Additive manufacturing (AM) has emerged as a transformative technology for producing complex geometries and high-performance metallic components, with applications spanning aerospace, defense, aviation, automotive and biomedical industries. However, the rapid thermal cycling and non-equilibrium solidification conditions intrinsic to AM processes often lead to unpredictable microstructures, phases and mechanical properties compared to conventionally processed materials. Understanding how both processing dynamics and compositional complexity influence solidification behavior is key to controlling material performance. This dissertation presents a progression of studies that is built upon a custom-developed platform for operando synchrotron X-ray diffraction, designed to investigate real-time phenomena during metal AM. The first study demonstrates the setup designed to mimic real-world metal AM processes such as directed energy deposition (DED) and powder bed fusion (PBF), enabling real-time observation of melt pool dynamics, phase transformations, and solidification pathways by capturing high-fidelity datasets in real time at the Cornell High Energy Synchrotron Source (CHESS). The second study leverages this custom setup to investigate solidification behavior in Inconel 625, a canonical nickel-based superalloy. By coupling high-speed synchrotron diffraction with postmortem electron backscatter diffraction (EBSD), this work reveals distinct solidification signatures and dendritic deformation modes, offering new insights into microstructural development under rapid solidification. Building upon this foundation, the third study investigates how introducing compositional heterogeneity through the concept of “dissimilar amalgamation” can influence solidification behavior. Specifically, small additions of stainless steel 316L are incorporated into Ti-6Al-4V, resulting in substantial microstructural modifications. Operando synchrotron studies conducted at the Advanced Photon Source (APS) reveal the emergence of coexisting α′ and β phases—an uncommon pairing not observed in the monolithic alloys. This compositional complexity with addition of 316L causes changes in the ’ lath grain sizes along with increased retention of metastable  phase. Transmission electron microscopy (TEM) captures fine-scale features that are not resolvable via synchrotron, offering deeper insights into the resulting microstructure. Together, these studies establish a framework for linking operando observations to final microstructural features, advancing our ability to design and control solidification in metal AM.