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Laser Assisted Machining
MECH-811
Master of Applied Science
Pranav Gupta
Student Number: 10158456
Supervisor: Dr. Jacob Jeswiet
April 2015
Declaration
I hereby declare:
that except where reference has clearly been made to work by others, all the work presented
in this report is my own work;
that it has not previously been submitted for assessment; and
that I have not knowingly allowed any of it to be copied by another student.
I understand that deceiving or attempting to deceive examiners by passing off the work of
another as my own is plagiarism. I also understand that plagiarising the work of another or
knowingly allowing another student to plagiarise from my work is against the University
regulations and that doing so will result in loss of marks and possible disciplinary proceedings
against me.
Signed…………………………
Date…………………………....
I
Abstract
The project aims to study the integration of laser assisted machining, a post manufacturing
technique in the industrial applications. A brief description of the laser parameters affecting
the process are presented. The project also presents a detailed description of the laser selection,
tool selection and temperature measurements. A review of the past researches are discussed
with the respective results for ceramics, titanium and alloys, nickel and alloys, ferrous alloys
and metal matrix composites. The results presented are presented in the form of a comparison
between the laser assisted machining and conventional machining under the categories of
cutting force, tool wear and surface integrity.
II
Acknowledgements
This investigation on laser assisted machining (LAM) was made possible by suggestions made
by my supervisor Dr. Jacob Jeswiet (Queen’s University). I would also like to thank Dr. I.S.
Jawahir (University of Kentucky, USA) for beneficial information on recent researches taking
place in the field of LAM.
III
Table of Contents
1.Introduction ........................................................................................................................... 1
1.1 Background ...................................................................................................................... 2
2. Theory ................................................................................................................................... 4
2.1 LAM Process .................................................................................................................... 4
2.2 Material Interactions ........................................................................................................ 5
2.2.1 Absorption of Laser Radiation .................................................................................. 6
2.2.2 Spot Diameter and Wavelength ................................................................................. 7
2.2.3 Effect of Laser Pulse Duration .................................................................................. 8
2.2.4 Surface Temperature ................................................................................................. 8
3. Device and Mechanism of LAM ......................................................................................... 9
3.1 Laser Selection ................................................................................................................. 9
3.2 Material Selection .......................................................................................................... 10
3.2.1 Ceramics .................................................................................................................. 10
3.2.2 Nickel Based Superalloys ........................................................................................ 10
3.2.3 Titanium and its alloys ............................................................................................ 10
3.2.4 Metal Matrix Composites (MMC)........................................................................... 11
3.3 Tool Selection ................................................................................................................ 11
3.4 Integration of Lasers with Traditional Techniques ........................................................ 12
3.4.1 Turning .................................................................................................................... 12
3.4.2 Milling ..................................................................................................................... 13
3.5 Temperature Measurement and Control ......................................................................... 14
3.5.1 Pyrometer ................................................................................................................ 14
3.5.2 Thermal Modelling .................................................................................................. 14
4. Effect of Introduction of Laser Energy in Machining .................................................... 17
4.1 Cutting Force .................................................................................................................. 17
4.2 Tool Wear ....................................................................................................................... 19
4.3 Surface Integrity ............................................................................................................. 22
5. Discussion and Conclusions .............................................................................................. 24
6. Future Work ....................................................................................................................... 25
7. Bibliography ....................................................................................................................... 26
8. Appendix-A ......................................................................................................................... 33
IV
1. Introduction
Light Amplification by Simulated Emission of Radiation (LASER) is a device that can
produce and amplify a coherent radiation particularly from the infrared to ultraviolet regions
of the electromagnetic spectrum. It was first demonstrated in 1960 by Theodore Maiman with
the aid of a ruby crystal. Since then, various developments have taken place and now
different types of materials and methods can be employed to generate a laser e.g. solid state
laser, semiconductor laser, dye laser, neutral laser, molecular gas laser and excimer laser.
Constant research and development in lasers over the time along with high achievable
temperatures providing exceptional accuracy has made lasers find their way in the industrial
applications like cutting, forming, melting, drilling and welding. They have also successfully
found way over the years in the surface metal treatment including heating, glazing and thin
film deposition (Shin 2011; Davim 2013).
The need to integrate lasers and other non-traditional processes in manufacturing has been
generated by the constant technological development over the years in the various
applications especially the aerospace and automotive industry. As a result, many new
materials have evolved with increased hardness, strength and stability at very high
temperatures to meet the needs of these demanding applications also making them difficult to
machine by conventional methods. This limitation is imposed by the absence of materials that
can be employed for the economical machining of materials such as titanium, advanced
ceramics and metal matrix composites (Armitage et al. 2004; Kalpakjian & Schmid 2006).
The two major methods that are employed to remove excess material from the workpiece by
making use of lasers are ablation and laser assisted machining (LAM). Ablation also known
as laser machining (LM) is a process of removing excess material by bringing it in direct
contact with laser and has been a part of the manufacturing industry for direct processing of
various materials such as metals, plastics etc.. However, advancements in the laser
technology over the years has led to emergence of LAM as an efficient machining technique
for advanced materials (K.-S. Kim et al. 2011).
