The paper presents a transient three-dimensional model of an anti-phase-synchronized pulsed tandem gas–metal arc welding process, which is used to analyse arc interactions and their influence on the gas shield flow. The shielding gases considered are pure argon and a mixture of argon with 18% CO₂. Comparison of the temperature fields predicted by the model with high-speed images indicates that the essential features of the interactions between the arcs are captured. The paper demonstrates strong arc deflection and kinking, especially during the low-current phase of the pulse, in agreement with experimental observations. These effects are more distinct for the argon mixture with 18% CO₂. The second part of the paper demonstrates the effects of arc deflection and instabilities on the shielding gas flow and the occurrence of air contamination in the process region. The results allow an improved understanding of the causes of periodic instabilities and weld seam imperfections such as porosity, spatter, heat-tint oxidation and fume deposits.