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Journal of Materials Processing Technology 153154 (2004) 10451050

Effect of high-pressure coolant supply when machining nickel-base,Inconel 718, alloy with coated carbide tools

E.O. Ezugwu, J. BonneyMachining Research Centre, Faculty of Engineering Science and Technology, South Bank University, London SE1 0AA, UK

Abstract

Inconel 718 was machined with a triple PVD coated (TiCN/Al 2O3/TiN) carbide tool at speeds up to 50 m min1 using conventional and

various high coolant pressures, up to 203 bar. Tool life, surface roughness (Ra), tool wear and component forces were recorded. The test

results show that acceptable surface finish and improved tool life can be achieved when machining Inconel 718 with high coolant pressures.Compared to conventional coolant supplies, tool life improved as much as 740%, when machining at 203 bar coolant pressure at a speed of

50mmin1. Tool life generally increased with increasing coolant supply pressure. This can be attributed to the ability of the high-pressure

coolant to lift the chip and gain access closer to the cutting interface. This action leads to a reduction of the seizure region, thus, lowering

the friction coefficient which in turn results in reduction in cutting temperature and component forces. Chip breakability during machining

is dependent on the depth of cut, feed rate and cutting speed employed as well as on the coolant pressure employed. Machining Inconel

718 with lower coolant pressures did not produce chip segmentation. Tool wear increased gradually with prolong machining with high

coolant pressures. Nose wear was the dominating tool failure mode due probably to a reduction in the toolchip and toolworkpiece contact

length/area.

2004 Elsevier B.V. All rights reserved.

Keywords: Tool life; Nose wear; Critical coolant pressure; Cutting temperature

1. Introduction

Advanced materials, such as nickel-base and titanium al-

loys as well as composites are widely used in the aerospace

and power industries. These materials are designed for high

temperature applications and at the same time maintain very

high strength to weight ratios. Nickel-based alloys have

high creep and corrosion resistance as well as the ability of

maintaining high strength-to-weight ratio, essential for the

economic exploitation of aerospace engines. Machining of

nickel-based alloys generate high temperatures at the tool

cutting edge which impair their performance, as they are

subjected to high compressive stresses acting on the tool

tip leading to plastic deformation of the tool edge, severe

notching and flank wear. The poor thermal conductivity of

nickel-based alloys raises temperature at the toolworkpiece

interface during machining, thus, accelerating tool wear.

Coolants play a significant role in improving lubrica-

tion as well as minimising temperature at the toolchip and

Corresponding author. Present address: South Bank University, School

Engineering Systems and Design, 103 Borough Road, London SE1 0AA,

UK. Fax: +44 171 815 7699.

E-mail address: [email protected] (E.O. Ezugwu).

toolworkpiece interfaces, consequently, minimising seizure

during machining. Flood cooling is not effective in terms of

lowering cutting temperature when machining exotic materi-

als. The coolant do not readily access the toolworkpiece and

toolchip interfaces that are under seizure condition as it is

vaporised by the high temperature generated close to the tool

edge. Machining of nickel-based alloys at high-speed condi-

tions can therefore, be achieved by a combination of the ap-

propriate tool material, machining technique and the choice

of a suitable cooling technology [1]. High-pressure assisted

cooling is one of the preferred technologies, currently, un-

der exploitation especially in the aerospace and power plant

industries for machining exotic materials. The credibility

of high-pressure coolant assisted machining had been thor-

oughly investigated over the years [25]. This system, not

only provides adequate cooling at the toolworkpiece in-

terface but also provides an effective removal (flushing) of

chips from the cutting area. The coolant jet under such

high-pressure is capable of creating a hydraulic wedge be-

tween the tool and the workpiece, penetrating the interface

deeply with a speed exceeding that necessary even for very

high-speed machining. This phenomenon also changes the

chip flow conditions [6]. The penetration of the high-energy

jet at the toolchip interface reduces the temperature gradi-

0924-0136/$ see front matter 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.jmatprotec.2004.04.329

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1046 E.O. Ezugwu, J. Bonney / Journal of Materials Processing Technology 153154 (2004) 10451050

Table 1

Chemical composition of Inconel 718 (wt.%)

