Laser Beam Welding (LBW)
Professor Pedro Vilaça *
Materials Joining and NDT
* Contacts
Address: P.O. Box 14200, FI-00076 Aalto, Finland
Visiting address: Puumiehenkuja 3, Espoo
[email protected] ; Skype: fsweldone
January 2015
Laser Beam Welding (LBW)
Process Fundaments
Definition and General Description
• A Laser is a device that produces a concentrated coherent light beam by stimulated
electronic or molecular transitions to lower energy levels.
• LASER is acronym for Light Amplification by Stimulated Emission of Radiation
• The laser is a device that produces monochromatic (all the radiation is in the
same wavelength), coherent (all the radiation waves are in phase) light. Therefore
electromagnetic radiation (light) can be amplified unidirectional and with
low divergence.
• From an engineering standpoint, a laser is an energy conversion device that simply
transforms energy from a primary source into a beam of electromagnetic radiation
at some specific frequency.
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1
Characteristics of a Laser Beam
Light
Amplification
Stimulated
Emission
Radiation
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Laser Welding
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Spontaneous
Emission
Laser Beam Welding (LBW)
2
Stimulated
Emission
E2
Process Fundamentals
h 21
2 h 21
Stimulated Emission of Radiation
E1
http://www.colorado.edu/physics/2000/lasers/lasers2.html
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2
Typical components of a laser system
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Historical scope
• Albert Einstein invented the laser beam in 1917
• Theodore Mainman the built 1st laser in 1960 - Ruby laser excited by a mercury
vapor lamp
• The first industrial applications arise in the 70's with gas lasers (CO2)
• Even today new main active media laser are under investigation and development
to best attend the needs of industry
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Historical scope:
 Segmentation of Laser Application in Industrial Materials Processing
Drilling
Microprocessing
Other
Cutting
Marking
Welding
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Principles of Operation
• Laser Beam Welding (LBW) is a fusion joining process that produces
coalescence of materials with the heat obtained from a concentrated beam of
coherent, monochromatic light impinging on the joint to be welded.
• In the LBW process, the laser beam is directed by flat optical elements, such
as mirrors, and then focused to a small spot at the workpiece using either
reflective focusing elements or lenses.
• LBW is a non-contact process, and thus requires that no pressure be applied.
Inert gas shielding is generally employed to prevent oxidation of the molten
puddle, and filler metal may occasionally be used.
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Principles of Operation: Energy of the electromagnetic radiation
E
hc

 h
c  3  10 8 m / s
h  6.6  10 34 J .s


-1
Frequency of the electromagnetic radiation [s ]
Wavelength of the electromagnetic radiation [m]
Fundamental condition…
…for a Laser to achieve continuous emission of radiation:
 inside the resonator the excited active medium must be a majority
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Wavelength of main active laser media
Ultraviolet
(0.1mm – 0.4mm)
Visible
(0.4mm – 0.7mm)
Infrared
(0.7mm – 100mm)
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Lasers Wavelength
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Energy distribution in laser beam:
 Transversal Electromagnetic Mode (TEM)
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Energy distribution in laser beam:
 Transversal Electromagnetic Mode (TEM)
Díodo
Gaussian
distribution
TEM00, TEM01, TEM02
CO2 - TEM00
CO2 - Multimode
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Quality of the Laser Beam
The smaller the Mode Factor (M2)
and the Beam Parameter Product (BPP)
the higher the quality of the laser beam
An M2 = 1 represents a laser beam
(with TEM00, which means a perfect Gaussian profile)
The BPP allow comparing the quality of the laser
beam for laser with different wavelengths
Legend:
dB – Diameter of the Raw Laser Beam (as out of power source)
df – Focal Diameter (at the processing spot)
lf – Focal Length
 – Wavelength of the Laser Beam
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Types of Lasers
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Some of the Most Relevant Active Laser media (1/5):
 CO2 Laser (Gas Laser) Operating with powers from 1-20kW is an extremely
mature laser technology and has been a mainstay of macro laser welding since
the late 1980’s
Axial slow flow CO2 laser
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Some of the Most Relevant Active Laser media (1/5):
 CO2 Laser
Key Features:
 Wavelength : 10600 nm
 Plug Efficiency : 10%
 Cost per kW : 50 k€
 Maximum Power : 20 kW
 Beam Quality M2 : 4.5
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Some of the Most Relevant Active Laser media (cont. 1/5):
 CO2 Laser
Transversal fast flow CO2 laser
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Some of the Most Relevant Active Laser media (cont. 1/5):
 CO2 Laser
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Some of the Most Relevant Active Laser media (2/5):
 Nd:YAG (Rod Laser) Pulse mode Laser (note: CW is superseded by CW Disk and
Fiber lasers) uses a single Nd:YAG rod that is pumped using flash lamps to produce
high peak and low average power for welding. E.g. a relatively low average power of
25W can provide up to 6kW of peak power. The correct high peak power/short pulse
combination provides a good solution comprising efficient material coupling and
precise energy input control.
