Parameter-Controlled Geometrically-Undefined Porosity in Laser Powder Bed Fusion of Metals: A Review

Abstract Porous metal structures provide unique capabilities for energy absorption, thermal management, and fluid transport, particularly for aerospace platforms. Laser powder bed fusion additive manufacturing has enabled unprecedented spatial control of porosity, surpassing the capabilities of conventional porous metal foam manufacturing processes. Two distinct approaches exist for fabricating porous metals via powder bed fusion: (1) geometrically-defined porosity, where porous structures are explicitly included in computer-aided design and manufactured with controlled lattice/pore features, and (2) geometrically-undefined porosity, where porosity arises as an artifact of strategically adjusted processing parameters during manufacturing. While geometrically-defined porosity fabrication of lattice structures via powder bed fusion has been extensively reviewed, parameter-based geometrically-undefined porosity fabrication remains comparatively underexplored despite its ability to generally achieve pore sizes ranging from 1 to 100 µ m and total porosity between 0 and 50%. By systematically adjusting key processing parameters, including laser power, scan speed, hatch spacing, and layer thickness, researchers can tailor pore density, distribution, and morphology within the fabricated structures. Geometrically-undefined porosity offers distinct advantages, including the ability to achieve finer feature sizes, gradient porosity transitions, and potentially reduce manufacturing complexity for certain applications. This review provides a comprehensive overview of parameter-based geometrically-undefined porosity formation in laser powder bed fusion, synthesizes experimental findings across multiple alloy systems, examines the influence of porosity on mechanical, thermal, electrical, and permeability properties, and discusses research gaps and potential applications for this emerging technique.