Cutting Performance and Wear/Damage Characteristics of PCBN Tool in Hard Milling

applied sciences

Article Cutting Performance and Wear/Damage Characteristics of PCBN Tool in Hard Milling

Haining Gao *, Xianli Liu and Zhitao Chen School of Mechanical and Power Engineering, Harbin University of Science and Technology, Harbin 150080, China; [email protected] (X.L.); [email protected] (Z.C.) * Correspondence: [email protected]; Tel.: +136-5466-2869

Received: 6 January 2019; Accepted: 19 February 2019; Published: 22 February 2019

Abstract: In the intermittent machining of hardened steel for the die and mold industry, determining how to reduce the wear of PCBN (Polycrystalline Cubic Boron Nitride) tools and improve their machining efficiency and quality is an important subject. This study investigated the intermittent machining of hardened steel (Cr12MoV, 59HRC (Rockwell hardness)) using uncoated PCBN tools to determine the cutting performance (cutting force, chip morphology, surface quality, tool life, cutting temperature) and the wear/damage characteristics of the tools. The results showed that the cutting performance of a PCBN tool was better than that of a cemented carbide tool. The wear mechanism on the PCBN tool flank was diffusion wear, adhesive wear, and oxidation wear. The main failure modes of the PCBN tool in the machining process of hardened steel at low speed were tool micro-chipping, the conchoidal damage of the rake face, and the larger damaged area of the flank face. The main failure modes of the PCBN tool in the machining process of hardened steel at high speed were flank wear and high-rate fatigue damage.

Keywords: cutting performance; wear characteristics; damage characteristics; PCBN tool; low-rate damage

1. Introduction PCBN is a cutting tool material most widely used in the machining of hardened steel due to its higher hardness, strength, thermal and chemical stability, and thermal conductivity, and lower friction coefficient than other tool materials at high temperature [1–3]. Cutting performance and wear characteristics of PCBN tools were extensively studied in the hardened-steel cutting process. During continuous and intermittent turning, the cutting performance and wear mechanism of PCBN tools were extensively studied. The cutting performance of PCBN tools with different CBN (Cubic Boron Nitride) content was investigated [4]. It was found that PCBN tools with low CBN content had better cutting performance than that of PCBN with high CBN content. The influence of chamfering angle on the cutting performance of PCBN tools was investigated [5–7]. It was found that the chamfering angle could effectively reduce cutting force, flank wear, and white layer thickness. Özel et al. [8] investigated the cutting performance of PCBN tools with variable micro-geometry in hard turning AISI4340 steel. It was found that the variable micro-geometry PCBN tool could effectively reduce cutting heat, plastic deformation of workpiece, and tool wear. The effect of the microstructure of hardened steel on the PCBN tool wear was investigated [9,10]. In addition, the effects of new tool materials [11] and high-pressure cooling technology [12] on the cutting performance of PCBN tools were studied. The main wear mechanism of PCBN tools in continuous and intermittent cutting of hardened steel (1117 steel, HRC62+1) was abrasive wear [13]. The main wear mechanism of PCBN tools in intermittent turning of bearing steel (AISIE-52100, HRC60) was abrasive wear and adhesion wear [14]. The main wear mechanism of PCBN tools in continuous and intermittent cutting

Appl. Sci. 2019, 9, 772; doi:10.3390/app9040772 www.mdpi.com/journal/applsci Appl. Sci. 2019, 9, 772 2 of 15 of hardened steel (AISI 4340, HRC56) was abrasive wear [15]. Under high-pressure cooling conditions, PCBN inserts primarily sustain adhesive wear, groove wear, and abrasive wear on the flank face when cutting GH4169 [12]. Tang et al. [16] studied the effect of quenching hardness on the wear properties of PCBN tools for dry hard turning. The results showed that the wear mechanism of tool flank was abrasive wear in the cases of 40–55 ± 1 HRC and abrasive and delamination wear in the case of 60 ± 1 HRC. The wear mechanism of PCBN tools was abrasive wear in the machining of high-chromium white cast iron [17,18]. There were a few studies on the cutting performance and wear mechanism of PCBN tools in the milling process. Gopalsamy et al. [19] carried out an experimental study on end milling of DIEVAR hardened steel (50 HRC) with PCBN tools. The results indicated that the PCBN tool gave excellent performance in terms of tool life and surface finish in comparison with carbide-coated tools. Wojciechowski et al. [20] investigated the life of PCBN tools in milling die steel. The results showed that the tool life was affected by the cutting speed. Okada et al. [21] studied the cutting force, cutting temperature, tool wear, and surface roughness of PCBN tools with different CBN content in hard milling hardened steel. It was found that PCBN tools with high CBN content had higher cutting performance. Elbestawi et al. [22] investigated the wear properties of PCBN ball-end milling cutters with different CBN content in high-speed milling hardened AISI H13 die steel. It was found that the main wear forms of PCBN ball-end milling cutters with 90% CBN and 65% CBN were flank wear and chipping, respectively. Boing et al. [23] proposed a three-dimensional wear parameter method based on a focal-length change microscope to evaluate PCBN tool wear and wear mechanism. It was found that there was a nonlinear relationship between abrasive wear, adhesive wear, and diffusion wear, and material hardness. Its calculated results were nearly the same as those given by Luo et al. [24]. Saketi et al. [25] investigated the wear properties of PCBN tool with high CBN content in milling hardened steel. It was found that tribochemical reactions, adhesive wear, and mild abrasive wear controlled the flank and crater wear. Cui et al. [26] investigated the failure mechanism of PCBN tools in milling hardened steel based on damage equivalent stress. By analyzing the above literature, it was found that chip morphology and cutting temperature were rarely used to evaluate the cutting performance of PCBN tools, whether in turning or milling of hardened steel. Two or three evaluation indicators were often selected to evaluate the cutting performance of PCBN tools in the existing literature. Cutting is a very complex process, and comprehensive evaluation indicators can more accurately reflect the cutting performance of PCBN tools. In the machining of hardened steel, the wear mechanism of PCBN tools varies with the material and hardness of the workpiece. At present, there is no research on the wear mechanism of PCBN tools in the intermittent machining of hardened steel (Cr12MoV, 59HRC). Meanwhile, the research on the damage characteristics of PCBN tools in the intermittent machining of hardened steel is still blank, which limits the application of PCBN tools. In this study, the cutting performances (cutting force, chip morphology, surface quality, tool life, cutting temperature) and the wear/damage characteristics of PCBN tools were investigated for the intermittent machining of hardened steel.

2. Experimental Set-Up

2.1. Experimental Conditions The hardened die steel material used in the experiment was Cr12MoV. The workpiece was 120 × 70 × 70 mm. The work material was hardened and tempered to 59 HRC. The chemical compositions of the die steel are shown in Table1. The physical properties of the die steel are shown in Table2. The cutter used in the experiment was the ball-type-head clamping milling cutter. The cemented carbide tool was BNM-200 JC5015 (Daijie Company, Osaka, Japan, 2018) with a diameter of 20 mm. The coating ﬁlm material of the cemented carbide tool was TiAlN. Physical properties of coating ﬁlm materials for cemented carbide tool are shown in Table3[ 21]. Physical properties of base materials (K10, Co content 6%, WC (Wolfram Carbide) grain size 2 µm) are shown in Table4. The uncoated Appl. Sci. 2019, 9, 772 3 of 15

PCBN tool used in the experiment was DCC500 (Element Six Company, Didcot, Britain, 2018) with a diameter of 20 mm. PCBN tools were composed of a CBN content of 45%, a TiC bonding agent, and a grain size of 1.5 µm. The geometric parameters of the two cutters were rake angle 0◦, clearance angle 13◦, chamfer width 0.1 mm, and chamfer angle 15◦. The cutting mode was down milling. The tool used in the experiment is shown in Figure1. The experiment was carried out on a three-axis machining center with the model VDL-1000E (Dalian machine tool group, Dalian, Liaoning, China, 2005). The experimental site is shown in Figure2.

Table 1. The chemical composition of the workpiece.

Element C Cr Mo Si Mn Ni V % 1.55 10.50 0.40 ≤0.4 ≤0.4 0 0.25

Table 2. Physical properties of the workpiece.

Density Thermal Conductivity Speciﬁc Heat Line Expansion Coefﬁcient Elasticity Modulus (g/cm3) (W/(m·K)) (J/(g·K)) (K−1) (GPa) 7.70 20.0 0.46 1.12 × 10−5 223

Table 3. Physical properties of the coating ﬁlm materials.

