Report on lbm

Its a seminar report on Laser Beam Machining.

1. MAHARISHI MARKANDESHWAR UNIVERSITY
SADOPUR, AMBALA

A
Seminar Report
on
LASER BEAM MACHINIG


Department of Mechanical Engineering

SUBMITTED TO: SUBMITTED BY:
Miss. Preeti Saini Mahesh Naagar
B. Tech. (Mech.)
7
th
Semester
75144001

2. CONTENTS
SR. NO. PARTICULARS PAGE NO.
01. Abstract 01
02. Introduction 02
03. The Lasing process 03
04. Lasing medium 06
05. Laser construction 07
06. Experimental Setup 10
07. Mechanism of material removal 12
08. Material removal 14
09.
Improvisation & Advancements of LBM
Process
16
10. Application 19
11. Advantages 21
12. Limitation 21
13. Conclusion 22
14. References 23

3. 1

ABSTRACT:
The high intensity which can be obtained by focusing the pulsed energy emitted by a
LASER can offer potential as a tool for nearly forceless machining. The method can
be used on any material, regardless of thermal properties, which can be evaporated
without decomposition, including almost all ceramics and metals.

With most substances, almost all of the material removed by LASER machining
leaves in the liquid state. Only a small fraction is vaporized, and the high rate of the
vaporization exerts forces which expel the liquid metal.

All features of LASER beam machining improve with increased intensity. The higher
the intensity, the less heat is resonant in the uncut material, an important
consideration with materials which are sensitive to heat shock, and the more efficient
the process is in terms of volume of material removed per unit of energy. The
intensities which are available with the LASER are high enough so that the heat
affected zone (HAZ) on a cut surface is too small to be detected and there is no
solidified liquid film residue on the cut surface.

4. 2

INTRODUCTION:
LASER BEAM MACHINING (LBM) is a valuable tool for drilling, cutting and
milling of almost any material.

The mechanism by which a LASER beam removes material from the surface being
worked usually involves a combination of melting and evaporation, although with
some materials, such as carbon and certain ceramics, the mechanism is purely one of
evaporation. Any solid material which can be melted without decomposition can be
cut with the LASER beam.

Advances in nanotechnology motivate the extension of LASER machining of
microstructures to the smaller dimensions of interest. Optical LASERs such as
RUBY LASERs and CO2 LASERs are widely used for micro-milling and micro-hole
drilling over a wide range of materials. The size of the smallest features that can be
created focusing intense LASER beams onto materials is limited mainly by the
LASER wavelength and by the diffusion of heat.

A variety of different techniques have been developed to overcome the limitations
imposed by the diffraction limit in order to produce ablation craters of sub-
wavelength size using optical and UV-LASERs.

Nowadays, there have been several experiments over a wide range of LASER
applications for material removal and cutting in which UV-LASERs and femto-
second LASERs are the most popular for industrial use. Also efforts have been made
to minimize the tapers and HAZ which result due to high temperature of the LASER
beam.

5. 3

LASER BEAM MACHINING – THE LASING PROCESS
Lasing process describes the basic operation of laser, i.e. generation of coherent (both
temporal and spatial) beam of light by “light amplification” using “stimulated
emission”.

In the model of atom, negatively charged electrons rotate around the positively
charged nucleus in some specified orbital paths. The geometry and radii of such
orbital paths depend on a variety of parameters like number of electrons, presence of
neighbouring atoms and their electron structure, presence of electromagnetic field
etc. Each of the orbital electrons is associated with unique energy levels. At absolute
zero temperature an atom is considered to be at ground level, when all the electrons
occupy their respective lowest potential energy. The electrons at ground state can be
excited to higher state of energy by absorbing energy form external sources like
increase in electronic vibration at elevated temperature, through chemical reaction as
well as via absorbing energy of the photon. Fig. depicts schematically the absorption
of a photon by an electron. The electron moves from a lower energy level to a higher
energy level.

On reaching the higher energy level, the electron reaches an unstable energy band.
And it comes back to its ground state within a very small time by releasing a photon.
This is called spontaneous emission. The spontaneously emitted photon would have
the same frequency as that of the “exciting” photon.