• Nd:YAG - Neodymium Yttrium Aluminum Garnet
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Some of the Most Relevant Active Laser media (cont. 2/5):
 Nd:YAG (Rod Laser)
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Some of the Most Relevant Active Laser media (cont. 2/5):
 Nd:YAG (Rod Laser)
Key Features:
 Wavelength : 1064 nm
 Plug Efficiency : 30%
 Cost per kW : 120 k€
 Maximum Peak Power : 10 kW
 Beam Quality : 12 mm.mrad
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Some of the Most Relevant Active Laser media (cont. 2/5):
 Nd:YAG (Rod Laser)
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Some of the Most Relevant Active Laser media (3/5):
 Yb:YAG (Disk Laser) A thin flat cristal disk (0.25mm) supported at one side by a
heat sink. The cooling from this design enables CW Power in excess of 10kW
with good beam quality.
• Ytterbium Yttrium Aluminum Garnet
Key Features:
 Wavelength : 1030 nm
 Plug Efficiency : 30%
 Cost per kW : 50 k€
 Maximum Peak Power : 16 kW
 Beam Quality : 12 mm.mrad
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Some of the Most Relevant Active Laser media (3/5):
 Yb:YAG (Disk Laser)
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Some of the Most Relevant Active Laser media (4/5):
 Diode Laser – The increasing power levels of single-emitter devices, new cooling
channel technology, and the development of micro optics to focus the beam from
arrays into sub-1000-micron core diameter fibers has given rise to the emergence of
the direct diode as a welding laser
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Some of the Most Relevant Active Laser media (4/5):
 Diode Laser
Key Features:
 Wavelength : 808 or 920 nm
 Plug Efficiency : 40%
 Cost per kW : 60 k€
 Maximum Power : 6 kW
 Beam Quality : 300 mm.mrad
 Very compact
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Some of the Most Relevant Active Laser media (5/5):
 Fiber Laser is an efficient diode pumped laser of a small core silicon fiber. Lasing
occurs within the fiber so there is no alignment requirement and when imaging the
small core fibers in the focus head, optical focus spot sizes down to 10 mm are
possible. This compact CW laser is provided in two configurations: single-mode for
low power welding (<3kW) and so-called multimode for higher power welding.
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Some of the Most Relevant Active Laser media (5/5):
 Fiber Laser
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Some of the Most Relevant Active Laser media (5/5):
 Fiber Laser




Multi-kilowatt fibre lasers introduced by IPG
Benefits:
 Rapid increase in power – up to 50 kW
 Wall Plug Efficiency 25%
 Very Compact and Easy to Install
 Estimated Diode Lifetime > 50,000 hrs
 BPP (4/5 kW) < 2.5 mm.mrad
 Diode Lifetimes (hours] > 100,000
 Floor Space (4/5 kW) < 1m2
 Low Maintenance
Limitations:
 Capital cost
Prognosis: Replace lamp-pumped Nd-YAG,
lower cost?
8 kW IPG fibre laser at
Cranfield University
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Laser Costs:
Courtesy of Trumpf
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Laser Costs:
Laser Operating Costs (8 year average)
45
$/hr
40
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Welding & Laser Gas
30
Floor Space
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Maintenance
20
Electric
15
Replacement Parts
10
Depr. & Interest
5
0
CO2
YAG
Fiber
Disc
Courtesy of EWI
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Robotic Style Beam Delivery
• While Laser Welding was initially performed in much the same manner as electron
beam welding, by manipulating the joint to be welded under stationary beam, it is
currently quite common to use robotic style device to manipulate the laser beams
over stationary joints.
• Highly flexible laser beam motion systems having the capability to perform three –
dimensional welding tasks are readily available today.
