International Journal of Machine Tools & Manufacture 50 (2010) 933 942
Contents lists available at ScienceDirect
International Journal of Machine Tools & Manufacture
journal homepage: www.elsevier.com/locate/ijmactool
Machining Ti 6Al 4V alloy with cryogenic compressed air cooling
a,d,e,n b,d,e c,d,e
S. Sun , M. Brandt , M.S. Dargusch
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 a b s t r a c t
Article history:
A new cooling approach with cryogenic compressed air has been developed in order to cool the cutting
Received 12 April 2010
tool edge during turning of Ti 6Al 4V alloy. The cutting forces, chip morphology and chip temperature
Received in revised form
were measured and compared with those measured during machining with compressed air cooling and
13 August 2010
dry cutting conditions. The chip temperature is lower with cryogenic compressed air cooling than those
Accepted 13 August 2010
with compressed air cooling and dry machining. The combined effects of reduced friction and chip
Available online 18 August 2010
bending away from the cutting zone as a result of the high-speed air produce a thinner chip with
Keywords:
cryogenic compressed air cooling and a thicker chip with compressed air cooling compared to dry
Cryogenic
machining alone. The marginally higher cutting force associated with the application of cryogenic
Compressed air
compressed air compared with dry machining is the result of lower chip temperatures and a higher
Cutting force
shear plane angle. The tendency to form a segmented chip is higher when machining with cryogenic
Chip temperature
compressed air than that with compressed air and dry machining only within the ranges of cutting speed
Chip roughness ratio
and feed when chip transitions from continuous to the segmented. The effect of cryogenic compressed
Tool wear
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 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 demand for titanium alloys in the aerospace industry has
the overall costs by increasing productivity without dramatic loss
dramatically increased in past decades due to their high strength
of tool life [2]. On the other hand, effective cooling methods can
to weight ratio, strong corrosion resistance and ability to retain
significantly contribute to improve tool life by minimizing the
high strength at high temperature. The high buy-to-fly ratio of
friction and lowering the cutting temperature.
titanium components for aerospace applications is a consequence
Conventional coolant delivery techniques are limited in their
of the large amount of material required to be removed by
ability to penetrate into the region adjacent to the cutting edge at
machining.
high cutting speed because coolants tend to vaporise at the high
Titanium and its alloys are classified as hard-to-machine
materials due to their inherent physical and mechanical proper- temperatures generated close to the tool edge, forming a high-
temperature blanket [3,4]. Various enhanced cooling approaches
ties. Because of the low thermal conductivity of titanium, the chip
have been developed in order to reduce the cutting temperature.
cannot effectively dissipate the heat generated by cutting with a
Cryogenic cooling uses liquid nitrogen (LN2) as coolant, which
large proportion (470%) of the heat conducted into the cutting
can be delivered through circulation to the tool cover, which is
tool [1], which leads to rapid increase in cutting temperatures
fixed on top of cutting insert [5] or jets impinging on the flank
with increase in cutting speed compared to that observed during
face, rake face or both [6 10]. The former technique only offers
machining of aluminium alloys. Combined with the high chemical
cooling on the tool, but the later not only reduces cutting
reactivity of titanium with almost all of the cutting tool materials
temperature, but also provides lubrication between the cutting
at high temperature, the tool life is substantially reduced with
tool and chip using high-pressure LN2 spray due to fluid gas
formation between the tool and chip as a result of evaporation of
n
Corresponding author at: IRIS, Faculty of Engineering and Industrial Sciences,
LN2 [8,11]. Therefore, tool wear is reported to be substantially
Swinburne University of Technology, Hawthorn, Victoria 3122, Australia.
reduced [9,10]. The benefits offered by LN2 decrease with cutting
Tel.: [phone]; fax: [phone].
speeds higher than 100 m/min due to inadequate penetration of
E-mail address: [e-mail] (S. Sun).
0890-6955/$ - see front matter & 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.ijmachtools.2010.08.003
934 S. Sun et al. / International Journal of Machine Tools & Manufacture 50 (2010) 933 942
LN2 into the chip tool interface [10]. However, the spray of
supercooled LN2 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).
Fig. 1. Optical microstructure of the workpiece material.
