laser interaction ii

7/29/2019 Laser Interaction II

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Laser Material Interaction


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General Scheme of Energy flow in Laser

treatment process

PL = PR + PA

PRPradPconv

Pchem

Pcon

Ppro

PL = PR + PA = R.PL + A. PL

PA

+ Pchem

= Ppro

+ Pred

+ Pconv

+ Pcon


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Dependence of Power coupling on

Laser Intensity

102 104 106 108

0

0.2

0.4

0.6

0.8

1.0

1

2

1 < 2

Hardening

Conduction

welding

Re-melting

Cutting

Deep

penetrationwelding

Drilling

Shock

hardening

Laser Intensity W/cm2

Pow

erCoupling

Plasma

formation


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Laser Material Interaction: Time scale dependence

Laser Pulse Duration : tL

Electron-Electron Thermalization Time : Te-e < 10-16 s

Electron-Ion Energy Transfer time : Te-iI ~ 10-12s (1ps)

Lattice Heating Time: Te-l (10-100ps) >> Te-i

++

Case-I: TL (>1ms, CW) >> Te-l>>Te-i

Heating via Electron- Lattice Thermalization

Absorption within skin depth (la)

Temperature rise: Heat conduction process

Classical Heat Transfer Laws

Typical power density: kW-MW/cm2 in this time scale

Process: Heating, Melting Heating: Surface Hardening

Material Removal Mechanism: Melting with

molten metal ejected by an assist gas

Most common machining process : Laser Cutting

Typical Lasers in this time scale: CO2 laser in a few kW range


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Laser Thermal EffectsLaser Heating: Temperature Depth & Time Profiles T (z,t)

Heating by direct laser beam & / or

Thermal Diffusion Process

Thermal Diffusion Length, = 2

- Thermal Diffusivity = K/.CP

K = Thermal conductivity, = Density,Cp = Specific heat, - Laser pulse duration

Case1: Attenuation length la (/4k) > >

I (z,t) = Io(t) e-z ( = 4k/)

T (z,t) = To(t) e-z

T (z,t) = Q/.CP = [H. . /.CP] e-z

H = PL (1-R) /.a2 Laser power absorbed per

unit area at z=0a- Laser beam diameter

I0

z

I,T

Q = Absorbed Laser

Energy Density at z

= PL(1-R). e-z/.a2


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Laser Thermal EffectsCase2: Attenuation length la


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Initial Condition

T(z,0) T0 =0 for 0 z , t =0where T0 = Initial temperature

Boundary Conditions

At the surface z = 0 laser power absorbed is

conducted in

-KT/z = H where H = PL (1-R) /.a2

Laser pulse of time duration is incident on

the surface

= 2 < 2a

2a


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Semi-infinite solid with uniform laser beamHeating

T(z,t ) = [H/ K] .ierfc(z/)

Ierfc(x) = (1/) {exp(-x2) x(1-erf(x)}

Where erf(x) = (2/) exp(-2)d

For small x, ierfc (x) = 1/ -x + x2/

Cooling

T(z,t ) = [H/ K] [ .ierfc(z/) - *

.ierfc(z/*

)] = 2t, * = 2(t- ),

At z = 0, During heating, neglecting

initial temperature

T(0,t) = 2(H/K).

(

t/

)

At z=0, t= ,

T0 = Tmax= 2(H/K).(/)

At z =Thermal Diffusion Length =

= 2 ,T(, ) = T0. .ierfc(1) 0.09 T0

x

0

x

ierfc(x)

ierfc(0)=0.564 &

ierfc(1) = 0.05


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Variation of calculated temperature increases with

time at various depths during laser irradiation

(Reprinted from Wilson &Hawkes 1987.

1. T(z=0) with t

T(z=0) = Tmax, at t=

T(z=0) with t >

Very Fast Cooling Rate:

106-108 K/s

2. At Z > 0

T(z=0) with t

Max. T reaches at t>

Tmax(Z > 0) < Tmax (z=0)


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Laser beam of Finite size with = 2 ~ 2a

Heat Conduction Loss in lateral

direction not negligible

T(z,t ) = [H/ K] .[ierfc(z/) ierfc{(z2+a2)1/2/}]

2a

For long irradiation time i.e. large surface temperature

T(0,) = H.a/K = Constant ( Steady state temperature)

t

T(0,)


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Semi-infinite solid with Gaussian Laser beam

