Functionally graded materials (FGMs) and functionally graded structures (FGSs) are special types of advanced composites with peculiar features and advantages. This article reviews the design criteria of functionally graded additive manufacturing (FGAM), which is capable of fabricating gradient components with versatile functional properties. Conventional geometrical‐based design concepts have limited potential for FGAM and multi‐scale design concepts (from geometrical patterning to microstructural design) are needed to develop gradient components with specific graded properties at different locations. FGMs and FGSs are of great interest to a larger range of industrial sectors and applications including aerospace, automotive, biomedical implants, optoelectronic devices, energy absorbing structures, geological models, and heat exchangers. This review presents an overview of various fabrication ideas and suggestions for future research in terms of design and creation of FGMs and FGSs, benefiting a wide variety of scientific fields.

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After a brief introduction on selective laser melting (SLM) and "ex situ" manufacture of metal matrix composites (MMCs), this paper reviews the capacities and benefits of SLM to activate and control various "in situ" reactions during fabrication of 3D parts. It introduces several systems (such as etc.) used to manufacture Al-based, Ti-based, and steel-based "in situ" MMCs. Then, it illustrates the novel microstructural characteristics of these SLM-made "in situ" MMCs for different cases, as they may appear from nano-particles to nano-whiskers and dendritic reinforcements homogeneously distributed in a metal matrix. It also focuses on SLM associated "in situ" mechanisms, explaining how an "in situ" reaction propagates based on "decomposition, diffusion, and reformation" and how the growth mechanisms turn into different morphologies such as rounded particles, whiskers, or polygonal block shapes. The influence of various SLM parameters (such as energy density, laser power/speed, powder layer thickness, and the size of initial powder particles) and the SLM "in situ" challenges are also discussed.

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