On the Mechanism of Formation of the Fine Structure of a Track in Selective Laser Melting

Experimental results of microstructural studies of refractory nickel alloys obtained by selective laser melting under different crystallization conditions are systematized. The dendritic (cellular) structure and the distribution of the alloying elements in the volume of dendrite arms are studied by scanning and transmission electron microscopy. The detected microsegregation matches a normal crystallization law and is caused by concentration supercooling. It is shown that the main factors responsible for formation of the fine structure of a track are the orientation growth (epitaxy) and the competitive growth of crystallites. The orientation of the crystallites is transferred not only by the first-order arms but also by the second-order arms, and a specific direction is implemented upon coincidence of the crystallographic orientation of the crystallite with the radial direction of the heat removal. The increasing complexity and the fragmentation of the fine structure are associated with the mechanism of stochastic binarization of the crystallographic orientation implemented upon partial remelting and opening of fragments of cells in the current and preceding layers.