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1 (a) Shielding with pure He: dγ/dT < 0 and dT/dl < 0, so dγ/dl > 0.

2 (b) Shielding with pure He: dγ/dl > 0 induces outward surface flow, and results in a low depth/width ratio.

3 (c) CO2 → CO + 1/2 O2 and 1/2 O2 → O or just CO2 → CO + OThe O content in the weld pool increases with increasing CO2 content of the shielding gas. If the O content is sufficiently high, dγ/dT can become positive. Since dγ/dT > 0 and dT/dl < 0, dγ/dl < 0.

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 3.2 Two 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 SO₂ gas. Which weld was deeper, and why?