Thermally enhanced machining of hard to machine materials

ARTICLE IN PRESS
International Journal of Machine Tools & Manufacture 50 (2010) 663 680
Contents lists available at ScienceDirect
International Journal of Machine Tools & Manufacture
journal homepage: www.elsevier.com/locate/ijmactool
Review
Thermally enhanced machining of hard-to-machine materials A review
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, Australia
b
School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University, Victoria, Australia
c
School of Mining and Mechanical Engineering, University of Queensland, Queensland, Australia
d
CAST Cooperative Research Centre, Australia
e
Defence Materials Technology Centre, Australia
a r t i c l e i n f o a b s t r a c t
Article history:
Thermally enhanced machining uses external heat sources to heat and soften the workpiece locally in
Received 16 February 2010
front of the cutting tool. The temperature rise at the shear zone reduces the yield strength and work
Received in revised form
hardening of the workpiece, which make the plastic deformation of hard-to-machine materials easier
19 April 2010
during machining.
Accepted 21 April 2010
This review summarizes the up-to-date progress and benefits of thermally enhanced machining
(with a focus on laser and plasma assistance) of ceramics, metals and metal matrix composites. It covers
Keywords:
the integration of the external heat source with cutting tools, analysis of temperature distribution
Thermally enhanced machining
around the cutting region, material removal mechanisms, tool wear mechanisms and the improvement
Heat source
in machinability of various engineering materials by the assistance of external heat source.
Machinability
& 2010 Elsevier Ltd. All rights reserved.
Material removal temperature
Cutting force
Tool wear
Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 664
2. Heat sources. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 664
3. Integration of a laser beam with machining applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 665
3.1. Turning. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 665
3.2. Milling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 665
3.3. Other machining operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 666
4. Temperature distribution due to external heating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 666
5. Thermally enhanced machining of hard-to-machine materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 669
5.1. Ceramics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 669
5.1.1. Material removal mechanisms and chip formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 669
5.1.2. Cutting forces and specific cutting energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 671
5.1.3. Tool materials and wear . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 671
5.1.4. Surface integrity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 672
5.2. Metals. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 673
5.2.1. Titanium alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 673
5.2.2. Nickel-based superalloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 673
5.2.3. Hardened steels and other metals. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 674
5.3. Metal matrix composites (MMCs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 675
6. Optimization and energy efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 676
7. Non-traditional laser assisted machining . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 677
7.1. Removal of material by the laser in front of the cutting tool [113,114]: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 677
7.2. Modification of microstructure in the to-be machined layer by the laser [115] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 677
8. Economic benefits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 677
n
Corresponding author at: IRIS, Faculty of Engineering and Industrial Sciences, Swinburne University of Technology, Hawthorn, Victoria, Australia.
E-mail address: ssun@groupwise.swin.edu.au (S. Sun).
0890-6955/$ - see front matter & 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.ijmachtools.2010.04.008
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9. Concluding remarks and suggested future research topics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 677
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 678
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 678
1. Introduction rise of the heated surface. The laser beam or plasma heats the
workpiece locally in front of the cutting tool, only the volume of
material to be removed is effectively heated (Fig. 2). Therefore, the
The demand for higher strength and heat resistant materials is
heat affected zone and thermal distortion by the laser beam
increasing particularly in aerospace applications. However these
during preheating are small due to the controllable spot size and
materials are often difficult to machine due to their physical and
high power density. However, the low absorption of the laser
mechanical properties such as high strength and low thermal
conductivity, which make the cutting forces and cutting tem- energy (especially the CO2 laser with a long wavelength of
10.6 mm) by the metallic workpiece, such as Inconel 718 [10,13]
perature very high and lead to a short tool life.
and steels [25,26], needs improving by applying a coating.
Because the flow stress and strain hardening rate of materials
A large thermal gradient through thickness exists due to the
normally decrease with increasing temperature due to thermal
local heating by the laser beam and plasma. It is hard to
softening (shown in Fig. 1 as the dependence of strength on
characterize the effect of preheating temperature on the im-
temperature), thermally enhanced machining (TEM) becomes a
provement of machinability, therefore, a comprehensive analysis
possibility when machining the hard-to-machine materials.
of the temperature distribution of the material before it enters the
Thermally enhanced machining is the process that uses an
cutting zone is required. The large thermal gradient enables the
external heat source to heat and soften the workpiece. As a result,
the yield strength, hardness and strain hardening of the work- process to take the benefit of high surface temperature without
affecting the integrity of the machined subsurface.
piece reduce and deformation behaviour of the hard-to-machine
This review of the thermally enhanced machining is focused on
materials (especially ceramics) changes from brittle to ductile.
the laser assisted machining (LAM) and plasma enhanced
This enables the difficult-to-machine materials to be machined
machining (PEM). Some results of the effect of temperature are
more easily [2,3] and with low machine power consumption,
from other heat sources.
which leads to increase in material removal rate and productivity.
2. Heat sources
Chip
In order for it to be applied effectively during the thermally
enhanced machining, the heat source should be local, rapid
heating and be controllable. The external heat source used in TEM
Cutting
Laser beam
to heat the workpiece has been plasma [4 9], laser beam [2,10 13],
tool
gas torch [14 16], induction heating [17 20], furnace preheating
[21], electrical current heating [22] or electric arc. A comparison
Heat affected zone
of the advantages and disadvantages of these heat sources can be
found in literature [23,24]. Inappropriate application of the heat
source may introduce undesirable microstructural change in the
workpiece after machining [6,14,16].
The advantages of laser beam and plasma arc heating over the
other heating methods are the rapid, high and local temperature
2000
ODS: Oxide Dispersion Strengthened
Workpiece
Ni-Cr-Mo-steel
1600
Inconel 718
Grinding Wheel
1200
Ti6Al4V
Laser
Si3N4-ceramics (HPSN)
beam
800
ODS-alloy
400
Stellite 6
200 400 600 800 1000 1200 1400
Workpiece
Temperature T (�C) Heat affected zone
Fig. 1. Effect of temperature on the ultimate tensile strength for various hard-to- Fig. 2. Illustration of laser assisted machining in (a) turning and (b) grinding
machine materials [1]. operations [2].
Tensile strength (MPa)
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3. Integration of a laser beam with machining applications machined surface and the tool is not in direct contact with the
laser heated workpiece surface.
The laser spot size is required to fully cover the chamfer
The laser beam is positioned in such a way so as to achieve a
surface to achieve uniform reduction in the cutting forces in the X,
uniform temperature distribution at the cutting edge without heating
Y and Z directions [32], however, even partial coverage of the
the cutting tool. This position depends on the machining operations.
chamfer surface by the laser beam close to the machined surface
can dramatically reduce tool wear [13].
3.1. Turning
Shin and colleagues [10,25,26,35] used multiple distributed
laser units simultaneously heating both the unmachined surface
For turning operations, it is relatively simple to integrate a
and the chamfer surface (as shown in Fig. 5) to create the desired
laser beam with the lathe due to the stationery nature of the
temperature distribution through the depth of cut in the
cutting tool. The laser beam variables are its position, spot size,
workpiece which is reported to result in longer tool life.
incident angle and tool beam distance.
The position of the laser beam relative to the tool is critical.
