur-face and tool wear analysis, only one insert was mounted on the cutter. Thepolycrystalline diamond (PCD) tool insert used is a square shaped end mill-ing insert SEEX09T3AFFN-L1, PCD05. The cutting rake angle of the insertis 12�. The polycrystalline diamond, with an average particle size of 1 mm, isembedded in tungsten carbide substrate.

Machining Tests and Operating Parameters

The machine used for the milling tests is a CNC vertical machining cen-ter (DAEWOO ACE-V500), equipped with variable spindle speed from80 rpm to 10,000 rpm, and a 15-kW motor drive. The feed direction is inthe 35-mm-length direction in the 100� 35mm surface. The workpiecewas mounted on a specially designed fixture.

The machining tests were carried out in the type of down millingoperation, and wet cutting using Blaser 2000 soluble cutting fluid with a

TABLE 1 Chemical Composition of Ti-6Al-4V Alloy (wt %)

Titanium Al V Fe Si C N H O

Balance 5.6 3.86 0.18 <0.01 0.02 0.023 <0.01 0.17

TABLE 2 Thermo-mechanical Properties of Titanium Alloy at Room Temperature

Density (kg=m3)Hardness(HRC)

Elastic modulusE (GPa)

Poisson’sratio n

Meltingtemperature

(�C)

4430 36 114 0.33 1668

Thermalconductivity(W=m �K)

Yield strength(MPa)

Tensilestrength (MPa)

Reduction inarea (%)

Elongation(%)

6.7 834 932 36 14

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15% volume concentration. Down milling results in a thick to thin chip,which is best practice for titanium milling, yielding better tool life than con-ventional milling. The thin chip on exit reduces shock load and the chanceof the built up edge breaking off part of the tool cutting edge. The coolantwas jetted out in the flow rate of 50L=min to the cutting zone by adjustingthe two nozzles’ position and angle. The parameters throughout these trialsare detailed in Table 3. Eight levels of cutting speed were selected, while thefeed, radial depth of cut and axial depth of cut were kept a constant value.

At the beginning of the experiments, to ensure measurement repeat-ability and fair comparison, a new sharp tool insert was mounted ontothe tool holder, the machined surface (with a dimension of 35� 25mm)were milled and marked corresponding to the experiment number forthe following measurements. Machining experiments at each conditionwere repeated till consistencies of the experimental values are obtained.The cutting length is 35mm, and each cutting condition was stopped aftermachining the same metal removal volume (437.5mm3). The workpieceafter machined is shown in Figure 3, and two rectangular blocks were usedfor the experiments under eight different cutting speeds.

Measuring Instruments

The tool wear morphologies showing the state of worn tools wereobserved and the maximum flank wear were accurately measured with a

FIGURE 2 The geometry of the milling cutter. (a) Tool holder. (b) Geometric dimensions of toolinsert. (Figure available in color online.)

TABLE 3 Cutting Parameters Used in the Milling Tests

Experimentalparameters Cutting speed v (m=min)

Feed per toothfz (mm=z)

Axial depth ofcut ap (mm)

Radial depthof cut ae (mm)

Value 250, 375, 500, 625, 750,1000, 1500, 2000

0.05 0.5 25

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Dino-Lite AM413ZT handheld digital microscope with polarizationcapabilities and high magnification up to 200�. The machined surfaceroughness and the machined surface topography were measured with awhite light interferometer WykoNT9300. The sample was cut from theworkpiece material and then prepared in the same size by cutting using adiamond blade with maintained speed and cooling system. Surface micro-hardness (HV) was measured using a microhardness indenter (ModelHL-600, China). Subsurface microhardness (HV0.05) was measured usinga microhardness indenter (Model MH-6, China) with the load of 50 g.