LAM is a hybrid process in which a high power laser is employed as a localised heat source
to soften the material before it can be machined by conventional methods. It has been found
to be a viable option to machine materials such as ceramics, nickel etc. that are difficult to
machine by conventional means in an efficient manner. This method has been found to be
1
responsible for an exceptional decrease in the processing cost as well as reductions in cutting
force and tool wear. In additional, LAM results in a very high surface finish of the machined
components similar to the finish obtained by grinding (Chryssolouris et al. 1997; Germain et
al. 2011; Shin 2011).
This report aims to develop an understanding of this post manufacturing laser based
technique and its viability on an industrial scale.
1.1 Background
Manufacturing is a collection of processes and operations to convert raw materials into useful
products. It is carried out by the a sequence of activities such as product design, selection of
raw material and material processing also refereed as process. A large number of
conventional manufacturing techniques have successfully been employed to manufacture
products or components over time. However, constant improvements in the conventional
techniques have led to development of new approaches to manufacture products with less
ambiguity, better quality and comparatively lower costs. An example of a non-traditional
technique that has proved to be beneficial in certain operations is the employment of lasers in
the manufacturing processes. Laser based manufacturing has shown to be a suitable
manufacturing technique for a large variety of materials with different properties such as
brittle, soft and thin. Flexibility and contact less nature are other advantages of this process
that make it more suitable in certain processes than the traditional manufacturing processes
(Dahotre & Harimkar 2008; Groover 2010).
Primary traditional manufacturing processes such as casting, joining and forming generally
require additional secondary processes like machining to convert manufactured components
into finished goods that are well within the final desired specifications. The removal of
excess material from the component that is obtained from the primary manufacturing process
in order to bring it down to appropriate size and tolerances is referred to as machining.
Hence, machining forms an integral part in deciding the total cost of the manufactured
component. Various machining processes have been developed over time and are generally
classified into traditional and non-traditional machining. Traditional machining processes
include the conventional material removal techniques such as turning, drilling, milling etc.
The excess material is removed in the form of chips by shearing action of the tool employed.
Non-traditional machining processes on the other hand, remove excess material by various
2
other means other than a sharp cutting tool e.g. photochemical machining, laser beam
machining etc. (Walker 1998; Dahotre & Harimkar 2008).
Traditional machining of superalloys and other advanced materials has been made possible
by grinding and diamond machining. However, these techniques are affected by low material
removal rate and lack of flexibility in producing complex geometries. In addition, these
techniques are expensive and can make upto 90% of the finished component cost. Cutting is
another traditional technique carried out by the tools made up of a very hard material such as
cubic boron nitride (CBN). However, the costs of materials like CBN are very high and
increase the overall price of finished components (Armitage et al. 2004; Anderson et al. 2006;
Skvarenina & Shin 2006; Ding & Shin 2010; Masood et al. 2011).
Non-traditional machining processes are necessary when the traditional processes fall short of
the desired specifications and become an impractical choice. Proper application of these
machining techniques has been found to provide better technical and economic efficiency
over the traditional processes. One such technique that is continuously being utilized in the
manufacturing industry is LM (Kalpakjian & Schmid 2006).
A number of advantages such as elimination of tool wear, tool breakage, noise, machine
deflection and mechanical damage are responsible for the successful integration of lasers in
manufacturing industry. However, a major disadvantage associated with LM is the material
evaporation and melting which further leads to micro-cracking. Direct contact of lasers with
the workpiece has also been found to affect the material composition due to high
temperatures requiring additional machining. Development of new economically feasible,
easy and transportable laser sources aided by the elimination of material evaporation has
generated a lot of interest in the LAM. In addition to the advantages stated above, this process
is carried out in the absence of any lubricants making process more environment friendly and
cost effective. As a result, it is being employed in the manufacturing of exhaust valves,
cylinder liners and ceramic disk brakes (Chryssolouris et al. 1997; Germain et al. 2011).
3
2. Theory
2.1 LAM Process
LAM is based on the principal of thermal assisted machining (TAM) i.e. to reduce the cutting
force required during the machining operation by increasing the temperature to a point where
strength reduction of the material is observed. It employs a laser as an intense heating source
to heat the workpiece surface and subsequently removing it by employing traditional
machining techniques (Fig. 2.1). The increase in temperature of the surface of work piece
occurs due to the absorption of the laser beam energy. This uniform heating of workpiece
only takes place in the region ahead of the cutting tool without raising the temperature of the
tool. The increased temperature in the shear zone softens the material by reducing the bulk
modulus. This in turn reduces the brittleness of the material by inducing ductile properties.
The extent of reduction in brittleness is a result of the maximum temperature that is achieved
during the heating carried out by the laser. This heating of the shear plane reduces the cutting
forces required during the cutting operation (Fig. 2.2). Other advantages of the localised
heating are the increase in material removal rate, increased tool life and better surface finish
(Chryssolouris et al. 1997; Dahotre & Harimkar 2008; Masood et al. 2011; Shin 2011).
