Additive manufacturing (AM) of metallic components promises many advantages over conventional manufacturing processes through high design flexibility across multiple length scales and precision coupled with an astonishing combination of mechanical properties. Characterizing the relationship between chemical environment, microstructure, and mechanical properties remains one of the major challenges for this novel technology. A natural precursor is identifying the influence of the processing path on the developing microstructure. We combine experimental studies of single-track laser powder-bed fusion (LPBF) scans of 316L stainless steel, finite element analyses, and large-scale three-dimensional discrete dislocation dynamics simulations to shed light on the complex interplay and transient nature of temperature, residual stresses, and chemical environment during evolution of the dislocation microstructure in the cool-down phase. Our results highlight the significant role of chemical segregation for the formation of characteristic microstructures observed in LPBF processing. We therefore provide a unique mechanistic perspective on the complex, multi-scale nature of LPBF processing.