Measurement 46 (2013) 2695–2704

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Measurement

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A comparative study on performance of multilayer coatedand uncoated carbide inserts when turning AISI D2 steelunder dry environment

0263-2241/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.measurement.2013.04.024

⇑ Corresponding author. Tel.: +91 674 6540805.E-mail address: aklala72@gmail.com (A.K. Sahoo).

Ashok Kumar Sahoo a,⇑, Bidyadhar Sahoo b

a School of Mechanical Engineering, KIIT University, Bhubaneswar-24, Odisha, Indiab Indira Gandhi Institute of Technology, Sarang, Talcher, Odisha, India

a r t i c l e i n f o a b s t r a c t

Article history:Received 7 April 2012Received in revised form 10 December 2012Accepted 16 April 2013Available online 28 April 2013

Keywords:Multilayer coated carbideFlank wearSurface roughnessRegressionEconomic analysis

The present work deals with a comparative study on flank wear, surface roughness, toollife, volume of chip removal and economical feasibility in turning high carbon high chro-mium AISI D2 steel with multilayer MTCVD coated [TiN/TiCN/Al2O3/TiN] and uncoated car-bide inserts under dry cutting environment. Higher micro hardness of TiN coated carbidesamples (1880 HV) compared to uncoated carbide (1430 HV) is observed and depicts betterresistance against abrasion. The low erosion rate was observed in TiN coated insert com-pared to uncoated carbide. The tool life of TiN coated insert is found to be approximately30 times higher than the uncoated carbide insert under similar cutting conditions and pro-duced lower surface roughness compared to uncoated carbide insert. The dominant wearmechanism was found to be abrasion and progression of wear was steady using multilayerTiN coated carbide insert. The developed regression model shows high determination coef-ficient i.e. R2 = 0.977 for flank wear and 0.94 for surface roughness and accurately explainsthe relationship between the responses and the independent variable. The machining costper part for uncoated carbide insert is found to be 10.5 times higher than the multilayerTiN coated carbide inserts. This indicates 90.5% cost savings using multilayer TiN coatedinserts by the adoption of a cutting speed of 200 m/min coupled with a tool feed rate of0.21 mm/rev and depth of cut of 0.4 mm. Thus, TiN coated carbide tools are capable ofreducing machining costs and performs better than uncoated carbide inserts in machiningD2 steel.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

The industry is the key driver for the development ofcutting tool materials to enhance productivity, to machinedifficult-to-cut materials and to improve high finishedproduct. Carbides, the popular cutting tool material devel-oped in 1928 to achieve high production rates. The toolmaterial should exhibit thermal conductivity-low at thesurface to resist incoming of heat and high at the core toquickly dissipate the heat entered. No such tool materials

have these dual properties of thermal conductivity. Thisconcept brings to the development of coated carbide insert.In recent years, wear resistant thin film hard coatings insingle layer or multilayer form developed which brings agreat breakthrough in metal cutting industry. Usually, theinternal layer ensures good adhesion to the substrate,one or more middle layers ensures hardness and strengthof the coating. The intermediate layers are chosen suchthat the transitional layer formed between any of themallows best mutual adhesion. While the external layer en-sures good tribological properties i.e. low coefficient offriction and therefore reduces adhering tendency on therake face [1]. The application of hard, wear resistantcoating on the cutting tools began in early 1970s and today

2696 A.K. Sahoo, B. Sahoo / Measurement 46 (2013) 2695–2704

nearly 50% HSS, 85% of carbide and 40% of super-hard toolsused in industry are coated and that CVD is found to be themajor coating process [2,3]. Combination of soft and hardcoating is suitable for dry machining in recent develop-ment and research stage. Moderate temperature CVD(MTCVD) is a further development in CVD technologywhere the leading coating is TiCN which can be appliedat 850 �C. This enhances the strength of the cutting edgesand making coated inserts suitable for interrupted cutting.

Khrais and Lin [4] found that dry cutting may be pre-ferred to wet cutting at high cutting speed i.e. from 200to 400 m/min for PVD TiAlN coating inserts during machin-ing AISI 4140 steel. Micro-abrasion and micro-fatigue werefound to be the principal wear mechanisms at higher cut-ting speeds (310–410 m/min) under dry cutting. Avila et al.[5] observed that the plasma assisted PVD monolayer TiNcoated carbide tool outperformed the uncoated carbidetool with regard to the crater wear resistance providinglower crater width and depth during continuous turningof AISI 8620 steel. Rech [6] investigated the tribologicalperformance of PVD TiN, TiAlN and TiAlN + MoS2 coatedand uncoated carbide insert. The TiN and (Ti,Al)N + MoS2