In most of the reported work on laser assisted turning, the
Tool beam distance along with cutting speed determines the time
laser beam is normal to the workpiece surface [12,27 31], as
interval between the laser heating and machining operation and
shown in Fig. 3. The advantage of this arrangement is that the
therefore the temperature distribution at the cutting zone. It is
machining is easy and does not result in heating of the machined
found that the larger reduction in the cutting force is achieved
surface, but the large temperature gradient through thickness
with the laser spot positioned closer to the cutting tool when
exists at the cutting edge and the temperature might not be high
cutting hardened steel [34], commercially pure titanium [32] and
enough at the depth of cut for a deep cut. Furthermore, the cutting
high chromium white cast iron [36]. However, if the tool beam
tool surface is exposed to the heated workpiece surface. This may
distance is too short, the tool may be damaged by overheating, the
be detrimental for cutting a workpiece material which has a high
chips may fly into the laser beam, become molten and drop onto
chemical reactivity with the cutting tool.
the machined surface [37]. Therefore, the tool must be kept at a
Alternatively, the laser beam can be incident and normal to the
minimum distance from the laser beam.
chamfer surface [3,13,32] (as shown in Fig. 4). Higher and uniform
reduction of cutting forces in the 3 directions is achieved with
this configuration [13,32 34]. With appropriate control of the
3.2. Milling
beam position on the chamfer surface, there is no change in
microstructure from the heat remaining in the vicinity of the
Integration of the laser beam with a milling machine as the
tool is rotating is a complicated task. Generally, the beam can be
arranged separately to the tool or integrated with the spindle.
For surface milling, the easiest way is to set the beam in front
of the tool in the feed direction as shown in Fig. 6. The limited
spot size of the external heat source limits the ability to cover the
width of cut by a single spot in most applications. Therefore, the
beam can only heat part of the cut width. This could be the middle
(position 1) [38] or tool entry point (position 2) or both entry and
Laser Spot
exit points (position 2 and position 3) by two beams [6,39]. A high
power laser, multiple beams or a line beam are required to cover
the width of the cutting zone (region 4) [1,39 41].
Since the dynamic impact on the cutting tool, as the rotating
Chip
tool intermittently engages and disengages with the workpiece at
the entry and exit points, respectively, causes significant vibration
and ultimately tool fracture during milling operations, laser beam
heating at the entry point is more significant for longer tool life
Tool
and a reduction in chatter.
If the beam is moved by an additional motor, flexibility can be
achieved by positioning it in alignment with the feed direction of
the cutting tool [40] or by scanning it to cover the width of the
Fig. 3. LAM set-up with the laser beam normal to the workpiece surface [30]. cutting zone.
L1
Beam

L2 Z
Z
Y
X
workpiece
workpiece
Tool
Fig. 4. The relative position of the laser beam, workpiece and cutting tool in laser assisted turning operation: (a) end-view and (b) side-view [32].
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Instead of heating the top surface of the workpiece with a surface integrity [52]. It is found that the temperature is the
beam separated from the spindle, two concepts were proposed to predominant variable affecting the laser assisted machining
integrate the beam with the machine tool [42 44]. Fig. 7 shows process, as a temperature which is too high may prematurely
the design proposed for in-situ laser materials processing degrade the cutting tool and may cause workpiece subsurface
(hardening, marking, etc.) during milling operations. The damage, while a temperature which is too low will not realize the
integration of the laser beam with the machining tool can be maximum benefit of LAM [25].
achieved by (1) coupling the beam in front of the spindle by an The material removal temperature, Tmr, which is defined as the
interface radial to the tool axis; (2) the laser beam is delivered average temperature of the material as it enters the shear
axially through the spindle and the end beam tool can be rotated deformation zone [38], plays a key role in the LAM process. This
with the spindle as shown in the figure. It is possible to perform needs to be controlled to achieve the full benefits of laser assisted
laser assisted milling with this integrated set-up.
machine tool
3.3. Other machining operations
main spindle
clamping device
laser tool
Laser preheating has been successfully integrated with planing
of Al2O3 [45], burnishing of steels shown in Fig. 8 [46], dressing
laser beam
[47 49], grinding [50] and drilling [51] in which the laser beam
process gas
locally heats the workpiece in front of the planing, burnishing and
dressing tools and through the centre-hollow drill tool. (axial)
(radial)
4. Temperature distribution due to external heating
laser beam
process gas
rotating union
machine tool
The operating conditions of LAM need to be controlled within
an optimum range for each given workpiece material to achieve
Fig. 7. Concept of radial and axial beam delivery in a milling machine [42,43].
the best machining results in terms of longer tool life and better
Optical Fiber
First laser heating
Burnished Surface
the workpiece
Second laser heating
the chamfer Laser Optic
Laser Beam
Tool and
VZ Unburnished
Surface
tool holder
Dynamometer
VZ
Embedded Springs
Workpiece
Workpiece
Chamfer
Burnishing Roller
Fig. 5. Laser assisted turning utilizing two laser beams [35].
Fig. 8. Set-up for laser assisted burnishing [46].
Vc spindle
beam
2
1
4
Vc
beam
tool
3
Vf
Vf
workpiece
top view
side view
Fig. 6. Illustration of the integration of the beam with the cutting tool for surface milling.
ARTICLE IN PRESS
S. Sun et al. / International Journal of Machine Tools & Manufacture 50 (2010) 663 680 667
machining without melting the workpiece or leaving undesirable where, k is the thermal couductivity; r is the density; cp is the
microstructural alteration in the machined workpiece. Because specific heat; o is the workpiece rotation speed; Vz is the feed
000
the deformation behaviour of the workpiece material is strongly speed and q is the volumetric heat generation associated with
dependent on the temperature, it is therefore essential to know material removal process and accounts for boundary conditions
the temperature distribution from the surface through the [63]. It includes the heat generated due to the friction between the
thickness up to the cutting edge and optimize it in accordance tool s flank face and workpiece and the heat generated due to
with the machining parameters. plastic deformation at primary shear zone. The heat generated at
The surface temperature of the workpiece subjected to laser tool s rake face due to the friction and plastic deformation at the
radiation can be measured by high speed and high-spatial secondary shear zone which is conducted into the workpiece is
resolution pyrometers [27,53 56] and infrared cameras negligible compared to the thermal energy advected away from the
[10,41,57 59]. The optimum surface heating temperature by workpiece with the heated chip [61]. However, the effect of heat
the laser is determined by the tool wear resistance and is about generated due to plastic deformation and friction on workpiece
1600 1800 K for Mo and 1550725 K for W [53]. temperature distribution is negligible compared to the energy
A transient, three dimensional heat transfer model of the deposited by laser radiation [28].
thermal response of the rotating opaque cylindrical workpiece External heating of the workpiece leads to high temperatures
subjected to laser radiation with and without the material removal at the shear zone, which results in a reduction in the shear flow
processes has been developed by Rozzi et al. [27,60 62] as shown stress of the workpiece. Therefore, heat generation due to plastic
in Fig. 9. The governing equation for transient heat transfer in a deformation in the shear zone during LAM is reduced compared
rotating cylinder in a cylindrical coordinate system is written as with conventional machining process [12].
By setting the appropriate boundary conditions (listed in
1 @ @T 1 @ @T @ @T
Table 1), the temperature fields as a result of laser radiation only
kr � k � k �q000
r@r @r r2@f @f @z @z
(without material removal), laser heating and cutting (with
material removal) can be calculated.
@T @T @T
źrcpo �rcpVz �rcp �1�
The model analyses were validated by comparing the pre-
@f @z @t
dicted surface temperature histories with the measured tempera-
ture using a pyrometer and infrared camera over a wide range of
operating conditions as shown in Fig. 10 for both with and
without material removal processes [27,61], or by comparing the
depth of the heat affected zone for some alloys [64,65].
The validated thermal model reveals the temperature distribu-
Focusing
tion on the surface and through thickness (Figs. 11 and 12). It shows
Optics
a large temperature gradient in all three coordinate directions. With
the beam incident on the workpiece surface, the temperature
gradient exists through the depth of cut and becomes larger with the
Vz
greater depth of cut at the cutting plane. The unmachined surface
with high temperature contacts directly with the cutting tool, which
Laser Spot
Workpiece may be detrimental for cutting chemically reactive materials with
(Ć = Ćc)
the cutting tool at high temperature.