In order to take advantage of both the extremely low
temperature of the LN2 and high pressure from the compressed
air, a new cooling approach has been developed using LN2 cooled
(cryogenic) compressed air to cool both the flank and rake faces of
Cryogenic
Compressed air
the tool. Cryogenic air at high pressure can easily penetrate into
compressed air
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.
Cu tube
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
LN2
microstructure of the workpiece material is shown in Fig. 1. It
contains primary alpha phase with a size of 773 mm and inter-
granular beta phase with average size of 1.06 mm. The hardness of
the workpiece is 351730 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 LN2 ( 196 1C) by
Air nozzles
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
Tool
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
Workpiece
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
Fig. 2. Schematic illustrations of (a) generation and (b) delivery of cryogenic
investigate the evolution of cutting forces with cutting time due
compressed air to the cutting edge.
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-
Table 1
nent force sensor (PCB Model 260A01) with an upper frequency
Chemical composition (in mass percentage) of workpiece alloy.
limit of 90 kHz. The force sensor was placed under the tool holder.
C Al O Fe V H N Ti Y
Details on the force sensor and data logging system have been
reported elsewhere [29]. The forces measured are those in the X, Y
0.010 5.86 0.120 0.200 4.02 0.0023 0.007 Bal. o0.0050
and Z directions as shown in Fig. 3.
S. Sun et al. / International Journal of Machine Tools & Manufacture 50 (2010) 933 942 935
900
800
700
600
Z
500
X
Tool and
Y tool holder 400
300
200
Dry machining
Air
nozzles
Workpiece Compressed air
100
Cryogenic compressed air
Fig. 3. Illustration of the position of air nozzle and directions of the measured
forces during turning.
0
0 30 60 90 120 150 180 210 240
Surface cutting speed (m/min)
790.4°C
Fig. 5. Effect of cutting speed on the maximum chip temperature under different
conditions at feed of 0.28 mm.
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 inthe cooling effect between the compressed air and
cryogenic compressed air increases with cutting speed and reaches a
<300.0°C
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
Fig. 4. Infra-red camera image during cutting showing the chip surface
compressed air and cryogenic compressed air is larger with a
temperature. The maximum chip temperature region is marked by arrow.
smaller feed (0.19 mm) than that with a larger feed (0.28 mm) at
the same cutting speed.
The temperature around the chip at the cutting zone was
monitored by an infra-red thermal camera, which was positioned
3.2. Influence of cryogenic compressed air on cutting forces
at 350 mm from the top of the cutting tool. Since the cutting edge
was covered by the chip, the maximum temperature reported in
Because the cutting tool is not significantly worn during a
this paper is the temperature measured at the chip surface at the
short cut, the effect of tool wear on the cutting forces is negligible.
cutting edge as shown by the arrow in Fig. 4. The emissivity of
The cutting force is mainly affected by the shear strength of the
the workpiece alloy was obtained through a calibration process,
workpiece, shear angle and the friction between the cutting tool
the calibration was made by heating the workpiece alloy sample,
and chip, workpiece during a short cut.
on which a thermocouple was spot-welded, in a furnace up to the
Fig. 6 shows the variation in cutting forces with cutting speeds
target temperature. The emissivity of the workpiece alloy was
during dry machining, compressed air and cryogenic compressed
calculated by calibrating the temperature from the infra-red
air cooling conditions. The cutting forces generally decrease
signal with the temperature measured by the thermocouple.
with cutting speeds up to about 50 m/min and are thereafter
roughly constant with any further increase in cutting speed up to
3. Results
210 m/min, which was the maximum cutting speed investigated
in this paper.
3.1. Effect of cryogenic compressed air on chip temperature
The difference in cutting forces with different cooling
approaches during a short cut is found only at cutting speeds
The measured maximum temperature of the chip during lower than 20 m/min below which the cutting forces during
cutting at a feed of 0.28 mm is shown in Fig. 5. Chip temperature cryogenic compressed air cooling are larger than those observed
increases rapidly with cutting speed up to 50 m/min following during both dry machining and compressed air cooling.
