Wear progressions and tool life enhancement with AlCrNcoated inserts in high-speed dry and wet steel lathing
Yueh-Jaw Lin ∗, Ashutosh Agrawal, Yunmei FangDepartment of Mechanical Engineering, The University of Akron, Akron, Ohio 44325-3903, USA
Received 22 October 2006; received in revised form 26 January 2007; accepted 9 March 2007Available online 1 May 2007
bstract
In this paper, the tool life enhancement with a PVD-applied aluminum chromium nitride (AlCrN) coating (named Alcrona) for cemented carbidenserts is investigated. Microstructure wear progressions such as the abrasive wear mechanisms of AlCrN coated cemented carbide tool insertsnder dry and wet machining at very high cutting speeds are fully analyzed based on experimental testings. The maximum cutting speed attainedn the testing for the turning operations is 410 m/min. Various progressive stages of abrasive wear are observed through the experimental results.hese wear results done on AlCrN coated carbide tool inserts are compared with other tool life data reported in the literature. It is found that at60 m/min, Alcrona performs near 95% better in tool wear than TiAlN coated carbide tool under the same machining conditions. Comparing theerformance of Alcrona coated tool inserts with that of TiN coated ones, the former can achieve approximately 33% more depth of cut and can attainigher cutting speed due to better thermal resistance of the coated inserts. This finding verifies the speculation that Alcrona coating enhances toolapability for metal cutting and improves tool life even under harsh cutting conditions. Types of microstructure wear phenomena captured during theourse of the experimental study are micro-abrasion, micro-tensile fracture, micro-fatigue, micro-thermal cracks, micro-adhesion, built up edge and
icro-attrition. The experimental cutting observations with the underlying tool inserts demonstrate that wear progresses with time and goes through
arious stages, namely, running into wear, steady state wear, and tool failure wear. It is also evidenced that the use of coolant emulsion increases theool life proportionally with respect to the cutting speeds and reduces the wear progress considerably. SEM and optical microscope images of the toolear in progress have been taken at various stages to delve into the morphology of the tool inserts. These are interpreted with reasoning in the paper.2007 Elsevier B.V. All rights reserved.
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eywords: Microstructure wear; Tool life; AlCrN coating; High speed cutting;
. Introduction
The objective of this research is to understand the micro wearechanisms of the uncoated and an Alcrona coated inserts for
igh speed machining under dry and wet conditions. The workoncentrates on investigating the performance of uncoated andoated carbide inserts under harsh cutting conditions. This studyives a better idea in understanding the properties of availableoatings in the marketplace and hence, helps machining prac-itioners to choose pertinent coatings for particular machiningrocesses.
Coatings are classified into hard coating and soft coatingn the basis of the hardness difference between the substratend the coating. Hard coatings are proved better and are used
ore than the soft coatings because of their improved proper-ies [1]. In high speed machining we require to coat the cementarbide tool bits with some type of coating which can reduceaterial losses, increase the lifetime of tool and machine parts,
nd most importantly, reduce wear [2]. Several researchers havestablished that hard coatings deposited on tool and machinearts by different physical vapor decomposition methods canramatically change the performance of the parts. These coatedaterials not only help reducing the wear and increasing the tool
ife but also improve strength and chemical inertness, reduceriction, and make the parts more stable at high temperatures3]. The use of surface coatings is beneficial in that the substrateaterial can be designed for strength and toughness while the
oating is responsible for the resistance to wear, thermal loads
nd corrosion [1].
In recent years, TiN-based coatings have been widely usedy industry for cutting tools protection. Among various alloyingiN-based coatings applied on tool inserts, TiAlN and TiAlCrN
There are many types of cutting fluid available today in themarketplace. The cutting fluid used in the underlying researchwas water based emulsion. It is mixed with water at a concen-tration of 10%. Its properties are listed in Tables 1 and 2.
re most commonly discussed by many researchers. Some recentelevant works reporting research results in the area can be foundn [4–9,18–21]. In particular, Luo et al. [7] investigated the tri-ological behaviors of TiAlN/CrN and TiAlCrN coatings byifferent deposition methods. They reported that the wear rate forll these coatings was at least ten times lower than the uncoatedool insert.