The subsurface microstructure of the samples was analyzed by a QuantaFEG 250 field-emission scanning electron microscope (SEM; FEI Ltd.,USA). For SEM observations, samples are mechanically polished usingSiO2 solution and microcloth, and then etched with Kroll reagent (2%HF, 4% HNO3, and 94% distilled water) for 20 s. The chemical compositionof machined surface and bulk Ti-6Al-4V alloy (free of oxidation layer) of thesamples was characterized by X-ray diffraction (XRD; a Rigaku (Japan)D=max-IIB diffractometer employing Cu Ka radiation).

RESULTS AND DISCUSSIONS

Surface Topography

Two-Dimensional TopographiesIn the milling operations, the characteristic of the machined surface

topography is different from that in turning operations. In the milling pro-cess, the rotating cutting tool moves back and forth across the workpiecesurfaces being machined with the cutting motion, and the tool trajectoryis more complex than turning process. Figure 4 illustrates the cutting pathsof the milling cutter used in the experimental trials. In the present study,the machined area in the dashed-line rectangular frame was chosen asthe surface topography measuring area. Taking the surface topographymeasuring area into consideration, there is much difference between thesurface patterns left after the milling cutter passing the position 1 and that

FIGURE 3 Photos of the machined workpiece surface. (Figure available in color online.)

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produced after the milling cutter overtaking position 2. The surfacetextures left after position 1 is perpendicular to that left after position 2.

Figure 5 shows the evolution of the maximum flank wear VBmax and cor-responding cutting time after removing the same metal volume 437.5mm3

under different cutting speeds. It can be seen that when the cutting speedis below 750m=min, the tool wear rate is relatively small. Rao et al. (2011)found that there is no significant change in the surface roughness left onthe surface machined by the cutting edge, which is gradually wearingout. So, the effect of tool wear on surface roughness can be ignored whenthe cutting speed is below 750m=min.

As the cutting speed surpassing 750m=min, the tool wear rate increasesrapidly with the cutting speed. The maximum flank wear of the PCD tool at

FIGURE 4 Schematic presentation of cutting paths of the milling cutter. (Figure available in coloronline.)

FIGURE 5 Maximum flank wear and cutting time vs. cutting speed (Metal removal volume¼ 437.5mm3

and the thickness of the PCD layer embedded in the insert is 0.5mm).

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cutting speeds higher than 1,000m=min even exceeded the thickness ofPCD layer (0.5mm) embedded in the insert, leading to catastrophic failureof the flank face. Because the cutting width is a fixed value along the feeddirection, the cutting time for each cutting speed shows a tendency ofreduction under the same metal removal volume. The photos showingthe state of worn tools are detailed in Figure 6. Due to the intensive influ-ence of thermal and mechanical loads on the cutting tools, cutting edgechipping, flaking on the rake face, and flank wear are the major tool wearpatterns. At cutting speeds higher than 1,000m=min, tool failure occurredin the form of excessive chipping=flaking and even catastrophic fracture ofthe cutting edge. The large and deep wear of the cutting edge will adverselyaffect the machined surface quality.

Two-dimensional (2-D) topographies of the Ti-6Al-4V machined sur-face produced at different cutting speeds with PCD tools are shown inFigure 7. Machining (feed) marks, which are a natural defect because offeeding, were observed at all machined surface samples. The machinedsurfaces generated consist of well-defined uniform machining marks run-ning parallel to the cutting speed direction. The machining marks gener-ated under the cutting speeds of 250–1000m=min are in the samedirection, while those produced at cutting speed of 1,500m=min and2,000m=min are in different direction. This is because the machined sur-face under cutting speed of 1,500m=min and 2,000m=min was producedafter the milling cutter moved across the position 1 in Figure 4, and it did

FIGURE 6 Photos of the worn tools after machined (Metal removal volume¼ 437.5mm3). (Figureavailable in color online.)

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not hold long before the cutting edge chipping and the large flank wearas shown in Figure 6.

Surface Roughness

Surface roughness, which can be characterized surface height statisticssuch as Ra, Rq, and Rt, etc., is a form of expression of surface topography.Ra is the average roughness as calculated over the entire measured array.

