Laser powder bed fusion (LPBF) for additive manufacturing (AM) of metals has garnered significant attention over the past decade, not only for its unique ability to fabricate complex geometries but also for the distinctive microstructural features it imparts, such as hierarchical grain and sub-grain structures. Notably, AM produces unusually rich sub-grain structures, primarily solidification or dislocation cells that lead to only low degrees of crystallographic misorientations yet are effective dislocation barriers. Such microstructural features, distinctly different from conventional thermomechanical processing, are largely responsible for the enhanced strength-ductility frequently reported for AM metals, but remain poorly understood. Key questions persist, including how solute segregation networks form, what mechanisms underlie the generation of dislocation cellular structures, and how the latter correlate with solidification cell structures. In this overview, we delve into the unusual solidification conditions during LPBF, focusing on the mechanisms underlying grain and sub-grain evolution, with the aid of computational simulations to gain insight into the experimental observations available. Overall, this overview aims to improve the foundational understanding of the microstructural evolution in LPBF-fabricated metals, illustrate the new aspects of sub-grain features different from conventional polygonised dislocation cells, and offer an outlook for future advances in the field from a metallurgy standpoint.

Link:Unconventional sub-grain structures formed in additively manufactured metals - ScienceDirect