Wear 300 (2013) 8–19

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Wear

0043-16

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An experimental investigation of the effect of coating material on toolwear in micro milling of Inconel 718 super alloy

_Irfan Ucun a, Kubilay Aslantas b,n, Fevzi Bedir c

a Faculty of Technical Education, Department of Mechanical Education, Afyon Kocatepe University, 03200 Afyonkarahisar, Turkeyb Faculty of Technology, Department of Mechanical Engineering, Afyon Kocatepe University, 03200 Afyonkarahisar, Turkeyc Faculty of Engineering and Architecture, Department of Mechanical Engineering, Suleyman Demirel University, 32000 Isparta, Turkey

a r t i c l e i n f o

Article history:

Received 25 July 2012

Received in revised form

16 January 2013

Accepted 22 January 2013Available online 8 February 2013

Keywords:

PVD coatings

Carbon-based coatings

Cutting tools

Surface analysis

Abrasion

48/$ - see front matter & 2013 Elsevier B.V. A

x.doi.org/10.1016/j.wear.2013.01.103

esponding author. Tel.: þ90 272 228 1319 47

ail address: aslantas@aku.edu.tr (K. Aslantas)

a b s t r a c t

In this study, milling of the 718 nickel super alloy in micro conditions and the effect of coating material

on tool wear are investigated. Within this context, coated and uncoated WC-Co micro milling tools

were used for cutting experiments under dry and lubricated (minimum quantity lubrication, MQL)

conditions. Tool wear occurred on the micro end mill, and the change in radius of the cutting tool and

the side-edge radius were determined in accordance with the processed slot geometry. The results

obtained showed that the cutting tools coated with AlTiN, TiAlNþAlCrN, and AlCrN displayed better

performances compared to those coated with TiAlNþWC/C and DLC. In addition, DLC and TiAlNþWC/C

coated tools showed better performance against built-up edge (BUE) formation. Furthermore, the MQL

method used during the cutting process significantly reduced the decrease in the radius.

& 2013 Elsevier B.V. All rights reserved.

1. Introduction

Miniaturized products in the dimension of a few micron to afew millimeter, has increased dramatically over the last decade inthe fields such as medical, transportation, bioengineering, micro-electronics environmental, and communication industries [1].There are different techniques (micro-layered manufacturing,micro-laser processing, micro-electro discharge machining, micro-forming and micro mechanical machining) for manufacturing ofminiaturized parts. Micro mechanical machining is derived fromconventional machining process by downscaling the cutting toolsizes which should lie within the range from 1 to 1000 mm [2,3].There is a substantial increase in the specific energy required witha decrease in chip size during machining [4].

Due to the capacity of nickel alloys to preserve their mechan-ical specifications in higher temperatures, they are widely utilizedin important fields of industry. Today, these materials are beingused in gas turbines, the aviation industry, nuclear reactors, andmany applications where high temperatures are used [5,6].Although these materials are widely preferred, their machinabilityis not at the desired level. Thermal and mechanical properties ofthe nickel alloys limit the machinability of these materials. Thehigh resistance of the material, deformation hardening, which

ll rights reserved.

2; fax: þ90 272 228 1319.

.

occurs during processing and a low thermal conductivity coeffi-cient affects the tool negatively during the cutting process [7].In order to solve the problems which are encountered during themachining of these materials, researchers have come up withseveral suggestions. The most significant of these solutions iscoating of the cutting tool. According to some researches, materi-als which are difficult in cutting are more easily machining withthe development of coating technology. The hardness values ofthe coatings, low friction coefficients, and thermal specificationsaffect the processing performance positively. Therefore, the usagelife of the tool, which directly affects the cost of machining andproduct quality, has been increased [8]. Among the studiesconducted within this context, Derrien and Vigneau [9] reportedthat TiN coated cutting tools have a higher corrosion resistanceand a better surface quality compared to uncoated carbide tools.