Sometimes such change of energy state puts the electrons in a meta-stable energy
band. Instead of coming back to its ground state immediately (within tens of ns) it
stays at the elevated energy state for micro to milliseconds. In a material, if more
number of electrons can be somehow pumped to the higher meta-stable energy state
as compared to number of atoms at ground state, then it is called “population
inversion”. Such electrons, at higher energy meta-stable state, can return to the

6. 4

ground state in the form of an avalanche provided stimulated by a photon of suitable
frequency or energy. This is called stimulated emission. Fig. shows one such higher
state electron in meta-stable orbit. If it is stimulated by a photon of suitable energy
then the electron will come down to the lower energy state and in turn one original
photon, another emitted photon by stimulation having some temporal and spatial
phase would be available. In this way coherent laser beam can be produced.


Energy bands in materials

7. 5


Spontaneous and stimulated emissions

Fig. schematically shows working of a laser. There is a gas in a cylindrical glass
vessel. This gas is called the lasing medium. One end of the glass is blocked with a
100% reflective mirror and the other end is having a partially reflective mirror.

8. 6

Population inversion can be carried out by exciting the gas atoms or molecules by
pumping it with flash lamps. Then stimulated emission would initiate lasing action.
Stimulated emission of photons could be in all directions. Most of the stimulated
photons, not along the longitudinal direction would be lost and generate waste heat.
The photons in the longitudinal direction would form coherent, highly directional,
intense laser beam.


Lasing action

LASING MEDIUM
Many materials can be used as the heart of the laser. Depending on the lasing
medium lasers are classified as solid state and gas laser. Solid-state lasers are
commonly of the following type

• Ruby which is a chromium – alumina alloy having a wavelength of 0.7
μm
• Nd-glass lasers having a wavelength of 1.64 μm
• Nd-YAG laser having a wavelength of 1.06 μm

These solid-state lasers are generally used in material processing.
The generally used gas lasers are

• Helium – Neon

9. 7

• Argon
• CO
2
etc.

Lasers can be operated in continuous mode or pulsed mode. Typically CO
2
gas laser
is operated in continuous mode and Nd – YAG laser is operated in pulsed mode.


LASER CONSTRUCTION
Fig. shows a typical Nd-YAG laser. Nd-YAG laser is pumped using flash tube. Flash
tubes can be helical, as shown in Fig. 9.6.10, or they can be flat. Typically the lasing
material is at the focal plane of the flash tube. Though helical flash tubes provide
better pumping, they are difficult to maintain.


Solid-state laser with its optical pumping unit
Fig. shows the electrical circuit for operation of a solid-state laser. The flash tube is
operated in pulsed mode by charging and discharging of the capacitor. Thus the pulse
on time is decided by the resistance on the flash tube side and pulse off time is

10. 8

decided by the charging resistance. There is also a high voltage switching supply for
initiation of pulses.
Fig. shows a CO
2
laser. Gas lasers can be axial flow, as shown in Fig., transverse flow
and folded axial flow as shown in Fig. The power of a CO
2
laser is typically around
100 Watt per metre of tube length. Thus to make a high power laser, a rather long
tube is required which is quite inconvenient. For optimal use of floor space, high-
powered CO
2
lasers are made of folded design.
In a CO
2
laser, a mixture of CO
2
, N
2
and He continuously circulate through the gas
tube. Such continuous recirculation of gas is done to minimize

Working of a solid-state laser

11. 9


Construction of a CO
2
laser
consumption of gases. CO
2
acts as the main lasing medium whereas Nitrogen helps in
sustaining the gas plasma. Helium on the other hand helps in cooling the gases.
As shown in Fig. high voltage is applied at the two ends leading to discharge and
formation of gas plasma. Energy of this discharge leads to population inversion and
lasing action. At the two ends of the laser we have one 100% reflector and one partial
reflector. The 100% reflector redirects the photons inside the gas tube and partial
reflector allows a part of the laser beam to be issued so that the same can be used for
material processing. Typically the laser tube is cooled externally as well.
As had been indicated earlier CO
2
lasers are folded to achieve high power. Fig. shows
a similar folded axial flow laser. In folded laser there would be a few 100% reflective

12. 10

turning mirrors for manoeuvring the laser beam from gas supply as well as high
voltage supply as shown in Fig

Construction of folded gas laser

EXPERIMENTAL SETUP:
Shown below is the experimental setup of Excimer LASER beam system.

