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The laser-matter interaction and solidification phenomena associated with laser additive manufacturing (LAM) remain unclear, limiting its applications and process optimisation. Here, through in situ and operando high-speed synchrotron X-ray imaging, we reveal the underlying physical phenomena during the deposition of the first and second layer melt tracks. We show that the laser-induced gas/vapour jet promotes the formation of melt tracks and denuded zones via spattering (at velocities of 1 m s-1). We also uncover new mechanisms of pore migration by Marangoni-driven flow (at velocities of 0.4 m s-1), pore dissolution and dispersion by laser re-melting. We develop a mechanism map for predicting the evolution of melt features, including changes in melt track morphology from a continuous hemi-cylindrical track to disconnected beads with decreasing linear energy density; and improved molten pool wetting with increasing laser power. Our results clarify aspects of the physics behind LAM, which are critical for its development.