Ć
Finite element analyses on temperature distribution have also
v
z=Lw been carried out on the laser assisted turning of the cylindrical
zc,0 workpiece of partially-stabilized zirconia (PSZ), a semi-transpar-
ent ceramic [28], mullite [56], Si3N4 ceramic [29], Si3N4 ceramics
with complex machined features [59], Inconel 718 alloy [10],
z
Plane of Interest (z=zc
)
austenitic stainless steel [25] and laser assisted milling of Si3N4
[41,66] and Inconel 718 alloy [41]. Leshock et al. [5] performed
r
the thermal model analysis of temperature distribution during
plasma enhanced machining. The effect of parameters on surface
rw
z=0
temperature can be identified with the model analysis.
Due to its importance for thermally enhanced machining, the
material removal temperature of the workpiece (Tmr), or surface
q'' + q''
conv rad
Area of Jet Interaction
temperature (Ts in K) as a function of processing parameters has
been obtained empirically with thermal model analysis and
Laser Spot (Ćc,zc )
experiments. It can be expressed as:
Ll Chamfer
for Inconel 718 alloy with plasma heating [8]:
Ćl
q'' + q''
conv rad
0:06
I0:58T0
Ć
Tsź80:3 �2�
0:2 0:2
Vc D0:41f
w
Vz
for Inconel 718 alloy with two lasers (CO2 and Nd:YAG lasers)
�
q'' + q'' [10]:
conv rad
r
q'' 0:086 0:031
z
mr
PCO PYAG
2
Tmrź27890 �3�
Material
0:32
D0:95f
w
Removal Plane
z = z (t > tp)
fe
for Ti-6Al-4V alloy with single laser (CO2 laser only) [67]:
Fig. 9. Coordinate systems for the rotating workpiece during (a) laser heating 0:66
e3:4PCO
2
without material removal [27] and (b) laser assisted machining with material
Tmrź �4�
0:31 0:31
f D0:34Vc
removal [61]. w
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668 S. Sun et al. / International Journal of Machine Tools & Manufacture 50 (2010) 663 680
Table 1
Boundary conditions for the thermal model analysis [27,61].
Without material removal With material removal
Workpiece surface
@T
k źq00 q00 E�T��asurGsur�Tsur�
l,abs conv
@r
rźrw
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2
for rw�f fc� ��z zc�2 rrl
@T
k ź q00 E�T��asurGsur�Tsur�
conv
@r
rźrw
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2
for rw�f fc� ��z zc�2 4rl
Workpiece surface unmachined
@T
k źalq00 q00 E�T�
l conv
@r
rźrw
for z4zch�f� and fl�r,z,f�r1 on laser spot
@T
k ź q00 E�T�
conv
@r
rźrw
for z4zch�f�and fl�r,z,f�41 off laser spot
Workpiece surface machined
@T
k źal,mq00 q00 E�T�
l conv
@r
rźrw,m
for zozch�f�and fl�r,z,f�r1 on laser spot
@T
k ź q00 E�T�
conv
@r
rźrw,m
for zozch�f�and fl�r,z,f�41 off laser spot
Centerline of workpiece
@T @T
ź0 ź0
@r @r
rź0 rź0
Interface between machined and unmachined material
@T
k źq00 �E�T�
conv
@z
zźzch�f�
for rw,m rr rrw and 0ofr2p fflank
@T
k ź q00
flank
@z
zźzch�f�
for rw,m rr rrw and 2p fflank ofr2p
End faces of workpiece
@T @T
k źq00 �E�T� asurGsur�Tsur� k zźzfe,0�t rtp�źq00 �E�T�
conv conv
@z @z
zź0 zźzfe�t 4tp�
@T
@T
k ź0
ź0
@z
zźLw @z
zźLcv
Material removal plane
k @T
źrcpro�T Tref�
r @f fź0
Circumferential direction T�r,f,z,t�źT�r,f�2p,z,t� T�r,f,z�źT�r,f�2p,z�
@T @T @T @T
ź ź
@f f @f f�2p @f f @f f�2p
Initiation T(r,f,z,0)źTNźTsur T(r,f,z,0)źTNźTsur
for hardened steel with single laser (CO2 laser only) [26]: mainly heats the machined chamfer surface where no absorptiv-
ity enhanced coating is applied, while CO2 laser (as the first laser
0:85
PCO
2
in Fig. 5) heats mainly the workpiece surface which is pre-coated
Tmrź1:8 �5�
0:47 0:47
Vc f
with an absorptivity enhanced coating.
for hardened steel with two lasers (CO2 and Nd:YAG lasers) The empirical expressions of material removal temperature
where PCO ź1100W [26]: show that the temperature increases with laser power in LAM or
2
plasma current in PEM and reduces with increasing cutting speed
0:29
PYAG
Tmrź152:7 �6� (except in the case of Inconel 718 where the material removal
0:45 0:47
Vc f
temperature is independent of cutting speed), diameter of
where, I is the plasma current (Amp), T0 is the initial workpiece workpiece and feed rate. The reduction in material removal
bulk temperature (K), PCO and PYAG are the laser power (W) for temperature with increasing cutting speed is due to the reduction
2
the CO2 laser (incident normal to the workpiece surface axially of beam workpiece interaction time, therefore, the benefit of
and 451 [10] or 551 to tool radially) and Nd: YAG laser (incident LAM, such as force reduction and longer tool life, reduces with
451 to the workpiece surface axially and 10 161 to tool radially), increasing cutting speed [12,32,34].
respectively as shown in Fig. 5, Dw is the diameter of the Since laser heating increases the temperature of the workpiece
workpiece (mm), f is the feed (mm) and Vc is the cutting speed before cutting, phase transformations may occur in the heated
(m/min). area. This area is detrimental if it remains after cutting for some
The contribution by the power of Nd:YAG laser to the material alloys for which the fatigue life is sensitive to the hardness and
removal temperature is smaller than that by the power of CO2 microstructure in the subsurface such as the Ti-6Al-4V alloy [6].
laser because the Nd:YAG laser beam (as the second laser in Fig. 5) Simulation can predict the thickness of the phase transformation
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S. Sun et al. / International Journal of Machine Tools & Manufacture 50 (2010) 663 680 669
Heating Start
VZ Heating End
Point Point
Machining End Machining Start
VZ
15 mm Point Point
CO Laser
2
Nozzle
10 mm
12.8 mm
Ll =1 mm
9.4 mm
7.8 mm
Cutting
Hj=10 mm
6.4 mm
2.8 mm
Tool
3.4 mm
M1 M2 M3
v
M4 Laser Pyrometer spot
N4
Diameter = 0.3 mm
M5
N3
N2 N1
1.4 mm
5.53 mm
1600
1800
Position Experimental Numerical Position Experimental Numerical
M1
N1
1600
1400
M2
N2
M3 N3
1400
1200
1200
1000
1000
800
800
600
v = 1000 rpm
600
VZ = 10 mm/min
Pl = 250 W
400
400
V =1000rpm
Dl = 2.5 mm
VZ =100mm/min d = 1 mm
200 Pl =500W Ll = 1 mm
200
Dl =3mm tp = 10 s
0
0
0 1 2 3 4 5 6 7 8 9 0 20406080
Time (s) Time (s)
Fig. 10. Comparison of the model predicted temperature and the measured temperature on the surface of Si3N4 cylinder in (d) laser assisted machining with material
removal [61] and (c) laser heating without material removal [27] at the selected locations shown in (b) and (a), respectively.
layer and determine the appropriate laser power to ensure that due to their high temperature strength, low density, thermal and
the layer associated with the phase transformation can be later chemical stability, and good wear resistance. Ceramics are
removed by machining [64,65,68]. difficult to machine using conventional machining techniques
because of their hardness and brittleness [69]. Machining of these
ceramics is a high cost process because of a short tool life and this
5. Thermally enhanced machining of hard-to-machine
often results in surface cracking and subsurface damage.
materials
It has been demonstrated that thermally enhanced machining
5.1.1. Material removal mechanisms and chip formation
improves the machinability of a variety of hard-to-machine
Generally, ceramics are brittle and plastic deformation does
materials in terms of a reduction in cutting forces and specific
not occur during material removal. However, both the strength
cutting energy, better surface finish and longer tool life. The
and brittleness of ceramics, such as silicon nitride, reduce at high
workpiece materials include ceramics, metals and metal matrix
temperature (as shown in Fig. 1) due to the softening of a glassy
composites.
phase at the grain boundaries. When the cutting tool is engaged
with the laser heated workpiece, the material is removed mainly
5.1. Ceramics
due to the combination of brittle fracture and plastic deformation
[45,63,70 72]. The material removed during LAM of silicon nitride
Advanced engineering ceramics are increasingly being used in is due to: (1) plastic deformation in the shear zone which is
automotive, aerospace, military, medical and other applications characterized by viscous flow of a glassy grain-boundary phase
Temperature (�C)
Surface Temperature (�C)
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670 S. Sun et al. / International Journal of Machine Tools & Manufacture 50 (2010) 663 680
Fig. 11. The predicted temperature (1C) distributions on (a) the surface and
through thickness at (b) the cutting plane (fź01) and (c) laser centre (fź551)
during LAM of PSZ [28].
