Industrial Lasers for Welding Ing. M. Muhshin Aziz Khan.

Industrial Lasers for Industrial Lasers for WeldingWelding

Ing. M. Muhshin Aziz KhanIng. M. Muhshin Aziz Khan

Facts About Laser:Facts About Laser: Laser BasicsLaser Basics

LLight ight AAmplification by mplification by SStimulated timulated EEmission of mission of RRadiationadiation

LaserLaser is essentially an optical amplifieroptical amplifier that

Generates and

AmplifiesStimulated Emission

Laser ComponentsLaser Components

Lasing Medium:Lasing Medium: Provides appropriate transition and Determines the wavelength (it must be in a metastable state)

Pump:Pump: Provides energy necessary for population inversion

Optical Cavity:Optical Cavity: Provides opportunity for amplification and Produces a directional beam (with defined length and transparency)

• Properties of LaserProperties of LaserCoherentCoherent (synchronized phase of light)

CollimatedCollimated (parallel nature of the beam)

MonochromaticMonochromatic (single wavelength)

High intensityHigh intensity (~1014W/m2)

Facts About Laser:Facts About Laser: Laser HistoryLaser History

Facts About Laser:Facts About Laser: Laser Material: Laser Material: Energy Levels of Atomic or Molecular SystemEnergy Levels of Atomic or Molecular System

Laser operation takes place via transitions between different energy levels of an atomic or molecular system

+- E3

E2

E1

Energy

Ground State

1° Excited State2° Excited State

Highly excited State

Ground State

Exci

tatio

n

Lase

r Tr

ansi

tion

Population Inversion

Two-Level

For material with Two-Level system Absorption and stimulated processes neutralize one another.

The material becomes transparent.

Population inversion is impossible

Facts About Laser:Facts About Laser: Laser Material: Laser Material: Energy Levels of Atomic or Molecular SystemEnergy Levels of Atomic or Molecular System

Three-Level

Exci

tatio

n

Lase

r Tr

ansi

tion

Population Inversion

Highly excited State

Ground State

Metastable State

Fast Decay Four-Level

Exci

tatio

n

Lase

r Tr

ansi

tion

Population Inversion

Highly excited State

Ground State

Higher Metastable State

Lower Metastable State

Fast Decay

Natural Depopulation

Facts About Laser:Facts About Laser: Optical Pumping: Optical Pumping: Population InversionPopulation Inversion

More atoms or molecules are in a higher energy state

Nonequilibrium distribution of atoms among the various energy level of atomic system

Process producing population inversion is called Pumping

Energy needed for population inversion is supplied by optical excitation with light source

Flash lamps (Pulsed laser), Arc lamp (CW Laser), Semiconductor Diode Excitation by electron collisions and resonant transfer of energy (Gaseous)

Is population inversion by thermal excitation possible!!??!!According to Boltzmann Ratio

Where E2 > E1

)(

expE E

KTNN

2 1

2

1

Is population inversion a necesary condition for laser operation!!??!

Light with intensity I(z) passing through a laser medium with densities of atoms N1 and N2 in higher and lower energy levelsAbsorption by atoms in Level 1 decreases the Intensity according to

I(z) = I(0) exp-(σz)

ln Ncf f

12 21

2

24

Amplification by stimulated emission increases the Intensity according to

I(z) = I(0) exp-(gz)

ln Ncg

f f

12 22

2

24

Net effect of passage of light through the material

ln( ) ( )exp

cI z I N N z

f f

12 2

2 12

2 10

4

Facts About Laser:Facts About Laser: Optical Cavity: Optical Cavity: Mirror ConfigurationMirror Configuration

Resonator cavity is formed by placing mirrors at the ends of the active medium

The mirrors are perpendicular to the axis along which the laser light travels

Acts as Positive feedback system Provides amplification and directionality to a laser beam via oscillation

The resonant cavity generally is much

longer than it’s width

Mirror configurations are judged on two criteria Stability Light rays bouncing back and forth between mirrors will be re-entrant. Filling of the active medium by light

