67896550-26938410-Laser-Beam-Machining-LBM

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Laser Beam Machining (LBM)By: Dhiman Johns M.E.(PIE), Thapar University, Patiala


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LASER stands for Light Amplification by Stimulated Emission of Radiation. The underline working principle of laser was first put forward by Albert Einstein in 1917 though the first industrial laser for experimentation was developed around 1960s. Laser beam can very easily be focused using optical lenses as their wavelength ranges from half micron to around 70 microns.

Laser Beam Machining An Introduction


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Focussed laser beam can have power density in excess of 1 MW/mm2. Laser Beam Machining or more broadly laser material processing deals with machining and material processing like heat treatment, alloying, cladding, sheet metal bending etc. Such processing is carried out utilizing the energy of coherent photons or laserbeam, which is mostly converted into thermal energy upon interaction with mostof the materials.


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As laser interacts with the material, the energy of the photon is absorbed by the work material leading to rapid substantial rise in local temperature. This inturn results in melting and vaporisation of the work material and finally material removal. Nowadays, laser is also finding application in regenerative machining or rapid prototyping as in processes like stereo-lithography, selective lasersintering etc.


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Laser Beam Machining The Lasing Process Lasing process describes the basic operation of laser, i.e. generation of coherent beam of light by light amplification using stimulated emission. In the model oftom, 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. Eac

h of the orbital electrons is associated with unique energy levels.


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At

absolute zero temperature an atom is considered to be at ground level, when allthe electrons occupy their respective lowest potential energy.

The electrons at ground state can be excited to higher state of energy by absorbing energy from external sources like increase in electronic vibration at elevat

ed temperature, through chemical reaction as well as via absorbing energy of thephoton. Fig. 1 depicts schematically the absorption of a photon by an electron.The electron moves from a lower energy level to a higher energy


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Figure 1 ,

Energy bands in materials


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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 aphoton. This is called spontaneous emission. Schematically the same is shown inFig. 1 and Fig. 2. The spontaneously emitted photon would have the same frequency as that of the exciting photon.


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Fig . 2 Spontaneous and Stimulated emissions


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Sometimes such change of energy state puts the electrons in a meta-stable energyband. Instead of coming back to its ground state immediately it stays at the elevated energy state for micro to milliseconds. In a material, if more number ofelectrons can be somehow pumped to the higher meta-stable energy state as compared to number of electrons at ground state, then it is called population inversion. Such electrons, at higher energy meta-stable state, can return to the ground state in the form of an avalanche provided stimulated by a photon of suitable freq

uency or energy. This is called stimulated emission. Fig.2 shows one such higherstate electron in meta-stable orbit.


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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 will be produced.In this way coherent laser beam can be produced.

Fig. 3 schematically shows working of a laser.


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Fig . 3

Lasing Action


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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 otherend is having a partially reflective mirror. Population inversion can be carriedout by exciting the gas atoms or molecules by pumping it with flash lamps. Thenstimulated 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 lon

gitudinal direction would form coherent, highly directional,


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Lasing Medium- Heart Of LASER 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 arecommonly of the following type Ruby which is a chromium alumina alloy having a wavelength of 0.7 m Nd-glass lasers having a wavelength of 1.64 m. Nd-YAG laser having a wavelength of 1.06 m.

(Nd-YAG stands for neodymium-doped yttrium aluminium garnet; Nd:Y3Al5O12 )

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


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The generally used gas lasers are: Helium Neon Argon CO2 etc. Lasers can beated in continuous mode or pulsed mode. Typically CO2 gas laser is operated in continuous mode and Nd YAG laser is operated in pulsed mode.


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Schematic diagram of Laser Beam Machine

Figure 4


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Material Removal Mechanism In LBM

Figure 5 Physical processes occurring during LBM


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As presented in Fig. 5, the unreflected light is absorbed, thus heating the surface of the workpiece. On sufficient heat the workpiece starts to melt and evaporates. The physics of laser machining is very complex due mainly to scattering and reflection losses at the machined surface. Additionally, heat diffusion into the bulk material causes phase change, melting, and/or vaporization. Depending onthe power density and time of beam interaction, the mechanism progresses from one of heat absorption and conduction to one of melting


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Machining by laser occurs when the power density of the beam is greater than what is lost by conduction, convection, and radiation, and moreover, the radiationmust penetrate and be absorbed into the material. The power density of the laserbeam, Pd, is given by

Pd =

4Lp

Fl22T

The size of the s ot dimeter ds is

ds = Fl


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The mchining r

te (mm/min) can be described as

ollows:

C L = l EP A h v b

A b o(F l 2 be

m

t foc

l oint, mm2 ) Where Ab =

re

l

ser 4 = = E v (F l 2 ) h

lLP 4C

Therefore,


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The volumetric removl r

te (VRR) (mm3/min) c

n be c

lcul

ted

s follows:

C lL P VRR= Ev h

where Pd = ower density, W/cm2 L = lser ower, W Fl = foc

l length of lens, c

m T =

ulse dur tion of l ser, s = be m divergence, r d Cl = const nt de

ending on the m

teri

l

nd conversion efficiency Ev = v

oriztion energy of the m

teri

l,

W/mm3 Ab =

re

of l

ser be

m

t foc

l

oint, mm2 h = thickness of m

teri

l, mm


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LASER Bem M

chining A lic

tion

Lser c

n be used in wide r

nge of m

nuf

cturing

lictions M

teri

l remov

l d

rilling, cutting

nd tre

nning Welding Cl

dding Alloying

Drilling micro-sized holes using lser in difficult to m

chine m

teri

ls is the

most domin

nt

lic

tion in industry. In l

ser drilling the l

ser be

m is focused over the desired s ot size. For thin sheets ulse l

ser c

n be used. For thic

ker ones continuous lser m

y be used.