Fig. 12. Temperature distribution through the thickness at (a) minimum and
(b) maximum depths of cut due to the complex features of the machined
material and reorientation of the b-Si3N4 grains and (2) segmenta-
component during LAM of Si3N4 ceramic [59].
tion of chips due to the initiation, coalescence and propagation
of intergranular microcracks [71,72]. Flow and redistribution of
the intergranular glassy phase is also found to hold the Al2O3
grains and maintain plastic deformation when the temperature is occurs during chip formation in addition to brittle fracture [63].
above 850 1C (glassy transition temperature) during LAM of Al2O3 The plastic deformation is found to be the result of high
ceramic [45]. temperature dislocation motion and dynamic recrystallization
There are 3 types of chip formed during LAM of silicon nitride that leads to a smaller grain size and preferred grain orientation
and mullite as shown in Fig. 13. It is found that the average near- in the machined surface produced by LAM [63,74].
chamfer surface temperature (Ts,ch) [73] or average material A multiscale finite element modelling of LAM of Si3N4 ceramic
removal temperature (Tmr,se) and ratio of feed force to main developed by Tian and Shin [75] showed that microcracks initiate
cutting force (Ff/Fc) [70] plays a key role in chip formation for and propagate under the loading of tool at material removal
silicon nitride and mullite as listed in Table 2. Plastic deformation temperature of 1200 1C. The microcracks propagate and coalesce
during chip formation occurs when the temperature is higher into a macrocrack in the shear zone, which produces a
than 1151 1C for silicon nitride and 1000 1C for mullite and force discontinuous chip with a sharp drop of main cutting force. The
ration Ff/Fc is lower than 1. discontinuous chip is formed mainly due to failure of the
Despite the fact that there is no continuous and semi- intergranular glassy phase, no grain reorientation in the chip
continuous chip formed with LAM of PSZ, plastic deformation and small amount of deformation are found in the chips.
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S. Sun et al. / International Journal of Machine Tools & Manufacture 50 (2010) 663 680 671
5.1.2. Cutting forces and specific cutting energy All three components of the cutting forces are independent of
The cutting zone stress is found to decrease with increasing cutting time (or tool wear), which is believed to be due to the
material removal temperature and feed when LAM of Si3N4 but is formation of thin glassy workpiece material on the wear land of
not significantly dependent on the cutting speed during LAM [71]. the tool flank face which acts as a lubricant on the wear land [71].
The cutting forces and specific cutting energy are reported to
decrease with increasing laser power or surface temperature
when cutting ceramics such as mullite [70], magnesia-partially
stabilized zirconia (PSZ) [63] and silicon nitride [38,72,73,76]
because of increasing material removal temperature with laser
power, but are not significantly affected by the laser-tool lead
distance Ll [73].
Not only the cutting forces but also the ratio of the feed force
(Ff) to the main cutting force (Fc) decreases with increasing laser
power during LAM of PSZ [63] and mullite [70]. The Ff/Fc ratio of
less than 1 achieved at high laser power proves that quasi-plastic
deformation in the chip occurred during machining due to the
softening of the workpiece.
5.1.3. Tool materials and wear
A polycrystalline diamond (PCD) tool is found not to be
suitable for LAM because of its low carburizing temperature
(900 1C)[63]. Polycrystalline cubic boron nitride (PCBN) has been
used for LAM of silicon nitride [72,73] and PSZ [63]. A carbide
insert has been used for LAM of mullite [70] and Al2O3 [77]
ceramics. A PCBN tool shows significantly longer tool life than a
tungsten carbide tool when LAM PSZ at the same cutting
conditions [63]. But Klocke and Bergs [76] showed that flank
wear is smaller using a PCD tool than that using a CBN tool when
laser assisted machining silicon nitride.
Abrasion, adhesion and diffusion mechanisms are attributed
to the tool wear during laser assisted turning of PSZ [63]. Abrasion
of the deposited workpiece material on both primary and
secondary flank faces results in the grooves on the flank face
(as shown in Figure 14b). Edge crater was not observed during
LAM of Si3N4 with PCBN insert at high temperature [72], but was
observed during LAM of PSZ at material removal temperature of
1000 1C (Figure 14d). This difference is inconclusively attributed
to the lower ductility and smaller thermal diffusivity of PSZ,
which lead to severer diffusive and abrasive wear during LAM of
PSZ compared with LAM of Si3N4 [63]. The tool life during LAM
of mullite is much longer than that during conventional
machining [70]. Flank wear is found to be the dominant tool
failure mode during laser assisted turning of Si3N4, which is
attributed to the adhesion of the glassy grain-boundary phase to
the cutting tool. The bonding between the glassy phase and
cutting tool may be broken during machining and this results in
the tearing of the cutting tool material [72].
Repeatedly, tool wear is strongly dependent on the material
removal temperature, and there is an optimum material removal
temperature for the longest tool life. A short tool life is due to an
insufficient reduction in workpiece strength when the material
Fig. 13. Morphology of chips formed during LAM of silicon nitride, (a) fragmented,
(b) semi-continuous and (c) continuous [73]. removal temperature is lower than the optimum temperature and
Table 2
Chip morphology, formation mechanisms and formation conditions for LAM of silicon nitride [73] and mullite [70].
Workpiece material Morphology Formation mechanisms Conditions
Silicon nitride Fragmented Brittle fracture Ts,cho1151 1C
Semi-continuous Local brittle fracture and plastic deformation o1151 1CoTs,cho1305 1C
Continuous Plastic deformation Ts,ch41329 1C
Mullite Brittle fracture, semi-continuous Brittle fracture and plastic deformation Ff=Fc 41 8001CoTmr,se o10001C
Semi-continuous Plastic deformation Ff=Fc o1 10001CoTmr,se o13001C
Continuous Plastic deformation Ff=Fc o1 Tmr,se 413001C
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672 S. Sun et al. / International Journal of Machine Tools & Manufacture 50 (2010) 663 680
Fig. 14. (a) Schematic of tool wear, wear on the (b) primary flank face, (c) nose and (d) rake face during LAM of PSZ [63].
a reduction in tool strength by overheating when the material by the built-up of irregular glassy phase debris and the formation
removal temperature is higher than the optimum temperature, of cavities due to b-Si3N4 grain pullout [72,73]. Similar to LAM
respectively [70]. Therefore, tool life can be optimized by maintain- of PSZ [63], the surface roughness is not sensitive to the material
ing the material removal temperature in the optimum region. removal temperature, but depends on the size and distribution of
The tool failure mode for laser assisted milling of Si3N4 is edge silicon nitride grains.
