Oscillatory Marangoni flow: A fundamental study by conduction-mode laser spot welding
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Publication Details
Author list: Kou S., Limmaneevichitr C., Wei P.S.
Publication year: 2011
Journal: Welding Journal (0043-2296)
Volume number: 90
Issue number: 12
Start page: 229s
End page: 240s
ISSN: 0043-2296
Languages: English-Great Britain (EN-GB)
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Abstract
Marangoni flow, a fundamental subject extensively studied in welding, was further studied by using conduction-mode (non-keyhole) laser spot welding of 304 stainless steels. It was discovered that a surface-active agent, such as sulfur (S) in stainless steels, can affect not only the weld pool depth as explained by Heiple and Roper's model, but also the pool-surface deformation, pool-surface oscillation, and ripple formation. With low S (42 ppm), the pool surface was concave and oscillatory, and the resultant weld was shallow with clear ripples. With high S (140 ppm), however, the pool surface was convex and nearly steady, and the resultant weld was deeper without clear ripples. A mechanism was proposed to explain these strikingly different phenomena. At low S the fast outward surface flow can make the pool surface concave, as shown by computer simulation. The raised surface near the pool edge is unstable; it can oscillate with oscillatory Marangoni flow, disturb solidification at the pool edge, and cause clear ripple formation. At high S, the fast inward surface flow can make the pool surface convex. Oscillation of the raised surface near the pool center, however, may not disturb solidification at the pool edge enough to cause clear ripple formation. The mechanism was verified by observing pool-surface oscillation and ripple formation immediately after turning off the laser. Furthermore, oscillatory Marangoni flow was demonstrated by flow visualization in simulated stationary weld pools of NaNO 3. Surface oscillation induced by oscillatory Marangoni flow was demonstrated with a NaNO 3 drop laser heated from above.
Keywords
Marangoni flow, Surface tension, Thermocapillary, Weld pool
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