Experimental studies on dry machining behavior of Ti-6Al ...

Experimental studies on dry machining behavior of Ti-6Al-4V usingcarbide, cermet, and SiAlON tools

SARTHAK PRASAD SAHOO and SAURAV DATTA*

Department of Mechanical Engineering, National Institute of Technology, Rourkela 769008, Odisha, India

e-mail: [email protected]

MS received 7 February 2021; revised 25 August 2021; accepted 11 October 2021

Abstract. Traditional dry machining of ‘difficult-to-cut’ titanium alloy Ti-6Al-4V has always been a chal-

lenging task. This is due to its lower thermal conductivity, strong work-hardening tendency, and extreme

chemical reactivity. These causes adverse machining effects including premature tool failure, evolution of huge

cutting temperature, machine tool chatter, and disappointing surface integrity of the machined work part.

Selection of compatible tool material, appropriate tool geometric parameters, and adequate control of cutting

parameters are of vital importance towards achieving satisfactory machining yield. In this context, performances

of MT-CVD TiCN/Al2O3 bi-layered coated carbide, PVD TiN/TiCN/TiN multi-layered coated cermet, and CVD

TiCN/Al2O3 bi-layered coated SiAlON inserts are studied during dry machining of Ti-6Al-4V within cutting

speed range 50-130 m/min; at constant feed * 0.1 mm/rev, and depth-of-cut * 0.35 mm. Approximate tool-tip

temperature (maximum value) attained during operation, magnitude of tangential cutting force, and width of

flank wear progression are measured. Detailed study on wear morphology of worn-out inserts, chip’s micro/

macro morphology, and surface integrity of the machined product are carried out. It is experienced that cermet

tool performs better than remaining two counterparts in purview of lower tool-tip temperature, reduced flank

wear, and better machined surface integrity.

Keywords. Dry machining; Ti-6Al-4V; tool-tip temperature; flank wear; wear morphology; surface integrity.

1. Research Background

Since the middle era of the ’90s when titanium was com-

mercially introduced in military and aircraft sectors, it

started playing an inevitable role in manufacturing industry.

As a prime element for manufacturing airframes and aero-

engine components; demand for titanium and its alloys is

rapidly rising. Inherent properties, including high strength-to-weight ratio, high fracture resistance and excellent

resistance against corrosive environments, make titanium

alloys suitable for working in the most dynamic operating

conditions with prolonged service period.

Beyond aerospace industry, these alloys also contribute

remarkably to petrochemical refineries, marine industries,

chemical and biomedical applications [1, 2]. Despite mas-

sive usage of titanium alloys, machining of these alloys is

still a challenging task as these are categorized as ‘difficult-to-cut’ alloys depicting poor machinability attributes which

were discussed in previous reporting [3]. Considering grade

5 titanium alloy (Ti-6A1-4V: alloy with both a- and b-phases); due to its adequate strength and hardness, it has

substantial usage in aerospace industry and also has sig-

nificant contribution to global titanium production. Hence,

investigating aspects of machinability of Ti-6Al-4V is a

primary concern for budding researchers.

As highlighted in literature, researchers utilized varied

machining environments: flood cooling [4–6], Minimum

Quantity Lubrication (MQL) [7–9], Minimum Quantity

Cooled Lubrication (MQCL) [10, 11], Nanofluid MQL

(NFMQL) [12, 13], etc. to improve machining performance

of ‘difficult-to-cut’ Ti–6Al–4V. Though improved tool life

(by reducing cutting zone temperature) was previously

reported by employing abovementioned cooling media,

huge procuring cost of coolants and associated post-use

disposal issues (especially in flood cooling), occupational

health hazards due to mist generation under MQL and

MQCL conditions, and arbitrary local agglomeration of

expensive nano-additives under NFMQL are challenging

issues for which desired cooling and lubrication effects of

applied coolants may get hampered. In view of such limi-

tations, researchers are still striving towards dry machining

(adherence to green manufacturing philosophy) with

appropriate cutting tool material. Hence, machinability of

Ti-6Al-4V in purview of various cutting tools appears a

critical research agenda in the present context.

Grzesik [14] reported that a suitable combination of

workpiece, cutting tool material, machine tool along with

fixtures, cooling environment and cutting parameters*For correspondence

Sådhanå (2021) 46:239 � Indian Academy of Sciences

https://doi.org/10.1007/s12046-021-01767-1Sadhana(0123456789().,-volV)FT3](0123456789().,-volV)

enables a machining process to perform smoothly and

efficiently. For this, cutting tool is expected to be 30-50 %

harder than workpiece with high hot hardness, high

toughness, superior wear resistance, excellent thermal

conductivity. Desired properties of a universal cutting tool

were already highlighted in literature [1, 15]. Figure 1

portraits hardness and toughness variations of few com-

monly used cutting tool materials [15]. Often, coating layer

(deposited over tool substrate) can also increase longevity

of tool insert by acting as barrier against standard wear

modes [16]. As WC-Co tool is traditionally practiced for

machining of Ti-6Al-4V alloy [2, 15], performance evalu-

ation of WC-Co tools (considering both coated and

uncoated grades) was carried out by many researchers

[17–21]. This indicates that substantial volume of work was

carried out on dry machining of Ti–6Al–4V alloy using

carbide inserts (with and without coating); however,

machining behavior of ‘difficult-to-cut’ alloys is still a

controversy. As properties of cutting tools play essential

role in deciding extent of machinability; it necessitates

selecting an appropriate cutting tool for improved

machining performance on Ti-6Al-4V work alloy.

During machining with coated carbide inserts, once

coating layer(s) get(s) peeled off, exposing the tool sub-

strate; bonding between tool and coating gets loosened as

the binder phase (Co) becomes unstable due to the trans-

mitted heat fluxes [22], and thus common wear mechanisms

(adhesion, abrasion, diffusion, etc.) are supposed to take

place rapidly. This enforces looking for harder materials

(harder than WC-Co) as cutting tools [23], especially, to

suit applications of High-Speed Machining (HSM) on

‘difficult-to-cut’ alloys.In ‘Advanced Cutting tools,’ a particular type of tool

material, comparatively harder and more chemically stable,

is introduced, which can withstand cutting temperature to a

greater extend. Such tools are referred as ceramic-basedcutting tools, which can preferably be used for continuous

turning, even under dry conditions. Ceramic tools possess

higher hot hardness than carbide tools, but toughness needs

to be compromised. Ceramic tools basically include silicon

nitride (Si3N4), alumina (Al2O3), and SiAlON as base

materials [24]. Applications of SiAlON tool were amply

documented in literature, especially for dry machining of

superalloys and different steel materials [25–27].

Another tool material denoted as Cermet, which is

basically composed of (Ni ? Co ? MoxC ? TiC) with

(TiC ? WC ? TaC ?NbC ? V) as additives, is introduced

as a breakthrough in the cutting tool industry. Cermet is

tougher than ceramics but has lower hot hardness. The

contribution of different elements (in its composition) with

their desired properties was reported by Porat and Ber

[28], followed by feasibility analysis of this particular

family of tools for a wide range of various machining

operations. Thus, cermet was described as a strong com-

petitor for the conventionally used coated tools to machine

a variety of work alloys. However, extensive applications

of cermet tool were found attempted by superalloys as well

as steel sectors [29–32].

Available literature resource is found scanty enough

focusing machining performance of Ti–6Al–4V with

harder tool materials. Motivated by substantial machining

applications of ceramic/ cermet tools, especially on

nickel-based super-alloys (for example Inconel 718);

present research articulates machining performance of Ti-

6Al-4V in purview of varied cutting tool material: car-

bide, cermet, and ceramic. As ‘difficult-to-cut’ work

alloys not only include nickel-based superalloys but also

titanium and its alloys. Machining challenges being

common to both Inconel 718 as well as Ti-6Al-4V;

hence, application feasibility of cermet/ ceramic tools

during dry machining of Ti-6Al-4V needs to be studied.