coatings was found to be best enhancement of tribologicalcharacteristics compared to uncoated tools i.e. reduction ofthe tool–chip contact area, reduction of secondary shearzone thickness and of the interface temperature duringthe machining of 27MnCr5 steels. Finally wear test showsthat the sliding ability of the TiN coating is a key parameterleading to a better wear resistance compared to a (Ti, Al) Ncoating. Su and Kao [7] found that multilayer PVD TiN/TiCN/TiN coatings exhibited better wear resistance thanthe binary-layer TiN/TiCN and the single-layer TiN coatingin milling operation. Scheerer et al. [8] investigated thepotentials of newly developed (Cr, Al) N coatings on car-bide insert with different percentage of Cr and Al duringturning spheroidal cast iron under dry environment. Thebest coatings in the tests were similar chromium and alu-minum contents in the region of 15%, the nitrogen contentwas about 60% respectively. High chromium content coat-ing deteriorated the performance of tool in dry turning. Inthe experiment conducted by Sahin [9] multi layer TiNCVD coated carbide tools outperformed than bilayer coatedtools during machining SiCp-reinforced composites. Tuffyet al. [10] observed that the wear of TiN coating with3.5 lm thickness was 40 times less than uncoated carbidedue to decrease of diffusion wear by application of thinfilm coating during turning AISI 1040 steel. Khalid [11]observed that, CVD multi-layered TiN/Al2O3 coating per-formed better owing to better structural integrity and lesssensitivity to brittle failure compared to single-layered TiNcoating. Ciftci [12] found that cutting force for CVD multi-layer TiC/TiCN/TiN coated carbide cutting tools was lowerthan CVD TiCN/TiC/Al2O3 coated tools during dry turningof austenitic stainless steels. It was because of lubricityprovided by TiN uppermost coating layer as coefficient offriction being very low. At low cutting speed, surfaceroughness was higher due to built-up-edge formation.Lim et al. [13] demonstrated that the application of singlelayer TiC coating exhibited better performance by increas-ing the wear resistance of carbide tools and could beoperated at high cutting speed and feed during machining

AISI 1045 steel. Gökkaya and Nalbant [14] investigated theeffects of different insert radii, depths of cut and feed rateson the surface quality of the work pieces during machiningof AISI 1030 steel without coolant by CVD multilayercoated carbide [TiC/Al2O3/TiN (outermost is TiN)] insert.It was observed that increase of insert radius decreasesthe surface roughness and increasing cutting speed anddepth of cut increases the surface roughness. Popescuet al. [15] investigated the drilling performance of TiNcoated HSS drill. Due to the special properties of the TiNcompound i.e. high microhardness, good resistance towear, abrasion and corrosion, chemical stability and lowfriction coefficient, coatings of this material were widelyused to increase the life of various machine parts and toolsunder severe working condition. GÖ KKAYA and Nalbant[16] observed that lower surface roughness was inducedusing a CVD multi layer coated tool (outermost with TiN)compared to uncoated, coated with AlTiN and coated withTiAlN using the PVD technique during dry turning of AISI1015 steel. Vikram Kumar et al. [17] discussed the deposi-tion and characterization of multilayer TiN/Al2O3 coatingson cemented tungsten carbide cutting tools using reactivesputtering. Yigit et al. [18] found that HTCVDTiCN + TiC + TiCN + Al2O3 + TiN multilayer coating with10.5 lm thick exhibited better performance of tool lifeduring machining nodular cast iron. Sahoo and Sahoo[19] optimized the flank wear and surface roughnesssimultaneously during turning hardened AISI 4340 steelusing multilayer TiN/TiCN/Al2O3/ZrCN coated carbide in-sert and developed mathematical model. Wang [20] ob-served that multilayer hard surface coatings CVD(TiC + Al2O3 + TiN) reduce the cutting forces, although thereduction is marginal under lighter cutting conditions thanuncoated carbide insert in turning mild steel. Sert et al.[21] compared the tool wear performance of coated car-bide inserts. It was observed that PVD TiAlN coated carbidetools are suitable under the cutting speed of 200 m/minduring machining AISI 5140 steel. CVD TiN coated carbidecutting tools were more suitable than TiAlN coated cuttingtool at higher cutting speed of 250 m/min and minimumtool wear was achieved. The higher wear rate wasobserved for cermet cutting tool at all cutting speed from100 to 250 m/min. This confirmed the suitability of CVDTiN coated carbide insert at high speed machining. Accord-ing to Grzesik [3], multilayer CVD (TiC/Al2O3/TiN) coatedcarbide insert performed well compared to single layerTiC coated carbide, binary coated carbide (TiC/TiN) and un-coated carbide during machining medium carbon steel.Neseli et al. [22] investigated the influence of tool geome-try on the surface finish during turning AISI 1040 steel.

Machinability of any combination of work-tool pair isusually characterized by (i) flank wear and tool life, (ii) sur-face roughness, (iii) cutting force and power consumptionand (iv) nature and type of chip forms. In metal cutting,wear of the cutting tool occurs by mechanical breakage,by plastic deformation and by gradual wear. Due to thecontinuous research and development of superhard toolmaterials like ceramic, CBN and diamond, the tool failureby mechanical breakage and plastic deformation are nowbeing controlled. However, wear is inevitable and cannotbe controlled. Flank wear is one of the most important

A.K. Sahoo, B. Sahoo / Measurement 46 (2013) 2695–2704 2697

responses that affect tool life and product quality inmachining and also raises power consumption. Hence,the rate of growth wear should be controlled so that thecutting tools can be utilized over a longer period of time.In recent years, dry machining is preferred choice overwet machining. The economic aspects and the enhancedecologic responsibility, as well as results of the latest re-search indicating a hazardous influence of emulsionsdraws the industries attention to alternative machiningtechnologies i.e. dry machining.

