NONTRADITIONAL MANUFACTURING PROCESSES

MF30604

Lecture:4

Lasers for Manufacturing Processes

Laser Processing Setup

Laser Processing Setup* Laser system* Beam Transport system &

Beam Delivery System* Workstation

Lasers used in Manufacturing: Lasers those can provide high 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

Lasers for Materials Processing Applications & their CharacteristicsCharacteristics CO2 Laser Nd:YAG

LaserDiode Lasers

Excimer Lasers

Fiber Laser

Wavelength µm 9.6-10.6 1.06 0.8-1.0 0.193-0.354 1.06

Laser power,cw Pulse energy

Upto 45kW1-20 J

50W- 2kW1-100J

Up to 4kW

--Avg.1kW1-10J,

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

Absorption in metals

Low ~2-15%

Moderate~5-30%

Moderate~5-30%

High>50%

Moderate~5-30%Life, CW Hrs.)

Pulsed (Shots)~ 1000s.~106

~200 Life of~106 lamps

~ 1000s. 104-107

(one gas fill)Size of lasers Large Moderate Compact Moderate Compact

Maintenance intervals (Hr)

1000-2000 500-1000 5,000-10,000

500-1000 5,000-10,000

Mode of operation

CW & Pulsed: ms-sub-µs

CW & Pulsed: ms-Sub-ps

CW & Modulated

Pulsed10’s ns

CW & Pulsed: ms- Sub-ps

Lasers: Types, Wavelength, Power

For Laser Material Processing: Laser Power Density at Focus of a Lens

= Laser Power / Focused beam areaFocused Beam Area

(i) Focal length of focusing lens- f,

(ii) Laser Beam Diameter - Dr

(iii) Laser Beam Quality– M2

Best quality Beam i.e. TEM00 M2 =1

(iv) Laser Beam Divergence of Laser Beam of beam quality factor M2

Half angle θr = 2M2.λ/π.Dr .

Focal spot size, dspot= 2.f. θr = 4.f.M2. λ/ πDr

DOF, d = +0.4 M2 . λ.f2 / Dr2

Depth of Focus, d by which beam size changes by 5%

Rayleigh Range Zr = πw0

2/ λ

For Multimode Laser Beam Zrm = πDr

2/ 4M2λ

Carbon Dioxide Laser

Laser Medium : Mixture of CO2, N2 & HeLaser Wavelength : 9.6-10.6 micron (Infrared) (Tunable)Mode of Operation : Continuous Wave (CW)& PulsedExcitation Method : Electrical Discharge: DC, RF, PulsedEfficiency : 10-15%Laser Power : CW- A Few Watts to Tens of kW

Pulsed-Rep. Rate : Single pulse - 100kHzAv. Power : kWPulse duration : ms- µsPulse Energy : 1-10’s Joules

Types of CO2 Laser : Conventional Diffusion Cooled LaserHi-power Fast Axial Flow LaserHi-power Transverse Flow Laser

CO2 Laser Excitation

e - Accelerated in Electric Discharge,

Electrons gain energy. e(Hi K.E.) + N2 → N2* + e (Low K E)

N2* + CO2 → N2 + CO2*

CO2* → CO2 + Laser Photon

CO2 N2

Lasing

+e+He

ULL

LLL

N2*

E

CO2 Laser System UsingThe Gas-Flow Discharge Principle

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

Maximum Laser Power Limited by Gas Heating

Conventional Diffusion Cooled CO2 Laser

Convective Cooled FAF CO2 Lasers

Convective Cooled Laser:

Laser gas cooled by circulating the gas through Heat Exchanger

Fast Axial Flow CO2 Laser

Flow Velocity = 100 - 300m/s

Laser Power ~ 1kW/m

DC, Radio Frequency Excitation

Excellent Beam Quality,

1- 5 kW Laser CommerciallyAvailable

P.S .