C Mn Si S Cr Fe Mo Nb & Ta Ti Al Cu Ni

0.08 0.35 0.35 0.15 18.6 17.8 3.1 5.0 0.9 0.5 0.3 bal

Table 2

Physical properties of Inconel 718

Tensile strength

(MPa)

Yield strength

(MPa)

Elastic modulus

(GPa)

Hardness

(HV150)

Density

(gcm3)

Melting

point (C)

Thermal conductivity

(W/mK)

1310 1110 206 370 8.19 1300 11.2

ent and minimises the seizure effect, offering an adequate

lubrication at the toolchip interface with a significant reduc-

tion in friction. Excellent chip breakability has been reported

when machining difficult-to-cut materials with high-pressure

coolant supply [7,8]. This is attributed to a coolant wedge

which forms between the chip and the tool forcing the chip

to bend upwards giving it a desirable up curl required for

segmentation.This paper investigates the effect of varying coolant pres-

sure on tool performance when machining Inconel 718 alloy

with coated carbide tools at high-speed conditions.

2. Experimental procedures

The machining trials were carried out on a CNC Cen-

tre Lathe with a speed range from 18 to 1800 rpm. The

lathe is driven by an 11 kW stepless motor which provides a

torque of 1411 Nm. Cast (200 mm diameter 300 mm long)

solution treated, vacuum induction melted and electroslagremelted Inconel 718 alloy bars were used for the machin-

ing trials. The chemical composition and physical properties

of the workpiece are given in Tables 1 and 2 respectively.

Up to 6 mm thickness of the top surface of each bar was re-

moved prior to actual machining trials in order to eliminate

any surface defect that can adversely affect the machining

result. A coolant containing alkanolamine salts of the fatty

acid dicyclohexylamine, specifically designed for delivery

at high pressures was used in the machining trials. Conven-

tional coolant was applied by flooding the cutting interface at

an average flow rate of 5 l min1. The high-pressure coolant

was supplied at an average flood rate of 2050 l min1 and

directed via a nozzle on the tool holder to the region wherethe chip breaks contact with the tool. PVD Coated carbide

insert with ISO tool designations SNMG120412 was used

for the machining trials. The nominal composition and phys-

ical properties of the inserts are given in Tables 3 and 4.

Table 3

Composition of the coated carbide inserts

Co

(% volume)

WC

(% volume)

TaC

(% volume)

NbC

(%volume)

17.1 81 1.2 0.6

Table 4

Mechanical properties of coated carbide inserts

Hardness

(HV3)

Grain size

(m)

K1C

[MPa (m1/2)]

Coating thickness (m)

TiCN Al2O3 TiN

2000 1.7 14 4 1 0.5

The following cutting conditions were employed in this

investigation:

Cutting speed (m min1): 20, 30, 50

Feed rate (mm rev1): 0.25, 0.3

Depth of cut (mm): 2.53.0 ramping tool path program-

ming

Coolant concentration (%): 6

Coolant supply pressure (bar): conventional, 110, 150, 203

The tool rejection criteria for roughing operation were

employed. These values were considered in relation to ISO

Standard 3685 for tool life testing. A cutting tool was re-

jected and further machining stopped based on one or a

combination of the following rejection criteria:

(1) Average flank wear (mm) 0.4mm

(2) Maximum flank wear (mm) 0.7mm

(3) Nose wear (mm) 0.5mm

(4) Notching at the depth of cut line (mm) 1.0mm

(5) Surface roughness (mm) 6.0m

(6) Excessive chipping (flaking) or

catastrophic fracture of the cutting edge

Cutting forces generated during the machining trials were

measured using a three component piezoelectric tool post

dynamometer. Tool wear was measured with a travelling mi-croscope connected to a digital readout device at a magnifi-

cation of 25. Surface roughness was measured at various

intervals with a stylus type instrument.

3. Results and discussions

Figs. 1 and 2 show that longer tool life was achieved

when machining with PVD coated carbide tool under

high-pressure coolant supplies than with conventional

coolant supply. Table 5 is a summary of the percentage

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E.O. Ezugwu, J. Bonney / Journal of Materials Processing Technology 153154 (2004) 10451050 1047

Fig. 1. Tool life recorded when machining Inconel 718 with various coolant pressures at a feed rate of 0.25mm rev1. (CM: Conventional Machining).