In conventional machining, it can found a lot of studies thatfocused on optimizing the cutting conditions of Inconel 718. Forthis purpose, the performance of different coating materials hasbeen investigated for longer tool life [10]. In addition, Gatto andIuliano [11] examined the performances of CrN and TiA1N coat-ings in the processing of super alloys. As a result of their study,the researchers stated that the characteristics of the coatingsprotect the main layer of the tool from the high temperature thatoccurs during the milling process. In another study, CrN andTiA1N coatings were compared, and the corrosion resistance ofthe CrN was observed to be lower than that of the TiA1N coating.The reason for this situation was found to be the low hardness

Table 3

_I. Ucun et al. / Wear 300 (2013) 8–19 9

value of the CrN coating. In addition, it was also stated that thismaterial was more interactive with Inconel 718 material [12]. Theaim of the nano-layers was to give the milling tool an extendedtool life. When the TiN/AlTiN and CrN/TiN nano-layer coatingswere compared to an uncoated tool, high corrosion enduranceand resistance against chip adherence could be observed [13].

The concept of tool wear in the micro milling process generallydifferentiates it from the conventional milling process. Hardmaterials, which are particularly used in micro pattern manufac-turing, lead to very short tool life. Failures affecting the tool life inmicro milling proccesing are generally shown as abrasive wear,chipping on the cutting surfaces, fatigue, and fractures due tooverstress [8]. Another study carried out to determine the failuremechanisms of micro-end-mills were studied during the machiningof aluminum, graphite electrodes and mild steel workpieces [14].In this study, the cutting force variation was monitored, i.e. therelationship between the utilization-related changes at the toolwear. Inspection of the cutting force variation patterns indicatedthat tool failure occurs with chip clogging, fatigue and wear-related excessive stress. The effects of the cutting parameters dueto the size of the process also vary compared to the conventionalmilling process [15,16]. Particularly in cases where the edgeradius is very small compared to the chip thickness, a uniformchip formation is not observed. This situation leads to an increasein the cutting forces that affect the cutting tool [17].

Coating of the micro milling tool helped to solve the currentproblems encountered regarding the tool life. In the studiesconducted, it was observed that due to the high corrosionresistance, both TiAlN and diamond coatings were particularlypreferred for the micro milling processes [18,19]. Due to thesuperior mechanical properties provided by the diamond coat-ings, they stand out as the most commonly preferred coatingtypes in recent years. Their high hardness values significantlyreduce tool wear during the metal cutting process. Additionally,they have a low friction coefficient which reduces the cuttingforce and heat occurrence during the milling process. In addition,the chemically stable structure of diamond coatings is significantin preventing chip adherence during the cutting process [20,21].Along this, in several studies regarding different coating compositions,TiN, TiCN, TiAlN, CrN, and CrTiAlN coatings were used [22,23].Aramcharon et al. [23] compared the performances of TiN, TiCN,TiAlN, CrN, and CrTiAlN for the milling of steel. The results

Table 1Some mechanical properties of WC-Co.

Type Grainsize(lm)

Density(g/cm3)

Hardness(HV30)

Transverserupturestrength (MPa)

Compressivestrength(MPa)

K20-K50 0.4 14.2 1680 4300 6500

Table 2Geometrical information on micro mill ends used in the experimental study.

d2 (mm) d1 (mm) L1 (mm) L2 (mm)

4 768 47 1.6

obtained showed that the performance of the TiN coating wasbetter in terms of corrosion and the quality of the machinedsurface than that of the uncoated tools and other coated tools.

In this study, micro milling experiments were carried out toinvestigate the effect of coating in the machining of Inconel 718nickel alloy. The most important difference of this study, in contrastto the studies considered above, is that it is focusing on wearbehavior of single layer (DLC, AlTiN and AlCrN) and multi-layer(TiAlNþAlCrN and TiAlNþWC/C) coated micro cutting tool. Addi-tionally wear mechanisms and effect of reduction of tool diameter/edge radius increasing on machined slot geometry were discussed.Micro milling tests were carried out under dry cutting conditions.Also, the Minimum Quantity Lubrication (MQL) method was used toinvestigate the effect of lubrication on tool wear of the AlCrN coatedtool. The decrease in diameter and change in radius are investigatedin each milling tool with identical cutting lengths.

2. Materials and method

Inconel 718 nickel-based super alloy is used in leading fields ofindustry (aviation and aerospace, medical, etc.) and because of itsprominent superior mechanical properties was chosen as thework material. For the cutting experiments, an ultra-fine graincarbide tool, 768 mm in diameter, of K20-K50 quality (Table 1),and a micro end mill with coatings of different compositions(TiAlNþAlCrN, DLC, AlTiN, TiAlNþWC/C, AlCrN) was used.