From the literature, studies related on exact tool wearmechanisms and surface finish during turning of high car-bon high chromium D2 cold work tool steel with increas-ing level of hardness using multilayer coated carbideinserts are limited. It is difficult to machine owing to theirhigh carbon and high chromium content and thus machin-ability is poor. At the same time it is also important toknow the results when using coated carbide tools, mainlyfor economical reasons though it is otherwise machinedby costly ceramic and CBN tools. Therefore, this paperattempts a comparative study on the performance ofmultilayer-coated carbide and uncoated inserts in machin-ing AISI D2 steel (22 ± 1 HRC) at higher cutting speed rangeunder dry environment. Some characterization test resultsof both workpiece and cutting inserts are presented for itseffectiveness. The evolution of flank wear and surfaceroughness with time have been analyzed and predictedthe tool life of both inserts. In order to predict flank wearand surface roughness in the turning process, mathemati-cal models have been developed using multiple linearregression analysis and predicted successfully. Lastly, acomparison between both inserts was made based on totalmachining cost per part to justify the economic viability.

2. Experimental details

The details of experimental set up, instrumentationsand cutting conditions adopted for the study are presentedbelow.

2.1. Test specimen

The workpiece material used for experimentation washigh carbon high chromium (AISI D2) steel in the form ofround bar of diameter 40 mm and 200 mm length. It isknown as cold work tool steel in the industry and suitablefor press molds, tools and dies. The hardness of as receivedworkpiece was found to be 22 ± 1 HRC. D2 tool steel is par-ticularly investigated because of its wide application inmanufacturing of molds and dies for automobile and elec-tronic components.

2.2. Cutting inserts and M/C tool

In the experimental study, commercially available un-coated (TTS) and medium temperature CVD coated carbide(MTCVD) insert manufactured by WIDIA were used. Themultilayer coated carbide insert possesses the substrate,a single layer of TiN to enhance adhesion with thesubstrate, an intermediate layer of TiCN and Al2O3 and a

TiN outer layer, designated as grade TN 2000 (TiN/TiCN/Al2O3/TiN) with application range of P15–P30. The two in-serts have the geometry of ISO designation CNMG 120408(80� diamond shaped insert) with negative rake angle andnose radius of 0.8 mm. The inserts were rigidly mountedon a tool holder designated by ISO as PCLNR 2525 M12.The type of the machine tool used for the turning testwas a high rigid CNC lathe model: JOBBER XL, AMS (ACEDesigners, INDIA) of maximum 3500 rpm and 16 kW max-imum spindle power with Sinumeric controller.

2.3. Measurement and cutting conditions

The experiments have been carried out using both un-coated and multilayer TiN coated carbide insert in plainturning of D2 steel under dry environment. The aim is toexplore the role of chosen parameter on the machinabilitycharacteristics mainly in terms of progression of flank wearand surface roughness and assess the tool life of bothinserts in dry machining conditions. The three surfaceroughness parameters were measured for surface character-istics i.e. arithmetic surface roughness average (Ra), maxi-mum peak-to-valley height within sampling length (Rz)and maximum peak-to-valley height within assessmentlength (Rt). To measure roughness of the surface formedwhile processing the workpiece, the cutoff length has beenfixed as 0.8 mm and 4 mm assessment length. The surfaceroughness tester (Taylor Hobson, Surtronic 25) was cali-brated using a standard calibration specimen prior to themeasurements. The measurement was taken at four loca-tions (90� apart) around the circumference of the workpiec-es and repeated twice at each point on the face of themachined surface and the average values were reported.Nikon Profile Projector, model V10AD with magnification20–0� was used to measure the wear on the flank surfaceat every successive runs. Images of flank surfaces of insertswere captured by stereo zoom microscope (Model RSM-8,Radical instrument, India). The tool life criteria were setbased on a maximum flank wear width of VBc = 0.3 mmmeasured at tool nose radius corner or when the tool isseverely broken due to occurrence of catastrophic failure.Cutting was stopped when the tool flank wear reached0.3 mm. The criterion is particularly chosen to achievedimensional accuracy and surface roughness requiredfor finish turning. Cutting parameters and conditions formachining of AISI D2 steel have been presented in Table 1.

3. Characterization test results

3.1. Chemical composition test

The chemical composition test of workpiece has beendone by Spectro Metal Analyzer (Model – Spectro Max).The principle of working is by comparing the wave lengthof emitted radiations from the test specimen as a result ofspark with that of different elements. The machinecomprises a metallic specimen table which is positivelycharged. The specimen to be tested is placed on the tableand a negatively charged electrode contained in anelectrode holder is brought closer to the test specimen.

Table 1Cutting conditions of successive runs in turning D2 steel.

Sl.no.