Spatial profile defined by the light rays fills all the volume of the active medium

Stability Diagram for laser resonators consisting of two mirrors

Condition for stability of a resonator

D DR R

1 2

0 1 1 1

Facts About Laser:Facts About Laser: Optical Resonator: Optical Resonator: Mirror ConfigurationsMirror Configurations

Plane paraller and Confocal Mirror Configurations Both mirror configurations have marginal or delicate stability Plane parallel mirrors have good filling whereas confocal mirrors offers poor filling of active medium For plane parallel mirrors, allignment is really crucial. However, for confocal mirrors even if the configuration is not exactly perfect, the light rays will still be reentrant.

Long-radius mirror configuration is most often used in modern comercial lasers

It falls within a region of good stability Beam spatial profile fills active medium reasonably well

Possible Mirror Configurations

Facts About Laser:Facts About Laser: Optical Resonator: Optical Resonator: GainGain

Laser turn-on and gain saturation Gain decreases as output power increases• Saturation

Laser gain and losses

Optical loss in resonant cavity r1 and r2 : mirror loss/coupling loss Due to non-unity reflectivity on the mirrors Loss is Independent of cavity length exp(-2αL) ~ 1-2αL : distributed loss/internal loss Due to absorption/scattering in the cavity material Loss is proportional to cavity length

Oscilation Condition: Gain ≥ Losses

Facts About Laser:Facts About Laser: Laser EfficiencyLaser Efficiency

Output versus input power for an optically pump power

Slope efficiency (or differential efficiency)The slope of the curve obtained by plotting the laser output versus the pump power.

Facts About Laser:Facts About Laser: Laser Quality and Its EffectLaser Quality and Its Effect

A measuremeasure of Lasers’ capabilitycapability to be☺ propagatedpropagated with low divergencedivergence and ☺ focusedfocused to a small spot by a lenslens or mirrormirror

BeamBeam Quality is measured by MM22 or BPPBPP (BBeam PProduct PParameter, mm.mradmm.mrad)

Ratio of divergencedivergence of actualactual beam to a theoretical diffractiontheoretical diffraction limited beam

with samesame waistwaist diameter MM22= 1= 1;; Ideal Gaussian BeamGaussian Beam, perfectly

diffraction limited ValueValue of M2 tends to increaseincrease with increasingincreasing laser powerpower

Effects of Beam QualityEffects of Beam QualityBeam QualityBeam Quality

SmallerSmaller focus at constantconstant aperture and focal length LongerLonger working distance at constantconstant aperture

and spot diameter SmallerSmaller aperture (‘slim optics’) at constantconstant

focal diameter and working distance

A higherhigher power density power density by a smallersmaller spot size with the same opticssame optics, or

The samesame power density at lowerlower laserlaser power

Facts About Laser:Facts About Laser: Primary Adjustable Parameters and Their EffectsPrimary Adjustable Parameters and Their Effects

Laser Beam Energy Output CharacteristicsLaser Beam Energy Output Characteristics(i) Voltage (ii) Pulse Duration

Laser Focus CharacteristicLaser Focus Characteristic(iii) Laser Beam Diameter

Primary Controllable ParametersPrimary Controllable Parameters

Change in VoltageChange in Voltage

IncreasedIncreased voltage results in deeperdeeper physical penetrationpenetration with lessless melting due to physicalphysical pressure

Change in Pulse DurationChange in Pulse Duration

IncreasedIncreased pulse duration results in deeperdeeper and widerwider melting

Change in Voltage and Pulse Change in Voltage and Pulse DurationDuration

SimultanousSimultanous increaseincrease in voltage and pulse duration results in deeperdeeper meltingChange in Beam DiameterChange in Beam Diameter

IncreasedIncreased beam diameter results in shallow softshallow soft penetration and widewide, but softsoft melting

Facts about lasers for weldingFacts about lasers for weldingLaser Characteristics, Quality and ApplicationLaser Characteristics, Quality and Application