Tables 2 and 3 give the geometrical information of the cuttingtool. In addition, Table 2 presents some characteristic features ofthe coatings. The cutting parameters were 20,000 rev/min (Vc¼48 m/min), a feed rate of 1.25, 2.5, 3.75, and 5 mm/flute, andcutting depths of 0.1, 0.15, and 0.2 mm. During all the cuttingexperiments, a constant cutting length of 120 mm was considered(Fig. 1). As a result of the cutting process carried out with eachcutting tool, the wear types and mechanisms were analyzed byscanning electron microscopy (SEM) and energy dispersive X-rayspectroscopy (EDX). Additionally, decreases in terms of thediameter of the tool were evaluated along with the changes inthe slot geometry.

Cutting flute number

2

Some characteristics of the coatings used.

Hardness(HV 0,05)

Frictioncoefficient

Oxidationtemperature (1C)

Coatingthickness(lm)

AlCrN 3200 0.35 1100 1.8

AlTiN 3300 0.4 900 3

DLC 2500 0.1–0.2 350 1

TiAlNþAlCrN 3300 0.35–0.4 1100 0.8

TiAlNþWC/C 3000 0.15–0.2 800 1.1

_I. Ucun et al. / Wear 300 (2013) 8–1910

Fig. 2 illustrates the measurement of effective tool diameterand tool edge radius of worn tool. Before cutting process, thediameter and edge radius of micro cutting tools were measuredusing scanning electron microscope (SEM). Moreover, the imagesof the worn micro-end mills were taken after 2 passes (120 mm inlength) using SEM. An optic USB microscope was used to measureslot geometry. Images taken from SEM (tool diameter and edgeradius) and USB microscope (slot geometry) were compared toestablish relationship between them.

In order to determine the effect of lubrication during the micromilling process, an extra cutting series was realized by using anAlCrN coated cutting tool. During the aforementioned additionalexperiments, minimum lubricating conditions (MQL) wereconsidered as the lubrication strategy. The cutting liquid usedfor lubrication was a herbal essence lubricant whose specifica-tions are given in Table 4. During the experiments, the operatingfrequency of the lubrication system was kept constant at200 pulse/min and the lubricant consumption at 150 ml/h.

3. Results and discussion

3.1. Evaluation of tool wear

According to the results obtained, the tool wear generallyoccurs as flank wear and fractures along the sides (Figs. 3 and 4).In the uncoated micro tool, generally chipping type damage wasobserved. It can be said that the coating material gave certaintoughness to the tool, because less chipping was observed in thecoated tool. Among the coated tools, the least chipping damageoccurred in the AlTiN and TiAlNþWC/C tools. In these tools, a

Micro end mill

n= 20000 rpm

Workpiece

Fig. 1. A schematic illustration of the micro end milling process.

OriginalDiameter

Fig. 2. Measurement of tool wear: effective tool

decrease in diameter due to the abrasive wear mechanism wasthe main effect observed. Apart from this, due to the interaction ofthe cutting tool with the workpiece, chip adherence on the sideswas observed. It is thought that the corrosion observed on thesurface of the workpiece was a result of the abrasive wearmechanism that occurred due to the corrosive particles in thechemical composition of the workpiece. In addition, the originalcutting edges of the cutting tools were significantly corrupted byabrasive wear and a new cutting edge towards the tool axis wasformed. This situation resulted in a decrease of the tool radius.Unlike in the turning process, in the milling process each of thecutting edges removes chips intermittently. Due to this situation,cutting edges are subject to repetitive loads. As a result, cracksdue to fatigue are observed particularly on the cutting edges andend parts where the resistance levels are comparatively low[24,25]. Due to the small feed per flute in the milling process,proper chip formation is not possible at all times. Instead, theworkpiece is become deformed as elastic–plastic. Deformationhardness which occurs as a result causes the cutting tools to wearmore rapidly [6].