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Robotic Style Beam Delivery
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Major Advantages of laser beam welding
• Heat input is close to the minimum required to melt the weld metal; thus,
metallurgical effects in heat-affected zones are reduced, and heat-induced work
piece distortion is minimized
• Single pass laser welding procedures have been qualified in materials of up to 32
mm thick, thus allowing the time to weld thick sections to be reduced and the need
for filler wire (and elaborate joint preparation) to be eliminated
• No electrodes are required; welding is performed with freedom from contamination,
indentation, or damage from high resistance welding currents
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• Laser beams are readily focused, aligned, and directed by optical elements. Thus
the laser can be located at a convenient distance from the work piece, and
redirected around tooling and obstacles in the work piece
• The workpiece can be located and hermetically welded in an enclosure that is
evacuated or that contains a controlled atmosphere
• The laser beam can be focused on a small area, permitting the joining of small,
closely spaced components with tiny welds
• A wide variety of materials can be welded, including various combinations of
different type materials
• The laser can be readily mechanized for automated, high-speed welding, including
numerical and computer control
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• Laser welds are not influenced by the presence of magnetic fields, as are arc and
electron beam welds
• Metals with dissimilar. physical properties; such as electrical resistance, can be
welded
• No vacuum or X-ray shielding is required
• Aspect ratios (i.e., depth-to-width ratios) on the order of 10:1 are attainable when
the weld is made by forming a cavity in the metal
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Limitations
• Joints must be accurately positioned laterally under the beam and at a controlled
position with respect to the beam focal point
• When weld surfaces must be forced together mechanically, the clamping
mechanisms must ensure that the final position of the joint is accurately aligned
with the beam impingement point
• The maximum joint thickness that can be laser beam welded is somewhat limited.
Thus weld penetration more than (19mm) are not presently considered to be
practical
• The high reflectivity and high thermal conductivity of some materials, such as
aluminium and copper alloys, can affect their weldability with lasers
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• When performing moderate to high power laser welding, an appropriate plasma
control device must be employed to ensure weld reproducibility
• Lasers tend to have a fairly low energy conversion efficiency, generally less than
10 %
• As a consequence of the rapid solidification characteristic of LBW, some weld
porosity and brittleness can be expected
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Characteristics of The Weld
Keyhole or Deep Penetration Welding
• When Beam Power densities on the order of (1.55 x 103 W /mm²) or greater are
achieved, deep penetration beam welding is accomplished by a keyhole energy
transfer mechanism.
• At this power density level, the energy input of the impinging beam is so intense
that it cannot be removed by the normal conduction, convection or radiation
processes.
• Thus the area upon which the beam is being impinged melts and vaporizes.
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Beam Power density
106 W / cm 2
Characteristics of The Weld
Keyhole or Deep Penetration Welding
Conduction mode
Keyhole mode
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Laser Welding Regimes
Transition = 106 W/cm2
Conduction Welding
Keyhole Welding
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Laser Welding Regimes
Constant Beam Diameter and
Welding Speed
Conduction – Welding, Brazing,
surface treatments
Key-Hole – Welding
Drilling – Cutting and drilling
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Applications of Conduction Mode Laser Welding
Stainless Steel
Welding Speed = 5 m/min
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Keyhole Welding
Low heat input process
(they can be almost 10 times lower than the
heat input required for TIG welding)
Does not require any surface finishing process
after welding
Good welding bead quality
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Keyhole Welding
•
Normally the welds produced
are narrow and precise
•
Very narrow HAZ
•
Low distortion and low residual
stresses
•
Allows welding complex
shapes at high welding speeds
Courtesy of Cranfield University
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Characteristics of The Weld
Position of Focal Point
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• This keyhole welding phenomena is common to both laser beam and electron
beam welding, indicating that it is primarily a function of power density, and not
dependent upon wavelength
• When the material at the interaction point melts and vaporizes, the vapour recoil
pressure creates a deep cavity or "keyhole“
• This keyhole is a vapour column surrounded by a thin cylinder of molten metal
• When the beam moves relative to the workpiece, the vapour pressure of the metal
sustains the keyhole, and the molten metal surrounding the keyhole flows opposite
to the weld direction where it rapidly solidifies, forming a narrow fusion zone or
weld
• The narrow, high depth-to-width ratio fusion zone formed by laser welds at
atmospheric pressure are similar to electron beam welds made in vacuum.