Fig. 2.1 Laser assisted machining (Chryssolouris et al. 1997)
The increase in the temperature of the workpiece should be enough to soften the material to
introduce ductile deformation during material removal. However, very high temperatures can
melt the material affect the surface quality. Surface quality is also affected inversely by the
4
softening of cutting tool caused by the undesirable heating. Excessive heating of the
workpiece can also introduce thermal expansion in the workpiece hence affecting the overall
quality of the finished component. As a result, it is of significant importance to carry out
localised and controlled heating of the workpiece essential for process optimization
(Armitage et al. 2004; Shin 2011).
Fig. 2.2 Variation of tensile strength upon increase in temperature of advanced materials (Sun
et al. 2010)
Precise control of local temperatures is essential to increase the efficiency of this process.
Thermal conductivity thus has a major role in determining the surface temperature and the
gradient generated due to heating. Through knowledge of the temperature progression is a
prerequisite for determining the optimum machining and laser beam parameters such as feed
rate, depth of cut, spot size and laser power for desired material removal rate. Beam intensity
and interaction time are selected such that acceptable temperature is achieved at the desired
depth of cut to avoid damage to the functional surface. The desired temperature is achieved
by precisely synchronising the motion of workpiece with respect to focal point of the laser
beam (Chryssolouris et al. 1997).
2.2 Material Interactions
Interaction of electromagnetic radiation generated from a laser has the capability to
permanently change the material properties (Brown & Arnold 2010). Material interactions
play an important part in determining the capabilities and drawbacks of the processes
5
occurring on the surface of the material to be machined. Whenever an electromagnetic
radiation is incident on the surface of a material, various phenomenon such as reflection,
refraction, absorption and transmission. Absorption is the most important phenomenon as the
absorbed laser energy is responsible for increasing the temperature of the material. This
absorption of radiation by material results in heating, melting, plasma formation and
evaporation. Hence, precise control of temperature is essential for the selection of appropriate
process parameters (Chryssolouris et al. 1997).
2.2.1 Absorption of Laser Radiation
Electromagnetic radiation with a wavelength range from infrared to ultraviolet spectrum
interacts with electrons of the material as the ions are much larger in size to respond to the
high frequencies. The absorption is characterized by the excitation of electrons from their
equilibrium state to an excited state by the absorption of photons. This absorbed energy is
shared with other electrons through electron-electron collisions and are then transferred to the
lattice which eventually leads to the heating of the target material. The absorption of the laser
radiation is hence an important parameter that influences the material-laser interaction and for
an opaque material is given by equations adopted from (Steen et al. 2010):
= 1
(2.1)
Where, is the absorptivity and is the reflectivity of the material. The reflectivity at the
normal incidence is given by:
=(1)+
(+ 1)+
(2.2)
Where, is the refractive index and is the extinction coefficient.
The absorptivity is given by:
=4
[(+ 1)+]
(2.3)
These parameters are highly dependent on wavelength and temperature thus making
reflectivity a function of wavelength and temperature. Once the beam is absorbed by the
material, there is a reduction in the intensity of the beam which is a function of depth and
absorption coefficient .
6
()=
(2.4)
Where, is the incident intensity and ()is the intensity at depth z. This expression explains
the weakening of laser radiation inside the material.
2.2.2 Spot Diameter and Wavelength
Laser spot diameter is an important parameter that affects the quality of the component in the
manufacturing industry. The laser power density at the focal point is determined by the laser
spot size which in turn depends on the spatial characteristic. An increase in power density is
observed upon decrease in the laser spot diameter . The relationship between the power
density and spot diameter is given by equations adopted from (Nath 2012):
=()
()
(2.5)
The spatial characteristic generally expressed by or beam product parameter.
=
2×
(2.6)
Where, is the wavelength of the beam.
=×
(2.7)
Where, is the beam diameter and
is the half-divergence angle of the beam given by:
= 2/
(2.8)
If the beam is focussed on the workpiece by using a lens of focal length , the focal spot
diameter is given by:
= 2
= 4/
(2.9)
For the TEM00 mode laser beam, = 1 and the minimum spot diameter is given by:
= 4/
(2.10)
Where,
=
(2.11)
7
Where, is the number of lens. It is assumed that the diameters of lens and the beam are
equal. The minimum number of lens that can be employed is nearly 1. The minimum focal
spot diameter is given by:
(2.12)
Hence, lasers of shorter wavelength are required to obtain a small spot size diameter in the
machining operations (Nath 2012). Lasers of shorter wavelength are also responsible in
increasing the absorptivity by decreasing the reflectivity of the incident beam on the material.
This phenomenon is observed due to the increased absorption of photons by the bound
electrons in the material (Steen et al. 2010).