The SEM analysis conducted on the damaged tools revealedthat the cracks observed along the lateral part of the milling toolswere due to the workpiece being stuck on the side edge (Fig. 5a).In the EDX analysis conducted, it was observed that the piecewhich was about to split from the tool surface contained exces-sive Ni, Cr, and Fe elements. Additionally, in Fig. 5(b) it can beseen that the flank of the cutting tool was literally smeared on theworkpiece. This situation was supported by EDX analysis. Unlessthere is full chip formation, this smearing is thought to formespecially on the part where new elastic–plastic deformationdominates. This situation causes excessive friction and localstressing on the parts where high accumulation of the workpieceis observed. Thus, the cracking type of tool loss is observed morefrequently in these areas. As a result, it can be said that thedamages observed at the cutting edges of the tools are directlyrelated to the cutting method, the cutting tool material, itsgeometric properties, and the structural properties of the work-piece material used.

The change in the radius of the tool due to the abrasive wearduring the micro milling process is an important parameter in

diameter (left) and tool edge radius (right).

Table 4Some properties of Coolube 2210 lubrication oil.

Ingredient Density (20 1C)(kg/m3)

Viscosity (40 1C)(mm2/s)

Flash point(1C)

Vegetable oil 890 10 4200

AlCrN

Fracture

Peeling off coating

Chip adhesion on cutting edge

AlTiN

Fracture

Peeling off coating

Edge chippingUncoated

Corner chippingThe newly formed

cutting edge

Edge chipping

Fig. 3. Wear shapes obtained for different coating types.

Flank wear

DLC

Flank wear

Fracture

Flank wear

TiAlN+AlCrN

Fracture

Corner chipping

Chipping

TiAlN+WC/C

Chip adhesion on cutting edge

Peeling off Coating

Fig. 4. Wear shapes obtained for different coating types.

_I. Ucun et al. / Wear 300 (2013) 8–19 11

terms of both the dimensional and surface quality of themachined product. The wear that occurred was also evaluateddepending on the change in the tool diameter (Fig. 6, Table 5). Themost significant factor in the graphs was the positive contributionof the coating material to the wear resistance of the tool. Certainproperties are required in cutting tools which are used for

materials which have hard metal cutting conditions, and thesemay be listed as good wear resistance, preservation of hardnessunder high temperatures, and good chemical stability [26]. Theseproperties provide better hardness, oxidation resistance, thermalconductivity, and friction coefficients [11,27]. When the variouscoated tools were compared, it was observed that the decrease in

Ti

W

Ni

Cr

Fe

Al

TiAlN+WC/C

AlTiN

Fig. 5. EDX analysis realized on cutting tool.

Fig. 6. Effect of coating materials and cutting parameters on change in tool diameter.

_I. Ucun et al. / Wear 300 (2013) 8–1912

the diameter depending on the abrasive wear was at its maximumlevel in the DLC and TiAlNþWC/C coated tools. Additionally,it can be said that the decreases in the radii of AlTiN, AlCrN and

TiAlNþAlCrN coated tools were comparatively lower and werequite similar to each other. This situation may be explainedby the mechanical properties of the coating materials.

Table 5Variation of diameter and edge radius of micro end mill.

Tool type Feed

rate

(mm/

flute)

Depth

of cut

(mm)

Original

diameter

(mm)

New

diameter

(mm)

Reduction

(%)

New

edge

radius

(mm)

AlTiN 1.25 0.1 768 696.85 9.26 27

2.5 671.74 12.53 29

3.75 715.21 6.87 26

5 0.1 712.11 7.28 32

0.15 698.65 9.03 35

0.2 743.26 3.22 35

DLC 1.25 0.1 768 656.11 14.57 64

2.5 649.04 15.49 77

3.75 704.34 8.29 52

5 0.1 706.95 7.95 29

0.15 714.44 6.97 30

0.2 720.51 6.18 35

TiAlNþAlCrN 1.25 0.1 768 670.00 12.76 33

2.5 706.15 8.05 37

3.75 716.46 6.71 52

5 0.1 698.36 9.07 30

0.15 732.86 4.58 69

0.2 745.21 2.97 50

TiAlNþWC-C 1.25 0.1 768 665.60 13.33 29

2.5 665.94 13.29 30

3.75 667.65 13.07 28

5 0.1 693.71 9.67 40

0.15 709.39 7.63 30

0.2 716.46 6.71 33

AlCrN 1.25 0.1 768 691.89 9.91 31

2.5 693.19 9.74 27

3.75 726.49 5.41 32

5 0.1 730.34 4.90 39

0.15 724.68 5.64 34

0.2 738.46 3.85 30

WC-Co 1.25 0.1 768 607.51 20.90 30

2.5 649.85 15.38 36

3.75 673.48 12.31 23

5 0.1 680.37 11.41 28

0.15 686.28 10.64 30

0.2 713.85 7.05 60

_I. Ucun et al. / Wear 300 (2013) 8–19 13

The difference between the hardness values of the coatingmaterials given in Table 3 coincides with the tendency shownin Fig. 6.