Gas Blower

Nd:YAG Solid State Laser

Laser Medium : Neodymium ion Nd3+ dopedYttrium Aluminum Garnet (Nd:YAG)

Laser Wavelength: 1.06 micron (Infrared)

Mode of Operation : Continuous Wave (CW)& Pulsed

Excitation Method : Optical PumpingXenon Flash Lamp for Pulsed ModeKrypton Arc Lamp for CW ModeDiode Laser

Efficiency : Lamp pumped lasers- 2-5%Diode pumped lasers- 15-20%

Laser Rod

Elliptical Reflecting Cavity

Output Mirror

Mirror

Power Supply

Flash LampLaser Beam

Q-switch / Mode-Lock Switch

λ→

Abs. Spe. of Laser Rod

Spectrum of Lamp

Spectrum of Diode Laser

*Lamp pumping: Low Efficiency *Diode pumping: High Efficiency

LasingE

Construction of a Nd:YAG Laser

Diode Pumped Solid State Laser

Axial Pumping

Diode Laser

Lens

Mirror

Laser Rod

Mirror

Laser Beam

Side Pumping

Diode Arrays

Fiber Laser

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

• Excitation / Pump source : Diode Lasers

• Most popular Fiber Laser for material processing applications: Ytterbiumdoped fiber operating at ~ 1µm wavelength, pumped by diode lasers

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

• Mode of operation: CW, modulated, pulsed

• Laser Beam quality: Excellent

• Laser Power : up to several kWs,

• Laser Efficiency: up to 30%

• Key advantage: High surface-area-to-volume, less thermal problems.

Diode Laser

Semiconductor Diode LasersActive medium: Semiconductor similar to a light-emitting diode.

Most common type of laser diode: p-n junctionExcitation: Injected electrical current.

In forward biased holes and electrons recombine at the p-n interfaceand they emit light. This is spontaneous emission like in LED.Under certain condition, population inversion is achieved andspontaneous 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 2-3D arrays• 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

Excimer Laser:Excimer :Excited Homo-nuclear ( e.g. Xe2

*) molecule- Dimer

Excited Hetro-nuclear (e.g. KrF*, XeCl*, ArF*) molecule - Excimer

Ground State repulsive or very weakly bound

Formed in the electric discharge in the excited state having very short lifetime.

Laser action between Excited bound state & Repulsive ground state

Due to short upper laser level lifetime: Very high input power density required for creating population inversion.

e + Kr → e + Kr*e+ F2 → e+ F +FKr* + F + M → KrF* + MThree body Collision: High Operating Pressure

Because of very short lived (10’s ns ) upper laser level & high operating gas pressure, Excimer lasers are operated in Pulsed mode only

Wavelength Active Gas Relative Power193nm Argon Fluoride 60248nm Krypton Fluoride 100308nm Xenon Chloride 50351nm Xenon Fluoride 45

Typical Operating ConditionsLaser Gas Mixture: Kr, He, F2Total Gas Pressure: 2-4 Atmospheric pressure,Discharge Voltage: 15-30kVPulse repetition rate: 1-1000HzPulse Energy : 50-500mJ;Efficiency : 2-3%Laser Pulse duration: 10-100ns

⇒ High Laser Peak PowersAverage Laser Power: up to several 100’s WAv. Power (W) = Energy (mJ) X Rep.Rate (Hz)/1000

Applications:Cutting - Diamond to Cornea of Eye, Biological tissues-Cold CuttingDrilling, Micromachining,Lithography Ablation- Dermatology

Applications of Laser in Manufacturing

Laser Cutting of Metal Sheets, Glass, Wood, Plastics, Textiles, Rubber, Ceramic, Marble etc.

Laser Welding of Similar & Dissimilar metals & Alloys.

Laser Surface Hardening

Laser Surface Alloying

Laser Surface Cladding

Laser Rapid Manufacturing

Laser Marking, Engraving

Laser Metal Forming