Cuttingconditions

Descriptions

1 Machining Time,Tc (min)

1, 3, 5, 8, 12, 22, 32

2 W/P D2 steel3 Hardness 22 ± 1 HRC4 Cutting speed (v) 200 m/min5 Feed (f) 0.21 mm/rev6 Depth of cut (d) 0.4 mm7 Cutting

environmentDry

8 Cutting tools 1. TiN/TiCN/Al2O3/TiN coated carbide(TN 2000) – [P15-30]2. Uncoated carbide (TTS) – [P20-30]

9 Tool geometry CNMG 12040810 Tool holder PCLNR 2525 M1211 Overhang length 30 mm12 Response Flank wear, surface roughness

Indentation of

uncoated

carbide insert

(a)

(b)Fig. 1. Indentation of (a) uncoated carbide insert and (b) TiN coatedcarbide insert.

2698 A.K. Sahoo, B. Sahoo / Measurement 46 (2013) 2695–2704

This results spark generation. The wave length of emittedradiations is compared with that of individual elementsand percentage of each element is recorded. Major compo-nents and their percent composition by weight have beenpresented in Table 2.

3.2. Microhardness

Microhardness of coated and uncoated carbide insertswere measured with Vickers test using a 1 kg load (HV1)by diamond indenter, Leco microhardness tester, USAequipped with optical microscope and monitor. Penetra-tion depth ranged from 0.85 to 1.2 mm. Four observationsare taken on each sample and the average value is reported.The indentation images for both coated and uncoated car-bide inserts are shown in Fig. 1. The microhardness (HV)value was found to be 1430 and 1880 for uncoated andmultilayer TiN coated carbide insert respectively. The high-er hardness of multilayer TiN coated insert compared to un-coated insert was probably due to the presence of thenitriding layer. From this test results, the wear resistanceof multilayer TiN coated insert is considered to be higherthan uncoated carbide insert.

3.3. Erosion wear behavior of inserts

The Air Jet Erosion Tester of model TR-471-600, Ducom,India was used to test the erosion resistance of coated anduncoated insert with a known abrasive particulate (Al2O3)utilizing compressed air as a propellant. In this work, roomtemperature solid particle erosion test on coated and un-coated inserts is carried out under 90� impingement angle,

Table 2Chemical composition test result of D2 steel specimen in percentage byweight.

C(%)

Si(%)

Mn(%)

P(%)

S(%)

Cr(%)

Mo(%)

V(%)

W(%)

1.55–1.75

0.25–0.40

0.20–0.40

0.03 0.03 11–12

0.5–0.7

0.1–0.5

0.4–0.6

particle velocity of 39 m/s, pressure of 0.2 bar, dischargerate of erodent being 8 gm/min, stand off distance 10 mm,size of erodent 50 lm and at ambient temperature (32 �C)of about 10 min. This parameter is constant for all two in-serts tested. Erosion rate, defined as the loss of weight ofcoated or uncoated material per unit weight of erodent(mg/kg) is calculated and shown in Table 3. It is done bymeasuring the weight of the samples at the beginning ofthe test and at regular intervals in the test duration. Aprecision electronic balance with ±0.1 mg accuracy is usedfor weighing. From the results, lowest erosion wear ratewas observed in multilayer TiN coated carbide insertcompared to uncoated carbide insert.

Table 3Readings of erosion wear test.

Insert Initialweight(gm)

Finalweight(gm)

Difference(gm)

Erosionrate in(mg/kg)

TiN coatedcarbide

8.963 8.96007 0.00293 36.625

Uncoated carbide 7.59246 7.58475 0.00771 96.375

A.K. Sahoo, B. Sahoo / Measurement 46 (2013) 2695–2704 2699

4. Evolution of flank wear and surface roughness

The experimental results for flank wear and surfaceroughness parameters (Ra, Rz and Rt) is shown in Table 4.Morphology of flank wear (VBc) as a function of machiningtime (Tc) for uncoated carbide inserts are presented inFig. 2. Figs. 3 and 4 show the progression of flank wearand surface roughness vs. machining time graphically.The summary of the findings are presented below.

1. For multilayer TiN coated carbide insert, flank weardevelops steadily which widens with machining timeand becomes irregular when approaches to 32 min.Gradual wear is the principal mechanism for the tool

Table 4Experimental observations of surface roughness and flank wear at successive runs

Inserts Parameters Machining time (Tc)

Run1 Run21 3

Multilayer TiN coated carbide Ra 0.78 1.04Rz 2.8 3.8Rt 3.1 4

Uncoated carbide Ra 3.68 4.082Rz 11.475 12.15Rt 12 12.55

Multilayer TiN coated carbide VBc 0.11 0.139Uncoated carbide VBc 0.295 0.541

(a) at 1 minute

(c) at 5 minute

Fig. 2. (a–d) Images of Flank wear u

failure noticed for coated carbide insert. No prematuretool failure by chipping and fracturing was observedand machining was steady compared to uncoated car-bide. The wear criterion (VBc = 0.3 mm) has beenreached at 30 min of machining whereas uncoated car-bide insert reached at approximately 1 min. The threezones of wear have been observed for multilayer TiNcoated carbide insert shown in Fig. 3 i.e. initial wear fol-lowed by gradual or steady wear and finally rapid stageof wear. The width of the wear increased rapidly,accompanied by the formation of severe abrasive markswith longer cutting time. The rapid wear zone is clearlynoticeable from images due to adhesion and diffusionand observed from Fig. 2.