which the increase becomes more gradual for all three cutting Significant differences in cutting forces with different cooling
conditions. Because the infra-red camera was set up to look at approaches are observed with long cuts for cutting lengths of
the top surface of the chip, the temperature measured is the 31 m at a cutting speed of 200 m/min (shown in Fig. 7). The forces
Maximum chip temperature (
°
C)
936 S. Sun et al. / International Journal of Machine Tools & Manufacture 50 (2010) 933 942
250
250
200
200
150 150
100 100
Dry machining
Dry machining
50
50
Compressed air
Compressed air
Cryogenic compressed air
Cryogenic compressed air
0
0
0 30 60 90 120 150 180 210 240 0 30 60 90 120 150 180 210 240
Surface cutting speed (m/min) Surface cutting speed (m/min)
900
800
700
600
500
400
300
200 Dry machining
Compressed air
100
Cryogenic compressed air
0
0 30 60 90 120 150 180 210 240
Surface cutting speed (m/min)
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.
with dry machining are smaller than those recorded with air segments are irregular and aperiodic. Instead, the average chip
cooling at the start of the cut and increase rapidly during machin- thickness and chip roughness ratio are used in this paper to
ing compared to the cutting forces with both compressed air and characterise the chip morphology. The chip roughness ratio is
cryogenic compressed air cooling. After cutting a length of 31 m, defined as the ratio of the standard deviation of chip thickness to
Fx, Fy and Fz increase, respectively, about 54%, 41% and 23% the average chip thickness.
during dry machining, 30%, 16% and 6% with compressed air cool- The changes in chip morphology with respect to cutting speed
ing and 17%, 7% and 4% with cryogenic compressed air cooling. and feed rate are plotted in Figs. 9 and 10. The cutting speed was set
Increases in cutting forces with cutting time is believed to be due at 80 m/min in Fig. 9 andthefeedwasfixedat 0.19mmasshownin
to the evolution of tool wear and the development of a built-up Fig. 10. It can be seen that chip thickness increases linearly with
edge. It can be seen clearly in Fig. 8 that flank and crater wear, along feed at constant cutting speed. The application of both compressed
with the chip built-up edge increase in the order of dry machining, and cryogenic compressed air did not significantly affect the chip
compressed air cooled and cryogenic compressed air cooled thickness at a cutting speed of 80 m/min (Fig. 9a).
machining. Furthermore, the black residue on the rake face Chip thickness was found to decrease dramatically with increase
indicates that local burning occurred on the tool s rake face during in cutting speed up to about 50 m/min and became steady with
machining under dry condition and with compressed air cooling. No further increases in cutting speed (Fig. 10a). This variation follows a
black residue was observed on the tool rake face during machining similar trend of cutting force variation with cutting speed. The chip
with cryogenic compressed air cooling, which further supports the is thickest with compressed air cooling, and thinnest with cryogenic
conclusion that the tool chip interface temperature was lower due compressed air cooling at cutting speeds lower than 50 m/min.
to more effective cooling from the cryogenic compressed air. 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
3.3. Chip formation when machining with cryogenic compressed air
speed or feed clearly shows the transition from an irregular seg-
mented chip to regular segmented chip with increase in cutting
The morphology of the segmented chip is normally charac- speed and feed. The ultimate chip roughness ratio is about 0.31
terised in terms of tooth depth and spacing. These parameters are when a regular and periodic segmented chip is obtained at cutting
useful in order to characterise the regular and periodic segmented speed greater than 100 m/min (feed of 0.19 mm) or feed greater
chip; however, these terms are hard to measure when the than 0.214 mm (cutting speed of 80 m/min). The significant
Feed force, Fx (N)
Thrust force, Fy (N)
Main cutting force, Fz (N)
S. Sun et al. / International Journal of Machine Tools & Manufacture 50 (2010) 933 942 937
250 200
Start
Start
Finish
Finish
200
150
150
100
100
50
50
0
0
Cutting conditions Cutting conditions
700
Start
Finish
600
500
400
300
200
100
0
Cutting conditions
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.
difference in chip roughness ratio due to the application of deformation in the primary shear plane and friction between
compressed air and cryogenic compressed air was only observed the cutting tool and chip [31,32]. The cutting temperature during
at feeds between 0.163 and 0.214 mm with a cutting speed of cutting of the workpiece with low thermal conductivity, such
80 m/min (Fig. 9b) and cutting speeds between 30 and 100 m/min as titanium alloys, is very high because the heat generated can-
with a constant feed of 0.19 mm (Fig. 10b). Within these speed not be effectively dissipated by the chip. The high cutting
and feed ranges the transition from a near-continuous chip to a temperature accelerates the tool wear rate and results in shorter
regular segmented chip occurred. The chip roughness ratio is the tool life.