Typical commercial lasers for welding Typical commercial lasers for welding

1. COCO22 Laser2. NdNd3+3+:YAG:YAG Lasers

Lamp-Lamp-pumped LD-LD-pumped

3.3. DiskDisk Laser4.4. DiodeDiode Laser5.5. FiberFiber Laser

COCO22 Laser: Laser: CharacteristicsCharacteristics

WavelengthWavelength 10.6 µm; far-infrared ray10.6 µm; far-infrared ray

Laser MediaLaser Media COCO22–N–N22–He mixed gas (gas)–He mixed gas (gas)

AverageAveragePower (CW)Power (CW)

45 kW (maximum)45 kW (maximum)(Normal) 500 W – 10 kW(Normal) 500 W – 10 kW

MeritsMerits Easier high power (efficiency: 10–Easier high power (efficiency: 10–20%)20%)

Output power Output power (W)(W)

MM22

<500<500 1.1-1.21.1-1.2

800-1000800-1000 1.2-21.2-2

1000-25001000-2500 1.2-31.2-3

50005000 2-52-5

10,00010,000 1010

COCO22 Laser: Laser: MM22 values [CW] values [CW]

Lamp-pumped YAG Laser: CharacteristicsLamp-pumped YAG Laser: Characteristics

WavelengthWavelength 1.06 µm; near-infrared ray1.06 µm; near-infrared ray

Laser MediaLaser Media NdNd3+3+: Y: Y33AlAl55OO1212 garnet (solid) garnet (solid)

AverageAveragePower [CW]Power [CW]

10 kW 10 kW (cascade type & fiber-(cascade type & fiber-coupling)coupling)

(Normal) 50 W–4 kW(Normal) 50 W–4 kW

MeritsMerits Fiber-delivery, and easier Fiber-delivery, and easier handling (efficiency: 1–4%)handling (efficiency: 1–4%)

LD-pumped YAG Laser: LD-pumped YAG Laser: CharacteristicsCharacteristics

WavelengthWavelength about 1 µm; near-infrared rayabout 1 µm; near-infrared ray

Laser MediaLaser Media NdNd3+3+ : Y : Y33AlAl55OO1212 garnet (solid) garnet (solid)

AverageAveragePowerPower

[CW] : 13.5 kW [CW] : 13.5 kW (fiber-coupling (fiber-coupling max.) max.)

[PW] : 6 kW [PW] : 6 kW (slab type max.)(slab type max.)

MeritsMerits Fiber-delivery, high brightness, Fiber-delivery, high brightness, and high efficiency (10–20%)and high efficiency (10–20%)

Output power Output power (W)(W)

MM22

0-20 0-20 1.1-5 1.1-5

20-50 20-50 20-50 20-50

50-150 50-150 50-75 50-75

150-500 150-500 75-150 75-150

500-4000 500-4000 75-150 75-150

YAG Laser: YAG Laser: MM22 values [CW & values [CW & PW] PW]

YAG Laser Application: YAG Laser Application: Automobile Automobile IndustriesIndustries

Lamp-Lamp-pumpedpumped

3 to 4.5 kW class; SI fiber 3 to 4.5 kW class; SI fiber delivered delivered (Mori, 2003)(Mori, 2003)

LD-pumped LD-pumped 2.5 to 6 kW2.5 to 6 kW

New New DevelopmentDevelopment

Rod-type:Rod-type: 8 and 10 kW; Laboratory 8 and 10 kW; Laboratory Prototype Prototype

Slab-type:Slab-type: 6 kW; Developed by 6 kW; Developed by Precision Laser Machining Precision Laser Machining Consortium, PLMConsortium, PLM

Facts about lasers for Welding:Facts about lasers for Welding: YAG LaserYAG LaserLaser Characteristics, Quality and ApplicationLaser Characteristics, Quality and Application

Disk Laser: Disk Laser: CharacteristicsCharacteristics

WavelengthWavelength 1.03 µm; near-infrared ray1.03 µm; near-infrared ray

Laser MediaLaser Media YbYb3+3+ : YAG or YVO : YAG or YVO44 (solid)(solid)

AverageAveragePower Power [CW] [CW]

6 kW 6 kW (cascade type max.)(cascade type max.)

