Machining Ti–6Al–4V alloy with cryogenic compressed air cooling

S. Sun

a

,

d

,

e

,

n

, M. Brandt

b

,

d

,

e

, M.S. Dargusch

c

,

d

,

e

a

IRIS, Faculty of Engineering and Industrial Sciences, Swinburne University of Technology, Hawthorn, Victoria 3122, Australia

b

School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University, Bundoora, Victoria 3083, Australia

c

School of Mining and Mechanical Engineering, University of Queensland, St. Lucia, Brisbane, Queensland 4072, Australia

d

Defence Materials Technology Centre, Australia

e

CAST Cooperative Research Centre, Australia

a r t i c l e

i n f o

Article history:
Received 12 April 2010
Received in revised form
13 August 2010
Accepted 13 August 2010

Available online 18 August 2010

Keywords:
Cryogenic
Compressed air
Cutting force
Chip temperature
Chip roughness ratio
Tool wear

a b s t r a c t

A new cooling approach with cryogenic compressed air has been developed in order to cool the cutting
tool edge during turning of Ti–6Al–4V alloy. The cutting forces, chip morphology and chip temperature
were measured and compared with those measured during machining with compressed air cooling and
dry cutting conditions. The chip temperature is lower with cryogenic compressed air cooling than those
with compressed air cooling and dry machining. The combined effects of reduced friction and chip
bending away from the cutting zone as a result of the high-speed air produce a thinner chip with
cryogenic compressed air cooling and a thicker chip with compressed air cooling compared to dry
machining alone. The marginally higher cutting force associated with the application of cryogenic
compressed air compared with dry machining is the result of lower chip temperatures and a higher
shear plane angle. The tendency to form a segmented chip is higher when machining with cryogenic
compressed air than that with compressed air and dry machining only within the ranges of cutting speed
and feed when chip transitions from continuous to the segmented. The effect of cryogenic compressed
air on the cutting force and chip formation diminishes with increase in cutting speed and feed rate. The
application of both compressed air and cryogenic compressed air reduced flank wear and the tendency
to form the chip built-up edge. This resulted in a smaller increase in cutting forces (more significantly in
the feed force) after cutting long distance compared with that observed in dry machining.

&

2010 Elsevier Ltd. All rights reserved.

1. Introduction

The demand for titanium alloys in the aerospace industry has

dramatically increased in past decades due to their high strength
to weight ratio, strong corrosion resistance and ability to retain
high strength at high temperature. The high buy-to-fly ratio of
titanium components for aerospace applications is a consequence
of the large amount of material required to be removed by
machining.

Titanium and its alloys are classified as hard-to-machine

materials due to their inherent physical and mechanical proper-
ties. Because of the low thermal conductivity of titanium, the chip
cannot effectively dissipate the heat generated by cutting with a
large proportion ( 470%) of the heat conducted into the cutting
tool

[1]

, which leads to rapid increase in cutting temperatures

with increase in cutting speed compared to that observed during
machining of aluminium alloys. Combined with the high chemical
reactivity of titanium with almost all of the cutting tool materials
at high temperature, the tool life is substantially reduced with

increase in cutting speed. Therefore, the machining of titanium
alloys is a high-cost process due to long cycle times and tool costs.
Optimization of the machining parameters can lead to minimize
the overall costs by increasing productivity without dramatic loss
of tool life

[2]

. On the other hand, effective cooling methods can

significantly contribute to improve tool life by minimizing the
friction and lowering the cutting temperature.

Conventional coolant delivery techniques are limited in their

ability to penetrate into the region adjacent to the cutting edge at
high cutting speed because coolants tend to vaporise at the high
temperatures generated close to the tool edge, forming a high-
temperature blanket

[3,4]

. Various enhanced cooling approaches

have been developed in order to reduce the cutting temperature.