2.2.3 Effect of Laser Pulse Duration
The way a material interacts with laser radiation changes significantly when the interaction
time changes to very small time durations. The distance over which thermal energy is
dissipated during a pulse is given by equation adopted from (Nath 2012):
= 2()
(2.13)
Where, is the thermal diffusion length, is the thermal diffusivity and is the laser pulse
duration. Long pulse duration increases the thermal effects around the area where laser
induced heating effects are required. However, the short pulse laser system heats only the
electron subsystem unlike continuous or long pulse where the thermal energy is transferred to
the lattice. From the Eq. (2.13) it is clear that the dissipation length is also less for the short
pulsed laser system.
2.2.4 Surface Temperature
Another important parameter that affects the absorption of laser radiation by a material is the
surface temperature of the material. Some metals exhibit an increased interaction with
photons due to increased energy exchange with electrons. This phenomenon results in a
reduction in the reflectivity and exhibits an increased absorptivity (Steen et al. 2010).
8
3. Device and Mechanism of LAM
3.1 Laser Selection
Various developments in the past have led to examination of various types of lasers such as
CO2, Nd: YAG and diode laser in the preheating process. CO2 lasers have been used in the
manufacturing industry for a long time and are well established. They are able to provide
high power continuous wave lasers with powers as high as 20kW with exceptional beam
quality. The operating wavelength of 10.6µm make them an ideal choice for ceramics.
However, CO2 lasers are responsible for reducing the flexibility of LAM as they employ
mirrors as the mode of beam transfer. These lasers also don’t make an ideal choice for the
thermally assisted machining of metals due to low absorption (Dumitrescu et al. 2006; K.-S.
Kim et al. 2011; Jeon & Lee 2012; Nath 2012).
Nd: Yag lasers make second highest number of lasers employed in the materials processing
applications. They have much shorter operating wavelength of 1.06µm offering high
absorption when compared to CO2 lasers. These lasers have the capability to make the
system flexible by making use of optical fibers for beam transfer. Even though ceramics
exhibit lower absorption to these lasers, easy beam transfer makes them a suitable choice for
LAM. However, some of the disadvantages of these lasers include the short lamp life, poor
beam quality and low efficiency (Sainte-Catherine 1991; Dumitrescu et al. 2006; K.-S. Kim
et al. 2011; Jeon & Lee 2012; Nath 2012).
Diode lasers, a result of recent developments are also being used in studies for the
optimization of LAM. These lasers use a number of diodes to generate the required output
power. A much lower wavelength of 0.8-1.0µm generated by the laser make it a suitable
option for preheating of metals. Short wavelength generated by diode lasers is also
advantageous in reducing the thermal damage to the surface of workpiece. In addition, these
lasers offer advantages such as high energy efficiency, easy integration with CNC, low
running cost and output per power supply. However, beam generated is of poor quality as the
output of diode lasers is multimodal and incoherent. Other disadvantages that have affected
their successful integration in LAM are low power density, low working distances, poor
focus and low working life (Dumitrescu et al. 2006; K.-S. Kim et al. 2011; Jeon & Lee 2012;
Nath 2012).
9
It is possible to increase the absorption of the laser by the materials by application of an
absorption enhancing coating and has been employed in various studies for CO2 lasers.
However, application of absorption enhancing coating also increases complexity and cost by
adding another operation to the machining operation (Sainte-Catherine 1991; Anderson et al.
2006; Dumitrescu et al. 2006).
3.2 Material Selection
3.2.1 Ceramics
Advanced ceramics have formed an integral part of various industries including aerospace,
military and automotive. Various desirable properties such as high temperature stability,
chemical resistance and wear resistance are responsible for their success in demanding
applications. However, high hardness and brittle nature of ceramics make their machining by
conventional techniques an expensive and complicated task (Sun et al. 2010; Brandt & Sun
2013).
3.2.2 Nickel Based Superalloys
Nickel based alloys properties such as high thermo-chemical stability, high strength and
toughness. LAM has been carried out on Inconel 718 is a nickel based superalloy being used
in the aerospace industry. This alloy is able to retain its strength at temperatures as high as
500oC. Another property of this alloy is rapid work hardening which makes this material very
hard to machine by increasing the cutting forces and tool wear (Sun et al. 2010; Lippold et al.
2011; Brandt & Sun 2013).
3.2.3 Titanium and its Alloys
Titanium and it alloys are employed in various industries due to their excellent properties
such as high strength, corrosion resistance and stability at high temperatures. As a result of
these properties, machining of titanium and its alloys is a difficult a difficult ad high cost
process. The cutting tool during the machining process is affected by high pressure and
temperature. There is a reduction in tool life due to high reactivity of titanium with a large
number of tool materials. The productivity of conventional machining is also affected by the
high vibrations involved in the process (Peter et al. 2006; Sun et al. 2010; Brandt & Sun
2013).
10
3.2.4 Metal Matrix Composites (MMC)
MMCs are materials that have particles or fibers acting as reinforcement in a metal matrix.
This combination results in increased hardness and wear resistance of the resulting material.
Presence of particles such as that of ceramics in the resulting MMC also make machining a
difficult and expensive process mainly due to the high tool wear. High abrasive action of the
ceramics is responsible for high tool wear observed in the machining of MMCs (Brandt &
Sun 2013).