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Pr

meters Affecting LBM

Figure 6


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Fig. 6 resents the fctors which

ffect the LBM rocess. The f

ctors c

n be rel

ted to LBM Drilling rocess

nd

re discussed below: Pulse Energy: It is recomm

ended tht the required e

k ower should be obt

ined by incre

sing the ulse en

ergy while kee ing the ulse durtion const

nt. Drilling of holes with longer u

lses cuses enl

rgement of the hole entr

nce. Pulse Dur

tion: The r

nge of ulse

dur

tions suit

ble for hole drilling is found to be from 0.1 to 2.5 millisecond. High ulse energy (20J)

nd short ulse dur

tion

re found suit

ble for dee h

ole drilling in eros

ce m teri ls.


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Assist Gses: The g

s jet is norm

lly directed with the l

ser be

m into the inte

rction region to remove the molten m

teri

l from the m

chining region

nd obt

i

ncle

n cut. Assist g

ses

lso shield the lens from the ex elled m

teri

l by s

etting u high ressure b

rrier

t the nozzle o ening. Pure oxygen c

uses r

idoxid

tion

nd exothermic re

ctions, c

using better rocess efficiency. The sele

ction of

ir, oxygen, or

n inert g

s de ends on the work iece m

teri

l

nd thickness. M

teri

l Pro erties

nd Environment: These include the surf

ce ch

r

cteri

stics such s reflectivity nd bsor

tion coefficient of the bulk m teri l. Addition

lly, therm

l conductivity

nd diffusivity, density, s ecific he

t,

nd l

te

nt he

t

re

lso considered.


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Lser Be

m Selection Guide


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Lser Be

m M

chining: New Develo ments

In 1994 Lu et

l., introduced the ultr

sonic

ssisted l

ser m

chining technique

not only to increse the hole de th but

lso to im rove the qu

lity of holes r

oduced inluminium-b

sed met

l m

trix com osites (MMC). Using such

method, th

e hole de th ws incre

sed by 20 ercent in

ddition to the reduced degree of ho

le t

ering. In 1995 Hsu

nd Moli

n, develo ed

l

ser m

chining technique th

tem loys du

l g

s jets to remove the viscous st

ge in the molten cutting front

n

d, thereby, llowing st inless steel to be cut f ster, cle ner, nd thicker.


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In 1997, Toddnd Co ley develo ed

rototy e l

ser rocessing system for sh

ing

dv

nced cer

mic m

teri

ls. This rototy e is

fully

utom

ted, five

xis, cl

osed-loo controlled lser sh

ing system tht

ccur

tely

nd cost effectively

roduces com lex sh

es in thebove-mentioned m

teri

l. L

ser Assisted EDM: In 1

997, Allennd Hu

ng develo ed

novel combin

tion of m

chining rocesses to f

b

ric

te sm

ll holes. Before the microEDM of holes, co er v

our l

ser r

di

tionw

s used to obt

in

n

rr

y of sm

ll holes first. These holes were then finished

by micro-EDM. Their method showed th t the m chining s

eed of microEDM h d beenincre

sed

nd electrode tool we

r w

s m

rkedly reduced while the surf

ce qu

lit

y rem

ined unch

nged.


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Lser Be

m M

chining Adv

nt

ges

Tool wer

nd bre

k

ge

re not encountered. Holes c

n be loc

ted

ccur

tely by u

singn o tic

l l

ser system for

lignment. Very sm

ll holes with

l

rge

s ect

rtio c

n be roduced. A wide v

riety of h

rd

nd difficult-to-m

chine m

teri

l

s cn be t

ckled. M

chining is extremely r

idnd the setu times

re economic

l. Holes c

n be drilled

t difficult entr

nce

ngles (10 to the surf

ce). Bec

useof its flexibility, the rocess c

n be

utom

ted e

sily such

s the on-the-fly

o

er tion for thin g uge m teri l, which requires one shot to

roduce hole. The o er

ting cost is low.


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Lser Be

m M

chining Limit

tions

High equi ment cost. T

ersre norm

lly encountered in the direct drilling of h

oles. A blind hole of recise de th is difficult tochieve with

l

ser. The th

ickness of the mteri

l th

t c

n be l

ser drilled is restricted to 50 mm. Adhere

nt mteri

ls, which

re found norm

lly

t the exit holes, need to be removed.


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References: Adv

nced M

chining Processes By H

ss

n Abdel-G

w

d El-Hofy Non Convention

l M

ch

ining By P.K. Mishr


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