Fig. 2. Tool life recorded when machining Inconel 718 with various coolant pressures at a feed rate of 0.3 mm rev1. (CM: Conventional Machining).

increase in tool life when machining at various coolant pres-

sures relative to conventional coolant supply. Up to 740%

improvement in tool life was achieved when machining with

carbide inserts most aggressive conditions (50 m min1

) us-ing 203 bar coolant pressure. Increase in coolant pressure,

generally increased in tool life when machining at speeds

in excess of 20 m min1.

The major cause of tool rejection when machining In-

conel 718 is high temperature generation at the toolchip and

toolworkpiece interfaces. The temperature is significantly

reduced by administering coolant under high pressures di-

rectly to the cutting interface. This could therefore, minimise

and/or completely eliminate thermally related wear mecha-

nisms. Therefore, tool performance tend to be primarily de-

pendent on mechanical wear phenomena. It can also be seen

in Figs. 1 and 2 that increasing coolant pressure did not in-

Table 5

Percentage improvement in tool life relative to conventional coolant supply

after machining Inconel 718 with coated carbide tool

Speed (m min1) Feed rate (mm rev1) 110 b ar 150 b ar 203 bar

20 0.25 8 9.8 33.8

30 0.25 87.7 50.6 64.1

50 0.25 335.0 411.1 462.8

20 0.3 8.6 11.5 43.9

30 0.3 27.05 95.2 104.5

50 0.3 517.6 647.2 739.8

crease tool life in cutting conditions investigated. This is ev-

ident from Table 5, where, a drop in tool life as much as 44%

is recorded when machining with 203 bar coolant pressure

at a speed of 20 m min1

and a feed rate of 0.3 mm rev1

.It has been established that at any speed condition, the

toolchip interface temperature initially decreased with an

increase in jet pressure, up to a critical pressure, above which

it rose to a relatively constant value for pressures in excess of

the critical pressure [2]. Cutting tools operate within a safety

temperature zone with minimal tool wear when machining at

the critical coolant pressure as thermal stresses are kept to a

minimum thereby prolonging tool life [1]. It is clear form this

that 203 bar coolant pressure is above the critical pressure

for machining at a speed of 20 m min1, hence, increased

toolchip contact temperature resulting in accelerated tool

wear and, hence, higher cutting forces.

Fig. 3 shows variation in component forces when ma-

chining Inconel 718 with PVD coated tool at various cut-

ting speeds and coolant pressures. The cutting forces gen-

erally decreased with increasing cutting speed as expected.

Reduction in cutting forces when machining at high coolant

pressure suggests that high-pressure jet is able to pene-

trate the cutting interface, thus, providing efficient cooling

as well as lubrication. The coolant water wedge created

at the toolchip interface reduces toolchip contact length

and also lowers the coefficient of friction and consequently

lowers cutting forces. Fig. 3 also shows that higher cutting

forces were, however, recorded when machining at a speed

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Fig. 3. Cutting forces recorded when machining Inconel 718 at various coolant supplies and machining conditions. (CM: Conventional Machining).

of 20mmin1 with 203 bar coolant pressure. This increase

in cutting force is due to increased friction coefficient and

higher nose wear rate generated by higher temperatures in-

duced by the hyper-critical coolant pressure (203 bar) at the

cutting interface.Nose wear is the dominant tool failure mode observed

when machining Inconel 718 at the conditions investigated.

Fig. 4 shows the nose wear plot when machining at a speed

of 20 m min1 with various coolant pressures and feed rates.

0

0.1

0.20.3

0.4

0.5

0.6

0.7

0 5 10 15 20 25 30 35 40

Cutting time (min)

Nosew

ear(mm)

CM (f=0.25) CM (f=0.3) 110 bar (f=0.25) 110 bar (f=0.3)

150 bar (f=0.25) 150 bar (f=0.3) 203 bar (f=0.25) 203 bar (f=0.3)

Fig. 4. Nose wear curves when machining Inconel 718 at a speed of 20m min1 under different coolant pressures and feed rate conditions. (CM:

Conventional Machining).