3.3 Tool Selection
Cutting tools must be made of those materials that are capable of withstanding high stresses
and temperatures. Depending on the machining operation they can be a combination of
certain properties such as high penetration hardness at elevated temperatures, deformation
resistance at high stresses, high fracture toughness, chemical stability, high thermal
conductivity, high fatigue resistance, shock resistance, high stiffness and must possess low
friction when in contact with material to be machined (Stephenson & Agapiou 2005).
In LAM, material at a very high temperature comes in direct contact with the cutting tool
(Fig. 3.1). This high temperature contact can be a drawback for the machining operation as
the materials in contact might be chemically reactive at those temperatures. Hence, the tool
life in the LAM process is highly dependent on the temperature as well as friction and cutting
speed. A suitable tool should possess properties such as small heat affected zones, high
material removal rate and stable thermo-chemical properties. There are certain materials like
poly crystalline diamond (PCD), cubic boron nitride (CBN), high speed steels (HSS), cobalt
enriched high speed steels (HSS-Co), sintered tungsten carbide (WC), single crystal natural
diamond and poly crystalline boron nitride (PCBN) that are used to machine hard to machine
materials (Stephenson & Agapiou 2005).
PCD is a stable material that can deliver high precision during the machining. However, the
material is not suitable for machining Fe or Fe based alloys due to chemical reactivity at high
temperatures. PCD has been found to be unsuitable for LAM process due to low carburizing
temperature of 900oC (Pfefferkorn et al. 2004). CBN is generally a favourable choice for
machining Fe and Fe alloys at high temperature because of the excellent thermo-chemical
properties. They have a high wear resistance and oxidise only at temperatures above 1370oC.
PCBN are also suitable option for machining Fe and Fe based alloys at elevated temperatures.
11
They are currently being employed in machining heat treated steels in the manufacturing
industry (Brandt & Sun 2013).
HSS make an inexpensive choice for the machining operations and offer desirable properties.
On the other hand, these tool materials are not suitable for temperatures above 540-600oC.
HSSs also are affected by their limited wear resistance, chemical resistance and cutting
speeds. Carbides also make an ideal choice for the machining operation with properties such
as high rupture strength, high fatigue strength and good hot hardness. They can also be
modified according to their use by modifying the grain size and cobalt content. However,
their major drawback is the rapid wear while machining ferrous materials at high speeds
(Stephenson & Agapiou 2005; Brandt & Sun 2013).
Fig. 3.1 Effect of temperature on the hardness of various tool materials (Brandt & Sun 2013)
3.4 Integration of Lasers with Traditional Techniques
3.4.1 Turning
Laser integration for the turning operation is much easier because of the stationary nature of
the turning tool (Fig. 3.2). Most of the research work that has taken place for the turning
based LAM has employed an incident beam normal to the workpiece. This setup is
advantageous as the temperature gradient is only obtained at material to be removed by
avoiding any temperature elevation at the machined surface. Another setup can involve a
12
laser beam normal to the chamfer surface. This setup has resulted in greater reduction in the
cutting forces. It is also advantageous in reduction of changes in the workpiece
microstructure due to elevation in temperature. The efficiency of the process can be increased
by reducing the distance between the beam spot and the cutting tool. However, it is important
to maintain tool at a minimum distance to prevent the tool damage due to overheating (Sun et
al. 2010).
Fig. 3.2 Integration of laser with turning operation (Wu & Guu 2006).
3.4.2 Milling
Implementation of a synchronous laser beam in the milling operation has not been successful
due to the complexity of the setup (Fig. 3.3). Another disadvantage of this setup is the limited
spot diameter that reduces the area that can be heated and machined. Therefore, a high power
laser or multiple beams are required to carry out the operation. Laser assisted milling can be
carried out by either integrating the laser with spindle or by using an additional motor to
move the laser in the feed direction (Sun et al. 2010; Wiedenmann & Zaeh 2015).
Fig. 3.3 Integration of laser with milling operation (a) side view (b) top view (Sun et al. 2010)
13
3.5 Temperature Measurement and Control
Thermal effect in the LAM is induced by the absorption of laser beam energy on surface of
the material. It is a complex process involving various thermal effects such as convection and
radiation from the workpiece, laser irradiation and penetration of absorbed heat flux,
conduction through the workpiece, heat generation due to deformation, energy transfer in the
form of chip removal, frictional and conductive energy transfer from chip to tool and
frictional energy transfer from tool to workpiece. Hence, a thorough knowledge of the
temperature at the material removal location and its subsequent effect on the cutting forces,
surface quality and tool wear is essential for process optimization (Chryssolouris et al. 1997).
3.5.1 Pyrometer
Pyrometers are the instruments that are used to measure very high temperatures. It is used in
the LAM to measure and control the temperature of the materials. The temperature
measurement is made possible by the measurement of infrared radiation emitted by the
material. The laser power is then adjusted depending on the variation of the temperature by
the pyrometer (K.-S. Kim et al. 2011).