chipping at workpiece temperatures lower than the softening Local cracks present in the heat affected zone without cutting
point of the glass phase due to the high and frequent dynamic indicate that the laser heating cycle introduces high thermal
impact on the cutting edge when the tool is intermittently stresses in PSZ [63]. The thickness of the cracking region increases
engaged with the workpiece during cutting. Gradual flank wear is with material removal temperature (laser power). If this thickness
the dominant tool failure mode at high workpiece temperature is greater than the depth of cut, the crack remains in the
and the flank wear is significantly reduced with increasing subsurface, which is detrimental for the properties of the
workpiece temperature up to a point, further increases in machined part. Therefore, the material removal temperature
temperature has less or a negative influence on the reduction in must be controlled to produce a damage-free component by LAM.
tool wear [38,41]. The maximum workpiece temperature allowed The sudden impact and stress release between cutting tool and
for reducing tool wear in laser assisted milling is higher than that workpiece when the cutting tool enters and leaves the workpiece
in laser assisted turning because of the lower temperature of the during milling of ceramics (Si3N4) result in workpiece edge
insert as a result of the shorter tool chip contact time during the chipping at both the entry and exit edges. The workpiece edge
milling operation [38]. chipping leads to poor dimensional and geometric accuracy and is
Similar to the milling process, the pressure on the cutting tool the source of cracking. Rise in workpiece temperature at the
when the tool is engaged with the workpiece during planing cutting zone during laser assisted milling of Si3N4 can eliminate
operation is high and often results in severe abrasion and larger both the macroscale entry and interior edge chippings, but the
area fracture on both the flank and rake faces. Laser heating prior macroscale exit edge chipping can not be avoided completely,
to cutting effectively softens the workpiece (Al2O3) and no however, it can be reduced significantly due to the increasing
fracture was observed on both the flank and rake faces of the workpiece temperature during laser assisted milling. Reduction in
cutting tool [45]. workpiece edge chipping is the result of the coupled effects of
softening and toughening mechanisms. Both softening and
toughening of the workpiece take positive effect when tempera-
ture is above the brittle/ductile transition temperature and below
5.1.4. Surface integrity
the global softening temperature (between 1300 and 1400 1C)
The integrity of the machined surface is determined by
[78].
measuring the surface roughness and surface or subsurface damage.
Compressive residual stress is observed on the laser assisted
Above a certain temperature, the machined surface of silicon
machined surface of Si3N4 in both the axial and hoop directions
nitride produced by the laser assisted machining is characterized
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S. Sun et al. / International Journal of Machine Tools & Manufacture 50 (2010) 663 680 673
[57]. However its magnitude is smaller than that produced by and the cobalt-diffusion controlled crater wear is minimized at the
conventional grinding [57,76] because the softening of the glassy cutting speed of 107 m/min [67,91]. However, higher temperatures
phase significantly relieves the stress at the material removal result in shorter tool life compared to conventional machining due
zone [57]. to the acceleration of Co diffusion. At this optimum material removal
Klocke and Bergs [76] reported that the bending strength and temperature, the tool life in terms of the volume of material
the Weibull modulus of Si3N4 components made by LAM are removed during LAM is shorter than that in conventional machining
better than those produced by conventional grinding. at cutting speed higher than 107 m/min. The hybrid machining, in
which a reservoir cap with liquid nitrogen (LN2) passage is built to
cool the tool s rake face, improves the tool life significantly at all the
5.2. Metals cutting speeds. Furthermore, the improvement of the tool life by
applying a TiAlN coating in the hybrid machining is more significant
5.2.1. Titanium alloys than that in the conventional machining. In addition to the local
Titanium and its alloys have seen increasing demand in the softening of workpiece by laser heating, the reduction in the specific
aerospace industry in recent years due to their superior properties, cutting energy during hybrid machining is attributed to (1) the
such as excellent strength-to-weight ratio, strong corrosion enhanced cooling of the cutting tool to maintain its hardness and
resistance and ability to retain high strength at high temperature strength; (2) the lower tool wear rate due to the lower tool-chip
[79,80]. This demand has resulted in the requirement to increase interface temperatures. The lower friction between the tool flank
machining speed and consequently the material removal rate. face and the machined surface improves the machined surface
The poor machinability of titanium alloys is mainly due roughness [67].
to their low thermal conductivity, high chemical reactivity to Tool life is reported to be shortened when milling Ti-6Al-4V
tool materials and low modulus of elasticity, all of which result alloy with plasma as the heat source because the cutting tool is
in high cutting temperatures, short tool life and high vibration exposed to higher temperatures and this results in rapid
during machining. These limit the cutting speed and productivity. degradation of the tool [6].
Flank wear and crater wear are the main tool failure modes However, Ginta et al. [20] reported that tool life increases with
as the results of dissolution diffusion, adhesion and attrition increasing workpiece temperature up to 650 1C by induction
mechanisms [81 83]. The cobalt diffusion leads to the pulling out heating when end milling with a PCD insert. Longer tool life at
and removing of WC particles which is much more deleterious to high temperatures is attributed to a reduction in cutting forces
the hardness and wear resistance than the diffusion of tungsten and the amplitude of vibration. The size of the built-up edge
and carbon and dominates the crater wear mechanism [84,85]. increases with preheating temperature up to 450 1C due to the
Instead of enhanced cooling strategies which have been high chemical reactivity between the titanium alloy and the
focused on improving the machinability of titanium alloys for cutting tool at elevated temperatures and decreases with further
years, laser assisted machining offers an alternative strategy to increase in preheating temperature.
improve the machinability by reducing the cutting pressure. The optimum preheating temperature is found to be that at which
Cutting forces in all three directions have been reduced the radial and axial cutting forces sharply increase [21]. Longer
dramatically with the assistance of a laser beam and the reduction chip tool contact length and minimum tool wear rate are achieved
of cutting forces strongly depends on the cutting speed (related when the workpiece is preheated at the optimum temperature.
laser energy input due to beam workpiece interaction time), LAM also produces the smoother machined surfaces with less
depth of cut, tool beam distance, laser spot size, laser power and grain pullout and smaller depth of the deformation zone [32].
the beam incident angle [13,32,65]. The significant effect of laser However, LAM reduces the compressive residual stresses at the
power on the force reduction during laser assisted turning of surface or transforms these stresses into tension with increasing
commercially pure titanium (a ductile workpiece) is only found at laser power [65,92]. This effect is more significant at low cutting
the beam incident direction [32]. speed and becomes negligible when the cutting speed is higher
When the laser beam is incident on the chamfer surface with than 54 m/min. Lower compressive or higher tensile residual
its minor axis along the workpiece rotation direction, the heat stress lowers the fatigue resistance.
penetration into the shear zone is minimized due to the short However, the heat source has to be carefully controlled to
beam interaction time. Therefore, heating of the cutting tool is ensure that the thickness of the heat affected layer associated
minimized and tool life is improved, because the strength of with Widmanstatten (needle-shaped) microstructure formation is
carbide tools used for machining titanium alloys is low at high within the cutting zone. This is difficult to achieve using plasma as
temperature [13]. the heat source due to the difficulty in predicting the depth of the
Critical cutting speed for the onset of chip segmentation is very heat affected zone during plasma heating [6]. The Widmanstatten
low during conventional machining. This causes dynamic cyclical microstructure that remains in the subsurface contributes to a
forces on the tool which leads to chatter and cutting tool tip reduction in its fatigue life [6,65]. One advantage of a laser beam
breakage [86,87]. With laser beam preheating, the critical cutting over plasma as the external heat source is its smaller and
speed for the onset of chip segmentation increases [88]. At accurately controlled spot size [6], there is no Widmanstatten
constant laser power, the chip morphology transitions from a microstructure observed in the machined subsurface after laser
sharp saw-tooth morphology to a continuous chip and back to a assisted machining [32,65,67].
saw-tooth shape with increasing cutting speed. The saw-tooth
chips formed at low and high cutting speeds have different
geometry ratios, which indicates the different formation mechan-
isms. The continuous chip transition speed increases with laser 5.2.2. Nickel-based superalloys
power [89,90]. Inconel 718, a nickel-based superalloy, has the ability to retain
Tool life during LAM of titanium alloys strongly depends on the its strength and toughness up to temperatures of around 500 1C.