The hardness value is an important parameter that determinesthe wear behavior of the tool. When the decrease in the toolradius is considered as a natural result of the abrasive wearmechanism, the reasons for the performances of AlTiN, AlCrN, andTiAlNþAlCrN (HV0,05¼3300, 3200, 3300) coated materials can bebetter understood. Another result obtained from Fig. 4 is thechange in the radius depending on the feed rate and depth of cut.When both the depth of cut and the feed rate increased, thechange in the cutting tool radius was observed to be compara-tively low. While the maximum wear for each tool was found tobe f¼1.25 mm/flute, the minimum wear at the maximum feedrate value of 5 mm/flute was found to be only half as much.A similar situation occurred for the cutting depth. While themaximum wear occurred at small cutting depths, it was observedthat the wear value decreased as the cutting depth increased.It was thought that the higher wear values that occurred with thelow feed rate were due to the chip formation process, because,as seen in Fig. 7, the chip thickness which varies along the cutmust be higher than the minimum chip thickness for a properchip formation to occur. Within the distance required to reach theminimum chip thickness, the plowing mechanism dominates. Thecircular movement of the tool for the cutting process is directly

related to the distance it covers. Thus, the axial position of thetool resulting in proper chip formation determines the distancecovered by the plowing mechanism. As known, the plowingmechanism significantly affects the micro milling process. WhenFig. 7 is considered, it is seen that a proper chip formation at theminimum feed rate can only be realized with a comparativelylarger rotation angle of the cutting tool. When this situation isconsidered in terms of other feed rates indicated in Fig. 7, it isseen that as the feed rate increases, a smaller rotation angle of thecutting tool is required for a proper chip formation. Therefore, thedistance where the plowing mechanism is observed is larger athigher feed rates than at low feed rates. Accordingly, a decrease inthe radius due to the wear observed at low feed rates.

In addition to these results, slot geometries which occur as theresult of the cutting process clearly reveal the effect of cuttingdepth on the decrease in radius (Fig. 8). As indicated in Fig. 7, forthe chip formation to be observed at low feed rates, it is essentialfor the cutting tool to cover a certain distance. From the momentthe cutting tools start removing metal to produce a proper chipformation; the plowing mechanism comes to the forefront as thedominant deformation type. By virtue of this mechanism, thefriction between the cutting tool and the workpiece increasesparticularly, and as a consequence the cutting tool wears rapidly[28,29]. As the micro tools wear more in parallel with thisexplanation, the difference between a1 and a0 in the slot geome-tries shown in Fig. 8 increase. It can be concluded from Fig. 8 thatthe feed rate is more important parameter for tool wear/tool lifeand geometrical tolerances than the depth of cut. In the micromilling process, the effects of cutting parameters and the chipformation process differ from the conventional milling process.In particular, the chip occurrence mechanism in the micro millingprocess significantly affects the tool life. While chip formation isobserved at values above the minimum chip thicknesses,a uniform stress distribution is maintained on the cutting tool.With regard to the chip thicknesses below the minimum chipdepths, chip formation does not fully occur and recovery isobserved. As a result, high local stresses occur as a result of thefriction forces [15]. In Fig. 8(a) the feed rate value and depth of cutis close to critical chip thickness. Therefore the slot geometry isdifferent from cutting tool geometry due to inappropriate chipformation mechanism. Additionally the feed per tooth is the mostinfluential parameter affecting burr formation (see Fig. 8a). Thelack of proper chip formation would result in the sticking/smearing of the workpiece material. The chip smeared on thetools would form an environment with additional friction forcesand the cutting edges would be subject to higher stresses.

As a result of the experiments conducted, the changes in thecutting edge radius were established as another criterion forevaluating the wear performances of the tools. The cutting edgesof the micro tools can be easily worn during the cutting process.As the aforementioned wear primarily occurs on the corner–edgepart of the tool, rolling occurs due to its circular movement. Thissituation is defined as the increase of the corner–edge radius.As the geometrical change on the cutting corner–edge wouldreduce the ratio of the minimum chip thickness to the edgeradius, it has a negative effect on the cutting process. As a result ofthe evaluations done, the characteristics of the edge-radius arethought to be related to the change in the diameter of the tool.

