Advanced Welding Processes: Technologies and Process Control

High-energy density processes 153
Most of the metallurgical problems experienced with laser welding are
common to other fusion welding processes; for example, cold cracking in
high-carbon and alloy steels, porosity and solidification cracking in aluminium
alloys, but, with suitable precautions, these problems may be restricted.
Some common application areas are described below.
Austenitic stainless steel
Austenitic stainless steels have been laser welded in a range of thicknesses.
Typical conditions for 13.3 mm keyhole welds are given in the figures above,
but speeds of up to 1 m min –1 can be achieved with a CO2 laser at 11 kW. As
with other welding techniques, care is needed to avoid solidification cracking
and sensitization although the thermal cycle may restrict thermal damage in
the HAZ.
Low-carbon steel
Low-carbon steel is readily joined with a range of common welding processes,
including GMAW and GTAW. The primary reasons for using high-capitalcost
processes such as lasers is to increase productivity and to improve
quality. There are no specific problems with the laser welding of uncoated
plain carbon steels and both CO 2 and Nd:YAG systems have been used
successfully as the following applications confirm.
In thin-section sheet material, laser welding has been used for fabrication
of high-precision pressings [166] to fabricate beams for the carriages for a
CNC punch press. A 5 kW CO 2 laser was used and the main objective was
to limit distortion and weld finishing operations.
Lasers are being adopted for many carbon steel welding applications in
the automotive industry, [167] including the welding of floor panels and
engine support frames. In most cases, robotic automation is involved and
integrated beam delivery systems have been developed. The use of 1 kW
Nd:YAG lasers with optical fibre delivery systems have also been applied to
robotic welding.
Coated steels, particularly zinc-coated or galvanized materials, are difficult
to weld and, even if satisfactory parameters are developed, they are prone to
batch variation. Some success has, however, been reported using a Nd:YAG
laser with a multiple laser (Multilase) system. [144, 168]
Laser welding has also been evaluated for fabrication of thicker section,
higher-strength steels such as ASTM A36 (0.29% C, 0.8–1.2% Mn, 0.15–
0.40% Si) [169] and it was found that welding speeds of up to 1 m min –1
could be achieved in 19 mm thick plate using a 15 kW CO 2 laser. A cost
analysis indicated a three-year payback period for the laser system in this
application.