3.5.2 Thermal Modelling
Prediction of temperature distribution within the material is an important aspect that is
considered for the utilization of lasers in the manufacturing industry. The temperature
distribution is calculated from the heat flow equation (one dimensional) given by equation
adopted from (Dutta Majumdar & Manna 2003):
..
.
=(,)
(3.1)
Where, is the mass density, is the specific heat, is the thermal conductivity and (,)
is the power density at a given depth of .
The thermal response of a laser beam incident on an opaque rotating cylindrical material can
be predicted by a three dimensional transient mathematical model given by (Rozzi, Incropera,
et al. 2000; Rozzi, Pfefferkorn, et al. 2000). This model is able to predict the temperature
distribution by a heat transfer model in cylindrical coordinates (Fig. 3.4) by:
14
1
+1
+
+
=
+
+
(3.2)
Where, is the thermal conductivity, is the density, is the specific heat, is the rotation
speed of the workpiece, is the feed rate and is the volumetric heat generation during
machining. It is possible to calculate the temperature distribution for laser heating with or
without material removal by setting appropriate boundary conditions (Appendix-A)
(Dandekar et al. 2010).
Fig. 3.4 Heat transfer model in cylindrical coordinate system (a) without material removal
(Rozzi et al. 1998) (b) with material removal (Rozzi, Incropera, et al. 2000)
Numerical modelling has taken place in the field to study the thermal effects during the
LAM. In the field of ceramics, a numerical modelling of a workpiece was performed to
predict the temperature at machining zone. This was done to optimize the machining for
15
mullite by eliminating the occurrence of cracking (Rebro et al. 2004). Another three
dimensional heat transfer model was developed to predict the temperature distribution during
LAM for partially stabilized zirconia (PSZ). It was found that the temperatures lie within the
sensitivity limits (Pfefferkorn et al. 2005). Thermal model has also been developed for LAM
of silicon nitride ceramics in various studies with successful validation (Tian & Shin 2006;
Wu et al. 2010). A validated model was also used to understand the effect of various
parameters on the temperature distribution of the compacted graphite iron (Skvarenina &
Shin 2006).
Numerical modelling has been carried out over the years for the nickel based alloys like
Inconel-718 as well. Thermal modelling was carried out to determine the effects of laser
radiation over the surface of the material. The determined temperatures were validated by the
use of infrared camera (Anderson et al. 2006). A thermal model was developed for the laser
assisted milling of Inconel-718. The developed model was able to predict the temperature
distribution for the changing geometries of the workpiece successfully (Tian et al. 2008).
A thermal model for LAM was used to study the temperature distribution for Ti-6Al-4V by
conducting finite element simulation for process optimization in separate researches
(Dandekar et al. 2010; Yang et al. 2010).
16
4. Effect of Introduction of Laser
Energy in Machining
4.1 Cutting force
The cutting force is found to decrease when the material temperature is elevated or by
increasing the laser power. The reduction in cutting force is due to the reduction in yield
strength at elevated temperature. The reduction in cutting force can aid to reduction in the
machine power consumption. This also improves the process efficiency by allowing higher
feed rate and depth of cut (Brandt & Sun 2013).
It was found that increasing the workpiece surface temperature, reduces the force required for
cut in ceramics such as silicon nitride (Rozzi, Incropera, et al. 2000; Lei et al. 2001; J.-D.
Kim et al. 2011). In addition, it was also found that the cutting zone stresses were had no
significant dependence on the cutting speeds (Lei et al. 2000). A similar trend was observed
while cutting other ceramics such as mullite and PSZ (Fig. 4.1b). A reduction in the ratio of
feed force to cutting force was recorded for LAM of mullite and PSZ (Rebro et al. 2002;
Pfefferkorn et al. 2004).
Various studies have been carried out to study the effect of temperature on the cutting forces
of metals and alloys such as Inconel-718, compacted graphite iron, hardened steel and Ti-
6Al-4V. A significant drop in in cutting forces was recorded for titanium alloy Ti555-3 due to
elevation in temperature. However, it was pointed out that the a decrease is the surface
temperature is expected upon increase of cutting depth, the feed and decrease in the cutting
speed (Braham-Bouchnak et al. 2013). A similar trend was observed for the machining of Ti-
6Cr-5Mo-5V-4Al where significant reductions were only obtained at high temperatures or
high laser power (Rahman Rashid et al. 2012b). In a separate study it was found that a lower
magnitude of forces were required to cut pure titanium at elevated temperatures. It was also
pointed out that this reduction in cutting forces was a function of temperature rise at cutting
zone (Sun et al. 2008). Laser beam of 1600 W was found to reduce the cutting forces while
machining Ti-10V-2Fe-3Al by 10-17% in a recent study (Rahman Rashid et al. 2014). A
cutting force reduction of 31.6% was observed while milling Ti-6Al-4V at a laser power of
1382 W whereas, a reduction of only 16.95% was recorded for milling BTi-6431S with the
assist of a laser (Fig. 4.1a) (Gao et al. 2015).