More recently, Luo et al. [8] further studied the wear mech-nism of low friction superlattice TiAlN/VN coatings. Basedn their TEM observations, it was found that the multilayeriAlN/VN coatings resulted in the formation of tribofilm on
he worn surface yielding tribo-oxidation other than tradi-ional isothermal oxidation, better for tool protection. Alonghe same line, Kovalev et al. [9] looked into the impact ofl and Cr alloying of TiN-based coatings on the final cut-
ing performance. They reported that the addition of Al to TiNoatings drops the chemical reactivity of the coating whichn turn, controls the crater wear intensity in turning ope-ations.
In addition in [9], it was concluded that the addition of Cr toiN coatings improved the plasticity of the coating. The authorsroved that the simultaneous addition of Al and Cr in the com-lex TiN-based nitride weaken the long range bonds, furthermprove the plasticity of the compound and prolong the cuttingool life under heavy wear conditions. Moreover, Koalas et al.eported that the wear rate of TiAlN/CrN was stable while thatf TiAlCrN was sensitive to change in cutting speed. Their con-lusion stated that both the nitride coatings increased the wearesistance of tool steel by up to 10 times.
Recently, physical vapor decomposition (PVD) is used exten-ively compared to other coating techniques. It involves thetomization or vaporization of material from a solid source andhe deposition of that material onto the substrate to form the coat-ng. The advantages of this process is the possibility to depositlloy compounds, compositions with multi-layer structure andhe ability to vary the coating characteristics continuously to getfunctionally graded coating [1]. Also as this process involves
emperatures ranging up to 500 ◦C, it overcomes the problem ofepositing brittle layer.
In addition, PVD process produces very smooth surface fin-sh of the coated product, resulting in good sliding of the chipsver the cutting inserts. Thus the contact zone temperatures reduced which further reduces the tendencies for thermalracking [4].
The paper is organized as follows. The coolant properties ofhe cutting fluid selected for the underlying machining opera-ions are provided in the next section. This is followed by thexperimental details of the research work. Then, the results ofhe experimental testing and the associated analytical discussionre presented. Finally, some concluding remarks are drawn andddressed in the last section of the paper.
. Tool wear mechanisms
Morphology of the wear mechanisms under high speedachining is discussed in this section. The morphology was
nvestigated and reported for the underlying Alcrona cutting
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4 (2008) 226–234 227
nsert material. The intention is to provide valuable insight fornderstanding, optimizing machining operations and obtainingufficient knowledge to improve coating materials and coatingechniques pertinent to unconventional high speed machiningperations.
Che Haron et al. [10] studied the wear phenomenon of coatednd uncoated carbides in turning tool for cutting steel under drynd wet conditions. The workpiece material was selected as ISO5MnCrW1 with hardness 23HRC. Coated carbide tools wereested at four different speeds, 200, 250, 300 and 350 m/min,espectively. The results showed that performance of coatedTiCN) carbide tools under wet cutting was significantly bet-er than under dry cutting for all the selected cutting speeds. Theool life measured was 52, 31, 16 and 14 min under wet cuttingor 200, 250, 300 and 350 m/min cutting speed, respectively. Theool lives for dry cutting at the above speeds was measured as9, 22, 12 and 7 min, respectively.
Jindal et al. [11] studied the performance of PVD appliediN, TiCN and TiAlN coated cemented carbide tools in turn-
ng. The experiment was done at two different speeds, 305 and96 m/min. For the wear criterion of 0.4 mm, the tool life for TiN,iCN and TiAlN was measured as 40, 50 and 60 min, respec-
ively, for 305 m/min, and 10, 15 and 28 min for 396 m/min,espectively.
D’Errico et al. [12] studied the influences of PVD coatings onemented tool life in continuous and interrupted turning. Mono-ayers of TiN and TiCN thin films were coated on the cementedool. It was stated that in terms of the tool life coated insertserformed better than uncoated inserts.
. Coolant properties
Coolant emulsion rapidly affects the temperature of the chipsnd can sometimes favorably influence chip breaking, particu-arly when large cross section chips are formed [13,23,24].