Ra ¼1

n

Xni¼1

Zi � �ZZj j ð1Þ

Zi is defined to be the distance from the measured point to the mean plane.Rq is the root-mean-squared roughness calculated over the entire

measured array.

Rq ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1

n

Xni¼1

Zi � �ZZð Þ2s

ð2Þ

Rt is the peak-to-valley difference calculated over the entire measured array.Comparing the machined surface obtained at different cutting speeds,

as shown in Figure 8, the average roughness Ra and root-mean-squaredroughness Rq captured from the three-dimensional surfaces exhibited thesame trend of variation, while the max profile height Rt has no much

FIGURE 7 Surface texture of the test workpiece at different cutting speeds (2.5�magnification).

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difference. In case of lower cutting speed (<500m=min), better surface fin-ish can be acquired by increasing cutting speed. However, the surfaceroughness increases when the cutting speed exceeds 500m=min. The cut-ting speed value of 500m=min appears to be optimal; it minimizes toolwear and improves the surface finish Ra in the milling conditions. At cut-ting speeds higher than 1000m=min, the tool insert could not hold longbefore catastrophic fracture of the cutting edge occurred. Although goodsurface finish was achieved, this should be attributed to the increase of toolwear with increase in cutting speed.

Three-Dimensional Topographies

Figure 9 presents three-dimensional (3-D) topographies of the Ti-6Al-4V machined surfaces at the various cutting conditions given in Table 3with PCD tools. A 3-D image of the surface topography in the figures

FIGURE 8 Machined surface roughness vs. cutting speed. (Figure available in color online.)

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contains more information than what can be obtained from the 2-D imageof the surface topography. The maximum profile height, surface rough-ness, detailed surface topography, and surface damages can be easilydistinguished from the 3-D images. We should note that a 3-D image covers

FIGURE 9 Three-dimensional topographies of the Ti-6Al-4V machined surface at different cuttingspeeds with PCD tools. (Figure available in color online.)

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2.5� 1.9mm of machined surface samples rather than a straight-linemeasurement. From the figures, it can be seen that the average values ofsurface roughness constructed by low-amplitude topography (betweenpeak and amplitude valley of the machined surface waves). The smallerthe amplitude observed by 3-D surface topography, the more uniformthe surface roughness and thus the better the surface roughness produced.In addition, there is evidence of surface roughness variation. The mor-phology at cutting speed of 250m=min is rougher than that at cuttingspeed of 375m=min, with the one at cutting speed of 500m=min followed.Moreover, it can be seen that the morphology under the cutting speeds(from 500m=min to 1,000m=min) turns rougher (Figures 9c–f).

Microhardness

Figure 10 shows the variation of machined surface roughness with thecutting speed. The hardness profiles indicate that a considerable variationof the hardness values measured. The hardness values measured near themachined surface of high-speed-milled Ti-6Al-4V alloy are found to beapproximately 5–20% higher (330–410 HV) than those of the bulk material(approximately 315 HV). Taking the developing trend of the surface hard-ness with cutting speed into consideration, it is inferred that when the cut-ting speed is below 750m=min, the effect of tool wear on machined surfacehardness is small (as can be seen in Figure 6).

Increasing the cutting speed created higher hardness values in the cut-ting speed range 250–625m=min, which can be explained to be due to thework-hardening effect on the surface. But as the cutting speed surpassing750m=min, the higher tool wear rate contributes greatly to the increaseof the machined surface hardness with the cutting speed. At cutting speed

FIGURE 10 Machined surface hardness vs. cutting speed. (Figure available in color online.)

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of 750m=min, relatively low surface hardness can be obtained, but themachined surface hardness values increase rapidly and due to theover-aging of the surface of the material due to very high cutting tempera-ture under high tool wear rates.

