The surface-tension-driven Benard conventions and unique sub-grain cellular microstructures in 316L steel selective laser melting

Abstract

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.