Fig. 5. (a) Worn tool after machining Inconel 718 with 203 bar coolant pressure at a speed of 20 m/min and a feed rate of 0.3mm/rev. (b) Enlarged view

on the flank face showing abrasive wear and coating delamination of coated carbide tool.

It can be seen from the graph that the nose wear rate in-

creased steadily with prolong machining. Rapid increases

in nose wear rate occurred when machining with 203 bar

coolant pressure at both feed rates, hence, the lower tool life

recorded.Fig. 5a shows a typical worn tool illustrating nose, flank

and rake face wears. The uniform flank wear observed

may be due to the low wear rate caused by tempera-

ture reduction at the cutting interface when machining

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E.O. Ezugwu, J. Bonney / Journal of Materials Processing Technology 153154 (2004) 10451050 1049

Fig. 6. (a) Chip generated when machining at coolant pressures up to 150 bar. (b) Segmented chips generated when machining with 203bar coolant pressure.

0

1

2

3

4

5

6

0 5 10 15 20 25 30 35 40

Cutting time (min)

SurfaceRoughness(um)

CM (f=0.25) CM (f=0.3) 110 bar (f=0.25) 110 bar (f=0.3)

150 bar (f=0.25) 150 bar (f=0.3) 203 bar (f=0.25) 203 bar (f=0.3)

Fig. 7. Surface roughness values recorded when machining with various coolant pressures and feed rates at a speed of 30 m min1

. (CM: ConventionalMachining).

with high coolant pressures. Fig. 5b shows that the wear

mechanism is mechanically related and typical of abrasion

wear.

Machining Inconel 718 with lower coolant pressures, up

to 150 bar, produced long continuous spiral chips (Fig. 6a),

while smaller segmented chips were produced when ma-

chining with higher coolant pressure of 203 bar (Fig. 6b).

Coolant supply at high-pressure tends to lift up the chip af-

ter passing through the deformation zone resulting to a re-

duction in the toolchip contact length/area. This tends to

enhance chip segmentation as the chip curl radius is reduced

significantly, hence, maximum coolant pressure is restricted

only to a smaller area on the chip. Similar observation with

chip segmentation, was made while machining steel. It was

observed that the power of the coolant jet and the lateral

position of the point where the jet hits the line where the

chip exits the tool rake face has significant influence on the

chip segmentation process with high-pressure coolant sup-

plies [9].

Fig. 7 is a plot of the surface roughness values recorded

when machining at a cutting speed of 30 m min1. The

curves show that lower surface roughness values (hence, im-

proved finish) were generated when machining at lower feed

rate of 0.25 mm rev1 while higher values were generated

at a higher feed rate of 0.3 mm rev1. The curves also show

that the surface roughness values varies marginally with pro-

long machining with high coolant pressures. This could be

attributed to the fact that the tool wear process was gradual

due to the significant reduction in temperature at the cutting

interface. The tool cutting edge may have been maintained

for longer periods, thereby, ensuring minimal variations in

recorded surface roughness values.

Surface roughness values recorded in all the cutting con-

ditions investigated are well below the stipulated rejection

criterion of 6m. This shows that the integrity of the ma-

chined surfaces may not be affected when machining Inconel

718 alloy under high coolant pressures.

4. Conclusions

1. Machining Inconel 718 with coated carbide tools under

high-pressure coolant supplies can improve tool life by

up to 7-folds, especially at higher speed conditions.

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2. Tool life tend to improve with increasing coolant pres-

sure. There is also evidence that once a critical pressure

has been reached any further increase, in coolant pres-

sure may only result to a marginal increase in tool life.

3. Chip segmentation depends on the cutting conditions

employed and to a greater extent on the coolant pres-

sure employed when machining Inconel 718. Machiningwith a 203 bar coolant pressure produced well segmented

C-shape chips.

4. Surface roughness values generated when machining

Inconel 718 alloy with the coated carbide tool vary

marginally with prolong machining due probably to the

gradual wear generated at the tool edge as well as tem-

perature reduction at the cutting interface by the high

coolant pressure employed.

Acknowledgements

The authors would like to thank Rolls-Royce plc andSandvik Coromant for their support that enabled this work

to be carried out.

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