Transcript Laser Welding of Stainless Steels

Industrial Lasers for
Welding
Ing. M. Muhshin Aziz Khan
Facts About Laser:
Laser Basics
Light Amplification by Stimulated Emission of Radiation
Laser is essentially an optical amplifier that
 Generates and
 Amplifies
Stimulated Emission
Laser Components
Lasing Medium:
• Properties of Laser
Coherent (synchronized phase of
light)
Collimated (parallel nature of the
beam)
Monochromatic (single wavelength)
High intensity (~1014W/m2)
Provides appropriate transition and
Determines the wavelength (it must be in a
metastable state)
Pump:
Provides energy necessary for population
inversion
Optical Cavity:
Provides opportunity for amplification and
Produces a directional beam (with defined
length and transparency)
Facts About Laser:
Laser History
Facts About Laser:
Laser Material: Energy Levels of Atomic or Molecular System
E1
Ground State
Laser
Transition
Excitation
Laser operation takes place via transitions between different energy levels of an
atomic or molecular system
Highly excited
TwoFor material with Two-Level system
State
Level
Energy
 Absorption and stimulated processes
2° Excited State
neutralize one another.
E
3
Population
1° Excited
State
 +The material becomes
transparent.
E2
Inversion
Ground
State
Population inversion is
impossible
Facts About Laser:
Laser Material: Energy Levels of Atomic or Molecular System
Ground
State
Laser
Transition
Population
Inversion
Highly excited State
Fast Decay
FourLevel
Excitation
Highly excited State
Fast Decay
Metastable
State
Laser
Transition
Excitation
ThreeLevel
Higher Metastable
State
Population
Inversion
Natural
Depopulation
Lower Metastable
State
Ground
State
Facts About Laser:
Optical Pumping: Population Inversion
More atoms or molecules are in a higher
energy state
Nonequilibrium distribution of
atoms among the various energy
level of atomic system
Process producing population inversion
is called Pumping
Energy needed for population
inversion is supplied by optical
excitation with light source
Flash lamps (Pulsed laser), Arc
lamp (CW Laser), Semiconductor
Diode
Excitation by electron collisions
and resonant transfer of energy
(Gaseous)
Is population inversion by thermal
excitation possible!!??!!
According to Boltzmann Ratio
N2
N1
 exp
(
E 2  E1 )
KT
Where E2 > E1
Is population inversion a necesary
condition for laser operation!!??!
Amplification
by I(z)
stimulated
emissiona
Light with intensity
passing through
increases
the Intensity
according
to N
laser medium
with densities
of atoms
1
-(gz)
I(0) exp
and NI(z)
and lower energy levels
2 in=higher
1
2


c
ln
2
 2N
Absorption by  atoms
in2  Level 1
g 

 to

2
 according
decreases
the
Intensity
4

f



f






= I(0)
exp-(σz) of light through the
Net I(z)
effect
of passage
1
material
2
2
 c
  ln 2  1  N 1 
2


   c 2  ln 2 2 1 

I ( z )  I ( 0 ) exp 4  f 2     f   N 2  N 1  z 
  
4

f


  f 






Facts About Laser:
Optical Cavity: Mirror Configuration
 Resonator cavity is formed by placing
mirrors at the ends of the active medium
The mirrors are perpendicular to the
axis along which the laser light travels
Mirror
configurations
are system
judged on two
 Acts as
Positive feedback
criteria
 Provides amplification and
 Stability
directionality to a laser beam via
Light rays bouncing back and forth
oscillation
between mirrors will be re-entrant.
 The resonant cavity generally is much
 Filling of the active medium by light
longer than it’s width
Spatial profile defined by the light rays fills
all the volume of the active medium
Stability Diagram for laser resonators consisting
of two mirrors
Condition for stability of a resonator
 D
 D