highest with cryogenic compressed air cooling and the lowest The cutting temperature can be reduced through the following
with compressed air cooling in these transition regions, which mechanisms.
means that the tendency to form a segmented chip is the highest
with cryogenic compressed air cooling and the lowest with
compressed air cooling. 4.1.1. 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
4. Discussion
lower friction, but also reduces the heat generated due to plastic
deformation because a larger shear plane angle and shorter shear
4.1. Reduction in chip temperature by utilising an air jet
plane are produced with lower friction [33].
Gaseous lubricants can form a film on the tool surface and this
During machining, the temperature rise in the cutting zone means that the chip flow over the tool surface is easier due
is the result of the heat generation mainly due to plastic to the lower friction at the tool workpiece chip interface [22].
Average feed force, Fx (N)
Average thrust force, Fy (N)
Average main cutting force, Fz (N)
Dry
Dry
machining
machining
Cryogenic
Cryogenic
air
air
Compressed
Compressed
compressed air
compressed air
Dry
machining
Cryogenic
air
Compressed
compressed air
938 S. Sun et al. / International Journal of Machine Tools & Manufacture 50 (2010) 933 942
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.
The reduction in friction force is significant with the application of heat-transfer coefficient and temperature of the coolant. A
high-pressure gases [34,35]. The mechanical effect of bending the dramatic increase in the heat convection coefficient with high-
chip by applying high-speed air onto the top surface of the chip pressure air (E2000 W/m2 K at pressure of 4 7 bar) over
increases the tool chip contact length and this leads to lower conventional dry machining (E20 W/m2 K) [37,38] can explain
pressure on the tool s rake face [36,37]. 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
4.1.2. Removal of heat from the cutting zone
LN2 (TNź 196 1C), which further increases the heat removal rate
A cooling media can only reduce tool flank and tool chip
and results in lower chip temperatures with cryogenic com-
interface temperature by heat convection if the coolant can
pressed air compared to compressed air.
directly access the primary cutting zone [23]. The heat flux (q)
The amount of heat removed from the cutting zone by the
from the hot surface to the coolant is given by
coolant increases by increasing the duration for which the heated
qźhðTw T1Þð1Þ
area is subjected to the coolant. With increase in cutting speed,
where, Tw and TN are the object surface temperature and the convection heat per length of chip decreases due to the
cooling-stream temperature, respectively, and h is the convec- shorter cold air chip interaction time. This leads to a diminishing
tion heat-transfer coefficient. The cooling effect of using the cooling effect on chip temperature using both compressed air and
conventional flood coolant is dramatically reduced at high cutting cryogenic compressed air with increase in cutting speed.
temperature because the convection heat-transfer coefficient is However, the air nozzle is virtually stationery relative to the
dramatically reduced as a result of boiling of the liquid coolant. cutting tool, which is continuously cooled by the cold air in order
When the heat from the cutting zone is removed by heat to remove the heat generated during machining. The reduction
convection, the rate of heat removal depends on the convection in tool temperature from the application of the cryogenic
S. Sun et al. / International Journal of Machine Tools & Manufacture 50 (2010) 933 942 939
0.25
0.19
0.18
Dry machining
Compressed air
0.2
0.17
Cryogenic compressed air
0.16
0.15
0.15
0.14
0.1
0.13
0.12
0.05
Dry machining
Compressed air
0.11
Cryogenic compressed air
0
0.1
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35
0 50 100 150 200 250 300
Feed (mm)
Surface cutting speed (m/min)
0.4
0.35
0.35
0.3
0.3
0.25
0.25
0.2
0.2
0.15
0.15
0.1
0.1 Dry machining
Dry machining
Compressed air
Compressed air
Cryogenic compressed air 0.05
0.05
Cryogenic compressed air
0
0
0 0.05 0.1 0.15 0.2 0.25 0.3
0 50 100 150 200 250 300
Feed (mm)
Surface cutting speed (m/min)
Fig. 9. Influence of feed on the (a) chip thickness and (b) chip roughness ratio at a
Fig. 10. Effect of cutting speed on the (a) chip thickness and (b) chip roughness
constant cutting speed of 80 m/min under different conditions.