Cryogenic cooling uses liquid nitrogen (LN

2

) as coolant, which

can be delivered through circulation to the tool cover, which is
fixed on top of cutting insert

[5]

or jets impinging on the flank

face, rake face or both

[6–10]

. The former technique only offers

cooling on the tool, but the later not only reduces cutting
temperature, but also provides lubrication between the cutting
tool and chip using high-pressure LN

2

spray due to fluid gas

formation between the tool and chip as a result of evaporation of
LN

2

[8,11]

. Therefore, tool wear is reported to be substantially

reduced

[9,10]

. The benefits offered by LN

2

decrease with cutting

speeds higher than 100 m/min due to inadequate penetration of

Contents lists available at

ScienceDirect

journal homepage:

www.elsevier.com/locate/ijmactool

International Journal of Machine Tools & Manufacture

0890-6955/$ - see front matter & 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.ijmachtools.2010.08.003

n

Corresponding author at: IRIS, Faculty of Engineering and Industrial Sciences,

Swinburne University of Technology, Hawthorn, Victoria 3122, Australia.
Tel.: [phone]; fax: + 61 3 92145050.

E-mail address: [e-mail] (S. Sun).

International Journal of Machine Tools & Manufacture 50 (2010) 933–942

LN

2

into the chip–tool interface

[10]

. However, the spray of

supercooled LN

2

may raise safety concerns for the machine

operator.

High-pressure coolant is another effective technique to

increase tool life during turning, drilling and milling of hard-to-
machining materials

[12–18]

and break the chip

[19–21]

. The

liquid coolant at high pressure can penetrate into the tool–
workpiece or tool–chip interfaces resulting in more effective
cooling of the cutting tool.

Considering the negative impact of liquid coolant on the

environment and operator health, alternative environmentally
friendly solutions such as gaseous lubricants have been devel-
oped, including oxygen, carbon dioxide, gaseous and liquid
nitrogen gas

[22–24]

, water vapour

[25,26]

and cold air generated

by vortex tube

[27]

or refrigerator

[28]

. Reductions in cutting

force, cutting temperature and tool wear have been reported with
gaseous lubricants. The reduction in cutting temperature using
cold air generated by vortex tube or refrigerator is limited because
the temperature at the cooling nozzle is subzero (  20 1C).

In order to take advantage of both the extremely low

temperature of the LN

2

and high pressure from the compressed

air, a new cooling approach has been developed using LN

2

cooled

(cryogenic) compressed air to cool both the flank and rake faces of
the tool. Cryogenic air at high pressure can easily penetrate into
the cutting edge to reduce the cutting temperature. The effects of
this cooling technique on the cutting force, chip temperature and
chip formation are investigated in this paper.

2. Workpiece alloy and experimental procedures

The workpiece material used in this study was mill annealed

Ti–6Al–4V alloy; its chemical composition is listed in

Table 1

. The

microstructure of the workpiece material is shown in

Fig. 1

. It

contains primary alpha phase with a size of 7

73

m

m and inter-

granular beta phase with average size of 1.06

m

m. The hardness of

the workpiece is 351

730 Hv.

The generation of cryogenic compressed air is shown schema-

tically in

Fig. 2

. The compressed air at a pressure of 7 bar passes

through a coiled copper tube inside a liquid nitrogen tank. The
compressed air was cooled to the temperature of LN

2

( 196 1C) by

the heat exchange in the coil and was directed to the rake and flank
faces through nozzles with dimensions of 1 mm  2 mm due to the
significant cooling effect on both faces reported by Hong et al.

[7,9]

.

Turning was conducted on a 3.5 hp Hafco Metal Master lathe

(Model AL540) by dry machining, with compressed air cooling
and cryogenic compressed air cooling respectively. The tool used
was a CNMX1204A2-SMH13A type tool supplied by Sandvik. The
rake angle was + 151, angle of inclination was  61 and the entry
angle was 451. Turning was performed with a length of cut of 2
and 31 m, respectively. The short cuts were made at: (1) constant
feeds of 0.19 and 0.28 mm, respectively, with cutting speeds up to
220 m/min and (2) constant cutting speed of 80 m/min with feed
ranges between 0.054 and 0.28 mm to check the effects of cutting
speed and feed on the cutting force, chip temperature and
morphology without severe tool wear. The long cut was perfor-
med at a high feed of 0.28 mm and cutting speed of 200 m/min to
investigate the evolution of cutting forces with cutting time due
to tool wear. The depth of cut was set at 1 mm for all the tests.

