Laser WeldingLaser beam
welding is increasingly being used
in industrial production ranging from microelectronics to
shipbuilding. Automotive manufacturing, however, is among the
industrial sectors, which have proven to be most outstanding
at developing applications that take advantage of the many
benefits of this technology:
- Low heat input
- Small heat affected zone
(HAZ)
- Low distortion rate
- High welding
speed
These characteristics
have made laser welding the process of choice for many
applications that used resistance welding in the past. By
adding the benefits of single-sided access, laser welding is
given another strategic advantage, allowing it to open the
door for a multitude of new applications.
Hybrid processes involving a combination of laser
and MIG arc welding are being developed to reduce fit-up
requirements on the parts to be joined, thus improving the
most critical aspect of laser welding. The addition of filler
wire in GMAW allows to substantially reduce the requirements
relating to weld-edge preparation. Alloying elements in the
filler wire may be used to refine the mechanical properties of
the seam. Beyond that, these combined processes can improve
the welding speed of the individual processes, weld
penetration depth and overall seam geometry. Recent
breakthroughs in the field of laser diodes present new
opportunities for solving manufacturing tasks. They will,
however, require thorough application-focused investigations
in order to convert them into reliable manufacturing
processes.
High-power CO2
lasers (2-10 kW) are currently being used in the welding of
car bodies, transmission components, heat exchangers and
tailored blanks. For many years, low-power Nd:YAG lasers
(<500 W) have been used to weld small components, for
example, medical instruments, electronic packages and razor
blades. Nd:YAG lasers with power levels in the multi-kW range
benefit from beam delivery via optical fibers. These are
easily manipulated by robots, thereby opening a large field of
3-D applications, such as laser cutting and welding of car
bodies.
The laser beam is focused
into a small spot, providing the intensity to melt and
evaporate the material. To focus the high CO2 laser powers,
water cooled mirrors are primarily used instead of lenses. The
welding is basically carried out in two ways. In conduction
mode welding the heat is transferred from the surface into the
material via thermal conduction. This is typical for low-power
Nd:YAG laser welding with relatively shallow welds. High-power
laser welding is characterized by keyhole welding. The laser
energy then melts and evaporates the metal. The pressure of
the vapor displaces the molten metal so that a cavity is
formed - the keyhole. The keyhole supports the transfer of the
laser energy into the metal and guides the laser beam deep
into the material. Keyhole welding thus allows very deep and
narrow welds to be obtained and is therefore also called deep
penetration welding.
Welding gas plays an
important role in laser welding and fulfill several demands -
shielding of the weld pool and the heat affected area,
protection of the optics against fumes and spatter, root
protection and 'plasma control' during CO2 laser
welding.
The weld plasma is a cloud
of ionized metal vapor and gases that can be formed above the
keyhole. This cloud affects laser radiation and thus has the
potential to interrupt the welding process. This effect
depends on the type of laser and the power applied, for which
reason different laser welding gases are used.
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