The schematic of subsurface microhardness measurement and theresults in terms of microhardness versus the depth beneath the machinedsurface are depicted in Figure 11. To acquire more detailed measurementresults the subsurface microhardness and to eliminate the influence errorsof the two adjacent measuring hardness values, the oblique-down measur-ing method was adopted, and the microhardness was tested at intervalsof 10mm beneath the machined surface. At least three different locationsat the same depth below the machined were chosen to find the averageof measuring results. The three locations of indentations were well spacedto avoid interference with each of them. The sub-surface exhibited workhardening due to the cyclic heating and cooling processes under the hightemperature. Deeper to the machined surface, the internal work hardeningeffect is gradually dissipated, so the micro-hardness values show a variationof decrease and reach the micro-hardness of the bulk matrix to a certaindepth. Moreover, with the increase of cutting speed, the subsurface micro-hardness shows a tendency of increase.

According to the variation of microharness versus the depth beneaththe machined surface, the depth where the microharness reaches the bulkhardness of Ti-6Al-4V alloy can be taken as the thickness of damaged ofdeformation layer. Figure 12 shows the variation of thickness of thedamaged layer depend on the cutting speed. Three ranges of cutting speedcan be distinguished according to the variation of damaged layer. Thethickness of damaged layer or work-hardening layer initially decreases with

FIGURE 11 Schematic of microhardness measurement of subsurface (a) and the microhardness vs.depth beneath the machined surface (b). (Figure available in color online.)

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increasing cutting speed under the cutting speed of 500m=min. Thefollowing reasons may be responsible for this variation.

First, as the cutting speed increases, the plastic deformation speedincreases, the first deformation zone turns narrower, the yield limit ofthe workpiece raises, and the plasticity of the material decreases. At thesame time, the increasing cutting speed shortens the contact time of thetool flank face and workpiece, making the work-hardening process inade-quately. In addition, cutting temperature increases with the increasing cut-ting speed, making the thermal softening of workpiece sufficiently.However, the thickness of damaged layer increases when the cutting speedincreases from 500m=min to 750m=min. This should be attributed to thedeformation speed overtaking the thermal softening speed and thermalsoftening of the work piece did not advance entirely. As discussed earlier,the higher tool wear rate contributes greatly to the machining processesat cutting speeds higher than 750m=min, the thickness of damaged layerdecreases.

Microstructure and Metallurgical Alterations

As explained previously, the samples were carefully prepared to avoidsmearing on the surface and the microstructure alternations were investi-gated. Figure 13 shows the etched cross-sections of machined surfaces per-pendicular to the tool feed direction. There was no evidence of subsurfacedefects such as cracks, laps, visible tears or shear deformation after machin-ing Ti-6Al-4V alloy under different cutting speeds. The figures enables us toconclude that the microstructure at the top region of subsurface trend toexhibit plastic deformation. The plastic deformation on the machined sur-face seems to follow the feed speed direction. This is caused by the high

FIGURE 12 The thickness of the damaged layer vs. cutting speed. (Figure available in color online.)

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FIGURE 13 Cross-section view of subsurface microstructure.

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cutting pressure applied by the cutting tool on the softer subsurface.During the wet milling process, the high mechanical and thermal loadsincrease the microstructure alternation effects. However, this deformationremains relatively moderate in comparison with the microstructuraldeformation observed by Ginting and Nouari (2009) after dry machiningTi-6Al-4V alloy with cemented carbide tools.

The microstructural characterization conducted in the cross-section ofthe machined surface reveals that no phase transformations near the sur-face when compared to the matrix texture presented in Figure 1. Thereis no white layer as mentioned by Che-Haron and Jawaid (2005) existamong all the samples. Because the surface was machined by the sharp cut-ting tool under good cooling conditions, the milling pressure and work-piece temperatures are too low to cause any phase transformations, sothe white layer cannot be observed under the machined surfaces.