In general, most turning and other machining applications useater based coolant emulsions. These contain a microscopic dis-ersion of the concentrate in water. The microscopic oil globulesre homogeneously dispersed throughout the coolant. The basicngredients of these emulsions are water, oil, and wetting agents15].
Description Hot rolled alloy steel bars, SAE 4140H(UNS H4140) having a surface hardness ofat least Rockwell 15N 89, and a corehardness of at least Rockwell C 25
Dimensions 15 cm diameter × 62 cm lengthHeat treatment Vacuum degassed/processed, Cal-Al treated,
annealed and special straightened,conforming to ASTM A322 and A304
There are thousands of coatings available in the marketplace.he selection depends on the work piece material and the speedf machining. BALINIT Alcrona was chosen for this experimen-al study on the basis of its extraordinary properties. Alcrona islCrN monolayer by structure. Its high hot hardness results in
xcellent abrasion resistance even at high cutting speeds [16].
ot hardness of Alcrona is compared with other commonly used
This study was conducted in accordance with ISO 3685 [17].he work piece material was SAE 4140 steel. Work piece spec-
fications are listed in Table 3. The chemical composition of theork piece is listed in Table 4. The workpiece was replacedhen length/diameter ratio reached 10 in accordance with ISO685 [17].
.3. Cutting inserts and tool holder geometry
Cemented carbide inserts with 6% cobalt were used in theurning tests. The tool holder used in the test has ISO designationf VBMT 160408. The cutting inserts assembled geometry isisted in Table 5.
Table 6 lists the two types of cutting inserts used in the exper-ment. The coating properties of the two inserts are listed inable 7. The inserts were mounted rigidly on a tool holder withSO designation of SVJBR 2525 M16.
.4. Cutting conditions
Continuous turning tests of SAE 4140 heat-treated steel barere performed on a (Clausing 1300) variable spindle speed
athe with a maximum power of 7.5 HP. The rotational speed
Substrate Grade Company
. . . KC313 KennametalKC313 . . . Balzers
Y.-J. Lin et al. / Wear 264 (2008) 226–234 229
Table 7Coating properties of the coated carbide insert
Structure AlCrN monolayer
Hardness HV 0.05 3200Residual Compressive stress
{GPA}−3.0
Max service temperature {◦C} 1100 (durability of thecoating in machining)
ttTbnmaway of the substrate material. As a result, micro-holes appear
eed rate (mm/rev) 0.14epth of cut (mm) 1.0
f the work piece was measured before every cut by a (HT-100) handheld digital tachometer to insure the work piececcurately running at the designated cutting speed. The cut-ing speeds and parameters are listed in Tables 8 and 9, respec-ively.
An optical microscope with magnification of 200 times wassed to measure the wear on the flank surface. Scanning electronicroscope (SEM) was utilized in obtaining images revealing
he initiation and micro-wear mechanisms at different stages ofool life. Five high cutting speeds were employed in the testanging from 210 m/min to the maximum speed of 410 m/min,efore the premature insert failure. The depth of cut and feed rateere kept constant during the test period with values of 1 mm,
nd 0.14 mm/rev, respectively.
. Experimental testing results and discussion
In this section experimental observations of micro wearechanism are summarized and discussed. Scanning electronicroscope (SEM) images and optical microscope images were
aken for obtaining zoom-in look at the wear and microstructurend to better understand the phenomenon involved behind theesult. Several wear mechanism occur simultaneously during theachining process, these include oxidation, diffusion wear and
atigue wear [25].In the various stages of wear from starting till the tool failure
everal stages of wear were noticed, these include welding onhe tip, nose breaking, micro abrasion, tip failure, nose failure,hip breaker loss, etc. which are observable and explained in
his section with the SEM and optical microscope images takenhrough the study.
oau
ig. 2. Optical microscope (200×) image showing the start of metal adhesionn the crater surface for uncoated carbide insert w/60 m/min speed under dryachining.
.1. Uncoated inserts
Uncoated carbide inserts have been the most frequentlysed cutting tool in the manufacturing industry. The materialf these carbide inserts is fabricated with powder metallurgi-al technology by sintering fine particles of carbide in metalinder. The advantage of the carbide insert over the earliersed high speed steel tool is its chemical stability at higherpeed leading to elevated temperatures [25]. The chemical com-osition of the cemented carbide usually includes tungstenarbide, titanium carbide and tantalum carbide with cobalt as ainder.