Coating Hardness Surface Roughness Rz Film Thickness Thermal Effusivity Oxidizing Film (HV) (µm) (µm) J/(s0.5 m2 K) Temperature (◦C) Appl.TiAlN Sci. 2018, 8,4353 x FOR PEER REVIEW 0.71 2.9 3027 800 3 of 15 Appl. Sci. 2018, 8, x FOR PEER REVIEW 3 of 15

a grain size of 1.5 um. The geometricTable 4. parametersPhysical properties of the two of base cutters materials. were rake angle 0°, clearance angle a13°, grain chamfer size of 1.5width um. 0.1 The mm, geometric and chamfer parameters angle of 15°. the Thetwo cutterscutting weremode rake was angle down 0°, milling. clearance The angle tool 13°, chamfer width 0.1 mm, and chamfer angle 15°. TheThermal cutting Conductivity mode was downThermal milling. Diffusivity The tool a used in the experimentRockwell is shownYoung’s in Figure Modulus 1. The experiment was carried out on a three-axis Base Materials k (W/m·K) at × 106 (m2/s) usedmachining in the centerexperimentHardness with the is (HRA) modelshown VDL-1000Ein× 10Figure4 (kg/mm 1.(Dalia 2The) nexperiment machine tool was group, carried Dalian, out Liaoning,on a three-axis China, 20/500/900 ◦C at 20/500/900 ◦C machining2005). The centerexperimental with the site model is shown VDL-1000E in Figure (Dalia 2. n machine tool group, Dalian, Liaoning, China, 2005). TheK10 experimental 91.1 site is shown in Figure 6.4 2. 91.7/74.4/68.1 31.9/18.9/15.0

(a) (b) (a) (b) Figure 1. Cutting tools for experiments: (a) carbide cutting tool; (b) PCBN tool. FigureFigure 1. 1. CuttingCutting tools tools for for experiments: experiments: ( (aa)) carbide carbide cutting cutting tool; tool; ( (bb)) PCBN PCBN tool. tool.

FigureFigure 2.2.Experimental Experimental site.site. Figure 2. Experimental site. Table 1. The chemical composition of the workpiece. Table 1. The chemical composition of the workpiece. Element C Cr Mo Si Mn Ni V Element% 1.55C 10.50 Cr Mo0.40 Si≤0.4 Mn≤0.4 Ni0 0.25 V % 1.55 10.50 0.40 ≤0.4 ≤0.4 0 0.25 Table 2. Physical properties of the workpiece. Table 2. Physical properties of the workpiece. Specific Line Density Thermal Elasticity SpecificHeat ExpansionLine Density(g/cm3) ConductivityThermal (W/(m ∙K)) ModulusElasticity (GPa) Heat(J/(g ∙K)) CoefficientExpansion (K −1) (g/cm3) Conductivity (W/(m∙K)) Modulus (GPa) 7.70 20.0 (J/(g 0.46∙K)) Coefficient 1.12 × (K 10−−15) 223 7.70 20.0 0.46 1.12 × 10−5 223 Table 3. Physical properties of the coating film materials. Table 3. Physical properties of the coating film materials. Surface Film Thermal Oxidizing Coating Hardness RoughnessSurface ThicknessFilm ThermalEffusivity TemperatureOxidizing CoatingFilm Hardness(HV) Roughness Thickness Effusivity0.5 2 Temperature Film (HV) Rz (um) (um) J/(s m K) (°C) TiAlN 4353 Rz 0.71(um) (um) 2.9 J/(s0.5 3027 m2 K) (°C) 800 TiAlN 4353 0.71 2.9 3027 800 Table 4. Physical properties of base materials. Table 4. Physical properties of base materials. Rockwell Young’s Thermal Conductivity Thermal Diffusivity a × Base RockwellHardness Young’sModulus Thermalk (W/m Conductivity∙K) at Thermal10 Diffusivity6 (m2/s) a × MaterialsBase Hardness Modulus4 2 k (W/m∙K) at 106 (m2/s) Materials (HRA) × 10 (kg/mm ) 20/500/900 °C at 20/500/900 °C K10 (HRA)91.1 × 104 (kg/mm 6.4 2) 20/500/900 91.7/74.4/68.1 °C at 31.9/18.9/15.020/500/900 °C K10 91.1 6.4 91.7/74.4/68.1 31.9/18.9/15.0 Appl. Sci. 20192018, 98, 772x FOR PEER REVIEW 44 of of 15

2.2. Experimental Scheme 2.2. Experimental Scheme The experimental scheme for exploring the cutting performance of the PCBN tool and cemented carbideThe tool experimental is shown in scheme Table for5. The exploring force in the the cutting cutting performance process was of themeasured PCBN toolusing and a three-way cemented carbidepiezoelectric tool isdynamometer shown in Table 9257B5. The (Swiss force KISTLER in the cutting company, process Winterthur, was measured Switzerland, using a three-way2015). The piezoelectricfree surface morphology dynamometer of 9257Bthe chip (Swiss was KISTLERobserved company,by SEM with Winterthur, SUPRA Switzerland,55 (Karl Zeiss, 2015). Oberkochen, The free surfaceGermany, morphology 2018). The ofmacroscopic the chip was morphology observed byof the SEM chip with and SUPRA the tool 55 wear (Karl state Zeiss, were Oberkochen, observed Germany,using a super-deep-scene 2018). The macroscopic three-dimensional morphology (3D) of the microscope chip and the with tool VHX-1000 wear state were(KEYENCE, observed Osaka, using aJapan, super-deep-scene 2009). A portable three-dimensional surface roughness (3D) measurin microscopeg instrument with VHX-1000 was used (KEYENCE, to measure the Osaka, roughness Japan, 2009).of the machined A portable surfaces surface roughnessof two kinds measuring of cutters instrument at the same was distance. used toFor measure cemented the carbide roughness tools, of the machinedcutting lengths surfaces of each of two cutting kinds parameter of cutters atin thetest same1 were distance. 16,720 mm, For cemented 27,600 mm carbide 76,000 tools, mm, theand cutting 33,600 lengthsmm. For of PCBN each tools, cutting the parameter cutting lengths in test of 1 each were cutting 16,720 mm,parameter 27,600 in mm test 76,0001 were mm, 400 mm, and 33,840 33,600 mm, mm. For98,400 PCBN mm, tools, and 154,560 the cutting mm. lengthsThe cutting of each length cutting of each parameter cutting inparameter test 1 were in test 400 4 mm,was 33,84020,000 mm. mm, 98,400The cutting mm, length and 154,560 of each mm. cutting The parameter cutting length in other of each tests cutting was 1200 parameter mm. in test 4 was 20,000 mm. The cutting length of each cutting parameter in other tests was 1200 mm. Table 5. Machining parameters. Table 5. Machining parameters. Cutting Speed Feed per Tooth Axial Depth of Cut Radial Depth Test Cutting(m/min) Speed Feed(mm/tooth) per Tooth Axial Depth(mm) of Radialof DepthCut (mm) of Test (m/min) (mm/tooth) Cut (mm) Cut (mm) 1 50, 150, 250, 350 0.05 0.05 0.3 1 50, 150, 250, 350 0.05 0.05 0.3 2 250 0.1, 0.15, 0.2, 0.25 0.05 0.3 2 250 0.1, 0.15, 0.2, 0.25 0.05 0.3 3 3 250 250 0.05 0.05 0.1, 0.1, 0.15, 0.15, 0.20, 0.20, 0.25 0.25 0.3 0.3 4 4300 300 0.15 0.15 0.3 0.3 0.3 0.3 5 5376 376 0.1 0.1 0.3 0.3 0.3 0.3 6 6 250, 250, 314, 314, 376, 376, 440, 440, 500 500 0.25 0.25 0.3 0.3 0.3 0.3

3. Experimental Analysis

3.1. Milling Force Thermal load and mechanics loads affect tool life and machined surface integrity. Because of the characteristics of milling, it is difﬁcultdifficult to measuremeasure the cutting heat, but the cuttingcutting force can be easily obtained. Therefore, the cutting force measured by experiment was selected to evaluate the cutting performance of the tool.tool. (1) Cutting Speed (Test(Test 1)1) The inﬂuenceinfluence of cutting speed on the cutting force of the two kinds of cutting tools is shown in Figure3 3..

