Interaction of near-infrared femtosecond laser pulses with ...
NTMP - 5 Laser Interaction & Processing
-
Upload
nishant-prabhakar -
Category
Documents
-
view
23 -
download
0
Transcript of NTMP - 5 Laser Interaction & Processing
-
Laser Material Processing
-
Heating Surface Transformation Hardening Metal Forming
Melting Welding,Metal Cutting Assist Gas
Surface Re-solidification, GlazingSurface Alloying External Surface Cladding Materials
Vaporization Drilling HolesCutting of non-metals e.g. Ceramics, Wood, Concrete, Glass etc.
Surface cleaning, Paint strippingAblation with Ultra short Laser Pulses Hi-precision
drilling & Cutting Plasma Formation Laser Peening
Laser Material ProcessingLaser Power = 1- 5kWFocal spot diameter = 10-5000micronLaser Power Density = 103- 1012 W/cm2
-
Laser beam incident on a metal surface
Reflection Scattering Absorption Transmission
Above phenomena will depend onType of material (metal, semiconductor, insulator), andits thermo-physical characterizes, surface condition,Laser parameters (wavelength, pulse duration, polarization, power density)
Laser Radiation Absorbed in metals, impurities in Semiconductor materialsby Free electrons & then transferred to ions & lattice ( Pyrolitic / Heating)
In Semiconductor materials* Interband absorption by electrons in valance band, h Eg
Lattice Vibration or Optical phonon excitation near Infrared (Si-O bonds): Glass, Quartz, etc at 10 m
In organic polymers , UV Laser: h > EBDirect bond breaking: Photolytic reaction
* Inhomogenity of wavelength scale: Ceramics, Multiple reflection & absorption at grain boundary
Eg
Conduction band
Valance band
Photon
Laser beam
-
Reflection and Absorption of Laser Radiation in a material :
Govern by refractive index which is usually complex
Reflectivity at normal incidenceR = [(n-1)2 +k2] / [(n+1)2 +k2]
Absorptivity, A = 1- R -T
Tansmitivity, T = 0 in Opaque materials
Absorption or Attenuation coefficient = 4k/ ;
Light intensity attenuated by 1/e in a length given by the attenuation length / Optical Skin Depth
la = 1/ = / 4k
Complex Refractive Index : n* = n + ik
I = I0 e -z
In most metals, k,> n R is large ( >90%), A is small (
-
Reflectivity of Different Metals
Au
Cu
NiTa
0.1 1.0 10.0
CO2Nd:YAG
ExcimerDiode
Wavelength m
100
80
60
40
20
Al
Room Temperature
0
Laser Wavelength Dependence
-
Reflectance & Absorption in different materials
Materials 0.25 m 0.5m 1.06 m 10.6 m
Aluminum laR
8nm92%
7nm92%
10nm94%
12nm98%
Germanium laR
7nm42%
15nm49%
200 m38%
>1cm36%
Silica, SiO2 la >1cm6%
>1cm4%
>1cm4%
40m20%R
-
Absorption with TemperatureAbsorption, A 0.365 / (0)0.5
Temperature , 0 Electric Conductivity , A
YAG
CO2
Melting Point Temperature
Boiling Point
100
80
60
40
20
0
-
Laser Interaction at High IntensityLaser Energy = 100mJ, Pulse duration = 1ns
Focal spot diameter = 50m,
Intensity I = 4.1012 W/cm2
Electric Field = 27.I1/2 = 50MV /cmIonization by Intense Electric FieldMultiple Ionization by Avalanche processMultiphoton Absorption & IonizationPlasma FormationLaser Absorption by PlasmaHeat conduction from plasma to the surface
Laser Processing of Dielectric Materials
-
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, and thus the material gets heated up. In 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 present in them. Interaction of Laser Beam depends upon laser wavelength, Polarization, Intensity and interaction time In semiconductors, laser radiation of photon energy (h) more than the band 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 ceramics At very high intensities laser beam can be absorbed by nonlinear processes in any material including transparent materials.
