Additive Manufacturing (AM) of metallic materials represents a transformative shift in materials processing, enabling the production of geometrically complex and functionally optimized components. However, the metallurgical complexity inherent in fusion-based AM, particularly in laser powder bed fusion (LPBF) and directed energy deposition (DED), poses significant challenges to achieving defect-free, high-performance structures. This review brings together the current understanding of the metallurgical and surface engineering principles governing fusion-based AM. It explores the relationship between processing parameters, molten pool dynamics, temperature distribution, and microstructural evolution, focusing on mechanisms of grain refinement, phase transformations, and defect formation. Additionally, the emerging role of surface engineering, including both in situ and post-processing techniques in improving surface integrity, fatigue resistance, and corrosion performance of metallic AM parts will be highlighted. By bridging metallurgical science and surface modification strategies, this work provides a unified framework for optimizing both bulk and surface properties. Future directions in hybrid processing, real time monitoring, alloy design, and computational intelligence are discussed to guide the advancement of AM toward performance-driven, sustainable production.

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