Evolving Process Dynamics Governing Porosity and Porosity-Microstructure Interactions in Powder Bed Fusion–Laser Beam Additive Manufacturing

Abstract

Powder Bed Fusion–Laser Beam (PBF-LB) enables fabrication of complex, near-net-shape metal parts, yet its adoption in fracture-critical applications remains limited by process-induced porosity. Conventional process windows, developed under nominally steady-state conditions, map porosity regimes such as balling, keyhole porosity, and lack-of-fusion (LoF). However, porosity also evolves dynamically during the build and depends on local geometric variations at the part, layer, and melt pool levels that are not captured by steady-state process windows. This review summarizes established porosity formation mechanisms comprising conventional process windows and then examines porosity evolution under transient build conditions. Focus is given to microstructure-driven shrinkage porosity associated with local preheat variations, LoF driven by melt pool geometry variability, and gas porosity governed by pore evolution within the melt pool. The role of remelting in modifying existing porosity is also discussed. In addition, interactions between porosity and the near-pore microstructure, a relatively unexplored topic, are examined. Translating this understanding into mitigation strategies requires closing the loop between process physics and porosity control. Opportunities include improved modeling of melt pool variability and spatter transport, integration of feed-forward and feedback control to account for local geometric variations, and post-processing routes such as hot isostatic pressing and surface machining. Overall, this review emphasizes that porosity develops in response to local process conditions, particularly in complex geometries where steady-state assumptions break down, and can further perturb the local microstructure. Understanding this behavior is essential as the field moves from coupon-scale studies to full parts, where spatially varying porosity and its interaction with microstructure govern fracture-critical performance.