LASER BEAM MACHINING CONTENTS - PowerPoint Presentation

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LASER BEAM MACHINING

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CONTENTS

IntroductionLaser action and Population inversionMethods to achieve Population inversionTypes of Laser

Process and Beam Parameters

Process Capabilities

Application ,Advantages, Disadvantages

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Introduction

In LBM work material is melted and evaporated by intense monochromatic beam of light called laser

LASER-Light Amplification by Stimulated Emission of Radiation

The laser is electromagnetic radiation .

LBM is used in machining difficult to machine material and with the ability to produce heat in a very small area where laser beam strikes can melt any of the known material

Light energy emitted by laser has several characteristics

1)Spectral Purity

(highly monochromatic permits simple lenses for focusing ,expensive color corrected lens not required)

2)Directivity

(highly collimated with less divergence angle)

3)High focused power density

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Lasing action and population inversion

The operation of laser depends on the naturally occurring transitions between energy levels

Planks assumption

(energy levels of atoms are equally spaced),

Einstein view point

(light is really photon i.e., particles of energy ),

Bohrs

pointing out

( system of atoms has energy levels but not spaced equally)

Let us consider an system of energy levels with horizontal lines and an atom condition in the excited state .

As we know that atom in any excited state ,sooner or later drop to lower energy levels radiating energy in the form of light

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The frequency of emitted light is determined by principle of conservation of energy between the difference of energy levels an quantum mechanics

When transition from energy E2

-E

0

= h

ν

Figure 26.0

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Einstein introduced the idea of interaction of radiation with the matter , Einstein proposed when such a atom has light of right frequency acting it , it can absorb energy

Figure 26.1

The probability that phenomenon occurs per second depends on :

1. Energy levels of two states between which transition takes place

2. Intensity of the light acting on the atom

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Absorption :

The phenomenon of the movement of atoms from lower to higher energy levels is known as absorption

Figure26.2

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Emission :

Movement of atom from higher energy level back to lower energy level is called Emission . Einstein proposed two theories by which such an emission can occurSpontaneous emissions:

in this no light is present at the higher energy level and atom in an excited state could fall to lower energy level by emitting a photon this process independent of light intensity

Stimulated emission:

the emission is influenced by presence of light ,if the light of right frequency acts on an atom , an increased rate of photons will be emitted and such emission are proportional to light intensity

Fiugure

26.3 Figure 26.4

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If the absorption rate for a given energy level is known ,we can mathematically calculate spontaneous emission rate and stimulated emission rate.

In the presence of light of appropriate frequency stimulated emission will occur in the upper energy level . When the atoms begin to emit , and a chain reaction will occur by causing more to emit and the whole avalanche would dump down together . This is called the lasing action. Stimulated emission is the basis of the laser operation.

Stimulated emission is in phase with the incident wave which is amplified but is also attenuated by absorption .

From Maxwell-Boltzmann relation it is known that the number of atoms of

q

th

level is greater than num of atoms in

p

th

level when energy

Ep> Eq In this situation ,with the shining of light the process of absorption predominates If number of

p

th

level is greater than number of the atoms in

q

th

level ,then population inversion occurs

.population inversion is required for stimulated emission to predominate

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Methods to achieve population inversion

Different methods to achieve population inversion

Optical pumping (example, ruby laser)

Direct electron

excitation(example , argon laser)

Inelastic atom-atom collisions(example, helium-neon laser)

Optical pumping

: An external light source is used to produce a high population of some particular energy in the laser medium by selective optical absorption . optical excitation is used in the solid state lasers such as ruby laser

figure optical pumping

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Direct electron excitation( argon laser

): in a gaseous ion (argon gas) ,direct electron excitation in a gaseous discharge is used to produce the population inversion ,the laser medium itself carries the discharge current ,for appropriate conditions of pressure and current , the electron in the discharge may directly excite the active atoms to produce higher populations in certain levels compared to lower ones

Inelastic atom-atom collisions (helium neon laser)

