This thesis presents an extensive experimental and numerical investigation into Wire Arc Additive Manufacturing (WAAM) and its integration with advanced joining and simulation strategies for steels and bimetallic alloys. The research encompasses the development, optimization, and characterization of WAAM processes for AISI 308L stainless steel, ER70S-6 low-alloy steel, and CuAl8 copper-based alloy, as well as hybrid laser welding of dissimilar WAAM-fabricated plates. Using Cold Metal Transfer (CMT) as the deposition technique, process parameters, including arc power, wire feed rate, and deposition speed, were systematically studied through full-factorial Design of Experiments (DOE) and validated via thermal imaging, microstructural, and mechanical analyses. The results show that optimal combinations of deposition speed (5–9 mm/s) and arc power (1200–1700 W for stainless steels; 1600–2800 W for low-alloy steels) significantly affect bead geometry, wetting angle, and aspect ratio, achieving values consistent with ideal ranges (aspect ratio ≈ 2.5, wetting ≈ 60°). Finite Element and Computational Fluid Dynamics (CFD) simulations, coupled with phase-field modeling, were developed to predict thermal distribution, melt pool dynamics, and residual stress fields. The models demonstrated strong agreement with experimental measurements, providing a robust predictive framework for multi-scale WAAM simulations. Microstructural analyses using SEM, EBSD, and XRD revealed refined dendritic morphologies and δ-ferrite/γ-austenite transformations in AISI 308L and ER70S-6 deposits, with grain sizes ranging from 5 μm in the heat-affected zone to 16 μm in weld centers. The CuAl8 and ER70S-6 bimetallic structures exhibited distinct phase transitions (α and β phases) with peak microhardness values of 137 HV (CuAl8) and 259 HV (ER70S-6), while hybrid laser welding of WAAM-fabricated dissimilar plates using 316L filler achieved enhanced interfacial bonding, maximum microhardness of 303 HV, and improved tensile performance with up to 43% strength increase at optimized parameters. Overall, this research advances the understanding of process–structure–property relationships in WAAM and laser-hybrid welded steels and bimetals. The integration of experimental, statistical, and numerical methods provides new insights into thermal-mechanical behavior and solidification phenomena, contributing to the design of defect-free, high-strength, and corrosion-resistant components for energy, aerospace, and marine applications.