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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 mech14.weebly.com
Laser Beam Machining: An Introduction
What is a LASER?
Acronym of Light Amplification by Stimulated of Radiation
Basically a source of light giving out a coherent and low divergent
beam
Difference from ordinary source: Emission in LASER is by stimulation
process & in ordinary source by spontaneous emission process
mech14.weebly.com
Construction of Atom
Probability of stimulated absorption = Probability of Stimulated Emission
Therefore, more population in upper energy level is required for
stimulated emission to dominate and laser action to take place mech14.weebly.com
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
* 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. mech14.weebly.com
Laser Processing Setup
Laser Processing Setup
* Laser system
* Beam Transport system &
Beam Delivery System
* Workstation
Lasers used in Manufacturing: Lasers those can provide high continuous
wave (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
mech14.weebly.com
Lasers for Materials Processing Applications & their Characteristics Characteristics
CO2 Laser
Nd:YAG
Laser
Diode
Lasers
Excimer
Lasers
Fiber
Laser
Wavelength m
9.6-10.6
1.06
0.8-1.0
0.193-0.354
1.06
Laser power,cw
Pulse energy
Upto 45kW
1-20 J
50W- 2kW
1-100J
Up to
6kW
Avg.1kW
1-10J,
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
Mode of
operation
CW &
Pulsed: ms-
sub-s
CW &
Pulsed: ms-
Sub-ps
CW &
Modulated
Pulsed
10’s ns
CW &
Pulsed:
ms- Sub-
ps
Applications CW: C,W, SH,
MF,
Pulsed: D
CW: C,W,
SH, MF,
Pulsed: D
W, SH,
MF,
CW: C,W,
SH, MF,
Micro-
machining
C- Cutting, W-Welding, SH- Surface Hardening, MF- Metal forming, D-Drilling mech14.weebly.com
Laser Rod
Elliptical Reflecting Cavity Output Mirror
100%
Reflecting
Mirror
Power Supply
Flash Lamp
Laser Beam
Nd: YAG (Neodymium doped Yttrium Aluminum Garnet) Laser
Optical Excitation
Laser Medium : Nd:YAG Rod (Typical diameter = 3-10 mm, Length = 50=150 mm)
Pumping (Excitation) Method: Optical pumping using either Flash lamp or Diode Lasers
Laser Power : CW- Few watts to kW level
Pulse- Pulse duration 0.1-20ms, 10-100 nm, 10-100ps
Pulse energy = 1- 100 J (ms pulse duration), ~ 1 J (ns pulses)
Laser Beam Parameter (M2) : 2-50
Laser Beam Delivery: Low Loss Optical Fiber mech14.weebly.com
CO2 Laser Excitation
e- Accelerated in Electric Discharge,
Electrons gain energy.
e- (Hi Energy) + N2 N2* + e -(Low E)
N2* + CO2 N2 + CO2*
CO2* CO2 + Laser Photon
Lasing
+e
+He
ULL
LLL
N2*
E
T 6000K
Role of N2 in CO2 Laser:
* N2 excited by electron (e-) to vibrational meta-stable state.
* Excites CO2 molecule to upper laser level (ULL) by resonant vibration-
vibration energy transfer, thus creates preferential excitation to ULL.
Role of Helium (He) in CO2 Laser:
* Maintaining uniform and stable electrical discharge
* De-excitation of the lower laser level through vibration-translation
(kinetic energy) energy transfer.
* Heat conduction from discharge zone.
CO2 N2
mech14.weebly.com
Conventional Diffusion Cooled CO2 Laser
Resistance Elec. Power
Supply
Laser
Beam
Water Jacket Discharge
Tube
Laser Power = 50W/ m of discharge length in Diffusion cooled laser,
Optimum Gas temperature in electrical discharge ~ 250C
Laser Power of 50W /m is limited by Gas Heating
Laser medium: A mixture of CO2:N2:He=1:2:8,
Gas Pressure ~ 50-100 mbar (1000 mbar = 1 atmospheric pressure)
Excitation: Electrical discharge, Typical, I = 50mA, V= 10-12kV/m,
Gold
coated
Mirror
ZnSe Partial
Reflector
mech14.weebly.com
Convective Cooled FAF CO2 Lasers
Fast Axial Flow CO2 Laser
Laser Gas mixture flow along the optic axis
and through heat exchanger
Heat removal by passing hot
gas through heat exchangers
Flow Velocity through discharge tubes =
100 - 300m/s
Laser Power ~ 1kW/m
DC, RF Excitation
Excellent Beam Quality,
1- 5 kW Laser Commercially
Available
Special Roots / Turbo Blower
P.S.
