NONTRADITIONAL MANUFACTURING PROCESSES

Lecture no. 2 &3

Basic NTM Process Groups:

* Thermal NTM Processes- Laser Beam Machining (LBM)- Electron Beam Machining (EBM) - Plasma Arc Machining (PAC)- Electrical Discharge Machining (EDM)

* Mechanical NTM Processes( Plastic deformation / Abrasive Erosion)- Abrasive Jet Machining (AJM)- Ultrasonic Machining (USM)- Water Jet Machining (WJM)- Abrasive Water Jet Machining (AWJM)

* Electrochemical NTM Processes- Electrochemical Machining (ECM)- Electrochemical Grinding (ECG)

* Chemical NTM Processes- Chemical Machining (CHM)- Thermo chemical Machining (TCM)

* Kinetic Energy ( Atom by atom knocking) - Ion beam

Laser Beam Machining: An Introduction

LASER MATERIAL PROCESSING

Laser basics

Properties of laser radiation

Different types of lasers in manufacturing

Laser processing system: Laser beam transport and delivery system

Laser beam absorption in metals, semiconductors and other non- metals

Different laser material processing specialties i.e. cutting, welding, drilling, marking, surface hardening, surface alloying, surface cladding, metal

forming, rapid prototyping and manufacturing

Parameters that influence the processes and their dependence

What are the advantages in using lasers in these processes

* What is LASER ?LASER: Light Amplification by Stimulated Emission of Radiation

* What is Light?Electromagnetic Radiation : Carries Energy in the direction of propagation

Velocity in vacuum or air, c= 3x108m/s, Frequency = ν ,Wavelength = λ = c/ ν

In a medium of refractive index n, Light velocity, v = c/nWavelength = c/ nν

Wave nature: Transverse Wave i.e. Oscillation of electric & magnetic fields are transverse to the direction of propagation.

Particle nature:Photon Energy = hν, Photon momentum p = hν/ch= Planck’s constant = 6.626x10-34 J-sec.

Polarization: Plane of Oscillation of Electric Vector :

Plane polarized light:

Circularly polarized light:

Elliptically polarized light:

Unpolarized light:

Randomly polarized light

.

What is Stimulated Process?Light Matter Interaction

Simplest Example : An Atom

Nucleus (protons & neutrons) and electron shell (electrons)

Bohr model of atom

Electrons can only have certain values of energy – not all values of energy allowed!– Electrons can switch between energy

levels by gaining or losing energy– Electrons can gain energy by absorbing

a little packet of light – photon: Absorption

– Can lose energy by emitting a photon: Spontaneous Emission

Photon Energy E = E2 – E1 = h ν

Planck constant h = 6.626068 × 10-34 m2 kg / s

Absorption & Spontaneous Emission of Photons

Ordinary Source of Light Emits Light by Spontaneous Emission Process

Stimulated Emission of Radiation

+c

Stimulated Emission

Photon

Laser Emits Radiation by Stimulated Process

Photons

In normal condition ( Thermal Equilibrium) more population in lower energy levels than higher energy levels,Absorption dominates over Stimulated Emission

E

Probability of Absorption = Probability of Stimulated emission

Stimulated Process to dominate over Absorption Process: More Population in Excited StateN2

N1> N2 N2> N1

N1Normal Population Population Inversion

N1> N2 N2> N1

Amplification

N2> N1

Laser BeamMirror1

Mirror2Laser

Absorption

Light Amplification by Stimulated Emission of Radiation

Characteristics of Stimulated Emission & Optical Resonator:* Stimulating & stimulated Photons have same

wavelength, phase, direction, and polarization* Optical resonator support waves parallel to its axis

Laser Beam Properties:-– Coherent– Monochromatic– Low Beam Divergence / Directional : Focusibility– High Brightness– High Power and High Power Density– Tunability– Ultra-short duration pulse

Laser Material Processing

Ordinary Source of lightOrdinary Source of Light :

Incoherent: No Phase relation between waves emitted by different atoms

Wide frequency spectrum, ∆ν≈ 1014Hz

Emits light in all (≈4π radian) direction

Laser beam : Coherent- Constant phase relation between waves in time and space

Laser emits rays of narrow frequency spectrum, ∆ν≈ 106-9Hz

Emits light in a small angle ( Divergence angle ≈ a few milli-radian)

Laser Power : mW – Several kW’s

Laser can operate in continuous wave (CW) and pulse mode; Pulse duration ranging from ms ( 10-3s ) to 10’s of fs (10-15s)

High Power & Low Divergence Exploited in Laser Material Processing

2. Excitation or Pump Source to produce population inversion in lasing medium.* Optical Pump ( Flash Lamp, Other Laser) : Solid State & Fiber Lasers* Electrical discharge (DC, AC, RF, Pulsed) : Gas Lasers* Current injection : Diode Lasers

Important components of a laser: 1.Active medium* Solid: Nd:YAG, Optical Fiber

* Liquid: Dye Laser

* Gas: He-Ne, CO2, Excimer Ar+ ion

* Semiconductor Diodes

3. Optical Resonator formed by a pair of parallel mirrors, one ~100% reflecting and other partial reflecting. They provide feedback into the active medium and facilitates laser beam to build up. Laser beam comes out through the partial reflecting mirror.