.1.1. Experimental findings for uncoated inserts on wearechanismsIn the experiment for machining the steel stocks, uncoated
arbide inserts with cutting speeds varying from 60 to 180 m/minere used for both the wet and dry machining. It is found thatetal adhesion started on the crater surface of the uncoated car-
ide insert in the very early stage of the turning, as is shown inig. 2. As the cutting speed increases the temperature increasesnd the hardness of the insert decreases. The loss of chip breakerechanism results in longest chips and accumulation of chips
round the cutting tool. This cluster of chips along with the newlyenerated surfaces rub the tool insert and catalyze the growth ofear on the flank surface.Slowly with the increase of the rubbing and temperature the
ip starts to break off. A typical sliding wear at the flank edge ofhe cutting insert starts growing parallel to the contact direction.he adhered metal layer is plucked away from the flank surfaceecause of the continuous rubbing between the flank side and theewly generated metal surface [27]. Therefore, micro-attritionechanism of wear is generated which involves the plucking
n the surface showing a characteristic feature of the micro-ttrition wear as demonstrated in Fig. 3 with 120 m/min speednder wet cutting.
230 Y.-J. Lin et al. / Wear 264 (2008) 226–234
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ig. 3. Optical microscope (200×) image showing micro holes on the craterurface of uncoated carbide insert at 120 m/min under wet machining.
With the loss of chip breaker the chips are not drifted awayrom the surface of the newly generated work piece and the chipow velocity across the cutting tool surface reduces. This brings
he built up edge formation on the surface as the chip that flowsver the crater surface part will move at a chip velocity of thepper layer, and the bottom layer sticks to the cutting tool craternd does not move. These phenomena of cutter wear are depictedn Fig. 4 with 120 m/min speed under dry cutting. The overallerformance of the cutting insert under wet machining improvesue to the average temperature reduction of the cutting tool edgend shear zone [26,27].
At the tool failure, i.e., when the wear reaches the cutoff valuef 0.6 mm the scanning electron microscope (SEM, 500 �m)mages depicted in Fig. 5 clearly showed the combination of tipreaking, nose breaking, crater wear and massive metal adhe-
ion on the flank side of the cutting insert, and the tool edgeracture can be seen in Fig. 6. This mechanical failure is causedy successive compression and tension on the cutting edge that
ig. 4. Optical microscope (200×) image showing built up edge formation forncoated carbide insert at 120 m/min under dry machining.
isac
ig. 5. SEM image showing complete loss of tip, chip breaker, crater wear andetal adhesion on the flank side for uncoated carbide insert at 90 m/min speed
nder dry machining.
eads to mechanical fatigue and thus inhibits any more effectiveemoval of the material from the work piece surface.
.2. Coated inserts
Coating increases the lubricity and reduces the affinity to theork piece material. This allows the coated inserts to performuch better than the uncoated inserts, especially at higher cut-
ing speeds. The coating provides a better thermal barrier so theemperature is reduced [22]. In this current experimental studylcrona coated inserts are investigated for the first time. This
oating is claimed to be the next generation coating capablef providing a longer tool life compared to the other titaniumoatings readily used today in the manufacturing industry. Thexperimental results are extremely meaningful to the coating
ndustry as well as machining practitioners for providing a factheet regarding the newly tested coating, Alcrona, most suit-ble for high speed cutting tool bits, among other hundreds ofoatings readily available at the present time (Fig. 7).
Fig. 6. SEM image showing tool’s edge fracture.
Y.-J. Lin et al. / Wear 264 (2008) 226–234 231
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In a recent study done by Khrais and Lin [14,22], high speed
ig. 7. Optical microscope (200×) image showing micro abrasion for Alcronat 410 m/min under wet machining.