material removal temperature. The optimum material removal Therefore it is an ideal material for the aerospace industry finding
temperature during LAM of Ti-6Al-4V alloy is found to be 250 1C applications in gas turbine components and jet engines [80]. Its
at which the balance between the heat generated due to plastic high strength is due to the presence of fine uniform metastable g0
deformation and the heat produced by laser energy input is achieved and g00 precipitates, which are the intermetallic phases Ni3(Al, Ti)
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674 S. Sun et al. / International Journal of Machine Tools & Manufacture 50 (2010) 663 680
and Ni3(Al, Ti, Nb), distributed throughout the matrix after age The location of the laser beam on the chamfer surface during
hardening. laser assisted turning has a more significant influence on the
In general, nickel-based superalloys are hard to machine reduction in the notch and nose wear than on the reduction in
because of their rapid work hardening during machining, chip flank wear. A greater reduction in notch wear is observed when
segmentation resulting in severe tool wear and their high the laser beam is incident on the top edge of the chamfer shoulder
tendency to form a built-up edge by welding to the tool material while a greater reduction in nose wear is obtained when the laser
at high cutting temperatures [93]. Adhesive and abrasive wear are beam is incident near the root of the chamfer shoulder [13].
dominant wear mechanisms, which lead to the flank wear and Thermal fatigue of the cutting tool during the heating/cooling
notching wear as the main tool failure modes in conventional cycle of its interrupted engagement with the workpiece during
machining of Inconel 718 [4,5,94]. milling is the main tool failure mechanism [6], which causes edge
Ceramic tool inserts, WG-300 by Greenleaf [4,5,10,41], CC670 chipping. However, the softening of the workpiece by the external
by Sandvik [6,95], K090 by Kennametal [13] (aluminum oxide heat source reduces the impact of the workpiece on the cutting
reinforced with silicon carbide whiskers or TiC) are the best tool tool, therefore, a reduction in chipping is more significant with
materials for cutting Inconel 718 because they are stable up to increasing material removal temperature than a reduction in flank
approximately 2000 1C [6]. wear during the milling operation. The amount of chipping is
With increasing temperature, the deformation in Inconel 718 dependent on the preheating cycle and beam position. Longer
transitions from being heterogenous, with slip being confined to heating time and beam position closer to the edge of workpiece
planar bands with dislocation pairs and non-dislocation structure (tool entrance or exit sides) favored the reduction in the tool edge
in the matrix at room temperature to being homogenously chipping during laser assisted milling of Inconel 718 [41].
distributed and comprised of a uniform tangle of dislocations at The shear angle in chip formation is reduced and chip
high temperature once the limit of stability of metastable g0 and thickness increases during LAM [11]. The tendency for chip
g00 precipitates is reached. This makes the deformation during segmentation increases during LAM of Inconel 718 compared
thermally enhanced machining much easier [96]. Reduction in with conventional machining because of the reduced strength of
cutting forces or specific cutting energy and longer tool life with the workpiece [96].
ceramic tooling has been reported with both laser [13,34,41,95] Compressive residual stress is measured in the axial direction,
and plasma [4 6,8] as external heat sources for both turning and which indicates that LAM does not produce any adverse effect on
milling operations. The cutting forces or specific cutting energy the resultant subsurface because a tensile residual stress is
decreases with increasing laser power [13,34,41,95] or tempera- usually produced by turning [10]. However, Germain et al. [95]
ture [5,10]. But the tool life using LAM is shorter than that reported that tensile residual stress is produced in both the axial
obtained by conventional cutting with carbide tools [95]. and tangential directions with and without laser assistance, and
Unlike the independence of cutting forces on cutting time the magnitude of residual stress produced by LAM is smaller than
when LAM of Si3N4 [71], the cutting forces in all directions that by conventional machining only in the axial direction when
gradually increase with cutting time in both plasma and laser machining with carbide tools.
assisted machining (for both turning and milling operations), but Surface roughness is significantly reduced by milling and
the rate of increase is much lower than that seen in conventional turning operations with the assistance of a laser beam and plasma
machining [4,41]. This can be attributed to (i) the lower strength, [4,8,95]. This improvement increases with surface temperature up
toughness and work hardening of the workpiece at the cutting to around 500 1C [5,10,41]. Beyond 530 1C, surface oxidation is
zone and (ii) reduction in tool wear during LAM. However, the sometimes observed, and this results in a slight deterioration in
laser heat source has a limited effect on the strain and strain rate surface quality. The improvement in surface integrity by plasma
field during LAM [96]. enhanced machining is because of (i) a lower chip rupture stress
Notch and the maximum flank wear are reduced with due to the lower workpiece hardness leading to the smoother chip
increasing material removal temperature up to 540 1C during removal; (ii) higher tool ductility due to the higher temperatures
laser assisted turning compared to conventional turning. preventing the secondary chipping [4].
Contrary to the increase in wear with cutting speed during LAM The subsurface after milling shows high strain hardening.
of Ti-6Al-4V alloy and conventional machining, the tool wear The depth and hardness of the deformation affected zone are
(both the notch and flank wear) decreases with increasing cutting 80 100 mm and 340 399 Hv0.2 at a depth of 24 mmwithassistance
speed and feed during LAM of Inconel 718 at the constant of plasma, these are smaller than those (180 mm and 435 Hv0.2 at
material removal temperature of 540 1C [10]. This is probably due depth of 22 mm) with conventional milling at the same cutting
to the independence of material removal temperature on cutting conditions. No carbide precipitation or new metallurgical phases
speed as shown in the empirical equation (3). Therefore, the are produced in the machined subsurface due to the external
benefit of LAM can be maximized at higher cutting speed. heating [6].
However, notch wear is still the main tool failure mode in laser
assisted turning.
A reduction in notch and flank wear has also been reported 5.2.3. Hardened steels and other metals
with plasma enhanced milling at low plasma intensity [6]. In the Thermally enhanced machining has been successfully applied
plasma enhanced turning operation with high arc current, the with other metals such as Co-based alloys [6,98,99], steels
reduction in notch wear is more significant. Deep notching is [7,12,14 19,25,26,33,34,65,100 102], 6061-T6 aluminum [100]
virtually eliminated, but the flank wear rate increases due to the and iron [36,58] in terms of lower cutting forces, increased tool
higher chip temperature compared to conventional machining life and the material removal rate.
[4,5,8,97]. The evolution of thrust force with cutting time in LAM of D2
Hybrid machining, developed by simultaneously preheating tool steel with rectangular laser beam is strongly dependent on
the workpiece using a plasma flame and cooling the tool using a the beam orientation. The thrust force is gradually reduced with
liquid nitrogen chamber that is attached to the tool, results in cutting time when the laser spot slow axis is perpendicular to the
increasing tool life and lowering surface roughness by 156% and feed direction because of the higher surface temperature due to (i)
250%, respectively over conventional machining and by 170% and the longer duration of the heating cycle and (ii) the laser power
33% over plasma enhanced machining [97]. distribution along its axis in this configuration [12].