As reported by de Lacalle et al [24], application of

advanced ceramic-based cutting tools during metal cut-

ting process may be beneficial for improved machin-

ability as they possess comparatively better properties

than conventionally used carbide inserts. Therefore, pre-

sent work carries out hazardless as well as environment-

friendly dry cutting tests on Ti–6Al–4V by using varied

tool materials: coated carbide, coated cermet, and coated

SiAlON. Conventional carbide tool is selected (as a basis

for comparison) since carbide tool possesses higher

toughness (but lower hot hardness) amongst three tools

selected. In addition, carbide has higher thermal con-

ductivity than cermet and ceramics. On the contrary,

ceramic tool (SiAlON) exhibits higher hot hardness but

lower toughness. Keeping carbide and SiAlON as

extreme cases; cermet tool is also chosen which corre-

sponds to intermediate properties (values) between car-

bide as well as SiAlON. Dry machining experiments on

Ti-6Al-4V, in consideration with aforesaid tool material

combination, have not been studied and documented in

earlier literature. In this work, machinability is evaluated

through a detailed study on cutting force, tool wear, tool-

tip temperature, evolved chip morphology (macro/

micro), and machined surface integrity. Results obtained

thereof, are interpreted in purview of process mechanics,

and tool/ work properties.

Figure 1. Hardness versus toughness plot for few commonly

used cutting tool materials.

239 of 23 Sådhanå (2021) 46:239

2. Problem Statement

The present work carries out hazardless as well as envi-

ronment-friendly dry machining tests on Ti–6Al–4V by

using varied tool materials: coated carbide, coated cermet,

and coated SiAlON. Conventional carbide tool is selected

(as a basis for comparison) since carbide tool possesses

higher toughness (but lower hot hardness) amongst three

tools selected. In addition, carbide has higher thermal

conductivity than cermet, and ceramics. On the contrary,

ceramic tool (SiAlON) exhibits higher hot hardness but

lower toughness. Keeping, carbide, and SiAlON as extreme

cases; cermet tool is also chosen which corresponds to

intermediate properties (values) between carbide as well as

SiAlON. Feasibility of dry machining on Ti-6Al-4V, in

consideration with aforesaid tool material combination, has

not been studied, and documented in earlier literature. In

this work, machinability is evaluated through detailed study

on cutting force, tool-tip temperature, tool wear, evolved

chip morphology (macro/ micro), and machined surface

integrity. Results obtained thereof, are interpreted in pur-

view of process mechanics, and tool/ work properties.

3. Material and Methods

A round bar of Ti–6Al–4V with an average initial working

diameter of 54 mm and length 490 mm is used as work-

piece. Ti-6Al-4V generally exists in two crystalline states:

a low temperature a-phase (with HCP structure), and a

high temperature b-phase (with BCC structure). Table 1

represents chemical composition of Ti-6Al-4V. Table 2

provides salient properties of this alloy. Turning tests are

carried out on a heavy duty precision lathe (NH26, HMT

Machine Tools Limited, Bangalore, India) with rated

spindle power of 11 kW. Details of three different cutting

inserts used for the present experimentation is given in

table 3. A tool holder (Kennametal made) having desig-

nation PSBNR-2020K12 is used to hold cutting inserts

rigidly.

All three inserts have identical geometry except nose

radius and chip-breaker type. In addition, there is also

variation in number/ type of coated layers as well as

method of coating material deposition. The present research

problem may be viewed as a decision-making problem to

investigate out of three available alternatives which insert

performs the best for given range of machining parameters

on Ti-6Al-4V work material. In this work, carbide, cermet

and SiAlON are selected as tool substrate materials which

possess wide difference in their properties. It is believed

that properties of tool substrate impose greater influence on

variation of machining performance indices than other

parameters which are not common for three alternative

inserts.

Each finish turning experiment is conducted for 30 s

machining duration. It is not continuous machining. For

each machining trial, a fresh cutting edge is used.

Longitudinal turning experiments are carried out with

inserts of different tool material by changing four different

settings of cutting speed (vc = 50 m/min, 80 m/min, 100

m/min, and 130 m/min); whereas, feed (f), and depth-of-cut

(doc) are maintained constant at 0.1 mm/rev, and 0.35 mm,

respectively. Aforementioned cutting speeds are achieved

by setting spindle speed at 275 RPM, 465 RPM, 605 RPM

and 787 RPM, respectively.

The workpiece indeed has to rotate at around 800 rpm at

v = 130 m/min; yet deflection of workpiece is taken care of

by length of the workpiece. The total length of the work-

piece is about 490 mm. Out of which machining is only

performed on a length of 210 mm and the rest are held

rigidly within three-jaw self-centered chuck. Also, other

part of the workpiece is firmly clamped by dead center of

the tailstock. The length (cutting length)-to-diameter ratio

is nearly 3.88 which agree with the findings of Yeh and Lai

[33]. Therefore, workpiece defection is expected not so

severe.

Though uncoated carbide is the most suitable tool

material for machining titanium-based alloys but such tool

also suffers from different tool wears (mostly abrasion,

adhesion, diffusion and edge chipping) during extreme

cutting conditions. Also, dry turning of titanium-alloys is

limited within cutting speed range of 60-75 m/min when

using uncoated type carbide insert. The present experiment

is planned to be conducted in a wide range of cutting speeds

(50-130 m/min) where the highest cutting speed selected

herein is twice the allowed cutting speed for uncoated

carbide tools. Also, combination of enhanced toughness

and hardness of coated carbide inserts (figure 1) is another

prompting concern which motivates authors’ selection to

choose coated carbide grades over uncoated ones for dry

machining of Ti-6Al-4V. Along with coated carbide, a

Table 1. Chemical constituents of Ti-6Al-4V.

Element Ti Al V Fe O C N H

Weight

(%)

Base 5.5-6.75 3.5-4.5 0.40 0.20 0.08 0.05 0.015

Table 2. Salient properties of Ti-6Al-4V.

Properties Unit Average value

Melting point (�C) 1649

Density (g/cc) 4.44

Specific heat (J/kg �C) 560

Thermal conductivity (at 23 �C) (W/m �C) 7.2

Hardness (HB) 241

Tensile strength, yield (MPa) 828

Tensile strength, ultimate (MPa) 895

Sådhanå (2021) 46:239 of 23 239

comparative study on tool performance is carried out using

cermet and SiAlON inserts.

As a general rule of thumb, during finish turning, depth-

of-cut smaller than 1/3 of nose radius should be avoided.

As machining experiments are conducted under dry con-

dition; hence, machining time is considered as 30 s for

finish turning with small depth-of-cut. This is because dry

machining causes aggressive cutting environment during

machining of low conductive work material. High depth-

of-cut results in huge heat generation which severely

affects cutting tool. Corresponding to a particular tool

material, for each cutting speed, experiential trials are

performed thrice. Average value of different output mea-

sures (in relation to machining performance) is considered

for analysis.

A four channel Force-Torque dynamometer (KISTLER

9272 type, Kistler Instruments AG, CH-8408 Winterthur,

Switzerland) is used to measure tangential component of

cutting force (mean value). Measured forces are displayed

with charge amplifier (5070 type) which is equipped with

the dynamometer. Machining induced temperatures, pri-

marily at the tool-work interface (or tool-tip) is recorded by

an infrared thermometer (AR882, Solarman Engineering

Project Pvt. Ltd., New Delhi, India) which is basically non-

contact type, and operates on black body radiation princi-

ple. Other facilities including optical microscope (Carl

Zeiss Microscopy; GmbH 37081 Gottingen; Germany),

Scanning Electron Microscope (Jeol; JSM 6480 LV; Japan)

and Field Emission Scanning Electron Microscope (Nova

NanoSEM 450; FEI; USA) with EDS-facility are utilized

for investigation of chip’s macro/ macro morphology, and

morphology of tool wear. Machined surface integrity is also

studied through scanning electron microscopy. In doing so,

specimens (5 94 95) mm3 are cut through wire-EDM.