0
 1 
 1  1
 R1
 R2

Facts About Laser:
Optical Resonator: Mirror Configurations
Possible Mirror Configurations
Plane paraller and Confocal Mirror Configurations
 Both mirror configurations have marginal or
delicate stability
 Plane parallel mirrors have good filling whereas
confocal mirrors offers poor filling of active
medium
 For plane parallel mirrors, allignment is really
crucial. However, for confocal mirrors even if the
configuration is not exactly perfect, the light rays
will still be reentrant.
Long-radius mirror configuration is most often used
in modern comercial lasers
 It falls within a region of good stability
 Beam spatial profile fills active medium
reasonably well
Facts About Laser:
Optical Resonator: Gain
Laser gain and losses
Oscilation Condition:
Gain ≥ Losses
Laser turn-on and gain saturation
Optical loss in resonant cavity
 r1 and r2 : mirror loss/coupling loss
Due to non-unity reflectivity on the
mirrors
Loss is Independent of cavity
length
 exp(-2αL) ~ 1-2αL : distributed
loss/internal loss
Due to absorption/scattering in
the cavity material
Loss is proportional to cavity
length
Gain decreases
as output power
increases
• Saturation
Facts About Laser:
Laser Efficiency
Output versus input power for an
optically pump power
Slope efficiency (or differential
efficiency)
The slope of the curve obtained by
plotting the laser output versus the
pump power.
Facts About Laser:
Laser Quality and Its Effect
Beam Quality
Effects of Beam Quality
 A measure of Lasers’ capability to be
☺ propagated with low divergence and
☺ focused to a small spot by a lens or
mirror
 Beam Quality is measured by M2 or BPP (Beam
Product Parameter, mm.mrad)
 Ratio of divergence of actual beam to a
theoretical diffraction limited beam with
same waist diameter
 M2= 1; Ideal Gaussian Beam, perfectly
diffraction limited
 Value of M2 tends to increase with increasing  Smaller focus at constant aperture and focal
length
laser power
 Longer working distance at constant aperture
A higher power density by a smaller spot size
and spot diameter
with the same optics, or
 Smaller aperture (‘slim optics’) at constant
The same power density at lower laser power
focal diameter and working distance
Facts About Laser:
Primary Adjustable Parameters and Their Effects
Primary Controllable Parameters
 Laser Beam Energy Output Characteristics
(i) Voltage
(ii) Pulse Duration
 Laser Focus Characteristic
(iii) Laser Beam Diameter
Change in Voltage
Change in Pulse Duration
Increased pulse duration results in
deeper and wider melting
Change in Voltage and Pulse
Duration
Increased voltage results in deeper
physical penetration with less melting
due to physical pressure
Change in Beam Diameter
Simultanous increase in voltage and
pulse duration results in deeper melting
Increased beam diameter results in
shallow soft penetration and wide, but
soft melting
Facts about lasers for welding
Laser Characteristics, Quality and Application