13. 11


(1) LASER unit
(2) LASER beam
(3) LASER shutter
(4) Attenuator
(5) & (6) LV1, LV2 (vertical lens)
(7) Mirror 1
(8) & (10) LH1, LH2 (horizontal lens)
(9) Mirror 2
(11) Scanning system
(12) Mirror 3
(13) Field lens
(14) Mask plane
(15) Projection lens
(16) Photo diode detector
(17) Diode LASER
(18) Z-axis
(19) X-axis
(20) Y-axis.

An Excimer LASER operates at 248 nm with 400 mJ maximum output pulse energy,
an average power of 100 Watt and 200 Hz maximum repetition rate. The beam
exiting the LASER is rectangular in shape and not of homogeneous intensity. To
correct the beam, the optics train made of cylindrical lenses (LV1-LV2 and LH1-
LH2) force the beam and makes it parallel with the square cross section in the vertical
and horizontal directions. Mirror 3 scans the beam across the mask plane to make it
homogeneous. The beam further passes through the mask plane/ aperture with a
maximum area of 15X15 mm2 before finally going through a projection lens that
gives 15 times linear reduction at the work piece. By changing the mask aperture,

14. 12

beam spots of different size and shape can be generated at the work piece. Automated
or manual focus control is achieved using a diode LASER beam reflected from the
work piece surface and a photo-diode array detector to provide positional
measurement.

MECHANISM OF MATERIAL REMOVAL:
The material removal by LBM process and vaporized energies are shown in the
figure below.


When LASER hits the material surface, it will have some recoil force. It can drive the
liquid away from the sides. Short pulsed LASERs generate higher recoil and it results
in farther liquid removal.
UV LASER will generate high temperature on material, and removed material gets
ionized. This will form plasma in the hole. Plasma can absorb further incoming
LASER energy. Part of it gets reemitted in wide spectrum and wide angle. It help the

15. 13

LASER energy coupling to material and also resulting in a larger "heat affected
zone".





LASER induced effects in the LBM process.
LASER beam machining is a thermal process with emphasis laid on heat
requirements and heat utilization. It is also important to determine physical properties
of the work piece material and their relationship to the operating characteristics of
optical LASERs.

The following factors have to be taken into account while LASER machining:
1. Part of the energy (Large part in case of highly reflective metal surfaces) is
reflected and lost.
2. Most of the energy which is not reflected is used for material removal.
3. A very small part of the energy is used to evaporate the liquid material.
4. Another small part of energy is conducted into the converted base material.

16. 14


The relative magnitudes of these four avenues of heat consumption depend strongly
upon the thermal and optical properties of the material being worked and the intensity
and pulse duration of the LASER beam. Time distribution of energy also plays an
important role.
The most prominent misconception in LBM is that the entire material being removed
is evaporated. But the large quantity of energy which would be required for this to
happen be not actually consumed which substantiates the argument. Most of the
material leaves the work piece surface in the liquid state and relatively high velocity.


MATERIAL REMOVA L:

The basic assumptions to analyse the material removal process are:
1. The intensity of LASER beam does not vary with time.
2. LASER beam is uniform over the entire area of the hotspot.
3. The material being removed is both melting and evaporating.
4. The steady state ablation is characterized by constant rate of material removal
and by the establishment of a steady temperature distribution.

According to the above assumptions, the steady temperature distribution is given by,
(T – To)/ (Tm – To) = e
-Vx/α

Where,
T = temperature at distance x below the ablating surface,
To = initial uniform temperature of the work piece,
Tm = melting point of the work piece
V = steady ablation velocity,

17. 15

a = thermal diffusivity of work piece, i.e., (K/)* Cp
K, , Cp= thermal conductivity, density, and specific heat, respectively, of the work
piece.