Transverse specimen section is polished, and properly

etched to reveal white layer just beneath the machined

surface.

Subsurface microhardness depth profile is obtained using

Vickers’s microhardness tester (LM-248AT, Leco Corpo-

ration, Michigan, USA). Machined specimens of appro-

priate size are cut through wire-EDM; cross section of

which are properly polished and finally etched for making

micro-indentations. All indentations are made using micro-

diamond indenter at 25 gf load and 10 s dwell time. The

indentation is measured optically and converted into a

hardness value.

4. Results and Discussion

Multi-layered coated WC-Co insert, used in the present

study, is manually polished with emery papers of different

grades along with running tap-water. Afterwards, the pol-

ished insert is viewed under scanning electron microscope

to distinctly identify coating layers, deposited over the tool

substrate. This results in clear visualization of two distinct

layers over carbide substrate (figure 2). EDS analysis is

carried out to confirm various constituents of coating layers

as well as the tool substrate. EDS reveals presence of Al2O3

upper layer; whereas, TiCN layer is deposited just above

the carbide substrate. Both coating layers are deposited

through CVD method. The upper coating of Al2O3 is

expected to provide better thermal and chemical stabilities

to the tool, especially, during dynamic cutting conditions

[34]. On the other hand, the bottom TiCN layer helps in

reducing flank wear by restricting abrasion at tool flank

face, and by strengthening the tool substrate [35].

Figure 3 depicts the microstructure of cermet tool sub-

strate. The manual polishing method, as stated before, is

followed herein, and finally, the substrate is viewed through

scanning electron microscopy. However, upper coating

layer of TiN, which is deposited through PVD technology,

is not visible under SEM. This may be due to presence of

tiny (very small thickness) coating layer which may be

Table 3. Details of cutting inserts used.

Insert type/particulars Carbide-based Ceramic-based (SiAlON) Cermet-based

Grade (Kennametal) KCM15B KYS25 KT315

Catalogue number (insert

specification)

SNMG 120408MP SNGA 120412T01020 SNMG 120408FN

Shape Square Square Square

Chip breaker type Medium Positive Chamfered edge with no chip breakerFinishing Negative

Coating layers TiCN/Al2O3 TiCN/Al2O3 TiN/TiCN/TiN

Coating method CVD CVD PVD

Tool designation as per

Orthogonal Rake System (ORS)

system

k; c0; a0; a00;u1;u; r ðmmÞ

-6�,-6�,6�,6�,15�,75�,0.8 (mm)-6�,-6�,6�,6�,15�,75�,1.2 (mm) -6�,-6�,6�,6�,15�,75�,0.8 (mm)

Here, k ¼ Inclination angle, c0 ¼ Orthogonal rake angle, a0 ¼ Orthogonal clearance angle, a00 ¼ Auxiliary orthogonal clearance angle, u1 ¼ Auxiliary

cutting edge angle, u ¼ Principal cutting edge angle, r ¼ Nose radius

239 of 23 Sådhanå (2021) 46:239

removed completely during mechanical polishing process.

Further, to have a better view of the microstructure, the

insert is etched. The etchant is prepared by mixing 1 ml HF,

5 ml HNO3, and 60 ml H3PO4. The darkest portions rep-

resent core which consist of raw Ti(C, N) particles. Inner

and outer rims can also be seen as greyish portions due to

presence of (Ti, W) (C, N). These hard ceramic grains are

well distributed by a thin layer of metallic binder phases of

Ni and Co. According to Monteverde et al [36], the amount

of TiC particles in the composition of cermet decides its

hardness; whereas, presence of higher amounts of TiN

particles makes the tool comparatively softer due to

extensive textured flaws incurred by the low sintering

temperature. Finally, features of cermet are greatly affected

Figure 2. Microstructure of MT-CVD TiCN/Al2O3 coated carbide insert.

Figure 3. Microstructure of cermet tool substrate (before, and after etching).

Sådhanå (2021) 46:239 of 23 239

by the metallic dopant, amount of C content, and binder

phases. The harder phase of Ti (C, N) imparts superior

thermal conductivity, hardness, and wear resistance;

whereas, the metallic binders boost up toughness. Again,

upper TiN coating is believed to provide better wear

resistance by minimizing friction.

Substrate microstructure along with deposited coating

layers over SiAlON insert are studied through optical

microscopy as well as scanning electron microscopy (fig-

ure 4). Though the substrate is viewed clearly under an

optical microscope, the coating layers are not much

prominent to clearly distinguish. Therefore, scanning

electron micrographs are analyzed along with EDS to

understand the composition of coating layers over the tool

substrate. The uppermost layer is identified as Al2O3;

beneath which, presence of TiCN layer is also confirmed.

The bottom layer (TiCN) protects the tool substrate from

abrasion action while the upper Al2O3 coating acts as a

thermal barrier due to its low thermal conductivity, and

ensures minimal crater wear. As these layers are CVD

coated, hence, coating thickness appears non-uniform

throughout the surface.

During machining of ‘difficult-to-cut’ materials, heat

generated at the cutting zone gets accumulated at tool-tip

because of poor thermal conductivity of the workpiece [3].

The recorded tool-tip temperature values are plotted with

respect to varying cutting speeds (figure 5). It is known that

increment in any one of the cutting parameters (cutting

speed, feed, or depth-of-cut) leads to an increase in tool-tip

temperature [37]. Carbide and cermet inserts exhibit almost

similar trend of temperature variation. At normal laboratory

condition, thermal conductivity of different coating layers

can be arranged in decreasing order as Al2O3 (*35 W/m.K)

[TiCN (*25 W/m.K)[TiN (*20 W/m.K). Hence, it is

expected that carbide insert would correspond to lower

tool-tip temperature due to presence of Al2O3 upper coating

Figure 4. Microstructure of SiAlON insert along with TiCN/Al2O3 coating layers: (a) FESEM micrograph and (b) Optical micrograph.

40 60 80 100 120 140

100

150

200

250

300

350

400

450

500

550

600

Tool

-tip

tem

pera

ture

[o C]

Cutting speed [m/min]

SiAlON tool Carbide tool Cermet tool

Figure 5. Effect of cutting speed on tool-tip temperature.

239 of 23 Sådhanå (2021) 46:239

layer, possessing higher thermal conductivity than upper-

most TiN layer of cermet insert. But Figure 5 exhibits a

reverse trend i.e. carbide tool-tip experiences higher tem-

perature than cermet. This is not only due to properties of

coatings but also tool substrate material. Though TiN cor-

responds to the lowest thermal conductivity amongst all

coating elements; Ti(C, N) substrate of the cermet tool

compensates for this thermal property of the coated layer.

Hence, both carbide, and cermet inserts experience near

equal temperature till vc = 100 m/min. Also, Bhat and

Woerner [38] published that Al2O3, and TiN layers possess

nearly equal hardness at room temperature. Al2O3 corre-

sponds to nano-indentation hardness of 15-19 GPa. On the

other hard TiN has 26 GPa nano-hardness. Beyond vc = 100

m/min, the interface (tool-work) temperature is likely to be

increased due to consumption of huge cutting energy with

high work material deformation rate [37, 39]. At elevated

temperature, Al2O3-coated layer starts losing its thermal

property; hence, heat gets accumulated at the tool-tip itself;

for which an upsurge in tool-tip temperature is witnessed in

case of carbide tool when compared to cermet tool, within

cutting speed range from vc = 100 m/min to vc = 130 m/min

[40]. Higher tool-tip temperature, in case of carbide tool

may also be due to higher coefficient of friction (* 0.5-0.7)

for upper Al2O3 coating layer than uppermost TiN coating

(* 0.4) of cermet insert when they come into contact with

work surface.

