  The unique sub-grain patterns have been found in some particular alloys
(316L, Al-Si, Co-Cr-Mo) selective laser melting (SLM), the submicron-scale
cellular, elongated cellular or even band structures are always coexisting
inside one single macro-solidified grain. Furthermore, the cellular structures
are symmetrical with hexagonal, pentagonal and square cellular patterns where
the cellular size is only around 1{\mu}m. Single-layer and bulk 316L SLM
experiments are presented that reveals the forming mechanism of these sub-grain
cellular microstructures. Complex cellular sub-micron patterns were formed by
the local convection and B\'enard Instabilities in front of the solid/liquid
(S/L) interface (so-called mushy zones) affected by intricate temperature and
surface tension gradients. In other words, this nonlinear self-organization
phenomenon (B\'enard Instability) occurring at the S/L interface is
superimposed on the macro-grain solidification process to form the sub-grain
patterns/structures and elemental microsegregations. This simple and unified
explanation can be expanded to other eutectic alloys formed by SLM, like the
Al-Si system.