A combination of gases is used ,say A and B are the gases and both have some excited states A* and B* . The transfer of excitation may occurs in the between the two atoms such that

A* + B A + B*

Excited helium atoms excites neon atoms and then laser transition occurs in the latter

Figure 26.6

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Types of laser

Depending on the lasing medium ,laser can be classified as

1)Solid state lasers

2)Gas lasers

Solid state laser can be further classified as

1)Ruby laser

2)

Nd

: glass laser

3)

Nd: YAG laserGas laser can be further classified as1)Helium-Neon lasers 2)Argon laser3)Carbon dioxide lasers

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Solid state laser

The laser material used in the process are ruby ,neodymium glass (

Nd

-glass) and Neodymium-

yetterium

aluminium

garnet(

Nd

-YAG)

Of these Nd-glass and Nd-YAG are widely used in machining applicationsThese laser material are fabricated into rods and their ends finished to high optical tolerances The method used to inject the energy into the material is by generating a very intense light flux which can be absorbed by these laser materials and then converted into a collimated laser beam . The light flux of high intensity is provided by Xenon filled flash lamps The life of this flash lamps is around 10,000-1,00,000 pulses Nd

-glass laser operates only in a pulsed mode with a pulse duration of a few billionth of a second to a max of 1 millionth of a second

Nd

-YAG can be continuous or pulsed wave to produce continuous power output

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Working of solid-state Nd

-YAG laser

solid state laser consists of following components

A solid ,crystalline lasing medium that can be pumped into a higher state of energy

A pumping system to pump energy into the lasing medium

A resonator( usually a pair of mirror mounted at each end of the laser) to bounce stimulated light through the lasing medium

Fig: Solid state laser

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Co2 Gas Laser

Co

2

laser 3 gases nitrogen , carbon dioxide and helium flowing through a glass discharge tube

Nitrogen functions as an intermediary between electrical energy and

vibrational

energy of co

2

molecules and nitrogen helps to sustain gas plasma

Helium cools the gas mixture so that it may be re-excited again .

Figure Co2

laser setup

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Advantages of co2

Relatively high and continuous output High conversion efficiency-Efficiency of ruby laser is around 1% for YAG laser is around 10%CO

2

the laser has few maintenance problems , unlike the solid lasers which require frequent replacement of flash lamps.

Divergence is small , it is available in many configurations.

It is compact.

High absorption of its output wavelength by many materials.

Higher power levels.

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The following gases are generally used for gas laser as lasing medium:

Helium- neon ArgonCarbondioxide

Helium-neon

Argon

Carbon dioxide

The most common

and inexpensive gas

lasser

the helium-neon laser is usually constructed to operate in the red at 632.8nm .it can also be constructed to produce Laser action in the green at 543.5nm and in the infrared at 1523nm

The argon laser can be operated as a continuous

gas laser at about 25 different wavelength in the visible b/w 408.9 and 686.1 nm ,but is best known for its most

effiecient

transition in the green at 488nm and 514.5nm .operate at much higher powers than the helium-neon gas laser. It is not uncommon to achieve 30 to 100 watts of continuous power using several transition .This output is produced in a hot plasma and takes extremely high power typically 9 to 12 kW ,so these are large and expensive devices

The carbon dioxide gas laser is capable of continuous output powers

above 10kilowatts . It is also capable of extremely high power pulse operation . It exhibits laser action several infrared frequencies but none in the visible

The co

2

laser is the most

effiecient

laser , capable of operating at more than 30% efficiency

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Process

A typical laser can have output energy of 20joules with a pulse duration of 0.001 seconds, corresponding to a peak power of 20 kW .A typical beam divergence for such a laser will be in the order of 0.002 radians . If the 25mm focal length lens is used to focus this energy , spot

dia

0.05mm. A power of 20kW is

focussed

on this small area of 0.05mm

2

a power densities of this magnitude is sufficient to

vapourize

any known metal and even a diamond , thus a laser is capable of drilling a hole in any metal .

figure 26.10

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Mechanics of material removal

