Advanced Welding Processes: Technologies and Process Control

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Advanced welding processes
remains axially symmetrical multimode beams are suitable for most welding
applications.
8.3.4 Welding with lasers
Welding modes
Lasers may be used for both melt-in (conduction-limited) welding in a similar
manner to GTAW, or in the keyhole mode described above. The beam energy
delivered to the workpiece will be dissipated by reflection and absorption. In
the case of laser welding, the reflected energy losses may be large, many
materials being capable of reflecting up to 90% 5 of the incident beam energy.
The reflectivity is reduced as the temperature of the surface is increased and
greater amounts of energy will be absorbed. The absorbed energy will be
conducted away from the metal surface and if sufficient energy is available
to establish a weld pool, convection within the pool may assist energy transfer.
The conduction-limited mode is often used for micro-welding applications
using low-power, pulsed lasers to ensure low heat input.
If a sufficient vaporisation pressure is developed in the weld pool the
reflection losses can be substantially reduced, the beam energy is absorbed
more efficiently, and a keyhole is formed. The process may be used with
single high-energy pulses to produce spot welds or with continuous or repeated
pulsing to produce butt welded seams as discussed above.
Conduction limited welding typically produces weld bead depth-to-width
ratios of around unity, whereas the keyhole mode can produce depth-towidth
ratios of 10:1.
Shielding gases
It is common to supply a shielding gas to the weld area to protect the molten
and solidifying metal from oxidation. Due to the high travel speeds involved
this often takes the form of an elongated shroud which trails behind the beam
as shown in Fig. 8.10. [159] It is also necessary to provide backing gas to the
rear of the joint in order to obtain clean penetration beads with satisfactory
profiles. The most common gases used for shielding are helium and argon
which are inert. Because of its high ionization potential, helium is more
resistant to plasma formation whilst argon, because of its density should
offer improved shielding efficiency, but some additional plasma control
measures may be necessary, particularly with CO2 lasers, as a result of its
5 The reflectivity of austenitic stainless steel to infrared radiation (1060 nm) is 92% whilst
that of copper is 98%.