17
(a) (b)
Fig. 4.1 Influence of laser power, feed rate and material removal rate on cutting force for (a)
BTi-6431S (Gao et al. 2015). (b) Magnesia partially stabilized zirconia (Pfefferkorn et al.
2004)
Cutting forces reduced by 25% in the nickel alloys such as Inconel-718 when the material
removal temperature was increased from 0oC to 620oC (Anderson et al. 2006). In another
study, the LAM was found to reduce the cutting force of Inconel-718 by up to 40% (Fig. 4.2)
(Germain et al. 2008). A similar trend was observed in another study where a significant
decrease in the cutting force was recorded at various feed rates (Attia et al. 2010; Venkatesan
et al. 2014). A cutting force reduction of 30% for Nickel 201 and 55% for Inconel 718 was
recorded in a recent study (Kim & Lee 2015).
Fig. 4.2 Influence of machining type and component length on cutting force (Germain et al.
2008).
18
A reduction in cutting force has been observed in LAM of ferrous alloys as well. A reduction
in the thrust component was observed as a result of laser assist during machining of AISI D2
(Dumitrescu et al. 2006). During the LAM of compacted graphite iron, reductions were
observed in the cutting force, feed force and the thrust force (Skvarenina & Shin 2006). A
20% cutting energy drop was observed for AISI 4130 when the material removal temperature
was increased above 200oC (Fig. 4.3a) (Ding & Shin 2010). LAM was found to be a suitable
option for machining high chromium white caste iron where a reduction of 24% was
observed in the cutting forces (Armitage et al. 2004). A reduction of 33% was observed in the
laser assisted milling of 17-4 precipitation hardened stainless steel (Bermingham et al. 2015).
In order to machine Ti MMC with an assist of lasers, it was found that there was decrease in
the cutting force required with the increase in temperature (Bejjani et al. 2011). Experimental
investigation was carried out for the LAM of Al-2% Cu/Al2O3 composite. It was found that
there were significant reduction in the cutting forces at material removal temperature of
300oC (Dandekar & Shin 2010). A 12% reduction in the specific cutting energy was observed
for the machining of A359 aluminum matrix composite reinforced with 20% by volume
fraction silicon carbide (SiC) particles (Fig. 4.3b) (Dandekar & Shin 2012).
(a) (b)
Fig. 4.3 Influence of temperature on cutting energy of (a) AISI 4130 (Ding & Shin 2010). (b)
Silicone carbide particle reinforced aluminium matrix composite (Dandekar & Shin 2012).
4.2 Tool Wear
Tool wear is highly dependent on the material removal temperature as the tool life is reduced
by the reduction of tool strength by overheating. The primary factors for the tool wear in
LAM of ceramics such as PSZ are abrasion, adhesion and diffusion. Edge craters were
observed for LAM of PSZ with PCBN at elevated temperatures but absent for LAM of Si3N4
19
(Fig. 4.4) (Pfefferkorn et al. 2004). It was also observed that an increased carbide tool life
when compared to conventional machining of mullite (Rebro et al. 2002).
Fig. 4.4 Flank wear over time for magnesia partially stabilized zirconia (Pfefferkorn et al.
2004).
It was reported that an improvement in tool life by a factor of 1.7 while machining Ti-6Al-4V
was obtained at a material removal temperature of 250oC with cobalt bound tungsten carbide
(WC/Co) (Dandekar et al. 2010). In another study, an appropriate material removal
temperature of 250oC was recommended to increase the tool life (Barnes et al. 2009). It has
been reported that cryogenic coolants such as liquid nitrogen are effective in increasing the
tool life while machining Ti-6Al-4V (Bermingham et al. 2011). LAM is found to have
significant increase in tool life was observed while milling BTi-6431S (Fig. 4.5) (Gao et al.
2015).
Fig. 4.5 Influence of laser power and material removal rate on flank wear (Gao et al. 2015).
20
It was suggested that tool failure during LAM of Inconel-718 occurs due to notch wear which
decreases with an increase in material removal temperature (Anderson et al. 2006). In another
study it was found that performance of the carbide tool drops significantly during the LAM of
Inconel-718 whereas, an improvement was observed for the ceramic tool. It was found that
there was an increase in the ceramic tool life during LAM when compared to that of
conventional machining (Germain et al. 2008). In order to get the least ceramic tool wear for
Inconel-718, a cutting speed of 300 m/min, feed of 0.4 mm/rev and cut depth of 0.25 mm was
suggested (Attia et al. 2010). An increased ceramic tool life of about 50% was recorded
during LAM of Waspaloy (Fig. 4.6) (Ding & Shin 2012).
Fig. 4.6 Influence of material removal rate and temperature on flank wear of Waspaloy (Ding
& Shin 2012).