On the contrary, SiAlON causes remarkably higher val-

ues of tool-tip temperature than carbide and cermet tools. In

addition, the rate of temperature rise is also high for SiA-

lON tool than other two inserts. For SiAlON insert, the

minimal tool-tip temperature is recorded as 275 �C (at vc =50 m/min); which finally reaches up to 509 �C at the

highest cutting speed. Higher temperature as evidenced in

case of SiAlON tool can be attributed to its geometry which

includes conventional flat rake surface (without chip-

breaker); whereas, other two inserts have integrated chip-

breaker (rake face grooved along the chip flow direction).

Thus, SiAlON insert is likely to produce higher frictional

interaction between chip, and tool rake face. On the other

hand, for remaining two inserts, chips can go through

restricted contact surface, and thus, face lower extent of

frictional resistance [41]. Again, for SiAlON tool, thermal

conductivity of uppermost Al2O3 coating layer gradually

declines with increase in temperature. This also contributes

towards evolution of excessive tool-tip temperature.

Though both carbide, and SiAlON inserts are coated with

similar coating layers; poor thermal conductivity of SiA-

lON substrate (*10 W/m.K) than carbide substrate (*70

W/m.K), causes dramatic difference in the tool-tip tem-

perature between these inserts [42]. At about 125 �C, bothAl2O3 and TiCN coatings possess nearly equal thermal

conductivity; afterwards, drastic drop in thermal conduc-

tivity can be witnessed for Al2O3 coated layer which

reaches around 15 W/m.K at 500 �C [40]. Such drop in

thermal conductivity also helps in temperature augmenta-

tion at the tool-tip of SiAlON tool at vc = 130 m/min.

As reported by Machado and Wallbank [2], energy

consumption during machining of titanium alloys is gen-

erally high enough as the material removal process is

associated with evolution of high magnitude cutting forces.

Cutting force generated during machining is influenced by

various factors such as: properties of work alloy, tool insert,

cutting parameters, tool wear modes etc. [43]. As a gov-

erning factor for power consumption as well as tool wear

rate; tangential cutting forces generated during application

of different cutting inserts are plotted in accordance with

varied cutting speeds (figure 6). It can clearly be noticed

that each of the cutting inserts exhibit different trend of

cutting force variation. Among three cutting inserts tested,

carbide tool exhibits higher cutting force; whereas, cutting

force attains minimal value for the SiAlON insert. An

intermediate value of cutting force is recorded when using

cermet insert. High toughness of carbide tool substrate

allows adequate plastic deformation of the tool. This in turn

necessitates higher cutting force for material removal. In

the beginning, when material removal is just started (re-

ferring to the lowest cutting speed), carbide tool tries to

intrude the workpiece, and generates huge cutting force.

Also, comparatively high toughness of carbide substrate

favors in sustaining such higher cutting force without any

breakage or edge chipping [15, 44]. The low magnitude of

cutting force experienced by SiAlON tool can be explained

by its relatively high thermal hardness (hot hardness) and

hot strength at elevated temperatures. Low toughness of

SiAlON tool substrate restricts detrimental plastic defor-

mation of the cutting edge. This facilitates material cutting

easier with lower cutting force.

As compared to carbide tool, SiAlON tool requires lower

magnitude of cutting force for all cutting speeds tested. If

40 60 80 100 120 14080

100

120

140

160

180

200

Cut

ting

forc

e [N

]


SiAlON tool Carbide tool Cermet tool

Figure 6. Effect of cutting speed on tangential cutting force.

Sådhanå (2021) 46:239 of 23 239

individual trend of cutting speed variation (while using

SiAlON tool) is considered, it is noticed that when vcincreases from 50-100 m/min, tool-tip temperature increa-

ses from 275-440 �C, FZ i.e. the main cutting force

increases from 120-128 N (figures 5-6). Slight increase in

main cutting force is attributed to formation of crater wear

which weakens tool-point. Significant crater wear is wit-

nessed at the rake face of SiAlON tool at vc= 50 m/min

(discussed in later section). Weak tool point requires higher

cutting force for material removal. On the other side, within

range of cutting speed 100-130 m/min, tool-tip temperature

rises from 440 �C to 509 �C. Under such situation, work

part thermal softening becomes highly dominant. Hence,

decreasing trend in cutting force is witnessed.

Thermal softening indicates reduction in shear strength

(flow strength) of work material ahead of tool cutting edge

due to evolution of high cutting temperature. Thermal

softening of work material reduces cutting force magnitude

and cutting power consumption. In the primary deformation

zone, evolution of heat is due to the plastic work done

(plastic deformation). Huge heat generated at this area

causes softening of work material and allows higher degree

of deformation by application of lower cutting force.

In addition, both carbide as well as SiAlON tool has

upper Al2O3 coating which gradually loses its thermal

conductivity with increase in temperature. Increase in cut-

ting speed in turn causes higher friction at tool-work

interfacial region. This increases temperature of the cutting

zone. Truncation in thermal conductivity of alumina coat-

ing further contributes to immense cutting zone tempera-

ture. Since, carbide tool possesses higher toughness than

SiAlON; carbide tool is severely affected by plastic

deformation which in turn causes altered tool geometry

(especially, sharpness of the cutting edge is lost). Hence,

higher cutting force is necessary for material removal. On

the contrary, higher hot hardness of SiAlON tool enables it

to retain its strength for prolonged machining duration.

For carbide insert, there is a gradual decrease in cutting

force throughout range of vc; however, it is noticed that

tool-tip temperature remains fairly constant and nearly

equal to that of cermet tool for vc up to 100 m/min speed

(figure 5). Apparently, it seems that thermal softening of

workpiece is not significant in case of carbide tool in pur-

view of recorded tool-tip temperature range. However, as

evidenced from figures 7a-c, wear of the tool flank face

exhibits increasing trend which is responsible for increase

in cutting heat generated for higher values of vc. Such

apparent discrepancy can well be explained. It is to be

noted that tool-tip temperature that is recorded by the

temperature indicator doesn’t indicate exact cutting zone

temperature for a particular instant. Evolution of cutting

heat is distributed through chips, cutting tool, workpiece

and environment. Carbide possesses higher thermal con-

ductivity than SiAlON (seven times than SiAlON). There-

fore, heat can easily be transferred through bulk of the

carbide tool at a faster rate than SiAlON tool. That means

temperature distribution is approximately uniform for the

carbide substrate. In contrary, non-uniform temperature

distribution is expected for the SiAlON substrate; peak

temperature being at the surface. Hence, effect of heat

accumulation near the tool-tip is expected to be more in

case of SiAlON tool. This is represented by high temper-

ature recordings at SiAlON tool-tip. In addition, enormous

chip sticking, accumulation of chip near the cutting zone,

manual error while operating hand held temperature-indi-

cator etc. may suppress actual cutting zone temperature

during machining by carbide tool. This is because work

material has strong chemical reactivity with tungsten car-

bide and Co binder. Such situation does not arise in case of

machining with SiAlON tool. Therefore, gradually

decreasing trend of cutting force, as observed in case of

carbide insert, may be correlated to adequate thermal

softening of work material as a result of advancement of

cutting speed causing extreme cutting heat generation [45].

Neither very hard nor very tough nature of cermet tool

substrate supports generation of cutting force values lying

between that obtained for other two counterparts. In their

study, Chen et al [46] found that Ti(C, N) based cermet tool

offered longer tool life as compared to carbide tool due to

its superior resistance to plastic deformation at elevated

temperature which also aided towards the generation of

reduced cutting forces. In the present study also, it is

experienced that cermet causes lower cutting force than

carbide tool.