260 300 DCC500 DCC500 240 BNM-200 BNM-200 250 220 200 200

180 150 Fy(N) Fx(N) 160 100 140 50 120

100 0 50 100 150 200 250 300 350 50 100 150 200 250 300 350 Cutting speed(m/min) Cutting speed(m/min) (a) (b)

Figure 3. Cont. Appl. Sci. 2019, 9, 772 5 of 15 Appl. Sci. 2018, 8, x FOR PEER REVIEW 5 of 15

220 DCC500 200 BNM-200 Appl. Sci. 2018, 8, x FOR PEER REVIEW 5 of 15 180

160220 DCC500 140200 BNM-200 180 Fz(N) 120 160 100 140 80

Fz(N) 120 60 100 40 8050 100 150 200 250 300 350 Cutting speed(m/min) 60

40 (c) 50 100 150 200 250 300 350 Cutting speed(m/min) Figure 3. The effect of milling speed on milling forces: (a)) Fx;Fx; ((b)) Fy;Fy; ((cc)) Fz.Fz. (c) When thethe cutting cuttingFigure speed speed 3. The exceeded effectexceeded of 150milling m/min,150 speed m/min, on the milling cutting the forces:cutting force (a) increased Fx;force (b) Fy; increased (c rapidly) Fz. forrapidly the cemented for the carbidecemented tool carbide (BNM-200). tool (BNM-200). The main The reason main was reason that, was when that, the when cutting thespeed cutting was speed high, was the high, cutting the When the cutting speed exceeded 150 m/min, the cutting force increased rapidly for the temperaturecutting temperature rose rapidly. rose The rapidly. cemented The carbidecemented tool ca cannotrbide withstandtool cannot high withstand temperature, high whichtemperature, results cemented carbide tool (BNM-200). The main reason was that, when the cutting speed was high, the inwhich a large results degree in a oflarge wear degree and evenof wear edge and chipping. even edge With chipping. the increase With the of cuttingincrease speed, of cutting the cutting speed, cutting temperature rose rapidly. The cemented carbide tool cannot withstand high temperature, performance of PCBN tools (DCC500) can be better reﬂected. The cutting force in three directions the cuttingwhich resultsperformance in a large of degree PCBN of tools wear (DCC500)and even ed cagen chipping.be better With reflected. the incr Theasee cutting of cutting force speed, in three decreaseddirectionsthe cutting graduallydecreased performance withgradually theof PCBN increase with tools the of (DCC500)increase cutting speed.caofn cuttingbe better The speed. reasonreflected. The was Th reasone that, cutting with was force thethat, in increase three with the of cuttingincreasedirections speed, of cutting thedecreased temperaturespeed, gradually the temperature of thewith cutter–chip the increaseof the cu contact oftter–chip cutting area speed.contact increased, The area reason whichincreased, was made that, which the with workpiece made the the materialworkpieceincrease produce material of cutting a thermalproduce speed,softening the a thermal temperature effect. softening of the effect.cutter–chip contact area increased, which made the (2)workpiece Feed per material Tooth produce (Test 2)2) a thermal softening effect. The influence(2) inﬂuence Feed per of Tooth feed (Testper tooth 2) on the cutting forceforce of the two kinds of cutting tools is shown in Figure4 4.. TheTheThe influence cuttingcutting forceforce of feed ofof theperthe tooth twotwo cutters cutterson the cutting increasedincrease forcd withewith of the thethe two increaseincrease kinds of of of cutting feed feed per per tools tooth.tooth. is shown TheThe reasoninreason Figure 4. The cutting force of the two cutters increased with the increase of feed per tooth. The reason waswas thatthat thethe metal removal raterate increasedincreased withwith thethe increaseincrease ofof feedfeed perper tooth.tooth. The cutting force of the was that the metal removal rate increased with the increase of feed per tooth. The cutting force of the PCBN tool (DCC500) waswas lessless thanthan thatthat ofof thethe carbidecarbide tooltool (BNM-200).(BNM-200). PCBN tool (DCC500) was less than that of the carbide tool (BNM-200). 100 220 100 220 DCC500 DCC500DCC500 DCC500 BNM-200BNM-200 BNM-200BNM-200 90 90 200200

80 80 180180

70 70 160160 Fx(N) Fy(N) Fx(N) Fy(N) 60 140 60 140

50 120 50 120

40 100 0.05 0.1 0.15 0.2 0.25 0.05 0.1 0.15 0.2 0.25 40 100 0.05 0.1 Feed per0.15 tooth(mm/tooth)0.2 0.25 0.05 Feed0.1 per tooth(mm/tooth)0.15 0.2 0.25 Feed per tooth(mm/tooth)(a) Feed( bper) tooth(mm/tooth) (a) 120 (b) DCC500 120 110 BNM-200 DCC500 110100 BNM-200

10090

9080 Fz(N) 70 80 Fz(N) 60 70 50 60 40 0.05 0.1 0.15 0.2 0.25 50 Feed per tooth(mm/tooth) 40 (c) 0.05 0.1 0.15 0.2 0.25 FigureFigure 4. The 4. The effect effect of of milling milling speedFeedspeed per ontooth(mm/tooth) milling forces: forces: (a) ( aFx;) Fx; (b) ( Fy;b) Fy;(c) Fz. (c) Fz. (c)

Figure 4. The effect of milling speed on milling forces: (a) Fx; (b) Fy; (c) Fz. Appl. Sci. 2019, 9, 772 6 of 15

Appl. Sci. 2018, 8, x FOR PEER REVIEW 6 of 15 (3) Axial Depth of Cut (Test 3) TheAppl. (3) inﬂuenceSci. 2018Axial, 8 , Depthx ofFOR axial PEER of Cut depthREVIEW (Test of 3) cut on the cutting force of the two kinds of cutting tools6 isof shown15 in Figure5The. The influence cutting of force axial ofdepth the of two cut cutters on the cutting increased force with of the the two increase kinds of of cutting axial tools depth is ofshown cut. The (3) Axial Depth of Cut (Test 3) reasonin wasFigure that 5. The the cutting metal removalforce of the rate two increased cutters incr witheased the with increase the increase of axial of axial depth depth of cut. of cut. Compared The The influence of axial depth of cut on the cutting force of the two kinds of cutting tools is shown reason was that the metal removal rate increased with the increase of axial depth of cut. Compared within cemented Figure 5. carbideThe cutting cutting force toolsof the(BNM-200), two cutters incr PCBNeased cutting with the tools increase (DCC500) of axial haddepth a gentleof cut. The trend of with cemented carbide cutting tools (BNM-200), PCBN cutting tools (DCC500) had a gentle trend of changereason in cutting was that force the andmetal higher removal stability. rate increased with the increase of axial depth of cut. Compared change in cutting force and higher stability. with cemented carbide cutting tools (BNM-200), PCBN cutting tools (DCC500) had a gentle trend of 120 250 change in cutting forceDCC500 and higher stability. DCC500 110 BNM-200 BNM-200 120 250 100 DCC500 200 DCC500 110 90 BNM-200 BNM-200 100 80 200 90 150 Fx(N) 70 Fy(N) 80 60 150 Fx(N) 70 Fy(N) 50 100 60 40 50 100 30 50 0.05 0.1 0.15 0.2 0.25 0.05 0.1 0.15 0.2 0.25 40 Axial depth of cut(mm) Axial depth of cut(mm) 30 50 0.05 0.1 (a0.15) 0.2 0.25 0.05 0.1 (b0.15) 0.2 0.25 Axial depth of cut(mm)130 Axial depth of cut(mm) (a) DCC500 (b) 120 BNM-200 130 110 DCC500 120 BNM-200 100 110 90

Fz(N) 100 80 90 Fz(N) 70 80 60 70 50 60 0.05 0.1 0.15 0.2 0.25 Axial depth of cut(mm) 50 0.05 0.1 (c0.15) 0.2 0.25 Axial depth of cut(mm) FigureFigure 5. The 5. The effect effect of of milling milling speedspeed( onc) milling forces: forces: (a) ( aFx;) Fx; (b) ( bFy;) Fy;(c) Fz. (c) Fz.