The XRD patterns of the Ti-6Al-4V machined surface under differentcutting speeds compared to surface before cutting (bulk material of Ti-6Al-4V) are shown in Figure 14. Comparing the XRD peak broadening andintensity radio of the machined surface under different cutting speeds, aqualitative assessment of microstructural modifications during high-speedmachining can be done. It is very interesting that the intensity under differ-ent cutting speeds is different. After machining, the XRD patterns of themachined surface exhibited a slightly broadening of all peaks.

This broadening should be attributed to the variation of crystallo-graphic texture induced by the intense plastic deformation in the surfaceand subsurface of the alloy. The dislocations created during the plasticdeformation produces microstrain and distortion in the crystal lattice caus-ing broadening of the diffraction peaks. The modifications of the peak

FIGURE 14 XRD patterns of bulk material of Ti-6Al-4V (a) and machined surface by PCD tools underdifferent cutting speeds (b). (Figure available in color online.)

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intensity radios in XRD patterns as well as higher peak broadening indicatethat grain refinement exist in the plastic deformation layer. Note that thepeak broadening in the obtained XRD patterns remains weak and Ti aand Ti b phases are clearly distinguished. Moreover, some modificationsof the peak intensity radios in XRD patterns of the machined surface werealso observed. A lower intensity of the (10.0) (00.2) and (20.0) Ti a peakand the (110) Ti b peak takes place, yet a higher intensity of the (11.0)and (11.2) Ti a peak and (200) Ti b peak takes place in the machined sur-face and even the subsurface. These modifications are mainly caused by theevolution of the crystallographic texture induced by the intense plasticdeformation in the machined surface. These experiments show that Ti aand Ti b phases are present in all the machined surface for all cuttingspeeds involved, so no obvious phase transformation was observed.

CONCLUSIONS

For a range of cutting speeds in high-speed milling of Ti-6Al-4V alloyusing PCD inserts under cooling conditions with emulsion, analyses ofmachined surface roughness, surface topography, microstructural observa-tions, metallurgical alteration, and microhardness were carried out. Thefollowing conclusions were drawn:

1. The machined surface integrity of milled Ti-6Al-4V alloy is sensitive tothe cutting speeds.

2. From the 2-D topographies of the machined surface, machining (feed)marks were observed at all machined surface samples under differentcutting speeds.

3. When the cutting speed is below 750m=min, tool wear has little effecton the surface quality. In case of low speed cutting (less than 500m=min), the surface roughness showed a downward trend with the increas-ing of cutting speed; yet, in high-speed cutting processes (more than500m=min), the surface roughness showed a little increase with theincreasing of cutting speed. At cutting speeds higher than 1000m=min, the cutting conditions were not recommended because the toolinsert could not hold long before catastrophic fracture of the cuttingedge occurred, in spite of receiving a good surface, which should beattributed to the increase of tool wear with the increase in cutting speed.

4. Among the involved cutting speed ranges, work hardening of themachined surface is important and the machined surface microhard-ness is nearly 5–20% higher than that of bulk material.

5. The thickness of damaged layer or work-hardening layer initiallydecreases and then increases with increasing cutting speed. At cutting

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speed higher than 750m=min, the thickness of damaged layer decreasesand was highly affected by the higher tool wear rate in the machiningprocesses.

6. In terms of the microstructure alteration of the machined surface and sub-surface, it trends to exhibit plastic deformation. The plastic deformation onthe machined surface seems to follow the feed speed direction. However,no phase transformation was observed under different cutting speeds.

7. After machining, the XRD patterns of the machined surface exhibited aslightly broadening of all peaks. This broadening can be attributed tothe evolution of crystallographic texture induced by the intense plasticdeformation in the surface and subsurface of the alloy. The evolutionof the crystallographic texture induced by the intense plastic defor-mation and grain refinement in the machined surface also leads tothe modifications of the peak intensity radios in XRD patterns. However,no phase transformation was observed.

ACKNOWLEDGMENTS

This work is sponsored by the National Basic Research Program ofChina (2009CB724402), the National Natural Science Foundation of China(51175310), and the Graduate Innovation Foundation of ShandongUniversity (yyX 10012).

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