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S. Sun et al. / International Journal of Machine Tools & Manufacture 50 (2010) 663 680 675
Not only is the magnitude of the cutting forces reduced, but stress due to the plastic deformation of the surface layer. With
also is the amplitude variation of the forces in LAM of D2 tool steel laser heating the workpiece surface in front of the burnishing
reduced [12]. This is attributed to the great reduction in vibration. roller (as shown in Fig. 8), larger plastic deformation in the
In LAM of some hardenable steels, such as 1090 steel, higher surface layer is achieved. This leads to a lower surface rough-
cutting forces are obtained because the phase transformation ness, higher hardness in the surface layer and a greater
hardening occurs when the laser preheated part of the workpiece compressive residual stress on the surface compared to
enters the cutting zone [103]. However, an improvement in conventional burnishing. Surface temperature by laser heating
surface integrity is obtained. should be controlled in order to prevent permanent phase
During LAM of a ductile 42CrMo4 steel, the effect of laser transformation in the surface layer [46].
power on the reduction of cutting forces is not significant Compressive residual stress in the axial direction is produced,
compared with that during LAM of hardened steel, but the and its magnitude and depth are smaller in the laser assisted
improvement in surface quality is significant with increasing laser machined bearing steel (100Cr6) than those made by conven-
power [102]. tional machining [65,92,102], this may be due to less deformation
The friction coefficient between the coated K10 tool and strain during LAM. However, Ding and Shin [26] reported that
hardened XC42 steel is lower in LAM than that during conven- LAM produces 150 MPa more compressive surface axial residual
tional machining under the same cutting conditions [34]. This stress compared to conventional cutting.
leads to a greater reduction in the thrust force during LAM [12].
But the friction coefficient remains constant at the material
5.3. Metal matrix composites (MMCs)
removal temperature from 25 to 450 1C during LAM of austenitic
stainless steel, which suggests that the shear angle may change
Composites are a combination of two or more materials made
during LAM [25].
by mixing or bonding in such a way that each maintains its
Both the evolution of flank wear and catastrophic failure of the
integrity. One part generally acts as a matrix and the other as the
tool during LAM of the tool steel have been reduced because of the
reinforcement which may be in the form of particles, whiskers
softening of the workpiece, which can be achieved with surface
or fibres. The properties of composites are achieved by the
temperatures above 300 400 1C for a thickness of the uncut
systematic combination of these constituents.
chip of 0.05 mm. The stable built-up edge is found to protect the
In particulate MMCs, the reinforcement is normally a ceramic
cutting edge during LAM, and this partially contributes to the
(Al2O3, SiC, etc.) which offers the matrix metal (normally ductile
reduction in tool wear. The chip morphology changes from a saw-
metal, such as aluminum alloys) extra hardness and wear
tooth chip produced with conventional cutting to a continuous
resistance [104].
chip with LAM at the same cutting speed [12]. The change in
However, the incorporated ceramic particles make the MMCs
the chip morphology due to laser preheating is the result that
hard to machine [105] because of rapid tool wear, which is
the material s failure mode changes from being predominantly
primarily attributed to the abrasive nature of the hard ceramic
a fracture at low temperature to plastic deformation at high
particles [106,107]. Tool wear is accelerated solely due to the
temperature. The chip reduction coefficient reduces with increas-
abrasive action of the ceramic particulates, and the built-up edge
ing preheating temperature which leads to improvements in the
to some extent protects the cutting tool from sliding against the
machinability [15].
abrasive particulates [108].
Unlike with laser assisted machining of titanium alloys, the
The cutting force consists of contributions from chip forma-
lower tool wear and more significant suppression of the formation
tion, ploughing, particle fracture and displacement [109]. The
of saw-tooth chips during LAM of tool steel are observed when
surface roughness is controlled by particle fracture and pullout at
the slow axis of the diode laser with a rectangular beam is
low feed [105].
perpendicular to the feed direction due to the higher surface
The metal matrix, which constitutes between 70% and 90% of
temperature [12].
most MMCs, also plays a significant role in machining. Softening
Tool life when machining steel is most significantly affected by
of the Al matrix by the laser beam prior to cutting leads to more
preheating temperature [7,14 16], and this enhancing effect may
significant force reduction in the X and Y directions (by nearly
be diminished with increasing cutting speed [17]. The maximum
preheating temperature allowed is limited by the recrystallization
Laser Power, Pl (W)
temperature of the workpiece in order to avoid any microstruc-
100 150 200 250 300
7
100 140
ture alteration in the machined subsurface [15].
The tool chip interface temperature (Tint), critical for dissolu-
Tool life
uc
tion-diffusion dominant tool wear, is much lower in LAM than 120
6
80
that in conventional machining of austenitic stainless steel when
the material removal temperature is above 120 1C and below 100
400 1C [25], which leads to a significant reduction in tool wear
5
60
80
within that temperature range, above which the accelerated tool
wear results in premature tool failure.
% Scrap
60
Segmentation during saw-tooth formation in machining is one
4
40
of the primary causes of chatter when the vibration frequency due
40
to chip segmentation coincides with the natural frequency of the
machine system [19]. It is found that preheating the workpiece 20 3
20
results in a dramatic reduction in the amplitude of acceleration
of vibration and chatter due to the reduction in instability of
0 0 2
chip formation and an increase in plasticity of the workpiece
400 600 800 1000 1200 1400
[12,19,101].
Material Removal Temperature, Tmr (oC)
Burnishing is a process involving pressing of the metallic
workpiece surface by a hard roller or ball. This process produces
Fig. 15. Effect of material removal temperature on specific cutting energy, tool life
a smooth surface with high hardness and compressive residual and scrap rate for LAM of PSZ [63].
3
c
l
% Scrap
Tool Life, t (min)
Specific Cutting Energy, u (J/mm )
ARTICLE IN PRESS
676 S. Sun et al. / International Journal of Machine Tools & Manufacture 50 (2010) 663 680
50%) compared to force reduction in the Z direction (only 10%) 1490 1C for silicon nitride [72], 900 1100 1C for partially stabi-
during LAM of Al2O3p/Al composites. The softened matrix is easily lized zirconia (PSZ) [63], 1043 and 1215 1C for mullite [56,70], 550
squeezed out while more Al2O3 particles are pushed in from the and 650 1C for Inconnel 718 alloy [10], 120 and 340 1C for
machined surface by the cutting tool, which produces a higher austenitic stainless steel P550 [25], around 250 and 400 1C for the
fraction of Al2O3 particles near the machined surface, and this Ti-6Al-4V alloy [67,91] and compacted graphite iron [58],
leads to an increase in the wear resistance of the machined respectively. The cutting conditions can be optimized using
surface. Further, the machined surface shows a higher compres- model analysis to ensure that the material removal temperature
sive residual stress (triple that observed in conventionally is within the appropriate range.
machined surfaces). Longer tool life is also reported during LAM During the LAM process, the energy absorbed by the workpiece
of Al2O3p/Al composites [110]. not only raises the temperature of the material to be removed to
However, Barnes et al. [111] found that the mode of tool the desired material removal temperature but also heats the
failure is flank wear by abrasion when cutting aluminum/SiC material near the removal zone. A preheating efficiency is defined
MMCs. The flank wear increases significantly with increasing as the ratio of the minimum power required to heat the material
preheating temperature. This is associated with a shift in the to be removed to the desired material removal temperature (Pmin)
stability range of the built-up edge to the lower cutting speeds, to the total amount of power absorbed from the external heat
combined with a change in the angle of the built-up edge s source (P) [112], i.e.:
leading edge which reduces its ability to protect the flank face
Pmin
from abrasive wear.
Zź �7�
P
This efficiency decreases with increasing material removal
6. Optimization and energy efficiency temperature because a large amount of energy is wasted in
heating the material outside the cutting zone. Improvement of
preheating efficiency significantly reduces the specific energy,
The optimum range of the material removal temperature is
utotal (including mechanical cutting energy, uc and thermal energy,
determined by comparing the specific cutting energy (uc), tool life,
ut). Therefore, the selection of the optimum material removal
scrap rate (determined by the damage on the machined surface in
temperature is critical in order to achieve not only the full benefit
the form of cracks and spalling) and surface roughness as shown
of LAM but also the preheating efficiency. As a metric, the
in Fig. 15. The optimum material removal temperature is found to
preheating efficiency can be used to guide the configuration of the
be in the range from approximately 900 to 1100 1C for PSZ [63].
laser beam, such as its focusing location and position relative
The optimum LAM operation (longer tool life, better surface
to the cutting tool in consideration of the workpiece material s
integrity and damage-free component) can be ensured when Tmr
thermo-physical properties.
is maintained in a proper range, which is between 1270 and
Objective
Pulsed
lens
O2
Laser
Objective Laser beam
lens
Pre-drilled
hole
Pre-drilled
hole
Chips
Cutting
Tool and
Chips
Workpiece
tool
tool holder
Pre-drilled holes
Workpiece
Cutting tool
Fig. 16. Illustration of turning operation with predrilled holes by laser beam (a) during and (b) before machining [113].