.2.1. Experimental findings for the Alcrona coated insertsBoth abrasion wear and stress concentration factor leavenon-uniform edge configuration on the micro-scale afterachining starts. Tensile fracture is less severe in the case of wetachining as compared to dry machining. This is due to the con-
ribution of the coolant in reducing the cutting temperature whichs very severe at higher speeds nearing 410 m/min. However,hile using coated inserts for machining under dry condition
t higher speeds, high levels of micro-abrasion is noticed, asemonstrated in Fig. 8.
A uniform droplet size is difficult to achieve therefore caus-ng inconsistent droplet sizes, which results in increased stresseseading to destructive coating wear [15]. Fig. 8 shows tip break-ng, crater wear and metal adhesion for the coated insert underet machining at 410 m/min. The coating is damaged and the
oating destruction accelerates the growth of crater wear on the
urface and leads to a rapid tool failure since the upper part ofhe cutting tool insert is left uncovered in this situation.
ig. 8. Optical microscope (200×) image showing tip failure, crater wear, metaldhesion for Alcrona at 410 m/min under wet machining.
mat
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ig. 9. SEM image showing numerous micro holes indicating micro wear andhe loss of coating.
Fig. 8 also revealed that part of adhered metal would belucked away taking grains of tungsten carbide and binder fromhe cutting inserts material. Therefore, thermal pitting, microdhesion and low levels of micro abrasion come into picture.n addition, it is involved with a loss of coating from the sur-ace and micro holes which catalyzes the growth of crater wearn the tool insert surface, as can be observed in the SEM50 �m) image of Fig. 9. The severe distortion of the binderlone occurs due to the activation of micro-adhesion and micro-brasion wear at the time of cutting conditions of speed changesnd coolant introduction, as evidenced in Fig. 10. Thus, micro-atigue, micro-abrasion, and micro-adhesion wear mechanismsre activated under wet condition, while high levels of micro-brasion can be observed under dry condition implicated inig. 11 [15].
achining tool wear micro structure was investigated for TiAlNnd TiN-TiCN-TiN coated cement tool insert. For the wear cri-erion of 0.4 mm, the tool life was measured as 70, 66, 34, 30 and
ig. 10. Optical microscope image (200×) showing start of the welding onurface for Alcrona wet machining at 360 m/min.
232 Y.-J. Lin et al. / Wear 264 (2008) 226–234
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oisons of tool lives before failure using 0.6 mm wear criterionare presented in the bar graph in Fig. 14. The tool life exten-sion of wet cutting against dry one in percentages at each high
ig. 11. Optical microscope image (200×) showing more welding on the craterurface for Alcrona at 360 m/min under dry machining.
8 min for TiN-TiCN-TiN coated tool under dry machining atutting speed 210, 260, 310, 360, and 410 m/min, respectively.he tool life for TiAlN tool was measured for the above wearriterion and cutting speeds as 50, 21, 10, 9 and 7 min, respec-ively for dry machining. For wet machining the tool life waseported to increase to 85, 70, 50, 45 and 22 min for TiN-TiCN-iN coated tool. It was also shown that in the case of TiAlN theet machining had negative effect and the tool life decreased to2, 19, 9, 8 and 6 min., respectively for the above wear criterionnd cutting speed combination.
For the current experimental study with Alcrona coating, forhe wear criteria 0.4 mm, the cutting tool life was measured to be4, 40, 15, 7 and 6 min under wet machining for cutting speedst 210, 260, 310, 360 and 410 m/min, respectively. For the sameear criteria and cutting speeds under dry machining, the cutting
ool life was recorded to be decreased to 51, 32, 11, 5 and 4 min,espectively. Changing the wear criteria to 0.6 mm and keepinghe range of cutting speeds as above, the tool life was obtainedo be 65, 47, 19, 10 and 7 min for wet machining and, 57, 38,5, 7 and 6 min for dry machining.
For the tool wear criterion of 0.4 mm and cutting speed at60 m/min, the tool life was reported by [14] as 19 min whilehe tool life for Alcrona coated tool is found to be 40 min, whichs about 95% percent increase in tool life. Jindal et al. [11]eported the tool lives at 0.75 mm depth of cut for TiN and TiCNoated inserts are 10 and 15 min at 396 m/min speed. However,ur current study at 410 m/min speed yielded the tool life to bepproximately 7 min. Considering the depth of cut ratio, Alcronaerformed excellent because the expected tool life for Alcronaill be about 10–15 min at smaller depth of cut and reduced
utting speed.Che Haron et al. [10] reported the tool life for coated (TiCn,
l2O3, TiN) carbide tools as 16 and 12 min for wet and dryachining at 300 m/min. The values for Alcrona is found to be
9 and 15 min, respectively. This is about 20% increase in theool life.