Summary
-
Physical phenomena at increasing Laser Intensity
Heating of Surface layer
Melting Formation of Keyhole
Formation of Plasma
Surface Hardening,
Metal Forming
Cutting,Conduction welding
Drilling,Deep penetration welding
Ionization of Vapor & gas,Shock hardening,Laser Peening
~107W/cm2~106W/cm2~103W/cm2 ~105W/cm2
-
Principle characteristics of laser material processing
Surface Alloying, Cladding
Keyhole welding
Metal Forming
-
Summary of Laser Power Density for Various Laser Material Processing:
Laser Processing Typical Laser Typical Laser Power Density Interaction Time
Laser Heat Transformation: 103 5.104 W/cm2 1-10-2sLaser Cutting: 5.104 - 107W/cm2 10-1-10-3s Laser Welding: 5.104 - 6.106W/cm2 10-1-10-3s Laser Drilling: 5.106-108 W/cm2 10-3-10-5s Laser Surafce Re-melting: 5.105- 107 W/cm2 10-4-10-7s Laser Alloying & Cladding: 5.104- 5.105 W/cm2 10-2-10-3s Laser Shock Hardening: 108- 5.109 W/cm2 10-6-10-8s
-
Laser Material Processing Parameters
Processing gas Type of gas & Pressure
Nozzle: Conical, Cylindrical, Supersonic
Laser Beam: Power (CW, Pulsed, Modulated) Mode, Polarization, Wavelength
Relative motion, Direction w.r.t. Beam Polarization
Material Properties , Surface condition Laser Material InteractionGas molten pool interaction Forces on molten pool: Marangoni type
Focal Spot Size Focal spot position Stand-off distance
Assist gas Composition Pressure Velocity
-
Laser Material ProcessingLaser Thermal Effects
A laser beam focused on a material generates very high power density capable of heating, melting & vaporizing any material.
The above feature has been utilized for various manufacturing operations,with unique advantages over conventional methods
Major Application Areas: Cutting: wide range of materials without regard to their hardness
Welding: autogenous welding of similar & dissimilar materials, narrow HAZ
Surface Hardening: localized treatment, little distortion, self-quenching Surface Alloying & Cladding: Modified microstructure with improved
characteristics, very little dilution in cladding
Drilling : Small holes in hard, brittle materials, heat sensitive alloys Marking: Finished products of any material- plastics, ceramics, metal
-
General Scheme of Energy flow in Laser treatment process
PL = PR + PA
PR PradPconvPchem
Pcon
Ppro
PL = PR + PA = R.PL + A. PLPA + Pchem = Ppro + Pred + Pconv + Pcon
-
Laser Cutting
Laser cutting dominants the industrial laser applications & has more than 75% of share of all LM applications.Basic Principle : Melting with a focused laser beam and molten material ejection by a high pressure gas jet.
CO2 Laser (10.6), NdYAG & Fiber Lasers (1.06)Laser Power = 500-5000WCircularly or randomly polarized laserFocal spot size ~ 0.1 0.3 mmPower density of 1kW power at focal spot
of 0.3mm ~ 1.4 X106W/cm2Effect on material
* Melting * Vaporization
Pressurized co-axial gas jet ejects the molten /vaporized material
-
Methods of Laser cutting: Melt and blow :
Inert gas cutting, e.g: Ti, SS, Al etc.Cutting realised by melting material by laser beam and blowing off the moltenmaterial by a high pressure gas jet
Melt, burn and blow :Oxygen assist gas cutting, e.g: MS, SS etc.)