Tool wear is a very sensitive factor that decides life of

the cutting tool. Nose wear is incurred at nose radius of the

cutting tool. Severe nose wear shortens the tool and cause

significant dimensional deviation during machining. It is

often treated as a part of flank wear since there is no dis-

tinguishing demarcation between them. Basically, nose is

the mating part of flank and face which faces combined

effects of flank and crater wear. It is true that when depth-

of-cut is less than nose radius, tool flank does not partici-

pate much in material removal. Hence, in this work, it is

experienced that ‘nose flank’ (as schematically represented

in Appendix) is affected by wear. Since there exists very

thin-line difference between nose wear and flank wear,

wear incurred at tool nose flank is denoted as ‘flank wear’

throughout the manuscript.

Tool-tips of worn-out inserts are first viewed through

optical microscopy to estimate average width of flank wear.

In accordance with ISO standard 3685, the criterion for tool

life is average flank wear (* 0.3 mm). It is also reported

that nose wearP0.5 mm (ISO Standard 3685 for tool life)

can be treated as the end of economic life of cutting insert

[47].

Figure 7 displays optical micrographs of worn-out tool

flank face of different cutting inserts. In order to obtain an

average flank wear estimate, width of flank wear is mea-

sured at three different locations; an average of three

measured values is considered for analysis. Values of

average flank wear width are plotted to demonstrate effect

239 of 23 Sådhanå (2021) 46:239

of cutting speed on flank wear progression, for respective

cutting tool inserts (figure 8). As reported in previous

studies [47–49], it is obvious that with increase in cutting

speed, progression of tool flank wear is also increased.

Similar trend is obtained in the present study. Growth of

tool flank wear can be attributed to increase in temperature

at the tool-tip (as discussed earlier) due to increased fric-

tion, and evolution of thermally induced stresses at the tool-

work interface. Again, it can be noticed that width of flank

wear appears minimal in case of cermet tool; whereas,

SiAlON insert suffers from severe flank face wear. An

intermediate value of flank wear is obtained for the carbide

insert. These observations are however similar to the results

of tool-tip temperature (figure 5). This confirms that tool-tip

temperature is a crucial factor affecting flank wear. How-

ever, beyond vc = 100 m/min, the flank wear curve is dis-

continued for SiAlON insert. This is because the tool is

found affected by severe macro-chipping (spalling), owing

to the generation of huge compressive stresses at the cutting

edge, when machining is carried out at high cutting speed

Figure 7. Optical micrographs exhibiting flank wear progression as affected by cutting speed (a) to (d) Carbide, (e) to (h) Cermet, and

(i) to (l) SiAlON inserts.

Sådhanå (2021) 46:239 of 23 239

(refer to figure 7(l)). Therefore, to avoid ambiguity, the

curve is made discontinuous. Lima et al (2017) [50] alsoexperienced spalling of SiAlON insert during machining of

Inconel 751 superalloy.

Afterwards, a detailed study on different tool wear

mechanisms is carried out through scanning electron

microscopy. Different wear mechanisms including abra-

sion, adhesion, Built-Up-Edge (BUE) formation, and tool

edge flaking, etc. are witnessed. Wear morphology of the

carbide tool at vc = 80 m/min is described in figure 9. Few

scratch marks, at the vicinity of tool-tip, are noticed

forming few groove-like structures. These marks can

essentially be stated as abrasion marks that are formed due

to continuous mechanical interaction between tool, and

workpiece, and evolved high stresses at the tool-tip

[17, 51]. In their reporting, when focusing on characteris-

tics of tool wear during machining of nickel-based alloys,

Zhu et al [52] explained that strain-hardening tendency of

workpiece at relatively low cutting speed produces some

hard asperities at the tool-work interface; and, relative

motion between these asperities cause abrasion action of

the cutting tool. But due to sufficient abrasion resistance,

and chemical stability, the upper Al2O3 layer of carbide

insert results in a lesser extent of abrasion at tool-tip [16].

Just above the abrasion zone, some chunks of metallic

materials are observed which can be identified by their

unique morphology. As explained by Badaluddin et al [16],these chunks can be termed as metal debris which is formed

over the tool surface due to dislocation of some macro-

scopic materials caused by the tribological alternation of

the tool-work interface as marked in figure 9. Formation of

such wear debris on surface of PVD coated cemented car-

bide tool was also observed by Hao et al [53] when ana-

lyzing tool wear modes during dry cutting of Inconel 718.

When such debris is accumulated over the coating layer,

they provide a suitable platform for the work material to get

adhered over them. This leads to adhesion wear. EDS

analysis, performed at the adhered layer, confirms presence

of work elements with high weight fraction of titanium

(nearly 43 %). Since adhered work material hinders chip

sliding over the rake face, chips stick with the adhered

layer. But relatively slow moving chips result in plucking

off coated layer at some portions on the rake face. Similar

observation was also reported by Joshi et al [54] while

performing dry turning of titanium alloys with coated car-

bide insert. As coating layer is dissipated, tool substrate is

exposed to outside. This is confirmed through EDS plot.

High magnitude cutting force and highly concentrated

stresses at the locality of cutting edge, leads to flaking of

cutting tool surface near the flank face. Similar wear modes

40 60 80 100 120 140

40

80

120

160

200

240

280

Wid

th o

f fla

nk w

ear [�m

]


Carbide tool Cermet tool SiAlON tool

Figure 8. Flank wear as affected by cutting speed.

Figure 9. Wear morphology of carbide tool at vc = 80 m/min.

239 of 23 Sådhanå (2021)46 239

of CVD coated carbide insert were documented in past

literature when turning of hard materials [35, 55].

Figure 10, and figure 11 exhibit different wear mecha-

nisms that are experienced by the carbide insert at vc = 100

m/min, and vc = 130 m/min, respectively. Similar to fig-

ure 9, following wear mechanisms: abrasion, adhesion, and

debris deposition, etc. are identified over the worn-out tool

face. Along with tool edge flaking, and deposition of

lamellar debris, dissipation of coating materials from rake

face, adjacent to the cutting edge, is also witnessed (fig-

ure 10). EDS plot provides necessary evidence of tool

substrate exposure, exhibiting major extent of substrate

elements (43.12 % of C, and 41.88 % of W). But coating

dissipation is not incurred at vc = 130 m/min; rather, stuck-



Sådhanå (2021) 46:239 of 23 239

out chips, and burnt chip fragments are evidenced, as

indicated in figure 11. Fast moving chips are responsible for

the above phenomena. EDS plot for the adhesion layer

reveals presence of work elements with maximum per-

centage of titanium (81.47 % by weight). Moreover, poor

thermal conductivity of uppermost Al2O3 coating layer

causes temperature gradient between chip surfaces (free,

and underside surface). Hence, chips tend to curl, and often,

get oxidized over tool face (chip burning). The phenomena

of chip burning, and sticking were also witnessed by past

researchers during machining of superalloys [19, 54, 56].

Wear mechanisms, as experienced by cermet insert at vc= 130 m/min, are displayed in figure 12, in which abrasion,

adhesion, and BUE formation are precisely highlighted. As

reported in previous studies [46, 57–59], during machining

of harder materials with Ti(C, N) based cermet inserts,

majority of wear modes can be attributed to adhesion, and

abrasion; which also holds good for the present study.

Relative motion of tool flank face with respect to newly

generated work surface contributes towards abrasive wear

at the flank face. This motion by the harder inclusions (such

as carbides, hardened fragments of work elements, and

some hard asperities from the hard coating layers of the

cutting tool trapped at the interfacial region) generates

parallel line-like appearance along tool flank face [46, 57].