The dynamic cutting forces were recorded using a 3-compo-

nent force sensor (PCB Model 260A01) with an upper frequency
limit of 90 kHz. The force sensor was placed under the tool holder.
Details on the force sensor and data logging system have been
reported elsewhere

[29]

. The forces measured are those in the X, Y

and Z directions as shown in

Fig. 3

.

Table 1
Chemical composition (in mass percentage) of workpiece alloy.

C

Al

O

Fe

V

H

N

Ti

Y

0.010

5.86

0.120

0.200

4.02

0.0023

0.007

Bal.

o0.0050

Fig. 1. Optical microstructure of the workpiece material.

Compressed air

Cryogenic

compressed air

LN

2

Cu tube

Workpiece

Tool

Air nozzles

Fig. 2. Schematic illustrations of (a) generation and (b) delivery of cryogenic
compressed air to the cutting edge.

S. Sun et al. / International Journal of Machine Tools & Manufacture 50 (2010) 933–942

934

The temperature around the chip at the cutting zone was

monitored by an infra-red thermal camera, which was positioned
at 350 mm from the top of the cutting tool. Since the cutting edge
was covered by the chip, the maximum temperature reported in
this paper is the temperature measured at the chip surface at the
cutting edge as shown by the arrow in

Fig. 4

. The emissivity of

the workpiece alloy was obtained through a calibration process,
the calibration was made by heating the workpiece alloy sample,
on which a thermocouple was spot-welded, in a furnace up to the
target temperature. The emissivity of the workpiece alloy was
calculated by calibrating the temperature from the infra-red
signal with the temperature measured by the thermocouple.

3. Results

3.1. Effect of cryogenic compressed air on chip temperature

The measured maximum temperature of the chip during

cutting at a feed of 0.28 mm is shown in

Fig. 5

. Chip temperature

increases rapidly with cutting speed up to 50 m/min following
which the increase becomes more gradual for all three cutting
conditions. Because the infra-red camera was set up to look at
the top surface of the chip, the temperature measured is the

temperature of the top surface of the chip, not the temperature at
the cutting zone. Therefore, temperatures shown in

Fig. 5

are

lower than those reported by other researchers

[30]

.

It should be noted that both the compressed air and cryogenic

compressed air show a significant cooling effect on the chip tempera-
ture, and the cooling effect diminishes with increase in cutting speed.
The difference in the cooling effect between the compressed air and
cryogenic compressed air increases with cutting speed and reaches a
maximum value (about 140 1C) at a cutting speed of 20 m/min and
slowly decreases with further increases in cutting speed.

It also should be pointed out that the cooling effect using both

compressed air and cryogenic compressed air is larger with a
smaller feed (0.19 mm) than that with a larger feed (0.28 mm) at
the same cutting speed.

3.2. Influence of cryogenic compressed air on cutting forces

Because the cutting tool is not significantly worn during a

short cut, the effect of tool wear on the cutting forces is negligible.
The cutting force is mainly affected by the shear strength of the
workpiece, shear angle and the friction between the cutting tool
and chip, workpiece during a short cut.

Fig. 6

shows the variation in cutting forces with cutting speeds

during dry machining, compressed air and cryogenic compressed
air cooling conditions. The cutting forces generally decrease
with cutting speeds up to about 50 m/min and are thereafter
roughly constant with any further increase in cutting speed up to
210 m/min, which was the maximum cutting speed investigated
in this paper.

The difference in cutting forces with different cooling

approaches during a short cut is found only at cutting speeds
lower than 20 m/min below which the cutting forces during
cryogenic compressed air cooling are larger than those observed
during both dry machining and compressed air cooling.

Significant differences in cutting forces with different cooling

approaches are observed with long cuts for cutting lengths of
31 m at a cutting speed of 200 m/min (shown in

Fig. 7

). The forces

Tool and

tool holder

Workpiece

Y

X

Z

Air

nozzles

Fig. 3. Illustration of the position of air nozzle and directions of the measured
forces during turning.