The experimental work verifies the increase in the tool lifeor Alcrona as anticipated earlier because of its higher oxidation
Fig. 12. Tool wear vs. cutting time under dry turning.
esistance giving it chemical stability and ability to withstandemperatures up to 1100 ◦C. Also its monolayer structure gives itigher hot hardness and results in excellent abrasion resistance.igs. 12 and 13 summarize with graphs all the experimentaleasurements of tool lives on Alcrona coated inserts for cuttingISI 4140 steel under high speed dry and wet turning operations,
espectively.It is noted that use of coolant exhibits positive effects for
oth uncoated carbide inserts and inserts coated with BALINITlcrona. The tool life of the inserts coated with Alcrona showsrastic improvements in both quality and the machining timeeduction. Coated inserts not only performed for longer time,ut also survived speeds up to 410 m/min.
Also the tool lives for dry and wet cutting are compared basedn the extreme results shown in Figs. 12 and 13. These compar-
Fig. 13. Tool wear vs. cutting time under wet turning.
Y.-J. Lin et al. / Wear 26
Fig. 14. Tool life comparison of high speed dry and wet turning with 0.6 mmwear criterion.
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ig. 15. Tool life extension due to coolant emulsion for high speed cutting.
utting speed is shown in Fig. 15. It can be seen that in theange of the cutting speeds (210–410 m/min) employed in oururning experiments, tool life extension due to coolant emulsionncreased with the cutting speeds. This finding implies that theigher the machining speed, the more advantageous due to these of cutting fluids.
. Conclusions
Tool life plays a critical role in an estimation of theroductivity level expected for specific cutting conditions inanufacturing. It becomes extremely important both econom-
cally and for good quality that a tool insert should be chosenn such a way that it wears out in a progressive manner ratherhan being unpredictable for its working life due to its uncertain
achining capability. The current experimental study provides
4 (2008) 226–234 233
new dimension on looking at the performance of the coatednd uncoated carbide tool inserts for metal cutting with desir-ble speeds. The findings of this research directly contribute tohe machining community in determining the optimal machin-ng cost and tool replacement policy associated with a particular
achining process.The experimental study began with the standards under ISO
685 1993 [16]. Five different speeds of cutting were chosen forsing both the coated and uncoated inserts. Machining was donender both dry and wet conditions. Cutting speeds tested werearying from 60 to 180 m/min for uncoated tools, and from 210o 410 m/min for coated tools. The scanning electron microscopend optical microscope images of the tool inserts with progres-ive wears were taken to better interrogate the tool health in theachining processes at various wear levels.The findings showed that the performance of the insert
ncreases drastically by applying coating to the carbide insert.he speed attained after coating was double compared to that of
he uncoated insert. The improvements achieved as a result ofoating were extending tool life, attaining higher cutting speeds,nd reducing production costs. It was also noted that the machin-ng performance was enhanced for both the Alcrona coated andncoated carbide inserts with the application of coolant emul-ion.
Flank wear was measured using the optical microscope atifferent stages of the machining process and the results wereresented graphically. In light of the experimental results, it cane summarized that at elevated temperatures, the newly testedlcrona coating performs much better than the other commonlysed coatings for tool inserts. The use of coolant emulsionrings a positive effect on the tool life for Alcrona, as notedn most other relevant literature. Conducting the experimentn a CNC lathe with effective coolant pump at higher speedsill be the suggestion for future research in this area. In addi-
ion, future research along the line can delve into precise wearehaviors modeling of new coated tool inserts for reducing theool life uncertainties to compliment similar research reportedn [24–27].
cknowledgements
This work was supported in part with an unrestricted researchrant by Allied Machine and Engineering Corp. which wasratefully acknowledged. The authors also thank Professor J.he for giving permission of using the microscope in his MEMS
ab.
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