Role of surface-active elements during keyhole-mode laser welding

During high power density laser welding of mild steel, the keyhole depth, liquid metal flow, weld geometry and weld integrity are affected by base-metal sulfur content and oxygen (O₂) present in the atmosphere or shielding gas. The role of these surface-active elements during keyhole-mode laser welding of steels is not well understood. In order to better understand their effects, welding of mild steel specimens containing various concentrations of oxygen and sulfur are examined. In addition, a numerical model is used to evaluate the influence of the surface-active elements on heat transfer and fluid flow in keyhole-mode laser welding. Increase in base-metal sulfur concentration or O₂ content of shielding gas results in decreased weld widths. Sulfur results in a negligible increase in penetration depth whereas the presence of O₂ in shielding gas significantly affects the weld penetration. It has earlier been proposed that oxygen, if present in the shielding gas, can get introduced into the weld pool resulting in formation of carbon monoxide (CO) at the keyhole surface and additional pressure from CO can result in increased penetration. Numerical modelling has been used in this work to understand the effects of formation of CO on the keyhole and weld geometries.