.

in min

Run3 Run4 Run5 Run6 Run75 8 12 22 32

1.25 1.29 1.32 1.835 2.064.2 4.8 5 6.9 8.54.9 5.2 5.7 7.3 10.4

4.295 5.05 – – –13.225 14.15 – – –13.65 14.95 – – –

0.147 0.156 0.186 0.25 0.3640.766 0.856 – – –

(b) at 3 minute

(d) at 8 minute

sing uncoated carbide inserts.

0 4 8 12 16 20 24 28 320.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

v = 200 m/minf = 0.21 mm/revd = 0.4 mm

Fla

nk w

ear,

VB

c (m

m)

Machining Time, Tc (min)

TiN coated carbide Uncoated carbide

Fig. 3. Flank wear vs. machining time for D2 steel turning.

0 5 10 15 20 25 30 35

2

4

6

8

10

12

14

16

v = 200 m/minf = 0.21 mm/revd = 0.4 mm

Surf

ace

roug

hnes

s (µ

m)

Machining Time, Tc (min)

Ra (TiN) Rz (TiN) Rt (TiN) Ra (uncoated) Rz (uncoated) Rt (uncoated)

Fig. 4. Surface roughness vs. machining time for D2 steel turning.

2700 A.K. Sahoo, B. Sahoo / Measurement 46 (2013) 2695–2704

2. Generally, cutting speed is considered as most importantparameter affecting the behavior of tool comparedto other parameters. Tool life can be deduced fromFig. 3 considering allowable wear criteria VBc = 0.3 mm.The tool life of multilayer TiN coated carbide insertis found to be approximately 30 times higher than theuncoated carbide insert under similar cutting conditions.This may be attributed to the presence of lubriciousTiN coating which helps to lower the temperature atthe flank surface. Consequently, it reduces thegrowth of abrasive wear by retaining the hardness at ele-vated temperature and also prevents adhesion and diffu-sion types of wear which are highly sensitive totemperature.

Table 5Machined chip volume for coated and uncoated carbide inserts.

Process parameters VBc criteria TT

T

v = 200 m/min, f = 0.21 mm/rev, d = 0.4 mm 0.3 mm 3

3. Multilayer TiN coated carbide tool does not exhibit cra-ter wear due to presence of oxide layer coating (Al2O3)which acts as a thermal barrier property. Thus thecoated tool with the oxide layer had a better tool lifeat higher cutting speed.

4. The rate of growth of wear for uncoated carbide insertwas rapid. It may be attributed due to its poor hot hard-ness property than multilayer coated carbide insert. Theuncoated tool failed primarily by rapid tool wear, quickdulling of the cutting edge due to plastic deformationunder extensive stress and temperature in machiningconditions. Chipping of the cutting edge was severeand degradation of cutting edge occurred within fewminute for the uncoated insert compared to the smoothgrowth of wear of coated tool. The uncoated carbidewas unable cut material just after 5 min and machiningwas stopped as it exceeds the flank wear limit.

5. From Fig. 4, it is clear that the machined surface rough-ness obtained by multilayer TiN coated carbide tool hasa lower surface roughness than that produced by theuncoated tool in all the runs. This might be due tolubricity offered by coated material which reduces thetemperature and built-up-edge formation. Improve-ment of surface finish is mainly due to reduction ofwear and damage at the tool tip by the use of multilayerTiN coated carbide insert at higher cutting speed andfeed. The Ra values for turning D2 steel is within 0.78–2 lm for TiN coated carbide insert whereas foruncoated carbide insert, it was 3.68–5.05 lm.

6. Cutting speed influences the productivity in industrialsector. The volume of the total machined chip is calcu-lated for both coated and uncoated carbide insert forlimit criteria of VBc = 0.3 mm and shown in Table 5. Itcan be revealed that, at VBc = 0.3 mm value, the toolcutting edge could produce 504 cm3 and 16.8 cm3 of chipduring 30 and 1 min respectively for multilayer TiNcoated and uncoated carbide insert. This indicates30 times higher chip volume removal utilizing coatedcarbide insert in machining and thus increasesproductivity.

7. Thus, the multilayer TiN coated carbide inserts outper-formed to uncoated carbide inserts at higher parametricconditions. However, uncoated carbide inserts may beacceptable at low or moderate range of cutting speed.This confirms the ability of multilayer coated carbideinsert to be implemented in hard turning.

5. Flank wear and surface roughness model usingregression analysis

Considering the flank wear (VBc) and arithmetic surfaceroughness average (Ra) parameters as output and machin-ing time (Tc) as input, a multiple linear regression model

ool life,(min)

Machined chip volumeV = v.f.d.T (cm3)

iN coated Uncoated TiN coated Uncoated

0 1 504 16.8

A.K. Sahoo, B. Sahoo / Measurement 46 (2013) 2695–2704 2701

have been developed at significance level of a = 0.05 i.e. fora 95% confidence level. The sources with a P-value less than0.05 are considered to have a statistically significant to theperformance characteristics. The following equations (Eqs.(1)–(4)) are attained for multilayer TiN coated carbideinsert.

