Examples
Example 3.1 It has been observed in GTAW shielded by a He gas containing CO2 that the depth/width ratio of the weld increases with increasing CO2 content [72]. Assume the Lorentz force is small because of the low welding current used. (a) Sketch the surface tension distribution along the pool surface with pure He shielding. (b) Sketch the flow pattern in the weld pool with pure He shielding. (c) How does CO2 affect the O content in the weld pool? Give the chemical reaction(s) involved. Sketch the surface tension distribution along the pool surface with He/CO2 shielding. (d) Sketch the flow pattern in the weld pool with He/CO2 shielding. Why does the depth/width ratio increase with increasing CO2 content in the shielding gas?
Answer
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?
Answer
The weld made with Ar plus 700 ppm SO2 gas was deeper because SO2 decomposed 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 SO2(g) in Ar shielding gas on weld shape.
For more details, see Heiple and Burgardt [31].
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