In addition to cracking, defects such as lack of fusion between weld beads or the weld bead and the side­wall, variable penetration, or second-phase inclusions can degrade weld quality. Lack of fusion defects is a notable concern when welding high-alloy nickel — based materials, which have notably ‘sluggish’ weld pools and are difficult to wet and tie into adjacent material (Figure 14(a)).

Inclusion-type defects are another concern and can be grouped into at least two types: (1) those that result from alloying additions or slag and (2) those that form via reaction with the environment. An example of the first type is given in Figure 14(b), which shows an unmelted iron-niobium Laves phase that is an intentional alloying addition to the flux coating of a shielded metal arc electrode. While alloying in this manner is a cost-effective way to tailor the composition of the electrode, it can lead to brittle second phases that also affect the local composition. In this case, the Nb-rich Laves phase is a strong melting point suppressant which can lead to either solidification — or liquation-type cracking.

The second type of inclusion is generally oxide or nitride-type particles that form via reaction with air. The corrosion-resistant alloys used in nuclear power systems (i. e., Fe-based stainless and Ni-based alloys) are especially prone to oxide-type defects as the nature of their corrosion resistance depends on the formation of stable, tenacious oxide films. An example of an aluminum-titanium-rich eutectic — type oxide that formed in a poorly shielded Alloy 690 fusion weld is shown in Figure 14(c). Another consideration of this oxide formation is that whatever metallurgical effect these alloying elements have is lost if they oxidize prior to solidification (e. g., the grain nucleating effect of Ti(C, N)-type particles).

Recent research shows that control of oxygen is critical to the weld puddle flow and wetting in nickel — based filler metals.10 In practice, this often translates into careful wire drawing practice so as to minimize the extent of embedded oxides or wire drawing lubri­cants into the filler metal. Figure 15 shows variability in the bead contour and tie-in of two filler metals welded under identical conditions, which was later traced to wire cleanliness. Additionally, separate test­ing shows that ^100 wt ppm levels of oxides can have macroscopic detriment on the regular flow and con­tour of Ni-30Cr-type filler metals.10