In total,In total, the the cutting cuttingFigure force force 5. The of effect the the PCBN PCBNof milling tool tool wasspeed was less on than lessmilling that than forces: of thatthe cemented(a of) Fx; the (b) cemented Fy;carbide (c) Fz. tool carbide under the tool same under the samecutting cutting conditions. conditions. The reason The was reason that the was friction that thecoefficient friction between coefﬁcient the cemented between carbide the cemented tool and the carbide workpieceIn total, was the larger cutting than force that of of the the PCBN PCBN tool tool was [19,20]. less than that of the cemented carbide tool under the same tool andcutting the conditions. workpiece The was reason larger was thanthat the that friction of the coefficient PCBN tool between [19,20 the]. cemented carbide tool and the 3.2.workpiece Surface was Quality larger than that of the PCBN tool [19,20]. 3.2. Surface Quality 3.2. SurfaceThe surface Quality topography of hardened steel with the carbide tool and PCBN tool under the same Thecutting surface parameters topography (Test 4) ofis hardenedshown in Figure steel with6. The the surface carbide texture tool of and the PCBNworkpiece tool machined under the by same The surface topography of hardened steel with the carbide tool and PCBN tool under the same cuttingthe parameters PCBN tool was (Test clear, 4) is and shown the ridge in Figure and the6. The scallop surface of the texture surface of texture the workpiece caused by machined the feed rate by the cutting parameters (Test 4) is shown in Figure 6. The surface texture of the workpiece machined by PCBNwere tool relatively was clear, low. and Therefore, the ridge better and surface the scallop topography of the surfacecould be texture obtained caused by machining by the feedhardened rate were the PCBN tool was clear, and the ridge and the scallop of the surface texture caused by the feed rate relativelysteel with low. the Therefore, PCBN cutter. better surface topography could be obtained by machining hardened steel were relatively low. Therefore, better surface topography could be obtained by machining hardened with the PCBN cutter. steel with the PCBN cutter.

(a) (b)

Figure 6. The machined(a) surface topography (Test 4): (a) carbide cutting(b) tool; (b) PCBN tool.

FigureFigure 6. The 6. The machined machined surface surface topography topography (Test 4): ( (aa) )carbide carbide cutting cutting tool; tool; (b) (PCBNb) PCBN tool. tool. Appl. Sci. 2019, 9, 772 7 of 15 Appl. Sci. 2018, 8, x FOR PEER REVIEW 7 of 15

The newnew PCBNPCBN tool tool and and carbide carbide tool tool were were selected sele tocted cut to the cut workpiece. the workpiece. The total The cutting total distancecutting wasdistance 20,000 was mm 20,000 (the tool mm flank (the weartool wasflank less wear than was 0.10 less mm). than Surface 0.10 roughnessmm). Surface was roughness measured oncewas everymeasured 2500 once mm. every Five different 2500 mm. positions Five different were selected positions for were each selected measurement, for each and measurement, the average valueand the of fiveaverage surface value roughness of five surface values roughness was taken asvalues the final was result.taken as The the surface final result. roughness The valuessurface obtained roughness at thevalues same obtained cutting at distance the same are cutting shown distance in Figure are7. shown in Figure 7.

2.5 2.5

2 2

1.5 1.5

1 1 Surface roughness(um) Surface 0.5 roughness(um) Surface 0.5

0 0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 0.25 0.5 0.75 1 1.25 1.5 1.75 2 Cutting path length(mm) 4 4 x 10 Cutting path length(mm) x 10 (a) (b)

Figure 7. The machined surface roughness (Test 4):4): (a)) PCBNPCBN tool;tool; ((b)) carbidecarbide cuttingcutting tool.tool.

ItIt was found that the surface quality of hardenedhardened steel machinedmachined by thethe cementedcemented carbidecarbide tooltool was significantlysignificantly higher than thatthat machinedmachined by thethe PCBNPCBN tool.tool. The surfacesurface roughness of hardenedhardened steel obtained by the carbide tool finishingfinishing waswas between 0.710.71 µumm and 2.19 µum.m. The surface roughness of hardened steel obtained byby the PCBN tool finishingfinishing waswas betweenbetween 0.210.21 µumm and 0.55 µum.m. The reasons for thethe differencedifference in in surface surface roughness roughness were were as as follows: follow (1)s: (1) the the friction friction coefficient coefficient of the of PCBNthe PCBN tool wastool relativelywas relatively small; small; (2) the (2) wear the wear of the of PCBN the PCBN tool wastool relativelywas relatively small small with with the processing. the processing.

3.3. Tool Life 3.3. Tool Life The wear morphology and the wear rate of tool ﬂankflank face at each cutting period were observed using a super-deep-scene 3D microscope. The wear curves of the PCBN tool andand cementedcemented carbidecarbide Appl.tool Sci. were 2018 obtainedobtained, 8, x FOR PEER asas shownshown REVIEW inin FigureFigure 8 8.. 8 of 15 When the cutting speed was 350 m/min and the cutting time was 60 min, the cemented carbide tool appeared damaged (as shown in Figure 8a). The reason was that the Co binder cannot withstand high temperature at high speed [27]. When the cutting speed was 150 m/min and 250 m/min, the cutting time before the cemented carbide tool reached the wear standard was significantly shorter than that of the PCBN tool. When the cutting speed was 50 m/min, the cutting performance of the cemented carbide tool was better than that of the PCBN tool. When the cutting speed was more than 150 m/min, the cutting performance of the PCBN tool was obviously higher than that of the cemented carbide tool. When the cutting speed was 50 m/min, the PCBN tool appeared damaged after 5 min of machining, as shown in Figure 8b.

(a) (b)

FigureFigure 8. 8. ToolTool wear wear curve curve (Test (Test 1): ( (aa)) carbide carbide cutting cutting tool; tool; ( (b)) PCBN PCBN tool. tool.

Therefore,When the cuttingit was found speed that was 150 350 m/min m/min was and the the critical cutting point time wasfor the 60 min,PCBN the tool cemented and cemented carbide carbidetool appeared tool for damaged intermittent (as shown machining in Figure of harden8a). Theed reasonsteel. When was that the thecutting Co binder speed cannotwas greater withstand than 150high m/min, temperature the performance at high speed of the [27 PCBN]. When tool the wa cuttings better speedthan that was of 150 the m/min cemented and carbide 250 m/min, tool; however, when the cutting speed was less than 150 m/min, the conclusion was just the opposite.

3.4. Chip Formation The friction and thermal conductivity between the tool and the chip are different due to the different tool materials in the process of high-speed milling of hardened steel, which leads to a difference of the contact surface between the chip and the tool. Because the majority of the heat generated during cutting is carried away by the chips, the high-temperature chips would react with oxygen in the air and produce temper colors of the chips. Therefore, chip color can be a characteristic indicator of cutting temperature [28–30]. The color of the chips can reflect the cutting temperature. The deeper the chip color is, the higher the cutting temperature was [30]. The chip temperature of the PCBN tool and carbide tool for the intermittent machining of hardened steel is shown in Figure 9. The overall temperature of chips was higher during the intermittent machining of hardened steel with the cemented carbide tool, and the contact surface between the tool and chips was rough.

Figure 9. Chip temperature distribution (Test 5): (a) PCBN tool; (b) carbide cutting tool.

When the PCBN tool was used to machine hardened steel, the cutting temperature was higher at the smaller chip thickness. The reason was that the action mechanism between the cutter and the workpiece was mainly ploughing and extrusion in the place where the chip thickness was small. Because the size of chip section was small, the excessive heat generated by ploughing and extrusion could not be discharged in time and, thus, the chip color was deepened. When the cutting thickness was large, the cutting heat was mainly caused by the shear action and the friction between the cutter and the chip. Appl. Sci. 2018, 8, x FOR PEER REVIEW 8 of 15

Appl. Sci. 2019, 9, 772 8 of 15 the cutting time before the cemented carbide tool reached the wear standard was signiﬁcantly shorter than that of the PCBN tool. When the cutting speed was 50 m/min, the cutting performance of the cemented carbide tool was better than that of the PCBN tool. When the cutting speed was more than 150 m/min, the cutting(a) performance of the PCBN tool was obviously higher(b) than that of the cemented carbide tool. WhenFigure the cutting8. Tool wear speed curve was (Test 50 m/min, 1): (a) carbide the PCBN cutting tool tool; appeared (b) PCBN damaged tool. after 5 min of machining, as shown in Figure8b. Therefore,Therefore, it it was found that 150 m/min was was the the critical critical point point for for the the PCBN tool and cemented carbide tooltool forfor intermittentintermittent machining machining of of hardened hardened steel. steel. When When the the cutting cutting speed speed was was greater greater than than 150 150m/min, m/min, the performancethe performance of the of PCBN the PCBN tool was tool better was thanbetter that than of thethat cemented of the cemented carbide tool;carbide however, tool; however,when the cuttingwhen the speed cutting was speed less than was 150less m/min, than 150 the m/min, conclusion the conclusion was just the was opposite. just the opposite.

