NONTRADITIONAL MANUFACTURING

PROCESSES

Lasers

&

Laser Beam Machining

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Basic NTM Process Groups:

* Thermal NTM Processes

- Laser Beam Machining (LBM)

- Electron Beam Machining (EBM)

- Plasma Arc Machining (PAC)

- Electrical Discharge Machining (EDM)

* Mechanical NTM Processes( Plastic deformation / Abrasive Erosion)

- Abrasive Jet Machining (AJM)

- Ultrasonic Machining (USM)

- Water Jet Machining (WJM)

- Abrasive Water Jet Machining (AWJM)

* Electrochemical NTM Processes

- Electrochemical Machining (ECM)

- Electrochemical Grinding (ECG)

* Chemical NTM Processes

- Chemical Machining (CHM)

- Thermo chemical Machining (TCM)

* Kinetic Energy ( Atom by atom knocking)

- Ion beam mech14.weebly.com

Laser Beam Machining: An Introduction

What is a LASER?

Acronym of Light Amplification by Stimulated of Radiation

Basically a source of light giving out a coherent and low divergent

beam

Difference from ordinary source: Emission in LASER is by stimulation

process & in ordinary source by spontaneous emission process

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Construction of Atom

Probability of stimulated absorption = Probability of Stimulated Emission

Therefore, more population in upper energy level is required for

stimulated emission to dominate and laser action to take place mech14.weebly.com

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2. Excitation or Pump Source to produce population inversion in lasing medium.

* Optical Pump ( Flash Lamp, Other Laser) : Solid State & Fiber Lasers

* Electrical discharge (DC, AC, RF, Pulsed) : Gas Lasers

* Current injection : Diode Lasers

Important components of a laser:

1.Active medium

* Solid: Nd:YAG,

Optical Fiber

* Gas: He-Ne, CO2,

Excimer Ar+ ion

* Semiconductor

Diodes

3. Optical Resonator formed by a pair of parallel mirrors, one ~100% reflecting and

other partial reflecting. They provide feedback into the active medium and

facilitates laser beam to build up. Laser beam comes out through the partial

reflecting mirror. mech14.weebly.com

Laser Processing Setup

Laser Processing Setup

* Laser system

* Beam Transport system &

Beam Delivery System

* Workstation

Lasers used in Manufacturing: Lasers those can provide high continuous

wave (CW) or average laser power required for material processing

* Solid State Laser : Nd:YAG Laser- Flash Lamp or Diode Laser Pumped

* CO2 Laser

* Diode Laser

* Fiber Laser

* Excimer Lasers

* He-Ne Laser for alignment, pointer, metrological applications

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Lasers for Materials Processing Applications & their Characteristics Characteristics

CO2 Laser

Nd:YAG

Laser

Diode

Lasers

Excimer

Lasers

Fiber

Laser

Wavelength m

9.6-10.6

1.06

0.8-1.0

0.193-0.354

1.06

Laser power,cw

Pulse energy

Upto 45kW

1-20 J

50W- 2kW

1-100J

Up to

6kW

Avg.1kW

1-10J,

10kW

Efficiency %

10-15

2–20 Diode

pump

20-40

2-3

30

Beam Diverg.

1-3 mr.

1 – 25 mr.

1x200 mr

2 – 6 mr.

1-2 mr.

Beam

Transportation

Reflecting

mirrors

Optical

fibers

Optical

fibers

Optical

fibers

Optical

fibers

Mode of

operation

CW &

Pulsed: ms-

sub-s

CW &

Pulsed: ms-

Sub-ps

CW &

Modulated

Pulsed

10’s ns

CW &

Pulsed:

ms- Sub-

ps

Applications CW: C,W, SH,

MF,

Pulsed: D

CW: C,W,

SH, MF,

Pulsed: D

W, SH,

MF,

CW: C,W,

SH, MF,

Micro-

machining

C- Cutting, W-Welding, SH- Surface Hardening, MF- Metal forming, D-Drilling mech14.weebly.com

Laser Rod

Elliptical Reflecting Cavity Output Mirror

100%

Reflecting

Mirror

Power Supply

Flash Lamp

Laser Beam

Nd: YAG (Neodymium doped Yttrium Aluminum Garnet) Laser

Optical Excitation

Laser Medium : Nd:YAG Rod (Typical diameter = 3-10 mm, Length = 50=150 mm)

Pumping (Excitation) Method: Optical pumping using either Flash lamp or Diode Lasers

Laser Power : CW- Few watts to kW level

Pulse- Pulse duration 0.1-20ms, 10-100 nm, 10-100ps

Pulse energy = 1- 100 J (ms pulse duration), ~ 1 J (ns pulses)

Laser Beam Parameter (M2) : 2-50

Laser Beam Delivery: Low Loss Optical Fiber mech14.weebly.com

CO2 Laser Excitation

e- Accelerated in Electric Discharge,

Electrons gain energy.

e- (Hi Energy) + N2 N2* + e -(Low E)

N2* + CO2 N2 + CO2*

CO2* CO2 + Laser Photon

Lasing

+e

+He

ULL

LLL

N2*

E

T 6000K

Role of N2 in CO2 Laser:

* N2 excited by electron (e-) to vibrational meta-stable state.