ratio at a constant feed of 0.19 mm under different conditions.
compressed air is expected to be substantial compared to the
application of compressed air. However, the increasing rigidity of
reduction in chip temperature.
the chip when cooled using cryogenic compressed air at low
cutting speeds makes the chip bending effect negligible; there-
4.2. Effect of air cooling on chip formation and cutting forces
fore, the reduction in friction is dominant and this leads to a
smaller chip thickness at cutting speeds lower than 50 m/min
Because the air was delivered to the tool chip interface, the with cryogenic compressed air cooling.
penetration of the high-speed air into the tool chip interface The difference in the chip roughness ratio in the transition
reduces the friction between the tool and the chip and this reduc- ranges (cutting speeds between 30 and 100 m/min at a feed of
tion in friction tends to increase the shear plane angle [33,39] and 0.19 mm or feeds between 0.163 and 0.214 mm at the cutting
produce smaller chip thickness. In the meantime the pressure speed of 80 m/min) by application of compressed air and
introduced by the high-speed air forces the chip to bend away and cryogenic compressed air has been investigated by comparing
separate from the cutting edge; this effect results in a marginally the plastic deformation in the chips. The formation of an irregular
lower shear plane angle and larger chip thickness with the segmented chip has been shown to be the result of the mixture of
Chip thickness (mm)
Chip thickness (mm)
Chip roughness ratio
Chip roughness ratio
940 S. Sun et al. / International Journal of Machine Tools & Manufacture 50 (2010) 933 942
continuous shear and localized shear [29]. The saw-tooth depth, at intermediate speed (15 m/min) and back to a segmented chip
which is associated with the chip roughness ratio is dependent on at high cutting speed (53 m/min) have been previously reported
the amount of continuous shear inside a segment. A regular seg- without explanation [31]. This study shows that the reduction
mented chip with a maximum saw-tooth depth or chip roughness in chip roughness ratio and greater continuous chip formation at
ratio is formed when the continuous shear is absent. a cutting speed of 14 m/min compared to cutting speed of
The comparison of the deformed structure in the cross-section 6 m/min under dry machining condition is most probably the
of the chips produced at a feed of 0.163 mm and cutting speed of result of thermal softening of workpiece at the primary shear zone
80 m/min with different cooling approaches is shown in Fig. 11.
The chip produced with cryogenic compressed air cooling shows
0.15
negligible plastic deformation inside the segment (Fig. 11c) while
significant plastic deformation occurred inside the segments of Dry machining
0.14
the chips made with dry machining (Fig. 11a) and compressed air
Compressed air
cooling (Fig. 11b). This plastic deformation within a segment leads
Cryogenic compressed air
to a smaller saw-tooth depth and chip roughness ratio under dry 0.13
and compressed air cooling conditions. Supercooling of the
workpiece using the cryogenic compressed air technique makes 0.12
plastic deformation more difficult during chip formation at the
cutting zone and this leads to a higher tendency for chip segmen- 0.11
tation compared with dry machining and compressed air cooling
at same cutting speed and feed within the continuous chip to
0.1
segmented chip transition range.
The relationship between the chip roughness ratio and cutting
0.09
speed in the low cutting speed region (Fig. 12) shows that the chip
roughness ratio is reduced with increase in cutting speed and
0.08
reaches a minimum value at 14 and 20 m/min for dry machining
and compressed air cooling, respectively, at which point a more
0.07
continuous chip was obtained. The chip roughness ratio increased
dramatically and a more segmented chip was formed with further
0.06
increases in cutting speed. However, the chip roughness ratio did
not change with cutting speeds up to 20 m/min with cryogenic
0.05
compressed air cooling. The changes in chip morphology with
0 10 20 30 40 50
increase in cutting speeds from 6 m/min to 14 m/min with
Surface cutting speed (m/min)
different cooling approaches are shown in Fig. 13.
Similar changes in chip morphology from a segmented chip at
Fig. 12. Effect of cooling conditions on the evolution of chip roughness ratio at low
extremely low cutting speed (0.025 m/min) to a continuous chip cutting speeds with constant feed of 0.19 mm.