3.4. Chip Formation The friction and thermal conductivity between between th thee tool tool and and the the chip are different due to the different tool materials in the process of high-speedhigh-speed milling of hardened steel, which leads to a difference of the contact surface between the chip and thethe tool.tool. Because the majority of the heat generated during cutting is carried away by the ch chips,ips, the high-temperature chips would react with oxygen in the air and produce temper colors of the ch chips.ips. Therefore, chip color can be a characteristic indicator of of cutting temperature temperature [2 [288–30].–30]. The The color color of of the the chips chips can can reflect reﬂect the the cutting cutting temperature. temperature. The deeper the the chip chip color color is, is, the the higher higher the the cutting cutting temperature temperature was was [30]. [30 The]. The chip chip temperature temperature of the of PCBNthe PCBN tool tool and and carbide carbide tool tool for forthe the intermittent intermittent ma machiningchining of ofhardened hardened steel steel is is shown shown in in Figure Figure 99.. The overalloverall temperaturetemperature of of chips chips was was higher higher during during the the intermittent intermittent machining machining of hardened of hardened steel steel with withthe cemented the cemented carbide carbide tool, andtool, theand contact the contact surface surface between between the tool the and tool chips and chips was rough. was rough.

Figure 9. ChipChip temperature temperature distribution (Test 5): ( a) PCBN tool; ( b) carbide cutting tool.

When the PCBN tool was used to machine hardened steel, the cutting temperature was higher at the the smaller smaller chip chip thickness. thickness. The The reason reason was was that that the the action action mechanism mechanism between between the thecutter cutter and andthe workpiecethe workpiece was was mainly mainly ploughing ploughing and and extrusion extrusion in inthe the place place where where the the chip chip thickness thickness was was small. small. Because the size of chip section was small, the ex excessivecessive heat generated by ploughing and extrusion could not be discharged in time and, thus, the chipchip color was deepened. When the cutting thickness was large, the cutting heat was ma mainlyinly caused by the shear action and the friction between the cutter and the chip. The microscopic characteristics of the chips of two tools under the same cutting conditions are shown in Figure 10. It was found that the chip contact surface of the PCBN cutter was smoother than that of the carbide cutter. The reason was that the PCBN tool had a smaller friction coefﬁcient, which decreased the friction between the tool and chip. There were pits in the middle of the chip contact surface machined by the PCBN tool. The chip contact surface processed by the carbide cutter involved mainly parallel scratches. Appl. Sci. 2018, 8, x FOR PEER REVIEW 9 of 15 Appl.Appl. Sci.Sci. 20182018,, 88,, xx FORFOR PEERPEER REVIEWREVIEW 99 ofof 1515 The microscopic characteristics of the chips of two tools under the same cutting conditions are shownTheThe in microscopicmicroscopicFigure 10. It characteristicswascharacteristics found that ofof the thethe chip chipschips co ntact ofof twotwo surface toolstools ofunderunder the PCBN thethe samesame cutter cuttingcutting was smootherconditionsconditions than areare thatshownshown of the inin FigureFigure carbide 10.10. cutter. ItIt waswas The foundfound reason thatthat was thethe chipthatchip the cocontactntact PCBN surfacesurface tool had ofof thethe a smaller PCBNPCBN frictioncuttercutter waswas coefficient, smoothersmoother which thanthan decreasedthatthat ofof thethe carbidethecarbide friction cutter.cutter. between TheThe reasonreason the tool waswas and thatthat chip. thethe PCBNPCBNThere tooltoolwere hadhad pits aa smallerinsmaller the middle frictionfriction of coefficient,coefficient, the chip contact whichwhich surfacedecreaseddecreased machined thethe frictionfriction by the betweenbetween PCBN tool. thethe tooltoolThe andchipand chip.contacchip. ThereTheret surface werewere processed pitspits inin thethe by middlethemiddle carbide ofof thethe cutter chipchip involved contactcontact mainlysurfaceAppl.surface Sci. parallelmachinedmachined2019, 9, 772 scratches. byby thethe PCBNPCBN tool.tool. TheThe chipchip contaccontactt surfacesurface processedprocessed byby thethe carbidecarbide cuttercutter involvedinvolved9 of 15 mainlymainly parallelparallel scratches.scratches.

(a) (b) ((aa)) ((bb)) Figure 10. Chip contact surface topography (Test 5): (a) PCBN tool; (b) carbide cutting tool. FigureFigure 10.10. ChipChip contactcontact surfacesurface topographytopography (Test(Test 5): 5): ( (aa)) PCBN PCBN tool; tool; ( (bb)) carbide carbide cutting cutting tool. tool. The free surface topologies of the chips processed by the two tools are shown in Figure 11. Pc representsTheThe freefree the surface surface surfacesawtooth topologiestopologies topologies pitch of chip.ofof of thethe the The chipschips chips sawtoot procesproces processedh seddegreesed byby by theandthe the two twothe two sawtoothtoolstools tools areare are shownpitchshown shown of inin chip FigureFigure in machined Figure 11.11. Pc11Pc . byrepresentsPcrepresents representsthe PCBN thethe thetool sawtoothsawtooth sawtooth were higher pitchpitch pitch ofof than of chip.chip. chip. those TheThe The ofsawtootsawtoot sawtooththe cahhrbide degreedegree degree cutting andand and the thetool. the sawtoothsawtooth sawtooth The reas pitchpitchon pitch was ofof of chipchipthat chip machinedthemachined larger flowbyby the the velocity PCBN PCBN of tool tool the were were chip higherhigherin the process than than those those using of of thethe the PC ca carbidecarbiderbideBN cutting cutting cutting tool tool. tool. led TheThe to a reas reasonreasstrongeronon was was separation that that the the larger largertrend inflowﬂowflow the velocity velocity process of ofof the thesawtooth chip chip in in chip thethe processprocess formation, using using which the the PCPCPCBN furtherBNBN cuttingcutting led to tool tool the increaseledled toto aa strongerstronger of sawtooth separationseparation pitch. trendtrend inin the the process process of ofof sawtoothsawtooth chip chip formation, formation, which which further further ledled toto thethe increaseincrease ofofof sawtoothsawtoothsawtooth pitch.pitch.pitch.

(a) (b) ((aa)) ((bb)) Figure 11. Chip free surface topography (Test 6, v = 500 m/min): (a) PCBN tool; (b) carbide cutting tool.FigureFigure 11. 11. ChipChip freefreefree surface surfacesurface topography topographytopography (Test (Test(Test 6, v6,6, = vv 500 == 500500 m/min): m/min):m/min): (a) (( PCBNaa)) PCBNPCBN tool; tool;tool; (b) carbide((bb)) carbidecarbide cutting cuttingcutting tool. tool.tool. TheThe free free surface morphology of of the the chip chip proce processedssed by by the the PCBN cutter with different cutting speedsspeedsTheThe is is freeshownfree shown surfacesurface in in Figure Figure morphologymorphology 12.12. It It was was ofof foun found thethed chipchip that that proceprocethe the sawtooth sawtoothssedssed byby pitchthethe pitch PCBNPCBN of of the the cuttercutter chip chip ( (PcwithPcwith)) increased increased differentdifferent with with cuttingcutting the the increasespeedsincreasespeeds isis in in shownshown cutting cutting inin speed. speed.FigureFigure The The12.12. ItItreason reason waswas founfoun was wasd dthat that thatthat the the thethe increase increase sawtoothsawtooth in in pitchcuttingpitch cutting ofof speed speedthethe chipchip made made ((PcPc) )the theincreasedincreased shear shear slip slip withwith more more thethe easilyincreaseeasilyincrease during during inin cuttingcutting chip chip formation,speed. speed. TheThe which reasonreason led waswas to thatthat the theincreasethe increaseincrease of sawtooth inin cuttingcutting degree. speedspeed mademade thethe shearshear slipslip moremore easilyeasily duringduring chipchip formation,formation, whichwhich ledled toto thethe increaseincrease ofof sawtoothsawtooth degree.degree.

(a) (b) (c) ((aa)) ((bb)) ((cc)) Figure 12. Cont. Appl. Sci. 2018, 8, x FOR PEER REVIEW 10 of 15

Appl. Sci. 2019, 9, 772 10 of 15 Appl. Sci. 2018, 8, x FOR PEER REVIEW 10 of 15

(d) (e)

Figure 12. Chip free surface topography machined by the PCBN tool with different cutting speeds (Test 6): (a) v = 250 m/min, Pc = 69 um; (b) v = 314 m/min, Pc = 76 um; (c) v = 376 m/min, Pc = 85 um; (d) v = 440 m/min, Pc = 92 um; (e) v = 500 m/min, Pc = 98 um.

