Visualization of Marangoni Convection in Simulated Weld Pools

摘要

A transparent pool of NaNO_3 (10 mm in diameter) was heated with a defocused CO_2 laser beam to simulate Marangoni convection in arc weld pools without a surface-active agent. The flow patterns were revealed clearly by flow vi-sualization with a laser light-cut technique, the surface temperature profiles were measured immediately below the pool surface, and a device for measuring the beam diameter was developed. The observed Marangoni convection was expected to resemble that in welding because the Marangoni number was close to those in welding. Two counterrotating cells were observed in the meridian plane of the pool. The maximum velocity was at the pool surface, the outward surface flow was much faster than the inward return flow and the centers of the cells were near the pool edge. These characteristics suggest Marangoni convection dominates in the pool over gravity-induced convection. Increasing the beam power (from 0.5 to 5.4 W) and reducing the beam diameter (from 5.9 to 1.5 mm) both made Marangoni convection stronger. The latter, however, had a significantly greater effect; the surface flow was so much stronger as to make the return flow penetrate deeper into the pool. The results of physical simulation provided interesting insights for understanding the significant effect of Marangoni convection on the weld pool shape, as will be presented in a follow-up report.