The wide applications of additive manufacturing are hindered by the lack of comprehensive understanding of process-structure-property relationships. To this end, we have developed and seamlessly integrated a series of high-fidelity multi-physics models. Specifically, multiphase flow models using the coupled computational fluid dynamics (CFD) and discrete element method (DEM) simulate the powder spreading procedure and powder spattering and denudation phenomena in the powder melting procedure. The powder melting model is powerful to reproduce the molten pool flow and relevant defects (e.g., lack-of-fusion and keyhole pores) by incorporating the major physical factors, e.g., the physically-informed heat source models, i.e., for an electron beam from micro-scale simulations of electron-atom interactions and for a laser incorporating multi-reflection and Fresnel absorption. The microstructure evolutions at both the grain- and dendrite- scales are modelled using the phase field and cellular automaton methods. The mechanical properties and thermal stresses are simulated using the crystal plasticity finite element (FE) model, which incorporates the realistic geometry (rough surfaces and voids), temperature profiles and microstructures. These models have proven to be useful in revealing the physical mechanisms, which have been validated against experiments.