<300.0

°C

790.4

°C

Fig. 4. Infra-red camera image during cutting showing the chip surface
temperature. The maximum chip temperature region is marked by arrow.

0

100

200

300

400

500

600

700

800

900

0

Surface cutting speed (m/min)

Maximum chip temperature (

°C)

Dry machining

Compressed air

Cryogenic compressed air

30

60

90

120

150

180

210

240

Fig. 5. Effect of cutting speed on the maximum chip temperature under different
conditions at feed of 0.28 mm.

S. Sun et al. / International Journal of Machine Tools & Manufacture 50 (2010) 933–942

935

with dry machining are smaller than those recorded with air
cooling at the start of the cut and increase rapidly during machin-
ing compared to the cutting forces with both compressed air and
cryogenic compressed air cooling. After cutting a length of 31 m,
Fx, Fy and Fz increase, respectively, about 54%, 41% and 23%
during dry machining, 30%, 16% and 6% with compressed air cool-
ing and 17%, 7% and 4% with cryogenic compressed air cooling.

Increases in cutting forces with cutting time is believed to be due

to the evolution of tool wear and the development of a built-up
edge. It can be seen clearly in

Fig. 8

that flank and crater wear, along

with the chip built-up edge increase in the order of dry machining,
compressed air cooled and cryogenic compressed air cooled
machining. Furthermore, the black residue on the rake face
indicates that local burning occurred on the tool’s rake face during
machining under dry condition and with compressed air cooling. No
black residue was observed on the tool rake face during machining
with cryogenic compressed air cooling, which further supports the
conclusion that the tool–chip interface temperature was lower due
to more effective cooling from the cryogenic compressed air.

3.3. Chip formation when machining with cryogenic compressed air

The morphology of the segmented chip is normally charac-

terised in terms of tooth depth and spacing. These parameters are
useful in order to characterise the regular and periodic segmented
chip; however, these terms are hard to measure when the

segments are irregular and aperiodic. Instead, the average chip
thickness and chip roughness ratio are used in this paper to
characterise the chip morphology. The chip roughness ratio is
defined as the ratio of the standard deviation of chip thickness to
the average chip thickness.

The changes in chip morphology with respect to cutting speed

and feed rate are plotted in Figs. 9 and 10. The cutting speed was set
at 80 m/min in

Fig. 9

and the feed was fixed at 0.19 mm as shown in

Fig. 10

. It can be seen that chip thickness increases linearly with

feed at constant cutting speed. The application of both compressed
and cryogenic compressed air did not significantly affect the chip
thickness at a cutting speed of 80 m/min (

Fig. 9

a).

Chip thickness was found to decrease dramatically with increase

in cutting speed up to about 50 m/min and became steady with
further increases in cutting speed (

Fig. 10

a). This variation follows a

similar trend of cutting force variation with cutting speed. The chip
is thickest with compressed air cooling, and thinnest with cryogenic
compressed air cooling at cutting speeds lower than 50 m/min.
There is no clear difference in chip thickness with respect to cooling
technique at cutting speeds higher than 50 m/min.

Increase in the chip roughness ratio with increase in cutting

speed or feed clearly shows the transition from an irregular seg-
mented chip to regular segmented chip with increase in cutting
speed and feed. The ultimate chip roughness ratio is about 0.31
when a regular and periodic segmented chip is obtained at cutting
speed greater than 100 m/min (feed of 0.19 mm) or feed greater
than 0.214 mm (cutting speed of 80 m/min). The significant

0

50

100

150

200

250

0

Surface cutting speed (m/min)

Feed force, Fx (N)

Dry machining
Compressed air
Cryogenic compressed air

0

50

100

150

200

250

Thrust force, Fy (N)

Dry machining
Compressed air
Cryogenic compressed air

0

100

200

300

400

500

600

700

800

900

Main cutting force, Fz (N)

Dry machining
Compressed air
Cryogenic compressed air

30

60

90 120 150 180 210 240

0

Surface cutting speed (m/min)

30

60

90 120 150 180 210 240

0

Surface cutting speed (m/min)

30

60

90 120 150 180 210 240

Fig. 6. Effect of cooling approaches on (a) feed force, (b) thrust force and (d) main cutting force in a cutting length of 2 m at various cutting speeds and feed of 0.28 mm.

