Perfecting densification in fusion-based additive manufacturing of metals: Current research status, challenges and perspectives

Additive manufacturing has emerged as a transformative technology for fabricating high-performance metallic materials, yet its widespread adoption is hindered by challenges in its reliability, repeatability, and cost-effectiveness. Achieving full densification remains critical because it unlocks the mechanical resistance inherent to distinctive additively manufactured microstructures. However, densification-related defects linked to melting and solidification, such as cracks and porosity, necessitate comprehensive strategies spanning pre-process, in-process, and post-process treatments. This review systematically examines the influence of densification on mechanical properties in fusion based additive manufacturing and proposes a three-tier classification: functionally unprintable, tensile-relevant near-full, and fatigue-critical full densification. The transition from unprintable printing to near-full densification can often be achieved through material modification and process optimization, which restore tensile integrity until continued defect reduction produces diminishing improvements in monotonic strength. Further progression to full densification remains challenging because individual fatigue critical defects may continue to control crack initiation even as the retained defect population is reduced. Overcoming this challenge requires novel research paradigms that extend beyond established strategies. This article highlights innovative attempts toward full densification, encompassing progressive process refinements and multifield integration to achieve robust stability, reliable post-processing ensuring defect elimination, and digital twin enabled dynamic adjustments to address stochastic defects. By providing a comprehensive analysis of the state-of-the-art and emerging trends, this paper outlines the upstream research and disruptive technology development for promising applications of fusion-based additive manufacturing.