Gas
Blower HX
HX
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Semiconductor Diode Lasers
Active medium: Semiconductor similar to that found in a light-emitting diode.
The most common and practical type of laser diode is formed from a p-n junction and powered by injected electrical current. When forward biased by applying + ve voltage at p-type, holes and electrons recombine at the p-n interface (depletion region) and they emit light. This is spontaneous emission like in LED. Under certain condition, population inversion is achieved and spontaneous emission leads to laser light by stimulated process.
Typical Materials used : GaAs, AlGaAs, GaInAsP
• Laser Wavelength: 600nm- 1.650micron range
• Power: Few mW to a few Ws in a single chip
and up to several kWs in 1-2D arrays
* Pumping: Electrical- Injection Current
• Efficiency: 30-50%
• Mode of operation: CW & Modulated up to 50kHz
• Beam quality: Poor than other lasers due to very small
area of emission
• Beam Delivery: Through Optical Fiber
Typical Beam Size:
1m x 3-10m,
1/d
Laser beam: Elliptical mech14.weebly.com
Fiber Laser
• The key advantage of a Fiber laser is its high surface-area-to-volume which effectively eliminates thermal problems.
• Active Medium: Optical (Silica Glass) Fiber with cladding, doped with either Ytterbium / Neodymium / Erbium or their combinations for different laser wavelength in 1-1.6 m range.
Most popular Fiber Laser for material processing applications: Ytterbium doped fiber operating at ~ 1 micron wavelength, pumped by diode lasers
• Size: Most common Single mode Fiber ~ -10micron dia. & 10’s m length
• Mode of operation: CW, modulated, pulsed
• Pumping: Optical pumping by Diode Lasers
• Laser Power : up to several kWs,
• Laser Efficiency: up to 30%
mech14.weebly.com
Active medium: Double cladded Yb doped optical fiber
Excitation: By a large number of low power diode lasers
Fiber Bragg gratings: Work like mirrors of optical resonator to reflect the laser
beam and also to couple out laser beam from one end
Fiber Laser: Excellent Beam Quality- Micromachining
Construction of a Fiber Laser
mech14.weebly.com
Main special properties that are utilized in material processing:
High power, Low divergence, Continuous wave / pulsed mode
of operation ( Pulse duration = ms, s, ns, ps, 10s of fs)
For laser beam machining, e.g. cutting, drilling, etc. beam is focused on the
wokpiece so that laser power density (power per unit area) is high enough to heat,
melt, evaporate or ablate the material.
1/2
db= 2w0 df
Laser
r
I
r
I
Laser Beam has a finite beam divergence.
Half divergence angle , 1/2 = M2/w0 , where = Laser wavelength ,
M2 = Beam quality parameter & w0 = Beam waist radius
M2 = 1 for laser beam of Gaussian intensity profile
M2 > 1 for other intensity profiles
When focused with a lens of focal length, f the focal spot diameter, df
df = 2f 1/2 = 2fM2 /w0 = 4f M2/db where db = Laser beam diameter mech14.weebly.com
Physical phenomena at increasing Laser Intensity
Heating of
Surface layer Melting
Formation of
Keyhole
Formation of
Plasma
Surface
Hardening,
Metal Forming
Conduction
welding,
Cutting,
Surface
alloying,
Cladding
Deep penetration
welding,
Drilling,
Shock
hardening;
Laser Peening
~107W/cm2 ~106W/cm2 ~103W/cm2 ~105W/cm2
mech14.weebly.com