3.5. Flank Wear (d) (e) FigureFigure(1) Oxidation 12. 12. ChipChip Wearfree free surfacesurface topography topography machined machined by by th thee PCBN PCBN tool tool with with different different cutting cutting speeds speeds (Test(TestDuring 6): 6): ( (thea)) v intermittent= 250 250 m/min, machiningPcPc == 69 69 um;µm; (of (bb) ) hardenedv v = = 314 314 m/min, m/min, stee l,PcPc at = = a76 76certain um;µm; ( (cc )temperature,) v v = = 376 376 m/min, m/min, some PcPc == 85elements 85 umµm;; in the tool((dd)) v vmaterial = = 440 440 m/min, m/min, reacted PcPc = =92with 92 um;µ m;O (e (in)e v) the v= =500 500air, m/min, m/min,which Pc Pcaccelerated= 98= 98um.µ m. the tool wear. The wear morphology and element analysis results of the PCBN tool flank are shown in Figure 13. Element analysis results 3.5.3.5. Flank Flank Wear Wear for unworn area A and worn area B are shown in Figure 13c,d. Compared with the elemental analysis at A,(1)(1) the Oxidation Oxidation content of Wear Wear oxygen at B was significantly increased. It could be seen from Figure 13b that a concaveDuringDuring shape the the was intermittent intermittent present for machining machining the CBN ofgrains, of hardened hardened resultin stee steel,g l,from at at a a possiblecertain certain temperature, temperature,oxidation of someboron some elements elementsnitride into in in the tool material reacted with O in the air, which accelerated the tool wear. The wear morphology and theB2O tool3. Therefore, material oxidationreacted with wear O occurredin the air, during which the accelerated intermittent the machiningtool wear. ofThe hardened wear morphology steel. andelement element(2) Diffusion analysis analysisresults Wear results of the of the PCBN PCBN tool tool ﬂank flan arek are shown shown in Figure in Figure 13. 13. Element Element analysis analysis results results for forunworn unwornDuring area area the A andAcutting and worn worn process, area area B B areunder are shown shown certain in in Figure Figureconditions, 13 13c,d.c,d. certain ComparedCompared elements withwith themay elemental spread from analysis analysis one atatregion A, A, the the to content contentanother, of of thereby oxygen oxygen causin at at B B was wasg diffusion significantly signiﬁcantly wear. inDiffusion increased.creased. wear It It could could can because be seen seen the from from chemical Figure Figure composition 13b13b that that a a concaveconcaveof the tool, shape shape thus was was weakening present present for forthe the the tool CBN CBN cutting grains, grains, perfor resultin resultingmance.g from from Figure possible possible 14 shows oxidation oxidation the change of of boron boron law nitride nitrideof element into into B O . Therefore, oxidation wear occurred during the intermittent machining of hardened steel. Bcontents22O3.3 Therefore, from the oxidation unworn wear area (A)occurred to the duringworn area the intermittent(B). machining of hardened steel. (2) Diffusion Wear During the cutting process, under certain conditions, certain elements may spread from one region to another, thereby causing diffusion wear. Diffusion wear can cause the chemical composition of the tool, thus weakening the tool cutting performance. Figure 14 shows the change law of element contents from the unworn area (A) to the worn area (B).

(a) (b)

Figure 13. Oxidation wear (Test 1 250 m/min): (a) wear surface morphology of ﬂank face; (b) local ampliﬁcation at point B; (c) elemental analysis at point A; (d) elemental analysis at point B.

(2) Diffusion Wear During the cutting process, under certain conditions, certain elements may spread from one Appl. Sci. 2018, 8, x FOR PEER REVIEW 11 of 15 region to another, thereby causing diffusion wear. Diffusion wear can cause the chemical composition of theFigure tool, thus13. Oxidation weakening wear the (Test tool 1 cutting250 m/min): performance. (a) wear surface Figure morphology 14 shows the of changeflank face; law (b of) local element contentsamplification from the at unworn point B; area(c) elemental (A) to the analysis worn at area point (B). A; (d) elemental analysis at point B.

(a)

100 Ti-KA Fe-KA Cr-KA

0 50100 150 200 Distance/um (b)

Figure 14. Diffusion wear (Test 11 250250 m/min):m/min): ( (aa)) the the range range of of elemental elemental analysis; analysis; ( (b)) the change law of element contents.

ItIt could could be be seen seen from Figure 14b14b that the cont contentsents of iron and chromium were significantlysignificantly increased, whichwhich indicated indicated that that iron iron and and chromium chromium diffused diffused from from the workpiece the workpiece material material to the cuttingto the tool.cutting Therefore, tool. Therefore, diffusion diffusion wear occurred wear occurred during theduring intermittent the intermittent machining machining of hardened of hardened steel. steel. (3) Adhesive Wear In the high-speed milling of hardened steel, due to the high temperature and pressure between the chipchip andand thethe flankflank face, face, chips chips were were observed observed to to adhere adhere to theto the cutting cutting edge edge easily, easily, and and chip chip and and tool weldingtool welding clearly clearly occurred, occurred, as shown as shown in Figure in Figure 15a. 15a. Chips Chips on the on cuttingthe cutting edge edge were were introduced introduced into theinto processing the processing region region and rubbed and rubbed or collided or collided with the with workpiece the workpiece surface before surface eventually before eventually falling off. Whenfalling chipsoff. When fell off, chips adhesive fell off,wear adhesive of the wear tool of occurred. the tool occurred. Elemental Elemental analysis wasanalysis performed was performed at point A,at point and the A, resultsand the are results shown are in Figureshown 15inb. Figure It was 15b. found It was that ironfound and that chromium iron and werechromium detected were on thedetected top and on bottomthe top surfacesand bottom of the surfaces chips. Therefore,of the chip thes. Therefore, wear mechanism the wear of mechanism the PCBN toolof the included PCBN adhesivetool included wear. adhesive wear. In total, a combination of oxidation wear, adhesive wear, and diffusion wear was believed to control the flank wear of the PCBN cutting tool.

(a) (b) Appl. Sci. 2018, 8, x FOR PEER REVIEW 11 of 15

Figure 13. Oxidation wear (Test 1 250 m/min): (a) wear surface morphology of flank face; (b) local amplification at point B; (c) elemental analysis at point A; (d) elemental analysis at point B.

(a)

100 Ti-KA Fe-KA Cr-KA

0 50100 150 200 Distance/um (b)

Figure 14. Diffusion wear (Test 1 250 m/min): (a) the range of elemental analysis; (b) the change law of element contents.