40-60 % energy from oxidation Vaporization :
High peak-power pulsed laser cutting or Materials which do not melt e.g: wood, plastics
Cut Kerf
* Melting* Melt ejection by gas jet
Cold cutting :* Cutting by high power Excimer laser in UV range* Bond energy of organic materials ~ few eV* If Photon energy > Bond Energy * Photon absorption leads to breaking of bonds * No heating
-
Operating Parameters in Laser Cutting:
Laser Beam Properties: Power, pulsed or CW, Spot size and mode, Polarization, Wavelength
Transport Properties: Speed, Focal position,
Cutting Nozzle: Type of nozzle (Cylindrical, Conical, Supersonic), Size of nozzle opening, Stand-off distance
Cutting Gas Properties: Composition (Inert or reactive), Pressure / velocity
Material Properties: Composition, Surface condition, Thickness & Thermo-physical Properties
-
Dependence of Laser Cutting Speed on material thickness at different laser power* High power : Fast cutting speed* High power : Higher Sheet Thickness
Cutting Speed mm/s0 20 40 60 80
Pow
er/U
nit T
hick
ness
W/m
m
0
200
4
00
600
800
100
0
Ti with Ar 14J/mm2
304SS with N2 8J/mm2
Ti with O23J/mm2
MS with O2 5J/mm2
-
Laser Fusion Cutting,
w
t
v
Energy balance equation: No conduction loss
P = w.t.v. (Cp.Tm + Lf + mLv)
Cutting speed, v = P / {w.t. (Cp.Tm + Lf )}
P/v.t = w. (Cp.Tm + Lf ) / = Constant for a constant w & a given material Called as Severance Energy (J/mm2)
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
= (1-R) Laser Power Coupling coefficientP = Laser powert = thicknessw = kerf widthv = cutting speedLf = latent heat of fusionLv = latent heat of vaporizationm=Fraction of metal evaporated = densityT = Temperature raiseCp = Specific Heat
-
Energy Balance: M.R.R.
t
t
Vw
wV
Material Lower value (J/ mm2)
Higher value (J/ mm2)
M.S + O2 4 13
M.S + N2 7 22
S.S + Ar 8 20
Cardboard 0.2 1.7
}{ vfp LmLTcwtVP ++=
Parametric dependence:* Laser cutting speed V increases with Laser power for a given job thickness t* With increased laser power thicker material can be cut at same speed.
-
Process Capability:
All most all materials e.g. metal, non-metals like ceramics, glass, concrete, rubber,fiber-glass, plastics, textile, lather etc. can be cut by lasers.
Steel sheets of thickness 25mm can be cut at 1-2m/min Speed with high power(2-4kW) CO2 , Nd:YAG and Fiber lasers and O2 gas assist.
Integrated with CNC machine it can cut any complex contour.Severance Energy gives an idea of the material removal rate for a given laser power.
Practical Applications:
Automobile industries, Rail-coach factory, Ordnance factory, Textile, Leather,Furniture, Ship-building, Nuclear and Aerospace industries, and many mechanical& metallurgical engineering job shops are using lasers in their production line to cutvariety of materials.
Cutting of diamonds is one of the most popular applications in India.
-
Advantages of the process: All most all type of materials Narrow Kerf width High Accuracy Low HAZ High productivity Low noise Low roughness Easy to cut hard materials No problem of tool wear Easy to produce complex Profiles Straight cut edges with sharp corners Cut edges can be welded without further machining Very low distortion High flexibility in 3D cutting
Major Limitations:
High capital cost Cutting of high reflective materials Cutting thicker metals > 1 inch.
-
Laser DrillingLaser drilling has found successful manufacturing applications in the automotive,aerospace, energy, electronics, medical, and consumer goods industries.Lasers make it possible to machine very small holes, unusual shaped holes, andprecisely tapered holes. They are used to drill holes at steep angles, and to processdifficult-to-machine materials. A single setup can produce hundreds of differentsizes over a 3D surface.
ECD and EDM have typically drilling speeds of 1-10 mm/min, but several holescan be drilled at the same time, using multiple electrodes. Electron Beam Drilling(EBD) is fast, but needs a vacuum chamber.Controlled energy input laser drilling offers fast drilling typically 1-10 mm/sec.
Cross section of a Rolls-Royce Tay aero-engine
-
Laser Drilling
Material Removal: Evaporation & Melt Ejection
Laser: Q-switched Nd:YAG Laser ( tp ~ 10-100ns) for most metals
CO2 laser for non-metalsExcimer laser for organic materials, plastics
etc. (by directly bond-breaking so-called cold process
Photon Energy h > Bond Energy of Materials
-
Lasers for Drilling Applications:
Pulsed Nd:YAG lasers are the predominant type used in laser drilling.Laser Pulse Energy = 1-100J, Pulse Duration = 2-20ms
Q-switched Nd:YAG laser pulses of 10s ns pulse duration are also used for precision hole drilling.
CO2 lasers have also found many successful applications, particularly in non-metals.