Surface Temperature,

T(0, t) = H{rf/ K. (2) } tan-1{ 2. /rf};

rf= Laser beam diameter

For Small interaction time, /rf >1,Final temperature is

T = H {(rf/ K). (/8) } = Constant

rf


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Calculation of temporal evolution of depth of melting:

(a) surface temperature as a function of time

Melting during the Laser Irradiation


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Calculated values

(b) Temperature as a

function of depthbelow the surface

during heating and

cooling,

(c) depth of melting asa function of time


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Schematic variation of

melt-depths

(a) effect of laser powerdensity at constant

pulse time, (b) effect

of laser pulse time at

constant laser powerdensity


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Moving Laser Beam: Asymmetric Temperature distribution

Analytical solution: Gaussian laser beam of diameter rfmoving

at velocity v along x direction on the surface

T(x,y,z) = (Hrf/4K) {1/(1+2)} exp(C)d

Where C = - {2/(1+ 2)}[{ - Pe/22}2+2] -22

= 2x/rf, = 2y/rf, = 2z/rf, = rf/2.,Pe = rf.v/22. (Peclet no.)


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Laser Beam: Line Heat Source

Laser Cutting

Temperature distribution:

T = P*. 1/ 2K . ev.x/2.K0[{v(x2+y2)1/2}/ 2]

P*= Laser Power absorbed per unit length across thickness

K0 =Zeroth order Bessel function, tables available

Depends on: Processing speed, v & Material properties, K,

Deep Penetration

Welding

X

Y


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Surface Heating by Laser for Transformation

Hardening during Laser Irradiation

Depth of heating limited by the Surface

Temperature reaching to melting point-Tm

What is the maximum depth ZP for phase transformation hardening?

Using analytical solution of 1D heat conduction equation

TP = (H/K) ierfc(ZP/)

Surface Temperature Tm = (H/K)/ = 2 t = Tm.K/H

TP/Tm = . ierfc(ZP/)

ierfc(H.ZP/ Tm.K ) = TP/Tm.

ZP & Time for reaching the melting point and achieving hardening

depth up to ZP can be calculated

T0 Tm

Tz1 TP ZP


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Laser Treatment Processes

Surface Transformation Hardening:

Suitable combination of Laser Power Density, H & Interaction

Time, ( t = Laser beam diameter / Laser Scan velocity) required

z1

T0 Tm

Tz1 TP

Tm

TTH

zPz1

[H1, t1 ][H2, t2]

H1 > H2, t1< t2,

[H1, t1 ] will raise T0 to Tm but depth

for T TP is less than desired zP H = 103-104 W/ cm2

t = 0.1- 1s.


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Temperature

0

500

1000

1500

t1 t2

Time duration t2-t1 of holding surface temperature above

TP can be estimated from heating and cooling curves.

Phase

Transformation

Tem.-TP


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Case-II: TL >1ns >>Te-l >>Te-iPulsed Laser in 1-100ns scaleHeating via Electron- Lattice Thermalization

Absorption within skin depth (la

)

Temperature rise: Heat conduction process

Classical Heat Transfer Laws

Typical Laser Power Densities : MW-GW/cm2

Process: Melting & Vaporization

Material Removal Mechanism : Vaporization &

Melt Ejection by recoil pressure of vapourMost common machining process: Laser drilling,

Grooving, Marking, Scribing

Heat Affected Zone (HAZ) less than that in CW

Laser processing

Typical Lasers in this time scale: Q-switchedNd:YAG (1.06m) Laser and their 2nd (0.53 m)3rd (0.355 m )& 4th (0.265 m ) harmonics,Excimer (193-248nm) Lasers


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Material removal rates due to melt

expulsion and vaporization : Typical in

Laser Drilling Process


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Vaporization during Laser Irradiation

Depth of Melting limited by the Surface Temperature reaching to

boiling point-Tb

What is the maximum melt depth ZMAX ?

T*m = (H/K) ierfc(ZMAX/) T*m = Tm + Lf/CP

Surface Temperature Tb = (H/K)/ = 2 t = Tb.K/H

Tm/Tb = . ierfc(ZMAX/)

ierfc(H.ZMAX/ Tb.K ) = Tm/Tb.

ZMAX

& Time for reaching the boiling point and achieving melting

up to ZMAX can be calculated


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Schematic of the variation of depth of melting with laser irradiation

time and power.