S. Sun et al. / International Journal of Machine Tools & Manufacture 50 (2010) 933–942

936

difference in chip roughness ratio due to the application of
compressed air and cryogenic compressed air was only observed
at feeds between 0.163 and 0.214 mm with a cutting speed of
80 m/min (

Fig. 9

b) and cutting speeds between 30 and 100 m/min

with a constant feed of 0.19 mm (

Fig. 10

b). Within these speed

and feed ranges the transition from a near-continuous chip to a
regular segmented chip occurred. The chip roughness ratio is the
highest with cryogenic compressed air cooling and the lowest
with compressed air cooling in these transition regions, which
means that the tendency to form a segmented chip is the highest
with cryogenic compressed air cooling and the lowest with
compressed air cooling.

4. Discussion

4.1. Reduction in chip temperature by utilising an air jet

During machining, the temperature rise in the cutting zone

is the result of the heat generation mainly due to plastic

deformation in the primary shear plane and friction between
the cutting tool and chip

[31,32]

. The cutting temperature during

cutting of the workpiece with low thermal conductivity, such
as titanium alloys, is very high because the heat generated can-
not be effectively dissipated by the chip. The high cutting
temperature accelerates the tool wear rate and results in shorter
tool life.

The cutting temperature can be reduced through the following

mechanisms.

4.1.1. Reduction in heat generation

Effective lubrication between the chip and cutting tool not

only results in the generation of lower amounts of heat due to the
lower friction, but also reduces the heat generated due to plastic
deformation because a larger shear plane angle and shorter shear
plane are produced with lower friction

[33]

.

Gaseous lubricants can form a film on the tool surface and this

means that the chip flow over the tool surface is easier due
to the lower friction at the tool–workpiece–chip interface

[22]

.

0

50

100

150

200

250

Dry

machining

Cutting conditions

Average feed force, Fx (N)

Start
Finish

0

50

100

150

200

Average thrust force, Fy (N)

Start
Finish

0

100

200

300

400

500

600

700

Average main cutting force, Fz (N)

Start
Finish

Compressed

air

Cryogenic

compressed air

Dry

machining

Cutting conditions

Compressed

air

Cryogenic

compressed air

Dry

machining

Cutting conditions

Compressed

air

Cryogenic

compressed air

Fig. 7. Effect of cutting conditions on the cutting forces at start and finish of cut for a cutting length of 31 m in (a) X, (b) Y and (c) Z directions at cutting speed of 200 m/min
and feed of 0.28 mm.

S. Sun et al. / International Journal of Machine Tools & Manufacture 50 (2010) 933–942

937

The reduction in friction force is significant with the application of
high-pressure gases

[34,35]

. The mechanical effect of bending the

chip by applying high-speed air onto the top surface of the chip
increases the tool–chip contact length and this leads to lower
pressure on the tool’s rake face

[36,37]

.

4.1.2. Removal of heat from the cutting zone

A cooling media can only reduce tool flank and tool–chip

interface temperature by heat convection if the coolant can
directly access the primary cutting zone

[23]

. The heat flux (q)

from the hot surface to the coolant is given by

q ¼ hðT

w



T

1

Þ

ð

1Þ

where, T

w

and T

N

are the object surface temperature and

cooling-stream temperature, respectively, and h is the convec-
tion heat-transfer coefficient. The cooling effect of using the
conventional flood coolant is dramatically reduced at high cutting
temperature because the convection heat-transfer coefficient is
dramatically reduced as a result of boiling of the liquid coolant.