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.
Chip roughness ratio
S. Sun et al. / International Journal of Machine Tools & Manufacture 50 (2010) 933 942 941
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.
during chip formation. This result agrees with the observation which is closely related to the frictional force between the chip
that an appropriate softening of the workpiece by laser beam and the cutting tool [8]) over the cutting time. Catastrophic
heating prior to cutting can lead to continuous chip formation breakage of the cutting tool tip may occur when the adhered
during laser assisted machining of Ti 6Al 4V alloy at higher workpiece material is removed by high cutting pressure.
cutting speed compared to conventional machining [40]. How- Fig. 8 clearly shows that cryogenic compressed air cooling
ever, enhanced cooling using cryogenic compressed air reduces reduces the size of the chip built-up edge and tool wear when
the effect of thermal softening of workpiece at the primary shear machining Ti 6Al 4V alloy. These are due to the lower cutting
zone during chip formation; therefore, no reduction in the chip temperature [41]. The lower tool wear and smaller chip built-up
roughness ratio is observed with increase in cutting speeds from edge lower the friction between the cutting tool and chip, which
6 to 20 m/min. leads to a significantly lower increase in the cutting forces during
The cutting force is a function of the yield strength of the the long cut, especially in the X and Y directions with application
workpiece, friction between the cutting tool, chip primary shear of cryogenic compressed air. A detailed quantitative study of the
plane and tool rake angle. The effect of cryogenic compressed air effect of cryogenic compressed air on the tool wear will be
on cutting force is the result of a reduction in friction and an reported in a follow-up publication.
increase in the yield strength of the workpiece. The smaller chip
thickness obtained by application of the cryogenic compressed air
5. Conclusions
compared to dry machining at low cutting speeds indicates that
the higher shear plane angle is obtained with cryogenic com-
The cryogenic compressed air cooling technique described in
pressed air cooling, which may be the result of a reduction in
this paper provides a safer and more readily applicable technique
friction between the chip and the tool [33,39].
for improving the machinability of Ti 6Al 4V alloy. The technique
The extremely low temperature of the cryogenic compressed
delivers high-speed air at extremely low temperatures into the
air not only cools the cutting tool, but also reduces the tempera-
cutting zone. Compared to dry machining, the cryogenic com-
ture of the workpiece before it enters the cutting zone. Similar
pressed air cooling makes the following changes in machining:
to the increase in the main cutting force during machining of
Ti 6Al 4V alloy with LN2 impingement cooling compared to that
(1) Reduction in chip thickness by approximately 9% at cutting
in dry machining [8], the marginally higher cutting force at low
speed lower than 50 m/min due to improvements in friction
cutting speeds with application of cryogenic compressed air
effects between the chip and the tool.
compared with dry machining can be attributed to the lower
(2) An increase in cutting forces by about 6% at cutting speed
temperature of the workpiece and higher shear plane angle. The
lower than 50 m/min because of a higher shear plane angle
cooling of the workpiece, reduction in chip thickness and increase
and lower chip temperature.
in the shear plane angle by cryogenic compressed air are reduced
(3) Changes in the characteristics of the segmented chip and the
with increase in cutting speed. Therefore, there is no significant
transition from irregular to regular segmented chips. The
difference in cutting forces observed between dry machining and
cutting speed is about 20 m/min lower and feed is about
machining with compressed air and cryogenic compressed air
0.04 mm smaller at the same chip roughness ratio of 0.25 in
cooling at high cutting speed for a short cut length.
the chip morphology transition region with cryogenic com-
The high temperatures at the cutting zone often result in the
pressed air compared with dry machining.
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 The dramatic reduction in chip temperature with both com-
wear; however, the oversized chip built-up edge may result in pressed air and cryogenic compressed air cooling at all cutting
accelerated attrition wear and high friction coefficient, which lead speeds indicates that there is a reduction in cutting temperature,
to a rapid increase in cutting forces (especially the feed force, which leads to a smaller chip built-up edge and lower tool wear.
942 S. Sun et al. / International Journal of Machine Tools & Manufacture 50 (2010) 933 942
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Research Centre and Defence Materials Technology Centre
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