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Laser Surface Melting: Welding, Fusion Cutting, Alloying, Cladding

T(0,) TbT(z1,) TmH = 104 106W/cm2 t = 10-2 10-4s

Resolidification, Glazing:H = 105 107W/cm2 t = 10-4 10-7s

w

t

v

Energy balance equation:

Negligible conduction loss

(Thermal Diffusion length < a /w )

P(1-R) = w.t.v. (Cp.Tm + Lf)

V = P(1-R) /{w.t. (Cp.Tm + Lf)}

P /t

Laser cutting

With conduction loss & oxidation energy

V = P(1-R) /{w.t. (Cp.Tm + Lf)} + .hox.vox/ 2t 1.2K .Tm/ w.t.

hox Oxidation enthalpy, vox- Oxidation speed

2a


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Laser Evaporation: Drilling

T(0,t) Tb

P(1-R) = a2.ve. (Cp.Tb + Lf+ Lv)v

e

- Evaporation speed of metal

H = 106-108W/cm2, t =10-5-10-7s

Escaping vapor produces Mbar pressure on

molten surface Keyhole formation / Shock

wave generation

Ve = P(1-R)/ [a2. (Cp.Tb + Lf+ Lv)]

= H / [ (Cp.Tb + Lf+ Lv)]

For Laser pulse duration tp depth ofdrilled hole d,

d = ve.tp =H.tp/ [ (Cp.Tb + Lf+ Lv)]

T Im t t P m t i th l i f Th m l Eff t


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Two Important Parameters in the analysis of Thermal Effect

Cooling Rate (z,t) = T/ t

Temperature Gradient G(z,t) = T/ z

Solidification Rate R = (z,t) / G(z,t)

Development of

Microstructure:

Dendritic, Cellular,

Planar growth


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Variation of calculated solidification) rate with fractional melt depth during laser

irradiation of nickel. (Q0, W/cm2) Melt depth ~0.025mm


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Temperature

dependence on

Temporal and

Special Laser

Beam Profile

(a) single pulse

(b) multipulse laser

irradiation of material

(dotted curve indicate

the average

temperature)

Case III: T


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Case-III: TL


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Material Removal Mechanisms:Spallation: Fast heating process could

generate high tensile pressure waves-

material removal due to mechanical

fracture of material following the

creation of defects induced by tensile

stresses;

Laser Fluence- Near Ablation Threshold

Phase Explosion: At higher intensities

Formation of homogeneous nucleation

of gas bubble inside a superheatedliquid- decomposition into a mixture of

liquid droplets and gas),

Fragmentation: At still higher energies

disintegration of a homogeneous

material (supercritical fluid) into clusters under the action of large strain rates

Vaporization : Collective ejection of monomers, or,

in most cases, a mixture of these.

Coulomb Explosion- At very high laser fluences: High Energy Electrons leave the

surface- High Electric Field sets in- Ions pulled out

Summary:


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Summary:

Laser Beam is reflected, scattered, absorbed, transmitted in a material

Laser radiation is first absorbed by free-electrons in a metal and their

energy and temperature increases.

Heated electrons share their energy with ions and lattice vibrations, andthus the material gets heated up.

In most metals laser radiation is absorbed within 10s nm depth of metal

surface

Further heating by thermal diffusion

In metals laser radiation of any wavelength is absorbed by free- electrons. Interaction of Laser Beam depends upon laser wavelength, Polarization,

Intensity and interaction time

In semiconductors, laser radiation of photon energy (h) more than theband gap energy ( between Valance & Conduction bands) is absorbed.

Si-O bonds in glass, quartz absorb around 10 m radiation Laser radiation could get absorbed during multiple reflections at grain-

boundaries in ceramicsTransparent / dielectric material can be processed by high intensity laser pulses.

Electron plasma is produced by either multi-photon or tunneling ionization process.

High intensity Ultra-short laser pulse ablates material with a little thermal effects.

Let us see the effect of conduction in lateral direction in cases of Laser Cutting & Drilling


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Let us see the effect of conduction in lateral direction in cases of Laser Cutting & Drilling

holes.

We take an example of steel cutting.

The typical laser power in laser cutting is 1-2kW and spot size is 200-300m.Thermo-physical properties of Steel are: Density = 8030 kg/m3, Tm = 14500C, Cp= 500J/kg.C,

Lf= 300kJ/kg, K= 20W/mCThermal diffusivity, = K/. Cp = 20/8030.500 = 5x10-6 m2/s

Laser Power density absorbed on the surface = 1000W / 4.10-8 m2 = 2.5x1010W/m2

We will like to find out time taken for surface temperature to reach to melting point.

