Sindo Kou - Welding Metallurgy

4 (d) Shielding with He/CO2: dγ/dl < 0 induces inward surface flow and hence downward axial flow, which increases the depth/width ratio. The more CO2 is in the shielding gas, the more O can be present in the weld pool to make dγ/dT > 0 and dγ/dl < 0, and hence the higher the depth/width ratio.

Figure E3.1 Distributions of surface tension on weld pool surfaces and flow patterns in weld pools.

For more details, see Lu et al. [72].

Example 3.2Two GTA welds of the same 304 stainless steel were made at 47 A current and 3.33 mm/s travel speed, one with a shielding gas of Ar and the other Ar plus 700 ppm SO 2gas. Which weld was deeper, and why?

The weld made with Ar plus 700 ppm SO 2gas was deeper because SO 2decomposed in the arc and the S absorbed by the weld pool became a surface‐active agent and reversed the Marangoni flow to deepen the weld.

Figure E3.2 Effect of SO 2(g) in Ar shielding gas on weld shape.

For more details, see Heiple and Burgardt [31].

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