Melt flow plays a critical role in determining the built quality during additive manufacturing (AM). External forces are usually applied to control melt flow. Thermoelectric (TE) Lorentz forces arise within the AM melt pool when an external magnetic field is applied; these forces offer the potential to control melt flow, removing pores and tailoring microstructures. However, the extent of TE forces, and even their underlying mechanisms, remain unclear. Here, we performed in situ high-speed synchrotron X-ray radiography to reveal the structure of TE force-induced flow using tungsten particle tracers during AM. We found that without an imposed magnetic field, natural convection dominates the flow and hence the tungsten particle trajectories. With a magnetic field applied perpendicular to the scan direction, a TE Lorentz force is induced and it alters the melt pool flow. Whereas when the laser scan direction is reversed, such force is introduced in an opposite direction.