Considering the one dimension heat flow

Tm = [2H/K] [(. tm/)]tm = {Tm.K/2H}2. / = {1450. 20 / 2.5x1010}2. {3.14/ 5x10-6} = 8x10-7 s. ~ 1s.

Thermal diffusion length = 2 (. tm/) = 2. (5x10-6. 1x10-6/ 3.14) = 2.5 m

This is too small compared to the laser spot diameter of 200 m, thus the heat loss by conduction

in lateral direction is very small and we can write the energy balance equation in case of suchprocesses neglecting the conduction loss.

Typical Cooling rate: 1kW laser beam of 1ms duration falling on steel plate

Cooling rate at the end of laser pulse

T/t = H.K.(/) . --1/2 = 75x106 0C/s Very high cooling rate unachievable by any

conventional processing Important in Laser Surface Hardening, Surface Glazing etc.

Eff f P


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Effect of Pressure

* Fast vaporization ( Pulse laser in drilling) exerts pressure on

molten pool: Increases evaporation temperature

* Escaping vapor at elevated temperatures exert pressure on molten

pool: Formation of keyhole- Deep penetration welding

Shock wave formation- Laser Peening for creating

compressive stress, improves fatigue life, surface properties

Rate of evaporation

Laser Intensity; Importance of TEM00 mode

Heating with Phase change


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Heating with Phase changeHeating Melting Vaporization Plasma formation

Phase change: Drastic change in thermo-mechanical properties

Alter boundary conditions for laser matter interaction

Solid Liquid (Fluid): Changes temperature distribution dueto Melt flow, convection of heat

Melt Flow: External factors-

Flow of shield or processing gases

Supply of powder or filler wire

Internal factors-

Temperature,T dependent Surface Tension,

Buoyancy, Back pressure of escaping vapor

Pure metals: T , Melt flows from centre to boundary,Marangoni Force, Wider melt pool, reduced central temperature.

Additives like Sulphur reverse the gradient of, Melt flows fromboundar to centre; Narrow melt ool, increased de th

Laser Drilling with UV Excimer Lasers


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Laser Drilling with UV Excimer Lasers

Laser Photon Energy h > Molecular Binding Energy (Organics,Plastics, Bio-molecules)

Ablation through Bond breaks Cold Ablation :

--Photolytic Ablation

Little Thermal Effects, High Precision & Better quality holes

Interaction of excimer laser radiation with solids. Left: PVC,

photolytic ablation. Middle: Al2O3 ceramic, combined photolytic

and pyrolytic process; Left: metal, melt.

Ul Sh P l L P i Sh P l L


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Ultra-Short Pulse Laser Processing Short Pulse LaserProcessing

Steel foil100 m in

thickness

L P i ith Ult h t L P l


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Laser Processing with Ultra-short Laser Pulse

*Femtosecond Laser : Pulse duration ~10-1310-14 sShorter than electron -lattice thermalization time

*Electrons in material get heated up to 10s thousand 0C butlittle transfer of heat to rest of the material during laser pulse.

*Thermal diffusion length = 2 t Attenuation length = la =1/,

*High temperature electrons exert pressure more than yieldstrength of material causing material ejection / ablation withoutrise in temperature of substrate.

No thermal effects

No burr, very clean processing

Micro-machining , drilling micro-holes,cutting of stents


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Laser Processing with Ultra-short Laser Pulse

Reproducible due to well defined ablation

thresholdMicro-drill diameter < , Diffraction limit,

Lower threshold fluence (Energy density due to

very short laser pulse duration)

Higher process efficiency

Minimal rise in substrate temperature

ns

100fs

Ith

I

Ablation(a.u.)

Ith

Summary


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SummaryLaser energy is coupled by e-s, which gain energy in radiation field.

Laser beam of shorter wavelength is better absorbed.

Laser beam absorption depends on beam polarization.

Absorption increases with rise in temperature.

In most metals laser beam is absorbed almost at the surface and heat

penetrates further by thermal diffusion process.

At high intensities laser produces e- plasma which absorbs laser

energy and couples to substrate: Coupling mechanism in dielectrics.

Escaping vapour produces keyhole : Deep penetration welding.Ultra-short laser pulse ablates material with a little thermal effects.

Controlling laser intensity and interaction time various material

processes e.g. cutting, welding, surface hardening, alloying, cladding,

glazing, shock hardening, drilling and marking are realised


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laser interaction ii

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