As seen in the scheme provided in Fig. 9, with the cuttingprocess of the cutting tools, the edge points, which are theweakest spots of the cutting edge, are observed to be the mostrapidly worn parts. Additionally, the effect of rotation leads torolling, and in a sense increases the edge radius. While the edgeradius is initially r1, it becomes r2 due to abrasive wear. As thecutting process continues, a new edge is formed with increasingwear. r3, which is the radius of the aforementioned new edge, is

DLC f=1.25 µm/flute- 0.1 mm DLC f=1.25 µm/flute- 0.2 mm

DLC f=1.25 µm/flute- 0.2 mm DLC f=3.75 µm/flute- 0.2 mm

a0

a1

b0

b1

c0

c1

d0

d1

Fig. 8. Variation of slots shape depending on change in tool diameter (a1/a04b1/b0, c1/c04d1/d0).

2 1 Workpiece Workpiece

Micro end mill Micro end mill

2 1

d d

mµ5.2mµ52.1

3 4

3.75 µm 5 µm

Micro endmill

Micro endmill

Workpiece Workpiece

3 4

dd

Fig. 7. Chip formation process depending on angular position of tool for different feed rates (f14f24f34f4, L14L24L34L4).

_I. Ucun et al. / Wear 300 (2013) 8–1914

smaller than the initial radius r2. In other words, the edge radius,which initially increased with the decrease in the tool diameter,showed a reducing tendency. With these explanations, thechanges in the edge radius of the cutting tools during the cuttingprocess are given in Fig. 10. As seen in Fig. 10, the size of the edgeradius value is inversely proportional to the change in the tooldiameter. As the change in the tool diameter increases, the radiusvalue tends to decrease.

An additional series of experiments were carried out in orderto expose this mechanism which occurs due to the wear of theedges. In the experimental study made with DLC coated cuttingtools, a constant feed rate and depth of cut were considered.In order to incrementally observe the above mentioned change inthe edge geometry, cutting processes were realized at certainstages and the geometrical changes after each cutting processwere determined via the measurements made in the microscopic

r1

r2

r3

Cutting edge Cutting edge

Flank weararea

Flank weararea

Originalcutting edge

Newly formedcutting edge

I. Phase II. Phase

Fig. 9. Change mechanism of edge radius depending on wear (r24r34r1).

Fig. 10. Relation between cutting edge radius and tool diameter change.

Fig. 11. Variation of tool diameter and edge radius depending on cutting length.

_I. Ucun et al. / Wear 300 (2013) 8–19 15

environment. The results obtained from the study conducted aregiven in Fig. 11. When the graph in the figure is considered, asignificant increase in the edge radius of the tool is observed aftera 20 mm cut. Conversely, the observed increase in the diameterchange is lower. This situation is an explicit indicator of the factthat the wear initially started at the end spots of the cutting edge,because it is a commonly known fact that wear initially starts onthe edges, which are the weakest spots of the cutting edges.Additionally, it was observed that with the increase in the cuttingdistance, the edge radius started to decrease, whereas thediameter of the cutting tools started to decrease rapidly.In consideration of these approaches, the graphs in both Figs. 10and 11 show an inverse relationship between the cutting edgeradius and changes in tool diameter.

In order to see the mechanism defining the relation betweenthe cutting edge radius and the diameter change more clearly,SEM analysis of the cutting tools used after each cutting distancewas carried out. In the analysis, the edge radius was defined as ‘‘r’’and the decrease in the diameter was defined as ‘‘a’’. At this point,‘‘a’’, which defines the decrease in the diameter, was expressedwith the equation below.

a1,2,3,4 ¼dnew�dwear

2ð1Þ

where ‘‘dnew’’ represents the diameter of the tool before use while‘‘dwear’’ represents the diameter of the worn tool. It can be seenfrom Fig. 12 that the wear which occurred after a 20 mm cuttingprocess caused deterioration primarily on the edge spots of thetool geometry. Conversely, it can be clearly observed that the ‘‘a1’’value, which represents the decrease in the diameter, increasedslightly. With the increasing cutting distance, it was observedfrom the SEM images that the cutting edge radius decreased,whereas the distance ‘‘a’’ started to increase. When a generalevaluation was done from Fig. 12, it was observed that therelation between the edge radii was ‘‘r14r24r34r4’’, whereasthe relation between the decrease in the tool diameters was as‘‘a1oa2oa3oa4oa5’’.

