During powder bed Additive Manufacturing (AM) metallic powders interact with directed energy sources such as laser or electron beam, varying partial pressures of process gasses, vaporized/condensed material, plasma and ejected liquids. This results in morphologies, compositions, and phases not observed in as-atomized feedstocks. Localized concentrations on the scale of powder and the melt pool have a profound impact on solidification, local composition, microstructure evolution, solid-state phase transformations and subsequently properties in surprising and unexpected ways. This is further complicated by AM thermal cycling which can form far-from-equilibrium interfaces. While this creates an additional level of complexity towards understanding, and is typically considered a shortcoming of AM to be eliminated, it also motivates research to elucidate these mechanisms and create new tools for the intentional interfacial control of these effects for exploitation of newly emerged properties. We will present several recent case studies of unique material interactions in LPBF/EB-PBF AM.