MeritsMerits Fiber-delivery, high Fiber-delivery, high brightness, high brightness, high efficiency(10–15%)efficiency(10–15%)

Recent DevelopmentRecent Development (Mann 2004; and Morris 2004): CommerciallyCommercially available diskdisk laser system: 11 and 44 kW class Beam deliverydelivery with 150150 and 200 µm

diameter fiberfiber Even a 1 kW1 kW class laser is ableable to produce

a deepdeep keyhole-typekeyhole-type weld bead extremelyextremely narrow width in stainlessstainless steel and aluminumaluminum alloy

Facts about lasers for welding:Facts about lasers for welding: Disk LaserDisk LaserLaser Characteristics, Quality and ApplicationLaser Characteristics, Quality and Application

Diode Laser: Diode Laser: CharacteristicsCharacteristics

WavelengthWavelength 0.8–0.95 mm; near-infrared ray

Laser MediaLaser Media InGaAsP, etc. (solid)

AverageAveragePower Power [[CW]CW]

10 kW (stack type max.) 5 kW (fiber-delivery max.)

MeritsMerits Compact, and high efficiency (20–50%)

Recent DevelopmentRecent Development (Hayashi 2004; and Zediker 2001): CommerciallyCommercially available DiodeDiode laser system: DirectDirect and/or fiber-coupledfiber-coupled modes Found suitablesuitable for welding of

plasticsplastics and thinthin sheets of aluminumaluminum or steelsteel at highhigh speed

Fiber-deliveredFiber-delivered laser is used for brazing Zn- Zn- coatedcoated steel using robotrobot.

Facts about lasers for welding:Facts about lasers for welding: Diode LaserDiode LaserLaser Characteristics, Quality and ApplicationLaser Characteristics, Quality and Application

Fiber Laser: CharacteristicsFiber Laser: Characteristics

WavelengthWavelength 1.07 µm; near-infrared ray

Laser Laser MediaMedia

Yb3+ : SiO2 (solid), etc.

AverageAveragePower Power [[CW]

20 kW (fiber-coupling max.)

MeritsMerits Fiber-delivery, high brightness, high efficiency(10–25%)

Recent DevelopmentRecent Development (Thomy et.al. 2004; and Ueda 2001): FiberFiber lasers of 10kW10kW or moremore are commerciallycommercially available Fiber lasers of 100kW100kW and moremore are scheduledscheduled FiberFiber laser at 6.9kW6.9kW is able to provide deeply penetrateddeeply penetrated weld at highhigh speed FiberFiber laser is able to replacereplace high quality (slab) COCO22 laser laser for remoteremote or scanningscanning welding

Facts about lasers for welding:Facts about lasers for welding: Fiber LaserFiber LaserLaser Characteristics, Quality and ApplicationLaser Characteristics, Quality and Application

CorrelationCorrelation ofof Beam QualityQuality to Laser PowerPower (Katayama 2001; O’Neil et. al. 2004; Shiner 2004; Lossen 2003): OverlaidOverlaid with conditioncondition regimes Beam qualityquality of a laser worsensworsens with an increaseincrease in powerpower LD-pumpedLD-pumped YAG, thin diskdisk, COCO22 and fiberfiber lasers can provide high-qualityhigh-quality beams The development of higherhigher power COCO22 or YAG lasers is fairlyfairly static and, hence Main focusfocus on developmentdevelopment::

i. high-powerhigh-power diode, ii. LD-pumpedLD-pumped YAG, iii. diskdisk and/or iv. fiberfiber lasers

Facts about lasers for weldingFacts about lasers for weldingComparison of different laser systemsComparison of different laser systems

Expanded portion of the electromagnetic spectrum showing the wavelengths at which several important lasers operate

Facts about lasers for weldingFacts about lasers for weldingWavelengths of some important laser sources for materials processingWavelengths of some important laser sources for materials processing

CO2 Laser

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