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