During dry machining of stainless tool steel with coated

cermet tool, Noordin et al [57], experienced removal of

coating from the tool surface through attrition mechanism;

but luckily, no attrition, and coating delamination are

pronounced in the present case. This may be due to rela-

tively low hardness of the work material (around 23 HRC

for Ti-6Al-4V) as compared to tool steel (nearly 45 HRC).

Again, PVD coated TiN layer is believed to be a barrier for

adhesion, and BUE formation [16]; but surprisingly, BUE

formation at the tool nose area as well as bulk adhesion of

work material on the rake face are prominently visualized

in the present work. This particular finding can be

explained as the dominance of chemical affinity of titanium

alloy towards tool/ coating materials at elevated cutting

temperatures which may suppress beneficial characteristics

of deposited coating layer [1, 3]. EDS analysis in the

basement of BUE also reveals presence of work materials

in abundance (figure 12). As rake face experiences much

higher temperature than flank surface, adhesion wear is

seemed to be much active on this surface, which in turn

assists sticking, and sliding tendency of chips, and results in

deposition of metallic chunks at the vicinity of tool nose

along the direction of chip-flow; those are marked as

lamellar debris in figure 12.

According to Aruna et al [60], ceramic inserts are prone

to adhesion, abrasion, plastic deformation, diffusion, and

tool fracture due to continuous interaction of chip, and tool

rake face, rubbing of machined surface with tool flank face,

and generation of high stress, and temperature at various

deformation zones which adversely affect cutting tool-

edge; and hence, machined surface integrity as well. Fig-

ure 13 depicts SiAlON insert that underwent several wear

modes: crater wear, flank wear, adhesion, and cutting edge

Figure 12. Wear morphology of cermet tool at vc = 130 m/min.

239 of 23 Sådhanå (2021) 46:239

flaking, at vc = 50 m/min. As per the previous discussion in

relation to figure 5, it is clear that extreme temperature is

generated at the tool-tip of SiAlON insert. Evolution of

such high temperature gradually degrades desired proper-

ties of coating layers which favor other wear modes (like

adhesion, diffusion, etc.) to take place at the rake face

causing formation of crater (preferably called as crater

wear). Also, strong chemical affinity of work material

accelerates chemical reaction between evolved chips, and

constituents of SiAlON insert resulting tribochemical wearto take place at tool rake face [60, 61]. This typical wear

can be affirmed by the presence of oxidized chip fragments

over tool rake face (figure 13). EDS plot in figure 13 for the

highlighted area over tool flank face confirms residues of

work elements (Ti, Al, V, N, Fe, etc.); hence, it can be

understood as the formation of adhesive layer. Adhesion

wear mechanism of SiAlON insert was amply reported in

previous research [48, 62]. Flaking of cutting edge is

caused due to plunging action of SiAlON insert during

machining of hard alloys [27, 50, 61].

Fracture of the cutting edge is evidenced for SiAlON

cutting tool at vc = 130 m/min, which is clearly identifi-

able to naked eyes, is shown in Figure 14. This fig-

ure clearly represents fractured cutting edge through

removal of coating layers. An average interface tempera-

ture of 509 �C is noted down at this high speed cutting

condition. As explained by Jawahir and van Luttervelt

[40], beyond a temperature range of 250 �C, Al2O3 coat-

ing layer exhibits the lowest thermal conductivity

(amongst TiC, TiCN, TiN, Si3N4, and Al2O3). According

to Liu et al [63], the combination of poor thermal con-

ductivity, and lower fracture toughness of Al2O3 layer

made it incapable to sustain such huge amount of gener-

ated heat; and hence, it fails. Then, the heat is transmitted

to the TiCN coating layer which is just beneath the Al2O3

layer. Though this layer possesses higher thermal con-

ductivity than Al2O3, but low thermal conductivity of

SiAlON substrate aids heat accumulation over TiCN

coating layer itself. As this layer has an inherent heat

stability capability up to 450 �C; intense heat accumula-

tion makes it thermally unstable to perform satisfactorily

[16]. Therefore, this layer also fails by exposing tool

substrate. EDS plot with residues of Si, N, O, Al clearly

validates exposed (bare) tool substrate without traces of

any coating elements. Once the coated layers are

detached, evolved hot chips tangle at places on tool rake

face which are marked as fused chips (figure 14). In

addition, congregated metallic chunks are also found at

the vicinity of cutting edge as deposited debris, which

may be formed due to continuous interaction between

exposed tool substrate, and strain hardened workpiece

elements [48, 60]. Formation of adhered layer over flank

Figure 13. Wear morphology of SiAlON tool at vc = 50 m/min.

Sådhanå (2021) 46:239 of 23 239

face is also observed; EDS analysis validates this through

detection of work elements within adhered layer.

During machining of titanium-alloys, apart from flank

wear of the cutting tool, formation of crater at tool rake face

(crater wear) is a common phenomenon. Crater formation

at tool rake face takes place due to enhanced tool-chip

interaction, accumulation of cutting heat over rake face

(adjacent to the cutting zone) and ineffectiveness of tool

material to sustain thermal loads that are induced during

machining. In the present study, significant crater formation

is detected only at the highest cutting speed (v = 130 m/

min) during usage of carbide and cermet tools, respectively

(figure 11 and figure 12). Beneficial aspects of improved

toughness of aforementioned coated tools can well be

understood because both coated tools absorb induced

stresses efficiently when operated at selected cutting speeds

(except v = 130 m/min). This causes crater wear not so

severe. In contrast, SiAlON tool is noticed to suffer from

Figure 14. Wear morphology of SiAlON tool at vc = 130 m/min.

Figure 15. Visual inspection of chips obtained by using (a) SiAlON, (b) Carbide, and (c) Cermet inserts at vc = 130 m/min.

239 of 23 Sådhanå (2021) 46:239

crater wear from the minimum cutting speed itself (v = 50

m/min) as shown in figure 13. The absence of chip breaker

in SiAlON insert is believed to amplify tool-chip interac-

tion at tool rake face. Huge cutting heat accumulation at

tool-tip also plays vital role in formation of crater.

In the present work, analysis of chip morphology starts

with macro-morphology of chips which includes naked-

eye inspection. Therefore, chips generated when using

cutting inserts at vc = 130 m/min are collected, and

displayed in figure 15. The figure clearly indicates that

continuous type of chips is evolved during machining,

regardless of the type of tool material employed. The ir-regularly arranged helical type chips can be seen when

carbide tool is used. This can be attributed to the wear of

cutting tool at the highest cutting speed. Similar type of

chip’s macro-morphology was also reported by Pradhan

et al [64] during dry turning of Ti-6Al-4V with uncoated

carbide insert at vc = 124 m/min. Again, use of PVD

Figure 16. Chip’s back (underside) surface produced using (a)-(b) Carbide, (c)-(d) Cermet, and (e)-(f) SiAlON inserts.

Sådhanå (2021) 46:239 of 23 239

coated cermet tool leads to formation of snarled ribbontype chips as a result of lower tool-tip temperature, and

anti-friction characteristics of TiN coating. During turning

of Ti-6Al-4V with PVD (Ti, Al)N/TiN coated carbide

tool, similar type of chips was witnessed both in dry as

well as MQL condition by Ramana et al [65]. Also,

SiAlON insert results in snarled ribbon type chips but

with relatively additional helicoid shape, owing to domi-

nant thermal softening of the work material due to

excessive temperature rise at tool-chip interface.

Notable color discrimination is not found from the col-

lected chips; chips appear as shiny-metallic.As continuous chips are witnessed, chances of interaction

between newly produced machined surface, and evolved

chips cannot be avoided. In order to understand this phe-

nomenon, chip’s back (underside) surface morphology is

visualized through scanning electron microscope as pre-

sented in figure 16. Figures 16 (a)-(b) indicate chip’s back

surface at vc = 50 m/min, and vc = 130 m/min, respectively,

when carbide tool is used. With negligible amount of

Figure 17. Chip thickness as observed through scanning electron microscopy: chips produced using (a) Carbide, (b) Cermet and (c)

SiAlON insert, at vc = 130 m/min.

