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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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