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Operational modes in laser welding
Pulsed Wave Laser
(PW)
Continuous Wave Laser
(CW)
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Continuous wave laser welding
Welding Parameters
Power (W) – Parameter from the laser
Welding Speed (m/min) – Parameter of
the system
Beam Diameter (mm) – Depends on the
optics, of the working distance, of the
beam quality and of the wavelength
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Continuous Wave Laser Welding Applications
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Pulsed Wave Laser Welding
Used when very small welding spots are required
Extremely tight tolerances
Pulsed lasers have a very high peak power
Used in high precision applications
Pulsed laser welding is more efficient than
continuous wave laser welding
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Pulsed Wave Laser Welding
Parameters of Pulsed wave laser welding
E – Pulse Energy (J, laser parameter)
t – pulse duration or interaction time
(ms, laser parameter)
f – Frequency (Hz, laser parameter)
v – Welding speed (mm/s, parameter
of system)
D – Beam Diameter (mm, depends on
the optics, on the working distance, on
the laser beam quality and on the
wavelength
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Pulsed Wave Laser Welding
Degree of overlap
50% to 60 % structural
welds
v – Welding speed(mm/s)
f – frequency(Hz)
D – Beam Diameter(mm)
t – Pulse duration (ms)
85% a 90 %
hermetic seals
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Plasma Suppression
• The intense heat generated by the laser beam melts the workpiece, and some of
the liquid metal is vaporized into a gaseous state.
• A fraction of this gas is ionized by the high energy learn and becomes a plasma.
• This is detrimental because it tends to absorb and attenuate the laser beam.
• Fortunately, plasma can be suppressed by blowing it away with a stream of gas
having a component of velocity transverse to the laser beam axis.
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Plasma Suppression
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Plasma Suppression… in CO2 laser welding
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Plasma Suppression
Laser beam
Coaxial gas
Transversal gas for
supressing the plasma
formation
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Plasma Suppression and porosity formation mechanisms:
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Porosity formation mechanisms:
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Welding defects
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Welding defects
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Applications
• Laser Beam Welding is being used for an extensive variety of applications such
as in the production of automotive transmission and air conditioner clutch
assemblies.
• Successful laser welding applications include welding transmission components
(synchro gears, drive gears and clutch housings).Materials welded are either
carbon or alloy steels. In some cases, such as the gear teeth, they have been
selectively hardened before welding.
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Metals Welded
• Laser Beam Welding can be used for joining most metals to themselves as well as
dissimilar metals that are metallurgical compatible. Low carbon steels are readily
weldable, but when the carbon content exceeds 0.25 percent, martensitic
transformation may cause brittle welds and cracking.
• Many stainless steels are considered good candidates for laser welding. Stainless
steel of the 300 series are readily weldable.
• Welds made in some of the 400 series stainless steels can be brittle and may
require post weld annealing. Many heat resistant nickel and iron based alloys are
being welded successfully with laser beams. Titanium alloys and other refractory
alloys can be welded in this way, but an inert atmosphere is always required to
prevent oxidation.
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Metals Welded
• Copper and brass are often welded to themselves and other materials with
specialized joint design used for conduction welding.
• Aluminium and its weld able alloys can be joint for partial penetration assembly
welds and are commonly joined by pulsed conduction welds for hermetically sealed
electronic package.
• Refractory metals such as tungsten are often construction welded in electronic
assemblies.
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Applications of lasers
Laser Brazing
Laser Welding with filler material
Hybrid Welding
Cladding
“Additive Layer Manufacturing”
Cutting
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Laser Brazing
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Laser Welding with Filler Metal
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Hybrid laser welding
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Hybrid laser welding
Mismatch
• Laser Beams are considerably more tolerant to mismatch than gap. Mismatch as
great as quarter the material thickness can be tolerated.