ARTICLE IN PRESS
S. Sun et al. / International Journal of Machine Tools & Manufacture 50 (2010) 663 680 677
7. Non-traditional laser assisted machining savings can be made on both the tool cost and tool changing
time due to the longer tool life achieved with LAM or PEM.
Traditional LAM benefits from the local temperature rise in the
It has been reported that almost a 50% saving can be made
workpiece in front of the cutting tool and the resultant local
when LAM of Inconel 718 [10] and P550 stainless steel [25]
softening of the workpiece in the cutting zone. However, other
and PEM of steel [7]. About 30% and 40% saving have been
studies on LAM take advantage of material removal and micro- reported when LAM and hybrid LAM of Ti-6Al-4V alloy with a
structure alteration by the laser.
coated tool [67].
In addition, coolant used in machining is generally toxic for the
operator and environment. Elimination of coolant in LAM shows
7.1. Removal of material by the laser in front of the cutting tool
that LAM is an environmentally friendly process.
[113,114]:
The laser is used as a heat source not only to soften the layer of
9. Concluding remarks and suggested future research topics
material which is about to be removed from the workpiece, but
also to remove part of material by laser drilling of Ti-6Al-4V alloy
Thermally enhanced machining uses external heat sources to
[113] or ablation of ceramics (laser ablation hybrid machining,
heat the workpiece, change microstructure or remove the work-
LAHM) [114]. The circumferentially spaced holes in a layer of
piece locally in front of the cutting tool to facilitate the machining
material to be removed are made in front of the cutting tool by
process due to softening, change in deformation behaviour and
material vaporization with or without oxygen flow (as shown in
thinning of the workpiece. Generally, it offers lower cutting forces,
Fig. 16a). The predrilled holes should have a depth not greater
longer tool life, a better machined surface and higher material
than the depth of cut, and a diameter greater than the feed rate
removal rate for a variety of workpiece materials, such as
and an expected chip width. The depth-to-diameter aspect ratio
ceramics, metals and composites.
of the pulsed laser drilled holes is between 1 and 2.
The local temperature of the material as it enters the shear
This can be done either in-situ during cutting, or before
deformation zone plays an important role in the thermally
cutting. The latter consists of two steps: the workpiece initially is
enhanced machining process. This temperature has to be main-
predrilled with circumferentially spaced, axially extending rows
tained in the optimum range, which is dependent on the actual
of holes in the layer of material to be machined by the laser, and
workpiece material.
then machined on a lathe (as shown in Fig. 16b).
Position and orientation of the incident beam are also critical
The predrilled holes ahead of the cutting process reduce the
to achieve the maximum benefit from thermally enhanced
cutting force with the breakage of chips to a manageable size, and
machining.
reduce tool wear (no crater wear is observed). When machining
Thermally enhanced machining is a complicated process.
Ti-6Al-4V alloy, as much as a 50% reduction in cutting force
The external local heating not only changes the flow stress,
is obtained. The reduction in the cutting energy is much
but also changes the deformation behaviour of the workpiece
greater than the reduction in volume of material due to laser
and the friction between the chip and tool. So far, intensive
predrilling [113].
experimental works and modelling of temperature distribu-
The cutting forces is significantly reduced with LAHM, the
tion have been reported. The authors suggest that future
reduction is more than 20% of the amount of force required to cut
research should be focused on the following topics in order to
a semi-transparent volume absorption material and 65% more
fully address the remaining issues in the thermally enhanced
than LAM. The smooth forces indicate the absence of any brittle
machining:
fracture and a continuous type chip is formed at this high
temperature range [114].
(1) Effective cooling of the cutting tool
Thermally enhanced machining introduces an external heat
7.2. Modification of microstructure in the to-be machined layer by
source to heat the workpiece locally in front of the cutting
the laser [115]
tool, which may result in high tool chip interface tempera-
tures. This high temperature may lead to shorter tool life
Laser pre-treatment of the to-be machined layer in alumi-
due to the premature degradation of the cutting tool
num/SiC composites results in a precipitate-free and recrystal-
and accelerated dissolution diffusion and adhesion wear.
lization layer due to the fast air cooling or liquid nitrogen
Therefore, it is clear that more effective methods for
quenching. This causes an increase in hardness at the surface
enhancing separation of the chip from the tool and cooling
followed by a zone of reduced hardness before a return to the
the cutting tool without affecting local heating of the work-
hardness as the as-received workpiece deep in the centre of
piece are required. These methods should be practically
workpiece. Under the selected preheat treatment conditions,
applicable for both milling and turning operations during
flank wear is dramatically reduced by nearly 50% in association
thermally assisted machining.
with a reduction in feed force. However, this reduction is not
(2) Application of the thermally enhanced machining to the
achieved by a simple reduction in the hardness of the matrix
ductile workpiece
materials.
Most of the thermally enhanced machining research to date
The improvement in tool wear by microstructure modification
has been focused on the hard-to-machine workpiece materi-
may be due to the change in the mechanism by which the chip
als which are characterized as having high hardness and
separates from the cutting tool, however, the authors did not
strength. However, some metals with high ductility are also
present analysis on chip formation in this work.
difficult to machine due to the high friction between the
cutting tool and chip and strong adhesion of the chip to the
8. Economic benefits
cutting edge, which often leads to poor surface finish.
Reduction in local ductility of the workpiece as a result of
To be adopted by industry, a technology has to be economic- phase transformations in the chip during thermally enhanced
ally competitive and environmentally friendly. Despite the capital machining may be a solution to improve the machinability of
and operating costs (power consumption) of a laser or plasma, workpiece materials with high ductility.
ARTICLE IN PRESS
678 S. Sun et al. / International Journal of Machine Tools & Manufacture 50 (2010) 663 680
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Additional processing parameters, such as, the position and
[4] J.W. Novak, Y.C. Shin, F.P. Incropera, Assessment of plasma enhanced
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both experimental and numerical modelling studies are
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Improved tool life in end milling Ti-6Al-4V through workpiece preheating,
ment of a shorter wavelength laser which has higher
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Acknowledgments
using electrical current heating, CIRP Annals 14 (1966) 145 151.
[23] E.J. Krabacher, M.E. Merchent, Basic factor of hot machining of metals,
Journal of Engineering for Industry, Transactions of the ASME 73 (1951)
The authors gratefully acknowledge the CAST Cooperative
761 776.
Research Centre and Defence Materials Technology Centre for the
[24] S. Lei, F. Pfefferkorn, A review of thermally assisted machining, in:
financial support and permission to publish this work. The CAST Proceedings of the ASME International Conference on Manufacturing
Science and Engineering, Atlanta, GA, October 15-7, 2007, pp. 1 12.
Cooperative Research Centre was established and is supported
[25] M. Anderson, Y.C. Shin, Laser-assisted machining of an austenitic stainless
under the Australian Government s Cooperative Research Centres
steel: P550, Proceedings of the Institution of Mechanical Engineers Part B:
Programme. The DMTC was established and is supported under Journal of Engineering Manufacture 220 (2006) 2055 2067.
[26] H. Ding, Y.C. Shin, Laser-assisted machining of hardened steel parts with
the Australian Government s Defence Future Capability Technol-
surface integrity analysis, International Journal of Machine Tools and
ogy Centres Programme.
Manufacture 50 (2010) 106 114.
[27] J.C. Rozzi, F.E. Pfefferkorn, F.P. Incropera, Y.C. Shin, Transient thermal
response of rotating cylindrical silicon nitride workpiece subjected to
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