Advanced Welding Processes: Technologies and Process Control

High-energy density processes 151
Pulsing of laser output
The output of both CO2 and Nd:YAG lasers may be pulsed to a high level to
achieve increased peak power output. A typical 5 kW rated CW CO2 laser
has a pulse frequency modulation of between 0 and 25 kHz, and can deliver
a peak pulse power of five times the continuous output. 6 Nd:YAG systems of
400 W average power are capable of delivering peak power levels of 5–20
kW over very short durations. The use of higher peak powers (at low average
power) enable pulsed lasers to weld a larger variety of materials than those
possible using the CW mode of operation. Overlapping pulses are used to
achieve a continuous weld seam.
Process control
The parameters which control laser welding may be classified as primary
and secondary variables as shown in Table 8.2.
Primary controls. The relationship between beam power, welding speed
and material thickness is common to most materials and laser types and is
illustrated in Fig. 8.12. [165] The secondary control variables have a more
complex effect on welding performance, but some attempts have been made
to develop operating envelopes, which describe the relationship between
welding speed and focus position as shown in Fig. 8.13. Shielding gas flow
can have a pronounced effect on process efficiency and with argon in particular
there is a possibility of shielding gas plasma formation at critical flow rates
in CO2 laser systems. 7
8.3.5 Applications
The range of materials that can be successfully welded by laser techniques
is, in the first instance, determined by their physical properties, including
Table 8.2 Control parameters – laser welding
Primary variables Secondary variables
Beam power Pulse parameters
Travel speed Plasma control
Spot size (and position) Shielding gases
Operating mode (CW or pulsed) Beam mode (normally fixed)
Wire addition
6 Rofin-Sinar RS 5000.
7 Shielding gas plasma plumes are particularly problematic since they persist in the laser
path even after absorption has decreased workpiece coupling. Metal vapour plasma will
tend to be suppressed when absorption attenuates the beam.