ABSTRACT
Over
the last 20 years, laser welding has been increasingly being accepted in the
field of manufacturing. There are many reasons for the acceptance; one of it is
because of its uniqueness of non contact autogenously welding process that is
not affected by electrical conductivity of magnetic properties of the material
being welded allowing it to be used extensively to weld a wide range of materials.
This paper reviews laser welding type, the implementation in manufacturing and
impact on manufacturing processes.
1.
INTRODUCTION
The term LASER is an acronym for “Light
Amplification by Stimulated Emission of Radiation”[1]. The principle of laser
was brought up by Albert Einstein when he described the theory of Stimulated
Emission [2]. Einstein proposed that an exited atom in isolation can return to
a lower energy state by emitting photons a process he dubbed spontaneous emission
[3]. The spontaneous emission leads to radioactive interactions, such as
absorption and stimulated emission. The atoms only absorb photons with the
proportionate wavelength, when the absorption happens the photon disappears and
the atoms will be at a higher energy state, setting the condition for the
spontaneous emission. He also predicted that as lights passes through a
substance, it could stimulate the emission of more lights [3].
In laser the atoms of lasing medium are
“pumped” to produce atoms with higher
energy levels than the ground state causing the sudden burst of coherent lights
as the atoms discharge in a rapid chain reaction, which we call “stimulated
emission”. The common active media for laser are, Nd:YAG (Rod laser) Neodymium
Yttrium Aluminium Garnet, Yb:YAG Ytterbium Aluminium Garnet (Disc laser) and CO2 (Gas Laser).
The basic components of laser consist of:
1) Laser Medium
2) Input “Pump” energy
3) Rear total reflecting mirror
4) Front partial reflecting mirror
5) Resonator
Figure
1
Figure 1 [4] shows the basic
construction of laser.
There are two kind of laser usage,
continuous wave (CW) which is similar to arc welding process and pulse which is
similar to resistance spot welding.
1.1 Laser
Pulse
The characteristic of a beam of laser
light are monochromatic (single wavelength) and collimated (parallel), which
give superior focus down to a very small spot. When the density of the photons
is sufficient, it can melt metal and alloys in a matter of seconds. The
commonly used for pulse welding is 1.064 micron Nd:YAG wavelength that has the
option of being transmitted through an optical fiber [5]. The basic
construction of pulse laser welding system comprises a number of elements:
1) Laser Beam
2) Beam delivery
3) Focusing Head
4) How a laser
welds
5) Key welding
parameters
Figure 2
Figure 2 demonstrates the elements of a
pulse laser welding system.
An instance for the pulse laser used for
welding, is a flash lamp pumped Nd:YAG crystal as it can handle the high power
generated and heat produces during laser process. Diodes are unsuited for laser
pulsing and Q-Switching does not provide sufficient energy for welding [1].
a) Coupling
Metals and
alloys basically are not transparent to laser light and are good reflector of
laser light in room temperature; hence the laser beam that hit material is
either absorbed or reflected. Nevertheless the photons that do get absorbed are
converted to heat and raise the local temperature of metal surface.
Consequently as the temperature increases the photon absorption increases
leading to a chain reaction. In a very short time practically all the photons
are absorbed and the weld zone reaches its melting point.
Figure 3
Figure 3
Schematic shows increase in laser absorption with the temperature; absorption
increases dramatically as the metal melts. The shape of absorption curve varies
for different metals but overall shape is similar [1]. The coupling process can
be affected by a number of factors, such as photon density, surface conditions
like color and roughness which affect the absorptive of photons. It can also be
affected by weld geometry for instance butt weld geometry.
The next step in
pulse laser is the fusion zone. At low power densities, the photons are absorb
only on the surface and heat generated is then dissipated into the interior of
the metal via conduction; this type of welding is called conduction mode
welding [6].
Figure 4
Figure 4 is the schematic for Conduction
mode.
Once the zone (fusion) molten the heat
by convection current is transferred to the interior, this mode tends to
produce weld fusion zone which are shallow and bowl shaped. As the power
density increase it will produce keyhole which allows the laser to go deeper
into metal. In pulse mode every time the pulse ends the keyhole will close,
causing the entrapment of high pressure plasma at the bottom of keyhole thus creating
pore. Excessive heating rate can cause weld to spatter. Since typical pulse
laser welds are of very short duration in the range of 1-10 milliseconds,
direct measurement is difficult and has to be estimated based on weld results
[5].
c) Cooling
As the molten metal surrounded by large
mass of metal the weld will cool rapidly when the pulse terminated abruptly
after establishing fusion. Hence issues such as trapped porosity, high residual
stress, cracks and excessive weld metal hardness would happen.
next we will discuss Continous Wave......
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