* Excites CO2 molecule to upper laser level (ULL) by resonant vibration-

vibration energy transfer, thus creates preferential excitation to ULL.

Role of Helium (He) in CO2 Laser:

* Maintaining uniform and stable electrical discharge

* De-excitation of the lower laser level through vibration-translation

(kinetic energy) energy transfer.

* Heat conduction from discharge zone.

CO2 N2

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Conventional Diffusion Cooled CO2 Laser

Resistance Elec. Power

Supply

Laser

Beam

Water Jacket Discharge

Tube

Laser Power = 50W/ m of discharge length in Diffusion cooled laser,

Optimum Gas temperature in electrical discharge ~ 250C

Laser Power of 50W /m is limited by Gas Heating

Laser medium: A mixture of CO2:N2:He=1:2:8,

Gas Pressure ~ 50-100 mbar (1000 mbar = 1 atmospheric pressure)

Excitation: Electrical discharge, Typical, I = 50mA, V= 10-12kV/m,

Gold

coated

Mirror

ZnSe Partial

Reflector

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Convective Cooled FAF CO2 Lasers

Fast Axial Flow CO2 Laser

Laser Gas mixture flow along the optic axis

and through heat exchanger

Heat removal by passing hot

gas through heat exchangers

Flow Velocity through discharge tubes =

100 - 300m/s

Laser Power ~ 1kW/m

DC, RF Excitation

Excellent Beam Quality,

1- 5 kW Laser Commercially

Available

Special Roots / Turbo Blower

P.S.

Gas

Blower HX

HX

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Semiconductor Diode Lasers

Active medium: Semiconductor similar to that found in a light-emitting diode.

The most common and practical type of laser diode is formed from a p-n junction and powered by injected electrical current. When forward biased by applying + ve voltage at p-type, holes and electrons recombine at the p-n interface (depletion region) and they emit light. This is spontaneous emission like in LED. Under certain condition, population inversion is achieved and spontaneous emission leads to laser light by stimulated process.

Typical Materials used : GaAs, AlGaAs, GaInAsP

• Laser Wavelength: 600nm- 1.650micron range

• Power: Few mW to a few Ws in a single chip

and up to several kWs in 1-2D arrays

* Pumping: Electrical- Injection Current

• Efficiency: 30-50%

• Mode of operation: CW & Modulated up to 50kHz

• Beam quality: Poor than other lasers due to very small

area of emission

• Beam Delivery: Through Optical Fiber

Typical Beam Size:

1m x 3-10m,

1/d

Laser beam: Elliptical mech14.weebly.com

Fiber Laser

• The key advantage of a Fiber laser is its high surface-area-to-volume which effectively eliminates thermal problems.

• Active Medium: Optical (Silica Glass) Fiber with cladding, doped with either Ytterbium / Neodymium / Erbium or their combinations for different laser wavelength in 1-1.6 m range.

Most popular Fiber Laser for material processing applications: Ytterbium doped fiber operating at ~ 1 micron wavelength, pumped by diode lasers

• Size: Most common Single mode Fiber ~ -10micron dia. & 10’s m length

• Mode of operation: CW, modulated, pulsed

• Pumping: Optical pumping by Diode Lasers

• Laser Power : up to several kWs,

• Laser Efficiency: up to 30%

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Active medium: Double cladded Yb doped optical fiber

Excitation: By a large number of low power diode lasers

Fiber Bragg gratings: Work like mirrors of optical resonator to reflect the laser

beam and also to couple out laser beam from one end

Fiber Laser: Excellent Beam Quality- Micromachining

Construction of a Fiber Laser

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Main special properties that are utilized in material processing:

High power, Low divergence, Continuous wave / pulsed mode

of operation ( Pulse duration = ms, s, ns, ps, 10s of fs)

For laser beam machining, e.g. cutting, drilling, etc. beam is focused on the

wokpiece so that laser power density (power per unit area) is high enough to heat,

melt, evaporate or ablate the material.

1/2

db= 2w0 df

Laser

r

I

r

I

Laser Beam has a finite beam divergence.

Half divergence angle , 1/2 = M2/w0 , where = Laser wavelength ,

M2 = Beam quality parameter & w0 = Beam waist radius

M2 = 1 for laser beam of Gaussian intensity profile

M2 > 1 for other intensity profiles

When focused with a lens of focal length, f the focal spot diameter, df

df = 2f 1/2 = 2fM2 /w0 = 4f M2/db where db = Laser beam diameter mech14.weebly.com

Physical phenomena at increasing Laser Intensity

Heating of

Surface layer Melting

Formation of

Keyhole

Formation of

Plasma

Surface

Hardening,

Metal Forming

Conduction

welding,

Cutting,

Surface

alloying,

Cladding

Deep penetration

welding,

Drilling,

Shock

hardening;

Laser Peening

~107W/cm2 ~106W/cm2 ~103W/cm2 ~105W/cm2

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