3.2. Effect of coating material on built-up edge formation

The effect of the coated tools on the built-up edge formationduring the cutting process is determined by the SEM images inFig. 13. The ductile structure and high chemical affinity of Inconel718 nickel, which was preferred as the workpiece material,caused the chips to generally stick on the cutting edge duringthe metal cutting process. When the images obtained wereexamined, a significant built-up edge formation was observedparticularly in TiAlNþAlCrN and AlCrN coated and uncoatedtools. With regard to the A1TiN coated cutting tool, a lower chipaccumulation was observed. Apart from these, it can be said thatDLC and TiAlNþWC/C coated cutting tools showed better built-upedge performances. The most important characteristic whichdistinguishes these coatings from the others is that both coatingmaterials are listed in the oil-free lubricants class. DLC coating(a: C–H) chemically comprises hydrogen and carbon elements.Due to its atomic packing, the carbon element may have differentstructures (graphite, diamond, etc.). The atomic packing of thecarbon in DLC coating is an amorphous structure formed with thedominant bond combination in both graphite and diamond[30,31]. This situation causes the coating to have the character-istics of both graphite and diamond. The aforementioned struc-tural properties cause the coating to become chemically stable.As a result, DLC coatings have a lower adherence property [32,33].

Lc = 90 mm Lc = 125 mm

Lc = 20 mm Lc = 45 mm Lc = 65 mm

a1

a2

a3

a4 a5

r1 r2r3

r4

r5

Fig. 12. Illustration of cutting edge change after each cutting processes.

DLC TiAlN+WC/C

TiAlN+AlCrN AlTiN

atedocnUNrClA

Fig. 13. Comparison of built-up edge performance of micro end mills (ap¼0.15 mm, f¼5 mm/flute).

_I. Ucun et al. / Wear 300 (2013) 8–1916

Fig. 14. Effect of MQL lubrication on tool diameter change.

Table 6Variation of diameter and edge radius of micro cutting tool for dry and MQL

cutting conditions.

Cutting

strategy

Tool

type

Feed

rate

(mm/

flute)

Depth

of cut

(mm)

Original

diameter

(mm)

New

diameter

(mm)

Reduction

(%)

New

edge

radius

(mm)

MQL AlCrN 1.25 0.15 768 732.52 4.62 42

2.5 746.69 2.77 41

3.75 0.1 737.66 3.95 32

0.15 738.09 3.89 25

0.2 746.30 2.83 62

5 0.15 739.24 3.75 34

DRY 1.25 0.15 768 702.76 8.49 42

2.5 719.13 6.36 41

3.75 0.1 726.49 5.41 32

0.15 721.42 6.06 25

0.2 751.15 2.19 62

5 0.15 724.68 5.64 34

_I. Ucun et al. / Wear 300 (2013) 8–19 17

Considering this information, it is thought that the lack of BUEformation on the cutting tool was a consequence of the fact thatthe coating comprised the characteristics of both graphite anddiamond. Additionally, the low friction coefficient of the afore-mentioned coating would contribute to the decrease of the loadderiving from the friction at the tool–chip interface. A similarsituation also exists for TiAlNþWC/C coating. The WC/C compo-nent in the coating composition is also listed within the oil-freelubricants class, similarly to the DLC coating [34]. Additionally,this coating type has a low friction coefficient. Therefore, WC/Cshows a good performance in terms of built-up edge formation.

The reason for the built-up edge formation observed inTiAlNþAlCrN coated cutting tools is thought to be the CrN phasein the composition of the A1CrN layer at the outer sole, because astudy by Sharman et al. [12] showed a good chemical interactionof CrN with Inconel 718 nickel alloy. Parallel to this result, anexcessive chip adherence was observed in the A1CrN coatedcutting tool. As for the AlTiN coated cutting tool, relatively lesschip adherence was observed. In another study [6], relativelyweak chemical interaction of A1TiN coating with Inconel 718nickel alloy was reported. Therefore, BUE in A1TiN coatingsoccurred in low amounts.

