| Strong temperature gradients below the laser generate temperature-dependent and complex hydrodynamic flows in the melt pool that generate spatter formation during build. |  Ref[1] - modelling Marangoni and recoil in short-lived melt pools |
Amongst these, surface tension gradient generate surface convection flows known as the Marangoni effect. It generally drives the melt flow from the hot laser spot toward the cold rear (although this may vary with the melt pool composition!). The result is an increase of the melt depth, recirculation of the melt flow and creation of spattering as high-speed surface liquid metal may break away from the more viscous (cooler) body of the melt pool.
Ref[1] - recoil pressure
In addition, the surface temperatures below the laser spot can easily reach boiling values. As the laser scans across the powder bed, liquid develops ahead of the laser spot: the powder is rapidly melted by conduction. At the contact point between powder bed and high power laser, a gas plume [22] is created: this is a high vapour surface flux. It exerts a pressure on the liquid of the melt pool directly under the laser spot and generates a depression in the molten pool. The vapor recoil pressure adds extra forces to the surface of the liquid that create a melt pool surface depression below the laser. The result is similar in nature to the “bow wave” that develops as a boat moves through the water, where the high vapour surface flux exerts a pressure force that ejects the melt liquidified ahead of the laser. Hot, low-viscosity liquid rapidly moves up the front wall of the depression and spills over onto the powder particles ahead of the laser beam. When the liquid metal elongates, it thins out and breaks up into small droplets that can be ejected and deposited as spatter particles in the powder bed.
References
S. A. Khairallah, A. T. Anderson, A. Rubenchik, W. E. King, Acta Materialia 108 (2016) 36-45
S. A. Khairallah, A. T. Anderson, A. Rubenchik, W. E. King, Acta Materialia 108 (2016) 36-45