When the heat from the cutting zone is removed by heat

convection, the rate of heat removal depends on the convection

heat-transfer coefficient and temperature of the coolant. A
dramatic increase in the heat convection coefficient with high-
pressure air (

E2000 W/m

2

K at pressure of 4–7 bar) over

conventional dry machining (

E20 W/m

2

K)

[37,38]

can explain

the reduction in chip temperature by applying compressed air at
the cutting zone. Furthermore, the air temperature is significantly
lower due to its stream passing through the coiled tube cooled by
LN

2

(T

N

¼ 

196 1C), which further increases the heat removal rate

and results in lower chip temperatures with cryogenic com-
pressed air compared to compressed air.

The amount of heat removed from the cutting zone by the

coolant increases by increasing the duration for which the heated
area is subjected to the coolant. With increase in cutting speed,
the convection heat per length of chip decreases due to the
shorter cold air–chip interaction time. This leads to a diminishing
cooling effect on chip temperature using both compressed air and
cryogenic compressed air with increase in cutting speed.

However, the air nozzle is virtually stationery relative to the

cutting tool, which is continuously cooled by the cold air in order
to remove the heat generated during machining. The reduction
in tool temperature from the application of the cryogenic

Fig. 8. Images of (a), (c) and (e) primary flank face, (b), (d) and (f) rake face of the cutting tool after cutting the length of 31 m at cutting speed of 200 m/min and feed of
0.28 mm under (a) and (b) dry machining, (c) and (d) machining with compressed air cooling, (e) and (f) with cryogenic compressed air cooling.

S. Sun et al. / International Journal of Machine Tools & Manufacture 50 (2010) 933–942

938

compressed air is expected to be substantial compared to the
reduction in chip temperature.

4.2. Effect of air cooling on chip formation and cutting forces

Because the air was delivered to the tool–chip interface, the

penetration of the high-speed air into the tool–chip interface
reduces the friction between the tool and the chip and this reduc-
tion in friction tends to increase the shear plane angle

[33,39]

and

produce smaller chip thickness. In the meantime the pressure
introduced by the high-speed air forces the chip to bend away and
separate from the cutting edge; this effect results in a marginally
lower shear plane angle and larger chip thickness with the

application of compressed air. However, the increasing rigidity of
the chip when cooled using cryogenic compressed air at low
cutting speeds makes the chip bending effect negligible; there-
fore, the reduction in friction is dominant and this leads to a
smaller chip thickness at cutting speeds lower than 50 m/min
with cryogenic compressed air cooling.

The difference in the chip roughness ratio in the transition

ranges (cutting speeds between 30 and 100 m/min at a feed of
0.19 mm or feeds between 0.163 and 0.214 mm at the cutting
speed of 80 m/min) by application of compressed air and
cryogenic compressed air has been investigated by comparing
the plastic deformation in the chips. The formation of an irregular
segmented chip has been shown to be the result of the mixture of

0

0.05

0.1

0.15

0.2

0.25

0

Feed (mm)

Chip thickness (mm)

Dry machining

Compressed air

Cryogenic compressed air

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0

Feed (mm)

Chip roughness ratio

Dry machining

Compressed air

Cryogenic compressed air

0.1

0.15

0.05

0.2

0.25

0.3

0.35

0.05

0.1

0.15

0.2

0.25

0.3

Fig. 9. Influence of feed on the (a) chip thickness and (b) chip roughness ratio at a
constant cutting speed of 80 m/min under different conditions.

0.1

0.11

0.12

0.13

0.14

0.15

0.16

0.17

0.18

0.19

0

Surface cutting speed (m/min)

Chip thickness (mm)

Dry machining

Compressed air

Cryogenic compressed air

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0

Surface cutting speed (m/min)

Chip roughness ratio

Dry machining

Compressed air

Cryogenic compressed air

50

100

150

200

250

300

50

100

150

200

250

300

Fig. 10. Effect of cutting speed on the (a) chip thickness and (b) chip roughness
ratio at a constant feed of 0.19 mm under different conditions.

S. Sun et al. / International Journal of Machine Tools & Manufacture 50 (2010) 933–942

939

continuous shear and localized shear

[29]

. The saw-tooth depth,

which is associated with the chip roughness ratio is dependent on
the amount of continuous shear inside a segment. A regular seg-
mented chip with a maximum saw-tooth depth or chip roughness
ratio is formed when the continuous shear is absent.

