Depth-dependent microstructural observations reveal the role of thermal cycling on the formation of a hierarchical dislocation cell structure during selective laser melting of 316L stainless steel

Abstract

Detailed microstructural observations as a function of depth from the surface of the final melt pool using both transmission electron microscopy (TEM) and electron back-scatter diffraction (EBSD) have been carried out. The observations show that a well-defined dislocation cell block structure is formed during selective laser melting (SLM) of 316L stainless steel, resulting in hierarchical dislocation cell structure in interior volumes. Specifically, orientation measurements using both TEM and EBSD, combined with high-angle annular dark-field imaging, show that the evolution of the dislocation cell-block structure in SLM-prepared 316L is a combined effect of deformation, solute segregation, and thermal cycling. A segregation network forms first during solidification and then stresses due to rapid solidification/cooling coupled with melt-pool constraints result in a high density of dislocations becoming trapped at the segregation network to form dislocation cells in each melt pool. Thermal cycling of volumes below each finally-formed melt pool, from over-printed layers, then additionally leads to the formation of well-defined cell-block structure as a result of biased-dislocation accumulation associated with nascent small orientation variations in just-solidified melt pools. Depth dependent hardness measurements confirm that these cell-block boundaries directly contribute to the mechanical strength of the microstructure observed in interior (bulk) volumes of additively manufactured samples.