Advanced Welding Processes: Technologies and Process Control

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Advanced welding processes
in resistance to fatigue as well as cost savings compared with pure CO 2
shielding (see also Chapter 7).
In some cases, the shielding gas mixture may improve the consistency of
fusion; argon/hydrogen mixtures have been found to have a beneficial effect
on cast-to-cast variation in the GTA welding of austenitic stainless steel (as
discussed in Chapter 6).
Joint properties
Fused area
Weld area
Reinforcement
5.3 Fusion geometry of fillet and square butt welds. The gas can
affect the ratio of fusion area to total area, the fusion profile and the
reinforcement geometry.
The mechanical properties of the weld will depend on freedom from defects
and the final weld metal microstructure, both of which are influenced by the
shielding gas. Porosity may be controlled by selecting an appropriate shielding
gas and ensuring that an efficient gas shield is maintained. Fusion defects
may be minimized by selecting a gas that gives increased heat input, and
oxide inclusions may be limited by controlling the oxidizing potential of the
gas.
The final weld microstructure may be influenced by the gas as a result of
its effect on heat input and weld metal composition. For example, it has been
found [60] that with ferritic steels an improvement in toughness may be
produced by increasing the oxidizing potential of the GMAW shielding gas
(by adding up to 2% oxygen and 15% CO 2 to argon) as shown in Fig. 5.4.
This is thought to be due to the nucleation of fine-grained acicular ferrite by
controlled levels of micro-inclusions. A further increase in the oxidizing
potential could, however, lead to the formation of coarse oxide inclusions
which would result in a deterioration of weld metal toughness. These effects
are relatively small and also rely on the composition of the welding wire and
the ability to maintain stable process performance.