Presentation on laser welding

1. Laser Beam Welding (LBW)

2. DEFINITION
• A LASER is a device that produces a concentrated coherent
light beam by simulated electronic or molecular transitions to
lower energy levels. Laser is an acronym for light
amplification by stimulated emission of radiation. Coherent
means that all the light waves are in phase.
GENERAL DESCRIPTION:
• Laser device consists of a medium placed between the end mirrors
of an optical resonator cavity. When the medium is “pumped” to
the point where a population inversion occurs , a condition where
in the majority of active atoms in this medium are put into a higher
than normal energy state, a source of coherent light that can then
reflect beck and forth between the end mirrors of the cavity will be
provided. This will results in cascade effect being included which
will cause the level of this coherent light to reach a threshold point,
there by allowing the device to start to emit a beam of laser light.

3. • Laser is an energy conversion device that simply transforms
energy from primary source into a beam of electromagnetic
radiation at some specific frequency. This transformation is
facilitated by certain, solid, liquid, or gaseous mediums which,
when excited on either a molecular or atomic scale , will
produce a very coherent and relatively monochromatic form
of light a beam of laser light. Because they are coherent and
monochromatic, both lo w power and high power laser light
beam have a very low divergence angle. Thus they can be
transported over relatively large distance before being highly
concentrated to provide the level of beam power density
needed to do a variety of material processing such as welding,
cutting and heat treating.

4. • The first laser beam produced in 1960 using a ruby crystal
pumped by a flash lamp. Solid state lasers of this type
produce only short pulses of light energy and at repetition
frequencies limited by heat capacity of the crystal.
• Consequently, even though individual pulses do exhibit
instantaneous peak power levels in the megawatt range,
pulse ruby laser are limited to low average power output
levels. Both pulsed and continuously operating solid state
lasers, capable of welding and cutting thin sheet metal, are
current y commercially available.
• Many of the latter utilize Neodymium Doped (ND), Yttrium
aluminium garnet (YAD) crystals rods to produce a continuous,
monochromatic beam output in the 1 to 2 kW power range.

5. PRINCIPLES OF OPERATION
• 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 LBW process, the laser beam is directed by flat optical, such
as mirrors and then focused to a small spot at the workpiece
using either reflective focusing elements 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.

6. • As described above, the lasers predominantly being used for
industrial material processing and welding tasks are the 1.06
µm wave length YAG laser and the 10.6 µm wave lengthCO2
laser, with the active element most commonly employed in
these two varieties of lasers being the Neo dynamic (Nd) and
the Co2 molecule (respectively).

7. Solid State Lasers
• Solid State Lasers utilize an impurity in a host material as the
active medium. Thus the neodymium ion (Nd+++) is used as a
“dopant” or purposely added impurity in either a glass or YAG
crystal and the 1.06 µm output wavelength is dictated by the
neodymium ion .
• Both ends of the cylinder are made flat and parallel to very
close tolerances, then polished to a good optical finish and
silvered to make a reflective surface.
• The crystal excited by means of an intense krypton or xenon
lamp. A simplified schematic arrangement of the rod, lamp
and mirrors is shown in Fig. 1.

8. Fig. 1. schematic view of a Solid State Lasers

9. • Selection of the host material for the neodymium ion depends
upon several factors. Like ability to produce large quantities of
good optical quality rods, efficiency, and optical absorption
bands.
• All of these factors influence the ability of the system to emit
reasonable amounts of energy in a single pulse, and
successful materials are those from which large amounts of
energy can be extracted. Since the YAG crystal processes all of
the ideal characteristics outlined, it makes an excellent host
material.
• The output characteristics of Nd: YAG lasers depend on the
excitation method, which may be either continuous or
repetitively pulsed in nature.
• In continuous operation , the laser4 is excited with either
xenon lamps
• For repetitively pulsed lasers, the output characteristics
depend on lamp configuration.

10. • Table 1 gives the characteristics of Nd: YAG lasers and offers
some idea of the capability for trade offs between the average
power, pulse energy, pulse duration and pulse repetition rates
for such lasers.

11. • The relatively narrow frequency band exhibited by Nd: YAG
lasers facilitates continuous waves operation at room
temperature , making the continuous waves Nd: YAG laser
second only to the CW gas lasers in terms of continuous
waves power generation. However its considerably lower
overall efficiency capability results in a lower power output.
• Fig. 2 shows the time relationship of the flash lamp and laser
output pulses of a typical pulsed Solid State Lasers
Fig. 2

12. • The beginning of the flashlmap pulses establishes a
population inversion in the active medium. When loop gain
reaches 1.0, lasing begins and continues as a series of closely
spaced spikes for the duration of the flashlmap pulse.
• These spikes are produced by gain switching in the active
medium . The gain rises quickly to a high value because of the
intense pumping level. This results in a high loop gain and a
high intensity standing wave in the optical cavity. This quickly
depletes the population inversion for that particular
wavelength , and lasing stops. Thus the laser switches itself off
momentarily by using up all of its gain.