239 of 23 Sådhanå (2021) 46:239

broken chip fragments, uneven edges, owing to fracture

growth due to moderate temperature, can clearly be traced

out in the chip profile. The underside surface of chips

generated by cermet insert at vc = 50 m/min, and vc = 130

m/min, are shown in figures 16(c) and (d), respectively.

With traces of fewer feed marks, comparatively smoother

surface is obtained as a result of evolution of lower tool-tip

temperature, and lower flank face wear [46]. With SiAlON

insert, morphology of chip’s back surface obtained is fur-

nished in figures 16 (e) and (f), respectively for vc = 50 m/

min, and vc = 130 m/min. Prominent feed marks long with

larger fractured depth propagation are witnessed due to

evolution of extreme tool-tip temperature for this ceramic

tool.

In order to understand formation of chip segments, pro-

duced by different tool inserts at vc = 130 m/min, thickness

of chips is closely viewed under scanning electron micro-

scope (figure 17). For carbide insert, chip segments are seen

to be trapezoidal shaped on which an elongated fractured

length can clearly be marked with a minute sheared area

[21]. This can be explained due to combined action of

thermal softening, and strain-hardening of work material

which allow formed chip segments to slide over one

another giving them a trapezoidal shape (figure 17(a)). As

discussed before, intense heat accumulation at the rake face

of SiAlON insert allow the work material to go through

adequate thermal softening without any fracture phenom-

ena; which aids in formation of irregular rectangular type

chip segments (figure 17(c)). Figure 17(b) depicts shorter

chip segments formed by cermet tool which can be attrib-

uted to minimal temperature rise at the tool-tip for which

effect of thermal softening may be suppressed by work part

strain-hardening. Similar type of highly serrated chip seg-

mentation profile was reported by Xu et al [59], when

machining high strength steel with Ti(C, N) based cermet

tool.

In order to validate above-explained segmentation phe-

nomena, depicted by various cutting tool inserts, chip’s

micro-morphology analysis is carried out by choosing few

suitable parameters. These parameters include maximum

chip height (H), minimum chip height (h), equivalent chipthickness (Tch), saw-tooth pitch (P), shear angle (h),included saw-tooth angle (u), width of the chip (wch), and

frequency of chip segmentation (fseg). Figure 18 provides

pictorial representation of these parameters corresponding

to the chip specimen taken at vc = 130 m/min, obtained by

using cermet insert. The figure clearly illustrates that the

maximum chip height (H) is nothing but the vertical dis-

tance of the peak of segments from the bottom edge of the

chip; whereas, the minimum chip height (h) indicates the

distance of undeformed chip surface from the bottom edge

(valley height). The saw-tooth pitch (also called segmen-

tation spacing) can be stated as the linear distance between

two consecutive teeth of saw-toothed chip profile. Again,

the angle made by saw-toothed profile at the peak of each

segment can be referred as included angle (u). The tangent

drawn at the shear band makes an angle with the vertical

axis, which is called shear angle (h).Quantification of aforesaid chip micro-morphological

parameters is done by on-site measurement of the required

data by taking some chip specimens produced by different

tool insert at the highest cutting speed (vc = 130 m/min).

The chosen chip samples are shown in figure 19. The

equivalent chip thickness (Tch) is calculated by taking data

of the maximum chip thickness (H), and the minimum chip

thickness (h), as per the following equation (eq. 1).

Tch ¼ H þ h

2; lm½ � ð1Þ

From figure 19, it is clear that the equivalent chip

thickness is the highest for the case of SiAlON insert

(* 64.7 lm); while that of cermet is the smallest one

(*51.4 lm). Excessive heat generated at the tool-tip of

SiAlON insert at vc = 130 m/min results in reduced chip

valley with increased chip height owing to the shear

instability at chip surfaces [66]. An intermediate tempera-

ture at tool rake face of carbide insert produces chips with

equivalent thickness of 55.3 lm. The lowest tool-tip tem-

perature, as experienced by cermet insert, causes formation

of the thinnest chip. Again, antifriction property of upper-

most TiN coated layer of cermet tool reduces frictional

force at tool-chip interface which aids to truncated chip

thickness. This reason for thinner chip formation was

elaborately explained by Uysal and Jawahir [67], when

establishing a slip-line model of serrated chip formation

under dry, and MQL machining of austenitic steel.

The pitch of chips is found to be the shortest when cer-

met insert is used as compared to remaining two inserts.

Lesser extent of tool wear of cermet insert may be

responsible for this minimum pitch. Aforesaid chip char-

acteristic features were also reported by Thakur and Gan-

gopadhyay [68], during dry turning of Inconel 825 with

Figure 18. Parameters describing chip’s micro-morphology:

specimen chip obtained using cermet insert at vc = 130 m/min.

Sådhanå (2021) 46:239 of 23 239

coated carbide insert. Again, measured shear angle val-

ues depict that shear angle on chip profile; obtained using

SiAlON insert, is narrower than other two counterparts.

According to Yen et al [69], shear angle is influenced by

nose radius of the cutting insert as it is directly related to

chip formation phenomena. Therefore, greater nose

radius of the SiAlON insert (1.2 mm) results in narrow

shear angle when compared to other two inserts (0.8

mm). Similar trend is also observed for the saw-tooth

included angle.

For mathematical quantification of chip segmentation

phenomenon, during machining of titanium alloy, Pawade

and Joshi [70] proposed a parameter called segmentation

frequency (fseg) which can be computed as per the follow-

ing equation (2).

fseg ¼ 100� Vchip

6� Pð2Þ

Here;Vchip ¼ vc � f � doc

Tch � wchð3Þ

where, fseg = chip segmentation frequency in kHz, Vchip =

chip flow velocity in m/min, P = saw-tooth pitch in lm,

f = feed rate in mm/rev, doc = depth of cut in mm, Tch =

equivalent chip height in lm, wch = width of the chip in lm.

In figure 19, tabulated values of chip segmentation fre-

quency elucidate that cermet tool produces chips with

maximum segmentation frequency among three cutting

inserts employed for machining. As it can be clearly

understood from (eq. 2) that segmentation frequency is

inversely varying with saw-tooth pitch. Therefore, it is

obvious that chip segmentation frequency in case of cermet

insert is expected to be the maximum as it corresponds to

the minimum saw-tooth pitch; whereas, the largest saw-

tooth pitch as witnessed by carbide insert results in lower

segmentation frequency. For the case of SiAlON tool, chip

segmentation frequency falls in between segmentation

frequency values obtained using other two inserts.

In order to investigate effects of dry turning on newly

generated finished surfaces of Ti-6Al-4V using three dif-

ferent tool inserts, specimen surfaces are studied through

Figure 19. Quantitative analysis on parameters of chip’s micro-morphology: specimen chip obtained using (a) Carbide, (b) Cermet and

(c) SiAlON inserts at vc = 130 m/min.

239 of 23 Sådhanå (2021) 46:239

scanning electron microscopy. Machined surface charac-

teristics are described in figure 20. Figures 20(a), (c) and (e)

represent machined surfaces cut at vc = 50 m/min by car-

bide, cermet, and SiAlON inserts, respectively; whereas,

surfaces produced by using three inserts at vc = 130 m/min

are shown in figures 20(b), (d) and (f), respectively.

