The rapidly developing additive manufacturing (AM) enables the fabrication of complex geometries that were previously considered infeasible . However, the highly localized energy input and rapid thermal cycles seen in beam-based metal AM are common causes of solidification cracking and anisotropic mechanical properties. Various studies have shown approaches to address the processing defects and material anisotropy in powder-based AM, such as creating AM-specific alloy materials or decorating the feedstock powder with grain modifiers. These solutions can be economically burdensome and do not fully leverage the in-situ material formation in AM. My work aims to expand the portfolio of AM capabilities through in-situ reactive printing (IRP). This advanced processing technique forms material with chemical reactions triggered by localized energy input and rigorous thermal-driven mixing during AM material solidification.In chapters two and three, I discuss the IRP of metal matrix composite focusing on the binary material system of aluminum (Al) and titanium (Ti). I begin with a feasibility study of IRP by fabricating in-situ TiAl3-reinforced aluminum matrix composites (AMC). It demonstrates improved processibility of AM aluminum and shows the capability of IRP to create compositional tunability with an extended ceiling of AMC intermetallic volume fraction. I further optimize the microstructure for enhanced tunable tensile properties and discuss the dual functions of the in-situ formed TiAl3 reinforcement phase: load transfer strengthening and grain refinement by providing heterogeneous nucleation sites. Extending the application of IRP, I fabricate porous metal and explain a post-process framework created to automate the collection of pore statistics from high-speed X-ray image sequences of operando synchrotron experiments elucidating the dynamic process of pore formation. Discussions on the effects of foaming agent type, content, and different processing parameters on porosity characteristics are presented in the fourth chapter. The findings shed light on AM enabling the fabrication of free-form porous metal structures and guide design choices for future AM porous metals.