Excimer Lasers are used to drill holes and micromachining in organic materials by directly bond-breaking so-called cold process
-
Energy balance considerations
za
Laser Energy =U
-
Drilling with Ultra-short (Femtoseconds) Laser Pulses:Laser Pulse Duration < Electron-Ion Thermalization Time
Without going throughmelting materials getablated
Electrons are stripped out& Plasma is formed. Highvelocity electrons drag outions along, effectingmaterial removal
-
EDM Mechanical drilling Laser drilling
Advantages
No taper, large depth and low equipment cost
Large diameter, large depth, low equipment cost
High throughput, no drill wear/breakage, noncontact, small HAZ, wide range of materials, low operating cost
Disadvantages
Slow drilling rate, long setup time, high operating cost, limited range of materials
Drill wear/breakage, low throughput, difficult to drill small holes, limited materials
Hole taper, limited depth and diameter, recast layer
Comparison of Laser Drilling with EDM & Other Mechanical Drilling Processes
-
Types of Laser Welding Process
Conduction welding
Keyhole welding
Laser Beam
Laser Welding of workpiece
-
Laser Welding of workpiece1. Laser Conduction Welding-
* Joining of thin metal sheets* Laser power densities: Relatively low < 5x105 W/cm2.* Two metal surfaces melt and
* Full thickness melts due to heat conduction from top hot surface.Laser Beam
IsothermsLiquid
Solid
Conduction Welding
In conduction welding the depth to width aspect ration is about 1.5.
t=2
- Thermal Diffusivity
- Laser Interaction Time = Laser pulse duration = Laser dwell time = Beam diameter d/Welding speed v
d
-
Scan velocity determines the shape of keyhole andcooling rate determines the microstructure of weldmet
2. Deep Penetration / Keyhole WeldingThicker sheets (>3mm):
Higher Laser Power
At intensities > 106 W/cm2, a smallamount of metal vaporizes & plasma isformed.
Escaping vapour exerts a recoil pressureon the molten pool creating a key hole
Laser beam is absorbed in the hole inmultiple reflections and in metal vapourplasma and heat is transmitted to thework-piece through the walls of the hole.
Deep penetratio Welding Aspect ratio in Keyhole welding = 3-5
-
Operating parameters Beam characteristics
Beam Power & Power Density
Beam Power distribution, i.e. Mode
Polarization
Mode of operation: CW, Pulsed
Process Parameters
Beam diameter & focus
Welding Speed
Shielding / Shroud gas
Material Properties
Joint Geometries: Butt, Lap
Gap Tolerance
-
Parameter effect: Laser Power Density
Conduction welding
Keyhole welding
-
Beam Power & Scan speed
1 kW of laser power per mm thickness is needed to weld at 1 m/min.Energy Balance Equation: CL.(1-R). PL =V.w.t. .(Cp.Tm + Lf)
where CL = 0.48 to account for conduction loss; V-Welding speed, w-weld-width, t-weld-thickness & others are material properties.
-
Laser Welding Advantages Can be used in open air Can be transmitted over long distances with a minimal power loss Narrow heat affected zone (HAZ) Low total thermal input Welds dissimilar metals No filler metals necessary: Autogenous Weld No secondary finishing necessary Extremely accurate Welds high alloy metals without difficulty
Laser Welding Limitations Rapid cooling rate may cause cracking in certain metals High capital cost High maintenance cost
-
Laser Welding ApplicationsAutomobile Sector (> 65% ) Tailored welded blanks for automobile
body blanks. Welding of Transmission components
gears, various coupling & differentials
Specialized applications Hydraulic bearing thrust units Joining of Diamond or WC impregnated steels to tool tips.Welding of thin fins to high finned tube heat exchangers.Welding of pipelinesWelding of bimetallic saw blades Repair of nuclear boiler from inside Spot welding in TV tubesWelding of heart-pacemaker
-
Laser Surface Treatment
*Laser Surface Transformation Hardening: Heating up to desired depth beyond
phase transformation temperature and rapid cooling by heat conduction (Self-quenching)-
Microstructure charges- Surface properties e.g. micro-hardness wear resistance increase
* Laser Melting & Re-solidification: Melting up to desired depth and rapid cooling-Grain refinement, homogenization of microstructure improves surface characteristics (wear,
corrosion resistance)
* Laser Surface Alloying: Surface melting along with alloying materials
* Laser Surface Cladding: Deposition of powder with metallurgical bond with surface
Need for Surface TreatmentTo improve
Hardness, Strength,Wear resistance, Corrosion resistance and Fatigue lifeParticular parts of surfaces which are vulnerable
-
Laser Surface Treatment- classifications
1) 2)
3) 4)
-
Transformation in steel : basics
Austenite () Ferrite () + Cementite (Fe3C)FCC BCC Orthorhombic0.8 % 0.02% 6.67%
At eutectoid temperature (727 C): Carbides and Ferrite dissolve into a single face-centered cubic phase called Austenite.