It can be seen that the exponential distribution represented by (1) confirms with
boundary conditions that T= To when x is very large and T = Tm when x=0.
The depth at which heat penetrates the ablating surface is of considerable practical
importance. It is reflected in the depth of the HAZ which will be left when the
ablation process is over. It is desirable to keep the HAZ as shallow as possible.

A simple way to identify the depth of heated layer is to define a characteristic depth
x:
Xc = α/V

The characteristic depth Xc is the depth during steady ablation which has experienced
a temperature rise 1/2.718 of the way from To to Tm. The characteristic heated depth
X. decreases with increasing ablation velocity and increases with increasing thermal
diffusivity.

During the initial transient period when ablation is just beginning, part of the heat
delivered to the work surface is being used to establish the temperature distribution
within the solid. Once steady conditions are obtained, the heat contained in the solid
does not increase any further, and the value of this steady heat content is given by:

(Q/A0)= ∫ p(T-To) dx = K(Tm – To)/V

After steady ablation is realized, the relationship between the intensity, exposure
time, thickness of material which has been removed, and thermal properties of the
material is:

18. 16


t= K(Tm – To )ρH/ + ρHd

Where, t is the exposure time.

IMPROVISATIONS AND ADVANCEMENTS IN LBM PROCESS:
Over the years there have been many research and advancements to improve various
parameters of LBM process like Material Removal Rate (MRR), Reducing HAZ,
Improving accuracy, Thermal Effect Characterization, analysis of ablation rate etc. A
few of them have been discussed below.
One of the major areas of research in the past few years has been Nano-Machining
through LASERs. Today LBM finds its applications in electronic equipments, micro-
machine devices, and also biochemical, medical and chemical fields.

LASER beams used in such fields are short wave length and ultra-short pulsed
beams. Beam quality is very important in micro-machining.

So in one of the experiments, space filtering of the beam was executed using a beam
expander and the diameter of focused beam was able to be minimized. LASER power
irradiated onto specimens was controlled by the attenuator. Debris attaches on the
surface of specimen by irradiation of LASER beams with very high power density.
So, the machining was carried out in flowing water to avoid attachment of debris.
Arbitrary shapes were machined using developed LASER machining system.

While, in another experiment, a UV-LASER was used but it was implemented using
different technologies and the results examined for finish. Technologies which were
used are:

19. 17

 Microvia Drilling:

The microvia technology brings about multiple benefits. It improves routing density
of the buildup layers, and also reduces layer count and chip spacing which leads to
significant cost reduction. It also improves electrical performance to meet the demand
for high frequency applications.
In via drilling, the photochemical process leads to remarkably clean via walls free of
carbonized debris or heat affected zones. The other materials, including copper, glass
and other inorganic materials, generally interact with UV photons through photo
thermal process. In such a case, materials are removed in the mixed form of
overheated melt and vapor. In order to obtain a perfectly clean feature, such as a
microvia, free of debris, it is expected that the materials are all driven rapidly through
the melt phase and into the vapor phase prior to expulsion from the interaction zone
by the gas dynamic effects.

 Direct Copper Structuring:

Since UV LASER couples with copper very well, the UV LASER drill systems can
be also used for direct copper structuring to make fine patterns. The beam positioners
accurately move the LASER beam based on electronic CAD layout data. After light
copper etching and cleaning, the well-defined features will remain present. This
process leads to remarkably clean features free of carbonized debris or heat affected
zones.

Efforts have been made to develop a LASER processing technology for High
Thermal Radiation Multilayer Module.

As short pulse LASER (pulse width: femto-nano seconds) is suited to decrease the
thermal damage of the material. The experiment was done using a DPSS UV LASER

20. 18

(λ = 355nm, pulse width 15nsec). The LASER beam diameter on the specimen is
changed by a metal mask, on which there are pinholes (0.45, 0.65, 0.75mm). Pulse
energy is adjusted by the diode current of the oscillator.



The configuration of the experimental setup.

Shown below are the processing results when resin with aluminum filler was
irradiated with a UV LASER using a metal mask with pinhole sizes of 0.45, 0.65 and
0.75 mm. In this graph, the X-axis indicates the LASER fluency [J/cm2] and Y-axis
is the diameter of via hole.