VBc ¼ 0:102549þ 0:00764 Tc;

R2 ¼ 97:7%; R2ðadjÞ ¼ 97:2% ð1Þ

Ra ¼ 0:91841þ 0:03791 Tc; R2 ¼ 94%;

R2ðadjÞ ¼ 92:8% ð2Þ

Rz ¼ 3:1244þ 0:1702 Tc; R2 ¼ 98%;

R2ðadjÞ ¼ 97:6% ð3Þ

Rt ¼ 3:3013þ 0:2107 Tc; R2 ¼ 97%;

R2ðadjÞ ¼ 96:4% ð4Þ

This quantitative relationship between the flank wearand machining time enables to calculate the expected va-lue of flank wear for a preset machining time. To checkthe adequacy of the models developed, analysis of variancetable is constructed. From ANOVA table of linear model(Tables 6 and 7) for flank wear and arithmetic surfaceroughness average (Ra), probability of significance (P-value) is found to be less than 0.05 and considered assignificant. The model shows high determination coeffi-cient R2 = 0.977 for flank wear and 0.94 for surface rough-ness respectively i.e. close to unity implying goodagreement with experimental results. It describes about97.7% and 94% of the variability in the responses. Thepredicted R2 value is very close and reasonably agreementto adjusted R2 value. It indicates that the model fits welland accurately explains the relationship between the re-sponses and the independent variable. The normal probabil-ity plot vs. residuals of linear models (Figs. 5 and 6) showthat the residuals lie reasonably close to a straight line. Thatmeans the errors are distributed normally and the termsmentioned in the model are significant. Also, the experi-mental vs. predicted values of flank wear (VBc) and arithme-tic roughness average (Ra) are found to be very close to each

Table 6Analysis of variance for flank wear (VBc) model.

Source DF Seq SS Adj SS

Regression 1 0.0447 0.0447Linear 1 0.0447 0.0447Residual error 5 0.001 0.001Total 6 0.0458

Table 7Analysis of variance for surface roughness (Ra) model.

Source DF Seq SS Adj SS

Regression 1 1.1018 1.1018Linear 1 1.1018 1.1018Residual error 5 0.0707 0.0707Total 6 1.1725

other (Figs. 7 and 8) and correlation between them is good[23,24]. The maximum residual for VBc and Ra are found tobe 0.021 and 0.142 respectively and within the reasonablelimit implying the significance of the model developed.Thus, the model developed using linear regression analysiscan be utilized to predict accurately the flank wear andsurface roughness in machining.

6. Economic analysis

Cost analysis with respect to the metal cutting process isan essential element in efficient manufacturing systembecause of large expenditure involved. The basic endeavorof any production process is to produce an acceptablecomponent at the minimum possible cost. Therefore, costanalysis based on total machining cost per part accordingto Gilbert’s approach [25] was performed for the compari-son of economical feasibility between multilayer TiN coatedand uncoated carbide inserts in turning. The cost analysiswas done for turning a cylindrical workpiece with a finisheddiameter (D) of 40 mm, length of cut (L) of 100 mm andcutting parameters (v = 200 m/min, f = 0.21 mm/rev, andd = 0.4 mm) considering flank wear criteria VBc = 0.3 mm.The study is based on measuring tool life using coated anduncoated carbide in the machining process and observedto be 30 min and 1 min for coated and uncoated carbideinserts respectively.

As per current machining practice, the labor charge, themachine charge and the overhead, the total cost of themachine time and labor (x) is estimated to be Rs 250 perhour (4.16 min�1). With these parameters, the machiningtime per part (Tc) can be calculated using the following for-mula [26]:

Tc ¼ ðpDLÞ=ð1000 vfÞ ð5Þ

where D is the finished diameter of workpiece (mm), L theaxial length of the work to be cut (mm), f the feed (mm/rev), and v is the cutting speed (m/min).

Therefore; the machining cost per part ¼ x � Tc ð6Þ

If Td is the downtime in minutes to change the tool andthe workpiece, and T is tool life for one cutting edge, thenthe tool changing cost per part is given by

Adj MS F P Remarks

0.0447 210.71 0.000 Significant0.0447 210.71 0.0000.0002

Adj MS F P Remarks

1.1018 77.84 0.000 Significant1.1018 77.84 0.0000.0141

Fig. 5. Normal probability plot of the residuals for flank wear (VBc).

Fig. 6. Normal probability plot of the residuals for surface roughness (Ra).

1 2 3 4 5 6 7

0.10

0.15

0.20

0.25

0.30

0.35

0.40

Flan

k w

ear,

VBc

(mm

)

Observations

Experimental Predicted

Fig. 7. Experimental vs. predicted values of flank wear.

2702 A.K. Sahoo, B. Sahoo / Measurement 46 (2013) 2695–2704

Tool changing cost per part ¼ xTdðTc=TÞ ð7Þ

The cost of commercially available, single tip uncoatedcarbide inserts (CNMG 120408) is Rs 180 per piece. Thecost of TiN coated tool inserts (CNMG 120408) is approxi-mately Rs 220 per piece. Therefore, the mean value of asingle cutting edge (y) is Rs 45 and Rs 55 for uncoatedand TiN coated carbide inserts respectively. The tool costper part is estimated by

Tool cost per part ¼ yðTc=TÞ ð8Þ

The total machining cost per part (C) is the sum ofmachining cost per part, the tool changing cost per partand the tool cost per part:

C ¼ xTc þ xTdðTc=TÞ þ yðTc=TÞ ð9Þ

1 2 3 4 5 6 70.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2Su

rfac

e ro

ughn

ess

(Ra)

, mic

rons

Observations

Experimental Predicted

Fig. 8. Experimental vs. predicted values of surface roughness.