Laser machining is basically a high speed ablation process . The evaporation of very small portion of a liquid metal takes place so rapidly under the high intensities of a focused laser beam that is substantial impulse is transmitted to the liquid .Material leaves the surface not only through evaporation but also in the liquid state at a relatively high velocity .physical process that occurs in laser beam is shown in fig

Figure 26.11

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Beam parameters

Lasing

material

Ruby

Nd

YAG

Nd

Glass

Co2

Type

Solid state

Solid

state

Solid

state

Gas

composition

0.03-0.07%

Nd

in Al

2

o

1%

Nd

doped with yttrium ,aluminum and

Garnet

2-6%

Nd

in glass

Co

2

+ He+N

2

(typically 3:8:4)

wavelength

0.69micron

1.066micron

1.064 micron

10.6

micron

Efficiency

1%

2%

3%

10-15%

Beam mode

pulsed

pulsed

pulsed

pulsed

Spot size

0.015mm

0.015mm

0.025mm

0.025mm

Beam output

10-100W

10-1000W

10-1000W

0.1-10kW

Peak power

D 200kW

400kW

200kW

D 100kW

Beam divergence

5-7mRad

1-5mRad

5-7mRad

0.1-10mRad

Pulse

rate

1-10pps

1-300pps

1-3pps

Continuous wave

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Process capabilities

Drilling diameter

0.005

to 1.25mm but larger diameter can be drilled using trepanning

Drilling depth

1.7mm

Drilling angle

15-90deg

Drilling taper

5-20 % of hole diameter

Drilling L/D ratio

Upto

50:1

Drilling length

Upto

6.5mm

Drilling tolerance

+/- 5-20% of hole diameter

Minimum corner radius

0.25mm

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Application of LBM

Material removal by drilling trimming and evaporatingMaterial shaping by cutting scribing and controlled fractureWelding of material

Thermo-kinetic changes like annealing ,photochemistry grain size control ,diffusion ,zone melting

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Laser drilling

Ruby drilling machine

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Drilling with multiple pulse

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Laser cutting

Laser cutting

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Laser cutting

Decrease in the speed of cutting with a

thickness of work material

1)Austenite Cr-Ni steel

11)St 37 steel

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Some other process

Laser weldingLaser heat treatmentLaser cladding Laser scribing

Controlled fracture

Laser trimming

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Advantages of LBM

No direct contact b/w tool and work pieceWill machine and weld through optically transparent materials.

Welding and machining of areas not readily accessible

Will melt and vaporize any known material

Easily melts dissimilar materials

Refractory materials are easy to work with

Machining hard brittle and non metallic materials

Welding machining in any desired atmospheric environment

Small heat affected zone and negligible thermal damage or effect on adjacent regions

Machining extremely small holes and precision welding of small sizes

Easy control of beam configuration and size of exposed areaThere is no problem of tool wear as tool is laser beamSoft material like rubber plastic can be machined

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Figure : Power densities of different processes

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Disadvantages of LBM

Overall efficiency is low Pulsed mode operation Practically limited to thin sheet plates

Holes machined not always round or straight

Control of hole size and

weldable

size is difficult

Repetition rate is slow

Durability and reliability are limited

Short life of flash lamp

Necessary for careful control of pulse length and power intensity to obtain the desired effect

Effective safety procedure is required High cost

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Comparison between EBM and LBM

Electron

beam machining

Laser beam machining

High

velocity electron strike the work surface and its kinetic energy is converted into heat energy

When the laser beam strikes the

workpiece

,

it melts and evaporates the work materials

Cuttng

tool is electron

beam

Cutting

tool is laser beam

Better surface

finish

Comparatively less surface finish

Metal

removal rate is high

Comparatively MRR

is less

Entire equipment must be maintained at

vaccum

Only laser producing unit must be in

vaccum

Only

metal can be machined

Any materials can be machined

except plastics

High power is required

Power requirement

is less than EBM

EBM can be used

for micro drilling

upto

0.002mm

dia

Can be used for micro drilling up to 0.005mm

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Thank you