An improvement in tool life by 100 % was recorded during LAM of AISI D2 steel during
LAM with a TiN coated carbide tool (Dumitrescu et al. 2006). An improvement of 60 % was
observed in the carbide tool life while machining compacted graphite iron at a feed of 0.150
mm/rev (Skvarenina & Shin 2006). A reduction in tool wear was observed in the
experimentation on AISI 4130 in another study (Ding & Shin 2010). Increased tool life was
also exhibited by laser assisted milling of 17-4 precipitation hardened stainless steel (Fig.
4.7a) (Bermingham et al. 2015).
Experimental investigations carried out on long fiber reinforced Al-2% Cu/Al2O3 exhibited
increased tool life of approximately 66% at a temperature of 400oC (Dandekar & Shin 2010).
An increased tool life was observed at high speed LAM of TiMMC with an unexpected
higher tool life at increased cutting speeds (Bejjani et al. 2011). LAM of A359 aluminum
matrix composite reinforced with 20% by volume fraction of SiC provided with an increased
21
tool life by a factor of 1.7-2.35 depending on the cutting speed (Fig. 4.7b) (Dandekar & Shin
2012).
(a) (b)
Fig. 4.7 Influence of material removal rate and machining type on flank wear during (a)
Milling of 17-4 precipitation hardened stainless steel (Bermingham et al. 2015). (b) Turning
of silicone carbide reinforced aluminum matrix composite (Dandekar & Shin 2012)
4.3 Surface Integrity
Surface roughness was found to decrease in the workpiece of mullite machined by assist of
laser (Rebro et al. 2002; Rebro et al. 2004). It was found in another study that preheating was
essential for machining PSZ for acceptable surface finish (Pfefferkorn et al. 2005).
LAM of Ti-6Al-4V saw a decrease in the surface roughness by 30% at temperature of 300oC
(Dandekar et al. 2010). Excessive heat energy can cause welding of chip to the tool which
inversely affects the surface integrity of Ti-6Cr-5Mo-5V-4Al. A speed of 25 m/min was
suggested to eliminate the welding of chip to the tool (Rahman Rashid et al. 2012a).
LAM was found to improve the surface roughness of Inconel-718 two fold at a temperature
of 540oC when compared to conventional machining at room temperature. A slight increase
in surface roughness was found associated with the increase in cutting speed (Anderson et al.
2006; Tian et al. 2008). Similarly, a significant decrease in the surface roughness was
observed for the LAM of Inconel-718 in another study (Germain et al. 2008; Attia et al. 2010;
Navas et al. 2013).
22
An improvement of 5% was achieved in the surface finish during the LAM of compacted
graphite iron at around 400oC (Skvarenina & Shin 2006). LAM of AISI 4130 exhibited better
surface finish at a temperature of 150oC (Ding & Shin 2010).
A significant decrease in the surface roughness was observed in the LAM of long fiber
reinforced Al-2% Cu/Al2O3. At a material removal temperature of 400oC, the surface
roughness was found to decrease by 70% (Dandekar & Shin 2010). However, an increase in
surface roughness by 15% was observed for LAM of TiMMC at 500oC (Bejjani et al. 2011).
A significant decrease in the surface roughness was recorded during LAM of A359 aluminum
matrix composite reinforced with 20% by volume fraction of SiC (Dandekar & Shin 2012).
23
5. Discussion and Conclusion
Results from various researches have proved that LAM is a suitable technique that can be
employed for the machining of advanced materials including ceramics, titanium and nickel
based superalloys, ferrous alloys and MMCs. LAM has also been found to reduce the
manufacturing costs substantially by decreasing the tool wear and increasing the material
removal rates at the same time. However, various studies in this field are required to
understand the effect of the laser and traditional machining parameters on material and
subsequent temperature distribution in this complex machining process for process
optimization.
24
6. Future Work
Various parameters are responsible for influencing the LAM process. This review covers the
influence of certain laser parameters and their subsequent effect on cutting forces, tool wear
and surface integrity of ceramics, titanium and alloys, nickel and alloys and MMCs. Future
work is recommended to study the developments taking place in the integration of LAM with
other machining techniques. It is suggested to include a detail of material science and its
reaction to the incident laser beam. Process optimization with the aid of finite element
analysis is another topic that should be included.
25
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32
Appendix-A
Boundary conditions for thermal model (Rozzi et al. 1998; Rozzi, Incropera, et al. 2000)
Without material removal
With material removal
Workpiece surface
=,
()
+()
for [( )]+( )
=
()+()
for
[( )]+( )>
Workpiece surface
unmachined
=
()
for >() and (,,)1 on laser spot
=
()
for >() and (,,)1 off laser
spot
Workpiece surface
machined
,
=,
()
for <() and (,,)1 on laser spot
,
=
()
for <() and (,,)1 off laser
spot
Centerline of
workpiece
= 0
= 0
Interface between
machined and
unmachined material
()
=
+()
for , and 0 < 2
()
=
for
,
and
2 < 2
End faces of
workpiece
=
+()()
= 0
,()
=
+()
= 0
Material removal plane
=( )
Circumferential
direction
(,,,)=(,+ 2,,)
=
(,,)=(,+ 2,)
=
Initiation
(,,, 0)==
(,,, 0)==
33
