Visualization of Marangoni Convection in Simulated Weld Pools Containing a Surface-Active Agent

摘要

The theory that a surface-active agent can cause weld pool Marangoni convection to reverse itself -to go inward along the pool surface and downward toward the pool bottom and result in deeper weld penetration — was proposed by Heiple and Roper in 1982 and studied worldwide since then. To actually observe such a flow pattern, a simulated transparent weld pool of NaNO_3 (10 mm in diameter) was heated with a defocused CO_2 laser beam to induce Marangoni convection, and C_2H_5COOK was used as a surface-active agent to cause flow reversal. A laser light-cut technique was used to reveal convection in the pool. With pure NaNO_3, convection was outward along the free surface and downward along the pool wall. However, when C_2H_5COOK was present (e.g., at 0.5 and 1 mole-%), convection slowed down. When more C_2H_5COOK was present (e.g., at 1.5 and 2 mole-%), convection was reversed to go inward along the pool surface and downward along the pool axis. Since this is in the opposite direction of gravity-induced buoyancy convection, Marangoni convection clearly dominated in the pools. In stationary laser beam welding, NaNO_3 containing 1 mole-% C_2H_5COOK showed deeper weld pools and inward surface flow, while pure NaNO_3 showed shallower weld pools and outward surface flow. This is consistent with the reversal of Marangoni convection caused by C_2H_5COOK observed in the flow visualization.