Table 8Cost comparisons for both inserts in turning D2 steel.

Sl.no.

Costs Uncoatedcarbide

Multilayer TiNcoated carbide

1 Operations cost, x, @ Rs250/h

Rs4.16 min�1

Rs 4.16 min�1

2 Machining cost per part(xTc)

Rs 1.25 Rs 1.25

3 Tool life for single edge (T) 1 min 30 min4 Tool changing cost per

part [xTd(Tc/T)]Rs 6.24 Rs 0.208

5 Mean value of singlecutting edge (y)

Rs 45 Rs 55

6 Tool cost per part [y(Tc/T)] Rs 13.5 Rs 0.557 Total machining cost per

part (C), (2 + 4 + 6)Rs 21 Rs 2

Cutting conditions: v = 200 m/min, f = 0.21 mm/rev, d = 0.4 mm,L = 100 mm, D = 40 mm, VBc = 0.3 mm, W/P = D2 steel (22 ± 1 HRC),Tc = 0.3 min, and Td = 5 min.

A.K. Sahoo, B. Sahoo / Measurement 46 (2013) 2695–2704 2703

Results of the cost analysis based on the above data aregiven in Table 8. It can be seen that the total machining costper part was found to be Rs 2 and Rs 21 for TiN coatedand uncoated carbide insert respectively. This indicates10.5 times higher machining cost per part for uncoatedcarbide insert than TiN coated insert. The savings inmachining costs using TiN coated inserts is 90.5% comparedto uncoated carbide insert. The cost analysis indicates thatthe TiN coated carbide tools are capable of reducingmachining costs, and therefore, will be an importantcomplement to uncoated tools for turning. The economicbenefit of carrying out finish turning operations with coatedcarbide insert is clearly established due to enhanced tool lifewhich minimizes the downtime and therefore providesmore savings.

7. Conclusions

In the present study, the performance of multilayercoated and uncoated carbide inserts have been assessedwith respect to flank wear and surface roughness. Regres-

sion models have been developed and economical compar-isons between both inserts have also been made. Theresults of the findings are presented.

1. Higher micro hardness of TiN coated carbide samples(1880 HV) compared to uncoated carbide (1430 HV) ispossibly due to the presence of the nitriding layer anddepicts better resistance against abrasion.

2. The erosion wear resistance of TiN coated carbide insertis observed to be higher i.e. 3 times better than uncoatedcarbide insert.

3. Lower machined surface roughness is observed in TiNcoated insert than uncoated carbide in turning of D2steel. This may be attributed due to high hardness, wearresistance, low coefficient of friction and high diffusionbarrier properties of the TiN coated material.

4. It is seen that the progression of flank wear for multi-layer TiN coated carbide insert was steady withoutany premature failure by chipping and fracturing. Abra-sion is found to be the dominant wear mechanism.

5. The rate of growth of wear for uncoated carbide insertwas rapid. The chipping was severe for uncoated insertand removed relatively large particles of tool materialfrom the cutting edge. Quick dulling of the cutting edgeby plastic deformation due to intensive stress and tem-perature has also been observed in the uncoated car-bide tool.

6. The tool life of TiN coated insert is approximately30 times higher than the uncoated carbide insert undersimilar cutting conditions. Machined chip volume re-moval is noticed to be 30 times higher than uncoated car-bide insert in machining and thus increases productivity.

7. The regression model shows high determination coeffi-cient i.e. R2 = 0.977 for flank wear and 0.94 for surfaceroughness. It indicates that the model fits well and accu-rately explains the relationship between the responsesand the independent variable. Also, the experimentalvalue and predicted values from the developed modelare found to be very close to each other.

8. It can be seen that the total machining cost per partusing TiN coated inserts is considerably lower than thatof uncoated carbide tools. Depending on the machinedown time, considerable savings in machining costsusing TiN coated inserts is 90.5% in machining takingflank wear limit of 0.3 mm. The enhanced tool life dueto the coating minimizes the downtime and thereforeprovides more savings.

9. Therefore the out performance of multilayer TiN coatedcarbide as compared to uncoated carbide inserts undersimilar cutting conditions clearly established and thuscan be implemented at extreme cutting conditionsand hard turning applications also.

Acknowledgements

The author would like to thank Central tool room andtraining centre (CTTC) and KIIT University, Bhubaneswar,India for providing their facilities to carry out experimentaland measurement works.

2704 A.K. Sahoo, B. Sahoo / Measurement 46 (2013) 2695–2704

References

[1] Recep Yigit, Erdal Celik, Fehim Findik, Sakip Koksal, Tool lifeperformance of multilayer hard coatings produced by HTCVD formachining of nodular cast iron, International Journal of RefractoryMetals and Hard Materials 26 (2008) 514–524.

[2] V.P. Astakhov, Tribology of Metal Cutting, Elsevier, London, 2006.[3] W. Grzesik, The role of coatings in controlling the cutting process

when turning with coated indexable inserts, Journal of MaterialsProcessing Technology 79 (1998) 133–143.