Prominent feed marks with few oxidized material deposi-

tion can be observed on the surfaces machined by carbide

tool due to higher magnitude of cutting force generated

[71]. As some wear debris are reported to be deposited over

the cutting edge region at the highest cutting speed; similar

type of debris is also found on the machined surface which

may be due to strong affinity of the work alloy towards

counter tool material. When machining is carried out with

SiAlON insert, rapidly moving chips get fused at some

places on the machined surface due to huge temperature

generated at the tool-tip (figure 20(e)). Comparatively

better surface morphology is witnessed for the case of

cermet insert at two extreme cutting velocities. With traces

of less significant feed marks; only presence of smeared

material is witnessed on the machined surface which can be

attributed to the lower tool-tip temperature, and lesser

severity degree of tool wear (figures 20(c)-(d)). These

pronounced surface defects were previously experienced by

few authors when working on titanium-based alloys

[64, 72].

As articulated in ‘Surface Integrity in Machining’ [73],an engineering surface refers to a surface that is produced

by any of the material removal processes (may be tradi-

tional or non-traditional methods), and acquires compara-

bly improved characteristics as that of the former one.

Hence, characteristics of the newly generated work part

surface seek proper investigation as it influences life of the

end product. Microscopic analysis of the machined surfaces

cut by different cutting tools reveals presence of a freshly

developed layer just beneath the machined surface which

appears to be white in color (under microscope) as pre-

sented in figure 21. As shown in figure, this layer stands out

from the bulk work alloy which is called as ‘white layer’(WL) or ‘hardened layer’, produced due to aggressive

cutting conditions followed by effects of rapid heating, and

cooling cycle. During dry turning of Ti-6Al-4V, severely

worn-out cutting insert, and high temperature produced at

the machining zone cause plastic deformation of the

machined surface along with its microstructural alternations

which altogether develop white layer [74, 75]. To elicit

effects of tool material on machined surface integrity,

thickness of white layer is estimated to be 1.04 lm, 0.41

lm, and 1.3 lm, when using carbide, cermet, and SiAlON

insert, respectively at vc = 80 m/min. Tiniest WL caused by

cermet tool is due to its low tool-tip temperature, reduced

degree of tool wear, and sustenance of intact (not dissipated

or delaminated) coated layer (TiN) which facilitate faster

dissipation of cutting heat through the tool itself. This in

turn reduces effects of heating, and work-hardening below

the machined surface (lesser depth of heat penetration). But

in case of SiAlON tool, excessive tool-tip temperature (due

to low thermal conductivity of ceramic substrate), severely

worn-out insert with detached coating layers (TiCN/Al2O3),

altogether, cause higher extent of work part thermal soft-

ening, and work-hardening (heat penetration depth is

more). This results in formation of the thickest white layer.

No other sub-surface detriments like micro-cracks, pits,

laps or visible tear marks are witnessed on the machined

surface except formation of smooth WL. Present findings

also support the observation made by Ibrahim et al [75],during dry turning of Ti-6Al-4V-ELI with coated cemented

carbide tool.

Formation of white layer at subsurface of the machined

product is indeed a complex phenomenon influenced by

excessive thermo-mechanical loading, severe plastic

deformation, development of huge temperature gradient

and local microstructure alteration of work material due to

abusive machining conditions (like high speed dry

machining). In order to investigate effects of white layer

Figure 20. Morphology of machined surfaces obtained by using

(a)-(b) Carbide, (c)-(d) Cermet, and (e)-(f) SiAlON insert,

operated at vc = 80 m/min.

Sådhanå (2021) 46:239 of 23 239

formation (as shown in figure 21), subsurface microhard-

ness depth profiles of machined Ti-6Al-4V specimens

obtained using different cutting inserts are provided in

figure 22. During microhardness test, just beneath the

machined surface (possibly within white layer), the first

indentation is made; corresponding position is considered

as origin of the microhardness depth profile. Afterwards,

indentations are made equidistance apart (approximately

* 30 lm) along an imaginary line towards interior of the

bulk specimen.

Just underneath the machined surface, white layer thus

formed exhibits much higher hardness than interior of the

bulk specimen. The highest subsurface hardness (* 462.7

� 9.12 HV) is obtained for the specimen machined by

SiAlON insert. Huge heat generation at tool-work interface

and accumulation of the same at cutting zone due to the

poor thermal conductivity of workpiece as well as SiAlON

tool substrate may be ascribed as the root cause to produce

wide white layer of high hardness. On the other hand,

specimens machined by carbide and cermet inserts also

exhibit higher hardness (than bulk material) just underneath

the machined surface; the lowest hardness of 401.2 � 5.29

HV is measured when cermet tool is used. This is because

minimum temperature at tool-tip due to comparatively

better thermal properties of cermet tool causes formation of

the thinnest white layer. Similar observations were reported

in some recently published articles [76, 77]. Though higher

hardness values are noticed at subsurface regions, micro-

hardness depth profiles afterwards follow gradual declining

curve towards reaching nominal bulk hardness of the work

alloy (340 - 350 HV) as locations of indentations move

farther from subsurface to interior of bulk material. This

indicates that only up to certain microns depth from the

machined surface, work material is affected by formation of

such hardened layer. Formation of tiny while layer having

low microhardness value clearly indicates better perfor-

mance of cermet insert over carbide and SiAlON

Figure 21. Depth of white layer as influenced by tool material:

(a) Carbide, (b) Cermet and (c) SiAlON insert, operated at vc = 80

m/min.

0 20 40 60 80 100 120 140 160 180300

315

330

345

360

375

390

405

420

435

450

465

480

Mic

roha

rdne

ss [H

V]

Depth [μm]

Carbide tool Cermet tool SiAlON tool

Figure 22. Subsurface microhardness depth profile (tools oper-

ated at vc = 80 m/min).

239 of 23 Sådhanå (2021) 46:239

counterparts. Consequently, machined surface integrity is

better in case of cermet tool usage.

5. Conclusions

• Amongst three inserts tested, SiAlON records maxi-

mum tool-tip temperature (*509 �C) while minimal

temperature of 323 �C is attained at cermet tool-tip at

vc = 130 m/min.

• Carbide tool experiences higher cutting force followed

by cermet and SiAlON inserts. Maximum cutting force

of 168 N is recorded at vc = 50 m/min, when using

carbide tool.

• Tool flank wear appears minimal in case of cermet

tool, even at maximum cutting speed (vc = 130 m/min),

than other two counterparts. At vc = 130 m/min, cermet

tool exhibits VB = 163.31 lm.

• Flaking, coating dissipation, chip sticking, and

burning are prominently visible in worn-out car-

bide insert. BUE formation is experienced in case

of cermet tool. On the contrary, SiAlON tool

suffers from adhesion, flanking, chip fusion, and

oxidation. At the highest cutting speed, edge of

SiAlON tool gets fractured; thus, tool substrate

gets exposed.

• Better back surface morphology of chips is observed in

case of cermet tool than carbide, and SiAlON coun-

terparts. Chips produced using SiAlON tool exhibits

presence of deep feed marks (friction tracks); intensity

of which is decreased with increment in the cutting

speed.

• Cermet tool produces thinnest chips with shortest

pitch, and maximum segmentation frequency as quan-

tified on chip specimens collected at the highest cutting

speed (vc = 130 m/min).

• Superior machined surface integrity is obtained when

using cermet tool than other two counterparts.

Machined surface produced by cermet tool exhibits

minimal thickness of white layer (* 0.41 lm) at vc =80 m/min. Subsurface microhardness of the machined

specimen appears the lowest in case of cermet tool

usage.

• In short, it is experienced that cermet tool produces

lower tool-tip temperature, medium cutting force,

truncated width of flank wear, and better machined

surface quality with tinier white layer, when compared

with remaining two counterparts.

6. Future work

In-depth analysis of crater wear of tool insert (focusing

crater depth, crater width, volume of material removed by

crater formation, etc.) is not carried out in the present study.

Detailed analysis of crater wear may be carried out in future

work.

Appendix A

Schematic representation of tool wear

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