Typical commercial lasers for welding
1. CO2 Laser
2. Nd3+:YAG Lasers
 Lamp-pumped
 LD-pumped
3. Disk Laser
4. Diode Laser
5. Fiber Laser
CO2 Laser: Characteristics
Wavelength
10.6 µm; far-infrared ray
Laser Media CO2–N2–He mixed gas (gas)
Average
45 kW (maximum)
Power (CW) (Normal) 500 W – 10 kW
Merits
Easier high power (efficiency: 10–
20%)
CO2 Laser: M2 values [CW]
Output power (W)
<500
M2
1.1-1.2
800-1000
1.2-2
1000-2500
1.2-3
5000
2-5
10,000
10
Facts about lasers for Welding: YAG Laser
Laser Characteristics, Quality and Application
Lamp-pumped YAG Laser: Characteristics
Wavelength
1.06 µm; near-infrared ray
Laser Media Nd3+: Y3Al5O12 garnet (solid)
YAG Laser: M
values
[CW &
Average
102 kW
(cascade
type & fiberPW]
Power [CW] coupling)
Output power (Normal) 50
M2W–4 kW
(W)
Fiber-delivery, and easier
Merits
0-20
1.1-5
handling (efficiency: 1–4%)
20-50
20-50
50-150
50-75
150-500
75-150
500-4000
75-150
YAG Laser Application: Automobile
Industries
Lamppumped
3 to 4.5 kW class; SI fiber
delivered (Mori, 2003)
LD-pumped
2.5 to 6 kW
New
Rod-type: 8 and 10 kW; Laboratory
LD-pumped
YAG Laser: Characteristics
Development
Prototype
Slab-type:
6 kW;
Developed
by
Wavelength
about
1 µm;
near-infrared
ray
Precision Laser Machining
Laser Media
Nd3+PLM
: Y3Al5O12 garnet (solid)
Consortium,
Average
Power
[CW] : 13.5 kW (fiber-coupling
max.)
[PW] : 6 kW (slab type max.)
Merits
Fiber-delivery, high brightness,
and high efficiency (10–20%)
Facts about lasers for welding: Disk Laser
Laser Characteristics, Quality and Application
Disk Laser: Characteristics
Wavelength
1.03 µm; near-infrared ray
Laser Media
Yb3+ : YAG or YVO4 (solid)
Average
Power [CW]
6 kW (cascade type max.)
Merits
Fiber-delivery, high
brightness, high
efficiency(10–15%)
Recent Development (Mann 2004; and
Morris 2004):
 Commercially available disk laser
system: 1 and 4 kW class
 Beam delivery with 150 and 200 µm
diameter fiber
 Even a 1 kW class laser is able to
produce
a deep keyhole-type weld bead
extremely narrow width
in stainless steel and aluminum alloy
Facts about lasers for welding: Diode Laser
Laser Characteristics, Quality and Application
Diode Laser: Characteristics
Wavelength
0.8–0.95 mm; near-infrared
ray
Laser Media
InGaAsP, etc. (solid)
Average
Power [CW]
10 kW (stack type max.)
5 kW (fiber-delivery max.)
Merits
Compact, and high efficiency
(20–50%)
Recent Development (Hayashi 2004; and
Zediker 2001):
 Commercially available Diode laser
system: Direct and/or fiber-coupled
modes
 Found suitable for welding of
 plastics and
 thin sheets of aluminum or steel
at high speed
 Fiber-delivered laser is used for brazing Zncoated steel using robot.
Facts about lasers for welding: Fiber Laser
Laser Characteristics, Quality and Application
Fiber Laser: Characteristics
Wavelength 1.07 µm; near-infrared ray
Laser
Media
Yb3+ : SiO2 (solid), etc.
Average
20 kW (fiber-coupling max.)
Power [CW]
Merits
Fiber-delivery, high
brightness, high
efficiency(10–25%)
Recent Development (Thomy et.al. 2004; and
Ueda 2001):
 Fiber lasers of 10kW or more are
commercially available
 Fiber lasers of 100kW and more are
scheduled
 Fiber laser at 6.9kW is able to provide
deeply penetrated weld at high speed
 Fiber laser is able to replace high quality
(slab) CO2 laser for remote or scanning
welding
Facts about lasers for welding
Comparison of different laser systems
Correlation of Beam Quality to Laser Power
(Katayama 2001; O’Neil et. al. 2004; Shiner 2004;
Lossen 2003):
 Overlaid with condition regimes
 Beam quality of a laser worsens with an
increase in power
 LD-pumped YAG, thin disk, CO2 and fiber
lasers can provide high-quality beams
 The development of higher power CO2 or YAG
lasers is fairly static and, hence
Main focus on development:
i.
high-power diode, ii. LD-pumped YAG, iii.
disk and/or
iv. fiber lasers
Facts about lasers for welding
Wavelengths of some important laser sources for materials processing
CO2 Laser
Expanded portion of the electromagnetic spectrum showing the
wavelengths at which several important lasers operate
Thank You for
Patience Hearing