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Hybrid: Laser + GMAW / GTAW / PAW to help bridge gaps and mismatches
Combination of laser light and an electrical arc into an amalgamated welding
process has been known since the 1970s, but has only recently been used in
industrial applications. There are three main types of hybrid welding process,
depending on the arc used; GTAW, PAW arc or GMAW augmented laser
welding. While GTAW augmented laser welding was the first to be researched,
GMAW is the first to go into industry and is commonly known as hybrid laser
welding
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Hybrid: Laser + GMAW / GTAW / PAW to help bridge gaps and mismatches
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Hybrid laser welding
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Hybrid: Laser + GMAW
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Hybrid laser welding
 Arc root stabilized by laser (especially at high speeds)
 Metal vaporised by laser forms part of arc plasma, increases
arc conductivity
 Arc reduces reflectivity (especially for CO2 laser welding of
aluminium)
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Hybrid laser welding
•
Top – conventional GMA pulse welding of titanium
•
Bottom: GMAW pulse + 200 W laser
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Hybrid laser welding
 Arc stabilised by laser
 Arc plasma concentrated and channelled into keyhole
 Extra penetration from arc plasma pressure at high arc
current, and use of V preps and root gaps
 More complex welding head
 Torch orientation must be maintained
 Smoke and spatter damage to lenses
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Hybrid laser welding
Kugler head
Cloos hybrid head
Fronius hybrid head
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Hybrid laser welding
 ILT integrating hybrid nozzle
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Hybrid laser welding
Approx. 900 km Weld Length
450 km Hybrid Welds (50%)
Laser-GMA Hybrid Welding
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Hybrid laser welding
Welding-parameters:
 Laserpower
PL= 3,8 kW
 welding speed
vw= 3,6 m/min
 wire feed speed vws= 4,5 m/min
Laser-GMA Hybrid Welding
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Hybrid laser welding
Axle component, 4.5
m/min, laser hybrid:
30% increase in welding
speed (compared to
Tandem MAG)
Automotive side cabin,
1.5 mm thick
aluminium, 4.5 m/min
laser hybrid
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Automotive
component, 4 mm
thick steel, laser
tandem, 3.5 m/min
Source: Fronius
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Hybrid laser welding
Application Aluminium-Profiles of High Speed Trains -
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Hybrid laser welding
Laser-GMA Hybrid Welding
GMA Welding
Laser-GMA Hybrid Welding
- Reduction of Distortion -
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Laser Cladding
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“Additive Layer Manufacturing”
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“Additive Layer Manufacturing”
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Sample of Industrial Applications: Suspension arm orbital laser welded joint
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Sample of Industrial Applications:
Wear components, e.g. Moulds, repair by laser precision welding
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Sample of Industrial Applications:
Ti alloy component in a “T”-joint welded both outside and inside
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Sample of Industrial Applications
The 0.4 mm iridium tip that generates the high performance of the IRIDIUM POWER is an
alloy with a very high melting point. Therefore, ordinary resistance welding cannot be used
because the iridium does not melt enough and an adequate weld strength cannot be
guaranteed.
"All-round laser welding", which employs a high-energy laser, is used in the IRIDIUM
POWER to melt and weld all around welding points.
Because all area to be welded is completely melted, the welding point is extremely reliable,
thus ensuring stable and quality response without changes in the electrode, even under
heavy driving conditions.
Department of Engineering Design
and Production
Engineering Materials
Materials Joining and NDT
90
Engineering Materials
Materials Joining and NDT
91
Sample of Industrial Applications:
Asphalt & Green Concrete Cutting Diamond Saw Blades
Department of Engineering Design
and Production
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Safety
• Misuse of laser equipment can result in permanent damage to the eyes and skin of
both operators and nearby personnel.
• Specific precautionary measures are needed to avoid other potential hazards
sometimes associated with using lasers such as dangers related to servicing highvoltage power sources, and harmful fumes that can be released when laser
processing certain materials.
Department of Engineering Design
and Production
Engineering Materials
Materials Joining and NDT
92
Beams – Related Hazards
• Class 1 – Denotes exempt lasers of laser systems that cannot, under normal
operating conditions, produce a hazard. Bar code reading lasers.
• Class 2 – Denotes low power visible lasers or laser systems which, because of
natural human aversion response bright light, do not normally present a hazard
but which may produce a hazard if viewed directly for extended periods.
• Class 3A - Laser system that would not produce a hazard under normal conditions
if viewed for only momentary periods with the unprotected eye.
• Class 3B – Produce a hazard if viewed directly, including intrabeam viewing of
specular reflections.
• Class 4 – Produce a hazard not only if the beam or specular reflections are
viewed directly, but also from direct viewing of diffuse reflection.
Department of Engineering Design
and Production
Engineering Materials
Materials Joining and NDT
93
47