Bubble Growth-Driven Spatter Generation in a TIG Weld Pool under Ar+N2 Shielding Gas

The scope of Ar+N2 TIG welding, for which a few percent of nitrogen gas contents are mixed into argon shielding gas, is expanding toward realization of a carbon-neutral society. However, spattering is sometimes generated during TIG welding under Ar+N2 shielding gas. Earlier studies have suggested that gas generation and rapid gas growth within the weld pool generate spatter. Nevertheless, no direct causal relation between gas generation in the weld pool and spatter generation has been clarified. Furthermore, the quantitative growth rate of the bubbles which generate spatter also remains unclear. For this study, we performed high-speed direct observation of the weld pool interior using quartz glass and quantitative analysis of the nitrogen content in the weld. The findings revealed the relation between gas generation and spatter in Ar+N2 TIG weld pool. During Ar+5%N2 TIG welding, a large amount of dissociated nitrogen was absorbed into the weld pool through the arc plasma space, exceeding the nitrogen solubility in the weld pool and leading to gasification. The apparent radius of the bubble grew transiently at approximately 0.5 m/s immediately before spatter ejection. Spattering then occurred when the grown bubbles were released into the atmosphere. These results indicate that the nitrogen solubility, which is material-dependent, serves as the threshold for spatter generation during TIG welding.