The comparison of the deformed structure in the cross-section

of the chips produced at a feed of 0.163 mm and cutting speed of
80 m/min with different cooling approaches is shown in

Fig. 11

.

The chip produced with cryogenic compressed air cooling shows
negligible plastic deformation inside the segment (

Fig. 11

c) while

significant plastic deformation occurred inside the segments of
the chips made with dry machining (

Fig. 11

a) and compressed air

cooling (

Fig. 11

b). This plastic deformation within a segment leads

to a smaller saw-tooth depth and chip roughness ratio under dry
and compressed air cooling conditions. Supercooling of the
workpiece using the cryogenic compressed air technique makes
plastic deformation more difficult during chip formation at the
cutting zone and this leads to a higher tendency for chip segmen-
tation compared with dry machining and compressed air cooling
at same cutting speed and feed within the continuous chip to
segmented chip transition range.

The relationship between the chip roughness ratio and cutting

speed in the low cutting speed region (

Fig. 12

) shows that the chip

roughness ratio is reduced with increase in cutting speed and
reaches a minimum value at 14 and 20 m/min for dry machining
and compressed air cooling, respectively, at which point a more
continuous chip was obtained. The chip roughness ratio increased
dramatically and a more segmented chip was formed with further
increases in cutting speed. However, the chip roughness ratio did
not change with cutting speeds up to 20 m/min with cryogenic
compressed air cooling. The changes in chip morphology with
increase in cutting speeds from 6 m/min to 14 m/min with
different cooling approaches are shown in

Fig. 13

.

Similar changes in chip morphology from a segmented chip at

extremely low cutting speed (0.025 m/min) to a continuous chip

at intermediate speed (15 m/min) and back to a segmented chip
at high cutting speed (53 m/min) have been previously reported
without explanation

[31]

. This study shows that the reduction

in chip roughness ratio and greater continuous chip formation at
a cutting speed of 14 m/min compared to cutting speed of
6 m/min under dry machining condition is most probably the
result of thermal softening of workpiece at the primary shear zone

Fig. 11. Cross-sections of chips made at cutting speed of 80 m/min and feed of 0.163 mm under (a) dry, (b) compressed air cooling and (c) cryogenic compressed
air cooling.

0.05

0.06

0.07

0.08

0.09

0.1

0.11

0.12

0.13

0.14

0.15

0

Chip roughness ratio

Dry machining

Compressed air

Cryogenic compressed air

Surface cutting speed (m/min)

10

20

30

40

50

Fig. 12. Effect of cooling conditions on the evolution of chip roughness ratio at low
cutting speeds with constant feed of 0.19 mm.

S. Sun et al. / International Journal of Machine Tools & Manufacture 50 (2010) 933–942

940

during chip formation. This result agrees with the observation
that an appropriate softening of the workpiece by laser beam
heating prior to cutting can lead to continuous chip formation
during laser assisted machining of Ti–6Al–4V alloy at higher
cutting speed compared to conventional machining

[40]

. How-

ever, enhanced cooling using cryogenic compressed air reduces
the effect of thermal softening of workpiece at the primary shear
zone during chip formation; therefore, no reduction in the chip
roughness ratio is observed with increase in cutting speeds from
6 to 20 m/min.

The cutting force is a function of the yield strength of the

workpiece, friction between the cutting tool, chip primary shear
plane and tool rake angle. The effect of cryogenic compressed air
on cutting force is the result of a reduction in friction and an
increase in the yield strength of the workpiece. The smaller chip
thickness obtained by application of the cryogenic compressed air
compared to dry machining at low cutting speeds indicates that
the higher shear plane angle is obtained with cryogenic com-
pressed air cooling, which may be the result of a reduction in
friction between the chip and the tool

[33,39]

.