It could be seen from Figure 14b that the contents of iron and chromium were significantly increased, which indicated that iron and chromium diffused from the workpiece material to the cutting tool. Therefore, diffusion wear occurred during the intermittent machining of hardened steel. Appl. (3)Sci. Adhesive2018, 8, x FOR Wear PEER REVIEW 12 of 15 In the high-speed milling of hardened steel, due to the high temperature and pressure between the chipFigure and 15. the Adhesive flank face, wear: chips (a) wear were surface observed morphology to adhere of flank to face;the cutting(b) elemental edge analysis easily, atand point chip A. and tool welding clearly occurred, as shown in Figure 15a. Chips on the cutting edge were introduced into theIn total,processing a combination region and of oxidationrubbed or wear, collided adhesive with wear,the workpiece and diffusion surface wear before was eventuallybelieved to fallingcontrol off. the When flank chipswear fellof the off, PCBN adhesive cutting wear tool. of the tool occurred. Elemental analysis was performed at point A, and the results are shown in Figure 15b. It was found that iron and chromium were detectedAppl.3.6. Tool Sci. 2019 Damageon ,the9, 772 top and bottom surfaces of the chips. Therefore, the wear mechanism of the PCBN12 of 15 tool includedThere are adhesive three types wear. of tool damage: the first type of damage is low-speed chipping; the second type of damage is crack-free fragmentation; the third type is high-speed damage [31]. Ten PCBN tools were used to study tool damage, five of which were used to study low-speed impact damage. The failure modes of PCBN tools at different cutting speeds are shown in Figure 16. The main failure modes of PCBN tools in the process of cutting hardened steel at low speed were tool micro-chipping, and low-rate impact damage of the rake face and the flank face. The main failure modes of PCBN tools in the process of cutting hardened steel at high speed were flank wear and high-rate fatigue damage. In the low-speed machining process of hardened steel, the rake face of the PCBN tool suffered micro-chipping, as shown in Figure 16a. When the tool rake face suffered micro-chipping, the extrusion effect of the tool on the machined surface increased, which led to a sharp increase in cutting temperature. Then, the chips were melted and adhered to the tool tip (as shown in Figure 17), which further led to the deterioration of the cutting position of the tool tip. There were two main forms of low-rate impact damage of PCBN tools in the intermittent (a) (b) machining of hardened steel: (1) the conchoidal damage of the rake face (as shown in Figure 16b); (2) the largerFigure damaged 15. Adhesive area wear: of the (a) wearflank surface face (as morphology shown in of Figure flank face; 16c). (b) The elemental conchoidal analysis damage at point A. of the rake face was mainly caused by the improper selection of feed and cutting depth at a small cutting 3.6. Tool Damage speed. This form of damage was mainly caused by mechanical impact. The larger damaged area of the flankThere face are was three caused types ofby tool thedamage: bending thestress first of type the ofcutter, damage which is low-speed was greater chipping; than the the bending second typestrength of damage of the tool is crack-freematerial itself fragmentation; [31]. the third type is high-speed damage [31]. Ten PCBN toolsThe were wear used mechanism to study tool of the damage, flank was five detailed of which in were Section used 3.5. to study low-speed impact damage. The failurePCBN modestools ofcan PCBN resist tools high-temperature at different cutting wear speeds at high are shown speed. in FigureHowever, 16. Thein mainintermittent failure modesmachining, of PCBN the tool tools was in thesubjected process to of alternating cutting hardened thermal steel load at and low impact speed load, were which tool micro-chipping, led to fatigue andcrack low-rate of the tool impact after damage a certain of period the rake of face stable and cutting. the flank The face. crack The extension main failure led to modes high-speed of PCBN fatigue tools indamage the process of the ofcutter, cutting as hardenedshown in Figure steel at 16e. high speed were flank wear and high-rate fatigue damage.

(a) (b) (c)

(d) (e)

Figure 16. MacroscopicMacroscopic damage damage morphology morphology of ofPCBN PCBN tool tool intermittent intermittent cutting cutting of hardened of hardened steelsteel tool tool(Test (Test 1): (a 1):) micro-chipping (a) micro-chipping (v = 50 (v m/min); = 50 m/min); (b) the (b rake) the face rake (v face = 50 (v m/min); = 50 m/min); (c) the flank (c) the face ﬂank (v face= 50 (vm/min); = 50 m/min); (d) flank ( dwear) ﬂank (v wear= 250 (vm/min); = 250 m/min);(e) high-rate (e) high-rate fatigue damage fatigue damage(v = 350 m/min). (v = 350 m/min).

In the low-speed machining process of hardened steel, the rake face of the PCBN tool suffered micro-chipping, as shown in Figure 16a. When the tool rake face suffered micro-chipping, the extrusion effect of the tool on the machined surface increased, which led to a sharp increase in cutting temperature. Then, the chips were melted and adhered to the tool tip (as shown in Figure 17), which further led to the deterioration of the cutting position of the tool tip. Appl. Sci. 2019, 9, 772 13 of 15 Appl. Sci. 2018, 8, x FOR PEER REVIEW 13 of 15

Figure 17. The macrostructure of the tool rake face.face.

4. ConclusionsThere were and two Outlook main forms of low-rate impact damage of PCBN tools in the intermittent machining of hardened steel: (1) the conchoidal damage of the rake face (as shown in Figure 16b); (2) the largerThe cutting damaged performances area of the flank (cutting face force, (as shown chip, in surface Figure quality,16c). The tool conchoidal life) and damage the wear/damage of the rake facecharacteristics was mainly of caused PCBN bytools the improperwere investigated selection for of feedthe andintermittent cutting depthmachining at a small of hardened cutting speed. steel This(Cr12MoV, form of 59HRC). damage The was main mainly conclusions caused by were mechanical as follows: impact. The larger damaged area of the flank face1. wasBased caused on the by evaluation the bending indexes stress ofof thecutting cutter, force, which surface wasgreater quality, than chip the morphology, bending strength and cutting of the tool materialtemperature, itself it [31 was]. concluded that the cutting performance of the PCBN tool was better than Thethat wearof the mechanism coated cemented of the flankcarbide was tool. detailed in Section 3.5.

2. PCBNA combination tools can of resist oxidation high-temperature wear, adhesive wear wear, at high and speed. diffusion However, wear was in intermittent believed to machining,control the the toolflank was wear subjected of the PCBN to alternating cutting thermaltool. load and impact load, which led to fatigue crack of the tool3. afterThe main a certain failure period modes of stableof the cutting.PCBN tool The in crack the machining extension ledof hardened to high-speed steel fatigueat low speed damage were of the cutter,tool micro-chipping, as shown in Figure and 16 low-ratee. impact damage of the rake face and the flank face. The main failure modes of the PCBN tool in the machining of hardened steel at high speed were flank 4. Conclusionswear and high-rate and Outlook fatigue damage. 4. The PCBN tool’s life is affected by the cutting speed. When the cutting speed was greater than The cutting performances (cutting force, chip, surface quality, tool life) and the wear/damage 150m/min, the PCBN tool’s life was better than that of the carbide tool. Meanwhile, PCBN tools characteristics of PCBN tools were investigated for the intermittent machining of hardened steel should be avoided at low cutting speeds. (Cr12MoV, 59HRC). The main conclusions were as follows: 5. The sawtooth pitch of chip increased with the increase in cutting speed. The reason was that the 1. increaseBased on in the cutting evaluation speed indexes made the of cuttingshear slip force, more surface easily quality, during chip chip morphology, formation, which and cutting led to thetemperature, increase in it sawtooth was concluded degree. that the cutting performance of the PCBN tool was better than 6. Whenthat of the the PCBN coated tool cemented was used carbide to machine tool. hardened steel, the cutting temperature was higher 2. atA the combination smaller chip of oxidation thickness. wear, The reason adhesive was wear, thatand the diffusionaction mechanism wear was between believed the to controlcutter and the theflank workpiece wear of the was PCBN mainly cutting ploughing tool. and extrusion in the place where the chip thickness was 3. small.The main Because failure the modes size of of chip the section PCBN toolwas insmall, the machiningthe excessive of hardenedheat generated steel atby low ploughing speed were and extrusiontool micro-chipping, could not be and discharged low-rate in impact time and, damage thus, of the the chip rake color face andwas thedeepened. flank face. The main Authorfailure Contributions: modes of theThe PCBNidea of tool this inproject the machining was conceived of hardened by H.G.; steelH.G. atconsulted high speed to relevant were flank high-level wear papersand on high-rateaspect of cutting fatigue performance damage. and wear/damage characteristics of PCBN tool in hard milling, and 4.wroteThe this PCBNproject; tool’sX.L. and life Z.C. is affected reviewed by this the project cutting and speed. proposed When constructive the cutting guidance speed wasto make greater the article than more 150complete. m/min, the PCBN tool’s life was better than that of the carbide tool. Meanwhile, PCBN tools Funding:should This be project avoided was at supported low cutting by speeds. the Projects of International Cooperation and Exchanges NSFC 5.(51720105009).The sawtooth pitch of chip increased with the increase in cutting speed. The reason was that the Conflictsincrease of Interest: in cutting The speedauthors made declare the no shear conflicts slip of more interest. easily during chip formation, which led to the increase in sawtooth degree. 6.ReferencesWhen the PCBN tool was used to machine hardened steel, the cutting temperature was higher at the smaller chip thickness. The reason was that the action mechanism between the cutter and 1. Huang, Y.; Chou, Y.K.; Liang, S.Y. CBN tool wear in hard turning: a survey on research progresses. Int. J. the workpiece was mainly ploughing and extrusion in the place where the chip thickness was Adv. Manuf. Technol. 2007, 35, 443–453. small. Because the size of chip section was small, the excessive heat generated by ploughing and 2. Monteiro, S.N.; Skury, A.L.; de Azevedo, M.G.; Bobrovnitchii, G.S. Cubic boron nitride competing with diamondextrusion as could a superhard not be engin dischargedeering material—An in time and, overview. thus, the J. chip Mater. color Res. was Technol. deepened. 2013, 2, 68–74. 3. Xi, Y.; Zhan, H.; Rashid, R.R.; Wang, G.; Sun, S.; Dargusch, M. Numerical modeling of laser assisted machining of a beta titanium alloy. Comp. Mater. Sci. 2014, 92, 149–156. Appl. Sci. 2019, 9, 772 14 of 15