[4] S.K. Khrais, Y.J. Lin, Wear mechanisms and tool performance of TiAlNPVD coated inserts during machining of AISI 4140 steel, Wear 262(2007) 64–69.

[5] R.F. Avila, A.M. Abrao, G.C. Godoy, The performance of TiN coatedcarbide tools when turning AISI 8620 steel, Journal of MaterialsProcessing Technology 179 (2006) 161–164.

[6] J. Rech, Influence of cutting tool coatings on the tribologicalphenomena at the tool–chip interface in orthogonal dry turning,Surface and Coatings Technology 200 (2006) 5132–5139.

[7] Y.L. Su, W.H. Kao, Tribological behavior and wear mechanisms ofTiN/TiCN/TiN multilayer coatings, Journal of Materials Engineeringand Performance 7 (1998) 601–612.

[8] H. Scheerer, T.H. Hoche, E. Broszeit, B. Schramm, E.C. Abele, Effects ofthe chromium to aluminum content on the tribology in drymachining using (Cr, Al) N coated tools, Surface and CoatingsTechnology 200 (2005) 203–207.

[9] Y. Sahin, The effects of various multilayer ceramic coatings on thewear of carbide cutting tools when machining metal matrixcomposites, Surface and Coatings Technology 199 (2005) 112–117.

[10] K. Tuffy, G. Byrne, D. Dowling, Determination of the optimum TiNcoating thickness on WC inserts for machining carbon steels, Journalof Materials Processing Technology 155–156 (2004) 1861–1866.

[11] F.A. Khalid, A note on the comparison of microstructure and coatingcharacteristics of hardmetal inserts, Journal of Materials ProcessingTechnology 122 (2002) 381–385.

[12] I. Ciftci, Machining of austenitic stainless steels using CVD multi-layer coated cemented carbide tools, Tribology International 39(2006) 565–569.

[13] C.Y.H. Lim, S.C. Lim, K.S. Lee, Wear of TiC-coated carbide tools in dryturning, Wear 225–229 (1999) 354–367.

[14] H. Gökkaya, M. Nalbant, The effects of cutting tool geometry andprocessing parameters on the surface roughness of AISI 1030 steel,Materials and Design 28 (2007) 717–721.

[15] A.A. Popescu, I. Tudor, M. Balaceanu, V. Braic, M. Braic, R. Rapeanu, A.Tudose, Wear resistance evaluation of TiN coated cutting tools, in:

Fifth General Conference of the Balkan Physical Union. VrnjackaBanja, Serbia and Montenegro, August 25–29, 2003, pp. 2237–2240.

[16] H. GÖ KKAYA, M. Nalbant, The effects of cutting tool coating on thesurface roughness of AISI 1015 steel depending on cuttingparameters, Turkish Journal Engineering Environmental Science 30(2006) 307–316.

[17] C.H.R. Vikram Kumar, P. Kesavan Nair, B. Ramamoorthy,Characterization of multilayer PVD nanocoatings deposited ontungsten carbide cutting tools’, International Journal AdvancedManufacturing Technology 38 (2007) 622–629.

[18] R. Yigit, E. Celik, F. Findik, S. Koksal, Tool life performance ofmultilayer hard coatings produced by HTCVD for machining ofnodular cast iron, International Journal of Refractory Metals andHard Materials 26 (2008) 514–524.

[19] A.K. Sahoo, B.D. Sahoo, Mathematical modeling and multi-responseoptimization using response surface methodology and Grey basedTaguchi method: an experimental investigation, InternationalJournal of Experimental Design and Process Optimisation 2 (3)(2011) 221–242.

[20] J. Wang, The effect of multi-layer surface coatings of carbide insertson the cutting forces in turning operations, Journal of MaterialsProcessing Technology 97 (2000) 114–119.

[21] H. Sert, A. Can, K. Habal, F. Okay, Wear behavior of PVD TiAlN, CVDTiN coated and Cermet cutting tools, Tribology in Industry 27 (2005)03–09.

[22] Süleyman Nes_eli, Süleyman Yaldız, Erol Türkes, Optimization oftool geometry parameters for turning operations based on theresponse surface methodology, Measurement 44 (2011) 580–587.

[23] C. Leone, D. D’Addona, R. Teti, Tool wear modelling throughregression analysis and intelligent methods for nickel base alloymachining, CIRP Journal of Manufacturing Science and Technology(2011), http://dx.doi.org/10.1016/j.cirpj.2011.03.009.

[24] N. Mandal, B. Doloi, B. Mondal, R. Das, Optimization of flank wearusing Zirconia Toughened Alumina (ZTA) cutting tool: Taguchimethod and regression analysis, Measurement 44 (2011) 2149–2155.

[25] M.C. Shaw, Metal Cutting Principles, Oxford University Press, NewYork, 2005. pp. 500–502.

[26] A.S. More, W. Jiang, W.D. Brown, A.P. Malshe, Tool wear andmachining performance of cBN–TiN coated carbide inserts andPCBN compact inserts in turning AISI 4340 hardened steel, Journal ofMaterials Processing Technology 180 (2006) 253–262.