The extremely low temperature of the cryogenic compressed

air not only cools the cutting tool, but also reduces the tempera-
ture of the workpiece before it enters the cutting zone. Similar
to the increase in the main cutting force during machining of
Ti–6Al–4V alloy with LN

2

impingement cooling compared to that

in dry machining

[8]

, the marginally higher cutting force at low

cutting speeds with application of cryogenic compressed air
compared with dry machining can be attributed to the lower
temperature of the workpiece and higher shear plane angle. The
cooling of the workpiece, reduction in chip thickness and increase
in the shear plane angle by cryogenic compressed air are reduced
with increase in cutting speed. Therefore, there is no significant
difference in cutting forces observed between dry machining and
machining with compressed air and cryogenic compressed air
cooling at high cutting speed for a short cut length.

The high temperatures at the cutting zone often result in the

formation a built-up edge when machining titanium alloys due to
the adhesion and welding of the chip to the cutting edge of the
tool. The stable built-up edge may protect the flank face from
wear; however, the oversized chip built-up edge may result in
accelerated attrition wear and high friction coefficient, which lead
to a rapid increase in cutting forces (especially the feed force,

which is closely related to the frictional force between the chip
and the cutting tool

[8]

) over the cutting time. Catastrophic

breakage of the cutting tool tip may occur when the adhered
workpiece material is removed by high cutting pressure.

Fig. 8

clearly shows that cryogenic compressed air cooling

reduces the size of the chip built-up edge and tool wear when
machining Ti–6Al–4V alloy. These are due to the lower cutting
temperature

[41]

. The lower tool wear and smaller chip built-up

edge lower the friction between the cutting tool and chip, which
leads to a significantly lower increase in the cutting forces during
the long cut, especially in the X and Y directions with application
of cryogenic compressed air. A detailed quantitative study of the
effect of cryogenic compressed air on the tool wear will be
reported in a follow-up publication.

5. Conclusions

The cryogenic compressed air cooling technique described in

this paper provides a safer and more readily applicable technique
for improving the machinability of Ti–6Al–4V alloy. The technique
delivers high-speed air at extremely low temperatures into the
cutting zone. Compared to dry machining, the cryogenic com-
pressed air cooling makes the following changes in machining:

(1) Reduction in chip thickness by approximately 9% at cutting

speed lower than 50 m/min due to improvements in friction
effects between the chip and the tool.

(2) An increase in cutting forces by about 6% at cutting speed

lower than 50 m/min because of a higher shear plane angle
and lower chip temperature.

(3) Changes in the characteristics of the segmented chip and the

transition from irregular to regular segmented chips. The
cutting speed is about 20 m/min lower and feed is about
0.04 mm smaller at the same chip roughness ratio of 0.25 in
the chip morphology transition region with cryogenic com-
pressed air compared with dry machining.

The dramatic reduction in chip temperature with both com-

pressed air and cryogenic compressed air cooling at all cutting
speeds indicates that there is a reduction in cutting temperature,
which leads to a smaller chip built-up edge and lower tool wear.

Fig. 13. Cross-section of chips made by (a) and (b) dry machining, (c) and (d) compressed air cooling and (e) and (f) cryogenic compressed air cooling at constant feed of
0.19 mm and cutting speeds of (a), (c) and (e) 6 m/min and (b), (d) and (f) 14 m/min.

S. Sun et al. / International Journal of Machine Tools & Manufacture 50 (2010) 933–942

941

Therefore, increase in cutting forces (especially the feed force)
with cryogenic compressed air cooling with respect to cutting
length is significantly lower than that when dry machining. Fx, Fy
and Fz increase by about 17%, 7% and 4%, respectively, during
cutting with cryogenic compressed air cooling, whereas those
increase by about 54%, 41% and 23%, respectively, during dry
machining after a cutting the length of 31 m.

Acknowledgments

The authors gratefully acknowledge the CAST Cooperative

Research Centre and Defence Materials Technology Centre
(DMTC) for the financial support and permission to publish this
work. The CAST Cooperative Research Centre was established and
is supported under the Australian Government’s Cooperative
Research Centres Programme. The DMTC was established and is
supported under the Australian Government’s Defence Future
Capability Technology Centres Programme.

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