If cooling rate is more than 103-104 K/s:Austenite () Martensite ()
FCC BCT 0.8 % 0.8%
No compositional
change or diffusion
At room temperature, plain carbon steels :a mixture of a body-centered cubic phase (Ferrite) and an iron carbide phase.
Under slow cooling conditions, high-temperature Austenite phase reverts to the ferrite and carbide structure.
In fast cooling Carbon tends to move- Distortion in Lattice Structure Compressive stress at the surface: Increased Hardness
-
Advantages of Laser Hardening
Limitation of Laser Hardening Limited depth of hardening : 0.1-2mm
Not enough time to become homogenous, equilibrium at temperature: Laser hardening-only of relatively homogeneous materials with narrow layers
Precise control of Heat Input to Localized Areas
Minimum Distortion
Hard to reach areas can be Heat Treated if a line of sight exists
No Quenchants required- Self Quenching
Time Efficient Process
No post processing required
-
Methodology ?
Ready to use Machining Annealing
Object + Imaging system Designer+ 3D CAD S/W Math data + Analysis
Slicing Job manipulation Matl deposition
CAD Model Making Laser Processing LRM Component
Laser Rapid ManufacturingLayer by layer powder deposition
1) Dynamic powder blowing (laser Cladding)
-
2) Preplaced powder bed type (Sintering)
Sintering Process
-
Summary: By controlling the laser power density and laser interaction time lasers are used in wide variety of manufacturing processes:
1. Laser Cutting
2. Laser Drilling
3. Laser Marking
4. Laser welding
5. Laser surface Modification
i. Laser Transformation Hardening
ii. Laser Surface Re-solidification
iii. Laser Surface Alloying
iv. Laser Surface Cladding
v. Laser Surface Cleaning
6. Laser Metal Forming
7. Laser Rapid manufacturing
Common Industrial Lasers:
1. High power CO2 laser
2. CW & Pulsed Nd:YAG lasers
3. Fiber laser
4. High power diode laser
5. Excimer laser
Laser Material ProcessingSlide Number 2Slide Number 3Slide Number 4Reflectivity of Different MetalsReflectance & Absorption in different materialsAbsorption with TemperatureLaser Interaction at High IntensitySummaryPhysical phenomena at increasing Laser IntensitySlide Number 11Slide Number 12Slide Number 13Laser Material ProcessingLaser Thermal EffectsGeneral Scheme of Energy flow in Laser treatment processSlide Number 16Slide Number 17Slide Number 18Slide Number 19Slide Number 20Slide Number 21Slide Number 22Slide Number 23Slide Number 24Slide Number 25Slide Number 26Slide Number 27Slide Number 28Slide Number 29Laser Welding of workpieceLaser Welding of workpieceSlide Number 32Operating parametersSlide Number 34Beam Power & Scan speedSlide Number 36Slide Number 37Laser Surface Treatment*Laser Surface Transformation Hardening: Heating up to desired depth beyond phase transformation temperature and rapid cooling by heat conduction (Self-quenching)- Microstructure charges- Surface properties e.g. micro-hardness wear resistance increase* Laser Melting & Re-solidification: Melting up to desired depth and rapid cooling-Grain refinement, homogenization of microstructure improves surface characteristics (wear, corrosion resistance) * Laser Surface Alloying: Surface melting along with alloying materials* Laser Surface Cladding: Deposition of powder with metallurgical bond with surface Laser Surface Treatment- classificationsTransformation in steel : basicsSlide Number 41Laser Rapid ManufacturingSlide Number 43Slide Number 44