Laser Processing Setup

Laser Processing Setup* Laser system* Beam Transport system &

Beam Delivery System* Workstation

Lasers used in Manufacturing: Lasers those can provide high CW or average laser power required for material processing

* Solid State Laser : Nd:YAG Laser- Flash Lamp or Diode Laser Pumped* CO2 Laser* Diode Laser* Fiber Laser* Excimer Lasers* He-Ne Laser for alignment, pointer, metrological applications

Lasers for Materials Processing Applications & their CharacteristicsCharacteristics CO2 Laser Nd:YAG

LaserDiode Lasers

Excimer Lasers

Fiber Laser

Wavelength µm 9.6-10.6 1.06 0.8-1.0 0.193-0.354 1.06

Laser power,cw Pulse energy

Upto 45kW1-20 J

50W- 2kW1-100J

Up to 4kW

--Avg.1kW1-10J,

10kW

Efficiency % 10-15 2–20 Diode pump

20-40 2-3 30

Beam Diverg. 1-3 mr. 1 – 25 mr. 1x200 mr 2 – 6 mr. 1-2 mr.

Beam Transportation

Reflecting mirrors

Optical fibers

Optical fibers

Optical fibers

Optical fibers

Absorption in metals

Low ~2-15%

Moderate~5-30%

Moderate~5-30%

High>50%

Moderate~5-30%Life, CW Hrs.)

Pulsed (Shots)~ 1000s.~106

~200 Life of~106 lamps

~ 1000s. 104-107

(one gas fill)Size of lasers Large Moderate Compact Moderate Compact

Maintenance intervals (Hr)

1000-2000 500-1000 5,000-10,000

500-1000 5,000-10,000

Mode of operation CW & Pulsed: ms-sub-µs

CW & Pulsed: ms- Sub-ps

CW & Modulated

Pulsed10’s ns

CW & Pulsed: ms- Sub-ps

Lowest order laser beam can be focused to smallest spot sizedspot =2 f.θg =2fλ / π w0

Real Beam : Usually Higher Order Mixed Modes; Beam Quality: M2 parameter M2 =1 for Lowest order beamFor real or higher order modes, M2 >1

TEM00 Mode Beam ⇒ Single point tool ⇒ Better Quality Machining High order Mode Beam ⇒ Multi-point ( Blunt) Tool ⇒ Fair Quality

Focal spot size of a real beam, dspot= 4M2. λ.f/ πDr Dr= Laser beam diameter at the lensdspot ∝ λ- Laser Wavelength, M2- Mode Quality Parameter, f-Focal length & 1/Dr

Focusing of Laser Beam: Laser spot diameter at focal point, dspot =2 f.θ1/2F= Focal length of lens, θ1/2 = as a finite divergence

Half Divergence Angle) = θ1/2 = λ / π w0

Laser Power, PL ⇒ Joule /s = W

Laser Energy, EL= Laser Power PL x Laser Pulse Duration tp⇒ Joule

Laser Power Density = Laser Intensity I = PL/ Area of beam ⇒ W/cm2

Laser Energy Density = EL / Area of beam ⇒ J/cm2 = Laser Fluence

PL

tp

Definitions:

Laser in material processing:Laser and target material interactions:

Laser Power Density = Laser Power / Laser Beam Area

Laser Power ⇒ Type of LaserLaser Beam Diameter at Focal point of lens

⇒ Laser Wavelength,λLaser Beam Divergence,θLaser Beam Quality, M2

Focal Length of Lens, f

Absorption of laser beam in the target the form of thermal energy ⇒ Laser Wavelength, λ,

Material Property, Surface ConditionSurface Temperature

Heating, Melting, Vaporization / Ablation Processes- Exploited for various processing specialties

Physical phenomena at increasing Laser Intensity

Heating of Surface layer

Melting of shallow zone

Formation of Keyhole

Formation of Plasma

Absorptivity, ASurface condition Wavelength, λSurface Hardening

Temperature dependent A Re-melting,Conduction welding

Strong absorption in keyhole, Weak dependence on λDrilling,Deep penetration welding

Ionization of Vapor & gas,Absorption in plasma, Shock hardening,Laser Peening

~107W/cm2~106W/cm2~103W/cm2 ~105W/cm2

Laser machining : Localized,Non-contact machining Greatly simplified FixturesHigh Precision Narrow kerf in laser cutting,Narrow Heat Affected Zone (HAZ), Minimum distortion of work-pieceProcess any material independent of its hardnessVery good process quality, Usually no further post- treatment required.Holes of high aspect ratio, impossible for other methods to machine, Highly flexibilityCan process at difficult to reach places

We learn more about the Lasers & their Applications in Manufacturing