21. 19

Next, we studied the causes that influence the fluency and beam diameter threshold.
The figure above (Right) shows the relationship between fluency and ablation depth
per pulse about sintered aluminum oxide. The horizontal axis indicates fluency with a
logarithmic scale and the vertical one the ablation speed. The relationship of these
satisfies the following formula.
Lp = Alog (F/Fth)


APPLICATIONS OF LBM:

A great advantage of LASER machining is capability to machine any kind of
material, not necessarily conductive, depending on LASER intensity and interaction
time. In contrast to some other processes, LASER operates using high energy photons
therefore there is not a typical tool as the LASER beam directly targets the work
pieces and machines breaking the work piece chemical bonds. LASER ablation
mechanism makes it possible to introduce the desired shape geometry of the work
piece without any prior preparations. This feature makes LASER machining
particularly feasible for wide range of applications.
Laser can be used in wide range of manufacturing applications
• Material removal – drilling, cutting and tre-panning
• Welding
• Cladding
• Alloying

Drilling micro-sized holes using laser in difficult – to – machine materials is the most
dominant application in industry. In laser drilling the laser beam is focused over the
desired spot size. For thin sheets pulse laser can be used. For thicker ones continuous
laser may be used.

22. 20


LASER beam machining processes in relationship to LASER intensity and interaction time.

Above figure shows the various processes for which LBM can be used.

The various range of traditional LASER applications are shown in the below figure
showing its uses in production of an automobile. The reasons of LASER use are
relatively high processing time compared to conventional processes, high flexibility
that enables easy automation for example using robot arms.

23. 21

ADVANTAGES

 In laser machining there is no physical tool. Thus no machining force or wear
of the tool takes place.
 Large aspect ratio in laser drilling can be achieved along with acceptable
accuracy or dimension, form or location
 Micro-holes can be drilled in difficult – to – machine materials
 Though laser processing is a thermal processing but heat affected zone
specially in pulse laser processing is not very significant due to shorter pulse
duration.


LIMITATIONS

 High initial capital cost
 High maintenance cost
 Not very efficient process
 Presence of Heat Affected Zone – specially in gas assist CO2 laser cutting
 Thermal process – not suitable for heat sensitive materials like aluminium glass
fibre laminate as shown in Fig.

Aluminium Glass Fibre Laminate – heat sensitive glass fibre layer due to presence of resin as binder

24. 22

CONCLUSIONS:
After studying various papers, it can be concluded that in a LBM operation:
1. When optimal focus position is centered in the work piece, there is an optimal
interaction between the number of required scans, the diameter on the LASER
beam input as well as on the output side, and the associated flank angle. The
further off-centered the focus position is, the more scans are required for a full
cut. The diameter on the LASER beam output side wanes, the deeper the focus
is positioned in the work piece.
2. The optimal feed rate obtained after conducting various experiments, amounts
to 8 mm/s. This results in a pulse overlap of 97.7 %. Even if higher pulse
overlap values reduce the required number of scans, they are not usable.
3. By analyzing the influence of the pulse overlap to the diameter on the LASER
beam output as well as on the input side, there was no dependency discovered.
4. The investigation of the track overlap found that the best value for the track
overlap amounts to 14.3 %.
5. To enlarge the kerf width the number of tracks need to be increased. The more
nested circles into each other, the less number of scans are required. However,
it is essential here to take into account the increasing required manufacturing
time. The optimal value of concentric circles is two. More than two circles are
not an efficient operation.
6. If a closed configuration on the LASER beam output side is required only low
wobbling frequencies are usable.

25. 23

REFERENCES
 www.google.co.in
 https://en.wikipedia.org/wiki/Laser_beam_machining
 https://basicmechanicalengineering.com/laser-beam-machining-lbm-principles-
and-applications/
 http://www.mech4study.com/2017/03/laser-beam-machining-principle-
working-equipment-application-advantages-and-disadvantages.html
 http://www.mechanicalbooster.com/2017/05/laser-beam-machining.html
 http://www.enggarena.net/2015/04/construction-and-working-of-laser-
beam.html
 http://www.123seminarsonly.com/EC/Laser-Beam-Machining.html