3.3. Effect of MQL on tool wear

In order to determine the effect of the lubrication factor duringthe micro milling process, experiments were carried out by usingA1CrN coated micro tools, and the diameter changes of thecutting tool are given in Fig. 14. Besides, all experimental valuesfor dry and MQL cutting conditions are given in Table 6. As seen inFig. 14, the MQL process significantly reduces the diameterchange observed in the tool. When MQL is used particularly forthe feed rate of f¼1.25 mm/flute, the decrease in the diameter isonly half as much. With increases in the feed rate, the differencebetween dry cutting and MQL cutting decreases. However, it canstill be said that the MQL system has a significant breakthrough interms of the wear performance of the tool. By virtue of thecompressed air and the nozzle used, the MQL system particu-larizes the fluid and transfers it to the cutting area.

As seen in Fig. 15, these steamy lubricating particles stick onthe interface between the cutting tool and the workpiece andform some kind of smooth film layer between them. Therefore,the intensity of the friction which occurs on the interface due tothe mechanical interaction is significantly reduced. This situationprovided by MQL is also thought to reduce the effects of plowingformation that are particularly observed at low feed rates, asillustrated in Fig. 7, because the difference between the diameterchanges during dry cutting and MQL cutting led to such animpression.

Along with its lubricant and coolant effect between the cuttingtool and the workpiece, the MQL process also prevents the formation

of a built-up edge during the milling process. The lubricantparticles placed at the tool–chip interface limit the interactionof the workpiece with the cutting tool. In the SEM images of thecutting tool given in Fig. 16, a significant amount of chipaccumulation was observed on the cutting edges during the drycutting conditions. However, with the MQL cutting, it wasobserved that the BUE formation decreased significantly. On theother hand, during a 0.2 mm deep cutting, a certain amount ofchip adherence was observed on the cutting edge. However, whencompared to the images of dry cutting conditions, it can be saidthat there was a significant decrease in the BUE formation.Additionally, in dry cutting, flank wear was observed over a largerarea. Furthermore, during a 0.15 mm deep cutting process, it wasobserved that a part of the cutting edge disappeared with theeffect of wear under dry cutting conditions.

4. Conclusion

In this study, the effects of the coating material and MQLsystem were examined in the milling of Inconel 718 nickel undermicro conditions. For this purpose, five different coating materialswere used: TiAlNþAlCrN, DLC, AlTiN, TiAlNþWC/C, and AlCrN. Inaddition, the MQL effect was examined solely for the AlCrNcoated tool.

As a result of the experimental study, flank wear was observeddue to the abrasive wear mechanism, which is the mostfrequently observed wear type. In addition, local fractures onthe cutting edges and sides of the cutting tools as a consequenceof fatigue and BUE formation were observed. It was observed thatlower levels of wear occurred in the coated tools compared to the

Workpiece

CuttingEdge

Oil ParticlesLubricatingnozzle

Micro end mill

Fig. 15. A schematic illustration of MQL lubrication.

0.1 mm 0.15 mm 0.2 mm Depth of cut

Condition

Adhered chip

Adhered chip Adhered chip

Flank wear area

Depth of cut

Condition

Flank wear area Flank wear area

Flank wear area

Flank wear area Flank wear area Flank wear area

Fig. 16. Effect of MQL lubrication on built-up edge formation.

_I. Ucun et al. / Wear 300 (2013) 8–1918

uncoated ones, and lower diameter changes were also observed inthe former. The reason for this situation is the high hardnessvalues and low friction coefficients of the coated materials.In addition, the BUE formation during the milling process varieddepending on the coating type. DLC and TiAlNþWC/C coatings inparticular showed a good performance within this context. At thispoint diamond coated cutting tool can be suggested for bothlonger tool life and surface roughness. Unlike in the conventionalmilling process, high wear rates were observed at low feed ratesand with small depths of cut. The edge radius values of the microtools were variable depending on wear. The radius value whichinitially increased showed a decreasing tendency with thedecrease in the diameter reduction. It was observed that thelubrication process during the milling process contributedsignificantly to the cutting performance. It was also observedthat the MQL process significantly increased the tool life and alsoprevented chip adherence. Further research needs to beconducted to investigate effect of flow rates and different oil.Future research also needs to address the effects of differentcooling technique (cryogenic, air cooling or cooling–MQL) onworkpiece surface integrity.

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