Author Contributions: The idea of this project was conceived by H.G.; H.G. consulted to relevant high-level papers on aspect of cutting performance and wear/damage characteristics of PCBN tool in hard milling, and wrote this project; X.L. and Z.C. reviewed this project and proposed constructive guidance to make the article more complete. Funding: This project was supported by the Projects of International Cooperation and Exchanges NSFC (51720105009). Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

1. Huang, Y.; Chou, Y.K.; Liang, S.Y. CBN tool wear in hard turning: a survey on research progresses. Int. J. Adv. Manuf. Technol. 2007, 35, 443–453. [CrossRef] 2. Monteiro, S.N.; Skury, A.L.; de Azevedo, M.G.; Bobrovnitchii, G.S. Cubic boron nitride competing with diamond as a superhard engineering material—An overview. J. Mater. Res. Technol. 2013, 2, 68–74. [CrossRef] 3. Xi, Y.; Zhan, H.; Rashid, R.R.; Wang, G.; Sun, S.; Dargusch, M. Numerical modeling of laser assisted machining of a beta titanium alloy. Comp. Mater. Sci. 2014, 92, 149–156. [CrossRef] 4. Taylan, F.; Çolak, O.; Kayacan, M.C. Investigation of TiN coated CBN and CBN cutting tool performance in hard milling application. Strojniski Vestnik J. Mech. Eng. 2011, 57, 417–424. [CrossRef] 5. Kurt, A.; ¸Seker, U. The effect of chamfer angle of polycrystalline cubic boron nitride cutting tool on the cutting forces and the tool stresses in finishing hard turning of AISI 52100 steel. Mater. Des. 2005, 26, 351–356. [CrossRef] 6. Li, S.; Chen, T.; Qiu, C.; Wang, D.; Liu, X. Experimental investigation of high-speed hard turning by PCBN tooling with strengthened edge. Int. J. Adv. Manuf. Technol. 2017, 92, 3785–3793. [CrossRef] 7. Zhou, J.M.; Walter, H.; Andersson, M.; Stahl, J.E. Effect of chamfer angle on wear of PCBN cutting tool. Int. J. Mach. Tools Manuf. 2003, 43, 301–305. [CrossRef] 8. Özel, T.; Karpat, Y.; Srivastava, A. Hard turning with variable micro-geometry PcBN tools. CIRP Ann. Manuf. Technol. 2008, 57, 73–76. [CrossRef] 9. Poulachon, G.; Bandyopadhyay, B.P.; Jawahir, I.S.; Pheulpin, S.; Seguin, E. The influence of the microstructure of hardened tool steel workpiece on the wear of PCBN cutting tools. Int. J. Mach. Tools Manuf. 2003, 43, 139–144. [CrossRef] 10. Khanna, N.; Rashid RA, R.; Palanisamy, S. Experimental evaluation of the effect of workpiece heat treatments and cutting parameters on the machinability of Ti-10V-2Fe-3Al β titanium alloy using Taguchi’s design of experiments. Int. J. Mach. Mach. Mater. 2017, 19, 374–393. 11. Zareena, A.R.; Rahman, M.; Wong, Y.S. Binderless CBN tools, a breakthrough for machining titanium alloys. J. Manuf. Sci. Eng. 2005, 127, 277–279. [CrossRef] 12. Li, L.; Wu, M.; Liu, X.; Cheng, Y.; Yu, Y. Experimental study of the wear behavior of PCBN inserts during cutting of GH4169 superalloys under high-pressure cooling. Int. J. Adv. Manuf. Technol. 2018, 95, 1941–1951. [CrossRef] 13. Pavel, R.; Marinescu, I.; Deis, M.; Pillar, J. Effect of tool wear on surface finish for a case of continuous and interrupted hard turning. J. Mater. Process. Technol. 2005, 170, 341–349. [CrossRef] 14. Ko, T.J.; Kim, H.S. Surface integrity and machineability in intermittent hard turning. Int. J. Adv. Manuf. Technol. 2001, 18, 168–175. [CrossRef] 15. Oliveira, A.J.; Diniz, A.E.; Ursolino, D.J. Hard turning in continuous and interrupted cut with PCBN and whisker-reinforced cutting tools. J. Mater. Process. Technol. 2009, 209, 5262–5270. [CrossRef] 16. Tang, L.; Sun, Y.; Li, B.; Shen, J.; Meng, G. Wear performance and mechanisms of PCBN tool in dry hard turning of AISI D2 hardened steel. Tribol. Int. 2018, 132, 228–236. [CrossRef] 17. Chen, L.; Stahl, J.E.; Zhao, W.; Zhou, J. Assessment on abrasiveness of high chromium cast iron material on the wear performance of PCBN cutting tools in dry machining. J. Mater. Process. Technol. 2018, 255, 110–120. [CrossRef] 18. Gutnichenko, O.; Bushlya, V.; Zhou, J.; Ståhl, J.E. Tool wear and machining dynamics when turning high chromium white cast iron with pcBN tools. Wear 2017, 390, 253–269. [CrossRef] 19. Gopalsamy, B.M.; Mondal, B.; Ghosh, S.; Arntz, K.; Klocke, F. Experimental investigations while hard machining of DIEVAR tool steel (50 HRC). Int. J. Adv. Manuf. Technol. 2010, 51, 853–869. [CrossRef] Appl. Sci. 2019, 9, 772 15 of 15

20. Wojciechowski, S.; Twardowski, P. Tool Life and Process Dynamics in High Speed Ball End Milling of Hardened Steel. Procedia CIRP 2012, 1, 289–294. [CrossRef] 21. Okada, M.; Hosokawa, A.; Tanaka, R.; Ueda, T. Cutting performance of PVD-coated carbide and CBN tools in hardmilling. Int. J. Mach. Tools Manuf. 2011, 51, 127–132. [CrossRef] 22. Elbestawi, M.A.; Chen, L.; Becze, C.E.; El-Wardany, T.I. High-Speed Milling of Dies and Molds in Their Hardened State. CIRP Ann. Manuf. Technol. 1997, 46, 57–62. [CrossRef] 23. Boing, D.; Schroeter, R.B.; de Oliveira, A.J. Three-dimensional wear parameters and wear mechanisms in turning hardened steels with PCBN tools. Wear 2018, 398, 69–78. [CrossRef] 24. Luo, S.Y.; Liao, Y.S.; Tsai, Y.Y. Wear characteristics in turning high hardness alloy steel by ceramic and CBN tools. J. Mater. Process. Technol. 1999, 88, 114–121. [CrossRef] 25. Saketi, S.; Sveen, S.; Gunnarsson, S.; M’Saoubi, R.; Olsson, M. Wear of a high cBN content PCBN cutting tool during hard milling of powder metallurgy cold work tool steels. Wear 2015, 332, 752–761. [CrossRef] 26. Cui, X.; Guo, J.; Zhao, J.; Yan, Y. Analysis of PCBN tool failure mechanisms in face milling of hardened steel using damage equivalent stress. Int. J. Adv. Manuf. Technol. 2015, 80, 1985–1994. [CrossRef] 27. Rashid, R.R.; Palanisamy, S.; Sun, S.; Dargusch, M. A case-study on the mechanism of ﬂank wear during laser-assisted machining of a titanium alloy. Int. J. Mach. Mach. Mater. 2017, 19, 538–553. [CrossRef] 28. Venkatesh, V.C.; Zhou, D.Q.; Xue, W.; Quinto, D.T. A study of chip surface characteristics during the machining of steel. CIRP Ann. 1993, 42, 631–636. [CrossRef] 29. Ning, Y.; Rahman, M.; Wong, Y.S. Investigation of chip formation in high speed end milling. J. Mater. Process. Technol. 2001, 113, 360–367. [CrossRef] 30. Zhang, S.; Guo, Y.B. An experimental and analytical analysis on chip morphology, phase transformation, oxidation, and their relationships in ﬁnish hard milling. Int. J. Mach. Tool. Manuf. 2009, 49, 805–813. [CrossRef] 31. Xing, G. Metal Cutting Technology; China Agriculture Press: Beijing, China, 1983; pp. 6–7.

© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).